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Applied Surface Science 258 (2012) 1016810174

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

journal homepage: www.elsevier .com/ locate /apsusc

Surface treatment ofpara-aramid fiber by argon dielectric barrier discharge

plasma at atmospheric pressure

Ruxi Gu a, Junrong Yu a,, Chengcheng Hu a, Lei Chen a,Jing Zhu a, Zuming Hu b

a State KeyLaboratory forModification of Chemical Fibers and Polymer Materials, Shanghai 201620, PR Chinab Key Laboratory of High-performance Fibers& Products,Ministry of Education, Donghua University, Shanghai 201620, PR China

a r t i c l e i n f o

Article history:

Received27 April 2012Receivedin revised form 26 June 2012

Accepted 27 June 2012

Available online 11 July 2012

Keywords:

Para-aramid fiber

Dielectric barrier discharge

Argon plasma

Adhesive performance

Wettability

a b s t r a c t

This paper is focused on influence ofargon dielectric barrier discharge (DBD) plasma onthe adhesive per-

formance and wettability ofpara-aramid fibers and three parameters including treated power, exposure

time and argon flux were detected. The interfacial shear strength (IFSS) was greatly increased by 28%with

300W, 60s, 2 Lmin1 argon flux plasma treatment. The content ofoxygen atom and oxygen-containing

polar functional groups were enhanced after the argon plasma treated, soas the surface roughness, which

contributed to the improvement ofsurface wettability and the decrease ofcontact angle with water. How-

ever, long-time exposure, exorbitant power or overlarge argon flux could partly destroy the prior effects

ofthe treatment and damage the mechanical properties offibers to some degree.

2012 Elsevier B.V. All rights reserved.

1. Introduction

Para-aramid fiber is a compound linear chain macromoleculematerial which has high crystallization and high orientation [1].

As it has the properties of low density, high strength, good tough-

ness, excellent thermal-resistance, chemical corrosion-resistance

andgood impact resistance,para-aramidfiber has beenwidely used

as an reinforce composite material in the fields of aviation, auto-

mobile, shipbuilding [2,3]. However, owing to the smooth surface

and chemical inertness,the fibers interfacial adhesive performance

with the resin is limited and the wettability properties of the fiber

are low, which are to disadvantage of its applying to the compos-

ite materials. So the surface modification of the para-aramid fiber

becomes a must [4].

The common methods of surface modification on the para-

aramid fiber are chemical and physical methods [57]. Chemical

methods include chemical etching, chemical surface grafting andpolymerization modification, which could efficiently enhance the

interfacial adhesive performance of the fiber and the effects are

highly durable. In spite of these merits, chemical treatments may

destroy the mechanical properties and produce large amount of

waste water and organic solvents, which leads the production pro-

cess environmentally unfriendly. In contrast, physical treatment

Correspondingauthor. Tel.: +86 21 6779 2945.

E-mail address: yjr@dhu.edu.cn (J.Yu).

would be more advocated. Low-temperature plasma modification

technique is one of the most important physical methods [8,9].

Low-temperature plasma technique has lots of discharge meth-ods, such as, direct current glow discharges [10], corona discharges

[1113], and radiofrequency glow discharges [1416]. In recent

years, dielectric barrier discharges (DBD) was more and more

favored with the advantages of high efficiency, flexible and envi-

ronmentally friendly. In contrast to other discharge methods, the

DBD technique could be used under atmosphere pressure without

the vacuum device, and maintain the possible of the large-scale

industrialization [17,18].

DBD treatment is the interaction of the oxidation and etching

effects. On the one hand, the bombardment of the atom, elec-

tron, UV radiation onto the fiber surface induces some new polar

functional groups; on the other hand, the plasmas are etching the

surface of fiber,making thesurface more rough to enhance thecon-

tact area of the fiber with resin. With the effects of oxidation andetching, the wettability and adhesive properties have been greatly

improved [19,20].

Despite of the fact that DBD surface modification technique has

been regarded as the most potential method to the para-aramid

fiber,the related reports arerelatively rare recently.Jia et al.[21,22]

had employed an air DBD plasma at atmospheric pressure for

Twaron and Armos fibers, the results revealed that the content of

polar functional groups such as C O,C O,O C O was obviously

increased. According to theprevious articles, theDBD plasmamod-

ification were all under the air atmosphere, the effects with the

argon atmosphere had not been reported. However, Liu et al. [23]

0169-4332/$ see front matter 2012 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.apsusc.2012.06.100

http://localhost/var/www/apps/conversion/tmp/scratch_3/dx.doi.org/10.1016/j.apsusc.2012.06.100http://localhost/var/www/apps/conversion/tmp/scratch_3/dx.doi.org/10.1016/j.apsusc.2012.06.100http://www.sciencedirect.com/science/journal/01694332http://www.elsevier.com/locate/apsuscmailto:yjr@dhu.edu.cnhttp://localhost/var/www/apps/conversion/tmp/scratch_3/dx.doi.org/10.1016/j.apsusc.2012.06.100http://localhost/var/www/apps/conversion/tmp/scratch_3/dx.doi.org/10.1016/j.apsusc.2012.06.100mailto:yjr@dhu.edu.cnhttp://www.elsevier.com/locate/apsuschttp://www.sciencedirect.com/science/journal/01694332http://localhost/var/www/apps/conversion/tmp/scratch_3/dx.doi.org/10.1016/j.apsusc.2012.06.100

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R. Gu et al. / Applied Surface Science258 (2012) 1016810174 10169

Fig. 1. Schematic of the argon DBD plasma apparatus.

usedtheargonDBDplasmatotreatonthePBOfiber,whichacquired

a more obviously effect in comparison to air, with a 40% improve-

ment of the interlinear shear strength (ILSS). In this article, theargon DBD plasma technique was used to modify Kevlar fiber and

the optimum treated parameters were obtained.

In addition, this article elaborately studied the effects of several

parameters, including treatment time, power and argon flux, and

usedthe interfacial shearstrength (IFSS) to characterize the interfa-

cial adhesive performance. The surface chemical composition was

examined by X-ray photoelectron spectroscopy (XPS). The surface

morphology and roughness were analyzed by scanning electron

microscopy (SEM) and atomic force microscopy (AFM); the wet-

tability of the fiber surface was inspected by the contact angle

measurements in this paper.

2. Experimental

2.1. Materials

Kevlar29 aramidfibers were supplied by DuPont Company with

the 1.5 cNdtex1 of average single filament. The samples were

cleaned in hot water bath at 100 C for 2h and ultrasound device

for 1h then dried in the room temperature for 3 days. The matrix

used in this paper was epoxy resin prepared with E-51 (A type of

epoxy), epoxy curing agent (No. 593) and acetone at the ratio of

10:3:2, all provided by Shanghai resin Co. Ltd., China.

2.2. Argon DBD plasma treatment

The DBD plasma device was a patent invited by Donghua Uni-

versity. The schematic diagram of the DBD apparatus is displayed

in Fig. 1. Kevlar29 fiber samples were treated by going through the

argon DBD plasma part under atmospheric pressure with different

experimental conditions. The treated powers of the DBD plasmawere 100400W and the treated time was at an interval of 30s

from30s to120s. The flux of argon was changed from 1 L min1 to

4Lmin1.

2.3. Interfacial shear strength

The interfacial shear strength (IFSS) was measured by micro-

bond test: the epoxy resin beads were first treated on the single

filament, then the samples were dried for over 36 h at 40 C.

The diameters of the fibers (D) and the embedded lengths (L)

were metered by the Olympus CH-2 microscope equipped with a

Panasonic WV-GP410/A digital photomicrography system. And the

interfacial shear peak load (F) was measured by XQ-1 fiber tensile

testing machine (Shanghai Lipu Research Institute, China).The IFSS (the schematic diagram was shown in Fig. 2) was cal-

culated by the following equation. In this paper, every samples had

been acquired the average value through over 20 times tests.

IFSS =F

DL

2.4. Contact angle measurements

The wettability of aramid fiber was measured by the water con-

tact angle (using the sessile drop method). The test was performed

by the OCA40 Contact Angle system (Data physics Instrument pro-

duced by Filderstadt, Germany). A small droplet of distilled water

(about 50L) was dropped onto the fiber with a matched syringe.

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10170 R.Gu et al. / Applied Surface Science258 (2012) 1016810174

Fig. 2. The schematic diagram of microdebondingtest.

A Nikon video camera was used to play the photograph of the pro-

cess and then the immediately contact angle was calculated with

SAC 20 software.

2.5. XPS analysis

The X-rayphotoelectron spectroscope (XPS) was detected using

PHI 5000 Versaprobe (ULVAC-PHI, Japan) equipped with an Al K

X-ray source. The base pressure in the chamber was about 2e8 Pa

and the energy of X-ray source was 40W at 15kV. Spectra were

acquired at a take-off angle of 90 and the non-linear least squares

fitting program was used for curving fitting of C1s spectra.

2.6. Surface morphology

Scanning electron microscope (SEM, JSW-5600LV, JEOL, Japan)

was selected to observe the surface morphologies of the Kevlar

fiber. The samples were jetted with gold onto the surface and the

magnification was set at 5000. Atomic force microscopy (AFM,

NanoScopeIV, Veeco,U.S.A.) wasusedto analyze thesurface rough-

ness and morphology of Kevlar fibers quantitatively. The images

with a 4m4m scan area were obtained under the tappingmode. The roughness of fiber surface was characterized by mean

square roughness (Rq) and arithmetic mean roughness (Ra) calcu-

lated automatically from Eqs. (1) and (2) by the software.

Rq =

1N2

Ni=1

Nj=1

(Zij Zav)2 (1)

Ra =1

N

Ni=1

Nj=1

|Zij Zcp| (2)

where N is the number of data points in the image, i and j are the

pixel locations on the AFM image,Zij is the height value at i and j

locations,Zavis the average height value within the given area andZcp is the height value from the center plane [24].

3. Results and discussion

3.1. Influence of argon plasma treatment on the IFSS

The treated power, exposure time and argon flux were

three important parameters that affected the adhesive properties

between fibersand resin.Under the mostsuitabletreatment power,

argon flux and exposure time, the IFSS between the fibermatrix

system could be significantly improved by producing the polar

functional groups and increasing roughness of the fibers.

Fig. 3 shows the values of IFSS for treated samples with chang-

ing treated power, time and argon flux. It can be observed that

Fig. 3. IFSS for untreated and treated Kevlar29 fibers at different treatment power,

time and argonflux.

the adhesion between aramid fiber and epoxy resin was obviously

improved after the plasma treatment. From Fig. 3a, the samples

were treated for 60 at 2 L min1 argon flux and when the power

was 300W, the best IFSS was acquired with a 28% improvement.

Under themostsuitable treated power of 300 W andfixedthe expo-

sure time at 60s, the IFSS changing with argon flux was shown in

Fig. 3b, the result showed that the optimum flux was 2 Lmin1. In

Fig. 3c, the treated power and argon flux were selected as 300 W

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10172 R.Gu et al. / Applied Surface Science258 (2012) 1016810174

Fig. 5. C1s spectra of Kevlarfibers: (a)untreated fibers,(b) treated for30 s, (c)treated for60 s, (d) treated for90 s.

content of carbon atoms and low contents of oxygen and nitrogen

atoms. However, after the argon plasma treated, the concentra-

tions of polar atoms increased. The O and N percentage on the

surface of untreated Kevlar fibers were 13.63% and 3.71%, and they

increased obviously after plasma treated. The atomic ratio of O/C

was improved from 16.49% to 39.70% after 60s argon DBD plasmatreated, then become lower with longer treatment. The same trend

had also shown with the N/C atomic ratio (Fig. 5).

The deconvolution analysis of C1s peaks was performed in

Table 2 and Fig. 5 to measure thechangeof functional groupsquan-

titatively. There are fourkindsof carbonstates: C C at284.8eV,

C N/C O at 286.3 eV, CONH at 287.7 eV and COOH at

289.0 eV. As shown in Table 3, the C C concentration decreased

sharply from 86.60% to 61.79% after 60s argon DBD plasma treat-

ment, while the percentages of the polar functional groups such

as C N/C O , CONH , COO had a dramatic increase from

10.72%, 1.35%, 1.34% to 26.88%, 5.08% and 6.24%, respectively.

Besides, it can be found that the C O / C N and COO con-

centration have an obviouslydrop after 90s treatmentas a resultof

theover etching effects, which agrees with thevariations of surfaceelement concentrations.

The results illustratedthat the argonplasmaprocess couldintro-

duce some new polar functional groups including C O/C N ,

CONH , and COO onto the surface of the fibers, which can

enhance the wettability and adhesive performance of aramid fibers.

The reason [25] was that the argon plasma composed of lots of

Table 3

Root mean square roughness (Rq) and arithmetic mean roughness (Ra) of Kevlar

fibers.

Fiber sample Rq (nm) Ra(nm)

(a) Untreated 197.56 171.07

(b) Treated for300 W, 60s, 2L min1 271.30 230.14

non-reactive particlescomparingwith oxygen and nitrogen plasma

and the energies were transferred by those particles to the fiber

surface, activating the surface layer by making the bombardment

of ions, electrons and UV radiation. Nevertheless, there existed an

optimum treated time as the longer treatment maybe etch the

formerly generated concave and convex and smooth the surfaceroughness, which probably result in the decrease in the polar func-

tional groups. The study of XPS analysis was corresponded to the

results of IFSS and contact angle measurement.

3.4. Influence of argon plasma treatment on surface roughness

According to the micro-bond test and contact angle measure-

ment, the optimum treatment condition was selected as 300W,

60s and 2L min1 argon flux. In order to detect the influence of the

argon plasmaon thesurface roughness, theSEM andAFM test were

applied on the original Kevlar29 fiber and the plasma treated fiber

at the optimum condition. Figs. 6 and 7 were the contrast of the

two samples of SEM and AFM, respectively.Thesurface morphology ofKevlar fiber wasinvestigated bySEM.

The comparison of the SEM images of the original and the argon

DBD treated fibers at 300W, 60 s and 2 L m in1 argon flux were

shown in Fig. 6. It can be obviously found that the fiber becomes

uneven and quite different in terms of the surface morphology

after plasma treatment. In Fig. 6a, the untreated fiber was clean

and smooth, but after plasma treated, a lot of apparent bulges and

ruts were introduced onto the fiber surface, which can be clearly

observed in Fig. 6b.

The AFM was used to investigate the surface roughness of the

Kevlar29 fibers before and after argon plasma treatment. In Fig. 7,

remarkable difference can be observed in the micrographs, which

could prove that theDBD plasmatreatment would changethe mor-

phology of the surface on a nanometer level.

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R. Gu et al. / Applied Surface Science258 (2012) 1016810174 10173

Fig. 6. Surface morphology of Kevlar29: (a)untreated and (b) plasmatreated argonplasma at 300W, 60s and 2L min1 argon flux.

Fig. 7. AFMimages of Kevlarfibers: (a)untreated; (b)plasma treated for 300W, 60s,2 Lmin1 argon flux.

The results of roughness were analyzed with the root mean

square roughness (Rq) and arithmetic mean roughness (Ra) whichwere summarized in Table 3. As shown in Table 3, the orig-

inal fiber had a relative lower Rq and Ra value of 197.56nm

and 171.07nm, respectively. After the argon plasma treatment,

the fiber roughness had a dramatically enhancement, with

the Rq and Ra value increasing to 273.10nm and 230.14nm,

respectively.

As a consequence of theincreasing roughnesson thesurface,the

interfacial area between the fiber and matrix would be expanded,

thereby devoted to more mechanical interlocking, which corre-

sponded to the results increase of IFSS.

4. Conclusion

The experimental results presented that the argon DBD plasma

at atmospheric pressure can enhance the surface adhesive perfor-

mance and wettability of Kevlar fiber. In this study, the plasma

treated power, exposure time and argon flux were detected and all

of thethreeparameters hada huge influence on modifyingthe fiber

surface properties. With the argon DBD plasma treatment, the IFSS

and wettability of Kevlar fiber would be greatly improved, the con-

tents of oxygen and oxygen containing polar functional groups on

the surface of Kevlar fibers were increased. Moreover, the surface

of argon plasma treated fibers became much rougher.

The results illustrated that plasma treated fibers would acquire

optimal interfacial properties when the treated power was 300W,

the exposure time was 60s and the argon flux was 2Lmin1.

Acknowledgements

This work was financially supported by China National 973

Project (No. 2011CB606103), the Fundamental Research Funds for

the Central Universities (No. 11D10625) and Shanghai Leading Aca-

demic Discipline Project (No. B603).

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