Curing of epoxy resin

Transformation of epoxy resins, which are viscous liquids or thermoplastic solids, into network polymers is a result of interaction with alkali or acid substances by means of to polyaddition and ionic polymerization mechanisms.10 A resin solidified by to the polyaddition mechanism, is a block copolymer consisting of alternating blocks of resin and a hardener or curing agent. A resin solidified by the ionic mechanism is a homopolymer. Molecules of both resin and hardener contain more than one active group. That is why block copolymer formation is a result of multiple reactions between an epoxy resin and a curing agent.11 

Epoxy resins are solidified by the polyaddition mechanism by primary and secondary amides di- and polyamides, polybasic acids and their anhydrides, monomer and oligomer isocyanates, poly- 

Typical recipes of compositions, widely used in the electrical industry and in the production of reinforced plastics, are given in Table 1.2, while the basic properties of epoxy plastics cured by amine and anhydride hardeners can be found in Table 1.1. 

Epoxy resins are an important class of polymers used for reactive processing. However, it is rather difficult to find a formulation which provides sufficiently high process rates to be useful in the modern processing equipment used for the RIM-process. 

---

Janke, C.J., Lopata, V.J., Havens, S.J., Dorsey, G.F. and Moulton, R.J., High energy electron beam curing of epoxy resin systems incorporating cationic photoinitiators, US Patent 5,877,229, 1999. 

Lopata, V.J., Chung, M., Janke, C.J. and Havens, S.J., Electron curing of epoxy resins initiator and concentration effects on curing dose and rheological properties. 28th Int. SAMPE Technical Conference, 901, 1996. 

DuSek, K. Network Formation in Curing of Epoxy Resins. Vol. 78, pp. 1—58. 

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. 

By contrast with tertiary amines used in catalytic quantities, primary and secondary amines or acid anhydrides may be used to bring about the cure of epoxy resins by reaction in stoichiometric proportions. A typical amine curing agent used at this level is diaminodiphenylmethane (DDM), which reacts with an individual epoxy-group in the way shown in Reaction 4.17. 

Curing of epoxy resins can also be achieved by ring-opening polymerization of epoxide groups using either Lewis acids (including those generated photochemically) or Lewis bases (Sec. 7-2). 

It easily forms double compds, such as boron trifluoride etherate. Another compd, boron tri fluoride-mono ethyl amine, BFa— C2H6NH2 is wh to pale tan flakes, melting at 88—90°. It released BFa above 110° and is used for elevated temp cure of epoxy resins Refs 1) Gmelin-Kraut Syst Number 13(1954), 168 2) CondChemDict (1961), I661L R ... 

Tertiary amines are used to accelerate both amine and anhydride cures of epoxy resins (B-67MI11501). Certain heterocyclic amines have been used for this purpose, including pyridine and piperidine. In the case of anhydride cures, the use of an amine catalyst not only accelerates the cure, but also improves the thermal stability of the cured resin. 

In Chapter 2 the DSC technique is discussed in terms of instruments, experimental methods, and ways of analysing the kinetic data. Chapter 3 provides a brief summary of epoxy resin curing reactions. Results of studies on the application of DSC to the cure of epoxy resins are reviewed and discussed in Chapter 4. These results are concerned with the use of carboxylic acid anhydrides, primary and secondary amines, dicyanodiamide, and imidazoles as curing agents. 

The results of DSC studies on the anhydride cure of epoxy resins are summarised in Table 2. These studies have confirmed that the cure mechanism is complex. The early stages show autocatalytic features while the later stages are complicated by the effects of diffusion control. Intermediate stages of cure can show an approximation to overall kinetic orders of 1 or 2. In general the isothermal DSC data are easier to... 

Wei, J., DeMuse, M. and Hawley, M.C., Kinetics modelling and time-temperature-transformation diagram of microwave and thermal cure of epoxy-resins, Polym. Eng. Sci., 1995, 35, 461. 

Kannebley 30) and Sorokin et al. 31,32) obtained second-order rate constants for the relevant model bimolecular reactions whereas Arnold 19) and Doszlop et al.37) consider curing of epoxy resin with anhydride or reactions of monoepoxides with various proton-donor compounds to be first order (Table 1). Considering the mechanism of individual reactions which proceed during curing or in the non-catalyzed... 

For iodides, the order of efficiency is tetrabutyl > tetraethyl > tetramethyl 56 A rise in the catalytic effect of initiator is observed for alkali salts and for ammonium salts with increasing diameter of the cation, hence with growing distance between charge centres. This is in agreement with an increase of the electropositivity of cations and the increasing ability of salts to dissociate 54), Despite of this, the remarkable efficiency of lithium salts in curing of epoxy resins 58) or in copolymerization reactions 411 was confirmed in some papers. 

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). 

If the proton donor is an alcohol or a phenol, the active centre is formed directly in reactions (56)—(58) without epoxide isomerization. The propagation steps are the same as in the previous mechanism (Eqs. (59) and (60)). Antoon and Koenig61) proposed a copolymerization scheme for the curing of epoxy resins by anhydrides as a refinement of the mechanisms suggested by Tanaka52) and Lustoft 45 74). However, copolymerization again occurs in the presence of proton donors. The complete sequence proposed by Antoon and Koenig 67) is as follows ... 

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... 

---