Reverse temperature profile rheology and recycle study of polyhydroxybutyrate copolymer within an injection molding machine

Polyhydroxyalkanoate (PHA) has gained significant attention as a substitute to conventional thermoplastics like Polypropylene (PP) and Polyethylene (PE) due to its biodegradability, biocompatibility, and renewability. PHA's have a great potential for durable applications as they possess properties comparable to those of Polyolefins'. Polyhydroxybutyrate (PHB), the most commonly available type of PHA, for example has high crystallinity and mechanical properties. The use of these polymers is limited due to their brittleness and narrow processing window. Various attempts have been made to overcome these drawbacks and improve the processing of PHB.;This research focuses on the reverse temperature profile rheology study of PHB copolymer within the injection molding machine, and also the stability of the material when used as regrind material. A modified nozzle with pressure and temperature measurement ports was installed on an injection molding machine. Capillary dies were installed within the modified nozzle and viscosity measurements at various shear rates and temperatures were conducted. An inline measurement method was used to quantify the effect of residence time, screw shear rate, and reverse temperature profile. This rheology data was compared to capillary rheology data. The inline rheology data was then curve fit with the Cross-WLF viscosity model used by Moldflow and obtained constants were then used for simulation studies. Simulated results were compared to actual cavity pressure results obtained from an instrumented tool. Simulation over predicted the pressures but predicted the trend with good replication to real time pressure data. These results were then used to develop the processing window for the material.;The material was also studied for its stability with the addition of regrind material. The ability of PHB copolymer to be processed a number of times and the use of different virgin to regrind ratios was investigated to understand the effect of regrind levels and heat history on material properties. The material was recycled for 10 regrind generations and also was studied for 7 regrind ratios with virgin material. A 79% reduction in viscosity and 10% reduction in ultimate tensile strength were observed after 10 regrind generations. Also a drop of 5% was observed in the viscosity and ultimate tensile strength with a 50:50 virgin to regrind ratio at generation two.