Crosslinking reactions of epoxy resins

Modeling with the Smoluchowski-like equation generalized to take into account FSSE is not limited to the simple RAf polymerization. A kinetics approach similar to that described in this section have been used to study crosslinking reactions of epoxy resins with components introduced into the system at different times [17]. Kinetic equations analogous to Eq. (101) have been derived [48] for an RA2 + R B2 system as well as for systems containing 3-functional monomers having functional groups of intrinsically different reactivities [49]. 

Several authors (1-6) have proposed that the crosslinking reactions of epoxy resins with anhydrides is a two or three step process formation of the monoester, formation of the diester and formation of ether crosslinks. It is hoped that this work may assist in a better understanding of this process. 

The DSC curves for thermoset epoxy with different weight fraction of organo-modified nano-clay are shown in Figure 9.11. The onset temperature of the curing and the temperature of the exothermal peak for neat resin are 115°C and 155 C, respectively. The addition of 5 wt% nano-clay in epoxy matrix reduces the onset temperature to 84°C and peak exothermal temperature to 143°C. The catalytic effect of the nano-clay on the crosslinking reaction of epoxy resin is responsible for the reduction. 

Finally, the Mannich reaction may be applied to the. synthesis of reactive amines used as crosslinking agents of epoxy resins. The process requires the availability of molecules possessing more than two NH groups, which are obtained by reaction of polyfunctional substrates with polyfunctional primary amines (oligomeric polyalkyle-ncamines, diamino cyclohexane, etc.) - (see also 422, Chap. Ill, C). 

The crosslinking reactions are illustrated in Reaction 1.8, and they demonstrate that, in principle, only a trace of curing agent is necessary to bring about cure of epoxy resins. Selection of curing agent depends on various considerations, such as cost, ease of handling, pot life, cure rates, and the mechanical, electrical, or thermal properties required in the final resin. 

The first moment of the distribution is Pt0T the total, cumulative molar concentration of polymeric material. As the molecular weight of polymeric species increases, branching and crosslinking reactions yield a thermoset resin. Chromatography analysis of epoxy resin extracts confirms the expected population density distribution described by Equation 4, as is shown in Figure 2. Formulations and cure cycles appear in Table II. 

Kinetic models determine the minimum time required to cure the resin (i.e., guarantee sufficient physical and mechanical properties). They also determine the heat of reaction of the resin for use by heat transfer models and the degree of crosslinking for use in viscosity submodels. The exothermic cure reaction for the transformation of the epoxy resin to the cured matrix polymer can be expressed as ... 

In the system epoxide (epoxy resin) — anhydride, we can thus expect the presence of anhydride, epoxy- and proton donor groups. In their study of the reaction mechanism, Fisch and Hofmann 20 22-24) proposed a sequence of reactions leading to the crosslinking of epoxy resins or to the formation of linear polyesters. The first step is the reaction of the anhydride with hydroxyl groups giving a monoester (Eq.(l))... 

The primary and secondary amines are discussed in this section. The secondary amines are derived from the reaction product of primary amines and epoxies. They have rates of reactivity and crosslinking characteristics that are different from those of primary amines. The secondary amines are generally more reactive toward the epoxy group than are the primary amines, because they are stronger bases. They do not always react first, however, due to steric hindrance. If they do react, they form tertiary amines. Tertiary amines are primarily used as catalysts for homopolymerization of epoxy resins and as accelerators with other curing agents. 

Monomeric Mannich bases of type A-B, yielding polymers 401 (Fig. 156), are less frequently used. Nevertheless, phenolic Mannich bases affording polymeric derivatives 412 (Fig. 158) are worth mentioning, as they arc particularly applied to the production of crosslinked material (Sec. C. 1). The reaction is made possible by the release of amine, as occurs, for example, during the baking of epoxy resins. ... 

DSC, middle IR and near IR data suggest that the crosslinking reaction of an epoxy resin with phthalic anhydride is a stepwise process as follows. 

Recently/ the crosslinking reactions of tetrafiinctional epoxy resins with aromatic primary diamines was investigated. The crosslinked polymers were characterized by UV visible and fluo> rescence spectroscopies after gelation. The amount of tertiary amine fluorescence intensity of the spectra shows significant amounts of such amines in the finished products. The infrared spectra confirm the overall reaction of epoxides with amines, but also show that ether formation becomes significant only late in the cure. In addition, during the cure, especially in air, some oxidations and degradations occur. This results in color formation. 

Discuss the chemistry of epoxy resins based on diglycidyl ethers of bisphenol A, their preparations, and crosslinking reactions with amines, dianhydrides, and dicyanodiamide. 

A very common reaction during processing is the polymerization or crosslinking reaction of thermoset materials to form a final polymeric product. This is a common, heat- induced reaction in the epoxy resins and phenol/methanal polymers discussed in this book. It is also common in mbbers undergoing vulcanization and in isocyanates undergoing transformation to urethanes. These reactions form molecules structurally similar to the crosslinked hydrocarbon shown in Figure 4. 

A study by Comyn et al. [8] indicated that low (or no) cure took place in the interphase between an amine cured epoxy and aluminum because the amine was preferentially adsorbed onto the aluminum oxide on the aluminum. Garton et al. [9] showed that the acidic surface of a carbon fiber selectively adsorbed amine and catalyzed the reaction between the amine and an epoxy resin. Nigro and Ishida [10] found that homopolymerization of epoxy resin was catalyzed by a steel surface. Zukas et al. [11] discovered, in a model system of an amine cured epoxy resin and an activated aluminum oxide, a change in the relative rates of the reactions leading to crosslinking of the epoxy, so that the material in the interphase was structurally different from that in the bulk. 

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