Commercial polyester polyol production by direct melt esterification proceeds through a step-growth condensation in which a dicarboxylic acid or anhydride is reacted with a molar excess of a glycol in a batch reactor at temperatures between 180°C and 235°C. The acid value, expressed in mg KOH/g, is the mass of potassium hydroxide required to neutralize the free carboxylic acid groups in 1 g of sample, and it is determined for polyurethane-grade polyester polyols by titrimetric or potentiometric methods under ASTM D4662-20 Procedure A or ISO 2114:2000. In a typical 10,000 L stainless-steel batch reactor equipped with a packed distillation column, a partial condenser, and a three-stage vacuum train capable of 5 mbar absolute, an initial charge of adipic acid and monoethylene glycol at a glycol-to-diacid molar ratio between 1.10:1 and 1.25:1 yields an initial acid value broadly in the range 180–240 mg KOH/g after melt-out at 120–140°C. The relationship between the number-average molecular weight of a linear polyester diol and the end-group concentrations is given by the expression Mn = 112,200/(AV + OHV), where AV is the acid value in mg KOH/g and OHV is the hydroxyl value in mg KOH/g. Consequently, reducing the acid value from 200 mg KOH/g to 0.5 mg KOH/g in a linear diol with a final hydroxyl value of 56 mg KOH/g corresponds to a molecular-weight increase from roughly 438 g/mol to approximately 1,986 g/mol. The atmospheric esterification stage is conducted at 210–220°C with nitrogen sparging and water removal through the packed column; the head temperature is held below 101°C at ambient pressure to prevent glycol carryover. Vacuum is applied only after the evolution of water has slowed, because premature vacuum can cause foam carryover into the column and produce a distillate with unacceptable glycol content. The batch is sampled through a bottom valve into a dry nitrogen-purged container, dissolved in a mixture of toluene and ethanol, and titrated with 0.1 N methanolic potassium hydroxide to a phenolphthalein endpoint when color permits; dark or high-viscosity samples are analyzed potentiometrically. Production campaigns that target a final acid value below 1.0 mg KOH/g typically require 4–8 h of vacuum finishing at 220–235°C and 10–30 mbar absolute after the atmospheric stage, but the floor is not set merely by the reaction time.

Why Does Vacuum Finishing Exhibit a Practical Acid Value Floor?

The practical floor for acid value in direct esterification is governed by the equilibrium concentration of water in the melt and by the rate at which water and volatile glycols are removed from the reacting mass. In a batch polycondensation with no catalyst and no vacuum, the equilibrium constant for the formation of an aliphatic ester bond is typically low enough that the acid value plateaus at 15–30 mg KOH/g at 220°C when water is retained in the reactor. Published technical literature on polyesterification equilibria reports that the number-average degree of polymerization increases as the square root of the product of the equilibrium constant and the removal efficiency, but only if the stoichiometric ratio of hydroxyl to carboxyl end groups is maintained. In a stirred 10,000 L reactor, the rate of water removal is determined by the surface renewal produced by a variable-speed anchor agitator, the swept surface area of the partial condenser, and the vapor-load capacity of the liquid-ring pump under the acidic distillate conditions. Once the pressure is reduced to 10–30 mbar, the mean free path of water in the vapor phase increases and the condenser inlet temperature falls, but the same vacuum also strips monoethylene glycol and 1,4-butanediol. Glycol loss distorts the molar ratio of hydroxyl to carboxyl groups, reduces the concentration of reactive chain ends, and creates a situation in which additional acid value reduction cannot proceed to the thermodynamic endpoint. Therefore, a polyester polyol that is not sampled and corrected by glycol addition may show an acid value plateau of 1.5–3.0 mg KOH/g even after prolonged vacuum finishing. Operators track overhead glycol content by gas chromatography with flame ionization detection and compare the integrated areas against calibrations prepared from the specific glycol. The acid value floor is further constrained by the accuracy of the titration itself: at 0.5 mg KOH/g, a 10 g sample consumes only 0.089 mmol of titrant, and potentiometric endpoint detection with a combined glass electrode is required because the color endpoint becomes unreliable. In some campaigns, the final acid value is not the sole termination criterion; the batch is stopped when the acid value falls below a target band and the melt viscosity at 75°C, measured under ASTM D4878-15, reaches a narrow specification window.

When 1,4-Butanediol Cyclization Accelerates Above 200°C

Polyester polyols based on 1,4-butanediol and adipic acid dominate high-performance cast elastomers and thermoplastic polyurethane feedstocks, but the same monomer introduces an acid value reduction limit through acid-catalyzed intramolecular cyclization to tetrahydrofuran. At temperatures above 200°C, especially when a strong Bronsted acid catalyst such as p-toluenesulfonic acid is present at 0.05–0.20 wt%, the conversion of 1,4-butanediol to tetrahydrofuran and water becomes kinetically significant. The tetrahydrofuran is volatile and leaves the reactor through the vacuum system, while the loss of hydroxyl-terminated chain ends leaves an excess of carboxyl groups that cannot be esterified. In a 5,000 L reactor fitted with a column overhead and a distillate receiver, the condensate from a butanediol-adipate campaign can contain tetrahydrofuran at concentrations that exceed 2 wt% of the total distillate during the late vacuum stage when the melt temperature is held at 225–235°C and the pressure is below 20 mbar. The acid value plateau in such campaigns is often observed at 2.0–4.0 mg KOH/g if the stoichiometric imbalance is not corrected by fresh glycol addition. Process control for this system therefore includes continuous or batch monitoring of the overhead composition by gas chromatography with a flame ionization detector, and the acid value is interpreted together with the hydroxyl value determined under ASTM D4274-21. If the hydroxyl value falls below the expected value for the target molecular weight, the batch is refilled with 1,4-butanediol through a dip leg under nitrogen pressure, and the vacuum is reduced temporarily to allow the glycol to react before reapplication of full vacuum. Published technical literature indicates that organotin catalysts suppress tetrahydrofuran formation relative to sulfonic-acid-catalyzed systems, though they do not eliminate it entirely; the available process data for this specific comparison is limited, but the retained hydroxyl content is consistently higher with tin-based catalysis. This monomer-specific side reaction creates an upper temperature boundary for acid value reduction: below 200°C the esterification rate is too low for commercial cycle time, while above 235°C the butanediol loss accelerates and color bodies form rapidly.

Rigid boardstock polyisocyanurate foams are produced with aromatic polyester polyols that tolerate and sometimes intentionally use residual acidity to modify catalyst activity. Commercial aromatic polyester polyols derived from dimethyl terephthalate residues, phthalic anhydride, and diethylene glycol typically have a hydroxyl value between 230 mg KOH/g and 350 mg KOH/g and an acid value between 1.5 mg KOH/g and 3.5 mg KOH/g. These materials are analyzed under ASTM D4662-20 and are not usually finished to the very low acid values required for aliphatic cast elastomers, because the excess polymeric methylene diphenyl diisocyanate used in foam formulations absorbs the stoichiometric effect of the acid groups. By contrast, aliphatic adipate polyester polyols for two-component polyurethane coatings, adhesives, and thermoplastic polyurethane prepolymers are typically specified with acid values below 0.5 mg KOH/g, and premium grades for high-durability thermoplastic polyurethane are controlled below 0.2 mg KOH/g. The table below summarizes acid value limits across typical downstream conversion routes, together with the standard test method and the operational consequence of exceeding the limit.

Catalyst Deactivation and Acid Value Reversion in Storage

Residual catalyst activity and moisture ingress after drumming create a distinction between the acid value recorded at reactor discharge and the acid value measured after storage. Tin catalysts such as butyltin tris(2-ethylhexanoate) and tin(II) octoate remain active at polyol storage temperatures between 40°C and 80°C, but their activity can be suppressed by hydrolysis products and by the carboxylic acid end groups themselves. Tetraalkyl titanates, including tetrabutyl titanate and tetraisopropyl titanate, are effective esterification catalysts at 50–150 ppm titanium based on total charge, but they are hydrolyzed by residual water to titanium dioxide, producing a visible haze and a loss of catalytic activity. In a production campaign using tetrabutyl titanate, the final acid value can be driven below 0.3 mg KOH/g at 230°C and 10 mbar, but the same polyol may show an acid value increase of 0.1–0.3 mg KOH/g after 6 months in a closed but not nitrogen-blanketed steel drum because the polyester backbone undergoes slow hydrolysis with water absorbed through the drum seal. Moisture absorption is particularly significant when the storage area exceeds 60% relative humidity at 25°C; under these conditions, pre-drying of the polyol by vacuum stripping or nitrogen sparging is mandatory before use in isocyanate-based systems. The acid value of stored polyol should be revalidated under ASTM D4662-20 Procedure A, and the water content should be measured by ASTM D4672-18 or an equivalent coulometric Karl Fischer method. Acid value reversion is not uniform across the drum: stratification can produce nonuniform acid values because moisture concentrates near the surface and acid-rich material may accumulate in lower layers. Production-scale bulk storage tanks equipped with heated coils, dry nitrogen padding, and recirculation loops reduce this stratification, but the recirculation pump must be sized to turn over the tank volume at least once every 4 h to prevent localized hydrolysis at stagnant zones. When tin catalysts are used, the combination of residual tin and free acid can slowly catalyze transesterification during storage, altering the molecular-weight distribution and shifting the acid value upward as low-molecular-weight oligomers are generated.

Thermal Oxidation Pathways That Raise Final Titration Values

At the high finishing temperatures required for low acid values, unsaturated diacids and oxygen ingress introduce side reactions that can increase the measured acid value or create colored impurities that interfere with endpoint detection. When maleic anhydride or fumaric acid is used in unsaturated polyester polyols, the cis-trans isomerization of maleate to fumarate is accelerated at temperatures above 200°C, and the fumarate ester is more prone to thermal oligomerization and gel formation. In a 10,000 L reactor with a nitrogen sparge rate of 2–5 m³/h, the headspace oxygen concentration must be maintained below 5 vol% to prevent oxidative degradation of the cooling condensate and the formation of low-molecular-weight aldehyde and ketone byproducts. The presence of these carbonyl compounds does not always change the acid value directly, but their reaction with residual acid groups can produce color bodies that obscure the phenolphthalein endpoint and require potentiometric titration under ISO 2114:2000. Color is measured as platinum-cobalt units under ASTM D1209-14, and polyester polyols finished above 230°C without an effective antioxidant can exceed 100 APHA, whereas water-white coating grades typically demand 50 APHA or below. Acid value measurements on oxidized polyols may be biased low when the sample is not fully dissolved in the titration solvent, because oxidized high-molecular-weight fractions and microgel particles remain undissolved and do not expose their carboxyl end groups to the titrant. For this reason, the sample preparation step in the method is not merely procedural; it must include heating and stirring at 60–70°C until the polymer is completely dissolved before the first titrant increment. The operational boundary for thermal finishing is therefore defined by the onset of color formation and microgel formation, not by the reaction equilibrium alone. Some production lines use a wiped-film evaporator as a terminal finishing device after the batch reactor, applying a residence time of 5–15 min at 220–230°C and 5–10 mbar to strip residual water and low-molecular-weight glycols while limiting the time at high temperature. This equipment configuration lowers the acid value of a 2,000 g/mol aliphatic polyester diol by approximately 0.2–0.5 mg KOH/g without increasing color when the feed is pre-dried to below 0.05 wt% moisture.

Viscosity Constraints at Low Acid Values in High-Functionality Polyols

For branched polyester polyols synthesized from trimethylolpropane, pentaerythritol, or glycerol, the acid value cannot be used as a single termination variable because molecular weight advancement and branching raise melt viscosity before the acid value falls to the level expected for linear systems. A trifunctional polyester polyol with a target hydroxyl value of 350 mg KOH/g and a target acid value of 1.0 mg KOH/g may have a viscosity at 25°C above 20,000 mPa·s, while a linear diol with the same acid value and a hydroxyl value of 56 mg KOH/g is usually below 2,000 mPa·s. The viscosity is measured under ASTM D4878-15 or ISO 3219, and the batch is terminated when the acid value and viscosity enter a narrow processing window rather than when the acid value alone reaches its lower limit. In a 5,000 L reactor, the anchor agitator and wall-scraping paddle draw a measurable increase in motor current as the hydroxyl value drops and the average functionality increases; the operator uses the agitator torque reading, calibrated against laboratory viscosity, to decide whether further vacuum stripping is safe. If vacuum finishing is continued past the viscosity limit, the polymer may undergo localized overheating near the heating jacket, producing gel bodies and a color increase that is not detected by the acid value titration. This situation is particularly severe in pentaerythritol-based polyols, where the tetrafunctional monomer introduces a gel point at a lower extent of reaction than is observed for trimethylolpropane-based materials. The relationship between acid value, hydroxyl value, and number-average functionality is given by the same end-group expression, but for branched systems the number-average molecular weight is no longer sufficient to describe the distribution; a gel permeation chromatography method under ASTM D5296-19 with a refractive index detector and polystyrene calibration standards is used to track the molecular-weight distribution and detect high-molecular-weight shoulders that precede microgel formation. Published data for this specific acid value–viscosity interaction in highly branched polyester polyols is limited, but the practical control range is frequently found to be 0.8–1.5 mg KOH/g for trifunctional polyols used in rigid polyurethane foam and 0.5–1.0 mg KOH/g for tetrafunctional grades intended for high-crosslink-density castings.

Potentiometric titration of dark or high-viscosity polyester polyols requires careful control of the solvent matrix because the acid value result can be biased by dissolved carbon dioxide, residual catalyst, and suspended titanium dioxide hydrolysis products. When tetrabutyl titanate is used as the finishing catalyst at 50–150 ppm titanium, the resulting titanium dioxide haze can adsorb acid groups or coat the electrode surface, causing slow endpoint response and a falsely high acid value if the electrode is not regenerated with acidified potassium chloride. Samples are dissolved in a mixture of toluene and isopropanol or toluene and ethanol, and the titration is performed with 0.1 N methanolic potassium hydroxide standardized daily against a potassium hydrogen phthalate primary standard. The blank value of the solvent mixture is titrated separately and subtracted, because solvent degradation products can consume small amounts of titrant. In a production laboratory attached to a polyester polyol line, the sampling frequency during vacuum finishing is usually every 1 h, and the sample is passed through a heated discharge line into a 250 mL glass bottle that has been purged with dry nitrogen. The sample must be cooled below 50°C before it is exposed to air to avoid moisture absorption and oxidative skin formation. If the melt is too viscous to discharge under gravity, a positive-displacement gear pump with heated jacketing is used to transfer the sample, and the first 100 mL of the sample is discarded to flush the line. The acid value result from the laboratory is compared with the acid value predicted by a kinetic model that uses reactor temperature, pressure, agitator torque, and overhead distillate mass as inputs. The model is reconciled to the laboratory value only when the sample preparation procedure is within control, because an undissolved high-molecular-weight fraction will make the acid value appear lower than the true end-group concentration. The operational limit for dark aromatic polyester polyols is often an acid value of 1.0 mg KOH/g, but the same value in a water-white aliphatic coating grade may already be unacceptable.

When Maleic Anhydride Isomerization Slows Acid Value Decline

Unsaturated polyester polyols that include maleic anhydride present a further acid value reduction limit because the maleate ester initially formed can isomerize to fumarate under the acidic reaction conditions. The isomerization does not change the total acid value directly, but it changes the reactivity of the unsaturated sites and can lead to side reactions that consume the hydroxyl-terminated chain ends or produce branched structures. In a 2,000 L pilot reactor producing a maleic-phthalic diethylene glycol polyester polyol for ultraviolet-curable coatings, the acid value decline follows a two-stage profile: an initial rapid drop from 80 mg KOH/g to 20 mg KOH/g during the atmospheric stage, followed by a slower decline to 5–10 mg KOH/g during vacuum finishing at 200–220°C and 15–20 mbar. The slow second stage is associated with the isomerization of maleate to fumarate and the reduced mobility of the growing chain ends in the increasingly viscous melt. The final acid value is therefore not reduced below 5 mg KOH/g in many unsaturated polyester polyol batches because the residual acidity is acceptable for the intended use and because further reduction would require temperatures that promote gelation. Process control for these materials includes viscosity measurement under ASTM D4878-15 and gel time determination by a hot-plate method, in addition to acid value titration under ISO 2114:2000. The acid value limit is set by the end-use formulation: unsaturated polyester resins for fiber-reinforced laminates are often reacted to acid values between 15 mg KOH/g and 35 mg KOH/g, while unsaturated polyester polyols for UV-curable oligomers are controlled below 10 mg KOH/g to avoid excessive inhibition of free-radical photoinitiators. Published kinetic studies report that the maleate-to-fumarate isomerization is accelerated by the presence of residual acid groups and by temperatures above 190°C, but the exact rate constant depends on the glycol structure, the catalyst type, and the presence of inhibitors such as hydroquinone. The production limit is therefore a balance between acid value, isomer content, viscosity, and color; no single titration value is sufficient to release the batch.