Development of Biomaterials from Renewable Resources

UNIVERSIDAD DE CASTILLA-LA MANCHA FACULTAD DE CIENCIAS Y TECNOLOGÍAS QUÍMICAS DEPARTAMENTO DE INGENIERÍA QUÍMICA TESIS DOCTORAL:

DEVELOPMENT OF BIOMATERIALS FROM RENEWABLE RESOURCES

Presentada por: JUAN CARLOS DE HARO SÁNCHEZ para optar al grado de doctor por la Universidad de Castilla-La Mancha

Dirigida por: Dr. Ángel Pérez Martínez Dr. Manuel Salvador Carmona Franco

Ciudad Real, 2018

Dr. Ángel Pérez Martínez Profesor titular de Universidad

Departamento de Ingeniería Química

Universidad de Castilla-La Mancha

Dr. Manuel Salvador Carmona Franco Profesor titular de Universidad

Departamento de Ingeniería Química

Universidad de Castilla-La Mancha

Certifican que:

Juan Carlos de Haro Sánchez ha realizado bajo su dirección el trabajo titulado “Development of biobased materials from renewable resources” en el Departamento de Ingeniería Química de la Facultad de Ciencias y Tecnologías Químicas de la Universidad de Castilla-La Mancha, considerando que dicho trabajo reúne los requisitos para ser presentado como Tesis Doctoral, expresan su conformidad con dicha presentación.

Ciudad Real, a 26 de septiembre de 2018

Dr. Ángel Pérez Martínez Dr. Manuel Salvador Carmona Franco

AGRADECIMIENTOS

En primer lugar, quiero expresar mi más sincero agradecimiento a mis directores de tesis, el Dr. Ángel Pérez y el Dr. Manuel Carmona. Sin su orientación científica y apoyo, esta investigación no podría haber sido completada con éxito. También me gustaría expresar especialmente mi gratitud al Dr. Juan Francisco Rodriguez, ya que sin su apoyo y confianza este trabajo no habría sido posible.

Así mismo, mi agradecimiento se extiende a todos los miembros del Departamento de Ingeniería Química de la Universidad de Castilla-La Mancha y del Instituto de Tecnología Química y Medioambiental (ITQUIMA) por brindarme la oportunidad de comenzar mi carrera investigadora.

My sincerest thanks to Dr. Gianmarco Griffini for the opportunity that he gave me to join the Laboratory of Chemistry and Characterization of Innovative Polymers at Politecnico di Milano and for the friendship of all the group members during my stay with them.

También me gustaría agradecer al Ministerio de Educación, Cultura y Deporte la financiación de esta investigación mediante la beca para la Formación de

Profesorado Universitario FPU014/00009.

A todos los compañeros de viaje durante estos años, especialmente a los que han trascendido del ámbito laboral. Al Equipo Chapuzas™ y en general a todos los “Cheskytos” y “Cabesonsitos” oficiales o que se sientan de corazón, imprescindibles en las paradas técnicas en el camino de la investigación por haber conseguido que este trabajo haya sido realizado siempre con una sonrisa en la cara.

A todos aquellos estudiantes que han tomado parte activa de esta investigación mediante la realización del Trabajo Fin de Grado o Trabajo Fin de Master, porque habéis compartido conmigo los momentos más complicados de la experimentación y me habéis brindado toda vuestra capacidad de trabajo.

Finalmente, a mis seres queridos y amigos, por su apoyo, comprensión, paciencia y ánimo en los momentos difíciles, por creer en mí y estar siempre a mi lado.

“We've arranged a global civilization in which most crucial elements profoundly depend on science and technology. We have also arranged things so that almost no one understands science and technology. This is a prescription for disaster. We might get away with it for a while, but sooner or later this combustible mixture of ignorance and power is going to blow up in our faces.”

Carl Sagan

Table of contents

Summary / Resumen Chapter 1. Introduction………………………………………………………………………………………1 1.1. Environmental situation………………………………………………………………………...3 1.2. The concept of biorefinery and biomass resources………………………………….4 1.3. Derived products from vegetable oils…………………………………………………...16 Chapter 2. Motivation, objectives and work plan………………………………………………..37 Chapter 3. Materials and methods…………………………………………………………………….43 3.1. Materials………………………………………………………………………...... 45 3.2. The concept of biorefinery and biomass resources………………………………..47 3.3. Experimental procedures…………………………………………………...... 54 3.4. Characterization techniques………………………………………………………………...58 SECTION 1. ALTERNATIVE BIOBASED CHEMICAL PERCURSORS Chapter 4. Epoxidation of grape seed oil…………………………………………………………...67 4.1. General background…………………………………………………………………………….71 4.2. Results………………………………………………………………………………………………..74 4.3. Conclusions…………………………………………………………………………………………84 Chapter 5. Production of free fatty acids via acid hydrolysis……………………………….89 5.1. General background…………………………………………………………………………….93 5.2. Results………………………………………………………………………………………………..96 5.3. Conclusions……………………………………………………………………………………….109 SECTION 2. SYNTHESIS OF BIOPOLYOLS AND POLYURETHANE FOAMS Chapter 6. Synthesis of rigid polyurethane foams of azidified biopolyols………….113 6.1. General background…………………………………………………………………………..117 6.2. Materials...…………………………………………………………………………………………118 6.3. Results………………………………………………………………………………………………118 6.4. Conclusions...……………………………………………………………………………………..126 Chapter 7. Production of rigid polyurethane foams from phosphorylated biopolyols 6.1. General background…………………………………………………………………………..135 6.2. Materials...…………………………………………………………………………………………136 6.3. Results………………………………………………………………………………………………136 6.4. Conclusions...……………………………………………………………………………………..148

Table of contents

SECTION 3. SYNTHESIS OF BIOLUBRICANTS DERIVED FROM FREE FATTY ACIDS Chapter 8. Synthesis and characterization of estolide-based biolubricants………..153 8.1. General background…………………………………………………………………………..157 8.2. Results…...…………………………………………………………………………………………159 8.3. Conclusions……………………………………………………………………………………….167 Chapter 9. Development of TMP-based biolubricants………………………………………171 9.1. General background…………………………………………………………………………..175 9.2. Results…...…………………………………………………………………………………………178 9.3. Conclusions……………………………………………………………………………………….190 Chapter 10. Lignin as additive in PEG-based lubricants 10.1. General background…………………………………………………………………………197 10.2. Results…...……………………………………………………………………………………….203 10.3. Conclusions……………………………………………………………………………………..220

Chapter 11. Conclusions and future work………………………………………………………..227 11.1. Conclusions…………..…………………………………………………………………………229 11.2. Future work…………………………………………………………………………………….230

Summary

Nowadays, the society is enormously dependent on fossil resources to produce derived chemicals and fuels, which makes that its consumption grows continuously. However, petroleum, carbon and natural gas have a finite character and its direct impact on the global warming by the emissions of greenhouse gases and other environmental issues is well known. Hence, the search and development of sustainable and renewable production routes of fuels and materials is mandatory. Biomass has been proposed as the most interesting alternative feedstock due to its renewable character and its potential for producing derived materials and fuels.

This research work is focused on the production of biobased materials by environmentally friendly processes using mainly vegetable oils as raw material. Biopolyols and biolubricants are the main targets because of its great consumption and the low biodegradability and high environmental impact of their non-renewable petroleum-based homologous.

In order to promote a versatile and economically interesting full process, the research started up with the transformation of two different vegetable oils (namely grape seed and high-oleic sunflower oil) into platform chemicals which would be used in subsequent steps. Firstly, grape seed oil was successfully epoxidized in presence of peracetic acid generated in situ. The obtained kinetic model indicated that moderate high temperatures (90 ºC) and short reaction times (1 h) are the optimal to minimize the effect of secondary reactions. Secondly, the viability of using DBSA as amphiphilic catalyst during the acid hydrolysis of vegetable oils was tested. The effect of temperature, reaction time and the concentrations of DBSA and water was determined through the experimental design technique. The results from the ANOVA test indicated the great influence of the temperature on the fatty acid content of the product. On the other hand, the influence of time was predicted to be almost negligible.

Once that the platform chemicals were successfully obtained, the transformation into high added value products was studied. Firstly, the viability of the ring-opening reaction of oxirane groups contained in epoxidized grape seed oil to produce biopolyols was studied. In a first approach, NaN3 was used as ring-opening reagent, finding a covalent linkage among the azide group and the biopolyol. In this case, the Table of contents obtention of a rigid polyurethane foam using exclusively the azidified polyol was possible, achieving a significant improvement of the thermal stability of the foam. In a second approach, H3PO4 was used as ring-opening agent. Similar to NaN3, the covalent linkage of phosphate groups was demonstrated by FTIR and 31P-RMN. Unfortunately, the obtention of polyurethane foams using exclusively the phosphorylated polyol was not possible and uniquely a 57 wt.% was successfully incorporated into the formulation. Finally, thermogravimetric and surface composition analyses revealed the formation of a phosphoro-carbonaceous protective layer.

Respecting to the production of biolubricants, free fatty acids were used as main raw material on its synthesis through two different processes. On the one hand, the oligomerization of free fatty acids conduced to the formation of estolides. Among the different catalysts tested, HClO4 reported the best activity. Nevertheless, a subsequent esterification process in presence of methanol was required to ensure a full transformation of unreacted oleic acid into derived products, improve the characteristics of the lubricant phase and facilitate the purification of the product. The positive effect on the process of incorporate an ionic liquid was also demonstrated. On the other hand, the synthesis of oleic-based trimethylolpropane (TMP) esters via esterification between oleic acid (OA) and TMP in presence of an acid catalyst using a reactive distillation process was explored. After determining the suitability of methanesulfonic acid as catalyst among many other homogeneous ones, the influence of the molar ratio OA:TMP was studied. A large amount of by- products were obtained for OA:TMP molar ratios lower than 2.5. Nevertheless, using an OA:TMP molar ratio of 3.0 only unreacted OA was obtained as impurity when the in situ formed water was removed by using the reactive distillation system. A predictive mathematical model was developed to determine the influence of the presence of by-products, and more specifically the presence of different functional groups, on the viscosity of the lubricant.

Finally, the viability of using Kraft lignin as additive in PEG-based lubricants was determined. A commercially available Kraft lignin (KL) was incorporated into poly(ethylene glycol)s (PEGs) of different molecular weights (200 and 300 Da) by means of an ultrasound-assisted treatment. Stable dispersions of PEG with a KL

content up to a 40 wt.% were successfully prepared through this method. The measurements of the dynamic viscosity and the Ostwald-de Waele model revealed a deviation from a Newtonian behaviour towards a pseudoplastic one at temperatures below 30 °C and lignin contents higher than 35 wt.% due to the temperature-reversible formation of lignin aggregates. Finally, the kinematic viscosity of the lignin-containing lubricants at 40 and 100 °C was determined and used to calculate their viscosity index, which was found to undergo a maximum improvement of nearly 50 % compared to neat PEG upon addition of an optimized amount of lignin

Resumen

Hoy en día, la sociedad es enormemente dependiente de los recursos fósiles para producir productos químicos y combustibles, lo que hace que su consumo siga creciendo continuamente. Sin embargo, el petróleo, el carbón y el gas natural tienen un carácter finito y su uso está directamente relacionado con el calentamiento global debido a las emisiones de gases de efecto invernadero. Por tanto, la búsqueda y desarrollo de rutas de producción renovables y sostenibles se ha convertido en tarea obligatoria. La biomasa ha sido propuesta como el recurso alternativo más interesante debido a su carácter renovable y su elevado potencial para producir productos derivados y combustibles.

Este trabajo de investigación se centra en la producción de biomateriales mediante procesos medioambientalmente sostenibles, empleando principalmente aceites vegetales como materia prima o algunos productos derivados de ellos. Los principales objetivos productivos son los biopolioles y los biolubricantes, debido a su elevado consumo y a la baja biodegradabilidad de los residuos generados después de su uso.

Para promover un proceso versátil y económicamente interesante, la investigación comenzó con la transformación de dos aceites vegetales diferentes (aceite de granilla de uva y aceite de girasol alto oleico) en “platform chemicals” para su posterior uso en la investigación. En primer lugar, el aceite de granilla de uva fue satisfactoriamente epoxidado en presencia de ácido peracético generado in situ. El modelo cinético obtenido indicó que para minimizar la presencia de las reacciones secundarias era necesario operar a temperaturas mediamente altas (90 °C) y tiempos de reacción cortos (1h). En segundo lugar, la viabilidad de emplear DBSA como catalizador anfifílico durante la hidrólisis ácida del DBSA fue estudiada. El efecto de la temperatura, del tiempo de reacción y las concentraciones de DBSA y agua fue determinado mediante diseño de experimentos. Los resultados del ANOVA indicaron la gran influencia de la temperatura en el contenido de ácidos grasos del producto.

Una vez que los productos intermedios fueron obtenidos satisfactoriamente, la transformación en productos de alto valor añadido fue estudiada. En primer lugar, determinó la viabilidad de la reacción de apertura de anillo de los grupos oxirano Table of contents presentes en el aceite de granilla de uva epoxidado para producir polioles. En una primera aproximación, el NaN3 fue empleado como agente de apertura de anillo, encontrando que el grupo azida quedaba enlazado covalentemente a la molécula de aceite vegetal. A partir de dicho producto, la producción de espumas rígidas de poliuretano fue posible, consiguiéndose un aumento de la estabilidad térmica de la espuma. En una segunda ruta, el H3PO4 fue empleado como agente de apertura de anillo. De forma similar al NaN3, se pudo observar mediante FTIR y 31P-RMN la unión covalente de los grupos fosfato. Desafortunadamente, la producción de espumas de poliuretano usando exclusivamente el poliol fosforilado no fue posible y únicamente se consiguió incorporar de forma exitosa un 57 % del mismo en la formulación de la espuma. Finalmente, los análisis termogravimétricos y de composición de la superficie demostraron la formación de una capa protectora fosforo-carbonosa.

Con respecto a la producción de biolubricantes, los ácidos grasos libres fueron empleados como materia prima principal en su producción mediante dos procesos diferentes. Por un lado, la oligomerización de los ácidos grasos libres condujo a la formación de estólidos. De entre los diferentes catalizadores empleados, el HClO4 mostró la mejor actividad. Sin embargo, una reacción consecutiva de esterificación en presencia de metanol fue requerida para asegurar la transformación total del ácido oleico no reaccionado en productos derivados, mejorar las características de la fase lubricante y facilitar la purificación del producto. El efecto positivo para el proceso de incorporar un líquido iónico fue también demostrado. Por otro lado, la síntesis de ésteres de trimetilolpropano basados en ácido oleico mediante la esterificación de TMP con ácido oleico (OA) en presencia de un catalizador ácido usando un proceso de destilación reactiva fue estudiado. Una gran cantidad de subproductos fue obtenida cuando se emplearon relaciones molares OA:TMP menores de 2.5. Sin embargo, al emplear una relación molar OA:TMP de 3, únicamente se obtuvo OA sin reaccionar como subproducto cuando el agua formada durante el proceso era retirada mediante el proceso de destilación reactiva. Un modelo matemático predictivo fue desarrollado para determinar la influencia de la presencia de subproductos, y más específicamente la presencia de diferentes grupos funcionales, en la viscosidad del lubricante.

Finalmente, fue estudiada la viabilidad del empleo de lignina Kraft como aditivo en lubricantes basados en PEG. Una lignina Kraft (KL) disponible comercialmente fue dispersada en polietilenglicol de diferente peso molecular (200 y 300 Da) mediante un proceso asistido por ultrasonido. Las dispersiones obtenidas con hasta un 40 % p/p de lignina fueron estables al ser preparadas mediante este método. Las medidas de la viscosidad dinámica y el modelo de Ostwald-de Waele revelaron una desviación con respecto al comportamiento Newtoniano hacia un comportamiento pseudoplástico a temperaturas inferiores a 30 °C y contenidos en lignina superiores al 35 %p/p debido a la formación reversible de agregados de lignina. Finalmente, la viscosidad cinemática de los lubricantes con lignina fue determinada a 40 y 100 °C y empleada para determinar el índice de viscosidad, observándose una mejora máxima de entorno al 50 % con respecto al PEG sin aditivar.

Introduction

CHAPTER 1. INTRODUCTION

Introduction

1.1. Environmental situation The actual society is enormously dependent on fossil resources to produce derived chemicals and fuels. Hence, it is necessary a constant search for petroleum, carbon and natural gas and it has associated a negative effect on society, economy and the environment. Moreover, the major concerns about the fossil resources are that the exploitation rate will not be enough to keep the actual global demand and the notation that fossil resources are a finite raw material. The main consequence of this facts is the increasing price of fossil resources over the last years, specially petroleum. In addition, it is well known the direct impact of consuming non- renewable fossil resources on the global warming by the emissions of CO2 and other greenhouse gases in the atmosphere. Hence, the urgency in the development of sustainable and renewable alternatives to fossil resources is clear.

Biomass is the most interesting alternative feedstock to non-renewable resources to produce derived chemicals and fuels. The main benefit of using biomass as raw materials on biorefineries is the faster renovation rate of the original resource. The

CO2 produced from the combustion of biobased compounds is rapidly transformed into biomass through the photosynthesis, meanwhile the transformation of CO2 into fossil resources can take over millions of years. These two alternative production systems are depicted in Figure 1.1.

Figure 1.1. Schematic diagram of the carbon cycle for A) the development of biobased products and B) the traditional petrochemical route.

Chapter 1

1.2. The concept of biorefinery and biomass resources

Due to its high versatility, biomass can be converted into a wide variety of chemicals, biomaterials and energy, increasing the value of biomass and reducing the generation of wastes. This integrational concept corresponds to the definition of biorefinery. In the same way that traditional petroleum-based refineries can produce a wide variety of chemicals and derived products, biorefineries will hypothetically be able to produce many different products from biomass. These wide variety of products will include from low-value commodities, like fuels (e.g. biodiesel or bioethanol); to high-value speciality chemicals, such as cosmetics or nutraceutical products.

Three different conceptual types of biorefineries have already been proposed in the literature:

• Phase I: This biorefineries use exclusively a single raw material which can be processed through a unique process and a single major product is generated. This type of biorefineries are already in operation and have proven to be profitable. The main processes implemented on a phase-I-type biorefinery are the transesterification of vegetable oils in presence of methanol into biodiesel and glycerine, and the production of bioethanol from corn. As an example of the operation of a phase I biorefinery, the transesterification process of a vegetable oil is shown in Figure 1.2.

Single process

Single raw material Single product Figure 1.2. Example of a phase I biorefinery producing biodiesel.

• Phase II: In the same way that phase I biorefineries, phase II ones can only process a single feedstock. Nevertheless, they are able to produce different products (energy, chemicals and/or materials). There are two main examples of phase II biorefineries operating at industrial scale. The first one is the Novamont plant in Italy, where mainly biodegradable polyesters and

Introduction

starch-derived thermoplastics are produced from corn. The second example is the Biorefinery Roquette, placed at Lestrem (France). This phase II biorefinery produces a wide range of carbohydrate derivatives using cereal grains as raw material (Roquette), as shown in Figure 1.3.

Single raw material Cereal grains

Physical separation Pre-treatment Vitamins

Polymers Chemical and/or Enzymatic process Amino acids biochemical catalysis Biopolyols

Biofuels Fermentation Multiproducts Multiprocess

Figure 1.3. Roquette productive diagram as an example of phase II biorefinery.

• Phase III: This type of biorefineries correspond to the most advance and versatile type of alternative. Phase III biorefineries can produce a variety of energy vectors and chemical products as phase II ones. Nevertheless, these biorefineries could also use various types of feedstocks and processing technologies. This high versatility gives them an advance position in case of changes in the market demands or to avoid problems in the supply of a certain raw material. Although no commercial phase III biorefineries are operative nowadays, a bien effort is being carried out through different projects, like the already finished research projects Sustoil and Biopol, or the ongoing one named Clamber in the Spanish region of Castilla-La Mancha.

Biomass can be defined as any plant-derived organic matter that is available on a renewable basis (Department of Energy). Hence, biomass feedstocks are typically provided from four different sectors:

1. Agricultural dedicated crops or residues from other activities 2. Forestry 3. Aquaculture, like algae and seaweeds

Chapter 1

4. Wastes from industries (process residues and leftovers) and households (municipal solid wastes and wastewaters)

Independently from the origin of the biomass, it is usually classified attending to the molecular structure into oils, carbohydrates, lignin, proteins and waxes. Their applications, technical viability and expected impact on biorefineries are discussed below.

1.2.1. Vegetable oils

A wide range of vegetable oils are produced for food and non-food uses. Table 1.1 shows the global production for the most abundant and commonly traded ones during the 2014.

Table 1.1. Global production of main vegetable oils. Global production Vegetable oil (million tonnes) Palm oil 57.33 Soybean oil 45.70 Rapeseed oil 25.94 Sunflower oil 15.85 Palm kernel oil 6.60 Cottonseed oil 5.04 Groundnut oil 5.03 Maize oil 3.19 Coconut oil 3.11 Olive oil 3.05 Sesame oil 1.63 Linseed oil 0.69

Vegetable oils are composed by triacylglycerides (usually referred to as triglycerides) in a 93-98 wt.%. The remaining composition of vegetable oils is based on monoglycerides, diglycerides and unsaponificable matter, like antioxidants. All vegetable and plant oils comprise a mixture of different triglyceride molecules, which differ between plant species. The presence of different fatty acids in the triglycerides, characterized by their carbon chain lengths and the degree of unsaturation, impart different physical and chemical characteristics to oils from different plant species. Long-chain fatty acids (C18) dominate in continental climate oil crops, while shorter fatty acid chains (C12–C16) dominate tropical commercial oil crops, like coconut and palm. The degree of saturation of fatty acids affects their

Introduction reactivity and resistance to oxidative degradation (which is faster with more unsaturation), while chain length affects melting point and influences characteristics such as viscosity at room temperature. Table 1.2 resumes the typical fatty acid profiles for a range of vegetable oils.

Table 1.2. Average fatty acid composition (wt. %) for the most used vegetable oils in industry.

Soy Sunflower Palm Coconut Linseed Grape seed Name Structurea oil oil oil oil oil oil Caproic 6:0 0–0.6 Caprylic 8:0 4.6–9.4 Capric 10:0 5.5–7.8 Lauric 12:0 0–0.1 0–0.1 0–0.4 45–50 0.5– Myristic 14:0 0–0.2 0–0.2 16–20 2.0 8.0– Palmitic 16:0 5.6–7.6 40–47 7.7–10 6.0 6.9 13.3 Palmitoleic 16:1 0–0.2 0–0.3 0–0.6 0.1 Stearic 18:0 2.4–5.4 2.7–6.5 3.5–6 2.3–3.5 2.5 4.0 Oleic 18:1 17–26 14–39 36–44 5.4–8.1 19.0 19.0 Linoleic 18:2 49–57 48–74 6.5–12 1.0–2.1 24-25 69.1 Linolenic 18:3 5.5–9.5 0–0.2 0–0.5 0–0.2 47-49 0-0.2 Arachidic 20:0 0.1–0.6 0.2–0.4 0–1.0 0–0.2 0.5 0.3 Gadoleic 20:1 0–0.3 0–0.2 0–0.2 Behenic 22:0 0–0.1 0.5–1.3 Erucic 22:1 0.3–0.7 0–0.2 Lignoceric 24:0 0–0.3 0.2–0.3 Nervonic 24:1 0–0.4 a The first number refers to the C atoms in the structure and the second one the number of unsaturations in the chain

Petroleum-derived organic acids are typically shorter, more branched than vegetable-derived ones and completely saturated compounds. These characteristics are good in high-temperature applications, but it makes them much less biodegradable than renewable ones. Hence, vegetable oils or its derived fatty acids are preferred in many different industrial applications:

• Lubricants: The base oil of a lubricant typically constitutes 90-99 wt.% of lubricant formulations. The physical properties of most unsaturated vegetable oils and its capability to be degraded in environmental-sensitive applications make them perfect to be used as base oil. To improve their lifetime and cold-temperature properties, additives like antioxidants and pour- point depressants might be used.

Chapter 1

• Fuels: the transesterification reaction of triglycerides in presence of methanol is a well-known procedure for the obtention of fatty acid methyl esters (FAME). FAME have an adequate flash point, good lubricative properties and a low sulphur content, making them an ideal substituent of diesel. FAME can also be obtained through the esterification reaction of free fatty acids. • Surfactants: Fatty acids have two well differenced parts on their structure. On the one hand a hydrophilic carboxy group and, on the other, a hydrophobic aliphatic chain. It makes them well suited for its incorporation as biosurfactant in detergent formulations. Based on the same principle, medium-chain saturated fatty acid salts are used as soaps. • Surface coatings: the paint industry has recently incorporated into formulations different products derived from vegetable oils. Typically, linseed and soy oil are used because of their average medium chain length and degree of unsaturation in gloss paints. • Polymers: unsaturated fatty-acid chains offer opportunities for polymerization that can be exploited. This application will be deeply reviewed in a subsequent section.

The socioeconomic activity of grape pressing for producing grape juice and wine imply the generation of a considerable amount of solid wastes, since approximately 30 wt.% of the material used become a waste. There are several environmental problems associated with wineries, including water pollution, soil degradation, damage to natural vegetation, odours and air emissions (Jin & Kelly, 2009). The main solid by-products and residues produced during wine production are vine shoots, grape marc or pomace (composed by skins, seeds and stems), and wine lees.

The legislative situation in Europe governing the by-products generated by the wine industries only indicates that wine lees have to be withdrawn once they have been denatured to make their use in winemaking impossible. Producers with a capacity of less than 25 hL/year of wine may be exempted by the Member State from this obligation. However, most of the Member States have specified the rules for the withdrawal and legal destinations of winery industries by-products (Spigno et al., 2017).

Introduction

Grape seeds are one of the main by-products from grape processing industries. An individual grape berry typically contains two/three seeds and it constitutes around 4 wt.% of grape marc. Grape seeds contain oil (13–19 wt.%), which is rich in essential fatty acids, protein (about 11 wt.%), 60–70 wt.% of non-digestible carbohydrates, non-phenolic antioxidants, such as tocopherols and beta-carotene (Yu & Ahmedna, 2013), and also phenolic compounds with antioxidant capacity (Jordão et al., 2001). Some investigations have focused on the use of grape seeds as a fuel via pyrolysis (Ferreira et al., 2016; Pǎrpǎriţǎ et al., 2014) or gasification (Fidalgo et al., 2015) processes. Nevertheless, the most interesting activity from an economic point of view consists on the extraction of the interesting compounds prior to the thermal or material exploitation.

Grape seed oil composition has been studied by different authors, focusing mainly on the fatty acid profile, the phytochemical composition, and antioxidant properties. Table 1.2 shows the fatty acid profile for grape seed oil found on a previous study (Ramos et al., 2009). However, slight differences on the fatty acid profile up to a 10 wt.% can be found due to the use of different grapes varieties (Beveridge et al., 2005) and/or extraction methods (Freitas et al., 2008; Lutterodt et al., 2011; Passos et al., 2010; Rubio et al., 2009).

With respect to nutraceutical compounds, most of the vegetable oils only contain a significant content of tocopherols, meanwhile tocotrienols are seldom. Virgin grape seed oil contains up to 10 mg α-tocopherol/100 g and different tocotrienols with a total amount of around 35 mg/100 g (Matthäus, 2008). Crews et al. investigated the total content of tocopherols and tocotrienols of 30 samples of grape seed oil, finding a total content between 63 and 1208 mg/kg (Crews et al., 2006). This range is much wider than the values given at the Codex Alimentarius (240–410 mg/kg). Hassanein and Abedel-Razek reported a value of 380 mg/kg of total tocopherol content (Hassanein & Abedel-Razek, 2009). These values are relatively low compared with other vegetable oils, indicating that most of vitamin E active compounds remain on the solid phase of the seed (grape seed flour). Phytosterols, which are natural sterols which occur in plants and vegetable oils, are contented in a range from 2580 to 11250 mg/kg. However, independently of the author, β-sitosterol is the main phytosterol found in grape seed oil (67–70 wt.% of phytosterols) (Crews et al., 2006;

Chapter 1

Hassanein & Abedel-Razek, 2009). The molecular structure of α-tocopherol and β- sitosterol are shown in Figure 1.4.

α-tocopherol

β-sitosterol

Figure 1.4. Molecular structures of the main nutraceutical substances in grape seed oil.

Different techniques have been proposed for grape seed oil extraction. The traditional method consists of pressing the whole seeds in a hydraulic press or the milled and heated seeds in screw press. Cold-pressing is a method of oil extraction that involves no heat or chemical treatment, and hence might retain more health beneficial components, such as natural nutraceutical substances mentioned previously. The cold-pressed oils may be a better source of beneficial components, such as antioxidative phenolic compounds, as well as other health-beneficial phytochemicals. Although the yield is usually lower than that with conventional solvent extraction, there is no concern about solvent residues in the oil, making for a safer and more consumer-desired product (Shinagawa et al., 2015). Using this technique, there were not significant differences on the fatty acid profile of cold- pressed grape seed oil, but an increment on the oxidative stability index, indicating a higher content of antioxidant compounds (Lutterodt et al., 2011).

Solvent extraction of grape seed oil is a diffusion process achieved by immersing the seed in solvent or by percolating solvent through a bed of seeds. Solvent is recovered from the oil-solvent mixture (known as micella) by an evaporator and recycled to the process. In order to determine the influence of the solvent, Fernández et al. compared hexane, acetone, pentane, acetonitrile, diethyl ether, methanol, and ethanol (Fernández et al., 2015). Among them, diethyl ether showed the highest

Introduction extraction yield (20.8 wt.%). However, its use was rejected due to its high flammability and hexane was chosen as ideal extraction agent. It was also found that the use of polar solvents enhanced the oxidation stability of the extracted grape seed oil, indicating a higher antioxidant content in those cases. In order to combine both effects, solvents mixtures were tested, and it was observed a maximum in oil yield (18.5 wt.%) and oxidation stability (16.3 h) by using a mixture hexane/acetone 1:1 (v/v). Moreover, Soxhlet and Soxtherm extraction technologies were compared without observing a significant difference on the extraction yield nor on the oxidation stability of the oil. Luque-Rodríguez et al. compared superheated hexane extraction with conventional Soxhlet system, finding that similar yields can be obtained in shorter extraction times (Luque-Rodríguez et al., 2005).

Supercritical fluid extraction of vegetable oils has been intensively studied in the last years. This procedure is not applied in large-scale plants because of the difficulties with continuous transport of seeds into, through and out of the high-pressure extractor. Smaller amounts of seeds can be extracted in semi-batch mode, with the supercritical fluid flowing through a fixed bed of material (Sovová et al., 1994). Molero Gómez et al. compared the supercritical extraction and the non-supercritical one using CO2 as extraction agent, showing a great increment on oil yield by working at supercritical conditions (Molero Gómez et al., 1996). Jokić et al. determined that the optimal conditions for obtaining the highest oil yield (14.5 wt.%) and antioxidant activity using supercritical CO2 as extracting agent were 400 bar and 41°C (Jokić et al., 2016). Fiori determined a break-even value of grape seed oil obtained at 550 bar and 60 °C of 5.9 €/kg using the same technology (Fiori, 2010). Freitas et al. pointed that supercritical propane is a more suitable solvent for grape seed oil extraction since a higher extraction yield and faster kinetic was observed (Freitas et al., 2008).

1.2.2. Lignin

Lignin is the second most abundant biopolymer after cellulose and is the largest source of aromatic groups in nature. Lignin is mainly found in plant cell walls and is the most important structural component of woody plants. The chemical structure of lignin consists on a random network of phenylpropane groups, as can be show in

Chapter 1

Figure 1.5. Nevertheless, the source from which lignin is obtained, how it is extracted and the different secondary treatments have a very significant influence on the physical and chemical properties of lignin (García et al., 2012).

Figure 1.5. Schematic model of lignin structure.

The pulp and paper industry are the main producers of commercial-grade lignin as a by-product during the cellulose extraction. Among the different processes used nowadays in the industry, the Kraft pulping process is the most commonly used one (Gellerstedt, 2015).

The global production of Kraft lignin is around of 131.2 Mt. During this process, different chemical reactions degrade and dissolve the lining in order to separate pure cellulose from the wood. One of the most characteristics steps of this process is the treatment of biomass with NaOH and Na2S to cleave the ether linkages (Smook, 1992). Kraft lignin is hydrophobic at neutral pH and contains approximately 1 % sulphur (measured in atomic %) in the form of aliphatic thiol. Other important functional groups in Kraft lignin are methoxy group (14 wt.%), aliphatic hydroxyl group (10 wt.%), phenolic hydroxyl group (2-5 wt.%), and carboxylic acid group with compositions that can vary depending on the plant source and processing conditions (4-7 wt.%).

Introduction

As a by-product in paper production, most lignin (>98 wt.% of the global production) is burned as a fuel to produce power in the pulp mill and to recover inorganic components, despite its huge production and chemical potential (Lora & Glasser, 2002). The applicability and future scope of lignin as a fuel in pyrolysis, gasification, hydrocracking and reductive or oxidative transformation has been recently reviewed in literature (Wang et al., 2018a). Lignin depolymerization has been also widely studied to produce chemicals, biofuels and phenolic chemical platforms. The conditions and the obtained products of the most interesting depolymerization processes are resumed in Table 1.3. Nevertheless, lignin depolymerization is an extremely inefficient process since it is based on the cleavage of chemical bonds for its further reconstitution to synthesize different materials. Hence, one of the main actual challenges consists on the direct application of lignin modifying its structure as little as possible (Stewart, 2008).

Among the different direct applications of lignin in the production of higher-added value products, the fields of phenolic and epoxy resins are the most remarkable ones (Stewart, 2008):

• Lignin is a clear possible phenol substitute in the field of phenol- formaldehyde resins. Nevertheless, its heterogeneous structure limits its incorporation in high percentages (Turunen et al., 2003). Alternatives such as the structural modification of lignin (Nada et al., 2003), the incorporation of different filler agents (Peng & Riedl, 1994) and the use of lignins with lower molecular weight than Kraft lignin (like organosolv and acetosolv lignins) has been proposed in literature (Çetin & Özmen, 2002). • There are different reports where lignin is crosslinked with different epoxy precursors (Baroncini et al., 2016; Simionescu et al., 1993). As an example, epoxy resins were obtained by using eucalyptus, cedar and bamboo lignin as a curing agent and its applicability in printed circuit boards was tested (Asada et al., 2015), and compared to traditional ones based on bisphenol A diglyceryl ether (DGEBA)-based ones. The effect of different catalyst on the preparation of lignin-based epoxy resins has also been reported, observing similar characteristics to the petroleum-based ones (Ferdosian et al., 2015).

Chapter 1

Table 1.3. Production of phenolic derivatives by oxidative and non-oxidative depolymerization. Yield Conditions Solvent H2 Catalyst Phenolics Ref. (%) Oxidative depolymerization of lignin 1.87 (batch) Vanillin 2.17 120 ºC, 2 h, (cont.) (Sales et al., 0.4-0.5 MPa NaOH solution - PdCl3·3H2O 1.67 2006) O2 (batch) Syringaldehyde 3.83 (cont.) 170 ºC, 2 h, Vanillin 13.4 (Wang et al., NaOH solution - Nitrobenzene 0.5 MPa O2 Syringaldehyde 14.2 2018b) Vanillin 3.1-5.3 120 ºC, (Deng et al., NaOH Solution - La/Co oxides Syringaldehyde 4.7-12.8 0.5 MPa O2 2009) Benzaldehyde 1.5-2.8 Production of phenolic derivatives by lignin depolymerization Guaiacol 11.4 TiO2 Other phenolics 25.4 (Nair & Vinu, 500 ºC - - Guaiacol 5.8 2016) - Other phenolics 12.2 170 ºC, 1 (Zhang et al., Water + Ni7Au3 Phenolics 14.2 MPa 2014) 225 ºC, Guaiacol 1.8 (Zakzeski et al., Ethanol/water + Pt/Al2O3 5.8 MPa Other monomers 15.8 2012) 300-330 ºC, Guaiacol 1.2-2.1 (Lavoie et al., Water - NaOH 9-13 MPa Catechol 0.1-3.2 2011) 260 ºC, (Long et al., Methanol + NaOH, Ru/C Phenolics 12.7 4 MPa 2015) 255 ºC, (Hepditch & Ether - NiCl2/FeCl3 Phenolics 2.9 4.3 MPa Thring, 2000) 250 ºC, SiO2/Al2O3, Aromatics 27 (Deepa & Dhepe, Methanol/water - 0.7 MPa Zeolite Monomers 60 2015) 400 ºC, Supercritical Phenol 11.8 (Wahyudiono et - - 25 MPa water Catechol 26.2 al., 2008)

Nevertheless, there are many different applications of lignin to substitute petroleum derivatives proposed in literature. As examples, lignin has been proposed for the controlled release of fertilizers (Wang & Zhao, 2013), as a base in the production of nanocomposites (Klapiszewski et al., 2013) and nanoparticles (Garcia Gonzalez et al., 2017), as a precursor in the synthesis of carbon fibres (Frank et al., 2014), as adsorbent for metal ions in solution (Guo et al., 2008), as a protective UV-absorbent (Zimniewska et al., 2008) and as renewable cathodic material in electrochemical applications (Milczarek & Inganäs, 2012).

1.2.3. Carbohydrates

Carbohydrates represent the greatest percentage of plant biomass and are typically divided into cellulose, hemicellulose and starches and sugars. Cellulose is a linear polymer of β-glucose which is mainly used in papermaking industry. The molecular

Introduction structure of cellulose is depicted in Figure 1.6. Therefore, cellulose esters are widely applied in many different fields, like the cellulose acetate in the production of cigarette filters or methyl, ethyl and propyl cellulose esters as detergents or adhesives in cosmetic and pharmaceutical products (Thakur & Thakur, 2014).

Figure 1.6. Molecular structure of cellulose.

Hemicellulose represents around the 25 wt.% of woody plant tissues. Unlike to cellulose, hemicellulose is a branched polymer with many different sugar monomers, like xylose, hexose and pentoses (Figure 1.7). Hemicellulose can be easily hydrolysed into its monomers, being a good feedstock for the production of ethanol or other superior alcohols. The most important application of hydrolysed hemicellulose consists on the production of furfural, an interesting product used on the synthesis of preservatives, disinfectants and herbicides, from the pentoses contained on its structure (Peng et al., 2012).

Figure 1.7. Molecular structure of hemicellulose.

1.2.4. Proteins

Legumes, oilseed cakes (i.e. the material generated after the vegetable oil extraction) and wheat grains are the most important vegetable sources of protein. There are

Chapter 1 many potential applications for such vegetable proteins, but the extraction of high quantities of pure protein compounds is still not possible without affecting their structural integrity. Nowadays, most used vegetable-derived proteins are the extracted from soya and wheat, which are used mainly in the food sector as animal supplements (Rodrigues et al., 2012).

1.2.5. Waxes

Waxes are organic compounds which consist on long alkyl chains. Plant waxes are usually concentrated on leaves and on fruit skins and typically consists on a mixture of esters from a wide variety of long carboxylic acids and fatty alcohols. The structure of a typical vegetable wax is depicted in Figure 1.8. The main application of vegetable waxes has been in the cosmetic sector, but nowadays there is an increasing interest in the use of these compounds as a dietary supplement to reduce the cholesterol formation (Fei & Wang, 2017; Petry et al., 2017).

Figure 1.8. Typical molecular structure of vegetable waxes.

1.3. Derived products from vegetable oils

As it was commented, vegetable oils are one of the most promising biomass compounds for the obtention of derived products to substitute petroleum-based ones. There are two main reactive points in the structure of the triglycerides, which are the main component of vegetable oils, the ester linkage to the glycerine structure and the unsaturations presented in the aliphatic fatty acid chain. The typical structure of a triglyceride and its main reactive sites are depicted if Figure 1.9.

Introduction

Figure 1.9. Structure of a triglyceride with reactive sites highlighted in blue (ester linkage) and red (double bond unsaturation).

These reactive sites in the structure of the triglycerides are the responsible for the obtention of the two most important platform chemicals from vegetable oils for the further production of derived products, namely epoxidized vegetable oils and free fatty acids (Biermann et al., 2011). From these compounds, a wide variety of products can be obtained, being the biopolyols and the biolubricants the most interesting ones.

1.3.1. Epoxidized vegetable oils and free fatty acids as platform chemicals

According to different experts, one of the most promising routes for the utilization of vegetable oils as raw material in biorefineries consists on their transformation into platform chemicals to be used downstream in different processes. The two main transformations that triglycerides can undergo are the epoxidation of double bonds and the hydrolysis of ester groups (Danov et al., 2017; Lligadas et al., 2010).

The epoxidation process mainly consists on the transformation of double bonds present along the fatty acid chain into oxirane rings in the presence of an oxidizing agent (Figure 1.10).

Figure 1.10. Reaction scheme of the epoxidation reaction.

Epoxidated vegetable oils are very promising substances due to their great applicability. For examples, epoxidated oils can be used as polyvinyl chloride (PVC) stabilizer, plasticizer for different plastics, lubricant with improved properties and

Chapter 1 as starting material in the synthesis of polyols (Danov et al., 2017; Lin et al., 2008; Tan & Chow, 2010).

The most common production method of epoxidated vegetable oils is based on the Prilezhaev method, in which the unsaturated oils are oxidized using carboxylic peracids. Usually, peracid are generated in situ through different procedures due to their low stability. The main procedure consists on the oxidation of the organic acid with H2O2 in the presence of an acid catalyst. Hence, H2O2 is consumed along the process meanwhile the organic acid is used as an oxygen carrier (Danov et al., 2017). Among the carboxylic acids used, performic and peracetic acid are typically preferred. In literature, perbenzoic and m-chloroperbenzoic acid have also been used. The order of reactivity of the peracids found was m-chloroperbenzoic > performic > perbenzoic > peracetic. Although many peracids present a higher activity, peracetic is the most used one due to its low cost and facility of separation from the oil. Respecting to the catalyst system, homogeneous acids are predominant in literature. In spite of the new trends about using complex acid materials, strong inorganic acids such as H2SO4, HCl, H3PO4 and HNO3 are the most used ones. As a resume, the most relevant catalytic systems proposed in literature are summarized in Table 1.4.

As it was previously commented, fatty acids are the main raw material for the preparation of a wide variety of products. Specially, the use of fatty acids in the industry is remarkable in the sectors of soaps, surfactants and plasticizers (Westfechtel, 2016). Among the different methods for the production of fatty acids, the hydrolysis of the ester linkages in presence of water is the most used one (Figure 1.11).

Figure 1.11. Hydrolysis of triglycerides into fatty acids and glycerine.

Introduction

Table 1.4. Production of phenolic derivatives by oxidative and non-oxidative depolymerization. Vegetable Oxidant Catalyst Conditions Solvent Ref. oil Homogeneous catalytic systems Rapeseed oil CH3COOH + Sulphuric acid 65 ºC, 6 h - (Milchert et al., H2O2 2010) Soybean oil H2O2 CH3ReO3 30 ºC, 2h - (Gerbase et al., 2002) TBHP MoO2(acaac)2 110 ºC, 2h Toluene (Farias et al., 2010) Karanja oil H2O2 Novozym 435 40 ºC, 10 h Toluene (Bajwa et al., 2016) Heterogeneous catalytic systems Cottonseed CH3COOOH Amberlite IR- 75 ºC, 10 h - (Dinda et al., 2011) oil 120 Castor oil 50 ºC, 10 h Benzene (Janković et al., 2014) Soybean oil H2O2 γ-Al2O3 80 ºC, 10 h Ethyl (Turco et al., 2016) acetate Sunflower oil TBHP CoCuAl layered 110 ºC, 4 h Toluene (Sankaranarayanan double et al., 2015) hydroxides

This process is characterized by its strong endothermicity and the low miscibility of water and vegetable oils. These problems are typically overcome by the increasing of the temperature and the pressure. For example, one of the most important industrial methods is the Colgate-Emery steam hydrolysis. In this process, the reaction is performed in a continuous column in the absence of catalysts at high temperature (250-330 ºC) and pressures (50-60 bar). Nevertheless, this process requires a huge amount of water (around 60 wt.% in the initial mixture) to displace the equilibrium towards the formation of fatty acids. In addition, its applicability to non-fresh vegetable oils, specially to those with multiple or conjugated unsaturations, is not recommended. At high temperatures, these triglycerides suffer various secondary reactions and might cause a deterioration in the quality of fatty acids. The Twitchell process operates at softer conditions (around 100 ºC and atmospheric pressure) but requires longer contact times between the aqueous and organic phases (12-24 h). Moreover, this process requires the presence of a strong acid as catalyst to promote the formation of fatty acids (Hartman, 1953). Some other alternatives, like the enzymatic catalysis (Linfield et al., 1984) and saponification (i.e. basic hydrolysis), have a low applicability at industrial scale because of the long reaction times (up to 16 hours in the case of enzymatic catalysis) and the low quality of the glycerine (Satyarthi et al., 2011).

Chapter 1

1.3.2. Biopolyols The increasing importance of polymeric materials has increased the attention towards biobased polyurethane. One out their main components are the polyols, which are nowadays mainly produced from petroleum-based monomers. However, many research works have been presented along the last decade about the obtention of vegetable-oil-based polyols.

Excepting castor oil which naturally contains hydroxylated fatty acids, vegetable oils have to be chemically modified to incorporate the -OH functionality for the further synthesis of polyurethanes. Among the different routes proposed in literature, the modifications of vegetable oils involving the C=C bond is the most common and efficient one. There are different alternatives to obtain the hydroxyl moieties from the aliphatic unsaturations, being the most relevant ones discussed above:

The air oxidation of vegetable oils is the cheapest and simple technique in terms of reactants. This procedure involves free radical intermediates because of the abstraction of allylic hydrogens and the subsequent formation of a delocalized free radical. The radicals can further react with oxygen and other triglycerides, forming polyols with a wide variety of molecular weights (Oyman et al., 2005). This technique has been applied using soybean oil (Fornof et al., 2006) and high-oleic sunflower oil (De Espinosa et al., 2009) as raw materials.

As it was previously commented, epoxidated vegetable oils are one of the main chemical platforms for the synthesis of derived biobased chemicals. The oxirane ring opening caused by nucleophilic compounds leads to the production of vegetable-oil- based polyols. A general reaction scheme for this system is presented in Figure 1.12.

Figure 1.12. Ring-opening reaction of epoxidated vegetable oils.

Introduction

Among the wide variety of chemicals used during the ring-opening reaction, special attention has been paid to polyfunctional alcohols. This products were mainly used on the production of resins and coatings (Zlatanić et al., 2004). Nevertheless, the obtention of polyurethane foams is a much more interesting and challenging application for vegetable-oil-based polyols (Chattopadhyay & Webster, 2009; Lligadas et al., 2010).

The obtained biopolyols can be decorated with different functionalities depending on the presence of other nucleophilic substances. Up to now, mainly halogenated groups has been covalently linked to the structure in order to confer flame retardant properties to the biopolyol and further to the polyurethane compound (Guo et al., 2000). Nevertheless, due to the environmental problems of halogenated flame retardants, many efforts have been done in the incorporation of other flame- retardant functionalities (i.e. nitrogenated and phosphorylated compounds) to biobased polyols (Javni et al., 2000).

1.3.3. Biolubricants

Lubrication is the process or technique used to reduce the wear of various surfaces with a moving relative to each another. This reduction is achieved by interposing a substance called lubricant between the surfaces. Apart from this, lubricants have other proposes, like the protection of the surfaces against corrosion and to act a sealing agent against dirty, dust and water.

Nowadays, petroleum is the main raw material on the production of lubricants due to great performance of their derived products. This great applicability and efficiency are caused not only by the base fluid, but by the wide range of non- renewable additives developed along the XX century (Furey et al., 2003). Nevertheless, it is estimated that between the 20 and the 40 wt. % of the 5.2 Mt of lubricant consumed in Europe every year is released into the environment (McNutt & He, 2016), polluting water and soils (Willing, 2001).

Vegetable oils and related products, like free fatty acids, have been used as lubricants for a long period of time before the discovery of petroleum-based ones. In general terms, vegetable oils possess high flash point, high viscosity index, low

Chapter 1 evaporative losses and good metal adherence (Panchal et al., 2017), making them good candidates as lubricants. However, vegetable oils present two main drawbacks on their use as lubricant, namely a low oxidative stability and bad low temperature properties (Fox & Stachowiak, 2007; Sharma et al., 2006). This deficiencies in the applicability of vegetable oils as lubricants are mainly caused by the presence of the β-hydrogen atom in glycerol and the presence of an inadequate amount of unsaturations in the fatty-acid structure. There are a number of methods to improve these bad properties of vegetable oils based on the addition of additives (Bisht et al., 1989), being the most important ones the incorporation of antioxidants, viscosity modifiers and pour point depressant. Nevertheless, the chemical modification of vegetable oils is nowadays considered as the most efficient route to correct their oxidative stability and low temperature properties (McNutt & He, 2016). The most interesting modifications are discussed below.

Estolides are formed, as shown in Figure 1.13, by the reaction between a fatty acid’s carboxylic acid functionality and the double bond of another fatty acid (Cermak & Isbell, 2004). The lubricants based on estolides can provide improved lubricity and oxidation stability.

Figure 1.13. Reaction scheme of the synthesis of estolides

Estolides can be produced from a very wide variety of fatty acids profiles and in a wide variety of conditions (temperature, catalyst type and concentration, etc.). Moreover, some previous or subsequent reaction steps (e.g. sulphur functionalization, esterification with linear and branched alcohols, etc.) have been

Introduction proposed in literature to improve even further the properties of these lubricants. Hence, the lubricant characteristics and the possible applications of estolide-based lubricants is enormous. As a resume, Table 1.5 shows some of the more relevant researches on the synthesis and characterization of estolides.

Table 1.5. Production of phenolic derivatives by oxidative and non-oxidative depolymerization. Viscosity Viscosity Reaction Viscosity Pour Fatty acids Catalyst 40 ºC 100 ºC Ref. conditions index point (cSt) (cSt) (García- 75.61- 9.51- Zapateiro Oleic acid - - H2SO4, HClO4, 415.6 17.02 et al., p- 50-100 ºC, 2013b) toluensulphonic 3-24 h (García- acid 581.56- 51.26- Zapateiro Ricinoleic acid - - 6712 232.62 et al., 2013a) (Cermak Pennycress 60 ºC, HClO4 494.7 42.2 134 -15 et al., FA 24 h 2015) Castor oil, (Gupta & esterified Tin (II) 2- 130 ºC, 51.4 9.9 183 <-54 Harsha, with lauric ethylhexanoate 24 h 2018) acid Oleic acid, 60-80 ºC, (Cermak esterified until (-33) BF3 55-108 10.2-15.3 163-184 et al., with linear complete – (-9) 2013) alcohol reaction

Esterification and transesterification processes are usually used to rearrange acyl groups in triglycerides, forming new esters with improved lubricative properties. Typically, esterification and transesterification modifications are considered as multistep processes. Firstly, unmodified triglycerides react through a transesterification with a short chain alcohol or a hydrolysis process, forming short chain new ester or a free fatty acid. During a second step, short chain esters or free fatty acids can react with a more complex alcohol in a transesterification or esterification process, respectively, forming new esters with improved lubricative properties compared to the original triglyceride. These two alternatives are resumed in Figure 1.14.

Chapter 1

Figure 1.14. Schemes of modifications of triglycerides.

In resume, as can be observed in the reaction scheme, the whole process consists on the change of the glycerol backbone of the triglyceride by another polyol with a similar structure. These polyols do not contain any β-hydrogen atoms and the most used ones are the trimethylolpropane (TMP), the neopentyl glycol (NPG) and the pentaerythritol (PE) (Sripada et al., 2013). The structures of these polyols can be observed and compared to glycerine in Figure 1.15. This enhances the thermal stability of the lubricant at high temperatures by preventing self-polymerization to form free fatty acids (Leung et al., 2010).

Introduction

a) b)

c) d) Figure 1.15. Structures of a) glycerine, b) NPG, c) TMP and d) PE.

According to the works reviewed in literature (Eychenne & Mouloungui, 1998), the number of hydroxyl groups affects the viscosity of the esters according to the following order: PE > TMP > NPG. The applicability of this transformations has been studied on different vegetable oils, namely soybean oil (Bokade & Yadav, 2007), jatropha oil (Gunam Resul et al., 2012), olive oil (Gryglewicz et al., 2003), canola oil (Sripada et al., 2013) , palm oil (Hamid et al., 2012) and rapeseed oil using NPG and TMP as novel polyol (Uosukainen et al., 1998). It was observed that olive-oil based lubricants presented the highest thermo-oxidative resistance due to their low polyunsaturated fatty acid profile. In addition, it was shown that the oxidation stability of TMP ester from Jatropha oil increased by a 100 % compared to the original oil due to the elimination of β-H groups (Zulkifli et al., 2014).

Chapter 1

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Introduction

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Chapter 1

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Chapter 1

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CHAPTER 2. MOTIVATION, OBJECTIVES AND WORK PLAN

Motivation, objectives and work plan

Motivation Nowadays, the society is enormously dependent on fossil resources to produce derived chemicals and fuels, which makes that its consumption grows continuously. However, petroleum, carbon and natural gas have a finite character and its direct impact on the global warming by the emissions of greenhouse gases and other environmental issues is well known. Hence, the search and development of sustainable and renewable production routes of fuels and materials is mandatory.

Biomass has been proposed as the most interesting alternative feedstock due to its renewable character and its potential for producing derived materials and fuels. Among the different types of biomass that can be found in the environment, vegetable oils constitute the most interesting ones to be used as raw materials. In particular, grape seed oil has been postulated as a promising feedstock due to two main factors. Firstly, the production of solid by-products in the grape processing industries have demonstrated to have a direct impact on the environment, like the contamination of subterraneous water and damage to natural vegetation. Secondly, due to its fatty acid profile and its content on nutraceutical substances, its potential for the synthesis of derived materials is highly interesting.

Considering all the previously commented, finding new solutions to the environmental problems based on the use of renewable raw materials for the synthesis of biobased products is a crucial issue. Among the different bioproducts that can be obtained, biopolyols and biolubricants are the most relevant ones in the international markets due to its great consumption and the high environmental impact of their petroleum-based alternatives.

Chapter 2

Objectives

Therefore, the main objective of the present Thesis is the transformation of different biomass resources into various alternative and environmentally-friendly biopolyols and biolubricants.

In order to achieve this general aim, the research work has been structured in the following particular objectives:

• Transformation of different vegetable oils into more reactive and versatile platform chemicals, namely epoxidized vegetable oils and free fatty acids. Study of the influence of various reaction conditions on the epoxidation and hydrolysis reactions. • Study the applicability of using epoxidized grape seed oil as raw material for the synthesis of biopolyols decorated with flame retardant groups. • Establish the most suitable conditions to perform the synthesis of rigid polyurethane foams with improved thermal properties from the previously synthesized biopolyols. • Analysis the most suitable conditions and operations required for the synthesis of an estolide-type biolubricant, transforming the entire raw material into higher-added value products • Determine the viability of the synthesis of an alternative lubricant based on the esterification of trimethylolpropane. Modell the negative effect of intermediate products on the properties of the product. • Study the viability of using lignin as additive on the formulation of PEG-based lubricants

Motivation, objectives and work plan

Work plan

According to the objectives and the main goal of the research vocation, the next work plan shown in Figure 2.1 was developed.

Biomass Vegetable oils Lignin

Transformation Platform chemicals Epoxidized Free fatty vegetable oils acids

Ring-opening Oligomerization Esterification Additivation reaction

Phosphorylated Azidified Estolide-based TMP-based PEG-based polyols polyols lubricants lubricants lubricants

Foaming Biopolyols Biolubricants

Polyurethane foams

Figure 2.1. Work plan developed along the thesis.

Thesis overview

Chapter 1 shows a general introduction about the actual environmental situation and the necessity of using biomass as alternative raw material to fossil resources. Nowadays, many different processes and derived bioproducts are being studied since the global solution to replace petrochemical feedstocks is still a challenge. In Chapter 2, the motivation and objectives of the research study are shown in order to provide the structure of the thesis. Next, Chapter 3 details the most relevant experimental setups, procedures and analytical techniques used in the subsequent sections.

The results have been divided in three different sections. Section 1 deals with the production of platform chemicals from vegetable oils. On the one hand, Chapter 4 is focused on the epoxidation of grape seed oil and a kinetic model to optimize the yield towards the formation of oxirane groups was developed. Alternatively, along

Chapter 2

Chapter 5 the acid hydrolysis of high-oleic sunflower oil using an amphiphilic acid catalyst (DBSA) for the production of free fatty acids was optimized.

Section 2 addresses the transformation of epoxidized grape seed oil into different polyols decorated with phosphate (Chapter 6) and azide (Chapter 7) groups. Along these chapters, the technical viability of the synthesis of rigid polyurethane foams with improved thermal properties from these polyols studied.

Section 3 is focussed on the obtention of biolubricants through three different approaches. Chapter 8 discusses the technical and economical viability of the full transformation of oleic acid into higher added-value products, being the main one an esterified estolide with excellent lubricant properties. Chapter 9 addresses the synthesis and characterization of TMP-based biolubricants obtained via esterification from oleic acid and TMP. Special attention on the influence of the intermediate reaction products in the properties of the lubricant was paid. Lastly, Chapter 10 details the use of lignin as additive on the formulation of PEG-based biolubricants and the improvement of the viscosity index in all the base fluids used.

Finally, the main conclusions and recommendations of the Thesis are collected in Chapter 11 and Chapter 12, respectively.

CHAPTER 3. MATERIALS AND EXPERIMENTAL METHODS

Materials and methods

3.1. Materials Chemical compounds utilised in each process as well as their purity and supplier are presented in Table 3.1.

Table 3.1. List of chemical substances employed in each process.

Reagent Purity Supplier Epoxidation of grape seed oil Grape seed oil (Airen Refined vegetable oil Crisve type) Acetic acid (CH3COOH) ⩾ 99 % Sigma-Aldrich Hydrogen peroxide 50 % Sigma-Aldrich (H2O2), aq. solution Sulphuric acid (H2SO4) 96 % Sigma-Aldrich Hydrolysis of high-oleic sunflower oil High-oleic sunflower oil Commercial oil Local grocery shop (HOSO) Dodecylbenzene sulfonic >95 % Sigma-Aldrich acid (DBSA, C18H30O3S) Methanol anhydrous 99.8 % Sigma-Aldrich (CH3OH) Diethyl ether >99.7 %, 1 ppm BHT as Sigma-Aldrich ((CH3CH2)2O) inhibitor Synthesis of azidified biopolyols Dimethylformamide 99.8 % Sigma-Aldrich ((CH3)NCHO) Sodium azide (NaN3) 99.5 % Sigma-Aldrich Ammonium chloride 99.5 % Sigma-Aldrich Synthesis of phosphorylated biopolyols Phosphoric acid (H3PO4), 85 % Sigma-Aldrich aq. solution Acetone (C3H6O) Ph. Eur. grade Sigma-Aldrich Oleic acid (C18H34O2) ⩾ 90 % Sigma-Aldrich Methanol (CH3OH) ⩾ 99 % Sigma-Aldrich Ultrapure water (1 μS Milli-Q - cm-1) Synthesis of rigid polyurethane foams Polyol Alcupol R4520 - Repsol Tegostab B8404 - Evonik Tegoamin BDE - Evonik Tegoamin 33 - Evonik Ultrapure water Milli-Q - (1 μS cm-1)

Chapter 3

Table 3.1. List of chemical substances employed in each process (continuation).

Reagent Purity Supplier Synthesis and purification of esterified estolides Oleic acid (C18H34O2) ⩾ 90 % Sigma-Aldrich Methanol (CH3OH) ⩾ 99 % Sigma-Aldrich Sulphuric acid (H2SO4) 96 % Sigma-Aldrich Nitric acid (HNO3) 65 % Sigma-Aldrich Methanesulfonic acid 100% BASF (CH3SO3H) Perchloric acid (HClO4) 70 % Panreac Choline hydroxide 45 % Sigma-Aldrich solution in methanol (C5H15NO2) Triethylamine ⩾ 99 % Sigma-Aldrich (N(CH2CH3)3) Synthesis and purification of trimethylolpropane trioleate Oleic acid (C18H34O2) ⩾ 90 % Sigma-Aldrich Trimethylolpropane 97 % Sigma-Aldrich (C6H14O3) Sulphuric acid (H2SO4) 96 % Sigma-Aldrich 4-dodecylbenzenesulfonic 95 % Sigma-Aldrich acid (C18H30O3S) Montmorillonite K-10 - Sigma-Aldrich Methanesulfonic acid 100% BASF (MSA, CH3SO3H) Phosphoric acid (H3PO4), 85 % Sigma-Aldrich aq. solution Perchloric acid (HClO4) 70 % Panreac Isooctane Reagent grade Sigma-Aldrich Choline hydroxide 45 % Sigma-Aldrich solution in methanol (C5H15NO2) Methanol (CH3OH) ⩾ 99 % Sigma-Aldrich Synthesis of PEG-based biolubricants additivated with lignin Poly(ethylene) glycol, MW - Sigma-Aldrich 200 (PEG200) Poly(ethylene) glycol, MW - Sigma-Aldrich 300 (PEG300) Softwood Kraft lignin, - Westvaco Corporation of INDULIN ® (KL) North Charleston

Materials and methods

3.2. Experimental setups 3.2.1. Epoxidation reaction The epoxidation reactions were carried out by the in situ generation of peracetic acid in the experimental setup depicted in Figure 3.1.

a) b)

Figure 3.1. Experimental setup for epoxidation reaction: a) a general view; b) schematic diagram: (1) reactor vessel, (2) thermostathic bath, (3) reflux condenser, (4) thermocouple, (5) temperature display, (6) overhead stirrer, (7) six-bladed Rushton stirrer and (8) reactor outlet.

The epoxidation reactions were carried out in a mechanically agitated glass- jacketed reactor with a 1-L of capacity equipped with a six-bladed Rushton stainless steel stirrer 5 cm in diameter. The temperature of the reactor was kept constant with an accuracy of 0.1° C by means of a thermostatic oil bath.

3.2.2. Hydrolysis of vegetable oils

The hydrolysis reactions of high-oleic sunflower oil were conducted in a sealed 500 mL high pressure Teflon reactor vessel using an Ethos 1 microwave unit (Milestone Inc., Connecticut, USA). The equipment was equipped with a magnetic stirrer bar, a pressure indicator and a temperature control system. An image of the microwave reactor is shown in Figure 3.2.

Chapter 3

(1)

(2)

(3)

(5)

(7)

(6)

(4)

Figure 3.2. Experimental setup for the hydrolysis reaction: (1) microwave reactor, (2) pressure sensor, (3) thermocouple, (4) thermocouple tube, (5) reactor vessel, (6) cooling system and (7) indicator screen.

3.2.3. Ring-opening reactions

The experimental set-up for the azidification and phosphorylation reaction is shown at Figure 3.3. It consists of a three-necked round-bottom flask with a capacity of 50 mL, equipped with a magnetic stirrer and placed in an oil bath. The reaction temperature was controlled at the desired value with an accuracy of ±1 °C. The central neck of the reaction flask was connected to a reflux condenser, and a nitrogen stream was introduced through one of the side necks in to maintain an inert atmosphere.

Materials and methods

a) b) Figure 3.3. Experimental setup for epoxidation reaction: a) a general view; b) schematic diagram: (1) reactor vessel, (2) stirring and heating plate, (3) reflux condenser, (4) oil bath and (5) thermocouple.

3.2.4. Synthesis of polyurethane foams

The polyurethane foam experimental setup consists of a precision balance and a stirrer system with a digital controller Heidolph RZR-2102 (Figure 3.4). The foaming vessel was a polypropylene cup of 100 mL.

Figure 3.4. Experimental setup for the synthesis of polyurethane foams.

Chapter 3

3.2.5. Synthesis of estolide ester

The synthesis of estolide esters was performed in a reactor Berghof BR-300 of 0.5 L (Figure 3.5.). A total of 5 connections are provided in the lid: an immersion tube for temperature probe, a pressure gauge, a rupture disc and two valves for gas and liquid sampling. The temperature is controlled by the BTC-3000 temperature controller unit.

a) b) Figure 3.5. Reactor Berghof BR-300: a) a general view; b) schematic diagram.

3.2.6. Synthesis of the ionic liquid

The synthesis of the ionic liquid was performed in a 0.5 L round bottom flask submerged in an iced water bath. The addition of the reactants was performed through a pressure-equalizing dropping funnel. A photography and a resumed scheme of the system can be observed in Figure 3.6.

Materials and methods

a) b) Figure 3.6. Experimental setup for the synthesis of ionic liquids: a) a general view; b) schematic diagram: (1) reactor vessel, (2) stirring plate, (3) Pressure- equalizing dropping funnel and (4) iced water bath.

3.2.7. Molecular distillation

The molecular distillation of temperature sensitive products was performed on a short-path molecular distiller KDL-5 from UIC (Figure 3.7). This laboratory plant is design for a feeding flow from 0.5 to 1.5 kg/h.

The feed is placed in the feeding vessel and its driven to the distilling chamber by a gear pump. The feed is distributed along the walls of the distilling chamber by two rollers, forming a thing film along it. The wall of the distilling chamber is heated by the oil coming from a thermostatic bath. The distillate is condensed in the coil and collected in one of the six distillate containers. The residue drips trough the wall and is collected in another carousel similar to the one where the distillate is placed. The vacuum is supplied by two pumps: a rotatory vane pump and an oil diffusion pump. The vacuum level can be observed in the pressure indicator placed on the installation. Finally, to protect the pumps from not-condensed products, there is a cold trap filled with liquid nitrogen.

Chapter 3

a) b) Figure 3.7. Molecular distiller UIC KDL-5: a) a general view: (1) feeding vessel, (2) feeding gear pump, (3) chamber, (4) product distribution motor, (5) thermostatic oil bath, (6) distillate containers, (7) residue containers, (8) rotary vane pump, (9) oil diffusion pump, (10) pressure indicator, (11) cold trap; b) schematic diagram of the distilling chamber.

3.2.8. Synthesis of trimethylolpropane esters

The synthesis of trimethylolpropane esters (TMP-esters) was carried out in a 0.5 L two-necked round bottom flask heated by an electric mantle. The reaction bulk temperature was controlled by a thermocouple with an accuracy of ±1 °C. When the influence of the oleic acid to trimethylolpropane was studied, a Dean-Stark apparatus combined with a reflux condenser was used for continuous removal of water produced during the esterification reaction. The experimental setup can be depicted in Figure 3.8.

Materials and methods

a) b) Figure 3.8. Experimental setup for the synthesis of TMP-esters: a) a general view; b) schematic diagram: (1) reactor vessel, (2) thermocouple, (3) Dean-Stark apparatus, (4) heating mantle and (5) reflux condenser.

3.2.9. Centrifuge

The centrifuge used along this research work was an Orto Alresa Digitor-C (Figure 3.9) with a maximum rotation speed of 2000 rpm.

Figure 3.9. Centrifuge Orto Alresa Digitor-C.

Chapter 3

3.2.10. Preparation of PEG-based lubricants additivated with Kraft lignin

The preparation of the high-lignin-content PEG-based fluids was performed in jacked-vessel with a capacity of 50 mL. An UP200S ultrasonic processor supplied by Hielscher was used for the induction of ultrasonic waves. The processor has a high frequency (24 kHz) and was operated at 100% amplitude with continuous energy supply. The ultrasonic processor was equipped with a sonotrode with a tip- diameter of 3 mm. The temperature of the process was controlled with an accuracy of ± 0.5 ºC by means of a thermostatic cooling bath. The experimental setup can be depicted in Figure 3.10.

a) b) Figure 3.10. Experimental setup for the preparation of lignin-additivated lubricants: a) a general view; b) schematic diagram: (1) ultrasonic processor, (2) refrigerating bath, (3) jacketed-vessel and (4) sonotrode.

3.3. Experimental procedures

3.3.1. Epoxidation of grape seed oil and methyl oleate

250 grams of grape seed oil or methyl oleate and 0.5 mol of acetic acid per mole of unsaturation were added to the reactor. At this time, the temperature and the agitation rate were fixed at the desired temperature and 900 rpm, respectively. Once the reaction temperature is reached, the catalyst H2SO4 (2 % wt. of the aqueous phase) and the required amount of aqueous H2O2 (2 mol per mole of unsaturation) were added dropwise over 30 min. The reaction was performed over 6 h at 2000

Materials and methods

rpm, with the completion of H2O2 considered the initial time. Samples were taken out at 0.5, 1, 2, 3, 4 and 6 h.

All the samples were separated into two phases, organic and aqueous, which settled at the bottom of the separating funnel. The organic phase was diluted in diethyl ether at a volumetric ratio 1:1 and further purified by ion exchange using the strong anionic resin Amberlite IRA402 to remove the traces of catalyst and acetic acid.

3.3.2. Hydrolysis of high-oleic sunflower oil

In all the experiments, 60 g of high-oleic sunflower oil were placed in the reactor.

Based on that amount, the required quantities of H2O and DBSA were incorporated. The experimental conditions (temperature, time, and the molar ratios DBSA:TG and

H2O:TG) were varied according to the values shown in Table 3.2 and the Draper- small composite experimental design. After the reaction time, the reaction mixture was quenched with cold water and diethyl ether, obtaining two different phases. Both aqueous and organic phases were separated by means of a centrifuge. The diethyl ether from the organic phase was evaporated using a rotatory evaporator and its acid value was determined.

Table 3.2. Design matrix of the optimization study of the hydrolysis of HOSO with DBSA. Levels Independent variables Symbol -1 0 1 Temperature (ºC) X1 80 140 200 Reaction time (h) X2 4 10 16 DBSA:TG (mol/mol) X3 0.01 0.055 0.01 H2O:TG (mol/mol) X4 3 26.5 50

3.3.3. Azidification of epoxidized grape seed oil

Ten grams of epoxidized grape seed oil (EGSO) and 30 mL of dimethylformamide (DMF) were added to the reactor. After reaching the desired reaction temperature, the required amounts of NH4Cl (1 mole per mole of epoxide) and the NaN3 (1 mole per mole of epoxide) were added to the reactor. The azidification reaction was performed at 3 different temperatures (50, 60 and 70ºC) for 24 hours. Samples were extracted at four points: 1, 2, 4 and 24 hours.

Chapter 3

The obtained samples were washed with 50 ml of ultrapure water to remove DMF and unreacted NaN3. The organic phase was separated by centrifugation at room temperature.

3.3.4. Phosphorylation of epoxidized grape seed oil and methyl oleate

The required amount of EGSO or methyl oleate (MO), acetone and water were placed in the round bottom flask at a mass ratio acetone/water/epoxidized product 5:1:1.

Once the temperature of the mixture reached the desired value, H3PO4 was added in a mass ratio epoxidized product 1:1. The mixture was kept under reflux at 56 °C for 6 h. After the reaction process, the organic phase was separated by centrifugation from the aqueous phase and the solvents were removed by vacuum evaporation.

3.3.5. Synthesis of rigid polyurethane foams

Rigid polyurethane (RPU) foams were synthesized by weighting and mixing the desired masses of polyols, silicone (Tegostab B8404), catalyst (Tegoamin 33 and Tegostamin BDE) and water. The mixture was agitated until a perfect homogenization was achieved. Them. The required amount of MDI was added and the resulting solution was stirred for 15 s, at which point the foam started to grow up. The required quantities of these reactants were calculated based on the hydroxyl number of the polyol used to synthesized.

3.3.6. Synthesis of estolide esters

Estolide esters were synthesized in a two-step reaction process performed consecutively. Estolides were synthesized from oleic acid (OA) using different acids as catalysts at 50 °C using a molar ratio H+:OA of 0.4. The OA was added to the reactor and, once its temperature reached the desired value, the reaction media was inerted with N2 prior to add the catalyst. After 12 h, a small sample was extracted from the reaction to determine the amount of unreacted carboxylic groups (CG).

The esterification process was performed subsequently during 3 h at 65 °C just by adding to the reaction media the required amount of methanol (MeOH) at a molar ratio MeOH:CG from 8 to 20 and, when indicated, a 50%wt. of triethylammonium hydrogen sulphate as ionic liquid (IL).

Materials and methods

At the end of the esterification reaction, the product was transferred to a separatory funnel and allowed to settle down. The aqueous bottom phase, composed by the unreacted methanol, water, IL and catalyst was removed. The upper organic phase was dried and characterized.

Finally, the remaining CG after the esterification process, were neutralized with methanolic choline hydroxide (ChOH) at a molar ratio ChOH:CG of 1.1 at room temperature during 15 min. To conclude the process, the methanolic solution of choline carboxylates was removed from the organic phase by means of a centrifugation process at 40 °C and 2000 rpm.

3.3.7. Synthesis of trimethylolpropane trioleate

On a typical experiment, 20 g of TMP and the required amount of oleic acid (OA) were added to the reactor. The OA:TMP molar ratio was varied from 1 to 3. When the mixture reached 105 °C, the catalyst was added (0.1 equivalents for homogeneous catalysts and 10 wt.% for heterogeneous one in respect to TMP). The reaction mixture was kept at a constant temperature under reflux during 6 h.

When the influence of OA:TMP molar ratio was investigated, 150 mL of isooctane were also added to the reactor prior to the addition of the catalyst. The reaction time and temperature were 105 °C and 6 h, respectively. Along the reaction time, the mixture of formed water and isooctane was continuously evaporated, condensed and recirculated. Most of the water removed and collected on the Dean-Stark trap due to its higher density and high immiscibility with isooctane.

At the end of the reaction, the product was washed with deionized water using a volumetric ratio of 1:1 and transferred to a separatory funnel. The aqueous layer, composed by unreacted TMP, the catalyst and the washing water was discarded. The upper organic layer was dried via rotatory evaporation under vacuum at 50 °C during 1 h.

When indicated, the previously obtained product was neutralized in presence of an ethanolic solution of choline hydroxide (ChOH) to remove unreacted OA at a molar ratio ChOH:OA of 1.1 during 15 minutes at room temperature. The mixture was centrifugated at 40 °C and 2000 rpm to remove the formed ethanolic solution of choline carboxylates. Finally, the product was washed with ultrapure water at a

Chapter 3 volumetric ratio reaction product:water of 10 to remove residual traces and concentrated via rotatory evaporation under vacuum at 40 °C during 1 h.

3.3.8. Synthesis of triethylammonium hydrogen sulphate

Triethylammonium hydrogen sulphate ([HNEt3][HSO4]) was prepared by combination of a Bronsted acid (H2SO4) with a Bronsted base (triethylamine). In this preparation process, stoichiometric amounts of acid and base were mixed to form the salt. For safety issues, due to the high exothermicity of the reaction, the acid was added dropwise to an aqueous solution of the base. The water was removed at the end of the reaction by a rotatory vacuum evaporator.

3.3.9. Preparation of the lignin-additivated PEG lubricants

PEG-based fluids with a specific concentration of KL were systematically prepared by dispersing a specific amount of KL in 20 g of PEG200 or PEG300. To get a uniform and stable product, the suspension was kept under ultrasonic vibration continuously for 6 h. During the process, the temperature of the media was maintained at 15 °C. The dispersion time was exceptionally increased to 8 h when the chemical stability of both PEG and PEG/KL under prolonged ultrasound treatment was studied.

3.4. Characterisation techniques

3.4.1. Gel permeation chromatography

Gel permeation chromatography (GPC) was used to determine the molecular weight distribution of the polyols. Measurements were performed with a Viscotek chromatograph GPCmax VE-2001 TDA 302 Detectors (Figure 3.11) with a refraction index detector and with two columns Styragel HR 0.5(500Å, 0 to 1000 g/mol) and Styragel HR 2 (50Å, 500 to 20000g/mol).

Materials and methods

Figure 3.11. Viscotek chromatograph GPCmax VE-2001 TDA 302 Detectors.

Table 3.3 shows the test conditions. The calibration curve for GPC analysis was obtained with poly(ethylene glycol) standards from Waters with an average molecular weight of 194, 420, 628, 970, 1500, 4450, 6240 and 21000 g/mol. The chromatogram data were acquired by using OmniSEC 4.5.6 software.

Table 3.3. Test conditions for GPC analysis. Temperature 40 °C Flow 1 mL/min Sample concentration 10 mg/mL Injection volume 100 μL Eluent THF

3.4.2. Fourier transform infrared spectroscopy Fourier transform infrared spectroscopy (FTIR) was utilised to identify the characteristic functional groups of the products and raw materials obtained along this research work. The spectra were obtained on a Varian 640-IR FTIR spectrometer (Figure 3.12) by using Resolution PRO™ software with 35 scans per analysis and a 4 cm-1 resolution in the range of 4000 to 400 cm-1.

Chapter 3

Figure 3.12. Varian 640-IR FTIR spectrometer.

3.4.3. Gas chromatography

The fatty acid profile of the vegetable oils and biodiesel was measured following the International and European Standards ISO 5509 (to prepare methyl esters) and EN 140103 (to determine the methyl ester profile) using a HP 6890 Series 2 gas chromatograph with a flame ionization detector (Figure 3.13). The capillary column was a DB-WX column with a length of 30 m, a film thickness of 0.25 μm and an internal diameter of 0.32 mm. Helium was used as a carrier gas.

Figure 3.13. HP 6890 Series 2 gas chromatograph.

Materials and methods

3.4.4. Thermogravimetric analysis

To determine the weight loss of the functionalized polyols and polyurethane foams due to the degradation process as a function of temperature, thermogravimetric analyses (TGA) were performed using a TA Instruments SDT Q600 Simultaneous DSC-TGA (Figure 3.14). Samples were heated from room temperature to 600 °C at a heating rate of 10 °C/min under a nitrogen or air atmosphere. The obtained data were analysed using TA Universal Analysis 2000 software.

Figure 3.14. TA Instrument SDT Q600 thermogravimetric analyser.

3.4.5. Scanning electron microscopy and energy dispersive X-ray spectrometer

The scanning electron microscopy (SEM) studies were performed on the polyurethane foams and its residues after the thermogravimetric analysis using a Quanta 250 (FEI Company) scanning electron microscope with a wolfram filament, using a large field detector (LFD) and a back scattered electron detector (BSED). This equipment is coupled to an energy dispersive X-ray (EDAX) spectrometer Apollo X to determine the elemental composition analysis. Motic Images 2.2 and Minitab 16 software were used to determine the average size of the cells. The SEM can be observed in Figure 3.15.

Chapter 3

Figure 3.15. Scanning electron microscopy Quanta 250.

3.4.6. Phosphorous-31 nuclear magnetic resonance

Phosphorous-31 nuclear magnetic resonance (P31-NMR) spectra were measured with a Varian Gemini FT-400 spectrometer using chloroform as solvent. H3PO4 (85%, aqueous solution) was used for calibrating the equipment.

3.4.7. Kinematic viscosimetry and viscosity index

The kinematic viscosity was measured using different capillary viscometers Cannon-Fenske according to EN ISO 3104 standard method by immersing the viscometers in a thermostatic bath TAMSON TV 2000 (Figure 3.16).

Materials and methods

Figure 3.16. Thermostatic bath TAMSON TV 2000.

The test procedure consists on measuring the time that takes to a specified volume of liquid to flow through the calibrated viscometer at the specified temperature.

The viscosity index (VI) of the biolubricants was determined from the kinematic viscosities at 40 and 100 ºC according to ASTM D2270 standard method.

3.4.8. Low temperature tests

The low temperature test where performed in an automatic tester ISL FPP 5Gs (Figure 3.17). The cloud point (CP), which refers to the temperature below a cloudy appearance in a liquid is observed, was determined according to the EN 23015. The pour point (PP) is the temperature below which the liquid loses its flow characteristics. The PP was determined following the ASTM D97 standard method. Finally, the cold filter plugging point (CFPP) is the lowest temperature at which a given volume of liquid still passes through a standardized filter. This test was performed according to EN 116 international standard.

Chapter 3

Figure 3.17. Authomathic tester ISL FPP 5Gs.

3.4.9. Dynamic viscosity measurements

The measurements of the rheological response (i.e. the shear stress dependence with shear rate) of the PEG-based lubricants in Chapter 10 were performed using a Bohlin Gemini HR 200 Nano-Rheometer with a Peltier plate assembly and parallel plate geometry (Figure 3.18). The study of the behaviour of the lubricant systems was performed in a temperature range from 10 to 50 °C. During the measurements, the shear rate was varied between 1 and 600 s-1.

Figure 3.18. Bohlin Gemini HR 200 Nano-Rheometer.

Materials and methods

3.4.10. Analytical titrations

Different analytical titrations methods were used along this research work. Duplicate analyses were performed on each sample in all cases and average values are reported.

i) Iodine value (IV) was determined according to AOCS Official Method Tg 1-64. The IV is a measure of the unsaturation of fatty acids and it is expressed in terms of the number of g of iodine absorbed per 100 g of test sample (% iodine absorbed). From these determinations, the conversion of unsaturated during the epoxidation process was calculated using the Equation 3.1.

퐼푉 −퐼푉 퐶표푛푣푒푟푠푖표푛 (%) = 0 푓 · 100 (3.1) 퐼푉0

ii) The percentage of oxirane oxygen (OO) was determine by a direct method using hydrobromic acid solution in glacial acetic acid according to the AOCS Official Method Cd 9-57. This method was used to determine the yield of the epoxidation process, according to Equation 3.2:

253.8 푂푂· 푌푖푒푙푑 (%) = 16 · 100 (3.2) 퐼푉0

iii) The hydroxyl value, defined as the mg of KOH equivalent to the hydroxyl content of 1 g of sample, was measured according to AOCS Official Method Tx 1a-66. iv) The determination of the acid value (AV, expressed in mg KOH/g of product) was developed according to EN 14104 standard method. This test method involves titration of the sample with ethanolic potassium hydroxide solution. Equation 3.3 was used to determine the conversion after the esterification reactions. 퐴푉 −퐴푉 퐶표푛푣푒푟푠푖표푛 (%) = 0 푓 · 100 (3.3) 퐴푉0

CHAPTER 4. EPOXIDATION OF GRAPE SEED OIL

Modelling the epoxidation reaction of grape seed oil

Abstract Around 20 wt.% of grape production typically becomes waste during wine production. Grape seed oil can be extracted from this winery by-product through pressing or solvent extraction. This oil constitutes an economic alternative for valuation of by-products obtained from wine manufacturing in the Spanish region of Castilla-La Mancha.

Epoxidized vegetable oils have become one of the main intermediates for synthesizing different biomaterials (biopolyols, plasticizers, etc). The most extended method for epoxidation is using organic peracids generated in situ from hydrogen peroxide and acid catalysts, and many studies have investigated the influence of different variables such as reaction temperature and time, type and amount of catalyst, etc. However, a complete kinetic model able to predict the selectivity and the formation of different by-products has not yet been developed previously to this thesis.

In this chapter, the in situ epoxidation of grape seed oil (iodine value of 141.52 g

I2/100 g of vegetable oil) with aqueous hydrogen peroxide and acetic acid in presence of sulphuric acid catalyst was studied. It was also proposed a kinetic model able to predict the formation and depletion of different species in the process, finding a maximum yield of oxirane groups at a reaction time of 1 hour at 90 °C.

The dependence of kinetic rate constants on temperature was evaluated through the Arrhenius equation. The activation energy for the epoxidation reaction of grape seed oil was 7.30 kcal mol-1. The obtained results, particularly the proposed kinetic model, will play an important role in developing a safer and more sustainable epoxidation process, minimizing its energy consumption and facilitating the scale- up.

Chapter 4

Resumen Alrededor del 20 % del peso de la producción de uva se convierte normalmente en residuo durante la elaboración del vino. El aceite de granilla de uva puede ser extraído de este residuo mediante procesos de prensado en frío o mediante el uso de disolventes. La extracción y posterior valorización de este aceite constituye una alternativa a la producción de derivados del petróleo, permitiendo aumentar la rentabilidad de los procesos de producción de vino en la región española de Castilla- la Mancha.

Los aceites vegetales transformados en aceites epoxidados se han convertido en productos intermedios de gran interés en la síntesis de diferentes biomateriales (biopolioles, plastificantes, etc.). El método más extendido para llevar a cabo la transformación de los dobles enlaces de los ácidos carboxílicos en anillos oxirano consiste en la generación de perácidos orgánicos in situ a partir de peróxido de hidrógeno y catalizadores ácidos. Varios estudios han investigado la influencia de diferentes variables, como la temperatura y el tiempo de reacción, tipo y cantidad de catalizador, etc. Sin embargo, aún no existe un modelo cinético capaz de predecir la formación de subproductos de reacción.

En este capítulo se ha estudiado la epoxidación in situ de aceite de granilla de uva

(índice de yodo de 141,52 g I2/100 g de aceite vegetal) a partir de una disolución acuosa de peróxido de hidrógeno y ácido acético en presencia de ácido sulfúrico como catalizador. Asimismo, ha sido propuesto un modelo cinético capaz de predecir la formación y consumo de diferentes especies relevantes en el proceso, encontrando un máximo en la selectividad hacia grupos oxirano en un tiempo de reacción de 1 hora a 90 °C.

La dependencia de las constantes de reacción con la temperatura fue evaluada mediante la ecuación de Arrhenius. La energía de activación para la reacción de epoxidación del aceite de granilla de uva fue de 7,30 kcal mol-1. Los resultados obtenidos, especialmente el modelo cinético propuesto, cumplirán un rol importante en el desarrollo de sistemas de epoxidación más sostenibles y seguros, minimizando el consumo energético y facilitando el escalado del proceso.

Modelling the epoxidation reaction of grape seed oil

4.1. General background The epoxidation reaction consists on the addition of an oxygen atom to a carbon- carbon double bond. This reaction has been established as one of the most important methods for the formation of carbon-oxygen bonds (Linker, 1997).

Epoxides are highly reactive and versatile functional groups which are commonly used as precursors for alcohols, glycols, carbonyls, alkanolamines, substituted olefins, polyester polyurethanes and epoxy resins (Petrović, 2008; Yadav & Manjula Devi, 2001).

Increasing environmental problems related to fossil sources overuse have made plant oils an attractive alternative for producing epoxy-based materials. In the industry, epoxidized vegetable oil, and more specifically soybean and linseed oil, are currently used as plasticizers for PVC and related resins, reaching an annual production of about 200,000 tons (Corma Canos et al., 2007). Indeed, the epoxides change the solubility and flexibility of the PVC resins and react with hydrochloric acid liberated from the PVC resins under the prolonged action of light and heat. Epoxidized vegetable oils are also used as lubricants (Adhvaryu & Erhan, 2002; Campanella et al., 2010) and as prepolymer in coating formulations (Shaker et al., 2008; Thames & Yu, 1999; Wan Rosli et al., 2003). The applicability of an epoxidized oil depends on its purity, oxirane number, and iodine number.

There are three different potential sources of vegetable oils containing epoxy groups. Firstly, there is a variety of natural occurring epoxy vegetable oil, mainly Vernonia galamensis (Perdue et al., 1986) and Euphorbia lagascae oil (Krewson & Scott, 1966), which contain up to 70% of vernolic acid (12S,13R-epoxy-9-cis- octadecenoic acid). The second option consist on the production of oil from genetically modified seeds which already containing vernolic acid in order to increase its content (Cahoon et al., 2002; Yu et al., 2008). However, none of these vegetable oils are commercially available at a competitive cost. Therefore, the only remaining solution consists on the chemical transformation of unsaturated vegetable oils, such us soybean (Petrović et al., 2002), sunflower (Benaniba et al., 2007), rapeseed (Milchert et al., 2010) or jatropha oil (Rios et al., 2011), on epoxidized vegetable oils.

Chapter 4

A wide variety of methods have been proposed for epoxidation of vegetable oils and related products (e.g. unsaturated free fatty acids or fatty acid methyl esters). The most important ones are listed below (Armylisasa et al., 2017):

• Chlorohydrin process: this indirect epoxidation process is based on the reaction of the unsaturation with HOCl, forming the chlorohydrin which subsequently yields the epoxide on alkaline treatment. However, this method is highly unfriendly environmentally. • Halcon reaction: epoxides can also be prepared by treating the unsaturations with tert-butyl hydroperoxide using vanadium, titanium of molybdenum complexes as catalysts. • Epoxidation with dioxirane: the enantioselective epoxidation in a neutral medium is made possible by using dioxirane and an optically active manganese (III) salt as catalyst. • Epoxidation with molecular oxygen: this process is catalysed by silver and is the cheapest and greenest route to epoxidize low molecular weight molecules (e.g. ethylene and butadiene). However, this process is not efficient to be applied to vegetable oils.

However, the selectivity and workup of these methods are not satisfactory for industrial applications, and currently, the industrial processes are based on the use of peracids. The Prilezhaev reaction consists on the epoxidation of alkenes with peracids, typically performic or peracetic acid, formed in situ from hydrogen peroxide and formic or acetic acid, respectively. Among them, peracetic acid is the most used one due to its low price, higher epoxidation efficiency and safety issues at ordinary temperatures (Danov et al., 2017). Acidic catalysts, either strong acids, acidic ion exchange resins (Musante et al., 2000) or enzymes (Vlcek & Petrovic, 2006), are required on this process, being mineral acids the most efficient ones. Dinda et al. found that the order of catalyst effectiveness for the vegetable oil epoxidation reaction based in mineral acids was headed by H2SO4 followed by

H3PO4, HNO3 and HCl (Dinda et al., 2008). Respecting to the concentration of homogeneous catalyst, typical values around 1 and 3 wt.% with regard to the amount of hydrogen peroxide and organic acid (Goud et al., 2006; Kousaalya et al., 2018; Santacesaria et al., 2011).

Modelling the epoxidation reaction of grape seed oil

During the in situ epoxidation process, different reactions take place. Firstly, the peracid must be formed from hydrogen peroxide and the carboxylic acid in the presence in the acid catalyst (Figure 4.1a). Secondly, the previously formed peracid attacks the alkene group, generating an oxirane group and the carboxylic acid as shown in Figure 4.1b. Hence, the carboxylic acid is not consumed during the epoxidation reaction and it just acts as an oxygen carrier. However, two side reactions might take place during the epoxidation process. Since oxirane rings are formed in aqueous acidic media, they can be hydrolysed into hydroxyl groups (Figure 4.1c). Besides, reactions between hydroxyl groups from different triglycerides led to the formation of oligomers linked by ether groups (Petrović et al., 2002).

Figure 4.1. Reaction scheme of the a) formation of the organic peracid, b) epoxidation reaction of double bonds, c) ring-opening reaction in presence of acidic water and d) reaction of oligomerization.

Respecting to the operating conditions of the epoxidation reaction of vegetable oils two different trends can be found in literature. On the one hand, some authors recommend soft temperatures and significant long reaction times in order to have a valid conversion of double bonds but a minimum formation of by-products, being the most typical conditions 35-65 °C and 4-16 h (do Valle et al., 2018; Kousaalya et al., 2018; Milchert et al., 2010). On the other hand, some others defend the

Chapter 4 performance of the in situ epoxidation reaction of vegetable oils at a moderately high temperatures (over 90 °C) using much shorter residence times (below 1 h) (Cortese et al., 2012; Naidir et al., 2012). Nevertheless, a kinetic model able to predict the reaction products based on the reaction conditions and independent from the vegetable oil used has not been reported yet in literature

Thus, the aim of this chapter is to study the effect of temperature on the epoxidation reaction of grape seed oil. The concentration of double bonds, oxirane rings, hydroxyl groups, dimers and trimers as function of the time and temperature (50, 60, 90 and 100 °C) were determined. A kinetic model able to reproduce the different species profiles with time and temperature is shown, being the kinetic constants determined by a non-linear regression method based on the Marquardt algorithm. The Arrhenius equation is further used to determine the variation of the previously determined kinetic constants with temperature. The proposed model could be used in a suitable way for scale-up purposes.

4.2. Results 4.2.1. Grape seed oil characterization Among the typical characteristics of a vegetable oil, fatty acid composition is one of the most relevant points. Table 4.1 shows the fatty acid profile of grape seed oil (GSO) determined in a previous study (Ramos et al., 2008).

Table 4.1. GSO fatty acid profile. Fatty acid wt.% Myristic acid C14:0 0.1 Palmitic acid C16:0 6.9 Palmitoleic acid C16:1 0.1 Stearic acid C18:0 4.0 Oleic acid C18:1 19.1 Linoleic acid C18:2 69.1 Linolenic acid C18:3 0.3 Arachidic acid C20:0 0.3 Others 0.1

The iodine value and average molecular weight (MW) of 141.52 g I2/100 g sample and 897 g/mol, respectively, of this raw material were determined in this study.

Modelling the epoxidation reaction of grape seed oil

The vegetables oils currently used for the production of epoxidized oils have a similar unsaturation level. Therefore, GSO must be considered as a potential raw material for its production (Benaniba et al., 2007; Sinadinovic-Fiser et al., 2001).

4.2.2. Effect of temperature on double bonds conversion

The epoxidation reactions were carried out at two different temperature ranges: low (50 and 60 °C) and high (90 and 100 °C). To evaluate the conversion and selectivity of the process, the concentration of different species was measured.

Table 4.2 shows the evolution of double bonds, epoxy groups and hydroxyl groups along the reaction time at different reaction temperatures.

Table 4.2. Presence of double bonds, epoxy groups and hydroxyl groups as function of temperature and reaction time. Double bonds Epoxy groups Hydroxyl groups Time (mol/l) (mol/l) (mol/l) (h) 50 °C 60 °C 90 °C 100 °C 50 °C 60 °C 90 °C 100 °C 50 °C 60 °C 90 °C 100 °C 0 2.947 0.000 0.000 0.5 1.758 2.098 1.083 0.880 0.970 0.784 1.511 1.383 0.055 0.077 0.225 0.258 1 1.622 1.576 0.193 0.100 1.067 1.125 2.186 1.683 0.137 0.105 0.295 0.412 2 1.434 1.130 0.168 0.095 1.235 1.571 2.088 1.142 0.132 0.124 0.535 0.924 3 0.987 0.756 0.150 0.042 1.502 1.867 1.519 0.391 0.189 0.140 1.162 1.988 4 0.879 0.487 0.128 0.000 1.606 1.880 0.452 0.027 0.205 0.174 1.664 2.220 6 0.759 0.217 0.101 0.000 1.673 2.023 0.010 0.008 0.229 0.269 1.941 1.989

The obtained data indicates that the reaction behaviour varies with temperature. At the high reaction temperature (reactions at 90 and 100 °C) there was a maximum epoxy groups yield after a reaction time of 1 h. After that, oxirane rings disappeared and tended to be completely eliminated within the reaction time. However, at low reaction temperature, the oxirane oxygen concentration increased with the reaction time, requiring long times to reach a high yield but without any depletion.

In the same way, the hydroxyl number increased sharply at a reaction temperature of 100 °C up to 4 hours. In this condition, the concentration of hydroxyl groups decreased in the bulk solution because the consumption rate was higher that the corresponding formation rate. This behaviour indicates that the formation of hydroxyl groups is dependent on the presence of oxirane rings, increasing while there are oxirane rings in the reaction media but decreasing in their absence. At low temperature, practically no differences are observed in the formation of these compounds. The profile obtained at 90 °C indicated a continued formation of the hydroxyl groups, although the net formation rate decreased during long reaction

Chapter 4 times. To evaluate the feasibility of this process, Figure 4.2 shows the conversion of double bounds with time and temperature.

100

30 Reaction temperature (ºC)

Double bond conversion (%) 20 50 60 10 90 100 0 0 1 2 3 4 5 6 Time (hour) Figure 4.2. Conversion of double bonds as a function of temperature and reaction time.

Clearly, there is an important gap in double bond conversion as function of temperature, with increased conversion at higher temperature. Conversions greater than 90% can be obtained in fewer than 1.5 hours for the high reaction temperature range. Over long reaction times, while the conversion at 90 °C tends to be stable at approximately 95 %, at 100 °C the conversion increases with time up to 100 % after 4 hours. On the other hand, if the reaction temperature is equal or lower than 60 °C, the reaction proceeds slower, requiring more than 6 hours for complete the double bond conversion. This long reaction times are usually undesirable at industrial scale attending to the large operating costs of the process. Nevertheless, a faster reaction does not ensure the desired conditions because the formation of a large amount of by-products could be promoted as noted above. At 100 °C, a diminishing in the concentration of hydroxyl groups was observed, indicating the formation of oligomers mainly when oxirane rings exist. The generation of oligomers in the reaction bulk was confirmed by gel permeation chromatography (GPC). The chromatographs are shown in Figure 4.3.

Modelling the epoxidation reaction of grape seed oil

a) Reaction time (h) b) 0.5 Reaction time (h) 1 0.5 2 1 3 2

4 3

6 4 6

10 11 12 13 14 15 16 17 10 11 12 13 14 15 16 17 Elution volume (ml) Elution volume (ml)

c) d) Reaction time (h) Reaction time (h) 0.5 0.5 1 1 2

2 3

3 4 4 6 6

10 11 12 13 14 15 16 17 10 11 12 13 14 15 16 17 Elution volume (ml) Elution volume (ml)

Figure 4.3. GPC chromatogram showing the evolution of the MW distribution with time at a) 50 °C, b) 60 °C, c) 90 °C and d) 100 °C. All chromatographs in Figure 4.3 present three peaks. The main product can be found at an elution volume between 13 and 14 mL, while the other two peaks represent dimers and trimers at 12.5 and 12 mL, respectively. The presence of oligomers with higher molecular weights was not observed.

When the epoxidation reaction was performed at 50 °C (Figure 4.3a), oligomer formation can be neglected. Nevertheless, a slight increase in molecular weight (MW) of the main product was observed due to the incorporation of the oxirane oxygen to the triglycerides. Figure 4.3b shows that the main peak relative intensity decreases with time, confirming that dimers and trimers are formed at 60 °C. However, if the reaction is carried out at higher temperatures (Figures 4.3c and 4.3d), the elution time of the main peak decreased with the reaction time due to the incorporation of oxirane and hydroxyl groups that replaced the unsaturations. The peaks corresponding to dimers and trimers increased in contrast to the behaviour of the main peak. Nevertheless, Figure 4.3d shows that dimers and trimers were stable once formed after 240 min of reaction. This result could be explained by the

Chapter 4 lack of oxirane ring groups in the reactor at this time, suggesting that the formation of oligomers requires the presence of both oxirane rings and hydroxyl groups.

Table 4.3 shows the average MW of the different peaks observed by GPC.

Table 4.3. Average MW for the peaks observed by GPC analysis. MW Reaction Peak Peak Peak Temperature time (h) 1 2 3 (°C) 0 897 - - 0.5 1184 2365 3269 1 1183 2356 3293 2 1196 2370 3418 50 3 1200 2359 3495 4 1203 2344 3525 6 1213 2353 3151 0.5 1199 2370 3456 1 1206 2341 3243 2 1222 2364 3258 60 3 1217 2350 3291 4 1255 2373 4075 6 1242 2355 3692 0.5 1195 2395 4045 1 1212 2383 3804 2 1228 2370 3643 90 3 1275 2432 3496 4 1368 2475 3566 6 1411 2469 3550 0.5 1205 2417 4275 1 1230 2372 3730 2 1279 2403 3189 100 3 1389 2499 3608 4 1440 2537 3682 6 1447 2537 3683

Considering all the reactions that are taking place in the presented system, a mass balance on the double bonds can be obtained (Equation 4.1). The initial amount of double bonds (DBt=0) is equal to the unreacted amount of double bonds (DBt) plus all its possible reaction products, which were found to be oxirane rings (OR), hydroxyl groups and oligomers generated from the condensation of -OH groups, taking into consideration its relative stoichiometry. As it was presented in Figure 4.1b, each double bond can only form a single oxirane ring. On the contrary, two -OH groups are formed by the cleavage of an oxirane ring (i.e. the stoichiometry

Modelling the epoxidation reaction of grape seed oil between DB and -OH is 1:2). Finally, the condensation of two hydroxyl groups is required for the formation of an ether linkage related to the presence of oligomers so, in a global balance, a single double bond is consumed to form an ether.

푂퐻 퐷퐵 = 퐷퐵 + 푂푅 + 푡 + 퐸푡ℎ푒푟 (4.1) 푡=0 푡 푡 2

A second mass balance related to the oligomers can be determined in order to calculate the concentration of both dimers and trimers individually. The stoichiometry shown in Figure 4.1d stated that one and two ether groups are present in a dimer (DIM) and trimer (DIM), respectively. On the base of that, Equation 4.2 can be stated.

퐸푡ℎ푒푟 = 퐷퐼푀 + 2 푇푅퐼푀 (4.2)

Finally, the portion of dimers (DIM) and trimers (TRIM) can be quantified using the area ratio of these compounds from the chromatographs previously presented. In this estimation, the same response factor for both dimers and trimers was assumed.

The obtained values for the concentrations of DIM and TRIM are shown in Table 4.4.

Table 4.4. Dimers and trimers concentration as function of temperature and reaction time. Dimers Trimers Time (mol/l) (mol/l) (h) 50 °C 60 °C 90 °C 100 °C 50 °C 60 °C 90 °C 100 °C 0.5 0.103 0.026 0.027 0.249 0.025 0.000 0.021 0.038 1 0.091 0.018 0.041 0.234 0.035 0.007 0.067 0.072 2 0.083 0.016 0.034 0.130 0.058 0.059 0.103 0.074 3 0.124 0.024 0.046 0.138 0.118 0.166 0.223 0.126 4 0.111 0.031 0.056 0.269 0.129 0.320 0.728 0.657 6 0.047 0.043 0.064 0.351 0.220 0.463 0.909 0.946

As expected, the concentrations determined for dimers and trimers at low temperatures are significantly lower than those presented for the other components previously presented at Table 4.2. This behaviour is completely inverted when the reaction was performed at higher temperatures, becoming the dimers and trimers together with hydroxides the main species founds at reaction times higher than 4 h.

From the point of view of its further application, the measured concentrations are valueless since it is exclusively based on discrete points. Nevertheless, this

Chapter 4 information can be used to generate a general kinetic model able to predict the evolution of the different functional groups along the reaction time

4.2.3. Kinetic model

Jankovic et al. have already postulated a kinetic model for the in situ formation of peracetic acid from acetic acid and hydrogen peroxide in the presence of a homogeneous catalyst. The mechanism can be summarised in the following steps: (i) formation of peracetic acid in the aqueous phase in the presence of the catalyst; (ii) transfer of peracetic acid from the aqueous phase to the oil phase; (iii) reaction between peracetic acid and the double bonds in the oil phase to produce oxirane rings, releasing acetic acid; (iv) diffusion of the acetic acid from the oil phase to the aqueous phase; and (v) opening of the epoxide ring by water in the oil-aqueous interface and oil phase (Jankovic & Sinadinovic-Fiser, 2004).

From the above mechanism the following reaction system is proposed (Equations from 4.3 to 4.5).

푘0,푘1 퐴퐴 + 퐻2푂2 ↔ 푃퐴퐴 + 퐻2푂 (4.3)

푘2 푃퐴퐴 + 퐷퐵 → 푂푅 + 퐴퐴 (4.4)

푘3 푂푅 + 퐻2푂 → 2푂퐻 (4.5) where AA, PAA, DB, OR and OH represent the acetic acid, peracetic acid, double bond, oxirane ring and hydroxyl groups, respectively. As it was previously demonstrated, oxirane rings can also be opened by the previously formed hydroxyl groups, forming dimers and, in a further stage, trimers (Equations 4.6 and 4.7).

푘4 푂푅 + 푂퐻 → 퐷퐼푀 (4.6)

푘5 푂푅 + 퐷퐼푀 → 푇푅퐼푀 (4.7) where DIM and TRIM are the concentration of dimers and trimers, respectively.

Finally, it is important to take into account the degradation of hydrogen peroxide in water and oxygen as it is shown in Equation 4.8 (Miller & Valentine, 1995).

푘6 퐻2푂2 → 퐻2푂 + 0.5푂2 (4.8)

Modelling the epoxidation reaction of grape seed oil

As can be observed in Equation 4.3, the absence of H2O2 in the media avoids the formation of peracetic acid and thus, the conversion of double bonds to oxirane rings.

To simplify the kinetic model, it was assumed that the epoxidation process takes place in a single phase (i.e. pseudo-homogeneous system). This assumption avoids using distribution constants for the quantification of the different species in the aqueous and oil phases. To minimise the effect of this assumption in the final prediction a high agitation rate (>2000 rpm) was used during the reaction. This high agitation rate favours the formation of small oil drops in the aqueous phase, increasing the specific external surface for mass transfer and minimising limitations to the mass transfer.

From the proposed reaction set, the corresponding differential equation system can be drawn (Equations from 4.9 to 4.17):

푑[퐴퐴] = −푘 [퐴퐴][퐻 푂 ] + 푘 [푃퐴퐴][퐻 푂] + 푘 [푃퐴퐴][퐷퐵] (4.9) 푑푡 0 2 2 1 2 2

푑[퐻 푂 ] 2 2 = −푘 [퐴퐴][퐻 푂 ] + 푘 [푃퐴퐴][퐻 푂] − 푘 [퐻 푂 ] (4.10) 푑푡 0 2 2 1 2 6 2 2 푑[푃퐴퐴] = 푘 [퐴퐴][퐻 푂 ] − 푘 [푃퐴퐴][퐻 푂] − 푘 [푃퐴퐴][퐷퐵] (4.11) 푑푡 0 2 2 1 2 2 푑[퐻 푂] 2 = 푘 [퐴퐴][퐻 푂 ] − 푘 [푃퐴퐴][퐻 푂] − 푘 [푂푅][퐻 푂] + 푘 [퐻 푂 ] (4.12) 푑푡 0 2 2 1 2 3 2 6 2 2 푑[퐷퐵] = −푘 [푃퐴퐴][퐷퐵] (4.13) 푑푡 2 푑[푂푅] = 푘 [푃퐴퐴][퐷퐵] − 푘 [푂푅][퐻 푂] − 푘 [푂푅][푂퐻] − 푘 [푂푅][퐷퐼푀] (4.14) 푑푡 2 3 2 4 5 푑[푂퐻] = 2푘 [푂푅][퐻 푂] − 푘 [푂푅][[푂퐻]] (4.15) 푑푡 3 2 4 푑[퐷퐼푀] = 푘 [푂푅][푂퐻] − 푘 [푂푅][퐷퐼푀] (4.16) 푑푡 4 5 푑[푇푅퐼푀] = 푘 [푂푅][퐷퐼푀] (4.17) 푑푡 5 The above set of differential equations was integrated numerically using the fourth- order Runge-Kutta method. The six unknown kinetic parameters (ki) were determined by fitting all of the experimental values of DB, OR, OH, DIM and TRIM to those obtained from Equations 4.9 to 4.17. For this purpose, an in-house spreadsheet in Excel using the solver tool to solve the non-linear fitting was implemented. This iterative method was solved using objective and constrained

Chapter 4 cells subjected to “minimum” and “zero” values, respectively. The objective cell (Equation 4.18) was obtained by summing all the squared residuals between the experimental and theoretical values for the studied dependent variables.

2 5 푛 푓 = ∑푗=1 ∑푖=1 [{(퐷푉푗) } − {(퐷푉푗) } ] (4.18) 푝푟푒푑 푖 푒푥푝푡 푖 where DV represents each dependent variable, and n is the number of samples taken for each reaction. Five of functional groups were analysed in the organic phase: DB, OR, OH, DIM and TRIM.

The constraint cell was stablished according to the double bound mass balance showed in Equation 4.1.

Figure 4.4 shows a comparison between the experimental data and theoretical values for the DB, OR, OH, DIM and TRIM concentrations.

3.5 3.5 [DB] [DB] [DB] [DB] a) Mod Exp b) Mod Exp [OR] [OR] [OR] [OR] 3.0 Mod Exp 3.0 Mod Exp [OH] [OH] [OH] [OH] Mod Exp Mod Exp [DIM] [DIM] [DIM] [DIM] 2.5 Mod Exp 2.5 Mod Exp [TRIM] [TRIM] [TRIM] [TRIM] Mod Exp Mod Exp

2.0 2.0

1.5 1.5

1.0 1.0

Concentration (mol/l) Concentration (mol/l)

0.5 0.5

0.0 0.0

0 50 100 150 200 250 300 350 0 50 100 150 200 250 300 350 time (min) time (min) 3.5 4.0 [DB] [DB] [DB] [DB] c) Mod Exp d) Mod Exp 3.0 [OR] [OR] 3.5 [OR] [OR] Mod Exp Mod Exp [OH] [OH] [OH] [OH] Mod Exp Mod Exp 3.0 2.5 [DIM] [DIM] [DIM] [DIM] Mod Exp Mod Exp [TRIM] [TRIM] [TRIM] [TRIM] Mod Exp 2.5 Mod Exp 2.0

2.0

1.5 1.5 1.0

Concentration (mol/l) 1.0 Concentration (mol/l) 0.5 0.5

0.0 0.0 0 50 100 150 200 250 300 350 0 50 100 150 200 250 300 350 time (min) time (min)

Figure 4.4. Theoretical and experimental profiles of functional groups at a) 50 C, b) 60 °C, c) 90 °C and d) 100 °C.

Good agreement between experimental and predicted data was obtained, with an error below a 0.5% in all cases.

Modelling the epoxidation reaction of grape seed oil

To determine whether the obtained results indicate statistical significance, Student’s t-test was carried out. Table 4.5 shows the calculated t values divided by the critical value (2.56). This test revealed that the model can be accepted with the 95% confidence interval.

Table 4.5. Student's t-test results. [DB] [OR] [OH] [DIM] [TRIM] T (°C) tstudent/tcrit tstudent/tcrit tstudent/tcrit tstudent/tcrit tstudent/tcrit 50 0.070 0.055 0.074 0.967 0.321 60 0.023 0.097 0.268 0.338 0.413 90 0.061 0.042 0.029 0.725 0.113 100 0.030 0.025 0.048 0.651 0.009

Table 4.6 shows the obtained values for the kinetic constants.

Table 4.6. Kinetic constants for the proposed model. Constant Units 50 °C 60 °C 90 °C 100 °C k0·102 1.35 1.20 1.01 0.990 k1·102 6.56 4.02 0.102 0.573 k2·10 1.29 1.78 4.85 5.60 (L mol-1 min-1) k3·104 0.247 0.288 1.38 1.75 k4·103 0.106 5.94 3.81 3.07 k5·102 1.04 1.05 1.50 1.70 k6·103 (min-1) 5.85 7.10 0.144 6.30

All the reaction constants have the same order of magnitude except k3, which is the lowest. This value could be related to the high concentration of water that was used in the process and practically neglected in this model. Thus, it is possible that the value of this constant would increase in a non-simplified model. The values of k0 and k1 indicate that peracetic formation is unfavourable at higher temperatures and this reaction is mainly reversible at low temperature, where k1 is higher than k0. Another important point is the higher value obtained for k5 respect to k4, indicating that trimer formation is favoured over dimer formation.

One of the most traditional representations of kinetic constants is the Arrhenius equation (Equation 4.19). To obtain the pre-exponential factor (A) and the activation energy (EA), all obtained constants for the chemical reactions were fitted by non-linear fitting by following the procedure describe above to obtain the unknown kinetic parameters.

−퐸퐴 푘 = 퐴 ∙ 푒 푅∙푇 (4.19)

Chapter 4

The activation energy, pre-exponential factor and coefficient of determination are shown in Table 4.7.

Table 4.7. Arrhenius model parameters. EA (kcal mol-1) A R2 k0 -1.46 1.35·10-3 0.972 k1 -16.51 4.35·10-13 0.738 k2 7.30 1.12·104 0.994 k3 10.19 171 0.979 k4 -5.33 2.27·10-6 0.937 k5 2.47 0.468 0.966 k6 5.33 23.18 0.996

A good fit between the kinetic constants and the Arrhenius equation was obtained for all reactions, illustrated by the high values of R2 values, except for the reverse reaction of peracetic acid synthesis. Different authors have studied the in situ epoxidation process using different vegetable oils as raw materials. Cai et al. and Mungroo et al. found a value of 10.3 kcal mol-1 (Cai et al., 2008) and 10.7 kcal mol-1 (Mungroo et al., 2011) using soybean and canola oil, respectively. There is a slight difference with respect to our determined value 7.3 kcal mol-1 because the rate of epoxidation of unsaturated fatty acids depends on the structure of the acid, the number and the position of the double bonds, their position with respect to the carboxylic group and the presence of cis- or trans- isomers (Jankovic & Sinadinovic-

Fiser, 2004). In addition, EA values are highly dependent on the simplifications and the chemical reactions taken into account along the proposed reaction system. Therefore, because our kinetic model considers secondary reactions that other models do not consider, slight differences in EA values are expected.

4.3. Conclusions

The epoxidation of grape seed oil by peracetic acid formed in situ from acetic acid and hydrogen peroxide can be accomplished using sulphuric acid as a catalyst. The conversion of double bonds increased with temperature and time, being temperature the most influent variable. The highest reaction yield for the formation of oxirane rings was obtained at 1 hour and 90 °C. Dimers and trimers formation was promoted by temperature, but their formation requires of both hydroxyl and oxirane groups.

Modelling the epoxidation reaction of grape seed oil

The developed mathematical model, which assumes a pseudo-homogeneous reaction system, describes perfectly the in situ epoxidation of unsaturated vegetable oils with organic peracids. The obtained kinetic rate constants were well fitted to the Arrhenius equation when investigating the reaction’s dependence on temperature. The obtained activation energy for the epoxidation reaction was compared with those from other authors and a good agreement was observed. The obtained results, and especially the proposed kinetic model, will play an important role in developing a safer and more sustainable epoxidation process, minimizing energy consumption and facilitating scale-up.

Chapter 4

References

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Modelling the epoxidation reaction of grape seed oil

Jankovic, M.R., Sinadinovic-Fiser, S. 2004. Kinetic models of reaction systems for the in situ epoxidation of unsaturated fatty acid esters and triglycerides. Chemistry & Industry, 58(12), 569-576. Kousaalya, A.B., Beyene, S.D., Gopal, V., Ayalew, B., Pilla, S. 2018. Green epoxy synthesized from Perilla frutescens: A study on epoxidation and oxirane cleavage kinetics of high-linolenic oil. Industrial Crops and Products, 123, 25-34. Krewson, C.F., Scott, W.E. 1966. Euphorbia lagascae Spreng., an abundant source of epoxyoleic acid; Seed extraction and oil composition. Journal of the American Oil Chemists Society, 43(3), 171-174. Linker, T. 1997. The Jacobsen-Katsuki epoxidation and its controversial mechanism. Angewandte Chemie (International Edition in English), 36(19), 2060-2062. Milchert, E., Smagowicz, A., Lewandowski, G. 2010. Optimization of the reaction parameters of epoxidation of rapeseed oil with peracetic acid. Journal of Chemical Technology and Biotechnology, 85(8), 1099-1107. Miller, C.M., Valentine, R.L. 1995. Hydrogen peroxide decomposition and quinoline degradation in the presence of aquifer material. Water Research, 29(10), 2353- 2359. Mungroo, R., Goud, V.V., Pradhan, N.C., Dalai, A.K. 2011. Modification of epoxidised canola oil. Asia-Pacific Journal of Chemical Engineering, 6(1), 14-22. Musante, R.L., Grau, R.J., Baltanás, M.A. 2000. Kinetic of liquid-phase reactions catalyzed by acidic resins: The formation of peracetic acid for vegetable oil epoxidation. Applied Catalysis A: General, 197(1), 165-173. Naidir, F., Yunus, R., Rashid, U., Masood, H., Ghazi, T.I.M., Ramli, I. 2012. The kinetics of epoxidation of trimethylolpropane ester. European Journal of Lipid Science and Technology, 114(7), 816-822. Perdue, R.E., Carlson, K.D., Gilbert, M.G. 1986. Vernonia galamensis, Potential new crop source of epoxy acid. Economic Botany, 40(1), 54-68. Petrović, Z.S. 2008. Polyurethanes from vegetable oils. Polymer Reviews, 48(1), 109-155. Petrović, Z.S., Zlatanić, A., Lava, C.C., Sinadinović-Fišer, S. 2002. Epoxidation of soybean oil in toluene with peroxoacetic and peroxoformic acids - Kinetics and side reactions. European Journal of Lipid Science and Technology, 104(5), 293-299. Ramos, M.J., Fernández, C.M., Casas, A., Rodríguez, L., Pérez, A. 2008. Influence of fatty acid composition of raw materials on biodiesel properties. Bioresource Technology, 100(1), 261-268.

Chapter 4

Rios, L.A., Echeverri, D.A., Franco, A. 2011. Epoxidation of jatropha oil using heterogeneous catalysts suitable for the Prileschajew reaction: Acidic resins and immobilized lipase. Applied Catalysis A: General, 394(1-2), 132-137. Santacesaria, E., Tesser, R., Di Serio, M., Turco, R., Russo, V., Verde, D. 2011. A biphasic model describing soybean oil epoxidation with H2O2 in a fed-batch reactor. Chemical Engineering Journal, 173(1), 198-209. Shaker, N.O., Kandeel, E.M., Badr, E.E., El-Sawy, M.M. 2008. Synthesis and properties of renewable environment-friendly epoxy resins for surface coatings. Journal of Dispersion Science and Technology, 29(3), 421-425. Sinadinovic-Fiser, S., Jankovic, M., Petrovic, Z.S. 2001. Kinetics of in situ epoxidation of soybean oil in bulk catalyzed by ion exchange resin. JAOCS, Journal of the American Oil Chemists' Society, 78(7), 725-731. Thames, S.F., Yu, H. 1999. Cationic UV-cured coatings of epoxide-containing vegetable oils. Surface and Coatings Technology, 115(2-3), 208-214. Vlcek, T., Petrovic, Z.S. 2006. Optimization of the chemoenzymatic epoxidation of soybean oil. JAOCS, Journal of the American Oil Chemists' Society, 83(3), 247-252. Wan Rosli, W.D., Kumar, R.N., Mek Zah, S., Hilmi, M.M. 2003. UV radiation curing of epoxidized palm oil-cycloaliphatic diepoxide system induced by cationic photoinitiators for surface coatings. European Polymer Journal, 39(3), 593-600. Yadav, G.D., Manjula Devi, K. 2001. A kinetic model for the enzyme-catalyzed self- epoxidation of oleic acid. JAOCS, Journal of the American Oil Chemists' Society, 78(4), 347-351. Yu, K., Li, R., Hatanaka, T., Hildebrand, D. 2008. Cloning and functional analysis of two type 1 diacylglycerol acyltransferases from Vernonia galamensis. Phytochemistry, 69(5), 1119-1127.

CHAPTER 5. PRODUCTION OF FREE FATTY ACIDS VIA ACID HYDROLYSIS

Production of free fatty acid via acid hydrolysis

Abstract In this chapter, the hydrolysis of high-oleic sunflower oil to produce free fatty acids (FFA) in presence of dodecylbenzene sulfonic acid (DBSA) as catalyst was studied in a microwave reactor. Response surface methodology was used to evaluate individual and interactive effects of four independent reaction parameters, namely temperature (T), time (t), DBSA to triglycerides (DBSA:TG) and water to triglycerides (H2O:TG) molar ratios.

A Draper-Lin small composite design was used to optimize the fatty acid content (FAC) of the reaction product, obtaining a second-order empirical relationship between FAC and the independent variables. The analysis of variance (ANOVA) demonstrated a high coefficient of determination value (R2=0.974). The response surface and contour plots revealed the negative effect of using a high H2O:TG molar ratios (dilution of DBSA) and high temperatures and long reaction times (carboxylic- group consumption on secondary reactions).

The model predicted various regions where the estimated FAC is 100 %. Two different reaction conditions in these regions (T=140 °C, t=10 h, DBSA:TG=0.0163 mol/mol, H2O:TG=30 mol/mol; and T=152 °C, t=4 h, DBSA:TG=0.055 mol/mol,

H2O:TG=35 mol/mol) were tested, observing FAC values close to the predicted ones (99.81 and 99.92 %, respectively). This study concluded that DBSA is a suitable catalyst for the hydrolysis of vegetable oils at softer reaction conditions than the industrial methods already implemented.

Chapter 5

Resumen En este capítulo ha sido estudiada la hidrólisis de aceite de girasol alto oleico para producir ácidos grasos libres (FFA) en presencia de ácido dodecilbenceno sulfónico (DBSA) en un reactor microondas. La metodología de superficie respuesta fue usada para evaluar los efectos individuales e interactivos de cuatro variables de reacción independientes: la temperatura (T), el tiempo (t) y las relaciones molares entre el

DBSA y los triglicéridos (DBSA:TG) y entre el agua y los triglicéridos (H2O:TG).

Un diseño de experimentos de Draper-Lin fue empleado para optimizar el contenido en ácidos grasos (FAC) del producto de reacción, obteniéndose una relación empírica de segundo orden entre el FAC y las variables independientes. El análisis de la varianza (ANOVA) demostró un alto coeficiente de determinación (R2=0.974). La superficie de respuesta y los gráficos de contorno revelaron un efecto negativo sobre el contenido en ácidos grasos cuando la relación molar H2O:TG es alta (dilución del DBSA), la temperatura es elevada y los tiempos de reacción son largos (consumo de FFA en reacciones secundarias).

El modelo predijo varias regiones donde el FAC es del 100 %. Se llevaron a cabo dos reacciones en las condiciones indicadas (T=140 °C, t=10 h, DBSA:TG=0.0163 mol/mol, H2O:TG=30 mol/mol; y T=152 °C, t=4 h, DBSA:TG=0.055 mol/mol,

H2O:TG=35 mol/mol), observándose contenidos de ácidos grasos cercanos a los predichos (99.81 y 99.92 %, respectivamente). Este estudio concluyó que el DBSA es un catalizador adecuado para la hidrólisis de aceites vegetales bajo unas condiciones de reacción más suaves que las que emplean los métodos industriales actuales.

Production of free fatty acid via acid hydrolysis

5.1. General background Free fatty acids (FFAs) are one of the most interesting types of biomass for producing biobased products. This term includes all saturated and unsaturated aliphatic carboxylic acids with carbon chain lengths in the range of C6-C24. The general structure of FFA is shown in Figure 5.1. FFA are used in very different industries as acids themselves or in the form of derivatives such as soaps, cosmetics, plastics, lubricants or rubbers (Beller et al., 2015).

Figure 5.1. General structure of a FFA.

FFAs are typically produced from vegetable oils and fats, which are mainly composed by fatty acid esters of glycerol. Hence, glycerol and FFAs can be obtained by splitting the glycerides. The most used splitting agents are water, methanol, sodium hydroxide and amines (Sonntag, 1979). The names for these processes are hydrolysis, methanolysis, saponification and aminolysis, respectively. Among them, hydrolysis is the most used one at industrial scale.

The vegetable oils and the water used as reactants in the hydrolysis reactions form a heterogeneous reaction system composed by two immiscible liquid phases. This endothermic reaction occurs mainly in the interface of the system, stating an equilibrium (Anneken et al., 2006). It is also remarkable that the hydrolysis reaction of vegetable oils takes place in three consecutive stages. Firstly, a diglyceride and a molecule of FFA are formed as products of the first hydrolysis reaction. This diglyceride may react with another molecule of water, generating another molecule of FFA and a monoglyceride. Finally, monoglycerides can be hydrolysed into glycerine and a third FFA. Globally, the reaction can be summarized as indicated in Figure 5.2.

Chapter 5

Figure 5.2. Global reaction scheme of the hydrolysis reaction. Nowadays, the hydrolysis process of vegetable oils is performed with superheated steam. Different variations of this technology have been proposed. On the Twitchell process, the hydrolysis is performed at around 100 °C using the Twitchell reagent (a mixture of aromatic hydrocarbons, oleic acid and concentrated sulfuric acid) as acid catalyst at atmospheric pressure using steam for 36-48 h. The low profitability of this process caused by the long reaction times and the high corrosiveness of the catalysts used makes that this process is no longer industrially applicable (Lascaray, 1952). The batch autoclave process is faster (6-10 h) than Twitchell process because of the higher operation pressure (10-13 bar) and temperature (180-230 °C). However, the requirement of a further distillation process to purify the product and the high consumption of high-pressure steam are the most important drawbacks of this process (Patil et al., 1988). The Colgate-Emery process is the most used one nowadays. The hydrolysis reaction is performed at a continuous countercurrent column without using any catalyst. The reaction temperature and pressure are typically around 240-315 °C and 48-52 bar, respectively (Barnebey & Brown, 1948). This severe reaction conditions allows a residence time between 1 and 6 h. Nevertheless, the high operative cost, the great amount of steam and the possibility of secondary reactions caused by polyunsaturated FFAs are the main drawbacks of this process. The use of sub- and supercritical water has also been proposed, but it presents the same disadvantages that the Colgate-Emery process (Holliday et al., 1997; King et al., 1999).

In order to avoid the secondary reactions and further purification steps, it is necessary to carry out the hydrolysis reaction at softer conditions. Since the hydrolysis reaction is limited by an equilibrium, the H2O:TG molar ratio and the presence of an acid catalyst play a key role on this reaction and it is common to work at molar ratios over 150 (Wang et al., 2011).

Production of free fatty acid via acid hydrolysis

Among the heterogeneous catalysts, tungstated zirconia and Nafion nanoparticles supported on mesoporous silica has reported conversions higher than 80 % after 12 hours with a moderate excess of water (Ngaosuwan et al., 2009). Fe−Zn double- metal cyanide complexes were also investigated as catalyst for hydrolysis of edible and nonedible vegetable oils and animal fats, reaching a yield of FFAs of 73 % at 190 °C and with a 5% of catalyst. The use of lipases as catalyst on this process has been widely discusses in literature reporting moderate conversions (Liu et al., 2008), even in organic solvents (Bilyk et al., 1991), after long reaction times. Nevertheless, the catalytical performance of these heterogeneous catalysts is less than ideal.

Among the homogeneous catalyst, various SO3H- Bronsted acidic ionic liquids reached conversions over 90 % at 180 °C during 12 h (Luo et al., 2014). However, the typical high price of the ionic liquids does not allow it use at industrial scale. The traditional acids like sulfuric and phosphoric acid, which are much more cost- competitive at industrial scale, reported acceptable results on the hydrolysis of palm oil (Anozie & Dzobo, 2006) and lard oil (Lv et al., 2018). Nevertheless, this reaction systems require high stirring speeds to increase the water-oil interface due to the immiscibility of both reagents and a high investment in stainless steel to avoid the corrosiveness.

The 4-dodecylbenzenesulfonic acid (DBSA) is a homogeneous acid organocatalyst with surfactant characteristics. It has already proposed as catalyst at different reaction systems, like the esterification of fatty acids (Alegría & Cuellar, 2015), the synthesis of dihydropyrimidinone derivatives (Bigdeli et al., 2011), acridine derivates (Luo et al., 2016) or the preparation of polyaniline microrods (Taş et al., 2015). The excellent catalytical activity of DBSA in multiphase systems is based in the formation of micelles, as it was previously reported in literature (Petrenko et al., 2010). Nevertheless, the effectiveness of this catalyst has not been tested on the hydrolysis of vegetable oils.

The study reported in this chapter is focused on the applicability of DBSA as catalyst for the hydrolysis of sunflower oil. The viability of this catalysts will be check at moderate reaction conditions of temperature (80-200 °C) and time (4-16 h) under microwave irradiation to reduce as much as possible the capital and operating costs.

Chapter 5

5.2. Results 5.2.1. High oleic sunflower oil characterization Different parameters of a commercial high-oleic sunflower (HOSO) oil were determined to ensure its quality and its viability to be used as raw material in the hydrolysis reaction. Firstly, the fatty acid (FA) profile of HOSO was determined according to the International Standard ISO 5509, as indicated in the Chapter 3. The results are shown in Table 5.1. Table 5.1. HOSO fatty acid profile. Fatty acid wt.% Palmitic acid C16:0 4.13 Stearic acid C18:0 4.02 Oleic acid C18:1 81.10 Linoleic acid C18:2 10.25 Linolenic acid C18:3 0.07 Arachidic acid C20:0 0.33 Others 0.1

From this table it can be seen that, as expected, oleic acid is the majoritarian fatty acid present in HOSO. It is also remarkable the relatively high content of linoleic, palmitic and stearic acid, as it typically happens in most varieties of sunflower oil. Then, an acid value of 0.09 mg KOH/g of HOSO was determined, indicating the low concentration of previously hydrolysed FA. All these characteristics are in good agreement with other studies previously reported (Belingheri et al., 2015; Moore & Akoh, 2017) and indicates the viability of using this HOSO as raw material for the hydrolysis reactions for the production of FA enriched in oleic acid.

5.2.2. Experimental design

Response surface methodology (RSM) is one of the most useful techniques for developing, improving and optimizing process. Minitab 18 (version 18.1) was used in this study to design the experiments and to optimize the reaction conditions. A Draper-Lin small composite design was used to optimize four variables which influence the hydrolysis of triglycerides (TG). Temperature (X1), reaction time (X2),

DBSA:TG molar ratio (X3) and H2O:TG molar ratio (X4) were the independent variables selected to optimize the fatty acid content (FAC) of sunflower oil after hydrolysis. The levels of variables are showed in Table 5.2.

Production of free fatty acid via acid hydrolysis

Table 5.2. Design matrix of the optimization study of the hydrolysis of HOSO with DBSA. Levels Independent variables Symbol -1 0 1 Temperature (°C) X1 80 140 200 Reaction time (h) X2 4 10 16 DBSA:TG (mol/mol) X3 0.01 0.055 0.1 H2O:TG (mol/mol) X4 3 26.5 50

From these conditions, the experiment matrix was obtained. The experiments, divided into factorial (F), axial (A) and central (C) runs, were performed in a random order. All the reaction products were characterized by means of their acid value (AV). The AV of the products were compared with the AV of the oleic acid

(AVOA=194.1 mg KOH/g) and the fatty acid content (FAC) of the product was determined according to Equation 5.1.

퐴푉 퐹퐴퐶 = · 100% (5.1) 퐴푉푂퐴

5.2.3. Fitting the model and analysis of experimental data The experiment matrix and the response obtained from the experimental runs are shown in Table 5.3. Table 5.3. Experimental and predicted values of FAC obtained at different reaction conditions. Run Type Independent variables FAC order of run X1 (°C) X2 (h) X3 (mol/mol) X4 (mol/mol) Exp. Pred. 3 F 80 (-1) 16 (1) 0.100 (1) 50.0 (1) 21.59 22.22 4 F 200 (1) 4 (-1) 0.100 (1) 50.0 (1) 76.87 77.50 5 F 80 (-1) 16 (1) 0.100 (1) 3.0 (-1) 28.09 26.65 6 Fl 200 (1) 16 (1) 0.010 (-1) 50.0 (1) 81.42 82.05 8 F 200 (1) 16 (1) 0.010 (-1) 3.0 (-1) 1.56 0.12 9 F 80 (-1) 4 (-1) 0.010 (-1) 3.0 (-1) 1.85 0.40 16 F 200 (1) 4 (-1) 0.100 (1) 3.0 (-1) 44.27 42.83 11 F 80 (-1) 4 (-1) 0.010 (-1) 50.0 (1) 0.25 0.88 1 A 140 (0) 4 (-1) 0.055 (0) 26.5 (0) 90.97 92.57 2 A 140 (0) 10 (0) 0.055 (0) 3.0 (-1) 50.91 56.65 10 A 140 (0) 10 (0) 0.010 (-1) 26.5 (0) 98.13 99.73 13 A 140 (0) 16 (1) 0.055 (0) 26.5 (0) 81.66 83.25 17 A 140 (0) 10 (0) 0.055 (0) 50.0 (1) 87.36 84.81 18 A 200 (1) 10 (0) 0.055 (0) 26.5 (0) 49.37 50.97 19 A 80 (-1) 10 (0) 0.055 (0) 26.5 (0) 12.83 14.43 21 A 140 (0) 10 (0) 0.100 (1) 26.5 (0) 62.13 63.73 7 C 140 (0) 10 (0) 0.055 (0) 26.5 (0) 85.06 80.50 12 C 140 (0) 10 (0) 0.055 (0) 26.5 (0) 84.55 80.50 14 C 140 (0) 10 (0) 0.055 (0) 26.5 (0) 87.61 80.50 15 C 140 (0) 10 (0) 0.055 (0) 26.5 (0) 78.42 80.50 20 C 140 (0) 10 (0) 0.055 (0) 26.5 (0) 76.50 80.50

Chapter 5

The experimental results were analysed via RSM in order to fit the second order polynomial equation (Equation 5.2).

2 푌 = 훽0 + ∑ 훽푖푋푖 + ∑ 훽푖푖푋푖 + ∑ ∑푖≠푗 훽푖푗푋푖푋푗 (5.2) where Y is the predicted response, β0 is the constant, βi are the linear factors, βii are the squared factors, βij are the interaction factors and Xi are the independent variables considered along this chapter.

The values of all the coefficients presented at the Equation 5.2 were calculated by fitting the experimental data using the software Minitab 18, obtaining the prediction equation for the FAC after the hydrolysis process (Equation 5.3).

2 퐹퐴퐶 = −287.7 + 4.655 · 푋1 + 5.44 · 푋2 + 28 · 푋3 + 0.004 · 푋4 − 0.013277 · (푋1) + 2 2 2 0.2059 · (푋2) + 609 · (푋3) − 0.01768 · (푋4) − 0.0798 · 푋1 · 푋2 − 2.16 · 푋1 · 푋3 +

0.01069 · 푋1 · 푋4 − 2.9 · 푋2 · 푋3 + 0.0375 · 푋2 · 푋4 − 6.17 · 푋3 · 푋4 (5.3) The statistical significance of the model and its coefficients was checked by F-test and the analysis of variance (ANOVA). The parameters of both analyses are shown at Table 5.4. Table 5.4. ANOVA test for response function. Source DF SS MS F-Value P Model 14 22174.5 1583.90 55.02 0.000 Linear 4 5797.9 835.46 29.02 0.000 β1 1 3567.9 667.56 23.19 0.003 β2 1 0.0 43.37 1.51 0.266NS β3 1 247.3 648.12 22.52 0.003 β4 1 1982.8 1982.77 68.88 0.000 Square 4 12620.5 3155.14 109.61 0.000 β11 1 12315.4 5831.89 202.60 0.000 β22 1 55.8 140.26 4.87 0.069NS β33 1 5.9 3.88 0.13 0.726NS β44 1 243.4 243.40 8.46 0.027 Interaction 6 3756.1 626.01 21.75 0.001 β12 1 1319.5 1319.53 45.84 0.001 β13 1 54.5 54.49 1.89 0.218NS β14 1 1816.9 1816.88 63.12 0.000 β23 1 1.0 0.96 0.03 0.861NS β24 1 224.1 224.07 7.78 0.032 β34 1 340.2 340.16 11.82 0.014 Residual (error) 6 172.7 28.79 Lack of fit 2 83.2 41.59 1.86 0.269NS Pure error 4 89.5 22.38 Total 20 22347.3 DF: Degree of freedom; SS: Sum of squares; MS: Mean square; NS: Non-significant

Production of free fatty acid via acid hydrolysis

The ANOVA showed that this model for the prediction of FAC after the hydrolysis of HOSO is highly significant (P<0.001). The P-value of the model for lack of fit (P>0.05) revealed that it is not statistically significant compared to the pure error. Another important parameter is the coefficient of determination (R2). The R2 for this model was found to be 0.974. R2 values close to 1 indicate a good correlation between the experimental and the predicted values. It means that only 2.6 % of the total variations of FAC could not be explained by the model and that it exists a good relation between the experimental and the predicted values. The statistical significance of each coefficient was determined by applying F-test and P-values. If the P-values of the coefficients are at 5 % (P<0.05), it indicates that the term is significant and show a quantitative effect of the response variable. Moreover, if P- values are at 1 % (P<0.01) it indicates that the influence of that term is extremely significant. As is summarized at Table 5.4, most terms of the indicated model are statistically significant, excepting the linear term of X2 (β2), the squared terms of X2

(β22) and X3 (β33) and the interaction terms of X1-X3 (β13) and X2-X3 (β23). However, the linear, square and interaction sources are globally significant.

The residues, which are defined as the difference between the experimental and modelled values for the FAC after the hydrolysis process, were also analysed. Figure 5.2a represents the obtained residues versus the theoretical FAC data. Ideally, if the model represented perfectly the proposed reaction system, all the residual values would be placed on the zero-horizontal line. However, it can be observed that all points are randomly scattered around that line, indicating that there was not a systematic error on the determination of the FAC at a specific analytical range. Figure 5.2b shows the residues on the order at which they were obtained. This plot is useful to determine if the observations were influenced by the order of the experiments. In this case, residues are also independents from the order and that no tending can be registered. All these graphs indicate that there is no evidence that the regression residues are correlated.

Chapter 5

Figure 5.2. Residue values versus a) predicted FAC values and

b) order of observation . a) 8

0 Residue -2

-4

-6

0 10 20 30 40 50 60 70 80 90 100 Predicted FAC value b) 8

0 Residue -2

-4

-6

0 2 4 6 8 10 12 14 16 18 20 Order of observation

Pareto analysis was performed to check the percentage effect of each factor. This analysis, which is a useful technique to interpret the results, calculates the effect of each factor following Equation 5.4.

2 푏푖 푃푖 = ( 2) · 100 (푖 ≠ 0) (5.4) ∑ 푏푖

Figure 5.3 shows the Pareto analysis results. This graph shows that there are great differences on the influence of all the independent variables on FAC after the hydrolysis process. As can be observed, temperatures effects (β11, β12, β1) are the most dominant ones. However, the square source of temperature (β11) and the interaction of time and temperature (β12) have a negative contribution on the FAC on the hydrolysis process. This is related to the presence of secondary reactions which are potentiated by high temperatures and long reaction times, like the formation of estolides which typically takes place at high concentration of FFA in presence of an acid catalyst (Cermak & Isbell, 2001) or the isomerization of FFA to

100

Production of free fatty acid via acid hydrolysis form lactones (Ftouni et al., 2017; Showell et al., 1968). On the other hand, time and catalyst load effects (β22, β13, β2, β33 and β23) are barely significant on the model.

Figure 5.3. Pareto analysis for FAC after hydrolysis process in presence of DBSA.

5.2.4. Response surface analysis

The three-dimensional (3D) response surface plots can provide valuable and helpful information on the relationships among the independent variables and the FAC after the hydrolysis process. The plots were designed by representing the quadratic model response (FAC) at the z-axis versus two independent variables, keeping the other two remaining independent variables fixed at their zero levels showed at Table 5.2. 2D Contour plots graphs, where the response of the model is represented in a colour scale, are also included. A circular contour of response surfaces with a few contour lines indicates that the interaction between the corresponding independent variables has a low effect on the response variable. On the contrary, an elliptical or saddle form indicates that the interaction is significant (Montgomery, 2008; Myers et al., 2009).

Figure 5.4 shows the response surface and contour plot for FAC as a function of temperature and H2O:TG molar ratio at reaction time of 10 hours and a DBAS:TG molar ratio of 0.05 mol/mol. As can be observed, the higher the temperature, the higher the FAC at a constant H2O:TG molar ratio. This is in good agreement with the

101

Chapter 5 results reported in literature when other catalysts are used (Ngaosuwan et al., 2009;

Wang et al., 2011). It is also remarkable that FAC presents a maximum at a H2O:TG molar ratio of 35 mol/mol and a higher amount of water implies a decrease on the FAC. Water is a necessary reagent on the TG hydrolysis, and so, and excess from the stoichiometric amount moves the equilibrium towards the formation of free fatty acids. However, an increment of water amount on the reaction media leads to a diminution of the concentration of catalyst, which also plays a key role on the reaction kinetics.

100

40 FAC (%) FAC

20 50 40 30 0 80 100 20 120

140 O:TG (mol/mol) 160 10 2 Temperature (ºC) 180 H 200

50 20.0 30.0 40.0 50.0 > 90.0 80.0 70.0 40 60.0 50.0 40.0 90.0 30.0 30 20.0 < 10.0 80.0 Hold values 70.0

O:TG (mol/mol) O:TG 20 Time - 10 h 2 DBSA:TG - 0.05 mol/mol H 60.0 50.0 10 30.0 40.0

10.0 20.0 10.0 80 100 120 140 160 180 200 Temperature (ºC) Figure 5.4. Response surface plot and contour plot of the FAC (%) as the function of T and H2O:TG molar ratio (time and DBSA:TG molar ratio values are held at 10 h and 0.05 mol/mol, respectively).

102

Production of free fatty acid via acid hydrolysis

Figure 5.5 illustrates the effect of DBSA:TG and H2O:TG molar ratios. The changes of the ratios of reactants might alter significantly the phase distribution of the intermediate products, modifying the reaction kinetics and the FAC content of the final product. As can be observed, an increase of DBSA concentration favours the hydrolysis reaction at the hold parameters of 10 h and 140 °C. In addition, the dilution effect of DBSA by increasing H2O:TG molar ratio or decreasing DBSA:TG molar ratio is more visibly in this graph since the predicted FAC decreases around a 20% on average.

100

FAC (%) FAC 40

0 0.10 10 0.08 20 DBSA:TG0.06 (mol/mol) 30 0.04 40 O:TG (mol/mol) 0.02 H 2 50

> 90.0 70.0 80.0 70.0 40 60.0 50.0 40.0 30.0 30 80.0 20.0 60.0 < 10.0

Hold values

20 90.0 Temperature - 140ºC O:TG (mol/mol) O:TG

2 Time - 10 h H

0.02 0.04 0.06 0.08 0.10 DBSA:TG (mol/mol) Figure 5.5. Response surface plot and contour plot of the FAC (%) as the function of DBSA:TG and H2O:TG molar ratios (temperature and time values are held at 140 °C and 10 h, respectively).

103

Chapter 5

Figure 5.6 shows the response surface and contour plots for FAC as a function of reaction time and H2O:TG molar ratio. Two different areas with a FAC higher than

90% are found, both at long reaction times. The first one can be found H2O:TG molar ratios below 10 mol/mol, which presence is justified by the high DBSA concentration at these conditions. On the other hand, the second high FAC region is found at a H2O:TG molar ratio of 50 mol/mol. At these conditions, the hydrolysis equilibrium displacement effect seems to be stronger than the DBSA dilution effect at reaction times higher than 13 hours.

100

40 FAC (%) FAC

16 0 14 10 12 20 H 10 O:TG (mol/mol) 2 30 8 Time (h) 40 6 50 4 16 90.0 90.0 > 90.0 80.0 14 70.0 60.0 50.0 12 40.0 30.0 20.0 10 < 10.0 80.0

Time(h) Hold values Temperature - 140ºC 8 DBSA:TG - 0.05 mol/mol

6 70.0

60.0 4 10 20 30 40 50 H O:TG (mol/mol) 2 Figure 5.6. Response surface plot and contour plot of the FAC (%) as the function of time and H2O:TG molar ratios (temperature and DBSA:TG molar ratio values are held at 140 °C and 0.05 mol/mol, respectively).

104

Production of free fatty acid via acid hydrolysis

Figure 5.7 illustrates the effect of reaction time and temperature upon FAC for a

DBSA:TG and H2O:TG molar ratios of 0.05 mol/mol and 26.5 mol/mol, respectively. As can be observed at this figure, two different regions are delimited by a maximum FAC line at a reaction time of approximately 10.5 hours. Along the first region, which is located at low reaction times, an increase of reaction temperature implies an increase of the FAC of the hydrolysis product. On the contrary, the second region depicted at high reaction times indicates that an increase of reaction temperature maintaining the reaction time at a constant value reduces the FAC of the organic phase. This fact clearly indicates the presence of secondary reactions where carboxylic groups are consumed in presence of acid catalysts, such us the production of estolides (Cermak & Isbell, 2001) or lactones (Ftouni et al., 2017; Showell et al., 1968).

100

60 FAC (%) FAC 40

16 0 80 14 100 12 Temperature120 (ºC) 10 140 8 160 6 Time (h) 180 4 200 16 40.0 90.0 > 90.0 50.0 80.0 14 60.0 70.0 60.0 70.0 50.0 12 40.0 80.0 30.0 20.0 10.0 10 80.0 <

Time(h) Hold values 70.0 DBSA:TG - 0.05 mol/mol 8 H O:TG - 26.5 mol/mol 2 60.0 50.0 6 40.0 30.0 20.0 10.0 4 80 100 120 140 160 180 200 Temperature (ºC) Figure 5.7. Response surface plot and contour plot of the FAC (%) as the function of time and temperature (DBSA:TG and H2O:TG molar ratios are held at 0.05 and 26.5 mol/mol, respectively).

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Figure 5.8 presents the response surface and contour plots of the FAC after the hydrolysis process as a function of DBSA:TG molar ratio and temperature. As can be observed, there is a maximum area located at low reaction temperatures (below 115 °C) and moderate DBSA:TG molar ratios. Hence, this graph indicates the need of a minimum acid catalyst concentration at the proposed conditions. However, an excess of catalyst will favour the previously discussed fatty-acid-consuming secondary reactions.

100

40 FAC (%) FAC

0 0.10 80 0.08 100 120 0.06 140 Temperature (ºC) 0.04 160 180 0.02 200 DBSA:TG (mol/mol) 0.10

40.0 > 90.0 50.0 80.0 60.0 70.0 0.08 60.0 90.0 70.0 50.0 40.0 30.0 0.06 80.0 20.0 < 10.0

Hold values Time - 10 h 0.04 H O:TG - 26.5 mol/mol

2 DBSA:TG (mol/mol) DBSA:TG

60.0 40.0 0.02 50.0 20.0 30.0 10.0 80 100 120 140 160 180 200 Temperature (ºC) Figure 5.8. Response surface plot and contour plot of the FAC (%) as the function of DBSA:TG molar ratio and temperature (time and H2O:TG molar ratios are held at 10 h and 26.5 mol/mol, respectively). Finally, Figure 5.9 shows the influence of reaction time and DBSA:TG molar ratio on the FAC after the hydrolysis reaction. H2O:TG molar ratio and temperature were fixed at their zero values (26.5 mol/mol and 140 oC, respectively). As can be seen in

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Production of free fatty acid via acid hydrolysis this figure, the FAC was greater than 60 % in all cases. As the Pareto analysis indicated in Figure 5.3, the influence of DBSA:TG molar ratio (X3) is positively significant. On the contrary, reaction time (X2) and its interaction with DBSA:TG molar ratio (X23) are not significant. Hence, no critical conclusions should be drawn from this figure.

100

40 FAC (%) FAC

0 4 0.10 6 8 0.08 Time (h) 10 0.06 12 0.04

14 0.02 DBSA:TG (mol/mol) 16 0.10

90.0 > 90.0 80.0 80.0 70.0 70.0 0.08 60.0 50.0 40.0 30.0 0.06 20.0 < 10.0

Hold values Temperature - 140ºC 0.04 H O:TG - 26.5 mol/mol

2 DBSA:TG (mol/mol) DBSA:TG

0.02

4 6 8 10 12 14 16 Time (h) Figure 5.9. Response surface plot and contour plot of the FAC (%) as the function of DBSA:TG molar ratio and time (temperature and H2O:TG molar ratios are held at 140 °C and 26.5 mol/mol, respectively).

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5.2.5. Determination of optimal conditions for the hydrolysis of HOSO

The desire FAC to determine the optimal reaction conditions was defined as “maximum” when the design of experiments was stablished. Nevertheless, the internal mathematical procedure of the software used was not capable to determine a unique set of independent variables as optimal since there are more than one conditions where a FAC=100 % (which was stablished as the maximum possible value for FAC) is predicted. Among the different conditions at which the model predicts a FAC=100 %, two of them were selected based on their low requirements of reactants or energy (i.e. reaction time and/or temperature) to check the reliability of the model.

Table 5.5 shows the optimal conditions and the theoretical and experimental values of FAC.

Table 5.5. Optimal conditions for maximum FAC. Temperature Time DBSA:TG H2O:TG Predicted Observed (°C) (h) (mol/mol) (mol/mol) FAC (%) FAC (%) 140 10 0.0163 30 100 99.81 152 4 0.055 35 100 99.92

As can be observed, a good agreement between theoretical and experimental values is observed in both cases. Moreover, the FAC found was higher than 99.8% in both cases, indicating that DBSA might be considered as a catalyst for the hydrolysis of vegetable oils with a potential application at industrial scale.

4.3. Conclusions

The efficiency of DBSA as catalyst along with temperature, reaction time and H2O:TG molar ratio was studied on the hydrolysis of HOSO. The relationship between these variables and the fatty acid content (FAC) of the product was expressed by a second- order polynomial equation based on the response surface methodology and a Draper-Lin small composite design of experiments. ANOVA showed a high coefficient of determination (R2=0.974), ensuring a good adjustment of the model to the experimental data. The residues among the experimental and theoretical values of FAC were not correlated neither with the analytical range nor the experiment order. The Pareto analysis indicates that temperature effects are the most significant

108

Production of free fatty acid via acid hydrolysis one meanwhile time effect is barely significant on the model. The response surface and contour plots indicated that a high excess of H2O is not recommended due to the dilution effect that it has over DBSA, overall at moderate temperatures and reaction times. These plots also confirmed the negative effect that high temperatures and long reaction times has on the FAC due to the presence of carboxylic-consuming secondary reactions. Nevertheless, this study concluded that DBSA is an appropriate catalyst for the hydrolysis of vegetable oils, reaching conversions of 100 % at moderate reaction conditions.

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References Alegría, A., Cuellar, J. 2015. Esterification of oleic acid for biodiesel production catalyzed by 4-dodecylbenzenesulfonic acid. Applied Catalysis B: Environmental, 179, 530-541. Anneken, D.J., Both, S., Chrostoph, R., Fieg, G., Steinberner, U., Westfechtel, A. 2006. Fatty Acids. in: Ullmann's Encyclopedia of Industrial Chemistry. Anozie, A.N., Dzobo, J.M. 2006. Kinetics of the hydrolysis of palm oil and palm kernel oil. Industrial and Engineering Chemistry Research, 45(5), 1604-1612. Barnebey, H.L., Brown, A.C. 1948. Continuous fat splitting plants using the colgate- emery process. Journal of the American Oil Chemists' Society, 25(3), 95-99. Belingheri, C., Giussani, B., Rodriguez-Estrada, M.T., Ferrillo, A., Vittadini, E. 2015. Oxidative stability of high-oleic sunflower oil in a porous starch carrier. Food Chemistry, 166, 346-351. Beller, H.R., Lee, T.S., Katz, L. 2015. Natural products as biofuels and bio-based chemicals: Fatty acids and isoprenoids. Natural Product Reports, 32(10), 1508-1526. Bigdeli, M.A., Gholami, G., Sheikhhosseini, E. 2011. P-dodecylbenzenesulfonic acid (DBSA), a Brønsted acid-surfactant catalyst for Biginelli reaction in water and under solvent free conditions. Chinese Chemical Letters, 22(8), 903-906. Bilyk, A., Bistline, R.G., Haas, M.J., Feairheller, S.H. 1991. Lipase-catalyzed triglyceride hydrolysis in organic solvent. Journal of the American Oil Chemists Society, 68(5), 320-323. Cermak, S.C., Isbell, T.A. 2001. Synthesis of estolides from oleic and saturated fatty acids. JAOCS, Journal of the American Oil Chemists' Society, 78(6), 557-565. Ftouni, J., Genuino, H.C., Muñoz-Murillo, A., Bruijnincx, P.C.A., Weckhuysen, B.M. 2017. Influence of Sulfuric Acid on the Performance of Ruthenium-based Catalysts in the Liquid-Phase Hydrogenation of Levulinic Acid to γ- Valerolactone. ChemSusChem, 10(14), 2891-2896. Holliday, R.L., King, J.W., List, G.R. 1997. Hydrolysis of Vegetable Oils in Sub- And Supercritical Water. Industrial and Engineering Chemistry Research, 36(3), 932-935. King, J.W., Holliday, R.L., List, G.R. 1999. Hydrolysis of soybean oil: In a subcritical water flow reactor. Green Chemistry, 1(6), 261-264.

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Lascaray, L. 1952. Industrial fat splitting. Journal of the American Oil Chemist’ Society, 29(9), 362-366. Liu, Y., Jin, Q., Shan, L., Liu, Y., Shen, W., Wang, X. 2008. The effect of ultrasound on lipase-catalyzed hydrolysis of soy oil in solvent-free system. Ultrasonics Sonochemistry, 15(4), 402-407. Luo, H., Xue, K., Fan, W., Li, C., Nan, G., Li, Z. 2014. Hydrolysis of vegetable oils to fatty acids using Brønsted acidic ionic liquids as catalysts. Industrial and Engineering Chemistry Research, 53(29), 11653-11658. Luo, X., Ge, X., Cui, S., Li, Y. 2016. Value-added processing of crude glycerol into chemicals and polymers. Bioresource Technology, 215, 144-154. Lv, E., Ding, S., Lu, J., Du, L., Li, Z., Li, J., Zhang, S., Ding, J. 2018. An integrated process of catalytic hydrolysis and membrane separation for fatty acids production from lard oil. Canadian Journal of Chemical Engineering. Montgomery, D.C. 2008. Design and Analysis of Experiments. John Wiley & Sons. Moore, M.A., Akoh, C.C. 2017. Enzymatic Interesterification of Coconut and High Oleic Sunflower Oils for Edible Film Application. JAOCS, Journal of the American Oil Chemists' Society, 94(4), 567-576. Myers, R.H., Montgomery, D.C., Anderson-Cook, C.M. 2009. Response Surface Methodology: Process and Product Optimization Using Designed Experiments. Wiley. Ngaosuwan, K., Lotero, E., Suwannakarn, K., Goodwin Jr, J.G., Praserthdam, P. 2009. Hydrolysis of triglycerides using solid acid catalysts. Industrial and Engineering Chemistry Research, 48(10), 4757-4767. Patil, T.A., Raghunathan, T.S., Shankar, H.S. 1988. Thermal hydrolysis of vegetable oils and fats. 2. Hydrolysis in continuous stirred tank reactor. Industrial and Engineering Chemistry Research, 27(5), 735-739. Petrenko, V.I., Avdeev, M.V., Garamus, V.M., Bulavin, L.A., Aksenov, V.L., Rosta, L. 2010. Micelle formation in aqueous solutions of dodecylbenzene sulfonic acid studied by small-angle neutron scattering. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 369(1-3), 160-164. Showell, J.S., Swern, D., Noble, W.R. 1968. Perchloric Acid Isomerization of Oleic Acid. Journal of Organic Chemistry, 33(7), 2697-2704.

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Sonntag, N.O.V. 1979. Fat splitting. Journal of the American Oil Chemists' Society, 56(11), 729A-732A. Taş, R., Can, M., Sönmezoʇlu, S. 2015. Preparation and characterization of polyaniline microrods synthesized by using dodecylbenzene sulfonic acid and periodic acid. Turkish Journal of Chemistry, 39(3), 589-599. Wang, C., Zhang, W., Zhou, X., Chen, Y., Yin, F., Li, J., Xu, R., Liu, S. 2011. Preparing fatty acid by hydrolysing waste oil from restaurants under normal pressure, Vol. 322, pp. 296-301.

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CHAPTER 6. SYNTHESIS OF RIGID POLYURETHANE FOAMS FROM AZIDIFIED BIOPOLYOLS

Synthesis of rigid polyurethane foams from azidified biopolyols

Abstract The synthesis of polyols from renewable resources, primarily unsaturated vegetable oils, is increasing in importance in the polyurethane market. In this chapter, previously epoxidized grape seed oil was transformed into azide-functionalized biopolyols through a ring-opening reaction with sodium azide.

The influence of temperature and reaction time on the incorporation of azide groups was investigated, finding a maximum oxirane ring conversion of 84.4 % after 24 hours of reaction at 70 °C. FTIR analyses confirmed the covalent linkage of azide groups to the biopolyol and the formation of hydroxyl groups.

A pseudo-first order model was proposed for predicting the evolution of epoxide groups, and a good fit trend was obtained. The dependence of the kinetic rate constant on temperature was evaluated using the Arrhenius equation, and an activation energy of 0.341 J/mol was obtained.

Finally, a rigid polyurethane foam was synthesized from an azidified biopolyol. No structural differences were observed between this foam and a commercial one synthesized from a petroleum-based polyol, but a considerable improvement in thermal stability was observed and attributed to the azide groups covalently linked to the polyurethane.

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Resumen La síntesis de polioles a partir de fuentes renovables, principalmente aceites vegetales insaturados, está ganando importancia en el mercado de los poliuretanos. En este capítulo, se describe la transformación de un aceite de granilla de uva previamente epoxidado en un biopoliol funcionalizado con grupos azida, mediante la reacción de apertura de anillo oxirano en presencia de azida sódica.

En primer lugar, se estudió el efecto de la temperatura y del tiempo de reacción sobre la incorporación de los grupos azida, encontrándose una conversión de oxígeno oxirano máxima del 84.4 % tras 24 horas de reacción, a 70 °C. Los análisis de FTIR confirmaron la unión covalente de los grupos azida al biopoliol y la formación de grupos hidroxilo.

Se ha propuesto un modelo cinético de pseudo-primer orden que reproduce satisfactoriamente la evolución de los grupos epóxido. La dependencia de la constante cinética con la temperatura fue evaluada mediante la ecuación de Arrhenius, encontrando una energía de activación de 0,341 J/mol.

Finalmente, se sintetizó una espuma de poliuretano rígida a partir del biopoliol funcionalizado. Al comparar esta espuma con la obtenida con un poliol de origen petroquímico mediante el mismo procedimiento, no se observó ninguna diferencia estructural. Sin embargo, sí se comprobó una mejora notable en la estabilidad térmica de la espuma debido a la presencia de grupos azida unidos covalentemente a la estructura del poliuretano.

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Synthesis of rigid polyurethane foams from azidified biopolyols

6.1. General background

Polyurethane derivatives have a considerable number of applications and their consumption have been increasing continuously during the last decades. Moreover, the future of their market appears to be very promising due to the substantial increase in demand in emerging economies in Eastern Europe, Asia and South America.

Currently, the polyols used in the polyurethane industry have been of petrochemical origin, but the industry tends towards the use of more sustainable and environmentally friendly raw materials (Kajaste, 2014; Zhao et al., 2014). This new trend could be considered as an excellent opportunity for the development of polyols derived from renewable resources to enter the polyurethane market (Hill, 2000).

Among the renewable raw materials, vegetable oils can be considered to be the most promising alternative to fossil resources for producing organic compounds (Mumtaz et al., 2010). These oils are primarily composed of triglycerides, which are esterified glycol compounds with generally unsaturated fatty acids (Petrovic, 2008).

As it was showed along the Chapter 4, the epoxidation of vegetable oils to convert them into intermediates for further chemical products is one of the most common processes used in biorefineries. These compounds are of great interest at the industrial level because they are precursors for a large number of synthetic products for diverse applications (Ahn et al., 2011; Borugadda & Goud, 2015; Guo et al., 2000).

Polyols are one of the products that can be obtained from epoxidized oils (Guo et al., 2000; Petrovic, 2008), and they are well accepted in the current polyurethane market because they can be green substitutes for fossil-based polyols. Furthermore, polyols obtained from epoxidized oils can be attributed with different functionalities when the oxirane ring is opened, being the flame retardancy one of the most popular ones (Zhang et al., 2014).

Nitrogen based flame retardants are becoming more popular as they are considered as an environment friendly and non-toxic alternative for existing halogen formulations. Among the possible pathways for obtaining polyols with enhanced

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Chapter 6 thermal properties from epoxidized oils, the ring-opening reaction with sodium azide could be one of the most interesting alternatives (Akintayo et al., 2006).

The presence of nitrogen atoms in the structure of the biopolyol resulting from the incorporation of azide groups enables the polyol to act as thermal stabilizer compound in polyurethane foams (Lu & Hamerton, 2002). Additionally, the azide group is highly polar, and when it is linked to a hydrocarbon chain, the hydrophobic character is modified, favouring its use as a surfactant (Zhong et al., 2002). Moreover, the azide group is one of the two main parts for performing the 1,3- dipolar cycloaddition reaction among azides and alkynes in the field of click chemistry (Kolb et al., 2001). The latter one is the most actively investigated approaches for preparing tailor made bioactive substances at this time.

The incorporation of azide groups into an epoxidized vegetable oil for the synthesis of polyurethane foams has not been previously reported in the literature. Hence, in this chapter, the influences of reaction time and temperature on the incorporation of azide groups into epoxidized grape seed oil were investigated. Finally, the synthesis of a polyurethane foam from the azidified biopolyols with enhanced thermal stability properties was accomplished.

6.2. Materials

Epoxidized grape seed oil (EGSO) synthesized following the procedure indicated in Chapter 3 and performed in Chapter 4, at 90 °C for 1 hour, was used to carried out the subsequent ring-opening reactions. The other reactants used in this chapter can be consulted in the Chapter 3 of this thesis.

6.3. Results

6.3.1. Ring-opening reaction with sodium azide

As it has been previously discussed, the incorporation of nitrogen atoms in biopolyols discovers a wide range of new applications and very interesting properties for the polyurethanes produced from them. Figure 6.1 depicts the general reaction scheme of the epoxide ring-opening reaction with sodium azide (NaN3) using NH4Cl as catalyst.

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Synthesis of rigid polyurethane foams from azidified biopolyols

Figure 6.1. Reaction scheme of the epoxide ring-opening reaction with NaN3.

The ring-opening reaction was performed at 50, 60 and 70 °C for 24 h extracting samples at reaction times of 1, 2 and 4 hours. Table 6.1 shows the oxirane oxygen (OO) content and the hydroxyl values (HV) of the azidified biopolyols. From the values of OO, the conversion of the reaction was calculated according to Equation

6.1 were OOEGSO is the oxirane oxygen content of epoxidized grape seed oil and OOt refers to the oxygen content of the reaction product.

푂푂 −푂푂 퐶표푛푣푒푟푠푖표푛 (%) = 퐸퐺푆푂 푡 · 100 (6.1) 푂푂퐸퐺푆푂

Table 6.1. Hydroxyl values (HV) and oxirane oxygen (OO) content for the azidified biopolyols. Temperature Time OO Conversion HV (mg (°C) (h) (%) (%) KOH/g) EGSO 5.47 - 15 1 5.42 0.91 16 2 5.18 5.30 19 50 4 5.04 7.86 26 24 2.9 46.98 70 1 4.84 11.52 19 2 4.47 18.28 30 60 4 4.19 23.40 43 24 1.49 72.76 122 1 4.68 14.44 25 2 3.92 28.34 41 70 4 3.16 42.23 74 24 0.83 84.83 152

As can be observed, either by increasing the temperature or the reaction time the OO of the azidified biopolyol decreases. As a consequence of the oxirane ring- opening, the hydroxyl groups are formed and an increasing HV was measured. The conversion of the system reached values up to 84.33 %, indicating that the extension of the reaction was not complete. Nevertheless, the uncompletion of the ring- opening process might not affect to the further synthesis of rigid polyurethane

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Chapter 6 foams process since the HV is in the adequate range for polyols with a molecular weight around 1000 g/mol, as will be shown below.

In order to ensure the linkage of azide groups into de biopolyol, FTIR analyses were performed to all the samples (Figure 6.2).

Temperature (°C) a) b) 50 60

4000 3500 3000 2500 2000 1500 1000 500 4000 3500 3000 2500 2000 1500 1000 500 Wavelength (cm-1) Wavelength (cm-1)

c) d)

4000 3500 3000 2500 2000 1500 1000 500 4000 3500 3000 2500 2000 1500 1000 500 -1 -1 Wavelength (cm ) Wavelength (cm ) Figure 6.2. FTIR spectra of the azidified biopolyols synthesized in a temperature range from 50 °C to 70 °C with reaction times of a) 1 h, b) 2 h, c) 4 h and d) 24 h. The FTIR spectra shows the characteristic bands of hydroxyl groups at 3480 cm-1;

C-H stretching vibrations at 2875-2970 cm-1, azide groups at 2095 cm-1; -CH2- and -

CH3 at around 1400 cm-1 and C-O-C at approximately 1250 cm-1.

As expected, the bands corresponding to hydroxyl and azide groups increase with time and/or reaction temperature. This result demonstrates that the oxirane ring generated from unsaturated fatty acids reacts with the NaN3 to generate hydroxyl and azide groups linked to the chain.

FTIR was also used to quantify the relationship between the evolution of hydroxyl and azide groups in the azidified biopolyol. Table 6.2 shows the ratio between the areas of hydroxyl and azide groups obtained at wavelengths of 3480 and 2095 cm-1, respectively.

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Synthesis of rigid polyurethane foams from azidified biopolyols

Table 6.2. Ratio of hydroxyl and azide area signals obtained by FTIR. Temperature Time AOH/AN (°C) (h) 3 1 1.43 2 1.91 50 4 1.11 24 1.61 1 1.16 2 1.20 60 4 0.94 24 1.53 1 1.82 2 1.14 70 4 0.94 24 1.04

As can be observed, this area ratio is very close to 1, which confirms that the main reaction mechanism is that shown in Figure 6.1 and that the presence of secondary reactions can be neglected. However, the formation of oligomers during the ring- opening of reaction epoxidized vegetable oils has been reported in literature (Guo et al., 2007). Hence, all the products were characterized by GPC, being the chromatograms shown in Figure 6.3.

Temperature (°C) a) b) 50 60

10 11 12 13 14 15 16 10 11 12 13 14 15 16 Elution time (min) Elution time (min)

c) d)

10 11 12 13 14 15 16 10 11 12 13 14 15 16 Elution time (min) Elution time (min) Figure 6.3. GPC analyses of the azidified biopolyols synthesized in a temperature range from 50 °C to 70 °C with reaction times of a) 1 h, b) 2 h, c) 4 h and d) 24 h.

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All chromatograms presented two main peaks. The main one at an elution time of 13.5 min corresponds to the main product, with a molecular weight around 1000 g/mol. The second one, with an elution time close to 12.5 min, indicates the presence of dimers formed by the combination of two different molecules of triglycerides. As can be seen, the main peak suffered a displacement to higher molecular weights by increasing reaction time and/or temperature due to the incorporation of -OH and

N3 groups to the molecule. On the other hand, the second signals remain almost invariable indicating that the oligomers which are in the azidified biopolyol were formed during the epoxidation reaction, as indicated in Chapter 4.

In this chapter, the effect of temperature was quantified by assuming that the reaction follows a pseudo-first-order kinetics. In this way, the kinetic constant (kobs) for the depletion of the oxirane ring concentration in the bulk solution (Cepox) at each temperature can be calculated from the slope of the curve ln(Cepox) vs time.

As shown in Figure 6.4, a good fit between the experimental data and the theoretical values of Cepox was obtained, with correlation coefficient (R2) values higher than 0.990 in all cases.

-5.4

-6.0

) -6.6

Epox

ln (C

-7.2 Temperature (°C) 50 60 -7.8 70

0 5 10 15 20 25 Time (h)

Figure 6.4. Kinetics of epoxide ring-opening reaction with NaN3 at different temperatures fitted to the pseudo-first-order kinetic equation.

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Synthesis of rigid polyurethane foams from azidified biopolyols

Kinetic constant values of 2.70·10-2, 5.05·10-2 and 7.18·10-2 s-1 were obtained for 50, 60 and 70 °C, respectively. As expected, it was observed that an increase in the reaction temperature resulted in a higher value of the kinetic constant, and thus, the higher the temperature, the greater the extent of the ring-opening reaction by the sodium azide.

To determine the activation energy (EA) and the pre-exponential factor (A), the linearized Arrhenius equation can be used (Equation 6.2):

−퐸 1 ln 푘 = 퐴 · + ln 퐴 (6.2) 표푏푠 푅 푇

As shown in Figure 6.5, a good correlation can be observed (R2=0.987). The values of EA and A can be calculated from the slope and y-intercept, respectively. Values of

EA=0.341 J/mol and A=0.121 were found.

-2.6 ln (K ) obs

-2.8

-3.0

)

obs

-3.2 ln (K

-3.4

-3.6

12 15 18 21 24 27 30 33 36 39

-1 1/T (K ) Figure 6.5. Fitting of experimental kinetic reaction constants to the linearized Arrhenius equation. 6.3.2. Synthesis of rigid polyurethane foams

Two different rigid polyurethane foams were synthesized by following the formulations shown in Table 6.3. The amounts of reactants were calculated based on the hydroxyl index of the polyols, following the procedure indicated in literature (Molero et al., 2008). Foam A was synthesized using a commercial polyether polyol

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(Alcupol R-4520, REPSOL), whereas Foam B was produced using the previously synthesized azidified biopolyol at 70 °C during 24 h.

Table 6.3. Weight percentages of raw materials for the synthesis of PU foams. Component Weight percentage Polyol 47.75 Water 0.75 Tegostab B8404 3 Tegoamin 33 0.37 Tegoamin BDE 0.37 MDI 47.75

No differences during the foaming process (growth start time, growth rate, and final external aspect of the foam) were observed between the two polyols. Figure 6.6 shows a visual comparison between Foam A (Figure 6.6a) and Foam B (Figure 6.6b).

a) b) Figure 6.6. Visual aspect of the rigid PU foams synthesized from a) a commercial petroleum-based polyol and b) the azidified biobased polyol. As can be observed, the only appreciable difference between the two PU foams was that the azidified foam was slightly darker than the commercial one due to the higher colouration of the azidified biopolyol.

The internal morphologies of both polyurethane foams were observed using a scanning electron microscope (SEM) (Figure 6.7).

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Synthesis of rigid polyurethane foams from azidified biopolyols

Figure 6.7. SEM images of a) Foam A (synthesized from a commercial polyol) and b) Foam B (synthesized from the azidified biopolyol at 70°C for 24 h). Both foams exhibited a polyhedral cell structure with faces containing nodes formed by the junction of three struts. An increase in the cell size of Foam B respect to that of Foam A can be observed, with average cell sizes of 248.5 and 303.4 µm for Foam A and Foam B, respectively. This increase can primarily be attributed to two effects: the different volumes of pendant groups and the unadjusted foam formulation, which modifies the nucleation rate.

Finally, thermogravimetric analyses (TGA) were used to investigate the effect of the incorporation of azide groups on the thermal stability of the biopolyols. Figure 6.8 compares the TGA and DTG curves for Foam A and Foam B under a nitrogen atmosphere.

The thermal degradation of both polyurethane foams occurred in two main steps. The first degradation step is attributed to the degradation of crosslinked polymer, including its hard and soft segments. The second step is assumed to be caused by the decomposition of the hydrocarbon chains that formed the main components of the foam (Chattopadhyay & Webster, 2009; Sacristán et al., 2011). It can be observed that the incorporation of approximately 0.09 g of nitrogen per gram of azidified biopolyol increased the temperature at which the maximum degradation of the PU foam occurs from 330 to 450 °C. Moreover, the residue yield obtained at 650°C increased from 6.28 to 8.11 wt.%, indicating that the Foam B is a 29.1 % more thermally stable than Foam A. These results confirm the enhancement in the

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Chapter 6 thermal stability of the PU foams as a result of incorporating nitrogen groups into their chemical structures by using azidified biopolyols as raw materials.

Figure 6.8. Comparison between thermogravimetric curves of Foam A (continuous line) and Foam B (discontinuous line). 6.4. Conclusions

The ring-opening reaction was successfully accomplished in a temperature range between 50 and 70 °C with reaction times up to 24 hours. A maximum oxirane ring conversion of 84.4 % was obtained after 24 hours of reaction at 70 °C. FTIR analyses confirmed the covalent linkage of azide groups to the biopolyol and the formation of hydroxyl groups. Moreover, reaction time and temperature favoured the incorporation of both functionalities to EGSO. GPC analyses confirmed a negligible formation of oligomers during the ring-opening reaction.

The kinetics of this reaction follows a pseudo-first reaction order, and the rate constants ranged between 2.70·10-2 at 50 °C and 7.18·10-2 s-1 at 70 °C. Arrhenius equation showed a good correlation between reaction temperature and rate constants (R2 = 0.987), finding values of 0.341 J/mol and 0.121 for the activation energy and pre-exponential factor, respectively.

Finally, a RPU foam was synthesized from the azidified biopolyol obtained at 70 °C during 24 hours. SEM images demonstrated that the azidified RPU foam exhibited a similar internal structure compared to a commercial one. Finally, TGA were used to

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Synthesis of rigid polyurethane foams from azidified biopolyols investigate the thermal stability of both foams, confirming the enhancement in the thermal stability of the azidified foam by shifting the maximum degradation temperature from 330 °C to 450 °C and increasing the residue yield at 650 °C a 29.1 %.

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References Ahn, B.K., Kraft, S., Wang, D., Sun, X.S. 2011. Thermally stable, transparent, pressure- sensitive adhesives from epoxidized and dihydroxyl soybean oil. Biomacromolecules, 12(5), 1839-1843. Akintayo, E.T., Thomas, Z., Akintayo, C.O. 2006. Characterization of the reaction products of epoxidised Adenopus breviflorus Benth. seed oil with sodium azide and trimethylsilyl azide. Industrial Crops and Products, 24(2), 166-170. Borugadda, V.B., Goud, V.V. 2015. Improved thermo-oxidative stability of structurally modified waste cooking oil methyl esters for bio-lubricant application. Journal of Cleaner Production. Chattopadhyay, D.K., Webster, D.C. 2009. Thermal stability and flame retardancy of polyurethanes. Progress in Polymer Science (Oxford), 34(10), 1068-1133. Guo, A., Cho, Y., Petrović, Z.S. 2000. Structure and properties of halogenated and nonhalogenated soy-based polyols. Journal of Polymer Science, Part A: Polymer Chemistry, 38(21), 3900-3910. Guo, Y., Hardesty, J.H., Mannari, V.M., Massingill Jr, J.L. 2007. Hydrolysis of epoxidized soybean oil in the presence of phosphoric acid. JAOCS, Journal of the American Oil Chemists' Society, 84(10), 929-935. Hill, K. 2000. Fats and oils as oleochemical raw materials. Pure and Applied Chemistry, 72(7), 1255-1264. Kajaste, R. 2014. Chemicals from biomass - Managing greenhouse gas emissions in biorefinery production chains - A review. Journal of Cleaner Production, 75, 1-10. Lu, S.Y., Hamerton, I. 2002. Recent developments in the chemistry of halogen-free flame retardant polymers. Progress in Polymer Science (Oxford), 27(8), 1661- 1712. Molero, C., De Lucas, A., Romero, F., Rodriguez, J.F. 2008. Influence of the use of recycled polyols obtained by glycolysis on the preparation and physical properties of flexible polyurethane. Journal of Applied Polymer Science, 109(1), 617-626. Mumtaz, T., Yahaya, N.A., Abd-Aziz, S., Abdul Rahman, N., Yee, P.L., Shirai, Y., Hassan, M.A. 2010. Turning waste to wealth-biodegradable plastics

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polyhydroxyalkanoates from palm oil mill effluent-a Malaysian perspective. Journal of Cleaner Production, 18(14), 1393-1402. Petrovic, Z.S. 2008. Polyurethanes from vegetable oils. Polymer Reviews, 48(1), 109- 155. Sacristán, M., Ronda, J.C., Galià, M., Cádiz, V. 2011. Phosphorus-containing soybean- oil copolymers: Cross-metathesis of fatty acid derivatives as an alternative to phosphorus-containing reactive flame retardants. Journal of Applied Polymer Science, 122(3), 1649-1658. Zhang, L., Zhang, M., Hu, L., Zhou, Y. 2014. Synthesis of rigid polyurethane foams with castor oil-based flame retardant polyols. Industrial Crops and Products, 52, 380-388. Zhao, Y., Xu, J., Xie, X., Yu, H. 2014. An integrated environmental impact assessment of corn-based polyols compared with petroleum-based polyols production. Journal of Cleaner Production, 68, 272-278.

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CHAPTER 7. PRODUCTION OF RIGID POLYURETHANE FOAMS FROM PHOSPHORYLATED BIOPOLYOLS

Production of rigid polyurethane foams from phosphorylated biopolyols

Abstract Renewable resources are playing a key role on the synthesis of biodegradable polyols. Moreover, the incorporation of covalently linked additives to the polyols is increasing in importance in the polyurethane (PU) market.

In this chapter, epoxidized grape seed oil (EGSO) and epoxidized methyl oleate (EMO) were transformed into phosphorylated biopolyols through an acid-catalysed ring-opening hydrolysis in the presence of H3PO4. The formation of phosphate polyesters was confirmed by GPC, FTIR and 31P-NMR.

However, the synthesis of a high-quality PU rigid foam was not possible using exclusively these polyols attending to their low hydroxyl value. As a consequence, different rigid PU foams were prepared from the phosphorylated biopolyols and the commercial petroleum-based polyol Alcupol R4520. It was observed that phosphorylated biopolyols can be incorporated up to a 57 wt.% in the synthesis of rigid PU foams without significant structural changes. Finally, TGA and EDAX analyses revealed an improvement of thermal stability of the foam by the formation of a protective phosphoro-carbonaceous char layer.

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Resumen Los recursos renovables están teniendo un papel clave en la síntesis de polioles biodegradables. Además, la incorporación de aditivos covalentemente unidos a los polioles está ganando importancia en el mercado de los poliuretanos (PU).

En este capítulo, el aceite de granilla de uva epoxidado y el oleato de metilo epoxidado fueron transformados en biopolioles fosforilados mediante un proceso de hidrólisis del anillo oxirano catalizado por ácidos en presencia de H3PO4. La formación de poliésteres fosfatados se confirmó mediante GPC, FTIR y 31P-NMR.

Sin embargo, debido al bajo índice de hidroxilo de estos polioles, no fue posible la síntesis de espumas de PU de alta calidad. Por este motivo, se prepararon diferentes espumas rígidas de PU a partir de mezclas de los biopolioles fosforilados y el poliol de origen petroquímico Alcupol R4520. Se observó que los biopolioles fosforilados pueden ser incorporados hasta en un 57% sin modificar significativamente la estructura interna de la espuma rígida de PU. Finalmente, los análisis de TGA y EDAX revelaron una importante mejora en la estabilidad térmica de la espuma debido a la formación de una capa fósforo-carbonosa protectora.

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7.1. General background

As it was discussed in the previous chapter, vegetable oils and related products (e.g., free fatty acids) are non-toxic, non-volatile, renewable, and biodegradable resources which represent the main biomaterial for the synthesis of biopolyols for the further production of rigid polyurethane foams (RPUF).

Different techniques such as thiolene coupling reaction (Samuelsson et al., 2004), ozonolysis (Petrović et al., 2005), hydroformylation (Petrović et al., 2008) and transesterification (Tavares et al., 2016) have been proposed to synthesize vegetable oil-based polyols. Nevertheless, the epoxidation of the double bonds and their further hydrolysis has become the most important pathway (Miao et al., 2014).

However, most PU are in general very flammable which is a highly undesirable characteristic under burning conditions because it represents a risk for the security. In that way, during the synthesis of a RPUF, beside the polyol and polyisocyanate, different additives are incorporated to improve the quality of the final product. Flame retardants, which are considered as one of the most important additives, inhibit or delay the spread of thermal decomposition by suppressing the combustion cycle on the surface of the material. There are two different approaches to incorporate flame retardants in PU foams either as free additives or as functional groups grafted covalently to the polyol/polyisocyanate structure. The latter one offers several advantages, such that functional groups are not susceptible to be lost through migration to the polymer surface of solvent leaching, what can become an environment problem (Chokwe et al., 2015; Kim et al., 2013; Pereira et al., 2015). Also, it can be homogeneously dispersed throughout the polymer and because of this may be required lower concentrations of additives than comparable not grafted ones (Chattopadhyay & Webster, 2009). Several reactive flame retardants have been proposed, being phosphorus compounds the most important group of environmentally friendly flame retardant due to its low toxicity, no release of poison gases and producing low smoke during the burning process (Levchik & Weil, 2006; van der Veen & de Boer, 2012).

Hence, the objective of this chapter is the synthesis of different phosphorylated biopolyols from vegetable biodegradable resources following the epoxidation and

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Chapter 7 further ring-opening reaction route. Grape seed oil and oleic acid were chosen as raw materials because they actually represent two of the main byproducts of the wine production and vegetable oil refining industries in the region of Castilla-La Mancha (Spain). Finally, the technical viability of synthesizing rigid PU foams with improved thermal stability using these biopolyols was studied.

7.2. Materials

Epoxidized grape seed oil (EGSO) was synthesized following the procedure indicated in Chapter 3 and performed in Chapter 4 at 90 °C during 1 hour. To reduce the presence of secondary reactions during the epoxidation procedure, oleic acid was firstly esterified in presence of methanol (Figure 7.1) following the procedure previously explained at Chapter 3. Epoxidation reaction was performed on methyl oleate following the same procedure than was used for grape seed oil.

Figure 7.1. Reaction scheme of the esterification reaction.

7.3. Results

7.3.1. Epoxidation of methyl oleate (MO)

In a first step, the viability of the epoxidation of methyl oleate (MO) was studied. Table 7.1 shows the iodine value, hydroxyl value and oxirane oxygen of MO and the epoxidated methyl oleate (EMO).

Table 7.1. Iodine value, oxirane oxygen content and hydroxyl values of MO and EMO. Iodine value OO Hydroxyl value Sample (g I2 /100 g) (%) (mg KOH/g) MO 88.37 0.0 4 EMO 23.77 3.60 17

Since the presence of enough double bonds is fundamental for the production of an epoxidized platform chemical, the iodine value of MO was measured finding a value of 88.37 g I2/100 g. This value is similar to that of other compounds previously used to produce epoxy-compounds as platform chemicals (Ji et al., 2015; Lu et al., 2010), indicating its suitability for this application. After the epoxidation process, the

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iodine value decreased to 23.77 g I2/100 g. As expected, the oxirane oxygen group was non-existent in MO before its epoxidation. After the epoxidation process, the oxirane oxygen content increased up to 3.60 %. The conversion of double bonds and yield towards epoxide groups were calculated as indicated at the Chapter 3, finding values of 73.10 and 64.62 %, respectively.

As it was discussed along the Chapter 6, during the epoxidation process different secondary reactions can take place, being the hydrolysis of oxirane rings towards the formation of hydroxyl groups the most common one (Petrović et al., 2002). Hence, the hydroxyl value of MO and EMO were determined, finding values of 4 and 17 mg KOH/g, respectively. In spite of this undesired secondary reaction, MO can still be considered as a suitable raw material for its epoxidation.

7.3.2. Ring-opening reaction with phosphoric acid (H3PO4)

Figure 7.2 shows a general reaction mechanism for the acid-catalysed ring-opening hydrolysis of oxirane compounds in presence of H3PO4. As can be observed, a wide variety of species can be obtained as products. Wu and Soucek indicated that this reaction can lead to the formation of 1,2-diol compounds (pathway A), polyether alcohols (pathway B), and ether alcohols when phosphate ion does not participate on the reaction progress (Wu & Soucek, 1998). However, Guo et al. have already reported that the use of H3PO4 as ring-opening agent might drive to the formation of phosphate mono-, di-, and triesters (pathway D) (Guo et al., 2007). Therefore, the four different pathways compete each other for the ring-opening reaction.

Both epoxidized products, EGSO and epoxidized methyl oleate (EMO), were hydrolysed using H3PO4 as ring-opening agent. The obtained biopolyols were named as phosphorylated grape seed oil (PGSO) and phosphorylated methyl oleate (PMO). In both cases, the obtained products had an oxirane oxygen content of 0.0 %, indicating a full conversion of the epoxy groups. The hydroxyl values of PGSO and PMO were found to be 123 and 45 mg KOH/g, respectively.

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Figure 7.2. Reaction scheme of the epoxide ring-opening reaction with H3PO4.

As expected, these values increased during the hydrolysis reaction compared with its epoxidized precursor. However, the increase observed in the hydroxyl value of PGSO was much greater than PMO one. This can be explained by the fact that GSO has a higher steric hindrance than MO, and therefore, oligomerization reactions described previously (Figure 7.2; pathways B, C, and D) are not favoured during EGSO hydrolysis reaction.

Molecular weight (MW) and its distribution is another critical parameter for the further production of rigid PU foams from the phosphorylated biopolyols. Figure 7.3 shows the GPC chromatograms of EGSO and EMO compared to their raw materials.

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GSO MO EGSO EMO

PGSO PMO

Intensity (a.u.) Intensity

9 10 11 12 13 14 15 16 11 12 13 14 15 16 17 18 Retention time (min) Retention time (min)

Figure 7.3. GPC chromatogram of the obtained products and their raw materials.

As it can be observed, EGSO and EMO contained oligomers which were not present at GSO and MO. As it was discussed along Chapter 4, oligomers can be formed during the epoxidation reaction as by-products during the epoxidation process due to the condensation of hydroxyl groups. Nevertheless, the extension of this reaction was almost negligible in both cases.

After the hydrolysis reaction in presence of H3PO4 the MW distribution become much wider in both cases. In the case of PGSO, the main specie after the hydrolysis process still has a very similar MW to the original triglyceride. The slight deviation of this peak to higher MW can be caused by the formation of a 1,2 diol (Figure 7.2; pathway A) or by the formation of a phosphate monoester (Figure 7.2; pathway D). On the other hand, respecting to PMO, the main product was the dimer, registered at an elution time of 15.7 minutes. This change in the MW distribution demonstrates that EGSO has a higher steric hindrance than EMO, justifying its lower hydroxyl value. It is also remarkable the high content of oligomeric species of PGSO and PMO. However, the identification of the different products was not possible due to the

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Chapter 7 wide variety of formed species and the overlapping of their MWs, as indicated at Figure 7.2.

In order to confirm the covalent bonding of phosphate groups, FTIR analyses were performed. Figure 7.4 shows the FTIR spectra of PGSO and PMO.

PGSO

PMO

1300 1200 1100 1000 900 800 700

Absorbance (a.u.)

4000 3700 3400 3100 2800 2500 2200 1900 1600 1300 1000 700 400 -1 Wavelength (cm )

Figure 7.4. FTIR spectra of the PGSO and PMO.

The FTIR spectra showed the characteristic bands of hydroxyl groups at 3480 cm−1,

C–H stretching vibrations at 2875– 2970 cm−1, −CH2 and −CH3 at 1350–1450 cm−1. As hydroxyl values predicted, the band of -OH groups is much more intense at PGSO than at PMO. The presence of a band at 1020 cm−1 indicated the presence of the phosphate ester groups (Guo et al., 2007; Pretsch et al., 2000), confirming the incorporation of phosphorous to the biopolyol.

31P-NMR spectroscopy, which is a more sensitive technique, was performed to corroborate the incorporation of phosphorous into the structure of the biopolyol. Figure 7.5 shows the 31P-NMR spectrum of PGSO (a) and EGSO (b).

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Intensity (u.a.) Intensity b)

60 50 40 30 20 10 0 -10  (ppm)

Figure 7.5. 31P-NMR spectra of a) PGSO and b) EGSO.

PGSO spectra present two signals at 1.02 and 17.23 ppm. This result indicates that

H3PO4 is not present as a free molecule but attached to the skeleton of the biopolyol and that it. Furthermore, the presence of two different chemical shifts indicates that two different phosphoric functional groups are linked. These signals were assigned to phosphate mono- and diester, according to Figure 7.2 (pathway D), in which these products are easier formed than phosphate triesters.

7.3.3. Assessment of PGSO and PMO in the synthesis of RPUF

Firstly, two different RPUF were synthesized from PGSO and PMO as biopolyols following the procedure indicated in Chapter 3. To compare the internal structure and thermal stability properties, the commercial polyol Alcupol R4520 was used as reference material to synthesize a third RPUF. The amounts of reactants used on the synthesis of these three foams are indicated in Table 7.2.

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Table 7.2. RPUF foam composition (expressed in wt.%). Foam R4520 PGSO PMO MDI Water Silicone Catalyst PU-PGSO 0.0 57.60 0.0 36.80 1.40 0.9 1.40 PU-PMO 0.0 0.0 64.90 30.88 1.62 0.9 1.62 PU-R4520 40.45 0.0 0.0 56.92 10.10 0.6 1.01

Unfortunately, the synthesis of a rigid PU foam using exclusively PMO was not possible due to its low hydroxyl value. Therefore, two rigid PU foams were obtained from PGSO (PU-PGSO) and Alcupol R4520 (PU-R4520). The internal structure of PU- PGSO and PU-R4520 are shown in Figure 7.6.

a) b)

Figure 7.6. SEM images of a) PU-R4520 and b) PU-PGSO.

The average cell size of PU-R4520 and PU-PGSO was found to be 0.511 and 50.805 μm, respectively, making noticeable the differences between both micrographs. It has been previously described in literature the great importance of the internal structure and cell size in the mechanical properties of RPUF (Goto et al., 2004). Hence, due to the significant differences among PU-R4520 and PU-PGSO, the use of exclusively PGSO on the synthesis of RPUF following the recipe indicated in Table 7.2 is not recommended.

Different RPUF were synthesized using exclusively PGSO as biopolyol but modifying up to ±10 wt.% the MDI, silicone, and water content. Nevertheless, the average cell size did not vary significantly from the previous value. This big difference in the internal structure might be justified by understanding the foaming process, which is carried out because of two parallel processes: the formation of the polymeric structure and the gas production. If one of these processes is accelerated in

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Production of rigid polyurethane foams from phosphorylated biopolyols comparison to the other, the morphology of the cell structure will be affected. Since the MW distribution and structure of both polyols are not similar, differences on the internal structure of PU foams might occur (Velencoso et al., 2015). However, the high influence of geometric shape and internal structure on the mechanical properties of PU foams has been demonstrated in literature (Goto et al., 2004; Hou et al., 2014). Therefore, PGSO cannot substitute totally Alcupol R4520 as polyol on the synthesis of RPUF.

As neither of the phosphorylated biopolyols are suitable to fully replace Alcupol R4520, 13 different mixtures of the polyols (A to M) were used to synthesize RPUF (PU-A to PU-M). This mixtures A to M were selected by stablishing a boundary condition of a hydroxyl value of 100 mg KOH/g to ensure the formation of RPUF in all cases. The composition of these foams is indicated in Table 7.3.

Table 7.3. Composition of RPUF PU-A to PU-M (expressed in wt.%).

Foam R4520 PGSO PMO MDI Water Silicone Catalyst PU-A 0.0 51.13 7.30 37.76 1.46 0.88 1.46 PU-B 13.02 39.07 0.0 44.52 1.30 0.78 1.30 PU-C 10.85 32.54 10.85 42.24 1.36 0.81 1.36 PU-D 0.0 42.83 16.66 36.64 1.49 0.89 1.49 PU-E 4.28 22.31 32.90 36.64 1.49 0.89 1.49 PU-F 11.66 18.32 25.54 40.86 1.39 0.83 1.39 PU-G 17.08 22.26 12.42 44.88 1.29 0.78 1.29 PU-H 28.46 17.08 0.0 51.51 1.14 0.68 1.14 PU-I 25.76 12.88 9.06 49.20 1.19 0.72 1.19 PU-J 21.72 8.61 20.26 46.07 1.27 0.76 1.27 PU-K 8.92 0.0 50.57 36.63 1.49 0.89 1.49 PU-L 19.73 0.0 33.24 43.59 1.32 0.79 1.32 PU-M 33.49 0.0 11.16 52.45 1.12 0.67 1.12

Figure 7.7 shows a ternary diagram where the polyol mixtures are depicted as well as the physical aspect of the 13 different RPUF (PU-A to PU-M).

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PGSOPGSO Chapter 7 0.00.0 1.01.0

PGSOA 0.0A 0.20.2 1.0 0.80.8 B D A B 0.2 D C 0.8 0.40.4 C B D 0.60.6 0.4 C 0.6 0.60.6 GG 0.40.4 0.6 EE G HH 0.4 E FF H F II 0.80.8 I 0.8 0.20.2 J 0.2 J J

LLL 1.01.0 1.0 K K M MM AlcupolAlcupolAlcupol K 0.0 0.00.0 PMOPMO PMO R4520 R4520R4520 0.00.0 0.0 0.20.20.2 0.40.40.4 0.60.60.6 0.8 0.80.8 1.0 1.01.0

A B C D E F G H I J K L M

Figure 7.7. Composition of polyol mixtures and physical aspect of the RPUF.

As expected, differences among the foams were observed depending on the polyol mixture composition used on their synthesis. For example, when Alcupol R4520 was not used (PU-A and PU-D), an irregular foaming process was observed. However, a higher content on PGSO leaded to better-looking foam due to its higher hydroxyl value. The same trend was observed on the binary mixtures PGSO-Alcupol R4520 (PU-B and PU-H) and PMO-Alcupol R4520 (PU-K, PU-L, and PU-M). It is also remarkable that the incorporation of small amounts of Alcupol R4520 to the polyol mixture improved notably the foaming process, as can be observed on PU-D, PU-E, and PU-K. Therefore, only the foams included on the highlighted region were considered for the subsequent analyses. Among them, PU-J, synthesized from a polyol mixture of 43% Alcupol R4520, 17% PGSO, and 40% PMO, presented the most regular and similar to PU-R4520 internal structure. Figure 7.8 shows the SEM image of foam PU-J.

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Figure 7.8. SEM images of PU-J. As can be observed, it exhibited a regular polyhedral cell structure with faces formed by the junction of three struts. The average cell size was found to be 0.641 μm. Therefore, a RPUF with an internal structure similar to the commercial one was synthesized substituting a 57% of the petroleum-based polyol by renewable phosphorylated biopolyols.

7.3.4. Thermal degradation behaviour of RPUF

It has been well recognized that phosphorus compounds can be used as flame retardants in polymers. In order to assess the improvement of the thermal stability due to the incorporation of phosphate ester groups to the biopolyol structure, TGAs under a synthetic air atmosphere were performed. Figure 7.9 shows the TGA and DTGA curves of PU-R4520 and PU-J.

From these curves, different important parameters can be withdrawn. These values are onset degradation temperature (Ton), which is defined as the temperature at which a 5% mass loss had occurred; the maximum rate degradation temperature

(Tmax); and the char yield at 700 °C (Yc). These values for both RPUF are summarized in Table 7.4.

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100 PU-J 1.4 90 PU-R4520 1.2 80

70 1.0

DTGA (% / ºC) / (% DTGA 60 0.8

50 0.6 40

Weight(%) 0.4 30 0.2 20

10 0.0

0 -0.2 50 100 150 200 250 300 350 400 450 500 550 600 650 700 T (ºC) Figure 7.9. TGA and DTGA curves under synthetic air atmosphere of PU-J and PU-R4520.

Table 7.4. Composition of RPUF PU-A to PU-M (expressed in wt.%).

Foam Ton (°C) Tmax (wt. %/°C) Yc (wt. %) PU-R4520 274.8 318.5 18.2 PU-J 188.2 327.7 21.3

As can be observed, the foam PU-J showed a degradation trend at a lower temperature than PU-R4520, implying a lower onset degradation temperature. This behaviour has been studied and attributed by some authors to the degradation of phosphoester groups, due to the fact that P–O–C bonds are less thermally stable than the predominant C–O–C bonds of petroleum-based polyether polyols (Mequanint et al., 2002; Troev et al., 1996). It is also remarkable that the weight loss of the phosphorylated PU foam (PU-J) in the temperature range of 360– 410 °C is around 60% whereas for PU-R4520, an increase of about 70% was registered. This is caused by the degradation of the phosphate groups, which leads to the formation of an organophosphorus char that prevents degradation of the remaining PU foam (Liu et al., 2012; Maiti et al., 1993; Price et al., 2007). However, when the temperature raised from 430 to 490 °C, the foam PU-J undergoes another weight loss step, indicating the decomposition of organophosphorus char layer. Finally, the residue

(Yc) was observed to be higher in the foam PU-J than in the foam PU-R4520, indicating a protective effect of the char, even at 700 °C.

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In contrast to nitrogen-based flame retardants, which typically acts on the gas phase during the combustion of a material releasing N2 and other non-flammable gases, phosphorous-based compounds form a barrier on the solid phase. This layer acts as a thermal protector of the non-burnt material. In addition, this char limits the mass transfer of oxygen and other flammable gases (Costes et al., 2017). To confirm that the phosphorous compounds were retained in the char and thus acted as a thermal- stabilizer barrier, the morphological structure of the char was studied by SEM. EDAX associated with SEM was used to obtain the elemental composition of the chars of foams PU-R4520 and PU-J. Figure 7.10 shows the SEM images of both chars generated after thermogravimetric analysis at 700 °C.

a) b)

Figure 7.10. SEM images of a) PU-R4520 and b) PU-J after the TGA.

As can be observed, a great cell size diminution was produced due to the thermal degradation process. Nevertheless, there is a more compact and denser structure on the surface of the burned PU-J, which indicates the presence of a protective char. Table 7.5 shows the surface composition of the chars obtained by EDAX.

Table 7.5. Composition of the residue after TGA of PU-R4520 and PU-J.

Foam C (wt. %) N (wt. %) O (wt. %) P (wt. %) Others (wt. %) PU-R4520 51.93 22.82 24.04 0.0 1.21 PU-J 45.66 16.54 22.05 15.31 0.44

As can be observed, C, N, and O were the main remaining elements. Besides, the residue of PU-J showed a phosphorous content of 15.31 wt.%, confirming the formation of a protective phosphorylated layer, as also was corroborated by the TGA results.

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3.4. Conclusions

The acid-catalysed ring-opening hydrolysis of EGSO and EMO was performed using an aqueous solution of H3PO4, reaching a 100 % conversion in 6 h. The formation of phosphate polyester was confirmed by GPC, FTIR and 31P-NMR.

Unfortunately, it was not possible to synthesize a high-quality rigid PU foam using exclusively the previously synthesized phosphorylated biopolyols. The incorporation of small amounts of Alcupol R4520 to the polyol mixture improved notably the quality of the RPUF. PU-J, synthesized from a polyol mixture of Alcupol R4520, PGSO, and PMO, having a composition of 43, 40, and 17 wt.%, respectively, presented the most regular and similar internal structure to the PU-R4520 foam. TGAs revealed that, in spite of PU-J was less stable than PU-R4520 at moderate temperatures due to the degradation of P-O-C bonds, the maximum degradation temperature and the residue at 700 °C increased significantly. Finally, EDAX analyses confirmed the presence of a 15.31 wt.% of P in the residue after the TGA, indicating the formation of a protective phosphorylated char layer on the foam.

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References Costes, L., Laoutid, F., Brohez, S., Dubois, P. 2017. Bio-based flame retardants: When nature meets fire protection. Materials Science and Engineering: R: Reports, 117, 1-25. Chattopadhyay, D.K., Webster, D.C. 2009. Thermal stability and flame retardancy of polyurethanes. Progress in Polymer Science (Oxford), 34(10), 1068-1133. Chokwe, T.B., Okonkwo, J.O., Sibali, L.L., Ncube, E.J. 2015. Alkylphenol ethoxylates and brominated flame retardants in water, fish (carp) and sediment samples from the Vaal River, South Africa. Environmental Science and Pollution Research, 22(15), 11922-11929. Goto, A., Yamashita, K., Nonomura, C., Yamaguchi, K. 2004. Modeling of cell structure in polyurethane foam. Journal of Cellular Plastics, 40(6), 481-488. Guo, Y., Hardesty, J.H., Mannari, V.M., Massingill Jr, J.L. 2007. Hydrolysis of epoxidized soybean oil in the presence of phosphoric acid. JAOCS, Journal of the American Oil Chemists' Society, 84(10), 929-935. Hou, C., Czubernat, K., Jin, S.Y., Altenhof, W., Maeva, E., Seviaryna, I., Bandyopadhyay- Ghosh, S., Sain, M., Gu, R. 2014. Mechanical response of hard bio-based PU foams under cyclic quasi-static compressive loading conditions. International Journal of Fatigue, 59, 76-89. Ji, D., Fang, Z., He, W., Zhang, K., Luo, Z., Wang, T., Guo, K. 2015. Synthesis of soy- polyols using a continuous microflow system and preparation of soy-based polyurethane rigid foams. ACS Sustainable Chemistry and Engineering, 3(6), 1197-1204. Kim, J.W., Isobe, T., Sudaryanto, A., Malarvannan, G., Chang, K.H., Muto, M., Prudente, M., Tanabe, S. 2013. Organophosphorus flame retardants in house dust from the Philippines: Occurrence and assessment of human exposure. Environmental Science and Pollution Research, 20(2), 812-822. Levchik, S.V., Weil, E.D. 2006. A review of recent progress in phosphorus-based flame retardants. Journal of Fire Sciences, 24(5), 345-364. Liu, H., Xu, K., Cai, H., Su, J., Liu, X., Fu, Z., Chen, M. 2012. Thermal properties and flame retardancy of novel epoxy based on phosphorus-modified Schiff-base. Polymers for Advanced Technologies, 23(1), 114-121.

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Lu, H., Sun, S., Bi, Y., Yang, G., Ma, R., Yang, H. 2010. Enzymatic epoxidation of soybean oil methyl esters in the presence of free fatty acids. European Journal of Lipid Science and Technology, 112(10), 1101-1105. Maiti, S., Banerjee, S., Palit, S.K. 1993. Phosphorus-containing polymers. Progress in Polymer Science, 18(2), 227-261. Mequanint, K., Sanderson, R., Pasch, H. 2002. Thermogravimetric study of phosphated polyurethane ionomers. Polymer Degradation and Stability, 77(1), 121-128. Miao, S., Wang, P., Su, Z., Zhang, S. 2014. Vegetable-oil-based polymers as future polymeric biomaterials. Acta Biomaterialia, 10(4), 1692-1704. Pereira, L.C., de Souza, A.O., Bernardes, M.F.F., Pazin, M., Tasso, M.J., Pereira, P.H., Dorta, D.J. 2015. A perspective on the potential risks of emerging contaminants to human and environmental health. Environmental Science and Pollution Research, 22(18), 13800-13823. Petrović, Z.S., Guo, A., Javni, I., Cvetković, I., Hong, D.P. 2008. Polyurethane networks from polyols obtained by hydroformylation of soybean oil. Polymer International, 57(2), 275-281. Petrović, Z.S., Zhang, W., Javni, I. 2005. Structure and properties of polyurethanes prepared from triglyceride polyols by ozonolysis. Biomacromolecules, 6(2), 713-719. Petrović, Z.S., Zlatanić, A., Lava, C.C., Sinadinović-Fišer, S. 2002. Epoxidation of soybean oil in toluene with peroxoacetic and peroxoformic acids - Kinetics and side reactions. European Journal of Lipid Science and Technology, 104(5), 293-299. Pretsch, E., Bühlmann, P., Affolter, C. 2000. Structure Determination of Organic Compounds. Tables of Spectral Data. Springer, Berlin. Price, D., Cunliffe, L.K., Bullett, K.J., Hull, T.R., Milnes, G.J., Ebdon, J.R., Hunt, B.J., Joseph, P. 2007. Thermal behaviour of covalently bonded phosphate and phosphonate flame retardant polystyrene systems. Polymer Degradation and Stability, 92(6), 1101-1114. Samuelsson, J., Jonsson, M., Brinck, T., Johtansson, M. 2004. Thiol-ene coupling reaction of fatty acid monomers. Journal of Polymer Science, Part A: Polymer Chemistry, 42(24), 6346-6352.

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Tavares, L.B., Boas, C.V., Schleder, G.R., Nacas, A.M., Rosa, D.S., Santos, D.J. 2016. Bio- based polyurethane prepared from Kraft lignin and modified castor oil. Express Polymer Letters, 10(11), 927-940. Troev, K., Tsevi, R., Bourova, T., Kobayashi, S., Uayama, H., Roundhill, D.M. 1996. Synthesis of phosphorus-containing polyurethanes without use of isocyanates. Journal of Polymer Science, Part A: Polymer Chemistry, 34(4), 621-631. van der Veen, I., de Boer, J. 2012. Phosphorus flame retardants: Properties, production, environmental occurrence, toxicity and analysis. Chemosphere, 88(10), 1119-1153. Velencoso, M.M., Ramos, M.J., Serrano, A., de Lucas, A., Rodríguez, J.F. 2015. Fire retardant functionalized polyol by phosphonate monomer insertion. Polymer International, 64(12), 1706-1714. Wu, S., Soucek, M.D. 1998. Oligomerization mechanism of cyclohexene oxide. Polymer, 39(15), 3583-3586.

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CHAPTER 8. SYNTHESIS AND CHARACTERIZATION OF ESTOLIDE-BASED LUBRICANTS

Synthesis and characterization of estolide-based biolubricants

Abstract In this chapter a new three step process has been developed and optimized to demonstrate that it is possible to achieve the complete conversion of free fatty acids (FA) into estolide esters (biolubricant), biodiesel and choline carboxylates

(biosurfactants). This new approach consists in the utilization of HClO4 as catalyst, a low-price and industrially available strong acid, to generate the estolides. Then, an esterification process with methanol in presence of a cheap ionic liquid will transform estolides into estolide esters and the unreacted FA into biodiesel. A neutralization step with choline hydroxide allowed to obtain biosurfactants from the unreacted FA, improving the lubricant properties of estolide esters and the quality of biodiesel.

Finally, estolide esters and biodiesel were optimally split by means of molecular distillation. Both products were found to have excellent properties, fulfilling the requirements of international standards SAE J306 and EN 14214 for lubricants and biodiesel, respectively. Hence, the industrial application of FA as raw material to produce higher added value products was demonstrated.

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Resumen En este capítulo un nuevo proceso en tres etapas ha sido desarrollado y optimizado para demostrar que es posible llevar a cabo una conversión completa de ácidos grasos en ésteres de estólidos (biolubricante), biodiesel y carboxilatos de colina

(biosurfactante). Este nuevo planteamiento se basa en la utilización de HClO4 como catalizador, un ácido fuerte económico e industrialmente disponible, para generar estólidos. Una posterior reacción de esterificación con metanol, en presencia de un líquido iónico, transformará los estólidos en estólidos esterificados y los ácidos grasos no reaccionados en biodiesel. Una etapa de neutralización adicional con hidróxido de colina permitirá la transformación de los ácidos grasos no reaccionados en biosurfactantes, mejorando el poder lubricante de los estólidos esterificados y la calidad del biodiesel.

Finalmente, los estólidos esterificados y el biodiesel fueron separados de forma óptima mediante destilación molecular. Ambos productos presentaron excelentes propiedades, cumpliendo los requisitos de los estándares internacionales SAE J306 y EN14214 para lubricantes y biodiesel, respectivamente. Por tanto, la aplicación industrial de los ácidos grasos como materia prima para producir productos de alto valor añadido ha sido demostrada.

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8.1. General background Estolides are products of the oligomerization reaction between a carboxylic acid functionality and an alkene group, forming an ester linkage, with good lubricant properties. These organic compounds are normally produced from unsaturated fatty acids (FA) in the presence of an acid catalyst. Homogeneous and heterogeneous catalysts have been used in estolides synthesis by Isbell et al. (Isbell et al., 1994). Among the homogeneous ones, perchloric and sulphuric acids have been specially used, whereas montmorillonites and supported homogeneous acids have been the most studied heterogeneous ones (Erhan & Isbell, 1997; Nordin et al., 2012). More recently, biological species such as Candida rugosa (Bódalo-Santoyo et al., 2005), Pseudomonas stutzeri (Todea et al., 2015) and other commercial ones (Aguieiras et al., 2011) have shown good catalytic activity. However, the reaction yield of the system is limited by the equilibrium and the complete transformation of the unsaturated FA into valuable products have not been reported yet.

This limited conversion makes necessary the separation of unreacted FA. Different alternatives have been proposed in literature. Isbell et al. (Isbell et al., 1994) proposed to cool down the reaction product in a separatory funnel up to room temperature. After that, the catalyst is neutralized by adding a solution of Na2HPO4. The organic phase is concentrated in vacuo to remove residual water. Lastly, the product is purified by Kugelrohr short-path vacuum distillation. Slight variations of this neutralization procedure have been studied in literature (Cermak et al., 2015b; Cermak & Isbell, 2004; Martin-Arjol et al., 2015). Other physical removal processes, like molecular distillation, have been proposed (Cermak et al., 2013; Cermak et al., 2015a). Molecular distillation technology is used to achieve the distillation of thermally unstable and sensitive materials. The high temperature exposure of the distilled liquid to the evaporating cylinder is limited to a few seconds and, thanks to the high vacuum, this method is often the most economically feasible method of purification (Cermak et al., 2007). However, neither the chemical nor the physical purification methods previously described in literature allow a full utilization of the unreacted raw material. This fact, combined with the limited yield of the estolide synthesis reaction, imply a significant reduction of the process profitability and demonstrates the low viability of this process at industrial scale. Hence, a new greener and more profitable industrial process is needed.

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Firstly, due to the limited efficiency of the estolide synthesis and the separation methods proposed in literature up to now, the conversion of unreacted FA into fatty acid methyl esters (FAME) is proposed together with the transformation of estolide into estolide esters. It has also been studied the esterification of estolide compounds in presence of branched alcohols, finding an improvement on the viscosity index and the low temperature properties (Harry-O'kuru et al., 2001). This improvement of the biolubricant properties of estolide esters have also been described in literature by other authors more recently (Cermak et al., 2015a; Cermak et al., 2015b; Doll et al., 2016). Moreover, esterify fatty acids to FAME in presence of an acid catalyst and methanol is one of the most important routes to produce biodiesel. Unfortunately, esterification reactions are limited by an equilibrium. Different alternatives have been discussed to improve the esterification yield, like the use of alcohol in large excess, using a dehydrating agent or the removal of water by physical means (Ma & Hanna, 1999). Among them, the use of green ionic liquids (ILs) has been accepted as the most interesting alternative due to their great hydrophilicity (Hayyan et al., 2014) and, in some cases, low cost (Chen et al., 2014).

FA can also be used as raw material on the synthesis of surfactants. In fact, sodium and potassium carboxylates are one of the most common surfactants due to their simple synthesis from FA and the respective hydroxide. However, these surfactants have a limited applicability due to their low solubility in water (Klein et al., 2008). Other counter-cations have been proposed to enhance water solubility and biodegradability of the obtained surfactant. Among them, choline carboxylates have demonstrated excellent surfactant properties at ambient temperatures. Moreover, choline carboxylates are easily biodegradable and are synthesized by a straightforward process with little or no heat requirement (Klein et al., 2008).

Hence, the main objective of this chapter is to produce estolides esters, biodiesel and choline carboxylates through a facile and innovative method, simplifying the purification step and ensuring a full conversion of the raw material into higher added value products to ensure the profitability of this process at industrial scale.

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8.2. Results 8.2.1. Oleic acid characterization Commercial oleic acid (OA) was characterized by means of GC analyses for quantifying the presence of FA. The fatty acid profile is shown in Table 8.1.

Table 8.1. OA fatty acid profile.

Fatty acid wt. % Palmitic acid C16:0 1.60 Stearic acid C18:0 2.29 Oleic acid C18:1 84.39 Linoleic acid C18:2 5.18 Others 3.44 Total 97.91

As it can be observed, the 97.91 wt.% of the sample is composed by FA and containing an 84.39 wt.% of oleic acid.

It was also determined the acidity value and kinematic viscosity at 40 °C of oleic acid, finding a value of 199.63 mg KOH/g and 18.96 cSt, respectively. Both values are in the standard range for FA.

8.2.2. Estolide synthesis

Four different mineral acids were tested as catalysts in the estolide synthesis reaction, namely HClO4, H2SO4, CH3SO3H and HNO3. Table 8.2 shows the effect of catalyst on the acidity reduction and the kinematic viscosity at 40 °C of the product.

Table 8.2. Acidity reduction and kinematic viscosity at 40 °C of estolide synthesis reaction products catalysed by different acids. Acidity reduction Kinematic viscosity Catalyst (%) at 40 °C (cSt) HClO4 65.38 161.81 H2SO4 12.00 31.83 CH3SO3H 8.02 26.87 HNO3 0.00 19.99

As it can be observed, HClO4 presented the highest depletion of the carboxylic group content, followed by H2SO4 and CH3SO3H, whereas HNO3 did not exhibited any catalytical activity. This catalytic activity order agrees with the results previously reported in literature (Isbell et al., 1994), which can be justified attending to the − − great stability of perchlorate anion (ClO4 ). The hydration energy of ClO4 in water is

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Chapter 8 among the lowest of common inorganic anions. This fact is caused because its negative charge is distributed equally over four oxygen atoms, originating a − delocalization of the negative charge. Hence, ClO4 has an enhanced transferability from aqueous to organic phases, increasing the concentration of H+ in the reaction media, which is the real catalyst of the estolide synthesis reaction (Brown & Gu, 2006). Besides, it was also found that the higher the catalytic activity, the higher the kinematic viscosity of the product. The viscosity measurements indicate the formation of compounds of high molecular weights. GPC analyses were performed to confirm the formation of these species. The chromatograms are shown in Figure 8.1.

Oleic acid

HNO

CH SO H 3 3

H SO

Intensity (a.u.) Intensity 2 4

HClO

10 11 12 13 14 15 16 17 18 Elution time (min)

Figure 8.1. GPC chromatograms of estolides obtained with different catalysts.

The chromatograms obtained were composed by discrete peaks at retention times of 15.4, 14.3, 13.6, 13.3 and 12.9 minutes, which correspond to products with a molecular weight of 282, 560, 845, 1120 and 1415 g/mol. In that way, H2SO4 and

CH3SO3H were only able to form dimers and a small amount of trimers. On the contrary, the high conversion of FA when using HClO4 as catalyst allowed to form up to pentamers. Finally, GPC analyses confirmed that HNO3 did not catalysed the reaction and thus the peak corresponding to oleic acid remained unchanged.

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FT-IR analyses were performed to confirm the formation of the estolide. Figure 8.2 shows the FT-IR spectra of OA and the product obtained using HClO4 as catalyst. This product was selected to be shown against the others because of its higher conversion.

HClO

Absorbance (a.u.) Absorbance

Oleic acid 4000 3500 3000 2500 2000 1500 1000 500 1800 1740 1680 Wavenumber (cm-1)

Figure 8.2. FT-IR spectra of OA and the product of estolide synthesis using HClO4 as catalyst.

Respecting to oleic acid spectra, signals at 2929 and 717 cm-1 revealed the presence of C=C and =C-H bonds, respectively. In addition, it was found the characteristic signal of C=O relative to carboxyl groups at a wavenumber of 1712 cm-1. Comparing this spectrum with estolide synthesis reaction, a decrease in the signal related to the unsaturation was observed. On the contrary, two new signals relative to C-O-C and C=O from ester groups appear at 1176 and 1732 cm-1, respectively. Hence, the formation of estolides was confirmed.

8.2.3. Esterification process

In this section, only those acids that had previously shown catalytic activity were used for esterification reactions just by adding the required amount of methanol and, when indicated, of ionic liquid (IL). Figure 8.3 shows the acidity reduction achieved after the estolide synthesis and the esterification process under three different reaction conditions.

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100 91.98 Estolide synthesis 90 88.00 Esterification 8:1 Esterification 20:1 80 Esterification 20:1 with IL

40 34.62 30

20 16.18

10 8.58 4.53 5.52

Acidity remainingAcidity compared to oleic acid (%) 3.91 1.45 1.18 0.72 1.32 0 H SO CH SO H HClO 2 4 3 3 4 Catalyst Figure 8.3. Acidity remaining (expressed as % from original OA) of different esterified products. It was observed that, independently of the catalyst, an increase in methanol:CG ratio promotes a higher acidity reduction due to esterification equilibrium displacement.

Moreover, HClO4 presented the lowest conversion of carboxylic groups to esters in absence of the ionic liquid. This is due to the high content of water in the commercial perchloric acid (30%). On the other hand, the most successful catalyst during esterification process was CH3SO3H, which achieved acidity reduction levels higher than 98.5% in all the cases. The excellent behaviour of this catalyst during esterification processes has been reported in literature (Aranda et al., 2008).

Nevertheless, the low oligomerization level that CH3SO3H presented during the estolide synthesis advice against using this catalyst in the reaction system proposed in this study.

It has been previously demonstrated that the presence of an IL in the esterification media improves the conversion of FA into fatty acid methyl esters (FAME's) (Zhao & Baker, 2013). The improvement of esterification reaction conversion obtained by using HClO4 in presence of the IL can be explained in terms of the high affinity of ionic liquids with water and its immiscibility with organic phase. Therefore, since water is removed from the organic phase, esterification equilibrium is displaced to the right and more esters are formed. Triethylammonium hydrogen sulphate was

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Synthesis and characterization of estolide-based biolubricants used for this propose due to its low cost and facile preparation (Chen et al., 2014). As can be observed at Figure 8.3, the presence of the IL improves the esterification conversion independently of the used catalyst.

Therefore, considering the high conversion level achieved in both processes, it can be stated that HClO4 is the most promising catalyst for the proposed system. Besides, the presence of the IL lead to a much efficient decantation process without the requirement of an extra washing step to remove the catalyst or the methanol.

8.2.4. Neutralization process

In spite of the great behaviour of HClO4 as catalyst in presence of triethylammonium hydrogen sulphate as IL at the esterification process, unreacted FA are still present at the product. The presence of FA at biodiesel avoid fulfilling the quality requirement indicated at EN 14214 meanwhile its presence at lubricant substances increases the oxidative wear and reduces its thermal stability (Rani et al., 2015). Different approaches have been proposed to reduce the acid value of biodiesel and biolubricants, being the neutralization with hot solutions of NaOH-H2O the most used at industrial scale (Chongkhong et al., 2009). However, this method creates additional wastewater and leads to higher energy consumption. Choline, formerly known as vitamin B4, is an essential nutrient for mammals which is present in most foods. Its combination with FA lead to the production of surfactants (choline carboxylates) completely made of materials naturally occurring in the human body (Rocha et al., 2014).

In that way, the organic phase obtained after the esterification reaction was neutralized with a choline hydroxide solution (45 wt.% in methanol), obtaining as result a product with an acid number < 0.05 mg KOH/g of sample when 1.1 mol of ChOH per carboxylic group were used. Nevertheless, the acid number of the neutralized product was 0.15 mg KOH/g when a stoichiometric amount (1:1) was used. Thus, a slight excess of ChOH during the neutralization process is required. The addition of a large amount of methanol during this step is not recommended due to the significant solubility of biodiesel on it (Zhou et al., 2006). In addition, the extremely low acid value of the phase composed by esterified estolide and biodiesel demonstrated that the hydrolysis of the products is not taken place.

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8.2.5. Estolide ester and biodiesel separation process by molecular distillation

To separate biodiesel and estolides esters formed during the esterification reaction, a molecular distillation process was proposed. Molecular distillation is a high vacuum process that allows the separation of different compounds at moderated temperatures due to the decrease in the boiling point. This fact, combined with a low residence time, allows the separation of mixtures without damaging the products (Maciel Filho et al., 2006).

Molecular distillation was performed to separate estolides ester (heavy component) and biodiesel (light component) phases at temperatures ranged between 110 and 145 °C. Figure 8.4 shows the distillate (DIS) to residue (RES) mass ratio and the kinematic viscosity at 40 °C of the residue.

0.50 90

0.45 85

0.40 80

75 0.35

70 0.30 65 0.25 60 Mass ratio DIST:RESMass 0.20 55 Mass ratio DIST:RES 0.15 Kinematic viscosity RES RES °C at (cSt)40 viscosity Kinematic 50 0.10 105 110 115 120 125 130 135 140 145 150 Temperature (ºC)

Figure 8.4. Influence of temperature on DIST:RES mass ratio and residue kinematic viscosity at 40 °C.

As it can be observed, an asymptotic behaviour is presented for both variables when temperature reaches 145 °C. Therefore, it is no required an increase in distillation temperature in order to separate both components. Both products obtained at 145 °C were characterized for checking their commercial applications.

Estolide ester phase presented an acid value < 0.05 mg KOH/g, a pour point (PP) of -24 °C, and a density of 0.907 g/cm3. Kinematic viscosity was determined at 40 and

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Synthesis and characterization of estolide-based biolubricants

100 °C, finding values of 82.89 and 13.41 cSt, respectively. Using these two values of kinematic viscosity, a viscosity index of 134 was determined attending to the ASTM D2270 standard. All the determined parameters indicate that the biolubricant phase has good properties for its use as lubricant. Moreover, this product can be classified as grade 85W, according to SAE J306 standard.

Regarding to biodiesel, firstly acidity value was measured, finding a value of < 0.05 mg KOH/g. Fatty acid methyl ester (FAME) content, density and the cold filter plug point (CFPP) were found to be 98.5%, 0.872 g/cm3 and -10 °C, respectively. All these parameters are in the range admitted by EN 14214, confirming the viability of using this secondary product as biodiesel. For a better comprehension, the main properties of the products obtained by molecular distillation after the neutralization process are summarized in Table 8.3.

Table 8.3. Summary of the physical and chemical properties of the products obtained by molecular distillation.

Biolubricant phase Biodiesel (estolide ester) Acid value (mg KOH/g) <0.05 <0.05 PP (°C) -24 - Density (g/cm3) 0.907 0.872 at 40 °C 82.89 - Viscosity (cSt) at 100 °C 13.41 - Viscosity index 134 - FAME content (%) - 98.5 CFPP (°C) - -10

These parameters confirm that it was possible to produce a high quality biolubricant and biodiesel in only two reaction steps making a full use of the raw material and without degrading them during the fractionation process.

8.2.6. Preliminary economical assessment

The deodorization of edible vegetable oils is a well-known industrial process to remove the FA generated during the vegetable oil extraction. The value of the distillate stream produced typically depends on the concentration of nutraceutical substances (tocopherols, sterols, squalene, etc.) and the FA profile of the saponificable fraction (Verleyen et al., 2001). In that way, the higher the concentration of nutraceutics and/or unsaturated FFA the higher the price of the

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Chapter 8 distillate stream. The range of prices typically goes from 350 (palm and palm kernel) up to 1100 €/t distillate (olive and sunflower oil). In the proposed process the presence of nutraceutical substances is not required, so an average soybean or high- oleic-acid sunflower oil distillate (500-800 €/t) can be used as raw material (OLEON, 2017).

The great demand of gear lubricants and diesel ensures a stable selling rate of both products. However, petroleum price changes, the incorporation of additives to enhance the quality of both products and geopolitical issues, like taxes, lead to a significant variation of their prices (Saif Ghouri, 2006). Nevertheless, an average selling price for grade 85W gear lubricant and biodiesel can be stablished at 2950 and 1800 €/t, respectively (Alibaba, 2017; Global Petrol Prices, 2017). In order to take into account the possible fluctuations of the selling price, a variation of ±15% was considered to stablish two more different scenarios.

Finally, examples of successfully commercialized biosurfactants exists, as has been demonstrated by different companies (AGAE Technologies, 2017; EVONIK Industries, 2017). A wide range of prices can be found depending on the raw materials, quality and concentration of the biosurfactant, varying from 1000 up to 3100 €/t (Alibaba, 2017). Table 8.4 summarizes the purchase and selling prices for the distillate fatty acids and biobased products at 3 different scenarios.

Table 8.4. Purchase and selling price (€/t) of the main chemicals at different scenarios. Scenario I Scenario II Scenario III Distilled FA (A) 800 650 500 Biolubricant (B) 2500 2950 3390 Biodiesel (C) 1530 1800 2070 Biosurfactant (D) 1000 2050 3100 Marginal benefit 1357 1905 2447

The marginal benefit, taking exclusively into account the selling and purchase price of the main raw materials and products (A, B, C, D given in Table 8.4), was determined from the mass balance of the products following the Equation 8.1.

푀푎푟푔푖푛푎푙 푏푒푛푒푓푖푡 = 0.654 · 퐵 + 0.333 · 퐶 + 0.013 · 퐷 − 퐴 (8.1)

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Synthesis and characterization of estolide-based biolubricants

As can be observed, a gap from 1357 to 2447 €/t was determined as marginal benefit depending on the scenario. These results should be read carefully since other raw materials (methanol, HClO4, ionic liquid, etc.) and the operation costs are not considered. However, the potential of this process at industrial scale is clearly demonstrated.

8.3. Conclusions

Estolide esters, biodiesel and choline carboxylates can be successfully synthesised from oleic acid performing a full use of the raw material using green unit operations and chemicals already available at industrial scale. The catalytic activity of HClO4 during the estolides synthesis was the highest, reaching a conversion of estolides of 65.38%. It was also found that a molar ratio methanol:CG of 20 and the presence of an ionic liquid allowed to transform the 96.18% of unreacted fatty acid into biodiesel during the subsequent esterification process. Moreover, the presence of an ionic liquid facilitated the separation of the organic and aqueous phases at the end of the esterification reaction and recovering the non-spent methanol. The remaining unreacted fatty acids were completely transformed into choline carboxylates, a biodegradable surfactant, by neutralization with choline hydroxide.

The separation of estolides esters (biolubricant) and biodiesel was successfully performed by molecular distillation at 145 °C and 10-2 mbar without degrading them. The quality of both products was found to be excellent, according to SAE J306 and EN 14214. Finally, a preliminary economical assessment indicated the industrial potential of the proposed process.

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vacuum residues using molecular distillation and optimization. Petroleum Science and Technology, 24(3-4), 275-283. Martin-Arjol, I., Isbell, T.A., Manresa, A. 2015. Mono-Estolide Synthesis from trans- 8-Hydroxy-Fatty Acids by Lipases in Solvent-Free Media and Their Physical Properties. JAOCS, Journal of the American Oil Chemists' Society, 92(8), 1125- 1141. Nordin, N.A.M., Adnan, N.F., Hamzah, N., Isahak, W.N.R.W., Ayatillah, A., Salimon, J., Yarmo, M.A., Kamaruddin, R.A. 2012. Comparison of different heterogeneous catalysts for the estolides synthesis from oleic acid, Vol. 364, pp. 288-292. OLEON. 2017. Distilled fatty acid producer web page. http://www.oleon.com/product/base-oleo/fatty-acids/distilled-fatty-acids (accessed 25/10/2017). Rani, S., Joy, M.L., Nair, K.P. 2015. Evaluation of physiochemical and tribological properties of rice bran oil - Biodegradable and potential base stoke for industrial lubricants. Industrial Crops and Products, 65, 328-333. Rocha, Â., Lourenço, N.M.T., Vidinha, P., Simões, P., Paiva, A. 2014. Separation of free fatty acids from deodorizer distillates using choline hydrogen carbonate and supercritical carbon dioxide. Separation and Purification Technology, 131, 14-18. Saif Ghouri, S. 2006. Assessment of the relationship between oil prices and US oil stocks. Energy Policy, 34(17), 3327-3333. Todea, A., Otten, L.G., Frissen, A.E., Arends, I.W.C.E., Peter, F., Boeriu, C.G. 2015. Selectivity of lipases for estolides synthesis. Pure and Applied Chemistry, 87(1), 51-58. Verleyen, T., Verhe, R., Garcia, L., Dewettinck, K., Huyghebaert, A., De Greyt, W. 2001. Gas chromatographic characterization of vegetable oil deodorization distillate. Journal of Chromatography A, 921(2), 277-285. Zhao, H., Baker, G.A. 2013. Ionic liquids and deep eutectic solvents for biodiesel synthesis: A review. Journal of Chemical Technology and Biotechnology, 88(1), 3-12. Zhou, H., Lu, H., Liang, B. 2006. Solubility of multicomponent systems in the biodiesel production by transesterification of Jatropha curcas L. oil with methanol. Journal of Chemical and Engineering Data, 51(3), 1130-1135.

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CHAPTER 9. DEVELOPMENT OF TMP- BASED BIOLUBRICANTS

Development of TMP-based biolubricants

Abstract The synthesis of oleic-based trimethylolpropane (TMP) esters via esterification between oleic acid (OA) and TMP in presence of an acid catalyst using a reactive distillation process was explored along this chapter. The performance of different acid catalysts, the OA:TMP molar ratio, the distillation method and the purification technique were studied to find the maximum OA conversion to TMP esters and the minimum by-products production.

Methanesufonic acid (MSA) presented a high catalytic activity, similar to the highest one reached when sulfuric acid was used. Hence, MSA was selected as catalyst due to its lower environmental impact. A large amount of by-products were obtained for OA:TMP molar ratios lower than 2.5. Nevertheless, using an OA:TMP molar ratio of 3.0 only unreacted OA was obtained as impurity when the in situ formed water was removed by using the reactive distillation system. A novel purification technique was proposed for the neutralization of unreacted OA using choline hydroxide (ChOH), obtaining choline carboxylates as product.

The relationship between the kinematic viscosity at 40 and 100 °C, molecular weight and composition for TMP-based biolubricants was accurately determined by combining the Mark-Houwink equation and the effect of different sensitivity parameters related to various functional groups. This model was able to quantify the deleterious effect that the presence of intermediate products (TMP mono- and diesteres) and unreacted OA had on the biolubricant viscosity index. The proposed synthesis and purification procedures led to a TMP-based lubricant with a viscosity index of 209 and a pour point of -70 °C, permitting the classification of the biolubricant as grade SAE 80W gear oil.

Finally, a preliminary economical assessment indicated the potential economical viability of the proposed synthetic and purification process along this chapter.

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Resumen En este capítulo se ha estudiado la síntesis de ésteres de trimetilolpropano (TMP) basados en ácido oleico (OA) mediante la esterificación de OA y TMP en presencia de un catalizador ácido empleando un proceso de destilación reactiva. El comportamiento de diferentes catalizadores ácidos, la relación molar OA:TMP, el proceso de destilación reactiva y la purificación del producto de reacción fueron estudiados para maximizar la conversión del OA en ésteres de TMP y minimizar la formación de subproductos.

El ácido metanosulfónico (MSA) presentó una gran actividad catalítica, similar a la máxima presentada por el ácido sulfúrico. Por tanto, el MSA fue seleccionado como catalizador debido a su menor impacto ambiental. Una gran cantidad de subproductos fueron obtenidos al emplear relaciones molares OA:TMP menores a 2,5. Sin embargo, al emplear una relación de 3,0 y el proceso de destilación reactiva, la única impureza observada en el producto de reacción fue ácido oleico son reacción. También se ha propuesto un proceso novedoso de neutralización del OA no reaccionado con hidróxido de colina (ChOH), obteniéndose carboxilatos de colina como subproductos.

La relación entre la viscosidad cinemática a 40 y 100 °C, el peso molecular y la composición del biolubricantes basados en ésteres de TMP fue determinada combinando la ecuación de Mark-Houwink y el efecto de diferentes parámetros de sensibilidad relacionados con la presencia de varios grupos funcionales. Este modelo es capaz de cuantificar el efecto negativo en el índice de viscosidad de la presencia de diferentes productos intermedios de reacción (mono- y diésteres de TMP) y el OA sin reaccionar. Los procedimientos de síntesis y purificación condujeron a la síntesis de un biolubricante con un índice de viscosidad de 209 y un punto de fluidez de -70 °C, permitiendo la clasificación de este biolubricante en la categoría SAE grado 80W para engranajes.

Finalmente, una evaluación económica preliminar indicó la viabilidad económica potencial de los procesos de síntesis y purificación propuestos a lo largo de este capítulo.

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9.1. General background The lubricant properties of vegetable oils can be chemically improved by modifying the ester linkage and the content of unsaturations, being the elimination of the β-H atom of the glycerol backbone one of the most important ones. The replacement of glycerol with other alcohols which do not contain β-hydrogens, such as neopentyl glycol, pentaerythriol or trimethylolpropane (TMP) could solve this problem. Among all these alcohols, TMP is the most interesting one due to its similar structure to glycerol.

The synthesis of TMP esters via esterification has been postulated as the most interesting alternative route. This method involves TMP and fatty acids (FA) using acids as catalysts. Figure 9.1 shows the schematic reaction process in three steps to synthesize TMP-based triesters via esterification.

Figure 9.1. Reaction scheme of the synthesis of TMP-esters from TMP and FA.

As can be observed, it consists on three-step consecutive esterification reactions where a molecule of water is formed at each step. Indeed, trimethylolpropane monooleate (TMPMO) is firstly formed, subsequently the trimethylolpropane dioleate (TMPDO) and finally, the desired trimethylolpropane trioleate (TMPTO) is obtained, with a global consumption of 3 moles of FA per mole of TMP. It is well known that the above esterification reactions are limited by the equilibrium, which

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Chapter 9 leads to uncompleted reactions. The most studied option for the proposed system is shifting the equilibrium by using non-stoichiometric amounts of the reactants. Tao et al. proposed an FA:TMP molar ratio up to 10 (Tao et al., 2012) meanwhile Cavalcanti et al. proposed a molar ratio up to 3.75 (Cavalcanti et al., 2018). However, the excess of any reactant is not efficient and implies a large number of complex purification steps. It is possible to shift the reaction towards higher reactants conversion by selective removal of formed water from the reaction mixture. Different techniques such as membrane processes (Izák et al., 2005), ionic liquids (Xing et al., 2005), molecular sieves (Geng et al., 2011) and reactive distillation systems (Lee et al., 2009) have been proposed. Nevertheless, the latter one is the easiest and less expensive option by using Dean-Stark apparatus (Rastegari et al., 2017).

Different heterogeneous and homogeneous acid catalysts have been reported in literature. Among the heterogeneous one, different solid acids (SO42-/MxOy, M=Ti, Zr, Sn) presented a conversion of 96.5 % after 3 h of reaction at 220 °C (Wu et al., 2015). However, their catalytic activity at moderate temperatures is negligible. Immobilized lipases presented high yields (around 97 %) towards the formation of TMP triesters. Nevertheless, its low thermal resistance and the long reaction time requirements (24 h) limit the economic feasibility of these catalysts (Åkerman et al., 2011). Strong cation exchange resins in H+-form are catalytically active in this reaction, but the TMP conversion was 3 times lower than that reached with H2SO4 using the same operating conditions of 4 h and 120 °C (Kuzminska et al., 2015).

In literature, the homogeneous acid esterification is recognized as the most efficient route for the production of TMP esters. Itsikon et al. achieved a yield to esters above 90 % in 6 hours at 160 °C using isovaleric acid as FA and sulfuric acid as catalyst (Itsikson et al., 1967). Arbain et al. compared 5 different homogeneous catalysts, namely perchloric, sulfuric, p-toluensulfonic, hydrochloric and nitric acid on the synthesis of TMP esters from a mixture of long chain FA. Among them, perchloric and sulfuric acid reported the highest yields (> 70 %) after a reaction time of 3 h at 150 °C (Arbain & Salimon, 2011). However, perchlorate is considered as a persistent inorganic pollutant with high water-solubility, diffusivity and stability (Xie et al., 2018); and the use of sulfuric acid is being avoided due to its high corrosiveness,

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Development of TMP-based biolubricants even against titanium which usually presents an outstanding corrosion resistance (de Souza & Robin, 2007). Environmentally friendly homogeneous catalysts such as methanesulfonic and dodecylbenzene sulfonic acids presented a good catalytic activity in other esterification processes (Aranda et al., 2008; Gang et al., 2007), exhibiting a reduced environmental impact, being easily degraded in soil (De Marco et al., 2000) and marine environments (Thompson et al., 1995). Hence, attending to the above properties and the lack of information related with their application for producing TMP esters, these kinds of catalysts must be studied for this purpose.

On the other hand, the negative effect on the viscosity index of the biolubricant caused by the presence of TMP mono- and diesters and unreacted FA has not been quantified, although some authors said that they are deleterious for lubricity properties (Yunus et al., 2005). Moreover, FA are susceptible to auto-oxidation as they contain acidic carboxylic groups (Fox & Stachowiak, 2007) and might cause corrosion problems on the lubricated surfaces and a decrease of the friction coefficient (Mannekote & Kailas, 2009). Hence, the study on a new method for the purification of the esterification product must be required. The molecular distillation has been used but it presents several drawbacks such as a high cost at industrial scale and the generation of a waste stream (Wang et al., 2014). The presence of the by-products (TMP mono- and diesters) in the biolubricant can be minimized by choosing the proper catalyst and operating conditions, reducing the purification process to a simple neutralization process. In that way, choline hydroxide, which chemically refers to (2- hydroxyethyl)trimethylammnonium, has been known to be essential to the mammalian organism and it can be found in foods from both the animal and plant kingdoms can be used. This bio-based compound can react with FA to form choline carboxylates, which have already been described in the literature as biodegradable surfactant of certain formulations (Klein et al., 2008).

Hence, the goal of this chapter is to synthesize TMP triesters with lubricant applications by using methanesulfonic and dodecylbenzene sulfonic acid as catalysts in the esterification process and to compare the results with those found with traditional ones. Further, FA:TMP molar ratios lower than the stoichiometric value were studied in order to know its influence on the product distribution (TMP

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Chapter 9 mono-, di- and triesters) and minimize the by-product formation and the advantages of using a reactive distillation system. Additionally, for reducing the negative effect of unreacted FA on the kinematic viscosity of the biolubricant, a novel neutralization process using choline hydroxide is proposed. Besides, the negative effect of by- products on the viscosity index of the biolubricant was quantified. A new kinematic viscosity model considering the temperature and the chemical characteristics of the products (molecular weight, presence and quantity of functional groups) as independent variables was developed based on the Mark-Houwink. Finally, the quality of the purified biolubricant has been compared with a commercial non- purified TMP-based one.

9.2. Results 9.2.1. Influence of catalyst nature

In this chapter, different homogeneous (H2SO4, MSA, DBSA and H3PO4) and heterogeneous (clay montmorillonite K10) acid catalysts were compared in the synthesis of TMP-esters. A reaction in the absence of catalyst was also carried out. Figure 9.2 shows the effect of the catalyst on the conversions of the esterification of TMP with oleic acid (OA).

30 Conversion (%)

0 H SO Montmorillonite H PO 2 4 MSA DBSA 3 4 No cat. K10 Figure 9.2. Influence of the catalyst nature on the esterification process (T=105 °C, t=6h, TMP:catalyst equivalent ratio=0.1, OA:TMP molar ratio=3, not using Dean- Stark trap).

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As can be observed, the reaction performed without a catalyst reached a conversion below 4 % so the presence of an acid catalyst is required. The using of the acid clay montmorillonite K10 reported a conversion of 29 %. Hence, the use of this acid clay is not recommended because its catalytic activity is not high enough at the temperature range studied (Reddy et al., 2005), although this kind of catalysts has exhibited good results at other less complex esterification reactions (Do Nascimento et al., 2015). Among the homogeneous catalysts, H3PO4 showed an OA conversion of 17 % meanwhile DBSA reached a 58%. These values are significantly lower than the conversions reached by sulfuric acid and MSA, 72.4 and 69.2 %, respectively. Although the conversion obtained by using the sulfuric acid was slightly higher, the MSA was selected as catalyst for the synthesis of TMP-esters from TMP and OA due to its lower environmental impact and reduced corrosiveness (Finšgar & Milošev, 2010).

9.2.2. Influence of the OA:TMP molar ratio

The product distribution in a reactive distillation system based on a Dean-Stark apparatus as function of the OA:TMP molar ratio between 1 to 3 was investigated. It is remarkable that according to the esterification reaction scheme the stoichiometric OA:TMP molar ratio should be 3. Figure 9.3 shows the OA conversion at different OA:TMP molar ratios.

100

75 Conversion (%)

60 1.0 1.5 2.0 2.5 3.0 OA:TMP molar ratio

Figure 9.3. Influence of the OA:TMP molar ratio on the esterification conversion (T=105 °C, t=6h, TMP:catalyst molar ratio=0.1, using Dean-Stark trap).

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As can be observed, at low OA:TMP molar ratios (1, 1.5 and 2), the OA conversion was almost complete, with conversions higher than 99%. An increase on the molar ratio up to the stoichiometric condition implied a decrease on OA conversion to an 81.0%. These results are in a similar range with those previously reported in literature but using a different catalyst, longer reaction times and not using a reactive distillation system (Li et al., 2012; Tao et al., 2012). In this case and as expected, the use of a Dean-Stark apparatus allowed to reach a displacement of the esterification reaction towards to the products, increasing the OA conversion from 69.2 to 81.0% at the same OA:TMP molar ration of 3.

As it was previously commented, there are three possible products that can be obtained during the reaction: TMPMO, TMPDO and TMPTO. In order to investigate the influence of OA:TMP molar ratio on the molecular weight distribution, GPC analyses of the obtained products were performed. All chromatograms presented four different signals as function of the elution time and corresponding to TMPTO, TMPDO, TMPMO and unreacted OA. Figure 9.4 shows the mass percentage of each component obtained by GPC and as function of the OA:TMP molar ratio.

100

Mass percentage (%) TMPTO 0 Component 3 TMPDO 2.5 2 TMPMO 1.5 OA 1 OA:TMP molar ratio

Figure 9.4. Product distribution obtained at different OA:TMP molar ratios.

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As can be observed, the distribution of products changed markedly with the OA:TMP molar ratio, obtaining a maximum content on TMPTO at an OA:TMP molar ratio of 2.5. However, this product presented a large amount of TMPDO as by-product (11.2 %) with similar contents of TMPMO and unreacted OA. Fortunately, it was observed that when an OA:TMP molar ratio of 3 is used, the product only contains TMPTO and unreacted OA as impurity. In that way, the unreacted OA can be separated from the product by neutralization, leading to a pure TMPTO that can be used as biolubricant.

9.2.3. Purification of the unreacted OA

In order to obtain a high quality biolubricant, the product obtained at an OA:TMP molar ratio of 3 was purified. This reaction product was selected because, as commented above, the unreacted OA was the unique impurity existing in the product. The proposed method consists on a neutralization process with a methanolic solution of ChOH, using 1.1 mole of ChOH per mole of carboxylic group. After the purification process, a product with an acid number <0.05 mg KOH/g was obtained, indicating the success of this neutralization technique. A GPC analysis of the neutralized product (Figure 9.5) was performed and a single signal corresponding to TMPTO was observed on the GPC chromatograph. This result confirmed the complete neutralization of OA by the ChOH and also that other by- products were not generated during the reaction, allowing to obtain a pure TMPTO as biolubricant.

Non-neutralized product

Neutralized product

Intensity (a.u.) Intensity

12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.0 16.5 17.0 Elution time (min)

Figure 9.5. GPC chromatograph of neutralized and non-neutralized products.

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9.2.4. Viscosity index of TMP-based lubricants

In order to check the workability and quality of the all synthesized TMP ester biolubricants, the influence of by-products and unreacted OA on their viscosity index (VI) was determined. Viscosity index indicates the influence of temperature changes on the kinematic viscosity of the product. In lubricant applications, high VI are desired because it designates a low dependence of viscosity with a given temperature increment. Figure 9.6 shows the viscosity index of the products synthesized at different OA:TMP molar ratios.

210 Non-purified products Neutralized product 200

190

180

170

160 Viscosity index Viscosity 150

140

130

1.0 1.5 2.0 2.5 3.0 OA:TMP molar ratio

Figure 9.6. Dependence of viscosity index of the products obtained at different OA:TMP molar ratios. It can be observed a noticeable increment of the VI among the products obtained at OA:TMP molar ratios from 1 to 2.5, with values between 133 and 203, respectively. This trend agrees with the composition of the biolubricant shown in Figure 9.4, where the lower the viscosity index is found at the higher contents of by-products and unreacted OA. However, the VI of the product obtained at an OA:TMP molar ratio of 3 was significantly lower than that obtained at 2.5, indicating that the unreacted OA also has a negative effect on the viscosity index. Moreover, the VI of the neutralized product was 209, 17 units greater than the non-purified one. Hence, the composition of the product presents a high influence on the VI of the biolubricant, and therefore, on its quality.

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9.2.5. Prediction of the kinematic viscosity

The two most important structural factors that have proved to play a key role on the viscosity of liquids are the molecular weight and the presence of different functional groups (Baled et al., 2018; Chen et al., 2003; Ramírez Verduzco, 2013).

The effect of the molecular weight (M) on the viscosity (ν) of different compounds have been extensively discussed in literature by means of the Mark-Houwink equation (Equation 9.1).

휈 = 푘 · 푀푎 (9.1) Where k and a are fitting constants. The value of a is usually related to the solubility and the mobility of the solute in the solvent where the measurement of the viscosity is performed. A value of a=0.5 indicates a theta solvent (i.e. the solute chains act like ideal chains and no interaction among them occur). Systems in theta conditions usually take place when either the concentration of the solute or its molecular weight is low. Values of a in the range of 0.8 indicate that a good solvent is being used. Nevertheless, values up to 2.0 can be found for systems at high temperatures. This potential-type equation has been used to model the viscosity of different molten materials and polymers like polyoxymethylene (Gonzalez-Gutierrez et al., 2014), different types of nylon (Khanna et al., 1996), poly(ɛ-caprolactone) (Grosvenor & Staniforth, 1996), ethylene-tetrafluoroethylene copolymers (Linliu & Chu, 1995) or perfluoropolyethers (Ajroldi et al., 1999).

From the composition determined by the previously shown GPC analyses, the average molecular weight (M) of the products obtained at different OA:TMP molar ratios can be calculated. These values together with the kinematic viscosities at 40 and 100 °C of the TMP-ester-based biolubricants are summarized in Table 9.1.

Table 9.1. Composition and characteristics of the TMP-ester-based biolubricants. OA:TMP molar 1 1.5 2 2.5 3 3 ratio Non-purified Purified xTMPTO 0.306 0.341 0.632 0.772 0.604 1 xTMPDO 0.251 0.371 0.231 0.141 0 0 xTMPMO 0.390 0.263 0.103 0.029 0 0 xOA 0.052 0.023 0.032 0.057 0.395 0 M (g/mol) 620 673 789 836 671 926 ν at 40 °C (cSt) 75 56 46 38 42 38 ν at 100 °C (cSt) 10.9 9.7 9.1 8.3 8.7 8.4

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As can be observed, the viscosity of the products decreases with the average molecular weight for both temperatures 40 and 100 °C. However, the Mark- Houwink equation indicates the opposite trend. Therefore, the effect of functional groups on the viscosity plays a key role in this case.

Respecting to the contribution of functional groups to the viscosity, Sastri et al. developed a correlation based on the structure of different organic compounds and some of its physical properties. Nevertheless, the prediction and consistency of this model with high molecular weight molecules is not good (Sastri & Rao, 1992). More recently, two different research works incorporated sensitivity parameters to express the efficacy of different functional groups to influence viscosity (Grayson et al., 2017; Rothfuss & Petters, 2017). It was found that this procedure is much more accurate since the non-functionalize compound is used as baseline which is displaced based on the sensitivity parameters. They were able to determine different viscosity sensitivity parameters (Sν) depending on molar change in the number of functional groups (ΔN) at a constant molecular weight, having the form showed by Equation 9.2.

∆푙표푔 휈 푆 = 10 (9.2) ν ∆푁

Hence, by using the logarithm properties Equation 9.2 can be rewritten in terms of the kinematic viscosities of the non-functionalized compound (νnf) and the functionalized one (νf) (Equation 9.3).

푆ν·∆푁 휈푓 = 휈푛푓 · 10 (9.3)

If the effect of adding more than one different functional group must be considered, Equation 9.3 can be generalized into Equation 9.4 by extending the contribution of each individual functional group to the viscosity of the non-functionalized one. It is remarkable that ΔN should be related with the molar fraction (xi).

푛 휈푓 − 휈푛푓 = 푣푛푓 · ∑푖 (푣푖 − 푣푛푓) (9.4)

Thus, the Equation 9.4 can be written in the form of Equation 9.5.

푛 푆ν,i·푥푖 휈푓 = 휈푛푓 · (1 + ∑푖 (10 ) − 푛) (9.5)

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In order to validate the model proposed at Equation 9.3, the viscosity at 25 °C of various compounds with different functional groups and molecular weights was collected in literature (Rothfuss & Petters, 2017). In this case, since pure compounds are being modelled, the term νnf from Equation 9.3 can be substituted by the definition indicated in Equation 9.1. Hence, Equation 9.6 can be drawn.

푎 푆ν·∆푁 휈푓 = 푘 · 푀 · 10 (9.6)

The mathematical fitting of experimental data to Equation 9.6 was performed at an in-house spreadsheet in Excel using the solver tool. The objective cell was obtained by summing all the squared residuals between the experimental and theoretical values (Equation 9.7) for each of the j experimental values.

2 휈 = ∑푗 {(휈 ) − (휈 ) } (9.7) 1 푚표푑푒푙 푗 푒푥푝 푗

Figure 9.7 compares the theoretical and experimental viscosities obtained of different groups of compounds with a molecular weight ranged from 45 to 310 g/mol containing 4 different functional groups: 1-alcohols (R-OH), carboxylic acids (R-COOH), methyl esters (R-COO-Me) and ethyl esters (R-COO-Et). Alkenes (R) where considered as non-functionalized compound in this validation case.

0.040 R exp 0.035 R-OH exp R-COOH exp R-COO-Me 0.030 exp R-COO-Et exp R 0.025 theo R-OH theo R-COOH 0.020 theo

R-COO-Me theo R-COO-Et theo

0.015 Viscosity (Pa·s) Viscosity

0.010

0.005

0.000 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 Molecular weight (g/mol)

Figure 9.7. Theoretical and experimental values of viscosity for different compounds.

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As can be observed, the differences between the experimental and theoretical values are negligible in the studied range of molecular weights. The values of the parameters of Mark-Houwink equation and the sensitivity parameters are resumed in Table 9.2.

Table 9.2. Mark-Houwink equation and the sensitivity parameters. Parameter T=25 °C k 1.662·10-9 a 2.660 Sν,R-OH 0.9840 Sν,R-COOH 0.8247 Sν,R-COO-Me 0.0421 Sν,R-COO-Et -0.1501

It can be clearly observed two groups of values for the sensitivity parameters. On the one hand, the more polar groups (i.e. hydroxy and carboxyl groups) present a sensitivity parameter close to one, confirming a great increase of the viscosity compared to an alkene with the same molecular weight. On the other hand, methyl esters showed a value close to zero indicating a small variation on the viscosity. Sensitivity parameters can take negative values, as in the case of ethyl esters, confirming a decrease on the viscosity of this compounds compared to the non- functionalized compounds used as reference (i.e. alkenes). Hence, it can be concluded that the proposed model is adequate for the determination of viscosities at 25 °C of pure compounds.

In the case of the TMP-based lubricants, Equation 9.5 can be particularized by considering that TMPTO acts as a solvent for the by-products of the reaction (i.e. TMPDO, TMPMO and OA).

푛 푆ν,i·푥푖 휈푓 = 휈푇푀푃푇푂 · (1 + ∑푖 (10 ) − 푛) (9.5)

Tus, the Equation 9.5 was used to predict the viscosity of the mixture of components present in the TMP-based lubricant as function of the presence of carboxylic acids (R-COOH), monoalcohols (R-OH) and dialcohols (HO-R-OH) into the TMPTO as “solvent” or reference viscosity.

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Development of TMP-based biolubricants

Figure 9.8 shows a comparison between the experimental data and theoretical values for the kinematic viscosity at 40 (Figure 9.8a) and 100 °C (Figure 9.8b). For a better understanding, the theoretical values were labelled with PX, where X refers to the OA:TMP molar ratio used on its synthesis. A good agreement was obtained in all cases, with a maximum error <6.5 % in all cases.

a) 88 Experimental 80 Modeled

P1 72

56 P1.5

48 Viscosity (cSt) Viscosity P2 P3 40 P2.5 P3 pur 32

600 650 700 750 800 850 900 950 M (g/mol)

b) 11.0 Experimental P1 Modeled

10.5

10.0

P1.5

9.5

Viscosity (cSt) Viscosity P2 9.0

P3 8.5 P3 pur P2.5

8.0 600 650 700 750 800 850 900 950 M (g/mol) Figure 9.8. Theoretical experimental values of kinematic viscosity at a) 40 °C and b)100 °C. Table 9.3 shows the results for k and the sensitivity parameters obtained (Sν,i) at each temperature.

Table 9.3. Values of the sensitivity parameters for 40 and 100 °C. Parameter T=40 °C T=100 °C Sν,R-COOH 0.113 0.034 Sν,R-OH 0.000 0.000 Sν,HO-R-OH 0.728 0.268

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As can be observed, a sharply decrease of the sensitivity parameters was obtained by increasing the temperature from 40 to 100 °C. This predicts that the kinematic viscosity of the biolubricant is strongly diminished with the temperature if neither M nor functional groups (i.e. composition) change.

Regarding the functional groups, diols and carboxylic acids present a high impact on the kinematic viscosity, meanwhile the predicted effect for the monol is negligible at both temperatures. However, it is remarkable the increase of the kinematic viscosity caused by diols; much higher than the increase caused by unreacted fatty acids. Therefore, the proposed model was able to predict the deleterious effect of TMPMO and unreacted OA on the biolubricant quality at the studied temperatures. In the case of lubricants, the viscosity index can be easily obtained from these values following the indications of the International Standard ASTM D 2270.

9.2.6. Properties of TMPTO

The chemical and physical properties of the neutralized TMPTO were measured and compared with another TMP-based lubricant (Wu et al., 2015). The results are listed in Table 9.4.

Table 9.4. Comparison of physicochemical properties of neutralized-TMPTO and a commercial product. Neutralized TMP-based lubricant Properties TMPTO (Wu et al., 2015) Acid value (mg KOH/g) <0.05 0.64 υ determined at 40 °C (cSt) 37.92 51.36 υ determined at 100 °C (cSt) 8.44 10.11 Viscosity index 208 188 Pour point (°C) -70 -41

As can be observed, the proposed neutralization process based on the neutralization of OA with ChOH removed the acidity of the biolubricant below the detection limit of the technique. The kinematic viscosity at 40 °C was noticeable lower due to the removal of OA. However, the kinematic viscosity at 100 °C presented a similar value in both compounds. Therefore, due to the lower dependence of kinematic viscosity with the temperature, the VI of the neutralized-TMPTO was 20 units greater than the non-purified TMP-based lubricant. Finally, the TMP-based lubricant reported a pour point of -41 °C, which is an acceptable value for commercial porpoises. However, the removal of OA by the proposed neutralization process improved the

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Development of TMP-based biolubricants pour point up to -70 °C. The enhancement of these critical quality parameters permitted the classification of the neutralized-TMPTO as grade 80W gear oil, according the Society of Automotive Engineers (SAE) standard method J306. Consequently, an increase in the selling price of the product and its applicability was produced because of the purification process. It is also remarkable that the product generated are choline carboxylates due to the neutralization process between OA and ChOH, which are considered as a green and harmless biosurfactant with a highly interest in the industry (Klein et al., 2008).

9.2.7 Preliminary economical assessment

The demand of lubricant is increasing year after year due to the increasing industrialization of different productive systems. However, effects like the variation of petroleum prices and the taxes associated to its consume lead to a variation on the marginal benefit of petroleum-based lubricants. Nevertheless, the price of a gear lubricant grade 80W is stablished around 13 €/kg (Alibaba, 2017; Global Petrol Prices, 2017). Respecting to the side product obtained during the purification process, there are examples of different biosurfactants that have been commercialized successfully during last years. A wide variety of selling prices between 1000 and 3100 €/t can be found depending on the concentration and the quality of the biosurfactant, being 1800 €/t an average one (Alibaba, 2017).

As opposed to traditional lubricants, the selling price of raw materials suffers less variations and it is even financed and supported by different administrations. In this case, FA are typically obtained by the deodorization of edible vegetable oils. Its price usually depends on the concentration of nutraceutical substances (i.e. tocopherols, sterols, squalene, etc.) and the FA profile of the saponificable fraction (de Haro et al. 2018). The range of prices usually goes from 350 €/t for palm and palm-kernel- based FA up to 1100 €/t for olive-oil-based ones. In the proposed process, nutraceutical elements are not required, so FA generated during the refining of soybean oil can be used as raw material since their price is around 500 €/t (OLEON, 2017). TMP is a common raw material which price is in the range from 1300 to 1600 €/t. For this preliminary assessment a price of 1450 €/t was considered.

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The marginal benefit, considering exclusively the selling and purchase prices of products and raw material respectively, can be calculated based on the product distribution obtained by GPC (Figure 4). This analysis revealed that the product obtained at a molar ratio OA:TMP of 3 was composed exclusively by an 83% of TMPTO and 17% of OA. The residual OA was totally transformed to choline carboxylates through the proposed neutralization process. Hence, the marginal benefit (MB) per ton of TMPTO produced can be determined by Equation 9.6, where SP refers to selling price and BP to buying price.

푀퐵 (€⁄푡) = 푆푃푇푀푃푇푂 + 0.210 · 푆푃퐶ℎ퐶 − 0.914 · 퐵푃퐹퐴 − 0.178 · 퐵푃푇푀푃 (9.6)

The obtained results of 13042 €/t should be read carefully since the fixed capital for the installation, the minority raw materials (catalyst and ChOH) and the operation costs were not considered. However, this preliminary analysis indicates the potential of this process at industrial scale.

9.3. Conclusions

The present chapter demonstrates the synthesis of oleic-based TMP-esters with lubricant applications via esterification using a reactive distillation process. MSA, an environmentally-friendly homogeneous catalyst, was found to have a similar efficiency to H2SO4 in this esterification process. The OA:TMP molar ratio was found to be a key process variable on the OA conversion and on the product composition, finding that a molar ratio of 2.5 lead to the highest conversion to towards TMPTO but with a high presence of by-products. On the contrary, the product obtained at a molar ratio of 3 only presented unreacted OA as impurity. The effectiveness of a novel purification method based on the neutralization of unreacted OA with ChOH obtaining choline carboxylates as by-product was demonstrated by obtaining a biolubricant with an acid number <0.05 mg KOH/g, a VI of 209 and a pour point of - 70 °C. These values that are significantly better than the previous ones reported for non-purified TMP-ester based lubricants. Also, in this chapter a predictive model of the kinematic viscosity based on the influence of the composition was developed. This model was able to predict perfectly kinematic viscosities of biolubricants considering the deleterious effect of intermediate reaction products having different functional groups. Finally, the proposed synthesis and purification methods

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Development of TMP-based biolubricants permitted to obtain a lubricant that can be classified as grade 80W gear oil, according to SAE standards.

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References

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Gang, L., Xinzong, L., Eli, W. 2007. Solvent-free esterification catalyzed by surfactant- combined catalysts at room temperature. New Journal of Chemistry, 31(3), 348-351. Geng, T., Li, Q., Jiang, Y., Wang, W. 2011. Esterification of stearic acid with triethanolamine over zirconium sulfate supported on SBA-15 mesoporous molecular sieve. Journal of Surfactants and Detergents, 14(1), 15-22. Global Petrol Prices. 2017. Diesel price online report. http://www.globalpetrolprices.com/diesel_prices/ (accessed 25/10/2017). Gonzalez-Gutierrez, J., Oblak, P., Von Bernstorff, B.S., Emri, I. 2014. Processability and mechanical properties of polyoxymethylene in powder injection molding. pp. 49-55. Grayson, J.W., Evoy, E., Song, M., Chu, Y., Maclean, A., Nguyen, A., Upshur, M.A., Ebrahimi, M., Chan, C.K., Geiger, F.M., Thomson, R.J., Bertram, A.K. 2017. The effect of hydroxyl functional groups and molar mass on the viscosity of non-crystalline organic and organic-water particles. Atmospheric Chemistry and Physics, 17(13), 8509-8524. Grosvenor, M.P., Staniforth, J.N. 1996. The effect of molecular weight on the rheological and tensile properties of poly(ε-caprolactone). International Journal of Pharmaceutics, 135(1-2), 103-109. Itsikson, T.M., Milovidova, N.V., Rapport, I.B. 1967. Esterification of trimethylolpropane with fatty acids. Chemistry and Technology of Fuels and Oils, 3(6), 398-400. Izák, P., Mateus, N.M.M., Afonso, C.A.M., Crespo, J.G. 2005. Enhanced esterification conversion in a room temperature ionic liquid by integrated water removal with pervaporation. Separation and Purification Technology, 41(2), 141-145. Khanna, Y.P., Han, P.K., Day, E.D. 1996. New developments in the melt rheology of nylons. I: Effect of moisture and molecular weight. Polymer Engineering and Science, 36(13), 1745-1754. Klein, R., Touraud, D., Kunz, W. 2008. Choline carboxylate surfactants: Biocompatible and highly soluble in water. Green Chemistry, 10(4), 433-435. Kuzminska, M., Backov, R., Gaigneaux, E.M. 2015. Behavior of cation-exchange resins employed as heterogeneous catalysts for esterification of oleic acid with trimethylolpropane. Applied Catalysis A: General, 504, 11-16. Lee, H.Y., Yen, L.T., Chien, I.L., Huang, H.P. 2009. Reactive distillation for esterification of an alcohol mixture containing n-butanol and n-amyl alcohol. Industrial and Engineering Chemistry Research, 48(15), 7186-7204. Li, R.J., Chen, L., Yan, Z.C. 2012. Synthesis of trimethylolpropane esters of oleic acid using a multi-SO 3H-functionalized ionic liquid as an efficient catalyst. JAOCS, Journal of the American Oil Chemists' Society, 89(4), 705-711.

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Linliu, K., Chu, B. 1995. Viscosity of ethylene/tetrafluoroethylene alternating copolymers. Polymer, 36(11), 2265-2269. Mannekote, J.K., Kailas, S.V. 2009. Studies on boundary lubrication properties of oxidised coconut and soy bean oils. Lubrication Science, 21(9), 355-365. OLEON. 2017. Distilled fatty acid producer web page. http://www.oleon.com/product/base-oleo/fatty-acids/distilled-fatty-acids (accessed 25/10/2017). Ramírez Verduzco, L.F. 2013. Density and viscosity of biodiesel as a function of temperature: Empirical models. Renewable and Sustainable Energy Reviews, 19, 652-665. Rastegari, H., Ghaziaskar, H.S., Yalpani, M., Shafiei, A. 2017. Development of a Continuous System Based on Azeotropic Reactive Distillation to Enhance Triacetin Selectivity in Glycerol Esterification with Acetic Acid. Energy and Fuels, 31(8), 8256-8262. Reddy, C.R., Iyengar, P., Nagendrappa, G., Prakash, B.S.J. 2005. Esterification of dicarboxylic acids to diesters over Mn+- montmorillonite clay catalysts. Catalysis Letters, 101(1- 2), 87-91. Rothfuss, N.E., Petters, M.D. 2017. Influence of Functional Groups on the Viscosity of Organic Aerosol. Environmental Science and Technology, 51(1), 271-279. Sastri, S.R.S., Rao, K.K. 1992. A new group contribution method for predicting viscosity of organic liquids. The Chemical Engineering Journal, 50(1), 9-25. Tao, Y., Chen, B., Liu, L., Tan, T. 2012. Synthesis of trimethylolpropane esters with immobilized lipase from Candida sp. 99-125. Journal of Molecular Catalysis B: Enzymatic, 74(3-4), 151-155. Thompson, A.S., Owens, N.J.P., Murrell, J.C. 1995. Isolation and characterization of methanesulfonic acid-degrading bacteria from the marine environment. Applied and Environmental Microbiology, 61(6), 2388-2393. Wang, E., Ma, X., Tang, S., Yan, R., Wang, Y., Riley, W.W., Reaney, M.J.T. 2014. Synthesis and oxidative stability of trimethylolpropane fatty acid triester as a biolubricant base oil from waste cooking oil. Biomass and Bioenergy, 66, 371-378. Wu, Y., Li, W., Wang, X. 2015. Synthesis and properties of trimethylolpropane trioleate as lubricating base oil. Lubrication Science, 27(6), 369-379. Xie, Y., Ren, L., Zhu, X., Gou, X., Chen, S. 2018. Physical and chemical treatments for removal of perchlorate from water–A review. Process Safety and Environmental Protection, 116, 180-198. Xing, H., Wang, T., Zhou, Z., Dai, Y. 2005. Novel Brønsted-acidic ionic liquids for esterifications. Industrial and Engineering Chemistry Research, 44(11), 4147-4150.

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Yunus, R., Fakhru'l-Razi, A., Ooi, T.L., Omar, R., Idris, A. 2005. Synthesis of palm oil based trimethylolpropane esters with improved pour points. Industrial and Engineering Chemistry Research, 44(22), 8178-8183.

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CHAPTER 10. LIGNIN AS ADDITIVE IN PEG-BASED LUBRICANTS

Lignin as additive in PEG-based lubricants

Abstract New lignin-additivated PEG-based biolubricants with improved viscosity index are presented in this chapter. A commercially available Kraft lignin (KL) was incorporated into poly(ethylene glycol)s (PEGs) of different molecular weights (200 and 300 Da) by means of an ultrasound-assisted treatment.

Stable dispersions of PEG with a KL content up to a 40 wt.% were successfully prepared through this method. The chemical stability of both PEG and KL during the ultrasonication process was monitored by Fourier-Transform infrared (FTIR) spectroscopy, and no chemical modifications were observed upon ultrasonic treatment on the processed materials.

The measurements of the dynamic viscosity and the Ostwald-de Waele model revealed a deviation from a Newtonian behaviour towards a pseudoplastic one at temperatures below 30 °C and KL contents higher than 35 wt.% due to the temperature-reversible formation of KL aggregates. Finally, the kinematic viscosity of the lignin-containing lubricants at 40 and 100 °C was determined and used to calculate their viscosity index, which was found to undergo a maximum improvement of nearly 50% compared to neat PEG upon addition of an optimized amount of lignin.

The results of this chapter provide a direct demonstration of the potential of unmodified lignin as viscosity index improver to obtain fully biodegradable PEG- based lubricants in a straightforward and industrially-scalable process with a wider temperature application range.

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Resumen En este capítulo se han sintetizado varios biolubricantes basados en polietilenglicol (PEG) aditivados con lignina con un índice de viscosidad mejorado. Una lignina Kraft (KL) disponible comercialmente fue incorporada en PEG de diferentes pesos moleculares (200 y 300 Da) mediante un tratamiento de ultrasonido.

Mediante este método fue posible obtener dispersiones estables de PEG con contenidos en KL de hasta el 40 % m/m. La estabilidad química del PEG y de la KL durante el proceso de ultrasonicación fue monitorizada mediante espectrometría infrarroja por Transformadas de Fourier (FTIR) y no fueron observando cambios durante el tratamiento de los materiales.

Las medidas de la viscosidad dinámica y el modelo de Ostwald-de Waele mostraron una clara desviación del comportamiento Newtoniano hacia un comportamiento pseudoplástico a temperaturas inferiores a 30 °C y contenidos en KL superiores al 35 % m/m debido a la formación de agregados de KL. Finalmente, se determinó la viscosidad cinemática a 40 y 100 °C de los todos los lubricantes aditivados. Estos valores fueron usados para determinar el índice de viscosidad de los fluidos, encontrando una mejora de casi el 50 % en algunos casos con respecto al PEG sin aditivar.

Los resultados de este capítulo demuestran el gran potencial de la lignina como aditivo mejorador del índice viscosidad para obtener lubricantes completamente biodegradables, utilizables en un amplio rango de temperaturas y obtenidos mediante un proceso sencillo y escalable industrialmente.

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10.1. General background In some occasions, additives are added to the lubricants in order to enhance the lubricative characteristics of the final product. Rudnick classified such additives on the basis of their ultimate effects on the lubricant fluid in anti-oxidants, dispersants, friction modifiers, anti-wear, cold-properties modifiers and viscosity modifiers (Rudnick, 2009). Among the different viscosity modifiers, the ones affecting the viscosity-temperature relationship (usually named viscosity index improvers) are the most interesting ones for industrial applications. Viscosity index indicates the influence of temperature changes on the kinematic viscosity of the product. In lubricant applications, high a viscosity index is desired because it designates a low dependence of viscosity with a given temperature increment (Martini et al., 2018). This fact will enlarge the range of applications for a specific base fluid, making it more profitable from an industrial point of view.

During their lifetime, lubricants are normally expelled to the environment through leakages, blown pipes and due to human errors (Zainal et al., 2018). Within this context, it has been estimated that around 40 wt.% of lubricants end up annually in the environment (Willing, 2001). For this reason, complete biodegradability of lubricants (i.e. base fluids and additives) is highly desirable (Battersby, 2000). This new trend placed biomass, and specially lignin, in a good position as additive for lubricant formulations. For example, chemically-modified lignin has been incorporated into lubricant systems based on castor oil (Cortés-Triviño et al., 2018), linear-low-density poly(ethylene) (Nadji et al., 2009) and ionic liquids (Mu et al., 2016a). On the other hand, pristine lignin has only been successfully additivated into ethylene glycol (Mu et al., 2018) and choline-based ionic liquids (Mu et al., 2015), improving the lubricative characteristics of the base fluids in all cases. However, despite such great potential, lignin is still very underutilized at industrial scale and it is typically used as a low-cost fuel for energy generation (Brebu & Vasile, 2010).

Poly(ethylene glycol)s, usually named as PEGs, are a very important group of non- toxic, non-ionic, water-soluble and synthetic polymers. One of the most important properties of PEGs is their biodegradability. The complete mineralization of PEG can be easily done in real soils (Abdalla et al., 2005) and river waters if its molecular weight (MW) is below 400 Da (Zgoła-Grześkowiak et al., 2006). Nevertheless, PEGs

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Chapter 10 with a MW up to 20000 can also be successfully degraded using some specific bacteria (Kawai, 2002). Some of the most recent applications of these polymers are the synthesis of copolymers for drug delivery applications (Zhang et al., 2014), tissue engineering for regenerative medicine (Tessmar & Göpferich, 2007) and their use as solvent in different coupling reactions (Vafaeezadeh & Hashemi, 2015).

PEG is also commonly used as lubricant due to its outstanding properties in different applications. For example, PEG showed better results than ultra-high molecular weight poly(ethylene) as an artificial lubricant for the longevity of total knee joint prostheses (Kobayashi et al., 2014) and it was used for the synthesis of an injectable composite gel used in the treatment of soft tissue restoration (Lu et al., 2018). PEG has also been used as base-fluid for the dispersion of different friction-reducing and anti-wear additives, like β-cyclodextrin (β-CD) and dialkyl pentasulfide (DPS) (Guan et al., 2014), silver nanoparticles (Ghaednia et al., 2016; Zhang et al., 2009), graphene oxide (Gupta et al., 2017), and ionic liquids (Gusain et al., 2014).

Lignin has been successfully incorporated to PEG with a content up to a 15 wt.%, showing a wear reduction of 94.6 %. Nevertheless, the method proposed in literature to incorporate lignin into PEG involve the use of high temperature (140

°C) and H2SO4 as a promoter of the cleavage of lignin molecules (Mu et al., 2016b).

In addition, a neutralization process of H2SO4 with triethanolamine was required to avoid the corrosiveness of the final lubricant. On the other hand, the preparation of dispersions and solutions by ultrasound-assisted methods have demonstrated to be effective in a wide range of both aqueous (Heidari et al., 2018) and non-aqueous solvents (Perelshtein et al., 2008) and solid matrixes, like carbon spheres (Alazemi et al., 2015), CuO nanorods (Gusain & Khatri, 2013), and even lignin (Garcia Gonzalez et al., 2017; Jesionowski et al., 2014).

Hence, the main objective of this chapter is to obtain stable high-lignin-content PEG- based lubricants through a facile and innovative method that does not require any previous chemical modification of lignin. For this purpose, the technical viability of using an ultrasound-assisted dispersion method was tested. The chemical stability of both PEG and lignin was verified by spectroscopic analysis. The lubricants were characterized by means of dynamic viscosity, which allowed to determine the rheological response of the fluids. Also, the dependence of dynamic viscosity on

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Lignin as additive in PEG-based lubricants temperature was evaluated through the Arrhenius equation. Finally, changes of the viscosity index of the PEG-based lubricants as a function of lignin content was also evaluated in this chapter based on kinematic viscosity measurements.

10.2. Results 10.2.1. Fluid homogeneity The dispersion homogeneity in a lubricant is probably the most important requirement for additivated lubricating systems. In a first attempt to prepare the dispersions of lignin in PEG, a mechanical stirrer was used. Nevertheless, neither stable nor homogeneous PEG-based fluids could be obtained even after up to 24 h of stirring at high speed (>2000 rpm) and using lignin concentrations below 2 wt.%. Figure 10.1 shows the progressive sedimentation process during 35 days of a PEG200-based lubricant additivated with a 2 wt. % of lignin.

a) b) c) Figure 10.1. Physical aspect of the PEG200 fluid incorporating 2 wt.% of lignin using a mechanical stirrer a) freshly prepared b) 60 days after and c) 35 days after. In a second approach, the effect of ultrasonic treatment on the homogeneity and stability of the additivated system was tested. Ultrasonic dispersion (USD) is a popular preparation technique because it facilitates disintegration of aggregates into submicron particles through the use of a widespread array of ultrasonic equipment (ultrasonic baths, high-probe sonicators, etc.) (Kusters et al., 1993; Retamal Marín et al., 2017). By using a probe sonicator as described in the previous section, stable suspensions of PEG200 and PEG300 could be prepared containing up to a 40 wt.% of KL. Further increasing the amount of KL in the lubricant fluid led to

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Chapter 10 a macroscopic increase of viscosity of the additivated fluid with subsequent problems in controlling the overall volume during ultrasound treatment. Therefore, higher KL concentrations were not considered further. As an example of the stable products, Figure 10.2 shows the physical aspect of PEG200 containing 40 wt.% of KL freshly prepared (Figure 10.2a) and the same sample 60 days after its preparation (Figure 10.2b). As it can be observed, there were not significant differences among their physical appearance and no signals of sedimentation were detected. Differences in the volume of both vials are due to the extraction of samples to its characterization.

a) b) Figure 10.2. Physical aspect of the PEG200 fluid incorporating 40 wt.% of KL a) freshly prepared and b) 60 days after. The great improvement of the dispersion stability experienced upon changing the dispersion technique relates to the unique properties of USD based on cavitation, which occurs in highly intense sound fields. Cavitation is described as the formation of gas bubbles inside a liquid which grow up to a critical point, at which they become unstable and implode causing a mechanical stress to any near particle. This mechanical stress is also coupled with an increase in the temperature on the edge of the bubble, with both effects causing a very fast and efficient fragmentation of lignin particle agglomerates (Raso et al., 1999). The propensity of lignin to form aggregates has already been described in literature and it was measured by gel permeation

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Lignin as additive in PEG-based lubricants chromatography (Bylin et al., 2014) and by multiple angle light scattering (Contreras et al., 2008), being a major problem during its dissolution and filtration from the black liquors of the Kraft process (Kannangara et al., 2016). Deng et al. demonstrated that hydrogen bonding is not the main responsible of this behaviour but rather the π- π stacking of aromatic structures (Deng et al., 2012). Hence, it can be established that USD is much more efficient compared to traditional mechanical stirring methods for preparing stable PEG-based dispersions of KL.

10.2.2. Chemical stability during the USD As it was previously discussed, an increase of temperature in the additivated lubricant system during the USD is typically taking place due to the cavitation process. In spite of the dispersion process was performed in a refrigerated vessel, this local increase of temperature might be harmful for the chemical stability either of PEG or to the mixture PEG/KL. Hence, a first test was performed just applying the ultrasonic treatment to PEG200 (i.e. no lignin was incorporated during this experiment) for 8 hours, extracting several samples along the process. These samples were analysed by FTIR (Figure 10.3a).

The FTIR spectra showed as main signals the characteristic bands of hydroxyl groups between 3630 and 3150 cm-1, the symmetric CH2 stretching vibrations at

2970-2745 cm-1 and the ether linkage CH2-O-CH2 between 1100 and 1076 cm-1.

Some other less intense signals like the in-plane bending of CH2 at 1465 cm-1, the in- plane scissoring of CH2 at 1341 cm-1, the asymmetric C-O-C stretching at 1279 cm-1 and the C-CH aliphatic deformation vibration at 841 cm-1 could also be observed (Pramanik et al., 2015). Nevertheless, the most important observation is that neither the signals nor their relative intensity changes during the USD treatment, ensuring that the chemical structure of PEG200 did not change along the process. In addition, the same experiment was repeated adding a 1 wt.% of KL to PEG200 to check if the presence of some other functional groups contained in lignin (phenolic and aliphatic hydroxyl groups, carboxylic acids, aryl-alkyl ethers, etc.) or the possible formation of radical species might affect its structure. The FTIR analyses performed on these samples (Figure 10.3b) presented the same absorption bands that the previous test because of the low concentration of KL. Once again, no differences on the spectra with time were appreciated during the USD process on the mixture of PEG200 and

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KL. Hence, it can be concluded that the dispersion process performed rely uniquely on physical aspects, namely the size reduction of lignin aggregates.

PEG-200 @0 wt.% KL

Time of USD (h)

841

3400

2865

1060 1279 8 1465 4 2 1

Intensity (a.u.) Intensity

4000 3500 3000 2500 2000 1500 1000 Wavelegth (cm-1)

b) PEG-200 @1 wt.% KL

Time of USD (h)

841

1060

3400

1465 2865 8 1279 4 2 1

Intensity (a.u.) Intensity

4000 3500 3000 2500 2000 1500 1000 -1 Wavelength (cm ) Figure 10.3. Evolution of FTIR spectra of a) PEG200 and b) PEG200 with a 1 wt.% of KL during 8 h under the USD process.

10.2.3. Dynamic viscosity measurements The dynamic viscosity of lubricants is another characteristic parameter that affects their application performance and it is defined as the resistance of a liquid against fluid when a tangential force is applied. To determine it, the dependence of shear stress on the shear rate was determined for both PEG200 (Figure 10.4) and PEG300 (Figure 10.5) with KL contents up to 40 wt.% at 10, 20, 30, 40 and 50 °C.

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5500 a) 60 b) 2400 30 5000 50 40 2100

30 20

4500

4000 10 1800 10 0 3500 0 20 40 60 80 100 120 140 0 1500 3000 0 20 40 60 80 100 120 140

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2000 900 Shear stress (Pa) stress Shear Shear stress (Pa) stress Shear 1500 KL content (wt.%) 600 KL content (wt.%) 1000 0 1 0 1 5 10 5 10 300 500 20 30 20 30 35 40 35 40 0 0 0 100 200 300 400 500 600 0 100 200 300 400 500 600

-1 -1 Shear rate (s ) Shear rate (s )

c) d) 1050 14 500 12 900 8

400 4 750 0 0 0 20 40 60 80 100 120 140 600 300 0 20 40 60 80 100 120 140

450

200 Shear stress (Pa) stress Shear Shear stress (Pa) stress Shear 300 KL content (wt.%) KL content (wt.%) 0 1 0 1 100 150 5 10 5 10 20 30 20 30 35 40 35 40 0 0 0 100 200 300 400 500 600 0 100 200 300 400 500 600

-1 -1 Shear rate (s ) Shear rate (s )

e) 300 10

250

200 0

0 20 40 60 80 100 120 140

150

100 Shear stress (Pa) stress Shear KL content (wt.%) 0 1 50 5 10 20 30 35 40 0

0 100 200 300 400 500 600 -1 Shear rate (s )

Figure 10.4. Influence of the shear rate on the shear stress for the KL-additivated systems based on PEG200 at a) 10 °C b) 20 °C c) 30 °C d) 40 °C and e) 50 °C.

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a) b) 5500 KL content (wt.%) 12000 90 KL content (wt.%) 0 1 0 1 40 5000 5 10 60 5 10

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c) 2700 d) KL content (wt.%) KL content (wt.%) 1200 15 0 1 2400 25 0 1 5 10 20 5 10 10 20 30

2100 15 20 30 1000

35 40

35 40 10 5 1800 5 800 0 0 1500 0 20 40 60 80 100 120 140 160 0 20 40 60 80 100 120 140 160

600

1200

900 400

Shear stress (Pa) stress Shear Shear stress (Pa) stress Shear

600 200 300

0 0 0 100 200 300 400 500 600 0 100 200 300 400 500 600 -1 -1 Shear rate (s ) Shear rate (s )

e) 700 12 KL content (wt.%) 0 1 600 9 5 10

6 20 30

35 40 500 3

0 400 0 20 40 60 80 100 120 140 160

300

Shear stress (Pa) stress Shear 200

100

0 0 100 200 300 400 500 600 -1 Shear rate (s ) Figure 10.5. Influence of the shear rate on the shear stress for the KL-additivated systems based on PEG300 at a) 10 °C b) 20 °C c) 30 °C d) 40 °C and e) 50 °C. Two very different behaviours could be observed in both additivated PEG200-based and PEG300-based lubricants. At low temperatures (up to 30 °C) and high KL contents (over 30 wt.%), a clear visual deviation from a linear response (i.e. non- Newtonian behaviour) was registered. In the remaining products, a Newtonian performance was apparently recorded.

In order to determine analytically when the fluids behaved as Newtonian or non- Newtonian, the Ostwald–de Waele equation was used (Wan Nik et al., 2005). This equation is commonly written as indicated in Equation 10.1.

휏 = 푘 · 훾푛 (10.1)

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Lignin as additive in PEG-based lubricants

Where τ is the shear stress, γ the shear rate, k the consistency index and n the flow behaviour index. Depending on the value of n, the fluids can be divided into pseudoplastic fluids (or shear-thinning fluids) if n < 1, Newtonian fluids if n = 1 and dilatant fluids (or shear-thickening fluids) if n > 1.

The previously obtained shear stress and shear rate values were fitted with the Ostwald–de Waele equation, obtaining a correlation coefficient (R2) close to one in all cases and flow behaviour index of 1 or slightly lower. This indicated that all the KL-additivated lubricants presented exclusively either a Newtonian or a pseudoplastic behaviour and that some other different deviations from the Newtonian behaviour (e.g. dilatant fluids or Bingham plastics) should not be considered. The results from the adjustment can be observed in Table 10.1 for PEG200-based fluids and Table 10.2 for the PEG300-based ones.

Figure 10.6 shows the dependence of the flow behaviour index on temperature and KL content using PEG200 (Figure 10.6a) and PEG 300 (Figure 10.6b) as base fluids.

1.05 a)

1.00

Temperature (ºC) 0.95 10 20

0.90 50 Flow behaviour index behaviour Flow 0.85

0.80 0 5 10 15 20 25 30 35 40 KL content (wt. %) b) 1.05 1.00

0.95 Temperature (ºC) 10 0.90 20 30 40 0.85 50

0.80

Flow behaviour index behaviour Flow 0.75

0.70 0.65 0 5 10 15 20 25 30 35 40 KL content (wt. %) Figure 10.6. Dependence of the flow behaviour index with the KL content for a) PEG200 and b) PEG300.

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Table 10.1. Adjustment parameters for PEG200-based lubricants to the Ostwald- de Waele equation. Temperature KL content k n R2 (°C) (wt.%) 0 0.107 1.027 0.996 1 0.120 1.019 0.997 5 0.192 1.007 0.999 10 0.397 1.000 0.999 10 20 2.482 0.994 0.999 30 3.952 0.988 0.999 35 9.308 0.895 0.998 40 26.35 0.822 0.995 0 0.0472 1.011 0.986 1 0.0548 1.009 0.987 5 0.097 1.016 0.995 10 0.193 1.012 0.999 20 20 1.021 0.999 0.999 30 1.550 0.996 0.999 35 2.659 0.955 0.999 40 5.869 0.924 0.998 0 0.0190 1.033 0.985 1 0.0225 1.008 0.986 5 0.0472 1.004 0.985 10 0.106 1.026 0.995 30 20 0.488 1.001 0.999 30 0.707 1.000 0.999 35 0.945 0.997 0.999 40 1.946 0.968 0.999 0 0.00961 1.009 0.985 1 0.0104 1.005 0.982 5 0.0181 1.015 0.987 10 0.0549 1.009 0.986 40 20 0.259 1.005 0.999 30 0.361 1.006 0.999 35 0.467 1.000 0.999 40 0.878 0.980 0.999 0 0.00471 1.006 0.986 1 0.0054 1.029 0.987 5 0.0115 1.025 0.980 10 0.0258 1.014 0.979 50 20 0.194 0.990 0.967 30 0.202 1.012 0.999 35 0.287 0.987 0.993 40 0.402 1.008 0.999

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Lignin as additive in PEG-based lubricants

Table 10.2. Adjustment parameters for PEG300-based lubricants to the Ostwald- de Waele equation. Temperature KL content k n R2 (°C) (wt.%) 0 0.156 1.012 0.998 1 0.190 1.006 0.999 5 0.276 1.007 0.999 10 0.629 0.988 0.999 10 20 2.552 0.947 0.999 30 14.212 0.867 0.997 35 16.498 0.856 0.996 40 110.684 0.687 0.980 0 0.0803 1.027 0.992 1 0.095 1.016 0.994 5 0.144 1.014 0.998 10 0.295 1.000 0.999 20 20 0.813 1.008 0.999 30 3.682 0.948 0.999 35 3.999 0.947 0.999 40 17.802 0.858 0.997 0 0.0358 1.008 0.984 1 0.0481 1.002 0.984 5 0.0753 1.010 0.991 10 0.156 1.013 0.998 30 20 0.424 0.997 0.999 30 1.460 0.969 0.999 35 1.414 0.967 0.999 40 4.999 0.933 0.999 0 0.0177 1.026 0.984 1 0.0196 1.020 0.986 5 0.0374 1.027 0.981 10 0.0931 1.029 0.992 40 20 0.177 1.024 0.999 30 0.664 0.986 0.999 35 0.753 0.974 0.998 40 1.875 0.969 0.999 0 0.00948 1.004 0.977 1 0.00999 1.013 0.987 5 0.0167 1.014 0.983 10 0.0469 1.007 0.985 50 20 0.0841 1.006 0.988 30 0.444 0.989 0.998 35 0.328 1.019 0.997 40 0.927 0.975 0.999

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Chapter 10

According to the Ostwald–de Waele equation, a pseudoplastic behaviour can be established if the flow behaviour index is below 1. In the case of PEG200 containing no KL, the dynamic viscosity measurement and the fitting with the Ostwald–de Waele equation was repeated 5 times and the flow index behaviour index was determined individually in all cases. The obtained values are shown in Table 10.3.

Table 10.3. Determined flow behaviour indexes for PEG200. Test Flow behaviour number index 1 1.069 2 0.99 3 0.992 4 1.065 5 1.024

The standard deviation (σ) of the flow behaviour indexes was determined according to Equation 10.2.

∑푛 (푥 −푥̅)2 휎 = √ 푖=1 푖 (10.2) 푁−1

Where σ is the standard deviation, xi the observed values, x the mean value and N the number of observations. A value of σ = 0.0411 was determined. This value represents the 4.0 % of the mean value and so this error was considered for the whole method. Hence, a marginal gap of the flow behaviour index from 0.96 to 1.04 (represented in the figures by dashed lines) was assumed when determining or not a deviation from the Newtonian behaviour. A pseudoplastic behaviour for the additivated lubricants was confirmed when the temperature was below 30 °C and the KL content was over 35 wt.% for PEG200 and over 20 wt.% for PEG300. The possible reasons for this change are related to the modification of the intermolecular forces at low temperatures and high contents of KL. The balance of acting forces (e.g. Van-der-Waals interaction, electrostatic interaction, viscous resistance, Brownian motion, etc.) between the different molecules of PEG and KL determines when the two phases in the system move independently (well dispersed KL molecules) or move together (agglomerates of KL). At high concentrations of KL, the formation of stable and randomly aligned agglomerates, which has been discussed and justified

212

Lignin as additive in PEG-based lubricants in the previous section, restricts the flow of PEG and, therefore, increases its dynamic viscosity. Nevertheless, by increasing the shear rate of the fluid, KL molecules start to get aligned in the direction of flow and a shear-thinning effect can be observed. This same behaviour has been previously described and analysed for other additives, like carbon nanotubes (Eken et al., 2012; Richter et al., 2009), and tungsten-based powder injection moulding feedstock (Suri et al., 2003). The progressive alignment phenomenon is also related to the thixotropic behaviour found in these cases (i.e. changes over time of the shear stress at a fixed shear rate). In these situations, the more time the fluid is under the shear force, the more aligned the KL molecules become and the lower the viscosity of the fluid.

Another approach to avoid the agglomeration of KL could be to increase the temperature of the fluid. Under these conditions, the KL molecules will move more freely and the formation of aggregates will not be favoured. This situation was also observed experimentally since the fluids that presented a strong non-Newtonian behaviour at low temperatures (e.g. PEG200-based fluids with a KL content over 30 wt.% at 10 and 20 °C) changed into Newtonian fluids at temperatures over 30 °C.

The consistency index is defined as the ratio between the dynamic viscosity of the fluid and the shear rate. As the consistency index decreases, the fluid becomes less viscous. Figure 10.7 shows the variation of the dynamic viscosity index with the temperature and the KL content for PEG200 (Figure 10.7a) and PEG300 (Figure 10.7b).

It can be seen from the figures that the consistency index is influenced by both the temperature and the particle concentration. The consistency index of the fluids increases with the KL concentration and decreases with an enhancement of temperature, independently of the base fluid used. Nevertheless, the impact of both variables on the consistency index was more marked when PEG300 was used, indicating that the apparent dynamic viscosity of PEG300 is more sensitive to the incorporation of KL. For instance, when the KL content was increased from 0 to 40 wt.% at a fixed temperature of 10 °C, the consistency index enhancement for PEG200 and PEG300 was 26.2 and 110.5, respectively. It is also remarkable that the impact of temperature on the consistency index was very low for both PEGs when the KL content was below 10 wt.%. As it was previously explained, the basis of this

213

Chapter 10 phenomenon is related with the formation of KL agglomerates at high concentrations and low temperatures. This same effect has been previously reported in literature using different additives for lubricant applications, such as polyaniline (Cao et al., 2018), SiO2 (Sui et al., 2018) and clay nanotubes (Peña-Parás et al., 2017), and even for the dispersion of lignin in other fluids for different applications like cutting fluids (Zhang & Jun, 2015), or the synthesis of biobased composites (Chung et al., 2013).

a) 30 Temperature (ºC) 10

25 20 7 30 6 40 5 50 20

3 2 15

0 0 5 10 15 20 25 30 35 40

10 Consistency index index Consistency

0 10 20 30 40 KL content (wt.%) b) 120 Temperature (ºC) 10 100 18 20 30 15 40 80 12 50

6 60 3 0 0 10 20 30 40

40 Consistency index index Consistency

0 0 10 20 30 40 KL content (wt.%) Figure 10.7. Dependence of the consistency index with the KL content for a) PEG200 and b) PEG300 at different temperatures.

214

Lignin as additive in PEG-based lubricants

As it was shown, the consistency index of these fluids, and therefore their dynamic viscosity, is highly dependent on temperature. This relation is quantifiable and can be described using the Arrhenius equation (Equation 10.3).

퐸푎 휂 = 퐴 · 푒 ⁄푅·푇 (10.3)

Where η is the dynamic viscosity, A is the pre-exponential factor, Ea is the activation energy, R is the gas constant and T is the temperature. It is worth pointing out that the dynamic viscosity of pseudoplastic fluids used for the fitting was that found in the region where no dependence on the shear rate was observed (shear rate > 550 s-1), as it was previously recommended in literature for non-Newtonian fluids (Quinchia et al., 2009). The dynamic viscosity values used for the fitting are shown in the Table 10.4.

Table 10.4. Viscosity values used for the Arrhenius fitting. Temperature (°C) KL content 10 20 30 40 50 (wt.%) 0 0.120 0.077 0.058 0.043 0.034 1 0.130 0.082 0.060 0.046 0.035 5 0.198 0.113 0.075 0.058 0.043 10 0.398 0.204 0.117 0.080 0.061 PEG200 20 2.341 1.006 0.490 0.266 0.160 30 3.672 1.515 0.712 0.373 0.214 35 4.610 1.983 0.936 0.469 0.267 40 8.042 3.554 1.607 0.784 0.421 0 0.163 0.097 0.070 0.054 0.040 1 0.195 0.113 0.075 0.058 0.046 5 0.286 0.154 0.096 0.070 0.057 10 0.597 0.295 0.166 0.104 0.075 PEG300 20 1.818 0.831 0.415 0.248 0.141 30 6.207 2.688 1.240 0.627 0.338 35 6.800 2.893 1.313 0.660 0.360 40 15.247 7.347 3.342 1.582 0.824

Then, the ln(η) vs 1/T was plotted for PEG200 (Figure 10.8a) and PEG300 (Figure

10.8b) and the values of A and Ea were calculated and shown in Table 10.5 together with correlation coefficients (R2).

215

Chapter 10

a) KL content (wt.%) 2 0 1 5 10 Equation y = a + b*x 20 30 Adj. R-Square 0.99177 0.98932 0.97879 0.9775 0.99551 0.99628 0.99838 0.99956 1 Value Standard Error 35 40 0 Intercept -12.25118 0.43007 0 Slope 2853.79419 129.88499 1 Intercept -12.43245 0.50257 1 Slope 2925.05627 151.78052 0 5 Intercept -13.79514 0.83175 5 Slope 3421.67957 251.19516 10 Intercept -16.27404 1.08128

10 Slope 4316.87134 326.55695  20 Intercept -20.92513 0.68261 ln ln -1 20 Slope 6144.14636 206.15395 30 Intercept -21.71136 0.65637 30 Slope 6494.57205 198.22873 35 Intercept -21.61764 0.43654 35 Slope 6540.01829 131.83847 -2 40 Intercept -21.88305 0.23535 40 Slope 6780.55536 71.078

-3

0.0031 0.0032 0.0033 0.0034 0.0035 0.0036 1/T (K-1) 3 b) KL content (wt.%) 0 1 2 5 10 20 30 35 40 1

 ln ln -1

-2

-3

0.0031 0.0032 0.0033 0.0034 0.0035 0.0036 1/T (K-1) Figure 10.8. Linear dependence of ln(η) with 1/T for the KL-additivated fluids of a) PEG200 and b) PEG300. Table 10.5. Fitting constants and correlation coefficients of Arrhenius equation.

KL content A Ea Base fluid R2 (wt.%) (Pa·s) (KJ/mol) 0 4.779E-06 23.728 0.992 1 3.987E-06 24.320 0.989 5 1.020E-06 28.449 0.979 10 8.556E-08 35.892 0.977 PEG200 20 8.172E-10 51.085 0.996 30 3.722E-10 53.998 0.996 35 4.088E-10 54.376 0.998 40 3.135E-10 56.376 0.999 0 2.572E-06 25.850 0.983 1 1.617E-06 27.316 0.973 5 5.196E-07 30.843 0.965 10 2.692E-08 39.599 0.986 PEG300 20 2.207E-09 48.179 0.997 30 3.642E-10 55.373 0.999 35 3.050E-10 55.987 0.999 40 7.206E-10 56.026 0.999

216

Lignin as additive in PEG-based lubricants

As can be observed, a good correlation (R2 > 0.96) was found in all cases, indicating that the Arrhenius equation is valid for predicting the dependence of the viscosity on temperature at steady state conditions of high shear rates for the KL-additivated PEG lubricants. The mobility of the molecules of the fluid is a temperature-activated process, and therefore related to the activation energy. This indicates that the higher the Ea, the higher the energy requirement for the flow of the lubricant (Torres et al.,

2004). In the case of KL-additivated PEG, an increase on the Ea was observed when increasing the KL content, independently of the molecular weight of the base fluid. This indicates, as expected, that the fluids containing high amounts of KL require a higher energy to be fluidized and hence present a higher apparent dynamic viscosity. A similar trend for the Ea was found in the case of high-oleic sunflower oil additivated with different polymers (Quinchia et al., 2009), for starch-filled poly(hydroxy ester ether) (Zhou et al., 2000) and coal-water slurries (Mishra et al., 2002).

10.2.4. Viscosity index determination

The viscosity index (VI) of a lubricant is a dimensionless number that describe its resistance to a change in kinematic viscosity when subjected to a change in temperature. The higher the VI, the lower the variation of kinematic viscosity with temperature, the wider the application temperature window for the given lubricant. This important characteristic is habitually determined by comparing the kinematic viscosity of lubricants at 40 and 100 °C. In contrast to dynamic viscosity, kinematic viscosity measures the resistance of a fluid to flow exclusively under the force of gravity. Table 10.6 shows the kinematic viscosities at 40 and 100 °C of all the synthesized KL-additivated PEG-based lubricants.

The values obtained for pristine PEG200 and PEG300 are in good agreement with the ones previously described (Cruz et al., 2000). Like in the case of dynamic viscosity, the higher the KL-content, the higher the kinematic viscosity, independently of the base fluid and the temperature, reaching values up to 1714.23 cSt for PEG300 with a content of KL of 40 wt.% at 40 °C. By comparing the additivated PEG200 and PEG300 at the same KL concentration, a higher viscosity was measured for PEG300-based fluids due to its higher molecular weight. It is also

217

Chapter 10 interesting to note that KL at moderate contents (between 5 and 10 wt.%) have a larger effect on the kinematic viscosity at 100 °C than at 40 °C.

Table 10.6. Kinematic viscosity of KL-additivated lubricants at 40 and 100 °C. KL content Kinematic viscosity, Kinematic viscosity, Base fluid (wt.%) 40 °C (cSt) 100 °C (cSt) 0 22.69 4.12 1 24.03 4.28 5 33.45 5.28 10 53.46 7.34 PEG200 20 71.13 8.80 30 319.15 22.79 35 447.99 28.07 40 789.26 38.22 0 29.86 5.41 1 33.17 5.76 5 45.86 7.46 10 61.90 8.72 PEG300 20 202.32 18.30 30 264.70 21.85 35 612.12 36.02 40 1714.23 66.44

VI values were calculated from the kinematic viscosities at 40 and 100 °C following the procedure previously indicated and were plotted against the KL content for both PEG200 and PEG300 (Figure 10.9).

130 125 PEG200 120 PEG300 115 110 105 100

90 85

80 Kinematic viscosity (cSt) Kinematicviscosity 75 70 65

0 5 10 15 20 25 30 35 40 KL content (wt.%) Figure 10.9. Variation of VI with the KL content of the PEG-based fluids.

218

Lignin as additive in PEG-based lubricants

As can be observed from Figure 10.9, the VI of the lubricants is clearly affected by KL content. In the case of both PEG200 and PEG300 a maximum in the VI was found, demonstrating the potential use of KL as a viscosity index modifier of biodegradable lubricant fluids. In particular, in the case of PEG200 the VI was increased from 65 to 96 (47 %), meanwhile for PEG300 a measurable enhancement from 113 to 127 (12 %) was determined. This increase on the VI indicates that PEG and PEG-based lubricants would utilizable in some applications where a higher temperature is reached, without losing it complete biodegradability. The most common mechanism on how random polymers like KL affect the VI is related to the expansion of the polymer molecules when increasing the temperature. As referred by Flory, the radius of gyration of polymer molecules varies with the interaction between other polymer molecules and the solvent (PEG in our case) itself (Flory, 1953). In the presence of bad solvents, the attractive forces between polymer segments dominate, causing a collapse of the polymer chains into compact polymer aggregates. On the other hand, when a good solvent is used, intermolecular repulsive forces govern, resulting in an expansion of the aggregate and forming random coils. Einstein modelled the increase on viscosity caused by adding ideal spherical particles to a liquid, being exclusively proportional to the volume fraction of spheres (Einstein, 1906). Hence, by increasing the volume fraction (i.e. passing from aggregates to coils) of the polymer, a bigger increment on the viscosity is expected. In some cases, like in the present work, the transition from a bad to a good solvent situation can be achieved by increasing the temperature, modifying the polymer chain structure and therefore the kinematic viscosity (Figure 10.10).

Low High Temperature

Figure 10.10. Schematic representation of the increment volume fraction of polymer chains by increasing the temperature according to Einstein’s model.

219

Chapter 10

10.3. Conclusions

New PEG-based lubricants with high KL content were developed and presented in this chapter. These systems were obtained without any chemical modification of either the KL or PEG, using exclusively a physical dispersion method based on the use of ultrasounds. This technique allowed the incorporation of up to 40 wt.% of KL in both PEG200 and PEG300, obtaining stable dispersions that did not sediment after over 60 days after their preparation. The lubricants were characterized through rheometric measurements and their Newtonian or non-Newtonian behaviour was determined by the Ostwal-de Waele equation. A significant deviation from a Newtonian to a pseudoplastic behaviour was observed for KL contents higher than 30% and temperatures below 30 °C due to the formation of lignin aggregates under those conditions. The Arrhenius equation was found to be appropriate to study the dependence of the dynamic viscosities of the lubricants on temperature, finding higher activation energies for higher KL contents. Finally, the kinematic viscosity of the PEG-based lubricants at 40 and 100 °C was determined and used to calculate their VI. A significant improvement in VI of 47 % and 12 % was found for PEG200- and PEG300-based fluids when incorporating 10 and 5 wt.% of KL, respectively. This indicates that the range of applications for PEG and PEG-based lubricants can be increased to those more exigent in terms of temperature, without losing the full biodegradability of the final product. Hence, the results of this study provide a clear demonstration of an easy and industrially-scalable process to incorporate lignin as viscosity index improver in PEG-based lubricants with a 100 % biodegradability.

220

Lignin as additive in PEG-based lubricants

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Sui, T., Ding, M., Ji, C., Yan, S., Wei, J., Wang, A., Zhao, F., Fei, J. 2018. Dispersibility and rheological behavior of functionalized silica nanoparticles as lubricant additives. Ceramics International, 44(15), 18438-18443. Suri, P., Atre, S.V., German, R.M., de Souza, J.P. 2003. Effect of mixing on the rheology and particle characteristics of tungsten-based powder injection molding feedstock. Materials Science and Engineering A, 356(1-2), 337-344. Tessmar, J.K., Göpferich, A.M. 2007. Customized PEG-derived copolymers for tissue- engineering applications. Macromolecular Bioscience, 7(1), 23-39. Torres, E., Dutta, N., Choudhury, N.R., Matisons, J. 2004. Effect of composition on the solution rheology of stearyl methacrylate-co-styrene-co-vinyl pyrrolidinone in paraffinic base oil. Polymer Engineering and Science, 44(4), 736-748. Vafaeezadeh, M., Hashemi, M.M. 2015. Polyethylene glycol (PEG) as a green solvent for carbon-carbon bond formation reactions. Journal of Molecular Liquids, 207, 73-79. Wan Nik, W.B., Ani, F.N., Masjuki, H.H., Eng Giap, S.G. 2005. Rheology of bio-edible oils according to several rheological models and its potential as hydraulic fluid. Industrial Crops and Products, 22(3), 249-255. Willing, A. 2001. Lubricants based on renewable resources – an environmentally compatible alternative to mineral oil products. Chemosphere, 43(1), 89-98. Zainal, N.A., Zulkifli, N.W.M., Gulzar, M., Masjuki, H.H. 2018. A review on the chemistry, production, and technological potential of bio-based lubricants. Renewable and Sustainable Energy Reviews, 82, 80-102. Zgoła-Grześkowiak, A., Grześkowiak, T., Zembrzuska, J., Łukaszewski, Z. 2006. Comparison of biodegradation of poly(ethylene glycol)s and poly(propylene glycol)s. Chemosphere, 64(5), 803-809. Zhang, K., Tang, X., Zhang, J., Lu, W., Lin, X., Zhang, Y., Tian, B., Yang, H., He, H. 2014. PEG-PLGA copolymers: Their structure and structure-influenced drug delivery applications. Journal of Controlled Release, 183(1), 77-86. Zhang, M., Wang, X., Fu, X., Liu, W. 2009. Investigation of electrical contact resistance of Ag nanoparticles as additives added to peg 300. Tribology Transactions, 52(2), 157-164. Zhang, Y., Jun, M.B.G. 2015. Feasibility of lignin as additive in metalworking fluids for micro-milling. Journal of Manufacturing Processes, 16(4), 503-510.

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CHAPTER 11. CONCLUSIONS AND FUTURE WORK

Conclusions and future work

11.1. Conclusions From the present research work related to the development of biobased materials from renewable resources, the following conclusions can be drawn.

• The epoxidation of grape seed oil by peracetic acid formed in situ from acetic acid and hydrogen peroxide is a good technique for the obtention of a platform chemical with a wide range of applications. A temperature of 90 ºC and a reaction time of 1 h was chosen as ideal conditions to avoid secondary reactions • The DBSA is an appropriate amphiphilic acid catalyst for the hydrolysis of vegetable oils, reaching conversions close to 100 % at moderate conditions. The response surface methodology indicated that temperature is much more significant than the other variables studied (time and the molar ratios

DBSA:TG and H2O:TG). • The ring-opening reaction of oxirane groups of epoxidized grape seed oil in

presence of NaN3 and H3PO4 is a good technique for the obtention of polyols with improved thermal stability. This thermal behaviour is transferred to the rigid polyurethane foams obtained from them, shifting the maximum degradation temperature and increasing the residue after the thermal degradation in both cases. • A full conversion of oleic acid into different high-added value products is feasible by combining the synthesis of estolides and the further esterification of unreacted carboxylic group. Also, the traces of -COOH groups after the esterification can be valorised by its transformation into choline carboxylates neutralized using a methanolic solution of choline hydroxide. The separation of the products can be performed adequately by molecular distillation, obtaining a lubricant and a biodiesel with excellent properties. • The synthesis of a high-quality TMP-based biolubricant via esterification is highly dependent on the OA:TMP molar ratio due to the wide variety of possible intermediate reaction products. At an OA:TMP molar ratio of 3, the only by-product is the unreacted OA. The neutralization of this unreacted OA in presence of choline hydroxide allowed the obtention of a high-quality product with a viscosity index of 209 and a pour point of -70 °C.

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• The incorporation of lignin as additive into PEG200 and PEG300 is feasible until a lignin content of 40 wt.%. These lubricants reported a marked deviation from the Newtonian behaviour at low temperatures and high concentrations of lignin, indicating the formation of aggregates at these conditions. Moreover, lignin demonstrated to have a possitve effect on the viscosity index of these based fluids.

11.2. Future work.

The following recommendations are interesting for future research:

• Extend the epoxidation model to other vegetable oils with a different unsaturation grade • Study the influence of other operative variables on the hydrolysis of vegetable oils using DBSA as catalyst, namely the agitation rate, the stirrer shape and the influence of microwave radiation • Perform the ring-opening reaction in presence of different products to obtain functionalized polyols with alternative applications. • Tribological characterization of the lubricants to study its applicability in different ranges.

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