February 1948
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Table I11 shows that 79% of the squash was lost during.fermentation. Removal of soluble carbohydrates by preboiling increased the total loss to %yo. Fermentation of the extracted pulp resulted in a thirteen fold concentration of carotene, as compared with a sixfold concentration obtained by fermentation of the unextracted pulp. Although C. roseurn can be considered as belonging to the acetone-butanol producing organisms, it is not so efficient in this regard as the acetobutylicum group; hence the carrot and sweet potato starches and sugars are not efficiently utilized. It is not unlikely that ‘an organism can be found that will possess the idlulose-digesting power of C. roseurn and will also be an effiBent solvent producer. No attempt was made to use a mixed cxlture of a good solvent producer together with C. roseurn, but w r h a procedure may be effective. ACKNOWLEDGMENT
The authors express their thanks to E. G. Kelley for supplytug some of the materials, to M. E. Wall for the carotene analyses, m d to C. 0. Willits for the other analyses. They are indebted to E. hlcCoy of the University of Wisconsin f o r the stock culture of C. rmeum A42
299
LJTERATURE CITED
(1) Assoc. Official Agr. Chem., “Official and Tentative iMethods of Analysis,” 5th ed. (1940). (2) Barnett, 13. M., U. S. Patent 2,348,443 (June 7, 1943). (3) Holmes, H. N., and Leicester, H. M., U. S. Patent 1,967,121 (July 17, 1934). (4) Hoover, 9. It., Diets, T. J., Naghski, J., and White, J. W., Jr. IND.ENG.CHEM.,37,803-9 (1945). (5) Huddleson, I. F., Du Frain, J., Barrons, K. C., and Giefel. M., J. Am. Vet. Med. Assoc., 105,394-7 (1944). (6) Morris, 1%. H., 3rd, Colker, D. A,, and Chernoff, M. F., U.8 Dept. Agr., Bur. Agr. Ind. Chem., AIC-51 (1944). (7) Naghski, J., White, J. W., Jr., Hoover, S. R., and Willaman, J. J., J . Bact.,49,563-74 (1945). (8) Slade, R. E., and Birkinshaw, J. H., British Patent 511.526 (August 21,1939). (9) Sullivan,J. T., Food Ind.,16 (3), 78 (1944). (10) Sullivan,J. T., Science,9 8 , 3 6 3 4 (1943). (11) Villere, J. F., Heinzelman, D. C., Pominski, J., and Wakeham. H. R. R., Ibid.,16 (I), 76-7,130-1 (1944). (12) Wall, M. E., and Kelley, E. G., IND.ENG.CHEM.,ANAL. ED.. 15, 18-20 (1943). (13) Zechmeister, L., and Cholnoky, L., “Principles and Practice of
Chromatography,” tr. by Bacharach snd Robinson, New York, John Wiley & Sons, 1941. R E C ~ I V EOotober D 16,1948.
Interior Coating of Tubular Containers L. P. HUEfsUCH AND W. C. JOHNSON A‘. I . dii Pont de Nemours & Company, Inc., Philadelphia, Pa.
‘l‘he coating of the interiors of containers for certain poison gases has been studied. Coatings based on straight 100% phenol-formaldehyde resins were found resistant enough to prevent the decomposition of the poison gases which occurs when they are stored in contact with iron or steel. A modification of the dip method of coating was worked out to provide uniform, thin continuous coatings on the interiors of tubular containers for subsequent storage of the poison gases.
T
HE storage of Levinstein mustard gas and mixtures of it with other similar poison gases in steel containers such as
cans, drums, and bombs has always presented certain stability difficulties. According to the literature (S), mustard gas and similar related chlorine compounds yield small amounts of hydrochloric acid which dissolve the iron. Catalytic amounts of iron chloride thus formed in the mustard greatly accelerate its polymerization to long-chain sulfonium compounds-products of a very viscous nature (often gelled) of considerably less blistering action and toxicity than the original material. This polymerization will take place over a period of one month to several years, depending upon storage conditions. Heat accelerates the polymerization, and as much as 80% of the original mustard gas may be polymerized in one month at 65 O C. The polymerization is usually, although not always, accompanied by the dcvclopment of pressure due to the liberation of hydrogen, hydrogen sulfide, and sulfur oxide gases ( I ) . One of the possible ways of avoiding such undesirable decomposition and/or polymerization is the coating of the interiors of containers carrying mustard gas or its mixtures with a coating composition, resistant and continuous enough to prevent the gases in question from reaching the iron
in the container even on prolonged storage a t elevated trrrrperatures. I t is not the purpose of the present paper to discuss in great detail the search for suitable coatings that would be resistant to the strong solvent action of chlorinated hydrocarbons coupled with the deteriorating action of strong hydrochloric acid, although some information along this line will be given. The paper is more concerned with methods of coating the interiors of storage containers, and the adequate bakirlg of the coatings to provide uniform, thin, continuous films. Offhand thie would look like a relatively simple spray operation, but unfortunately the coatinge that are resistant enough to the poison gases are of such a type that unevenness of the film results in (a) popping and blistering in the thick portions and ( b ) incomplete protection in the thin portions. The present paper describes several variations of a method of coating the interiors of tubular containers to give uniform, thin coatings of a resistant composition. The variation@ were tried both in the laboratory and in plant scale operations, although in the latter connection not very extensive work W&F performed. From the investigation of a variety of available coating cornpositions (Table I), it was found that the only really satisfactory coatings that would withstand the severe requirements involved in the storage of poison gases over prolonged periods of time were those based on 100% straight phenol-formaldehyde o resins substituted resins. Even those 1 0 0 ~phenol-formaldehyde with alkyl or aryl groups do not appear to give the necessary resistance. These 100% straight phenol-formaldehyde resins, due to the fact that they are not completely condensed, undergo further condensation a t baking temperatures with the liberation of water. In thick films this results in blistering or popping or
298
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rate of flon of the solution wher it is at the top of the box &i Agent \'isoosity, Cp. compared to when it is at the bot. Sample Type of Coating 30 days tom of the box. A panel 6 incheP KO. Vehicle typea Pigment Initially a t 6 5 O C. Aftercondition of Coating long and 2 inches nide with a Resistanoe of Coatings t o Mustard Gas None i None 5.1 5.1 No steel present perforation near the top edge 1' 2 None iione 5.1 16.2 Steel present hung by means of an indcntrd 3 100% straight P.P Kone 5.1 5.3 Reddened b u t intact 4 100% straight P . F Brown 5.1 5.2 Blackened but intact cross bar arrangement in the box 5 U.F./alkyd, 50/50 6.1 White 16.1 Darkened and partly removed 6 Kylon &one 5.1 7.4 eoftened and sticky The box and tube down to thc or1 7 *1i Hydrolyzed P,V.A None 5.7 Blackened and slightly swolleu fice are filled with coating coni Long oil varnish 8 None 5.1 Removed completely 9.8 9 Short oil varnish None 9.1 5.1 Removed completely position so that the whole (11 Vinvl resin LO Black 5.1 34.5 Blistered and partly removed 11 5.1 5,lh Kone Removed completely the panel is completely coverPC1 I2 None 5.1 Swollen and lifted 12.8 13 None 35.8 The clamp near the orifice is I(' 5.1 Removed below liquid level 14 Sone 5.1 7.2 Blackened and swollen leased and the solution allowed t ( x 15 None 5.1 7.7 Blackened and lifted 16 None 5.1 6.5 Blackened and lifted flow from the box, leaving a dv Resistance of Coatings t o Lewisite posit of coating composition o f L None None 2.2 2.2 No steel present the panel. The time interval kw 2 kone Kone 2.2 2.3 Steel present taeen the point when the Coalr a 100% straight P.F. None 2.2 2.3 Intact, no change Brown 4 100% straight P.F. 2.2 2.3 Intact, b u t darkened ing composition passes the top of 5 U.F.,'alkyd, 50/50 2.2 White 2.3 Darkened and partly removed Kylon 6 2.2 None 2.3 Softened and sticky the panel and the point when it Hydrolyzed P.V.A 2.2 7 Xone 2.4 Softened and partly removed Long oil varnish 2.4 2.2 8 None passes the bottom of the panel. ic Intact, b u t darkened Sliort oil varnish Sone 2.3 2.2 9 Removed completely noted by meanr of a stop mat& Vinyl resin 10 Black 2.3 2.2 Removed completely Sulfidr resin 11 iione 2.3 2.2 Removed completely The panel is given a superficin 2.3 2.2 12 hlrlainine/alkyd, 50/50 None Badly blistered 2.2 13 Lortic acid resin None 2.3 Removed below liquid level bake (about 10% o f the normal 1009'o cresol F. resin Sone 2.2 14 2.3 Very finely b!istered bake) t o prevent subscquenr Uu taci tc/plast., 75/25 None 2.2 15 2.3 Completely removed 2.2 16 Kitrocell./plast., 63/37 Yone Completely removed 2.3 dissolving of the film, and is then 5 P.F. = phenol-formaldehyde, U.F = urea-formaldehyde, P.V.A. = polyvinyl acetate. recoated. It is then measured b Large amount of sludge. for film thickness a t the top and a t the bottom of the panc! The Der cent variation in filrr pinholing. These resins contain no plasticizing ingredient and thickness between the ton and the bottom of the Dane1 can be would probably not be satisfactory from a repistance standpoint calculated. The rate of flow can be expressed in inches per miri if they did. As a net result, films of such coatings aic brittle and lack adhcsion unless applied a t rather low film thickncsses. For GRAVITY PRESSURE VCCUUM clear films the bcst compromise bctnccn resistance and flexibility METHOD METHODS METHOD IS about 0.4 mil film thickness. Optimum enamel film thickness is about 1 mil. To be able to apply on the interior of bombs, shells, ctc., a uniform coating a t a low film thickness, it was decided to investigate a variant of the dip method of coating. Since in the case of cans, drums, bombs, and shells ono end of tiic container is closed, it is necessary, instcad of dipping the container, to fill the container with a coating composition and withdraw this coating composition a t a controlled rate through a definite size orifice by any one of three methods: (a) gravity flow, ( b ) presI I sure application, (c) the application of vacuum. Diagrammatic representations of these methods are given in Figure 1. Prior to the use of any of these methods, it was necessary t o determine certain variables controlling the deposition of uniform film of a definite film thickness. These variables are ( a ) the CONSTANT CONSTANT .rate of withdrawal of the liquid, ( b ) the solids, and indirectly AIR P R E S S U R E VACUUM the viscosity, of the coating composition. There are several mays in which this can be done. -4machine has already been developed bv Pavne ( 4 ) for dipping panels and viithdrawing thein a t a eontrollod rate from a solution of a coating coinposition CROSS SECTION Kowever, it \vas found simpler to construct a homemade apOF ORIFICE paratus in which the solution was withdrawn a t a controlled rate away from the panel. This apparatus is shown in Figure 2. A small rectangular box about 8 inches high, 2.5 inches wide, and 1 inch thick can be conveniently made from tin plate. The top is left open and the bottom carries a tube of metal. This short tube of metal is connected to a 32-inch-long glass tubc. the lower end of which terminates in a piece of rubber tubing, a clamp, and a special orifice. A number of different orifices can readily be made either by using different size pieces of capillary tubing or SCREEN S P E C I A L S; I Z E OPENING by melting d o m the end of ordinary glass tubing to give a series of different size openings. The purpose of the long glass tube is to minimize the effect of the difference in head and, therefore, Figure 1. Flow Coating \Iethods
TABLE I. RESISTAXCE OF COATINGS
,
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INDUSTRIAL AND ENGINEERING CHEMISTRY
/
299
the valve closed, the filled bomb is inverted over an empty bomb and is allowed to drain through an orifice of definite size. Since the bomb is about 18 inches long and has a volume of 105 cubic inches, it is necessary, in order to realize a film-deposition rate of 3 inches per minute, to withdraw the coating solution a t the rate of 17.5 cubic inches per minute or to empty the bomb in 6 minutes. After sevcral preliminary tests it is possible to select an orifice that will do this. It should be noted that, a t the start of the draining operation, material will recede away from the closed bottom at an accelerated rate, causing the deposition of an unusually heavy irregular film at this point. To minimize but not entirely eliminate this effect, a procedure is adopted whereby a small amount of the coating composition is withdrawn, the withdrawal process is interrupted for about 10 to 15 seconds to permit the thick coating on the closed bottom to drain donrn, and at this point the withdrawal process is resumed. Application of two coats with a short bake of 10 minutes a t 350" F. between coats and a final bake of 35 minutes a t 350" F. results in a film of 0.4 + 0.05 mil thick over widely separated areas of the container walls.
TABLE11. FILMTHICKNESS us. RATEOF FLOW AND SOLIDS R ~ +of ,~ Solids % o f Cdar
Figure 2.
Flow Coating Outfit
By coating separate psnels with a coating solution of certain solids a t three different coating rates (which is accomplished by simply changing the size of the orifice), a range of variations between the tops and the bottom of the panels is obtained. Changing to the same enamel but at a different solids by reduction of the enamel with thinner permits three additional panels to be coated at different rates of flow, and the variations m film thickness between the tops and bottom of the paneh are noted (Table 11). Inspection of these data permits selection of the solids of the enamel and the rate of flow that will give the thickest possible film showing uniformity from top to bottom of the panel. Attempts to attain thicker film than this by faster rates of flow will result in nonuniformity of the film. The selcction is made by establishing a graph for the three different solids enamels, (a) the hypothetical one at 0% solids, ( b ) one at intermediate solids, and (c) one at high solids. These plots are shown m Figure 3A. Since the data indicate that a flow rate commensurate with uniformity of film must not be eseecded, the maximum flow rate that will still observe this restriction is chosen for replotting purposes, as it will give the thickest film. At this definite rate of flow the film thickness from the three different solids enamels are read off and replotted as film thickness os. per cent solids of enamel. Inspection of this curve permits selection of the solids of the enamel or clear that will give a definite film thickness under definite flow rate conditions (Figure 3B). Having established, with this particular clew coating, that a rate of flow about 3 inches per minute is the maximum that will give uniform film thickness, and that a coating solids of 40% will give a film thickness of 0.4 mil in two coats, it is possible to translate this information to the problem of coating the interiors of tubular objects with substantially parallel sides. Suppose it 18 desired to coat the interior of a IO-pound incendiary bomb by the gravity method with this particular clear coating. An arrangement eurh as that shown in Figure 4A may be used. A bomb is filled with coating solution, and a suitable plug carrying an airvent tube anti R long draw-off tube with valve and orifice at the end opposite that in the plug is inserted in the filled bomb. With
~
l
~
IrdMih.
43.0 43.0 43.0 38.6 38.5 38.5
,
Panel Film Thicknesa of Two ~ Coats, a Mils Top Bottom
1.5
0.34 0.60 0.75 0.19 0.32 0.38
3.0 4.0 1.8 3.5 4.3
0.36 0.64 1.13
0.20 0.42 0.61
Variation,
Av.
%
0.35 0.62 0 94 0.20 0.37 0.50
5.9 6.7 50.7 5.3 31.3 60.5
a Plot indicates t h a t a rats of flow of 3 inches per minute will not give more than 10% variation.
Ute
The pressure method of emptying the container is perhaps easier and gives better film uniformity. Suppose it is desired to coat the interior of a 100-pound gas bomb of light gagc metal with a black phenolic enamel to a film thickness of 1.0 mil. From preliminary test data it is determined that an enamel of 42.5% solids and a rate of flow of 4.2 inches per minute will give the desired thickness and uniformity in two costs. An arrangement such as that shown in Figure 4B may be used. The bomb to be coated is filled with coating enamel and a suitable plug is inserted, this plug carrying a tube for the application of air pressure to the receding surface of the enamel, and a long tube having an orifice near the bottom of the container being coated, this lab ter tube being fitted with a valve near its other end and extending over into an empty bomb. Since the bomb is about 38 inches long and has a volume of 1685 cubic inches, it is necessary, in order to deposit a film a t the rate of 4.2 inches per minute, to force out the coating solution a t the rate of 186.3 cubic inches
A
1.0,
B
4 3 % SOLIDS
I
m'O.6 67 w
z Y 0 0.4 I
-
3 IN./MIN.
-
L
0 % SOLIDS
00
1.0 R A T E OF
Figure 3.
2.0 FLOW,
3.0
4.0
IN./MIN.
0
20 PERCENT
40
60
SOLIDS
Film Thickness m. Rate of Flow and Solids
INDUSTRIAL AND ENGINEERING CHEMISTRY
300
per minute or to empty the bomb in 9 minutes. With an orifice of 7/82 inch in diameter, a pressure of about 4.7 pounds (obtained by use of a reducing valve) will give the desired rate of removal of coating solution. The closed bottom of the container will, efter the draining operation, have on it a small amount of residual
I A
Figure 4.
Flow Coating Applications
oultting composition which, if not removed, will result in a thick, irregular film at this point. To minimize this edcct, this excess coating composition is sucked from the bottom by means of a tong, pointed tube connected to a suitably arranged trap and vacuum system. Application of two coats with a short bake of 8 minutes at 350" F. between coats and a final bake of 28 minutes et 350' F. results in a film of 1.08 * 0.10 mil thick over widely separated arcas of the container walls. The vacuum method of emptying the container is extremely convenient for coating the interiors of small tubular objects. Suppose it is desired to coat with a white enamel, to a film thickuess of 1.0 mil, the interior hollow of a 75-mm. shell, without coating the threads at the top opening which subscquent,ly receive the explosive head. From preliminary test data it is determined that an enamel of 51% solids and a rate of flow of 4 inches per minute will give the desired result in t\vo coats. An arrangement such as that shorvn in Figure 4C may be used. A shell is filled Kith coating enamel. To this shell is connected another shell equipped Kith a plug, this plug carrying a tube for evacuating the empty shell at a constant rate, and a second tube, equipped with a valve, extending over to and down to the bottom of the filled shell and terminating in an orifice a t that point. Since the portion of the shell to be coatcd is 10 inches deep and has a volume of 29.6 cubic inches, it is necessary, in order to deposit a film at the rate of 4 inches per mindte, to suck out the coating solution a t the rate of 11.82 cubic inches per minute or inch in to empty the shell in 2.5 minut,es. Vith an orifice of diameter, a vacuum of about, 19 inches of mercury (obtained by use of a vacuum pump and vacuum regulat,or) \Till give the desired rate of removal of coating solution, As indicated under the pressure method of the coating, the excess coating composition is sucked from the closed bottom of the container by means of a pointed tube-trap-vacuum arrangement. Application of two cxmtx with a short bake of 20 minutes of 350" F. between coats
Vol. 40, No. 2
and a final bake of 50 minutes at 350" P. results in a film 1.0 * 0.10 mil thick over widely separated areas of the shell walls. These methods of coating have been applied to quite a num. ber of different coating compositions, both clear and pigmentod, and have been found to work satisfactorily. The only precauliorl in the case of some (but not all) coating compositions is to be sure that each coat is set up sufficiently (but not necessarily coinpletely) EO that the application of a succeeding coat will not IT off the first coat. Short baking schedules will usually permit to be done, since one of the difficult parts of coating work of this type li" to obtain the propcr bake on the coating, the proccd(irc3 applied were t,hose outlincd tiy Graves @), concerning the estimation of the degree of baking obtained on finishes on metal object? during their time in coming to temperature and their time a i tempcraturc. These procedures involve ( a ) determining t,hi. rate of heat-up of t,he met,al objects in an oven by lastenipg B thermocouple thereto, ( b ) integrating these heat-up curves with reference to the per cent of reaction obtained a t equal inure ments of time but at diffcrens temperatures, and (c) selecting the time of the object in the ovcn when the summat,ion of tllew reaction values quais 100%. This method has given good re. sults with a varic,ty of diffcrent objects coated. Refwencs t,u Table I11 will give some indication as to how these baking t,imer vary xith the differcnt objects coated but with the same coating composition baked to the same degree in each case. A considerable part of this work was utilized in tho develop nicnt of fuel and lubricant container coatings where the sttmc: problems of resistance to solvents, uniformity of coating, and thin coating for flexibility purpoxs exist.
TABLE 111. BAKING Trims FQH DIFFERENT OBJECTS Time t o Obtain 100% Rake on Coating Objeo? s t 3.50" F.a, M i n 24-gag~panel 6 100-lb. hornbb 2E 10-lh. hombC 14 250-!h. botnbb 25 75-mm. shell 50 Steel test cups 27 Using a 100% straight p h e n o l - i ~ ~ ~ i ~ lcoatina. ~l~liyd~ 6 Thin gage metal. A t the diaphragm which is thin and located about 2 iliohes frorii twtbori,
The coating of the insides of contaiuers of ;he type prcviou4, mentioned can be accomplished by witable spray apparai 11" It is felt, that the nierhod of coating outlined in 'ihiii paper is, generailv, not, so commercially applicable ils the spiny nictJrr,d In certain cases whcre suitable spray equipment is not availxi& or whcre expensive masking operations of portions of the C I I I I tainer interiors are to be avoided, the methods of c o s h g by C O I I trolled vithdraival of t,he coating composiiion ma!. b(s i i s 1 ~ ~I O1 advantagc. DISCUSSION OF K E S U L T S
Coatings based upon 100% st,raight plienol-forIrialdctlvtii, ~ ' 1 ' hides have been found satisfactory for coating the interiors (11 containers t,o prevent the deterioration of must,ard gas arid 1 1 mixtures when stored therein. Several modifications of the basic. dip niethod have bt3:ii 118 vestigated for coating t,he interior 01' tuhular-t,ypc eonhiners The results indicate that it is feasible to coat the interior of C O I I tainers xith substantially parallel sides by withdra~~ing thc CXJBT ing composition a t a predetermined, contro1ii:d rate t,o deposit t+ uniform film of coating composition. The removal of t,he coaririp liquid from the interior of one container and its transfer to a *utisequent container to be coated may be accomplished l);? gruvit) , pressure, or vacuum. Certain information concerning a siniple mrthod of establishing t,hc cnrreot miiditions of (,a) coating
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February 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
composition solids and ( b ) rate of flow of a coating composition to give specified uniform film thicknesses has been given. Although no extensive plant experiments have been conducted on this method of interiorly coating objects, it is believed that, in certain cases, a production line setup could be established to accomplish unique results by this method, particularly where it is desired to coat certain sections of the containers without resorting to expensive masking operations.
OEM-sr-796. Permission to publish tjhe resultts of this gation is gratefully acknowledged.
301 iirvrrti-
LITERATURE CITED
(1) Chemical Warfare Service, unpublished work, Edgewood Ameiial Edgewood, Md. (2) Graves, Stuart, IND. ENG.CHEM., ANAL.ED., 16, 599-602 (1944, (3) Marvel, C. S., unpublished work, Univ. of Ill., Uibana, Ill. (4) Payne, H. F., IND. E m . CHEM.,ANAL.ED.,15, 48-56 (1943)
ACKNOWLEDGMENT
This article is based upon work performed for the Office of Srirntifir Research and Development under Contract No.
RECEIVED July 17, 1946. Presented before the Division of Paint, Varnish. and Plaatica Chemistry a t the 110th Meeting of the AMERICAN CHEMroai SOCIETY,Chicago, Ill.
STABLE RED PHOSPHORUS M. S. SILVFXSTEIN, G. F. NORDBLOM, C. W. DITTRICH, AND J. J. JAKABCIN Frankford Arsenal, Philadelphia, Pa.
ED phosphorus, or amorphous phosphorus, as the commercial product is sometimes called, is notcd for its dcterioration when cxposed to air a t normal temperaturcs and humidities. The acidic, hygroscopic, and poisonous propertics of the products of oxidation and the spontaneous combustian hazards of commercial red phosphorus provcd highly objectionable in certain applications of this material and resulted in a wartime develop ment of a stable type of red phosphorus. This article summarizes the more important aspects of this invcatigation. The oxygcn and water vapor of the air combinc with red phosphorus to form a mixture of phosphorus acids-namcly, phosphoric, phosphorous, and hypophosphorous acids and a small amount of phosphine. The mole fractions of tho acids vary slightly with oxidizing conditions and duration of oxidation, and are approxirnatcly 0.40: 0.55: 0.05, respectivcly. Thcsc acids are hygroscopic and are rcsponsible for the sticky, nonflowing character of tho present commercial product after cxposure. The amount of phosphine formcd in the oxidation of red phosphorus is rclativcly small, thc ratio of phosphorus combined in phosphorus acids to that in phosphine being approximately 13 to 1. Neverthcless, phosphine formation is important, for the gas is quite poisonous, and care must bc exercised where largc quantities of red phosphorus are cxposed or where relatively small quantities are exposed for long periods of time. Henderson and Haggard' state that the symptoms of phosphine poisoning resemble those of food poisoning. The maximum concentration of this gas that can be inhaled for one hour without serious results is given as 100 to 200 parts of phosphine pcr million parts of air. The handling of large quantities of red phosphorus raises the problem of spontaneous combustion. I t is to he cxpected that the exothermic oxidation which occurs when rod phosphorus is exposed to air will result in a tempcrature gradient with ita highest point toward the center laycr of red phosphorus, since the surrounding phosphorus acts as a heat insulator. However, the higher the temperature the greatcr the rate of oxidation; conscquently more heat is produccd, and this causes the tomperature to rise still further. Considering a laycr of red phosphorus with a cross-sectional area much greater than the thickness, therc is a maximum thickness of laycr below which this temperature rise is brought to a halt by the accompanying increase in rate of hcat transfcr before the ignition tempcrature is rcachcd. But with thickncsses greater than this crilical laycr thickness the temperature mounb in a pyramiding fashion and the vapor 1 Renderaon and Haggard, "Noxious Gama," A.C.S. Monograph 86. Chemical Catalog Go., 1927.
above the mass begins to glow until the whole mass of phosphorus bursts into flame. The critical thickness of the layer is a function of the ambient tcmperaturc, heat transfer coefficient, and rate of generation of heat. This function takes the form:
where Y = critical thickness of layer, cm., above which spontencous ignition occurs X = distance from plane of maximum temperature, cm K = hcat transfcr coefficient, cal./cm./' C./sec. To = autogcnous temperaturk, C. 2'. = ambienttemperature, C. Q = hcat of reaction, cal./cc./sec. Equation 1 shows that the thickness of layer above which spontaneous ignition occurs is inverscly proportional to the square root of the rate of generation of heat, which is directly proportional to the oxidation rate of the red phosphorus. Thus it is evident that the principle disadvantugcous charaateristics of commercial red phosphorus-namcly, its acidic, hygroscopic, poisonous, and spontancous combustion propertiesare a result of, and incrcase with, its rate of oxidation. The problem, thcrcfore, consisted of stabilizing red phosphorus-that is, drastically reducing its oxidation rate. Thc solution of the problem was resolved into the identification and classification of the accclerators and inhibitors of the oxidation, followed by the development of methods for removing the principal acccleratom and methods for applying the best inhibitor. The effects of a number of classes of inorganic and organic substanccs on the oxidation rate were investigatcd. I t w a ~ found that the most important effects are obtained with certain mctals, in clcmentary form or as the oxides or salts. The effect on the oxidation rate a t 60" C. and 90% relative humidity of samples of rcd phosphorus containing 5% powdercd metals ie prcsented in Table 1. Table I shows that copper, bismuth, silver, iron, and nickel increase the oxidation rate drastically, that cadmium and tin increase the rate moderately, that lead and chromium have very littlc effcct on the rate, and that aluminum and zinc decrease the rate. Thus, the mctals present in red phosphorus are an extremely important stability consideration. Of all the matcrials tested, the best inhibitors of oxidation were found to be the hydroxides of the inhibiting metals of Table I, of which aluminum hydroxide w&s by far superior. Tho effect on
