MSc Chemistry Science for Energy and Sustainability
Literature Thesis
High-performance polymeric applications for FDCA beyond PEF
by
Clara Kamel ID: 11714182
February 2020 12 EC
Supervisor and Examiner: 2nd Examiner: Prof. Dr. Gert-Jan Gruter Dr. Chris Slootweg
HIMS: Industrial Sustainable Chemistry Avantium: Renewable Chemistries Abstract
With increasing environmental concerns and the depletion of fossil reserves, the search for performance competent bio-based polymers has significantly increased in the last decade. 2,5-Furandicarboxylic acid represents a promising building block for next generation polymers due to its renewable source as well as its structural similarity to well-known terephthalic acid. This monomer is currently being produced at pilot scale through Avantium’s YXY® technology. Today, FDCA is being used in the production of poly(ethylene 2,5-furandicarboxylate) (PEF), the so-called ‘furan counterpart of the engineering polyester, poly(ethylene terephthalate)’ (PET), for packaging and bottle applications. With a limited FDCA annual production capacity, Avantium is focusing on high-value as opposed to high- volume markets where FDCA can compete on performance with its terephthalate counterparts. The goal is to enable further scale-up of FDCA and allow for cost- competitiveness of its resulting polymers. A series of furanic polyesters and polyamides were discussed in previous reports, however no review covered the totality of high performance FDCA polymers for specialty and engineering applications. In this perspective, the following review will identify the different high-value polycondensates incorporating FDCA and assess their thermo-mechanical properties, commercial applications, technical requirements and future challenges.
1 Table of Contents FDCA, a versatile building block in next-generation polymers ...... 3 The roadmap to a sustainable polymer industry ...... 3 FDCA, a versatile renewable building block ...... 4 Production of FDCA: state-of-the-art...... 6 Chemical-catalytic oxidative conversion of HMF to FDCA ...... 6 Non-chemical oxidative conversion of HMF to FDCA ...... 8 Industrial production of FDCA ...... 9 FDCA in the synthesis of polyesters ...... 11 Furanic-aliphatic polyesters and their applications ...... 11 Furanic-aromatic polyesters for novel material and liquid crystalline character ...... 22 Other FDCA-based polymeric applications...... 24 Furanic-poly(ether-ester)s for high-performance engineering materials ...... 24 Furanic polyamides as sustainable alternatives to high-performance polyphthalamides ...... 26 Other high-performance furanic polycondensates ...... 29 Performance assessment of FDCA-based polymers ...... 32 Performance assessment of FDCA-based thermoplastics ...... 32 Performance assessment of FDCA-based thermoplastic elastomers and thermosets ...... 33 Conclusion ...... 35 Future outlook ...... 35 References ...... 36
2 FDCA, a versatile building block in next-generation polymers
The roadmap to a sustainable polymer industry
In recent years, the search for greener alternatives to petroleum-derived monomers has gained momentum. There is a paradigm shift towards biomass-derived monomers that is stimulated by the depletion of fossil reserves, the associated environmental concerns and a fragile economy.[1–4] With the aim of developing a more sustainable bio-based economy, the European and American legislative landscapes are shifting in favor of renewable feedstock.[1] In this regard, multiple biomass components like carbohydrates, lignin, lipids, proteins and vegetable oils are currently being sourced in biorefinery systems, yielding a set of renewable building block for polymer synthesis.[4] These polymers are referred to as “bio-based polymers” and are defined as natural polymers formed by microorganisms, plants or animals. They can also be chemically-modified according to the desired physical properties and functionalities.[5] Many bio-polymers have already been synthetized and successfully commercialized, like: polylactic acid (PLA), poly(hydroxyalkanoates) (PHAs), poly(butylene succinate) (PBS) and bio-polyethylene (PE). However, when compared to their petroleum counterpart, poly(ethylene terephthalate) (PET), polycarbonate (PC) and poly(butylene terephthalate) (PBT) their mechanical and thermal properties are still subject to improvements, especially for engineering materials (figure 1). The weaker properties of bio- based polymers can be explained by the lack of aromaticity and rigidity in the chemical structure.[2] Thus, bio-based aromatic and cyclic compounds are essential for the production of rigid next-generation bio-polymers with high glass transition (Tg) and melting temperature (Tm) and excellent mechanical properties. In the coming years, the strong social interest in a more sustainable society will become a main factor in the development and industrialization of bio-based polymers for general and engineering applications. [2,5]
Figure 1. Structural differences between bio-based and petroleum polymers
3 FDCA, a versatile renewable building block
In this framework, 2,5-furandicarboxylic acid (FDCA), which can be derived from carbohydrates through the oxidation of hydroxymethylfurfural (HMF), has been placed as one of the top 12 higher-value platform chemicals by the U.S. Department of Energy.[6] FDCA is one of the most promising candidates amongst the furan family because of its multifunctionality based on the aromatic and di-carboxylic acid moieties. The molecule has structural similarities to that of terephthalic acid (TPA) and has therefore witnessed a rapid growth to large-scale production, especially as its substitute in different polymers.[7] FDCA is also a promising building block because of it can yield numerous other monomers essential for the production of polyesters, polyamides, polyurethanes and plasticizers. Derivatives from FDCA include the dichloride- (FDCDCl), dimethyl- (DMFDC), diethyl- (DEFDC) and bis(hydroxyethyl)- (BHEFDC) (figure 2). [8]
Figure 2. Potential monomers derived from 2,5-furandicarboxylic acid[1]
According to a search on ISI Web of Knowledge, there has been an increasing amount of publications and patents over the last decade, on the incorporation of FDCA units in polymers (figure 3). The growing interest in FDCA-derived polymers can be attributed to multiple factors: their renewable and abundant origin, lignocellulosic biomass; the chemical features of the furan heterocycle to prepare materials with a wide array of properties, such as self- healing, thermal reverse-cross-linking and recyclability; the synthesis of the furan counterpart of the most important commercial polyester PET and the gradual commodification of HMF, rendering the production of FDCA more sizeable and economical.[9– 12] In addition to its widespread application within polymers, FDCA has also been used for the production of biochemicals like succinic acid [13] and fungicides [14], as a corrosion inhibitor, pharmaceutical intermediate and crosslinker for polyvinyl alcohols. [8]
200
150
100
50
0 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 Figure 3. Number of publications on FDCA-derived polymers in the last decade
4 Today, the most important FDCA-derived polymer is poly(ethylene 2,5-furandicarboxylate) (PEF), the presumed furan counterpart of PET. PEF is sourced entirely from renewable resources, namely FDCA and ethylene glycol (EG) and it has proven to have wide range of applications within films, fibers and plastic bottles.[1,4,8] Another highlighting aspect is the enhanced performance of PEF bottles compared to PET bottles. The superior properties include: 11 times better O2 barrier, 19 times better CO2 barrier, 1.6 higher tensile modulus, roughly 70% lower CO2 emission and 65% lower non-renewable energy use for PEF production.[11] These reductions in energy use and carbon emissions are also higher than that of other bio-based polymers like PLA or bio-PE. The annual market size of plastic PET bottles produced on a global scale is 15 million metric tonnes (Mt), representing roughly 6% of global plastics production. [15] The complete replacement of PET from FDCA-based PEF bottles would [11] result in greenhouse gas emission reductions of 45% to 55% (20-35 Mt CO2 equivalents).
Accordingly, in view of the shifting polymer industry, FDCA and its derivatives have a growing place in the market for bio-based monomers. Despite PEF being the lead application of FDCA, it will be left out of the scope of this paper as its properties and applications have been studied and researched extensively. Prior to the work published by Sousa et. Al [1], there was no comprehensive review summarizing the state-of-the-art use of FDCA in polyesters and other polycondensates. The authors highlighted the renewed industrial and academic interest in FDCA for polymeric applications as well as the diversity of research activity around it, including; processes for production, improvements of mechanical and thermal properties, biodegradability amongst others. Although a thorough analysis of FDCA-based polyesters was conducted, applications in other polymers and sectors seemed to be incomplete.[1] The following review will explore the most recent FDCA applications within polyesters, polyamides and other polycondensates that have been reported in the last decade. The aforementioned applications will be assessed based on their thermo-mechanical properties, type of application and performance in comparison to their petroleum-counterpart.
5 Production of FDCA: state-of-the-art
Chemical-catalytic oxidative conversion of HMF to FDCA
FDCA was first synthesized in 1876 by Fitting and Heinzelman from mucic acid using an aqueous solution of hydrogen bromide (fig 4).[16] This dehydration route did not go on to become a large-scale process due to the severe reaction parameters and high cost. Other starting material were also investigated including pentose-derived furfural. The oxidation included the use of inorganic oxidants (HNO3) proceeded by the esterification of 2-furoic acid with methanol which is converted to FDCA through multiple subsequent steps. This two-step synthetic route also proved to be complex, resulting in many intermediates and thus lowering the overall FDCA selectivity and yield.[17]
Figure 4. FDCA synthesis through the dehydration of mucic acid [8]
To date, the oxidative production of FDCA from HMF is the most promising as it can be done with homogeneous catalysts, heterogenous catalysts, biocatalysts, electrochemical oxidation and without the presence of a catalyst.[1,8] Today, many production processes have been thoroughly studied on a lab-scale and some are currently growing industrially. The industrial production of FDCA largely depends on two important steps: the dehydration of hexoses followed by the oxidation of HMF into FDCA (figure 5). According to multiple sources and patents, the production of HMF has been known since the 19th century and has been researched extensively, making it a suitable feedstock for large-scale production of FDCA. As previously mentioned, the oxidation of HMF can be achieved in multiple ways, however, it is governed by three approaches: chemical-catalysis, bio-catalysis and electrochemistry.
Figure 5. Oxidative dehydration of hexoses to FDCA through HMF
The chemical-catalytic conversion of HMF with the use of heterogeneous and homogeneous, notably noble and transition metal catalysts has attracted the most attention.[8] According to recent reports, the use of homogenous catalysis for the oxidative production of FDCA suffers from multiple hurdles; the lower FDCA yield (60%), the formation of by-products, purification and catalyst recycling.[18,19] In contrast, the heterogenous catalytic route appears to be eco- friendlier with the use of inexpensive and natural oxidants, non-toxic solvents, the easy
6 product separation and recycling of the catalyst. Metal catalysts like platinum supported on [20] [21] carbon (Pt/C) or alumina (Pt/Al2O3) have been reported to afford high FDCA yields (99%), in water and under ambient oxygen pressure. In this case, the only drawback would be the need for a high catalyst/substrate ratio, rendering the process more costly. Gold nanoparticles doped on cerium oxide (CeO2) and titanium oxide (TiO2) have also proven to be very efficient with very high FDCA yields (<99%) and operating in water or basic aqueous solutions under mild conditions. Although these catalysts seem to be presenting good results, they still suffer from instability and degradation.[22–24] The design of the catalyst, notably particle size, alloys, support, active phase amongst other factors, is key to overcome these technical barriers.
Table 1. Best reported heterogeneous catalysts for the oxidative production of FDCA from HMF HMF FDCA Time Temperature Catalyst Base Oxidant conversion yield Ref. (h) (ºC) (%) (%) 1.25 equiv. Pt-Pb/C 1 bar O 2 25 100 99 [20] NaOH 2 [21] Pt/Al2O3 pH = 9 0.2 bar O2 6 60 100 99 [20] Pt/C 1.25 equiv. NaOH 1 bar O2 2 25 100 81 [24] Au/TiO2 5 equiv. NaOH 0.3 bar O2 6 60 100 85 Au-Pd/CNT Base free 10 bar air 12 100 100 96 [25] [22] Au/HT Base free 1 bar O2 7 95 100 >99 [8] Ru(OH)x/HT Base free 2.5 bar O2 6 140 100 100 [8] Pd/HT Base free 1 bar O2 7 100 100 >99
Moreover, environmental and financial concerns were established concerning the use of alkaline solution, FDCA salt neutralization and the separation of additional by-products.[8] The development of carbon nanotubes (CNT) to support metals like Au, Pt, Pd, Ru or bimetallic systems has rendered base-free FDCA production systems competitive with alkaline medium oxidation processes. However, further research is still required to improve the base-free process FDCA yield and selectivity.[25] Another environmentally benign FDCA synthesis was published highlighting the use of hydrotalcite-supported gold nanoparticles catalyst (Au/HT). It has also demonstrated high activity and FDCA selectivity (>99%) in base-free aqueous solution under atmospheric oxygen pressure (table 1). Catalyst degradation, namely Mg2+ leaching due to the FDCA-HT interactions, still limits this process. [22]
7 Non-chemical oxidative conversion of HMF to FDCA
The biocatalytic production of FDCA from HMF has also been of high interest because of the attractive process parameters but nevertheless challenging.[8] Recent reports have shown that enzymatically-catalysed production of FDCA requires that the enzyme be reactive enough to oxidize both the aldehyde and alcohol groups of HMF. Until today, only one enzyme, hexamethyl furfural oxidase (HMFO), being part of the glucose-methanol-choline (GMC) oxidoreductase family has successfully converted all HMF to FDCA, through the 2,5‐ diformylfuran (DFF) intermediate.[26] Despite the high FDCA yield (<95%), these biocatalysts have shown to be very sensitive to the feed concentration and pH of the medium and therefore require precise process control. To overcome this, robust biocatalysts with genetically engineered strains like HMFO and HmfH have been developed to undergo the oxidation of both aldehyde and alcohol groups. These biocatalysts are most promising for large-scale applications due to their higher stability however still suffer from low FDCA yield.[27,28] Future research should be focused on genetic engineering of biocatalysts that can sustain in fluctuating process conditions, digest high HMF concentrations while maintaining a high catalytic activity.
The electrochemical oxidation of HMF can be a good alternative route for FDCA production
with the benefit of H2 production, which can be used as a green source of energy. This technology also eliminates the need of an external oxidant.[8] Different electrochemical processes have been reported however a noteworthy research is the one done by Choi with the use of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as a mediator (figure 6). The TEMPO-mediated oxidation greatly reduces the potential requirements for HMF oxidation as it suppresses the competing water oxidation. [29] The gold electrode oxidized TEMPO to TEMPO+ and gave a high FDCA yield (99%) with a high faradaic efficiency (93%) in alkaline solution. The limiting factors of this technology are the difficult separation and purification of FDCA from TEMPO and the electrolyte as well as the need for greater proportion of TEMPO to obtain high FDCA yield. [29,30]
Figure 6. TEMPO-mediated electrolytic synthesis of FDCA
All in all, the biocatalytic and electrochemical oxidative routes from HMF to FDCA still require thorough research in order to become economically and technologically competitive with the chemical-catalytic route.
8 Industrial production of FDCA
The industrial production of FDCA has been placed on the forefront of large-scale bio-based monomers by Avantium. In 2011, the company built the first FDCA pilot plant in the Chemelot campus in Geleen, Holand. [31] The nameplate capacity was set at 40 tonnes per annum. Avantium developed and patented the YXY technology to produce plant-to-plastic FDCA and PEF. It consists of the dehydration of carbohydrates to alkoxymethylfurfural (RMF) and methoxymethylfurfural (MMF) followed by a subsequent catalytic oxidation to FDCA. [32] In 2016, the company formed a joint venture with BASF, called Synvina, to commercialize the YXY technology and scale-up their plant capacity to 5 000 tons per annum in order supply enough FDCA for a multi-ton scale PEF production. In January 2019, Avantium took full ownership of the Synvina business unit and renamed it Avantium Renewable Polymers.[33] In recent events, Avantium announced that they will be locating their 5-kiloton FDCA flagship plant at Chemie Park Delfzijl in Groningen, which is predicted to start-up in 2023 (figure 7).[34]
Figure 7. Towards Avantium’s FDCA industrial production
The industrialization of FDCA has also been the core of other research and development (R&D) institutes and chemical corporations such as Corbion[35], Braskem[36], Eastman Chemical Company[37], DuPont and ADM[38] amongst others.[8] Most industrial processes employ heterogenous catalytic oxidation however some, like Corbion’s process, utilize the microbial route.[35] A summary of the commercialized FDCA production processes and operating parameters are listed in table 2. According to the table, most industrial processes can achieve high FDCA yield, however, still suffer from problems linked to product recovery, catalyst deactivation and recycling and overall production cost-competitiveness with petroleum-derived counterparts.
9
Table 2. Industrial production of FDCA FDCA Temperature Time Conversion Company Substrate Catalyst Oxidant yield Ref. (ºC) (h) (%) (%) 100 Bar [39] BASF HMF Pt/C air 100 20 100 95.2 Avantium: Renewable HMF Pt/C 100 Bar 100 18 100 78.3 [40] Polymers air 8.96 Bar [41] Eastman HMF Mn/Co/Br air 132 2 100 89.4 ADM/ Metal Salt [8] HMF 42 Bar O2 115 15 100 40 DuPont CoBr2 [8] Braskem Furfural Au/TiO2 3 Bar O2 265 4 94.9 89
[8] NOVAMONT HMF Pt/C 5 Bar O2 100 4 100 95
With a limited FDCA annual production capacity, Avantium is focusing on high-value (5.000 - 10.000 €/ ton) as opposed to high-volume markets (1.500 - 2.000 €/ ton). The objective of this review is to identify the high-value applications of FDCA that go beyond PEF, as to enable the successful commercialization.[42] A PEF market study was already conducted, where the initial targeted markets of high-value where PEF competes on performance rather than price. Eventually, further scale-up and production of PEF will lower its cost, thereby enabling its use in high-volume markets. The high-value applications identified for PEF include; multilayer packaging, enhanced bottles and optical films (figure 8). [42]
Figure 8. PEF’s market place amongst high-value applications
10 FDCA in the synthesis of polyesters
The creation and history of FDCA or DMF-derived polyesters has been known for more than 70 years, it has however only gained widespread interest in the last decade. [1–4] The first PEF patent application was filed in 1946 by the Celanese Corporation of America, only five years after the first PET patent application. In 1958, an Osaka report on PEF’s melting behaviour was published. Moore and Kelly reported the synthesis of poly(hexylene 2,5- furandicarboxylate) (PHF) and its fibrous film-forming properties in 1978. Since then, there was very limited literature on PEF or any other FDCA-based polyester. This interest resurged in the late 1990s when Gandini and coworkers published a thorough review on polymers bearing furan rings.[1,8] The renewed interest in polyesters from FDCA arose with the concern of the gradual depletion of fossil raw materials and the simultaneous emergence of biorefineries. Moreover, furanic polyesters like poly(ethylene 2,5-furandicarboxylate) (PEF), poly(propylene 2,5-furandicarboxylate) (PPF) and poly(butylene 2,5-furandicarboxylate) (PBF), present similar if not better thermal and mechanical properties than their petroleum counterparts. [43]
The polyester family can be subcategorized into three groups; linear-aliphatic, cyclo-aliphatic and entirely aromatic furanoates. To date noteworthy research activity has been focused on the aliphatic-furanic polyesters as they present more sustainable alternative to current aromatic-aliphatic polyesters, which are presently used as commodity thermoplastic polymers (PET/PBT/PPT).[44] On the other hand, fully aromatic-furanic polyesters have enhanced glass transition temperature, melting temperature, chemical stability and heat resistance which are more suitable for engineering applications.
Furanic-aliphatic polyesters and their applications
Furanic-linear aliphatic polyesters and their applications
The most researched class of furanic polymers are the linear-aliphatic polyesters. They are synthesized from biomass-derived linear aliphatic diols and FDCA or its derivatives. Their roles as future generation bio-polymers are very promising as they present more sustainable analogues to commercial terephthalic acid-derived thermoplastics like PET, PPT and PBT, which have dominated the market. Although the production of bio-based terephthalic acid from terpenes (limonene) or isobutylene is now a reality, the processes are still far from being commercialized.[45,46] The worldwide production of these three polyesters amounts to 50 Mt per year, which incentivizes the work on the improvement of the properties of their furan counterparts within industrial and academic communities. PEF, PPF and PBF, are synthesized from the FDCA dimethyl ester and a biomass-derived diol: ethylene glycol (EG), 1,3-propane diol and 1,4-butane diol, respectively. Their production and basic thermal and mechanical properties have been heavily investigated over the last decade. According to scattered literature, they have demonstrated comparable thermo-mechanical properties and superior barrier properties. Despite the advantageous properties of these FDCA-based polyesters, some limitations are still hindering their industrializations, such as their medium glass transition temperature (Tg), poor crystallizability and non-biodegradability. [43]
11 a. Furanic linear aliphatic homopolyesters
Noteworthy research has been conducted to tailor the properties of poly(2,5-furandicarboxylate) homopolyesters by increasing the carbon chain length of the diol comonomer. A series of poly(2,5-furandicarboxylate)s were synthesized and studied by Zhou et al., including, PEF, PPF PBF, poly(1,6-hexylene 2,5-furandicarboxylate) (PHF) and poly(1,8-octylene 2,5- furandicarboxylate) (POF).[47] Other linear FDCA-based homopolyesters were also prepared by multiple groups; poly(1,6-hexylene 2,5-furandi- carboxylate) (PHF) and poly(1,8-octylene 2,5-furandicarboxy- late) (POF) poly(2,3-butylene 2,5-furandicarboxylate) (P23BF)[48], poly(1,9-nonylene 2,5-furandicarboxylate) (PNF), poly(1,10-decylene 2,5-furandicarboxylate) (PDF) and poly(1,12-dodecylene 2,5-furandicarboxylate) (PDDF).[49] The thermal properties of these homopolyesters are summarized in table 3. According to these results, the glass transition (Tg) and melting (Tm) temperatures decrease as the number of methylene groups in the diol increases. This trend does not apply to P23BF, as this low-molecular weight polymer was completely amorphous, and therefore suitable for (powder) coating applications.[48]
Table 3. Thermal properties of poly(2,5-furandicarboxylate) homopolyesters with an odd and even number of methylene groups M x 103 Homopolyester w T (ºC) T (ºC) T (ºC) Ref. (g/mol) g m d PPF 22-60 50 - 58 174-175 300-375 [1] PBF 16-65 31 - 40 168-173 304-373 [1] P23BF 2.5-13 71 - 113 - 276-299 [1] PHF 13.1-32.1 7 - 28 141-148 350-375 [1] [1] POF 20.7-34.5 -5 - 22 140-149 375 [1] PNF 40.0 -30 69 - [1] PDF 36.7 -8 116 - PDDF 25.3-39.4 -22 109-111 - [1]
PPeF IV = 0.53 dL/g 19 94 394 [50]
PHepF IV = 0.38 dL/g 5 82.5 401.12 [50]
A new class of homopolyesters have been synthesized by Tsanaktsis et al., with an odd number of methylene groups, namely, 1,5-pentanediol and 1,7-heptanediol. They were only reported in 2016 because the diols were not available in high purity in the market. The resulting poly(pentylene furanoate) (PPeF) and poly(heptylene furanoate) (PHepF) were synthesized via the two-step melt polycondensation method with a resulting intrinsic viscosity of IV= 0.53 and 0.38 gL/g. This new class of bio-based polyester display high thermal stability, with a degradation temperature above 300ºC which facilitates their production (table 3).[50] Similar to the previously mentioned poly(alkylene furanotes), they are suitable for packaging applications as well as films and fibers. The mechanical properties of PPeF were then improved with the incorporation of isosorbide and succinic acid (PPeFIS) in the polyester chain. The molecular weight, flexibility and impact resistance were improved with an isosorbide concentration of 30 mol %, an FDCA concentration of 70 mol %, and a succinic acid concentration of 30 mol %. As a result, PPeFIS became ideal for coil coating applications, allowing the use of fully bio-based resins in that sector.[51]
12 b. Enhancement of PEF and other linear aliphatic FDCA-polyesters via copolymerization with different diols/acids for high performance applications
In this regard, one way to tune the properties of these homopolyesters is through the use of more than one type of comonomer, i.e., copolymerisation. Over the years, the number and type of linear chain diols and diacids used in conjunction with FDCA for the synthesis of polyesters has drastically increased. In order to address the slow melt crystallization, brittleness, and low elongation at break (1-5%) of PEF, many groups have reported its copolymerization with a wide array of commoners (figure 9).
Figure 9. List of linear aliphatic diols copolymerized with FDCA
Despite the fact that PEF showed extremely high elongation at break (35-115%) after biaxial orientation in a recent report, it usually displays brittleness due to its restricted elongation at break (1-5%).[52] For instance, to toughen PEF, different blends and random and block copolymers have been reported. When compared to other poly(alkylene 2,5- furandicarboxylate)s, higher elongation at break has been reported for PPF (46%), PBF (256%) and PHF (210%). Therefore, copolymerizing PEF with an aliphatic diol with a corresponding alkylene chain length may enable the balance between ductility and toughness. Typically, by increasing the alkylene chain length, the strength and modulus of poly(alkylene 2,5- furandicarboxylate)s are reduced but the ductility or tensile toughness increases.[53] To tackle the poor toughness, Xie et al., synthesized a series of high molecular weight poly(ethylene- co-pentylene 2,5-furandicarboxylate) (PEPeF) copolyesters with FDCA, EG and 1,5- pentanediol via melt condensation.[53] By incorporating a PeF unit as low as 9 mol% into PEF, significant improvements with tensile strength (72-83 MPa) and modulus were observed (2.8 – 3.3 GPa). Additionally, when PEPeF is compared to bottle-grade PEF, it possessed similar Tg and elongation at break. The thermal and mechanical properties obtained were very promising for applications in eco-packaging. Another series of aliphatic copolyesters poly(ethylene-co-hexamethylene 2,5-furandicarboxylate) (PEHF) were prepared from FDCA, EG and 1,6-hexanediol (HDO) via a two-step melt polycondensation.[54] The resulting semicrystalline copolymers exhibited high intrinsic viscosity, varying from 0.76 to 1.12 dL/g and superior mechanical properties than PEF. The copolyesters with 28-36 mol% hexamethylene 2,5-furandicarboxylate (HF) exhibited enhanced elongation at break (54−160%) and retained their tensile modulus (2.6−2.2 GPa) and yielding strength (71−65 MPa). The successful improvement of the tensile ductility can widen the applications of PEHF from eco-packaging to specialty packaging, thereby qualifying as high-performance applications.[54]
As previously mentioned, PEF is not only promising copolyester derived from FDCA. Another novel copolymer with two types of linear aliphatic diols has been reported by Geng et al. The
13 resulting PHPF, made from FDCA, 1,6-HDO and 1,2-propane diol (PDO), displayed high thermal stability and mechanical performance beyond that of polylactides (PLA). With a concentration of 50 mol% FDCA, 40 mol% 1,6-HDO and 10 mol% 1,2-PDO the mechanical properties were significantly enhanced when compared to the alternative PLA. The PHPF showed improvements in tensile strength of 42.7 MPa, impact strength of 35.62 KJ/m2, and elongation at break of 515 %. By increasing the content of 1,2-propanediol, the Tg was also increased from 10.0 to 92.2 ºC. Owing to its renewable source, a high thermal stability and adequate toughness, this material is of high interest for 3D-printing material through fused deposition modeling (FDM).[55]
Lately, the objective has been to develop furanic-polyesters suitable for specialty and engineering applications. For instance, thermoplastic elastomers are considered high- performance material because they fuse the rubber-like properties of elastomers and good processability of thermoplastics. [56] They are usually block copolymers with alternating rigid and elastic properties. Thermoplastic polyester elastomer (TPE) represent a new branch of the thermoplastic elastomer family. They are characterized by their high strength, elasticity and excellent abrasion resistance. Their properties are easily tunable by varying the ratio of hard to soft segment. The esters of terephthalic acid (TPA) are widely used as monomers for the hard segment of TPE, like for polybutylene terephthalate (PBT). In this regard, FDCA, with its analogous structure, has the potential to replace fossil-derived TPA. A fully bio-based TPE, poly(ethylene dodecanedioate‐2,5‐furandicarboxylate) (PEDF), has recently been synthesized with FDCA, dodecanedioic acid (DDCA), and ethylene glycol (EG) by a two-step polycondensation. As a result of its long soft aliphatic chain, DDCA served as the soft segment of the TPE. By varying the molar ratio of FDCA to DDCA, the structure of the copolyester PEDF was changed from semicrystalline plastics to completely amorphous elastomers. PEDFs with a DDCA concentration of 30 and 40 mol% showed high recovery ratios (above 90%) after cycle tensile testing, superior than some polyether-esters reported in literature. Also, biodegradability was reported, however it decreases with increasing FDCA content. PEDFs are most useful for applications in electronics, wires and cables, adhesives, athletic shoe soles and automotive components.[57]
Table 4. Thermal and mechanical properties of FDCA-based linear aliphatic copolyesters
a b c Copolyester Mw (g/mol) Tg (ºC) Tm (ºC) E (MPa) σm (MPa) εb (%) Ref.
IV= 0.78 − PEPeF 53 – 80.6 189.7 2.8 - 3.3 72 – 83 115 [53] 1.03 dL/g IV= 0.76 – PEHF 70 – 32 132.1 2.6 – 2.2 71 – 65 54 - 160 [54] 1.12 dL/g PHPF 17300 - 31700 17.5 – 41.1 - - 24 – 42.7 216 - 515 [55] IV = 0.87 – 53.5 – PEDF 19.2 – 60.7 210 - 2340 7 – 68 380 - 1480 [57] 1.20 dL/g 76.1 a Young’s modulus. b Tensile strength. c Elongation at break.
14 c. Biodegradable FDCA linear aliphatic polyesters
With regards to sustainability, the shift to fully bio-based and biodegradable polymers is of high importance. Although most poly(alkylene 2,5-furandicarboxylate)s are bio-based, they are nondegradable and resistant to hydrolysis and most bacterial or fungal attack.[58] The lack of biodegradability in most furanic aliphatic copolyesters (PEF and PBF) can be tackled by the incorporation of biodegradable aliphatic comonomers. Biobased alkanedioic acids like succinic and adipic acid or hydroxy acids like lactic acid have been used as comonomers for the preparation of FDCA biodegradable polyesters (figure 10).[1,58] Sousa et al. prepared a promising series of poly(ethylene 2,5-furandicarboxylate)-co-poly(lactic acid) (PEF-co-PLA) copolyesters. It was shown that with the incorporation of 8 mol% of lactyl units, the biodegradability of the copolyester was substantially improved, while retaining high Tg and [58] Tm values. Other interesting copolymerization with bio-based succinic acid was done with FDCA and EG resulting in poly(ethylene 2,5-furandicarboxylate-co-ethylene succinate) (PEF- co-PES) copolyester through a two-stage melt polycondensation. The copolyesters represented random structures with 12-92 mol% FDCA and molecular weights ranging from 25 600 to 57 400 g/mol. As a result of their high thermal stability with a maximum degradation temperature at 400ºC, they can be used as shape memory material.
Figure 10. Chemical structures of the monomers for the synthesis of biodegradable FDCA-copolyesters
Similarly to PEF, to address the lack of biodegradability of PBF, fully biobased poly(butylene 2,5-furandicarboxylate)-co-poly(lactic acid)s (PBF-co-PLA) were made from DMFD, 1,4- butanediol and polylactide diols through melting polycondensation. The resulting PBF-co-PLA displayed significantly superior mechanical and barrier properties than the respective homopolyester and most biodegradable materials (table 5). The incorporation of LA (10-40 mol%) did not change the thermal properties of PBF, which is vital asset in plastic processing. The ester bonds between LA and BF units were broken down via hydrolysis. These novel FDCA copolyesters are suitable for applications in eco-packaging and containers as well as bio- films.[59]
15 Table 5. Thermal and mechanical properties of biodegradable FDCA-based linear aliphatic copolyesters
M Copolyester w Tg (ºC) Tm (ºC) E (MPa) σ (MPa) ε (%) Ref. (g/mol) m b
73 500 – PBF-co-PLA 37 - 38 90 - 162 1044 - 1220 38 – 52.7 233 - 311 [59] 87 500 6 900 – PEF-co-PLA 76 119.6 - - - [58] 9 000 26000 - PEF-co-PES - 21 - 173 - - - [1] 41000 28 000 – PESF -33 - 58 39 - 70 170 - 1160 25 - 46 4 - 950 [60] 138 000 35 000 – PBSF 6 - 28 144 - 166 - 33 - 50 100 - 780 [61] 46 000
A new bio-based comonomer, sebacic acid, was reported by Wang et al. as an ideal candidate to address the biodegradability of furanic aliphatic copolyesters. It is more suitable for the synthesis of polyesters as opposed to short chain diacids due to its long carbon chain which prevents intramolecular cyclization. Additionally, the long sequence of CH2 in sebacic acid can enhance the liquidity and crystal quality of polyesters. The aliphatic poly(ethylene sebacate) (PES) synthesized from EG and sebacic acid, has attracted a lot of attention as a result of its biobased origin and biodegradability. However, its durability, thermal properties as well as mechanical properties are still lacking in some applications. By introducing the rigid FDCA into PES, the thermal and mechanical properties of the ensuing PESF were tunable depending on the composition. By increasing FDCA content, Tg and the degradation temperature increase. Although the FDCA-rich copolyesters have much higher Young modulus (1160 MPa) and strength (46 MPa) than PES, the elongation at break (4%) is sacrificed due to brittle nature of PEF. [60] The most suitable balance within the mechanical properties was achieved with 10 mol% ethylene 2,5-furandicarboxylate (EF). Another series of FDCA-polyester were copolymerized with sebacic acid and 1,4-butanediol via melt polycondensation to yield high molecular weight poly(butylene sebacate-co-butylene furandicarboxylate) (PBSFs). All PBSFs showed excellent thermal stability, however when compared to the parent homologue PBF, their Tg and crystallization is lower. These properties make them suitable for applications requiring high ductility as they display superior elongation at break (100 to 780%).[61]
With regards to high performance biobased polyesters, there is an emphasis on performance and endurance in contrast to biodegradability. Therefore, these copolyesters will not be reviewed in detail as Avantium should focus on high-performance applications for the successful first stage commercialization of FDCA. That being said, biodegradable applications for FDCA are of high importance as the chemical industry is progressively relying on plant- based raw material. FDCA has high potential within biodegradable polymers, as its main application is packaging material, thereby aiding the reduction of volume of waste generated.
16 Furanic-cycloaliphatic polyesters and their applications
a. Enhancement of furanic aliphatic polyesters via copolymerization with cyclic diols
Up until recently, most scientific literature focused on the copolymerization of FDCA with linear aliphatic polyesters. The introduction of cyclic comonomers (cyclic diols, diacids, esters) into furanic polyesters is very recent. However, this approach is very attractive as the cyclic comonomer introduces more toughness to the aliphatic polyester. According to Sousa et al., the literature is centered around the use of sugar-based 1,4:3,6-dianhydrohexitols (DAHs). The DAHs, namely, isosorbide, isomannide and isoidide, are bicyclic dihydroxyethers derived from C6 sugar alcohol in corn starch, with two hydroxyl groups placed on C2 and C5 positions with either endo or exo stereochemistry (figure 11).[1]
Figure 11. The chemical structures of the 1,4:3,6-dianhydrohexitols
Amongst them, isosorbide (Is) is the most popular one in the polymer community as it is the only isomer produced on an industrial scale at a satisfactory purity level. As a result of attractive properties like rigidity and chirality, the DAHs can be copolymerized with FDCA or its derivatives to produce polyesters with high glass transition temperature or optical features. However, the insertion of Is in the polymer backbone can hinder the chain’s regularity thereby affecting crystallinity and leading to amorphous material. Moreover, the low reactivity of the secondary hydroxyl groups can hamper the polycondensation reaction, yielding low molecular weight polymers. Copolymerization with other diols is the most well- known approach to solve these issues. This approach was taken by Chebbi et al. to synthesize a novel series of biobased poly(decamethylene-co-isosorbide 2,5-furandicarboxylate)s (PDIsFs) copolyesters from dimethylfuran-2,5-dicarboxylate (DMFD), isosorbide (Is), and 1,10-decanediol (1,10-DD) by melt polycondensation. With the incorporation of Is unit, the copolyesters presented satisfactory molecular weights and excellent thermal stability up to 405ºC (table 6).[62] Additionally, PDIsFs showed similar crystal structure than its parent homopolyester PDF as well as superior Tg and mechanical properties. By increasing Is unit to 15 mol% the copolyester becomes more susceptible to biodegradation in soil. These attractive features make this copolyester suitable for sustainable plastic packaging and related fields.[62] Other noteworthy work including the copolymerization of FDCA with Is is done by Lomeli-Rodriguez et al., for the production of biomass-derived coil coatings. The group was able to enhance the thermo-mechanical properties of copolyester resins from FDCA (15 - 30 mol%), succinic acid and either 1,3-propanediol or 1,5-pentanediol by adding isosorbide (30 – 70 mol%). With an isosorbide concentration of 30 mol% and an FDCA concentration of 70% in PPeFIS, there was a significant improvement in flexibility (2.5 T) with
respect to the reference resin (3 T), Tg on specification of 34 ºC and better impact resistance as it presented a slight cracking. By being able to tune molecular weight, glass transition
17 temperature with the Is content, these bio-based resins can potentially substitute the petroleum-based coatings. [51]
The incorporation of isosorbide as a cyclic diol in FDCA-derived polyesters has attracted attention due to its ability to tune thermo-mechanical properties to target values. In this regard, Dow Global Technologies patented the production of FDCA-based polyesters made with isosorbide for film applications requiring balanced properties like improved gas barrier, higher Tg and improved toughness, impact resistance and high temperature sealability. The films from the invention contain 5 - 10 mol% FDCA and 10 – 90 mol% isosorbide. They can be used to form multilayer packaging systems for food or other products. Additionally, they can be employed for shrink sleeve labels, bottles for beverages, high-barrier film application and long shelf-life packaging for pharmaceuticals.[63]
Table 6. Thermal and mechanical properties of cyclo-aliphatic FDCA-based copolyesters
Copolyester Mw (g/mol) Tg (ºC) Tm (ºC) E (MPa) σm (MPa) εb (%) Ref.
55300 - -1.2 – 105 - 284.5 - 205% - PDIsFs 16.66 – 20.53 [62] 84500 20.6 110 558 265% PPFIS 700 - 1100 - - - - - [51] PPeFIS 1900 - 5400 7 – 35 - - - - [51] 80.6 - 206 – 2300 - 50 – PECF 46900-76500 59-75 [64] 83 225 1740 186% 38000 - PCF 87 291 1820 52 - [65] 44700 43400 - 80.3 – 223 - 1820 - PECTF 75 – 88 186 – 67 [66] 63600 105.7 230 2140
Following the successful incorporation of isosorbide into furanoates, polyester producers have investigated the effect of other cyclic diols as comonomers. Due to the low reactivity of the secondary hydroxyl groups of isosorbides, a primary cyclic diol, 1.4- cyclohexanedimethanol (CHDM), was selected by Wang et al., to copolymerize with FDCA and EG and thereby improving the toughness of PEF. [64] CHDM is already a well-established comonomer of EG for the synthesis of polyesters and it is known to enhance their thermal properties. The group synthesized a series of high molecular weight poly(ethylene-co-1,4- cyclohexanedimethylene 2,5-furandicarboxylate)s (PECFs) with different compositions through melt-polycondensation. The incorporation of CHDM did enhance the toughness without sacrificing its Tg and barrier properties. As the content of CHDM (0-59 mol%) increased and FDCA decreased, the modulus and strength are decreased and the elongation at break was increased contrastingly. The reason is the difference in molecular flexibility between FDCA and CHDM. [64] The same observations were also published in a 2013 patent filed by Eastman Chemical, in which the resulting polyesters were suitable for specialty packaging applications.[67]
These results were also confirmed by another study which proved the enhancement of PEF crystallizability when the molar ratio of CHDM to EG is higher. This can be explained by the conformational transition of the cyclohexane ring (ring flip) which improves the polyester’s chain mobility thereby generating a much faster crystallization.[52] Since the copolymerization of FDCA with CHDM has demonstrated enhanced rigidity and crystallizability, these comonomers can be used to synthesize high performance engineering
18 bioplastics where high crystallization rate is essential. Many aliphatic and cyclic diols have been used to copolymerize with FDCA as to improve crystallizability, however the mechanical properties were greatly reduced. As an example, a series of poly(1,4- cyclohexanedimethylene furandicarboxylate) (PCF) were synthesized from FDCA and CHDM, wherein the stereochemical structure of CHDM (cis/trans) was altered to obtain high performance polyesters (figure 12). The presence of a higher trans-CHDM (98 mol%) content in PCFs, lead to a significant the increment of Tg and Tm as well as the crystallization rate, rendering these copolyesters suitable for engineering plastic applications.[65]
Figure 12. Synthesis of poly(1,4-cyclohexanedimethylene furandicarboxylate) from DMFD and CHDM.[64]
Contrastingly, completely amorphous furanic copolyesters with high Tg, transparency and good barrier properties and mechanical properties are still needed, especially for packaging material. In this vein, another interesting cyclic diol for the introduction of rigidity in FDCA- based polyesters is 2,2,4,4-tetramethyl-1,3-cyclobutanediol (CBDO). It has previously been used as a substitute for bisphenol-A in polyesters since it can cause developmental and [68] reproductive problems in humans. CBDO’s C4 ring is rigid enough to inhibit the two hydroxyl groups from forming cyclic structures. Wang et al. showed that the introduction of CBDO in PEF, PPF and PBF showed an increase in their glass transition temperature, tensile modulus, strength and a decrease in their crystallizability.[43] When the CBDO composition was at 10 mol% the semicrystalline polyesters became completely amorphous while retaining high transparency and better barrier properties to O2. When increasing the CBDO content to 18 mol%, the barrier properties to CO2 were also improved when compared to their parent homologue. The good barrier properties, mechanical performance and high transparency make these copolyesters suitable for plastic packaging applications. Eastman Chemical Company has even patented their work on FDCA-polyesters with CBDO. The polyester composition displayed constant Tg, good thermo-mechanical properties and biodegradability with different FDCA (70 -100 mol%) and CBDO (1 – 99 mol%) content.[69] The aforementioned characteristics make these cyclic copolyesters ideal in fibres, films, coatings, bottles and sheets.
Although the copolymerization of FDCA with cyclic diols like flexible CHDM and rigid CBDO seemed to enhance thermo-mechanical properties of furanic polyesters, the reported glass transition temperatures aren’t sufficiently high (below 100ºC) for high clarity and heat resistance applications. The alternative BPA polycarbonate has a Tg of 145ºC however its use
19 has been forbidden in many developed countries due to its endocrine disruptive abilities. Therefore, bio-based furanic polyesters with high thermo-mechanical performance and high transparency are still necessary. In this context, Wang et al. reported the copolymerization of both CBDO and CHDM with FDCA and EG.[66] The ensuing polyesters poly(ethylene-1,4- cyclohexyldimethylene-2,2,4,4-tetramethyl-1,3-cyclobutanediol 2,5-furandicarboxylate)s (PECTFs) will benefit from the trade-off between the two cyclic diols (figure 13). As a result of CBDO’s rigidity and CHDM’s high reactivity, the group was able to obtain polyesters with increased Tg (103.1ºC) and molecular weights (43400 – 63600 g/mol). When the CBDO composition was 45%, the reported Tg was 103.1ºC, tensile strength (88 MPa), tensile modulus (2140 MPa) and higher elongation at break (67%) than PEF’s. Additionally, PECTFs displayed good transparency as the CBDO contributed to poor crystallizability and amorphous characteristics, which are of high relevance for high transparent packaging applications.[66]
Figure 13. Synthesis of PECTFs from FDCA, EG, CHDM and CBDO[66]
b. Enhancement of furanic aliphatic polyesters via copolymerization with cyclic diacids and diesters
The incorporation of other type of cyclic comonomers in FDCA-based polyesters to tune the thermo-mechanical performance to target values has also gained popularity in the last couple of years. Although copolymerization of FDCA with CHDM and CBDO lead to satisfactory performance, the polycondensation reaction temperature exceeded 240ºC as a result of CHDM’s high boiling point. These elevated temperatures can be problematic as they tend to promote discoloration and degradation of the ensuing polyesters. In view of this, CHDM’s dicarboxylic acid, 1,4-cyclohexanedicarboxylic acid (CHDA), was selected as a commoner of FDCA to introduce better toughness of PEF under milder polycondensation temperatures. The resulting poly(ethylene 2,5-furandicarboxylate-co-ethylene 1,4- cyclohexanedicarboxylate)s (PEFCs) copolyesters were synthesized from FDCA, EG and CHDA through an esterification-polycondensation reaction at 190ºC (figure). PEFCs displayed a single glass transition temperature which increased with FDCA content. With a 30 mol% FDCA
20 composition in the diacid feed, a very high elongation at break is achieved (667%) to the expense of a lower tensile modulus and tensile strength (18 MPa). When introducing CHDA units in the polymer chain, the thermal properties remained unchanged when compared to PEF. Therefore, as a result of their significantly elevated elongation at break, PEFCs can be suitable for thermoplastics involved in stretch blow molding.[70]
Table 7. Thermal and mechanical properties of cyclo-aliphatic FDCA-based copolyesters
Copolyester Mw (g/mol) Tg (ºC) Tm (ºC) E (MPa) σm (MPa) εb (%) Ref.
33090- PEFC 32 - 76 187 800-1880 18 - 46 16 - 667 [70] 43900 36000 - -11 - PEFCL - 32 - 1632 2 - 116 32 - 2216 [71] 93000 59 15000 - -24 - 73 - 398 - PBFCL 15 - 188 24 - 33 [72] 40700 7.2 125 1300
Another interesting cyclic comonomer, which has previously been incorporated into PET, is ε-caprolactone (CL). This cyclic ester is already produced at an industrial scale, as a precursor to caprolactam for the manufacture of Nylon 6. CL has recently been incorporated in PBF through a combination of esterification, ring-opening and polycondensation reactions to yield a thermoplastic polyester elastomer (TPE). The reported TPE, namely poly(butylene 2,5- furandicarboxylate-ε-caprolactone) (PBFCL), is a multiblock copolyester composed of random hard FDCA segments and soft CL segments. With a 20 mol% CL content, PBFCL displayed a Young’s modulus of 188 MPa, a tensile strength of 33.8 MPa, and an elongation at 398%. Therefore, ε-caprolactone was able to enhance the toughness of the parent homologue PBF and maintained a higher Young’s modulus. In addition to the high strength, biodegradability was also reported. This property was inherited from aliphatic CL, respectively whereby the ester linkages are susceptible to enzyme attacks due to hydrolysis. With the excellent mechanical properties and biodegradability, these TPE are ideal candidates for the biomedical field, the furniture industry as well as sporting goods. [72]
Following the successful incorporation of ε-caprolactone in PBF, another group reported the copolymerization of this comonomer with PEF prepared by ring-opening polymerization. The resulting series of poly(ethylene 2,5-furandicarboxylate-co-ε-caprolactone) (PEFCL) copolyesters displayed superior mechanical performance when compared to PEF.[71] This improvement can be explained by the introduction of CL units which impart more flexibility and improve chain mobility. With a CL composition from 20 mol% to 70 mol% the properties of the copolyesters vary from that of brittle thermoplastics to elastomers. The reported thermal stability is slightly inferior to that of PEF, as it decreases with increasing CL content. The PEFCLs are therefore promising candidates for biobased thermoplastic applications such as packaging, consumer products, and sporting goods.[71]
21 Furanic-aromatic polyesters for novel material and liquid crystalline character
This class of polyester are formed upon polycondensation reaction between a furanic dicarboxylic acid and an aromatic diol. Over the last decade, the synthesis of fully aromatic polymers has been extensively researched due to their enhanced thermo-mechanical properties and liquid crystalline feature, which are fit for high-performance applications. In their review, Sousa et al. have briefly summarized the renewable comonomers involved in FDCA-based polyesters (figure 14). Some of them are derived from lignin (vanillic, syringic and salicylic acid) and other from sugar (2,5-bis(hydroxymethyl)furan). [1]
Figure 14. Chemical structures of the aromatic monomers present in FDCA-polyesters
The most interesting class of aromatic FDCA copolyesters are the ones with thermotropic liquid crystalline behaviour.[1] The liquid crystalline (LC) polymers exhibit characteristics of both liquid and solids and are divided into two categories: thermotropic and lyotropic polymers. Liquid crystalline thermotropic polymers (TLCPs) display LC behavior above the melting point of their crystals. Contrastingly, the ones that exhibit LC behavior in solution are the lyotropic polymers.[73] TLCPs are characterized by their excellent thermal and mechanical properties, good chemical resistance as well as ease of processing in their LC phase. Commercial examples of high performance TLCPs are DuPont’s Kevlar® polyamides and Celanese’s Vectra® polyesters.[74] In this vein, TLCPs have high melting temperatures and therefore require structural modification to lower the processing temperature. The melting point can be decreased by copolymerization with FDCA, vanillic acid or syringic acid. In the case of FDCA, the crystallization is hindered due to the introduction of bend in the polymer backbone as a result of its 123º angle between its carboxylic groups.[75]
In a recent study, Wilsens et al. developed a series of TLCPs based on FDCA, 4,4′- diacetoxybiphenyl (DABP), 4-acetoxybenzoic acid (ABA), and 4-acetylvanillic acid (AVA) via the thin-film polymerisation method (TFP).[75] The copolymerization of AVA (10 mol%) in the polymer chain prevented crystallization and thus lowered the melting temperature of the LC phase. Concomitantly, the AVA-based synthesized polymers had a higher molecular weight and lower processing temperatures.[75] In another study by the same group, a series of TLCPs with different amounts of FDCA and AVA were copolymerized with ABA, suberic acid (SuA) and 1,4-diacetoxybenzene (HQ).[74] The aromatic copolyesters were synthesized via melt polycondensation. The maximum amount of FDCA that was incorporated in the polymer chain was 16 mol% due to its solidification at low reaction temperature. Results show that
22 the introduction of FDCA into the polymer backbone lead to block polymers. The thermal properties (Tg and Tm) were improved upon SuA substitution with more rigid FDCA. Upon combination of AVA and FDCA, polymers with high aromatic content, lower melting temperature and stable melts up to 300ºC were obtained. The elongation at break were expectedly low as a result of the LC nature of the polyesters which does not allow elongation due to a lack of entanglements. FDCA is therefore a promising monomer for aliphatic- aromatic polyesters, especially when combined with AVA. Additionally, polyesters with FDCA and VA moieties require more thermal energy to become mobile, which improves their dimensional stability and overall performance at higher temperatures when compared to the TPA-based polyesters. These polymers are therefore viable for applications such as electrical and mechanical automotive parts, high strength fiber, films and tapes.[74,75]
An important aspect regarding the design of thermotropic polyesters is the balance between mechanical performance and mild processing conditions. Although the study of the incorporation of FDCA in thermotropic polyesters is limited it has been patented since 1988 by Vriesema.[76] The invention demonstrated that the substitution of TPA by FDCA causes a decrease in the polyesters melting temperatures without hindering their thermotropic character. Later, Fujioka synthesized heat resistant TLCPs by copolymerizing FDCA with hydrobenzoic acid and a phosphorous containing diol. [77]
More recently, FDCA thermotropic polymers and their preparation via were patented by the Stichting Dutch Polymer Institute of Eindhoven in 2015.[78] As mentioned previously, due to their high melting temperature it is difficult to obtain thermotropic polymers with a high average molecular mass (greater than 10 000 g/mol). The patent disclosed polyester incorporating FDCA, or its ester form (8 – 20 mol%), another hydroxycarboxylic acid with a non-cyclic group (15 mol%), a dihydric alcohol (20 – 50 mol%) and an aromatic hydroxycarboxylic acid (1 – 25 mol%). The polymerization process in the invention and the presence of a flexible monomer allows for the synthesis of polyester with an average molecular mass greater than 10 000 g/mol and products that are less prone to crystallization. As a result of their pre-oriented LC domains, the patented polymers can be applied for the formation of oriented structures like fibers, films and tapes. [78]
23 Other FDCA-based polymeric applications
Furanic-poly(ether-ester)s for high-performance engineering materials
The preparation of furan-based block poly(ether-ester) copolymers has been a hot topic in the last couple of years. They are thermoplastic elastomers consisting of a semicrystalline alkylene terephthalate hard segment with a high melting point and crystallization, and a soft polyether segment with a low glass transition temperature (figure 15).[79] They are of high commercial relevance and categorized as engineering-type thermoplastic elastomers. This class of polymers exhibits properties like combined elasticity and toughness, low temperature flexibility and strength at 150ºC.[80] Recently, the incorporation of poly(ethylene glycol) (PEG) as the soft segment, into furan-polyesters has attracted a lot of attention because it leads to copolymers with good thermal, mechanical and degradation properties.[81]
Figure 15. General chemical structure of a poly(ether-ester) with x = soft segment and y = hard segment [82]
PBT-based thermoplastic elastomers have been reported since the 1970s, like Hytrel® RS (DuPont), as a result of PBT’s high melting point and crystallization ability. Recently, PBT’s furan analogue, PBF, has been successfully incorporated in poly(ether-esters) with PEG as a result of its relatively similar thermal and crystallization properties. This novel copolymer was first synthesized by Sousa et al. showing its higher thermal stability (352-380 ºC) and lower melting point (107ºC) than PBF. With the incorporation of increasing amounts of soft PEG units, a noticeable decrease in stiffness (tensile strength and Young’s modulus) was observed. However, the elongation at break is significantly improved even with 10 wt% PEG (565 %), as opposed to PBF (289 %).[83] Another group also studied PBF-PEG’s mechanical properties and water degradation.[81] Impact toughness and elongation at break were better than that of PBF. The results showed that with increasing content of PEG to 60 wt%, the hydrophilicity of the copolymer improved, by maintaining good mechanical properties even after water uptake. These bio-based PBF-PEG copolymers are therefore suitable in fields like biomedical industry.[81] Generally, these thermoplastic elastomers have can be potentially used in applications, including tubing for automotive and industrial purposes, furniture and specialty packaging.[79]
A new family of polyether-esters based on PBF and poly(tetramethylene glycol) (PTMG) has also emerged in the last years.[84] PBF-PTMG are synthesized from FDCA, 1,4-butanediol and PTMG (1000 g/mol) through an esterification and polycondensation process. The copolymer demonstrated typical characteristics of elastomers, with good stress at break (16 – 26 MPa) and excellent elongation at break (381–832 %) and thermal stability with decomposition temperature above 360 °C.[84] Another group reported the substitution of PBF by poly(neopentyl glycol 2,5-furandicarboxylate) (PNF), showing superior performance to the existing PBT-PTMG (figure 16). By adjusting the PTMG soft segment from 50 to 70 wt% and
24 80 to 60 mol % PNF with molecular weights ranging from 13 200 to 13 800 g/mol, the melting temperature, tensile modulus, and elongation at the break change from 180 to 134 °C, 738 to 56 MPa, and 38 to 1089%, respectively. On the basis of these results, PNF-PTMG has proven to have similar thermal properties and better mechanical properties than PBF-PTMG and PBT-PTMG. Therefore furanic PNF is a suitable candidate for the hard segment in thermoplastic elastomers with PTMG and a promising substitute to its petroleum analogue PBT-PTMG.[85] The novel bio-based copolymers PBF-PTMG and PNF-PTMG can find applications in the automotive and electronic industries.
Figure 16. Synthesis of PNF-PTMG through melt-polycondensation[85]
Another type of application for FDCA-based poly(ether-esters) has been identified by Wang et al., who synthesized intelligent material with shape memory effect (SME). The group reported a series of copolymers poly(ethylene 2,5-furandicarboxylate)-poly(ethylene glycol) (PEF-PEG) from the polycondensation of FDCA, EG and poly(ethylene glycol).[86] Previously synthesized PEG-based copolymers with the same SME property include, PET, PLA[87] and PBS.[88] The authors reported PEF-PEGs with comparable thermal stability and superior elongation at break (61%) when compared to PEF, which usually display high tensile strength (72 MPa) but a low elongation at break (3%). The SME was measured with a bending test. Shape memory features were only observed with copolymers with PEG molecular weight from 6000 g/mol to 2000 g/mol, increasing with PEG content. [86] PEF-PEGs are therefore suitable for applications in biomedical science, packaging material, assembling devices, textiles and membranes. [89]
Table 8. Summarized thermo-mechanical properties of FDCA-based poly(ether-ester)s
Mw Polymer Tg ( C) Tm ( C) E (MPa) σm (MPa) εb (%) Ref. (g/mol) ° °
30 600 – PBF-PEG -34 - 35 121 - 169 29 - 476 16 - 66 567 - 1085 [81] 46 900 59 300 – PBF-PTMG -16 - 12 126 - 159 12 - 288 16 - 26 381 - 832 [84] 85 900 PNF-PTMG 94 000 – 37 134 - 180 17 - 738 34 - 40 38 - 1281 [85] 138 000 96 800 – PEG: 47 – 58 PEF-PEG PEF: 78 - 84 - 8 - 27 4 - 61 [86] 106 000 PEF: 177 - 209
25 Furanic polyamides as sustainable alternatives to high-performance polyphthalamides
a. Overview of the existing furanic polyamides and their limitations
Polyphthalamides are considered high performance materials as they exhibit high heat, chemical and corrosion resistance and excellent mechanical properties (high impact strength and modulus). They are stronger than aliphatic polyamides like Nylon 66. In addition to their favorable processing characteristics, they are a very versatile polymer family with applications in the automotive industry, marine, medical devices, personal care, electronics amongst others. They consist of an aromatic diacid (usually terephthalic (TPA) or isophtalic acid (IA)) and an aliphatic diamine.[90] Typical (co)polyphthalamides include PA6T/66, PA6T/6I, 6T/M5T, PA9T, and PA10T. The market-share leaders are the polyamides derived from hexamethylenediamine owing to their cheap cost. DSM commercialized the only aliphatic polyphthalamide, PA46 (Stanyl®), with high crystallinity and stiffness and high-water absorption due to the short chain and high amido ratio.[91]
According to several studies, FDCA, with its analogous structure to TPA, has potential in aiding the transition to more sustainable bio-polyamides, which can also be applied as high- performance material. Despite their great commercial interest, recent research activity on the synthesis and characterization of FDCA-based polyamides has been scanty. The main cause being the decarboxylation of FDCA at high temperatures (>200ºC). Instead, dimethyl 2,5-furandicarboxylate (DMFDC) or 2,5-furandicarbonyl dichloride (FDCDCl) have been employed as starting materials in the synthesis of polyphthalamides (figure 2). So far, Hopff and Krieger reported the synthesis of poly(hexamethylene furanamide) (PA6F), poly(octamethylene furanamide) (PA8F), and poly(decamethylene furanamide) (PA10F) through melt and interfacial polycondensation.[92] Another group produced poly(butylene furanamide) (PA4F), PA6F, and PA8F via melt, solution, and interfacial polycondensation. PA6F, PA8F, PA10F, and poly(dodecamethylene furanamide) (PA12F) were also prepared via melt polycondensation with the use of organometallic catalysts.[1] In general, the furanic polyamides synthesized displayed low number average molecular weights (Mn) from 6000 to 10000 g/mol.[90] Moreover, the incorporation of FDCA units in polyamides has led to lower crystallinities or amorphousness, as opposed to highly crystalline phthalic acid polyamides. According to molecular dynamics simulations, bio-based furan polyamides showed overall weaker hydrogen bonding than nylons, however they exhibited superior van der Waals cohesive energy densities and retained more rigid structures.[93]
26 b. Synthesis of heat-resistant furanic copolyamides
So far, the only commercial heat-resistant bio-based polyamide is PA10T, with the diamine segment sourced from castor oil. Nonetheless, the bio-carbon content of this polyamide is only at 55.5%.[91] Although the first FDCA-based polyamides date back to the twentieth century, their synthesis still remains problematic. FDCA degrades at 271ºC, and its respective polyamides are subject to decarboxylation at high temperatures (>200 ºC).[94] The reaction temperatures required for the synthesis of heat-resistant polyamides in the industry through melt polymerization and solid state polymerization (SSP) are 300 ºC and 250 ºC, respectively. Therefore, it is of great importance to find a viable route for the commercial production of furanic heat-resistant polyamides. In this regard, semi-biobased poly(decamethylene terephthalamide/furanamide) (PA10T/10F) was synthesized from FDCA, TPA, benzoic acid and 1,10-decanediamine through SSP. A series of copolyamides with a phenylene to furan moiety ratio of 10T/10F= 9/1, 8/2 and 7/3 were synthesized. A noteworthy observation indicated that an increment of FDCA concentration limited the formation of high molecular weight polyamides (3100 g/mol) due to its decarboxylation (figure 18). Similarly, the melting point of the ensuing polyamides decreased with an increasing FDCA concentration. However, all the PA10T/10F displayed melting points above 280ºC and satisfactory thermal stabilities when compared to the homopolymer PA10T, making them suitable for heat-resistant fields.[91]
Figure 18. The decarboxylation of 2,5-furandicarboxylic acid at high temperatures [91]
In 2014, Furanix filed a patent covering an improved method to produce high-molecular weight (Mw=30 000-35 000 g/mol) FDCA-based polyamides. The ensuing polyamides are made of 15-80 mol% FDCA, 20-85 mol% aromatic dicarboxylic acid and 50-100 mol% of an aliphatic diamine with six to seven carbon atoms. Comparing to prior art, the new preparation method suggests the use of DMFDC and a 10-50 mol% excess of a diamine to avoid unwanted N-methylation. The resulting oligomers were then coupled to FDCDCl, which acts as a bifunctional linker, in the presence of a hindered amine.[1] The second step was completed in an anhydrous polar aprotic solvent, such as DMF or NMP.[95] By varying the bifunctional linker, different homo- or co-polyamides can be obtained. When the linker is an oligomer, multiblock copolyamides are usually ensued.
Another group synthesized a series of biobased furanic aromatic polyamides from FDCA and different aromatic diamines: p-phenylenediamine (PPD), m-phenylenediamine (MPD), 4,4- diaminodiphenyl ether (ODA), 4,4-diaminodiphenyl sulphone (DDS) and 4,4- methylenedianiline (DDM), named PPF, PMF, POF, PSF and PCF (figure 19). When compared to prior studies, the group achieved higher weight average molecular weight (Mw) than ever reported, via direct solution polycondensation (table 9). PPF displayed the best thermo- mechanical properties amongst the five furanic polyamides, showing adequate properties for
27 engineering plastic applications (table 9). In comparison to its petroleum homologue poly(p- phenylene terephthalamide) (PPTA), also known as trade marketed Kevlar®, the thermal stability of bio PPF is inferior but still high enough for some high-temperature applications like automotive powertrain components or the housing for electrical connectors. These results can be explained by the easier degradation of the furan moiety than that of benzene, the disrupted hydrogen bonds amongst amide groups which are accountable for the strong interchain interactions and the lower molecular weight of PPF.[96]
Figure 19. Synthesis of furanic aromatic polyamides with FDCA and a diamine monomer [94]
Table 9. Summarized thermo-mechanical properties of FDCA-based polyphthalamides [96]
Polyamide Mw (g/mol) Tg (ºC) Tm (ºC) E (MPa) σm (MPa) εb (%)
PPF a 100 000 302.4 - 1635 94.77 9.13 PMF b 118 000 274.4 - 1504 87.5 9.46 POF c 140 000 218.8 - 1361 83.9 11.12 PSF d 58 800 234.7 - 998 71.4 10.97 PCF e 70 500 217.4 - 1283 77.7 7.45 Formed upon the polycondensation of FDCA and a p-phenylenediamine, b m-phenylenediamine, c 4,4- diaminodiphenyl ether, d 4,4-diaminodiphenyl sulphone, e 4,4-methylenedianiline.
28 Other high-performance furanic polycondensates
a. FDCA-based poly(aryl ether ketone)s for high-performance engineering plastics
Poly(aryl ether ketone)s (PAEK) are a family of semicrystalline thermoplastics with a polymeric backbone consisting of alternating ketone and ether groups linked by aryl groups. They are characterized by their high thermal stability, excellent mechanical properties and resistance to hydrolysis and are therefore applicable in engineering fields such as oil drilling components and aerospace electronics or in medicine for the production of surgical implants.[97] So far, the industrially relevant PAEK were synthesized from petroleum based aromatic monomers. In this context, Bao et al., reported the synthesis of two partially bio- based PAEK from an FDCA derivative, 2,5-bis(4-fluorobenzoyl) furan (BFBF) (figure 20).[98]
Figure 20. The synthesis of the 2,5-bis(4-fluorobenzoyl) furan (BFBF) monomer from FDCA[98]
The polymers were prepared by nucleophilic aromatic substitution polymerization of petroleum-based monomers 4-(4-hydroxyphenyl) phthalazin-1(2H)-one (DHPZ) and 9,9- bis(4-hydroxyphenyl) fluorine (BHPF) with BFBF, forming partial bio-based poly(aryl ether [98] ketone) resins (PFBEK and PFDEK) (figure 21). The measured Tg of PFBEK and PFDEK was approximately 12 °C less than that of their petroleum counterparts. This is due to the substitution of the phenyl moiety by a non-linear furan ring which increases the free volume in the polymer backbone. Nevertheless, the bio-based PAEK show excellent thermal stability, mechanical properties as well as reduced melt viscosities which allows for lower temperature processing (table 10).[98]
Figure 21. The synthesis of poly(aryl ether ketone)s PFBEK and PFDEK from BFBF monomer [98]
Table 10. Summarized thermo-mechanical properties of FDCA-based poly(aryl ether ketone)s
M Polymer w Tg (ºC) Tm (ºC) E (MPa) σ (MPa) ε (%) Ref. (g/mol) m b
PFBEK 65 000 226 - 1170 88 11 [98] PFDEK 69 400 234 - 1160 92 13 [98]
29 b. FDCA-based poly(amide imide) for high-performance films
Poly(amide imide)s (PAI) are classified as high performance amorphous engineering thermoplastics. They are synthesized by the polycondensation of an aromatic diamine such as methylene dianiline and an anhydride such as trimellitic anhydride (figure 22).[99] PAIs are characterized by high thermal stability (high Tg around 275 ºC and resistance to heat), high tensile strength over a wide range of temperatures, good chemical and UV resistance.[100] They are fit for applications demanding high mechanical strength and chemical properties such as machined parts for the automotive and aerospace industries. However, the limiting factors hindering their commercialization is their cost and high processing temperatures. [101,102]
Figure 22. Synthesis of a poly(amide imide) from the condensation of trimellitic anhydride and methylene dianiline [99]
In 2019, bio-based FDCA poly(amide imide)s were patented by a Chinese university for film applications. The ensuing polymer is formed by reacting the FDCA diamine monomer with an aromatic dianhydride in a DMF solution.[101] The introduction of the furan ring improved the strength, modulus and visible light transmittance. When compared to terephthalic acid polyamide imide film, the tensile strength, modulus and elongation at break were significantly improved (table 11). The prepared TPA film was a dark yellow film with an intrinsic viscosity of 0.89 dL/g, a Tg of 207ºC a tensile modulus of 5.7 GPa, a tensile strength of 187 MPa and an elongation at break of 3.3%. The resulting polyamide-imide can meet the requirement of applications such as packaging material, fibers and engineering plastics. It is also deemed suitable for high-performance heat resistant films.[101]
Table 11. Thermo-mechanical properties of FDCA-based poly(amide imide) patented by SKC
Polymer IV (dL/g) Tg (ºC) Tm (ºC) E (MPa) σm (MPa) εb (%) Ref.
6500 – 10 PAI 0.87 180 - 250 - 140 - 320 2.6 – 7.9 [101] 300
30 c. FDCA in the synthesis of high-performance bio-epoxy resins
Epoxy resins are characterized by their excellent mechanical properties, chemical resistance and shape integrity under extreme conditions. These outstanding properties result from their crosslinked 3D network structure formed upon the reaction between an epoxy monomer and a curing agent.[103] The use of epoxy thermosets has experienced a rapid surge in recent years especially in the fields of aerospace, coatings, solar cells, adhesives and composites.[2] Today, the most important commercial epoxy resin is the diglycidyl ether of bisphenol A (DGEBA), covering 90% of global use. Despite its superior thermo-mechanical properties, it has been proved to have severe health effects and the use of BPA in packaging material has been banned by the US Federal Drug Administration.[104] To date, most of the bio-based epoxy resin displayed poor thermo-mechanical properties and low curing potential.[2,103] In this regard, Deng et al., synthesized the first epoxy monomer from FDCA, diglycidyl ester of 2,5-furandicarboxylic acid (DGF). The mechanical properties of DGF were compared to that of its petroleum counterpart, DGT. Both epoxy monomers were cured by the same agent, methylhexahydrophthalic anhydride (MHHPA). Results showed that DGF has tremendous potential to replace TPA in the synthesis of bio-epoxy resins due to its enhanced curing activity, glass transition temperature and comparable mechanical performance. The intermolecular hydrogen bonds between the oxygen atoms of the furan ring and the hydroxyl groups during the curing reaction and the more difficult rotation of the heterocycle explain [2] the enhanced properties (Tg, strength and modulus) of DGF.
Figure 23. Chemical structures of TPA-based DGT and FDCA-based DGF[2]
Table 12. Thermo-mechanical properties of FDCA and TPA-based epoxies cured with the same agent MHHPA [2]
Polymer Tg (ºC) Tm (ºC) E (MPa) σm (MPa) εb (%)
DGF/MHHPA 152 - 3000 96 - DGT/MHHPA 128.8 - 3080 90 -
Following this study, a Chinese university patented their work on FDCA-containing bio-epoxy resins and their preparation.[104] Prior art[2] obtained cured FDCA epoxy resin, however, only with 62.5% biomass content. The goal of the invention was to synthesize epoxy resins with full bio-content and excellent thermo-mechanical performance. FDCA is first acetylated with thionyl chloride, and eugenol and a tertiary amine are separately dissolved in dichloromethane. The acetylated FDCA solution is then added to the eugenol solution dropwise to obtain bis(4-allyl-2-methoxyphenyl)furan-2,5-dicarboxylic acid ester. By adding
31 m-chloroperoxybenzoic acid, the double bond is epoxidized to form a fully bio-based epoxy resin. The Chinese patent stood out as a result of the simple preparation process, the use of green and non-toxic raw materials as well as the excellent thermal and mechanical properties of the ensuing epoxy resin.[104]
Performance assessment of FDCA-based polymers
Performance assessment of FDCA-based thermoplastics
Following the increasing research activity on FDCA-based polymers in the last decade, several other applications have also surfaced within poly(ester amide)s, polyurethanes, plasticizer additives. The bulk of the activity remains within thermoplastics. FDCA thermoplastics can be categorized into three groups. The first one consists of commodity thermoplastics. This group usually requires larger volumes for mass-production and is dedicated to consumer use as packaging material, bottles, films and fibers. Most of the furanic linear aliphatic homopolyesters and copolyesters make up for this class (PPeF[50], PEHF[54], PDISF[62], PECF[64]...). In this case, these polymers have enhanced properties when compared to PEF. Notwithstanding its superior barrier and thermal properties, PEF still shows brittle fracture behaviors as a result of its low elongation at break and low crystallization rate, which causes difficulties in processability (stretch blow molding) and solid-state polymerization (SSP) for the production of high-molecular-weight PEF.[52] Therefore, the performance of these newly reported FDCA copolyesters is considered higher than that of PEF as their reported mechanical properties were successfully improved with the incorporation of other monomers (linear and cyclic diols), with young moduli, impact strengths and elongation at breaks reaching 2340 MPa, 83 MPa and 780%, respectively (figure 24).
The engineering thermoplastics are designed for superior performance according to at least one metric like, chemical resistance, shock absorbance, durability or flame resistance.[105] Common products that include engineering thermoplastics are heat and chemical resistant units or housings, electrical connectors, heat resistant films, machined parts for the automotive industry or other technical parts for machine-building. For such applications, relatively small quantities are required in comparison to commodity thermoplastics. In this review, this class refers to most of the furanic polyphthalamides (PA10T/10F[91], PPF, PMF and POF[94]). They are characterized by their high heat resistance and excellent mechanical properties and processing flexibility (figure 24).[96]
The high-performance engineering thermoplastics (HPET) which are valued for their excellent thermal stability, high and low temperature performance and mechanical properties as well as good chemical and hydrolysis resistance. They can withstand extreme conditions and are suitable for applications in aerospace, aircraft, medical devices, oil drilling components amongst others. With their current production capacity set at 40 tonnes of FDCA per year, Avantium should mainly focus on high performance thermoplastics, namely poly(amide imide)s, poly(aryl ether ketone)s and the aromatic thermotropic copolyesters with liquid crystalline (TLCP) character as these applications require small volumes and tend to be highly priced. Despite their outstanding properties, the commercialization of FDCA-based HPET may be limited by their cost and difficulties related to FDCA decarboxylation at high-processing
32 temperatures (figure 18). The transportation field (aircraft, aerospace and automotive) for FDCA-based HPET seems to be promising because it is driven by the gradual replacement of metals by thermoplastics due to the high demand for weight reduction, increase in fuel efficiency and reduction in carbon footprints of vehicles and planes. In the biomedical field, the use of furanic PFDEK and PFBEK for surgical implants will be propelled by their biocompatibility and enhanced processability.[98]
Figure 24. Performance assessment of FDCA-based thermoplastics covered in this review
Performance assessment of FDCA-based thermoplastic elastomers and thermosets
The other portion of the FDCA-based polymeric applications can be grouped within thermosets and thermoplastic elastomers (TPE). High-performance thermosets (HPT) are characterized by their high thermal resistance and excellent bond performance. Today, epoxy resins are the most popular class of HPT as a result of their affordability, good heat and chemical resistance. These pre‐polymers can be found in a variety of industries, such as adhesives, coatings, insulations, and high-performance composites.[106] The FDCA-based monomer, diglycidyl ester of 2,5-furandicarboxylic acid, can be a potential bio-replacement to commercial diglycidyl ether of bisphenol A (DGEBA) due to its suitable thermo-mechanical properties as a result of the hindered rotation of the furan ring in the polymer network.[2]
Thermoplastic elastomers (TPE) also constitute their own category as they combine the rubber-like properties of elastomers and good processability of thermoplastics. In this study, many TPE were uncovered such as thermoplastic elastomer copolyesters (linear and cyclic), such as PEDF and PBCL, and poly(ether-ester)s like PEF-PEG, PBF-PEF and more novel ones like PBF-PTMG and PNF-PTMG. They all display high strength, elasticity and excellent abrasion resistance with reported moduli, elongation at breaks up to 2340 MPa and 1480%,
33 respectively. It is important to mention that their mechanical performance is superior than that of their parent homologues, PEF and PBF. These reported properties render these TPE appropriate for applications in electronics, wires and cables, tubing and sporting goods. A noteworthy novel application for PEF-PEG is in the synthesis of intelligent material with shape memory effect which can be applied in high-performance fields like biomedical science for stents of filters or even aerospace in deployable structures and morphing wings.[86] Recently, there has been a shift from shape memory alloys to shape memory polymers. From an economical and engineering standpoint, tailoring the properties of TPE polymers is much more facile and inexpensive than that of metals and alloys.[88]
34 Conclusion An important aspect to highlight is the renewed and more diversified interest in FDCA-based polymers. The furanic compound will play an essential role in next generation bio-polymers due to its renewable source as well as its structural similarity to terephthalic acid. Indeed, furanoate-based polyesters have shown tremendous potential in comparison to their terephthalate counterparts, which propelled scientific curiosity to pursue topics like the incorporation of FDCA in high performant polymers and the enhancement of their thermo- mechanical properties. In this vein, the slow melt crystallization and brittleness of PEF were addressed through copolymerization with monomers such as 1.6-hexanediol or 1.4- cyclohexanedimethanol. These PEF copolyesters may compete with other engineering type thermoplastics in terms of performance, however they are still not cost competitive alternatives.
With a limited FDCA production capacity, Avantium should focus on high performance engineering thermoplastic applications such as poly(amide imide)s, poly(aryl ether ketone)s, thermotropic liquid crystal polyesters and polyphthalamides, which are highly priced and require small volume. Moreover, innovative domains are gradually emerging for furanic polymers such as 3D printing and shape memory material. Expectedly, not all polymeric applications will reach practical realizations. Some of the challenges impeding on their commercialization include; obtaining the raw materials in bulk, the stability of the production and processability of the polymers as well as their cost-property ratio. In the near future, the most promising FDCA applications are the ones where the polymer possesses several properties with superior performance than the commercial counterpart.
Future outlook
As discussed in this review, substantial work has been done in regard to the incorporation of FDCA or its derivative forms in thermoplastic polymers for high performance applications. In this regard, targeted properties like high glass transition temperature, high crystallinity and a balance between toughness and stiffness were successfully achieved through the copolymerization with respective monomers and variation of compositions. A challenge that needs to be addressed for the development of high molecular weight furanic polymers for engineering applications, is the instability of pristine FDCA in the melt polycondensation reactions above 200 ºC, which leads to decarboxylation and discoloration issues. Therefore, further research needs to focus on the use of thermally stable compounds such as the protected FDCA monomers like its dimethyl or dichloride forms. Furthermore, following the successful synthesis of high performance FDCA-based epoxy resins, future research should be directed towards thermosetting resins as to widen the potential applications.
Another challenge that may need to be addressed in the near future will be the implementation of an economy of scale around FDCA. The scale-up of FDCA production will establish a non-interrupted supply chain and will slowly enable these bio-polymers to become cost-competitive alternatives. The development of high performant engineering furanic polymers that is partly driven by the shift of metal to plastic use in aerospace, automotive and medical industries can accelerate the industrialization of the furan compound.
35 References
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