Improved electrostatic pinning method

A process for improving adhesion of a thermoplastic polymeric resin on a casting surface by using an electrostatic pinning wire in conjunction with a conductive shield. The method includes the steps of extrusion casting a sheet of molten polymer from a die, directing the cast sheet onto a moving, chilled and electrically grounded casting surface to solidify the polymer and extending a conductive wire transverse to a longitudinal axis of the sheet with the sheet between the wire and the chilled surface before the sheet contacts the chilled surface. An elongated conductive uninsulated shield parallel to the wire with the wire between the shield and the sheet is provided and a bias voltage is applied to the wire and the shield. The bias voltage of the wire is in the range of 6 to 15 kV, and the bias voltage of the shield is adjusted to be at least 5 kV lower than the voltage of the wire to maintain a desired current flowing from the wire and the shield to the sheet.

FIELD OF THE INVENTION 
The present Invention relates to the casting of polymeric resins using an 
electrostatic pinning process. More particularly, the present invention 
uses an electrically biased conductive shield in conjunction with a 
pinning wire, to improve the pinning of a polymeric resin onto a casting 
surface. 
BACKGROUND OF THE INVENTION 
Electrostatic pinning is one method used in the casting of an extruded 
plastic resin to a surface. A pinning wire is raised to a high voltage 
such that it ionizes the air around it, creating a corona discharge. Ions 
of the same polarity as the wire are repelled towards the resin and, 
hence, electrostatically charge the resin. The electrostatic charge, in 
combination with the ground plane provided by the casting surface, exerts 
an electrostatic force on the resin that improves its contact with the 
casting surface, resulting in films that are rapidly quenched and free of 
defects. This is especially important at increased casting speeds where 
air entrainment between the resin and the casting surface would otherwise 
result in film nonuniformities and defects. 
The use of an electrostatic pinning wire is described in U.S. Pat. No. 
3,223,757. In the process described in this patent, a film forming 
polymeric material is extruded onto an electrically grounded quenching 
surface while being passed in close proximity to an electrode. The 
electrode deposits an electrostatic charge onto the polymeric material 
before the material has solidified which causes the material to adhere 
firmly across the width to the quenching surface. 
In U.S. Pat. Nos. 3,655,307; 3,660,549; and 3,686,374, a method is 
disclosed for improving the high speed performance of electrostatic 
pinning by using a grounded shield in the shape of a semi-cylinder that is 
covered with an insulating layer. The second electrode gets charged from 
the pinning wire tending to push the ions towards the resin. In addition, 
a gas may be introduced that serves to raise the sparkover voltage, the 
voltage at which the electrode discharges onto the wheel or the die. 
In U.S. Pat. No. 3,820,929, a second wire is electrically connected to the 
pinning wire. It is of a larger diameter so as not to emit ions, hence, it 
serves in a similar capacity as the dielectric shield mentioned in the 
above paragraph except the geometry is more limited. 
In U.S. Pat. No. 4,129,630 a non-grounded shield, conductive or 
non-conductive and of variable geometry, is interposed between the pinning 
wire and the extrusion die, and claimed to improve pinning due to improved 
electric field uniformity resulting from decreased contamination of the 
pinning wire with sublimate from the die. 
A semi-cylindrical, grounded conductive shield is disclosed in U.S. Pat. 
No. 4,244,894. This patent asserts that at a given pinning wire voltage, 
more charge is delivered to the resin with a grounded shield than with 
other configurations. 
In U.S. Pat. No. 4,534,918, a second electrode consisting of grounded pins 
extended towards the pinning wire is disclosed. This patent claims that 
these pins improve the uniformity of the charge laid out on the resin 
resulting in better pinning. 
In U.S. Pat. No. 5,030,393, a set of floating conductive electrodes are 
disposed to either side of the pinning wire. These electrodes perform 
similarly to the insulating shields mentioned earlier, such as in U.S. 
Pat. No. 3,655,307. 
Finally, another method that is used is vacuum pinning. However, it is 
believed that vacuum pinning is very dependent on the usage of beads on 
either side of the extruded resin. For applications requiring minimal 
beads, beads on only one surface or no beads at all, vacuum pinning is 
difficult and may not work at all. 
It has been observed that pinning at higher speeds requires higher current 
output by the pinning wire. This can be increased by raising the wire 
voltage, however, above a certain voltage arcing will occur between the 
wire and either the casting surface or the die. The above described 
methods have been employed to circumvent this limitation. However, none 
have been completely successful. 
The present invention is a method that solves the above-described problems 
by using an electrically biased conductive shield in conjunction with a 
pinning wire so that the charging efficiency of the pinning apparatus can 
be controlled, resulting in improved pinning latitude with respect to 
polymer formulation and process speed. 
SUMMARY OF THE INVENTION 
The present invention is a method for pinning a cast sheet of polymeric 
film. The method includes the steps of extrusion casting a sheet of molten 
polymer from a die, directing the cast sheet onto a moving, chilled and 
electrically grounded surface to solidify the polymer and extending a 
conductive wire parallel to the width of the sheet with the sheet between 
the wire and the chilled surface before the sheet contacts the chilled 
surface. The method further includes extending an elongated conductive 
uninsulated shield parallel to the wire with the wire between the shield 
and the sheet, applying a bias voltage to the wire in the range of 6 to 15 
kV; and adjusting a bias voltage of the shield at least 5 kV below the 
bias voltage of the wire to maintain a desired current flowing from the 
wire and the shield to the sheet. 
In an alternate embodiment of the present invention, the method includes 
the further step of positioning the shield between the wire and the die. 
The present invention has the advantage of providing a greater amount of 
net current or charge to the resin than with prior art methods. The 
present invention also has the advantage as opposed to the use of an 
insulated shield in the elimination of the dielectric insulation material 
and its requisite constraints of having a long life and high dielectric 
breakdown strength. Finally, the present invention has the advantage of 
greater control than prior art methods.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Shown in FIG. 1 is a prior art device used for electrostatic pinning to 
improve adhesion of an extruded resin onto a grounded casting wheel in the 
production of plastic films such as polyethylene terephthalate (PET). The 
concept of electrostatic pinning is to spray electrical charge of one 
polarity onto the resin as it is extruded from the die. This is shown 
schematically in FIG. 1. An extrusion die 10 extrudes the polymeric resin 
11 onto a casting wheel 12. A voltage is applied to pinning wire 14 which 
creates an electrostatic force between the resin and the casting wheel 
thereby preventing air entrainment in enabling the production of high 
quality films of polymeric resin. Key factors in this process are the 
pinning hardware used to charge the resin and the electrical resistivity 
of the resin. 
FIG. 2 shows an alternate method for pinning of the polymeric resin 11. A 
vacuum box 20 is used to create a vacuum behind the resin 11 as it 
extrudes from die 10. This vacuum eliminates the air boundary layer 
entrained by the casting wheel 12 allowing more uniform contact between 
the resin 11 and wheel 12. It is believed that vacuum pinning is very 
dependent on the usage of the beads on either side of the extruded resin 
11. For applications requiring minimal beads, beads on only one surface or 
no beads at all, vacuum pinning is difficult and may not work at all. 
FIG. 3 shows the apparatus used in the present invention. An extrusion die 
10 extrudes the polymeric resin 11 onto the casting wheel 12. A pinning 
wire 14 is located between the resin 11 and the casting wheel. A 
conductive shield 15 is located between the extrusion die and the pinning 
wire. However, rather than grounding or floating this shield 15, a power 
supply 16 is attached to the shield so as to be able to control the bias 
voltage of the shield 15. If necessary, a resistor 17 to ground may be 
placed in parallel with the power supply so as to sink current from the 
pinning wire 14. 
The advantage of this approach as opposed to a grounded shield such as 
described in U.S. Pat. No. 4,244,894, is the greater amount of net current 
(charge) available to the resin with a biased shield. The parameter of 
import is the charge deposited on the resin and a biased shield can 
deliver more charge, and therefore, operate more efficiently, than a 
grounded shield. This is demonstrated in FIG. 4, a plot of net current to 
the resin as a function of the bias voltage of the shield. For this 
experiment, a variety of polymeric resin formulations were each cast at 45 
feet per minute at a thickness of 5 mil with a pinning wire less than or 
equal to 10 kV held at a constant current output of 2000 .mu.A. The 
voltage to the wire was less than or equal to 10 kV. Typically, a grounded 
shield is only 10 percent efficient (roughly 200 .mu.A) whereas a biased 
shield can be as high as 50 percent efficient (@4 kV). The bias shield 
should be physically isolated to protect operators. As shown in FIG. 4, 
the increase in charging efficiency occurs regardless of resin 
formulation. Therefore, a resin formulation that causes low net current 
values can be compensated for by increasing Vshield. Higher casting speeds 
require higher net currents and this is more easily achieved using the 
bias shield of the present invention rather than a grounded one. 
The novel feature of the present invention is the realization that there is 
an optimal bias voltage for the shield that depends upon the pinning wire 
voltage but is significantly different from the pinning wire voltage and 
the ground potential. This is demonstrated by the following experiment. 
The apparatus of FIG. 3, with shield 15 was used to pin a polymeric resin 
at 45 fpm at a 5 mil thickness. The pinning wire current was monitored 
with a Spellman high voltage power supply. Another Spellman high voltage 
power supply 16 was used to bias the shield 15 in parallel with a 600 
k.OMEGA. resistor to ground. The current from the shield's power supply 
was also monitored. The current supplied by the power supply to the shield 
with the pinning wire off was used as a baseline and values obtained with 
the pinning wire energized provided a measure of the current to shield. 
The net current to the resin was computed as the difference between the 
pinning wire current and the previously computed current to shield. 
The two parameters varied were pinning wire voltage and shield bias 
voltage. For a given shield bias, the pinning wire voltage was varied from 
7 kV (a little above corona turn on) to 9.5 kV (limited value due to 
arcing) in 0.5 kV increments. This was done for shield bias voltages of 1, 
1.5, 2, 3 and 4 kV. A contour plot of the net resin current data was 
generated. It is very clear that an optimal shield bias voltage (Vshield) 
exists for a given pinning wire voltage (Vwire) and this optimal voltage 
lies between 1 and 4 kV for the configuration tested. Also, the optimal 
Vshield appears to be a function of Vwire, increasing in a nonlinear 
fashion with increasing Vwire. 
For a given resin, process speed and film thickness, there are two major 
effects of Vwire and Vshield in determining the net current to the resin. 
One is the amount of current produced by the pinning wire, the other is 
the number of electric field lines emanating from the wire and terminating 
on the resin. For a given Vwire, a low Vshield results in high wire 
current production but also a large number of field lines terminating on 
the shield rather than the resin. As Vshield is increased, wire current is 
reduced but more field lines terminate on the resin, tending to drive most 
of the current to the shield. If Vshield is too high wire current 
production is shut down completely. At the optimal Vshield the balance 
between shutting down wire current production and increasing the number of 
field lines terminating on the resin is maximized. 
Pinning quality is directly related to net current going to the resin as 
demonstrated using a biased shield. At 45 fpm, 5 mil thick films are well 
pinned at net current levels of 4 to 8 .mu.A/cm, depending upon resin 
formulation. There is variation among different resin formulations, 
presumably due to resin resistivity variations. 
Increasing the bias voltage on a shield placed adjacent to the pinning wire 
improves the charging efficiency of the apparatus and can improve the 
robustness of the pinning device to resin resistivity variations. There is 
an optimal shield bias voltage that depends upon pinning wire voltage. 
Furthermore, it was demonstrated that there is an optimal shield bias 
voltage that is a function of pinning wire voltage. 
The present invention is applicable to casting polymeric formulations at a 
variety of speeds. The invention works best if the bias voltage applied to 
the pinning wire is from 6 kV to 15 kV and the bias voltage applied to the 
shield is at least 5 kV lower than the bias voltage applied to the pinning 
wire. 
While there has been shown and described what are at present considered the 
preferred embodiments of the present invention, it will be obvious to 
those skilled in the arts that various changes, alterations and 
modifications may be made therein without departing from the scope of the 
invention as defined by the following claims.