HARD-CORE REVERSE MICELLES IN TRIBOFILM FORMATION AND SOLUBILIZATION PROCESSES

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1 HARD-CORE REVERSE MICELLES IN TRIBOFILM FORMATION AND SOLUBILIZATION PROCESSES IN ENGINE OIL Zenon PAWLAK 1) and Barney E. KLAMECKI 2) 1) University of Technology and Agriculture, Faculty of Technology and Chemical Engineering, Bydgoszcz, Seminaryjna 3; and Utah Department of Health, Environmental Chemistry, Salt Lake City. 46 Medical Drive, UT 84113, USA 2) University of Minnesota, Department of Mechanical Engineering, 111 Church Street SE, Minneapolis, MN , USA. ABSTRACT The tribofilm formation requires that tribochemical reaction occur between the solubilized zinc dialkyldithiophosphate () by soft-core reverse micelles (RMs) and the metallic surfaces under boundary lubrication. In the case of hard-core reverse micelles, the colloidal core (CaCO 3 ) x or (CuO) x is directly introduced in the sliding contact, and undergoes physical or tribochemical changes during the film build-up. Hard-core RMs in hydrocarbon formulation is one or two orders of magnitude larger than that of the soft-core RMs. Discussion of hard-core reverse micelles formation, solubilization, acidbase interaction and related phenomena are presented in this paper. INTRODUCTION Interactions between additives are of practical importance in hydrocarbon and synthetic oil formulation. Engine oil additives are dissolved or dispersed in an oil formulation, some occur in a micellar form as hard-core reverse micelles, RM's, or micellar aggregates [1,2,3]. Considering tribochemical mechanism for the action of the hard-core, RM's are under intensive investigation. The (Cu metallic ) tribofilm formation requires the tribochemical reaction to occur between the (CuO) x(oleic acid)n, RMs and the metallic surfaces. (CuO) x (oleic acid)n (RMs) + Triboelectrons (e - tribo) (Cu metallic ) x Tribofilm (1) 128

2 In the case of the calcium hard-core reverse micelles, the colloidal (CaCO 3 ) x or (CaBorate) x is directly introduced in the sliding contact and undergoes physicochemical changes during the film build-up. Consequently, no tribochemical reaction with the hard-core RMs take place [3]. (CaCO 3 ) x(sulfonate)n (RMs) + Friction (CaCO 3 ) x calcite surface layer (2) To improve antiwear and extreme-pressure performance of micellar carbonate (borate), a chemical modification of these colloidal species with zinc dialkyldithiophosphate (), and with sulfurized carboxylic acids was performed [5,7]. In case of modified hard-core RMs better performance is observed in comparison to Ca borate, RMs. (CaBorate) x(sulfonate)n- (RMs) + Friction Ca(PO 3 ) 2 (Calcium phosphate, tribofilm) (3) This suggests that under rubbing conditions, the borate-sulfonate micelles interact more effectively than on the surface. and the boratesulfonate micelles interact with each other under rubbing conditions and form calcium phosphate [7]. RESULTS AND DISCUSSION Hard-core Reverse Micelles Formation. Hard-core reversed micelles are composed of detergent molecules, e.g. sulfonate strongly bonded to the calcium carbonate core (CaCO 3 ) x(sulfonate)n or oleic acid to the copper oxide core (CuO) x(oleic acid)n (Fig.1). The sizes of the carbonate-sulfonate RMs and surfactant layer thickness were investigated using small angle neutron scattering method (SAXS) [8,9,10]. Typical hard-core RMs sizes (Å): (a) calcium sulfonate surfactant layer 21.5, (CaCO 3 ) x hard-core diameter 39, total diameter 82 and (b) calcium phenolate surfactant layer 15, (CaCO 3 ) x core 29 and total diameter 59. In industrial formulations a variety of surfactants is used to coat calcium, barium and magnesium carbonate, or borate particles. Colloidal dispersions of inorganic carbonates can fulfill most requirements as effective acid-neutralizing additives with the following features: strong base, stable after reaction with acid, not harmful neutralization product, clear and stable oil solution. Calcium may be preferred for specialized diesel engine applications and magnesium for rust control in gasoline car engines [2]. Tribochemical Nature of Antiwear Films. A "Tribological Tree" shown in Fig. 2 will summarize our knowledge of some most important processes proceeding in the bulk oil and the effects of those processes on the mechanically activated surfaces. The "Tribological Tree" consists of additive mixtures which interact in a variety of ways, both in the bulk oil and on 129

3 surfaces. The most important factor governing the tribochemical reactions under boundary friction is due to the action of RMs [2]. Fig. 1. Schematic represenation of normal micele (M) in water, soft-core reverse micelle (RM), and hard-core reverse micelle (RM) in hydeocarbon formulation ( O) detergent molecule. Fig. 2. Tribological Tree 130

4 The core RMs of calcium carbonate (or borate) core surrounded by calcium benzensulfonate, phenolate or salicylate molecules [3] have been recognized as an efficient multifunction class of antiwear/anticorrosive additives. The analytical results obtained on the wear particles allow to conclude that during friction micelles undergo chemical and structural changes. Hard-core RMs under the wear conditions lead to the loss of the organic shell. Simultaneously the metastable amorphous calcium carbonate undergoes crystallization into the calcite structure. The following tribochemical mechanism for the film build-up can be proposed [3] to confirm the good antiwear properties of the micelles: the feeding of the rubbing surface is achieved by adsorption of the hard-core RMs onto the friction surfaces, the micelles lose their organic shell and form a polycrystalline film adherent to the metallic surfaces, tribological film delamination leads to protection of the rubbing surfaces. The combination of zinc dialkyldithiophosphate () and hard-core RMs leads to a synergistic effect of metallic detergents on the degradation of. These phenomena are observed in many tests and can be explained in terms of: the acid neutralization property of hard-core RMs leads to the prevention of decomposition of (in the valve train wear test and the thin film oxygen uptake test), the competitive solubilizaton reduces the effective concentration of on the metal surface (in the four-ball test), mixed borophosphate tribofilms are formed on the metal surface (in the Falex wear test). Calcium borate hard-core micelles have recently been recognized as an efficient multifunction class of anticorrosive-antiwear additives. The additive acts as an antiwear agent by the formation of a calcium borate glass tribofilm material. The boron behaves as a glass former like phosphorus does with the additive. The combination of and calcium borate-salicylate micelles is expected to be synergistic, due to the formation of a mixed phosphate-borate glass tribofilm. Micellar borate-salicylate (BS) and produce long chain oxide glasses as antiwear tribofilm. The friction coefficient in the steady-state conditions is 0.12 for, 0.09 for (BS) and 0.11 for /BS. There is an evidence for P-O-B bonding in this glass, indicating an atomic scale mixing of the two additives in the tribocontact. Iron is not seen in the tribofilm composition [3,5,11,12]. Hard-core RMs (CuO) x -(oleic acid) n. The formation of a copper tribofilm takes place during the friction process and disintegration of the RMs takes place in a tribochemical reaction. The colloidal particle diameters are about 10 to 40 nm and the particles are easily dispersible in oils, fuel, and water forming a stable micelle solution [13]. Transfer and adhesion of the 131

5 132 micellar particles accelerate surface modification forming of a fine copper tribofilm. Boundary friction and the film decrease wear and the friction coefficient induces the formation of a thin copper tribofilm. Since tribofilm formation takes place during the friction process, disintegration of the RMs takes place in a tribochemical reaction: Cu e - tribo Cu 0 (4) A thin copper tribofilm, which exhibits considerable compressive strength and a low tangential shear strength is formed on the tool surfaces. If hard-core RMs are delivered to the friction zone during cutting, the tribofilm formed will be present on the tool over the whole period of operation. The tribofilm reduces the coefficient of friction, the extent of direct contact between the cutting tool and the workpiece, the temperature in the cutting zone and hence tool wear. In water-based fluids a different tribochemical mechanism is proposed for the action of hard-core reversed micelles (see Fig.1) of (CaCO 3 ) x -(sulfonate) n. During the rubbing process, the micelles lose their organic shell and their mineral cores crystallize and a polycrystalline film adherent to the metallic surface is formed [2, 3]. The tribochemical reactions during the friction process of hard-core reverse micelles copper oxide, (CuO) x -(oleic acid) n and oil-based soft-core (RMs) (see Fig.1) are very similar. Tribofilm formation requires a tribochemical reaction to occur between the micellar additive and metallic surface during the friction processes. It is known that under boundary friction conditions and/or surface damage caused by fatigue processes, triboemisions can lead to the release of triboelectrons (see reaction 4), charged particles and photons. When the friction condition become severe, for example in heavy load, high speed machining or drilling, RMs decompose and the useful atoms or compounds produced, eg., Cu metallic or polyphosphate, and the original structure of the rubbing surface is replaced by a tribofilm (see reaction 1) [2, 3]. Micellar Solubilization in Lubrication. Several mechanisms of action of RMs are generally recognized: solubilization of organic acids, solubilization of guest molecules, e.g.,, steric and electrostatic stabilization of contaminants. The solubilized compounds go into micelles interior cavity which consist of ionic or polar groups. The RMs interface solubilize or deactivate polar compounds, such as water, catalytic metal ions, organic acids and sludge, Fig. 3 [2]. Processes of solubilization and interaction of water, polar molecules and acidic materials with reverse micelles has been expressed in terms of the adsorption of the substrate into the micellar surface. The acid deactivation mechanism in hydrocarbon media is supplementary to the neutralizing action of detergents. Traces of strong sulfur, nitrogen or halogen acids are scavenged by colloidal (CaCO 3 ) x and by neutral detergents. The strong acid reacting with

6 carboxylate, sulfonate or carbonate micelles to form mineral salts and long chain acids which will be retained in the micelle [9,10,11]. HA HA (H 2 O)n HA HA CaCO 3 Soft-core reverse micelle metal alkyl phenate HA sludge particle organic acid zinc dialkyldithiophosphate Hard-core reverse micelle metal alkyl benzenesulfonate CaCO 3 colloidal core sulfur acid, nitrogen acid Fig. 3. Solubilization of oxidation products, e.g., organic acids, and inorganic acids () by hard-core reverse micelles in oil formulation Reverse micelles manage the prevention of agglomeration and electrostatic stabilization mechanisms. The steric stabilization mechanism provides a physical barrier to attraction between soot particles (P) and surfactant molecules by RMs formation. This effect contributes to the internal cleanliness of engine by minimizing the agglomeration and subsequent deposition of sludge particles. REFERENCES [1] H.A. Spikes, Lubrication Science, 1989, 2, 1. [2] Z. Pawlak, Tribologia, 2003, 2, [3] J.L. Mansot, M. Hallouis, J.M. Martin, Colloids Surfaces, 1993, 75, 25. [5] K. Inoue, Lubr. Eng., 1993, 49, 263. [6] B.L. Papke, L.D. Rubin, SAE Paper, No (1992). [7] K. Varlot, M. Kasrai, M.M. Bancroft, E.S. Yamaguchi, P.R. Rayson, J. Igarashi, Wear, 2001, 249, [8] J.F. Marsh, Chem. Ind., 1987, 20 July, 470. [9] S. Giasson, D. Espinat, T. Palermo, R. Ober, M. Passah, M.F. Morizur, J. Colloid. Inter. Sci., 1992, 153, 355. [10] I. Markovic, R.H. Ottevill, D.J. Cebula, I. Field, J.F. Marsh, Coll. Polym. Sci., 1984, 262, 648. [11] K. Varlot, J.M. Martin, B. Vacher, K. Inoue, in: Lubrication at the Frontier, D. Dowson et. al., (Eds.), Elsevier, 1999, [12] S. Shirahama, M. Hirata, Lubr. Sci., 1989, 1, 365. [13] G.P. Shpenkov, Friction Surface Phenomena, Elsevier,

A Study of Wear Properties of Different Soybean Oils

Available online at www.sciencedirect.com ScienceDirect Energy Procedia 42 (2013 ) 633 639 The Mediterranean Green Energy Forum 2013, MGEF-13 A Study of Wear Properties of Different Soybean Oils Kraipat