Degradable bags or compostable bags? What ARE they?

With legislation regarding compostable bags appearing in South Australia, Northern Territory and the ACT, it has become important to understand the meaning of the different terms which exist for the various materials available. The terms “biodegradable bags”, “degradable bags”and “compostable bags” are often misused. To add to the confusion, other terms such as hydro-degradable and oxo-degradable have also been introduced to the vernacular. No wonder people are left bewildered! It feels like we all need to get a polymer science degree just to understand the terms, and this leads to a great deal of confusion in the market place.

What does it all mean?

ASTM D883-99 defines “Degradable Plastic” as “a plastic designed to undergo a significant change in its chemical structure under certain conditions resulting in a loss of some properties that may vary as measured by standard test methods appropriate to the plastic and the application in a period of time that determines its classification.”

Degradable plastics, also known as “oxo-degradable” plastics describe when traditional polyethylene is treated with pro – degradent additives at the point of extrusion. This allows the finished product to undergo a two stage process after being triggered by sufficient exposure to a combination of UV light, heat and mechanical stress. The process firstly causes chemical reactions within the film to become extremely brittle – losing substantial tensile strength, elasticity and molecular weight. The plastic film ends up resembling minute “dust” particles. Secondly, these smaller molecules are ingested by micro organisms – causing the film to be biodegraded into carbon dioxide, water and bio mass.

Compostable plastics, also known as “hydro – degradable” plastics are developed from a different raw material – typically a fusion of traditional polymers and starch based components. The “hydro – degradation” process is quite similar to the “oxo – degradation” process. Both undergo initial chemical reactions which cause deterioration in the molecular weight of the film, followed by biodegradation by micro organisms. However the trigger for the process for Compostable plastics is being in the presence of high bacterial / microbial environment. The end result is identical: carbon dioxide, water and biomass.

With the processes sounding similar, and the end results being the same – are there any other differences? Well, yes …

  • Compostable bags, if they enter the right a high microbial environment, will complete the degradation process faster than Degradable bags.
  • Compostable bags must be collected and put into a commercial composting facilities with high levels of bacteria and microbes in order to degrade. Degradable bags can degrade in a variety of environments – so long as they are exposed to UV light, heat or mechanical stress.
  • Compostable bags emit methane in the anaerobic conditions of a landfill- where Degradable bags are safely inert.

“Biodegradable Plastics” – what are they? According to ASTM D883-99 a “Biodegradable Plastic” is “a degradable plastic in which the degradation results from the action of naturally-occurring micro-organisms such as bacteria, fungi, and algae.” With such a definition, it can be inferred that both “Degradable” and “Compostable” plastic bags are ultimately “Biodegradable” at their end point.