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 highrnenergy electron beam. The primary goal was to study the development of glass transitionrntemperature in resin systems that undergo a significant amount of cure in the solid state. Thernresins included an epoxy resin catalyzed with a cationic initiator, and a vinyl ester/styrenerncopolymer resin. The latter was known to cure by free radical polymerization, so no initiatorrnwas required, i.e. the electron beam served as a free radical generator and direct source of freernelectrons. Resin sample composition was varied by changing the level of initiator (for cationicrnepoxy resin) and by varying the level of styrene (for vinyl ester resin). Resin samples were castrninto a rectangular mold to a depth of about 1-2mm and cured near ambient temperature (50-rn70℃) with e-beam doses of up to 150 kGy. These resins are known to undergo gelation in thernearly stages of dose accumulation, followed by continued cure and vitrification in the solid staternduring the remainder of the exposure. Differential Scanning Calorimetry (DSC) was used tornquantify the amount of residual cure in the samples. Thermomechanical Analysis (TMA) andrnDynamic Mechanical Analysis (DMA) were used to measure Tg. Results indicated that the Tgrnwas higher than the cure temperature, with some cases demonstrating over 50℃ increase. TMArnresults indicated an unusual step change in thermal expansion at Tg (i.e. apparent first orderrntransition), rather than a simple change in slope that is normally expected at Tg (i.e. second orderrntransition). This feature was not reversible upon cooling. We believe this to be similar to wellrnknown hysteresis effects that occur when polymers are cooled and heated at different rates,rnand/or physical aging phenomena.