Mooney scorch curve

Figure 24 Typical Mooney scorch curve for a rubber compound with curatives.

Figure 9.3 Mooney Scorch curve for a rubber compound at a set temperature.

Parallel plate compression plastimeters have been used for measuring the rate of cure at various times and temperatures. The test pieces are heated for various times and then tested in the plastimeter. The change in plasticity is then plotted against time of heating to give a scorch curve. The Mooney viscometer offers a more convenient way of measuring scorch, and a standard method is given in ISO 667 [7]. 

As discussed earlier, the Mooney scorch test can measure time to scorch and provide some information regarding the rate of cure. However, because the Mooney rotor rotates continuously. slipping and tearing of the specimen occur well before the ultimate state of cure is reached. It is not possible to observe the entire cure curve with the Mooney viscometer. 

A development of plasticity determination by means of the Mooney viscometer. The same instrument is used but the temperature of operation is higher and the test is continued until the sample vulcanises. A curve of Mooney reading against time is drawn, from which the scorch and cure characteristics are estimated (BS 903-A58). 

In all the above methods, it is necessary to cure specimens of test samples for each of a series of curing times and then perform the desired test on the vulcanizate. However, in the test for continuous measurement of vulcanization complete information could be obtained with saving in time. The mooney viscometer test approaches this objective. However a weakness of the mooney viscometer test is that the test is completed before a measurable modulus value after the scorch point has been obtained. This is because the test sample is destroyed after the induction period is passed due to tearing by continuous rotation of the rotor whether small or large. To overcome this deficiency and to provide a total cure curve for the entire vulcanization cycle, a series of instruments called cure meters was developed. In each of these instruments the stiffness or modulus of the compound was chosen as parameters for vulcanization continuously. The Vulkameter developed by Bayers, Germany was the first of the cure meters developed. 

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