Hydroxyl value is one of the most important analytical parameters used to characterize polyester polyols intended for polyurethane applications. In rigid polyurethane foam, the hydroxyl value provides an indication of the concentration of hydroxyl groups available to react with isocyanate.

However, hydroxyl value should not be treated as an isolated specification. The final performance of a rigid PU foam depends on the interaction between hydroxyl value, functionality, molecular weight, polyester structure, isocyanate index, catalysts, blowing system, surfactants and processing conditions.

This becomes particularly important when selecting polyester polyols for insulation applications. A change in hydroxyl value can alter the balance between molecular chain length, reactive group concentration and polymer network formation.

Research on rigid polyurethane foams has demonstrated that changes in polyol hydroxyl value can influence crosslink density, closed cell content, compressive strength, dimensional stability and thermal conductivity.

What Does Hydroxyl Value Mean in a Polyester Polyol?

Hydroxyl value, commonly expressed as mg KOH/g, is a measure of the hydroxyl functionality present in a polyol material. It indicates the amount of potassium hydroxide equivalent to the hydroxyl groups contained in one gram of polyol.

From a polyurethane formulation perspective, hydroxyl value is important because the hydroxyl groups react with isocyanate groups to form urethane linkages.

A higher hydroxyl value generally indicates a greater concentration of hydroxyl groups per unit mass. However, this does not automatically mean that a higher-OH polyol will always produce a better rigid foam.

The correct hydroxyl value depends on the molecular architecture and functionality of the polyol and on the target formulation.

Hydroxyl Value and Molecular Weight

Hydroxyl value is closely related to the molecular weight and functionality of a polyester polyol. For a simplified multifunctional polyol, increasing the number of hydroxyl groups relative to molecular mass generally increases the hydroxyl value.

This relationship is important because two polyester polyols can have similar hydroxyl values while possessing different molecular structures, functionality and molecular-weight distributions.

Consequently, hydroxyl value alone cannot completely describe how a polyester polyol will behave during polyurethane foam formation.

Parameters That Should Be Evaluated Together

• Hydroxyl value.
• Average functionality.
• Molecular weight.
• Acid value.
• Viscosity.
• Water content.
• Aromatic or aliphatic structure.
• Reaction profile.

Why Hydroxyl Value Matters in Rigid PU Foam

Rigid polyurethane foam requires a sufficiently crosslinked polymer structure to maintain its shape and mechanical integrity. The hydroxyl functionality available in the polyol contributes directly to the formation of this three-dimensional polymer network.

When hydroxyl value and functionality are increased within an appropriate formulation range, the concentration of reactive sites can increase. This can contribute to a higher degree of network formation.

Published experimental work on rigid polyurethane foam found that increasing polyol hydroxyl value increased crosslink density, accompanied by increases in closed-cell content, compression strength and dimensional stability.

The relationship is nevertheless formulation-dependent. Excessive changes in hydroxyl value can also alter reaction kinetics, viscosity and foam processing behaviour.

Hydroxyl Value and Crosslink Density

Crosslink density is one of the key structural characteristics of a rigid polyurethane network. A higher degree of crosslinking generally produces a more tightly connected polymer structure.

Polyester polyols with suitable functionality and hydroxyl concentration can therefore contribute to the development of a rigid network capable of supporting mechanical loads and resisting dimensional changes.

Hydroxyl Value and Rigid Foam Compressive Strength

Compressive strength is an important property for rigid polyurethane insulation used in sandwich panels, refrigeration systems, roofing, pipe insulation and other structural insulation applications.

A more highly crosslinked polymer network can provide greater resistance to deformation under compression. Therefore, changes in hydroxyl value can influence the mechanical response of the final foam.

Experimental research has reported an increase in compression strength with increasing hydroxyl value in rigid polyurethane foam, attributed in part to increased crosslink density.

Nevertheless, compressive strength is also strongly affected by foam density, cell structure, isocyanate index, formulation additives and processing conditions.

Hydroxyl Value and Foam Cell Structure

Rigid polyurethane insulation does not obtain its thermal performance from the polymer matrix alone. The cellular structure of the foam plays a major role in determining thermal conductivity and dimensional stability.

Polyol chemistry can influence the reaction profile, viscosity and development of the polymer network during foam formation. These factors can affect cell formation and stabilization.

Research investigating hydroxyl value in rigid polyurethane foam has reported changes in closed-cell content as hydroxyl value increased.

This illustrates why polyol selection should be evaluated together with the blowing system, surfactant and catalyst package rather than considering hydroxyl value independently.

Hydroxyl Value and Thermal Insulation Performance

Thermal conductivity is a critical performance parameter for rigid PU insulation. Lower thermal conductivity allows thinner insulation systems to achieve a given thermal resistance.

Hydroxyl value can indirectly influence thermal performance through its effects on polymer network formation and foam cell structure.

In the published study of rigid polyurethane foams using polyols with different hydroxyl values, thermal conductivity exhibited a small minimum around the tested 500 mg KOH/g formulation.

This is an important formulation lesson: the relationship between hydroxyl value and insulation performance is not necessarily linear. An optimum formulation window may exist rather than a simple rule that higher hydroxyl value always produces better insulation.

Typical Hydroxyl Value Ranges for Rigid Foam Polyols

Rigid PU foam polyols commonly operate in substantially higher hydroxyl-value ranges than many polyols designed for flexible polyurethane applications. Published technical literature describes rigid-foam polyols with total hydroxyl numbers broadly ranging from approximately 200 to 800 mg KOH/g, although the appropriate value depends strongly on the chemistry and application.

Commercial and patent literature also describes polyester polyol compositions for rigid polyurethane foam in ranges such as 250–550 mg KOH/g, with functionality ranges tailored to the intended foam system.

These figures should be viewed as formulation-development ranges, not as universal specifications. The correct target depends on the complete formulation and required foam performance.

Why the Highest OH Value Is Not Always the Best Choice

It can be tempting to assume that a higher hydroxyl value will automatically produce a stronger or better rigid foam. In practice, polyurethane formulation is a balance between chemical reactivity, network structure, foam morphology and processing.

Increasing hydroxyl value may change the concentration of reactive groups, viscosity and reaction behaviour. These changes can affect cream time, gel time, foam rise and curing.

The published hydroxyl-value study observed non-linear behaviour in several processing and foam properties, with some parameters reaching a minimum around 500 mg KOH/g.

Therefore, polyol selection should focus on identifying the formulation window that delivers the required combination of processing and final properties.

Hydroxyl Value Must Be Considered With Functionality

Two polyester polyols can have similar hydroxyl values but behave differently if their functionality and molecular architecture are different.

Functionality determines how many reactive sites are available per molecule, while hydroxyl value describes the overall hydroxyl concentration per unit mass.

This distinction is particularly important for rigid PU foam, where network architecture has a major influence on rigidity, dimensional stability and mechanical performance.

For this reason, formulators should evaluate hydroxyl value and functionality together when comparing polyester polyols.

Hydroxyl Value in PET-Based Polyester Polyols

PET-based polyester polyols are particularly interesting for circular polyurethane technologies because PET waste can be chemically converted into polyol intermediates suitable for polyurethane formulations.

For a PET-derived polyester polyol, hydroxyl value becomes an important indicator of the reactive functionality available after glycolysis and subsequent formulation or modification.

However, the target hydroxyl value must be considered alongside aromatic content, molecular structure, functionality, viscosity, acid value and residual components from the recycling process.

The objective should therefore be to develop a PET-derived polyol with a controlled and reproducible specification rather than simply maximizing hydroxyl value.

How Formulators Should Evaluate Polyester Polyol OH Value

1. Define the target rigid foam application.
2. Establish the required density and thermal conductivity.
3. Define the required compressive strength and dimensional stability.
4. Select an appropriate hydroxyl-value window.
5. Evaluate functionality and molecular structure.
6. Check viscosity and processing compatibility.
7. Evaluate acid value and moisture content.
8. Test compatibility with the selected isocyanate and additives.
9. Conduct laboratory foam trials.
10. Validate the selected polyol under representative production conditions.

Enviol's Approach to Controlled Polyester Polyols

Enviol is developing polyester polyols from recycled feedstocks, including PET waste, for demanding polyurethane applications such as rigid PU and PIR insulation.

Our development approach focuses on controlling application-relevant parameters including hydroxyl value, functionality, viscosity, acid value, moisture and molecular structure.

The goal is to develop recycled polyester polyols that can be evaluated based on actual formulation performance rather than recycled content alone.

Application-specific OH-value development can allow foam manufacturers to evaluate different formulation windows and identify the appropriate balance between processing behaviour, mechanical performance and insulation properties.

Conclusion

Hydroxyl value is a fundamental parameter in polyester polyol selection for rigid polyurethane foam. It influences the concentration of reactive hydroxyl groups and can affect network formation, crosslink density, foam structure, mechanical properties and dimensional stability.

However, hydroxyl value should never be considered independently. Functionality, molecular weight, aromatic structure, viscosity, acid value, moisture and the complete formulation all contribute to the behaviour of the final rigid foam.

For recycled polyester polyols, controlled hydroxyl value is especially important because consistency in chemical properties is essential for reproducible industrial foam processing.