Dielectric heating, and

Since a large majority uf drying equipment involves the use of hot gases (usually air), other means of heating can be overlooked. Dielectric heating and freeze-drying, for example, are other means where practical. 

Chemicals and the containment materials for chemical reaction do not interact equally with the commonly used microwave frequencies for dielectric heating and consequently selective heating may be achieved. Specifically, it is possible to cool the outside of the vessel with a coolant that is transparent to microwaves (solid C02 or liquid N2) and thereby have cold walls that still allowthe microwave energy to penetrate and heat the reactants, which are microwave active, in the vessel. Also for solid-state reactions contamination from the crucible walls may be minimised. 

Westman, J., An efficient combination of microwave dielectric heating and the use of solid-supported triph-enylphosphine for Wittig Reaction, Org. Lett., 2001, 3745—3747. 

From an applications point of view, it may be that the filler yields an unexpected advantage reduction of dielectric heating, and concomitant reduction in power loss. As the tan 8 peak is reduced, less energy would be transferred from an AC source to the polymer in the range of near -10 to 0°C. 

The rubber industries use alkanolamines as secondary vulcanization accelerators to give improved mechanical properties and better heat resistance. Alkanolamines help stabilize rubber dispersions and may also act as an antioxidant for some rubbers. Alkanolamines are activators for the dielectric heating and microwave vulcanization processes. The various alkanolamines are used in the cold cure of certain rubbers and can act as chain termination agents in the chloroprene polymerization. 

A common cause of RF contact burns is from the metal electrode jigs of dielectric heating and plastics welding apparatus, operating at radio frequencies. Unless the output is pulsed there is unlikely to be a shock risk, but an accidental hand contact, for example, will draw out an arc when the hand is removed and a high frequency burn will result. The burning is a function of the arc energy, and for low power apparatus only a minor burn is likely. More powerful equipment, above about 1 kW, can inflict more serious burns. 

In 2007 a very important article was published by Kappe [35], a well-known specialist in the field of the microwave research. His group re-examined the Marmich and aldol reactions presented above and carefully investigated the differences between conventional and microwave heating (results presented in Schemes 21.10-21.12). Moreover, the Mannich reaction of acetone with the PMP-protected imino ester of ethyl glyoxylate was studied (Scheme 21.13). In this case the obtained results, under MW (49 W or 270 W with Uquid cooling) at 60 C for lOmin, were very similar in terms of yield and enantioselectivity to those obtained for the reaction performed in an oil bath at the same temperature and reaction time (90-92%, >99% ee). Similar results with microwave dielectric heating and conventional thermal heating were also obtained for the Mannich and aldol reactions presented in Schemes 21.10-21.12 [35]. 

Fluorinated aryltin, heteroaiyltin, and allyltin reactants were treated with organic halides in the presence of lithium chloride and a catalytic amount of bis(triphenylphosphine)palladium (11) chloride in DMF under microwave irradiation. The reaction was completed in less than 2 min with microwave dielectric heating and a microwave power of 50-70 W (Larhed et al., 1997). 

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