What is oxo-biodegradable plastic?

Hengky Wibawa, Argha Karya Indonesia

Unlike "normal" plastics, that degrade and disintegrate very slowly, oxo-biodegradable plastics are manufactured to accelerate the destruction of the plastic product, preferably extending the destruction to the extent where the plastic is mineralized into its basic component elements.
There are many different types of oxo-biodegradable plastics being introduced into the Australian market at present, resulting in confusion about their impacts and benefits.
When considering packaging and bags, the most important distinction needs to be made between hydro-biodegradable plastics and oxo-biodegradable plastics.
Oxo-biodegradable plastics (also referred to as oxo-degradable plastics) are normal petrochemical based polymers which have a small amount of prodegradant added during the production of the plastic. The prodegradants act as a catalyst to speed up the degradation process of the plastic and is normally a transition metal. Some of these metals are considered trace elements and are essential for life, such as cobalt which is essential for the formation of vitamin B 12.
These bags retain all of the advantages of normal plastic bags but provide effective solutions to the compost and landfill industries with minimal disruption of consumer behavioural patterns.
The degradation process involves the oxidation of the long chain polymer molecules at random points along the molecule. At each oxidation point, the molecule will break into two shorter molecular lengths, releasing the catalyst to attack another part of the plastic and attaching oxygen molecules at the break.
This process of shortening the polymer molecules commences from the time of production of the plastic and continues throughout the life of the plastic. As the molecular length of the polymer is reduced, so the plastic becomes less flexible and ultimately becomes brittle and easily disintegrates into small fragile pieces.
With continued oxidation and reduction in polymer length the molecular weight of the plastic is reduced to below 40,000, at which point the molecules can be attacked and processed by the enzymic activities of fungi and bacteria. This results in the complete biodegradation of the plastic.
The rate at which this entire process is completed varies with the design of the plastic i.e. which type of polymer is used, the addition rate of the prodegradants and the type of active ingredient in the prodegradants.
A further important variable is that should external energy be applied to the plastic, for example, if plastic is littered and thereby receives direct ultra violet light, the disintegration process is hastened, causing the film to break down within weeks. Thereafter biodegradation occurs.
In an industrial compost environment operating at approximately 60 °C, the generated heat will ensure disintegration will be complete within the normal compost processing time of 12 weeks, followed by biodegradation and bio assimilation when mixed into soil.
In a dry landfill operating at approximately 35 °C to 40 ° C the process will take from a few months for specifically designed day covers, to 18 months to 2 years for shopping bags and waste bags.
Agricultural mulch films are designed to take account of local conditions and to exist for a specified time prior to rapid disintegration followed by biodegradation and bio assimilation in soil.
The rate of the biodegradation stage will depend on the type of environment the disintegrated particles end up in. If placed in soil, the relatively large molecules of polymer are only assimilated by certain microbes. These are the same organisms which assimilate lignin (which is comprised of large naturally occurring polymer molecules). Lignin makes up majority of the humus content of compost, thus the rate of degradation depends on the availability of those microbes and it can take 4 to 5 years for the carbon in the polymer to be completely absorbed back into the environment. Because of this slow absorption rate much of the carbon in the plastic is converted to biomass with only a portion being converted to carbon dioxide.
If disposed of in a landfill, shopping bag type plastic will fragment but will only partially biodegrade, as the microbes which process large molecules cannot exist with the low oxygen levels normally found in landfills. This is advantageous, as the plastic will only make a minimal contribution to greenhouse gasses, especially methane, formed by anaerobic biodegradation, and which will trap 56 times more atmospheric heat than carbon dioxide over a 20 year period.
Landfill day covers are specifically designed to disintegrate quickly and biodegrade during the aerobic stage of landfill operation.
If included with other petrochemical products for recycling, prior to the onset of degradation, oxo-biodegradable plastics will have no effect of the quality of recycled material produced. Should high levels of prodegradant materials be included in a batch, the recycling facility can either dilute the levels with non-degradable material or process it as a degradable recycled material or include additional levels of anti-oxidant which will counter the prodegradants.
Oxo-biodegradable bags can be used for numerous products and applications because they retain all of the properties of existing plastic such as strength, clarity, water resistance and low cost.
Hydro Bio-degradable plastics are those plastics that contain a percentage of both petrochemical derived polymers and bio-polymers (normally starch based when used to produce bags for example Starch-polycaprolactone (PCL) blends produced by Novamont in Italy and branded as Mater-Bi ) These polymers are complex in structure and need to be initially hydrolyzed by water which breaks them into shorter lengths and they are then destroyed by the enzymic action of microbes. Originating from organic materials there are numerous species of microbes which will assimilate these products.
Although starch based bags are ostensibly derived from renewable resources they do require large amounts of fossil fuels to generate the power they consume during plant growth, polymer production and the transport they require. In some cases the amount of fossil fuel used to produce starch polymers exceeds that of petrochemical derived plastics , inclusive of the plastic itself and processing needs (How Green are Green Plastics - Scientific America 2000 - Gerngross and Slater).
Growing the plants required to produce the starch also has environmental impacts such as use of arable land for purposes other than food production, water usage and run off, fertilizer production and usage, transportation pollution and disposal issues associated with the remainder of the plant.
Starch based plastics are designed to degrade very quickly with 80 to 90 percent of their carbon content being converted to carbon dioxide during the normal 12 weeks required for industrial composting. Unfortunately this means that much of that carbon is not recycled back to biomass by the microbes but has to go through the whole process of contributing to greenhouse gases prior to photosynthesis by plants.
This fast mineralization rate also means that the composting facility is not able to benefit from the weight of the plastic it processes.
Starch based plastic is not recommended to be disposed of in landfill site for a number of reasons:
- because they do not have the strength of petrochemical bags, they are thicker and occupy more space in landfill
- in dry landfills they do not degrade very quickly and can remain in the landfill until it turns anaerobic and then degrade into more environmentally harmful methane;
- in landfills consisting of large amounts of non-organic material there may not be the microbial population to biodegrade the plastic;
- there is no recycling of the carbon content of the plastic;
- there is little chance of their contributing to the benefit of possible future mining of landfills for resources.
If starch based polymers are used for energy recovery by combustion they do not generate the levels of energy produced by petrochemical products, one of the reasons being that they absorb moisture very easily and therefore do not generate high heat.
If littered, starch based plastics need both the presence of water and a microbial population in order to disintegrate and reduce the danger they present to wild life. Modifications to the plastic to assist with water resistance during its normal use also mean that they do not disintegrate immediately upon being littered but can last for months in the environment.
These polymers are not normally photodegradable.
Starch based polymers are not able to be recycled as they are not compatible with normal petrochemical based plastics.
If included with waste for recycling they coagulate forming lumps which prevent the entire batch from being used. The use of starch based shopping bags will require considerable public education to ensure they are excluded from bag material collected for recycling.
Starch based plastics are opaque and thus have limited uses aside from shopping bags and waste bags. Even bags destined to be used to transport waste to compost facilities can benefit from being transparent so that staff are able to easily see any contaminants the bag may contain.
Although starch based polymers have been in the market place for almost 2 decades they have not, as yet, been able to capture as much market share, of the packaging film and bag market, as oxo-degradable plastics have done since their launch 4-5 years ago.
This is possibly due to the high cost of starch based raw materials ( 3.75 times as expensive) as well as the limited usefulness of the plastic produced, which has poor clarity, gloss and surface finish as well as poor water resistance and requires increased thickness of plastic (nearly twice) when attempting to achieve similar mechanical properties to normal plastic bags.