Plastics are a diverse technological wonder. And as with all technological advances, polymers need some sort of regulation to guide their usage and disposal. The ISO (International Organization for Standardization) started by defining six types of degradable plastics.
The first four types are degradable, photodegradable, oxidatively degradable and hydrolytically degradable. Degradable plastics are simply plastics that degrade in some measurable way. Photodegradable plastics are broken down by light. Oxidatively degradables are degraded by oxidation; rust is a type of oxidation, and the same kind of process can happen to polymers. Oxo-degradable plastics have an additive that speeds this "rusting" process. And hydrolytically degradable plastics are broken down by the interaction of the polymer and water.
For example, a plastic bag that degrades due to sunlight or oxygen exposure might fall apart into tiny, microscopic pieces, which aren't necessarily benign. That leftover particulate matter could be absorbed by small creatures and work its way up through the food chain, affecting the body chemistry of each organism along the way – with unknown consequences.
The final two types of ISO-defined degradable plastics are biodegradable and compostable. A biodegradable plastic is simply one that our aforementioned microbe friends can dismantle to water and carbon dioxide, but on a timeline that's not necessarily well-defined. Compostable plastics degrade at a rate that's similar to other types of compostable materials, and they result, again, in water, carbon dioxide, humus, and inorganic compounds.
One big difference between compostable and biodegradable plastics is that the former require the high heat of a professionally-managed compost pile or landfill in order to rot. This distinction is vital, because 10 to 15 billion pounds, or 75 percent, of all plastics wind up in landfills [Source: PEC]. Truly biodegradable plastics will break down best in a landfill, but they'll also degrade in a roadside ditch.
It's not such a bad thing if biodegradables wind up in a landfill instead of a recycling center. In a landfill, the methane they release can be captured and combusted for our energy needs. Of the 1,200 operating landfills in the United States, about half capture methane. In 2008, those landfill gas energy (LFG) projects generated around 12 billion kilowatt hours of electricity in just one year.
Now, let's get the down-low on the dirty work of plastics additives.