Polyols for polyurethane are the hydroxyl-bearing component that reacts with an isocyanate to build the polymer. They are the larger half of most formulations, and their structure sets most of the properties a customer actually notices: softness or rigidity, resilience, chemical resistance and how the part ages. This guide explains the main polyol families, the numbers on a technical data sheet and how to match a polyol to an application.
A polyol is a molecule with two or more hydroxyl (-OH) groups that reacts with an isocyanate to form polyurethane. The main families are polyether polyols, polyester polyols, polymer (filled) polyols and bio-based polyols. Hydroxyl value and functionality decide whether the result is flexible or rigid: low OH value and functionality give soft foams and elastomers, high values give rigid foams.
- Polyether polyols are the volume workhorse: low viscosity, good hydrolysis resistance, used from mattresses to insulation panels.
- Polyester polyols give higher strength, abrasion and solvent resistance; aromatic polyesters improve fire behaviour in PIR foam.
- Hydroxyl value (mg KOH/g) and functionality are the two numbers that most predict flexibility versus rigidity.
- Equivalent weight is calculated as EW = 56,100 / OH value and drives the isocyanate demand of the formulation.
- Polyols pick up moisture; dry storage under nitrogen protects density and reactivity.
01What A Polyol Is And What It Does
A polyol is an alcohol with several hydroxyl groups per molecule. In a polyurethane system each -OH group reacts with an -NCO group of the isocyanate to form a urethane link, so the polyol becomes the flexible or rigid "backbone" of the polymer network.
Three structural features matter most. Chain length (expressed through molecular weight or hydroxyl value) controls how far apart the crosslinks sit: long chains give soft, elastic materials, short chains give stiff ones. Functionality, the average number of -OH groups per molecule, controls how branched the network becomes: diols build linear chains, while polyols with three to eight groups build three-dimensional networks. Chemistry of the backbone (ether or ester links, aromatic or aliphatic segments) controls resistance to water, solvents, heat and fire.
Because the polyol side is usually blended with catalysts, surfactants, blowing agents and other additives before it meets the isocyanate, many processors buy it as a fully formulated "polyol blend" or system rather than as a single raw material. The additives guide covers that side of the blend, and the isocyanates guide covers the other reactive component.
02Polyether Polyols
Polyether polyols are made by adding propylene oxide (PO) and often ethylene oxide (EO) to a starter molecule called an initiator. They are the most widely used polyols because they combine low viscosity, good hydrolysis resistance and competitive cost.
Initiators Set The Functionality
The initiator determines how many chains grow from the core and therefore the functionality of the polyol:
- Glycols (propylene glycol, dipropylene glycol) give difunctional polyols for elastomers, coatings and sealants.
- Glycerol gives trifunctional polyols, the standard for flexible slabstock and moulded foam.
- Sucrose and sorbitol give high-functionality polyols (often five to eight) for rigid insulation foam, where heat resistance and dimensional stability matter.
- Amines such as ethylenediamine or toluenediamine give polyols with built-in catalytic activity, useful for fast-reacting spray and rigid systems.
- Mannich bases (from phenol, formaldehyde and diethanolamine) give reactive, aromatic-rich polyols that improve hardness, strength and fire behaviour in rigid foam.
Flexible And Rigid Grades
Flexible polyether polyols are long-chain, low-OH-value triols, typically with molecular weights of 3,000 to 6,000. A small EO cap at the chain ends raises the share of primary hydroxyl groups, which makes the polyol more reactive. Such high-reactivity, glycerine-initiated grades are the basis of high-resilience (HR) moulded foam for seating and automotive applications. Rigid polyether polyols are the opposite: short chains, high OH value and high functionality.
Pluskim supplies standard flexible polyether polyols for slabstock and glycerine-initiated high-reactivity grades for HR moulded foam, each with a manufacturer data sheet; the range is listed on its polyether polyols page.
03Polyester Polyols
Polyester polyols are made by polycondensation of dicarboxylic acids (or anhydrides) with glycols. They form tougher, more solvent-resistant polyurethanes than polyethers, at the cost of higher viscosity and greater sensitivity to hydrolysis.
Saturated Aliphatic Polyesters
The classic aliphatic polyester polyol is based on adipic acid reacted with glycols such as ethylene glycol, diethylene glycol or 1,4-butanediol. Linear grades (made from diols only) are used in shoe soles, elastomers, adhesives and synthetic leather, where tensile strength, abrasion resistance and oil resistance are needed. Branched grades, made with a small amount of a triol, give crosslinked networks for coatings and some foams. Pluskim lists both adipic acid and glycols among its raw materials, and produces its own polyester polyol series under the PLUSTEROL name.
Aromatic Polyester Polyols
Aromatic polyester polyols are based on phthalic anhydride, terephthalic acid or chemically recycled PET. Their aromatic rings char rather than burn, which is why they are the preferred polyol for polyisocyanurate (PIR) insulation boards and other rigid foams with demanding fire performance. They also tend to be cost-effective. Polyols made from recycled PET bottles and fibres add a circular-economy argument; Pluskim describes its approach in an article on PET-recycled polyester polyols.
For a side-by-side view of the two main families, see polyether vs polyester polyols.
04Polymer Polyols And Bio-Based Polyols
Polymer polyols are base polyols that carry finely dispersed solid particles, most often styrene-acrylonitrile (SAN) copolymer. The particles reinforce the foam: they raise hardness and load-bearing capacity and help open the cells, without requiring a lower density. They are standard in slabstock and HR foam for furniture, mattresses and car seats. SAN-filled polyester polyols also exist for systems that need the mechanical profile of a polyester.
Bio-based polyols replace part of the petrochemical content with renewable feedstock. The common routes are natural-oil polyols derived from castor, soybean or other vegetable oils, and polyester polyols made with bio-based acids or glycols. Castor oil is naturally hydroxyl-functional; most other oils need chemical modification first. Bio-based content is usually blended in at a partial level so that reactivity and foam properties remain within the processing window.
05Key Specification Parameters
Seven numbers on a polyol technical data sheet tell a processor most of what they need to know. The table gives typical ranges; actual limits are always those on the supplier's data sheet and certificate of analysis.
| Parameter | Unit | Flexible foam polyether | Rigid foam polyether | Aliphatic polyester | Why it matters |
|---|---|---|---|---|---|
| Hydroxyl value | mg KOH/g | 25-60 | 300-800 | 35-120 | Chain length, isocyanate demand, flexibility |
| Functionality | -OH per molecule | 2.5-3 | 3-8 | 2-3 | Degree of crosslinking |
| Molecular weight | g/mol | 3,000-6,000 | 300-1,000 | 1,000-3,000 | Softness, elongation |
| Viscosity at 25 °C | mPa·s | 400-1,500 | 1,000-20,000+ | often solid; measured hot | Pumping, mixing, metering |
| Acid value | mg KOH/g | < 0.1 | < 0.1 | < 1-2 | Catalyst activity, hydrolysis stability |
| Water content | % by weight | < 0.05-0.1 | < 0.1-0.2 | < 0.05 | Unplanned CO₂, density drift |
| Colour | APHA / Gardner | low | varies, often darker | low to medium | Appearance of light-coloured parts |
Hydroxyl Value And Equivalent Weight
The hydroxyl value is the most important single number. It states how many reactive hydroxyl groups a gram of polyol contains, expressed as the milligrams of potassium hydroxide equivalent to them, and is determined by standard methods such as ASTM D4274. From it the equivalent weight follows directly:
Equivalent weight (g/eq) = 56,100 / OH value (mg KOH/g)
The constant 56,100 is the molar mass of KOH (56.1 g/mol) multiplied by 1,000 to convert milligrams. A flexible polyol with an OH value of 56 therefore has an equivalent weight of about 1,000, and as a triol a molecular weight of about 3,000. A rigid polyol with an OH value of 450 has an equivalent weight of about 125. Formulators add up the equivalents of every hydroxyl-bearing ingredient, including water, to calculate how much isocyanate the system needs at a given index.
Acid Value And Water Content
A raised acid value in a polyester polyol indicates incomplete reaction or hydrolysis; free acid slows amine catalysts and can shorten the service life of the finished part. Water reacts with isocyanate to form carbon dioxide and urea, so even a few hundred ppm more than expected changes density, cell structure and the heat of reaction. Both should be on every certificate of analysis.
06How To Choose A Polyol By Application
Start from the end product and its service conditions, then narrow down by process. The table shows the usual starting points; most commercial systems blend two or more polyols to tune reactivity, viscosity and final properties.
| Application | Typical polyol choice | Main reason |
|---|---|---|
| Flexible slabstock foam (mattress, furniture) | Glycerol-initiated polyether triol, MW about 3,000; SAN polymer polyol for hardness | Elasticity, low cost, load bearing |
| HR moulded foam (seating, automotive) | EO-capped, high-reactivity polyether triol, MW 4,800-6,000; polymer polyol | Fast cure, resilience, comfort |
| Rigid PU insulation (panels, appliances, pipes) | Sucrose/sorbitol polyethers, amine or Mannich polyethers | Crosslink density, dimensional stability |
| PIR boards and fire-rated panels | Aromatic polyester polyols (phthalic anhydride or PET based) | Fire behaviour, char formation |
| Spray foam | Mannich and amine polyethers, aromatic polyesters | Reactivity, adhesion, fire behaviour |
| Shoe soles | Linear adipate polyester polyols; polyether for some lightweight systems | Abrasion, flex fatigue, strength |
| Coatings, adhesives, sealants, elastomers (CASE) | Polyether diols/triols or polyester polyols depending on resistance needs | Elongation, adhesion, chemical resistance |
The applications guide follows the same logic for the complete raw material set, including isocyanates and additives. Pluskim's polyols range covers sorbitol, amine, Mannich and glycerol-initiated polyethers as well as saturated polyester polyols, so a processor can source rigid and flexible polyols from one producer.
07Storage And Handling
Polyols are relatively easy to handle compared with isocyanates, but moisture and temperature still need control. Most quality complaints traced to a polyol turn out to be water uptake or incorrect temperature at the point of use.
- Keep containers closed and dry. Polyols, especially EO-rich grades, are hygroscopic. Bulk tanks benefit from a dry nitrogen blanket or a desiccant breather.
- Control temperature. Store within the range given on the data sheet, typically protected from frost and from prolonged heat. Many aliphatic polyester polyols are waxy solids at room temperature and must be melted gently and homogenised before use; avoid local overheating.
- Mix before sampling and use. Formulated blends and polymer polyols can stratify or settle; stir or recirculate drums and IBCs.
- Respect shelf life. Pure polyols are stable for long periods in closed packaging; formulated blends containing catalysts and blowing agents have shorter, supplier-defined shelf lives.
- Check the certificate of analysis against the data sheet for each batch: OH value, water content, acid value and viscosity.
08Sources And Further Reading
- ASTM D4274, Standard Test Methods for Testing Polyurethane Raw Materials: Determination of Hydroxyl Numbers of Polyols - store.astm.org/d4274-21.html
- ISOPA, European Diisocyanate and Polyol Producers Association: applications of polyurethanes - isopa.org/applications
- ISOPA, circular economy and polyurethanes - isopa.org/circular-economy
- American Chemistry Council, Center for the Polyurethanes Industry - americanchemistry.com
- ISOPA, product information on diisocyanates and polyols - isopa.org/product-information
09Frequently Asked Questions
What Is A Polyol In Polyurethane?
A polyol is a molecule carrying two or more hydroxyl (-OH) groups. In polyurethane production it reacts with an isocyanate to form urethane links, building the polymer network. The polyol usually makes up the larger part of a formulation by weight and largely sets flexibility, hardness, chemical resistance and ageing behaviour of the finished product.
What Is The Difference Between Polyether And Polyester Polyols?
Polyether polyols are made by adding propylene oxide and ethylene oxide to an initiator; they give good hydrolysis resistance, low viscosity and low cost. Polyester polyols are made by condensing acids with glycols; they give higher mechanical strength, abrasion and solvent resistance, but are more sensitive to moisture over time. The choice depends on the application and service conditions.
What Does The Hydroxyl Value Of A Polyol Mean?
The hydroxyl value (OH value or OHV) expresses how many reactive -OH groups a polyol contains, in mg KOH per gram. A high OH value (typically 300-800) means short chains and a rigid, highly crosslinked product; a low value (typically 25-60) means long chains and flexible foams or elastomers. It is measured by methods such as ASTM D4274.
How Do You Calculate The Equivalent Weight Of A Polyol?
Equivalent weight equals 56,100 divided by the hydroxyl value: EW = 56,100 / OHV. A polyol with an OH value of 56 mg KOH/g therefore has an equivalent weight of about 1,000 g per reactive group. Multiplying equivalent weight by functionality gives the approximate molecular weight, and the equivalent weight is used to calculate how much isocyanate the formulation needs.
Which Polyol Is Used For Rigid Insulation Foam?
Rigid polyurethane and PIR insulation foams use high-functionality, high-OH-value polyols: sucrose or sorbitol initiated polyether polyols, amine and Mannich polyethers for reactivity and strength, and aromatic polyester polyols based on phthalic anhydride or recycled PET, which improve fire behaviour in PIR boards. Blends of several polyols are normal in rigid systems.
How Should Polyols Be Stored?
Keep polyols in closed, dry containers, ideally under a dry nitrogen blanket, at moderate temperature as stated on the technical data sheet. Polyols absorb moisture from the air, and water reacts with isocyanate to release carbon dioxide, which shifts density and reactivity. Some polyester polyols need gentle warming before use because they are solid or very viscous at room temperature.
10Related Reading
- Isocyanates For Polyurethane: MDI, TDI And PrepolymersIsocyanates for polyurethane: MDI, polymeric MDI, TDI, aliphatics and prepolymers, NCO content, isocyanate index, safe storage and EU training rules.
- Polyether Vs Polyester Polyol: Which One For Your Application?Polyether vs polyester polyol compared on hydrolysis, strength, oil resistance, cold flexibility and cost, with a quick decision list for each application.
- Polyurethane Raw Materials By ApplicationWhich polyols, isocyanates and additives each application uses: rigid and flexible foam, footwear, coatings, adhesives and binders, in one clear table.
Sourcing These Raw Materials
Comparing offers for a polyol, an isocyanate or a formulated system? The supplier checklist covers the documents to request, what to test on delivery and the red flags to watch for.