JPS6245050B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for re-stretching a biaxially stretched film made of a thermoplastic resin. In general, thermoplastic resins are melted, extruded into a sheet, cooled and solidified, then biaxially stretched in both the longitudinal and lateral directions, and then re-stretched in the longitudinal direction or in both the longitudinal and lateral directions to create a shape suitable for each application. Various films are manufactured. However, in such conventional re-stretching methods, since the film to be re-stretched has already been stretched in both the longitudinal and transverse directions, the film is stretched in the transverse direction due to the effects of heating and longitudinal tension during re-stretching. Shrinkage (hereinafter referred to as neckdown) was unavoidable. As a result of this neckdown, the machine-direction re-stretched film had reduced transverse strength.
At the same time, the edge cutting rate (the rate at which thick waste portions due to shrinkage of both edges of the film are cut) increased, and the product yield also decreased. The object of the present invention is to overcome the drawbacks of the conventional re-stretching method, suppress neckdown, thereby suppressing a decrease in transverse strength and product yield. The object of the present invention is to provide a re-stretching method. To achieve this object, the present invention provides a method for re-stretching a thermoplastic resin film that has been biaxially stretched in the machine and transverse directions in the machine direction between at least one pair of supply rolls and a stretching roll. This method is characterized in that only the contacting edges of the film are re-stretched while being electrostatically charged using electrodes arranged so as to have approximately the same curvature as the supply roll. Hereinafter, the present invention will be specifically explained with reference to the drawings. FIG. 1 is a perspective view of the vicinity of a re-stretching section for explaining a re-stretching method according to an embodiment of the present invention, and FIG. 2 is a perspective view of an electrode viewed from below. In Fig. 1, 1 is a thermoplastic resin film biaxially stretched in the longitudinal and lateral directions, 2 is a preheating roll, 3 is a heating/supplying roll (grounded), and 4 is a cooling/supplying roll.
Stretching rolls, 1' are films that have been re-stretched in the longitudinal direction, 5A and 5B are electrode supports attached above both edges of the supply roll 3, and 6A and 6B are supported by electrode supports 5A and 5B, respectively. Needle electrode, 7
A and 7B are support rods for fixing the electrode supports 5A and 5B to a stand (not shown), 8 is a lead wire, and 9 is a DC high voltage power source (ground). To explain the electrodes in more detail, as shown in FIG. 2, needle-like electrodes 6 are embedded in each of a plurality of holes drilled in the lower surface 10 of an electrode support 5 made of an insulating material such as hard polyvinyl chloride. The tip of the needle electrode 6 has a curvature approximately equal to the curvature of the supply roll 3. Note that the plurality of needle-like electrodes 6 are connected to lead wires 8 inside the electrode support 5. The re-stretching method of the present invention can be carried out using such an apparatus as follows. As shown in FIG. 1, the biaxially stretched film 1 has electrode supports 5A and 5B above both ends of the heating/supply roll 3 passing through its upper roll surface.
Set up. That is, the electrode support 5 is placed above only both edges of the biaxially stretched film 1 so that the tips of the needle electrodes 6A and 6B embedded in the lower surfaces 10A and 10B are approximately curved with the supply roll 3, respectively.
Decide the mounting position of A and 5B. At this time, the needle-shaped electrodes 6A and 6B are located slightly inside and above both edges of the film 1. When the DC high voltage power supply 9 is turned on in this state, a high voltage is applied between each needle electrode 6A, 6B and the supply roll 3 (a type of electrode) through the lead wire 8, and the corona A discharge circuit is formed. Due to this corona discharge, an electrostatic charge is applied only to both edges of the biaxially stretched film 1 between the needle electrodes 6A, 6B and the supply roll 3, and due to the action of the electrostatic charge, the biaxially stretched film 1 The paper runs in the direction of the arrow with only both ends tightly adsorbed onto the surface of the supply roll 3, and is re-stretched in the longitudinal direction between the heating/supply roll 3 and the cooling/stretching roll 4. Therefore, since only both edges of the biaxially stretched film 1 running on the supply roll 3 surface are tightly attracted to the supply roll 3 surface due to the action of electrostatic charge, it is not possible to re-stretch it in the longitudinal direction. Even if a force is exerted to force the biaxially stretched film 1 to bend down due to heating and tension, the biaxially stretched film 1 can run and be re-stretched while keeping its width approximately at the specified dimension. . Therefore, if the method of the present invention is used, neck down is suppressed, so that a decrease in lateral strength is suppressed, and a decrease in product yield is also suppressed. In the present invention, it is necessary to apply electrostatic charge only to both edges of the biaxially stretched film that are in contact with the supply roll. Since the adhesion force due to electrostatic adsorption is greater in the lower part, the occurrence of scratches due to the difference in circumferential speed between the film and the roll is more likely to occur. In the present invention, since only both edges of the film are electrostatically attracted, it is possible to prevent scratches caused by the difference in circumferential speed. In addition, the amount of electrostatic charge applied must be large enough to tightly attract both edges of the biaxially stretched film onto the supply roll surface, and for this purpose, the amount of static charge applied must be sufficient to tightly attract both edges of the biaxially stretched film onto the supply roll surface. It is most preferable to apply a high voltage just before spark discharge occurs. The arc angle of the needle electrode embedded in the lower surface of the electrode support is preferably in the range of 30° to 120°. Each needle-shaped electrode is preferably implanted toward the center of the curvature of the concave surface, and the length of the needle protruding from the lower surface of the electrode support is preferably about 10 to 40 mm.
The distance between each needle in the longitudinal direction of the support is preferably 5 mm or more, and the width of the electrode in the width direction of the support (roll axis direction) is preferably about 10 to 200 mm, more preferably 20 to 100 mm. . The number of needles, number of rows (in FIG. 2, five needles are lined up in two rows), spacing, length, etc. can be changed as appropriate depending on the situation.
For example, by adjusting the screw 11, the length of the needle-like electrode 6 that extends above the concave surface 10 can be changed. The shortest distance (vertical distance) between the tip of each needle electrode and the supply roll surface is preferably about 10 to 20 mm. Also,
Appropriately, the applied DC voltage is about 5 to 15 KV. Among these, the most effective voltage value is a voltage value slightly lower than the spark discharge starting voltage as explained earlier, but it depends on conditions such as the distance between the electrode and the roll and the running speed of the film. It can be selected each time. Although FIG. 1 shows an example of a re-stretching section in which three preheating rolls 2, heating/supply rolls 3, and cooling/stretching rolls 4 are horizontally adjacent to each other, there may be other re-stretching parts, such as preheating rolls. A multistage heating/supply roll 3' is provided at intervals from 2 (in this case, each supply roll is provided with a corona discharge circuit),
The cooling/stretching rolls 4 may be provided at further intervals. In short, it is sufficient that the biaxially stretched film can be re-stretched in the longitudinal direction between at least one pair of "supply roll and stretching roll". The re-stretching method of the present invention can be applied to films made of thermoplastic resin in general, but preferably polyesters such as polyethylene terephthalate and polyethylene-2,6 naphthalate, polyethylene,
Films made of thermoplastic resins such as polyolefins such as polypropylene and polyamides such as nylon 6 and nylon 66 are preferred, and among them, polyester films are most preferably applied. EXAMPLES Hereinafter, the effects of the present invention will be specifically explained with reference to Examples. [Example 1] Polyethylene terephthalate was melt-extruded at 280°C and then rapidly cooled to form an amorphous film. This amorphous film was stretched 3.5 times in the machine direction at 95°C, and further stretched 5.1 times in the cross direction at 110°C. This biaxially stretched film was re-stretched in the longitudinal direction at a film feed speed of 80 m/min while being electrostatically attracted onto the heating/supply roll surface using an apparatus as shown in FIG. At this time, needle-like electrodes arranged in two rows of five needles as shown in Figure 2 (the needle-to-needle distance in the longitudinal and width directions of the support are both 30 mm) were used.
The vertical distance between the tip of the needle electrode and the supply roll surface is 15
mm, and a DC voltage of 8KV was applied. By changing the longitudinal stretching ratio again, F5 in the longitudinal and transverse directions
Changes in film strength (strength of the film at 5% elongation) and neck down ratio (percentage of decrease in film width after longitudinal re-stretching relative to film width before longitudinal re-stretching) were measured. As Comparative Example 1, the above-mentioned biaxially stretched film was longitudinally stretched again in the conventional manner without being electrostatically attracted.
The F5 value and net down rate were measured. The above measurement results are shown in Table 1.

[Example 2]

The biaxially stretched film was longitudinally stretched again in the same manner as in Example 1. However, the re-longitudinal stretching magnification is 1.4 times (1 constant),
Changes in F5 value and neckdown rate were measured by changing the vertical distance between the tip of the needle electrode and the supply roll surface. As Comparative Example 2, measurements were also conducted on a film that was longitudinally stretched again (1.4 times) in the conventional manner without electrostatic adsorption. The measurement results are shown in Table 2.

【table】

[Table] As can be seen from Table 2, when the vertical distance between the tip of the needle electrode and the supply roll surface was 10 to 20 mm, the high strength and low neckdown effects were large. However, if the above distance is too short (for example, 5
mm), which is not desirable because the film surface was scratched due to the direct influence of corona discharge. Comparative Example 3 and Comparative Example 4 In Example 1, in which the longitudinal re-stretching ratio was 1.3 times, the electrode for electrostatic adsorption was changed to a tungsten wire electrode, which was arranged over the entire surface in the width direction of the film, and then supplied. Vertical distance between roll and electrode 15mm
8KV was applied (Comparative Example 3). In addition, as an electrode,
Wire electrodes that were straight in the longitudinal direction of the film and had no curvature were provided only at both ends of the film, and 8 KV was applied at a distance of 15 mm perpendicular to the supply roll (Comparative Example 4). Scratches at the center of the film were examined using these films and the film of Example 1 (re-stretching ratio: 1.3 times), neck-down ratio, and halogen light. The results are shown in Table 3, and when the film was electrostatically adsorbed over the entire width, scratches occurred in the center. Further, unless the electrodes are arranged as in the present invention, the effect of reducing the neckdown rate will be small.

【table】 [Brief explanation of the drawing]

FIG. 1 is a perspective view of the vicinity of a re-stretching section for explaining a re-stretching method according to an embodiment of the present invention, and FIG. 2 is a perspective view of an electrode viewed from below. 1... Thermoplastic biaxially stretched film, 1'... Re-longitudinal stretched film, 3... Heating/supply roll, 4... Cooling/
Stretching roll, 5, 5A, 5B... electrode support, 6,
6A, 6B...acicular electrode, 9...DC high voltage power supply.

Claims (1)

[Claims] 1. When re-stretching a thermoplastic resin film that has been biaxially stretched in the longitudinal and transverse directions in the longitudinal direction between at least a pair of supply rolls and a stretching roll,
Only the edges of the film that are in contact with the supply roll are
A method for re-stretching a thermoplastic biaxially stretched film, characterized in that re-stretching is performed while applying an electrostatic charge using an electrode arranged to have approximately the same curvature as the supply roll.