Microwave technologies: development, simulation, and application in material processing and recycling

Microwave technology has been established in industry and households for decades; nevertheless, its use in production is still limited to some relatively specific applications. For providing an overview – even for non-specialists – the authors will cover several aspects of microwave technologies: The development of new microwave-oven concepts, simulation of electromagnetic field distributions within microwave systems, measurement of dielectric properties using dielectric spectroscopy, and various applications of microwave technologies. The authors have developed and patented a continuous microwave oven with a maximum sample width of 800 mm, utilizing mono-mode and multi-mode technology at frequencies of 2.45 and 5.7 GHz, with the option of additional heating via infrared irradiation. This device can be used for continuous processing of bulk goods as well as for the continuous microwave treatment of individual items. We demonstrated a combination of horizontal impregnation and coating plant with the continuous microwave oven. To analyze materials including thermosetting resins and composites regarding their complex dielectric function changes depending on frequency, temperature, and changes in composition or degree of curing, dielectric spectroscopy, which ranges from μHz to GHz (10-6 to 3 x 109 Hz), can be used. The dielectric data provides input for subsequent simulations of the influence of different materials on the electromagnetic fields of the microwave ovens. Finally we present exemplary results of different applications of microwave technology, starting with the microwave vulcanization of injection-molded elastomer sealing rings, through experiments on microwave sintering of additive inkjet-manufactured conductor tracks on different substrates, to the mechanical and color modification of European woods for tropical wood substitution in high-priced stringed instruments. In addition the authors demonstrate the applicability of microwave technologies for fast and energy-efficient recycling of polyurethane foams and established a scale-up to up to 90 liters in the microwave oven.