2.3 Rheology
The melt flow index (MFI) is a measure of molecular weight and molecular weight distribution characteristics of plastics in industry. The plastics with higher MFI have shorter chains and therefore lower molecular weight and easier flow (Strong, 2006). The opposite is also true. A low melt index means longer chains and high molecular weight. An increase in molecular weight leads to an increase in melt viscosity and impact strength, but leads to a lower yield strength, lower hardness, lower stiffness and lower softening point. Generally for linear polymers, when MFI is plotted against molecular weight, third order lines are obtained (Tzoganakis et al., 1988; Brember et al., 1990). An increase in melt flow index of recycled plastics improves the impregnation of plastic on lignocellulosic fillers.