Epoxy/anhydride reactions

The reactivity of the epoxy-anhydride reaction is slow therefore, an accelerator is often used at 0.5 to 3 percent to speed the gel time and cure. Most often the accelerator is a tertiary amine, and the optimum concentration is dependent on the anhydride, the resin used, and the cure conditions. The accelerator concentration, like the anhydride concentration, is usually determined experimentally based on a specific set of end properties. 

Recently, phosphine compounds have been vised as catalysts in the epoxy-phenolic or epoxy-anhydride reactions. There is indication that the mechanism does not involve the decomposition of the phosphonium compound to the free phosphine species. The... 

Epoxy/anhydride reactions vary significantly depending on the presence of a catalyst. In uncatalyzed systems, the anhydride reacts withhydroxyl... 

The cure reaction illustrated above is typical of epoxy-anhydride reactions. A catalytic amount of tertiary amine is an effective accelerator for the process, and the product is a tough, rigid solid which has the appearance of a typical cured epoxy. 

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). 

Shortly thereafter, Aiken and coworkers also reported that quinine (4) could be used as a catalyst (50mol%) to promote the methanolytic desymmetrization of the meso-epoxy anhydride 8a to give the lactone 9a in 57% yield and 76% ee (Scheme 11.6) [4]. Lowering the reaction temperature to 0 or —30 °C did not result in any increase in selectivity. meso-Aziridine anhydride 8b was also tested under similar reaction conditions, but a lower enantioselectivity (40% ee) was obtained (Scheme 11.6). 

The Initiation Mechanism with Phosphonium Compounds. Although further experimental data are needed to give a fuller understanding of the reaction mechanisms involved in the latent acceleration effect of these quaternary phosphonium compounds in epoxyanhydride resins, there is definite indication, at this stage, that the mechanism does not involve the decomposition of the phosphonium compound to the free phosphine species (16). The initiation mechanism probably involves the formation of hydrogen-bonded phosphonium-epoxy or phosphonium-anhydride complexes which rearrange on the application of heat to form activated species resulting in polymerization of the epoxy-anhydride components ( > ... 

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) ... 

It was later found that Lewis acids and bases were catalysts for the epoxide-anhydride reaction. The preferred stoichiometry with respect to anhydride and epoxy proved to be 1 1. This relationship led Fischer (3) to propose a mechanism wherein the first step was a reaction between the anhydride and the accelerator to form a carboxyl anion. This anion in turn reacts with an epoxide, and the reaction repeats itself. 

Figure 3.4 Anhydride-curing mechanism for epoxies (a) reaction of hydroxyl group (from epoxy resin) with anhydride (b) reaction of carboxylic acid group with epoxy (c) reaction of...

The interphase in PE fibre/epoxy resin matrix composites was studied by FTIR microspectroscopy using a set up for investigation of the matrix as close to the fibre as a few microns or less. It was shown that moisture present on the fibre surface could influence the polymerisation reaction of the epoxy/anhydride matrix in an irreversible manner (249). 

The interphase in PE fibre/epoxy resin matrix composites was studied by FTIR microspectroscopy using a set-up for investigation of the matrix as close to the fibre as a few microns or less. It was shown that moisture present on the fibre surface could influence the polymerisation reaction of the epoxy/anhydride matrix in an irreversible manner. This effect was enhanced for composites from the more hydrophilic PVAl fibre. The fibre/matrix interaction in these thermoplastic fibre composites was also studied by DSC through characterisation of the fibre melting. A decreased DSC interaction parameter was found if the composition of the interphase was changed by moisture. For a composite with an epoxy/amine matrix, on the other hand, the DSC interaction parameter was unaffected by moisture from the fibre surface. 22 refs. (Pt.I, ibid, p.83-100)... 

In contrast to epoxy-amines, water really participates in the cure chemistry of the epoxy-anhydride system and alters the living polymerisation mechanism because of interfering termination reactions [34]. A less dense network structure with altered properties results [35]. 

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

The experiment of Figure 2.2 will now be considered in more detail as a typical example of isothermal cure with vitrification. It shows the nonreversing heat flow (Figure 2.2a), the heat capacity (Figure 2.2b) and the heat flow phase (Figure 2.2c) as a function of reaction time for the quasi-isothermal cure of an epoxy-anhydride resin at 100°C for 200 min. The reaction exotherm obeys an auto-catalytic behaviour the heat flow increases at... 

Figure 2.15. Comparison of the heat capacity change as a function of reaction conversion for an epoxy(/ = 2)-amine(/ = 4), epoxy-anhydride, unsaturated polyester and melamine-formaldehyde system.

Figure 2.20. Reaction rate for the quasi-isothermal cure of an epoxy-anhydride system.

Polyamine and organic acid anhydride hardeners serve as co-reactive hardeners, which become incorporated into the epoxy resin, whereas tertiary amines, such as 2,4,6-tris-(dimethylaminomethyl)phenol (tris-DMP) and N,N-dimethylbenzylamine, imidazoles, and boron trihalide amine complexes are catalyst-type curing agents, which may not be chemicaUy bound to the resin molecules during epoxy curing reactions (Guin and Work 1995 Muskopf and McCoUister 1987). 

---