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EC number: 941-496-7
CAS number: 1689576-89-3
The following table shows the huge number of data in a broad temperature range from 38 to 85°C as results for the 4,4’-MDI grade ISONATE M 125 MDI. The lowest temperature was chosen to detect the data for liquid material as the melting point is – depending on its purity - in the range of 37 to 39 °C typically. Measured data for 4,4’-MDI in mmHg has been recalculated to Pa:
P [10E-4 mmHg]
Chakrabarti estimated the following equation for inter- and extrapolation (< 38°C) with the Clausius Clapeyron equation (or simplified Antoine equation):
Log(10) (P [mmHg]) = 11.15 – 4809,9/ T [K]
This was re-calculated into the following equation for Pa and K:
Ln (P [Pa]) = 30,60809 – 11086,8414 / T [K]
The values at 20, 40 and 70 °C were calculated with Clausius Clapeyron equation above.
Extrapolation and Interpolation (between 38 and 85°C) of the measured values resulted into the following table of vapor pressures:
Saturated Vapor Conc.
Further MDI grades have been investigated with less than 100 % monomeric MDI isomers. The composition of these products is not well documented and can’t be used for detailed comparison. But it is known that they all contained only minor amount of 2,4’-MDI, typically below 5%.
Overall, the results demonstrated (in the absence of a high 2,4’-MDI containing substance) that the 4,4’-MDI grade M 125 with 100% MDI isomers shows the highest vapor pressure in comparison to all other MDI grades with a monomeric MDI isomer content of less than 100%.
The study investigated the vapor pressure of 12 proprietary MDI grades which are partially still available on the market. The best documented sample of this series is ISONATE M 125 which represents the registered 4,4’-MDI.
The vapor pressure of all other grades with less content of monomeric MDI isomers follows the same temperature-related trend of 4,4’-MDI’s vapor pressure with overall lower MDI vapor pressure respectively.
Chakrabarti measured 24 data points for 4,4’-MDI in the range of 38 to 85°C. The following equation describes well the vapor pressure of 4,4’-MDI::
Extrapolation (below 38°C) and Interpolation (between 38 and 85°C) of the measured values resulted into the following table of vapor pressures:
The newer study by Gerbig and Jamin, 2018 again measured the vapor pressure of 4,4’-MDI and resulted in comparable results with fewer data.
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 188.8.131.52 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 Chakrabarti (1989) or the 4,4'-MDI vapour pressure is used as key study for 4,4'-MDI/DPG as a worst-case approach (see also category justification document in Appendix 28 IUCLID section 13), since the substance is produced using 4,4'-MDI 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 1989 using the effusion method in a Knudsen cell, which was extrapolated from the regression equation of 4,4'-MDI, is:
Vapour pressure at 20°C: 0.00074 Pa
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