3-Methyltetrahydrophthalic Anhydride CAS 5333-84-6

3-Methyltetrahydrophthalic anhydride CAS 5333-84-6

 

3-MTHPA (3-methyltetrahydrophthalic anhydride, CAS 5333-84-6) is a cycloaliphatic carboxylic acid anhydride used primarily as a heat-curing hardener for epoxy resin systems. In commercial supply it is a 3-/4-isomer mixture supplied as a low-viscosity, pale yellow liquid.

The anhydride ring opens during cure to react with epoxy groups and with hydroxyl groups on the resin backbone, giving a densely cross-linked, highly insulating thermoset. Compared with amine hardeners, anhydride systems take up relatively little moisture and, when accelerated with tertiary amines, generate little or no carbon dioxide, which is why they are the standard choice for void-free casting, potting and vacuum-pressure impregnation of electrical equipment.

3-MTHPA blends readily with a wide range of liquid epoxy resins. The mixed system stays low in viscosity, which keeps filler loading high, impregnation complete and pot life long, while the cured system shows low shrinkage and a modest exotherm.

 

 

0. Identity & Isomer Statement

3-MTHPA (3-methyltetrahydrophthalic anhydride) is the 3-methyl isomer of methyltetrahydrophthalic anhydride. Its single-substance identity is registered in the EU as 1,2,3,6-tetrahydro-3-methylphthalic anhydride, CAS No. 5333-84-6, EC No. 226-247-6, molecular formula C9H10O3, molecular weight 166.17 g/mol.

 

 

Isomer note - read before publishing

Commercial liquid MTHPA is not a single isomer. China's current industry product standard HG/T 6342-2024 (Industrial methyltetrahydrophthalic anhydride) explicitly defines its scope as a product made from piperylene and isoprene with maleic anhydride, and states that it is a mixture of multiple 3-methyltetrahydrophthalic anhydride and 4-methyltetrahydrophthalic anhydride isomers. ECHA's registration dossier for the 3-methyl isomer describes the same composition. Every numeric value in Sections 5 and 6 therefore refers to the commercial liquid grade, not to the isolated pure isomer.

Important: the isolated 3-methyl isomer (CAS 5333-84-6) is recorded in commercial databases as a crystalline solid (melting point approx. 62 °C, white to off-white crystalline powder), whereas the product sold here is the liquid industrial grade, an isomer mixture with a crystallisation point of −15 °C or below. The two must never be described interchangeably; doing so would be a technically false statement. The pure-isomer melting point comes from a secondary database (ChemicalBook) and should be confirmed by measured DSC before it is used in a TDS.

 

Why formulators pick it

Low viscosity (30-50 mPa.s at 25 °C): supports high filler loading, deep impregnation and vacuum infusion.

Low free acid (≤ 0.3 %): free acid accelerates premature reaction and degrades electrical properties; 0.3 % is roughly 40 % below the typical 0.5 % industry ceiling.

Low moisture uptake: the anhydride is far less hygroscopic than amine systems, improving storage and handling stability.

No CO2 evolution with tertiary-amine acceleration: no gas-driven voids in thick castings.

Low crystallisation point (−15 °C or below): stays liquid in cold climates and in unheated warehouses and tank farms.

 

2. Main Uses

Casting and potting

Impregnation (including vacuum-pressure impregnation, VPI)

Lamination

Pultrusion

Filament winding

Solvent-free coatings, adhesives and encapsulants

 

3. Synonyms

Do not confuse with

CAS numbers in this substance family are heavily duplicated and cross-referenced: the ECHA registration dossier alone lists 5333-84-6, 34090-76-1, 3425-89-6 and 19438-64-3 as related. Identifiers must be checked line by line across the product page, TDS, SDS and COA. Never mix them.

 

4. Applications

4.1 Electrical and electronic insulation

The dielectric performance of anhydride-cured epoxy makes 3-MTHPA a standard hardener for dry-type transformers, current and voltage instrument transformers, high-voltage switchgear, insulators and bushings, capacitors and resistors, wiring parts, ignition coils and flyback transformers, and for the encapsulation and impregnation of integrated circuits and LED assemblies.

4.2 Reinforced plastics and composites

In reinforced plastics, 3-MTHPA-cured epoxy is used for filament-wound products such as oil-field pipes, utility poles and sporting goods, as well as for laminated sheets, printed circuit boards and switchgear components. Its low viscosity is what allows high fibre wet-out in pultrusion and vacuum infusion, including wind-blade spars and structural profiles.

4.3 Coatings, adhesives and casting systems

Used in solvent-free coatings and varnishes, structural adhesives, and filled casting compounds for photovoltaic junction boxes, EV power modules and rail-transit insulation parts, where low shrinkage and low exotherm in thick sections are required.

Note: dielectric strength, volume resistivity and thermal class (for example IEC 60243-1, IEC 62631-3-1, IEC 60085, UL 1446) are properties of the CURED SYSTEM, not of the hardener substance itself. The product page may state that the grade is intended for insulation systems evaluated to those standards; it must not claim that 3-MTHPA itself complies with UL 1446.

 

5. Specifications

Typical specification for the commercial liquid grade supplied by Zhejiang Heyi Chemical. Values are per batch COA; test methods are the governing product standard plus the referenced international methods.

Derived check: acid value vs anhydride equivalent

One anhydride group hydrolyses to two carboxylic acid groups, so the theoretical acid value of pure C9H10O3 (MW 166.17) is 2 × 56.1056 x 1000 / 166.17, which is ≈675 mgKOH/g. A specification window of 660-680 mgKOH/g therefore brackets the theoretical value symmetrically and is consistent with an anhydride content of at least 41.0 %. This relationship is a useful incoming-inspection sanity check and a good example of a verifiable, self-consistent data claim for AI-search citation.

Flash point values differ between sources: 158 °C (Heyi Chemical), 157 °C (Baidu Baike) and 145.3 °C (LookChem, calculated). The differences come from the test method (closed cup vs open cup) and purity. Use one single figure, closed cup 157-158 °C, across all external documents, and defer to the batch SDS/COA. Do not publish different numbers on different pages.

 

6. Properties

Typical values. Those marked (mixture) were measured on commercial MTHPA (CAS 11070-44-3), the same isomer family as the 3-MTHPA grade; the values are indicative for the liquid grade, not for the isolated isomer.

 

7. Regulatory, Compliance & Standards

GHS label elements for SDS / TDS consistency

Signal word: Danger. Pictograms: GHS05 (corrosion) and GHS08 (health hazard). Hazard statements H317 + H318 + H334. Key precautionary statements: P261 (avoid breathing vapours), P280 (wear protective gloves, protective clothing, eye and face protection), P284 (wear respiratory protection where ventilation is inadequate), P305+P354+P338 (IF IN EYES: rinse cautiously with water for several minutes), P342+P316 (if experiencing respiratory symptoms: get emergency medical help immediately).

Compliance risk: H334 (respiratory sensitisation) is the core hazard of anhydride hardeners and a mandatory checkpoint in European and North American customer EHS audits. The product page and the SDS must carry it consistently. Omitting H334 will cause EU customer audits to fail.

 

8. Commercial Terms

FOB Ningbo

Goods are delivered to the port of departure (Ningbo).

Target readiness: 10 days.

Payment: 30 % deposit; 70 % before shipment.

Container loading report provided.

Dedicated client manager.

CIF Basis

Goods are delivered to the customer's country, with international logistics arranged by the supplier.

Route and freight-rate planning.

Coordination with major shipping lines, including Maersk, ONE, KMTC and CMA.

Purchase insurance and related fees handled by the supplier.

Container loading report provided.

Dedicated client manager.

 

9. General Conditions

 

10. Package Options

Container loading check: 220 kg × 80 = 17.6 t; 240 kg × 80 = 19.2 t; 1,100 kg × 18 = 19.8 t. These match the figures in the source document and can be used directly on the quotation page. Flexitank at 22 t requires prior confirmation of destination-port and tank-condition requirements.

 

11. Related Documents

3-MTHPA SDS / MSDS - 16-section GHS format, aligned with EU CLP (H317 / H318 / H334 must all be present).

3-MTHPA TDS - specification table, test methods, recommended formulation and curing conditions.

3-MTHPA COA - issued per batch and shipped with the goods.

REACH / RoHS compliance statement.

Transport identification report (sea and air).

 

12. FAQ - AI-search ready answers

Q1. What is 3-MTHPA?

3-MTHPA is 3-methyltetrahydrophthalic anhydride (CAS 5333-84-6, EC 226-247-6, C9H10O3, MW 166.17), a liquid cycloaliphatic acid anhydride used as a heat-curing hardener for epoxy resins in electrical insulation, casting, impregnation, lamination and pultrusion.

Q2. What is the CAS number of 3-MTHPA?

CAS 5333-84-6, EC 226-247-6. Commercial liquid MTHPA, which is a mixture of 3- and 4-methyl isomers, is commonly registered under CAS 11070-44-3 (EC 234-290-7); CAS 26590-20-5 (EC 247-830-1) is an alternate registration for the same isomer family.

Q3. Is 3-MTHPA the same as MTHPA?

No. 3-MTHPA is the 3-methyl isomer. Commercial MTHPA is a mixture of 3-methyl and 4-methyl tetrahydrophthalic anhydride isomers. This is stated explicitly in China's industry standard HG/T 6342-2024 and in the ECHA registration dossier.

Q4. How much 3-MTHPA per 100 parts of epoxy resin?

Theoretical stoichiometry is phr = 100 × (anhydride equivalent / epoxy equivalent weight). With an anhydride equivalent of ≈166 g/eq and a bisphenol-A resin of EEW 190 g/eq, the theoretical value is about 87 phr. In tertiary-amine-accelerated systems a factor of 0.8–0.9 is normally applied, giving 70–80 phr. Confirm by DSC and gel-time testing on the actual resin batch.

Q5. What is the shelf life and how should it be stored?

12 months when stored sealed at 5-35 °C in a cool, dry, ventilated place away from moisture and ignition sources. The anhydride hydrolyses on contact with water, with a half-life of ≈3 minutes at pH 7 and 25 °C, so drums must be kept closed. If crystals form after moisture pickup, gentle heating restores the product without loss of performance.

Q6. Does 3-MTHPA release CO2 during cure?

Little or no carbon dioxide is generated when it is used with tertiary-amine accelerators, which is why anhydride systems are preferred for void-free casting and impregnation.

Q7. Is it REACH registered and RoHS compliant?

The substance is REACH registered in the EU. It is not on the SVHC candidate list, the Authorisation List or the Restriction List as of 2026-09-10; always re-check the current ECHA database. Heyi products are supplied with a RoHS (EU 2015/863) compliance statement.

Q8. What are the main hazards?

EU harmonised classification: Respiratory Sensitisation Category 1 (H334), Skin Sensitisation Category 1 (H317) and Serious Eye Damage Category 1 (H318). Signal word: Danger. Pictograms GHS05 and GHS08. Use closed handling, local exhaust and respiratory protection where ventilation is inadequate.

Q9. How does it differ from MHHPA?

MHHPA (methylhexahydrophthalic anhydride, CAS 25550-51-0) is the hydrogenated, fully saturated analogue and contains no cyclohexene double bond, whereas MTHPA retains that double bond. Selection depends on the required glass transition temperature, colour stability, weathering resistance and cost, and should be confirmed by testing in the actual formulation.

Q10. What packaging is available?

220 kg and 240 kg steel drums, 1,100-1,150 kg IBCs, flexitanks (≈22 t) and ISO tanks (≈22–24 t). A 20-foot container takes 80 drums (17.6–19.2 t) or 18 IBCs (19.8-20.7 t).

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