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

弹性体的交联度是橡胶加工中的关键质量参数。除了胶料配方外，它还主要取决于材料在硫化过程中的温度分布。特别是在用于生产橡胶型材和硫化片材的连续硫化的情况下，还不可能在线测量硫化材料的温度或交联度。因此，连续硫化是一个“黑箱”过程。如果没有质量参数，加工商就有可能产生不被注意的废料。测量交联度的既定方法是基于化学、量热或机械方法。这些方法中没有一种可以集成过程并在真实的时间内实现，因为测量持续时间太长或测量不能自动化。声共振频率是与半成品橡胶产品的交联程度相关的机械参数。测量是用激光测振仪进行的。测量时间小于1秒。因此，这种测量系统原则上可以集成到硫化生产线中用于过程监控。在提交的研究项目中，将开发和验证这种新的声学测量方法。为了从测量的共振频率中得出关于交联度的明确结论，必须量化所有相关影响变量（如材料温度）对结构声行为的影响。为此，通过改变密度、增塑剂含量、几何形状以及交联度在热压机中生产条形样品，并在不同温度下测定其共振频率。在这些基础研究中，使用了带有温度箱的固定式结构声试验台，其中样品被固定在电动振动台上，以获得规定的激励。基于这些测量数据，使用机器学习方法来确定各种影响变量与测量的共振频率之间的关系。此外，开发了一个回归模型，与该模型的交联度可以计算以下的声学测量。最后，新的测量方法用于监测交联状态的适用性进行了评估。因此，与已建立的机械测量方法的再现性，准确性，测量速度以及测量系统的限制的分析进行比较。

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.