Polyurethane (TDI, MDI): raw materials, properties and applications
Polyurethane (PU) is one of the most versatile and widely used polymers in modern industry. First synthesised by Otto Bayer and his colleagues in the late 1930s, polyurethane today occupies key positions in the production of flexible and rigid foams, elastomers, coatings, adhesives, sealants and many other materials. Aromatic isocyanates such as TDI and MDI represent around 95% of the diisocyanate market for the production of polyurethanes.
Chemistry of Polyurethanes
The chemistry of polyurethanes is based on the polyaddition reaction between polyisocyanates and polyols (compounds containing two or more hydroxyl groups). During this reaction, the hydroxyl group of the polyol attacks the electrophilic carbon atom of the isocyanate group (–N=C=O), forming a urethane bond. Catalysts play a key role in this process, accelerating the reaction and regulating the balance between two primary reactions: the gelling (polymerisation) reaction and the blowing (gas-producing) reaction. If the blowing reaction occurs faster than the gelling reaction, the foam may collapse; conversely, if polymerisation occurs faster, the foam cells remain closed, causing shrinkage. Two main types of catalysts are used to balance these reactions: metal-based catalysts (e.g., stannous octoate) and amine-based catalysts.
TDI (Toluene Diisocyanate)
TDI is a mixture of two main isomers — 2,4-TDI and 2,6-TDI, with the most common commercial grade TDI 80/20 containing 80% 2,4-isomer and 20% 2,6-isomer. The melting point of TDI is approximately 14 °C, making it a liquid at room temperature. The molecular weight of TDI is 174.20 g/mol. TDI is characterised by lower viscosity compared to MDI, higher reactivity, but also higher toxicity due to greater vapour pressure.
The synthesis of TDI involves several stages. Toluene is first nitrated to dinitrotoluene, which is then reduced to toluene diamine. In the final step, the diamine reacts with phosgene (COCl₂) to form TDI.
MDI (Methylene Diphenyl Diisocyanate)
MDI is synthesised in two stages: first, aniline and formaldehyde are condensed in the presence of an acid catalyst (e.g., hydrochloric acid) to form methylene diphenyl diamines (MDA). Depending on the reaction conditions, three principal isomers are formed: 2,2'-MDA, 2,4'-MDA and 4,4'-MDA. In the second stage, the mixture of MDA isomers is phosgenated using chlorobenzene as a solvent.
The molecular weight of MDI is 250.25 g/mol. MDI has higher viscosity and lower volatility compared to TDI, making it less toxic and safer to process. The melting point of pure MDI is approximately 38 °C, making it a solid at room temperature. The symmetric molecular structure of MDI promotes more ordered packing of polymer chains and provides higher tensile strength — up to 23.4 MPa.
MDI comes in three main forms: pure monomeric MDI (4,4'-MDI), polymeric MDI (pMDI, a mixture of oligomers), and modified liquid MDI grades (carbodiimide- or uretonimine-modified) designed to remain liquid at ambient temperature for easier processing.
Comparative Characteristics of TDI and MDI
Comparison of physical properties of TDI and MDI:
| Characteristic | TDI | MDI |
|---|---|---|
| Apparent colour | Colourless to pale yellow | Brown |
| Molecular weight, g/mol | 174.20 | 250.25 |
| Structure | Mixture of 2,4- and 2,6-isomers | Mixture of 2,2'-, 2,4'- and 4,4'-isomers |
| Viscosity | Lower | Higher |
| Reactivity | More reactive | Less reactive |
| Toxicity | More toxic | Less toxic |
| Density | Moderate | High |
| Main application | Flexible foams | Semi-rigid and rigid foams |
Structural differences also determine differences in the properties of the final polyurethanes. MDI's symmetric structure promotes more ordered packing and better microphase separation of hard and soft segments, providing higher tensile strength and abrasion resistance. TDI's asymmetric structure (the 2,4-isomer has different NCO group reactivities at the 4- and 2-positions) leads to less order but greater flexibility of the polymer network.
Global Capacity and Production
The isocyanate market is characterised by a high degree of concentration: the five largest producers control more than 90% of global capacity. As of 2026, global MDI capacity reached 1,112 ktpa, and TDI capacity stood at 368 ktpa. China is the world's largest producer and supplier of isocyanates, accounting for approximately 50% of global TDI production.
Major MDI Producers (2026) :
| Producer | Capacity, ktpa | Market Share |
|---|---|---|
| Wanhua Chemical | 450 | ~34% |
| BASF | 210 | ~19% |
| Covestro | 177 | ~16% |
| Huntsman | 137 | ~12% |
| Dow | 111 | ~10% |
Major TDI Producers (2026) :
| Producer | Capacity, ktpa | Market Share |
|---|---|---|
| Wanhua Chemical | 147 | ~40% |
| BASF | 92 | ~25% |
| Covestro | 67 | ~18% |
| Cangzhou Dahua | 15 | ~4% |
| Gansu Yinguang | 10 | ~3% |
By 2025, China has strengthened its position as the world's largest producer and supplier of isocyanates. The market is expected to grow at a compound annual growth rate of 5.38% by 2030.
Applications
Polyurethanes are used in almost all industries.
TDI is used almost exclusively to produce flexible polyurethane slabstock foam — the material used in mattresses, upholstered furniture, automotive seating and bedding. Flexible foam held approximately 58.7% of the global polyurethane foam market by revenue in 2023. TDI also finds some use in coatings, sealants and elastomers.
MDI is the primary raw material for rigid and semi-rigid foams widely used in building insulation and refrigeration equipment. Rigid polyurethane foam based on MDI accounts for approximately 36.8% of global MDI demand. In 2024, more than 1.2 billion square metres of building surfaces were insulated with MDI-based materials. MDI also dominates reaction injection moulding (RIM), cast elastomers, thermoplastic polyurethanes (TPU) and integral skin mouldings.
Polyurethane elastomers, coatings, adhesives and sealants (the CASE segment) constitute a significant share of the polyurethane market. Both TDI and MDI are used in this segment, with MDI-rich systems preferred for high-performance applications.
Emerging Trends
In recent years, the use of polyurethanes in new industries has been actively developing: electric vehicles, wind energy, photovoltaic panels and biomedical products. Active research is underway in the areas of thermoplastic polyurethanes (TPU), nanocomposites and dynamic/dissociative networks (vitrimers) that enable reprocessing and self-healing. More environmentally friendly technologies are also being developed — bio-based polyols, non-phosgene and greener isocyanate synthesis routes, and chemical recycling strategies.
Conclusion
TDI and MDI remain the two key raw material components for polyurethane production, each with its own unique properties and applications. The choice between them is determined by the specific requirements for the final product — from soft elastic foams to rigid thermal insulation materials. Growing capacities in China and other regions, as well as the development of more environmentally friendly production and recycling technologies, will determine the future of this critical industry.