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Aviation History
1962
1962 - 1529.PDF
FLIGHT International supplement, 23 August 1962 Air-Cushion Vehicles ACVs Give Paint its Biggest Test This article, contributed by a specialist in the paint industry, shows how new finishes of great durability are being developed for use on air-cushion vehicles. AT FIRST SIGHT the part played by paint in the development of air-cushion vehicles would appear to be both humble and commonplace, but the value of its contribution is actually very great, because the thin film of the finishes is the first line of defence against corrosive elements which attack costly structures, especially at sea. Paint has never before been subjected to such severe tests as the operation of ACVs demands. There is a combined assault of weather—sun, rain, wind; a combination of sea and air similar to the highly-corrosive conditions affecting the waterlines of ships; bombardments of sand and pebbles; engine heat; the solvent onslaught from petrol, kerosine and hydraulic fluids; and oil contamin ation. Separately these conditions are arch-enemies of paint, but united in a formidable concerted attack on finishes measured in thousandths of an inch, their impact is so very great that paints of exceptional toughness and flexibility have to be produced. Research and development in the laboratories of the paint manufacturers never ceases. There is no time to sit back complacently on past achieve ments, because competition is world wide. So the restless experimenting goes on to improve the product and develop new techniques, methods and appli cations. Air-cushion vehicles presented a new and exciting challenge to the paint industry. The finishes available have emerged successfully from the trials of toughness, elasticity, resistance to abrasives and impact. The types selected have a resin base, like the polyurethanes v«th their highly-complex chemical structure. Household paints dry by the evaporation and oxidation of oils in the atmosphere; cellulose finishes dry o the evaporation of a solvent; but Polyurethanes achieve drying by their own chemical reaction—the result of mixing two components. Basically, the resin component is contained in the OH (oxygen-hydrogen) group. The linking is an NCO (nitro- gen-carbon-oxygen)/isocyanate bond which attacks the OH bond in the resin and links itself on. If there is an NCO group at both ends of the curing agent it can form a series of interlinking chains all round the final product. Expressed simply, the components form chemically a cubic lattice-work which gives the essential flexibility. The longer the chain lengths of the original resin, the more flexible the resin becomes. This is a finish to which ACV tests gave the "full treatment." It came through its severe trials unscathed, after what one of the technologists described as "tremendous turbulence and a shattering impact of sand and pebbles before the craft reached the sea." The finish was unmarked, and approval is to be sought from the Ministry of Supply for its use on air craft. The paint manufacturers' approach to their products is almost masochistic. Having made something they believe to be really good they attempt to break it down by series of exacting controlled laboratory tests. All schemes aim at a film as tough and thin as possible, as the weight/load ratio is an important factor, particularly in aircraft. Primers vary in thickness from 0.0005in to 0.0015in; fillers, if they are used, from O.OOlin to 0.0025in; finishes from O.OOlin to 0.002in. There are some of the tests employed before a can of paint is allowed out of the works:— A thinly-coated metal sheet is kept in a refrigerator for two hours at freezing point. It is bent back in a hinged mandrel to an angle of 180°. If there is the slightest evidence of cracking under microscopic examination the scheme is rejected. To test for surface hardness, the trial plate is scratched by a tiny hardened- steel sphere, no bigger than that in a ball-point pen, under a load of up to 51b. Some finishes have stood up to 161b. If there is a break through the paint, a voltmeter needle indicates that the metal is exposed. There are various tests for corrosion resistance. A test-piece is put into a cabinet with a fluctuating temperature and high degree of humidity. Other pieces are subjected to the heat from an arc lamp while water is sprayed on the finish. Some of the more conventional materials blister and discolour under this treatment and the inevitable break through to the metal follows. Other test-pieces are immersed in synthetic sea-water for l,000hr; some of the indoor tests last 2,000hr. They must show no signs of blistering, loss of colour or gloss if the product is to be passed. Tests are made on resistance to oil, petrol and fuels like methanol and kerosine. The test plates are immersed in beakers of oil at extreme tempera tures—the conditions they would have to withstand near an engine or exhaust pipes. Another important test is based on the powerful solvents in certain types of hydraulic fluids. One of them, the manufacturers say wryly, is a first-class paint stripper; but it leaves the polyure- thane finishes untouched. Apart from accelerated artificial weathering, there are natural weathering and ageing tests covering periods up to two years. Treated panels are exposed on roof-tops to all the vagaries of the English climate plus air pollution. Im pact tests range from sand-blasting to dropping a heavy hard steel ball on to a metal strip. With chemically cured materials like polyurethane, reaction takes place in a given time, so it is important to know the "pot life"—for how long the material is usable after mixing. Par ticulars are given on every can or drum. For the pot life test a sensitively- mounted "swizzle-stick" moves up and down a beaker of paint at high speed. The instrument records the time the paint remains fluid and, therefore, suit able to use. When the paint reaches a predetermined consistency the mixer is stopped. One of the MoS tests for Service air craft is for half the paint to be removed from the test-piece to discover if there is any corrosion or pitting of the metal. The retention of high gloss and colour are important pointers to quality. The essential qualities of the fine film of paint are toughness, flexibility, hardness and durability under all conditions. As is well known, a high gloss reduces skin friction. Hovercraft with speeds of 140kt are envisaged in the foreseeable future, and the glass-smoothness of good, light finishes will help to achieve this goal. 48
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