470 



are two methods of producing the desired result, known as the 

 heat cure and cold cure, respectively. 



IN THE HEAT CURE. 



23. The raw rubber and finely powdered sulphur are mixed 

 together intimately on a mixing or a masticating machine. If other 

 ingredients are to be added to the rubber it is done at the same 

 time that the sulphur is incorporated. Chemical union between 

 the sulphur and the rubber takes place neither during this mixing 

 nor afterwards, as long as the mixture is kept cold. If however, 

 it be heated to about 300 F. chemical union takes place slowly 

 and the new product, vulcanised rubber, is formed. By far the 

 greater bulk of rubber is vulcanised in this way. The hot cham- 

 bers in which the actual heating and vulcanisation are carried out 

 are of several types, and differ in the way in which the heat is 

 applied. Where pressure has to be exerted on the rubber during 

 vulcanisation the goods are vulcanised in moulds, between large 

 plates of iron, which are hollow and heated by steam. In other 

 cases, large chambers heated by steam are used and into these the 

 rubber goods, placed on trays and smothered in French chalk, are 

 taken. Fabrics coated with rubber — such as sheeting and mack- 

 intosh cloth — are wound round a large iron drum and immersed in 

 water, which under pressure is heated to the required temperature. 

 Long tunnels, 50 or 60 feet long, dry heated by steam, are used 

 for vulcanising hose pipe and lengths of tubing which cannot be 

 coiled. The temperature is regulated so as to slowly rise to about 

 300 F., and after maintenance at that point for a period varying 

 from half to three hours, it is slowly allowed to drop again. 

 During vulcanisation a portion of the sulphur combines with the 

 rubber and forms the new addition compound, which is quite 

 distinct from raw india-rubber, and from which the sulphur 

 cannot be removed by any known process. Although the whole 

 of the rubber is acted upon by the sulphur to greater or lesser 

 degree, the action is slow and the whole of the sulphur present 

 is not used up during the short period that the vulcanisation lasts, 

 and free uncombined sulphur remains disseminated throughout the 

 vulcanised product. A prolonged period of heating during 

 vulcanisation diminishes this excess of sulphur, and leads to the 

 production of more highly vulcanised rubber. The more sulphur 

 which vulcanised rubber has used and actually combined with, the 

 darker and harder the product until the extremes of vulcanite and 

 ebonite are reached. From partially vulcanised goods the excess 

 of free sulphur can be chemically extracted, and this is one of the 

 operations in " recovered " vulcanised rubber : the combined 

 sulphur, however, remains always in the recovered rubber. The 

 recovery of rubber, therefore, is an operation by which the mecha- 

 nically mixed substances, such as the excess of sulphur and the 

 fillings with which the rubber was mixed in manufacture, are 

 wholly or partially removed, and the residue resulting is worked 

 up into a form in which it can be blended with new rubber, and 

 act as a substitute for a portion. 



