suAgeing of LDPE surfaces modified by low temperature plasma torch
P. Leroy, S. Abou Rich and F. Reniers
Université Libre de Bruxelles, Faculty of Sciences, cp 255, Bld Triomphe, 2-B-1050 Bruxelles, Belgium
Abstract: Surface modification treatments were performed on Low Density
PolyEthylene (LDPE) using a low temperature plasma torch of argon (Ar) with or
without adding oxygen as reactive gas at atmospheric pressure. The composition
of the gas mixture, the plasma power and the exposure time to the plasma were
varied. Due to the “hydrophobic recovery” effect, the induced modification of the
surface is not permanent. This phenomenon was investigated using Water Contact
Angle measurements (WCA) and X-ray Photoelectron Spectroscopy (XPS).
Keywords: polymer modification, atmospheric plasma, polyethylene
1. Introduction
Polymers such as polyethylene have excellent bulk
physical and chemical properties but exhibit poor
adhesion abilities. Therefore, in order to improve the
deposition of a subsequent layer onto such
compounds, a pretreatment of the surface is usually
performed. Plasma treatments have a lot of
advantages compared with other methods used to
treat and modify such polymer like chemical, thermal
or mechanical processes. Since the plasma process is
solvent-free and dry, therefore environmental
friendly. Moreover, atmospheric pressure plasma is
attractive for industrial applications. The costs of a
vacuum installation are avoided. Another advantage
of plasma processes is that the modifications only
affect the top layer of the material without changing
the bulk properties [1, 2].
Many studies have been done on the treatment of
polyethylene by plasma [2, 3]. Currently, it is well
known that the enhancement of the hydrophilic
character of the polyethylene treated by plasma
processes is mainly due to the surface oxidation,
creating at the interface new oxygen-based
functionalities. Some authors like Encinas et al. [3]
report, in addition to the surface oxidation, an etching
effect responsible for changes in the surface
roughness.
Due to the “hydrophobic recovery” effect, the
induced modification of the surface is not permanent.
This ageing effect is caused by the reorientation of
induced polar chemical groups into the bulk of the
material in order to minimize the interfacial free
energy between the surface of the polymer and its
environment, and the migration of polar chemical
groups into the polymer matrix [1].
It is shown that the treated polymer undergoes some
surface reorganization after treatment, but the sample
never goes back to its native state. Ageing of plasmatreated polymer surface is influenced by many
parameters as the working gas [4], the storage
conditions [5] and the cristallinity degree of the
treated material [6, 7]. The hydrophobic recovery
mostly happens in the 2-3 days after treatment, the
further ageing effect being much less important [2].
In this paper, surface modification treatments were
performed on Low Density PolyEthylene (LDPE)
using an atmospheric pressure plasma torch of argon
(Ar) with or without adding oxygen as reactive gas.
The composition of the gas mixture, the plasma
power and the treatment time to the plasma were
varied.
2. Experimental
Plasma torch set-up
The polymer has been treated using an AtomfloTM250D plasma source provided by SurfX
Technologies LLC (Fig 1). The plasma was
maintained by supplying a RF power at 27.12 MHz
to the top electrode, while the bottom electrode was
grounded. The argon flow was fixed to 30L/min.
Material and analyses equipments
The treatments have been performed on low density
polyethylene (LDPE, 40 µm thick, PackOPlast
Belgium). For ageing, samples have been kept in
Petri dishes during storage. The gap between the
LDPE sample surface and the torch was fixed at 0.9
cm.
a pure argon plasma, the contact angle reaches up
51.5°± 2.6° just after treatment (pristine value is
94.2°±1.7°) and returns to 56.9° ± 1.4°after 3 days of
ageing.
Figure 1. Atmospheric pressure plasma torch set-up.
The contact angle values were measured using a
Krüss DSA 100 (Drop Shape Analysis system). The
static WCA measurements have been performed in a
climatized room, using milli-Q water as working
liquid. The obtained values were the mean of 10
drops measurements randomly deposited on the
sample surface with the dimension of 1x1 cm. Each
drop had a size of 3 µl.
XPS measurements were performed with a PHI 5600
photoelectron spectrometer system, operating at 300
W and under a vacuum of about 10-9 Torr. Pass
energy of 93.90 eV was used for the survey spectra
and a pass energy of 23.5 eV for high resolution
spectra. The hydrocarbon C1s core level was set to
285.0 eV and was used as a calibration of the energy
scale. The deconvolution of C1s components was
done by CasaXPS software.
3. Results and discussion
In this paper, three parameters of the plasma torch
and their effects on the ageing of the LDPE plasma
treated have been studied: treatment time, power and
amount of oxygen in the feeding flux.
Fig. 2 (a) shows as expected an improvement of the
LDPE surface wettability with the exposure time to
the plasma post-descharge. These results are similar
with those obtained in the literature with other
plasma configurations with other polymers [8, 9].
After three days of ageing, it is noted that the plasma
treatment is not permanent but the improvement of
the wettability is not completely lost during the
ageing process. The plasma treatment on the low
density polyethylene seems to be relatively stable.
For a sample treated at 60 W during 60 seconds with
Figure 2. (a) Evolution of the WCA values with the treatment times and
3 days of ageing for sample treated at 60 W and with 20mL/min of
oxygen or with pure argon plasma. (b) Evolution of the WCA values with
the plasma power and 3 days of ageing for samples treated 60 s with a
pure argon plasma. (c) Evolution of the WCA values with oxygen flux.
The polyethylene treated with argon and oxygen
plasma gives better results in term of wettability
immediately after treatment (43.2°± 1.4°) but the
ageing process seems to be more important. After 3
days, the WCA value is equal to 53.3° ± 1.8°. Even if
this result is lower than in the previous condition,
about 10° are lost in comparison with about 5° for
sample modified with pure argon plasma. For longer
treatment times (2 and 5 minutes) the contact angles
of the polyethylene treated with and without oxygen
tend, after ageing, to have a same value. Fig. 2 (b)
emphasizes the non influence of the power on the
increase of the hydrophilic character after plasma
treatment and on the ageing of the sample plasma
treated. The WCA values are around 52.8° just after
treatment and around 58.5°after 3 days of ageing.
Fig. 2 (c) shows the effect of the addition of oxygen
in the feeding flux on the ageing of the polyethylene
plasma treated. No specific behaviour is observed
when oxygen is added to the working gas in different
quantities (5 mL/min to 25 mL/min). The values of
the contact angle after 3 days of ageing are relatively
close. It is not necessary to add a large amount of
reactive gas in the feeding flux for having an
improvement of the wettability.
XPS analyses have been done to complete the contact
angle measurements in order to understand the
involving phenomenon. The O/C atomic ratio related
to the previous WCA results are presented in Fig. 3.
The first behaviour which can be noted is the large
decrease of O/C mainly during the first day of
ageing. Fig. 3 (a) emphasizes that longer the
exposure time to plasma, the more oxygen species
are integrated into the polymer surface. But, for 2 or
5 minutes of treatment, the O/C ratio after 7 days of
ageing tends to have a same value. This confirms the
previous results obtained for the wettability. Fig. 3
(b) confirms the non influence of the plasma power
on the ageing effect. No logical sequence appears. It
can be noted on Fig. 3 (c) that immediately after
treatment the O/C atomic ratio is more important for
samples exposed to plasma containing more oxygen
in the feeding flux. But, after one day of ageing, the
trend is reversed, the ageing effect is more effective
on samples treated with plasma containing a greater
amount of oxygen. In view of these results, the
generation of oxygen species at the polymer surface
seems to be responsible to the increase of the
wettability of samples plasma treated.
Table 1 and 2 shows the atomic percentage of the
C1s and O1s. It is observed for both plasmas that the
oxygen content at the film surface decreases with
ageing time (from 23.7 %at to 19.3%at after 7 days
of ageing for a film treated 60 s at 60 W with a pure
argon plasma) whiles the carbon content increases
(from 76.3 %at to 80.7 %at after 7 days of ageing for
the same treatment conditions). It is noted that more
oxygen is embedded in the polyethylene surface
immediately after treatment with argon and oxygen
plasma (25.9%at) than with a pure argon one
(23.7%at). This correlates the contact angle and the
previous XPS results. After 3 days of ageing, the
sample treated with argon and oxygen presents lower
oxygen content (17.989 %at instead of 20.989 %at
for sample modified with pure argon plasma).
Figure 3. (a) Evolution of the O/C atomic ratio with the treatment times
and 0, 1, 3 and 7 days of ageing for sample treated at 60 W and with pure
argon plasma. (b) Evolution of the O/C atomic ratio with the plasma
power and 0, 1, 3 and 7 days of ageing for samples treated 60 s with a
pure argon plasma. (c) Evolution of the O/C atomic ratio with the flow of
oxygen in the feeding gas and 0, 1, 3 and 7 days of ageing for samples
treated during 60 s at 60 W.
The decomposition of the C1s peaks has been done
in order to estimate the evolution of the chemical
functionalities during ageing. Table 1 and 2 show the
values of the C1s band fitting in four peaks. These
peaks correspond to the hydrocarbon signal (1), the
C-O bonds (2), the C=O bonds (3) and carboxyl
carbons (4). These tables reveal that the amount of
the oxygen species created during plasma treatment
is progressively lost during ageing while the
hydrocarbon signal increases, confirming the
reorientation of the polar groups into the bulk after
treatment. This observation has been done for both
plasma treatments. With ageing time, the percentage
of oxygen species decreases but seems to stabilize 3
days after treatment. After 7 days of ageing the
LDPE film treated with pure argon plasma lose about
17.4% of oxygen species created after plasma
exposition instead of a lost of 30.8% for sample
treated with argon and oxygen plasma.
Table 1: C1s binding energies and surface composition of the treated
(60s, 60W, 0mL.min-1) and the aged LDPE. Fraction of element (%)
vs aging time.
Peak no.
BE (eV)
Assignment
Untreated
0 day
1day
3 days
1
285.0
C-C
94.74
76.5
79.05
79.82
80.6
2
286.5
C-O
3.47
12.48
11.48
10.98
10.41
3
288
C=O
1.79
5.99
5.21
5.08
4.6
4
288.9
O-C=O
-
5.03
4.26
4.12
4.39
are lower than those for samples treated with pure
argon plasma. We suggest that this phenomenon is
due to the etching effect happenings when oxygen is
added to the working gas. The hydrophilic gain
would be attributed, in this case, to the surface
oxidation and to a modification of the roughness of
the LDPE surface [3].
Conclusion
This paper emphasizes that the working gas use for
plasma treatment influences the evolution of the
chemical composition of the surface and the resulting
ageing of the films plasma treated. Seven days after
treatment, the O/C atomic ratio decreases in
comparison to the O/C ratio obtained immediately
after plasma treatment but it remains much lower
than the pristine value. Therefore, all oxygen
functions created during plasma treatment are not
lost during ageing process. In the case of argon and
oxygen
plasma
treatment,
the
wettability
improvement is due to surface oxidation but also
probably to a modification of the surface roughness.
7 days
C %at
76.3
79.05 79.711 80.655
O %at
23.7
20.95 20.289 19.345
Table 2: C1s binding energies and surface composition of the treated
(60s, 60W, 25mL.min-1) and the aged LDPE. Fraction of element (%)
vs aging time.
Peak no.
BE (eV)
Assignment
Untreated
0 day
1day
3 days
7 days
1
285.0
C-C
94.74
74.13
81.05
82.06
82.11
2
286.5
C-O
3.47
13.26
9.02
8.62
8.5
3
288
C=O
1.79
6.24
5.28
5.19
5.15
4
288.9
O-C=O
-
4.13
6.37
4.65
C %at
74.1
81.05 82.011
81.95
4.24
O %at
25.9
18.95 17.989
18.05
Immediately after treatment, an addition of oxygen in
the feeding flux gives better results, the WCA value
is lower and XPS results emphasize a higher
oxygenation of the surface but the following ageing
process is more significant with this kind of
treatment. Nevertheless, despite a lower percentage
of oxygen species after ageing for sample treated
with a plasma containing oxygen, the WCA values
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