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Decomposing Plastics

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Author: Randall Frost, Ph.D.

Most plastics are not readily biodegradable, especially when buried in the ground away from sources of oxygen. Many can be decomposed partially by photodecomposition in the presence of sunlight, however. This results in the plastic materials breaking up into tiny fragments.

Although standard polyethylene bags don’t biodegrade, the molecules in them become brittle and start to crack when the bags are exposed to ultraviolet light. But it could take hundreds to thousands of years for this to happen under normal sunlight conditions.

Oxo-biodegradable plastic bags are made from petroleum products, just like polyethylene bags. They initially fragment when exposed to air and sunlight. This typically takes three to four months. Later, when placed in a landfill, the fragments are decomposed by bacteria in the soil. It takes from 12 to 18 months for the fragments to be converted into carbon dioxide, water, and humus.

Problem:

How long does it take for a conventional polyethylene plastic bag and a biodegradable plastic bag to decompose?

Materials:

  • Oxo-biodegradable plastic bags, polyethylene plastic bags
  • Oxo-biodegrable plastic bags are available in quantities of 500 from Costco ($15) and are used by some retailers. Traditional high density polyethylene (HDPE) plastic bags are still used by most retail stores – look for the designation HDPE on the bag.
  • Materials are readily available.

Procedure:

  1. Start with six plastic bags: three oxo-biodegradable plastic bags and three polyethylene plastic bags.
  2. Store one of the oxo-biodegradable bags and one of the polyethylene bags away from sunlight and soil. These will be controls.
  3. Compare the physical toughness – the ability to experience deformation before breaking, of one of the oxo-biodegradable bags and one of the polyethylene bags.
  4. Based on the observations of the toughness of the two bags, formulate a hypothesis about the relative resistances of the bags to degradation in the environment.
  5. Subject the remaining oxo-biodegradable bag and the remaining polyethylene bag to full sunlight for two months. Regularly monitor the bags for signs of cracking or fragmentation. Record your observations.
  6. Place the sun-exposed oxo-biodegradable bag (along with any fragments) and the polyethylene bag in a bed of soil for one month. At the end of this period monitor the two bags for signs of biodegradation (conversion to carbon dioxide, water, and humus).
  7. Compare the physical conditions of the degraded bags with those of the control samples.
  8. Based on the percent decompositions observed, extrapolate your findings to come up with predictions for the time it would take for the two types of plastic bags to completely decompose.
  9. Compare your extrapolations with your initial hypothesis. Revise it if necessary and propose additional experiments to test it.
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