Journal of the American Oil Chemists' Society

, Volume 94, Issue 2, pp 169–186

Sustainable Synthetic Approaches for the Preparation of Plant Oil-Based Thermosets

Review

DOI: 10.1007/s11746-016-2932-4

Cite this article as:
Llevot, A. J Am Oil Chem Soc (2017) 94: 169. doi:10.1007/s11746-016-2932-4
  • 346 Downloads

Abstract

The good availability and high degree of functionalization possibilities of plant oils entitles them to be one of the most intensively studied renewable resources, especially in polymer science. However, in line with the principles of green chemistry, the use of renewable resources should be accompanied with catalytic procedures, comparably less or non-toxic chemicals, as well as reduction of waste and energy consumption to achieve an overall sustainable process. In this review, these aspects are addressed using the example of plant oil-based thermoset materials, which bear the advantage, in terms of sustainability, of not requiring a separation or purification step prior to polymerization. The direct homopolymerization of plant oils, as well as copolymerization, exclusively with other renewable resources, are highlighted. The sustainability of the synthesis of a broad range of thermosets including epoxy resins, polyurethane networks, polybenzoxazines and unsaturated polyesters is discussed.

Keywords

Vegetable oil Thermoset Sustainability Renewable resources Polymer 

Introduction

Nowadays, the awareness about environmental deterioration and about our dependency on depleting fossil feedstocks forces research to find solutions in order to design a more sustainable future. Due to an ever-increasing population on our planet and industrial developments, a major focus is on energy and fuel supply, as well as chemical and polymer production. In search for alternatives to the finite fossil feedstocks, the renewability and structural diversity of biomass makes it an excellent candidate [1, 2, 3, 4, 5, 6]. Especially in polymer science, the use of renewable resources has been intensively investigated and discussed in several reviews over the last years [7, 8, 9, 10, 11]. Amongst biomass, vegetable oils constitute a platform of aliphatic hydrocarbons, particularly studied as polymer precursors due to their good availability, low price, inherent biodegradability and potential for chemical modifications [12, 13, 14, 15, 16]. Vegetable oils are composed of triglycerides, which consist of three fatty acid chains esterified with glycerol. Fatty acids vary in their chain length, as well as degree and location of their unsaturations. The fatty acid structures of the plant oils discussed in this review are shown in Scheme 1 and the composition of these oils displayed in Table 1. The fatty acid composition of the triglycerides, depending on the plant origin, influences their properties significantly [14]. Well-defined fatty acids can be recovered after hydrolysis of triglycerides and further separation and purification [17]. The latter can be transformed into difunctional monomers for the synthesis of a broad range of thermoplastic polymers [18]. However, this additional step decreases the sustainability of the synthesis. Thus, the direct polymerization of poly-functional vegetable oils into thermoset polymers represents a “greener” approach towards polymeric materials.
Scheme 1

a General structures of triglycerides, b of some fatty acid chains (palmitic acid 1, stearic acid 2, oleic acid 3, linoleic acid 4, linolenic acid 5, ricinoleic acid 6, vernolic acid 7, behenic acid 8)

Table 1

Fatty acid composition of the vegetable oils mentioned in this review [16, 30, 31, 32]

Name

Palmitic 1

Stearic 2

Oleic 3

Linoleic 4

Linolenic 5

Ricinoleic 6

Vernolic 7

Behenic 8

Soybean

14.0

4.0

23.3

52.2

5.6

Castor

1.5

0.5

5

4

0.5

87.5

Linseed

5

4

22

17

52

Vernonia

3.0

3.0

5.0

14.0

74.0

Karanja

11.6

7.5

51.5

16.0

2.6

4.3

Rapeseed

4

2

56

26

10

Grapeseed

7

4

15.8

69.6

Thermosets are cross-linked polymeric materials known for their good thermomechanical properties and chemical resistance. The production of these polymers represents ~20% of the total annual polymer production [19]. Thermosets can be synthesized in different chemical ways and the production of biobased thermosets has been recently reviewed extensively [20, 21, 22, 23, 24, 25, 26]. The multiple unsaturations of the triglycerides constitute polymerizable moieties for the direct synthesis of thermosets, as well as bases for poly-functionalization and further polymerization. This versatility makes vegetable oils excellent substrates for the production of cross-linked materials.

Renewability is one of the twelve principles of green chemistry, as introduced by Anastas and Warner in 1998 [27, 28]. For a more sustainable future, it is crucial to implement as many green chemistry principles as possible for the production of polymers, including the use of safer synthetic procedures and catalytic reagents, as well as the consideration of factors such as energy consumption and waste prevention [29]. In this context, this review is dedicated to the sustainable synthesis of fully biobased thermosets from vegetable oils. One of the main advantages of vegetable oils in terms of sustainability is their liquid character, which generally enables the use of solvent-free procedures. The objective of this manuscript is not to provide an exhaustive list of all the works reported in the literature, but rather to discuss the sustainability of different polymerization strategies and synthetic advances, which enable us to circumvent the use of hazardous and toxic materials. All copolymers with petroleum-based comonomers are therefore not discussed.

Discussion

Direct Polymerization Methods

The inherent structure of many triglycerides enables their direct polymerization, depending on their degree of unsaturation. The degree of unsaturation is expressed by the iodine value, which corresponds to the amount of iodine (in g) that can react with the double bonds present in 100 g of sample. The oxidation of vegetable oils is a natural phenomenon that can be accelerated by several parameters, such as temperature, oxygen, driers or catalysts. This property is exploited for the production of alkyd resins, in which fatty acid derivatives are added to polyester resins in order to give them air drying properties [30]. Although alkyd resins are used for coating, oxidized linseed oil is also including in the formulation of linoleum for floor covering applications [31]. The mechanism of this oxidative polymerization involves the formation of radicals by homolytic cleavage and abstraction of a hydrogen radical on the allylic position in a polyunsaturated fatty acid chain. On the one hand, conjugated hydroperoxides are produced by oxygen uptake and on the other hand, recombination of the radicals leads to cross-linking between the triglyceride chains (Scheme 2). Further decomposition of hydroperoxides yields alkoxy radicals or oxidation products. Consequently, numerous reactions are involved in the formation of cross-linked triglycerides. The mechanism as well as the influence of driers, catalysts and structure of the starting vegetable oil was investigated [32, 33, 34]. The cross-linking efficiency affects the physical properties of the resulting polymers, which are decisive for the final application. For instance, Hillmyer et al. polymerized soybean oil and conjugated soybean oil directly in the presence and absence of a free radical initiator, yielding materials with different physical properties. In contrast to unpolymerized soybean oil, this polymer was successfully blended with PLLA resulting in improvement in the tensile strength compared to native PLLA. The gel fraction of the polymer, initially tuned by the cross-linking reaction conditions of soybean oils, proved to be a key parameter to control the blend morphology [35]. However, mainly oligomers are produced by oxidative homopolymerization. In order to tackle this issue and to improve the thermomechanical properties of the obtained polymers, triglycerides are copolymerized with more reactive monomers, such as styrene or methylmethacrylate. These approaches are outside the scope of the review but are discussed elsewhere [36]. Materials with similar film properties as styrene-vegetable oil systems were prepared by copolymerization of condensed tannin-fatty acid esters with vegetable oils. The similar structural motifs between the fatty acid chains of these esters and the vegetable oil comonomer ensures a consistency in polymerization rate leading to a single copolymer phase. The tannin moiety offers rigidity and enables the production of polymer films ranging from soft rubbers to rigid thermosets by tuning the comonomer ratio [37]. Even if 100% biobased thermosets are produced, the sustainability of this study is affected by the esterification procedure, using acyl chloride to prepare the condensed tannin-fatty acid esters.
Scheme 2

Direct polymerization methods of plant oils: a initial radical formation during autoxidation of vegetable oils, further recombination leads to cross-linking, b solvent-free grinding polymerization of cardanol 9 by FeCl3

Other aromatic naturally occurring compounds were also investigated in such an approach. Cardanol 9 is a component of cashew nut shell liquid, which is a viscous liquid contained in the honeycomb structure of the shell of cashew nuts. This by-product from the cashew industry exhibits a phenolic structure containing, in meta position, a C15 aliphatic chain exhibiting between 1 and 3 unsaturations. Plant oil-based polymers, were directly prepared by (co)polymerization of cardanol 9. The solvent-free grinding of cardanol 9 in presence of FeCl3 was investigated and proved to consist of a combination of oxidative polymerizations similar to triglycerides, Friedel–Crafts reactions and etherification reactions [38]. By directly utilizing the raw product without further functionalization, the direct polymerization methods are the “greenest” approaches, but suffer from the limited reactivity of the internal double bonds of the fatty acid chains. In order to overcome this problem, most authors describe chemical modifications of vegetable oils prior to their polymerization.

Polymerization of Epoxy Precursors

One of the most straightforward approaches for the synthesis of thermosets from vegetable oil derivatives is realized by the preparation of epoxy resins. Indeed, the presence of several double bounds in their chemical structure can be chemically exploited enabling the preparation of poly(epoxy) precursors. In order to produce thermosets, the crosslinking reactions can be performed by either homopolymerization including cationic and radical mechanisms or by addition of a hardeners, such as diamine, diacid or anhydride [39]. The suitability of vegetable oils for the synthesis of epoxy resins is of high interest, as epoxy resins represent 70% of the thermoset materials and are employed in many fields, such as aeronautics, construction, electronics, etc. [19]. Epoxidized vegetable oils have nowadays already been cross-linked employing different strategies.

Synthesis of Epoxy Precursors

Vegetable oil epoxy precursors are obtained by peroxidation of the carbon–carbon double bonds. Epoxidized vegetable oils, such as epoxidized soybean oil, are produced by the Prilezhaev reaction on an industrial scale, due to their use as plasticizers and as intermediate for the synthesis of polyols for the manufacture of polyurethanes [40]. This synthesis involves the in-situ production of peracids, generated from acetic acid and hydrogen peroxide in the presence of strong mineral acids [41]. This process conflicts with several principles of green chemistry, especially in terms of safety and waste, respectively due to the use of large amounts of peracids and strong acids, which require neutralization and removal from the final products. In addition, undesirable acid-catalyzed epoxy ring-opening reactions are observed. In order to address this problem, several protocols with different degrees of sustainability and industrial suitability were described in the literature previously. Several catalytic systems have been investigated, such as homogeneous and heterogeneous, inorganic, organometallic and enzymatic systems [25, 42, 43]. For instance, chemical-catalyzed epoxidations were reported with tungsten-based catalysts, titanium-grafted silica or molybdenum(IV) complex and enzymatic ones with oxygenase/peroxygenase or lipase biocatalysts. Despite the high costs of most catalysts, which normally limit their industrial use, these procedures are preferable to the classic route, in terms of sustainability and safety.

Homopolymerization

The homopolymerization of epoxidized vegetable oils, involving cationic or radial mechanisms, enables the synthesis of 100% biobased materials and avoids the use of hardener (e,g,, toxic amines or anhydride, Scheme 3). These thermosets mainly find applications in coating and pressure-sensitive adhesives, due to their good flexibility resulting from the long aliphatic chains of the employed vegetable oils [13, 44, 45]. Direct homopolymerization of epoxidized vegetable oils can be achieved by ring opening polymerization of the epoxy moieties in the presence of acid catalysts. Super acids [e.g., fluorosulfonic acid, hydrated or anhydrous fluoroantimonic acids (around 0.05 mmol of catalyst per gram of epoxidized vegetable oil)], were successfully established for the polymerization of epoxidized soybean oil in ethyl acetate. Only very recently, a mechanism on epoxidized soybean oil has been proposed by Biswas et al. [46, 47, 48]. The propagation is a classic cationic polymerization of epoxides. Depending on the temperature and amount of initiator, the formation of ketone by rearrangement, as well as the hydrolysis of the triglyceride esters was observed. Additionally, furan moieties could be detected due to backbiting reactions of the propagating cation and further elimination/isomerization. Lewis acids, such as boron trifluoride, were also reported for the polymerization of epoxidized vegetable oils in dichloromethane and in liquid and supercritical carbon dioxide (scCO2) in order to reduce the environmental impact of the procedure [49, 50, 51, 52, 53]. Indeed, scCO2 is considered as a green reaction medium and good substitute to volatile organic solvents due to its non-flammability and lack of any (toxic) residues in the final products [54].
Scheme 3

Different homopolymerization approaches of epoxy precursors towards thermoset synthesis

An even more sustainable approach for the synthesis of epoxy resins from vegetable oils employs solvent-free procedures with latent catalysts, which do not require separation from the final product. The use of latent initiators, which are inert under normal conditions but activated under heat or light, enables the control of the polymerization and thus increases the storage stability and handling of the epoxy resins [55, 56]. Cationic polymerization of epoxidized vegetable oils can be initiated by thermally latent initiators, such as benzylpyrazinium salts, which are non-active at room temperature. Park et al. reported the cationic polymerization of epoxidized soybean oil and castor oil by N-benzylpyrazinium hexafluoroantimonate. Polymerization occurred at relatively low temperatures (e.g., 50 and 80 °C for epoxidized soybean oils and epoxidized castor oil) compared to the curing of epoxy resins employing hardener, thus lowering the energy consumption. However, the application of higher temperatures increased the polymerization rate drastically. The difference in reactivity between the two epoxidized vegetable oils can be explained by a difference in epoxy contents, i.e. 4.6 for epoxidized soybean oil against 2.8 for epoxidized castor oil. Thus, a higher cross-linking density is reached for resins prepared from epoxidized soybean oil [57]. The initiating temperature corresponds to the cleavage of a heteroatom and a carbon atom of the initiator [58]. As the thermal latency can be tuned by the structure of the initiator, Park et al. demonstrated that the replacement of N-benzylpyrazinium hexafluoroantimonate initiator by N-benzylquinoxalinium hexafluoroantimonate could decrease the polymerization temperature [59]. More recently, Sharma et al. employed N-benzylpyrazinium hexafluoroantimonate as initiator for epoxidation and polymerization of a variety of vegetable oils, namely linseed, soybean, oilseed radish, cottonseed, peanut and canola oils. The composition of the oil and therewith varying fatty acid composition (and thus epoxy content) directly influenced the thermomechanical properties of the final material. For instance, linseed oil, with the highest number of unsaturations, exhibits the highest modulus and good impact resistance properties [60]. In another examples, Tsujimoto et al. recently reported the shape memory properties of a biobased polymeric material prepared by thermal curing with a latent catalyst of a mixture of epoxidized soybean oil and poly(lactic acid) (PLA) [61]. PLA mainly did not react during the polymerization, but was dispersed into the network, hence improving the mechanical properties of the native vegetable oil-based polymer and leading to excellent shape memory recovery properties. This work highlights the potential of coupling different biomass derived compounds in order to design entirely new materials with remarkable properties, especially comparable to petroleum-based materials.

Furthermore, curing can also be achieved by radiation with UV or visible light in the presence of a photoinitiator. This process exhibits several advantages in terms of sustainability, such as the absence of solvent, mild conditions and high curing efficiency reached with a relatively low curing energy [62, 63]. To the best of our knowledge, the first photoinitiated polymerization of epoxidized triglyceride was reported in 1992 by Crivello et al. [64]. This cationic polymerization was enabled by the use of diaryliodonium and triarylsulfonium salts bearing long alkoxy chains to ensure the good miscibility with the epoxidized vegetable oils. The polymerization rate was studied depending on the structure of the cation and anion of the photoinitiator. Films with good adhesion and mechanical properties were synthesized from vegetable oils issued from different plant species. The influence of the degree of epoxidation on the cationic homopolymerization of vegetable oils was further investigated by et al. in 1994 [65]. For instance, vernonia oil, a naturally occurring vegetable oil containing epoxy groups, proved to be more suitable for homopolymerization than fully epoxidized soybean and linseed oils, which are not fully liquid and too viscous at room temperature. However, a decrease in the degree of epoxidation of soybean and linseed oils led to a decrease in their melting point, consequently enabling a more efficient homopolymerization. The presence of hydroxyl groups in the structure of the vegetable oils also influenced the polymerization rate, as the alcohol competes with the epoxide for the ring opening of the oxiranium cation [66]. Indeed, epoxidized castor oil, bearing up to three hydroxyl groups in its triglycerol ester, displayed excellent reactivity, compared to other vegetable oils. An activated monomer mechanism involving hydroxyl groups was proposed by Crivello et al. [67]. Over the years, different photoiniating systems were investigated for the curing of epoxidized vegetable oils, such as onium salts containing tetrakis(pentafluorophenyl)gallate anion and the combination of substituted benzyl alcohols and classic onium salts [68, 69]. However, cycloaliphatic epoxies are the most used epoxies for cationic UV curing due to faster curing and better coating properties than acyclic epoxides. In this context, epoxidized vegetable oils were formulated with non-renewable cycloaliphatic epoxies [70]. Another strategy to keep a high biobased content consists of incorporating cycloaliphatic moieties by the Diels–Alder reaction, then to further epoxidize them [71, 72]. This additional synthetic step in combination with the incorporation of a non-biobased moiety affects the sustainability of this approach, which will not be described further in the scope of this review. As the cationic UV curing efficiency is also affected by the relative humidity, some “humidity blocker” can be used in material formulations [73, 74]. Epoxidized cardanol proved to be a good candidate with the respective properties, in order to incorporate more hydrophobic or rigid units in the formulations of biobased cationic UV curable materials [75]. In this context, Sun et al. prepared copolymers of epoxidized soybean oil, dihydroxyl soybean oil and rosin esters for pressure sensitive adhesive applications [76]. The incorporation of rosin esters led to adhesives with improved adhesion compared to fully vegetable oil-based formulations and final properties similar to commercial petroleum-based equivalent. In a similar approach, the same group copolymerized epoxidized soybean oil with lactic acid oligomers of different chain length in order to adjust the copolymers thermal, mechanical, viscoelastic and adhesion properties [77]. In order to even lower the energy requirement of cationic polymerization of epoxidized vegetable oil, Crivello et al. performed the reaction with visible light. The addition of curcumin to the system acts as an electron transfer agent for the photosensitized decomposition of an iodonium hexafluoroantimonate in laminate [78]. In 2010, Lalevée et al. went one step further and polymerized epoxidized soybean oil with sunlight, at air, via free-radical-promoted cationic polymerization process. The photoinitiating systems were composed of different phosphine oxide derivatives as radical sources, diphenyliodonium hexafluorophosphate and tris(trimethylsilyl)silane (TTMSS). The phosphonyl radical is converted into a silyl radical by hydrogen transfer reaction, which is further oxidized by iodonium salts leading to cations able to start the polymerization. Tack free films with a conversion of 80% were obtained after 1 h of outdoor exposure. The addition of TTMSS was crucial to obtain a good curing. Indeed, silyl radicals are more easily oxidized and the resulting silylium cations are good polymerization initiators [79]. Over the years, the above-mentioned developments in terms of initiating systems enabled us to increase the sustainability of the polymerization of epoxidized vegetable oils. However, even though the photocurable polymerization of epoxidized vegetable oils presents several advantages in terms of sustainability and furthermore shows tolerance towards oxygen, its industrial application remains limited due to a low reactivity, compared to already existing systems.

Copolymerization with Hardeners

Epoxy precursors can also be cured by addition of hardeners, such as anhydrides, amines, acids or alcohols. This copolymerization enables the formation of a cross-linked network. To stay within the scope of green chemistry, the comonomer should be non-toxic and biobased. Although extensive research has been carried out on the synthesis of sustainable epoxy precursors, the number of biobased curing agents remains limited and most of the biobased epoxy precursors are cured with petroleum-based, often toxic hardeners [21, 22]. These partially biobased epoxy resins are not discussed in the scope of this review, which focuses on 100% renewable resource-based thermoset polymers.

Anhydrides were synthesized from different bioresources and used to cure epoxidized vegetable oils (Scheme 4). First, anhydride hardeners were produced from soybean, rapeseed and linseed oils by maleinization of the vegetable oil double bonds by Diels–Alder or Alder-Ene reactions with maleic anhydride [80]. Even though catalytic processes were developed to synthesize maleic anhydride from bioresources (furfural and 5-hydroxymethylfurfural), this molecule remains toxic [81]. Epoxidized vegetable oils were cured with the maleinated vegetable oils employing acetylacetonate as accelerator. The long aliphatic chains of maleinated vegetable oils offer more flexibility to the resulting epoxy resins in comparison to pure maleic anhydride. The thermomechanical properties of these materials were influenced by the epoxy to anhydride ratio and the number of unsaturations of the used vegetable oils. For instance, the Tg of the epoxidized soybean oil/maleinated soybean oil with a 1/1 ratio is 16.4 °C, whereas the Tg of the epoxidized rapeseed oil/maleinated linseed oil with a ratio 1/1.5 is 41.6 °C [82]. Additionally, higher curing efficiency of epoxidized linseed oil with maleinated linseed oil 11 was observed for a higher degree of epoxidation [83]. Epoxidized soybean oil 12 was also cured with a terpene-based acid anhydride 10 and the properties of the polymers were compared with the material cured with maleinated linseed oil and thermal latent catalyst. The introduction of this new hardener enables the synthesis of materials with higher Tg and storage modulus but lower biodegradability than the other polymers [84].
Scheme 4

Synthesis of thermoset material by reaction of biobased anhydride curing agents derived from terpenes 10 and linseed oil 11 with epoxidized soybean oil 12

Additionally, amine hardeners are used as curing agents due to their high reactivity towards epoxides. However, amines often exhibit high toxicity and high volatility and are not the greenest option to cure epoxy precursors. Biobased amine hardeners were synthesized from fatty acids, grapeseed oil and cardanol (Scheme 5). Different strategies for the amination of unsaturated fatty acids were reviewed by Metzger et al. [85]. Most of the described approaches involve multistep procedures, which do not respect several of the principles of green chemistry. In 2011, Robin et al. prepared polyamines from grapeseed oil 13 in a one-step synthesis by UV initiated thiol-ene chemistry. The atom economy and mild conditions of this methodology represents an improvement in terms of sustainability for the synthesis of amines from vegetable oils. The employed thiol, cysteamine, is a degradation product of the amino acid cysteine. The highest functionalization reached 4.13 amine groups per triglyceride. The polyamine was investigated as curing agent for epoxidized linseed oil leading to fully biobased epoxy resins with a low Tg of −38 °C [86]. In a further work, the same group compared the curing properties of this polyamine with Priamine 14 and a newly synthesized fatty acid derived diamine 15 (details on this renewable diamine below) [87]. The latter was synthesized by reaction of fatty acids with allyl amine and further thiol-ene addition of cysteamine hydrochloride. In comparison to the previously reported polyamine, the synthesis of this fatty acid derived diamine suffers from the use of toxic petroleum-based allylamine and the additional reaction steps both for the production of the fatty acid from triglycerides and the allylation. Priamine is a diamine commercialized by Croda and obtained after dimerization of fatty acids. The Tg of the Priamine-based and fatty acid-based epoxy resins are respectively, −31 and −9 °C. These Tg values, as well as the more rigid aspect of epoxy resin cured with the fatty acid based diamine, can be attributed to the lack of dangling chains in contrary to Priamine and vegetable oil based polyamines. Aromatic compounds are crucial chemicals for the synthesis of epoxy resins, giving good thermomechanical properties to the resulting material. Amine curing agents from cardanol, called Cardolite phenalkamines 16, are commercially available and exhibit a lot of advantages, such as fast and low temperature curing and approval for use in food contact and potable water [88]. This curing agent is synthesized by the Mannich reaction between cardanol, toxic formaldehyde and different diamines. Another approach for the synthesis of amine from cardanol was reported by Caillol et al. [89]. Cardanol was allylated with allyl bromide and further reacted with cysteamine by thiol-ene coupling. More sustainable allylation methods were developed on biobased phenols and should replace this classic pathway employing toxic allyl bromide and dimethylformamide as often as possible [90]. The obtained aminated cardanol 17 exhibits an amine functionality of 1.85. This curing agent was used to cure commercially available epoxidized cardanol, leading to epoxy resins fully based on cardanol. The epoxy resins cured with Cardolite phenalkamines 16 and thiol-ene derived cardanol 17 exhibit similar thermostability, but the Cardolite phenalkamines-based network showed higher Tg and crosslink density (30 °C against 19 °C and 280.0 mol/m3 against 20.3 mol/m3, respectively).
Scheme 5

a Biobased amine curing agents synthesized from grapeseed oils 13, Priamine 14, fatty acids 15 and cardanol 16 and 17, b ring-opening of epoxides with amines

A green alternative to cure epoxy resins, circumventing the use and synthesis of amines and anhydrides, entails in the investigation of curing efficiency of naturally occurring carboxylic acids. Biobased carboxylic acids can be produced by fermentation of carbohydrates or modifications of vegetable oil derivatives. The industrial availability of some of these biobased dicarboxylic acids drives research towards their use as hardeners. The reaction of epoxy with carboxylic acids forms β-hydroxy ester linkages, which can be accelerated by catalysts, such as amines (Scheme 6). To the best of our knowledge, the first report of a fully biobased epoxy resin investigated the curing of epoxidized linseed oil with Pripol 18, employing different amine catalysts [91]. Flexible transparent films with Tg between −18 and −10 °C were obtained. The mechanical properties of the films were improved by the addition of a catalyst, especially by 4-dimethylaminopyridine, which acts both as nucleophile and base, thus deprotonating the carboxylic acid. Following this study, the same group also investigated the influence of the chain length (from C6 to C18) of different biobased diacids 19 on the properties of the final network [92]. The dicarboxylic acids with shorter chains exhibited better reactivity towards the epoxies and the resulting resins showed higher Tg (up to 7 °C), better mechanical properties, but displayed a poorer thermal stability. The synthesis of such epoxy resins involving the production of ester linkages motivated several researchers to investigate the degradability, biodegradability and self-healing behavior of these materials. Degradability and recyclability are key requirements for the synthesis of environmentally benign materials, as illustrated by the first and tenth principles of green chemistry, which advice waste prevention and design of products that degrade after use [93, 94]. Yemul et al. prepared epoxy resins from epoxidized Karanja oil (composed of 53% oleic acid) and naturally occurring acids, especially citric acid 20 and tartaric acid 21, and investigated their biodegradability. Citric acid and tartaric acid are naturally occurring acids in citrus fruits including grapefruit. produced on an industrial scale and used in the food industry [95, 96]. Both epoxy resins exhibited high Tg at around 110 °C and biodegradable behavior. The biodegradation is the decomposition of a substance into CO2 and H2O, under the action of a microorganism [97]. Indeed, after respectively 69 and 259 days in bacterial media, the citric acid and tartaric acid-based epoxy resins were 82 and 89% biodegraded. Recently, Webster et al. reported the water-assisted synthesis of epoxy resins from epoxidized sucrose soyate and naturally occurring carboxylic acids, such as tartaric and citric acid, in order to limit the emission of volatile organic compounds [98]. The epoxy resins, produced with excellent properties for coating applications, could be degraded in basic media very rapidly. The concept of self-healing materials also constitutes an approach to address the issue of material end life by a solution of reprocessability [99]. In this context, Altuna et al. produced biobased self-healing polymer networks by reaction between epoxidized soybean oil and aqueous citric acid [100]. The transesterification reactions enabled the self-healing behavior thermally, without the addition of external catalyst.
Scheme 6

a Biobased and naturally occurring fatty acids: Pripol 18, alkyl diacids 19, citric acid 20, tartaric acid 21, b ring-opening of epoxides by carboxylic acids

Epoxides can also be ring-opened by hydroxyl groups under high temperature or in presence of a catalyst (Scheme 7). Exemplary, tannic acid 22 is a polyphenol extracted from different plant leaves and pods, belonging to the class of tannin, the second most abundant aromatic biomolecule after lignin [101]. In order to take advantage of the aromatic properties of tannin and of the flexibility of the vegetable oil alkyl chains, epoxidized soybean oil was cured with tannic acid at 210 °C and formulated in composites with microfibrillated cellulose [102]. Another example of combining biomass resources with different structures was described by Mija et al., who investigated the copolymerization of furfuryl alcohol 23 and epoxidized linseed oil. Furan-based monomers derive from C5 or C6 lignocellulose biomasses and are one of the major renewable-based monomers [103]. Mija et al. reported the cationic polymerization using a Lewis acid boron trifluoride ethylamine complex catalyst system [104]. The mechanism was deeply investigated and revealed the polymerization of furfuryl alcohol oligomers including polycondensation reactions and cross-linking by Diels–Alder reactions. Furthermore, the formation of covalent bonds between the hydroxyl groups of furfuryl alcohol and/or its oligomers and epoxidized linseed oil was observed. The final material exhibited a macroscopic homogeneity and a semi-ductile behavior, thus addressing the brittleness issue of furan-derived thermosets.
Scheme 7

a Tannic acid 22, b ring-opening of epoxides by hydroxyl groups, c reactions occurring in the use of furfuryl alcohol 23 as curing agent for epoxidized vegetable oils

Polyurethane Resins

Vegetable oils can be easily transformed into polyols, which provide important building blocks for polymer science. Many routes to functionalize vegetable oils with hydroxyl groups, such as thiol-ene coupling reactions, ozonolysis followed by reduction, hydroformylation, photochemical oxidation followed by reduction, epoxidation followed by ring-opening reaction, transesterification or transamidation of the triglyceride ester moiety were investigated and are summarized elsewhere [12]. Such polyols were especially employed for the synthesis of polyurethanes, one of the most important polymeric materials, finding numerous applications in our everyday life [105]. Classically, the polymerization occurs by reaction of the polyols with toxic and hazardous isocyanates. In order to address this toxicity issue, alternative routes for polyurethane synthesis are intensively investigated and in line with that, the concept of non-isocyanate-based polyurethanes (NIPU) was developed [106, 107, 108, 109]. An attractive route to avoid the use of isocyanates is the synthesis of polyhydroxyurethanes by polyaddition of diamines with cyclic carbonates (Scheme 8) [110]. Carbonated linseed and soybean oils containing five-membered cyclic carbonates were prepared by reactions of epoxide precursors with CO2 employing tert-butylammonium bromide as catalyst. These were further reacted with petroleum-based diamines to produce thermoset polyurethanes [111, 112, 113, 114]. In 2008, Javni et al. reported the synthesis of NIPU from carbonated soybean oil with several diamines, and amongst 1,4-butanediamine, which is obtained by transformation of lignocellulosic biomass-derived succinic acid (Scheme 8) [114, 115]. A stoichiometric carbonate-to-amine ratio gave the network with the best thermomechanical properties (Tg of 19.3 °C). During the formation of urethane linkages, ester cleavage and amidation were also observed. Poussard et al. investigated the synthesis of similar polymers and additionally explored the reaction of the carbonated soybean oil with aminated oligomers with a molar mass of 4880 g/mol produced from dimerized fatty acid and the toxic ethylene diamine. Although the use of 1,4-butanediamine led to thermoset material, a material with properties of a partially cross-linked elastomer was obtained with aminated oligomers [116]. Fully fatty-acid derived NIPU were synthesized by Averous et al. [117]. Bis(cyclic carbonate)s were synthesized from dimerized fatty acid by halogenation employing toxic and corrosive thionyl chloride, followed by esterification with glycerol carbonate. The obtained bis(cyclic carbonate)s reacted under solvent and catalyst-free conditions with Priamine, a mixture of aminated fatty acid dimer and trimer (75/25) inducing the formation of a NIPU cross-linked network. The polymers exhibited good thermal stability and low Tg of around −20 °C. Although the synthesis of NIPU is a greener approach for the polyurethane production, the toxic nature or chemical synthesis of the starting monomers reported in literature still has to be addressed.
Scheme 8

Non isocyanate-based polyurethane route for vegetable oil-based thermosets

Polybenzoxazine

High industrial and commercial interest is given to phenolic resins, a class of high-performance thermoset materials exhibiting excellent thermomechanical and chemical properties [118]. Common phenol/formaldehyde resins are produced under strong acidic or alkaline conditions, which corrode the processing equipment and release by-products, such as water or ammonia during curing. A relatively new class of phenolic resins, polybenzoxazines, was developed with the advantage of being cured additionally at high temperature, thus avoiding corrosive conditions and by-product releases. Benzoxazine monomers are synthesized in a one-pot method from phenolic derivatives, primary amines, and paraformaldehyde. Although the toxicity of formaldehyde remains an issue to be addressed, the synthesis of polybenzoxazine represents a “greener” approach for the production of phenolic resins. Additionally, fully biobased polybenzoxazines with tunable thermomechanical properties depending on the nature of the renewable resource were prepared (Scheme 9) [119]. Paraformaldehyde is the polymerization product of formaldehyde, potentially obtained via catalytic oxidation of biomethanol [120]. Cardanol was explored as phenolic starting compound. In a first attempt, cardanol benzoxazine 24 was prepared by a condensation reaction of cardanol with formaldehyde in the presence of ammonia [121]. Thermal curing yielded a thermoset with a Tg of 36 °C, which was used further for the preparation of composite materials reinforced with natural jute fibers. Cardanol benzoxazines were also synthesized employing several amines, including biobased furfurylamine 25, and their curing kinetic and final properties were discussed [122]. In comparison to other thermosets, polybenzoxazines suffer from relatively low cross-linking densities. The presence of furan moieties accelerates the curing process by participating in the ring-opening polymerization of benzoxazines via electrophilic aromatic substitution, thus introducing further cross-linking points. This polybenzoxazine displayed a Tg of 100 °C. An increase in cross-linking density can also be achieved by the design and polymerization of dibenzoxazines. Cardanol-based dibenzoxazines were also prepared from cardanol and ethylenediamine and from cardanol, ethylenediamine and vanillin. The networks obtained from the unsymmetric benzoxazine exhibits a Tg of 129 °C. Although the design of unsymmetric biobased dibenzoxazine is an efficient strategy to combine the thermomechanical properties of different biomasses, the use of toxic ethylenediamine affects the sustainability of this study [123]. Stearylamine, a fatty acid derivative, was investigated as amine compound for the synthesis of benzoxazines in combination with guaiacol 27, eugenol 28 and triphenols prepared from guaiacol and vanillin [124, 125, 126]. After thermal curing, the materials respectively displayed Tg of 82, 101 and 58 °C. However, the materials show low cross-linking densities, which was, as mentioned before, enhanced by the replacement of stearylamine by furfuryl amine.
Scheme 9

a Benzoxazine synthesis from cardanol and ammonia 24, cardanol and furfurylamine 26, b structures of the benzoxazines prepared from guaiacol and stearylamine 27 and eugenol and stearylamine 28

Unsaturated Polyesters

Well-defined platform chemicals with high purity are industrially produced by chemical transformations of vegetable oils [5]. Difunctional derivatives, such as 10-undecenoic acid, oleic acid and sebacic acid are widely used for the synthesis of linear thermoplastic polymers [24]. Sebacic acid, a C10 diacid derived from the oxidation of castor oil, was also employed for the synthesis of unsaturated polyester resins in combination with unsaturated biobased comonomers. This relatively long diacid is very often incorporated as a soft segment and compared to shorter diacids, such as succinic acid. Unsaturated polyester resins, benefitting from their simple processing and versatility, represent an important part of the plastic market [127]. Guo et al. reported the first incorporation of sebacic acid in the formulation of shape memory polymers [128]. Sebacic acid was copolymerized with the 1,3-propanediol and itaconic acid, both issued from lignocellulosic biomass [115]. The resulting unsaturated polyesters were cross-linked at 150 °C employing a radical initiator, yielding materials with excellent shape recovery and fixity (Scheme 10). In vitro experiments revealed that these networks exhibit potential biocompatibility and biodegradability making them suitable for biomedical applications. Unsaturated polyesters incorporating itaconic and sebacic units were also produced by enzymatic polymerization [129]. Biocatalysis contributes to sustainability by its efficiency under mild conditions and the avoidance of toxic or metal catalyst [130, 131].
Scheme 10

Unsaturated polyester synthesis and further cross-linking from sebacic acid 29, propane-1,3-diol 30 and itaconic acid 31

Other Polymerization Strategies: Insertion of a Polymerizable/Cross-Linkable Moiety

Another approach for the synthesis of vegetable oil-based thermosets involves the insertion of cross-linkable or polymerizable moieties into triglycerides. The biobased content of this strategy suffers if the introduced moiety is derived from petroleum. For instance, soybean oil carrying alkyne moieties was prepared and further reacted with several azidated derivatives of vegetable oils to form a network [132, 133]. Different norbornenyl-functionalized vegetable oils were polymerized by ring opening metathesis polymerization (ROMP), yielding thermosets with varied cross-linked densities [134, 135]. These examples are not discussed further for being not completely biobased. Acrylated vegetable oils can be free radically polymerized and find applications in pressure-sensitive adhesives and coatings due to higher reactivity in comparison to epoxidized vegetable oils. Acrylic acid can, in principle, be biobased. Indeed, alternative sustainable processes are investigated for the synthesis of acrylic acid from biomass and are reviewed elsewhere [136]. These routes include the conversion of glycerol and 3-hydroxypropionic acid, respectively produced as a biodiesel by-product from the transesterification of triglycerides and derived from glucose [137, 138]. Although several acrylated vegetable oils are synthesized in multistep procedures, including reaction with toxic acryloyl chloride, “greener” approaches were developed. Wool et al. described and studied the kinetic of an acid-catalyzed acrylation of epoxidized oils with acrylic acid (Scheme 11a) [139]. A more sustainable direct acrylation of soybean oil with acrylic acid using boron trifluoride etherate catalyst was reported by Zhang et al. (Scheme 11b) [140, 141]. The recovery and recycling of both the catalyst and excess of acrylic acid was also studied. Several studies reporting the copolymerization of acrylated vegetable oils with petroleum-based comonomers or the synthesis and further curing of poly(urethane acrylate) or poly(ester acrylate) employing non-biobased abundant moieties are discussed in the literature but are not mentioned in this review due to a reduced sustainability. Acrylated vegetable oils from different sources were homopolymerized by UV or thermal curing [142, 143]. Similarly to the epoxidized vegetable oils described above, the properties of the resulting material are related to their cross-linking density and can therefore be tuned depending on the nature of the vegetable oils and degree of acrylation [144]. This versatility enables the coverage of a broad range of thermomechanical and adhesion properties [145]. The curing speed is also influenced by the degree of acrylation. For example, acrylated epoxidized soybean oil, which is commercially available, offered cross-linked material within few seconds by UV irradiation [146]. In order to enhance the thermoset rigidity, the acrylated vegetable oils were copolymerized with aromatic and cycloaliphatic biobased molecules such as vanillin, furan, gallic acid, eugenol and rosin [147, 148, 149, 150, 151].
Scheme 11

Acrylation of vegetable oils: a 2 step-route including epoxidation and further reaction with acrylic acid, b direct route by reaction with acrylic acid

Conclusion

Plant oils are very suitable bioresources for the synthesis of thermosets due to their multiple unsaturations, which enable their direct polymerization or poly-functionalization, as well as further polymerizations. A broad range of cross-linked materials, including common epoxy resins, non-isocyanate-based polyurethanes, polybenzoxazines and unsaturated polyesters may be synthesized from vegetable oils. The properties of the resulting thermosets depend greatly on the cross-linking density, which is directly related to the number of unsaturations and the degree of functionalization of the plant oils. Thus, the properties of the thermoset can be tuned by selective use of oils from different plant origins. Additionally to their biobased nature, a higher degree of sustainability is provided due to their liquid character at room temperature, often allowing solvent-free syntheses. Nonetheless, only a limited number of examples reported the synthesis of fully biobased thermosets from vegetable oils up to date. This goal was reached either by homopolymerization of the vegetable oils or by combination with different biomass derivatives, such as lignocellulosic-based comonomers, in order to broaden the thermomechanical properties of the resulting material. The emphasis of this review was to highlight the improvements in sustainability of chemical modifications and polymerizations approaches. These advances include: (1) the avoidance of toxic reagents and corrosive acids, (2) waste prevention by investigating the biodegradability or self-healing behavior or limiting the reaction steps and catalyst separation (latent catalysts), and (3) reducing energy consumption by lowering the reaction temperature and employing photo-curing at room temperature. Most of these progresses were published recently and show the increasing conscious of the researchers to develop a greener and sustainable future.

Copyright information

© AOCS 2016

Authors and Affiliations

  1. 1.Karlsruhe Institute of Technology (KIT), Institute of Organic Chemistry (IOC)KarlsruheGermany