ARTICLE pubs.acs.org/IECR
Hydrogen Chloride and Ammonia Permeation Resistance of Tetrafluoroethylene-Perfluoroalkoxy Copolymers Sung In Moon* and C. W. Extrand Entegris, Inc., 101 Peavey Road, Chaska, Minnesota 55318, United States ABSTRACT: Hydrogen chloride and ammonia permeation resistance were measured through two grades of tetrafluoroethyleneperfluoroalkoxy copolymers using standard manometric procedures. Hydrogen chloride showed a lower permeation coefficient than ammonia due to the combined effect of different molecular size and interaction with perfluoroalkoxy (PFA). The permeability coefficients of these two gases were comparable to those of oxygen and nitrogen. The two grades of PFA differed only in molecular weight or chain length. The molecular weight had little affect on permeation when the samples were cooled slowly. Slowly cooled samples always showed better permeation resistance than those cooled quickly. The influence of sample preparation was quite pronounced, demonstrating that process can be as important as polymer grade in determining the ultimate permeation resistance of various PFA grades.
’ INTRODUCTION Fluoropolymers provide a unique combination of toughness, purity, and chemical inertness. Among the melt-processable fluoropolymers, tetrafluoroethylene-perfluoroalkoxy copolymers, often abbreviated simply as PFA, have the highest service temperature and therefore have been used broadly in semiconductor fabrication.1,2 While much is known about the purity, mechanical, and thermal properties of polytetrafluoroethylene (PTFE) and its copolymers,3-5 less information is available regarding their permeation characteristics. Some permeation testing has been performed on PFA,6-13 but these studies have not addressed two of the most widely used process chemicals, hydrochloric acid and ammonium hydroxide. Since the active ingredient in both of these aqueous chemicals is a dissolved gas, there have been concerns about unwanted permeation potentially causing cross-contamination or corrosion. Therefore, this paper explores the hydrogen chloride and ammonia permeation resistance of PFA. ’ ANALYSIS The crystalline mass fraction (xc) of the fluoropolymers was calculated as14 xc ¼ ΔH=Hf
ð1Þ
where ΔH is the measured melting enthalpy and Hf is the melting enthalpy of the polymer in a 100% crystalline state. Vapor permeates through homogeneous materials by first dissolving and then diffusing.15 The downstream pressure (pl) of the permeant can be converted to an equivalent volume of gas (V) at standard temperature and pressure (STP), V ¼ ðpl =po ÞðTo =TÞVs
ð2Þ
where T is the measurement temperature, Vs is the volume of the downstream side of the permeation apparatus, To is standard temperature (0 °C = 273 K), and po is standard pressure (1 atm = 76 cm Hg). The volume (V) of gas that permeates through a film r 2011 American Chemical Society
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)15,16 V ¼ PAph t=B
ð3Þ
’ EXPERIMENTAL SECTION Two grades of DuPont Teflon PFA were selected for this study: 440HP and 450HP. These grades are widely used for many semiconductor fluid-handling components. The two grades are chemically identical, except that 450HP has higher molecular weight than 440HP. Thin sheet specimens were compression-molded from pellets using a PHI Bench Design, Hydraulic Compression Press, at a temperature of 343 °C (650 °F). A specified amount of resin was weighed, poured into the center of a brass plaque mold, and then sandwiched between thin aluminum sheets. This sandwich was placed on the preheated lower platen of the press and brought into contact with the top platen and held for 5 min, after which a load of 1700 kg was applied for 2 min then increased to 3400 kg for 2 min. At this point, two different methods of cooling were employed that produced quite different polymer morphology and permeation resistance for a given grade. These methods were the following: Ice water quench: The sandwich was quickly cooled by dropping it in ice water for approximately 1 min, then placing it between two chilled aluminum blocks for 10-15 min. Press cooled: The sandwich was left in press (under 3400 kg load). The heat was turned off, and the sandwich was allowed to cool slowly overnight with the platens intact (approximately 16-18 h). Received: October 8, 2010 Accepted: January 6, 2011 Revised: December 14, 2010 Published: January 31, 2011 2905
dx.doi.org/10.1021/ie1020443 | Ind. Eng. Chem. Res. 2011, 50, 2905–2909
Industrial & Engineering Chemistry Research Melt flow rates (MFRs) were measured using a Kayness Galaxy I Melt Flow Indexer. The die radius was 0.1048 cm, and its length was 0.80 cm. Specimens were cut into small pieces, loaded into to a 372 °C melt indexer, and allowed to preheat for 6 min; then, a 5000 g load was applied. Results were reported as deci grams/minute (dg/min). Triplicate measurements were made for each material. Melt temperatures (Tm) and melt enthalpies (ΔH) of the various 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 350 °C (662 °F), cooled from 350 to 50 °C, and then heated again from 50 to 350 °C at a rate of 10 °C/min (18 °F/min). Triplicate DSC scans were performed for each material. Using the software resident in the DSC7, the resulting DSC scans were analyzed. The density of the compression-molded PFA films (Fp) was measured with an apparatus for density determination (Denver Instrument, Density Determination Kit) and a microbalance (Mettler Toledo, AG245), using isopropanol (IPA, Brenntag, FIPA = 786 kg/m3). Using the density apparatus, films were massed both above the isopropanol (mair) and immersed (mim) in it. Measurements were performed in triplicate, and Fp values were calculated as Fp = (mair/mim)FIPA. The permeant gases were industrial grade anhydrous hydrogen chloride and ammonia (Toll Co., Minneapolis, MN). The gas permeation apparatus consisted of a sample holder inside of a temperature-controlled chamber, a series of valves, an upstream ballast tank, a pressure transducer (5000 Torr MKS Baratron Type 628B) for the upstream gas, and a 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. Permeation was measured according to standard manometric procedures17,18 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 gas permeation apparatus. Unless noted otherwise, specimens had a thickness of B ≈ 0.25-0.30 mm (10-12 mil). The apparatus was pumped down to approximately 3 10-3 cm Hg and held for a predetermined time to remove volatile constituents from the apparatus as well as from the specimen. The predetermined time was estimated using previously measured permeation properties, assuming the sample was initially saturated with atmospheric air and humidity. This was usually 3 days for a 0.25 mm (10 mil) thick sample. After that period, the apparatus was leak tested. If the leak rate was sufficiently low, the upstream was charged with HCl or ammonia gas at pressures of 4-30 cm Hg for HCl and 40-60 cm Hg for ammonia. (The vapor pressures of HCl and ammonia in hydrochloric acid and ammonium hydroxide solutions are given in the Appendix.) After the pressure and temperature were allowed to equilibrate, 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.) All measurements were made at T = 25 °C (77 °F).
’ RESULTS AND DISCUSSION Melt Flow Rates of PFA. Flow properties are summarized in Table 1. Melt flow rates were performed on the resin and on
ARTICLE
Table 1. Melt Flow Rate (MFR) for PFA Resins and Films MFR supplier
grade
resin (dg/min)
film (dg/min)
DuPont
Teflon 440HP
16.0 ( 0.10
16.0 ( 0.60
Teflon 450HP
1.74 ( 0.03
1.86 ( 0.16
Table 2. Overall Averages of Thermal Properties for the PFA Films As a Function of Grade and Cooling Methoda grade 440HP 450HP
cooling method
Tm (°C)
ΔH (J/g)
xc
ice quench
307 ( 1
23.9 ( 0.6
0.36 ( 0.02
press cool
315 ( 1
32.7 ( 1.6
0.42 ( 0.02
ice quench
308 ( 1
20.6 ( 1.0
0.31 ( 0.02
press cool
314 ( 1
26.4 ( 1.3
0.40 ( 0.02
a
Each entry represents the overall average and standard deviation of triplicate measurements.
Table 3. Overall Averages of Densities and Fractional Free Volume (FFV) for the PFA Filmsa grade 440HP 450HP a
cooling method
density (g/cm3)
FFV
ice quench
2.13 ( 0.01
0.11
press cool
2.17 ( 0.01
0.095
ice quench
2.12 ( 0.01
0.12
press cool
2.16 ( 0.02
0.10
Each entry represents the overall average and standard deviation of triplicate measurements.
0.25 mm (10 mil) compression molded films prior to testing to ensure that processing had not degraded them. The increase of MFR in converting the resin pellets to film was 40%) are possible, but the evaporation rate is so high that these super concentrated versions require special storage and handling. Bulk industrial grade is therefore 30-34%, optimized for effective transport and limited loss by HCl vapors.25,26 Figure A1a shows the partial pressures of hydrogen chloride gas (pHCl) and water vapor (pw) associated with room temperature hydrochloric acid as a function of HCl concentration (c). For the more dilute solutions, c < 20%, the vapor pressure from hydrochloric acid is negligible and that vapor is mostly water with a trace of hydrogen chloride gas. For the highest concentrations, the “vapor” is almost all hydrogen chloride gas. Figure A1b shows the partial pressures of ammonia gas (pNH3) and water vapor (pw) associated with ammonium hydroxide at 80 °F (26.7 °C) as a function of NH3 concentration (c).26 For the dilute solutions, c < 15%, the both vapor pressures from water and ammonia contribute significantly. For the concentrations higher than 25%, the “vapor” from water becomes less than 5%. 2908
dx.doi.org/10.1021/ie1020443 |Ind. Eng. Chem. Res. 2011, 50, 2905–2909
Industrial & Engineering Chemistry Research
’ AUTHOR INFORMATION Corresponding Author
*Tel.: 952-556-1074. E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank Entegris management for supporting this work and allowing publication. Also, thanks to B. Arriola, D. Brettingen, M. Caulfield, C. Duston, L. Goedecke, J. Goodman, T. King, J. Leys, R. Lindblom, L. Monson, S. Moroney, J. Pillion, B. Reichow, and S. Tison for their suggestions on the technical content and text. ’ REFERENCES (1) Khaladkar, P. R. Fluoropolymers In Chemical Handling Applications. In Modern Fluoropolymers; Scheirs, J., Ed.; Wiley: New York, 1997; pp 311-326. (2) Extrand, C. W. The Use of Fluoropolymers to Protect Semiconductor Materials. J. Fluor. Chem. 2003, 122, 121–124. (3) Goodman, J. B.; Andrews, S. J. Fluoride Contamination from Fluoropolymers in Semiconductor Manufacture. Solid State Technol. 1990, 33 (7), 65–68. (4) Goodman, J. B.; Van Sickle, P. M. Extracting Ionic Contaminants from PFA Polymeric Materials. Microcontamination 1991, 9 (11), 21–26. (5) Kerbow, D. L.; Sperati, C. A. Physical Constants of Fluoropolymers. In Polymer Handbook, 4th ed.; Brandrup, J., Immergut, E. H., Grulke, E. A., Eds.; Wiley: New York, 1999; pp V/31-V/48. (6) Giacobbe, F. W. Oxygen Permeability of Teflon-PFA Tubing. J. Appl. Polym. Sci. 1990, 39 (5), 1121–1132. (7) Giacobbe, F. W. Oxygen Permeation Through Teflon-PFA Tubing into Flowing Helium. J. Appl. Polym. Sci. 1991, 42 (8), 2361– 2364. (8) Giacobbe, F. W. Oxygen Permeation through Teflon-PFA Tubing into Flowing Argon. J. Appl. Polym. Sci. 1992, 466, 1113–1116. (9) Pauly, S. Permeation and Diffusion Data. In Polymer Handbook, 4th ed.; Brandrup, J., Immergut, E. H., Grulke, E. A., Eds.; Wiley: New York, 1999; pp VI/543-VI/569. (10) Monson, L.; Moon, S. I.; Extrand, C. W. Gas Permeation Resistance of Various Grades of Perfluoroalkoxy-Polytetrafluoroethylene Copolymers. J. Appl. Polym. Sci. 2009, 111, 141. (11) Prabhakar, R. S.; De Angelis, M. G.; Sarti, G. C.; Freeman, B. D.; Coughlin, M. C. Gas and Vapor Sorption, Permeation, and Diffusion in Poly(tetrafluoroethylene-co-perfluoromethyl vinyl ether). Macromolecules 2005, 38, 7043–7055. (12) De Angelis, M. G.; Sarti, G. C.; Sanguineti, A.; Maccone, P. Permeation, Diffusion, and Sorption of Dimethyl Ether in Fluoroelastomers. J. Polym. Sci. Part B: Polym. Phys. 2004, 42, 1987–2006. (13) Aminabhavi, T. M.; Naidu, B. V. K. Experimental, Simulation Studies on Molecular Transport of Substituted Monocyclic Aromatic Liquids into Fluoropolymer Sheet Membranes: Liquid StructureDiffusion, -Sorption, -Permeation Relationships. J. Appl. Polym. Sci. 2004, 92, 991–996. (14) Runt, J. P. Crystallinity Determination. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Mark, H. F., Kroschwitz, J. I., Eds.; Wiley: New York, 1986; Vol. 4, pp 482-519. (15) Osswald, T. A.; Menges, G. Materials Science of Polymers for Engineers; Hanser: New York, 1995; Chapter 12. (16) Crank, J. The Mathematics of Diffusion; Oxford University Press: London, 1970; Chapter 4. (17) (a) 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. (b) Test Method for Gas Transmission Rate through Plastic Film and Sheeting; Japanese Industrial Standard: Tokyo, Japan, 1987, JIS K7126. (18) Daynes, H. A. The Process of Diffusion through a Rubber Membrane. Proc. R. Soc. London, Ser. A 1920, 97, 286.
ARTICLE
(19) Pozzoli, M.; Vita, G.; Arcella, V. In Modern Fluoropolymers, Scheirs, J., Ed.; Wiley: New York, 1997. (20) Wang, X.-Y.; Lee, K. M.; Lu, Y.; Stone, M. T.; Sanchez, I. C.; Freeman, B. D. Cavity Size Distribution in High Free Volume Glassy Polymers by Molecular Simulation. Polymer 2004, 45, 3907–3912. (21) van Krevelen, D. W. Properties of Polymers: Their Correlation with Chemical Structure; Their Numerical Estimation and Prediction from Additive Group Contributions, 3rd ed.; Elsevier: Amsterdam, 1990. (22) Shishatskii, A. M.; Yampol’skii, Y. P.; Peinemann, K.-V. Effects of Film Thickness on Density and Gas Permeation Parameters of Glassy Polymers. J. Membr. Sci. 1996, 112, 275–285. (23) Perry, R. H., Green, D. W., Eds. Perry’s Chemical Engineers’ Handbook, 7th ed.; McGraw-Hill: New York, 1997; pp 2-139. (24) Churakov, S. V.; Gottschalk, M. Perturbation Theory Based Equation of State for Polar Molecular Fluids: I. Pure Fluids. Geochim. Cosmochim. Acta 2003, 67, 2397. (25) http://en.wikipedia.org/wiki/Hydrochloric_acid (Accessed December 10, 2010). (26) Perry, R. H., Green, D. W., Eds. Perry’s Chemical Engineers’ Handbook, 7th ed.; McGraw-Hill: New York, 1997; pp 2-49.
2909
dx.doi.org/10.1021/ie1020443 |Ind. Eng. Chem. Res. 2011, 50, 2905–2909
