Wire corona charging apparatus

An improved wire corona charging apparatus and processes for treating webs and films, to improve surface wettability characteristics and other surface properties, and to create permanently charged electrets. Corona produced by wires results in more evenly distributed ion flux under a high level of control, than possible with commonly used bar chargers. Wire chargers are not used in applications that involve webs and films of large width, since at these widths wires can slacken and break, resulting in possible electric shorting that can be a safety hazard in most non-batch processes. The embodiments disclosed overcome these reliability and safety issues.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to treatment of webs and films of fibrous 
material more particularly, to wire corona charging apparatus and 
processes. 
2. Description of Background Art 
Corona processes are used for surface treatment of fiber webs and films. 
Although surface treatment can have many other objectives, the most 
important or common objectives are to increase wettability for printing, 
to increase absorptive characteristics (see, for example, Dinter et al., 
U.S. Pat. No. 5,135,724), and to produce permanently charged materials 
that are typically referred to as electrets (see, for example, Wadsworth 
and Hersh, U.S. Pat. No. 4,375,718). Such processes involve apparatus for 
causing corona discharge. Corona producing apparatus that can be used in 
these surface treatment processes are commonly referred to as "ionizers", 
"corona treaters" or "corona devices" or "chargers". 
Thin wires produce a highly controlled and well distributed ion flux, when 
compared to more commonly used corona devices such as charge bars, rods, 
and needle points, principally because of the small diameter of the wires. 
Corona streamers produced from smaller diameter wires are more uniform 
across the treatment surface than those produced by using larger diameter 
rods or sharp edges or needle points (White, Industrial Electrostatic 
Precipitation, Addison-Wesley Publishing Company, Inc. 1963). 
Additionally, the amount of ozone, a pollutant, produced by small wire 
corona chargers is lower than the amount of ozone produced by larger 
diameter wires, rods, etc. (Whit, 1963). Another advantage of wire corona 
chargers is that due to the highly distributed ion flux, (i.e., uniform 
streamers) there is a lower possibility of producing violent, high density 
sparks that can cause pitting in counter potential rollers (see, for 
example, Schuster U.S. Pat. No. 4,281,247) that are coated with a 
dielectric layer (typically a ceramic coating). Corona produced from other 
devices, such as charge bars, rods, or sharp points results in such sparks 
and pitting under many conditions. Such rollers are expensive, and thus 
pitting can substantially increase operating cost when such corona charge 
bars are used. 
Although wires are commonly suggested as options for use in corona charging 
equipment, we have found that wires are seldom, if at all, used in corona 
devices for treatment of webs or films that are over 24-30 inches wide 
because wires require high axial tension when strung over wide widths, in 
order to prevent slack from occurring in the middle of the wires. In 
contrast, bars, rods and sharp points do not require axial tension for 
mounting in wide ionizers. By way of explanation, rods and bars, 
regardless of their specific cross-sectional shape (with that 
cross-sectional shape taken within a plane dividing the rod or bar and 
defining an orthogonal angle with the longitudinal axis of the rod or bar) 
are elongate elements having sufficiently large dimensions within that 
plane that the linear measurement of deflection of the centroid of a bar 
or rod over a span where the unloaded bar or rod is simply supported only 
at its opposite ends is significantly less than the greatest value of a 
cross-sectional dimension of the bar or rod taken along a line parallel to 
the path of the centroid during the deflection. In contradistinction, 
although a wire is also an elongate element, the centroid if an unloaded 
wire simply supported only at its opposite ends will freely trace a path 
during deflection of the wire that is many times greater than the greatest 
cross-sectional dimension of the wire taken along a line parallel to that 
path, even while the opposite ends of the wire are held under tension. 
Consequently, to restrict the deflection of a centroid of a wire to a 
value that is comparable to that of a bar or rod of the same length, it is 
necessary to hold the opposite ends of the wire with such a high degree of 
tension that substantial risk exists that the wire will break. Wires used 
in wide corona chargers can therefore easily break and cause safety 
hazards in these surface treatment processes, which are typically 
continuous processes running at high speeds. We have also found that wires 
can get snagged with the film or roll moving at high speed, thus causing 
safety problems, and damage to process components. In order to alleviate 
this breakage problem, tungsten wires have been suggested, chiefly due to 
the high tensile strength of tungsten. Typically, tungsten wires with 
0.2-1 millimeter diameters are preferred for corona charging processes 
(cf. Nakao, U.S. Pat No. 4,582,815). Many web and film processes however 
involve web or film widths between 50-120 inches. Over such lengths, even 
tungsten wires of these small diameters can break while under tension. 
We have noticed that another reason for the lack of use of wires in 
contemporary wide corona chargers is that over time, the wire can relax 
and, when the wire lengths are long, the slack in the middle part of the 
wire can produce field strength variations and, in some cases, may even 
become tangled with the web or film. 
Primarily due to the safety issues due to breakage of wires, typically 
charge bars (as is suggested by Wadswo and Hersh, U.S. Pat. Nos. 4,375,718 
and by Dinter et al., 5,135,724) are preferred in such applications, even 
though corona produced by thin wires have distinct advantages that are not 
available with charge bars. Additionally, charge bars are preferred in 
contemporary chargers because many charge bar designs enable the 
introduction of gases or aerosols (as, for example, Dinter et at., 
5,135,724 and Kubik and Davis, 4,215,682) into the corona- these gases or 
aerosols are thought to enable better or specific surface treatment of the 
fibrous materials, often through surface chemical reactions induced by 
corona treatment. Currently, there are no wire based chargers on the 
market that facilitate the introduction of aerosols and or gases into the 
corona region. 
SUMMARY OF THE INVENTION 
It is therefore one object of the present invention to provide an improved 
wire charger for corona production in the treatment of fiber webs and 
films. 
It is another object to provide a wire charger for corona production in the 
treatment of fiber webs and films facilitating automatic shutdown of the 
process and interruption of the application of high voltage in case of 
wire breakage. 
It is still another object to enable the use of shorter wires, with higher 
resistance to breakage, in a wide corona treater, without creating zones 
of the fibrous material that are not corona treated during a process. 
It is yet another object to provide a process and a corona wire charger 
that enables the introduction of treatment aerosols and gases into the 
corona produced by the thin wires of the wire charger. 
It is still yet another object to enable increasing the corona current 
density in a uniform manner, by avoiding the use of liquid aerosols in the 
corona producing region. 
It is a further object to provide a process and a corona wire charger 
achieving an enhanced corona density and uniformity in distribution within 
the corona producing region by pumping high humidity ambient air into the 
corona producing region. 
It is a still further object to provide a process and a corona wire charger 
achieving an increased level of surface treatment and induced corona 
current production. 
It is a yet further object to provide a corona charger that minimizes the 
risk of pitting of dielectric or other coatings on the counter potential 
rolls used in web and film treatment processes. 
It is also an object to provide a process and a corona wire charger that 
enables production of a uniform electric field between the wires and the 
counter potential rolls, and thus provide for uniformly distributed corona 
treatment. 
These and other objects may be achieved with a wire ionizer constructed 
according to the principles of the present invention with a high 
dielectric strength framework having a channel for holding a plurality of 
ionizing wires and other components. The ionizing wires are connected by 
means of springs, on one end to ceramic insulators attached to levers 
connected to the framework, and to a power distribution bar attached to 
ceramic insulators which are in turn attached, in the case of shorter 
ionizer widths, to the other end of the framework, or, in the case of 
significantly larger ionizer widths, to a thin mid section of framework. 
Spring loaded electric switches are positioned in contact with one arm of 
the levers so that upon breakage of one of the wires, the corresponding 
lever does not exert any pressure on the contact arm of the switches, thus 
opening the electric circuit across the switch and thereby stopping the 
process. A shield insulates a desired portion of the length of the wires 
and thus adjusts the width of the corona field so as to adapt the wire 
ionizer for different widths of web and film. A channel accommodates 
passage of compressed gas or air or aerosol into the ionizing zone and a 
netting or highly perforated dielectric material is attached to the front 
face of the ionizer frame to trap the ionizing wires inside the channel 
within the frame, in case of wire breakage. A high voltage power supply is 
connected to the power distribution bar, thus enabling ionization, when 
the wire ionizer is in the near vicinity of the counter potential web or 
film processing roller. Electrical power for the entire web or film drive 
is routed through a controller for the process drive power relay, which is 
controlled by power supplied via the switches in a break detecting lever 
switch bank. These switches may be connected in series to the electric 
power. As long as current flows through the switches (i.e., while all of 
the ionizing wires are unbroken and intact), the process drive power relay 
is enabled. Should one of the ionizing wires break, the power through the 
switches is disrupted and the process drive power relay is opened, thus 
stopping the process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following description is of the best mode presently contemplated for 
carrying out the invention. This description is not to be taken in a 
limiting sense, but is made merely for the purpose of describing the 
general principles of the invention. The scope of the invention should be 
determined with reference to the claims. 
Referring now to the drawings, the wire ionizer or corona discharge device 
for application in continuous treatment of film and web, is generally 
indicated by reference numeral 1. Referring to FIGS. 1, 2, 3, 4 and 5, the 
primary components of wire ionizer 1 are the channel frame 2, the wire 
tension lever 8 and switch block assembly 3, plurality of ionizing wires 4 
and the high voltage (hereinafter referred to as HV) bus assembly 5. 
The channel frame 2 is typically made of a high arc resistance material 
such as acrylic plastic. It has a "dug out" channel 26 in a block of 
acrylic inside of which the assemblies 3, 4, and 5 are mounted. The 
channel frame may be constructed of one block of material or, preferably, 
as is shown in FIG. 5, made out of two thick plates of acrylic, one of 
which, the front plate 27, has the channel 26 cut within it (i.e., the 
"dug out" section) along its entire length. This front plate 27 is 
attached to the back plate 28 as, for example, by means of threaded 
fasteners such as screws (typically plastic screws) 14 to form the channel 
frame 2. The back plate 28 has an air flow channel 17 machined throughout 
in the center as shown in FIG. 5. The air flow channel 17 is machined out, 
typically as a rectangular groove within the back plate 28. A plurality of 
branch conduits 17a extended upwardly from channel 17, through front plate 
27, and open in a series of orifices within channel 26 that are preferably 
aligned in a single row beneath the central one of the ionizing wires 4. 
When assembled together with the plate 27, the groove in the back plate 28 
forms a substantially enclosed channel 17 for air or other gas flow as 
shown in FIGS. 1 to 5. The front plate has the series of small apertures 
formed at the terminal ends of conduit ducts 17a at locations such that 
these holes form gas outlets from channel 17 into the ionizing region 
within front plate 27, as is illustrated in FIG. 5. A pipe fitting 18 is 
connected to the opening in the back plate 28 by means of providing a 
threaded hole of the appropriate size on back plate 28. This pipe fitting 
18 connects to the air flow channel 17. The advantages of using two plates 
instead of one block of material for the construction of the ionizer frame 
2 are as follows. Firstly, use of two plates 27 and 28 allows for the 
construction of ionizers with large widths. If one block of material is 
used, the air channel 17 must be formed by a drilling operation. Such an 
operation is difficult, if not impossible, when the block width is as 
large as 150 inches. Fiber and film processes often use web and film 
widths between 90-150 inches. This drilling operation becomes even more 
difficult when the ionizer frame material is as brittle as acrylic. 
Secondly, this two plate approach allows for easier machining for the 
creation of the channel in the front plate 27. A third advantage of the 
two plate approach is that extremely wide ionizers may be constructed in 
multiple sections formed by the front plates and back plates, such that 
the joints in the front plate 27 are at different sections than the joints 
of back plate 28. Thus the electric leakage paths, if any, occurring due 
to the joints, are increased substantially, thereby minimizing the risk of 
arcing through such joints. 
Referring collectively to FIGS. 1 to 5, the front plate has two spaced 
apart, oppositely facing sliding grooves 29 near the top surface, at both 
ends of the plate, such that insulating material (typically acrylic) 
exposure shields 12 can be slid into and out of the plate. By sliding the 
exposure shields in, the width of the ionizing zone is reduced and vice 
versa. This is one important feature of this invention because it allows 
for the treatment of different web widths provided that the web widths are 
smaller than the maximum ionizing exposure width of the ionizer. The 
maximum ionizing exposure width of the ionizer is achieved when the 
exposure shields are pulled out to the maximum limit, such that the 
springs 7 are still shielded or covered by the shields 12. It should be 
noted that operation of any ionizer such that the ionizing exposure width 
is greater than the web or film width, will result in wasted electrical 
energy, since a disproportionately high current will be transferred to the 
counter potential roll that is not covered by the dielectric web or film. 
This will occur to a greater extent if the counter potential roll is not 
coated with a dielectric layer. Not only would energy be wasted, but the 
charge density or charge flux into the web or film would be significantly 
reduced, because a disproportionately high amount of the ionization 
streamers would be transported to the exposed part of the counter 
potential roll. Thus the effectiveness of corona treatment would be 
drastically reduced. Hence, the adjustable exposure shield mechanism, 
described above, allows variation of the ionizing exposure width to 
accommodate for the treatment of smaller width webs or films, without 
energy loss and without requiring ionizers with the same size ionizing 
exposure width as the webs or films. 
Referring now to FIGS. 1 and 2, a netting or screen 16 made of a dielectric 
and non conductive material, such as polypropylene, is attached by means 
of fasteners such as preferably plastic threaded screws to the front plate 
27. The netting 16 typically has a linear opening dimension of about one 
inch. Its purpose is to prevent wires 4 from snagging with the web or 
film, should one or more ionizing wires 4 happen to break. The netting 
opening size should be such that a broken ionizing wire is retained inside 
the ionizer frame 2. Typically a one inch opening netting is used. This is 
a secondary level safety feature that will come into play if the wire 
break detection switches 10, described below, fail or for some reason and 
do not cause shutdown of the process drive mechanism. Neither the safety 
netting nor screen are shown in the embodiment shown in FIGS. 3 and 4 in 
order to better illustrate the various components. 
Referring now to FIGS. 1 through 4, wire ionizer 1 is attached to a plate 
or other mounting assembly (not shown), that is a part of the treatment 
process equipment, by means of using mounting bolts 22 and nuts 30 through 
mounting bolt holes 15 that are drilled through the back plate 28. A 
process mounting plate or other mounting assembly 36 must have similar 
size holes at the corresponding locations for mounting the ionizer. 
Referring now to FIG. 11, in some cases, due to space constraints, it is 
not permissible to have back plate 28 of a larger height than front plate 
27 of ionizer frame 2 assembly. In such a design the front and back plates 
are designed to be of equal height. Any mounting of back plate 28 to 
process mounting assembly 36 would mean that the back of plate 28 would 
have to be drilled and tapped for screwing onto the plate. This is not 
advisable however, for brittle materials, such as acrylic, because such 
drilled and tapped holes tend to develop star shaped fractures over time, 
especially if the screws are periodically removed for ionizer removal and 
reinstallation or if the ionizer 1 has substantial weight. In order to 
circumvent this from occurring, in such cases, a metal plate 31 may be 
attached by a high density of small screws 14 that are not subject to 
periodic removal. The metal plate 31 has the same or smaller height than 
the ionizer frame 2, and has a minimum thickness such that it is possible 
to drill and tap mounting holes in the metal plate. The back plate 28 of 
the ionizer frame 2 has corresponding holes that are larger than the 
mounting bolt holes on metal plate 31, so as to allow the ends of the 
mounting threaded bolts 22 to penetrate partially in these holes. This 
mounting arrangement thus effectively removes or reduces the screw thread 
stress on the plastic components of the ionizer frame 2. 
The ionizing wires 4 utilized for ionization are preferably tungsten wires 
with diameters between 0.2-1 min. Tungsten wires are preferred since they 
have high tensile strength even with a small diameter. If the width of the 
ionizer is about sixty inches or less, it is possible to use continuous 
ionizing wires 4 that span the width of ionizer 1 without concern 
regarding wire sagging breakage under tension. This case is illustrated in 
FIGS. 1, 2 and 8. Even in this case, some center support may be necessary. 
Ceramic supports 21 are used to prevent sagging of the wires, as shown in 
FIGS. 1 and 2. For higher widths split (i.e., two or more) wires may be 
used to cover this high width. This case is illustrated in FIGS. 3, 4 and 
9. The wires 4 are attached to springs 7 by means of loops 6 on the end of 
the wires. The springs 7 are in turn attached, via metal screws 32 to a 
high voltage distribution bus or metal strip 24 at one end of the ionizer 
frame 2, in the embodiment shown in FIGS. 1 and 2 with continuous wires 
that span the width of the ionizer. 
Referring now to the alternative embodiment shown by FIGS. 3 and 4, in the 
case of extremely wide ionizers, two or more (split) wires electrically 
coupled end-to-end in series and are used to span the width of the 
ionizer. Continuous electrical conduction occurs through bus 24, springs 
7, and ionizing wires 4. In the case of two wire spanning the width, the 
distribution bus 24 is in the center of the ionizer frame as shown in 
FIGS. 3, 4 and 9. In this case the two split wires 4 share a common high 
voltage distribution bus 24. Hooks 11 are attached to the distribution bus 
24 and the wire loops are attached to the hooks. Each hook supports each 
set of the split wires. Although the frame 2 material, typically acrylic, 
is a good insulator, it has been our experience that it is essential to 
further isolate the high voltage contact regions by means of a better 
insulator for longevity for continuous operation of the ionizer. Hence, 
the distribution bus 24 is mounted on double end threaded cylindrical 
ceramic or other high insulating material standoffs 13 via screws 14. The 
ceramic insulators are mounted via screws 14 on to the frame 2 material. 
In the case of split wires spanning the width of the ionizer, it is 
essential that the distribution bus 24 and the width associated with the 
hooks 11 and springs 7, be shielded from the web or film and counter 
potential roll 35, in order to prevent high density streamers forming at 
this section. The high density streamers can occur at the distribution bus 
24, hooks 11, and springs 7, due to their larger size (than the wires) 
and, thus, their closer proximity to the counter potential roller 35. 
Hence, it is necessary to provide a small acrylic or similar material 
shield 23 as part of the ionizer frame 2. This shield should be as wide as 
the combined distance between the farthest end of the springs 7 under 
tension. It is important to note that the width of the shield 23 (and thus 
the distance between the stretched out springs 7 on either side of the 
common high voltage distribution bus 24) be as small as possible, and 
preferably smaller that the spacing between the wires and the counter 
potential roll. If this width is less than the above-mentioned wire to 
counter potential roll spacing, then the section of web or film being 
treated that receives a lower density of corona treatment is negligible, 
since the ionizing field spreads outward from each wire. 
Referring again to FIGS. 1 and 2, in the case of the continuous wires, the 
other end of the wires are attached to the tension levers 8 mounted 
adjacent to the switch block assembly 3. Referring to FIGS. 3 and 4, in 
the case of the two split wires, the other end of each of the split wires 
are attached via springs 7 to the ceramic insulators or standoffs 13. The 
ceramic standoffs 13 are in turn attached to the tension levers 8 mounted 
opposite the two switch block assemblies 3. 
Referring now to FIG. 7, the switch block assembly 3 is a device for 
detection of wire breakage that can enable shutdown of the treatment 
process, if the process is connected to the switches 10 as indicated in 
FIG. 6. The switches 10 are conventional safety disconnect switches that 
have spring loaded levers 25 which protrude out of the body of the spring. 
Switches 10 are mounted on to the switch block assembly 3, which is in 
turn mounted on the ionizer frame 2. The levers of spring loaded switches 
10 are aligned against tension levers 8 which are mounted on lever pin 9 
which is mounted to ionizer frame 2. Tension levers 8 rotate freely about 
lever pin 9. A ceramic insulator 13 is mounted on the top end of each of 
tension levers 8 and a tension spring 7 is mounted on each of these 
ceramic insulators 13 using screws 32. One end of ionizing wires 4 are 
mounted on these springs 7 using loops 6. The tension between the wire 
suspension ends rotates the top end of the tension levers 8 inwards and 
the bottom end of the levers 8 pushes against levers 25 of the spring 
loaded switches 10, thereby turning on each of the switches. If a wire 
breaks, then the tension pulling one of the tension levers 8 is removed, 
thereby enabling a corresponding one of spring loaded levers 25 of switch 
10 to push out against the corresponding one of the now freely rotating 
tension levers 8, thereby opening that switch 10 and the entire process 
circuit which is wired through that switch. 
Referring now to FIG. 10, although a single ionizing wire 4 may be used, 
typically three wires (continuous or split) are used in one ionizer. 
Ionizing wires 4 are positioned in a manner such that the distance between 
the distance of a radial line through the center of each ionizing wire and 
through the center of the counter potential roll 35 is the same for each 
wire. This is done by positioning center wire attachment spring 7 onto the 
distribution bus 24 and the corresponding center tension lever 8 deeper 
into the channel 26 in the ionizer frame 2. Thus an arc through the 
centers of the wires 4 is parallel to the circular center potential roll 
35. This results in similar fields around each wire. This assures an equal 
ion flux through each wire. 
Referring to again FIG. 6, when any one of switches 10 is opened, the power 
to a relay or other process controller 33 from a high voltage power source 
HVPS, which enables power to the process drive, is removed, thereby 
stopping the process according to the logic of the controller. 
Additionally, an alarm may be connected to the switch block 3 such that it 
turns on in the event of wire breakage. 
Recent efforts such as represented by Dintner et al., U.S. Pat. No. 
5,135,724 suggest an apparatus for increased surface modification by 
introduction of aerosols in the corona region such that these aerosols 
chemically react with the material that is being treated in the presence 
of the corona. In the case of forming permanently charged electrets, 
better results are obtained if the corona density is increased without 
reducing the uniformity of the corona flux. If conductive aerosols are 
introduced into the corona region, then the corona current will tend to 
follow the aerosol droplets and this can result in the formation of 
localized corona and a concomitant reduction in corona current flux 
uniformity. Hence, it is preferable to use high humidity air which can be 
produced by conventional methods such as evaporative heating and mixing 
with dry air. In order to ensure that no droplets enter the ionizing 
region, this humid air may be filtered by conventional air filters. This 
humid air is pumped through the air flow channel 17 and conduit ducts 17a, 
into the ionizing region of the channel frame 2. This allows for a uniform 
and intense corona treatment. For example, an increase in relative 
humidity from about 40% to 55% results in a doubling of corona current; 
increasing relative humidity from 40% to 65% will produce even higher 
corona current. Probably, relative humidity can be increased 
satisfactorily to 85% to 90%. Since water vapor is always uniformly 
distributed in the air, unlike the case for aerosol introduction, the 
uniformity of the corona current is maintained.