22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Adhesion improvement between polyurethane paint and aluminium alloy
AA1100 treated by atmospheric pressure plasma
T.S.M. Mui1, L.L. Gonçalves da Silva1,2, V. Prysiazhnyi1 and K.G. Kostov1
1
Faculty of Engineering – FEG, Universidade Estadual Paulista - UNESP, Guaratinguetá, SP, Brazil
2
Technological Faculty of Pindamonhangaba – FATEC, Pindamonhangaba, SP, Brazil
Abstract: The effect of atmospheric pressure plasma treatment on the adhesion between a
protective coating and AA1100 alloy was investigated. Two plasma sources were used for
surface modifications: atmospheric pressure plasma jet (APPJ) and dielectric barrier
discharge (DBD). A significant improvement of surface wettability and adhesion was
obtained after plasma treatments. The paint coating was tested using the adhesion tape test
(ASTM D3359).
Keywords: aluminium alloy, DBD, atmospheric pressure plasma jet, adhesion
1. Introduction
Aluminium alloys are widely employed in many
industries. In the most applications, initial surface
processing is required, as, for example, pre-cleaning or
activation. The chemical pre-treatments, which are
frequently used for those purposes are not
environmentally friendly due to the production of
hazardous by-products. In opposite, atmospheric pressure
plasma technology can be an alternative technology for
initial surface processing.
Nowadays, two major
discharge systems are investigated: atmospheric pressure
plasma jet and dielectric barrier discharge [1-5].
2. Experimental
2.1. Materials
The material used in this work was 0.3 mm thick
aluminium alloy AA1100 (Al content 99 %). Samples
were pre-cleaned in ultrasonic bath by isopropanol and
stored in air for at least 24 hours prior any measurements
or plasma treatments. The reproducibility of treatments
was verified by performing each treatment on at least
three samples.
2.2. APPJ treatment
A plasma jet terminating with a wide (horn-like) nozzle
was used to treat the aluminium samples. The use of such
nozzle has not been reported before. The jet system
consists of Pyrex tube with wide nozzle (Ø 18 mm), HV
electrode placed inside the tube and a glass table with a
grounded electrode beneath it.
Plasma was generated with an AC power supply
operating at 19 kHz and applied voltage of 12 kV (plasma
power: 9.72 ± 0.2 W). The device was flushed with argon
flow of 1.2 L/min. Samples were exposed to plasma for
40 s on the nozzle-to-sample distance of 1 mm.
The experimental set-up of APPJ is illustrated in Fig. 1.
2.3. DBD treatment
The DBD reactor was a double barrier parallel plate
P-II-4-5
volume DBD (dielectric made of 1 mm Mylar). Plasma
was generated by an AC power supply (60 Hz) in air (8
L/min). The inter-electrode gap was fixed to 1 mm and
the applied voltage used was 30 kV (plasma power: 0.65
± 0.04 W). Samples were exposed to air-DBD plasma for
16 min. Time was chosen to get the same value of energy
per unit area in order to compare both treatments. The
DBD reactor configuration is shown in Fig. 2.
Fig. 1. Experimental set-up of the plasma jet.
Fig. 2. Experimental set-up of DBD.
2.4. Contact angle
Water contact angle measurements were performed on a
Rame-Hart 300 goniometer, using the sessile-drop
method.
1
2.5. Roughness
A Leica DCM 3D confocal microscope was used to
assess the surface roughness. The topography was
measured using 5x magnification objective with field of
view 2.55 x 1.91 mm2. The surface roughness (R rms )
value was obtained from a background-subtracted surface
topography using provided by producer software.
2.6. Painting
After plasma treatment all samples were painted with
polyurethane paint. The painting was performed no more
than 10 min after the treatments. Samples were dried in
ambient temperature for 48 hrs.
2.7. Adhesion tape test
The adhesion tape test was performed according to Test
Method B of ASTM D3359. The classification of the
adhesion was made by comparison with the adhesion test
table results available in ASTM D3359 [6].
3. Results and Discussion
3.1. Contact angle
A decrease of water contact angle from 87˚ (value for
the untreated Al surface) to less than 10° (after the
treatment) was obtained. Surface modifications using
both plasma systems led to a super-hydrophilic surface.
Fig. 3 shows the contact angle for the untreated sample
and for one treated with DBD.
Fig. 3. (a) untreated (b) treated with DBD.
3.2. Roughness
The mean roughness R rms of the untreated samples was
2.89 ± 0.57 µm. Both plasmas treatments showed
insignificant changes in the surface roughness, which
means that the improved wettability was not caused by
surface roughening.
3.3. Adhesion Tape Test
Fig. 4 shows adhesion tape test results on painted
samples. As can be seen in Fig. 4a, the paint inside the
grid came off completely after the tape test applied to the
untreated Al sample.
Surface treatment using APPJ (Fig. 4b) resulted in
perfect adhesion (no paint was removed inside the grid).
There was only paint detachment on the areas that were
outside the jet nozzle.
The treatment using DBD plasma (Fig. 4c) showed a
slight paint removal (less than 5% of the painted area
inside the grid).
2
Fig. 4. Adhesion Tape Test results for AA1100
samples: (a) untreated, (b) APPJ treated, and (c) DBD
treated.
4. Conclusions
AA1100 alloy was treated with two commonly used
atmospheric pressure plasma sources (APPJ and DBD).
The effect of plasma on surface wettability, roughness
and adhesion of polyurethane paint were investigated.
1) The contact angle decreased from 87° (nearly
hydrophobic) to less than 10° (super-hydrophilic) after
both plasmas treatments.
2) The plasma modifications did not induce significant
changes in topography and surface roughness. Thus,
the main effect of the plasma is in surface activation
and cleaning.
3) With the adhesion tape test was possible to verify that
both plasmas treatments were very efficient in
improving the adhesion of the polyurethane paint to
the aluminium substrate when compared to the
untreated sample.
5. References
[1] L. Bárdos and H. Baránková. Thin Solid Films, 518,
6705-6713 (2010)
[2] C. Tendero, et al. Spectrochim. Acta B, 61, 2-30
(2006)
[3] V. Prysiazhnyi, et al. Surf. Coatings Technol., 206,
4140-4145 (2012)
[4] K.G. Kostov, et al. Surf. Coatings Technol., 234,
60-66 (2013)
[5] E. Ritz, et al. Surf. Coatings Technol., 251, 64-68
(2014)
[6] ASTM D3359-09. Standard Test Methods for
Measuring Adhesion by Tape Test.
P-II-4-5