EFFECT OF GLASS TRANSITION TEMPERATURE ON VOLATILE EMISSIONS FROM POLYMER MATERIALS

Transcription

1 EFFECT OF GLASS TRANSITION TEMPERATURE ON VOLATILE EMISSIONS FROM POLYMER MATERIALS SS Cox 1, JC. Little 1*, and AT Hodgson 2 1 Department of Civil and Environmental Engineering, Virginia Tech, 418 Durham Hall, Blacksburg, VA 24061, USA 2 Indoor Environment Department, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA ABSTRACT It has been shown that diffusion coefficients, partition coefficients, and initial concentrations of volatile organic compounds (VOCs) as well as ventilation rates affect the rate at which building materials emit these compounds. There is now evidence that glass transition temperature could be a key parameter affecting emission rates of VOCs from polymeric building materials. This study shows that temperature relative to the glass transition temperature influences the rate of emissions from vinyl flooring. Thermal desorption was used to extract VOCs from samples at temperatures ranging from 35 oc to 75 oc. The glass transition temperature of the vinyl flooring was determined to be 49 oc. The emission rate of n-pentadecane, was essentially constant at temperatures between 35 oc and 45 oc but increased by 430% from 35 oc to 75 oc. This finding is consistent with the free volume theory of polymers. INDEX TERMS Emission rate, glass transition temperature, volatile organic compound (VOC), polymers, building materials INTRODUCTION Many building materials and indoor furnishings contain volatile organic compounds (VOCs). When installed in an indoor environment these materials emit VOCs, resulting in degraded indoor air quality. When evaluating a material for use indoors it is desirable to possess some knowledge of the material s VOC emission characteristics. It has been shown that the initial material-phase VOC concentration (C0), the material/air partition coefficient (K), and the material-phase diffusion coefficient (D) are key parameters affecting the emissions characteristics of indoor materials and furnishings (Cox et al. 2002). Due to the behavior of polymeric materials, measurement of K, D and C0, and consequently the emissions characteristics, may be strongly affected by temperature. Free-volume theory and glass transition behavior of polymers can be used to describe how the structure and consequently the emission characteristics of a polymeric material can be affected by temperature. The properties of amorphous polymers such as polyvinyl chloride (PVC) are profoundly different depending on temperature relative to the glass transition temperature (Tg) of the material. At temperatures below Tg, the polymer is relatively hard and brittle, referred to as the glassy state. At temperatures above Tg, the polymer is relatively soft and ductile, referred to as the rubbery state. At temperatures above Tg polymer molecules possess sufficient energy so that large-scale polymer chain movement becomes possible (Rudin 1999). Large-scale polymer chain movement increases ductility as well as the pore space between chains. With respect to polymeric materials, free-volume is the sum of the pore space that exists between the imperfectly arranged carbon chains (Sperlin 1992). Because the rate of diffusion through a solid material depends of the number and size of pores through which a volatile molecule can travel, an increase in free volume greatly increases the rate of diffusion through the material. At temperatures below Tg, free volume is relatively small and is largely independent of temperature. However at temperatures above Tg, free volume and consequently the rate of diffusion through the polymer is relatively high. Depending on the size of a diffusant molecule relative to the pore space between adjacent carbon chains of a * Corresponding author jcl@vt.edu 1845

2 polymeric material, the diffusion of a volatile molecule may be either hindered or unhindered. In the hindered state, diffusion is limited by relatively low free volume. In the unhindered state a volatile molecule diffuses relatively easily through the greater free volume created by an increase in size and number of void spaces between polymer chains. The material temperature relative to Tg therefore can affect D and C0 and consequently the emission characteristics of the material. The objective of this study is to investigate the relationship between temperature and the emission characteristics of VF, an exemplary polymeric building material. VF is comprised primarily of PVC, is present in many residential and commercial buildings, and has been shown to emit organic chemicals of concern. Total North American production of PVC equaled kg in 2003 (American Plastics Council 2003). RESEARCH METHODS The VF used in this study was a monolayer sheet vinyl obtained from a wholesale distributor of flooring products. The 12 ft2 specimen was cut from a bulk spool, tightly rolled, and wrapped in packaging film. The nominal thickness was 2 mm, the density was approximately 1.5 g cm-3. This particular type of VF contained approximately 50% by weight CaCO3, 50% by weight PVC as well as plasticizers, pigments, and stabilizers (Tshudy 1998). A microtome was used to cut samples from larger pieces of VF. A VF sample was transferred to a 4-mm ID 20-cm long borosilicate glass thermal desorption tube. The glass tube was then inserted in a sleeve heater connected to a temperature controller. A mass-flow controller was used to regulate the flow of N2 through the desorption tube. A fraction of the outlet gas stream was drawn through a Tenax TA packed sorbent tube using an SKC air sampling pump. Target VOCs were extracted from the sorbent tubes and quantitatively analyzed by gas chromatography/mass spectrometry (GC/MS) following U.S. Environmental Protection Agency (EPA) Method TO-1 (USEPA 1984). Target VOCs were n-tridecane (n-c13), n-tetradecane (n-c14), n-pentadecane (n-c15), phenol, and 1-methyl-2-pyrrolidinone (NMP). A Perkin Elmer Pyris 1 differential scanning calorimeter (DSC) was used to estimate the VF Tg. DSC measures the amount of energy absorbed by a material sample as it is heated. When a polymer is heated to the glass transition temperature, an inflection occurs in the rate at which the polymer absorbs energy as large-scale chain movement commences. The temperature at which the energy absorption inflection point occurs is Tg. The inflection point measured by DSC occurred at approximately 49oC as shown in Figure Tan delta Figure 1. Tg measurement by differential scanning calorimetry RESULTS To assess the influence of temperature on emission rate, six sampling events were conducted at temperatures of 35, 40, 45, 55, 65, and 75 o C. VF sample mass in each event ranged from approximately 34.2 mg to 43.8 mg. Sampling commenced 10 minutes after the desorption tube was placed in the sleeve heater. Sampling duration 1846

3 ranged from 10 to 25 minutes. The results are summarized in Figures 2 and Emission Rate (ng/mg-min) n-c13 n-c14 n-c Figure 2. Alkane emission rate Emission Rate (ng/mg-min) Phenol NMP Figure 3. Phenol and 1-methyl-2-pyrrolidinone (NMP) emission rate 1847

4 25 20 C 0 (ng/mg) n-c15 30 o C n-c15 85 o C Figure 4. n-pentadecane C 0 To assess the influence of temperature on C0, target VOCs were extracted from two VF samples at temperatures of 30 and 85 oc. Sampling duration was 53 hours for the 30 oc extraction and 5.4 hours for the 85 oc extraction. The results for n-c15 are summarized in Figure 4. The diffusion coefficient, D, can be determined by fitting a diffusion model to the emissions data shown in Figure 4. The VF sample conforms to the geometry of a thin slab. Under the experimental conditions, the rate of change in mass due to Fickian diffusion is given by (Crank, 1975), = Mt 8 D(2n 1) π t = 1 exp M n 0(2n 1) π 4L (1) where M t is the total mass of a VOC emitted from the sample in time t, M is the corresponding mass after equilibrium has been reached, and L is the thickness of the VF sample. D estimation results are summarized in Figures 5 and 6. Measured Model Measured Model M t /M T D =7.0 x m 2 /sec M t /M T D = 2.7 x m 2 /sec Figure 5. D for n-c15 at 85 o C Figure 6. D for n-c15 at 30 o CDISCUSSION The data summarized in Figures 2 and 3 show that the VOC emission rate increase at temperatures below and above the glass transition region is relatively small compared to the emission rate increase within the glass transition region. The emission rate increase at temperatures outside the glass transition region can be explained by considering the temperature effect on D. However as shown in Figures 5 and 6, D increased by a factor of 25 between 30 and 85 oc. This relatively large increase in D, and consequently emission rate, is likely due to an increase in free volume as the VF transitions from the glassy state to the rubbery state. Within the alkane target compounds, the effect of temperature on emission rate appears to positively correlate with 1848

5 molecular weight as shown in Table 1. Table 1. Measured emission rates of target alkanes in VF. Target VOC Emission Rate (ng VOC / mg VF min) Emission Rate Increase Compound 35 o C 75 o C n-c % n-c % n-c % Larger diffusing molecules are more greatly affected by temperature than smaller molecules. This observation is consistent with the free volume theory of polymers. When in the glassy state, the diffusion of relatively large compounds is hindered by the relatively low pore volume within the VF matrix. At temperatures above Tg, the size of the pores increase, reducing the degree of diffusion hindrance, especially for larger molecules. CONCLUSIONS AND IMPLICATIONS Knowledge of the mass transfer characteristics of a building material is crucial for assessing the emissions behavior of the material and its potential to contaminate indoor air. Polymeric building materials exhibit significantly different emissions behavior depending on material temperature relative to Tg. Because polymeric materials of possessing varying Tg characteristics could be installed in relatively high temperature environments such as utility rooms, attics, and kitchens, assessing the mass transfer characteristics of polymeric materials presents additional challenges. Glass transition behavior of polymers should also be taken into account when developing material testing methods and interpreting test results. The results of VF concentration and emission rate measurement by direct thermal desorption are consistent with free volume theory and glass transition behavior for polymers. Although additional work is needed to validate this theory in terms of the emission characteristics of polymeric materials, at temperatures below Tg, diffusion appears to be more greatly hindered due to the physical properties of the material than at temperatures greater than Tg, especially for relatively large molecules. ACKNOWLEGEMENTS Financial support for the work at Virginia Tech was provided by the National Science Foundation (NSF) through an NSF PATH Award (Grant No. CMS ). Work at Lawrence Berkeley National Laboratory was sponsored by the Assistant Secretary for Energy Efficiency and Renewable Energy, Building Technology Program of the U.S. Dept. of Energy under Contract No. DE-AC03-76F REFERENCES American Plastics Council. (2003). Cox SS., Little JC. and Hodgson AT Predicting the emission rate of volatile organic compounds from vinyl flooring, Environmental Science & Technology, (36): Crank J The Mathematics of Diffusion, 2nd Edition, pp Oxford University Press, New York. Rudin A The elements of Polymer Science and Engineering, 2nd Edition; Academic Press: San Diego. Sperling L Introduction to Physical Polymer Science, 2nd Edition; John Wiley & Sons: New York. Tshudy JA. 1998, Personal communication. U.S. Environmental Protection Agency Method TO-1, Revision : Method For The Determination Of Volatile Organic Compounds in Ambient Air Using Tenax Adsorption and Gas Chromatography/Mass Spectrometry (GC/MS); Center for Environmental Research Information, Office of Research and Development. 1849