Apparatus for corona discharge treatment of an article

An apparatus for corona discharge treatment of an article, and in particular an article comprising a conductive composite, includes a first electrode, which is a rotating, high-frequency voltage electrode and a second electrode, which is preferably a translation table, disposed with respect to the high-frequency voltage electrode so as to provide a return path to ground. The translation table supports the article to be treated, and oscillates with respect to the first electrode. Since translation table supports the article to be treated, the article itself is grounded, which is important when treating conductive composite articles. An adjustment mechanism provides a constant gap between the high-frequency voltage electrode and the article, so that the voltage does not seek the article. This enhances the efficiency of the corona treatment of the article. The first electrode rotates continuously and independently of the motion of the article in order to dissipate heat from the high-frequency voltage electrode. This arrangement makes the apparatus particularly useful for treating articles comprising conductive fibers, since such articles have relatively high power requirements.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for corona discharge 
treatment of an article. In particular, the invention relates to an 
apparatus for corona discharge treatment of a surface of an article 
comprising a conductive composite in order to enhance the surface adhesion 
of the article. 
2. Description of the Related Art 
Corona discharge treatment of surfaces of articles made of thermoplastic 
polymers is a well-known technique to enhance the surface adhesion of such 
articles. A number of applications and modifications to corona discharge 
technology for treating fiber composites based upon thermoplastic 
polymeric matrices are described in the literature. See, for example, 
Kodokian et al., "Surface pretreatment and adhesion of thermoplastic 
fibre-composites", Journal of Materials Science Letters 7 (1988) 625-627, 
which discloses treating thermoplastic-composite substrates placed on an 
automatically controlled table which travels horizontally backwards and 
forwards under a high-frequency voltage, discharge electrode. 
Corona discharge devices for treating sheets of material generally comprise 
a pair of electrodes, at least one of which rotates. For example, U.S. 
Pat. No. 3,973,132 to Prinz discloses a corona discharge apparatus for 
treating non-conductive foils comprising a rotating electrode pair, the 
high-frequency voltage electrode having a profiled cross section, and the 
ground electrode being a smooth cylinder. When high voltage is applied to 
one of the electrodes, a corona discharge takes place through the air gap 
between the electrodes and onto the surfaces of the foil. U.S. Pat. No. 
4,273,635 to Beraud et al. discloses a process for corona treatment of 
bulky fibrous webs derived at least partially from thermoplastic fibers to 
impart cohesion to the webs. The process includes passing the webs between 
a pair of rotating cylindrical metallic electrodes. U.S. Pat. No. 
4,392,178 to Radice discloses an apparatus for enhancing the piezoelectric 
properties of polymeric films by corona treatment using a roller electrode 
mounted for movement along the circumference of a motorized, rotating drum 
which propels the film. The roller electrode moves in an oscillatory 
motion normal to the axis of rotation of the drum. U.S. Pat. No. 3,435,190 
to Schirmer discloses a corona discharge apparatus used to perforate films 
of dielectric material. The apparatus includes a stationary, blade-like 
electrode covered with electrically insulating materials along its length 
and another elongated, rotatable electrode, between which a sheet of the 
dielectric material passes. Canadian Patent No. 790,038 to Adams discloses 
an apparatus for corona treatment of plastic films which conveys a sheet 
of such plastic film on a cylindrical, rotating roller which acts as one 
electrode, and passes close to a similarly contoured stationary electrode 
covered with a layer of dielectric material. The apparatus also has a 
locknut for adjusting electrode spacing. 
In order to obtain uniform corona treatment of an article, surface sparking 
must be evenly distributed, or the voltage applied by the electrodes will 
be concentrated in one area and fail to uniformly treat the surface. 
Achieving uniform corona discharge across the high-frequency voltage 
electrode requires exacting spacing tolerances, not only between the two 
electrodes, but in the article-electrode gap as well. Through continued 
use, gap tolerances are often exceeded, and the discharge uniformity along 
the high-frequency voltage electrode is destroyed, as evidenced by 
sparking and pitting of the high-frequency voltage electrode surface. 
Moreover, most corona discharge devices are not able to maintain even gap 
spacing when there are thickness variations in the article. 
A problem especially arises in the corona treatment of composite articles 
comprising conductive fibers. In this situation, conventional devices and 
methods will not be effective, due to increased sparking caused by loose 
or exposed conductive fibers in the composite. To treat conductive 
composite articles, high power must be supplied to the high-frequency 
voltage electrode, since the voltage will seek the conductive fibers in 
the composite article. This lessens the efficiency of the corona treatment 
of the surface of the article. Since high power must be supplied, 
overheating of the electrode, as well as the articles to be treated, 
becomes a serious obstacle. If the problem is not addressed, the surface 
of the article may be destroyed by overheating and ablating before the 
desired surface corona treatment is achieved. Alternately, the increased 
incidence of sparking can render the corona treatment ineffective. 
Additionally, at high power levels, any non-uniformity in the electrode is 
magnified, which further increases the chances that sparking will occur. 
SUMMARY OF THE INVENTION 
The present invention solves the problems associated with the prior art by 
providing an apparatus for corona discharge treatment of an article 
comprising a conductive composite which is able to maintain uniform corona 
discharge along the high-frequency voltage electrode by providing a 
constant spacing between the high-frequency voltage electrode and the 
article. 
The apparatus of the present invention also reduces the incidence of 
sparking and thus provides more effective corona discharge treatment and 
less chance of non-uniformity in the electrode than devices of the prior 
art. 
Also, the apparatus of the present invention is able to supply 
high-frequency voltage to the discharge electrode without overheating the 
electrode, thereby reducing the possibility of degradation of the 
electrode. This prevention of overheating the electrode also decreases the 
occurrence of non-uniformity of the electrode and consequent sparking. 
To achieve the foregoing solutions and in accordance with the purpose of 
the invention, as embodied and broadly described herein, the invention 
comprises an apparatus for corona discharge treatment of a surface of an 
article. The apparatus comprises a first electrode for supplying a 
high-frequency voltage to the article; a second electrode for providing a 
return path to ground; and means for adjusting the spacing between the 
first electrode and the article in order to provide a constant gap 
therebetween. 
Further in accordance with the present invention, there is provided an 
apparatus for corona discharge treatment of a surface of an article, 
comprising: a high-frequency voltage electrode rotatable about a shaft 
disposed along the central axis of the cylindrical electrode for applying 
a high-frequency voltage to the article; a timing belt drive system for 
rotating the rotatable electrode; a translation table for holding the 
article, the translation table being disposed with respect to the 
high-frequency voltage electrode so that as the voltage is supplied to the 
article, the translation table provides a return path to ground; and a 
rider wheel connected to the shaft of the rotatable electrode for 
adjusting the spacing between the high-frequency voltage electrode and the 
article in order to provide a constant gap therebetween.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference will now be made in detail to the present preferred embodiment of 
the invention, an example of which is illustrated in the accompanying 
drawing. 
In accordance with the invention, there is provided an apparatus for corona 
discharge treatment of a surface of an article. The apparatus of the 
present invention is shown generally at 10 in FIG. 1. The apparatus of the 
present invention is particularly suited for treating articles made of 
conductive composites, such as thermoplastic composite articles used for 
adhesive bonding. As noted above, since the voltage will seek the 
conductive fibers in the composite article, relatively high power must be 
supplied to the high-frequency voltage electrode. As used herein, the term 
"high power" means up to 24 kV. In addition, when treating conductive 
composites, loose or exposed conductive fibers in the composite material 
increase the incidence of sparking. The apparatus of the present invention 
is capable of treating such conductive composite articles by virtue of its 
elements and configuration as will be described below. 
As embodied herein, the apparatus of the present invention comprises a 
first, rotating electrode for applying a high-frequency voltage to a 
surface of the article. A first electrode is shown as a cylinder 12 as 
shown in FIG. 1. The first electrode may take any form, such as a blade, 
as long as it is capable of being supplied with high voltage. High voltage 
is supplied through a cable 14 as shown in FIG. 1 and is applied to first 
electrode 12 via a brush contact (not shown). First electrode 12 is 
supported by a pair of insulating spacer blocks 16 as shown in FIG. 1. 
Blocks 16 are connected to a cylinder mounting apparatus which comprises a 
support beam 18, a pair of polymeric insulators 20 and 22, a bar 24, a bar 
25 having a plurality of screws 31 and a pivoting assembly, shown 
generally at 27. Support beam 18 supports spacer blocks 16, insulators 20 
and 22, bars 24 and 25, pivoting assembly 27 and first electrode 12. 
Support beam 18 is connected to a pair of vertical supports 19, which 
provide support for beam and thus all the elements the beam supports. Bar 
24 supports spacer blocks 16. In addition, bar 24 acts as an insulator and 
a support for the first electrode. Insulators 20 and 22 are preferably 
polymeric and keep the high-frequency voltage from jumping from first 
electrode 12 to support beam 18. Insulators 20 and 22 support bar 24, 
blocks 16 and first electrode 12. Bar 25 supports insulators 20, 22, 
spacer blocks 16 and electrode 12. Pivoting assembly 27 comprises an upper 
block 27a and a lower block 27b and a plurality of springs 29 disposed 
therebetween. Pivoting assembly is provided with a plurality of screws 33 
and 35. Pivoting assembly 27 also comprises a middle block 27c, which 
supports the pivoting assembly with respect to the support beam by virtue 
of screws 35. Screws 33 adjust the tension of springs 29 and thus the 
distance between upper block 27a and lower block 27b. The apparatus of the 
present invention is adjustable according to the surface contour and the 
position of the article being treated. Specifically, pivoting assembly 27 
allows for vertical motion of bars 24 and 25, insulators 20 and 22, spacer 
blocks 16 and electrode 12, should the article be curved or have varying 
elevations. Screws 31 of bar 25 allow electrode 12, spacer blocks 16, 
insulators 20 and 22 and bars 24 and 25 to tilt, should the article be 
disposed in a position other than generally parallel to the closest side 
of electrode 12 as shown in FIG. 1. 
First electrode 12 is preferably made of aluminum, though other conductive 
materials are suitable. The first electrode is preferably covered with a 
dielectric material, which changes the difference in the dielectric 
constant of the first electrode and the article. The dielectric material 
is preferably silicone rubber. However, any material having a dielectric 
constant of about 3.75 to about 3.95 (at 25.degree. C. and a frequency 
ranging from 1.times.10.sup.3 to 1.times.10.sup.8) would be suitable for 
use with the present invention. 
The apparatus of the present invention further comprises a timing belt 
drive system for rotating the first electrode. It should be understood 
that it is within the scope of the invention to use any electrical, 
mechanical or electro-mechanical system to rotate the first electrode. As 
shown in FIG. 1, timing belt drive system comprises a motor 26 for driving 
the belt drive system. Motor 26 is preferably an AC motor. A stepper motor 
is not preferred, since interference cause by corona discharge is likely 
with such a motor. A suitable AC motor for driving the timing belt drive 
system is Type KC1-24A2 #749NG7055, made by Bodine Electric Co., of 
Chicago, Ill. This motor is 115 V continuous duty, 1 phase, 60 Hz, 100 ma, 
40 RPM, 1.2 V .mu.farad capacitor, 44 oz./in. torque, can be powered from 
a variable AC source, and is reversible. Belt drive system also comprises 
a motor bracket 28 which supports motor 26 and a pulley assembly including 
a pulley 30, an optional guard bracket (not shown) which protects the 
operator from pinch points, or rotating hazards, an insulated timing belt 
32 supported by pulley 30, which drives the first electrode and an 
optional guard covering (not shown) which also protects the operator from 
pinch points. The timing belt drive system also acts as an insulator for 
the first electrode. 
The apparatus of the present invention also comprises a second electrode 
for providing a return path to ground as the high-frequency voltage is 
supplied to the first electrode and is subsequently applied to the 
article. In a preferred embodiment, the second electrode comprises a 
translation table, shown generally at 40 in FIG. 1, which holds an article 
42. Translation table 40 also imparts oscillatory motion to the article. 
This oscillatory motion can be in the horizontal planar direction as shown 
by arrow 43 in FIG. 1, the vertical planar direction, or a planar 
direction which is oblique with respect to either the horizontal or the 
vertical planar directions, just as long as an air gap can be maintained 
between the first, high-frequency voltage electrode and the second 
electrode, or ground. The translation table may also be referred to as an 
X-Y table, and can be purchased commercially, for example, from Daedal, of 
Harrison City, Pa. As shown in FIG. 1, translation table 40 includes a 
platform 44 which supports the article to be treated. Translation table 40 
also includes a track 46 for horizontal oscillation of the table, a ground 
wire 48 for grounding the table and a stepper motor 50, including a 
controller 52, which imparts oscillatory motion to the table. 
In operation, high-frequency voltage is applied to the first electrode, and 
the stepper motor is turned on. Platform 44 makes multiple passes beneath, 
or next to, the first electrode, depending on how the table is disposed 
with respect to the first electrode, to achieve the desired amount of 
corona discharge treatment. The oscillation of the table ensures uniform 
treatment across the article, since, for each oscillation, the article 
passes fully under the first electrode, and all the surface area of the 
article is equally treated. Since the surface at any particular point is 
not subjected to continuous corona treatment, it has time to cool during 
each oscillation. This is especially important when treating conductive 
articles, since they are more susceptible to sparking and overheating. 
Since the table supports the article to be treated, the article itself is 
also grounded, which is important when treating conductive composite 
articles. 
With the present invention, the first electrode rotates independently of 
the velocity of the second electrode and consequently, the article held 
thereon. This independent rotation allows high power to be supplied 
thereto without overheating the high-frequency voltage electrode. As noted 
above, high power, which causes the electrode to overheat, is necessary 
for treating articles comprising conductive composites as compared to 
treating articles made of other materials. For example, an article which 
comprises polyethylene requires only about 0.2-0.5 J/mm..sup.2 of energy 
per surface area. In contrast, the apparatus of the present invention 
allows 20 J/mm..sup.2 of energy per unit area to be supplied to the 
surface of a composite known as AS-4 PEEK, where AS-4 is a type of carbon 
fiber and PEEK is a poly ether ether ketone, commercially available from 
ICI of Wilmington, Del. Preventing overheating of the high-frequency 
voltage electrode extends the life of the dielectric material covering and 
hence extends the life of the electrode. In addition, sparking effects are 
distributed evenly over the surface of the first electrode, with the 
result that (1) degradation of the silicone rubber cover from thermal and 
UV effects is mitigated; and (2) pin-hole breakthroughs from the cover to 
the first electrode are prevented, thus also extending the life of both 
the cover and the electrode. 
In accordance with the present invention, the apparatus of the present 
invention further includes adjusting means for adjusting the spacing 
between the article and the first electrode in order to provide a constant 
gap therebetween. In a preferred embodiment, the adjusting means comprises 
a rider wheel 54, which is connected to the shaft of rotating first 
electrode 12. Since rider wheel 54 is connected to the shaft of the 
rotating first electrode, it rotates with the first electrode. The rider 
wheel has a diameter which is slightly greater than the diameter of the 
first electrode, depending on how much air gap is desired. This causes a 
constant gap to be maintained between the first electrode and the article. 
Since the rider wheel has a larger diameter than the first electrode, if 
the rider wheel were metallic, this configuration would cause the wheel to 
short to ground. Therefore, it is preferable that the rider wheel is made 
of a non-conductive material. This keeps the voltage from the 
high-frequency voltage electrode from shorting to ground. 
In operation, as the first electrode rotates, rider wheel 54 rides along 
the article and provides a constant gap between the first electrode and 
the article at every point on the article. This constant gap enables 
uniform application of corona discharge, since the voltage does not seek 
the article in concentrated areas. The rider wheel is especially useful 
when the article's surface is curved and/or has varying elevations. It 
should be noted that it is within the scope of the present invention that 
the rider wheel can move independently of the first electrode, as long as 
a constant air gap is maintained. 
It should be noted that the gap between the first electrode and the article 
must be properly chosen. If this gap is too large or too small, sparking 
can also occur, since the preferred path for the voltage will again be to 
ground. Hence, the article will not be effectively treated under such 
circumstances. The proper gap between the first electrode and the article 
is determined based on the type of material to be treated, the thickness 
of the article, as well as the relationship between the gap between the 
first electrode and the article, and the gap between the first and second 
electrodes. Moreover, the relationship between the gap between the first 
electrode and the article and the gap between the first electrode and the 
second electrode is important to avoid sparking. If these gaps are 
comparable, there will be a higher incidence of sparking, since the corona 
discharge voltage will go to ground (the second electrode), rather than to 
the surface of the article. 
It will be apparent to those skilled in the art that various modifications 
and variations can be made in the arrangement of the rider wheel of the 
present invention without departing from the scope or spirit of the 
invention. For instance, the rider wheel can be located at either or both 
ends of the first electrode. The rider wheel could even be located at the 
center of the first electrode, along the longitudinal length thereof if a 
split electrode is employed as the first electrode. 
Alternatively, the means for adjusting the spacing between the first 
electrode and the article may comprise a robot for controlling the 
position of the first electrode. In addition, a laser sensor could be used 
to determine the geometry of the article, and the robot could be used to 
adjust the position of the article, as well as to control the position of 
the first electrode. 
Other embodiments of the invention will be apparent to those skilled in the 
art from consideration of the specification and practice of the invention 
disclosed herein. It is intended that the specification be considered as 
exemplary only, with a true scope and spirit of the invention being 
indicated by the following claims.