Powerful property averages of polypropylene regrind

Abstract

A nine-pass recycling study was conducted on polypropylene to determine the effects of degradation. Two resins which differed in molecular weight distribution and heat stabilization were reprocessed by injection molding. Recycle effects were characterized by typical industrial tests to measure processing, molecular, thermal, and mechanical property changes. The experiments were adjusted so that the results included processing variations which regularly occur in a production environment. A mathematical analysis was chosen to show how easily material changes can be evaluated. The method utilized property averaging to determine the percent change from the initial property value. The final analysis was concluded by calculating the difference of two means.

The investigation showed that recycling did not severely degrade either material but that the resin considered least stable was affected the most.

Polydispersity index tests confirm that only minimal degradation of the molecular chain occurs and that chemicals used for stabilization must be removed before chain scission can occur. A polymer of broader molecular weight distribution experiences greater reductions in chain length.

Other findings of the study were correlated to net changes in melt flow rate, particularly mechanical performance. However, density and some thermal properties showed no change. The analysis method identified significant recycle effects as distinct from statistical variations in data.