New Development in Rubber Reclaiming

Abstract

Abstract The history of rubber reclaiming has been given by Alexander, and it is therefore intended only to make a brief reference to the well-known acid and alkali processes, the latter having proved to be by far the most successful method of reclaiming vulcanized rubber. The alkali process, introduced by Marks in 1899 had the outstanding advantage of simultaneously removing, from the waste, free sulfur, of which from 3 to 5 per cent was present, of destroying the fabric and of plasticizing the waste, this being accomplished by heating in a closed vessel under steam pressure with caustic soda. The acid process destroyed only the fabric, and plasticization by heat was performed in another operation in a separate vessel: by this method little or no free sulfur was removed from the waste. This latter disadvantage has since disappeared, for, with modern rubber compounding, there is little free sulfur left in vulcanized rubber. In recent years there have been widespread changes in the manufacture of rubber goods, particularly in the case of pneumatic motor tires, which are now the main source of raw material for reclaimed rubber. Organic accelerators, anti-oxidants and carbon black have made vulcanized rubber a very tough, resilient article, extremely resistant to aging and to heat, one of the chief agents used in rubber reclaiming. For example, modern motor tire treads, which are heavily reinforced with carbon black, are not so amenable to alkali and heat treatment, and tire treads reclaimed by this process have finally to be plasticized and made homogeneous by a much longer mechanical milling and refining treatment than was formerly necessary; in spite of this the treads still produce a proportion of rough, unrefinable material which has to be separated and rejected from the final reclaim. This increase in milling time, combined with lower finished production due to rejected unrefinable material, has had the effect of greatly increasing the manufacturing cost of the reclaim. Research was therefore started with the object of evolving a process which would successfully offset these difficulties caused by the changes in the properties and characteristics of the initial waste. The result of this work has been the creation of reclaimed rubber from which the disadvantages mentioned have been largely eliminated.