December, 1929
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I S D CSTRIAL A S D ENGIlVEERING CHELVISTRY
1215
Permeability of Rubber to Air I-Eff ect of Temperature, Pressure, and Humidity' V. N. Morris and J. N. Street F I R E S T OTIRE ~ E 8; RVBBERCOMPANY, A K R O N OHIO .
LTHOUGH it has been nearly a century since Mitchell Pickering (S), Schumaclier and Ferguson (I.:), and others. (I.$) and Cagniard-Latour ( 2 ) reported their experi- Certain interesting obserrations made by Lowry and Kohments on the passage of gases through rubber, it y a s man ( I S ) on the mechanism of the absorption of water by not until relatively recent years that any practical interest rubber are mentioned later in this report. I n regard t o the i n the phenomenon was aroused. This interest was first possible effect of the humidity of the atmosphere on the peroccasioned by observations on the tendency for hydrogen to meability of rubber to air, Dubosc (6) quoted Victor Henry diffuse through the rubber membranes of balloons. At pres- to the effect that humidity influences the permealdity of ent the permeability of rubber to gases is of much more gen- balloon fabrics. Edwards ( 7 ) , on the other hand, found eral inteiest, since it is a factor of prime importance in con- humidity to be of very little influence. The observations of Edwards and Pickcring (8) on the nection x i t h inany of our modern products, such as automobile inner tubes, diiigibles, gas masks, tennis balls, other balls for effect of pressure up t o 100 mm. of water are of little significance in connection with the athletic contrsts, etc. present study. Wroblemski As a preliininary step to (16) stated that the diffua more comprehensive study Apparatus and procedure are described for investision of carbon dioxide or of the permeability of rubgating the influence of temperature, pressure, and hydrogen through r u b b e r ber s t o c k s t o air, it was humidity upon the capacity of rubber membranes to w a s p r o p o r t i on a 1 to the considered desirable to esretain air under pressure. pressure of the gaq. This tablish the effect of experiThe relation between total pressure and permeability observation was later verimental conditions, such as has been found to be almost linear, the permeability fied by Kayser (12). Dewar temperature, pressure, and being approximately proportional to pressure. The (.?) found, however, that as humidity of the atmosphere, temperature coefficient of permeability is very high, p r e s s u r e s were increased upon permeability. thus indicating that the pressures of inner tubes from 1 to 20 atmospheres should be checked much more frequently in summer Previous Work the rate of diffusion of these than in winter. Moisture has been found to decrease t w o gases increased a t a Previous investigations in the permeability of rubber to air slightly under cermore rapid rate than did the this field h a v e n o t been tain circumstances. Prolonged immersion of the rubpressure. e x t e n s i v e . Much of the ber in water, however, resulted in an appreciable inIt is not intended in this work that has been d o n e crease in permeability. first report to go very prohas been carried out in confoundly into the theory of nection with the hydrogenpermeability of r u b b e r t o retaining capacity of balloon fabrics, -and thirefore does not contribute any conclusive gases. It mayibe mentioned that Graham (9) considered the evidence regarding the air-retaining capacity of rubber mem- phenomenon to be one involving solution of the gas in the branes. As has been recently mentioned by Daynes ( 4 ) in rubber, diffusion of the dissolved or liquefied gas, and evapohis excellent review of the permeability of rubber to gases, ration on the opposite side. Later investigators have, in the textiles in balloon fabrics exert such a marked influence general, accepted Graham's hypothesis. That an actual poon Permeability that conclusions froin work on balloon fabrics rosity of a type may also be a contributing factor in the caie of certain rubber stocks seems probable to the author%. cannot safely be extended to rubber films. In regard to the effect of temperature, Graham (9) obExperimental Method and Apparatus serred in 1866 that the permeabilities of a given rubber specimen a t 4",14", and 60" were in the ratio of 1 : 4 : 11.8. I n The apparatus used in this study represents an alteration of more modern tiines Daynes (3) and Edwards and Pickering (8)have also observed a marked temperature coefficient for an earlier one developed by R. W. Brown, of the Experimental permeability. Kayser (12) has reported results for the dif- Engineering Department of this company. The method fusion of hydrogen and carbon dioxide, which, when extrapo- of measurement was essentially a manometric one. Air. lated, are capable of leading to the conclusion that the under pressure, in one chamber was allowed to permeate into permeability of rubber to these two gases is zero at 0" C. another chamber through the rubber slab being tested. 4 Dewar (j),however, has observed a definite diffusion of gases manometer sealed to the slab allon ed the change in pressure to be followed. through rubber at temperatures well below 0 " C. The essential features of the apparatus, \vhich has been I n his curves coordinating temperatures and logarithms of rate of diffusion through rubber, Dewar ( 7 ) found such a designated as a permeameter, are s h o m in Figures 1 and 2 break a t 0" C. that he was naturally led to associate the mois- (glass part not completely shown in Figure 2). The two air ture content of the rubber with its permeability. TF'hen he chambers, set in rather heavy iron castings, are indicated by continued his experiments to include water yapor itself, he 4 and B. A screw clamping arrangement permitted the tn-o found this substance t o diffuse through rubber a t a very rapid chambers to be drawn together as tightly as desired on the rate. This observation has been verified by Edwards and rubber slab, RR'. The latter was also held in position at its ends by clamps (Figure 2), whose distance apart was adjust1 Pre-ented before the meeting of the Division of Rubber Chemistry able. It was thus possible to stretch the rubber in one direcof the American Chemical Society, Atlantic City, N. J , September 26 to 28, tion t o the extent of about 40 per cent. -4pressure gage, 1929
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1216
registering from 0 to 100 pounds per square inch (0 to 7.03 kg. per sq. cm.), is designated by P. The brass tube, D, from the chamber B was sealed with wax a t E to the glass manometer system, F. As the changes in pressure during any experiment were very small, it was possible to use mineral oil as a manometer fluid. I n order to avoid any difficulty from leaking stopcocks, the system was closed by the mercury seal a t G prior to the beginning of any determination. The drying tube, S, contained soda lime. A wooden case, within which i t was possible to maintain a fairly constant tempera-
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Figure 1-Apparatus for Measuring Permeability
ture by means of heating units, a fan, and a suitable thermostatic device, is indicated by LMNO. For runs of short duration, such as those reported below, hand operation of the heating device was found to give satisfactory control of temperature. The front of the case was provided with gIass windows, arm holes, and rubber sleeves, so that it was possible to manipulate the apparatus without disturbing the temperature control. The rubber membranes used in most of this study were prepared from a pure-gum mixture containing 2.89 per cent of both sulfur and zinc oxide, and 0.63 per cent of di-o-tolylguanidine. They were cured in specially made molds in a press a t 149' C. for 30 minutes, unless otherwise specified. The test slabs had an average thickness of 0.07 cm. In making a measurement the rubber slab, which had usually been dried over soda lime for several days, was placed in the clamps. The rubber was ordinarily stretched as much as it was possible to stretch it by adjusting the position of the clamps. The effect of stretch was counterbalanced somewhat by the contraction which tended to occur a t right angles to the direction of stretch. The two chambers were then drawn together over the rubber slab. A gasket of rubber was used around the base of the upper chamber to prevent injury to the slab being tested. Compressed air, dried as completely as possible by passage through a metal tube filled with soda lime, was introduced slowly through the valve, C. A rather arbitrarily selected gage pressure of 46 pounds per square inch (3.23 kg. per sq. em.) was used in chamber A during most of the experiments. A fairly heavy metal gauze across the base of the low-pressure chamber, B , served as a support and thus prevented too great a distension and possible rupture of the rubber slab. This gauze, by being in contact with the rubber membrane, undoubtedly exerted some influence on the absolute value of permeability obtained. With K and G both open, a slight suction was applied through S and the manometer fluid lifted to a definite height. K was then closed and H lifted until the mercury rose to a definite point, J . Before reading the manometer a t any time thereafter, the mercury level was always brought
Vol. 21, KO. 12
exactly to the point ,J. The usual procedure consisted in making readings of the manometer every 3 minutes. When the rate of fall in the manometer became constant, the average for several successive readings was taken as the value to be used in calculating permeability. Strictly speaking, the rate of fall should never become constant, as the pressure difference on the two sides of the rubber slab is constantly decreasing, and the composition of the air on the high-pressure side is constantly changing as a consequence of the greater permeability of rubber towards oxygen than towards nitrogen. The influence of these factors is negligible, however, during the course of any single measurement. The factors which tend to militate against a high accuracy in these experiments are rather numerous. Even in the case of a test specimen which is tested several times while standing undisturbed in the permeameter, small variations in the temperature, pressure, time subsequent to stretching, and degree of stretch (slippage) may influence the values. If the slab is removed from the permeameter between tests, possible variation in the exact portion being tested is of considerable importance. I n the case of a blooming slab, interference by the bloom with a proper seal a t the gaskets is a possible source of error. I n the case of slabs tested from day to day, variations in aging, humidity, etc., are added to the above-mentioned factors. Several additional possibilities are encountered in the cases of slabs cured a t different times or from different mixes. I n general, however, it has been possible t o check results within 5 per cent. Although it is possible under the best conditions to obtain values which are reproducible within 1 or 2 per cent, it is improbable that the absolute accuracy of the measurements is ever much under 5 per cent. Two calculations were made throughout the experiments. One was that necessary to convert the actual reading taken into units of volume per unit of time per unit of surface exposed. Calibrations of the apparatus indicated that the factor 0.90 will convert centimeters per minute on the manometer to liters of air per hour per square meter of rubber exposed, the volume being that existing under a pressure equivalent to that of the atmosphere. The volume obtained in the above calculation was then corrected to standard barometric pressure. All values shown in the experimental part have already been corrected to a barometric pressure of 760 111111. Effect of Pressure on Permeability
For the purpose of studying the effect of pressure on permeability, a rubber slab was put in the apparatus, stretched, and left stretched overnight previous to starting the experiments. The measurements reported in Table I were all made on the same day without moving the slab from the permeameter. Since the slab had been cured 25 days before use, any change in permeability due to aging during the course of the experiments was undoubtedly negligible. The average thickness of the slab used, when measured unstretched, was 0.066 cm. (0.026 inch). A temperature of 35' C. (95' F.) was maintained. The pressure difference as given in Table I represents the gage pressure plus the slight vacuum on the lowpressure side. Pressures given in all subsequent work refer to gage pressures only. The results are shown in Table I and graphically in Figure 3. It is apparent from Figure 3 that the relation between pressure and permeability is almost linear. The rate of increase of permeability with pressure becomes slightly greater, however, as the pressure increases. Another series of measurements made on a different slab gave a similar curve, but one showing a slightly greater change in rate as the pressure increases. The results thus obtained tend to confirm those of Dewar (6) rather than those of other investigators who concluded that permeability is an exactly linear function of
'NEE'RIA'G C'IIEMI~STl1Y
1217
ti~iiipiwiturci s lowereil ta 5' I?.:or 11 tu :12"E'. The above coiisi,lel.at,ioas Iiold for a tube t,tiat, is not otlterwiso heated
ljy atmospheric conditions. Certain experiments perionned by our laboratories iiidicate that Iieavy-dnt,). inner trilics may attain a teniperatirrc soriiewliat hhove tile boiling point of water uiider extreme coridit,ioiis
