Nanotechnology enable developing of new dielectrics which show not only better mechanical, thermal and barrier properties but also display the better dielectric properties. If the size of the particle approaches that of the polymer chain length the particles stop to behave like foreign inclusions. The micron scale fillers produces significant Maxwell-Wagner polarization which is negligible in a case of nanofillers. This paper presents the results of testing of polyester (PE) and polyesterimide (PEI) varnishes modified by adding of various nanoparticles: fumed nanosilica, nanosilica and titanium dioxide obtained via sol-gel process, zinc oxide and montmorillonite (MMT) at loading 1- 3 wt %. The processing, mechanical, electrical and thermal properties have been examined for both basic pure varnishes and varnishes with nanofillers. The incorporation of nanoparticles into varnishes increased temperature of varnish degradation from 5 to 40 ^oC and also the bond strength by about 20% to 40%. The varnishes PE contained of fumed SiO₂, ZnO and TiO₂ showed the decrease in dielectric permittivity and maintained almost the same thermal dependence of dielectric loss factor and electrical resistivity as pure varnish. In the case of PE varnish modified with nanosilica the electric strength significantly grew at higher temperatures. The varnish PEI with fumed nanosilica exhibited much better resistance to impulse voltage and partial dischargers. Improving of dielectric properties of electroinsulating varnishes is possible by incorporation of nanoparticles. Nanofilled varnishes exhibit also the increased thermal endurance and bond strength and less water absorption.

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