Effect of Wax Esters as Friction modifiers in petroleum base stock

Transcription

1 Journal of Oleo Science Copyright 2012 by Japan Oil Chemists Society Effect of Wax Esters as Friction modifiers in petroleum base stock Sharmishtha Khalkar, DiptiNarayan Bhowmick and Amit Pratap * Department of Oils, Oleochemicals and Surfactants Technology, Institute of Chemical Technology (University under Section 3 of UGC Act 1956; Formerly UDCT/ UICT), Nathalal Parekh Road, Matunga (East), Mumbai, INDIA Abstract: Condensation product of fatty acids and fatty alcohols is termed as wax esters. A series of fatty acids and fatty alcohols both saturated & unsaturated ranging from C 12 to C 22 were synthesised using p-tsa as a catalyst. Carbonyl functionality was confirmed by infrared spectrophotometer. Wax esters were used as friction modifier additives at 1%, 3%, 5% in 150N and 500N commercial petroleum base stocks. These base stocks were characterized by tribological properties like wear scar, weld load and coefficient of friction. All the tribological properties were better for saturated products than the unsaturated ones. Key words: Wax esters, tribological properties, wear scar, weld load 1 INTRODUCTION Most of the lubricants currently in daily use originates from petroleum base stocks those are toxic to the environment, making it increasingly difficult for safe and dispose. It is claimed that 50 of all lubricants sold worldwide end up in the environment via accidental spillage, non recoverable usage, volatility, industrial and municipal waste, urban runoff, refinery processes, and total loss applications, such as chainsaw oils, two-stroke engines, concrete mold release oils, exhaust fumes in engines, and metal cutting and forming processes 1, 2. Estimates for the loss of hydraulic fluids are as high as , 4. Environmental protection from pollution can best be performed by preventing undesirable lubricant losses and by reclaiming and reusing mineral oil lubricants. Because of pollution concerns, strict government regulations, and uncertainty in petroleum supply, there has been an increasing demand for environmentally friendly lubricants 5, 6 in recent years. Even the price of mineral oil is increasing over the years, with crude oil prices hovering around $130/barrel, and has already surpassed the vegetable oil prices. Lubricants based on vegetable oils are biodegradable. These are derived from renewable resources and are lowcost alternatives to synthetic fluids 7. Ecological and economical advantages, the present market share of these lubricants is relatively small 2 worldwide, with an estimated growth rate of , 9.This hydrogenated FAME showed higher oxidation stability 10. Industries have been working to increase the yields and lower the cost of higher molecular weight fatty esters/wax esters. Recently, much effort has been focused on research and development of new types of lubricating oil additives to reduce wear and friction in the tribological systems. It has been noted that the use of additives to improve the lubricating capacity and durability of oil plays an important role in the wear and friction process of materials. Due to the environmental problems, many researchers are embarking on the viability of the vegetable-based lubricants 11. Present research work is aimed at synthesis of fatty acid esters/wax esters as a friction modifier additive. The objectives were to develop basic information concerning esterification reaction of fatty acids and fatty alcohols, by acid catalyzed reaction and to check the effect of antiwear AW, extreme pressure EP additives. Fatty esters were characterized by various physicochemical and tribological properties like wear scar, weld load. It was observed that fatty acid esters have high good lubricity. 2 EXPERIMENTAL SECTION 2.1 Materials All chemicals were purchased from commercial sources and used as received, unless otherwise stated. Stearic acid 99 pure, oleic acid 70 pure, erucic acid 92 pure were obtained as a gift sample from M/s. Godrej Industries Ltd., Mumbai. Fatty alcohols lauryl, myristyl, pal- * Correspondence to: Amit Pratap, Department of Oils, Oleochemicals and Surfactants Technology, Institute of Chemical Technology (University under Section 3 of UGC Act 1956; Formerly UDCT/ UICT), Nathalal Parekh Road, Matunga (East), Mumbai, INDIA amitpratap0101@rediffmail.com, ap.pratap@ictmumbai.edu.in Accepted June 29, 2012 (recieved for review February 10, 2012) Journal of Oleo Science ISSN print / ISSN online

2 S. Khalkar, D. Bhowmick A. Pratap mityl, staeryl 99 pure were obtained as gift samples from M/s. Godrej Industries Ltd., Mumbai. Toluene AR grade, p-tsa AR grade, sodium hydroxide AR grade were procured from M/s. Thomas Bakers, Mumbai. Commercial base oils 150N, 500N were procured from M/s Savita Chemicals Ltd., Mumbai. 2.2 Methods Synthesis of fatty acid esters Fatty acid esters were prepared as shown in the following pathway: 1 Saturated fatty acid esters: 2 Unsaturated fatty acid esters: Esterification reactions were conducted in 250 ml threenecked round bottom flask equipped with magnetic stirrer, thermocouple and Dean and Stark along with condenser. Toluene was used as a solvent to remove the water of reaction azeotropically. Fatty acids and fatty alcohols were reacted at 120 for 6 h using 1 p-tsa para-toluene sulfonic acid as a catalyst. Slight excess 0.1 acid was added in excess to drive the equilibrium towards ester formation. Catalyst was neutralized by equivalent quantity of sodium hydroxide NaOH and washed by warm water until the ph 7 was obtained. Traces of moisture were removed under vacuum. Methanol, toluene, petroleum ether and chloroform were used for crystallization 12. FT-IR was done; which showed spectra at cm 1. The reaction was monitored by acid value AV and thin layer chromatography TLC The tribological performance of fatty esters as an additive Four-Ball Method was done.wear scar and weld load characteristics were determined by ASTM D 4172 using a four ball tester. This experiment was designed to study the antiwear properties of ester derivatives under sliding contact using Four-Ball Tester Make Ducom; Model: TR- 30L. The balls steel, 12.7 mm in diameter, Rc hardness, and extreme polish were thoroughly cleaned with methylene chloride and hexane before each experiment. Three balls were clamped together to make threepoint contact with the top ball. Test fluid 15 ml of 5 w/ w oil sample dissolved in hexadecane was poured in the test cup to cover the stationary balls. For wear scar detection, the test sequence allowed the speed to attain a set rpm of 1200 before a normal load of 40 kg 88 lb. was applied at room temperature for 60 min, temperature maintained at 75. For weld load detection, the temperature of the test fluid was 22, which increased to at the end of the 15 min run. Duplicate tests were performed with a new set of balls, and the scar diameter varied within 0.04 mm. An important property of lubricants is their ability to maintain a stable lubricating film at the metal contact zone. Vegetable oils are known to provide excellent lubricity because of their ester functionality. The four ball wear and weld test perform to demonstrate their effectiveness in reducing wear and weld 13. The results are given as the means of three replicates standard deviations High frequency reciprocating friction and wear tester HFRR Boundary lubrication properties of fatty acid esters were studied using a ball-on-disk configuration on the instrument TR-282. The balls and the disks were thoroughly cleaned with fresh reagent-grade methylene chloride and hexane prior to each experiment. The balls were AISI E steel, hardened 62 HR, Ra 0.05 mm, size: 6 mm dia. Disc material was AISI E steel, hardness 200 HV, annealed, Ra: 0.02 mm,size: dia 10 4 mm wide. A total of 3-5 ml of the test fluid 5 w/w was poured in the cup to totally immerse the ball and disk. The duration of the friction test was 15 min at a sliding speed of 6.22 mm/s 5 rpm and normal load of 200 gm at room temperature. The temperature of the specimen and test fluid was 25 25, which increased by 2-3 at the end of the test period. Friction and other data were recorded until the set time elapsed. The coefficient of friction CoF values reported are averages of three independent experiments with the standard deviation of and scar diameter on balls is measured using an optical microscope attached to a digitized moving platform. Five measurements were recorded at different positions of the wear track, and the average value in millimeters was taken in each case. For wear scar diameter in millimeters, two measurements, perpendicular to each other, were recorded for each scar on a ball. The average of six measurements for three balls was taken in each case with a standard deviation of RESULT AND DISCUSSION 3.1 Load-carrying property On addition of fatty acid ester as additive, base stock 150 N and 500 N compatibility increases listed in Table 1. Weld load found to be of 150 N base stock was 165 Kg and of

3 Wax Esters as Friction modifiers Table 1 Wear scar and weld load of esters with 1%, 3%, 5% blend in 500 N and 150 N base stock. Sr. No. Esters Blend Wear scar Wear scar Weld load (Kg) (mm) 500 N (mm) 150 N in 500 N in 150 N 1% 0.52± ± Lauryl stearate 3% 0.49± ± % 0.46± ± % 0.53± ± Myristyl stearate 3% 0.47± ± % 0.44± ± % 0.50± ± Palmityl stearate 3% 0.45± ± % 0.42± ± % 0.48± ± Stearyl stearate 3% 0.44± ± % 0.39± ± % 0.57± ± Lauryl oleate 3% 0.51± ± % 0.46± ± % 0.55± ± Myristyl oleate 3% 0.49± ± % 0.44± ± % 0.52± ± Palmityl oleate 3% 0.47± ± % 0.42± ± % 0.49± ± Staeryl oleate 3% 0.44± ± % 0.39± ± % 0.55± ± Lauryl erucate 3% 0.50± ± % 0.45± ± % 0.52± ± Myristyl erucate 3% 0.48± ± % 0.42± ± % 0.49± ± Palmityl erucate 3% 0.45± ± % 0.40± ± % 0.47± ± Staeryl erucate 3% 0.42± ± % 0.37± ± The values given are averages of three independent experiments±standard deviations. N base stock was 195 Kg, listed in Table 3. Weld load capacity increases with increase in chain length whereas with unsaturation found decreasing. Due to polar structure of esters, these additive form 725

4 S. Khalkar, D. Bhowmick A. Pratap Table 2 Coefficient if friction (CoF) of esters with 1%, 3%, 5% blend in 500 N and 150 N base stock. Sr. No. Esters Blend Coefficient of friction in 500 N in 150 N 1% Lauryl stearate 3% % % Myristyl stearate 3% % % Palmityl stearate 3% % % Stearyl stearate 3% % % Lauryl oleate 3% % % Myristyl oleate 3% % % Palmityl oleate 3% % % Staeryl oleate 3% % % Lauryl erucate 3% % % Myristyl erucate 3% % % Palmityl erucate 3% % % Staeryl erucate 3% % The values given are averages of three independent experiments±0.02 standard deviations. 726

5 Wax Esters as Friction modifiers layers on the metal surface by adsorption or chemisorptions that guarantees their immediate availability in the case of mixed friction conditions. When hydrodynamic lubricating film is not present extreme pressure EP additive can react with the metal surface forming tribochemical reaction layers that will prevent direct contact between the sliding metals. Layers formed by only physically adsorbed polar substances fatty acid esters, which exhibit moderate high pressure properties. EP additives are much more reactive and used when the stress of the system is very high in order to prevent the welding of the moving parts that otherwise would lead to severe damage Antiwear properties Wear scar diameter is an indication of extent of wear when sliding contact occurs. We found, the compatibility of fatty acid esters as an additive more than a plan base stock. Wear scar of plan base stock 150 N found to be 0.68 mm and of 500 N 0.62 mm listed in Table 3. on addition additive fatty acid ester wear scar diameter decreases. Wear scar diameter decreased with an increase in chain length and increases with an increase in unsaturation listed in Table 1. During the tribochemical process, these molecules undergo chemical transformation at the metal contact zone and develop a stable tribochemical film to protect further wear of the metal. These compounds demonstrate excellent antifriction and antiwear properties using four ball test. The thermal stability increases with the chain length of the alkyl groups, so the weld load capacity found to be more. 3.3 Friction performance The coefficient of friction CoF of plane base stock 150 N found to be 0.13 and of 500 N 0.11, listed in Table 3. The relationship between the concentration of the additive in plane base stock and the friction coefficient is given in Table 2; it indicates that the friction coefficient of the additive becomes lower with increasing additive concentration when it is less than 3 wt. The friction coefficient shows little difference with increasing additive concentration when it is above 3 wt. Beyond 3 wt concentration, the additive did not show decrease in CoF. 4 CONCLUSION Weld load of stearic acid saturated esters was observed more than oleic and erucic acid unsaturated esters. Wear scar of stearic acid ester was smaller than oleic and erucic acid esters. Load baring capacity of esters increased with chain length no. of carbons and decreased with unsaturation. Wear scar decreased with chain length of fatty acid esters and increased with unsaturation. Coefficient of friction decreased with chain length and decreased with unsaturation. The same was lower at increased additive concentration and the change was not significant beyond 3 wt. Overall the wax esters proved to be good friction modifier additives in commercial petroleum base stocks 150 N and 500 N. REFERENCE 1 Horner, D. Recent trends in environmentally friendly lubricants. J. Synth. Lubr. 18, Gawrilow, I. Vegetable oil usage in lubricants. IN- FORM. 11, Carnes, K. Offroad hydraulic fluids: Beyond biodegradability. TribolLubr. Techno. 60, Battersby, N. S. The biodegradability and microbial toxicity testing of lubricants Some recommendations. Chemosphere 41, Rhee, I. S. Evaluation of environmentally acceptable hydraulic fluids. NLGI Spokesman. National Lubricating Grease Institute 60, Bergstra, R. Green means go. Lubes-n-Greases.10, Table 3 Physicochemical properties of 500 N and 150 N base stock. Base stock 150 N 500 N Wear scar (μm) 0.68± ±0.013 Weld load (kg) Coefficient of friction (mm) 0.13± ±0.01 Flash point ( ) 320±2 356±2 Pour point ( ) -26 ±2-22 ±2 Viscosity at ± ±0.02 Viscosity at ± ±0.02 Viscosity index ± ±2 727

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