Atmospheric DBD plasma processes for production of lightweight composites
D. Vangeneugden1*, B. Verheyde1, J. Wielant2
1
VITO, Sustainable Materials Management, Boeretang 200, 2400 Mol, Belgium
2
OCAS, Pres. J.F. Kennedylaan 3, 9060 Zelzate, Belgium
*
[email protected]
Abstract: An innovative atmospheric dielectric barrier discharge (DBD) plasma system was used
to increase the adhesive strength of lightweight steel-polymer composite panels. Surface treatment
of pre-painted steel skins using the reactive afterglow of a N2/CO2 (5%) DBD plasma increases the
peel force significantly towards laminated polyolefin core materials. However, best results are
obtained by addition of aminopropylthiethoxysilane (APEO) to the discharge afterglow.
Keywords: adhesion, sandwich panel, DBD, plasma treatment
1. Introduction
In the framework of the joint European research project
Nano2Production (FP7 “N2P” 2008-2012), a new
atmospheric dielectric barrier discharge (DBD) plasma
system was developed by VITO called “PlasmaLine”
(Fig. 1). The system enables low temperature plasma
assisted surface modification and coating of flat and
slightly textured (e.g. embossed) materials. A wide
variety of gas mixtures can be used and it is possible to
to inject liquid or dispersed chemistries in the form of
nano-sized aerosols [1-3].
Fig.2: Schematic drawing of a typical lightweight composite
structure with a steel skin and a plastic honeycomb core.
2. Experimental
Fig. 1: VITO’s DBD plasma system “PlasmaLine”
Plasma treatments were performed in a lab setup at
VITO as shown in Fig. 3. The PlasmaLine reactor is
mounted in a ventilated hood with electromagnetic
shielding. Substrates to be treated are placed on an Xmoving table which passes under the plasma reactor.
The distance between the substrate surface and the
reactor is typically 2 to 10 mm. The substrate holder
can move at speeds between 1 and 40 m/min. Any kind
of flat or slightly textured substrate can be treated up to
300x1200 mm. In principle there are no limitations
regarding substrate thickness but for practical reasons
the maximum is about 300 mm in the lab set-up.
The DBD plasma system was evaluated for adhesion
improvement of lightweight steel-plastic composite
panels. A schematic 3D drawing of a typical composite
panel is presented in Fig. 2. Usually, a painted steel
skin is laminated to a plastic honeycomb core with a
polymer film for adhesive bonding. However, the latter
solution
isn’t
providing
sufficient
adhesive
performance for all envisioned applications. These
include for instance indoor decorative elements,
outdoor lightweight constructions or construction
elements for public transport. Key driver for the latter
application field is reduced fuel consumption [4,5].
Fig.3: Lab set-up for DBD plasma treatment
A schematic presentation (crosscut) of the DBD reactor
is shown in Fig. 4 as well as a side view picture.
Important elements in the reactor construction are the
L-shaped dielectrics which prevent arcing to the central
grounded electrode. These also allow placing substrates
very close to the slits were plasma activated gas exits
the reactor. Chemicals can be introduced in the reactive
afterglow via the central grounded electrode without
any risk to contaminate the plasma discharge area. The
latter is important to limit maintenance intervals during
continuous operation. The reactor has a segmented
design for discharge gas and chemistry injection which
enables scale-up in steps op 100 mm. The reactor used
in the above described set-up has a width of 400 mm
(PlasmaLine 400).
Netherlands) were used as adhesion promoting
chemicals. Average aerosol particle size is 30-50 nm
and precursor flow per atomizer is typically between
20 and 50 ml/h depending on feed gas flow, precursor
viscosity and vapour pressure.
The plasma system is fully automated and operates via
an in-house developed software program (LabVIEW).
All experimental settings are saved in a logbook and
can be uploaded to repeat experiments under exactly
the same conditions. Typical conditions for plasma
treatment were the following: 600 slm discharge gas
flow (N2/CO2 5%), 80 ml/h APEO precursor injection,
2 kW discharge power, line speed 10 m/min, two
treatment passes.
Initial tests to evaluate and optimize plasma treatments
were performed on A4 size (200x300 mm) polyester
pre-painted steel sheets (ArcelorMittal, Belgium).
Lamination at lab scale was performed under static
conditions in an oven at 160°C for 30 minutes with a
load of 1 kg. A schematic presentation of the sandwich
structure with indication of the plasma treated surface
is presented in Fig. 5.
Fig. 5: Multilayer structure of lightweight steel-plastic
composite panel with indication of plasma treated surface
Fig. 4: Schematic of crosscut of the DBD reactor (top) and
picture of similar side view (bottom)
The system power supply, a 5 kW G50SE (AFS,
Germany), is developed to generate DBD plasma
discharges in a broad frequency range (1-100 kHz) and
at various power settings by means of load matching.
The main discharge gas used to operate the plasma
system is nitrogen (Linde Gas, Belgium). Other gasses
such as carbon dioxide (Air Products, The Netherlands)
can be admixed in low concentrations (few %).
However, the unique advantage of the system is that it
enables injection of liquid chemicals or solutions in the
form of nano-sized aerosols. The system is equipped
with two in-house designed pressure atomizers for each
reactor segment of 100 mm. Aminopropylthiethoxysilane (APEO) and acrylic acid (Sigma-Aldrich, The
Initial lab-scale adhesion tests were performed by
tensile lap-shear tests according to ISO 4587. Adhesion
tests on sandwich panels were performed by floating
roller peel tests according to the ISO 1464 standard.
Contact angle measurements were performed on a
Krüss goniometer using both ultrapure water and
diiodomethane. Surface energy levels were calculated
according to the Owens-Wendt model.
Chemical surface characterisation was performed by
means of X-ray photoelectron spectroscopy (XPS)
using a PHI Quantera SXM microprobe (Physical
Electronics GmbH, Germany).
3. Results and discussion
Initial screening of adhesion improvement of prepainted steel substrates by DBD plasma treatment was
performed by direct lamination of two treated steel
skins with a thermoplastic polyethylene (PE) foil.
Plasma treatments were performed using N2 or N2/CO2
(5%) as the main discharge gas without and with
addition of additional precursor chemistry to the
discharge afterglow. Both aminopropyltriethoxysilane
(APEO) and acrylic acid (AA) were evaluated for
reactive DBD plasma treatment. Adhesive strength was
evaluated by lap-shear tests. Tests were performed one
day after lamination (no ageing) as well as upon 2
weeks ageing at 40°C and 100% relative humidity
(RH) (aging 1) and same conditions plus an additional
22 hours at 80°C followed by 2 hours at -20°C and 0%
RH (aging 2). (Fig. 6)
16
14
Force (N/mm2)
12
10
8
6
4
2
0
no plasma
treatment
N2 + CO2, no
precursor
reference, no ageing
N2 + APEO
N2 + CO2 +
APEO
ageing 1
N2 + CO2 +
acrylic acid
ageing 2
Fig. 6: Lap shear adhesion tests on laminated pre-painted
steel sheets upon various DBD plasma treatments
A significant increase in adhesion forces was observed
for all DBD plasma treatments. Taking into account
initial adhesion performance and resistance to aging,
plasma treatments with N2/CO2 and N2/CO2 + APEO
were selected for lamination tests using honeycomb
and full plastic core materials.
The uniformity of plasma treatments proved to be very
high as indicated by contact angle and XPS
measurements over the full width (300 mm) of A4
treated samples along the plasma system
(perpendicular to treatment direction). Contact angle
measurements (Fig. 7) were performed every 2.5 mm
using ultrapure water and diiodomethane. While
contact angles with diiodomethane remain at about 50°
before and after plasma treatments, contact angles with
ultrapure water are reduced from ~90° to ~75°. XPS
measurements were performed every 20 mm (Fig.8)
and show uniform chemical surface modification over
the full width of treated substrates (15 measurement
points over 300 mm). Plasma treatment using N2/CO2
results in an increase of the surface atomic
concentration (at%) of oxygen from 15 at% to 20 at%.
When APEO is injected into the plasma afterglow, the
surface concentration of oxygen is slightly further
increased to ~23 at%. In addition, there is incorporation
of nitrogen (~4 at%) and silicon (~6 at%).
To simulate industrial production conditions, prepainted steel skins (300x1200 mm) were transferred
immediately after plasma treatment at VITO to a
nearby industrial sandwich production line from
OCAS. Metal skins were joined to lightweight
polyolefin honeycomb and full core materials by means
of a thermoplastic PE foil. Upon lamination, samples
were transported to OCAS lab facilities where they
were cut into pieces of 20x300 mm for adhesion testing
using a floating roller setup (ISO 1464).
Fig. 7: Contact angle measurements before and after DBD
plasma treatment using N2/CO2 and N2/CO2 + APEO
Fig. 8: XPS measurements over 300 mm panel width before
and after DBD plasma treatment using N2/CO2 + APEO
Evaluation of adhesive strength for laminated
polyolefin honeycomb core structures showed little
difference in adhesive peel force values compared to
untreated samples. However, visual inspection of the
fractured samples showed much less adhesive defects
as shown in Fig. 9. This was reflected in lower
deviations in peel force values for plasma treated
materials (from ~10% to ~5%) and hence improved
product uniformity.
Fig. 11: Adhesion tests (ISO 1464) on full core composite
panels with and without N2/CO2 plasma treatment upon
accelerated ageing.
Fig. 9: Pictures of pre-painted steel surface upon adhesion
testing of honeycomb laminates. Untreated surfaces show
adhesive defects.
For further evaluation of adhesion improvement by
plasma surface treatments of pre-painted steel,
sandwich panels with full polyolefin core material were
subsequently produced on the industrial lamination
line. In such sandwich structures there is a much larger
contact area between the adhesive film and the core.
Adhesion tests show ~50% increase in peel force for
the N2/CO2 DBD plasma treatment and ~100% increase
for plasma treatment using N 2/CO2 + APEO (Fig. 10).
Reference samples were always taken from an
untreated area of the same composite panel to assure
exposure to similar treatment conditions. This explains
the different reference values for untreated samples.
The initial gain in bonding strength remains upon aging
at 40°C and 100% RH and even slightly improves as
shown by the increased distance between the two
graphs in Fig. 11. This is attributed to rearrangement of
polar groups in the adhesive film as well as on the prepainted steel surface.
4. Conclusions
An innovative DBD plasma source (PlasmaLine) was
used for adhesion promotion of pre-painted steel for
use in lightweight composite structures. The plasma
system can operate with various gas mixtures and
enables injection of liquid chemistry in the form of
nano-sized aerosols. Uniformity of the plasma
treatments is very high, resulting in reproducible
surface modifications over the full width of large test
panels. For polyolefin honeycomb sandwich panels, a
strong reduction of variation in peel values is observed
which indicates a more stable product quality. A
significant increase in adhesion strength (50-100%) is
obtained for full polyolefin core sandwich materials,
especially with injection of aminopropylthiethoxysilane (APEO). Increased bonding strength remains
upon accelerated aging.
5 Acknowledgements
The authors wish to thank the support of the European
Commission through the FP7 NMP program (N2P
2008-2012, grant agreement number 214134) in which
framework this work has been carried out.
Fig. 10: Floating roller adhesion test (ISO 1464) on full core
composite panels with and without DBD plasma treatment.
Adhesion testing upon accelerated aging indicated that
plasma treatments have a long term effect on the
bonding performance of laminated panels (Fig. 11).
Tests were performed one day after lamination (no
ageing), after 1 week ageing at 40°C and 100% relative
humidity (RH) (CH-test) and after 2 weeks of CHtesting.
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