Clearing the myths: Compostable, biodegradable, oxo-biodegradable and bio-based plastics - what are they?

Gordon Thompson, Technical Manager at the Renewable Energy Association, writes for BP&R on the definitions, differences and distinguishing factors in order to clarify an often-misunderstood area. 

Compostable, biodegradable, oxo-biodegradable and bio-based plastics are now in increasingly common use. Unfortunately in many cases the definitions are confused and misused. The subject of these plastics is probably one of the most highly defined and yet misunderstood subjects in the industry. In order to bring some clarity to this situation, it is necessary to understand what the various terms mean and how they relate to the management of waste plastic and packaging materials. 

In order for any plastic or packaging material to be accepted for composting on a site producing compost to the PAS 100 standard and Compost Quality Protocol, the plastic or packaging material must be independently certified as meeting the EN 14995 (plastic) standard, EN 13432 (packaging) standard (or another similar, accepted standard), or criteria for home compostability. Currently, DIN CERTCO (in Berlin) and Vinçotte (in Belgium) are the leading European organisations that assess and certify material as complying with these standards (in the UK, Renewable Energy Assurance Limited provides certification services in partnership with DIN CERTCO and Vinçotte). 

The certification schemes aligned to the standards can award two levels of recognition – registration and certification. Registration applies to non-finished items in three categories: raw materials (e.g. plastic granules), intermediates (e.g. plastic film), and additives (e.g. inks, dyes, pigments, adhesives and coating substances). Certification applies solely to finished products, such as bags and kitchen caddie liners.

In order gain recognition, non-finished items and finished products must pass a series of tests to demonstrate their compostability. These tests measure the biodegradability, disintegration, ecotoxicity and the levels of 11 chemical elements contained in the sample. Biodegradability is a measure of how much the sample is broken down by microbial action into CO₂, water and biomass. Disintegration quantifies the extent to which the sample falls apart into pieces greater and less than 2mm. Ecotoxicity investigates whether the residue from the disintegration test has any adverse effects on plant growth and finally, chemical analysis determines the concentrations of 11 elements which have the potential to cause harm the environment. As can be seen from the above, biodegradability is only one element of the tests and assessments carried out to determine whether a non-finished item or finished product is compostable. 

In addition to the standards referred to above, there are ‘OK compost HOME’ criteria defined and published by Vinçotte and this organisation operates a certification scheme geared to these criteria. Although similar to the EN 13432 standard, parameters for the biodegradability and disintegration tests are different, being designed to reflect the smaller mass and volume of home composting heaps and their less intensive management (compared with commercial scale composting).

It should be noted that in order for a finished product to be certified as (commercially) compostable or home compostable, all of the components of that product must themselves be compostable/home compostable. (Components are #)

Whilst any item that gains recognition under the certification schemes referred to above will be given a unique identification number, only finished products that have been certified as (commercially) compostable or home compostable are allowed to display the relevant certification mark.   

Oxo-biodegradable plastics are oil-based (polyolefin) plastics to which metal salts, filler and stabiliser substances are added. The metal salts play a key role in early stage break down of the polymer chains, reducing the discarded product from a single, large item to a marginally lower total mass of macroscopic fragments. Further degradation will occur over time, the rate being influenced by temperature, ultraviolet light, oxygen, moisture and microbes (species diversity and population size) in the degradation environment.  It should be noted that although oxo-biodegadable plastics are eligible for testing for compliance with standards for (commercially) compostable and home compostable criteria, they tend to take much too long to reach a highly biodegraded state, and certification bodies are not aware of any oxo-biodegradable plastic product that has complied with all of the criteria.  

Bio-based plastics are plastics, potentially manufactured from fossil derived materials that contain a bio-based constituent. This could potentially be a percentage of bio-polymers or bio-based filler within the plastic. Bio-based plastics are defined and certified according to their non-fossil carbon content, on a percentage basis. Non-fossil carbon content is quantified by measurement of two carbon isotopes: Carbon 12 and Carbon 14.  All living things absorb both isotopes throughout their lives and absorption ceases at death.  The amount of Carbon 12 contained within the body of an organism will remain unchanged after death but the amount of Carbon 14 will gradually decrease after death. By measuring the ratio between these isotopes it is possible, in most cases, to establish the time since death of the organism. The exceptions to this are where the living organism has received its carbon intake mainly from fossil, lentic or deep marine sources.

It is possible for there to be a bio-based constituent in almost any product, for example a car steering wheel could have a significant amount of bio-based content (e.g. 85 percent).  Whilst bio-based plastics can justifiably claim that they contain an identifiable content derived from renewable carbon sources, they are not necessarily compostable.  For example, crop-derived carbon can be made into polyethylene, which is not compostable. 

Renewable Energy Assurance Limited works in close cooperation with DIN CERTCO and Vinçotte, the certification bodies that manage schemes for the recognition of non-finished items and finished products as compostable and home compostable. REAL and DIN CERTCO are currently setting up a cooperation agreement to provide a service for assessing and certifying bio-based materials.