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Physical & Chemical properties

Vapour pressure

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vapour pressure
Type of information:
experimental study
Adequacy of study:
key study
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
guideline study with acceptable restrictions
according to guideline
OECD Guideline 104 (Vapour Pressure Curve)
GLP compliance:
Type of method:
effusion method: Knudsen cell
Key result
20 °C
Vapour pressure:
0 hPa
Remarks on result:
other: extrapolated value
30.65 °C
Vapour pressure:
0 hPa
40 °C
Vapour pressure:
0 hPa
Remarks on result:
other: interpolated value
70 °C
Vapour pressure:
0.001 hPa
Remarks on result:
other: interpolated value

Measured values in detail:

Temperature (°C) Vapour pressure (hPa)
30.65 2.29e-05
31.40 2.01e-05
44.15 9.94e-05
58.95 4.83e-04
73.15 1.90e-03

The values at 20, 40 and 70 °C were calculated with Antoine equation as follows:

ln(p/bar} = 19,2203 - 11216,06 / (273, 15 + t)

(t) in °C;

Value at 20 °C is a extrapolated value.

Executive summary:

Executive summary for all 6 MDI`s

The vapour pressures of six different BASF methylenediphenyl diisocyanate (MDI) based or MDI-modified substances were investigated in this summary of 6 reports.
The MDI-based substances were commercially available samples of BASF products: 4,4’-MDI (CAS 101-68-8 , EC 202-966-0), MDI mixed isomers (MI; EC 905-806-4; mixture of 4,4’-MDI and 2,4’-MDI) and polymeric MDI (pMDI; CAS 9016-87-9, EC n/a; based on 4,4'-MDI containing oligomers of high functionality and isomers with an typical average functionality of 2.7; viscosity at 25 °C of approx. 200 mPa*s).

The MDI-modified substances are commercially available BASF “Prepolymers”: 4,4'-MDI/DPG/HMWP (modified 4,4’-MDI with dipropylenglycole; CAS n/a, EC 701-041-3), MDI-BD/DEG/PD (modified 4,4’-MDI with butadiole, dietehylenglycole and propanediole; CAS 158885-29-1, former EC 500-415-1, new EC 701-276-1) and 4,4'-MDI homopolymer (CAS 25686-28-6, EC 500-040-3). All investigated substances contain a significant amount of free 4,4’-MDI (m-MDI).

The following table shows the proportion by weight of monomeric MDI in
the test substances:

Test substance4,4-MDIMDI mixed isomerPMDIMDI-DPGMDI-BD/DEG/PDMDI-Homopolymer
m-MDIca. 100 %ca. 100 %ca. 40 %ca. 50 %ca. 60 %ca. 75 %

For measuring the vapour pressures the effusion method with a Knudsen cell was used according to the procedure of OECD 104. The method is based on the estimation of the mass of test substance flowing out per unit of time of a Knudsen cell in the form of vapour, through a rnicro-orifice under ultra-vacuum conditions. The mass of effused vapour was obtained by determining by means of the temporal mass loss of the test item at a constant temperature generated in a Knudsen cell under vacuum. The vapour pressure was calculated by applying
the Hertz-Knudsen relation. Measurements were conducted from lower to higher temperatures to prevent excessive depletion of the most volatile compounds. For most of the samples, the measurements were conducted in the temperature range of 30 to 70°C.

The regression parameters of the Antoine equation are given in the substance data
measurement reports. Using this equation, the following vapour pressures can be
calculated (*values of 20°C are extrapolared values):

Test substance4,4-MDIMDI mixed IsomerPMDIMDI-DPGMDI-BD/DEG/PDMDI-Homopolymer
vapour pressure at 20 °C (hPa)7.04e-069.16e-064.05e-066.47e-065.93e-065.38e-06
vapour pressure at 40 °C (hPa)8.11e-051.08e-043.78e-056.18e-056.73e-056.20e-05
vapour pressure at 70 °C (hPa)1.86e-032.52e-036.60e-041.11e-031.51e-031.42e-03


These “Prepolymers” should better be described as “Mixtures of MDI Monomers with small amounts of some larger monomers” having a molecular weight below 1000 g/mol. The total monomer contents of the investigated substances start with approx. 40% (w/w) and increase to 100 % (w/w). The monomer 4,4’-MDI is always the major component in all MDI based products, except in PMDI, which reveals besides considerable 3 ring monomers, some higher oligomers, larger than the 4 ring moieties. PMDI therefore has the lowest vapour pressure of all MDI compounds. For the “MDI-Prepolymers” it must be stated: The 4,4’-MDI
monomer is always the key molecule with a concentration for all REACH registered substances around 50% (w/w) and above.
The 2,4’-MDI monomer is in the substances either not present or only at a much lower concentration. The majority of the constituents, the so called “higher oligomers”, are just simple diol-MDI adducts, having a molecular weight below 1000 g/mol and < 1500 g/mol respectively, depending on the used diol. The MDI mixed isomers contain both, the 4,4’-MDI and the 2,4’- MDI to each 50% (w/w).
The measured vapour pressures are completely in line with the composition of MDI-modified “Prepolymers”. From the regression parameters of the Antoine equation given in the substance data measurement reports, vapour pressures can be calculated for 4,4’-MDI as well as for MDI mixed isomers. By means of the Raoult law the vapour pressure of 2,4’-MDI can be estimated
mathematically, since the proportion of 4,4’-MDI and 2,4’-MDI is known from the composition of the used MDI mixed isomers substance sample. The partial pressure of the 2,2’-MDI is neglected due to its low proportion.
At a temperature of 40°C a vapour pressure of the respective test substance without heavy boiling components is calculated by means of the Raoul law by normalising the content of 2,4’- MDI and 4,4’-MDI in the test substance to 1 and calculating the respective partial pressures by means of the pure substance vapour pressure of the corresponding MDI multiplied by the weight component of the corresponding MDI, since the substance quantity component is not
accessible. Due to the very low proportion of 2,2’-MDI in the test substance, this partial pressure fraction is neglected. The vapour pressures of the test substances calculated in this way represent a reference pressure which the test substance would have without the influence of the heavy boilers. The ratio of the vapour pressure calculated at the same temperature from the regression parameters of the Antoine equation of the measurements to this reference
pressure shows the influence of the respective heavy boilers on the reduction of the vapour pressure. Only for MDI mixed isomers was the vapour pressure of the 4.4-MDI calculated from the regression parameters of the Antoine equation of the measurement chosen as the reference pressure in order to demonstrate the influence of the 2,4’-MDI.

Test substance4.4-MDIMDI mixed IsomerPMDIMDI-DPGMDI-BD/DEG/PDMDI-Homopolymer
Reference pressure (hPa)8.11e-058.11e-058.63e-058.17e-058.94e-058.47e-05
Vapour pressure 40 °C (hPa)8.11e-051.08e-043.78e-056.18e-056.73e-056.20e-05

For all test substances other than 4,4’-MDI and 2,4’-MDI, the ratio of measured vapour pressure to reference pressure is less than one. In particular, PMDI and MDI homopolymers show an almost linear relationship between the ratio of measured vapour pressure to reference pressure and the proportion of free monomeric MDI in the test substance.

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 (shown elsewhere) 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 cited there).


In a substance specific study according to the study design of OECD Guideline 104 (Vapour pressure curve) in 2016 using the effusion method in a Knudsen cell the following estimated vapour pressure at 20 °C was extrapolated from the regression equation:


Vapour pressure at 20°C: 0.00054 Pa

Key value for chemical safety assessment

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

Additional information