Thermal Analysis of Thermoset Resins Cured in the Solid State by Electron Beam Irradiation

摘要

This study involved thermal analysis of neat thermoset resin samples cured with a high energy electron beam. The primary goal was to study the development of glass transition temperature in resin systems that undergo a significant amount of cure in the solid state. The resins included an epoxy resin catalyzed with a cationic initiator, and a vinyl ester/styrene copolymer resin. The latter was known to cure by free radical polymerization, so no initiator was required, i.e. the electron beam served as a free radical generator and direct source of free electrons. Resin sample composition was varied by changing the level of initiator (for cationic epoxy resin) and by varying the level of styrene (for vinyl ester resin). Resin samples were cast into a rectangular mold to a depth of about 1-2mm and cured near ambient temperature (50- 70°C) with e-beam doses of up to 150 kGy. These resins are known to undergo gelation in the early stages of dose accumulation, followed by continued cure and vitrification in the solid state during the remainder of the exposure. Differential Scanning Calorimetry (DSC) was used to quantify the amount of residual cure in the samples. Thermomechanical Analysis (TMA) and Dynamic Mechanical Analysis (DMA) were used to measure Tg. Results indicated that the Tg was higher than the cure temperature, with some cases demonstrating over 50°C increase. TMA results indicated an unusual step change in thermal expansion at Tg (i.e. apparent first order transition), rather than a simple change in slope that is normally expected at Tg (i.e. second order transition). This feature was not reversible upon cooling. We believe this to be similar to well known hysteresis effects that occur when polymers are cooled and heated at different rates, and/or physical aging phenomena.