22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Optimising the surface activation of polyethylene using different argon plasmas
A. Van Deynse1, P. Cools2, C. Leys2, R. Morent2 and N. De Geyter2
1
Department of Industrial Technology and Construction, Ghent University, Ghent, Belgium
2
Department of Applied Physics, Ghent University, Ghent, Belgium
Abstract: In this work, a comparison is made between different plasma treatments of
LDPE: an Ar DBD treatment operating at medium pressure and an Ar APPJ at atmospheric
pressure. LDPE surface activation is examined using WCA, XPS and AFM. The surface
activation of the DBD can be enhanced by adding water vapour to the precursor gas while
better APPJ treatment efficiency can be obtained by decreasing the distance foil-capillary.
Keywords: plasma treatment, LDPE, DBD, APPJ, argon plasma, water addition, ageing
1. Introduction
Low density polyethylene (LDPE) is one of the most
widespread polymers used for industrial and medical
applications due to its excellent material properties (low
density, high flexibility and high chemical resistance) [1].
However, despite these excellent characteristics, LDPE is
often unsuitable for use due to its low surface free energy,
leading to poor wettability and poor adhesion [2]. An
increase in surface free energy can be obtained using wet
chemical processes; however, ecological requirements
force the industry to search for alternative
environmentally friendly methods [3]. Plasma treatment
of polymers has been gaining popularity as an
environmentally benign surface modification technique
since it does not require the use of solvents and chemicals
[4]. Next to the ecological aspect, plasma surface
treatment has many more advantages, including
modification of just the outermost atomic layers of a
substrate [5], selection of desired chemical pathways,
minimization of thermal degradation and rapid treatment
[6]. This method has already shown its effectiveness and
different types of non-thermal plasmas operating at low,
medium or atmospheric pressure have been used for
polymer surface modification [3, 7-10]. In the case of
LDPE, surface modification has been mostly performed in
a single discharge gas, such as oxygen, air, nitrogen [1, 2,
8, 11-13] or discharge mixtures like He/O 2 and Ar/O 2
[14]. Recent studies have shown the beneficial effect of
water vapour addition to the plasma gas on LDPE surface
modification [12, 14, 15].
In this work, plasma treatment of LDPE is studied using
a medium pressure dielectric barrier discharge (DBD) and
an atmospheric pressure plasma jet (APPJ) operating in
argon (Ar). Both systems have their own advantages and
disadvantages.
The Ar DBD works in a closed
environment and the microdischarges are uniformly
distributed over the complete dielectric surface. At
medium pressure, a large plasma volume can thus be
created which results in an overall higher production
efficiency. An Ar APPJ on the other hand is not limited
to flat and thin substrates but can be used for large three
dimensional structures. In addition, specific parts of a
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sample can be selectively treated with an APPJ. Since the
jet operates at atmospheric pressure it can also be easily
integrated into existing production lines. A comparison
between the surface modification of LDPE using both
plasma systems is provided in this work using water
contact angle measurements (WCA, for wettability
determination), X-ray photoelectron spectroscopy (XPS,
for chemical composition determination) and atomic force
microscopy
(AFM,
for
surface
morphology
determination).
The plasma treatments have been
optimised for both plasma systems in order to provide us
with an efficient tool to activate LDPE surfaces. Finally,
the LDPE ageing processes are also examined in this
study. A detailed description of the set-ups, plasma
treatment methods and measuring devices can be found
elsewhere [15, 16].
2. WCA results
Different samples of LDPE foil are exposed to an Ar
plasma using a DBD or an APPJ. For the DBD, a 1.15 W
Ar plasma operating at medium pressure is used with an
Ar flow rate of 1.0 slm. The power and flow rate of the
DBD is selected in that way that the plasma volume is as
large as possible but remains between the two electrodes.
For the APPJ, a 3.7 W Ar plasma is used at a 20 mm foil
– capillary distance and a flow rate of 1.25 slm. For the
APPJ the power is optimised to avoid changes in the bulk
properties and this flow rate is selected to avoid
turbulence in the flow [16]. By changing the exposure
time, it is possible to treat samples with different energy
densities. The energy density can be calculated by
multiplying the plasma exposure time with the plasma
power and by dividing this value by the area of the
plasma. First, the WCA values are measured as a
function of energy density after plasma treatment using
the above mentioned DBD and APPJ specifications. The
results are presented in Fig. 1. From Fig. 1, it can be seen
that the WCA decreases when the energy density
increases until a saturation level is reached. Using an Ar
DBD, the WCA decreases very fast and at an energy
density of 5.5 J/cm², the saturation WCA of 51.9 ± 1.6° is
already reached. This means a reduction in WCA value
1
110
100
APPJ
DBD
90
WCA (°)
80
70
60
50
40
30
20
0
10
20
30
40
50
60
70
80
Energy density (J/cm²)
Fig. 1. Comparison of LDPE after Ar plasma treatment
with a DBD and an APPJ.
In an effort to enhance the LDPE wettability after a
DBD plasma treatment, we opted to add 41% water
vapour to the plasma precursor gas. The total flow rate
during exposure is kept constant at 1 slm, the same as
during the pure Ar treatment, however, the power is much
higher (8.0 W) since electrical breakdown is more
difficult when water vapour is added. A comparison of
the DBD plasma treatment in pure argon and argon/water
is presented in Fig. 2. From this figure, it can be seen that
the saturation WCA is 32.6 ± 1.6° after an exposure of
minimum 1 J/cm² which means a reduction in WCA of
69%. This saturation WCA value is now comparable to
the value reached using the APPJ and the minimum
energy density to reach this saturation value is lower than
when the DBD or APPJ are used with pure Ar. As a
result, adding water vapour to the Ar precursor gas gives
a beneficial effect on the LDPE wettability.
110
Argon
Argon+water
100
90
WCA (°)
80
70
60
50
40
30
20
0
0.5
1
1.5
2
2.5
3
3.5
Energy density (J/cm²)
Fig. 2. Optimisation of LDPE plasma treatment using a
DBD.
2
As the required energy density to reach the saturation
WCA value is much higher when the Ar APPJ is used, we
also did some efforts to optimise the APPJ treatment. For
this purpose, the distance between the capillary and the
foil z has been reduced from 20 mm to 10 mm. For the
two applied distances, the results of the WCA evolution
as a function of energy density are presented in Fig. 3.
From this figure, it can be seen that the WCA of LDPE
after plasma treatment at the same energy density is lower
when the distance between the capillary and the foil
decreases. However, at a capillary-foil distance of 10
mm, saturation is still not reached after an expose at
22 J/cm². The lowest WCA is in this case 26.5 ± 1.3°, so
comparable to the saturation value at a distance of 20 mm.
However, from practical point of view, it is not possible
to expose the LDPE to higher energy densities at
z = 10 mm since this leads to deformations of the LDPE
films. Due to the close contact between the plasma and
the LDPE surface, the LDPE structure extremely changes
and even starts to melt when high energy densities are
applied. These effects should of course be avoided as the
purpose of surface activation is to enhance the
hydrophilicity without changing the LDPE bulk
properties. So, by reducing the capillary – foil distance,
the WCA decreases faster as a function of energy density
but it is not possible to expose to high energy densities.
110
100
90
z = 10 mm
80
WCA (°)
of 50% as the WCA of the untreated sample is
103.8 ± 0.8°. When using an Ar APPJ, the WCA
decreases slower and saturation is only reached after an
exposure at an energy density of 21.8 J/cm². The
saturation WCA is however 29.6 ± 3.8° which means that
the reduction of WCA is 70%, much higher than in the
case of the DBD plasma treatment.
z = 20 mm
70
60
50
40
30
20
0
5
10
15
Energy density (J/cm²)
20
25
Fig. 3. Optimisation of LDPE plasma treatment using an
APPJ.
3. XPS results
Besides WCA analysis, XPS measurements are also
performed on untreated and plasma treated LDPE samples
to get an insight into the chemical composition of the
samples. LDPE foils are plasma treated (1) using an Ar
APPJ at an energy density of 21.8 J/cm² with a distance
foil-capillary of 10 and 20 mm and (2) using a pure Ar
and an Ar/41% water vapour DBD at an energy density of
20 J/cm². At these applied energy densities, the samples
are saturated but still not deformed. Based on the XPS
survey scans, the atomic compositions of the LDPE
samples can be determined and from these data, the O/C
ratios can be calculated. The results are shown in
Table 1. From this table, it can be seen that the O/C ratio
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Table 1. XPS and roughness results after plasma
treatments with a DBD and an APPJ.
untreated
APPJ –
z=10 mm
APPJ –
z=20 mm
DBD –
Ar
DBD –
Ar+water
O/C
(%)
C-C/C-H
(%)
C-O
(%)
C=O
(%)
O-C=O
(%)
3
96.5
3.5
0
0
42
66.1
13.4
6.6
10.9
21
85.1
8.5
3.7
2.7
13
88.1
7.9
2.5
1.6
24
80.7
9.9
5.6
3.9
R rms
(nm)
36.7
± 1.8
35.5
± 6.2
40.7
± 3.1
58.5
± 2.3
40.3
± 5.2
To determine which chemical groups are present on the
surface foils, curve-fitting of high resolution C1s peaks is
also performed. The C1s envelope of the LDPE samples
can be decomposed into 4 distinct peaks: a peak at
285.0 eV corresponding to C-C and C-H bonds, a peak at
286.5 eV due to C-O functional groups, a peak at 288 eV
attributed to C=O and O-C-O groups and a peak at
289.2 eV due to O-C=O groups [17]. Based on the
deconvoluted C1s peaks, the concentration of the different
chemical bonds can be calculated and the obtained results
are given in Table 1. This table clearly shows that the
concentration of C-C and/or C-H bonds decreases after
the applied plasma treatments while the concentration of
all O-containing groups consequently increases.
Introducing water vapour in the plasma using the DBD
increases all O-containing groups equally compared to the
pure argon DBD plasma. Reducing the foil – capillary
distance also enhances all O-containing groups but
especially an increase of the high energetic O-C=O
groups can be noticed, resulting in an even better
wettability, but a more reactive plasma.
4. AFM results
The morphology of untreated and plasma treated LDPE
samples is examined using AFM. For this purpose,
samples are treated under the above mentioned conditions
using the APPJ at an energy density of 21.8 J/cm² and
using the DBD at an energy density of 20 J/cm².
Morphology changes can be accurately quantified by
root-mean-square roughness values (R rms ) and the
obtained values, averaged over 3 different AFM images
are shown in Table 1. These values show that the
roughness only increases after plasma treatment in the
pure Ar DBD. In the other cases, no significant increase
of the roughness can be noticed. As a result, the
improved LDPE hydrophilicity after plasma modification
can be fully attributed to the incorporation of oxygen
groups as shown in the XPS results section.
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5. Ageing
Different authors [18-20] have stated that the increase
in surface hydrophilicity is only temporary: if an LDPE
surface which has become hydrophilic after plasma
treatment is left under suitable conditions, the surface can
regain its original hydrophobicity. This process is
referred to as hydrophobic recovery or ageing process. In
most cases, ageing should be minimized and therefore the
ageing behaviour of the plasma treated LDPE films is also
investigated. Different LDPE samples are plasma treated
in an Ar and an Ar/41% water vapour DBD at an energy
density of 20 J/cm² and with an APPJ at an energy density
of 27.3 J/cm² at a capillary – foil distance of 20 mm.
These treatment conditions are selected to provide us with
saturated LDPE samples with unmodified bulk properties.
After plasma treatment, the LDPE samples are stored in
air at room temperature for a period up to 14 days. Fig. 4
shows the evolution of the WCA as a function of storage
time. The WCA of the untreated sample is 103.8 ± 0.8 as
previously mentioned, however, this value is not included
in the figure. As can be seen in Fig. 4, the ageing process
of the LDPE film is characterised by a quick increase in
WCA during the first hours of storage. At longer storage
times, the WCA increases more slowly and finally reach a
plateau value after approximately 2 days of storage. For
the APPJ treated samples, the WCA increases from 29.4°
immediately after treatment to a plateau value of 58.4°.
This means a loss in treatment efficiency (L) of 39%. The
WCA of the Ar DBD treated samples increases from
55.0° to 69.1° or an L of 29%. The WCA of the Ar/41%
water vapour DBD samples increases from 32.7° to 59.4°
or an L of 37.5%. From these data can be seen that the
loss in treatment efficiency is the lowest for the pure Ar
DBD treated samples, while the ageing processes of the
APPJ treated samples and the Ar/41% water vapour DBD
are comparable. It is also worthwhile to mention that the
maximal WCA after ageing is still much lower than the
WCA of the untreated LDPE surfaces which means that
the major part of the surface wettability is maintained
after plasma treatment.
75
70
65
60
WCA (°)
increases when the samples are plasma treated. The
higher the wettability of the LDPE or the lower the WCA,
the higher the O/C ratio.
55
50
45
APPJ
40
DBD-Ar
35
DBD-Ar+water
30
0
50
100
150
200
250
300
350
Ageing Time (h)
Fig. 4. Ageing behaviour of LDPE after different plasma
treatments (APPJ and DBD).
3
6. Conclusions
The wettability of LDPE foils can successfully be
enhanced using plasma treatment.
Two different
techniques are proposed each with their own advantages
and disadvantages. An Ar DBD operating at medium
pressure works in a closed environment and distributes
the microdischarges uniformly over the whole dielectric
surface. At medium pressure, a large plasma volume can
thus be created which results in an overall higher
production efficiency as saturation is reached at low
energy densities. An Ar APPJ on the other hand is not
limited to flat and thin substrates but can also be used for
large three dimensional structures.
It works at
atmospheric pressure and can therefore easily be
integrated into existing production lines.
Besides these practical considerations, it can be
concluded that an Ar DBD increases the wettability of
LDPE very fast, so even after exposure to low energy
densities saturation is already reached. However, the
saturation WCA remains higher than after plasma
treatment using an Ar APPJ. The WCA can be further
reduced when water vapour is added to the precursor gas
during DBD plasma treatment. This results in an extra
30% increase in wettability. By doing so, the saturation
WCA is comparable to the one obtained after APPJ
treatment, but this value is however reached at lower
energy densities with the DBD. The APPJ can be
optimised by reducing the distance between the capillary
and the foil. This reduces the WCA at lower energy
densities but deforms the LDPE foil at higher energy
densities. XPS and AFM measurements reveal that the
enhanced wettability can be fully attributed to the
increased oxygen content at the LDPE surface since the
roughness is not significantly changed upon plasma
treatment.
Ageing processes show that the wettability decreases if
the LDPE is stored in air at room temperature. However,
the loss in treatment efficiency remains less than 40% for
all applied plasma methods. It can thus be concluded that
the major part of the surface wettability is maintained
during storage. The loss in treatment efficiency is the
lowest for LDPE treated with the Ar DBD, while the
WCA after ageing remains the lowest for the Ar APPJ
and Ar/water vapour DBD treatments. The results
obtained in this work thus make it possible to select the
optimal settings for plasma surface modification of
LDPE.
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7. Acknowledgments
The research leading to these results has received
funding from the European Research Council under the
European Union's Seventh Framework Program
(FP/2007-2013) / ERC Grant Agreement n. 279022.
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