A factorial design applied to the extrusion foaming of polypropylene/wood-flour composites

A factorial design was performed to determine the statistical effects of ma terial compositions and extrusion processing variables on the foamability o f polypropylene (PP)/wood-flour composites. Two levels with centrepoint val ues of wood flour content, chemical foaming agent (CFA) content, extruder's die temperature and screw speed were employed. The isolated main and inter action 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 s ignificant effects (values of "Prob > F" less than 0.0001) on the void frac tion. Wood flour content/CFA content and wood flour content/die temperature constitute the important interaction effects. The experimental results ind icate that void fraction of extrusion foamed composites is a strong functio n of the extruder's die temperature. A large amount of gas molecules availa ble for the cell growth is not the only requirement for the production of c omposite foams with a high void Fraction. Processing at a high die temperat ure is also very important for the development of proper viscoelastic prope rties of the matrix suitable for cell growth.