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Abstract: Insulating varnish formulations should not be evaluated only through labels such as polyester, epoxy, or waterborne. Information that is more useful for R&D and production evaluation includes how the base resin and reactive components, active or non-volatile content, dilution and working viscosity, and gelation and curing conditions are reflected in the TDS. Product parameters can also be measured at different temperatures and heating durations. Formulation variables therefore cannot be reduced to one value that is always better when higher, and a complete formulation cannot be reverse-engineered from a TDS.

Why Insulating Varnish Formulations Are Not an Ingredient-Ratio Table

Discussions of insulating varnish formulations often move toward two extremes: comparing only resin names or attempting to reverse-engineer exact ingredient ratios from a TDS. Neither approach is sufficient to support material qualification.

A TDS primarily describes the product’s composition category, technical specifications, and recommended application conditions. It does not disclose the complete formulation. If specific resin grades, addition ratios, accelerator levels, or production processes are not listed, they should be marked as “not specified in the TDS” rather than completed through assumptions.

For practical material evaluation, it is more useful to answer four questions:

1. How are the base resin and reactive components described?
2. Under which test conditions were the active or non-volatile content values measured?
3. How are dilution and working viscosity recorded?
4. How do gel time, thick-layer curing, and production curing conditions correspond to one another?

For a broader comparison of resin chemistry, see Electrical Varnish: 5 Resin Systems and Their Processing Differences.

Four Formulation Variables and Their Corresponding TDS Items

Variable 1: Base Resin and Reactive Components

The base resin establishes the product’s system category, but the complete product can also contain a reactive diluent, stabilizer, accelerator, curing agent, and other additives. During evaluation, review how the TDS describes these components rather than focusing only on the word polyester or epoxy in the product name.

E57A: Alkyd-Amino System

The E57A TDS describes the product as a baking insulating varnish based on an alkyd-amino resin system and specifies the X-7 Series designated thinner. For process evaluation, the resin system and baking requirement should be recorded together rather than retaining only the alkyd label.

E57-81H: Unsaturated Polyester and Reactive Diluent

E57-81H is formulated from a modified heat-resistant unsaturated polyester resin, a reactive diluent, a stabilizer, and additives. It is a one-component solventless system. Its reactive diluent should be recorded separately and should not be treated as the same formulation concept as a conventional designated thinner.

E57-89H: Epoxy-Polyester and Curing Components

E57-89H is formulated with a modified heat-resistant solventless epoxy-polyester resin, an accelerator, and a curing agent. It uses an epoxy-polyester system rather than a pure epoxy system. Its gel time and staged baking conditions should be evaluated together with this composition description.

CX936: Waterborne Epoxy System

The CX936 TDS describes the product as a waterborne epoxy insulating varnish. Use as supplied is generally recommended, with deionized-water dilution available when required by the application. “Waterborne” describes characteristics of the liquid carrier and dilution method, but it does not independently determine cured electrical performance or thermal class.

These four examples show that formulation evaluation begins with reading the complete composition description rather than reducing different products to a single resin label.

Variable 2: Active and Non-Volatile Content Must Be Interpreted with Test Conditions

Active content, non-volatile content, and solids content are common specification terms, but they should not automatically be treated as identical definitions across all documents.

ISO 3251 covers the determination of non-volatile matter in paints, varnishes, resins, and related products. The standard specifically notes that non-volatile matter is not an absolute quantity independent of conditions. The result depends on the heating temperature and duration used, while retained solvent, thermal decomposition, and volatilization of low-molecular components can affect the measurement.

The Changxian TDS documents provide the following values:

E962 and E962-788 use the same 105°C-for-3-hours condition. Their values can therefore be compared more directly after the remaining test conditions have been reviewed.

E57A uses 120°C for 2 hours, which differs from the conditions used for the other two products. The three values—45–55%, 40.0 ± 2.0%, and 37 ± 3%—cannot be used alone to calculate retained resin on an actual winding.

The 100% active-ingredient statement for E57-89H should not be rewritten as “100% non-volatile content measured under the same conditions used for E57A or E962.” Each statement must retain the meaning used in its own TDS.

Is Higher Non-Volatile Content Always Better?

Not necessarily. Practical evaluation also needs to consider:

• Working-liquid viscosity;
• Dilution method;
• Impregnation and draining conditions;
• Drying and curing schedule;
• Material distribution and validation results on actual workpieces.

Non-volatile or active content alone cannot establish material consumption per workpiece, film thickness, or total production cost.

Variable 3: Dilution Method and Working Viscosity Must Be Recorded Together

The formulation supplies the original varnish, while production uses a controlled working liquid. Preparing that working liquid can involve a designated thinner, reactive diluent, deionized water, or viscosity adjustment under specific conditions.

This table cannot be converted directly into a penetration ranking because:

• The test temperatures are not completely identical;
• Supplied-varnish viscosity is not the same as actual working-liquid viscosity;
• Dilution methods differ;
• Workpiece designs and impregnation processes differ;
• Tu-4 cup seconds and rotational viscosity are not the same measurement result.

ISO 2431 specifies the use of flow cups to determine the flow time of paints, varnishes, and related products. It notes that the method is primarily applicable to materials with Newtonian or near-Newtonian flow behavior. For thixotropic materials, mechanical action such as stirring can affect flow time, so all cup-flow seconds should not be treated as one absolute viscosity value.

Cup-flow seconds therefore should not be converted directly into mPa·s without a valid conversion basis. A lower flow-time value also cannot guarantee better penetration into a winding.

Variable 4: Gelation, Thick-Layer Curing, and Production Curing Are Different Levels

Gel time helps characterize reaction conditions. Thick-layer curing capability describes curing performance under a specified material test. A production curing schedule applies to a particular workpiece and equipment setup. These three levels cannot replace one another.

Example from the E57-81H TDS

• Gel time: 6 ± 3 minutes at 140 ± 2°C;
• Thick-layer curing capability: S1, U1, I2.1 at 140 ± 5°C for at least 2 hours;
• Reference production curing: Bake at 145–150°C for 4–8 hours. For more demanding applications, the TDS provides options to extend the baking time or use another listed condition.

Example from the E57-89H TDS

• Gel time: 5–15 minutes at 120 ± 2°C;
• Thick-layer curing capability: No worse than S1-U1-I2.2 under the specified specimen and 135°C-for-3-hours conditions;
• Reference curing after vacuum impregnation: Bake at 120–130°C for 30–60 minutes, followed by 135–150°C for 2–4 hours, or until winding resistance stabilizes.

The two products use different gel-test temperatures. Their 6 ± 3-minute and 5–15-minute values therefore cannot be used alone to determine which product cures faster in production. Thick-layer curing results also should not be converted into a performance ranking without understanding the test method and rating definitions.

Production records should separately identify:

1. Gel-test conditions;
2. Thick-layer curing test conditions;
3. Actual workpiece preheating, impregnation, and draining conditions;
4. Oven settings and actual workpiece heating;
5. The final basis for determining adequate curing.

For a process-level comparison, see Varnishing Technologies for Windings.

How to Turn the Four Variables into a Sample-Validation Table

For insulating varnish formulations, the value of these variables is to guide validation rather than replace it. A sample record can be organized as follows:

When adjusting the process, clearly identify which condition was changed and retain a comparison record. If dilution ratio, impregnation time, and curing schedule are all changed simultaneously, it becomes difficult to determine which variable caused the result.

How to Handle Formulation Information Not Disclosed in the TDS

If the following information is not listed, mark it as “not specified in the TDS” rather than estimating it:

• Specific resin grades;
• Ingredient percentages by mass;
• Chemical identities and levels of accelerators and curing agents;
• Complete composition of stabilizers and other additives;
• Resin-synthesis conditions;
• Complete production formulations and manufacturing steps.

If certain information is necessary for formulation evaluation or import review, it should be confirmed through formal technical or compliance channels, including appropriate confidential-document arrangements. A “complete formulation table” should not be completed with unsupported data from outside the TDS.

FAQ

Is an insulating varnish with higher non-volatile content always better?

Not necessarily. Non-volatile content must be interpreted together with heating temperature, heating duration, working viscosity, and actual processing conditions. It cannot independently determine material distribution, curing condition, or complete-equipment performance.

Can every solventless product be described as containing 100% active ingredients?

No. This statement should be used only when the specific TDS explicitly provides it. The E57-89H TDS specifies 100% active ingredients, but that statement cannot automatically be extended to every solventless insulating varnish.

Does lower supplied-varnish viscosity guarantee better winding penetration?

This cannot be determined from supplied-varnish viscosity alone. Measurement conditions, working-liquid condition, workpiece design, and impregnation process must also be considered. Tu-4 cup seconds also cannot be converted directly into rotational viscosity without a valid basis.

Does a shorter gel time always mean a faster production cycle?

Not necessarily. Gel time is only one indicator of the reaction process. Draining, thick-layer curing, workpiece heating, and the final curing endpoint must also be considered. Gel values measured at different temperatures cannot be ranked directly.

Can non-volatile content from different products be compared directly?

Test conditions should be compared first. ISO 3251 explicitly states that the non-volatile result depends on heating temperature and duration. Values measured under different conditions cannot be used directly to calculate retained resin in an actual application.

Can a complete formulation be reverse-engineered from a TDS?

No. A TDS provides composition categories, technical specifications, and application information; it is not a complete formulation disclosure. Undisclosed ingredient grades, ratios, and process conditions should remain marked as unspecified rather than being completed through assumptions.

Conclusion

Insulating varnish formulations should be evaluated through four variables: base resin and reactive components, active or non-volatile content, dilution and working viscosity, and gelation and curing conditions. Every variable needs to be read together with its corresponding TDS test conditions.

A more reliable technical pathway is to retain the original TDS definitions, distinguish supplied varnish from the working liquid, align comparable test conditions, and then validate the relationship between formulation variables and production results using actual workpieces. Formulation details not disclosed in the TDS should remain clearly outside the available evidence rather than being filled with assumptions.

If you provide your current material TDS, working-liquid conditions, impregnation process, and curing schedule, our engineers can help you prepare a comparison of candidate Changxian models and a sample-validation table.

About Changxian

than 30 years. We are listed on China’s NEEQ under stock code 831323 and recognized as a national-level specialized and innovative “Little Giant” enterprise. Our approximately 40,000 m² production base in Zhuhai’s Gaolan Port area supports in-house resin synthesis. Our management systems include ISO 9001, ISO 14001, ISO 45001, and IATF 16949. We also hold relevant UL files for insulating varnishes and conformal coatings; coverage for specific models is subject to the applicable certificates.

Changxian’s position is straightforward: performance benchmarked item by item against international brands, with factory-direct pricing. You can contact our engineers for TDS documents, selection comparisons, and free sample support for evaluation under your actual application and process conditions.

Sources

• E57A Baking Insulating Varnish Product Instruction
• E962 Insulating Varnish Product Instruction
• E57-81H Environmentally Friendly Heat-Resistant Modified Unsaturated Polyester Resin Product Instruction
• E57-89H Environmentally Friendly Solventless Insulating Varnish Product Instruction
• E962-788 Environmentally Friendly Insulating Varnish Product Instruction
• CX936 Waterborne Insulating Varnish Product Instruction
• ISO 3251:2019 — Paints, Varnishes and Plastics: Determination of Non-Volatile-Matter Content
• ISO 2431:2019 — Paints and Varnishes: Determination of Flow Time by Use of Flow Cups