1,1'-Methylenebis(4-isocyanatobenzene) and 1,1'-methylenebis(4-isocyanatobenzene) homopolymer and their reaction products with [(methylethylene)bis(oxy)]dipropanol and butane-1,3-diol and propane-1,2-diol

Vapour pressure

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

vapour pressure

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Qualifier:

according to guideline

Guideline:

OECD Guideline 104 (Vapour Pressure Curve)

Deviations:

no

GLP compliance:

no

Type of method:

effusion method: Knudsen cell

Key result

Temp.:

20 °C

Vapour pressure:

0.001 Pa

Remarks on result:

other: extrapolated value

Temp.:

31.05 °C

Vapour pressure:

0.003 Pa

Temp.:

40 °C

Vapour pressure:

0.008 Pa

Remarks on result:

other: interpolated value

Temp.:

70 °C

Vapour pressure:

0.186 Pa

Remarks on result:

other: interpolated value

Measured values in detail:

Temperature (°C)	Vapour pressure (hPa)
31.05	3.21e-05
31.80	2.98e-05
45.25	1.30e-04
60.40	6.97e-04
75.35	3.30e-03

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

ln(P [Pa]) = 31,0200 -11221.525 /  T[K]

(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; CAS 26447-40-5, 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 substance	4,4-MDI	MDI mixed isomer	PMDI	MDI-DPG	MDI-BD/DEG/PD	MDI-Homopolymer
m-MDI	ca. 100 %	ca. 100 %	ca. 40 %	ca. 45 %	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 Clausius Clapeyrone 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 substance	4,4-MDI	MDI mixed Isomer	PMDI	MDI-DPG	MDI-BD/DEG/PD	MDI-Homopolymer
vapour pressure at 20 °C (Pa)	7.04e-04	9.16e-04	4.05e-04	6.47e-04	5.93e-04	5.38e-04
vapour pressure at 40 °C (Pa)	8.11e-03	1.08e-02	3.78e-03	6.18e-03	6.73e-03	6.20e-03
vapour pressure at 70 °C (Pa)	1.86e-01	2.52e-01	6.60e-02	1.11e-01	1.51e-01	1.42e-01

 

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 Clausius Clapeyrone 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 Clausius Clapeyrone equation of the measurement chosen as the reference pressure in order to demonstrate the influence of the 2,4’-MDI.

Test substance	4.4-MDI	MDI mixed Isomer	PMDI	MDI-DPG	MDI-BD/DEG/PD	MDI-Homopolymer
Reference pressure (hPa)	8.11e-05	8.11e-05	8.63e-05	8.17e-05	8.94e-05	8.47e-05
Vapour pressure 40 °C (hPa)	8.11e-05	1.08e-04	3.78e-05	6.18e-05	6.73e-05	6.20e-05
Ratio	1.00	1.33	0.44	0.76	0.75	0.73

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.

The older study by Chakrabarti, 1989 also measured the vapor pressure of 4,4’-MDI and resulted in comparable results with a more huge data set. It also showed that modified MDI grades or oligomeric MDI grades have lower vapor pressure as 4,4’-MDI if the contained 2,4’-MDI isomers remains low.

Reference 2

Endpoint:

vapour pressure

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: Guideline study without restrictions

Qualifier:

according to guideline

Guideline:

other: effusion method: by loss of weight or by trapping vaporisate; analog to OECD Guideline 104 (Vapour Pressure Curve)

Deviations:

no

Principles of method if other than guideline:

effusion method: by loss of weight or by trapping vaporisate

GLP compliance:

no

Type of method:

effusion method: by loss of weight or by trapping vaporisate

Key result

Temp.:

20 °C

Vapour pressure:

0.001 Pa

Remarks on result:

other: extrapolated value

Temp.:

40 °C

Vapour pressure:

0.008 Pa

Remarks on result:

other: interpolated value

Temp.:

70 °C

Vapour pressure:

0.183 Pa

Remarks on result:

other: interpolated value

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:

 

Temperature	Vapor Pressure
T [°C]	P [10E-4 mmHg]	P [Pa]
59,97	4,04	0,053
53	2,49	0,033
38,98	0,65	0,009
57,98	3,36	0,044
51	2	0,026
48,98	1,74	0,023
42	0,77	0,010
37,98	0,43	0,006
37,98	0,55	0,007
75,01	18,99	0,250
68,03	10,32	0,136
84,98	52,23	0,687
79,98	34,7	0,457
74,95	20,82	0,274
69,97	14,01	0,184
83,96	47,45	0,624
78,91	31,37	0,413
73,77	27,05	0,356
73,73	21,1	0,278
73,9	20,8	0,274
68,97	11,92	0,157
68,96	11,57	0,152
63,97	9,1	0,120
59,01	5,1	0,067

 

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:

 

T	Pressure	Saturated Vapor Conc.
°C	Pa	mg/m³
20	0,0007	0,076
25	0,0014	0,14
30	0,0026	0,26
40	0,0083	0,79
50	0,025	2,30
60	0,069	6,25
70	0,183	16,0

 

 

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%.

Executive summary:

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::

Ln (P [Pa]) = 30,60809 – 11086,8414 / T [K]

Extrapolation (below 38°C) and Interpolation (between 38 and 85°C) of the measured values resulted into the following table of vapor pressures:

 

T	Pressure
°C	Pa
20	0,0007
25	0,0014
30	0,0026
40	0,0083
50	0,025
60	0,069
70	0,183

 

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.

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 1.3.2.2 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/4,4'-MDI Homopolymer/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 mainly 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

Key value for chemical safety assessment

Vapour pressure:

0.001 Pa

at the temperature of:

20 °C