Thermal Degradation of Aviation Synthetic Lubricating Base Oil

Article abstract of DOI:10.1134/S0965544118030179

The thermal degradation, under oxidative pyrolysis conditions, of two synthetic lubricating base oils, poly-α-olefin (PAO) and di-ester (DE), was investigated. The main objective of the study was to characterize their behavior in simulated “areo-engine” conditions, i.e. compared the thermal stability and identified the products of thermal decomposition as a function of exposure temperature. Detailed characterizations of products were performed with Fourier transform infrared spectrometry (FTIR), gas chromatography/ mass spectrometry (GC/MS), viscosity experiments and four-ball tests. The results showed that PAO had the lower thermal stability, being degraded at 200°C different from 300°C for DE. The degradation also effected the tribological properties of lubricating oil. Several by-products were identified during the thermal degradation of two lubricants. The majority of PAO products consisted of alkanes and olefins, while more oxygen-containing organic compounds were detected in DE samples according to the observation of GC/MS analysis. The related reaction mechanisms were discussed according to the experimental results.

Full text of DOI:10.1134/S0965544118030179

ISSN 0965-5441, Petroleum Chemistry, 2018, Vol. 58, No. 3, pp. 250–257. © Pleiades Publishing, Ltd., 2018.
Thermal Degradation of Aviation Synthetic Lubricating Base Oil1
Nan Wua, b, Zhimin Zonga, *, Yiwei Feib, Jun Mab, and Feng Guob
aKey Laboratory of Coal Processing and Efficient Utilization, Ministry of Education,
China University of Mining and Technology, Jiangsu, Xuzhou, 221116 China
bDepartment of Aviation Oil and Material, Air Force Logistics Institute, Jiangsu, Xuzhou, 221000 China
*e-mail: wunan_china@163.com
Received March 17, 2017
Abstract—The thermal degradation, under oxidative pyrolysis conditions, of two synthetic lubricating base
oils, poly-α-olefin (PAO) and di-ester (DE), was investigated. The main objective of the study was to char-
acterize their behavior in simulated “areo-engine” conditions, i.e. compared the thermal stability and iden-
tified the products of thermal decomposition as a function of exposure temperature. Detailed characteriza-
tions of products were performed with Fourier transform infrared spectrometry (FTIR), gas chromatogra-
phy/mass spectrometry (GC/MS), viscosity experiments and four-ball tests. The results showed that PAO
had the lower thermal stability, being degraded at 200°C different from 300°C for DE. The degradation also
effected the tribological properties of lubricating oil. Several by-products were identified during the thermal
degradation of two lubricants. The majority of PAO products consisted of alkanes and olefins, while more
oxygen-containing organic compounds were detected in DE samples according to the observation of GC/MS
analysis. The related reaction mechanisms were discussed according to the experimental results.
Keywords: synthetic lubricating oil, poly-α-olefin, di-ester, thermal stability, viscosity degradation, tribolog-
ical properties
DOI: 10.1134/S0965544118030179
INTRODUCTION
matters are included in the degraded ones, which
influence lubricants’ comprehensive properties
severely, such as lubricity, viscosity and volatility [4].
For this purpose, understanding molecular composi-
tion of organic matter in in-use SALO is crucially
important.
As the most widely used synthetic lubricating base
oil, poly-α-olefin (PAO) and synthetic di-ester (DE)
are formulated to suit with the automotive and indus-
trial applications [1]. Compared to mineral oils, PAO
and DE possess unique properties such as higher vis-
cosity, lower corrosively, lower volatility and toxicity,
and wider operating range [2]. The high viscosity
index of PAO and DE allow these synthetic lubricants
to be used under extremely hot or cold temperature,
while its high mechanical and thermal stability ensures
operation of equipment in severe performance appli-
cations. However, with the development of high-per-
formance aero-engine, PAO and DE could not meet
the demands of harsh terms, including the ultra-tem-
perature, time, revolving speed and catalyst of transi-
tion metal ions, and so on [3]. In particular, viscosity
thinning and color darkening of in-use oil not only
embarrass the normal usage of the aero-engine, but
also present the abnormal phenomenon in an instant.
Hence, synthetic aviation lubricating oil (SALO) must
be replaced frequently to match the high performance
demands. In comparison to the non-degraded lubri-
cant oils, more micro-molecules and other organic
The determination of lubricant application associ-
ated with the degradation processes is an interesting
topic of research. Many studies have described how
the molecular composition of base oil influences its
physical and chemical properties, especially its ther-
mal stability which is connected to both the initial
temperature and the rate of degradation of polymers
[5–13].
Significant advances in analytical instrumentation
have given a big boost to the process of detecting lubri-
cants. For example, as a commonly used instrument,
gas chromatography/mass spectrometer (GC/MS) is
effective for analyzing some volatile species by passing
test fluids over oil-coated chromosorb packed in GC
column and measuring the retentions index to monitor
oil degradation, through which we could monitor the
thermal-oxidative stability of lubricants. GC/MS has
been successfully applied in many fields [14–17], how-
ever, to the best of our knowledge, few studies on ther-
1
The article is published in the original.
250

THERMAL DEGRADATION OF AVIATION SYNTHETIC LUBRICATING
Table 1. Typical pproerties of PAO
Properties
251
PAO
Test Method
Kinematic viscosity (40°C)/(mm2/s)
17.94
4.018
ASTM D445
ASTM D445
Kinematic viscosity (100°C)/(mm2/s)
Viscosity index
131
201
ASTM D2270
ASTM D92
ASTM D97
ASTM D974
Flash point(COC)/(°C)
Pour point/(°C)
<–68
0.009
Acid number/(mg KOH/g)
mal-oxidation of lubricants and their derivatives cov- dized samples under 170, 200, and 300°C were abbre-
ered the application of this technique.
viated as P0, P1, P2, P3 and D0, D1, D2, D3, separately.
Both PAO and DE are typical synthetic lubricating
base oils in China and were extensively investigated [9,
13, 18–22]. Nevertheless, more investigations are
needed to probe the complex chemical changes using
analytical techniques including a number of spectro-
scopic studies such as ultraviolet (UV), infrared (IR)
and GC/MS. In the present study, structural features
of PAO, DE and their reacted samples were character-
ized by traditional viscosity experiment and analyzed
with GC/MS. Influence of degradation products on
the tribological properties of PAO and DE was also
discussed.
Analytical Methods
PAO, DOA and their degraded samples were ana-
lyzed with a Nicolet Magna IR-560 Fourier transform
infrared (FTIR) spectrometer by collecting 32 scans at
a resolution of 4 cm−1 in reflectance mode with mea-
suring region of 4000–400 cm−1.
Afterwards, oil samples were analyzed with a Hew-
lett-Packard 6890/5973 GC/MS, which was equipped
with a capillary column coated with HP-5MS (cross-
link 0.5% PH ME siloxane, 60 m length, 0.25 mm
inner diameter, 0.25 m film thickness) and a quadru-
pole analyzer with a m/z range from 33 to 500 and
operated in electron impact (70 eV) mode. The capil-
lary column was heated from 120 to 274°C at a rate of
13°C min–1 and held for 2 min, then raised to
281°C min–1 at a rate of 0.5°C min–1 and held for
2 min, finally raised to 300°C at a rate of 12°C min–1
and held at 300°C for 3 min. Data were acquired and
processed using software of Agilent MSD Productivity
Chemstation. Compounds were identified by compar-
ing mass spectra with NIST05a library data.
EXPERIMENTAL
Oil Sample and Reagents
PAO and Di(2-ethylhexyl)adipate (DOA) (a kind
of DE) were collected from Air Force Fuel Research
Institute. The typical properies of PAO were shown in
Table 1. Petroleum ether (90−120°C), acetone and n-
hexane used in the experiments were analytical
reagents, and all the organic solvents were distilled
before use with a Büchi R-210 rotary evaporator.
Thermal Degradation Experiments
And using MQ-800 four-ball tester produced by
Jinan Testers Factory to evaluate the wear and friction
characteristics of degradation samples with the condi-
tions of rotating rate 1450 r/min, test duration 30 min,
and load 392N. The balls used in the tests were made
of GCr 15 (AISI 52100) bearing steel with HRC 59-61.
The wear scar diameters (WSDs) were measured by
using an optical microscope. Parallel experiments
Both PAO and DOA were heat-treated at 170, 200,
and 300°C, respectively, in a 500 mL stainless-steel,
magnetically stirred autoclave with the stirring rate of
800 r/min. After reaction for 2 h, the reacted samples
were withdrawn at room temperature. And then, the
kinematic viscosity (KV) and total acid number (TAN)
of the reaction product were measured according to
ASTM D445 and ASTM D974 respectively. For con-
venience in description, the PAO, DOA, and their oxi- were done to ensure the relative error less than 5%.
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NAN WU et al.
(a)
(b)
D0
D1
P0
P1
P2
P3
D2
D3
2854
2925
10%
10%
3500 3000 2500 2000 1500 1000 500
Wavenumber, cm–1
3500 3000 2500 2000 1500 1000 500
Wavenumber, cm–1
1000
1000
Fig. 1. Infrared spectra of PAO (a), DOA (b), and their reacted samples.
RESULTS AND DISUSSION
of D3 is appreciably stronger than that in D0, which
indicates that DOA has been pyrolysis under thermal
conditions.
FTIR Analysis
During thermal degradation, synthetic lubricant
oils may go under changes in compositions. Products,
which are harmful to the high-performance of these
oils, may be formed due to cracking process, changing
its properties [23].
GC/MS Analysis
There are 101 micro-compounds (MCs) were iden-
tified from Pn (n = 0−3) by GC/MS analysis, includ-
ing 13 n-alkanes (NAs), 26 isoparaffins (IPs), 51 ole-
fins (OFs), and 11 other compounds (OCs). While for
DOA samples, there are 53 MCs were determined,
including 24 single esters (SEs), 13 micro-di-esters
(MDEs), 9 ketones, 1 alkane, 2 ethers, 1 organic acids
(OAs), and 3 alcohol.
Obvious differences in the FTIR spectra among
PAO and DOA and their reacted samples can be
observed from Fig. 1. The absorbance of aliphatic
moieties (AMs) from Pn (n
= 0−3) around
2850−2960 cm–1 and 1465 cm–1 are much stronger
than those from Dn, indicating that AM-rich species in
Pn are predominant products, which could be ascribed
to the special structure of PAO, multi-aligned comb-
likely side-chains and skeleton bones of straight chain
alkynes, too. The stretching vibration of –OH group
around 3429 and 3459 cm–1 from Dn is much stronger
than from Pn, which suggesting that species containing
–OH were produced in Dn. The weak absorbance
As illustrated in Fig. 2a, NAs, IPs and OFs are
main products from pyrolysis of PAO. The range of
carbon number in NAs is from 12 to 24 and eicosane is
predominant individual component. The relative con-
tent (RC) of OFs is equivalent to that of NAs and car-
bon number is from 12 to 23, implying that further
molecular recombination happened in the pyrolysis
condition. This result is in agreement with the molec-
ular composition of PAO, less oxygen-containing
organic compounds (OCOCs), but a high content of
long-chain macromolecular species. The detection of
OCOCs is in consistent with FTIR analysis stated
above. However, RC of OCOCs is 1.02% and all of
them only appeared in P3. As we all know, RC of
OCOCs represents the oxidation degree of PAO and it
is obvious that breakage rather than oxidation is dom-
inantly under thermal atmosphere, as depicted in
Scheme 1.
around 1641, 1633, 966, and 888 cm–1 are assigned to
C=C bending vibration, indicating that olefins may
exist in P3.
Remarkably stronger absorbance at 1738 cm–1
assigned to –C=O bond of carboxyl groups in Dn than
that in Pn proved that esters are dominating com-
pounds. The characteristic absorbance of the carbonyl
group form aldehydes, ketones and carboxylic acids is
around 1716 cm–1. Such absorbance from P3 is very
weak, suggesting that fractional substances are oxyge-
nized. The absorbance of more −(CH2)n− at 754 cm–1
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THERMAL DEGRADATION OF AVIATION SYNTHETIC LUBRICATING
253
+
+
5
4
2
1
3
1
2
+
+
7
3
+
+
+
6
8
9
+
+
Scheme 1. Mechanism for the oxidative pyrolysis reactions of PAO.
PAO first broke weak covalent bonds such as
‒CH2– bonds to produce large amounts of MCs and
then some of them gradually rearranged by further
reaction to afford OFs. According to the supposition,
the formation of NAs and/or IPs could be ascribed to
the cleavage of weak covalent bonds at the tertiary car-
bon atom.
Further decomposition of OFs and OCOCs can
consequently lead to the formation of di-alkenes or
vinyl radicals vial dehydrogenation. However, at tem-
peratures higher than 300°C, such dialkenes and vinyl
radicals are efficiently destroyed leading to a very
small benzene production, which makes the oils color
deepened.
As Fig. 2b exhibits, the RC of SEs decreases in the
order: D3 @ D2 > D1 > D0. RC of SEs from D3 (1.35%)
is triple more than that from D2 (0.49%), whereas SEs
in D1, D2, and D3 are mainly short-chain species with
carbon number from 13 to 16, indicating that the ther-
mal rupture dramatically increased the yields of short-
chain SEs. In addition, there are few significant differ-
ences in the RC of MDEs (0.25%), ketone (0.12%),
OAs (0.018%), ether (0.005%) and alcohol (0.05%)
between D1 and D2, but much lower than that from D3.
6-(2-ethylhexyloxy)-6-oxohexanoic acid is predomi-
nant in OCOCs from D3 with 0.02% of RC, which is
related to the fact that the acid value of D3 is 12.597 mg
(a)
(b)
2.4
20
15
10
5
NAs
IPs
OFs
OCOCs
Alcohols
OAs
ethers
alkanes
ketones
MDEs
1.8
1.2
SEs
0.6
0
0
P1
P2
P3
D1
D2
D3
P0
D0
Fig. 2. Relative abundance of reacted samples of PAO (a) and DOA (b).
PETROLEUM CHEMISTRY Vol. 58 No. 3 2018

254
NAN WU et al.
KOH/g, much higher than that of P3. 2-Cyclopen- to predict actual lubricant service life in high tempera-
ture and other extreme applications. Noteworthy, the
more resistant a lubricant is to thermal-oxidation, the
fewer tendencies it has to viscosity increases during
use [3].
tenylethenylpentanone is the most abundant in ketone
from D1, D2, and D3, contributing to the deepened
color in reacted samples.
The formation of OCOCs from DOA could be
explained that under high temperature conditions the
O−C bond is likely to rupture after the formation of
hydrogen peroxide, and then giving rise to format a
phenyl radical. This radical can, in turn, be quenched
by a hydrogen atom leading, finally, to the formation
of chromophore in large quantities. The higher
OCOCs production of DOA compared to that of PAO
may be due to the different chemical composition of
the two samples.
In addition, as demonstrated in Fig. 3, 7 OCOCs
were confirmed in the products from DOA degrada-
tion. Various compounds were identified from oil oxi-
dation, but to our knowledge, few reports have been
issued on the detection of OCOCs in the products
from DOA degradation. Compared to those obtained
under pyrolysis conditions (where a plateau of ben-
zene formation was obtained at temperatures higher
than 300°C for all lubricants), the results clearly show
the positive effect due to the presence of oxygen, lead-
ing to the formation of reactive oxygenated species.
The thermal stability profiles of the PAO and DOA
can be depicted in Fig. 4, each point representing the
viscosity of degraded oils respectively. From this fig-
ure, viscosity for the two synthetic lubricants follows
the order PAO > DOA, which might be corresponding
to the fact that PAO has a higher content of long-chain
AMs than DOA. However, PAO has the lower thermal
stability and its degradation starts at temperatures
lower than 200°C, with 23.8% of RC in MCs. DOA, in
fact, is made of OCOCs being constituted of methyl
and ethyl di-ester, which makes it more thermal stable
than PAO with a high NAs content. In addition, the
higher thermal stability of DOA, with respect to PAO,
can be attributed to the higher content of stable species
which is quiet a resistant compound thermally.
Total Acid Number (TAN) Analysis
Acid value is often used to measure the corrosive
properties of lubricating oil. The high TAN will accel-
erate corrosion and shorten the service life of the
engine. It can be seen from Fig. 5 that TAN of PAO
This results in oxidation reactions of both benzyl and base oil and its high temperature reaction samples
increased with the temperature going up. When the
temperature was higher than 200°C, the acid value
increased sharply, but still at a low level. Even if the
temperature rose to 300°C, the acid value was only
0.493 mg KOH/g. Meanwhile, TAN of DOA and its
tested samples had the similar trend, but the values
were higher comparing with PAOs. TAN of the DOA
tested oil at 200°C raised to1.568 mg KOH/g, which
can cause serious corrosion to the engine lubrication
system. From 200 to 300°C, the TAN increased by
11.029 mg KOH/g to 12.597 mg KOH/g, indicating
that the oil sample had been seriously deteriorated.
The difference between the two SALO can be
attributed to the fact that the ester lubricating oil pro-
duces more acidic substances after high temperature
oxidation and increases the acid value.
alkyl radicals which, ultimately, give rise to the
observed increase in OCOCs formation at higher tem-
peratures. This again reflects a different formation
mechanism that has to proceed, as already mentioned,
via the intermediate formation of ketone by means of
dialkenes and vinyl radical displacement and cycliza-
tion reactions.
Ketone, then, may further polycondense and
polymerize with the same radicals giving rise to aro-
matic radicals that, acting as building blocks, generate
heavy species by consecutive reactions and, eventu-
ally, to highly condensed groups (non-volatile) that
are included into the black char deposit.
However, these compounds do not represent the
totality of by-products formed because at the end of
the test, i.e. after the experiments at high temperature,
there is evidence of a light black solid deposit (char)
left in the samples, probably due to high molecular
weight (or polymerized) compounds. It is not possible
to characterize the compounds responsible for form-
ing this solid deposit because the analytical technique
employed (GC/MS) only allowed volatile and semi-
volatile by-products to be detected.
Wear and Friction Performance Analysis
Figure 6 showed the variation of WSDs with tem-
perature of PAO and DOA oxidation products. It can
be seen that the WSDs of the oil oxidation products
reached maximum values of 1.10 mm to PAO oxida-
tion products, and 0.79 mm to DOA oxidation prod-
ucts, respectively. It may be due to a large number of
small molecules produced by the high temperature
oxidation, which weakened the anti-wear properties of
oil and damaged the friction surface seriously. With
Viscosity Analysis
In general, oils high in long straight-chained con- the increasing of temperature, the WSDs of PAO and
tent and certain sulfur and nitrogen containing species DOA oxidation products decreased, but still higher
exhibit faster degradation. Nowadays, KV is often used than that of non-oxidized oil, which indicated the oxi-
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THERMAL DEGRADATION OF AVIATION SYNTHETIC LUBRICATING
255
100
80
60
40
20
0
150
79
O
O
150
79
4
11
121
94
107
93
67
39
41
67
53
135
106
121
55
60
80
100
120
140
160
60
80
100
120
140
160
40
40
100
80
60
40
20
0
110
84
O
O
N
95
N
H
81
43
21
25
67
138
55
194
97
41
55
137
196
166
111
124
68
165
60 80 100 120 140 160 180 200
60 80 100 120 140 160 180 200
40
40
100
80
60
40
20
0
O
110
97
O
O
O
O
O
28
57
110
123
70
O
43
55
43
29
96
145
67
127
83
84
164
149
195
194
242
283
80
120
160
200
240
80
120 160 200 240 280
40
40
100
80
60
40
20
0
m/z
143
O
O
O
O
O
111
38
57
83
70
43
129
97
161
329
241
199
281
90
140
190
m/z
240
290
340
40
Fig. 3. Mass spectrums of some chromophore detected from DOA oxidized samples.
dation temperature had a great effect on the anti-wear approach for understanding the pyrolysis of PAO and
properties of synthetic lubricants. And under the same
conditions, PAO oxidation products had larger WSDs
than DOAs, which may be due to formation of the
acidic substances produced during the oxidation pro-
cess to improve the friction and wear resistance of the
lubricating oils.
DOA. According to the investigations, NAs, IPs and
OFs are important moieties and short-chain AMs are
predominant in P1, P2, and P3. The products from
thermal degradation in DOA are rich in SEs, MDEs,
ketone, ether, OAs, and alcohol, which leads to higher
acid value than that of PAO reacted samples. Viscosity
experiment is expected to be an effective method for
inquiring degradation extent of aero-engine lubri-
cants. The significant decrease of viscosity in PAO
reveals its lower thermal stability compared to DOA.
CONCLUSIONS
Thermal treatments along with subsequent prod-
ucts separation and analyses prove to be effective The small molecules formation in the thermal degra-
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NAN WU et al.
14
20
15
10
5
PAOs
12
10
8
DOAs
PAOs
DOAs
6
4
2
170
200
300
20
0
Temperature, °C
170
200
300
20
Temperature, °C
Fig. 4. Variations in KV values (40°C) with temperature of
Fig. 5. Variations in TAN values with temperature of PAO
and DOA tested samples.
PAO and DOA tested samples.
120
100
80
60
40
20
0
PAOs
DOAs
170
200
300
20
Temperature, °C
Fig. 6. Variations in WSDs with temperature of PAO and DOA tested samples.
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ACKNOWLEDGMENTS
This work was subsidized by the Jiangsu Provincial
Natural Science Foundation of China (Grant
BK20161187), Xuzhou Science and Technology
Development Funds (Grant KC16SG269), and a
Project Funded by the Priority Academic Program
Development of Jiangsu Higher Education Institu-
tions.
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