Anhydride-cured epoxy reaction

Anhydride Cured Epoxy Reaction Mechanism. In the case of anhydride cured epoxy reaction, catalysts will promote ring opening of the anhydride to provide carboxylic group for reaction with epoxide. Without catalysts, the reactions are slow and accompanied by extensive epoxide homopolymerization at elevated temperatures. 

In the non-catalyzed anhydride cured epoxy reaction, Tanaka and Kakiuchi (5, 6) found that the reaction rate was proportional to the concentration of epoxide, acid anhydride, and hydroxyl compound, suggesting a termolecular transition state for the rate determining step. 

For Lewis base and Lewis base salt catalyzed anhydride cured epoxy reactions, Fischer (7) proposed that the initial step of an anhydride, epoxide, and tertiary amine system was the activation of the anhydride (reaction 1) ... 

Tanaka and Kakiuchi (6) proposed catalyst activation via a hydrogen donor such as an alcohol as a refinement to the mechanism discussed by Fischer (7) for anhydride cured epoxies in the presence of a tertiary amine. The basic catalyst eliminates esterification reactions (8). Shechter and Wynstra ( ) further observed that at reaction conditions BDMA does not produce a homopolymerization of oxiranes. 

These compounds do not readily react with epoxy resins except in the presence of water, alcohol, or some other base, called an accelerator. Tertiary amines, metallic salts, and imidazoles often act as accelerators for anhydride cured epoxy systems. The reaction between acid anhydride and epoxy resins is illustrated in Fig. 5.7. 

Since the reaction between an epoxy resin and an acid anhydride curing agent also produces an ester linkage, anhydride cured epoxies have poorer hydrolytic stability than do... 

Gel Tg. the temperature at which gelation and vitrification occm simultaneously Tgo. the Tg of the mixed reactants, corresponding to a minimiun cure In isothermal cures, at temperatures between the gel Tg and Tgo, the resin undergoes gelation followed by vitrification. Since Tg is a function of both the degree of cure and the cross-link density, it increases to a point at which aU polymer reaction sites have been consumed. At that point, the Tg reaches a plateau and does not increase further with temperature until the decomposition temperature where a char region occurs (Fig. 2.17). An example of the effect of cure conditions on Tg and CTE values was shown by Konarski " who varied the cure cycles of an anhydride-cured epoxy from 130 to 175 °C as shown in Table 2.10. The value of Tg continued to... 

A simplified mechanistic model for the epoxy-anhydride cure of Reaction scheme 3 is given in Reaction scheme 5. 

Before vitrification, a heat capacity change as a result of chemical reaction, ACp,react, is noticcd. For the anhydride-cured epoxy and the polyester-styrene resin a minor, but reproducible, and almost linear decrease of Cp with conversion is observed. The former system is supposed to be an anionic chain-growth living polymerisation (without termination), the latter is a chain-growth copolymerisation with termination. 

The use of di- or poly-carboxylic acid anhydrides to cure epoxy resins is based on the reaction of these materials to produce ester links, in the presence of acidic or basic... 

After the amines, acid anhydrides constitute the next most commonly used reagents for curing epoxy monomers. The epoxy-acid reaction proceeds through a stepwise mechanism (Sec. 2.2.4) while the reaction of epoxides with cyclic anhydrides, initiated by Lewis bases, proceeds through a chain-wise polymerization, comprising initiation, propagation, and termination or chain transfer steps. Some of the postulated reactions are shown in Table 2.25 (Matejka et al., 1985b Mauri et al., 1997). 

As with amine cured epoxies, frequently the optimum properties of anhydride-epoxy systems are obtained at other than the calculated stoichiometric amount. This is because competing reactions occur, such as the reaction of the anhydride group with a hydroxyl group. Typically, optimum anhydride concentrations occur at 85 to 95% of stoichiometric concentrations. 

Another usual family of hardeners employed to cure epoxy monomers are cyclic anhydrides, with the reaction initiated by tertiary amines or ammonium salts. The reaction proceeds through an alternating chainwise copolymerization, as shown in Figure 28.3. 

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