DEPARTMENT OF COMMERCE Scientific Papers OP THE Bureau of Standards 3. W. STRATTON. Director No. 387 PERMEABILITY OF RUBBER TO GASES BY JUNIUS DAVID EDWARDS, Associate Chemist S. F. PICKERING, Associate Chemist Bureau of Standards JULY 12, 1920 PRICE, 10 CENTS Sold only by the Superintendent of Documents. Government Printing Office Washington, D. C. WASHINGTON GOVERNMENT PRINTING OFFICE 1920 DEPARTMENT OF COMMERCE Scientific Papers OP THE Bureau of Standards S. W. STRATTON, Director No. 387 PERMEABILITY OF RUBBER TO GASES BY JUNIUS DAVID EDWARDS, Associate Chemist S. F. PICKERING, Associate Chemist Bureau of Standards JULY 12. 1920 PRICE, 10 CENTS Sold only by the Superintendent ol Documents, Government Printing Office Washington, D. C. WASHINGTON GOVERNMENT PRINTING OFFICE 1920 ©v of D, SEP 23 J920 ■p / Ik ah PERMEABILITY OF RUBBER TO GASES By Junius David Edwards and S. F. Pickering CONTENTS Page I. Introduction 327 II. Nature of permeability process 328 III. Methods of determining permeability and characteristics of rubber sam- ples employed 328 1. Methods 329 2. Characteristics of rubber samples employed 330 IV. Relatiorfpf permeability to composition of rubber 332 V. Relation of permeability to experimental conditions 338 1. Relation of permeability to pressure 338 2. Relation of thickness of rubber to permeability 342 3. Time of penetration of rubber 344 4. Relation of permeability to temperature 345 VI. Permeability of rubber to various gases 347 1. Permeability of rubber to hydrogen 347 2. Permeability of rubber to oxygen 348 3. Permeability of rubber to nitrogen 340 4. Permeability of rubber to argon 350 5. Permeability of rubber to air 350 6. Permeability of rubber to carbon dioxide 351 7. Permeability of rubber to helium 352 8. Permeability of rubber to ammonia 354 9. Permeability of rubber to ethyl chloride 356 10. Permeability of rubber to methyl chloride 356 11. Permeability of rubber to water vapor 357 VII. Theory of permeability 360 VIII. Summary 36! I. INTRODUCTION Rubber has been in everyday use as a gas-retaining material for a great many years. Nevertheless, until the recent development of the modern rubberized balloon fabric, comparatively little advance was made in our knowledge of the permeability of rubber to gases. With the development of fabrics for lighter-than-air craft came the demand for accurate methods of measuring per- meability, together with a demand for the most varied kinds of information regarding the permeability relations of rubber and gases. The Bureau of Standards has already, in its Technologic 327 328 Scientific Papers of the Bureau of Standards \yu.z6 Paper No. 1 13 , l published the results of an investigation of methods for the determination of the permeability of rubber to hydrogen. The present investigation of the factors involved in the passage of gas through rubber and the permeability of rubber to different gases has been correlated with that work. The experimental work extended from 191 7 to 1919; its publication has been de- layed for obvious reasons. II. NATURE OF PERMEABILITY PROCESS Graham, 2 in his work on the " Dialytic Separation of Gases by Colloid Septa," was the first to point out that the characteristic passage of gas through rubber took place by solution in the rubber and not by diffusion through microscopic openings. If gases passed through rubber by the process of diffusion, as through a porous plate, their rates of penetration should be approximately inversely proportional to their viscosities. As pointed out by Graham, the relative rates of penetration of different gases bear no relation to their densities or viscosities. In fact, it is difficult to correlate the permeability with any of the well-known proper- ties of the gases. It is quite obvious from a consideration of the facts that some phenomenon other than that of diffusion through small openings is concerned and that the properties of both rubber and gas determine the rate of penetration. Before enter- ing on a discussion of this point the experimental facts which bear on the case will be presented. III. METHODS OF DETERMINING PERMEABILITY AND CHARACTERISTICS OF RUBBER SAMPLES EMPLOYED The permeability of a rubber film may be defined as the rate at which it is penetrated by a certain gas. Permeability will be expressed in terms of liters of gas per square meter per 24 hours, the volume of gas being corrected to the standard conditions of o°C and 760 mm mercury pressure. Unless stated otherwise, all determinations are made under the following conditions, which are adopted as standard for this work: The fabric is held at a temperature of 25 ° C, with air at atmospheric pressure (760 mm of mercury) on one side of the fabric and the gas in question at an excess pressure of 30 mm of water on the other side. 'J. D. Edwards, Determination of Permeability of Balloon Fabrics, B. S. Tech. Paper No. 113; 1918. 2 Phil. Mag., 82, p. 401; 1866. Edwards 1 Picktringl Permeability of Rubber to Gases 1. METHODS 329 Most of the different types of apparatus available for the deter- mination of permeability have been described in Technologic Paper No. 113, to which reference has been made. Certain other apparatus developed recently will be mentioned in connection with the experimental work. What may be called the standard apparatus of the Bureau of Standards is shown in diagram in Fig. 1. The rubber sample to be tested is held in the permeability cell a, which is maintained at a constant temperature in an air or water bath h. The cell con- sists of two circular plates with a shallow chamber in each. The test piece is held between the flanges of the cell and separates the Fig. 1 . — Standard apparatus for determining permeability of rubber to gases two chambers; it is supported by a series of crossed wires in the form of a screen. A constant concentration of the gas whose permeability is to be measured is maintained in one chamber. The gas which penetrates the exposed area of rubber passes into the other chamber, from which it is continuously removed by a stream of air or other gas and determined quantitatively. Because of the common use of hydrogen in balloons, the per- meability to hydrogen is the property most often determined in the case of balloon fabrics. For this reason, and because of the accuracy with which the permeability to hydrogen can be deter- mined, the permeability to any other gas will be referred to its permeability to hydrogen as the standard of comparison. 33° Scientific Papers of the Bureau of Standards [va. 16 In determining the permeability to hydrogen a current of pure, dry hydrogen is passed over one side of the fabric and out through a water seal. Dry air under carefully regulated pressure is passed over the other side of the fabric through a drying tube d 1 and into one chamber of a gas interferometer, where the percentage of hydrogen in the air is determined optically. The gas then passes out through the drying tube d 2 , which prevents diffusion of water vapor into the interferometer, through the saturator / filled with glass beads partly covered with water, and then through the wet meter ra. The saturator is employed to prevent loss of water from the meter by evaporation into the gas which is being meas- ured. Arrangements are made for by-passing the air stream from the interferometer to the meter when the interferometer is being read and for supplying the comparison chamber of the interferometer with pure, dry air. The gas interferometer 3 of the Rayleigh type measures the difference in refractivity of the two samples of gas contained in the gas chambers of the instrument. Several interferometers were used, and their sensitivity was such that each scale division indicated from 0.007 to °- 01 P er cent hydrogen in air. The average of 10 settings of the instrument gave a reading which was good to somewhat better than 1 scale division ; this gives ample precision in the determination of the hydrogen. The calibration of the interferometer, both for hydrogen and other gases, was accom- plished by the method described by one of the present authors in the Journal of the American Chemical Society. 4 By the use of this method the utility of the interferometer was greatly extended, and we were enabled to handle accurately such mixtures as helium and air, which are difficult to analyze by other methods. The interferometer furnishes a rapid and accurate means of analyzing many gas mixtures, and its use will be discussed further in that connection in this paper. 2. CHARACTERISTICS OF RUBBER SAMPLES EMPLOYED The greater part of the determinations recorded were made with rubber films as they are contained in balloon fabrics. This was done not only because of the immediate application of the results in that field, but also because balloon fabrics of great variety were readily available. Rubber films of satisfactory uniformity and low permeability are also most easily secured in the form of balloon » For detailed description see L. H. Adams, J. Am. Chem. Soc., 83, p. 1181: i»ij. ' Edwards, J. Am. Chem. Soc., 39, p. 3383: 1917. ffSnJ Permeability of Rubber to Gases 331 fabrics. The support given the rubber film by the cloth on which it is spread simplifies the handling and testing of the material. The question might be raised as to whether in some cases the results might not be influenced by the cloth on which the rubber is spread. To test this point, determinations were also made on thin sheet rubber in those instances. The absolute permeability of the rubber is profoundly modified by the cloth, as will be shown later ; its relative permeability to different gases is apparently not affected thereby. The presence of the cloth, however, introduces a factor which may lead to serious errors in testing if not properly taken account of. Most balloon fabrics are constructed of two plies of cloth with a film of rubber between the plies and a thinner coating of rubber on the inside and outside for the purpose of protection; these inner and outer coatings have little effect in reducing the permeability of the fabric. The rubber does not penetrate very thoroughly into the interstices between the threads, and, as a result, hydrogen is able to diffuse laterally along the cloth as well as directly through the rubber film. Hydrogen can there- fore diffuse along the textile and into the area clamped between the edges of the cell which, it might be assumed, is not active in the test. Here it can pass through the main layer of rubber, back through the textile on the other side, and into the air chamber. The exposed or "active" area of fabric is, then, larger than the area defined by the edges of the cell, and the results are correspondingly high. If there be no rubber on either side or only on one side, the interstices in the cloth can be satisfactorily sealed with vaseline or soft wax applied hot, which fills up the openings between the threads and prevents lateral diffusion of the hydrogen. If the fabric has a rubber coating on both sides, the vaseline can not penetrate this rubber into the cloth under- neath; no satisfactory method of sealing such fabrics is available. The best procedure in that case, is to reduce the margin of the fabric to as small an area as possible and put hot wax on the edge. The possible error, if the whole margin is active, can then be estimated. The " edge effect" can be illustrated by the results of a series of experiments on limiting the area of a test piece (see Table 1). Two samples of two-ply fabric were tested, one having an outside rubber coating on one side only and the other being rubber coated on both sides. The total area of each test piece was about 130 332 Scientific Papers of the Bureau of Standards [Vol. 16 cm 2 , but the exposed area was reduced to ioo, 90, and 70 cm 2 by coating with grease. With the fabric having one cloth surface it is seen that the area is accurately defined in each test. With the fabric having rubber on both surfaces, practically the whole area of the test piece is effective. TABLE 1.— Effect of Limiting Area of Test Piece Apparent permeability — Liters; exposed areas limited to — 100 cm* 90 cm ! 70 cm* 12.1 11.8 12.2 15.8 12.1 16.6 °The exposed area was used in calculating the permeability per square meter. The fabric (No. 50313) with which a great deal of the experi- mental data were obtained in succeeding experiments was a two- ply fabric without rubber coating on either side. Where it was necessary for some reason or other to use fabrics having rubber on both sides, the edge effect was made as small as possible by reducing the margin to a minimum ; its effect on the relative values of tests was then without significance. IV. RELATION OF PERMEABILITY TO COMPOSITION OF RUBBER Crude rubber, as well as vulcanized rubber, may vary so widely in composition and physical characteristics that one can hardly expect to find or define such a constant as the specific permeability of rubber. Part of the disagreement between previous experi- menters has been ascribed to differences in the samples of rubber which were tested. Nevertheless, certain regularity of behavior has been noted and certain observations made on the relation between permeability and composition which are of interest and value. For the present purpose rubber may be considered to be a mixture of "polyprene" (CsHg)! in different stages of polymeriza- tion, together with resins, nitrogenous matter, water, and inorganic material in varying proportions. Vulcanized rubber, which we will hereinafter refer to simply as rubber, contains, in addition, varying proportions of sulphur, combined with or adsorbed by the polyprene, together with some free sulphur. Compounding materials in great variety may also be added to the rubber to Edwards 1 Pickering! Permeability of Rubber to Gases 333 give it desirable characteristics, but where imperviousness to gases is desired their use is usually restricted. The effect of sulphur upon permeability may be considered in connection with the effect of vulcanization, since the two factors are interrelated. The effect of different degrees of vulcanization or "cure" upon permeability is shown, for one compound, by the series of tests given in Table 2. The samples were taken from a roll of two-ply balloon fabric, different sections of which had been given different degress of vulcanization, as indicated. Except for variations in the uniformity of spreading, the temperature and time of heating were the only variables. TABLE 2.— Effect of Time and Temperature of Vulcanization Upon Permeability 1 Sulphur Acetone extract (sulphur free) Permea- bility Sample No. (steam beat) Com- bined Free Hours None 0.5 1.0 1.0 1.25 1.0 1.5 °F Per cent 0.3 .5 1.4 1.3 1.6 2.5 2.3 Per cent 4.3 3.2 2.5 1.8 2.1 2.1 1.8 3.8 3.2 3.2 3.0 3.0 3.2 3.0 Liters 12.8 270 270 284 284 288 284 11.6 11.5 12.7 15.5 12.2 12.8 a The chemical analysis "was made about eight months after the permeability determinations, free sulphur may. therefore, be somewhat lower than that originally present. The This series of fabrics shows no significant variation in permea- bility which can not be ascribed to lack of complete uniformity in the fabric. The combined sulphur varied from practically nothing to 2.5 per cent. In a similar series of tests samples were taken from adjacent portions of 13 different rolls of fabric before and after vulcaniza- tion. In two cases the permeability was the same before and after vulcanization; in five cases the permeability of the uncured sample was the highest and in the remaining six cases the reverse was true. The average difference was only 1 liter. The average combined sulphur before and after vulcanization was 0.76 and 1.07 per cent, respectively ; similarly the average acetone extract was 3.8 and 3.7 per cent. In Table 3 are shown the results of another series of tests in which the time and temperature of vulcanization were varied. The permeability and chemical characteristics are given both for 181118°— 20 2 334 Scientific Papers of the Bureau of Standards [Vol. i« the fabric as received and after storage under ideal conditions (in a cool, dark place) for 1 2 months. It may be remarked, to begin TABLE 3. — Relation of Composition to Permeability Composition and permeability as received Composition and permeability alter storage (12 months) Sample No. temperature of cure Acetone extract Free sulphur Com- bined sulphur Per- mea- bility Acetone extract Free sulphur Com- bined sulphur Per- mea- bility Hours 3 4 3 4 3 4 3 4 4 4 4 4 •F 270 270 290 290 270 270 290 290 270 290 270 290 Percent 3.1 5.1 5.2 5.3 6.1 5.7 5.5 5.8 5.8 6.9 5.3 6.3 Percent 2.9 3.0 .9 1.1 5.7 4.4 1.7 1.9 4.9 1.6 3.8 1.7 Percent 1.6 1.9 3.7 4.0 1.8 2.0 3.6 4.8 2.1 4.6 1.7 5.0 Liters 23.5 20.4 18.8 15.8 20.6 19.9 16.8 14.9 23.8 15.9 20.5 10.6 Percent 5.4 6.0 6.3 7.6 5.9 4.2 12.0 16.8 5.3 13.6 16.6 17.7 Percent 2.2 3.2 .7 .6 3.6 4.6 .6 .5 3.5 .5 .4 .5 Percent 1.6 2.6 4.1 4.7 3.6 2.9 5.2 6.3 2.8 5.7 6.8 8.0 Liters 17.7 18.8 13.2 9.5 26998 15.4 26997 14.9 26999 4.0 26996 6.0 14.0 26994 2.4 26992 6.8 26993 1.3 a All fabrics were two-ply construction. They varied in weight and distribution of rubber compound. The percentage of sulphur was varied in two different proportions, but this was the only change in the composition of the rubber compound. Fabrics grouped together were of identical construction except for variations in cure. The analyses were calculated on the basis of the rubber compound contained and not on the weight of rubber plus fabric. with, that practically all of these fabrics were somewhat over- cured. The rubber had the characteristic odor of overcured balloon fabric, and many of the samples became quite stiff with time ; some reached the stage where the rubber compound was brittle and cracked on bending. The most noticeable facts which these results show are that with these fabrics the permeability decreased during storage and that there was a concomitant increase of combined sulphur and acetone extract. If the percentages of combined sulphur are plotted as abscissae and the permeabilities as ordinates, the graph shown in Fig. 2 is obtained. As shown by the legend, data on fabrics which have been exposed outdoors for 30 days are also included. There apparently is some relation in this series between the permeability and the percentage of combined sulphur. The acetone extract also increases at the same time, but there is no such striking relation between these two variables as that shown in Fig. 2. The original acetone extract is about the same on all the fabrics, but the permeability shows a considerable variation. Edwards "| Pickering} Permeability of Rubber to Gases 335 Similar decreases in permeability are observed when fabrics are exposed to the weather. In Fig. 3 are shown the relations between permeability and period of exposure for three different fabrics. The periods of testing were not frequent enough to locate the lowest point on each curve, but the curves indicate that the permeability reaches very low values. This lower permeability is accompanied 22 3 e e orj,30a J8 I • 1 »e e • • j e • « » < > • £ t e > • • — — 1 1 Z 3 + 5 6 7 3 9 Combined Sulfur - Par cent /o Fig. 2. — Graph showing decrease in permeability of rubber with increase of combined sulphur by a characteristic hardening of the rubber. When the rubber film becomes sufficiently brittle it cracks easily and thereafter shows a very high permeability. The changes in permeability shown by the graphs were accompanied by the following changes in chemical characteristics at the time the lowest permeability was observed. 336 Scientific Papers of the Bureau of Standards [Voi.16 TABLE 4.— Change in Chemical Characteristics of Rubber in Three Fabrics of Fig. 3 Fabric No. 10650 Fabric No. 10652 Fabric No. 23990 Composition Original 45 days Original 60 days Original 150 days Per cent 1.4 .5 3.1 Per cent 1.7 .2 50.0 Per cent 3.0 2.8 3.2 Per cent 4.2 .2 16.2 Per cent 1.5 1.2 4.0 Per cent 2.5 .2 42.9 In the case of fabrics exposed outdoors there is sometimes an increase in permeability during the initial period of exposure, which is followed later by the customary decrease. This increase is not accompanied, apparently, by any significant change in chemical characteristics. taaw •.«■ i zo / ;oeso 101,52 ft i33C IB '-. / '•- ■- / 16 \ \ / J> \ / $14 \ / +*> v / \ ' ^ 10 \ \ S> 3 \ V •» ""** / \ I "5 s / 0. 6 1 ) \ s / \ \ v \ \ \ 4 \ > 1 \ / 1 2 \ \ V O 30 60 90 ZO /SO JBO £X3 £40 ttO Time- of Exposure -Ctiyj Fig. 3. — Effect upon permeability of exposure of rubber to the weather Preliminary to drawing any conclusions from them it may be desirable to summarize the observed facts in regard to permea- bility and composition as shown by the preceding tests. 1. The aging of rubber in thin films is accompanied by a char- acteristic decrease in permeability. 2. The aging of rubber is usually accompanied by a decrease in the percentage of total sulphur; the combined sulphur increases by varying amounts and the free sulphur decreases eventually to a low value. Scientific Papers of the Bureau of Standards, Vol. 16 Fig. 4. — Section of balloon fabric, showing crystals of sulphur. (Xi\j8) PkkZfno] Permeability of Rubber to Gases 337 3. In one series of fabrics where the degree of cure was varied no significant change in permeability was observed. In this case the percentage of combined sulphur varied from 0.3 to 2.5 per cent. In another series large changes in permeability were noted with change in the degree of cure; the combined sulphur varied from 1.6 to 5 per cent. The original acetone extract was approxi- mately constant in each series. Because of the number of factors involved and because of the relatively small number of data presented, it would be unwise to draw any very extensive conclusions. The view is quite widely held by manufacturers and others that the permeability of a fabric can be reduced by increasing the degree of cure. Between certain limits this is true. That this reduction in permeability is caused entirely by the increase in combined sulphur is not at all certain. Opposed to this latter view is the fact that as great and greater decreases in permeability are noted on exposed fabrics where there are relatively small changes in combined sulphur. (See Fig. 3 and Table 4.) The most striking change in exposed fabrics is the increase in acetone extract, which increase is a meas- ure of the resinification and oxidation of the rubber. It appears reasonable to believe, therefore, that an increase in both the com- bined sulphur and acetone extract causes a decrease in permea- bility. This would be the natural result if hydrogen was insoluble in both the acetone-soluble material and the " polyprene sulphide." It has been thought by some that the free sulphur plays an important part in determining the permeability. The free sulphur which is present in the colloidal condition in the rubber after vul- canization frequently crystallizes out. This is strikingly shown by the microsection of a sample of ballonet fabric illustrated in Fig. 4. The sulphur crystals are seen as dark dendritic masses in the rubber between the two plies of cloth. A certain amount of this sulphur eventually penetrates to the surface and evaporates. This process might possibly produce a certain porosity which would increase the permeability. Tests made on portions of the fabric of Fig. 4, where crystallization was extensive, showed no significant difference in permeability as compared with portions where crystallization had not occurred. Certainly our tests and experimental methods have not been of sufficient delicacy to detect any effect on the permeability which can be ascribed to this blooming out of sulphur. 338 Scientific Papers of the Bureau of Standards IVo/.ks Compounding materials may be added to the rubber either to make it more impervious to gases or to give it greater durability. Paraffin and glue are two substances which are said to lower the permeability of rubber to hydrogen. It is known that either of them alone will give a film of very low permeability, provided it is nonporous. Their use is not essential, however, to the produc- tion of a satisfactory coating for balloon fabrics. Lampblack, zinc oxide, or litharge may be incorporated in the rubber to give it greater life by protecting the rubber from the injurious action of light. No systematic investigation of the effect of these sub- stances has been undertaken; our work has been confined almost exclusively to rubber compounds of the simplest composition. V. RELATION OF PERMEABILITY TO EXPERIMENTAL CONDITIONS 1. RELATION OF PERMEABILITY TO PRESSURE In considering the effect of pressure, a distinction should be made between the total pressure and the partial pressure of any constituent. A difference in the total pressure on the two sides will produce tension in the rubber film and a change in thickness or physical properties may result. The effect of a change in the total pressure will be influenced by the support given the rubber film, such as when it is held between cloth of one or more plies, as in the case of a balloon fabric. The work of previous investigators indicates that the perme- ability of rubber to any gas is about proportional to the partial pressure of that gas. The agreement on this point is not unani- mous, however, and the methods and data recorded are not satisfactory in all particulars. Almost all of the recorded meas- urements were made with apparatus of the volume-loss type; that is, they measured the loss in volume of a mass of gas con- fined by the rubber. The diffusion took place under varying pressure, and the back diffusion of air was seldom corrected for. In some cases the total pressure and partial pressure varied simultaneously, a condition which is obviously undesirable. In Fig. 5 is shown the relation between permeability and difference in partial pressure of hydrogen, as shown by tests on six different test pieces of the same fabric. The percentages of hydrogen in the air were determined by means of the interfer- rometer. The permeability of the different test pieces with ioo per cent hydrogen varied from 9.4 to 10 liters; each result was, Edwards 1 Pickering! Permeability of Rubber to Gases 339 therefore, multiplied by the ratio of 10 to the observed permea- bility at ioo per cent, so that the ioo per cent value became 10 in each case and all the other values Avere directly comparable. It may be concluded from these results that the permeability is directly proportional to the partial pressure, within the limits of experimental error. Similar results were obtained with carbon dioxide, as shown in Fig. 6. In addition to the balloon fabric (No. 50313) a sample of thin rubber known as "dental dam" was also tested. The permeability in each case was directly proportional to the partial pressure, any deviation being reasonably ascribed to experimental error. The values indicated for fabric No. 50313 are the averages of tests on seven different test pieces. To make the results iz /o h h r /O ZO 30-10 SO 60 70 SO 90/00% hydrogen m d/r -Per cent Fig. 5. — Relation between permeability and partial pressure of hydrogen directly comparable and save plotting a graph for each of the seven test pieces, the results were corrected, as in the case of the experiments with hydrogen, by reducing the 100 per cent values to the same figure by direct proportion and changing the other values proportionally. In accordance with the conclusion that the permeability was directly proportional to the partial pressure, the results of all permeability determinations have been corrected to the standard condition of a partial pressure of 760 mm in the following man- ner: In one determination with carbon dioxide (99.9 per cent pure) there was 0.6 per cent carbon dioxide in the air on the opposite side of the fabric; the barometric pressure was 750 mm and the observed permeability 20 liters. The corrected perme- 1 — ^ ability is then equal to 20.0 x 1 00.0 760 , 750 "99.9-0-6 , or 20.4 liters. 34Q Scientific Papers of the Bureau of Standards [Vol. 1(5 The change in permeability when the difference in the total pressure on the two sides of the sample is varied follows no simple law. In this case, not only does the permeability change with the change in partial pressure, but also it may change with any variation in thickness caused by the tension on the rubber. Ob- viously, the effect will vary with the support given the rubber. In the case of a balloon fabric the rubber film is given very inti- mate support by the cloth on which it is spread. The cloth may to some extent be prevented from stretching by the manner in 44 40 / / 3Z # y^ 4* OjS ^- IZ 6 4 y y r. / 10 20 JO ■&> SO 60 70 30 &O Carbon d/ox/de m d/r- Per cent /oo Fig. 6. — Relation between permeability and partial pressure of carbon dioxide which it is held in the cell during a test. In the case of a sheet of rubber such as dental dam, the only support the rubber receives is from the screen on which it rests in the cell and the fact that it is clamped at the edges. In Fig. 7 are shown the graphs of several experiments where the pressure of the hydrogen was varied. Four different pieces of fabric No. 50313 were tested, and, to make the results more nearly comparable in the graph, the values were reduced by direct proportion, so that the values at 100 mm were identical; the 100 mm point, therefore, repre- Edwards 1 Pickering] Permeability of Rubber to Gases 34i sents four determinations. The two balloon fabrics (Nos. 50313 and 47174) show about the same small rate of increase of per- meability with increase of pressure. The two samples of dental dam show a slightly higher rate. The sample of dental dam show- ing the higher permeability was supported in the cell during the test by a screen having openings 4 cm 2 in area ; the other sample was supported by wire gauze of 28 mesh. At least part of the difference in permeability can be ascribed to the greater stretch- ing of the rubber in the case of the first sample mentioned. The extensibility of rub'ber and cloth may vary so greatly that no /8 17 '6,^- fS, 1 I. SoilS Fig. 7. /O £0 SO 40 SO 60 70 SO 90 /00 // 8 Loc. cit. 30 Barr British Advisory Comm. for Aeronautics, 191s. Edwards "| Pickering^ Permeability of Rubber to Gases 355 In determining the permeability of rubber to ammonia it was necessary to use a special permeability cell made of steel, which would be unattacked by the ammonia. All connecting tubes coming in contact with the ammonia were either steel or glass. The ammonia which passed through the fabric into the air stream was absorbed in a measured volume of tenth-normal sulphuric acid and the excess acid determined by titration. Two small wash bottles were always used in series, but never more than a negligible amount of ammonia escaped absorption in the first wash bottle. Two sets of wash bottles were used, and they were connected to the cell alternately through a three-way cock. They were attached to the system by a mercury seal so that they could be easily detached. Each value recorded in the table is the average of a number of observations, each covering a half- hour period. The ammonia was taken from a small steel cylinder, which had been evacuated to a very low pressure before filling with liquid ammonia. The gas can be considered to be free from air and water vapor; in fact, the total amount of impurity in this sample, which was carefully purified by fractionation, was shown by tests of C. S. Taylor, of this Bureau, to be less than i part per ioo ooo. The results of a series of experiments are given in Table 12. It was noted that it took considerable time to remove all the ammonia from the rubber, so that it was necessary to deter- mine the permeability to hydrogen first in each case. The average ratio of the permeability to ammonia and hydrogen is probably very close to 8. TABLE 12. — Permeability of Rubber to Ammonia and Hydrogen Fabric No. Permeability to ammonia Permeability to hydrogen Ratio o! permeabilities, ammonia to hydrogen 50313 Liters per m= per 24 hours f 71.9 1 3" 1 3 « i a - r ii9. 6i J \ 119.7 I 119. 8] Liters per m- per 24 hours 9.0 9.1 10.0 1 S3 ■" 7.99 50313 8.02 50313 8.04 7.93 Average ratio of permeabilities, ammonia to hydrogen, 8. a These two results which were obtained on two succeeding days indicated that some change had oc- curred in the sample, and they are omitted from the average. 356 Scientific Papers of the Bureau of Standards [Voi.16 9. PERMEABILITY OF RUBBER TO ETHYL CHLORIDE The permeability of rubber to ethyl chloride (C 2 H 5 C1) is prin- cipally of interest because of the high value found and its relation to their mutual solubility. An interferometer of the portable type was used for determining the percentage of ethyl chloride passing through the fabric into the air stream. The interferometer was calibrated by the method previously referred to; for the purposes of this calibration the refractivity of ethyl- chloride vapor was calculated from values for the refractive index of the saturated vapor as recently determined at this Bureau. In making this calculation the refractivity of ethyl-chloride vapor at the partial pressures at which it was measured (4 to 5 per cent) was estimated from the density, which was calculated by means of Berthelot's equation of state. In doing this it was assumed that the partial pressure of the ethyl-chloride vapor in air followed Dalton's law. The ethyl chloride was contained in a glass flask fitted with a steel valve, and the vapor could be readily withdrawn under its own pressure. It was prepared by C. S. Taylor, of this Bureau, from very pure materials and further purified by fractionation. The total impurity in the vapor phase was undoubtedly less than 1 part in 10 000, a purity far beyond that required for the present work. TABLE 13. — Permeability to Ethyl Chloride and to Hydrogen Fabric Nc. Permeability to etbyl chloride Permeability to hydrogen Ratio of permeabilities, ethyl chloride to hydrogen 50313. 50313. 50313. 50313. Liters per m 2 per 24 hours 1717 1851 1816 1763 Liters per m= per 24 hours 8.80 9.76 8.31 9.44 195 190 218 187 Average rates of permeabilities, ethyl chloride to hydrogen, 198. The permeability of rubber to ethyl chloride as shown by these few tests is approximately 190 to 200 times its permeability to hydrogen. 10. PERMEABILITY OF RUBBER TO METHYL CHLORIDE For the purpose of comparison with ethyl chloride, the permea- bility of rubber to methyl chloride (CH S C1) was also determined. The interferometer was employed in estimating the percentage of methyl chloride in the same manner as with ethyl chloride. The only available data on the refractivity of methyl chloride are those of Mascart (N-d, o° C, 760 mm = 1 .000870) ; 21 the calibration « From Landolt-Bornstein Phys. Tabellen. Edwards 1 Pickering! Permeability of Rubber to Gases 357 is based on this value. The sample of methyl chloride used had been carefully purified by fractionation by C. S. Taylor. Three tests were also made with a sample of methyl chloride, the purity of which was unknown. These gave a ratio of permeabilities, methyl chloride to hydrogen, of 16.8, 17.6, and 17.8, but they are not included in the average. The other results are given in Table 14. The permeability of rubber to methyl chloride is approxi- mately 18.5 times its permeability to hydrogen. TABLE 14.— Permeability of Rubber to Methyl Chloride and Hydrogen Fabric No. Permeability lo methyl chloride Permeability to hydrogen Ratio of permeabilities, methyl chloride to bydrogen 50313 Liters per Ta- per 24 hours 173.8 185.8 174.8 180.3 Liters per m- per 24 hours 9.43 9.58 9.68 9.86 50313 50313 50313 Average rates of permeabilities, methyl chloride to hydrogen, 18.5. 11. PERMEABILITY OF RUBBER TO WATER VAPOR The permeability of rubber to water vapor is interesting for a number of reasons. In view of the popular conception of rubber as a "waterproof" material, it might be thought that it was quite impermeable to water vapor, whereas the opposite is true — its permeability is relatively high. This fact is of great importance in many instances where rubber is used as a gas container; such as, for example, the use of rubber tubing in chemical and physical work. The use of rubber connections in any apparatus where the water content of the gas is important may introduce more or less serious errors. The high permeability of rubber to water vapor renders its determination rather difficult. The first method employed in its determination was to pass a current of air saturated with water vapor at a temperature slightly below 25 C over the fabric, which was maintained in the cell at 25 C. A stream of air, pre- viously dried over phosphorus pentoxide, was passed over the other face of the fabric and thence through an absorption tube filled with phosphorus pentoxide in which the water vapor could be absorbed and weighed. The results were very erratic, prob- ably because of the low partial pressure of the water vapor in the air (about 3 per cent) and the large effect on the difference in partial pressure produced by small variations in the rate of passage of the dry air. The results, however, are confirmatory of those secured by the following method: 358 Scientific Papers of the Bureau of Standards [Vol. 16 A shallow, crystallizing dish, 8 cm in diameter, was partly filled with phosphorus pentoxide and the top closed by a sheet of rubber, such as dental dam, which was fastened at the edge with rubber cement. The dish was then placed in an atmosphere saturated with water vapor and the rate of increase in weight determined. The results are shown in Table 15; obviously they only give an approximate figure, and no claim of accuracy is made for them. Lack of time prevented carrying this phase of the work farther. In connection with this table and the succeeding one, attention should be called to the fact that the permeability to water vapor is calculated for the assumed case of a difference in partial pressure of water vapor of 760 mm. This is done to make the results comparable with the hydrogen value. In any test the partial pressure of water vapor was about 20 mm. TABLE 15.— Permeability of Rubber to Water Vapor and Hydrogen [Air saturated with water vapor in contact with rubber] Sample No. Permeability to water vapor (100 per cent partial pressure) Permeability to hydrogen Ratio of per- meabilities, water vapor to hydrogen Liters per m : per 24 hours 953. 969 1270 1001 1108 978 1130 1174 975 920 1021 875 970 890 1030 1019 1262 1075 Liters per m 2 per 24 hours 1034 22.0 47.0 905 905 726 1118 860 765 905 930 889 14.3 £2.0 Edwards 1 Pickering! Permeability of Rubber to Gases 359 A few experiments were also made with liquid water in con- tact with the rubber film. In these tests instead of cementing the rubber to the dish containing the phosphorus pentoxide, the rubber was cemented to the top of another exactly similar dish from which the bottom had been removed. The edges of both dishes were ground plane. The dish with the rubber film across the bottom was partially filled with water and placed on top of the dish containing the phosphorus pentoxide. When it was desired to weigh the lower dish, the upper dish was replaced by a watch glass. The results of these tests are shown in Table 16. In calculating the results the partial pressure of water vapor used was that corresponding to the temperature of the water in contact with the rubber. TABLE 16.— Permeability of Rubber to Water Vapor [Liquid in contact with rubber] Sample No. Permeability to water vapor Permeability to hydrogen Ratio of per- meabilities, water vapor to hydrogen A-1K— Thirlrni-Qc,