Plasticizers protein-based materials

Protein-based polymers have the potential to surpass the polyesters and other polymers because they can be directly produced in microorganisms and plants by recombinant DNA technology resulting in the capacity for diverse and precisely controlled composition and sequence. This is not possible with any other polymer, and it increases range of properties and the numbers of applications. Remarkably, with the proper design of composition, protein-based materials can be thermoplastics, melting at temperatures as much as 100°C below their decomposition temperatures. Therefore, they can be molded, extruded, or drawn into shapes as desired. Aspects of protein-based materials as plastics is also considered below. 

Certain compositions of plastic protein-based polymers can be prepared that melt rather than decompose, as protein materials normally do... 

With a worldwide production estimated at about 33 million metric tormes, cottonseed is the most important source of plant proteins after soybeans [68]. The viscoelastic behaviom of cottonseed protein isolate, plasticized with glycerol, was characterized in order to determine the temperature range within which cottonseed protein-based materials can be formed by extrusion or thermo-monlding [68]. The results indicated that cottonseed proteins are thermoplastics with a Tg ranging from 80 to 200°C when the glycerol content varies from 0% to 40% (w/w, dry basis). 

A downside to the sustainable and extensive use of soy protein-based materials is their intrinsic reactivity and thus lower inertia when compared to most conventional petrochemical-based plastics. They are known to be sensitive to microbial spoilage and also to water due to hydrophilic nature of many amino acids constituting their primary structure and to the substantial amount of hydrophilic plasticizer required to impart thermo-processability and film flexibility. As a consequence, their mechanical properties and water vapor barrier properties in high moisture conditions are poor compared to synthetic films such as low-density polyethylene. 

Plasticizers are generally required for the formation of protein-based materials (] 1,14-18). These agents modify the raw material formation conditions and the functional properties of these protein-based materials (i.e. a decrease in resistance, rigidity and barrier properties and an increase in flexibility and maximal elongation of the materials). Polyols (e.g. glycerol and sorbitol), amines (e.g. tri-ethanolamine) and organic acids (e.g. lactic acid) are the most common plasticizers for such applications. Completely or partially water insoluble amphipolar plasticizers such as short-chain fatty acids (e.g. octanoic acid) can be used since some protein chain domains are markedly apolar. 

It is essential to determine the phase equilibrium patterns (Figure 1) of protein-based materials according to the moisture (or plasticizer) contents in order to be able to control the material formation conditions and predict variations in the properties of the end products under different usage conditions (temperature and relative humidity) 6,10,19JO),... 

The mechanical properties of protein-based materials closely depend on the plasticizer content, temperature and ambient relative humidity (16,34,35). At constant temperature and composition, an increase in relative humidity leads to a major change in the material properties, with a sharp drop in mechanical strength and a concomitant sharp rise in distortion. These modifications occur when the Tg of the material is surpassed (Figure 1). These variations can be reduced by implementing crosslinking treatments (physical or chemical) or using high cellulose or mineral loads (22). 

The gas barrier properties (O2, CO2 and ethylene) of protein-based materials are very attractive since they are minimal under low relative humidity conditions (38). O2 permeability (around 1 amol.m . s .Pa ) is comparable to the EVOH properties (0.2 amol.m s Pa ) and much lower than the properties of low-density polyethylene (1000 amol.m" s Pa Table IV). The O2 permeability of protein films is about 10-fold higher that of EVOH-based films, mainly due to the high plasticizer content. 

Several studies reviewed formulations, barrier properties and possible application of edible protein-based films (Table 23.3) (Gennadios et al. 1994 Krochta and Me Hugh 1997 Torres 1994). Overall, similarly to polysaccharide films, proteins exhibit relatively low moisture barrier properties, two to four times lower than conventional polymeric packaging materials (McHugh and Krochta 1994d). The limited resistance of protein films to water vapour transmission is attributed to their substantial hydro-philicity and to the amounts of plasticizers, such as glycerol and sorbitol, incorpo-... 

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. 

Despite the early enthusiasm and fanfare, soy plastics have not measured up to alternative materials. As potential began to grow for protein-based plastics after World War II, inexpensive and adequate supplies of petroleum and better-performing synthetic materials were realized. Widespread availability of synthetic polymers discouraged attempts to make improvements. For a short period (1935—1943), H. Ford used soy plastics in his popular, inexpensive automobiles, but his plan to use over 1.4 million kg (4 million lb) of soybean meal for every million cars produced never materialized. 

Professor, Department of Chemical Engineering and Materials Science, Biological Process Technology Institute, University of Minnesota, St. Paul Elastic, plastic, and hydrogel-forming protein-based pol)nners... 

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