Ind. Eng. Chem. Res. 2001, 40, 2183-2187
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RESEARCH NOTES Apparatus for Studying the Effect of Mechanical Deformation on the Permeation of Organics through Polymeric Films Juan Hinestroza, Daniel De Kee,*,† and Peter N. Pintauro*,‡ Department of Chemical Engineering, Tulane University, New Orleans, Louisiana 70118
A new experimental apparatus for studying the effect of mechanical deformation on the permeation of organic compounds through polymeric membranes is described. The apparatus consists of a custom-built biaxial membrane stretching device and an ASTM F739 permeation cell. Infrared spectroscopy was used as the analytical technique to measure, in real time, the downstream concentration of organic compounds that passed through the membrane. The equipment was used to monitor the permeation of single organic liquids and organic mixtures through uniaxially and biaxially elongated membranes composed of polyisoprene rubber and carbon black. The steady-state flux of acetone through a 40% uniaxially deformed membrane increased by 100%, as compared to that for a nonelongated film, whereas the acetone flux decreased by 25% when the membrane was deformed biaxially 40 × 40%. Variations in membrane thickness with stretching were small and could not account for the observed change in acetone flux with uniaxial elongation. For an equivolume feed mixture of acetone, benzene, and hexane, the steady-state permeation of all three compounds increased when the membrane was elongated 40 × 40%, as compared to the nonstretched case. Some deterioration of the membrane occurred during permeation of pure benzene with biaxial stretching (40 × 40%), as evidenced by a stepjump increase in the benzene flux. No deterioration was observed with nonelongated and uniaxially stretched films. Introduction In recent years, because of increasing awareness of the risks associated with exposure to toxic and harmful chemicals in the environment, there has been increased activity to develop and test new types of protective clothing. Clothing items are generally manufactured with a polymeric coating that may be composed of poly(tetrafluoroethylene) or rubber reinforced with fillers.1 In addition to providing a permeation barrier to challenging chemical agents, the polymer coating must also maintain its structural integrity against mechanical stresses and excessive dimensional change.1,2 Because gloves and boots as well as clothing areas at the elbows and knees will be elongated during use, their ability to block organic agent permeation in a stretched state could be compromised. Similarly, the permeation properties of geomembranes and packaging films may be altered when such barriers are under stress. It is, therefore, important to understand how the transport of organic chemicals through a polymer film is affected by mechanical deformation. It is generally recognized that stresses (both internal and external) play a major role in regulating the diffusion and sorption of organic compounds in polymeric barrier films, where the simultaneous action of * Corresponding author. † E-mail:
[email protected]. Voice: (504) 8655620. Fax: (504) 865-6744. ‡ E-mail:
[email protected]. Voice: (504) 865-5872. Fax: (504) 865-6744.
permeation and deformation can lead to pronounced polymer swelling, a deterioration in the mechanical properties of the polymer, and an increase in the permeation rate of a chemical penetrant.1,3-9 There have been some reports in the literature dealing with the effect of stress on organic species transport through polymer films, but these studies were limited to theoretical analyses4-9 and experimental investigations that focused primarily on either uniaxially stretched films or single-component permeation.1,3 Despite this prior work, the effect of externally applied uniaxial or biaxial stresses on the resistance of protective polymeric materials to chemical agent permeation is still not well understood, due in part to the difficulty in simulating realistically the simultaneous processes of mechanical deformation and permeation. The present paper describes a new experimental apparatus for the continuous monitoring of organic species permeation through a mechanically deformed membrane. An infrared spectroscopic method is employed to measure downstream organic species concentrations, so that permeation rates in real time can be determined quickly and accurately. The IR analytical technique also allowed us to measure individual component transport rates for upstream feeds of organic mixtures. To demonstrate the use of the apparatus and to show that the equipment can detect the complex behavior of organic species transport through a membrane under deformation, permeation data are presented for acetone and an acetone/hexane/benzene
10.1021/ie000879r CCC: $20.00 © 2001 American Chemical Society Published on Web 04/06/2001
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Figure 1. Experimental apparatus for deforming a membrane sample and monitoring organic species permeation by FTIR.
mixture through a uniaxially and biaxially stretched polyisoprene-rubber/carbon-black membrane. Experimental Section A block diagram of the apparatus used to deform a membrane sample and to measure permeation through the elongated film is shown in Figure 1. The major components of the equipment are an ASTM F-739 permeation cell, a custom-built uniaxial/biaxial membrane stretching device, and a Mattson Genesis FTIR connected to a personal computer. A spherical 51 mm diameter two-chambered ASTM F-739 test cell10,11 was used to expose one side of the membrane to a challenging single-component organic liquid or liquid organic mixture. The challenge chamber had a volume of approximately 0.045 L and was equipped with a stoppered nozzle for addition of liquids. An additional nozzle was installed to allow for the possible use of flowing upstream vapors as challenging chemicals. The downstream collection chamber had a volume of approximately 0.1 L and was equipped with inlet and outlet ports. The concentration of permeating organic species was found using a nitrogen sweep gas that carried the organic(s) to a Mattson Genesis FTIR equipped with a multiple-pass 2.4 optical path length absorption cell (Infrared Analysis, Anaheim, CA) and a zinc-selenide window. The apparatus to execute uniaxial or biaxial deformation (shown in Figure 2) was constructed of aluminum and has four mobile heads, four fixed heads, and a frame. The movement of the heads was controlled using a screw mechanism that allowed for a maximum elongation of 200% in two perpendicular directions. The degree of membrane deformation was monitored using a ruler attached to the sides of the frame. The lower chamber of the permeation cell was fixed to the center of the aluminum frame, while the upper chamber was attached to a clamping device that pressed the feed and permeation compartments to the test membrane. The entire stretching apparatus is 24 in. long and 15 in. wide, which allowed it to fit inside a conventional laboratory oven. Permeation tests were performed with 0.648 mm thick membranes and a liquid feed of either acetone or an equivolume mixture of hexane, acetone, and benzene (27.3 wt % hexane, 33.0 wt % acetone, and 39.6 wt % benzene). All chemicals were of reagent grade. The membrane (provided by the Canadian Defense Research
Figure 2. Apparatus for elongating polymeric membranes: 1, mobile heads; 2, fixed heads; 3, screw system; 4, inlet and outlet ports of the permeation cell; 5, bottom hemisphere of the permeation cell; 6, membrane test sample.
Establishment Suffield, Alberta, Canada) was composed of cis-1,4-polyisoprene with 1% carbon black.1,12 The membrane density was found to be 1.14 g/cm3, with an ultimate tensile strength of 0.17 N/m2 and a glass transition temperature of 211.5 K. For tests under uniaxial deformation, the membrane was clamped between the two mobile heads of the stretching apparatus and elongated in one direction. For biaxial stretching, the membrane was initially elongated in one direction, as in the uniaxial case, and then it was clamped to the remaining mobile heads and stretched in a direction perpendicular to the initial elongation. After the membrane was deformed, the upper and lower chambers of the permeation cell were aligned and clamped into place using two Teflon gaskets and the membrane itself as a seal. After the entire apparatus was placed into a constant-temperature oven (Fisher Scientific model 655F) maintained at 298 ( 1 K, a liquid organic mixture was introduced into the upper (challenging) chamber of the permeation cell. Contact of the membrane with the organic chemical designated zero time for a permeation experiment. Periodically, over the next 4-6 h, the downstream nitrogen sweep gas was analyzed for concentration of organic(s) by FTIR. To ensure meaningful comparisons of permeation data, a new membrane was used in each experiment. Results and Discussion Results for acetone permeation through the rubber membrane at various degrees of biaxial and uniaxial deformation are presented in Figure 3. Measured downstream concentrations were multiplied by the flow rate of the carrier gas (2200 cm3/min) and divided by the area of the membrane exposed to the organic chemical (20.3 cm2) in order to obtain a flux (µg/cm2‚s) versus time profile. All experiments were repeated three times, and average downstream concentrations from multiple runs were used to compute fluxes. The reproducibility of the apparatus and procedure was excellent, with a maximum variation of 7% in measured concentrations from repeated experiments. When the membrane was stretched uniaxially, the steady-state acetone flux increased with increasing elongation (e.g., the acetone flux increased by about 100% when the membrane was stretched uniaxially by 40%). Qualitatively similar
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Figure 3. Flux versus time data for liquid acetone permeation through a polyisoprene/carbon-black membrane at 298 K. Elongation: 9, 0%; 1, 20% uniaxial; [ 40% uniaxial; 0, 20 × 20% biaxial; 4, 20 × 40% biaxial; O, 40 × 40% biaxial. Table 1. Acetone Breakthrough Times and Steady-State Fluxes for Uniaxially and Biaxially Elongated Polyisoprene/Carbon-Black Membranes at 298 K elongation (%)
breakthrough time (min)
steady-state permeation rate (µg/cm2‚s)
0
Nonelongated Film 28.2
2.3
20 40
Uniaxial Elongation 21.8 19.4
3.0 4.1
20 × 20 20 × 40 40 × 40
Biaxial Elongation 16.3 13.6 8.1
3.5 2.3 1.8
results have been reported previously1,12 for uniaxially elongated membranes composed of cis-1,4-polyisoprene (with no carbon loading), bromobutyl rubber, nitrile rubber, poly(vinyl chloride), and high-density polyethylene. In the biaxial stretching tests, a different permeation behavior was found, with steady-state fluxes that increased with small/moderate elongations (20 × 20% and 20 × 40%) and decreased with a 40 × 40% stretch. Attainment of steady-state flux conditions decreased with increasing film elongation (from ca. 120 min for the nonelongated film case to
