Acid value (AV) and hydroxyl value (OHV) are the two titration endpoints that tell a process engineer whether a polyester resin will actually cure the way the datasheet promises, with AV expressed in mg KOH per gram of sample and OHV carrying the same units or the modern mg OH/g equivalent [S1][S3].
Both numbers are referenced to the same KOH normality (typically 0.1 mol/L in ethanol or methanol) and both are required to characterise a polyester batch, because AV measures residual carboxylic acid groups and OHV measures the hydroxyl groups that will later react with isocyanate or melamine crosslinkers [S1][S3][S5].
What the two numbers actually mean on a datasheet
Acid value is defined as the number of milligrams of KOH equivalent to the carboxylic acid groups in alkyl resins, polyester acrylate resins, or resin mixtures, and it is also used for evaluating plasticizers, where AV must be as low as possible [S1][S5]. For saturated and unsaturated polyester polyols destined for polyurethane, the AV is the correction factor that must be subtracted from the apparent OHV to obtain the true hydroxyl number, which is why most datasheets list both side by side [S1].
Hydroxyl value is the weight of KOH in milligrams equivalent to the acetic acid consumed during acetylation of one gram of resin, or equivalently the milligrams of OH groups per gram when converted through OHV (mg OH/g) = OHV (mg KOH/g) × 17/56.1 [S3]. ISO 2554:1997 makes the dependence explicit: the method determines the difference between hydroxyl value and acid value, and the total acid value (ISO 2114:1996) must be measured separately to back-calculate the true OHV [S2].
Methods, standards, and the sample-size table that drives bench work
For polyurethane raw materials, AV is measured per ASTM D4662 and ASTM D7253, OHV per ASTM E1899 and DIN 53240-3, and isocyanate content per EN ISO 14896 method A, ASTM D5155 method A, and ASTM D2572 [S1]. For paint and varnish binders the more relevant OHV reference is ISO 4629-2:2016, with ASTM D4274-23 covering polyols and GB/T 6743-2008 as the Chinese equivalent [S3]. ISO 2554:1997 (second edition) is the dedicated standard for hydroxyl value on unsaturated polyester resins, prepared by ISO/TC 61/SC 12 and superseding the 1974 first edition [S2].
The Metrohm Application Bulletin 200/3e sample-size table is the practical rule of thumb: at expected AV 0 to 1 mg KOH/g use 20 g (±0.05 g), at 1 to 4 use 10 g, at 4 to 15 use 2.5 g, at 15 to 75 use 0.5 g, and above 75 use 0.2 g, all dissolved in 50 to 100 mL of an ethanol/diethyl-ether or ethanol/toluene mixture (Φ EtOH = 50% v/v) and titrated with alcoholic 0.1 mol/L KOH to the first equivalence point, with a benzoic-acid titer (dried in a desiccator overnight) used to standardise the titrant [S1][S5]. This single table is the most-copied piece of datasheet advice in resin QC labs because it converts the customer's expected AV band straight into a weighable sample. The hydroxyl-value titration by contrast uses an acetylation step: excess acetic anhydride esterifies the -OH groups, water hydrolyses the unreacted anhydride, and the resulting acetic acid is back-titrated with standardised KOH, with a parallel blank run to give the ΔV that defines OHV [S3].
AV vs OHV vs acid number: a side-by-side reading guide
On a polyester datasheet the three columns that get confused are acid value, acid number, and hydroxyl value. Acid value and acid number are the same quantity (mg KOH/g); hydroxyl value is also mg KOH/g but reports -OH groups, not -COOH [S1][S5]. The acid value quantifies residual carboxyl ends left from incomplete esterification, so a high AV signals a low-molecular-weight, high-chain-end resin, while a low AV (often <5 mg KOH/g for coating-grade polyester polyols) signals the reaction has been driven close to stoichiometric completion and the resin will behave predictably in a 2K system [S1][S8].
OHV sets the isocyanate demand: a 2K polyurethane clear formulated at OHV = 110 mg KOH/g (roughly 33.3 mg OH/g by the 17/56.1 conversion) needs the NCO:OH ratio checked against the datasheet's isocyanate value, and the hydroxyl equivalent weight 56100/OHV ≈ 510 g/eq is the number to plug into stoichiometry, not the OHV itself [S3]. If only OHV is given without AV, beware: the apparent OHV may be inflated by the unreacted carboxylic acid contribution and the real hydroxyl number will be lower than the datasheet claims [S1][S2].
Who needs both numbers, and who can get away with one
Polyester polyol producers selling into 2K PU coatings, PU adhesives, and TPU must report both AV and OHV on every batch CoA, because downstream formulators need the isocyanate demand (from OHV) and the catalytic acid content (from AV, which also drives corrosion of metal packaging and pot-life drift) [S1][S3][S8]. Fiberglass-reinforced plastics (FRP) shops using unsaturated polyester for laminating resins are less interested in OHV and more interested in AV as a cure monitor, since the styrene crosslinker reacts with the C=C double bonds rather than the -OH ends [S2][S4].
Resin buyers specifying powder-coating polyesters (typically neopentyl glycol/terephthalic acid based) need both, because the end-of-reaction AV and OHV together fix molecular weight, Tg, and melt viscosity, and the ASTM-recommended reaction monitoring uses the AV/OHV pair as the primary control loop [S4]. Researchers using NIR spectroscopy as a faster alternative still build their PLS models against the titrimetric AV and OHV, which remains the reference method that the new rapid technique must match [S4].
How the two values are used during production and on a CoA
In a batch esterification the operator pulls samples, titrates AV, and tracks the curve as it drops from a starting value (often 30 to 50 mg KOH/g for an early reaction) toward the target (commonly 5 to 10 mg KOH/g for a high-molecular-weight PET-type resin); OHV is titrated less frequently because the acetylation step is slower, but it sets the polyol identity for the next stage [S3][S4]. When the reaction overshoots and AV goes too low, the resin has been over-pushed and may be too high in molecular weight to dissolve cleanly; when AV stalls above target, the reaction is starving for heat, catalyst, or water removal, and the OHV reading will be unreliable until the AV is corrected [S4].
On the certificate of analysis the two numbers appear as separate line items with method references (e.g., "AV = 2.1 mg KOH/g, ISO 2114" and "OHV = 56 mg KOH/g, ISO 2554:1997"), and a third number, the isocyanate value in %NCO, is often added for the isocyanate-side counterpart [S1][S2]. When a [synthetic-resin](synthetic-resin) supplier ships a polyester with AV <7 and OHV 50 to 250 mg KOH/g (typical coating-grade polyol ranges), the datasheet should also state the KOH normality, the solvent system, and the indicator, otherwise the buyer's incoming-QC titration will not reproduce the supplier's number [S1][S3][S5].
Limits, failure modes, and what a datasheet does not tell you
Acid value and hydroxyl value are bulk, weight-averaged numbers; they cannot tell you whether the -OH groups are clustered on one end-cap (functionality 1, no crosslink potential) or distributed along the chain (functionality 2 to 3, real crosslinker) [S3]. A resin with the same OHV but lower functionality will under-cure, and a resin with the same AV but different molecular-weight distribution will behave differently in viscosity and pot life, which is why datasheets that quote only AV and OHV without molecular weight or viscosity are inadequate [S3].
ISO 2554:1997 itself flags a constraint: the method determines (OHV - AV), not OHV directly, so any error in the AV titration propagates into the reported OHV, and saturated polyesters, certain alkyd resins, and unsaturated polyesters all sit inside the same standard's scope with a footnote rather than a separate method [S2]. Titration methods also produce solvent waste and require 30 to 60 minutes per sample, which is why NIR and FT-NIR multivariate calibration against AV/OHV has been published as a faster, greener alternative for process control, although titration remains the reference [S4]. Track signals: any new ISO 4629-2 revision, ASTM D4274 update, or extension of NIR/FT-NIR model-transfer protocols between resin plants, and any tightening of low-AV specifications (<2 mg KOH/g) for high-solids or waterborne polyester dispersions, would be the next concrete changes worth following on the PEEK and high-performance polymer landscape and broader synthetic-resin datasheet practice.
Spec-level background on the components involved: checking and testing apparatus.
Background reading: Acetone vs MEK: Solvent Strength and Evaporation Rate Compared.