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AIRCRAF T ENGINEERIN G 163 B y G. FitzGerald-Lee, F.R.S.A., A.M.I.E.I., A.RAe.s. H E object of this paper is to trace, with also leads to acetyl salicylic acid, or what is as few technical terms as possible, the commonly known as "aspirin". main points of the various plastics belong I n this article the author Finally, from coal and lime, through calcium ing to the acetylene group, so that designers carbide, acetylene or ethylene, aldol or acetal describe s for engineers and technicians who have not the good fortune dehyde, and acetic acid and its anhydride, we the composition and general to be chemists may more easily understand the arrive at the major plastic known as "cellulose practical applications of these comparatively acetate" . characteristic s of one of the new materials. Cellulose acetate is cellulose treated with mai n classes of plastic ma To begin with, it must be understood that acetic acid and acetic anhydride, dissolved with plastics are generally divided into two groups: solvents and plasticizers. It may with benefit terial s which is widely used The thermoplastics—which are formed by be mentioned here that a cellulosic plastic has In aeroplanes to-day heating, can be re-heated after forming, and three constituents: The "ester" (cellulose re-formed almost ad lib—such as celluloid, acetate), the "solvent", and the "plasticizer" xylonite, rhodoid, cellophane, and perspex; and (a very important constituent affecting the the thermosetting plastics—which . are also resultant physical, mechanical, and electrical formed by heating bu t cannot yet be re-formed mouldings, and is more generally useful than properties). It may further be explained that by th e application of heat or any other means— vinyl acetate; and finally the "polyvinyl plas cellulose acetate is said to be the "acetic acid probably th e best known of which is th e thermo ester" of cellulose, an "ester" being a compound tics", truly thermoplastic materials used for setting variety of bakelite. Further, one must formed by the condensation of an alcohol and disc records, floor coverings, and tubes. beware of the acetylene plastics being given in a n acid, with the elimination of water. The Acetylene can be readily "reduced" to one's mind undue prominence in comparison cellulose is obtained from cotton "linters" (the "ethylene " (C H ), or "carburetted hydrogen", 2 4 with other equally important groups such as second cut of fibre from cotton seeds after the which is a colourless gas with a sweet smell; th e synthetic rubbers, the resins, the bitumen removal of the spinning fibre). Cellulose is ethylene is generally, however, obtained by group, and the casein group. heating alcohol with sulphuric acid. Ethylene is richest in flax and hemp, rathe r less in jute, and a very important gas, being in itself the basis still less in bamboo and esparto; the greatest The acetylene thermoplastics are based on of polyethylene, styrene, vinyl chloride, and cotton producers are the United States, India, two substances: coal, and lime. and Egypt. vinylidene chloride, all important products in The story of coal is known too well to spend plastics, but rather off the acetylene track. many words on, but it may be pointed out for The two methods generally used for the our present purpose tha t it contains from about Ethylene directly produces "ethyl benzine" manufacture vary according to the type of 60 per cent to 90 per cent of carbon (C) accord (C H ), which is the basis of the styrenes pro cellulose acetate required. The first, which pro 8 10 ing to its quality, and when chemically treated, duced by its thermal decomposition; also "vinyl duces the far more important variety known as it produces three important commodities, viz. benzine", and the "polystyrene plastics". "acetone soluble cellulose acetate" is manu coal-gas, coal-tar, and coal-oil. But for the These last are widely used for electrical equip factured by the cellulose being acetylated in purpose with which we are here concerned, the ment, being resistant to acids and alkalis, th e presence of a catalyst, and a diluent (which main use of coal is for the reduction to coke. accurately mouldable, either transparent or dissolves the acetate as it is being formed). The When coal is heated out of contact with the air opaque, and of many colours. second variety, known as "fibrous cellulose it produces, amongst other things, coke; and acetate " is a process in which the catalytic From acetylene also are produced "aldol" th e quality of this coke depends on the nature acetylation takes place in a non-solvent diluent, (and from that "croton aldehyde" and "butyl and quality of the original coal, and the temper th e esterification occurring without any appa alcohol") and "acetaldehyde". Acetaldehyde atur e and the rapidity of the coking. Apar t from rent change in the fibrous structure of the cellu (C H O) is made by the oxidation of ethyl 2 4 the type of coal which softens greatly on heat lose itself. alcohol, and is itself oxidizable into acetic acid. ing, th e best coke is produced by the most rapid 'Aldehydes " are formed by the oxidation of Cellulose acetate was discovered in 1865, obtaining of th e highest temperatures. alcohols; acetaldehyde is also produced cata- bu t did not become really commercial until the lytically from acetylene, catalysis being the The second fundamental of the acetylene beginning of the Four Years' War, when it acceleration or retardation of the decomposition plastics is lime, or "calcium oxide" (CaO), began to be used extensively in the manufac and recomposition of a compound by means of which is obtained by heating limestone (or tur e of lacquers for aircraft dopes. an unchanging substance known as a "catalyst". "marble"); it is powdered by the atmospheric As we know it to-day, cellulose acetate is a moisture, and becomes th e commercial CaH O , "Ethy l alcohol" (C H O), allied to acetalde tough thermoplastic, with an impact value 2 2 2 6 but it is the calcium oxide itself which directly hyde, is the "spirits of wine" which is the main (1·5 ft/lb.) equal to cast aluminium alloys, an concerns us here. constituent of "methylated spirits"; it is the ultimate tensile strength (5 tons/sq. in.) equal basis of the preparation of many organic When these two fundamentals, coke an d lime, to cast aluminium, and a compressive strength substances, and was in 1933 granted its own (100 tons/sq. in.) equal to twice that of iron. are fused together in an electric furnace, they British Standards Institution specification, I t is half the weight of aluminium, its specific produce the crystalline, grey substance known as "calcium carbide" (CaC ), or simply "car No. 507. gravity being only 1·3, and it is non-inflamma bide", and this substance is the basis of the ble. I t is easy to work, has no smell, is good for Th e next stage towards the higher acetylene acetylene and the ethylenes. plastics is Acetic Acid (C H O ), which can be machining, and is a good insulator. It may be 2 4 2 produced by the oxidation of the fermentation made in any colour, and may be transparent, Acetylene (C H ) is made by mixing calcium 2 2 of ethyl alcohol, or by the oxidation of acetalde translucent, or opaque; and although it is carbide and water, that is, CaC and H O, the 2 2 hyde (C H O+O); it is universally known, in its decomposed by strong acids and alkalis, it equation being as follows: CaC +2(H O)→ 2 2 2 4 C H + Ca(OH) ; thus the acetylene is formed, highly diluted form, as "vinegar". resists weak acids and alkalis. 2 2 2 with the elimination of the calcium. This Acetic acid leads to the useful solvent known I t is used in aircraft, so far, for cowlings and "hydrocarbon", acetylene, is a colourless gas, as "acetone" (C H O). Acetone is made cataly- fairings, air intakes, fillets, windows, boxes, 3 6 with a distinctive smell vaguely like garlic; tically from alcohol, or by the dry distillation etc., and is th e subject material of three aircraft it is highly inflammable. It combines readily of calcium acetate or barium acetate. I t is non specifications (B.S.S. D.50, B.S.S. F.56, and with hydrogen, giving (+H ) ethylene, and toxic, and is used, amongst other things, to D.T.D. 315). Plasticized cellulose acetates in (+H ) ethane. It may not be out of place to make chloroform, cordite, and hold dissolved commerce include, among many lesser known mention here that acetylene should never be acetylene (for fusion welding). In 1933 acetone ones, Bexoid (sheet, rod, tube, film, moulding passed through copper tubing, as it thereby was accepted by the British Standards Institu powders), and Isoflex (heat insulation material), forms "copper acetylide" (CuC ), a violently tion as specification No. 509. both of BX Plastics, Ltd.; Cellomold moulding plosive gas. From this branc h ar e also derived th e "metha- powders, of F . A. Hughes, Ltd;, and the sheets, From acetylene are produced the "vinyl crylate " plastics, which arc non-corrosive to rods, tubes, and moulding powders of Erino- Metals", which arc the bases of an important fluids, and therefore useful in the making of fort (Erinoid, Ltd.) and Celastoid (British Celanese, Ltd.). A very well-known cellulose ss of vinyl resins; also "vinyl acetate" such things as dentures. acetate is the "rayon fabric" produced by etylene+aceti c acid), which is a glass-clear I n rather the same way as acetic acid, "acetic British Celanese, Ltd. , known a s "Celanese".Th e inous solid, very stable to heat, light, and anhydride " can be produced; this leads to American cellulose acetates include Plastacele, rather action; thirdly, "vinyl chloride" various "ester solvents" which have low volati Tenite, and the thermoplastic variety of Bake cetylene + hydrochloric acid), which is a non- lities, and "plasticizers" used in the manu lite; France has Rhodoid, and Germany Trolit. plammable material used for extrusions and facture of plastics (as we shall see later). It
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Jun 1, 1944
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