PLATINUM METALS REVIEW A quurterly survey of reseurch on the platinum metuls urrd of dwelopments in their applications in industry VOL. 5 JANUARY 1961 NO. 1 Contents Platinum Alloys in the Production of Viscose Rayon 2 Platinum Reforming Catalysts 9 Cathodic Protection of Water Heaters I2 Rhodium Plating to Specification I3 A History of Platinum 18 The Refractory Noble Metals and Rhenium I9 The Hydrides of Palladium and Palladium Alloys 21 Faraday’s Lecture on Platinum 26 Abstracts 30 New Patents 38 Communications should be addressed to The Editor, Platinum Metals Review Johnson, Mutthey & Co., Limited, Hatton Garden, London, E.C.1 Platinum Alloys in the Production of Viscose Rayon J THE SELECTION OF MATERIALS FOR SPINNING JETS By J. w. s. Hearle, MA., Ph.D., and A. Johnson, M.Sc.Tech., Ph.n. Manchester College of Science and Technology In 1884 Count Hilaire de Chardonnet continuous filament yarn has doubled again. patented a process for making artificial fibres The technical improvements in ordinary by the extrusion of a solution of nitro- textile filament rayon during these forty cellulose derived from mulberry leaves, the years were marginal, but the versatility of natural food of the silkworm. These were viscose rayon was being exploited extensively the first commercially successful man-made in other directions. The production of staple fibres, and the production of rayon by this fibre began in 1930, and by 1940 production method continued until the last nitro-cellulose was greater than that of continuous filament factory was destroyed by fire in Brazil in yarn. Diversification here included the mak- 1949. ing of fibres covering a wide range of lengths Other developments followed rapidly in the and thicknesses, so that they could be spun closing years of the last century. The cupram- on cotton, wool, worsted, flax, jute and monium process, which is still used to a small other spinning machinery. The addition of extent was patented in 1890, and the patents titanium dioxide could be used to dull the for the viscose process followed in 1891.In naturally bright appearance, and the introduc- this process, cellulose-usually obtained from tion of pigments into the spinning solution wood-pulp-is steeped in caustic soda, aged, gave fibres which could be blended into an and reacted with carbon bisulphide to give infinite range of colours. cellulose xanthate which is dissolved in Another important development was the caustic soda. The resulting viscose solution production of Tenasco-a strong industrial is then squirted into an acid bath where it yarn-in 1935. By the end of the war, fibres coagulates as the cellulose is regenerated. of this type had almost completely replaced The filaments so produced are soft and weak, cotton in the big market for tyre-cords. but stretching them increases their strength, The years after the war were devoted to and after washing and drying they are ready increasing the depleted production facilities. for use. However, by 1953, improvements in the The production of viscose rayon was strength of rayon were needed to meet the started in Britain in 1905 by Courtaulds. threat of competition from nylon in tyre- Initially it was an expensive fibre made as a cords. Fortunately earlier academic work had continuous filament yarn which competed provided a basis on which to work. with silk. By 1920 world production was Rayon is unique among man-made fibres 33 million lb per annum, but the price was in that its production is accompanied by a zoo pence per lb: twenty years later produc- chemical reaction-the change from cellulose tion had increased to 1,200 million lb per xanthate to cellulose. By varying the condi- annum and the price had fallen to 33 pence tions under which the reaction proceeds, it is per lb. Since then the world production of possible to vary the fine structure of the Platinum Metals Rev., 1961, 5, (l), 2-8 2 Many thousands of platinum alloy spinning jets are in use in n modern viscose rayon plant. The illustration shows one unit engaged in spinning continuous $lament in a Courtaulds factory fibres. Ordinarily, rayon has a structure com- carpets, blankets, non-woven fabrics for posed of separate skin and core. In the skin interlinings, surgical swabs, filter fabrics, the texture of the arrangement of the long- ropes, hose-pipes, conveyor belts-the list is chain molecules in crystalline and non- too long to complete. Rayon is now second crystalline regions is finer than in the core, only to cotton in its scale of production and and by increasing the proportion of the skin diversity of applications. structure, stronger fibres can be made. This and other improvements have resulted in a The Heart of the Process rise in the strength of rayon tyre-cords from The heart of the rayon production process 25 lb to 40 lb, and have succeeded in main- is the spinning jet through which the viscose taining the price advantage of rayon over solution is extruded into the coagulating nylon in tyres. solution. Similar developments are taking place, The compositions of these two solutions although more slowly, in textile rayons. All- vary somewhat according to the particular skin fibres should have greater durability, type of viscose rayon being made, but a and their round cross-section changes the reasonably typical viscose would contain appearance and reduces soiling. All-core about 6.5 per cent of alkali in the form of fibres such as the so-called polynosic fibres sodium hydroxide and about 7.5 per cent of have much better dimensional stability, cellulose (present, of course, as the xanthate). especially on wetting. And fibres with an The coagulating solution is essentially dilute asymmetrical skin have a permanently built- sulphuric acid containing about 10 per cent in crimp. Other techniques give a collection by weight of the acid, together with about of fancy yarns. 20 per cent by weight of sodium sulphate Viscose rayon is not really one type of and smaller amounts of zinc sulphate and fibre: it is a great variety of fibres which have glucose, the remainder being water. Thus the a vast range of uses-dress-wear, furnishings, spinneret is subject on the one side to about Platinum Metals Rev., 1961, 5, (l), 3 A simple demonstration of the formation of a multi-jihment yarn by pumping viscose solution through the holes in a jet immersed in an acid bath (Courtaulds Ltd) 1.5 N sodium hydroxide and on the other to In this case a solution of cellulose in cu- about 1.5 N sulphuric acid. prammonium hydroxide is used, but although These are both severe conditions, which the final coagulation of the filaments requires bring about chemical attack of most metals. an acidic bath they are first spun simply into The use of steels, for example, is immediately water. Thus, the spinneret is not in direct precluded for the manufacture of spinnerets contact with acid, and so may be fabricated for viscose rayon. In this respect, viscose from steel. Another difference between the differs from other manufactured fibres where cuprammonium and viscose processes which steel spinnerets may be used. permits steel to be used for the former con- Thus, for the manufacture of cellulose cerns the diameter of the holes in the spin- acetate filaments, the spinning solution con- neret. In the cuprammonium process this sists simply of the acetate dissolved in either is of the order of 0.8 to 1.00 mm, whereas acetone or methylene chloride, both of which in a viscose spinneret the diameter may be as are without chemical action on steels. The small as 0.03 mm, so that the removal of solution is extruded into warm air, when the metal by corrosion is of greater significance. solvent evaporates, so again there is no corrosive action corresponding to that of the Resistance to Chemical Action acid coagulating bath in viscose rayon To return to the viscose process, we can manufacture. see that spinnerets must be fashioned from A process more akin to viscose spinning is materials that are capable of withstanding the manufacture of cuprammonium rayon. the simultaneous chemical action of 1.5 N Platinum Metals Rev., 1961, 5, (l), 4 caustic soda and 1.5 N sulphuric acid. So behaves as an electrical cell, most simply far as metals are concerned, this at once regarded as a form of hydrogen ion concen- limits the choice to the noble metals, platinum tration cell. As with all concentration cells, and gold, and some of their alloys, and also the electrode in contact with the more con- tantalum protected by an oxide film. centrated solution will assume a positive Two alloys in particular have been widely charge, so that the spinneret becomes used; 10 per cent rhodium-platinum and polarised, the surface in contact with the 30 per cent platinum-gold. The former, alkaline solution bearing a negative charge because of its higher cost, is normally re- and that on the acid side a positive charge. stricted to use where the conditions are most With steel spinnerets, of course, these exacting and the cheaper one is used where electrochemical effects would in themselves the conditions are less critical. cause corrosion, enhancing the deleterious However, although such alloys will resist effects of direct chemical attack. The noble direct chemical attack by acid and by alkali, metals are not corroded, but nevertheless a and on that account are suitable for spinneret current will flow, resulting in electrolysis of manufacture, other factors also have to be the viscose solution passing through the jet. taken into consideration. One of these, and The most serious result is that the negatively one which mitigates against the use of any charged xanthate ions will be attracted to the metal for spinneret manufacture, is the positive surface of the spinneret, where they occurrence of electrochemical phenomena.