Test for the uniformity of gum rubber

Because the Mooney rheometer is a rotational instrument, the deformation is shear. The torque is proportional to shear stress. Since the rotational speed is constant, the time is proportional to shear strain. Therefore, it is a shear stress-strain curve, which is analogous to the more familiar tensile stress-strain curve. 

In addition to the torque-rise behaviour the steepness of the curve may be expressed with the torque/time ratio, which is the stress/strain ratio, i.e., modulus. Between the two gum-rubbers in question, there is a significant difference in modulus. The gum rubber giving a steeper curve is stiffer than the other one. The peak may be interpreted as the failure point [4]. The stiffer rubber has a smaller strain to break and the softer rubber is more deformable. The former tends to fall in Region I of mill processability and the latter in Region II. The former may contain macrogel in a significant amount, if it is an emulsion-polymerised diene rubber. See Chapters 4 and 6 for additional information. 

Reprinted with permission from N. Nakajima, Poiymer international 1995, 36, 2, 105. 

The importance here is to recognise the limitation of the test. In addition to the nonequilibrium temperature of the test and the complex distribution of strain, the curve is measured at 2 rpm, which is an arbitrary time-scale. The material behaviour may be different at other time-scales this situation is exactly like a blind man touching an elephant . In addition, at this time scale the Mooney rheometer is not very sensitive to a small difference between the gum rubbers. Therefore, this method is for the screening and catching only a gross difference among the samples. The Mooney test is performed for the specification in any event, and therefore, it is proposed here to pay attention to the shape of the curves also. 

Realising the present test is for assisting production, there is a time-limit for testing, which may arbitrarily be set to less than ten minutes. 

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