Gas MAsk Absorbents

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Vol. rch N o . 5

I5 L

CHEMICAL WARFARE, SERVICE, U.S . A.

GAS MASK ABSORBENTS B y ARTHURB. LAMB,ROBERT E WILSONA N D N. K. CHANEY Received March 17, 1919

INTRODUCTION

The development of absorbents for gases has naturally taken an enormous stride forward during the recent war. T h e present gas mask absorbents are unquestionably several times as efficient in every way as any materials which were used for like purposes before the war. Such an advance has, of course, been made possible only by intensive study of a,ll the factors which combine t o make a n efficient absorbent, and by the trial of many thousands of mixtures of different absorbent materials. The development of these absorbents has been further complicated by the fact t h a t the goal aimed a t has not been a fixed one, but instead has gradually passed through many different stages, as new gases came into use, as new field conditions arose, and as the relative importance of t h e different factors had t o be re-balanced in t h e light of new information coming from the front. The absorbent used in both t h e British and American gas mask canisters, which afforded a degree of protection far superior t o t h a t of any other allied or enemy nation, consisted of a m&;%gfchart@ and sod&me. The mixture used in the American canisters for t h e last g months of the war contained &-.per c_en; 6 t o 14 mesh coconut shell charcoal (or other shell charcoal) and 40 per cenl 8 t o 14 mesh soda-lime-permanganate granules. This absorbent, commonly referred t o as t h e “war gas mixture,” has been generally recognized t o be distinctly more stable and efficient t h a n t h a t used by any foreign country. Even better t h a n this, however, was a new combination which was t o have been put into production very shortly, a mixture of 7 5 per cent specially impregnated coconut charcoal and 2 5 per cent of soda-lime containing no permanganate. This mixture would have had a distinctly greater all-round efficiency than the one which was actually used. I n order t o make clear the reasons underlying the choice and composition of the above two absorbents, i t is necessary t o discuss t h e general requirements which must be met by a gas mask absorbent. Each of these requirements will accordingly be taken up in turn.

I

violently breathes about 60 liters of air per minute and, since inhalation occupies b u t slightly more t h a n half a breathing cycle, the actual rate a t which gas passes through the canister during inhalation is about I O O liters per minute. Calculated on t h e basis of the regular army canister, this corresponds t o an average Iinear air velocity of about 80 cm. per sec. On the average, therefore, a given small portion of the air remains in contact with the gas absorbent for only about 0.1 sec. This is obviously a very brief interval in which t o remove toxic materials from the air. Furthermore, this removal of the toxic rnakrials must be surprisingly complete. Though the aoncentration entering the canister may occasionally (though very seldom) be as high as one-half or one per aent, even the momentary leakage of 0.001 per cent ( I O parts of gas per million of air) would cause serious discomfort, while the prolonged leakage of as much as 0.0001per cent (one part per million) would h a r e Iserious results in the case of many of the extraordkarily toxic gases used in modern warfare. It is evident, therefore, t h a t an absorbent OF w m bination of absorbents for use in a gas mask must be capable of reducing the concentration of gas from say 1000 p. p. m. t o I p. p. m. or less, within t h e 0.1 sec. This is, however, accomplished with a safe margin by t h e present gas mask materials. I n fact, it has been shown t h a t charcoal will reduce a concentration of 7000 parts of chlorpicrin (CClaNO2) per million of air in a rapidly moving current t o less t h a n 0.5 part per million in less than 0.03 sec. Nevertheless, a great many other absorbents, which give excellent sesults under normal conditions of low concentrations and ordinary rates of breathing, must be rejected on account of insufficient activity to meet high eoncentrations or high rates of breathing. I t is essential, therefore, t h a t any absorbent, t o be finally adopted for use in a gas mask, must have a very high rate of absorption or, as i t is commonly termed, a high degree of adsorptive activity. ABSORPTIVE

CAPACITY

Of almost equal importance, from a military point of view, is the absorptive c a p a c i t y of an absorbent. The difficulty of transporting large numbers of fresh canisters t o the front-line trenches makes it imperative t h a t an absorbent shall have a long life-indeed, one measured in months against ordinary concentrations GENERAL REQUIREMENTS FOR QAS MASK ABSORBENTS of gas-and in addition, be able t o survive occasional ABSORPTIVE ACTIVITY exposure for shorter lengths of time t o very high conAbsorptive activity, or a very high rate of absorption, centrations. The absorbent must, therefore, be able is one of the most important properties of a satisfactory t o absorb and hold large amounts of gas per unit weight gas mask absorbent. A normal man when exercising of absorbent. It is, of course, possible t o obtain 1 This article, one of a series of three on the army gas mask, is pubalmost any desired capacity by using very large conlished by permission of the Director of Chemical Warfare Service, tainers, but the need for conserving every cubic inch U. S. A. It is based on investigations conducted by the Research Division of that Service, t o which many different individuals have contributed. No of space and every ounce of weight in a soldier’s attempt has been made t o describe the details of the large-scale manu equipment makes i t imperative t o secure this large facturing processes, which will be fully covered in another article in THIS capacity with t h e smallest possible weight of absorbent JOURNAL by Major J. C Woodruff, of the Gas Defense Division, C. W. S.

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T H E J O U R N A L OF 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

material. The American canister, as a matter of fact, contained considerably less t h a n a pound of war gas mixture. Furthermore, in view of t h e fact t h a t even extremely small amounts of the war gases are dangerously toxic, it is necessary t h a t the gases be held firmly, and not in any loose combination which might give up even the minutest traces of gas when air is for long periods of time breathed in through a canister which has previously been exposed t o gas. This requirement rules gut many absorbents which seem t o have very good rapacity on high concentrations of gas, but which do not hold these gases with sufficient tenacity. This applim particularly t o the type of absorbents which hold the gas partly by e d s o r p t i o n a n d partly by condense$irn in extremely small capillaries, t h e latter form ef absorption being practically valueless for field use, although i t appears t o give very good results on high sencentration tests. VERSATILITY

It is obviously impracticable t o have more t h a n one type of canister filling for general military use. Any other arrangement would not only introduce complications and increase transport difficulties, b u t would also require a more rapid and certain identification of toxic gases t h a n is now available, or t h a n is reasonably t o be hoped for. Furthermore, there is always t h e possibility of meeting some new, and perhaps entirely unknown gas. It follows, therefore, t h a t B single t y p e of canister filling must not only present very high activity a n d capacity for t h e ordinary toxic gases now in general use, b u t i t must be of a t y p e which can be relied upon t o give protection agailast practically any kind of toxic gas. It is fortunately true, however, t h a t all gases which are highly toxic, and therefore, adapted for use in warfare, are either very reactive chemically, or else have relatively high boiling points a n d can therefore be adsorbed in large amounts by a material such as charcoal. MECHANICAL STRENGTH

A canister during transport over rough roads in motor trucks and in actual field use is subjected t o extremely rough handling a n d jolting. Gas mask absorbents must, therefore, be mechanically strong, in order t o retain their structure a n d porosity under such conditions. Not only must they be not easily crushed or deformed by continuous pressure, b u t neither must they be subject t o abrasion, with the production of any considerable amount of fines. The size of the granules, which, on account of other conditions, must be used in the gas mask, is such t h a t t h e production of a relatively small amount of fines would tend t o plug up t h e canister or cause serious channeling. CHEMICAL STABILITY

An absorbent canister is ordinarily filled several months before i t is first used in t h e trenches. Furthermore, air containing more or less gas is breathed through the canister for long periods of time, frequently extending over several months, before t h e canister is discarded or withdrawn from service,

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Obviously, therefore, i t is not t h e initial efficiency of a n absorbent which determines its value under actual service conditions, but rather t h e ultimate amount of activity and capacity for t h e absorption of poison gases, which will be retained after this long period of storage and exposure t o air of varying humidity, and temper? ure. T o give satisfactory service, therefore, an absorbent must possess a very considerable degree of chemical stability. I n t h e first place, i t must not be subject t o any considerable chemical deterioration or slow reaction proceeding within t h e absorbent itself. The two absorbents used in any mixture must, furthermore, have no tendency t o react with one another. I n order t o withstand t h e exposure t o air of varying a n d sometimes very high humidity, i t is essential t h a t t h e absorbent contain no hygroscopic or efflorescent material, nor one which is easily oxidized. TLe absorbent must not be seriously deteriorated by taking up carbon dioxide (which means, in general, t h a t it should not absorb very large amounts of this gas). I t must not disintegrate or become deliquescent even after gassing with any of t h e ordinary war gases. It must .also have no tendency t o corrode its metal container. It is obvious t h a t t h e above-mentioned requirements place very serious limitations on t h e materials which can be used with satisfaction in any gas mask absorbent. L O W BREATHING RESISTANCE

I n order t o obtain a high degree of activity, it is necessary t o have t h e absorbent present t h e largest possible surface t o the gas, and t o have t h e distance through which t h e gas must diffuse t o reach t h e absorbent as small as possible. A finely granular absorbent gives extremely good results in these respects. On the other hand, troops, particularly those in action, must be able t o breathe freely and deeply. T h e size a n d arrangement of t h e absorbent granules must accordingly be such t h a t t h e resistance t o the passage of air is very small, as otherwise t h e soldier will be forced either t o cease active movement or remove his mask. From the standpoint of breathing resistance, therefore, it is important t o use granules of a relatively large size. This again places a fresh burden upon t h e activity of t h e chemical absorbent. It also requires t h a t t h e granules be sufficiently porous to permit the penetration of gas well into t h e interior so t h a t their capacity may be fairly well utilized. T h e necessity for having low breathing resistance is also the basic reason for the insistence upon the two previously mentioned requirements, namely, t h e mechanical strength of t h e absorbent and its non-deliquescent properties, since both of these would tend t o cause a marked increase in the breathing resistance. Very elaborate investigations have been made t o determine t h e most desirable size of granules, depth of layer, a n d area of cross section in order to strike the best possible balance between low breathing resistance a n d good all-round absorptive efficiency. These investigations have tended t o show, in general, t h a t the use of large cross-sectional areas of' relatively fine

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granules gives the best all-round results. Such investigations have also indicated t h a t where two absorbents, such as charcoal and soda-lime, are used, as in t h e present war gas mixture, i t is better t o mix t h e components rather than t o place them in separate layers. This obviously simplifies t h e problem of canister filling. E A S E O F MANUFACTURE, CHEAPNESS AND AVAILABILITY O F R A W MATERIALS

The need for adequate protection against war gases is so transcendent t h a t no difficulty would be too great nor any price too dear t o provide such protection. On the other hand, t h e scale of military requirements in our modern armies is so colossal t h a t an expensive absorbent would represent a severe economic strain. Furthermore, the setting up of a very complicated plant with a great deal of special machinery, under t h e stress of a war emergency, is a difficult and time-consuming task. Cheapness a n d ease of manufacture are therefore important considerations which must be given due weight in deciding upon an absorbent for war gases. The availability of the raw materials, without putting too much drain on any other essential war industry, is naturally a n absolutely essential prerequisite t o the choice of any absorbent. P R O P E R BALANCE

While a single type of canister filling is essential for military use, it is impossible t o combine in any single absorbent or combination of absorbents the best solution of each of t h e above requirements. For example, it is only possible t o obtain adequate mechanical strength in t h e soda-lime absorbent by the use of certain binding agents which militate markedly against both t h e activity and t h e capacity of t h e resulting absorbent. T h e most active absorbents are also in general too unstable, or have a tendency t o t a k e up carbon dioxide or t o deliquesce in the presence of moisture. Certain desirable components for use in absorbents are also too expensive or too difficult t o obtain t o be recommended for such purposes. It is obvious, therefore, t h a t in a good general absorbent there must be t h e proper balance between its various essential qualities. Accordingly, t h e present war gas mixture is a t best a compromise. It nevertheless fulfills these requirements t o a much greater degree t h a n would previously have been considered possible. It should be emphasized, however, that t h e best balance of these factors for military purposes is obviously not t h e one which *will give t h e best results for industrial uses. CHARCOAL ADVANTAGES AS A N ABSORBENT

The only single substance which even approximately fulfills all t h e above requirements is charcoal. This absorbent, when properly prepared, has almost unique qualifications for this purpose as will appear from t h e following summary of its characteristics. I t s absorptive activity is extremely high. Active charcoal a t t h e temperature of liquid air will bring a hundred times its own volume of air a t atmospheric pressure down t o a pressure of a few millimeters of

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mercury in a few seconds. Similarly, when air containing an easily absorbed gas is passed over the charcoal, t h e velocity of adsorption is apparently limited only by t h e velocity of t h e diffusion of the gas. I t s absorptive capacity is very great. Charcoal has been made on a large scale, which will absorb about half of its own weight of certain toxic gases, while samples have been prepared in the laboratory which will absorb more t h a n their own weight. I t s mechanical strength, a t least when prepared by proper treatment from suitable raw materials, is very great. I t s chemical stability is practically complete. Caref u l investigations have indeed shown t h a t a very slow oxidation of the charcoal by the air is continually proceeding, but this produces no measurable effect on the absorptive value or apparent density of t h e charcoa.1 over a period of several months. Very small amounts of carbon dioxide are taken up from the air, but produce no harmful effect. The adsorption of water from moist air causes a considerable decrease in its absorptive capacity for many other gases, but even this cannot be said t o be due to any chemical instability. Charcoal is reasonably cheap and not particularly difficult t o manufacture after t h e development stage has been passed. I t s raw materials are cheap and accessible, though the quantities required a n d t h e scarcity of shipping caused some difficulty during the war. Its versatility is pronounced; indeed, since all gases and vapors are somewhat adsorbed by charcoal, i t is a universal absorbent. Permanent gases, t o be sure, are only slightly adsorbed, but none of them is highly toxic. A majority of the so-called war gases are really liquids boiling above ordinary temperatures, and all these are very firmly held b y charcoal. Only in t h e case of a relatively few, very volatile, toxic gases, such as arsine and cyanogen chloride, does charcoal possess inadequate adsorptive capacity. ADSORPTION B Y CHARCOAL D E T E R M I N I N G ADSORPTIOX-In discussing the properties of charcoal, i t is necessary t o distinguish carefully between charcoals of different degrees of “activation.” I n determining t h e adsorptive capacity o€ a sample of charcoal, i t is necessary t o choose a gas which is rather inactive chemically, and which is therefore held b y adsorption only. T h e standard test used for rating charcoals was known as t h e accelerated chlorpicrin test, and consisted essentially in passing a stream of d r y air, containing 7 0 0 0 p. p. m. of chlorpicrin, a t the rate of 1000 cc. per sq. cm. through a I O cm. layer of charcoal until t h e first trace began t o come through. The minutes t o breakdown measured t h e value of the charcoal, and was customarily referred t o as the service time of the charcoal. No pre-war material in the country lasted over 1 5 sec. on this test. The regular plant product during 1918 averaged about 18 min.; the last z months‘ operation gave material averaging 35 min. Largescale units have frequently produced so-min. material, and t h e Research Division has made material in METHODS

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the laboratory running as long as 7 0 min. on t h e foregoing test. Tests against other gases are generally conducted at a rate of flow of 500 co. per sq. cm. cross section, I O cm. layer, and concentrations varying widely so as t o give a reasonable length of test time. T h e foregoing type of tests is very valuable and practical, in t h a t it is rapid, reasonably accurate, and approximates fairly closely (except for t h e high concentrations) actual field conditions. Such tests do not, however, separate the factors of activity and capacity, o r represent any very definite physical constant of t h e material, since equilibrium is by no means obtained. This is especially t r u e if t h e charcoal contains moisture. I n such accelerated tests, t h e first or upper portion of the absorbent is generally more t h a n saturated, the middle portion partially saturated, and the lower portion contains very little gas when t h e breakdown occurs. The length of these various portions depends, of course, on the activity or rate of adsorption of t h e absorbent. The test does, however, weight activity a n d capacity in about t h e proper ratio for actual service conditions. Where investigations are made of the effect of pressure or other factors on adsorption, t h e test is usually conducted in an evacuated chamber where only t h e gas being studied is present, and pressures and weights absorbed are carefully determined. VARIATION WITH KIND OF GAS-AS indicated previously, t h e adsorption of gases on charcoal, a n d indeed on all substances, is roughly parallel t o t h e case with which they are condensed t o t h e liquid state. I n other words, their adsorption decreases with their boiling points, or probably more vigorously with their critical temperatures. This is shown b y the figures in the following table: TABLE1-ADSORPTIONOF GASESAT O o BOILING POINT GAS Deg. C. Helium... -268 Hydrogen.. -253 Nitrogen. -196 Carbon monoxide. -190 Oxygen.. -183 Argon -186 Carbon dioxide. -78 Ammonia -33.5

....... ...... ........

........ ........... ..

........

CHARCOAL GAS ADSORBED CC. PER G.CHARCOAL CRITICAL Wood CharCoconut TEMPERATURE coal at Charcoal Deg. C. 10 cm. at 76 cm. -267 2 -241 0.3 4 -149 1.5 15 -1 36 21 -119 2 18 -117 ... 12 4-131 20 f130 50

less of t h e adsorbing agent, nevertheless specific chemical forces between the charcoal and the gas play no small part in the action of the adsorbent. T h e service time of a variety of different charcoals against all t h e commoner war gases is shown later in Table 5. VARIATION

WITH THE CONCENTRATION

OF T H E G A S

-Adsorption represents a balanced condition between t h e free gas space and t h e surface film. The higher the concentration of t h e free gas, t h a t is, the higher the gas pressure, the greater will be the amount adsorbed. T h e amount of adsorption is not, however, strictly proportional t o the pressure; t h e relationship is shown clearly b y means of the typical Curve ( a ) , Fig. I, for t h e adsorption of a gas by charcoal a t constant temperature.

ON

...

...

..

Carbon monoxide with a critical temperature about higher t h a n hydrogen is adsorbed about j times as much on t h e same charcoal; ammonia with its critical temperature 270' higher still is adsorbed about zoo times as much. This parallelism is not, however, perfect; argon, for example, is adsorbed much less t h a n would be expected from its boiling point and critical temperature. This divergence is unquestionably due t o t h e extreme weakness of its s t r a y fields of force. Also, substances with a relatively high critical temperature a n d which are highly adsorbed, when compared among themselves, show great irregularities in this respect, t h e more reactive gases generally being held the more firmly. These facts indicate t h a t although variation in t h e forces of attraction between t h e gas molecules themselves is t h e main factor in causing differences in the adsorption of different gases, regard100'

4 23

FIG. I-VARIATION

OF

ABSORPTION OF A GASWITH ITS CONCENTRATION CONSTANT TEMPERATURE

AT

Each p a r t of t h e foregoing curve may unquestionably be referred back t o certain physical relationships which occur as larger a n d larger amounts of gas are absorbed. Thus the lower p a r t of t h e curve, where a considerable amount of gas is absorbed with practically zero vapor pressure, very probably represents t h e amount of gas adsorbed in a layer one molecule deep covering t h e active surface of t h e carbon. This p a r t of the gas is held extremely firm and is unquestionably the most valuable p a r t of the absorption curve. A great difference between charcoal and metallic oxide a n d gel-like absorbents lies in t h e fact t h a t this first layer of molecules is held with such extraordinary tenacity, whereas in t h e case of most other absorbents, even t h e first layer of molecules exerts appreciable vapor tension. T h a t p a r t of t h e curve which begins t o round off


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away from t h e zero pressure axis very probably represents t h e amount of gas which is held by t h e less firm a n d more variable adsorption of molecules in t h e second and third layers. T h e tenacity of this adsorption depends t o a considerable extent upon the curvature of t h e carbon surface of t h e capillaries. Thus, if t h e curvature is very sharp, a molecule might be held rather firmly even on a second-layer film, being partially held by t h e attraction of t h e carbon and gas molecules on either side. This second a n d third layer adsorption, however, passes indistinguishably into a third kind of absorption, which, in general, exerts still higher vapor pressures. This more loosely held gas is not really adsorbed but held merely by condensation in t h e fine capillaries, where, due t o a surface tension effect, it exerts a very much lower vapor pressure t h a n does t h e free surface:of t h e liquid. As the capillaries become larger and larger, t h e vapor pressure also increases. If t h e absorption curve shows a n even upward slope free from points of inflection with increasing pressure, i t indicates t h e progressive filling up of larger a n d larger capillaries. On t h e other hand, a sharp upward slope followed b y a horizontal one indicates t h a t most of t h e capillary pores have a fairly uniform definite size a n d t h a t condensation in them proceeds with scarcely any rise in pressure until all of this particular size of pore has been filled. T h e majority of charcoals exhibit t h e former type of curve, which can be explained only by t h e assumption of gradation in the sizes of t h e capillaries. As t h e curve approaches t h e value of t h e vapor pressure representing saturation, i t invariably shows another sharp rise which is unquestionably filling up t h e larger capillaries which have a very considerable volume, b u t whose effect in lowering vapor pressure is so small t h a t they do n o t condense any gas until saturation is practically reached. Such absorption has, of course, absolutely no practical value in t h e field, where t h e concentrations are low. V A R I A T I O N W I T H THE TEMPERATURE-Adsorption invariably decreases with rising temperature. This temperature effect is slight a t higher pressures where t h e adsorption is nearly independent of t h e pressure, b u t much more pronounced a t lower pressures. T h e variation of adsorption with temperature also increases as t h e boiling point of t h e gas is approached. T h e effect of temperature on the efficiency of charcoal in gas masks, where low pressures are of special concern is, therefore, decidedly important. This is illustrated by t h e results in Table 2 and Fig. I1 which show t h e service time of sample of charcoal against chlorpicrin and cyanogen chloride a t different temperatures. TABLE Z-EFaECT TBMPELUTURE Deg. C. 0 15 25

40

OF

.

TEMPERATURE ON ADSORPTION SERVICE TIME, MINUTES

Chlorpicrin 7000 P . P . ~ . 65 59 53

47

Cyanogen Chloride 1000 P. D. m.

..

87

55

38

It is evident t h a t against chlorpicrin t h e same charcoal is nearly 5 0 per cent more efficient a t o o

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t h a n a t 40'. I n t h e case of cyanogen chloride, which boils much lower (IS" insteadof 112' C.), t h e effect of temperature is very much more marked, t h e service time more t h a n doubling in going from 40' t o 15'. It is, therefore, apparent that, other things being equal, offensive gas warfare can be more effectively waged in summer. Also gas concentrations are on t h e average higher during the summer, due t o their greater rate of vaporization.

Temperature FIG.

11-EFFECT OF TEMPERATURE O N SERVICE T

~ TO E 99 PER CENT AT 0 . 7 P E R CENT CONCENTRATION O B DRYAIR AND 0.1 P E R CENT CONCENTRATION O F C Y A N O G e N CHLORIDE. AIR-GASwrrn 5 0 PERCENT RELATIVE

EFPICIWNCY O F CHLORPICRIN

HUMIDITY

ADSORPTION OF MIXTURES O F GASES-A gas or vapor, when passed over charcoal on which another gas or vapor is adsorbed, will displace a portion of it; a gas which, by itself, is strongly adsorbed on charcoal, will in time displace a less strongly adsorbed gas almost completely. I n general, then, a high boiling gas will displace a lower boiling gas which has previously been adsorbed. If a mixture of gases is passed over charcoal, a final equilibrium condition is established, depending upon the partial pressures of the two gases which are maintained. Substantially t h e same equilibrium is obtained under t h e same conditions, no matter which gas was t h e first adsorbed. It follows from these considerations t h a t t h e adsorption of a gaseous mixture will, in general, be less complete t h a n t h a t of either constituent of t h e mixt u r e b y itself. I n other words, charcoal would have a certain approximately fixed adsorptive capacity, a n d if p a r t of this were utilized for one gas, less would be available for another.

Slay, 1 9 1 9


Another factor, however, certainly comes into play. If either or both t h e vapors is soluble in t h e liquid phase of t h e other, charcoal will adsorb more of the solution t h a n it will of either component separately. If either or both of t h e vapors is v e r y soluble in t h e liquid phase of the other, then a n adsorbed film of one vapor may adsorb t h e vapor of the other even more extensively t h a n t h e fresh charcoal surface would adsorb i t ; in other words, t h e vapor pressure of one or both substances may be lower from the film of adsorbed solution than from a n adsorbed film containing the same fractional amount per square centimeter of the pure solute or solvent. Tests of the service time of charcoal confirm the above conclusions. Thus, stannic chloride, for instance, if mixed in equivalent amounts with chlorpicrin, will decrease t h e service time against chlorpicrin by about 50 per cent; chlorine and also water similarly 'hasten the breakdown of charcoal toward chlorpicrin. These substances all show only a moderate or small mutual solubility. Admixture of 1/5 a n equivalent amount of cyanogen chloride with an air-chlorpicrin mixture has b u t little effect on the breakdown of charcoal against chlorpicrin, though it shortens decidedly the life of the charcoal t o the 90 per cent point. This illustrates the fact t h a t i t is t h e capacity rather than the activity of the charcoal which is affected by this treatment. On t h e other hand, t h e presence of small amounts of water in charcoal actually inzreases its absorptive power for ammonia or for hydrogen chloride, while small amounts of ammonia will similarly increase t h e service time of charcoal against arsine. I n these cases we have great mutual solubility, and, therefore, enhanced adsorption. EFFECT O F M O I S T U R E O N ADSORPTION-Moisture is more highly adsorbed a t ordin,zry temperatures by charcoal t h a n is a n y other of the normal constituents of the air. On t h e other hand, it is much less adsorbed t h a n such toxic gases as chorpicrin for instance. Thus an active charcoal which mdl adsorb and retain z j per cent of its own weight of chlorpicrin a t zoo, will adsorb a n approximately equal amount of water if it is exposed t o saturated water vapor a t t h e same temperature, but it holds this water very feebly, losing all b u t a per cent or two of it when it is exposed t o dry air for some time. Charcoal in the canister will, when in use, evidently adjust itself t o the humidity of t h e air drawn through it. As this humidity is usually high under t h e climatic conditions prevailing in northern France, i t will, after prolonged exposure, contain perhaps I O per cent of water, the exact amount, of course, depending upon its own degree of activation, as well as upon t h e prevailing humidity. I n accordance with what was stated in a previous section regarding adsorption from mixtures, this adsorbed moisture will diminish somewhat the adsorptive efficiency of charcoal against other gases, except those gases which are very soluble in water, and gases such as phosgene, which react with water in the presence of charcoal. This decrease, however, is not in general serious, for water, compared with most of t h e

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toxic gases, is relatively but slightly adsorbed, and is easily displaced by gases of higher boiling point. This is illustrated by t h e curve in Fig. 111, showing t h e effect of humidity on t h e adsorption of chlorpicrin b y a rather poor sample of charcoal a t z o o .

Percent Relative Humidity FIG.111-EFFECT OF BRATED

HUMIDITY OF GAS-AIRON SERVICE TIMEOR EQUILICHARCOAL AGAINST CHLORPICRIN. CONCENTRATION, 0.7 P E R CENT. D E P T H OF LAYER, 10 CM.

The results shown in this diagram were obtained by conducting the standard 0.1 per cent chorpicrin test with an air of varying humidities against charcoal which had previously been equilibrated with pure air of the same humidity as t h a t used in t h e test. T h e water content of the charcoal, therefore, increased roughly in proportion t o the humidity. The line is brought down t o zero service a t I O O per cent humidity, because any charcoal which is actually wet with more water than it will take up from saturated air shows a n immediate break on this test. I n this case, the large as well as the small capillaries are filled with moisture, and the chlorpicrin cannot get in t o displace t h e water, except very slowly. Adsorbed moisture has a similar and even more pronounced effect upon t h e adsorption of cyanogen chloride. A sample of charcoal which, when dry, ran 1 2 0 min. t o the break against this gas, lasted b u t 69 min. after equilibration with air of 7 0 per cent relative humidity. This is what would be expected, for cyanogen chloride, being less firmly adsorbed t h a n chlorpicrin, would less readily displace the water film. The effect of moisture on t h e adsorption of phosgene by charcoal is more complicated. Dry charcoal adsorbs phosgene freely and constitutes a satisfactory absorbent for it, provided the air-phosgene mixture t o be purified is also dry. If, however, the mixture is partially humidified, t h e apparent service time of the dry charcoal drops t o one-third of its former value. Careful tests indicate, however, t h a t the gas which


426

comes through under such circumstances is hydrogen chloride and t h a t t h e decrease in service time is due primarily not t o any decreased adsorption of phosgene, but to t h e catalytic action of charcoal on t h e hydrolysis of phosgene. This and similar special cases are, therefore, discussed under the later heading, “Chemical Action of Charcoal.” VARIATION WITH THE K I N D OF CHARCOAL-The same charcoal activated t o different degrees will show similar but somewhat displaced adsorption curves. Curve ( b ) , Fig. I, for instance, represents a less highly activated sample of t h e same coconut charcoal as was used in obtaining Curve (a). With a charcoal of different origin and composition, t h e form of t h e adsorption curve may be quite different, as is illustrated b y Curve (c), which represents t h e adsorption curve of wood charcoal. The wood charcoal used for Curve (c) was evidently a better adsorbent at high pressures than t h e coconut charcoal, but for use under war conditions would be inferior t o it, for a t low pressures, which are then of primary importance, i t is a distinctly poorer absorbent. The effect of the kind of charcoal is further discussed in connection with t h e structure of active charcoal.

Vol.

11,

No. 5

Charcoal containing moisture lasts somewhat longer t h a n dry charcoal against moist air-phosgene mixtures (but is still far inferior t o dry charcoal against dry air-phosgene mixtures). The reason for this increase is t h a t t h e hydrogen chloride gas is very soluble in t h e water held in t h e charcoal. The same effect would have been produced in t h e above-mentioned case of dry charcoal against moist air-phosgene mixtures had t h e test lasted long enough t o permit t h e absorption of considerable amounts of t h e water vapor b y t h e charcoal.

CHEMICAL ACTION O F CHARCOAL

I n addition t o its adsorptive action, charcoal acts catalytically upon a number of reactions t o which certain war gases are subject. The most conspicuous instance of this effect is its catalytic action upon t h e hydrolysis of phosgene into hydrogen chloride and carbon dioxide. Superpalite (trichlormethylchlorformate) behaves in almost identically t h e same way as phosgene with regard t o its hydrolysis into hydrogen chloride and carbon dioxide. Certain other hydrolyzable gases also show evidence of an accelerated hydrolysis. Some similar reaction appears during the absorption of chlorine by moist charcoal, in which case hydrogen chloride is formed and oxygen is evolved. Arsine is t o some extent catalytically oxidized by t h e charcoal, although this suffices t o take care of only a fraction of t h e pas. I n fact, charcoal has such a Dronounced chemical action on most of the active toxic gases t h a t there are relatively few of t h e war gases with regard t o which it can be stated with certainty t h a t they are held by adsorption alone, and t h a t accelerated chemical reactions play no part in their absorption. The importance and characteristics of t h e chemical action of charcoal can best be brought out by t h e de‘tailed discussion of a specific case-the hydrolysis of phosgene. As previously pointed out, in t h e entire absence of moisture phosgene is fairly well absorbed. When, however, any moisture is present, either in t h e air or in the charcoal, the break point comes much sooner, though hydrogen chloride rather t h a n phosgene comes through at first, indicating t h a t t h e hydrolysis of t h e phosgene has been greatly accelerated by t h e charcoal. The catalysis undoubtedly proceeds by t h e simultaneous absorption of phosgene and water vapor on the charcoal surfaces. The hydrogen chloride gas thus formed is only very slightly adsorbed b y dry charcoal and, therefore, comes through very rapidly.

-

Percent H,O FIG.IV-EFFECT

in Charcoal

MOISTURE CONTENT OF C H A R C O A L ON SERVICE TIME AGAINST PHOSGENE 1 PER CENT CONCENTRATION, 10 CM. LAYER OF

The behavior of a typical sample of charcoal against phosgene is shown in Fig. IV. The point on t h e ordinate axis indicates t h e very high service time of dry charcoal against dry air mixtures. The points on t h e curve indicate t h e service time of charcoal with varying percentages of moisture on phosgene-air mixtures of 5 0 per cent humidity. It will be noted t h a t these conditions are somewhat different from those prevailing in the above-mentioned chlorpicrin tests. I n this case, t h e humidity is constant during t h e test, while t h e original water content varies. The general effect is t h e same, however, since t h e test time is much too short t o reach equilibrium. As would be expected from t h e foregoing explanation, t h e service time for t h e moist air mixture against perfectly dry charcoal is very low, b u t increases rapidly with increasing water content, due t o t h e tendency of t h e water t o hold t h e hydrogen chloride formed by t h e hydrolysis of t h e phosgene. I n the case of all the

May, 1919


foregoing points on t h e curve, t h e gas which comes through at t h e breakpoint a n d for some time thereafter is hydrogen chloride only. When very high water contents are reached, however, another striking change takes place in the service t’me curve, in t h a t i t rapidly drops off and soon reaches zero. Furthermore, beyond the maximum of the curve t h e gas which comes through first is always found t o be phosgene. The effect is unquestionably due t o t h e fact t h a t by t h e time these high water contents are reached practically all t h e capillaries are filled up with water, and t h e simultaneous absorption of phosgene and water vapor on carbon surfaces, which causes t h e hydrolysis, is thereby very greatly slowed up. Part of t h e phosgene is unquestionably still hydrolyzed. The hydrogen chloride thus produced is easily held b y the large amount of water present. A considerable amount of unhydrolyzed phosgene comes through, however, and gives t h e earlier break. As t h e water c m t e n t goes still higher, the charcoal becomes, in effect, merely an inert material covered entirely with a film of water and in this case phosgene comes through from almost t h e first moment, since i t is impossible t o hydrolyze it rapidly enough on any such relatively small amount of water surface. It should be stated t h a t the higher water contents (those beyond t h e maximum part of the curve) cannot be obtained by passing even 98 per cent saturated air through t h e charcoal, b u t only by adding water as such t o t h e charcoal. This part of the curve is, tlierefore, of theoretical rather t h a n practical interest as affecting field service conditions. The behavior of charcoal against chlorine and superpalite is very similar t o t h a t described for phosgene. I n t h e case of gases where chemical reactions play a large. part in their absorption, two charcoals which differ very greatly in their true adsorptive capacity (as measured b y their chorpicrin service time) show much less difference in their service time against these gases. I n general, however, the charcoals which are best against chlorpicrin are a t least slightly better against practically all gases. BEHAVIOR O F CHARCOAL I N T H E CANISTER

I n t h e light of t h e foregoing discussion, t h e behavior of charcoal in a gas mask canister can be readily explained. The air entering a t t h e bottom may be assumed t o contain, under average battlefield conditions, perhaps one part of toxic gas per 1000 parts of air. When t h e wearer is moderately active, his inhalation will be a t the rate of about 30 liters per min., or with a canister of normal dimensions about 500 cc. per sq. cm. of cross section. Under these conditions, ordinarily active charcoal will adsorb practically all the war gases very rapidly and very compIetely, so t h a t only the lower layers are a t first operative. If the supply of the toxic gas continues, the zone of adsorption moves upward, and, of course, t h e toxic gas will finally begin t o escape from t h e upper layers in detectable quantities. I n t h e case of most gases the amount leaking through t h e canister increases very slowly from the time t h e first trace appears until I or 2 per cent of the affluent gas concentration begins

427

t o pass through. Just when t h e apparent breakdown occurs will depend on the sensitiveness of t h e tests used for the toxic gas. Tests are available however, sufficiently sensitive t o detect amounts considerably below the toxic or harmful limit of practically every gas. The layer of charcoal in the bottom of the canister becomes highly saturated with toxic gas before the breakdown occurs. There is, however, no great advantage in having a charcoal which holds a great deal of gas loosely in this way (that is, a t a high equilibrium pressure), because as was pointed out previously, even if the breakdown during exposure t o toxic gas is delayed on this account, this gas will be rapidly passed along in t h e canister even when pure air alone is breathed through it. STRUCTURE O F ACTIVE CHARCQAL

Since t h e functioning of charcoal depends primarily upon its adsorptive power, t h e greater the ratio of its surface t o its mass, t h a t is, the more highly developed and fine-grained its porosity, the greater its value. Since increase in porosity involves t h e removal of material from the interior of the charcoal, t h e activation of charcoal is necessarily accompanied by a decrease in its apparent density; and starting with any given inactive charcoal, the increase in adsorptive capacity is approximately parallel t o this decrease. The porosity of t h e most active charcoals is very great. By a comparison of their true and apparent densities, it has been found t h a t about one-half t h e volume of such charcoal consists of capillary pores. This does not, however, include t h e grosser pores which must be relatively inactive in adsorption, though they are necessary t o serve as passages t o t h e interior of the charcoal grains. Most of t h e pores are extremely minute. From a study of the slope of the vapor pressure curves of liquids adsorbed upon such charcoal, the indications are t h a t t h e pores have, if a cylindrical form be assumed, an average diameter of about 5 X I O cm. On this basis, I cc. of active charcoal would contain about 1000 sq. m. of surface. The amount of surface exposed by charcoal does not, however, appear t o be the only determining factor, for it has been found t h a t active charcoal can be obtained only from amorphous carbon formed or deposited a t relatively low temperatures by thermal or chemical decomposition. Carbon deposited at high temperatures, presumably more graphite in nature, will not become activated. This fact might conceivably be explained by assuming t h a t t h e graphitic carbon does not become so pitted and porous during the activation process as does the low temperature amorphous carbon. I t seems much more reasonable, however, t o assume t h a t f h e amorphous carbon has a greater specific adsorptive power t h a n the graphitic carbon. I n strong support of this latter explanation is the fact t h a t a n active amorphous carbon can be ruined for adsorptive purposes by the deposition of a thin film of graphitic carbon on its surface. The carbon formed or deposited by t h e destructive

’

~I-~WCT UP~IARBUNL?.BD ON Cocohllir SHBL~.MI\oLrPrPD 14h'/a DrAlliiTzXs

low temperatures (as in the charring of woods, or thc distillation of coal-tar, pitches, etc ), is spoken of as L'primarycarbon." This primary carbon has little or no apparent adsorptive power. It is evidently a complex materiai consisting of hydroca of active carbon These hydr

charged into the retorts in thin layers, so that the contact of the hydrocarbon vapors with hot charcoal is avoided as much as possihie. Furthermore, most of t h e hydrocarbon 12 removed before dangerous tcm-

oduce an inactive

Th? essential char then are. i--It must have hi Z-Tt must consist of amorphous base carbon. 3-It must be free from adsorbed h&ocarbons PREPARATION

Or' ACTIVI: CIIARCOAT,

On the basts of the above discussion, the preparation of active charcoal will evidently involve two steps: First-The formation of a porous, amorphous base carbon a t a relatively low temperature Second-The removal of t h e adsorbed hydrocar. bons from the primary carbon. and the increase of its porosity. The first step presents no ve It involves, in the case of woods a piocess of destructive distilla temperatures. The deposition resulting from the cracking of temperatures, must be avoided

highly resistant t o oxidation

and distillation, whereby the hydrocarbons of high broken down into more volatile substdnces emoved a t lower temperatures, or tinder s less likely t o result in subsequent tive carbon. Thin layers of charcurrents are used so t h a t contact ilized hydrocarbons and the bot active charcoal may be aS brief as pOSSl way cracking of the hydrocarbons at h

hfay.

1919

T H E J 0 U K N . A L OF I N U U S I ' K I A L A N D E N G I N E E R I N G C H E M I S T R Y

rcoal of the highest activity cannot he air activation process css has the disadvantage

already present in the charcoal, thus total surface exposed. Moreover, th the capiiiary pores and fissures mu vided to the inner portions of the charcoal. ever, as soon as the eating away of the carbon wall begins to unite cavities. it decreases, rather than increases, the surface of the charcoal, and a consequent drop in volume activity, that is in the service time, of the charcoal, i s found to result. I t is obvious, therefore, that condition' must he so chosen and regulated hydrocarbons rapidly and the prim Such a differential oxidation is not to secure since the hydrocarbons involved have a very low hydrogen content. and are not much more easiiy oxidized than t,he primary carbon itself. Furthermore, most of the hydrocarbons to he removed are shut up in the interior of t h e granule. On the one hand, a high enough temperature must be maintained t o oxidize thc hydrocarbons with reasonable speed; on the other hand, too high a temperature must not bc employed, else the primary carbon will be unduly consumed. The permissible range is a relatively narrow one, only about j o to 7 j '. The location of the optimum activating temperature depends upon the oxidizing agent employed and upon other variables as well; :tor air, it has been found t o lie somewhere between 3 j o and 4j0°, and for steam b oo and 1000~. The air activation process e advantage of operating a t a conveniently low temperature. It has the disadvantage, since the oxidation of the hydrocarbons and the primary carbon by oxygen is an exothermic process, that local heating and an excessive consumption of primary carbon occur, so that a drop i n volume activity results from that cause before the hydrocarbons have been completely eliminated. As

429

advantage t h a t since the reaction between carbon a

. .

t

This Der

nremium unon initial density of a primary carbo preponderance of very fin tively large amount of pr this denser material will

n

densitv

in surface hegins admit of a mor bons carbon dioxide-steam actiThe air, s t vation orocesses ha moloved on a laree scale by the Chemical Warfare Service for the manufacture of gas mask carbon.

ise to the long axis of t h e

. VI

i s a photograph of a the same coconut shell a

r voids, and de

430

*


black, while the charcoal itself appears white. It is clear from this photograph t h a t much of the original grosser structure of t h e shell persists in t h e carbonized products. Figs. VI1 and VI11 are more highly magnified photographs of a carbonized charcoal before and after activation, respectively. As before, all the dark areas represent voids of little or no importance in t h e adsorptive activity of t h e charcoal, while t h e white areas represent t h e charcoal itself. I n Fig. VI1 (unactivated) t h e charcoal itserf between t h e voids is seen t o be relatively compact, while in Fig. VI11 (activated) it is decidedly granular. This granular structure, just visible a t this high magnification (1000 diameters), probably represents t h e grosser porous structure on which t h e adsorption really depends. These photographs, therefore, show how the porosity is increased by activation. Another more recently developed method of activation should be mentioned as having distinct promise. The original wood or nut cellulose structure consists essentially of carbon, oxygen, and hydrogen, the latter two being in t h e proportion t o form water. The distillation method of making a primary carbon has t h e disadvantage of driving off carbon along with both the hydrogen and oxygen (as carbon monoxide, carbon dioxide, hydrocarbons, etc.), thus greatly reducing the density of the product. Furthermore, a considerable amount of nonvolatile hydrocarbon remains which must be removed by one of t h e high temperature activation processes. It has been found, however, t h a t by certain chemical processes, such as heating t h e wood up with strong sulfuric acid, hydrochloric acid and zinc chloride, etc., i t is possible t o remove practically all of t h e hydrogen and oxygen as water, leaving behind a very dense carbon which, since there is practically no hydrocarbon present, needs relatively little treatment (except washing out t h e chemicals) t o make a satisfactory charcoal. The sulfuric acid method has been successfully used in England, the hydrochloric acid and zinc chloride in Germany, both employing wood as a raw material. The resulting product is distinc$ly better t h a n charcoals made from similar woods b y t h e regular activation process, but by no means as good as coconut charcoal made by the regular process. The application of t h e chemical treatment t o coconuts, etc., has not as yet been developed t o give entirely satisfactory results, and all American production charcoal has, therefore, been made by t h e other methods. COMPARISON

OF

CHARCOALS SOURCES

PROM

DIBFERENT

A great number of charcoals have been prepared in the laboratory from the most varied raw materials. The results obtained with representative samples of a few typical materials are collected in Table 3. I n all cases, t h e method of activation has been identical and the times of treatment have been approximately those giving t h e highest service time. The results against chlorpicrin, therefore, represent roughly the relative excellence of t h e charcoals ob-

Vol.

11,

No. 5

tainable from these various raw materials, using this method of activation. Too much significance must not, however, be attached t o t h e precise figures or t h e exact order of excellence thus established, as a more careful regulation and adjustment of t h e activation treatment might easily have altered t h e service times by perhaps I j per cent. TABLE3-COMPARISON

VARIOUS ACTIVE CHARCOALS ACTIVATED IN LABORATORY STEAM ACCELERATED TREATMENTCHLORPICRIN AT 900' TESTRESULTS APPARENT DENSITY Weight Weight Service Primary Activated Time Loss Absorbed Time BASE MATERIAL Carbon Carbon Min. Per cent Per cent Min. 18 Sycamore.. 0.158 41 7.3 60 Cedar.. 0.223 16.0 78 60 Mountain mahogany 0.420 16.3 32 60 20.8 31 Ironwood.. 0.465 120 32.2 46 Brazil nut.. 0.520 120 47.0 48 Ivory n u t . . 0.700 120 53.4 51 Cohune nut. 0.659 210 58.7 Babassu n u t . . 0.540 85 120 58.4 61 Coconut.. 0.710 180 64.4 72 Coconut.. 0.710 BRiQUETTED MATERIALS Sawdust 0.542 0.365 120 66 53 40.0 Carbon black 0.769 0.444 240 64.3 53 50.5 Bituminous coal 0.789 0.430 165 61 58.3 46.8 0.371 480 81 53 40.7 Anthracite coal. 0.830 OP

......... ............ ......... ........ ........ ........ ......

.......... ......... ............ ....... ..... ....

These figures illustrate clearly t h e points brought out in t h e above discussion of the method of preparation of active charcoal. The dense primary carbons permit of a more extensive activation treatment before their carbon is too extensively eaten away, as would be disclosed by t h e figure for t h e apparent density. The hydrocarbons can, therefore, be more completely removed. The results indicate clearly t h a t t h e higher t h e original density of t h e wood, t h e longer the time required for proper activation, b u t t h e latter the final adsorptive capacity, especially on a volume basis (service time.) Anthracite and bituminous coals contain considerable amounts of refractory hydrocarbons, and very few large pores t o permit access of t h e oxidizing agent. They require very long activation treatment, with at least some consequent deposition of inactive carbon, and, hence, a lessened final activity. Nuts, with their high initial density and relatively easy activation, give t h e highest service time. Artificially dense primary carbons may be made b y briquetting almost any carbonaceous material with a suitable hydrocarbon binder. Sawdust, charcoal fines, powdered anthracite and bituminous coal, and petroleum black have all been converted into reasonably good absorptive charcoal by this means. These charcoals are more difficult t o activate than woods or nuts but easier t h a n plain anthracite or bituminous coals, as the burning away of t h e hydrocarbon binder provides an adequate amount of the larger voids, which afford ready access for t h e oxidizing agent t o the interior of t h e granules. None of t h e briquetted materials has yielded as high maximum capacity as is possessed by charcoal made from the denser nuts. I n conclusion, it will be of interest t o compare t h e charcoals manufactured and used by the principal belligerent nations, both with one another and with t h e above-mentioned laboratory preparations. D a t a on these charcoals are given in the following table:

May, 1919


TABLBCOMPARISON

O F TYPICALPRODUCTION CHARCOALS PRINCIPAL BELLIGERENT NATIONS

OP

THE

Appar- Service ent Time Den- Corr. to Country Date Raw Material sity 8-14 Mesh Remarks United States Nov. 1917 Coconut 0.60 10 Air activated United States June 1918 Mixed nuts, etc. 0 . 5 8 18 Steam activated United States Nov. 1918 Coconut 0.51 34 Steam activated England 1917 Wood 0.27 6 Long distillation Enaland Aug. 1918 Peach stones, etc. 0 . 5 4 16 0.23 2 Frahce 1912-18 Wood Germanv Earlv Wood ? 3 Chemical and steam treatment Germany June 191 7 Wood 0.25 33 Chemical and steam treatment Germany June 1918 Wood 0.24 42 Chemical and steam treatment

............... ...............

It is at once evident t h a t t h e service time of most of these charcoals is very much less t h a n was obtained with t h e laboratory samples. However, in t h e emergency production of this material on a large scale, quantity and speed were far more important t h a n t h e absolute excellence of t h e product. It will be noted, for instance, t h a t t h e coconut charcoal manufactured b y t h e United States, even in November 1918, was still very much inferior t o t h e laboratory samples made from t h e same raw material. This was not because a very active charcoal could not be produced on a large scale, for even in May 1918 t h e possibility of manufacturing a so-min. charcoal on a large scale had been conclusively demonstrated, but this activation would have required two or three times as much raw material and five times as much apparatus as was then available, due t o t h e much longer time of heating,and t h e greater losses of carbon occasioned thereby. It should furthermore be pointed out t h a t t h e increase in t h e chlorpicrin service time of charcoal from 18 t o jo min. does not represent anything like a proportionate increase in its value under field service conditions. This is partly due t o t h e fact t h a t t h e increased absorption on t h e high concentration tests is in reality due t o condensation in t h e capillaries, which, as has been pointed out, is not of much real value. More important t h a n this, however, is t h e fact t h a t most of the important gases used in warfare are not held b y adsorption only, b u t b y combined adsorption and chemical reaction, for which purpose an 18-min. charcoal is, in general, almost as good as a so-min. charcoal. It will be noted t h a t t h e American charcoals, produced a t any given time, show up better t h a n either t h e French or British absorbents of similar periods, in spite of the fact t h a t t h e work on this problem was started here much later. On t h e other hand, German charcoals, since t h e middle of 1917, have shown up better on t h e accelerated chlorpicrin test t h a n the American production charcoals. This advantage is entirely illusory, however, because it is produced b y capillary condensation in a wood charcoal and is held b u t very loosely. I n fact, experiment has shown t h a t drawing a stream of air through such charcoal which has been subjected t o the accelerated test removes over 7 0 per cent of t h e adsorbed gas, whereas, in t h e case of t h e 35-min. American coconut charcoal, it displaces less t h a n I O per cent. The American charcoal is, therefore, very much superior for field use, where t h e concentrations are low and air is drawn through the mask over long periods of time. It will also be of interest t o compare t h e behavior of these charcoals against a variety of toxic gases.

43 I

This can readily be done by means of t h e following table where the service times of the various charcoals against a concentration of 0.1 per cent of the more important war gases are given. TABLES-TYPICAL ABSORPTIVE VALUESOF DIFFERENTCHARCOALS AGAINST VARIOUS GASES 0

E3.8

No. Charcoal Nation 1 Poorcoconut U.S.A. 0 2 MediumcoconutU.S.A. 0 3 Goodcoconut U.S.A. 0 4 Same as No. 2 U . S . A . 12 but met 5 No. 2 impregnated U.S.A. 0 6 Wood French 0 7 Wood British 0 8 Peachstone British 0 9 Treated.wood German 0 10 No. 9 *mnt.co.__r_ German 30 natec1

-~

Service Time, Minutes -Standard Conditions--

10 30 60

120 175 20 350 260 25 620 310 27

18

320

330

35 400 2 . 5 25 6 70 16 190 42 230

700 75 90 135 105

9

90

STANDARD CONDITIONS

18 25 30

55 65 75

35

16

35

70 9 18 30 20

400 0 4 25 20

70

320

16

1

OF

TESTS

1

5

65

22

50 270 65 370 70 420 95

... ... ...

190 510 20

30 60 25

110 120

... ...

...

Mesh of absorbent.. ...................... 8-14 10 cm. Depth of absorbent layer.. Rate of flow per sq. cm. per min.. 500 cc. 0 . 1 per cent Concentration of toxic gas.. 50 per cent Relative humidity.. Temperature 20’ Results expressed in minutes to the 99 per cent efficiency points Results corrected to uniform concentrations and size of particles

................ .......... ............... ...................... .............................

While this concentration is not as high as is occasionally met with in field service, particularly in projector attacks, it is still many times greater t h a n t h e average. The rate of flow employed in the test was always the same, and corresponds t o t h e breathing rate of a moderately exercising man. It will be seen t h a t the service times of t h e best American and German productions run into hundreds of minutes in t h e cases of the gases chlorpicrin and phosgene, even at this high concentration. This means, of course, t h a t under average service conditions, t h e absorbent will function satisfactorily for months at a time without any necessity for renewal. I n general, the charcoals show the shortest service times toward arsine and hydrocyanic acid, but fortunately both of these gases are fairly well adsorbed by t h e permanganate soda-lime. Moreover, neither of these gases has been found well-adapted for use in gas warfare. As a matter of general interest, i t may be mentioned t h a t even relatively poor charcoal will furnish indefinite protection against field concentrations of the vapor of high boiling vesicant substances such as “mustard gas.” SODA-LIME N E E D FOR SODA-LINE I N WAR-GAS MIXTURE

While charcoal unquestionably comes surprisingly close t o fulfilling all of t h e requirements which have been outlined, nevertheless, plain charcoal alone cannot be considered a satisfactory all-round absorbent. I t s principal deficiencies are as follows: I-It has too little capacity for certain highly volatile acid gases, such as phosgene, hydrocyanic acid, etc., which are used t o a fairly large extent in warfare.

43 2


a-Certain other gases are best handled by oxidation and it is impossible t o accomplish this with the aid of charcoal alone (although certain impregnated charcoals have been found t o be very satisfactory in this respect). 3-While charcoal has a very high activity for all gases and a fairly high capacity for most gases, in general it does not hold t h e more volatile gases quite as firmly as would be desirable, and there is a tendency for such gases t o be slowly given off t o a current of air. This is because, as previously pointed out, the gases in charcoal are held almost entirely by physical adsorption, and they, therefore, exert a certain small, b u t nevertheless, in t h e case of t h e more volatile gases, appreciable vapor pressure, when any considerable amount of gas has been taken up. I n order t o counterbalance these deficiencies, the use of a n alkaline oxidizing agent in Combination with t h e charcoal has been found advisable. The material actually used for this purpose by both the British and t h e Americans has been granules of soda-lime containing sodium permanganate. Such an absorbent when properly prepared has a remarkably high capacity for the acid gases, and will oxidize easily oxidizable gases. Since t h e soda-lime in all cases holds the gases in very stable chemical combination, it has no tendency t o release even the smallest amounts when air is breathed through the mask. It has, in fact, been shown t h a t after a canister filled with a mixture of charcoal and soda-lime has been exposed t o certain gases, such as phosgene, a slow transfer of gas from t h e charcoal t o t h e soda-lime takes place continually, thus leaving t h e charcoal free t o pick up more gas the next t;me t h e canister is used. It may, therefore, be said t h a t t h e principal function of t h e soda-lime is t o act as a reservoir of large capacity for t h e permanent fixation of the more volatile acid and oxidizable gases, while t h e charcoal serves t o furnish t h e required degree of activity for all gases as well as the storage capacity for the more stable and less volatile ones. In the case of phosgene and superpalite, two widely used war gases, t h e soda-lime has a special function. Both of these gases, especially t h e latter, are very well adsorbed by dry charcoal, but, as previously pointed out, in the presence of moisture they are very rapidly hydrolyzed into carbon dioxide- and hydrogen chloride, the reaction being catalyzed by the charcoal. While t h e latter gas is not extremely toxic, it nevertheless causes serious discomfort, and if allowed t o pass through, causes t h e wearer t o lose confidence in his mask. Since ordinary charcoal can hold but little of this very volatile gas, there is great need for the presence of the alkaline soda-lime, which very readily adsorbs large amounts of it. I n fact, soda-lime is a much better absorbent for hydrogen chloride than for a n equivalent amount of phosgene, so while t h e charcoal holds but little of t h e phosgene, it nevertheless greatly aids in its absorption by accelerating its hydrolysis. Another reason for t h e use of the combination absorbent rather than charcoal alone is its well-balanced

1701.

11,

No.

j

behavior toward variations in temperature and humidity. Thus as the temperature increases, charcoal adsorbs more poorly, while soda-lime reacts more readily. High humidity also tends t o decrease the value of t h e charcoal, but in general favors absorption by the soda-lime. The absorptive efficiency of the combination is, therefore, surprisingly little affected by all ordinary variations in temperature and humidity. SPECIAL

FOR S 0D A - LI 1M E

REQUIREMENTS

A

SATISFACTORY

Due t o the inherent nature of soda-lime, it is obvious t h a t the problem of determining t h e best balance of the requirements is a very difficult and complicated one. The principal requirements may with profit be reconsidered briefly in this connection. In the first place, since t h e alkaline absorbent is mixed with charcoal before use, its activity is not of vital importance, as t h e charcoal is able t o take up t h e gas with extreme rapidity and then later give i t off more slowly t o the soda-lime. Absorptive activity is, nevertheless, a very desirable property, especially if high concentrations of phosgene are t o be dealt with. Adsorptive capacity is of the greatest importance since the soda-lime is relied upon t o hold in chemical combination a very large amount of toxic gas. Chemical stability is obviously much more difficult t o attain in the case of soda-lime than in t h a t of charcoal, especially when a strong and somewhat unstable oxidizing agent, such as sodium permanganate, is present. The deliquescent tendency of sodium hydroxide is another great source of difficulty. Mechaizical strength combined with absorptive activity and capacity is also difficult to attain, and this problem had never been solved until the war made some solution absolutely imperative. Methods had t o be worked out by which t h e hardness and mechanical strength required t o withstand the severe conditions of usage could be secured without any serious sacrifice of activity and capacity. I n order t h a t t h e following discussion of t h e various components and their functions shall, be clear, it is desirable a t this point t o outline very briefly the steps in t h e manufacture of the army soda-lime. Within limits, the method of manufacture is more important t h a n the composition of other variables, and has been t h e subject of a great deal of research work even on apparently minor details. The process finally adopted consists essentially in making a plastic mass of lime, cement, kieselguhr, caustic soda, and water, spreading in slabs on wire-bottomed trays, allowing t o set for z or 3 days under carefully controlled conditions, drying, grinding, and screening t o 8-14 mesh, and finally spraying with a strong solution of sodium permanganate with a specially designed spray nozzle. T h e spraying process is a recent development, most of t h e soda-lime having been made by putting t h e sodium permanganate into the original wet mix. Many difficulties had t o be overcome in developing t h e spraying process, b u t it eventually gave a better final product, and resulted in a large' saving of permanga-

X a y , 1919

T H E J0CyR~’7.AL O F I Y D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

n a t e which was formerly lost during drying, in fines, etc. C O M P O S I T I O N O F R E G U L A R AR-MY SODA-LIME The exact composition of t h e a r m y soda-lime has undergone considerable modification from time t o time as i t has been found desirable t o change t h e raw materials or t h e method of manufacture. A rough average formula which will serve t o bring out t h e interrelation between t h e different constituents is as follows : TABLE6 - ~ o ~ ~ o s 1 T 1 oOiF~ W E T MIX Per cent Hydrated lime.. . . . . . . . . . . . . . . . . . . . . . 45 Cement.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Kieselguhr., , . . ................ 6 Sodium hydroxide., . . . . . . . . . . . . . . . . . . 1 Wateq. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 (approx.) AFTER DRYING Moisture c o n t e n t . . . . . . . . . . . . . . . . . . . . . 8 (approx.) AFTER SPRAYING Moisture c o n t e n t . . . . . . . . . . . . . . . . . . . . . 13 (approx.) , , 3 (approx.)

Sodium permanganate content, . , , , . .

F C N C T l O N S O F THE DIFFERENT

COMPONENTS

Having previously pointed out in detail t h e requirements which gas mask absorbents must meet, it is well t o show exactly what part is contributed b y each of t h e components in t h e soda-lime toward making a well-balanced absorbent granule for military purposes. I t must be emphasized a t t h e start of this discussion t h a t t h e statements hereinafter made with regard t o t h e effect and optimum amount of each of t h e constituents apply with accuracy only t o a soda-lime made essentially according t o t h e method above described. Different methods of preparation may profoundly modify the effect of any of t h e abovementioned components. H Y D R A T E D LIME-This material furnishes t h e backbone of t h e absorptive properties of t h e soda-lime. It constitutes over 50 per cent of t h e finished dry granule and is responsible in a chemical sense for practically all of the gas absorption. CExENT-The sole function of t h e cement in t h e soda-lime formula is t o furnish a degree of hardness adequate t o withstand service conditions. It interferes considerably with the absorption properties of the soda-limeJ especially if no kieselguhr is present. On t h e other hand, a great many other binding agents have been very thoroughly tried out, practically all of which seem t o form an impenetrable film over t h e particles of lime and prevent their functioning satisfactorily as an absorbent. Cement has therefore been chosen as t h e least objectionable binder which will give satisfactory results. It is, however, still an open question as t o whether t h e gain in hardness produced b y its use is valuable enough t o compensate for the decreased absorption which results. KIESELGUHR-AS stated above, the introduction of cement into an ordinary soda-lime formula makes a very distinct decrease in t h e porosity and hence in t h e absorptive capacity of t h e soda-lime. It has been found possible, however, t o counterbalance this loss i n porosity by t h e simultaneous introduction of a rel-

433

atively small weight, though considerable bulk, of kieselguhr. It is rather surprising t o find t h a t this addition of kieselguhr does not cut down the hardness. I n fact, with carefully controlled methods of preparation, a reaction appears t o take place between the lime and t h e kieselguhr, which results in some increase in hardness. The main function of the kieselguhr, however, is t o produce increased porosity and hence increased absorptive activity in t h e finished cement granule. It is interesting t o note in this connection t h a t in a cement-free granule t h e addition of kieselguhr serves t o reduce rather t h a n increase the absorptive capacity. This reduction is not serious, however, and t h e resulting increase in hardness may for some purpose be worth while. SODIUM HYDROXIDE-This material has two primary functions in t h e soda-lime granule. I n t h e first place, a small amount of it serves t o give t h e granule considerably more activity. I n other words, t h e sodium hydroxide tends t o react with very small traces of toxic gases somewhat more rapidly t h a n does the calcium hydroxide and hence delays the breakdown and gives t h e latter time t o react and take up t h e bulk of t h e gas. This greater reactivity produced b y the alkali is almost certainly due t o t h e fact t h a t the gas first comes in contact with an aqueous film on t h e surface of t h e soda-lime, and t h e presence of t h e sodium hydroxide in this solution keeps t h e hydroxylion concentration of this film high enough t o react readily with t h e gas t h e instant t h e two come into contact. During t h e process of manufacture and use in t h e field t h e small amount of free sodium hydroxide present must certainly become fully carbonated by t h e carbon dioxide of t h e air. There is every indication, however, t h a t in soda-lime of t h e proper moisture content t h e carbonate thus formed is fairly rapidly re-causticized by t h e great excess of lime which is present, and t h a t there is actually free sodium hydroxide present even after t h e soda-lime has taken up several times t h e sodium hydroxide equivalent of carbon dioxide. If large amounts of sodium hydroxide are used, t h e effect is quite t h e reverse of t h a t observed with small amounts. The activity of t h e absorbent is greatly decreased, apparently due t o t h e fact t h a t t h e pores become plugged with a concentrated solution of alkali. The best absorbent seems t o be one in which the structure is very porous and yet each particle of lime in t h e granules is covered with a thin film of a dilute sodium hydroxide solution. One of the most striking things which has been brought out of the careful investigation of t h e sodalime composition is t h e fact t h a t in practically all gases t h e optimum content of sodium hydroxide is between 2 and 8 per cent, t h e remainder being lime, and t h a t a material with I per cent caustic in general works much better t h a n a material with ~j per cent or more. This is rather surprising, in view of t h e fact t h a t practically all industrial soda-limes now on t h e market contain between 30 and j o per cent of sodium hydroxide. Such granules are, of course,

434


very deliquescent, hard t o handle, and could never be considered for any gas mask purposes. The second function of sodium hydroxide in the granule is to maintain roughly the proper moisture content. I n most of the absorbents of the soda-lime type, the water content is the most important single factor in their composition. If only a very little caustic soda is used in the granules they dry out very rapidly and become more friable when air of ordinary humidity is passed through them. If, on the other hand, too much caustic soda is used, the material will tend t o take up water rapidly from air of average humidity and form a deliquescent product, increasing the resistance through the canister and greatly cutting down the porosity and value of the absorbent. For the majority of gases the optimum water content is in the general neighborhood of 11 per cent. The proportions of caustic and any other material tending to take up water must, therefore, be adjusted in such a way as t o keep the water content between say g and 13 per cent under the humidity conditions prevailing where the box respirator is likely t o be used. This adjustment cannot, of course, be very precise even under the best conditions. If there were no need for sodium permanganate in the finished granule, t h e recommended alkali content wouldIbe 4 per cent. On the other hand, when sodium permanganate is .to be sprayed on the dry, finished granules there are several other factors which must be considered, and which finally fix the alkali content a t the rather low figure of I per cent. I n the first place, t h e low alkali content in t h e original white granule greatly increases the ease of drying the granule down t o the low moisture content which is necessary before spraying. This easy dehydration would, of course, be a serious objection if the granules were t o be used without further treatment. Fortunately, however, the sodium permanganate, which is later added, is in itself deliquescent and tends t o replace almost a n equivalent amount of sodium hydroxide in holding the proper moisture content a t ordinary humidities. A third reason for the use of the low alkali content is t h a t in the presence of more alkali there is a very marked tendency t o reduce the sodium permanganate t o sodium manganate, which latter is of more or less value as an oxidizing agent. Furthermore, there is no question but t h a t t h e sodium permanganate, when introduced into the granule, has an activating action resembling t h a t of caustic. This may be partially due to t h e small amount of sodium carbonate which is always present in the sodium permanganate solution. Again, as t h e permanganate is decomposed into manganese dioxide, either spontaneously or by oxidizing other substances, more caustic is automatically liberated. A t any rate, the sodium permanganate tends to replace an equivalent amount of sodium hydroxide in its activating as well as its moisture-retaining action. WATER-The water content of absorbents is very frequently overlooked in studying their behavior, but as a matter of fact, not only is the final water

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content of great importance, as pointed out above, but it is also one of the most important variables a t practically every stage in the manufacture of sodalime, and must be kept under very careful control. A soda-lime-kieselguhr “wet mix” changes its properties very markedly as the water content changes from about 2 5 or 30 per cent t o about 35 or 40 per cent, the exact limits depending upon the raw materials. A t the lower limit it is moist but will not form a coherent plastic mass which can be thoroughly mixed in a kneading type of mixer. A t the upper limit it becomes a thick but fluid cream. I n order t o get a satisfactory product, especially from the standpoint of the hardness of the final product, it is absolutely essential t h a t the water content be kept very close t o the minimum which can be used and yet make a mass which is plastic and workable. There should be absolutely no tendency toward fluidity or the finished granules will be very soft. The exact description of the proper consistency of this wet mix is Very difficult, but experience will soon indicate just what water content is necessary t o give the best results. A consistometer has been used with success in keeping this property constant. The water content of the wet mix should, of course, not be considered to be rigidly fixed a t the 33 per cent specified in the table, but should be varied, depending upon the exact qualities of the raw materials. The water content after drying must be made low enough so t h a t when the permanganate solution is sprayed onto the finished white granules there will be no necessity for further drying to bring the granules t o the proper final water content. On the other hand, it is important not t o dry the material too far or the hardness will be injured. About 7 or 8 per cent water gives the best results. The water content of the granules after spraying should be 13 t o 14 per cent. This water content is somewhat higher than t h e precise optimum value a t which the soda-lime has the maximum efficiency against most gases, but the specified figure is determined by the fact t h a t when the soda-lime is mixed with the dry charcoal in the canister it is immediately robbed of 3 or 4 per cent of its water. This brings the moisture content down t o slightly below the optimum for gas absorption. It is not considered wise to increase the water content after spraying still further, however, as the large amount of water which would then be taken up by the charcoal would considerably injure its adsorptive properties. Careful determinations have been made of t h e vapor pressure-water content equilibria of the army soda-lime. The curves in Fig. I X show these vapor pressure curves a t 4 different temperatures. Rather surprisingly, the logarithms of these vapor pressures when plotted against the reciprocals of the absolute temperatures give the perfectly straight line characteristic of pure substances, and with a slope very nearly t h a t for pure water. The relationships prevailing are brought out even more clearly in Fig. X, in which the per cent of the saturation pressure a t each temperature is plotted

May, 1 9 1 9


against the water content. It will be noted t h a t t h e curves for t h e different temperatures agree very closely with %one another, although t h e higher temperatures consistently give slightly higher relative humidities.

435

sium and barium, the two most stable and easily manufactured, were found t o be insufficiently active as oxidizing agents, and so slightly soluble t h a t they crystallized on drying, thus clogging t h e pores. Magnesium and calcium permanganates both gave slightly better results than sodium, but the former was too unstable, and both of them were too difficult to manufacture on a large scale t o permit their use. Sodium permanganate was therefore used and when its impurities had been eliminated, it gave very satisfactory results. The exact amount of sodium permanganate to be used in t h e granule depends upon the degree of protection deemed desirable, taking into account the great cost of the permanganate. I t is possible t o use up t o 5 or 6 per cent without interfering seriously with the physical properties of the granule. Careful consideration of t h e amounts of oxidizable gases likely to be met with in the field led t o fixing t h e sodium permanganate content of t h e granules for t h e Army a t about 3 per cent. S E L E C T I O N OP R A W M A T E R I A L S

Percent H,O in Sample FIG.IX-VAPOR PRESSURE OF WATERIN ARMYSODA-LIME WITH VARYING WATERCONTENT

The relationships indicated by t h e above figure are of considerable value in determining the proper conditions in t h e setting room and drying tunnel where definite moisture contents are t o be produced. By controlling t h e wet and dry bulb temperatures, i t is possible t o dry t o any given point and then stop. The figure also shows very clearly t h a t the sodium hydroxide and sodium permanganate contents are correctly adjusted t o give almost exactly t h e optimum water content (11 t o 1 2 per cent) under the humidity conditions prevailing in France (about 7 0 per cent relative) regardless of the temperature. The exact shape of t h e curve does not accord exactly with what would be expected if t h e vapor pressure lowering were entirely due t o t h e dissolved sodium hydroxide and t h e sodium permanganate. S O D I U M PERMANGANATE-The function of t h e sodium permanganate is t o oxidize certain gases, such as arsine,, this gas being especially difficult t o absorb with charcoal or plain soda-lime. When the permanganate was first introduced i t was the only satisfactory absorbent which had been found for this gas. Later several impregnated charcoals were perfected which would handle arsine even better t h a n permanganate soda-lime, but t h e value of an active alkaline oxidizing agent as an assurance of protection against possible new gases was deemed sufficient t o justify the very considerable expense of its continued use in the war-gas absorbent. Of the five permanganates tried c u t (including all those which would be commercially available) potas-

The selection of t h e various raw materials for the soda-lime must be very carefully made in order t o obtain a satisfactory product. The most important requirements for each material will be briefly discussed. H Y D R A T E D LIME-It has been definitely shown t h a t t o make a highly absorbent soda-lime i t is necessary t o have a completely hydrated, high calcium lime with

FIG. X-VAPOR PRESSURE O R WATERINZARMY SODA-LIME WITH VARYING WATERC O N T E N T PLOTTED AS PERCENT OB SATURATION PRESSURE AT CORRESPONDING TEMPERATURES

although i t is entirely satisfactory from a chemical standpoint. The Bureau of Standards “soundness test” for hydrated lime has not been found t o be a reliable indication of the completeness of hydration, a t least for these purposes. The presence of magnesia makes t h e resulting soda-lime considerably harder,

43 6


but very inefficient chemically; carbon dioxide has a similar harmful effect. Altogether about 1 3 different brands of lime have been tried out, and no sample which contained over 5 per cent magnesium oxide or which was incompletely hydrated has given good results. It ' i s apparent, however, t h a t not all the factors which go t o make a satisfactory lime have been fully determined. While the three above-mentioned variables are unquestionably the most important, nevertheless four brands of lime, all of which meet the above specifications very satisfactorily, have been thoroughly tested out, and only two have been found t o give consistently good results when used in soda-lime. It is quite possible t h a t t h e difference between t h e highcalcium, completely hydrated brands may lie in the size of the ultimate lime particles. KO adequate study has been made of t h e effect of this property, except t h a t it has been found t h a t pure precipitated calcium hydroxide does not give as satisfactory results as some of the commercial brands of lime, and t h a t especially fine, air-separated lime and cement give poor results on absorption tests. It may well be, therefore, t h a t within certain limits the larger t h e ultimate particles of lime t h e better the results. The following set of specifications for t h e lime would serve t o eliminate most of t h e unsatisfactory grades. It should not contain over 4 per cent carbon dioxide and not over 5 per cent combined magnesium Oxide, Oxide and ferric Oxide' There be enough combined water present to hydrate at least 98 per cent of the Oxide present. Not Over 4 per cent should be held on a Ioo-mesh sieve.

CEMENT-practicallY all t h e s ~ ~ n d a r grades d of Portland cement Seem t o give satisfactory results and t o be much better t h a n any of the Special cements (natural, quick-setting, etc.) which have been tried. A slow-setting cement is preferable in order t o decrease t h e difficulty of handling t h e Product during t h e slabbing stage. The cement should Pass t h e skmdard cement specifications of t h e American Society for Testing Materials. KIESELGUHR-A very careful study has been made of t h e different brands of kieselguhr available on the market, and there is no question but t h a t t h e lightest grades are the best. There are only two commercial brands which give satisfactory results. The apparent density of t h e kieselguhr after packing in a wooden box, tapping and refilling until it does not settle any further, should in no case exceed 0.3. It should not contain appreciable amounts of organic matter. SODIUM HYDROXIDE-Any grade of material reasonably free from sodium chloride will give satisfactory results. S O D I U M PERMANGANATE-The purity of t h e sodium permanganate solution, used has been found t o be one of t h e most important factors in making stable soda-lime. Apparently any considerable amount of soluble salts greatly increases the rate of decomposition of the sodium permanganate. It is therefore necessary t o carry on the method of manufacture in such a way t h a t the amount of chloride, chlorate,

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and sulfate shall be a minimum. I n no case should the total amount of these three salts exceed 2 j per cent of the amount of sodium permanganate in t h e solution. The production of such a solution on a commercial scale has necessitated the working out of a new process for its manufacture. Material of a satisfactory purity containing about 30 per cent sodium permanganate is now being produced on a large scale by a process involving t h e customary fusion of sodium hydroxide and manganese dioxide, leaching, and chlorination of t h e sodium manganate in t h e presence of a catalyst which decomposes all excess sodium hypochlorite into sodium chloride and oxygen and prevents t h e formation of sodium chlorate. Further purification is then effected b y evaporation, during which most of t h e sodium chloride crystallizes out and is removed. The resulting 30 per cent solution contains about 6 per cent sodium chloride and practically no other impurities. This is far superior t o any similar material previously obtainable on t h e market. An electrolytic method for producing sodium permanganate has also been worked out by the Research Division and found t o be entirely practicable, reasonably cheap, and to yield a very pure product. THE S T R U C T U R E O F S O D A - L I M E

The structure of army soda-lime is probably the of its great superiority most important single Over t h e types used before t h e war. This structure depends, of course, t o a considerable extent, upon the raw materials and the proportionsin which they are used. Even important than these factors, however, are three details in the method of preparation. I n t h e first place t h e plasticity of t h e wet mix before slabbing has a marked influence-on the final structure. This has already been discussed in connection with t h e water content of t h e wet mix. I n t h e second place, in order t o get a product with a satisfactory degree of hardness and resistance to abrasion, it is necessary t o allow the material t o take a sort of preliminary set b y allowing it t o stand several days and lose water very slowly, in a room where t h e temperature and humidity conditions are carefully controlled. Thus if t h e material is dried out within a day after it has been mixed, a very soft, crumbly product invariably results, but, allowing it t o stand 2 or 3 days and slowly lose 8 or I O per cent of water before t h e final drying, gives a hard and yet porous material. The exact mechanism by which the material t h u s gains hardness during the time of setting is not definitely understood. Several hypotheses have been advanced t o explain it. Unquestionably t h e slow setting of t h e cement cannot be realized if t h e sodalime is dried too soon after mixing. On t h e other hand, however, much the same effects are observed with soda-limes containing no cement. It may be t h a t t h e rapid drying of material which contains a large amount of water serves t o disrupt t h e structure which would otherwise be produced. Again, the setting effect might conceivably depend on the slow deposition of a small amount of calcium hydroxide

M a y , 1919

T H E J O U R N A L O F I , V D U S T R I A L A N D E N G I N E E RI N G C H E &?IS T R Y

which was dissolved in t h e excess water. Another plausible explanation is t h a t t h e very slow b u t steady decrease in t h e volume and water content of t h e slab gives t h e cohesive force of surface tension full opportunity t o pack together t h e individual particles into a very dense, strong material which is, nevertheless, porous after t h e water has been still further evaporated. At any rate, the fact remains t h a t a better material can be made with no binding agents b y t h e slowsetting process t h a n b y using considerable amounts of kieselguhr and cement and drying shortly after mixing. Furthermore, no matter what binding agent is used, t h e slow-setting process seems t o be of great advantage in developing t h e maximum hardness for any given formula. The third very important factor in the structure of soda-lime influences t h e efficiency of t h e material rather t h a n its mechanical strength. This is t h e necessity of having t h e outer surface of a granule composed entirely of freshly broken surfaces. A great variety of experiments in which granules were made from soda-lime molded in pill machines, rolled into pellets, squirted through macaroni machines, etc., have shown t h a t such surfaces produced before drying, or which have been smeared over while t h e material was still plastic, have very little value for absorptive purposes. Material made in this way seems t o be coated with a sort of skin through which t h e gas can penetrate but very slowly. This effect is not difficult t o understand because an examination shows t h a t such outer surfaces of such material are more dense and of distinctly different texture from t h e interior of t h e granule, which is porous and highly absorptive. I t is obvious, therefore, t h a t t h e material must be dried in relatively large, thick slabs and then ground up in such a way as t o produce t h e maximum amount of freshly broken surface. The slabs must not be made too thick or t h e drying process will be greatly impeded. A slab I'/~ in. thick has been found t o yield very good material, and can be dried without serious difficulty. SUGGESTED MODIFICATIONS O F SODA-LIME F O R I K D U S T R I A L USE:

The conditions which the soda-lime must meet when used for industrial purposes are very markedly different from those prevailing in gas warfare. It is therefore necessary t o strike a new balance between t h e various factors, and specify a different formula if t h e new conditions are t o be adequately met. I n general, a soda-lime for industrial use will meet higher concentrations of gas and be subjected t o much less severe handling t h a n under field service conditions. It is advisable therefore t o increase t h e capacity and especially t h e activity of t h e soda-lime and i t is entirely justifiable t o decrease its hardness markedly. It is furthermore almost certainly unnecessary t o use sodium permanganate as it is of value only against a limited number of gases, and these are not ordinarily met with in t h e industries. Furthermore, if these gases must be provided for in special cases, a special

~

43 7

absorbent can be used which will handle them much more efficiently t h a n sodium permanganate granules. A thoroughgoing investigation as t o t h e extent t o which hardness of t h e soda-lime might be sacrificed and its chemical efficiency increased for use in industrial masks was started by t h e Chemical Warfare Service, b u t not completed before t h e experimental work had t o be stopped. An investigation was also partially completed covering t h e effect of variations in composition on t h e absorption of certain gases, such as sulfur dioxide, which are met with in industries but not on t h e battlefield. While these investigations are not complete nor entirely conclusive, t h e indications are t h a t a soda-lime containing 4 per cent sodium hydroxide, 4 per cent kieselguhr, and t h e remainder lime, with a final water content of about 1 2 per cent, would be an adequately hard and very efficient all-round absorbent for practically all industrial gases. The use of more kieselguhr and less caustic would give a harder product, while t h e use of less kieselguhr and slightly more caustic would give softer b u t somewhat more absorptive material. The method of preparation would be similar t o t h e above-described process now used b y t h e Army. As a special case of t h e industrial use of soda-lime may be mentioned t h e need for a satisfactory carbon dioxide absorbent. Such absorbents are very much used in self-contained oxygen respirator apparatus where t h e air is circulated in a closed circuit, carbon dioxide being absorbed and oxygen led in from a t a n k as needed. A special investigation of carbon dioxide absorbents has therefore been made b y t h e Research Division. It has been found t h a t t h e great difference between a good absorbent for carbon dioxide and absorbents for t h e more strongly acid gases lies in t h e need for a very high moisture content t o give complete carbon dioxide absorption. A formula which has been found t o give very satisfactory results contains about 4 per cent alkali, and should have a moisture content between 16 and 18 per cent. The reason this high moisture content gives so much better results is n o t entirely clear, but it may be necessary in order t o facilit a t e the rapid recausticization of the sodium hydroxide by the lime, which unquestionably plays a considerable part in t h e rapid and complete absorption of carbon dioxide by soda-lime. The simplest way t o obtain material with this moisture content is t o dry t o I O or 1 2 per cent water preferably in vacuum (to prevent the absorption of carbon dioxide as in air drying), grind and screen, and then increase the water content t o 16 or 18 per cent b y spraying t h e granules with a specially designed atomizing nozzle, which gives an extremely fine, uniform mist. This spraying must, of course, be very carefully controlled so as t o make it quite uniform. The reason for adopting this method is t h e great difficulty of grinding and screening material containing 16 or 18 per cent water. At best, the grinding yields are rather low. The use of even a small amount of any binding agent has been found t o decrease very markedly t h e efficiency of t h e carbon dioxide absorption. T h e

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438

methods of handling t h e product throughout are of t h e greatest importance. If properly prepared, t h e sodalime should absorb in t h e neighborhood of 0.3 g. of carbon dioxide per gram of soda-lime at high rates of flow before its efficiency drops off seriously. I t is reasonably hard, granular, and non-deliquescent. T h e material possesses a further great advantage over t h e ordinary high caustic-absorbing materials for respirators in t h a t it gives off much less heat due t o t h e fact t h a t it evaporates off rather t h a n absorbs moisture. The suggestions made in this section as t o t h e modifications of the a r m y soda-lime for industrial purposes are based on investigations which are as yet incomplete. T h a t t h e recommended procedure will produce an absorbent several times as efficient as any soda-lime now on t h e market has been verified by thoroughgoing tests on all available commercial soda-limes. It should have considerable value for use in steel analyses, where present types of soda-lime do not give satisfactory service, and for other industrial purposes. RESEARCH DIVISION,C. W. S., U. S. A. AMERICANUNIVERSITY EXPERIMENT STATION WASHINGTON, D. C.

EFFECT OF EXPOSURE TO WEATHER ON RUBBER GAS MASK FABRICS' By G.

ST. J.

PERROTT AND

A. E. PLUMB

Received March 18, 1919

The purpose of this investigation was t o determine t h e relative desirability for gas mask use of a number of rubber coated fabrics submitted by different rnanufacturers, particularly with regard t o their resistance t o t h e deteriorating effect of exposure t o t h e elements. The desirable qualities in a rubber fabric for gas mask use are: 1-Resistivity to war gases 2-Flexibility and comparative lightness 3-Resistance to deterioration by weather 4-Resistance to deterioration due to exposure to gas

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ing roof with southern exposure. At successive periods samples were cut t o be examined for permeability, acetone extract, and general physical properties.

Acefone Ewtrac+ Per cent.

FIG. I-FABRICS

EXPOSED TO WEATHER, FEBRUARY TO JUNE,

1918.

ACETONE EXTRACT

At t h e time when t h e investigation was started (early in February) it was thought t h a t exposing samples in Florida would give quicker deterioration due t o greater amount of heat, sunshine, and moisture. Accordingly, Samples P-198, P-199, P-197, and P-205 were exposed in Florida. Later other samples were exposed in Washington, of which P-197 and P-200 are duplicates of P-198 and P-199, respectively. I n order t o determine t h e effect of ultraviolet light in t h e sun's rays on t h e rate of deterioration, a portion of each fabric exposed was covered with a sheet of glass in. thick. This shielded t h e fabric underneath from t h a t portion of the sunlight which is of short wave length.

MATERIALS USED

Gas mask fabrics consisting of a finely woven cotton sheeting covered with a rubber layer varying from 0 . 0 1 0 to 3 . 0 2 5 in. in thickness were used for t h e tests. FABRIC No. P-190 P-196 P-197 P-198 P-298 P-199 P-200 P-299 P-205 P-366 P-297

Weight Manufacturer Oz./Sa . - Yd 29.06 Plymouth Co. 18.9 U. S. Rubber Co. 15.0 Kenyon Co. 13.9 Kenyon Co. 15.4 Kenyon Co. 19.1 Goodrich Co. 15.3 Goodrich Co. 15.5 Goodrich Co. 16.0 Goodyear Co. 13.8 Goodyear Co. 15.9 Plymouth Co.

Thickness of Rubber Layer Inch 0.025

0.015 0.012 0.011 0.011

0.018

0.016 0.012 0.019 0.012 0.010

Date

Received Jan. 16, 1918 Jan. 21, 1918 Jan. 21, 1918 Jan. 21, 1918 April 19, 1918 Jan. 21, 1918 Jan. 21, 1918 April 19 1918 Feb. 14: 1918 May 13, 1918 April 19, 1918

METHOD O F EXPOSURE

The effe-t of exposure t o weather (sun and rain) and the effect of exposure t o heat alone have been investigated. For exposure to weather, t h e samples were loosely stretched on wooden frames placed on a slightly slop1 Approved for publication by the Director of the Chemical Warfare Service.

FIG.2-FABRICSEXPOSED TO

WEATHER,

FEBRUARY TO JUNE, 1918.

PERMEABILITY

The early samples were exposed rubber side up. Surface cracking occurred so quickly t h a t later tests conducted in Washington were run with samples both rubber up and cloth up. The accelerated aging tests were made in a Freas electric drying oven, heated t o 130' C., well ventilated by an electric fan. Temperature did not vary more t h a n a few degrees in any part of t h e oven. No attempt was made t o control humidity. Samples were cut a t successive intervals over a period of 18 hrs.

Gas MAsk Absorbents

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