The average molecular weights ðM n and M w ) and the molecular weight distributions (MWDs) of the WPC at 30:70 mixing formulation after each reprocessing pass were obtained by GPC as shown in Fig. 6 and Table 2. Both the M n and the M w evidently decreased with increasing re-processing passes.
A gradual increase in the polydispersity index (PDI) was essentially due to the broadening of the GPC curves toward longer retention time (corresponding to lower molecular weights), as shown in Fig. 6 and Table 2. Moreover, the shoulder observed at the far side of the molecular weight distribution after the first to the third re-processing pass gradually diminished and shifted to longer retention time. It finally formed a sub-distribution as the recycling passes progressed. The disappearance of such shoulders implies that chain scission of the long PVC molecules must have occurred as a consequence of the shear stress
introduced repeatedly on the PVC melt during re-processing. Both the number-averaged and the weight-averaged molecular weight were also found to decrease with further re-processing. Their reduction was gradual and small at the initial re-processing. After seven passes of re-extrusion, the M n lowered by 35% while the M w did by 30%. The relatively unchanged mechanical properties, coupled with the minute drop in the average molecular weights, verified that the mixture of WPC scrap to fresh feed at the ratio of 30:70 can be recycled close-looped type repeatedly up to seven passes without critically affecting the performance of the recycled WPC profiles.