Imidazole catalysis in the curing of epoxy resins

Abstract

SynopsisThe high catalytic activity of imidazoles and particularly of 2-ethyl-4-methylimidazole (EMI) for the curing of epoxy resins and the properties of the resulting resins prompted this study concerned with the nature of the curing reaction. Epon 828 epoxy resin and the model compound phenyl glycidyl ether were used BS starting materials with EMI, 2-methylimidazole, and dimethylbenzylamine as catalysts. During the curing of the resin at 50°C., the decrease in the infrared absorption of the epoxy band with time is accompanied by a decrease in the intensity of the imine band of the imidazole moiety, indicating its reaction with the epoxy group and its incorporation into the resin. The measurement of the residual epoxy content after curing for 24 hr. at 50 and 140°C. showed that the imidazoles were not more efficient in completing the epoxy reaction than dimethylbenzylamine. In the experiments with phenyl glycidyl ether the rate of reaction of the epoxy group with EM1 was faster than the rate of polymerization, proving that the imidazole becomes permanently attached to the polymer chain. These results also suggest that the true catalytic species is not EM1 but some addition product thereof. In comparative rate measurements the compound formed from equimolar quantities of EM1 and phenyl glycidyl ether was found to be an excellent catalyst. The NMR analysis of the 1: 1 and 1 : 2 adducts of EM1 and phenyl glycidyl ether has shown that the second mole of phenyl glycidyl ether reacts with the ring nitrogen in the 3 position and not with the hydroxyl group of the mono adduct. By forming the bis adduct in this way the imidazole molecule acts as a crosslinking agent and at the same time introduces an alkoxide ion which can initiate further polymerization. It is very likely that this crosslinking is the process that leads to the superior physical and chemical properties (high heat deflection temperature, resistance to chemicals and oxidation) of the resins prepared with imidazoles as catalysts.

Mechanism and Kinetics of Epoxy-Imidazole Cure Studied with Two Kinetic Methods

“…[1][2][3] In this work, as shown in Figure 4, each curve analyzed is typically parabolic, which is also indicative of an autocatalytic kinetics of the studied DGEBA/EMI-2,4 system. 9 For the thermosetting resins that follow autocatalytic kinetics, the expression of f ðÞ, which excludes diffusion-controlled, fusion, and inversion-type reactions, is:…”

“…[1][2][3] The adducts were believed to be the catalyst that initiated the etherification reactions which cross-link the epoxy resins and determine the final properties of the network. Previous work 5 showed that the formation of adduct was necessary prior to the etherification reaction.…”

“…16 As far as the curing kinetics of DGEBA initiated with EMI-2,4 is concerned, the process of cure has been demonstrated to be autocatalytic. [1][2][3] The first step in the curing process of DGEBA/EMI-2,4 system is the formation of both 1:1 and 2:1 adducts. The 2:1 adduct (A2), which contains the nucleophilic alkoxide ion, was believed to be the catalyst that initiated the etherification reaction.…”

“…The kinetics of imidazole/resin systems has been widely studied by using isothermal or dynamic DSC technique. [1][2][3][4][5] But their studies were all based on the assumption of a constant E and the whole reaction of curing was considered as a single kinetic process (Method I). Therefore, the calculated E is the apparent activation energy of the global reaction.…”

Imidazoles-Intercalated α-Zirconium Phosphate as Latent Thermal Initiators in the Reaction of Glycidyl Phenyl Ether (GPE) and Hexahydro-4-Methylphthalic Anhydride (MHHPA)

“…Aside from DBU and DABCO, imidazoles have been widely used for curing epoxy resins as cured epoxy resins give good thermal resistance and physical properties because of the reactivity in chain polymerization with epoxide [12,13]. Latent thermal initiators of imidazoles have been developed.…”