Protein-Based Plastics and Composites as Smart Green Materials

摘要

Plant proteins are generally inexpensive, widely available and relatively easy to process. Animal proteins are more expensive, but sometimes have no functional substitutes. Protein-based materials are generally homogeneous, transparent, grease and water resistant and water insoluble. Solvent processes are tailored for fiber extrusion, production of coatings for seeds, drug pills and foods, for making cosmetic masks or varnishes and pharmaceutical capsules. Heat casting of protein-based materials by techniques usually applied for synthetic thermoplastic polymers (extrusion, injection, molding, etc.) is more cost-effective. This process is often applied for making fibers, resins and flexible films (e.g. films for agricultural applications, packaging films, and cardboard coatings) or objects (e.g. biodegradable materials) that are sometimes reinforced with fibers (composite bioplastics for construction, automobile parts, etc.). Material protein properties can generally be modified in a wide range of ways via raw material choices and combinations, the proper use of fractioning techniques and rheological modifying additives, and also by adjusting the product formation process variables. Proteins are thus especially sensitive to thermomechanical treatments, so carefully controlling these treatments will in turn facilitate control of the material forming process and especially the properties—to even obtain highly crosslinked materials that are very strong and relatively resistant to water (swelling rate of less than 30% after 24 h). The functional properties (especially optical, barrier and mechanical) of these protein-based materials are often specific and unique, and they could thus be used as raw materials for bioplastics with a wide range of agricultural, agrifood, pharmaceutical and medical industry applications. Their high moisture permeability is especially attractive for cheese, fruit and vegetable packaging, and for agricultural material and cosmetic applications. Protein-based materials have slightly lower mechanical properties than reference materials such as low-density polyethylene or plasticized PVC, but the addition of fibers (composite materials) can considerably improve them. The thermoplastic properties of proteins and their water resistance (for insoluble proteins) are especially interesting for natural resin uses to produce particleboard type materials. Gas barrier properties (O_2, CO_2 and ethylene) of protein-based materials can be utilized in designing selective or active materials for modified atmosphere packaging of fresh products such as fruit, vegetables, cheese, etc. Solute retention properties (especially antimicrobial and antioxidant agents) are attractive for designing controlled-release systems for useful agents in food (e.g. active coatings, encapsulation), agriculture (e.g. coated seed), pharmacy (drug delivery) and cosmetic industries. Since protein based materials have very unique properties in this concern, they can be used for the design of "smart" materials with properties (mechanical, barrier and degradation) controlled by environmental conditions (temperature and relative humidity).