Coil impregnation varnish is a type of silicone insulating varnish. It is mainly used for impregnating coil windings, glass fiber-covered wires, glass cloth, sleeves, and insulating parts in electrical equipment such as motors (above Class H), electrical appliances, and transformers. It can fill gaps and microholes. When heated and cured, the impregnation varnish forms a continuous and smooth film on the surface of the impregnated object. This bonds the coil into a solid whole, improving the heat resistance, insulation, moisture resistance, thermal conductivity, dielectric strength, and mechanical strength of the insulation structure.
1. Coil Impregnation Varnish and Its Types
Coil impregnation varnish is mainly methyl toluene silicone resin. It has two main product forms: solvent-based impregnation varnish and solvent-free impregnation varnish.
| Solvent-based System | Solvent-free System |
|---|---|
| Silicone resin has large molecular weight and poor fluidity | Silicone resin has small molecular weight and good fluidity |
| Its molecular structure usually contains groups like SI-OH or SI-OR | Its molecular structure does not contain groups like SI-OH or SI-OR; instead, it has SI-VI or SI-H groups |
| Uses toluene or xylene as solvent, which pollutes the environment | Does not use any organic solvent as medium, so it is green and environmentally friendly |
| Undergoes condensation reaction; low-molecular substances like water are released during curing, which affects electrical performance | Undergoes addition reaction; no low-molecular substances are released during curing, so it does not affect electrical performance |
| Not suitable for vacuum pressure impregnation insulation treatment; cannot form a good air-gap-free integral system | Suitable for vacuum pressure impregnation insulation treatment; can form a good air-gap-free integral system |
| Suitable for low-end coil impregnation varnish and sleeve impregnation varnish | Suitable for high-end motor insulation impregnation varnish and laminates |
During curing, solvent-based silicone impregnation varnish releases solvents. It also releases low-molecular substances due to condensation reaction, leading to small air gaps. So it needs multiple impregnations. Even with this special treatment, the integrity of the paint film is affected, and the insulation performance of the paint film is also affected. Therefore, condensed solvent-based silicone impregnation varnish can only be used for low-end motors, such as coils with low voltage and low process requirements for impregnation insulation treatment.
Solvent-free silicone impregnation varnish, however, has a different curing mechanism. It contains no organic solvents and no low-molecular substances are precipitated during curing. So it is suitable for vacuum pressure impregnation insulation treatment of motors. The impregnation process is simplified, efficient, and can be completed in one step. It forms an insulation structure with excellent integrity and insulation performance. Currently, this solvent-free silicone impregnation varnish is widely used for insulation impregnation treatment of high-end motors, such as coil impregnation treatment of aviation motors and high-speed railway traction motors.
2. Performance Advantages of Silicone-Based Impregnation Varnishes
- Compared with traditional insulating materials, silicone coil impregnation varnish reduces weight by 35%-40%. It also reduces the consumption of materials like copper and silicon steel sheets.
- Silicone resin insulating materials have high high-temperature insulation. They have a service temperature of 180~200℃ and can be used for a long time at 180℃. Their heat resistance grade is 180℃, so they belong to Class H insulating materials.
- Silicone insulating varnishes have excellent electrical insulation performance. Their dielectric strength is 50kV/mm, volume resistivity is 10¹¹~10¹⁵ Ω·cm, dielectric constant is 3, and dielectric loss tangent is about 10⁻⁸. These properties change little over a wide temperature range (-50~250℃) and can be used in both high and low frequency ranges.
- Silicone insulating varnishes have excellent properties such as moisture resistance, water resistance, acid and alkali resistance, radiation resistance, ozone resistance, corona resistance, flame retardancy, and non-toxicity. Their electrical performance is better than other organic coatings not only at high temperatures but also at low temperatures. When damaged at high temperatures, they usually do not burn. Their residue is SiO₂, which will not form coke and cause conduction.
3. Silicone Resins as the Key Raw Material
Silicone resin is not just an additive in coil impregnation varnish. It is the core framework that determines the heat resistance, electrical insulation, and long-term stability of the coating. For manufacturers of solvent-based or solvent-free coil impregnation varnish, the quality of the silicone resin they choose directly affects the performance of downstream products in harsh electrical environments.
- The Si–O–Si main chain structure of silicone resin gives the material excellent heat resistance. It can withstand working temperatures of 180–200 °C or even higher. Without high-performance silicone resin, impregnation varnish cannot meet the insulation grade requirements of Class H or higher.
- The electrical performance of insulating varnish highly depends on the purity and structural stability of the resin. Suitable silicone resin can reduce dielectric loss and maintain high breakdown voltage. This is crucial for motor coils, transformers, and generators.
- In coil impregnation varnish, the resin needs good permeability and film-forming properties. It should be able to coat the coil and form a uniform and dense coating. Silicone resin not only has strong adhesion to copper wires and inorganic substrates but also maintains flexibility during thermal cycles to avoid cracking.
- Solvent-based systems require resins to have appropriate molecular weight and solubility. Silicone resin can be modified with structures like alkyd, epoxy, and polyester. This balances curing speed, hardness, and flexibility, allowing the formula to meet different process requirements.
- End users expect insulating varnish to be used for a long time under electrical and thermal loads. The stability of silicone resin ensures that the impregnation varnish can resist oxidation, moisture penetration, and thermal degradation during decades of service.
- It is clear that silicone resin is not just one of many raw materials. It is the core key raw material that determines the performance of impregnation varnish. For formula developers, choosing the right resin supplier is often the key to distinguishing ordinary products from high-performance insulating varnishes.
Quote:
- https://patents.google.com/patent/CN108368344A/zh
- https://books.google.co.jp/books/about/%E6%9C%89%E6%9C%BA%E7%A1%85%E6%A0%91%E8%84%82%E5%8F%8A%E5%85%B6%E5%BA%94%E7%94%A8.html?id=zZXFoQEACAAJ&redir_esc=y