V O L U M E 1 9 , NO. 6
396 Actually, because of uncontrollable variables, the probable error in gas permeability determinations is usually at least as large as 1 5 t o *IO%, so that for all determinations a t , say, 25" * 5' C'., 0 to 1007, relative humidity, with P = 760 * 20 mm., and manometer readings up to 50 or 60 mm., the calculation may be simplified t o
ments on hand, it is probably bimpler to increase the length of the test period to several days, or even weeks, if necessary. In this case, even the most minute leak in the system would become very significant, but with all ground-glass joints properly fitted and lubricated, the wax seal around the test specimen carefully made, and the test specimen free from any holes, several tests have been run for periods of 4 to 6 weeks without any evidence of leaks.
sq. meter per 24 hours per atmosphere For a typical instrument of the type described, A = 70 sq. cm. and V = 0.58 cc.; therefore Po = 2.5 ( p z - p J / H . Thus, the senjitivity, as previously calculated for the other types of permeability testers discussed, is 0.052 N.T.P. cc. per sq. meter per 24 hours per atmosphere. This is about seven times the sensitivity of Shuman's instrument, six times that of Todd's instrument, and one third greater than that of the modified Todd apparatus described.
.
For testing materials of very low permeability it would be possible t o increase the sensitivity of this type of instrument thirteenfold by using dibutyl phthalate in the manometer instead of mercury, but because the simplicity and low cost of the instrument make it feasihle t o have a considerable number of initru-
iCKNOWLEDGMENT
The author wishes to express his appreciation to the Fisher Scientific Company for its cooperation in executing his design into a number of instruments of the type described. LITERATURE CITED
(1) Davis, Donald W.,Paper Trade J . , 123, No. 9, 33-40 (1946). ( 2 ) Doty, P. M., Aiken, W. H., and ,Mark, Hermann, ISD. h t 7 CHEY., ANAL.ED.,16, 686-90 (1944). (3) Elder, L. W., Modern Packaging, 16, No. 11, 69-72 (1943). ANAL,ED.,16, 58-60 (1944). (4) Shuman, A. C., IND.EKG.CHEM., ( 5 ) Smith, F. R., and Kleiber, Max, Ibid., 16, 586-7 (1944). (6) Todd, H. R., P a p e r Trade J . , 118, No. 10, 32-5 (1944).
.
Determination of Gas Permeability of Saran Films T. W. SIRGE, Saran Development Laborcrtory, ?'ha Dow Chemical C o m p ~ nMidland, ~, Mich. A modified manometric apparatus for measuring gas pernieabilities of films having extremely low transmission characteristics is described. Experimental results of equilibrium transmission for Saran films measured by a variable pressure technique are reported. Oter-all results for Saran film gas transmissions are lower than those generally encountered in the literature for any organic film material. The self-consistency of results obserbed with the present apparatus, the relationships obsened, and the extended examination of several gases under sarioils pressure differentials appear to justify the present exposition.
T
H E extremely low water vapor transmission characteristic of Saran films was recognized early, as evidenced by widespread application t o packaging of metal parts ( 2 , 6). Detailed properties were given and extended application of the films to gas impervious packaging was suggested, following preliminary data on gas transmission (8). The present research was undertaken to obtain extended information on the permeability of Saran (trade mark, Dow Chemical Company, for polymers and copolymers of vinylidene chloride) films to the gases of the atmosphere, as well as to certain lower aliphatic hydrocarbon gases. A technique based on manometric methods was employed, and transmission rates were obtained for helium, hydrogen, oxygen, air, nitrogen, carbon dioxide, methane, ethane, propane, ethylene, and acetylene at 25 ' C. and several pressure differentials. . A P P ~ R A T UA S N D PROCEDURE
Several methods of measuring gas transmission through thin films are known. These methods involve a t least one of the following measurements: refractive index, thermal conductivity, and pressure or volume. The Sational Burkau of Standards general y employs the measurement of refractive index to obtain transmission, while the Shakespeare fabric permeameter (Cambridge Instrument Company) measures the thermal conductivity of gases. Neither the refractometer nor the permeameter has yet been extensively applied to the measurement of extremely low gas transmission as found among certain plastic films because of the time-consuming calibrations which are required. Techniques involving volume (11 ) and pressure (6, 10) have recently been
described. The two latter methods appear to be the most satisfactory for experimental determinations, in view of the difEculties mentioned previously, and it would seem that any lack of time sensitivity which may be attributed t o the methods is probably compensated by a closer approach t o true equilibrium transmission. The apparatus which was selected for the present work is an adaptation of the manometric instrument described briefly bg Elder (6) and in detail by Shuman (10). Only the essential differences will be pointed out here. 1. I n thc apparatus originally described, a drying agcrit (IlFhydrite with indicating Ilrierite) ability cell on the measuring side of t apparatus omits the dehydrating ag nating a volume correction for the d quant,ityof drying agent outside the whichare attached to t h e two holw provided for the entering :i11t1 emerging gas (Figure 1). 2. The original appitratus a-: described maintains an atmosphere of gas other than air above the sample by passing,@* through a glass tube inserted by means of a rubber st'opper into one of the holes mentioned above and allowing it to leave by thc other. The present apparatus maintains an atmosphere,of tht. test gas (including air) by a "static" method. The entire apparatus is transferred t o a tall bell jar on a polished metal plate, vacuum-sealed, fast,ened, evacuated, and then filled lr-ith the ga.\ t o be measured. The entire evacuated syst,em is enclosed in a wooden box having a safety glass observation window. I n addition, gas cylinders, a sulfuric acid gas drying bottle, a vacuuni pump, and a 1-meter mercury vacuum gage (not shown) complete the equipment necessary for the determinations (Figures 2 and 3). 3. The total volume on the manometer side of the ~1.11W W P reduced approximately fourfold (2 c r . to 0.5 cc.).
J U N E 1947
397 For crample: = nK?'a, standard condition&(0" C . , 760 mm.
of mercury)
PfVf =.nRT,, test conditions
(2) (3) (4)
01
v . = v , x gPtx -
To TI
(5)
Figure 1. Assembly of Gas Transmission Apparatus
Figure 2.
Apparatus
4. Minor modifications of the originel apparatus includc the mounting of a small thermometer to one leg of the msnomcter and tho support oi the manometer base by ccmentakion int,o a cork stoppcr t o avoid accidental breakage. The above modifications were necessitated by the fact that the permeability of Saran films is so low that initially even the extremely sensitive original apparatus gave little indication of success. The original values submitted by Elder (6) far air and carbon dioxide permeability ai polyvinylidene chloride film 1 mil (0,001 inch 3 0.0254 mm.) thick are air, 0; carbon dioxide, 0. TREATMENT OF DATA AND CALCULATIONS
Record of the following da,ta,is taken throughout the coume of left arm imm.): center (mm.): difference
EBS transmission tests:
im. ot mercuryl; aate. asion rates are calculated simply by applicati 1s Isw
t o the data. obtained.
Figure 3.
Gas Transmission Apparatus and Connections
V O L U M E 19, NO. 6
398 Table I.
Dry Gas Transmission through Saran Film at 23" C. Film Thickness, 0.0005 Inch (0.01270N m . ) Absolute Permeability Constant
Transmission
Gas Helium Hydrogen Oxygen Acetylene Air Propane Ethylene Ethane Methane Nitrogen Carbon dioxide
Formula He H-H
o=o
Liters/sq. m./B4 hre. 355 mm. 735 mm.
35 mm. 0.0015
0.1819 0.0494 0.0020
0.0058
0,000055
n c=cH
CH8CH*'CH8 H~C=CHI CH3CHD CHI N N -
o=c=o
....
...... ...... ...... ...... ...... ...... ...... ......
.... .... . . I .
.... .... .... ....
0.3500 0.0792 0.0049 0.0024 0.0022 0.0018 0,0017 0.0015 0,0013 0.0010 0.0007
For use in making conversions, the following extensions of Equation 7 are made:
V 2(liters per 100 square inches per 24 hours)
V1 (liters per square meter per 24 hours)
V 2
=
(8)
1000
V , X 15.5
=
(9)
TRANSMISSION RATES AND PERMEABILITY CONSTANTS
Table I gives tabulated data for transmission and absolute permeability constants for Saran film, Type M, and several gases at different pressures. Conversion factors for liters per square meter per 24 hours to absolute permeability constant units are as follows: a t 35 mm., P = rate in liters per square meter per 24 hours X
'
0.307 x
10-7
(10)
Film Thickness,O.O028 Inch (0.07112hlm.) Absolute PermeTransmission ability Constant P X IOs, Cc. gas ( S T P ) - m m . p X lo6, Liters/sq. m./B4 thick/sq. cm./sec./ Cc. gas ( S T P ) - m m . thick/sq. c m . / s e c . / c m .Hg at 26' C . hrs. em. HQ at 26' C. 35 nim. 355 mm. 735 mm. 760 * 30 mm. 760 * 30 mm 0,0445 0.0 71 0.0915 0.0 975 0.O419 0.0426 O.Oi452 0.042 O.Ot78 0.0007 0.06742 0 . 0620
.... .... .... ....
.... .... .... ....
....
.... 0.0005
.... ....
.... .... ....
.... ....
.... ....
....
0.0004 0,0003
....
..... 0.06530
.....
.....
.....
.....
nique involved. The following limitations of the technique undoubtedly affect the absolute value of gas transmission rate and permeability constants, and certain characteristics of polymeric films (such as specific gas solubility) undoubtedly affect relative transmission and permeabilities: 1. The present technique requires evacuation of the film sample for rate determinations. This certainly results in the loss of some plasticizer, and perhaps all by the time a series of determinations is made. Ordinarily 1 to 5 minutes of evacuation take place prior to placing the transmission cell under the bell jar, followed by 15 to 30 minutes of further evacuation in the bell jar vacuum chamber. 2. The present technique is variable pressure-Le., the pressure inside the manometer rises during the course of transmission-and this certainly must affect the rate of transmission. However, the total pressure change is small compared to the external pressure or "driving" force. For example, the total measuring range is approximately a maximum of 50 to 60 mm. of mercury, while the external pressure (a large excess of test gas in the vacuum chamber) is 735, 355, or 35 mm. The variable
a t 355 mm., P = rate in liters per square meter per 24 hours X
t 3.07
x
10-7
(11)
at 735 mm., P = rate in liters per square met,er per 24 hours X 8.57
x
10-7
(12)
t = thickness of film in mm.
where
The units selected for expressing the absolute permeability constant are those suggested by Barrer (1) and employed by Doty, Aiken, and Mark (4), and may be obtained from the transmission data as follows:
A Pt Transmission (S.T.P.) = P X 1 where
T P
(13)
= total transmission in cc. of gas a t standard tem=
A = Ap = t = 1 =
perature and pressure (0' C. and 760 mm. of mercury) absolute permeability constant as in Table I film sample area in square centimeters pressure differential in cm. of mercury time interval corresponding to T in seconds thickness of film sample in mm.
Converting Equation 13 gives
p
T X 1 --
At A p
(14)
DISCUSSION O F RESULTS
I n the discussion which follows, it should be kept in mind that the conclusions are valid only in view of the apparatus and tech-
D Molecular Figure k.
0
in
A.
From IndW of Rafraction
Relation of Transmission Rate to Molecular Diameter
JUNE 1947
399
I PRESSURE VI TIME SARAN FILM, TYPE M,OOO5 735MM Hg, 25°C:
I
50
F. 4 0
e 30
t
0 (Y
-
20
3
s 2 IO 0 0
50 IO0 I50 Time in Hours HYDROCARBON GAS TRANSMISSION (T.
Figure 5 .
hz
200
I
Dry G a s Transmission t h r o u g h S a r a n Film
Pressure Differential in mm. Hg.
Figure 6 . Relation of AP a n d Transmission pressure method is admittedly not ideal, yet it simulates actual conditions in film applications inasmuch as there, too, the inside pressure creates a resistance to transmission by virtue of the lower pressure differential or driving force.
An examination of Table I brings to light some interesting facts on the nature of transmission through Saran films. Relation of Permeability Constant (or Transmission Rate) to Molecular Diameter of Permeating Gas. I t has bt,en pointed out several times in the literature that there is little or no relation between the molecular weight (1) and permeability, or between molecular diameter ( 3 ) and permeability. This observation appears to be valid, as judged by data submitted on the various natural and synthetic film materials. However, the data in Table I indicate a dependence of transmission rate upon molecular diameter in the case of helium, hydrogen, oxygen, nitrogen, and carbon dioxide. This dependence is shown in Figure 4, where log molecular diameter of the gases is plotted versus log transmission, indicating that I
demonstrated. Some correlation in this direction was indicated by hIuller (9) for polystyrene, polyvinyl chloride, and cellulose triacetate films with reference to the noble gases. One reason for this behavior in Saran film may be the already well-recognized inertness or solvent resistance of polymers and copolymers of vinylidene chloride. Among the gases discussed are included monatomic (helium), diatomic (hydrogen, oxygen, nitrogen), and triatomic (carbon dioxide) gases and therefore, excluding solubility or reactivity, it is reasonable t o suppose that transmission rate and permeability constant should decrease with increasing molecular diameter and increasing molecular complexity. The transmission characteristic under these circumstances appears to be purely a case of effusion through molecular “canals” in the polymer chain structure of polyvinylidene chloride. As a corollary, it would also appear that “inert” gases or vapors of m2lecular diameter (by refractive index) greater than about 3.5 A. would not permeate ‘Saran films. Relation of Permeability Constant to Solubility of Permeating Gas. h comparison of the transmission rates of the lower aliphatic hydrocarbon gases given in Figure 5 shows in general an increase in rate for an increase in molecular diameter or complexity (as shown in Table I) where time in hours is plotted os. absolute manometer pressure in millimeters of mercury. (TransAP mission T is proportional to the rat,e, or T = k -, where t is At time.) This phenomenon would appear to be due to the increasing solubility effect of an increasing carbon chain length on the “insoluble” polyvinylidene chloride chain. On the other hand, for an equal number of carbon atoms, it is observed that transmission increases with increasing unsaturation, w seen from Figure 5, where the order for transmission is acetylene > ethylene > ethane. From these observations, the following generalizations appear to be valid for gas transmission through Saran film: 1. For inert., relatively insoluble gases, transmission decreases with increase of molecular diameter-Le., helium > hydrogen > oxygen > air > nit,rogen > carbon dioxide. 2. For a given homologous hydrocarbon gas series, permeability increases wit,h increasing molecular weight, (also effective molecular diameter)-Le., propane > ethane > methane. 3. For the same number of carbon atoms, permeability increases with an increasing degree of unsaturation (smaller effective molecular diameter)-Le., acetylene > ethylene > ethane. Relation between Transmission and Permeability Constant and Gas Pressure DBerential. In general, for the transmission of inert gases the relation T = k A p from Equation 13 is assumed. The data is Table I show that this relationship holds reasonably well for Saran film. Figure 6 gives the result when A p (initial) is plotted os. T for helium, hydrogen, and oxygen. The constancy of the permeability constant, P , is seen in Table I to be reasonably good, considering the extreme range examined.
200 Time in Hours
Figure 7.
Considering the uncertainties in both absolute gas transmission values and molecular diameters (7), the dependence is reasonably
Pressure us. Time for G a s Transmission through Saran Type M , 0.0005 inch thick, 35 mm., 25’ C. Air nitrogen, and carbon dioxide transmissions are too low at 35
md.to be premented.
V O L U M E 19, NO. 6
50
0
Figure 8.
Pressure
100 Tm0 in Hours
I50
solubility of carbon dioxide in the mercury of the manometer. If the absorption is extremely high, the pressure change would indicate only a low rate of transmission. This would undoubtedly result in a transmission lowering, yet it hardly seems likely that the time us. p plot for carbon dioxide in Figure 9 would remain constant as indicated by the curve. Further experimental work which was completed after the preparation of this paper but before publication shows that the iolubility of carbon dioxide in mercury a t 25’ C. is too low (0.037%) to account for the observed low permeability. Moreover, data publiqhed by van Smerongen (12) show the Same tendency-namely, that the permeability of 1 ubber and rubberlike materials to carbon dioxide decieases as the solubility of c w h n dioxide in the mate-
200
z’s. Time
for Gas Transmission through Saran
Type M, 0.005 inch thick, 355 mrn., 25’ C.
Relation between Transmission and Permeability Constant and Film Thickness. I n general, for the transmission of inert k gases T = -, where t is film thickness. The data in Table I
e
t
show that this relationship holds reasonably well for Saran film. Relation between Pressure and Time during Transmission. Figures 7, 8, and 9 show t,he relative rate constancies or the “equilibrium” transmission for the various permanent gases measured over long periods of time on the modified variable pressure apparatus. I t is interesting to observe the position of air with respect to oxygen and nitrogen. As might be expected for permeability which is not significantly influenced by solvency or reactivity, the permeability or transmission rate of air corresponds to the concentration of partial vapor pressure of the constituents in air-namely, 80% nitrogen and 207& oxygen.
6
:. *$ 5 0 40
3
I
3
CONCLUSION
A modified manometric apparatus for measuring gas permeabilities of films having extremely low transmission characteristics is described. Experimental results of equilibrium transmission for Saran films measured by a variable pressure technique are reported and discussed. The over-all results for Saran film gas transmissions reported here are lower than those generally encountered in the liternture for Saran or any other organic film material. The self-consistency of results observed with the present apparatus, the relationships observed, and the extended examination of sevei al gases under various pressure differentials appear to justify the present exposition. Among the more important questions to be resolved by further work may be included the relation of deplasticization to absolute gas permeability, and the unexpected position of carbon dioxide in the permeability “spectrum” of Saran film. Edwards and Strohm ( 5 ) report a permeability of 0.038 gram per 100 square inches per 24 hours for carbon dioxide towards 0.002 inch thick Saran film a t 760 mm. of mercury, using a constant pressure technique. The present paper ivould give approximately 0.00004 gram per 100 square inches per 24 hours for carbon dioxide, the film thickness being slightly greater (0.0028 inch) and the pressure being 735 mm. of mercury. The difference is about lo3 and therefore represents a serious discrepancy. It is known that the carbon dioxide permeability of rubber and after treated rubber is extremely high compared with oxygen, nitrogen, and air, but it appears that the known high solubility of carbon dioxide in rubber may well account for this phenomenon. Whether t h e highly crystalline vinylidene chloride polymers and copolymers exhibit such selective solubility is not known to the author. The results appear to indicate otherwise. One factor in the experimental technique may contribute to the low permeability of carbon dioxide observed in this work-the
0
50
Figure 9.
100 Time in Hours
200
I50
Gas Transmission through Saran
Type M, 0.005 inch thick, 735 mm., 25’
6
lo C.
rial decreases, and also as vinyl and vinylidene-type “rubber” structures are approached. ACKYOWLEDGMENT
It is a pleasure to acknowledge the assistance of J. E. Ritzer of the Dow Glass Laboratory in modifying the permeability cell, and of H. L. Schaefer and R. D. Lowry for their suggestions and criticiqms during the course of this -ork. LITERATURE CITED
(1) Barrer, R. M., “Diffusion in and thiough Solida”, New York, Macmillan Co., 1941. (2) Dixon, W. R., and Schaefer, H. L., Modern Packaging, 16, 72 (1943). (3) Doty, P. M., and diken, W. H., J . Chem. Phys., 14, 244 (1946). (4) Doty, P. M.,A4iken,R. H., and Mark, H., I n d . Eng. Chem., 16, 686 (1944). (5) Edwards, J. D., and Strohm, D. B., -1fodern Packaging, 19, 157 (1945). (6) Elder, L. W., Ibid., 16, 69 (1943). (7) Glasstone, S. S., “Physical Chemistry”, 2nd ed., New York, D. Tan Nostrand Co., 1946. (8) MacRae, F. J., and Schaefer, H. L., Modern Packaging, 17, 95 (1943). (9) Muller, F AH., Kolloid-Z., 100, 355 (1942). (10) Shuman, A . C., IND. ESG. CHEM.,ANAL.ED., 16, 58 (1944). (11) Todd, H. R., Modern Packaging, 18, 124 (1944). (12) van Amerongen, G. J., J . Applied Phvs., 17, 972-85 (1946). PRESENTED before the Division of Paint, Varnish, and Plastics Chemistry a t the 110th Meeting of the AMERICANCHEMICAL SOCIETY, Chicago, Ill.
