On the increase of viscoelastic modulus with advancement of reaction of an epoxy resin

Log G* = f(X) isothermal variation curves were determined at various temperatures for two epoxy-aromatic amine systems widely used in industry. G* = G′ + jG″ is the viscoelastic modulus, determined by sinusoidal shearing at constant frequency between parallel plates; X represents the degree of reaction, determined by calorimetry. These curves clearly show the phenomena of gelation and glass transition. In the temperature range studied, the value of X at the gel point varies between 0.7 and 0.9 for the DGEBA-DDS system, and from 0.3 to 0.8 for the TGDDM-DDS system. These variations reflect a major modification of the reaction mechanism, particularly before the gel point.