Anhydride-cured epoxy polymerization

A Kinetic Study of an Anhydride-Cured Epoxy Polymerization... 

Some polymeric materials, notably certain anhydride cured epoxies and ester-based polyurethanes, will chemically change or revert when exposed to humid conditions for a... 

The viscosity of liquid adhesives decreases with increasing temperature until polymerization begins to occur, then increases. The viscosities of epoxies are reported to decrease 20% to as much as 80% for a 10°C rise in temperature above room temperature.Figure 2.4 shows this viscosity reduction as a function of temperature for two anhydride-cured epoxy adhesives. For low-molecular-weight polymers, the temperature dependence of viscosity follows the Arrhenius equation ... 

The rate of reversion or hydrolytic instability depends on the chemical stracture of the base adhesive, the type and amount of catalyst used, and the flexibility of the adhesive. Certain chemical linkages, such as ester, urethane, amide, and urea, can be hydrolyzed. The rate of attack is fastest for ester-based linkages. Ester linkages are present in certain types of polyurethanes and anhydride-cured epoxies. Generally, amine-cured epoxies offer better hydrolytic stability than anhydride-cured types. Figure 7.34 illustrates the hydrolytic stability of various polymeric materials determined by a hardness measurement Reversion is usually much faster in flexible materials because water permeates more easily. 

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). 

For example, the strain-to-failure of the network formed by curing DGEBA with nadic methyl anhydride (NMA) and the catalyst benzyldimethylamine (BDMA) decreases from 12 % to 6 % after 56 days at 23 °C [5]. Physical aging has also been observed in amine cured epoxy resin systems and in an uncross-linked 28,000 g/mol linear epoxy resin tested under wet conditions [6]. In contrast, DGEBA homo-polymerized with piperidine (subsequently... 

Even the earliest reports discuss the use of components such as polymer syrups bearing carboxylic acid functionality as a minor component to improve adhesion [21]. Later, methacrylic acid was specifically added to adhesive compositions to increase the rate of cure [22]. Maleic acid (or dibasic acids capable of cyclic tautomerism) have also been reported to increase both cure rate and bond strength [23]. Maleic acid has also been reported to improve adhesion to polymeric substrates such as Nylon and epoxies [24]. Adducts of 2-hydroxyethyl methacrylate and various anhydrides (such as phthalic) have also been reported as acid-bearing monomers [25]. Organic acids have a specific role in the cure of some blocked organoboranes, as will be discussed later. 

An interesting approach to maleimide-terminated phenoxy resin has recently has described (42). para-Maleimidobenzoic acid was reacted with diglyci-dylbisphenol-A epoxy resin in the presence of catalyst to provide the bismale-imide of Fig. 13. Instead of diglycidyl bisphenol-A, linear epoxy resin pre-polymers can be used in this reaction to form a maleimide terminated phenoxy resin. Another suitable functionalized monomaleimide is m- or p- N-(hydroxyphenyl) maleimide which is synthesized from maleic anhydride and m-aminophenol in DMF as a solvent at 70 °C. The purified hydroxyphenyl maleimide was reacted with epoxy resin to form novel BMIs as outlined in Fig. 14. The new BMI and phenoxy oligomers polymerize at temperatures of 200-220 °C, but the cure temperatures can be significantly lowered when catalysts such as imidazoles or triphenylphosphine are added. The cured homopolymers show Tg of 140 and 230 °C for the n = 2 and the n = 1 polymer, respectively(43). 

The curing mechanism of an epoxy resin or the type of functional group of a hardener is the most essential factor determining the structure of the cured resin. The well-known hardeners are polyamines, acid anhydrides, and polymerization catalysts. 

Photocationic initiators. Epoxy resins can be cross-linked by compounds containing active hydrogen, e.g., carboxylic acids, anhydrides, amines, phenols etc., or by the ionic polymerization process. Lewis acids such as BF3 and usually a crystalline complex of BF3 with amines, e.g., BF3 NH2C2H5, can be used for curing reaction at 80-100° C [124]. 

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