Improving Impact Properties of Novatein through Polymer Blending and Reactive Extrusion
Novatein is a brittle thermoplastic made from protein and requires impact strength modification before it can be used in a wider variety of applications, such as in the agricultural, horticultural and meat processing industries. Polymer blending is often used for improving material properties cost effectively. This study aimed to improve the energy absorbing properties of Novatein through manipulating the morphology of blends and assessing the factors that are important during this process; these were, the choice of polymer, composition, viscosity ratio, interfacial tension and chemical interactions. Novatein was blended with either elastomeric core-shell particles, polyethylene (PE), or biodegradable polybutylene adipate-co-terephthalate (PBAT) using twin-screw extrusion to allow for sufficient mixing and residence time for chemical interactions or reactions to form. Materials were characterized for mechanical (tensile and impact testing), thermal (dynamic mechanical analysis and differential scanning calorimetry) and morphological (microscopy and solvent extraction) properties. The greatest increase in energy absorption occurred in blends containing core-shell particles, but only above a critical particle concentration. This was attributed to good particle dispersion, efficient stress transfer to the particles and a decrease in interparticle distance, allowing the matrix to elongate freely during fracture, forming fibrillar structures. When the particle content was high, the protein secondary structure became more ordered in these fibrils during fracture. It was suggested that the change in protein conformation contributed to the increase in energy absorption, along with matrix yielding. In contrast, in-situ formation of morphologies required for impact strength modification through reactive extrusion of Novatein and modified polyethylene proved difficult due to the viscosity ratio and interfacial tension. In most Novatein/PE blends these factors were not favorable for PE to form stable, dispersed droplets. However, blends containing ethylene-based zinc ionomer performed well in mechanical testing, with high elongation and greatly increased impact strength attributed to strong ionic interactions at the interface. However, no changes in protein secondary structure were detected, suggesting that improvements to impact properties were as a result of morphological changes in these blends. To preserve the biodegradability of Novatein, PBAT was used instead of PE to achieve the same goal. The compatibilization of Novatein/PBAT blends was explored using dual compatibilizer systems whereby it was found that a compatibilizer system of epoxy functionalized chain extender (Joncryl ADR4368) and imidazole catalyst brought about increased energy absorption compared to poly-2-ethyl-2-oxazoline (PEOX) with polymeric diphenyl methane diisocyanate (pMDI). Compatibilization dramatically reduced PBAT’s phase size and led to a well distributed PBAT dispersion, required for improved impact properties. However, the dispersed phase coalesced at very low PBAT concentration (~ 5 wt. % PBAT) due to inappropriate viscosity ratio and interfacial tension. The onset of coalescence can typically be modified by manipulating these factors. The viscosity ratio and interfacial tension were altered by increasing the water content in Novatein, which is included as a processing aid, leading to a reduction in blend viscosity ratio and an increase in interfacial tension. As water was removed after injection moulding, water was not considered a plasticizer at this stage. Coalescence of PBAT was reduced when the viscosity ratio decreased and interfacial tension increased in uncompatibilized blends, with a further reduction in domain size upon compatibilization with Joncryl/imidazole. Impact strength was higher in compatibilized blends attributed to this fine domain size and improved interfacial adhesion. Increasing PBAT content to 30 wt. % showed higher impact strength, however a co-continuous morphology formed at this composition, demonstrating that composition can override the effect of viscosity ratio and interfacial tension. Novatein thermoplastic protein can be impact strength modified through polymer blending or the incorporation of elastomeric particles, however this process is not straightforward. The unique nature of Novatein, whereby the viscosity is exceptionally high during processing and the lack of a traditional melt causes viscosity to be insensitive to temperature, make blending particularly difficult. Interfacial interaction and good particle dispersion are crucial when incorporating pre-synthesized impact modifiers into Novatein, whilst careful consideration is needed of factors such as viscosity ratio, interfacial tension and chemical interactions when blending Novatein with reactive polymers. The morphology formed during blending, along with composition and interfacial interaction, heavily influenced energy absorption. The theories presented for impact strength modification in conventional polymers can be applied here, yet additional considerations need to be made when aiming to modify impact strength in unconventional polymers such as Novatein.
Biopolymer, Novatein, Impact strength, Polymer Blending, Reactive Extrusion
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