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
