June 1952
1465
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
pumpkin, concentrated soup, etc.) has a substantial effect on the lethality manifested at the can center, especially at the higher processing temperatures. Maintaining pressure on the cans after shutting off the steam prevents the can contents in cans which contain a t least standard legal fills from boiling and mixing in the can. When boiling is permitted in the can by release of the external pressure, the cooling of the contents is relatively rapid with consequent reduction in sterilizing value effective at the can center. Data are presented giving the cooling times and water consumption of various methods of cooling cans outside of retorts for cream-style corn, brine-packed peas, and apple juice, which are representative of canned foods of various consistencies. ACKNOWLEDGMENT
Sincere appreciation is expressed to the Mayville Canning Co., Clyman Canning Co., Oconomowoc Canning Co., and Illinois Canning Co., whose assistance and cooperation made the cooling tests involving canal and continuous pressure coolers possible.
LITERATURE CITED
(1) Ball, C. O., Canner, 64, No. 5, 27 (Jan. 22, 1927). (2) Ball, C. O., Univ. of Calif., Public Health, 1, No. 2 , 15-245
(1928). (3) Benjamin, H. A., and Jackson, J. M., Canner, 88, No. 12, Pt. 2, 75 (Feb. 25, 1939). (4) Bigelow, W. D., Bohart, G. S., Richardson, A. C., and Ball, C. O., Natl. Canners Assoc. Research Lab., Bull. 16L (1920). (5) Bitting, A. W., “Appertizing, or the Art of Canning,” San Francisco, The Trade Pressroom, 1937. (6) Ecklund, 0. F., Food Technol., 3, No. 7, 231-33 (1949). (7) Ecklund, 0. F., andBenjamin,H. A,, FoodInds., 14, No. 3, 40-42 (March 1942). (8) Jackson, J. M., Proc. I n s t . Food Technol., 39 (1940). (9) Jackson, J. M., and Olson, F. C. W., Food Research, 5 , No. 4,409 (1940). (10) Schultz, 0. T., and Olson, F. C. W., Ibid., 2, No. 5, 399 (1940). (11) Seaman, M. L., American Can Co., General Research Laboratory, private communication. (12) Somers, I . I., Food Inds., 16, No. 2, 80-83 (February 1944). RECEIVEDfor review April 16, 1951. ACCEPTEDFebruary 4, 1952. Presented a s part of the Symposium on Chemical Engineering Aspects of Food Technology before the Divisions of Industrial a n d Engineering Chemistry and Agricultural a n d Food Chemistry at t h e 119th Meeting of t h e AMERICAN CHEMICAL SOCIETY, Boston, Mass., April 1951.
Separation of Gases by Means of Permeable Membranes
Engfinnedering Bocess development
PERMEABILITY OF PLASTIC MEMBRANES TO GASES
I
DAVID WILLIAM BRUBAKER
AND KARL KAMMERMEYER STATE UNIVERSITY OF I O W A , I O W A CITY, I O W A
U
K T I L the advent of commercial production of plastic films little attention had been paid to the gas permeation behavior of these materials. The extensive developments in the use of such films for packaging purposes introduced problems of gas and vapor permeation, and investigations were initiated during the past 12 years. While some work, especially on natural rubber films, was published by Graham ( I S ) as early as 1866, the majority of references are of very recent origin. The possibility of utilizing plastic films in the separation of gas mixtures is probably one of the most recent applications. While the present paper does not touch upon this subject specifically, the findings which are presented contribute t o the basic knowledge required for the study of gas separation by means of such membranes. It was the primary object of this research to obtain data on the rate of gas permeation through a number of selected plastic membranes a t different temperatures. I n addition, ‘some observations were made on the effect of varying amounts of plasticizer in the membrane upon the rate of gas permeation. The findings on the effect of varying amounts of plasticizer are presented to a limited extent only, as this work will be published separately in the near future. THEORY AND LITERATURE SURVEY
The process of diffusion in polymers may be conveniently divided into two parts: first, the flow of gases and vapors which are not readily condensable a t or near standard conditions (hydrogen, carbon dioxide, sulfur dioxide, and ammonia) in plastic organic compounds; and second, the flow of water and organic liquids as well as their vapors through membranes.
This division can be made because the process of gas diffusion obeys Fick’s law dC/&
=
Kd2C/dX2
(1)
fairly rigorously, while vapor diffusion does not usually follow this law. Simril and Hershberger (21, 22) found that the permeability rates of gases are about 10,000 times smaller than the permeation rates of vapors. The present work will only be concerned with the first type-that is, the flow of gases. As early as 1866 Graham ( I S ) studied the flow of gases through rubber. He regarded this permeation process as a sequence of solution, diffusion, and re-evaporation of the diffusing gas, a viewpoint which is held today. The fact that Fick’s law of diffusion applies within a plastic membrane was first pointed out by Wroblewski (28) in 1879 and has been verified since then by many investigators ( 2 , S, 5 ) . The pressure, area of membrane, thickness of membrane, and the temperature are the possible variables in the kinetics of permeation. If Henry’s law,
s
= kp
(2)
can be applied to the solubility, S , of the gas in the polymer substance, and Fick’s law, Equation 1, is taken to represent t h e diffusion process within the material, the permeation rate for steady state flow is directly proportional to the pressure difference and to the area of the membrane, and is inversely proportional t o the thickness of the membrane. The rate of transfer of a gas across a film of unit thickness per unit area per unit pressure difference is known as the permeability constant and characterizes the effectiveness of the film as a harrier. The permeability coefficient, P , follows from
INDUSTRIAL AND ENGINEERING CHEMISTRY
1466
P = PAt(pi - p z ) / d
(3)
In steady state diffusion the velocity of diffusion for a given constituent of a gas mixture is proportional to the difference in pressure of that constituent between the ingoing and outgoing surfaces of the membrane. Barrer ( 3 ) has shown that this behavior holds for pressures which are not too high (less than about 10 atmospheres for carbon dioxide). Also, in steady state flow the velocity is inversely proportional to the thickness of the membrane.
Vol. 44, No. 6
elastomers is attracting increasing attention. Some aspects of this problem for monomeric solvents have been considered by Horiuti ( 1 4 ) and 'others (2-4) and theoretical treatments of particular solvents, such as water, have been given by Eley ( 1 1 ) . Gee ( l a ) and Barrer (4)give a semiempirical treatment of gaselastomer equilibrium. EFFECTOF MOLECUL.AR STRUCTURE.Sager (18)considered that permeability was governed by the chemical nature of the film. The films vihich he used included cryst,alline, fibrous, and amorphous substances, and the degree of polymerization varied over a wide range. Hon-ever, Sager was not able to observe any simple connect,ion betn-een permeability and cryst'alline structure, nor the degree of polymerization, or molecular weight (18). Reitinger ( 1 7 ) and others ( 3 ) have recently made additional attempts to find a relation bet,ween permeability and the microstructure of polymers. Their conclusions xvere all of a qualitative nature as it !vas not possible to obtain quantitative results. G.4s SEPARATIOS.Since these organic films have the marked property of showing a much higher permeability toward some gases than toward others, they should be quite effective as a means of gas separation. This has been demonstrakd by \Teller and St,einer (26, 27) in their investigation of films for use in the separation of air into nitrogen and oxygen and also for the separation of helium from natural gas. APPARATUS AXD PROCEDURE
The experimental work required a method of determining the permeabilities of various films and a method of separating binary mixtures of gases.
Figure 1. Permeability Apparatus c
EFFECTOF TEMPERATURE. The most interesting relation-
2
ship is that of the influence of temperature on the permeation rate. It has been shown that the temperature coefficient of the permeation rate is independent of the differences of partial presmres and the thickness of the membrane. Graham (13) found that the temperature coefficient is very large. Correlation of experimental data indicates that the permeabilities can be expressed by the equation
P = POexp.( -E,/Rl')
(4)
over a moderate temperature range, a relation first pointed out by Barrer. Barrer (3) also stated that the permeability constant, P , may be resolved into the product of the diffusion constant, D , and the solubility coefficient, S , so that
P = DS
(5)
The dimensions of D are area per unit time and S has the dimensions of volume of gas per unit volume of polymer a t one atniosphere pressure. This relation has been verified for a number of polymeric substances ( 4 , 6, 19). A I of the investigators u-hose work has been reviewed agree that the diffusivity constant, D, and the solubility coefficient, S , as well as the permeability constant, P , vary exponentially with temperature as follows:
P = POexp.( - E , / R T )
(4)
D = Do exp.( -Ed/R2')
(6)
S
(7)
=
So exp.(AH/RT)
for a moderate temperature range. On the basis of these mathematical relations an expression which relates the values E p j E d , and AH may be shown t o be ( 1 )
E,.=
Ed
$- AH
(8)
The solubility of gases in liquids and EFFECT OF SOLUBILITY.
U
Figure 2.
Plastic Film Testing Apparatus
600-pound pressure gage Gas storage tanks Rotameter 6 X 11 inch blind flange 5. Rubber gasket 6 . Thermometer 7. Gas-sample collection bulb 8. Liquid displacement bottle 9. Plastic film 10. Filter paper film support 11. Conneotion for glass capillary tube or gas sample collection bulb 12. 100-pound-pressure gage i n 1-pound graduations 13. 50-inch manometer 14. 3Iotor-driven t ibrator 15. Glass capillary tube 1. 2. 3. 4.
Methods of deterniining the permeability of plastic membranes as described in the literature are of two general types: Rleasuring a change in pressure undar conditions of constant volume and temperature; and measuring a change in volume under conditions of constant temperatwe and pressure, as a gas passes through a membrane ( 4 , 7 , 19, 20, 2 3 ) . Measuring pressure change has the advantage that the determination of the permeability constant permits the simultaneous determination of the diffusivlty, while the other method does not permit this determination. One of the disadvantages of measur-
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1952
1467
ing pressure change is that, in general, a rather thick film (approximately 50 t o 100 mils or 0.05 t o 0.1 inch) is required to ensure the desired accuracy. This is especially true in the case of membranes which have a large permeability constant and/or a small diffusivity constant. The apparatus used in the present experimental work is shown in Figures 1 and 2. The apparatus was similar t o the one described by Todd (93),and made use of the method measuring the rate of change of volume of permeated gas in determining the permeability constant of the plastic films. This apparatus was chosen because of the desire to use thin (1- to 3-mil thick) fast films that were more readily available. Such films also would be suitable for gas separation, and therefore the same apparatus would be readily adaptable for application to the study of gas separation. I n operation, the plastic film was placed between two rubber gaskets, which were inserted between two 6 X 11 inch blank pipe flanges; the flanges were held together by twelve 6/s-inch SAE bolts and nuts. It was assumed that the rate of permeation through the rubber gasket would be negligible because the ratio of the area t o the thickness was small. The film was supported by filter paper, which also acted as a diffuser for the gas, since its permeability is considerably higher than t h a t of the plastic film. A 50-inch manometer or a 100-pound pressure gage was used t o measure the pressure drop across the film and a thermometer was used t o measure the temperature of the gas leaving the low pressure side of the film. This temperature was assumed t o be the temperature of the film since the temperature was controlled by placing t h e equipment in a constant temperature room. All connections on the low pressure side of the cell were filled with lead and drilled with a Number 44 drill (0.086 inch) so as t o increase the accuracy of the experimental results by reducing the downstream volume. After equilibrium was reached, the rate at which a short column of mercury moved through a section of a
Figure 3.
Secondary Permeability Apparatus
30-inch length of 1-mm. glass capillary tubing was observed to determine the gas rate of flow. The capillary tube was vibrated t o prevent the mercury from sticking and t o ensure a rather uniform rate of travel of the mercury plug. The thickness of the membrane was determined with a n Ames micrometer gage t o within a n accuracy of 1 0 . 2 mil. The area of the membrane was assumed to be the same as that of the gasket opening. For gas separation experiments a gas collection bulb was substituted for the length of 1-mm. capillary tubing and a gas collection bulb and a FlowTABLEI. PLASTIC FILMSUSEDIN THE EXPERIMENTAL WORK rator were placed on the upstream side of the film. ThickComposition ness, A photograph of an apparatus which utilizes t h e Source Film Code Number Chemical Parts In. method of measuring pressure change is shown in Figure 3. This apparatus was constructed for 1007 VBa resin . . 0 001 Bakelite Co., division of VB-1300 8 0 9 VB resin . . . 0 001 checking the results obtained on the equipment Union Carbide a n d VB-1920 2 0 g D O P ~(plasticizer) ., Carbon Corp. used in this experimental work. VB-1925 75% VB resin . . . 0.001 VB-1930
T h e Daw Chemical Co.
T y p e 610 Ethocel 064 1
25Y0 D O P (plasticizer) 70% VB resin 30% D O P (plasticizer)
Ethylcellulose Polystyrene Resin Geon 101-EPC Plasticizer DOP
012261 012206
Plasticizer Additives Resin Geon 202 Plasticizer Additives
Naugatuok Chemical (division of U. S. Rubber Co.) Ross and Roberts
PressPolish 88-127 196-25 Clear 100
.
0.0035 0.001
. 100 0 100 5 100 20 100 35 6.5
0.0016
0.0012 0.0015 0.0125
100 33 2
0 0015
100 31
0.0015
Marvinol VR-lOf Plasticizer Additives
100 10 2.5
0.002
Marvinol VR-10 DOP Stabilizer and lubricant Vinyl film
100 30
0,001
Commercial vinyl film
Vinyl film
Visking Corp.
Visqueen Visqueen
Polyethylene Polyethylene
Vinyl chloride, 9 5 % ; vinyl acetate, 5%. Di-2-ethylhexylphthalate. 100% vinyl, chloride. d Vinyl chloride, 97 vinylidene chloride, 3%. e Now Naugatuck &Rkmical Co. 1 Vinyl chloride type. C
0.001
0.0015
Naugatuck Chemical (division of U. S. Rubber Co.)
b
.
100 33
012241 Glenn L. Martin Co., Chemical Divisione
-
6.5
4
... . ..
0.0026 0.00225
0,001
0,0075
EXPERIMENTAL WORK AND DISCUSSION O F RESULTS
All of the gas permeability data obtained in this investigation are reported in terms of the permeability constant, P, which is defined as the number of standard cubic centimeters (0' C., 1 atmosphere) of gas passing through 1 square cm. of film (1 cm. thick) per second per centimeter of mercury partial pressure difference across the film. The plastic films which were used in the experimental work are listed in Table I, along with their composition and the supplying company. Polystyrene, polyethylene, and ethyl cellulose were selected for the initial permeability tests because of their relatively high gas permeation rates and because some work had been done on them previously. Utilization of these films permitted a means of comparing the apparatus as well as the experimental technique with those used by other investigators. Two series of films were selected in order t o include data on films which had a varying plasticizer content. A series of films of known composition was also obtained from the B. F. Goodrich Co. and these films were used t o determine the effect of additives on the rate of gag, permeation.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1468
0.5
-
- -~
04
I
Vol. 44, No. 6
- ..
A B
0.3
c
r igure
p
4.
0.2' 3.0
Figure 6.
3.1 bas
3.2 3.3 (VT) x 103 O K : ~
-
3.4
3.5
rermeation through Polyethylene Film
I
3.1
I
I
3.2 3.3 ( V T ) x lo3 O K ?
-
1
3.4
-
3.0
I G O 2 __ H2
o2 N2
__
___
I
0.1 3.0
D
__
0.2
-
3.I
3.2 W T ) x 103
- 3.3
3.4
3.5
OK:~
Figure 5.
Gas Permeation through Ethyl Cellulose Film
Figure 7.
Time Lag of Diffusion for Nitrogen-Ethyl Cellulose Film
J
3.5
Gas Permeation through Polystyrene Film
The gases used in this experiment work were nitrogen, oxygen, and hydrogen (supplied by Linde $ir Products Co.) and carbon dioxide (supplied by Pure Carbonic Co.). I n the initial work carbon dioxide, hydrogen, nitrogen, and oxygen mere used, Carbon dioxide and hydrogen mere selected t o be used in the remainder of the experimental work because it w,s believed that
the! aould be representative in indicating the effects of film plasticizer content on the rate of gas permeation of the other gases and also because of their relatively high permeation rates. APPLRATVS. The accuracy of the apparatus used in this experimental 11ork as checked by t\\ o methods-by comparing
June 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
the data obtained on it with those obtained on a n apparatus similar in construction t o that used by other investigators (this apparatus is shown in Figure 3) and by comparing the data obtained with results reported by other investigators. In all cases the logarithm of the permeability is plotted versus the reciprocal of the absolute temperature as is indicated by Equation 5 , since the resulting plot will be a straight line, provided that PO and Ep are not functions of temperature. The results of the comparison are shown in Table 11. These results, which were obtained on the same film samples in each case, show that the selected apparatus was capable of producing results equivalent t o those obtained previously by other investigators on the second type of apparatus. The degree of variation encountered between the results in the two apparatus is t o be expected. Additional tests of this type were not made since the agreement was considered satisfactory and because of the difficulty in obtaining gas-film combinations that had permeation constants in the range of both apparatus. Further checks were made on the validity of the experimental results by comparing these data with those previously reported by other investigators. I n this second method of testing the equipment and experimental technique, the values of P , the permeability constant for gases, including carbon dioxide,
1469
hydrogen, oxygen, and nitrogen, have been obtained at a variety of temperatures and pressures for the Visking Corporation's
TABLE IT. PERXEABILITY DATAOBTAINED ON OF
THE
APPARATUS
T y p e Aa
T w o TYPES
T y p e Bb
liitrogen-Ethyl Cellulose Gas-Film Combination Temp.,
P x 108
C.
30.2,42.2,49.7 0.84, 1.07,1.25
30.2,42.2,49.7 0.86, 1.05,1.20
Hydrogen-Polyethylene0 Gas-Film Combination Temp., C. 23.4,30,40 P x 100 .1.0,1.61,2.0 a Apparatus used in the present work. b Apparatus described in literature. C 7.5 mils.
23.4,30,40 1.1,1.81,2.12
TABLE111. EXPERIMENTAL PERMEABILITY DATA COMPARED WITH
LITERATURE VALUES P x
Experimental Values
Gas
Du Pont's (81) Values
logo
Weller's (86) Values
Dow's (10) Values
Viskinp's (26) Values
Ethyl Cellulose (25" C . )
coz H2 02
N2
4.43 3.34 2.69 0.82
4.75 0,778 I .
12.10 12.50 2.85
1.135 2.06 1.04
..
0,837
Polyethylene (25O C.) 1.67 0.615 .. .. 1.56 1.52 0,495 .. 0.415 0,1590 0.482 .. 0,337 NZ 0.140 0,0505 0.193 .. 0.0933 a Standard cc. per second per square cm. per cm. of mercury pressure differential.
Con
Hz
01
2.
c
i Figure 8. 1. 2. 3. 4. 5. 6. 7.
8. 9. 10.
Gas Permeability Range of Plastic Films
He, vinyl film commercial calendered, no directional pattern COP,vinyl fil& commercial calendered, no directional pattern HP.vinyl film, experimental press polished, no directionalpattern COz, vinyl film, experimental press polished, no directional pattern Ha, vinyl film, commercial calendered, no directional pattern COS,vinyl film, commercial calendered, no directional pattern Hz, polyethylene film, commercial, extruded, directional pattern COe, polyethylene film, commercial, extruded, directional pattern R z , vinyl film, experimental, extruded, no strain pattern COz, vinyl film, experimental, extruded, no strain pattern
=!
B-GP-261- 5 C-GP-261- 0
1.0
m
$
0.5
I a W
" ,0.2
% *
n.
0.1 3.I
Figure 9.
3.2
(vi)x
3.3 103-
3.4
3.5
OK:'
Carbon Dioxide and Hydrogen Permeation through Vinyl Films
INDUSTRIAL AND ENGINEERING CHEMISTRY
1470
TABLEIV. EXPERIMENTAL VALUES OF LITERATURE VALUES %J
Ep,
Gas
Experimental Values
COMPARED WITH
Calories/Gram Mole Weller's (16) Barrer's (S) Values Values
E t h y l Cellulose
Con Hz 0 2
Nz
1,394 3,436 3,769 4,141
... 4180 4480 4920 Polyethylene
COZ H P 0 2
sz
7,348 6,954 9,485 10,495
...
... ... ....
... ... ...
...
... ... ...
9130
reported by Weller and Steiner (26) and by Barrer (3). T h e results obtained agree with the previously published data. Deviations of the magnitude found are t o be expected since different film samples made by different companies at different times viere used. VALIDITYOF RESULTS.Statistical methods were used in t h e determination of the best line for the data for each of the gas-film combinations. The best line was calculated by means of t h e method of least squares and the following relation was used for the determination of the confidence interval~ofEp, a t a 0.005 level of significance (8):
[
P bn -
tou1,,g
uz
dl - 1.2 dw-2
Hz
0 2
?-z COZ
Hz 0 2
Nz COz
Ht COz
Hz Ez'
COz COz COz CO1
Hz Hz Hz
Hz COz COz
COP Hz
HP H z
COZ H2
COz
Hz
CO2
Hz COz
Hz COz COz COZ COZ
Hz
Hz Hz Hz
L
Px
L
bP1
+
~ O ' I o ~4 ~ UD
Visqueen (polyethylene) film, the Dow Chemical Company's Ethocel (ethyl cellulose) film and their polystyrene film. I n Figures 4, 5, and 6 the logarithm of the permeabilities of these films is plotted versus the reciprocal of absolute temperature ( " K.). Table I11 gives a comparison of the values obtained for P with those reported by Simril and Hershberger @ I ) , Weller and Steiner (g6,27), the Dow Chemical Co. (IO), and the Visking Corp. (25). Table I V compares the values of E p with the values
Gas COZ
Vol. 44, No. 6
1
- Y2]
d,v - 2
= 99.5
(9)
There P = probability that the line will lie within t'he interval; bP1= slope of the least squares line; to is evaluated from table of student's "tJ' distribution for p = 99.5; P21 = slope of the population line; r = correlation coefficient; mOg = standard deviation of logy; u0 = standard deviation of x; and N = number of degrees of freedom. The permeability data report,ed here have a deviation on the average of approximately 4%. It can be seen in Table V that some of the values of Ep have TESTED TABLE V. VALUESOF P,Po,A 4EP~FOR~ ALL Gas-FILlu: COMBINATIONS deviations considerably grester Actual than 4%. These greater deviTotal Number of ~ ~ t ~ Permeability ~ ~ X i 100 ~ ~ ationa occur, however, where theFilm Po x 108 EP 20" c. 300 c. 400 C * values of are very nearly zero Ethocel 44 1 , 3 9 4 i 187 111 and, hence, their effect on the Ethocel 991 3 , 4 3 6 i 247 165 Ethocel 1,383 3 , 7 6 9 f 166 74 2.15 2.65 3.40 calculated permeability constant Ethocel 805 4 , 1 4 1 zt 525 74 0.66 0.84 1.05 T'isqueen 431,800 7 , 3 4 8 3z 203 97 1.45 2.20 3.20 is actually much smaller. 6,954 i 152 89 0.90 1.33 1.93 138,000 Visqueen A review of the literature in9,485 i 2 0 5 141 0,325 0.55 0.92 Visqueen 3 . 8 3 X 106 6 . 8 1 X 106 Visqueen 1 0 , 4 9 5 f 502 95 01 ,, 12 12 0 01 ,. g188 3 0 ,. 3 1 20 dicates that the accuracy 01)7,480 60 Visqueen, 7.5-inil 463,000 tained is greater than that presample Visqueen, 7.5-mil 440,000 7,740 60 0.84 1.30 2.95 viously reported. Shunian (20) sample Polystyrene 19.81 1 , 0 0 3 i 58 351 39 ., 50 0 39,10 .70 3 ., 92 00 9 states that measurements on hi5 Polystyrene 15.72 328.8 32 147 419 Polystyrene 2.26 - 4 4 . 5 t 130 193 2.42 2.40 2.33 apparatus are accurate to within -81.6 f l 5 , l 105 0.78 0.78 0.77 0,680 Polystyrene 15 % and that the precision or re3 , 6 5 6 f 243 120 0.074 0.091 0.11 38.99 VB-1300 VB-1920 50.65 1 , 9 5 0 f 203 105 o 1 ,. 8 4 08 2 o ., 0 7 07 2 ., 1280 producibilityof measurements on 100 1,857,000 8 835 f 2 2 7 VB-1925 91437 i 2 7 7 110 0.68 1.18 1.90 duplicate samples of the same 7,437,000 VB-1930 2 , 1 8 8 i 545 90 0.422 0.48 0.02 17.86 VB-1300 material was generally about 150 4.99 5.00 5.05 6.319 1 3 8 . 6 f 191 VB-1520 156,000 VB-1925 7,417 i 273 120 0.46 o,6, o,96 0.70 1.03 1,36 10%. Todd (23) has stated in 44,690 6 , 4 7 6 f 294 120 VB-1930 (100 0.537 992 f 98 70 0.096 0.102 0.109 hispublicationthatresuitsoftrip101-EP parts), GP-261 licat,e determinations for thicker (0 part) -640 i 1 7 . 7 110 101-EP (100 1.358 4.00 3.85 3.70 sheets checked t o within 8%. parts), GP-261 140,00 130,0 118.00 Cartwright (7') found that t h e (5 parts) -1,521 f 2 0 4 101-EP (100 10.35 parts), GP-261 probable error on his apparatus (20 parts) for gas permeation determina101-EP (100 8,644 1 , 9 1 4 zt 103 90 0.32 0.36 0.40 parts), GP-261 tions mas usually a t least as ( 0 part) 101-EP, (100 1.153 - 1 , 3 8 6 zt 106 70 12.20 11.30 10.80 large as rrt5 to &lo%. In an parts), GP-261 ( 5 parts) article by Simril and Hershber8 . 5 0 8 . 6 0 8 . 8 0 101-EP (100 46,58 990 =t174 70 ger (21)it was stated that values parts), GP-261 (20 parts) 3 . 9 6 X 10-8 -11,107 32373 112 4,70 2,55 ,40 of the permeability constant for Martin 88-127 Martin 88-127 .... 288 1 9 ,335 .00 18.00 o , 560 1o2,.9o 00 check determinations varying Naugatuck 196-25 232,000' ' 7,820 60 Naugatuck 196-25 .... .... 60 0.450 0.60 0.88 more than 10% were not, inRoss a n d Roberts 6.55 - 708 60 21.00 20.50 20.10 cluded in their calculations. Clear 100 13.5 - 868 60 60.0 57.00 54.00 Ross a n d Roberts As a rule, the determination of Clear 100 Commercial vinyl 8,250 4,550 60 3.15 4.22 5.60 the thickness of the membrane film Commercial vinyl 19.15 505 60 8.00 8.25 8.50 x a s the least accurate part of t h e film 012206 101.4 X 106 11,502 f 428 100 0.262 0.505 0.92 permeation test,. I n addition Goodrich Goodrich 012214 826,100 X 106 16,314 f 530 160 t o accurately measuring a film 93 2 3 0 . 1 X 106 12,302 z t 6 5 3 Goodrich 012241 5 0 . 5 X 105 Goodrich 012261 11,203 32281 130 0.22 0.41 0.76 having a thickness of 1 t o 3 mils, 9 , 0 8 7 i 3 1 0 90 0.253 0 , 4 2 2 0 . 6 7 1 . 5 3 X 10s Goodrjch 012206 Goodrich012214 165,300 X 106 1 6 , 7 5 8 i 682 130 0,053 0,134 0.335 it is very difficult t o obtain memGoodrich 012241 27,570 X 106 14,072 f 413 110 O 'Og2 o 00 0 branes of uniform t,hickness a t Goodrich 012261 38.4 X 106 1 1 , 2 6 5 + 280 94 0.153 0 ,' 2 29 0 .' 45 01 every point.
$;;!
i::: 4":;
:!&
;::yo
g:g:o
lune 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
erin Figure 10.
Carbon Dioxide Permeation Characteristics of Vinyl Films
The other main possible error in the determination of the permeability constant was due t o the time lag of diffusion-that is, the time required t o arrive at steady state conditions. Slight changes in t h e pressure and/or the temperature lead t o a state of nonequilibrium between the inlet and outlet sides of the membrane; the time lag data obtained for the nitrogen-Ethocel-gasfilm combination are shown in Figure 7. This length of time was found t o be small enough and experimental operation was adjusted accordingly so t h a t errors of this type should be negligible in most cases. EFFECTOF TEMYERATURE ON PERMEABILTTY. While a fair amount of permeability data has been published by a number of investigators using independent techniques, t h e data in many instances are for one particular temperature only. It soon became apparent in the present work t h a t permeability data reported for only one temperature are practically meaningless. Inspection of all graphs which present plots of the logarithm of P versus the reciprocal of T brings out forcefully that the change of permeability with temperature is essentially unpredictable. Not only does t h e permeability vary t o a different extent for different films or different gases, but it also may increase, remain essentially unchanged, or decrease. It is important here t o state that these comments refer t o a rather limited temperature interval from about 20" to 40' C. However, this limitation is not so serious as it might appear because the useful temperature range of plastic films is normally near these temperatures. Nevertheless, it would be very desirable t o cover a wider range and in particular t o determine permeability behavior at lower temperatures. Because of the strength characteristic of plasticized films a n upper temperature limit would soon be reached, probably not far above 40" C. For unplasticized films or films containing relatively small amounts of plasticizer i t may be possible t o operate for extended periods at temperatures as high as 60' C. or perhaps somewhat above (16). Such considerations would primarily enter the picture if films were t o be used in gas separation cells. Most of t h e available data can be represented b y a straightline relationship over the temperature interval which waa covered in this work, as shown b y the data presented in Figures 4,5,6,8,9, and 12. However, at least two films were encountered where the
1411
straight-line relationship definitely did not hold, a s shown in Figure 8. These cases will be discussed in a later section. For the predominant case of t h e straight-line relationship it is possible t o characterize the line by means of two values, the permeability at infinite temperature. PO(extrapolated value), and the activation energy of permeation, Ep, in accordance with Equation 4. Values of these parameters are given in Table V for all the films which were tested. While these values are of theoretical importance, it is obvious that their variations in magnitude, and in the case of E , ale0 the variation in sign, makes them impractical for general use. Therefore, values of the actual permeabilities are also reported for three different temperatures. It is strongly recommended that permeability values always be reported for at least two temperatures, and preferably a t three temperatures. This recommendation is obvious in the light of t h e great variation in behavior of plastic films; the data in Figures 8 and 9 emphasize the variations which are encountered in the magnitude of permeability and in the magnitude and direction of the temperature coefficient. Some of the gas-film combinations exhibited a positive slope for the temperature coefficient rather than the much morecommon negative slope. The explanation of this phenomenon may be due t o the relative importance of Ed and A H , related by Equation 8 Considering the usual thermodynamic use of the positive and negative signs, it can be seen t h a t in the ordinary course of events A H will have a negative value and the value of Ed will be positive. Thus, if the value of AH is larger than Ed, the slope of the line will be positive, and if Ed is larger than A H the slope will be negative. This analysis and explanation of the phenomenon conform to that used by Muller (16) to explain the positive slopes he encountered in his investigations using water vapor and plastic films. I n a like manner the order of permeability of different gases, for example the carbon dioxide-hydrogen inversion shown by a comparison of Figures 4 and 5 with Figure 6, can be explained by
1 resin Figure 11. Hydrogen Permeation Characteristics of Vinyl Films
Vol. 44, No. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
1472
3.0 Figure 12.
3.I
3.2 3.3 ( v T j x to3- O K - '
3.4
3.5
Carbon Dioxide Permeation through Vinyl Films
t h e relative importance of the diffusivity and the soiubilit,y coefficient. EFFECT OF PLASTICIZER. Very little information has been published regarding the effect of the degree of plasticization of a film on the rate of gas permeation of the film. A few statements, with limited experimental evidence, can be found in the literature. Doty (0) has pointed out t h a t the rate of permeation is in general increased by the addition of plasticizers to t,he membranes, while van Amerongen ( 2 ) has stated t h a t the addition of plasticizers has little effect on the rate of gas permeation. If the former is t m e it seems plausible t'hat a film could be compounded which might possess most of the desired separating charact,erist,ics of the polymeric film-forming material and a t the same time would be fast enough t o be used commercially in gas separation. None of the journals which mere reviewed contained any references t o investigations of the permeation of films containing varying amounts of the same plasticizer. However, one industrial catalog ( 2 4 ) contains a plot of permeability versus plasticizer content for one series of films a t one temperature. Plasticizers are utilized in the composition of plast,ic films t,o increase the toughness and the flexihility of the film. Upon careful formulation, such desirable properties as high tensile, strength, specific chemical resistance, freedom from odor and taste, and electrical properties may be intensified for special uses. I n order t o determine the effect of plasticization and composition on the rate of gas permeation, experiment,s were performed on two series of vinyl composition films. Temperature coefficient data for one of tjhe films containing 0, 5 , and 20 parts of di-2ethyl hexyl phthalate (DOP) plasticizer per 100 parts of resin ale presented in Figure 9, for both hydrogen and carbon dioxide permeation. A cross plot of the data for carbon dioxide a t t,hree different temperat.ures is presented in Figure 10. The behavior sho1vvn by these films would perhaps appear t o be typical and in the manner one might expect. However, reference to Figure 11 shows a radically different behavior for a different set of films. So far in this work four ca.ses have been encountered where the
permeability did not progress uniformly with increasing p l : ~ ticizer content. The possibility t h a t errors in plasticizer contcnt, values are responsible in all four cases is rat,her remote. Thc information now available a,nd resulting from investigations under way, was considered too extensive for inclusion in this paper and a separate publication on this subject will appear i n the near future. For the time being, it is sufficient to state that, as in the case of the permcability, more than one condition should be investigated. I t probably is advisable to examine at least three different plast.icizer contents at not less thtw two different temperatures. OTHEREFFECTS.Because of the relative complexity of such :t heterogeneous system as is represented by a plasticized resin film, it is to be expected that a number of variables can exci,t pronounced effects upon the permeability behavior. I n Figurcs 12 and 13 data are presented for four different films and two different gases. As shown in Table I, the films contain somewhat different resins and total plasticizer t o about the same extent. The plasticizer in all of the films is different and in two of tlic films actually consists of a mixture of plasticizers. Furthermore, two of the films also cont,ain small amounts of the usual additivcs, such as stabilizers, lubricants, et,c. The exact compositions of the films are only of relative importance, because so far it has not, been possible to establish any general relationship between type of plasticizer and permeability. The purpose of presenting data is primarily to establish the fact t h a t differences in plwticizer type may result in as much as a t,enfold difference in the magnitude of permeability, and also, that the temperature coefficient' for a given gas-film combination may be appreciably affected by differences in type of plasticizer. These facts are further substantiated by the behavior of csperimental films 3 and 9 shown in Figure 8. The permeabilitytemperature relationship of these films with hydrogen reproscii t., a t present, the only instances where a straight-line relationship did not hold and a, definite curve was obtained. This may indicate that Po or En, or both, did vary with the temperature,. but there is no evidence available t o permit any definit,c con-
:0.023.0 Figure 13.
3.1
32
33
3.4
3.5
Hydrogen Permeation through Vinyl Films
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1952
clusion. Nor is it known a t present if some other factor is involved which caused this anomalous behavior. I n addition, it should be kept in mind that the possibility exists that films which exhibit the straight-line relationship in the temperature ranges covered by this work may exhibit a nonlinear behavior outside of this temperature range. Films 3 and 9 (Figure 8) represented a press polished and extruded film, respectively, and it was thought at first that the mechanical processing might have been responsible for the anomalous behavior. All of the other films which had been investigated were prepared by casting. Therefore, additional films which had been prepared by mechanical processing were checked. These films are represented by numbers 1 and 2, 5 and 6 , and 7 and 8 in Figure 8. Observations regarding the existence of 4
2 0
- 2 - 4
e c)
0 -I
-6 - 8 -IO
- 12
of P versus the reciprocal of the absolute temperature. Hence by use of the above-mentioned principle concerning the passing of lines through a common point, it is obvious that the line resulting from a plot of log POversus E, should closely approximate a straight line. Consequently, it is concluded that this correlation has no significance. CONCLUSIONS
Permeabilities for a variety of gas-film combinations have been obtained. The plastic films which were included in the study covered experimental and commercial films and the basic film materials covered most of the commercially important compounds. The effect of temperature upon the permeation of gases through plastic films is not readily predictable and it is recommended that permeability data be reported for at least two and preferably three temperatures. While the temperature effect can in many cases be expressed by means of two parameters-Le., the permeability a t infinite temperature and the activation energy of permeation-the values of these parameters show such wide variations in magnitude for different gas-film combinations that it is considered impractical to use them as a ready means of characterization. The reporting of actual permeabilities is to be preferred. The effect of plasticizer in the film formulation may or may not result in a uniform change in permeability. Both the amount and the t y e of plasticizer can produce pronounced changes in the permeagility behavior of films. Proper choice of compounds and of processing technique should permit the development of films having the desired gas permeability characteristics,
- 14
ACKNOWLEDGMENT
-16 -18 -14 -12 -10 -8
1473
-6 - 4 -2
0 2 4 Ep x Id
6
8
IO 12 14 16 18
Figlire 14. Plot of Log PO versiis Ep for All Gas-Film Combinatioiis
dirertional patterns when viewed under polarized light are liiited for these films. Only one film, the polyethylene film, showed a directional pattern and this is believed to be due to the fact that this film had a thickness of 7.5 mils. In general, it seems that directional patterns do not show up to any extent until a film thickness of about 4 mils is reached. Obviously, the results on mechanically processed films do not give any indication that such processing would be responsible for anomalous permeability temperature curves. While a systematic investigation of the effects of compounding and processing has not yet been undertaken. it is evident that the effects of such variables can be quite pronounced. This is clearly indicated by the data shown in Figure 8. The films presented in this figure represent a variety of compounds and processing conditions resulting in permeabilities which, in the extreme cases, show a hundredfold difference. Therefore, the conclusion is justified that it should be possible t o design fast films, that is, films having high permeability rates, which also would possess adequate selectivities for use in gas separation processes. APPARENTRELATION BETWEEN Po AND E;. A plot of Po vcrsus Ea for all gas-film combinations investigated resulted in a straight line, as shown in Figure 14. Doty (9) has reported this phenomenon previously, but he could not find a suitable theoretical explanation for its existence. It seems entirely plausible tbat this relationship can be explained in the following manner. When the intercepts of many lines which pass through a common point are plotted against their respective slopes, the result will be a straight line. I n the case considered above, the range of data utilized in the making of such a plot as that of Figure 14 was extremely small as compared with that of the temperature range employed in determining the value of PO. Thus, all of the data used effectively represent little more than a point on the entire plot of the logarithm
The authors wish t o express their appreciation to the United States Atomic Energy Commission for sponsoring the project which made this work possible. Furthermore, they are very grateful to the many industrial concerns for their help in supplying film materials so essential to the investigation. The names of the firms whose products were included in the present study are listed in Table I. NOMENCLATURE
A
= = = =
S
= solubility coefficient, cc. of gas per cc. of film per em. of
area of membrane, square em. C concentration, cc. of gas per cc. of film D diffusion constant, square em. per second DO diffusion constant a t infinite temperature, square em. per second d = thickness of membrane, em. E , = activation energy of permeability, calories per gram mole Ed = activation energy of diffusivity, calories per gram mole AH = molar heat of solution K or k = proportionality constants P = permeability constant, cc. of gas passing through 1 square em. of film per second per em. of mercury partial pressure differential Po = permeability constant a t infinite temperature, cc. of gas passing through 1 square em. of film per second per em. of mercury partial pressure differential p , and p , = partial pressures on both sides of the membrane p = pressure, em. of mercury. = quantity of gas permeated, cc. q R = universal gas constant, calories per gram mole per degree
I
