Occupational exposure to airborne isocyanates during brush/roller application of 2-pack polyurethane paints in a tropical climate.
Abstract Shipboard and laboratory trials were conducted to determine airborne occupational exposure to isocyanates by brush/roller application of 2-pack polyurethane paints. Airborne concentrations in the breathing zones of the operators were found to be <0.1 [micro]g/[m.sup.3] during the paint application to a vessel deck, in a tropical climate, in the open air. Similarly, brush/roller application of the paints in an enclosed space generated airborne isocyanate concentrations <3 [micro]g/[m.sup.3]. The dust generated during light sanding of the polyurethane paint, 24 h after curing, did not show detectable levels of isocyanates.
Keywords Polyurethane paints, Isocyanates, Air sampling, Analysis, Occupational exposure, Brush, Roller
Isocyanates are a major contributing cause of asthma in the Australian occupational environment. (1) Polyurethane paints (PUP) contain low volatility isocyanate prepolymers in addition to small quantities of volatile isocyanate monomers, (2), (3) both are recognized as a major source of exposure to isocyanates. (2) Because of their reactivity, airborne isocyanates can irritate the respiratory tract and may result in asthma attacks either immediately or some hours after exposure. (4), (5) Repeated exposures may result in impairment of lung functions. Personnel may also become sensitized to airborne isocyanates at concentrations below the maximum exposure limits (6); hence, there are strict controls on occupational exposure to airborne isocyanates. In response, paint and resin manufacturers have minimized the concentration of oligomeric isocyanates and volatile unreacted monomeric isocyanates in paints. (7)
During paint spraying, the low volatility, partially cured oligomers become airborne as aerosols while the unreacted monomeric isocyanates vaporize. However, during brush/roller applications, there is less opportunity for the generation of aerosols, and the principal source of airborne isocyanates will be the small quantities of unreacted volatile monomeric species present in the paint. The high ambient temperatures experienced in a tropical climate are likely to enhance vaporization of the latter. Furthermore, the curing rate of the paint is likely to be accelerated by the elevated temperatures and high humidity.
While studies under tropical conditions have not been reported, the few studies of brush/roller applications in temperate climates have either shown that airborne isocyanate concentrations are below the level of detection (0.01 [micro]g/[m.sup.3]) (8) or at low concentrations. (2)
Sanding of cured PUP can also result in personnel exposure to isocyanates, both dermal and respiratory. This is due to the slow formation of the polymer film with the isocyanate groups remaining unreacted for prolonged periods (days and weeks) (9), (10) and from thermal decomposition of the polymer as a result of mechanical abrasion. (11) However, not all sanding procedures result in the emission of aerosols containing unreacted isocyanates. (11), (12)
Polyurethane paints have wide application in the Australian Defence Forces (ADF) and industry. There are guidelines for personnel protection during the spray painting of polyurethanes, (13-16) but the extent of personnel protection required during brush/roller application and light manual sanding is unclear.
General industrial applications require wearing of appropriate personnel protective equipment to avoid skin contact and ensuring good ventilation without specifically requiring the use of respirators. (14-16) These recommendations may not address the potential effects of elevated ambient and substrate temperatures on airborne isocyanate concentrations. Such conditions are generally experienced during the painting of Royal Australian Navy (RAN) vessels in northern Australia. Although it is important to provide the painter with protection from potentially adverse health effects of isocyanates, burdensome respirators should not be imposed unnecessarily, particularly in hot and humid climatic conditions.
This study sets out to assess the hazard to personnel exposed to isocyanates during brush/roller painting with isocyanate paints by the following means (Fig. 1):
[FIGURE 1 OMITTED]
1. determining whether personnel are being exposed to excessive airborne concentrations of isocyanates released from 2-pack PUP during mixing and application by brush/roller, to ship superstructures, in an open-air environment in a tropical climate (e.g., northern Australia) and
2. determining whether personnel are likely to be exposed to excessive levels of airborne isocyanates during the application of the specified isocyanate paints under simulated conditions, and
3. measuring the airborne concentrations of incompletely cured isocyanates in the form of oligomers (aerosol) and isocyanate monomer (vapor) when removing recently cured paint by light abrasion (manual sanding).
This study was initially intended to be a preliminary survey. However, the opportunity for a more comprehensive field trial did not arise; hence, only a relatively small number of samples were collected. Nevertheless, the results were probably indicative of exposures and form a basis for a future comprehensive study.
In this study, two paints were evaluated: the low gloss Interthane 987 and the nonskid Interthane 864. According to the information provided by the manufacturer, the Part B packs contain the curing agent (Material Safety Data Sheet, Interthane 990 Part B (17) and Interthane 864-863 NSP Curing Agent (18)) with <1% monomeric isocyanate (hexamethylene diisocyanate, HDI) (Fig. 2).
[FIGURE 2 OMITTED]
Polyurethane 2-pack paints, when mixed, cure by the reaction of a polyalcohol in Part A with an isocyanate resin to form a urethane polymer (equation 1):
Most of the isocyanates are present as the less volatile oligomers such as the isocyanurate trimer (Fig. 3) and pentamer (Fig. 4) with the trimer being the predominant isocyanate species. This composition is fairly typical of a commercial 2-pack polyurethane paint. (7), (19), (20)
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Exposure limits of isocyanates
The Australian National Occupational Health and Safety Commission (NOHSC) (21) and the UK Health and Safety Executive (HSE) have set an 8-h exposure limit (TWA) of 20 [micro]g/[m.sup.3] (15) based on the total reactive isocyanate groups (TRIG) which encompasses all the isocyanate-terminated compounds including oligomers. The American Conference of Government and Industrial Hygienists (ACGTH) (22) and the US National Institute for Occupational Safety and Health (NIOSH) (23) specify individual isocyanates with a TWA for airborne HDI vapor of 35 [micro]g/[m.sup.3].
During the application of PUP, personnel exposure to isocyanates vapors may occur through the evaporation of HDI from the open paint cans and during application by brush/roller or spray (Fig. 1). Emissions from the paint cans arc likely to be low because of the small surface area of the liquid paint and particularly after the two components are mixed allowing the HDI monomer to react. The surface area is significantly increased on application of the paint which greatly enhances evaporation of unreacted HDI monomer. Substantially higher quantities of airborne solvents and isocyanates are likely to be generated during spray application both in the form of vapors and aerosols. However, this study is only concerned with brush/roller application where airborne emissions are likely to be restricted to HDI vapors.
The duration of the ship-painting process may vary from a couple of hours to the full working day depending on the requirement. This could involve small areas to an entire deck of a patrol boat. The painters mix the paint mechanically, on site, in 10- to 20-L batches. Protective clothing is worn in the form of coveralls (in this case cotton) together with long gloves and a respirator with a carbon filter.
The following 2pack polyurethane paint systems were used: (1) Superstructure coating; Interthane 987 LSALGP (low solar absorbing, low gloss paint) Part A (PLA297)/Interthane (990) 987 Curing Agent, Part B (PHA046) and (2) Nonskid deck coating; Interthane (Pewter) 864 LSANSP (nonslip paint) Part A (PLA786)/Interthane 864 NSP HP Curing Agent, Part B (PLA047). These are marine protective paint systems that are widely used on naval vessels in the ADF. They are intended for brush/roller applications. Cure rates are typical for brush/roller applications.
Hexamethylene diisocyanate derivatives of 9-(N-methylaminomethyl) anthracene (MAMA) and methoxy-2-phenyl-1-piperazine (MOPIP) were prepared according to the Occupational Safety and Health Agency (US) Method 42, (24) a modified method from Goldberg et al. (25) MAMA (Fig. 5) and MOPIP (Fig. 6) were obtained from Aldrich with a purity of 99% and 98%, respectively, and HDI (Fig. 2), of an unknown purity, was obtained from Sigma.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
A 2-stage filter sampler was used, with the first stage designed to trap the aerosols and the second stage for the vapors in accordance with ASTM Standard Test Method D 6562-00 (26) and D 6561-00 (27) for isocyanates (monomer and oligomers, respectively). A 37-mm SKC preloaded cassette was used with twin filters (Omega Instrument Company, ISO-CHEK[R], 37 mm, Dual Filter, 225-9022A) containing a polytetrafluoroethylene (PTFE) top filter and a lower glass fiber filter impregnated with MAMA.
The total sampling time was 15 min giving a total of 15 L of air sampled at a flow rate of 1 L/min. (26), (27) The sampling period was sufficient to establish if the exposure limit was exceeded (method LOQ, 0.1 [micro]g/[m.sup.3] cf. TWA, 3 [micro]g/[m.sup.3]). The results were adjusted for 8 h (TWA) exposures.
Air sampling for peak exposure levels was conducted at a flow rate of 3 L/min for 5 min with a limited number of samples. This was the highest attainable flow with a low risk of separating the top filter. This was done to discern whether the HDI monomer was present during the initial 5-min application period or persisted throughout the entire sampling time.
The filter sampling allowed the simultaneous collection (in the one filter cassette) of airborne isocyanates in solid and liquid aerosol (a) and vapor (b) form using the following standard procedures:
(a) The vapor fraction was collected on a glass fiber filter impregnated with MAMA. Trapped HDI urea derivatives were desorbed with a mixture containing dimethylformamide/acetonitrile/triethylammonium phosphate buffer and analyzed by reversed phase high-performance liquid chromatography (HPLC) with fluorescence detection. (26)
(b) The aerosol fraction was collected on a PTFE filter. Immediately after sampling, the filter was transferred into a jar containing a 5 mL solution of MOPIP in toluene (0.1 mg/mL). The toluene was evaporated and the residue dissolved in acetonitrile containing 0.5% acetic anhydride. The resulting solution was analyzed by reversed phase HPLC using gradient elution with fluorescence detection. (27)
The isocyanate monomer content of Interthane (990) 987 Curing Agent Part B (PHA046) was determined by diluting 0.1 g of Interthane 990 Curing Agent in 5 mL of dry tetrahydrofuran (THF) in a 50-mL volumetric flask followed by the addition of 0.1 g of MOPIP in 5 mL dry THF. The mixture was allowed to stand at room temperature for 2 h. Dimethylformamide was then added to the mark and aliquots of the resultant solution analyzed by HPLC using fluorescence detection (excitation wavelength: 254 nm; emission wavelength: 412 nm).
The HPLC system was calibrated with working standard solutions (in the range 0-250 [micro]g/mL) prepared from the MOPIP urea derivative of HDI (m.p. 198-200[degrees]C).
The shipboard trial was conducted aboard the decommissioned patrol boat in the dry dock at Fleet Intermediate Maintenance Activity (FIMA), Larrake-yah Barracks, Darwin, Australia. The trial was restricted to brush/roller application of the PUPs specifically designed for this purpose. Spray painting applications were carried out under different conditions using different PUPs. The duration of the brush/ roller applications could vary from 0.5 to 6 h (per day) depending on the requirements.
Two operators were fitted with Aircheck 2000 personal sampling pumps and air sampling filters were placed in their breathing zone (Fig. 7). Additional air sampling was also conducted at the following positions:
[FIGURE 7 OMITTED]
1. Downwind of the painting (~70 cm above the deck),
2. Above the painted deck in the operator breathing zone (~70 cm above the deck), and
3. Above the paint pot (~40 cm).
The ambient air temperature was 43[degrees]C in the sun and 33[degrees]C in the shade with a deck temperature of 57[degrees]C. The wind speed was estimated to be 2.5-4.0 m [s.sup.-1] as measured with an air velocity meter (VelociCalc Plus, TSI). FIMA personnel indicated that they regularly paint in these conditions. The surface area painted was approximately 25 [m.sup.2].
A laboratory trial was conducted under conditions of elevated temperatures and poor ventilation. A test bed was setup in a fume cupboard (approx. 1 [m.sup.3]) to allow adequate ventilation before and after the experiment while maintaining an enclosed volume during painting. An aluminum plate (650 mm x 650 mm x 13 mm) was maintained at a surface temperature of 50[degrees]C while resting on three thermostated hotplates (BTL Thermoplate, Industrial Equipment and Control Pty. Ltd., Cat No. 2092.001) and irradiated (from above) by two 250-W reflector light globes (Fig. 8). The air temperature in the enclosure was 30[degrees]C. A film of Melinex[R] polyester (50 [micro]m, 700 mm x 550 mm) was placed over the aluminum plate.
[FIGURE 8 OMITTED]
The paints were mixed in accordance to the manufacturer's requirements. They were applied to the polyester surface with a paint roller to an average dry thickness of approximately 300 [micro]m (Interthane 987, LSALGP) and 800 [micro]m (Interthane 864, LSANSP). The manufacturer's recommended dry paint thicknesses were 50-75 [micro]m for spray application and 400 [micro]m for brush application. (28), (29) The high film thickness for brush/roller application was expected to enhance emissions of isocyanates from the coating surface. Air sampling was conducted approximately 7.5 cm above the paint surface.
A trial was also conducted in the open space (outdoors) at the Defence Science and Technology Organisation (DSTO) laboratories, Melbourne, Victoria. The paint (Interthane 987 LSALGP) was applied with a roller to the polyester sheet on a heated aluminum plate (vide supra) with the surface temperature maintained at 55[degrees]C. Air sampling was conducted 7.5 cm and 75 cm above the paint surface with an air temperature of 20[degrees]C and wind speed of 1 m/s.
Air monitoring for isocyanates after sanding was carried out indoors. The two paints were applied to a polyester sheet (Melinex[R]) by roller and allowed to cure for 24 h. The surface was sanded lightly by hand. The edges were folded inward to form a bag and an air sample (15 L) was taken within the bag to maximize the concentration of any trapped isocyanate vapor.
Results and discussion
The limit of detection (LOD), for the analysis of isocyanates by HPLC, was based on the isocyanate concentration corresponding to three times the peak height over the interfering peaks of a blank sample and was found to be 0.03 [micro]g/[m.sup.3]. The limit of quantitation (LOQ) was based on the isocyanate concentration corresponding to nine times greater than the peak height over the interfering peaks of a blank sample and corresponded to 0.1 [micro]g/[m.sup.3]. These criteria were derived from a commonly accepted methodology in analytical chemistry. (30)
Liquid Interthane 987 Curing Agent Part B (PHA046), Batch No. DL3781AB (Akzo Nobel Pty Ltd.), was analyzed for HDI monomer and found to contain 0.15% HDI. The product label indicated a concentration of <0.4%.
During the trial undertaken aboard a Navy patrol boat, personnel were fitted with air sampling devices in the breathing zone under conditions of relatively high ambient temperatures (surface and air) and low wind speeds which would be conducive to the volatilization of the free isocyanate monomer from the painted surface. Despite these conditions, the concentrations of airborne isocyanates were below the LOQ (<0.1 [micro]g/[m.sup.3], Table 1). Likewise, the concentrations of airborne isocyanates above the paint pot were below the LOQ. By comparison, spray application of automotive isocyanate paints outdoors generated airborne isocyanate concentrations in the range <5-328 [micro]g/[m.sup.3]. (31)
Table 1: Summary of isocyanate monitoring data Trial Application Paint Sampling Isocyanate height concentration (cm) [micro]g/ [m.sup.3]) (a) Patrol Roller-painted Interthane 987 70 [less than or boat surface (air LSALGP (personal) equal to]0.1, temp. [less than or 33[degrees]C, equal to]0.1 surface temp. 57[degrees]C) 70 [less than or downwind equal to]0.1 Interthane 864 70 [less than or LSANSP (personal) equal to]0.1, [less than or equal to]0.1 70 [less than or downwind equal to]0.1 Above the paint Interthane 987 40 [less than or pot (air temp. LSALGP equal to]0.1 35[degrees]C) Interthane 864 40 [less than or LSANSP equal to]0.1 Indoor Roller-painted Interthane 987 10 5.2, 7.9 laboratory surface (air Curing Agent temp. only 30[degrees]C, surface temp. 55[degrees]C) Brush-painted Interthane 987 15 7.1 surface (air Curing Agent temp. only 30[degrees]C, surface temp. 55[degrees]C) Roller-painted Interthane 987 10 0.7, 0.6, 1.0, surface (air LSALGP 1.0 (b), 1.8 temp. (c) 30[degrees]C, surface temp. 55[degrees]C) Interthane 864 10 1.4, 2.7 LSANSP Paint pot (air Interthane 987 10 0.2 temp. LSALGP 20[degrees]C) Interthane 864 10 [less than or LSANSP equal to]0.1 Outdoor Paint pot (air Interthane 987 7.5 [less than or laboratory temp. LSALGP equal to]0.1, 20[degrees]C, [less than or surface temp. equal to]0.1 55[degrees]C) (b) 75 [less than or equal to]0.1 Sanding 18[degrees]C Interthane 987 - [less than or LSALGP equal to]0.1 Interthane 864 - [less than or LSANSP equal to]0.1 (a) Sampling flow rate = 1 L/min for 15 min; only HDI monomer detected (b) Sampling flow rate = 3 L/min for 5 min (c) Air sampling flow rate = 3 L/min for 12 min, LOD = 0.03 [micro]g/[m.sup.3], and LOQ = 0.1 [mivto]g/[m.dup.3]
Airborne isocyanate concentrations were monitored for aerosols (oligomers) and vapors (HDI) using 2-stage filter cassettes. Painting conditions without ventilation in an enclosed space and elevated temperatures were simulated in laboratory trials and air samples were taken above the surface of the paint pot (Table 1). Although these conditions were extreme and not recommended in practice without adequate respiratory protection, the airborne isocyanate concentrations (TRIG) were found to be substantially below the 8-h TWA exposure (20 [micro]g/[m.sup.3]). In all the laboratory measurements made, only HDI (monomer) was detected.
Two measurements were undertaken at shorter sampling times (5 and 12 min), in a confined space, to capture initial (peak) concentrations compared to the 8-h TWA. Although these gave higher concentrations of total airborne isocyanate (TRIG), the values (< 2 [micro]g/[m.sup.3]) at 10 cm above the painted surface were still well below the maximum allowable exposure limit.
Airborne HDI concentrations 7.5 cm above the paint pot (containing the premixed paint) were found to be below the level of quantitation but of a measurable concentration (0.6-2.7 [micro]g/[m.sup.3]) above the freshly painted surface (at an air temperature of 30[degrees]C and surface temperature of 55[degrees]C) without exceeding the 8-h TWA exposure limits (Table 1).
When Part B (the curing agent-containing component alone) was applied to the surface under the above conditions, the airborne HDI concentrations measured 10 cm above the paint surface were substantially higher (5.2-7.9 [micro]g/[m.sup.3]) than for the premixed paint (Table 1). This indicated that a significant amount of the monomeric isocyanate had reacted during the mixing of the two packs prior to the application of the mixed paint. It also illustrated the exposure to airborne HDI vapor under extreme conditions (large surface area of paint containing unreacted HDI) did not exceed the TWA (TRIG) in an unventilated space. Such a situation could arise if Part B was accidentally spilled or inadvertently mixed with another can of Part B rather than Part A.
The experiments were repeated at the DSTO in outdoor conditions with roller application of the paint. The results showed very low isocyanate concentrations (below the quantitation limit) just above the surface (7.5 cm) and in the breathing zone of an applicator (70 cm above the paint surface). These measurements correlate well with the data from the shipboard trial despite the differences in air temperatures (20[degrees]C vs 43[degreees]C).
A study of brush/roller application of isocyanate paints to a car door and at UK petrol stations showed that airborne isocyanate concentrations were below the LOD (0.01 [micro]g/[m.sup.3]) (8) while a Netherlands study of industrial painting companies reported concentrations in the range 0.01-1 [micro]g/[m.sup.3] during the brush/roller application of isocyanate paints. (2) Although ambient temperatures were not cited, given the geographic location of the studies, it is unlikely that they approached those of a tropical climate.
Airborne monomeric and oligomeric isocyanate concentrations were also monitored from the surface of the paints after "drying" for 24 h and light sanding. The isocyanate levels were found to be below the quantitation concentrations. The measurement of airborne particulates containing reactive isocyanates generated by sanding (approx. 0.1-3 [micro]g/[m.sup.3]) (2) has been previously reported but this involved mechanical sanding which, unlike light manual sanding, will remove a greater amount of the paint film and potentially cause thermal degradation of the paint. (11) In another study, where manual sanding of fully cured automotive isocyanate paints was examined, airborne isocyanate concentrations were below the level of detection. (12)
Even though respiratory exposures to isocyanate monomers are very low or nondetectable, isocyanate-induced asthma continues to be a problem as long as skin exposure occurs. (9) Thus, it has been suggested that dermal exposure contributes to isocyanate sensitization and asthma. Dermal exposures were not assessed in this study due to the preliminary nature and the restricted requirement of this exercise but they should be included in any further studies of personnel exposure to airborne isocyanates. The semiquantitative SWYPE[TM] technique could be deployed to this end. (10), (32)
Measurements of human exposure to isocyanates during brush/roller application of Interthane 987 LSALGP and Interthane 864 LSANSP were conducted on a RAN patrol boat during a hot day with a slight wind. The isocyanates were present as the monomeric HDI. Concentrations were measured close to the surface of the deck in the open air. These conditions were considered to be conducive to producing airborne isocyanate vapors. During the application of the paints, the airborne concentrations of isocyanates, in the breathing zone of the operator, were found to be less than < 0.1 [micro]g/[m.sup.3], compared with 20 [micro]g/[m.sup.3] for the UK and Australian (TRIG) exposure limits and 35 [micro]g/[m.sup.3] for the US (TDI) occupational exposure limits.
The brush/roller application of the paints in a confined space with air and surface temperatures of 30 and 50[degrees]C, respectively, did not generate concentrations of isocyanate vapors in excess of the exposure limits. The results are in broad agreement with a study of brush/roller application carried out in the UK (presumably undertaken at lower ambient temperatures than this study), where airborne isocyanate concentrations (monomer vapor and oligomeric aerosols) were found to be below the level of detection during painting indoors and outdoors. (15)
On manual sanding of the cured paints (after 24 h curing), airborne isocyanate concentrations (vapor and aerosol) were found to be below the level of quantitation.
Acknowledgments The authors wish to acknowledge the advice and assistance provided by Dr. Lindsay Wake, Dr. Chris Lyons, and Mr Neil McKenzie who provided advice on the application of the paints.
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[C] FSCT and OCCA 2009
J. Kibby was on contract to the Defence Science and Technology Organisation.
G. DeNola, J. Kibby, P. J. Hanhela, T.-H. Gan, W. Mazurek ([*])
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