78 



TACKINESS OF RUBBER 



not ver}^ convincing, and in certain cases a bacteriological 

 examination of tacky rubber has given negative results. 



Another suggestion, similar to the above, is that tacki- 

 ness is produced by the action of certain enzymes, whilst 

 other views are that the method of preparation and the 

 coagulant employed may predispose the rubber to become 

 tacky. The development of stickiness in rubber, other 

 than that produced by heat or sunlight, is, however, 

 occasional rather than general, and it is difficult to ex- 

 plain why one of two pieces of rubber prepared in exactly 

 the same way should develop tackiness whilst the other 

 remains sound, or why one portion of a sample should 

 be affected and another not. 



In certain cases apparently the development of tackiness 

 is not associated with any change in the chemical com- 

 position of the rubber, as wiU be seen from the following 

 analyses by Bamber : 



In these specimens the sound and tacky rubbers were 

 practically identical in composition, whilst the " very 

 tacky " rubber only contained about 1 per cent, more 

 resin and protem than the others. 



Onthe other hand, an examination made at the Imperial 

 Institute of the sound and tacky portions of a specimen 

 of crepe rubber from the Federated INIalay States showed 

 that the sound portion contained only 2-9 per cent, of 

 resin (acetone extract), whUst the tacky portion contained 

 no less than 8-2 per cent. In this case, therefore, the 

 development of tackiness was associated with a large 

 increase in the amount of resin. No evidence of the 

 presence of volatile or fixed oil in the acetone extract from 

 the tacky portion could be obtained, but it was found 

 to contain a considerable quantity of free acids of resinous 

 nature. 



It has been stated that it is the rubber obtained from 



