Biodegradable Plastic Film Developed from Soybeans

Biodegradable plastic film, developed by Ramani Narayan at Michigan State University with support from the United Soybean Board using soybean protein and oil, can be used in applications like lawn and leaf bags, carryout bags, trash bags and agricultural mulch film, representing a new market opportunity for soybeans as well as a significant advancement in environmentally friendly packaging.

Bioplastics can be composted along with biodegradable paper waste and other organic compostable materials to generate such needed products as carbon-rich compost (humic material). Bioplastics made of cornstarch-polyester blends are already being commercialized. Narayan believes soy proteins offer functionalities and property enhancement that can be even more beneficial than starch.

"Proteins have reactive nitrogen functional groups, like amino and carboxyl acids, which react with polyesters and compatibilize the protein-polyester blend," explains Narayan. Moreover, nitrogen is an important nutrient for plants and microbial activity. Soy protein polymers also have the ability to blend well with polyesters, enhancing biodegradability.

Conventional protein plastic degrades when heated in plastics processing equipment. Narayan has developed thermoplastic protein-polyester blends that are water-repellent. The new protein plastic melts and forms a viscous fluid that is injected into a mold and takes the shape of the mold before cooling and forming the part. The melted plastic can also be blown or cast into film.

"Soy proteins are not easy to work with," says Narayan. "If they are subjected to high temperatures, they break down and degrade. We are modifying the soy protein so that instead of degrading, it will melt and flow."

These types of films are normally weak and brittle, with the best elongation to breaks reported around 40%-70%. "Elongation means how far it will elongate. Typically 200%-300% is required for many film applications," says Narayan. "When you pull on the ends of a garbage bag, it stretches. But protein films don't stretch — they break."

Proteins were presumed unsuitable for heavy-duty trash and mulch film applications. Early soy-protein films did not perform well when compared with typical non-degradable, LDPE film used in trash bags.

But researchers from the Chemical Engineering Department at Michigan State have made great strides. "We have successfully extrusion-blended hydrophilic soy proteins with selected aliphatic polyesters at 30%-40% levels to produce fully biodegradable thermoplastic compositions that can be blown into film. These blends will elongate up to 500% with tensile strengths around 2,000 psi," says Narayan.

Extrusion blending is a process by which plastic pellets are made for the manufacture of plastic products. Narayan's team uses standard plastics operation machinery to mix the protein with a polyester that imparts the thermoplastic properties they are looking for in injection-molded products, like plastic forks and containers for seedlings.

Adapted from the United Soybean Board's newsletter 'Feedstocks'