BIODEGRADABLE 'SUGAR DERIVED' PLASTIC BAGS
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Introduction - Biodegradable Plastics
plastics made with plant-based materials have been available for many years.
Their high cost, however, has meant they have never replaced traditional
non-degradable plastics in the mass market. A new Australian venture is
producing affordable biodegradable plastics that might change all that.
Our whole world
seems to be wrapped in plastic. Almost every product we buy, most of the food we
eat and many of the liquids we drink come encased in plastic. In Australia
around 1 million tonnes of plastic materials are produced each year and a
further 587,000 tonnes are imported. Packaging is the largest market for
plastics, accounting for over a third of the consumption of raw plastic
materials – Australians use 6 billion plastic bags every year!
packaging provides excellent protection for the product, it is cheap to
manufacture and seems to last forever. Lasting forever, however, is proving to
be a major environmental problem. Another problem is that traditional plastics
are manufactured from non-renewable resources – oil, coal and natural gas.
Plastics that break down
In an effort to
overcome these shortcomings, biochemical researchers and engineers have long
been seeking to develop biodegradable plastics that are made from renewable
resources, such as plants.
biodegradable means that a substance is able to be broken down into simpler
substances by the activities of living organisms, and therefore is unlikely to
persist in the environment. There are many different standards used to measure
biodegradability, with each country having its own. The requirements range from
90 per cent to 60 per cent decomposition of the product within 60 to 180 days of
being placed in a standard composting environment.
traditional plastics are not biodegradable is because their long polymer
molecules are too large and too tightly bonded together to be broken apart and
assimilated by decomposer organisms. However, plastics based on natural plant
polymers derived from wheat or corn-starch have molecules that are readily
attacked and broken down by microbes.
Plastics can be produced from starch
Starch is a
natural polymer. It is a white, granular carbohydrate produced by plants during
photosynthesis and it serves as the plant's energy store. Cereal plants and
tubers normally contain starch in large proportions. Starch can be processed
directly into a bioplastic but, because it is soluble in water, articles made
from starch will swell and deform when exposed to moisture, limiting its use.
This problem can be overcome by modifying the starch into a different polymer.
First, starch is harvested from corn, wheat or potatoes, then micro-organisms
transform it into lactic acid, a monomer. Finally, the lactic acid is chemically
treated to cause the molecules of lactic acid to link up into long chains or
polymers, which bond together to form a plastic called polylactide (PLA).
PLA can be used
for products such as plant pots and disposable nappies. It has been commercially
available since 1990, and certain blends have proved successful in medical
implants, sutures and drug delivery systems because of their capacity to
dissolve away over time. However, because PLA is significantly more expensive
than conventional plastics it has failed to win widespread consumer acceptance.
Plastics can also be produced by bacteria
Another way of making biodegradable polymers involves getting bacteria to produce granules of a plastic called polyhydroxyalkanoate (PHA) inside their cells. Bacteria are simply grown in culture, and the plastic is then harvested. Going one step further, scientists have taken genes from this kind of bacteria and stitched them into corn plants, which then manufacture the plastic in their own cells.
as with PLA, PHA is significantly more expensive to produce and, as yet, it is
not having any success in replacing the widespread use of traditional
biodegradable plastic products currently on the market are from 2 to 10 times
more expensive than traditional plastics. But environmentalists argue that the
cheaper price of traditional plastics does not reflect their true cost when
their full impact is considered. For example, when we buy a plastic bag we
don’t pay for its collection and waste disposal after we use it. If we added
up these sorts of associated costs, traditional plastics would cost more and
biodegradable plastics might be more competitive.
If cost is a
major barrier to the uptake of biodegradable plastics, then the solution lies in
investigating low-cost options to produce them. In Australia, the Cooperative
Research Centre (CRC) for International Food Manufacture and Packaging Science
is looking at ways of using basic starch, which is cheap to produce, in a
variety of blends with other more expensive biodegradable polymers to produce a
variety of flexible and rigid plastics. These are being made into ‘film’ and
‘injection moulded’ products such as plastic wrapping, shopping bags, bread
bags, mulch films and plant pots.
Mulch film from biodegradable plastics
The CRC has
developed a mulch film for farmers. Mulch films are laid over the ground around
crops, to control weed growth and retain moisture. Normally, farmers use
polyethylene black plastic that is pulled up after harvest and trucked away to a
landfill (taking with it topsoil humus that sticks to it). However, field trials
using the biodegradable mulch film on tomato and capsicum crops have shown it
performs just as well as polyethylene film but can simply be ploughed into the
ground after harvest. It’s easier, cheaper and it enriches the soil with
Pots you can plant
biodegradable plastic product is a plant pot produced by injection moulding.
Gardeners and farmers can place potted plants directly into the ground, and
forget them. The pots will break down to carbon dioxide and water, eliminating
double handling and recycling of conventional plastic containers.
Different polymer blends for different products
Depending on the
application, scientists can alter polymer mixtures to enhance the properties of
the final product. For example, an almost pure starch product will dissolve upon
contact with water and then biodegrade rapidly. By blending quantities of other
biodegradable plastics into the starch, scientists can make a waterproof product
that degrades within 4 weeks after it has been buried in the soil or composted.
Landfill sites aren't compost heaps
To maximise the
benefit of the new bioplastics we’ll have to modify the way we throw away our
garbage – to simply substitute new plastics for old won’t be saving space in
is a popular misconception that biodegradable materials break down in landfill
sites, they don't. Rubbish deposited in landfill is compressed and sealed under
tonnes of soil. This minimises oxygen and moisture, which are essential
requirements for microbial decomposition. For biodegradable plastics to
effectively decompose they need to be treated like compost.
Composting the packaging with its contents
Compost may be
the key to maximising the real environmental benefit of biodegradable plastics.
One of the big impediments to composting our organic waste is that it is so
mixed up with non-degradable plastic packaging that it is uneconomic to separate
them. Consequently, the entire mixed waste-stream ends up in landfill. Organic
waste makes up almost half the components of landfill in Australia.
By ensuring that
biodegradable plastics are used to package all our organic produce, it may well
be possible in the near future to set up large-scale composting lines in which
packaging and the material it contains can be composted as one. The resulting
compost could be channelled into plant production, which in turn might be
redirected into growing the starch to produce more biodegradable plastics.
Olympic effort – recycling 76 per cent of waste
For anyone who
thinks such schemes aren’t feasible, you only have to look at the recycling
success of the Sydney 2000 Olympics to see that where there’s a will,
there’s a way. More than 660 tonnes of waste was generated each day at its
many venues. Of this, an impressive 76 per cent was collected and recycled. Part
of this success was due to the use of biodegradable plastics used in the
packaging of fast food, making the composting of food scraps an economic
proposition as it eliminated the need for expensive separation of packaging
waste prior to processing.
use, these new plastics have the potential to reduce plastic litter, decrease
the quantities of plastic waste going into landfills and increase the recycling
of other organic components that would normally end up in landfills.
Source: © Australian Academy of Science