ABSTRACT
The thickening reaction, the free radical polymerization reaction, and the mechanism of mold fill have been studied for an unsaturated polyester sheet molding compound (SMC). The thickening reaction, which is a reaction between polyester acidity and magnesium oxide, has been studied by monitoring the changes in viscoelastic response using Dynamic Mechanical and Brookfield Viscosity techniques. The extent of reaction is well correlated to the degree of neutralization of the acid groups of the polyester resin, supporting the chain extension theory of polyester thickening reactions as proposed by previous researchers. The free radical polymerization of an SMC formulation was studied by dynamic dielectric spectroscopy and differential scanning calorimetry techniques. The dielectric analysis was done on a compression molded sample in non-isothermal experiments while the DSC characterization was done at a series of isothermal temperatures which permitted calculation of the reaction rates for the SMC. Both the thickening and cure reactions have a tremendous influence on the end product performance properties of molded SMC composites; therefore, experiments were conducted to demonstrate the interaction of these two reactions with the process conditions used to mold the SMC. These experiments employed differently pigmented SMC layers in the molded charge mass which permitted easy visual inspection of the influence of process and reaction rate variables on the ply morphology of the molded SMC. These data indicate that very precise control over the viscoelastic properties and the process conditions are essential for reproducible SMC moldings.
