A Factorial Design Applied to the Extrusion Foaming of Polypropylene/Wood-Flour Composites

Abstract

A factorial design was performed to determine the statistical effects of material compositions and extrusion processing variables on the foamability of polypropylene (PP)/wood-flour composites. Two levels with centrepoint values of wood flour content, chemical foaming agent (CFA) content, extruder's die temperature and screw speed were employed. The isolated main and interaction effects of these variables on the void fraction of foamed composite samples were analysed using Design Expert software. Statistical analysis of data revealed that the void fraction data was best fit with a linear model. The extruder's screw speed showed no discernible effect within the narrow range studied (20 to 50 rpm) whereas the other three main factors showed significant effects (values of “Prob > F” less than 0.0001) on the void fraction. Wood flour content/CFA content and wood flour content/die temperature constitute the important interaction effects. The experimental results indicate that void fraction of extrusion foamed composites is a strong function of the extruder's die temperature. A large amount of gas molecules available for the cell growth is not the only requirement for the production of composite foams with a high void fraction. Processing at a high die temperature is also very important for the development of proper viscoelastic properties of the matrix suitable for cell growth.