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Vol. 16, No. 1
INDUSTRIAL AND ENGINEERING CHEMISTRY Geiger, H., and Scheel, K., “Handbuch der Physik”, Vol. 2, p. 160, Berlin, Julius Springer, 1926. Goske-Mulheim Ruhr, 2. Untersuch. Nahr. Genussm., 24, 245 (1912).
Hare, J. Inst. Brewing, 40, 92 (1934). Hartley, H., and Barrett, W. H., J. Chem. SOC.,99, 1072 (1911). Heard, L., J. Chem. Education, 7, 1910 (1930). Hennion, G. F., IND. ENQ.CHEM.,ANAL.ED.,9,479 (1937). Houben, J., “Methoden der organischen Chemie”, Vol. 1, p. 891, Leipzig, G. Thieme, 1925. Irving, H., Science Progress, 31, 654 (1937). Kalahne, A., BeT. deut. physik. Ges., 16, 81-92 (1914). Lipkin, M.R., and Kurtz, S. S., Jr., IND.ENQ. CHEM.,ANAL. ED., 13, 291 (1941). Ostwald, IT7., J . p7akt. Chem., 16, 396 (1877). Ostwald, W.. Luther, R., and Drucker, C., “Hand- und Hilfsbuch zu. Ausfiihrung physiko-chemischer Messungen”, pp. 234-7, Leipzig, Akademische Verlagsgesellschaft, 1931. Parker, H. C., and Parker, E. W., J. Phys. Chem., 29, 130 (1925).
(22) Perkin, W. H., J. Chem. SOC.,45, I, 443-5 (1884). (23) Reidel, R., 2. physik. Chem., 56, 245 (1906). (24) Reilly, J., and Rae, W. N., “Physical Chemicd Methods”, New York, D. Van Nostrand Co., 1939. (25) Robertson, G. R., IND.ENQ.CHEM.,ANAL.ED., 11, 464 (1939). (26) Shedlovsky, T., and Brown, A. S., J . Am. Chem. SOC.,56, 1066 (1934). (27) Sprengel, H., Pogg. Ann., 150,459 (1873). (28) Timmermans, J., and Martin, F., J . chim. phys., 23, 747 (1926). (29) Ward, A. L., Kurtz, S. S., Jr., and Fulweiler, W. H., in “Science
of Petroleum”, ed. by Dunstan, Nash, Brooks, and Tizard, Vol. 11, p. 1137, New York, Oxford University Press, 1938. (30) Washburn, E. W.. and Smith, E. R., Bur. Standards J . ReS C U T C ~ ,12, 305 (1934). (31) Westberg, J., Tek. Fdren. Finland Fdrh., 58, 314 (1938). (32) Wojciechowski, M., J. Research Nutl. Bur. Standards, 19, 347 (1937). (33) Yuster, S. T., and Reyerson, L. H., IND.ENG.CHEM.,ANAL ED., 8, 61 (1936).
Apparatus for Measuring the
Gas Pbrmeability
of
Film Materials of Low Permeability A. CORNWELL SHUMAN Central Laboratories, General Foods Corp., Hoboken,
Apparatus is desyibed for measuring the gas permeability of film materials having permeabilities as low as 0.001 cc. (at standard temperature and pressure) per 100 square inches per 24 hours. The low range for previously reported methods (water vapor permeability and balloon fabric permeability) is about 100 cc. per 100 square inches per 24 hours. The apparatus combines simplicity of design, simplicity of manipulation, and high sensitivity in a unit which can b e fabricated in most machine shops. The measurements are made under conditions of one atmosphere pressure differential.
THE
packaging of some foods, drugs, chemicals, etc., requires flexible films and paper coatings of very low permeability to fixed gases, particularly oxygen. The need for film having a permeability of not more than 0.25 cc. per 645 sq. cm. (100 square inches) per 24 hours has been pointed out by Elder (S). This figure is entirely out of the range usually measured for balloon fabrics (6), where permeabilities range from 100 cc. per 100 square inches per hour and up. The customary measurements of water vapor permeability (1, 9,4) of packaging films are also in a relatively high range-for example, a “good” water vapor barrier will have a permeability figure of 0.1 gram or 125 cc. per 100 square inches per 24 hours. The apparatus described in this paper is designed for measurements in the range 0.001 to 1000 cc. per 100 square inches per 24 hours, and primarily for the measurement of the fixed gases, although with some modification i t might be also used for water vapor permeabilities. The purpose of this publication is to make available to others the means which the author has developed for testing pachging films of very low permeability. The mechanism of gas transmission through solids has not been explained thoroughly. Oswin discusses this briefly (4). If the gas passed through small pores or openings in the solid, the situation would be a relatively simple one; the permeability of a film to various gases could be predicted from the known laws of gas flow through orifices. However, such is not the case. It is known that polyvinyl alcohol fdm is permeable to water vapor but not to oxygen, and Pliofilm is permeable to carbon dioxide but
N. J.
relatively impermeable to oxygen. Perhaps a process of solution of gas in the film on one side, diffusion through the film, and evaporation from the film on the other side takes place in some cases. . -4.
-1
ee”
r
Figure I. Diagram of Apparatus
ANALYTICAL EDITION
January 15, 1944
The meohanism of gas transmission through film determines to under a film be tested. Two factors are involved: the difference in total pressure on the two sides of the film and the difference in partial pressure of the gas being tested on the two sides of the film.If permeability is due to flowt b o u e h ofifices in the fib. the difference in total p r e m m is of primary importance and the difference in partial SOmeextentthe
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59
stopper with a glass tube ia inserted into each of these holes. The Space between E and F is then flushed out with gas to be measured by passing the gas in one opening and out the other. It has heen shown that the moisture of fixed gases markedly affeots their rate of transmission through some film materials (4).This factor may be controlled by conditioning the gas to he tested hefore it is passed through the apparatus.
on the other side The appsratus, shown in Figures 1 and 2, combines simplicity of design, simplicity of manipulation, and high sensitivity in a unit which can he fabricated in most machine shops. Essentially the method involves measuring change? in pressure inside a small evacuated space due to gas passing into that space through the film sample being tested. CONSTRUCTlON OF APPARANS The manometer. M , is held in d a c e hv the nut.
N,to which it
seal.' The ;ut is screw& up against the thin rubber wisher, W , and the entire joint is coated with shellac to ensure a vacuumtight md. Manometer M serves the dual purpose of recording the pressure change and of evacuating the space inside the apparatus. For evacuation, the entire apparatus is tipped 80 that the mercury runs over into the reservoir, R; then the apparatus is evacuated through tube B with stopcock S in the open position. After evacuation, S is closed and the apparatus is tipped again so that the mercury runs from R into M . I n case sir leaks through S during a test, the mercury in that arm of the manometer will be depressed, thus serving as an indicator of leakage at this point without spoiling the test, The center arm of M is made of capillary tubing of about 1.5-mm. internal diameter for the purpose of reducing the volume of gas space inside the apparatus and thereby increasing its sensitivity. The rest of the manometer may be made of tubing of any convenient size. The scale used in measuring the height of the mercury is not shown. A piece of millimeter cross-section Daoer held behind the manometer serves very well for the purpose. The upper part of the hole in the center of of the disk, K, is covered with a s m Il 1 metal disk, D, which has four small holes about 1 mm. in diameter for passing gas !nto the manometer svstem. t O D surface of the small disk IS 1s flush wrth the tor, top system. The top &face of the large l a k e disk, K,presenting a continuous smooth sursurface
.
measurement. behvarite (anhydrous m a m e s i b perchlorate)
Courtw "Modem Packagine"
Figure
P.
Apparatus
I n mounting multi-ply film having one or more plies of a porous material-far example a film laminated between two pieces of paper-it is practicall; impossible to obtain a vacuum-tight seal using stopcock gnase between the paper surface of the sample and K . I n mounting such films, use has been made of a plasticiaed tar as shown in Figure 3. An edge coating extending about 0.6 em. (0.25 inch) from the circumference is applied to both sides of the disk of film to he tested by dipping iqthe plasticized tar (Fiymre 3). The tar-coated disk of matenal I S mounted directly on K without the use of stopcock grease, When cap G is screwed into place, the plasticized tar is spread out by the compressive forces and makes a vmuum seal between F and K . Carbowax 1500 is a suitable plasticizer for the tar when used a t the level of about 5 per cent. CALCULATIONS
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size. I n general, ftny drying agent whioh will reduce the water vapor pressure inside the apparatus to less than 0.5 mm. of mercury will serve. A small plug of cotton over the top of the manometer tube will prevent particles of the drying agent from falling into themanometer.
Gas trammission measurements are conveniently recorded
BS
cubic centimeters of gas a t standard conditions of temperature and pressure transmitted per 100 square inohes of surface per 24 hours. Such a figure is given by the following expression:
MOUNTING SAMPLES
The film sample, F, is mounted on the apparatus as shown in the diagram. A piece of filter paper, P,is placed between the film and K to provide a porous medium for gas leaking through the filmto travel to the openings in D aud pass through the dlying agent into M . The part of F overlapping P and in direct contact with K is sealed to the disk by means of a thin film of heavy stopcock grease having a low vapor pressure. This area of the film is held in place by the rubber gasket, C: Disk E has two small holes dlametncally opposite one another and near the inside circumference of the metal ring, G. When measuring air transmission, these openings are left open so that the space between E and F is filled with air. When i t is desired to measure the gas transmission for any other gas, a small rubber
where P = the absolute pressure inside the apparatus as measured on the manometer in millimeters V = total volume of the inside of the apparatus in cubic centimeters T = tem erature in degrees absolute A = ,urP,ce mea of test sample (area of filter paper P in square inches) hours = time of test The value of V may be calculated with sufficient accuracy (within 5 per cent) from the h o w n dimensions of the apparatus, making a correction for the volume of the drying agent used.
60
Vol. 16, No. 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
It is equal to the sum of the voldme of the holes in D, the volume of the circular opening containing the drying agent minus the volume of the drying agent, and the volume of the capillary bore of the center stem of the manometer down to the mercury level in this stem. The volume of the pore space in P amounts to about 0.05 cc. and the volume change due to a mercury level change of 20 mm. in the center stem of the manometer amounts to about 0.04 cc. These errors in volume are neglected.
urements with this apparatus are accurate to within 15 per cent. The temperature, time, and manometer reading may be determined within a few per cent. The principal factors affecting the accuracy are the volume of the apparatus and the area of the test sample. It is estimated that these are easily determined in the manner described within 10 per cent accuracy. The precision or reproducibility of measurements on duplicate samples.of the same material is generally about 10 per cent. EXAMPLES
The following examples will serve to typify the data which may be obtained with this apparatus:
Figure 3
For the apparatus diagrammed in Figure 1, the value of V is about 2 cc., and the value of A is 23 sq. em. (5 square inches). A t a temperature of 30" C., 2' = 303" absolute. Substituting these values in the above equation, we get the following working equation: P/hours X 1.14 = cc. of gas a t standard conditions of temperature and pressure transmitted/lOO square inches/24 hours SENSITIVITY, ACCURACY, AND PRECISION
The actual time required for a test with this apparatus varies with the permeability of the film being tested and the limits within which it is desired to determine the gas transmission rate. For example, it might be desired to know whether a film sample has a transmission rate greater or less than 0.25 cc. per 100 square inches per 24 hours. Using the above equation and constants for the apparatus herein described, this transmission rate will produce a pressure change in the manometer of 0.5 mm. in about 2.5 hours: 0.5-mm. change is easily read on the manometer; therefore, the failure of the manometer to change 0.5 mm. in 2.5 hours' time is sufficient evidence that the gas transmission rate is less than 0.25 cc. per 100 square inches per 24 hours. Should the time of the test be extended five times as long, to 12.5 houfs, the failure of the manometer to change 0.5 mm. is sufficient evidence that the gas transmission rate is less than 0.05 (0.25/5) cc. per 100 square inches per 24 hours. Further extending the time of the test thus further reduces the figure for the maximum transmission rate of the sample.
*
'
With films of high permeability, the mercury in the manometer will drop several millimeters in an hour's time. With such films, therefore, the test need not be extended more than a few hours. Occasionally film samples contain material, such as plasticizers, which have appreciable vapor pressures. Such materials will vaporize into the apparatus and depress the mercury in the manometer. This may be mistaken for gas transmission through the film,but may be distinguished from gas transmission by the fact that the pressure will reach a constant value, the rate of change of pressure decreasing as this value is approached. I t is therefore necessary to make readings of the manometer at intervala during the test and calculate the rate of manometer change for each interval. If the rate of change is constant, it is due to gas transmission through the film, but if it is a decreasing rate of change, the change is due a t least partly to vaporization of some volatile material from the film. The required accuracy is not very great for measurements of permeability of film packaging materials where values might vary a millionfold among various materials and a thousandfold among various samples of the same material. It is estimated that meas-
1. Air transmission for an impermeable sample (a sample of laminated glassine a t 60 to 80 per cent relative humidity). I n a period of 36 hours, no change was observed in the manometer. Assuming that a minimum change of 0.5 mm. can be detected on the manometer, the rate of change of the manometer was not greater than 0.5/36 or 0.0139 mm. per hour. The gas transmission was, therefore, not greater than 0.0139 X 1.14, or 0.016 cc. per 100 square inches per 24 hours. 2. Air transmission for an impermeable sample containing a volatile plasticizer (a polyvinyl alcohol coating a t low humidity). The manometer changes during the indicated time intervals were as follows: Time Interval
Manometer Change Mm. 2 1.5 2 3.5 0.5 Total 9.5
Hours 0-1
1-2 2-4 4-10 10-40
Rate of Change Mm./hour 2.0 1.6 1.0 0.58 0.017
It is evident from the above data that a material having a vapor pressure of about 9.5 mm. is evaporating from the sample and that equilibrium is nearly established after 10 hours. The rate of change of the manometer during the last 30 hours of the test was 0.017 mm. per hour. Therefore, the gas transmission rate for the sample was not more than 0.017 X 1.14 or 0.019 cc. per 100 square inches per 24 hours. 3. Air transmission for a permeable sample (a thin coating of polyvinyl alcohol). The manometer changes during the indicated time intervals were as follows: Time Interval
Nanometer Change Mm. 1.5 1.5 3.0 9.5 11.5 27.0
Hours
0-1 1-2 2-4 4-10 10-18 Total 18
Rate of Change Mm./hour
1.5 1.5 1.5 1.58 1.45 Av. 1.5
The constancy of the rate of change figures indicates that this sample is permeable to air. The rate of gas transmission is 1.5 X 1.14, or 1.71 cc. per 100 square inches per 24 hours. LITERATURE CITED
(1) Anon., ModernPaokaging, 16,78-82,100(1942). (2) Carson, F. T.,Nstl. Bur. Standards, Misc. Publ. M127 (1937). (3) Elder, L. W.,Modern Packaging, 16,69-71 (1943). (4) Oswin, C. R.,J. 800. Chem. Id.,62,45-8 (1943). (5) Sager, T.P., J . Research Natl. Bur. Standards, 25, 309-13 (1940).
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Bemuse of critical shortages, the AMERICANCHEMICAL
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