Secondary Secondary Science

Superstuffs – Polyethene

Bits of coloured plastic caught in hedgerows and pictures of plastic debris accumulating in the Pacific Ocean have given polyethene a bad name. The supermarket carrier bag is now a problem rather than the free, throwaway, item it once was. Can we still call polyethene a superstuff?

Polyethene was discovered in 1898. German chemist, Hans von Pechmann was doing an experiment with highly dangerous diazomethane and accidentally made a white compound. Friends found it to be a hydrocarbon with a high molecular mass. Nothing seems to have come of Pechmann’s discovery, possibly because he died shortly after.

In 1933 another accidental discovery occurred at the ICI research laboratories at Northwich in Cheshire. There, Reginald Gibson and Eric Fawcett were carrying out reactions at high pressure. They obtained the strange white substance after heating a mixture containing ethene. They tried to repeat the experiment but failed to make the polymer. Eventually they realised that the original apparatus had leaked. Oxygen was needed to catalyse the reaction. ICI decided to manufacture polyethene but at the start of the Second World War, the work became top secret. Polyethene was used to insulate the cables in the new radar devices fitted to aircraft.

High pressure polyethene was soft and had a low melting point. Earl Tupper an American chemist realised that, nevertheless, it would be suitable for making containers for storing food in fridges. They became a hit when sold directly to housewives in home “Tupperware parties”.

There are in fact many types of polyethene although all are simple compounds with a chain of carbon atoms and hydrogen atoms attached. High pressure polyethene had relatively short chains of a few thousand carbon atoms with many branches of different lengths.

In 1951 Robert Banks and Paul Hogan, working for Phillips Petroleum, found a new way of making polyethene using a lower pressure and a catalyst. They produced High Density Polyethene (HDPE). In 1953 Carl Ziegler discovered a different catalyst to produce the same material. HDPE has long chains without branches and forms a denser, crystalline material that is tougher and has a higher melting point.

It was thought that HDPE would be ideal for making water pipes. Factories were built and manufacture began but it was found that after a few months the material became brittle. Suddenly there were tonnes of useless HDPE. Someone had the bright idea of turning the pipes into hoops and the 1958 craze of the Hula Hoop was born – and no-one was bothered when the hopes wore out after a few months of hula-ing. The problems were solved and HDPE began to be used in pipes, packaging and the many other uses that made the 1960s the start of the Age of Plastics

In 1976, German chemists Walter Kaminsky and Hansjorg Sinn found another group of catalysts which allow better control of the polymerisation process. Now the length of the polymer chain and the size and number of branches can be planned to provide types of polyethene with a wide variety of properties.

• The carrier bag that causes so much problem is made from low density polyethene (LDPE). With a number of short branches it is quite strong and will stretch without breaking.

• HDPE, as well as being used for pipes, is found in the harder, more rigid containers used for milk and spreads, and for “crinkly” bags.

• Medium density polyethene (MDPE) has similar uses to LDPE and HDPE with properties in between.

• Very low density polyethene (VLDPE) is used for freezer bags.

• Linear low density polyethene (LLDPE) is a tougher type used for lids.

• The toughest of the lot, and the longest name, is ultra high molecular weight polyethene (UHMWPE) which has a molecular mass of up to 5 million. It is extremely tough and is used for hip joints, machine bearings and chopping boards.

• There is even an elastic polyethene called cross linked polyethene (XLPE or PEX).

Probably one of the commonest uses of polyethene today is for the polytunnels which cover acres of the countryside allowing fruit and vegetables to be grown out of season. This is made from MDPE with extra coatings to filter out uv light, stop condensation and prevent growth of algae.

With its variety of properties, polyethene is very useful but there are two problems. The first is that petroleum has been the raw material for manufacturing polyethene but it is becoming scarce. Polyethene can however be made from sugars.

The second problem is more serious. As it is nonbiodegradeable, polyethene persists in the environment becoming broken up into smaller and smaller bits that suffocate animals. In the oceans it is eaten in mistake for plankton but has no food value and blocks the guts of animals. Polyethene can be recycled far more easily than most polymers so it should never be simply thrown away. Perhaps we need to appreciate this superstuff rather more and not let it turn into waste.

Activities:

1 Carry out a survey of the uses of different types of polyethene (Objects should be stamped with the recycling code – 2 for HDPE and 4 for LDPE)

2 Compare the properties of HDPE and LDPE. Collect samples of each and test them for ease of stretching(toughness), ease of scratching and cutting (hardness). Put samples in boiling water and note what happens (melting point). Find out why the different types of polyethene have different properties.

3 Investigate the availability of plastic recycling in your area. Find out if all types of polyethene are accepted and recycled.

4 Investigate the amount of plastic waste in your neighbourhood. Collect waste plastics (wear gloves and avoid handling waste). See if you can identify what the waste plastic was used for and try to identify if the polymer is polyethene.
Plan a campaign to alert people to the dangers of plastic waste.

5 Find out more about the people mentioned in this article.

6 Discuss whether the benefits of polyethene outweigh the problems.

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