st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Influence of dissipated discharge power on polymer powder treatment in
atmospheric pressure plasmas
G. Oberbossel1, D. Butscher1, C. Roth1, P. Rudolf von Rohr1
1
ETH Zurich, Institute of Process Engineering, Zurich, Switzerland
Abstract: A remote atmospheric pressure plasma reactor to increase the wettability of high
density polyethylene powder is investigated. Short, square, high-voltage pulses are used to ignite the discharge. The benzyl alcohol contact angle of the treated powder is a function of the
gas composition, total gas flow rate and applied pulse frequency. The achieved wettability
rises with increasing pulse frequency and thus higher effective discharge power.
Keywords: Particle Surface Modification, Wettability, Atmospheric Pressure Plasma.
1.
Introduction
About 60 % of all products and intermediates in the
chemical, pharmaceutical or food industry are powders.
Hence, the handling and processing of powders are common unit operations.
However, most polymers have a water contact angle
over 90° and are not wettable with water. Environmentally harmful surfactants are typically necessary to build
stable emulsions or pastes out of these powders.
Fine-grained powders own a very high surfaceto-volume ratio and the macroscopic behaviour of the
powder mainly depends on the surface structure and surface chemistry of the single powder particle. Therefore,
powder properties can be tailored by only minimal surface
modifications of the single particles while keeping the
particle bulk properties unaffected [1].
Plasma processes allow to efficiently increase the surface free energy of polymer particles and thus to increase
their wettability. An oxygen containing plasma enables to
incorporate polar groups into the particle surface, which
leads to an increased wettability of the treated powders.
At our institute, we have successfully implemented a
low-pressure plasma system to treat fine-grained powders.
This process enables to reduce the water contact angle of
high-density polyethylene (HDPE) powder from over 90°
to approximately 65° within only 0.1 s treatment time [2].
Nevertheless the low-pressure plasma system reveals
some drawbacks like high expenses for vacuum equipment and clogging problems due to particle-plasma interactions. We are currently developing a non-thermal atmospheric pressure plasma process to overcome these
drawbacks. In this reactor concept the powder is treated in
the afterglow of dielectric barrier discharges to avoid particle-plasma interactions and hence clogging of the reactor system.
Experimental
The remote treatment dielectric barrier discharge device,
designed for the treatment of powders consists of 64 discharge channels, which are concentrically arranged
around a cylindrical treatment zone (see Fig. 1). Eight
discharge channels with a depth of 0.5 mm, directing towards the treatment zone, are incorporated into a circular
polymethylmetacrylate (PMMA) plate with a thickness of
1.5 mm. This plate is bonded to a second PMMA plate
(thickness: 0.5 mm) and placed between the high voltage
and ground electrodes. Eight of these units are placed on
top of each other and build the multichannel plasma device. The discharge channels measure 37 mm in length
and converge from 10 mm at the entrance to 2 mm at the
outlet. The whole setup is casted in an epoxy resin to provide electrical and mechanical strength. Further details
regarding the setup are reported by Reichen et al. [3].
The multichannel plasma module is placed into the powder processing unit shown in Fig. 2. A screw-conveyer
carries the powder from a storage container to a nozzle,
where it is dispersed into the carrier gas flow and carried
through the treatment zone.
Argon and oxygen flowing through the discharge
channels are used as process gases. The dispersed particles pass the treatment zone in tens of milliseconds. After
the treatment zone a cyclone separates the powder particles from the gas flow.
2.
Fig. 1: Design of multichannel plasma module,
1 : Discharge channel, 2: Ground electrode,
3: High voltage electrode, 4: Treatment zone.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
The treatment of non-water soluble HDPE polymer
powder (Schaetti Fix 1820, Schaetti, Switzerland) is investigated in this study. The particle size distribution of
the dry powder is measured by laser diffraction
(Sympatec Helos KF) and the investigated HDPE batch
has a median particle diameter of 66.4 m. The analysis
of powder samples treated with the plasma process shows
no difference in the particle size distributions compared to
the untreated powder. Hence, the size of the powder particles is not affected during the process.
The measurement of a contact angle allows a quantitative statement of the wettability of a powder sample.
Here, the benzyl alcohol contact angle (BACA) of the
polymer powders is measured using the capillary penetration method [4]. A tensiometer (K100, Krüss GmbH,
Germany) measures the increase of mass over time, when
a compacted powder sample gets into contact with a test
liquid. The penetration rate (m2/t) is correlated with the
contact angle based on Washburn’s theory [5] according
to equation (1).
m2
t
Ccp
2
l
l
cos
(1)
l
The powder is wetted with hexane to measure the capillary constant Ccp. Hexane wets nearly every surface
with a contact angle of approximately 0° (cos = 1) and
thus, allows to calculate Ccp. Based on the measurement
of the capillary constant, the powder contact angle with
benzyl alcohol as wetting liquid can be measured. The
density l, dynamic viscosity l, and the surface free energy of the used liquids are given in Table 1.
Table 1: Properties of the used liquids at 25 °C
Property
Density l
Dynamic viscosity
Surface free energy
l
l
Hexane
0.661 g/cm3
0.326 mPa∙s
18.4 mN/m
Benzyl alcohol
1.041 g/cm3
5.651 mPa∙s
38.7 mN/m
A stainless steel cylinder with a filter paper (SH0621,
Krüss GmbH, Germany) on the bottom is filled with
1000 ± 5 mg of powder. The cylinder is tapped 250 times
using a jolting volumeter (STAV II, Engelsmann AG,
Germany). The tapping facilitates a uniform and reproducible compaction of the particles, which is essential for
reproducible contact angle measurements using the capillary penetration method.
The temperatures of both, the benzyl alcohol and the
stainless steel cylinder are kept constant at 25 °C during
the analysis. The BACA of each powder sample is measured at least three times to derive the mean value of the
contact angle.
Fig. 2: Powder treatment unit with atmosphericpressure multichannel reactor.
The plasma is ignited by means of short, square, unipolar, high-voltage pulses. The pulses are generated out of a
DC source using a fast high-voltage transistor switch. The
applied voltage is measured with a 1:1000 high voltage
probe (Tektronix, P6015) and a current probe (Pearson
electronics, 2877) is used to monitor the external total
current. Voltage and current form are recorded by a digital
oscilloscope (LeCroy WaveRunner 64Xi, 600 MHz).
Experiment are carried out at a pulse width of 550 ns
and a voltage amplitude of 2.5 kV. The pulse frequency
varies between 1 and 8 kHz. During the powder treatment
experiments the carrier gas flow is kept constant at 2 slm
Ar. The process gas flow rate amounts 5-40 slm Ar with
0-10 vol. % O2 admixture.
3.
Results and Discussion
The form of the applied voltage and external measured
current for the ignited multichannel plasma reactor are
shown in Fig. 3. The applied voltage amplitude measures
2.5 kV and the pulse frequency is adjusted to 1 kHz. The
total process gas flow consists of 40 slm (98 vol. % Ar,
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
2 vol. % O2). Rising and falling times of a single pulse
amount to approximately 42 ns. Two current pulses at the
rising and falling flank are detected, which indicate two
discharges per voltage pulse.
To estimate the effective discharge power, the time delay between current and voltage probe is determined first.
This was achieved by calculating the theoretical current
form out of the applied voltage in case of a non-ignited
multichannel device. The time-delay is obtained by comparing the maximum of the theoretical with the measured
current form.
The calculation of the effective discharge power out of
the shifted current and voltage forms follows the approach
developed by Liu and Neiger [6]. Discharge current and
voltage drop over the gas gap are calculated from the externally measured characteristics as also explained by
Williamson et al. [7].
At a pulse frequency of 1 kHz the calculation reveals an
effective discharge power of 0.4 W. The voltage and current form for a single pulse remain constant within the
explored frequency range of 1-8 kHz. Therefore, we suppose that the power input scales linearly with the applied
frequency.
59.3 ± 1.4° at the maximum applied total gas flow of
40 slm.
We suppose that more reactive species are transported
to the afterglow and thus to the powder treatment zone at
elevated gas flow rates. In contrast, many reactive species
recombine within the channel before reaching the treatment zone at low gas flow rates and thus low gas velocities.
Fig. 4: BACA as a function of the total gas flow
rate. O2 concentration: 0.25 vol. %,
pulse frequency: 5 kHz.
The influence of the O2 concentration in the total gas
flow on the resulting BACA is shown in Fig. 5. The O2
fraction is varied between 0 and 10 vol. % at a total constant gas flow rate of 40 slm. Here, the BACA decreases
also for all plasma treated samples. Pure Ar plasma leads
to a contact angle reduction of about 8.5° to 61.4 ± 0.5°.
Even small amounts of admixed O2 (≤ 2 %) decrease the
BACA further to approximately 59°. With increasing O 2
fraction in the process gas, the BACA rises again and
reaches 62.8 ± 0.8° for an O2 concentration of 10 vol. %.
Fig. 3: Voltage and current form for the multichannel device. Applied voltage: 2.5 kV,
frequency: 1 kHz, total gas flow rate: 40 slm,
O2 concentration: 2 vol. %.
The mean BACA of untreated HDPE powder measures
69 ± 1.1° and is used as reference value to estimate the
treatment efficiency of the remote multichannel plasma
device. No change of the BACA is detected for powders
transported through the treatment unit without plasma.
Fig. 4 shows the BACA of plasma treated powders in
dependence on the total gas flow rate. The O2 concentration is 0.25 vol. % and the pulse frequency is set to 5 kHz.
Compared to the untreated powder, a decrease of the contact angle is achieved for all plasma treated samples. For
the lowest applied total gas flow of 2.5 slm a decrease of
approximately 6° is measured. By increasing the total gas
flow rate the BACA is further reduced and achieves
Fig. 5: BACA as a function of the O2 fraction in
the total gas flow. Total gas flow rate: 40 slm,
pulse frequency: 5 kHz.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
We assume that the preferred formation of oxygen
radicals at low O2 concentrations leads to the drop of the
BACA, while an increase in the formation of ozone may
lead to the observed increase of the BACA at higher O2
fractions. A similar trend has been detected when treating
solid substrates with the remote multichannel device [3].
To investigate the influence of the discharge power on
the powder treatment, the pulse frequency is varied at a
constant total gas flow rate of 40 slm with an O2 fraction
of 2 vol. %. The BACA drops with increasing pulse frequency as shown in Fig. 6. A minimum mean BACA
value of 56.3 ± 0.7° is detected at the maximum applied
pulse frequency of 8 kHz. The effective discharge power
rises with increasing pulse frequency as explained previously and also indicated in Fig. 6.
More species are most probably produced and carried
to the treatment zone at higher pulse frequencies and thus
higher discharge powers.
The O2 concentration and the total gas flow rate influences the treatment, whereas best results are found for
low O2 concentrations (≤ 2 vol. %) and high total gas flow
rates (up to 40 slm).
5.
References
[1]
C. Roth, Nanoscale plasma surface modification of
powders, Dissertation, ETH Zürich, (2012).
[2]
C. Arpagaus, A. Rossi and P. Rudolf von Rohr,
Applied Surface Science, 252, 5, (2005).
[3]
P. Reichen, A. Sonnenfeld and P. Rudolf von Rohr,
Plasma Processes and Polymers, 6, S1, (2009).
[4]
G. Buckton, Journal of Adhesion Science and
Technology, 7, 3, (1993).
[5]
E. W. Washburn, Physical Review, 17, 3, (1921).
[6]
S. H. Liu and M. Neiger, Journal of Physics
D-Applied Physics, 36, 24, (2003).
[7]
J. M. Williamson, D. Trump, P. Bletzinger and B. N.
Ganguly, Journal of Physics D-Applied Physics, 39,
20, (2006).
6.
Acknowledgements
We would like to thank the Swiss National Fond (SNF)
and the Foundation Claude & Giuliana for the financial
support.
Fig. 6: BACA and average discharge power as a
function of the pulse frequency. Total gas flow
rate: 40 slm, O2 fraction: 2 vol. %.
4.
Conclusions
A remote atmospheric pressure plasma rector, designed
to increase the wettability of high density polyethylene
powder, is investigated. The discharge is ignited by means
of unipolar, fast-rising, high-voltage pulses. Benzyl alcohol contact angle measurements are performed to monitor
the wettability improvement of the plasma treated powder
samples.
Although the residence time of the powder particles in
the treatment zone measures only about 0.01 s, an effect
of the plasma treatment on the powder is confirmed.
The obtained benzyl alcohol contact angle decreases
with rising pulse frequency and thus higher effective discharge power. This is attributed to the increased production of reactive species, which are required to increase the
surface free energy of the polymer powders.