August, 1929
INDUSTRIAL A N D ENGINEERING CHE:MISTRY
acid is the hydroxyl as in an alcohol. From the above relation stearic acid should have a cohering voltage of 8.9 volts, since it has an equivalent length of 19 carbon atoms. The molar heat of vaporization may be estimated quite accurately for stearic acid a t 13,930 calories by Trouton's rule. Its dielectric constant was measured in this laboratory at 1000 cycles a t 25" C. and found to be 2.27. The valuesoof A and d are-given by Langmuir (4) as 22 and 12.5 square Angstrom and Angstrom units, respectively. From these values the total heat of desorption which equals the heat of adsorpt'ion is calculated to be 64.5 calories per gram of stearic acid. Using this value, it may be calculated that if rubber contains 2.2 per cent fatty acids and 30 volumes of carbon black may be successfully dispersed in 100 volumes, the heat of wetting of the carbon black by stearic acid is 4.7 calories per gram of carbon. Thus the heat of dispersion as calculated from cohering voltages checks Hock's experimental value of 5.1 calories and in turn is checked by Gaudechon's value of 6.0 calories for the wetting of carbon by acid. Additional evidence is thus given that the heat of wetting of carbon by rubber is the heat of adsorption of fatty acids by carbon. The agents that disperse carbon black in rubber are the naturally occurring fatty acids, principally stearic. Conclusion
The equations used in the calculations are based on the assumption that the adsorbed molecules are dipoles and are adsorbed in an oriented condition. That the values check is evidence that the proposed theory of dispersion is correct and that the dispersing agent is in an oriented condition on the surface of the filler particle. It is peossible to calculate that the center of the dipole axis is 4.2 Angstroms from the filler surface. Since this value is but 20 per cent of the length of the stearic acid molecule, the carboxyl group is the chief contributor to its dipole moment. This would imply that the length of the hydrocarbon chain is probably not critical so far as the dispersing properties of a fatty acid are concerned. The mechanism of the successful dispersion of fillers other than carbon black should be essentially the same. The whole theory throws much light on the essential part that the naturally occurring fatt,y acids and added stearic acid play in the dispersion of carbon black in rubber and explains why the addition of small amounts of stearic acid to rubber, especially to rubbers deficient in the naturally occurring dispersing agents, tends to produce a better and more uniform product. L i t e r a t u r e Cited Blake. I s m ENG.CHEM., 20, 1084 (1928). Gaudechon, Compl. r e n d . , 157, 209 (1913). (3) Hock, I n d i a Rubber J . , 7 4 , 454 (1927). (1) (2)
(4) (5)
719
Langmuir, J . A m . Chcm. Soc., 39, 1848 (1917). Palmer. Pmc: Roy. SOC.(London) 115, 227 (1921).
Discussion C. R. Boggs Doctor Blake has suggested a theory for the mechanism of filler dispersion and rekinforcement. However, he deals only with carbon black in rubber. Carbon black is the most important rehforcing ingredient we have for a tire-tread compound. Being a conductor, it cannot be used in appreciable quantities in insulation. We might consider the possibility of finding a non-conducting rehforcing filler for insulation and also what should be done when the present supply of natural gas is depleted. What is needed is a material similar to clay which is cheap and has a small particle size. With present dispersing agents, clay does not, however, reenforce rubber to anywhere near the extent that carbon black does. For a a l e r to rebforce satisfactorily it must be well dispersed and adhere firmly to the rubber. The correct dispersing agent should bring this about. This agent should be a polar compound, One portion of the molecule should be soluble in the rubber hydrocarbon and another portion should be capable of being adsorbed by and adhering strongly to the surface of the clay. It would be laborious to mix clay in rubber with every material that might fulfil the above requirements for a dispersing agent and then test the product. What is needed is a relatively quick laboratory method of testing that will parallel the action of the filler in rubber. Two methods suggest themselves. The first one is that of determining the cohering voltages of various polar compounds on a clay surface. Unfortunately, clay is a non-conductor and is not adapted to use as electrodes in a coherer. The second method would consist of making a stiff paste of the filler with kerosene. The addition of a small amount of a suitable dispersing agent would thin the paste to a mobile liquid. Kerosene, being a long-chain hydrocarbon, should resemble rubber as a dispersing medium and the experiment should be analogous to the dispersion of the filler in rubber. We have made a number of mixes of clay and kerosene and added various dispersing agents. Seventy-five per cent nitrobenzene by weight on the clay did not produce a satisfactory dispersion. Although the nitrobenzene is a strongly polar compound, no part of the molecule is apparently adsorbed by the clay. Twenty-two per cent stearic acid did produce a good dispersion, while only 2 per cent wool grease was required to reduce the stiff paste to a thin liquid. These results indicate that a small quantity of wool grease would transform clay to a strong reenforcing filler in a rubber compound. Rubber-clay compounds containing 2 per cent wool grease on the filler were mixed and cured. Their physical properties were not appreciably different from the same compounds without the wool grease. I n other words, the kerosene experiment is not parallel to conditions in the rubber compound. Present laboratory methods for examining dispersing agents are inadequate for the development of new ones and improving the ones we have now. New methods should produce materials that would do the same thing for other fillers that stearic acid does for carbon black. More theoretical work on adsorption and dispersion should develop new reenforcing fillers and be of great benefit to the rubber industry as a whole and the wire trade in particular.
Nature, Manufacture, and General Use of Stearic Acid BINNEY8 z
D. F. Cranor EAST4 2 ST.,~ NEW ~
SMITH CO., 41
TEARIC acid is a colorless, waxlike material, melting a t 69.3" C. and having a specific gravity of 0.847 a t this temperature. It belongs t o the saturated series of fatty acids of the type indicated empirically by the formula C,Ha,02, being the member having 18 carbon and 36 hydrogen atoms, and is probably best written CH3(CH2)16COOH.
S
Occurrence
Stearic acid occurs in nature as the triglyceride principally in the solid animal fats commonly called tallows, and, in
YORK,
x. Y.
fact, derives its name from the Greek word meaning tallow. It is associated with the glyceride of oleic acid, which i s the member of the unsaturated class containing the same number of carbon atoms, and to a lesser extent with palmitic acid, which stands immediately below stearic in the saturated series. I n the article of commerce stearic acid very largely predominates, but i t follows from its natural occurrence that appreciable amounts of oleic and some palmitic acid are also present in proportions which vary somewhat according to the grade.
