A series of thermal insulating EPG composite with high performance and light wight were prepared through solvent-free BMC thermocompression technique using multifunctional group epoxy resin (MF4101) as matrix, methyl tetrahydrophthalic anhydride as curing agent, and hollow glass beads with high strength as filler and thermal insulating phase. And the structure, mechanical properties, and thermal properties of the composite were studied. The results show that the average particle size of hollow glass beads is 25.6 μm, and it remains intact in the composite. When the amount of hollow glass beads is 80, 120, and 140 phr, the density of EPG composite is 0.82, 0.72, and 0.71 g/cm³, respectively; the compressive strength is 105.7, 68.1, and 67.4 MPa, respectively; the thermal conductivity is low, the value is 0.012 9, 0.085 4, and 0.0826 W/(m·K), respectively. Besides, with high glass transition temperatures and epitaxial degradation temperatures (>300℃), the EGP composite material can be used under high temperature and high pressure for a long time.

In this paper, the withstand voltage characteristics of cross-linked polyethylene (XLPE) and its nanocomposites at different temperatures was systematically measured by step voltage rising method. A new method for calculating the key parameters of the Crine model was proposed, and the activation energy and charge acceleration distance of XLPE and its nanocomposites at different temperatures were calculated on the basis of the experimental results of step voltage rising method. The results show that the characteristic breakdown time of both the materials decreases with the increase of temperature. At the same temperature and time step, the characteristic breakdown voltage and breakdown time of XLPE nanocomposites are higher than that of XLPE, and the electrical ageing life of XLPE nanocomposites is longer than that of XLPE under high voltage. With the increase of temperature, the ageing activation energy and charge acceleration distance of the two materials increase. Under the same temperature, both the ageing activation energy and charge acceleration distance of XLPE nanocomposite are smaller than that of XLPE. It is more intuitive to reflect the ageing resistance of insulating materials by activation energy and charge acceleration distance on basis of Crine model than the ageing life index on basis of the inverse power model or exponential model.

Insulating paper is an important insulating material that produced by papermaking technology and widely used in electrical equipment such as transformers, capacitors, motors, wires and cables. According to its composition, insulating paper can be divided into cellulose paper and non-cellulose paper. With the sustainable development of China’s power industry, motors and other electric equipment are gradually getting more efficient and smaller. Therefore, higher property requirements are put forward to insulating paper, especially high-temperature resistant performance. Although cellulose papers have a long history, their heat resistance is not high enough to meet the operating requirements of electrical insulation systems, which need the thermal class of F or above. Non-cellulose papers with higher heat resistance have sprung up. This article briefly introduces the development history, performance characteristics, main uses, production status, and development trends of various high-temperature resistant non-cellulose insulating papers in China.

In order to improve the thermal conductivity of polyimide film and maintain its insulating properties, SiO₂ was used to coat AgNWs to obtain AgNWs@SiO₂ core-shell structure. At First, the AgNWs@SiO₂ was dispersed in polyamide acid (PAA) discharge fiber by electrospinning technology, and the heat conduction path and the dispersity of AgNWs@SiO₂ in PI matrix was planned and improved by electrospinning technology, respectively. Then the PAA glue containing AgNWs@SiO₂ was used to impregnate PAA electrospun film, and an E-AgNWs@SiO₂/PI composite film was obtained after thermal imidization. The effects of filler modification and content on the thermal conductivity and insulating properties of the composite films were studied. The results show that when the mass fraction of filler is 25%, the thermal conductivity of E-AgNWs/PI and E-AgNWs@SiO₂/PI composite films is 2.92 W/(m·K) and 2.80 W/(m·K), which are 14.6 and 14 times higher than those of pure PI films, respectively. The dielectric constant of E-AgNWs@SiO₂/PI composite film decreases to below 5, the dielectric loss factor maintains below 0.015, and the volume resistivity increases to 1.79×10¹³ Ω·m.

A series of black polyimide (PI) films were prepared by using 4,4′-diaminodiphenyl ether (ODA) as diamine, pyromellitic dianhydride (PMDA) as dianhydride, and carbon black treated with different types of dispersants as shading filler, respectively, and the effects of dispersants on the surface morphology and properties of the carbon black/PI films were investigated. The results show that the dispersibility of carbon black treated with dispersants in PI matrix and the organic-inorganic interface compatibility are improved, the agglomeration degree of carbon black decreases, and the properties of the films increase. When the carbon black is treated by YK-3 (modified block polymer), the surface morphology and properties of the film are the best, its electric strength, tensile strength, and elongation at break are 145 kV/mm, 146 MPa, and 38%, respectively, and the light transmittance is close to 0.

As a special engineering plastic, polyimide is widely used in electrical insulation, electronics, and other fields because of its excellent dielectric properties, mechanical properties, and thermal stability. The polyimide films with excellent thermal stability and dielectric properties can be obtained by adjusting the molecular chain structure of polyimide through molecular structure design and monomer optimization. In this paper, the molecular structure design strategies for adjusting the dielectric properties of polyimide and the influencing mechanism of polyimide structure on its dielectric properties were reviewed, and the research direction of dielectric properties adjusting was prospected.

Under different environmental relative humidity (RH), the corona resistance of polyimide film presents significant differences. In order to understand the effects of RH on the surface corona characteristics and ageing process of film, we monitored the corona discharge process, electrical properties, and surface morphology of the film under different RH, and obtained the effects of RH on the corona ageing of film. The results show that with the increase of RH, the changing trends of the discharge activity at positive and negative half cycle are opposite. When the RH is equal or less than 69%, both the discharge quantity and discharge frequency show rapid growth stage, stationary stage, and secondary development stage with corona time, and when the RH attains 84%, both of them increase linearly with time. The changing rate of moisture content (Δwt%) of the film increases significantly at the beginning of corona ageing and tends to be stable at the middle and later periods, and when the RH attains 84%, the Δwt% increases linearly with the ageing time. It is analyzed that the surface electrothermal damage of film caused by corona discharge is the main cause of the rapid decrease of surface resistivity (ρ_s). With the corona erosion, water penetrate into the film gradually, eventually leading to the sharp decrease of ρ_v till breakdown.

At First, a poly(amic acid) (PAA) based on pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) was prepared by polycondensation procedure, and the PAA/SiO₂ composite solutions were obtained by adding different mass fraction of colloidal SiO₂/N,N-dimethylacetamide (DMAc) during the polymerization. Then a series of PI/SiO₂ composite films were prepared by thermal imidization of the PAA/SiO₂ composite solutions in a clean oven from room temperature to 350℃ in nitrogen. The PI composite films were characterized by the attenuated total reflectance Fourier transform infrared (ATR-FTIR), scanning electron microscopy (SEM), thermal-gravimetric analysis (TGA), dynamic mechanical analysis (DMA), and thermo-mechanical analysis (TMA), and the effect of SiO₂ on the dielectric properties of composite film was tested by the impedance analysis measurement. The results show that the composite films with expected structure are prepared, and the SiO₂ disperses in PI matrix uniformly. The incorporation of SiO₂ improves the heat resistance and dimensional stability at high temperature of the PI composite films. The 5% weight loss temperature (T_5%) and 750℃ residual weight ratio (R_w750) of the PI-25 film with 25% of SiO₂ are 611.3℃ and 73.1%, respectively, which are 14.7℃ and 9.2% higher than those of the PI-0 film (without silica). The PI composite films exhibit stable dielectric constant (D_k) and dielectric loss factor (D_f) in the frequency range of 10³-10⁶ Hz. The incorporation of nano-silica slightly increases the D_k of composite films, and the D_k of the PI-25 film at 1 MHz is 3.58, which is a bit higher than that of the PI-0 film (D_k is 3.20).

A graphene oxide (GO) was synthesized by modified Hummers’ method, and a graphene oxide/polyimide (GO/PI) composite film was prepared by solution blending method. The structure and the properties of the composite film were analyzed by XRD, TMA, TGA. The results show that compared with the PI film without GO, when the mass fraction of GO in composite film is 0.2%, the mechanical properties of the composite film increase obviously, the tensile strength increases by 26.77%, the elongation at break increases by 76.47%, and the elastic modulus changes little basically. When the mass fraction of GO is 0.1%, the maximum value of T₅ and T₁₀ of the composite film are 587.3℃ and 603.3℃, which increase by 2.44% and 1.69% compared with that of the PI film without GO, respectively. It is indicated that the addition of GO with appropriate amount can enhance the mechanical properties and thermal properties of PI films.

With the development of electronics and microelectronics technology, thermally conductive polyimide (PI) film is facing new application requirements. The control of thermal conductivity and preparation of PI film have drawn much attention. However, there is a lack of systematic research on the analytical methods of its thermal conductivity. In this paper, the analytical methods of thermal conductivity for PI film at home and abroad were summarized. The basic principles, main features, and application scope of the transient methods, steady-state methods, and temperature wave analysis were introduced in detail. In addition, the comparison results of thermal conductivity along the out-of-plane direction and in-plane direction and thermal diffusivity of PI films tested by different methods were reviewed. The existing problems and future development trends of the thermal conductivity analytical methods for PI films were summarized and prospected.