What is water-based insulating paint?

Comprehensive Analysis of the Characteristics and Technological Development of Waterborne Insulating Varnishes

In the field of electrical equipment manufacturing, the performance of insulating materials directly affects the safe operation and service life of equipment. As a type of special coating, insulating varnish, with its excellent electrical insulation, thermal stability, and mechanical strength, has become a core material for the insulation treatment of motor and electrical appliance windings. Traditional solvent-based insulating varnishes contain large amounts of volatile organic compounds (VOCs) such as benzenes and ketoesters, which not only pose fire hazards during production and application but also seriously threaten the health of operators and the ecological environment. With increasingly stringent national environmental regulations, waterborne insulating varnishes, which use water as the main dispersion medium, have gradually become an inevitable choice for industry upgrading. Their low VOC emissions, safety, and environmental friendliness are leading technological innovation in the field of insulating materials.

Core Definition and Classification System of Waterborne Insulating Varnishes

Waterborne insulating varnishes are insulating coating systems constructed using water-soluble polymers or water-emulsifiable polymers with water as the dispersion medium. They are mainly divided into two categories:

Water-soluble insulating varnish: Relies on polar groups (such as hydroxyl and carboxyl groups) in the molecular chain to form hydrogen bonds with water molecules, enabling the polymer to dissolve in water and form a uniform, transparent aqueous solution.

Water-emulsifiable insulating varnish: Uses emulsifiers to disperse polymer resins into nanoscale droplets, forming a stable emulsion system with a dispersed phase particle size typically in the range of 50–500 nm.

These coatings inherit the core functions of traditional insulating varnishes—forming a continuous, dense insulating film under specific curing conditions to block current leakage paths, while withstanding harsh environments such as high temperatures and vibrations during electrical equipment operation. Compared to solvent-based products, the innovations of waterborne insulating varnishes lie in: Solvent substitution: Replacing traditional organic solvents with water (VOC content nearly zero), reducing VOC emissions by over 80%; Safety improvement: Eliminating the risk of flammability and explosion caused by solvent volatilization, lowering the fire hazard level of the application environment from Class A to Class C; Cost optimization: The cost of water as a dispersion medium is only 1/20 that of organic solvents, significantly reducing raw material costs.

Currently, mainstream waterborne insulating varnishes use high molecular polymers as the base, supplemented by components such as flame retardants (e.g., aluminum hydroxide, decabromodiphenyl ether), curing agents (isocyanates, amine compounds), and pigments/fillers (titanium dioxide, talc), forming a multiphase composite system. The curing process essentially involves cross-linking reactions between polymer chains to build a three-dimensional network structure, thereby endowing the film with excellent mechanical strength and insulating properties.

Technical Bottlenecks and Breakthrough Directions for Waterborne Insulating Varnishes

Despite significant environmental advantages, waterborne insulating varnishes still face three core challenges in practical applications. Solving these problems directly determines whether their performance can meet industrial application standards: Dual Impact of Polar Groups Waterborne polymers require the introduction of a large number of polar groups (such as carboxyl and sulfonic acid groups) or the use of emulsifiers to achieve water dispersion. If these groups remain excessive after curing, they become "hidden danger points" in the film:

Water absorption risk: Polar groups are highly hydrophilic and easily absorb moisture in high-humidity environments, causing volume resistivity to drop from 10¹⁴ Ω·cm to below 10⁹ Ω·cm;

Mold prevention failure: Moisture penetration can breed mold, forming conductive channels on the film surface, especially problematic in tropical climates.

The industry currently uses two technical approaches to mitigate this issue: First, developing self-crosslinking polymers so that polar groups are consumed during the curing process; Second, adding hydrophobic silane coupling agents (e.g., KH-570) to form a waterproof protective layer on the film surface. Experimental data show that after 1000 hours in an 85% humidity environment, waterborne insulating varnish modified with silane still maintains over 70% of its initial insulation resistance.

Metal Corrosion and Protection Balance

Water molecules and residual ionic emulsifiers can cause electrochemical corrosion of metal windings such as copper and aluminum, manifested as: Green copper rust (basic copper carbonate) on the surface of copper windings; Pitting corrosion on aluminum components, forming corrosion pits with diameters of 0.1–0.5 mm in severe cases. The application of anti-flash corrosion technology has partially solved this problem. By adding corrosion inhibitors like benzotriazole (BTA), a passivation film can form on the metal surface. However, studies show that adding corrosion inhibitors reduces the breakdown voltage of the film by 5–10%. Finding a balance between corrosion prevention and insulation performance remains a challenge in formulation design. Practices at a motor factory show that when the BTA addition is controlled at 0.5–1.0%, the corrosion rate can be ≤ 0.01 mm/year while maintaining a breakdown voltage above 30 kV/mm. 

Contradiction in High-Temperature Mechanical Performance

During operation, the winding temperature of electrical equipment can reach 120–180°C, requiring the film to maintain sufficient bonding strength (≥ 5 MPa) and tensile strength (≥ 20 MPa) at this temperature. However, water-soluble polymers typically have strong polarity and weak intermolecular forces, leading to: High-temperature softening: Low glass transition temperature (Tg), with significant softening above 150°C; Bonding failure: Adhesion to metal drops from 10 MPa at room temperature to below 3 MPa at high temperatures. To overcome this limitation, the industry is adopting composite modification strategies: Introducing aromatic monomers (e.g., phthalic anhydride) to increase molecular chain rigidity; Adding nano-montmorillonite to form a reinforcing phase. Experimental data indicate that nano-composite modification can increase the bonding strength retention rate of waterborne insulating varnish at 200°C to 80%.

Technical Characteristics of Mainstream Waterborne Insulating Varnish Varieties

After years of development, waterborne insulating varnishes have formed a pattern of multi-variety collaborative development, with each type having unique performance focuses to meet application needs in different scenarios: Alkyd Resin-Based Waterborne Insulating Varnish As the most used category (about 45% of the market share), its core advantages are readily available raw materials and low cost: Synthesis route: Polyols such as trimethylolpropane and neopentyl glycol undergo esterification with oleic acid and phthalic anhydride to produce the base resin, which is then cross-linked and cured with methanol-etherified melamine-formaldehyde resin;

Performance shortcomings: Heat resistance rating is only Class B (130°C), with breakdown voltage typically 20–25 kV/mm, limiting its use in high-voltage equipment; Typical applications: Impregnation insulation treatment for small and medium-sized low-voltage motors (power ≤ 100 kW) and household appliance motors. In recent years, performance has been improved through silicon modification technology. For example, the electrical strength of tung oil-based silicon-modified alkyd insulating varnish can reach 130 MV/m, expanding its applicability in high-temperature environments.

Waterborne Polyester Insulating Varnish

By introducing aromatic diacids (e.g., terephthalic acid), its heat resistance and insulation properties are significantly improved compared to alkyd types:

Synthesis process: Two-step melt polymerization is used, where diacids and diols first form oligomers, then trimellitic anhydride is added to introduce carboxyl groups, which are neutralized to achieve water dispersion; Performance advantages: Heat resistance rating reaches Class F (155°C) or above, dielectric constant remains stable at 3.5–4.0, suitable for high-frequency motors; Typical product: Silicon steel sheet insulating varnish, with film thickness controllable at 5–10 Мm and adhesion rating reaching Level 0 (cross-cut method).

A case study shows that when used in variable frequency motors, this type of varnish can reduce core loss by 15–20%, demonstrating excellent electrical performance.

Waterborne Epoxy Insulating Varnish

The inherent properties of epoxy resin make it a preferred choice for high-performance insulating materials: Performance matrix: Outstanding mechanical strength (tensile strength ≥ 30 MPa), excellent chemical corrosion resistance (no change after 1000 hours of immersion in engine oil), low shrinkage (≤ 0.5%); Technical difficulty: Dispersing high molecular weight solid epoxy resin in water requires a large amount of emulsifier, leading to reduced water resistance of the film; Application scenarios: High-voltage motor windings (voltage ≥ 10 kV), transformer insulating bushings, and other components with extremely high reliability requirements. To solve the water-based dispersion challenge, the industry has developed self-emulsifying epoxy resins, which achieve self-dispersion by introducing carboxyl groups into the molecular chain, reducing emulsifier usage from 5% to below 1%.

Waterborne Polyimide Insulating Varnish

As a representative of high-performance polymers, its performance reaches the top level of insulating materials: Core advantages: Heat resistance rating reaches Class H (180°C) or above, maintaining 70% mechanical strength at 250°C, dielectric strength ≥ 40 kV/mm; Technical bottleneck: The rigid chain structure of polyimide makes it very poorly soluble in water, requiring chemical imidization or the introduction of flexible chain segments to improve dispersibility;

Development prospects: Special motor insulation in extreme environments such as aerospace and nuclear energy, currently in the transition stage from laboratory to industrialization.

Technological Development Trends and Future Outlook

With the advancement of the "dual carbon" goals, the development of waterborne insulating varnishes is showing three major directions:

High performance: Through molecular design and composite modification, break through the current bottlenecks in high-temperature mechanical performance and water resistance, aiming to reach the performance level of solvent-based products; Functional integration: Develop composite coatings with multiple functions such as insulation, thermal conductivity, and corrosion prevention, e.g., adding graphene to improve thermal conductivity (from 0.2 W/m·K to 1.0 W/m·K); Process adaptability: Optimize curing conditions, develop low-temperature rapid curing systems (60°C/30 min) to meet the needs of continuous production lines.

Currently, domestic research institutions have made breakthroughs in several key technologies: A self-crosslinking waterborne epoxy varnish developed by a university achieved a breakdown voltage of 45 kV/mm; A nano-composite polyester varnish developed by a company increased the heat resistance rating to Class H. These advancements indicate that waterborne insulating varnishes are expanding from low-voltage, room-temperature applications to high-voltage, high-temperature scenarios, and are expected to fully replace traditional solvent-based products in the future, promoting the green transformation and upgrading of the electrical manufacturing industry.