Paper to the 39th International Symposium of the Society for the Advancement of Material and Process Engineering (SAMPE), 11th-14th April 1994.

S F Bush with J M Methven

Abstract

There is much published evidence that certain chemical reactions proceed faster when heated in a microwave applicator than when heated in a conventional oven or press. Recent work in this laboratory on the pultrusion of polyester/glass fibre rods indicates a reaction rate enhancement of about ten times. To account for these observations this paper considers reactions in which one of the reactants has a significantly higher dielectric loss factor than the others.

Two cases are considered. One is the peroxide initiator system relevant to pultrusion and the other a bimolecular reaction relevant to epoxy systems. It is shown that a necessary condition for rate enhancement is that a key reaction step proceeds at least as fast as thermal equilibration by collision, and that this condition is likely to have been fulfilled in the pultrusion case. Under this condition the high loss species (the peroxide in the case of pultrusion) has, in effect a non-equilibrated temperature (NET significantly above (ΔT) that of the bulk of the material (T). For the bimolecular reaction the analysis shows that the rate of a non equilibrated temperature reaction (NETR) would be enhanced by up to 20 times for a reaction with ΔT of some tens of degrees, a bulk temperature T of 400K and activation energy of 150kJ/mol.