WO9321250 Curing agent for epoxy resins

Abstract

The sol-gel-glass transformations were examined in the thermosets of diglycidyl ether of bisphenol A (DGEBA) and diglycidylaniline (DGA) cured with 4,4′-diaminodiphenylmethane (DDM) at 70°C by using time-resolved fluorescence, Fourier-transform (FT) Raman spectroscopy, an ultrasonic technique and torque measurements. The rotational correlation times of substituted styryl and cyanine dyes showing a twisted intramolecular charge-transfer increase as the isothermal cure reaction proceeds and are sensitive to gelation. The chromophores detect the local viscosity of the surroundings. In the case of DGEBA/DDM, the extent of the epoxide ring reaction, obtained from FT-Raman studies over the whole curing process, was used to determine the variation of the glass transition temperature and the reduced free volume during crosslinking. Thus, it was possible to relate quantitatively the increase of the rotational correlation time of the dyes to the decrease of the reduced free volume. The variation of the ultrasonic velocity and absorption in the course of curing indicates vitrification (dynamic glass transition), but shows no characteristics with respect to gelation. Differences in the curing behaviour of DGEBA/DDM and DGA/DDM were clearly evident.