Semimicro Gas Permeability Apparatus for Sheet Material W. R. R. Park1, Case Institute of Technology, Cleveland 6, Ohio LITERATURE
search for an apparatus
A for rapid yet accurate determina-
tion of the gas permeability constants of small pieces of plastic or paint film showed no device that met requirements. The devices described by Barrer (d), Cartwright (4), Shuman ( 5 ) , Amerongen ( I ) , and others use a vacuum system in which temperature, pressure, volume, and time are recorded to obtain the gas permeation rate. These methods, while accurate and fairly simple, require considerable time pci determination. The instrument developed by Brubaker and Kammermeyer (3) seemed much better suited to requirements. except for the size of sheet sample required. The films available are seldom greater than about 4 sq. cm., while Brubaker’s instrument requires 300 sq. cm. As a result, a semimicro film holder was designed and built on the principle of Brubaker’s. This device measures the volume of permeating gas under conditions of constant pressure and temperature. Leaks on the high preesure side of the membrane cause no error in permeability determinations, as the pressure remains constant. Leaks on the low pressure side are unlikely, as it is held essentially a t atmospheric pressure. As the apparatus described is accurate to about *5$!&, the fact that it employs a small piece of film may be an advantage. Areas of unequal thickness and pinholes are less likely to be present in a small piece of film; thus results may 1
be more representative than those obtained on larger film pieces. CONSTRUCTION
The film holder shown in Figure 1 is custom made of aluminum and is approximately 3 inches long and, a t its widest dimension, 2 inches in diameter. The left or upstream side is equipped with a needle valve to flush air from the system; the right or downstream side is filled with paraffin except for a central hole 0.5 mm. in diameter. This ensures a total downstream volume as low as possible and improves the accuracy of subsequent measurements of downstream gas volume. Two rubber O-rings 0.5 inch in inside diameter fit into recesses on the faces of both halves. These serve a dual purpose. First, they hold snugly in place, the dished disks of 200-mesh stainless steel wire gauze, which act as gas diffusers and film supports. Secondly, the O-rings hold the film in a gas-tight seal when the unit is assembled. When compressed, the 0rings become flush with the surface of the stainless wire disks. If any gas diffuses through the O-ring seal instead of through the film, a gas vent, F, open to the space between halves, causes it to disperse into the atmosphere rather than around the O-rings. Accurate positioning of the two halves is ensured by two eccentrically mounted retaining pins in the left half, which fit into matching holes in the right half when the retaining ring is tightened. They also eliminate torsional tearing forces on the film during tightening of the retaining ring.
OPERATION OF PERMEABILITY APPARATUS
I n operation, a piece of film, which can be as small as 3/4 inch in diameter, is placed horizontally on one half of the film holder. The other half is positioned; then the retaining ring is tightened as hard as possible by hand pressure. The film holder, D (Figure 2), is then mounted between stopcocks B and C, with C and D held flush inside the Tygon tubing. A gas-proof seal is ensured by the use of a small quantity of silicone grease on the Tygon tubing. The stopcock positions should be A-1, B-4, and C-4. The gas cylinder is opened slowly, using a sensitive needle valve, and left open until the level of mercury in J has become stationary. The gas needle valve is now shut; if there are leaks in the upstream side, the level in J will fall rapidly. I n the absence of gas leaks, E is opened until most of the gas has streamed into the atmosphere. E is closed and the cycle is repeated three times to ensure that all entrapped air is removed from the upstream part of the apparatus. Finally E is shut and the gas pressure is adjusted to the desired value and left on. The upstream pressure will come to equilibrium in about 5 minutes and remain constant as long as the temperature does not change. Now F is removed and a small drop of methyl isobutyl ketone containing 1% methyl violet is inserted into the bore. The length of this liquid slug in the bore is adjusted to 1.5 to 2.0 mm. Shorter slugs tend to be too rapidly consumed in wetting the capillary walls, while longer slugs have much greater
Present address, The Dow Chemical
Co., Midland, Mich. E
Stopcock Positions
p
@ ‘ G 2 B 3
I F
Figure 1. Cross-sectional view of film holder for permeability apparatus
A. B. C. D. E. F.
Rubber O-rings Dished disks of 200-mesh stainless steel wire gauze Cavity filled with paraffin except for small central hole Tightening ring Needle valve Gasvent
Figure 2.
Assembled permeability apparatus
A , B. Stopcocks, 2-mm. bore C . Stopcock, 0.5-mm. bore
D. Film holder
E . Needle valve F. Uniform bore capillary tubing, 0.59-mm. diameter G. Millimeter scale H . Tygon tubing J. Mercury reservoir VOL. 29, NO. 12, DECEMBER 1957
1897
frictional resistance to movement along the bore. Mercury slugs have too much frictional resistance to movement along the tube and require a powerful vibrator in connection with F. The liquid slug, on the other hand, moves smoothly and uniformly without auxiliary vibration. When the slug has been adjusted to the desired size, usually by absorbing the excess on tissue, F is remounted and rotated carefully into position until F and C are flush. The seal is again made gas-proof by small quantities of silicone grease. C is turned rapidly clockwise to C-2 and the progress of the slug of liquid along the capillary bore is noted every 60 seconds. These readings are continued until about 10 consecutive readings give a linear plot of millimeters moved us. time-i.e., until a state of equilibrium is attained. Thus the time required for a determination depends upon the pressure differential, the gas being used, temperature, thickness, and permeability of film. When readings are concluded, C is turned to C-3, gas is turned off, pressure is released a t A-2, and D is removed. A new piece of film is positioned in D, C is turned to C-1 and A is turned to A-1, and the upstream side is flushed as before. Now C is turned to C-4 and the liquid slug is drawn back to its starting position by applying gentle suction to the open end of C. C is turned clockwise to C-2 and readings are taken as before. Repeat determinations on the same piece of film can readily be made a t the end of a run by turning C to C-4, returning slug to starting position, and returning C to C-2. The equilibrium permeation through the film is not disturbed and readings may be taken immediately. Similarly, repeat determinations on one piece of film a t different partial pressures may be run by turning C to C-4, adjusting pressure, returning slug, and moving C to (2-2. LIMITATIONS OF APPARATUS
Two design features inevitably affect the accuracy of this device. First, permeability values are directly proportional to partial pressure differences across the film. The initial partial pressure difference is known; but as gas, say helium, diffuses through the film and into the space between the film and the liquid slug, the partial pressure difference across the film decreases. This decrease is kept constant by taking readings between fixed limits, 0 to 20 or 0 to 10 em., along the capillary, but it is probably impossible to define the error accurately. At the same time, the air that is present initially between the liquid slug and the film also tends to diffuse into the upstream side and decrease the downstream volume. This effect also cannot be accurately defined. Finally, the slug of liquid that moves along the capillary has a finite frictional resistance to movement, so 1898
ANALYTICAL CHEMISTRY
I
I
I
5
10
15
I
mm./min.
Figure 3. Relation of movement of 1.5- to 2.0-mm. liquid slug in capillary tubing to initial pressure of helium
1.5-mil. polyethylene film in holder that when it is moving smoothly along the tube there is a slight difference in pressure on the two sides of the slug. This increment of pressure is kept constant by keeping the slug size constant. Although none of these effects may be conveniently corrected for individually, they may be corrected for in total. Figure 3 shows the rate of movement of a 1.5 to 2.0-mm. liquid slug from 0 to 20 cm. under different initial pressures of helium when a 1.5-mil film of polyethylene is mounted in the film holder. The fact that the plot is linear indicates that at pressures up to 225 em. of mercury no loss of film area being permeated is caused by increased compression of the film against the stainless steel wire gauze. Therefore, the design of the film holder is fundamentally sound. Secondly, when the line is extrapolated to zero it intersects the y axis a t 22 cm. of mercury pressure. Ideally, if the aforementioned three effects were not operative, the line should intersect the origin. Thus the correction to be applied, in the case of permeation of helium gas through polyethylene film, is -22 em. of mercury pressure. On a percentage error basis this correction results in only a 10% difference in permeability constants when pressures greater than 200 cm. of mercury are used but becomes much more significant a t lower upstream pressures. Another unavoidable drawback to the use of this device is its high temperature sensitivity. As very small volumes of gas are being measured, slight temperature changes affect readings disproportionately. Where possible, temperatures should be controlled to within +O.O5O C. during a run in order to obtain good reproducibility. Where temperature fluctuations exceed these limits, it is advisable to obtain temperature readings to the nearest 0.01' C. every minute during a run. If the temperature increases or decreases uniformly during a run, a temperature correction factor may be applied to the volume
measurement. This is obtained empirically by observing the movement of a slug of liquid in the capillary tube when the temperature changes by 0.1 O C., C is at C-2, gas is off, A is a t A-2, and only air is present on both sides of the film. This correction, in millimeters moved per 0.1" C., may be added or subtracted, as appropriate, from the scale readings on G. Finally, the device cannot be used ta determine the diffusion constant of various gases in film materials, as the induction period between the time pressure is applied and permeation starta cannot be accurately found. However, the apparatus has proved convenient and accurate for relatively rapid determinations (10 to 30 minutes) of the gaseous permeability of film materials of thicknesses up t o 10 mils. CALCULATIONS
The permeability constant, P , is recorded in the same units as those proposed by Brubaker and Kammermeyer, where P 273 P =-xvxx -A1x - 1tx - A pd- e 760 T where p = barometric pressure, mm. of mercury A p = pressure differential of gaa across film, cm. of Hg C = pressure correction (see Figure 3) V = total volume of gas transmitted, cc. T = absolute temperature, "K. A = surface area of test sample, sq. em. d = thickness of test sample, em. t = time, seconds P = cc. of gas a t standard conditions of temperature and pressure that pass through a film 1 em. thick, per second, per square centimeter, per centimeter of mercury partial pressure difference across the film The volume, V , of transmitted gas may be calculated from the length of travel of the liquid slug in the uniform, bore capillary over the test time.
EXAMPLES
The reproducibility of permeability constants that can be obtained on dif-
ferent samples of the same material is demonstrated below for a polyethylene film.
Sample 1 Sample 2 7.50 750 Atmospheric pressure, mm. Hg .- Cm./min. moved by slug 0.827 0.847 Volume of gas permeating, cc./min. 0.00233 0.00239 Temperature, C. 24.85 f 0.04 24.74 =!= 0.05 Area of film, sq. cm. 1.60 1.60 Time, seconds 60 60 Thickness of film, cm.0 0.00432 0.00450 Initial helium pressure, cm. Hg 53.8 54.0 p - C, em. Hg 31.8 32.0 Permeability constants, cc.-cm./sec.-sq. cm.-cm. Hg 3.11 X 10-e 3.03 X a Average of six readings on American Instrument Co. Magne Gauge. O
ACKNOWLEDGMENT
Thanks are due to Herman Braun for having built and helped in the design of the film holder. LITERATURE CITED
Amerongen, G. J. van, J. Polymer Sci. 5 , 307-32 (1950).
Barrer, R. N., “Diffusion 3iffusion in and through Solids,” Macmillan, New
York, 1941. (3) Brubaker, D. W.,, Kammermeyer, K., ANAL. CHEM.25. 2 5 , 424-6 (1953). (4) Cartwright, L. C., Ibid., 19, 393 f19471. (5) ShLman,. A. C., IND. ENG. CHEX., h A L . ED.16, 58 (1944).
Fraction Cutter for Gas Chromatography Allan Weinsteinl, Radiological Warfare Division, U.
S. Army,
Dugway Proving Ground, Dugway, Utah
A
vapor phase or gas chromatography is a powerful tool for separation and identification of complex mixtures of unknown composition, other analytical procedures must often be used as adjuncts for the complete elucidation of such mixtures. For instance, the high intensity gamma radiolysis of ethyl alcohol produced mixtures that could be analyzed conveniently only through mass spectrometric examination of the separate fractions resulting from gas chromatographic separation. It was thus necessary to devise some form of apparatus for collection and removal of the fractions as they emerged from the sensing element of the gas chromatograph. Of the fraction cutters described (1-3, 5-7), two are designed for this purpose (2, 5 ) . They are a multiple series of fixed U-tubes, through vhich the sample vapors are directed by stopcocks. I n both, the samples must be transferred within a short time to storage vessels, or the array of U-tubes must be long enough to accommodate the number of fractions expected The device shown in Figure 1 has several advantages over these cutters. LTHOUGH
Figure
1.
Fraction cutter
A . Top view B. Side view, tilted
It is a compact unit, in which all four collecting traps (Figure 2) can be accommodated in a single Dewar flask. This eliminates the need for a long series of traps, with danger of breakage and inconvenience of operation. As many fractions can be collected as there are collecting tubes, without altering or enlarging the unit. While one tube is used to collect a sample, the others may be preflushed with carrier or any other gas. This is especially important when oxygensensitive compounds are being collected; Present address, Department of Fuel Technology, The Pennsylvania State University, University Park, Pa.
Figure 2.
Collecting tube
condensation of oxygen is prevented in the tubes prior to their use with a liquid nitrogen bath. The samples, after collection, may be stored directly in the tubes, and later transferred and the carrier gas removed, through the ground-glass joint, or through the simple apparatus, A , shown in dotted lines in Figure 2. A may be attached conveniently anywhere on a n independent vacuum line, or directly to a pump via the right-hand side arm. The spectrometric or other vessel is attached to the ground joint of A . After suitable freezing and evacuation, the sample may be transferred into the new vessel by immersing the vessel in a freezing bath and subsequently warming the collection tube. The collecting tubes may be converted into sample vessels for direct use with the spectrometer by blowing a vacuum stopcock at B, Figure 2 . As an alternative, ordinary spectrometric sample flasks may be used after blowing on an additional stopcock and bending to a U shape. Such flasks, however. would add much undesirable bulk to the unit. The time interval between emergence of a fraction from the sensing element and its entrance into the collecting tube is the same for all four positions. This is convenient for closely emerging fractions. This time interval may be determined by a number of simple methods, such as measurement of the volume between the sensing element and joint A of the fraction cutter, and the flow rate of the gas. I n other forms of cutters, this interval is different for each unit. K i t h chromatographs having provision for introduction of gas or vapor samples via a U-tube, collecting tubes may serve also as sample tubes. The chief value of such a n arrangement, aside from saving of materials, is the ease with which collected fractions may be reintroduced on the same or a different column. The unit is simple to operate. K i t h the stopcocks in positions shown in VOL. 29,
NO. 12,
DECEMBER 1957
1899
