CC BY-NC-ND
0Journal of Saudi Chemical Society (2011) 15, 195-201
King Saud University Journal of Saudi Chemical Society
www.ksu.edu.sa www.sciencedirect.com
ORIGINAL ARTICLE
Chemically modified biolubricant basestocks from epoxidized oleic acid: Improved low temperature properties and oxidative stability
Jumat Salimon a *, Nadia Salih a, Emad Yousif b
a School of Chemical Sciences & Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
b Department of Chemistry, College of Science, Al-Nahrain University, Baghdad, Iraq
Received 20 June 2010; accepted 31 August 2010 Available online 6 September 2010
KEYWORDS
Plant oils;
Environmentally friendly
biolubricants;
Oleic acid;
Pressurized DSC;
Cold flow property;
Oxidation onset temperature
Abstract Synthetic biolubricant basestocks with improved low temperature and oxidative stability were prepared by chemical modification of epoxidized oleic acid (EOA). Preparation, characterization and physico-chemical properties of mono, di and triester derivatives of 9,10-dihydroxyocta-decanoic acid after the epoxidation of oleic acid, opening of the formed oxirane ring in suitable medium, esterification of carboxylic acid hydroxyl group and acetylation of free hydroxyl group is discussed in this paper. Removal of the double bond from fatty acid acyl group, increase of the molar weight and change of molecular structure resulted in the increase of viscosity index and oxidation stability of synthetic esters.
© 2010 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.
1. Introduction
Sustainable development has become the key ideal of the 21st century. In the search for sustainable chemistry, considerable
* Corresponding author. Tel.: +60 3 8921 5412; fax: +60 3 8921 5410.
E-mail address: jumat@ukm.my (J. Salimon).
1319-6103 © 2010 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.
Peer review under responsibility of King Saud University. doi:10.1016/j.jscs.2010.08.004
importance is being attached to renewable raw materials, which exploit the synthetic capabilities of nature and may eventually substitute for fossil, depleting feedstocks (Metzger and Eissen, 2004). The encouragement of the environmentally sound and sustainable use of renewable natural resources is an important aim of Agenda 21 (Eissen et al., 2002). Oils and fats of plant and animal origin make up the greatest proportion of the current consumption of renewable raw materials in the chemical industry because they offer chemistry a large number of possibilities for applications that can be rarely met by petrochemistry.
Plant oils as biolubricants are preferred because they are biodegradable and non-toxic, unlike conventional mineral-based oils (Randles and Wright, 1992; Battersby et al., 1998). They have very low volatility due to the high molecular weight of the triacylglycerol molecule and have a narrow range of viscosity changes with temperature. Polar ester groups are able to adhere to metal surfaces, and therefore, possess good boundary
lubrication properties. In addition, plant oils have high solubi-lizing power for polar contaminants and additive molecules.
On the other hand, plant oils have poor oxidative stability (Gapinski et al., 1994; Becker and Knorr, 1996) primarily due to the presence of bis allylic protons and are highly susceptible to radical attack and subsequently undergo oxidative degradation to form polar oxy compounds. This phenomena result in insoluble deposits and increases in oil acidity and viscosity. Plant oils also show poor corrosion protection (Ohkawa et al., 1995). The presence of ester functionality renders these oils susceptible to hydrolytic breakdown (Rhodes et al., 1995). Therefore, contamination with water in the form of emulsion must be prevented at every stage. Low temperature study has also shown that most plant oils undergo cloudiness, precipitation, poor flow, and solidification at —10 0C upon long-term exposure to cold temperature (Rhee et al., 1995; Kassfeldt and Goran, 1997) in sharp contrast to mineral oil-based fluids.
Epoxidation is one of the most important functionalization reactions of the C-C double bond to improve plant oils oxidative stability. The chemistry of the Prileshajev epoxidation of unsaturated fatty compounds is well known (Findley et al., 1945). A short-chain peroxy acid is prepared from hydrogen peroxide and also the corresponding acid either in a separate step or in situ (Rangarajan et al., 1995). Other methods for the epoxidation include the use of dioxiranes (Sonnet et al., 1995), the generation of peracids from aldehydes and molecular oxygen (Kuo and Chou, 1987), and the use of alkyl hydroperoxides with transition-metal catalysts (Ucciani et al., 1993). Furthermore, a convenient method for the chemoenzymatic self-epoxidation of unsaturated fatty acids was developed (Riisch and Warwel, 1999). Unsaturated fatty compounds are preferably epoxidized on an industrial scale by the in situ performic acid procedure (Baumann et al., 1988). Epoxidized fatty acids are precursors for ring opening reactions (Crivello and Narayan, 1992).
The presented paper focuses on the preparation and characterization of new potential ester-type biolubricants. Epoxidized oleic acid (EOA) was prepared by starting oleic acid (OA). Subsequent opening of the oxirane ring in a suitable medium resulted in the formation of the 9-hydroxy-10-acyloxyoctadeca-noic acid (HYAODA). Esterification reaction for these seven mono esters was carried out using isobutanol to yield isobutyl 9-hydroxy-10-acyloxyoctadecanoate (IBHYAOD) and finally isobutyl 9-(hexanoyloxy)-10-(acyloxy) octadecanoate (IB-HOAOD) were obtained by the modification of the diesters (IB-HYAOD) with chloride of hexanoic acid. The paper discusses some physico-chemical parameters of these products.
2. Experimental
2.1. Materials
Formic acid (88%) was obtained from Fisher Scientific (Pittsburgh, PA) and oleic acid (90%) from Sigma-Aldrich Chemical Company. All other chemicals and reagents were obtained from commercial sources. Solvents were dried and purified with known conventional methods.
2.2. Methods
2.2.1. Characterization
The percentage compositions of the elements (CHNS) for the compounds were determined using an elemental analyzer
CHNS Model Fison EA 1108. *H and 13C NMR spectra were recorded using a JEOL JNM-ECP 400 spectrometer operating at a frequency of 400.13 and 100.77 MHz, respectively, using a 5-mm broadband inverse Z-gradient probe in DMSO-d6 (Cambridge Isotope Laboratories, Andover, MA) as solvent. Each spectrum was Fourier-transformed, phase-corrected, and integrated using MestRe-C 2.3a (Magnetic Resonance Companion, Santiago de Compostela, Spain) software. FTIR spectra were recorded neat on a Thermo Nicolet Nexus 470 FTIR system (Madison, WI) with a Smart ARK accessory containing a 45 Ze Se trough in a scanning range of 6504000 cm—1 for 32 scans at a spectral resolution of 4 cm—'.
2.2.2. Low temperature operability
The pour point is defined as the lowest temperature at which a liquid remains pourable (meaning it still behaves as a fluid). This method is routinely used to determine the low temperature flow properties of fluids. Pour point values were measured according to the ASTM D5949 method (ASTM Standard D5949) using a phase Technology Analyzer, Model PSA-70 S (Hammersmith Gate, Richmond, B.C., Canada). Each sample was run in triplicate and average values rounded to the nearest whole degree are reported. For a greater degree of accuracy, PP measurements were done with a resolution of 1 0C instead of the specified 3 0C increment. Generally, materials with lower PP exhibit improved fluidity at low temperatures than those with higher PP.
2.2.3. Flash point values
The flash point of a volatile liquid is the lowest temperature at which it can vaporize to form an ignitable mixture in air. Flash point determination was run according to the American National Standard Method using a Tag Closed Tester (ASTM D 56-79) (ASTM Standard D 56-79). Each sample was run in triplicate and the average values rounded to the nearest whole degree are reported.
2.2.4. Viscosity index measurements
Viscosity index (VI) is an arbitrary measure for the change of kinematic viscosity with temperature. It is used to characterize the lubricating oil in the automotive industry. Automated multi range viscometer tubes HV M472 obtained from Walter Herzog (Germany) were used to measure viscosity. Measurements were run in a Temp-Trol (Precision Scientific, Chicago, IL, USA) viscometer bath set at 40.0 and 100.0 0C. The viscosity and viscosity index were calculated using ASTM methods D 445-97 (ASTM D 445-97) and ASTM D 2270-93 (ASTM D 2270-93), respectively. Triplicate measurements were made and the average values were reported.
2.2.5. Oxidative stability
Pressurized DSC (PDSC) experiments were accomplished using a DSC 2910 thermal analyzer from TA Instruments (Newcastle, DE). Typically, a 2-^L sample, resulting in a film thickness of <1 mm, was placed in an aluminum pan hermetically sealed with a pinhole lid and oxidized in the presence of dry air (Gateway Airgas, St. Louis, MO), which was pressurized in the module at a constant pressure of 1378.95 kPa (200 psi). A 10 0Cmin—1 heating rate from 50 to 350 0C was used during each experiment. The oxidation onset (OT, oc)
and signal maximum temperatures (SMT, 0C) were calculated from a plot of heat flow (W/g) versus temperature for each experiment. Each sample was run in triplicate and average values rounded to the nearest whole degree are reported.
2.3. Synthesis
2.3.1. Epoxidized oleic acid (EOA, 1)
Hydrogen peroxide solution (30% in H2O, 8.0 mL) was added slowly into a stirred solution of oleic acid (OA) (90%, 15 g) in formic acid (88%, 14 mL) at 4 0C (ice bath). The reaction proceeded at room temperature with vigorous stirring (900 rpm) until the formation of a powdery solid was noticed in the reaction vessel (2-5 h). The solid was collected via vacuum filtration, washed with H2O (chilled, 3 x 10 mL), and placed for 12 h under high vacuum to provide epoxidized oleic acid (EOA) as a colorless, powdery solid.
2.3.2. 9-Hydroxy-10-acyloxyoctadecanoic acid (HYAODA, 2-8)
To a mixture of EOA (31 g), 5 g of p-toluenesulfonic acid (PTSA) and toluene, fatty acids (6 g) were added during 1.5 h in order to keep the reaction mixture temperature under 70-80 0C. The reaction mixture was subsequently heated to 90-100 0C and refluxed for 3 h. After the reaction termination, the heating was stopped and the mixture was left to stand overnight at ambient room temperature. The mixture was washed with the water next day. The organic layer was dried over anhydrous magnesium sulfate and the solvent was removed using the vacuum evaporator.
2.3.3. Isobutyl 9-hydroxy-10-acyloxyoctadecanoate (IBHYAOD, 9-15)
Sulfuric acid (conc. H2SO4, 10 mol-%) was added into a stirred suspension of HYAODA (3.35 mmol) in the isobutanol (3.35 mL). The suspension was heated with stirring at 60 0C for 10 h. Hexanes (5 mL) was then added, and the solution was washed with NaHCO3 (sat. aq., 1 x 0.5 mL) and brine (2 x 1 mL), dried (MgSO4), filtered, concentrated in vacuo and placed for 6 h under vacuum to yield the title products.
2.3.4. Isobutyl 9-(hexanoyloxy)-10-(acyloxy)octadecanoate (IBHOAOD, 16-22)
The reaction scheme of triesters formation is shown in Fig. 1. Appropriate amounts of IBHYAOD, pyridine and CCl4 were weighed into the 500-mL three-neck flask equipped with a cooler, dropping funnel and thermometer. The mixture was heated to 50 0C, with suitable aliquots of hexanoyl chloride added during 1 h, and the reaction mixture was subsequently refluxed for 4 h. On completion, the mixture stood overnight at ambient temperature. After washing with water, the solvent extract was dried over anhydrous sodium sulfate, further filtered and vacuum distilled to remove solvent.
3. Results and discussion
3.1. Epoxidation, esterification and acetylation
(FA) chain of plant oils. This reaction is carried out in four stages and the final products have significantly improved oxi-dative stability and low temperature property compared with the starting materials (Fig. 1). The straightforward epoxida-tion of oleic acid was closely monitored to avoid the synthesis of the undesired 9,10-dihydroxyoctadecanoate, which will form if the reaction temperature is elevated or the reaction is allowed to progress for too long.
The removal of unsaturation in the oleic acid by converting them to epoxy-groups 1, improves the oxidative stability. It has already been established that the presence of multiple double bonds in the plant oil FA chains accelerates oxidative degradation (Adhvaryu and Erhan, 2002). However, the low temperature fluidity of 1 is poor and found to solidify at 0 0C. This would limit the application of plant oil at low operating temperature especially as automotive and industrial fluids. A suitable approach to improve the low temperature flow behavior of 1 is to attach branching sites at the epoxy carbons. This was achieved by careful ring opening to obtain the 9-hydroxy-10-acyloxyoctadecanoic acid (HYAODA) products 2-8. Then, esterification of these products was carried out using isobutanol and sulfuric acid as catalyst to yield isobutyl 9-hydroxy-10-acyloxyoctadecanoate (IBHYAOD) 9-15. The seven prepared isobutyl esters were used as feed for the synthesis of modified triester derivatives by acetylation in an aprotic solvent.
3.2. Characterization
In the FTIR spectra of compounds 2-22, the absorption due to the C-O of epoxy group (822 and 842 cm-1) is not observed. This fact suggests that 1 undergoes complete ring opening under the reaction condition. Bands representing C=O groups (725, 1743 cm-1), CH3 groups (1373-1461 cm-1), OH groups (3475-3440 cm-1) and also C-O-C bands in esters (9981100 cm-1) are clearly visible in the spectra (Salimon and Sal-ih, 2010a).
All synthesized compounds were verified by 1H and 13C NMR spectroscopy. Significant signals in the 1H spectrum of EOA 1 between 2.5 and 2.7 ppm correspond to quaternary carbons of the oxiran ring and the doublet in the 13C spectrum between 56.86 and 56.90 ppm correspond to carbons of the oxirane ring. Furthermore, 1H spectrum of EOA showed singlet signal at 9.20 ppm due to OH group. A signal in the area around 9.15-9.27 ppm, representing an OH group, and the bands at 2.05-3.66 ppm, corresponding to -CH2- groups, are present in the 1H spectra of monoesters, HYAODA 2-8. The 1H spectra of synthesized diesters, IBHYAOD 9-15, and tries-ters, IBAHOAOD 16-22, consist of signals of low intensity at about 9.22-9.40 and 2.10-3.65 ppm due to OH and -CH2-groups' hydrogen peaks, respectively. On the other hand, in the 13C NMR spectra significant bands at about 174 ppm are present, which exhibit the characteristic signals attributed to C=O ester groups (Sliverstien et al., 2005). The elemental analysis data are in agreement with the proposed structures (Table 1).
3.3. Products parameters
Preparation of the triesters (IBHOAOD) 16-22 from EOA is an effective way of introducing branching on the fatty acid
Physico-chemical properties of prepared compounds are summarized in Table 2. The ability of a substance to remain as a
Oleic acid (OA)
PTSA, RCOOH
9-Hydroxy-10-acyloxyoctadecanoic acid (HYAODA, 2-8)
10 mol% H2S04, isobutanol
Isobutyl 9-hydroxy-10-acyloxyoctadecanoate (IBHYAOD, 9-15)
hexanoyl chloride, pyridine, CCI4 O
O-C-(CH2)4CH3
H2 / o—c—CH
•C—R CH=
Isobutyl 9-(hexanoyloxy)-10-(acyloxy)octadecanoate (IBHOAOD, 16-22)
Figure. 1 Triester formation. RCOOH - octanoic, nonanoic, lauric, myristic, palmitic, stearic and behenic acids.
liquid at low temperatures is an important attribute for a number of industrial materials, such as biolubricants, surfactants and fuels. The cold flow property of plant oils is extremely poor and this limits their use at low operating temperature especially as automotive and industrial fluids. Plant oils have a tendency to form macrocrystalline structures at low temperatures through uniform stacking of the 'bend' triglyceride backbone. Such macrocrystals restrict the easy flow of the system due to loss of kinetic energy of individual molecules during
self-stacking (Salimon and Salih, 2009). Cold flow properties of these samples were determined using their pour points. In practice, the usable liquid range is limited by the pour point (PP) at low temperatures and the flash point at high temperatures.
An improvement in the cold flow behavior of diesters IBHYAOD and triesters IBHOAOD was obtained over that of their monoester precursors HYAODA. Actually there are two reasons for this behavior. The first reason is that the
Table 1 Elemental analysis data of Prepared Products.
Samples Formula Elemental analysis Calc. (Found)
%C %H %N %S
HYOODA C26H50O5 70.54 (70.55) 11.38 (11.37) - -
HYNODA C27H52O5 71.01 (71.02) 11.48 (11.49) - -
HYLODA C30H58O5 72.24 (72.25) 11.72 (11.73) - -
HYMODA C32H62O5 72.95 (72.96) 11.86 (11.87) - -
HYPODA C34H66O5 73.59 (73.60) 11.99 (11.98) - -
HYSODA C36H70O5 74.17 (74.18) 12.10 (12.11) - -
HYBODA C40H78O5 75.18 (75.19) 12.30 (12.29) - -
IBHYOOD C30H58O5 72.24 (72.25) 11.72 (11.73) - -
IBHYNOD C31H60O5 72.61 (72.60) 11.79 (11.78) - -
IBHYLOD C34H66O5 73.59 (73.58) 11.99 (11.98) - -
IBHYMOD C36H70O5 74.17 (74.16) 12.10 (12.09) - -
IBHYPOD C38H74O5 74.70 (74.71) 12.21 (12.20) - -
IBHYSOD C40H78O5 75.18 (75.19) 12.30 (12.31) - -
IBHYBOD C44H86O5 76.02 (76.03) 12.47 (12.48) - -
IBHOOOD C36H68O6 72.44 (72.45) 11.48 (11.49) - -
IBHONOD C37H70O6 72.74 (72.75) 11.55 (11.56) - -
IBHOLOD C40H76O6 73.57 (73.58) 11.73 (11.72) - -
IBHOMOD C42H80O6 74.07 (74.08) 11.84 (11.85) - -
IBHOPOD C44H84O6 74.52 (74.51) 11.94 (11.93) - -
IBHOSOD C46H88O6 74.95 (74.94) 12.03 (12.04) - -
IBHOBOD C50H96O6 75.70 (75.71) 12.20 (12.19) - -
HYOODA, 9-hydroxy-10-octyloxyoctadecanoic acid; HYNODA, 9-hydroxy-10-nonanoxyoctadecanoic acid; HYLODA, 9-hydroxy-10-lauroxyoctadecanoic acid; HYMODA, 9-hydroxy-10-myristoxyoctadecanoic acid; HYPODA, 9-hydroxy-10-palmitoxyoctadecanoic acid; HYSODA, 9-hydroxy-10-stearoxyoctadecanoic acid; HYBODA, 9-hydroxy-10-behenoxyoctadecanoic acid; IBHYOOD, isobutyl 9-hydroxy-10-octyloctadecanoate; IBHYNOD, isobutyl 9-hydroxy-10-nonanoxyoctadecanoate; IBHYLOD, isobutyl 9-hydroxy-10-lauroxyoctadecanoate; IBHYMOD, isobutyl 9-hydroxy-10-myristoxyoctadecanoate; IBHYPOD, isobutyl 9-hydroxy-10-palmitoxyoctadecanoate; IBHYSOD,isobutyl 9-hydroxy-10-stearoxyoctadecanoate; IBHYBOD, isobutyl 9-hydroxy-10-behenoxyoctadecanoate; IBHOOOD, isobutyl 9-(hexanoyloxy)-10-(octyloxy)octadecanoate; IBHONOD, isobutyl 9-(hexanoyloxy)-10-(nonanoxy)octadecanoate; IBHOLOD, isobutyl 9-(hexanoyloxy)-10-(laur-oxy)octadecanoate; IBHOMOD, isobutyl 9-(hexanoyloxy)-10-(myristoxy)octadecanoate; IBHOPOD, isobutyl 9-(hexanoyloxy)-10-(palmitoxy) octadecanoate; IBHOSOD, isobutyl 9-(hexanoyloxy)-10-(stearoxy)octadecanoate; IBHOBOD, isobutyl 9-(hexanoyloxy)-10-(behenoxy) octadecanoate.
presence of a side chain attached to the FA backbone does not allow individual molecules to come close for easy stacking due to steric interactions. This results in the formation of micro-crystalline structures rather than macro structures. At lower temperatures, such microcrystalline structures can easily tumble and glide over one another resulting in better fluidity of the total matrix. Secondly, the lack of one hydroxyl group in diesters and then the absence of it in triester structures means that the number of hydrogen bonds decrease, which could cause the molecules to stack together.
Another important factor in determining how well oil will behave as a potential biolubricant is to evaluate the oil flash point (Salimonand Salih, 2010a). The flash point is often used as a descriptive characteristic of oil fuel, and it is also used to describe oils that are not normally used as fuels. Flash point refers to both flammable oils and combustible oils. There are various international standards for defining each, but most agree that oils with a flash point less than 43 0C are flammable, while those having a flash point above this temperature are combustible (Salimon and Salih, 2010b).
The efficiency of the biolubricant in reducing friction and wear is greatly influenced by its viscosity, the viscosity of oils decreases as temperature increases. Generally, the least viscous biolubricant which still forces the two moving surfaces apart is desired. If the biolubricant is too viscous, it will require a large
amount of energy to move; if is too thin, the surfaces will rub and friction will increase. The viscosity index highlights how a biolubricant's viscosity changes with variations in temperature. Many biolubricant applications require performing across a wide range of conditions, ex., in an engine. Automotive biolubricants must reduce friction between engine components when it is started from cold (relative to engine operating temperatures) as well as when it is running (up to 200 0C). The best oils (with the highest VI) will not vary much in viscosity over such a temperature range and therefore will perform well throughout. In this work, increased viscosity index (VI) of tri-esters is the result of their higher molar weight, and especially the altered structure of their molecules (Table 2).
The ability of a substance to resist oxidative degradation is another important property for biolubricants. Therefore, EOA, HYAODA, IBHYAOD and IBHOAOD were screened to measure their oxidation stability using PDSC through determination of OT and SMT. PDSC is an effective method for measuring oxidation stability of oleochemicals in an accelerated mode (Du et al., 2003). The OT is the temperature at which a rapid increase in the rate of oxidation is observed at a constant, high pressure (200 psi). A high OT would suggest high oxidation stability of the material. The SMT is the temperature at which maximum heat output is noted from the sample during oxidative degradation. A higher SMT does
Table 2 Pour point, Flash point, Viscosity index, OT, SMT and percentage yield of Prepared Products.
Samples Pour pointa (°C) Flash pointa (°C) Viscosity Indexa OTa (°C) SMTa (°C) %
EOA 0 113 45 75 164 91
Monoesters 2-8
HYOODA -20 250 71 113 123 70
HYNODA -30 305 80 101 256 63
HYLODA -33 176 84 91 189 80
HYMODA -35 199 89 83 213 56
HYPODA -39 123 93 76 209 92
HYSODA -41 194 102 70 243 85
HYBODA -43 232 123 64 175 76
Diesters 9-15
IBHYOOD -22 125 113 123 239 90
IBHYNOD -32 112 119 115 215 82
IBHYLOD -35 188 122 104 210 70
IBHYMOD -37 174 131 94 134 85
IBHYPOD -42 123 139 89 187 72
IBHYSOD -44 285 145 82 164 90
IBHYBOD -60 297 167 78 193 75
Triesters 16-22
IBHOOOD -24 156 169 207 215 65
IBHONOD -33 173 183 200 234 53
IBHOLOD -36 201 191 196 218 74
IBHOMOD -38 182 203 187 220 80
IBHOPOD -44 191 212 172 199 75
IBHOSOD -46 213 218 168 186 55
IBHOBOD -62 200 222 155 165 78
% Yield
Mean n = 3, SE ± 1 °C.
not necessarily correlate with improved oxidation stability. Both OT and SMT were calculated from a plot of heat flow (W/g) versus temperature that was generated by the sample upon degradation and, by definition, SMT > OT.
In the present study, as the chain length of the mid-chain ester is decreased, a corresponding improvement in oxidation stability was observed, which is because longer chains are more susceptible to oxidative cleavage than shorter chains. These results are in agreement with other studies on synthetic esters (Randals, 1999). For example, when comparing monoesters (HYAODA), diesters (IBHYAOD) and triesters (IBHOAOD), an improvement in OT was noticed as the mid-chain ester length (R) was decreased (Table 2).
4. Conclusions
The process describes a systematic approach to modify chemically the oleic acid oil to yield basestocks capable of operating at low temperatures. Preparation was based on the epoxidation of acyl double bond, opening of the formed oxi-rane ring in an appropriate medium and acetylation of free hydroxyl group. Based on the results obtained, an increase in the chain length of the mid-chain ester had a positive influence on the low temperature properties of the products because they create a steric barrier around the individual molecules and inhibits crystallization, resulting in lower pour point. But the trends for PP run counter to that of OT, i.e., increasing chain length is a benefit to PP, but a detriment to OT. Also it is evident that hydrogen bonding is a critical parameter influencing the low temperature properties and
oxidation stability of synthetic esters. An increase in hydrogen bond amount will lead to an increase in pour point (PP) and decreases the oxidation stability of these compounds. Removal of the unstable double bonds from fatty acid acyls, increased the molar weight and change in the molecular structure results in increased viscosity index of the products.
Acknowledgements
The authors acknowledge the Universiti Kebangsaan Malaysia for the funding ("Code UKM-GUP-NBT-08-27-113'' and "UKM-OUP-NBT-29-150/2010"), and the direct contributions of the support staff from the School of Chemical Sciences and Food Technology, the Faculty of Science and Technology, Universiti Kebangsaan Malaysia. Special thank to SRF, IIE.
References
Adhvaryu, A., Erhan, S.Z., 2002. Ind. Crops Prod. 15, 247. ASTM Standard D5949: Standard test method for pour point of petroleum (automatic pressure pulsing method). ASTM, West Conshohocken, PA (USA). ASTM Standard D 56-79: Standard test method for flash point of liquids with a viscosity less than, 45 Saybolt Universal Seconds (SUS) at 37.8 °C (that don't contain suspended solids and don't tend to form a surface film under test). ASTM D 445-97: Standard test method for kinematic viscosity of transparent and opaque liquids. ASTM, West Conshohocken, PA (USA).
ASTM D 2270-93: Standard practice for calculating viscosity index from kinematic viscosity at 40 and 100 C. ASTM, West Cons-hohocken, PA (USA). Battersby, N.S., Pack, S.E., Watkinson, R.J., 1998. Chemosphere 24, 1998.
Baumann, H., Bühler, M., Fochem, H., Hisrsinger, F., Zoeblein, H.,
Falbe, 1988. J. Angew. Chem. Int. Ed. Engl. 27, 41. Becker, R., Knorr, A., 1996. Lubr. Sci. 8, 95. Crivello, J.V., Narayan, R., 1992. Chem. Mater. 4, 692. Du, D., Kim, S.-S., Moon, W.-S., Jin, S.-B., Kwon, W.-S., 2003.
Thermochim. Acta 407, 17. Eissen, M., Metzger, J.O., Schmidt, E., Schneidewind, U., 2002.
Angew. Chem. Int. Ed. 41, 414. Findley, T.W., Swern, D., Scalan, J.T., 1945. J. Am. Chem. Soc. 67,412. Gapinski, R.E., Joseph, I.E., Layzell, B.D., 1994. SAE Tech. Pap.
941785, 1-9. Kassfeldt, E., Goran, D., 1997. Wear 207, 41. Kuo, M.C., Chou, T.C., 1987. Ind. Eng. Chem. Res. 26, 277. Metzger, J.O., Eissen, M.C., 2004. Chemie 7, 569. Ohkawa, S.A., Konishi, H., Hatano, K., Tanaka, K., Iwamura, M., 1995. SAE Tech. Pap. 951038, 55-63.
Randals, S.J., 1999. Esters in Synthetic Lubricants and High-Performance Functional Fluids. In: Rudnick, L.R., Shubkin, R.L. (Eds.). Marcel Dekker, New York, p. 63. Randles, S.J., Wright, M., 1992. J. Synth. Lubr. 9, 145. Rangarajan, B., Havey, A., Grulke, E.A., Culnan, P.D., 1995. J. Am.
Oil Chem. Soc. 72, 1161. Rhee, I.S., Valez, C., Bernewitz, K., 1995. Warren, MI, 1. Rhodes, B.N., Mammel, P., Landis, P., Erikson, F.L., 1995. SAE
Tech. Pap. 952073, 1-4. Rusch, M., Warwel, S., 1999. Ind. Crops Prod. 9, 125. Salimon, J., Salih, N., 2009. Eur. J. Sci. Res. 32, 216. Salimon, J., Salih, N., 2010a. Asian J. Chem. 22, 5468. Salimon, J., Salih, N., Yousif, E., 2010b. Eur. J. Lipid Sci. Technol. 112, 519.
Sliverstien, R., Bassler, G., Morrill, T., 2005. Spectrometric Identification of Organic Compounds, 7th ed. John-Wiley, New York.
Sonnet, P.E., Lankin, M.E., McNeill, G.P., 1995. J. Am. Oil Chem. Soc. 72, 199.
Ucciani, E., Debal, A., Rafaralahitsimba, J., 1993. Fat Sci. Technol. 95, 236.
