Improvement of Hydrophilicity of Polypropylene Film by Dielectric Barrier Discharge Generated in Air at Atmospheric Pressure
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Improvement of Hydrophilicity of
Polypropylene Film by Dielectric
Barrier Discharge Generated in
Air at Atmospheric Pressure
Rajesh Prakash Guragain1*, Hom Bahadur Baniya1,2†, Santosh Dhungana1,
Ganesh Kuwar Chhetri1, Saurav Gautam1, Bishnu Prasad Pandey3,
Ujjwal Man Joshi1 and Deepak Prasad Subedi1
1
Department of Physics, School of Science, Kathmandu University, Dhulikhel, Nepal
2
Department of Physics, Tri-Chandra Multiple Campus, Tribhuvan University, Kathmandu, Nepal
3
Department of Chemical Science and Engineering, Kathmandu University, Dhulikhel, Nepal
Abstract The industrial use of polypropylene (PP) films is constrained due to their
unattractive properties like poor wettability, printability and adhesion.
In the present study, a dielectric barrier discharge (DBD) is employed to
enhance the surface properties of PP. A sinusoidal power supply with
discharge voltage of 13 kV (r.m.s), and frequency of 50 Hz was used for
the generation of the discharge. The change in wettability of treated PP
film surface was measured in terms of water contact angle (WCA). In
addition, changes in morphological composition of control and treated films
were inspected by scanning electron microscopy (SEM) and atomic force
microscopy (AFM). SEM and AFM observations on the polymer samples
showed increment in roughness of the surface due to DBD operation. After
the plasma treatment, WCA was found to change from 90.8° ± 1.8° to 73.6° ±
0.9°, which indicated that the surface had changed to a hydrophilic state
caused by an increase in the surface roughness and incorporation of polar
functional groups into PP surface.
*Corresponding author: rayessprakash@gmail.com
†
Corresponding author: hombaniya@gmail.com
DOI: 10.7569/RAA.2021.097303 CC BY-NC- Creative Commons Attribution License
This license allows users to copy, distribute and transmit an article,
adapt the article as long as the author is attributed. The CCBY
license permits commercial and non-commercial reuse. © 2021 by
Rajesh Prakash Guragain, Hom Bahadur Baniya, Santosh Dhungana,
Ganesh Kuwar Chhetri, Saurav Gautam, Bishnu Prasad Pandey,
Rev. Adhesion Adhesives, Vol. 9, Ujjwal Man Joshi, and Deepak Prasad Subedi. This work is published
No. 1, March 2021 and licensed by Scrivener Publishing LLC. 153
Rajesh Prakash Guragain and et al.: Improvement of Hydrophilicity of Polypropylene Film
Keywords: Dielectric barrier discharge (DBD), polypropylene (PP), optical emission
spectra (OES), water contact angle (WCA), surface free energy (SFE), surface
morphology, surface modification
1 Introduction
Polypropylene (PP) is a thermoplastic material having high dielectric strength,
and good heat, fatigue and resistance to chemicals. It is extensively used in man-
ufacturing industries due to its useful properties like superb mechanical strength
[1], being lightweight, low manufacturing cost and high recyclability [2, 3]. Being
partially crystalline, it is translucent in nature, which makes it very useful in fields
that are related to the manufacture of products like food containers, textiles, pack-
aging, and surgical implants [4]. However, PP being a highly non-polar material
[5] is known for its innately hydrophobic nature and inherently low surface free
energy [6]. These properties make PP difficult to adhere to other materials and,
therefore, limit it from being used in industries where painting, coating, bonding
and metallization on the polymer surface are required. There are several ways to
combat this limitation and to promote the surface properties like wettability, adhe-
sion, barrier properties and dyeability, for example, by grafting polar monomers
onto the substrate, surface oxidation and DBD treatment [7–9].
Non-thermal plasmas usually generated at room temperature [10] can widely
be used for the treatment of polymer surfaces [11]. Treating polymer surface with
non-thermal plasmas only alters the surface characteristics of treated materials
without modifying their bulk properties [12, 13].
Although the ion temperature in a non-thermal plasma is barely above room tem-
perature, the electrons are very energetic [14] and are responsible for the alteration of
the surface properties of material being treated. Polymers have low surface free energy
due to the lack of polar groups in the external surfaces. Treating polymer surfaces with
non-thermal plasmas increases their surface free energy by incorporating polar func-
tional groups such as carbonyl (C = O) and carboxyl (-COOH) groups [15, 16].
DBD reactors operating at kilohertz frequencies are more commonly used for
surface modification applications as compared to those operating at 50 Hz as the
plasmas generated at higher frequencies are more stable than those generated at
lower frequencies [17]. But generating plasmas at higher frequencies requires an
additional high frequency converting device. One of the goals of our research is
to demonstrate that surface modification of polymers can still be done using the
plasmas generated at 50 Hz. This should help in minimizing the cost related to
additional frequency converting device.
In the present investigation, a non-thermal plasma generated by an ac source
operating at 50 Hz was used to modify PP surface. The surface properties of
treated materials/samples were studied by contact angle and surface free energy
determinations. It was found that both contact angle and surface free energy of the
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Rajesh Prakash Guragain and et al.: Improvement of Hydrophilicity of Polypropylene Film
material changed after being treated with the non-thermal plasmas. In addition,
results of SEM and AFM analyses showed that DBD treatment causes an incre-
ment in roughness of the polymer surface.
2 Materials and Methods
The typical laboratory arrangement and the photograph of discharge used in the
present study are shown in Figure 1. The entire reactor system is fixed on a mov-
able table. The reactor consists of a transparent polycarbonate cylinder of height
10 cm, diameter 10 cm and 0.5 cm thickness. The edges of the cylinder are smoothed
using the lathe machine. An orifice is made on the cylinder and a fiber optic cable
is inserted and sealed. The fiber optic helps to collect signal from the discharge and
send it to an optical emission spectrometer. Both the upper and bottom electrodes
are made of brass (5.1 cm × 5.1 cm × 1.0 cm). A polycarbonate (PC) sheet of 2 mm
thickness is inserted between the two electrodes which serves as the dielectric bar-
rier. The reactor consists of two pipes.
One of the pipes is connected to a vacuum pump while the other is connected to
the analogue pressure gauge. The reactor is designed in such a way that it can be
made to operate at both atmospheric and reduced pressures. The inter-electrode
separation was maintained at 3.5 mm for all treatments. A high voltage probe
(PINTEX HVP-28HF) was employed to estimate the discharge voltage. Similarly,
a voltage probe was fed across 10 kΩ shunt resistors for the estimation of current.
The current and voltage waveforms were monitored and analyzed using a digital
oscilloscope (Tektronix TDS 2002, 60MHz). Spectrometer from Ocean Optics, Inc.
(USB 2000+) was used for the measurement of emission spectra. In this work, the
operating ac supply was kept at 13 kV (r.m.s).
8 12
9
6 5 13
1 3
16
2
14
7
4 11
10
15
(1,2) Electrodes, (3) Dielectric Sheet (PC), (4) Ballast Resistor, (5) Shunt Resistor, (6) High
Voltage Probe, (7) Current Probe, (8) Oscilloscope, (9) Reaction Chamber, (10) Vacuum
Pump, (11,12) Pipes, (13) Pressure Gauge, (14) High Voltage Transformer, (15) Ground,
(16) Computer Interface
Figure 1 Experimental arrangement (left) and photograph of discharge (right).
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Rajesh Prakash Guragain and et al.: Improvement of Hydrophilicity of Polypropylene Film
Before DBD treatment, PP films of dimensions (51 mm × 16 mm × 0.05 mm), pro-
vided by Goodfellow, U.K., were washed in double distilled water for 30 minutes
in an ultrasonicator. To get rid of the organic contaminants, the films were washed
by soaking in isopropyl alcohol for 15 min and then dried at ambient conditions.
The contact angles were acquired by the sessile drop technique on a Rame-Hart
goniometer, model 200 using DROP Image software. The measurements were
done at ambient temperature. The contact angle assessments were performed
within 20 min after the treatment. A minimum of five drops were placed at differ-
ent positions on the PP surface and the mean value of the contact angles was taken.
The maximum error in contact angle determination did not exceed 2°. A scanning
electron microscope (JEOL JSM-7001F) was used for the study of surface morphol-
ogy. An atomic force microscope; (FlexAFM, Nanosurf AG) was employed for the
estimation of surface roughness.
3 Results and Discussion
3.1 Electrical Characterization
Figure 2 illustrates typical current and voltage waveforms of DBD generated in air
with electrode gap 3.5 mm, applied voltage 13 kV (r.m.s) and ballast resistance 20 MΩ
at atmospheric conditions. The discharge current peaks appear on increasing and
decreasing the applied voltage. The current peaks on the increasing part correspond
to positive polarity where some of the charges accumulate on the dielectric barrier.
Current density J can simply be obtained by taking the ratio of discharge cur-
rent I and the cross-sectional area of the plasma A.
20 Applied voltage Discharge Current
40
15
30
10
Discharge current (mA)
Applied voltage (kV)
5 20
0 10
-5 0
-10
-10
-15
-20
-20
-0.06 -0.04 -0.02 0.00 0.02 0.04 0.06
Time (sec)
Figure 2 Typical current and voltage waveforms for 50Hz DBD.
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Rajesh Prakash Guragain and et al.: Improvement of Hydrophilicity of Polypropylene Film
I
J= (1)
A
The electron density ne can be estimated as [18]:
J
ne = (2)
e µe E
In the present case, the cross-sectional area of the electrodes = 20.41 cm2, dis-
charge current = 10.60 mA (r.m.s), applied voltage = 13 kV (r.m.s), inter-electrode
distance (d) = 0.35 cm, electronic charge (e) = 1.6 × 10−19 C and electron mobility,
µe = 552 cm2/Vs [19]. Here, E is the electric field in the discharge region. Using
these values in Eq. (2), the electron density turned out to be 1.59 × 108 per cm3.
Discharge power is determined by the integration of instantaneous voltage V(t)
and current I(t) [20] as:
T
∫
Discharge power, P(w ) = f V (t )I (t )dt
0
(3)
Where f stands for the frequency and T is the period of the cycle. Using Eq. (3),
the power consumed is found to be 24.3 W per cycle.
Figure 3 shows the charge(Q)-voltage(V) plot, also called Lissajous figure, at
atmospheric conditions. The energy dissipated per cycle was found to be 0.4184 mJ.
3.2 Optical Characterization
Figure 4 illustrates the discharge spectrum at atmospheric pressure conditions.
From the discharge spectrum, two lines of NI (413.76 nm, 439.24 nm) and two of
N II (411.10 nm, 437.92 nm) were taken.
Line intensity ratio method was employed for estimating the electron tempera-
ture [21, 22].
R1 I1 /I2 Apq g p λrs Auv g u λxy E p − Er − Ex + Ev
= = exp −
R2 I 3 /I2 Ars g r λ pq Axy g x λuv kTe
(4)
In Eq. (4), R and I are the ratio and intensity of the spectral lines, Aji, gi, λ, Ei are
the transition probability, statistical weight, wavelength and energy of the spectral
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Rajesh Prakash Guragain and et al.: Improvement of Hydrophilicity of Polypropylene Film
0.06
0.05
0.04
Charge (µC)
0.03
0.02
0.01
0.00
-0.01
-20 -15 -10 -5 0 5 10 15 20
Applied Voltage (kV)
Figure 3 Charge(Q)-Voltage(V) plot of the discharge at a frequency of 50 Hz.
405.26
398.96
4000
3000
Intensity (au)
426.32
433.32
2000
420.05
449.04
411.10
413.76
437.92
439.24
447.71
1000
400 410 420 430 440 450
Wavelength (nm)
Figure 4 Spectrum of the discharge at a frequency of 50 Hz.
lines, respectively. These values are taken from NIST database [23]. Also, k rep-
resents the Boltzmann constant and λ and I are taken from the discharge spectrum
to estimate the electron temperature (Te).
The ratios of spectral lines (R1/R2) for various electron temperatures (Te) are
presented in Table 1.
From Figure 5, the electron temperature is estimated to be 1.01 eV (1 eV = 11600 K).
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Rajesh Prakash Guragain and et al.: Improvement of Hydrophilicity of Polypropylene Film
TABLE 1 Ratios of spectral lines for various electron temperatures.
Electron Temperature (Te) (eV) Ratio of Spectral lines (R1/R2)
0.8 2.00
0.9 1.05
1.0 0.63
1.1 0.41
1.2 0.29
1.3 0.22
1.4 0.17
1.5 0.13
1.6 0.11
1.7 0.09
1.8 0.08
1.9 0.07
2.0 0.06
Te=1.01eV
2.0
1.5
R1/R2
1.0
0.5
0.0 1.01eV
0.8 1.0 1.2 1.4 1.6 1.8 2.0
Te(eV)
Figure 5 Plot of R1/R2 as a function of Te.
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Rajesh Prakash Guragain and et al.: Improvement of Hydrophilicity of Polypropylene Film
3.3 Polymer Wettability
Wettability of a polymeric film is enhanced with the existence of hydrophilic
groups on its surface [8, 17]. There are various approaches to determine the surface
free energy of the solids from measured contact angles [24, 25] but here we have
adopted the commonly used Owens-Wendt approach [26].
For two liquids j and k,
1 1
(
γ lj (1 + cosθ j = 2 γ ljdγ sd ) 2
(
+ 2 γ ljpγ sp ) 2
(5)
1 1
d d 2 p p 2
γ lk (1 + cosθ k ) = 2(γ γ ) + 2(γ γ )
lk s lk s (6)
Here we have used water and glycerol as test liquids. Their surface tension and
its components values are given in Table 2.
Using the values of surface tension as well as polar and dispersion components
of test liquids, γ sd and γ sp can be determined by solving (5) and (6). Total surface
free energy is obtained by adding these two components.
Figure 6 demonstrates the images of water drops on control and PP film treated
for 10 seconds.
Figure 7 shows WCAs on DBD-treated PP samples for various treatment times.
The effect of treatment time on wettability was examined utilizing water and gly-
cerol as test liquids on the surface of PP. Here, the WCA on control sample of
PP was found to be 90.8°. After 10 sec of DBD treatment, the WCA decreased to
73.6° as depicted in Figure 6 exhibiting that DBD treatment can be used for the
TABLE 2 Surface tension of the test liquids.
Liquid Total (mN/m) Dispersion (mN/m) Polar (mN/m)
Water 72.8 21.8 51.0
Glycerol 63.9 37.5 26.4
(a) (b)
Figure 6 Photographs of WCA of (a) control and (b) PP treated for 10 sec.
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95
Water
Glycerol
90
Contact angle (degree)
85
80
75
70
65
0 20 40 60 80 100
Treatment time (sec)
Figure 7 Contact angle as a function of treatment time.
improvement of surface wettability. The decrease in the WCA was achieved in a
few seconds of DBD - polymer interaction. This might be due to the introduction
of new oxygen containing groups [27, 28]. However, further increase in exposure
time did not show further enhancement of hydrophilicity. WCAs on treated PP
films attained a saturation value of about 72°.
Figure 8 illustrates the variation in surface free energy with treatment time. The
surface free energy of the control sample was found to be 24 mJ/m2. We are well
40
Polar
Dispersion
35 Total
Surface free energy (mJ/m2)
30
25
20
15
10
5
0
0 5 10 30 60 90
Treatment time (sec)
Figure 8 Surface free energy at various exposure times.
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aware that the surface free energy of pure PP is 35 mJ/m2. Apparently, the surface
of the PP we used was contaminated by low energy material.
The surface free energy was found to increase up to 10 sec of plasma treatment
and thereafter it remained almost constant. This could be because of no further
increase in the oxygen content incorporated into PP surface. Surface free energy is
found to saturate at 34.9 mJ/m2. Identical trend is seen for the polar component.
It might be due to the addition of hydrophilic groups like C=O, -OH, -COO, etc.,
[4, 29, 30, 31, 32]. The dispersion component is found to be almost constant. Thus,
the increment in surface free energy is solely due to the addition of hydrophilic
groups at the PP surface.
3.4 Surface Morphology
During plasma treatment, excited reactive species strike and cause roughening
of PP films [33]. Figure 9 shows SEM images of control and polypropylene film
treated for 90 sec. The surface of the control sample is relatively smoother than
that of treated one. The enhancement in surface roughness of PP could have been
caused by the excited particles from the discharge plasma, thus resulting in a
smaller contact angle.
Figure 10 shows the AFM micrographs of control and treated (90 sec) PP film.
The surface of control PP film is found to be comparatively smooth with a mean
surface roughness Ra = 1.43 nm in an area of 5 × 5 µm2. After 90 sec treatment, the
roughness was found to be 3.07 nm. The morphological changes may be because
of removal of a few top monolayers of the film caused by bombardment of plasma
particles on the surface. Many researchers have reported similar roughness trends
for different plasma treated polymers [4, 30, 33, 34, 35].
1 µm JEOL 2/8/2019 1 µm JEOL 2/8/2019
x 15,000 15.0kV SEI SEM WD 8.4mm 8:07:40 x 15,000 15.0kV SEI SEM WD 8.4mm 6:01:15
(a) (b)
Figure 9 SEM micrographs of the (a) control and (b) PP film treated for 90 sec.
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24 nm
24 nm
0 nm
0 nm
µm
µm
5.0
5.0
y: 5
x:
.0 µ y: 5
x:
m .0 µ
m
(a) (b)
Figure 10 AFM micrographs of the (a) control and (b) treated (90 sec) PP film.
4 Conclusions
In this work, custom-made DBD operating at 50 Hz was utilized to modify PP
films. Electron temperature and density of the discharge were found to be 1.01
eV and 1.59 × 108/cm3 respectively. The treatment led to significant changes in
the chemistry and morphology of the surface of polypropylene film. The sur-
face morphology measurements revealed that roughness of the plasma treated
surface increases with increase in treatment time. The decrease in WCA on the
treated surface can be ascribed to the higher surface roughness produced after
treatment. Thus, the present work showed that the atmospheric pressure plasma
treatment operating at 50 Hz can be considered as a useful technology to enhance
the hydrophilicity of PP films. This plasma system should be of interest to many
industrial applications due to its reduced cost and ease of operation. From the
present study, it is evident that more than 40% decrease in the WCA value is
achieved in just 30 sec of exposure to plasma. Thus, a quick hydrophilization
of PP using this cost-effective plasma device constitutes a novel feature of this
research.
Data Availability
The data, figures and articles that support findings of our study can be obtained
from the author upon request.
Conflict of Interest
The authors declare no conflict of interest for publishing this research work.
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Acknowledgments
The authors are very thankful to Prof. Dr. Eun Ha Choi and Dr. Bhagirath
Ghimire, Kwangwoon University, Korea for their valuable help and support. The
corresponding authors were supported by the Nepal Academy of Science and
Technology (NAST), Lalitpur, Nepal by providing Ph.D. fellowships. The authors
are also grateful to all the researchers of the Department of Physics, Kathmandu
University who provided valuable suggestions for the completion of this work.
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