CURE OF UNSATURATED POLYESTER RESINS .1. HEAT OF COPOLYMERIZATION AND GLASS-TRANSITION TEMPERATURE

An unsaturated polyester (UP) resin based on maleic anhydride, isophthalic acid, propylene glycol and dipropylene glycol was cured with variable amounts of styrene, using benzoyl peroxide as initiator. Differential scanning calorimetry was used to determine heats of copolymerization, thermal mechanical analysis was employed to measure glass transition temperatures and Fourier-transform infra-red spectroscopy was used to follow the individual conversions of styrene (S) and UP double bonds (E). A heat of copolymerization equal to 66 kJ mol-1 was obtained, independently of the initial S/E ratio. Taking the overall conversion into account, the theoretical value for complete conversion of double bonds was estimated as 69 kJ mol-1. These values are in good agreement with literature values for the copolymerization of diethyl fumarate and styrene (65-70 kJ mol-1) and styrene homopolymerization (68.5-73 kJ mol-1). The maximum glass transition temperature (T(g)) was obtained for formulations with S/E between 2 and 3, undergoing a near-azeotropic copolymerization. A single relationship between T(g) and overall conversion was obtained for different cure temperatures.