Evaluation of Cellulose Laurate Esters for Application as Green

Article pubs.acs.org/IECR

Evaluation of Cellulose Laurate Esters for Application as Green Biolubricant Additives Raj K. Singh,* Om P. Sharma, and Arun K. Singh* Chemical Science Division, CSIR-Indian Institute of Petroleum, Mohkampur, Dehradun 248 005, India ABSTRACT: Decreasing mineral oil resources, increasing costs, and the need for environmental protection in the use and disposal of lubricants have forced researchers to develop green lubricant formulations. Although many green lubricants consisting of biodegradable base stocks that exhibit excellent properties have been developed, further improvements in their friction and wear performance are required for them to become replacements for mineral-oil-based lubricants. Therefore, in addition to the introduction of biolubricants, the development of new environmentally benign lubricant additives is gaining attention to address the environmental issues. Along these lines, the present investigation involved the synthesis of cellulose laurate by an esterification reaction between microcrystalline cellulose and lauroyl chloride [CH3(CH2)10COOCl] using dimethylacetamide/ lithium chloride (DMAc/LiCl) as the solvent and 4-(dimethylamino)pyridine (DMAP) as the catalyst. Different reactions were carried out with varying concentrations of lauroyl chloride. Three samples (Cell Lau-A, -B, and -C) with different degrees of substitution (DS) were then characterized using IR spectroscopy, NMR spectroscopy, TG, SEM, CHN analysis, and XRD to confirm the conversion of the cellulose into cellulose laurate ester. The degrees of substitution (DS) of the synthesized samples were determined using the NMR method. The lubricating efficiencies of the cellulose laurate samples were estimated using a high-frequency reciprocating rig (HFRR) by measuring the wear scar diameters (WSDs) of spherical specimens and the coefficient of friction. The lubricity was found to increase with increasing concentration of cellulose laurate ester in n-butyl palmitate/stearate. Regarding the lubricity of the cellulose laurate esters with different values of DS, the lubricity increased as the DS increased.

1. INTRODUCTION Most of the lubricants used in the automotive and manufacturing industries are based on mineral oils. They are not environmentally friendly or biodegradable, and they can cause pollution of the environment during use or at the time of disposal. The ultimate consequence of the prolonged use of such lubricants might be detrimental to human beings, plants, animals, and aquatic life.1−3 Because of the damaging potential of conventional lubricants, the U.S. Environmental Protection Agency and other government agencies have introduced stringent regulations on the use, containment, and disposal of lubricants. These regulations have altered the market for lubricants. Now, new ecofriendly lubricants need to be developed to address the environmental, health, and economic issues, as well as the need for high performance. Because the disposal costs of conventional lubricants have also increased, it has become necessary to develop lubricants from natural resources.4 A lubricant typically consists of a base oil (>85%) and additives. Lubricant additives are important constituents for enhancing the inherent properties of the base oil and also adding new characteristics to improve their performance. Major objectives for the use of additives include reducing wear, preventing destructive metal-to-metal contact, reducing oxidative or thermal degradation, preventing rust and corrosion, and lessening the deposition of harmful deposits on lubricated parts.5,6 Among additives, those used for lubricity represent an important and integral part of any engine oil formulation. For a lubricant formulation to be ecofriendly, both the base stock and any additives must be ecofriendly.7−9 In terms of ecofriendly base oils, vegetable oil esters and synthetic © 2014 American Chemical Society

lubricants have been introduced to replace traditional mineral oils as base stocks in practice.4,10−12 In the case of additives, however, traditional lubricant additives containing harmful components, such as Cl, P, and some heavy metals, are still in use because they perform well in the biolubricant bases as well and no alternative, competitive ecofriendly additives are available. Therefore, the current trend in lubricant additive research is to explore new environment-friendly additives that suit these evolving environment-friendly base stocks.13,14 Many reports have been published on the use of modified vegetable oils as lubricant oil additives.15,16 Additives have even been made by sulfurization of soybean, sunflower, cottonseed, higherucic rapeseed, canola, Limnanthes (meadowfoam), and prime lard oils.17 Some dibasic acid esters have also been tested in terms of their suitability as synthetic engine oil additives. Examples include didecyl carbonate, didecyl adipate, and didecyl sebacate. It was found that addition of 10% of the respective esters to oils based on poly(α-olefin)s led to decreases in both the pour point and low-temperature viscosity. The viscosity index rose, and oil lubricity improved. Esters of oligomeric structures synthesized by the transesterification of dimethyl adipate or dimethyl sebacate with a mixture of neopentyl glycol and decanol showed particularly suitable properties.18 Received: Revised: Accepted: Published: 10276

March 14, 2014 May 25, 2014 May 28, 2014 May 28, 2014 dx.doi.org/10.1021/ie501093j | Ind. Eng. Chem. Res. 2014, 53, 10276−10284

Industrial & Engineering Chemistry Research

Article

2.2. Synthesis of Cellulose Laurate Esters. For each synthesis, 1.8 g of LiCl and 20 mL of DMAc were taken into a three-necked, round-bottom flask provided with a condenser and equipped with a magnetic stirrer. The solvent was stirred at 80 °C for a few minutes. Then, 1.0 g of microcrystalline cellulose (6.17 mmol) was added at 120 °C, and the mixture was stirred until it became a homogeneous transparent solution. After that, the slurry was cooled again to 80 °C. Then, for synthesis of three different samples, lauroyl chloride was added dropwise into the flask using a dropping funnel in three different quantities: 12.34, 18.51, and 30.85 mmol for samples Cell Lau-A, -B, and -C, respectively. DMAP was also added in equimolar ratio to lauroyl chloride. Each mixture was stirred for an additional 6 h at 80 °C. After that, the reaction mixture was cooled, and distilled water was poured into the mixture to precipitate the product, which was then washed twice successively with 0.2 M NaHCO3 and ethanol. The brownish solid product was filtered and dried in a vacuum oven at 60 °C overnight. 2.3. FT-IR Spectroscopy. All measurements were carried out using the KBr method. The samples were dried in an oven at 60 °C. About 0.2 mg of sample and 2 mg of KBr were mixed and finely ground, and the mixture was compressed to a form a transparent disk. The infrared spectra of these samples were recorded with a Perkin-Elmer spectrometer between 400 and 4000 cm−1. 2.4. NMR Spectroscopy. The synthesized cellulose ester samples were also characterized by NMR spectroscopy. 1H and 13 C NMR measurements were carried out on a Bruker Avance 500 spectrometer in the proton noise-decoupling mode with a standard 5-mm probe. 2.5. Thermogravimetry (TG). Thermogravimetry curves were recorded on a Perkin-Elmer EXSTAR TG/DTA 6300 instrument, using aluminum pans. The experiments were carried out under a continuous nitrogen flow of 200 mL min−1, and the temperature ramp was set at 10 °C min−1. The mass loss was recorded from 30 to 900 °C. 2.6. X-ray Diffraction. A Bruker AXS D-8 Advance diffractometer (Karlsruhe, Germany) was operated at a Cu Kα wavelength of 1.54 Å, 30 mA, and 40 keV. The spectra were recorded at a scan rate of 0.028 2q/s. 2.7. CHN Analysis. CHN analysis was performed on a Perkin-Elmer Series II CHNS/O 2400 analyzer. 2.8. SEM Measurements. The morphologies of the synthesized cellulose laurate samples were examined with an FEI Quanta 200F scanning electron microscope (FEI, Hillsboro, OR). The parameters used were as follows: chamber pressure, 10 Pa; high voltage, 20.00 kV; tilt, 0.00; takeoff, 35.00; amplitude time (AMPT), 102.4; resolution, 133.44. The powdered samples were analyzed without coating and carbon cement as adhesive. 2.9. Tribological Studies. The performance of the compounds as green lubricity additives was evaluated according to standard method ASTM G133-0526 using a high-frequency reciprocating test rig (HFRR) with a ball-on-plate geometry in n-butyl palmitate/stearate. The fixed plate specimen materials had a composition of C, 0.85−1.00%; Si, CO) group. Other bands could be assigned easily, such as the band at 1649 cm−1 in the 10279

dx.doi.org/10.1021/ie501093j | Ind. Eng. Chem. Res. 2014, 53, 10276−10284


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Figure 4. Detailed 13C NMR spectra of carbonyl carbons in cellulose laurates.

Figure 2. 1H NMR spectrum of Cell Lau-A (DS 1.37) in CDCl3.

Table 2. DS Determination from 1H NMR Spectral Data sample Cell Lau-A Cell Lau-B Cell Lau-C

integral of methyl protons (0.89 ppm)

integral of carbohydrate backbone protons (5.5−2.75 ppm)

DS

1

2.0963

1.371 949

1

1.1211

2.291 843

1

0.8937

2.716 579

Figure 5. TGA curves of cellulose and cellulose laurate samples.

methods are also available for calculating DS values, such as methods based on saponification followed by titration of excess alkali,46,47 elemental analysis,48 NIR (near-infrared) spectroscopy,49 alkaline hydrolysis followed by derivatization of the liquid products and analysis by gas chromatography (GC),50 alkaline hydrolysis followed by either capillary electrophoresis51 or reverse-phase liquid chromatography,52 and also pyrrolidinolysis followed by GC analysis.39 Among all methods, 1H NMR spectroscopy is comparatively convenient. Figure 2 shows the 1H NMR spectrum of Cell Lau-A. The DS was calculated by integrating the characteristic signals of CH3 protons of fatty alkyl chains from 0.89 to 2.34 ppm and cellulosic backbone (carbohydrate) protons from 3.0 to 5.5 ppm using the equation 10ICH3 DS = 3IC + ICH3

Figure 3. 13C NMR spectrum of Cell Lau-C (DS 2.72).

cellulose FT-IR spectrum corresponding to the bending mode of absorbed water molecules that weakens in the cellulose laureate samples. This is further evidence of the successful introduction of fatty alkyl chains, which confirms the hydrophobic nature of the cellulose ester and hence reduces the tendency to retain water.36 The FT-IR study revealed that cellulose laureate samples have DS values in the order Cell LauA < Cell Lau-B < Cell Lau-C, although the absolute DS values were not determined by IR spectroscopy in our study. 3.3. NMR Spectroscopy. NMR analysis of cellulose laureates can be used not only for characterization but also for determining the absolute DS value.45 A number of other

where ICH3 is the integral of methyl protons and IC is the integral of carbohydrate protons. The DS values obtained for Cell Lau-A, -B, and -C were 1.37, 2.29, and 2.71 respectively (Table 2). 10280



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Table 3. HFRR Test Conditions parameter

value

test duration frequency stroke length applied load temperature

33 min 20 s 10 Hz 12 mm 200 N 100 °C

Figure 6. SEM images of (A) cellulose, (B) Cell Lau-A (DS 1.37), (C) Cell Lau-B (DS 2.29), and (D) Cell Lau-C (DS 2.72).

Figure 9. Effects of concentration on lubricity properties: (A) reduction in WSD with increasing concentration of Cell Lau-A in nbutyl palmitate/stearate base oil at a load of 200 N, a frequency of 10 Hz, and a temperature of 100 °C and (B) coefficient of friction vs contact time for different concentrations of Cell Lau-A in n-butyl palmitate/stearate base oil.

Figure 7. XRD spectra of (A) cellulose and (B) Cell Lau-C (DS 2.72).

C4, and C1 carbons exhibit signals at 62, 71.1−73.1, 76.2, and 101.6 ppm, respectively. The most important downfield signal corresponding to the ester carbonyl group (C7) is seen at 171.8−173.2 ppm, which confirms the acylation of the hydroxyl group. Comparison of the C7 carbon signals of all three cellulose laurate samples (Figure 4) clearly indicates that the number of peaks increased as the DS increased because of the presence of different types of ester carbonyl groups made by substitution of hydroxyl groups at different positions (C2-OH, C3-OH, and C6-OH). In the 13C NMR spectra of cellulose laruates, the signals at 173.2, 172.4, and 171.8 ppm correspond to carbonyl carbon at C6, C3, and C2, respectively. Further, by integration of the carbonyl carbons, it was found that the order of preferred positions for esterification is C6-OH > C3-OH > C2-OH. All of these results are similar to those in previous

Figure 8. Schematic diagram of the HFRR. 13 C NMR spectra were also recorded, as shown in Figure 3 for the Cell Lau-C sample. The peak at 14.2 ppm corresponds to C18. The C8 carbon atom gives a signal at 33.5 ppm. The signals between 22 and 30 ppm can be attributed to C9−C17 carbons. Regarding the AGU backbone carbons, the C6, C2−5,

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Figure 11. Effects of variations in temperature and load on the lubricity properties of Cell Lau-C: (A) WSD with increasing temperature in n-butyl palmitate/stearate base oil at a load of 200 N and a frequency of 10 Hz and (B) WSD with increasing load in n-butyl palmitate/stearate base oil at a temperature of 100 °C.

Figure 10. Effects of DS on lubricity properties: (A) reduction in WSD with increasing DS of cellulose laurates in n-butyl palmitate/ stearate base oil at a load of 200 N, a frequency of 10 Hz, and a temperature 100 °C and (B) coefficient of friction vs contact time for 1% concentrations of cellulose laurate samples in n-butyl palmitate/ stearate base oil.

3.5. Elemental Analysis. Elemental analysis with a CHN analyzer was used to determine the DS value in previous reports.48 However, in this work, we determined absolute DS values using NMR spectroscopy. Yet, CHN analysis was also performed, and the results are reported in Table 1. The increasing percentage of C with increasing DS in cellulose laurates confirms the increasing incorporation of fatty alkyl chains in the cellulose backbone. 3.6. Scanning Electron Microscopy. The morphologies of cellulose laurate esters having different DS values in comparison to cellulose were studied by scanning electron microscopy, as shown in Figure 6. The cellulose SEM image clearly shows its fibers with a smooth surface. The images of the Cell Lau-A, -B, and -C samples show that esterification increases the surface roughness as the DS increases. The results are similar to those of previous studies on acylated cellulose.34−46 3.7. X-ray Diffraction. The XRD patterns of microcrystalline cellulose and the synthesized esters were recorded and are shown in Figure 7. Figure 7A displays the typical diffraction peaks at 2θ = 14.64°, 16.45°, 22.85°, and 34.39°, which correspond to the cellulose diffraction planes 101, 101̅, 002, and 040, respectively. All of these peaks weakened or almost disappeared in the case of cellulose laurates, as seen in Figure

cellulose acylation reports.34−44 Overall, the NMR study confirmed the successful acylation reaction between lauroyl chloride and cellulose in DMAc/LiCl. 3.4. Thermogravimetry. The acylation of cellulose influences the thermal stability properties.34−44 Therefore, the TG curves of cellulose and cellulose laurates with different DS values (1.37, 2.29, and 2.71) were recorded in N2 atmosphere as shown in Figure 5. Cellulose exhibited an initial weight loss of moisture at 100−120 °C, whereas the esters did not show such identifiable losses, which also indicates the formation of hydrophobic ester products. The major decomposition in cellulose and cellulose esters with DS values of 1.37, 2.29, and 2.71 was found to start at about 279, 225, 262, and 295 °C, respectively. The thermal stability of cellulose is greater than that of Cell Lau-A and Cell Lau-B. Because the crystallinity of cellulose was greatly reduced by the acylation reaction, they can be easily decomposed by thermal treatment. One other reason might be the decrease in the molecular weight during cellulose esterification. However, among esters, a high DS makes an ester increasingly thermally stable. From the TG curves, it can be clearly noticed that Cell Lau-C with a DS of 2.71 had the highest thermal stability, even higher than that of cellulose. 10282



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2.29, and 2.71 for Cell Lau-A, -B, and -C, respectively. The tribological performance of these samples as lubricity additives was evaluated using an HFRR according to standard method ASTM G133-05. All three samples were found to be soluble in n-butyl palmitate/stearate, which was taken as the biolubricant reference base fluid. It was observed that the WSD and average friction coefficient decreased as the concentration cellulose laurate esters in the base oil increased and as the DS of the cellulose fatty ester increased. Therefore, a cellulose laurate sample with a high DS is desirable to achieve high lubricity additive properties.

7B (Cell Lau-C). A new broad diffraction peak appeared at around 20.38° attributed to 021 planes. The higher the DS value of the cellulose laurate sample, the more pronounced this effect. These results are similar to those reported previously.34−44 3.8. Tribological Studies. After characterization of the synthesized cellulose laurate samples, their tribological properties as additives in terms of WSD (wear scar diameter) and friction coefficient in n-butyl palmitate/stearate were evaluated using a high-frequency reciprocating rig (HFRR; Figure 8) following ASTM G133-0526 under the standard conditions listed in Table 3. All three samples with DS values of 1.37, 2.29, and 2.71 were found to have good solubilities in the n-butyl palmitate/stearate mixture used as the biolubricant reference base fluid. At first, an effect of increasing concentration of cellulose ester in the base fluid was observed; for example, Figure 9A shows that, when the concentration of Cell Lau-A was sequentially increased from 0.2% to 0.5% and then 1%, the WSD of the base fluid (575 μm) decreased to 570, 560, and 505 μm, respectively. The base-fluid average friction coefficient (0.0699) also decreased to values of 0.0689, 0.0634, and 0.0616, respectively. Figure 9B shows the friction coefficient as a function of contact time. With time, the coefficient of friction decreased, which suggests that the ester might form a stable film over the metal surface. The polar ester carbonyl groups along with unesterified hydroxyl groups might have an affinity toward the metallic surface, so that they act as heads sticking to the surface while the long lauroyl fatty carbon chains work as arms that remain toward the bulk of the second interacting surface. Consequently, this helps to avoid the direct contact of metallic surfaces. An effect of the cellulose ester DS on the lubricity at a particular concentration was also observed, as shown in Figure10. At a 1% concentration, samples Cell Lau-A (DS 1.37), Cell Lau-B (DS 2.29), and Cell Lau-C (DS 2.71) exhibited decreases in WSD to values of 505, 463, and 460 μm, respectively, as the average friction coefficients decreased to values of 0.0616, 0.0589, and 0.0564, respectively (Figure 10A). We thus conclude that both the friction- and wear-reducing tendency increase as the DS of esters increases. Cellulose esters with high DS values are more stable thermally and have more ester polar functionalities, so they might have more wear- and friction-reducing tendencies. Some experiments varying the temperature (80, 100, and 120 °C) keeping the other parameters fixed were also performed, as shown in Figure 11A. An obvious trend of increased WSD with temperature was observed. Experiments with varying load (100, 150, and 200 N) were also performed. Again, an obvious trend of increasing WSD with load was observed (Figure 11B). Therefore, when a 1% concentration of cellulose laurate ester with the highest DS (2.71) was used, a 20% drop in WSD was observed (Figure 10A). This is not very encouraging for the commercial exploitation of cellulose laurates as additives.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +91-135-2525708. Fax: +91-135-2660202. E-mail: [email protected] (R.K.S.). *Tel.: +91-135-2525708. Fax: +91-135-2660202. E-mail: [email protected] (A.K.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We kindly acknowledge the Director of CSIR-IIP for his permission to publish these results. We are thankful to the Analytical Science Division of CSIR-IIP for providing help with the analyses of the samples. The authors are grateful to IIT Roorkee for its support with the CHN analysis.

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REFERENCES

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4. CONCLUSIONS In the present work, three cellulose laurate samples were prepared through homogeneous acylation using different lauroyl chloride molar ratios in the DMAc/LiCl solvent system with DMAP as the catalyst. The samples were characterized using FT-IR spectroscopy, NMR spectroscopy, TG, CHN analysis, SEM, and XRD. The degree of substitution was determined using 1H NMR spectroscopy and found to be 1.37, 10283



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