Quick answer. Polyol functionality is the average number of reactive hydroxyl groups on each polyol molecule. Raising functionality generally increases crosslink density, shortens the molecular distance between network junctions and produces a stiffer foam, but final hardness also depends on OH value, molecular weight, foam density, isocyanate index, catalysts, water and cell structure.
A purchasing specification based only on “functionality 4” or “high-hardness polyol” is incomplete. Industrial buyers should define the raw-material chemistry, processing window and finished-foam test together, using newtons for flexible-foam indentation or kilopascals for rigid-foam compression.
What Polyol Functionality Actually Measures
Functionality, commonly written as f, is the average number of hydroxyl groups available per polyol molecule. A nominal difunctional polyol has about 2 OH groups per molecule, a trifunctional polyol about 3, and a sucrose- or sorbitol-initiated rigid-foam polyether may have a nominal functionality between roughly 4 and 8.
Functionality is an average because commercial polyols contain molecular populations rather than one perfectly uniform molecule. Side reactions, unsaturation and incomplete initiation can make effective functionality lower than the nominal initiator functionality.
The distinction matters during sourcing. Two products described as “f = 3” can produce different networks if one has an OH value of 35 mg KOH/g and a molecular weight near 5,000 g/mol while the other has an OH value above 300 mg KOH/g and a molecular weight below 1,000 g/mol.
OH value measures hydroxyl concentration per unit mass, expressed in mg KOH/g. Functionality measures hydroxyl groups per molecule; neither value can replace the other in an isocyanate calculation.
For a simple hydroxyl-terminated polyol, buyers can use the approximate relationship: number-average molecular weight equals 56,100 multiplied by functionality and divided by OH value. A trifunctional polyol at 56 mg KOH/g therefore corresponds approximately to 3,005 g/mol, while a functionality-4 polyol at 400 mg KOH/g corresponds to about 561 g/mol.
This calculation is a screening tool, not a release test. Amine-initiated polyols, mixed polyol packages and materials containing water or other active-hydrogen compounds require the supplier’s equivalent-weight calculation for correct MDI or TDI dosing.
Hydroxyl number can be checked under ASTM D4274. A buyer comparing quotations should record the method, result in mg KOH/g, test date and batch number because a bare numerical value without a method may conceal differences in sample preparation or calculation.
How Functionality Changes Crosslinking and Hardness
Each hydroxyl group can react with an isocyanate group to form a urethane linkage. Molecules with more than two reactive sites can connect several polymer chains, creating network junctions instead of predominantly linear chains.
As the number of effective junctions rises, chain movement becomes more restricted. The cured polymer usually shows a higher modulus, lower elongation and greater resistance to indentation or compression at the same foam density and test condition.
The relationship is directional rather than one-to-one. Increasing average functionality from 3 to 4 does not guarantee a 33% increase in hardness because crosslink formation also depends on conversion, stoichiometry, steric accessibility and the mix of urethane, urea, allophanate and isocyanurate structures.
A ScienceDirect overview of polyurethane crosslink density describes flexible foams as low-crosslink-density structures based on high-molar-mass polyols, while shorter, higher-functionality polyols promote the more restricted networks used in semi-rigid and rigid products.
Water adds another reaction route. One mole of water consumes two isocyanate equivalents through the CO2-forming sequence and creates urea-containing hard segments; a change of only 0.10 percentage point in formulation water can affect blowing, density and hardness simultaneously.
Isocyanate index must therefore be calculated from all active-hydrogen sources, not from the base-polyol OH number alone. An index of 100 represents stoichiometric NCO-to-active-hydrogen equivalence; values above or below 100 alter residual chemistry and network development.
More crosslinking can raise green strength and reduce post-demould deformation. Excessive network density may instead cause brittleness, friable edges, poor skin elongation or cracking around inserts, especially when the foam is demoulded hot or the part contains sharp changes in section thickness.
In flexible seating foam, excessive crosslinker can also reduce airflow and recovery or raise compression set. In rigid insulation, insufficient crosslinking can appear as panel shrinkage, weak facer adhesion, edge crushing or dimensional change during hot and humid exposure.
Comparing Typical Polyol Design Windows
The table gives formulation-screening ranges used across polyurethane foam families. They are not universal purchase limits; the accepted numbers must come from the tested formulation and the customer’s mould, machine and density target.
| Foam design | Typical polyol functionality | Typical OH value | Network and hardness behavior | Acceptance data to request |
|---|---|---|---|---|
| Conventional flexible slabstock | About 2–3 | About 20–60 mg KOH/g | Long soft segments and relatively low crosslink density support compression and recovery. Hardness is normally reported as IFD or indentation force in N at a stated deflection. | Density in kg/m³; 25%, 40% or 65% IFD; airflow; rebound; tensile strength in kPa; elongation in %; compression set in %. |
| High-resilience or moulded flexible foam | Commonly near 3, often with crosslinker | Base polyol commonly about 20–40 mg KOH/g; blended OH value may be higher | Primary-OH-rich polyols and controlled crosslinking support faster cure and load bearing without converting the cushion into a rigid network. | Core density; 25% and 65% IFD in N; comfort factor; ball rebound in %; demould time in seconds or minutes; compression set. |
| Semi-rigid or integral-skin foam | Often 3–5 in the complete polyol package | Frequently about 100–400 mg KOH/g | Shorter segments and additional junctions raise modulus; skin formation, mould temperature and part density strongly influence measured hardness. | Overall density and core density; Shore hardness where applicable; tensile strength in MPa; elongation in %; tear strength; moulded-part weight. |
| Rigid insulation foam | Commonly about 3.5–8 | Commonly about 300–550 mg KOH/g | Closely spaced junctions restrict segment motion and help the cellular structure resist compressive load and dimensional change. | Core density in kg/m³; compressive strength in kPa; closed-cell content in %; dimensional stability in %; thermal conductivity in W/(m·K). |
| Low-molecular-weight crosslinker | Usually 3 or more | Often several hundred to above 1,000 mg KOH/g | Small additions can change equivalent weight and cure sharply. Excess can produce brittleness, shrinkage or an impractically short cream-to-gel window. | Dosage in parts per hundred polyol; blend OH value; viscosity at 25°C; cream, gel, rise and tack-free times in seconds. |
The flexible-versus-rigid comparison shows why functionality must be read with molecular size. A 5,000 g/mol triol places three OH groups along a large molecule, while a 500 g/mol functionality-4 polyol packs four groups into one-tenth of that molecular mass.
That tenfold molecular-weight difference changes the average distance between crosslinks. It also changes the required isocyanate mass, heat release, viscosity and hard-segment content, even before catalysts or blowing agents are adjusted.
Buyers working on seats, mattresses or moulded cushions can review SPC’s flexible polyurethane foam systems. Insulation, refrigeration and sandwich-panel producers should instead start with the rigid foam polyol range, where higher-functionality networks and closed-cell performance are the relevant design route.
Why Foam Hardness Cannot Be Predicted from Functionality Alone
Density can overpower a chemistry comparison
Foam is a cellular composite of polymer and gas. Raising moulded density from 35 to 45 kg/m³ places about 29% more polymer mass in the same volume, so a higher indentation or compression result may come from density rather than a change in functionality.
A supplier trial must therefore compare samples at the same conditioned density. A hardness value without density, conditioning temperature, specimen thickness and test method is unsuitable for bid comparison.
Flexible and rigid foam use different hardness language
Flexible foam hardness is commonly expressed as indentation force deflection. ISO 2439 includes procedures at 25%, 40% and 65% indentation and reports force in newtons; its methods also specify dwell conditions such as 30 seconds.
ASTM D3574 covers slab, bonded and moulded flexible polyurethane foam tests. A purchase order should name the exact method and deflection rather than state “hardness 150” because 150 N at 25% deflection is not equivalent to 150 N at 40% deflection.
Rigid foam is normally evaluated by compression rather than cushion IFD. ASTM D1621 derives compressive stress and modulus information from a load-deformation curve, with results commonly reported in kPa or MPa.
Cells, catalysts and temperature modify the result
Cell size, open-cell fraction and anisotropy affect load transfer. A rigid panel tested parallel to foam rise can produce a different compressive result from a specimen cut perpendicular to rise, so orientation belongs on the test report.
Silicone surfactant controls cell stabilization, while the balance between blowing and gelling catalysts determines whether gas generation is supported by sufficient polymer strength. Too much blowing relative to gel can cause collapse or coarse cells; too much gel can trap closed cells prematurely and increase shrinkage risk.
Temperature also changes reaction rate and measured feel. A formulation trial run with components at 18°C should not be compared casually with production material processed at 25°C, and viscoelastic foam requires especially careful conditioning because its modulus changes around its service-temperature region.
A Procurement Specification That Prevents Expensive Mismatches
Begin with the finished part rather than a preferred functionality number. Define foam family, application, target density, hardness test, specimen direction, service temperature and the failure that must be prevented.
For flexible foam, specify density in kg/m³ plus IFD in N at the required deflection and dwell time. Add 65%/25% support or comfort factor, rebound in %, airflow, compression set after a stated time and temperature, and any flammability method required by the end product.
For rigid foam, specify free-rise and moulded core density separately. Add compressive strength in kPa at a stated strain and direction, closed-cell content in %, dimensional stability under stated temperature and humidity, thermal conductivity in W/(m·K), and adhesion to the actual facer where panels are produced.
The raw-material section should state polyol OH value in mg KOH/g, nominal or effective functionality, viscosity in mPa·s at a stated temperature, water in %, acid value where relevant, and the calculation basis for the recommended polyol-to-isocyanate ratio. Record acceptable tolerances rather than accepting a typical value as a release limit.
Require a batch Certificate of Analysis linked to the lot number, plus TDS and SDS revision dates. For EU imports, verify the actual substances, use and supply-chain duties against ECHA’s REACH guidance; the word “REACH” on a quotation does not establish registration coverage or mixture compliance.
A practical qualification sequence is a 1–10 kg laboratory sample, followed by a pilot quantity sufficient for the customer’s mixing head, mould or continuous line. SPC publishes 1–10 kg samples and trials from 200 kg for its formulated systems, allowing the customer to compare lab foam, machine foam and bulk-production behavior before container-volume purchasing.
During the trial, log component temperature in °C, mixing ratio by mass, output in kg/min, cream time, gel time, rise time, tack-free time, mould temperature, demould time and finished density. Retain labelled foam and liquid samples so a later drift can be separated into raw-material, machine and ambient causes.
Manufacturer-direct custom blending is valuable when the target cannot be reached by swapping one base polyol. The formulator can adjust base-polyol functionality, crosslinker dosage, catalyst balance, silicone surfactant, water and flame retardant as one system, then recalculate equivalent weight and MDI or TDI demand.
Price comparison should use cost per accepted foam part or per cubic metre, not polyol price per tonne alone. A lower-priced blend that raises reject rate from 2% to 6%, increases density by 3 kg/m³ or adds 60 seconds to demould time can erase the raw-material saving on a production line.
The purchase contract should freeze the approved formulation or require written change control. Define which CoA properties are tested per batch, which finished-foam tests are periodic, how out-of-spec material is quarantined and how retained samples will be used during a claim.
FAQ
Q: Does higher polyol functionality always make polyurethane foam harder?
No. Higher effective functionality generally increases crosslink density, but a lower-density foam, lower isocyanate index, longer polyol chain, different cell structure or changed water level can reverse the apparent hardness result. Compare samples at equal density under the same ISO or ASTM procedure.
Q: What functionality is normally used for flexible foam?
Many flexible polyether foam systems use base polyols near functionality 2–3 with molecular weights around 3,000–6,000 g/mol and OH values commonly around 20–60 mg KOH/g. Crosslinkers and graft polymer polyols may be added to raise load bearing without replacing the entire soft-segment structure.
Q: What functionality is normally used for rigid polyurethane foam?
Rigid systems commonly use polyols or polyol mixtures with average functionality around 3.5–8 and OH values around 300–550 mg KOH/g. The final selection depends on blowing agent, density, isocyanate index, flame-retardant loading, viscosity and dimensional-stability target.
Q: Is OH value the same as polyol functionality?
No. OH value is hydroxyl concentration by mass in mg KOH/g, while functionality is the average number of OH groups per molecule. A formulation needs both, plus molecular-weight and active-hydrogen information, to estimate network structure and isocyanate demand.
Q: Should a buyer specify Shore hardness for every PU foam?
No. Flexible cushioning is normally compared using indentation force in N under ISO 2439 or ASTM D3574. Rigid cellular plastics are commonly compared using compressive stress in kPa under ASTM D1621; Shore methods may suit compact elastomers or some integral-skin parts but do not replace foam-specific tests.
Q: Can a blended-polyol manufacturer customize hardness without changing density?
Yes, within a validated processing window. The formulator can adjust average functionality, OH value, crosslinker, polymer polyol, catalyst balance and isocyanate index, but the customer should confirm density, cell structure, compression set, dimensional stability and cure time after every meaningful formulation change.
Q: What documents should accompany an export shipment?
Request a lot-linked CoA, current TDS, SDS, packing list, transport classification and the market-specific compliance evidence agreed in the contract. A certificate should identify the tested batch and numerical limits; a generic declaration cannot demonstrate that the delivered blend meets the approved OH value, water or viscosity range.
