Development of a silicone modified unsaturated polyester varnish for rated electrical insulation application

We report on the development of a silicone modified unsaturated polyester (SUP) using a novel combination of well known principles and rigorous calculation and control of mole fraction to produce what we believe the first SUP varnish which can be used in all insulation systems including those requiring application in vacuum (solvent free, single component). Silicone modified polyesters are available but not in solvent free and unsaturated varnish form and their chemistry is different. We believe that the nature of the chemical structure generated by the manufacturing protocol is critical to the properties of the cured SUP varnish. The properties of SUP are consistently better than those of unsaturated polyesters (UP) with respect to chemical resistance as defined by ASTM tests to moisture, acids, bases, toluene, transformer oil and silicone oil. IEC comparative tracking resistance was 300 drops vs 160-240 for UPs. Results of electrical properties vs temperature tests and heat deflection temperature suggested application to high voltage electric motors and other high temperature uses. Heat aging stability of bond strength of SUP treated helical coils was tested for tensile strength (ts) (using Stecker) and for flexural strength (fs) to determine the temperature index (TI) of the varnish alone. The end point was taken to be 14 kgf (50% loss for both ts and fs). Also, motorettes insulated with materials of high temperature class were treated with SUP for TI evaluation. The TI on a motorette was sufficient (208°C) to suggest application at 200°C rating level (20,000 h level). The TIs of helical and Stecker coils were 194 and 193°C, respectively. The literature values of TIs of UPs on motorettes with similar insulating materials never exceeded 180°C. These properties are consistently better than the properties of conventional unsaturated polyester.