Increasing interest in competitive, sustainable, and biodegradable alternatives to petroleum‐based plastics has encouraged the developmentof protein‐based plastics. The formation of a homogeneous protein melt during extrusion occurs through: denaturation, dissociation, unraveling, and alignment of polymer chains. The presence of covalent cross‐links is unfavorable, decreasing chain mobility, increasing viscosity and preventing homogenization. Proteins have high softening temperatures, often above their decomposition temperatures. To avoid degradation, the required chain mobility is achieved by plasticizers. By understanding a protein's physiochemical nature, additives can be selected that lead to a bioplastic with good processability. The final structural and functional properties are highly dependent on the protein and processing conditions.

No abstract

No abstract

Protein‐based thermoplastics resemble semi‐crystalline polymers, suggesting the occurrence of a glass transition (Tg) and melting point. Denaturing a protein's native structure is often called melting, but this does not necessarily imply complete unfolding into a fully amorphous structure as true melting would. Protein secondary structures, such as α‐helices and β‐sheets, can remain after denaturing, stay intact above the Tg and do not necessarily melt at typical processing temperatures. This implies that consolidation of aggregated protein particles into a macroscopically monolithic material depends on inter‐chain interactions in the amorphous phase and on newly formed secondary structures. Structural relaxations and transition temperatures of the amorphous phase are influenced and constrained by the presence of these secondary structures as well as heavily influenced by plasticizers.

Blood meal is a high protein, low value by‐product of the meat processing industry that can be converted into a thermoplastic material by extrusion with a combination of a surfactant, urea, a reducing agent, water, and plasticiser. Changes in protein structure after each processing step (mixing with additives, extrusion, injection molding, and conditioning) were explored using synchrotron FTIR microspectroscopy. Blood meal particles were found to have higher β‐sheet content around the perimeter with a randomly structured core. α‐Helices were either located near the core or were evenly distributed throughout the particle. Structural rearrangement consistent with consolidation into a thermoplastic was seen after extrusion with processing additives, resulting in reduced α‐helices and increased β‐sheets. Including triethylene glycol as a plasticiser reduced α‐helices and β‐sheets in all processing steps. At all processing stages, regions with increased β‐sheets could be identified suggesting blood meal‐based thermoplastics should be considered as a semicrystalline polymer where clusters of crystalline regions are distributed throughout the disordered material. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013