Tuesday, 5 May 2009

Planter, heal thyself!

Most of us are familiar with the term glass fibre and associate it with car body repairs or roof insulation. Glass fibre actually refers to the fine glass strands that make up the matting that is combined with a resin for car repairs or made in to a thick blanket for insulation. The reason that users are advised to wear gloves and eye protection is that the glass fibres are so thin and sharp they can and will penetrate the skin causing irritation. The complete material used for car repair an parts is properly known as GRP (glass reinforced plastic).

A further development of this has been AC or advanced composite materials. As with GRP, a strong mesh is combined with resin to form a shape but the mesh is a high tech material such as carbon fibre. This allows for the manufacture of intricate shapes. Advanced composite materials are used in undersea exploration and in space. They are also used in lower tech manufacturing such as car parts ( bumpers, door skins), ATM machine fronts, fairground rides (vehicles, animal shapes, pay kiosks) specialist vehicle bodies, building construction forms, building accessories ( canopies, dormer roofs), water tanks and transportation and plant containers. Both types of composite materials are durable, strong and lightweight.

It is inevitable, however, that impact damage will result at some stage especially with composites used in vehicles. The damage can simply be cosmetic, though if the damaged area is large enough the impact can have structural implications. Repairing damage to GRP and AC in vehicles is possible with the damaged area being replaced with new mesh or matting coated with the resin. The repaired section is abraded and filled until a smooth finish is achieved. Access to the affected vehicle part is usually easy. Impact damage undersea or in space is a different matter. A small meteorite travelling at thousands of miles per hour will penetrate an AC component on a spacecraft and may cause stress fractures. If the stress fracture is not repaired it will almost certainly enlarge and cause further problems.

An astronaut on a spacewalk may be able to perform a repair but it would be a great deal safer if the component could fix itself. A development of composite materials based on biological repair mechanisms may have achieved that aim. With repairs to animal tissue, the blood hardens over damaged tissue to enable new skin to form underneath. Replicating that concept, a small percentage of the strands in the self healing composite material is hollow and contains the uncured resin. The resin carrying strand has to be of a material that will break upon impact and so glass was chosen. With animal tissue the blood reacts with air to harden but in space or undersea there is no air to effect curing. Instead a matrix of alternate hollow fibres contain a resin and a hardener and the combining of both liquids allows the repair solution to fill the cracks and harden. A consideration for use in space has been the performance of the liquids at extremely low temperatures. Other repair methods have been investigated such as microscopic capsules containing the resin and hardener and a matting that will repair itself when heated. It may be some time, however, until a self repairing compound is a reality.

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