Are all Bioplastics Compostable? - Complexities and Realities

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Bioplastics, which not long ago were a niche offering, are now being used as material for a wide range of products and touted as the eco-friendly way to go. However, the term ‘bioplastics’ is a broad and often confusing to the consumer as their properties vary depending on the feedstock used and the way the plastics are put together.

The feedstock of bio-based polymers is derived from a readily renewable source – such as corn or starch – instead of fossil-based sources of traditional plastics. Biodegradable, meanwhile, refers to materials that are compostable. But one quality doesn’t always precede the other. You can have oil-based plastics that are compostable and plant-based plastics that are not compostable.

Bioplastics are not always truly compostable.  

  • Bioplastics, if they meet the international standards that define compostable materials, can be included in composting.
  • Compostability and degradability are not based on the feedstock. It’s literally based on the chemical signature, that is the way the plastics are put together.
  • Different compost facilities use different processes. Some have trouble even with bioplastics.
  • In some facilities, the plastic-coated packaging get separated and no visible plastic is left in the compost, while others are concerned that the nearly invisible tiny fragments of plastic will ultimately ends up contaminating the environment .
  • Screening out the compostable items means extra steps and costs.
  • A product is either 100% compostable or it is not as most composting facilities cannot separate the biomass from the plastic effectively.
  • If compostable plastics do not end up in the composting facility, then there is nothing green about them. In fact they end up in landfills and produce methane emissions.
  • To confuse matters further, many compostable plastics are made to look like real plastics and end up in the recycling bin where they cause problems for the plastic recycling process.
  • Most coffee cups and to-go food containers may look/claim to be recyclable and compostable. But they are lined with very thin plastic to hold food and drink. This plastic coating breaks down into tiny plastic fragments which do not disappear. Consequently, they contaminate the finished compost as well as the soil where that compost is used.

Standard for “compostable plastics”.

ASTM D6400 and D6868 are the basis for certifying that products are ‘compostable’ in professionally managed facilities,” according to ASTM Subcommittee D20.96-Environmentally Degradable Plastics. The Biodegradable Products Institute (BPI) has, since D6400 was first published in 1999, applied D20.96 standards to certify items such as plastic cups, cutlery and bags; plastic-coated paper items; and even food service items based on bagasse, or sugar cane. (Source: http://www.bpiworld.org/BPI-Public/)

According to ASTM D6400 and D6868 (and the European equivalent EN13432), compostable plastics must meet the following three criteria:

  • Biodegradability– Determined by measuring the amount of CO2 produced over a certain time period by the biodegrading plastic. The standards require 60% (90% for EN13432) conversion of carbon into carbon dioxide within 180 days for resins made from single polymer and 90% conversion of carbon into carbon dioxide for co-polymers or polymer mixes.
  • Disintegration– Measured by sieving the material to determine the biodegraded size and that less than 10% remains on a 2mm screen within 120 days.
  • Eco-Toxicity– Measured by testing the concentrations of heavy metals to ensure that they are below the limits set by the standards and by testing plant growth by mixing the compost with soil in different concentrations and comparing plant growth in test and controlled compost samples. There must be no difference from the control compost.