Effect of long-chain branches of polypropylene on rheological properties and foam-extrusion performances

The effect of modifying polypropylene by the addition of long-chain branches on the rheological properties and performance of foam extrusion was studied. Three polypropylenes, two long-chain-branched polypropylenes and a linear polypropylene, were compared in this study. The modification was performed with a reactive-extrusion process with the addition of a multifunctional monomer and peroxide. The rheological properties were measured with a parallel-plate and elongational rheometer to characterize the branching degree. The change from a linear structure to a long-chain-branched nonlinear structure increased the melt strength and elasticity of polypropylene. Also, there was a significant improvement in the melt tension and sag resistance for branched polypropylenes. Foaming extrusion was performed, and the effect of the process variables on the foam density was analyzed with Taguchi's experimental design method. For this study, an L-18(2(1)3(5)) orthogonal array was used on six parameters at two or three levels of variation. The considered parameters were the polypropylene type, the blowing agent type, the blowing agent content, the die temperature, the screw speed (rpm), and the capillary die length/diameter ratio. As a result, the most significant factor that influenced the foam density was the degree of long-chain branching of polypropylene. (c) 2005 Wiley Periodicals, Inc.