Observing changes in nonlinear viscoelastic properties occurring throughout the vulcanization process

Using a purposely modified Rubber Process Analyzer, a technique has been developed to record the complete torque signal during a vulcanization test, and to split it into a sequence of single oscillations, which are treated through Fourier transform calculations. Various EPDM‐based compounds were tested to investigate the capabilities of the technique. It was found that the progressive variation of the harmonic torque signal when vulcanization occurs reflects in singularities in the manner that all harmonic components vary during cure. Contrary to the main torque component (i.e., the torque amplitude) that exhibits a smooth variation as vulcanization proceeds, harmonic components exhibit sharp up and down changes at precise times that seem to be related to critical steps in the curing process. For instance, pair harmonic components passes through extremes (maximum or minimum) when scorch time is reached. Odd harmonic components show a singularity that is likely associated with the precise moment when the vulcanizing material passes from a still “liquid” state to an essentially “solid” one. This event does not coincide with the time for the maximum cure rate to be reached but occurs either before or after it, depending on the compound or the curing parameters. Such results support the view that rubber vulcanization presents some analogy with a nucleation process and underline the promising capabilities of the test technique. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1140–1150, 2006