The use of additives that control the degradation behavior of conventional thermoplastics and inherently biodegradable plastics is becoming popular due to price competition. These additives, known as prodegradant concentrates, are typically based on catalytic transition metal compounds such as cobalt stearate or manganese stearate and are used at levels of 1-3%, resulting in additional costs of 10-35% over polyethylene.
Plastic products manufactured with prodegradant additives such as TDPA™ or degradable and compostable polymer (DCP) additives gradually degrade to lower molecular weights, becoming brittle and disintegrating. They are ultimately digested by microorganisms back to basic elements with no harmful residues. These additives have been shown not to affect bacteria, fungi, or earthworms, and can control the degradation rates of plastics from a few weeks to several years at a competitive cost.
Prodegradant-containing films represent a significant development over earlier biodegradable films based on starch-filled polyethylene. The addition of a masterbatch, such as AMCOR Flexibles (a metal chelate), initiates chain scission processes that cause the polymer chain to be reduced to 20% of the original molecular weight, at which point bacterial degradation takes over. Polyethylene containing 3% of the additive is claimed to degrade to a 95% weight loss after four weeks at 60°C.
These environmentally degradable plastic compositions are not strictly biodegradable but rather “bioerodable.”
DCP-containing polymers do not biodegrade initially. Instead, they undergo chemical oxidation, resulting in lower molecular weights, making them brittle and fragmented. These fragments are then gradually ingested by microorganisms, leading to the production of carbon dioxide, water, and biomass. The additives facilitate an oxidation process that does not require light or heat and can function at ambient temperatures. Once the plastic’s molecular weight has decreased to a sufficient extent, it is converted into wax, allowing it to biodegrade naturally.