Eco-friendly Multifunctional Biolubricant Additive

1. 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 tem-
perature, 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 environ-
mental impact caused by industrial waste materials. Regulatory
agencies are also making more and more stringent specifica-
tions 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 alter-
native 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
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 multifunc-
tional additives.12
Tribological evaluation reveals that the
Cys2-derived additives exhibited comparable antiwear proper-
ties 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, friction-
modifying, 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
Article
pubs.acs.org/IECR
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2. 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 ethyl-
cellulose 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 biopoly-
meric 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 (glucos-
amine) 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-hexam-
ethylene 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-tert-
butyl-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-hydroxybenzal-
dehyde, and 4-(dimethylamino)pyridine (DMAP) were pur-
chased from Sigma-Aldrich. N,N-Dimethylacetamide (DMAc),
lauroyl chloride, N-butyl palmitate/stearate, and ethanol were
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 vacuum-
dried 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 manufac-
tured 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
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3. 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
cor-
responds 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
Scheme 1. Reaction Scheme for Synthesizing CSB and
ACSBs
Figure 1. (a) Chitosan, (b) chitosan Schiff base (CSB) with 3,5-di-tert-
butyl-4-hydroxybenzaldehyde, and (c) acylated chitosan Schiff base
(ACSB-1).
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4. 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 Deacetyla-
tion (DD). The DD of crab chitosan obtained from Sigma-
Aldrich was determined using infrared spectroscopy. The
absorbances at 1651 and 3433 cm−1
were used to calculate the
DD according to the following equation36
= −
×A A
DD 100
( / ) 100
1.33
1651 3433
Figure 2. FT-IR spectra of chitosan, chitosan Schiff base (CSB), and acylated chitosan Schiff base samples (ACSB-1 and ACSB-2).
Industrial & Engineering Chemistry Research Article
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5. 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 (Imini-
zation) 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 substitu-
tion (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
chlorides have different ester substitutions as evident from the
FT-IR spectra in Figure 2. However, it is difficult to deter-
mine 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,
(C42H77O7N)0.669-(C32H57O7N)0.1545-(C45H75O8N)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 com-
paratively 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-tert-
butyl-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
Table 1. Elemental Analysis of Chitosan, CSB, and ACSBs
content %
sample
DD
(%)
DS
(%) C H N O
chitosan 84.55 44.98 6.52 8.65 39.85
chitosan Schiff base (CSB) 17.70 52.27 6.88 7.54 33.31
acylated chitosan Schiff
base (ACSB-1)
37.96 59.95 8.65 4.49 26.91
acylated chitosan Schiff
base (ACSB-2)
51.70 64.37 9.43 3.58 22.62
Figure 3. TG curves of chitosan, chitosan Schiff base, and acylated
chitosan Schiff base samples.
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6. 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-hydroxybenzalde-
hyde 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 Pres-
sure Vessel Oxidation Test (RPVOT).33
N-Butyl palmitate/stearate
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 anti-
oxidant 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.
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.
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7. 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
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 con-
ditions. 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 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.
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.
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8. 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
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.
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9. 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 pro-
perties. 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 anti-
wear 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 multifunc-
tional 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.
■ 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.
■ AUTHOR INFORMATION
Corresponding Author
*Phone: +91-135-2525708. Fax: +91-135-2660202. E-mail:
rksing@iip.res.in.
Notes
The authors declare no competing financial interest.
■ 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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