Use of an Acylated Chitosan Schiff Base as an Ecofriendly

Article pubs.acs.org/IECR

Use of an Acylated Chitosan Schiff Base as an Ecofriendly Multifunctional Biolubricant Additive Raj K. Singh,*,† Aruna Kukrety,† Alok K. Chatterjee,† Gananath D. Thakre,‡ Gajendra M. Bahuguna,§ Sandeep Saran,§ Dilip K. Adhikari,∥ and Neeraj Atray∥ †

Chemical Science Division, ‡Tribology Division, §Analytical Science Division, and ∥Bio Fuels Division, CSIR-Indian Institute of Petroleum, Dehradun, Uttarakhand 248 005, India S Supporting Information *

ABSTRACT: Two acylated chitosan Schiff base samples ACSB-1 and -2 were synthesized via a two-step reaction pathway. First the chitosan Schiff base (CSB) was prepared utilizing 3,5-di-tert-butyl-4-hydroxybenzaldehyde. In the second step, esterification with lauroyl chloride catalyzed by 4-(dimethylamino)pyridine (DMAP) in N,N-dimethylacetamide (DMAc) solvent affords the final product acylated chitosan Schiff base (ACSB-1 and -2). The products were identified and characterized by Fourier transform infrared (FT-IR) spectroscopy, CHN analysis, thermogravimetry (TG), X-ray diffraction (XRD), etc. The synthesized compounds were evaluated as multifunctional additives for antioxidant and lubricity properties in N-butyl palmitate/stearate. A rotating pressure vessel oxidation test (ASTM D2272) was used for evaluating antioxidant property. The thermo-oxidative stability of the N-butyl palmitate/stearate oil was increased 1.5 times by using this additive in 3000 ppm concentration of ACSB-2 at 150 °C. Lubricity property was evaluated by using the four ball test (ASTM D4172A) which was performed at 75 °C temperature, frequency of 1200 rpm, and 198 N load for 60 min. The lubricating efficiency of the synthesized sample was estimated by measuring the average wear scar diameter (WSD) of the spherical specimen. The WSD is also found to be decreased significantly by adding these compounds as additives in N-butyl palmitate/stearate. Both samples passed the copper strip corrosion test (ASTM D130) too.

1. INTRODUCTION The environment is threatened due to exerted negative effects by almost 10 million tons of engine, industrial, and hydraulic oils releasing into it every year.1 Water and soil are affected directly by disposed lubricant or loss in lubrication systems, while the air is affected by volatile lubricants or lubricant haze.2 The demand of the lubricant is also growing despite the depleting crude oil reserves, the main source of lube oils.3 Nowadays, the interest is increasing in the replacement of nonrenewable raw materials by renewable resources not only to meet the growing demand but also to minimize the environmental impact caused by industrial waste materials. Regulatory agencies are also making more and more stringent specifications toward the use of toxic materials in industrial products as happens with the lubricants products too.4,5 The major component of a lubricant is the base oil. Some additional components, called additives, are normally included in order to improve the specific required properties.6 As far as the base oils are concerned, the vegetable oil esters have come up strongly as the substitute for the mineral base oils;7,8 but, in the case of additives, conventional ones like zinc dialkyldithiophosphate (ZnDDP) containing harmful components, such as Cl, P, and some heavy metals, are still in use, since they perform well in the newly introduced base oil, and there is no competitive alternative ecofriendly additive technology available for the last few decades.9,10 There are only some companies like Lubrizol and RheinChemie (Lanxess) that have just launched ecofriendly additives in the market. Although a comparatively low amount of additive is used in lubes, their toxicity could not be avoided for making lube formulation completely ecofriendly.11 © XXXX American Chemical Society

Some efforts have been made to develop the ecofriendly additives from renewable resources (e.g., long chain esters of cystine (Cys2)), which is an essential amino acid obtained from natural sources. They have been tested as multifunctional additives.12 Tribological evaluation reveals that the Cys2-derived additives exhibited comparable antiwear properties to the conventional additive zinc dialkyldithiophosphate. The new additives reduced also the friction coefficient of poly alpha-olefin and synthetic esters. Some condensation products were prepared using various amines with di(alkylphenyl)phosphorodithioic acid, derived from cashew nutshell liquid, which is a renewable, biodegradable, low cost, naturally occurring vegetable product. Then these compounds have been evaluated as ashless antioxidant, antiwear, frictionmodifying, and extreme-pressure additives in lubricants.13 Soybean lecithin obtained from soybean seeds, which is a mixture of various phospholipids, is used for synthesizing environmentally friendly boron-containing friction-reducing, antiwear, and extreme pressure additives in synthetic base fluids by the reaction of boric acid.14 Tribological properties of the methanol esterified bio-oil from Spirulina have been evaluated, and it has been found that the friction coefficient of the esterified bio-oil is decreased by 21.6% compared with the unesterified bio-oil. The wear is also decreased by esterification.15 The homopolymer of sunflower oil (SFO) Received: June 17, 2014 Revised: November 5, 2014 Accepted: November 7, 2014

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dx.doi.org/10.1021/ie502441z | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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purchased from E-Merck, Darmstadt, Germany. Acetic acid and methanol was purchased from RFCL (formerly Ranbaxy Fine Chemicals Limited, India). All other chemicals were of the highest available grade and were used without further purification. 2.2. Synthesis of Chitosan Schiff Base (CSB). Chitosan (0.59 g, 3.5 mmol monomer units) was dispersed into 10 mL of 95% aqueous methanol having a catalytic amount of acetic acid (1% w/v), and then the solution of 2.55 g (∼10.5 mmol) of 3,5-di-tert-butyl-4-hydroxybenzaldehyde in 15 mL of methanol was added dropwise with magnetic stirring. The mixture was refluxed for 10 h, and then the product was obtained in the form of a yellow powder which was filtered. Unreacted aldehyde was extracted in a Soxhlet apparatus using ethanol/ ether as eluent for 2−3 days. The final product was vacuumdried at 50 °C. Yield: 0.70 g. 2.3. Synthesis of Acylated Chitosan Schiff Base (ACSB). 0.70 g (3.5 mmol) of the above synthesized CSB was taken in a round-bottom flask, and then 10 mL of DMAc was added into it. It was stirred at 80 °C for 30 min and then cooled to 50 °C. Lauroyl chloride (2.19 g, ∼10.5 mmol) for ACSB-1 and 3.0 g (∼14 mmol) for ACSB-2, respectively, dissolved in DMAc (10.5 mL) was added dropwise into the reaction mixture within 1 h, and then 0.25 g of DMAP was added into it. The temperature was gradually increased to 90 °C with stirring. The reaction was carried out during 3 h. Afterward, the content was cooled down to room temperature without stirring and poured into 100 mL of a cooled aqueous ethanolic solution taken in a beaker. The dark brown product was filtered and then washed twice with 0.2 M NaHCO3 and several times with ethanol. The semisolid light brown product was dried in vacuum oven at 60 °C overnight. Yield obtained is 1.25 and 1.50 g for ACSB-1 and ACSB-2, respectively. 2.4. Characterization. The synthesized compounds were characterized using various analytical techniques. At first the FT-IR spectra were recorded by the KBr method with a PerkinElmer spectrometer between 400 and 4000 cm−1. Thermogravimetry curves of the synthesized samples were also recorded with a PerkinElmer EXSTAR TG/DTA 6300, using aluminum pans. The experiments were carried out under continuous nitrogen flow of 200 mL min−1, and the temperature ramp was set at 10 °C min−1. Then, X-ray diffraction patterns were also obtained using a Bruker AXS D-8 advance diffractometer (Karlsruhe, Germany), which was operated at 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−1 from 4 to 60°. CHN analysis was performed on the PerkinElmer Series II CHNS/O 2400 analyzer. 2.5. Antioxidant Performance Analysis. The RPVOT (Rotating Pressure Vessel Oxidation Test) apparatus manufactured by Stan-hope Seta, U.K. was used for conducting performance evaluation tests of compounds as antioxidants following ASTM Method D 2272-11.33 All tests were performed at 150 °C on an oil/additive blend at different concentrations as 1000, 2000, and 3000 ppm. Near to 50.0 ± 0.5 g samples were measured, and 5.0 mL of water was added into it. The copper wire was cleaned with 220 grit silicon carbide sand paper and was used immediately as catalyst in the form of a spring-coil shape having an outside diameter of 44−48 mm, weight of 55.6 ± 0.3 g, and height of 40−42 mm. Then the bomb was charged with oxygen at 90.0 ± 0.5 psi (620 kPa) pressure. For ensuring that there is no leakage, the bomb was immersed in water and checked. The test was run

and soybean oil (SBO) was synthesized and tested as pour point depressant (PPD) and viscosity index improver (VII) or modifier (VM) green additive for lube oil.16 Cellulose, the most abundant biopolymer on earth, has been used as an antioxidant additive for the vegetable oil.17 One of its ether derivatives, i.e. carboxymethylcellulose, is used as an ingredient of drilling mud, where it acts as a viscosity modifier and water retention agent.18 Many environmentally acceptable thickeners from renewable sources for grease formulations have been reported in the literature. Oleo-gels that can be prepared by dispersing sorbitan monostearate (SMS) in castor oil can be used as a substitute for the metallic soap thickener.19 Methylcellulose and ethylcellulose have also been used successfully as thickener for preparing castor oil based environmentally friendly lubricating greases.20,21 In addition to the above-mentioned renewable materials for additive development, chitosan can be an interesting biopolymeric feedstock which is obtained by full or partial deacetylation of the chitin.22 Chitin is a natural polysaccharide, synthesized by a large number of living organisms, mostly exoskeletons of crustaceans such as shrimps, crabs, and lobsters. Chitin is considered the most abundant biopolymer in nature after cellulose. Chitosan consists of 2-N-acetyl-2-deoxyglucose (N-acetylglucosamine) and 2-amino-2-deoxyglucose (glucosamine) units linked with β-1,4-linkages and considered as biocompatible, biodegradable, and nontoxic natural polymeric material, having inherent antioxidant activity and enormous applications, including waste treatment, chromatography, cosmetics, textiles, photographic papers, biodegradable films, biomedical devices, drug delivery agent, and in the food industry such as antimicrobial, emulsifying, thickening, and stabilizing agents.23,24 Some efforts have been made to use them in the lubricant area. Recently, acylated derivatives of chitin and chitosan have been used as thickener agents for vegetable oils.25 Isocyanate-functionalized chitin and chitosan polymers were obtained by their reaction with 1,6-hexamethylene diisocyanate and used as thickeners for castor oil too.26 A water-soluble acylated chitosan derivative having alkylated amine groups was evaluated as reactive clays inhibitors, rheological modifiers, and filtrate loss reducer for water-based drilling fluids.27 As far as exploiting the chitosan inherent antioxidant property is concerned, chitosan and carboxymethylchitosan Schiff bases have been extensively studied as antioxidants for food and medicinal purposes.28 Chitosan gallate synthesized by gallic acid grafting on chitosan through the esterification reaction also shows good antioxidant property.29−32 In this work, a new chitosan Schiff base ester was synthesized in two steps: first imine derivatization using the 3,5-di-tertbutyl-4-hydroxybenzaldehyde followed by the acylation using the lauroyl chloride in the second step. The compound was characterized using FT-IR, CHN, TG, XRD, etc. The applicability of this derivative as a green multifunctional lubricating oil additive was explored by testing the antioxidant, anticorrosion, antiwear, and antifriction properties in N-butyl palmitate/stearate which was taken as a biolubricant reference base fluid.

2. MATERIALS AND METHODS 2.1. Materials. Chitosan, 3,5-di-tert-butyl-4-hydroxybenzaldehyde, and 4-(dimethylamino)pyridine (DMAP) were purchased from Sigma-Aldrich. N,N-Dimethylacetamide (DMAc), lauroyl chloride, N-butyl palmitate/stearate, and ethanol were B



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Scheme 1. Reaction Scheme for Synthesizing CSB and ACSBs

and considered completed after the pressure dropped more than 175 kPa from the original pressure. All samples were run in triplicate, and the average time was reported. 2.6. Anticorrosion Test. The oil/additive blends in different concentrations (1000, 2000, and 3000 ppm) were tested for corrosion characteristics by copper strip corrosion test (ASTM D130-12).34 A polished copper strip is immersed in the 30 mL test fluid taken in a 25 × 150 mm test tube, and the test tube is placed in a heated bath at 100 °C temperature. The test was done for 3 h time. The copper strips are taken out and washed with hexane to remove the adhered sample oil. After that the corrosion is rated by visual comparison to the ASTM Copper Strip Corrosion Standards. 2.7. Tribological Test. A four-ball test machine from Ducom, India was used for evaluating the tribological properties in terms of the friction coefficient and the wear scar diameter (WSD) as per the ASTM D4172A standard test method.35 For these tests, the typical 12.7 mm steel balls were used where one upper ball under the load is rotated against three stationary steel balls clamped in the holder. Different samples were prepared by adding different concentrations of additives in the N-butyl palmitate/stearate reference base oil, and four balls were covered by them; tests were performed at a rotating speed of 1200 rpm; 198 N load; 75 °C temperature; and for 60 min duration. The surfaces of the four ball test specimens were examined by FEI Quanta 200F SEM (FEI, Hillsboro, OR) equipped with EDX analysis. The parameters used are 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.

3. RESULTS AND DISCUSSION The synthesis of acylated chitosan Schiff base samples (ACSB-1 and ACSB-2) was done by using two different molar ratios of chitosan:aldehyde (1:3 for ACSB-1 and 1:4 for ACSB-2) following the route as shown in Scheme 1. The off-white chitosan color changed to yellow in the chitosan Schiff base (CSB) gives direct evidence of the successful imine bond formation as shown in Figure 1. 3.1. FT-IR Spectroscopy. The synthesized compounds were characterized using FT-IR as shown in Figure 2. The FT-IR spectrum of chitosan showed the characteristic CO stretching (amide I) bands at 1651 cm−1, N−H angular deformation band of amino groups at 1601 cm−1, −CH2 bending vibration at 1421 cm−1, C−H (in plane) bending at 1382 cm−1, C−O stretching (secondary alcoholic groups) band at 1154 cm−1, and amide III band at 1320 cm−1, as well as the band at 1060 cm−1 corresponding to the of C−O stretching (primary alcoholic groups). The C−N stretching band is observed at 1154 cm−1. The broad band at 3433 cm−1 corresponds to −OH and −NH stretching absorption, whereas the aliphatic C−H symmetric and asymmetric stretching band can be observed at 2849 and 2923 cm−1, respectively. Now the successful Schiff base formation by the reaction of chitosan with the 3,5-di-tert-butyl-4-hydroxybenzaldehyde can be proved by the appeared characteristic imine bond (CN) stretching band at 1633 cm−1 in the case of the CSB sample. Aromatic CC and C−H stretching bands also appeared at 1539 and 2956 cm−1, respectively. A band at 1217 cm−1 is also observed attributed to the C−O stretching band of the hindered phenolic group. As far as the acylation of chitosan Schiff base is concerned, the characteristic strong CO stretching peak at

Figure 1. (a) Chitosan, (b) chitosan Schiff base (CSB) with 3,5-di-tertbutyl-4-hydroxybenzaldehyde, and (c) acylated chitosan Schiff base (ACSB-1). C



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Figure 2. FT-IR spectra of chitosan, chitosan Schiff base (CSB), and acylated chitosan Schiff base samples (ACSB-1 and ACSB-2).

1743 cm−1 is observed in the case of ACSB-1 and -2 implying the successful ester linkages between chitosan and lauroyl chain. The other significant evidence observed is the increased intensity of the asymmetric and symmetric C−H stretching (CH2 and CH3 groups) band at 2924 and 2853 cm−1 along with the reduced intensity of the −OH and −NH stretching band at 3468 and 3292 cm−1, respectively. Here, to quantify how significant esterification was, the comparison of these bands can be done. It was found that the ratios of A2924/A3468 and A2853/A3468 are higher for ACSB-2 than ACSB-1 revealing that the ACSB-2 is a comparatively more substituted ester.

However, it is difficult to determine the absolute value by FT-IR, so the CHN analysis method was used. 3.2. Determination of Chitosan Degree of Deacetylation (DD). The DD of crab chitosan obtained from SigmaAldrich was determined using infrared spectroscopy. The absorbances at 1651 and 3433 cm−1 were used to calculate the DD according to the following equation36 DD = 100 − D

(A1651/A3433) × 100 1.33



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Table 1. Elemental Analysis of Chitosan, CSB, and ACSBs

chlorides have different ester substitutions as evident from the FT-IR spectra in Figure 2. However, it is difficult to determine the absolute value of the degree of substitution by IR. So % DS was evaluated by elemental analysis using the C/N ratio. 29 Anal. Calcd for the fully acylated CSB, (C42 H77O7N)0.669-(C32 H57O7N)0.1545-(C 45H75O 8N) 0.177: C, 70.80; H, 10.59; N, 2.01; O, 16.60. Found for ACSB-1 (DS ∼ 37.96%) C, 59.95; H, 8.65; N, 4.49; O, 26.91. Found for ACSB-2 (DS ∼ 51.7%) C, 64.37; H, 9.43; N, 3.58; O, 22.62 (Table 1). 3.5. TG. TG curves of chitosan with 84.55% degree of deacetylation along with the CSB, ACSB-1, and ACSB-2 are shown in Figure 3. In the chitosan TG curve, two weight losses are observed. The 4% initial weight loss at 50−100 °C is due to the moisture vaporization due to its hygroscopic nature. Due to strong inter- and intramolecular hydrogen bonds leading to close packing of polysaccharide chains, chitosan shows the thermal degradation (Td) at 268 ± 4 °C attributing to a complex process including dehydration of saccharide rings, depolymerization along with decomposition of the acetylated and deacetylated units of the polymer. At the end of the experiment at 700 °C, the chitosan shows a residual mass of about 23 ± 0.5% of the starting mass similar to the reported studies.37 It is obvious that the introduction of a functional group obstructs the chain packing causing loosening of packing structure, thus the degradation temperature (Td) will decrease. The same is observed in the case of CSB and ACSBs. The Td for CSB is found to be 229 ± 2 °C which is lower than that of chitosan. For ACSB-1 and -2 the Td is found to be 136 ± 2 and 156 ± 2 °C, respectively (Figure 3). The ACSB-2 is a comparatively more substituted ester, but it is more stable than ACSB-1 revealing that more extensive esterified samples are a little bit more stable than that of less substituted ones. Higher thermal stability of the highly esterified polysaccharide is a well established fact as observed in the case of cellulose fatty esters too.38 3.6. X-ray. The chitosan sample shows two sharp peaks approximately at 2θ 10.25° (d 8.62) and 20° (d 4.43). The strong reflections correspond to 020 and 110 planes of chitosan,39 whereas that of CSB shows the peak in the vicinity of 13.33° 2θ (d 6.63) with reduced intensity and the peak at 19.76° (d 4.49) becomes wide and a little bit stronger.40 Most importantly, the new crystallinity has happened at 5° which is mainly attributed to the formation of imine groups and the cleavage of intramolecular hydrogen bonds of chitosan41 (Figure 4). The appearance of this new spacing (d 17.65) clearly gives the strong evidence of successful introduction of the 3,5-di-tert-butyl-4-hydroxybenzaldehyde group through the imine bond in chitosan. The chitosan indicates high crystallinity. The increase in the amorphous phase in the case of CSB also confirms the successful conjugation of 3,5-di-tertbutyl-4-hydroxybenzaldehyde onto chitosan. 3.7. Antioxidant Property. Chitosan is considered a good antioxidant, a scavenger for hydroxyl radicals, and a chelator for ferrous ions and may be used as a source of antioxidants along with its application as a food supplement or ingredient in the pharmaceutical industry.42 A strong hydrogen-donating ability of chitosan provides the chitosan inherent antioxidant activity. Quaternized chitosan was also evaluated as radical scavengers for hydroxyl radical and superoxide radicals using established methods.43 Many phenolic and polyphenolic compounds with antioxidant effects are condensed with chitosan to form mutual prodrugs too.44,28−32 None of the chitosan derivatives have

content % sample chitosan chitosan Schiff base (CSB) acylated chitosan Schiff base (ACSB-1) acylated chitosan Schiff base (ACSB-2)

DD (%)

DS (%)

C

H

N

O

17.70 37.96

44.98 52.27 59.95

6.52 6.88 8.65

8.65 7.54 4.49

39.85 33.31 26.91

51.70

64.37

9.43

3.58

22.62

84.55

Figure 3. TG curves of chitosan, chitosan Schiff base, and acylated chitosan Schiff base samples.

where DD is the deacetylation degree, and A1651 and A3433 are the absolute absorbance of amide and hydroxyl groups stretching band at 1633 and 3433 cm−1, respectively. DD (%) was found to be 84.55 for the procured chitosan. The CHN results also support this finding. Anal. Calcd for (C6H11O4N)0.8455-(C8H13O5N)0.1545: C, 45.16; H, 6.75; N, 8.35; O, 39.74. Found for chitosan (DD ∼ 84.55%) C, 44.98; H, 6.52; N, 8.65; O, 39.85 (Table 1). 3.3. Determination of Degree of Substitution (Iminization) for Chitosan Schiff Base (CSB). It is expected that there is no loss of the CSB product during the reaction workup as sufficient care has been taken to avoid the losses during it. The obtained product yield was used to calculate the degree of iminization. 0.59 g of the chitosan gives 0.70 g of the CSB product, i.e. an 18.65% weight increase in chitosan weight. So the CSB monomer unit molecular weight also will be 18.65% higher than the chitosan monomer unit, i.e. (C6H11O4N)0.8455(C8H13O5N)0.1545 (molecular weight 167.65 g/mol). So the molecular weight of CSB monomer unit having the empirical formula (C6H11O4N)0.8455‑x-(C8H13O5N)0.1545-(C18H27O5N)x will be 198.92 g/mol where x represents the degree of substitution (iminization). Solving it, the value of DS is found to be 17.7%. The result was also supported by the CHN analysis. Anal. Calcd for (C6H11O4N)0.669-(C8H13O5N)0.1545-(C18H27O5N)0.177: C, 50.89; H, 7.11; N, 7.04; O, 34.96. Found for CSB (DS ∼ 17.7%) C, 52.27; H, 6.88; N, 7.54; O, 33.31 (Table 1). 3.4. Determination of Degree of Substitution for Acylated Chitosan Schiff Base Samples (ACSB-1 and ACSB-2). The two acylated chitosan Schiff base samples, ACSB-1 and ACSB-2, synthesized using different molar ratios of lauroyl E



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Figure 4. XRD patterns of (a) chitosan and (b) chitosan Schiff base.

Figure 6. Effect of increasing (a) ACSB-1 and (b) ACSB-2 concentration in the base oil on the RPVOT time.

was taken as the biolubricant reference base fluid in which the compounds are found to have very good solubility (Figure 5). Results are shown in Figure 6a and 6b. Both compounds show antioxidant activity and the activity increases with the increasing additive concentration, but ACSB-2 is a more effective antioxidant than others particularly at 3000 ppm concentration. At 3000 ppm concentration ACSB-2 increases the RPVOT time of the reference oil from 30.70 to 46.63 min (Figure 6b), while ACSB-1 increases the RPVOT time to 35.88 min only (Figure 6a). Both additives are synthesized from the same intermediate (CSB) having the DS 17.70%; this means that the hindered phenolic groups attached over the chitosan backbone will be the same in both ACSB-1 and ACSB-2. Still, the activity found is higher in the case of ACSB-2. The most probable reason would be the higher thermal stability of ACSB-2 than that of ACSB-1 as indicated by TG analysis along with the higher dispersing power of the acylated chitosan Schiff base sample having high DS. The RPVOT tests were performed at a temperature of 150 °C so thermal stability will be a critical parameter. 3.8. Anticorrosion Test. The ACSB-1 and ACSB-2 in N-butyl palmitate/stearate samples in different concentrations (1000, 2000, and 3000 ppm) were also tested for corrosion tendencies by the copper strip corrosion test (ASTM D130-12).34

Figure 5. High DS acylated chitosan Schiff base (ACSB-2) solubility in N-butyl palmitate/stearate.

been used as lubricant additives so far. Hindered phenols are well-known antioxidants for lubricants.6 In the present research work, we have tried to exploit the inherent antioxidant ability of chitosan in conjugation with the hindered phenols. The coupling of chitosan with 3,5-di-tert-butyl-4-hydroxybenzaldehyde will not only introduce the hindered phenol to the chitosan framework but also create an imine bond which could increase the metal chelating abilities of the native chitosan molecule. Acylation in the second step will make the Schiff compound soluble in the oils. So the synthesized ACSB-1 and -2 with DS 37.96 and 51.70%, respectively, were evaluated as antioxidant additives following ASTM Method D 2272-11 using the Rotating Pressure Vessel Oxidation Test (RPVOT).33 N-Butyl palmitate/stearate F



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Figure 7. Anticorrosion test: (a) Plate A before test; (b) Plate A after test with 1000 ppm ACSB-2; (c) Plate B after test with 2000 ppm ACSB-2; and (d) Plate C after test with 3000 ppm ACSB-2. Figure 10. Reduction in the WSD and the average friction coefficient with increasing concentration of acylated chitosan Schiff base sample ACSB-2 in N-butyl palmitate/stearate.

Figure 8. Reduction in the WSD and the average friction coefficient with increasing DS of acylated chitosan Schiff base samples in N-butyl palmitate/stearate. Figure 11. Plot of friction coefficient vs time for blank (N-butyl palmitate/stearate) and ACSB-2 sample in different concentrations.

of an imine bond in the compounds which provides the corrosion resistance to the additives as reported in the literature too.45−48 3.9. Tribological Properties. Anticorrosion tendency particularly for steel−steel contact is well realized in the case of Schiff bases.45−48 Some studies have also been carried out to use the organic Schiff compounds as friction reducing and antiwear additives. The Schiff base reacts with the metal surface to form a surface-complex film leading to the hindered metal contact.49 In the present work the synthesized additives ACSB-1 and ACSB-2 have also the imine bond along with the polar ester groups. Some underivatized OH and NH2 polar groups may also contribute to the metal interaction. In view of this, both additives were tested for the antifriction and antiwear properties in terms of friction coefficient and wear scar diameter using the four ball test machine at standard conditions. At first the effect of the increasing DS (acylation) was evaluated. The tests were carried out at 3000 ppm concentration of ACSB-1 and -2. It was found that ACSB-2 is more effective as an antiwear and antifriction additive. The values of the WSD and the average friction coefficient for the base oil, i.e. 507.5 μm and 0.104, reduce to a value of 432 μm

Figure 9. Plot of friction coefficient vs time for blank (N-butyl palmitate/stearate) and 3000 ppm acylated chitosan Schiff base samples having different DS.

Figure 7 shows the real pictures of the copper strips before and after the test with ACSB-2 samples. All the samples pass the test with no. 1a. These tests reveal that synthesized additives do not have any corrosive tendencies. The reason may be the presence G



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Figure 12. SEM micrographs of the worn out ball test specimens lubricated with (a) the N-butyl palmitate/stearate base (b) 3000 ppm ACSB-2; EDX results for the worn out ball test specimens lubricated with (c) the N-butyl palmitate/stearate base (d) 3000 ppm ACSB-2.

and 0.084, respectively, at 3000 ppm concentration of ACSB-2. While in the case of ACSB-1 the value of the WSD and the average friction coefficient decrease to 463.5 μm and 0.087 (Figure 8). The reason for the higher lubricity in the case of ACSB-2 may be due its higher thermal stability, higher solubility in the base oil, and existence of more polar ester groups than that of ACSB-1. Figure 9 shows the relationship between contact time and friction coefficient. As time of contact increases the friction coefficient also decreases. Now to see the effect of increasing concentration of additives over the tribological properties, the four ball tests were performed varying the concentration as 1000, 2000, and 3000 ppm. It was also observed that the lubricity increases as the concentration increases. At 1000, 2000, and 3000 ppm ACSB-2 concentration of the values of the WSD obtained is 457.33, 435.67, and 432 μm, respectively, while the value of the average friction coefficient is 0.102, 0.086, and 0.084, respectively (Figure 10). At lower concentration the sufficient interaction of additives

does not take place. At higher concentration the good interaction of additives with surface takes place as evidenced by the linear decrease in the friction coefficient with contact time (Figure 11). The morphology of the ball worn surface is also observed using SEM and EDX to describe tribological mechanisms. Figure 12a and 12b shows the SEM micrographs of the worn out test specimens lubricated with N-butyl palmitate/stearate base and 3000 ppm ACSB-2, respectively. Clear contour fluctuation and many furrows due to wear can be found after lubrication by the N-butyl palmitate/stearate base oil. Some of the wear debris was also seen in it. The wear mechanism is adhesive wear, as the wear tracks seen are smooth and the surfaces too are very smooth. The rubbed surface lubricated by 3000 ppm had few shallow furrows. No signs of corrosive pits were observed in both specimens. So the worn surface lubricated by additive ACSB-2 is clearly found to be smoother than the N-butyl palmitate/stearate base indicating that the H



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(3) Fuchs, M. The world lubricants market, current situation and outlook. In 12th international colloquium on Tribology, 2000. (4) Woods, M. Think green: Biodegradable lubes glow with promise. Lubes‘n’Greases 1997, 3, 14. (5) Nagendramma, P.; Kaul, S. Development of ecofriendly/ biodegradable lubricants: An overview. Renewable Sustainable Energy Rev. 2012, 16, 764. (6) Rudnick, L. R. Lubricant additives: chemistry and applications, 2nd ed.; CRC Press, Taylor & Francis: 2009. (7) Erhan, S. Z.; Asadaukas, S. Lubricants base stocks from vegetable oils. Ind. Crops Prod. 2000, 11, 277. (8) Willing, A. Lubricants based on renewable resources: An environmentally compatible alternative to mineral oil products. Chemosphere 2001, 43, 89. (9) Spikes, H. A. The history and mechanisms of ZDDP. Tribol. Lett. 2004, 17, 469. (10) Nicholls, M. A.; Do, T.; Norton, P. R.; Kasrai, M.; Bancroft, G. M. Review of the lubrication of metallic surfaces by zinc dialkyldithiophosphates. Tribol. Int. 2005, 38, 15. (11) Hewstone, R. K. Environmental health aspects of lubricant additives. Sci. Total Environ. 1994, 156, 243. (12) Minami, I.; Mori, S.; Isogai, Y.; Hiyoshi, S.; Inayama, T.; Nakayama, S. Molecular design of environmentally adapted lubricants: antiwear additives derived from natural amino acids. Tribol. Trans. 2010, 53, 713. (13) Swami, K. K.; Prakash, S.; Sarin, R.; Tuli, D. K.; Bhatnagar, A. K. Development of ashless multifunctional additives from cashew nutshell liquid. Lubr. Sci. 2003, 15, 361. (14) Li, W.; Wu, Y.; Wang, X.; Liu, W. Tribological study of boroncontaining soybean lecithin as environmentally friendly lubricant additive in synthetic base fluids. Tribol. Lett. 2012, 47, 381. (15) Xu, Y.; Hu, X.; Yuan, K.; Zhu, G.; Wang, W. Friction and wear behaviors of catalytic methylesterified bio-oil. Tribol. Int. 2014, 71, 168. (16) Karmakar, G.; Ghosh, P. Green Additives for Lubricating Oil. ACS Sustainable Chem. Eng. 2013, 1, 1364. (17) Saga, L. C.; Rukke, E. O.; Liland, K. H.; Kirkhus, B.; Egelandsdal, B.; Karlsen, J.; Volden, J. Oxidative stability of polyunsaturated edible oils mixed with microcrystalline cellulose. J. Am. Oil Chem. Soc. 2011, 88, 1883. (18) Roper, L. E.; Sauber, C. A. Drilling mud containing sodium carboxymethylcellulose and sodium carboxymethyl starch. US Patent no. 4123366 A, 1978. (19) Sánchez, R.; Franco, J. M.; Delgado, M. A.; Valencia, C.; Gallegos, C. Effect of thermo-mechanical processing on the rheology of oleogels potentially applicable as biodegradable lubricating greases. Chem. Eng. Res. Des. 2008, 86, 1073. (20) Sánchez, R.; Franco, J. M.; Delgado, M. A.; Valencia, C.; Gallegos, G. Development of new green lubricating grease formulations based on cellulosic derivatives and castor oil. Green Chem. 2009, 11, 686. (21) Sánchez, R.; Franco, J. M.; Delgado, M. A.; Valencia, C.; Gallegos, C. Thermal and mechanical characterization of cellulosic derivatives-based oleogels potentially applicable as bio-lubricating greases: Influence of ethyl cellulose molecular weight. Carbohydr. Polym. 2011, 83, 151. (22) Yen, M.-T.; Yang, J.-H.; Mau, J.-L. Physicochemical characterization of chitin and chitosan from crab shells. Carbohydr. Polym. 2009, 75, 15−21. (23) Pillai, C. K. S.; Paul, W.; Sharma, C. P. Chitin and chitosan polymers: Chemistry, solubility and fiber formation. Prog. Polym. Sci. 2009, 34, 641. (24) Ravikumar, M. N. V. A review of chitin and chitosan applications. React. Funct. Polym. 2000, 46, 1. (25) Sánchez, R.; Stringari, G. B.; Franco, J. M.; Valencia, C.; Gallegos, C. Use of chitin, chitosan and acylated derivatives as thickener agents of vegetable oils for bio-lubricant applications. Carbohydr. Polym. 2011, 85, 705.

synthesized additives had a boundary lubrication function avoiding direct contact of the frictional pairs. This result is in accordance with the findings based on the average friction coefficient and wear scar diameter. The EDX analysis shows that carbon, iron, chromium, and oxygen are prominent on the surface owing to the steel surface (Figure 12c and 12d). However, no strong evidence is observed for contribution from the additive in film formation on the surface except a higher percentage of carbon on the surface revealing some interaction with the additives. Finally we can say that both additives ACSB-1 and ACSB-2 have the antifriction and antiwear properties. ACSB-2 at 3000 ppm concentration decreases the WSD of the base oil to 14.88%. Although it is still insufficient for commercial applicability, the work is a significant breakthrough in the direction of developing the environmentally benign multifunctional additives.

4. CONCLUSION In summary, two acylated chitosan Schiff base samples ACSB-1 and ACSB-2 having DS 37.96% and 51.70%, respectively, were synthesized. FT-IR, CHN, TG, SEM, and XRD characterization confirmed the synthesis. Thermal stability and solubility of the ACSB-2 is found to be greater than that of ACSB-1. Both compounds were evaluated as multifunctional lubricant additives in biolubricant reference fluid (N-butyl palmitate/ stearate) for antioxidant, anticorrosion, antifriction, and antiwear properties following ASTM D 2272-11, ASTM D130-12, and ASTM D4172A. Both compounds were found to have all four properties, but ACSB-2 is more effective as a multifunctional additive than ACSB-1. ACSB-2 increases the RPVOT time of the reference base oil from 30.70 min to a value of 46.63 min at 3000 ppm concentration. At this concentration the value of the WSD of the base oil decreases from 507.5 to 432 μm. The average friction coefficient of the reference oil also decreases from 0.104 to 0.084.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectrum of additive ACSB-2 and details of panel coker test. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +91-135-2525708. Fax: +91-135-2660202. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We kindly acknowledge Director IIP for his kind permission to publish these results. The Analytical Division of the Institute is kindly acknowledged for providing analysis of samples. The Tribology Division is acknowledged for extending support in tribological studies.

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