Recent Developments in the Microwave Processing of Polymers
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MRS BULLETIN/NOVEMBER 1993
the selective absorption of energy by one phase or part of a material—leading to a custom temperature profile within a part. Although microwave processing offers excellent uniformity throughout the thickness of a part, achieving uniformity of the microwave flux is a major challenge and has been a limitation of the technology. The simplest microwave applicator to design and build is a multimode cavity, similar in design to a home microwave oven. The disadvantage of this system is that heating is non-uniform, despite the use of mode-stirring fans and the movement of the part through the chamber. Other microwave applicator designs can provide a much more uniform, predictable electric field, but the penalty is complex design
Figure 1. Schematic showing a dipole (amine group) moving in an alternating electric field.
and the need for real-time tuning or process control to allow the microwave radiation to reach the part being processed because the dielectric properties of the material change during processing. Some of these limitations are being overcome through innovative design and the use of advanced process control algorithms. Over the last five years, significant improvements in microwave applicator design and in the associated hardware and control loops have enabled the successful application of this technology to both polymers and ceramics. An added challenge is that the process temperature for most polymers is close to the degradation temperature, requiring excellent process control. This problem is further increased for microwave processing because the microwave absorption increases markedly in the rubbery phase, when most of the processing is completed. These problems, however, can be readily overcome by understanding the processes which occur and by using suitable process control methods. Microwave technology was first used to process polymeric materials in the late 1960s. However, commercialization of the technology is somewhat limited, due partly to the complexity of the technology and traceable to (1) non-uniformity of microwave applicators; (2) limited process control (partly due to the non-uniformity of the applicators); (3) thermal runaway in which the temperature increases uncontrollably; (4) limited understanding of the processes occurring; and (5) inability of this technology to directly replace other processing methods without additional development. Despite these early difficulties, microwave processing has become the preferred method for foaming and preheating rubber extrudate used in the weather seals of automobiles and the like. Commercial success was due to lower equipment costs, substantially shortened process lines (due to greater heating rates), enhanced product quality (more uniform thermal cross section), and significant environmental advantages over previous technologies (replacing salt baths, talc coating, etc., used in the conventional heat transfer processes). The technology was successfully implemented through careful process development and the selection of
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