3.2. Effect of screw speed
Fig. 8 shows the mechanical properties of the sample blends ex-truded in different screw speeds (Table 3) using the optimum screw configuration C. The PP/WF composites extruded at 150 rpm showed the maximum mechanical properties compared to 50 rpm, 100 rpm, 150 rpm and 200 rpm. This is due to higher shear rate obtained at 150 rpm screw speed than 50 rpm and 100 rpm, and higher residence time than the screw speed of 200 rpm, as shown in Fig. 9. In the case of immiscible polymer blends, its morphology determines the physical properties. Mor-phological changes are very dependent on local flow conditions (shear rate, temperature and residence time) which will modify
the material properties (viscosity, interfacial tension coefficient) and that the final distribution obtained at the die exit is the result of a long and complex process. Fig. 10 shows the morphology of the samples prepared at different screw speed in screw configuration C, it shows different rates of reaction and WF dispersion which had occurred at different screw speeds. The morphology at screw rpm of 150 was the best than other screw speed. The technical
importance of reaction to make the PP/WF composite needs high shear stress at PP melt state and suitable residence time. Based on experimental results, the best screw speed is 150 rpm for screw configuration C because this is the best condition for reaction to oc-cur. These results indicate that the properties strongly depend on the reaction between maleic anhydride groups of compatibilizers and OH groups on WF particles during processing.
Batch foaming experiments were performed to study the effect of screw speed on the foamed cell morphology. Fig. 11 shows the scanning electron micrographs for foamed PP/WF composites that were saturated with CO2 at 155 °C and 16 MPa for 2 h and then the pressure was quenched atmospheric pressure within 3 s. The composite foams were analyzed in terms of cell size distribution. From
Fig. 11, it is can be seen that screw speed has not too much effect on cell morphology. Fig. 12 shows the effect of screw configuration on the cell size distribution. Under similar foaming conditions, the size of PP/WF composite produced at screw speed of 150 is a little smaller than that of composite foams produced at other screw speeds, and composite foams produced at screw speed of 150 and 200 show narrower cell size distribution, namely, the cell morphology of composite foamed at screw speed of 150 and 200 improved. This improvement can be attributed to better dispersion of wood fiber in the polymer matrix. As aforementioned, the WF particles are believed to act like nucleating agent, thus promoting heterogeneous nucleation. At the same foaming conditions, the amount of gas available for foaming is constant. Increasing the number cell through heterogeneous nucleation leads to smaller sizes.