Film Treating Method

Abstract

A method is disclosed for the surface treatment of a plastic body by exposure to a high intensity voltage accompanied by corona discharge wherein said voltage is a sequence of alternating-directional, sonic frequency pulses of electrical voltage.

Parent Case Text

This application is a continuation-in-part of application Ser. No. 862,412, filed Sept. 30, 1969 and now abandoned.

Description

BACKGROUND OF THE INVENTION

Exposing the surface of a polymer body, such as polyethylene film, to a high voltage gaseous discharge having corona characteristics is known to improve the affinity of the surface for adhesives, inks and other polar substrates. The treatment zone of a typical system comprises a relatively large ground electrode separated from one or more relatively sharp high voltage electrodes by two and preferably three dielectrics. The essential dielectrics are an ionizable gaseous dielectric, normally air, and the polymeric body to be treated. Normally, the ground electrode is covered with a "buffer" dielectric, such as rubber or a polyester film, which acts to preclude an arc from bridging the gap at weak points in the polymer body. The high voltage electrode, which may consist of one or more treater bars in series or in parallel, runs the length of the ground electrode and is in circuit with a high voltage generator.

Most commercial treating systems employ alternating current supplied at frequencies up to 500 kHz or more. Gap voltages up to 15 kv or more are employed to effectively treat a polymer film which is continuously passed through the gap at speeds up to 500 feet per minute or more. In practice, an energy density-to-film surface of the order of about 1 watt-minute per square foot of film surface or more is sought to achieve good surface adhesion characteristics.

While every component of a film treating system has come under investigation from time to time, the waveform of the high voltage employed in the treating system has generally been neglected. The spark-gap generators and motor alternators now in use are inefficient and suffer from many inherent deficiencies.

In addition to interfering with radio reception due to the presence of radio frequencies in the spark-gap generator output wave, that generator has a short duty cycle. The range of output power for a given generator is severely limited since the gap breakdown voltage sets the minimum voltage.

The motor alternator, on the other hand, is cumbersome in size and subject to frequent mechanical failure. Further, its output is sinusoidal which is far from the ideal waveform.

In a typical high voltage film treating system, an alternating current line voltage is fed to a high voltage generator and the generator alternating current output is fed through an output transformer to the treating circuit load.

The load should be viewed as a lossy capacitor wherein the electrodes, in their area and spacing, define the capacitance and the dielectric is a composite made up of an air gap, the film and the buffer dielectric all in series. As the corona voltage threshold level is reached, the losses of this system vary in a nonlinear manner. It is the loss component which is effective in treatment and the recognition of the capacitive reactive behavior of the load is important.

The concept of variable frequency has been only recently recognized as the all important parameter for load adjustment and optimization in film treating operations. Looking at the corona treating region as a lossy capacitor system, the power would be proportional to frequency just as, for a given input voltage, the current entering a capacitor is linear with frequency. This concept is disclosed and claimed in our copending application, filed of even date herewith, and entitled "Film Treating Process."

SUMMARY OF THE INVENTION

The present invention relates to the high voltage surface treatment of a plastic body with an alternating voltage of high intensity accompanied by corona discharge, wherein the treatment zone constitutes a capacitive load, comprising exposing said surface to an alternating-directional pulse waveform electrical voltage in the sonic frequency range and accompanying corona discharge.

It has been found that a broad range of sonic frequency (20-20,000 Hz) treating voltages may be employed, where frequency is varied to effect surface treatment under optimum load conditions. Accordingly, a treating system providing a broad frequency variation of treating voltage over a range of 20 to 5,000 Hz is desired.

The pulse waveform is desired since corona current flows only during the time of voltage change according to i = C(dv/dt). For example, a pulse with its rapid positive and negative change will result in corona current. The square wave during the flat top region results in no corona current and merely applies an electrical stress on all components. The nature of a corona load is such that charges residing on the dielectric surfaces inhibit further corona discharge during the constant voltage region. Thus, the square wave is of no value and only the swing from positive to negative and negative to positive extremes is useful in generating corona. Similar arguments can be applied to the sine wave waveform. It has been discovered that the transient aspect is the essential part of any waveform and the most desirable from the point of view of efficacy and utilization.

In the drawings:

FIG. 1 is a schematic view of apparatus circuitry capable of use in the practice of the process of the invention;

FIG. 2 (a) and (b) are schematic representations of the treating circuit voltage and load current waveforms, respectively, for apparatus of the type shown in FIG. 1;

FIG. 3 is a graphical representation of the relationship between treating load power and frequency employed for varying electrode lengths in the process of the present invention.

An improved film treating system is shown schematically in FIG. 1 of the drawings. As there shown, a suitable variable direct current source is provided comprising a variable autotransformer 10 having an alternating current supply, the output of which is rectified by a full wave rectifier 12 and filtered by capacitor 14 connected across the output terminals of rectifier 12. The dc voltage output E.sub.dc which is a direct function of the applied auto transformer voltage is fed to the high voltage pulse output circuit 15. Polyphase rectifiers and the like can also be used to provide adjustable dc voltages. It should be noted, however, that the employment of means for varying input voltage and consequent selected output voltage to a desired constant level constitutes merely an apparatus convenience but does not constitute a point of criticality or novelty in the present invention.

The high voltage pulse output circuit 15 comprises a high voltage transformer 16 having a high voltage secondary winding and a low voltage primary center tapped at 18 where voltage E.sub.dc is applied. At least two power thyristors 20 and 22 are coupled at their cathodes and respectively connected at their anodes to the end taps 24 and 26 of the primary of the transformer 16. As described in the article "Thyristors: Semiconductors for Power Control" by V.W. Wigotsky in Design News, Vol. 22, No. 18, page 26, which is incorporated by reference, thyristors are super switches for electrical power as is their function in the solid state high voltage generator of this invention. The preferred power thyristors are silicon controlled rectifiers but any solid state device or combination of devices which function equivalent to a thyristor or switch can be used. Ordinarily, a thyristor, particularly a silicon-controlled rectifier in a high conductive state, continues to conduct after the gate signal is removed until the anode current is interrupted or diverted for a time sufficient to permit the rectifier to regain its forward blocking condition.

At least one capacitor 28 is connected across the end taps 24 and 26 of the primary of the transformer and consequently between thyristors 20 and 22.

The high voltage transformer 16 is an important integral part of the high voltage pulse generator circuit. It is center-tapped with end return taps in the primary while the secondary is a high potential winding. Its core must not saturate at operating frequencies and voltages.

At least one pair of diodes 34 and 36 are, respectively, connected at their cathodes to the end taps 24 and 26 of the primary of the transformer 16. The anodes of diodes 34 and 36 are commonly connected to the cathodes of thyristors 20 and 22. Inductor 38 is positioned between filtering capacitor 14 and pulse output circuit 15. The diodes 34 and 36 act as anti-parallel or reverse conduction diodes to allow for reversed current flow.

The rate at which power thyristors undergo gating is controlled by a timing circuit 40 which is typically a multi-vibrator, preferably a free-running, astable, solid state oscillator or a unijunction, astable oscillator which generates trigger pulses of any desired frequency. If coupled with another triggering circuit, monostable and bistable oscillators may also be used. The multivibrator 40 is coupled to the gate of thyristor 20 by a capacitor 42 and resistor 44 and to the gate of thyristor 22 by capacitor 46 and resistor 48 networks, respectively.

Variation of the output frequency of the multivibrator circuit 40 is obtained by the employment of variable resistors 49 and 50 (ganged at 51) which are, respectively, positioned in each of the base circuits of the transistors. Such variable control of multivibrator output frequency produces a consequent controllable output from the pulse output circuit 15 which results in output frequency control of power delivered to the treating load circuit 52.

The output of the transformer thyristor section of the high voltage generator is essentially a pulsed wave of variable frequency. Such output is produced by sequentially gating thyristors 20 and 22 by timing pulses applied to the gates thereof by the timing circuit 40. More particularly, when thyristor 20 is closed, thyristor 22 is maintained in a blocked or open condition and current from the power supply will then flow through the inductor 38 and one half of the transformer. The capacitor 28 is across the whole transformer. This series combination of inductor 38 and capacitor 28 oscillate (at a frequency higher than the gating frequency) to provide a single cycle of oscillation. The thyristor 20 is self-extinguished during the time that diode 34 is conducting (i.e., the negative portion of the cycle). Each thyristor is independently turned off by this procedure.

When thyristor 22 is closed, the same sequence occurs using the other half of the transformer in a sequential manner. By this action, current from the power source alternately flows through the two sides of the transformer primary as the thyristors are sequentially fired.

Since the direction of current flow through the two halves of the primary is opposed, an alternating, variable frequency, pulse wave output having an amplitude of about [N.sub.2 /N.sub.1 ] 2 E.sub.dc, wherein N.sub.2 is the number of windings on the secondary of the transformer and N.sub.1 is the number of windings on each half of the primary, will be created in the secondary having the waveform shown schematically in FIG. 2(a) of the drawings. This voltage is applied to the treater circuit load and produces a treater load current having a pulse waveform as shown schematically in FIG. 2(b) of the drawings.

The waveshape of the voltage output from the secondary of the transformer is an alternating pulse superimposed on a residual pedestal. This pedestal is due to the charge remaining on the system capacitance at the end of each pulse. The load current in the treater circuit has the waveform of a series of alternating-directional, sonic frequency, single oscillation pulses. There is a natural resonant ring due to the transformer following the useful load current burst. This ring does not contribute to corona. Comparing the waveforms of FIGS. 2a and 2b in proper time sequence one can see that the current (2b) is a derivative function of the voltage (2a).

The solid state high voltage generating system disclosed herein is especially suited for use in polymer film treating systems. As shown schematically in FIG. 1, the system as a whole consists of the high voltage generator whose output is connected to the film treating work cell 52 comprising a treater electrode 54 which is normally separated from ground electrode 56 by an air gap 58, the polymeric film 60 and a buffer dielectric 62.

To effectively modify or treat the surface of a polymeric film, the solid state, variable frequency, high voltage generator must cause a rapid sequence of high voltage gaseous discharges to occur in gap 58 during passage of a polymeric film therethrough.

In carrying out treating tests in accordance with the present invention, a high slip polyethylene film 70 inches wide, 1.5 mil thick, traveling at 50 feet per minute, was exposed to the corona discharge provided by the pulse generator of FIG. 1. It was possible to operate over an extremely wide frequency range. The voltage fed to the corona generator was maintained at 120 volts dc and the input current varied with variations in frequency as a criterion of loading. By taking the product of dc voltage and dc current, the power input to the generator is indicative of loading.

The observed data is presented as the curves of FIG. 3. In the curves, the number associated with each curve indicates the length (in inches) of the electrode employed for treating.

The load is continuously controllable down to essentially zero. The impulsive waveshape has resulted in lower harmonic currents and their associated resonances. It would be expected that in avoiding any resonance absorption, circulating currents and the associated internal heating would be reduced and it was noted that certain components operated cooler.

Tests were carried out at desired energy densities and the treatment was satisfactory for ink adhesion at commercial levels.

The term "high voltage gaseous discharge," as used herein, applies to the discharge phenomenon observed during the treatment of polymer films. Although essentially a suppressed arc which possesses aspects of corona glow and arc discharges, the predominant visual indicia is the corona which has caused the art to term the phenomenon a "corona discharge."

To generate the high voltage discharge in the gap 58, the high voltage generator is capable of supplying to a sharp knife-edge electrode at least 2,000 volts ac. Commercial units with larger radius electrodes require from about 5,000 to 50,000 volts or more ac which for a dc power supply having an output up to about 120 volts dc will require a transformer having at least 20, preferably 70 or more, windings for each half primary winding. It will be appreciated, however, that the number of secondary windings could vary depending on the magnitude of the selected supply voltage. The solid state high voltage generator should also be capable of providing a power output of from about 5 to about 25 watts per linear inch of electrode 54 to effectively treat the surface of a polymeric film.

Since polymer film treating systems operate at gap film speeds in order of about 100 to 200 feet per minute or more, the astable timing circuit should preferably operate at a frequency of about 20 to 5,000 Hz to closely space the discharges on the film surface. As used herein, the term "Hz," or Hertz, is the currently accepted abbreviation for cycles per second.

While not critical to the operation of a polymer film treating system, gap spacings in the order of about one-sixteenth to three-sixteenth inch are most commonly employed and contemplated within the ambit of this invention.

In addition to an efficient duty cycle, and the ability to obtain maximum loading conditions through frequency control over an extremely broad range of frequency, the solid state high voltage generator of the invention possesses several characteristics which are deficient in prior generators.

Radio frequency interference is essentially nonexistent because the fundamental waveshape is lacking in radio frequency components. This avoids the use of expensive shielding devices and allows its use in areas where regulations have forbidden the use of other generators.

dc Input voltage variation offers a convenience over existing units. Since the timing circuit operates independently of the voltage supply, output voltage is not dependent on the frequency of the timing circuit and any desired output voltage is available at any selected frequency of operation of the timing circuit by variation in dc input voltage. Therefore, for any selected dc input voltage, the output voltage of the generator will be constant and independent of frequency variation.

Claims

What is claimed is:

1. A process for the surface treatment of a plastic body with an alternating voltage of high intensity accompanied by corona discharge, wherein the treatment zone constitutes a capacitive load, comprising exposing said surface to an alternating-directional sequence of pulse waveform electrical voltage in the sonic frequency range and accompanying corona discharge, whereby the rapid change of electrical voltage is maximized and the application of electric stress on all circuit components is minimized.

2. The process in accordance with claim 1, wherein said sequence of pulse waveform electrical voltage has a frequency in the range 20-5,000 Hz.