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Vegetable oils, as a material of natural origin and from renewable sources, are the subject of numerous studies aimed at their application for the synthesis or modification of various polymers [1]. Soybean, castor, linseed, rapeseed, sunflower, cotton, peanut, and palm oils are primarily used on a larger scale depending on the type of oil produced in a given region [2]. From the chemical point of view, plant‐based oils are a mixture of esters derived from glycerol and free fatty acids, mainly unsaturated acids (primarily oleic, linoleic, linolenic, ricinoleic, and erucic acids) and in a small amount of saturated acids (stearic and palmitic acids) (Figure 1.1), depending on the type of oil.

When choosing vegetable oil for use in the synthesis of polymers, first of all, its structure should be taken into account: the presence of unsaturated bonds and possibly other functional groups (e.g. hydroxyl in castor oil or epoxy in vernonia oil), the amount of unsaturated bonds present in the molecule (referred as the oil functionality), and chain length alkyl derived from fatty acids (Table 1.1).

Figure 1.1 Schematic structure of triglycerides.

Table 1.1 The content of various fatty acids in selected vegetable oils.

Fatty acid	Vegetable oil, content of individual acids (wt%)
Soybean	Rapeseed	Linseed	Sunflower	Castor	Palm
Palmitic	12	4	5	6	1.5	39
Stearic	4	2	4	4	0.5	5
Oleic	24	56	22	42	5	45
Linoleic	53	26	17	47	4	9
Linolenic	7	10	52	1	0.5	—
Castor	—	—	—	—	87.5	—
Other	—	2	—	—	—	2
Functionality	4.6	3.8	6.6	4.6	2.8	1.8

The functionality of oils (understood as the content of unsaturated bonds) primarily determines the cross‐linking density of oil‐based chemosetting polymers or polymers obtained by free radical polymerization as well as oil‐modified polymeric materials. In turn, the final polymer properties such as mechanical strength, thermal stability, and chemical resistance strongly depend on the cross‐linking density. The elasticity of the polymers with the addition of vegetable oil or based on them depends on the length of the alkyl chains in the oil molecule derived from fatty acids.

Vegetable oils can be easily and efficiently converted into epoxy derivatives by oxidizing unsaturated bonds present in fatty acid residues. Several methods of double bond oxidation in triglyceride molecules are known and commonly used [3]: the method based on the Prilezhaev reaction, the radical oxidation, the Wacker‐type oxidation, dihydroxylation of oils and fats, and enzyme–catalyst oxidation. The Prilezhaev reaction is the most often used method for natural oil epoxidation, commonly applied in the industry. In this method, the process of epoxidation of natural fatty acids and triglycerols is carried out in the system consisting of hydrogen peroxide, an aliphatic carboxylic acid, and an acidic catalyst. The organic peracid formed in situ by the reaction of acid with hydrogen peroxide is the real oxidizing agent in this method (Figure 1.2).

Carboxylic acids with one to seven carbon atoms are the most commonly used (in practice, mainly acetic acid). Inorganic or organic acids and their salts, as well as acidic esters, can be used as catalysts; however, sulfuric and phosphoric acid are the most often used in industrial practice. A promising method is oxidation in the presence of enzymes [4], heteropolyacids [5], and even ion exchange resins [6] as catalysts. The most commonly used oxidizing agent is hydrogen peroxide in the form of solution with a concentration of 35–90% (usually 50%). Epoxidation of plant oils in ionic liquids, as well as in supercritical carbon dioxide, is also described [7].

The earliest epoxidized esters of higher fatty acids have found wide applications as both plasticizers and stabilizers for thermoplastics, mostly poly(vinyl chloride), poly(vinylidene chloride), their copolymers, and poly(vinyl acetate) and chlorinated rubber [8, 9]. Epoxidized fatty acids containing oleic acid are used as a valuable intermediate in the production of lubricants and textile oils [10, 11]. It seems that epoxidized vegetable oils could also be used as hydraulic liquids [12]. However, primarily, they can also act as reactive diluents of bisphenol‐based epoxy resins [13], which are usually highly viscous. They have oxirane groups, although less reactive because of their central location in triglyceride chains (compared to terminal glycidyl groups), but also capable of reacting with polyamines or carboxylic anhydrides. By building into the structure of the cured resin in the process of co‐cross‐linking with it, they affect its final properties – improving flexibility and impact strength. In this way, embedded triglycerides not only facilitate the processing of resins with high intrinsic viscosity but also allow limiting their typical disadvantages (high brittleness, low impact strength, and flexibility) resulting from the rigid structure they owe because of the structure of bisphenols [14].

Figure 1.2 The reaction of triglyceride epoxidation with organic peracids.

However, the first, logically implied possibility of use of epoxidized vegetable oils is their application as stand‐alone materials: the networks cross‐linked with bifunctional compounds such as dicarboxylic acids or aliphatic and aromatic diamines, which are typically used as hardeners for the epoxy resins. Because of the content of more than one epoxy group in the molecule, epoxidized triglycerides may, according to the generally accepted definition, be treated as epoxy resins. However, curing of, e.g. epoxidized soybean oil [15] or vernonia oil (natural epoxidized oil mainly obtained from plant Vernonia galamensis), dicarboxylic acids [16] resulted in obtaining only soft elastomers. Materials with higher mechanical strength are synthesized by reacting epoxidized oils first with polyhydric alcohols (e.g. resorcinol) and phenols or bisphenols and then cross‐linking the obtained modified oil with partially reacted epoxidized rings [17]. Finally, curing by photopolymerization or polymerization with latent initiators allows to obtain from modified vegetable oils, without the addition of the bisphenol‐based or cycloaliphatic resins, coating materials with satisfactory mechanical properties. It was found [18] that the properties of hardened vegetable oils also depend on the type of used thermal latent initiator. The properties of epoxidized castor oil cross‐linked with N‐benzylpyrazine (BPH) and N‐benzylquinoxaline (BQH) were studied (Figure 1.3).

It was found that materials characterized by a higher glass transition temperature, a higher value of the coefficient of thermal expansion, and greater thermal stability are obtained using BPH as a photoinitiator. Nevertheless, the composition cross‐linked with BQH is characterized by better mechanical properties. Most likely, the better final properties result from the higher cross‐linking density of cured with BPH composition. Anhydrides of various carboxylic acids are used to cure epoxidized linseed oil [19], and cross‐linking reactions are catalyzed by various tertiary amines and imidazoles. The materials obtained with phthalic anhydride and methylendomethylenetetrahydrophthalic anhydride hardeners exhibit a lower cross‐linking density than those obtained with cis‐1,2,3,6‐tetrahydrophthalic anhydride. It was found that a greater degree of oil–anhydride conversion and thus higher cross‐linking density and greater rigidity of the cured material are obtained using imidazoles. The best properties are achieved for the composition cured with cis‐1,2,3,6‐tetrahydrophthalic anhydride as the hardener and 2‐methylimidazole as the catalyst.

Figure 1.3 Chemical structure of cationic photoinitiators.

High‐molecular‐weight epoxies are a special group of very important epoxy resins commonly used as coating materials, especially for powder, can and coil coatings mainly in automotive industry. Theoretically, they can be obtained in the traditional way in the Taffy process with epichlorohydrin and bisphenol. However, even the use of a slight excess of epichlorohydrin does not provide high‐molecular‐weight solid resins. Therefore, industrially, they are synthesized from low or moderate molecular weight epoxy resins and bisphenol A by the epoxy fusion process. It is the method of polyaddition carried out in bulk, in the molten state of reagents, and without the use of solvents. In this way, it is possible to obtain resins with a softening temperature of 100–150 °C, characterized by an epoxy value of 0.020–0.150 mol/100 g, and an average molecular weight of 1.5–10 thousands of Daltons. The application of epoxidized vegetable oils in place of low/medium molar mass resins, as well as hydroxylated oils in place of bisphenols, in the epoxy fusion process with bisphenol A (BPA) or BPA‐based epoxy resin was proposed [20, 21]. Hydroxylated oils are obtained from epoxidized oils in the reaction of opening of oxirane rings using diols and the most often glycols. Depending on the type of starting oil, catalyst used, and reaction time, the products of the epoxy fusion process using modified oils (Figure 1.4) contain a large amount of hydroxyl groups (hydroxyl value 120–160 mg KOH/g), some free epoxy groups (epoxy value 0.050–0.150 mol/100 g), and are characterized by weight‐average molecular weight even above 30 000 g/mol.

Therefore, for the cross‐linking of these products, diisocyanates or blocked diisocyanates can be applied (Figure 1.5).

Figure 1.4 The synthesis of high‐molecular‐weight epoxy resins based on modified vegetable oil: (a) epoxidized or (b) hydroxylated soybean oil.

Figure 1.5 Cross‐linking reactions of epoxy fusion process products.

The resins cross‐linked with polyisocyanates are characterized by differential mechanical properties, which depend on the type of used isocyanate, and are higher than the one of the low‐molecular‐weight bisphenol A‐based resin crosslinked with methyl‐tetrahydrophthalic anhydride, however lower while cured with isophoronediamine [22]. Moreover, the presence of epoxy groups in the polyaddition products can be used to obtain two‐layer materials [23], in which one layer is cured with polyamine epoxy resin and the other is a polyaddition product cross‐linked with diisocyanate. The reaction of the amine hardener with the free epoxy groups that are present within the polyaddition product ensures a very good interlayer bonding.

Because of the usually unsatisfactory properties of the oils cured with amines or acid anhydrides, epoxidized vegetable oils began to be used as one of the components of epoxy compositions [24]. The compositions consisting of epoxidized esters of higher fatty acids obtained by the transesterification of various vegetable oils and natural or hydrocarbon resin acids can be used as an ingredient in, among others, epoxy adhesives with reduced crystallization tendency [25]. Compositions of modified vegetable oils with epoxy resins based on various bisphenols can generally be prepared via two methods. One of them is the homogenization of the components of the composition and their simultaneous co‐cross‐linking. In this way, compositions of bisphenol F diglycidyl ether with epoxidized linseed oil are prepared [26] and cured with methyltetrahydrophthalic anhydride in the presence of 1‐methylimidazole or polyoxypropylenetriamine [27]. It turned out that with an increase in the content of epoxidized linseed oil in the anhydride‐cured compositions, the storage modulus, glass transition temperature, and heat resistance under load decrease, while the impact strength measured by the Izod method does not change, but above 70 wt% of oil content increases the cross‐linking density. In contrast, compositions cured with the use of amine are characterized by an almost fivefold increase in impact strength at the oil content of 30% by weight. Other discussed cured parameters change in the same way as in the case of anhydride cross‐linked materials. In turn, comparison [28] of the properties of the composition with epoxidized linseed oil and soybean oil shows significant differences between the materials based on both oils. It was found that, due to the greater compatibility of linseed oil with the epoxy resin and better oil solubility in the resin (resulting from greater polarity and functionality and lower molecular weight), linseed oil does not tend to form a separate phase. However, the two‐phase structure, observed in the case of epoxidized soybean oil, is responsible for improving the impact strength and fracture toughness of the epoxy resin composition. A decrease in cross‐linking density is also observed in the compositions of 4,4′‐tetraglycidyldiamino‐diphenylmethane with epoxidized soybean oil cured with diaminodiphenylmethane [29]. Also in this case, besides the improvement in impact strength, as the effect of reducing the cross‐linking density, a decrease in the heat resistance and the glass transition temperature is observed. Using the example of a bisphenol‐based epoxy resin compositions with different contents of epoxidized castor [30] or soybean oil [31], cured with thermal latent initiator BPH, it was proven that the final properties of cross‐linked materials are determined not only by the polarity, functionality, or structure of the used oil but also by its content, ensuring the optimal amount of flexible fragments embedded in the rigid epoxy resin structure, and the most favorable phase composition of the material.

Bisphenol‐based epoxy resin compositions with modified vegetable oils might also be prepared in the two‐step method. The first stage is the initial cross‐linking of oil so that free functional groups capable of co‐cross‐linking with the epoxy resin remain in it. In this way, a prepolymer or, as it is called in some publications, a rubber is obtained from the modified oil. Only then, the prepared prepolymer is mixed in appropriate proportions with epoxy resin, and finally, the composition is cured. The cross‐linked composition is characterized by a two‐phase structure, analogous to that of epoxy resins modified with liquid acrylonitrile butadiene copolymers with reactive carboxyl or amine end groups and acrylic elastomers. The two‐phase structure of the composition determines their postcuring properties. Using the two‐step method, composition of diethylene epoxy resin with epoxidized soybean oil was prepared [32]. Initially, both the oil and then the composition with the epoxy resin were cross‐linked with 2,4,6‐tri(N,N‐dimethylaminomethyl)phenol. The soybean prepolymer, cross‐linked for 12–84 hours, is a highly viscous liquid that mixes well with the epoxy resin [33]. The formation of the two‐phase structure of the cured composition was confirmed by DSC and DMA analyses. The adhesive joint prepared with the use of the tested composition shows a significant improvement in the impact strength and the strength. It has been found that the properties of the composition depend on both the content of soybean prepolymer and the time of its pre‐cross‐linking. The best results are obtained using the addition of 20 wt% of prepolimer pre‐cross‐linked for 60 hours. Compositions characterized by greater cross‐linking density and mechanical strength than the networks with epoxidized soybean oil were obtained using methyl and allyl esters, synthesized by the transesterification of soybean oil [34]. The esters were epoxidated and then precured with p‐aminocyclohexylmethane, which showed the highest reactivity to soybean oil derivatives among the tested polyamines. The curing conditions were selected in such a way that cross‐linking of both esters and epoxidized oil, which was chosen for the comparison purposes, terminates at the gelation stage. The bisphenol‐based epoxy resin compositions, with the content of prepolymers of 10–30 wt%, were cured using various polyamines, and their mechanical properties were compared with those of the samples of analogous composition but obtained via the one‐step method. Generally, the mixed compositions with various soybean oil derivatives obtained by the two‐stage method are characterized by the best strength parameters, definitely better than the networks synthesized only with epoxidized oil. In particular, the addition of epoxidized allyl ester increases the glass transition temperature and provides greater rigidity and mechanical strength of the composition.

Additionally [35], the process of cross‐linking of the above‐described materials with acid anhydride (the commercial product called Lindride LS 56V produced by Lindau Chemicals, USA) was studied. Based on the results of DSC and viscometric measurements, models describing the course of curing reactions have been developed, which might be applied in the industrial processing of the described compositions. The DMTA analysis showed [36] that the conservative modulus of elasticity and glass transition temperature increase with an increase in the content of epoxidized allyl ester in the case of anhydride cross‐linking while decrease for polyamine‐cured materials. Moreover, the value of the loss factor decreases in the case of anhydride cross‐linking, but it is definitely higher for polyamine‐cured compositions. That kind of formation of dynamic mechanical properties results from a greater degree of cross‐linking of anhydride‐cured compositions. The epoxidized palm oil was prepolymerized in a reaction with isophorone diamine [37]. The resulting palm oil derivatives were used as modifiers of a bisphenol A‐based low molecular weight epoxy resin. The prepared compositions and the pure unmodified epoxy resin were cured with isophorone diamine. It was found that the palm oil derivatives led to a decrease in the mechanical strength of the resin, but on the other hand, they contributed to an increase in relative elongation at break and significant improvement (even twice) in impact strength of the cross‐linked products. A two‐phase structure of the compositions studied, responsible for the increase of their impact strength, was observed.

Figure 1.6 Chemical structure of the cycloaliphatic resin (3,4‐epoxycyclohexylmethyl‐3′,4′‐epoxycyclohexane carboxylate).

One of the most important areas of application of epoxidized vegetable oils are compositions with epoxy resins, capable of cross‐linking with UV or visible light. Photoinitiated polymerization is a commonly used industrial method of cross‐linking of coating materials. Throughout this method, the cured coating is obtained in a short time and above all at the room temperature. Modified natural oils are a very interesting alternative to acrylic monomers, commonly used to obtain photosetting coatings, and starting from the first reports [38] are the subject of the research performed by scientific teams around the world. Compositions consisting of vernonia oil or epoxidized soybean oil and cycloaliphatic epoxy resin were tested [39] (Figure 1.6).

The compositions were cross‐linked by photopolymerization using a cationic UV initiator, which was a mixture of triarylsulfonium salts of hexafluoroantimone with a trifunctional primary triol based on ε‐caprolactone. Coatings with the addition of epoxidized vegetable oils are characterized by excellent adhesion to the surface, high impact strength, UV stability, corrosion resistance, and long‐lasting shine. It was also found that pencil hardness and tensile strength of coating films decrease with increasing oil content. Similarly, the glycidyl castor oil derivative [40], added in an amount of up to 60 wt% to the same cycloaliphatic resin and cross‐linked with it using triarylsulfonium salts as cationic initiators (Figure 1.7), leads to a significant improvement of epoxy coating properties: increasing its elasticity and gloss as well as reducing water absorption.

Flexible coatings characterized by high tensile strength and hardness are obtained by adding epoxidized palm oil to cycloaliphatic resin (Figure 1.6) [41]. Additionally, in the described research, the possibility of photopolymerization of prepared compositions with UV light in the presence of various initiators, radical, cationic, and hybrid ones, was tested. Because of the low solubility of triarylsulfonium salts in oil, divinyl ethers of various structures were also added to the composition, which, as it was found in the course of the study, did not affect the mechanical properties of cured coatings. The photocuring process of highly branched resins obtained from modified vernonia oil was also investigated [42]. For the reason that the final properties of cross‐linked compositions with modified vegetable oils depend not only on the amount of oil but also on their structure, the authors decided to study the photopolymerization of epoxy resin with a strictly defined composition and structure. For this purpose, obtained in the oil transesterification reaction of Euphorbia lagascae, methyl vernolate was reacted with trimethylol propane to give the compounds depicted in Figure 1.8.

Figure 1.7 Triarylsulfonium salts applied as the cationic photoinitiators.

Figure 1.8 Structure of vernolic acid methyl ester and product of its reaction with trimethylol propane.

The obtained derivatives, including the hyperbranched polyether, were used to prepare compositions with different contents of individual components, with methyl vernolate acting as a reactive diluent. The compositions were polymerized with a cationic photoinitiator (octyloxydiphenyliodine hexafluoroantimonate). The application of methyl vernolate reduces the viscosity of the polyether as well as significantly decreases the glass transition temperature. An interesting example of the synthesis of epoxy resin based on vegetable oil, hardened later by the photopolymerization, is the attachment of bicyclo[2.2.1]heptane to linseed oil [43]. The derivative, which is shown in Figure 1.9, was obtained by the Diels–Alder reaction of cyclopentadiene with linseed oil, carried out at the temperature of 240 °C and a pressure of 1.4 MPa.

Compositions consisting of a epoxidized derivative, the addition of various divinyl ethers of epoxidized linseed oil, and cycloaliphatic epoxy resin (Figure 1.6) have been cured using the already mentioned triarylsulfonium salts. Divinyl monomers fulfilled the role of reactive diluents and compatibilizers primarily of oil and photoinitiator. It is also known that the presence of this type of monomers accelerates photocuring of cycloaliphatic epoxy resins. It has been observed that the cross‐linking of the cycloaliphatic linseed oil derivative proceeds at a lower rate than the cycloaliphatic epoxy resin, but with a higher rate than epoxidized oil. The addition of divinyl monomers accelerates the speed of curing and increases the elasticity of the cured materials. A similar relationship was also observed during kinetic studies of the cationic photopolymerization reaction of a cycloaliphatic linseed oil derivative [44]. It was found that the photo‐cross‐linking rate is controlled by the diffusion of active macromolecules, which depends on the viscosity of the environment. The different reactivity of the cycloaliphatic and epoxidized oil derivative in the main chains results from the differences in the diffusion of the molecules of both compounds and depends on the presence of divinyl monomers in the reaction environment. The improvement of the final properties of the described compositions was obtained by adding up to 20 wt% of tetraethyl orthosilane (TEOS) [45]. The organic–inorganic hybrid materials obtained in this way, containing the optimum amount of TEOS oligomers, amounting to about 10 wt%, were characterized by the highest value of the elastic modulus, the highest glass transition temperature, and the highest cross‐linking density. Although the incorporation of TEOS oligomers in the structure of a cured cycloaliphatic linseed oil derivative simultaneously reduces the relative elongation at break and fracture toughness, it should be remembered that the biggest disadvantages of modified vegetable oils as materials susceptible to photocuring are low glass transition temperature and low speed of cross‐linking. Another example of a cycloaliphatic linseed oil derivative, also intended for photocuring, is the product of a Diels–Alder reaction of linseed oil with 1,3‐butadiene [46] (Figure 1.10).

Figure 1.9 Structure of norbornyl epoxidized linseed oil.

Figure 1.10 Structure of epoxidized cyclohexene‐derivatized linseed oil.

Compositions based on modified vegetable oils, hardened by photopolymerization, are mainly intended for coating materials. However, it has been shown that it is also possible to use epoxidized soybean and linseed oils together with cycloaliphatic epoxy resin as binders for glass fiber‐reinforced composites and cross‐linked with visible or UV light [47].