23 2 RUBBER AND 



becomes soft and plastic at the temperature employed, 

 and must therefore be supported in order to preserve 

 its shape. Powdered French chalk is largely employed 

 as a bed for the rubber articles to lie on. Closed objects 

 such as balls, india-rubber dolls and other toys, are 

 vulcanised in moulds, and before the cavity of the 

 rubber is closed up some substance is introduced which 

 will volatilise at the temperature of vulcanisation, and 

 so press the object firmly against the mould. The 

 outer covers of motor tyres are vulcanised in heavy 

 presses, in order that their substance may be firmly 

 compacted during the process. The most complicated 

 of all vulcanising machines is the autoclave press, which 

 consists of a hydraulic press completely enclosed in a 

 steam pressure chamber. In this way the difficulties of 

 obtaining an even temperature in a press heated by 

 steam pipes are overcome. Vulcanisation by steam is 

 generally carried out at a pressure of three to four 

 atmospheres (45 to 60 Ibs. per square inch) corre- 

 sponding to a temperature of I34C. to 144 C. The 

 temperature and pressure are first raised gradually, 

 and afterwards kept high for three or four hours. Self- 

 recording thermometers and pressure gauges are fitted 

 to the apparatus in order that the process may be kept 

 under complete control. The time required for vulcani- 

 sation varies according to the source of the rubber, 

 being shortest in the case of Hevea rubber. The 

 previous treatment of the rubber also affects the 

 process. Thick articles naturally require to be heated 



