A PDC Problem?
Biodegradable plastics look and feel exactly like the plastic products we're encouraged to recycle. So what happens if we accidentally recycle those biodegradable bags? Well, the consequences are potentially catastrophic-- recycled polyethylene irrigation pumps that are contaminated with PDC additives aren't likely to last very long. In fact, plastic recyclers in South Africa feel so strongly about the inability to keep PDC-containing biodegradables out of recycling streams that they want to ban their use in that country.
While some people are busy developing plastic substitutes, others are bent on making conventional thermoplastics biodegradable. How? By throwing in additives called prodegradant concentrates (PDCs). PDCs are usually metal compounds, such as cobalt stearate or manganese stearate. They promote oxidation processes that break the plastic down into brittle, low-molecular-weight fragments. Microorganisms gobble up the fragments as they disintegrate, turning them into carbon dioxide, water and biomass, which reportedly contains no harmful residues.
Search around for additive technologies and you'll come across the trade names TDPA (an acronym for Totally Degradable Plastic Additives) or MasterBatch Pellets (MBP). They're used to manufacture single-use plastics such as thin plastic shopping bags, disposable diapers, trash bags, landfill covers and food containers (including fast-food containers).
When added to polyethylene (the standard plastic bag material) at levels of 3 percent, PDCs can promote nearly complete degradation; 95 percent of the plastic is in bacteria-friendly fragments within four weeks [source: Nolan-ITU Pty]. While not strictly biodegradable ('bioerodable' is more like it), PDC-containing polymers are more environmentally friendly than their purer polymer cousins, which sit in landfills for hundreds of years.