What is polyol? Types, production and uses

Polyol is the common name for compounds carrying more than one hydroxyl (–OH) group in their molecule. In polyurethane chemistry, polyol is one of two fundamental components alongside isocyanate, and it is polyol that largely determines the character of the final product: the softness of a foam, the flexibility of a coating and the recovery behavior of an elastomer are all established through polyol selection.

What is a polyol?

Three numbers define a polyol:

• Functionality: the number of –OH groups per molecule. Two produces a linear chain, while three or more produces a cross-linked network structure. Values of 2–3 are typical for flexible foam, and 4–8 for rigid foam.
• Hydroxyl number (OH value): measured in mg KOH/g; it is inversely proportional to molecular weight and forms the basis of formulation calculations.
• Molecular weight: high molecular weight polyols yield flexible products, while low molecular weight polyols yield rigid ones.

Once these three values are known, the required amount of isocyanate (index) can be calculated; this is the backbone of any polyurethane formulation.

How is it produced?

• Polyether polyols: produced by adding propylene oxide and/or ethylene oxide to an initiator (glycerin, sorbitol, sucrose, propylene glycol) in the presence of a catalyst. They make up the largest share of the market; they are hydrolysis-resistant and economical.
• Polyester polyols: obtained by esterifying a diacid (adipic acid, phthalic anhydride) with a glycol. They offer higher mechanical strength and oil resistance, but are more prone to hydrolysis in humid environments. See the polyester page for details.
• Natural oil-based polyols: produced through epoxidation and ring-opening of soybean, canola or castor oil; used to increase bio-based content ratios.
• Recycled polyols: obtained by breaking down PET waste through glycolysis or by chemically recovering polyurethane foam.

Types and varieties

Which sectors use it?

• Polyurethane and composites: the main component of flexible and rigid foam production.
• Furniture and bedding: the comfort of flexible foam is directly adjusted through polyol selection.
• Construction and refrigeration: rigid foam insulation panels, spray polyurethane.
• Paints and coatings: the hydroxyl component of two-component polyurethane systems.
• Adhesives and sealants: the prepolymer side of PU adhesives.
• Automotive: seats, headliners, acoustic insulation, bumpers.

The Turkish market and production

Türkiye is predominantly an importer on the polyol side; polyether polyol and isocyanate (TDI, MDI) are sourced from abroad. However, the country has a regionally strong manufacturing base in flexible and rigid foam production — mattress, furniture, white goods and insulation panel manufacturing form the main source of this demand.

Key issues on the sector's agenda include: volatility in raw material prices, the growing share of bio-based and recycled polyols (particularly for manufacturers exporting to Europe), the impact of insulation regulations on rigid foam demand, and research into isocyanate-free polyurethane. Current developments are listed in the news section below.

Technical specifications

Formulation calculations are derived from this table: once the hydroxyl number and polyol quantity are known, the required isocyanate amount and index can be calculated. Water content is tracked as a separate item because water also reacts with isocyanate, and if not accounted for, it results in both carbon-dioxide-induced bubbling and insufficient cross-linking.

Points to consider in selection and sourcing

• Request specifications with all three numbers. A polyol cannot be properly compared without the hydroxyl number, functionality and viscosity.
• Measure water content in every batch. Moisture consumes isocyanate and changes foam density in unexpected ways.
• Evaluate the catalyst and surfactant package together. When the polyol is changed, the rest of the formulation usually needs to be rebalanced.
• Back up bio-based or recycled content claims with documentation. For customers selling into Europe, certification is as important as performance.
• Control storage temperature. High-functionality polyols increase viscosity rapidly in cold conditions and strain dosing pumps.

Frequently asked questions

How should one choose between polyether polyol and polyester polyol?

Polyether is preferred when humid conditions, hydrolysis risk and cost are priorities; polyester is preferred when mechanical strength, abrasion and oil resistance are priorities. Polyester dominates in shoe soles and industrial elastomers, while polyether dominates in mattress and furniture foam.

What does the hydroxyl number (OH value) indicate?

It expresses the hydroxyl group content of one gram of polyol in terms of the equivalent amount of KOH. The required amount of isocyanate in a formulation is calculated from this value. A high OH value means the molecule is small and densely cross-linked — resulting in a rigid product; a low value means a long, flexible molecule and product.

What is the isocyanate index?

It shows what percentage of the stoichiometrically required amount the isocyanate used represents. 100 means an exact match; values such as 105–110 indicate excess isocyanate, giving additional cross-linking and a harder, more stable structure. An excessively high index produces brittleness.

Do polyols absorb moisture, and why does it matter?

Polyols are hygroscopic. Water reacts with isocyanate and releases carbon dioxide — in foam, this can serve as a desired blowing mechanism, but in coatings or elastomers it means bubbling and surface defects. This is why storage conditions and moisture content measurement are routine quality control steps.

Do bio-based polyols compromise performance?

Not up to certain ratios. Natural oil-based polyols can replace a portion of the petrochemical polyol in most foam formulations. At very high substitution ratios, odor, color and curing profile may change; therefore the formulation is rebalanced accordingly.