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INDUSTRIAL AND ENGINEERING CHEMISTRY
solvents. The tendency is to yield slightly lower viscosities with the more powerful solvents. 2. The distillation ranges of petroleum solvents and hydrogenated petroleum solvents have an effect on the viscosities of Batavia dammar solutions; the solvents with higher distillation ranges give Batavia dammar solutions of higher viscosities. 3. The plots show that the viscosities of Batavia dammar solutions increase logarithmically with increase in concentration. 4. Examination of the average viscosity curves of Figure 5 shows that toluene is the best solvent of the three for Batavia dammar, with Solvesso 1 second, and solvent naphtha 55 third. This is shown by the lower viscosities on the toluene curve of corresponding points, and also by the differences in the slopes of the three curves. 5. Batavia dammars F and dust increase in viscosity enormously after the 50 per cent concentration point is reached. This may be due to the effect of solution of the resin in the solvent and the increasing amount of solvent dissolved by the resin. 6. The curves may be used to determine what co6centration of solution should be made to obtain a desired viscosity, or to determine the concentration of a solution by its viscosity. 7. The average curves may be used but not with nearly as great accuracy as the individual curves. The degrees of accuracy with which the average curves may be used vary with the concentration and are indicated by the per cent average deviations given for each concentration in Table VI.
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Examination of the viscosities of dammar solutions in toluene indicate that in low coilcentrations there is very little difference in viscosity, irrespective of the grade used. This condition indicates that there is little change in chemical characteristics with decreasing size of resin particles. I n this respect it might be assumed that smaller size particles with increased surface area would be subject to oxidation, resinification, or deresinification reactions on the surface. Above 50 per cent and higher ranges, not only the solubility of the resin in the solvent but also the solubility of the solvent in the resin is important. Higher grades of dammar give lower viscosities, and in general lower grades, as indicated by decreasing size and increasing impurities, give higher viscosities. A similar situation holds for petroleum solvents of the hydrogenated as well as the paraffinic type. To state this in another way, since the viscosity-concentration relation is exponential, the differences in viscosities are more apparent a t higher concentrations. However, with clean resin, the variation between grades is of a magnitude only slightly greater than the magnitude of experimental error.
Literature Cited (1) Am. Gum Importers Assoc., Inc., "Natural Resins," 1936. (2) Mantell, C. L., Allen, C. H., and Sprinkel, K. M., OficiaE Digest Federation Paint & Varnish Production Clubs, March, 1937. (3) Mantell, C. L., and Rubenkoenig, H. L., Paint, Oil Chem. Rev., June 11, 1936. RECEIVED September 23, 1937. Presented before the Division of Paint and Varnish Chemistry at the 94th Meeting of the American Chemical Society, Rochester, N. Y., September 6 to 10,1937.
HYDRAULIC BRAKE FLUIDS ROBERT R. FULTON Mellon Institute, Pittsburgh, Pa.
T
HE hydraulic principle of actuating brakes was first used in 1922 on the Dusenberg automobile. At about the same time Stutz employed hydraulic brakes, but their design was entirely unlike those of the present. The Lockheed hydraulic brake system, invented by Malcolm Loughead, was first used in 1924 on Chrysler cars. Other automobiles subsequently equipped with Lockheed brakes were the Moon, Jewett, Meteor, Wills-Sainte Claire, Peerless, Dort, Stearns, Jordan, Davis, and others too numerous to list here. Many of these cars have passed into history. Today every American-made automobile, except the Ford, is equipped with hydraulic brakes. It is not the purpose of this article to describe the mechanical features or the advantages and disadvantages of the various old and new hydraulic brake systems. The movable parts of the brake system operate to transmit fluid pressure to move forcibly the brake shoes into engagement with their drums. This arrangement involves the use of both metals and rubber, which are exposed to the brake fluid, and have working frictional contact surfaces which, if lubricated a t all, must be lubricated through the medium of the fluid itself. Because of their environment-that is, their outdoor use as well as location with respect to motor block and brake drums-hydraulic brakes must remain serviceable and dependable under all sorts of weather or atmospheric conditions
involving extreme changes, especially with respect to temperature; under normal service conditions the latter varies between extremes of overheated brake drums or motor and the subzero weather of -40" or -50" C.
Analysis of Commercial Brake Fluids At present there are numerous brands of brake fluid on the market, but most of them fall within the following classes, as found by analysis : Castor oil-ethyl alcohol mixtures Castor oil-methyl alcohol-glycol mixtures. Castor oil-diacetone mixtures. Castor oil-Cellosolve (ethylene glycol monoethyl ether) mixtures, with and without glycerol and glycols. Castor oil-butyl alcohol-glycol mixtures. (In at least one such roduct the vehicle for the castor oil seems t o be a mixture of gutyl and amyl alcohols, probably by-products of methanol synthesis.) Special processed fluids having unusual properties and complex secret com osition Glycerof-ethyl alcohol mixtures Glucose-ethyl alcohol-water mixtures
Present Status of Brake Fluids The last two types probably make up an insignificant proportion of all brake fluid sold and fall far short of meeting the
APRIL, 1938
materials that will withstand the action of a t least the standard fluids. This is not just the plea of a chemist who wishes to have his work made easy for him. Anyone familiar with the difficulties surrounding the formulation of a good brake fluid must realize the need for cooperation from the engineer and manufacturer. Nor can the latter much longer deny the responsibility. For many years the castor oil-ethyl alcohol brake fluid was
INDUSTRIAL AND ENGINEERING CHEMISTRY
The requirements, composition, methods of testing, and evaluation of hydraulic brake fluids are discussed. A list of the important patents and their basic claims are given, and the present status of brake fluids is defined. The chemical composition of many commercially important fluids is presented.
Patented Hydraulic Brake Fluids Many materials and mixtures have been patented by individuals who have plainly had little or no knowledge of the
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sively for many years, it possessed two serious disadvantages: It was very volatile and was corrosive to the aluminum alloy, copper, and brass parts of t h e b r a k e mechanism. To overcome the latter difficulty, Sherbino (19) used an alkali metal phosphate, and Tseng (21) used potassium hydroxide and potassium arsenate. It is doubtful, whether either the phosphate or arsenate, which are very slightly soluble, if a t all, in the castor oil-alcohol
(10) patented a brake fluid consisting of furfural, or some other furan derivative, and glycol. Many other patents covering hydraulic pressure transmitting fluids, damping liquids, etc., have been omitted from
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.. .
Phantom View of the System
THE HYDRAULIC BRAKE: SYSTEM
PRINCIPLE OF
Courtesy, Chrysler Corporation
0
the list of patent citations a t the end of this article, but not a single one omitted could possibly be considered s u i t a b l e for use in any m o d e r n hydraulic brake system.
General Requirements
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namely, the wheel cylin ders; while transmitting pressure without perceptible friction or resistance, it must not be sufficiently penetrating to permit leakage between working surfaces-for instance, between the wheel cylinder walls and the rubber piston cups. It must not crystallize or solidify a t low temperatures; it should, in fact, remain fluid at temperatures as low as -50" C., if its capabilities and usefulness are not to b e seriously impaired. It must lubricate t h e rubber and metal workingsurfaces. It should have minimum effect upon any of the m a t e r i a l s of whichthe brake system is f a b r i c a t e d . It should not evaporate, volatilize, separate, or change i t s physical or chemical. structure a t temperatures. to which it may be s u b j e c t e d w h i l e in storage or in use in the hydraulic brake systems of pleas, ure cars, m o t o r b u s e s and trucks. It should be readily available for extensive commercial use a t r e a s o n a b l e cost. These requirements may be summarized as follows:
The requirements for a brake fluid are diverse and e x a c t i n g . So far as is k n o w n , no one has attempted to define a satisf a c t o r y hydraulic brake fluid except in accordance with his own accomplishments in f o r m u l a t i o n . Methods of testing have not been published, and in the light of the recent and almost universal use of hyCross Section of a Wheel Assembly Illustrating the Internal Mechanism of the Whkel Cylinder draulic brakes on pleasure cars and small trucks, and 1. Brake shoe return sprin.g 6 Wheel brake cylinder piston 2 Wheel brake cylinder iston CUP spring the coincidental appearance 6. Wheel brake cylinder bpdy 3: Wheel brake cyljnder gpot 4. Wheel brake cylinder piston 7. Wheel brake oylinder ptston cup on the market of numerous CUP 8. Wheel brake cylinder piston brands of brake fluids, it seems opportune to report such information. It is the purpose of this paper to introduce and stimulate a discussion of this highly important subject. The following criteria of excellence are the results of almost five years of effort to formulate the ideal brake fluid and are believed to cover all essential requirements. The satisfactory fluid must meet a number of varied conditions and remain unchanged in its capabilities and actions. I t s viscosity throughout the range of working temperatures should persist with in reasonably restricted margins; it must have and maintain just the proper viscosity or state of fluidity to permit it to flow or to transmit txessure readilv to the remotest parts of t
