Rheology and Processing of Novatein Thermoplastic Protein

Abstract

Biopolymers have become suitable alternatives to petro-chemical polymers as they can biodegrade and are considered environmentally friendly. Novatein Thermoplastic Protein (NTP) is a newly developed plastic material using bovine bloodmeal. Knowledge of the rheology of NTP is required to assess processability and to optimise process design. The objective of this research was to use capillary rheometry and batch mixing to determine the rheology and processing behaviour of NTP. These were evaluated at constant plasticiser content, but using three different ratios of water to plasticiser (triethylene glycol, TEG). Each of these was evaluated at 115, 120 and 125 °C. It was shown that NTP is a non-Newtonian, shear thinning fluid with similar behaviour compared to linear low density polyethylene. It was found that viscosity is highly dependent on water content; decreasing with increasing water content. At a shear rate of 15 s-1, the apparent viscosity for the standard formulation (60 parts water per hundred parts bloodmeal) was 2000 Pa.s compared to 7000 Pa.s for the formulation containing 30 parts water [water (30) : TEG (30)], measured at 115 °C. Viscosity decreased slightly with increasing temperature and the degree of non-Newtonian behaviour was mostly unaffected by temperature. The flow behaviour index, n, was found to be in the range 0.11 to 0.17, with no discernable temperature dependence. In the standard formulation, the total amount of plasticiser and ratio water to TEG was higher, which resulted in different flow behaviour with respect to temperature. Batch mixing was used to determine the processing window (∆t) by monitoring torque changes over time during mixing. Processing window for standard NTP decreased from 260 to 220 seconds when the mixing speed was increased from 75 to 95 RPM. The processing window was shortened with reducing water content or an increase in temperature. At 125 °C and 95 RPM the processing window was only 67 seconds for the formulation with 30 parts water and 30 parts TEG. It was concluded that crosslinking was accelerated with an increase in shear and temperature or a reduction in moisture content. Thermal or mechanical energy activates crosslinking, while water plasticises the polymer which decreases the rate of crosslinking. Processing NTP required a delicate balance of supplying enough mechanical and thermal energy for chain rearrangement and consolidation, but preventing fast crosslinking. Crosslinking can be retarded using larger amounts of water, but excessive water may lead to problems after product moulding. Replacing water with TEG does not prevent crosslinking, but does lower the apparent viscosity during processing.