Protein materials thermoplastic process

The formation of a 3D network stabilised by new interactions or bonds, after removal of the intermolecular bond scission agent. Two different technological strategies can be used to make protein-based materials the wet process or solvent process involving a protein solution or dispersion, and the dry process or thermoplastic process using the thermoplastic properties of the proteins under low-hydration conditions (Figure 11.4). 

Thus, protein-based bioplastics can be obtained by extrusion, calendering, extrusion blow moulding, injection and thermoforming processes. These thermoplastic processes are derived from synthetic material production processes. 

The Tg of proteins is partly reversible depending on the density of covalent interactions (usually disulfide bonds) established as a result of heating treatments or variations in redox potential. Heat treatments associated with thermoplastic processing of filmforming materials facilitate formation of covalent bonds [152, 153]. For wheat gluten. 

Guilbert, S. (2002) Protein-based Bio-Plastics formulation, thermoplastic processing and main applications International Congress Trade Show The Industrial Applications of Bioplastics, 3rd, 4th and 5th February Gunatillake P.A. and Adhikari R. (2003) Biodegradable synthetic polymers for tissue engineering , European Cells and Materials, 5, 1-16. 

Extrusion is a cost effective manufacturing process. Extrusion is popularly used in large scale production of food, plastics and composite materials. Most widely used thermoplastics are processed by extrusion method. Many biopolymers and their composite materials with petroleum-based polymers can also be extruded. These include pectin/starch/poly(vinyl alcohol) (Fishman et al. 2004), poly(lactic acid)/sugar beet pulp (Liu et al. 2005c), and starch/poly(hydroxyl ester ether) (Otey et al. 1980), etc. In this study, composite films of pectin, soybean flour protein and an edible synthetic hydrocolloid, poly(ethylene oxide), were extruded using a twin-screw extruder, palletized and then processed into films by compression molding process or blown film extrusion. The films were analyzed for mechanical and structural properties, as well as antimicrobial activity. 

The thermoplasticity of material proteins has been utilised to produce materials using thermal or thermomechanical processes under low hydration conditions, as already employed for starch- or polyolefin-based materials [136, 152]. According to the thermoplastic behaviour of synthetic polymers, the Tg of the proteins involves sudden variations in their physical properties (thermal, mechanical, dielectric properties and so on). The molecular response associated with the transition from the glassy to the rubbery state involves an overall increase in the free volume and macromolecule mobility [153,154]. As for synthetic polymers, the Tg of the proteins is affected by the MW, chain rigidity, size and polarity of the lateral groups, presence of intermolecular bonds or crystalline zones, and also by the plasticiser type and concentration [155,156]. 

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