Method and apparatus for uniform corona discharge bonding

A method and apparatus for providing uniform corona bonding between and across the width of a plurality of layers of material each having a generally uniform thickness and width whereby, if desired, the plurality of layers may be easily and uniformly separated. Uniformity of bond is achieved by configuring the outer ends of an otherwise straight electrode, placed at an optimum effective bonding position, in proportion to the higher corona energy levels found to occur at the ends of the corona electrode bar.

DESCRIPTION 
1. Technical Field 
This invention generally relates to corona bonding of layers of plastic 
material by means of a corona discharge, and more particularly, concerns a 
method and apparatus for providing a substiantially uniform bond between 
the layers of the material across the width of the materials being bonded. 
2. Background Prior Art 
U.S. Pat. Nos. 3,823,061 and 4,096,013 disclose the manufacture of 
multilayered material by subjecting the various layers of material to a 
corona discharge and bringing them into contact with each other and are 
incorporated herein by reference. Both are commonly assigned with the 
instant application. 
Such corona bonded multilayered plastic material may be used, among other 
things, to produce an intermediate liner for the manufacture of 
Co-Pak.RTM. brand multilayered containers. In that process, the liners are 
thermo-formed and cut from strips of the corona bonded multilayered 
plastic material after which the remaining portion of these strips are 
delaminated, reground and reused in the manufacture of new material. Such 
recovery of the layers of material in the strip has a significant impact 
on the economic viability and competitiveness in the marketplace of the 
Co-Pak.RTM. brand container. 
In the process of delaminating the strip of material bonded together 
through the use of known prior art apparatus and methods, it was observed 
that the edge portion of these strips of material extending along opposite 
outside edges had significantly greater adhesion to each other than did 
the inward, central portions of the strip. Consequently, on occasion, the 
edge portions of these strips did not delaminate but rather the central 
portion, where the adhesion permitted delamination, tore away from the 
edge causing disruption of the delaminating process. Prior to the 
development of the invention disclosed herein, when this situation 
occurred, the energy level of the corona discharge was reduced to provide 
the desired delaminable bond at the outer edges. In such instances, 
however, it was noted that the adhesion of the inward central portions 
became weaker than desired and such nonuniform adhesion created problems 
in the manufacture of the Co-Pak.RTM. brand containers. Thus, the process 
required a uniform bonding force between the layers of material across the 
entire width of the strip. To applicant's knowledge, an apparatus and 
method for continuous uniform bonding force across the width of the strip 
was not known. 
It would be understood from a review of the patents above mentioned, that 
the adhesion between the layers of material is subject to many variables 
including, without limitation, the composition, angle, thickness and speed 
of the plastic material being extruded; the size, temperature and 
conductivity of a chrome roll on to which the plastic material is 
extruded; the size, configuration, position, and energy level of the 
electrodes; as well as the spatial relationship between the various 
elements in the process. For additional details of these variables, 
reference may be had to U.S. Pat. No. 4,096,013, and particularly column 
13, et seq. Although there are many different ways to vary the 
effectiveness of the adhesion, it has been found that there is generally 
one preferred, optimum relationship of all factors involved which produces 
maximum bonding of specific materials in a specific manufacturing 
situation. For purposes of discussion, the point at which the various 
variables come together in a relationship that produces maximum 
effectiveness shall be referred to as the "optimum point or line". 
Nevertheless, at this optimum point, the higher level of bond along the 
opposite outer edges of the strip of material remained a matter of 
concern. 
The higher edge bonding strength was found to be caused by a concentration 
of electrical energy at the sharp points of the straight corona discharge 
electrode that extended across the width of the strip of material being 
formed. Rounding the sharp points helped a little but still left a very 
noticeable stronger bonding force at the edge. Moving the ends of the 
straight electrode inwardly from the edge also was not effective as it 
left an area along the edge with no bond. Moreover, moving the ends in the 
direction away from the material tended to produce sparking or streamers 
which could result in a nonuniform bonding in those areas. Thickening of 
the material at the edges was found effective for producing a more uniform 
bond; however, it was unacceptable in the specific thermoforming process 
to which the material was subsequently subjected. Accordingly, it was not 
satisfactory. 
SUMMARY OF THE INVENTION 
According to the invention herein a method and apparatus are disclosed for 
providing uniform corona bonding between and across the width of a 
plurality of layers of generally uniform thickness and width whereby, if 
desired, the plurality of layers may be uniformly separated. Uniformity of 
bond is achieved by configuring the outer ends of an otherwise straight 
electrode, placed at the optimum point, in proportion to the higher corona 
energy levels found to occur at the ends of the electrode. 
In one aspect of this invention, an apparatus for progressively and 
uniformly bonding together at least two layers of material by passing the 
width of the materials through a corona energy field produced by an 
electrode includes the improvement wherein the opposite end portions of 
the electrode are bent at an angle to the remainder of the otherwise 
straight electrode. Bending of the end portions occurs in a plane 
generally perpendicular to the plane extending through the axis of a 
ground and the straight portion of the electrode. In practice it has been 
found that the bent end portions may be maintained straight and are 
preferably bent at one point at an angle in the range of about 8 to 12 
degrees from the remainder of the straight electrode. 
According to another aspect of the invention, there is provided a method of 
producing material of a plurality of layers having predetermined desired 
adhesion between the adjacent layers of material comprising the steps of 
subjecting the plurality of layers to a uniform corona bond across its 
width. The method may comprise the further steps of forming a plurality of 
intermediate components from the uniformly bonded material and separating 
the materials by breaking the uniform corona bond between the materials, 
as for example, by rolling them onto rollers on opposite sides of the 
laminated material.

DETAILED DESCRIPTION 
Referring to FIG. 1, there is shown a pair of extruders 10, 12, producing 
strips of hot plastic material 14, 16, respectively, which strips pass 
through a corona discharge formed around the electrode generally indicated 
at 18 by an electrical energy source 38. The strips of plastic material, 
which may be for example, polypropylene and barex, respectively, then 
proceed around a chrome surface chilled roll 40 grounded at 50 before 
moving to a point 60 where the thus bonded material may be used as for 
example by forming a separate item or items therefrom. It will be 
understood that the strip may, of course, be rolled, cut and stored prior 
to this schematic illustrated use step. After use, the material remaining 
will pass to a delaminating station 80 where it is separated into its 
original components 14, 16, by breaking the corona bond between the 
layers. Such breaking of the corona bond may be accomplished mechanically 
as by rolling the materials 14, 16, onto rollers as shown. 
Details of the extrusion and prior bonding methods and apparatus may be 
understood by reference to the U.S. patents herein. 
The electrode generally indicated at 18 in FIG. 1 is of a general bar 
configuration and movable in three dimensions. It is connected to a 
suitable electrical energy source schematically indicated at 38. As shown, 
the electrode has a straight central portion 21 and two end portions 22, 
24, of equal length and bent at an angle to the straight portion 21. As 
previously mentioned, when the straight portion 21 is at the optimum point 
in relation to all of the other factors necessary to produce a desired 
bond, the strength of the bond between the layers of multilayered material 
may be varied by changing the energy level of the corona electrode. Thus, 
by increasing the energy level, the strength of the bond is increased and 
vice versa. 
It was found that any movement of the electrode or portions thereof away 
from the optimum point resulted in a decrease at that point of the bonding 
strength between the materials passing through the corona discharge. 
Accordingly, it was discovered the higher electrical energy concentration 
levels found at the end of the electrode could be compensated for by 
moving the higher energy end portions away from the optimum point to a 
position where they were less effective. Accordingly, the net result was 
that there occurred across the width of the material passing through the 
corona discharge a uniform bond between the layers. 
In the embodiment illustrated in FIG. 1, the higher end concentrations are 
compensated for by cutting the generally flat electrode bar 18, positioned 
with its inner edge adjacent to the material 14, inwardly from about the 
midpoint of each end lengthwise a predetermined distance, and bending the 
thus formed portions 22, 24, adjacent the material 14, downwardly through 
a predetermined angle alpha (.alpha.) out of the line of the axis of the 
straight portion 21 of the bar. With this apparatus construction, a 
satisfactory uniform delaminable bond was produced. The remaining straight 
portions 23 and 25 of electrode 18 respectively, have been found to 
produce no adverse effect. 
FIG. 2 illustrates a further embodiment in which angled end portions 32, 
34, are produced without the severing step used in the embodiment of FIG. 
1 but rather by simply bending the ends of a bar generally indicated at 28 
at an angle alpha (.alpha.) a predetermined distance A and C respectively 
from each end. The electrode generally indicated at 28 thus comprises a 
central straight portion 31 and two straight end portions 32, 34. In 
practice, it has been found that the distance A and C may be equal and may 
be in the order of 13/4" where the "W" width of the material being bonded 
is 12" and in the order of 2" where the width of the material being bonded 
is 28". Under the same conditions, angle alpha (.alpha.) is in the range 
of about 8 to 12 degrees. The same angle range of 8 to 12 degrees works 
for both the 12 and 28" widths and, although not tested, is expected to 
hold true for wider widths. This same angle range applies to the 
displacements of the portions 22 and 24 in FIG. 1. 
Such less effective positioning of the end portions 22, 24, in FIG. 1 and 
32 and 34 in FIG. 2, is outside the optimum point or optimum line of 
maximum effectiveness. According to the preferred embodiments, the bending 
from that optimum point or line is generally in a plane perpendicular to 
the plane extending through the axis of the grounded roll 40 and the 
straight portions 21 in FIG. 1 and 31 in FIG. 2. While the field strength 
will also be weakened by moving the electrodes 18 and 28 away from the 
roll 40 in a plane passing through the axis of the roll 40, it has been 
found that such movement leads to the breakdown of the corona and produces 
uneven bonding. 
In FIG. 2 the width of the material 14, 16, is slightly greater than the 
overall effective length D of the corona electrode 28 a predetermined 
distance so that the corona will not short out directly to the roll 40. 
The width W, of the material, may be equal to or less than the width R of 
roll 40. 
It will be understood from the referenced patents and the disclosure herein 
that a great number of variables must be considered in obtaining maximum 
effectiveness of the corona bonding process. In the prior art a straight 
electrode located at the line of greatest effectiveness produced a higher 
bond along the edges of the material. Thus, when proper bonding occurred 
in the central inward portion between the opposite edges of the material 
14 and 16, the bond at the edge was substantially greater so at the 
separation station 80 it occurred that the edges in some cases did not 
separate but instead tore the material lengthwise along the line inwardly 
from the edge. This, of course, disrupted the reclamation process and 
wasted material. 
The illustrated solution to the higher edge bond levels involve 
compensating for the higher energy levels experienced at the ends of the 
electrode by configuring the end of the electrode porportionately to the 
higher energy levels so the electrode ends were in a less effective 
position out of the optimum point or line. 
Although only preferred embodiments of the inventions have been 
specifically illustrated and described herein, it is to be understood that 
minor variations may be made in the methods and apparatus without 
departing from the spirit and scope of the invention, as defined by the 
appended claims.