Oxidative Stability of Base Lubricant Oil Monitored by Gas

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Oxidative Stability of Base Lubricant Oil Monitored by Gas Chromatography−Mass Spectrometry: Influence of Sawdust-Derived Antioxidants Imtiaz Ahmad,*,† Jan Ullah,† M. Ishaq,† Hizbullah Khan,‡ Razia Khan,† Waqas Ahmad,† and Kashif Gul† †

Institute of Chemical Sciences and ‡Department of Environmental Sciences, University of Peshawar, 25120 Peshawar, Khyber Pakhtunkhwa, Pakistan ABSTRACT: This paper focuses on evaluating the potential of sawdust-derived antioxidants in oxidative stability of the lubricant oil samples monitored through gas chromatography−mass spectrometry (GC−MS). The plain and antioxidants-spiked oil samples were subjected to artificial aging at 100 °C for 6 h in a flow of air (10 L h−1). The results indicate that the antioxidants under study imparted thermo-oxidative stabilities to the oil compared to the plain sample and exhibited good antioxidants potential at 100 °C.

1. INTRODUCTION Engine oil undergoes oxidation in service and suffered from thermo-oxidative degradation. The oxidized products, i.e., acids, aldehydes, ketones, esters, and lactones, alter the chemical composition of the oil, which causes a change in desired properties. As a result, the oil becomes spent and needs frequent replacement with a new oil to avoid engine malfunction.1,2 To increase the life span of the oil and retain its integrity during service under the hostile engine conditions, several performance improvement additives are added.3−6 Among these, the focus has been made on new and novel antioxidants. Many synthetic antioxidants have been studied up to date.7−11 However, most of them are petroleum-derived and possess process and environmental problems.12−15 Moreover, they suffer from antagonism in the presence of impurities, such as water, ethylene glycol, fuel, soot, and wear metals, and lose potential as antioxidants.16,17 On the contrary to synthetic antioxidants, the bioantioxidants are gaining importance, owing to their abundant availability, cost effectiveness, resistance to antagonism, and minor environmental issues. Many biomass materials have been studied in the recent past;18−20 however, with the derived antioxidants being alkaline, they suffer from antagonistic effects and readily deactivate as a result of acidic oxidized products.21 Further, the hostile conditions in a machinery can influence the integrity and performance of antioxidants. The activity of most bio-antioxidants depends upon hydrogen donation, because they are sparingly soluble in oils, evaporate easily at high temperatures, and thus cannot withstand oxidation for a long duration.22 Therefore, concerted efforts are needed to develop new, novel, environmentally friendly, and cost-effective bioantioxidants. Owing to the presence of higher levels of phenolic and amininc constituents, the potentials of rice husk and sawdust (SD) as sources of antioxidants have been reported in our previous study carried out at a high temperature (200 °C),23 where the SD-derived antioxidants did not prove effective compared to the rice husk. Lubricant oils can be used in lowtemperature applications; hence, they can be formulated with © XXXX American Chemical Society

antioxidants that are effective in maintaining oil stability at low temperatures. In the present work, we aimed to evaluate the antioxidants character of the studied antioxidants at a low temperature, i.e., 100 °C. We employed the gas chromatography−mass spectrometry (GC−MS) to monitor the changes in chemical composition of the oil samples.24

2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. The lubricant oil was supplied by the Hydrocarbon Development Institute, Pakistan, in a plastic can. The SD of Pinus longifolia was collected from a saw machine and dried in an oven. A representative sample was sieved through a 750 μm sieve and stored in a desiccator at room temperature for further use. Elemental (CHNSO) analysis of the SD is provided in Table 1. Methanol and other chemicals and reagents were of analytical grade, procured from Merck (Germany), and used without further purification.

Table 1. Elemental Analysis of SD element

level (wt %)

carbon hydrogen nitrogen oxygen

38.44 4.98 0.45 36.84

2.2. Extraction of SD. The dried SD was extracted through maceration by soaking in 1 L of methanol and kept in contact for a few days at room temperature. The supernatant was then decanted and subsequently concentrated by rotary evaporation using a rotary evaporator (RV-05-ST, Janke & Kunkel IKA-Labortechnik Company, Germany). The crude extract was stored in a glass vial for analysis and further use as a source of antioxidants. The percentage extract yield was determined to be 10 ± 1%. 2.3. Oxidation Tests. The Institute of Petroleum (IP) 48 method25 was used in the oxidation study, which was carried out in a laboratory oxidation apparatus (Figure 1). In a typical run, an aliquot Received: March 1, 2017 Revised: June 5, 2017

A

DOI: 10.1021/acs.energyfuels.7b00555 Energy Fuels XXXX, XXX, XXX−XXX

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3. RESULTS AND DISCUSSION 3.1. Antioxidants Potential and Composition of the SD Extract. The ferric thiocyanate (FTC) method was used to measure the absorbance of the blank and that of the sample. A high absorbance value in the blank indicates greater linoleic acid oxidation, which lead to the formation of peroxides. The peroxides thus formed lead to oxidation of Fe2+ to Fe3+ ions, which form a red-colored complex with thiocyanate that can be estimated by measuring the absorbance at 500 nm. The antioxidants potential of the SD extract was determined in the concentration range of 1−5 μg/mL. The results are compiled in Table 2. The percent inhibition calculated from the difference Table 2. Antioxidants Character of the SD-Derived Extract Figure 1. Schematic of the oxidation reactor. of 5 g of the lubricant oil was summed into the oxidation reactor, which was connected to an air supply. The reactor was immersed into an oil bath. The paraffin oil was used as bath liquid. The bath was, in turn, heated by a hot plate. The air was continuously blown during the experiment. After the oil was subjected to oxidation for a definite duration of time, the residual oil was carefully collected and stored in a stoppered vial for further analysis. The experimental conditions used were as follows: temperature of oxidation, 100 °C; oxidation time, 6 h; antioxidants concentration, 3% (w/w); and air flow rate, 10 L h−1. 2.4. Characterization. The elemental analysis of the SD was carried out using a CHNS analyzer (Elementar model vario EL II). The antioxidants ability of the SD-derived antioxidants was determined by a thiocyanate method described elsewhere.26,27 In a typical method, the following reagents were used: (1) 4.0 mg of SD extract in 4 mL of absolute ethanol, (2) 4.1 mL of 2.5% linolenic acid in absolute ethanol, (3) 8.0 mL of 0.05 M phosphate buffer (pH 7.0), (4) 3.9 mL of distilled water, (5) 9.7 mL of 75% ethanol, (6) 0.1 mL of 30% ammonium thiocyanate, and (7) 0.1 mL of 0.02 M ferrous chloride in 3.5% HCl. Reagents listed as numbers 1−4 were mixed and poured into a glass vial provided with a screw cap and then placed in an oven at 40 °C in the dark. To 0.1 mL of this mixture was added reagents listed as numbers 5 and 6. Precisely 3 min after the addition of reagent listed as sample 7 to the reaction mixture, the absorbance of the resultant red color was measured at 500 nm for 24 h until the absorbance of the control reached a maximum. The mixture without the test sample was used as the control. The inhibition of lipid peroxidation in percentage was calculated as

concentration (μg/mL)

inhibition (%)

1 2 3 4 5

37.24 43.56 62.33 83.72 82.37

in absorption of the control and extract was found to be increased with the increase in the concentration and observed to be the highest (83.72%) at the concentration of 4 μg/mL. The chemical compounds in the SD extract were determined by GC−MS and are reported in our earlier study.23 Most of the compounds determined are phenols and their derivatives and aromatic amines (Table 3). The phenols and amines have been Table 3. Phenolics and Other Antioxidants Determined in the SD-Derived Extract antioxidants

concentration (%)

phenolics others

61.310 38.69

reported to be associated with excellent antioxidants characteristics, owing to their radical-scavenging abilities.28−30 The compositional analysis of the lubricant oil shows aliphatic, naphthenic, and aromatic hydrocarbons, with a preponderance of aliphatic components (Figure 2a and Tables 4−7). 3.2. TG Analysis of the SD Extract. The TG study of the SD extract was carried out. The thermogram is displayed in Figure 3, and the corresponding data are provided in Table 8. A significant mass loss can be observed with the increase in the temperature, which reveals the presence of different chemical constituents in the extract. Several mass losses/disintegration steps can be observed throughout the thermogram. A mass loss of about 6% can be observed at the temperature range of 105− 171 °C, which may be due to evaporation of inherent moisture and other volatile constituents.31 The Tonset and Toffset are observed to be 100 and 155 °C, respectively. The second degradation step indicates 65% mass loss, which may be due to the thermal degradation of the phenolic constituents. Tonset in this step is observed to be 156 °C, and Toffset in this step is observed to be 226 °C. The antioxidants that frequently decompose in this step are reported to be naphthoquinone, ascorbic acid, phenols, methylphenols, aminophenol, bromophenol, methoxyphenols (anisol), etc.32,33 The results indicate their presence in the derived extract. The third step is the medium step that involves the thermal splitting of some excellent antioxidants, namely, pyrogallol

percent inhibition (%) = 100 − [(A s − Ac) × 100] where Ac is the absorbance recorded for the control and As is the absorbance recorded for the sample. The SD methanolic extract and original unoxidized and oxidized oil samples were analyzed by a gas chromatograph coupled with a mass analyzer (Shimadzu, Japan). The experimental conditions used were as follows: carrier gas, helium; flow rate of carrier gas, 1.3 mL min−1; split ratio, 50; injector temperature, 300 °C; sample injection volume, 1 μL; and initial oven temperature, 35 °C. The mass spectrum library of the National Institute of Standards and Technology (NIST) was consulted for identification of product peaks in the chromatograms. A thermogravimetric (TG) analyzer (model TGA/SDT 85e, Mettler-Toledo, Switzerland) was used for thermal analysis. The experimental conditions used were as follows: heating rate, 10 °C min−1; O2 atmosphere, 50 mL min−1; and temperature, from ambient to 700 °C. The physicochemical analyses were carried out using ASTM-/IPdesignated methods (kinematic viscosity at 40 °C, ASTM D445; kinematic viscosity at 100 °C and viscosity index, ASTM D2270; viscosity ratio, ASTM D445/IP 71; Conradson carbon residue, ASTM D189; acid number, ASTM D664; and iodine number, ASTM D1959). B


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Figure 2. GC−MS chromatograms of original and various oxidized lubricant oil samples: (a) original unoxidized lubricant oil, (b) plain oxidized lubricant oil, and (c) SD-derived antioxidants-spiked oxidized lubricant oil. C


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Energy & Fuels Table 4. Paraffins Determined in Various Lubricant Oils original unoxidized oil

plain oxidized oil

antioxidants-spiked oxidized oil

compound

tR

concentration (%)

tR

concentration (%)

tR

concentration (%)

heptane octane, 2,5-dimethylheptane, 3-methyloctane octane, 4-methyloctane, 3-ethyl, 2,7-dimethylnonane pentane, 3-ethyl-2,4-dimethyloctane, 2,6-dimethylnonane, 2-methylnonane, 3-methyldecane octane, 2,3,6-trimethylheptane, 2,5,5-trimethylnonane, 3,7-dimethylundecane undecane, 5-methyldodecane tridecane tridecane, 6-methyltridecane, 4-methyltridecane, 3-methyltetradecane tetradecane, 3-methylpentadecane pentadecane, 4-methyldecane, 5-propylhexadecane dodecane, 2,6,11-trimethylheptadecane, 2,6,10,15-tetramethylheptadecane heptadecane, 2-methylhexadecane, 3-methyloctadecane hexadecane, 2,6,10,14-tetramethyltetratriacontane nonadecane eicosane octadecane, 1-iodo henicosane hentriacontane pentatriacontane hexaheptane, 2,6,9,15-tetramethyldocosane tricosane tetraacosane pentacosane hexacosane heptacosane octacosane total concentration

2.964 4.883

2.045 0.186

6.190 8.706

0.729 0.248

10.181 10.794 11.436 12.501 12.862 13.631 14.684 15.991

0.358 14.952 3.831 0.264 0.130 0.582 0.424 0.074

17.321 19.450 20.529 22.789 23.683

0.649 0.062 0.678 0.537 0.136

2.010 1.277 0.566 1.515 0.328 0.591 0.311 13.627 3.022 0.215 0.105 0.451 0.382 0.125 0.545 0.483 0.096 0.869 0.692 0.234

0.882 0.138 2.747

24.228 24.479 26.025 26.630

1.025 0.801 0.220 2.439

28.534 29.833

0.360 3.233

34.614

5.363

2.565 1.317 0.602 1.555 0.678 0.638 1.090 5.227 4.185 1.378 0.731 2.410 1.930 0.508 0.540 2.281 0.658 2.375 4.697 0.537 0.343 1.199 5.292 0.381 4.25 0.199 0.499 0.236 0.090 0.696 4.800 0.640

3.154 4.861 5.025 6.162 8.527 8.485 10.136 10.427 11.522 12.284 12.321 13.921 14.652 15.834 16.229 17.285 19.577 20.631 22.925 23.735

24.575 26.006 26.721

3.154 4.870 5.136 6.178 8.691 8.771 10.167 10.789 11.440 12.494 12.857 13.924 14.678 15.985 16.334 17.315 19.457 20.524 22.785 23.687 23.828 24.119 24.574 26.004 26.719 28.332 28.145 30.048 32.283 32.969 34.612 38.380

38.394 39.913 40.200 43.315 44.688 49.042 51.325 53.058

0.300 6.498 1.821 0.427 4.618 5.918 0.375 6.692

39.901 40.192 43.310 44.676 49.032

5.816 1.599 0.130 5.674 5.582

55.811

0.051

11.455 4.455 1.495 2.550 0.955 1.233 2.840 0.664

5.441 0.389 0.031 0.411 5.977 0.844 4.979 0.055

0.397 3.111 0.053 0.196 5.998 0.041 0.209 8.213 0.989 0.412 7.534 8.922 0.339 8.141

54.458 54.814 58.642 63.325 64.125 64.852 66.709 70.103

53.054 55.741 55.933 56.234 56.809 59.341 63.699 70.103

28.255 30.754 32.112 32.969 34.537 38.377 38.257 39.826 40.148 43.316 44.609 49.031 51.321 53.033

56.726 59.339 63.714

7.812 1.117 4.865

70.087

0.602

92.719

87.892

90.931

the thermal decomposition of constituents, such as 7-hydroxy4-methylcoumarin and phenylazo-β-naphthol in the SD extract,35 which corresponds to a mass loss of 6%. The last, fifth step involves the combustion of fixed carbon or carbon residue of the SD extract. Tonset and Toffset are observed to be 298 and 400 °C, respectively, with a mass loss of 3%.

(1,2,6-trihydroxybenzene) and naphthalene diols and alkylated benzene diol nitro-naphthol and methylphenyl acetamide.34 The degradation of these constituents corresponds to a mass loss of about 20%. Tonset in this step is observed to be 227 °C, while Toffset is observed to be 264 °C. In the fourth step, Tonset is observed to be 247 °C, while Toffset is observed to be 297 °C. This degradation may be due to D


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Energy & Fuels Table 5. Olefins Determined in Various Lubricant Oils original unoxidized oil

plain oxidized oil


compound

tR

concentration (%)

tR

concentration (%)

tR

concentration (%)

1-hexene, 2-methyl2-heptene, 2-methyl1-hexene, 2,3-dimethyl2-pentene, 2,3,4-trimethyl4-nonene 2-nonene 2-decene, 5-methyl-, (Z) 7-tetradecene, (Z) 9-octadecene, (E) 1-tetradecene total concentration

2.964

2.045

5.409

0.087

2.957 6.386 4.113 5.493 10.530

0.782 0.076 0.175 0.410 0.307

2.964 4.113 6.412 5.427 10.534

0.851 0.051 0.041 0.097 0.085

10.726 19.089

0.171 0.086

19.086 22.558 24.808 26.422

0.950 0.099 0.112 0.061

11.863 19.098 22.562

0.025 0.093 0.055

2.389

2.890

2.648

Table 6. Naphthenes Determined in Various Lubricant Oils original unoxidized oil

plain oxidized oil


compound

tR

concentration (%)

tR

concentration (%)

tR

concentration (%)

cylohexene cylohexane, 1,1,3-trimethylcylohexane, propylcyclopentene, 1-(2-methylpropyl)cylohexane, 1-methyl, 3-propylcylohexane, butylcycloheptanone, 2-methyl, 3-isopentylcyclododecane undecane, 2-cyclohexyltotal concentration

2.858 7.654 11.297

0.264 0.086 0.237

0.308 0.141 0.299

0.236 0.147 1.011 0.713

0.093 0.486 1.054 0.070 0.506 0.809

2.876 7.683 11.249

13.302 15.017 58.417 24.123

2.850 7.641 11.286 11.882 13.436 15.012

13.417 15.122

0.292 0.195

28.263

0.392

24.181 28.179

0.833 0.991

2.694

2.694

3.059

plain oxidized oil


Table 7. Aromatics and Esters Determined in Various Lubricant Oils original unoxidized oil compound

tR

concentration (%)

tR

concentration (%)

tR

concentration (%)

toluene ethylbenzene benzene, 1,3-dimethylbenzene, 1-ethyl-3-methylbenzene, 1,3,5-trimethylbenzene, 1,2,4-trimethylbenzene, 1-ethyl-2,3-dimethylbenzene, 1-methyl-3-(1-methylethyl) benzene, (1,1-dimethylpropyl)naphthalene, 2,6-dimethyl 0-undecanoic acid, 3-methylbutyl ester total concentration

4.969 8.706 8.954 13.094

0.633 0.330 0.421 0.328

4.951 8.567 8.941 13.083 13.676

1.160 0.188 1.386 0.208 1.444

14.623 16.564

1.290 0.196

4.957 8.573 8.986 13.257 13.682 14.749

0.758 0.229 0.298 0.168 1.104 0.212

16.591 19.776 25.198 64.792

0.301 0.171 0.490 0.281

16.591

0.508

2.198

5.729

64.812

0.095 3.322

provided in Figure 2b, and the related data are compiled in Tables 4−7. Some intense peaks (10−52 min) characteristic of lubricant oil range hydrocarbons, i.e., C10−C26, can be observed. The presence of some new peaks as a result of degraded products can also be seen. It is well-established that the degree of degradation is assessed by the increase in the concentration of naphthenes, olefins, aromatics, and carbonyl compounds, such as aldehydes, ketones, carboxylic acids, or esters.36 The composition of the plain base oil after oxidation is found to be similar to that of the original base oil (unoxidized), with some minor changes in the concentration of aliphatic and aromatic hydrocarbons. The hydrocarbon-group-type distribution of different compounds shows the relative abundance of paraffins, olefins, naphthenes, and esters in the plain sample, where their proportions are found to be 87.892, 2.966, 5.448,

In a nut shell, the thermal analysis shows that the SD extract is composed of high volatile matter (6%), thermally stable phenol-derived antioxidants below 200 °C (65%), and other thermally stable natural antioxidants (20%) above 200 °C as medium volatiles. The natural thermally stable antioxidants are found to be phenolic compounds, including naphthoquinone, alkylated hydroquinone, anisole pyrogallol, and coumarin, in a high concentration. Thus, the SD extract can be employed as an additive in base oil, owing to the presence of some excellent antioxidants. 3.3. Oxidative Stability of Lubricant Oil. The oxidative stability of the lubricant oil was evaluated from the changes in chemical composition before and after oxidation. GC−MS was used to monitor these changes during oil oxidation. The chromatogram of the plain lubricant oil oxidized at 100 °C is E


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Figure 3. Thermogram of the SD extract.

Table 8. Thermo-oxidative Degradation Data of the SD Extract and Various Lubricant Oils after Oxidation at 100 °C sample

atmosphere

SD extract

O2

plain oxidized base oil

O2

SD-extract-additized oxidized base oil

O2

degradation step

Tonset (°C)

mass before degradation step (wt %)

Toffset (°C)

mass after degradation step (wt %)

mass loss per degradation step (wt %)

1 2 3 4 5 1 2 3 1 2 3

100 156 227 247 298 100 206 401 100 225 431

100 94 29 9 3 100 88 8 100 91 5

155 226 246 297 400 205 400 627 224 430 616

94 29 9 3 0 88 8 3 90 5 3

6 65 20 6 3 12 80 5 10 88 2

naphthenes. The peaks corresponding to the original unoxidized and oxidized oils are found to be similar to some distinct difference in paraffins, as evident from the peak size. The size of the peak corresponding to lower hydrocarbons (C6−C11) in unoxidized oil is quite large compared to the oxidized oil. Hydrocarbons of this range may have been converted to aromatics via cyclization and dehydrogenation reactions and may have caused a significant percent increase in the aromatic contents of the oxidized sample. The removal of labile H gives the ring enclosure to form naphthene, followed by dehydrogenation reactions that may have promoted the aromatic formation, as reported earlier.37 The lubricant oil spiked with the SD extract was oxidized under similar experimental conditions. The GC−MS data are presented in Tables 4−7, and the corresponding chromatogram is displayed in Figure 2c. The results indicate that the spiked oil retains the chemical composition closer to the original oil (unoxidized). The hydrocarbon group types and degraded products are confirmed from the change in their concentrations in comparison to the plain lubricant oil, which are found to be 90.972, 2.607, 3.277, 3.059, and 0.095%, respectively. The hydrocarbon carbon range compounds (C6−C11, C12−C17, and C18−C35) are found to be 29.879, 19.514, and 50.607%, respectively. As seen from the results, the lubricant oil range compounds (C18−C35) increase, while the lower range hydrocarbons (C6−C10 and C11−C20) decrease in comparison to the plain sample. The C18−C35 range compounds have been reported to be lubricant range compounds.38 The effectiveness of the antioxidants under study can be evaluated from the fact that the lubricant oil range (C18−C35) hydrocarbons withstood

3.410, and 0.281%, respectively. The carbon number distributions, i.e., C6−C11, C12−C17, and C18−C35, are found to be in concentrations of 34.683, 29.317, and 36.00%, respectively. It can be observed that the concentration of C18−C35 range hydrocarbons (the lubricant range) is less than that observed in the original unoxidized lubricant oil. Moreover, the concentration of C18−C35 hydrocarbons is found to be C6−C10 > C11−C20

The overall PD in composition of the plain base oil oxidized at 100 °C in comparison to the control sample is found to be 9.370%, which indicates that the sample has undergone compositional changes, which lead to 9.370% degradation. The results show a decline in the concentration of the paraffins with a corresponding increase in the olefins, aromatics, and F


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Figure 4. Thermograms of lubricant oil samples oxidized at 100 °C: (a) plain oil and (b) SD-extract-additized oil.

sample is found to be 0.281%, while that in the SD is 0.095%. As clear from the results, the antioxidants cause a reduction of the concentration of the degraded products (esters), which indicates the efficacy as antioxidants. The sum of the PDs (overall PD) determined for all fractions with respect to the control sample is found to be 4.124%. The overall PD for all of the hydrocarbon group types along with the ester content calculated in the plain oxidized oil in comparison to the original sample is significant (9.370%). The results reveal that the overall degradation of 9.370% (change in composition) occurs in plain base oil, which declines to 4.124% in the presence of antioxidants under study. Thus, the SDderived antioxidants reduce the degradation by 5.246%. This significant reduction in the percent change reveals the antioxidants behavior. 3.4. TG Analysis of the Oxidized Oil. The thermograms of the plain and SD-extract-additized base oil samples oxidized at 100 °C for 6 h are provided in panels a and b of Figure 4, and the corresponding data are provided in Table 8. The thermooxidative stabilities of the samples were evaluated in terms of the change in Tonset, concentration of primary degradation products, and weight gain. Tonset of the plain base oil sample after oxidation at 100 °C is observed to be 205 °C, while that of the SD-extract-additized sample is observed to be 224 °C. The result indicates that the SD extract caused an appreciable increase in Tonset. The higher the Tonset for a substance, the better its antioxidants behavior will be, as reported elsewhere.40

oxidation. The aliphatic and aromatic hydrocarbons determined in the spiked samples indicate a decline in the concentration of paraffins with a corresponding increase in olefins, aromatics, and naphthenes. The hydrocarbon net fractional composition of the spiked sample follows the order of paraffins > aromatics > naphthenes > olefins > ester

To find out the antioxidants efficacy of the antioxidants under study, the PDs in various C fractions in comparison to the original base oil is also determined, which are found to be 1.747, 0.246, 1.079, and 0.957%, respectively. Similarly, the PDs of the various carbon range compounds, i.e., C6−C11, C12−C17, and C18−C35, determined in comparison to the original unoxidized sample are found to be 1.140, 4.074, and 2.934%, respectively. The results exhibit that the values in the spiked sample are very much close to the original oil, which indicates the antioxidants character. The results further indicate that the antioxidants under study increase the lubricant oil desired range by 14.607% in comparison to the plain oil. The overall results provide evidence of the antioxidants potential of the SD extract. It has been reported earlier that ester is formed because of paraffin oxidation through the formation of alcohols, aldehydes/ketones, and carboxylic acids as intermediate oxygenates.39 The level of degradation inhibited by SD can be evaluated from the percent decrease in the concentration of esters among the spiked and plain oxidized lubricant oil samples. The ester concentration determined in the plain G


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Energy & Fuels Table 9. Physicochemical Properties of Various Lubricant Oils parameter kinematic viscosity at 40 °C kinematic viscosity at 100 °C viscosity index viscosity ratio Conradson carbon residue total acid number iodine number

unit mm2/s mm2/s

wt % mg of KOH/g mg/g

original base oil

plain oxidized oil (100 °C)

SD-extract-additized oil (100 °C)

110 15.50 150.2 1.00 0.20 0.40 17

119.60 14.26 120.20 1.081 0.330 2.50 102.0

115.08 14.87 133.94 1.038 0.245 1.850 62.00

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The degree of degradation can also be evaluated from the concentration of the fast volatiles as mass loss in the first degradation step. For the unadditized base oil, the fast volatiles are found to be 12%, while for the SD-extract-additized oil, the fast volatiles are found to be 10%. The weight gain can be seen in the oxidized samples, which may be due to the formation of polymeric oxidation products.41 The weight gain observed in the plain oxidized sample is quite significant compared to that in the SD-additized lubricant oil. Thus, the SD extract reduced the fast volatiles as well as weight gain, which indicate its effectiveness as a source of antioxidants. The results inferred a good antioxidants character of the SD extract at 100 °C. 3.5. Physicochemical Properties of the Oxidized Oil Samples. The physicochemical properties, such as kinematic viscosity determined at 40 and 100 °C, viscosity index, and viscosity ratio, and complementary properties, such as Conradson carbon residue, total acidity number, and iodine number, of the original unoxidized and oxidized plain and SDextract-additized base oil samples were determined using standard ASTM methods and are listed in Table 9. It can be seen that oxidation of plain samples cause some properties to alter, while the SD-extract-additized sample exhibits no significant change, and most of the properties are comparable to the original unoxidized oil.

4. CONCLUSION It is concluded from the results that the lubricant oil provided with the additive under study attained thermo-oxidative stability compared to the plain oil. The antioxidants character of the SD extract is established at 100 °C as good.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +92-91-9216652. E-mail: patwar2001@ yahoo.co.in. ORCID

Imtiaz Ahmad: 0000-0003-2056-3540 Notes

The authors declare no competing financial interest.

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REFERENCES

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Article

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Oxidative Stability of Base Lubricant Oil Monitored by Gas

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