India's polyurethane demand is growing fast. Grand View Research estimates the market at USD 5,991.9 million in 2025, with a forecast 14.5% CAGR from 2026 to 2033. More demand means more formulators asking the same questions.
This guide covers what each component does, how they react, why the right ratio depends on equivalents rather than a rule of thumb, and how to choose a combination for your application.
Key Takeaways
- Polyols supply hydroxyl groups and shape flexibility, resilience and viscosity.
- Isocyanates supply NCO groups and drive cure, crosslinking, hardness and strength.
- Mixing ratio comes from OH value, NCO content, solids, moisture and target index, not a fixed rule.
- Aromatic and aliphatic isocyanates trade off cost, reactivity and weathering.
- Confirm every choice with TDS, SDS, COA and trials.
What Are Polyols and Isocyanates?
Polyurethane forms when polyisocyanates react with polyols. Each component has two or more reactive groups, which is what lets chains grow and link together.
What is a polyurethane polyol?
A polyol contains multiple hydroxyl (-OH) groups. Those groups react with isocyanate groups to form urethane linkages.
Several properties determine how a polyol behaves in the final network:
- Molecular weight and hydroxyl value
- Functionality (OH groups per molecule)
- Backbone chemistry
- Viscosity
- Distribution of hydroxyl groups along the chain
Common polyol families and typical strengths:
| Family | Typical strengths | Watch-outs |
|---|---|---|
| Polyether | Hydrolysis resistance, low-temperature flexibility, microbial resistance | Lower mechanical, heat and mineral-oil resistance than polyester in BASF's TPU comparison |
| Polyester | Mechanical strength, heat and mineral-oil resistance | Prolonged hot water, moisture vapour or tropical exposure can hydrolyse chains |
| Acrylic | Fast cure, early hardness, long pot life (BASF Joncryl RPD 950-AC/P, solventborne 2K) | Profile is grade-specific; not every acrylic polyol matches this cure and pot-life balance |
| Polycarbonate | Flexibility, hydrolysis, oxidative and thermal resistance; weather-stable coatings and sealants | Grade-dependent |
| Bio-based / natural-oil | Renewable feedstock options; Sovermol 750/760 with HDI gives UV, weathering and chemical resistance | Often polyester or ether structures underneath |
What is an isocyanate?
The isocyanate group (-NCO) is the reactive partner. Diisocyanates and polyisocyanates carry multiple NCO groups, enabling chain extension or crosslinking.
Common chemistries are TDI, MDI, PMDI, HDI and IPDI. NCO content varies widely, and only the current data sheet for your grade is reliable:
| Type | Class | Example grade | NCO, wt.% |
|---|---|---|---|
| TDI | Aromatic | Desmodur T 80 | ≥48 (typical) |
| MDI, monomeric | Aromatic | Desmodur LS 2424 | 33.6 (theoretical) |
| PMDI | Polymeric MDI | Desmodur 44 V 20 L | 30.5-32.5 |
| HDI, monomeric | Aliphatic | Desmodur H | ≥49.7 |
| IPDI | Cycloaliphatic | VESTANAT IPDI | 37.5-37.8 |
| HDI trimer | Aliphatic | Desmodur N 3300 | 21.8 ± 0.3 |
Note the gap between HDI monomer and its trimer. Their equivalent weights are not interchangeable.
How do the two components work together?
The polyol usually provides the hydroxyl-bearing resin or soft segment. The isocyanate acts as hardener, connecting chains. Change either one and the whole system shifts, so evaluate them as a matched pair.
How Do Polyols and Isocyanates React?
What happens during the isocyanate–polyol reaction?
An NCO group meets an OH group and forms a urethane linkage:
R-NCO + R'-OH → R-NH-CO-O-R'
With multifunctional reactants, the result can be linear, branched or crosslinked.
In segmented polyurethanes, flexible segments come from polyol and diisocyanate, while rigid segments come from diisocyanate and short-chain diol. Higher crosslink density and higher hard-segment content generally mean more hardness and chemical resistance, with less elasticity.
Why does moisture matter?
Water reacts with isocyanate to give an amine and carbon dioxide. The amine then reacts quickly with more isocyanate to form urea linkages.
Practical consequences:
- Bubbles and foam defects from CO₂
- Viscosity drift and altered cure
- Poor surface appearance
- Inconsistent mechanical properties
- Pressure build-up in closed TDI/MDI containers if contaminated
Treat intentional water as its own reactive component in the formulation. It is not just dilution.
How is the mixing ratio determined?
Work in reactive equivalents, not grams. The standard relationships, as published by the Polyurethane Manufacturers Association:
EW_OH = 56,100 / OH value
EW_NCO = 4,200 / NCO%
OH equivalents = m_polyol / EW_OH
NCO equivalents = m_isocyanate / EW_NCO
Index = NCO equivalents / OH equivalents
m_isocyanate = Index x EW_NCO x (m_polyol / EW_OH)
These cover OH-only systems. Add water and other reactive components separately. Solids content matters too: adjust component mass to the reporting basis.
A stated ratio by weight or volume does not transfer between grades unless concentration, density, OH value, NCO content and basis are comparable.
Why is there no universal "ideal" ratio?
Application, target hardness, crosslink density, cure conditions, moisture, catalysts, fillers and additives all affect which index is right.
Worked example (hypothetical assumptions only):
- Polyol: OH value 280 mg KOH/g, 100 g, 100% solids
- Isocyanate: NCO 31%
- Target index: 1.05
Steps:
- EW_OH = 56,100 / 280 = 200.4 g/eq
- OH equivalents = 100 / 200.4 = 0.499
- EW_NCO = 4,200 / 31 = 135.5 g/eq
- Isocyanate mass = 1.05 × 135.5 × 0.499 ≈ 71 g
So about 71 g isocyanate per 100 g polyol, for these numbers only. Swap either grade and the answer changes. Always follow supplier guidance and testing.
Verify inputs against titration standards:
- ASTM D4274 for hydroxyl numbers
- ASTM D2572 for NCO in prepolymers
- ISO 14896 for aromatic isocyanate NCO content
- ISO 4629-1 for hydroxyl value of paint binders
What formulation problems should be monitored?
- Incomplete mixing or weighing errors
- Moisture contamination
- Incompatible components
- Excessive reaction speed or short pot life
- Bubbles, shrinkage and surface defects
- Inadequate cure
Run controlled trials comparing viscosity, pot life, cure development, adhesion, hardness, flexibility, and abrasion or chemical resistance.
How Do You Choose Polyols and Isocyanates for Different Applications?
Start with the end-use performance requirements
List the requirements first:
- Substrate and service environment
- Processing method
- Target hardness or flexibility
- Water, chemical, temperature and UV exposure
- Appearance and cure time
- Regulatory or sustainability needs
Then split your framework into two columns. Material properties (OH value, NCO content, functionality) sit on one side. Purchasing factors (supply continuity, documentation, support) sit on the other.
Flexible and rigid polyurethane foams
Polyether polyol with TDI is the classic flexible foam pairing, per BASF, which also identifies MDI alternatives. Rigid foam typically uses polyether or polyester polyols with PMDI. Polyester grades suit higher strength, adhesion or chemical resistance needs.
Tie the choice to cell structure, density, dimensional stability, insulation, resilience and your equipment. No single chemistry fits every foam process.
Coatings, adhesives, sealants and elastomers
- Coatings: Acrylic or polyester polyols with HDI-derived hardeners such as N 3300. IPDI is another light-stable option.
- Elastomers: Polyether or polyester polyols with TDI/MDI prepolymer systems.
- Adhesives: Polyether or polyester with aromatic or aliphatic isocyanates, forming NCO-terminated prepolymers.
- Sealants: Polyether for flexible, hydrolysis-resistant systems; polycarbonate for weather-stable ones.
Aliphatic systems are usually investigated where light exposure and yellowing matter. Aromatic systems suit cost-driven, reactive or indoor uses. Aromatic grade data sheets often omit a UV rating, so test rather than assume.
Industrial and sector-specific applications
| Sector | What should guide formulation |
|---|---|
| Automotive | Cabin air quality, tested by VDA 278 or chamber methods |
| Wire and cable | Abrasion, tear, weather, ozone and chemical resistance; hydrolysis and cold flexibility |
| Flooring | Hydrolysis and chemical resistance, elasticity, crack bridging |
| Insulation | Thermal conductivity, dimensions, workmanship (see BIS IS 12436 product manual) |
| Packaging | Food-contact rules and heat or sterilisation exposure |
| Healthcare | Device manufacturer owns suitability testing and sterilisation choice |
The same pair will not serve all of these.
Build a technical comparison before purchasing
Compare these parameters across candidate grades:
- OH value, functionality and NCO content
- Viscosity, solids and colour
- Moisture specification and cure profile
- Storage conditions and compatibility guidance
- Available grades
Before approval, request:
- Current TDS, SDS and COA
- Samples
- Storage and processing recommendations
- Application references
Then verify supplier claims in application-specific lab trials, and pilot or production runs where relevant.
Quality, Safety, and Sourcing Considerations
Handle isocyanates and reactive components responsibly
Isocyanates can irritate eyes and airways and inflame skin. NIOSH notes that respiratory and skin exposure can both cause sensitisation, and re-exposure can trigger severe asthma attacks. Closed systems and ventilation are the main controls.
- Read the current SDS for your grade
- Use engineering controls, ventilation, PPE and containment
- Train operators and prepare spill procedures
- Review OSHWC Code duties (in force from 21 November 2025)
- Note MSIHC Rules 1989 list MDI and TDI in Schedule 3
Confirm applicability with your EHS advisor.
Control raw-material and batch quality
On arrival, verify:
- Identity, appearance and viscosity
- Solids or NCO/OH data, moisture and COA consistency
Store in sealed original containers, away from moisture and heat. Shelf life is grade-specific: Covestro states 6 months for 44 V 20 L and 9 months for N 3300 under stated conditions.
Record mixing sequence, temperature, batch size, equipment, cure conditions and test results. That trail makes scale-up and troubleshooting traceable.
Evaluate a supplier beyond price
Look at:
- Technical support and application trials
- Supply continuity and logistics
- Batch-yield visibility and scale-up capability
- Quality systems and responsiveness
- IP protection
Distil is an R&D-led, asset-light speciality chemicals platform. Its dedicated R&D team manages application-specific tuning and trials. A network of 20+ manufacturing partners supports flexible manufacturing and scale-up from development to commercial production, with a unified quality system for batch consistency. Specific polyol or isocyanate grades should be confirmed with the team directly.
Frequently Asked Questions
What is the reaction between isocyanate and polyol?
An NCO group reacts with an OH group to form a urethane linkage. Water and other active-hydrogen compounds can trigger competing reactions, such as urea formation.
What is the ideal mixing ratio for polyol and isocyanate?
There is no universal ratio. Calculate NCO and OH equivalents from your grades' NCO content, OH value, solids basis and target index, then validate by testing.
Is isocyanate hazardous?
Yes. Isocyanates can cause irritation and sensitisation. Follow the SDS, with ventilation, suitable PPE, trained operators and exposure controls for your grade and process.
What is the difference between polyether and polyester polyols?
Polyethers generally offer better hydrolysis resistance and low-temperature flexibility. Polyesters tend to give stronger mechanical, heat and oil resistance. Grade data should drive the final choice.
How do I choose between aromatic and aliphatic isocyanates?
Base it on UV exposure, colour stability, reactivity, hardness, flexibility, cost and cure needs. Confirm with your chosen polyol and application tests.
