658 RUBBER 



quently creped. If used in minimum quantity, the 

 mineral acids, especially sulphuric acid, produce a 

 rubber not inferior to that coagulated by means of acetic 

 acid. The chief advantage of sulphuric acid is its cheap- 

 ness, while its disadvantages are its dangerous and 

 corrosive action and the liability to damage the rubber 

 if the coagulation is not properly controlled. 



Other Coagulants. A number of salts have been 

 found to act as coagulants, but the quantities required 

 are comparatively large, the rubber does not appear to 

 be superior, and the cost of the coagulant is greater. 



Hydrofluoric acid (also sold in dilute solution under 

 the title " Purub " for coagulation) acts as a coagulant, 

 and produces a pale rubber due to inhibition of natural 

 oxidation. This acid is, however, expensive and not 

 convenient to handle, and the pale colour desired in raw 

 rubber for specific purposes may now be obtained by 

 other methods. 



Various combinations of salts, etc., have been patented 

 as coagulants and boomed at certain periods, but these 

 have died a natural death. 



Formic acid deserves special mention, as, provided 

 the present method of coagulation of Hevea latex by 

 means of acid continues, it appears ppobable that formic 

 acid may replace acetic acid. As a coagulant it is more 

 powerful, i.e., smaller quantities per unit volume of latex 

 may be used, and the resultant rubber is apparently not 

 inferior to that prepared by means of acetic acid. It has 

 been stated that formic acid can now be prepared at 

 about half the cost of acetic acid, owing to the discovery 

 of new methods and sources of manufacture. At the 

 present time, in the Federated Malay States, it costs 

 slightly more than acetic acid, and I am informed by the 

 firm which first imported this acid that the freight is 

 high, as the shipping companies consider it is a dangerous 

 chemical, hence the higher price charged in this country. 

 It also corrodes the corks of the carboys in which it is 

 contained, and thus the strength of the acid is liable to 

 diminish on storage, especially in the tropics. One 

 estate manager in the Federated Malay States prepares 

 an excellent pale crepe by using formic acid as a 

 coagulant. 



