The Rise of RAYON
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Methods by Which Viscose Process Yarns, Now Used Extensively in Commercial Tyres, are Produced
WHEN the ordinary man thinks of rayon, li:s mind turns to stockings and underwear, and it would be surprising if there had not been a number of initial misgivings when the use of such yarn for heavyduty tyres was announced. In fact, compared with those used for dress goods, tyre yarns are ordinarily composed of many more filaments and are of a much higher dernier. The " demier " is the standard unit of measurement for rayon yarns, and is based on the weight, in grammes, of a 9,000-metre length.
Rayon is, of course, being used in increasing quantities as a reinforcement material for tyres, conveyer belting, transmission ropes, electrical insulating fabric and similar materials. For these purposes, high-tenacity yarns, which are stronger than those used for domestic fabric, are employed. These high-tenacity viscose rayon yarns are produced at a number of the factories of Courtaulds, Ltd., under the trade name of Tenasco.
Stronger Than Cotton
An important feature claimed for rayon in tyre yarns is that at high-running temperatures its strength becomes proportionally greater than that of cotton, so that designers can afford to make the tyre walls thinner. This helps to keep down the running temperature and the rate of degradation due to heating. Severe and successful tests in this respect were carried out during the war with heavy-duty and aeroplane tyres.
Rayon has been used in industrial materials for a number of years, but it was not until the 1939-45 war that its application in this field was widened to any great extent_ The specialized needs of war-time brought an increase in the demand, but rayon was not generally accepted until it had proved its worth under operational conditions.
In the rayon industry, incidentally, a ' poor view" is taken of people who describe the material as "artificial silk," for " rayon " is the accepted term of the Textile Institute for "all fibres for textile use which are not of natural occurrence" and, further, the yarn produces many fabrics which have nothing in common with silk as most of us know it.
60 Years of Development
In the 1880s a French concern started commercial production of a rayon fibre in France, England, Switzerland and Belgium, but it was not until the early days of the 201h century, following upon the inventions of two British chemists—C. F. Cross and E. J. Bevan —that the commercial manufacture of rayon became a success. Cross and %van had invented viscose solution, whilst spinning into rayon thread was developed by C. H. Steam and C. F. Topham; the latter was also the patentee of the spinneret.
Courtaulds, Ltd., purchased the viscose processes in 1904 and viscose now accounts for the greater part of the rayon output both in Britain and abroad. Other well-known processes are those working with cellulose acetate and cuprammonium.
Spruce wood from Scandinavia or
Canada is the basic material behind viscose rayon, but it goes through many treatments in the pulp mills overseas before being transformed into cellulose. That material arrives in this country looking something like thick blotting paper and, after being cut to standard dimensions at the factory, is immersed in a solution of caustic soda which, having dissolved the unnecessary elements, merges to form what is known as alkali cellulose.
By means of hydraulic presses and grinders, this material is reduced to crumbs, which are carefully stored under regulated atmospheric conditions, so as to allow the chemical action of the caustic soda to become complete.
In due time these particles are placed in large hexagonal churns and a quantity of carbon bisulphide is added, and when the churns turn, the creamy-white material changes to an orange red sub
stance which is soluble in water. The contents have been transformed into cellulose xanthate.
Emptied into mixing vessels where paddles keep the liquid continuously circulating, the cellulose xanthate is dissolved in a caustic soda solution. Once again its nature changes, appearance and consistency becoming similar to honey—and this is viscose.
During the storage period of several days in specially conditioned vaults, the viscose receives a thorough filtration which removes undissolved cellulose or other solids. To make certain that all is well, several filterings take place and vacuum pipes extract any air or gas that may be present in the solution.
Next, the viscose is forced through the spinneret. This is shaped something like a top hat and is about the size of a thimble, and it may contain as many as 500 minute holes. The spinneret is usually made of a platinum-gold alloy and the number of holes decides the number of filaments in the viscose continuous-filament rayon yarn which emerges. Great care is necessary ir filtering,
Rayon "Cakes'
Leaving the spinneret, the filaments run over glass wheels and down a tube into a revolving container. By movement of the tube and box, the thread is twisted to a predetermined extent and it is then wound into what are known as "cakes," afterwards being dried and tested.
Bases for cuprammonium rayon are cotton linters or wood pulp, which by various treatments are reduced to pure cellulose, afterwards being mixed in a solution of copper sulphate and ammonia. After being forced through a spinneret, the solution comes into contact with water and partly sets into a filament, at the same time being stretched to reduce the diameter. Setting is completed by means of an acid bath.
In the cellulose acetate process the cellulose obtained from cotton linters or spruce wood is converted by chemical action to a heavy liquid. In this way cellulose acetate Rake is produced. Flakes are then dissolved in acetone, the mixture afterwards being forced through a spinneret into a hut air chamber, where the heat evaporates the acetone, leaving solid filaments which are drawn together and wound on a bobbin as finished yarn.
Spun RayOn Yarn
Whilst rayon yarn is a continuous filament, the textile industry also use rayon staple, manufacturing methods being the same up to the spinneret treatment. Filaments from several spinnerets are drawn together, and not twisted, into a finger-thick rope, which is chopped into lengths before final treatment and dispatch to spinning mills. After spinning on the conventional machinery, the product is known as spun rayon yarn.
To-day, rayon is in as great demand for industrial purposes as it is for dress and domestic materials, and commercial use has more than proved that it is not merely a substitute, but is a fibre with its own particular merits.