Epoxy reactions homopolymerization

The catalytic curing agents commonly used include tertiary amines, Lewis acids and bases, and dicyandiamide. Since their function is truly catalytic, the catalyst is added at relatively low concentrations (0 to 5% by weight) to the epoxy formulation. Homopolymerization generally requires both the presence of catalysts and elevated temperatures for the reaction to proceed. Like the polyaddition reaction, the homopolymerization reaction is accelerated by hydroxyl groups or tertiary amines. 

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. 

DSC thermograms for the uncured epoxies are shown in Figure 3. The uncatalyzed system exhibits two reaction peaks, as observed in our earlier work (12). The lower temperature shoulder is attributable to amine-epoxy reaction and the higher temperature peak is due to hydroxyl and homopolymerization reactions. The... 

Catalytic curing agents are a group of compounds which promote epoxy to epoxy reactions without being consumed in the process. A typical epoxy homopolymerization using a tertiary amine is shown below ... 

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. 

Recently, FTIR spectroscopy studies have been reported which support the above observations. Moacanin et al 3) concluded that two reactions dominate the TC3fDA/DDS cure epoxy-primary amine addition is the principal reaction occurring during the early stage of cure followed by the epoxy-hydroxyl addition reaction. Indeed they find that the rate of epoxy-hydroxyl addition is at least an order of magnitude slower than for the epoxy-primary amine reaction at 177 C. Furthermore, Morgan et al (4) report that the epoxysecondary amine addition and epoxy-epoxy homopolymerization reactions also occur at 177°C but at rates that are approximately 10 and 200 times slower, respectively, than the epoxy-primary amine react ion. 

Other possible reactions, such as homopolymerization (epoxide+epoxide) and epox-ide+hydroxyl group (in the latter stages of cure), can be neglected when the ratio of epoxide to amine is stoichiometric and in the absence of catalyst or accelerator [194], For TGDDM/DDS resins, the homopolymerization reaction may be neglected at cure temperature below 180°C [84], At temperatures between 177°C and 300°C, dehydration and/or network oxidation occur, which results in formation of ether cross-linkings with loss of water. Decomposition of the epoxy-OH cure reaction can also take place, which results in propenal... 

Tertiary amines catalyze the homopolymerization of epoxy resins in the presence of hydroxyl groups, a condition which generally exists since most commercial resins contain varying amounts of hydroxyl functionality (B-68MI11501). The efficiency of the catalyst depends on its basicity and steric requirements (B-67MI11501) in the way already discussed for amine-catalyzed isocyanate reactions. A number of heterocyclic amines have been used as catalytic curatives pyridine, pyrazine, iV,A-dimethylpiperazine, (V-methylmorpholine and DABCO. Mild heat is usually required to achieve optimum performance which, however, is limited due to the low molecular weight polymers obtained by this type of cure. 

The nature of the cure reactions in these epoxies can be confirmed by monitoring the epoxide consumption via near infra-red spectroscopy for a series of epoxide-amine mixtures containing a range of amine contents. A plot of % epoxide consumption vs. amine concentration for DGEBA-T403 epoxies is illustrated in Fig. 2. This plot confirms that the DGEBA-T403 epoxy system forms exclusively from epoxide-amine addition reactions, because (i) 100% epoxide consumption is attained at the stoichiometric amine concentration associated with exclusive epoxide-amine addition cure reactions and (ii) extrapolation of this plot to zero amine content indicates there is no epoxide consumption i.e. there are no epoxide homopolymerization reactions. 

This reaction is in fact the homopolymerization of the epoxide. The reactivity of the secondary hydroxy group with respect to the epoxy group is intermediate between that of tertiary and primary hydroxy group 26). 

For r < 1/3, no gel can be produced by epoxy-amine reactions. However, the homopolymerization of the epoxy excess (Eq. 3.37) may take place, finally leading to gelation. So, it is not convenient to use an epoxy excess to synthesize stable epoxy-amine prepolymers. Commercial epoxy-amine adducts are based on an amine excess. 

However, for several epoxy-amine systems, the simple kinetic model expressed by the set of Eqs (5.18) (5.21) does not provide a good fitting with experimental results. Reaction mechanisms, including the formation of different kinds of complexes, have been postulated to improve the kinetic description (Flammersheim, 1998). Also, a more general treatment of the kinetics of epoxy-amine reactions would have to include the possibility of the homopolymerization of epoxy groups in the reaction path. Sets of kinetic equations including this reaction have been reported (Riccardi and Williams, 1986 Chiao, 1990 Cole, 1991). 

Chemical clusters can be obtained also with two monomers, when two reaction mechanisms are in competition, favoring formation of regions of higher and lower crosslink densities. This situation is more complex and more difficult to control. It is certainly the case for dicyanodiamide (Dicy)-cured epoxies with this hardener an accelerator is always used and a competition between step (epoxy-amine addition) and chain (epoxy homopolymerization) occurs (Chapter 2), leading to inhomogeneous networks. 

When the curing process is based on the reaction between the reactive epoxy molecules only, the reaction is homopolymerization (see Fig. 2.12). The cured structure is essentially made up only of the original epoxy molecules linked together through their own reactive sites. The reactive compound that initiates the homopolymerization reactions is generally known as a catalyst. (Note Sometimes the term hardener is used interchangeably for catalyst, although this is not the convention followed in this book.)... 

The catalyst does not make up part of the final epoxy network structure or have a significant effect on the final properties of the cured resin. Thus, the final cured properties of the epoxy system are primarily due to the nature of the epoxy resin alone. Homopolymerization normally provides better heat and environmental resistance than polyaddition reactions. However, it also provides a more rigidly cured system, so that toughening agents or flexibilizers must often be used. In adhesive systems, homopolymerization reactions are generally utilized for heat cured, one-component formulations. 

Tetraglycidyl ether of tetraphenolethane is an epoxy resin that is noted for high-temperature and high-humidity resistance. It has a functionality of 3.5 and thus exhibits a very dense crosslink structure. It is useful in the preparation of high-temperature adhesives. The resin is commercially available as a solid (e.g., EPON Resin 1031, Resolution Performance Polymers). It can be crosslinked with an aromatic amine or a catalytic curing agent to induce epoxy-to-epoxy homopolymerization. High temperatures are required for these reactions to occur. 

The primary and secondary amines are discussed in this section. The secondary amines are derived from the reaction product of primary amines and epoxies. They have rates of reactivity and crosslinking characteristics that are different from those of primary amines. The secondary amines are generally more reactive toward the epoxy group than are the primary amines, because they are stronger bases. They do not always react first, however, due to steric hindrance. If they do react, they form tertiary amines. Tertiary amines are primarily used as catalysts for homopolymerization of epoxy resins and as accelerators with other curing agents. 

DICY is considered a catalyst and polymerizes epoxy resin through the homopolymerization mechanism. But DICY has also shown behavior with epoxies that indicates some breakdown at cure temperatures to produce a curing agent that contributes to the polyaddition reaction mechanism. 

Since epoxy homopolymerization may be neglected in the absence of catalysts (T), the major cure reactions can be assumed to be the reactions between epoxide and amine groups as expressed in Scheme I. 

The methods are at hand to distinguish which mechanism is responsible for the increase in EEW during isothermal aging. The epoxide homopolymerization should have a second order dependence on epoxide concentration and no dependence on secondary alcohol concentration, whereas the epoxide-alcohol reaction should display a first order dependence on both epoxide and secondary alcohol concentration. Therefore, a study of isothermal aging kinetics versus EEW of the epoxy resin will distinguish these mechanisms. 

An homologous series of epoxy resins with various EEWs was synthesized by standard advancement techniques and subsequently esterified with 15 wt% 1300X13. Isothermal aging kinetics were followed at 175C. Aliquots of resin were withdrawn hourly, rapidly cooled to room temperature, and titrated for EEW. No precautions were taken to exclude air during the isothermal aging. Table Xll summarizes the formulations, reactive moiety equivalent weights, and kinetic calculations for the epoxide-alcohol addition reaction and the epoxide homopolymerization reaction. 

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