Ind. Eng. Chem. Res. 2009, 48, 8961–8965
8961
Water Vapor Permeation Resistance of Polycarbonate at Various Temperatures Sung In Moon and C. W. Extrand* Entegris, Inc., 3500 Lyman BouleVard, Chaska, Minnesota 55318
The permeation rate of water vapor through polycarbonate (PC) was examined. Permeability (P), diffusion (D), and solubility (S) coefficients were measured at temperatures ranging from 25 to 80 °C. P values remained almost constant over this temperature range. On the other hand, D values increased and S values decreased exponentially with temperature. These data represent a broad map of the mass-transport characteristics of water through PC, which can be used to estimate break-though times, permeation rates, absorption, desorption/ outgassing, and moisture content over a wide range of temperature and humidity. Introduction Polycarbonate (PC) is an amorphous engineering thermoplastic. With excellent clarity, toughness, and a high softening temperature, it is used in a wide variety of semiconductor, medical, aerospace, and consumer applications. In the semiconductor arena, PC is used for containers or “microenvironments” that protect and transport critical materials, such as silicon and ceramics, as they are converted from raw wafers or disks into integrated circuits or rotating memory. In the past, moderate humidity and oxygen found in ambient air were usually benign. Today, some semiconductor manufacturing technologies have advanced to the point where even the water and oxygen in air can be detrimental, causing time-dependent haze or corrosion.1,2 Purging with an inert gas is one method for lowering water or oxygen levels inside microenvironments. However, water can permeate through or desorb from the microenvironments, causing levels to quickly rise. Engineering a solution to minimize water and oxygen inside microenvironments requires knowledge of the mass-transport characteristics of the materials of construction. While data exist for the permeation of gases such as oxygen or nitrogen through PC,3-6 data for water are much scarcer. The water permeation data for PC that do exist are limited to room temperature and moderate humidity levels.4,7 Therefore, in this study, we have used a manometric technique to measure the mass transport of water through PC over a broad range of humidity and temperature. Analysis The following analysis assumes one-dimensional diffusion through a flat sheet. Relative humidity (RH) was calculated as ratio of water vapor pressure (pv) to saturation water vapor pressure (psat) at a given temperature: RH ) 100pv/psat
(1)
Vapor permeates through homogeneous materials by first dissolving and then diffusing.8 The downstream pressure (pl) of the permeant can be converted to an equivalent volume of gas (V) at standard temperature and pressure (STP): V ) (pl/po)(To/T)Vs
(2)
where T is the measurement temperature, Vs is the volume of * To whom correspondence should be addressed. Tel.: (952) 5568619. E-mail:
[email protected].
the downstream side of the permeation apparatus, To is standard temperature (0 °C ) 273 K), and po is standard pressure (1 atm ) 76 cmHg). The volume (V) of gas that permeates through a film with time (t) under steady-state conditions depends on the permeability coefficient (P), as well as film thickness (B), film area (A), and the applied upstream pressure (ph):8,9 V ) P·A·ph·t/B
(3)
The time required for a permeant to break though a film (tb) depends on the film thickness (B) and the diffusion coefficient of the material:9 tb ) B2/6D
(4)
Solubility coefficients were calculated from permeability and diffusion coefficients as: S ) P/D
(5)
The temperature dependence of the mass-transport coefficients usually is described by the following exponential functions:10 P ) Po exp(-EP/RT)
(6)
D ) Do exp(-ED/RT)
(7)
S ) So exp(-ES/RT)
(8)
and
where Po, Do, and So are pre-exponential factors; EP, ED, and ES are activation energies; R is the ideal gas constant; and T is absolute temperature. Experimental Details The PC evaluated in this study was extruded film from GE (now Sabic) (8010-MC-112). Glass transition temperatures of the films were determined using differential scanning calorimetry (Perkin-Elmer DSC7). Samples ranging in mass from 4 to 8 mg were cut from specimens, heated from 50 °C (122 °F) to 200 °C (392 °F), cooled from 200 to 50 °C, and then heated again from 50 to 200 °C at a rate of 10 °C/min (18 °F/min). Using the software resident in the DSC7, the resulting DSC scans were analyzed. Triplicate DSC scans were performed. The vapor permeation apparatus consisted of a sample holder inside of a temperature-controlled chamber, a series of valves, a 12 L upstream ballast tank, a solid-state manometer (1000 Torr MKS Baratron type 628B) for the upstream vapor, and a
10.1021/ie900842t CCC: $40.75 2009 American Chemical Society Published on Web 09/18/2009
8962
Ind. Eng. Chem. Res., Vol. 48, No. 19, 2009
Table 1. Saturation Vapor Pressure (psat) of Water at Different Temperatures (T) T
psat (cmHg)
°C
K
from ref
25 30 40 50 60 70 80
298 303 313 323 333 343 353
2.38 3.18 5.53 9.25 14.94 23.37 35.51
15
from measurements 2.35 3.26 5.63 9.32 14.95 23.39 34.94
downstream solid-state manometer (10 Torr MKS Baratron type 628B). The apparatus was constructed from stainless steel. Connections were made by welding or with VCR flanges to minimize leaks. Data acquisition and control were performed remotely with a personal computer. The instrument design is similar to that of Schult and Paul.11 Permeation was measured according to standard manometric procedures12,13 as described below. A circular specimen with a diameter of 4.6 cm and an effective area (A) of 13.7 cm2 was placed in the vapor permeation apparatus. The apparatus was pumped down to approximately 3 × 10-3 cmHg and held overnight to remove volatile constituents from the apparatus as well as from the specimen. The next day, the apparatus was leak tested. If the leak rate was sufficiently low (typically ∼2 × 10-8 cmHg/s), then the upstream side of the apparatus, including the 12 L ballast tank, was charged with the vapor of 18 Mohm deionized (DI) liquid water from a 150 mL stainless steel bottle. A solenoid valve was opened until the vapor pressure on the upstream side rose to the desired level. After pressure and temperatures were allowed to equilibrate for a few minutes, the test was started. The downstream pressure rise (pl) was recorded with the passage of time. Temperature and upstream pressure (ph) also were monitored over the duration of the experiment to ensure their constancy. Measurements were made with samples having a thickness of B ) 0.25 mm at 25 °C (77 °F), 30 °C (96 °F), 40 °C (104 °F), 50 °C (122 °F), 60 °C (140 °F), 70 °C (158 °F), and 80 °C (176 °F). Given the uncertainty of the temperature, the pressures, the downstream volume, and the dimensions of the polymer specimen, the uncertainty of our permeation measurement was 3-5%. Results and Discussion Thermal Characteristics. The glass transition temperature of the PC films was 148 °C (298 °F). These values generally agreed with previously reported data.14 Relative Humidity and Water Vapor Pressure. In gas permeation, pressure is typically expressed in units such as psi, Torr, cmHg, or atm. Water vapor pressure also can be quantified using the same units. However, it is more commonly reported as relative humidity (RH). Once the temperature and vapor pressure are known, RH can be easily calculated from eq 1. Or vice versa, if we know the ambient conditions of a real application (i.e., temperature and RH), the vapor pressure can be estimated. For example, many PC products are used in semiconductor chip fabrication facilities where the temperature and humidity are maintained at 25 °C and 45% RH. Because water at 25 °C has a saturation vapor pressure of psat ) 2.38 cmHg, a RH of 45% equates to water exerting a vapor pressure of 1.07 cmHg. Table 1 shows the measured and literature values15 of the saturation vapor pressure (psat) of water for the different temperatures used in this study. These values correspond to
Figure 1. Downstream pressure (pl) versus time (t) for water permeating through a 0.25 mm PC film at 25 °C from three separate trials, where the upstream vapor pressures were ph ) 0.60, 1.07, and 2.38 cmHg, corresponding to RH values of 25%, 45%, and 100%.
Figure 2. A plot of V · B/A · ph versus time (t) for water vapor permeating through a 0.25 mm PC film at 25 °C under three different RH values: 25%, 45%, and 100%.
100% RH at their respective temperatures. Vapor pressure increases with temperature, modestly for lower temperatures and more rapidly for higher temperatures. Even though measured and reported values generally showed excellent agreement, small temperature fluctuations ((0.5 °C) during measurements caused measured psat values to vary by as much as 3%. Therefore, relative humidity listed throughout the result was estimated from literature psat values. We included the data for saturation pressure versus temperature as a way of assuring that our pressure and temperature measurements were reasonably accurate. Had either the pressure transducer or the thermocouple been erroneous, we would have observed significant deviations from the literature values. Mass-Transport Properties. For a given specimen, permeation rates were proportional to the applied upstream pressure and inversely proportional to thickness. Figure 1 shows an example of raw data obtained from a 0.25 mm thick PC specimen exposed to three different upstream pressures, ph ) 0.60, 1.07, and 2.38 cmHg. These upstream vapor pressures correspond to 25%, 45%, and 100% relative humidity, eq 1. Initially, the downstream pressure did not increase until the water vapor reached the downstream side. Once water vapor broke through, the downstream pressure increased at a rate proportional to the upstream pressure. Thus, the greater was the upstream vapor pressure, the faster was the rise of water vapor downstream after breakthrough. These data can be rearranged to calculate permeation properties using eq 3. The results are shown in Figure 2. The points are experimental data, and the solid line represents linear regression from longer times. The slope of the line in Figure 2
Ind. Eng. Chem. Res., Vol. 48, No. 19, 2009
Figure 3. The permeation coefficient (P), diffusion coefficient (D), and solubility coefficient (S) for water vapor through PC at different relative humidities (RH) and temperatures.
is equal to the permeability coefficient (P), which, for this film, has a regression value of P ) 1.1 × 10-7 cm3 · cm/cm2 · s · cmHg. The break-through or lag times (tb) were estimated from the intersection of two lines: the first is defined by the permeability coefficient, and the second is a zero slope dashed line passing through the initial pl at t ) 0 s. For the data in Figure 2, tb ) 1900 s (about 31 min), corresponding to a diffusion coefficient of D ) 5.7 × 10-8 cm2/s. From the quotient of P and D [eq 5], the solubility coefficient was estimated as S ) 1.9 cm3/ cm3 · cmHg. If analyzed individually, data from each of the pressures gave nearly identical values of P, D, and S. Pressure Dependency of Water Permeation. Figure 3 shows permeability, diffusion, and solubility coefficients of water vapor through PC as a function of relative humidity. Generally, these coefficients did not show much dependence on RH. P is an inherent material property that describes the normalized flow rate through a material. The permeability coefficient for water through PC was essentially constant up to RH ) 80%. At RH ) 100%, it showed a slight upswing. Stannett and co-workers16 also observed this increase at high humidity for water in polyacrylonitrile. The diffusion coefficient (D) increased slightly with RH. However, the change was more pronounced at lower temperatures and always 0.99). On the other hand, correlation for P values was poor, as P did not depend on temperature. We also tried the same calculation using data only from RH ) 100%. The difference in the pre-exponential factors and activation energies was less than 3%. Comparison to Previously Reported Data. A few investigators have published data for water vapor permeation through PC.4,7,21 These data are limited to permeability coefficients (P) or water vapor transmission rates (WVTRs); D and S values largely do not exist. Water vapor transmission rates report the quantity of water vapor as a mass rather than volume and have units such as g · mm/m2 · day. WTVR values taken from the
literature21 were converted to Barrer (Ba) for comparison (1 Ba ) 10-10 cm3 · cm/cm2 · s · cmHg). Previously reported P values ranged from 1000 to 1400 Ba and generally agreed with the values measured here. The data generated in this study, which also includes diffusion and solubility coefficients, represent a broad map of the mass-transport characteristics of water through PC that can be used not only to estimate permeation rates, but also break-though times, absorption, desorption/outgassing, and moisture content over a wide range of temperature and humidity. Conclusions Permeability, diffusion, and solubility coefficients of water vapor were measured for PC at temperatures ranging from 25 to 80 °C. The diffusion coefficient (D) increased exponentially with temperature, and the solubility coefficient (S) decreased exponentially. These opposite trends in D and S coincidentally canceled each other and made the permeability coefficient temperature invariant in this case. Curve-fitting parameters extracted from these data can be used to estimate mass-transport characteristics of products constructed from PC. Acknowledgment We thank Entegris management for supporting this work and allowing publication. Also, thanks to B. Arriola, C. Duston, J. Goodman, T. King, L. Monson, S. Moroney, J. Pillion, S. Tison, and B. Waldridge for their suggestions on the technical content and text. Literature Cited (1) Shrive, L. W.; Blank, R. E.; Lamb, K. H. Investigating the Formation of Time-Dependent Haze on Stored Wafers. Micro 2001, 19, 59. (2) Extrand, C. The Permeation Resistance of Polymers. Semicond. Fabtech 2008, 3, 43. (3) Kim, C. K.; Aguilar-Vega, M.; Paul, D. R. Dynamic Mechanical and Gas Transport Properties of Blends and Random Copolymers of Bisphenol-A Polycarbonate and Tetramethyl Bisphenol-A Polycarbonate. J. Polym. Sci., Part B: Polym. Phys. 1992, 30, 1131. (4) Norton, F. J. Gas Permeation through Lexan Polycarbonate Resin. J. Appl. Polym. Sci. 1963, 7, 1649. (5) McHattie, J. S.; Koros, W. J.; Paul, D. R. Effect of Isopropylidene Replacement on Gas Transport Properties of Polycarbonates. J. Polym. Sci., Part B: Polym. Phys. 1991, 29, 731. (6) Pinnau, I.; Hellums, M. W.; Koros, W. J. Gas Transport through Homogeneous and Asymmetric Polyestercarbonate Membranes. Polymer 1991, 32, 2612. (7) Ruvolo-Filho, A.; Murakami, M. M. Transport Properties of Water in Glassy Polycarbonate Films. Effects of the Processing and Thickness. J. Macromol. Sci., Phys. 1998, B37, 627. (8) Osswald, T. A.; Menges, G. Materials Science of Polymers for Engineers; Hanser: New York, 1995; Chapter 12. (9) Crank, J. The Mathematics of Diffusion; Oxford University Press: London, 1970; Chapter 4.
Ind. Eng. Chem. Res., Vol. 48, No. 19, 2009 (10) Pauly, S. In Polymer Handbook, 4th ed.; Brandrup, J., Immergut, E. H., Grulke, E. A., Eds.; Wiley: New York, 1999. (11) Schult, K. A.; Paul, D. R. Techniques for Measurement of Water Vapor Sorption and Permeation in Polymer Films. J. Appl. Polym. Sci. 1996, 61, 1865. (12) Standard Test Method for Determining Gas Permeability Characteristics of Plastic Film and Sheeting; American Society for the Testing of Materials: West Conshohocken, PA, 1998; ASTM D1434-82. Test Method for Gas Transmission Rate through Plastic Film and Sheeting; Japanese Industrial Standard: Tokyo, Japan, 1987; JIS K7126. (13) Daynes, H. A. The Process of Diffusion through a Rubber Membrane. Proc. R. Soc. London, Ser. A 1920, 97, 286. (14) Kerbow, D. L.; Sperati, C. A. In Polymer Handbook, 4th ed.; Brandrup, J., Immergut, E. H., Grulke, E. A., Eds.; Wiley: New York, 1999. (15) Perry, R. H., Green, D. W., Eds. Perry’s Chemical Engineers’ Handbook, 7th ed.; McGraw-Hill: New York, 1997; pp 2-49. (16) Stannett, V.; Haider, M.; Koros, W. J.; Hopfenberg, H. B. Sorption and Transport of Water Vapor in Glassy Poly(Acrylonitrile). Polym. Eng. Sci. 1980, 20, 300.
8965
(17) Meares, P. The Diffusion of Gases Through Polyvinyl Acetate. J. Am. Chem. Soc. 1954, 76, 3415. (18) Michaels, A. S.; Vieth, W. R.; Barrie, J. A. Solution of Gases in Polyethylene Terephthalate. J. Appl. Phys. 1963, 34, 1. (19) Michaels, A. S.; Vieth, W. R.; Barrie, J. A. Diffusion of Gases in Polyethylene Terephthalate. J. Appl. Phys. 1963, 34, 13. (20) Paul, D. R.; Koros, W. J. Effect of Partially Immobilizing Sorption on Permeability and the Diffusion Time Lag. J. Polym. Sci., Part B: Polym. Phys. 1976, 14, 675. (21) Massey, L. K., Ed. Permeability Properties of Plastics and Elastomers, 2nd ed.; Plastics Design Library: Norwich, NY, 2003.
ReceiVed for reView May 22, 2009 ReVised manuscript receiVed August 27, 2009 Accepted August 31, 2009 IE900842T
