Investigation of the curing reaction mechanism of polyester and polyesterimide wire enamels

Basic wire enamels for magnet wire manufacturing are polyesters and polyesterimides. These tris-(2-hydroxyethyl)-isocyanurate modified polymers have as end groups ethylene glycol half esters. The polymers are usually solved in a mixture of technical cresol and naphtha. During processing of a polyester or polyesterimide wire enamel solvents are evaporated and the resin is crosslinking releasing ethylene glycol and is forming a film. The transesterification reaction, in which ethylene glycol is formed, is catalysed by a titanium catalyst, e.g. cresyltitanate. When a GC-MS analysis of the volatile transesterification reaction products is done, beside the ethylene glycol, other molecules, like methanol and therephtalic acid monomethylester are identified. The concentration of the fragments is low and they have been found when dimethylterephthalate was used in resin synthesis. They are formed from methylterephthalate end groups reactions. Besides solvents and the small molecules described, there is an additional material release during the curing process of an wire enamel, not analysable by GC-MS. This material has a polymeric structure and is known as stack loss. The material evaporated as stack loss condenses on cold parts of the enamelling machines forming the condensate which requires cleaning operations. This material analysed by infrared spectroscopy shows the absorption of polyesters, also in the case when polyesterimides are used in the process. Therefore a simple evaporation of molecular fragments is not occurring.