RUBBER 369 



Vulcanisation is effected by two principal processes. In the 

 one the rubber, made into a stiff semi-solid solution in coal-tar 

 naphtha, is mixed with the requisite quantity of flowers of sulphur 

 together with certain other materials, such as litharge (lead 

 oxide), zinc oxide, magnesia, and antimony sulphide, some of 

 which seem to accelerate combination. The mixture is exposed 

 to a steam heat and is then usually spread by means of rollers 

 on a cotton cloth, whereby a sheet is formed from which the 

 majority of rubber articles are made. The cloth is finally hung 

 in chambers heated by steam pipes, and the solvent naphtha 

 employed at the beginning of the process dries off. 



The other process, called cold vulcanisation, consists first in 

 spreading a thin layer of rubber paste on cloth, and then by 

 means of rollers passing the coated material through a trough 

 containing a mixture of sulphur chloride, S 2 C1 2 , and carbon 

 bisulphide. In this case it is not merely the sulphur which is 

 added on to the rubber molecule, but the chlorine as well. The 

 formula of the compound produced is (C 10 H 16 )wS 2 Cl 2 , but there 

 is great difference of opinion as to the value of n in the formula, 

 that is as to the number of molecules of sulphur chloride which 

 are associated chemically with the rubber molecule, and whether 

 the compound so formed is mechanically united with more 

 rubber. In fact the exact nature of the vulcanised product of 

 the cold curing process is still a subject for further investigation. 



Several " substitutes " for rubber have long been used for 

 incorporation with true rubber in order to cheapen the material. 

 Of these the most interesting and important is a peculiar tough 

 substance produced by the action of sulphur chloride on various 

 vegetable oils. But several other additions to rubber are em- 

 ployed when toughness and tensile strength is not the most 

 important quality looked for in the material. When, for example, 

 rubber is required as an electrically insulatory material, as in 

 coating cables, various resins, nitrocellulose, bitumen, and other 

 substances are used. 



A considerable quantity of rubber is reclaimed from vulcanised 

 waste by heating it with an alkaline solution, and subsequently 

 washing the desulphurised mass. 



This, however, is not to be regarded as a treatise on the 

 manufacture of rubber, and those who are interested in the 

 scientific principles of the industry would consult with advantage 

 such a work as Dr. Schidrowitz's Rubber (Methuen and Co.). 



2B 



