Electrical Insulating Varnish: UL 1446 Thermal Classes

Start with your task

Match class and systemCertification. Read the data sheetProcess.

Match the thermal class to your insulation system

UL's guide to insulating varnishes and impregnating resins states that they increase the mechanical integrity of windings, improve dielectric and thermal conductivity properties and protect the insulation system from the environment; varnish coats the winding wires and glues them together when cured, but an improperly selected varnish can increase thermal degradation of the magnet wire, leading to turn insulation breakdown and failure, so it must be compatible with the magnet wire and other components of the electrical insulation system (EIS). [1]

UL certifies the temperature class of a varnish when applied to specific ANSI magnet wire (MW) types, through the three-temperature thermal ageing programme of UL 1446 using twisted pair (electrical), helical coil (mechanical) and curved electrode (flexibility) specimens; a class rating is assigned for each property and wire type. Its class letters are E 120°C, B 130°C, F 155°C, H 180°C, N 200°C, R 220°C, S 240°C and C over 240°C. [1]

If a varnish was part of the EIS thermal ageing (required in the EIS), a substitute must be a UL-recognized varnish with twisted pair and helical coil class ratings for the applicable magnet wire type at least equal to the original, and must pass UL 1446's sealed tube chemical compatibility test; a varnish listed as optional was not part of the full ageing. [1]

Editorial note: give the supplier your EIS designation and magnet wire type, and ask for its UL recognition and the class ratings for that wire, not just a general thermal class.

Give the supplier the equipment (motor, generator, transformer, coil), EIS designation and whether varnish is required or optional in it, magnet wire type and enamel class, required thermal class, application method (dip and bake, trickle, VPI, spray), oven and cure capacity, solvent or solventless preference and VOC limits, viscosity and solids needed for penetration, dielectric and bond strength needs, packaging, shelf life and storage.

Read the data sheet against your process

As an example of a data sheet, one producer's class H air-drying impregnation varnish based on modified polyester, for windings, stators and transformers and applied by immersion, brush or spray, lists viscosity of 30–45 s in a Ford cup No. 4 (ASTM D1200), specific gravity 0.88 ± 0.02 g/cm³, solids 36 ± 4% by weight, thermal class 180°C, dielectric strength 60–90 kV/mm (IEC 60464-2), VOC content 230.48 g/L, shelf life 9 months, and a cure of 30–60 minutes at 50–80°C, with a 200 L drum package and UL certification. [2]

Editorial calculation: at 0.88 g/cm³ and 36% solids by weight, each litre of that varnish carries about 0.88 × 0.36 ≈ 0.32 kg of resin solids; compare solids per litre, not only viscosity, when estimating coating build and consumption. [2]

Editorial note: ask the supplier for viscosity and solids control limits per lot, the cure schedule for your winding mass, and how the varnish behaves with your magnet wire in a compatibility test.

Ask for trickle and roll-dip resin data for small motors

Axalta's catalogue page for Voltatex 4310 describes a one-pack, ready-to-use, thermosetting impregnating resin on a polyesterimide base, vinyltoluene-free and low-emission, for trickle feed and roll dip application on power tool stators and rotors in the low-voltage market. It lists a 180 °C thermal class and a viscosity of 800–1,200 mPa·s at 25 °C measured to DIN 53019, describes the resin as highly reactive with high mechanical strength, and marks it UL-approved. [4]

The page states compliance with REACH, Regulation (EC) No 1907/2006, and RoHS, Directive 2011/65/EU, and releases the full technical data sheet only on request through a form. [4] Editorial note: the air-drying varnish above states viscosity as a Ford cup No. 4 flow time of 30–45 s [2], while this page gives a rotational viscosity in mPa·s; ask each supplier to state viscosity by one method and temperature, and to supply the data sheet with gel time and cure schedule for your trickle or roll-dip line.

Item	Voltatex 4310	Source
Application	Trickle feed; roll dip	[4]
Chemical base	Polyesterimide	[4]
Thermal class	180 °C	[4]
Viscosity at 25 °C (DIN 53019)	800–1,200 mPa·s	[4]

Compare a solventless VPI resin with an air-drying varnish

ELANTAS PDG's data sheet for Ripley E 468-2-7FC-55F describes a single-component, heat-cured, 100% solids, mineral-filled epoxy impregnating resin for transformers, applied by vacuum-pressure or vacuum impregnation, needing no catalyst, and used in UL-recognised insulation systems from Class 130 up to Class 240. [3]

For VPI tank management, the sheet advises storing below 25 °C (6 months shelf life in sealed containers, longer refrigerated at 5 °C), replenishing the tank by 10% or more per month and cooling the resin to 20 °C or below, and letting preheated units cool below 30 °C before immersion for good penetration. [3] Compare these handling needs with an air-drying solvent varnish applied by dip, brush or spray. [2]

Editorial calculation: 2,000 V/mil ≈ 79 kV/mm and 1,400 V/mil ≈ 55 kV/mm; 2,400 psi ≈ 16.5 MPa; 12.8–13.2 lb/gal ≈ 1.53–1.58 g/cm³.

Property (ELANTAS data sheet)	Condition	Value
Viscosity	25 °C	4,000–8,000 cP
Weight per gallon	25 °C	12.8–13.2 lb
Sunshine gel time	135 °C	12–20 min
Flash point (ASTM D93)	—	Above 94 °C
Cure	After unit reaches temperature	4 h at 135 °C or 3 h at 150 °C
Dielectric strength (ASTM D149)	0.5 mil, 25 °C; after 24 h in water	2,000 V/mil; 1,400 V/mil
Dissipation factor / dielectric constant	1 kHz, 25 °C	0.01 / 3.4
Volume resistivity (ASTM D257)	25 °C	7.0 × 10¹⁵ Ω·cm
Tensile strength / elongation (ASTM D638)	25 °C	2,400 psi / 20%
Glass transition (TMA)	—	47–52 °C

State the impregnation method (dip, trickle, VPI), cure oven capacity, the UL insulation system you must match, and ask for viscosity, gel time at your cure temperature, tank stability and replenishment advice.

Insulation system, thermal class, chemistry, application, cure, viscosity and VOC on a varnish order

The first two rows come from UL's certification rules and decide whether a varnish may be used in your system at all; the rest are process values read from a data sheet such as WEG's. [1] [2]

Item	What it covers	Fix in the order
EIS and magnet wire	Compatibility basis. [1]	EIS; MW type.
Thermal class	Letter class from UL 1446 ageing. [1]	Rating per wire.
Chemistry	E.g. modified polyester. [2]	Type.
Application	Immersion, brush, spray. [2]	Method.
Cure	Time and temperature. [2]	Schedule.
Viscosity and solids	Penetration and build. [2]	Limits.
VOC and storage	Solvent content; shelf life. [2]	Limits.

Dielectric strength, thermal class, viscosity, cure, solids and shelf life for a winding varnish

Viscosity is stated by a different method on each sheet reviewed (Ford cup flow time, mPa·s at 25 °C, cP), so fix one method and temperature in the order. [2] [4] [3]

Field	What it means and published values	Ask the supplier for
Dielectric Strength (kV/mm)	E.g. 60–90 kV/mm (IEC 60464-2) in one data sheet. [2]	Value; method.
Volume Resistivity (Ω·cm)	Insulation property.	Value.
Thermal Class (Class)	E 120 to H 180°C and above (UL letters). [1]	Rating for your MW.
Viscosity (cP)	E.g. 30–45 s Ford cup 4. [2]	Value in your units.
Cure Temperature (°C)	E.g. 50–80°C, 30–60 min. [2]	Schedule.
Glass Transition Temperature (°C)	Cured resin.	Value.
Solvent Content (%)	100% minus solids; e.g. 36% solids. [2]	Value; VOC.
Flash Point (°C)	Storage and oven safety.	Value; SDS.
Density (g/cm³)	E.g. 0.88. [2]	Value.
Shelf Life (months)	E.g. 9 months. [2]	Value.
Moisture Resistance (MΩ)	Insulation after humidity.	Test method.
Gel time and cure (VPI resin)	12–20 min at 135 °C; cure 4 h at 135 °C or 3 h at 150 °C in one epoxy resin. [3]	Gel time at your temperature; cure schedule.

Send the system, wire and process data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Equipment: [motor; generator; transformer; coil]
EIS: [designation; varnish required/optional]
Magnet wire: [MW type; class]
Class: [required]
Application: [dip; trickle; VPI; spray]
Cure: [oven; time; temperature]
Limits: [viscosity; solids; VOC]
Documents: [UL recognition; TDS; SDS]

See manufacturer records Send an enquiry through CNFX

Buyer questions

What does a UL thermal class mean for varnish?

UL 1446 ageing rates the varnish on a specific ANSI magnet wire type; classes run from E (120°C) through H (180°C) to C (over 240°C). [1]

Can I swap one varnish for another in a certified motor?

If the varnish is required in the EIS, the substitute needs equal or higher UL class ratings on that wire and must pass the sealed tube compatibility test. [1]

What does a varnish data sheet list?

For example viscosity, density, solids, thermal class, dielectric strength, VOC, shelf life and cure schedule. [2]

How is a solventless VPI epoxy resin cured?

One ELANTAS data sheet gives 4 hours at 135 °C or 3 hours at 150 °C, timed after the unit reaches temperature, with a 12–20 minute gel time at 135 °C. [3]

What does a resin for trickle or roll-dip impregnation look like on paper?

Axalta's Voltatex 4310, for example, is a one-pack polyesterimide resin for trickle feed or roll dip on power tool stators and rotors, listed at a 180 °C thermal class and 800–1,200 mPa·s viscosity at 25 °C. [4]

Sources and what still needs a direct check

Sources checked 2026-10-12. The UL document explains UL's own certification programme (2016); the WEG and ELANTAS data sheets and Axalta's catalogue page each describe that producer's own product, cited as examples, not endorsements.

UL — Insulating Varnishes and Impregnating Resins for Electrical Insulation Systems (2016) ↗ Certification body guidance Return to text ↑
WEG — LACKTHERM 1351 H impregnation varnish technical data sheet ↗ Manufacturer data Return to text ↑
ELANTAS PDG — Ripley E 468-2-7FC-55F single-component epoxy VPI resin, technical data sheet (PDF) ↗ Maker data sheet Return to text ↑
Axalta Coating Systems — Voltatex 4310 trickle resin, electrical insulation product catalogue page ↗ Maker technical page Return to text ↑

Still to check yourself

Name the EIS and magnet wire.
Check UL class ratings.
Match cure to your ovens.
Set viscosity and solids limits.
Verify the legal supplier and factory.

Electrical insulation varnish compound