Polyurethane Formulation: the Chemistry of Isocyanate and Polyols

8th November 2023

Polyols and isocyanates are two essential building blocks for the formulation of polyurethanes. What exact roles and functions do they provide during the Isocyanate Polyol Reaction? How they affect the final products? Let’s dive in to understand the chemistry between isocyanates and polyols.

What is isocyanates?

Isocyanates are chemical compounds containing NCO functional group (R−N=C=O), which are also the essential raw material to produce polyurethane products. It can see a wide range of applications in various industries such as automotive, construction, furniture and more.

Isocyanate chemical structure

Learn more at our comprehensive guide of Isocyanates.


What is polyols?

Polyols are high-reactive compounds containing multiple hydroxyl (-OH) groups in their molecules. They can be derived from various sources, including petrochemicals and natural oils. Polyols are generally soluble in water and appear as viscous liquids or crystalline solids with high boiling points. Most polyols exhibit strong dissolving capabilities for low toxicity and volatility. Their boiling points, viscosity, relative density, and melting points increase with molecular weight. There are mainly two types of polyols including polyether polyols and polyester polyols.

Types of polyols

In the synthesis of polyurethane, polyols are primarily classified into two main categories: polyester polyols and polyether polyols. Polyurethane synthesized from different polyols and isocyanates can deliver distinct properties and applications.

Polyether polyols

Polyether polyols are synthesized through etherification reactions, commonly involving the reaction of epoxides like ethylene oxide with hydroxyl compounds. Polyether polyols find extensive use in the production of flexible foams, elastomers (such as elastic foam), sealants, coatings, and adhesives.

Polyether Chemical StructurePolyether Chemical Structure

Polyester polyols

Polyester polyols are prepared through esterification reactions, typically involving the reaction of dicarboxylic acids (such as adipic acid) with polyhydric alcohols (such as glycols). Polyester polyols is primarily utilized in the production of rigid foams, coatings, adhesives, and sealants.

Polyester Chemical Structure

Polyester Chemical Structure

Reaction of isocyanate and polyols

The reaction of isocyanates and polyols is the primary process for the production of polyurethane products, also known as Polyurethane Curing or Isocyanate Polyol Reaction. Isocyanates and polyols are mixed according to specific chemical formulations and molar ratios under specific conditions with the presence of catalyst and additives.

In this reaction, the NCO functional group (-NCO) in isocyanates react with the hydroxyl groups (-OH) in polyols to form urethane linkages (-NHCOO-), as depicted in the chemical equation:


Doxu supplies a comprehensive range of  polyisocyanate curing agents/ crosslinkers for polyurethane coating formulations. Click and check the detailed specifications.

Isocyanate and polyols ratio

Different isocyanate and polyols ratio can form different types of polyurethanes products. For instance, polyols can adjust the flexibility and resilience of the resulting polyurethane, while isocyanates serve as the crosslinking agent that impart strength and stability to the polyurethane materials. Generally, a higher ratio of isocyanates can form a more densely packed molecular structure, which make the resulted polyurethane harder and more rigid. Higher polyol content usually leads to a softer and more flexible polyurethane.

Types of these isocyanate and polyols also matter. Polyurethanes made from polyester polyols exhibit higher strength and hardness, whereas those derived from the reaction of polyether polyols demonstrate superior flexibility and resistance to hydrolysis.

Isocyanate and polyol resin

Polyurethane products are typically formulated as two-component systems, containing isocyanate or isocyanate pre-polymers (component A) and polyol resin blend (Component B).  Polyol resin blend are achieved by the blending of polyol and additional ingredients which can bring desired properties in polyurethane, such as catalyst, solvents, colorants, defoamer and dispersant. In the formulation of polyurethane systems, isocyanate and polyol resin blends are kept separate and mixed together during applications. To achieve the desired properties of the final polyurethane product, the formulators would adjust the proportions of these two components and mix them in the specific ratio.