A polyol change that looks harmless on paper—same nominal hardness, same target density, same supplier—can still shift gel time, shrinkage, demold behavior, and the amount of isocyanate the machine has to pump. In many reformulation problems, the hidden variable is functionality. It is not a specification to scan and forget; it is a structural property that controls how the polymer network forms.
What Functionality Means in Polyurethane
In polyurethane chemistry, the concept shows up most directly on a raw-material certificate: a polyol’s hydroxyl number is measured according to ISO 14900, and that value feeds directly into the functionality calculation [1]. Functionality, usually written f, is the average number of reactive chemical groups per molecule [2]. For a polyol, those groups are hydroxyl (OH) sites. For an isocyanate, they are isocyanate (NCO) sites. A pure diol has a nominal functionality of 2; a triol has 3; a polymeric MDI may have an average functionality above 2.5. The word “average” matters because most commercial raw materials are mixtures of homologues and oligomers, so the quoted value is a number-average functionality, not a single molecular species.
| Raw material | Reactive group | Typical nominal functionality |
|---|---|---|
| Polyether diol | OH | 2.0 |
| Polyether triol | OH | 3.0 |
| Polyester polyol, slightly branched | OH | 2.0–2.8 |
| Castor oil | OH | approx. 2.7 |
| Pure MDI / TDI | NCO | 2.0 |
| Polymeric MDI | NCO | approx. 2.5–2.9 |
Representative values vary by grade and supplier; treat the table as a starting point, not a specification [2].
Why a One-Digit Change in Functionality Matters
Functionality changes network architecture. A difunctional system builds mostly linear chains. A triol or higher-functionality polyol introduces branch points, and those branch points become permanent chemical crosslinks when they react with isocyanate. More branch points mean higher crosslink density, which usually raises hardness, reduces swelling, and improves chemical resistance—but it also increases viscosity, shortens gel time, and can raise internal stress before demold. If a formulator replaces a triol with a diol and only adjusts the ratio to match OH value, the part may still reach the same hardness on a durometer, but it will respond differently to solvents, heat, and load.
Crosslink density also determines how the part feels under load and how it resists indentation. Shore A Vs Shore D Hardness In Polyurethane covers the test methods and what the two scales mean in practice.
How to Read Functionality from a TDS
A polyol TDS usually reports hydroxyl value (OHV) in mg KOH/g and number-average molecular weight (Mn). Functionality is calculated as:
f = OHV × Mn / 56,100
The constant 56,100 is the milliequivalent weight of KOH expressed in mg. The same relationship is visible in the equivalent weight equation: equivalent weight = Mn / f, and OHV = 56,100 / equivalent weight [3].
For example, a 3,000 g/mol polyether triol with an OHV of 56 mg KOH/g has:
f = 56 × 3,000 / 56,100 = 3.0
This is the clean way to check a polyol. If the TDS shows OHV = 28 mg KOH/g on a 3,000 g/mol material, functionality is not 3; it is 1.5—a very different material unless the supplier actually blended a monofunctional or high-molecular-weight species.
| Example | Mn (g/mol) | OHV (mg KOH/g) | Calculated f |
|---|---|---|---|
| Polyether diol | 2000 | 56 | 2.0 |
| Polyether triol | 3000 | 56 | 3.0 |
| Polyether tetraol | 2000 | 112 | 4.0 |
Isocyanate functionality is read the same way from NCO content. The NCO content in weight percent relates to functionality as NCO% = (42 × f / Mn) × 100, where 42 is the molecular weight of the NCO group [4]. NCO content is measured by ISO 14896 [4] and, for aromatic isocyanates, by ASTM D5155 [5].
If you are comparing two polyol alternatives and the TDS math does not line up, send both certificates to Info@chinahaifeng.com. A quick OH-value check will show whether the change is a true reformulation or a supplier-side spec shift.
How Functionality Reaches the Part
For a fixed hydroxyl value, changing functionality does not directly change the amount of isocyanate needed by simple stoichiometry. What changes is where those reactions lead: a higher-functionality polyol creates branch points instead of only extending chains, so the same nominal index can produce a much tighter network and a faster viscosity build.
Nominal functionality and actual functionality are not the same. Polyether polyols can contain monol by-products, and a small monol fraction lowers the real average functionality more than the nominal value suggests [6]. That is why two 56 mg KOH/g triols from different suppliers can behave differently on the same line.
Functionality also interacts with the isocyanate index. If a formulator raises functionality while keeping ratio and index fixed, the gel point arrives earlier. If functionality drops, the system may remain more thermoplastic-like and release from the mold differently. Crosslink density, gelation, and final mechanical response are linked through the functionality of both sides of the reaction [7].
Once functionality changes, the required A/B ratio and shot size change with it. How To Calculate Shot Weight In Polyurethane covers the calculation step by step.
Where It Shows Up on the Production Line
Higher functionality tends to raise material viscosity, especially when the polyol is branched or has a narrow molecular weight distribution. The practical result is that metering accuracy, tank temperature control, and mixing head energy all become more important. A low-viscosity diol can be metered with a wider operating window; a chemically similar triol or tetraol may need more precise temperature control before it thins enough for stable flow.
On a pouring line, functionality-driven viscosity changes often show up as ratio drift, poor mixing, or trapped air before they show up as hardness defects. Closed-loop servo metering helps because it holds ratio constancy even when viscosity moves during a shift.
Functionality also shifts the balance between chain extension and blowing, which shows up in the foam’s rise profile. The Role Of Rise Time In Polyurethane Foaming explains how to read that behavior on the line.
If you are mid-reformulation and the gel time or ratio no longer matches the machine’s current recipe, do not tune around the symptom yet. Check the functionality arithmetic first. For a direct review of your formula, TDS data, and machine settings, contact Haifeng at Info@chinahaifeng.com or WhatsApp 86 13566296633.
Common Mistakes When Functionality Is Ignored
- Reading hydroxyl value as functionality. Hydroxyl value tells you the concentration of OH groups; functionality also requires molecular weight.
- Replacing a triol with a diol at the same OHV and expecting the same part performance.
- Ignoring monol content. A small monofunctional fraction can lower real functionality enough to change demold and physical properties.
- Ignoring isocyanate functionality when choosing between pure MDI and polymeric MDI.
- Assuming higher functionality is always better. It usually improves chemical resistance, but it can also increase viscosity, shorten working time, and create more internal stress.
Get a Second Read on Your Formulation
Functionality is one of those raw-material properties that is easy to calculate but expensive to get wrong. If the line is showing gel-time scatter, shrinkage, hardness drift, or ratio instability after a material change, check the functionality arithmetic before changing machine settings.
- Send the polyol and isocyanate TDS values to Info@chinahaifeng.com
- Include target hardness, density, and demold time
- WhatsApp 86 13566296633 for a direct technical review
FAQ
What does functionality mean in polyurethane?
Functionality is the average number of reactive chemical groups per molecule. For polyols, it refers to hydroxyl groups; for isocyanates, it refers to NCO groups. A functionality of 2 gives mostly linear chains, while a functionality of 3 or higher introduces branch points and crosslinks.
What is the difference between functionality and hydroxyl value?
Hydroxyl value measures the concentration of OH groups in mg KOH/g. Functionality also requires molecular weight. Two polyols can have the same hydroxyl value but different functionality if their molecular weights differ.
What functionality do common polyols have?
Polyether diols have a nominal functionality of 2, polyether triols about 3, and tetraols about 4. Castor oil is roughly 2.7, and slightly branched polyester polyols often fall between 2 and 3.
How does isocyanate functionality affect polyurethane?
A difunctional isocyanate such as pure MDI or TDI builds linear extension. Polymeric MDI has an average functionality above 2.5, so it contributes additional crosslink density, higher stiffness, and often faster cure.
Why does a small functionality change alter gel time?
Higher functionality creates more branch points and faster network formation, so the system reaches its gel point sooner. Lower functionality extends the working window and produces a more linear structure.
How do I calculate functionality from a TDS?
Use f = OHV × Mn / 56,100 for polyols. For isocyanates, use f = NCO% × Mn / 4,200 when NCO content is expressed as a percentage.
Does higher functionality always make a stronger polyurethane?
Not necessarily. Higher functionality often increases hardness and chemical resistance, but it can also raise viscosity, shorten working time, increase internal stress, and reduce elongation. The correct functionality depends on the part design and production constraints.
References
[1] ISO 14900:2017, Plastics — Polyols for use in the production of polyurethanes — Determination of hydroxyl number, International Organization for Standardization, 2017.
[2] G. Oertel, ed., Polyurethane Handbook, 2nd ed., Munich: Hanser Publishers, 1994.
[3] ASTM D4274-21, Standard Test Methods for Testing Polyurethane Raw Materials: Determination of Hydroxyl Numbers of Polyols, ASTM International, 2021.
[4] ISO 14896:2009, Plastics — Polyurethane raw materials — Determination of isocyanate content, International Organization for Standardization, 2009.
[5] ASTM D5155-19, Standard Test Methods for Polyurethane Raw Materials: Determination of the Isocyanate Content of Aromatic Isocyanates, ASTM International, 2019.
[6] M. Ionescu, Chemistry and Technology of Polyols for Polyurethanes, 2nd ed., Shawbury: Smithers Rapra, 2016.
[7] D. Randall and S. Lee, eds., The Polyurethanes Book, New York: Wiley, 2002.
If you’re interested, check out these related articles:
Causes Of Shrinkage In Polyurethane Foam How To Control It
The Role Of Rise Time In Polyurethane Foaming
