TOUGH LOW-PROFILE ADDITIVES IN SHEET MOLDING COMPOUND

Thermosetting polyester in a sheet molding compound (SMC) shrinks by 7 to 10% and causes fiber show-through and poor surface quality. Adding certain thermoplastics results in zero shrinkage and smooth surfaces; these are called low-profile additives (LPA). Poly(vinyl acetate) (PV Ac), a highly effective LPA, decreases the fracture energy from 66.3 t o 35.1 J/m(2) when present as 14 wt% of the matrix, even though the PV Ac is tough. It produces a nodular three dimensional network; the cros slinked polyester nodules are interconnected by weak polyester bridges and the PVAc forms a thin coating on each nodule. The macroscopic cro sslinking shrinkage is offset by void formation in the PVAc phase. Cra ck follows the void structure and cleaves the bridges between the nodu les. Low matrix fracture toughness leads to high microcracking and ear ly gross failure of the SMC. This paper presents alternative low profi le additives that give excellent low profile effect with improved matr ix fracture energy and SMC strength, Commercially available styrene-bu tadiene block copolymers were studied. These are less polar and form d iscrete cavitated domains of a few 100 mu m in the solid polyester mat rix. With this morphology, good low profile and toughening usually are contradictory, since the former requires small LPA particles for smoo th surfaces and the latter requires large particles for crack bridging by the rubber. However, a good compromise was found in Epoxidized Kra ton D1300X (a 50% diblock and 50% triblock copolymer of styrene and bu tadiene) with the compatibilizer Ricon 131MA17 (a maleic anhydride-pol ybutadiene adduct). This produces a low profile effect equivalent to P VAc, but with a higher matrix fracture energy of 70.7 J/m(2) and highe r matrix modulus. With the epoxidized rubber, the flexural strength of the SMC increases by 13% over that with PVAc, with no sacrifice of th e modulus or strain-to-failure.