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Cracking the secret of clearer aircraft windows

CSIRO : 21 March, 2000  (Technical Article)
Australian scientists have developed a new way to keep aircraft windows clear and crack-free, extending their life and saving money for airlines. 'Most people have looked through aircraft windows and noticed that they have very fine cracks on the surface,' says CSIRO scientist, Dr Hans Griesser.
'These cracks, which look like scratches don't pose any threat to safety, but they do mean a major expense for airline companies, because the windows have to be removed and polished at regular intervals.'

'This is a costly operation. Not only that, each polishing reduces the thickness of the window, and after a number of polishings, the window must be discarded and replaced with a new one if it no longer meets the thickness requirements,' Dr Griesser says

'We realised that longer intervals between polishings would be of considerable economic benefit to airlines. This can help to keep the costs of air travel down.'

CSIRO and Aeroclear Pty Ltd, an Australian company specialising in the refurbishment of aircraft windows, have worked together to find a way to slow down the rate of cracking.

The researchers set out to discover why aircraft windows develop the very fine cracks (called crazing) in the first place. They found that the crazing is due to stresses in the material from which aircraft windows are manufactured, caused by changes in the water content of the plastic.

'The water content of the windows varies with altitude', explains Aeroclear's Geoff Thomas. 'At sea level it is about 2 per cent. As an aircraft climbs to its cruising altitude, the water in the plastic is forced out. As it descends the window takes up water again'.

This frequent change in the water content is enough to cause stress that cannot evenly dissipate, hence the window crazes, says Thomas.

'We thought that the problem could be lessened by adding a coating that would reduce or eliminate the water transfer. It is extremely difficult to produce a transparent coating that completely eliminated water vapour transport because water molecules are so tiny. However, we recognised that even slowing down the transport would help reduce the stress crazing.'

The project team has developed a process for coating the windows with a water repellent polymer, flexible enough to accommodate changes in shape that a window undergoes without itself crazing or coming unstuck.

The technology involves plasma coating the window with a thin protective polymer. It has to be applied with precision so that the thickness is uniform and it adheres strongly to the window.

Aeroclear and CSIRO, in association with the Belgian company EUROPLASMA have developed a specially designed plasma reactor to apply the coating to passenger windows.

The process has been successfully trialled and is covered by patents.

'This treatment should result in massive cost saving for the aero industry. The time between refurbishment is trebled, and the need for replacement windows reduced accordingly,' Dr Griesser says. 'In addition, other industries that need to eliminate crazing of windows or transparent plastic sheets should likewise benefit from this invention. For example, boat windows become hazy from exposure to salt water and a protective coating could eliminate this,' he says.
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