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Vapour pressure

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Description of key information

All MDI substances have extremely low vapour pressures at room temperature (<0.01 Pa).  Only special laboratories with highest precision could apply the mass-loss Knudsen effusion method for MDI substances at elevated temperatures from 30 to 90°C in order to extrapolate to room temperature. Due to this fact, measurements are difficult to perform and only the most reliable will be taken into account for assessment.


Substances of the ‘Monomeric MDI’ subgroup (4,4’-MDI, 2,4’-MDI, 2,2’-MDI and MDI Mixed Isomers) have the highest vapour pressure, ranging from 0.7 to 8.05 mPa at 20°C. All modified MDI substances of the subgroups ‘Oligomeric MDI’, ‘MDI reaction products with glycols’ and ‘MDI condensation products’ have lower values compared to the basic monomers they are made from.


The overall content of monomeric MDI isomers in all substances and the ratio of 2,4’-MDI and 4,4’-MDI are the main driver of air exposure (Gerbig and Jamin, 2018; Chakrabarti 1989) within the MDI category. The higher molecular weight constituents, i.e. MDI oligomers, condensation adducts or glycol adducts, all have much higher molecular weight and therefore much lower vapour pressure. These higher molecular weight constituents do not contribute to the overall vapour pressure of the MDI substances. Theoretical vapour pressure calculations support this hypothesis (see Chapter of the Category Justification Document and supporting studies of Sadler 2019).

Chakrabarti (1989) used the mass-loss Knudsen effusion method which is the most applicable method for measuring the vapour pressure of the MDI substances and generated a huge data set for some MDI substances. This study is therefore rated with a Klimisch score of 1. A new study was performed (Gerbig and Jamin, 2018) using the same mass-loss Knudsen effusion method which is rated with a Klimisch score of 2 but with a smaller data set per substance. Therefore, the Chakrabarti (1989) study is chosen as KEY and the Gerbig and Jamin, 2018 study as support for 4,4’-MDI. Both studies match in their results. A graph in the additional information illustrates the findings of both studies. The Chakrabarti formular fits also the newer results of Gerbig et al. 2018.


As a read-across Gerbig and Jarmin (2018)  and the respective mesaurement of BASF in 2016 of the MDI mixed isomer vapour pressure is used as key study for MDI mixed isomer/1,3-BD/TPG/PG as a worst-case approach (see also category justification document in Appendix 28 IUCLID section 13), since the substance is produced using MDI mixed isomer as starting material and as descirbed before the other constituents all have much higher molecular weight and do not contribute to the overall vapour pressure of the substance.


Therefore, the vapour pressure according to the study design of OECD Guideline 104 (Vapour pressure curve) in 2016 using the effusion method in a Knudsen cell, which was extrapolated from the regression equation of MDI mixed isomer, is:


Vapour pressure at 20°C: 0.00092 Pa

Key value for chemical safety assessment

Vapour pressure:
0.001 Pa
at the temperature of:
20 °C

Additional information