224 RUBBER AND 



rollers. The whole process thus calls for considerable 

 skill and judgment on the part of the operator in 

 regulating both the temperature and the action of the 

 machine. 



After undergoing a certain amount of mastication 

 the rubber is judged to be ready for mixing. In the 

 mixing process, the sulphur required for vulcanisation 

 is incorporated with the rubber, together with other 

 substances known as fillers. The rubber is passed 

 repeatedly between the rollers and the sulphur, and 

 filling materials are gradually sprinkled over it in the 

 form of a fine powder. The process of rolling is then 

 continued until the fillers are evenly distributed through 

 the rubber, and the whole substance has become nearly 

 homogeneous. 



Sulphur is introduced either in the form of flowers 

 of sulphur or as precipitated sulphur. Other substances 

 are used in the mixing for several definite purposes. 

 Thus various sulphides aid in the process of vulcanisa- 

 tion. These and other chemicals are also introduced 

 in order to impart definite qualities of toughness and 

 durability to the finished product. Among the most 

 important of these substances are zinc oxide, magnesia, 

 antimony sulphide and litharge (lead sulphide). Some 

 of these chemicals impart characteristic colours to the 

 rubber, whilst a further series of materials is used simply 

 for colouring. Thus red rubber may contain vermilion, 

 red lead or antimony sulphide ; whilst white or grey 

 rubbers usually contain zinc oxide, and black rubber 



