How Insulating Varnish Is Tested: The IEC 60464-2 Methods

The thing that makes varnish testing different from most insulation testing is that you are testing two different materials.

Before it cures, the varnish is a liquid. You care about how it flows, how it stores, how much solid it leaves behind, and whether it will catch fire in your shop.

After it cures, it is a thin solid film on a substrate. Now you care about how it bends, how it bonds, how it survives heat and chemicals, and how it behaves electrically.

IEC 60464-2 splits along exactly that line. Clause 5 covers the undried and uncured material. Clause 6 covers the dried and cured coating. Almost every misunderstanding about varnish data sheets comes from mixing the two up, or from comparing numbers that came from different methods within one of them.

Before anything else: the conditions

Test conditions are not a footnote here. They are the reason two figures can disagree while both are correct.

Unless otherwise specified in the relevant specification standard or in the test method, tests are carried out at (25 ± 4) °C and 45 % to 70 % relative humidity, with the sample or specimen pre-conditioned under those conditions until it reaches stability. Sampling of liquid and paste material follows ISO 15528, and preparation for testing follows ISO 1513.

Two things worth carrying away:

Viscosity is measured at a tighter temperature than everything else — 23 °C, held to a fraction of a degree. Viscosity moves fast with temperature, so this matters.

Where the requirements or methods in Part 2 are inconsistent with the specification sheets of IEC 60464-3, the applicable specification sheet takes precedence. Part 2 states this directly.

That precedence rule points at the single most useful distinction in the series, and it is worth fixing before you read any further:

• IEC 60464-2 is the methods document. It tells you how a property is measured.
• IEC 60464-3 holds the requirements. Its sheets tell you what value a given varnish type has to achieve.

So the bend test procedure is in Part 2. The requirement to pass it around a 3 mm mandrel is in IEC 60464-3-2. Mixing the two up is how people end up quoting a “standard requirement” that does not apply to the product they are buying.

Tests on the varnish as delivered

Flash point

Two different standards depending on where the flash point falls. For flash points of 79 °C and above, ISO 2592 is used. For flash points below 79 °C, ISO 1523 is used with a closed-cup apparatus, read in conjunction with ISO 3679. Two measurements on two separate samples.

This is a safety and storage number before it is a performance number. If your national regulations set a minimum flash point for the application, that minimum applies whatever you and the supplier agreed.

Viscosity

Determined at 23 °C, using one of three routes: a Brookfield-type rotational device, a rotational device working at a defined shear rate, or an efflux flow cup.

This is the single most misread number on a varnish data sheet. A flow cup gives an efflux time in seconds; a rotational method gives a viscosity value. Both are valid standardised results, but they are not the same quantity. Rotational measurements also depend on the instrument and its operating conditions. Two suppliers quoting “viscosity” without stating the method cannot be compared. When you write a nominal viscosity into a purchase contract, write the method next to it.

Content of non-volatile matter

How much solid is left after the volatiles go. This is what actually stays in your winding. A varnish with low solids needs more passes to build the same film, which changes your process time and your cost per machine more than the price per litre does.

Dilution ability

A simple, useful test: take a measured sample, add solvent or diluent in defined portions, stir and let it settle between additions, and record how much you can add before cloudiness or separation appears.

The result is reported as the type of solvent and the percentage added without the mixture going cloudy. That tells you how much room you have to adjust viscosity in the tank before the product stops being a solution.

Stability in an open vessel

Viscosity is measured at 23 °C, a weighed sample is held at (50 ± 2) °C for (96 ± 1) hours with solvent or diluent added every 24 hours to compensate for evaporation, and viscosity is measured again at 23 °C.

The test is particularly relevant to open-tank processing, because it deliberately exposes the varnish to elevated temperature and evaporation while allowing solvent replacement. For hot curing impregnating varnishes, IEC 60464-3-2 limits the viscosity increase to a defined multiple of the nominal value. If your own tank thickens faster than that, the varnish is probably not the variable — ventilation and top-up discipline usually are.

Drying and curing in a thick layer

This is the test that tells you whether the product cures properly in bulk, not just as a thin film.

A weighed mass of varnish is cured in a small aluminium foil mould, producing a thick specimen of approximately 4 mm. The mass is calculated from the density and the non-volatile content rather than poured to a nominal volume, so the resulting thickness stays consistent across products. Cure temperature and time are agreed between supplier and purchaser.

The specimen is then described on three axes:

Code	Meaning
S 1 / S 2	Top side smooth / wrinkled
U 1 / U 2	Underside non-tacky / tacky
I 1 to I 6	Interior: rigid, horny machinable, leather-like, rubber-like, gel-like, liquid
.1 / .2 / .3	No voids / not more than five voids / more than five voids

Plus a statement of whether the interior is uniform.

So a report reading S 1 – U 1 – I 3.2 – uniform means: smooth on top, not tacky underneath, leather-like interior, not more than five voids, uniform through the section. The interior code carries two separate pieces of information — the condition category, then the void classification after the point.

For reference, IEC 60464-3-2 requires hot curing impregnating varnishes to reach S 1, U 1, no worse than I 4.2, and uniform. The example above is therefore better than that minimum: a leather-like interior sits above a rubber-like one on the scale.

Why it matters: the bottom of a slot is a thick section. A product that skins over on top and stays gel-like inside is telling you something about what will happen in the deepest part of your winding.

pH, for water-based and emulsion products

Types W and E get their own test. Calibrated pH meter, material held at 23 °C, repeat measurements required to agree closely. Solvent-based products skip this entirely.

Making the specimens

This step gets skipped in most summaries, and it shouldn’t, because the substrate is part of the result.

Steel panels are the default. Thin, defined dimensions, prepared and cleaned to a standard procedure. Panels are what the mechanical, chemical and electrical film tests run on.

One note worth knowing if you ever run this in-house: copper panels were historically proposed as a substrate, but IEC 60464-2 uses steel panels as the default. Copper can interact with certain resin systems, so it is not the default substrate for these tests.

Textile glass fabric is the other substrate, specified by weave, yarn count per warp and weft, mass per square metre and yarn type. Glass fabric specimens are what the temperature index work uses for the breakdown voltage and mass loss criteria.

Coating thickness is measured and reported, because most of the film results depend on it.

Tests on the cured film

Mechanical

Bend test. The method — bending the coated panel around a cylindrical mandrel and examining it for cracks under normal vision — is given in IEC 60464-2, 6.2.1. The requirement is separate: for hot curing impregnating varnishes, IEC 60464-3-2 calls for bending around a 3 mm diameter mandrel with no cracks visible under normal vision. It is a flexibility check, and a brittle film cracks where a winding moves.

Cupping test. A deformation test on the coated panel, from the paint and varnish standards.

Bond strength at ambient temperature. The property that holds the winding rigid.

Thermal

Bond strength at elevated temperature. The same property, hot. This is the more important of the two for a running machine, because bond strength generally falls as temperature rises, and the winding is not sitting at 23 °C when it is working.

If you take one thing from the mechanical and thermal group: bond strength is not a fixed requirement in IEC 60464-3-2. It sits among the properties to be agreed between supplier and purchaser. For anything with vibration, frequent starting, or high short-circuit forces, it goes in the contract or you do not get it.

Chemical

Tackiness. Filter paper is pressed onto the cured surface under a defined weight for one minute. If the paper falls away under gravity or slight vibration, or lifts cleanly without leaving fibres, the surface is non-tacky. If fibres stay stuck to the specimen, it is tacky. Tacky means undercured or wrong cure schedule, and a tacky winding collects every bit of dust in the plant.

Resistance to liquids including water. Immersion for seven days at 23 °C, then examination for appearance change, blistering, tackiness or other deterioration. The test liquid is reported with the result, so “passes resistance to liquids” without naming the liquid is meaningless.

Resistance to solvent vapour. Specimens are suspended above a test solvent in a closed glass container for seven days at 23 °C. The named solvents are acetone, xylene, hexane, methanol and carbon disulphide. Specimens are then examined for loss of adherence, peeling, draining, blistering, tackiness and appearance change.

Two practical points. First, this is a vapour-exposure test rather than an immersion test. Second, if the solvent environment in your application differs from the solvents specified by the applicable requirement, an additional compatibility test may be needed.

Resistance to mould growth. Runs to the environmental testing method for mould growth. Relevant in humid climates and food or agricultural plants.

Electrical

Part 2 defines its terms up front here, which helps: volume resistance is the part of insulation resistance due to conduction through the volume, excluding surface current. Volume resistivity is that reduced to a unit cube. Dissipation factor is the ratio of the imaginary to the real part of the complex permittivity. Relative permittivity is the ratio of absolute permittivity to the electric constant.

Effect of water immersion on volume resistivity. The default is the established volume resistivity method; Part 2 offers an alternative where that does not suit the material. Either way the shape of the test is the same: measure, immerse in demineralised water for (168 ± 1) hours at (23 ± 2) °C, blot, re-measure. Test field strength is 1 000 V/mm DC, the measurement is made a fixed interval after the set-up, and the reading is taken a fixed interval after electrification. Three specimens, and the result is normally expressed as volume resistivity.

The before-and-after pair is the whole point. For the applicable varnish type, IEC 60464-3-2 sets a minimum volume resistivity before immersion and a lower minimum after it. This is a moisture-resistance test wearing a resistivity measurement’s clothes, and the drop between the two numbers tells you more than either number alone.

Dissipation factor and relative permittivity. Measured at 23 °C with a sinusoidal test voltage at 1 kHz, using the panel’s metal sheet as the bottom electrode. Two specimens. Frequency matters — tan δ is frequency dependent, so a figure quoted without a frequency is incomplete.

Breakdown voltage and electric strength. The default is the established electric strength method; Part 2 gives an alternative arrangement. That alternative is ball-to-plate: a polished steel ball as the high-voltage electrode, the panel’s metal sheet as earth, voltage raised at a controlled rate, the whole assembly under a dielectric fluid — unused mineral insulating oil or unused synthetic ester. Five specimens, and the thickness at the point of breakdown is reported with each result.

Part 2 notes that a ball-to-plate arrangement reads slightly higher than plate-to-plate. So a breakdown figure obtained one way is not directly comparable with one obtained the other way, and the reported electric strength depends on the specimen thickness at the puncture, which is why that thickness is part of the result rather than a detail.

Temperature index, in more detail

This is the slowest and most expensive test in the series. It is the test behind the temperature index associated with a varnish type classification — for hot curing impregnating varnishes, IEC 60464-3-2 covers types with temperature indices of 130, 155, 180, 200 and 220.

The work follows established thermal endurance principles — specimens are aged at several elevated temperatures, a chosen property is tracked to a defined end point, and the times to failure are used to derive an index.

Part 2 provides the method. The requirement comes from the applicable Part 3 specification: for hot curing impregnating varnishes, IEC 60464-3-2 requires the temperature index to be established using any two of four criteria, agreed between supplier and purchaser — bond strength, proof voltage on an enamelled wire substrate, breakdown voltage on glass fabric, and loss of mass on glass fabric. That specification also defines the end points for each, including 22 N for bond strength, 3 kV for breakdown voltage and 30 % for mass loss.

The practicalities tell you why this is a periodic conformance test rather than a routine one. Exposure periods run out to many weeks — the standard’s own example sequence doubles from one week to thirty-two. Loss of mass work uses several specimens at each exposure temperature. The report has to carry the specimen details, the exposure temperatures, the times to failure at each, a plot of property against log time, the thermal endurance graph, the index itself, and the correlation coefficient.

So when a supplier quotes a type number, the useful follow-up is which two criteria it was established on. A product qualified on bond strength and proof voltage has been demonstrated on a different axis from one qualified on breakdown voltage and mass loss. Both are compliant. They are not equivalent.

Tests that apply only to W and E types

Amendment 1 brought water-based and emulsion varnishes into the series, and with them four tests that solvent-based products never see:

Test	What it covers
pH	On the liquid varnish, calibrated meter, repeat measurements must agree closely
Flash rusting of steel	Panels examined for rust or discoloration immediately after drying, reported present or absent
Volatile organic compound content	Two ISO methods, chosen according to whether content is above or below 15 %
Water content	Karl Fischer determination

Flash rusting is the one people do not expect. A water-carried varnish can rust a steel substrate during the drying process itself, which is a straightforward pass or fail rather than a measured value.

Reading a test report

Pulling the above together, a report is only useful if it carries the conditions with the numbers. Check for:

• The method used, especially for viscosity and for breakdown voltage
• The substrate and coating thickness — most film results depend on it
• The test liquid or solvent named, for the chemical resistance results
• Temperature and frequency, for tan δ and permittivity
• Before and after values, for the water immersion resistivity
• The cure schedule used, since it was agreed between the parties rather than fixed by the standard
• The number of specimens, since the standard sets a minimum for each test

A number without its conditions is not a result. It is a claim.

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