In AC power applications, the corona resistance of AC mylar sheet directly impacts its reliability in applications such as variable-frequency motors and power cables. Due to its molecular structure, conventional AC mylar sheet is susceptible to insulation degradation due to localized discharge in high-frequency electric fields, manifesting as surface roughening, crack propagation, and even breakdown. Improving its corona resistance through process improvements requires a comprehensive approach encompassing material modification, surface treatment, and structural optimization.
Material modification is a fundamental approach to improving the corona resistance of AC mylar sheet. Incorporating inorganic nanoparticles into the polyester matrix is a common approach, such as nano-silica, alumina, or layered silicates. These particles slow down insulation degradation by dissipating electric field energy and absorbing active particles generated by discharge. For example, nano-silica particles create a reverse electric field on the film surface, offsetting some of the impact energy of corona discharge. Alumina particles, through their high thermal conductivity, quickly dissipate localized heat, preventing chain degradation reactions caused by heat accumulation. During process implementation, the uniformity of nanoparticle dispersion must be controlled to prevent agglomeration, which can lead to performance degradation.
Surface treatment can significantly improve the corona resistance of AC mylar sheet. Corona treatment uses high-frequency, high-voltage discharges to break molecular chains on the film's surface, creating a microporous structure and introducing polar groups. This enhances surface energy and improves the adsorption capacity of corona discharge products. Moderate corona treatment can significantly increase the film's surface wet tension, promoting adhesion to subsequent coatings. However, the treatment intensity must be carefully controlled to avoid excessive corona treatment leading to surface hardening or ink sticking caused by reverse treatment.
Coating is a direct means of enhancing the corona resistance of AC mylar sheet. Applying a corona-resistant coating containing inorganic nanoparticles, such as a silicone or fluorocarbon resin composite, to the film surface creates a physical barrier to prevent corona discharge from eroding the substrate. The nanoparticles in the coating absorb discharge energy, inhibiting the development of partial discharge and extending the film's service life. For example, a coating containing nano-alumina can significantly increase the film's breakdown voltage under high-frequency electric fields while reducing the amount of partial discharge.
Co-modification can achieve complementary properties by blending polymers with excellent corona resistance with polyester. Polyimide, due to its excellent high-temperature and corona resistance, is an ideal blending component. Through melt blending or solution blending, polyimide is uniformly dispersed in a polyester matrix to form an interpenetrating network structure. This structure retains the processing properties of polyester while incorporating the corona resistance advantages of polyimide, significantly extending the life of the composite film under high-frequency electric fields.
Multilayer composite structures achieve synergistic enhancement of corona resistance by stacking different functional layers. For example, a sandwich structure consisting of a "corona-resistant layer-polyester matrix layer-corona-resistant layer" allows the outer corona-resistant layer to preferentially withstand corona discharge shocks, protecting the inner polyester matrix layer. The corona-resistant layer can be made of a composite material containing mica flakes or nanoparticles, leveraging its high dielectric constant and resistance to partial discharge to create an electric field shielding effect. This structure has been used in rail transit traction motor insulation, significantly improving the film's reliability under high-frequency pulse voltages.
Optimizing process parameters is key to ensuring stable performance. During film manufacturing, precise control of extrusion temperature, draw ratio, and annealing is crucial. Excessively high extrusion temperatures can cause nanoparticle agglomeration, reducing dispersion effectiveness. Improper stretch ratios can lead to uneven molecular chain orientation, affecting the isotropy of corona resistance. Inadequate annealing can result in residual internal stress, accelerating corona aging. Optimizing process parameters through orthogonal experiments can achieve an optimal balance between mechanical strength and corona resistance.
Corona resistance processing for AC mylar sheet is evolving towards intelligent and environmentally friendly approaches. Online monitoring technology provides real-time feedback on corona treatment effectiveness, enabling dynamic adjustment of process parameters. The introduction of bio-based nanomaterials will reduce environmental impact. Research on self-healing coatings may enable active repair of insulation damage. These innovations will continue to drive the widespread application of AC mylar sheet in high-voltage, high-frequency electrical applications.
