Section
| HS Code | 914535 |
| Name | Maleic Acid |
| Iupac Name | (2Z)-but-2-enedioic acid |
| Chemical Formula | C4H4O4 |
| Molar Mass | 116.07 g/mol |
| Cas Number | 110-16-7 |
| Ec Number | 203-742-5 |
| Appearance | White crystalline solid |
| Odor | Odorless |
| Density | 1.59 g/cm3 |
| Melting Point | 135 °C (decomposes) |
| Boiling Point | Decomposes before boiling |
| Water Solubility | 788 g/L at 20 °C |
| Flash Point | 127 °C |
As an accredited Maleic Acid factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | Maleic Acid is supplied in 25 kg moisture-resistant, double-lined polyethylene bags with sealed inner liners and clear hazard labeling. |
| Container Loading (20′ FCL) | Maleic Acid is palletized in 25 kg bags and loaded into a 20′ FCL container, secured, evenly distributed, and documented for safe transport. |
| Shipping | Ship Maleic Acid as UN 3261, Corrosive solid, acidic, organic, n.o.s. (maleic acid), Class 8, Packing Group III, in sealed, corrosion-resistant packaging with correct labels and documentation. Keep dry, away from alkalis and oxidizers, and follow DOT/IMDG/IATA segregation and emergency-response requirements. |
| Storage | Store maleic acid in a cool, dry, well-ventilated area away from direct sunlight, heat, and ignition sources. Keep containers tightly closed, properly labeled, and corrosion-resistant. Segregate from bases, oxidizers, metals, and foodstuffs. Protect from moisture and dust; provide spill containment and eyewash/safety shower. Use PPE, inspect containers regularly, and follow local hazardous-chemical storage regulations. |
| Shelf Life | Maleic acid is stable for several years when kept sealed in a cool, dry, well-ventilated area, away from moisture and incompatible materials. |
Manufacturing of unsaturated polyester resin uses maleic acid as the α,β-unsaturated diacid component in step-growth polycondensation with propylene glycol, ethylene glycol, or neopentyl glycol. Maleic acid charged at 28–42 mol% of total dibasic acid load, equivalent to 16–29 wt% of initial reactor charge depending on glycol molar mass and phthalic anhydride balance, introduces unsaturation that is later crosslinked with styrene. Because the maleate ester has a cis configuration, polycondensation at 180–210 °C induces partial isomerization to fumarate ester sequences; this drift raises cured network crosslink density, tensile modulus, and glass transition temperature. Process control of this isomerization drift is critical because excessive fumarate sequences shorten gel time during styrenation and can produce microgel particles. The replacement of maleic anhydride by maleic acid increases the water of esterification by 1 mol water per mol maleic acid and requires xylene recycle capacity to be raised by 12–18% relative to an anhydride-based formulation. In a 6,300 L to 12,000 L 316L stainless steel reactor equipped with a Dowtherm A jacket, anchor agitator at 8–14 rpm, packed column, and decanter, atmospheric esterification proceeds with xylene at 8–12 wt% of total batch until the acid value falls below 45 mg KOH/g. Vacuum is then applied at 8–16 kPa absolute and the temperature is held at 190–210 °C until the finished acid value reaches 18–28 mg KOH/g; cone-and-plate melt viscosity at 125 °C is typically 350–650 mPa·s. Styrene letdown at 30–36 wt% is conducted below 85 °C with hydroquinone inhibitor at 80–150 ppm, and the final resin is stored under air to preserve inhibitor activity.
Production-scale failure modes are concentrated in two regions. First, if the reactor wall temperature exceeds 220 °C, decarboxylation and color body formation accelerate and the isomerization drift can exceed 42 mol% of total unsaturation, producing gel particles during styrene letdown. Second, if the wall temperature remains below 170 °C, esterification rate declines and the acid value plateau remains above 35 mg KOH/g, yielding a resin with poor compatibility with styrene and low final molecular weight. Composite fabricators convert the resin into filament-wound storage tanks, pultruded structural profiles, SMC and BMC automotive panels, gel-coated marine hulls, and corrosion-resistant process piping. Acceptance testing uses ISO 527-4:2023 for tensile properties of orthotropic FRP composites, ISO 14125:1998/Amd 1:2011 for flexural properties, ASTM D638-22 for cast resin tensile elongation, ISO 75-2:2024 for HDT at 1.8 MPa, and ASTM D792-20 for density. GRP chemical tanks may require EN 13121-3:2016 design verification. EU REACH registration for maleic acid and styrene monomer is mandatory; residual styrene in cold-cured laminates is typically specified below 0.01 wt% by end-use specification for enclosed-space applications.
| Standard | Scope | Typical acceptance criterion |
|---|---|---|
| ISO 527-4:2023 | Tensile properties of FRP composites | CSM laminate tensile strength ≥ 60 MPa; tensile modulus ≥ 6.0 GPa |
| ISO 14125:1998/Amd 1:2011 | Flexural properties of FRP composites | Flexural strength ≥ 120 MPa; flexural modulus ≥ 7.0 GPa |
| ASTM D638-22 | Unreinforced cast resin tensile properties | Elongation at break 1.5–3.0% |
| ISO 75-2:2024 | Heat deflection temperature at 1.8 MPa | 65–85 °C depending on styrene content |
| ASTM D792-20 | Density of cured resin and composite | 1.10–1.25 g/cm³ |
In circulating cooling water and reverse osmosis antiscalant production, maleic acid is copolymerized with acrylic acid in aqueous solution to yield a low-molecular-weight carboxylated dispersant that inhibits calcium carbonate, calcium sulfate, and barium sulfate scale through threshold inhibition and crystal distortion. The monomer charge is 10–35 wt% maleic acid on total monomer mass, and total monomer concentration is maintained at 25–45 wt% in deionized water. Polymerization is run in a 2,000–5,000 L 316L continuous stirred-tank reactor at 82–95 °C, using ammonium persulfate initiator at 1.0–3.0 wt% based on total monomer and sodium hypophosphite chain-transfer agent at 2–8 wt% to control weight-average molecular weight to 3,000–15,000 g/mol. The maleic acid feed is preheated to 60–70 °C because maleic acid solubility at 25 °C is 78.8 g/100 g water, and feed lines colder than 55 °C allow undissolved monomer to accumulate, causing oscillating redox initiator consumption and batch-to-batch viscosity drift. The resulting acid copolymer is partially neutralized with 30 wt% NaOH to pH 4.0–5.5 and finished as a 40–50 wt% solids liquid. Static scale inhibition is evaluated according to NACE TM0374-2007 or GB/T 16632-2019, with typical calcium carbonate inhibition of 80–95% at 10 mg/L active polymer in 800 mg/L CaCO₃ hardness at 60–80 °C for 24 h. For potable water contact, NSF/ANSI/CAN 60 certification is required; for reverse osmosis antiscalant qualification, membrane tests may use ASTM D8002-18 to standardize permeate flow loss. Finished product types include phosphonate-free cooling tower scale inhibitors, brackish water reverse osmosis antiscalants, and dispersant blends for seawater desalination. The material is not suitable for continuous chlorine residuals above 2 mg/L at pH below 6.5 because oxidative chain scission reduces molecular weight. Published data for high-molecular-weight grades beyond 20,000 g/mol is limited; the practical production ceiling is 20,000 g/mol before cross-batch viscosity drift exceeds ±10%.
| Standard or method | Applies to | Typical specification point |
|---|---|---|
| NACE TM0374-2007 | Laboratory screening of scale inhibitors for cooling water | Calcium carbonate inhibition ≥ 85% at 10 mg/L active |
| GB/T 16632-2019 | Determination of inhibition performance of scale inhibitors | Calcium carbonate inhibition ≥ 90% for formulated product |
| NSF/ANSI/CAN 60 | Drinking water treatment chemicals | Maximum certified dose per manufacturer label |
| ASTM D8002-18 | RO membrane fouling data standardization | Permeate flow loss ≤ 10% during antiscalant screening |
Offset ink vehicles and rosin-based paper-sizing applications require rosin to be first reacted with maleic acid to create a higher-functionality adduct before esterification with polyols. Maleic acid is charged at 6–12 wt% of rosin solids, and the Diels-Alder addition is carried out at 180–200 °C under nitrogen in a 5,000–15,000 L stainless steel or glass-lined reactor fitted with a condenser and decanter. The equipment load pattern is distinctly different from unsaturated polyester processing: maleic acid is added slowly over 30–90 min to avoid local acid accumulation, which generates insoluble fumaric acid byproduct and reduces adduct yield. After the addition, the reaction mass is held for 60–120 min until the acid value stabilizes. Pentaerythritol or glycerol is then charged, and esterification proceeds at 250–270 °C under CO₂ sparge until the acid value reaches 15–25 mg KOH/g. The finished maleic-modified rosin ester has a softening point of 95–125 °C by ring-and-ball method and is used in sheet-fed offset ink vehicles where it provides tack and gloss development; in paper sizing, the adduct is neutralized with sodium or potassium hydroxide to form a rosin soap size. Compliance for food-contact paper and board relies on 21 CFR 176.170 and 21 CFR 175.105 where the rosin derivative is part of a coating or adhesive; offset ink vehicles are tested against ISO 2846-1:2017 for color and transparency. Terminal finished products include quickset sheet-fed offset inks, heat-set web offset inks, rosin soap sizes for fine paper and board, and tackifier dispersions for water-based inks. The operational boundary is non-negotiable: the adduction stage must not exceed 210 °C, because decarboxylation of rosin acids and maleic acid becomes measurable, and the acid value drops without the expected adduct formation, producing lower-molecular-weight resin with reduced tack stability.
When food-grade fumaric acid is required, maleic acid is isomerized in aqueous hydrochloric acid with a thiourea catalyst, exploiting the large solubility difference between the cis and trans isomers. Maleic acid is charged as a 25–40 wt% aqueous solution, with hydrochloric acid at 2–5 wt% of reaction medium and thiourea at 0.05–0.5 wt% of maleic acid feed. The reaction is carried out in a glass-lined stirred tank at 90–105 °C for 1–3 h. Fumaric acid precipitates during the hold because its solubility at 25 °C is 0.63 g/100 mL, while maleic acid is 78.8 g/100 mL; this crystallization pulls the equilibrium toward the trans isomer and yields 85–95% isolated yield after centrifugation. The crude fumaric acid is reslurried in deionized water and recrystallized to reduce thiourea and chloride carryover below food-grade limits. Equipment includes a glass-lined reactor, Hastelloy C276 heat exchanger, and bottom-discharge centrifuge; rubber-lined steel is not recommended because the acid conditions and abrasive crystals reduce service life. Compliance is established against FCC 14, USP-NF 2024, EU 231/2012, and JECFA 2006 monographs for fumaric acid; heavy metals, chloride, and residual maleic acid are controlled by the recrystallization step. Terminal finished products include dry beverage acidulants, bakery acidulant blends, meat-curing acidifiers, and unsaturated polyester resins made from fumaric acid where higher crosslink density is needed. Fumaric acid also serves as a raw material for L-aspartic acid through enzymatic amination using immobilized aspartase at 37 °C and pH 7.5. The critical process conflict is temperature control: below 85 °C the isomerization rate falls sharply and batch time extends beyond 8 h; above 110 °C condensation byproducts and color body generation increase, and fumaric acid losses through decarboxylation become measurable. Published data for continuous fixed-bed isomerization of maleic acid is limited; industrial practice remains batch crystallization.
Ammonia-based thermal polyaspartic acid production converts maleic acid into a biodegradable polyamino acid antiscalant through a two-stage molten condensation and alkaline hydrolysis sequence. Maleic acid is charged at 60–70 wt% of dry reaction mass, and ammonia is fed at 1.05–1.25 molar ratio to maleic acid. The condensation is run in a 3,000 L 316L high-shear kneader reactor at 170–230 °C for 2–6 h, with a vacuum ramp from atmospheric pressure to 10–20 kPa absolute to remove water and residual ammonia while minimizing unreacted maleic acid carryover. On production lines, batch-to-batch variance in polysuccinimide melt viscosity is the dominant bottleneck; at temperatures above 220 °C the melt viscosity can exceed the kneader torque limit quickly if ammonia flow is interrupted, while below 170 °C the conversion stalls. The intermediate polysuccinimide is then hydrolyzed with 10–20 wt% NaOH at 60–80 °C for 1–3 h, producing a 20–40 wt% solids solution with pH 8.5–10.0. Biodegradability is evaluated by OECD 301B, with typical ready biodegradability above 60% ThOD, and scale inhibition is tested by NACE TM0374-2007. The product is used as a biodegradable cooling tower antiscalant, oilfield scale inhibitor package, and agricultural fertilizer synergist. The hydrolysis stage is not compatible with acid pH below 7.0, as free polyaspartic acid precipitates and reduces the active polymer content in the final liquid.
Catalytic hydration of maleic acid to DL-malic acid is conducted in a high-pressure reactor under acid catalysis, producing the racemic hydroxyacid used in food, pharmaceutical, and buffering applications. Maleic acid is charged as a 35–50 wt% aqueous solution, with sulfuric acid catalyst at 0.5–2.0 wt% of maleic acid feed. The reaction is held at 160–200 °C and 0.6–1.5 MPa for 3–8 h; conversion is thermodynamically limited to 65–80% per pass, with residual maleic acid and fumaric acid recovered from the mother liquor. The equipment train includes a Hastelloy C276 stirred pressure reactor, graphite block heat exchanger, and forced-circulation crystallizer; 316L stainless steel is acceptable only at catalyst concentrations below 1.0 wt% and temperatures below 180 °C. Crude DL-malic acid is recrystallized from deionized water to reduce fumaric acid byproduct to 1–5 wt%, followed by a second recrystallization if USP-NF purity is specified. Compliance is established against FCC 14, USP-NF 2024, EU 231/2012, and JECFA 2006 monographs for DL-malic acid. Terminal finished products include confectionery acidulants, fruit-flavored beverage acidifiers, pharmaceutical effervescent buffers, and synthetic flavor ester precursors. The critical operational boundary is the isomerization/hydration balance: above 200 °C, fumaric acid formation and decarboxylation products increase at the expense of malic acid selectivity, while below 160 °C the hydration rate is too low to achieve acceptable cycle time. Published data for continuous maleic acid hydration is limited; industrial batch hydration remains the standard because of the narrow temperature window and the need to recycle the mother liquor.
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Maleic acid (CAS 110-16-7; EINECS 203-742-5; (2Z)-but-2-enedioic acid; C4H4O4; molar mass 116.07 g mol−1) is the cis butenedioic acid isomer, supplied as a white crystalline solid with a bulk density that varies with crystal habit but generally falls between 0.80 and 0.95 g cm−3. The compound has pKa1 1.90 and pKa2 6.07 at 25 °C, and aqueous solubility of approximately 79 g per 100 mL at 25 °C. Thermal analysis of commercial material often shows a melting range of 130–139 °C with simultaneous dehydration to maleic anhydride; the cyclodehydration rate increases above 150 °C. There is no unified ISO model designation for this product; commercial models are producer-specific codes that distinguish assay, fumaric acid content, moisture, iron content, and packaging configuration. Standard packaging includes 25 kg multi-wall paper bags with a polyethylene liner and 500 kg or 1000 kg fabric bulk bags. Food-grade maleic acid is not the dominant market form; industrial technical grade is the primary material traded, with downstream food-use compliance applied to fumaric acid and malic acid rather than to technical maleic acid itself.
The most operationally significant comparison is between maleic acid, maleic anhydride, and fumaric acid. Maleic acid and fumaric acid share the same molecular formula and molar mass, but their geometric isomerism changes melting behaviour, water solubility, acid strength, and condensation reactivity. Maleic acid is a cis diacid with high water solubility; fumaric acid is the trans isomer with low water solubility and higher thermal stability. Maleic anhydride is the intramolecular dehydration product of maleic acid and is preferred where water-free processing and reactive anhydride functionality reduce esterification time and energy input.
| Property | Maleic acid | Maleic anhydride | Fumaric acid |
|---|---|---|---|
| CAS registry | 110-16-7 | 108-31-6 | 110-17-8 |
| Geometric configuration | cis diacid | cis anhydride | trans diacid |
| Melting behaviour | 130–139 °C with dehydration | 52–53 °C | 287 °C sealed tube; sublimes |
| Water solubility at 25 °C | approximately 79 g per 100 mL | reacts to maleic acid | approximately 0.63 g per 100 mL |
| Acid strength at 25 °C | pKa1 1.90, pKa2 6.07 | — | pKa1 3.03, pKa2 4.44 |
In supplier quality agreements, maleic acid is released against producer-specific model codes rather than a harmonized international standard. A representative high-purity technical-grade certificate of analysis includes assay ≥99.0% calculated as C4H4O4 on a dry basis by non-aqueous titration using sodium hydroxide, fumaric acid ≤0.5% by reversed-phase HPLC with photodiode array detection at 210 nm, water ≤0.5% by Karl Fischer coulometry, ash ≤0.1% by combustion at 800 ± 25 °C, and iron ≤10 mg kg−1 by ICP-OES. Some electronic or polymer applications add chloride ≤20 mg kg−1 and sulfate ≤50 mg kg−1 limits. The moisture ceiling is not merely a purity issue; water in the solid increases bridging in silo extraction and increases the heat load for anhydride regeneration. Iron above 10 mg kg−1 can catalyse colour formation in unsaturated polyester resin and reduce optical clarity in water-based copolymer solutions. Because no harmonized model code exists, purchase specifications should always reference CAS 110-16-7, assay method, fumaric acid limit, and moisture limit. A model designation such as “MA99” without the accompanying certificate of analysis is insufficient for critical applications.
Replacement of maleic anhydride with maleic acid in an unsaturated polyester resin cook requires a molar correction, not a weight-for-weight substitution. On a C4 backbone basis, 1.00 kg maleic anhydride is equivalent to 1.18 kg maleic acid because the molar masses are 98.06 g mol−1 and 116.07 g mol−1, respectively. The diacid route generates one mole of condensation water per carboxyl group, increasing water evolution and requiring more heat input than the anhydride route. Industrial cooks are normally run in 5–20 m3 stainless-steel or glass-lined reactors with helical or anchor agitation at 15–40 rpm, a partial condenser held at 98–105 °C, and a Dean-Stark separator when xylene is used. After charging propylene glycol, maleic acid, phthalic anhydride, and a phosphite stabilizer, the batch is heated under inert gas to 180–215 °C. The acid value is sampled at 30 min intervals; the endpoint is normally 20–35 mg KOH g−1. After cooling to 80–100 °C, inhibited styrene is added to a final viscosity of 350–550 mPa·s at 25 °C.
The critical processing window is narrow at the end of the cook. If the acid value is driven below 15 mg KOH g−1 or if the kettle wall temperature exceeds 220 °C, the cis maleate units partially isomerize to trans fumarate units. This shift increases the crosslink density potential with styrene but also raises the resin viscosity at a given styrene content and can reduce ambient storage stability. Resin producers typically quantify the maleate-to-fumarate ratio by 1H NMR at 250 MHz or higher during process qualification. Gel time of the final styrenated resin is measured according to ISO 2535:2001, with typical laminating resin values of 8–15 min at 25 °C using a standard peroxide initiation system; cured castings are additionally tested under ISO 527-2:2012 for tensile properties. Published data for the isomerization rate in a specific plant formulation is limited, so process qualification is usually performed with the plant’s own reactor configuration and heat-transfer parameters.
| Application | Input form | Relevant standard or code |
|---|---|---|
| Unsaturated polyester resin gel time | Styrenated resin from maleic acid cook | ISO 2535:2001 |
| Tensile properties of cured resin castings | Cured resin plate | ISO 527-2:2012 |
| Food-contact coating raw material | Maleic acid-derived polyester | 21 CFR 175.300 |
| Combustible dust hazard analysis | Dry maleic acid powder | NFPA 652 |
| Aqueous copolymer pH and solids | Neutralized copolymer solution | No single ISO method; plant-specific |
When maleic acid is used as a comonomer in aqueous-phase free-radical polymerization, the carboxylic acid groups and the cis double bond impose different reactivity limitations than maleic anhydride. Homopolymerization is not a practical industrial route because termination and chain transfer dominate; therefore commercial products are copolymers with acrylic acid, vinyl acetate, acrylamide, or sulfonated styrene. A typical water-treatment polymer cook runs in a 2–10 m3 glass-lined or 316L jacketed reactor with an agitator power input of 50–120 W m−3. The monomer solution is held at pH 4.0–6.5 with sodium hydroxide to maintain solubility while limiting base-catalysed isomerization; ammonium persulfate or sodium persulfate initiator is fed at 80–95 °C over several hours. The resulting anionic copolymer is neutralized to pH 7.0–8.5 and diluted to 30–40% solids for storage.
In open recirculating cooling systems, the maleic acid-containing copolymer is metered at 5–15 mg L−1 active solids to control calcium carbonate, calcium sulfate, and barium sulfate scale. Monitoring uses carbon steel and copper-alloy corrosion coupons in the recirculating loop and differential-pressure readings across plate-and-frame heat exchangers. Static precipitation tests at 60 °C for 24 h are used for formulation screening because water chemistry varies across make-up sources; no single ISO method covers all cooling-water scale inhibitor evaluation protocols. Field dose adjustment is typically based on plant-specific scaling indices and heat-exchanger pressure drop.
Maleic acid is an electron-poor dienophile in Diels-Alder cycloadditions with butadiene, cyclopentadiene, and rosin acids. The cis acid geometry differentiates it from fumaric acid; fumaric acid, with trans carboxyl groups, shows lower reactivity in some endo-selective cycloadditions, while maleic anhydride is often preferred industrially because the anhydride ring is more electrophilic and the reaction does not release water. Maleic acid is nonetheless used in maleated rosin size and in certain corrosion-inhibitor intermediates. The reaction is normally performed at 170–220 °C under nitrogen in a stirred reactor with an acid value endpoint; the diacid should be pre-dried at 60–70 °C under vacuum when water exceeds 0.3%. Paper-size emulsification uses a high-shear homogenizer at 5–10 MPa to produce a stable aqueous dispersion, although exact pressure settings depend on the rosin adduct’s Brookfield viscosity and acid value.
Maleic acid is hygroscopic and forms acidic aqueous solutions with pH 1.5–2.0 at 10% solids. Carbon steel storage is not recommended when moisture is present; 316L stainless steel, polypropylene, and fluoropolymer-lined piping are used. Dry-solid handling in pneumatic conveying, bucket elevators, and silos requires a dust-hazard analysis under NFPA 652, and the solids should be stored at or below 40 °C and below 60% relative humidity to reduce caking. In humid climates, bags should be kept in a dehumidified transfer room or conveyed with dry air. The solid must not be mixed with concentrated alkalis or aqueous amines because the neutralization exotherm can locally exceed 130 °C and initiate dehydration to maleic anhydride. Strong oxidizers are incompatible; thermal decomposition products include carbon oxides and maleic anhydride vapour.
For downstream conversion, maleic acid is isomerized to fumaric acid in hot aqueous solution at temperatures above 150 °C under pressure; the trans isomer precipitates because its water solubility is only 0.63 g per 100 mL at 25 °C. Fumaric acid is then hydrated enzymatically to L-malic acid. In this chain, the food-grade monographs of the FCC and JECFA apply to fumaric acid and malic acid, not to the technical maleic acid starting material. For food-contact coatings, maleic acid-derived unsaturated polyester resins may be evaluated as raw materials under 21 CFR 175.300; migration limits are article-specific and cannot be inferred from the maleic acid specification alone. Published data for maleic acid-specific migration under this regulation is limited, so end users typically confirm compliance through extraction testing of the cured coating rather than from the monomer certificate of analysis.
