PREPARATION OF ENVIRONMENTAL FRIENDLY BENZENE-FREE METAL SULFONATE SURFACTANT FOR LUBRICANT ADDITIVE FORMULATION

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1 Proceedings of the 6th International Conference on Mechanics and Materials in Design, Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, July 2015 PAPER REF: 5449 PREPARATION OF ENVIRONMENTAL FRIENDLY BENZENE-FREE METAL SULFONATE SURFACTANT FOR LUBRICANT ADDITIVE FORMULATION EunMin Song 1, DoWon Kim 1, ByungJo Kim 2, JongChoo Lim 1(*) 1 Department of Chemical and Biochemical Engineering, Dongguk University-Seoul, Korea 2 AK ChemTech Central Research Lab., DaeJeon, Korea (*) jongchoo@dongguk.edu ABSTRACT In this study, environmental friendly benzene-free metal sulfonate surfactants were synthesized for lubricant formulation and the structure of intermediates and final products were elucidated by 1 H NMR, 13 C-NMR and FT-IR. Also, the interfacial properties including critical micelle concentration (CMC), surface tension, contact angle, interfacial tension, foam stability and emulsion stability were measured. The biodegradability and long-term stability of the resulting products were characterized and the performances of prepared lubricants were measured such as oil stain protection, rust inhibition and lubrication and compared with that of lubricant based on LAS (linear alkyl benzene sulfonate). Keywords: Benzene-free metal sulfonate surfactant, Lubricant additive, Interfacial property, Lubrication property INTRODUCTION Colloidal additives have been widely used as detergents for many years in lubricant formulations to prevent the formation of varnish lacquer in the combustion engine and to neutralize acidic products formed during combustion, which can be the source of corrosion and lubricant degradation. In addition, the detergents are also known to play an important role as antiwear, extreme pressure and antioxidant additives. Colloidal lubricant additives mainly consist of high molecular-weight surfactant, colloidal particles of metal carbonate, and diluent oil. In the composition of lubricant detergents, colloidal particles of metal carbonate provide the required total base number (TBN) as a neutralizing agent, while the diluent oil acts as a compatible agent and surfactants provide steric stabilization around the colloidal particles in a non-polar oil medium. Surfactants have been known to play a very important role as cleaning, solubilizing, wetting, dispersing, emulsifying, stabilizing, foaming, conditioning and anti-foaming agents in many practical industrial applications. For example, surfactants have a wide variety of applications ranging from being the active ingredients in household cleaning formulations such as laundry detergents, soaps, and shampoos to industrial use in petroleum production and textile processing. Various surfactants such as calcium (or magnesium or barium) sulfonate (or phenate, salicylate, or phosphonate) have been used to stabilize the colloidal particles of metal carbonate in the lubricants. Among them, anionic surfactants such as LAS (linear alkyl benzene sulfonate) and DBSA (dodecyl benzene sulfonic acid) have been widely used in the formulation of lubricants since the surfaces of metal carbonate particles are predominantly positively charged. Thus, the adsorption of anionic surfactant molecules on the surface of -465-

2 Track_F Tribology, Gears and Transmissions metal carbonate particles with a head-on configuration decreases the hydrophilicity of particles since the external surfaces are covered by the hydrophobic chains of the adsorbed surfactant molecules. As a result, surface activity at the metal carbonate particles is promoted by in situ surface activation by their interaction with negatively charged anionic surfactant molecules and the colloidal particle can be stabilized in a non-polar oil medium. As mentioned above, anionic surfactants such as LAS and DBSA have been widely as detergent, dispersant, anti-oxidant and corrosion inhibitor mainly due to excellent performance and relatively low cost. However, these products based on LAS or DBSA have serious biodegradation problems mainly owing to the benzene group contained in LAS and DBSA, which could lead to the environmental pollution. In this study, two kinds of benzenefree metal sulfonate surfactants such as neutralized calcium sulfonate (NCS) and OCS (overbased calcium sulfonate) were synthesized to replace LAS or DBSA for lubricant formulations and the structure of the synthesized products were elucidated by 1 H NMR, 13 C- NMR and FT-IR. The term overbased indicates that the quantity of colloidal calcium carbonate in the particle cores is greater than that is needed to neutralize the acidic surfactant, otherwise it is described as neutral, indicating that the overbased detergents have a greater acid neutralizing capacity than their neutral salts. The biodegradability and long-term stability of the resulting products were characterized and the interfacial properties were also measured. The performances of prepared lubricants prepared with NCS and OCS respectively were measured such as oil stain protection, rust inhibition and lubrication and compared with that of lubricant based on LAS. EXPERIMENTAL Lauryl alcohol with a purity of greater than 98% and sulfuric acid with a purity of greater than 99% were purchased from Samchun Pure Chemical Co. and were used without any further purification. NaCl, KOH, anhydrous sodium sulfate, chloroform and ethyl alcohol were also received from Samchun Pure Chemical Co. and were used as received. Calcium carbonate (CaCO 3 ) nanoparticles with a purity of greater than 98.3%, the whiteness of 97.0 L and the average particle size of 78.4 nm were supplied by Dongyang M&M Industry Co., Korea. Hydrochloric acid solution (1.0 M HCl) and sodium hydroxide solution (0.1 M NaOH) with a purity of greater than 98% were purchased from Sigma-Aldrich Co. and n-decane of purity greater than 99% was obtained from Sigma-Aldrich. Water used for sample preparation was ultrapure, which have been double distilled and passed through a Nanopure (Sybron- Brinkman Inc.) ion exchange system. In this study, NCS and OCS were synthesized through the following steps (Schemes 1 and 2). The benzene-free sulfonic acid intermediate was prepared via the sulfonation of 1 mole of lauryl alcohol by sulfuric acid and then followed the neutralization of sulfonic acid intermediate using 1 mole of Ca(OH) 2 for 3 hrs. The yield and TBN of NCS were found to be 98% and 99 mg KOH/g respectively. For the preparation of OCS, the sulfonic acid intermediate was prepared via the sulfonation of 1 mole of lauryl alcohol by sulfuric acid and then followed the neutralization of sulfonic acid intermediate using 4 moles of Ca(OH) 2. A small amount of acetic acid was added as a polar promoter to the reaction mixture prior to bubbling carbon dioxide into the reaction vessel. Finally, the overbased products went through the carbonation using CO 2 gas to produce OCS. The yield and TBN of OCS were found to be 97% and 310 mg KOH/g respectively

3 Proceedings of the 6th International Conference on Mechanics and Materials in Design, Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, July 2015 C 12 H 25 OH + H 2 SO 4 /SO 3 C 12 H 25 SO 3 H 2 C 12 H 25 SO 3 H + Ca(OH) 2 (C 12 H 25 SO 3 ) 2 Ca + 2 H 2 O Scheme 1 Synthesis route of neutralized calcium sulfonate (NCS) Ca(OH)2 CO2 gas 2 C 12 H 25 SO 3 H [C 12 H 25 SO - 3 ] 2 Ca amorphous CaCO 3 Scheme 2 Synthesis route of overbased calcium sulfonate (OCS) The structure of intermediates and final products were elucidated by 1 H-NMR, 13 C-NMR and FT-IR spectrophotometer. 1 H-NMR and 13 C-NMR spectra were recordedd on a Bruker DPX 400 and expressed as δ units at room temperature in CD 3 OD. Digilab's FT-IR IR spectra of NCS and OCS surfactants. Table 1 shows FTS-165 FT-IR spectrometer was used to obtain molecular structure, molecular weight, ph and viscosity of NCS and OCS surfactants. The properties of LAS were also included in Table 1 for comparison. Table 1 - Summary of the properties of LAS, NCS and OCS Molecular Structure MW (g/mol) ph Viscosity (cp) LAS NCS OCS O S Ca 2+ O O NCS + amorphous CaCO During this study, the CMC of surfactants was determined by measuring the surface tension of a surfactant using a Du Noüy ring tensiometer (Sigma702, Biolin Scientific) at 25 C. The CMC was considered to reach when there was no further decrease in surface tension with an increase in surfactant concentration. Dynamic surface tension was measured by a maximum bubble pressure tensiometer (Kruss BP2, Germany), where the range of bubble life time used was from 10 to 60,000 ms. The interfacial tension between 1 wt% surfactant solution and n- decane oil was measured at 25 C using a spinning drop tensiometer (SITE 100HS, Kruss) equipped with a video cameraa (Sony SSC-DC374, Japan). Drop shape analysis system (Kruss DSA100, Germany) has been used to measure a contact angle at 25 by forming a drop of surfactant solution on a glasss micro slide. The DV-II+ digital viscometer was utilized to measure the viscosity of a surfactant solution. The stability of aqueous surfactant solutions was evaluated by using an emulsion stability tester (DualCon ITEC, Germany). The electrical conductivities of top and bottom portions of a sample bottle of a surfactant solution were measured at 25 and the difference between two conductivity values was used to estimate stability of surfactant solutions. The biodegradability test of NCS and OCS was conducted according to the OECD 301 E test for ready biodegradability. The performances of prepared lubricants weree measured such as oil stain protection and rust inhibition (salt spray, wetting), where the lubricant was formulated using 90% of naphthenic base oil, 3% of surfactant and 7% of paraffin mineral oil -467-

4 Track_F Tribology, Gears and Transmissions on a weight basis. The tribological properties were evaluated on a four-ball machine. The antiwear and friction-reducing properties were determined under the following conditions: speed 1200 rpm, load 500 N and room temperature. The surface of a lubricated specimen made of stainless steel (5cmx5cm) was observed by an optical microscope (Nikon LV100 Pol). RESULTS AND DISCUSSION Two kinds of benzene-free metal sulfonate surfactants such as neutralized calcium sulfonate (NCS) and OCS (overbaesd calcium sulfonate) were synthesized to replace LAS or DBSA in lubricant formulation. The structure of NCS and OCS was characterized by 1 H-NMR, 13 C- NMR and FT-IR and the results are shown in Figs. 1 and 2. In this study, the CMC was determined by measuring the surface tension of a surfactant as a function of concentration. The CMC was taken as the concentration beyond which the surface tension of the aqueous solution does not decrease any more. In addition to CMC, other interfacial properties such as surface tension, interfacial tension, contact angle and foam stability were measured at 25 and the results are summarized in Table 2. CMC (mol/l) Table 2 - Summary of interfacial properties of LAS, NCS and OCS Interfacial Tension (mn/m) Contact Angle ( ) LAS 5.00e NCS 6.69e OCS 9.82e As shown in Table 2, the CMCs of LAS, NCS and OCS surfactant systems in mol/l are 5.00x10-4, 6.69x10-3 and 9.82x10-3 respectively. The CMCs of NCS and OCS surfactant systems are found to be larger than that of LAS mainly due to the large molecular weight of NCS and OCS. Surface tensions of aqueous surfactant solution at CMC condition are summarized in Table 2. As shown in Table 2, the surface tensions of LAS, NCS and OCS are 34.15, and mn/m respectively. It is noticeable that the surface tensions of NCS and OCS are lower than that of LAS. As shown in Figs. 3, 4 and 5, dynamic surface tension measurement using a maximum bubble pressure tensiometer indicated a sharp decrease in the surface tension of the aqueous surfactant solution with an increase in concentration of the surfactant solution. In addition, all of LAS, NCS and OCS surfactant systems required relatively shorter time to reach an equilibrium value presumably due to the high mobility rate of surfactant molecule. This result indicates that any depletion of surfactant molecules from the air/water interface will be replenished by an instantaneous diffusion of molecules from the bulk aqueous solution. Interfacial tensions were measured as a function of time for n-decane drops brought into contact with 1 wt% surfactant solutions at 25. As Figs. 3, 4 and 5 indicates, the interfacial tension between an aqueous surfactant solution and n-decane dropped over a period of about 20~30 min to an equilibrium value. The equilibrium values of LAS, NCS and OCS surfactant systems are 0.034, and mn/m respectively. It is worthy pointing out that the interfacial tensions measured between surfactant solution and n-decane oil are in the same order of magnitude as those exhibited between micellar solutions and nonpolar hydrocarbon oils. The contact angle measured for 1 wt% of LAS, NCS and OCS surfactant systems were found to be 14.89, and respectively

5 Proceedings of the 6th International Conference on Mechanics and Materials in Design, Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, July 2015 a b c S O Ca 2+ O O- 2 (a) a b c O S Ca 2+ O O- 2 (b) S O Ca 2+ O O- 2 (c) Fig. 1 - Spectral data of NCS; (a) 1 H NMR spectrum, (b) 13 C NMR spectrum, (c) FT-IR spectrum -469-

6 Track_F Tribology, Gears and Transmissions (a) (b) (c) Fig. 2 - Spectral data of OCS; (a) 1 H NMR spectrum (b) 13 C NMR spectrum, (c) FT-IR spectrum -470-

7 Proceedings of the 6th International Conference on Mechanics and Materials in Design, Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, July 2015 Interface Tension (mn/m) e-5 1e-4 1e-3 1e-2 1e-1 Concentration (mol/l) 5.0 E-6 (mol/l) 5.0 E-5 (mol/l) 1.0 E-4 (mol/l) 5.0 E-4 (mol/l) 1.0 E-3 (mol/l) 5.0 E-3 (mol/l) (a) 1e+1 1e+2 1e+3 1e+4 1e+5 Surface Age (ms) (b) Time (min) (c) Fig. 3 - Interfacial property measurement for LAS surfactant; (a) static surface tension, (b) dynamic surface tension, (c) interfacial tension -471-

8 Track_F Tribology, Gears and Transmissions e-5 1e-4 1e-3 1e-2 1e-1 Concentration (mol/l) (a) E-5 (mol/l) 1.34 E-4 (mol/l) 6.69 E-4 (mol/l) 1.34 E-3 (mol/l) 6.69 E-3 (mol/l) 1.34 E-2 (mol/l) 6.69 E-2 (mol/l) 1e+1 1e+2 1e+3 1e+4 1e+5 Surface Age (ms) Interface Tension (mn/m) (b) Time (min) (c) Fig. 4 - Interfacial property measurement for NCS surfactant; (a) static surface tension, (b) dynamic surface tension, (c) interfacial tension -472-

9 Proceedings of the 6th International Conference on Mechanics and Materials in Design, Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, July Concentration (mol/l) (a) E-3 (mol/l) 3.27 E-3 (mol/l) 6.55 E-3 (mol/l) 9.82 E-3 (mol/l) 1.31 E-2 (mol/l) 1.64 E-2 (mol/l) 1e+1 1e+2 1e+3 1e+4 1e+5 Surface Age (ms) (b) Interface Tension (mn/m) Time (min) (c) Fig. 5 - Interfacial property measurement for OCS surfactant; (a) static surface tension, (b) dynamic surface tension, (c) interfacial tension -473-

10 Track_F Tribology, Gears and Transmissions The biodegradability of NCS and OCS surfactants was found to be 86% and 93% respectively, indicating excellent biodegradation. In order to measure the performance of lubricant based on NCS and OCS, oil stain protection, anticorrosion and lubrication properties were evaluated. The performance of lubricant prepared with LAS surfactant was also measured for comparison. The lubricant was made of 90% of naphthenic base oil, 3% of surfactant and 7% of paraffin mineral oil on a weight basis. The results of lubricant performances are shown in Figs. 6, 7 and 8. The result was also summarized in Table 3 for performance comparison of each surfactant. The performance of oil stain protection was measured for lubricants prepared with LAS, NCS and OCS respectively by observing the surface of a lubricated specimen made of stainless steel by using an image analyzer. As exhibited in Fig. 6, no oil stain was observed in all of three surfactant systems, suggesting that oil stain protection capacity of NCS and OCS is comparable to that of LAS. The result of rust inhibition test by a salt spray method was shown in Fig. 7. No corrosion was observed with OCS while the extents of corrosion for LAS and NCS were observed to be 4% and 2 % respectively. On the other hand, the rust inhibition test by a wetting method revealed no corrosion in all of three surfactant systems. The tribological property was evaluated on a fourball machine under the following conditions: speed 1200 rpm, load 500 N and room temperature. As summarized in Table 3, the load-wear index (LWI) values measured for the lubricant detergents prepared with LAS, NCS and OCS were found to be 21%, 28% and 31% respectively. This result implies superior lubrication efficiency of the products based on NCS and OCS. Table 3 - Summary of the performance of lubricant additives prepared with NCS and OCS Test Items Unit LAS (Reference) NCS OCS Methods Oil Stain (24 hrs) - Not occur Not occur Not occur MIL-C-22235A Rust Inhibit (24 hrs) Salt spray % 4 2 No corrosion KSM 2109 Wetting - No corrosion No corrosion No corrosion KSM 2109 Lubricating (Four-ball) *LWI KSM 2026 CONCLUSIONS In this study, environmental friendly benzene-free metal sulfonate surfactants were synthesized for lubricant formulation and the structure of intermediates and final products were elucidated by 1 H NMR, 13 C-NMR and FT-IR. Also, the interfacial properties such as critical micelle concentration, surface tension, contact angle, interfacial tension, foam stability and emulsion stability were measured. The biodegradability and long-term stability of the resulting products were characterized and the performances of prepared lubricants using synthesized surfactants were measured such as oil stain protection, rust inhibition and lubrication. The performances test of prepared lubricants have shown that both lubricant formulations prepared with NCS and OCS exhibited excellent performances such as oil stain protection and rust inhibition compared with that prepared with LAS. This result indicates -474-

11 Proceedings of the 6th International Conference on Mechanics and Materials in Design, Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, July 2015 that the newly synthesized surfactants, such as NCS and OCS, are potential candidates to replace LAS and DBSA in lubricant formulation. (a) (b) (c) Fig. 6 - Oil stain test result; (a) LAS, (b) NCS, (c) OCS (a) (b) (c) Fig. 7 - Rust inhibition test result by a salt spray method; (a) LAS, (b) NCS, (c) OCS (a) (b) (c) Fig. 8 - Rust inhibition test result by a wetting method; (a) LAS, (b) NCS, (c) OCS -475-

12 Track_F Tribology, Gears and Transmissions REFERENCES [1]-Delfort B, Chive A, Daoudal B, Lacome T, Phosphosulfuration of colloidal calcium carbonate evaluation of antiwear and extreme-pressure properties. Tribology Transactions, 1998, 41, p [2]-Vipper AB, Cook SJ, Karaulov AK, Leahy R, Antifriction properties of calcium phenatetype detergents used in marine cylinder lubricants. Lubrication Science, 1998, 10, p [3]-Delfor B, Chiv A, Daoudal B, Lacôme T, Phosphosulfuration of colloidal calcium carbonate-evaluation of antiwear and extreme-pressure properties. Tribology Transactions, 1998, 41, p [4]-Vipper AB, Antioxidant properties of engine oil detergent additives. Lubrication Science, 1996, 9, p [5]-Besüergil B, Akın A, Celik S, Determination of synthesis conditions of medium, high, and overbased alkali calcium sulfonate. Industrial & Engineering Chemistry Research, 2007, 46, p [6]-Yonglei W, Wumanjiang E, Recent advances in colloidal lubricant detergents. China Petroleum Processing and Petrochemical Technology, 2010, 12, p [7]-Boschkova K, Kronberg B, Stålgren JJR, Persson K, Salagean MR. Lubrication in aqueous solutions using cationic surfactants a study of static and dynamic forces. Langmuir, 2002, 18, p [8]-Serreau L, Beauvais M, Heitz C, Barthel E. Adsorption and onset of lubrication by a double-chained cationic surfactant on silica surfaces, Journal of Colloid and Interface Science, 2009, 332, p [9]-Nehal SA, Nassar AM. Lubricating oil additives. In: Kuo PH (ed) Tribology-lubricants and lubrication. InTech, Croatia 2011, p