ORIGINAL PAPER

Synthesis and Tribological Properties of Air Plasma PolymerizedSoybean Oil with N-Containing Structures

Xiaoyun Zhao • Jingyi Yang •

Dehua Tao • Xinru Xu

Received: 4 May 2013 / Revised: 12 January 2014 / Accepted: 24 January 2014

� AOCS 2014

Abstract In this study, polymerized oils with different

viscosity grades were synthesized by the polymerization of

soybean oil (SO) in an air plasma environment. The results

of the elemental analysis, infrared spectroscopy, and

pyrolysis gas chromatography revealed that the carbonyl,

organic amine, and nitrogen heterocyclic groups were

incorporated into the molecule of the polymerized oil

(PAIR9); the GPC chromatogram of this oil revealed that

the product consisted of dimers and oligomers with higher

molecular weights. The tribological behaviors of the

polymerized oils were evaluated using a four-ball friction

and wear tester. The maximum non-seizure loads of all the

polymerized oils surpassed that of SO, and the PB value of

PAIR9 reached 1,186 N. Meanwhile, PAIR9 exhibited

much better anti-wear performance when the tested loads

were lower than 350 N. The worn surfaces lubricated by

SO and PAIR9 were analyzed using the X-ray photoelec-

tron spectra (XPS). The results of the XPS analysis proved

that during the frictional process, the polymerized oil could

not only promote the adsorption of the oil on the metal

surface because of the oxygen-containing species (such as

esters and carbonyl groups) with higher polarities but also

promote the interactions with the metallic iron to form

compact and stable tribochemical films containing organic

nitrogen complexes.

Keywords Soybean oil � Plasma polymer � Viscosity �Organic amine � Lubricant � Tribology � XPS �Coordination complexes

Abbreviations

GPC Gel permeation chromatography

XPS X-ray photoelectron spectroscopy

Introduction

The development of environmentally friendly lubricants to

reduce our reliance on petroleum resources and protect the

ecology environment has garnered a global consensus.

Vegetable oils, which are renewable, biodegradable, and

less toxic to the environment as compared to mineral oils,

are important materials in preparing green lubricants.

Fernandez Rico [1] developed a method to enhance the

lubricity of polyalphaolefin by adding sunflower oil. Ratoi

[2] explained the film-forming behavior of fatty acids when

used as oiliness additives. In addition, vegetable oils have

higher dissolving capacities for additives as compared to

mineral oils. Ren [3–5] found that triazine derivatives when

used as additives show excellent lubricating behaviors in

rapeseed oil.

However, vegetable oils persist in having certain

drawbacks that need to be overcome, namely, poor

oxidative stability and poor low-temperature perfor-

mance; this complicates the use of vegetable oils as

biodegradable lubricants. Moreover, the viscosity ranges

of most vegetable oils (except castor oil) are very nar-

row, namely, ISO viscosity grades (VG) of 32 and 46

[6–8]. Several types of lubricant oils, such as gear oils

and hydraulic oils, need higher viscosities. Lathi [9] and

Erhan [10, 11] developed the ring-opening process of

X. Zhao � J. Yang � X. Xu (&)

Research Institute of Petroleum Processing, East China

University of Science and Technology, Meilong Road 130,

Shanghai 200237, People’s Republic of China

e-mail: xrxu86@ecust.edu.cn

D. Tao

School of Electromechanical Engineering and Automation,

Shanghai University, Yanchang Road 149, Shanghai 200072,

People’s Republic of China

123

J Am Oil Chem Soc

DOI 10.1007/s11746-014-2424-3

epoxidized soybean oil with alcohols and anhydrides to

improve the oxidative stability. Cermak [12, 13] syn-

thesized estolides from various fatty acids, which yielded

better low-temperature characteristics. By using anthra-

quinone as the catalyst, Guner [14] increased the vis-

cosity of anchovy oil through thermal polymerization.

Biswas [15] evaluated the lubricity of microwave-irra-

diated soybean oil with higher viscosity.

In recent years, the use of plasma polymerization for

fabricating special polymer materials has received

increased attention. Yasuda [16] studied the polymeri-

zation of acetylene using gases and vapors of H2O, N2,

and CO. Dilsiz [17] investigated the effects of plasma

discharge power, different atmospheres, and monomers

having different functional groups on the structures of

plasma polymers. With regard to biomedical applications,

the deposition of polyacrylic acid films using nitrogen

plasma jets at atmospheric pressure has been investigated

[18]. In this paper, air plasma produced in an original

reactor was employed to open the double bonds of SO

for preparing polymerized oils with higher molecular

weights and viscosities. The structure of the oil poly-

merized by air plasma was analyzed using elemental

analyzer, infrared (IR) spectroscopy, pyrolysis gas chro-

matography (Py-GC), and gel permeation chromatogra-

phy (GPC). The tribological behaviors of the

polymerized oils were evaluated on a four-ball friction

and wear tester. The lubricating mechanism of the

polymerized oil was discussed by analyzing the X-ray

photoelectron spectra (XPS) of the worn surface.

Experimental

Materials

In this study, the commercially available SO employed in

the polymerization process was provided by Shanghai

Liangyou Haishi Oils and Fats Industry Company, China.

The fatty acids of SO were primarily composed of unsat-

urated linolenic acids (C18:3, 2.69 %), linoleic acids (C18:2,

46.3 %), and oleic acids (C18:1, 26.6 %), which displayed a

higher degree of unsaturation; they also comprised satu-

rated stearic acids (C18:0, 6.78 %) and palmitic acids (C16:0,

15.0 %). The characteristics of SO were listed in Table 1.

Synthesis of the Polymerized Oils by Air Plasma

The plasma reactor employed to prepare the polymerized

oils used in this study is shown in Fig. 1. The major

components of this equipment were a non-pressurized

vessel containing two parallel graphite electrodes

(150 9 25 9 200 mm); the electrodes were separated

using two quartz glass plates (separation: 6 mm) that acted

as the dielectric medium. The plasma reactor was activated

using a plasma generator (CTP-2000K, Nanjing Suman

Electronics Co., Ltd.) that operated at a frequency of

7.30 kHz and had a deliverable power range from 0 to

500 W.

Approximately 300 mL of SO placed in a storage tank

at a constant temperature (90 �C) was pumped into the

reactor filled with air; the pumping was controlled to

obtain a circular flow at 160 mL/min. In the polymeri-

zation process, the plasma power was maintained at

120 W, and the frequency was adjusted to obtain glow

discharges. The polymerized oil samples were withdrawn

at predetermined times and the viscosities of the samples

were calculated.

Testing the Physicochemical Properties

The viscosities (at 40 and 100 �C) and viscosity index (VI)

were measured and calculated according to ASTM D445

and ASTM D2270, respectively. The iodine values, a

measure of the degree of unsaturation in fatty acids, were

determined according to ISO 3961:1996.

Structural Analysis of Polymerized Oil

Elemental Analysis

The elemental measurement of carbon (C) and hydrogen

(H) for the oil samples were conducted using the vario EL

cube elemental analyzer (Elementar Analysensysteme

GmbH, Germany). The Antek 9000 sulfur and nitrogen

fluorescence analyzer (Petroleum Analyzer Company,

USA) was used to determine the mass fraction of nitrogen

(N) in the polymerized oil, which was diluted by 100 times

using di(2-ethylhexyl)phthalate before the test. The per-

centage composition of oxygen (O) was calculated from

the analysis results of C, H, and N.

Table 1 Characteristics of air plasma polymerized soybean oil at

different reaction times

Sample Reaction

time (h)

Viscosity (cSt) Viscosity

index

Iodine value

(g I2/100 g)40 �C 100 �C

SO 0 33.8 7.84 215 130

PAIR1 1 48.4 10.1 203 125

PAIR3 3 111 16.6 162 104

PAIR5 5 307 40.5 186 79.8

PAIR7 7 645 49.2 130 74.3

PAIR9 9 1,136 68.6 123 69.8

J Am Oil Chem Soc

123

IR Spectroscopy

The polymerized oil of SO was analyzed using a Nicolet

6700 Fourier transform infrared spectrometer (Thermo

Fisher Scientific Inc., USA). The IR spectra were recorded

from 4,000 to 400 cm-1 at a resolution of 0.09 cm-1.

Pyrolysis of Oil Polymerized by Air Plasma

The oil sample was pyrolyzed using a PY-2020iD double-

shot pyrolyzer (Frontier Lab, Japan) coupled to a

GC7089A gas chromatograph (Py-GC). The detection was

carried out using a MS5975C mass selective detector

(Agilent, USA) (MSD, qualitative analyses). In the exper-

iment, the pyrolysis temperature was maintained at 500 �C

and the temperature was maintained constant for 20 s. The

GC oven temperatures were changed as follows: main-

tained at 40 �C for 2 min, increased at a rate of 10 �C/min

up to 310 �C, and finally maintained at 310 �C for 40 min.

Helium was used as the carrier gas as well as to create an

inert atmosphere in the pyrolysis interface.

GPC

The GPC was obtained using a Waters 1515 gel permeation

chromatograph equipped with a column (dimensions

300 9 7.5 mm), a manual injector and a Waters 2414

refractive index detector. The molecular weights were

calculated and calibrated using the polystyrene standards

obtained from the Waters Corp., USA. The polymerized

samples were dissolved in tetrahydrofuran, which was used

as the eluent with a flow rate of 1.00 mL/min at 40 �C and

an injection volume was 100 lL.

Friction and Wear Test

The tribological properties were determined at ambient

temperature on the MS-800 four-ball friction and wear

tester (Xiamen Tenkey Automation Company Limited,

China). GCr15-grade steel ball bearing (diameter:

12.7 mm; hardness: HRC 59–61; chemical composition: C,

0.95–1.05 %; Si, 0.15–0.35 %; Mn, 0.20–0.40 %;

P \ 0.027 %; S \ 0.020 %; Cr, 1.30–1.65 %; Ni \ 0.30

%; Cu \ 0.25 %) was used as the test material; it was

supplied by the Shanghai Steel Ball Plant, China.

Under the test conditions of a rotating speed of

1,450 ± 50 rpm for the duration of 10 s at room temper-

ature, the maximum non-seizure loads (PB value) were

measured according to GB 3142-82 (Chinese National

Standard) similar to ASTM D-2783. The anti-wear tests

were carried out at a rotating speed of 1,450 ± 50 rpm and

different loads of 98, 196, 294, and 392 N for a duration of

30 min. The wear scar diameters (WSD; accuracy:

±0.01 mm) of the three lower test balls were determined

Oil Inlet

Oil OutletGas Inlet

Gas Outlet

Oil Chamber

Oil Chamber

Discharge Gap

Gas Blast Pipe

Electrode Plate

Earth Terminal

High VoltageTransmissionLine Terminal

Fig. 1 The schematic of the plasma reactor

J Am Oil Chem Soc

123

using an optical microscope. In this paper, the average

WSD values of the three lower balls were noted. The

friction coefficients were measured and recorded indirectly

using a friction torque tester. Each test was repeated at least

three times.

Friction Surface Analysis

The original non-wear (NW) and wear surfaces of the steel

balls were analyzed using a PHI-5702 multi-functional

X-ray photoelectron spectroscope (PerkinElmer, USA).

Before conducting the XPS analysis, the upper balls

lubricated by SO and PAIR9 under a load of 294 N for the

duration of 30 min were ultrasonically washed with

petroleum ether. The binding energies and compositions of

C1s, O1s, N1s, and Fe2p on the worn surfaces of the upper

balls were detected by using the Al Ka X-rays as the

excitation source with a pass energy of 29.35 eV. The

binding energy of the contaminated C (C1s: 284.60 eV)

was used as a reference, and the vacuum degree during

testing was maintained lower than 1 9 10-8 Torr.

Results and Discussion

Effect of Reaction Time on the Properties of Oils

Polymerized by Air Plasma

The effect of the reaction time on the characteristics of

polymerized oils was investigated in Table 1, keeping all

other plasma operational parameters constant. The results

revealed the expected effects of the air plasma on the

viscosity and unsaturated degree of SO. The viscosities (at

both 40 and 100 �C) of the polymerized oils increased with

rising reaction time, while the iodine values followed an

opposite trend. All the characteristics of the polymerized

oils were listed in Table 1. Although the VI value of the

polymerized oil (PAIR9), with a reaction time of 9 h,

decreased from 215 to 123, its viscosity at 40 �C increased

from 33.8 to 1136 cSt and that at 100 �C increased from

7.84 to 68.6 cSt. Meanwhile, its iodine value decreased

from 130 to 69.8 g I2/100 g (Table 1). This result indicated

that the opening of the double bonds in SO by the air

plasma caused an increase in the viscosities of polymerized

oils.

The change in the elemental compositions of the poly-

merized oil with the reaction time for polymerization is

shown in Fig. 2. It was noteworthy that the contents of both

C and H decreased with an increase in the reaction time,

while the O and N contents increased with the reaction

time. Furthermore, within the same reaction time, the

increase in the O content of the polymerized oil surpassed

that in the N content. In particular, the O content of PAIR9

increased from 12.65 to 19.07 %, while its N content

reached only 0.52 %. This significant difference between

the elemental increments of the O and N contents can be

attributed to the dissociation energies of O2 and N2. As we

know, N2 possesses a higher dissociation energy (9.8 eV),

but that of O2 is only 5.1 eV [19]. Therefore, in the pre-

sence of air plasma, the O2 molecule can easily participate

in the polymerization process. In other words, it can be

concluded that N2 and O2 in air, which were originally used

in the experiment to function as the reactive gases, par-

ticipated in the plasma polymerization of SO; conse-

quently, nitrogen and oxygen atoms were incorporated into

the molecules of SO during the polymerization process.

0 2 4 6 8 1012.0

13.6

15.2

16.8

18.4

20.0

O atom

Con

tent

s of

oxy

gen

(%)

Reaction time (h)

0 2 4 6 8 1070.0

71.8

73.6

75.4

77.2

79.0

C atom

Con

tent

s of

car

bon

(%)

Reaction time (h)

9.00

9.36

9.72

10.08

10.44

10.80

H atom

Con

tent

s of

hyd

roge

n (%

)

0.00

0.17

0.34

0.51

0.68

0.85

N atom

Con

tent

s of

nitr

ogen

(%

)

Fig. 2 Contents of all elements

of air plasma polymer versus the

reaction time

J Am Oil Chem Soc

123

IR Characterization of Air Plasma Polymerized Oil

The IR spectra of PAIR9 prepared under the air plasma

conditions are compared with those of SO in Fig. 3. It was

shown that the IR spectra of SO exhibited a few peaks at

3,008.8 cm-1 (=C–H stretching vibration), 2,925.6 and

2,854.6 cm-1 (CH2 asymmetric stretching), 1,745.5 cm-1

(C=O triglycerides carbonyl stretching), 1,653.1 cm-1

(C=C stretching vibration), 1,461.0 cm-1 (CH2 bending

vibration), 1,374.5 cm-1 (CH3 symmetrical bending

vibration), and 722.7 cm-1 (CH2 rocking vibrations);

additional peaks were observed at 1,236.7, 1,163.3, and

1,098.8 cm-1 due to the stretching vibrations of the C–O

group in the esters.

However, the two absorption peaks at 1,653.1 (C=C)

and 3,008.8 cm-1 (=C–H) in the IR spectra of PAIR9

exhibited a marked decay, probably because of the opening

of the double bonds in the triacylglycerol molecules.

Meanwhile, the formation of new groups in the polymer-

ized oil, which was induced by the air plasma, was

observed in the IR spectra of PAIR9. The region with the

broad and fairly strong signal at 3,470.7 cm-1 can be

attributed to the N–H stretching vibration. In addition, the

two sharp characteristic peaks at 1,630.8 and 1,552.5 cm-1

can be identified as the N–H bending vibration of the fatty

primary and secondary amines, respectively. The peak at

1,274.1 cm-1 can be assigned to the C–C stretching

vibration of the C–C(=O)–C groups in the aliphatic ketone

molecule. The peak at 862.6 cm-1 can be identified as the

N–H rocking vibration. Therefore, it can be concluded that

the free radicals in the long-chain molecules, which were

formed by the opening of the double bonds under the

plasma conditions, could capture the N2 and O2 molecules

to produce and incorporate ‘‘amine-like’’ and carbonyl

groups into the polymerized oils.

Moreover, in the IR spectra of PAIR9, a small charac-

teristic peak was visible at 972.1 cm-1, which was the

=CH bending vibration of the trans-conjugated system. It

means that under the effect of air plasma, the isolated cis-

double bonds in the hydrocarbon chains of the linolenic

and linoleic acids migrate to the conjugated position and

yield the structure of the conjugated dienes.

Pyrolysis of Air Plasma Polymerized Oil

To additionally clarify the structure of the nitrogen-con-

taining groups, PAIR9 was pyrolyzed by pyrolysis gas

chromatography along with a mass selective detector (Py-

GC/MSD). The nitrogen-containing compounds in the

pyrolysis products of PAIR9 were mainly characterized by

primary and secondary amines, with relative contents

(calculated by normalizing the peak areas) of 4.07 and

2.89 % (Table 2), respectively. These results were almost

consistent with the above explanations regarding the IR

characterization of PAIR9. Meanwhile, minor amounts of

quaternary amines, acid amides and nitrogen heterocyclic

compounds were also observed. Therefore, organic amines

and nitrogen heterocyclic species were identified as the

nitrogen-containing groups.

In addition, 2.64 % cyclic compounds with side chains

were observed in the pyrolysis products of PAIR9

(Table 3). These cyclic compounds were produced by the

Diels–Alder addition between the isolated double bonds

and conjugated dienes.

GPC Analysis

Air plasma polymerized products of SO yielded complex

mixtures that might comprise monomers, dimers, trimers,

and oligomers with higher molecular weights; therefore,

1745

.5

4000 3400 2800 2200 1600 1000 400

722.

7

1098

.811

63.312

36.713

74.5

1461

.0

1653

.1

2854

.629

25.6

3008

.8

SO

Wavenumbers (cm-1)

862.

6

972.

1

1274

.1

1552

.516

30.834

70.7

PAIR 9

Fig. 3 The IR spectra of SO

and PAIR9

J Am Oil Chem Soc

123

GPC was performed on PAIR9 to determine the relative

content of each component and to determine the extent of

plasma polymerization (Fig. 4). Three fractions were sep-

arated from the polymerized product. The molecular

characteristics of these fractions are summarized in

Table 4. Fraction 1, which had a similar molecular weight

and retention time as that of SO, was identified as the

unreacted monomer, which was probably due to the pre-

sence of triglyceride molecules containing fatty acids

without double bonds (palmitic and stearic acids) and with

only one weakly polymerizable double bond (oleic acid).

Apart from the monomer, PAIR9 comprised 17.1 % dimers

(fraction 2) and 44.6 % oligomers (fraction 3), with num-

ber average molecular weights (Mn) of 2,201 and 5,964,

respectively. Obviously, the retention time of the polymer

peak on the GPC chromatogram decreased with increasing

molecular weight of the product. The higher viscosity of

PAIR9 can be attributed to the presence of oligomers with

the broad molecular weight distribution and polydispersity

(Mw/Mn) of 1.54. Therefore, it was concluded that the

presence of air plasma promoted the polymerization pro-

cess of SO.

Tribological Performances

The tribological characteristics of polymerized oils with

different viscosity grades were evaluated using a four-ball

friction and wear tester and the obtained results were com-

pared with those of SO. The PB values of the polymerized

products are shown in Fig. 5. It was interesting to note that all

the products obtained through the treatment of SO by air

plasma exhibited better load-carrying capacities as com-

pared to the untreated SO. Further, the higher the viscosity of

the product, the higher the PB value. Moreover, The PB value

of PAIR9 reached 1,186 N. The difference in the load-car-

rying capacity could be related to the molecular structure and

active elements of the polymerized products under air

plasma conditions. First, the higher viscosity indicated that

the product possessed a longer molecular chain, which could

maintain a sufficiently thick lubricant film on the metal

surface. Moreover, the incorporated O- and N-containing

species were often used as lubricant additives [20, 21] such

that the polymerized oils were able to withstand extreme

pressures and exhibit excellent anti-wear capabilities. It was

known that the polarity of the ester, carbonyl and amines

Table 2 Major peaks of N-containing compounds in pyrolysis mass spectra of PAIR9

GC number Pyrolysis products Formula Relative intensity (% of total)

2/3 NH2 C3H9N 3.25

5 HN

C4H9N 2.73

15 N C6H11N 0.34

33

NO C5H9NO 0.25

52 HN

C4H9N 0.16

55HN

O C7H11NO 0.43

64

O

NH2 O C5H9NO2 0.23

72

H2NOH

O

O C4H5NO3 0.09

73

N NH

N NH2

HN

C3H7N5 0.22

83NH2H2N

C6H14N2 0.59

J Am Oil Chem Soc

123

groups could be arranged in the following order: amine [carbonyl [ ester. Therefore, the presence of the carbonyl

and amine structures with considerably stronger polarities in

the polymerized products could get easily adsorbed on the

metal surface, and react with the metal surface to develop an

additionally durable tribochemical film during the rubbing

process.

The WSD values of the steel balls lubricated with SO,

PAIR7, and PAIR9 as a function of the tested load are shown

in Fig. 6a. It was evident that the WSD values of all these

base stocks increased with increasing tested load; when the

tested load was maintained at a value lower than 350 N, both

PAIR7 and PAIR9 demonstrated much better anti-wear

abilities than that of SO. The WSD values (294 N) of PAIR7

and PAIR9 were only 0.41 and 0.42 mm, respectively, while

that of SO reached 0.57 mm. The O- and N-containing

structures of the polymerized oils played a key role in

improving the anti-wear behavior. However, the anti-wear

abilities of the products deteriorated when the tested load

surpassed 350 N: the WSD values (at 392 N) of PAIR7 and

PAIR9 reached 0.68 and 0.72 mm, respectively, which were

higher than that of SO, namely, 0.60 mm. This can be

attributed to the fact that under the boundary lubrication

conditions, the competitive nature of the adsorption char-

acteristics of the carbonyl and nitrogen-containing groups on

the metal surface might render the boundary lubricating film

unstable and no longer compact.

The friction-reducing properties of these lubricant base

stocks are shown in Fig. 6b. It was observed that the fric-

tion coefficient (f) of SO increased with the applied load,

Table 3 Major peaks of cyclic structures in pyrolysis mass spectra of PAIR9

GC number Pyrolysis products Formula Relative intensity (% of total)

17 O C6H10O 0.22

29 O C7H12O 0.26

42 C8H14 0.21

44 C14H28 0.40

56O

C9H14O 0.20

82 C16H30 0.18

104 HO C10H18O 0.06

106

HO

HO C10H20O2 0.08

100/108/114 C11H20 0.41

110 HO C11H22O 0.23

146 C20H40 0.24

147 C29H48 0.15

J Am Oil Chem Soc

123

while the friction coefficients of both the polymerized oils

followed an opposite trend. At lower loads, both these oils

exhibited higher friction coefficients, which can be attrib-

uted to the higher viscosities of the polymerized oils; this

was not suitable to reduce the energy loss from the lubri-

cation system. However, the friction-reducing perfor-

mances of the polymerized oils were superior to that of SO

at higher loads. In particular, the f294N and f392N of PAIR7

were 0.0682 and 0.0736, respectively. These results indi-

cated that at higher loads, the active elements (O and N) in

the polymer molecules formed an effective tribochemical

adsorption film with low shear strength, thereby resulting

in the lower friction coefficients.

Worn Surface Analysis

To study the lubricating mechanism of the air plasma

polymerized oil comprising carbonyl and N-containing

groups, the XPS analysis of the worn surfaces of the

steel balls lubricated by PAIR9 and SO was carried out

and the obtained results were compared with the original

NW surface of the steel ball. The binding energies of

C1s, O1s, Fe2p, and N1s on the worn surface are shown in

Fig. 7. It was observed that the XPS spectra of C1s and

Fe2p exhibited peaks at the binding energies of 284.8,

707.3, and 710.3 eV, which were attributed to the pre-

sence of C*–C, iron (Fe), and Fe2O3 [22–24], respec-

tively. The above facts indicated during the frictional

process, both SO and PAIR9 adsorbed on the metal

surface to form lubricant films with certain thicknesses.

Meanwhile, the relative concentrations of the C atom on

the worn surface lubricated by SO and PAIR9 increased

from 10.6 to 44.1 % and 86.4 %, respectively, while the

relative concentrations of Fe on the worn surface of SO

and PAIR9 decreased from 39.2 to 28.9 % and 3.20 %

(Table 5), respectively. Obviously, the film formed using

PAIR9 was sufficiently thick to cover the metal surface

for reducing the consequent wear; this advantage can be

attributed to the longer hydrocarbon chain of the PAIR9

molecules.

The O1s spectra of the original NW surface exhibited a

peak at 530.2 eV; this peak was present in the binding

energy range of a metal oxide (529.8–530.2 eV) [25], such

as those of Fe2O3 and Fe3O4. Therefore, this peak can be

attributed to iron oxides. However, it was obvious that the

intensities of O1s peaks on the worn surfaces lubricated by

both SO and PAIR9 decreased. The binding energy

(531.4 eV) of the distinctive peak in the O1s spectra of SO

0.0 6.6 13.2 19.8 26.4 33.0-30

-15

0

15

30

45

SO PAIR9

Res

pons

e

Minutes

Monomer

Fraction 1

Fraction 2

Fraction 3

Fig. 4 GPC chromatogram of SO and PAIR9

Table 4 The molecular weight distribution of SO and PAIR9

Sample Distribution Mna Mw

b Polydispersityc Relative composition (%)

SO Monomer 1,396 1,440 1.03 100

Fraction 1 1,051 1,093 1.04 38.3

PAIR9 Fraction 2 2,201 2,243 1.02 17.1

Fraction 3 5,964 9,158 1.54 44.6

a Number average molecular weightb Weight average molecular weightc Polydispersity: Mw/Mn

SO PAIR1 PAIR3 PAIR5 PAIR7 PAIR90

280

560

840

1120

1400

1186

1049

882.0803.6

774.2

Base stocks

Loa

d-ca

rryi

ng c

apac

ity (

P B v

alue

, N)

646.8

Fig. 5 The load-carrying capacities of air plasma polymers with

different viscosity

J Am Oil Chem Soc

123

75 145 215 285 355 4250.065

0.074

0.083

0.092

0.101

0.110 SO PAIR7 PAIR9

Fric

tion

coef

fici

ent

Load (N)

75 145 215 285 355 4250.20

0.32

0.44

0.56

0.68

0.80

(b)

SO PAIR7 PAIR9

Wea

r sc

ar d

iam

eter

(m

m)

Load (N)

(a)Fig. 6 The wear and friction

properties of SO, PAIR7 and

PAIR9

300 296 292 288 284 2800

600

1200

1800

2400

3000

(c)

(b)

284.8 C1s

Inte

nsity

Binding energy (eV)

(a)

(a)

(b)

(c)

740 732 724 716 708 7001500

2900

4300

5700

7100

8500

(c)

Fe2p

Inte

nsity

Binding energy (eV)

710.3

707.3(a)

(b)

(c)

545 541 537 533 529 5250

1200

2400

3600

4800

6000

O1s

Inte

nsity

Binding energy (eV)

532.3531.4

529.9

414.0 409.8 405.6 401.4 397.2 393.0375

387

399

411

423

435

N1s

Inte

nsity

Binding energy (eV)

399.8

398.5

Fig. 7 XPS spectra of typical elements in tribofilms: a NW, b SO, c PAIR9

J Am Oil Chem Soc

123

did not exist in the range of that of a purely organic oxygen

(532.0–533.9 eV) [26] and was consistent with the O1s

binding energy of metal complexes, such as Fe(1-H-2,4-

pentanedione)3 (531.7 eV) and Fe(1-phenyl-1,3-butanedi-

onate)3 (531.6 eV) [27, 28]; this indicated that the oxygen

in SO and Fe combined together in a coordination bond

similar to the aforementioned two metal complexes.

Therefore, it can be concluded that the breakdown of SO

molecules during the tribochemical process resulted in the

formation of fatty acids, which could interact with the

metallic iron to produce iron carboxylate complexes.

However, iron carboxylate complexes were not observed in

the O1s spectra of PAIR9, and only one peak corresponding

to O*=C–O or O*=C appeared at 532.3 eV. The results

suggested that the O-containing species in PAIR9 mole-

cules exist only on the worn surface in the adsorbed forms.

In addition, the binding energies of N1s on the worn sur-

face of PAIR9 were in two forms with the binding energies of

398.5 and 399.8 eV: the binding energy of 398.5 eV was

assigned to the adsorbed forms of organic amine groups in

the PAIR9 molecules, whereas the binding energy of

399.8 eV was identified to exist because of the presence of

organic nitrogen complex compounds of metallic iron with

N-containing compounds [29, 30]. The relative concentra-

tion of the N atoms on the worn surface lubricated by PAIR9

also reached approximately 1.27 % (Table 5). In other

words, the N-containing species of the air plasma polymer-

ized oil could adsorb onto the metal surface prior to the

O-containing species. Simultaneously, the N atoms with the

lone pair electrons in the organic amine and nitrogen het-

erocyclic structures of PAIR9 possessed a strong coordinate

capacity, and these electrons could combine with the empty

orbital of the metal Fe to form a stable and compact coor-

dinated complex film on the metal surface during the rubbing

process. The insights were consistent with those concluded

by previous studies [31, 32].

Conclusions

The effects of air plasma on the physicochemical prop-

erties and the structure of SO were investigated. It was

proven that the air plasma could not only increase the

molecular weight to yield polymers with higher viscos-

ities by opening the double bonds of SO but also assist

in the incorporation of the carbonyl, organic amine and

nitrogen heterocyclic functional groups with stronger

polarities into the molecules of the obtained polymerized

oil. The tribological tests indicated that the polymerized

oil displayed significantly improved extreme pressure

properties and anti-wear characteristics. The results of

the XPS analysis proved that a majority of the oxygen-

containing groups in the plasma polymer, such as esters

or carbonyl groups, existed in the adsorbed form in the

boundary film. Meanwhile, the nitrogen-containing

structures with the lone pair electrons could combine

with the metal Fe to form complex films containing

organic nitrogen on the metal surface. The synergistic

effect of the carbonyl and nitrogen-containing groups in

the plasma polymer promoted the effective covering of

the inner iron oxide by the chemical adsorbed film,

thereby reducing the wear on the metal surface.

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