15. Thermosets from Renewable Sources

15 Biobased Thermosets

Ana Dotan Pernick Faculty of Engineering, Shenkar College of Engineering and Design, Ramat-Gan, Israel

OUTLINE

Polymers from Renewable Sources 577 Thermoset from Renewable Sources 594 Determination of Bio-Based Content in References 615 Polymers 578 Raw Materials for Renewable Sources Polymers 579

Polymers from Renewable Bio-based polymers are derived from renewable Sources resources such as plant and animal mass from CO2 recently fixed via photosynthesis [2]. Bio-based poly- In the past few decades concern about the environ- mers can be natural or synthetic. Natural bio-based ment, climate changes, and limited fossil resources polymers are polymers synthesized by living organ- has led to an intensive research of alternatives to isms such as animals, plants, algae, and microorgan- fossil-based polymers. From a historical point of isms. The most abundant bio-based polymers in view, the first polymers used by mankind were from nature are polysaccharides [3]. Cellulose and starch renewable sources, bio-based polymers, long before are natural polymers based on polysaccharides, and the birth of synthetic polymers. Celluloid, a bio- are abundant in nature. Proteins and bacterial polyhy- based man-made polymer was invented in the 1860s, droxyalkanoates are also natural bio-based polymers and since then, many other bio-based polymers have [4]. Bio-based polymers are not necessarily sustain- been developed. However, the development of the able; this depends on a variety of issues, including crude oil industry in the 20th century transformed the the source material, production process, and how the world of polymers, leading to the use of synthetic material is managed at the end of its useful life. Sus- polymers as a replacement for bio-based polymers. tainable produced bio-based polymers are those The increasing use of synthetic polymers as a result grown without genetically modified organisms of the growing human population and standard of liv- (GMOs), hazardous pesticides, certified as sustain- ing in the next decades will result in higher demands able for the soil and ecosystems, and compostable. on oil production and will contribute to a possible Sustainability also depends on the reduction of depletion of crude oil before the end of the 21st cen- impacts to occupational and public health as well as tury. It is estimated that by 2015 the worldwide the environment throughout their life cycles [2].Bio- annual production of plastics is very likely to reach based materials are important candidates for sustain- 300 million tons (1418% are thermosets), which able development since they present the potential to will require large amounts of petroleum and will reduce greenhouse emissions by sequestering CO2,to result in the emissions of hundreds of millions of tons reduce raw material costs, and to create opportunities of CO2 to the atmosphere [1]. A return to bio-based for growth and employment in agriculture [5]. polymers will reduce the dependency of the polymers Life cycle assessment (LCA) is the most widely and plastics industry on petroleum, thus creating applied and accepted method to quantitatively assess more sustainable alternatives. the environmental impact of a given material

Handbook of Thermoset Plastics. DOI: http://dx.doi.org/10.1016/B978-1-4557-3107-7.00015-4 © 2014 Elsevier Inc. All rights reserved. 577 578 HANDBOOK OF THERMOSET PLASTICS throughout its life cycle; its principles and frame- carbon dating), because ancient petroleum has lost work are described in ISO 14040 [6]. Usually the its 14C through radioactive decay whereas feedstock impact categories considered in LCA are global derived from recently living organisms have a 14C warming, acidification of soil, ozone layer depletion, content related to the current equilibrium concen- aquatic eutrophication, respiratory organics, respira- tration in the atmosphere. tory inorganics, land occupation, non-renewable ASTM D6866-11 has developed a protocol to energy, and aquatic ecotoxicity [7]. Additional envi- quantify the bio-based content in materials by com- ronmental indicators are resource depletion and paring the 14C/12C ratio to that of a standard speci- human toxicity [5]. men typical of living organisms [11]. CEN/TS Global warming impact is measured by the 16137 is the equivalent European for the ASTM amount of CO2 that is liberated from the material standard test [3]. ASTM D6866-11 is the Standard throughout its life cycle. CO2 is the principal anthro- Test Methods for Determining the Bio-based pogenic gas that is thought to affect the Earth’s radia- Content of Solid, Liquid, and Gaseous Samples tive balance. For this reason it is believed that there Using Radiocarbon Analysis. It defines bio-based is a close correlation between CO2 and the change of content as the amount of bio-based carbon in the the Earth’s temperature [8].TheCO2 footprint of a material or product as a percent of the weight material is assessed by the carbon emissions conse- (mass) of the total organic carbon in the product. quent on the creation of a unit mass of material, This standard utilizes two methods to quantify the including those associated with transport, generation bio-based content of a given product: (a) Accelera- of the electric power used by the plant, and that of tor Mass Spectrometry (AMS) along with Isotope feedstocks and hydrocarbon fuels. The CO2 footprint, Ratio Mass Spectrometry (IRMS); or (b) Liquid which is measured in units of kg of CO2/kg material, Scintillation Counters (LSC) using sample carbon is the sum of all contributions per unit mass of mate- that has been converted to benzene. Those methods rial. Renewable resource polymers have a low CO2 directly discriminate between product carbon result- footprint since plants grow by absorbing CO2 from ing from new carbon input and that derived from the atmosphere and thus sequester carbon [9]. fossil-based input. A measurement of the 14C/12C ratio is determined relative to the modern carbon- based oxalic acid radiocarbon Standard Reference Material (SRM) 4990c (referred to as HOxII), as Determination of Bio-Based the oxalic acid standard is 100% bio-based. The Content in Polymers percent new carbon can be slightly greater than 100% due to the continuing (but diminishing) Bio-based polymers can be made totally or par- effects of the 1950s nuclear testing programs. tially from renewable source raw materials, pro- Because all sample 14C activities are referenced to duced from photosynthesis and CO2. In order to a “pre-bomb” standard, all modern carbon values determine the bio-based content of the polymer, the must be multiplied by 0.95 to correct for the bomb “new” carbon content must be measured. A renew- carbon and to subsequently obtain the true bio- able source is replenished by natural processes at a based content of the sample [11]. rate comparable to its exploitation rate. The carbon CEN/TS 16137 specifies the calculation method content of such polymers is derived from the so- for the determination of bio-based carbon content called short carbon cycle within an expected time in monomers, polymers, plastics materials and pro- frame between 1 to 10 years. Most industrial poly- ducts using the 14C method based on three test mers and plastics are presently produced from fossil methods: (a) Proportional scintillation-counter resources that are non-renewable as they cannot be method (PSM); (b) Beta-ionization (BI); and replenished at a rate comparable to the exploitation (c) Accelerator mass spectrometry (AMS). The ana- rate. Fossil resources have a long carbon cycle, lytical test methods specified in this Technical with an expected time frame to convert biomass to Specification are compatible with those described petroleum, gas, and coal of greater than 106 years in ASTM D6866-11. The bio-based carbon content [10]. The accepted measure of bio-based content is is expressed by a fraction of sample mass, as a frac- the level of 14C isotope in the feedstock (basically, tion of the total carbon content, or as a fraction of 15: BIOBASED THERMOSETS 579 the total organic carbon content. This calculation hydrophobic or amphiphilic (both hydrophilic and method is applicable to any polymers containing lipophilic) small molecules. Different plant species organic carbon, including bio-composites [3]. contain lipids with different fatty acid compositions Calculation of percentage of bio-based content and distributions. Lipids help form a hydrophobic according to CN/TS 16137:2011 is based on the biological membrane that separates cells from their calculation of carbon content as a fraction of the surroundings and keeps chloroplasts, mitochondria, total organic carbon content (TOC) expressed as a and cytoplasm apart, thus preventing or regulating percentage, using Equation (15.1): diffusion of chemicals [14]. Plant oil is a mixture of various triglycerides XB;TOC 5 ðXBÞ=ðXTOCÞ (15.1) (also called triacylglycerol). One glycerol is attached to three different fatty acids to form a tri- Here, XB 5 is the bio-based carbon content by glyceride. Glycerolipid and fatty acids are synthe- mass, expressed as a percentage; and XTOC 5 is the sized in the oilseed simultaneously during seed total organic carbon content, expressed as a per- development, before forming diacylglycerol and centage, of the sample. subsequently triacylglycerols. Triglycerides are composed of three fatty acids joined at a glycerol juncture, as can be seen in Figure 15.1. Fatty acids Raw Materials for Renewable account for approximately 95% of the total weight of triglycerides and their content and chemistry are Sources Polymers characteristics of each plant oil and geographical Natural Oils conditions [15]. The most common oils contain fatty acids that vary from 14 to 22 carbons in Natural oils, which can be derived from both length with 0 to 3 double bonds per fatty acid. plant and animal sources, are considered to be one Fatty acids derived from nature have even an num- of the most important classes of renewable sources ber of carbons due to their biosynthesis (acetyl because of the wide variety of possibilities for coenzyme A, two carbon carrier). Some of the fatty chemical transformations, worldwide availability, acids may have additional functional groups like and relatively low price [12]. Vegetable oils, fish hydroxyl (castor oil), epoxide (vernonia oil), or oils, and oils from algae can be transformed in raw ketone (licania oil), as well as triple bonds [14,16]. materials for polymers. Vegetable oils are inexpen- Although fatty acid pattern varies between crops, sive and offer different degrees of unsaturation growth conditions, seasons, and purification meth- while fish oils have a high degree of unsaturation ods, each of the triglyceride oils has a unique fatty [13]. Plant oil is a type of lipid, stored in an organ- acid distribution, as can be seen in Table 15.1 [17]. elle in the form of triglycerides, during the oilseed Hydrolysis of vegetable oils provides about 15 dif- development. Lipids may be defined as ferent fatty acids (some of them can be seen in

1 H2C O O 5

HC O O 91215 ω H2C O α

2 3 4 6

Figure 15.1 A triglyceride molecule (from top to bottom: saturated palmitic acid and unsaturated oleic acid and alpha-linolenic acid), glycerol linkage (1), ester group (2), α-position of ester group (3), double bonds (4), monoallylic position (5), and bisallylic position (6). Table 15.1 Fatty Acid Composition of Different Plant Oils [14,16]

Fatty Acid Composition (X:Y, where X is the number of carbon atoms and Y is the number of double bonds) % Palmitic Stearic Oleic Linoleic Linolenic Gadoleic Erucic Ricinoleic Oil 16:0 18:0 18:1 18:2 18:3 20:1 22:1 18:1 Linseed 5.5 3.5 19.1 15.3 56.6 Soybean 11 4 23.4 53.2 7.8 Palm 44.4 4.1 39.3 10 0.4 Rapeseed 3 1 13.2 13.2 9 9 49.2 Castor 1.5 0.5 5 4 0.5 87.5 Sunflower 6 4 42 47 1 15: BIOBASED THERMOSETS 581

Table 15.1). Triglyceride oils have been used extensively to produce coatings, inks, plasticizers, lubricants, and agrochemicals.

The stereochemistry of the double bonds, the O2 degree of unsaturation, and the length of the carbon chain are important parameters affecting physical and chemical properties. The degree of unsaturation can be expressed by the iodine value (the amount of iodine in grams that can react with double bonds present in 100 grams of sample). According to iodine O OH value, oils can be divided into three categories: (1) drying oils (iodine value above 130, e.g. linseed oil), (2) semi-drying oils (iodine value between 90 and 130, e.g. sunflower or soybean oil), and (3) non- drying oils (iodine value below 90, e.g. palm kernel oil) [17]. Triglycerides contain active sites amenable O to chemical reaction: the double bond, the allylic carbons, the ester group, and the carbons alpha to the ester group. These active sites can be used to introduce polymerizable groups on the triglyceride using Figure 15.2 Oxypolymerization of triglycerides. similar techniques applied in the synthesis of petrochemical-based polymers. The key step is to reach a higher level of MW and cross-link density, as well as to incorporate chemical functionalities known follows: an initial hydrogen abstraction on the methy- to impart stiffness in a polymer network (e.g. aro- lene group between two double bonds that leads to the matic or cyclic structures) [14]. formation of conjugated peroxides, and then, radical There are three main routes to obtain polymers recombination that produces cross-linking (alkyl, from plant oils: (1) direct polymerization through ether, or peroxy bridges [1921]. Oils with high the double bonds, or through other reactive func- iodine value can also be polymerized directly via cat- tional group present in the fatty acid chain, ionic polymerization using boron trifluoride diethyl (2) chemical modification of the double bonds, etherate as initiator [20]. which introduces functional groups to facilitate the Direct cationic homogenous copolymerization of polymerization, and (3) chemical transformation of some vegetable oils with olefinic copolymers such plant oils to produce platform chemicals that can be as styrene, divinylbenzene, norbornadiene, or dicy- used to produce monomers for the polymer synthe- clopentadiene can be achieved using BF3OEt2 mod- sis, usually through the conversion of the triglycer- ified with methyl oleate or Norway fish oil ethyl ides into mono/diglycerides or into simple fatty ester as initiator [12]. Ronda et al. [12] has shown acids [12]. that a cationic polymerization of soybean oil with styrene, divinylbenzene, and trimethylsilylstyrene Direct Polymerization of Natural Oils can lead to flame retardant bio-based thermosets. In order to accelerate the reaction that usually pro- Double bonds present in fatty acids can be poly- ceeds for long periods of time due to low reactivity merized through a free radical or cationic mechanism. of the internal double bonds, microwave irradiation Drying oils, such as linseed or tung oil, are was used as an alternative to a heat source. vegetable oils which polymerize through a free-radical mechanism [18]. The double bonds react with atmo- Chemical Modification of the Double spheric oxygen, which leads to the formation of a network in a reaction called oxypolymerization, as can be Bonds of Natural Oils seen in Figure 15.2. These oils are film-formers and There are two different sites in triglycerides aremostlyusedinpaints,coatings,inks,andresins. available to chemical modification: ester groups, The reaction mechanism can be summarized as which can be readily hydrolyzed or trans-esterified, 582 HANDBOOK OF THERMOSET PLASTICS double bonds along the aliphatic chains, and with percarboxylic acid using Acidic Ion Exchange hydroxyl groups [22]. Some of the important chem- Resin (AIER) as the catalyst [24]. It is possible to ical pathways of functionalization are described in use either acetic acid or formic acid as the carbox- the following sections. ylic acid in the epoxidation process. Peroxy-acid is created by the reaction between hydrogen peroxide Epoxidation of Triglycerides and the carboxylic acid. The process can be con- Epoxidized triglycerides can be found in natural trolled by the quantitative analysis of oxirane rings oils, such as vernonia (see vernolic acid, and iodine number [25]. Different catalysts have Figure 15.3), or can be obtained from unsaturated been studied, including ion-exchange resins, phos- oils by a standard epoxidation reaction. Different photungstic acids, rhenium catalysts, titanium cata- pathways of epoxidation can be found in the litera- lysts, and enzyme catalysts [26,27]. A schematic ture. Epoxides of fat and oils and their ester deriva- example of a triglyceride epoxidation process can tives are usually used as plasticizes, toughening be seen in Figure 15.4. Chemo-enzymatic epoxida- agents, and stabilizers in plastics industry, espe- tion is a relatively new process and has the advan- cially as alternative plasticizers for the polyvinyl tage of suppressing undesirable ring opening of the chloride (PVC) industry. Linseed oil is the most epoxide. In this method, unsaturated fatty acid or epoxidized oil due to the high content of double ester is initially converted into unsaturated percar- bonds (linolenic acid), and is commercially avail- boxylic acid by a lipase-catalyzed reaction with able. Epoxidized soybean oil is also commercially hydrogen peroxide and is then self-epoxidized via available, usually with a functionality of 4.1 4.6 an intermolecular reaction [28,29]. The epoxidation epoxy rings per trygliceride [14]. The oxirane ring of triglycerides makes them capable of reacting via is useful for further chemical modifications and for ring opening [30]. Arkema is already commercializ- the synthesis of thermosetting resins by cross- ing a line of epoxy plasticizers from renewable linking with anhydride or amine compounds or by sources under the trade name of Vokoflex® based homopolymerization of the oxirane rings initiated on epoxidized linseed oil, soybeans, and tall oil of by catalysts [23]. Epoxidation can be done using fatty acids [31]. The preparation of bio-based epoxy organic and inorganic peroxides together with a resins from epoxidized vegetable oils will be dis- metal catalyst; it can also be obtained using halohy- cussed later. drins (haloalcohols), molecular oxygen, and by in situ epoxidation with percarboxylic acid. Acrylation of Epoxidized Oils A suitable technique for clean and efficient epoxi- Acrylation of epoxidized oil can be achieved dation of vegetable oils is the epoxidation in situ from the synthesis reaction of acrylic acid with epoxidized triglycerides [14,32]. Acrylated triglycerides can react via additional polymerization. CO H 2 Acrylated epoxidized soybean oil is commercially available for the surface coatings industry. The reaction of acrylic acid with epoxidized soybean oil occurs through a standard substitution, as can be seen schematically in Figure 15.5. Although the O reaction of epoxidized soybean oil with acrylic acid Figure 15.3 Vernolic acid. is partially catalyzed by the acrylic acid, the use of additional catalysts such as tertiary amines or

O O O O Formic/acetic acid O O O O O O O O Catalyst O O O O Peroxide

Figure 15.4 Epoxidation process. 15: BIOBASED THERMOSETS 583

Figure 15.5 Acrylation of epoxidized triglyceride oil (soybean oil). organometallic catalysts is common. The resulting [30]. The cross-linking density of thermoset poly- polymer properties can be controlled by changing mers obtained from hydroxylated oils depends on the molecular weight of the monomer or the func- the number of hydroxyl groups in the oil and the tionality of the acrylated triglyceride, residual position in the fatty acid (end or middle of the fatty amounts of unreacted epoxy rings, as well as newly acid chain) [33]. The hydroxylation process can be formed hydroxyl groups, both of which can be used controlled, measuring the hydroxyl values accord- to further modify the triglyceride by reaction with a ing to the ASTM titration method (D 1957-86). The number of chemical species (such as diacids, dia- reactivity of the polyol obtained is relatively low mines, anhydrides, and isocyanates). The polymer due to the nature of the secondary alcohol. can be stiffened by the introduction of cyclic or Furthermore, multiple numbers of hydroxyl groups aromatic groups [14]. with varied reactivity are also obtained, which leads to premature gelation [34]. Maleinization of Acrylated Epoxidized Oils Hydroformylation is another method for prepar- According to Wool [14], oligomers of malei- ing polyols; hydrogenation of the formyl group pro- nizated acrylated epoxidized oils can be obtained duces hydroxyl, using cobalt or rhodium as by reacting the acrylated epoxidized oil with maleic catalysts, at high temperatures [35]. In this process anhydride (Figure 15.6). The maleinization reaction aldehydes are formed and an extra carbon is intro- introduces more double bonds in the triglycerides. duced per double bond. Hydroformylation creates The reaction, if not controlled, can lead to an primary hydroxyl groups which are more reactive increase in the viscosity, which leads to gelation of than the secondary polyols created via epoxidation the oligomer. After maleinization the oligomer can (Figure 15.7) [13]. be cured with styrene. Ozonolysis is another alternative chemical process utilized to synthesize polyols with terminal Hydroxylation of Triglycerides hydroxyl groups from vegetable oils. Ozone, a very The introduction of hydroxyl groups at the posi- powerful oxidation agent, is used to cleave and oxi- tion of double bonds creates polyols that can be dize the double bonds in the vegetable oil and then used in the polyurethane and polyester industry, as the ozonides formed are reduced to alcohols using well as for creating new pathways for triglyceride NABH4 or similar catalysts. Tran et al. [34] devel- functionalization [33]. Natural occurring hydroxyl oped a method to synthesize vegetable oil-based groups can be found in ricinoleic acid (castor oil). primary polyol (soybean oil) in a single-step ozono- The hydroxylation can be done by reacting an lysis procedure. When the oil is exposed to ozone epoxidized triglyceride with an acid, or alterna- in the presence of ethylene glycol and an alkaline tively by the hydroxylation of the double bonds. catalyst, double bonds are cleaved, yielding a mix- Ring opening of epoxidized triglycerides can be ture of polyols that is dependent on the relative obtained by reacting with hydrochloric or hydrobro- concentration of the unsaturated fatty acids present mic acid or by an acid-catalyzed ring-opening reac- (Figure 15.8). In this type of ozonolysis, the ozo- tion with methanol (yielding methoxylated polyol), nides produced react with the hydroxyl group of or reacting with water forming vicinal hydroxyl the glycol to form an ester linkage with a terminal groups, or even through a catalytic hydrogenation hydroxyl group. 584 HANDBOOK OF THERMOSET PLASTICS

H2COH

CH H2COH 2 H COH CH2 2 CH2 OH H COH HO CH CH 2 OCH3 2 CH HCO CH3 CH3O CH3O CH O CH HCOH CH3O HC (Carbohydrate) H2COH HOC CH CH2OH OH OCH3 HC O OCH3 O H COH HC O 2 CH3O H2COH CH3O OCH3 HC OH O CH CH3O O CH

H2COH HC O HC O C2H

OCH3 HC O HC HC CH H2COH CH3O HCOH HOCH H C O CH O CH 2 OH 2 H2COH OCH HOCH 3 HOCH HC O CHO HC H2COH CHO OCH3 CH3O O CH H2C O CH O CH CH3O HOC CH CH2OH CH O CH HC O O HOCH H COH CH3O 2 HOCH2 CH3O O CH HC OCH OCH3 3 CH3O HCOH2 HC O CO OH

H2OCH CO O CH CH O HC CH 3 HC O CH CH3O 2 OCH HOCH2 3 HC CH HO CH HC O O 2

H2OCH HCOH HCOH

HCOH CH3O O OCH H2CO 3 OH

Figure 15.6 Maleinization of acrylated epoxidized triglyceride oil (soybean oil).

H CH O H CH2 OH Catalyst H2 CC + CO + H2 CC CC Pressure Catalyst

Figure 15.7 Hydroformylation process [13]. 15: BIOBASED THERMOSETS 585

(a) O O

− O O O3 OH O O OH O OH O HO + O O O HO R

(b) O O O O − O 3 OH O O OH O O OH HO + O O O O HO OH O O

+ O HO O (c) O O O O OH− O 3 O O OH O OH O HO + O O OO HO OH O O

+ O HO O

Figure 15.8 Alkaline-catalyzed ozonolysis of soybean oil with ethylene glycol using triglyceride containing different fatty acids: (a) oleic acid, (b) linoleic acid, and (c) linolenic acid [34].

Maleinization of Hydroxylated Oils to obtain enone-containing triglycerides from high The synthesis of maleinized hydroxylated oil is oleic sunflower oil. Enone-containing triglycerides similar to that used to obtain acrylated epoxidized are interesting alternatives to epoxidized oils for oil. After the hydroxylation, the oil is reacted with the production of thermosets by cross-linking with maleic anhydride in order to functionalize the tri- amines via aza-Michael addition [12]. glyceride with maleate half-ester, as can be seen in Figure 15.9. The reaction can be catalyzed with Conversion of Triglycerides into Mono/ N,N-dimethylbenzylamine [14]. Diglycerides/Simple Fatty Acids Enone-Containing Triglycerides The conversion of the triglycerides into mono/ According to Ronda et al. [12], allylic hydroper- diglycerides or into simple fatty acids can produce oxides can be readily prepared by reacting platform chemicals for producing monomers toward alkenes with photochemically generated singlet polymer synthesis. Mono or diglycerides can be oxygen, which results in a mixture of enones obtained using a transesterification reaction with (Figure 15.10). The researchers utilized this process glycerol (glycerolysis). Standard glycerolysis 586 HANDBOOK OF THERMOSET PLASTICS

Figure 15.9 Maleinization of hydroxylated triglycerides.

OO O O O High oleic sulflower oil (85% oleic acid) O

ν O2, TPP, h , CH2CI2, 6h

OOHO O OOH O O O OOH O

Ac2O/Et3N, 4h O O O O O O O Mixture of isomers. 2.6 enone O O groups per triglyceride molecule

H2NNH2 90°C to 120°C, 12h

O O O O O O O HN

Aza Michael cross-linked triglycerides Soft rubbers Tg (DMTA) = 16°C

O HN OO O O O O

120°C to 160°C, 12h

Tough thermosets Tg (DMTA) = 64°C

Figure 15.10 Synthesis of enone-containing triglycerides from high oleic sunflower oil, cross-linked with 4,40- diaminodiphenylmethane [12]. 15: BIOBASED THERMOSETS 587

O C OH 230-240°C O

2 OH %1 Ca(OH)2 1 O C + O OH O C z Glycerol Soybean oil O O O C O C O

O C OH +

OH OH Diglyceride Monoglyceride

CH2 C C O CCCH2 ° 55 C CH CH Pyridine, 3 3 Hydroquinone Methacrylic anhydride

O O

O C O C O O O CH3 C CH C C OH C O + 2 O C CH2 + CH3 CH O 3 O CH 3 Methacrylic acid O CCCH2 O CCCH2

Figure 15.11 Glycerolysis reaction of soybean oil followed by methacrylation. requires high temperatures and the use of an inor- via addition polymerization. Maleates are relatively ganic homogeneous catalyst such as sodium, potas- unreactive with each other and the addition of sty- sium, or calcium hydroxide. Although this process is rene increases the polymerization conversion rate energy consuming it has high conversion rates and and the stiffness of the resultant polymer. relatively short reaction times [36]. Enzymatic glycerolysis of fats and oils at atmospheric pressure at Methacrylation of Monoglycerides nearly ambient temperatures has been investigated in Methacrylation of monoglycerides can be per- the last decade and has been found to be an attractive formed after the glycerolysis of the triglyceride (soy- and advantageous route compared to the chemical bean oil). The glycerolysis product can be reacted process [37,38]. Usually the product of glycerolysis with methacrylic anhydride to form the methacrylic is a mixture of monoglycerides, diglycerides, and ester of the glycerides and methacrylic acid using glycerol. According to Can et al. [39] maleinization pyridine as catalyst and hydroquinone as inhibitor of of soybean oil and castor oil monoglycerides can the radical polymerization of the methacrylic esters produce maleate half-esters which in turn can react (Figure 15.11). 588 HANDBOOK OF THERMOSET PLASTICS

Glycerol starch and cellulose being the most abundant. Starch is a branched polymer while cellulose is a Glycerol is a non-toxic, edible, biodegradable long, rigid molecule. Sugar-based monomers can be compound. It is an extremely versatile building introduced into polymer architecture as follows: block. Glycerol is usually found in pharmaceuticals, (1) Addition reactions involving vinyl-type sacchar- cosmetics and personal care products, alkyd resins, ides; (2) functionalization based on appending the etc. Glycerol is produced in two forms: natural carbohydrate to a reactive backbone; and (3) polyglycerol, as a by-product of the oleochemical and condensation reactions of sugar-based monomers biodiesel industries, and as synthetic glycerol, from [41]. Sugars are basically polyols with a high num- propylene. Glycerol forms the backbone of trigly- ber of hydroxyl groups along the chain. In order to cerides and is mainly produced by saponification of control the reactivity of the many different oils as a by-product of the soap industry. Around hydroxyl groups on carbohydrates, simple mole- 75% of the glycerol produced in the United States cules with two hydroxyl groups are desired. is derived from natural sources [40]. Glycerol can Dianhydrohexytols, such as isosorbide, isomannide, be used as oligomers or co-monomers in copoly- and isoidide (Figure 15.12), are a result of intramo- mers. Catalyzed self-condensation of glycerol lecular dehydratation; having two reactive hydroxyl yields a mixture of linear and branched oligomers. groups, they can be used as raw material for poly- Linear growth involves only the primary hydroxyl condensation, which leads to polyester, polyether, groups while the secondary ones can have conse- or polyurethane chiral polymers. Isosorbide is pre- quences. The preparation of hydroxyesters or pared from starch, isomannide from D-mannose, hydroxyacids from glycerol and their polyconden- and isoidide from isosorbide [41]. Isosorbide, or sation by transesterification can lead to exquisite 1,2,3,6-dianhydrosorbitol, is the product of a multi- polymers such as hyperbranched polycarbonates step process, starting with starch, and passing and polyesters [41]. In recent years the amount of through D-glucose and sorbitol. It is produced from natural glycerol-based monomers increased consid- biomass in a combination of enzymatic and chemi- erably, and includes diols, diacids, hydroxyacids, cal technologies [44]. Due to steric effects and oxiranes, acrolein, and acrylic acid, among others. hydrogen bonding, isomannide with two endo Ethylene and propylene glycol are particularly rele- hydroxyl groups is the least reactive compound vant for the polyester market (polyethylene tere- compared with isoidide, which has two exo phthalate and polytrimethylene terephthalate). hydroxyl groups. However, isoidide is rare in nature while isosorbide is widely available [45]. Saccharides Isosorbide, which is water soluble and non-toxic, can be a substitute for bisphenol A in different Carbohydrates are considered a very important polymers, especially in epoxy polymers. It can be renewable source of monomers for the preparation attached to glycidyl ether or allyl ether to make of a variety of polymers since they are the most cross-linkable epoxy monomer with similar proper- abundant class of organic compounds found in liv- ties to bisphenol A-diglycidyl ether [46]. According ing organisms. Sugars and starches are carbohy- to Feng et al. [45] the bisisosorbide diglycidyl ether drates used as sources of metabolic energy in plants can be prepared by heating isosorbide with sodium and animals, while cellulose, also a carbohydrate, hydroxide solution with a large excess of epichloro- serves as structural material. Carbohydrates are also hydrin. Two equivalents isosorbide are linked to called saccharides, and small molecules of saccharides are called sugars. They can be divided into three categories: (1) monosaccharides (glucose, galactose, and fructose); (2) oligosaccharides, a combination of two to ten monosaccharides; and (3) polysaccharides such as cellulose, starch, glyco- gen, and hemicellulose [42]. The most abundant oligosaccharide is sucrose, a disaccharide of glu- Figure 15.12 Structures of isosorbide, isomannide, cose and fructose. The vast majority of carbohy- and isoidide [43]. (Reprinted with permission from drates present in nature are polysaccharides, with the National Academy of Engineering.) 15: BIOBASED THERMOSETS 589 three molecules of epichlorohydrin to form the centers accommodate the inter-flavonoid bonds. A- epoxide dimer (Figure 15.13). rings in tannins are highly reactive to aldehydes Xylitol, sorbitol, mannitol, and maltitol (formaldehyde) in wood adhesive compositions and (Figure 15.14) are additional naturally occurring the reaction kinetics can be controlled by the addi- sugar alcohols; polyols that can be polycondensated tion of alcohols to the system. Self-condensation with dicarboxylic acids such as sebacic acid, citric reactions of polyflavonoid tannins are used to pre- acid, glutaric acid, and others [47]. The polymer pare adhesives in the absence of aldehydes. The obtained can be made photo-cross-linkable by add- reaction is based on the opening under alkaline or ing methacrylate units to the polymer. acid conditions of the flavonoid repeating unit and the subsequent condensation of the reactive center Polyphenols with free sites of a flavonoid unit on another tannin chain [48]. Naturally occurring polyphenols are characterized by the presence of multiple phenol structural units. Tannins are a naturally occurring broad class of polyphenols present in trees and shrubs. Tannins can appear in nature as condensed tannins (polyfla- vonoids) or hydrolyzable tannins [41]. Condensed tannins are oligomeric in nature while hydrolyzable tannins are non-polymeric. Tannin has been used for centuries in leather treatment. Tanning leather involves a process which permanently alters the protein structure of leather, thus increasing its durability. Condensed tannin constitutes more that 90% of the total world production of commercial tannins (Figure 15.15). Hydrolyzable tannins are a mixture of phenols based on complex substances built with simple molecules such as gallic acid, ellagic acid, flavogallonic acid, valoneic acid, etc. Hydrolyzable tannins have relatively low reactivity. In condensed tannins the main polyphenolic pattern is represented by flavonoid moieties based on resorcinol A-rings and pyrogallol B-rings. A-rings contain one highly reactive nucleophilic center while the other reactive Figure 15.14 Naturally occurring polyols.

HO O NaOH CI 3 + H2O O O OH

Epichlorhydrin Isosorbide

O O O O O O O O OH O O

Bisisosorbide diglycidyl ether

Figure 15.13 Bisisosorbide diglycidyl ether preparation [45]. 590 HANDBOOK OF THERMOSET PLASTICS

lignin sub-products [52]. All chemical pulping processes are associated with the chemical splicing of natural lignin producing fragments with high molecular weights [41]. Polymeric lignins can be used in thermosetting resins and as additives in thermoplastic polymers. Lignin can be chemically modified to improve polymer-lignin compatibility and to introduce reactive sites. The phenyl propane units in lignin are generally bonded through carbon and ether bonds, where only ether bonds can be easily dis- rupted. The methoxy group substituted at the ortho positions also influences the reactivity and solubility of the lignin [49]. Lignin and its degradation products can originate various polymers such as phenolformaldehyde, poly(azophenylenes), polyi- mides, polyurethanes, polyesters, polyamides, poly- Figure 15.15 Condensed tannin. phenylene oxides, polyphenylene sulfide, and others. Due to the high number of reactive hydroxyl groups present, esterification and etherification are the most Lignin is the second most abundant organic poly- common reactions used for chemical modification of mer on Earth, after cellulose, and the most abun- the lignin. According to Lora et al. [51], etherifica- dant polyphenol in nature. It is most commonly tion with alkylene oxides (ethylene oxide, propylene derived from wood. Lignin is an integral part of the oxide, and butylene oxide [53,54]) results in hydro- secondary cell wall of plants and some algae and xyalkyl lignin derivatives with aliphatic hydroxyl has the function of bonding cells together in the groups. Nonphenolic hydroxypropyl lignins are woody stems, giving the stem rigidity and impact already commercially available in the market. resistance [14]. Lignin accounts for 24 to 33% of Extended alkyl ether chain lignins can be also pre- the dry matter of softwood and 16 to 24% of the pared, although propylation through heterogeneous hardwood [49]. Lignin chemical structure is based reaction of lignin with propylene oxide can lead to on syrigyl, guaiacyl, and p-hydroxyphenol, bonded unpredictable and even dangerous reactions [51]. together by a set of linkages to form a complex Etherification and acetylation of lignin provide solu- matrix [50], as can be seen by the schematic bility in a variety of organic solvents such as ace- structure in Figure 15.16. Lignin is a complex, tone, tetrahydrofurane, and chloroform. Lignins can three-dimensional, amorphous, cross-linked pheno- also be sulfonated, sulfomethylated, aminated, halo- lic-based polymer. It is a by-product of paper pulping genated, and nitrated. and biorefineries, and is considered a waste product. According to Wool et al. [14], grafting of lignin Lignin is separated from wood during pulping and is another possible chemical modification, despite papermaking operations where it serves as fuel for the the inhibiting effect of the phenolic hydroxyl process. Major differences exist between lignin groups, which reduces the efficiency of the process. derived from different pulping processes [51].Sulfite The addition of polar solvents, such as alcohols, processes generate water-soluble sulfonate lignin can increase the efficiency though swelling the lig- while kraft processes generate alkaline soluble lignin. nin molecules improves the accessibility of the Hydrolysis lignin is produced by strong hydrolysis, reagents. Lignin has reportedly been grafted with organosolv lignins are produced from different methyl methacrylate, vinyl acetate, styrene, acrylo- organic solvent-based systems, and steam-explosion nitrile, acrylic acid, acrylamide, maleic anhydride, lignin is obtained through high temperature/pressure and others. Methacrylated lignin can form cross- treatment with steam [42]. Recent pulping trends linked networks when copolymerized with methyl involving organic solvents (organosolv) produce less- methacrylate. Other grafting techniques such as modified, sulfur-free lignins, which simplifies the ionic chain polymerization and chemo-enzymatic pyrolysis process for obtaining low-molecular-weight grafting have also been reported [14]. A great deal 15: BIOBASED THERMOSETS 591

H2COH

H2COH HC O HC O C2H

H2OCH CO O CH CH O HC CH 3 HC O CH CH3O 2 OCH HOCH2 3 HC CH HO CH HC O O 2

H2OCH HCOH HCOH

HCOH CH3O O OCH H2CO 3 OH

Figure 15.16 Generic structure of lignin. of effort and research has been done to determine lignin with propylene oxide following capping of the possible effects of lignin addition to polymers, OH functional groups with aliphatic ethers. introducing functional groups to increase chemical Polyurethane films were prepared by Kelley et al. interactions. Lignin can be used as a co-monomer [56] based on hydroxypropyl lignin. In order to in phenolic thermosetting polymers in wood adhe- reduce rigidity of the network polyethylene glycol sive applications. Lignin can also be co-reacted and polybutadiene glycol extended lignin-based with epoxy resins and polyurethane precursors [14]. polyurethanes were prepared mixing two polyol Star-like macromers from lignin were obtained by components prior to cross-linking. Feldman et al. Oliveira et al. [55] through a combination of chain [50] modified a bisphenol A-based epoxy adhesive extension and etherification reaction, synthesizing poly-blending with Kraft lignin. 592 HANDBOOK OF THERMOSET PLASTICS

Cardanol is the main constituent (97%) of thermally treated cashew nut shell liquid and is based on phenolic compounds (Figure 15.17). In the last few decades several new methodologies were developed for the preparation of polymers using cardanol [16]. Cardanol is a phenol derivative with a meta-substituent of a C15 unsaturated hydrocarbon chain with one to three double bonds; it has potential applications in surface coatings and resins. Cardanol is used to industrially produce phenolic resins with formaldehyde (novolac) for coatings applications, generating high-gloss films for indoor use [57]. Ikeda et al. [57,58] studied oxidative poly- Figure 15.17 Structure of cardanol. merization of cashew nut shell liquid using iron-N, N0-ethylenebis(salicylideneamine) (Fe-salen) as catalyst, in bulk and at room temperature. A soluble cross-linkable polymer was obtained. The polymer more polypeptides, based on amino acids (up to 19 was cross-linked using heat and a hard, glossy different amino acids) bonded together by peptide film was obtained. Gopalakrishnan et al. [59] devel- bonds. Polypeptides are usually folded into globular oped a novel polyurethane based on cardanol, con- or fibrous form according to their biological func- densing cardanol with formaldehyde using sebacic tion [63]. Proteins hold together, protect, and pro- acid as catalyst. The resulting resin (novolac vide structure to the body of living organisms in phenolformaldehyde resin) was epoxidized, fol- the form of skin, hair, calluses, cartilage, muscles, lowed by hydrolysis in order to obtain a hydroxyl tendons, and ligaments. Proteins can catalyze, regu- alkylated derivative, a polyol. The resulting polyol late, and protect the body chemistry as enzymes, was reacted with hexamethylene diisocyanates, hormones, antibodies, and globulins [64]. Most pro- generating rigid polyurethanes. Benedetti et al. [60] teins fold into 3-dimensional structures. The pri- developed cardanol-based polyurethanes by react- mary structure of proteins is defined by the ing cardanol derivatives with diisocyanates or poly- sequence of amino acids in the chain; the folding isocyanates and blowing agents in order to obtain pattern defines the secondary structure, resulting foamed polyurethanes. Cardanol was derivatized by from the rigidity of the amide bond and from intra- condensing cardanol with alkylic aldehydes or molecular hydrogen bonding. Coiling and aggrega- acrylic aldehydes with an acid catalyst. The product tion of polypeptides creates the tertiary structure. was epoxidized and hydrolyzed to obtain polyols, The quaternary structure refers to the spatial which in turn were reacted with isocyanates using arrangement of a protein, as an oligomeric structure blowing agents. Cardanyl acrylate prepolymers containing several subunits [42]. Covalent disulfide with controlled molecular weight and polydispersity cross-linking bonds stabilize the tertiary and quater- can be used in the formulations of inks, coatings, nary structures. The shape into which a protein nat- and adhesives with good adhesion and gloss [16]. urally folds is known as its native conformation. Campaner et al. [61] investigated the use of Denaturation is the breakdown of the tertiary cardanol-based novolac resins as curing agents of structure of a protein, and involves structural or commercial epoxy resins. The novolacs were pre- conformational changes to native structure, such as pared by the condensation reaction of cardanol and unfolding of the protein molecule [14]. These paraformaldehyde using oxalic acid as catalyst. changes can be induced by pH, detergents, heat, etc., and are accompanied by irreversible enthalpy Proteins changes [63]. Plasticization of proteins can be obtained by the interaction between molecules of Proteins are readily available raw materials from plasticizer and protein macromolecules via polar plants (wheat gluten, soy, sunflower, and zein) and interactions (with hydroxyl groups) that increase, as animals (collagen, keratin, casein, and whey) [62]. a result, the free volume. Through plasticization, Proteins are macromolecules consisting of one or proteins can be transformed using various 15: BIOBASED THERMOSETS 593

processing methods [42]. According to Verbeek Collagen, a triple helical, self-organizing protein, [62], melt extrusion of proteins can occur through is the main component of connective tissue and is denaturation, dissociation, unraveling, and align- the most abundant protein in mammals. Mano et al. ment of polymer chains. The presence of disulfide [71] has shown that collagen can be effectively cross-links in this case is unfavorable, and cross-linked with glutaraldehyde in order to over- decreases chain mobility, increases viscosity, and come the drawbacks of fast biodegradation and prevents homogenization. Proteins usually decom- lack of mechanical properties. pose at temperatures below the softening tempera- Albumin usually refers to any protein that is tures, and the introduction of plasticizers helps to water soluble and denatures by heat. Oss-Ronen avoid degradation. Animal and vegetable proteins et al. [72] conjugated serum albumin to polyethyl- can be cross-linked by the reaction of tannic acid, ene glycol (PEG) and cross-linked it to form chromic acid, or formaldehyde. The use of proteins PEGylated albumin hydrogels. The main applica- as raw materials for polymers is not new. Some of tion is as scaffolds for controlled drug release. the earliest plastics were based on casein, phospho- Gluten is a protein composite based on gliadin and proteins found in abundance in cow milk. Casein- glutenin, forming, together with starch, the endo- based paints were used until the late 1960s, when sperm of grass-related grains. The term gluten comes they were replaced by acrylic-based paints. from Latin, meaning “glue,” due to its adhesive prop- Another important cow milk protein is the whey erties. Gliadin and glutenin comprise around 80% of protein, isolated from the whey, a liquid by-product the overall protein in wheat grains. Wheat gluten is of cheese production. The protein in cow’s milk is an important resource for bio-based polymers due to 20% whey protein and 80% casein protein. Whey its viscoelastic properties, mechanical strength, excel- protein films and coatings have been the focus of lent gas barrier properties, low price, and large-scale recent research as potential materials for high oxy- availability [73]. Gliadin and glutenin form a cross- gen barrier packaging [65,66]. Whey protein can be linked network that contributes to the extensibility of plasticized using glycerol and sorbitol [66]. Gao mixed dough. In order to reduce the strong intramo- et al. developed whey protein-based aqueous lecular interactions, plasticizers are usually required, polymer-isocyanate adhesives. The adhesive prop- which leads to an undesired reduction in mechanical erties improved cross-linking of the resulting poly- properties. Gluten has found applications in the mer with methylene bisphenyl diisocyanate [67]. pressure-sensitive adhesives field [74].Agluten- Zein, a class of prolamine protein found in corn, based PSA, with internal plasticizer, was developed was historically used as polymeric resins and coat- by Aranyi et al. [75]. An acetic acid soluble hydroly- ings in a variety of applications such as fibers and sate was obtained using a hydrochloric and acetic tough, glossy, and grease-resistant coatings. Zein acid mixture. The gluten hydrolysate was then epoxi- has also antimicrobial resistance and can be cured dized using ethylene oxide, and generated a water with formaldehyde. Zein has regained interest in soluble, epoxidized gluten hydrolysate. This hydroly- the last few years, and the water resistance was sate was then reacted with hydroxyethyl methacrylate improved by chemical modification where hydro- giving rise to an acrylic copolymer of gluten with phylic groups such as aNH2, aOH, aCOOH, and PSA properties. A reduction in water vapor transmis- aSH were modified by hydrophobic groups or sion of gluten coatings was obtained by Cho et al. polymer segments. Plasticizers can be introduced in [76] using glycerol as plasticizer and oleic acid as a order to flexibilize zein films. Wu et al. [68] intro- hydrophobic component. Kim et al. [77] developed a duced dibutyl L-tartarate (DBT), which contains gluten/zein composite with flexural properties similar ester and hydroxyl moieties to form potential to those of polypropylene. hydrogen bonds with zein. Sessa et al. [69] intro- Soy protein is the major co-product of soybean duced isocyantes and diisocyanates to reduce the oil extraction comprising around 50% of the hydrophilicity of zein and, as a consequence, defatted soy flour [42]. Soy protein is a storage pro- decrease the water moisture uptake. Cross-linking tein held in discrete particles called protein bodies through isocyanate moieties should improve dimen- which provide amino acids during soybean seed sional stability. Lai et al. [70] used oleic acid to germination. Soy protein comprises a high percent- plasticize zein, thus creating more flexible and age of water soluble albumins and salt solution sol- tough films. uble globulins. Water is considered a major 594 HANDBOOK OF THERMOSET PLASTICS plasticizer in the processing of soy proteins. High Soy protein can be modified using polycapro- quantities of water content reduce the denaturation lactone/hexamethylene diisocyanate prepolymer temperature of soy proteins. Soybean protein plas- [68]. It was found that HDI-modified PCL forms tics can be prepared using a variety of different ureaurethane linkages with the amino acids in the processes: compression molding, injection molding, protein, increasing substantially the water resistance and lamination [78]. According to Sun [14], smooth and the elongation. Grafting can be obtained with surface soy protein plastics can be prepared by hot acrylate-based polymers via free radical reaction, pressing soy protein at 150°C, the temperature at although branching is created, disrupting the strong which soy protein molecules melt and unfold, lead- inter and intramolecular interactions [80]. Grafting ing to a homogeneous distribution. A posterior with polyurethane pre-polymer has been proved to interaction between the molecules leads to entan- enhance toughness and water resistance [42]. glement upon curing. Soy can be used as an adhesive and the perfor- Plastics based exclusively on soy protein are mance can be controlled depending on the particle rigid and brittle and plasticizers are usually intro- size and structure of the protein, viscosity, and pH duced in order to increase the flexibility and tough- [83]. In order to enhance the adhesion strength of ness. Polyols have often been used as plasticizers. soy protein, a chemical modification is needed to Plasticizers disrupt molecular interactions, decreas- break the internal bonds and uncoil or disperse the ing the forces holding the chains. According to Sun polar molecules. Dispersion and unfolding of pro- [14], polypropylene glycol and glycerol appear to tein is obtained by hydrolysis or by an alkali be more compatible with soybean protein and eas- environment. ier to introduce between protein chains. Mo et al. [78] studied the effect on soy protein properties of different plasticizers, such as glycerol, polyethylene Thermoset from Renewable glycol, and butanediols (1,2- and 1,3-substituted). Ethylene glycerol, propylene glycerol, sorghum Sources wax, and sorbitol have also been studied for this Epoxy purpose [42,79]. Addition of cross-linking agents such as formaldehyde, glutaraldehyde, and adipic/ Approximately 90% of all non-bio-based epoxy acetic anhydride improve mechanical properties as in the market is based on diglycidyl ether of well as the water resistance of the resulting soy bisphenol A (DGEBA) derived from epichlorohy- protein plastic [80]. Gonzalez et al. [81] used the drin and bisphenol A. Epichlorohydrin is an epox- naturally occurring cross-linker genipin, a chemical ide. The conventional, petrochemical process of compound found in gardenia fruit extract, in order producing epichlorohydrin is the chlorohydrination to tailor mechanical properties and biodegradabil- of allyl chloride, which in turn is made by chlorina- ity. Denaturants such as sodium dodecyl sulfate and tion of propylene. Until recently, epichlorhydrin urea also act as plasticizers, reducing the glass tran- has also been used to produce glycerol (glycerine). sition temperature and increasing tensile strength, The large availability of bio-based glycerol, elongation, and water resistance. A high degree of obtained as a by-product of biodiesel production, entanglements and cross-links is expected when a has made the production of glycerol using epichlo- high degree of denaturation is obtained, leading to rohydrin superfluous [4]. Epichlorohydrin can be an increase in stiffness and strength. Su et al. [82] produced using glycerol from renewable feedstock. investigated the moisture barrier properties of The glycerin-to-epichlorohydrin (GTE) is a process blends of soy protein with polyvinyl alcohol using based on two chemical steps: (1) hydrochlorination glycerol as plasticizer. Water vapor transmission of glycerin with hydrogen chloride gas at elevated was significantly affected by the PVA content. temperature and pressure using a carboxylic acid as Studies on chemical modification of soy proteins catalyst, and (2) conversion of the dichlorohydrin with monomers or oligomers have been done based formed in the first step to epichlorohydrin with a on functional groups able to react with hydroxyl or base [84]. This new glycerin-to-epichlorohydrin amino groups in the protein. Acetylation and esteri- process reduces energy consumption by about one- fication are known pathways for chemical modifi- third, generates less than one-tenth the waste water, cation [80]. and produces less chlorinated organics, when 15: BIOBASED THERMOSETS 595 compared to conventional processes [85]. Solvay On the other hand, the polymer obtained is poten- Chemicals produces epichlorohydrin from bio- tially biodegradable via hydrolytic cleavage of based glycerol, a by-product of biodiesel produc- glycerol ester bonds present in the triglyceride oils. tion using rapeseed, according to EPICEROL® Blends of epoxidized oils and fossil epoxy resins technology [86]. The Dow Chemical Company, have also been studied. Epoxidized soybean oil currently the world’s largest producer of epichloro- (EBSO) is considered to be the second largest hydrin, also announced its intent to manufacture epoxide following epichlorohydrin, and it is pre- epichlorohydrin via a novel, acid-catalyzed hydro- pared commercially by epoxidation with percar- chlorination process using bio-based glycerin [4]. boxylic acids. Epoxidized linseed oil (ELSO) can Krafft et al. [87] developed a process from glyc- be produced by epoxidation with formic acid and erol via 1,3-dichloropropanol. Bio-based DGEBA is hydrogen peroxide [4]. chemically identical to the fossil one, and the bio- Tan et al. [92] added epoxidized palm oil to a based epichlorohydrin accounts for approximately fossil-based epoxy blend of diglycidyl ether of 20% of the molecular weight of DEGBA. Glycerol- bisphenol A/cycloaliphatic epoxide resin/epoxy based epoxy resins such as glycerol polyglycidyl novolac resin in order to obtain a thermal curable ether and polyglycerol polyglycidyl ether are indus- partially bio-based epoxy system. The epoxidized trially available, and have been used in the textile palm oil acts as plasticizer, thus reducing the glass and paper industry as processing agents, tackifiers, transition temperature of the system. In order to coatings, etc. Shibata et al. [88] developed a bio- overcome those limitations, Tan et al. [93] synthe- based epoxy system based on a mixture of commer- sized a thermally curable epoxidized soybean oil in cially available sorbitol polyglycidyl ether (SPE, the presence of tetraethylammonium bromide cata- DENACOL EX-614B) [89] and glycerol polyglyci- lyst. Increasing catalyst concentrations led to a dyl ether (GPE,DENACOL EX-313) [89] cured reduction of curing cycles and temperatures, and to with e-poly(L-lysine) as the bio-based curing agent. higher degrees of conversion and cross-linking den- Sorbitol polyglycidyl ether is a multifunctional sity. As a result, higher storage modulus and glass epoxy resin obtained by the reaction of epichloro- transition temperatures were obtained. hydrin and sorbitol, which in turn is obtained by Chemical modification of the epoxidized oils is the reduction of glucose. Tannic acid was used as another method used to improve final properties. the bio-based curing agent. Transesterification of soybean oil with allyl alcohol Bio-based epoxy can also be obtained using followed by epoxidation in the presence of benzoyl plant oils and fatty acids. Unsaturated vegetable peroxide, and cured with anhydride, resulted in oils can be converted into epoxidized oils using highly cross-linked polymers with better mechanical performic acid and peroxide (see the section on nat- properties. cis-9,10-Epoxy-18-hydroxyoctadecanoic ural oils) or through enzymatic processes [90]. acid is a naturally occurring epoxidized monomer Natural epoxidized oil can be found in vernolic present in birch tree. Lipase-catalyzed condensation acid, present in Vernonia species. Epoxidized UV- polymerization of this naturally occurring epoxi- curable resins can be synthesized via transesterifi- dized acid creates epoxidized oligomers [91]. cation of vernonia fatty acids with hyperbranched Lu et al. [94] synthesized high stiffness polymers hydroxyl functional polyether. The resin can then from epoxidized linseed oils as a result of the be cationically polymerized in the presence of ver- high degree of unsaturation of the linseed oil. nolic acid and methyl ester as diluents [17]. Chandrashekhara et al. [95] developed a soy-based Epoxidized vegetable oils (castor oil, soybean epoxy resin consisting of mixtures of epoxidized oil, linseed oil, etc.) are currently used in epoxy fatty acid esters, specifically, epoxidized allyl compositions [91]. Epoxidized oils can be polymer- soyate. Epon 9500 and Epicure 9550 from Shell ized in the presence of latent catalysts or in the Chemical Company were used as base epoxy resin. presence of curing agents, such as anhydrides. The Epoxidized allyl soyate (EAS) was synthesized low reactivity of the epoxy groups together with a through a lab-scale process from food grade soybean tendency for intramolecular bonding lead to a low oil. The epoxidized allyl soyate was prepared by degree of cross-linking. As a result, poor mechani- transesterification of triglycerides, and yielded fatty cal and thermal properties are obtained with higher acid methyl ester and allyl ester using methyl alco- water uptakes, compared to fossil epoxy systems. hol and allyl alcohol, respectively. The fatty acid 596 HANDBOOK OF THERMOSET PLASTICS

esters were then epoxidized and yielded soyate 80 epoxy resins. The resin formulations were prepared 10% EAS by directly mixing the epoxidized soyate resins into 60 base Epon resin and curing in one step. The following ratios of Epon/epoxidized soyate resins were Pure epon used: 100% Epon resin, 90/10%, 80/20%, and 70/ 40 20% EAS 30%. Tensile tests results can be seen in Stress (MPa) Figure 15.18. An increase in the ductility could be 20 noticed with the increase of the epoxidized soyate resin content, accompanied by a corresponding decrease in the ultimate strength and modulus. The 0 increase in the ductility was related to the higher 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 molecular weight of the soybean oil. It could be con- Strain (m/m) cluded that epoxidized allyl soyate provided better Figure 15.18 Mechanical properties of soy-based intermolecular cross-linking, yielding tougher mate- resin systems [95]. rials as compared to commercially available epoxidized soybean oil. The addition of epoxidized allyl soyate to a commercial epoxy resin led to a viable increasing ester-carboxylic acid derivative content, low-cost, high-performance thermoset product with suggesting that lignin indeed acts as the hard seg- improved properties when used with glass fibers in ment (reaching a maximum value of 211°C). the pultrusion process. The pultruded composites, Sugar-based epoxy systems were also developed. based on soy-based resin systems, have shown com- East et al. [98] developed substitutes for bisphenol parable mechanical properties compared to neat A based on bisglycidyl ethers of anhydro-sugars, resin. The presence of soybean oil increased the such as isosorbide, isomannide, and isoidide lubricity, thus reducing the pull force during the pul- (Figure 15.12). The curing agent used could be trusion process, an additional bonus. either bio-based polyamines or polycarboxylic Espana et al. [96] cured epoxidized soybean oil acids. The novel sugar-based epoxy system devel- of commercial grade (Traquisa S.A.Barcelona, oped can be synthesized to be water soluble. Spain, EEW of 238 g/equivalent) with maleic anhy- Isosorbide epoxy with equivalent weight 230 g/eq dride (AEW index of 98.06 g/equivalent) with the was cured with methylenedianiline (MDA, with aid of a mixture of catalysts (1,3-butanediol anhy- equivalent weight of 49.6 g/eq) at 80°C for 2 hours drous and benzyldimethylamine). The mixture was and 16 hours at 120°C. The glass transition temper- cured at various temperatures for 5 hours using dif- ature obtained for the cured epoxy was 89°C. ferent epoxidized soybean oil (EBSO):anhydride Boutevin et al. [99] developed epoxy pre- (AEW) ratios. The best-balanced mechanical and polymers from phenol compounds based on flavo- thermal properties were obtained for a ratio of 1:1, noids, condensed tannin, or hydrolyzable tannin, representing a high level of cross-linking. and derivatives of catechin (Figure 15.19), epicate- Maximum values of flexural modulus (432 MPa) chin, gallocatechin, epigaloctechin, etc. Those fla- and Shore D (70) were obtained for an EBSO:AEW vonoids can be epoxidized with epichlorohydrin ratio of 1:1, while maximum glass transition tem- according to the schematic reaction shown in perature (42.6°C) was obtained for a ratio of 1:0.9. Figure 15.19. The resulting epoxidized flavonoid Lignin can be used as the hard segment in epoxy can be cross-linked with amine or anhydride, or resin networks, increasing the glass transition of the mixed with resorcinol diglycidyl ether for posterior resultant polymer. Hirose et al. [97] investigated cure. Epoxidized catechin was added to DGEBA at the properties of ester-type epoxy resins derived different ratios: 25:75% and 40:60%. The mixtures from lignin. An ester-carboxylic acid derivative of were cured using cycloaliphatic Epamine PC 19 lignin was previously obtained from the alcoholysis (PO.INT.ER SRL, Italy). Glass transition tempera- of lignin with succinic acid anhydride. The ester- tures obtained for the cured resin containing 25% carboxylic acid obtained can be reacted with ethyl- epoxidized catechin mixed with 75% DGEBA ene glycol diglycidyl ether to form epoxy resins. (Epikote 828 Resolution Performance Products) Glass transition temperature increased with was 48°C, while the commercial DGEBA/Epamine 15: BIOBASED THERMOSETS 597

Figure 15.19 Reaction of epoxidation of catechin using epichlorohydrin [99].

O NH2 O S R O O O HO N N OH O O O O S NH2 O HO N O

O S S H2N NH2 O S AGSO O + O O

SS O O O O O R N R O O O O HO N N OH O O O O O O O O O O OH O ELO N

Figure 15.20 Cross-linking reaction between epoxidized linseed oil (ELO) and amine grapeseed oil (AGSO) [90].

PC 19 system Tg is 47°C. For the ratio of 40:60%, A terpene-derived acid anhydride (TPAn) was syn- aTgof32°C was obtained. thesized by Diels-Alder reaction of maleic anhy- Stemmelen et al. [90] developed cross-linking dride and allo-ocimene obtained by the materials from renewable resources and created isomerization of α-pinene, a terpene found in conif- fully bio-based epoxy systems. Amines are consid- erous trees. ered the most popular cross-linking agent due to The authors compared the thermal and mechani- their nucleophilicity, which makes possible reactiv- cal properties of epoxidized soybean oil (ESO) ity at room temperature. Functionalization of natu- cured with three different curing agents: terpene- ral oils, such as grapeseed oil, with amine groups based acid anhydride (TPAn), hexahydrophthalic involving the reaction of cysteamine chloride and anhydride (HPAn), and maleinated linseed oil UV initiated thiol-ene chemistry created bio-based (LOAn). ESO-TPAn showed a higher glass transi- cross-linking agents. The curing reaction with a tion temperature (67.2°C) and a higher tensile mod- commercial epoxidized linseed oil can be seen in ulus and strength. Figure 15.20. An additional bio-based curing agent Wang et al. [101] used rosin as a bio-based cur- was developed by Takahashi et al. [100]. ing agent. Rosin is a natural and abundant product 598 HANDBOOK OF THERMOSET PLASTICS

using 2-ethyl 4-methylimidzole as accelerator. Epoxide/anhydride ratio affects the mechanical properties. At a ratio of 3:2, the cured resin exhibited clear stress yield. As the ratio increased to 5:2, the cured resin became less ductile, stronger, and stiffer. These changes can be attributed to an increase in the amounts of rigid polyether segments formed. The length of the soft segment in the curing agent affected the cross-linking density. As the molecular weight of the PCL segment increased, the corresponding cured epoxy became more ductile and less strong. It can be concluded that the obstacles to replace petroleum-based epoxy resins with bio-based poly- Figure 15.21 Structure of abietic acid, one of the mers in commercial applications are mainly due to main components of rosin. inferior mechanical and thermo-physical properties. In the meantime, bio-based epoxy systems can be used to partially replace conventional epoxy obtained from coniferous trees. The acidic compo- systems. nent of rosin is mainly a mixture of isomeric abietic-type acids (Figure 15.21). Rosin and its Bio-Based Unsaturated Polyester derivatives have been used as tackifiers for adhesives, varnishes, paints, etc. Unsaturated polyester resins are widely used as The researchers have studied a rosin-based, the matrix in commodity composite materials, usu- anhydride-type curing agent and rosin-based glyci- ally reinforced with fiber glass. Unsaturated polye- dyl ether-type epoxies. Glycidyl ether of abietic sters are produced by polycondensation of alcohol was synthesized, reducing first the carboxyl unsaturated and saturated dicarboxylic acids with group of rosin acid to a hydroxyl group, and then diols. Curing reactions are usually performed reacting with epihalohydrin to obtain glycidyl through radical or thermal processes in the presence ether. of vinyl monomers, such as styrene. Polyester resins Cimteclab [102] developed a series of products are usually classified as: (1) Ortho resins, (2) Iso- based on the phenolic structure currently derived resins, (3) Bisphenol A-Fumarates, (4) Chlorendics, from cashew shell oil, a by-product of the cashew and (5) Vinyl ester [105]. The most widely used diol nut industry. Novocardt is a curing agent devel- for standard unsaturated polyester is propylene glycol oped for epoxy systems based on the reaction of (1,2-propanediol). Additional polyols are dipentaery- cardanol and paraformaldehyde using oxalic acid as thritol, glycerol, ethylene glycol, trimethylpropane, catalyst [61]. Using Novocardt (18%) as a curing and neopentylglycol [91]. Maleic anhydride and agent for a composite matrix based on DGEBA fumaric acid are among the most common unsaturated epoxy (EC01, Camattini, Spa, Italy), a Tg of 110°C acid monomers used. Phthalic acid (iso or ortho acid) was obtained, compared to the 122°C obtained using is the saturated dicarboxylic acid used in all standard a regular amine hardener. Mechanical properties unsaturated polyester resins [4]. Ortho resin is the obtained were similar to the conventional system least expensive among all polyester resins. Monomers except for the impact strength. The bio-based hard- of styrene are usually used as cross-linking agents, ener increased the impact strength from 9.2 kJ/m2 to and solutions of unsaturated polyesters and styrene 24.3 kJ/m2 [103]. vinyl monomers (reactive diluents) are known as Wang et al. [104] synthesized rosin-based flexi- unsaturated polyester resins (UPR). The curing reac- ble anhydride-type curing agents based on tion of UPR is a free-radical chain growth polymeri- maleopimarate-terminated polycaprolactone (MPA- zation between reactive diluent (styrene) and the terminated PCL). Commercial epoxy resin (DER resin. Curing procedures are considered versatile, 332 Epoxy resin, DOW) was cured with (MPA- from room temperature to elevated temperature, terminated PCL) in different stoichiometric ratios, according to the catalyst used [105]. 15: BIOBASED THERMOSETS 599

These building blocks can be substituted by bio- O O O based analogs, either partially or even totally, in some cases. Bio-based propylene glycol (1,2-propa- O O O nediol) and 1,3-propanediol (PDO) are currently + + being commercially produced from glycerol. In the presence of metallic catalysts and hydrogen, glyc- Maleic Phthalic erol can be hydrogenated to propylene glycol (1,2- anhydride anhydride propanediol), 1,3-propanediol, or ethylene glycol. (MA) (PA) 1,3-Propanediol (PDO) is also being used for production of bio-based unsaturated polyesters [86]. HO OH Several entities are working to develop and/or com- 1,3-propanediol mercialize glycerin-to-propylene glycol technology: (1,3-PDO) Senergy/Suppes (University of Missouri), Cargill/ Ashland, Archer Daniels Midland (ADM), UOP/ O O O Pacific Northwest National Laboratory (PNNL), Virent Technologies (University of Wisconsin), O O O Huntsman, and Dow Chemical. The cost of produc- O O tion for propylene glycol made from crude O OO biodiesel-based glycerin is compared to conventional propylene oxide-based propylene glycol [106]. Page et al. [107] detailed the polymerization of unsaturated polyester resins based on biologi- O cally derived 1,3-propanediol. PDO monomers can be obtained via a fermentation process of corn feed Fumaric stocks, using bacterial strains able to convert glyc- fragment erol into 1,3-propanediol (Susterrat, DuPont). (FA) Maleic anhydride was chosen as the unsaturated Figure 15.22 Unsaturated polyesters based on bio- diacid, and anhydride was added in order to based 1,3-propanediol [107]. increase the solubility in styrene since aromatic diacids increase the solubility of unsaturated polyesters in vinyl monomers (Figure 15.22). Solubility in sty- anhydride. Maleic anhydride was added as the rene enables storage, handling, and processing of unsaturated diacid and ortho/isoanhydride was the unsaturated polyester/vinyl monomer solution. added as the aromatic saturated diacid building The solubility is dependent on the ratio between the block. A styrene or methacrylate-containing com- saturated and unsaturated diacids, e.g. the ratio pound was used as reactive diluent. Radical inhibi- between phthalic anhydride (PA) and maleic anhy- tors based on phenolic groups such as dride (MA). Compositions containing ortho- hydroquinones, catechols, or phenothiazines may phthalic acid, maleic anhydride, and Susterrat be added. A tertiary aromatic amine was used as 1,3-PDO were compared to compositions containing co-initiator for the free radical polymerization. The ortho-phthalic acid, maleic anhydride, and 1,2-pro- composition containing bio-based itaconic acid and pylene glycol (instead of 1,3-PDO) using the same 1,3-propanediol were compared to a fossil-based ratio between the components. The curing was composition containing maleic anhydride and 1,2- obtained using styrene (60%). The bio-based com- propylene glycol (with similar ratio between the position has shown higher tensile strength (70 MPa components). The compositions were cured using compared to 44 MPa), higher flexural strength the same amount of styrene (65%). The bio-based (112 MPa compared to 67 MPa), but slightly lower unsaturated polyester compositions showed compa- HDT (67°C compared to 75°C) [107]. rable mechanical properties to similar fossil compo- Szkudlarek et al. [108] synthesized low-viscosity sitions with higher elongation and slightly lower unsaturated polyester resins based on bio-based 1,3- HDT (105°C compared to 109°C). Fatty acids or propanediol and C5-C10 unsaturated dicarboxylic oils can be used as polyacids, while rigid carbohy- building blocks, such as bio-based itaconic acid or drates, such as isosorbide, can be used as polyols. 600 HANDBOOK OF THERMOSET PLASTICS

Szkudlarek et al. [109] also synthesized unsaturated of 28% was observed for polyester resins contain- polyester resins using corn-based isosorbide and ing 10% EMS in the blend, and 42% for 20% 1,3-propanediol, corn-based itaconic acid/anhy- EMS, relative to neat UPE. The failure strain dride, and maleic anhydride in styrene solution. increased, while the tensile strength decreased, Isosorbide provides the aromatic building block which led to increased toughness with increasing needed to obtain the solubility in styrene. Low ther- EMS content. The blend containing 10% EMS mal stability obtained by Szkudlarek et al. was showed 44% higher toughness relative to neat UPE. improved by introducing bio-based itaconic, citra- The addition of EMS also increased the moisture conic, and mesaconic ester units [110]. The unsatu- absorption of the resulting bio-based resin. rated polyester described by the researchers can be Acrylated epoxidized vegetable oils can be cured obtained by polycondensation of a polyol (bio- alone or mixed with unsaturated polyester resins. based 1,3-propanediol or 1,2-propanediol) and Grishchuk et al. [114] developed hybrid thermosets itaconic, citraconic, and/or mesaconic acid or anhy- with interpenetrating network (IPN) structures dride as unsaturated dicarboxylic acids. The heat based on vinyl ester/acrylated epoxidized soybean deflection temperature of this new composition was oil hybrids with IPN structure, cross-linked with considerably improved (from 70 to 105°C). Citric styrene, and anhydride as an additional cross-linker. acid can also be converted into bio-based polyol. The styrene diluted (B30 wt%) bisphenol A-type Kraft lignin, esterified with anhydrides, is soluble vinyl ester (VE; Daron-XP-45-A2) was obtained in styrene, and can be used as an additive in unsatu- from DSM Composite, Nederland, and the acry- rated polyesters, which improves toughness and lated epoxidized soybean oil (AESO) containing connectivity in the polymer network [91]. monomethyl ether hydroquinone as inhibitor, was Epoxidized vegetable oils can be used as a replace- obtained from Sigma-Aldrich Chemie GmbH. The ment for polyester resins. Robert et al. [111] following vinyl ester (VE)/AESO combinations reported a new strategy, based on tandem (serial) were synthesized: 75/25, 50/50, and 25/75 wt.%. catalysis, to obtain alternating polyesters from Lower storage moduli were obtained for combina- renewable sources, using available complexes to tions containing AESO. Two Tg were detected for catalyze the cyclization of dicarboxylic acids fol- the polymerized AESO (at 250 and 15°C), proba- lowed by alternating copolymerization of the result- bly due to the multi-functionality and high unsa- ing anhydrides with epoxides. Rosh et al. [112] turation level of the AESO generating hard and soft prepared cross-linked partially bio-based polyesters segments. The presence of the acrylated epoxidized by curing epoxidized soybean oil with various soybean oil in the hybrid system led to a reduction dicarboxylic acid anhydrides in the presence of of the flexural modulus (around 50% lower for the cure catalysts such as tertiary amines, imidazoles, 50/50 composition) and to an increase in the tough- or aluminum acetylacetonate. The anhydride dic- ness of the system. The resistance to thermal tates the final thermal and mechanical properties. decomposition of the hybrid resin system was Anhydrides of hexahydrophthalic acid, succinic improved due to the interpenetrating network struc- acid, and norbornene dicarboxylic acid have led to ture created. high flexibity with glass transition temperatures Liu et al. [115] obtained unsaturated polyester- below room temperature. Maleic anhydrides gave like resins from functionalized tung oil. Tung oil is rise to more rigid, stiff polyesters with higher glass extracted from the seeds of tung trees. The princi- transition temperatures between 43 and 73°C. pal compound of this oil is a glyceride based on Haq et al. [113] replaced partially petroleum- alpha-elaeostearic acid (cis-9, trans-11, trans-13- based unsaturated polyester with functionalized octadecatrienoic acid). This compound is a highly vegetable oils such as epoxidized methyl soyate. unsaturated conjugated system that is used as a dry- Ortho-unsaturated polyester resin (UPE, Polylite ing oil, mostly for coatings, paints, and varnishes, 32570, Reichhold Inc., USA) containing 33.5 wt.% but cannot compete with the properties of a general styrene was used in the research. The bio-resin purpose unsaturated polyester. In order to obtain based on epoxidized methyl soyate (EMS) was unsaturated polyester-like resin with enhanced obtained from Arkema Inc., USA (Vikotex 7010). properties, tung oil was functionalized in two steps: The amount of bio-based portion in the resin varied (a) alcoholysis with pentaerythritol to produce tung from 0 to 20%. A reduction in the tensile modulus oil pentaeritritol; and (b) maleination to produce 15: BIOBASED THERMOSETS 601 tung oil pentaerithritol (maleinated). The product the acid or anhydride. The half ester is then reacted was blended with styrene, and cross-linking with a polyol to form the polyester. Soybean oil is took place via a free radical polymerization, as can introduced in the reaction step of the half ester and be seen in Figure 15.23. Promising mechanical the poyol [117]. A comparison between mechanical properties were obtained: tensile strength was properties of one of Envirezt grades and a standard 35.9 MPa, tensile modulus was 1.94 GPa, flexural UPR can be seen in Table 15.2. strength was 46.2 MPa, and flexural modulus was 2.08 GPa. Bio-Based Polyurethanes Ashland Performance Materials, a commercial unit of Ashland Inc., developed the first commer- Polyurethanes are extremely versatile polymers cially available bio-based unsaturated polyester with a great variety of applications: flexible and resin (Envirezt) based on soybean oil triglycerides. rigid foams, elastomers, coatings, adhesives, and Different grades of Envirezt are available that con- sealants. Polyurethanes can be thermoplastic or tain from 8 to 22% bio-based content [116]. thermoset. Polyurethanes are synthesized by the Envirezt is obtained by a process where a carbox- reaction of a polyol and a diisocyanate. The func- ylic acid or corresponding anhydride containing an tionality of the polyol determines the properties of ethylenic unsaturation is first reacted with a satu- the final polyurethane. Diols lead to linear thermo- rated, monohydric alcohol to form the half ester of plastic polyurethane whereas polyols with three or

O O C CH2OH O (1) 200–210°C CH OH CO + 2 HOH2C 2 1 O Ca(OH)2 CH2OH O C

Tung oil (TO) Pentaerythritol (PER)

O O O CH2OH CH2OH (2) O O + O C N,N-Di methyl benzylemine HOH2C CH2 O C Hydroquionone CH OH 2 T = 95°C CH2OH

Pentaerythritol alcoholysis products (TOPER)

O O

C H2C O C CH CH COOH H2C O CH CH COOH

O O O O C HOOC CH CHC O CH2 CH2 O C +

O O H C O C CH CH COOH H2C O C CH CH COOH 2

Malinated products (TOPERMA)

Styrene unt (St) (3) Rigid thermoset polymer Free radical copolymerization

Figure 15.23 Synthesis of tung oil-based unsaturated polyester-like polymer [115]. 602 HANDBOOK OF THERMOSET PLASTICS more hydroxyl groups are required to prepare ther- epoxidation of the double bonds with further moset, polyurethane networks. oxirane ring opening with alcohols or other The terminology “bio-based polyurethanes” usu- nucleophiles [120], transesterification with ally refers to polyurethanes based on renewable multifunctional alcohols, and the combination source polyols [42]. Bio-based isocyanates have of hydroformylation or ozonolysis with subse- been introduced only recently and the results have quent reduction of carbonyl groups (see the not yet been conclusive. Bio-based content of poly- section on natural oils for more details). Fatty ols can range from 30 to 100%, and as a result, acids can be easily isolated from triglycerides polyurethane bio-based content varies from 8 to and can be used to prepare diols and polyols. 70%, depending on the building blocks chosen [4]. Triglycerides of castor oil and lesquerella oils Bio-based polyols available in the market for poly- are characterized by the presence of ricinoleic urethane production are divided into three groups: and lesquerolic fatty acids, respectively, both polyether polyol, polyester polyol, and oleochem- presenting hydroxyl groups on their backbones ical polyols from vegetable oils. [42]. Both oils are important sources of naturally occurring polyols, however, the number a. Bio-based polyether polyols: Sucrose and sor- of hydroxyl groups in castor oil are substan- bitol are short-chain polyether polyols used for tially higher compared to lesquerella oil. rigid foams [4]. Polyether polyols can also be obtained by condensation of 1,3-propanediol Polyurethanes from Vegetable Oils synthesized for bio-based glycerol [118].Bio- based 1,3-propanediol can be used to produce Most of the bio-based polyols for polyurethanes polytrimethylene ether glycol as the soft seg- are synthetized from vegetable oils. The hydroxyl ment in elastomers and spandex fibers. groups present in oils can react with isocyanates to b. Bio-based polyester polyols: These can be form branched polyurethanes. Natural oils vary obtained by polycondensation of bio-based greatly regarding the type, composition, and distri- dicarboxylic acids such as adipic or succinic bution of fatty acids in the triglycerides molecules. acid with bio-based polyols (1,3-propanediol) The principal variation in fatty acid composition of [118]. Polyester-based polyurethanes have the oils results from variations in chain length, better mechanical properties and are more degree of unsaturation, and position of the double resistant to oil, grease, solvents, and oxida- bond in the fatty acid chains. As a result, there is a tion, compared to polyether-based. Polyester- great variation in the length of elastically active based polyurethanes are more sensitive to network chains (related to the double bonds/ hydrolysis and microorganism attack [119]. hydroxyl groups) and dangling chains (the soft seg- Longer and hydrophobic chain polyols can ments resulting from the saturated portions) in result in greater flexibility and hydrolytic sta- polyurethane networks obtained from vegetable oil- bility of the resulting polyurethane. based polyols [33]. The functionality of the polyol determines the properties of the final polyurethane c. Vegetable oil-based polyols: These can be prepolymers. Diols lead to linear thermoplastic polyur- pared using distinct methods such as ethanes, whereas polyols with three or more

Table 15.2 Mechanical Properties of Envirezt 70302 [116]

Property Bio-based Envirezt 70302 Aropol S 542 H (Std ref. resin) Tensile Strength (MPa) 73 75 Tensile Modulus (MPa) 2500 4300 Elongation at Break (%) 4.0 2.9 Heat Deflection Temperature (°C) 90 94

Envirezt 7030(Ashland) is an isophthalic-based resin with 22% bio-content used for pultrusion. Aropolt S 542 H (Ashland) is a standard reference unsaturated polyester resin used for pultrusion. 15: BIOBASED THERMOSETS 603 hydroxyl groups are required to obtain thermoset hydroxyl equivalent (molecular weight divided by polyurethane networks [120]. Petroleum-based the hydroxyl functionality) varies from 200 to 300, polyols usually present hydroxyl groups as primary making them suitable for rigid and semi-rigid appli- alcohols while the majority of functional groups in cations, rigid foams, cast resins, coatings, and adhe- vegetable oils are secondary. The reaction rate of sives [13]. Rigid polyurethane foams can be obtained primary alcohols with isocyanate is about 3.3 times from 100% vegetable oil-based polyols, while flexi- faster than that of the secondary ones [121]. ble foams are obtained by mixing vegetable oil- Additionally, when polyurethane foams are created based polyols with petroleum-based polyols. from secondary polyols, slower reaction rates dur- Castor oil, for example, is a naturally occurring ing gas expansion might weaken the three- polyol with a functionality of 2.7 hydroxyls per dimensional network of the polyurethane foam, molecule. Polyurethane obtained from castor oil thus creating more open cells with lower carbon with diphenylmethane diisocyanate is a hard elasto- dioxide content and reduced thermal isolation prop- mer with a glass transition temperature around 7°C. erties [121]. According to Petrovic [13], in order to obtain flexi- Castor oil can be used in the synthesis of cross- ble foams and elastomers, polyols with molecular linked polyurethanes and interpenetrating networks. weight greater than 3000 Da and hydroxyl equiva- Soybean oil, palm oil, and rapeseed oil are more lent weight of 1000 and higher are required. These popular and cheaper oils compared to castor and polyols were synthesized by Petrovic from ricino- lesquerella oils (natural polyols). In order to obtain leic acid via transesterification, which produced polyols from these oils, chemical modifications polyricinoleic acids (Figure 15.24). The dangling have to be made (refer to the section on natural oils chains act as plasticizers, and inhibit crystallization. for more details). Polyurethanes produced from Elastomers with glass transition temperatures rang- vegetable oil-based polyols include elastomers ing from 233°Cto258°C were obtained when obtained from oils with low hydroxyl content and polyricinoleic acids were polymerized with diphe- rigid foam and plastics from high hydroxyl content. nylmethane diisocyanate. According to Petrovic, The extent of unsaturation conversion to hydroxyl lower glass transition temperatures and higher elon- groups can tailor the final properties of the polyure- gations can be achieved using a triol polyricinoleic thane since the hydroxyl content controls the degree acid obtained by introducing a triol during the of cross-linking and the resulting stiffness of the transesterification step. polymer. The polymer structure must be highly Zlatanic et al. [33] synthesized polyurethanes cross-linked when a rigid foam is required, whereas from different vegetable oils midoleic sunflower, less cross-linking gives rise to flexible foams [115]. canola, soybean, sunflower, corn, and linseed oil The degree of cross-linking is also dependent on the with 4,4-diphenylmethane diisocyanate. The func- NCO/OH ratio. Highly cross-linked and stiffer poly- tionality of the polyols ranged from 3.0 for the mid- urethanes are obtained when the ratio is high [122]. oleic sunflower polyol to 5.2 for the linseed oil. Most vegetable oil-based polyols are of relatively The conversion of the double bonds to epoxy low molar mass (around 1000 Da) with a functional- groups during the epoxidation reaction was rela- ity distribution that ranges from 1 to 8 hydroxyl tively high for all the oils, and ranged from 91 to groups per molecule. As a result, the average 94%. The polyols were obtained by epoxidation

Figure 15.24 Polyester diol obtained from ricinoleic acid [13]. 604 HANDBOOK OF THERMOSET PLASTICS followed by ring opening. Polyurethanes were Bio-based triols were also obtained by transition obtained by reacting the polyols with MDI. Linseed metal-catalyzed cyclotrimerization of methyl oil-based polyurethanes presented higher cross- 10-undecynoated and methyl 9-octadecynoated linking density, better mechanical properties, and compounds that can be synthesized from oleic and higher glass transition temperatures. The lowest Tg undecylenic acid via bromination and further elimi- was obtained for the polyurethane from midoleic nation to alkyne functionality, and then subsequent sunflower oil (33°C), and the highest was observed reduction of carboxylate groups to obtain primary for the linseed oil-based polyurethane (77°C). hydroxyl groups (Figure 15.26). The resulting poly- Tensile strengths of all polyurethanes ranged from ol was reacted with methylene diphenyl isocyanate 1523 MPa, except for the polyurethane based on (MDI) using 1,4-butanediol as the chain extender. linseed oil, which showed tensile strength three Desroches et al. [125] synthesized ester- times higher than the others (56 MPa). The tensile containing diols of fatty acids via transesterification modulus of linseed oil-based polyurethane was near with diol compounds followed by thiol-ene radical four times higher (2.0 GPa) compared to other oil- coupling. Polyurethanes were prepared from the based polyurethanes. The variation in properties synthesized oleochemical pseudo-telechelic diols, resulted primarily from the different cross-linking which were reacted with methylene diphenyl-4,4 densities and less from the position of the reactive diisocyanate (MDI). In Desroches’ study, soybean sites in fatty acids chains. oil was used as raw material, and contained differ- Dwan’Isa et al. [122] used soy phosphate ester ent fatty acids with 0 to 3 double bonds. The soft polyol with hydroxyl content ranging from 122 to segments of vegetable oils were based on ester 145 mg KOH/g and diphenylmethane diisocianate groups or amide groups with various spacer lengths to prepare highly cross-linked bio-based in between. These were obtained through the trans- polyurethanes. esterification with a diol or amidification with Del Rio et al. [124] obtained polyols with differ- hydroxylamine or through thio-ene radical cou- ent hydroxyl contents using the synthesis of poly- pling. Amide groups containing polyurethanes ether polyols through the combination of cationic exhibited the highest glass transition temperatures ring-opening polymerization of epoxidized methyl (62°C) due to hydrogen bonding enhancement. oleate and the reduction of carboxylate groups to Miao et al. [126] developed a polyol with high hydroxyl groups. Polyurethanes were obtained from hydroxyl value from epoxidized soybean oil and iso- the reaction of the polyols with MDI or L-lysine propanolamine (Figure 15.27). Both ester groups and diisocyanate (LDI), a non-toxic diisocyanate. It was epoxide groups in epoxidized soybean oil reacted observed (as expected) that the higher the function- with amino group generating hydroxyls, leading to a ality of the polyol, the higher the degree of cross- hydroxyl value of 317 mg KOH/g. The resulting poly- linking that occurred, which in turn led to higher ol was reacted with 1,6-diisocyanato-hexane to obtain Tg values. Additionally, the aromatic MDI led to polyurethane, and additionally used 1,3-propanediol higher Tg values compared to the aliphatic LDI. (PDO) as the chain extender. A single glass transition Lligadas et al. [120] synthesized diols and polyols was noticed (24.4 to 28.7°C). from oleic acid (C18 fatty acid found mostly in olive Natural oil polyols are produced commercially oil) and undecylenic acid (C11 fatty acid derivative by several companies Agribusiness Cargill with a terminal double bond obtained from castor (BiOH, soybean-based polyol), Dow Chemical oil) using click chemistry. Click chemistry was tai- (Renuva soybean-based polyols), Urethane Soy lored to generate substances quickly and reliably, Systems Company, and BioBased Technologies mimicking nature, and was designed to generate sub- (Agrol) BASF (BALANCE, castor oil-based poly- stances by joining small units together. ol), Bayer (BAYDUR, castor oil-based polyol) and Undecylenic acid was obtained by heating rici- Mitsui Chemicals (castor oil-based polyols) for noleic acid under vacuum pyrolysis to create unde- making polyurethane foams for the automotive, fur- cylenic acid and heptaldehyde. Using thiol-ene niture, spray insulation, and other industries. click chemistry, Lligadas et al. [120] applied photo- Polyurethane rigid foams are widely used as insula- initiated coupling of 2-mercaptoethanol and methyl tion and structural materials for construction, trans- esters of oleic and undecylenic acids, followed by portation, decoration, and appliances, which reduction (Figure 15.25). accounts for approximately one-third of the 15: BIOBASED THERMOSETS 605

OH (a)

SH O HO S O O HO O O S OH 7 DMPA/hν 7 7 p-TSA/reflux UD 80% 85% UDA UDA-diol OH

CH3OH p-TSA/reflux 85%

OH OH SH O HO LiAIH /THF S O 4 S O 7 DMPA/hν O 84% OH 7 7 UDM 95% UDM-diol

(b) OH O O SH O HO OH HO 4 O S O 4 4 DMPA/hν 6 p-TSA/reflux 6 S 70% 6 OL 80% HO OLA OLA-diol

CH3OH p-TSA/reflux 85%

OH OH O SH HO O LiAIH4/THF O S S 4 O OH DMPA/hν 4 75% 4 6 80% 6 6

OLM OLM-diol

Figure 15.25 Preparation of undecylenic and oleic acid-derived diols using thiol-ene click chemistry [120].

polyurethane market. Flexible polyurethane foams application in cushioning and vibration-damping are used in automobile seating, upholstered furni- materials [127]. The visco-elastic effect is obtained ture, carpet backing, and bedding (mattresses and due to small open cells (with little opening, which pillows). Bayer recently developed a visco-elastic delays the air re-entrance after compression and polyurethane foam based on castor oil polyol for leads to a slow-down recovery). A summary of bio- 606 HANDBOOK OF THERMOSET PLASTICS

Figure 15.26 Preparation of undecylenic aromatic triol (UDT) and oleic aromatic triol (OLT) using cyclotrimerization process [120]. based polyols, raw materials, and producers can be for polyurethane preparation [137]. In order to seen in Table 15.3. obtain polyols from lignocellulosic materials, they must first be liquefied by chemical or thermochemical treatments at high temperatures and high pres- Polyols from Lignocellulosic Materials sure [42,138,139]. In the presence of alcohols such Lignocellulosic materials including wood, agri- as ethylene glycol, liquefied wood with hydroxyl cultural, or forestry wastes are a mixture of natural content suitable to reaction with isocyanate is polymers based on lignin, cellulose, and hemicellu- obtained. Foam with varied densities can be lose, and tannins with more than two hydroxyl obtained with properties similar to conventional groups per molecule, and can be used as polyols rigid polyurethane foams. 15: BIOBASED THERMOSETS 607

Polyols from Natural Polyphenols As detailed in the section on polyphenols, naturally occurring polyphenols such as tannins and lignins are characterized by the presence of multiple phenol structural units. Polyurethanes having mechanical properties ranging from soft to hard were synthesized by varying the kraft lignin content. High content (3035%) of kraft lignin results in rigid and brittle materials, while lower amounts provide soft polyurethanes [122]. Kelley et al. [140] synthesized polyurethanes from kraft hydroxypropyl lignin with varied molecular weights and hexamethylene diisocyanate. The mechanical properties of the polyurethane obtained improved as the molecular weight of the hydroxypropyl lignin increased. Kelley et al. [56] tried to increase the elongation and toughness of polyurethanes based on lignins by introducing chain extenders to the lignin. Polyethylene glycol and polybutadiene glycol were used as chain extenders. In order to eliminate phase separation, a polyether soft segment was attached to the lignin derivative by propylene oxide chain extension, which created a star-like copolymer with rigid aromatic core and flexible polyether arms. Saraf et al. [141] compared the properties of polyurethanes synthesized from two types of lignin: kraft and steam explosion lignin. Kraft lignin showed inferior mechanical properties. Cashew nut-based phenols, especially cardanol, are considered an attractive raw material for poly- Figure 15.27 Synthesis of bio-based polyurethane urethanes. Cardanol can be recovered from cashew from epoxidized soybean oil and isopropanolamine nut shell liquid by double vacuum distillation and [126]. can be converted into a diol to be reacted with a diisocyanate [60]. According to Mohanty et al. [123], cardanol can be reacted with glycol in the presence of phosphoric acid as catalyst. The result- Polyols from Carbohydrates ing mixture can be reacted with aromatic diisocyanate to obtain films of polyurethane. Mixtures of Sugars and starches are carbohydrates that are cardanol, formaldehyde, and diethanolamine, considered a very important renewable resource for blended first with polyethylene glycol and then polyols (see the saccharides section for more with methylene diphenyl diisocyanates; MDI and details). Sugars are basically polyols with a high hexmethylene diisocyanate led to rigid polyure- number of hydroxyl groups along the chain, and thane foams [59]. therefore are potential raw materials for polyurethanes [22,41]. Starches and cellulose can be liquefied in the presence of alcohols in order to obtain Isocyanate-Free Polyurethanes and polyols to be used in the preparation of polyurethanes. The process includes the use of liquefac- Bio-Based Isocyanates tion solvents based on polyethylene glycol, Isocyanates are the derivatives of isocyanic acid glycerol, and sulfuric acid. The solution obtained is (H-NQCQO). The functionality of the isocyanate then neutralized with caustic soda [122]. (R-NQCQO) group is highly reactive toward 608 HANDBOOK OF THERMOSET PLASTICS

Table 15.3 Summary of Bio-Based Polyols for Polyurethanes [4]

Trade Source Material Name Producer Application Reference Soybean oil BiOH Cargill Flexible foams [128] Renuva Dow Flexible foams and CASE [129] SoyOil Urethane Soy Flexible and rigid foams, [4] Systems spray foams, elastomers Baydur BAYER Rigid and flexible foams [130] Agrol BioBased CASE, molded foams [131] Technologies Castor oil Lupranol BASF Rigid foams, mattresses [132] BALANCE Polycin Vertellus Coatings [133] Mistui Rigid and flexible foams [134] Chemicals Renewable and Enviropol IFL Chemicals Rigid foams for insulation [135] recycled natural oil and refrigeration Rapeseed/ Metzeler- Flexible foam [136] Sunflower oil Schaum GmbH

CASE: Coatings, Adhesives, Sealants, and Elastomers.

proton-bearing nucleophiles, and the reaction of vegetable oils. Mahendran et al. [143] developed isocyanate proceeds with addition to the carbon- a new bio-based non-isocyanate urethane by the nitrogen bond [142]. Important isocyanates used reaction of a cyclic carbonate synthesized from a in polyurethane manufacturing include 2,4-toluene modified linseed oil and an alkylated phenolic diisocyanate, 2,6-toluene diisocyanate, 1,6-hexam- polyamine from cashew nut shell liquid ethylene diisocyanate, and 1,5-naphthalene diiso- (Figure 15.28). Cyclic carbonates can be synthe- cyanate, among others (all petroleum-derived). The sized from any epoxy monomers. The cyclic car- reactivity of isocyanates depends on their chemical bonate groups were added to the triglyceride by structures. Aromatic isocyanates are usually more reacting epoxidized linseed oil with carbon dioxide reactive than their aliphatic counterparts. The pres- in the presence of a catalyst. ence of electron-withdrawing substituents on iso- C¸ ayli et al. [144] synthesized isocyanate deriva- cyanates increases the partial positive charge on the tives from unsaturated plant oil triglycerides. The carbon atom and moves the negative charge further triglyceride was first brominated at the allylic posi- away from the reaction site, which results in a fast tions by a reaction with N-bromosuccinimide, and reaction. Isocyanate is obtained from the reaction then, the brominated species were reacted with between amines and phosgene, a hazardous chemi- AgNCO to convert them to isocyanate-derivative cal that requires special precautions, is toxic, and is triglycerides (Figure 15.29). an irritant to mucous membranes. Polyurethanes are Polyurethanes and polyureas were synthesized currently being produced from bio-based polyols curing the fatty isocyanates obtained with alcohol with toxic isocyanate. Environmental and public and amines, respectively. Glycerin polyurethane health concerns motivate the research for alterna- exhibited a glass transition temperature of 19°C, tive routes to create bio-based, non-toxic isocya- castor oil polyurethane showed two glass transition nates. Isocyanate-free, environmentally friendly temperatures at 24 and 36°C, and triethylene tetraa- polyurethane systems can be obtained from mine polyurea showed a glass transition 15: BIOBASED THERMOSETS 609

Figure 15.28 Isocyanate free urethane: Reaction of cyclic carbonate with phenylalkylamine [143].

Br O

C C CH O O H H

O O

AgNCO in THF R.T.

NCO O

C C CH O H H O

O O

Figure 15.29 Bio-based isocyanate from vegetable oil [144]. temperature of 31°C. The polyurethanes synthe- polyurethane showed excellent elongation, 410% sized demonstrated similar mechanical properties and 353%, respectively. (tensile modulus around 50 kPa and tensile strength Hojabri et al. [145] described the synthesis of around 100 kPa). Despite the low tensile strength linear saturated terminal aliphatic diisocyanates and modulus, castor oil polyurethane and glycerin from fatty acids via Curtis rearrangement, a thermal 610 HANDBOOK OF THERMOSET PLASTICS

Figure 15.30 Synthesis of bio-based isocyanate from oleic acid [145]. decomposition of acyl azide. Diacids such as azea- such as introducing chain stopping agents, phenolic lic acid, can be produced from oleic acid, as in resins, acrylic monomers, styrene, vinyl toluene, Figure 15.30. silicones, isocyanates, etc. Alkyd resins are compat- Saturated diacids can be prepared by ozonolysis ible with a vast number of polymers, making them of oleic acid to the corresponding aldehyde fol- very versatile to produce coatings, binders, paints, lowed by purification and oxidation of the aldehyde lacquers, and varnishes, both transparent and semi- to obtain the diacid. Hojabri et al. compared physi- transparent [147]. Fatty acids are produced from cal properties of polyurethanes obtained using the vegetable oil. The common polyols are synthetic same polyol with fatty acid-derived diisocyanate glycol or glycerol, although bio-based glycerol can (1,7-heptamethylene diisocyanate, HPMDI) and a be used instead, which increases the bio-based con- similar but petroleum-derived commercially avail- tent of the final polymer [4] and extends up to 70% able diisocyanate: 1,6-hexamethylene diisocyanate of the bio-based content [147]. Fossil-based phtha- (HDI). Canola polyols were synthesized using ozo- lic acid and maleic acid (and anhydrides) are the nolysis and hydrogenation. Desmophen 800 (Bayer) most commonly used organic acids; glutaric and was used as the commercial polyol. The polyur- succinic anhydrides can also be used in alkyd for- ethanes obtained had comparable properties to mulations [17]. Drying times can be controlled by fossil-based diisocyanates. the type and amount of anhydride, with drying times increasing with the amount of anhydride. An Bio-Based Alkyd Resins illustrative reaction between glycerol and phthalic anhydride is shown in Figure 15.31. Alkyds are polyesters obtained by polycondensa- Alkyd resins can be classified according to oil tion of three monomers: polyols, dicarboxylic acids, length, which refers to the oil percentage in the or anhydrides, and natural fatty acids or triglycer- alkyd. Short oil alkyds contain below 40% oil; ides. The term alkyd is a modification of the origi- between 40 and 60% they are called medium, and nal name “alcid” coined by R. H. Kienle, meaning above 60%, long oil alkyds [17]. The oil length from alcohol and organic acids [146]. Since alkyd affects the properties of the final product. A typical resins emerged in the market in the late 1920s, they long oil alkyd is made of 60% soybean fatty acids, have always had a substantial bio-based content 21.5% polyol (pentaerythritol), and 25.4% phthalic [4]. These resins are used in paints and coatings. In anhydride [4]. Most alkyd-based coatings are used 1927 Kienle combined fatty acids with unsaturated for industrial goods (vehicles, wood products, etc.) esters while searching for a better insulating resin and infrastructure (traffic control striping, bridges, for General Electric [146]. Double bonds present in etc). Alkyd coatings are inexpensive, durable, and the fatty acids are capable of oxidative coupling heat resistant. Durability and abrasion resistance reactions that lead to “air drying” of plant oils, thus can be increased by modifying alkyd with rosin creating film coatings. This is the chemistry behind (pine resin) [146]. Phenolic and epoxy resins the alkyd resins [14]. When the resin is applied to a improve hardness and resistance to chemicals and substrate, heat or oxidizing agents are achieved by water. Styrene extends flexibility of coatings and changing the oil length or chemical modification, can be used to cross-link alkyd resins (using 15: BIOBASED THERMOSETS 611

O O O CH2-OH OC COCH CH CH O O 2 2 CH-O-CO-R n O O CH2-OH C O R

Figure 15.31 Synthesis of alkyd resin [17]. peroxides as initiators). Thermoset alkyds can be acrylic-modified alkyd resin based on the polymeri- used to produce billiard balls, appliance housings, zation of a sulfonated alkyd resin with an acrylated motor cases, switches, electronic encapsulations, fatty acid. After polymerization, the polymer could etc. More ecological friendly versions of alkyds be dissolved in water. were produced based on the synthesis of alkyd Hofland [147] described an ultimate low carbon resins with relatively high acid value, and neutral- footprint alkyd resin that had not only a low con- ized using amines to form colloidal solutions in a tent of fossil carbon, but also low energy content mixture of water and water miscible solvents such (i.e. low energy bio-based raw materials such as as glycol ether [147]. Nevertheless, solvents present succinic acid, colophonium (rosin), glycerol, and in alkyd formulations at an amount of 20 to 30% isosorbide). Water-based alkyd and high-solid alkyd present an environmental problem. High-quality resins can be ecological friendly solutions. High- alkyd coatings with low solvent amounts are an solid alkyd solvents can be replaced by bio-based important target in the coating industry. High solid ones like ethanol, methyl ester of soybean fatty content alkyd resins can be obtained by reducing acids, or methyl lactate. polymer viscosities using lower-molecular-weight Kemwerke (Philippines) developed eco-friendly polymers, thus enabling a reduction in the solvent short oil coconut alkyd resins for paint formula- content [17]. The same effect can be obtained by tions. Coconut alkyds are a range of tough resinous synthesizing highly branched and star-structured products formed by reacting polybasic organic resins [148]. Hyperbranched alkyd resins can be acids or anhydrides with polyhydric alcohols in the obtained using trimethylpropane and dimethylolpro- presence of coconut fatty acid or the monoglyceride pionic acid. Saturated polyesters with hydroxyl end state of the refined coconut oil [150]. Soy groups are first obtained. The alkyd resin is Technologies LCC (KY, EUA) produces soy-based obtained by the esterification of the polyester alkyd emulsions for coating formulations (Soyanol) obtained in the first step, with unsaturated fatty [151]. Perstop (Sweden) produces bio-based glyc- acids. According to Gruner [17], star-like resins erol in medium oil-length, soybean oil-based, air- with three or four arms can be formed by the esteri- drying alkyd resin formulations [103]. fication of dipentaerythritol with fatty acids. In the 1980s and 1990s, environmentally friendly, zero VOC (Volatile Organic Contents) Bio-Based Phenolic Resins versions of alkyd waterborne resins were developed. In order to obtain water-based resins, alkyd Phenolformaldehyde is one of the oldest com- resins with high acid numbers were prepared and mercial synthetic polymers, first introduced by Leo neutralized by amines. Amines were avoided by Hendrik Bakeland in 1907 [152]. The polymer is using suitable surfactants. According to Hofland the result of a step-growth polymerization of two [147], surfactants can also be eliminated by in situ simple chemicals: phenol or a mixture of phenols polymerization of hydrophilic monomers within and formaldehyde using an acidic or basic catalyst solvent-less alkyd resin, followed by inverse-phase [153]. Phenol is reactive towards formaldehyde at emulsification. Water-soluble alkyd resins were the ortho and para sites, allowing up to three units synthesized by the copolymerization of acrylic of formaldehyde to attach to the aromatic ring. The acid, glycidyl methacrylate esterified with unsatu- main product of the reaction between them is the rated fatty acid, styrene, and methyl methacrylate production of methylene bridges between aromatic [17]. Kuhneweg [149] developed a waterborne rings, as can be seen in Figure 15.32. 612 HANDBOOK OF THERMOSET PLASTICS

Figure 15.32 Structure of phenol formaldehyde.

The ratio between formaldehyde and phenol deter- Resoles are used as adhesives (plywood, oriented mines the degree of cross-linking. When the molar strand boards, laminated composite lumber, etc.) and ratio is one, theoretically, every phenol can be in composite materials (glass and carbon compo- linked together via methylene bridges, enabling total sites). Resole and novolac are soluble and fusible cross-linking. Novolacs (novolaks originally) are low-molecular-weight products at A-stage, rubbery phenolformaldehyde resins where the molar ratio at B-stage, and rigid, hard, and insoluble at C-stage of formaldehyde to phenol is less than one. The poly- [153]. One of the most common applications of merization is catalyzed by acids such as oxalic acid, novolac phenol resins is as photoresists. hydrochloric acid, or sulfonate acids; the reaction Since phenolformaldehyde is one of the major is slow and relatively low molecular weights are adhesive resins in the manufacture of plywood, and obtained [153]. In a second step, a hardener such as the raw materials are petroleum-based, their hexamethylenetetramine can be added to cross-link replacement by bio-based, non-toxic substitutes has the resin. The hexamine forms methylene and become a necessity. In the polyphenols section, dimethylene amino bridges between the aromatic bio-based polyphenols were presented as well as phenol rings at high temperatures. Resoles are the chemical pathways used to transform them into one-stage phenolformaldehyde resins obtained raw materials for polymers. Lignin, a renewable, with formaldehyde/phenol ratios greater than one non-toxic, widely available, low-cost raw material (B1.5) using a basic catalyst. Hydroxymethyl was presented as having a high potential to be used phenols and benzylic ether groups are formed in the as a phenol substitute in phenolformaldehyde first step of the reaction, at around 70°C. Cross- resins; it is a relatively expensive petroleum-based linking occurs when the temperature reaches around chemical [154]. 120°C to form methylene and methyl ester bridges Lignin is highly cross-linked; three-dimensional through the elimination of water molecules. A aromatic polymers with phenylpropane units are three-dimensional, cross-linked network is formed. linked together by carbon-carbon or ether bonds Resole-type phenolformaldehyde resin becomes with phenolic and hydroxyl groups (Figure 15.16). hard, and heat and chemically stable, after curing. Lignin is the second most abundant polymer in 15: BIOBASED THERMOSETS 613 nature, after cellulose, and it is produced as a by- product of the paper industry. One of the functions of lignin in plants, together with hemicellulose, is to bind cellulose fibers, although isolated lignins have proven to be poor adhesives for wood composites [155]. Lignin is not structurally equivalent to phenol. Phenol has five free sites on the aromatic ring and no ortho and para substituents around the hydroxyl group. In lignin, the aromatic ring is para- substituted by the propyl chain of the propylphen- 4-ol(coumaryl) structural unit, which affects lignin reactivity during cross-linking with formaldehyde [155]. Lignin presents most ortho and para sites as Figure 15.33 A- and B-ring in tannin, where R1 blocked by functional groups, which leads to can be resorcinol or phlorogucinol and R2 can be slower reaction with formaldehyde when compared pyrocathecol or pyrogallol [91]. to phenol. For this reason, lignin is used to replace phenol in a limited amount, between 40 to 70% in adhesives formulations. Brosse et al. [156] added enables one to obtain natural resins from a ligninic lignin to phenol-formaldehyde resins without dete- fraction of the liquid pitch produced, all without riorating the mechanical and adhesion properties, having to extract the phenol fraction using solvents. and observed that an additional environmental ben- The total phenolic content obtained ranged from 30 efit was obtained: the presence of lignin reduced to 80 wt.% and was considered highly reactive and the formaldehyde emissions for both the resin and suitable for use within resin formulations without the finished products. requiring any further fractionation procedure. Phenolic precursors can also be prepared by liq- Resole-type resins were used in the production of uefying wood, bark, and forest and wood industry oriented strand board and plywood, with similar residues using a fast pyrolysis process. Monomeric results compared to fossil-based resins [154]. phenols can be produced by thermochemical con- Vergopoulo-Markessini et al. [160] used a mix- version of biomass using fast and vacuum pyrolysis ture of phenol compounds obtained from pyrolysis [51,147]. Liquefation, phenolysis, and fractionation and cashew nut shell liquid. A synergistic effect methods can be also used. Fast pyrolysis is based between phenolic-based pyrolysis products and on fast heating rates using temperatures between cashew nut shell liquid was noticed, which enabled 400 and 600°C. Chum et al. [157] synthesized the substitution of up to 80 wt.% of the phenolic novolac and resole resins using phenol derivatives component of a standard formaldehyde-based resin. obtained by fast pyrolysis from softwood, hard- Cashew nut shell liquid contains cardanol, a pheno- wood, and bark residue. Reactive components for lic compound (Figure 15.17) that can be used to resin synthesis were obtained by fractioning the substitute phenol in novolac or resole resins. pyrolysis oils and isolating phenolic and neutral Cardanolformaldehyde resins have shown fractions, which were directly polymerized improved flexibility due to an internal plasticization [154,157]. These pyrolytic oils contained a complex effect of the long chain and better processability. mixture of compounds including phenolics, guaia- Additionally, hydrophobic behavior was noticed col, syringol and para-substituted derivatives, car- due to side chains, making the resin water-repellent bohydrate fragments, polyols, organic acids, and resistant to weathering [91]. formaldehyde, acetaldehyde, furfuraldehyde, and Tannins are also polyphenolic compounds other oligomeric products. The fraction containing (Figure 15.15), with good reactivity towards form- phenolic and neutral components substituted for not aldehyde. Tannins are composed mostly of flavan- only the phenol, but also some of the formaldehyde 3-ol units, for which two phenolic rings are joined present in the fraction. together by a heterocyclic ring, as can be seen in Giroux et al. [158] developed a technology Figure 15.33 [91]. The high reactivity of the tannin called Rapid Thermal Processing [159] based on towards formaldehyde is due to the A-ring. In fast pyrolysis or rapid destructive distillation that novolac resins, the presence of tannins enable 614 HANDBOOK OF THERMOSET PLASTICS

NaOH + CH2O

CH3O CH OH CH3O 2

OH – + O Na Coniferyl monolignols

Figure 15.34 Methylolation reaction of bio-oil with formaldehyde [167].

reactions with hexamine at lower temperatures, compared to fossil-based phenol formulations. However, the high reactivity and the large molecules of tannin can result in premature gelation, which leads to brittleness [91]. Pizzi et al. [161] developed polymers derived from the cross-linking of condensed tannins by polycondensation reactions Figure 15.35 Guaiacol and syringol chemical with furfuryl alcohol and small amounts of formal- structures (in lignin) [154]. dehyde. The addition of blowing agents created tannin-based rigid foams with fire resistance similar to synthetic phenolic foams. Lignin has fewer reactive sites compared to [162]. Cetin et al. [164,165] utilized phenolated- petroleum-based phenol. More than half of the lignin (2030% of overall phenol) instead of potentially reactive aromatic hydroxyl groups in unmodified lignin in phenolformaldehyde resins kraft lignin, for example, are blocked by methyl to achieve an improvement in mechanical and groups, which hinders the aromatic hydroxyl group physical properties when utilized as wood adhesive [162]. Sulfur-mediated demethylation can be used in particleboard. Hu et al. [166] modified a foamed to remove methyl groups on the aromatic ring, thus resole resin using phenolated lignosulfonate (lignin increasing the reactivity. Methylolation or hydroxy- from the sulfite process) to achieve mechanical and methylolation can be used to introduce hydroxy- physical properties similar to fossil-based phenol- methyl groups (aCH2OH) to lignin molecules formaldehyde foams. Perez et al. [167] synthesized using formaldehyde in an alkaline medium. two types of novolac resins based on lignin: Methylolated lignin can be directly incorporated in (a) ammonium lignosulfonate, which was used phenolformaldehyde resin, replacing part of the directly, as filler, and (b) ammonium lignosulfonate phenol, in wood adhesives compositions [155,162]. modified by methylolation. Cheng et al. [168] El-Mansouri et al. [163] replaced the formaldehyde obtained bio-oil from hydrothermal liquefaction of in the methylolation process by glyoxal, a bio- pine sawdust. The bio-oil obtained was methylo- based, non-toxic dialdehyde. The process was lated with formaldehyde (Figure 15.34) in the pres- called glyoxalation. ence of sodium hydroxide. The methylolated bio- Lignin can be treated with phenol in the presence oil was polymerized with formaldehyde, replacing of organic solvents such as methanol or ethanol in up to 75 wt.% of phenol in the resin for the produc- a process called phenolation or phenolysis [154]. tion of plywood. The methylolation treatment The phenolation process is based on a thermal improved the thermal stability of the bio-based phe- treatment of lignin with phenol in an acidic nolic resin. Phenolation and methylolation have medium, which leads to the condensation of phenol shown the best results in terms of number of reac- with the aromatic ring of the lignin and side chains. tive sites created. The origin of the lignin may A reduction of the molecular weight of the lignin is affect the reactivity. Softwoods may yield more also observed, as a result of ester bond cleavage reactive phenolics than hardwoods due to the 15: BIOBASED THERMOSETS 615 relatively high presence of guaiacol with one meth- biopolymers, and biocomposites. Boca Raton, oxy group (Figure 15.35) [154]. FL, USA: Taylor and Francis Group; 2005. Formaldehyde-containing resins (ureaformal- p. 83353. dehyde, melamineformaldehyde, and phenol [6] ISO 14040. 2006 Environmental management- formaldehyde) for applications as wood adhesives Life cycle assessment-Principles and frame- and mineral fiber binders for thermal insulation are work, 2006. under intense pressure to reduce or eliminate form- [7] Madival S, Auras R, Singh SP, Narayan R. aldehyde content due to ecological concerns and Assessment of the environmental profile of toxicity issues [152]. There is a need to develop non- PLA, PET and PS clamshell containers using toxic, bio-based formaldehyde substitutes. Ramires LCA methodology. J Cleaner Prod 2009;17 et al. [169,170] utilized glyoxal (OCHaCHO), a bio- (13):118394. based chemical obtained by the oxidation of lipids, a [8] Florides G, Christodoulides P. Global warm- non-toxic and non-volatile dialdehyde that can be used ing and carbon dioxide through sciences. as a substitute for formaldehyde in phenolic resins. Environ Int 2009;35(2):390401. Both resole and novolac resins based on glyoxal- [9] Ashby MF. Materials and the environment phenol resins were developed for application in com- eco-informed material choice. Canada: posite materials using sisal fibers as reinforcement, and Butterworth-Heinemann; 2009. showed improved results for novolac-type resins in [10] Narayan R. Biobased (Carbon) content of terms of mechanical and physical properties. The novo- complex assemblies. In: USDA biopreferred lac glyoxal-phenol composite showed higher impact public meeting, Riverside, CA; 2010. strength (237 J m21) than the resole glyoxalphenol [11] ASTM Standard. ASTM D6866-11 Standard composite (118 J m21), indicating a better fiber/matrix test methods for determining the biobased interaction. The novolac glyoxalphenol composite content of solid, liquid, and gaseous samples exhibited a higher storage modulus (E’), and conse- using radiocarbon analysis; 2011. quently was more rigid than the resole composite. In [12] Ronda JC, Lligadas G, Galia` M, Ca´diz V. addition to glyoxal, oxazolidine, solid resole, epoxy, Vegetable oils as platform chemicals for poly- resorcinol, and tannins are being examined to substitute mer synthesis. Eur J Lipid Sci Technol formaldehyde in phenol resins [152]. 2011;113(1):4658. [13] Petrovic´ ZS. Polymers from biological oils. Contemp Mater 2010;1(1):3950. [14] Wool RP, Sun XS. Bio-based polymers and References composites, July. Elsevier Science & Technology Books; 2005. [1] Chen G-Q, Patel MK. Plastics derived from [15] Montero de Espinosa L, Meier MR. 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