Gums, Resins, 



6 



[July, 1910. 



somie dilution to facilitate the straining. 

 Consequently speedier methods of pre- 

 paration have been devised. These 

 depend for the most part on taking 

 advantage of the phenomenon known as 

 coagulation. But before treating of this 

 important subject, the sepn ration of the 

 rubber by centrifugal force requires 

 consideration. 



Centrifdgalisation, 



Centrifugal force was first applied to 

 the separation of the caoutchouc glo- 

 bules from the latex by Prof. Biffen. 

 In 1897 he accompanied as scientific 

 adviser a small rubber-exploiting expe- 

 dition to tropical America, and when 

 there experimented on various latices 

 with a cream separator. He found that 

 the machine answered admirably for 

 Castilloa, and claimed also that he 

 effected the separation of the caoutchouc 

 of Hevea latex by this means. The 

 advantage of the centrifugal method 

 lies in the fact that the rubber can be 

 extracted from the latex in a state of 

 great purity. 



The success of the centrifugal separ- 

 ator in the case of Castilloa has been 

 repeatedly confirmed. This latex, in- 

 deed, creams readily on standing, and 

 good rubber can be prepared in this 

 way. With Hevea, however, confirm- 

 ation has not been forthcoming. No 

 one in the East, as yet, has been able to 

 effect the separation of the caoutchouc 

 globules of this latex by centrifugal 

 force, even with a speed of 11,000 revolu- 

 tions per minute. This latex, further, 

 never shows the least sign of creaming. 

 It remains homogeneous as long as 

 the slightest acidity is prevented from 

 arising. The difference in the behaviour 

 of the two latices is probably due to the 

 size of the caoutchouc globules. These 

 are most likely much larger in Castilloa 

 latex than in that of Hevea. 



Biffen's ingenious application appeared 

 most promising twelve years ago as a 

 handy means of preparing first-class 

 commercial rubber. Since it has been 

 proved to be impracticable with the 

 latex of cultivated Para rubber trees in 

 the East, it is now of minor importance. 



Centrifugally separated caoutchouc is 

 no doubt the purest, as it is practically 

 free from albuminous matter (protein) 

 which forms a small percentage of all 

 other raw rubbers. Recent investigation 

 suggests, however, that the small 

 quantity of cured protein contained in 

 Para rubber is not *vithout an advant- 

 ageous influence. The texture of the 

 raw rubber appears to depend to a large 

 extent on the manner of coagulation— a 

 phenomenon depending upon the pro- 

 tein present, 



Coagulation. 

 To Biffen is also due the first adequate 

 explanation of the nature of latex-coagu- 

 lation. His view has been challenged 

 since, but in the writer's opinion the 

 adverse critisisms are due to the con- 

 fusing distinct phenomena ; in fact, the 

 experiments brought forward in refu- 

 tation of the theory, in reality, am- 

 plify it. 



The explanation he propounded is as 

 follows. When the protein dissolved in 

 the latex comes out of solution it forms 

 a delicate network throughout the latex, 

 entangling the caoutchouc globules in 

 the meshes. This network shrinks upon 

 itself, forming a spongy clot, which can 

 be removed from the mother liquid and 

 compressed into a solid mass of rubber. 



Latices behave differently as regards 

 the means to be employed to bring 

 about coagulation. Manihot latex is 

 easily clotted by boiling ; not so that 

 of Hevea. A trace of acid, however, 

 coagulates the latter. In the first case 

 Biffen considered the protein to belong 

 to the globulin class, and in the second 

 instance to the albumins. He applied 

 his theory to the explanation of the 

 means employed in the Amazon region 

 of preparing Para rubber by smoking, 

 and gave strong reasons for believing 

 that this excellent empirical native 

 method is due to coagulation. The 

 smoke arising from the smouldering tire 

 made of palm-nuts contains both acetic 

 acid and creosote. The former coagu- 

 lates the latex as it is passed over the 

 rotating wooden paddle held in the 

 smoke ; the latter impregnate? the 

 rubber as it is formed layer upon layer, 

 thus preventing any subsequent mould- 

 ing or putrefaction. The wet system 

 of coagulation, now in use on the Hevea 

 estates of the Eastern Tropics, was 

 largely suggested by Biffen's explan- 

 ation of the Amazon smoking method. 



My attention was early directed, in 

 the Ceylon experiments of 1898-9, to 

 this question of coagulation as it affects 

 Hevea latex. Several acids and some 

 salts were tried, with the result that 

 acetic acid was recommended as the 

 best reagent to be used on the estates 

 to bring about coagulation, and so for 

 the preparation of the rubber from the 

 latex. This acid is now generally em- 

 ployed. 



The following table gives the weight 

 required of the different reagents to 

 coagulate completely 100 cubic centi- 

 metres of Hevea latex. Since these 

 Ceylon experiments, a few other coagu- ■ 

 lants have been recommended, and these 

 are appended below the horizontal line :— 



