Homopolymerization, of epoxy resin

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 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. 

Lewis acids initiate polymerization through the formation of carbonium ions, and Plesch (17) has proposed that a suitable coinitiator is necessary to produce these ions. The mechanism for the initiation of the homopolymerization of epoxy resins by BF3 complexes has been proposed by Arnold (11) to proceed as follows ... 

It is a Lewis acid t e catalyst, which initiates the homopolymerization of epoxy resins that predominantly form ether linkages [25]. The complexes are normally used at 3-5 phr and when used as an accelerator in systems involving anhydrides or amines, 1 phr or less. At room temperature, it is stable when mixed into epoxies, resin soluble salt that has essentially no catalytic activity. Due to this latency, most resin systems incorporating BF3MEA can be stored at room temperature for several months without any appreciable change in viscosity. The effect of BF3MEA on the gel time of an 828/DDS resin system at different temperatures is shown in Figure 13.3. 

A study by Comyn et al. [8] indicated that low (or no) cure took place in the interphase between an amine cured epoxy and aluminum because the amine was preferentially adsorbed onto the aluminum oxide on the aluminum. Garton et al. [9] showed that the acidic surface of a carbon fiber selectively adsorbed amine and catalyzed the reaction between the amine and an epoxy resin. Nigro and Ishida [10] found that homopolymerization of epoxy resin was catalyzed by a steel surface. Zukas et al. [11] discovered, in a model system of an amine cured epoxy resin and an activated aluminum oxide, a change in the relative rates of the reactions leading to crosslinking of the epoxy, so that the material in the interphase was structurally different from that in the bulk. 

Therefore, substituted ureas may be used as latent initiators for anionic homopolymerization of epoxy resins. Pearce and Morris [73] reported the use of 1,1-pentamethylene-3-phenyl urea, prepared by the reaction of phenyl isocyanate with piperidine in dry benzene, to cure a tetraglycidyl methylenedianiline resin toughened with carboxyl-terminated butyl rubber. These formulations showed an excellent stability at 23°C and led to a high Tg product when cured at 170°C. 

The mechanism of homopolymerization of epoxy resins initiated by imidazoles is now well established. A first nucleophilic attack by the unsubstituted nitrogen atom of the imidazole ring forms zwitterion 57 (Fig. 7, path C), which rearranges to an adduct by internal proton transfer. This is followed by a nucleophilic reaction of the newly formed unsubstituted nitrogen, opening a second epoxy group to give the 2 1 adduct that promotes the anionic polymerization of the epoxy. FTIR... 

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. 

The rate of cure of epoxy resins with tertiary amines depends primarily upon the extent to which the nitrogen is sterically blocked. The homopolymerization reaction depends on the temperature as well as the concentration and type of tertiary amine. Benzyldimethylamine (BDMA) and TDMAMP are mainly used as accelerators for other curing agents, in the cining of anhydride-and dicyandiamide-based systems. Other tertiary amine catalysts include 1,4-diazabicyclo(2,2,2)octane (DABCO) and diazabicycloundecene (DBU). 

The catalytic process uses Lewis acids, such as boron trifluoride as the etherate, or a tertiary amine, or the more reactive imidazoles. Amine complexes of boron trifluoride are stable in epoxy resins until heated, when rapid homopolymerization of the resin takes place. The reaction mechanism for tertiary amines has been represented by Garnish [3] (see Figure 3). 

Sergei V Levchik, GiovcUini Camino, Mcuia Paola Ludci, et al. MechcUiistic study of thermal behavior md combustion performcmce of epoxy resins I homopolymerized TGDDM. Polymers for Advanced Technologies, 6(2) 53-62,1995. 

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. 

Epoxy resins may be cured in the manner of polyadditions, i. e., homogeneously catalyzed by multifunctional amines and isocyanates, or cyclic anhydride, dicyan-diamide, or biguanide derivatives. On the other hand epoxy resins are also subject to homopolymerization. The catalysts represent Lewis bases, preferably tertiary amines, imidazoles, or ureas (the latter exclusively for the dicyandiamide curing)... 

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. 

Stoichiometric ratios can be calculated similarly for hardeners. In principle, each active hydrogen will react with one epoxy group. Thus a low-molecular-weight aliphatic polyamine such as diethylene triamine (DETA) has a molecular weight of 103 and five active hydrogens. The hydrogen equivalent is thus 20.6. The stoichiometrically correct ratio with an epoxy resin of EEW 200 would thus be 100 parts resin to 10.3 parts of DETA. In practice there is always a percentage of homopolymerization, especially at the temperature... 

A number of other strategies to manipulate epoxy nanocomposite formation have been discussed in the literature. Chin et al. [45] have investigated the influence of the stoichiometric resin/hardener ratio on exfoliation of a MPDA/DCEBA/octadecylammonium montmorillonite using in situ small-angle X-ray scattering. It was found that resin cure with under-stoichiometric amounts of MPDA and the homopolymerization of DCEBA without any hard-... 

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