Aerosol / Vapor Partitioning of Monmeric Isocyanates

 

Halet G. Poovey, Roy J. Rando and Rachele A. Gibson

 

Tulane University School of Public Health and Tropical Medicine

1430 Tulane Ave. SL 09 New Orleans, La 70112

Abstract

The physical state of semi-volatile toxicants has become an important topic of research in the field of industrial hygiene. A toxicant’s physical state will affect its site of deposition and uptake in the respiratory tract upon inhalation, in addition to influencing the techniques most appropriate for collection, analysis, as well as exposure control. Isocyanates are commonly used in applications in which aerosolization is part of the application process. This aerosolization yields a mixture of vapor and aerosol isocyanate. It is expected that the isocyanates would partition in a manner related to their vapor pressures. However, there is evidence that the monomeric isocyanates remain in the aerosol phase even when present at levels significantly below their saturated vapor pressure. A series of atmospheres were generated by nebulization in a laboratory testing chamber and sampled with a dichotomous sampler. These atmospheres consisted of MDI monomer, three pre-polymer solutions, CMDI, PMPPI and Desmodur N100 spiked with HDI and a commercial coating, Rexthane (TDI-based single component coating). MDI was generated by flash evaporation at 250 0C and by nebulization. The atmosphere generated by flash evaporation followed the predicted pattern of vapor / aerosol partitioning; however the atmosphere generated by nebulization showed an enhancement of the aerosol fraction. The CMDI atmosphere showed vapor and aerosol levels of isocyanate monomer near that predicted by the saturated vapor pressure of MDI. Whereas the PMPPI, Demodur N100 and Rexthane atmospheres showed the vapor and aerosol phases at levels different from that which would be predicted by vapor pressure alone. In addition, field sampling was conducted at a spray paint booth and polyurethane foam plant. The field data showed some monomeric isocyanate in the aerosol phase at concentrations that were significantly below the saturated vapor pressure. Overall the data suggests the vapor / aerosol partitioning of isocyanates is dependent on factors other than simply vapor pressure.

 

Saturated Vapor Pressures of Monomeric Isocyanates

 

Isocyanate

Saturated

Vapor Pressure

At 25 0C

(mm Hg)

Expected Vapor Phase Concentration

At Saturation

ppb

g/M3

HDI

5.0 x 10-2

66,000

450,000

4,4' MDI

3.8 x 10-6

5

47

2, 4' & 4,4' MDI

6.8 x 10-6

9

90

2,4 TDI

1.7 x 10-2

22,000

160,000

2,6 TDI

2.5 x 10-2

33,000

230,000

 

 

Isocyanates Used to Generate Test Atmospheres

Test Atmosphere Generation

Aerosol test atmospheres were generated with a Devilbiss Model 40 glass nebulizer. Test solution was continuously fed into the nebulizer with a syringe pump. Nitrogen flow through the nebulizer was continuously monitored with a mass flow meter. Generated aerosols were feed into an aerosol test chamber. The aerosol chamber operated under positive pressure, and was flushed at a flow rate of about 100 cubic feet per minute (cfm),with room air, resulting in an average flow velocity of 100 feet /minute in the sampling cross-section. Test atmosphere and dilution air were mixed by passing through a series of perforated plate diffusers. The atmosphere then passed through a honeycomb flow straightener (tubular cells of 28-mm diameter and 155-mm length) before entering the sampling zone. The chamber was placed in a walk in fume hood during use.

Schematic of Nebulizer

Sample Collection and Analysis

The dichotomous sampler was obtained from University Research Glassware (Model 2000, URG, Carrboro, North Carolina). The device consisted of an annular denuder in series with an aluminum cyclone inlet and an aerosol filter backup. All components were made of aluminum, borosilicate glass, Teflon, or stainless steel. The cyclone has a D50 = 3.5 m at a flow rate of 2.0 L/min. The cyclone was followed by the annular denuder section, consisting of inner and outer glass cylinders with an annular spacing of 0.1 cm in between. The outer diameter of the denuder tube was 2.6 cm, and the length was 24 cm. The final stage of the sampler was a 37-mm Teflon filter holder containing a treated glass fiber filter. Each of the denuder walls and backup filter were coated with a mixture of MAMA reagent (2 mg) and tributylphosphate (30 mg). Immediately after collection, the cyclone inlet was washed with a solution of MAMA in dimethyl sulfoxide. The denuder and filter were then desorbed with in dimethyl sulfoxide. All samples were analyzed by HPLC. Chromatographic determination of the isocyanate-MAMA urea derivatives from the Dichotomous samplers was accomplished with a Waters model 6000a pump connected to a Alcott 738R autosampler. Three detectors were utilized to detect peaks. Two variable wavelength ultraviolet absorbance detectors, a Perkin-Elmer Model LC90 and Waters Model 450 UV were set to 245 nm and 370 nm respectively. The third detector, a Shimadzu Model RF551 fluorescence detector was set at an excitation wavelength of 245 nm and an emission wave length of 414 nm. Sample injection volumes were 20l with chromatographic run times varied depending on the isocyanate being analyzed. The samples of isocyanate – MAMA ureas were chromatographed on a Supelcosil LC8-DB octyl bonded base deactivated silica column, 5 m particle size (Supelco, Belafonte PA). The column dimensions were 4.6 mm i.d. by 5 cm long. The mobile phase consisted of a mixture of acetonitrile and aqueous triethylammonium phosphate buffer, ( 3% Triethylamine, Aldrich catalog #13,206-3 pH adjusted to 3.0 with phosphoric acid, EM Science (Houston, TX) catalog # PX0995-14). Actual composition of the mobile phase varied depending on the isocyanate being analyzed. The mobile phase flow rate was maintained at a constant 1.0 mL per minute.

The ability of the dichotomous sampler to separate MDI vapor and condensation aerosol in the laboratory is shown in the figure above. The points represent MDI generated by flash evaporation over a range of 25 to 515 g / M3. Overlaid on the data is a model based upon the saturated vapor concentration of 4,4'-MDI as reported by Brochhagen and Shal. There is excellent correlation between the data and the model.

MDI Nebulization Test Atmosphere

Sample Time

(Minutes)

N

Total MDI

(g/M3)

% MDI

Vapor

% MDI Vapor Expected

30

3

249 11

12 0.1

19

30

3

83 3

7 2

57

30

3

49 7

18 6

96

 

For MDI Monomer Test Atmospheres

If [MD] < 47 g/m3, then % Vapor = 100;

If [MDI] > 47 g/m3, then % Vapor = (1- ({[MDI] - 47} / [MDI] )) x 100

Where [MDI] is the concentration of MDI in the test atmosphere, and % vapor is the predicted percentage of MDI present in the vapor fraction. Saturated vapor concentration is based on the assumption that the monomer was pure 4,4' MDI

 

CMDI Test Atmospheres

Sample Time

(Minutes)

N

Total MDI

(g/M3)

% MDI

Vapor

% MDI Vapor Expected

30

6

277 49

28 12

32

 

 

 

 

PMPPI Test Atmospheres

Sample Time

(Minutes)

N

Total MDI

(g/M3)

% MDI

Vapor

% MDI Vapor Expected

15

6

590 180

7.8 1.1

15

180

6

50 5

41 7

100

 

 

For Polymeric MDI Test Atmospheres

If [MDI] < 90 g/m3, then % Vapor = 100;

If [MDI] > 90 g/m3, then % Vapor = (1- ({[MDI] - 41} / [MDI] )) x 100

Where [MDI] is the concentration of MDI in the test atmosphere, and % vapor is the predicted percentage of MDI present in the vapor fraction. Saturated vapor concentration is based on the assumption that the polymeric isocyanates have a mixture of 2, 4' & 4,4' MDI

 

Rexthane Test Atmospheres

Sample Time

(Minutes)

N

Total 2,6 TDI

(g/M3)

% 2,6 TDI

Vapor

% 2,6 TDI Vapor Expected

15

6

47 8

61 4

100

180

6

5 4

66 9

100

Sample Time

(Minutes)

N

Total 2,4 TDI

(g/M3)

% 2,4 TDI

Vapor

% 2,4 TDI Vapor Expected

15

6

15 3

40 7

100

180

6

1.2 .5

40 8

100

 

 

 

Desmodur N100 Test Atmospheres

Sample Time

(Minutes)

N

Total HDI

(g/M3)

% HDI

Vapor

% HDI Vapor Expected

10

24

188 91

82 7

100

30

6

203 32

78 8

100

 

Vapor/Aerosol Distribution of Collected Isocyanates at a Polyurethane Foam Manufacture

Sample

Area

Phase

2,4 TDI (g/M3)

2,6 TDI (g/M3)

Foaming

Vapor

20.0

109.4

Respirable Particulate

ND

ND

Non-Respirable Particulate

ND

ND

Foaming

Vapor

3.2

11.6

Respirable Particulate

ND

ND

Non-Respirable Particulate

ND

ND

Flame*

Laminating

Vapor

6.3

12.2

Respirable Particulate

ND

ND

Non-Respirable Particulate

ND

1.3

Zapping

Vapor

0.8

ND

Respirable Particulate

ND

ND

Non-Respirable Particulate

ND

ND

Flame*

Laminating

Vapor

0.7

6.6

Respirable Particulate

ND

8.2

Non-Respirable Particulate

0.7

ND

Zapping*

Vapor

3.8

6.9

Respirable Particulate

1.3

1.5

Non-Respirable Particulate

ND

ND

Foaming*

Vapor

1.7

5.0

Respirable Particulate

ND

ND

Non-Respirable Particulate

0.4

2.4

Foaming

Vapor

3.7

29.9

Respirable Particulate

ND

ND

Non-Respirable Particulate

ND

ND

Flame

Laminating

Vapor

4.5

5.4

Respirable Particulate

ND

ND

Non-Respirable Particulate

ND

ND

*Samples showing aerosol phase monomeric TDI. All levels were 10,000 times or

more below the saturated vapor pressure of 2,4 and 2,6 TDI

 

Sampling was conducted in a factory producing flexible polyurethane foam products. Both TDI and MDI were used in the polyurethane foam formulations. While no MDI was detected in any of the samples taken in the foam factory, levels of TDI up to about 128 g / M3 were measured along the foam lines. The majority of the TDI was found in the vapor phase although four of the samples showed some TDI in the aerosol phase.

HDI Sampling in Spray Paint Booth

 

SAMPLE

VAPOR PHASE

(m g/M3)

AEROSOL PHASE (m g/M3)

ACTIVITY

1*

17

3

PAINTING

2*

14

5

PAINTING

3

ND

ND

MIXING

4

ND

ND

MIXING

5

7

ND

PAINTING

6

10

ND

PAINTING

7

2

ND

CLEANUP/

DRYING

8

5

ND

CLEANUP/

DRYING

9

ND

ND

CLEANUP/

DRYING

10

ND

ND

CLEANUP/

DRYING

*Samples showing aerosol phase HDI. All levels were >10,000X below the saturated vapor pressure of HDI

Samples were collected in a paint spray booth during application of a polyurethane paint. The paint spray operations utilized a two part paint based on HDI. Concentrations of HDI averaged only 14 6.5 g/m3. The majority of the HDI was measured in the vapor phase, but significant (15 - 26%) amounts were measured in the aerosol fraction of the paint overspray. After spraying was finished, small amounts of HDI were measured from offgassing of the newly painted surfaces. However, in this case all of the HDI was present in the vapor phase.

 

 

Conclusion

The data suggests that when monomeric isocyanates are generated by aerosolization the level of isocyanate in the aerosol phase is higher than that predicted by vapor pressure alone. This enhancement of the aerosol phase needs to be considered when assessing engineering controls and the use of personal protective equipment.