Green materials

Plastics manufacturers are increasingly concerned with improving their environmental performance.

Generally, their improvements are similar to those seen elsewhere in the process industries - use less power, produce less waste, substitute harmful and hazardous materials for more innocuous alternatives where possible. However, some plastics makers and polymer scientists are taking a different approach.

One aspect of environmentally-friendly production is concerned with making sure that the plastic is easy to recycle, with minimal processing needed to return used materials to a virgin state, or as near as possible.

One such plastic is currently being developed by BP Amoco Fabrics under the name Curv. Developed at Leeds University, Curv is an apparent contradiction in terms - a composite which has only one component. It's made entirely from polypropylene, but has similar properties to glass fibre-reinforced composites.

The technology to make Curv, known as hot compaction, was invented by Ian Ward, Peter Hine and Keith Norris of the Interdisciplinary Research Centre in Polymer Science and Technology in the early 1990s. The first stage of the process is to draw the polypropylene into tapes. This lines up the polymer chains in the same direction, creating areas of crystallinity within the structure which increase its strength in the direction of orientation. These tapes are woven together, and the woven assembly is heated to a critical temperature which, according to Peter Hine, melts 'a chosen fraction of the skin of each element'. When the assembly cools down, the molten material recrystallises to form a matrix which binds the structure together. But as the heating procedure only melts part of the assembly, most of the crystalline oriented structure remains - as does its strength.

Hot compacted polypropylene has high failure stress and is exceptionally resistant to impact and abrasion, but is lighter than glass fibre-reinforced composites and, Hine claims, has lowerlife cycle costs. Moreover, its properties are retained at very low temperatures, even down to -40 degrees C. Recycling is a simple matter as the compacted PP can be melted down and re-drawn into new oriented tapes without any loss of properties.

Commercial composite

Ward, Hine and Norris patented their invention in 1993, and formed a company called Vantage Polymers, under the auspices of Leeds University, to commercialise it. BP's interest was drawn by the material's potential for automotive applications. Components made from Curv can be just half the weight of glass-reinforced composites, and can be moulded and formed at lower temperatures and pressures. Moreover, as it contains no glass, it is safer to handle, easier on moulds and tools, and forms smooth surfaces that don't need extra coatings.

Possible uses include the undershield component, which protects the bottom of the car. Another use under investigation is in sporting protective clothing, such as shinpads.

As well as being an opportunity for materials producer, cars are an ever-growing problem for recyclers. Of particular concern are tyres, which are clogging landfills because they are extremely difficult to recycle. 'It's chemically cross-linked, and those links will not melt and will not dissolve,' says Richard Farris of the University of Massachusetts, who is working on just this problem.

At its heart, Farris's solution to the problem is a refinement of a process more than a century old and originally developed by Goodyear - grinding up vulcanised scrap rubber to a powder, mixing it with unvulcanised rubber and re-vulcanising the whole mixture with additives to encourage the formation of new cross-links. But the original process has major limitations, Farris says. If the mixture contains more than 15 per cent recycled powder, the strength of the resulting material is greatly reduced.

The answer appears to lie in the addition of energy. Heating the cross-linked powder to 200 degrees C and subjecting it to around 70bar pressure breaks and reforms the chemical bonds in the rubber in a sintering process, forming a lump of solid, cross-linked rubber which retains 50-90 per cent of the strength and elasticity of the original material, depending on the type of rubber used. The addition of other chemicals may suppress unwanted reactions in the sintering process, further increasing the strength, Farris says.

One of Farris's team, Drew Williams, is working on another method for recycling rubber, combining it with asphalt to produce a material for road-building. On its own, asphalt tends to melt and become sticky in hot weather, and becomes very brittle at low temperatures - neither of which make for safe driving conditions.

Williams has found that freezing the asphalt, grinding it to a fine powder, mixing this with ground-up rubber from tyres to make a 15-40 per cent rubber mixture, and sintering this mixture as before, creates a material which outperforms traditional asphalt. It does not become sticky when hot, and retains its flexibility when cold, he claims. 'These projects represent green chemistry at its best,' says Farris. 'We're generating lower amounts of waste, and reclaiming used materials, and all we're adding is heat and pressure.'

At the higher-tech end of the industry, the human body is producing some problems for polymer scientists. Biologically-compatible plastics, used in surgical implants, need to have chemical groups on their surfaces to trick the body's defences into accepting them. Sulphate and phosphate groups are particularly important, but the usual methods for adding these to polymer surfaces involves treating the materials with concentrated sulphuric and phosphoric acids, leading to environmental problems with disposal of wastes.

Hans Griesser and colleagues at the Jan Wark Research Institute at the University of South Australia are looking at the use of low-temperature gas plasmas to add the chemical groups. In theory, this would allow the placement of sulphate and phosphate on the surface without the addition of biologically incompatible carboxylic acid groups, as happens with acid treatment, and to vary the density of the added groups. Importantly, it avoids the disposal problems, as only tiny amounts of reactants are needed.

However, there are huge problems to be surmounted. Nobody has done this type of work before, so the team is having to work from basics - designing how to produce the right sort of plasma chemistry, investigating the reaction mechanisms that take place at the surface, and looking at the composition of the surfaces formed.