Quick answer. Hydroxyl value measures the concentration of hydroxyl groups in a polyol or formulated polyol blend, expressed as mg KOH/g. It determines hydroxyl equivalent weight and therefore how much isocyanate is required at a selected NCO index. A higher value can support a more highly crosslinked polyurethane network, but functionality, water, catalyst balance and process conditions decide the actual foam result.
For buyers assessing the hydroxyl value role in blend polyol formulation, the commercial question is not simply whether the number is high or low. The useful questions are how it was measured, which ingredients are included in the reported value, how tightly each batch is controlled and what foam properties were verified at the stated mixing ratio.
What Hydroxyl Value Measures—and What It Does Not
Hydroxyl value, also called hydroxyl number or OH number, is the milligrams of potassium hydroxide chemically equivalent to the hydroxyl groups in 1 g of sample. A result of 400 mg KOH/g does not mean that the product contains 400 mg of potassium hydroxide; KOH is the reporting convention used to express reactive hydroxyl content.
The current ASTM D4274-23 covers hydroxyl-number testing for polyester, polyether, polymer and amine-initiated polyols. Its five procedures use different esterification conditions, and ASTM identifies specific methods for particular polyol families.
ISO 14900:2023 specifies two methods for polyurethane polyols. ISO states that Method B can produce low results for sterically hindered polyols, so a purchase specification should name the method rather than state only “OH value tested.”
ASTM also warns that ASTM D4274, ISO 14900 and ISO 6796 address the same subject but differ technically. Results produced by different procedures should not be treated as automatically interchangeable during supplier qualification or dispute testing.
The relationship between hydroxyl number and hydroxyl equivalent weight is:
Hydroxyl equivalent weight, g/eq = 56,100 ÷ hydroxyl value, mg KOH/g.
Equivalent weight describes how many grams of material contain one equivalent of hydroxyl groups. Increasing the OH value from 280 to 400 mg KOH/g reduces equivalent weight from approximately 200.4 to 140.3 g/eq, meaning the same mass presents more reactive OH equivalents.
| Illustrative OH value | Calculated OH equivalent weight | OH equivalents in 100 kg | 31.5% NCO isocyanate at index 110 | Typical formulation implication |
|---|---|---|---|---|
| 56 mg KOH/g | 1,002 g/eq | 99.8 eq | 14.6 kg | Low OH concentration; common among higher-molecular-weight flexible-foam polyols |
| 110 mg KOH/g | 510 g/eq | 196.1 eq | 28.8 kg | More NCO demand, subject to functionality and other reactive ingredients |
| 280 mg KOH/g | 200.4 g/eq | 499.1 eq | 73.2 kg | Intermediate equivalent weight; application and network design remain decisive |
| 400 mg KOH/g | 140.3 g/eq | 713.0 eq | 104.6 kg | Within a commonly screened region for rigid polyurethane systems |
| 500 mg KOH/g | 112.2 g/eq | 891.3 eq | 130.7 kg | High NCO demand and potential for a tightly crosslinked rigid network |
The isocyanate quantities in the table use an NCO equivalent weight of 133.3 g/eq, calculated as 4,200 divided by 31.5% NCO. They exclude water, amines and other active-hydrogen ingredients, so they are calculation examples rather than production mixing instructions.
OH value does not reveal hydroxyl functionality by itself. A difunctional polyol and a higher-functionality polyol can share the same OH value but create different network architectures, gel behavior, modulus and dimensional response.
How Hydroxyl Value Sets the Isocyanate Requirement
Polyurethane stoichiometry is based on equivalents, not on matching kilograms of polyol and isocyanate. For a dry polyol, hydroxyl equivalents equal polyol mass in grams multiplied by OH value and divided by 56,100.
Consider 100 kg of polyol at 400 mg KOH/g. It contains about 713 OH equivalents. With an isocyanate containing 31.5% NCO, the theoretical requirement at index 100 is approximately 95.1 kg; at index 110, it becomes approximately 104.6 kg.
If the delivered polyol is actually 420 mg KOH/g while production continues using the 400 mg KOH/g calculation, its hydroxyl content rises to approximately 748 equivalents. The corrected index-110 isocyanate demand becomes about 109.7 kg, a difference of 5.1 kg per 100 kg of polyol.
That deviation can move a rigid system away from its validated index. An NCO shortage can leave residual hydroxyl groups, slower demould strength, soft zones or dimensional instability; an unintended NCO excess can alter friability, adhesion, exotherm and surface cure.
A formulated blend requires a component-by-component balance. If a simplified base-polyol package contains 70 parts of a 450 OH polyol and 30 parts of a 35 OH polyol, its calculated value is 325.5 mg KOH/g before considering catalysts, surfactants, flame retardants, blowing agents or other reactive additives.
Non-reactive additives dilute the OH value when the result is reported on the total resin-side mass. Reactive flame retardants, chain extenders and crosslinkers add active-hydrogen equivalents. A purchase order must therefore state whether the target applies to a neat polyol, the polyol mixture or the complete formulated resin side.
Water needs a separate line in the stoichiometric calculation. Water reacts with isocyanate to form carbon dioxide and an amine, followed by urea formation; its commonly used active-hydrogen equivalent weight is 9 g/eq.
One kilogram of water represents about 111.1 active-hydrogen equivalents and theoretically requires about 14.8 kg of a 31.5% NCO isocyanate at index 100. It also generates approximately 1.24 m³ of CO2 at 0°C and 1 atm, showing why a seemingly small moisture change can alter density, pressure and cell structure.
ASTM D4672-24 measures water in polyurethane polyols and explicitly identifies water content as significant because isocyanates react with water. The equivalent international method, ISO 14897:2023, includes manual and automated Karl Fischer procedures.
How OH Value Changes Processing and Foam Performance
Raising OH value while holding polyol structure and functionality reasonably comparable increases reactive groups per kilogram and reduces equivalent weight. After the isocyanate quantity is corrected, the change can increase hard-segment content, crosslink density, glass-transition temperature, modulus and early demould strength.
A published polyurethane-coating study illustrates the magnitude. Formulated polyol blends at OH 261 and 692 mg KOH/g had equivalent weights of 215 and 81 g/eq; the reported calculated crosslink densities were 2.24 and 5.31 mmol/g, while measured glass-transition temperatures were 15.2°C and 128.7°C. The chemistries differed, so the data demonstrate direction and scale rather than a universal conversion rule; see the ScienceDirect study.
For rigid insulation foam, formulators commonly screen polyols or finished blends in a higher-OH region. SPC’s rigid-foam polyol selection guide identifies approximately 300–550 mg KOH/g as a practical starting band, paired with functionality around 3.5–8.
Those figures are not a universal acceptance limit. Refrigerator injection, continuous sandwich-panel lamination, discontinuous panels and spray foam impose different demands for flow length, cream time, gel time, adhesion, density distribution and demould cycle.
An OH value that is too high for a specific process can increase isocyanate consumption and produce early gel, restricted cavity flow, local exotherm or a brittle core. Faster network formation can trap uneven cells before the mixture fills panel corners or refrigerator cabinet sections.
An OH value that is too low, without a redesigned functionality and index, can delay green strength or create a softer network. In rigid foam, the observed result may be edge shrinkage, post-expansion, weak facer adhesion or deformation after demoulding.
Flexible foam follows another design logic. Higher-molecular-weight polyether polyols frequently have lower OH values because long chains separate their hydroxyl end groups. Elastic recovery, indentation force deflection or ILD, airflow and compression set still depend on functionality, polymer-polyol solids, water, TDI or MDI choice, index and cell-opening package.
OH value also does not predict reaction speed. Primary hydroxyl groups generally react differently from sterically hindered secondary hydroxyl groups, while amine and metal catalysts shift blowing and gelling rates. Two blends carrying the same CoA hydroxyl number can show different cream, rise, tack-free and demould times.
Why an OH Specification Cannot Stand Alone
A useful incoming specification links OH value to the failure modes that matter on the buyer’s equipment. At minimum, the data package should cover water, viscosity at a stated temperature, density or specific gravity, appearance, storage conditions, test methods and batch identification.
| Control item | What it detects | Procurement requirement | Possible production symptom if uncontrolled |
|---|---|---|---|
| Hydroxyl value, mg KOH/g | Reactive OH concentration | Target range, sample basis and ASTM D4274 or ISO 14900 method | Incorrect NCO demand, shifted cure or changed network density |
| Water, mass % | Intentional and accidental chemical blowing load | ASTM D4672 or ISO 14897 result, with water-blown systems clearly identified | Density loss, excess CO2, coarse cells, pressure or voids |
| Viscosity, mPa·s | Pumpability and mixing resistance | Test temperature such as 25°C and named spindle or method | Ratio drift, poor impingement mixing or incomplete cavity fill |
| Reactivity profile, seconds | Combined catalyst, temperature and raw-material response | Cream, gel, rise and tack-free definitions with cup temperature and mix method | Premature gel, slow demould or surface defects |
| Free-rise and core density, kg/m³ | Blowing balance and moulded result | Separate free-rise from moulded/core density; define sample conditioning | False material-yield calculations or insufficient strength |
| Finished-foam properties | Performance after reaction | Relevant methods such as ISO 845 density or ASTM D1621 compression | A liquid CoA passes while the produced panel or moulding fails |
An OH tolerance should be connected to dosing sensitivity. For example, a 390–410 mg KOH/g acceptance band around a nominal 400 represents a ±2.5% variation in hydroxyl equivalents and calculated isocyanate demand before water is counted.
That example is not a default industry tolerance. The workable limit must come from laboratory screening and line trials because a continuous laminator with a narrow gel window may tolerate less variation than a hand-poured moulding process.
The supplier’s Certificate of Analysis should report the actual batch result, not merely repeat a sales-sheet typical value. Buyers should retain lot numbers and compare incoming OH, water and viscosity trends against cream time, density and scrap rate.
Raw-material compliance documents and finished-product certifications must remain separate. REACH documentation and an SDS address chemical-market obligations, while tests such as FMVSS 302, ASTM E84 or a building-system approval depend on the cured article, density, thickness, substrate and complete formulation.
How to Qualify a Direct Blend Polyol Manufacturer
A direct manufacturer should be able to adjust the complete resin side rather than substitute a single base polyol and leave the rest of the package unchanged. Changing OH value affects isocyanate demand, so catalyst concentration, surfactant selection, blowing level, flame-retardant loading and viscosity may all require rebalancing.
Begin the RFQ with the actual process: foam type, MDI or TDI grade and %NCO, mixing equipment, component temperatures, desired weight ratio, target density in kg/m³, cream and gel times in seconds, mould temperature in °C, demould time and required finished-part tests.
Ask the supplier to return a calculation basis showing blend OH value, water contribution, assumed isocyanate equivalent weight and recommended index. If the proposed ratio is 1:1 by mass, request the chemistry behind that convenient ratio; equal drum weights do not prove stoichiometric balance.
SPC formulates the polyol, catalyst, silicone surfactant, blowing agent and flame-retardant package in-house for rigid, flexible, spray, integral-skin and elastomer systems. Its published qualification route offers 1–10 kg samples, batch CoA documentation and a stated 2–4 week custom-formulation cycle; production MOQ and export lead time should be confirmed for the destination and package size.
A sensible approval sequence is a laboratory cup test, a small machine trial and then a controlled production lot. Record raw-material temperatures, actual A:B output by mass, foam density and reaction times so a later batch can be compared against the same operating window.
For rigid insulation, qualify thermal conductivity in mW/(m·K), closed-cell content in %, compressive strength in kPa and dimensional change after thermal or humidity exposure. For flexible foam, use density, ILD or indentation hardness, tensile strength, elongation, tear strength, resilience and compression set as applicable.
Manufacturer-direct customization has value only when the revised formulation is traceable. The final agreement should identify the approved formula revision, specification limits, test methods, packaging, shelf-life conditions, change-notification requirement and the corrective-action process for an out-of-spec shipment.
FAQ
Q: Is a higher hydroxyl value always better for rigid polyurethane foam?
No. A higher OH value reduces equivalent weight and can support greater crosslink density, but it also raises isocyanate demand and may shorten flow time or increase brittleness. The correct value is the one validated against cavity fill, density, compression, dimensional stability and thermal conductivity on the buyer’s process.
Q: Can hydroxyl value be estimated from viscosity?
No. Viscosity reflects molecular structure, temperature, intermolecular forces, fillers and additive loading. Two products can have similar viscosity at 25°C yet carry different OH values, so titration by ASTM D4274 or ISO 14900 is required.
Q: Should the buyer test every delivered lot?
Review an actual batch CoA for every lot and perform risk-based incoming verification. OH value, Karl Fischer water and viscosity are strong screening controls; audit frequency can be increased after a formula change, deviation, long shipping exposure or unexplained process drift.
Q: Does the hydroxyl value include water?
A reported titration result and a formulation’s total active-hydrogen balance are not necessarily the same quantity. Water must be measured and entered separately in the stoichiometric calculation because 1 kg of water can consume about 14.8 kg of 31.5% NCO isocyanate at index 100.
Q: Why can two blend polyols with the same OH value produce different foam?
They may differ in functionality, polyether or polyester backbone, primary versus secondary OH groups, water, catalysts, surfactants, flame retardants and blowing agents. Those differences change reaction speed, cell stabilization, phase structure, flow, adhesion and cured mechanical properties.
Q: What information is needed for a custom blend-polyol quotation?
Provide the application, annual volume, trial quantity, required density and mechanical data, fire-test target, isocyanate identity and %NCO, machine ratio, component temperatures, reaction-time window, packaging and destination port. This allows the manufacturer to quote a formulation and qualification plan instead of matching one isolated OH number.
