JP2014030933A - Method for producing plastic film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing industrially at high speed, a plastic film excellent in thickness uniformity, capable of producing stably a cast sheet having uniform crystallization even when a casting roll is operated at high speed, by maintaining stable pinning discharge at a low voltage by removing discharge interference which is characteristic to a belt-like blade electrode.SOLUTION: In a sheet cooling molding method in which a resin 2 formed by melt extrusion molding in a sheet shape from a die 1 is extruded along the surface of a rotary cooling roll 3, and allowed to adhere to the surface of the rotary cooling roll 3 by applying a high voltage to a blade-shaped discharge electrode 4, discharge is prevented from an upward end edge 4-2 on the farther side from the cooling roll 3 of the discharge electrode 4.

Description

  The present invention relates to a method for producing a plastic film. More specifically, the present invention relates to a method for producing a plastic film characterized by a cooling and solidifying process after extruding a thermoplastic resin from a die into a sheet shape, and in particular, the plastic film without impairing the quality of the sheet in the electrostatic adhesion method. The present invention relates to a method for producing a plastic film capable of dramatically improving productivity.

  Usually, in the film forming process of the cast sheet by the T-die method, the molten resin is extruded from the extruder with a T-die, and the extruded sheet-like melt is driven and rotated by a cooling roll called a casting roll (hereinafter referred to as “ The cast sheet is formed by a method of cooling and solidifying by pressing on the surface of CR). Next, the cast sheet formed by cooling is stretched uniaxially or biaxially or biaxially in the longitudinal and lateral directions in a stretching process, and heat-treated to finally obtain a stretched plastic film.

  As a method of pressing the sheet-like melt on the CR surface, a method in which a high voltage application electrode is provided in the width direction of the sheet, an electrostatic charge is deposited on the sheet-like melt and electrostatically adhered to the CR (hereinafter referred to as “electrostatic adhesion”). Law ”is generally adopted.

  When the CR rotation speed is relatively slow, the above-mentioned electrostatic contact method can obtain a cast sheet having a desired quality. However, for the purpose of increasing production efficiency, the CR rotation speed is increased to increase the sheet deposition speed. Attempting to achieve this will cause problems due to a decrease in the adhesion of the sheet-like melt and adhesion spots as the CR rotational speed increases.

Usually, at the contact point where the sheet-shaped melt contacts the CR surface, the accompanying air flow pressure and the component of the melt tension act as levitation force on the sheet-shaped melt as the CR rotates, Accompanying air is slightly trapped in between, and minute bubble-shaped or tunnel-shaped air dust is generated. Since the air rubbed part has a lower heat conduction, that is, a cooling rate, with other CR contact molded parts, the crystallinity of the resin becomes high. The local crystallization spots in the film forming process of this sheet appear as surface defects of the cast sheet, and the uniform stretching property is impaired in the next stretching process, and the stretched spots are enlarged, and the quality of the plastic film product is increased. In particular, this is an important factor that deteriorates the thickness accuracy.
For these reasons, the upper limit of sheet deposition speed that does not impair quality has been a bottleneck for increasing production efficiency.

  Conventionally, a wire electrode is generally used in the electrostatic contact method, but as a means for improving the speed, for example, a method using a belt-like blade electrode as described in Patent Document 1 has been proposed.

  However, there is a problem inherent to the use of a blade electrode as a discharge electrode, that is, a strip having a thin cross-sectional shape.

  The band-shaped blade electrode is used in a state where one of both end edges of the blade faces the CR surface with respect to the CR surface, and when a high voltage is applied, the downward edge of the side close to the CR and the CR surface A strong electric field is formed between them, and pinning discharge occurs.

  However, in this blade electrode, the radius of curvature R of the leading edge (for example, 0.025 mm) is overwhelmingly smaller than the radius R of the wire electrode (for example, 0.75 mm), and the blade electrode has a lower voltage easy discharge capability. There is a contradictory fact that a high applied voltage is required despite having Furthermore, there is a troublesome problem that weak spark discharge frequently occurs even at a low voltage setting.

  This is because a pinning discharge is generated between the downward end edge near the CR and the CR surface, and an air discharge is generated from the upward end edge far from the CR. It is interpreted that the space charge released into the atmosphere prevents pinning discharge at the downward edge near the CR. When a discharge phenomenon occurs from the blade electrode toward the CR, a strong ion wind with a wind speed of several tens of meters induced by the discharge blows toward the CR. The space charge released from the air into the atmosphere is blown to the electric field around the downward edge near the CR, and this space charge of the same polarity disturbs the electric field and causes a decrease in discharge capability. I think.

  As a result, a high applied voltage is required, spark discharge troubles are likely to occur frequently, and high-speed stable production cannot be achieved.

Japanese Patent Publication No. 63-20688

  The present invention eliminates the discharge obstruction inherent to the belt-like blade electrode and maintains a stable pinning discharge at a low voltage, thereby stably forming a cast sheet having uniform crystallization even when the CR speed is increased. Therefore, the present invention provides a method for industrially producing a plastic film excellent in thickness uniformity at a high speed.

  As a result of intensive studies to solve such problems, the present inventors have investigated the discharge phenomenon inherent to the blade electrode in the electrostatic contact method, and occurred at the upper edge of the blade electrode far from the CR. The inventors have found the fact that by blocking (or suppressing as much as possible) the air discharge, the blade electrode's inherent low-voltage easy-discharging capability can be exhibited and the adhesion force corresponding to the high-speed CR can be secured, and the present invention has been achieved.

  That is, the gist of the present invention is a sheet cooling molding in which a resin melt-extruded in a sheet form from a die is extruded onto the surface of a rotating cooling roll, and a high voltage is applied to a blade-shaped discharge electrode to adhere to the surface of the rotating cooling roll. In the method, the discharge electrode is not discharged from the upward end edge on the side far from the cooling roll.

  According to the present invention, the inclination angle of the center line along the width direction in the cross section of the discharge electrode is in the range of 0 to ± 20 ° with respect to the normal line of the cooling roll passing through the contact point where the sheet-like melt contacts the cooling roll. It is preferable that

  Further, according to the present invention, the temperature of the discharge electrode is preferably in the range of 350 to 400 ° C.

  A plastic film can be obtained by stretching the cast sheet cooled and molded by the above method.

  According to the present invention, by preventing air discharge from the upper end edge of the blade electrode, the pinning discharge capability of the lower end edge can be greatly improved, and charge deposition corresponding to CR speeding up can be achieved. It becomes possible.

  Therefore, uniform cooling molding of the sheet-like melt can be performed, and a cast sheet having uniform crystallization can be formed at high speed. As a result, it is possible to improve the physical properties such as high productivity and operational stability of the stretched plastic film, and further high quality, particularly thickness uniformity, transparency, surface smoothness, shrinkage characteristics and the like.

It is a side view which shows the manufacturing method of the plastic film of embodiment of this invention. It is a side view which shows arrangement | positioning of the blade electrode and block electrode in this invention.

  Hereinafter, the present invention will be described in detail.

  FIG. 1 is a side view showing an embodiment of a method for producing a plastic film of the present invention. A sheet-like melt 2 is extruded from the T-die 1 onto the surface of the CR 3, and a high voltage is applied to the strip-shaped discharge electrode (blade electrode) 4 disposed in the width direction of the sheet-like melt 2 near the contact point in contact with the CR 3. Thus, an electrostatic charge is imparted to the sheet-like melt 2 and pressed against the CR3 surface to be cooled and solidified. The strip-shaped discharge electrode 4 has a downward end edge 4-1 at one end in the width direction and an upward end edge 4-2 at the other end.

  In the present invention, it is most important that the discharge is performed only from the downward end edge 4-1 on the side close to CR3 of the discharge electrode 4 and not from the upward end edge 4-2 on the side far from CR3. In the present invention, “do not discharge” is a concept including both not discharging at all and allowing a slight discharge within a range in which the object of the present invention can be achieved.

  As a measure not to discharge from the upward edge 4-2 on the side far from the CR3, as shown in FIG. 1, the upward edge 4-2 of the discharge electrode 4 is sandwiched between the block electrodes 5 and brought into contact with the discharge electrode 4 as shown in FIG. And the same potential. As a result, discharge from the upward edge 4-2 can be prevented.

  Examples of the block electrode 5 used in the present invention include, but are not particularly limited to, a cross-sectional shape that is circular, semicircular, elliptical, oval, streamlined, round U-shaped, or polygonal. It is necessary to select a shape with rounded corners so that there are no sharp corners on the surface of the block electrode 5. In addition, by making the radius of curvature of the cross section of the block electrode 5 larger than the radius of curvature of both end edges 4-1 and 4-2 of the discharge electrode 4, discharge can be more effectively prevented.

  If the radius of curvature of the block electrode 5 is too large, the cross section itself of the block electrode 5 becomes large, so that the discharge from the block electrode 5 can be prevented, but the flow of wind in the vicinity of the electrode 5 is hindered, and the monomer gas This is not preferable because it causes a stagnation. Theoretically, the discharge capacity decreases in inverse proportion to the square of the radius of curvature, so it is not necessary to make it too large. The radius of curvature of the block electrode 5 is preferably at least 10 times, more preferably at least 30 times the radius of curvature of the ends of the end edges 4-1 and 4-2. The block electrode 5 may be selected in an appropriate shape and size for engineering design for fixing and supporting the block electrode 5.

  Examples of the metal material used for the block electrode 5 include, but are not limited to, copper alloy, nickel, steel, stainless steel, tungsten, and amorphous alloy. However, a material whose conductivity is significantly reduced by surface oxidation is not preferable.

  The discharge electrode 4 used in the present invention is a blade electrode having a thin and wide cross-sectional shape.

  The thickness of the discharge electrode 4 is optimally 0.02 mm or more and 0.20 mm or less. The thinner the thickness, the better the easy discharge capability. However, if the thickness is less than 0.02 mm, the edge is easily scratched and the handling becomes difficult. On the other hand, if the thickness exceeds 0.20 mm, the discharge starting voltage is high, and the discharge capability of the discharge electrode 4 itself is low, so that the CR film forming speed cannot be increased.

  The width of the discharge electrode 4 is preferably 5 mm or more and 20 mm or less. If the width is less than 5 mm, the discharge in the low voltage region becomes unstable due to the influence of the electric field formed by the block electrode 5, which is not preferable. On the other hand, if the width is larger than 20 mm, an apparatus that applies tension to the discharge electrode 4 evenly and installs it becomes undesirably large.

  As for the discharge electrode 4, the both ends of the downward end 4-1 and the upward end 4-2 should be rounded without corners. It is advisable to use an extremely fine flat round wire that has been rolled and has excellent processing dimensional accuracy at both ends. If the belt is cut into a strip shape, it is not preferable because there is a return to the corner and burrs, so that concentrated discharge occurs locally and metal sublimation and spark fusing occur at the leading edge.

  2 shows a structure in which the upper end edge 4-2 of the discharge electrode 4 is inserted into the block electrode 5 [FIGS. 2 (a) and (c)], and the tip of the upper end edge 4-2 is the block electrode 5. [FIG. 2 (b) (c)]. The air discharge can be sufficiently prevented by bringing the tip of the upward edge 4-2 into contact with the block electrode 5. The block electrode 5 is insulated from the earth ground and is not electrically grounded.

  Next, in the present invention, the inclination angle θ of the center line 6 along the width direction in the cross section of the discharge electrode 4 is 0 to ±± with respect to the normal line 7 of CR3 passing through the contact point 8 where the sheet-like melt 2 is in contact with CR3. A range of 20 ° is preferred.

  The discharge characteristics between the discharge electrode 4 which is a blade electrode and CR3 depend on the inclination angle θ formed by the center line 6 in the width direction of the cross section of the discharge electrode 4 and the normal line 7 of CR3.

  As shown in FIGS. 2A and 2C, the structure in which the upper end edge 4-2 of the discharge electrode 4 is inserted into the block electrode 5 has a symmetric structure with respect to the normal line 7 of CR3, and the inclination angle θ The state in which the discharge electrode 4 of 0 ° is perpendicular to the CR3 surface shows the maximum discharge capability at the lowest voltage and high discharge current. It has a characteristic of decreasing and attenuating. If the inclination angle θ is in the range of 0 to ± 20 °, the discharge capacity can be maintained at 95% or more, which is acceptable since it is not substantially a problem.

  On the other hand, as shown in FIGS. 2B and 2D, the structure in which the tip of the upper end edge 4-2 of the discharge electrode 4 is in contact with the block electrode 5 is as shown in FIG. Therefore, there is an angle indicating the maximum discharge capacity at a position slightly inclined with respect to the normal line of CR3 with the tip of the downward end edge 4-1 on the side close to CR3 as a base point. It is most desirable to use at an inclination angle θ that provides the maximum discharge capability. This inclination angle θ is also in the range of 0 to ± 20 °. This is considered to be due to the influence of the intensity and direction of the ion wind that merges at the tip of the downward edge 4-1 of the discharge electrode 4.

  In the present invention, the discharge electrode 4 is preferably heated to a temperature of 350 ° C. or higher and lower than 400 ° C.

  In order to prevent electrode condensation of low molecular weight substances such as monomers and oligomers, it is preferable to heat at least 350 ° C. or higher, and in order to obtain low voltage and easy discharge characteristics, a higher temperature range is preferable, but 400 ° C. or higher. Then, the oxidative deterioration of the metal constituting the electrode 4 becomes remarkable, and conversely, the life of the electrode 4 is shortened.

  As a heating method of the discharge electrode 4, for example, a method in which a sheathed heater in which a filler such as a nichrome wire and a magnesium compound, which is a hot body, is covered with a special metal tube is used directly as a guard electrode, or a sheathed heater is used as a guard electrode. There are methods such as charging and heating. Thereby, the discharge electrode 4 can be maintained at a high temperature.

  In the present invention, a stretched plastic film can be obtained by stretching and heat-treating a cast sheet that has been cooled and molded.

  Examples of the stretching method include longitudinal or lateral uniaxial stretching, sequential biaxial stretching, and simultaneous biaxial stretching. In particular, it is preferable to apply sequential biaxial stretching and simultaneous biaxial stretching to obtain an oriented crystallized film.

  The plastic used in the present invention is not particularly limited as long as it is a resin to which the electrostatic contact method can be applied.

  Examples thereof include homopolymers such as polyester resins typified by polyethylene terephthalate, polyolefin resins such as polyethylene and polypropylene, polyamide resins such as nylon 6 and nylon 66, and mixtures and copolymers thereof.

  These resins may contain known additives such as stabilizers, antioxidants, fillers, lubricants, antistatic agents, antiblocking agents, colorants and the like.

  The method of the present invention is extremely useful for forming an unstretched sheet for a polyester biaxially stretched film, which has particularly strict requirements for thickness accuracy and optical distortion.

  The following characteristic values used in the following examples and comparative examples were measured by the following methods, respectively.

(1) Film thickness fluctuation amount The width direction of the film is referred to as the TD direction, and the flow direction of the film is referred to as the MD direction.

  About the running film, the thickness of the TD direction was measured with the width | variety 25mm pitch with the infrared thickness meter, and also measured 40 times at 100m pitch in MD direction. The maximum deviation value at 40 points in the MD direction at each measurement point in the TD direction was defined as the thickness unevenness at that point.

  In addition, NDC infra red engineering company make (infrared transmission type sensor TFG710) was used for the infrared thickness meter.

(2) Evaluation of Air Claw Defects Using a Polarization Distortion Inspection Device A cast sheet was inserted between crossed polarizing plates (polarizer / analyzer) and observed while rotating the analyzer.
Evaluation was performed according to the following criteria.
◯: Air flaw defects are not observed at all and good △: Observation limit of air flaw defects ×: Air flaw defects are observed and defective

  Next, the present invention will be specifically described with reference to examples.

Example 1
A T-die having a width of 600 mm was attached to the extruder, and a polyethylene terephthalate resin (relative viscosity 1.38) containing 0.1% by mass of silica having an average particle diameter of 1.0 μm was extruded into a sheet at an extrusion temperature of 280 ° C. A non-oriented cast sheet having a width of 520 mm and a thickness of 195 μm was cooled and formed on the CR adjusted to a temperature of 20 ° C. by an electrostatic adhesion method.

  At this time, a flat round wire made of stainless steel having a thickness of 0.05 mm and a width of 7 mm as a discharge electrode was installed near the CR contact point of the sheet-like melt in parallel with the width direction of the upper part of the sheet-like melt. At this time, the inclination angle θ of the center line of the discharge electrode cross section was set to 0 °, and the distance from the lower end of the discharge electrode to the CR surface was set to 5 mm. The upper end of the discharge electrode was brought into contact with the slit of a stainless φ5 mm block electrode. Further, the block electrode was heated with a heater, and the temperature of the discharge electrode was controlled at 400 ° C.

  A DC high-voltage stabilized power source was connected to the discharge electrode, a DC voltage of +7 kV was applied, and the CR surface speed was 80 m / min. At this time, air film was not generated between the sheet and the CR surface, and stable film formation was possible without generating spark discharge.

  Next, the cast sheet was biaxially stretched in the longitudinal and lateral directions at a stretching ratio of 4 times, and then heat-set at 220 ° C. to obtain a biaxially stretched polyethylene terephthalate film having a thickness of 12 μm. A film excellent in thickness and optical uniformity was obtained.

  Table 1 shows the results of observation of the air sheet defect of the cast sheet and the measurement result of the MD thickness variation.

Comparative Example A film was formed under the same conditions as in Example 1 except that the block electrode was removed. The occurrence of air nick defects was remarkable and spark discharge occurred frequently, so the CR speed had to be reduced to 60 m / min. The results at this time are shown in comparison with Table 1.

Example 2
A film was formed under the same conditions as in Example 1 except that the inclination angle θ of the center line of the discharge electrode cross section was inclined + 25 ° toward the T die. Air flaw defects were observed intermittently and had to be lowered to a CR speed of 70 m / min. The results at this time are shown in comparison with Table 1.

Example 3
A film was formed under the same conditions as in Example 1 except that the heater heating of the block electrode was stopped. Although stable at the beginning of production, monomer foreign matter defects frequently occurred after the lapse of 16 hours, and the production line had to be temporarily stopped to clean the monomer around the electrodes.

1 T-die 2 Sheet-like melt 3 Casting roll (CR)
4 Discharge electrode (blade electrode)
4-1 Downward edge 4-2 Upward edge 5 Block electrode

Claims (4)

  In a cooling method for a sheet in which a resin melt-extruded in a sheet form from a die is extruded onto the surface of a rotating cooling roll, and a high voltage is applied to the blade-shaped discharge electrode so as to adhere to the surface of the rotating cooling roll. A method for producing a plastic film, characterized by not discharging from an upward end edge far from the roll.   The inclination angle of the center line along the width direction in the cross section of the discharge electrode is in the range of 0 to ± 20 ° with respect to the normal line of the cooling roll passing through the contact point where the sheet-like melt contacts the cooling roll. The method for producing a plastic film according to claim 1.   The method for producing a plastic film according to claim 1 or 2, wherein the temperature of the discharge electrode is in the range of 350 to 400 ° C.   A method for producing a plastic film, comprising: further stretching a cast sheet cooled and molded by the method according to any one of claims 1 to 3.

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