Effects of chain extender on properties and foaming behavior of polypropylene foam.

Polypropylene (PP) foam offers superior thermal and mechanical properties and versatile applications. However, the linear structure of PP hinders the fabrication of a uniform and fine foam, owing to changes in the melt strength with variations in the temperature. The foamability of the material can be improved by fabricating modified PP by introducing long-chain branches by grafting and chain extension reactions, using glycidyl methacrylate (GMA) and adipic acid (AA). Adding 5 phr GMA to PP optimizes the graft ratio. AA as a chain extender forms a long-chain branched structure, as confirmed by the melt flow index, morphology, and thermal and dynamic viscoelastic properties. Variations in foaming characteristics according to the AA content, temperature, volume expansion ratio, and oven residence time have also been observed. An optimal volume expansion ratio of ∼14 is obtained at an AA content of 1.5 phr and foaming for 11 min at 240 °C.