Curing with Cyclic Anhydrides

In spite of a great number of studies devoted to curing with cyclic anhydrides, there existed a number of contraversial views concerning the reaction mechanism. Recent studies have revealedthat in the presence of tertiary amines the reaction can proceed also in the absence of proton donors. The tertiary amine reacts first with epoxide most probably via a zwitterion The simplified mechanism is as follows... 

This article summarizes and analyzes the results obtained for the anionic copolymerization of cyclic ethers with cyclic anhydrides. This reaction is of great practical importance, especially as curing reaction of epoxy resins and is also used for the preparation of linear polyesters with special functional pendant groups. 

For the copolymerization of epoxides with cyclic anhydrides and curing of epoxy resins, Lewis bases such as tertiary amines are most frequently used as initiators. In this case, terminal epoxides react with cyclic anhydrides at equimolar ratios. The time dependence of the consumption of epoxide and anhydride is almost the same for curing 35-36> and for model copolymerizations 39,40,45). The reaction is specific 39,40) to at least 99 %. In contrast, the copolymerization with non-terminal epoxides does not exhibit this high specificity, probably because of steric hindrances. The copolymerization of vinylcyclohexene oxide or cyclohexene oxide is specific only to 75-80 % and internal epoxides such as alkylepoxy stearates react with anhydrides only to 60-65 %. On the other hand, in the reaction of epoxy resins with maleic anhydride the consumption of anhydride is faster 65the products are discoloured and the gel is formed at a low anhydride conversion 39). Fischer 39) assumes that the other resonance form of maleic anhydride is involved in the reaction according to Eq. (33). 

A reaction order of 1/2 was determined by Malavasic et al.91) for the curing of epoxy resins with cyclic anhydrides over the conversion range 18-79 %. At 86-98.5 % conversion, the authors established a first-order curing reaction. Booss and Hau-schildt90) regard copolymerization and curing as a zero-order process with respect... 

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 the copolymerization of epoxides with cyclic anhydrides and curing of epoxy resins, Lewis bases such as tertiary amines are most frequently used as initiators. In this case, terminal epoxides react with cyclic anhydrides at equimolar ratios. The time dependence of the consumption of epoxide and anhydride is almost the same for curing copolymerizations jjjg reaction is specific... 

The kinetics of copolymerization or curing of epoxy resins with cyclic anhydrides initiated by tertiary amines was investigated by chemical analysis 52,65,73,74,90) differential scanning calorimetry isothermal methods electric methods , dynamic differential thermal analysis , IR spectroscopy dilatometry or viscometry Results of kinetic measurements and their interpretation differ most authors agree, however, that the copolymerization is of first order with respect to the tertiary amine. 

Curing with anhydride hardeners requires a long curing time and a high temperature. The cured resins are excellent in mechanical and particularly in electrical properties at temperatures above Tg. Anhydride hardeners are extensively used. Many practical studies on mechanical and electrical properties 381 but few systematic studies on the structures of the cured resins and physical properties have been reported 27,39 41). The reasons why such systematic studies are scarce are as follows Most of the acid anhydride hardeners are cyclic anhydrides of dibasic acids. The main structure of the cured resins is such as shown in Fig. 1 b. Differences among the structure of the cured resins with different hardeners are only based on the differences in the R segment and consequently, cured resins with rather different mechanical properties cannot... 

Curing of epoxy resins by polycarboxylic acids and cyclic anhydrides is also important in applications, but it is much less understood due to more complex reaction mechanism. Also, the statistical treatment is less developed and partly requires a revision. In this section, the statistics of curing of epoxy resins with polycarboxylic acids and cyclic anhydrides is discussed. 

Permanent-press fabrics are often cured with TV-methy-lol compounds, such as 4.18, and tend to release formaldehyde after the cure. (Formaldehyde [a carcinogen] from various formaldehyde resins is a problem in some homes.115) These can be replaced with polycarboxylic acids, such as the 1,2,3,4-butanetetracarboxylic acid (4.19), which cure via the cyclic anhydride.116 Both dyeing and durable-press treatments can be combined in a single step, if desired.117... 

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. 

Polyesters contain the ester group —COO— in the main chain. Many methods are suitable for their synthesis self-condensation of a,o>-hydroxy acids, ring-opening polymerization of lactones, the polycondensation of dicarboxylic acids with diols, transesterification, the polycondensation of diacyl chlorides with diols, polymerization of O-carboxy anhydrides of a- and jS-hydroxycarboxylic acids, and the copolymerization of acid anhydrides with cyclic ethers. The last reaction is commercially used in the curing of epoxides with anhydrides. 

Cyclic anhydrides are the second most important class of comonomers for cure of epoxy resins (Fig. 3.34). Whereas amine cure usually leaves linear segments and hydrophilic -OH groups, anhydride cure can also react with the -OH groups to produce many more cross-links, thus increasing molecular rigidity and water resistance. These cure reactions generally require heat and catalysis. 

The curing reaction between an epoxy resin and an acid anhydride with or without catalyst has been studied by Tanaka and Kazinschi [78], Fisch and coworkers [80,81], Wegler and coworkers [82], Dearborn and coworkers [83,84], Fischer [85], and Schechter and Wynstra [79J. Anhydride curing agents are widely used with epoxy resins. The majority of these are cyclic anhydrides. 

Cyclic anhydrides of carboxylic acid are also used as cross-linkers for epoxy resins. Curing is initiated by reaction of a hydroxyl with an anhydride to yield a half ester and a carboxylic acid group. The newly formed carboxylic acid will in turn react with the epoxy to generate an ester and a new hydroxyl group. These complementary reactions result in a cross-linked network. Etherification reactions... 

The mechanism of cyclic anhydride curing or crosslinking of epoxy resins has received substantial attention, " using several analytical techniques.In order for the cyclic anhydride to react with the epoxide, it must first be converted to the monoester/monoacid, as follows ... 

As a result of the ring opening, the carboxylic acid group becomes available for reaction with the epoxide moiety. The new hydroxyl residue produced from the acid-epoxide reaction reacts with a new cyclic anhydride group, etc., to give cured resins. The overall reaction may be visualized as follows ... 

A cyclic anhydride, such as phthalic anhydride or maleic anhydride, will form a nearly 1 1 alternating copolymer with alkylene oxides. This reaction is the same one used in the curing (hardening) of epoxy resins. A large number of initiators can be used for this reaction, including tertiary-amines, various Lewis acids, and bases including alkoxides. The uncatalyzed reaction invokes initiation with a hydroxyl group (125,126). This reaction can be used to introduce a polyester unit into a polyether polymer chain. With phthalic anhydride, the reaction proceeds as follows ... 

Cyclic acid anhydrides such as maleic, dodecylsuccinic, hexahydrophthalic, phthalic, phyromellitic, etc. are widely employed as curing agents for epoxy resins. They form esters with epoxy resins. These resins have better thermal stability and good electrical insulation and chemical resistance expect to alkalis. 

Martini DS, Braga BA, Samios D. On the curing of hnseed oil epoxidized methyl esters with different cyclic dicarboxylic anhydrides. Polymer 2009 50 2919-2925. 

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