Surface morphologies of composites based on unsaturated polyester pre-polymer

We report the study of surfaces of bulk molding compounds (BMC) based on miscible polymeric thermoset blends (TB)—unsaturated polyester, styrene and low profile additive (LPA)—containing fillers and glass fibers. In contrast to scanning electron microscopy (SEM) that identified a continuous organic layer at the BMC surface, atomic force microscopy (AFM) showed the existence of aggregates linked together to form a network at the micrometric scale. This indisputably demonstrated that phase separation took place at the surface of the BMC. The influence of TB was examined by comparing the surface morphologies of BMC and corresponding TB. Several cases were distinguished as a function of the TB composition. (1) Without LPA, the surface of the TB was continuous (no phase separation took place during curing) and the surface of BMC revealed the presence of aggregates resulting from a phase separation induced by the fillers. (2) For very low molecular weight LPA, aggregates randomly spread on islands surrounded by large holes were observed on the TB surface. These holes were shown to result from surface deformations induced by absence of shrinkage compensation. The corresponding BMC presented particles randomly spread on the surface. (3) The general case (higher molecular weight LPA) corresponded to similar TB and BMC surfaces morphologies with aggregates randomly spread on the surface. In this case, BMC roughness and morphology reflected the TB roughness and morphology. These observations led to the proposal of some considerations concerning the control of surface aspects of BMC.