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Biodegradable Plastics

Types of Biodegradable Plastics

It is important to distinguish between the different types of biodegradable plastic, as their costs and uses are very different. The two main types are oxo-biodegradable and hydro-biodegradable. In both cases degradation begins with a chemical process (oxidation and hydrolysis respectively), followed by a biological process. Both types emit CO2 as they degrade, but hydro-biodegradable can also emit methane. Both types are compostable, but only oxo-biodegradable can be economically recycled. Hydro-biodegradable is much more expensive than oxo-biodegradable.

OXO-BIODEGRADABLE PLASTIC - This new technology produces plastic which degrades by a process of OXO-degradation. The technology is based on a very small amount of pro-degradant additive being introduced into the manufacturing process, thereby changing the behaviour of the plastic. Degradation begins when the programmed service life is over (as controlled by the additive formulation) and the product is no longer required.

There is little or no additional cost involved in products made with this technology, which can be made with the same machinery and workforce as conventional plastic products.

The plastic does not just fragment, but will be consumed by bacteria and fungi after the additive has reduced the molecular structure to a level which permits living micro-organisms access to the carbon and hydrogen. It is therefore “biodegradable.” This process continues until the material has biodegraded to nothing more than CO2, water, and humus, and it does not leave fragments of petro-polymers in the soil. Oxo-biodegradable plastic passes all the usual ecotoxicity tests, including seed germination, plant growth and organism survival (daphnia, earthworms) tests carried out in accordance with ON S 2200 and ON S 2300 national standards.

The length of time it takes for oxo-biodegradable products to degrade can be ‘programmed’ at the time of manufacture and can be as little as a few months or as much as a few years. They are protected from degradation by special antioxidants until ready for use, and storage-life will be extended if the products are kept in cool, dark conditions.

Unlike PVC, the polymers from which oxo-biodegradable plastics are made do not contain organo-chlorine. Nor do oxo-biodegradable polymers contain PCBs, nor do they emit methane or nitrous oxide even under anaerobic conditions.

HYDRO-BIODEGRADABLE PLASTICS - Hydro-biodegradation is initiated by hydrolysis. Some plastics in this category have a high starch content and it is sometimes said that this justifies the claim that they are made from renewable resources. However, many of them contain up to 50% of synthetic plastic derived from oil, and others (e.g. some aliphatic polyesters) are entirely based on oil-derived intermediates. Genetically-modified crops may also have been used in the manufacture of hydro-biodegradable plastics.

Hydro-biodegradable plastics are not genuinely “renewable” because the process of making them from crops is itself a significant user of fossil-fuel energy and a producer therefore of greenhouse gases. Fossil fuels are burned in the autoclaves used to ferment and polymerise material synthesised from biochemically produced intermediates (e.g. polylactic acid from carbohydrates etc); and by the agricultural machinery and road vehicles employed; also by the manufacture and transport of fertilisers and pesticides. They are sometimes described as made from “non-food” crops, but are in fact usually made from food crops.

A disproportionate amount of land would be required to produce sufficient raw material to replace conventional plastic products, and a huge amount of water, which is in such short supply in so many parts of the world.

Residues from some native starches can be seriously toxic; bitter cassava for example (tapioca) has a high level of hydro-cyanic glucoside present, which has to be removed by careful washing. During growth the plant is toxic to wildlife. Cassava is exhaustive of potash .

Three recent articles in the international press have drawn attention to the danger of using “renewable” resources derived from plants as a substitute for petroleum products. They focus on the use of corn and palm oil to make “biofuels” for motor vehicles, but the same danger arises from the use of corn and other agricultural products to make hydro-biodegradable plastics.

The International Herald Tribune wrote on 31st January 2007 “Just a few years ago politicians and green groups in the Netherlands were thrilled by the country’s adoption of “sustainable energy” by coaxing electricity plants to use biofuel. Spurred by government subsidies, energy companies designed generators that ran exclusively on this fuel, which in theory would be cleaner than fossil fuels because it is derived from plants.

Plastics made from crops, are up to 400% more expensive, they are not strong enough for use in high-speed machinery, and they emit methane (a powerful greenhouse gas) in landfill. Also, it is wrong to use land, water and fertilizers to grow crops for bioplastics and biofuels, which drives up the cost of food for the poorest people

Business Week 5 Feb 2007 edition “The rise in the price of corn that's hurting US pig farmers isn't caused by any big dip in the overall supply. In the U.S., last year's harvest was 10.5 billion bushels, the third-largest crop ever. But instead of going into the mouths of pigs or cattle or people, an increasing slice is being transformed into fuel for cars. The roughly 5 billion gallons of ethanol made in 2006 by 112 U.S. plants consumed nearly one-fifth of the corn crop.” US chicken producers are also being hit. The industry's feed costs are already up $1.5 billion per year. Ultimately, these increases will be passed on to consumers, and there could be dramatic inflation in food costs.

Oxo-bio plastics degrade in the upper layers of a landfill, but they are completely inert deeper in the landfill in the absence of oxygen. They do not emit methane at any stage.

Paper bags use 300% more energy to produce, they are bulky and heavy and are not strong enough, especially when wet. They will also emit methane in landfill.

Comparison of Oxo-Biodegradable and Hydro-Biodegradable Plastics

OXO

HYDRO

Usually made from a by-product of oil-refining

Usually made from starch

Can be recycled as part of a normal plastic waste-stream

Damages recyclate unless extracted from feedstock

Can be made from recyclate

Cannot be made from recyclate

Emits CO2 slowly while degrading

Emits CO2 rapidly while degrading

Inert deep in landfill

Emits methane deep in landfill

Can use same machinery and workforce as for conventional plastic

Needs special machinery and worforce

Suitable for use in high-speed machinery

Not suitable

Compostable in-vessel

Compostable

Little or no on-cost

Four or five times more expensive than conventional plastic

Same strength as conventional plastic

Weaker than conventional plastic

Same weight as conventional plastic

Thicker and Heavier

Leak-proof

Prone to leakage

Degrades anywhere on land or sea

Degrades only in high-microbial environment

Time to degrade can be set at manufacture

Cannot be controlled

No genetically modified ingredients

Possibility of GM ingredients

Safe for food contact

Safe for food contact

No PCB's Organo-chlorines, or "heavy metals"

No PCB's Organo-chlorines, or "heavy metals"

Can be incinerated with high energy-recovery

Can be incinerated, but lower calorific value

Production uses no fertilisers, pesticides or water

Production uses fertilisers, pesticides and water

No limit on availability of feedstock

Limited availability of feedstock

Demand for oxo-biodegradable plastics does not drive up cost of fuel for vehicles

Demand for hydro-biodegradable plastics drives up price of human and animal foodstuffs


**The above information has been reprinted with permission of the Oxo-Biodegradable Plastics Association   www.biodeg.org

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