An experimental investigation of the effect of prepolymer molecular weight and stoichiometry on thermal and tensile properties of epoxy resins

An experimental study was undertaken to investigate the thermomechanical properties of a certain epoxy/amine configuration. The basic structure of all the epoxies was the same—DGEBA—and the curing agent used was PACM 20. By varying the epoxy prepolymer molecular weight and the stoichiometry between epoxy and amine, a range of different epoxy networks were produced. Glass transition temperatures were evaluated by using differentil scanning calorimetry (DSC). Modulus values as well as an alternative Tg determination were provided by dynamic mechanical analysis (DMA). Coefficients of thermal expansion were obtained from thermomechanical analysis (TMA). The tensile tests conducted at room and elevated temperatures provided additional modulus data along with the yield point, tensile strength, and elongation at break data. Property vs. stoichiometry curves exhibited a maximum for the glass transition temperature and the over the Tg modulus at the stoichiometric point. On the other hand, the under Tg modulus showed a minimum at the stoichiometric point. The results of the yield strength show remarkable similarity with the results of the modulus. Strength and elongation at break do not show clear trends, but a much different behavior is exhibited between room and elevated temperatures.