Quick answer. The isocyanate index is the ratio of isocyanate actually used to the stoichiometric amount required by all active hydrogens in the formulation, multiplied by 100. Most rigid polyurethane and PIR foams run at an index of 105-130 (a slight isocyanate excess); pure PIR insulation boards run 200-350. If foam is friable and crumbly you are usually over-index or under-catalyzed on trimerization; if it is soft, shrinking, or tacky you are typically under-index or off-ratio on the polyol side.
What the isocyanate index actually measures
Rigid foam chemistry is a competition between reactions, but the index question is simple: did you supply enough NCO groups to react with every OH group (and water) in the system, and how much extra did you add on purpose? An index of 100 means exact stoichiometric balance. An index of 110 means you dosed 10% more isocyanate than theory requires, leaving free NCO available for allophanate, biuret, and — critically for PIR — isocyanurate (trimer) crosslinks that raise thermal stability and fire performance.
For B2B buyers of rigid foam polyol systems, the index is not an academic number. It sets your MDI consumption per board-foot, your dimensional stability in the field, and whether your panels pass reaction-to-fire testing. Getting it wrong costs raw material margin on one side and warranty claims on the other.
How to calculate the NCO/OH ratio
The working formula every plant technician should have on the wall is:
Isocyanate Index = (actual parts isocyanate / theoretical parts isocyanate) × 100
The theoretical (stoichiometric) isocyanate requirement is driven by the total equivalents of active hydrogen in the B-side. Do not forget water — each mole of water consumes two NCO equivalents (one to form the amine, one to form urea) and is often the single biggest hidden NCO sink in a water-blown rigid formulation.
- Polyol OH equivalents = (OH value × mass of polyol) / 56,100
- Water equivalents = mass of water / 9.0 (water behaves as if it had an equivalent weight of 9)
- Isocyanate equivalents supplied = (%NCO × mass of ISO) / 4,202
Divide isocyanate equivalents supplied by the sum of all active-hydrogen equivalents, multiply by 100, and you have the index. In practice, most producers work from a supplier-provided formulation sheet and adjust the ISO parts up or down a few points; the math above lets you audit that sheet independently instead of trusting it blindly.
Target index by foam type
There is no single "correct" index — it depends on the product you are making. Use the table below as a starting window, then fine-tune against your own core density, friability, and dimensional-stability results.
| Foam / application | Typical index | Why this range |
|---|---|---|
| Flexible slabstock (context) | 95-105 | Softness and comfort; slight under- to near-balance |
| Rigid appliance & panel PUR | 105-120 | Excess NCO for crosslink density and dimensional stability |
| Rigid spray foam (SPF) | 105-115 | Balance of cure speed, adhesion, and low friability |
| PIR boardstock (polyisocyanurate) | 200-350 | Large NCO excess drives isocyanurate trimer for fire performance |
| Rigid pour-in-place / discontinuous panel | 110-130 | Robust cure margin against mixing and metering variation |
Notice the jump to PIR. Isocyanurate rings only form in meaningful quantity when there is a large NCO excess plus a dedicated trimerization catalyst (potassium octoate / potassium acetate). Running a PIR-style index without the right catalyst package gives you the worst of both worlds: expensive foam that is brittle and still under-performs on fire.
Why manufacturers rarely run exactly 100
Real production lines have metering drift, temperature-dependent viscosity, moisture pickup in the polyol, and batch-to-batch %NCO variation in the isocyanate. Running a small planned excess (index 105-115) is a deliberate safety margin so that normal process noise never pushes you below index 100, where unreacted polyol leaves you with soft, tacky, dimensionally unstable foam.
Troubleshooting: reading defects back to the index
Most rigid-foam defects that get blamed on "bad polyol" are actually index or ratio problems. Use this diagnostic table before you reject a raw-material lot.
| Symptom | Likely index / ratio cause | First corrective action |
|---|---|---|
| Friable, crumbly, dusty surface | Over-index and/or too little gelling catalyst; trimer forming too fast | Lower index 5-10 points; rebalance gel vs. trimer catalyst |
| Soft, spongy, poor compressive strength | Under-index; polyol OH under-reacted | Raise ISO parts; verify %NCO of current ISO drum |
| Post-demold shrinkage / concave panels | Under-index or insufficient crosslink density | Increase index toward 110-115; check core density |
| Tacky, uncured core | Off-ratio (ISO short) or cold components | Verify pump calibration; condition A/B to 20-25 °C |
| PIR foam fails fire test | Index too low for trimerization; weak trimer catalyst | Raise index above 250; boost potassium catalyst |
| Cracks / voids with strong amine odor | Excess water (over-index effect from CO2) or moisture contamination | Dry polyol; re-check water content in B-side |
A repeatable rule: confirm the metering ratio physically (bucket test both streams) before you touch the formulation. An index calculated on paper is meaningless if your ISO pump is delivering 8% low.
Health, safety, and the case for a controlled supply chain
Isocyanates are potent respiratory sensitizers, and running a high-index PIR process means handling substantial free MDI. Occupational exposure limits and safe-handling practices are non-negotiable: review the U.S. OSHA guidance on isocyanates and the NIOSH exposure and medical-surveillance recommendations from the CDC/NIOSH isocyanates topic page. European buyers should also confirm restriction and registration status through ECHA, since diisocyanate training requirements now apply across the EU.
This is where sourcing matters. Index control depends on a stable, accurately certified %NCO on the isocyanate and a stable OH value on the polyol. Batch-to-batch drift in either number forces you to re-tune the index constantly — burning material and QC time. As a direct manufacturer of polyol systems, catalysts, surfactants, and flame retardants, we supply each shipment with a certificate of analysis (OH value, viscosity, water content, moisture), offer formulation packages tuned to a target index rather than generic drums, and can customize the catalyst balance for your exact line speed and mold temperature. That traceability is what keeps your index inside a 3-point window instead of a 15-point one.
A practical index-setting workflow
- Step 1 — Baseline. Start at the supplier-recommended index for your foam type from the table above.
- Step 2 — Verify inputs. Confirm current-lot %NCO and OH value from the COA; recalculate stoichiometry including water.
- Step 3 — Trial ±5. Pour at index, index+5, and index-5. Measure core density, friability, and 24 h dimensional stability.
- Step 4 — Lock and log. Choose the index with the best stability/friability trade-off, then hold pump calibration and component temperatures constant.
- Step 5 — Monitor drift. Re-audit index whenever you change an ISO or polyol lot, or when ambient humidity shifts seasonally.
FAQ
Q: What is a good isocyanate index for rigid foam?
For standard water- or blend-blown rigid PUR (appliances, panels, pour-in-place), an index of 105-120 is typical. It provides enough NCO excess for full polyol conversion and crosslink density while keeping friability low. PIR insulation is a different regime entirely, running 200-350.
Q: What happens if the isocyanate index is too high?
Excess NCO drives fast trimer and allophanate formation, which can make foam brittle, friable, and prone to surface dusting. You also waste expensive isocyanate. Without a matching trimer catalyst, a high index in a non-PIR system simply degrades quality rather than improving fire performance.
Q: What happens if the index is too low?
Below index 100 there is not enough isocyanate to react with all the polyol and water. The result is soft, tacky, dimensionally unstable foam that shrinks after demold and has poor compressive strength. Low index is the most common cause of "the polyol went bad" complaints that are actually metering or formulation errors.
Q: How does water affect the NCO/OH ratio?
Water is an active-hydrogen source: each part of water consumes about the equivalent of an OH group with an equivalent weight of only 9, so small water changes swing your isocyanate demand sharply. Undried polyol that picks up moisture effectively lowers your real index and generates extra CO2, causing splits, voids, and higher exotherm.
Q: Do I need a different catalyst package to run a high (PIR) index?
Yes. Isocyanurate crosslinks that give PIR its fire and thermal performance require a dedicated trimerization catalyst (typically potassium octoate or potassium acetate) balanced against your gelling and blowing catalysts. Raising the index without the right catalyst produces brittle, under-cured foam. Ask your supplier for a catalyst package matched to your target index and line conditions.
Dialing in the isocyanate index is one of the highest-leverage adjustments in rigid foam production: it touches raw-material cost, mechanical performance, dimensional stability, and fire rating all at once. Get your %NCO and OH values certified, calculate honestly including water, trial around the target, and lock the process. If you want formulation support tuned to a specific index target, our technical team can build a matched polyol-plus-catalyst package for your line.
