Friction Properties of C18-Fatty Acids Seiichiro HIRONAKA Department of Chemical Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152 (Received June 29, 1987) The friction properties of C18-fatty acids (stearic acid, oleic acid and isostearic acid) were investigated using a pendulum type friction tester. These fatty acids gave different antifriction properties owing to their different molecular structures, in spite of their having the same polar group (carboxylic group) and the C18-hydrocarbon chain. The order of antifriction properties of these C18-fatty acids was: stearic acid >oleic acid>isostearic acid. This suggests the difference of the state of the adsorbed films formed on the frictional surfaces as lubricating films. The states of the adsorbed films were examined from the surface pressure-area isotherms obtained for these fatty acid monolayers at the air/water interface. 1. Introduction The mechanism of action of oiliness agents, such as fatty acids and aliphatic alcohols, is explained with the formation of their adsorbed films on frictional surfaces. Under boundary lubrication conditions, oiliness agents adsorb on or react with metal surfaces and form lubricating films to minimize metallic contact, contributing to friction and wear reduction. The lubricities of oiliness agents depend greatly on their molecular structures and their heats of adsorption onto frictional surfaces.1)-4) The static friction coefficients and film strengths of fatty acids and their esters are dependent on the hydrocarbon chain lengths and the cohesion among them.1) These compounds have the same polar group, but longer hydrocarbon chain compound gives lower friction and stronger film strength. On the contrary, additives with stronger polar group give higher heats of adsorption and better lubricities even in case the respective, hydrocarbon chain lengths are the same.2)-4) In general, higher heat of adsorption gives stronger film strength and better lubricity. Stearic acid forms the close packing adsorbed film to give the higher heat of adsorption and greater antiwear properties than isostearic acid.2) The effect of molecular structure of additives on their lubricities is confirmed with pyridine derivatives2) and fluorinated compounds.5) The lubricities and heats of adsorption of the alkyl derivatives of pyridine are dependent on the position of substituent on the ring. n- C7F15COOH gives better friction properties as compared with n-c7f15ch2oh and n-c7f15ch- (OH)2 In the present work, the friction properties of oiliness agents were investigated with fatty acids which have a same polar group (carboxylic group) and C18-hydrocarbon chain, but having different molecular structures. The adsorbed films of these fatty acids formed on frictional surfaces were discussed from the standpoint of two-dimensional behaviors of the spread monolayers at the air/water interface. A new explanation, using the data from surface pressure-area isotherms, was made on the friction properties of fatty acids with different chemical structures. 2. Experimental 2.1 Materials Available highest-purity stearic acid (m.p. 69- n80d 1.4299), oleic acid (d2525 0.895, n18d 1.463) and isostearic acid (d2020 0.880, n20d 1.449) were used as C18-fatty acids. These chemical structures are shown in Fig. 1. Squalane was used as the base oil, which properties are as follows: viscosity specific gravity d2020 0.811, refractive index n20d 1.4535. 2.2 Friction Tests The friction coefficients were measured as a function of concentration of added fatty acid or oil temperature, using a pendulum type friction Fig. 1 Chemical Structures of C18-Fatty Acids
tester.5) All experiments were repeated at least three times and average values, respectively, were reported. Prior to the use, the ball and roller pin (AISI 52100 steel) were thoroughly washed in benzene and acetone, and then cleaned with benzene reflux in a Soxhlet apparatus, and finally dried in vacuum. 2.3 Surface Pressure Measurements A Wilhelmy-type film balance was used to measure the surface pressure. The balance was in the chamber to allow thermostatic control at distilled water. Benzene was used as the spreading solvent. The rate of compression was 10cm2/min. The details of the experiments are described in the previous paper.6) The effect of concentration of stearic acid and oleic acid on friction coefficients is shown in Figs. 3 and 4, respectively. As is clear from these figures, these fatty acids showed lower friction coefficients with increasing concentration at every oil temperature, giving almost constant values at a concentration above 0.5wt%. Stearic acid showed the better friction reducing effect than oleic acid. Isostearic acid had no remarkable effect on its friction properties even at 1.0wt%. Therefore, the effect of concentration was not examined for Isostearic acid. Figures 5 and 6 show the effect of oil temperature on the friction properties of stearic acid and oleic acid, respectively. At higher 3. Results and Discussion Figure 2 shows the friction properties of squalane and squalane containing 1.0wt% of various C18-fatty acids in the oil temperature range performed with squalane to lower the friction coefficient. Stearic acid with long unbranched chain gave much lower friction coefficient as compared with oleic acid and isostearic acid. This suggests that stearic acid adsorbs closely on the frictional surfaces to form a strong lubricating film. Oleic acid and isostearic acid cannot form such close packing adsorbed films because of the double bond in hydrocarbon chain and branched chains, respectively. Especially, the side chains of isostearic acid have the effect of steric hindrance on the adsorption of its molecules on the frictional surfaces. Fig. 3 The Effect of Concentration of Stearic Acid on Its Friction Properties Fig. 2 Friction Properties of C18-Fatty Acids Additive concentration: 1.0wt% Fig. 4 The Effect of Concentration of Oleic Acid on Its Friction Properties
concentrations, stearic acid gave much lower friction coefficient in the oil temperature range applied. At 0.1wt% (2.9mmol/l), however, there was little difference between the friction properties of stearic acid and oleic acid because of its Fig. 5 The Effect of Temperature on Friction Properties of Stearic Acid being lower than the saturated adsorption concentration (about 0.14wt% (4.1mmol/l) for stearic acid3)). Oleic acid showed higher friction with increasing oil temperature in like manner at any concentrations. The friction properties of oleic acid are influenced more easily by temperature as compared with stearic acid because oleic acid cannot form perfectly the close-packing adsorbed film due to the double bond in the hydrocarbon chain, and furthermore oleic acid film is more unstable since it is apt to resolve in the base oil. In most boundary lubrications in which oiliness agents act effectively to reduce friction and wear, their monolayers adsorbed on the frictional surfaces are responsible for the effects of such additives on friction and wear reduction. However, much remains to be investigated about the orientation and packing-state of molecules in the adsorbed films. The surface pressure-area isotherms of monolayers which are spread at air/ water interface are sensitive to delicate differences in chemical structure, orientation and packingstate of molecules. Ries and Cook7) have investigated about the spread monolayers of fatty acids and phosphate ester and their mixtures to provide Fig. 6 The Effect of Temperature on Friction Properties of Oleic Acid Fig. 7 The F-A Diagrams of C18-Fatty Acids on Distilled Table 1 Properties of F-A Curves of C18-Fatty Acids
a basis for interpreting the behavior of complex lubricant additives. The surface pressure-area isotherms obtained with the C18-fatty acid monolayers which are spread on distilled water and the cross-sectional area per molecule at each surface pressure are shown in Fig. 7 and Table 1, respectively. Stearic acid with long linear hydrocarbon chain gave easily the close packing film behavior at lower surface pressure by two-dimensional compression. At zero pressure, the cross-sectional area of stearic obtained by the extrapolation of the steep upper portion of the isotherm to zero surface pressure, in Fig. 7. The collapse pressure was not obtained in the pressure range applied (reported collapse pressure is 43dyne/cm7)). These facts suggest that stearic acid is able to form strong condensed film due to the cohesion among long linear hydrocarbon chains. The better friction properties of stearic acid and its adsorbed film formation on the frictional surfaces may be related well to the behaviors of its spread monolayer. Oleic acid showed the behavior of liquidexpanded film due to the double bond in its hydrocarbon chain, giving larger molecular areas than stearic acid at each pressure. Oleic acid has the peculiar chemical structure, namely, its molecule is bent at the center of the molecule due to the double bond. Oleic acid, therefore, could not form a close packing film such as by stearic acid, even under two-dimensional compressible condition. The collapse pressures of oleic acid and isostearic acid obtained from Fig. 7 were 23.6 and 20.5dyne/cm, respectively. The collapse pressure of oleic acid was much lower than that of stearic acid (43dyne/cm7)). These facts suggest that the order of the film strengths of these fatty acids is: stearic acid>oleic acid>isostearic acid. Isostearic acid with some branched chains as shown in Fig. 1 gave much larger molecular areas and lower collapse pressure (20.5dyne/cm) than those by oleic acid. This explains the great sensitivity of monolayer to branched chains. The much expanded film with isostearic acid, even under two-dimensional compressible condition, is related to its lower friction properties. Probably, isostearic acid cannot form the effective adsorbed film on frictional surfaces as a lubricating film, because of the steric hinderance among side chains. The friction properties of the C18-fatty acids with the different chemical structures were discussed from the behaviors of monolayers spread at the air/water interface. The friction properties of the adsorbed films formed on frictional surfaces, which had been explained by the heat of adsorption,2)-4) were related well to the behaviors of the spread monolayers. The molecular area and collapse pressure and the condensed or expanded state of monolayer obtained in surface pressurearea isotherms give an important information to analyze the friction properties of oiliness agents. 4. Conclusion From the facts described above, it is concluded in this paper that, (1) the frictional properties of oiliness agents such as fatty acids are greatly influenced by their chemical structures, (2) the orientations of their adsorbed molecules and the state of their adsorbed films on frictional surfaces can be discussed from the behaviors of monolayers spread at the air/water interface. Acknowledgement The author would like to thank Dr. K. Suga and Prof. M. Fujihira of Tokyo Institute of Technology for their support of this study. References 1) Sakurai, T., Furusawa, A., Baba, T., Kogyo Kagakushi, 56, 193 (1953). 2) Hironaka, S., Yahagi, Y., Sakurai, T., Sekiyu Gakkaishi, 17, (2), 201 (1975). 3) Hironaka, S., Yahagi, Y., Sakurai, T., ASLE Trans., 21, 231 (1978). 4) Groszek, A. J., ASLE Trans., 5, 105 (1962). 5) Sekiya, A., Hironaka, S., Sekiyu Gakkaishi, 29, (2), 183 (1986). 6) Hironaka, S., Meguro, K., J. Colloid Interface Sci., 35, (3), 367 (1971). 7) Ries, H. E. Jr., Cook, H. D., J. Colloid Sci., 9, 535 (1954).
Keywords Boundary lubrication, Fatty acid, Friction, Friction modifier, Lubricant, Spread-monolayer behavior