Tris(2-hydroxyethyl) Isocyanurate (THEIC) Purchasing Guide

Tris(2-hydroxyethyl) isocyanurate, usually called THEIC, is a trifunctional polyol used mainly in heat-resistant polyester resins, magnet wire enamels and electrical insulating materials.

When purchasing THEIC, purity is only one specification to check. Hydroxyl value, acid value, moisture, melting point and impurities can also matter, especially when THEIC is used to make polyester electrical insulation.

This guide focuses on the specifications and application differences that buyers should understand before selecting a THEIC grade.

What Is THEIC?

THEIC (CAS 839-90-7) contains an isocyanurate ring and three hydroxyethyl groups.

The three hydroxyl groups allow THEIC to react as a trifunctional polyol during polyester synthesis. The isocyanurate structure is one reason THEIC is commonly selected for polyester systems that need good performance at elevated temperatures.

Commercial specifications vary between grades and manufacturers. Buyers should therefore compare the actual specification and COA rather than judging THEIC only by a stated purity.

Why Is Hydroxyl Value Important for THEIC?

THEIC contains three hydroxyl groups per molecule.

Based on its molecular weight of about 261.23 g/mol, pure THEIC has a theoretical hydroxyl value of approximately:

3 × 56,100 ÷ 261.23 ≈ 644 mg KOH/g

This is very close to the 630–650 mg KOH/g range commonly seen in commercial THEIC specifications.

This explains why hydroxyl value is a useful purchasing parameter.

THEIC is normally purchased as a reactive polyol, not simply as an inert additive. The hydroxyl groups participate in polyester formation. A stable hydroxyl value therefore helps resin manufacturers control raw-material calculations and batch consistency.

For this reason, two products both sold as “THEIC 99%” may still need to be compared by hydroxyl value.

Where Is THEIC Mainly Used?

Magnet Wire Enamels

Magnet wire insulation is one of the best-established applications of THEIC.

THEIC reacts with glycols and aromatic diacids or diesters to form polyester resins that can be applied to copper or aluminum wire.

The reason for using THEIC here is not simply “heat resistance.” THEIC polyester insulation has been developed for a combination of properties including:

• dielectric performance;
• adhesion to wire;
• flexibility;
• heat shock resistance;
• heat aging resistance.

These properties are important because an enameled wire coating must remain electrically insulating while also surviving winding, bending and elevated operating temperatures.

Historical magnet-wire technology also shows that THEIC polyesters can be formulated into relatively high-solids enamels. This reduces the amount of solvent required compared with some older low-solids polyester enamel systems.

Electrical Insulating Varnishes

THEIC is also used as a polyol raw material for heat-resistant electrical insulating varnishes.

For this application, raw-material consistency deserves more attention than it might in a less demanding resin system.

In particular, ionic and metal impurities should be considered.

Technical literature on THEIC polyester insulation notes that cationic impurities can increase electrical conductivity in the insulation. They may also interfere with polyester formation.

This means that a THEIC buyer producing electrical insulation should not rely on purity alone.

If the application is sensitive, ask the supplier whether metal or ionic impurity data are available.

Heat-Resistant Polyester Resins

THEIC can be used as the trifunctional alcohol during polyester synthesis.

Its three reactive hydroxyl groups introduce branching into the polyester structure. The resulting resin design can be adjusted for properties such as:

• heat resistance;
• hardness;
• adhesion;
• chemical resistance;
• electrical insulation.

THEIC is therefore better understood as a resin-building raw material rather than a simple additive added at the end of coating production.

The required amount depends on the complete polyester formulation, including the diol, acid or ester, OH/COOH balance and target resin properties.

Other Resin Systems

THEIC is not limited to conventional magnet-wire polyester.

Commercial material suppliers also list THEIC for insulating varnishes, UV-curing resin raw materials and PVC stabilizer applications.

Liquid THEIC derivatives are also commercially available for polyurethane and polyester systems.

However, buyers should distinguish these materials from standard crystalline THEIC. Liquid THEIC grades can have different molecular weights and hydroxyl values and should not automatically be treated as the same raw material in formulation calculations.

THEIC Powder vs. Granules

Standard THEIC is commonly supplied as either white crystalline powder or granules.

Chemically, both forms can meet the same basic THEIC specification. The difference mainly affects handling.

THEIC Powder

Powder provides a larger surface area and may dissolve or melt faster during processing.

However, fine material can create more dust during:

• bag opening;
• weighing;
• conveying;
• reactor charging.

THEIC Granules

Granular THEIC produces less fine dust and may be easier to handle in some production lines.

For large-scale resin production, physical form can therefore be a practical purchasing parameter even when the chemical specification is the same.

Ask for particle size information if your feeding or dissolution process is sensitive to material form.

Some commercial specifications, for example, control THEIC particle size around a defined mesh range rather than simply stating “powder.”

THEIC Solubility: An Often Misunderstood Property

THEIC is highly soluble in water.

Reference data report water solubility around 1,190 g/L at 25°C. Commercial technical data also report approximately 119 g per 100 g of water at 25°C.

Its solubility in common organic solvents is much lower. One commercial grade reports approximately:

This is useful when evaluating handling and formulation.

THEIC should therefore not simply be described as “insoluble in ethanol and acetone.” It is more accurate to say that its solubility in these solvents is much lower than in water.

THEIC vs. TMP

THEIC and trimethylolpropane (TMP, CAS 77-99-6) are both trifunctional polyols, but they have very different molecular structures.

The difference in OH equivalent weight is particularly important.

One kilogram of TMP and one kilogram of THEIC do not provide the same amount of hydroxyl functionality.

This is why replacing TMP with THEIC by weight can change:

• OH/COOH ratio;
• resin molecular structure;
• branching;
• viscosity;
• reaction behavior;
• final resin performance.

THEIC and TMP should therefore not be treated as direct 1:1 weight replacements.

If a formulation is changed, first recalculate the polyol amount on an OH-equivalent basis, then verify the resin through a laboratory or pilot batch.

What Specifications Should You Check When Buying THEIC?

1. Hydroxyl Value

A typical commercial range is around:

630–650 mg KOH/g

or

640 ±10 mg KOH/g

This is one of the most useful parameters for buyers using THEIC in polyester synthesis.

2. Melting Point

Commercial THEIC commonly has a melting range around:

133.5–138°C

A result outside the normal range does not automatically mean the material is unusable, but it should be checked against the supplier’s specification.

3. Moisture / Loss on Drying

Commercial specifications often control moisture or loss on drying to approximately:

≤0.2–0.3%

Moisture control is especially useful for maintaining stable resin production.

Do not assume that every THEIC grade uses the same limit. Confirm the actual specification before purchasing.

4. Acid Value

Acid value is often overlooked in online THEIC product descriptions.

A common commercial specification is:

≤1.0 mg KOH/g

Since THEIC is being used as a polyol in resin synthesis, acid value provides another useful check on raw-material consistency.

5. pH

Commercial THEIC specifications commonly show a near-neutral pH, often around:

6.5–7.5

The test concentration and method should also be checked when comparing data from different suppliers.

6. Turbidity

Some industrial THEIC grades include a turbidity limit for an aqueous solution.

This can help identify insoluble material or contamination that may not be obvious from the basic purity number.

For applications where resin clarity or electrical properties matter, this can be a useful additional QC parameter.

7. Metal and Ionic Impurities

This is particularly important for electrical insulation applications.

THEIC manufacturing can involve catalysts and purification steps. Residual ionic impurities can affect electrical properties and polyester synthesis.

If THEIC will be used for magnet wire enamel or another electrical insulation system, ask whether the supplier can provide information on:

• Fe;
• Cu;
• other metal ions;
• ash or ionic residue, where applicable.

Not every industrial grade controls these parameters, so this requirement should be discussed before ordering.

8. Particle Size

Particle size normally does not change the chemical identity of THEIC, but it can change handling and dissolution behavior.

For automated feeding or large reactors, confirm whether you need:

• fine powder;
• controlled powder;
• granules.

A More Useful Way to Compare Two THEIC Suppliers

Suppose Supplier A offers:

THEIC ≥99%

while Supplier B provides:

• hydroxyl value: 640 ±10 mg KOH/g;
• melting point: 133.5–137°C;
• acid value: ≤1.0 mg KOH/g;
• moisture: ≤0.2%;
• pH: 6.5–7.5;
• controlled particle size.

The first product is not necessarily worse.

But Supplier B has given the buyer more information to determine whether the material will remain consistent in an existing resin process.

For industrial THEIC procurement, this is why a detailed specification + batch COA is usually more useful than a purity number alone.

What Should You Ask for Before a Bulk Order?

Before approving a new THEIC supplier, request:

• product specification;
• recent batch COA;
• SDS;
• hydroxyl value;
• acid value;
• moisture or loss on drying;
• melting point;
• physical form and particle size;
• packaging information;
• metal impurity data if required for electrical insulation;
• production sample for formulation testing.

Then test the sample in your actual resin process.

Check whether it changes:

• dissolution or melting time;
• polyesterification behavior;
• resin viscosity;
• resin color;
• final coating performance.

A specification tells you whether the raw material meets defined limits. A production trial tells you whether it works in your process.

Packaging and Storage

Foconsci’s THEICis supplied in 25 kg bags, while 500 kg or larger bulk bags are also available.

Keep the package tightly sealed and store it in a dry area.

Because THEIC is highly water-soluble and commercial grades often have controlled moisture specifications, protecting opened material from moisture is especially important.

Powder grades should also be handled in a way that limits unnecessary dust generation.

FAQ

Is THEIC mainly a flame retardant?

THEIC contains nitrogen and can be used in some flame-retardant systems or as a reactive intermediate.

However, its best-established industrial role is as a trifunctional polyol, especially in heat-resistant polyester and electrical insulation systems.

It should not automatically be treated as a direct replacement for a conventional flame retardant.

What is the theoretical hydroxyl value of THEIC?

Based on three hydroxyl groups and a molecular weight of approximately 261.23 g/mol, the theoretical value is about 644 mg KOH/g.

Is THEIC soluble in water?

Yes.

THEIC is highly soluble in water. Reference data report approximately 1,190 g/L at 25°C.

Its solubility in ethanol and acetone is much lower.

Can THEIC replace TMP?

Both have three hydroxyl groups, but their molecular weights and OH equivalent weights differ substantially.

Any substitution should be recalculated on an OH-equivalent basis and confirmed through resin testing.

Why should electrical insulation buyers care about impurities?

Ionic impurities can increase electrical conductivity and may interfere with polyester formation.

For magnet wire enamel and other electrical insulation applications, impurity control can therefore matter beyond the basic THEIC purity number.

Should I buy THEIC powder or granules?

Choose according to your production equipment.

Powder may dissolve or melt faster, while granules can reduce dust during handling and feeding.

For automated production, particle size consistency may be more important than simply choosing “powder” or “granules.”

Final Purchasing Check

For industrial resin production, do not compare THEIC suppliers using only:

CAS 839-90-7 + 99% purity + price/kg.

A better comparison is:

Hydroxyl value + acid value + moisture + melting point + impurities + particle form + batch consistency + price.

For general polyester production, hydroxyl value and moisture may be enough to screen a grade.

For magnet wire enamel and electrical insulating resin, impurity control deserves more attention because the raw material ultimately becomes part of an electrical insulation system.

Foconsci supplies Tris(2-hydroxyethyl) Isocyanurate (THEIC), CAS 839-90-7 in industrial quantities. Buyers can request specifications, COA, packaging information and samples for resin testing before bulk purchasing.