Parametric study on the effectiveness of treatment of polyethylene (PE) foils for
pharmaceutical packaging with a large area atmospheric pressure plasma source
Marco Boselli2, Vittorio Colombo1,2, Emanuele Ghedini1,2, Matteo Gherardi1,
Romolo Laurita1, Anna Liguori1, Filippo Marani1, Paolo Sanibondi1 and Augusto Stancampiano1
Alma Mater Studiorum – Università of Bologna
1
Department of industrial engineering (DIN)
2
Industrial Research Center for Advanced Mechanics and Materials (C.I.R.I.-M.A.M.)
Via Saragozza 8-10, 40123 Bologna, Italy
Abstract: The effects of a Dielectric Barrier Discharge plasma source operating at
atmospheric pressure in ambient air on a polyethylene polymer foil commonly used for
pharmaceutical packaging are investigated. To evaluate the uniformity and the effectiveness
of the treatment the variation induced on water contact angle and the preservation of polymer
foil weldability are considered. Results show that it is possible to achieve a uniform treatment
while maintaining a good weldability of the treated material.
Keywords: Atmospheric pressure plasma, DBD, polyethylene, wettability, weldability,
treatment uniformity
1. Introduction
Atmospheric pressure plasmas have been proven an
effective tool for many material treatments such as
cleaning [1] and activation of surfaces [2], polymer
deposition [3] and interaction with in-vivo and in vitro
tissues [4] and disinfection and sterilization of surfaces
[5][6][7].
The wide and constantly expanding range of possible
applications is due to the great variety of sources and
operating conditions for the plasma generation [8].
Moreover plasma technology is environmentally friendly,
as no chemical compounds are required [9].
Though low pressure non-thermal plasma is already an
established technology in the field of material
modification, atmospheric pressure plasmas are rising
interest because of their easy handling, effectiveness and
low operational costs.
An interesting use of non-thermal atmospheric pressure
plasma is the sterilization of polymer foil for packaging
applications.
One of the most employed polymers worldwide is
polyethylene [10] due to its limited cost, easy processing,
chemical inertness and durability.
The sterilization of polymer foil is an attested potential
of atmospheric cold plasma [11][12]. However as plasma
has the potential to induce surface modification of the top
layer (1-10 nm) of polymeric materials [13] is essential to
investigate the possible reduction of the weldability of the
polymer because is an essential property in many
industrial processes.
The future challenge is therefore to develop a uniform
treatment able to sterilize while maintaining a good
weldability of the polymer foil.
In this work we have focused on establishing the
effectiveness of a plasma treatment with a DBD source on
a polyethylene foil by considering the induced
modifications
on
wettability
and
weldability
characteristics. The first has been evaluated, through
water contact angle (WCA) measurements, to establish
the uniformity of the treatment. The second has been
investigated by mean of T-peel test to establish the
feasibility of introducing the plasma treatment in an
online industrial process including a welding stage.
2. Experimental setup
The plasma source adopted in this study is designed for
the treatment of foils with dimension up to 30x20 cm and
can be easily scaled up to support the treatment of larger
foils. The plasma source (Fig. 1) is a planar Dielectric
Barrier Discharge (DBD) with a steel high voltage
electrode, a 1 mm dielectric layer of polyoxymethylene
(pom-c) covering a grounded aluminum electrode and an
air gap of 2 mm.
Fig.1 Schematic of the DBD plasma source (a); picture
of the DBD plasma source during operation (b)
Fig.2 High voltage bipolar square wave powering the
plasma source
The plasma source is powered with a high voltage
bipolar square wave with a slew rate of 750V/µs (Fig. 2)
and operated in ambient air at atmospheric pressure.
The treated material is a polymer foil, commonly
employed in the manufacturing of pharmaceutical
packaging, composed of three different layers:
- Polyvinyl chloride - PVC (200 µm)
- Polyvinylidene chloride - PVDC (23 µm)
- Polyethylene - PE (50 µm)
The treated sample dimensions are 30x16 cm and the
plasma treatment was performed on the PE side of the
foil..
To evaluate treatment uniformity, wettability of the PE
substrate after the plasma treatment is evaluated at
different positions; advancing and receding water contact
angles (WCA) is measured with the sessile drop method
using a Kruss Drop Shape Analysis System DSA 30. First,
a drop of distilled water is deposited on the sample
surface by a micrometric syringe; then the volume of the
drop is increased, forcing the drop to advance on the
sample, until a constant contact angle is observed,
representing the advancing water contact angle (A-WCA).
Then the drop is progressively shrunk until a drop radius
decrease is observed and a constant angle is measured,
representing the receding water contact angle (R-WCA)
[14].
The weldability of the PE foil is evaluated by means of
T peel test measurement (Instron 4465 with load cell 1kN
and speed of 110 mm/min). To prepare the test specimens
(Fig. 3) the treated samples have been folded so that the
PE layer faced itself and then welded using a laboratory
welding machine (130°C, 2s, 230 kPa).
Fig.3 T-peel test specimens (80x6mm, welded over
60 mm)
3. Results
The effects of plasma treatment on PE foil are
evaluated by means of WCA measurements and welding
tests with the aim to investigate the variation of polymer
weldability and wettability after plasma exposition. The
investigation of weldability modification is carried out
because it is a fundamental property in many industrial
processes including packaging applications. WCA
measurements are used to investigate the change of
polymer wettability which is a good indicator of the
plasma effect and allows consideration on the uniformity
of the treatment and therefor of the polymer sterilization.
Advancing and receding WCA are measured in different
points of the same sample to test the uniformity of plasma
treatment and, then, of the sample sterilization.
Different operating conditions are tested in order to
relate the modification of PE wettability and weldability
properties with the electric operating parameters (peak
voltage, repetition frequency) and the treatment time.
Aging tests to evaluate the persistence of plasma
treatment effects on PE samples are also carried out.
The most important results of wettability and
weldability analysis are highlighted and discussed in the
two following subparagraphs.
Wettability analysis
The advancing and receding water contact angle values
for the unmodified PE sample obtained with the sessile
drop dynamic method are 107°±2° and 90°±3°
respectively. To evaluate the uniformity of plasma
treatment on the polymer film, advancing and receding
WCA are measured on five samples, having a size of
2x2cm, extracted from five different positions of the
treated PE film as reported in Fig. 4.
Fig.4 Schematic representation of the five samples
considered for the WCA measurements
To investigate the effects of peak voltage and repetition
frequency on the modification of the WCA values,
treatment time is fixed to 3 s, peak voltage is increased
from 10.5 kV to 15 kV while repetition frequency from
100 Hz to 300 Hz.
Fixing the repetition frequency to 300 Hz and
increasing the peak voltage from 10.5 kV to 15 kV for a
treatment time of 3 s any important differences can’t be
noticed comparing the advancing and receding WCA
measured on the samples of the two different plasma
treated films. For a peak voltage of 15 kV and a treatment
time of 3 s, the increase of pulse frequency from 100 Hz
to 300 Hz doesn’t cause a relevant variation of the WCA
values, as observable comparing the WCA values
measured on the samples of the two different treated
films.
Comparing the advancing and receding WCA obtained
on the different samples of the PE films treated for 3 s
with those of untreated samples a drastic decrease of the
angle values is registered and therefore a relevant increase
of polymer hydrophilicity is achieved also for very short
treatment time. Moreover a relevant hysteresis of the
material is noticed after only 3 s of plasma treatment.
In Table 1 the advancing and receding WCA measured
on the five samples of the each treated film are reported.
Three different operating conditions are tested.
Table 1. Advancing WCA (A-WCA) and receding
WCA (R-WCA) for 5 points of each treated film.
Point 1
Point 2
Center
10.5kV 300Hz
Point 3
To investigate the persistence of plasma treatment
effects the same samples considered for the evaluation of
the treatment time influence are used for aging tests.
Measurements are carried out after 1 hour and 4 hours
from plasma treatment and the results are compared with
those obtained after 10 min from the exposition. From the
comparison the persistence of plasma treatment effects
after 4 hours from the exposition is demonstrated
observing the unimportant differences of WCA values
obtained after 10 minutes and 4 hours from the plasma
treatments.
The results for different treatment time and elapsed
time are reported in Table 2.
Table 2. Advancing WCA (A-WCA) and receding
WCA (R-WCA) for different treatment time and elapsed
time
TREATMENT
ELAPSED
TIME
TIME
Point 4
t=3s
3sec
A-WCA
R-WCA
10 min
63° ± 2°
20° ± 2°
1h
65° ± 3°
19° ± 4°
A-WCA
67° ± 2°
66° ± 2°
69° ± 2°
66° ± 2°
64° ± 2°
4h
67° ± 2°
22° ± 2°
R-WCA
26° ± 3°
32° ± 5°
21° ± 4°
19° ± 2°
19° ± 5°
10 min
57° ± 3°
16° ± 3°
1h
59° ± 3°
10° ± 2°
15kV 100Hz
t=3s
30sec
A-WCA
64° ± 3°
65° ± 2°
68° ± 2°
66° ± 2°
67° ± 2°
4h
57° ± 3°
13° ± 4°
R-WCA
17° ± 3°
18° ± 2°
19° ± 3°
18° ± 2°
18° ± 3°
10 min
51° ± 4°
12° ± 3°
1h
49° ± 4°
11° ± 3°
4h
54° ± 3°
14° ± 5°
15kV 300Hz
t=3s
60 sec
A-WCA
64° ± 1°
63° ± 2°
63° ± 2°
65° ± 2°
63° ± 2°
R-WCA
19° ± 3°
19° ± 2°
20° ± 2°
22° ± 3°
17° ± 3°
The influence of treatment time on the measurements of
advancing and receding WCA is evaluated fixing peak
voltage to 15 kV and pulse frequency to 300 Hz and
varying the treatment time (3, 30 and 60 s). WCA are
measured only on sample (2x2 cm) extracted from the
center of PE treated film as the uniformity of the
treatment as already been proven for these conditions.
Comparing the advancing and receding WCA
measurements obtained after 10 minutes of plasma
exposition on the samples subjected to different treatment
times, an additional decrease, not significant if compared
with that obtained for a treatment of 3 s, of the values is
verified increasing treatment time to 30 s or 60 s.
Weldability analysis
To evaluate the effects of the plasma treatment on the
weldability of PE, polymeric specimens with a size of 80
mm x 6 mm are welded and subjected to T-peel tests. The
percentage change of treated samples weldability from
that of untreated polymer is highlighted in Fig.5.
Different operating conditions are experimented fixing
treatment time to 3 s, varying peak voltage from 10.5 kV
to 15 kV and pulse frequency from 100 Hz to 300 Hz.
Any relevant correlations between weldability resistance
and operating parameters aren’t noticed.
A decrease of welding resistance for treated samples in
a range between 21% and 39% of that of the untreated
polymer is registered.
Fig.5 Percentage change of treated sample weldability
from that of untreated sample
Because of the obtained results have an uncertainty of
20%, a comparison between the weldability resistance of
the samples subjected to plasma discharges generated
with different operating conditions is not feasible or
significant.
To investigate the aging of welding properties after
plasma treatment, different samples treated in the same
operating conditions (15 kV, 300 Hz, 3 s) are welded after
4 hours and 28 hours from the plasma treatment and
subjected to T-peel test after 4 hours and 28 hour from the
plasma exposition. Results are highlighted in Fig.6
Fig.6 Aging of welding properties after plasma treatment
4. Conclusions
The planar DBD plasma source has demonstrated to be
a reliable mean to treat uniformly a polymer foil with a
surface up to 30x16 cm. The plasma treatment induces a
hydrophilic behavior on the previously hydrophobic
polyethylene layer already after just 3 s of treatment.
There is an evident weldability reduction induced by
the plasma treatment but it remains confined in a range
between 21 and 39 % of the untreated welding resistance.
There is no evident correlation between the weldability
reduction and the treatment time or operating conditions.
In conclusion we may affirm that the tested planar large
area DBD plasma source is a feasible mean for online
polyethylene polymer foil disinfection in assembly chain
as long as the weldability reduction induced is acceptable
for the final product characteristics.
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