Quick answer. To choose a polyol for rigid polyurethane foam, target a high-functionality (3.5–8), high-OH-value (300–550 mg KOH/g) polyether or polyester polyol so the cured matrix is densely crosslinked and dimensionally stable. Match the OH value to your target density and isocyanate index, keep water content below ~0.10% (unless it is your chemical blowing agent), hold acid value low (≤2 mg KOH/g for polyester), and confirm viscosity fits your dispensing equipment. Always validate against a supplier Certificate of Analysis (CoA) and your own foam trials before committing to volume.
Why OH value and functionality decide rigid-foam performance
Rigid foam is stiff because the polymer network is heavily crosslinked. Two polyol parameters drive that crosslink density more than any others: hydroxyl (OH) value and functionality.
The OH value (expressed in mg KOH/g) tells you how many reactive hydroxyl groups are available per gram of polyol. A higher OH value means more reaction sites for the isocyanate, more urethane linkages, and a harder, more thermally stable foam. Functionality is the average number of those OH groups per molecule. For rigid foam you want functionality typically between 3.5 and 8 — sucrose-, sorbitol- and aromatic-amine-initiated polyethers sit at the high end and give the best compressive strength and heat resistance. OH value is measured by standardized wet-chemistry titration, and buyers should ask suppliers which method (for example ASTM D4274 for hydroxyl number) was used so values are comparable lot to lot.
The practical equation buyers care about: OH value feeds directly into stoichiometry. The amount of isocyanate (and therefore your isocyanate index) is calculated from the combined OH value of the polyol blend plus the water content. Get the OH value wrong on the CoA and your A:B mix ratio, free-rise density and dimensional stability all drift.
The four specs every rigid-foam buyer must read on a CoA
Before you compare price, compare the spec sheet. These four numbers separate a foam-grade polyol from a generic one:
- OH value (mg KOH/g) — sets crosslink density and the isocyanate dose. Rigid grades usually 300–550.
- Functionality — average OH groups per molecule. 3.5–8 for rigid; below 3 trends toward semi-rigid or flexible.
- Water content (%) — water reacts with isocyanate to form CO₂ (a blowing agent) and urea hard segments. Even small variation shifts density and free rise. Measured by Karl Fischer titration.
- Acid value (mg KOH/g) — residual acidity, especially relevant for polyester polyols. High acid value slows the gel reaction, attacks amine catalysts and can cause storage instability and hydrolysis.
Two secondary specs round out the decision: viscosity (must match your high-pressure or low-pressure dispensing machine) and color (APHA/Gardner) if appearance matters for the finished part.
Buyer selection table: OH value vs. application
Use this table as a first-pass screen. It maps typical OH value and functionality bands to common rigid-foam applications so a procurement engineer can shortlist grades quickly.
| Application | OH value (mg KOH/g) | Functionality | Why this band |
|---|---|---|---|
| Spray foam insulation (wall/roof) | 350–450 | 4–5 | Fast cream/gel, good adhesion, stable closed cells |
| Appliance / refrigerator panels | 400–500 | 4.5–6 | Low thermal conductivity, high dimensional stability |
| Sandwich / metal-faced panels (PIR) | 240–320 (aromatic polyester) | 2–3 + high index | Fire performance via isocyanurate ring, char stability |
| Pipe-in-pipe / district heating | 380–480 | 4–6 | Compressive strength at depth, low creep |
| Block / discontinuous rigid foam | 300–400 | 3.5–5 | Balanced reactivity and demold time |
For board and panel lines running PIR chemistry, aromatic polyester polyols at a high isocyanate index (often 250–350) are the norm because the excess isocyanate forms the isocyanurate ring that drives fire performance — a different lever than simply raising OH value.
Reading water and acid value: the limits that cause field failures
Water is a double-edged sword. As a co-blowing agent it generates CO₂ and reduces reliance on physical blowing agents, which matters as the industry moves away from high-GWP HFCs. But uncontrolled water means uncontrolled density and friable, brittle foam from excess urea. The table below shows typical buyer limits.
| Spec | Typical rigid-grade limit | Test method | Risk if out of spec |
|---|---|---|---|
| Water content | ≤ 0.10% (unless water-blown formula) | Karl Fischer | Density drift, brittleness, mix-ratio error |
| Acid value (polyester) | ≤ 2.0 mg KOH/g | ASTM D4662 / ISO 2114 | Slow cure, catalyst poisoning, hydrolysis |
| OH value tolerance | ± 10–15 mg KOH/g lot-to-lot | ASTM D4274 | Stoichiometry shift, inconsistent rise |
| Viscosity @ 25°C | Per machine spec | ISO 3219 / ASTM D445 | Poor mixing, dosing error, voids |
Acid value deserves attention with polyester polyols because hydrolytic stability degrades as acidity rises. The standardized titration for hydroxyl and acid numbers is described under ISO 14900 and related methods; insist that your supplier's CoA cites a recognized method rather than an in-house shortcut.
Safety, REACH and handling the buyer must verify
The polyol side is generally lower-hazard than the isocyanate side, but a complete sourcing decision still depends on regulatory documentation. Catalysts (tertiary amines, organometallics), flame retardants and surfactants in a system polyol all carry their own hazard and registration profiles. For EU-bound shipments, confirm that every component is REACH-registered and that you receive compliant Safety Data Sheets; the European Chemicals Agency maintains the searchable substance database at echa.europa.eu. The reactive isocyanate (MDI) you pair with the polyol is also subject to worker training requirements, and exposure guidance for diisocyanates is published by OSHA. Build these checks into your supplier qualification, not after the first container lands.
How to run a sourcing trial before you commit volume
Spec sheets shortlist; trials decide. A disciplined buyer evaluation looks like this:
- Request representative samples with a full CoA covering OH value, functionality, water, acid value, viscosity and color.
- Hand-mix or machine-trial at your real isocyanate index and check cream time, gel time, tack-free time, free-rise density and core density.
- Cure and condition, then test compressive strength, dimensional stability (heat and cold aging), closed-cell content and, for insulation, thermal conductivity (lambda/k-factor).
- Audit lot consistency across at least two or three production lots — a polyol that meets spec once but drifts ±30 mg KOH/g lot to lot will wreck a continuous line.
- Confirm supply chain: lead time, MOQ, packaging (IBC vs. drum vs. ISO tank), and whether the supplier can pre-blend a system polyol to your formulation.
This last point is where a direct manufacturer adds the most value. Buying a tailored rigid-foam polyol system — pre-blended with the right catalyst package, surfactant and flame retardant to your target OH value and reactivity — removes formulation risk and saves your team the blending step. As a SPC Foam Material manufacturer supplying polyol, catalysts, silicone surfactants and flame retardants directly, we can match an existing formulation, supply individual components, or develop a custom combined polyol with documented CoA values and REACH-compliant documentation for export.
Direct-supply advantages for rigid-foam buyers
Sourcing from the original manufacturer rather than a trading layer changes the economics and the technical support you get:
- Custom OH value and reactivity tuned to your line speed and demold target, not a fixed catalog grade.
- Lot-to-lot consistency backed by in-house titration and Karl Fischer QC on every batch.
- Full documentation: CoA, SDS and REACH/registration support for EU, North American and other export markets.
- Component flexibility: buy the base polyol, the catalyst, the surfactant or a complete system polyol — your choice.
- Technical collaboration on density, thermal conductivity and fire performance trade-offs before you scale.
FAQ
Q: What OH value should I use for rigid polyurethane foam?
Most rigid-foam polyols fall in the 300–550 mg KOH/g range, with appliance and high-stability insulation foams toward the higher end. The exact value depends on your target density, isocyanate index and required compressive strength, so confirm with a foam trial rather than the catalog band alone.
Q: What is the difference between polyether and polyester polyol for rigid foam?
Polyether polyols offer better hydrolytic stability, lower viscosity and good processing for spray and appliance foam. Aromatic polyester polyols deliver superior fire performance and char stability and are standard for PIR board and metal-faced sandwich panels run at high isocyanate index.
Q: Why does water content matter so much in a foam-grade polyol?
Water reacts with isocyanate to produce CO₂ (a blowing agent) and urea hard segments. Variation in water content shifts foam density, free rise and friability and changes the isocyanate you must dose, so foam-grade polyols are typically held at or below 0.10% water unless water is your intended chemical blowing agent.
Q: What acid value is acceptable for a polyester polyol?
Keep acid value low — typically ≤ 2.0 mg KOH/g for foam-grade polyester polyols. High acidity slows the gel reaction, can poison amine catalysts and accelerates hydrolysis during storage, hurting both processing and shelf life.
Q: Can a supplier custom-blend a rigid-foam system polyol to my formulation?
Yes. As a direct manufacturer we can match an existing formulation or develop a new combined polyol — polyol, catalyst, surfactant and flame retardant pre-blended to your target OH value and reactivity — and supply it with full CoA and REACH-compliant documentation. Request a sample and we will trial it against your isocyanate index and density target.
