Molecule of the month: polyurethane

From condoms to cladding, this useful material is finding an ever- expanding market. John Emsley reports

This month, London International Group (LIG), the world's leading condom manufacturer, announced a return to profitability after years of drooping because of the recession. Last year it closed its UK factory making Durex, but now LIG is back in production at a plant in Cambridge Science Park, turning out a new type of condom, Avanti. These are now all being exported to the US, where they sell for as much as $1.50 (pounds 1) each, but next year they should be on sale here.

Avanti is revolutionary because it is made from polyurethane, the plastic better known for lightweight insulation and upholstery padding. LIG's scientific affairs director, Dr Bill Potter, says the new condoms have taken five years to develop, at a cost of more than pounds 10m. "The polyurethane is twice as strong as the traditional latex, so condoms can be made much thinner; they are completely transparent, and slightly larger. Tests showed that 80 per cent of users preferred them, reporting increased sensitivity," says Dr Potter. The new condoms are non-allergenic, unaffected by lubricants and provide an effective barrier against sperm and sexually transmitted bacteria and viruses, including HIV.

Most people's idea of polyurethane is of a porous material for use in cushions, mattresses and insulation panels. Our cars are full of it and we save fuel because it saves weight. It provides comfort as the padding of seats, and safety as cushioned dashboards and steering wheels. It can also be a rubbery material suitable not only for condoms, but also for Lycra swimwear and hosiery.

Chemists make polyurethane by reacting molecules that contain alcohol groups with those that have isocyanate groups. As they mix, a strong chemical bond quickly forms, binding them together and giving off heat. If a volatile liquid is present, this will form gas bubbles in the plastic, expanding it as it sets, like a sponge cake in an oven. Polyurethane foams weigh so little because they can be 95 per cent gas.

This month a South African company, Tech Edge, has unveiled a stop-gap plan to solve the local housing crisis. For those who have to live in shacks of corrugated iron and plywood, a temporary answer is to spray them with polyurethane. Russell Moore, Tech Edge's managing director, says this makes them habitable, as it keeps out the sun's heat and insects while also making the shacks soundproof.

Polyurethane cladding is sprayed on, then painted with a fire-retardant resin, which also protects the plastic from damaging UV rays of the sun. A shanty can be treated for as little as pounds 120. And the investment will not be wasted when people are rehoused. They can cut the polyurethane into panels with a knife and use it as insulation in their new homes.

ICI Polyurethanes, based at Everberg in Belgium, is one of Europe's largest producers. The company has recently developed a flexible foam using water, which reacts with a little of the isocyanate to generate CO2, which then forms the bubbles that make the foam. The product, christened Waterlily, will be launched in the UK later this year. ICI has also designed a new generation of insulation based on polyurethane foam suitable for use under vacuum. This "super insulation" is up to three times more effective than ordinary polyurethane, meaning fridges can be built smaller or have larger interiors.

However, it is the non-foam polyurethane that is providing the new markets for some of the 5 million tonnes produced each year. Lycra, the stretch fabric, is expanding into sports clothing and fashion in addition to its more conventional swimwear outlets. In the 1994 World Cup footballs were for the first time coated with polyurethane, making play faster, or so it was claimed. Polyurethane can also be used as an adhesive to bind together other materials, and in this way old tyres are shredded and bonded to form athletic tracks and surfaces for children's playgrounds.

Nor when its useful life is over need the polymer be wasted. It can be burned as fuel and has the calorific content of coal, or it can be reduced to its constituent chemicals and reconstituted as a new batch of polyurethane.

The writer is science writer in residence at Imperial College, London.