Which SDS figures should you compare for IPDI, H12MDI, HDI and MDI before the show?

Line up five fields from each supplier’s Safety Data Sheet: vapor pressure with its temperature, the exposure limits in Section 8, molecular weight (the basis for NCO content), flash point, and the hazard classification in Section 2 of the product page SDS. The four diisocyanates are IPDI CAS 4098-71-9, H12MDI CAS 5124-30-1, HDI CAS 822-06-0 and MDI CAS 101-68-8. All four fall under REACH Annex XVII Entry 74, and all four sit inside EPA’s ongoing TSCA Section 6 diisocyanate evaluation.

The field that separates them is volatility. PubChem lists HDI monomer at 0.05 mmHg at 75 °F and IPDI at 0.0003 mmHg at 68 °F.

Picture three quotes on the show floor for “aliphatic isocyanate for a non-yellowing topcoat.” One is IPDI, one is HDI monomer, and one is an HDI trimer. The price per kilogram is close enough that lead time becomes the tiebreaker. That is where buyers get burned. Two of those offers can go through the same drum-handling area. The third may need engineering controls your plant has not budgeted.

Does the exposure limit actually separate these chemistries?

Start with the numbers most buyers look up first. OSHA’s permissible exposure limit is 0.02 ppm as a ceiling for MDI and for HDI. ACGIH’s TLV is 0.005 ppm TWA for both. NIOSH’s REL for diisocyanates is 0.005 ppm TWA with a 0.02 ppm ceiling. On Section 8 of the SDS, HDI and MDI look interchangeable.

The limits are identical. The rate at which each material puts vapor into the room is not. The limits in this pack cover HDI and MDI; for IPDI and H12MDI, take the Section 8 values from each supplier’s SDS and check that they cite a named body (OSHA, ACGIH, NIOSH or an EU national limit) rather than “not established.”

Sources: OSHA isocyanates page; NIOSH isocyanates topic page; ACGIH; EU Regulation 2020/1149; EU CLP Regulation (EC) No 1272/2008.

Regulatory status, by named instrument

EU. Regulation (EU) 2020/1149 amended REACH Annex XVII to add Entry 74, the diisocyanate restriction. Its training obligation for industrial and professional users took effect in August 2023. ISOPA publishes the industry guidance on how that training is delivered. The restriction applies to IPDI, H12MDI, HDI and MDI alike, so it does not help you rank offers.

It does give you a document to request. If the material is bound for an EU site, ask each supplier how its SDS or label carries the Entry 74 training statement. An offer that cannot answer that question will cost you time at the customer’s EHS review. Classification sits under CLP, Regulation (EC) No 1272/2008.

US. EPA is running a TSCA Section 6 risk evaluation of diisocyanates. It is not concluded. Any supplier claim about what TSCA will require of you after that evaluation is a forecast, not a status. OSHA’s 0.02 ppm ceiling for HDI and MDI is enforceable today.

Hazard classification. Do not assume the four SDS documents are the same in Section 2; the approved hazard text for each substance is on its product page and SDS, and that is where to read it.

Read each one line by line and flag any supplier SDS for the same CAS whose Section 2 lists fewer codes than another’s. Two SDS documents for one substance that disagree on classification mean one of them is out of date.

The side-by-side sheet

This is the table to fill in before you walk the floor. Where the cell says “SDS,” the pack carries no public figure; the supplier’s own SDS is the source, and a blank from them is a finding.

Sources: PubChem compound record for MDI, citing NTP 1992 and USCG 1999; PubChem records for IPDI and HDI.

Three rows in that table change what you pay beyond the invoice.

Vapor pressure. HDI monomer at 75 °F reads 0.05 mmHg. IPDI at a higher temperature, 122 °F, reads only 0.0007 mmHg. Against identical exposure limits, that gap is what drives ventilation design, respirator selection and air-monitoring frequency. Watch the units and the temperature. Normalize every entry to one unit and one temperature before you compare. An unnormalized column looks precise and misleads.

Molecular weight. You buy isocyanate to deliver NCO groups. For diisocyanates, the ratio of molecular weights is the ratio of mass you need to deliver the same NCO equivalents. A lower price per kilogram has to beat that ratio before it is cheaper per equivalent. Ask each supplier for NCO content as percent by weight on the CoA. Compare price per equivalent, never price per kilogram, across chemistries.

Physical state. Confirm from each SDS whether the material is liquid or solid at your warehouse temperature. A solid means drum heating, melt-out time and a question to ask about how the material was stored before shipment.

Why aliphatic or aromatic, and where volatility ranking fails

The coatings formulator reading over your shoulder chose the chemistry before you got the RFQ. IPDI’s role is set out in its applications record: non-yellowing polyurethane paints and varnishes, electrostatic powder coatings, 1K enamels and elastomers. H12MDI is cycloaliphatic and is valued for UV stability over aromatic isocyanates. MDI is aromatic, made from aniline, and goes into foams and elastomers where yellowing does not matter. Procurement does not swap an aliphatic for an aromatic to save money; the topcoat will tell you within a season.

The decision procurement does influence is which aliphatic, and in what form. That is where the vapor-pressure argument carries weight, and where it can be wrong.

It is wrong in two places. First, process temperature: the PubChem figures are at 68 to 122 °F, and if your line heats the isocyanate or sprays it, aerosol and elevated temperature can overwhelm the room-temperature ranking. A low-vapor-pressure material can still exceed the 0.02 ppm ceiling. Second, oligomers: most coatings buyers of HDI buy the biuret or isocyanurate trimer, not the monomer, because the monomer is volatile. A trimer’s exposure profile depends on residual free HDI, which the supplier must state on the SDS or CoA.

The trade-off is real. HDI-based systems are widely formulated for the properties they give the film. Choosing a lower-volatility alternative to cut ventilation cost can cost you reformulation and requalification that the formulator will have to sign off on. The SDS comparison tells you what the handling cost is. It does not tell you whether the film properties are worth it.

What to check before you accept an offer

• Normalize vapor pressure to one unit and one temperature. Any SDS that gives a vapor pressure without a temperature goes back to the supplier.
• For HDI trimers and polymeric MDI, require residual monomer on the SDS or lot CoA. That number sets your exposure control, not the oligomer’s name.
• Ask for NCO content as weight percent on the CoA and compare three consecutive lots. If NCO drifts outside the tolerance the supplier stated on the qualification sample, your formulation stoichiometry drifts with it.
• Confirm the Section 8 limits cite a named body. For HDI and MDI, anything above OSHA’s 0.02 ppm ceiling or ACGIH’s 0.005 ppm TWA means the SDS is out of date.
• For EU-bound material, ask how Entry 74 training is referenced, and point the supplier to the ISOPA guidance if they hesitate.
• Check Section 10 for decomposition products. For IPDI, PubChem names nitrogen oxides and hydrogen cyanide; a supplier SDS silent on this is incomplete.

Price and lead time for all four move with feedstock and producer capacity. The pack carries no dated figures, so ask for a current quote rather than trusting a show-floor number past the week it was given. The same discipline applies to any material bought by a loose name; see our piece on why ordering silicone oil without a cSt target goes wrong.

Questions the tables do not answer

Does an HDI trimer share HDI monomer’s vapor pressure? No. The 0.05 mmHg at 75 °F figure is for monomer. The trimer’s volatility is governed by its residual monomer, which the supplier must state.

Is TSCA settled? No. EPA’s Section 6 evaluation of diisocyanates is ongoing.

Request pricing and specification

Request current pricing, availability, the product-page SDS and a recent lot CoA stating NCO content and, for oligomers, residual monomer, for the IPDI, H12MDI, HDI or MDI grade your formulator has specified.

Whether that request goes to the lowest quote or the second-lowest comes down to one comparison: the vapor pressure on the SDS, normalized to your process temperature, against the controls your plant already has in place for a 0.02 ppm ceiling.

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