A few grams of isocyanate error per 100 parts of polyol can change a dense, wear-resistant elastomer into a part that cures slowly at the center or fails to reach demold hardness. On the production floor, the problem is often blamed on pump ratio, mold temperature, or catalyst level. Before any of those, the first number worth checking is the equivalent weight data used to build the recipe.
Equivalent weight (EW) defines how many grams of a material supply one reactive group. In polyurethane systems, that means grams per hydroxyl equivalent for polyols and chain extenders, and grams per NCO equivalent for isocyanates and prepolymers. The math is simple, but small mistakes propagate through the isocyanate index and the A/B ratio, then show up as shrinkage, missed hardness, foam collapse, or dimensional instability.
What Equivalent Weight Controls in a PU Recipe
Equivalent weight is the bridge between a raw material data sheet and a workable machine recipe. It controls four practical decisions in sequence:
- Whether the formula is chemically balanced before the machine ever starts.
- Whether the target isocyanate index is real or only nominal.
- How many parts by weight of isocyanate must be metered against the polyol side.
- Whether the dispensed mass in each mold matches the shot weight the tool was designed for.
A formulation can have excellent mixing, precise temperature control, and a stable pump. If the equivalent weight entered into the recipe is wrong, the part will still be wrong.
How to Calculate Polyol Equivalent Weight
For polyols, the standard conversion comes from the hydroxyl value (OHV), expressed in mg KOH/g:
Polyol equivalent weight = 56,100 ÷ hydroxyl value [1]
Written more directly:
EW polyol (g/eq) = 56,100 / OHV
The 56,100 factor converts milligrams of KOH per gram into molecular equivalents. It exists because the molar mass of KOH is 56.1 g/mol.
For example, a 3,000 molecular weight polyether triol with a published hydroxyl value of 56.1 mg KOH/g has:
EW = 56,100 ÷ 56.1 = 1,000 g/eq
A lower molecular weight polyol with a hydroxyl value of 112.2 mg KOH/g has an equivalent weight of 500 g/eq.
For blended polyols, do not average the equivalent weights directly. Calculate the blend equivalent weight from equivalents:
EW blend = 100 ÷ [(parts A ÷ EW A) + (parts B ÷ EW B)]
If a blend contains 80 parts of a 1,000 EW polyol and 20 parts of a 500 EW polyol:
Equivalents = 80 ÷ 1,000 + 20 ÷ 500 = 0.08 + 0.04 = 0.12 equivalent per 100 parts
EW blend = 100 ÷ 0.12 = 833.3 g/eq
How to Calculate Isocyanate Equivalent Weight
For isocyanates and prepolymers, the starting point is NCO content as a weight percentage:
Isocyanate equivalent weight = 4,200 ÷ %NCO [2]
The more exact conversion uses the NCO group molar mass of 42.02 g/mol:
EW isocyanate (g/eq) = 4,202 / %NCO
Pure MDI with an NCO content of 33.6% therefore has:
EW = 4,202 ÷ 33.6 = 125.1 g/eq
A quasi-prepolymer with 16.0% NCO has:
EW = 4,202 ÷ 16.0 = 262.6 g/eq
For shop-floor work, 4,200 is usually accurate enough. The larger issue is not the conversion factor; it is whether the NCO value being used is current. Prepolymers drift with storage temperature, moisture exposure, and batch age, so the measured NCO from titration should govern critical recipes.
Calculating the Isocyanate Index from Equivalent Weights
Equivalent weights become useful when the isocyanate index is calculated correctly. The index compares actual NCO equivalents to theoretical active-hydrogen equivalents [3]:
Isocyanate index = (actual NCO equivalents ÷ theoretical active-hydrogen equivalents) × 100
Water must be included because one mole of water consumes two NCO groups. Water has a molecular weight of 18 g/mol and two active hydrogens, so its equivalent weight is 9 g/eq.
Consider this simple formulation:
| Component | Parts by weight | Equivalent weight | Equivalents |
|---|---|---|---|
| Polyol | 100.0 | 1,000 | 0.100 |
| BDO chain extender | 8.0 | 45 | 0.178 |
| Water | 0.2 | 9 | 0.022 |
| Total active hydrogen | 108.2 | — | 0.300 |
At a 1.05 index with an MDI equivalent weight of 125:
Isocyanate parts = 1.05 × 0.300 × 125 = 39.4 parts
Without the 0.2 parts of water, the same formula would require only about 36.5 parts of MDI. The water looks negligible by weight but adds nearly 3 parts of isocyanate demand. That is exactly where many operator-built spreadsheets drift away from the lab formulation.
Errors That Change the Real Index
Using hydroxyl value as equivalent weight
A hydroxyl value is not an equivalent weight. A polyol with an OHV of 56 is not 56 g/eq. The correct value is 56,100 ÷ 56, or about 1,002 g/eq. Skipping the 56,100 conversion creates a grossly unbalanced recipe.
Averaging equivalent weights without weighting
An 800 EW and a 1,000 EW polyol do not necessarily produce a 900 EW blend. The equivalent contributions must be calculated from parts by weight, as shown above. The error grows when one polyol is present at a much higher fraction.
Using nominal NCO instead of measured NCO
A supplier datasheet is a starting point, not a live quality certificate. Moisture ingress during storage can lower effective NCO and shift the true equivalent weight. For tight elastomer or structural foam work, use measured NCO from titration.
Ignoring water and moisture
Water is not just a blowing agent. It is a reactive component with an equivalent weight of 9. Wet polyol, humid filler, or small water additions all consume isocyanate and change the effective index. If the recipe does not account for them, the machine can hold a perfect ratio while the part still comes out soft or friable.
Index errors from equivalent weight mistakes often appear first as dimensional or surface defects. Causes Of Shrinkage In Polyurethane Foam How To Control It covers the shrinkage mechanisms operators see when the real ratio drifts away from the recipe.
From Recipe Math to Machine Metering
A correct spreadsheet only matters if the machine reproduces the ratio as mass throughput. The isocyanate parts and polyol parts calculated from equivalent weights become the weight- or volume-based ratio setpoint on the metering system.
In practice, three machine-level factors determine whether the calculation survives contact with production:
- Metering accuracy. A closed-loop servo or mass-flow system holding ±0.3% full-range accuracy is far more likely to preserve a 1.05 index than an open-loop gear pump subject to viscosity drift.
- Temperature control. Viscosity change affects flow through fixed orifices, especially when polyol and isocyanate temperatures are not held independently.
- Recipe integrity. When a line switches from BDO to MOCA, the chain extender equivalent weight changes materially. The machine must switch more than temperatures; it must switch the active mass-flow calibration.
A deviation of one percentage point in the component ratio can shift the effective index enough to produce measurable changes in hardness, cure time, or foam density. The formula does not need to be off by large numbers to create a production problem.
Equivalent weight sets the balance; the next calculation is how much of that balanced mixture belongs in each mold. How To Calculate Shot Weight In Polyurethane covers cavity volume, overpack, and shot calibration for consistent parts.
Worked Example: From TDS to Dispense Ratio
Assume a rigid foam polyol blend with a reported hydroxyl value of 450 mg KOH/g:
Polyol EW = 56,100 ÷ 450 = 124.7 g/eq
The polymeric MDI reports 31.0% NCO:
Isocyanate EW = 4,202 ÷ 31.0 = 135.5 g/eq
At a target index of 1.10 and 100 parts by weight of polyol:
Isocyanate parts = 1.10 × (100 ÷ 124.7) × 135.5 = 119.5 parts
The machine ratio is therefore 100:119.5 by weight. If the mixing head delivers 150 g/s total output, the A-side and B-side flow rates follow directly from that ratio. This is the difference between a recipe calculated from chemistry and a recipe assembled from trial-and-error memory.
Need Help Turning These Numbers Into a Dispense Recipe?
Haifeng Polyurethane Machinery works with production teams to convert TDS hydroxyl values, measured NCO, and target indexes into metering settings for low-pressure, high-pressure, and elastomer casting lines. Send the following to Info@chinahaifeng.com or WhatsApp +86 13566296633:
- Polyol and isocyanate TDS or measured OHV/NCO values
- Target isocyanate index and component temperatures
- Required shot weight or mold cavity volume
- Current output target and cycle time
We will help you check the recipe before it reaches the mixing head.
Frequently Asked Questions
Can I calculate equivalent weight from molecular weight instead of hydroxyl value?
Yes, if the true functionality is known. Equivalent weight equals molecular weight divided by functionality. This works well for defined diols or triols. Formulated blends may contain monofunctional species or side products, so a measured hydroxyl value is usually more reliable for production recipes.
Is 4,200 always correct for isocyanate equivalent weight?
It is a practical conversion from NCO weight percentage to grams per equivalent. The more exact factor is 4,202 because the NCO group molar mass is 42.02 g/mol. For most shop-floor work, 4,200 is close enough, but the NCO percentage itself should be measured, not assumed.
What happens if I enter the wrong polyol equivalent weight?
The calculated isocyanate demand changes in direct proportion. If the equivalent weight is entered too high, the recipe will under-dose isocyanate and produce soft, under-cured, or hydrolytically weak parts. If it is entered too low, the recipe will over-dose isocyanate and may create brittleness, excessive exotherm, or higher free-NCO levels.
Does water really matter in the calculation?
Yes. Water has an equivalent weight of 9 because one mole of water supplies two reactive hydrogens. Even 0.2 parts of water in a total formula can require roughly 3 additional parts of MDI per 100 parts of polyol. Ignoring it changes the true index.
How does equivalent weight relate to A/B mixing ratio?
Equivalent weights plus the target index determine the parts by weight of each component. Those parts become the weight-based ratio setpoint for the metering pumps. The calculation is the same for low-pressure and high-pressure equipment; only the machine’s ability to hold the ratio changes.
References
[1] ASTM D4274-21, Standard Test Method for Testing Polyurethane Raw Materials: Determination of Hydroxyl Numbers of Polyols, ASTM International, 2021.
[2] ASTM D2572-19, Standard Test Method for Isocyanate Groups in Urethane Materials or Prepolymers, ASTM International, 2019.
[3] G. Oertel, Polyurethane Handbook, 2nd ed., Hanser Publishers, 1994.
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