Hyperbranched unsaturated polyester resin for application in impregnation coatings

A new kind of water-soluble hyperbranched unsaturated polyester resin (HBUP) was prepared to enhance the rate of curing, to increase the adhesive strength and to decrease the dielectric constant of the water-soluble insulating impregnation coating comprising water-soluble unsaturated alkyd resin and amino resin. The HBUP was synthesized from pentaerythritol as the core molecule, which was then esterified with dimethylol propionic acid (DMPA). In addition, the terminal hydroxyl groups of HBUP were modified by acrylic acid and maleic anhydride. The hyperbranched structure and C=C double bonds of HBUP resin were investigated by Fourier transform infrared (FTIR) and nuclear magnetic resonance (C-13-NMR and H-1-NMR) spectroscopies. The reaction rate of the water-soluble insulating impregnation coating was accelerated by addition of HBUP which was demonstrated by the surface drying test and differential scanning calorimetry (DSC). The electrical and mechanical properties of the water-soluble insulating impregnation coating were also studied. It was shown that the addition of HBUP resin could reduce the permittivity of the coating which was at its lowest level when the HBUP content was 5 wt%, whereas the breakdown voltage, volume resistivity and adhesive strength of the coating were improved.