Plastics are important materials which contribute significantly to environmental protection. When compared to alternatives in typical applications they can:
- reduce energy costs by up to 40%
- reduce waste by 75 - 80%
- reduce emissions by 70%
- reduce water pollution by up to 90%
However, due to recent concerns about fossil resources depletion, efforts have been made to replace conventional oil and gas-based plastics with others based on hydrocarbons derived from renewable resources such as biomass.
Besides crude oil, natural gas and coal, plastics can be derived from natural and renewable sources such as wood (cellulose), vegetable oils, sugar and starch and can be defined as 'bio-based' plastics but are often termed 'biopolymers' or 'bioplastics'.
The terms 'biopolymer' and 'bioplastic' are currently widely used but with some confusion. They are often used collectively to describe two different concepts at the same time. The two concepts can be differentiated through;
- materials source, i.e. renewable resource based
- functionality, e.g. biodegradable and/or compostable
The dual use of this term is a cause of concern as it can be the source of misleading information and confusion for the consumer. It is important, particularly with regards to the end-of-life stage, to differentiate between biodegradable plastics and durable renewable or biomass derived plastics.
To avoid ambiguity the BPF is keen to avoid the use of the term 'biopolymer' or 'bioplastic' except where necessary. Ideally they should be substituted by more accurate and informative equivalents.
Currently, most industrial polymers and plastics are produced from non-renewable, oil or gas-based resources. However, due to recent concerns about fossil resources depletion, efforts have been made to replace conventional oil and gas-based plastics with others based on hydrocarbons derived from renewable resources such as biomass.
Natural bio-based polymers
These polymers are synthesised by living organisms, essentially in the form in which they are finally used. Examples of naturally produced bio-based polymers include;
- cellulose / starch
- bacterial polyhydroxyalkanoates
After extraction and purification, direct industrial exploitation is possible.
Synthetic bio-based polymers
Polymers whose monomers derive from renewable resources but which require a chemical transformation for conversion to a polymer.
Many conventional polymers can, in principle, be synthesised from renewable feedstock. For example, corn starch can be hydrolysed and used as the fermentation feedstock for bio-conversion into lactic acid from which poly(lactic acid), PLA, can be produced through chemical processing. Although it's orgin is renewable the polymer cannot be consider 'natural' as it is synthesised within a chemical plant.
Degradability, Biodegradability & Compostability
A plastic can be described as degradable when it undergoes a significant change in initial properties due to chemical cleavage of the macromolecules forming a polymeric item regardless of the mechanism of chain cleavage i.e. there is no requirement for the plastics to degrade due to the action of naturally occurring micro-organisms. Examples of degradable plastics include, oxo-degradables and UV-degradables which break down when exposed to oxygen or light and are primarily oil-based.
Biodegradability can be described as "the degradation of a polymeric item due, at least in part, to cell-mediated phenomena. As a result of the action of micro-organisms the material is ultimately converted to water, carbon dioxide, biomass and possibly methane."
The ability of a polymer to biodegrade is independent of the origin of its raw material. Instead it strongly depends upon the structure of the polymer. For example, whilst some bio-based plastics may be biodegradable ( e.g. polyhydroxyalkanoates) others are not (e.g. polyethylene derived from sugar cane).
Source; Frost & Sullivan, 1998
Some polymers degrade in only a few weeks, while others take several months.
In comparison with conventional commodity polymers, biodegradable polymers are niche market materials finding focused applications within a diverse range of market sectors, including;
- Medical Devices: orthopaedic, dental, drug release and tissue engineering
- Agriculture: mulch films, flowerpots and encapsulation of fertilisers for controlled release
- Packaging: carrier bags, waste bags and food wrapping and containers
For a plastic to be considered compostable it must meet the following criteria:
- Biodegrade; break down into carbon dioxide, water and biomass. 90% of the organic materials is converted into CO2 within 6 months.
- Disintegrate; After 3 months' composting and subsequent sifting through a 2mm sieve, no more than 10% residue may remain
- Eco-toxicity: the biodegradation does not produce any toxic material and the compost can support plant growth.
A plastic therefore may be degradable but not biodegradable or it may be biodegradable but not compostable (i.e. it breaks down too slowly or leaves toxic residues).
Standard Test Methods
Biodegradability and Compostability as material properties are regulated by a number of international standards including:
- EN 13432
- EN 14995
- ASTM D6400
To comply with these standards a product must posess the ability to undergo a complete biological decomposition due solely to the action of naturally occurring micro-organisms.