Sustainable Synthetic Approaches for the Preparation of Plant Oil-Based Thermosets
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- Llevot, A. J Am Oil Chem Soc (2017) 94: 169. doi:10.1007/s11746-016-2932-4
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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.
KeywordsVegetable oil Thermoset Sustainability Renewable resources Polymer
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 . 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 . 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.
Direct Polymerization Methods
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 . 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 . 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. . 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 . This synthesis involves the in-situ production of peracids, generated from acetic acid and hydrogen peroxide in the presence of strong mineral acids . 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.
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 . The initiating temperature corresponds to the cleavage of a heteroatom and a carbon atom of the initiator . 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 . 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 . 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) . 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. . 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 . 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 . 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. . 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 . 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 . In this context, Sun et al. prepared copolymers of epoxidized soybean oil, dihydroxyl soybean oil and rosin esters for pressure sensitive adhesive applications . 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 . 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 . 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 . 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.
Other Polymerization Strategies: Insertion of a Polymerizable/Cross-Linkable Moiety
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.