Study of the kinetic and crosslinking reaction of novolak with epoxy resin

Novolak and epoxy reaction was studied by means of differential scanning calorimetry. Two reactions can take place in the mixture: one between OH groups of the novolak and epoxy groups, and the other between secondary OH groups (coming from the opening of the epoxy ring by the first reaction) and epoxy groups. Since only one peak was detected in dynamic thermograms obtained at different heating rates, both reactions occurred simultaneously. A global kinetic model with two kinetic constants was developed. It agreed with dynamic and isothermal experiences. The reaction heat was determined by an average of several runs as 23.1 kcal equiv.(-1) (96.7 kJ/equiv.epoxy). The lass transition temperature was determined for different novolak/epoxy stoichiometric ratios from 0.04 to 2.2 The maximum glass transition was obtained for the 1:1 ratio and it was 99 degrees C. The activation energy obtained from gel-time measures was 15.8 kcal equiv.(-1) (66 kJ equiv.(-1)). A statistical structural model was used in order to describe this epoxy-novolak cure. Equal reactivity between OH groups of the novolak, equal reactivity between epoxy groups and no-substitution effect are taken into account. Intramolecular reaction in the pre-gel stage is neglected. Expression of the average molecular weight and gel conversion are derived. It is shown from the model that for the stoichiometric ratio of OH groups to oxirane groups, the predicted gel conversion agreed with the experimental value. (C) 1998 Elsevier Science Ltd. All rights reserved.