Abstract
Imidazoles are widely used as epoxy curing agents, producing resins with good high temperature properties. This thesis is concerned with certain aspects of the mechanism by which cure occurs and is divided into five chapters, as follows: Chapter 1 contains a general review of epoxy resin chemistry and the use of imidazoles as curing agents. Chapter 2 deals with the preparation of a number of substrates used in this study, and also with the tritiation of a range of imidazoles and epoxides. Labelling patterns are interpreted using 3Hnmr spectroscopy. In Chapter 3 details of a radio-tracer kinetic method for following epoxide cure are presented. The induction period seen in the reaction between phenylglycidyl ether and a number of imidazoles is discussed in terms of the ability of 1-unsubstituted imidazoles to form hydrogen bonds and also in terms of the formation of a cocatalyst, with the 1:1 adduct being shown to be an effective catalyst for the reaction. The observed first order rate constant for 1:1 adduct formation is shown to vary with the initial concentration of imidazole used over a range of temperatures. Rate o constants obtained at 95°C are interpreted in terms of the electron donating and withdrawing characteristics of substituents on the imidazole ring. The reaction between the 1:1 adduct of phenylglycidyl ether and 2-ethyl-4-methylimidazole is also discussed, and data obtained using an ultra violet spectrophotometric method interpreted in terms of a transition state involving attack of both the tertiary nitrogen and the hydroxyl hydrogen on the epoxide ring. The reaction at 95°C between imidazole and a number of model epoxides is discussed in Chapter 4. It is shown that, for a number of these epoxides, the observed first order rate constant for formation of the 1:1 adduct varies with the initial concentration of imidazole used. Results are interpreted in terms of the electronegativity of the groups adjacent to the epoxide ring. In Chapter 5, the use of metal-1:1 adduct complexes as curing agents is explored. Results obtained using an ultra violet spectrophotometric technique are interpreted in terms of the stability of the complexes at the reaction temperature.