It's confusing when a plastic is labeled as biodegradable, because often they are made of a combination of starch derivatives and synthetic polymers. In such instances although the bacteria in the soil is able to breakdown the starch part, the polymer part that is of petroleum origin remains intact, and furthermore these fine particles are not easily noticed. So in reality, the product may not be entirely biodegradable. Therefore, when we talk of biodegradable plastics we should actually mean green plastics---for instance the ones that degrade completely into say water and carbon dioxide and more importantly doesn't leave any non-biodegradable substances even if it were minute substances.
Biopolymers made from starch or cellulose or more specifically from cornstarch have the feel and look of ordinary plastics and are biodegradable completely as bacteria can feed on them. So perhaps a biodegradable plastic has to be made from potential food material so that the bacteria can eat them or else it may remain in the soil for ages.
Paradoxically microbial action can decompose some polymers of petroleum origin too. Polycaprolactone is one such synthetic polymer. However polycaprolactone does not qualify as a renewable biodegradable plastic. That's because renewable means you should be able to grow it from plants or brew it from microorganisms.
Bioplastics from bacteria
Most of the bacteria store energy in a substance that they make themselves called as polyhydroxybutyrate or PHB. This PHB is a biodegradable plastic by itself, and can be produced in genetically modified potatoes or by making the right bacteria feed on sugar etc. However the costs involved in making it in large scale, makes it unviable for mass scale use.
Nevertheless, it has been established that the bacterium R. eutropha, can produce polyhydroxyalkanoate (a naturally occurring form of polyester) with just sunlight, water and a carbon source as raw material. Sinskey and Oliver Peoples have been studying several ways to augment the bacteria's natural plastic production capabilities, as that would mean a sure way of making biodegradable plastics. Oliver Peoples and other scientists have managed to reengineer a strain of E.coli to produce seven times more PHA than other bacteria, by putting into E.coli several genes from different bacteria.
Polylactic acid (PLA) made from lactic acid is also reckoned as a biopolymer, and bioplastics made from PLA have been in use for some time. PLA is made from corn but has the drawback of being less heat resistant.
Examples of Bioplastics products
Starch-based packagings that dissolve on sustained contact with warm water are made from corn or potatoes. For example, Earthshell® makes proprietory plates and bowls that are biodegradable and made from a combination of potato starch and limestone. Corn based bioplastics are also made by Cargill, BAS, and Novamont SpA
DuPont makes water-soluble polyethylene terephthalate. But what if this water passes on to streams and rivers and increases the biological oxygen demand and chemical oxygen demand and poses difficulties to aquatic ecosystem?
Novomer (one of the 50 most innovative companies in the world) makes biodegradable plastics from carbon dioxide and carbon monoxide by making them react using a catalyst. In this case, using carbon dioxide as a feedstock is advantageous, simply because carbon credits are accruable by using it, which is as good as getting money to use a raw material The process involves mixing a liquid metal with carbon dioxide or carbon monoxide in a reactor at low pressure. Effectively this means converting waste carbon dioxide to a high performance, low cost polymer.
Scientists at Agricultural Research Service have succeeded in making biodegradable plastic from chicken feathers. Poultry production generates billions of pounds of chicken feather and disposing this waste is a serious eco- problem. Now these scientists at ARS have succeeded in making biodegradable plastic from chicken feathers.
Why the need for biodegradable plastic?
It is perfectly understandable if the need for seeking biodegradable plastics is to make a conscious decision to shift from oil as a feedstock in view of rising costs. Other reasons like trying to use biodegradable plastic material to solve the littering problem caused by conventional plastic materials don't make much sense. Even a biodegradable sachet will take a while to degrade, so that's not going to solve the littering problem which is essentially a question of attitude. As a matter of fact, making biodegradable plastic bags and other items, will increase the littering problem as people might throw these things into the open thinking it will assimilate quickly in the soil. That's far from true. For example, biodegradable packaging takes weeks and months to degrade completely and depends upon temperature and other soil conditions. Biodegradable plastics of polyhydroxyalkanoates variety take close to half a year to break up in the soil.
Bioplastics and emissions
And what is the contribution of biodegradable plastics, towards preventing climate change and lessening carbon dioxide emissions? This is a highly debatable issue but let's nevertheless have a look at it.
The plastics in everyday use today is made from oil---and that means carbon is taken from oil and stored in the plastics and when the plastic doesn't degrade it remains that way,although there is emission of carbon dioxide to the atmosphere during the making of conventional plastics.
Biodegradable plastics made from natural sources, meaning corn etc would take carbon from the atmosphere (when corn is grown) and store it. This would be released to the atmosphere when the plastic degrades, but there is no net addition of carbon dioxide to the atmosphere. But we are missing a point. Growing sustained crop of wheat, maize, and potato etc for making the plastics itself, releases nitrous oxide (N2O) which is more potent than carbon dioxide. Isn't that toxic?
Synthetic biodegradable plastic made from oil stores carbon, and when this synthetic biodegradable plastic degrades it means more carbon dioxide is released to the atmosphere. So is it worthwhile? That apart, oxo-biodegradable plastics if sent to landfills, due to anerobic activity start releasing methane which means even more greenhouse gases.
Nevertheless the fact that 101.5 billion pounds of plastics were made in the U.S during 2008 and several billion pounds elsewhere in the world, effective technologies for making biodegradable plastics when successful, will surely lessen the requirement of oil in making plastics.
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