Effect of conversion on the structure-property relationships of amine-cured epoxy thermosets.

The effects of conversion beyond the gel point on the structure-property relationships of epoxy thermosets using formulations representative of the most commonly used epoxy resin and amine curing agents at balanced stoichiometry with an emphasis on the thermal, tensile, and fracture properties were studied. The range of T(g) from just beyond the gel point to full conversion typically is >100 degrees C. Fracture toughness (as K(1c)) of the epoxy thermosets cured with relatively flexible amines such as ethylenediamine (EDA), diethylenetriamine (DETA), and m-xylylenediamine (MXDA) reaches near its full cure value at only approximately 65-70% conversion. The maximum in K(1c) for these types of epoxy thermosets is at approximately 90% conversion for EDA and DETA but just below its full cure for MXDA. Isophoronediamine represents a special case for fracture behavior because of its apparent substantial cyclization during cure. In the 4,4'-diaminodiphenylsulfone series, K(1c) generally increases with conversion as the concentrations of their strongly antiplasticizing soluble and pendant fractions decrease. A uniform trend of decreasing tensile modulus with increasing conversion was observed in each formulation and is consistent with the expected decrease in the cohesive energy density as monomer glass is transformed into polymer glass.