Effect of processing variables on thermal and viscoelastic properties in TGDDM/DDS epoxy system

Abstract

Abstract The effects of processing temperature and mechanical mixing speed on glass transition temperature, and pot life, activation energies for chemical reaction and molecular relaxation of diaminodiphenyl sulphone (DDS) cured tetraglycidyl diaminodiphenyl methane (TGDDM) epoxy have been evaluated. Viscometry study indicates that the pot life of TGDDM/DDS resin mixtures is significantly affected by the cure temperature and mixing speed. The anomalous behavior at the mixing speed of 900 rpm is likely to be due to a result of poor mixing. Differential scanning calorimetry (DSC) studies show that the glass transition temperature of uncured resin exhibits a Tgo of 5 ‐ 19 °C, which decreases with increasing mixing speed. In general, a higher mixing speed would create a higher mixing power of the system, is a result of increasing fluid molecular friction and shear stress. This may be a consequence of molecular chain scissions of oligomer that probably reduce the average molecular weight, and therefore reduce the glass transition temperature. The activation energy for the curing reaction of this epoxy system, around 91 ‐ 98 kJ/mole, was found to be only slightly affected by the mixing speed. Dynamic mechanical analysis (DMA) studies show that two main relaxations can be ob‐ served for this epoxy system. The low temperature relaxation at ‐20 ‐ ‐50 °C is the β transition, with a low activation energy of about 56 ‐ 59 kJ/mole under increasing mixing speed, whereas the high temperature relaxa‐tion at 230 ‐ 260 °C is the a transition, which is associated with the glass transition temperature, with an activation energy of 240 ‐ 400 kJ/mole, and was strongly affected by the mixing speed. It is believed that these activation energies provide a unique method of characterizing the molecular segmental motion of epoxy networks which were affected by the mixing process.