Solvent-Based vs Water-Based Insulating Varnish

IEC 60464-1 designates a varnish on three separate axes: what it is for, what resin it is built on, and what carries it.

That third axis is the one that concerns us:

Type	Code
Organic solvent based	S
Water based	W
Emulsion	E

All three are inside the scope of the same standard. A water-based impregnating varnish is not a lesser product or a special case — it is a type with a code, tested by the same methods document, specified against the same sheets.

That is worth stating plainly because the older literature does not reflect it.

Why the standard changed

The 1998 text of Part 1 contained a sentence saying that all varnishes contain solvent. Amendment 1:2006 deleted that sentence and introduced the three type codes above.

The same amendment added the supporting vocabulary: definitions for emulsion (a stable colloidal mixture of two immiscible liquids), for co-solvent (a polar or hydrophilic liquid used in small concentrations to bridge between the resin and the main solvent, usually water, and help emulsification), and for volatile organic compound content, expressed relative to the mass of resin in the varnish.

So the current definition of a varnish is a solution or emulsion of one or more resins in a solvent or carrier liquid. Water is a carrier liquid. That is how W and E types belong.

One practical warning: the deleted sentence still appears in abstracts and listings that describe the 1998 base publication rather than the consolidated text. For this question, use the consolidated IEC 60464-1:1998+AMD1:2006 text rather than relying on an abstract of the base edition.

The tell: four tests only W and E products take

Here is the most useful way to see the real difference between the families. IEC 60464-2 contains four additional determinations specifically for water based (Type W) and emulsion (Type E) varnishes:

Test	What it is watching for
pH	Stability and handling of the liquid product
Flash rusting of steel	Corrosion of the substrate during drying
Volatile organic compound content	How much organic material still leaves during drying and curing
Water content	How much water the product actually holds

IEC 60464-3-2 reflects the same split: for hot curing impregnating varnishes, pH, volatile organic content and water content sit among the liquid-state properties to be agreed between supplier and purchaser for Type W and Type E products, and flash rusting of steel panels sits among the cured-state properties.

Solvent-based products take none of these four. That asymmetry is the standard telling you where the different risks are.

pH

Measured with a calibrated meter, with the material at 23 °C, and the repeat measurements have to agree closely.

A water based system has a pH, and that pH is tied to its stability. Organic solvent based products have no meaningful equivalent, which is why the test does not apply to them.

Flash rusting

Steel panels are examined immediately after drying for rust or discoloration. The result is reported simply as present or absent.

This is the failure mode people do not expect from a protective coating: a water based varnish can rust a steel substrate during the drying process itself, before it has finished becoming a protective film. Pass or fail, no measured value.

It also explains why substrate preparation and drying conditions deserve particular attention with Type W and Type E products.

Volatile organic compound content

Determined by one of two ISO methods, chosen according to whether the content is above or below 15 %.

Note the implication. Water based does not mean zero organic content — the co-solvent definition in Part 1 exists precisely because a polar liquid is often needed alongside the water. The VOC determination exists to quantify what is still leaving, rather than to confirm that nothing is.

Water content

Determined by Karl Fischer titration.

Straightforwardly a composition check on the delivered product.

What the standard treats the same

Just as informative. Everything else in Part 2 applies to all three types:

• Flash point, viscosity, non-volatile content, dilution ability, open vessel stability, thick layer cure
• Effect on enamelled wire — the compatibility check
• Bend and cupping tests, bond strength at ambient and elevated temperature
• Tackiness, resistance to liquids, resistance to solvent vapour, mould growth
• Volume resistivity before and after water immersion, dissipation factor and permittivity, breakdown voltage
• Temperature index

So the same Part 2 test methods cover the electrical, mechanical, thermal and compatibility properties across the three varnish types, although the applicable requirements and agreed property values depend on the relevant Part 3 specification.

That distinction matters. Part 2 is a methods document — shared methods do not mean identical acceptance limits. A water based Type 155 impregnating varnish is still evaluated under the same IEC 60464 framework for properties such as viscosity, electrical performance, mechanical performance and temperature index, while the applicable limits and the agreed properties come from the relevant specification. Where the two are useful to state directly: IEC 60464-3-2 identifies Type 155 with property levels specified at 155 °C and a minimum temperature index of 155 °C, whatever the carrier.

The carrier is primarily a process and environment question, not by itself a performance-class ranking.

Flash point is the interesting one in that list. It applies to all types, and it is agreed between supplier and purchaser subject to national safety requirements. For organic solvent based products it is a live constraint on storage and handling. For water based products it is usually less of a concern in practice — but the requirement structure is the same, and co-solvent content is the reason the determination still applies.

What the standard does not decide for you

Three things sit outside IEC 60464 and belong in your own assessment.

Regulation. VOC limits for industrial coatings vary by country and by application. The standard gives you a way to measure the content; it does not tell you what you are allowed to emit. Check your national requirements, because in some jurisdictions this is the whole reason the question comes up.

Your process. Drying behaviour, oven loading, ventilation requirements and cycle time differ between an organic solvent based system and a water based one. An oven and extraction arrangement set up around solvent evaporation is not automatically suited to driving off water. This is a shop-by-shop assessment, and the supplier’s process data governs.

Storage. Do not assume a water based product can be stored like any other water-based coating. The applicable storage temperature and the final date for use come from the product specification and the container marking — Part 1 requires that storage information to be marked on the container. Read the marking rather than assuming a shelf in the corner will do.

Choosing between them

A short way through it:

Start with regulation. If VOC limits in your jurisdiction constrain the application, that may settle the question before any technical comparison begins.

Then check your process can run it. Oven, extraction, cycle time, and whether your staff have run the type before. A product you cannot cure properly is worse than a product with a slightly higher VOC figure.

Then specify normally. Application (FV or IV), type number, resin type if you need to constrain the chemistry, nominal viscosity with its method, nominal non-volatile content, and the agreed properties. The type code just becomes one more field: IEC 60464-3-2 type 155, varnish type W.

Add the Type W and Type E specifics if that is what you are buying. pH, volatile organic compound content and water content on the liquid side; flash rusting on the cured side. They sit on the agree-with-the-supplier list, which means they do not happen unless you ask.

Do not assume a performance penalty. The same test framework applies across the three types, and the type number carries the same meaning whatever the carrier. If a supplier implies a water based product is inherently weaker, ask which specified requirement they think differs.

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