Polyurethane Mix Ratio: How To Calculate It, Step By Step

A panel line starts the Monday shift after a cold weekend. The first panels feel softer than usual, the core shrinks slightly at the edges and nobody has touched the recipe. In most cases like this the chemistry is fine and the polyurethane mix ratio is not: the components are colder, more viscous, and the pumps are no longer delivering the proportions the formulation was designed for. Understanding where that ratio comes from is the fastest way to find the fault.

01What The Mix Ratio Represents

The mix ratio expresses how many grams of isocyanate must meet each 100 g of polyol blend so that the reactive groups find each other in the intended proportion. Polyurethane forms when an isocyanate group (NCO) reacts with a hydroxyl group (OH); in foam systems, isocyanate also reacts with water to release the carbon dioxide that blows the foam.

The data sheet of a formulated system already states the ratio, for example "100 : 130 by weight". That figure is not arbitrary: it is the result of the calculation below, made with the target values of the components. Every real batch deviates slightly from target, which is why a technician who can repeat the calculation with the values on the certificate of analysis can explain, and correct, most ratio problems.

02The Four Inputs You Need

Everything comes from four values, all of which appear on technical data sheets and certificates of analysis.

• Hydroxyl value (OH value): in mg KOH/g, measured by methods such as ASTM D4274. For a formulated blend, use the blend value, which already includes the polyols, chain extenders and other reactive additives.
• Water content: in parts per hundred of blend (phr). Water reacts with two NCO groups and must be counted separately unless the blend value already includes it; the data sheet states which convention it uses.
• NCO content: in per cent by weight, measured by methods such as ASTM D5155. Polymeric MDI is typically about 31 %, TDI about 48 %, prepolymers anywhere from a few per cent to about 30 %.
• Isocyanate index: the chosen excess of isocyanate over stoichiometry, set by the formulator for the application.

03The Calculation, Step By Step

The method is the same for foams, elastomers, coatings and binders. Only the numbers change.

1. Equivalent Weight Of The Polyol Blend

Equivalent weight is the mass that carries one mole of reactive groups. For a polyol it is derived from the OH value, because 56,100 is the mass of potassium hydroxide in milligrams per mole:

EW polyol = 56,100 / OH value

2. Equivalents In 100 Parts Of Blend, Plus Water

Divide the parts of each reactive ingredient by its equivalent weight and add them up. Water has a molar mass of 18 and reacts with two isocyanate groups, so its equivalent weight is 9.

Equivalents = 100 / EW polyol + water parts / 9

3. Equivalent Weight Of The Isocyanate

For the isocyanate the same idea uses the NCO content. 42 is the mass of one NCO group, so:

EW isocyanate = 4,200 / %NCO

4. Stoichiometric Isocyanate And Index

Multiply the total equivalents by the isocyanate equivalent weight to get the stoichiometric amount, then by the index divided by 100:

Isocyanate parts = equivalents x EW isocyanate x index / 100

04Worked Example: A Rigid Foam System

The table follows the four steps for a rigid foam blend. The values are illustrative round numbers chosen to show the method, not the data of any commercial system.

So 101.5 kg of fully formulated blend (100 kg blend plus 1.5 kg water) needs about 131.1 kg of polymeric MDI. Expressed per 100 parts of the complete blend, the weight ratio is 100 : 129.2.

From Weight Ratio To Volume Ratio

Spray rigs and many low-pressure machines meter by volume, so the weight ratio has to be converted with the densities of the two liquids. With typical densities of 1.10 g/cm3 for the blend and 1.23 g/cm3 for polymeric MDI:

• Blend: 101.5 / 1.10 = 92.3 volume units.
• MDI: 131.1 / 1.23 = 106.6 volume units.
• Volume ratio: 100 : 115.5 (blend : MDI).

Density changes with temperature too, which is one more reason to measure it at the processing temperature rather than rely on a value at 25 °C.

05Choosing The Index

The index is a design decision, not a calculation result. It balances hardness, dimensional stability, fire behaviour and cost.

• Flexible foam: typically about 100-115; higher values give firmer foam.
• Rigid polyurethane foam: typically slightly above 100, often 105-130.
• PIR insulation: often 250 or more, so that excess isocyanate trimerises into heat-stable rings.
• Elastomers, coatings and binders: usually close to 100-105, where an excess quickly makes the material brittle.

Speed and index interact. A formulator who raises the index without adjusting the catalysts changes the reactivity profile as well; the discussion of balancing reaction speed and performance shows why a faster system is not automatically a better one on the line.

06When The Real Ratio Drifts From The Set Ratio

A correct calculation does not guarantee a correct ratio in the mix head. Four causes account for most deviations.

1. Temperature: a colder component is more viscous; a gear or piston pump then delivers less of it at the same setting. Keep components and lines within the range on the data sheet and record their temperatures at shift start.
2. Batch variation: an isocyanate at 30.5 % instead of 31 % NCO, or a blend at 410 instead of 400 mg KOH/g, shifts the real index even if the machine is unchanged. Recalculate with certificate values when a new lot arrives.
3. Moisture pick-up: water absorbed by the polyol consumes isocyanate and adds unplanned blowing; isocyanate exposed to humid air loses NCO groups.
4. Wear and settings: worn pumps, partially blocked filters and changed pressures alter delivery over time.

The effect is easy to underestimate. If the isocyanate pump delivers 5 % less than set on the example system, the real index falls from 110 to about 104.5. In a rigid foam that can be the difference between a stable panel and edge shrinkage.

07Checking The Ratio On The Machine

A ratio check takes a few minutes and should be part of every start-up and every change of batch.

• Shot weight check: dispense each component separately into tared containers for the same time, weigh both and compare the ratio with the calculated value. Most machines have a calibration mode for this.
• Cup test: mix a defined amount by hand or from the machine and record cream, gel and tack-free times and free-rise density; compare with the approved reference.
• Visual and tactile signs: soft or tacky spots point to too little isocyanate, a brittle or dusty surface to too much.
• Records: keep component temperatures, batch numbers and shot weights together, so a later complaint can be traced to its cause.

Buying components that are developed together removes one source of error: ready-matched polyol and MDI systems are formulated so that the stated ratio fits the reactivity of the specific isocyanate, rather than leaving the processor to reconcile two suppliers' data.

08Estimating Consumption: Kilograms And Drums

Once the ratio is known, the quantity for a job follows from simple geometry. The steps are the same for a poured, sprayed or laminated rigid foam layer.

1. Volume: area x thickness, in cubic metres.
2. Mass of foam: volume x target core density in kg/m3.
3. Allowance: add a percentage for overspray, trimming, purging and start-up waste, often 5-15 % depending on the process.
4. Split: divide the total by the weight ratio to get the kilograms of each component.
5. Drums: divide by the drum size and round up.

Drum sizes vary by supplier and product, so check the actual net weight before ordering. Rounding up to full drums also means planning what happens to the remainder; an opened isocyanate drum needs a dry nitrogen blanket if it is to wait, and the remaining shelf life should be noted on the drum.

Consumption is also where cost comparisons go wrong: a system that needs higher density to reach the same performance uses more kilograms per square metre, whatever its price per kilogram.

09Conclusion

The mix ratio on a data sheet is the visible result of four values: OH value, water, NCO content and index. Recalculating it with the certificate values of each batch, converting it correctly to a volume ratio and checking shot weights at start-up prevents most of the soft, brittle or shrinking parts that are blamed on the formulation. The same arithmetic, extended by area, thickness and density, gives a reliable order quantity. For a job-specific estimate of material and drums, Pluskim's consumption calculator is a practical next step.

10Sources And Further Reading

1. ASTM D4274, standard test methods for hydroxyl numbers of polyols: store.astm.org/d4274-21.html
2. ASTM D5155, standard test methods for isocyanate content: store.astm.org/d5155-19.html
3. ISOPA, product information on diisocyanates and polyols: isopa.org/product-information

11Frequently Asked Questions