Polyol and Isocyanate: What They Are, How They React and the Mix Ratio

A polyol is a molecule with two or more hydroxyl (OH) groups; the isocyanates used in polyurethane have two or more reactive NCO groups. When they are mixed, NCO groups react with OH groups to form urethane bonds, and polyurethane is built from these bonds. The polyol largely decides whether the plastic ends up soft or rigid, the isocyanate (usually MDI or TDI) links the chains, and the mix ratio keeps the two groups in balance. Isocyanates react with moisture and can cause allergic sensitisation, so they need careful handling.

1. Polyol and isocyanate: the two raw materials

Every industrial polyurethane (PU) system starts from the same two raw material classes: a polyol (a molecule carrying hydroxyl groups, OH) and an isocyanate (a molecule carrying reactive isocyanate groups, N=C=O, written NCO). When the two meet, an NCO group reacts with an OH group and forms a urethane bond. Repeated over and over across the mix, that single bond builds the polymer. For an overview of the whole material family, see What Is Polyurethane?.

Everything else in a formulation (catalysts, surfactants, blowing agents, flame retardants) fine-tunes this reaction and the finished product; the behaviour of a foam, coating or moulded part is decided first by the polyol, the isocyanate and the ratio between them. Flexible foam usually pairs a polyether polyol with TDI, rigid foam a high-functionality polyol with polymeric MDI. See Rigid vs Flexible Polyurethane Systems and What Is Polyurethane Foam?.

2. What is a polyol?

A polyol is an organic compound that carries several hydroxyl (OH) groups. In polyurethane the polyols are oligomers or polymers with on average two or more OH groups per molecule, usually viscous liquids at room temperature. The OH groups are where the isocyanate attaches.

Polyether and polyester polyols

Polyether polyols are made by adding propylene oxide and/or ethylene oxide to a starter molecule; they are very widely used and usually cheaper, easier to handle and more resistant to water than polyester polyols. Polyester polyols come from the polycondensation of carboxylic acids with glycols; they are usually more viscous and more expensive but give better solvent, abrasion and cut resistance. Aromatic polyester polyols are used in rigid foam; some are made from recycled PET.

Functionality, hydroxyl number and molecular weight

Functionality is the average number of OH groups per molecule. The hydroxyl number (mg KOH/g, measured for example to ISO 14900) shows how many OH groups there are per gram. Both are tied to the molecular weight. More OH per gram means more isocyanate is needed. The values of a specific polyol are given in its technical data sheet (TDS).

Flexible or rigid: what the polyol decides

Long chains with few OH groups (a low hydroxyl number) give a loosely crosslinked, elastic network (flexible foam); short chains with many OH groups (a high hydroxyl number) a densely crosslinked, rigid one (rigid foam). Which polyols are used for which product depends on the formulation of the system.

Special polyols: polymer, bio-based and recycled

Polymer polyols carry fine dispersed polymer particles (for example SAN, styrene acrylonitrile) and are used to give flexible foam a higher load-bearing capacity, that is, a firmer feel.

Bio-based polyols (from castor, soy or rapeseed oil) and recycled polyols (from glycolysis of PET or polyurethane waste) can differ in hydroxyl number and viscosity from petrochemical grades. Renewable-content work has so far focused mostly on polyols. Products sold on a “mass balance” (biomass balance) basis receive their renewable share through certified bookkeeping, not necessarily as renewable molecules in the product itself, so check whether a stated renewable content is physically in the product or only allocated. See Polyurethane Recycling Methods.

3. What is an isocyanate?

An isocyanate is an organic compound with the group N=C=O (NCO). Polyurethane uses diisocyanates and polyisocyanates, with two or more NCO groups per molecule, which link the chains.

MDI, polymeric MDI and TDI

MDI (methylene diphenyl diisocyanate) is the main isocyanate for rigid foam, insulation and spray foam. Polymeric MDI (pMDI), despite its name, is not a polymer but a liquid mixture of monomeric MDI and larger molecules. TDI (toluene diisocyanate) is made mostly as an 80:20 mixture of the 2,4- and 2,6-isomers and is the main isocyanate for flexible foam in furniture and bedding. TDI has a higher vapour pressure than MDI, so it needs stricter exposure control.

Aliphatic isocyanates

Aliphatic isocyanates (including the cycloaliphatic ones) such as HDI, IPDI and H₁₂MDI are used mostly in coatings where colour and transparency matter, because polyurethanes made with aromatic isocyanates (MDI, TDI) tend to darken in light.

NCO content

NCO content is the percentage by weight of NCO groups in the isocyanate. The higher it is, the less isocyanate by weight is needed for the same number of OH groups. Every isocyanate product has its own NCO content, which is stated in its technical data sheet. The table compares three common types.

4. How do a polyol and an isocyanate react?

An NCO group and an OH group join and form a urethane bond (RNCO + R'OH → RNHCOOR'), releasing heat. With two groups per molecule on each side, chains form; when one side has three or more, a crosslinked network forms. In a foam, several more reactions run alongside:

Gelling and blowing must stay in balance, which strongly affects the cell structure and the density. Catalysts (amine catalysts and metal compounds) control the speed of the reactions; the supplier normally blends them into the polyol component. In rigid foam a physical blowing agent adds gas as well.

5. Isocyanate index and mix ratio: how is the ratio set?

The isocyanate index compares the NCO actually used with the NCO that would react exactly with all reactive groups in the mix (mainly the OH groups of the polyols and any water). An index of 100 means exact balance; above 100 the isocyanate is in excess, below 100 it is short. PIR (polyisocyanurate) foams are made with a much larger excess of isocyanate than ordinary PUR foams; see PIR vs PUR Insulation Panels.

How is the mix ratio set?

The ratio of polyol blend to isocyanate is part of the system. It follows from the reactive groups of the two components and from the target index, and the formulator sets it when the system is developed. The ratio of a ready system is therefore fixed: it is given in the system's technical data sheet, together with the processing conditions, and should be used as stated there. For questions about the ratio or index of a specific system, JiTPOL's technical team can help.

Too much or too little isocyanate

In general, too little isocyanate leaves OH groups unreacted and gives a softer polymer with fewer crosslinks, while too much creates additional crosslinks and a harder, stiffer one; the exact effect depends on the system, so the ratio on the technical data sheet should be followed.

6. What are component A and component B?

Industrial polyurethane is usually delivered as two components in separate containers that are mixed only when they are used, for example in the mix head of a machine or in a spray gun. In Türkiye and Europe, A is usually the polyol blend and B the isocyanate, which is also JiTPOL's usage. In North America the letters are often reversed. They are a trade habit, not a standard, and practice differs from company to company, so always check the label and the data sheet.

The polyol blend is mostly polyol, plus the additives the supplier has balanced in advance: catalysts, silicone surfactant, blowing agent and, in some systems, flame retardant and colourant.

7. How are polyols and isocyanates stored and handled?

The main enemy is moisture. Water reacts with isocyanate and releases carbon dioxide; in a closed drum the gas can build up enough pressure to bulge or rupture it. So containers are kept closed and protected from moisture, and a container that has held even a little water must not be refilled. Many polyols are hygroscopic too: a polyol left open takes up water from humid air. That water later uses up isocyanate and releases carbon dioxide, which changes the density and cell structure of the foam.

Storage temperature, shelf life and the handling of containers differ from product to product. They are given in the technical data sheet and the safety data sheet (SDS) of each material, so follow them and your supplier's instructions. In general, store in an enclosed, ventilated area and keep isocyanates apart from water, alcohols, amines and polyols. ISOPA, the European association of diisocyanate and polyol producers, publishes storage and transport guidelines for TDI and MDI. For advice on a specific system, contact JiTPOL's technical team.

8. Are isocyanates harmful?

This section covers the liquid raw materials; the cured product depends on its type (see What Is Polyurethane?). Polyols themselves are generally regarded as low-hazard, but a ready polyol blend also contains catalysts (a few amines can sensitise the skin), flame retardants and blowing agents, so it has its own safety data sheet.

Isocyanates need the most care. Breathing vapour, mist or aerosol can irritate the eyes, nose and lungs and cause respiratory sensitisation (an allergy) that may lead to occupational asthma; once sensitised, a person can react to very low concentrations. Skin contact can cause irritation and allergic dermatitis and may contribute to respiratory sensitisation, so avoid it. MDI has a low vapour pressure at room temperature, so airborne MDI is mainly a concern when heated or sprayed; TDI needs controls such as local exhaust ventilation. Work only with the controls and protective equipment named in the safety data sheet (SDS).

In the EU, Regulation (EU) 2020/1149 (REACH Annex XVII, entry 74) requires industrial and professional users of substances or mixtures that contain 0.1 percent by weight or more of diisocyanates (individually or combined) to complete training before use. It has applied since 24 August 2023 and the training must be renewed at least every five years. Since February 2022 suppliers have had to print on the packaging: “As from 24 August 2023 adequate training is required before industrial or professional use”. In Türkiye, KKDİK (the Regulation on Registration, Evaluation, Authorisation and Restriction of Chemicals, similar to REACH) and the SEA regulation on classification and labelling apply, and under KKDİK Article 27 the safety data sheet is supplied in Turkish.

9. Ready system or separate raw materials? How a system house helps

Large chemical groups make MDI, TDI and polyols at industrial scale. A usable foam still needs the right polyols, catalysts, surfactant, blowing agent and flame retardant, balanced and matched to an isocyanate. Companies that do this work and supply a ready two-part package are called system houses. Manufacturers choose ready systems because:

• One matched pair: blend and isocyanate are balanced in advance, so there is one ratio and one index to manage.
• Fewer chemicals on site: catalysts, surfactants, flame retardants and blowing agents arrive pre-blended; users typically do not handle them separately.
• Formulation know-how: a panel, a cushion and a spray job each need a different balance of reactivity, flow and cure.
• Documents and support: TDS, SDS and help with trials.

JiTPOL is a polyurethane system house in Bursa. We formulate, develop and supply two-component systems for rigid, flexible, viscoelastic, integral skin and spray applications, together with mould release agents of our own production and auxiliary chemicals. Tell us about your product and process, and we will suggest a system family and send the TDS and SDS. Buyers abroad can start with Buying Polyurethane Systems from Türkiye.

Frequently asked questions

Sources

1. Wikipedia: Polyurethane
2. Wikipedia: Polyol
3. Wikipedia: Isocyanate
4. Makowska et al. (2024), Materials: environmentally friendly rigid polyurethane foams
5. Ivdre et al. (2020), Polymers: isocyanate index in rigid foams
6. Kurańska et al. (2025), Materials: repolyols from glycolysis
7. Olszewski et al. (2022), Polymers: stoichiometry in flexible foams
8. ISO 14900: hydroxyl number of polyols
9. Allport, Gilbert and Outterside (eds.), MDI and TDI: Safety, Health and the Environment (Wiley, 2003), Part 1
10. Allport, Gilbert and Outterside (eds.), MDI and TDI: Safety, Health and the Environment (Wiley, 2003), Part 5: Supporting sciences
11. ACC / CPI: Polyol Resin Blends Safety and Handling (2022)
12. ACC / CPI: Health Effects of Diisocyanates (2013)
13. ACC / CPI: Safe Handling of MDI (2020)
14. Hanna Instruments: Moisture in Polyols
15. ISCC: Mass Balance Explained
16. OSHA: isocyanates overview
17. US EPA: chemicals in spray polyurethane foam
18. ISOPA: logistics and product stewardship for MDI and TDI
19. Commission Regulation (EU) 2020/1149 (diisocyanates, REACH Annex XVII), Official Journal L 252, 4.8.2020
20. KKDİK Regulation, Article 27 (in Turkish; Official Gazette of 23.6.2017, no. 30105, repeated issue)