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The characteristics of particle charging and deposition during powder coating processes with ultrafine powder1

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The characteristics of particle charging and deposition during powder coating processes with
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2009 J. Phys. D: Appl. Phys. 42 065201
(http://iopscience.iop.org/0022-3727/42/6/065201)
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IOP PUBLISHING
JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 42 (2009) 065201 (12pp)
doi:10.1088/0022-3727/42/6/065201
The characteristics of particle charging
and deposition during powder coating
processes with ultrafine powder
Xiangbo Meng, Jingxu (Jesse) Zhu and Hui Zhang
Department of Chemical and Biochemical Engineering, The University of Western Ontario, London,
Ontario, N6A 5B8, Canada
Received 18 August 2008, in final form 21 January 2009
Published 25 February 2009
Online at stacks.iop.org/JPhysD/42/065201
Abstract
In a preceding work, the mechanisms of particle charging and deposition during powder
coating processes were explored with coarse polyurethane powder. In this paper, the
developed mechanisms were further examined with ultrafine polyurethane powder in order to
meet the growing needs for ultrafine powder in finishing industries. This study first verified the
previous findings in particle deposition, which account for a cone-shaped pattern formed by
deposited particles on the substrate and a rise in particle accumulation in the fringe region. It
was further demonstrated with ultrafine powder that, as disclosed by using coarse powder, the
primary charging of in-flight particles competes with back corona in particle deposition
processes, and the highest deposition efficiency is a compromise by balancing their effects. In
comparison with coarse powder, ultrafine powder presents a faster reduction in the deposition
rate with extended spraying duration, but shows some superiority in the uniformity of the
deposited layer. In the case of charging characteristics of the deposited particles, it was further
substantiated with ultrafine powder that the secondary charging mechanism takes
predominance in determining the distribution of local charge-to-mass ratios. It was also
disclosed that ultrafine powder shows a decreasing charge-to-mass ratio with increased
charging voltage in the deposited layer, opposite to the increasing tendency of coarse powder.
However, it was commonly demonstrated by both coarse and ultrafine powders that the
charge-to-mass ratio of the deposited particles decreases with the extended spraying durations.
In comparison, ultrafine powder is more likely to produce uniform charge-to-mass ratio
distributions in the deposited layer, which contrast sharply with the ones associated with the
coarse powder. In conclusion, it is believed that this study supplements the preceding study
and is of great help in providing a comprehensive understanding of the mechanisms of corona
charging processes of different powders.
(Some figures in this article are in colour only in the electronic version)
the finishing industries emerged in the USA in the 1950s as
electrostatic powder coating [10]. Since then, powder coating
has promptly occupied an important place in the finishing
industries in place of conventional liquid coatings, due to its
overwhelming advantages in saving energy and cost, resisting
corrosion and not releasing any volatile organic compounds
(VOCs) [10].
A typical powder coating system consists of a spray gun
and a metal substrate in a point-to-plane geometry, and usually
negative high voltages are supplied to the gun tip in order
1. Introduction
Since the studies on point-to-plane corona discharge started
from the end of the nineteenth century, it is now involved in a
number of commercial and industrial applications [1–10]. Of
these applications, corona discharge is often used to provide
ions for charging materials, and corona charging performs
more reliably and controllably than induction charging and
tribo-charging [6–10]. After its first successful practice with
an electrostatic precipitator in 1907 [5], corona charging in
0022-3727/09/065201+12$30.00
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© 2009 IOP Publishing Ltd
Printed in the UK
J. Phys. D: Appl. Phys. 42 (2009) 065201
X Meng et al
Q/M of the overall deposited particles, but they overlooked the
characteristics of the non-uniform inter-electrode electric field,
which may produce some influence on the charging behaviour
of the local deposited particles.
In the case of deposited particles, the accumulated charge
in the deposited layer often incurs an abnormal discharge,
named back corona. Masuda and Mizuno [27–29] defined the
initiation of back corona by using the following correlation:
to initiate coronas. During the coating processes, powder
particles of high resistivity are entrained in transportation
gas streams and thereby sprayed out from the gun. Usually
particles bypass a deflector at the gun outlet to form a dispersed
powder cloud in the inter-electrode space. Wu [11] pointed out
that the charging of in-flight particles is mainly within 50 mm
of the corona electrode, named primary charging. Earlier
studies disclosed that the charge obtained by in-flight particles
only accounts for less than 10% of the total corona current
while free ions take 90% [8, 10, 11]. Due to the convergence
of free ions on the substrate, the deposited particles may
receive extra charge by the so-called secondary charging.
During the trip to the substrate, charged in-flight particles are
subjected to several kinds of forces (gravitational, aerodynamic
and electrostatic forces), and their precise trajectories mainly
depend on the balance between electrostatic and aerodynamic
forces [10]. Adamiak [12, 13] numerically predicted that
the powder cloud exhibits a dispersing tendency, and both
the charge and sizes of in-flight particles are important in
determining their trajectories. However, aerodynamic forces
are dominant in the region close to the gun and electrostatic
forces become increasingly important as charged particles
approach the substrate [10]. In particular, the motions of
in-flight particles are only dominated by electrostatic forces
in the vicinity of the substrate of about 10 mm [7], and the
deposited particles mainly depend on their image forces to
adhere onto the substrate [10, 11]. Thus, a powder coating
process in essence is both a particle charging and a deposition
process [8].
In the case of particle charging, the Pautheniner limit was
widely referred to in the literature to interpret the maximal
(saturation) charge, which is proportional to the electric field
and the square of the particle radius [11]. Furthermore,
the time to reach the saturation charge is proportional to
the electric field but inversely proportional to the corona
current [7, 10]. However, in order to evaluate the charging
efficiency of a particle, its charge-to-mass ratio (Q/M) is
usually employed, which is proportional to the electric field
but inversely proportional to the particle radius [11]:
Qmax /M ∝ E/r.
Ed = ρd J Eb ,
(2)
where ρd is the resistivity of the dielectric and Ed and Eb
the electrical field across the layer and the breakdown field of
the layer, respectively. Because back corona produces ions
of opposite polarity with respect to the fore corona at the
gun, the charge of the deposited and the in-flight particles is
possibly reduced by neutralization. Tachibana [30] observed
that arriving particles changed their paths inversely in the
vicinity of the substrate and were charged oppositely under the
onset of back corona. Thus, back corona is commonly regarded
as the main cause of reduced deposition efficiency and makes
particle deposition a self-limiting process [7, 10, 27–29]. In
particular, as reviewed by Bailey [10], Basset et al illustrated
that free ions play a very important role in limiting the thickness
of the deposited layer and concluded that the self-limiting
process during powder coating is caused by back corona rather
than the repulsion of the already deposited charged particles
to arriving particles. In practice, due to the difficulties in
directly detecting free ions of both polarities, an increase in the
current density often serves as evidence of back corona [4, 31].
With reference to particle deposition during powder coating
processes, some earlier efforts [26, 32, 33] simply inspected the
first-pass-transfer-efficiency (FPTE), a mass ratio of the overall
deposited particles to the overall sprayed particles. Although
Ye et al [34, 35] noticed cone-shaped particle accumulation on
a substrate, no further explanations were provided.
Based on the above knowledge, it is clear that, partially due
to its complexity, few studies correlated the particle charging
and the deposition process to provide a deep insight into the
powder coating process. Motivated to explore the underlying
mechanisms in powder coating processes, recently the authors
conducted a series of investigations using two powder systems
(coarse and ultrafine, as defined by Zhu and Zhang [36], the
former larger than 30 µm and the latter smaller than 25 µm
in their mean particle sizes). In a preceding work [37], the
authors demonstrated that ultrafine powder would produce
a stronger suppression on the corona current in comparison
with the coarse powder, but both powders resulted in distorted
electric fields between the electrodes. Thus, the properties
of the applied powders influence the characteristics of the
powder coating processes. In another preceding work [38],
the characteristics of particle charging and deposition with the
coarse powder were disclosed. In this paper, the characteristics
of particle charging and deposition with ultrafine powder are
revealed, and the differences in the two powders are discussed.
More importantly, in the current state of art powder
coating is suffering from its extensively applied coarse powder,
whose mean particle size is typically in the range 30–40 µm,
because many aesthetic problems (such as thick film, orange
(1)
Obviously, charging behaviour of particles is closely related
to the characteristics of the inter-electrode electric field. For
point-to-plane corona discharges, several empirical formulae
were proposed to describe the current–voltage relation
[2, 14, 15], and Warburg’s law was extensively applied to
predict the current density distribution [16–18]. However,
the particle charging process is even more complex in the
powder coating processes, for charged in-flight particles not
only distort the inter-electrode electric field but also incur
corona quenching [19], a drop in the corona current. In earlier
studies, some works [20–22] were dedicated to investigating
the charging behaviour of in-flight particles and a suction-type
Faraday pail with an insulator pipe attached to its inlet [20] was
found to be more reliable in providing precise measurements.
Unfortunately, those studies provided no clues to correlate the
primary charging of particles with their deposition. Some
other works [23–26] were conducted to examine the average
2
J. Phys. D: Appl. Phys. 42 (2009) 065201
X Meng et al
Booth
Substrate
Corona Gun
Electrometer
Current
Analog Signal
Screw Feeder
Negative
Voltage
Digital
A/D Board Data
Powder
Supply
LabVIEW
Computer
On/Off
Signal
Pulse Signal
Venturi Pump
Feeding Gas
Gun Control
Unit
(a)
G
On/Off
Signal
A
P
System
Controller
A/D
T
PC
(b)
V
Figure 1. Experimental setup: (a) schematic diagram of the experimental system; (b) the nominal circuit followed by the corona current:
V—voltage supply, G—corona gun, P—corona electrode, T—substrate, A—electrometer, A/D—A/D board and PC—computer.
in a Nordson® Model 902 booth. An electrometer (Model
6514 Keithley® ) was used to receive currents, which were
then transferred to a computer for storage via an A/D board
(NI Lab-PC-1200). The nominal circuit of the current is
illustrated in figure 1(b). The corona gun equipped with a coneshaped deflector (as illustrated in figure 2(a)) was mounted
on a support stand 300 mm away from the substrate. The
configuration of the gun tip was introduced previously [14],
and the tip was supplied with high voltages to induce corona
discharges. In this study, three charging voltages (30, 60 and
90 kV) and spraying durations (5, 10 and 20 s) were applied.
In addition, the air relative humidity was controlled at 50 ± 2%
and room temperature of approximately 23 ◦ C.
peel, pinholes and craters) are incurred on the coated films
[10, 36, 39]. These problems are preventing powder coating
from widening into the core of liquid coatings. For instance, all
the applications of powder coating in automotive industries are
limited to underbody, trim components, steel and aluminium
car wheels [39]. However, ultrafine powder was successfully
demonstrated in the literature of being capable of improving
the film appearance and decreasing the film thickness [36, 39–
41]. Therefore, ultrafine powder is attracting more and more
attention from the finishing industries due to its superior
properties and is considered as the next generation of powder
coating [8, 36, 39, 42, 43]. In this case, this study is important
not only for understanding the related physical mechanisms
but is also informative for practical applications. This paper is
a strong supplement for gaining comprehensive knowledge of
powder coating processes.
2.2. Measurement techniques and materials
As shown in figure 2(b), the substrate (i.e. the collecting
electrode) was a 300 mm diameter blank printed circuit board
covered with a 0.2 mm thick layer of copper. It was divided into
ten 15 mm wide annular regions by nine marked borderlines in
order to facilitate the study of the local charging and deposition
characteristics of the deposited particles. The regions were
named as A1, A2, . . . , A10 in sequence outwards. Nine tiny
protruding points (as shown in figure 2(b) by dark dots) made
of glue help discriminate different regions of the substrate after
a coating cycle.
In the investigation of the charging and deposition
characteristics of the deposited particles, a suction-type
Faraday pail, as illustrated in figure 3, was used to collect
particles deposited in each region. The inner pail was
connected with a polyethelyne (HDPE) insulator suction pipe,
2. Experimental
2.1. Apparatus
The experimental setup in this study is the same as that used
in [38] and illustrated in figure 1(a). A system controller
synchronized the powder feeding, voltage supply and current
data collecting by sending three signals simultaneously to
the gun control unit, the screw feeder and the A/D board,
respectively. The powder was accurately fed into a Venturi
pump by a SCHENCK AccuRate® screw feeder at 1.0 g s−1
and then pneumatically transported to a Nordson Surecoat™
negative corona gun by a feeding gas of 1.5 bar. The powder
particles were charged and then coated on a metallic substrate
3
J. Phys. D: Appl. Phys. 42 (2009) 065201
X Meng et al
On the other hand, the charging behaviour of in-flight particles
was investigated selectively. The method is illustrated in
figure 4(b): the extension pipe of a Faraday pail passed through
the centre of the substrate from its backside and sucked in inflight particles into the pail at different locations along the
centreline. In this case, the pail was free of the outer bass
pipe used in figure 4(a), and the extension pipe was 500 mm
in length. In this study in-flight particles were collected in
the vicinities (within 50 mm) of the gun tip and the substrate,
respectively, and their charges were compared.
Furthermore, another substrate was applied to measure the
local current densities associated with the coating processes.
Its dimensions were the same as the ones in figure 2(b), but
annular regions were physically insulated by 0.6 mm air gaps,
instead of marked borderlines.
The powder used in this study was black polyurethane
paint with a mean particle diameter (D50 ) of around 12 µm
and supplied by Links Coatings (London, Ontario). Its
representative particle size distribution (PSD) determined by
Malvern Mastersizer® was compared with the coarse powder
used in the preceding work [38] and is shown in figure 5.
D10 , D50 and D90 represent the volume percentages of powder
with the diameter less than the stated diameter, e.g. D50 is
the equivalent volume diameter where 50% of the particles in
volume have a diameter smaller than the stated diameter.
Gun Tube
Gun Tip
(a)
Deflector
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10
3. Results and discussion
3.1. Characteristics of the particle deposition
At a spraying duration of 20 s and various charging voltages
(30, 60 and 90 kV), the profiles of the local particle mass-tosurface ratio (M/S) are illustrated in figure 6, in which particle
deposition was assumed centrosymmetric. It is obvious that
the deposited particles of the ultrafine powder present a coneshaped pattern in the internal regions (A1–A9) and a M/S rise
in the fringe region (A10) in all the cases, both of which are
identical to the previous observation of the coarse powder in
[38]. In the preceding work [38], it was believed that the coneshaped distribution of deposited particles was mainly attributed
to inhomogeneous concentrations of in-flight charged particles
in the powder cloud whereas the M/S rise in the fringe region
was due to the edge effect.
It is well known that electrostatic forces are the main
reasons for the adherence of the employed particles to the
substrate in powder coating processes. So, the primary
charging of in-flight particles becomes the premise of
particle deposition. On the other hand, the particles form
a powder cloud initially in the vicinity of the corona
electrode while receiving their primary charges. Thereafter
the cloud evolves more dispersed in travelling towards the
substrate, for the newly incoming charged particles suffer
retardation from the foregoing space charge (consisting of
charged particles and free ions) and repulsion in the radial
direction from the surrounding space charge. Earlier studies
[10, 12, 13] disclosed that the trajectories of in-flight particles
strongly depended on the balance between aerodynamic and
electrostatic forces, which was particle size dependent. In
(b)
Figure 2. (a) Schematic view of the gun tip and deflector; (b) the
configuration of the substrate.
which was 6.5 mm in inner diameter (I.D.), 9.5 mm in outer
diameter (O.D.) and 300 mm in length. The outer pail was
attached with a brass pipe (10 mm I.D. and 11 mm O.D.)
used to level the inner insulator pipe and electrically shield
the device. The sampling method of the deposited particles is
shown in figure 4(a): the corona gun was removed promptly
after a coating cycle and replaced with a compass; the holes on
the compass’s arm helped direct the suction pipe of the pail in
a certain region of the substrate for each hole corresponding to
one annular region, and the deposited particles were sucked
into the pail by vacuum in the rotating processes of the
compass’s arm. No particles remained in the pipe during
samplings. The collected particles were then determined
with their charge and weight by a Model 6514 Keithley
electrometer and a digital balance (with an accuracy of 0.001
in grams), respectively. In this case, the mass-to-surface ratio
(M/S: g m−2 ) and the charge-to-mass ratio (Q/M: µC g−1 ) of
a certain region would be known and used to characterize the
particle deposition and the charging, respectively. To alleviate
the influence of the charge decay during the samplings, only
one annular region was sampled right after a coating cycle and
the procedures were repeated at least twice for each region.
4
J. Phys. D: Appl. Phys. 42 (2009) 065201
X Meng et al
Outer Metallic Faraday Pail
Inner Metallic Faraday Pail
Metallic Extension Pipe
Insulator Suction Pipe
Vacuum Suction
Porous Thimble Filter
BNC Connector to Electrometer
Figure 3. Schematic configuration of the suction-type Faraday pail with an extension pipe.
Earthed Plane
7
Ultrafine Powder
D10 = 2.65 µm
6
Extension Pipe
D50 = 12.36 µm
Vacuum
Volume (%)
Faraday
Pail
5
D90 = 33.72 µm
4
Coarse Powder
D10 = 11.24 µm
3
D50 = 35.13 µm
D90 = 90.78 µm
2
(a)
Compass
1
Earthed Plane
0
1
10
Particle Size (µm)
100
Figure 5. The PSDs of the applied ultrafine and coarse powder.
Corona Gun
Extension Pipe
140
Spraying Duration 20 s:
60 kV;
30 kV;
90 kV
Vacuum
Faraday
Pail
120
(M/S)i (g/m2)
100
(b)
Figure 4. Schematic diagrams of the samplings: (a) samplings of
deposited particles; (b) samplings of in-flight particles.
80
60
40
20
particular, it was demonstrated in the preceding work [38] that
small particles had stronger propensities to move towards the
peripheries of the powder cloud, due to their higher charging
capability and less inertia (both evaluated by their mass).
In this study, the characteristics of the powder cloud were
examined and shown in figures 7(a) and (b). First, by applying
the method shown in figure 4(b), the in-flight particles were
sampled in the vicinities of the gun tip and the substrate for a
spraying duration of 20 s. The results in figure 7(a) imply that
the powder cloud evolved more dispersed while approaching
the substrate, for fewer particles were collected in the vicinity
of the substrate and higher charging voltages promoted the
dispersing progress. Furthermore, the deposited particles in
each region were collected by applying the method illustrated
in figure 4(a) and their particle sizes are shown in figure 7(b).
0
10
8
6
4
2
0
2
4
6
8
10
Annular Region
10 9 8 7 6 5 4
3 2 1
1 2
3 4 5 6 7 8 9 10
Figure 6. The distributions of the local mass-to-surface ratio
((M/S)i ) for deposited particles.
5
J. Phys. D: Appl. Phys. 42 (2009) 065201
X Meng et al
50
2.0
20 s:
50 mm to the Gun Tip;
50 mm to the Substrate
45
Spraying Duration 5 s:
30 kV
60 kV
90 kV
40
35
(M/S)i (g/m2)
M (g)
1.5
1.0
0.5
30
25
20
15
10
5
0.0
0
30
60
Charging Voltage (kV)
(a)
90
10
8
6
4
(a)
2
0
2
4
6
8
10
Annular Region
70
24
20 s:
Original Ultrafine Powder
30 kV;
60 kV;
Spraying Duration 10 s:
30 kV;
60 kV;
90 kV
60
90 kV
22
50
(M/S)i (g/m2)
D50 (µm)
20
18
16
40
30
20
14
10
12
0
10
A1-A4
(b)
A5-A7
A8-A10
(b)
10
8
6
4
2
0
2
4
6
8
10
Annular Region
Annular Region
Figure 8. The effects of charging voltage on the local
mass-to-surface ratio ((M/S)i ) distributions: (a) in a spraying
duration of 5 s; (b) in a spraying duration of 10 s.
Figure 7. (a) The number (in mass: M) of in-flight particles
collected in a spraying duration of 20 s in the vicinity (50 mm) of the
gun tip and in the vicinity (50 mm) of the substrate; (b) the size
evolutions of deposited particles in local regions.
in figures 6, 8(a) and (b), it is easy to learn that there are
more particles to deposit on the substrate with an extended
spraying duration, and the effects of the charging voltage
account for an improvement in the deposition efficiency with
the voltage increased from 30 to 60 kV but a comparable
efficiency between 60 and 90 kV.
To clarify the characteristics of particle deposition due
to different powders, the deposition rates of the ultrafine
powder were illustrated in figure 9(a) in comparison with
those induced by the coarse powder of the preceding work
[38]. It is evident that the rates with ultrafine powder show
a faster decrease with time, except for a slight increase in the
segment between 10 and 20 s for the voltage of 30 kV. It can be
further observed that there exists an optimal voltage to achieve
the highest deposition efficiency, which is around 60 kV for
both powders. Exceeding this optimal voltage, the deposition
efficiency exhibits a comparable value for the ultrafine powder
but a decrease for the coarse powder. Furthermore, the
uniformity of the deposited layers was evaluated by using
It is obvious that the deposited particles present a sizedecreasing tendency along the radial direction, which accounts
for the variations in the trajectories of different particles.
Thus, it is easy to understand that particle concentrations are
locally inhomogeneous in the powder cloud. As a result, the
discrepancy in local particle fluxes is responsible for the coneshaped pattern formed by the deposited particles. However,
the exception in the M/S of the fringe region is thought
to be mainly due to a much stronger electric field between
the fringe and the corona electrode, which compensates the
correspondingly lower particle concentration by capturing
particles with a higher efficiency and thereby contributes to
the M/S rise in the fringe region. This is called the edge effect
and will be further demonstrated later in this paper.
Similarly, cone-shaped distributions of the internal M/S
and the edge effects of the fringe M/S were observed at
spraying durations of 5 s and 10 s, which are illustrated in
figures 8(a) and (b) respectively. Combining the M/S results
6
J. Phys. D: Appl. Phys. 42 (2009) 065201
X Meng et al
0.6
4.0
Ultrafine Powder Coarse Powder
30 kV
30 kV
60 kV
60 kV
90 kV
90 kV
0.3
2.0
1.5
1.0
0.0
0.5
0.0
5
10
15
20
35
30
Ultrafine Powder
30 kV
60 kV
90 kV
15
10
5
10
15
90
increases with the charging voltage for both powders, for
higher voltages produce higher electric fields and higher corona
currents. However, the ultrafine powder shows a lower Q/M
with respect to the coarse powder, which may be partly due to a
shorter exposure of the ultrafine powder to the electric field of
the gun tip but more possibly due to a weaker electric field and
much more severe corona quenching induced by the ultrafine
powder. As for corona quenching effects of both powders,
more details were discussed in a preceding work [37] and the
ultrafine powder showed a stronger propensity in weakening
the electric field and in reducing the corona current. More
importantly, the Q/M of in-flight particles becomes lower in
the vicinity of the substrate for both powders. In particular, the
higher the charging voltage is, the more severe the reduction
in Q/M is induced. The reason is that, as disclosed by
equation (2), the back corona has a propensity to intensify with
the charging voltage, due to higher current densities produced
by higher voltages. In addition, it can be seen from figure 10
that in the vicinity of the substrate the Q/M values with the
voltages of 60 and 90 kV are at a comparable level for both
powders. On the other hand, it was demonstrated in [38]
that the back corona showed an intensifying tendency with
time, which was evaluated by the increased part of the corona
current and is verified in figure 11 in this study. The initial
drops in the current profiles are due to the effect of corona
quenching, and thereafter the increasing currents account for
the intensifying back corona. In addition, it was also observed
in this study that some particles could be ejected from the
deposited layer with some severe occurrences of back corona
and left pinholes or craters in the deposited layer. In figure 12,
the appearance of the coated film is exemplified in the case
of 90 kV in an elongated spraying duration of 60 s in order to
get clear information on pinholes. If the charging voltage of
30 kV was applied at the same conditions, the film appearance
became fluffy and loosened without evident pinholes.
As a result, it was demonstrated by the results of
figures 10–12 that increasing the charging voltage improves
20
5
60
Figure 10. Comparisons of the charge-to-mass ratio (Q/M) of
in-flight particles in the vicinity (50 mm) of the gun tip with those in
the vicinity of the substrate (50 mm) for different powders.
Coarse Powder
30 kV
60 kV
90 kV
25
0
30
Charging Voltage (kV)
Spraying Duration (s)
(a)
Standard Deviation σ
2.5
0.2
0.1
(b)
50 mm to the Gun Tip 50 mm to the Substrate
Ultrafine Powder
Ultrafine Powder
Coarse Powder
Coarse Powder
3.0
0.4
Q/M (µC/g)
Deposition Rate (g/s)
0.5
3.5
20
Charging Voltage (kV)
Figure 9. (a) Comparisons of deposition rates due to different
powders; (b) Comparisons of standard deviations (σ ) of the local
mass-to-surface ratio ((M/S)i ) distributions due to different
powders.
standard deviations of local M/S distributions, as shown in
figure 9(b). Remarkably, the ultrafine powder shows a more
uniform deposited layer under the same conditions, and the
uniformity of the deposited layer is opposite to the deposition
efficiency.
With reference to the characteristics disclosed in
figure 9(a), the preceding work in [38] attributed them to the
competing result between the primary charging of in-flight
particles and the occurrences of back corona in the deposited
layer. As the underlying mechanisms, the primary charging
and the back corona exert opposite influences on the ultimate
charges of in-flight particles and thereby combine to determine
their electrostatic forces in the vicinity of the substrate. To
demonstrate the so-called competition between the primary
charging and the back corona, the charge of the in-flight
particles was investigated in both the vicinities of the gun tip
and the substrate by using the method described in figure 4(b)
in a spraying duration of 20 s. As shown in figure 10, in
the vicinity of the gun tip the Q/M of in-flight particles
7
350
300
250
200
150
100
50
6
5
4
3
2
1
0
X Meng et al
2.0
Spraying Duration 20 s:
30 kV;
60 kV;
90 kV
1.5
90 kV
(Q/M)i (µC/g)
JOverall (µA/m2)
J. Phys. D: Appl. Phys. 42 (2009) 065201
60 kV
30 kV
1.0
0.5
0
5
10
Spraying Duration (s)
15
20
0.0
Figure 11. The dependence of overall current densities on the
charging voltage.
10
8
6
4
2
2
0
Annular Region
4
6
8
10
Figure 13. The distributions of local charge-to-mass ratio ((Q/M)i )
for deposited particles.
90 kV
additional charges by the secondary charging mechanism due
to the convergence of ions on the substrate. In the meantime,
the back corona induced by the accumulated charge of the
deposited layer may discharge the deposited particles with an
intensifying tendency. Therefore, back corona and secondary
charging also compete in determining the particle charging
characteristics of the deposited layer. In the preceding work
[38] it was demonstrated with the coarse powder that the
secondary charging of deposited particles was predominant
in determining the characteristics of local Q/M distributions,
and the back corona reduced the Q/M of the deposited layer
with the extended spraying durations. In the preceding work
[38] secondary charging was interpreted and evaluated by
employing the concept of the current-to-mass ratio (A/M).
In this study, the local Q/M distribution profiles of the
deposited particles are first illustrated for ultrafine powder
in figure 13 under a spraying duration of 20 s and various
charging voltages (30, 60 and 90 kV). It is clear that the
Q/M distributions of the ultrafine powder are fairly uniform,
despite a rising tendency with 90 kV in some external regions
(A8–A10). In order to explore the effects of secondary
charging on the Q/M distributions of the ultrafine powder,
the profiles of local current densities are shown in figure 14
under a spraying duration of 20 s and various voltages, whose
characteristics have been discussed in [37]. It is evident that
there is a remarkable rise in the current density of the fringe
region, which is called the edge effect and is thought to be
responsible for the M/S rise in the fringe region (as shown in
figures 6, 8(a) and (b)). By further dividing the local current
density (Ji : µA m−2 , see figure 14) by the corresponding M/S
(g m−2 , see figure 6) of a certain region, the profiles of local
A/M for a spraying duration of 20 s are obtained and shown in
figure 15. Comparing the Q/M profiles in figure 13 with the
A/M profiles in figure 15, it is noticeable that the uniformity
of the A/M profiles underlies the uniform Q/M distributions,
despite some variance between them existing in the fringe
region at the voltages of 60 and 90 kV. The variance possibly
implies that the local current of the fringe region concentrates
more at the edge of the fringe region rather than distributes
Figure 12. The surface defects incurred by back corona.
the primary charging efficiency of the in-flight particles
in the vicinity of the gun tip, but will also induce a
severe back corona in the deposited layer at the same
time. Thus, the characteristics of the ultrafine powder in
the deposition efficiency can be interpreted as follows: the
improved deposition efficiency from 30 to 60 kV was due to the
improved primary charging efficiency of the in-flight particles,
which was dominant in competing with the occurrences of back
corona; but a further increase to 90 kV induced a much severe
back corona, which resulted in a more remarkable reduction in
the charge of in-flight particles and thereby suppressed further
improvement in the deposition efficiency. On the other hand,
the intensifying back corona was responsible for the decreasing
deposition rate in powder coating processes. In particular,
the ultrafine powder showed some inferiority in its primary
charging efficiency with respect to the coarse powder, which
might make it more sensitive to the charge loss incurred by
back corona, and thereby results in a faster decrease in the
deposition rate.
3.2. Characteristics of particle charging
In the above section, it is clearly demonstrated that, as the
premise of particle deposition, the primary charging of in-flight
particles competes with back corona during the powder coating
processes. However, the deposited particles may accept
8
J. Phys. D: Appl. Phys. 42 (2009) 065201
Spraying Duration 20 s:
X Meng et al
30 kV;
1000
60 kV;
4.0
90 kV
3.5
100
Q/M(µC/g)
Ji (µA/m 2 )
3.0
10
2.5
2.0
1.5
1.0
0.5
1
8
10
6
4
2
2
0
Annular Region
4
6
8
0.0
10
Spraying Duration 5 s:
10
Spraying Duration 20 s:
60 kV;
2
Spraying Duration 5 s:
0
2
4
6
90 kV
8
10
30 kV;
60 kV;
90 kV
10
90 kV
1
1
0.1
0.1
10
(b)
0.01
4
60 kV;
Annular Region
(A/M)i (µA/g)
(A/M)i (µA/g)
30 kV;
6
(a)
Figure 14. The dependences of local current density (Ji )
distributions on charging voltages during powder coating processes.
10
8
30 kV;
10
8
6
4
2
0
2
4
6
8
8
6
4
2
0
2
4
6
8
10
Annular Region
Figure 16. Comparisons between the local charge-to-mass ratio
((Q/M)i ) and the local current-to-mass ratio ((A/M)i ) at a spraying
duration of 5 s: (a) local charge-to-mass ratio distributions; (b) local
current-to-mass ratio distributions.
10
Annular Region
Figure 15. The dependences of local current-to-mass ratio ((A/M)i )
distributions on charging voltages during powder coating processes.
for the ultrafine powder while an increasing tendency for the
coarse powder. The characteristics of the average Q/M for
both powders were supported by the behaviour of their average
A/M, as shown in figure 18(b). Obviously, the different Q/M
tendencies of the two powders are simply due to the differences
in their secondary charging efficiencies with the increasing
voltages. The different secondary charging efficiencies lie
in the differences in their particle sizes of the two powders,
which lead to the differences in their particle number and
specific surface. Further, different powders incur variations
in the suppression of corona currents (as disclosed in [37]), in
their primary charging efficiencies (as shown in figure 10) and
in the current density distributions (as disclosed in [37]). As
a result, the deposition efficiency and the secondary charging
efficiency vary with different powders. On the other hand,
the average Q/M of both powders in figure 18(a) decreases
with the extended spraying duration. This should be attributed
to the intensifying back corona in powder coating processes,
which produces more positive ions and thereby neutralizes the
negative charge of the deposited particles more severely with
the extended spraying duration. The deposited particles in the
evenly in the whole fringe region. More importantly, it can
be further observed from figures 13 and 15 that the Q/M and
A/M profiles nearly change synchronously with the charging
voltage. Thus, the dominance of secondary charging on
the Q/M characteristics of deposited particles is witnessed
again with the ultrafine powder. Furthermore, the dominance
of secondary charging on the characteristics of local Q/M
distribution is supported at the spraying durations of 5 s and
10 s by comparing the local Q/M profiles with the local A/M
profiles, as shown in figures 16(a) and (b) and figures 17(a)
and (b) respectively.
In addition, the results in figures 13, 16(a) and 17(a)
commonly disclosed that the Q/M roughly shows a decreasing
tendency in most of the local regions with the increase
in voltage. To contrast the charging characteristics of the
deposited layers due to the two powders, the average Q/M
of the deposited particles in the internal regions of A1–A9 was
illustrated in figure 18(a). It is obvious that the average Q/M
presents a decreasing tendency with the increase in voltage
9
J. Phys. D: Appl. Phys. 42 (2009) 065201
X Meng et al
2.5
10
Spraying Duration 10 s:
30 kV;
60 kV;
90 kV
Ultrafine Powder
5s
10 s
20 s
(Q/M)AV. (µC/g)
(Q/M)i (µC/g)
2.0
Coarse Powder
5s
10 s
20 s
1.5
1.0
1
0.5
0.0
30
10
8
6
4
(a)
2
0
2
4
6
8
Annular Region
30 kV;
60 kV;
Ultrafine Powder Coarse Powder
5s
5s
10 s
10 s
20 s
20 s
90 kV
10
(A/M)i (µA/g)
90
Charging Voltage (kV)
10
(A/M)AV. (µA/g)
Spraying Duration 10 s:
60
(a)
10
1
1
0.1
0.1
30
10
(b)
8
6
4
2
0
2
4
6
8
60
(b)
10
Annular Region
90
Charging Voltage (kV)
Figure 18. (a) The dependences of the average charge-to-mass ratio
((Q/M)AV ) on charging voltages and spraying durations; (b) the
dependences of the average current-to-mass ratio ((A/M)AV ) on
charging voltages and spraying durations.
Figure 17. Comparisons between the local charge-to-mass ratio
((Q/M)i ) and the local current-to-mass ratio ((A/M)i ) at a spraying
duration of 10 s: (a) local charge-to-mass ratio distributions;
(b) local current-to-mass ratio distributions.
1.4
fringe region (A10) were not taken into account in figures 18(a)
and (b), due to the difficulty in interpreting the part of the
local current of the fringe region working on its local deposited
particles.
Nevertheless, the Q/M distributions of deposited particles
with both powders were compared by evaluating their standard
deviations, as shown in figure 19. It is implied by standard
deviations that the Q/M distributions with the ultrafine powder
are more uniform than those with the coarse powder, except
for the case of 30 kV under the spraying durations of 5 and
10 s. In addition, the results of standard deviations indicate
that the Q/M uniformity with the coarse powder gets much
worse with increasing voltage while the one with the ultrafine
powder suffers a small influence from the changing voltage
and mostly has a less deviation. The results in figure 19 still
imply that the extended spraying duration can improve the
Q/M uniformity of both powders, but there is an exception in
the case of 90 kV of the ultrafine powder.
Ultrafine Powder
5s
10 s
20 s
Standard Deviation σ
1.2
Coarse Powder
5s
10 s
20 s
1.0
0.8
0.6
0.4
0.2
0.0
30
60
90
Charging Voltage (kV)
Figure 19. Comparisons of standard deviations (σ ) of the local
charge-to-mass ratio ((Q/M)i ) distributions due to different
powders.
10
J. Phys. D: Appl. Phys. 42 (2009) 065201
X Meng et al
The authors are grateful to Links Coatings (London, Ontario)
for supplying the paint powder.
4. Conclusion
The understanding of powder coating processes is not yet
fully clear, due to the many physical mechanisms involved.
As an essential supplement to the studies in exploring the
mechanisms of particle charging and deposition during powder
coating processes, this study employed an ultrafine powder
with respect to the coarse powder used in the preceding work
[38]. By investigating the charge-to-mass ratio and the massto-surface ratio of the deposited particles, the characteristics
of particle charging and deposition with ultrafine powder were
revealed for the first time in this paper. It was disclosed
that the ultrafine powder behaves similarly in many ways as
the coarse powder studied in the preceding work. First, a
cone-shaped pattern of deposited particles across the substrate
and a rise in particle accumulation in the fringe region were
observed with both powders. It was demonstrated again in
this study that the inhomogeneous concentrations of charged
in-flight particles contribute to the former, and the edge effect
is responsible for the latter. In addition, it was verified by
the ultrafine powder that the particle deposition efficiency has
a strong dependence on the primary charging of the in-flight
particles but suffers a severe influence from the back corona.
Thus, the highest efficiency is a competing result between the
primary charging of in-flight particles and the back corona
in the deposited layer and can be realized by compromising
the effects from primary charging and back corona. It was
indicated in this study that the optimal voltage for the highest
efficiency is around 60 kV for both powders. However, the
ultrafine powder presents a faster reduction in the deposition
rate and has some superiority in improving the uniformity
of the deposited layer. As for the charging characteristics
of the deposited particles, it was demonstrated again with
the ultrafine powder that secondary charging dominates the
characteristics of the local charge-to-mass ratio distribution
across the substrate. The ultrafine powder exhibits a decreasing
Q/M of the deposited particles with increasing voltage, which
is opposite to the increasing tendency of the coarse powder. In
particular, the ultrafine powder is more likely to produce more
uniform Q/M distributions in the deposited layer with respect
to the coarse powder. However, the charge-to-mass ratio of the
ultrafine powder decreases with extended spraying durations
due to the intensifying back corona, as is also the case for the
coarse powder.
Significantly, the characteristics of particle charging
and deposition with ultrafine powder are disclosed in this
study. By combining the findings from both coarse and
ultrafine powders together, comprehensive knowledge on
powder coating processes is possible. It is believed that the
outcome will be of great help in understanding the mechanisms
of corona charging processes of powder coating as well as
improving the applications of powder coating in finishing
industries.
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