CURING OF UNSATURATED POLYESTER RESINS - EFFECTS OF COMONOMER COMPOSITION .1. LOW-TEMPERATURE REACTIONS

The effects of comonomer composition on the curing kinetics of unsaturated polyester (UP) resins at 40-degrees-C were studied by differential scanning calorimetry (DSC) and infrared spectroscopy (IR) over the entire conversion range. Two commercial UP resins, including UP2821 and UP536B, which on the average contained 6.82 and 4.16 unsaturated C=C bonds per polyester molecule, respectively, by H-1-NMR analysis, were used. Experimental results showed that for UP536B reactions initiated by BPO (benzoyl peroxide)/DMA (N,N-dimethyl aniline) amine accelerated system, at higher molar ratio of styrene to polyester C=C bonds, the DSC rate profile at 40-degrees-C exhibited a shoulder after the peak, which could be caused by the grafting reaction of styrene on C=C bonds of the polyester chain inside the microgel particles. In contrast, for UP2821 resin, no shoulder was observed presumably because UP2821 reaction systems would lead to a higher crosslinking density during the cure, and styrene molecule could not readily diffuse into the microgel structure at the later stage of the reaction for the reaction shoulder to appear. A microgel-based reaction mechanism was thus proposed due to the experimental results.