High Purity Adipic Acid (ADA) for Nylon 66 & Polyurethane Production is delivered as a white crystalline solid with a dicarboxylic acid content of 99.7% by weight or greater and a melting range of 151.5–153.5 °C as measured by ASTM E324. The material is produced by oxidation of cyclohexane or cyclohexanol-cyclohexanone mixtures; residual monobasic acids, moisture, iron, and colour bodies are controlled because each interacting impurity shifts downstream polymer-building behaviour. At production scale, bulk rail hopper cars and pneumatic transfer systems maintain moisture below 0.20% by weight, with Karl Fischer titration per ISO 760 used for certificate-of-analysis verification. The product is registered under EU REACH as chemical identity 124-04-9, but the polymer-grade value is defined by the narrower high-purity profile rather than by the general industrial grade specification.

In downstream interpretation of these limits, assay, moisture, iron, and melting-range data are not independent. A lot with a wide melt range often contains increased glutaric acid and succinic acid fractions from incomplete oxidation. In nylon salt preparation, these dibasic homologues incorporate into the chain but alter hydrogen-bond geometry and reduce crystallinity. In polyester polyols, they shift the molar balance and broadens the molecular weight distribution. The specification is therefore handled as a system of mutually dependent limits rather than a set of isolated pass/fail values.

How Does Residual Monobasic Acid Content Limit Nylon 66 Molecular Weight?

During Nylon 66 salt manufacture, ADA is reacted with hexamethylenediamine in demineralized water. A 52–60 wt% salt solution is prepared by metering ADA into a stirred reactor containing hexamethylenediamine at 45–55 °C. The reaction is exothermic; cooling water at 30–35 °C maintains the batch below 70 °C. Stoichiometry is controlled by pH, with the target pH range for a 25 wt% diluted salt solution at 25 °C held between 7.6 and 8.0. If the pH shifts above 8.2, excess free amine accelerates oxidative colour formation and leads to volatile amine loss in the evaporator. If the pH remains below 7.5, free adipic acid persists and the salt is acid-terminated, reducing the final degree of polymerization.

Monobasic acids such as valeric acid or caproic acid, present at trace levels from incomplete oxidation, react with one amine group and terminate chain propagation. Their concentration is therefore limited to not more than 0.05% of total acidity in high-purity ADA; above this value, the number-average molecular weight of the finished polyamide decreases even when the pH titration is on target. Iron above 0.5 ppm acts as a catalytic centre for oxidative degradation during melt finishing. Batch-to-batch variation in iron loading has been observed on twin-screw finishing lines with L/D ratios of 32:1 to 40:1 as increased yellowing and faster pressure rise at the die plate. Molecular-weight build-up is tracked by relative viscosity in 90% formic acid per ISO 307; acid-terminated material typically exhibits lower relative viscosity at the same finisher vacuum level.

Following salt crystallization and dissolution, concentrated AH salt solution is transferred to a batch autoclave where water removal and condensation proceed in two stages. The initial pressure stage is held at 1.70–1.80 MPa at 210–220 °C for 2–3 h; the pressure is then vented to atmospheric while the mass temperature is raised at 1–2 °C/min to 270–280 °C. At temperatures above 285 °C, thermal decomposition accelerates, generating carbon dioxide and ammonia; bubble formation in the molten polymer becomes visible at the finisher discharge. Vacuum finishing at 5–10 kPa removes water and drives molecular-weight build-up. Polymer-grade ADA is specified with a melt crystallization range no wider than 2 °C because wide melting ranges correlate with isomer impurities that depress polyamide crystallinity. Published data for exact degradation-rate constants in full-scale autoclave hot spots is limited, but production records correlate low-iron ADA with reduced torque increases during the final 30 min of finishing.

When Esterification Is Driven to Low Acid Values for Urethane-Grade Polyols

Polyurethane-grade polyester polyols are produced by reacting High Purity Adipic Acid (ADA) with a controlled molar excess of diol. For a 2000 g/mol diethylene glycol adipate diol, the theoretical carboxyl-to-hydroxyl ratio is approximately 0.80:1.00, which corresponds to a 25 mol% diol excess. The reaction is carried out in a stainless-steel batch reactor with a 3:1 height-to-diameter ratio and a packed column operated at an overhead temperature of 100–105 °C; diol is returned to the reactor while water of esterification is removed. The batch temperature is stepped from 150 °C to 220 °C over 8–12 h, with nitrogen injection at 0.5–1.0 m³/h per tonne to limit oxidative discolouration.

Titanate or tin alkoxide catalyst is charged at 0.01–0.05% of total mass. Excess catalyst increases dehydration side reactions and can produce vinyl unsaturation when 1,4-butanediol is used. Acid number by ASTM D4662 and hydroxyl number by ISO 14900 are measured at 2 h intervals once the batch reaches 200 °C. Endpoint control is based on both values rather than viscosity alone because viscosity responds to branched by-products that may not contribute proportionally to isocyanate demand.

Hydroxyl number is calculated as 112,200 /M_n for linear diols; the repeat unit masses include the dicarboxylate residue and the dialkoxy residue after loss of water. In flexible slabstock and cast elastomer systems, an acid number above 1.0 mg KOH/g consumes isocyanate and liberates carbon dioxide, causing pinholes and unpredictable gel times. For high-hardness thermoplastic urethanes, the acid number is driven below 0.5 mg KOH/g, and water is stripped to below 0.05% before drumming.

After reactor discharge, molten polyester polyol is cooled to 70–80 °C and filtered through a 10 µm bag filter. Storage in nitrogen-blanketed stainless-steel tanks is limited to 72 h at 70 °C because residual acidity hydrolyzes ester linkages and regenerates adipic acid, shifting the effective isocyanate demand. Contact with copper, brass, or zinc-containing fittings is avoided; dissolved copper ions above 1 ppm accelerate oxidative viscosity rise and darkening. If opened totes are reheated to 90 °C without dry-air sweep, acid number can increase by 0.2–0.4 mg KOH/g after 48 h and the moisture specification is no longer met.

Thermal Degradation Thresholds in Molten ADA and Polyol Reactors

Because molten ADA transfer is required for continuous Nylon 66 lines, hot-oil jacketed transfer lines are operated at 160–170 °C. Above 170 °C, adipic acid undergoes gradual anhydride and cyclopentanone-related degradation pathways that consume carboxylate functionality and shift AH salt pH. Residence time in the molten transfer system is maintained below 30 min; wall temperatures above 180 °C increase APHA colour from 5 to above 15 in exposed laminar boundary layers. Published data for exact degradation kinetics in molten ADA transfer lines is limited, but production lines therefore specify maximum heating-medium temperatures rather than bulk temperatures alone.

In polyester polyol reactors, wall temperatures above 240 °C accelerate decarboxylation and can generate allyl ether unsaturation from 1,4-butanediol. The resulting polyol shows a reduction in average functionality from 2.0 toward 1.90; this is detected by a combination of ISO 14900 hydroxyl number and gel-permeation chromatography, and it reduces the plateau modulus of the cured polyurethane network. Stainless steel 316L is used for long-term hold tanks because dissolved transition metals otherwise become degradation accelerators at these temperatures.