114 PARA RUBBER. 



For the present, therefore, the application of the correct quantity of 



acetic acid followed by thorough washing and rolling may be adopted, 



but care must be exercised not to add excess, and every effort be made 



to subsequently expel the reagent by suitable mechanical processes. 



Components of Coagulated Rubber. 



Whenever the rubber is prepared by the coagulation of the proteid, 

 either by the smoking method or the use of familiar chemical reagents, 

 hot or cold, it is obvious that the rubber must contain the precipita- 

 ted proteids together with the suspended globules of caoutchouc, 

 resin, &c. Analyses of well-dried Para rubber show only a small per- 

 centage of substances other than caoutchouc — practically from 4 to 

 5 per cent. — and it may at first sight appear unnecessary to draw 

 attention to the desirability of extracting them. If one compares 

 the analyses of latex and rubber from Hevea brasiliensis, it is 

 surprising to find that when chemical reagents have been used 

 the percentage of proteid matter in the rubber shows that the whole 

 of that in the latex was not precipitated, and B amber and Parkin 

 proved that the clear liquid remaining after coagulation with acetic 

 acid often gave re-actions with the tests for proteids. The amount 

 of proteid in the clear liquor may, according to Bamber , be as much as 

 50 per cent, of the original. It may be asserted that a great part of the 

 proteids generally occurs in the prepared rubber, and their presence 

 along with other substances leads in many cases to putrefaction. 

 Putrefaction and Tacky or Heated Rubber. 



The proteid matter is responsible for much of the " tackiness" or 

 the " heating," which is seen in many rubber samples. The rubber 

 first becomes sticky, and sooner or later appears to melt as if due to 

 excessive heat. It often emits a strong odour when in this stage. 

 This is due to the inclusion of the proteids and perhaps the sugary 

 and gummy constituents in the rubber and the subsequent develop- 

 ment of micro-organisms on these substances. If the rubber is 

 free from these materials it will not undergo such a change, and the 

 removal of the latter from rubber takes us into several important 

 methods of purification. The chemical change which takes place in 

 tacky rubber is indicated in the analyses , made by Mr. M. Kelway 

 Bamber, of sound rubber and material in various degrees of tacki- 

 ness. They are here quoted in full : — 



Analyses of Sound and Tacky Para Rubber. * 



♦Committee of Agricultural Experiments, Peradeniya, Sept., 1905. 



