Rainforests Being Cleared to Grow Crops for Bioplastics

Bioplastics Some people believe that making plastic shopping bags biodegradable is one way to try to allow plastic pollution to degrade. However biodegradable bags pose many environmental problems. Problems which are similar to plastic bags themselves!

Bioplastics need to be kept away from the usual recycling stream to prevent contamination. They can take up to two years to degrade which means they still have a lot of time to potentially harm marine life.

Types of biodegradable polythene film

Polythene film cannot biodegrade naturally. There are two methods the industry has tried to resolve this problem. One is to modify the carbon chain of polyethylene to improve its degradability and at some point its biodegradability; the other is to make a film with similar properties to polyethylene from a biodegradable substance such as starch.

Starch based bags

These are made from corn, potatoes or wheat. This form of biodegradable film meets the the American Standard for Testing Materials and European norm EN13432 for compostability. It degrades at least 90% within 180 days or less under specified conditions. Heat, moisture and aeration in a compost pile is required for this type of biodegradable bag to biodegrade.

Pros & cons of starch bags

Pros

  • Litter is biodegradable and compostable. Biodegradable means that, under certain conditions, the material will be degraded into small pieces that can be absorbed by microorganisms and transformed into CO2, H2O, energy and neutral residue.
  • Reduced fossil fuel content (depending on loading of filler)
  • Faster degradation of litter
  • No net increase of carbon dioxide in global ecosystem

Cons

  • Source of starch is a problem, it competes with land for food use and rainforests are being cleared to grow crops for bioplastics
  • Poorer mechanical strength than additive based example – filling a starch bag with wet leaves and placing it curbside can result in the bottom falling out when a hauler picks it up. However, some biodegradable and compostable films are now very close to polyethylene or polypropylene, depending on the starch used.
  • Degradation in a sealed landfill takes at least 6 months which means it still causes harm to marine life.
  • Limited Shelf life. In order to be stable, some conditions must be respected while in stockage.
  • Some need to be composted in industrial facilities because the temperature of the compost needs to be at 58°C. Others ( OK-compost) are home composting (temperature 20°C).
  • Should not be mixed with traditional plastic bags for recycling. If they are, the value of recycling is reduced.
  • For plastic recycling, resin identification code 7 is applicable.

Additive based bags

These films are made by blending an additive to provide a UV oxidative and/or biological mechanism to degrade them. This typically takes 6 months to 2 years in a landfill site if adequate exposure to oxygen and heat over 140°F/60°C.

If put in a compost, the oxo degradables which remain after decomposition could be considered a form of pollution even though it may contribute to improved soil fertility.

Pros & cons of additive based bags

Pros

  • Much cheaper than starch-based plastics
  • Materials are well known
  • Does not compete against food use
  • These films look, act and perform just like their non-degradable counterparts, except that they are weaker and break down after being discarded.

Cons

  • Made using fossil fuel (contributes to global warming through the release of carbon dioxide)
  • Degradation in a sealed landfill is very slow and no conclusive testing shows biodegradation will occur in all cases.
  • Degradation depends on conditions of heat, light, stress, air etc.
  • No biodegradability established, which means it cannot be absorbed by micro-organisms. Pollution is not visible but exists.
  • They do not comply with European or American (D6400) Norms on compostable products
  • Proprietary additives often used to include Cobalt Stearate, which dissociates into toxic cobalt free metal during the degradation process recent testing showed that it is no longer used.

References

  1. ^ Law suit on Oxo-degradable bags

BBC News: "All Tesco bags 'to be degradable dt. 10th May'06" http://news.bbc.co.uk/1/hi/uk/4758419.stm

BBC News: "Degradable carrier bags launched dt. 2nd Sep'02" http://news.bbc.co.uk/1/hi/uk/2229698.stm

Manuel Martinez

Project GreenBag, 2200 Market St, San Francisco, CA, 94114, United States