Isocyanates For Polyurethane: MDI, TDI And Prepolymers

Isocyanates for polyurethane supply the reactive -NCO groups that join with polyol hydroxyl groups to build the polymer. Two aromatic families, MDI and TDI, account for the great majority of global polyurethane, with aliphatic isocyanates and prepolymers serving specialised coating, elastomer and adhesive markets. This guide covers the grades, the numbers that define them, how to dose them and how to store and handle them safely.

01The NCO Group And The Urethane Reaction

The isocyanate group, -N=C=O, reacts readily with any molecule carrying an active hydrogen. With the hydroxyl group of a polyol it forms a urethane link, which gives polyurethane its name:

R-N=C=O + R'-OH → R-NH-CO-O-R' (urethane)

Two side reactions matter in practice. With water, the isocyanate forms an unstable carbamic acid that splits into an amine and carbon dioxide; the CO₂ blows foam, and the amine reacts with more isocyanate to form urea. Under suitable catalysts and a large isocyanate excess, three NCO groups also combine into an isocyanurate ring, the heat-stable structure behind PIR foam:

R-N=C=O + H₂O → R-NH₂ + CO₂ ; R-NH₂ + R-N=C=O → R-NH-CO-NH-R (urea)

The balance between the gelling (urethane) and blowing (water) reactions is steered by catalysts, which are covered in the additives guide.

Aromatic And Aliphatic Isocyanates

In aromatic isocyanates (MDI, TDI) the NCO group sits on a benzene ring. They are highly reactive and economical, and they form the backbone of foams, elastomers and adhesives. Their drawback is UV yellowing. Aliphatic isocyanates have no aromatic ring; they react more slowly and cost more, but keep their colour and gloss outdoors, which is why they dominate light-stable top coats.

02The MDI Family

MDI (methylene diphenyl diisocyanate) is the largest-volume isocyanate and the most versatile. It is supplied in three main forms, and Pluskim offers polymeric, monomeric and modified grades on its isocyanates (MDI and TDI) page.

Monomeric MDI

Pure 4,4'-MDI contains about 33.6 % NCO and has a functionality of exactly two. It is a solid at room temperature, melting at roughly 38-40 °C. It is used where a linear, well-defined structure is needed: thermoplastic polyurethane (TPU), cast and microcellular elastomers, spandex fibres and high-performance adhesives. Mixtures richer in the 2,4'-isomer are liquid and somewhat less reactive.

Polymeric MDI

Polymeric MDI (often written pMDI or PMDI) is a dark brown liquid mixture of MDI monomer and higher-ring oligomers. Its average functionality is typically 2.6 to 3.0 and its NCO content typically about 30.5-32 %. The higher functionality builds dense networks, so pMDI is the base isocyanate for rigid insulation foam in sandwich panels, spray foam, refrigerators and pipe insulation, and it is also used in wood binders and some adhesives.

Modified MDI

Modified MDIs are chemically adjusted to stay liquid and to process more easily. Carbodiimide- or uretonimine-modified MDI is a liquid with about 29 % NCO that does not crystallise like the pure monomer. Other modifications pre-react a small amount of glycol or polyol with the MDI. Modified grades are common in integral skin foam, elastomers, shoe sole systems and moulded flexible foam.

03TDI: Toluene Diisocyanate

TDI is a low-viscosity liquid with about 48 % NCO, the highest of the common aromatic isocyanates because its molecule is small. It is the traditional isocyanate for flexible slabstock (block) foam used in mattresses, furniture and packaging.

TDI is sold as isomer blends. TDI 80/20 (80 % 2,4-isomer, 20 % 2,6-isomer) is the most common; TDI 65/35 is used where a different cure profile or firmer foam is wanted. TDI is also used in some prepolymers, coatings and adhesives. Because its vapour pressure is higher than MDI's, TDI requires closed handling systems and good ventilation. For a fuller comparison, see MDI vs TDI.

04Aliphatic Isocyanates For Light-Stable Coatings

Aliphatic isocyanates are chosen when appearance must survive sunlight. The main monomers are HDI (hexamethylene diisocyanate) and IPDI (isophorone diisocyanate), with H12MDI used in some elastomers and coatings.

Monomeric HDI is rarely used directly. It is converted into oligomers such as biurets and isocyanurate trimers, which have much lower volatility and form the hardener component of two-component polyurethane top coats for vehicles, floors, roofs and industrial structures. The European aliphatic producers' association ALIPA publishes guidance on the safe use of these products.

05Prepolymers

A prepolymer is made by reacting a diisocyanate with a polyol under controlled conditions, leaving free NCO groups at the chain ends. Depending on the formulation, prepolymers carry roughly 1 % to 30 % free NCO.

Pre-reacting part of the system brings several advantages: lower vapour pressure and often lower free monomer, a controlled molecular structure, more predictable reactivity and pot life, and better mechanical properties in the finished part. Quasi-prepolymers with high NCO content are common in shoe soles and spray polyurea; low-NCO prepolymers are used in cast elastomers, one-component moisture-curing adhesives, sealants and coatings. Pluskim has produced prepolymers since 2017 and offers MDI-based grades under its PLUSNATE series; see its polyurethane prepolymers page for the range.

06NCO Content And The Isocyanate Index

Two numbers decide how much isocyanate goes into a formulation: the NCO content of the isocyanate and the target isocyanate index of the system.

NCO content is the weight percentage of isocyanate groups, measured by standard titration methods such as ASTM D5155. From it, the isocyanate equivalent weight is 4,200 divided by the NCO percentage (42 being the mass of an NCO group). Polymeric MDI at 31 % NCO therefore has an equivalent weight of about 135.

Index = (isocyanate used / isocyanate required for stoichiometry) × 100

At index 100 the isocyanate exactly matches all hydroxyl groups and water. Running above 100 raises hardness and crosslinking; running below gives softer, weaker products. Very high indices, combined with trimerisation catalysts, produce isocyanurate structures for PIR foam.

Values are typical industry ranges for orientation only; the certificate of analysis for each batch is the reference for calculation.

07How To Choose An Isocyanate Grade

The right isocyanate follows from the product, the process and the polyol it will meet. Four questions narrow the choice quickly.

• Rigid or flexible? Rigid insulation foams need the crosslink density of polymeric MDI. Flexible slabstock traditionally uses TDI, while moulded flexible foam increasingly uses MDI or MDI/TDI blends.
• Does it see sunlight? Exposed coatings and top coats need an aliphatic hardener; aromatic grades are fine where a later coat or the final assembly protects the polyurethane.
• How is it processed? High-pressure spray and panel lines suit low-viscosity liquid grades. Casting and adhesive production often favour prepolymers, which give longer pot life and lower vapour pressure.
• What does the polyol side demand? Functionality, hydroxyl value and water content of the polyol blend set the stoichiometric isocyanate demand, so the two components are always specified together. The polyols guide explains those parameters.

Where several grades could work, a trial on the processor's own line is the reliable way to decide, because reactivity, flow and demould time depend on equipment and conditions as much as on chemistry.

08Quality And Acceptance Parameters

Incoming inspection of an isocyanate usually checks a short list of parameters against the supplier's data sheet:

• NCO content, the basis of every dosing calculation.
• Viscosity at a stated temperature, which affects metering and mixing.
• Acidity (often expressed as ppm HCl) and hydrolysable chlorine, which influence reactivity and catalyst demand.
• Isomer ratio for TDI and monomeric MDI blends.
• Colour and clarity; turbidity or sediment can indicate moisture contamination or dimer formation.

Keeping isocyanate and polyol from matched, well-documented sources reduces variation. That is one reason processors buy pre-formulated systems; Pluskim's polyol and MDI systems pair both components for a given application.

09Storage And Handling

Isocyanates are reactive chemicals, and most storage problems come from moisture or temperature. Follow the supplier's data sheet; the points below are general good practice.

• Exclude moisture. Keep containers sealed and blanket bulk tanks with dry air or nitrogen. Water forms insoluble urea and CO₂, which can pressurise a closed drum and leave solids that block filters and mix heads.
• Control temperature. TDI 80/20 solidifies at roughly 12-15 °C, so it must be kept above that range. Polymeric MDI can thicken or crystallise when cold. Pure monomeric MDI is a solid at room temperature and is typically stored either molten within a narrow temperature window for a limited time or frozen.
• Avoid prolonged heat. MDI slowly forms a dimer, faster at elevated temperature; the dimer is insoluble and appears as turbidity or sediment.
• Use dedicated equipment for isocyanates and clean lines with suitable solvents rather than water.

Transport of bulk MDI and TDI in Europe follows ISOPA's logistics guidelines, covering carriers, driver training, discharge and emergency response.

10Safety And The EU Training Requirement

Diisocyanates can cause skin and respiratory sensitisation, so exposure must be prevented through closed processes, local exhaust ventilation and suitable personal protective equipment as specified in the safety data sheet.

In the European Union, Regulation (EU) 2020/1149 added entry 74 to REACH Annex XVII. Since 24 August 2023, industrial and professional users of products containing more than 0.1 % monomeric diisocyanates, alone or in combination, must complete adequate training before use, refreshed at least every five years. Suppliers must indicate the requirement on the packaging. ISOPA summarises the restriction on its REACH restriction page, and courses are available through the industry portal safeusediisocyanates.eu.

11Sources And Further Reading

1. ISOPA, REACH restriction on diisocyanates - isopa.org/reach-restriction-on-diisocyanates
2. EUR-Lex, Commission Regulation (EU) 2020/1149 - eur-lex.europa.eu/eli/reg/2020/1149/oj
3. Safe use of diisocyanates, training portal - safeusediisocyanates.eu
4. ALIPA, European Aliphatic Isocyanates Producers Association - alipa.org
5. ASTM D5155, Standard Test Methods for Polyurethane Raw Materials: Determination of the Isocyanate Content of Aromatic Isocyanates - store.astm.org/d5155-19.html
6. US EPA, Methylene diphenyl diisocyanate (MDI) and related compounds - epa.gov

12Frequently Asked Questions

13Related Reading

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• MDI Vs TDI: Choosing The Right IsocyanateMDI vs TDI compared: structure, NCO content, volatility, handling, grades and typical applications, with a checklist for choosing the right isocyanate.→
• Polyurethane AdditivesPolyurethane additives explained: catalysts, silicone surfactants, blowing agents, flame retardants and chain extenders, with typical dosages and fixes.→