Plastic decompose?

Since plastics belong to a chemical family of high polymers, they are essentially made up of a long chain of molecules containing repeated units of carbon atoms. Because of this inherent molecular stability (high molecular weight), plastics do not easily breakdown into simpler components.

North America, touched landscape 
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Plastics do decompose, though not fully, over a very long period of time (in average 100 to 500 years). Commercially available plastics (polyolefins like polyethylene, polypropylene, etc.) have been further made resistant to decomposition by means of additional stabilizers like antioxidants. Thus, unless the plastic is specially designed to decompose in the soil, such materials can last a very long time because the chemical bonds that hold the molecules together are often stronger than nature’s power to take them apart. This means that soil microorganisms that can easily attack and decompose things like wood and other formerly living materials cannot break the various kinds of strong bonds that are common to most plastics. This depends upon the plastic (polymer) and the environment to which it is exposed.

The Marine Conservancy has published that the estimated decomposition rates of most plastic debris found on coasts are:

  • Foamed plastic cups: 50 years
  • Plastic beverage holder: 400 years
  • Disposable diapers: 450 year
  • Plastic bottle: 450
  • Fishing line: 600 years.

Until Dr. Saido’s report, no studies had been conducted on plastic decomposition at low temperature in the marine environment, owing to the mistaken conception that plastic does practically not decompose in such condition. In the first study to look at what happens over the years to the billions of pounds of plastic waste drifting in the world’s oceans, researchers, lead by Katsuhiko Saido, PhD, reported that plastic does “decompose with surprising speed (as little as a year) and release potentially toxic substances into the water.”

These findings were reported on August 19, 2009, at the 238th National Meeting of the American Chemical Society (ACS). The scientists there termed the discovery “surprising.”

Dr. Saido described a new method to simulate the breakdown of plastic products at low temperatures (30º Celsius, 86º F), such as those found in some oceans. David Barnes, marine ecologist from the British Antarctic Survey, expressed that the Japanese’s team lab results cannot be applied uniformly across the ocean. However, even though the decomposition process would not occur in much cooler seawater as Barnes mentioned, the oceans are vast, currents are constant and permanent, nothing stays static and furthermore, it seems that garbage patches where plastics accumulate, are to be found in even greater dimension in the South Gyres, in the tropical and sub tropical zones with very warm waters. One of the researchers stated: “Even at 30 degrees Celsius, the plastic decomposes. In natural conditions, the tide comes in and sunlight heats the plastics [which increases decomposition].”

The type of plastic studied by Saido’s team was polystyrene, a white foamed plastic, commonly known by the trademark Styrofoam.

The process involved modeling plastic decomposition at room temperature, removing heat from the plastic and then using a liquid to extract the BPA and PS Oligomer that are not found naturally, thus must have been created through the decomposition of the plastic. Once degraded, the plastic was shown to release three new compounds not found in nature: styrene monomer (SM), styrene dimer (SD) and styrene trimer (ST). While SM is already a known carcinogen, SD and ST are suspected to be as well.

Plastics are not metabolized subsequent to ingestion since they are polymers. On the other hand, low molecular compounds such as PS oligomer or BPA from plastic decomposition are toxic and can be metabolized!

Samples of sea sand and seawater collected from Europe, India, Japan and the Pacific Ocean were found to be contaminated, with up to 150 parts per million of some of these components of plastic decomposition. “Plastics in daily use are generally assumed to be quite stable,” said study lead researcher Katsuhiko Saido, Ph.D. “We found that plastic in the ocean actually decomposes as it is exposed to the rain and sun and other environmental conditions, giving rise to yet another source of global contamination that will continue into the future.”

This latest study clearly shows new micro-pollution by compounds generated by plastic decomposition to be taking place out of sight in the ocean, leaching toxic chemicals such as Bisphenol A (BPA) and derivatives of polystyrene.

Even though present in seawater and sands, the pollutants are found in highest concentration in areas heavily littered with plastic debris, such as ocean vortices, which bring us to define more specifically the notion of gyres and “garbage patches”.

Information source: http://coastalcare.org