Developing a Reference Material for Diffusion-Controlled

Oct 8, 2013 - Formaldehyde, a known human carcinogen and mucous membrane irritant, is emitted from a variety of building materials and indoor furnishi...
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Developing a Reference Material for Diffusion-Controlled Formaldehyde Emissions Testing Zhe Liu,† Xiaoyu Liu,‡ Xiaomin Zhao,† Steven S. Cox,† and John C. Little§,* †

Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, Virginia 24061, United States National Risk Management Research Laboratory, Environmental Protection Agency, Research Triangle Park, North Carolina 27711, United States § School of Civil Engineering, University of Sydney, Sydney, New South Wales 2006, Australia ‡

S Supporting Information *

ABSTRACT: Formaldehyde, a known human carcinogen and mucous membrane irritant, is emitted from a variety of building materials and indoor furnishings. The drive to improve building energy efficiency by decreasing ventilation rates increases the need to better understand emissions from indoor products and to identify and develop lower emitting materials. To help meet this need, formaldehyde emissions from indoor materials are typically measured using environmental chambers. However, chamber testing results are frequently inconsistent and provide little insight into the mechanisms governing emissions. This research addresses these problems by (1) developing a reference formaldehyde emissions source that can be used to validate chamber testing methods for characterization of dynamic sources of formaldehyde emissions and (2) demonstrating that emissions from finite formaldehyde sources can be predicted using a fundamental mass-transfer model. Formaldehyde mass-transfer mechanisms are elucidated, providing practical approaches for developing diffusion-controlled reference materials that mimic actual sources. The fundamental understanding of emissions mechanisms can be used to improve emissions testing and guide future risk reduction actions.

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INTRODUCTION Formaldehyde (H2CO) is a flammable colorless gas with a pungent odor at room temperature and is classified by the U.S. Department of Health and Human Services National Toxicology Program as a known human carcinogen and mucous membrane irritant. One of the primary uses of formaldehyde is for the production of synthetic resins (including urea-formaldehyde, phenol-formaldehyde, melamine-formaldehyde, and polyacetal resins), which are used as adhesives, impregnating resins, and curable molding products in the wood, textile, leather, rubber, and cement industries.1 These products and many other indoor materials, including natural wood, can emit formaldehyde during the use phase, although emission rates vary greatly.2−4 Concentrations of formaldehyde in indoor air typically range from 10 to 4000 μg/m3.1,5,6 While exposure to formaldehyde can occur through multiple pathways, indoor air is the largest source of formaldehyde exposure for the general population, although occupational exposures are important for specific populations such as employees in formaldehyde-related industries.7 Due to the potential health risks associated with indoor formaldehyde exposure, various guidelines, standards, and recommendations have been established.5 In the United States, the Formaldehyde Standards for Composite Wood Products Act, enacted as Title VI of the Toxic Substances Control Act © 2013 American Chemical Society

(TSCA), was signed into law in July 2010. TSCA Title VI requires formaldehyde emissions testing in chambers to demonstrate compliance with these standards. Emissions testing performance is often evaluated through interlaboratory studies, but these are costly and timeconsuming and may lead to inconclusive results. A wellcharacterized reference formaldehyde emissions source would be a valuable tool for verifying and validating emissions testing procedures. A recently published study describes a semi-infinite, steady-state formaldehyde emissions source that would be useful for validating single-point or steady-state chamber testing procedures such as those described in ASTM E1333-10, ASTM D6007-02, and EN 717-1.8−11 The subject of this research is a dynamic formaldehyde emissions reference material with an emissions profile similar to the emissions profile of finite sources of formaldehyde. A dynamic reference emissions source would be useful for validating full emissions profile characterization tests such as those described in ASTM D5116-10.12 In collaboration with the National Institute of Standards and Technology (NIST), researchers at Virginia Tech (VT) previously developed a reference material for volatile organic Received: Revised: Accepted: Published: 12946

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the dry-gas flow rate. As a check, the formaldehyde concentration in the gas stream was also measured by visible absorption spectrometry, following NIOSH Analytical Method 350017 (see the SI). Mass-Transfer Parameter Measurement. Mass-transfer parameters of VOCs in polymers include the diffusion coefficient of the VOC in the material (D), the partition coefficient of the VOC between the material and air (K), and the initial material-phase concentration (C0).13,18,19 To determine D, K, and C0, a microbalance was employed.20 The mass of a polymer film was continuously measured at 25 °C and 0% RH using a high-resolution dynamic recording microbalance (see the SI). During the sorption test, mass gain of the film was recorded as air containing a known concentration of formaldehyde was passed through the filmcontaining chamber of the microbalance. At the conclusion of the sorption test, a desorption curve was created by passing clean air across the film while again using the microbalance to monitor formaldehyde mass loss. When Fickian diffusion controls the sorption and desorption process, D can be determined by fitting a simple diffusion model to the sorption and desorption curves. For the experimental configuration, the mass change caused by diffusion of formaldehyde inside the film is given by the following:21

compound (VOC) emissions testing.13,14 That reference material consists of a thin polymethylpentene (PMP) film that is loaded to equilibrium with toluene. Extensive chamber tests at NIST and other emissions testing laboratories have shown that emissions from the reference material closely resemble emissions from actual homogeneous building materials and that the emission profiles can be accurately predicted by a mechanistic model. The model-predicted concentrations therefore serve as true reference values and can be compared to concentrations measured during chamber testing. Knowing the true emission profile in advance is clearly of considerable benefit when validating laboratory performance as well as for diagnosing causes of chamber testing variability. In this project, a similar procedure was employed to create a reference material for formaldehyde by (1) identifying a suitable polymer substrate and determining its mass-transfer properties; (2) diffusing formaldehyde into the polymer film; (3) predicting formaldehyde emissions from the preloaded polymer films employing a fundamental emission model; (4) measuring formaldehyde emissions from the preloaded films in small-scale environmental chambers; and (5) comparing the predicted emission profiles to the measured results.

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MATERIALS AND METHODS Substrate Selection. An ideal substrate should be uniform, stable, and free of additives that may confound mass transfer of formaldehyde within the material. In addition, formaldehyde needs to be sufficiently soluble in the substrate so that an adequate amount of formaldehyde can be diffused into the substrate prior to the chamber test. The diffusion of formaldehyde in the substrate should preferably be Fickian (diffusion under ideal conditions where the process is independent of concentration and sorption and desorption are symmetrical). Although the encouraging application of PMP for toluene suggests that diffusion of formaldehyde within PMP, a nonpolar polymer, may be ideal, the solubility of formaldehyde in PMP is rather low because formaldehyde is quite polar. Formaldehyde is expected to have higher solubility in polar matrices. For example, the solubility of formaldehyde in polycarbonate (PC) is reported to be 150 times higher than in polypropylene.15 Therefore, PMP and PC were tested as candidate substrates (see the Supporting Information, SI). Formaldehyde-Containing Gas Stream Synthesis. A continuous gas stream with a constant formaldehyde concentration was used to diffuse formaldehyde into the substrates and to characterize mass transfer properties of the polymer/formaldehyde systems. The formaldehyde gas generating system consisted of a paraformaldehyde-containing diffusion vial placed in a temperature-controlled calibration gas generator with a purge dry air flow regulated by a mass flow controller. At elevated temperatures in the oven of the calibration gas generator, paraformaldehyde depolymerized to monomeric formaldehyde gas that then diffused into the dry air stream.16 While maintaining a constant gas flow rate, the formaldehyde concentration in the gas stream was varied by adjusting the oven temperature and using vials with different diffusion path lengths. To determine the gas-phase concentration of formaldehyde, each diffusion vial was weighed using a high-precision mechanical balance (10 μg readability) over appropriate time intervals. The formaldehyde concentration in the gas stream can be calculated by dividing the formaldehyde release rate by

Mt =1− M∞

∞

∑ n=0

⎧ −D(2n + 1)2 π 2t ⎫ 8 ⎬ × exp⎨ 2 2 (2n + 1) π 4H2 ⎭ ⎩ ⎪

⎪

(1)

where Mt is the total formaldehyde mass that has entered or left the film via diffusion in time t, M∞ is the formaldehyde mass in the film when equilibrium is reached between the film and the air, and H is the diffusion path length. K can be derived by dividing M∞ by the volume of the film sample and the gasphase formaldehyde concentration. Furthermore, C0 can be determined by dividing the VOC mass gain at equilibrium by the film volume. Reference Material Preparation. Potential reference materials were created by placing one film sample on the microbalance and several film samples in a loading vessel (see the SI). Air containing formaldehyde was passed through the entire system. After equilibrium had been reached, films were removed from the loading vessel, wrapped in aluminum foil, sealed in zip-loc bags, and placed in dry ice-packed insulated containers. The containers were shipped to one of two participating laboratories for emissions testing. Once received, the films were retained in the original package and stored at −12 °C prior to chamber testing. One laboratory was operated by the Environmental Protection Agency (Laboratory A), while the other laboratory remains anonymous (Laboratory B). The fundamental emissions model used to predict gas-phase formaldehyde concentrations during chamber testing has been previously derived (see the SI). Chamber testing parameters are also included in the SI.

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RESULTS AND DISCUSSION Gas-Phase Formaldehyde Concentration Measurement. The measured weight change over time of the paraformaldehyde-containing diffusion vials was linear in all cases. When linear regression was performed R2 was larger than 0.999 for each data set (see the SI). The formaldehyde concentration in the gas stream measured using visible absorption spectrometry was compared to the 12947

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Figure 1. Sorption/desorption data and analysis for PMP.

Figure 2. Sorption/desorption data and analysis for PC.

the slope of the total mass gain after 200 h. Assuming that the rate of slowly reversible sorption is constant and begins at t = 0, the mass gain due to slowly reversible sorption can be subtracted from the total mass gain, yielding the net mass gain due to reversible diffusion that can be described by the Fickian diffusion model given in eq 1. Using the method described above, D and K for the reversible fraction were found to be (3.5 ± 0.2) × 10−14 m2/s and 40 ± 5, respectively. K for gas-phase formaldehyde and PMP is much smaller than K for gas-phase toluene (500 ± 30),14 indicating that formaldehyde has low solubility in PMP. During the desorption period, the mass decrease should be due to the reversible fraction. Indeed, as shown in Figure 1(a), the diffusion model, with D and K obtained from the sorption test, predicts the overall mass decrease well. The slowly reversible fraction appears to contribute little to the total mass decrease during the desorption test. PC was next evaluated for use as a formaldehyde emission reference material. Figure 2 shows microbalance sorption/ desorption results for PC with a gas-phase formaldehyde concentration of 0.86 g/m3. The mass increase of the film during the sorption period followed a trend similar to PMP, as a possible result of the combined effect of reversible and slowly reversible sorption. On the basis of the net mass gain due to diffusion during the sorption period, D and K were found to be (1.9 ± 0.3) × 10−13 m2/s and 230 ± 40 respectively. Therefore, PC has much greater formaldehyde solubility than PMP. As before, desorption curves can be predicted using the diffusion model with D and K obtained from the sorption tests. In summary, sorption/desorption tests of the two polymer candidates suggest that the mass uptake during the sorption period was a combined result of reversible sorption plus irreversible or slowly reversible sorption. In both PMP and PC, the mass of formaldehyde emitted from the film could be

formaldehyde concentration calculated using the diffusion vial weight change. The gas-phase formaldehyde concentrations obtained by both methods were equivalent (see the SI), demonstrating that a gas stream with a controllable formaldehyde concentration could be synthesized using the formaldehyde gas generating system. Evaluating Mass Transfer Properties of Selected Polymer Films. Considering the previously described selection criteria, one PMP film and one PC film with thicknesses of 0.025 cm were chosen as candidate substrates. To determine their mass transfer properties, 3.6 × 3.6 cm2 film samples were subjected to microbalance sorption/desorption testing. The measured mass of a PMP sample during a sorption/desorption test is shown as blue circles in Figure 1(a). The gas-phase formaldehyde concentration for the sorption test was 1.70 g/m3. If Fickian diffusion governs the sorption process, then the measured mass would stabilize upon reaching equilibrium, as demonstrated for toluene in PMP and phenol in vinyl flooring.13,20 In contrast, the mass of the PMP film continued to increase in almost linear fashion as shown in Figure 1(a). The mass desorbed from the film during the desorption period was less than the mass sorbed by the film during the sorption period. A possible explanation is that the formaldehyde behaves in two distinct ways. One fraction undergoes reversible (Fickian) diffusion while the other fraction experiences some interaction between the formaldehyde molecules themselves, or between formaldehyde molecules and the film, that impede the diffusion process rendering it irreversible or perhaps very slowly reversible. According to this simple assumption, total mass gain during the sorption cycle would be the sum of the reversible fraction plus the irreversible or slowly reversible fraction. As shown in Figure 1(b), the rate of mass gain due to the slowly reversible fraction is linear and can be estimated from 12948

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predicted using the simple Fickian diffusion model. Due to the higher K value, PC was selected as a suitable substrate for a formaldehyde emissions reference material. Validating Packaging and Storage Methods by SmallScale Chamber Testing. A batch of PC films was prepared to investigate the effectiveness of packaging and storage methods. Films (10 ×10 cm2) were cut from a PC sheet and loaded using a gas stream containing ∼0.91 g/m3 formaldehyde. The formaldehyde concentration in the films was determined to be 160 ± 9 g/m3. Emphasis was placed on minimizing the time loaded films were in contact with air. The films were then shipped to Laboratory A for small-scale chamber testing. First, small-scale chamber emissions tests were conducted at 0% RH using films with and without the original aluminum foil packaging. As shown in Figure 3, the gas-phase formaldehyde

Figure 4. Measured formaldehyde emission profiles of different storage durations and the model prediction.

Table 1. Statistical Analysis Summary

Figure 3. Measured formaldehyde emission profiles from films with and without foil wrapping.

concentration without the foil wrapping is obviously much higher than the gas-phase formaldehyde concentration with the foil wrapping. The results clearly show that films wrapped in foil do lose formaldehyde at room temperature, although the foil wrapping reduces the formaldehyde loss rate. To study formaldehyde emission profiles with respect to film storage duration, a series of chamber tests was conducted using PC films that had been stored for time periods from 0 to 10 weeks. Tests were conducted in duplicate. Because the emitted mass of formaldehyde from the PC films was assumed to be due primarily to diffusion, the emission model based on diffusion introduced earlier is applicable to formaldehyde emissions estimation using measured D and K values. Therefore, the emissions model was used to predict the gas-phase formaldehyde concentration profile during emissions testing. The Monte Carlo method 22 was used to estimate uncertainties in model predicted concentrations associated with D, K, and C0 (see the SI). Figure 4 shows the model prediction, with the black solid line indicating the mean of the transient gas-phase formaldehyde concentration in the chamber air and the shaded area indicating the range of mean ± one standard deviation of the transient gas-phase concentration obtained using the Monte Carlo method. Figure 4 summarizes storage duration test results compared to the model prediction, with the points showing the average value of duplicate tests and error bars showing the deviation of duplicate tests. All gas-phase formaldehyde concentration measurements fit the model very well (Table 1). However, emissions from films with shorter storage duration fit the model better during the first 20 h of testing. The gray shaded area

batcha-test

laboratory

r

NMSE

1−1 1−2 1−3 1−4 1−5 1−6 1−7 1−8 1−9 1−10 2−1 2−2 2−3 2−4 2−5 2−6 2−7 2−8 2−9

A A A A A A A A A A A A A A B B B B B

0.97 0.97 0.91 0.92 0.92 0.92 0.89 0.88 0.91 0.92 0.96 0.98 0.90 0.99 0.96 0.96 0.96 0.96 0.92

0.058 0.061 0.20 0.17 0.20 0.17 0.29 0.52 0.23 0.19 0.34 0.15 0.23 0.14 3.0 0.9 2.1 0.93 0.76

a

(1) Packaging and storage testing, no storage duration; (2) ILS testing.

provides an expression of the uncertainty of the mean ± one standard deviation associated with estimates of D, K, and C0. The peak values of gas-phase formaldehyde concentration generally decrease with longer storage duration. Results suggest that some formaldehyde is lost from films even during storage at −12 °C. The detailed view in Figure 4 also shows that the gas-phase formaldehyde concentrations after the first 40 h were somewhat higher than the model predictions. A possible reason could be release of slowly reversible formaldehyde from the film. Inter-Laboratory Study (ILS). To further evaluate the reference material, a third series of chamber tests was conducted by two different laboratories at 50% RH. PC films were loaded using a gas stream containing ∼0.91 g/m3 formaldehyde. The formaldehyde concentration in the films was determined to be 170 ± 20 g/m3. Two 10 ×10 cm2 films were tested by Laboratory A and five 8.5 ×8.5 cm2 films were tested by Laboratory B. Figure 5 shows that the measured formaldehyde concentrations during the first 10 h of testing are lower than model predictions, while measured formaldehyde concentrations after that period tend to be higher than the model predictions. 12949

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Figure 5. Emission profiles measured by Laboratory A and Laboratory B and the model prediction.

SI). This difference could be explained by slow release of some of the slowly reversible fraction during chamber testing. Statistical Evaluation of Agreement between Measured and Predicted Concentrations. Differences between chamber-measured and model-predicted gas-phase formaldehyde concentrations can result from (1) errors associated with the measurement procedure, (2) model assumptions, and (3) inaccuracies in model parameters. Two statistical analyses were used to assess the degree of agreement between model predictions and chamber measurements. In the first analysis, the correlation coefficient, r, was calculated for each data set. The correlation coefficient is a measure of the strength of the relationship between measured and predicted concentrations. The second analysis, normalized mean square error (NMSE), provides a measure of the magnitude of the prediction error relative to measured and predicted concentrations. ASTM D5157-9726 advises that an r value of 0.9 or greater and an NMSE of 0.25 or lower indicate acceptable agreement between model-predicted and chamber-measured concentrations. The results of the analysis are summarized in Table 1. The statistical analysis shows that the agreement between the model and measured concentrations represented by r is relatively strong, indicating that model predictions compare acceptably to experimental observations. The prediction error represented by NSME is within the acceptable range for all testing of films stored for a duration of four weeks or less conducted by Laboratory A at 0% RH (the condition for which model parameters were measured). The high NSME values for tests 2/5−2/9 could be due to incomplete mixing in the chambers during the measurement process. Overall, results indicate that it is possible to create a formaldehyde emissions reference material with a dynamic emissions profile that is predictable within acceptable accuracy as described by ASTM guidelines.

The lower concentrations during the early period could again be explained by some loss of formaldehyde from the films during handling, while higher concentrations during the later period could be explained by slow release of the slowly reversible formaldehyde during the chamber test. Some deviation might be explained by the impact of RH on formaldehyde emission profiles as chamber tests were conducted at 50% RH while the D and K values used in the model were obtained at 0% RH. It is well-known that RH affects formaldehyde emissions from composite wood products manufactured with urea/formaldehyde (UF).23 Effect of Humidity on Formaldehyde Emissions. To investigate the effect of relative humidity (RH) on formaldehyde emissions, four films prepared for the ILS were subjected to additional chamber testing at three RH conditions by Laboratory A. Test results are shown in Figure 6.

Figure 6. Emission profiles at different RH levels and the model prediction.

Although formaldehyde concentrations measured at 0% RH are higher in early times and lower at later times when compared to concentrations measured at 70% RH, the statistical analysis shows no relationship between formaldehyde emissions and humidity. Previous chamber studies have indicated that humidity has an effect on VOC emissions for some materials.24,25 Assessment of Validation Process by Mass Balance. As an additional check of the overall approach, a formaldehyde mass balance analysis was conducted using data obtained during chamber testing. The emission rate was integrated over the duration of each chamber test and compared to the initial reversible formaldehyde mass in each tested film. Measured formaldehyde mass emitted from each film was 30% ± 16% greater than the mass of reversible formaldehyde estimated to have diffused into the film during loading (see the

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ASSOCIATED CONTENT

S Supporting Information *

Gas-phase formaldehyde concentration measurement by VAS; gravimetric gas-phase formaldehyde concentration measurement; measured weight decrease of diffusion vials over time (Figure S1); comparison of formaldehyde concentration measured by VAS (Figure S2); gas-phase formaldehyde concentration measurement method comparison; reference material substrates; microbalance sorption/desorption test and microbalance and loading vessel system (Figure S3); emissions model; schematic of a formaldehyde-containing source (Figure S4); chamber testing parameters; Monte Carlo method; formaldehyde mass balance analysis (Table S1); and additional 12950

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testing: Pilot inter-laboratory study. Build. Environ. 2011, 46 (7), 1504−1511. (15) Hennebert, P. Solubility and diffusion coefficients of gaseous formaldehyde in polymers. Biomaterials 1988, 9 (2), 162−167. (16) Rö ck, F.; Barsan, N.; Weimar, U. System for dosing formaldehyde vapor at the ppb level. Meas. Sci. Technol. 2010, 21 (11), 115201. (17) NIOSH. Formaldehyde by VIS 3500. In NIOSH Manual of Analytical Methods (NMAM), Fourth edition; NIOSH: Washington, DC, 1994. (18) Xiong, J.; Yan, W.; Zhang, Y. Variable volume loading method: A convenient and rapid method for measuring the initial emittable concentration and partition coefficient of formaldehyde and other aldehydes in building materials. Environ. Sci. Technol. 2011, 45 (23), 10111−10116. (19) Xiong, J.; Yao, Y.; Zhang, Y. C-history method: Rapid measurement of the initial emittable concentration, diffusion and partition coefficients for formaldehyde and VOCs in building materials. Environ. Sci. Technol. 2011, 45 (8), 3584−3590. (20) Cox, S. S.; Zhao, D.; Little, J. C. Measuring partition and diffusion coefficients for volatile organic compounds in vinyl flooring. Atmos. Environ. 2001, 35 (22), 3823−3830. (21) Crank, J. The Mathematics of Diffusion, 2nd ed.; Clarendon Press: Oxford, England, 1975. (22) Kim, E.; Little, J. C.; Chiu, N. Estimating exposure to chemical contaminants in drinking water. Environ. Sci. Technol. 2004, 38 (6), 1799−1806. (23) Parthasarathy, S.; Maddalena, R. L.; Russell, M. L.; Apte, M. G. Effect of temperature and humidity on formaldehyde emissions in temporary housing units. J. Air Waste Manage. Assoc. 2011, 61 (6), 689−695. (24) Fang, L.; Clausen, G.; Fanger, P. O. Impact of temperature and humidity on chemical and sensory emissions from building materials. Indoor Air 1999, 9 (3), 193−201. (25) Wolkoff, P. Impact of air velocity, temperature, humidity, and air on long-term VOC emissions from building products. Atmos. Environ. 1998, 32 (14−15), 2659−2668. (26) ASTM. ASTM D5157−97 Standard Guide for Statistical Evaluation of Indoor Air Quality Models; ASTM International: West Conshohocken, PA, 2008.

references. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +61 2 9351 7569; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Technical and financial support was provided by the United States Environmental Protection Agency (Contract No. EP11C000135). Disclaimer: The U.S. Environmental Protection Agency through its Office of Research and Development funded and collaborated in the research described here under Contract EP-11-C-00135 to Virginia Tech. It has not been subject to Agency review and therefore does not necessarily reflect the views of the Agency. No official endorsement should be inferred.

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REFERENCES

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