Influence of microwave irradiation on a polyesterification reaction

The paper focuses on selective microwave heating and its influence on a polyesterification process. As a model reaction, the polyesterification reaction of adipic acid with neopentyl glycol is investigated. Non-catalyzed and tin (II) chloride catalyzed reaction experiments have been performed with microwave and conventional heating in an open, stirred vessel under a nitrogen atmosphere. Except faster heating times obtained under microwave heating, no different effects on the polyesterification reaction were found between the two heating modes. After 3-h experiments the conversion of the acid groups was similar under microwave heating and conventional heating. Particular emphasis was given on elucidating the important, though scarcely reported in the microwave literature, issues of energy consumption and efficiency. Approximately 20–30% of the electric energy consumed by the microwave oven is converted to thermal energy in the vessel during (non-reactive) heating of the individual components of the polyesterification process. This fraction drops to ∼5% in the event of isothermal reaction experiments. A vast amount of energy is lost in the magnetron and the multimode cavity; these losses do not hamper conventional (conductive) heating, which is currently more economical. Finally, different ways of improving the thermal efficiency and thus the economic prospects of microwave technology are discussed.