Tag: Raw Materials

Why Changing One PU Foam Raw Material Changes Everything

Introduction

PU foam raw material changes rarely affect only one property. A foam plant has a hardness drift. The engineer adjusts the catalyst. Hardness improves slightly, but compression set gets worse. The water level is reduced. Density changes, but the foam now feels different. Surfactant is increased to fix the new cell structure. The cells become tighter, but airflow drops.

Three changes, three new problems.

This pattern is not unusual. It is the result of treating a polyurethane foam formula as if each raw material can be adjusted independently. It cannot.

A polyurethane foam formula behaves as one connected system. Changing one raw material can move several properties at the same time. Polyol affects flexibility, hardness, network architecture, and isocyanate demand. Water affects density, urea hard segments, exotherm, and the index. Catalyst affects timing and the gel/rise balance. Surfactant affects cell size, cell opening, and rise stability. Crosslinker affects network density, hardness, and cell tightness.

These effects overlap. A single adjustment intended to fix one property often moves three or four others.

This article explains why single-variable troubleshooting often fails in PU foam, what a connected formula change actually looks like, and how to use a structured change-control checklist to avoid making one problem into several.

Why Single-Variable Troubleshooting Fails

In many engineering disciplines, single-variable adjustment works. Change the input, observe the output, refine. In polyurethane foam, that approach often fails because the inputs are not independent.

A foam formula is a chemical and physical system where:

Polyol provides the polymer backbone
Isocyanate reacts with polyol, water, and crosslinker
Water generates CO₂ and forms urea hard segments
Catalyst sets the relative timing of competing reactions
Surfactant stabilizes cells while the network forms
Crosslinker adds reactive equivalents and network junctions

Each material participates in multiple physical and chemical roles. When one is changed, the others must respond. The catalyst that worked at the original water level may not work at a higher water level. The surfactant that gave fine cells at the original index may give tight cells at a higher index. The DEOA dosage that supported compression set at the original isocyanate %NCO may overshoot if the new drum has higher %NCO.

This is why isolated adjustments produce unpredictable results. The variable being controlled is not the only thing that moved.

PU foam formula shown as interconnected system where each raw material affects multiple properties

What Each Raw Material Change Actually Does

A polyurethane foam formula behaves as one system. Changing one raw material can move several properties at once.

Change the polyol

You may change OHV, equivalent weight, functionality, viscosity, flexibility, compression set, network structure, and isocyanate demand. A polyol substitution at “the same OHV” can still change functionality and supplier-specific impurities, which means the network architecture is not actually the same.

Change the isocyanate

You may change %NCO, equivalent weight, the index calculation, cure speed, hardness, exotherm, and moisture sensitivity. A new isocyanate supplier with the same nominal grade can deliver a different actual %NCO from the CoA, shifting the running index without any visible formula change.

Change the water level

You may change density, CO₂ generation, urea content, hardness, exotherm, NCO demand, and compression set. Water has EW = 9 g/eq, so even a 0.3-part change is a meaningful stoichiometric movement, not a small adjustment.

Change the catalyst

You may change cream time, gel time, rise time, tack-free time, the gelling-versus-blowing balance, surface cure, and processing window. Catalyst does not change the index, but it can mask an index error temporarily by altering when the foam appears to set or open.

Change the surfactant

You may change cell size, cell opening, cell uniformity, foam stability, airflow, and surface quality. Surfactant changes do not affect the index, but they can mask a chemistry-driven cell problem if cell tightness is actually caused by over-crosslinking.

Change the crosslinker

You may change reactive equivalents, the index calculation, hardness, ILD, compression set, cell tightness, and network strength. DEOA’s low EW means small additions have large index impact — and the index must be recalculated every time crosslinker dosage moves.

Most raw material changes affect more than one property. That is the central reality of PU foam troubleshooting.

Single-Variable Failure: A Worked Example

Consider a flexible foam plant troubleshooting a hardness drift. The original formula targets ILD 40 N at 25 kg/m³ density.

Step 1. Hardness reads low. Engineer increases catalyst, expecting cure to improve. Cure improves slightly, but compression set increases (catalyst shifted gel/rise balance and changed cell opening).

Step 2. Engineer reduces water by 0.3 parts to firm up the foam. Density rises, but the actual running index drops because water reduction lowered reactive hydrogen equivalents while isocyanate parts stayed constant. Hardness moves the wrong way again.

Step 3. Engineer increases isocyanate to recover the index. Cure now overshoots, exotherm increases, and the foam shows scorch in larger blocks.

Step 4. Engineer increases surfactant to recover cell structure after the firmer foam. Cells become tight, airflow drops, and the foam fails ventilation testing.

Each adjustment was logical in isolation. The combined result is a formula that no longer matches any of the original targets.

The root cause may have been simple — for example, an isocyanate drum with a CoA %NCO 1.0 point lower than the formula assumed, shifting the actual running index from 105 to 102. A single index recalculation at the start would have solved it. Instead, four adjustments compounded into a worse formula.

This is the cost of single-variable troubleshooting in a connected system.

Single variable PU foam troubleshooting failure pattern showing four sequential adjustments compounding into worse formula

Practical Raw Material Change-Control Checklist

Before changing any raw material, ask the following questions. This checklist forces a structured review instead of an ad-hoc adjustment.

Raw Material Change	What Must Be Checked
Polyol grade change	OHV, EW, functionality, viscosity, index, compression set
Isocyanate supplier change	CoA %NCO, EW, index, moisture exposure, reaction profile
Water level change	Density, NCO demand, exotherm, urea formation, index
Catalyst change	Cream time, gel time, rise time, gel/rise gap
Surfactant change	Cell size, cell opening, airflow, collapse risk
Crosslinker change	EW, reactive equivalents, index, compression set, cell tightness

Do not approve raw material changes only by name, price, or TDS comparison. Use current CoA values where possible. Recalculate reactive components. Run a controlled trial. Record timing, density, hardness, cell structure, and recovery.

The Reactive vs Non-Reactive Discipline in Troubleshooting

When a foam quality problem appears, the first question is not “what should I change?” The first question is “is this a stoichiometric problem or a process problem?”

Stoichiometric / index symptoms

These suggest a problem in the reactive components — polyol, water, crosslinker, or isocyanate.

Hardness drift after a raw material change
Compression set failure that does not respond to catalyst adjustment
Cure inconsistency after a supplier change
Density-versus-hardness mismatch
Foam properties that drifted gradually over many production runs

Process / timing / cell-structure symptoms

These suggest a problem in catalyst, surfactant, mixing, temperature, or machine variables.

Cream/gel/rise timing drift
Surface defects, splits, or voids
Cell collapse or coarse cells
Fine cells with poor airflow
Rise instability or surface tackiness

These two symptom groups call for different troubleshooting pathways. Stoichiometric problems require recalculation of reactive equivalents and the index. Process problems require catalyst and surfactant review. Mixing the pathways — for example, changing surfactant when the real problem is index drift — produces the compounding pattern shown in the worked example above.

Two PU foam symptom groups showing stoichiometric problems versus process and cell structure problems

When to Change One Variable at a Time

Despite everything above, single-variable trials are still the right tool — when used in the right context.

Good single-variable use:

Validating a deliberate, calculated change (e.g., adjusting isocyanate by a known amount to recover index)
Confirming a hypothesis after stoichiometric review (e.g., testing if the problem is really catalyst, not chemistry)
Comparing two specific options (e.g., surfactant grade A vs grade B)
Designed-experiment scoping where one variable is controlled at a time

Poor single-variable use:

Reactive adjustments without prior calculation
Trying to fix a multi-symptom problem with one change
Adjusting a non-reactive component (catalyst, surfactant) to compensate for a reactive-component error
Substituting raw materials based only on TDS or product name

The difference is whether the adjustment is calculated and predicted, or whether it is a guess. Calculated adjustments make use of the connected nature of the formula. Guesses fight against it.

A Better Troubleshooting Workflow

When a foam quality problem appears, use this sequence instead of ad-hoc single-variable adjustments:

Define the symptom. What property is off, and by how much?
Group the symptom. Is it stoichiometric or process?
Verify the reactive components first. Polyol OHV (current), water level, crosslinker EW, isocyanate %NCO from CoA. Recalculate the index.
If the index is correct, review the process variables. Catalyst dosage, surfactant grade, gel/rise balance, mixing, temperature.
Make one calculated change. Predict the expected outcome. Run a controlled trial.
Measure all relevant properties, not just the one being targeted. Density, hardness, cell structure, recovery, timing.
Document the change and the result. This builds a formula history that supports future troubleshooting.

This workflow is slower than reactive adjustment, but it is reliable. It avoids the compounding failure pattern that comes from changing variables without calculation.

PU foam troubleshooting workflow with seven steps from symptom definition to documentation

Recognizing Compound Errors in Existing Formulas

Some formulas in production today are the result of years of single-variable adjustments. The current formula sheet may be the accumulated result of dozens of small changes, each made to fix a previous adjustment.

Signs that a formula has accumulated compound errors include:

Multiple historical revisions with no clear logic
Catalyst dosages that drifted higher over time
Surfactant levels that look unusually high or low
Crosslinker dosage that has been adjusted without index recalculation
Water level changes that were not paired with isocyanate changes
Index values that no longer match the equivalents column when recalculated
Foam properties that drift in ways that the formula sheet does not explain

When these signs appear, the most useful step is often a full formulation audit. Reset the formula by recalculating reactive components from current CoA values, identifying which historical adjustments were corrections to real problems and which were corrections to previous corrections. This usually simplifies the formula and improves consistency.

A formula does not need to be redesigned from scratch. It needs to be calibrated back to its actual chemistry.

Use the PolymersIQ Calculators

The PolymerIQ Equivalent Weight Calculator helps verify EW values for every reactive component before any formula change. Use it when reviewing a polyol grade switch, updating water level, adjusting crosslinker, or auditing an inherited formula.

Open the Equivalent Weight Calculator →

The PolymerIQ NCO / TDI Index Calculator helps confirm whether the formula is actually running at the intended index after a raw material change. Use it before approving any change to polyol, water, crosslinker, or isocyanate.

Open the NCO / TDI Index Calculator →

Water level and density move together. The PolymersIQ Foam Density Estimator helps predict density impact before a water-level change reaches production.

Open the Foam Density Estimator →

For the foundation article on the six raw materials, read The Six Raw Materials Behind Every Polyurethane Foam.

For the technical article on which materials enter the index, read Reactive vs Non-Reactive Components in PU Foam: Which Raw Materials Enter the Index Calculation.

For the article on water level effects, read How Water Level Affects PU Foam Density, Hardness, Exotherm, and Compression Set.

For the article on common water mistakes, read 4 Water Adjustment Mistakes That Affect PU Foam Properties.

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

For the article on common index calculation mistakes, read 5 Isocyanate Index Calculation Mistakes That Affect Foam Quality.

FAQs

Why does single-variable troubleshooting often fail in PU foam?

Because PU foam raw materials are not independent. Each component participates in multiple physical and chemical roles. Polyol affects flexibility, network architecture, and isocyanate demand. Water affects density, urea content, exotherm, and index. Catalyst affects timing and the gel/rise balance. When one variable is changed, the others must respond. The result is that an adjustment intended to fix one property often moves three or four others, sometimes in directions that compound the original problem.

How can a single isocyanate drum cause four troubleshooting steps?

A drum with CoA %NCO 1.0 point lower than the formula assumed shifts the actual running index downward — for example, from 105 to 102. The lower index produces softer foam. If the engineer reads this as a hardness or cure problem and starts adjusting catalyst, water, or surfactant, each adjustment introduces new side effects without addressing the actual root cause. A single index recalculation at the start would have identified the drum as the source.

What’s the difference between a calculated change and a reactive adjustment?

A calculated change is based on equivalent-weight math, index calculation, and predicted outcome. The formulator knows in advance what the change should do and verifies the result against the prediction. A reactive adjustment is a guess based on symptoms — increase catalyst because the foam is soft, decrease water because density is low. Reactive adjustments treat the formula as if each variable is independent, which produces the compounding pattern described in this article.

Should I always recalculate the index when changing water?

Yes. Water has EW = 9 g/eq, which means even a 0.3-part change is a meaningful stoichiometric movement. If isocyanate parts stay constant, the index will shift every time water changes. Recalculate the index before approving any water-level change, and adjust isocyanate quantity to maintain the target index unless the index shift is intentional.

What’s the most common compounding mistake in PU foam troubleshooting?

Increasing catalyst to compensate for a stoichiometric problem. The symptoms of an index error — soft foam, slow cure, compression set drift — overlap with timing problems, so it is tempting to adjust catalyst first. Catalyst can mask the symptoms briefly by changing when the foam appears to set, but it cannot fix the index. The underlying problem persists and often resurfaces after the next raw material change.

How do I tell whether a problem is stoichiometric or process-related?

Stoichiometric problems usually involve hardness drift, compression set failure, cure inconsistency, density-hardness mismatch, or gradual drift over many production runs after a raw material change. Process problems usually involve cream/gel/rise timing drift, surface defects, splits, voids, cell collapse, fine closed cells, or rise instability. Stoichiometric problems require reactive-component review and index recalculation. Process problems require catalyst, surfactant, mixing, and temperature review.

Can a formulation accumulate errors over years of single-variable changes?

Yes. Many production formulas in active use today are the result of years of small adjustments, where each change was made to fix the previous change rather than the original problem. The accumulated formula often contains catalyst dosages that drifted higher, surfactant levels that look unusual, crosslinker adjustments without corresponding index recalculations, and an index that no longer matches the equivalents column when verified. A formulation audit can identify and unwind these compound errors.

When is single-variable adjustment actually appropriate?

When the change is calculated, predicted, and used to validate a hypothesis. Examples: adjusting isocyanate by a known amount to recover a known index shift; testing surfactant grade A vs grade B at a fixed formula; comparing two catalyst options under controlled conditions. Single-variable adjustment is a tool — it works when used to test a specific question, not as a general approach to troubleshooting unknown problems.

How does the change-control checklist prevent compounding errors?

The checklist forces a structured review before any raw material change. It requires checking the reactive components first (polyol OHV, water EW, crosslinker EW, isocyanate %NCO), recalculating the index, and predicting the expected outcome of the change. This catches the most common cause of compounding — making a non-reactive adjustment (catalyst, surfactant) to fix a reactive-component problem. By forcing the index check up front, the checklist routes problems to the correct troubleshooting pathway.

What should I do if I inherited a formula that already has compound errors?

Run a formulation audit. Recalculate equivalent weights for all reactive components using current CoA values, verify the index against the formula sheet, and review the historical change log if available. Identify which adjustments were corrections to real problems and which were corrections to previous corrections. The goal is not to redesign the formula from scratch, but to calibrate it back to its actual chemistry. This often simplifies the formula, improves consistency, and reveals the original root cause that the compounding adjustments were trying to address.

Key Takeaways

A polyurethane foam formula is a connected system. Changing one raw material can move several properties at once.

Polyol changes affect OHV, functionality, network, viscosity, and isocyanate demand.
Isocyanate changes affect %NCO, EW, index, cure, and exotherm.
Water changes affect density, urea content, hardness, exotherm, and index.
Catalyst changes affect cream/gel/rise timing and the gel/rise balance.
Surfactant changes affect cell size, cell opening, and rise stability.
Crosslinker changes affect reactive equivalents, index, hardness, and cell tightness.

Single-variable troubleshooting often fails because the variables are not independent. Sequential ad-hoc adjustments compound into formulas that no longer match the original targets.

The discipline that prevents compounding is the reactive vs non-reactive line:

Stoichiometric problems → review polyol, water, crosslinker, isocyanate, and the index.
Process problems → review catalyst, surfactant, gel/rise balance, and mixing.

Use a structured workflow: define symptom, group it, verify reactive components first, then process variables, make one calculated change, measure all relevant properties, and document the result.

Inherited formulas with years of single-variable adjustments may contain accumulated compound errors. A formulation audit calibrates the formula back to its actual chemistry, which often simplifies the system and reveals the root cause that the compounding adjustments were trying to address.

Conclusion

If your foam quality problem has resisted multiple adjustments, the issue may not be one raw material in isolation — it may be the interaction between polyol, isocyanate, water, catalyst, surfactant, and crosslinker, and possibly the accumulation of compound errors from past troubleshooting.

PolymersIQ can help review your formula at the system level: which adjustments addressed the real cause, which created new side effects, and how to calibrate the formula back to its actual chemistry.

To get accurate support, please share:

Polyol grade, OHV, functionality, and supplier
Isocyanate type and current CoA %NCO
Water level and recent changes
Catalyst package and dosages
Surfactant grade and level
Crosslinker type and dosage
Target foam properties (density, hardness, compression set)
Historical formula adjustments and the order they were made
Description of the persistent issue

Contact PolymerIQ for a formulation audit →

May 12, 2026

Reactive vs Non-Reactive Components in PU Foam

Introduction

Reactive components in polyurethane foam are the raw materials that chemically enter the isocyanate index calculation. A polyurethane foam formula contains six raw-material families, but not all of them belong in the same calculation.

This is one of the most important distinctions in PU foam formulation: some raw materials enter the isocyanate index calculation, and some do not.

The reactive components — polyol, water, crosslinker, and isocyanate — contribute reactive equivalents to the stoichiometric balance of the foam. Their parts, equivalent weights, and equivalents must be tracked carefully because every change to one of them can shift the index.

The non-reactive components — catalyst and surfactant — control reaction timing and cell structure. They are essential for foam quality, but they do not contribute reactive equivalents. They cannot fix a wrong index, a wrong water level, or a wrong %NCO.

When this distinction is not understood, troubleshooting often goes in the wrong direction. A formulator may try to fix a hardness problem by adjusting catalyst, when the actual cause is index drift. A production team may increase surfactant when the real problem is a missing crosslinker correction. Catalyst and surfactant get blamed for problems they cannot solve.

This article explains how each of the four key non-foundation components — water, catalyst, surfactant, and crosslinker — fits into the formulation calculation, and why the reactive vs non-reactive line is the most important distinction in PU foam stoichiometry.

The Reactive vs Non-Reactive Line

The line between reactive and non-reactive components is set by chemistry. A material is reactive in PU foam stoichiometry if it contributes hydroxyl groups, amine hydrogens, water hydrogens, or NCO groups that participate in the polyurethane reaction.

Component	Enters Index Calculation?	Why
Polyol	Yes	Contains reactive OH groups
Isocyanate	Yes	Contains reactive NCO groups
Water	Yes	Reacts with NCO and contributes reactive hydrogen equivalents
Crosslinker	Yes	Contains reactive OH or amine groups
Catalyst	Normally no	Controls reaction speed, not stoichiometric equivalents
Surfactant	Normally no	Controls cell structure, not index

This table is the single most useful tool for separating two different types of foam troubleshooting:

Stoichiometric / index troubleshooting — review polyol, water, crosslinker, and isocyanate. Recalculate equivalents. Verify the index.
Timing and cell-structure troubleshooting — review catalyst balance and surfactant package. Adjust gel/rise timing. Tune cell size and cell opening.

These are different problems with different solutions. Mixing them — for example, adjusting catalyst to fix a stoichiometric error — does not work.

Water: A Reactive Component, Not Just a Blowing Agent

Water is one of the smallest ingredients by weight in many flexible foam formulas. But chemically, it is one of the most powerful.

Water is the main chemical blowing agent in many flexible polyurethane foam systems. It reacts with isocyanate to generate CO₂ gas. That CO₂ expands the foam and creates the cellular structure.

But water does more than reduce density. Water affects four major properties at the same time:

Density
Hardness
Exotherm
Compression set behavior

The reason is that water has two linked roles. First, it reacts with NCO to generate CO₂. Second, the reaction forms amine, which reacts with more NCO to create urea linkages. Those urea linkages act as hard segments in the foam structure.

That is why changing water level is not just a density adjustment. It changes the chemistry.

Water has a very low equivalent weight:

Water EW = 9 g/eq

That means a small amount of water can contribute a large share of reactive hydrogen equivalents in a flexible foam formula. In one common flexible foam example, water may be only a few percent by weight but contribute the majority of reactive hydrogen equivalents.

This is why every water change must be recalculated.

Water Increase Usually Causes	Why
Lower density	More CO₂ generation
Higher urea content	More water-NCO reaction
Higher exotherm	More reactive heat generation
Higher NCO demand	More reactive hydrogen equivalents
Possible scorch risk at high levels	Higher internal heat in large blocks
Compression set change	Urea network and balance shift

Water is not only a blowing agent. It is a reactive component, and it must enter the index calculation alongside polyol and crosslinker.

Water as reactive component in polyurethane foam contributing CO2 urea hard segments and equivalent hydrogen

Catalyst: The Timing Controller, Not an Index Component

Catalyst controls reaction timing. It does not change the stoichiometric ratio of the formula.

This distinction is critical. A catalyst changes how fast reactions happen. It does not correct wrong equivalent weight, wrong water level, wrong %NCO, or wrong index.

In polyurethane foam, catalyst balance controls cream time, gel time, rise time, tack-free time, gelling reaction speed, blowing reaction speed, surface cure, foam stability, and processing window.

There are two major catalyst categories in flexible foam:

Amine catalysts

Amine catalysts can accelerate the blowing reaction, the gelling reaction, or both, depending on the grade.

Blowing amines push the water-isocyanate reaction and move cream and rise behavior.
Gelling amines support urethane formation and viscosity build.

Tin catalysts

Tin catalysts mainly accelerate the gelling reaction. They help move gel time earlier and build network strength faster. This gives formulators another way to adjust the gel/rise balance.

Catalyst does not enter the index calculation in normal PU foam formulation. It is still extremely important:

If gelling runs too slowly, the foam may rise before the network can hold it.
If gelling runs too fast, the foam may lock before full expansion.
If blowing runs too fast, collapse risk increases.
If blowing runs too slowly, the foam may under-rise or become tight.

The best catalyst adjustment starts with timing data. Do not adjust catalyst before checking index, water level, equivalent weight, raw material temperature, and gel/rise balance.

Catalyst controlling polyurethane foam cream time gel time rise time and reaction balance

Surfactant: The Cell Structure Architect, Not an Index Component

Surfactant controls foam cell structure. In polyurethane foam, the surfactant is usually a silicone-based additive. It stabilizes bubbles while the foam rises.

Without surfactant, CO₂ bubbles can merge, rupture, or escape before the polymer network has enough strength to hold the foam. That can lead to collapse, coarse cells, holes, or irregular structure.

Surfactant controls three key cell variables: cell size, cell uniformity, and cell opening.

Too little surfactant can cause:

Collapse
Coarse cells
Voids
Poor rise stability
Irregular cell structure
Weak surface quality

Too much surfactant can cause:

Very fine cells
Closed cells
Poor airflow
Higher apparent tightness
Dense or poorly ventilated foam
Comfort performance problems

Surfactant grade selection matters. A slabstock surfactant is not automatically suitable for molded foam. A rigid foam surfactant is not automatically suitable for flexible foam. A spray foam surfactant is chosen for a different processing and cell-stability requirement.

Surfactant normally does not enter the index calculation. It changes cell structure, foam stability, and surface behavior. That makes it different from reactive components such as polyol, water, crosslinker, and isocyanate.

Surfactant is invisible when it works. It is obvious when it fails.

Silicone surfactant controlling cell size cell uniformity and cell opening in polyurethane foam

Crosslinker: A Reactive Component That Must Be Counted

A crosslinker is a small reactive molecule that adds network junctions. It usually contains multiple hydroxyl or amine groups. Because it has reactive groups, it must be included in equivalent and index calculations.

Crosslinker can improve:

Hardness
ILD
Compression set
Load-bearing
Network strength
Cure response
Dimensional stability

The most common flexible foam crosslinker is DEOA (diethanolamine). DEOA has three reactive groups (two OH and one NH). Its equivalent weight is commonly calculated as:

DEOA EW = 105.14 ÷ 3 = 35.0 g/eq

This matters because using the wrong EW creates an index error. If a formulator counts only the two OH groups and uses EW = 52.6 instead of 35.0, the index calculation under-counts DEOA’s contribution and the actual running index drifts.

Crosslinker is powerful because small additions can change network architecture. Typical flexible foam dosage may be low, but the effect can be meaningful because the equivalent weight is low.

However, crosslinker has a limit. Too little crosslinker may not give enough network support. Too much crosslinker can make the network too tight.

Over-crosslinking can cause:

Tight cells
Reduced elasticity
Harsh feel
Poor recovery
Higher compression set in some systems
Processing sensitivity
Cell structure problems

A crosslinker is not just an additive. It is a reactive component. Every crosslinker addition should be recalculated in the formula.

Crosslinker DEOA in PU foam adding network junctions and contributing to the index calculation

How These Components Connect to the Index Calculation

The isocyanate index is calculated from reactive equivalents only.

Index = NCO equivalents ÷ Total reactive H equivalents × 100

Where total reactive H equivalents is the sum of:

Polyol equivalents (parts ÷ polyol EW)
Water equivalents (parts ÷ 9)
Crosslinker equivalents (parts ÷ crosslinker EW)
Any other reactive hydrogen contributors (chain extenders, amine modifiers, reactive flame retardants if present)

Catalyst and surfactant do not appear anywhere in this calculation.

That means:

Adding more catalyst does not change the index — only the timing.
Adding more surfactant does not change the index — only the cell structure.
But adding more water, more crosslinker, more polyol, or changing the isocyanate level always changes the index.

A common mistake is treating catalyst dosage as a control variable for cure or hardness. Cure is partly a timing issue (which catalyst affects) but also an index issue (which catalyst does not affect). Hardness is partly a network density issue (which involves polyol functionality, water, and crosslinker — all reactive) and partly a cell-structure issue (which surfactant affects). Mixing the two leads to long, frustrating troubleshooting cycles.

Two Different Troubleshooting Pathways

Once the reactive vs non-reactive line is clear, foam quality problems can be sorted into two pathways:

Pathway 1: Stoichiometric / Index Troubleshooting

Symptoms include hardness drift, compression set failure, cure inconsistency, density-vs-hardness mismatch, and unexpected foam behavior after a raw material change.

Variables to check:

Polyol OHV (current, not historical)
Polyol functionality
Water level (and whether it has drifted)
Crosslinker dosage and EW
Isocyanate %NCO (from CoA, not TDS)
Calculated index vs target index
Equivalent weight values for every reactive component

Pathway 2: Timing and Cell-Structure Troubleshooting

Symptoms include cream/gel/rise timing drift, surface defects, splits, voids, collapse, coarse cells, fine cells, poor airflow, or rise instability.

Variables to check:

Amine catalyst type and dosage
Tin catalyst dosage and balance with amine
Gel/rise balance
Silicone surfactant grade and dosage
Mix temperature and raw material temperature
Mixing efficiency

These pathways are different because they correspond to different chemistry. Use the right one based on the symptom. Catalyst and surfactant changes will not solve a stoichiometric problem. Index recalculation will not solve a cell-collapse problem.

Two PU foam troubleshooting pathways stoichiometric index versus timing and cell structure

Practical Component Reference

This table consolidates the role of each non-foundation component for quick reference during formulation review.

Component	Type	EW	Index Role	Main Property Controlled
Water	Reactive	9 g/eq	Yes — enters index	Density, urea, hardness, exotherm
Crosslinker (DEOA)	Reactive	35.0 g/eq	Yes — enters index	Network junctions, hardness, compression set
Amine catalyst	Non-reactive	—	No	Cream/gel/rise timing, blowing-gelling balance
Tin catalyst	Non-reactive	—	No	Gel time, network strength build
Silicone surfactant	Non-reactive	—	No	Cell size, cell uniformity, cell opening

The four reactive components (polyol, water, crosslinker, isocyanate) together define the chemistry. The two non-reactive components (catalyst, surfactant) together define the process and structure. Both are essential. Neither group can substitute for the other.

PU foam component reference table showing reactive and non-reactive components with EW and index role

Use the PolymersIQ Calculators

Equivalent weight values are the foundation of correct index calculation. The PolymersIQ Equivalent Weight Calculator helps verify EW for water (9), DEOA (35.0), polyol (56,100 ÷ OHV), and isocyanate (4,200 ÷ %NCO). Use it when reviewing crosslinker dosage, updating water level, or verifying that the formula sheet uses correct EW values.

Open the Equivalent Weight Calculator →

The PolymerIQ NCO / TDI Index Calculator helps confirm the index calculation includes all reactive components. Use it when changing water level, adjusting crosslinker dosage, switching isocyanate grade, or auditing whether catalyst and surfactant adjustments have masked an index error.

Open the NCO / TDI Index Calculator →

Water level and foam density are tightly connected. Use the PolymersIQ Foam Density Estimator when changing water level, reviewing density targets, or checking density impact before a production trial.

Open the Foam Density Estimator →

For the foundation article on the six raw materials, read The Six Raw Materials Behind Every Polyurethane Foam.

For the troubleshooting article on raw material interactions, read Why Changing One PU Foam Raw Material Changes the Whole Formula.

For the deep guide on water, read The Dual Role of Water in Polyurethane Foam: Blowing Agent and Urea Network Builder.

For the article on water level effects, read How Water Level Affects PU Foam Density, Hardness, Exotherm, and Compression Set.

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

For the formulation sheet guide, read How to Read a Polyurethane Formulation Sheet.

FAQs

Which PU foam raw materials enter the isocyanate index calculation?

Polyol, water, crosslinker, and isocyanate enter the index calculation because they contribute reactive equivalents — hydroxyl groups (polyol, crosslinker), water hydrogens, amine hydrogens (DEOA, amine modifiers), or NCO groups (isocyanate). Catalyst and surfactant normally do not enter the index calculation. They control reaction timing and cell structure but do not contribute reactive equivalents.

Why is water considered a reactive component and not just a blowing agent?

Water reacts directly with isocyanate (NCO + H₂O → CO₂ + amine), and the amine reacts again with NCO to form urea linkages. Both reactions consume NCO equivalents. Water has an equivalent weight of 9 g/eq — far lower than polyol — so even a few parts of water can contribute the majority of reactive hydrogen equivalents in a flexible foam formula. Every water-level change must be recalculated in the index.

Why doesn’t catalyst affect the isocyanate index?

Catalyst accelerates chemical reactions but does not contribute the OH, NH, or NCO groups that the index calculation tracks. Amine catalysts and tin catalysts change reaction speed and gel/rise balance, but they do not provide reactive equivalents. Catalyst is essential for foam quality and timing, but it cannot fix a wrong index, wrong water level, or wrong %NCO. The most common formulation mistake is increasing catalyst to compensate for a stoichiometric problem.

What is DEOA’s equivalent weight and why does it matter?

DEOA (diethanolamine) has a molecular weight of 105.14 g/mol and three reactive hydrogens — two OH groups and one NH group. Its equivalent weight is 105.14 ÷ 3 = 35.0 g/eq. Using the correct EW matters because if a formulator counts only the two OH groups, EW becomes 52.6 instead of 35.0, and the index calculation under-counts DEOA’s reactive contribution. The actual running index then drifts away from the target.

Can I fix a hardness problem by adjusting the catalyst package?

Hardness has both a chemistry component (network density, polyol functionality, water level, crosslinker) and a process component (cure timing, cell structure). Catalyst affects timing and cure, so it can change apparent hardness slightly. But if the underlying network is wrong — wrong index, low functionality, missing crosslinker contribution — catalyst tweaks treat the symptom while leaving the cause in place. Always check stoichiometry first.

How does surfactant affect foam cell structure if it doesn’t react?

Silicone surfactant works by reducing surface tension and stabilizing the polymer-air interface during foam rise. It is physically active but chemically inert in the polyurethane reaction. Too little surfactant and bubbles merge or rupture (collapse, voids, coarse cells). Too much surfactant and bubbles become very small and remain closed (fine cells, poor airflow). The surfactant grade must match the application — slabstock, molded, rigid, or spray foam each require different surfactants.

Should I include catalyst or surfactant when calculating the formula’s reactive equivalents?

No. The formal index calculation includes only reactive components — polyol, water, crosslinker, and isocyanate. Catalyst and surfactant should be listed in the formula sheet (with parts and weight percentage) but excluded from the equivalents and equivalent percentage columns. This separation makes the formula sheet self-verifying: if the calculated index from reactive components alone matches the target index, the stoichiometry is correct.

What’s the difference between stoichiometric troubleshooting and process troubleshooting?

Stoichiometric troubleshooting focuses on reactive components: polyol OHV, water level, crosslinker EW, isocyanate %NCO, and the index calculation. It addresses problems like hardness drift, compression set failure, cure inconsistency, and unexpected behavior after a raw material change. Process troubleshooting focuses on non-reactive components and machine variables: catalyst type and dosage, surfactant grade and dosage, gel/rise balance, mixing, and temperature. It addresses problems like timing drift, surface defects, splits, voids, and cell-structure issues. Different symptoms point to different pathways.

What happens if I increase water without adjusting isocyanate?

The actual running index drops. Water has EW = 9 g/eq, so each part of water added increases reactive hydrogen equivalents substantially. If the isocyanate level stays the same, NCO equivalents are now divided by a larger reactive H denominator — the index falls. The foam may run at a lower index than the formula sheet states, with possible consequences for hardness, cure, and compression set. Every water change requires an isocyanate adjustment to maintain the target index.

Why is this distinction so important for PU foam formulation?

Because most foam quality problems are misdiagnosed when this line is unclear. A formulator who does not separate reactive from non-reactive components may keep adjusting catalyst or surfactant for a stoichiometric problem, getting partial improvement but never solving it. Recognizing that polyol, water, crosslinker, and isocyanate are the four index components — and that catalyst and surfactant control different things — is the foundation of disciplined PU foam troubleshooting.

Key Takeaways

The isocyanate index calculation includes only reactive components:

Polyol — provides OH groups
Water — provides reactive hydrogen (EW = 9)
Crosslinker (DEOA) — provides reactive OH and NH groups (EW = 35.0)
Isocyanate — provides NCO groups

Catalyst and surfactant normally do not enter the index calculation. They control reaction timing and cell structure but do not contribute reactive equivalents.

This distinction creates two different troubleshooting pathways:

Stoichiometric / index troubleshooting — review polyol, water, crosslinker, isocyanate, and the index calculation.
Timing and cell-structure troubleshooting — review catalyst balance and surfactant package.

Mixing the two pathways — for example, adjusting catalyst to fix a stoichiometric error — wastes time and produces inconsistent results.

Crosslinker must always be included in the index calculation. Its low equivalent weight (35.0 g/eq for DEOA) means small dosage changes have meaningful index impact.

Water must always be included in the index calculation. Its very low equivalent weight (9 g/eq) means it dominates the reactive hydrogen side of the formula even at low parts.

A correct formulation sheet treats reactive and non-reactive components differently — both are essential, but neither can substitute for the other.

Conclusion

If your foam quality problem has resisted catalyst and surfactant adjustments, the cause may be in the reactive components — water level drift, missing crosslinker contribution, wrong polyol EW, or outdated isocyanate %NCO.

PolymesrIQ can help review which components are entering your index calculation correctly, verify equivalent weights, and identify whether catalyst or surfactant changes have masked a stoichiometric error.

To get accurate support, please share:

Polyol grade, OHV, and supplier
Isocyanate type and current CoA %NCO
Water level (current and recent)
Crosslinker type, dosage, and EW used in the formula
Catalyst package and dosages
Surfactant grade and level
Target and observed foam properties
Description of the issue and adjustments already tried

Contact PolymerIQ for a reactive component audit →

May 11, 2026

PU Foam Raw Materials: The Six Families Explained

Introduction

PU foam raw materials are the foundation of every polyurethane foam formulation. Whether the final product is a soft mattress foam or a rigid insulation foam, the performance starts with the same six material families.

Six raw materials can create millions of different polyurethane foams.

The foam in a luxury mattress and the foam insulating a cold storage warehouse do not look the same. One is soft and flexible. The other is rigid and closed-cell. One is designed for comfort. The other is designed to stop heat transfer.

But the raw-material architecture is similar.

Most polyurethane foam systems are built from six main material families:

Polyol
Isocyanate
Water
Catalyst
Surfactant
Crosslinker

What makes one foam different from another is not only which ingredients are present. It is the grade of each ingredient, the amount used, the equivalent weight, the functionality, the index, the catalyst balance, the surfactant package, and the processing window.

This article covers the foundation: what each of the six raw materials is, what each one does, and how the two main reactive components — polyol and isocyanate — build the polyurethane foam network. The technical distinctions between reactive and non-reactive components, and how raw materials interact in production troubleshooting, are covered in the next articles in this series.

The Six Raw Materials in a PU Foam Formula

A polyurethane foam formula is not just a list of ingredients. It is a connected chemical and physical system.

Each raw material has a different job.

Raw Material	Main Function	Main Property It Controls
Polyol	Builds the polymer backbone	Foam type, flexibility, rigidity, compression set, durability
Isocyanate	Reacts with OH and water	Index, network formation, hardness, cure, exotherm
Water	Chemical blowing agent	Density, CO₂ generation, urea formation, hardness, exotherm
Catalyst	Controls reaction speed	Cream time, gel time, rise time, cure timing
Surfactant	Stabilizes cells	Cell size, cell uniformity, cell opening, collapse resistance
Crosslinker	Adds network junctions	Hardness, compression set, load-bearing, network strength

The first three materials define most of the chemical foundation: polyol, isocyanate, and water. Crosslinker is also a reactive component when present. These components must be included in equivalent and index calculations.

Catalyst and surfactant are different. Catalyst controls timing. Surfactant controls foam stability and cell structure. They are critical, but they normally do not contribute reactive equivalents to the index calculation.

This article focuses on the two main reactive partners — polyol and isocyanate — and provides a brief introduction to the remaining four. Each receives deeper treatment in the next articles in this sub-cluster.

Overview of six polyurethane foam raw materials and the foam properties each one controls

1. Polyol: The Foundation of the Foam Network

Polyol is the foundation of polyurethane foam.

It is a molecule with multiple hydroxyl groups that react with isocyanate to form urethane linkages. These linkages build the polymer network that defines whether the foam is flexible, rigid, soft, firm, elastic, brittle, durable, or weak.

Polyol is usually the largest component by weight in flexible foam formulas. That is why polyol selection has such a large effect on final foam performance.

Polyol controls:

Foam type
Flexibility / rigidity
Softness / hardness potential
Compression set
Recovery
Network durability
Load-bearing behavior
Hydrolysis resistance
Processing viscosity

The first key polyol number is OHV (hydroxyl value). OHV tells you how many reactive hydroxyl groups are present per gram of polyol.

Higher OHV usually means shorter chain length, lower equivalent weight, and a more rigid network.
Lower OHV usually means longer chain length, higher equivalent weight, and a more flexible foam.

Foam Type	Typical Polyol OHV Direction
Flexible foam	Lower OHV, often around 28–56 mg KOH/g
Rigid foam	Higher OHV, often around 300–600 mg KOH/g

The second key polyol number is functionality. Functionality tells you how many reactive OH groups exist per molecule.

Two polyols can have the same OHV but different functionality. That means the index calculation may look similar, but the foam network can behave differently.

This is why polyol cannot be substituted based only on OHV. A polyol change is a network change.

Polyol OHV and functionality controlling polyurethane foam network architecture

2. Isocyanate: The Reactive Partner That Makes the Foam

Isocyanate is the reactive partner in polyurethane foam. It contains NCO groups that react with hydroxyl groups from polyol, with water, and with reactive crosslinkers. Without isocyanate, the foam does not form.

Isocyanate participates in several reactions at the same time:

NCO + polyol OH forms urethane linkages
NCO + water generates CO₂ and amine
Amine + NCO forms urea linkages
NCO + crosslinker creates additional network junctions

These reactions compete for the same NCO pool. That is why isocyanate level is controlled through the isocyanate index.

The isocyanate index compares NCO equivalents to reactive hydrogen equivalents:

Index 100 = stoichiometric balance

Index 105 = 5% excess NCO

The index affects hardness, cure, network formation, compression set, exotherm, cell stability, and final properties.

The key isocyanate specification is %NCO. For TDI, a common value is around 48.3% NCO.

Isocyanate EW = 4,200 ÷ %NCO

For 48.3% NCO: EW = 4,200 ÷ 48.3 = 86.96 g/eq

Use g/eq, not g/mol, when using equivalent weight in formulation calculations. The %NCO value should be taken from the current CoA whenever possible, not blindly from an old TDS.

Isocyanate is also moisture sensitive. If an isocyanate drum is exposed to humidity, NCO groups can react with water. Over time, this can reduce active %NCO and shift the real index, depending on exposure conditions, humidity, time, and handling.

A formula does not run at the intended index. It runs at the actual index created by the raw materials on the production floor.

Isocyanate NCO reactions with polyol water and crosslinker in polyurethane foam

3. Water (Brief Introduction)

Water is one of the smallest ingredients by weight in many flexible foam formulas. But chemically, it is one of the most powerful.

Water is the main chemical blowing agent in many flexible polyurethane foam systems. It reacts with isocyanate to generate CO₂ gas, which expands the foam and creates the cellular structure.

Water has a very low equivalent weight:

Water EW = 9 g/eq

That means a small amount of water can contribute a large share of reactive hydrogen equivalents in a flexible foam formula. Water is treated in detail in the next article in this sub-cluster, where its dual role as blowing agent and urea network builder is explained.

4. Catalyst (Brief Introduction)

Catalyst controls reaction timing. It does not change the stoichiometric ratio of the formula.

In polyurethane foam, catalyst balance controls cream time, gel time, rise time, tack-free time, and the gelling-versus-blowing balance. Two main categories are used: amine catalysts (which can favour blowing or gelling depending on grade) and tin catalysts (which mainly accelerate gelling).

Catalyst does not enter the index calculation in normal PU foam formulation. It is critical for foam quality but cannot fix wrong stoichiometry. Catalyst is covered in detail alongside water in the next article.

5. Surfactant (Brief Introduction)

Surfactant controls foam cell structure. In polyurethane foam, the surfactant is usually a silicone-based additive that stabilizes bubbles while the foam rises.

Without surfactant, CO₂ bubbles can merge, rupture, or escape before the polymer network has enough strength to hold the foam — leading to collapse, coarse cells, or holes. Surfactant controls cell size, cell uniformity, and cell opening.

Surfactant normally does not enter the index calculation. Surfactant grade selection and dosage limits are covered alongside crosslinker in the next article.

6. Crosslinker (Brief Introduction)

The most common flexible foam crosslinker is DEOA (diethanolamine). DEOA has three reactive groups, giving an equivalent weight of 35.0 g/eq:

DEOA EW = 105.14 ÷ 3 = 35.0 g/eq

Crosslinker is reactive, low in equivalent weight, and powerful even at small dosages. It is treated in detail alongside surfactant in the next article in this series.

Polyol and isocyanate as the reactive pair building the polyurethane foam polymer network

How Polyol and Isocyanate Build the Foam Network

Polyol and isocyanate are the two main reactive partners. The polymer network is built from urethane linkages formed when polyol OH groups react with isocyanate NCO groups.

The basic reaction:

Polyol OH + Isocyanate NCO → Urethane linkage

When this reaction repeats across thousands of molecules, the result is a three-dimensional polymer network — the polyurethane backbone of the foam.

The character of that backbone depends on:

Polyol OHV — controls equivalent weight and isocyanate demand
Polyol functionality — controls how many connection points each molecule contributes
Isocyanate %NCO — controls how many reactive NCO groups are available per gram
Isocyanate index — controls whether NCO is in stoichiometric balance, deficit, or excess
Polyol type (polyether or polyester) — affects hydrolysis resistance, durability, and feel

A correctly designed reactive pair gives the foam its fundamental performance: flexibility or rigidity, resilience, hardness potential, compression set behavior, and durability. A mismatched pair — wrong OHV for the application, wrong index, or incompatible functionality — limits everything else, regardless of catalyst, surfactant, or process tuning.

This is why polyol and isocyanate are reviewed first when reformulating, qualifying a new product, or troubleshooting persistent foam quality problems.

Polyol and Isocyanate at a Glance

Specification	Polyol	Isocyanate
Reactive group	Hydroxyl (OH)	Isocyanate (NCO)
Key spec value	OHV (mg KOH/g)	%NCO
Equivalent weight formula	EW = 56,100 ÷ OHV	EW = 4,200 ÷ %NCO
Architecture variable	Functionality (OH per molecule)	Functionality (NCO per molecule)
Index role	Provides reactive H equivalents	Provides NCO equivalents
Storage sensitivity	Hygroscopic in some grades	Strongly moisture sensitive
Common grades	Polyether triol, polymer polyol, polyester polyol	TDI 80/20, MDI, polymeric MDI
Source of variation	OHV drift, functionality, supplier change	%NCO drift, moisture exposure, supplier change

The two columns share a structure: both have an equivalent weight formula tied to a key specification (OHV or %NCO), both have a functionality value, and both contribute to the index calculation. Reading them as a paired system is more accurate than reading them in isolation.

Side-by-side comparison of polyol and isocyanate specifications including OHV NCO content equivalent weight and functionality

Why Polyol-Isocyanate Selection Matters Most

Polyol and isocyanate together represent most of the formula by weight, and they define almost every foam property that depends on the polymer backbone.

A polyol change can affect:

Foam flexibility or rigidity
Hardness potential
Compression set
Recovery and resilience
Hydrolysis resistance
Processing viscosity
Cell wall strength
Long-term durability

An isocyanate change can affect:

Reaction rate
Cure profile
Hardness
Index calculation
Exotherm
Moisture sensitivity
Compatibility with catalyst package
Network completeness

When both move together — for example, switching to a different polyol grade with a new isocyanate supplier at the same time — production troubleshooting becomes very difficult because the variables are entangled.

The rule is simple: change one major raw material at a time, and verify the index, equivalent weight, and foam performance before changing another.

Use the PolymerIQ Calculators

Polyol and isocyanate calculations depend on accurate specification values. The PolymerIQ Equivalent Weight Calculator helps verify polyol EW from current OHV and isocyanate EW from current %NCO. Use it when reviewing a new grade, comparing CoA values, updating raw material data, or auditing a formulation sheet.

Open the Equivalent Weight Calculator →

The PolymerIQ NCO / TDI Index Calculator helps confirm whether the polyol-isocyanate pair is balanced at the intended index. Use it when changing polyol OHV, switching isocyanate supplier, updating %NCO from CoA, or validating actual index against target index.

Open the NCO / TDI Index Calculator →

For the technical article on which raw materials enter the index calculation, read Reactive vs Non-Reactive Components in PU Foam: Which Raw Materials Enter the Index Calculation.

For the troubleshooting article on raw material interactions, read Why Changing One PU Foam Raw Material Changes the Whole Formula.

For the deep guide on hydroxyl value and equivalent weight, read Hydroxyl Value in Polyurethane Foam: What OHV Means and How to Calculate Equivalent Weight.

For the foundation article on NCO content, read NCO Content in Isocyanate: What %NCO Means in PU Foam Formulation.

For the polyol functionality guide, read Polyol Functionality in Polyurethane Foam: What It Means and Why It Matters.

For the formulation sheet guide, read How to Read a Polyurethane Formulation Sheet.

FAQs

What are the six main raw materials in polyurethane foam?

Polyurethane foam systems are built from six main raw-material families: polyol, isocyanate, water, catalyst, surfactant, and crosslinker. Polyol and isocyanate form the polymer network. Water acts as a chemical blowing agent and contributes to urea hard segments. Catalyst controls reaction timing. Surfactant controls cell structure. Crosslinker adds network junctions.

Why are polyol and isocyanate considered the main reactive pair?

Polyol provides hydroxyl (OH) groups and isocyanate provides isocyanate (NCO) groups. When OH and NCO react, they form urethane linkages — the chemical backbone of polyurethane. This reaction repeated thousands of times builds the three-dimensional polymer network that defines whether the foam is flexible, rigid, soft, firm, durable, or short-lived. Without this reactive pair, polyurethane foam does not form.

What is OHV and how is it used in polyol selection?

OHV (hydroxyl value) is the concentration of reactive hydroxyl groups per gram of polyol, measured in mg KOH/g. Higher OHV usually means shorter polyol chains and a more rigid network. Lower OHV usually means longer chains and more flexible foam. OHV is also used to calculate polyol equivalent weight: EW = 56,100 ÷ OHV. Different foam types require different OHV ranges — flexible slabstock typically uses 28–56, while rigid foam uses 300–600.

What is %NCO and why does it matter for isocyanate?

%NCO is the mass percentage of reactive NCO groups in the isocyanate material. It controls how many reactive NCO groups are available per gram of material. The equivalent weight formula is EW = 4,200 ÷ %NCO. For TDI 80/20 with 48.3% NCO, the equivalent weight is 86.96 g/eq. Always use the actual CoA %NCO, not the TDS midpoint, because drum-to-drum variation can shift the real running index.

Can two polyols with the same OHV behave differently in foam?

Yes. OHV measures concentration of reactive groups per gram, but two polyols with the same OHV can have different functionality (number of OH groups per molecule), different molecular weight, different polyether vs polyester structure, or different impurity profiles. They will calculate the same equivalent weight but can build different foam networks. This is why polyol cannot be substituted on OHV alone — supplier and grade matter.

Why is isocyanate moisture sensitive?

NCO groups react chemically with water. That is the same reaction used inside the foam (water + NCO → CO₂ + amine), so atmospheric moisture exposure during storage can consume some of the active NCO before the material reaches production. Moisture exposure can occur through poor drum sealing, damaged bungs, humid storage, or repeated opening. A drum’s CoA may have been correct at the supplier, but the active %NCO entering the mixing head can be lower if storage is poor.

What is the difference between polyether and polyester polyols?

Polyether polyols (the most common type in flexible foam) are based on propylene oxide and ethylene oxide chemistry. They offer good hydrolysis resistance, low viscosity, and broad processing windows. Polyester polyols are based on diacid-diol chemistry, offer higher tensile strength and certain durability advantages, but are more sensitive to hydrolysis. The choice depends on the application — automotive, comfort, technical, or specialty foam.

How is the isocyanate index calculated?

Index = NCO equivalents ÷ Total reactive H equivalents × 100. Index 100 means stoichiometric balance — exactly enough NCO to react with all reactive hydrogens. Index 105 means 5% excess NCO. Reactive H equivalents include polyol OH, water (EW = 9), and any crosslinker reactive groups. The index controls hardness, cure, network completeness, compression set, exotherm, and overall foam properties.

What’s the most common mistake when changing polyol or isocyanate supplier?

The most common mistake is approving the change based on the product name or TDS specification, without verifying the actual delivered material. A “same grade” polyol from another supplier can have different functionality, different impurity profile, or a CoA at a different end of the OHV range. A “same grade” isocyanate can have a different CoA %NCO. Both changes shift the index calculation. Approval should require CoA review, EW recalculation, index verification, and a controlled production trial.

Should I change polyol and isocyanate at the same time?

No — change one major reactive component at a time and verify foam performance before changing another. If polyol and isocyanate change simultaneously, troubleshooting becomes much harder because the variables are entangled. Production teams cannot tell whether a hardness drift, compression set change, or cure problem comes from the polyol shift, the isocyanate shift, or the interaction. The cleanest approach is sequential change with full validation between each step.

Key Takeaways

Every polyurethane foam formula is built from six raw-material families: polyol, isocyanate, water, catalyst, surfactant, and crosslinker.

The two main reactive partners are polyol and isocyanate. Together they form the urethane linkages that build the polyurethane polymer network.

Polyol provides reactive OH groups, controls flexibility/rigidity, and is characterized by OHV and functionality.
Isocyanate provides reactive NCO groups, controls cure and network completeness, and is characterized by %NCO and the index.

Both share a paired structure: each has a key specification, an equivalent weight formula, and a functionality value.

Equivalent weight formulas:

Polyol EW = 56,100 ÷ OHV

Isocyanate EW = 4,200 ÷ %NCO

Polyol and isocyanate together represent most of the formula by weight and define the polymer backbone. Polyol-isocyanate selection should be reviewed first when reformulating, qualifying new products, or troubleshooting persistent foam problems.

Conclusion

If your foam quality problem starts with the polymer network — flexibility, hardness, cure, or compression set — the polyol-isocyanate pair is where troubleshooting should begin.

PolymersIQ can help review your polyol grade, OHV, functionality, isocyanate type, current %NCO, and index calculation to identify whether the reactive pair is correctly specified for your foam target.

To get accurate support, please share:

Polyol grade, OHV, functionality, and supplier
Isocyanate type and current CoA %NCO
Target foam type, density, and hardness
Current isocyanate index
Description of the foam quality issue and adjustments already tried

Contact PolymersIQ for a polyol and isocyanate review →

May 10, 2026