Annals of Occupational Hygiene Advance Access published online on April 30, 2008
Annals of Occupational Hygiene, doi:10.1093/annhyg/men018
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Size-Separated Sampling and Analysis of Isocyanates in Workplace Aerosols—Part II: Aging of Aerosols from Thermal Degradation of Polyurethane
Work Environment Chemistry, Stockholm University, PO Box 460, S-281 24 Hässleholm, Sweden
* Author to whom correspondence should be addressed. Tel: +46-451-385251; fax: +46-451-385260; e-mail: gunnar.skarping{at}anchem.su.se
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMETHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
|---|
A new type of isocyanate sampler has been used to investigate aging aerosols generated during thermal degradation of polyurethane (PUR). The sampler consists of a denuder connected in series with a three-stage cascade impactor and a filter. The denuder collects gas-phase isocyanates. The three impactor stages had cut-off diameters (d50) of 2.5, 1.0 and 0.5 µm, respectively. The end filter collects particles <0.5 µm. For derivatization of isocyanates in the sampler, di-n-butylamine mixed with an equimolar amount of acetic acid was used for impregnation of the sampler stages. Consecutive sampling using three denuder–impactor samplers was performed in a test chamber, with a total sampling time of 9 min. Analysis of air samples was performed using liquid chromatography-mass spectrometry (LC-MS)/MS. Particle size measurements were performed using a scanning mobility particle sizer (SMPS). A time-dependent behavior was observed for aromatic diisocyanates during aging of the aerosol. Thermal degradation of different PUR materials showed different distribution of isocyanates between gas and particles. Aromatic diisocyanates (toluene diisocyanate (TDI) and methylene diphenyl diisocyanate) were initially in gas phase and associated to very small particles. After a few minutes most of these isocyanates were associated with particles <1 µm. Monoisocyanates and hexamethylene diisocyanate (HDI) were not found to be associated with particles.
aerosol • air sampling • DBA • denuder • impactor • isocyanate • LC-MS • PUR • thermal degradation
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
|---|
Isocyanates are a group of reactive organic compounds that may cause airway irritation and occupational asthma (Baur et al., 1994). Exposure to isocyanates is very common in several different industries. Isocyanates occur both as gaseous compounds and associated to particles. Depending on the size, the particles in an aerosol reach different parts of the human airways. Different models have been proposed as estimations of the deposition of particles in the airways (Hinds, 1999). Generally speaking, particles larger than
5 µm are deposited in the nose and mouth; the fraction between 1 and 5 µm reach the tracheobronchial region, while particles smaller than 1 µm are able to penetrate into the alveolar region of the lung. Gases are absorbed in the upper parts of the airways due to diffusion (Witschi and Last, 1996). Isocyanates are used in the production of polyurethane (PUR), a polymer that is produced in large amounts, and during manufacture of the polymer, there is a risk for isocyanate exposure. PUR products are produced by foaming, casting, spraying and other processes. During a foaming or casting process, exposure to monomers mainly takes place by evaporation, while spraying produces small airborne droplets containing isocyanates (Marand et al., 2004). Exposure to isocyanates can also occur when the polymer is processed after production. Thermal degradation of a PUR causes breakdown of the polymer, and isocyanates are released to the surrounding air. Heat cutting of foam blocks; flame lamination of PUR with textile (Tinnerberg et al., 1997); and welding, cutting or grinding of PUR-coated metal (Karlsson et al., 2000), which is very common in car workshops, are examples of processes where there is a risk for airborne exposure. The isocyanates formed when PUR is subjected to heat are both monomers, used in production of the polymer, and new isocyanates, often monoisocyanates, formed during the polymer degradation (Tinnerberg et al., 1996, 1997; Boutin et al., 2004, 2006). Diamines and aminoisocyanates corresponding to the diisocyanates are also formed by different reactions during thermal degradation of PUR (Karlsson et al., 2002).
The composition and size distribution of an isocyanate aerosol can vary. Depending on volatility, molecular weight or generation process, isocyanates occur either as gaseous substances or associated to particles. When isocyanate exposure is studied, it is important to use air-sampling methods with capability to collect both gas and particle phases in a representative way (Streicher et al., 2000). Isocyanates are also very reactive, and derivatization immediately upon collection is necessary, in order to avoid underestimation of the levels.
Different sampling methods, which are able to estimate the amount of isocyanates in the gas and particle phase, have been presented. They consist either of series of different filters (Lesage et al., 1992; Tsai et al., 2003), a denuder in series with a filter or impinger in series with a filter (Rando and Poovey, 1994, 1999; Karlsson et al., 2000). In Part I in this series, a sampling method for isocyanates that in addition to total air concentrations provides information about the distribution of isocyanates between gas and particles and gives an estimate of the particle size distribution was presented (Dahlin et al., 2008). The sampler consists of a denuder for collection of the gas-phase isocyanates, connected in series with three impaction stages and a filter for collection of the particles in different sizes. The denuder–impactor (DI) sampler was used for air sampling of isocyanate aerosols. The distribution for the studied isocyanates between gas and particles was different. The sampler is based on a dry sampling technique using di-n-butylamine (DBA) on impregnated filters (Marand et al., 2005). When comparing the results with an impinger–filter (IF) sampling system using the same reagent dissolved in toluene, good correlation was obtained.
In this work, the composition and particle size distribution of isocyanate aerosols formed during thermal degradation of different PUR materials were studied using the DI sampler. Isocyanate aerosols were generated in a chamber with controlled airflows and humidity. The time-dependent behavior and aging of aerosols was studied.
|
METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
|---|
Apparatus
Determination of isocyanates in air samples was performed using liquid chromatography-mass spectrometry (LC-MS)/MS and multiple reaction monitoring. The instrument used was a quattro micro triple quadrupole mass spectrometer (Waters, Altrincham, England). The MS instrument was equipped with an electrospray interface and positive ionization was used. The capillary voltage was set to 4 kV. Molecular ions of the DBA derivatives were selected in the first quadrupole, and after collision with argon gas, the second quadrupole monitored the DBA fragment (m/z = 130). The pressure in the collision cell was 3 x 10–3 mbar. Sampler cone voltage and collision energies were optimized individually for the analyzed derivatives. Deuterium-labeled isocyanate–DBA derivatives were used as internal standards
A micro-LC system was connected to the MS instrument, for separation of the analyzed derivatives. Gradient elution was performed using 2 Shimadzu LC10ADvp (Shimadzu Corporation, Kyoto, Japan) micro-LC pumps. The total flow of mobile phase was 70 µl min–1. Water and acetonitrile were used as eluents. The gradient was linear from 40 to 95% acetonitrile in 10 min. Formic acid (0.05%) was also present in the eluents. Samples were injected using an LC-PAL injector (CTC Analytics, Zwingen, Switzerland). On-column focusing was used for sample injection. A 2.5-µl sample, surrounded by 17.5 µl of 95/5 H2O/acetonitrile, was injected. The analytical column used was a Waters Xterra, 50 x 1 mm, with 2.5-µm particles.
Aerosol characterization (10–800 nm, electrical mobility diameter) was made by monitoring thermal degradation products, using a scanning mobility particle sizer (SMPS) Model 3936, consisting of a differential mobility analyzer Model 3081 and a condensation particle counter, Model 3775 (TSI Inc., St Paul, MN, USA).
Chemicals
DBA, ethyl isocyanate (EIC), isophorone diisocyanate (IPDI) and 2,6-toluene diisocyanate (TDI) were obtained from Merck-Schuchardt (Hohenbrunn, Germany); acetic acid, formic acid, methanol, toluene and acetonitrile were obtained from Merck (Darmstadt, Germany). All solvents were of high pressure liquid chromatography (HPLC) grade. Propyl isocyanate (PIC), phenyl isocyanate (PhI), hexamethylene diisocyanate (HDI), methylene diphenyl diisocyanate (MDI) and 1,3-dimethylurea were obtained from Acros Organics (Morris Plains, NJ, USA). 2,4-TDI was from TCI (Tokyo Kasei Kogyo Co. Ltd, Tokyo, Japan), DBA derivatives of isocyanic acid (ICA) and methyl isocyanate (MIC) were synthesized by thermal degradation of urea (ICN Biomedicals Inc., Aurora, OH, USA) or 1,3-dimethylurea and collecting the degradation products in toluene containing DBA (Karlsson et al., 2001). Nona-deuterium-labeled DBA for synthesis of internal standards was obtained from Synthelec (Lund, Sweden).
DI sampler
The DI sampler used consisted of five stages analyzed separately. The incoming air entered the sampler via a channel-plate denuder consisting of eight parallel glass fiber plates (filter type MGF, Munktell, Falun, Sweden). The distance between the plates was 2 mm. The plates were mounted in a holder constructed from polypropylene plastic and held together by six stainless steel bolts. The amount of impregnation used on each denuder plate was 1.5 ml of a 1.4 M solution of DBA and acetic acid in methanol. The isocyanate reagent DBA was mixed with acetic acid to form an ion pair.The volatility of DBA was greatly reduced in this way.
The denuder was followed by three single-jet impaction stages with circular nozzles. The three individual stages had cut-off diameters (d50) of 2.5, 1.0 and 0.5 µm, respectively. The impaction stages were made from aluminum. As impactor substrates for the three stages, thin glass fiber plates (filter type MGC, Munktell) with a diameter of 15 mm were mounted on the substrate holders. The impactor glass fiber plates were placed in the impactor substrate holders and impregnated with 65 µl of a saturated solution of DBA and acetic acid, obtained by mixing of 2 ml DBA, 673 µl of acetic acid and 50 µl of H2O. In this way a highly viscous impregnation was obtained. The impregnation needs to fill the pores of the impaction substrates to avoid filtration through the substrates. To keep this impregnation solution from becoming sticky during application, it was kept in a water bath with a temperature of 50°C.
The impactors and the channel-plate denuder were manufactured at a local workshop.
After the last impaction stage a glass fiber filter impregnated with 300 µl of 0.7 M DBA–acetic acid is placed. The filter, with a pore size of 0.3 µm (type MG 160, Munktell), had a diameter of 25 mm and was placed in a Swinnex 25 mm polypropylene cassette (Millipore, Bedford, MA, USA).
The airflow through the sampler (5 l min–1) was maintained by a Laboport twin-diaphragm vacuum pump (KNF Neuberger GmbH, Freiburg, Germany)
IF sampler
An IF sampling system was used for comparison of air levels measured with the DI sampler. Midget impinger flasks (Werner Glas & Instrument, Stockholm, Sweden) were filled with 10 ml 0.01 M DBA in toluene and connected in series with a 13-mm glass fiber filter (type AE, SKC Inc., Eighty Four, PA, USA) with a pore size of 0.3 µm. The filter was placed in a polypropylene holder (Swinnex 13 mm). The airflow through the IF system was 1 l min–1.
Sample handling and work-up procedure
DI sampler.
After sampling, the different sampler parts were treated separately for extraction of the isocyanate DBA derivatives.
The denuder plates were cut out from the holder, folded using tweezers and placed in test tubes. Each filter plate was placed in a separate test tube. Three milliliters of 1 mM H2SO4, 3 ml MeOH and 6 ml toluene and 50 µl of internal standard (1 µg ml–1 deuterium-labeled DBA derivatives) was added to the test tube. Single samples were shaken for 5 min in a shaker, sonicated for 10 min in an ultrasonic bath and shaken again for 20 min. After centrifugation, 5.5 ml of the toluene was transferred to a new test tube. The extraction was repeated by addition of an aliquot of 5.5 ml toluene. A total amount of 11 ml toluene was evaporated to dryness in a vacuum centrifuge, and the residue was dissolved in 0.5 ml acetonitrile, for analysis using LC-MS/MS.
The standards for the denuder samples were prepared in the same way as the samples, but spiked with known amounts of isocyanate–DBA derivatives in toluene. A volume of 1.5 ml of the methanol was replaced with 1.5 ml 1.4 M DBA–acetic acid, to introduce the same amount of impregnation solution in the standards as in the samples.
The impactor substrates and the end filter were placed in 5 ml toluene in test tubes. Two milliliters of 1 mM H2SO4 and 50 µl of internal standard were added. The samples were shaken for 5 min, sonicated for 10 min and the shaken again for 10 min. After centrifugation, the toluene phases were transferred to new test tubes and evaporated to dryness. The residues were dissolved in 0.5 ml acetonitrile before the LC-MS/MS analysis. Standards were prepared as the samples, but spiked with proper amounts of isocyanate–DBA derivatives. To introduce the same amount of DBA to the standards as to samples, an amount of 150 µl of 1.4 M DBA in acetic acid was added to each standard sample.
IF sampler.
After air sampling, the impinger solutions were transferred to test tubes before the addition of 50 µl of internal standard (IS) (1 µg ml–1 deuterium-labeled DBA derivatives). The end filters were transferred to test tubes containing 10 ml 0.01 M DBA in toluene for separate analysis. The filter samples were shaken for 10 min and sonicated for 10 min to extract the isocyanate–DBA derivative from the filters. After centrifugation, the toluene was transferred to new test tubes. The toluene solutions were evaporated to dryness and dissolved in 0.5 ml acetonitrile before LC-MS analysis. Standards were prepared in 10 ml 0.01 toluene–DBA by adding isocyanate–DBA standard solution (0–0.7 ml) and 50 µl of IS.
Characterization of the generation system
Particle size and concentration measurements were made in the range 10–480 nm. For the aerosol generation, the same experimental parameters as described below for DI measurements of thermal decomposition of rigid MDI-based foam were used. Measurements were also performed both with and without airflow through the chamber to estimate the deposition and dilution, respectively. Mixing fans in the chamber were running in both experiments.
Sample generation and air sampling
Air sampling was performed in a 0.300-m3 test chamber made from stainless steel and glass. A small negative pressure was maintained in the chamber, via the laboratory exhaust ventilation connected to the top of the chamber. Humidified air was supplied at the bottom of the chamber, with a flow of 40 l min–1. The relative humidity was maintained at 60%. In the humidifier, a dry air stream is mixed with an air stream bubbled through temperature-controlled water, to obtain the desired humidity. Inside the chamber, four small mixing fans were placed to get an efficient mixing of incoming air and isocyanate aerosol. Isocyanate aerosol from thermal degradation of different PUR samples was introduced close to the inlet air supply of the chamber. The aerosol was generated by placing a PUR sample in a glass tube, between two wads of glass wool to keep the sample in place during the generation process. The tube was 150 mm long and had an inner diameter of 7 mm. A heated air stream (400°C) was blown through the tube with a flow rate of 15 l min–1.
To study the composition of an aging aerosol three DI samplers (denoted A, B and C) were placed in the chamber. The Sampler A reflects the concentration during the generation of the aerosol. Sampling with the impactors was performed during 9 min according to the schedule in Table 1.
|
In the first series of experiments, a mix of three different PUR materials was used for generation. The materials were 40 mg of soft TDI-based flexible foam taken from an orthopedic pillow, 25 mg of hard MDI-based foam cut out from the insulation of a heat-pipe and 20 mg of a HDI-based car coating. The foams were cut out from larger blocks. The coating was scraped from a piece of metal from a car. Three measurements were performed according to the schedule in Table 1.
In the second series of experiments, the PUR materials were studied separately. The amounts used were increased to 50 mg of soft foam in Measurement 1, 30 mg of hard foam in Measurement 2 and 30 mg of coating in Measurement 3. Samples were collected as described in Table 1. Samples were also collected with three IF samplers, during the sampling period for DI Sampler B, to study total air concentrations and distribution between the impinger flask and the glass fiber filter.
|
RESULTS AND DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
|---|
Characterization of the generation system
The thermal degradation products of PUR, in the test chamber, are influenced by the ventilation, agglomeration, deposition and chemical reactions. The ventilation influences the gas phase and the different studied particle size fractions in the same way. Naturally, particle agglomeration influences the number of airborne particles but not the mass. Agglomeration velocity is concentration dependent. The concentration of small particles is more affected by deposition as they have higher diffusion velocity. Previous knowledge regarding chemical reactions of isocyanates in the aerosol is essentially missing. Settling is also a deposition process for particles >2.5 µm, but particles formed during thermal degradation are too small to be affected by this process.
The size of the chamber limits the number of DI samplers that can be used in each experiment to three. The ventilation results in dilution of the aerosol. The ventilation rate was eight air exchanges per hour. If a general ventilation equation is applied, it can be calculated that a gaseous, nonreactive compound has a half-life of 5.3 min in the generation system. For the thermal decomposition of MDI foam (Table 2), we have compared the calculated dilution due to ventilation with the decrease in concentration actually found. For Samplers B and C, it can be calculated that PhI and ICA do not follow the ideal exponential decrease in concentration. PhI has a lower concentration than expected, indicating that some PhI may be lost in chemical reaction. ICA on the other hand shows a slightly higher concentration than expected. The data are insufficient to clearly explain why ICA behaves different as compared to the other studied isocyanates. We will study this in more detail in forthcoming papers.
|
The concentration of MDI is also slightly lower than that expected if the general ventilation equation is applied. However, since MDI is associated to particles, a deposition term must be added to the ventilation equation. SMPS measurements of the particle size distribution after thermal degradation of 30 mg MDI foam in the chamber (Fig. 1), with and without ventilation, show that the deposition part of the decrease in particle mass concentration over time is 12%. In Fig. 1, a bimodal size distribution can be seen for the first sampling period. The primary condensation particles formed by homogenous nucleation can be seen as a concentration peak at 70 nm. They quickly undergo agglomeration to larger particles (the second peak at 165 nm). The deposition was turbulent due to the mixing fans in the chamber and it was measured to be
16% of the total decrease in concentration between the samples B and C. From the SMPS data (Fig. 1) an exponential decline in particle mass concentration was calculated. The MDI concentration follows this calculated exponential decrease (Table 2). This indicates that no chemical reactions occur for MDI.
|
Aerosol generation with mixed PUR materials
The aerosols formed during the thermal decomposition of PUR contain very reactive compounds. Experience gained from previous studies indicated that the aerosol was in a very reactive form. Efforts have been made to develop derivatization reagents that react instantly with isocyanates to minimize side reactions. Clearly, the total isocyanate content and the physical form varied with time. To find out details about what happens with an aging isocyanate aerosol, we have sampled at different time intervals with a DI sampler (Tables 2–5).
Several different isocyanates were found and some can be seen in Table 3. In addition, EIC, PIC, PhI and 2,4-TDI were found. The distributions of EIC and PhI were similar to MIC and the distribution of 2,4-TDI was similar to 2,6-TDI. In Part I of this series, the same type of isocyanate aerosol was studied (Dahlin et al., 2008). Small isocyanates (monoisocyanates) were predominately found in the denuder, indicating that they are in the gas phase. Diisocyanates were present both in the gas phase and associated to particles.
|
In the present study consecutive sampling was performed. Small isocyanates such as ICA and MIC remained in the gas phase during the whole 9-min sampling time. The isocyanate distributions did not change with time (see Table 3). 2,6-TDI shows a changing distribution over time. During the first 3 min (Sampler A, Table 3) most of the TDI was found in the gas phase (
50% in the denuder). For the next sampling period (Sampler B, Table 3) the gas-phase TDI was much lower (15%). It was even lower in Sampler C (%). Hence, the amount on the impaction stages and the end filter was increased over time. Clearly, TDI becomes associated with particles over time. The relative amount of TDI on impaction Stage 2 increases from 5 to 25% during the investigated time period. HDI is different and the distribution is about the same in all DI samplers and HDI does not seem to become associated to particles. IPDI is at start predominantly in the gas phase. The distribution does not change with time. About 30% is associated to particles, which is different as compared with HDI.
MDI shows a time-dependent behavior, but it never appears in the gas phase. Instead MDI immediately becomes associated to particles. In Sampler A 50% is found on the end filter and it decreased in Samplers B and C, to 30%. In Sampler B 50% is found on the third impaction stage and in Sampler C it decreased to 40%. At the same time the relative amount on impaction Stage 2 increased from 18 to 28%.
Two different processes for particle growth are involved. The first process is condensation that occurs after the PUR is thermally decomposed and gaseous compounds are released. During cooling, small particles are formed by condensation. The second process is agglomeration of small particles to large particles (Fig. 1). The volatility of the monoisocyanates is high and they are predominately found in the gas phase during the whole experiment, as can be seen in Table 3. Aromatic diisocyanates like TDI are initially gaseous, but after a few minutes they are found to be associated to particles. MDI immediately becomes associated to particles. The amount of TDI and MDI found on impaction Stage 2 (d50 = 1.0 µm) increased during the sampling period. For the high temperature used for generation, it can be assumed that most of the isocyanates are released from the PUR as gaseous compounds that condense onto particles upon cooling of the gases. Surprisingly, HDI condenses much slower than TDI. Most of the HDI was found in the denuder, while TDI was distributed in several sampler parts. One explanation may be the slightly higher vapor pressure for HDI (7 Pa at 20°C), as compared to 2,6-TDI (2 Pa at 20°C) (Dobbs and Daems, 2003), but with such a small difference this seems unlikely. It was also expected to find a greater amount of IPDI on the impaction stages, due to the low volatility, but only 30% of the IPDI is found associated to particles and the distribution changes very little during the time period studied. Probably some other factor than the vapor pressure of the different compounds is more important for association of the isocyanate to particles. One thing clearly seen from the results is that aromatic diisocyanates are found on particles, and the relative amount in the gas phase is decreasing with time. For the aliphatic isocyanates, the relative amount in the gas phase remains fairly constant during the whole sampling time. An equilibrium between gas-phase and particle-associated isocyanates seems to exist, where aromatic isocyanates have a greater affinity for particles than aliphatic isocyanates. Aliphatic isocyanates quickly reach this equilibrium, while aromatic isocyanates are slower. Similar distributions have been observed for alkanes and polycyclic aromatic hydrocarbons in tobacco smoke (Liang and Pankow, 1996).
The total amounts of the isocyanates are decreasing with time. This is due to dilution and deposition as described earlier above.
Aerosol generation with separate PUR materials
In the second series of experiments, different PUR samples were studied individually. The aim was to have a smaller load of organic material present during the generation to see if this affected the distribution of the isocyanates.
For the soft TDI foam, the same results were obtained as when different PUR materials were mixed (Table 4). TDI shows as above a changing distribution with time. MIC, PhI, 2,4-TDI and a small amount of MDI could also be observed (not in table). The distributions of MIC and PhI were similar to ICA. 2,4-TDI was distributed as 2,6-TDI. MDI showed a similar distribution as in Table 3.
|
The levels in DI Sampler B were about the same as compared to the three IF samplers (Table 4). The IF sampler collects particles in the size range 0.01–1.5 µm on the end filter and both gaseous isocyanates and large and very small particles in the impinger flask. The IF sampler filter reflects impactor Stage 2, 3 and the end filter.
In Table 2 it can be seen that MDI shows a changing distribution with time, but the monoisocyanates remain in gas phase. MIC was also detected and the distribution was similar as for ICA. In IF samples the same concentrations were found as for the DI samples. In the IF sampler, most of the MDI was found on the filter whereas monoisocyanates were found in the impinger flask.
After the initial phase (Sampler A), HDI and IPDI have no tendency to become associated with particles (Table 5). EIC, PIC, PhI were also detected in the samples. Distributions were similar to ICA. The total amount in the DI sampler was the same as for the IF samplers.
|
The results presented in this paper show that isocyanates generated during thermal degradation remain airborne for a long time. Side reactions consuming the isocyanates are limited, despite the high reactivity of isocyanates. The way of isocyanate generation used in these experiments is not very ‘clean’, and a lot of other reactive organic compounds are formed at the same time as the isocyanates. Several of these are probably able to react with the isocyanates present. In addition, the experiment is performed with humid air, which further increases the possibility that isocyanates should be lost by reaction with water vapor.
However, this is what to expect at workplaces involved in hot work with PUR. In Part I of this series isocyanates in the gas phase were generated (Dahlin et al., 2008). The isocyanates studied were HDI, TDI and IPDI. No particle-associated isocyanates were then detected. Instead, the isocyanates remained in the gas phase for the whole 10 min of sampling. These measurements were performed with different levels of humidity, but this had no effect on the distribution of gaseous isocyanates. When particle generation occurs, aromatic isocyanates become associated to particles. Condensation may be important, but other factors may also affect the distribution. Aliphatic isocyanates do not become particle associated, even though they have similarities with aromatic isocyanates.
The changes in the distribution of the isocyanates in aging aerosols raise questions regarding possible exposure in working life. Isocyanates associated to particles are able to penetrate deep in the airways, while gaseous isocyanates generally are deposited by diffusion in the upper parts (nose, mouth and throat) of the airways. Extra precautions should be made if there is knowledge of the presence of isocyanates that may penetrate in the lower airways. These isocyanate fractions are found in the impactor Stage 2, 3 and on the end filter, or on the end filter of the IF sampler.
Exposure to thermal degradation products of PUR can occur in all parts of the industry, but not everyone is aware of the possible risks involved. If safety precautions are not satisfactory, exposure to isocyanates in the particle size that penetrates the lower airways may be very high.
Further studies are needed to reveal data on other degradation products and how they are formed from different materials.
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
|---|
Thermal degradation of different PUR materials showed different distribution of isocyanates between gas and particles. Aromatic diisocyanates (TDI and MDI) are initially in gas phase or associated to very small particles. After a few minutes most of these isocyanates are associated with particles <1 µm. Monoisocyanates and HDI were not found to condense onto particles.
|
ACKNOWLEDGEMENTS |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
|---|
Monica Hassgård is acknowledged for skilful laboratory assistance.
Received August 27, 2007; in final form March 10, 2008
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
|---|
Baur X, Marek W, Ammon J, et al. Respiratory and other hazards of isocyanates. Int Arch Occup Environ Health (1994) 66:141–52.[CrossRef][Web of Science][Medline]
Boutin M, Dufresne A, Ostiguy C, et al. Determination of airborne isocyanates generated during the thermal degradation of car paint in body repair shops. Ann Occup Hyg (2006) 50:385–93.
Boutin M, Lesage J, Ostiguy C, et al. Identification of the isocyanates generated during the thermal degradation of a polyurethane car paint. J Anal Appl Pyrolysis (2004) 71:791–802.[CrossRef]
Dahlin J, Spanne M, Karlsson D, et al. Size-separated sampling and analysis of isocyanates in workplace aerosols. Part I. Denuder-cascade impactor sampler. Ann Occup Hyg (2008) doi:10.1093/annhyg/men017.
Dobbs J, Daems D. Product stewardship. In: The polyurethanes book—Randall D, Lee S, eds. (2003) Everberg, Belgium: Huntsman International LLC. 43.
Hinds WC. Respiratory deposition. Aerosol technology: properties, behavior, and measurement of airborne particles. In: (1999) 2nd edn. New York: Wiley-Interscience. 233–59.
Karlsson D, Dalene M, Skarping G, et al. Determination of isocyanic acid in air. J Environ Monit (2001) 3:432–6.[CrossRef][Web of Science][Medline]
Karlsson D, Dahlin J, Skarping G, et al. Determination of isocyanates, aminoisocyanates and amines in air formed during the thermal degradation of polyurethane. J Environ Monit (2002) 4:216–22.[CrossRef][Web of Science][Medline]
Karlsson D, Spanne M, Dalene M, et al. Airborne thermal degradation products of polyurethene coatings in car repair shops. J Environ Monit (2000) 2:462–69.[CrossRef][Web of Science][Medline]
Lesage J, Goyer N, Desjardins F, et al. Workers exposure to isocyanates. Am Ind Hyg Assoc J (1992) 53:146–53.[Web of Science][Medline]
Liang CK, Pankow JF. Gas/particle partitioning of organic compounds to environmental tobacco smoke: partition coefficient measurements by desorption and comparison to urban particulate material. Environ Sci Technol (1996) 30:2800–5.
Marand A, Dahlin J, Karlsson D, et al. Determination of technical grade isocyanates used in the production of polyurethane plastics. J Environ Monit (2004) 6:606–14.[CrossRef][Web of Science][Medline]
Marand A, Karlsson D, Dalene M, et al. Solvent-free sampling with di-n-butylamine for monitoring of isocyanates in air. J Environ Monit (2005) 7:335–43.[CrossRef][Web of Science][Medline]
Rando RJ, Poovey HG. Dichotomous sampling of vapor and aerosol of methylene-bis-(phenylisocyanate) [MDI] with an annular diffusional denuder. Am Ind Hyg Assoc J (1994) 55:716–21.[Web of Science][Medline]
Rando RJ, Poovey HG. Development and application of a dichotomous vapor/aerosol sampler for HDI-derived total reactive isocyanate group. Am Ind Hyg Assoc J (1999) 60:737–46.[Web of Science][Medline]
Streicher RP, Reh CM, Key-Schwartz RJ, et al. Determination of airborne isocyanate exposure: considerations in method selection. Am Ind Hyg Assoc J (2000) 61:544–56.
Tinnerberg H, Karlsson D, Dalene M, et al. Determination of toluene diisocyanate in air using di-n-butylamine and 9-n-methylaminomethyl-anthracene as derivatization reagents. J Liq Chromatogr Relat Technol (1997) 20:2207–19.[CrossRef]
Tinnerberg H, Spanne M, Dalene M, et al. Determination of complex mixtures of airborne isocyanates and amines. 2. Toluene diisocyanate and aminoisocyanate and toluenediamine after thermal degradation of a toluene diisocyanate-polyurethane. Analyst (1996) 121:1101–6.[CrossRef]
Tsai CJ, Cheng KC, Aggarwal SG, et al. Simultaneous sampling of gas- and aerosol-phase TDI with a triple filter system. J Air Waste Manag Assoc (2003) 53:1265–72.[Web of Science][Medline]
Witschi HR, Last JA. Toxic responses of the respiratory system. In: Casarett and Doull's toxicology—Klaassen CD, ed. (1996) 5th edn. New York: McGraw Hill. 446–9.
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||