The MDI vs TDI question comes up whenever a plant designs a new line, qualifies a second supplier or moves into a new application. Both are aromatic diisocyanates that react with polyols to form polyurethane, but they differ in structure, volatility, grade range and where they perform best. This article sets out those differences in practical terms.
TDI (toluene diisocyanate) is a volatile, high-NCO liquid (typically about 48 % NCO) used mainly for flexible slabstock foam. MDI (methylene diphenyl diisocyanate) is far less volatile, is supplied in polymeric, monomeric and modified grades, and is the standard isocyanate for rigid foam, elastomers, footwear, binders and most coatings and adhesives.
- Volume split: TDI is the workhorse of flexible block foam; MDI covers almost everything else, from insulation panels to shoe soles.
- Volatility: TDI has a much higher vapour pressure than MDI, so exposure control at room temperature is more demanding.
- Grades: MDI comes as polymeric, monomeric and modified grades plus prepolymers; TDI is mainly sold as the 80/20 and 65/35 isomer blends.
- Regulation: both fall under the EU REACH diisocyanate training restriction in force since 24 August 2023.
- Choice: the isocyanate is chosen together with the polyol system, density target and process, never in isolation.
01Two Molecules, Two Structures
MDI and TDI both carry two isocyanate groups (-N=C=O), which is what lets them build a polymer network with polyols. Their backbones are different, and that difference explains most of their behaviour.
TDI: One Aromatic Ring
TDI is toluene with two isocyanate groups attached. It is produced as a mixture of the 2,4- and 2,6-isomers. The two NCO groups on the 2,4-isomer react at different speeds, which gives formulators a useful two-stage reactivity in flexible foam. The small molecule means a high share of reactive NCO groups by weight and a relatively high vapour pressure.
MDI: Two Aromatic Rings
MDI links two benzene rings through a methylene (-CH2-) bridge, with one isocyanate group on each ring. The larger molecule is much less volatile. Pure 4,4'-MDI is a solid at room temperature, so most MDI sold is either a liquid polymeric mixture or a modified grade that stays liquid. This flexibility in form is why MDI appears in so many different systems.
R-N=C=O + R'-OH → R-NH-CO-O-R' (urethane link)
02Grades On The Market
TDI is a short list of isomer blends; MDI is a family of products. Knowing the grade names avoids confusion when comparing offers.
- TDI 80/20: about 80 % 2,4-isomer and 20 % 2,6-isomer. The standard grade for flexible slabstock foam.
- TDI 65/35: a higher share of the 2,6-isomer, used for specific foam grades and processing behaviour.
- Polymeric MDI (pMDI, crude MDI): a dark brown liquid mixture of MDI isomers and higher oligomers, with an average functionality typically around 2.7. The base isocyanate for rigid foam.
- Monomeric MDI (pure 4,4'-MDI): a solid that melts at about 38-40 °C, used for elastomers, thermoplastic polyurethane and prepolymers. It slowly forms dimer during storage, so shelf life and storage temperature matter.
- Modified MDI: carbodiimide- or uretonimine-modified grades that keep pure MDI liquid at room temperature, plus isomer-rich and prepolymer-modified variants for integral skin, footwear and elastomer systems.
Pluskim supplies MDI- and TDI-based isocyanates, including polymeric and modified MDI grades, and offers its own MDI-based systems under the PLUSNATE series; the isocyanate grades Pluskim supplies are listed with their typical applications.
03Side-By-Side Comparison
The table summarises typical values and uses. Figures are general industry ranges for orientation, not specifications for any particular product.
| Property | TDI | MDI |
|---|---|---|
| Backbone | Toluene, one aromatic ring | Diphenylmethane, two aromatic rings |
| Main grades | 80/20, 65/35 isomer blends | Polymeric, monomeric, modified, prepolymers |
| NCO content | about 48 % | about 30.5-32 % (pMDI), 33.6 % (pure 4,4') |
| Form at 25 °C | Clear to pale liquid | Brown liquid (pMDI), solid (pure 4,4'), liquid (modified) |
| Vapour pressure | Relatively high | Several orders of magnitude lower |
| Average functionality | 2.0 | 2.0 (monomeric) to about 2.7 (polymeric) |
| Typical applications | Flexible slabstock foam, some moulded foam, TDI prepolymers for elastomers and coatings | Rigid insulation foam, HR moulded foam, elastomers, footwear, binders, coatings, adhesives, sealants |
| Storage concern | Can crystallise in cold conditions; keep dry | pMDI: keep dry and within the recommended range; pure MDI: dimer formation if stored warm |
04Volatility, Handling And Regulation
The most practical difference on the shop floor is volatility. TDI evaporates far more readily than MDI at room temperature, so it reaches measurable airborne concentrations faster and needs tighter engineering controls.
That does not make MDI harmless. Both are respiratory and skin sensitisers. MDI becomes an inhalation risk when it is heated or atomised, which is exactly what happens in spray foam, spray coatings and some high-temperature processes. Both react with water and release carbon dioxide, so moisture in a closed drum can build up pressure.
In the United States, OSHA publishes exposure limits and hazard guidance for isocyanates, and the EPA maintains an overview of MDI and related compounds. For logistics, ISOPA's guidelines cover bulk transport, unloading and storage of both isocyanates. Whatever the region, the safety data sheet of the specific product is the governing document.
05Where Each One Is Used
The split follows the chemistry: TDI's two-stage reactivity and low viscosity suit open-cell flexible foam, while MDI's range of functionalities suits everything from highly crosslinked insulation to tough elastomers.
Typical TDI Applications
- Flexible slabstock (block) foam for mattresses, furniture and packaging.
- Some moulded flexible foam, often as TDI/MDI blends.
- TDI-based prepolymers for cast elastomers, coatings and adhesives.
Typical MDI Applications
- Rigid PUR and PIR insulation: sandwich panels, cold rooms, refrigerators, pipe insulation and spray foam.
- High-resilience moulded foam and automotive seating.
- Footwear soles, integral skin foam and microcellular elastomers.
- Binders for rebonded foam, rubber granulate and sports surfaces.
- CASE: coatings, adhesives, sealants and elastomers.
A fuller map of which isocyanate and polyol go into each end use is on our raw materials by application page, and the general overview is in the isocyanates guide.
06Processing Differences
MDI and TDI rarely compete on the same line, because each process has been engineered around one of them. The differences that matter in practice are viscosity, reactivity, index and storage.
- Isocyanate index: flexible TDI foams typically run close to stoichiometric, around 100-115. Rigid PUR foams with polymeric MDI often run around 110-130, and PIR formulations use much higher indices, typically above 200, to form isocyanurate rings.
- Mix ratio: because of its higher NCO content, less TDI by weight is needed to react with a given amount of polyol than MDI. Metering equipment and ratios are set for one isocyanate.
- Viscosity and temperature: TDI is thin and flows easily; polymeric MDI is more viscous and is usually conditioned to a set temperature before dispensing. Pure MDI must be melted and handled warm.
- Formulation package: catalysts, surfactants and polyol blends are tuned to the isocyanate. Exchanging one for the other means reformulating, as the additives guide explains.
07Where Prepolymers Fit
A prepolymer is an isocyanate that has been partly reacted with a polyol, leaving NCO-terminated chains. It is a way of tailoring an isocyanate rather than a third option beside MDI and TDI.
Prepolymers can be made from either. MDI prepolymers dominate footwear, elastomers, one-component moisture-curing adhesives and sealants; TDI prepolymers are used in some cast elastomers and coatings. Depending on formulation, the free NCO content ranges widely, from about 1 % to about 30 %. Prepolymers lower vapour pressure and give controlled reactivity, easier mix ratios and better mechanical properties. Pluskim has produced MDI-based prepolymers since 2017 and also supplies ready polyol and MDI systems where a plant prefers a matched pair to separate components.
08Decision Checklist
For a new line, a new product or a supplier change, these questions usually settle the choice quickly.
- Product type: open-cell flexible block foam points to TDI; rigid insulation, elastomers, footwear and binders point to MDI.
- Existing equipment: metering ratios, temperature conditioning and ventilation are designed for one isocyanate. Changing means requalifying the line.
- Exposure control: can the site manage TDI vapour, or is a lower-volatility MDI system the more realistic choice?
- Target properties: density, hardness, resilience, compressive strength, thermal conductivity and fire behaviour decide the grade within each family.
- Polyol system: the isocyanate must match the polyol functionality and reactivity; see polyols for polyurethane.
- Training and compliance: confirm REACH training status for every user and the regional rules for export markets.
- Supply security: two qualified grades or suppliers, with a certificate of analysis per batch, reduce the risk of a stopped line.
09Sources And Further Reading
- ISOPA, REACH restriction on diisocyanates: isopa.org/reach-restriction-on-diisocyanates
- Safe use of diisocyanates, training portal: safeusediisocyanates.eu
- Regulation (EU) 2020/1149, EUR-Lex: eur-lex.europa.eu/eli/reg/2020/1149/oj
- US OSHA, isocyanates: osha.gov/isocyanates
- US EPA, methylene diphenyl diisocyanate (MDI) and related compounds: epa.gov
- ISOPA, logistics guidelines: isopa.org/logistics
10Frequently Asked Questions
What Is The Main Difference Between MDI And TDI?
MDI (methylene diphenyl diisocyanate) is built on two benzene rings linked by a methylene bridge, while TDI (toluene diisocyanate) is built on a single toluene ring. In practice, TDI has a higher NCO content (typically about 48 %) and is far more volatile, and it is used mainly for flexible slabstock foam. MDI is less volatile, comes in many grades and dominates rigid foam, elastomers, footwear and binders.
Is MDI Safer Than TDI?
Both are respiratory sensitisers and require the same basic discipline: closed handling, ventilation, protective equipment and trained staff. TDI has a much higher vapour pressure, so it reaches airborne concentrations more easily at room temperature and needs tighter exposure control. MDI is less volatile but can still be inhaled as aerosol during spraying or when heated. Neither should be treated as harmless.
Can MDI Replace TDI In Flexible Foam?
Partly. MDI-based and TDI/MDI blend systems are widely used for high-resilience moulded foam and automotive seating. Conventional flexible slabstock foam is still produced mostly with TDI, because the established foam lines, polyol systems and processing windows are built around it. Switching requires reformulating the polyol blend, catalysts and surfactant, not just exchanging the isocyanate.
What NCO Content Do MDI And TDI Have?
Typical values are about 48 % NCO for TDI 80/20, about 33.6 % for pure monomeric 4,4'-MDI and about 30.5-32 % for polymeric MDI. Modified MDI grades and prepolymers sit lower, often between 18 % and 29 %. The exact figure for a specific product is stated on its technical data sheet and confirmed on the certificate of analysis for each batch.
Do MDI And TDI Fall Under The EU Training Requirement?
Yes. Under REACH Annex XVII entry 74, introduced by Regulation (EU) 2020/1149, industrial and professional users of products containing more than 0.1 % monomeric diisocyanates must have completed adequate training since 24 August 2023. This covers most TDI and MDI products and many prepolymers. Training is available through the industry portal safeusediisocyanates.eu.
What Does TDI 80/20 Mean?
TDI is produced as a mixture of two isomers, 2,4-TDI and 2,6-TDI. TDI 80/20 contains about 80 % of the 2,4-isomer and 20 % of the 2,6-isomer and is the standard grade for flexible slabstock foam. TDI 65/35 contains more of the slower-reacting 2,6-isomer and is used where a more open cell structure or different processing behaviour is needed.
11Related Reading
- Isocyanates For Polyurethane: MDI, TDI And PrepolymersIsocyanates for polyurethane: MDI, polymeric MDI, TDI, aliphatics and prepolymers, NCO content, isocyanate index, safe storage and EU training rules.
- Polyether Vs Polyester Polyol: Which One For Your Application?Polyether vs polyester polyol compared on hydrolysis, strength, oil resistance, cold flexibility and cost, with a quick decision list for each application.
- Polyurethane Raw Materials By ApplicationWhich polyols, isocyanates and additives each application uses: rigid and flexible foam, footwear, coatings, adhesives and binders, in one clear table.
Sourcing These Raw Materials
Comparing offers for a polyol, an isocyanate or a formulated system? The supplier checklist covers the documents to request, what to test on delivery and the red flags to watch for.