MECHANICAL TESTING OF RUBBER 89 



A considerable amount of work on these lines has been 

 conducted by several investigators,* but although inter- 

 esting results have been obtained, it has been found 

 that the classification of crude rubbers according to the 

 viscosities of theii' solutions does not always agree with 

 that based on the tensile tests of the vulcanised rubbers. 

 Schidrowitz considers that so far as the rubber of one 

 particular species is concerned, the order of the viscosity 

 figures can be accepted as an indication of the relative 

 strengths of different specimens, but he admits that this 

 conclusion does not hold good for rubbers derived from 

 different species. It therefore appears doubtful at 

 present whether the viscosity method will prove to be 

 of general application, although it may be useful in 

 dealing with any one variety of rubber. 



It has been found by Schidrowitz that the viscosity 

 of rubber solutions is affected by the temperature at 

 which the determination is made ; that the effect of 

 heating the solution is sometimes to increase and some- 

 times to diminish the viscosity ; that the removal of the 

 resin from rubber usually increases the viscosity ; and 

 that with rubber from the same species the method of 

 coagulation has considerable influence on the viscosity. 



Mechanical Testing of Rubber. — In view of the fact that 

 the technical value of rubber depends on the physical 

 properties of the vulcanised product, considerable atten- 

 tion has been devoted recently to the mechanical testing 

 of vulcanised rubber as a means of accurately determining 

 the comparative qualities of different specimens. The 

 suggestion has been made that the crude rubbers might 

 be submitted to mechanical tests, but it would be ex- 

 ceedingly difficult in most cases to obtain suitable homo- 

 geneous test pieces, and, moreover, it would have to be 

 shown that the results thus obtained from the crude 

 rubbers held good for the vulcanised products. The 

 plan of making the tests on the vulcanised rubber has 

 therefore been generaUj'- adopted. 



The tests usually made involve the determination of 

 (1) the tensile strength ; (2) the resistance to compres- 

 sion ; (3) the resistance to abrasion ; (4) the hardness ; 

 and (5) the resiliency. 



* See paper by J. G. Fol in The India-rubber Journal, March 22, 

 29, April 5, 12, 1913. 



