ROY 6. BOSSERT Ohio Wesleyan University, Delaware, Ohio
A
SOAP, as the term is commonly used, is the sodium or potassium salt of the acids of higher molecular weight, such as sodium palmitate, potassium stearate, sodium oleate, etc. These soaps are "soluble" in water and have been used as detergents for centuries. The so-called metallic soaps, which have been known for several hundred years, are the heavy metal salts (the metals other than the alkali metals, sodium or potassium) of the acids of high molecular weight, such as calcium stearate, copper oleate, cobalt naphthenate, etc. In short, the metallic soaps are those soaps which are insoluble in water. They differ from the ordinary soaps not only in water solubility but also in their increased solubility in nonpolar (organic) solvents. They possess properties which make them very useful in a great number of special applications. The annual production of the metallic soaps, although increased greatly in the last two decades, is not indicative of the importance of the metallic soaps in industry. Historically, the uses for the metallic soaps developed with the search for better lubricants since it was found that the gels formed from the metallic soaps and petroleum hydrocarbons served as good lubricants. Since that time, hundreds of applications for the metallic soaps have developed. From a practical standpoint,
Presented before Section H, Chemistry, Ohio Academy of Science, Marietta, Ohio, Msy 2, 1947.
their increased solubility in nonpolar solvents and their comparatively high metal content lend to them rather uniciue properties. CHIEF METALS AND ACIDS
There are twenty-seven of the metals and about twelve acids (or mixtures) which are used commonly in the production of the metallic soaps. The chief metals, the chief acids, and the sources of the latter are presented in Table 1. The table shows that the most important metals, from the standpoint of commercial production of metallic soaps, are Al, Ca, Zn, Mg, Pb, Mn, Cu, and Co. The most outstanding acids are stearic, palmitic, oleic, linoleic, naphthenic, the tall oil acids, and the rosin acids. The animal and vegetable fats and oils serve as good sources for the wellknown acids of commerce, such as stearic, palmitic, oleic, linoleic, and ricinoleic. The lesser known acids, such as naphthenic, the rosin acids, and tall oil acids have increased in importance within the last few decades and deserve special mention. Naphthenic Acids. The naphthenic acids are mixtures of the monocarboxylic derivatives of the cylcoparaffin hydrocarbons which are isolated from various petroleum fractions. The original crudes in which they are found chiefly are the Russian, Rumanian, Louisianan, Texan, and Californian. The naphthenic acid
TABLE 1 Chief Metals and Acids i n Metallic Soaps* Periodic q"JuP I Cu, Ag
I1
I11
Chief metals
Chief acids 1. Saturated
Laurie Alyristic Palmitic Be, Ca, Sr, Ba, Mg, Zn, Hg, Cd Stesric 2. Unsaturated Oleic Linoleic Al, TI, Ce Linolenic
Natural sources of acids
Metallic soaps
Animal and vegetable fats:
Laurates Tallow Myristates Lard Palmitates Steamtes Palm Oil Animal and vegetable oils: Olestos Olive Linoleat,en Linseed Linolenates Cottonseed Sovbertn
tives of oyolopamffins) Naphthenic VII M n 5. Rosin (Abietic-typical) Rosin VIII Fe, Co, Xi 6. Tall Oil Tall Oil (pine) * The most outstanding metals and acids are in hold face type.
Naphthenates Resinates (Abietates) Tallates
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JANUARY. 1950
The precipitated soap is washed well in several changes of hot water to remove the soluble salts. The water content is lowered to 0.1 to 1 per cent (depends upon soap) either in the original kettle, in drying ovens, or by air-drying. The higher melting soaps are then reduced to proper consistency in a hammer mill. The reverse addition (the addition of sodium soap to metal salt solution) also requires carefully controlled conditions and produces a soap which may contain occluded metal salt and the free acid as the chief impurities. The simultaneous addition of reagents (both Where n is 0 to 3, in general (8-12 carbon atoms). The solutions added a t the same rate to a third container of molecular weights run from 170 to 200. The bicyclic water) produces a metallic soap of high purity and conacids, C.Hz,-,02, range from 13 to 23 carbon atoms tains a minimum of occluded materials. From a practiper molecule and have higher molecular weights (I). cal standpoint, the preparation by the simultaneous The metallic soaps produced from the naphthenic addition of reagents is a costlier one. In the final analyacids are known as naphthenates. The metal naph- sis, the method of precipitation employed is governed thenates form one of the most important classes of by the requirements of the soap desired. metallic soaps. Fusion Method. The fusion method results from The Rosin and Tall Oil Acids. These acids are ob- the interaction of the organic acid (fat or oil in some tained from the gummy products of the pine tree. The instances) and the oxide, hydroxide, carbonate, or Chief rosins, in view of metallic resinates, are gum rosin acetate of the desired metal at a somewhat elevated from the oleoresin and wood rosin from stumps. Al- temperature (180-300°C.). This method is illustrated though over 30 different rosin acids are reported, the as follows: chief acid associated with these rosins is abietic acid, 2RCOOH MO (RC00)JvI + FLO CzoH3002,a monocarboxylated derivative of hydroAoid Metal Oxide Metallic Soap genated (terpene family) phenanthrene. Whereas the ZRCOOH M(om (RCOO)~ 211,o rosins are the products remaining from the distillation Acid Metal Metallic Soap Hydroxide of turpentine from pine, the tall oil is a product isolated from the pine tree during the production of paper pulp MCO, (RCOO)*M + H,O + CO 2RCOOH Acid Metal Metdlic Soap by the alkaline paper pulp process (8, 3). The acids Carbonate from tall oil are mixtures of rosin acids and fatty acids The reaction proceeds in the absence of water. The (oleic and linoleic), together with sterols and alcohols. method requires simpler apparatus and fewer steps METHODS OF PREPARATION than that of the precipitation technique. The product Two chief methods of preparation of metallic soaps contains a higher free acid content and a correspondingly lower metal content, as compared with precipihave developed, namely, precipitation and fusion. tated soaps. The higher acid content increases soluPrecipitation Method. This method results from the bility (dispersability) in some solvents and adds certain metathetic reaction between a solution of a soluble physical characteristics which may be desirable. The metal salt and a soluble soap of the acid. We may resulting effectsof high temperatures in the fusion procrepresent this reaction as follows: ess may produce a material of inferior quality. HowMSO. 2RCOONa (RC00)JvI + Na.SO+ ever, metal soaps which hydrolyze easily can be preMetal Salt SodiumSaap Metdlie Sodium S d t pared to advantage by this method. The presence of Soap unreacted metal compounds adds a further objection to The sodium soap is prepared by the neutralization of this method of preparation. the fatty acid or by the saponification of a fat or oil. Although most metallic soaps are prepared either by The metallic soap forms as a heavy precipitate which is the precipitation or fusion technique, modified methods greatly hydrated. The success of the method depends are available. For the most part, they employ either upon carefully controlled conditions although the proc- the precipitation or fusion technique with special soless appears to be a simple one. Important factors vents for extraction of the metallic soap upon formation. include temperature of precipitation, manner of addition In some instances, the sodium soap may be dissolved in of reagents, concentrations of reagents, and stirring. either ethyl or butyl alcohol before precipitation. In common practice, an equivalent amount of the hot The alcohol may replace most of the water as the presoluble metallic salt (in dilute solution) is added to an cipitating medium. equivalent amount of a hot, dilute solution of the sodium soap, under vigorous stirring. This manner of addition CHEMICAL AND PHYSICAL PROPERTIES produces a heavy metal soap which contains certain The metallic soaps of the saturated acids are amorquantities of occluded sodium soap. The method also phous solids and most of them are fluffy, unctuous submakes possible the formation of the basic metal soaps. stances when finely divided. They can be produced in contents of these crudes vary with the crude but run somewhere between 0.3 and 0.7 per cent. The general formula, C,Hzn-zOz, holds for typical monocyclic naphthenic acids, such as:
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crystalline form from appropriate solvents. The colors of the metallic soaps are similar to the colors of many of the common salts of the cations. For example, the nickel soaps are green, the copper soaps are bluishgreen, the cobalt soaps are lavender, the iron soaps are red-brown, the aluminum, zinc, lead, and alkaline earth soaps are white, etc. The metallic soaps do not possess sharp melting points. The melting point values found in the literature are given as "melting ranges" since most of them are not pure substances; furthermore, the values reported by various workers are not always in agreement ( 4 4 ) . The combination of a polar and nonpolar group within the same molecule lends to soaps many of their unique properties. The metallic soaps are insoluble in water (with few exceptions) but show various degrees of solubility (dispersability) in the different organic solvents, especially the hydrocarbon solvents. The metallic soaps retain large quantities of water when freshly precipitated but lose it readily and become highly water repellent. Most of themetallic soaps form gels with organic solvents. For example, 7 per cent aluminum di-stearate dispersed by heat in a mediumweight lubricating oil (S. A. E. Number 20) will produce a 6i-m grease on cooling. Many applications of the metallic soaps depend upon their insolubility in water and their capacity to thicken oils and set to gels. The heavy metal soaps stabilize water-in-oil emulsions, whereas the soaps of the alkali metals stabilize oil-inwater emulsions.
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acteristics and the physical effects produced when they are mixed or incorporated with other materials. Their insolubility in water and nonwettability make them useful in waterproofing materials, textile finishing, leather dressing, special detergents, flotation, and cosmetics. The presence of the metallic cation makes them useful in catalysis, paint driers, antioxidants, oil additives, antifouling paints, fungicides, pharmaceuticals, and ceramics. Other important uses, which may depend more completely upon the unique combination of groups within the same molecule, are found in their application in flatting agents, thickeners, lubricants of various kinds (extreme pressure, greases, molds, rubber articles) dispersing agents, detergents, hardening agents, etc. The uses of the various metallic soaps are given in Table 2. The table brings out the most important soaps as well as the industries involved and the specific uses for a given soap. These data were taken from various sources and include suggested uses as well as longestablished uses. The most important uses for given soaps are contained in bold face type. The figures for the production of the metallic soaps are not readily available in the literature. The sources of information are few and the values may be incomplete. Many users of metallic soaps produce them in their processes of manufacture and, consequently, do not report them separately. Table 3 presents the figures for information on the stearates of aluminum, calcium, magnesium, and zinc for the years 194245 (9).
ANALYSIS
The analysis of metallic soaps is frequently of importance to the manufacturing consumer. The presence of certain kinds and amounts of impurities may obviate a given use for a metallic soap. Most metallic soaps are not pure, individual chemical compounds and therefore, the analysis for metal or acid radical cont.ent gives a value of comparative use only. For this reason the producers of metallic soaps give specificationswhich state the metal soluble salts, free acid, and moisture contents within certain allowable limits. Oftentimes, the specifications include melting ranges and bulkiness. Although the method of analysis may vary from one metal soap to another, the usual items of analysis include: (1) moisture content; (2) total ash, washed ash, and soluble ash; (3) free fatty acid content; (4) fineness; (5) bulkiness; (6) gel-forming properties. The American Society for Testing Materials has investigated methods for testing metallic soaps associated with driers (7). The tentative specifications for the testing of driers was reported (8) and methods of analysis presented for lead, manganese, and zinc. USES AND APPLICATIONS
The physical characteristics of the metallic soaps, as indicated previously, are rather unique and result from the combination of a polar and nonpolar group within the same molecule. The practical applications of the metallic soaps depend chiefly upon these char-
TABLE 3 Production of Metallic Stearates Production, Metallic stearate Year lb. .4luminum stearate 1912 832,000 1943 5,381,000 1944 4,16!1,000 1945 9,182,000 Calcium stearate 1942 317,000 1943 2,242,000 1944 2,354,000 1945 2,144,000 Magnesium sksrrate 1942 70,000 1943 406,000 1944 663,000 1!346 730,000 Zinc stearate 1942 808,000 1943 3,144,000 1944 4,384,000 1945 8,910,000
Prieellb. S0.21 0.23 0.23 0.23 0.25 0.26 0.26 0.26 0.29 0.31 0.31 0.25 0.24 0.29 0.30 0.29
REFERENCES
Two books and many journal articles have appeared in the literature on metallic soaps over the last three decades. Most of the articles have appeared in the industrial journals. Appending the literature cited are references to some of the most pertinent literature on metallic soaps. LITERATURE CITED (1) HARKNESS, R. W., AND J. H. 499 (1940).
BRUUN,Ind. Eng. Chem., 32,
JANUARY, 1950 (2) EDWARDS, K. B., Chemistry and Industn~,61,233 (1942). Trade J . , 118, Part 2, No. 16, 30, (3) . . HASSELSTROM. . T... Paper . (1944). (4) KOENIG,A. E., J. Am. C h m . Sac., 36, 951 (1914). W. F., l i M.~ L ~A ~ oInd. , Eng. Chem., 22,646 (5) WHITMORE, flQ.ZO\. ~----,(6) SILMH, H., Soap and Sanit. Chemicals, 12, 36 (1936). (7) A.S.T.M. Bulletin, No. 106, 15 (1940). (8) A.S.T.M. Designation, D56440T. (9) "Chemical Facts and Figures," 2nd ed., Manufacturing Chemists' Association of the United States, Washington, D. C., 1946, p. 93
R m R E N C E S TO METALLIC SOAPS A.
Rooks 1. E ~ r . r o n ,S. B., "The Alkaline-Earth and Heavy-Metal Soaps," IEeinhold Pub. Corp., New York, 1946. An excellent and complete treatment from an industrial standpoint. 2. BRA^, H. J., "Die Mehllseifeu," 0.Spamer, Leipzig 1932. In German; thorough treatment.
History 1. LICATA,J. F., Am. Paint J., 15, 68 (1931). P ~ e p a ~ a t iand m Propmlies 1. KOENIG,A. E.-See reference (4). H. I., C h m . Eng. 28,489 (1923). 2. JONES, W. F.. AND M. L h m w S e e reference (5). 3. WHITMORE. S I L M ~H.,-See , reference (6). LAND,H., Chem. I d , 39, 593 (1936). LAWRENCE, A. S. C., Trans. Faraday Soc., 34, 660 (1938). Applications 1. ELLIOTT,S. B., Oil Gas. J., 46, NO. 26, 63, 81 (1947). 2. BONER,C. J., Ind. Eng. Chem., 29, 58 (1937). 3. LESSER,M. A,, Soap and Sanit. Chemicals, 21, No. 7, 36 (1945): ibid.. No. 8., 40 (1945): . .. ibid.. 20 (No. . 7). . 29 i1944j: 3. J., "Protective and Decorative Coatings," 4. MAWIELLO, Vol. 11, J. Wiley & Sons, New York, 1941, Chapter 28, pp. 61341. 5. KLEMGARD, E. N., "Lubricating Greases: Their Manufacture and Use," Reinhold Publ. Corp., New York, 1937.
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