Tough low profile additives in sheet molding compound

Abstract 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) (PVAc), a highly effective LPA, decreases the fracture energy from 66.3 to 35.1 J/m² when present as 14 wt% of the matrix, even though the PVAc is tough. It produces a nodular three dimensional network; the crosslinked polyester nodules are interconnected by weak polyester bridges and the PVAc forms a thin coating on each nodule. The macroscopic crosslinking shrinkage is offset by void formation in the PVAc phase. Crack follows the void structure and cleaves the bridges between the nodules. Low matrix fracture toughness leads to high microcracking and early gross failure of the SMC. This paper presents alternative low profile additives that give excellent low profile effect with improved matrix fracture energy and SMC strength. Commercially available styrene‐butadiene block copolymers were studied. These are less polar and form discrete cavitated domains of a few 100 μm in the solid polyester matrix. With this morphology, good low profile and toughening usually are contradictory, since the former requires small LPA particles for smooth surfaces and the latter requires large particles for crack bridging by the rubber. However, a good compromise was found in Epoxidized Kraton D1300X (a 50% diblock and 50% triblock copolymer of styrene and butadiene) with the compatibilizer Ricon 131MA17 (a maleic anhydride‐polybutadiene adduct). This produces a low profile effect equivalent to PVAc, but with a higher matrix fracture energy of 70.7 J/m² and higher matrix modulus. With the epoxidized rubber, the flexural strength of the SMC increases by 13% over that with PVAc, with no sacrifice of the modulus or strain‐to‐failure.