Zinc Oxide in Rubber H ~ R L A A. N
T
DEPEW,American Zinc Sales Company, Columbus, Ohio
HERE are few
is a matter of first importance. The a m o u n t of rubber 5m~0c used in i n d u s t r y has increased rapidly in the past 4mpc 2 two decades (Figure l ) , but the a m o u n t of zinc oxide Joo,oc U S CRUDE R & K R COhSUMPT ON p r o d u c e d h a s increased 5 comparatively l i t t 1e . Of this zinc oxide, perhaps half 2mpc or two-thirds has been used U S ZINC OXIDE P R O O U C T I O N I w,cc by the rubber industry and the balance by the paint industry which u t i l i z e s aptxoximatelv 20 Der cent adl e c t e d patents. The vaF~~~~~1. cRUDE R~~~~~ c~~~~~~~~~~ AVD zINCoXIDE hitional1ea"ded hnc oxide. The consumption t r e n d PRODUCTION FOR U. S. lidity of many of these is has continually c h a n g e d , questionable, s i n c e t h e y have never been litigated, and no claims are made regarding working one way for one rubber product and the reverse directheir value except that they describe the processes used in the tion for others. Today the number of rubber products that contain zinc oxide has increased until practically all contain industry more satisfactorily than can be found elsewhere. The methods of making zinc oxide can be only outlined in some; but the percentages have decreased to comparatively the space available, and it is to be hoped that additional in- small amounts, although in the case of solid tires (which are formation will be presented by those who find the discussion gradually disappearing from the highway because of pneumatic tire competition) the percentage has increased to as much as incomplete. There is considerable misunderstanding among nonrubber G5 per cent by weight. Against the general trend to lower technologists regarding the use of zinc oxide. It is hardly zinc contents, the tendency has been to increase the amount necessary a t the present time to remind rubber technologists of zinc oxide in the carcass compounds of heavy-duty pneuthat zinc oxide is not a filler, inasmuch as the cost on volume matic truck tires to 30 or 40 per cent by weight. It seems basis is approximately ten times that of rubber, without con- probable that the percentages used are reaching a point of sidering the cost of incorporating it into rubber. It is used stability, and that in the future the consumption of zinc oxide largely as an essential chemical ingredient and its chemistry will follow that of rubber. s e c r e t s in the zinc industry, as in most industries, but t h e r e h a s been a reticence to permit the publication of a r t i c l e s regarding the industry that has r e a c t e d to its detriment. There is, a c c o r d ingly, comparatively little l i t e r a t u r e , and much of that D a r t of t h i s DaDer
e....
I. Chemical and Physical Properties of Zinc Oxide and Their Relation to Rubber-Making Properties INC OXIDE is manufactured by two general methods. zinc oxide particles i t exists as adsorbed sulfur trioxide or In the first, it is made directly from ore and the process chemically combined as zinc sulfate, and as a basic zinc sulis known as the American. I n the second, or French fate. Since sulfur trioxide and zinc sulfate are acidic, the inprocess, zinc metal is first produced and the oxide is made by corporation of this sulfated zinc oxide makes the rubber slightly volatilizing and burning the vapor in air. The outstanding acid, rather than slightly alkaline or neutral as in the case of a difference in the oxides made by these two methods lies in nonsulfated zinc oxide. This acidity mevents the zinc oxide from reacting with the proteins the amount of sulfur adsorbedwhich e x i s t in a p p r e c i a b l e on, or combined with, the zinc Zinc oxide for rubber compounding is essena m o u n t s in the crude rubber oxide; the American p r o c e s s and thereby improves f a c t or y tially a chemical ingredient and the acidity (sulfur oxide contains sulfur, and the processing. Stocks containing French process oxide is substantrioxide) plays an important part in the reactions A m e r i c a n process zinc oxide tially free from it. that occur during vulcanization, the acidity prem i l l relatively smoothly, and venting undesirable protein reactions. Other CHEMICAL PROPERTIES tube and calender with smooth chemical ingredients to be considered are small s u r f a c e s and clean edges. It Sulfur is necessary in many has been stated that plant foreamounts of lead, cadmium, and fatty acid. The i n d u s t r i e s , including rubber men can see this improvement manufacturing, and it is an inphysical properties that affect the behavior of zinc when A m e r i c a n process zinc jurious impurity in other prodoxide in rubber are particle size, uniformity of oxide has been used in a tread ucts. When p r e s e n t in u n i size, and particle shape; these injuence the instock c o n t a i n i n g o n l y 4 to form amounts in zinc oxide, it corporation of the zinc oxide in rubber and the 5 per cent z i n c oxide. Proch a s b o t h advantages and disessing is one reason that acid reenforcement when large amounts are used. advantages. On the surface of
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371
I N 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
April, 1933
zinc oxide is frequently chosen for inner tube manufacture. When a stock is alkaline, zinc oxide reacts with protein in the rubber, and a semi-rigid structure develops that makes the stock heterogeneous and harsh, and accounts for the inferior factory working properties. In no instance is homogeneity more desirable than where a stock has to be flexed; Gibbons (11, 12, 16) recommends the addition of strongly acid materials such as zinc chloride to rubber compounds with extra
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Rubber Sulfur Stearic acid Zinc oxide Diphenylguanidine
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sorb water even when compounded in a rubber stock. Accordingly, a stock may swell too greatly to be suitable for a packing in a water pump, although the same packing may show no deleterious effects when used in nonhygroscopic solvents. Similarly, in the case of electrical insulation, the dielectric constant of water is 80 and of rubber is 2.5. The absorption of water will accordingly cause undesirably high dielectric constants, and it will also increase the electrical leakage. In commercial practice, however, where the amount of zinc oxide added is not great and where lime or litharge is used in the compound, the injurious effect of the hygroscopic material may be largely or wholly overcome. The addition of 10 parts of litharge to the following compound containing American process zinc oxide reduced the swelling in mater of thin sheet rubber from 50 to less than 20 per cent, which is comparable with the swelling developed by thin sheets of rubber containing French process zinc oxide:
50 P A R T SO F i Y C
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FIGURE2. EFFECTOF ACIDITY OF ZINC OXIDE ON RATEOF CURE UITH BOTH ACID AND ALKALINE ACCELER.4TORS FoRMULaS
Rubber Sulfur .Mercaptobensothiazole Butyraldehyde aniline Stearic acid Zinc oxide
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accelerator to overcome the retardation of cure caused by the acid. I n the case of a certain compound he obtained more than double the flexing life through the addition of acid material. An acid zinc oxide will furnish enough acidity to prevent the zinc protein reaction in most stocks, and comparatively large amounts of acid zinc oxide may cwercome the alkalinity caused by alkaline compounding ingredients. Acidity has been shown to improve not only the factory working and flexing of stocks but also the scorch resistance (17 ) . The cure of rubber stocks is faster in alkaline stocks than in acid stocks. Accordingly, as might be expected, rubber containing French process oxides tends to cure more rapidly than rubber containing American process oxides, and the difference in cure increases with the amount of zinc oxide in the compound; the difference in curing rate is negligible when only 10 to 15 per cent is used. In stocks accelerated with organic accelerators, the effect depends on the kind of accelerator used. In the case of alkaline accelerators, such as diphenylguanidine or the butyraldehyde aniline accelerators, acid material retards the cure more than it does for acid accelerators, such as mercaptobenzothiazole and the thiurams. Irrespective of the amount of zinc oxide and, therefore, the amount of acid material within limits, the time to the optimum cure is approximately the same when mercaptobenzothiazole is used; but with butyraldehyde aniline the time required to reach the optimum increases with the amount of acid zinc oxide (Figure 2). The effect of adding more accelerator is to increase the accelerator cost, but, since accelerators often have antioxidant properties, the antioxidant cost may be correspondingly decreased. The acid effect can be largely overcome by adding alkaline materials, but in this case the advantages of the acid material are lost. I n one phase of rubber compounding, acid zinc oxide may be undesirable. Zinc sulfate is hygroscopic and tends to ab-
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It is evident either that the zinc sulfate migrated to the litharge or that lead soaps migrated to the zinc sulfate. Migration of the zinc sulfate is improbable; it has been shown ( 7 ) that sulfuric acid stored in rubber-lined tanks will not leach zinc oxide compounded in the rubber. As another example of the reaction in rubber between litharge and zinc sulfate, an oxide with an excess of litharge that gave a gray stock was mixed with another containing an excess of sulfur trioxide, and a white stock was obtained. The sulfur trioxide in the one oxide reacted ITith the litharge in the other to form inert lead sulfate, and this reaction must have taken place during milling. Zinc oxides containing excessive amounts of acid material may form pellets of unmixed pigment when incorporated in rubber. This pellet formation does not develop except in internal mixers unless the amount of acid material is much greater than is present in the oxides used in the rubber indus-
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try today. Some years ago, oxides with high water-soIuble salt contents were sold t o the rubber trade in the same car with oxides of low water-soluble content, and the amounts of acid were so great that unskilled workmen in the plant could separate the lots by noting the unmixed pellets that formed during milling. The amounts of acid were so large that tensile strength determinations in a rate-of-cure formula that was used showed variations from 400 to 2800 pounds per square inch among the lots. Today the variation is rarely more than a couple of hundred pounds in the same formula, and the acid American process zinc oxide mixes a t least as well as the low-acidity French process zinc oxide on the mills. When inclosed mixers are used, special procedures have been
31.
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worked out by wlricli Airiericaii pruccss zinc oxide lias lteeii satisfactorily lrandled in certain plants. Some technologists have gone so Ear as to say that the wliolo problem of Ilanbiiry mixing is one of mixing procedure rather than of oxide charttcteristics. Other teclinolrrgists take the viewpoint that. no zinc oxides mix satisfactwily in the internal mixer, aiid they prefer to a.dd tlie oxide on tlic rolls. In a few eases, rubber mixed mith French process oxides in an inclosed mixer has i!til\wized and cotild not lx made to coalcsce. The same p l ~ c ~ n i ~ m m lias ~m also becii 01 mixing on tlie mills mlme stocks appear to be biirned aitlroat any sulfur being present. The most reasonable exlrlanat,ion is that the protein in the crude rubber (high in protein and low- in fatty acid) reacted with the zinc o d e to give it tlin burned appearance. In addition to sulfur, aiiic oxide may contain various other irigredients such as lead and cadmium, hut these are as likely to be found in oxides made by tho oue prncess as by t.tre other. As a matter of interest, one of the zinc oxides analyzing liigliest in zinc oxide content, with tlie exception of tlre sulfur, is
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proce inc oxide, atid one that analyzes lowest is a French process zinc oxide. Copper aiid chlorine, especially tlie Eormw, are iindesirable ingredients in zinc oxides ljecausc of their influenee in causing rubber to deteriorate (age). At the present time nolie of the sbandard zinc oxides on tlie market has enough of either of these materiah so that they need be seriously considered. Lead exist.s in American process zinc oxide as lead sulfate, tirid in French process oxides as free lead oxide that will react with proteins to form prodiicts that cause fast cures with merraptobenzotliiazole. It is questionable whether t.his rnethod of obtaining acccleration is desirable since Dinsmore (8) reports lower abrasion resistance Cor mercaptobenzotlii:~nole-diphenyl guanidine ncceleratcd stocks. Free lead oxide (19) will also retard tliiuram-accelerated compounds. Lead as lead sulfate is inert except in alkaline stocks when some of tlie lead may go into solution and prevent a desirable white color. Tliornpssnn and Brathy (18) recommend as a matter of salety not over 0.1 per cent lead and preferably lower. It prulral~lydoes iittle liarrn in black stocks. Lead obtained
April, 1933
INDUSTRIlL AND ENGINEERING CHEMISTRY
its bad name largely because it was associated with variable amounts of sulfur rather than with uniform amounts, and, accordingly, the rate of cure mas nonuniform. Cadmium exists in American process oxide a t least partly as cadmium sulfate, which is water-soluble and retards thiuram stocks ( 9 ) , but there is evidence that it enables other accelerators t o cure more satisfactorily (possibly by absorbing hydrogen sulfide that tends to decompose the zinc accelerator compound) ; it is accordingly considered desiral ]le by some rubber manufacturers and undesirable by others. Cadmium is also of value in retarding scorching (4)in the case of thiuram accelerators. Some oxides have been offered t o the trade, surface-treated with fatty acid that is deposited on the surface and that exists as free acid or as the zinc soap. The purpose of adding this material has been t o change the surface characteristics from inorganic to organic, similar to rubber. Fatty acid is soluble in rubber, and a surface coating of this material is intended to pull the zinc oxide particle into the rubber during mixing by virtue of the solution pressure of the stearic acid molecule. The fatty acids most frequently found a t present are commercial stearic consisting largely of stearic [CHa (CH& COOH] and palmitic [CH, ( C H Q )COOH] ~~ acids, and the fatty acids from coconut oil that consists of lauric [CH3 (CH2)io COOH] acid with some other acids of the same series of both higher and lower molecular weights. Fatty acid on the surface of zinc oxide is conceded t o act chemically about the same as an equivalent amount of fatty acid added to the rubber. I n general, the fatty acid treatment has failed to achieve all that had been hoped for it in the way of improved Banbury mixing and shorter mixing time. Some surface-treated zinc oxides that appear to disperse well in rubber form many small unmixed pellets that are seen when the stock is examined microscopically. In the case of certain mixing schedules, horn-ever, there are reports of faster mixing. I n other cases difficulties have been reported due to fatty acid lubrication of sides of the mixer that allowed slippage and a t times caused the mix to pulverize. The use of acid surface treatment is fairly old. Zinc oxide was surface-treated in the nineteenth century (14) with fatty acid to improve its mixing qualities in oil. Without going into the large number of patents on this subject, it should be noted that the process has become commercial in the case of lithopone Tvhich is treated commercially with soaps ( 2 ) and with free fatty acids (15) to improve the mixing in paint oils.
PHYSICAL PROPERTIES Particle size embraces not only the average particle size but the distribution of sizes (Figure 3). Two oxides could be of the same average particle size-for example, 0.2 microns-and yet 90 per cent of the particles in the one pigment might have a particle size within 20 per cent of the average, n-hereas in the other case not over 40 per cent would have particle sizes within this range. There has been considerable discussion as to whether a uniform pigment or a nonuniform pigment is desirable. It would seem, however, that the weight of evidence strongly favors a uniform particle size. Fine particles ( I O ) mix into rubber with more difficulty than the coarser ones; they develop less resilient stocks and are more likely to cause scorching. Accordingly, it would seem desirable to eliminate them. The poor mixing of fines in zinc oxide can be compared directly with the frequently noted poor mixing of sulfur and of accelerators that contain too much fine material. It has long been thought that fine pigments activated better than coarser ones, but recent data (5) have indicated that particle size has little influence, as might have been expected on a theoretical basis. Activation is the reaction of zinc with
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the accelerator to form the zinc accelerator compound; the reactions between fatty acid present in the rubber and zinc oxide, and, in turn, between the zinc soap so formed and the accelerator, are so rapid that, irrespective of the particle size of the zinc oxide, there is sufficient time for the zinc salt of the accelerator to be formed during processing. Accordingly, it is necessary only for the zinc present to act as a reservoir to replace any zinc that has been precipitated by hydrogen sulfide during curing and to form zinc soaps which, by mass action, will keep a large percentage of the accelerator in the form of the zinc compound. Relatively coarse zinc oxide is as capable of forming the accelerating zinc compounds as ?24
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CURVESCOMPARING ACICULAR WITH FIGURE 5. STRESS-STRAIN NONACICUL~R ZIXC OXIDES
Several years ago when a zinc oxide of fine particle size was offered t o the trade, there \vas considerable speculation as to whether small amounts of this oxide could be used in the place of larger amounts of ordinary oxide, but laboratory tests and commercial experience have shown that the finer particles do not make it possible to reduce the quantity. In fact, in certain cases the fine zinc oxide actually has tested unsatisfactorily, owing to adsorption of some of the curing ingredients. I t seems reasonable that the differences that have been noted in the activating properties of zinc oxides are due to differences in the kind and amount of nonzinc constituents, such as sulfur, lead, and cadmium, rather than in the physical characteristics. When larger amounts of zinc oxide (50 per cent by weight) are used, the reenforcing properties of the oxide come into play, and the finer particles tend t o develop higher tensile strengths. It is frequently necessary, however, from the factory working viewpoint to add more fatty acid in the case of the finer zinc oxides; this addition lowers the reenforcement of the compound to a point where it is little better than if coarser zinc oxides had been used. It is obvious that, when only 5 per cent of zinc oxide is used in a carbon black stock and part of this zinc oxide goes into solution, the change of reenforcement that this one volume of zinc oxide will make in a mix containing 100 volumes of rubber and perhaps 25 volumes of carbon black will not be measurable. I n the case of American process oxides, particle size has little influence on the rate of cure, but in the case of French process oxides, the finer particle sizes accelerate the cure when.large amounts are used ( 3 ) . Although this paper is intended to be a summary of facts rather than theories, it may be helpful to point out that zinc polysulfides ( I ) are
I N 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 lbf I S T R Y
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curing ingredients, and that, when large amounts of zinc surfaces are present, sufficient amounts of zinc polysulfides may be formed in alkaline stocks to account for the acceleration. The oxides that have been discussed approximate spheres; that is, no one dimension tends to predominate. Oxides of unusual shape can be made, however, as shown in Figure 4. The particles in one of the photomicrographs consist almost entirely of needles. These needles apparently mill into rubber and develop a strawstack structure, and thereby stiffen the rubber as shown by the stress-strain curves in Figure 5 , Acicular zinc oxide is desirable where specifications require stiffness; it is also desirable where stiffness will develop superior service life, possibly in air bags. At present this grade of oxide is used largely in paint where it reduces checking and cracking. With reference t o grit, nothing can be said in its favor. It has no virtues and serves only to lower tear resistance and to increase the possibility of flexing failure. Optical properties are determined basically by the refractive index which is fixed as definitely as specific gravity. The actual whiteness of rubber containing different zinc oxides, however, depends on particle size, chemical properties, and cleanliness. Often the color of the compounded rubber depends on the chemical ingredients in the zinc oxide more than on the whiteness of the zinc oxide itself. However, if the zinc oxide is too dirty, caused, for example, by carbon from electrodes, it will not be possible to obtain the best whites. The whitening strength, which refers to the ability of the pigment to overcome the dark color of the rubber-sulfur compound, depends on the particle size of the zinc oxide, and it
Vol. 25, No.4
has been shown that the optimum whitening in rubber (6) is obtained with zinc oxides of about 0.15 micron (Goodyear scale) which is about the size of most commercial zinc oxides. Referring again to the question of uniformity of particle size, an oxide containing a range of particle sizes, some extremely small and some coarse, will have a much lower whitening strength than an oxide consisting largely of uniform particles, in the neighborhood of 0.15 micron in diameter. LITERATURE CITED (1) Bedford, C. TT., U. S.Patent 1,532,645 (April 5, 1925). (2) Booge, J. E., and Hanahan, M. L., U. S. Patent 1,722,174 (July 23, 1929). (3) Breyer, F. G., Gaskill, E. C., and Singmaster, J. il., U. S. Patent 1,522,098 (Jan. 6, 1926). (4) Bridgewater, E. R., U. S.Patent 1,822,561 (Sept. 8, 1931). (5) Depew, H. A , IND. ENG.CHEM.,24, 565 (1932). (6) Depew, H. A., A'ew J e r s e y Zinc Co. Bull., p. 17 (Jan., 1931). (7) Depew, H. A., and Lewis, A. R . , India Rubber World, 75, 129 (1926). (8) Dinsmore, R. P., Inst. Rubber Ind. Trans., 7,406 (1932). (9) Easley, M. K., and Eide, A. C., ISD. ENG.CHEM.,24, 568 (1932). (10) Gaskill, E . C., U. S.Patent 1,743,946 (Jan. 14, 1930). (11) Gibbons, W. C., British Patent Application 367,183 (1932). (12) Gibbons, W. C., Canadian Patent 321,430 (1932). (13) Jones, H. C., and Depew, H. A,, IND.ENG. CHEM.,23, 1467 (1931). (14) Laurie, A. P., British Patent 4870 (Jan. 13, 1894). (15) O'Brien, W.J., U. S.Patent 1,832,416 (Nov. 17, 1931). (16) Schidrowita, P., India Rubber J., 83, 376 (1932). (17) Thies, H. R., IXD. ENG.CHEM.,23, 1357 (1931). (18) Thompson and Bratby, J . Oil C O ~ O UChem. T Assoc., 14, 218 (1931).
RECEIVED October 31, 1932. Presented before t h e meetings of the Boston and Chicago Rubber Groups of t h e American Chemical Society, November 2, 1932, a n d January 13, 1933, respectively.
Proposed Manufacture of Monopotassium Phosphate at Green River, Wyo. 111. Manufacture of Potash from Leach Solutions ROBERTD. PIKE,4069 Hollis St., Emeryville, Calif. amount will be kept small. The following mixture is practical: 3 parts wyomingite, 1 part sodium chloride, and 4 parts water. At 200" C. (392' F.) for 3 hours this mixture will take into solution about 8.0 pounds KzO per 100 pounds wyomingite. The leach solution from the autoclave, not including wash water used in filtration or injected steam, is made up of 17.55 grams of sodium chloride and 9.51 grams of potassium chloride per 100 grams of water. R e f e r r i n g to Figure 1 from Blasdale (1) the concentration of the leach solution is represented by t h e p o i n t m a r k e d ml. If the solution is e v a p o r a t e d by -B E boiling a t 100" C. (212" F.), the SEPaRATION O F SODIUM CHLORIDE c o n c e n t r a t i o n of the residual rn AND POTASSIUM CHLORIDE solution c h a n g e s continuously, 5;-" the concentration b e i n g repreIt has b e e n s h o w n that the ? sented by points along the line amount of water present in the Om,, p r o d u c e d . W h e n t h e Lemberg r e a c t i o n r e l a t i v e to e v a p o r a t i o n has c a r r i e d the the w y o m i n g i t e and salt does G R A " 6 KC I PER 100 G U M S &O OF poTAssIUM CHLORIDE AND c o n c e n t r a t i o n to t h e p o i n t not in itself affect the extrac- F~~~~~1. sOLUBILITY A , which is the i n t e r s e c t i o n SODIUM CHLORIDEAT VARIOUSTEMPERATURES (1) tion of potash. Therefore, the
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H E extraction of potassium from wyomingite by the Lemberg reaction which employs base exchange with sodium has been described in Parts I and I1 of this series ( 3 ) . I n this paper will be described the separation of salts of potassium from the leach solutions which result from the Lemberg reaction, and estimates of costs of production. Two principal cases will be considered: the solution of sodium and potassium chloride 00 resulting from the use of common salt for base exchange, a n d t h e solution containing the chlorides and c a r b o n a t e s of sodium and ps potassium resulting from the use of Green River soda brine. 8
9 2 . I
