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
