Low profile additive, LPA

Low profile plastics are added to reduce shrinkage during cure. They are normally thermoplastics that include polyvinyl acetates, polymethyl methacrylate, and copolymers with other acrylate, vinyl chloride-vinyl acetate copolymers, polyurethane, polystyrene, polycaprolactone, cellulose acetate butyrate, saturated polyester, and styrene butadiene copolymers. More details about the low profile additive (LPA) mechanism are published in the literature. ... 

Bulk molding compounds (BMC) and sheet molding compounds (SMC) have gained increased commercial importance in recent years, particularly in the automotive industry. Every yeeir new applications are introduced. BMC and SMC products consist of a thermosetting resin, such as an unsaturated polyester, containing a filler and reinforced with glass fibers. Often the recipe includes a thermoplastic additive known as a low profile additive (LPA) to eliminate cure shrinkage and improve surface quality. Other additives may also be present to control viscosity, add color, and promote mold release. 

The level of measured improvements in toughness attributed to liquid rubber additives is somewhat dependent on the type of test performed as well as the specific SMC recipe. The toughening mechanisms in effect for rubber modified SMC materials have not been well documented. The presence of a low profile additive (LPA), mineral filler, and glass fibers affects the dispersion of the rubbery additive and its effectiveness in toughening the polyester matrix. Interactions between the rubbery additive and each of the typical SMC components have not been well researched. 

Fig. 22 Scheme of the shrinkage control mechanism of the low temperature cure a induction stage b spinodal decomposition c coarsening to (i) the low profile additives (LPA)-rich phase dominated structure, or (ii) the UP-rich phase dominated structure d coalescence and growth, (i) coarse co-continuous structure, or (ii) LPA-rich phase dominated structure e gelation and f microvoid formation. Reprinted from (2000) Polymer 41 697 [213] with permission... 

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