Adipic acid, CAS 124-04-9, EINECS 204-673-3, is a linear six-carbon saturated dicarboxylic acid with the molecular formula HOOC(CH₂)₄COOH and molar mass 146.14 g/mol. Industrial material is supplied as white crystalline powder or fine granular solid with a typical bulk density of 0.80–0.90 g/cm³; particle-size distribution is controlled by screen residue on 0.500 mm, 0.150 mm, and 0.075 mm sieves. Polymerisation-grade certificate-of-analysis boundaries include purity ≥99.8% (w/w) by alkalimetric titration, melting range 151–153 °C by differential scanning calorimetry at 10 K/min, water content ≤0.20 wt%, ash ≤5 mg/kg, iron ≤0.5 mg/kg, and methanolic colour ≤5 APHA. Food-grade material is controlled in the United States under 21 CFR 184.1009 and in the European Union under Regulation (EC) No 1333/2008 Annex II as E355; reported food-grade assay is ≥99.7% with residue on ignition ≤0.01% and arsenic ≤1 mg/kg. No universal model-number system exists across suppliers; grade codes are supplier-specific, so cross-supplier qualification is performed against certificate-of-analysis parameters rather than the product name. Published aqueous solubility at 15 °C is 14.4 g/L, rising to more than 1,000 g/L near the normal boiling point. The dominant industrial route is nitric acid oxidation of a cyclohexanol/cyclohexanone mixture; the crude product contains glutaric acid and succinic acid, which are removed by crystallisation. Bio-based adipic acid produced from fermentation-derived muconic acid is chemically identical, but carbon-14 analysis under ASTM D6866-20 can distinguish fossil-derived from bio-derived material for sustainable sourcing verification.

Which Specification Boundaries Separate Polymerisation-Grade from Food-Grade Material?

The differentiation between polymerisation-grade and food-grade material appears mainly in trace-metal, ash, and residue-on-ignition limits, because polymerisation catalysts and melt-phase polyamide quality are sensitive to iron and non-volatile residues. The table below lists representative certificate-of-analysis boundaries from published supplier data; individual lot data may be tighter.

For fibre-grade polyamide production the iron limit is not merely cosmetic. Soluble iron in the melt phase reduces colour stability and can participate in redox side reactions that shift the end-group balance, so polymerisation-grade material is qualified by lot-based spectrochemical data rather than by total purity alone.

In continuous polyamide 66 polymerisation, adipic acid is first dissolved with hexamethylenediamine in demineralised water to form the 1:1 nylon salt; the salt solution is usually maintained at 50–60 wt% and pH 7.6–8.2. A deviation of ±0.3 mol% from the stoichiometric acid–amine ratio changes the end-group balance and lowers the molecular weight ceiling measured as viscosity number under ISO 307. The salt is concentrated and polymerised in two-stage equipment at 210–280 °C; below 210 °C conversion rate becomes the bottleneck, while sustained operation above 285 °C increases nitrogenous volatile formation and gel content. Textile-grade relative viscosity is typically 2.4–2.7; industrial-yarn grades run 3.0–3.3 in sulfuric acid solution. The low ash and iron specifications of polymerisation-grade adipic acid are therefore linked directly to stable melt spinning and downstream drawing. In fibre-grade production, insoluble char particles above 10 µm increase spinneret pack changes and draw-line breaks; in resin-grade production, moisture in the acid feed must still be controlled to avoid hydrolysis of the polyamide during melt processing.

Esterification and Polyol Synthesis Parameters for Adipate-Based Urethane Systems

For polyester polyol production, adipic acid is reacted with excess glycol—commonly 1,4-butanediol, diethylene glycol, or mixed glycols—under nitrogen at 180–230 °C. The diacid-to-glycol molar ratio is typically 1:1.10 to 1:1.25, producing hydroxyl numbers of 50–60 mg KOH/g measured according to ISO 14900-1. Esterification is monitored by acid value; discharge is normally below 2 mg KOH/g. Water removal is performed through a packed column with top temperature 95–105 °C. The critical processing fault is sublimation of adipic acid into vent lines and condenser internals; overhead surfaces below 120 °C accumulate crystalline deposits, raising back-pressure and causing batch-to-batch hydroxyl-number drift. Heated overhead lines and condensate returns kept above 120 °C reduce this failure mode. Operation above 235 °C increases colour and branching, while operation below 170 °C gives insufficient esterification rate for standard batch cycles. In semi-continuous plants, the diacid addition profile is set to keep top temperature below 105 °C and to avoid separator flooding. Final polyester polyols for cast elastomer or flexible foam systems are dried to water contents below 0.05 wt% before reaction with isocyanate; residual moisture above 0.08 wt% can generate carbon dioxide and reduce Shore A hardness under ISO 868.

Conversion to di-2-ethylhexyl adipate, commonly DEHA, is a major non-polyamide application. Esterification with 2-ethylhexanol proceeds under acid catalysis; after neutralisation the residual acid value is controlled at ≤0.1 mg KOH/g, and residual alcohol is reduced to ≤100 mg/kg for low-volatile-loss plasticiser performance. DEHA produces lower low-temperature stiffening than ortho-phthalate esters because the linear adipate backbone increases molecular mobility; comparative stiffness is characterised under ASTM D1043-16. In flexible PVC, DEHA is typically used at 20–35 phr, but published data for drop-in substitution ratios across all plastisol and calendering formulations is limited; formulation-specific fusion and Shore D hardness under ISO 868 must be confirmed. The higher water extraction of DEHA compared with polymeric plasticisers is an operational boundary in humid applications.

When Adipic Acid Replaces Aromatic Diacids in Non-Phthalate Plasticiser Formulations

When technical requirements exclude ortho-phthalate plasticisers, adipate esters are compared with citrate, sebacate, and terephthalate systems. The six-carbon adipic acid itself yields esters with lower intrinsic viscosity and higher volatility than sebacate esters, which carry a ten-carbon chain; however, adipate esters show better PVC solvency than azelates in plastisol fusion, as measured by minimum fusion temperature on a torque rheometer. In flexible PVC compounds, direct replacement is not stoichiometric: plasticiser loading is commonly adjusted upward to compensate for lower plasticising efficiency, and dry-blend handling requires cooled mixer settings below 60 °C to prevent early additive absorption. REACH registration dossiers list end-use exposure scenarios for the specific ester CAS number; formulators must verify food-contact clearance such as 21 CFR 175.105 or 177.2600 before use in packaging or article-contact applications.

Food-grade adipic acid is used as acidulant E355 in dry beverage bases, gelatin desserts, and chemical leavening systems. Its acidification profile differs from citric acid because the first acid dissociation constant pK_a1 is 4.44, compared with 3.13 for citric acid, producing a less sharp initial pH drop in aqueous solution. In chemical leavening, reaction with sodium bicarbonate is controlled by the dissolution rate of the acid; therefore the crystalline particle-size distribution is specified by residue on 0.150 mm and 0.075 mm sieves. Regulatory status in the United States appears in 21 CFR 184.1009; European Union food-additive use appears in Regulation (EC) No 1333/2008 Annex II as E355. At high pH, the adipate dianion may precipitate as insoluble calcium salts, which limits its use in hard-water systems and requires buffering in liquid formulations.

For bulk handling, adipic acid dust forms combustible organic dust clouds; equipment should be grounded, and dust extraction should be designed to avoid explosive concentrations. Minimum ignition energy is reported in safety data sheets but varies with particle size and moisture. Storage in silos or closed bags at 10–30 °C prevents caking, and the product is stable under normal conditions. Prolonged heating above 265 °C causes decarboxylation to cyclopentanone and carbon dioxide; this is an operational limit for melt processing and for dryers. In downstream plants where hot-air dryers above 120 °C are used to pre-dry material, stagnant zones must be avoided because local overheating can produce odorous decomposition products and reduce assay.

Compared with other saturated linear dicarboxylic acids used in polycondensation, adipic acid occupies an intermediate position between the shorter-chain succinic acid and the longer-chain azelaic or sebacic acids. The following physical constants are representative values from published safety data sheets and supplier specifications.

Adipic acid differs from succinic acid in crystal packing, reflected in a lower melting range despite higher molecular weight, because the additional methylene units reduce crystal lattice energy. Compared with sebacic acid, adipic acid provides higher ester-group density in polyester polyols, which raises tensile modulus and Shore A hardness under ISO 868, while sebacate systems show lower glass-transition temperatures and better low-temperature flexibility. In polyamide condensation, adipic acid with hexamethylenediamine yields nylon 66, whereas sebacic acid with hexamethylenediamine yields nylon 6,10, which absorbs less moisture and has lower modulus; selection between them is therefore governed by the required balance of mechanical stiffness, water uptake, and low-temperature behaviour rather than by purity considerations alone.