Category: Formulation Guides

Practical formulation guides for polyurethane foam systems, material selection, processing balance, performance tuning, and production optimization.

TDS %NCO vs CoA %NCO: Use the Drum Value in PU Foam

Introduction

Every drum of isocyanate that arrives at a foam plant can have a different %NCO value.

Most plants still use the same number for all of them.

That number usually comes from the Technical Data Sheet. It is entered into the formulation spreadsheet, used in the equivalent weight calculation, and treated as if it is a fixed property of the isocyanate grade.

But the TDS value is not the actual value inside the drum.

The Technical Data Sheet gives a specification range or typical value. It tells you what the supplier considers acceptable for that grade. The Certificate of Analysis gives the actual %NCO value for the specific batch or drum delivered to your plant.

That difference matters.

The %NCO value controls isocyanate equivalent weight. Equivalent weight controls NCO equivalents. NCO equivalents control the isocyanate index. And the index affects foam hardness, compression set, resilience, cure behaviour, and batch consistency.

If your formula uses the TDS midpoint while the drum has a different CoA value, the formula may not be running at the index shown on the sheet.

This article explains why the CoA %NCO value belongs in your formulation calculation, why the TDS value is not enough, and how drum-to-drum variation creates real PU foam quality drift.

What Is the Difference Between TDS %NCO and CoA %NCO?

The Technical Data Sheet gives a general specification for the isocyanate grade. It may show typical %NCO value, acceptable %NCO range, viscosity range, appearance, storage guidance, and general product properties.

The TDS is useful for understanding the product grade. But it does not tell you the exact %NCO value of the drum sitting in your plant.

The Certificate of Analysis gives the batch-specific or drum-specific value. That is the number measured for the actual material delivered.

For formulation calculation, the difference is simple:

Document	What It Gives	How It Should Be Used
TDS	Specification range or typical value	Product reference only
CoA	Actual batch or drum value	Formulation calculation input

The isocyanate index calculation needs one specific %NCO value. A range is not enough. A midpoint is only an assumption. The CoA value is the correct production input.

Technical Data Sheet range compared with Certificate of Analysis actual NCO value

Why the TDS %NCO Value Is Not a Formulation Input

The TDS %NCO range is a commercial specification window.

It defines the range within which the supplier considers the product acceptable. It does not define the exact value that should be used in your foam formula.

For example, a TDI grade may have a %NCO specification range. A drum at the high end and a drum at the low end can both be accepted. Both can be within specification. Both can be shipped correctly.

But they will not have the same equivalent weight. They will not deliver the same NCO equivalents per part. And if the same isocyanate quantity is used for both drums, they will not produce the same actual index.

That is why the TDS midpoint should not be locked into the formulation spreadsheet as a permanent constant.

The TDS helps identify the product. The CoA controls the calculation.

How %NCO Controls Isocyanate Equivalent Weight

The isocyanate equivalent weight formula is:

Isocyanate EW = 4,200 ÷ %NCO

The constant 4,200 comes from the molecular weight of the NCO group (42 g/mol) multiplied by 100. The only variable is %NCO.

If %NCO changes, equivalent weight changes. If equivalent weight changes, the same TDI or MDI parts deliver different NCO equivalents.

%NCO Value	Isocyanate EW (g/eq)
49.8	84.34
48.3	86.96
46.8	89.74

Higher %NCO gives lower equivalent weight. Lower %NCO gives higher equivalent weight.

This is the reason drum-to-drum %NCO variation becomes index variation.

Percent NCO controlling isocyanate equivalent weight in polyurethane foam formulation

The Index Swing Caused by Using One TDI Value for Every Drum

Now look at what happens when the formula uses the same TDI quantity for drums with different actual %NCO values.

Example TDI range:

%NCO Value	EW (g/eq)	TDI Quantity	Actual Index Running
49.8 (high-end CoA)	84.34	50.22 parts	108.3
48.3 (midpoint / design)	86.96	50.22 parts	105.0
46.8 (low-end CoA)	89.74	50.22 parts	101.8

Same TDI quantity. Same foam formula. Same machine settings. Different actual index.

From 101.8 to 108.3, the formula can experience about a 6.5 index point total swing only because the actual %NCO value changed.

That is enough to affect foam properties.

At the high %NCO end, the same TDI parts deliver more NCO equivalents. The foam may run firmer than expected.
At the low %NCO end, the same TDI parts deliver fewer NCO equivalents. The foam may run softer than expected.

If the plant is using only the TDS midpoint, this variation can be misdiagnosed as a process issue. But the cause is inside the raw material data.

Drum-to-drum NCO variation causing isocyanate index swing in polyurethane foam

How This Shows Up in Foam Quality

A wrong %NCO assumption can appear as ordinary foam quality variation.

If the actual %NCO is higher than the formula assumption, the real index can rise. Possible symptoms include:

Higher hardness
Firmer hand feel
Higher ILD
Tighter compression set
Reduced softness
Possible brittleness if the shift is large

If the actual %NCO is lower than the formula assumption, the real index can drop. Possible symptoms include:

Softer foam
Lower ILD
Weaker recovery
Compression set risk
Lower network development
Customer complaints about feel or durability

The foam plant may look for the problem in catalyst, silicone, temperature, or machine calibration. Those checks are useful, but they do not answer the first question:

Was the actual CoA %NCO value used in the formula?

If the answer is no, troubleshooting is starting from an uncertain index baseline.

High and low NCO content effects on polyurethane foam hardness and compression set

Why CoA Logging Builds Better Formulation Control

Using the CoA value for one drum is good. Logging CoA values over time is better.

Every isocyanate delivery should be recorded with:

Supplier name
Product grade
Drum or batch number
Date received
CoA %NCO
Calculated equivalent weight
Formula or production run used
Any foam quality observation

After 20 to 30 drums, patterns become visible. Some suppliers may deliver very tight values close to the design target. Others may move across a wider part of the specification range. This supplier profile helps the foam plant understand real delivery behaviour, not just published specification limits.

A supplier profile can answer questions like:

Is this supplier consistently high or low in %NCO?
Does the value drift by production batch?
Are quality issues linked to certain %NCO ranges?
Does a supplier switch require formula adjustment?
Is the formula using a realistic design value?

This turns raw material data into production knowledge.

Correct Workflow: How to Use Drum CoA %NCO

Correct %NCO handling is a simple production workflow.

Read the CoA. Before the drum enters production, check the actual %NCO value on the Certificate of Analysis.
Record the value. Log supplier, grade, drum number, date, and %NCO value.
Calculate isocyanate EW. Use EW = 4,200 ÷ %NCO.
Compare with design EW. If the drum EW is close to the formula design value, no major change may be needed. If the difference is meaningful, review the index impact.
Recalculate the index. Use the actual EW value in the full index calculation.
Adjust isocyanate parts if required. If the index shift is significant, correct the TDI or MDI quantity before production.
Document the formula decision. Record whether the run used the original formula or a corrected value based on CoA %NCO.

This workflow prevents a TDS assumption from becoming a production quality issue.

Workflow for using drum CoA percent NCO in polyurethane foam formulation

Practical Decision Thresholds for %NCO Variation

Not every %NCO difference requires a formula adjustment. The decision depends on the effect on equivalent weight and index.

A practical guide:

%NCO Deviation from Design	Typical Index Shift	Action Required
Less than ±0.5%	Around 1 point	Record and monitor
±0.5% to ±1.0%	Around 1–2 points	Recalculate index and review adjustment
More than ±1.0%	3+ points	Adjust isocyanate quantity before run

These are practical production thresholds, not universal laws. High-specification products may require tighter control.

The key point is that the decision should be based on calculation, not assumption.

Use the PolymersIQ NCO / TDI Index Calculator

The PolymersIQ NCO / TDI Index Calculator helps you calculate the correct isocyanate quantity using the actual CoA %NCO value.

Use it when a new TDI or MDI drum arrives, the CoA %NCO differs from the design value, you switch isocyanate supplier, foam hardness shifts without a clear process reason, compression set changes after a new isocyanate batch, or you want to confirm the real index before production.

Open the NCO / TDI Index Calculator →

For the foundation explanation of %NCO, read NCO Content in Isocyanate: What %NCO Means in PU Foam Formulation.

For common NCO handling mistakes, read 4 NCO Content Mistakes That Corrupt PU Foam Index Calculations.

For the complete equivalent weight guide, read Equivalent Weight in Polyurethane Foam: Complete Calculation Guide.

For the full isocyanate index calculation method, read Isocyanate Index Calculation Guide for PU Foam Engineers.

FAQs

What is the difference between TDS %NCO and CoA %NCO?

The TDS (Technical Data Sheet) gives a general specification range or typical value for the isocyanate grade — it tells you what the supplier considers acceptable for that product. The CoA (Certificate of Analysis) gives the actual measured %NCO value for the specific batch or drum delivered to your plant. The TDS is for product reference; the CoA is for formulation calculation.

Why shouldn’t I just use the TDS midpoint in my formula?

The TDS midpoint is an assumption, not a measurement. A drum at the high end and a drum at the low end of the TDS range can both be inside specification, but they will have different equivalent weights and deliver different NCO equivalents per part. Using a fixed midpoint locks in an error every time the actual drum value differs from that midpoint.

How much can the actual index swing due to drum-to-drum %NCO variation?

For a typical TDI grade with a normal specification range, using one fixed TDI quantity across drums at the low and high ends of %NCO can produce about a 6.5-point total index swing. For example, the same 50.22 parts of TDI can deliver an actual index of 101.8 with a low-%NCO drum and 108.3 with a high-%NCO drum. That’s a meaningful difference in foam properties.

Can a drum within TDS specification still cause foam quality problems?

Yes. Being within specification means the supplier delivered acceptable material, but it does not mean the material matches your formulation baseline. If your formula was designed around one %NCO value and the delivered drum has a different value (still within range), the same isocyanate quantity will produce a different running index. Foam properties can drift even though the raw material is technically compliant.

How do I calculate the index impact when %NCO changes?

Use EW = 4,200 ÷ %NCO to calculate the new equivalent weight, then plug it into the full index calculation. The change in EW alters how many NCO equivalents the same TDI or MDI parts deliver, which moves the actual running index. If the index shift is significant, the isocyanate quantity should be adjusted before production.

Should I always recalculate the index when a new drum arrives?

For meaningful changes in CoA %NCO, yes. Small variations within ±0.5% of the design value can typically be monitored without immediate adjustment. Variations of ±0.5% to ±1.0% should be reviewed for index impact. Variations greater than ±1.0% generally justify adjusting the isocyanate quantity before the production run.

What’s the benefit of logging CoA %NCO values over time?

A supplier profile reveals delivery patterns that are not visible from a single drum. Some suppliers consistently deliver near the high end of the specification range, others near the low end, others at the midpoint. After 20–30 drums, you can see whether your formula’s design value matches what your supplier actually delivers — or whether the design value should be updated to match real delivery behaviour.

Can supplier switches cause foam quality problems even with the same product grade?

Yes. The same generic product grade from different suppliers can have different actual %NCO ranges, different batch-to-batch variation patterns, and different real delivery values. Switching suppliers without verifying CoA %NCO and recalculating the index can introduce unexpected foam quality drift. A supplier change should always trigger a formulation review.

How does this rule apply to MDI and polymeric MDI?

The same rule applies — only the %NCO range is different. MDI typically has %NCO around 31.5, polymeric MDI may have different values, and modified isocyanates have their own ranges. The formula EW = 4,200 ÷ %NCO is universal, and the same logic about TDS vs CoA applies to all isocyanate types.

What’s the simplest QC change a foam plant can make to prevent this problem?

Add one step to incoming QC: when a new isocyanate drum arrives, record the actual CoA %NCO, calculate the equivalent weight, and compare it to the formula design value. If the difference is significant, recalculate the index before the drum enters production. This single discipline prevents most %NCO-related index drift.

Key Takeaways

The TDS %NCO value is not the same as the CoA %NCO value. The TDS gives a specification range or typical value. The CoA gives the actual value for the delivered drum or batch.

PU foam formulas should use the actual CoA %NCO value whenever available.

%NCO controls isocyanate equivalent weight through:

EW = 4,200 ÷ %NCO

If %NCO changes, equivalent weight changes. If equivalent weight changes, the same isocyanate parts deliver different NCO equivalents. That changes the actual running index.

Across a typical TDI range, using one fixed TDI quantity can create about a 6.5-point total index swing from low-end to high-end %NCO. This can appear as hardness drift, compression set variation, or inconsistent foam feel.

The solution is simple: read the CoA, record the actual %NCO, calculate EW, recalculate index, and adjust isocyanate quantity when required.

Conclusion

If your foam quality is varying and the process data does not explain it, the cause may be in the isocyanate CoA.

A formulation audit can identify whether the %NCO value in your formula matches what your supplier is actually delivering.

PolymersIQ can help review your CoA data, calculate the true equivalent weight, and correct the index baseline before more production runs compound the error.

To get accurate support, please share:

Isocyanate type, supplier, and grade
Recent CoA %NCO values (last 5–10 drums if available)
Design %NCO used in your original formulation
TDI or MDI quantity in current formula
Target index and observed foam properties (ILD, compression set)
Description of the production issue you are facing

Contact PolymerIQ for an isocyanate formulation audit →

May 6, 2026

NCO Content in Isocyanate: What %NCO Means in PU Foam
Introduction

NCO content is one of the most important raw material values in polyurethane foam formulation.

It tells you how much reactive isocyanate functionality is available in a given isocyanate material. That value directly affects equivalent weight, isocyanate demand, index calculation, and final foam properties.

Most foam plants understand that TDI or MDI reacts with polyol, water, crosslinkers, and chain extenders. But many plants treat the %NCO value as if it is fixed for a grade.

It is not fixed.

Every drum or batch can have a specific %NCO value. That value is normally reported on the Certificate of Analysis. If the formulation uses a general Technical Data Sheet value instead of the actual drum value, the index calculation may not reflect what is really being fed to the mixing head.

A small %NCO difference can change the isocyanate equivalent weight. Once equivalent weight changes, the same isocyanate parts no longer deliver exactly the same NCO equivalents.

This article explains what NCO content means, how to calculate isocyanate equivalent weight, why %NCO varies, and why this value must be treated as a live formulation input.

What Is NCO Content?

NCO content is the mass percentage of reactive isocyanate groups present in an isocyanate material. It is usually written as %NCO.

In practical terms, %NCO tells you how much of the isocyanate material is chemically available to react with active hydrogen components in the foam formula. Those reactive components may include:
- Polyol hydroxyl groups
- Water
- Crosslinkers
- Chain extenders
- Amine-functional additives
- Other active hydrogen sources
A higher %NCO means more reactive NCO groups per gram of material. A lower %NCO means fewer reactive NCO groups per gram of material.

This matters because polyurethane formulation is not only about how many parts of TDI or MDI are added. It is about how many reactive NCO equivalents are delivered to the system.

Two isocyanate batches can have the same product name and still carry slightly different %NCO values. If the formula does not reflect that difference, the foam may not run at the intended index.

Why %NCO Matters in PU Foam Formulation

The isocyanate index depends on the relationship between NCO equivalents and reactive hydrogen equivalents.

If %NCO changes, the equivalent weight of the isocyanate changes. If equivalent weight changes, the number of NCO equivalents delivered by the same isocyanate parts changes.

This can affect:
- Actual isocyanate index
- Foam hardness
- Compression set
- Resilience
- Cure behaviour
- Crosslink density
- Batch-to-batch consistency
- Foam feel and performance
For example, if the %NCO is higher than the value used in the formula, the same weight of isocyanate delivers more NCO equivalents than expected. If the %NCO is lower than the value used in the formula, the same weight of isocyanate delivers fewer NCO equivalents than expected.

This is why %NCO is not only a supplier data point — it is a formulation control value.

Isocyanate Equivalent Weight Formula

Isocyanate equivalent weight is calculated from %NCO. The formula is:

Isocyanate Equivalent Weight = 4,200 ÷ %NCO

Where:
- Equivalent weight is expressed in g/eq
- %NCO is the actual NCO content of the isocyanate
- 4,200 is the molecular weight of the NCO group (42 g/mol) multiplied by 100
The constant 4,200 does not change. The variable is %NCO.

This formula applies to TDI, MDI, polymeric MDI, and modified isocyanates, as long as the actual %NCO value is known.

Worked Examples: TDI and MDI Equivalent Weight

Example 1: TDI 80/20

If a TDI drum has %NCO = 48.3:

EW = 4,200 ÷ 48.3 = 86.96 g/eq

So the isocyanate equivalent weight is approximately 87 g/eq.

Example 2: MDI

If an MDI material has %NCO = 31.5:

EW = 4,200 ÷ 31.5 = 133.33 g/eq

So the isocyanate equivalent weight is approximately 133 g/eq.

The calculation method is the same. Only the %NCO value changes.

This is why the actual %NCO value from the drum or batch is important. The formula should not assume that every drum has exactly the same reactive content.

How %NCO Changes Isocyanate Equivalent Weight

The relationship between %NCO and equivalent weight is inverse:
- If %NCO increases, equivalent weight decreases.
- If %NCO decreases, equivalent weight increases.
That means higher %NCO material delivers more reactive NCO per gram. Lower %NCO material delivers less reactive NCO per gram.

%NCO Value Isocyanate EW (g/eq)
49.8 84.34
48.3 86.96
46.8 89.74

These numbers show why %NCO variation matters. The isocyanate material may still be inside supplier specification, but the equivalent weight is not identical across the range.

If the same isocyanate parts are used for every drum, the actual NCO equivalents delivered to the formula can shift. That shift can move the real running index away from the target.

Why Every Drum Can Have a Different %NCO Value

%NCO can vary from drum to drum even when the product grade is the same. This does not automatically mean the material is defective. It usually means the material is inside the supplier’s allowed specification range, but the exact reactive content is not identical.

Common reasons include:

1. Manufacturing batch variation

Isocyanate production depends on feedstock quality, reactor conditions, process control, and final product handling. Even well-controlled production can produce small %NCO variation within specification.

2. Moisture exposure

NCO groups react with water. If isocyanate is exposed to atmospheric moisture, some reactive NCO groups may be consumed before the material reaches the mixing head. This can lower active %NCO.

Moisture exposure can occur through poor drum sealing, damaged bungs, humid storage conditions, repeated opening and closing, and improper handling during transfer.

3. Storage temperature and aging

Storage conditions can affect reactive isocyanate quality over time. Elevated temperature and long storage periods can contribute to chemical changes that reduce active NCO availability. The degree of change depends on material type, storage conditions, handling history, and supplier guidance.

The practical point is simple: the %NCO value should be checked as a drum-specific or batch-specific value, not treated as a permanent constant.

Why the Certificate of Analysis Matters

The Technical Data Sheet usually gives a specification range or typical value. The Certificate of Analysis gives the actual value for a specific batch or drum.

For formulation control, the CoA value is the more important number. The formula calculation needs one actual value, not a broad specification range.

If the formulation uses a general TDS value but the drum’s CoA value is different, the equivalent weight calculation may be wrong. That can shift the real running index.

The proper production habit is:
1. Read the drum or batch CoA.
2. Record the actual %NCO value.
3. Calculate isocyanate EW using 4,200 ÷ %NCO.
4. Recalculate the isocyanate index if the EW differs from the design value.
5. Adjust isocyanate quantity if the index shift is significant.
This does not mean every tiny %NCO movement requires a major formula change. It means the plant should know the effect before production starts.

How %NCO Affects Foam Properties

%NCO does not affect foam properties directly by itself. It affects foam properties through the index calculation.

If the formula assumes the wrong %NCO value, the same isocyanate parts may deliver a different number of NCO equivalents than expected. That can shift the actual index.

A higher actual index can move the foam toward:
- Higher hardness
- Higher crosslink density
- Firmer feel
- Lower softness
- Possible brittleness if excessive
A lower actual index can move the foam toward:
- Softer hardness
- Lower ILD
- Weaker recovery
- Compression set risk
- Lower network development
This is why %NCO should be treated as part of foam property control. A small raw material value can become a visible foam quality issue.

Practical Rules for Using %NCO Correctly

Use these rules in production:
1. Do not treat %NCO as fixed. It can vary drum to drum or batch to batch.
2. Use CoA %NCO for calculation. The CoA value is the specific value for the delivered material.
3. Calculate isocyanate EW from the actual value. Use EW = 4,200 ÷ %NCO.
4. Recalculate index when %NCO changes meaningfully. The same isocyanate parts may not deliver the same index if EW changes.
5. Be careful after supplier changes. The same grade from a different supplier can have a different actual %NCO value.
6. Protect isocyanate from moisture. Moisture consumes NCO and can reduce active reactive content.
7. Check aged or suspect drums before production. If storage or sealing was poor, verify before using the material in critical foam.
Use the PolymerIQ NCO / TDI Index Calculator

The PolymeraIQ NCO / TDI Index Calculator helps you use the actual %NCO value in the index calculation.

Use it when a new TDI or MDI drum arrives, the CoA %NCO differs from the design value, you switch isocyanate supplier, foam hardness changes without a clear process reason, a drum has been stored for a long period, or you need to confirm required isocyanate parts for target index.

Open the NCO / TDI Index Calculator →

For the deeper article on TDS versus CoA values, read TDS %NCO vs CoA %NCO: Why Your PU Foam Formula Must Use the Drum Value.

For common NCO handling mistakes, read 4 NCO Content Mistakes That Corrupt PU Foam Index Calculations.

For the complete equivalent weight guide, read Equivalent Weight in Polyurethane Foam: Complete Calculation Guide.

For the full index calculation method, read Isocyanate Index Calculation Guide for PU Foam Engineers.

FAQs

What is NCO content in polyurethane foam formulation?

NCO content is the mass percentage of reactive isocyanate groups in an isocyanate material, written as %NCO. It tells you how much of the isocyanate is chemically available to react with polyol, water, crosslinkers, and chain extenders during foam formation. Higher %NCO means more reactive NCO groups per gram of material.

How is isocyanate equivalent weight calculated?

Use EW = 4,200 ÷ %NCO, where %NCO is the actual NCO content from the Certificate of Analysis. The constant 4,200 comes from the NCO group molecular weight (42 g/mol) multiplied by 100. This formula applies to TDI, MDI, polymeric MDI, and modified isocyanates.

What is the typical %NCO for TDI and MDI?

TDI 80/20 typically has %NCO around 48.3, giving an equivalent weight of about 87 g/eq. MDI typically has %NCO around 31.5, giving an equivalent weight of about 133 g/eq. Polymeric MDI and modified isocyanates have their own typical ranges. The exact value for any specific drum should always be taken from its Certificate of Analysis.

Why does %NCO vary between drums of the same product?

Three main reasons: manufacturing batch variation (small differences in feedstock, reactor conditions, and process control), moisture exposure during storage or handling (NCO reacts with water), and storage temperature and aging. Even drums with the same product name can have slightly different %NCO values, all within the supplier’s specification range.

Should I use %NCO from the TDS or the Certificate of Analysis?

Always use the actual %NCO from the Certificate of Analysis for the specific drum or batch in production. The TDS gives a specification range, which is a commercial conformance window, not a precise formulation input. Equivalent weight is calculated directly from %NCO, so using a wrong %NCO creates a wrong EW and a wrong isocyanate balance.

How does %NCO affect foam hardness?

%NCO affects hardness indirectly through the index calculation. If the actual %NCO is higher than the formula assumes, the same isocyanate parts deliver more NCO equivalents than expected, the actual running index rises, and foam can become harder. If %NCO is lower than assumed, the index drops and foam can become softer. The effect on foam properties always goes through the index.

Can moisture exposure really change %NCO?

Yes. NCO groups react with water — that’s the same blowing reaction used inside the foam. If isocyanate is exposed to atmospheric moisture through poor drum sealing, damaged bungs, humid storage, or repeated opening and closing, some NCO groups can be consumed before the material reaches production. The active %NCO reaching the mixing head is then lower than the original CoA value.

What happens if I keep using the same %NCO value when the drum changes?

The formula sheet still shows the design index, but the actual running index drifts every time the new drum’s %NCO differs from the assumed value. Over many drums, this can produce inconsistent foam properties, batch-to-batch hardness drift, compression set variation, and confusing troubleshooting. The fix is to recalculate isocyanate EW for each drum’s actual %NCO.

Should I recalculate the isocyanate index every time %NCO changes?

For meaningful changes — yes. A small %NCO variation may produce a small index shift that’s within normal production variation. But a larger %NCO change (for example, after switching suppliers, opening a drum from long storage, or receiving a batch at the edge of the specification range) can produce a meaningful index shift that justifies recalculating the isocyanate quantity before production.

Does the same rule apply to TDI, MDI, and polymeric MDI?

Yes. The formula EW = 4,200 ÷ %NCO applies to all standard isocyanates because the constant 4,200 is the NCO group’s molecular weight contribution, which doesn’t depend on the specific isocyanate type. Only the %NCO value differs between TDI, MDI, polymeric MDI, and modified grades.

Key Takeaways

NCO content is the mass percentage of reactive isocyanate groups in an isocyanate material, usually written as %NCO.
- Higher %NCO means more reactive NCO groups per gram.
- Lower %NCO means fewer reactive NCO groups per gram.
Isocyanate equivalent weight is calculated as:

EW = 4,200 ÷ %NCO

The %NCO value should be taken from the actual Certificate of Analysis when available, not treated as a fixed value from the TDS.

Every drum or batch can carry a slightly different %NCO value. That variation changes equivalent weight, which can change the actual running index. If the actual index changes, foam hardness, compression set, recovery, and consistency can also change.

Correct %NCO handling is a basic part of polyurethane foam formulation control.

Conclusion

If your foam properties are shifting from batch to batch and the process looks stable, the issue may be in the raw material data.

The isocyanate drum may not be delivering the same %NCO value your formula assumes.

PolymersIQ can help review your CoA data, calculate the correct isocyanate equivalent weight, and identify whether %NCO variation is shifting your index baseline.

To get accurate support, please share:
- Isocyanate type, supplier, and grade
- Recent CoA %NCO values (last 5–10 drums if available)
- Design %NCO used in your original formulation
- Polyol grade, OHV, water level, and any crosslinkers
- Target index and observed foam properties (ILD, compression set)
- Description of the production issue you are facing
Contact PolymerIQ for an isocyanate formulation audit →
May 5, 2026

How Water Level Effects PU Foam Properties

Introduction

Water level is one of the most powerful variables in flexible polyurethane foam formulation.

Most engineers understand its effect on density. Increase water — more carbon dioxide is generated, more gas expands the foam matrix, density usually decreases. Reduce water — less CO₂, less expansion, density usually increases.

That part is simple.

The problem is that water does not control only density.

Water also affects urea formation, hardness, compression set, resilience, exotherm, and isocyanate demand. A water adjustment made for one reason can quietly create a second problem somewhere else in the foam. This is why a density correction can later become a compression set complaint.

Water level changes four major properties at the same time:

Density
Hardness / ILD
Compression set
Exotherm

This article explains how each property responds to water level and why water should never be treated as a single-function density control variable.

Water Controls More Than Density

Water is often adjusted to control foam density. That is understandable, because water reacts with isocyanate to generate carbon dioxide. The CO₂ expands the foam and helps create the cellular structure.

But water also produces an amine intermediate, which reacts with another isocyanate group to form urea linkages. Those urea linkages become hard segments in the foam network.

So every water change has two chemical consequences:

It changes gas generation (affecting density).
It changes urea formation (affecting hardness, recovery, compression set, and heat generation).

This means water is not just a blowing-agent variable — it is also a structure-building variable.

A plant that watches only density after a water change is only watching half of the effect.

Water level in polyurethane foam changing CO2 generation and urea network formation

Property 1: Density

Density is the most visible property affected by water level.

When water reacts with isocyanate, carbon dioxide is released. This gas expands the foam mass and creates the cellular structure.

More water → more CO₂ → more expansion → lower density
Less water → less CO₂ → less expansion → higher density

In flexible slabstock foam, a water increase can noticeably reduce density. As a practical rule, each 0.5 part increase in water may produce a meaningful density reduction, often in the range of several percent depending on the full formulation and process conditions.

However, density response is not controlled by water alone. It also depends on:

Polyol type
Isocyanate index
Catalyst balance
Silicone surfactant
Cream time and rise profile
Cell opening
Block height
Production temperature
Machine mixing efficiency

Water can be used to adjust density, but it should not be treated as a simple linear dial. A density correction must also be checked against the other property changes caused by water.

Water level effect on polyurethane foam density through CO2 generation

Property 2: Hardness / ILD

Water also affects foam hardness. This is where troubleshooting often becomes confusing.

When water level increases, density usually decreases — and lower density often tends to reduce load-bearing. But water also increases urea formation, and urea hard segments can stiffen the polymer network and raise hardness or ILD.

So water can create two opposing effects:

More CO₂ → lower density (tends to reduce hardness)
More urea formation → stiffer network (tends to increase hardness)

Which effect dominates depends on the formulation. The final hardness response depends on:

Index
Polyol functionality
Water level
Crosslinker level
Catalyst balance
Foam density
Cell structure
Cure condition

This is why two formulas may respond differently to the same water adjustment. In one formula, increasing water may mainly reduce density and soften the foam. In another formula, the increased urea formation may partially offset the density effect and keep ILD higher than expected.

Hardness should be tested after every meaningful water adjustment. Do not assume density movement alone predicts hardness movement.

Water level effect on PU foam hardness and ILD through density and urea formation

Property 3: Compression Set

Compression set is one of the most important long-term performance properties affected by water level.

Compression set measures how well foam recovers after being held under compression for a defined time and condition. Water affects compression set because water contributes to urea hard-segment formation. Urea linkages help build the foam network, and a stronger network usually improves resistance to permanent deformation.

If water is reduced to fix a density issue, urea formation is also reduced. The foam may meet density target, and it may look acceptable during production, but the network may be weaker than intended.

That weakness may appear later as:

Higher permanent deformation
Poorer recovery
Mattress or cushion complaints
Field returns after sustained load
Compression set values above specification

This is why water reductions should be reviewed carefully. A water reduction can solve a density problem today and create a compression set problem later. The two problems may appear separated by weeks or months, but chemically they are connected.

Water level affecting compression set in polyurethane foam through urea network formation

Property 4: Exotherm

Water also affects exotherm.

The reaction sequence that forms urea linkages releases heat. As water level increases, the amount of urea formation increases, and the thermal load in the foam block can increase as well.

This becomes especially important in high-water flexible slabstock formulas. At higher water levels — particularly around and above 4.5 parts — the risk of excessive core temperature becomes more serious, depending on block size, density, formulation, and ventilation.

High exotherm can contribute to:

Core discoloration
Scorch risk
Cell structure irregularities
Reduced tensile strength in the block center
Internal property variation
Processing instability

Large slabstock blocks are especially sensitive because heat dissipates slowly from the core. The outside of the block may look normal while the center experiences a much higher thermal load.

This is why high-water formulas require thermal management, not only density calculation. Important factors include water level, block height, pour profile, catalyst package, foam density, ambient temperature, ventilation, cooling time, and raw material temperature.

Water level is therefore also a heat-management variable.

High water level causing exotherm and scorch risk in polyurethane foam slabstock core

Why One Water Adjustment Moves Four Properties

The four effects of water are connected because they come from the same chemistry.

Property	Main Reason
Density	CO₂ generation changes foam expansion
Hardness / ILD	Urea hard segments change network stiffness
Compression set	Urea network affects long-term recovery
Exotherm	Urea-forming reaction increases heat generation

This is why “just changing water” is never just changing water. A small adjustment may be necessary and correct, but it should be treated as a full formulation change.

When water is increased, check: Density reduction, index impact, hardness response, compression set, exotherm risk, and cell structure stability.

When water is reduced, check: Density increase, index impact, urea network reduction, compression set risk, recovery and resilience, and customer performance requirements.

Water can solve one production issue and create another if only one property is monitored.

One water adjustment changing four polyurethane foam properties at once

Practical Water Adjustment Checklist

Before changing water level in a PU foam formula, review the full formulation impact.

Checkpoint	Question
Density	What density change is expected?
Index	Has the isocyanate requirement been recalculated?
Water EW	Is water treated as EW = 9?
Hardness / ILD	Will the urea change affect hardness?
Compression set	Will the network still meet recovery requirements?
Exotherm	Is the water level high enough to create core heat risk?
Cell structure	Will the surfactant and catalyst package still support stable cells?
Production validation	Will the trial include hardness, density, compression set, and core inspection?

This checklist prevents a common production mistake: fixing the visible issue while creating a hidden performance problem.

Example: A Density Fix That Creates Compression Set Risk

A production team reduces water by 0.3 parts to correct a density issue.

The next run looks better. Density is closer to target. The adjustment is considered successful.

But the water reduction also reduces urea formation. If the formula is not rebalanced, the foam network may become weaker. The effect may not show up immediately during production.

Weeks later, compression set complaints appear.

The team may treat this as a new problem, but it is connected to the earlier water adjustment. This is why water changes should be documented, recalculated, and validated against more than density.

A water adjustment should be accepted only after checking density, ILD, compression set, index, cure behaviour, exotherm, and customer application requirement.

Use the PolymerIQ Calculators

The PolymerIQ Foam Density Estimator can help estimate the density impact of water level changes before they reach production. Use it when increasing or reducing water, comparing different water levels, reviewing low-density foam formulas, or checking whether a density correction may create other risks.

Open the Foam Density Estimator →

Because water consumes NCO, every water change affects the index. The PolymerIQ Isocyanate Index Calculator helps verify the corrected isocyanate requirement after a water adjustment. Use it when water level changes, TDI or MDI quantity needs recalculation, compression set changes after a water adjustment, foam hardness changes unexpectedly, or a formula has been adjusted without full recalculation.

Open the Isocyanate Index Calculator →

For the chemistry behind water’s dual role, read The Dual Role of Water in Polyurethane Foam: Blowing Agent and Urea Network Builder.

For the water equivalent weight calculation, read Why the Equivalent Weight of Water Is 9 in Polyurethane Foam.

For the full index calculation guide, read Isocyanate Index Calculation Guide for PU Foam Engineers.

FAQs

How does water level affect PU foam density?

Water reacts with isocyanate to generate CO₂. More water generates more CO₂, which increases foam expansion and lowers density. As a practical rule, each 0.5 part increase in water can produce a meaningful density reduction, but the exact response depends on polyol type, index, catalyst balance, silicone, block height, and process conditions.

Why does water affect foam hardness in two directions?

Water has two opposing effects. More water lowers density (which tends to reduce hardness), but more water also increases urea hard-segment formation (which tends to stiffen the network and raise hardness). Which effect dominates depends on the full formulation. This is why hardness should be tested after every water adjustment — density alone does not predict it.

How does water level affect compression set?

Water contributes to urea linkage formation, and urea hard segments help build the foam network. A stronger network resists permanent deformation better. If water is reduced to fix density, urea formation also drops, and the network may be weaker. Compression set problems can appear weeks or months later as a result.

Why does high water level increase exotherm risk?

The reaction sequence that forms urea linkages releases heat. More water means more urea formation, which means more heat generated during the rise and cure. In large slabstock blocks, this heat dissipates slowly from the core, and at high water levels (particularly around or above 4.5 parts) the core can reach temperatures that risk discoloration, scorch, or cell structure problems.

Can a water reduction cause compression set failure?

Yes. Reducing water reduces both CO₂ generation and urea formation. If the isocyanate level and overall formulation are not rebalanced, the foam network can be weaker than intended. The density may meet target, but compression set, recovery, and long-term performance may suffer. This is one of the most common hidden consequences of a “simple” density correction.

Should I recalculate the isocyanate index every time I change water?

Yes. Water is a reactive component. Every water change alters the total reactive hydrogen equivalents in the formula, which means the isocyanate demand changes. If the isocyanate quantity is not recalculated, the actual running index drifts away from the target — even if the formula sheet still shows the original number.

Why does the same water adjustment behave differently in different formulas?

Because water’s effects depend on the rest of the formulation. Polyol type, functionality, crosslinker level, catalyst balance, silicone, density, and cure conditions all influence how the foam responds to a water change. A formula that softens with more water may stiffen in another system where urea formation dominates.

What’s the maximum safe water level in flexible slabstock?

There is no universal limit — it depends on block size, density target, formulation, ventilation, and process conditions. Many flexible slabstock formulas operate up to around 4.0–4.5 parts water without major exotherm concerns. Above this level, thermal management becomes increasingly important. The combination of high water, large block size, and low density poses the highest scorch risk.

How do I troubleshoot foam that’s too soft after a water change?

First, check whether the actual running index is correct after the water adjustment — water consumes NCO, so an unadjusted isocyanate quantity creates an under-indexed system. Second, check whether the urea contribution change is large enough to affect network stiffness. Third, verify that catalyst, silicone, and crosslinker levels still match the new water level.

What should I check before increasing water to lower density?

Check density target, expected index after recalculation, hardness response, compression set requirements, exotherm and core heat risk, surfactant and catalyst compatibility at the new water level, and customer performance specifications. A water increase is rarely a single-property change — it should be approached as a full formulation review.

Key Takeaways

Water level affects much more than foam density. It controls four major foam properties at the same time:

Density — through CO₂ generation
Hardness / ILD — through urea hard-segment formation
Compression set — through urea network contribution
Exotherm — through heat from the urea-forming reaction

Water generates CO₂, which affects foam expansion and density. Water also creates urea linkages, which affect hardness, recovery, compression set, and heat generation.

A water adjustment made only for density can change foam performance in ways that appear later. Higher water can reduce density but increase urea formation and exotherm risk. Lower water can increase density but reduce urea network contribution and increase compression set risk if the formula is not rebalanced.

Every water adjustment should include index recalculation, density review, hardness testing, compression set validation, and exotherm awareness.

Conclusion

If your plant has used water adjustments to fix density but later faced hardness drift, compression set failure, or core heat problems, the issue may not be random.

Water moves several properties at once.

PolymersIQ can help review your water level, index balance, density target, compression set performance, and exotherm risk to identify where the formulation balance is off.

To get accurate support, please share:

Current and target foam density
Water level (recent and historical)
Polyol grade, OHV, and isocyanate %NCO
Target index and observed foam properties (ILD, compression set)
Block size and any core temperature observations
Description of the production issue and any adjustments already tried

Contact PolymerIQ for a water-level formulation audit →

May 3, 2026

Why Water Equivalent Weight Is 9 in Polyurethane Foam
Introduction

The equivalent weight of water in polyurethane foam is 9, not 18.

This is one of the most important rules in PU foam formulation — and one of the most damaging mistakes when entered incorrectly.

Water has a molecular weight of 18 g/mol. Because of that, many engineers assume the equivalent weight of water is also 18. That assumption is wrong in polyurethane chemistry.

In PU foam, one water molecule ultimately consumes two NCO groups through the blowing reaction sequence. That is why the equivalent weight is calculated as:

Water EW = 18 ÷ 2 = 9 g/eq

If a formula spreadsheet uses 18 instead of 9, the water contribution is cut in half. The total reactive hydrogen equivalents become wrong. The calculated isocyanate demand becomes wrong. The index shown on the formula sheet no longer matches the chemistry in the reactor.

This article explains why water EW is 9, how the water-isocyanate reaction works, what happens when 18 is used by mistake, and how this error appears in foam production.

Why Water Equivalent Weight Is Not 18

Water has a molecular weight of 18 g/mol. But equivalent weight is not always the same as molecular weight.

Equivalent weight means the mass of material that contains one equivalent of reactive functionality.

In polyurethane foam, water has two reactive hydrogens involved in the isocyanate reaction sequence. That means one mole of water provides two equivalents of reactivity toward NCO.

So the calculation is:

Water EW = Molecular Weight ÷ Reactive Hydrogen Count = 18 ÷ 2 = 9 g/eq

For polyurethane foam index calculation, the correct value is Water EW = 9, not 18.

Using 18 treats water as if it had only one reactive hydrogen. That cuts the water contribution in half and corrupts the index calculation.

Water molecular weight is 18, but its equivalent weight in PU foam is 9 because it provides two reactive equivalents.

How Water Reacts with Isocyanate in PU Foam

Water reacts with isocyanate in two main stages.

Stage 1: Water reacts with NCO. Water reacts with an isocyanate group to form an unstable carbamic acid intermediate. This intermediate quickly decomposes, releasing carbon dioxide (which blows the foam and forms cells) and producing a primary amine.

Stage 2: The amine reacts with another NCO group. The amine formed in Stage 1 is reactive. It reacts with a second isocyanate group to form a urea linkage.

This means one water molecule ultimately consumes two NCO groups:
- One NCO in the initial water reaction
- One NCO in the amine-to-urea reaction
This is the chemical reason water equivalent weight is 9. It is not an approximation or a rule of thumb — it comes directly from the reaction mechanism.

One water molecule reacts through a sequence that ultimately consumes two NCO groups.

The Correct Water EW Calculation

The calculation is simple:
- Water molecular weight = 18 g/mol
- Reactive hydrogens = 2
- Water EW = 18 ÷ 2 = 9 g/eq
This means 9 grams of water contain one equivalent of reactive hydrogen functionality for the PU foam index calculation.

When calculating water equivalents in a formulation:

Water Equivalents = Water Parts ÷ 9

For example, if a flexible foam formula contains 4.0 parts water:

4.0 ÷ 9 = 0.44444 equivalents

If the formula uses EW = 18 instead:

4.0 ÷ 18 = 0.22222 equivalents

That is exactly half the correct value. The formula spreadsheet now believes there is much less reactive hydrogen demand than the chemistry actually has.

Using EW = 18 cuts the calculated water equivalents in half compared with the correct EW = 9.

Worked Example: How EW Water = 18 Corrupts the Index

Let’s see how this mistake changes the full formula calculation.

Example flexible slabstock formula:

Component Parts Correct EW Correct Equiv. Wrong EW Wrong Equiv.
Polyol 100 1,100 0.09091 1,100 0.09091
Water 4.0 9 0.44444 18 0.22222
DEOA 0.5 31 0.01613 31 0.01613
Total H equiv. 0.55148 0.32926

The correct total reactive hydrogen equivalents are 0.55148. Using water EW = 18 gives 0.32926.

Now assume the engineer targets Index 105 using the wrong equivalent system. NCO equivalents calculated from the wrong system:

0.32926 × 1.05 = 0.34572

But the actual correct reactive hydrogen equivalents are 0.55148. So the real running index is:

0.34572 ÷ 0.55148 × 100 = 62.7

The formula sheet says Index 105. The chemistry is running at approximately Index 62.7.

This is not a small error. It is a completely wrong stoichiometric foundation.

Using water EW = 18 can make the formula sheet show Index 105 while the actual chemistry runs much lower.

What This Error Looks Like in Production

A water equivalent weight error does not always create a dramatic visual failure. The foam may still rise. The block may still form. Operators may not immediately see the problem at the machine.

But the properties can be seriously wrong.

If water is entered as 18 instead of 9 and the isocyanate quantity is calculated from that wrong value, the foam can be severely under-indexed.

Common symptoms include:
- Softer foam than expected
- ILD below target
- Poor compression set
- Weak recovery
- Slower or weaker cure
- Tacky feel during early cure
- Poor aging performance
- Customer complaints after use
- Confusing response to catalyst adjustments
This kind of problem can be difficult to diagnose because it looks like a process issue. The team may adjust catalyst, silicone, cure temperature, water level, or crosslinker dosage. Some changes may improve one symptom temporarily. But the root cause remains inside the calculation.

The spreadsheet must be checked.

Wrong water EW can appear as soft foam, poor compression set, weak recovery, and confusing process variation.

Why This Mistake Stays Hidden

The water EW mistake stays hidden because the formula sheet often looks internally consistent.

The numbers may be formatted correctly. The index cell may show the target value. The spreadsheet may have been used for years.

But the spreadsheet is only as accurate as the assumptions inside it. If water EW is entered as 18, every downstream calculation built on that value becomes wrong.

This mistake is especially common in legacy formulas because:
- Water molecular weight is commonly remembered as 18
- Engineers may copy old spreadsheets without checking the chemistry
- The formula may have been empirically adjusted over time
- Production teams may trust a formula because it has been used for years
- Troubleshooting often focuses on machine and process variables first
- The equivalent weight layer is rarely audited
This is why a formula can carry the same error for months or years. The plant may keep adding practical corrections on top of a wrong calculation foundation. That creates a formula that works only by accident — and becomes difficult to transfer, scale, or troubleshoot.

How to Check Your Formula Today

Checking for this mistake is simple.

Open your formula sheet and find the equivalent weight value used for water. It should be 9, not 18.

Then check how the water equivalents are calculated:
- Correct: Water equivalents = Water parts ÷ 9 (e.g., 4.0 ÷ 9 = 0.44444)
- Wrong: Water equivalents = Water parts ÷ 18 (e.g., 4.0 ÷ 18 = 0.22222)
After correcting the water equivalent weight, the full index must be recalculated. Do not change only the water EW cell and assume the formula is now production-ready. The isocyanate quantity may also need to be recalculated based on the correct total reactive hydrogen equivalents and target index.

A safe review should include:
1. Confirm water EW = 9
2. Confirm polyol EW from actual OHV
3. Confirm isocyanate EW from actual %NCO
4. Confirm all crosslinkers and chain extenders are included
5. Recalculate total reactive hydrogen equivalents
6. Recalculate required NCO equivalents
7. Recalculate TDI or MDI parts
8. Compare the corrected formula against current production results
The first check is simple: water equivalent weight must be 9 in PU foam index calculations.

Use the PolymerIQ Isocyanate Index Calculator

Manual calculation is important because engineers should understand why water EW is 9. But in production, the calculation must also be checked quickly and consistently.

The PolymersIQ Isocyanate Index Calculator can help verify whether your formula is using the correct equivalent weights and delivering the intended index.

Use it to check water equivalent weight, total reactive hydrogen equivalents, required NCO equivalents, TDI or MDI parts, actual running index, and the effect of correcting EW errors.

Open the Isocyanate Index Calculator →

For the complete equivalent weight calculation guide, read Equivalent Weight in Polyurethane Foam: Complete Calculation Guide.

For common production spreadsheet mistakes, read 5 Equivalent Weight Mistakes That Damage PU Foam Production.

For the full isocyanate index calculation method, read Isocyanate Index Calculation Guide for PU Foam Engineers.

FAQs

Why is the equivalent weight of water 9 and not 18?

Water has a molecular weight of 18 g/mol, but each water molecule has two reactive hydrogens and consumes two NCO groups during the blowing reaction. So the equivalent weight is 18 ÷ 2 = 9 g/eq. Equivalent weight measures the mass per reactive equivalent — not the mass per molecule — so the divisor matters.

How does water actually consume two NCO groups?

The reaction happens in two stages. First, water reacts with one NCO group to form an unstable carbamic acid that releases CO₂ (the blowing gas) and forms a primary amine. Second, the amine reacts with another NCO group to form a urea linkage. The result: one water molecule consumes two NCO groups.

What happens if I use water EW = 18 by mistake?

Using 18 cuts the calculated water equivalents in half. The total reactive hydrogen equivalents become wrong, the calculated isocyanate demand becomes wrong, and the actual running index can be much lower than the formula sheet shows. The foam may still rise but will likely be under-indexed.

What does under-indexed foam look like in production?

Common symptoms include softer foam than expected, ILD below target, poor compression set, weak recovery, slower or weaker cure, tacky feel during early cure, and poor aging performance. The foam may rise normally, which is why this error often stays hidden for a long time.

Can this error explain unexplained compression set failures?

Yes. If water EW is wrong and the foam is under-indexed, crosslink density is lower than designed, which directly affects compression set, recovery, and aging stability. Compression set problems that don’t respond to catalyst or silicone changes should trigger an EW audit.

How do I check if my formula has this mistake?

Open your formula sheet and find the equivalent weight value used for water. If it shows 18 instead of 9, the calculation is wrong. Also check the equivalents formula: it should be water parts ÷ 9, not water parts ÷ 18.

Should I just change the water EW cell from 18 to 9?

No — that alone is not enough. After correcting water EW, the entire index must be recalculated, and the isocyanate quantity may need to change as well. Changing only the water EW cell without recalculating the rest of the formula may create new imbalances.

Why has this mistake stayed in some formula sheets for years?

Because the formula sheet looks internally consistent. The index cell shows the target value, the math is formatted correctly, and the spreadsheet has been used for a long time. Troubleshooting usually focuses on machines and process variables, and the equivalent weight layer is rarely audited.

Does this rule apply to flexible foam, rigid foam, and elastomers?

Yes. Water has the same chemistry — two reactive hydrogens, two NCO groups consumed — regardless of the polyurethane system. Water EW = 9 applies to flexible slabstock, HR foam, rigid foam, elastomers, and any PU system that uses water as a blowing agent or reactive component.

Does the water purity or temperature change the equivalent weight?

No. The equivalent weight comes from the reaction stoichiometry, not from physical conditions. As long as the water is participating in the standard PU blowing reaction, EW = 9 is the correct value to use.

Key Takeaways

The equivalent weight of water in polyurethane foam is 9, not 18.

Water has a molecular weight of 18, but it has two reactive hydrogens involved in the isocyanate reaction sequence:

Water EW = 18 ÷ 2 = 9 g/eq

Using 18 instead of 9 cuts the calculated water equivalents in half. This can severely corrupt the isocyanate index calculation and cause the actual running index to be much lower than the formula sheet suggests.

The foam may still rise and look normal, but it can show soft hardness, poor compression set, weak recovery, and confusing production behaviour.

Every PU foam formulation spreadsheet should be checked to confirm that water equivalent weight is entered as 9. A single wrong number can silently damage the entire stoichiometric calculation.

Conclusion

If your foam is consistently soft, failing compression set, or responding unpredictably to catalyst and process adjustments, the problem may not be the machine.

It may be the equivalent weight foundation inside the formula sheet.

PolymersIQ can help audit your formulation, verify water EW, recalculate the true index, and identify whether a hidden stoichiometric error is affecting production.

To get accurate support, please share:
- A screenshot or copy of your current formula sheet (with EW values)
- Polyol OHV and isocyanate %NCO values currently in use
- Water level and any crosslinkers or chain extenders
- Target index and actual foam properties (ILD, compression set, density)
- Description of the production issue you are facing
Contact PolymerIQ for a stoichiometric formulation audit →
April 29, 2026

Component	Parts	Correct EW	Correct Equiv.	Wrong EW	Wrong Equiv.
Polyol	100	1,100	0.09091	1,100	0.09091
Water	4.0	9	0.44444	18	0.22222
DEOA	0.5	31	0.01613	31	0.01613
Total H equiv.			0.55148		0.32926

Equivalent Weight in PU Foam: Calculation Guide

Introduction

Equivalent weight is one of the most important calculation values in polyurethane foam formulation.

It is also one of the most common sources of hidden formulation errors.

A foam formula can look correct on paper. The index may appear correct. The raw material parts may look familiar. The production team may check catalysts, silicone, temperature, density, and machine settings. But if even one equivalent weight value is wrong, the entire stoichiometric balance can be wrong.

This is why equivalent weight matters.

Equivalent weight is the value that connects raw material data to polyurethane chemistry. It converts each reactive component into a common basis so the formulator can calculate isocyanate demand correctly.

Polyol, isocyanate, water, and crosslinkers all have different structures and different reactive groups. Equivalent weight allows all of them to be compared on the same chemical basis.

This guide explains what equivalent weight means, how it differs from molecular weight, and how to calculate equivalent weight for every major PU foam component.

What Is Equivalent Weight?

Equivalent weight answers one simple question:

How many grams of this material contain one equivalent of reactive groups?

In polyurethane formulation, equivalent weight is not just a theoretical value. It is the foundation of stoichiometric balance. It tells the formulator how much of a material is required to provide one mole-equivalent of reactive functionality.

For example:

Polyol provides hydroxyl groups.
Isocyanate provides NCO groups.
Water provides reactive hydrogens.
Crosslinkers provide hydroxyl, amine, or other active hydrogen groups.

Each of these materials has a different molecular weight and a different number of reactive groups. Equivalent weight normalizes them so they can be used in the same calculation system.

Without equivalent weight, the isocyanate index calculation has no reliable foundation.

Equivalent Weight vs Molecular Weight

A common mistake is confusing equivalent weight with molecular weight. They are not always the same.

Molecular weight is the mass of one mole of complete molecules.
Equivalent weight is the mass that contains one mole-equivalent of reactive groups.

For a monofunctional material, molecular weight and equivalent weight can be the same. But for materials with more than one reactive group, equivalent weight is lower than molecular weight.

The general relationship is:

Equivalent Weight = Molecular Weight ÷ Functionality

For example, a trifunctional polyol with molecular weight 3,000 g/mol has three reactive hydroxyl groups per molecule.

So:

EW = 3,000 ÷ 3 = 1,000 g/eq

This means 1,000 grams of that polyol contains one equivalent of hydroxyl reactivity.

The same principle explains why water has an equivalent weight of 9, not 18. Water has a molecular weight of 18, but it has two reactive hydrogens involved in the isocyanate reaction.

So:

EW water = 18 ÷ 2 = 9 g/eq

This distinction is critical. A formulation that uses molecular weight where equivalent weight is required can produce a completely wrong index calculation.

Diagram explaining equivalent weight versus molecular weight in polyurethane formulation — Molecular weight measures the whole molecule. Equivalent weight measures the mass per reactive group.

Why Equivalent Weight Matters in PU Foam Formulation

Polyurethane foam chemistry is based on the reaction between isocyanate groups and active hydrogen groups.

The key reaction balance is:

NCO groups from isocyanate
OH groups from polyol
Reactive hydrogens from water
Reactive groups from crosslinkers or chain extenders

The isocyanate index depends on these equivalent relationships.

If the equivalent weight of one component is wrong, the calculated number of reactive equivalents is wrong. If the reactive equivalents are wrong, the isocyanate requirement is wrong. If the isocyanate requirement is wrong, the actual foam properties can shift.

This can affect:

Foam hardness
Compression set
Resilience
Crosslink density
Cure behaviour
Aging stability
Batch-to-batch consistency

Equivalent weight errors are dangerous because the foam may still rise and look normal. The problem usually appears later in physical testing or customer use.

How to Calculate Polyol Equivalent Weight

For polyols, equivalent weight is calculated from hydroxyl value.

The formula is:

Polyol EW = 56,100 ÷ OHV

Where:

EW = equivalent weight in g/eq
OHV = hydroxyl value in mg KOH/g
56,100 = conversion constant from the KOH titration basis

The constant 56,100 comes from the molecular weight of potassium hydroxide (56.1 g/mol) multiplied by 1,000 for unit conversion.

Example

If a polyol has an OHV of 51 mg KOH/g:

EW = 56,100 ÷ 51 = 1,100 g/eq

So a polyol with OHV 51 has an equivalent weight of approximately 1,100 g/eq. This means 1,100 grams of that polyol contains one equivalent of reactive hydroxyl groups.

This calculation should be done using the actual OHV from the Certificate of Analysis, not only the nominal value from the Technical Data Sheet.

Polyol equivalent weight formula using hydroxyl value in polyurethane foam formulation — Polyol equivalent weight is calculated from hydroxyl value using EW = 56,100 ÷ OHV.

How to Calculate Isocyanate Equivalent Weight

For isocyanates, equivalent weight is calculated from the percentage of NCO.

The formula is:

Isocyanate EW = 4,200 ÷ %NCO

Where:

EW = equivalent weight in g/eq
%NCO = actual NCO percentage from the Certificate of Analysis
4,200 = molecular weight of the NCO group (42 g/mol) multiplied by 100

Example 1: TDI 80/20

If TDI has a %NCO of 48.3:

EW = 4,200 ÷ 48.3 = 86.96 g/eq

So the TDI equivalent weight is approximately 87 g/eq.

Example 2: MDI

If MDI has a %NCO of 31.5:

EW = 4,200 ÷ 31.5 = 133.33 g/eq

So the MDI equivalent weight is approximately 133 g/eq.

The same formula applies to TDI, MDI, polymeric MDI, and modified isocyanates. The constant does not change. The variable is the actual %NCO value.

For production calculation, use the %NCO from the Certificate of Analysis, not only the general TDS range.

Isocyanate equivalent weight formula using percent NCO for TDI and MDI — Isocyanate equivalent weight is calculated from actual %NCO using EW = 4,200 ÷ %NCO.

How to Calculate Water Equivalent Weight

Water is one of the most important components in flexible polyurethane foam formulation. It is also one of the easiest to calculate incorrectly.

Water has a molecular weight of 18 g/mol. But its equivalent weight in polyurethane formulation is not 18.

Water has two reactive hydrogens involved in the isocyanate reaction sequence. One water molecule consumes two NCO groups.

Therefore:

Water EW = 18 ÷ 2 = 9 g/eq

This value is fixed.

For PU foam index calculation: water equivalent weight is 9, not 18.

Using 18 instead of 9 cuts the calculated water contribution in half and can severely distort the isocyanate index calculation.

The detailed water equivalent weight error and its production consequences are covered in a separate article — the water EW mistake is one of the most damaging single-number errors in PU foam formulation.

Water equivalent weight is 9 not 18 in polyurethane foam formulation — Water has two reactive hydrogens, so its equivalent weight in polyurethane formulation is 9 g/eq.

How to Calculate Crosslinker Equivalent Weight

Crosslinkers and chain extenders must also be included in equivalent weight calculations if they contain reactive groups.

For hydroxyl-based crosslinkers, the same formula used for polyols can often be applied:

Crosslinker EW = 56,100 ÷ OHV

Example: Glycerol

If glycerol has an OHV of approximately 1,827 mg KOH/g:

EW = 56,100 ÷ 1,827 = 30.7 g/eq

So the equivalent weight is approximately 31 g/eq.

This is much lower than the equivalent weight of a typical flexible foam polyol. That means even small quantities of crosslinker can contribute meaningful reactive equivalents.

Important note about amine-functional crosslinkers

Some crosslinkers or chain extenders contain more than hydroxyl groups. For example, some amine-functional materials include reactive amine hydrogens as well. In those cases, an OHV-only calculation may not capture all reactive functionality.

The correct approach is to account for all active hydrogen groups that react with isocyanate.

This topic is covered in more depth in a separate article on equivalent weight mistakes, because missing reactive groups in crosslinkers can quietly distort index and network structure.

Crosslinker equivalent weight calculation using hydroxyl value in polyurethane foam formulation — Hydroxyl-based crosslinkers use the same EW formula as polyols, but their low EW can strongly affect reactive balance.

Complete Equivalent Weight Reference Table

The table below summarizes the main equivalent weight formulas used in PU foam formulation.

Component	EW Formula	Key Variable	Worked Example
Polyol	56,100 ÷ OHV	OHV from CoA	OHV 51 → EW 1,100
Isocyanate	4,200 ÷ %NCO	%NCO from CoA	48.3% NCO → EW 86.96
Water	18 ÷ 2	Fixed value	EW = 9
Hydroxyl crosslinker	56,100 ÷ OHV	OHV of crosslinker	OHV 1,827 → EW 30.7

Every number in this table can feed into the isocyanate index calculation.

If one EW value is wrong, the index becomes unreliable. If multiple EW values are wrong, the production symptoms can become confusing and difficult to diagnose.

How Equivalent Weight Feeds Into Isocyanate Index

Equivalent weight is used to calculate the number of reactive equivalents in the formula.

The general formula is:

Reactive Equivalents = Parts by Weight ÷ Equivalent Weight

For example, if a formulation contains 100 parts of polyol with EW 1,100:

Polyol equivalents = 100 ÷ 1,100 = 0.09091

If the formula contains 4 parts of water with EW 9:

Water equivalents = 4 ÷ 9 = 0.44444

Each reactive component is converted into equivalents. Then all reactive hydrogen equivalents are added together. The isocyanate required is calculated from that total and the target index.

This is why equivalent weight is not an isolated calculation. It is part of the full stoichiometric system.

Wrong EW → wrong equivalents → wrong index → wrong foam properties.

Workflow showing equivalent weight calculation feeding into isocyanate index calculation in PU foam formulation — Equivalent weight is the first step in calculating reactive equivalents and isocyanate index

Practical Rules for Equivalent Weight Calculation

Use these rules to avoid common formulation mistakes:

Do not confuse molecular weight with equivalent weight. Molecular weight describes the whole molecule. Equivalent weight describes the mass per reactive group.
Use actual CoA values when available. Polyol OHV and isocyanate %NCO can vary by batch.
Use water EW = 9. Water has two reactive hydrogens and consumes two NCO groups.
Recalculate EW when OHV changes. Polyol equivalent weight is not fixed if OHV changes.
Recalculate isocyanate EW when %NCO changes. The isocyanate equivalent weight depends on actual %NCO.
Include crosslinkers and chain extenders. Any reactive component must be included in the stoichiometric calculation.
Check all active hydrogens. Some materials contain amine groups or other reactive functionality not captured by simple OHV alone.
Audit old formula sheets. Legacy spreadsheets often contain copied EW values that may no longer match current raw material data.

Use the PolymerIQ Equivalent Weight Calculator

Manual calculation is useful because every foam engineer should understand the chemistry behind equivalent weight. But in production, the calculation must also be fast and consistent.

The PolymersIQ Equivalent Weight Calculator helps you calculate equivalent weight from OHV quickly.

Use it when:

A new polyol batch arrives
The CoA OHV is different from the design value
You are checking a formulation before production
You are preparing an isocyanate index calculation
You are auditing an old formula sheet

Open the Equivalent Weight Calculator →

For a deeper article on the water calculation error, read Why the Equivalent Weight of Water Is 9 in Polyurethane Foam.

For common production mistakes, read 5 Equivalent Weight Mistakes That Damage PU Foam Production.

For the full isocyanate index method, read Isocyanate Index Calculation Guide for PU Foam Engineers.

FAQs

What is equivalent weight in polyurethane foam formulation?

Equivalent weight is the mass of material that contains one mole-equivalent of reactive groups. In polyurethane foam, it is used to convert each reactive component (polyol, isocyanate, water, crosslinker) into a common basis so the formulator can calculate isocyanate demand and index correctly.

How is equivalent weight different from molecular weight?

Molecular weight is the mass of one mole of complete molecules. Equivalent weight is the mass per reactive group. For monofunctional materials they can be the same, but for multifunctional materials, equivalent weight is lower than molecular weight. The relationship is EW = Molecular Weight ÷ Functionality.

How do I calculate polyol equivalent weight?

Use EW = 56,100 ÷ OHV, where OHV is the hydroxyl value in mg KOH/g. The constant 56,100 comes from the molecular weight of potassium hydroxide (56.1 g/mol) multiplied by 1,000 for unit conversion. Always use the actual OHV from the Certificate of Analysis, not the nominal TDS value.

How do I calculate isocyanate equivalent weight?

Use EW = 4,200 ÷ %NCO, where %NCO is the percentage of NCO groups by weight. The constant 4,200 comes from the NCO group molecular weight (42 g/mol) multiplied by 100. The same formula applies to TDI, MDI, polymeric MDI, and modified isocyanates — only the %NCO value changes.

Why is the equivalent weight of water 9 and not 18?

Water has a molecular weight of 18, but each water molecule has two reactive hydrogens and consumes two NCO groups during the blowing reaction. So the equivalent weight is 18 ÷ 2 = 9 g/eq. Using 18 instead of 9 cuts the calculated water contribution in half and severely distorts the isocyanate index.

Do I need to calculate equivalent weight for crosslinkers?

Yes. Hydroxyl-based crosslinkers use the same formula as polyols (EW = 56,100 ÷ OHV). Glycerol, for example, has an OHV around 1,827 mg KOH/g, giving an EW of about 31 g/eq. Because crosslinker EW is much lower than polyol EW, even small amounts contribute meaningful reactive equivalents to the calculation.

What about amine-functional crosslinkers and chain extenders?

Materials with amine groups or other active hydrogens cannot be captured by an OHV-only calculation. The correct approach is to account for all active hydrogen groups that react with isocyanate. Missing reactive groups in crosslinkers can silently distort the index and the polymer network.

How does equivalent weight feed into the isocyanate index?

Reactive equivalents are calculated as Parts ÷ Equivalent Weight for each component. All reactive hydrogen equivalents are summed, then multiplied by the target index to determine required NCO equivalents. The isocyanate quantity is then calculated as Required NCO equivalents × Isocyanate EW. Wrong EW values create wrong equivalents and wrong index.

Should I recalculate equivalent weight when raw material batches change?

Yes. Polyol EW changes when OHV changes. Isocyanate EW changes when %NCO changes. Treating EW as a fixed value copied from an old formula sheet is one of the most common causes of hidden formulation drift.

What’s the most common equivalent weight mistake in PU foam formulation?

Using water EW as 18 instead of 9. Because water is usually one of the largest contributors to reactive hydrogen equivalents in flexible foam, getting this single value wrong can shift the running index by many points and produce foam that is significantly harder than expected.

Key Takeaways

Equivalent weight is the mass of material that contains one equivalent of reactive groups. It is not always the same as molecular weight.

In polyurethane foam formulation, equivalent weight is needed for every reactive component because the isocyanate index depends on reactive equivalents.

The main formulas are:

Polyol EW = 56,100 ÷ OHV
Isocyanate EW = 4,200 ÷ %NCO
Water EW = 18 ÷ 2 = 9
Hydroxyl crosslinker EW = 56,100 ÷ OHV

Equivalent weight should be treated as a live calculation, not a fixed value copied from an old formula sheet.

If OHV changes, polyol EW changes.
If %NCO changes, isocyanate EW changes.
If water is entered as 18 instead of 9, the index calculation becomes seriously wrong.

A correct equivalent weight system is the foundation of a correct isocyanate index calculation.

Conclusion

If your foam formula has been adjusted many times over the years, the equivalent weight values in the spreadsheet may no longer be correct.

PolymersIQ can help review your formulation, check every equivalent weight value, and identify whether hidden stoichiometric errors are affecting foam quality.

To get accurate support, please share:

Polyol grade, OHV, and supplier
Isocyanate type and %NCO from the Certificate of Analysis
Water level and any crosslinkers or chain extenders in use
Current EW values used in the formula sheet
Description of the foam quality issue (if any)

Contact PolymerIQ for a stoichiometric formulation audit →

April 29, 2026

Hydroxyl Value in Polyurethane Foam: What OHV Means and How to Calculate Equivalent Weight
Introduction

Most polyurethane foam quality problems are blamed on the machine.

The machine is not always the problem.

In many foam plants, the real cause is sitting inside the raw material data — especially the hydroxyl value of the incoming polyol batch.

A formula may be developed using one polyol OHV value, but the next delivery may arrive with a slightly different OHV. The value may still be inside the supplier’s specification range. It may pass incoming QC. It may not trigger any alarm.

But if nobody recalculates the equivalent weight, the formulation is no longer running at the same chemical balance. The formula looks the same on paper, but it behaves differently in production.

This is why hydroxyl value is one of the most important numbers in polyurethane foam formulation. It controls the equivalent weight of the polyol, affects isocyanate demand, and directly influences the final foam properties.

This article explains what hydroxyl value means, how it relates to equivalent weight, and how to calculate it correctly for PU foam production.

What Is Hydroxyl Value?

Hydroxyl value, often written as OHV, measures how many reactive hydroxyl groups are present in one gram of polyol.

It is expressed as:

mg KOH/g

This means milligrams of potassium hydroxide equivalent per gram of sample.

The potassium hydroxide is not actually inside the polyol. It is part of the measurement convention used in titration chemistry. The value gives formulators a standard way to compare the hydroxyl content of different polyols.

In practical terms:
- Higher OHV means more reactive hydroxyl sites per gram.
- Lower OHV means fewer reactive hydroxyl sites per gram.
- Higher OHV usually means shorter polyol chains.
- Lower OHV usually means longer polyol chains.
- Higher OHV generally produces stiffer foam behaviour.
- Lower OHV generally produces softer, more flexible behaviour.
This is why OHV is not just a laboratory number — it is a formulation control value.

If OHV changes, the polyol equivalent weight changes. If equivalent weight changes, the isocyanate requirement changes. If the isocyanate requirement changes but the formulation is not recalculated, foam properties can shift.

Hydroxyl value represents the concentration of reactive OH groups in the polyol.

Typical OHV Ranges for Different Foam Types

Different polyurethane foam systems use polyols with very different hydroxyl value ranges.

A flexible slabstock foam polyol is not the same as a rigid insulation foam polyol. The OHV range reflects the type of polymer network the formulation is designed to create.

Foam Type Typical OHV Range
HR flexible foam 28–35 mg KOH/g
Flexible slabstock foam 45–56 mg KOH/g
Semi-rigid foam 100–200 mg KOH/g
Rigid / insulation foam 350–550 mg KOH/g

Flexible foams usually use lower-OHV polyols because they need longer, more elastic polymer chains.

Rigid foams use much higher-OHV polyols because they require a dense, highly crosslinked structure.

This is why OHV immediately tells you something about the intended application of a polyol. A polyol with OHV around 50 belongs to a very different formulation world than a polyol with OHV around 450.

Different PU foam systems use different OHV ranges depending on flexibility, stiffness, and crosslink density.

How Hydroxyl Value Is Measured

Hydroxyl value is commonly measured using standard titration methods such as ASTM D4274 or ISO 14900. These are acetylation-based titration methods used to determine hydroxyl content in polyols.

In production, the OHV value usually appears on the supplier’s Certificate of Analysis. For serious formulation control, the incoming CoA value should not be ignored or treated as a fixed number.

The OHV value from each batch matters because every batch can have a slightly different hydroxyl value. Even if the value remains inside the supplier’s TDS specification range, it can still change the formulation balance.

OHV and Equivalent Weight: The Critical Link

Equivalent weight is the bridge between hydroxyl value and isocyanate stoichiometry.

The formula is:

Equivalent Weight = 56,100 ÷ OHV

Where:
- Equivalent weight is expressed in g/eq
- OHV is expressed in mg KOH/g
- 56,100 is the conversion constant based on potassium hydroxide molecular weight
Equivalent weight tells you how many grams of polyol contain one equivalent of reactive hydroxyl groups.

This value is essential because polyurethane formulation is based on equivalent relationships, not simply weight relationships.
- A polyol with a lower OHV has a higher equivalent weight.
- A polyol with a higher OHV has a lower equivalent weight.
That matters because isocyanate demand is calculated from reactive equivalents.

[IMAGE 4 — OHV TO EQUIVALENT WEIGHT FORMULA] Placement: After the section “OHV and Equivalent Weight”, before “Worked Example”. Filename: ohv-equivalent-weight-formula-polyurethane.jpg ALT text: Hydroxyl value to equivalent weight formula for polyurethane polyol calculation Caption: Equivalent weight is calculated from hydroxyl value using the formula EW = 56,100 ÷ OHV. ChatGPT image prompt: “Create a clean technical formula infographic on a white background showing the relationship between hydroxyl value and equivalent weight in polyurethane formulation. Display the formula: Equivalent Weight = 56,100 / OHV. Add simple labels: OHV in mg KOH/g, EW in g/eq, used for isocyanate stoichiometry. Include a polyol drum icon, calculator icon, and small OH group symbols. Professional engineering style, blue and grey color palette, clean and readable. No logos. No brand names.”

Equivalent weight is calculated from hydroxyl value using the formula EW = 56,100 ÷ OHV

Worked Example: Calculating Polyol Equivalent Weight

Let’s calculate equivalent weight using a polyol OHV of 51 mg KOH/g.

Formula: EW = 56,100 ÷ OHV

Calculation: EW = 56,100 ÷ 51 = 1,100 g/eq

So a polyol with OHV 51 has an equivalent weight of approximately 1,100 g/eq. This means 1,100 grams of that polyol contains one equivalent of reactive hydroxyl groups.

Now compare that to different OHV values within a typical flexible foam range:

OHV (mg KOH/g) Equivalent Weight (g/eq)
45 1,247
47 1,194
51 1,100
53 1,058
55 1,020

This table shows why OHV cannot be ignored.

A shift from OHV 45 to OHV 55 creates an equivalent weight change of more than 200 g/eq.

That is a large stoichiometric difference, even though the polyol may still be inside a normal supplier specification range.

Small OHV changes can create large equivalent weight shifts, affecting the formulation balance.

Why OHV Changes Foam Behaviour

OHV affects foam behaviour because it changes the number of reactive sites available in the polyol.

If OHV is higher:
- There are more reactive sites per gram.
- Equivalent weight is lower.
- Isocyanate demand increases.
- The foam may trend softer if isocyanate is not adjusted correctly.
- The final network balance may shift.
If OHV is lower:
- There are fewer reactive sites per gram.
- Equivalent weight is higher.
- Isocyanate demand decreases.
- If isocyanate is not adjusted, the actual index can rise.
- The foam may become harder than expected.
This is one of the most important diagnostic relationships in PU foam formulation:

OHV low → equivalent weight high → actual index can increase → foam can become harder

OHV high → equivalent weight low → actual index can decrease → foam can become softer

This does not mean OHV is the only factor controlling hardness. Catalyst, water, silicone, crosslinker, density, temperature, and machine delivery also matter.

But if hardness changes batch to batch and the formulation looks unchanged, OHV should be checked early.

The direction of OHV drift helps diagnose whether foam may trend harder or softer.

Why TDS OHV Is Not Enough

A polyol Technical Data Sheet gives a specification range.

That range tells you what the supplier considers acceptable for the product grade. It does not tell you the actual OHV of the batch sitting in your plant today.

For example, a TDS may show:

OHV range: 45–55 mg KOH/g

If the engineer uses the midpoint forever, the calculation may be wrong when the actual delivered batch is 47 or 55.

The Certificate of Analysis gives the batch-specific OHV value. That is the number that should be used for equivalent weight calculation.

The difference matters because equivalent weight is calculated directly from OHV. Using the wrong OHV means using the wrong equivalent weight. Using the wrong equivalent weight means the isocyanate requirement may not match the actual reactive demand.

The TDS gives the allowed OHV range, but the CoA gives the actual batch value needed for calculation.

Practical Calculation Workflow for Foam Plants

A simple OHV workflow can prevent many formulation errors.

Use this process for every incoming polyol batch:
1. Receive the polyol Certificate of Analysis.
2. Record the actual batch OHV.
3. Calculate equivalent weight using EW = 56,100 ÷ OHV.
4. Compare the new EW with your design value.
5. Recalculate the isocyanate index if the difference is meaningful.
6. Adjust the formula if required before production.
7. Keep a batch-by-batch OHV log for each supplier and grade.
This workflow is simple, but it prevents one of the most common sources of hidden formulation drift.

The most important point is this: equivalent weight is not a one-time value. It changes when OHV changes.

Use the PolymerIQ Equivalent Weight Calculator

Manual calculation is useful because every foam engineer should understand the relationship between OHV and equivalent weight.

But in production, the calculation must be fast, repeatable, and error-free.

The PolymerIQ Equivalent Weight Calculator helps you convert OHV into equivalent weight instantly.

Use it when:
- A new polyol batch arrives
- The CoA OHV differs from your design value
- A formulation is being checked before production
- Foam hardness changes without a clear process reason
- You are preparing an isocyanate index calculation
Open the Equivalent Weight Calculator →

Hydroxyl value and equivalent weight are directly connected to isocyanate index. After calculating equivalent weight, the next step is to use it in the index calculation. For the full index calculation method, read Isocyanate Index Calculation Guide for PU Foam Engineers.

FAQs

What is hydroxyl value in polyurethane foam?

Hydroxyl value (OHV) measures how many reactive hydroxyl groups are present in one gram of polyol. It is expressed in mg KOH/g (milligrams of potassium hydroxide equivalent per gram of sample). The KOH is not actually in the polyol — it is part of the titration measurement convention. OHV is a key formulation control value because it determines polyol equivalent weight and isocyanate demand.

How is hydroxyl value measured?

OHV is commonly measured using standard titration methods such as ASTM D4274 or ISO 14900, which are acetylation-based titration techniques. The value is reported on the supplier’s Certificate of Analysis for each batch.

What is the difference between OHV and equivalent weight?

OHV expresses hydroxyl content in mg KOH/g. Equivalent weight expresses how many grams of polyol contain one equivalent of reactive hydroxyl groups (g/eq). They describe the same chemistry but in different units. The conversion is EW = 56,100 ÷ OHV.

Why is the equivalent weight formula 56,100 ÷ OHV?

The constant 56,100 comes from the molecular weight of potassium hydroxide (56.1 g/mol) multiplied by 1,000 for unit conversion. Since OHV is reported in mg KOH/g, dividing 56,100 by OHV gives the grams of polyol that contain one equivalent of OH groups.

What is the typical OHV range for flexible foam polyols?

Standard flexible slabstock foam polyols typically have OHV in the range of 45–56 mg KOH/g. HR flexible foam polyols are usually 28–35 mg KOH/g. Semi-rigid foam polyols sit at 100–200 mg KOH/g, and rigid insulation foam polyols are much higher at 350–550 mg KOH/g.

Should I use OHV from the TDS or the Certificate of Analysis?

Always use the actual OHV from the Certificate of Analysis for the specific batch in production. The TDS gives a specification range, and using the midpoint can introduce calculation error when the actual batch sits at the edge of the range. Equivalent weight is calculated directly from OHV, so a wrong OHV means a wrong EW.

How does OHV affect foam hardness?

OHV affects hardness indirectly through equivalent weight and isocyanate stoichiometry. If OHV is lower than design (and isocyanate is not adjusted), the actual running index can rise and foam may become harder. If OHV is higher than design, the actual index can drop and foam may become softer. This is why OHV should be checked early when batch-to-batch hardness variation appears.

What happens if I don’t recalculate equivalent weight when polyol batch changes?

The formula sheet will look correct, but the real reactive equivalents in the system will be different from what the calculation assumes. The isocyanate amount may no longer match the actual reactive demand, and the running index will drift away from the target. This can cause hidden hardness, compression set, or recovery problems that are hard to trace.

Can OHV variation cause batch-to-batch foam quality problems?

Yes. Even when OHV stays inside the supplier’s TDS range, batch-to-batch variation can shift the equivalent weight by tens or hundreds of g/eq. If the formula is not recalculated for each batch, the actual running index changes silently and foam properties can drift between deliveries.

How does OHV differ between flexible and rigid foam polyols?

Flexible foam polyols have low OHV (typically 28–56 mg KOH/g), which gives long, elastic polymer chains and a flexible network. Rigid foam polyols have high OHV (typically 350–550 mg KOH/g), which produces a dense, highly crosslinked network with stiff structural properties. The OHV range tells you immediately what kind of foam the polyol is designed for.

Key Takeaways
- Hydroxyl value (OHV) measures the concentration of reactive hydroxyl groups in a polyol.
- Higher OHV means more reactive sites per gram and lower equivalent weight.
- Lower OHV means fewer reactive sites per gram and higher equivalent weight.
- The equivalent weight formula is EW = 56,100 ÷ OHV.
- A change in OHV changes equivalent weight. A change in equivalent weight changes the isocyanate demand. If the formula is not recalculated, the actual running index can shift.
- The TDS range should not be used as a fixed formulation value. The batch-specific OHV from the Certificate of Analysis should be used for production calculation.
- For consistent PU foam production, every incoming polyol batch should have its OHV recorded, equivalent weight recalculated, and formulation impact reviewed before production.
Conclusion

If your foam hardness is changing from batch to batch and the machine settings look stable, the incoming polyol OHV may be one of the first values to check.

PolymersIQ can help review your formulation, calculate equivalent weight correctly, and identify whether raw material variation is affecting your production baseline.

To get accurate support, please share:
- Polyol grade and supplier
- Current OHV from the Certificate of Analysis
- Design OHV used in your original formulation
- Water level, crosslinker, and any other reactive components
- Isocyanate type and %NCO
- Description of the quality issue you are facing
Contact PolymerIQ for a formulation audit →
April 28, 2026

Foam Type	Typical OHV Range
HR flexible foam	28–35 mg KOH/g
Flexible slabstock foam	45–56 mg KOH/g
Semi-rigid foam	100–200 mg KOH/g
Rigid / insulation foam	350–550 mg KOH/g

OHV (mg KOH/g)	Equivalent Weight (g/eq)
45	1,247
47	1,194
51	1,100
53	1,058
55	1,020

How Isocyanate Index Affects PU Foam Properties

Introduction

The isocyanate index is not just a calculation number. It directly affects the physical behaviour of polyurethane foam.

When the index changes, the foam does not only become slightly harder or softer. The entire polymer network changes. Crosslink density changes. Elastic recovery changes. Compression set changes. Aging behaviour changes. In extreme cases, the foam can become unstable, brittle, weak, or outside customer specification.

This is why experienced polyurethane formulators do not treat index as a simple recipe value. They treat it as a production control parameter.

A foam block may look normal after rise. It may cut normally. It may even pass basic visual inspection. But if the index is outside the correct window, the final foam properties can fail during testing or during customer use.

This article explains how different isocyanate index ranges affect PU foam properties, and why you should always design around an index window, not a single target point.

Why Isocyanate Index Changes Foam Properties

The isocyanate index changes the chemical balance between NCO groups and reactive hydrogen groups in the formulation.

At a basic level:

Lower index means less available NCO compared with reactive hydrogen demand.
Higher index means more NCO is available than the theoretical requirement.
Correct index means the foam network develops with the intended balance of flexibility, strength, recovery, and durability.

In flexible PU foam, the polymer network is built through urethane and urea linkages. These linkages form the structure that gives foam its physical properties.

When the index is too low, the foam network can be underdeveloped. When the index is too high, the network can become too dense and rigid. This is why the same foam formula can produce very different results when the index is changed.

The Molecular Mechanism Behind Index Effects

At Index 100, every NCO group theoretically has one reactive hydrogen partner. The system is close to stoichiometric balance.

In a simplified view, the polymer network contains:

Urethane linkages from polyol and isocyanate reaction
Urea linkages from water and isocyanate reaction
A balanced crosslink structure

But real foam chemistry is more complex than the simple formula.

Below Index 100, there may not be enough NCO to fully react with all active hydrogen sources. This can leave unreacted hydroxyl groups and reduce network continuity. The foam may become softer, weaker, more moisture-sensitive, and less stable during aging.

Above Index 100, excess NCO does not simply stay unused. It can participate in secondary reactions. It can react with already-formed urethane or urea groups and create additional crosslinks. These extra crosslinks can increase hardness and improve compression set up to a useful point. But if the index becomes too high, the foam can lose elasticity and become brittle.

This is why index effects are not always perfectly linear. A small increase may improve performance, but a large increase can create a different failure mode.

Diagram comparing low index, balanced index, and high index PU foam polymer networks — Low, balanced, and high isocyanate index levels create different polymer network structures inside PU foam.

Low Isocyanate Index: Soft Foam and Weak Network Formation

A low isocyanate index means the foam has insufficient NCO compared with the reactive hydrogen demand.

In practical foam production, this can cause:

Lower hardness
Reduced load-bearing
Poorer compression set
Lower recovery
Moisture sensitivity
Aging-related hardness loss
Higher batch-to-batch variation

The foam may feel soft at first, but the problem is not only softness. The deeper issue is weak network development.

When the polymer network is underbuilt, the foam may not hold its properties over time. It may lose shape more easily under compression, recover more slowly, or show poor durability after aging.

Low-index foam can be used intentionally in some specialty systems, but it must be designed carefully. If low index occurs unintentionally in standard flexible slabstock foam, it is usually a production or calculation problem.

Common causes of unintentional low index:

Missing reactive components from the index calculation
Increasing water without recalculating isocyanate
Excluding crosslinkers or chain extenders
Using wrong polyol OH value
Isocyanate pump under-delivery
Incorrect %NCO value in the formula

Low index should not be corrected blindly by changing catalysts. First, verify the actual index calculation and machine delivery.

Low isocyanate index causing soft PU foam, poor recovery, and compression set problems — Low isocyanate index can reduce crosslink density, causing softer foam and weaker recovery.

Balanced Index: The Practical Operating Zone for Flexible Foam

For many flexible foam systems, the most stable production zone is slightly above theoretical stoichiometric balance.

This does not mean every formula should use the same index. The correct target depends on:

Foam density
Polyol type
Polyol functionality
Water level
Crosslinker level
Catalyst system
Required hardness
Compression set specification
Customer application

However, standard flexible slabstock systems often operate in a practical index range where the foam has enough crosslink density for strength and recovery, without becoming overly rigid or brittle.

In this balanced range, the foam typically shows:

Target hardness
Good compression set
Stable recovery
Acceptable resilience
Consistent cutting and handling
Better long-term property retention

This is the zone where the index supports the intended foam grade instead of fighting against it.

For many standard flexible foam grades, the useful range is often around Index 103 to 108, while higher-load grades may move higher depending on the application. The key is not to copy a number from another formula — the key is to validate the index against actual foam testing.

Elevated isocyanate index increasing PU foam hardness and compression set performance — A balanced index window helps maintain hardness, recovery, and compression set within specification.

Elevated Index: Higher Hardness and Better Compression Set

As the isocyanate index increases above the balanced zone, crosslink density increases.

This can be useful when the foam needs:

Higher load-bearing
Higher hardness
Better compression set
Improved dimensional stability
Stronger support under repeated loading

This is why some high-load seating, automotive, or industrial grades may use a higher index than standard comfort foam.

But elevated index must be controlled carefully.

If the index rises unintentionally, the foam may become harder than the customer specification. The foam can feel too stiff, even if density and dimensions are correct.

Possible symptoms include:

ILD above target
Stiffer hand feel
Lower comfort
Customer complaints after unpacking
Edge brittleness in severe cases
Reduced elongation if pushed too high

A higher index is not automatically better. It improves some properties while risking others.

The correct question is not “Can we raise the index?” The correct question is “Does the application need the property changes created by a higher index?”

High and Excessive Index: Brittleness and Elasticity Loss

When the index becomes too high, the foam can move beyond useful firmness and into over-indexed behaviour.

The polymer network becomes too dense. Elastic recovery becomes limited. Instead of behaving like flexible foam, the material may become brittle or friable.

In flexible foam applications, excessive index can cause:

Edge brittleness
Crumbling under repeated compression
Poor elongation
Harsh hand feel
Reduced flexibility
Possible dimensional instability
Higher risk of customer rejection

This type of failure can sometimes be confused with over-catalysis, poor mixing, raw material contamination, or curing issues. But the root cause may simply be that the foam is over-indexed.

This is why troubleshooting should always include index verification before making multiple process changes.

If the foam is too hard, brittle, or failing elongation, check:

Actual TDI or MDI delivery
Water level
Crosslinker level
%NCO from CoA
Pump calibration
Whether the formula was recently adjusted
Whether the same formula was transferred from another line

High index is not only a formulation choice. It can also be created by metering error.

Over-indexed PU foam showing brittleness and reduced elastic recovery risk — Excessive isocyanate index can create a dense network that reduces elastic recovery and increases brittleness risk.

Isocyanate Index Reference Table

The table below gives a practical reference for how different index ranges can affect flexible polyether-based polyurethane foam.

Actual results depend on formulation design, polyol type, functionality, molecular weight, water level, catalyst package, and production equipment. Use this table as a technical guide, not as a replacement for formulation testing.

Index Range	Crosslink Density	ILD / Hardness Effect	Compression Set	Typical Application	Risk if Unintentional
Below 90	Very low / deficient	Significantly below target	Poor	Avoid in normal flexible foam	Collapse, weak structure, aging problems
90–98	Low	Soft, often below target	Marginal	Specialty soft or HR systems only when designed	Short service life, moisture sensitivity
98–103	Near-stoichiometric	At or slightly below target	Acceptable but sensitive	Limited use in slabstock	High batch-to-batch variation
103–108	Balanced	Usually on target	Good	Standard flexible slabstock, furniture foam	Generally stable if controlled
108–115	Elevated	Often 10–20% above target	Excellent	High-load seating, automotive foam	Stiff feel, customer complaints
115–125	High	Significantly firm	Very good	Industrial grades, carpet underlay	Edge brittleness, reduced comfort
125–160	Very high	Semi-rigid behaviour	Excellent	Packaging, acoustics, structural uses	Friability, elongation failure
Above 200	Full rigid network	Rigid foam behaviour	Not applicable to flexible foam	PIR/PUR insulation boards	Dimensional instability if uneven

This table is especially useful during troubleshooting. If foam is soft and compression set is weak, the actual index may be lower than expected. If foam is too hard, stiff, or brittle, the actual index may be higher than expected.

Why You Should Target an Index Window, Not a Single Point

In real production, the index is not perfectly fixed.

Even if the formula sheet says Index 105, the actual running index may move during the day. This can happen because of:

Metering pump variation
Pump calibration drift
Isocyanate temperature changes
Polyol temperature changes
Viscosity changes
Drum-to-drum %NCO variation
Small weighing or delivery errors
Mixing head condition
Production line differences

On a well-controlled line, the actual index may vary by a few points around the target. On an older or poorly controlled line, the variation can be larger.

This is why a target index should not be selected too close to the failure boundary.

For example, if a foam grade needs at least Index 103 to consistently pass compression set, targeting exactly 103 is risky. If normal production variation moves the actual index down to 101 or 102, some batches may fail.

A better approach is to design a practical safety margin.

If the minimum acceptable index is 103, a production target around 106 or 107 may be more stable, depending on the line variation and customer specification.

This is called designing around an index window.

The goal is not to hit a perfect number on paper. The goal is to keep real production inside the acceptable property window.

Isocyanate index window showing production tolerance around target index in PU foam manufacturing — Real production index behaves like a tolerance window, not a single fixed point.

Practical Troubleshooting Guide by Index Direction

When foam properties are outside specification, index direction can help guide the investigation.

If the foam is softer than expected

Check whether the actual index is lower than the formula target.

Possible causes:

Isocyanate under-delivery
Polyol over-delivery
Water increase without recalculation
Crosslinker excluded from calculation
Chain extender excluded from calculation
Wrong %NCO value
Wrong polyol OH value
Formula copied from another machine without validation

If the foam is harder than expected

Check whether the actual index is higher than the formula target.

Possible causes:

Isocyanate over-delivery
Polyol under-delivery
Water equivalent weight entered incorrectly
Formula recalculated using wrong reactive component values
Actual %NCO higher than assumed
Line-specific metering drift
Uncontrolled process temperature effects

If compression set is failing

Check whether the index is too low or too close to the lower specification boundary.

Possible causes:

Target index selected without safety margin
Actual production variation dipping below the acceptable range
Crosslinker or chain extender not included correctly
Isocyanate delivery instability
Formula adjusted over time without full recalculation

The main point is simple: do not troubleshoot foam properties only by changing catalysts or silicone. First, verify whether the foam is actually running at the intended index.

Use the PolymerIQ Isocyanate Index Calculator

The PolymerIQ Isocyanate Index Calculator can help verify whether the formulation is running at the intended index.

Use it to check:

Target index
Actual running index
Required TDI or MDI parts
Polyol equivalent weight
Water contribution
Crosslinker contribution
Effect of %NCO changes
Effect of formulation adjustments

This is especially useful before changing catalysts, replacing raw materials, or blaming machine conditions.

Open the Isocyanate Index Calculator →

For the full calculation method and worked example, read Isocyanate Index Calculation Guide for PU Foam Engineers.

For common production mistakes, read 5 Isocyanate Index Calculation Mistakes That Cause PU Foam Quality Problems.

FAQs

How does isocyanate index affect PU foam hardness?

Higher isocyanate index generally increases crosslink density, which increases foam hardness and load-bearing capacity. Lower index reduces crosslink density, producing softer foam. The relationship is not perfectly linear — at very high index, the foam can become brittle and lose elasticity, while at very low index it becomes weak and unstable.

What happens if the isocyanate index is too low?

Low index means there is not enough NCO to fully react with all active hydrogen sources. The polymer network is underdeveloped, leading to softer foam, poor compression set, weaker recovery, moisture sensitivity, and aging-related hardness loss. In extreme cases, the foam can collapse or fail structurally.

What happens if the isocyanate index is too high?

Excessive index creates a dense, over-crosslinked polymer network. The foam can become brittle, friable, harsh in feel, and lose elongation. Edge brittleness, crumbling under repeated compression, and dimensional instability are common symptoms of over-indexed flexible foam.

What is the typical isocyanate index range for flexible foam?

Standard flexible slabstock foam often operates around Index 103 to 108, while high-load grades for seating, automotive, or industrial applications may run higher (108–115 or above). The exact target depends on density, polyol type, water level, crosslinker, catalyst system, and required foam properties. There is no universal index — it must be validated for each formulation.

Why should I target an index window instead of a single point?

Real production index varies because of pump calibration drift, %NCO variation between drums, temperature changes, viscosity changes, and metering accuracy. If the target is set exactly at the lower acceptance boundary, normal variation can push some batches below specification. Designing a safety margin keeps actual production inside the acceptable property window.

How does isocyanate index affect compression set?

Compression set generally improves as crosslink density increases, up to a point. A balanced or slightly elevated index usually gives the best compression set performance. Very low index produces poor compression set due to weak network development, while very high index can also reduce performance if the foam becomes too rigid.

Can over-indexed foam become brittle?

Yes. When index is significantly above the useful range, the polymer network becomes too dense and elastic recovery is limited. The foam can show edge brittleness, crumbling, harsh feel, and elongation failure. This is sometimes mistaken for over-catalysis or curing problems, when the actual root cause is the index.

Does isocyanate index affect foam aging?

Yes. Foam at low index often shows aging-related hardness loss and moisture sensitivity because the polymer network is underbuilt. Foam at very high index can also age poorly due to brittleness. Foam at a balanced index typically shows the best long-term property retention.

How do I troubleshoot foam that is harder than expected?

First, check whether the actual running index is higher than the formula target. Possible causes include isocyanate over-delivery, polyol under-delivery, wrong water equivalent weight (using 18 instead of 9), higher actual %NCO than assumed, or line-specific pump drift. Verify pump delivery and recalculate the index before making catalyst or silicone changes.

Should I change catalysts first when foam properties are off-spec?

No. Index verification should come first. Changing catalysts or silicones without confirming the actual running index can mask the real problem and create new ones. Confirm the index calculation and pump delivery, then move on to process variables if the index is correct.

Key Takeaways

The isocyanate index affects foam properties because it changes the polymer network inside the foam.

A low index can produce softer foam, weaker recovery, poorer compression set, and reduced aging stability.
A balanced index helps the foam achieve the intended hardness, recovery, and durability.
An elevated index can improve load-bearing and compression set, but may make the foam too stiff if uncontrolled.
An excessive index can create brittleness, friability, poor elongation, and reduced flexibility.

The best production practice is not to target a single index point. The better approach is to design a safe index window that accounts for real production variation.

When foam hardness, compression set, or resilience is outside specification, the actual isocyanate index should be checked early in the troubleshooting process.

Conclusion

If your foam plant is facing unexplained hardness variation, compression set failure, poor recovery, or different results between production lines, the actual running index may not match the formula sheet.

PolymersIQ can help review your formulation, check index sensitivity, and identify whether the foam is operating inside the correct property window.

To get accurate support, please share:

Polyol grade and OH value
Water level and any other reactive components
Isocyanate type and %NCO from the Certificate of Analysis
Target index and observed foam properties (ILD, compression set, density)
Description of the quality issue you are facing
Production line conditions and any recent formula adjustments

Contact PolymerIQ for a formulation audit →

April 28, 2026

Isocyanate Index Calculation Guide for PU Foam Engineers

Introduction

A polyurethane foam plant had been running the same flexible slabstock formula for months. The foam looked normal. It rose properly, the block size was within tolerance, and the machine settings had not changed. But the final product was consistently harder than the target specification.

ILD values were coming out 15 to 20 percent above the design grade. Compression set was marginal. Customers started complaining that the foam felt too stiff after unpacking.

The production team investigated the usual suspects. They changed the polyol lot. They adjusted amine catalyst levels. They reviewed silicone performance. They checked temperature conditions. Nothing solved the problem.

The formula sheet said the foam was running at Index 105. In reality, it was running at Index 112.

The issue was not the polyol, the catalyst, or the silicone. The problem was the isocyanate index calculation. One reactive component had been missed, and the isocyanate quantity had not been recalculated after a water adjustment.

This is why the isocyanate index is one of the most important control numbers in polyurethane foam production. It affects hardness, compression set, resilience, aging behaviour, dimensional stability, and batch consistency. When it is calculated incorrectly, the foam may still rise and look acceptable, but the final properties can move far outside specification.

What Is Isocyanate Index in Polyurethane Foam?

The isocyanate index is the ratio between the actual NCO equivalents used in a formulation and the theoretical NCO equivalents required for exact stoichiometric balance.

In simple terms:

Isocyanate Index = (Actual NCO equivalents ÷ Theoretical NCO equivalents required) × 100

At Index 100, the formulation has exactly enough NCO groups to react with all active hydrogen groups in the system. In theory, every NCO group has a matching reactive hydrogen partner.

But in real polyurethane foam production, Index 100 is rarely the practical target.

Simple diagram showing actual NCO equivalents versus theoretical required NCO equivalents in PU foam — The isocyanate index compares actual NCO used to the theoretical NCO required for stoichiometric balance.

Why Index 100 Is Usually Not the Target

Perfect stoichiometric balance sounds logical, but polyurethane chemistry does not stop at the main polyol-isocyanate reaction.

During foam formation, NCO groups can also react with:

Water
Urea linkages
Urethane linkages
Crosslinkers
Chain extenders
Atmospheric moisture
Other NCO groups under heat

These secondary reactions consume additional NCO beyond the basic theoretical requirement.

If a flexible foam formula is run exactly at Index 100, these extra reactions may effectively pull the system below the desired balance. The result can be lower crosslink density, softer foam, poorer compression set, and weaker aging performance.

This is why flexible slabstock foam commonly runs above Index 100. Many flexible foam systems are developed in the range of approximately Index 105 to 115, depending on the required hardness, density, resilience, and compression set performance.

The correct target index is not selected from theory alone. It is established through practical formulation trials and production validation.

Diagram showing primary and secondary reactions consuming NCO in polyurethane foam chemistry — Secondary reactions consume extra NCO during foaming, which is why many flexible foam systems run above Index 100.

What the Index Is Actually Measuring

The isocyanate index measures how much NCO is available compared with all reactive hydrogen sources in the formulation.

Reactive hydrogen sources include:

Hydroxyl groups from polyol
Hydrogen atoms from water
Hydroxyl groups from crosslinkers
Amine groups from chain extenders
Any additive with active hydrogen functionality

This is where many calculation errors happen.

Most engineers include polyol and water because they are the main reactive components. But crosslinkers, chain extenders, and other active hydrogen additives are sometimes missed. Even small quantities can shift the real index by several points.

For example, a crosslinker at only 0.5 to 1.5 parts per hundred polyol can create a meaningful index difference if it is excluded from the denominator.

That difference may not be visible during foaming, but it can appear later as hardness drift, compression set failure, poor recovery, or batch-to-batch inconsistency.

Isocyanate Index Is a Control Parameter, Not Just a Recipe Number

A formulation sheet may show polyol, water, catalyst, silicone, crosslinker, and TDI or MDI levels. These are recipe components.

The index is different.

The index describes the chemical balance between reactive components. It is a control parameter.

This means the index changes whenever any reactive component changes:

Change in Formulation	Effect on Index
Increase water	Index changes
Add or remove crosslinker	Index changes
Change polyol OH value	Index changes
New isocyanate batch with different %NCO	Index changes
Adjust chain extender	Index changes

The isocyanate parts cannot remain fixed after reactive formulation changes. Every adjustment to a reactive component requires a fresh index calculation.

This is one of the most important rules in polyurethane formulation discipline.

Infographic showing that changes in water, polyol, crosslinker, and %NCO affect isocyanate index Caption: The isocyanate index changes whenever any reactive component in the formulation changes. — The isocyanate index changes whenever any reactive component in the formulation changes.

The Basic Calculation Method

To calculate isocyanate index correctly, you need to calculate the equivalent contribution of each reactive component.

The process is:

Calculate the equivalent weight of the polyol.
Calculate the equivalent weight of water.
Calculate the equivalent weight of the isocyanate.
Calculate the equivalent weight of crosslinkers or chain extenders.
Convert each component into equivalents.
Add the total reactive hydrogen equivalents.
Calculate the NCO required for the target index.
Convert the required NCO equivalents into isocyanate parts.

The calculation is not difficult, but every reactive component must be included.

Step 1: Calculate Polyol Equivalent Weight

For a polyol, equivalent weight is calculated from the hydroxyl value.

Polyol Equivalent Weight = 56,100 ÷ OH Value

Where OH value is measured in mg KOH/g.

Example:

Polyol OH value = 56 mg KOH/g
Calculation: 56,100 ÷ 56 = 1,001.8 g/eq

The number 56,100 comes from the molecular weight of potassium hydroxide multiplied by 1,000 for unit conversion.

Step 2: Calculate Water Equivalent Weight

Water is the component most often calculated incorrectly.

Water has a molecular weight of 18 g/mol, but its equivalent weight in polyurethane formulation is not 18.

Water has two reactive hydrogen atoms. One water molecule consumes two NCO groups during the polyurethane blowing reaction.

Therefore:

Water Equivalent Weight = 18 ÷ 2 = 9 g/eq

So the correct equivalent weight of water is always 9 g/eq.

This is a critical rule. Using 18 instead of 9 cuts the water contribution in half and can create a major index error. In flexible foam, this mistake can shift the real running index by many points and produce foam that is much harder than expected.

Technical illustration showing why water equivalent weight in PU foam is 9 instead of 18 — In polyurethane formulation, water consumes two NCO groups, so its equivalent weight is 9, not 18.

Step 3: Calculate Isocyanate Equivalent Weight

For isocyanate, equivalent weight is calculated from the %NCO value.

Isocyanate Equivalent Weight = 4,200 ÷ %NCO

Example using TDI 80/20:

TDI %NCO = 48.3%
Calculation: 4,200 ÷ 48.3 = 86.96 g/eq

Important note: Use the actual %NCO from the Certificate of Analysis for the drum or batch being used. Do not simply use the general range from the Technical Data Sheet.

Step 4: Calculate Crosslinker or Chain Extender Equivalent Weight

Any reactive crosslinker or chain extender must also be included.

For hydroxyl-based crosslinkers, the same formula used for polyol can be applied:

Equivalent Weight = 56,100 ÷ OH Value

Example using DEOA:

DEOA OH value = approximately 1,260 mg KOH/g
Calculation: 56,100 ÷ 1,260 = 44.5 g/eq

Even when used at low levels, crosslinkers can strongly affect the index calculation because their equivalent weight is much lower than that of the main polyol.

Step 5: Convert Each Component Into Equivalents

Now convert each reactive component into equivalents.

Equivalents = Parts in Formula ÷ Equivalent Weight

Example formulation:

Component	Parts	Equivalent Weight	Equivalents
Polyol	100.0	1,001.8	0.09982
Water	3.5	9.0	0.38889
DEOA crosslinker	0.5	44.5	0.01124
Total Reactive H			0.49995

Total reactive hydrogen equivalents: 0.49995

This total becomes the denominator for the isocyanate index calculation.

Step 6: Calculate Required NCO Equivalents for Target Index

Now apply the target index.

Required NCO Equivalents = Total Reactive H Equivalents × Target Index ÷ 100

Target Index = 105

Calculation: 0.49995 × 105 ÷ 100 = 0.52495 eq

So the formulation requires 0.52495 NCO equivalents to run at Index 105.

Step 7: Convert NCO Equivalents Into Isocyanate Parts

Finally, multiply the required NCO equivalents by the equivalent weight of the isocyanate.

Isocyanate Parts = Required NCO Equivalents × Isocyanate Equivalent Weight

Using TDI EW = 86.96:

0.52495 × 86.96 = 45.64 parts

So the correct TDI quantity is 45.64 PPHP.

Final formula at Index 105:

Component	Parts
Polyol	100.00
Water	3.50
DEOA	0.50
TDI 80/20	45.64

Step-by-step worked example of isocyanate index calculation in PU foam formulation — A worked example helps translate equivalent weights and formulation parts into the correct isocyanate requirement

What Happens If You Miss a Reactive Component?

Now let’s see what happens if the DEOA crosslinker is excluded from the calculation.

Without DEOA, the reactive hydrogen total becomes:

Component	Equivalents
Polyol	0.09982
Water	0.38889
DEOA	Excluded
Total Reactive H	0.48871

Using the incorrect total:

0.48871 × 1.05 × 86.96 = 44.62 PPHP TDI

But the correct TDI amount is 45.64 PPHP TDI.

That difference may look small, but chemically it matters.

The formulator believes the foam is running at Index 105. In reality, the actual index is lower because the reactive crosslinker was not included.

This can affect:

Foam hardness
Compression set
Recovery
Crosslink density
Aging behaviour
Batch consistency

At higher crosslinker levels, the error becomes much larger. A formula with 1.0 to 1.5 parts of crosslinker can shift several index points if the component is missed.

This is why every active hydrogen source must be included in the calculation.

Comparison graphic showing correct versus incorrect isocyanate index calculation when a crosslinker is excluded — Excluding a reactive crosslinker from the denominator causes the real running index to drift away from the intended target.

Common Signs of Index Calculation Problems

A wrong isocyanate index can create symptoms that look like other production problems.

Common signs include:

Foam consistently harder than target
Foam consistently softer than target
ILD variation between batches
Compression set failure
Poor resilience
Brittleness at higher index
Moisture sensitivity at lower index
Different foam properties on different machines
No clear improvement after catalyst or silicone adjustments

When foam properties are wrong but the process looks normal, the index calculation should be one of the first things checked.

Practical Rules for Production

Use these rules for safer formulation control:

Use water equivalent weight as 9, not 18. This is one of the most important calculation rules in PU foam.
Use actual %NCO from the Certificate of Analysis. Do not rely only on the TDS range.
Include every reactive component. Polyol, water, crosslinkers, chain extenders, and active hydrogen additives must be included.
Recalculate after every formulation change. Any change in water, polyol, crosslinker, chain extender, or isocyanate quality changes the index.
Do not treat index as a fixed recipe number. Index is a stoichiometric control parameter and must be managed like one.

Use the PolymerIQ Isocyanate Index Calculator

Manual calculations are useful for understanding the chemistry, but production teams need a fast way to verify formulas.

The PolymerIQ Isocyanate Index Calculator helps you check:

Polyol equivalent weight
Water contribution
Isocyanate equivalent weight
Crosslinker contribution
Required TDI or MDI parts
Actual running index

Use it to verify new formulations, check existing formula sheets, or audit production adjustments before they create quality problems.

Open the Isocyanate Index Calculator →

FAQs

What is the isocyanate index in polyurethane foam?

The isocyanate index is the ratio of actual NCO equivalents used in a formulation to the theoretical NCO equivalents required for stoichiometric balance, multiplied by 100. It is a control parameter that describes the chemical balance between NCO groups and all reactive hydrogen sources in the system.

Why is the equivalent weight of water 9 and not 18?

Water has a molecular weight of 18, but each water molecule has two reactive hydrogen atoms and consumes two NCO groups during the blowing reaction. So the equivalent weight is 18 ÷ 2 = 9 g/eq. Using 18 in the calculation cuts the water contribution in half and can shift the real index by many points.

What is the typical isocyanate index for flexible foam?

Flexible slabstock foam is commonly developed in the range of approximately Index 105 to 115, depending on the required hardness, density, resilience, and compression set performance. The exact target should be established through formulation trials and production validation, not selected from theory alone.

Should I use %NCO from the TDS or the Certificate of Analysis?

Always use the actual %NCO from the Certificate of Analysis for the specific drum or batch being used. The Technical Data Sheet typically shows a range, and using the range value instead of the actual COA value can introduce calculation errors when the batch %NCO sits at the edge of the range.

Do I need to include crosslinkers in the index calculation?

Yes. Crosslinkers, chain extenders, and any additive with active hydrogen functionality must be included in the reactive hydrogen total. Even small amounts (0.5 to 1.5 parts per hundred polyol) can shift the real index by several points if excluded.

What happens if I run a formula at exactly Index 100?

Index 100 represents theoretical stoichiometric balance, but in real foam chemistry, NCO groups are also consumed by secondary reactions (urea, urethane, atmospheric moisture, crosslinkers). Running at Index 100 can effectively pull the system below balance, leading to lower crosslink density, softer foam, and weaker aging performance.

How do I calculate polyol equivalent weight?

Polyol equivalent weight is calculated from the hydroxyl value: Equivalent Weight = 56,100 ÷ OH Value (mg KOH/g). For a polyol with OH value of 56 mg KOH/g, the equivalent weight is 56,100 ÷ 56 = 1,001.8 g/eq.

Why does my foam keep coming out harder than target even though the formula has not changed?

If the foam is consistently harder than expected and process variables are normal, the running index is likely higher than the formula sheet shows. Common causes include: a reactive component (such as a crosslinker) was added but not included in a fresh index calculation, the %NCO of the new isocyanate batch is higher than the previous one, or the water level was adjusted without recalculating the TDI quantity.

How often should I recalculate the index?

Every time any reactive component changes — water, polyol OH value, crosslinker, chain extender, or isocyanate %NCO. The isocyanate index is not a fixed recipe number and cannot be treated as one.

Key Takeaways

The isocyanate index is one of the most important control parameters in polyurethane foam formulation. It is not just a number written at the top of a formula sheet — it represents the chemical balance between NCO groups and all reactive hydrogen sources in the system.

The most important points are:

Index 100 means theoretical stoichiometric balance.
Flexible foam often runs above Index 100 because of secondary NCO reactions.
Water equivalent weight is 9, not 18.
Every reactive component must be included in the denominator.
Crosslinkers and chain extenders are not passive additives.
The %NCO should come from the Certificate of Analysis.
Any formulation adjustment requires a fresh index calculation.

If a foam plant is facing unexplained hardness, compression set, or batch variation problems, the isocyanate index calculation is one of the first places to investigate.

A small calculation error can silently create months of off-spec production.

Conclusion

If your formulation sheet has been adjusted over time without recalculating the isocyanate index, the number written on the sheet may no longer reflect production reality.

PolymerIQ can help review your formulation, check your index calculation, and identify whether stoichiometric imbalance is contributing to foam quality problems.

To get accurate support, please share:

Polyol grade and OH value
Water level and any other reactive components
Isocyanate type and %NCO from the Certificate of Analysis
Target index and observed foam properties (ILD, compression set, density)
Description of the quality issue you are facing

Contact PolymerIQ for a formulation audit →

April 28, 2026

Category: Formulation Guides

TDS %NCO vs CoA %NCO: Use the Drum Value in PU Foam

Introduction

What Is the Difference Between TDS %NCO and CoA %NCO?

Why the TDS %NCO Value Is Not a Formulation Input

How %NCO Controls Isocyanate Equivalent Weight

The Index Swing Caused by Using One TDI Value for Every Drum

How This Shows Up in Foam Quality

Why CoA Logging Builds Better Formulation Control

Correct Workflow: How to Use Drum CoA %NCO

Practical Decision Thresholds for %NCO Variation

Use the PolymersIQ NCO / TDI Index Calculator

FAQs

What is the difference between TDS %NCO and CoA %NCO?

Why shouldn’t I just use the TDS midpoint in my formula?

How much can the actual index swing due to drum-to-drum %NCO variation?

Can a drum within TDS specification still cause foam quality problems?

How do I calculate the index impact when %NCO changes?

Should I always recalculate the index when a new drum arrives?

What’s the benefit of logging CoA %NCO values over time?

Can supplier switches cause foam quality problems even with the same product grade?

How does this rule apply to MDI and polymeric MDI?

What’s the simplest QC change a foam plant can make to prevent this problem?

Key Takeaways

Conclusion

NCO Content in Isocyanate: What %NCO Means in PU Foam

Introduction

What Is NCO Content?

Why %NCO Matters in PU Foam Formulation

Isocyanate Equivalent Weight Formula

Worked Examples: TDI and MDI Equivalent Weight

Example 1: TDI 80/20

Example 2: MDI

How %NCO Changes Isocyanate Equivalent Weight

Why Every Drum Can Have a Different %NCO Value

1. Manufacturing batch variation

2. Moisture exposure

3. Storage temperature and aging

Why the Certificate of Analysis Matters

How %NCO Affects Foam Properties

Practical Rules for Using %NCO Correctly

Use the PolymerIQ NCO / TDI Index Calculator

FAQs

What is NCO content in polyurethane foam formulation?

How is isocyanate equivalent weight calculated?

What is the typical %NCO for TDI and MDI?

Why does %NCO vary between drums of the same product?

Should I use %NCO from the TDS or the Certificate of Analysis?

How does %NCO affect foam hardness?

Can moisture exposure really change %NCO?

What happens if I keep using the same %NCO value when the drum changes?

Should I recalculate the isocyanate index every time %NCO changes?

Does the same rule apply to TDI, MDI, and polymeric MDI?

Key Takeaways

Conclusion

How Water Level Effects PU Foam Properties

Introduction

Water Controls More Than Density

Property 1: Density

Property 2: Hardness / ILD

Property 3: Compression Set

Property 4: Exotherm

Why One Water Adjustment Moves Four Properties

Practical Water Adjustment Checklist

Example: A Density Fix That Creates Compression Set Risk

Use the PolymerIQ Calculators

FAQs

How does water level affect PU foam density?

Why does water affect foam hardness in two directions?

How does water level affect compression set?

Why does high water level increase exotherm risk?

Can a water reduction cause compression set failure?

Should I recalculate the isocyanate index every time I change water?

Why does the same water adjustment behave differently in different formulas?

What’s the maximum safe water level in flexible slabstock?

How do I troubleshoot foam that’s too soft after a water change?

What should I check before increasing water to lower density?

Key Takeaways

Conclusion

Why Water Equivalent Weight Is 9 in Polyurethane Foam