to prepare a biodegradable plastic , biodegradable materials such as starches and a non - biodegradable polymer such as a polystyrene , polyurethane , polyethylene , polypropylene , or polycarbonate are treated : under heat , pressure and reagents to break the polymers ; and by adding to them an oxidizing agent . this treatment forms and / or makes available reactive groups for bonding : on the biodegradable material groups such as aldehyde or hydroxyl groups in the case of the carbohydrates and amine groups in the case of proteins and certain other compounds such as urea ; and on the non - biodegradable plastic groups such as aldehydes , methyl , propyl , ethyl , benzyl or hyroxyl groups . in one embodiment , plastic and starch are processed in an extruder by : mixing a starch in a range of between 15 percent and 80 percent , an oxidizing agent and an agent to break up the starch and the plastics ; and subjecting the combination to sufficient heat and / or pressure to break the plastic into shorter chains and bond monosaccharides to monomers from the non - biodegradable polymer .