Synthesis of Fatty Alcohol based Sterically Hindered Esters as Potential Antioxidants for Bioadditives

Article abstract of DOI:10.1134/S0965544120110110

Abstract: Four hindered phenolic esters were prepared by esterification between 3-(3,5-Di-tert-butyl-hydroxyphenyl)propionic acid and fatty alcohols. The synthesized esters were characterised by NMR, IR and ESI-MS and analysed for their antioxidant proper

Full text of DOI:10.1134/S0965544120110110

ISSN 0965-5441, Petroleum Chemistry, 2020, Vol. 60, No. 11, pp. 1309–1316. © Pleiades Publishing, Ltd., 2020.
Published in Russian in Neftekhimiya, 2020, Vol. 60, No. 6, pp. 853–861.
Synthesis of Fatty Alcohol based Sterically Hindered Esters
as Potential Antioxidants for Bioadditives
Venkateshwarlu Konthama, b, Korlipara V. Padmajaa, b, *, and Devarapaga Madhua, c, **
aCentre for Lipid Science and Technology, CSIR-Indian Institute of Chemical Technology,
Hyderabad, Telangana, 500007 India
bAcademy of Scientific and Innovative Research, New Delhi, India
cDepartment of Chemistry, Indian Institute of Technology (BHU) Varanasi, Varanasi, 221005 India
*e-mail: deverapaga.rs.chy14@itbhu.ac.in
**e-mail: v52621@gmail.com
Received March 3, 2020; revised June 30, 2020; accepted July 10, 2020
Abstract—Four hindered phenolic esters were prepared by esterification between 3-(3,5-Di-tert-butyl-
hydroxyphenyl)propionic acid and fatty alcohols. The synthesized esters were characterised by NMR, IR and
ESI-MS and analysed for their antioxidant properties and thermal stability. Differential scanning calorimetry
(DSC) and Rancimat studies were conducted to explore the effect of synthesized compounds on oxidation
stability of base oil. The results from both the studies were correlated well. Thermogravimetric analysis indi-
cates that all the esters have significantly high thermal stability. Among all the prepared compounds myristyl
alcohol ester showed better antioxidant properties whereas, stearyl alcohol ester exhibited higher thermal sta-
bility. The synthesized antioxidants can be potential bio additives for lubricant formulations.
Keywords: differential scanning calorimetry, thermal stability, oxidation induction time, rancimat, surface-
volume ratio
DOI: 10.1134/S0965544120110110
INTRODUCTION
Lubricating fluids are widely used in industrial and
automobile applications to improve the service life of
the oxidation process of lubricant by inhibiting the for-
mation of free radicals [11–13].
Hindered phenols and arylamines are well known
machineries by providing protection against wear and antioxidant additives in lubricating oils [10, 14]. Hin-
friction [1, 2]. Despite their many advantages, lubri- dered phenolic compounds act as hydrogen donors
cating oils possess poor oxidation and thermal stabili- and can donate hydrogen atom to the alkoxy or alkyl
ties. The continuous exposure to elevated tempera- peroxy free radicals to neutralize [10, 15]. Some petro-
tures and environmental oxygen leads to the formation chemical derived sterically hindered phenolic (SHP)
of free radicals [3, 4]. Additives are introduced to compounds are well known commercial antioxidants,
improve the lubricant performance and increase the such as BHT (butylated hydroxytoluene), BHA
shelf life by improving anti corrosion, antioxidant (butylated hydroxyanisole) and TBHQ (tert-butylhy-
properties and also to enhance the tribological proper- droquinone) [16, 17]. Amidation and esterification of
ties such as antiwear and antifriction [5, 6]. As per the phenolic acids and alkyl phenols are practical tech-
environmental regulations low levels of sulfur, phos- niques to enhance their solubility and emulsification
phate ash and metal free additives have been encour- properties as well as oxidation inhibition performance
aged in lubrication applications [7–9]. In this regard, [18, 19]. Yu et al. synthesized Schiff base bridged hin-
researchers synthesized some metal free additives and dered phenolic diphenylamine antioxidants and found
successfully characterised their tribological properties that the compounds have better thermal stability and
and their performance is comparable to conventional antioxidant efficiency in 150SN base oil [20]. How-
multifunctional additive Zinc dialkyldithiophosphates ever, there has been growing interest for bio based
(ZDDP). Oxidative breakdown of lubricants usually lubricants and additives due to concerns about envi-
results in the formation of primary and secondary oxi- ronmental pollution caused by mineral oil based lubri-
dation products (peroxides, carboxylic acids, alcohols, cants and additives [21, 22]. Oleochemical based addi-
aldehydes and ketones) which alter the physical and tives will have better biodegradability compared to tra-
chemical properties of lubricant basestock [10]. ditional petroleum derived additives due to the
Therefore, antioxidants are needed to delay or prevent presence of natural components and the lack of metal
1309

1310
VENKATESHWARLU KONTHAM et al.
ions. Singh et al. synthesized two phenolic esters of (CDCl3, ppm) δ 0.88 (t, 3H, –CH3), 1.25–1.33 (m,
pentaerythritol monooleate and evaluated their anti-
oxidant and tribology properties [14]. The oxidative
stability of lubricant base fluid was greatly improved
with each additive along with effective tribology per-
formance. In the present study we report synthesis of
sterically hindered esters with long chain fatty alcohols
(C14, C11:1, C18, and C18:1), their characterization using
NMR, IR and ESI-MS and their oxidation and ther-
mal stability properties enhancement in an eco
friendly base oil.
20H, 10x–CH2), 1.42 (s, 18H, 6x–CH3), 1.54–1.64
(m, 2H, –CH2), 2.55–2.64 (t, 2H, –CH2CO), 2.84–
2.90 (t, 2H, –CH2–Ar), 4.0–4.11 (t, 2H, –OCH2),
6.99 (s, 1H), 7.25 (s, 2H): 13C NMR (CDCl3, ppm)
δ 14.1, 22.7, 25, 29.3, 29.6, 31.6, 34.2, 36.5, 64.7,
124.7, 131.1, 135.9, 152.0, 173.4: IR (cm–1, CHCl3):
3641, 2927, 2860, 1732, 1443, 1232, 1160, 879: ESI
MS: m/z 497 [M + Na]+.
Synthesis of undec-10-ene-yl 3-(3,5-di-tert-butyl-
4-hydroxy phenyl)propionate (2b). Quantities of sub-
strates taken: 3-(3,5-Di-tert-butyl-4-hydroxyphe-
nyl)propionic acid (1.0 g, 3.6 mmol), 10-undecenol
(0.92 g, 5.4 mmol) and EDC ⋅ HCl (0.67 g, 4.3 mmol)
DMAP (0.43 g, 3.6 mmol), yield obtained 95%
(1.48 g) of pale yellow color solid. Mp 32–33°C;
1H NMR (CDCl3, ppm) δ 0.88 (t, 3H, –CH3), 1.24–
EXPERIMENTAL
Materials.
3-(3,5-Di-tert-butyl-4-hydroxyphe-
nyl)propionic acid, myristyl alcohol, stearyl alcohol,
10-undecenol, oleyl alcohol, 1-ethyl-3(3-dimethyl-
aminopropyl) carbodimide hydrochloride (EDC ⋅
HCl) and 4-dimethylaminopyridine (DMAP) were 1.34 (m, 14H, 7x–CH2), 1.39–1.45 (s, 18H, 6x3–
purchased from Sigma-Aldrich (St. Louis, USA). Sil-
ica gel (60–120 mesh) for column chromatography
was purchased from M/s Acme Synthetic Chemicals
(Mumbai, India) and pre-coated TLC plates (silica gel
60 F254) were purchased from M/s Merck (Darm-
stadt, Germany). All the solvents were purchased from
M/s SD Fine Chemicals (Mumbai, India) and were of
the highest grade of purity.
CH3), 1.56–1.66 (m, 2H, –CH2), 2.57–2.64 (t, 2H
‒CH2CO), 2.86–2.94 (t, 2H, –CH2–Ar), 4.08–4.12
(t, 2H, –OCH2), 4.95 (m, 2H), 5.80 (m, 1H), 6.84 (s,
1H), 7.26 (s, 2H), 13C NMR (CDCl3, ppm) δ 14.1,
22.7, 25, 29.3, 29.6, 32.6, 34.4, 36.2, 64.8, 115.7, 123.4,
131.3, 135.6, 152.7, 173.6: IR (cm–1, CHCl3): 3640,
3068, 2925, 2864, 1730, 1639, 1453, 1261, 1177, 802:
ESI MS: m/z 453 [M + Na]+.
1
Characterization. The H NMR and 13C NMR
Synthesis of octa decyl 3-(3,5-di-tert-butyl-4-
hydroxy phenyl)propionate (2c). Quantities of sub-
strates taken: 3-(3,5-Di-tert-butyl-4-hydroxyphe-
nyl)propionic acid (1.0 g, 3.6 mmol), stearyl alcohol
(1.45 g, 5.4 mmol) and EDC ⋅ HCl (0.67 g, 4.3 mmol)
DMAP (0.45 g, 3.6 mmol), yield obtained 97% (1.84 g) of
pale yellow color solid. Mp 55–56°C; 1H NMR
(CDCl3, ppm) δ 0.88 (t, 3H, –CH3), 1.25–1.33 (m,
30H, 15x–CH2), 1.44 (s, 18H, 6x–CH3), 1.56–1.64
(m, 2H, –CH2), 2.56–2.63 (t, 2H, –CH2CO), 2.83–
2.90 (t, 2H, –CH2–Ar), 4.0–4.11 (t, 2H, –OCH2),
spectra were recorded on Varian 300 and 75 MHz,
respectively and TMS was used as an internal stan-
dard. Mass spectra were recorded using electron spray
ionization on Waters e2695 Separators module
(Waters, Milford, MA, USA) mass spectrometer. IR
spectra were recorded in dichloromethane on a Per-
kin-Elmer Fourier Transform Infrared (FT-IR) spec-
trum BX instrument (model: Spectrum BX; Con-
necticut, USA).
General procedure for the synthesis of esters (2a-
2d). A mixture of 3-(3,5-Di-tert-butyl-4-hydroxyphe-
nyl)propionic acid (1.0 g, 3.6 mmol), fatty alcohol (5.4
mmol) and EDC ⋅ HCl (0.67 g, 4.3 mmol) were dis-
solved in dry DCM (20 mL), stirred at room tempera-
ture for 20 min and added DMAP (0.43 g, 3.6 mmol)
to the reaction mixture and the stirring was continued
for 6 h. The formation of product was monitored with
TLC (hexane/ethyl acetate 90 : 10, v/v). After comple-
tion of reaction the reaction mixture was extracted in
to ethyl acetate and washed with water and dried over
sodium sulphate and the resulted crude product was
subjected to column chromatography using hexane
and ethyl acetate (90 : 10, v/v) to get pure product.
7.01 (s, 1H), 7.27 (s, 2H): 13C NMR (CDCl3, ppm)
δ 14.1, 22.6, 25, 29.3, 29.6, 31.7, 34.2, 36.7, 64.6,
124.6, 131.8, 135.7, 152.0, 173.2: IR (cm–1, CHCl3):
3642, 2924, 2857, 1732, 1444, 1360, 1235, 1160, 874:
ESI MS: m/z 553 [M + Na]+.
Synthesis of (Z) octa dec-9-ene-yl 3-(3,5-di-tert-
butyl-4-hydroxy phenyl)propionate (2d). Quantities of
substrates taken: 3-(3,5-Di-tert-butyl-4-hydroxyphe-
nyl)propionic acid (1.0 g, 3.6 mmol), oleyl alcohol
(1.44 g, 5.4 mmol) and EDC ⋅ HCl (0.67 g, 4.3 mmol)
DMAP (0.43 g, 3.6 mmol), yield obtained 95% (1.80 g)
1
of pale yellow color solid. Mp 38–39°C; H NMR
Synthesis of tetra decyl 3-(3,5-di-tert-butyl-4-
hydroxy phenyl)propionate (2a). Quantities of sub-
strates taken: 3-(3,5-Di-tert-butyl-4-hydroxyphe-
nyl)propionic acid (1.0 g, 3.6 mmol), myristyl alcohol
(1.15 g, 5.4 mmol), EDC ⋅ HCl (0.67 g, 4.3 mmol) and
DMAP (0.43 g, 3.6 mmol), yield obtained 93% (1.6 g)
(CDCl3, ppm) δ 0.87 (t, 3H, –CH3), 1.25–1.33 (m,
22H, 11x–CH2), 1.43 (s, 18H, 6x–CH3), 1.56–1.63
(m, 2H, –CH2), 1.99–2.04 (m, 4H, 2x–CH2), 2.56–
2.61 (t, 2H, –CH2CO), 2.84–2.90 (t, 2H, –CH2–Ar),
4.0–4.10 (t, 2H, –OCH2), 5.36 (m, 2H, –HC=CH),
6.99 (s, 1H), 7.27 (s, 2H), 13C NMR (CDCl3, ppm)
1
of pale yellow color solid. Mp 47–48°C; H NMR
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SYNTHESIS OF FATTY ALCOHOL BASED STERICALLY HINDERED ESTERS
1311
δ 14.1, 22.7, 25, 29.3, 29.6, 32.8, 34.2, 36.8, 64.7,
RESULTS AND DISCUSSION
124.8, 129.3, 131.2, 135.2, 152.2, 173.4: IR (cm–1,
CHCl3): 3644, 2924, 2862, 1736, 1466, 1360, 1233,
Synthesis and spectral analysis. Sterically hindered
phenolic esters were synthesised with various fatty
alcohols using EDC ⋅ HCl and DMAP as catalyst with
93 to 97% yield (Scheme 1). The prepared esters were
1162, 876: ESI MS: m/z 551 [M + Na]+.
Rancimat analysis. The oxidative induction time
(OIT) was investigated by a Metrohm Rancimat
model 743 (Herisau/Switzerland) as per AOCS offi-
cial method Cd 12b-92. The conductivity of distilled
water (60 mL) was measured by continuous air flow
(20 L/h) bubbled into base oil (5 g) at 110 and 140°C.
The resulted volatile components were collected in
water and the time taken to reach the conductivity
inflection was recorded as OIT [23]. The analysis was
performed in duplicate and mean value is reported.
1
characterised by H NMR, 13C NMR, IR and mass
spectral studies. The characteristic triplet peaks
appeared in 1H NMR at 4.08–4.13 and 2.54–
2.64 ppm correspond to the methylene protons of
alkyl chain attached to the carbonyl oxygen and the
protons α to the carbonyl carbon respectively. The
peaks present at 1.24–1.32 ppm are due to fatty alkyl
chain protons whereas the chemical shift values
observed at 4.95–5.80 ppm indicate the presence of
unsaturated protons (for the products 2b and 2d) and
the signals appeared at ∼7.26 ppm confirm the pres-
ence of phenyl ring. In 13C NMR the prominent peak
appeared at 173.2–173.6 ppm is due to carbonyl car-
bon and the peak observed at 64.6–64.8 ppm corre-
sponds to the alkyl chain carbon attached to carbonyl
group. The peaks observed at 22.7–34.4 ppm and
123.4–152.9 ppm corresponds to alkyl chain carbons
and aromatic carbon signals respectively. In the FT-IR
spectra a significant peak appeared at 3640–3644 cm–1
indicating the presence of phenolic OH and the char-
acteristic peaks observed at 1732–1736 cm–1 and
1160–1177 cm–1 are attributed to ester group. The
peaks appeared at ∼2927 and ∼2864 cm–1 are due to
the symmetric and asymmetric stretching bands of
alkyl chain. Moreover, the molecular ions m/z at 497,
453, 553, 551 [M + Na]+ obtained by positive mode of
ESI spectra further confirm the formation of target
molecules. The compound octa decyl 3-(3,5-di-tert-
butyl-4-hydroxy phenyl)propionate (2c) is commer-
cially known as Irganox 1076 and act as antioxidant
[24, 25]. In the present study the similar class of new
Differential scanning calorimetry analysis. The oxi-
dative induction time (OIT) and oxidation onset tem-
perature (OT) were measured with differential scan-
ning calorimeter (DSC) according to ASTM E 2009-
08. DSC analyses were conducted on Q-100 thermal
analyzer from TA Instruments under continuous oxy-
gen flow at 50 mL/min in isothermal mode. Tempera-
ture was raised from 40°C to specified temperature
140°C. Typically, 5
0.5 mg of tested sample was
weighed in sealed aluminum pan and placed in the
equipment chamber. Similarly the non isothermal
tests also conducted in oxygen environment with
increasing the temperature at a rate of 10°C/min up to
appearance of exothermic peak. The oxidation induc-
tion time and oxidation onset temperatures were ana-
lyzed by using TA universal analysis software. The tan-
gent was drawn by extrapolating of baseline to the
maximum exothermic peak.
Thermogravimetric analysis. TA Q500 (TA Instru-
ments, Inc., New Castle, DE, USA.) instrument was
used to determine the thermal stability in non-isother-
mal mode by taking 4 0.5 mg sample in α-Al2O3 cru-
cible operated at the temperature range about 40– Irganox 1076 compounds was synthesized by varying
400°C with a heating rate of 10°C/min in continuous the fatty alkyl chain and studied the effect of alkyl
nitrogen gas flow of 60 mL/min.
chain on the oxidation and thermal stabilities.
HO
HO
EDC · HCl, DMAP
+ R−OH
Room temperature, 6 h
OH
OR
1
O
2 (a−d)
O
2a R =
2b R =
2c R =
2d R =
Scheme 1. Synthesis of 3-(3,5-di-tert-butyl-4-hydroxy phenyl)propionic acid, fatty alcohol esters.
PETROLEUM CHEMISTRY Vol. 60 No. 11 2020

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VENKATESHWARLU KONTHAM et al.
Table 1. Oxidation induction time of base oil and base oil containing synthesized antioxidants evaluated by Rancimat and
DSC isothermal method
Rancimat OIT values, h
DSC OIT values, h
Additive concentration
Sample
100 ppm
200 ppm
200 ppm
110°C
140°C
110°C
140°C
140°C
1.55 0.21ns
4.63 0.61ns
4.16 0.39ns
4.48 0.19ns
4.16 0.28ns
3.26 0.36ns
Base oil
26.96 0.72***
43.24 0.79***
30.47 0.56***
42.52 0.74***
29.37 0.38***
28.46 0.61***
–
–
2.96 0.34
4.33 0.10
4.10 0.48
4.24 0.32
4.18 0.35
3.96 0.57
B + 2a
B + 2b
B + 2c
B + 2d
B + BHT
49.23 1.68***
32.42 0.88***
46.41 0.98***
37.46 0.68***
30.24 0.85***
12.36 0.36***
7.52 0.60***
11.45 0.21***
8.94 0.11***
8.56 0.47***
(B = base oil, 2a = tetra decyl 3-(3,5-di-tert-butyl-4-hydroxy phenyl)propionate, 2b = undec-10-ene-yl 3-(3,5-di-tert-butyl-4-hydroxy
phenyl)propionate, 2c = octa decyl 3-(3,5-di-tert-butyl-4-hydroxy phenyl)propionate, 2d = octa dec-9-ene-yl 3-(3,5-di-tert-butyl-4-
hydroxy phenyl)propionate).
Antioxidant activity by Rancimat. Antioxidant
activity of the base fluid and base fluid containing syn-
thesized additives was evaluated by measuring the oxi-
dative induction time (OIT) by conducting rancimat
tests. The OIT value was obtained by plotting graph
conductivity against time, a high OIT value indicates
greater resistant towards oxidation [26]. The analysis
was carried out by blending 100 and 200 ppm concen-
tration of each additive in base oil and base oil without
additive in the presence of atmospheric air at both 110
and 140°C temperature. The experiment was carried
out in triplicate and the values are given as mean SD
of three tests. The results found significantly (p <
0.001) higher OIT values in rancimat compared to
DSC (One-way ANOVA test). The resulted OIT val-
ues were reported in Table 1 (***P < 0.001; ns: non-
significant). It illustrates that at 100 ppm concentra-
tion all the additives enhanced the oxidation stability
of base oil. Typically the base oil induction period of
25.96 h at 110°C and 1.55 h at 140°C were further
increased to 43.24 h at 110°C and 4.63 h at 140°C in
the case of base oil spiked with 2a. Further increasing
the additive concentration to 200 ppm at both the tem-
perature resulted in improvement of oxidation stability
of base oil. The OIT values of base oil improved to
49.23 h at 110°C and 12.36 h at 140°C with additive 2a,
it is a significant increase of more than 1.9 times com-
paring to 100 ppm additive dosage. All the synthesized
additives exhibited improved stability however, the
variation in the results is due to the nature of alkyl
chain. Unsaturation in alkyl chain has significant
effect on OIT values of SHP esters. Unsaturated alkyl
chain containing esters showed lower oxidation inhi-
bition ability than the saturated esters. This is due to
the fact that the allylic positions to the double bonds
are prone to oxidation [27, 28].
Antioxidant activity by DSC (isothermal). Isother-
mal differential scanning calorimetry studies were
employed to determine the oxidation induction time.
The time taken for the appearance of exothermic peak
in the isothermal experiment is recorded as oxidation
induction time [29, 30]. Antioxidant activity of syn-
thesized compounds was examined by blending with
base oil at 200 ppm concentration. Table 1 shows
increase in OIT of base oil from 2.96 h to 4.33 h with
additive 2a. The delay in oxidation clearly explains the
efficiency of blended additive. Among all the deriva-
tives, 2a showed superior performance followed by 2c
and 2d whereas, the lower OIT observed with additive
2b may be due to the presence of terminal unsatura-
tion. All the synthesised compounds exhibited supe-
rior performance compared to BHT. The experiment
was carried out in triplicate and the values are given as
mean SD of three tests. Figure 1a compares the OIT
of Rancimat and OIT of DSC, there observed signifi-
cantly (p < 0.001) higher OIT values for all the test
samples in Rancimat than DSC according to Two-way
ANOVA test (Tukey’s multiple comparison test).
However, comparing the results OIT of all the synthe-
sized additives there observed significantly (p < 0.001)
higher OIT values compared to BHT (Fig. 1b). The
higher OIT value observed in Rancimat tests than
DSC is mainly due to the amount of test sample,
exclusively a low quantity of sample nearly 3–5 mg was
used in DSC whereas, in Rancimat 5 g of sample is
required. Another important factor is surface-volume
ratio between tested sample and oxygen. Typically in
Rancimat the surface-volume ratio of sample placed
in a test tube is smaller than DSC analysis [31]. Usu-
ally the high surface-volume ratio or a small quantity
of sample is more effectively react with oxygen. More-
over, the Rancimat analysis was conducted with air
flow however, the DSC tests were conducted with
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SYNTHESIS OF FATTY ALCOHOL BASED STERICALLY HINDERED ESTERS
1313
Base oil
60
40
20
0
(a)
***
B + 2a
B + 2b
B + 2c
B + 2d
B + BHT
***
***
D
нс
B
C
E
A
A: 110°C Raneimat 100 ppm
B: 110°C Raneimat 100 ppm
C: 110°C Raneimat 200 ppm
D: 140°C Raneimat 200 ppm
E: 140°C DSC 200 ppm
Base oil
B + 2a
(b)
60
40
20
0
B + 2b
B + 2c
***
***
B + 2d
B + BHT
ns
*
ns
*
***
D
Multiple comparison test D: 140°C Raneimat 200 ppm
E: 140°C DSC 200 ppm; ***P < 0.001; *P < 0.05 for B + BHT
E
Fig. 1. (a) Antioxidant performance of synthesized compounds. (b) Comparison between DSC induction time and Rancimat
induction time at 140°C and 200 ppm additive concentration.
pure oxygen hence the OIT reached by DSC is more improved to 231, 238, and 231°C respectively. All the
rapid than Rancimat test [28]. A good correlation additives exhibited improved oxidation stability of
existed between DSC OIT and Rancimat OIT values base oil than the commercially available antioxidant
as shown in Fig. 2. The correlation between DSC and
Rancimat OIT is calculated from pearson correlation
coefficient [26] and there exist a positive linear cor-
relation (R2 = 0.875).
BHT (229°C). In addition to OT values SM tempera-
ture values also improved with tested additives ranging
from 268 to 291°C (Table 2) compared to base oil SM
value at 205°C. However, commercial additive BHT
could improve the base oil SM value to 257°C only.
The mean differences of oxidation stability tempera-
tures (OT and SM) of synthesized additives (One-way
ANOVA test) showed that there was statistically signif-
icant higher OT and SM values compared to BHT
(P < 0.001). The improvements in OT and SM tem-
peratures of additive blended base oil confirm the high
antioxidant efficacy of synthesized SHP esters (***P <
0.001 compared with B + BHT; ns: non-significant).
Oxidation onset temperature by DSC (non isother-
mal). Non isothermal DSC tests were employed to
determine the onset oxidation temperature (OT) and
signal maximum temperature (SM) within a short
time. OT values were recorded when the appearance
of exothermic peak with respect to heat flow and it
indicates the initiation of oxidation. The experiment
was carried out in triplicate and the values are given as
mean
SD of three tests. Figure 3 illustrates the
Thermogravimetric analysis. Thermal stabilities of
enhancement in oxidation stability of base oil blended
at 200 ppm concentration of synthesized additives. synthesized SHP esters were examined by using ther-
Additive 2a improved the initial oxidation stability of mogravimetric analysis (TGA) to describe the working
base oil from 180 to 241°C whereas 2b, 2c and 2d temperature range of synthesized additives. Thermo-
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1314
VENKATESHWARLU KONTHAM et al.
6
5
4
3
2
1
0
Rancimat
DSC
B + 2a
B + 2b
B + 2c
B + 2d B + BHT
Base oil
Fig. 2. Correlation between isothermal DSC induction time and Rancimat induction time.
***
300
200
100
0
***
***
ns
ns
Base oil
B + 2a
***
B + 2b
B + 2c
B + 2d
B + BHT
Signal
maximum, °С
Oxidation, °С
Fig. 3. Non-isothermal DSC test (onset temperature).
grams were obtained by plotting a graph temperature tert-butyl-4-hydroxyphenyl)propionic acid (1) shows
against weight loss (wt %) as presented in Fig. 4. The
experiment was carried out in triplicate and the values
are given as mean SD of three tests. Figure 4 and
Table 3 indicates good onset decomposition tempera-
tures (Td) and signal maximum decomposition tem-
perature (Td max) for all the synthesized additives. The
178°C. The mean differences of thermal degradation
temperatures (Td and Td max) of synthesized additives
(One-way ANOVA test) showed that there was statis-
tically significant higher thermal degradation tem-
peratures compared with sample 1 (P < 0.001) (***P <
0.001). The variation in thermal stability is due to alkyl
stability ranges from 243 to 290°C whereas, 3-(3,5-di- chain length and unsaturation [32].
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SYNTHESIS OF FATTY ALCOHOL BASED STERICALLY HINDERED ESTERS
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Table 2. Antioxidant activity of SHP esters on base oil at
FUNDING
200 ppm concentration evaluated by DSC non-isothermal
method
Venkateshwarlu Kontham thanks the University Grants
Commission (UGC), New Delhi, India, for financial sup-
port through a Senior Research Fellowship (SRF). Manu-
script Communication Number: IICT/Pubs./2019/129.
Sample
OIT, °C
SM, °C
Base oil
B + 2a
B + 2b
180 0.81***
241 0.63***
231 1.13ns
205 0.92***
291 1.25***
268 2.28***
CONFLICT OF INTEREST
The authors declare no conflict of interest to this work.
B + 2c
B + 2d
238 1.81***
231 0.81ns
286 1.40***
285 3.18***
ADDITIONAL INFORMATION
B + BHT
229 1.23
257 3.46
Venkateshwarlu Kontham; ORCID: 0000-0001-9437-
5628
Korlipara V. Padmaja; ORCID: 0000-0002-8014-2361
Devarapaga Madhu; ORCID: 0000-0002-6448-0619
Table 3. Thermal stability of SHP esters Td and Td max
Td
Td max
Sample
1
178 1.58
300 4.21
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2b
2c
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283 2.26***
243 2.50***
290 3.04***
260 0.94***
355 4.16***
331 2.54***
360 3.54***
341 2.48***
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Fig. 4. Thermal stability of SHP esters.
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2020