Sheet extrusion and plasticisation of Novatein thermoplastic protein

Abstract

The purpose of this study was to develop a relationship between protein structure, rheological properties and plasticization of Novatein. Novatein is a biomass-based thermoplastic in which the main constituent is blood meal, a highly aggregated protein-rich biopolymer, which is a by-product from the meat industry. It can be processed with industrially scalable thermo-mechanical processing methods such as extrusion and injection molding. However, its properties makes more challenging processing methods such as sheet extrusion limited. This is considered a rheology problem and is related to the protein’s structural characteristics that can be modified by using polyol plasticizers such as ethylene glycol (EG), glycerol (GLY), propylene glycol (PG) and triethylene glycol (TEG).

Rheological characterisation revealed the apparent shear viscosity of highly plasticized Novatein and polypropylene (PP) to be very similar even though only PP can be sheet extruded. Novatein’s extensional viscosity was significantly higher and its entrance pressure accounted for up to 80% of the total pressure drop with no-polyol plasticized Novatein. Increasing polyol content and temperature were found to decrease the extensional viscosity but simultaneously increased the shear viscosity due to better flow development in the capillary. In other words, poor elongational properties of the no-polyol composition led to flow behavior closer to plug flow. Thus, with longer capillaries apparent shear viscosity of polyol plasticized samples (at the same water content) could become even higher but also raises the question whether fully developed flow is a desired property for processing Novatein.

Synchrotron-based FT-IR measurements with support of XRD explained the difference in the rheological performance further. Blood meal’s protein structure was highly aggregated, consisting of up to 50% -sheets that do not melt into a fully amorphous state at suitable processing temperatures. The behavior of Novatein was considered to be closer to filled polymers, consisting of nano-crystallite aggregates, rather than a semi-crystalline polymer that has reached a molten state. This made sufficient plasticization of the amorphous fraction crucial for processability.

Plasticization was classified into primary and secondary plasticization. In primary plasticization, the plasticizer interacts directly with the protein network by replacing the protein’s hydrogen bonding sites with water. In secondary plasticization, the polymer network becomes saturated, leading to phase separation and increases hygroscopicity. The effect is substrate and plasticizer dependant and secondary plasticization was dominant with Novatein due to its aggregated structure. The plasticizer content at which the equilibrated moisture content (EMC) became equal to that of no-plasticizer compositions was called the point of equivalence (POE). This is also the point at which primary plasticization turns into secondary plasticization.

The POE is unique for all plasticizers and dependant on molecular characteristics such as size and theoretical hydrogen bonding sites. The EMC is a result of these changes (including secondary structure) and strongly correlates with mechanical properties and the brittle-to-ductile transformation. Water provided the ability to form ideally mixed phases, explaining the applicability of the free volume-based plasticizer theory. The Novatein network consisted of protein-rich, plasticizer-rich and an intermediate phase, and the fractional composition and the relative magnitude of each phase was determined by using the Couchman-Karasz model. The role of the intermediate phase was found to make the biggest difference in plasticizer performance and behaved in accordance to the observed secondary structure changes.

Of the selected plasticizers, GLY clearly showed the highest tendency to phase separate followed by TEG, PG, and EG. Of the tested plasticizers the POE values varied between 20 pphBM to 29 pphBM in the order of GLY, TEG, PG, and EG. However, TEG had the lowest POE on a molar basis and nearly three times the molar amount of EG was required to reach the POE in comparison to TEG, whereas for GLY it was only 1.3 times. TEG’s plasticization is based on its ability to interact efficiently with the protein network and modify the secondary structure sufficiently. GLY, instead, showed a strong tendency to phase separate leading to the highest hydrogen bonding potential above the POE, and therefore also raised the EMC above TEG level. Despite the increase in ordered secondary structures, GLY led to the highest strain at break, which was attributed to phase separation. EG and PG as smaller sized molecules were able to diffuse into polymer network more efficiently.

Even though the plasticization mechanism varied significantly, EMC was the dominant factor determining mechanical properties with a brittle-to-ductile transformation observed at 8%. However, it is important to understand that an EMC of 8% required different amounts of plasticizer for each polyol. In accordance to the constraint theory, the amount of theoretical hydrogen bonding sites of plasticizer was most appropriate to predict changes EMC. The role of water was significant in forming a ternary system that behaves in accordance to the free volume theory despite the protein’s heterogeneous structure. For dried samples, TEG and GLY formed clusters in the polymer network in the absence of water, whereas for hydrated samples the plasticizer was well distributed through the polymer network, albeit in micro-separated regions.

The concept of primary and secondary plasticization provided a better understanding of rheological behavior as well. Elongational flow was dominated by primary plasticization of the protein-rich and intermediate phases whereas secondary plasticization played a significant role in the reduction of the shear viscosity. Flow without secondary plasticization was characterised as a plug flow. PG showed the most efficient plasticization in both shear and elongational viscosity, which was attributed to the combination of its small molecular size and ability to enhance both primary and secondary plasticization. GLY acted mostly as a secondary plasticizer and had the higher elongational and the lowest shear viscosity. TEG as an efficient primary plasticizer with an ability to modify secondary structure performed exceptionally well in terms of extensional viscosity considering its high molecular weight. Higher shear viscosity levels were comparable to EG that was shown to diffuse very efficiently in the polymer network.

With a fundamental understanding of plasticization and rheology, significant process improvements were made by increasing temperature and combining PG and TEG leading to efficient sheet extrusion.