22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Multichannel reactor optimization for the activation of fine-grained polymer
powders
G. Oberbossel, S. Zihlmann, R. Wallimann and Ph. Rudolf von Rohr
Institute of Process Engineering, ETH Zurich, Zurich, Switzerland
Abstract: Polymer powder is activated in the afterglow of several dielectric barrier
discharges. To improve the activation efficiency of the used device, single micro channel
geometries are investigated regarding their ability to activate flat polymer substrates.
Results received from water contact angle measurements on these substrates are then used
to improve the design of the reactor applied for powder activation.
Keywords: polymer activation, powder, contact angle, dielectric barrier discharge
1. Introduction
Polymer powders are used in large quantities. Most
polymers own a low surface free energy. Environmentally
harmful surfactants are typically necessary to build stable
emulsions or pastes out of these powders. As alternative,
plasma processes effectively increase the surface free
energy of powder particles and thus lead to increased
powder wettability [1].
At our institute we have successfully implemented a
low-pressure plasma system to activate fine-grained
powders, which enables to reduce the water contact angle
(WCA) of high density polyethylene powder from over
90° to approximately 63° within only 0.1 s treatment time
[1].
Nevertheless, the low-pressure plasma system reveals
some drawbacks as for example high expenses for
vacuum equipment and clogging problems due to particlewall interactions within the plasma zone. Therefore, we
are currently developing a plasma process working at
atmospheric pressure. In this reactor the powder passes
the afterglow of 64 dielectric barrier discharges, which
are concentrically arranged around the cylindrical
activation region (see figure 1). A carrier gas stream
transports powder particles through the treatment region
of this multichannel plasma device (MCD). Particles pass
only the afterglow of the plasma discharges, and direct
particle-plasma interaction, often connected to a system
clogging, is avoided.
Powder particles pass the treatment zone within very
short time of about 0.01 s. The reduction of the contact
angle within this short residence time has already been
shown in a previous study [3]. To further increase the
surface free energy of particles we incorporated the
plasma device into a circulating fluidized bed reactor,
allowing multiple passes of the particles through the
treatment zone [4].
In this study we would like to discuss the influence of
the relevant geometry of the single micro channels on the
surface activation efficiency. Thereby an improved MCD
is developed allowing more efficient powder processing.
O-23-5
Fig. 1. Multichannel device, 1: Discharge channel,
2: Ground electrode, 3: High voltage electrode,
4: Treatment zone. Adapted from [2].
2. Experimental
The construction of several MCDs with different micro
channel geometries as well as the processing and analysis
of powder samples is time-consuming. Therefore, we
decided to study the activation of flat PMMA samples
using single micro channels as a first step. In this way,
channel geometry can be changed easily and the surface
activation is analyzed using WCAs determined from
sessile drops placed on the substrate surface.
The distance from the channel outlet to the sample
covers 10 mm. Before the experiments PMMA samples
are cleaned with isopropanol and dried for at least 30
minutes at ambient conditions.
Plasma was ignited with a sinusoidal high voltage at a
frequency of 8 kHz and amplitude of 6.4 kV pp . Unless
otherwise mentioned the total gas flow rate was set to
2 slm argon with 0.25 vol.-% oxygen admixed.
After plasma treatment the contact angle of deionized
water was measured within 3 minutes. To gain a better
statistics two droplets of water were positioned 6 mm left
and right from the treatment center. The contact angles of
1
the sessile drops were recorded using a goniometer
(Krüss, Contact angle measuring system G10).
We analyzed the decay of contact angle depending on
treatment time for each channel using a simple empirical
model. A contact angle decay constant was defined for all
geometries investigated, describing the efficiency in
activating the PMMA substrate.
3. Results
Figure 2 shows the measured contact angles depending
on plasma afterglow treatment time at standard process
conditions. A channel with two dielectric barriers, total
barrier thickness of 1.5 mm and aspect ratio (ratio of
channel height to width) of 0.375 was used during the
experiments.
Figure 4 presents contact angles obtained for the same
channel as in figure 3 but using a reduced total gas flow
rate of 1 l/min. A time constant of 66.8 s was determined
in this case. The slower activation was most probably
caused by the slower transport of active species from the
plasma zone to the afterglow and sample surface.
Apart of the influence of gas velocity on contact angle
decay, we investigated also the influence of micro
channel aspect ratio, total barrier thickness, number of
barriers, electrode length and distance between channel
outlet and substrate on the resulting WCAs.
Fig. 4. Measured WCA depending on plasma afterglow
treatment for the second channel geometry at a reduced
total gas flow rate of 1 l/min
Fig. 2. Measured WCA depending on plasma afterglow
treatment time at standard process conditions and fitted
regression curve.
As expected a fast drop of WCA for short treatment
times is observed. Subsequently the WCA approaches
asymptotically a saturation value, which is measured to
44 ± 1°. The fitted regression curve reveals a contact
angle decay constant τ of 43.6 s.
The WCA data for a micro channel with a total barrier
thickness of 3 mm and an aspect ratio of 1.5 is presented
in figure 3 for standard gas flow conditions. The obtained
time constant amounts 31.2 s.
Beside a high gas velocity, we found a thin total barrier
thickness as well as a high aspect ratio beneficial for fast
decay of contact angle. The electrode length didn’t have a
significant influence on WCA decay constant for the
investigated range between 10 and 40 mm. Also, time
constants for different distances between channel outlet
and sample didn’t change significantly as long as the gap
was below 10 mm. WCA time constant increased for
greater distances.
On the basis of these experiments a new MCD design
was developed. The total electrode length was decreased
to gain a lower overall capacity, and the dielectric
thickness was reduced compared to the old design,
whereas the channel aspect ratio was increased. First
powder activation experiments with the improved MCD
reveal better powder activation efficiency than the
previous one.
4. Conclusion
We found an effective method to optimize the MCD
used for powder treatment. Results obtained from the
activation of flat polymer substrates were transferred to
the microchannel geometry of the MCD. The whole
discharge unit was successfully redesigned resulting in
improved powder activation efficiency.
Fig. 3. Measured WCA depending on plasma afterglow
treatment time for a second micro channel geometry at a
total gas flow rate of 2 l/min.
2
5. Acknowledgements
We would like to thank the Swiss National Fond (SNF)
and the Foundation Claude & Giuliana for financial
support.
O-23-5
6. References
[1] C. Arpagaus, A. Rossi and P. Rudolf von Rohr,
Applied Surface Science, 252, 5, (2005)
[2] P. Reichen, Phd thesis, ETH Zürich, (2009).
[3] G. Oberbossel, D. Butscher, C. Roth and P. Rudolf
von Rohr, Proceedings of ISPC 21, (2013).
[4] G. Oberbossel. A.T. Güntner, L. Kündig, C. Roth
and P. Rudolf von Rohr, Plasma Processes and
Polymers, DOI: 10.1002/ppap.201400124, (2014).
O-23-5
3