UNSATURATED POLYESTERS - INFLUENCE OF THE MOLAR-MASS ON THE CURE WITHSTYRENE AND THE PROPERTIES OF THE RESULTING NETWORKS

Fractions of an unsaturated polyester resin (UP), based on 1 mol malei c anhydride, 1 mol isophthalic acid, 1 mol propylene glycol, and 1 mol diethylene glycol, were obtained from an industrial reactor at variou s times during polyesterification. They were characterized by H-1-NMR and chemical titration. Increasing the reaction time from 1 to 9 h led to an increase in (a) the number-average molar mass (M-n) in the rang e 484-1712 g/mol, (b) the maleic acid-fumaric acid isomerization from 85 to 95%, (c) the glass transition temperature (T-g) from 251 to 267 K, and (d) the compatibility with styrene. Several of these fractions were cured with styrene, in amounts ranging from S/E = 2-5 (molar rati o of C=C double bonds in styrene, S, and unsaturated polyester, E), us ing benzoyl peroxide as an initiator. Increasing M-n led to an increas e in the T-g value and a decrease in the amount of moisture absorption . Both effects were explained by the decrease in the concentration of the polar end groups present in the UP chains. However, using a relati vely high styrene fraction in the formulation (i.e., S/E = 5) led to c ured products exhibiting reasonably high T-g values (approaching the p ure polystyrene value) combined with a low moisture absorption. This c onstitutes a practical way to revalorize batches of UP resins with mol ar masses out of specification. (C) 1997 John Wiley & Sons, Inc.