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Aviation History
1953
1953 - 1579.PDF
FLIGHT, 4 December 1953 733 A VERSATILE PLASTIC Polyvinyl Chloride: Some of its Applications in Aircraft Construction and Operation IT was about twenty years ago that methods of softening and processing polyvinyl chloride were first discovered. During the past two decades tremendous strides have been made in the production of improved compounds and the development of new applications for this very important group of plastics. By varying the compounding materials and the type of resin, numerous combinations of desirable properties can be obtained. The primary vinyl materials are made from carbide, chloride and hydrogen, and are produced in the form of fine white powders or milky fluids. These are processed by plastics manufacturers into a number of characteristic secondary forms from which, in turn, are produced a variety of articles used in the aircraft industry. P.V.C. can be calendered to produce thin, flexible sheeting; it can be applied as a coating to cloth or paper; it can be extruded to produce cable, wire or flexible tubing; and it can be moulded by injection processes. Products made from P.V.C. have properties which are derived from the basic vinyl polymer. In general, they are extremely fire-resistant, practically non-ageing, and unaffected by ozone. They are also highly resistant to weather exposure, sunlight, most acids, alkalis, oils, and solvents, as well as to salt spray. They are resistant to abrasion and are not attacked in tropical climates. In its plasticized form P.V.C. has varying degrees of flexibility, but it can be compounded without a plasticizer to produce rigid materials. Unplasticized P.V.C. possesses many of the properties of the flexible grades to an enhanced degree. Because of its high resistance to chemical attack, dimensional stability and good mechanical properties, it is replacing metals in many appli cations. It is easily moulded and can also be calendered into sheets or extruded into piping or conduit. Less dian one-fifth as heavy as steel, it has a high strength-weight ratio, and its other attrac tions include permanence and low maintenance costs. Apart from its low specific gravity, two of the most important properties of P.V.C. from the standpoint of employment in air craft are low inflammability and low flame-spread. Although the material will soften under heat, unlike rubber it will not propagate flame. This property is illustrated in a recent test in which a piece of triple-core electric cable, insulated and sheathed with a Geon 101 composition, was suspended for ten minutes in a Bunsen flame at about 1,800 deg C. The conductor became red- hot, but the fire did not spread along the P.V.C. covering. This experiment clearly shows that, while P.V.C. insulation can be burned if very fierce heat is applied, it does not support combus tion of its own accord. There is no spreading of the fire beyond the immediate influence of the applied flame. Results of Fire Tests A comparison of the flame-retardant properties of plastic- insulated and rubber-insulated electric cable was carried out at the Fire Research Station of the Joint Fire Research Organization on behalf of the British Plastics Federation. Part of the investigation related to the application of an external source of heat and was carried out substantially in accordance with the test specified by the Underwriters' Laboratories Inc. of America in their Standard for Thermo-Plastic Insulated Wires (3rd edition, March 1945). This test shows that plastic-insulated cables of the type tested have a greatly superior resistance to external heating than have corre sponding rubber-insulated cables. All the plastic-insulated cables reached the standard flame-retardant properties as defined by Underwriters' Laboratories Inc., while all the comparable rubber- insulated specimens failed to reach this standard when subjected to the same test. It follows that fire hazards in aircraft can be greatly reduced by using P.V.C.-covered cables and wires, passed through conduits constructed of rigid P.V.C. Though its resilience is not so great as that of rubber, the sheathing is tough and flexible and will stand up to most cable applications. An additional advantage in certain situations is its resistance to oil and grease. Improved techniques have been developed for depositing vinyl materials on cloth, and P.V.C.-coated fabrics can now be very economically produced. The risk of fire can be further reduced by using materials made by applying P.V.C. to Fibreglass instead of to an ordinary cloth backing. Such coated fabrics have been officially approved for use in Service aircraft and are superior to many other kinds of synthetic seat-upholstery materials in their high degree of resistance to cracking, their excellent abrasion- resistance, and their exceedingly long life. Other advantages are that they are hygienic, can be washed with soap and water, do not scratch or stain readily, and are resistant to most acids as well as to grease and ink. They are obtainable in a variety of different colours—creams, reds, greens and blues— and also in multi-colour effects. These properties render them highly suitable for use as decorative coverings for walls, interior window frames and fair ings. Cloth treated with P.V.C. can also be used in aircraft as a covering for wood or metal stands and tables. For panelling their Comet, Argonaut and Hermes aircraft, B.O.A.C. specified vinyl leathercloth because it meets their requirements of toughness and high strength, combined with low weight, precise finish, consistency of colour, and low fire-hazard. An example of leathercloth approved by B.O.A.C. is Duranide, made with Geon P.V.C. Rubber latex foam seating, as such, absorbs water. This tendency can be prevented by a two-coat treatment with a P.V.C. material. Two hundred and fifty foam seats, used with slip covers, were recently supplied for furnishing a Constellation order. Tests of fire-resistant finishes for aircraft have been described.* A fireproof coating used experimentally as a sealant for fire walls, hot air ducts, electrical junction boxes and oil tanks con sisted of a primer coat of Buna N base lacquer, a layer of putty, a wrapping of Fibreglass cloth, a second layer of putty, and a top coat of a Buna N lacquer applied after three days' curing to release all solvents. Disadvantages of this coating were excessive weight, complexity of application, and loss of adhesion at high temperatures. Paint-type coatings for less critical applications, such as plywood partirions, are designed to "puff" and form an insulating shield in the event of fire. A top coat of vinyl resin- base lacquer is applied over the fireproof paint to increase resist ance to water and hydraulic oil. A rigid vinyl sheet is used in B.E.A. Pionair Leopards, which are quickly adaptable for either passenger or freight work. For this dual function fitments must be strong and attractive. The sheeting is therefore specified for window coamings, bulkhead panels, kicking plates, luggage compartment and toilet linings, roof-trims and labels. Pressure-Sealing Application Cabin pressurization necessitates use of a special type of airtight bung when cables are passed through a pressure bulkhead. A suitable fitting has been produced in P.V.C. The holes through which the cables are passed are of a strictly controlled size and over these a P.V.C. membrane is moulded, this membrane being punctured just before the cable is passed through. British Standard Sp.48, Plastics Grommets for Aeronautical Purposes, specifies requirements for the material, dimensions and finish of a range of grommets. The standard includes tests for the material from which the grommets are required to be manu factured, namely, plasticized P.V.C. or copolymers having vinyl chloride as their major constituent. An interesting application in the United States is the use of a vinyl-coated fabric in die manufacture of a quick.y erectaDle shelter for fighters. The shelter is designed primarily for use in the Arctic. It weighs 4,824 lb and will withstand a 70 m.p.h. wind and a snow-load of 30 lb/sq in. When erected, it provides enough room to house the largest type of fighter in use and gives sufficient working space for maintenance. Also from the United States come reports of a special vinyl- proofed duck fabric which is placed beneath steel or aluminium landing mats to keep down dust on runways used by jet aircraft. Besides being flame-resistant, the coated duck is impervious to fuels ari"cl chemicals. Foul-weather clothing made from fabrics impregnated with P.V.C. is replacing conventional oilskins in the Royal Navy. These lightweight P.V.C. "oilskins" are also very suitable for use by aircraft ground staff. They withstand prolonged exposure to fierce sun and Arctic frost, tropical storms, abrasion, oil and flame, and they can be stored away without deterioration. The rime cannot be far distant when the crews of aircraft will wear flying suits made from cotton impregnated with P.V.C., the fabric being completely encased. Suits of this material are as hard-wearing as those of the conventional type and can be produced at very much lower cost. In both its flexible and rigid forms, P.V.C. is unquestionably making an important and timely contribution to the progress of the British aircraft industry. Yet this plastic is still a relatively new material, particularly in its rigid form. Research and development are still continuing. A.G.T. *Jones, J. A., and Nisioander, R. V. "A.S.T.M. Bulletin," 19S0.
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