22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Effect of applied voltage on coating process of zinc coated PMMA with
combined gliding arc discharge and spouted bed condition using solid precursor
W. Ua-amnueychai, S. Kodama and H. Sekiguchi
Department of Chemical Engineering, Tokyo Institute of Technology, Tokyo, Japan
Abstract: A novel coating process of combining solid precursor, spouted bed condition
and gliding arc discharge was used to produce zinc coated PMMA particles. Different
applied voltages, 1.5, 3.0, 4.5, 6.0 and 7.5 kV, were supplied to generate the arc discharge.
Depending on the applied voltage, the amount of zinc coating varied between 9.8 and
14.2 mg per 1 g of PMMA. The coated samples exhibited significant electrical
conductivity, approximately 2.3 x 10-2 S·m-1, independent of the applied voltage.
Keywords: gliding arc discharge, solid precursor, applied voltage, zinc coated PMMA
1. Introduction
Particle coating process is one of the most important
industrial operations in many industries, such as
pharmaceutical, food and electronic industries. Many
techniques are improvised to achieve additional resistance
or properties to the substrate [1, 2]. Some pursue novel
process by utilizing the combination of plasma and
fluidized/spouted bed condition [2, 3]. Uniformity and
flexibility of the system are some of the possible
advantages of this combined process. Due to the perfect
mixing and excellent rate of heat and mass transfer of the
fluidized/spouted bed condition, the coated particles are
expected to be uniform [3, 4]. Additionally, plasma
allows most of its energy to excite electrons, thus it
features high energy density and high reactivity, which
offers high selectivity and energy efficiency [5].
Moreover, plasma can also provide rapid coating process.
Based on our previous studies, particles coating process
was carried out using a novel system by combining the
gliding arc discharge and the spouted bed condition. Both
vapor precursor [6] and solid precursor [7] were used to
provide coating to the substrate particles. The flexibility
of the precursor is one of the benefits of this combined
gliding arc discharge and spouted bed system. Simplicity
and inexpensiveness of the gliding arc discharge also add
to its advantages.
In the case of combined solid precursor, spouted bed
condition and gliding arc discharge particle coating
process, zinc solid precursor was used to coat on to
PMMA particles [7].
A fine layer of zinc was
successfully coated onto the surface of the PMMA
particles with additional provision of electrical
conductivity to the polymer particles.
With the
continuation of the previous works, this present research
aims to investigate further in detail of this novel particle
coating process using solid precursor. The effect of
applied voltage on the amount of zinc coating on PMMA
particle, percentage surface coverage and the average
thickness of the coating layer were investigated.
Moreover, the relationship between the applied voltage
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and electrical conductivity of the coated samples was also
examined.
2. Methodology
A schematic of the gliding arc discharge reactor is
shown in Fig. 1. PMMA and zinc particles, 0.3 and
0.18 mm in diameter, respectively, were loaded inside the
reactor with a specific zinc/PMMA mass ratio of 0.10 in
the bed. They were pre-mixed by an argon gas, injected
from the bottom of the reactor, with a flow rate of
2 L·min-1. At this flow rate, a spouting condition was
achieved inside the reactor bed. The terminal settling
velocities of PMMA and zinc particles are 12.1 and
23.1 cm·s-1, while the minimum spouting velocity of the
system is 46.5 cm·s-1.
Fig. 1. Schematic of gliding arc discharge reactor.
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After the particles inside the reactor were pre-mixed, an
AC voltage was supplied to two diverging zinc wire
electrodes having a diameter of 1.0 mm. They made a
30° angle from each other. The supplied voltage resulted
in an initiation of arc discharge at the shortest gap
between the electrodes. With the help of the argon gas
flow, the arc discharge elongated along the electrodes
resulting in gliding arc discharge. The coating process
were allowed to proceed for 10 min and the coated
samples were collected for further analysis. Various
applied voltages, 1.5, 3.0, 4.5, 6.0 and 7.5 kV, were used
to study its effect on the coated particles.
The coated samples were separated from unused zinc
by taking advantage of the difference in their density,
1.18 and 7.14 g·cm-1 for PMMA and zinc particles,
respectively. The surface of the coated samples was
observed by scanning electron microscope (SEM), while
the percentage coverage of zinc coating layer is measured
by energy-dispersive X-ray spectroscopy (EDS). The
electrical conductivity of the coated sample was also
measured by suspending the coated sample between two
cylindrical aluminum rods. A constant pressure was
applied to both ends of the rods. The resistance across the
coated sample can be measured by a multimeter. Since,
the electrical conductivity is inversely proportional to the
resistance, therefore the electrical conductivity can be
calculated from the obtained resistance value.
The amount of zinc coating was also measured by
bleaching out the zinc coating. A mixture of 1 mL nitric
acid (1 mol·L-1) and 1 mL hydrogen peroxide was used as
a bleaching agent. Coated zinc underwent oxidative
reduction and dissolved in the solution, while PMMA
particles remained undissolved. Inductively coupled
plasma optical emission spectrometry (ICP-OES) was
used to measure the concentration of zinc dissolved in the
solution, which gave the amount of zinc coated on
PMMA particles.
The surface temperature and temperature profile of the
reactor were investigated using an infra-red (IR) camera.
The initiation point of the arc discharge was selected as
the focal point of the IR camera.
3. Results and Discussion
After 10 min of coating process, a fine layer was
observed on the PMMA surface for all experiments
conducted at 5 different applied voltages. The images of
zinc coating layer is shown in Fig. 2 for the sample
obtained at the applied voltage of 1.5 and 7.5 kV.
As observed from IR camera, it showed that the applied
voltage has a linear relationship with the temperature at
the surface of the reactor as shown in Fig. 3. As the
applied voltage increased, the maximum surface
temperature observed at the arc initiation point increased.
The temperature increased from 45.5 to 71.3oC for the
applied voltage of 1.5 and 7.5 kV, respectively. This
suggests that the power in the reactor bed was increased
by the increase in the applied voltage.
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Fig. 2. SEM images of the surface of the coated sample at
an applied voltage of a) 1.5, and b) 7.5 kV.
Fig. 3. Amount of zinc coating and the bulk surface
temperature of the reactor at the arc initiation point as a
function of applied voltage.
On the other hand, the temperature profile is
independent of the applied voltage. Surface temperature
was measured to be highest at the arc initiation point. The
temperature also remained relatively high along the
elongation path of the gliding arc discharge. The
temperature decreased as it moved further away from the
initiation point making a temperature profile as shown in
Fig. 4b.
For the amount of zinc coating calculated based from
ICP-OES result, it varied between 9.8 and 14.2 mg per 1 g
of PMMA, depending on the applied voltage as shown in
Fig. 3. At relatively low applied voltages (1.5 and
3.0 kV), the result showed higher amount of zinc coating
when compared to that at relatively higher voltages (4.5,
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6.0 and 7.5 kV). The greater amount of zinc coating
the average thickness of the coating layer when increasing
the applied voltage as shown in Fig. 5. Since the
percentage coverage remained constant, while the coating
amount
Fig. 4. Images of the surface of the reactor at an applied
voltage of 4.5 kV taken by a) high speed camera, and
b) infra-red camera.
observed at lower applied voltage is due to the
contribution of the thick rough structure observed on the
coating layer as shown in Fig. 2a. This could be
explained by the effect of the applied voltage as follows.
In the previous work [7], two coating mechanisms was
proposed, which are the coating by vapor deposition and
the coating by contact with the partially melted zinc
particle. At high applied voltage, large amount of energy
was available to vaporize zinc particles. The vaporized
zinc was able to move up along with the upwards flow of
argon gas and was allowed to coat onto the surface of
PMMA. This resulted in a smooth surface of the zinc
coating layer as shown in Fig. 2b. This smooth coating
probably was obtained by vapor deposition coating
mechanism as discussed previously. On the other hand, at
low applied voltage, there are lesser energy to vaporize
the zinc particle. This resulted in a fraction of zinc
particles being partially melted by the lower energy
gliding arc discharge instead of being vaporized. The
partially melted zinc contacted the PMMA surface
forming a coating layer. This effect can be observed in
Fig. 2a, where there are observable amount of thick rough
structure attached on the polymer surface resulted from
coating by contact with the partially melted zinc.
Only zinc particle was vaporized or partially melted by
the gliding arc discharge. This is possibly due to its
conductive nature. This potentially could allow the
excited electrons in the surrounding to excite zinc
particles, allowing them to vaporize or melt, while
non-conductive PMMA particle would remain relatively
unaffected.
Fig. 5 shows the thickness, calculated based from the
ICP-OES result, and the surface coverage of the coating
layer obtained by varying the applied voltage used to
generate the gliding arc discharge. The percentage
coverage of the coated sample remained relatively
constant. The surface coverage was possibly fixed at the
early stage of the coating process. This could be
determined by the coating rate at the beginning of the
process, which has no relationship with the applied
voltage. On the other hand, there was a slight decrease in
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Fig. 5. Surface coverage and thickness of the zinc coating
layer as a function of applied voltage.
decreased as the applied voltage was increased, this
resulted in the decreased in the average thickness of the
coating layer. At low applied voltages (1.5 and 3.0 kV),
the thickness of the coating layer was approximately 170
nm, while at high applied voltages (4.5, 6.0 and 7.5 kV),
the thickness of the coting layer was roughly 140 nm.
The change in the average thickness of the coating layer
was probably due to the different of the coating
mechanism as mentioned earlier. At low applied voltage,
the portion of coating by contact with the partially melted
zinc particle had increased, which resulted in a thick
rough portion of coating layer. This thick coating layer
contributed to the increase in the average coating
thickness; however, the thickness of the majority of the
coating layer obtained from coating by vapor deposition
should still be equal to that of the coated sample obtained
at high applied voltages.
The increase in the applied voltage did not cause any
change to the electrical conductivity of the coated sample
as shown in Fig. 6. The electrical conductivity of the
3
[4]
[5]
[6]
[7]
G.Z. Martins, C.R Souza, T.J. Shankar and
W.P. Oliveira. Chem. Engng. Process.: Process
Intensification, 47, 2238-2246 (2008)
A. Fridman, A. Chirokov and A. Gutsol. J. Phys.
D: Appl. Phys., 38, 2, R1 (2005)
H. Lee, S. Kodama and H. Sekiguchi. in: 12th
European Plasma Conference. (2012)
W.
Ua-amnueychai,
S.
Kodama,
W. Tanthapanichakoon and H. Sekiguchi. Chem.
Engng. J., in press (2014)
Fig. 6. Electrical conductivity of the coated sample as a
function of applied voltage.
2.3 × 10-2 S·m-1. This is because the percentage surface
coverage of the zinc coating remained constant, thus
having the same amount of established continuous
conductive pathways for electrons to travel. Moreover,
even though the average thickness of the coating layer
had increased, it is only due to the small portion of the
thick coating layer obtained by physical contact with the
partially melted zinc particle as discussed previously.
Hence, it did not promote the flow of electric charge as
the actual thickness of the majority of the coating layer
still remained unchanged.
4. Conclusions
The amount of zinc coating on the PMMA surface is
dependent on the applied voltage used to generate the
gliding arc discharge. At low applied voltage, there is a
significant amount of coating obtained by contact
between PMMA particles and the partially melted zinc
particle, which results in thick rough structure, thus
contributing to higher amount of zinc coating. For the
coating thickness, the average thickness tends to be higher
at low voltage. However the majority of the thickness
obtained from coating by vapor deposition still remained
the same as that observed in the coated samples obtained
at high voltage. There is no significant relationship
between applied voltage and surface coverage of the
coating layer as well as the electrical conductivity of the
coated sample.
5. References
[1] R.G. Szafran, W. Ludwig and A. Kmiec. Powder
Technol., 225, 52-57 (2012)
[2] Ph. Rodriguez, B. Caussat, X. Iltis, C. Ablitzer and
M. Brothier. Chem. Engng. J., 211, 68-76 (2012)
[3] I. Sanchez, G. Flamant, D. Gauthier, R. Flamand,
J.M. Badie and G. Mazza. Powder Technol., 120,
134-140 (2001)
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