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| HS Code | 683597 |
| Productname | Trimellitic Anhydride (TMA) |
| Chemicalname | 1,2,4-Benzenetricarboxylic anhydride |
| Casnumber | 552-30-7 |
| Ecnumber | 209-008-0 |
| Molecularformula | C9H4O5 |
| Molecularweight | 192.13 g/mol |
| Appearance | White to off-white crystalline solid, flakes, or powder |
| Odor | Mild characteristic odor |
| Meltingpoint | 161-163 °C |
| Boilingpoint | 390 °C at 760 mmHg |
| Flashpoint | 227 °C (closed cup) |
| Density | 1.54 g/cm³ at 20 °C |
| Vaporpressure | Very low; approximately 1.7E-6 mmHg at 25 °C |
| Solubilityinwater | Reacts with water/hydrolyzes to trimellitic acid |
| Solubilityinorganicsolvents | Soluble in acetone, ethyl acetate, acetic acid, and dioxane |
| Purity | ≥99.0% (typical commercial grade) |
| Acidity | Acidic; forms trimellitic acid upon hydrolysis |
| Reactivity | Reacts with water, alcohols, amines, and bases |
As an accredited Trimellitic Anhydride TMA factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
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During high-temperature PVC compounding for automotive engine compartment wiring, TMA-derived trioctyl trimellitate is metered from heated bulk storage at 60–80°C into a high-intensity hot mixer after the PVC resin reaches 90–110°C. The TMA-derived ester is synthesized upstream in a stainless steel batch reactor by reacting TMA with 2-ethylhexanol at 180–230°C under a rectification column; tetrabutyl titanate catalyst at 0.1–0.3 wt% of charge accelerates esterification, and vacuum stripping continues until residual acid number falls below 0.5 mg KOH/g. In the PVC dry blend, TMA-derived trioctyl trimellitate is incorporated at 35–70 phr based on PVC resin mass, with higher loadings used for 105°C service wires and lower loadings for stiffer conduit compounds. The dry blend is transferred to a counter-rotating twin-screw extruder equipped with an L/D ratio of 28:1–44:1 and barrel zones set between 150°C and 175°C, then pelletized and subsequently extruded onto copper conductors through a crosshead die at melt temperatures of 170–190°C. Production lines experience plate-out on screw elements and die lips when liquid plasticizer is introduced too rapidly or when stabilizer consumption is elevated by shear heat; batch-to-batch variation in TMA ester acid value above 0.5 mg KOH/g shifts fusion time and increases insulation resistance failures after wet ageing. Compliance is tested under UL 1581 for flame propagation, ISO 6722:2011 for thermal class up to 125°C, SAE J1128 for automotive primary wire, and RoHS Directive 2011/65/EU for restricted phthalate plasticizers; TMA-derived trimellitate esters are selected because they do not depend on ortho-phthalate chemistry and exhibit low migration relative to standard phthalates. Terminal products include engine compartment cable, appliance wiring, medical device power cords, and industrial control cables where prolonged heat exposure would volatilize lighter esters.
In carboxyl-functional polyester resins intended for TGIC or β-hydroxyalkylamide crosslinking, TMA is charged as a polyacid chain extender during the second-stage esterification after the resin clears. TMA content between 3–12 wt% of total monomer mass raises acid value and functionality; TGIC systems typically target 30–40 mg KOH/g acid value and glass transition temperatures of 55–70°C, while HAA systems operate near 20–35 mg KOH/g to reduce cure-related pinhole defects. Synthesis occurs in a stainless steel batch reactor with xylene azeotropic reflux at 230–250°C, followed by vacuum stripping at 180–220°C to remove unreacted glycol and condensation water; the resin is then discharged onto flaking belts and crushed to flakes. In downstream powder coating production, resin, TiO2, flow agent, and curing agent are pre-mixed in a high-intensity mixer and melt-extruded through a co-rotating twin-screw extruder with L/D 16:1–20:1 at barrel temperatures 90–110°C, followed by chill-roll cooling, ACM milling, and air classification to a median particle size of 20–40 µm. The critical processing window is narrow: over-reaction in the reactor produces gel particles that survive extrusion and appear as film defects; under-reaction leaves unreacted tin catalyst residues that reduce overbake yellowing resistance. Gel time measured on a hot plate at 180°C shifts by 20–60 s across a 5 mg KOH/g acid-value drift, and electrostatic application efficiency falls when particle size distribution exceeds ±8 µm from target. Compliance testing under Qualicoat Class 2, AAMA 2604-22, ASTM D523-20, ASTM B117-19, and ASTM D3359-17 documents gloss retention, filiform corrosion resistance, and adhesion after 1000 h salt spray exposure. Terminal products include architectural aluminum profiles, automotive alloy wheels, agricultural equipment housings, and HVAC cabinets.
TMA is introduced at 4–12 wt% of finished resin solids in medium-oil alkyd cooking, producing carboxylic acid groups that are neutralized with dimethylethanolamine at 80–100°C before water dispersion. The resin is cooked by single-stage azeotropic polycondensation in a reinforced stainless steel reactor with xylene reflux at 180–230°C; after clearing, TMA is added at 150–170°C to limit anhydride losses, and the reaction is continued until acid value reaches 35–60 mg KOH/g. Vacuum stripping removes xylene to 0.5 wt% residual, and cooling to 80–100°C is followed by neutralization with dimethylethanolamine at 80–95% of the calculated acid equivalent. A rotor-stator disperser then incorporates deionized water under high shear to yield a translucent to opaque emulsion with solids 40–50 wt%. Processing failures occur when the acid value falls below 30 mg KOH/g, causing viscosity spikes and phase separation during water letdown; storage hydrolysis of ester linkages, accelerated at pH greater than 8.5, reduces film hardness and re-coat adhesion. Performance is evaluated by ASTM D1640 for dry time, ASTM D522 for mandrel flexibility, ASTM D3359-17 for adhesion, and ISO 2811-1 for density; VOC limits align with Directive 2004/42/EC. Terminal products include direct-to-metal machinery primers, steel furniture topcoats, and agricultural equipment enamels.
Because rotating equipment insulation must resist thermal ageing at 180–200°C while surviving high-speed winding, TMA-derived polyesterimide varnishes are applied on copper or aluminum conductors in multi-pass enamelling ovens. In resin synthesis, TMA is reacted with aromatic diamines and polyols in cresol/NMP solvent at 180–220°C; the TMA charge typically occupies 25–45 mol% of the acid/anhydride monomer composition, and the condensation continues until dynamic viscosity reaches 0.5–2.0 Pa·s at 30°C and solids reach 30–40 wt%. The varnish is diluted with higher aromatic solvents to application viscosity and pumped through multi-pass dies onto wire moving through vertical or horizontal ovens with zone temperatures between 400°C and 550°C; line speed depends on wire diameter and film build, commonly 50–300 m/min. The main production bottleneck is pinhole formation caused by solvent boil at the first oven zone; if the first zone exceeds 450°C before film degree, dielectric faults increase, while an under-cured film fails flexibility and heat shock tests. Batch-to-batch variation in TMA moisture content above 0.5 wt% shifts imidization extent and alters tan δ, making dielectric loss tangent measurements under IEC 60250 a release test. Thermal class conformity is governed by IEC 60317-8 for class 180 and IEC 60317-13 for class 200, with NEMA MW 1000 MW 30-C and UL 1446 insulation system recognition; heat shock, cut-through, and retention of tap adhesion are tested after ageing under IEC 60216-1. Terminal products include motor windings, automotive alternators, refrigeration compressor motors, and power tool armatures.
Trimellitate ester base stocks synthesized from TMA and linear or oxo C8–C10 alcohols provide higher thermal stability than conventional polyol esters, but their low-temperature viscosity and pour point are sensitive to alcohol branching. Kinematic viscosity at 100°C ranges from 4–9 mm²/s depending on alcohol structure, and pour point falls between -40°C and -20°C; these properties are measured under ASTM D445-19a and ASTM D97. In aviation turbine lubricants qualified to MIL-PRF-23699G or SAE AS5780, trimellitate esters replace 20–100 wt% of the ester base component, with additive packages containing antiwear, antioxidant, and metal deactivators blended in heated stainless steel vessels at 60–80°C. Production of the ester itself uses a batch reactor with nitrogen sparging, followed by thin-film evaporation at 160–190°C under 1–10 mbar vacuum and activated bentonite filtration; residual acidity below 0.1 mg KOH/g is a release criterion. The substitution is limited by seal compatibility and hydrolytic stability under ASTM D2619; if water content exceeds 200 ppm during blending, ester hydrolysis raises acid number and degrades oxidative stability. Terminal products include aircraft gas turbine lubricants, rotary screw compressor fluids, and high-temperature hydraulic fluids.
In polyester polyol synthesis for polyisocyanurate insulation, TMA partially replaces phthalic anhydride to increase aromatic content and char formation under fire exposure. TMA is charged at 5–20 wt% of total polyol raw materials with diethylene glycol and aromatic acids in a stainless steel batch reactor, cooked at 200–240°C under nitrogen, and vacuum-stripped until hydroxyl number reaches 150–300 mg KOH/g and water content falls below 0.05 wt%. The resulting polyol viscosity at 25°C spans 2000–8000 mPa·s; higher TMA loadings require heated storage at 50–70°C to maintain pumpability in continuous lamination. During PIR board production, polyol is blended with catalysts, flame retardants, and pentane blowing agent, then mixed with PMDI in a high-pressure impingement mixer and deposited between facers on a double-belt laminator. The main processing constraint is the viscosity rise at low line speeds; if the blend temperature falls below 40°C, wet-out on aluminum facings decreases and closed-cell content under ISO 4590 falls below 90%. Fire performance is assessed by ASTM E84 or UL 723, with insulation classification under EN 13165:2012 and ASTM C1289-20; smoke development is measured by ISO 5660-1. Terminal products include PIR roof boards, insulated metal panels for cold storage, and HVAC duct panels.
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Trimellitic anhydride (TMA) is supplied as a white to off-white flake or powder with CAS registry number 552-30-7, molecular formula C9H4O5, and molecular weight 192.13 g/mol. The anhydride ring and free carboxylic acid group create a trifunctional aromatic intermediate, which distinguishes TMA from phthalic anhydride and maleic anhydride in polyester synthesis. Commercial bulk material is typically specified at purity not less than 96.0 wt%, melting point 165–168 °C, and moisture content not exceeding 0.20 wt% when shipped in sealed polyethylene-lined fibre drums or bulk containers. No unified model-number system exists across global suppliers; product technical data sheets differentiate flake and powder forms by particle size, bulk density, and packaging configuration. Flake grades reduce dust generation during reactor charging, while powder grades are preferred for faster dissolution in hot esterification media. The material hydrolyses on contact with water to trimellitic acid, and the theoretical acid number after complete hydrolysis is approximately 876 mg KOH/g.
TMA provides three reactive carboxyl groups per molecule after ring opening; phthalic anhydride provides two and pyromellitic dianhydride provides four. In saturated polyester resin design, TMA introduces chain branching and terminal acid functionality without the immediate gel risk associated with pyromellitic dianhydride. Differential scanning calorimetry of cured powder clearcoats measured according to ISO 11357-2 shows a higher glass transition temperature for TMA-based polyester resins than for phthalic anhydride-based resins at equivalent formulation stoichiometry, although the magnitude is governed by diol composition and cure schedule. The comparison table below summarises the structural differences relevant to resin formulation.
| Chemical | Molecular weight | Carboxylic functionality | Polymer architecture | Principal use |
|---|---|---|---|---|
| Trimellitic anhydride | 192.13 g/mol | 3 | Branched saturated polyester or polyamideimide | TOTM plasticizers, powder coatings, wire enamels |
| Phthalic anhydride | 148.12 g/mol | 2 | Linear polyester | General-purpose plasticizers and resins |
| Pyromellitic dianhydride | 218.12 g/mol | 4 | Highly branched or network aromatic polyimide | Polyimide films and high-Tg resins |
| Maleic anhydride | 98.06 g/mol | 2 (unsaturated) | Unsaturated polyester | Fibreglass-reinforced plastics |
Trimellitic acid is the hydrolysed free acid form and is preferred when anhydride ring opening in aqueous media must be avoided. TMA is specified for melt esterification because ring opening consumes hydroxyl groups and liberates one equivalent of water, while the free carboxyl group remains available for branching. The anhydride form also has a lower molecular weight and lower acid equivalent weight than the free acid, which improves reactor loading efficiency on a mole basis. This distinction is important when incoming material has been exposed to moisture; partially hydrolysed TMA contains trimellitic acid and shifts apparent acid number upward.
In liquid epoxy systems, TMA is evaluated as an aromatic anhydride curing agent with a reaction profile between phthalic anhydride and pyromellitic dianhydride. Cure is accelerated by tertiary amine or imidazole catalysts at 0.1–0.5 phr; gelation occurs in the range 150–180 °C and is followed by post-cure for 2–4 h. TMA dissolves in liquid epoxy at 90–120 °C, whereas pyromellitic dianhydride may require higher temperature and can recrystallise on cooling. Glass transition temperature is measured by ISO 11357-2; TMA-based networks typically fall between phthalic anhydride and pyromellitic dianhydride systems. Published data for this specific configuration is limited, and pot-life should be confirmed by viscosity profiling according to ISO 3219 before production-scale mixing.
TMA is first esterified with 2-ethylhexanol under titanium alkoxide catalysis at 200–230 °C to produce tris(2-ethylhexyl) trimellitate (TOTM). Esterification is driven to an acid number below 0.5 mg KOH/g by vacuum stripping. TOTM has a molecular weight of 546.8 g/mol and is used as a primary plasticizer in flexible PVC insulation at 50–70 phr, where its branched aromatic structure reduces volatility and extractive loss. Tensile property retention on compression-moulded specimens is evaluated according to ASTM D638-14; published comparative data show that TOTM compounds retain more tensile elongation after 7 days at 136 °C than dioctyl phthalate compounds of equal plasticizer concentration. Formulations for UL 1581-rated insulation are designed with TOTM where dioctyl phthalate cannot maintain post-ageing elongation above the specified minima.
The lower volatility of TMA-derived TOTM compared with dioctyl phthalate is based on its higher molecular weight and branched triester structure. Plasticizer loss in PVC compounds proceeds through surface evaporation, extraction, and migration; higher molecular weight and lower diffusion coefficient reduce all three mechanisms. Activated carbon volatility testing according to ISO 176:2005 quantifies the evaporation component under forced conditions. Published industrial data show that TOTM mass loss values are lower than dioctyl phthalate under 100 °C test conditions at 24 h intervals; exact ratios depend on compound hardness, filler type, and specimen geometry. The branched aromatic structure also reduces extraction by soap solutions and white mineral oil, which is relevant for building-wire and automotive under-hood jackets exposed to lubricants. This difference is process-relevant only when insulation ageing requirements exceed the retention capability of general-purpose phthalate plasticizers.
Trimellitic anhydride triesters of linear C7–C10 alcohols are also used as high-temperature lubricant basestocks and specialty plasticizers. The branched triester gives lower volatility and higher flash point than linear adipate diesters of equivalent molecular weight. Oxidation-corrosion stability is evaluated by ASTM D4636-17; viscosity increase and deposit formation depend on alcohol chain length and esterification completeness. Production of these triesters uses the same vacuum esterification route as TOTM but replaces 2-ethylhexanol with linear alcohols and may require post-treatment with activated clay to reduce acid number below 0.1 mg KOH/g.
Certificates of analysis for flake-grade material commonly report the parameters listed below. The product is hygroscopic, and surface hydrolysis accelerates above 60% RH, producing free trimellitic acid and reducing esterification or polyester reactor conversion. Bulk storage silos are therefore padded with dry nitrogen, and discharge systems are fitted with heated tracing to maintain a molten handling temperature just above the melting point. Local exhaust ventilation is required during bag dumping and reactor charging because TMA is classified as Respiratory Sensitiser Category 1 under CLP with hazard statement H334. Direct contact with water or steam causes ring opening and exothermic heat release; drainage systems should be designed to prevent molten material solidification and pipe blockage.
| Parameter | Typical specification | Method |
|---|---|---|
| Appearance | White to off-white flake or powder | Visual |
| Purity | ≥ 96.0 wt% | Gas chromatography after derivatisation |
| Melting point | 165–168 °C | Capillary method |
| Moisture | ≤ 0.20 wt% | ASTM E203-16 |
| Acid number after hydrolysis | 870–880 mg KOH/g | ISO 2114:2000 |
| Molten colour, APHA | ≤ 100 | ASTM D1209-05 |
For water-reducible coil coating resins, TMA-derived carboxyl functionality is used to produce anionic dispersions after neutralisation with amines. The free carboxyl group remains pendant after condensation, and resin acid numbers in the range 35–50 mg KOH/g are targeted by titration according to ISO 2114:2000. TMA is charged as part of the acid monomer mixture during two-stage polycondensation at 180–230 °C; the final resin is cut in glycol ether solvent and neutralised with dimethylethanolamine. Increasing TMA content raises acid number and water dilutability but may reduce cured-film chemical resistance and cause micro-foaming during spray application. Commercial waterborne polyester formulations therefore use TMA as a portion of the aromatic acid monomer, not as the sole aromatic source.
TMA is also incorporated into high-solids polyester baking enamels for metal packaging and automotive refinish. A partial condenser with xylene azeotrope maintains condensation at 200–230 °C. Replacement of 5–12 mol% of phthalic anhydride with TMA raises branched content and lowers solvent demand at application viscosity. Cure with hexamethoxymethylmelamine at 130–150 °C gives rapid crosslink response; excess TMA can generate retained acid groups and reduce storage stability with basic pigments. Published data for this specific formulation is limited.
In polyester/epoxy hybrid powder coatings, replacement of phthalic anhydride with TMA at 5–15 mol% of the acid monomer charge increases resin functionality and acid number, typically into the range 50–80 mg KOH/g. The higher functionality shortens hot-plate gel time measured at 180 °C according to ISO 8130-6. Extrusion is performed on a twin-screw extruder with barrel set point 90–110 °C, screw speed adjusted to keep melt temperature below 120 °C, and chill rolls at 10–20 °C. Powder is air-milled to a median particle size of 30–50 μm. Coating films cured at 180 °C for 10 min show higher methyl ethyl ketone solvent resistance in ASTM D5402-19 double-rub testing relative to low-functional phthalic anhydride resins. Excess TMA above the indicated range increases melt viscosity and can produce orange peel; starting-point loadings should be confirmed by factorial experimentation on the specific extruder line.
TMA-containing polyester powder coatings are applied with corona charging guns at 60–90 kV, using powder resistivity and particle shape to control transfer efficiency. Fine particles below 10 μm reduce reclaim efficiency and can cause back-ionisation on conductive substrates; TMA-based formulations are therefore milled to a median size of 35–45 μm. Flow, gloss, and appearance are assessed after cure at 180–200 °C using ISO 2813, and abrasion resistance is measured by ASTM D4060-19. In comparison with linear phthalic anhydride polyester powders, TMA-based powders show higher edge coverage because of higher melt viscosity retention during early cure, but require longer degassing time when thick films exceed 80 μm.
Before application to copper magnet wire, TMA-derived polyamideimide is prepared as a viscous cresylic solution and applied through multi-pass vertical dies. Cure ovens are operated with zone temperatures from 350–550 °C, depending on conductor diameter and line speed. The imide linkages formed from TMA raise thermal endurance and solvent resistance compared with polyester imide and polyamide enamel grades. Thermal class is determined by twist-pair ageing according to IEC 60172; TMA-based polyamideimide wire enamels are used in motors and transformers requiring class 200 or higher performance. The main processing constraint is solution viscosity increase on standing due to residual anhydride reactivity with solvent water; closed solvent handling and viscosity monitoring are required.
Solvent-free laminating adhesive formulations can incorporate TMA-modified polyester polyols prepared from adipic acid, diethylene glycol, and TMA. The reactor is heated incrementally to 220–240 °C under nitrogen, with condensate withdrawal until acid number falls below 1.0 mg KOH/g. Hydroxyl value is then adjusted to 20–40 mg KOH/g for subsequent reaction with aromatic isocyanate prepolymer. The branched aromatic structure raises polyol viscosity compared with linear adipate polyols at equal hydroxyl value, and application at 60–80 °C is required for solvent-free lamination. TMA-modified polyols contribute cohesive strength and heat resistance after full isocyanate cure; final adhesive shear strength is measured according to ASTM D3163-01 on rigid substrates, while peel strength is measured according to ASTM D1876-08 for flexible laminates. Formulators limit TMA content because excessive branching increases glass transition temperature beyond the substrate flexibility window.
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