Microwave heating of rubber extrudates

The increasing demand for energy-efficient methods for vulcanizing rubber extrudates requires the optimization and further development of existing vulcanization processes. Vulcanization by microwave irradiation makes it possible to introduce the energy required for heating directly into the material by means of dielectric losses. Microwave heating requires the polarity of the rubber compound so that the electromagnetic wave can vibrate the polar components in the compound. This oscillation results in internal friction, which leads to an increase in the temperature of the rubber compound. However, the polarity of rubber compounds is reduced due to corrosion protection, especially in the automotive sector. This requires optimization of the process parameters in the microwave crosslinking process. For this purpose, it is necessary to specifically adapt the setting variables of the electromagnetic wave as well as the geometry of the heating chamber to the different rubber compounds. The heating studies presented in this paper were performed with a solid-state microwave device using a nitrile butadiene rubber. The results show a material specific excitation frequency of the rubber compound in the range of 2436 MHz and 2478 MHz. In addition, investigations of the waveguide length of the microwave unit show that the geometry of the waveguide has a significant influence on the absorption behavior of the electromagnetic energy. In laboratory tests with optimized settings, 90% of the applied electromagnetic energy was converted into heat in the extrudate. In comparison, conventional processes such as hot air vulcanization have an efficiency of only 33%. In subsequent temperature tests of the extrudate surface, a temperature maximum was found in the center of the extrudate. This makes microwave technology an ideal complement to hot air vulcanization.