EP0034878A1

EP0034878A1 - Process of and apparatus for the electrostatic charging of continuous filaments of a synthetic organic polymeric material by means of a corona discharge

Info

Publication number: EP0034878A1
Application number: EP81300180A
Authority: EP
Inventors: Colin Morgan Bowden
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1980-02-26
Filing date: 1981-01-15
Publication date: 1981-09-02

Abstract

Process of and apparatus for the electrostatic charging of continuous filaments of a synthetic organic polymeric material, the process comprising forwarding the filaments (4) towards and into contact with a first, earthed, electrode (2) and subjecting the filaments to a corona discharge maintained between the first electrode (2) and one or more second electrodes (1) carrying a high potential electric charge characterised in that the current inthe corona discharge is constant.

Description

This invention relates to a process and apparatus for placing electrostatic charges on continuous filaments of a synthetic organic polymeric material by means of a corona discharge.
The placing of electrostatic charges on a plurality of filaments serves to keep them apart so that they do not interfere with each other when they are subsequently, and simultaneously, being processed. For example when the filaments are being laid down side by side on a conveyor, it has been found that the uniformity of laying, ie the spacing between adjourning filaments and the flatness of the individual filaments on the conveyor, is much improved if the filaments are electrostatically charged.
A corona discharge is the electrical discharge occurring in a gas surrounding a conductor when the potential gradient at a point in a non-uniform electric field exceeds the critical value for ionisation causing a local self-sustaining discharge. It is a phenomenon particularly associated with sharply curved electrodes and essentially occurs in a limited region near the electrodes. The magnitude of the electric field gradient required to produce a corona discharge is the . same as that required to produce arcing; the difference between the two being that in corona, the critical value has been reached only in a limited region of the breakdown path between the electrodes.
It is already known to place an electrostatic charge on continuous filaments of a synthetic organic polymeric material by forwarding the filaments continuously through a corona discharge zone.
This is achieved by contacting the filaments with an earthed electrode which is spaced from one or more other electrodes, a high potential difference being maintained between the earthed electrode and the other electrodes. Such systems have been energised by an uncontrolled or constant voltage source. However, because of varying conditions, eg ambient moisture, this can lead to a varying corona current and varying charging of the filaments which effects the uniformity of products made from the filaments. Furthermore considerable erosion of the high potential electrodes occurs when operating under constant voltage conditions.
We have now found that if a constant current supply is used, a more consistent corona current is obtained which is largely unaffected by time, variations of humidity and electrode erosion. Further by using a constant current supply,electrode wear is reduced because the possibility of high discharge currents, which were prevalent with a constant voltage system, is eliminated.
According to our invention, therefore, we provide a process for providing continuous filaments of a synthetic organic polymeric material with an electrostatic charge comprising forwarding the filaments towards and into contact with a first, earthed, electrode and subjecting the filaments to a corona discharge maintained between the first electrode and one or more second electrodes carrying a high potential electric charge characterised in that the current in the corona discharge is constant.
We also provide an apparatus for placing electrostatic charges on continuous filaments of a synthetic organic polymeric material comprising, in combination, a first, earthed, electrode and, spaced therefrom,one or more second electrodes, an electric circuit in operative association with the one or more second electrodes for energising such electrodes to a high electrical potential, the one or more second electrodes being of such a shape and being so positioned with respect to the first electrode that when such second electrodes receive said high potential electrical charge a'corona discharge is established between said second and first electrodes, and means for forwarding the filaments towards and into contact with the first electrode at a location where said filaments cross the path of said corona discharge, characterised in that the electric circuit is adapted to ensure that the current in the corona discharge is constant. The invention will now be described with reference to the accompanying drawings in which:-

Figure 1 shows, diagrammatically, the layout of the apparatus used in the invention in its simplest form.
Figure 2 shows, diagrammatically, a modification of the layout shown in Figure 1.
Figure 3 shows the circuit diagram of a typical constant current supply unit for generating a negative charge for use with the invention.
Figure 4 shows a similar unit for generating a positive charge for use with the invention.

In Figure 1 is shown by a broken line the corona discharge between a shaped electrode 1 and a stationary cylindrical electrode 2, electrode 2 being earthed and electrode 1 being connected to a high voltage unit 3. Filaments 4 are passed in the direction indicated across the path of the corona discharge over guide 5 and cylindrical electrode 2o Provided that the guide and cylindrical electrode are arranged correctly to bring the filaments into the vicinity of the corona, the discharge is filament seeking and position to position uniformity of charging is likely to occur if the current in each position is constant.
The high voltage supply 3 is a transistor oscillator type, where the oscillations are transformed up on a transformer and then rectified in a quadrupler circuit. The kilovolt output is proportional to the low direct voltage input. Assuming no leakage current, this corona discharge current can be obtained by measuring the voltage across the resistor R. This voltage is amplified by amplifier 6 and then compared with a reference voltage 7 by a comparator 8. The comparator can only tolerate a millivolt difference in voltage between its two inputs. The output of the comparator will therefore adjust the input to the high voltage unit to a value, where the kilovolt output will cause sufficient corona current to make the output from the amplifier equal the set current reference voltage within a few millivolts. The corona current will then depend only on the resistor R, the set current reference voltage and the gain on the resistor R. The set current reference voltage and the gain of the amplifier in turn depends only on the values of resistors.
Referring to Figure 2, the output from the amplifier may also be compared with high limit reference voltage 9 and low limit reference voltage 10 by means of high limit comparator 11 and low limit comparator 12. Also incorporated in the circuit are lights Ll, which goes out if the corona current becomes too high, and L2, if the corona current becomes too low, for example a short circuit or circuit failure for Ll and L2 respectively.
In Fig 3 is shown the circuit diagram of a typical constant current supply unit which generates a negative charge.
Referring to.Fig 3, provided there is no other path to earth from the high voltage point of the circuit the current in the corona discharge is the same as the earth return current through R22. The current of 25 microamperes through a 10 kilohm resistor gives a 0.25 V signal which is amplified to 2.5 V in IC3. This signal which is a measure of the corona current is compared to a 2.5V reference signal, derived from R12/R13, in the comparator IC4. Any difference between these two signals results_' in an error voltage on the output of IC4 which is then applied via transistor TR1 to the input of the EHT unit. This signal changes the output of the EHT unit to make the corona discharge current come to 25 microamperes since IC4 will only tolerate a few millivolts difference in the signals applied to its inputs. The Zener clamp across the output of IC4 is used to prevent the voltage at TP4 rising to more than + 3.9 V higher than the +2.5V at TP3. This limits the input voltage to the EHT unit to 15.9V which in turn limits the EHT at the output to about 13 kv. This protection prevents the unit driving too hard to maintain 25 microamperes when the plug and socket Pl/Sl (Fig 2) is disconnected. The diode D10 is necessary to prevent the Zener diode conducting in the forward direction when the output of IC4 is less than +2.5V.
The amplifiers IC1 and IC2 are used as comparators. The 2.5 volt signal which represents 25 microamperes is compared to 3.0V and 2.0V which represent 30 microamperes and 20 microamperes respectively. The reference voltage of 3V on IC1 will cause the output of IC1 to be -12V so that light Ll will be ON. Should the discharge current rise above 30 microamperes the discharge current signal will rise above 3V and the output of IC1 will change to +12V and the light Ll will go out. Similarly the reference voltage of 2.0V on IC2 will result in the output of IC2 being +12V provided the discharge current signal is +2.5V. Should this signal drop below 2.0V then the output of IC2 will change to -12V and the light L2 will go out.
The EHT unit consists of a 15 k Hz oscillator build around the transistor TR2. The output of this oscillator is transformed to a high voltage by the transformer Tl. This output is rectified and increased in a voltage quadrupler before being fed to the output socket via a 10 megohm resistor. A 2 gigohm resistor across the output discharges the capacitors when the unit is switched off. The capacitor C12 smoothes out the 15 k Hz ripple across R22 and the newwork R19/C7 together with the diodes D3 and D4 give protection to the control circuitry against transient voltages.
With a 2.5V reference signal the value of the corona discharge current can be varied by altering the value of R22. The corona current must cause a voltage drop of 0.25V across R22 so that if R22 = 10 kilohms then the corona discharge is 25 microamperes. If R22 = 25 kilohms then the discharge current is 10 microamperes.
Similarly, in Fig 4 is shown the circuit diagram of a typical constant current supply unit which generates a positive charge. It is the same as for the negative charge unit except that the output polarity of the voltage quadrupler is changed and the phasing of the signals to the comparator IC4 is also changed. In general, the reference voltage must drive the EHT unit on and the earth return current signal must drive it off. Similarly the phasing between the return current signal and the reference voltages to the comparators is reversed. The reference voltages are negative in this case.

Claims

1. A process for providing continuous filaments of a synthetic organic polymeric material with an electrostatic charge comprising forwarding the filaments towards and into contact with a first, earthed, electrode and subjecting the filaments to a corona discharge maintained between the first electrode and one or more second electrodes carrying a high potential electric charge characterised in that the current in the corona discharge is constant.

2. An apparatus for placing electrostatic charges, on continuous filaments of a synthetic organic polymeric material comprising, in combination, a first, earthed, electrode and, spaced therefrom, one or more second electrodes, an electric circuit in operative association with the one or more second electrodes for energising such electrodes to a high electrical potential, the one or more second electrodes being of such a shape and being so positioned with respect to the first electrode that when such second electrodes receive said high potential electrical charge a corona discharge is established between said second and first electrodes, and means for forwarding the filaments towards and into contact with the first electrode at a location where said filaments cross the path of said corona discharge, characterised in that the electric circuit is adapted to ensure that the current in the corona discharge is constant.