Turning sugar beet pulp into biodegradable plastic

More than one million tonnes of sugar beet pulp are generated annually by US beet sugar industries. Until recently, a profitable use had yet to be found for the biodegradable pulp, which is leftover residue from sugar extraction. Now, scientists from the USDA’s Agricultural Research Service (ARS) have developed the pulp into a biodegradable plastic for use in disposable food containers. The thermoplastic was developed by the ARS’s LinShu Liu and Arland Hotchkiss in collaboration with Professor Jinwen Zhang of Washington State University.

The bioplastic is manufactured from both sugar beet pulp and a biodegradable polymer called polylactic acid (PLA) using a twin screw extruder. PLA is a polymer derived from the sugars in corn, sugar beet, sugarcane, switchgrass and other plants. Extrusion is a cost-effective manufacturing process that is used in large-scale production of food, plastics and composite materials.

The scientists showed that up to 50% sugar beet pulp can be incorporated with PLA and the resulting thermoplastic composites retain mechanical properties similar to those of polystyrene and polypropylene - the compounds used to make white, spongy food packages. The thermoplastic is said to be cost-competitive with commonly used petrochemical plastics.

ARS-WSU researchers also developed other sugar beet pulp materials. Under extrusion compounding, sugar beet pulp was turned into a thermoplastic-like material with the assistance of water and/or glycerol. This material can be subsequently processed by extrusion or injection moulding to produce pure sugar beet pulp products.

The resulting thermoplastic possesses mechanical properties that are similar to those of low-density polyethylene - the materials used for opaque plastic containers, bags and film coverings. It can also be blended with PLA and other biodegradable polymers for enhanced water resistance. The composite could function as a light-weight-bearing material comprising up to 98% sugar beet pulp.

The continued development of the sugar beet pulp plastic - for example as yoghurt cups, cottage cheese tubs or other thin, opaque plastic containers - could benefit sugar beet growers and beet sugar processors.