Category Science

If you removed everything from your home that contained plastic, how much would be left? Many kitchens would be almost bare. Most carpets and rugs would go, many clothes and perhaps the curtains would vanish. There would certainly be no telephone, hi-fi or television.

And think of all the other things made of plastic, such as riot shields, credit cards, artificial snow and hip joints. Now Australians are even buying their plastic goods with plastic banknotes.

The term ‘plastics’ covers a wide range of materials man-made from two basic ingredients: carbon and hydrogen. By adding extra chemicals, plastics can be given special properties like extra strength, heat-resistance, slipperiness and flexibility.

There is almost no end to the number of plastics that can be created by combining chemicals in different ratios and patterns. Scientists are already trying to develop a plastic as tough as steel, as clear and waterproof as glass and as cheap as paper.

Plastics are made up of large molecules called polymers, which are formed by smaller molecules joining together in long chains. These chains become tangled, giving plastic its strength – considerable force is needed to pull the chains apart.

When most plastics – called thermoplastics – are heated to about 3900ºF (2000ºC) the chains stay intact but move apart enough to slide over one another. This allows thermoplastics to be repeatedly heated and moulded into new shapes. Once the plastic has cooled it holds its neew shape and maintains its strength.

However, there are other plastics which, once moulded, remain hard and keep their shape even when reheard. These are thermosetting plastics.

The process of getting small molecules to join up and form larger ones, called polymerization , differs from one plastic to another. But it often involves high pressures and the use of special agents, called catalysts, to encourage the small molecules to link up.

The carbon and hydrogen atoms that form the base of all plastics come from crude oil. Oil consists of hydrocarbons – hydrogen and carbon molecules bonded together. Hydrocarbons range from simple molecules like methane (a gas made up of one hydrogen atom combined with four carbon atoms) to tars and asphalts, which may have hundreds of atoms.

In the process of refining crude oil many different hydrocarbons are produced, one of them is the gas ethane (two carbon and six hydrogen atoms) which can be converted to another gas, ethylene, and then polymerized to make polyethylene (polythene). Similarly, propane gas becomes polypropylene. These two plastics are used to make bottles, pipes and plastic bags.

PVC – polyvinyl chloride – is chemically similar to polythene, but its hydrogen atom is replaced by a chlorine atom. This slight change makes PVC ‘flame retardant’, making it safer to use in the home. If four fluorine atoms are used rather than the chlorine atom, polytetrafluoroethylene, PTFE, is made. This, known as Teflon, is used for nonstick frying pans and bearings.

Many polymers have been made in the laboratory, but only those with the most useful qualities, like polystyrene, PTFE and nylon, are produced industrially.

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One of the advantages of plastic is that it does not rust or rot. But this can also be a problem – plastic cups, bags, wrappers and containers litter the countryside and beaches all over the world. Unless they are picked up, they go on accumulating year after year.

To deal with the problem, various forms of degradable plastic have been developed. The secret is to incorporate into the plastic a chemical that can be attacked by light, bacteria or other chemicals.

Biodegradable plastics can be made by adding starch. If the plastics are buried, bacteria that feed on starch will gradually break them up into tiny pieces that disappear harmlessly into the soil.

Chemically degradable plastics can be broken up by spraying them with a solution that causes them to dissolve. They can be used, for example, as a protective waxy covering for new cars, and washed off at the dealer’s garage by a specially formulated spray. This reacts with one of the components in the plastic and causes it dissolve into harmless materials which can be flushed down the drain.

One of the most successful uses of degradable plastics is in surgery, where stitches are now often made using plastics which dissolve slowly in body fluids, saving the patient the anxiety of having the stitches removed. Drugs are often prescribed in plastic capsules which dissolve slowly, releasing the rug into the bloodstream at a controlled rate.

Photodegradable plastics contain chemicals that slowly disintegrate when exposed to light. In France, strips of photodegradable plastic about 3ft (1m) wide are used in the fields to retain heat in the soil and produce early crops. They last for between one and three years before rotting into the soil. But they have to be used in a country with a consistent amount of sunshine so they decay at a predictable speed.

In the USA, about one-quarter of the plastic ‘yokes’ that link beer cans in a six-pack are made of a plastic called Ecolyte, which is photodegradable. But to stop them decaying too early they must be stored away from direct sunlight, which can be an inconvenience for the retailer.

Degradable plastic has other problems. For example, it cannot be recycled because there is no easy way to measure its remaining life span. The biggest drawback has been the cost of producing it, but Japanese scientists believe they will soon be able to produce a much cheaper multipurpose biodegradable plastic.

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High-level radioactive waste is lethal and it remains dangerous for thousands of years. If someone were to stand 30ft (9m) away from a small amount of fresh waste from a nuclear reactor for ten minutes, he would have only a 50 per cent chance of living. A nuclear reactor’s spent fuel contains a deadly cocktail of radioactive products, like plutonium, strontium and caesium.

Fortunately the volume of high-level nuclear waste is small. A typical plant, generating 1000 megawatts of electricity, produces about two and a half cubic yards (two cubic metres) of waste a year.

Storage methods vary. In the USA, some processed waste is stored in double-walled stainless-steel tanks surrounded by 3ft (1m) thick concrete cladding. But most is immersed in special pools near the nuclear plants, in the form of spent fuel rods still inside the original cladding. Unfortunately this is not a long-term solution.

In Britain the waste is stored as a liquid, the colour of strong tea, in steel tanks encased in concrete, similar to those used in America. The waste generates hear as the radioactive atoms decay, so the tanks have to be cooled to prevent the liquid boiling dry, which could eventually cause a radioactive leak. Cold water is pumped through coils inside the tanks.

However, although they have already been used for 40 years, tanks are also only a temporary storage solution.

Possibly the best answer at the moment is to fuse the waste into glass cylinders to be stored deep underground. A demonstration plant in Marcoule, France, has been carrying out this process since 1978.

The waste is dried and reduced to a solid residue by heating it inside a rotating drum. It is then mixed with silica and boron, and other glass-making materials, poured through a vertical chamber and heated to  ( . A stream of molten glass emerges from the bottom, to be cast into stainless-steel containers about twice the size of an old-fashioned milk churn. A year’s output from a 1000 megawatt plant fills 15 of these canisters. After the glass has solidified, the lids are welded on.

The canisters are stored in special ‘pits’ in a neighbouring building at Marcoule. Each consider produces 1.5 kilowatts of heat and is cooled by air. The British and the Americans are also beginning to adopt this process. The waste is safe so long as it is monitored, but ultimately it should be put where it can remain without further human intervention.

One proposal is to surround the canisters with a jacket cast iron or copper, and then store them in underground caverns. The canisters would be placed in holes or trenches, then covered with concrete or a clay called bentonite, which absorbs escaping radioactive material.

The canisters should last up to 1000 years before they become corroded and let any radioactivity escape. After 500 years the radioactivity will have dropped to about the level of the original uranium ore. Experts believe that as long as the caverns are well suited and sufficiently deep – several hundred metres – it would take a million years before any material could seep to the surface, and by that time all but the tiniest traces of the radioactive waste would have decayed. The areas chosen for the ‘dumps’ should contain no valuable minerals; in case some future civilization should stumble across the waste while mining. Eventually the caverns could be sealed off and forgotten. The waste would be sealed behind so many barriers that escape in any imaginable time scale would be impossible.

The difficulty is finding sites where local people agree to have nuclear waste stored. Nobody relishes the idea of a nuclear dump close to their home. In the end, the nuclear waste authorities may well be forced to drill caverns beneath existing reprocessing facilities, or under the sea, rather than try to find new sites on land.

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Recycling rubbish is not only makes economic sense – it also helps the environment. Pollution created by burning rubbish is reduced and valuable resources are saved. Some 75,000 tress would be spared every week just by recycling the Sunday edition of the New York Times.

Many countries encourage recycling new technology allows more and more waste to be reprocessed. Most of the world’s rubbish can be reused – paper, metals, glass, even some plastics.

Plastic is one of the most difficult substances to recycle, because it comes in so many varieties. A plastic tomato-ketchup bottle, for example, consists of six layers of different plastics, each designed to give the bottle certain qualities – shape, strength, flexibility. And as yet there is no simple way to turn an old plastic bottle into a new one.

Plastic can only be turned into a product of lower quality – a plastic lemonade bottle might be cleaned, shredded and used to stuff seat cushions or insulate sleeping bags. A mixture of plastic waste can be recycled into plastic ‘timber’ and used to make durable fencing. But a lot of plastic waste still has to be thrown away because its value as scrap is so low.

Metals are different. Any car on the road today will consist, in part, of earlier cars that have been scrapped and recycled into new steel and other metals.

The more valuable the metal, like gold and silver, the more it pays to recycle it. Aluminium is worth recycling because extracting it from bauxite consumes a huge amount of electricity. Largely thanks to recycling programmes the energy used to make aluminium has fallen by a quarter since the early 1970s.

More than 70 billion canned drinks are bought in America every year, and all the cans are made of aluminium. About half are remelted after use and within six weeks they have been made into new tins and are back on the supermarket shelves.

Glass is worth recovering. The most sensible method is to use glass bottles as often as possible. The average British milk bottle makes about 30 trips to and from the dairy.

Many countries now have compulsory deposit schemes to make people return bottles to shops. When such a law was passed in the state of New York in 1983, it was estimated that within two years it had saved $50 million on rubbish collection, $19 million on waste disposal costs, and about $50 million in energy costs.

Some supermarkets now have machines that accept glass bottles and aluminium cans and give cash or redeemable vouchers to the customer. They read the computer codes on the containers to work out how much to pay.

Broken glass, known as ‘cullet’, can also be recycled, and many countries have bottles banks depend on people’s goodwill. The success of bottle banks varies widely from country to country. The Swiss and Dutch recover 50 per cent of their glass, while in Britain only 12 per cent is recovered.

Glass is best separated by colour, since cullet of mixed colours can be used only to make green glass. Broken glass can be remelted in furnaces and then it can easily be shaped into new bottles or other objects.

Half the world’s waste consists of paper. Many countries import waste paper rather than new pulp for their paper mills. The waste is pulped, cleaned and bleached to remove most of the ink and dirt, before it is turned into new paper in the same way as wood pulp or rags. Japan now makes half its paper by recycling.

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