Research and validation of a novel dynamic mechanical measuring method for determining the degree of cross-linking of semi-finished rubber products

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The degree of cross-linking of elastomers is a key quality parameter in rubber processing. Besides the compound formulation, it depends primarily on the temperature profile of the material during vulcanisation. Particularly in the case of continuous vulcanisation for the production of rubber profiles and vulcanised sheeting, it has not yet been possible to measure the temperature or the degree of cross-linking of the vulcanised material inline. Therefore, continuous vulcanisation is a "black box" process. The processors bear the risk of producing scrap unnoticed as long as no quality parameters are available.The established methods for measuring the degree of cross-linking are based on chemical, calorimetric or mechanical approaches. None of these methods can be implemented process-integrated and in real time, because the measurement duration is too long or the measurement cannot be automated. The acoustic resonance frequency is a mechanical parameter that correlates with the degree of cross-linking of semi-finished rubber products. The measurement is carried out contactlessly with a laser vibrometer. The measuring time is less than one second. Thus, such a measuring system can in principle be integrated into a vulcanisation line for process monitoring. In the research project submitted, this new acoustic measurement method is to be developed and validated.In order to allow clear conclusions to be drawn from the measured resonance frequencies on the degree of cross-linking, the effects of all relevant influencing variables, such as the material temperature, on the structure-borne sound behaviour must be quantified. For this purpose, bar-shaped samples are produced in a heating press by varying the density, the plasticiser content, the geometry as well as the degree of cross-linking and their resonance frequencies are determined at different temperatures. For these fundamental investigations, a stationary structure-borne sound test rig with a temperature chamber is used, in which the samples are fixed to an electrodynamic shaker for a defined excitation.Machine learning methods are used to determine the relationships between the various influencing variables and the measured resonance frequency based on these measurement data. In addition, a regression model is developed with which the degree of cross-linking can be calculated following the acoustic measurement. Finally, the applicability of the new measurement methodology for monitoring the cross-linking state is evaluated. Therefore a comparison with the established mechanical measuring methods regarding their reproducibility, accuracy, measuring speed as well as an analysis of the limits of the measuring system is carried out.