RUBBER MANUFACTURE. 97 



resiliency, and therefore their value for specific purposes, after 

 vulcanisation. 



Experiments conducted by Ditmar, to test the effects of 

 resin on the vulcanisation of rubber, tend to show that, provided 

 the resin content of rubber does not exceed 7 per cent, it has no 

 injurious effect upon the vulcanisation of rubber ; but that the 

 strength of rubber is considered to be diminished and exhibits a 

 marked tendency to oxidation by containing a higher percentage 

 of resin. 



Beadle and Stevens state :* " Now without denying the exist- 

 ence of a correlation in properties between a sample of caoutchouc 

 and vulcanised rubber prepared from it, it must be remembered 

 that the properties of caoutchouc are profoundly altered by 

 vulcanisation, and the properties of the vulcanised product are 

 largely within the control of the manufacturer. For this reason 

 we should rather interpret the word ' nerve ' to be an expression 

 applied to those physical properties of caoutchouc which are 

 preserved in the vulcanised product. It is evident that whatever 

 excellent qualities raw rubber may possess, these will be value- 

 less unless they outlive the vulcanising process." 



On arrival at the manufactory crude rubber is immersed in 

 hot water for periods varying between ten and twenty-four hours. 

 It is then passed through heavy iron grooved rollers over which 

 a constant supply of water flows. The rollers revolve at different 

 rates of speed which tears the rubber to pieces and the water 

 washes away mechanical impurities such as sand, dirt, bark, &c. 



The loss in weight which rubber suffers in this process varies 

 from i to 3 per cent, in plantation Para rubber to as much as 

 60 per cent, in inferior grades. 



The rubber is eventually turned out in sheets resembling in 

 structure the lace plantation rubber which will be referred to 

 later. These are usually hung up to dry in well-ventilated, 

 darkened, artificially heated drying rooms. Vacuum drying 

 apparatus is also used for this purpose. 



The temperature is, however, regulated in accordance with 

 the class of rubber under treatment, as inferior grades of rubber, 

 which contain a large percentage of resins and similar bodies 

 liable to be affected by heat, would soften and melt if exposed 



* India-rubber Journal, p. 321, gth Sept. 1907. 

 G 



