Development of Bloodmeal Protein Thermoplastic Blends

Abstract

This study investigated the compatibilization of Novatein® Thermoplastic Protein (NTP) blends with other polymers. NTP was blended with three different types of polymers;a petroleum-based polyolefin, (low-linear density polyethylene, LLDPE); a biodegradable synthetic polyester, (polybutylene succinate, PBS) and a bioderived, compostable polyester, (poly (lactic acid), PLA). It was a relatively straightforward process to produce a compatible blend of LLDPE and NTP using a compatibilizer, regardless of the obvious difference in chemical structure between these polymers. Blending NTP with PBS, on the other hand, was much more challenging. It required two compatibilizers, added at different stages of blending to produce a compatible blend. For blends with PLA, a novel copolymer, itaconic anhydride grafted PLA (PLA-g-IA) was produced to be used as a compatibilizer and initial results suggested that PLA-g-IA may be a successful approach. Two different methods of compatibilization pathways were explored: the addition of a graft copolymer based on one of the components (in the case of NTP/LLDPE blends) or using compatibilizers that are not chemically the same as either component (in the case of NTP/PBS blends). Polyethylene grafted maleic anhydride (PE-g-MAH) was added to NTP/LLDPE blends and produced a blend with synergistic mechanical properties (the elongation at break exceeded the raw LLDPE properties). The water resistance of NTP was improved after blending with LLDPE, but it may compromise the compostability of the material. For NTP/PBS blends, two compatibilizers with different functional end-groups were used; these were (poly-2-ethyl-2-oxazoline)(PEOX) and (polymeric diphenyl diisocyanate)(pMDI). Using either of the compatibilizers individually in blends containing 50 % NTP resulted in poor mechanical properties. Compatibilization was accomplished with the addition of both two compatibilizers at different stages of blend preparation; PEOX dissolved in water and added during NTP production, while pMDI was added during injection moulding. This approach led to a tensile strength greater than that of pure PBS (24 MPa compared to 22 MPa). Dissolving PEOX in water improved the dispersion of the NTP phase throughout the PBS matrix via hydrogen bonding between water, PEOX and NTP. The addition of pMDI during injection moulding stabilized and strengthened the interactions between PBS and NTP, thereby leading to a superior blend. The compatibility between NTP and LLDPE or NTP and PBS were characterized using thermal and morphological properties as well as water resistance. Two Tgs were obtained for all blends, however, the addition of compatibilizers improved the adhesion between phases, evident from broader and lower height of Tg peaks. The morphology provided evidence of a homogenous dispersion of NTP in LLDPE. In NTP/PBS blends, the fracture mechanism changed from brittle to ductile with the addition of PEOX. Both NTP/LLDPE and NTP/PBS blends showed a phase inversion from a particle- dispersed morphology to a co-continuous morphology at compositions greater than 50% NTP. The water resistance also improved with the addition of LLDPE and PBS. Reactive extrusion was used to produce a copolymer, itaconic anhydride grafted poly (lactic acid) (PLA-g-IA). Different initiator (dicumyl peroxide) and monomer (IA) concentrations were used to optimize the degree of grafting. 0.75% was the highest degree of grafting and showed minimal chain scission evident from the polymer’s intrinsic viscosity. The reaction kinetics of grafting and the effect of grafting on thermal and mechanical properties were investigated. Grafting increased the crystallization rate of PLA, increased the crystallinity and also raised the thermal decomposition temperature. The mechanical properties of PLA-g-IA blended with PLA were also improved. Crystallization of PLA and PLA-g-IA were investigated during annealing at different annealing temperatures and durations using differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS). The rate of crystallization increased after grafting and affected the formation of PLA crystals by increasing the lattice spacing at the (110) plane, suggesting an expanded helical structure of PLA. The crystallinity of PLA-g-IA was also higher than that of neat PLA. Although polymer blending offers an attractive route to modify selected polymer properties, it is not always successful without the addition of compatibilizers. Despite the subtle differences in methods to incorporate compatibilizers, development of NTP blends with different polymers, whether a petroleum based polymer, synthetic biodegradable polyester or bioderived compostable polyester, offers material with improved mechanical properties and thermal stability. Water resistance is also improved, however, biodegradability will likely be compromised if the second polymer is not also biodegradable. These blends offer a potentially wider range of commercial NTP grades.