Effect of organophosphate antioxidant on the thermo-oxidative degradation of a mineral oil

Article abstract of DOI:10.1007/s10973-011-2160-y

This study shows the synthesis, characterization, and evaluation of the effect of organophosphate antioxidant on the thermo-oxidative degradation of a mineral oil. The organophosphate was synthesized by nucleophilic substitution (S_N2) of hydrogenated cardanol. For this study, were employed thermogravimetry, derivative thermogravimetry, differential scanning calorimetry, and differential thermal analysis techniques. The results showed that organophosphate contributed for thermo-oxidative stability of mineral oil (initial decomposition temperature (IDT) mineral oil: 91.28 C < IDT mineral oil + organophosphate (1%): 156.42 C). The organophosphate obtained shows significant thermal stability when compared with other compound of the same class (diphenyl phosphate).

Full text of DOI:10.1007/s10973-011-2160-y

J Therm Anal Calorim (2013) 111:553–559
DOI 10.1007/s10973-011-2160-y
Effect of organophosphate antioxidant on the thermo-oxidative
degradation of a mineral oil
•
Maria Alexsandra de Sousa Rios
Selma Elaine Mazzetto
Received: 14 November 2011 / Accepted: 13 December 2011 / Published online: 7 January 2012
´
´
Ó Akademiai Kiado, Budapest, Hungary 2012
Abstract This study shows the synthesis, characterization,
and evaluation of the effect of organophosphate antioxidant
on the thermo-oxidative degradation of a mineral oil. The
organophosphate was synthesized by nucleophilic substitu-
tion (SN2) of hydrogenated cardanol. For this study, were
employed thermogravimetry, derivative thermogravimetry,
differential scanning calorimetry, and differential thermal
analysis techniques. The results showed that organophos-
phate contributed for thermo-oxidative stability of min-
eral oil (initial decomposition temperature (IDT) mineral
oil: 91.28 °C \ IDT mineral oil ? organophosphate (1%):
156.42 °C). The organophosphate obtained shows signifi-
cant thermal stability when compared with other compound
of the same class (diphenyl phosphate).
materials such as oils, fuels, biofuels, and polymers [1–4]. In
particular, when an automotive lubricant or a lubricant for
any mechanical system is being used, this oxidizes more
quickly, depending on its type (paraffinic, naphthenic, or
aromatic), operational conditions, environmental, and other
parameters [5, 6]. Therefore, the study of the thermo-oxi-
dation mechanism of this product can help find means of
overcoming this disadvantage [7].
According to the literature [8–10], oxidation is a complex
chemical reaction and the hydrocarbon degradation process
can be divided into three phases: an initial oxidative phase, a
medium oxidative phase, and a deep oxidative phase [7]. In a
naphthenic lubricant oil, saturated hydrocarbons account for
more than 95% of its composition. Therefore, in the oxida-
tive phases, this oil will yield many products of oxidation
such as carbonyl compounds, peroxy and hydroxyl radicals,
hydroperoxides, and others [7, 11–13].
Keywords Cashew nut shell liquid Á Alkylphenol Á
Diphenyl phosphate
For this reason, the oxidation of the lubricant is unde-
sirable because in certain cases it can increase lubricant
viscosity; hence its ability to be pumped to parts needing
lubrication. In this context, a good thermo-oxidative sta-
bility is important so that the lubricant does not lose its
integrity or form corrosive products and deposits during
service, or both [7]. Among several possibilities to reduce
and or prevent the speed of the oxidation reactions,
chemical industries have been adding antioxidants in their
formulations to improve the oxidative stability of these
lubricant oils.
Introduction
Oxidative degradation is a common problem in several
industrial processes, especially those involving hydrocarbon
M. A. de Sousa Rios (&)
´
Departamento de Quımica – Grupo de Inovac¸o
˜es Tecnologicas e
Especialidades Quımicas - GRINTEQUI, Universidade Federal
´
do Piauı, Campus Ministro Petronio Portella, Teresina,
PI CEP 64.049-550, Brazil
e-mail: maria.alexsandra@terra.com.br
´
´
ˆ
Antioxidants are organic compounds that are added to
oxidizable organic materials to retard oxidation and, in
general, to prolong the useful life of the substrates [14–17].
These additives are classified as either radical trapping
(chain breaking) or peroxide decomposing, terms that
describe the mechanisms by which they function. The
radical trapping antioxidants such as hindered phenols and
S. E. Mazzetto
Departamento de Quımica Organica e Inorganica – Laboratorio
´
de Produtos e Tecnologia em Processos - LPT, Universidade
ˆ
ˆ
´
´
Federal do Ceara, Campus do Pici, Bloco 935, Fortaleza,
CE CEP 60.455-760, Brazil
123

554
M. A. de Sousa Rios, S. E. Mazzetto
secondary aromatic amines react with oxygen radicals
(peroxy and alkoxy), while phosphites and phosphates
function as peroxide decomposing by abstracting peroxidic
oxygen from hydroperoxides and peroxides, and reducing
them [11, 12, 18].
of hydrogenated cardanol with tert-amyl chloride in the
presence of Lewis acid (zinc chloride) [14, 24], and the
organophosphate was obtained by nucleophilic substitution
(SN2) with diethyl chlorophosphate (DECP). The lubricant
oil was supplied by Petrobras (Brazilian Oil Company,
Brazil) with no further distilled procedure. The reagents
and solvents were supplied by Aldrich (analytical grade).
Among the peroxide decomposing antioxidants, the
organophosphate is one of the most important types of
compound of this class. Thus, this study involved the
synthesis, characterization, and investigation of the anti-
oxidant potentiality of an organophosphate derived from
cashew nut shell liquid (CNSL) when added to mineral oil.
CNSL is constituted of anacardic acid, cardanol, and
cardols which are alkyl-substituted phenolic compounds
(Fig. 1) [19–22].
Physical measurements
NMR (1H, 13C, and 31P) spectra were recorded by AVANCE
DRX 500 BRUKER spectrometer, at a frequency of
500 MHz under the following conditions: solvent, CDCl3;
tetramethylsilane (TMS) as an internal standard for 1H and
13C, and phosphoric acid (H3PO4) for 31P.
For the characterization of the new molecule, the authors
1
used H, 13C, 31P RMN, GC–MS, and its stability, and
GC–MS analyses were performed by the use of Hewlett-
Packard 5890 gas-chromatograph and by a Hewlett-Pack-
ard 5971A spectrometer equipped with a DB-5 column
(0.25 mm, 30 m, 0.25 lm film) used an oven temperature
program that initiated data collection at a temperature of
100 °C and ramped at 10 °C min-1 to 300 °C, holding this
temperature for the remaining duration of the data collec-
tion. Electron impact (EI, 70 eV) mode was used. Sample
of 1 lL was injected into the column.
antioxidant potentiality was evaluated using thermogravi-
metric analyses (TG–DTG, DSC, and DTA). Thermal
parameters such as number of degradation steps, onset
(Tonset) and maximum (Tmax) degradation temperatures,
percentage of loss of mass (Dm (%)), initial decomposition
temperature (IDT), and the integral procedure decomposi-
tion temperature (IPDT) [23] were determined in this study.
IR spectra were obtained by the use of a FT-IR spec-
trophotometer Shimadzu, model 8300 (KBr window).
Thermoanalytical measurements were obtained in a
SHIMADZU TGA-50H thermobalance, using alumina
pans and a heating rate of 10 °C min-1, in the temperature
range 25–900 °C and samples of approximately 10 mg.
The samples were carried out in nitrogen and synthetic
air atmospheres (50 mL min-1). The DSC curve was
obtained with a SHIMADZU DSC-50 differential scanning
calorimeter in the temperature range 25–500 °C, using
Materials and methods
Materials
Cashew nut shell liquid and hydrogenated cardanol were
´
supplied by Laboratorio de Produtos e Tecnologia em
Processos, Brazil. The hydrogenated cardanol was purified
by column chromatography on silica gel using hexane as
eluent [12]. The alkylphenol was synthesized by alkylation
Fig. 1 Chemical structures of
CNSL constituents
anacardic acid
cardanol
cardol
2-methyl-cardol
R =
C15H31
C15H29
C15H27
C15H25
123

Effect of organophosphate antioxidant
555
aluminum pans, at a heating rate of 10 °C min-1 and
samples of approximately 10 mg. A nitrogen atmosphere
was used at a flow rate of 50 mL min-1. The differential
thermal analysis (DTA) curves were obtained with a
SHIMADZU DTA-50H DTA in the temperature range
25–900 °C, using platinum pans, at a heating rate of 10 °C/
min and samples of approximately 10 mg. Synthetic air
–(CH2)12–); 1.45 (6H, singlet, –(CH3)(CH2)(CH3)2); 2.60
(2H, triplet, –CH2–Ar); 6.53 (1H, sharp singlet, Ar–OH);
6.77–7.26 (aromatic protons signals). IR (v, KBr, cm-1):
3,364 (OH, hydroxyl group of phenol); 2,929 (vasCH2,
asymmetrical stretching); and 2,854 (vsCH2, symmetrical
stretching) methylene groups; 1,625 and 1,576 (C=C ring
stretch); 983, 818, 723 (angular C–H deformation of the
aromatic ring).
atmosphere was used at a flow rate of 50 mL min-1
.
Methods
1H, 13C, 31P NMR, GC–MS, TG–DTG,
and DSC-organophosphate
Synthesis of alkylphenol
Physical state and color: viscous yellow oil. The analysis
by 1H NMR showed the following (Acetone-d6, ppm): 0.64
(3H, triplet, H3CCH2(CH3)2C–); 0.88 (3H, triplet, –CH2–
(CH2)12–CH3); 1.30 (6H, triplet, (–O–CH2CH3)2); 1.34
(24H, multiplet, –(CH2)12–); 1.38 (6H, singlet, –(CH3)2);
2.58 (2H, triplet, –CH2–Ar); 4.22 (4H, quintet, (–O–
CH2CH3)2); 6.94 (1H, duplet, Ar–H); 7.19 (1H, duplet Ar–
H); 7.21 (1H, duplet Ar–H); 7.33 (1H, singlet, Ar–H).
13C{1H}NMR, showed the following (Acetone-d6, ppm):
14.98 (1C, –CH2–(CH2)12–CH3); 17.07 (2C, (–O–
CH2CH3)2); 34.90 (1C, H3CCH2(CH3)2C); 39.26 (1C,
H3CCH2(CH3)2C–); 65.43 (2C, (–O–CH2CH3)2); 120.62
(1C, Ar–C3), 125.44 (1C, Ar–C4), 129.88 (1C, Ar–C6),
136.00 (1C, Ar–C5), 143.40 (1C, Ar–C2), 151.50 (1C, Ar–
The alkylphenol was synthesized by alkylation of hydro-
genated cardanol (3.3 mmol) with tert-amyl chloride (3.4
mmol) in the presence of Lewis acid (zinc chloride).
The reagents were dissolved in chloroform (20 mL), and
the mixture was heated under reflux during 4 h until the
reaction was completed (monitored by thin-layer chroma-
tography). After 4 h, when no more significant changes in
the reaction were observed, the mixture was poured into a
separatory funnel, washed with 5% aqueous sodium
bicarbonate and then with water. The organic phase was
separated, dried with anhydrous sodium sulfate, and
evaporated under reduced pressure. A yield of 82% was
reached, and the product was purified through a silica gel
chromatographic column using solution of hexane/chloro-
form (80:20). The product was characterized using 1H
NMR and IR [14, 25].
C1), in accordance with the literature data [12, 25, 27]. 31
P
{1H} NMR (Acetone-d6, ppm): -5.58 (1P, singlet, O = P–
(OC2H5)2), in accordance with the literature data [25, 27].
GC–MS analysis showed the molecular ion at m/z 510 (5,
[M? = C30H55O4P?]), 482 (100, C28H51O4P?), 453
(10, C26H46O4P?), 425 (12, C24H42O4P?), 201 (8, C8H10
O4P?), 187 (10, C8H12O3P?), 91 (5, C7H7?), compatible
with mass spectrum fragments of phosphorus compounds
(Fig. 2).
Synthesis of organophosphate
The organophosphate was synthesized by nucleophilic sub-
stitution (SN2). The stoichiometry ratio of the reaction sys-
tem was of 1.0 mol of alkylphenol:1.0 mol of sodium
hydroxide:1.0 mol of DECP, respectively. The reagents
were dissolved in chloroform (20 mL), and the mixture was
heated under the reflux system with constant agitation at
60 °C ( 1 °C) for 1 h and 35 min. After the reaction time, a
viscous yellow oil was obtained [12, 18]. A yield of 74% was
reached, and the product was purified through a silica gel
chromatographic column using solution of hexane/chloro-
form (70:30). The product was characterized using 1H, 13C,
31P NMR, GC–MS, TG–DTG, and DSC [26].
Figures 3 and 4 show thermogravimetric analysis of
organophosphate. According to the results, it is noteworthy
that organophosphate exhibits thermal and oxidative sta-
bilities smaller than diphenyl phosphate. Organophosphate
shows a Tonset of oxidation at 37 °C, which is 235 °C lower
than diphenylphosphate. This trend suggest that as the ratio
of alkyl (alkoxy: –OC2H5)/aryl (phenoxy: –OC6H5) groups
increases in the organophosphate molecules, oxidative
stability decreases [28]. However, after the first thermal
degradation event (47 °C (N2) and 37 °C (air)), stable
species were formed with degradation temperatures higher
than diphenyl phosphate. In previous work, similar behav-
ior was observed in molecules of the same class [27]. The
values of IPDT calculated by Doyle’s [23] method is in the
range of 360–429 °C. The value of IPDT represents an
overall thermal stability of the materials. Table 1 summa-
rizes the thermal parameters of organophosphate and
diphenyl phosphate.
Results and discussion
1H NMR and IR-alkylphenol
1
Physical state and color: yellow oil. The analysis by H
NMR showed the following (CDCl3, ppm): 0.98 (3H,
triplet, –CH2–(CH2)12–CH3); 1.32 (24H, multiplet,
123

556
M. A. de Sousa Rios, S. E. Mazzetto
Height
85000000
Fig. 2 GC–MS of
organophosphate
80000000
75000000
70000000
65000000
60000000
55000000
50000000
45000000
40000000
35000000
30000000
25000000
20000000
15000000
10000000
5000000
0
OP(O)(OC2H5)2
C15H31
TIC
10
20
min
100
482
43
41
57
187
215227
201
425
129
99 117
453
440
467
71
91
143
241
510
495
159 175
255
284 297 325
271
311
345
385 411
400
30
367
360
440
480
560
0
40
80
120
160
200
240
280
320
520
aromatic ring and the tert-amyl group attached on the
aromatic ring present in organophosphate exhibit a great
thermal stability [29]. In an inert gas atmosphere, if TG
(weight loss) data are seen at a temperature below the Tonset
of DSC measurements (thermal degradation), it would
suggest that this weight loss is due to volatility and not due
to thermal degradation. Similarly, weight loss above ther-
mal degradation onset temperature would be a result of
degradation along with volatility [29].
–1.4
–1.2
–1.0
–0.8
–0.6
–0.4
–0.2
0.0
110
Organophosphate
atmosphere:N2
Tonset = 47 °C
T = 385 °C
100
T = 318 °C
6 %
90
80
70
60
50
40
30
20
90 %
As seen in Table 2, DSC profile of organophosphate
shows four peaks and the first peak, with onset temperature
occurring above weight loss onset temperature (Fig. 3),
hence, this weight loss, would be due to volatility alone. As
shown in Fig. 5, organophosphate presented four thermal
events: one exothermic (first event) and three endothermic
(second, third, and fourth events). The first event occurring
probably due to polymerization reaction and the others due
to thermal degradation [28, 29].
T
= 67 °C
max
10
0
0
100 200 300 400 500 600 700 800 900
Temperature/°C
Fig. 3 TG–DTG curves of organophosphate in nitrogen atmosphere
–1.0
–0.8
–0.6
–0.4
–0.2
0.0
110
100
90
80
70
60
50
40
30
20
10
0
T
= 37 °C
6%
Organophosphate
atmosphere:air
onset
T = 356 °C
T = 290 °C
Differential thermal analysis
For evaluation of antioxidant effectiveness of organo-
phosphate comparing results with alkylphenol and diphenyl
phosphate was used a thermoanalytical technique—DTA.
This technique permits to predict the initial decomposition
temperature—IDT, which precedes the main oxidation
process. The existence of the reactions occurring during the
IDT is not detected by the experimental technique used so
that seemingly no reaction takes place. In fact, IDT is a
preparatory stage in which the entities needed for the full
development of autoxidation are formed. The high IDT is
considered a relative measure of the stability of materials
[26].
78%
T = 486 °C
T = 63 °C
12%
0
100 200 300 400 500 600 700 800 900
Temperature/°C
Fig. 4 TG–DTG curves of organophosphate in air atmosphere
Thermal degradation DSC profile of organophosphate is
shown in Fig. 5, and data are given in Table 2. From
results presented in Table 2, it is apparent that organo-
phosphate shows a good thermal stability, with Tonset of
degradation at 178 °C for the first peak. This thermal
behavior can be related to its basic structure, once that the
Differential thermal analysis measurements are shown
in Figs. 6, 7, 8 and given in Table 3. According to the
results, antioxidant effectiveness of organophosphate can
be proven by the difference between IDT for mineral oil
(91.28 °C) and mineral oil ? organophosphate (1%)
123

Effect of organophosphate antioxidant
557
Table 1 Thermal parameters of organophosphate and diphenyl phosphate
Sample
Degradation event
Tonset/°C
Tmax/°C
Dm/%
IPDT/°C
Air
III
N2
II
Air
N2
Air
N2
Air
N2
Air
N2
Organophosphate
37 (I)
47 (I)
63 (I)
67 (I)
6 (I)
6 (I)
360
371
290 (II)
318 (II)
356 (II)
680 (III)
301 (I)
352 (II)
604 (III)
385 (II)
78 (II)
90 (II)
486 (III)
252 (I ? II)
12 (III)
87 (I ? II)
Diphenyl phosphate
III
–
–
–
–
429
–
550 (III)
3 (III)
7.5
uV
Organophosphate
Baseline
Peak 238.70C
Onset 178.36 C
Endset274.03 C
Heat –1.48J
–147.94J/g
5.00
Atmosphere: N
2
6.0
4.5
1
1– T
= 178 °C
4
onset
2
3
T
= 191 °C
0.00
peak
4– T
= 453 °C
= 497 °C
Integrated = 173 mW °C g–1
onset
T
3,0
peak
2– T
= 290 °C
= 291 °C
Integrated = 211 mW °C g–1
onset
–5.00
–10.00
T
peak
1.5
Integrated = 79 mW °C g–1
3– T
onset
= 308 °C
= 338 °C
0.0
T
0.00
1000.00
500.00
peak
Integrated = 109 mW °C g–1
Temperature/°C
–1.5
Fig. 7 DTA curve of mineral oil ? alkylphenol (1%) in air
atmosphere
0
100
200
300
400
500
600
Temperature/°C
Fig. 5 DSC curve of organophosphate in nitrogen atmosphere
uV
Peak 203.85C
Onset 156.42C
Endset274.53C
5.00
Table 2 DSC measurements of organophosphate (N2 atmosphere)
Heat –2.24J
–221.45J/g
Sample
Thermal
transitions
Tonset/°C
T
peak/°C Integrated/
0.00
mW °C g-1
Peak 478.98C
Onset 358.02C
Organophosphate
I
178
290
308
453
191
291
338
497
173
79
–5.00
Endset547.54C
II
Heat –2.51J
–248.39J/g
III
IV
109
211
–10.00
0.00
200.00
400.00
600.00
800.00
Tpeak peak temperature, Integrated involved energy
Temperature/°C
Fig. 8 DTA curve of mineral oil ? organophosphate (1%) in air
atmosphere
uV
10.00
Peak 225.75C
Onset 91.28C
Endset287.44C
Heat –9.43 J
(156.42 °C). This behavior can be explained partly by the
substituent presence at the meta and/or ortho positions. The
bulkier substituents such as tert-amyl on ortho- and/or
para-positions have been recommended to inhibit coupling
reaction of phenoxy radicals, resulting in the decrease of
oxidation rate [14, 24]. For example, organophosphate has
bulkier substituent on both ortho and meta positions and,
consequently, shows a good thermo-oxidative stability,
because this molecule possesses extensively conjugated
structure that is efficient in scavenging free radicals. The
–952.80 J/g
0.00
Peak 396.28C
Onset 300.42 C
Endset434.40C
Heat –2.94 J
–296.90J/g
–10.00
–20.00
0.00
200.00
400.00
Temperature/°C
600.00
800.00
Fig. 6 DTA curve of mineral oil in air atmosphere
123

558
M. A. de Sousa Rios, S. E. Mazzetto
´
Acknowledgements The authors acknowledge Petroleo Brasileiro
Table 3 DTA measurements of mineral oil with and without addi-
tives (air atmosphere)
S.A. (PETROBRAS) and CNPq (Process number 472009/2007-9) for
the financial support, the Organic and Inorganic Chemistry Depart-
ment of Federal University of Ceara by chemical and thermal anal-
yses, and the Northeastern Center of Application and Use of Nuclear
Magnetic Resonance—CENAUREM/UFC by 1H, 13C, and 31P NMR.
Sample
IDT/°C
Air
T
max/°C Heat/J g-1 °C-1
Air
´
Mineral oil
91.28
225.75
238.70
-952.80
-147.94
Mineral oil ?
alkylphenol (1%)
178.38
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