TDI vs MDI vs HDI — Which Isocyanate for Your Polyurethane Application?
Isocyanates are the essential reactive component in polyurethane chemistry — they define the crosslink density, flexibility, UV resistance, and handling characteristics of the final PU product. In the GCC, where polyurethane foam is used in furniture manufacturing across Saudi Arabia and the UAE, PU rigid insulation is specified on construction projects, and two-pack PU coatings protect structural steel on NEOM and ARAMCO sites, choosing the right isocyanate is a fundamental formulation and procurement decision. This guide compares TDI, MDI, and HDI across every parameter relevant to GCC manufacturers.
The Three Main Isocyanates — At a Glance
| Property | TDI 80/20 | Polymeric MDI | Pure MDI | HDI Trimer |
|---|---|---|---|---|
| Type | Aromatic | Aromatic (polymeric) | Aromatic (pure) | Aliphatic |
| CAS Number | 26471-62-5 | 9016-87-9 | 101-68-8 | 28182-81-2 |
| NCO content (%) | ~48 | ~31 | ~33.6 | ~21 |
| Functionality | 2 (difunctional) | 2.6–3.0 (average) | 2.0 | 3.0 (trimer) |
| Viscosity at 25°C | Low (~3 cPs) | Medium (~150–200 cPs) | High (solid at 20°C) | High (~3,000 cPs) |
| UV resistance | Poor — yellows rapidly | Poor — yellows | Poor — yellows | Excellent — colour stable |
| Vapour pressure | Higher (safety concern) | Very low (safer) | Moderate | Very low |
| Cost | Low | Low | Medium | High |
TDI 80/20 — Flexible PU Foam and Some Coatings
TDI 80/20 (a mixture of 80% 2,4-TDI and 20% 2,6-TDI isomers) is the dominant isocyanate for flexible polyurethane foam manufacturing — the largest PU application globally. In Saudi Arabia and UAE, it is used primarily for:
- Flexible PU foam: Furniture cushions, mattresses, automotive seating, packaging foam. This is the primary market for TDI in the GCC, where Saudi Arabia has a substantial domestic furniture manufacturing sector.
- Slabstock foam production: Continuous block foam for the furniture and bedding industry.
- Some 2K PU coatings: Interior coatings where UV exposure is not a concern — TDI-based hardeners can be used for interior floor coatings, industrial interior coatings, and adhesives.
Do NOT use TDI for exterior coatings in the GCC — the aromatic structure yellows rapidly under Saudi Arabia's intense UV radiation. Any exterior coating using TDI as the isocyanate component will show significant yellowing within months.
Polymeric MDI (Crude MDI) — Rigid Foam and Construction
Polymeric MDI (also called crude MDI or PMDI) is the most widely used isocyanate globally by volume, primarily for rigid polyurethane foam:
- Rigid PU insulation foam: Sandwich panels for construction, refrigerated containers, cold room panels. GCC demand is growing as Saudi Arabia's construction programme (NEOM, Vision 2030 residential) specifies foam insulation for energy efficiency.
- Spray foam: Applied insulation for roofs and walls in GCC industrial and commercial buildings.
- Composite wood binding: MDI-based binders for wood composites and particleboard.
The higher functionality of Polymeric MDI (average ~2.7) compared to pure MDI (2.0) provides greater crosslink density in rigid foam, giving better compressive strength and dimensional stability — critical for GCC construction applications where foam may be exposed to 55–60°C roof surface temperatures.
HDI — Exterior Coatings Where Nothing Else Will Do
HDI (hexamethylene diisocyanate) and its derivatives (HDI trimer, HDI biuret) are aliphatic isocyanates with excellent UV stability. They are the premium choice for any exterior coating application in the GCC:
- 2K PU topcoats for structural steel: The standard choice for exterior protective coating systems on NEOM, Saudi Aramco, and major GCC infrastructure projects. HDI trimer Part B combined with an acrylic polyol Part A gives 10+ year gloss and colour retention under Saudi UV.
- Automotive refinish: HDI-based clear coats are standard in automotive OEM and refinish.
- Marine topcoats: For vessels and offshore structures in the Arabian Gulf, HDI topcoats outperform aromatic alternatives by a wide margin in gloss retention over time.
Sourcing Isocyanates in the GCC from Raykem
Raykem supplies TDI 80/20 and Polymeric MDI to foam manufacturers, construction chemical producers, and coating manufacturers across the UAE and Saudi Arabia. Available in 250 kg drums and ISO tank quantities with full SDS, COA, and GHS-compliant labelling. Contact our technical team for isocyanate selection guidance for your specific application, current pricing, and lead times from origin.
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Request a Quote →Frequently Asked Questions
While TDI can technically be used for rigid foam, MDI (specifically Polymeric MDI / Crude MDI) is strongly preferred for rigid PU foam insulation panels used in construction. Polymeric MDI provides higher crosslink density (due to its multi-functional nature), lower vapour pressure than TDI (safer handling), better compatibility with polyol blends designed for rigid foam, and generally lower cost per unit foam volume. TDI is primarily optimised for flexible foam chemistry. Switching from MDI to TDI for rigid foam would require significant formulation changes and would likely result in inferior physical properties. Raykem supplies both TDI 80/20 and Polymeric MDI — contact our technical team for grade selection.
Raykem can source HDI (hexamethylene diisocyanate) and HDI-based polyisocyanates (trimerised HDI, HDI biuret) for coating applications. HDI is primarily used as Part B (isocyanate hardener component) in two-pack aliphatic polyurethane topcoats where UV resistance and colour stability are critical — for example, exterior structural steel coatings on NEOM projects, marine topcoats, and automotive refinish. HDI is not used in foam applications. If you are formulating an exterior 2K PU topcoat for GCC conditions, always specify HDI or IPDI-based aliphatic isocyanates — never TDI, which is aromatic and will yellow under UV exposure.
MDI has a significantly lower vapour pressure than TDI, making it considerably safer from an inhalation exposure perspective. TDI has a vapour pressure of approximately 2 Pa at 20°C, meaning it readily evaporates and creates inhalation hazard when handled in open systems. Polymeric MDI has a vapour pressure of <0.001 Pa — essentially no evaporation at ambient conditions. This is why MDI is preferred for hand-pour foam applications and pourforms, while TDI requires enclosed mixing systems and exhaust ventilation. Both are classified as sensitisers (respiratory sensitisation risk with repeated exposure), but TDI's higher vapour pressure makes inhalation exposure management more critical. Always follow the SDS and COSHH assessment for your specific isocyanate — Raykem provides full GHS SDS for all isocyanate products.
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