JP2924123B2 - Method for producing thermoplastic polymer sheet - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a thermoplastic polymer sheet. More specifically, the present invention relates to a method for producing a thermoplastic polymer sheet having a small unevenness in sheet thickness in the longitudinal direction.

[Prior art] A thermoplastic polymer sheet is a base sheet of a magnetic recording material,
It is used as a material for various industrial applications such as a photographic base sheet and a dielectric material of a capacitor. In these applications, a high dimensional accuracy is required for the thickness of the sheet, particularly, the thickness of the sheet in the longitudinal direction.

Conventionally, when extracting and molding a molten thermoplastic polymer into a sheet on a moving cooling body, if a discharge electrode to which a high voltage is applied is installed above the polymer sheet, the polymer sheet cooling body is electrostatically charged. It is known that the degree of adhesion to the resin can be improved and a good molded state can be obtained (Japanese Patent Laid-Open No.
55759).

In this method, the position of the discharge electrode is an important condition for obtaining a good molding state.

Particularly, the position of the discharge electrode in the longitudinal direction of the polymer is important, and a method has been disclosed in which the movement of the discharge electrode can be easily set while keeping the distance to the moving cooling body constant (Japanese Patent Laid-Open No. 57-8116).

Also disclosed is a method of adjusting the position to an optimum position at which a good formability can be obtained in a shorter time, and it is shown that the speed can be increased by automatically controlling the quantified position of the maximum current value to the position ( JP-A-60-120028).

With respect to the position of the discharge electrode and the thickness of the sheet in the longitudinal direction, a method of measuring the sheet thickness after molding, performing frequency analysis and calculation, and controlling the position to minimize the thickness unevenness is disclosed (Japanese Patent Application Laid-Open No. H10-163873). 63-63723).

[Problems to be Solved by the Invention] However, in the study of the present inventors, it has been found that the unevenness in the thickness of the sheet in the longitudinal direction is not sufficiently reduced by the conventional method.

In other words, at the position of the maximum current value disclosed in Japanese Patent Application Laid-Open No. Sho. do not become. Also, JP-A-63-6272
The method disclosed in No. 3 requires a long time for analysis and calculation, and there is a time lag for a non-stationarily fluctuating thickness, so that the method cannot be sufficiently controlled.

It is an object of the present invention to overcome the disadvantages of the prior art and to a method for producing a sheet with a small thickness variation in the longitudinal direction.

[Means for Solving the Problems] In the present invention, a molten thermoplastic polymer is formed into a sheet from a die and extruded onto a moving cooling body, and an electrostatic charge is applied by a discharge electrode provided above the polymer sheet, In the method for producing a thermoplastic polymer sheet that is tightly adhered to the moving cooling body, a discharge current fluctuation is detected, and the distance between the discharge electrode and the moving cooling body surface is kept constant so that the fluctuation width is minimized. The present invention relates to a method for producing a thermoplastic polymer sheet, wherein the discharge electrode is controlled to an optimum position by moving the discharge electrode while maintaining the same.

The thermoplastic polymer in the present invention is a well-known polymer which can be formed as a sheet of polyolefins such as polyethylene and polypropylene, polyesters, polyamides, polyimides, polystyrenes and polyvinyls, and copolymers and mixtures thereof. However, it may contain other additives such as an antistatic agent, a weathering agent, a lubricant composed of organic particles and inorganic particles, and the like.

The sheet extracted from the die may be a single layer or a multilayer. In the present invention, a known apparatus method is used for imparting an electrostatic charge to the molten polymer by a discharge electrode.

 Hereinafter, the present invention will be described in detail with reference to the drawings.

In FIG. 1, a polymer sheet 2 melt-extracted from a die 1 is brought into contact with a moving cooling body 3 (in this case, a cooling roller), cooled and solidified, and is continuously passed through a separation roller 11 to a post-treatment step. Sent to The discharge electrode 4 is applied with a high voltage by the high voltage generator 5 and is maintained at a high potential with respect to the moving cooling body.

The discharge electrode 4 is connected to the cooling body 3 by a motor 6 and gears 7 and 8.
It can move so that the distance from the surface is constant.

Using the device configured as described above, the method of the present invention is performed as follows. First, the distance between the discharge electrode 4 and the surface of the cooling body 3 is set to an appropriate constant value so that an appropriate potential gradient is generated between the electrodes and the polymer sheet 2 does not cause dielectric breakdown.

Next, position control of the discharge electrode 4 in the circumferential direction of the cooling body 3 will be described. The relationship between the position in the circumferential direction (see FIG. 2) and the current flowing from the discharge electrode 4 to the cooling body 3 is shown in FIG. 3, and the relationship between the position in the circumferential direction of the discharge electrode and the thickness unevenness in the longitudinal direction of the sheet is as follows. As shown in FIG.

In FIG. 3, the width indicated by R indicates the current fluctuation width measured by the ammeter 9. Also, 1 indicates the electrode position. In FIG. 3, a region B corresponds to a region centered on a landing point of the polymer sheet 2 on the cooling roller 3, a region C corresponds to a downstream of the landing point, and a region A corresponds to an upstream of the landing point. C
In the area, the solidification of the polymer sheet is progressing, the adhesion of the sheet is weak, and air is trapped between the sheet and the cooling body, so the current value is low, the fluctuation is large, and the sheet thickness unevenness is good Absent. In the region A, the current value is high but the current fluctuation is large and the thickness unevenness is large. The region B has the smallest current fluctuation, and particularly at the position b, the fluctuation width is the smallest and the thickness unevenness is also small. Therefore, in order to make the discharge electrode as close as possible to the position b, the current fluctuation width is monitored, and the fluctuation width fluctuation is differentiated, and the position where the differential coefficient becomes 0 is controlled by the control device 10. That is, the current fluctuation width at the initial position (arbitrary) is compared with the current fluctuation width at the position where the discharge electrode is displaced by a very small distance, and the discharge electrode is moved in a direction in which a smaller fluctuation width can be obtained. . For control, feedback control may always be performed,
Further, more practically, an optimum position may be set when the sheet forming conditions (thickness, speed, etc.) are changed.

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1 Using the apparatus shown in FIG. 1, molten polyethylene terephthalate extruded from a die into a sheet was molded. At this time, the sheet thickness was 100 μm, the sheet width was 720 mm, the surface speed of the cooling roller was 40 m / min, and the interval between the die and the cooling roller was 20 m / m. The discharge electrode was a SUS wire with a diameter of 0.2 mm, the distance to the surface of the cooling body was set to 8 mm, and the applied voltage was set to 6 KV, and constant voltage control was performed. When the relationship between the electrode position l, the current value A, and the current fluctuation width R was examined, the current fluctuation width R showed a minimum of 0.08 mA when l = 33 mm. Therefore, a sheet was formed with l = 33 mm. At this time, the current value was 2.0 mA. The thickness unevenness in the longitudinal direction of the obtained sheet was 2 μm in total length and 2.1 μm.

Comparative Example-1 When l = 30 mm under the same apparatus conditions as in the example, the current value increased to 2.1 mA, but the fluctuation range R also increased to 0.20 mA. At this time, the thickness unevenness in the longitudinal direction of the obtained sheet was 4.0 μm.

Comparative Example 2 Similarly, when l = 36 mm, the current value was reduced to 1.8 mA, and the fluctuation range was 0.12 mA. The thickness unevenness of the sheet obtained at this time was 2.4 μm.

[Effects of the Invention] As described above, according to the production method of the present invention, unevenness in the thickness of the obtained sheet in the longitudinal direction can be significantly reduced.

[Brief description of the drawings]

FIG. 1 is a schematic view of an embodiment of an apparatus for performing a manufacturing method according to the present invention. FIG. 2 is a schematic view showing the base part of FIG. 1, and shows the electrode position l. FIG. 3 is a diagram showing the relationship between the electrode position and the current value and the current fluctuation width, and FIG. 4 is a diagram showing the relationship between the electrode position and the thickness unevenness. 1: Cap 2: Thermoplastic polymer sheet 3: Cooling roller 4: Discharge electrode 5: High voltage power supply 6: Motor 7: Gear 8: Gear 9: Ammeter 10: Control device 11: Pull-off roller

Claims (1)

(57) [Claims] 1. A molten thermoplastic polymer is formed into a sheet from a die and extruded onto a moving cooling body, and an electrostatic charge is applied by a discharge electrode provided above the polymer sheet to solidify and solidify the moving cooling body. In the method for producing a thermoplastic polymer sheet, a discharge current fluctuation is detected, and the discharge electrode is moved while maintaining a constant distance from a moving cooling body surface so that the fluctuation width is minimized. A method for producing a thermoplastic polymer sheet, comprising controlling an electrode to an optimum position.