DE2534761A1 - METHOD OF CASTING POLYMER FILMS - Google Patents

Description

TlEDTKE - BüHLING "KlNNE Patent attorneys:

Dipl.-Ing. Tiedtke Dipl.-Chem. Bühling Dipl.-Ing. Chins

8 Munich 2, PO Box 202403 Bavarlaring 4

Tel .: (0 89) 53 9653-56 Telex: 5 24845 tipat

cable: Germaniapatent Munich

B 6757 / ICI case Pf.27198 August 4, 1975

Imperial Chemical Industries Ltd, London, Great Britain

Process for casting polymer films

The invention relates to the production of films from organic thermoplastic polymers, in particular a Method and apparatus for electrostatically adhering a film to a casting surface.

Polymeric films can be made by extrusion from a slot die onto a casting surface, for example a coolable, rotating drum or a cool- -able, moving belt so that the melted film un-

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ter solidification is quenched as it is for a subsequent Foil production is appropriate. In the case of certain crystallizable polymeric materials, such as polyethylene terephthalate, it is important to quickly cool the extruded film to a temperature below its freezing temperature, so as to the onset of excessive crystal formation in your To reduce the film to a minimum, as this would cause embrittlement and the subsequent operations interferes with film production. A well-known method of deterrence of a molten film uses a technique * commonly known as electrostatic tacking. In this case, electrostatic charges are applied to the molten film when it reaches the casting surface, so that .the film from the casting surface, which is usually grounded, is electrostatic is attracted.

According to the present invention, the method exists for quenching a molten polymeric film thereinj that one forms a molten polymeric film and on one cooled, electrically grounded casting surface quenched, with the molten. Film is guided past an electrode, which, at a distance from the film surface and near the first point of contact between the melted Film and the casting surface is located. The method is according to the invention characterized in that one is attached to the electrode

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applies alternating voltage of a symmetrical or asymmetrical waveform exceeding 4.0 kilovolts from tip to tip, the frequency of which does not exceed the value at which positive and negative electrostatic charges emitted by the electrode "fill the space between the electrode and the molten PiIm in front of the Polarity change of the electrode voltage can no longer cross, so that the electrostatic charges emitted by the electrode reach the molten film by the applied electric field and cause it to adhere to the casting surface, the improvement being the use of an alternating voltage waveform in which the Electrode lying voltage, is kept above the voltage of the electrode arrangement for triggering a corona discharge for at least * 75% of the duration of each voltage half cycle, and in that a period of at least 10 microseconds is maintained in each half cycle in which there is no discharge current is procreated.

The present invention can serve to quench all polymer materials which can be shaped and quenched by extrusion into a flat film, such as polycarbonates, polyamides, e.g. polyhexamethylene adipamide and polycaprolactam, polysulfones, polymers and copolymers of oc -olefins, e.g. ethylene, Propylem, butene and 4 ~ methylpenteri-1, polymers and. Mixed polymers of vinyl monomers »eg

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Vinyl chloride, linear polyesters and copolyesters, for example those obtained by condensation of one or more dicarboxylic acids or their lower alkyl diesters, such as terephthalic acid, isophthalic acid, phthalic acid, 2,5-, 2,6- or 2,7-naphthalenedicarboxylic acid, succinic acid, sebacic acid , Adipic acid, azelaic acid, diphenyldicarboxylic acid. Or He3cahydrOterephthalic acid ^: or bis-p-carboxyphenoxyethanes, optionally with a monocarboxylic acid, such as pivalic acid, with one or more glyco-1s under ethylene glycol, 1,3-propanediol, 1,4-butylenediol ·· and 1 ^ -cyclohexanedimethanol The present invention is particularly suitable for the production of polyethylene terephthalate film.

In the arrangement according to the invention, in which one If an alternating current source is applied to the electrode, the adhesion to the casting surface will be electrostatic Charges by a corona discharge current in the electrode circuit that is nominally in phase with the AC voltage source. The corona discharge and consequently the Discharge current only occurs when the applied voltage is the trigger voltage for the corona discharge of the electrode assembly exceeds. The actual trigger voltage of an electrode arrangement is determined by the properties of the electrode and their location in space with respect to the molten film

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certainly. For example, the corona discharge occurs on a pointed electrode with a lower voltage than on a blunt one Electrode. The trigger voltage for a point electrode is whose head has a small radius, lower than for an electrode with a large radius, and the same for a ' Wire electrode with a smaller radius is lower than with one those with a larger radius. For most electrodes is the trigger voltage is at least 6.0 kilovolts from tip to tip, and the voltage applied to the electrode from tip to tip The peak should generally exceed the actual trigger voltage by at least 1 kilovolt. The trigger voltages can from tip to tip can even be around 20.0 kilovolts. For example, has a wire electrode with a diameter from 0.127 to 0.178 "mm, a tip-to-tip release voltage of about 6.0 to 11.0 kilovolts depending on their relative location to the melted egg. Operation of the electrode above its trigger voltage at a peak-to-peak potential about 6.0 to 14.0 kilovolts, preferably 8.0 to 12.0 kilovolts, will generally produce satisfactory attachment. With a given electrode arrangement, the triggering voltage for the negative discharge is arithmetically smaller than the triggering voltage, for. the positive discharge, the one required in operation actual voltages depend on the geometry of the electrode arrangement. Furthermore, there is the negative corona discharge generally through intense point discharges characterized, which are distributed over the length of the electrode.

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If the negative voltage spike is too large, the local negative corona discharge becomes too strong, which leads to pitting and crater voids in the molten film, The positive discharge does not have such properties. Accordingly, the applied voltage is preferably a symmetrical, where the positive voltage spike is the negative voltage spike exceeds. In addition, since defects in the film surface are not permissible in many cases of use, it is preferable to work with a negative voltage peak below that at which such damage would occur. It it has also been found that the use of an appropriately chosen asymmetrical 'applied voltage more closely matched Discharge currents and consequently more closely matched Causes attachment forces in the negative and positive voltage excursions.

In order for the discharge to occur during the positive and negative areas of the period of the applied voltage, the voltage from tip to tip must be equal to and preferably greater than the AC corona discharge trip voltage the electrode assembly. So for most typical electrodes should be the positive and negative Peak voltage, the mean potential of the polymer film around Exceed 2.0 kilovolts.

'When working with an adjoining asymmetrical

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Voltage at which the positive voltage peak is greater than the If the voltage spike is negative, the additional voltage should be positive compared to. a balanced, symmetrical state preferably 1 to 4 kilovolts, in particular 1.5 ^ is 3 Be kilovolts.

The ones on the slide after any 'on the deterrent Loads remaining in the following processing operations can lead to numerous difficulties e.g. poor handling due to sticking together of neighboring Layers of a roll of film, as well as problems caused by the release of charges, which, for example, in film coating processes, where the coating is applied from an organic medium creates a fire hazard comes. It has been found that it can be difficult to disperse a residual negative charge on the film. Preferably is therefore carried out the electricity supply in an asymmetrical form, so that it is ensured that the finished film is essentially neutral or overall weakly positively charged, in the latter case because there are positive charges dissipate more easily than negative charges.

The root mean square value of the sticking current flowing in phase and fed to the electrode is expediently in the range of 0.2 to 20 milliampsie j _e ^ ₃ Ci ₁ size and speed of the casting device. The square co-

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The mean value (rms value) of the current represents the current consumption for pinning including the scattering losses.

According to the invention, the frequency applied to the electrode applied voltage must not be greater than a value at which the positive and negative emitted by the electrode electrostatic charges the space between the electrode ' and the molten film before the electrode voltage is reversed can no longer traverse. If the frequency exceeds this value, the sticking is impaired as the discharge does not reach the cast film and recombination of the ionized air can occur between the electrode and the film. Increasing speed at which the melted Film is fed to and removed from the casting surface, increases the lower frequency limit at which casting can take place without impairing the casting quality. The frequencies used can be in the range of 500 Hz to 10 kHz and even below 500 Hz. .

According to the invention, the discharge current-free Dead time, i.e. when the applied voltage is lower than the Coroma discharge trigger voltage and the electrode does not emit electrostatic charges, at least 10 microseconds. Preferably the dead time is at least 20 microseconds. The dead time can fall in the voltage rise, the voltage drop or in the voltage drop and drop. It

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it is essential that there is a dead time in each half period provides to ensure that the in the gap between the electrode and the molten film for a half period built-up electrostatic charges are dissipated before being in the gap as a result of the voltage having changed Polarity Build up charges of opposite polarity, thus preventing the formation of a conductive plasma in the gap will. As a result, there are consecutive corona current pulses alternating. Polarity from each other by a period disconnected without discharge current.

The corona discharge trigger voltage and even the optimum peak operating voltage are preferably reached as quickly as possible after the desired dead time has ended. In the ideal case, the optimum peak operating voltage is then kept essentially constant over the remainder of the half cycle. In any case, according to the invention, the operating voltage is kept above the corona discharge triggering voltage for at least 75% of the voltage half-cycle. The operating voltage is preferably kept over 90 to 95 % of the voltage half-period, '• de;

Suitable "trapezoidal" or essentially "Square" waves can be generated by means of a pulse generator, a power amplifier or a transformer. Low-voltage pulse generators are commercially available, in which the rise and fall times of the pulse \

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can be varied from 30 χ 10 " ⁹ Ms 3 seconds at frequencies from 0.3 Hz" to 10 MHz.

The usefulness of AC voltages with a sinusoidal waveform is limited in practice, since the electrostatic charges are only emitted by the sticking electrode when the instantaneous voltage on the electrode exceeds the corona discharge triggering voltage of the electrode arrangement. In a typical arrangement, with a sinusoidal voltage source, this condition can only be satisfied for 50% of the total voltage period, with the result that no tack voltages are generated during a substantial portion of the period. In the case of an alternating voltage supply according to the invention, this difficulty is reduced in that the voltage on the electrode is kept above the corona discharge initiation voltage for a greater proportion of the period. .

AC voltage waveforms that. According to the invention can be used, are in the accompanying drawing ". Figure 1 shows a" square "waveform in which the voltage built up momentarily drops to zero and increases the voltage in the next half cycle only builds up again after a "dead time of at least 10 microseconds. In FIG the voltage varies steadily between the positive and negative operating voltage, whereby during the transition it is for

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is kept at least 10 microseconds below the corona release voltage. FIG. 3 shows a waveform which increases to a maximum operating value during each harvest period and which maintains the corona triggering voltage for at least 10 microseconds during this increase. Figure 4- shows one
Waveform that instantly rises to the maximum operating value in each half cycle and then falls to zero, being at least 10 microseconds below the corona triggering voltage. · · '■' ..

Various modifications of the waveforms shown in the drawing can also be used For example, the flat wave tip can with an increase and / or decrease in size above the corona trip voltage vary.

The molten film can be produced in a known manner by extrusion through a slot die, which is shown in FIG
is arranged a small distance from the casting surface. There
the melted film between the nozzle and the. Casting surface generally tends to constriction, it is desirable
to arrange the nozzle as close as possible to the casting surface, so that the extent of the constriction to a minimum
is decreased.

It is important that between the nozzle and the elec-

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electrode placed close to the molten film, no electrical arcing or excessive current losses appear. Therefore, in practice, the actual distance of the nozzle from the casting surface is a matter of caution measures that must be observed to avoid arcing and power loss. .

In a continuous process, the casting surface can be a conveyor belt or preferably a rotating roller or drum be. It is desirable that the casting surface be smooth, highly polished Outside has to prevent the formation of defects on the Film surface is avoided. The pouring surface is expedient a highly polished metal, e.g. steel, roller or drum, which by passing a coolant, e.g. water, .can be cooled to the desired quenching temperature.

The electrostatic charges deposited on the molten film by the electrode serve to separate the film to adhere to the electrically grounded casting surface ' or to attach to it; the electrode can therefore be used as a "Staple electrode" are designated.

The tack electrode is generally close to the surface of the molten film, for example about 1.77 up to 25.4 mm above the film surface, around the film to supply electrostatic charges and thus by electro-

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static attraction to the casting surface '.

The sticking electrode can be arranged across the path of the molten film and conducts the electrostatic Charges preferentially to the side of the film which is remote from the casting surface when the film comes onto this surface.

The sticking electrode should have a pointed surface capable of discharging electrostatic charges and can be made of a knife edge, a wire, or a series of probes or needles. Most expediently, the electrode is made from a wire made of "a metal that is expediently resistant to attack and contamination by the molten Vapors emitted from the film or materials is resistant. Suitable metals are stainless steel, tungsten, nickel alloys, e.g. nickel-iron alloys and nickel-chromium alloys. It can be a fine wire with a diameter of up to 2.04 mm, expediently from 0.127 to 0.254 mm, use.

Often times, the edges of the film do not reach the edges of the casting surface and there is therefore a risk that they will an arc is formed between the tack electrode and the exposed edges of the casting surface. As a precaution The parts of the sticking electrode located above the edges of the casting surface can prevent the formation of an arc a suitable dielectric material.

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For example, if the staple electrode - the shape of a wire has tubular insulator sleeves made of "for example polytetrafluoroethylene, which is commercially available under the trade name Fluon, on the corresponding parts of the Wire arranged.

After prolonged use, volatile materials tend to escape from the molten polymeric material and condense on the sticking electrode surface, this increases the effectiveness the fastening and thus the deterrent effect. To avoid this problem, a Adhesive wire electrode from a supply of fresh wire, pulled and slowly moved forward across the film web. According to another embodiment, an endless' tacking wire across the film and through a cleaning bath or be passed over a scraper to remove any deposited material. For another alternative • the deposition can be avoided by using a gas or Air flow to distribute the vapors between the sticking. electrode and the molten film conducts or the sticking electrode to a temperature above the condensation temperature the vapors are heated.

The invention is illustrated in more detail by the following example.

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example

It became a molten polyethylene enter ephthalate film on one with a peripheral speed of 21.3 m / min ' poured rotating casting drum. An electrostatic sticking electrode made from a nickel-chromium-stainless steel wire with a 0.1 - 8 mm diameter was about 5 jams from the casting drum surface removed near the first point of contact of the ge "-melted Film arranged with the casting surface. ■. .

The pouring took place in two separate runs, where- " when attached first with a sinusoidal alternating current and then with a current with a waveform according to the invention became.

When attached to the sinusoidal current was' the supply voltage connected to the tack wire as follows: Asymmetrical voltage: 7.3 kV positive peak

2.3 kV negative peak. Frequency:. 1200 Hz ■.

The molten film was adhered to the casting drum by the attachment charges emitted from the wire and frightened into the amorphous form. However, the film showed surface defects that were observed during the casting process Blistering. The quenched film was approximately 23.5 cm wide and 0.15 mm thick.

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When tacking according to the invention, the waveform was ~ the electrical supply voltage applied to the sticking electrode is the same as the form shown in FIG. 2 of the drawing with the difference that the flat apex of each wave was hard had a slight drop. The supply voltage was as follows:

Asymmetrical voltage: - 7 * 1 kV positive peak

2.1 kV negative peak

Maximum. at the flat crown

6.75 kV positive peak) Minimum 1.75 kV negative peak> ^a £ _en " ^la ~

-'S parting

Frequency: '1 200 Hz

Time of voltage rise approximately 80 µ seconds. from the negative
to the positive and from '
the positive to the negative peak ■:

The molten film adhered to the casting drum due to the applied electrostatic charges. He was on a width and a thickness quenched that using a sinusoidal stitch were the same. The movie that was deterred was and was of good quality free from surface imperfections caused by blistering.

Briefly summarized, the invention relates to the generation of more efficient attachment forces during AC electrostatic attachment in the casting and quenching of polymeric films, particularly polyethylene terephthalate films, using "square" voltage waveforms. *.

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Claims (1)

Claims . A method of quenching a molten polymeric film by forming the film and quenching it on a cooled, electrically grounded casting surface, wherein the molten film is moved past an electrode spaced from the film surface in the vicinity of the first contact zone between the molten film and the casting surface, characterized in that an alternating voltage exceeding 4.0 kilovolts from tip to tip and having a symmetrical or asymmetrical waveform is applied to the electrode, the frequency of which is not higher than the frequency at which the positive and the emitted by the electrode negative electrostatic charges can no longer cross the space between the electrode and the molten film before the electrode voltage is reversed, so that the electrostatic charges emitted by the electrode reach the molten film through the applied electric field and cause it to adhere to the casting surface, wherein the undulating voltage applied to the electrode is above for at least 75% of the duration of each voltage half cycle 60 9809/07 13 the corona discharge trigger voltage of the electrode arrangement . holds and in each half cycle a dead time of at least 10 Observes microseconds in which no discharge current is generated will. • 2. The method according to claim 1, characterized in that a dead time of at least 20 microseconds is observed. . · '. 3 .. The method according to claim 1 or 2, characterized in that ' that the operating voltage during 90 to 95% of the Voltage half-cycle maintained. 4-. Method according to one of Claims 1 to 3> characterized in that a voltage is applied to the electrode which reduces the trigger voltage of the electrode by at least Exceeds 1 kilovolt. 5- The method according to any one of claims 1 to 4, characterized characterized in that a voltage is applied to the electrode • which exceeds the trigger voltage of the electrode and in the range of 8.0 to 12.0 kilovolts from peak to Tip lies. · 6. The method according to any one of claims 1 to 5 »dacLurch marked that the. Frequency of the applied voltage 6 09809/0713 is at least 500 Hz. 7. Method according to one of Claims 1 to 6, characterized in that the resultant root mean square value of the sticking current supplied to the electrode is the which includes leakage currents in the range of 0.2 to milliamps. ■. . 8. The method according to any one of claims 1 to 7 »thereby characterized in that the quenched polymeric film is a polyethylene terephthalate film. - 9 · Method according to one of claims 1 to 8, characterized characterized in that one · voltage with asymmetrical Waveform applies ^ whose positive voltage peak is greater than their negative voltage peak is. U b 8 0 8 / C / ι 3 Blank page