Methods of testing vulcanized rubber. Determination of creep and stress relaxation

1958 ◽  
Keyword(s):  
Stress Relaxation ◽  
Vulcanized Rubber ◽  
Creep And Stress Relaxation

Relaxation Processes in Vulcanized Rubber. III. Relaxation at Large Strains and the Effect of Fillers

1963 ◽  
Vol 36 (3) ◽  
pp. 697-708 ◽  
Author(s):  
A. N. Gent
Keyword(s):  
Stress Relaxation ◽  
Carbon Black ◽  
Natural Rubber ◽  
Progressive Failure ◽  
Large Strains ◽  
Relaxation Processes ◽  
Vulcanized Rubber ◽  
Rubber Vulcanizate ◽  
Finite Extensibility ◽  
Creep And Stress Relaxation

Abstract Some experimental measurements are described of stress relaxation and creep at room temperatures in vulcanizates of natural rubber, butyl, and SBR. In an unfilled natural rubber vulcanizate the rate of stress relaxation is found to rise sharply for extensions of more than about 200%. Reasons are given for attributing this to the growth of a crystalline phase. Similar rates are observed at all extensions for a carbon black filled natural rubber vulcanizate. This is shown to be in satisfactory accord with the Mullins-Tobin model structure for filled vulcanizates, when the whole of the observed relaxation occurs in “softened” regions at rates appropriate to the high local deformations. The failure of rubber-carbon black associations with time does not appear to constitute a major relaxation process. In noncrystallizing unfilled vulcanizates the rate of relaxation is found to decrease somewhat with extension, possibly due to finite-extensibility effects. Preliminary measurements on a filled SBR vulcanizate suggest that a significant contribution to the observed relaxation arises from progressive failure of rubber-filler associations in this case. The relation derived previously between the rates of creep and stress relaxation at equivalent deformations is confirmed in all cases, within experimental error. Its validity in highly-irreversible systems is thus established experimentally.

Apparatus for the Continuous Measurement of Stress Relaxation in Vulcanized Rubber

1956 ◽  
Vol 29 (3) ◽  
pp. 834-837 ◽  
Author(s):  
J. C. Danjard
Keyword(s):  
Stress Relaxation ◽  
Reaction Rate ◽  
Continuous Measurement ◽  
Reaction Rate Constant ◽  
Approximate Determination ◽  
Relaxation Theory ◽  
Chain Molecules ◽  
Vulcanized Rubber ◽  
Free Activation Energy

Abstract The measurement of stress relaxation in vulcanized rubber is based on the principle that, when kept at constant elongation, a stretched sample of rubber shows a progressive decrease of stress with time and that this decrease depends both on the nature of the compound and on the temperature of testing. The resultant curve is a decreasing potential, which is expressed approximately by the equation: σt=σ0⋅e−kt, where σt is the instantaneous stress at time t, σ0 is the initial stress, and k is a reaction rate constant which depends on the free activation energy of the rubber chain molecules. Tobolsky was the first to apply the relaxation theory to the study of the oxidation behavior of compounds and to the approximate determination of the nature of vulcanization structures. Certain types of apparatus are available for the discontinuous measurement of this stress relaxation, but we have designed, with the aid of the French Rubber Institute technical service, a relaxometer for the continuous recording of this phenomenon.

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