Microwave processing

Microwaves (0.3-300 GHz) lie between radiowave (RF) and infrared (IR) frequencies in the electromagnetic radiation spectrum. Microwaves can be reflected, absorbed and/or transmitted by materials. Microwaves can interact with materials through either polarization or conduction processes. Polarization involves the short-range displacement of charge through 

Microwave processing has been used to process thermoset polymers and polymer composites, including polyesters, polyurethanes polyimides and epoxies, and in most studies it has been concluded that the curing speed is faster using microwave energy (Clark and Sutton, 1996). The effects of continuous and pulsed microwave irradiation on the polymerization rate and final properties also have been studied, and it was demonstrated that, for certain epoxies, a pulsed microwave cure resulted in improvements in mechanical properties, better temperature uniformity and a faster polymerization mte (Thuillier and Jullien, 1989). 

Microwave processing has also been suggested to be able to aid pultrusion and autoclave processing (Clark and Sutton, 1996). 

Quality-control tests and important process variables 

As previously mentioned many studies have been performed on microwave curing of thermosets, including polyesters, polyurethanes polyimides and epoxies. 

The use of miaowave heating to carry out chemical reactions between organic compounds has been studied since the mid-1980s and several reviews of this technology have been published (Caddick, 1995 Corsaro et al., 2004 Gabriel et al, 1998 Galema, 1997 Lidstrbm et al., 2002). The modification of starch by microwave radiation was reviewed by Tomasik and Zaranyika (1995). 

Microwave heating was also used to prepare inorganic derivatives of starch. Potato starch was sulfated with pyridine. SO3 complex in a microwave-assisted soUd state process, and a maximum DS of 1.05 was obtained (Staroszczyk et al., 2007a). Although conventional oven 

In summary, microwave heating allows reactions to be carried out with shorter reaction times, more uniform heating, and less chance for overheating, since microwave radiation heats only the reactants and solvent and not the reaction vessel itself. In contrast, heating 

---

C. Shibata and H. Tamai, Proceedings of the International Conference on High Frequeny j Microwave Processing and Heating, Tokyo, Japan, 1989, pp. 5.2.1-5.2.5. 

W. H. Sutton and co-workers, eds.. Microwave Processing of Materials Materials Research S ociety Proceedings, Vol. 124, Pittsburgh, Pa., 1988. 

Plastics Southern Africa 28,No.5,Nov.l998,p.l8/22 BACKGROUND TO MICROWAVE PROCESS FOR RECOVERY OF PMMA WASTE... 

The microwave technique is widely applied to process polymer materials, e.g. in microwave cure [429], Microwave processing is a developing technology. 

A particularly difficult problem in microwave processing is the correct measurement of the reaction temperature during the irradiation phase. Classical temperature sensors (thermometers, thermocouples) will fail since they will couple with the electromagnetic field. Temperature measurement can be achieved either by means of an immersed temperature probe (fiber-optic or gas-balloon thermometer) or on the outer surface of the reaction vessels by means of a remote IR sensor. Due to the volumetric character of microwave heating, the surface temperature of the reaction vessel will not always reflect the actual temperature inside the vessel [7]. 

Scheme 4.30 Hydrodechlorination of chlorobenzene using active flow cells in a continuous-flow microwave process.

An even simpler protocol for performing nucleophilic substitutions (aminations) and Suzuki reactions in one pot was reported by the Organ group for the generation of a 42-member library of styrene-based nicotinic acetylcholine receptor (nAChR) antagonists (Scheme 6.21) [49]. After considerable experimentation, the authors found that simultaneous nucleophilic displacement and Suzuki coupling could be carried out very effectively by charging the microwave process vessel with the palladium catalyst (0.5 mol% palladium-on-charcoal), the boronic acid [R1B(OH)2], the... 

Most importantly, microwave processing frequently leads to dramatically reduced reaction times, higher yields, and cleaner reaction profiles. In many cases, the observed rate enhancements may be simply a consequence of the high reaction temperatures that can rapidly be obtained using this non-classical heating method, or may result from the involvement of so-called specific or non-thermal microwave effects (see Section 2.5). 

High power microwaves are generated by vacuum tubes. The magnetron and klystron are the most commonly used tubes for the generation of continuous waves power for microwave processing. Power is normally launched from the microwave tube into a transmission line or waveguide, where it travels to a load or termination such an antenna or a microwave heating applicator. 

J. Wei,V. Adegbite, Mater. Res. Soc. Symp. Proc. 1994, 347 (Microwave Processing of Materials IV), 669, and references cited therein. 

---