JPS6251730B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a polyester resin film with excellent thickness uniformity and low crystallinity with high efficiency.

In the process of extruding polyester resin from a die and receiving it on the rotating cooling body surface to form a film, a wire or knife-shaped electrode to which a high voltage is applied is provided on the opposite side of the film from the side that contacts the rotating cooling body surface. A method of forming a film while rapidly cooling the film by depositing an electrostatic charge on the top of the rotating cooling surface and bringing the film into close contact with the rotating cooling surface using electrostatic attraction acting between the film and the grounded rotating cooling surface (this method is called the electrostatic pinning method). ) is an effective method for improving thickness uniformity, transparency, etc., and is widely used. However, this method becomes more difficult as the speed of the rotating cooling surface increases, and as the film forming speed increases, air bubbles tend to become trapped between the molten film and the rotating cooling surface. When the electrostatic pinning method is used, the molten film is strongly pressed against the surface of the rotating cooling body by electrostatic attraction and is rapidly cooled. Therefore, if air bubbles are caught between the molten film and the surface of the rotary cooling body, cooling will be inhibited and not only will a homogeneous film not be obtained, but also unevenness will occur on the surface. Therefore, entrainment of air bubbles is a fatal defect in this film forming process, which limits the film forming speed.

Conventional improvement proposals made to increase the film forming speed in the electrostatic pinning method are related to electrodes, such as the one shown in Japanese Patent Publication No. 53-6180, which uses an auxiliary electrode to increase the binding force to generate a large amount of ions. Methods of strengthening, things related to atmosphere, such as JP-A-53
- A method of increasing the temperature near the electrode to make it easier to ionize the air, and applying a large amount of static charge to the film surface to strengthen the binding force, as shown in Japanese Patent No. 143659. As shown in Japanese Patent Publication No. 50-28108, the vicinity of the electrode is covered with a gas that has a large discharge current before dielectric breakdown, and by passing a large amount of current, a large amount of static charge is applied to the film surface to strengthen the binding force. how to.
Regarding raw materials, for example, as shown in Japanese Patent Publication No. 53-40231, etc., it is possible to increase the speed by improving the electrical conductivity of molten polyester resin. Others use a suction type air knife as shown in Japanese Patent Publication No. 55-22257. Although there are methods that use infrared rays as shown in Japanese Patent Publication No. 55-10365, there is no improvement in the film forming technology itself.

The present inventors believe that when applying the electrostatic pinning method to polyester resin films, there are limits to the conventional film-forming method with the above-mentioned improvement proposals. We thought that it should be improved to be more suitable for this purpose, and performed extrusion film formation using electrostatic pinning using various equipment and resins, and investigated the cause of air bubble entrainment.
As a result, we learned that controlling the curl at the end of the molten film is extremely important in order to increase the film forming rate without entraining bubbles. This curl includes: (a) When the molten film is divided into the center and the edges, the deflection at the edges is smaller than the deflection at the center, resulting in a curl in which the edges appear to be lifted up.

(b) A curl in which the end of the molten film is clearly located above the straight line connecting the first point of contact with the extrusion die lip and the rotating cooling surface.

etc. exist. It is difficult to completely explain the curls in (a) and (b) theoretically.

These curls are thought to be subtly influenced by transverse melt tension directed toward the center of the molten film, surface tension, memory phenomenon, etc., and increase as the film forming speed increases. As a result, the edges of the film are tied in toward the center, increasing the thickness. Also, the water reaches the rotating cooling surface later than the central part. Therefore, if the electrodes are installed in a straight line, it is not possible to obtain the best binding force over the entire width of the film. This is because the electrostatic charge that creates the binding force on the film is determined by the distance between the film and the electrode. For example, if the electrodes are placed to provide the best binding force relative to the center of the film, the ends of the film will be extremely close together. As a result, a spark discharge occurs between the electrode and the end portion of the film, which inhibits the deposition of charges, hinders the binding operation, and entrains air bubbles. To avoid this, it is necessary to place the electrodes at a sufficient distance from the curled film edges, since such spacing is greater than that required to produce the best binding force in the central portion. Air bubbles will become trapped between the film and the rotating cooling surface.

In order to solve this difficulty, as shown in Japanese Patent Application Laid-open No. 56-53037, a blade electrode is used to curve the blade electrode at a position corresponding to the edge of the film, so that the film cross section crosses the entire width of the film. A method of bending the blade electrode at a position corresponding to the edge of the film toward the center of the film to prevent the blade electrode from extending beyond the edge of the film, etc. Proposed.

However, in these methods, first, it is difficult to determine the degree of curvature to be applied to the electrode, which makes manufacturing difficult.
Second, once the electrode is completed, it is almost impossible to apply it to films of different film widths, and it is extremely difficult to use. Thirdly, there are difficulties in positioning and attaching the electrodes to the film.

The inventors of the present invention have made extensive studies on improvements to the prior art, and have found that there is a close relationship between the curl of the film edge, the film take-up angle, viscosity, draft ratio, etc. Through mechanical considerations and repeated detailed experiments on the factors mentioned above, we improved the film-forming conditions and found a method that enables high-speed electrostatic pinning film-forming without special devising of electrodes. Ta.

That is, the density ρ (g/cm ³ ) of the molten film, the air gap S (cm) given by the length measured along the molten film from the extrusion die lip to the point where the molten film first contacts the rotating cooling surface, and the extrusion The net in δ is the transverse amount (1/2 of the difference between the die lip width and the film width at the point of contact) that the molten film extruded from the die shrinks at each edge before it first contacts the rotating cooling surface. (cm),
The draft ratio λ(-) is given by the average discharge speed in the transverse direction at the exit of the extrusion die divided by the rotational cooling surface speed, the temperature of the resin passing through the extrusion die lip, and the melt viscosity μ(g), which corresponds to the shear rate.・
sec/cm ² ), the angle α (degrees) between the vertical direction and the tangent to the flow direction at the midpoint between the flow direction and the transverse direction of the molten film between the extrusion die and the first contact point with the rotating cooling surface. make use of Curling is less likely to occur if the film is formed under conditions such that the value P shown by is 25.0 (cm ² /sec) or more, and if the electrostatic pinning method is applied under these conditions, the film formation speed can be relatively easily increased. I discovered that it is possible. The meaning of P will be explained next.

In general, a fluid with a tensile viscosity η is strained at a strain rate dv/
When the fluid is deformed by dx (v is the moving speed and x is the length measured along the streamline of the fluid to the point of consideration), the tension acting per unit area of the fluid is given by the following equation.

=ηdv/dx Since the polyester resin for film molding can be regarded as a Newtonian fluid under normal molding conditions, the tensile viscosity η has approximately the following relationship with the general melt viscosity (shear viscosity) μ.

η=3μ Since the tension per unit width is balanced at any point in the flow direction at the center where there is no influence from the edge of the film, and the flow rate per unit width is constant, 3μTdv/dx=constant Tv= Q (constant) (T is the film thickness at the point of consideration, Q is the flow rate per unit width) Therefore, the film thickness at the die lip discharge point To,
Letting air gap length S and draft ratio λ, the following relationship is obtained.

=3μQl _o λ/S The above relationships were determined for the center portion of the film, but applying these relationships to the edge portions of the film should not result in any serious errors. The part that is strongly affected by the free edge of the film,
That is, if the width of the end is considered to be B (this concept is shown in FIG. 2), the tension at that portion can be approximated by the following equation.

B=3θQBl _o λ/S The weight W of the part at this time is approximately W=ρ・B・T′√ ² + ² (T′ is the average thickness of the edge, δ is the net-in amount on one side of the film, and ρ is (film density) Therefore, the force that tries to bend the film edge
W″ is W′ = ρ・B・T′√ ² + ² sinα (α is the angle between the film's tangent in the flow direction and the vertical direction, and it is approximated that the center and edges are the same) The amount of deflection is D. Then, the following approximate mechanical balance should be established at the edge of the film (the above relationship is shown in FIG. 3; F is the tension at the edge).

Since Q/T' corresponds to speed, the deflection index p is given by the following equation.

As described in detail above, it is clear that although the parameter P cannot represent the actual amount of deflection, it is at least a characteristic value related to the amount of deflection at the edge of the film.

In other words, what P means is that if P is large, the deflection at the end will be large and the curl will be small, assuming the same take-up speed, and if the same deflection, that is, the same curl, is considered, the larger P is, the larger the curl will be. This value indicates that the take-up speed v can be increased as much as

The present inventors took up P as an important parameter for determining film forming conditions suitable for the electrostatic pinning method, and investigated various raw materials, extrusion dies with various shapes, extrusion temperature, extrusion conditions, withdrawal conditions, etc. in detail. I conducted an experiment. As a result, air gap: 20m/m
~150m/m, Draft ratio: 5-20, Melt viscosity:
It was found that under film forming conditions ranging from 1500 poise to 5000 poise and a take-up angle of 30° to 90°, P was quite good as shown in FIG. 1, indicating the state of bubble entrainment.

In FIG. 1, cases where bubble entrainment occurred under each film forming condition are indicated by an x mark, and cases where no bubble entrainment occurred are indicated by an ○ mark. This figure shows that in the range where the curl is small and air bubbles are not entrained, the higher the take-up speed, the larger the P value must be.
This shows that the larger the value, the faster the collection speed can be. When P≧25.0 (cm ² /sec), a take-up speed of 40 m/min or more, which has been difficult to achieve using conventional wire electrodes, can be easily achieved.

The parameters P are the density ρ, which is mainly determined from the raw material conditions, the viscosity μ, the extrusion die shape and the thickness of the product film, the draft ratio λ, which is determined by the stretching ratio, etc., and the net-win δ, which is strongly controlled by the draft ratio.
The constituent factors are the air gap S and the angle α, which can be controlled by the positional relationship between the extrusion die lip and the rotating cooling roll. In the present invention, the positional relationship between the extrusion die lip and the rotating cooling roll is selected so that the parameter P is 25 cm ² /sec or more for the density ρ, viscosity μ, draft ratio λ, and net-in δ given as film manufacturing conditions. Adjust the combination of air gap S and angle α.
By achieving a positional relationship in which the parameter P is 25 (cm ² /sec) or more, the shape of the film edge becomes suitable for high-speed pinning film formation, and unlike conventional technology, it is possible to It becomes possible to form polyester film at high speed without making any additional efforts.

The term "polyester" as used herein refers to a polyester having a film-forming ability obtained from a dibasic acid and a dihydric alcohol, or a copolymer thereof. Of course the third
Polyesters copolymerized with dibasic acids such as isophthalic acid, adipic acid, and triethylene glycol or dihydric alcohols may be used as components.
Additionally, additives such as stabilizers and colorants may be added.

The low-crystalline unstretched film with excellent thickness uniformity produced in this way is particularly useful when making biaxially stretched polyester films that have strict requirements for thickness accuracy, such as stretchability, toughness, and optical properties. This makes it the perfect material.

In order to obtain the best effect of the method of the present invention on the entire line under general conditions, one more consideration becomes important. That is, it is the discharge angle of the extrusion die. Conventionally, an extrusion die was installed near the top of the rotary cooling body, and the extrusion film was formed with the discharge direction vertical or at an angle close to vertical. This method is advantageous in achieving mechanical accuracy, thickness accuracy, etc., and has therefore been a common method in the past. As a conventional technique for tilting the direction of the extrusion die, for example, there is Japanese Patent Publication No. 51-31267, but this method simply shows tilting the extrusion die to prevent vibration of the molten film, and the present invention This paper does not describe a method for reducing contamination of an extrusion die lip, that is, "eye mucus", nor does it describe the angle between the extrusion direction and the flow direction of the molten film at the tip of the extrusion die. If the method of the present invention is specifically carried out to find conditions in which curling is less likely to occur, as the drawing speed becomes higher, the drawing angle becomes considerably larger. In such a case, if the extrusion die is installed with its lip direction vertical or close to vertical, the direction of discharge of the die and the flow direction of the molten film at the tip of the die lip will inevitably have a large angle (this relationship is schematically shown below). (shown in Figure 4). If film forming operations are continued under these conditions, molten resin and its carbide, known as "eye mucus", will adhere to the die lip, causing uneven thickness and surface irregularities on the film, making it impossible to continue film forming operations. become. Frequent cleaning of the die lip significantly reduces efficiency. As a result of various studies on this problem, we found that the angle between the discharge direction at the die lip and the flow direction of the molten film at the tip of the die lip has a great deal to do with the adhesion of "eye mucus", and if this angle is made smaller, the adhesion of "eye mucus" will be reduced. (The arrangement at this time is schematically shown in FIG. 5). This seems to be related to the surface tension, viscosity, density, and sharpness of the die lip of the molten resin, but when using polyester resin and a die lip with normal sharpness, the angle formed by the die lip is smaller than 60°. It has been found that if the size is smaller, the adhesion of "eye mucus" is significantly reduced. This is shown in Figure 6 (showing the shape when the angle between the discharge direction and the flow direction of the molten film at the die lip tip is small) and Figure 7 (showing the shape when the angle between the discharge direction and the flow direction of the molten film at the die lip tip is large). If the angle between the discharge direction of the die lip and the flow direction of the molten film at the tip of the die lip is less than 60 degrees, the molten resin will not pass around the edge of the die lip and adhere, but if it exceeds 60 degrees, This is thought to be due to the fact that it passes around the edges of the die lip, attaches or stays there, and carbonizes.
Further, under the film forming and taking-off conditions under which the effects of the present invention appear, the flow direction of the molten film at the tip of the die lip can be considered to be approximately the same as α.

As shown above, the present invention improves the film forming method itself, which has not been done in the past, in a film forming method using electrostatic pinning, and thereby very easily achieves a high speed of film forming. Therefore, it has great industrial value.

Examples and comparative examples are shown below.

Example 1 20 with a 1:1 mixed solution of phenol and carbon tetrachloride
Polyethylene phthalate having a relative viscosity of 1.41 as measured at °C was extruded into a film at 280 °C. At this time, the melt viscosity of the resin passing through the die lip was 3450 poise. Other film forming conditions include air gap: 50
m/m, netquin 38m/m, take-up speed: 42m/
min, draft ratio: 6.7, film thickness: 120μ,
The film was formed at a take-off angle of 52°. At this time, the electrostatic pinning conditions were to use a 150μ SUS steel wire in diameter as the electrode, and the voltage:
The temperature was 7.0 Kv, current: 0.035 mA/cm, and distance between electrode and film: 5.0 m/m. At this time P value 25.7
(cm ² /sec). At this time, no curling occurred and no air bubbles were observed.

Example 2 The same polyester resin as in Example 1 was extruded into a film at 290°C. The melt viscosity of the resin passing through this die lip was 2930 poise. Other film forming conditions include air gap: 50m/m, network:
45m/m, take-up speed: 45m/min, draft ratio:
8.4, film thickness: 120μ, take-off angle: 50°. In addition, the electrostatic pinning conditions were as follows: using the same electrode as in Example 1, voltage: 6.5 Kv, current: 0.03 mA/cm,
The distance between the electrode and the film was 5.0 mm. The P value at this time is 28 (cm ² /sec). At this time, no curling occurred and no air bubbles were observed.

Example 3 Polyethylene terephthalate having a relative viscosity of 1.38 measured in the same manner as in Example 1 was extruded into a film at 290°C. At this time, the melt viscosity of the resin passing through the die lip was 2040 poise. Other film forming conditions include air gap: 45 m/m, net film: 42 m/m, take-up speed: 50 m/min, draft ratio: 9.1, film thickness: 90μ, take-off angle: 55
It was warm at ゜. At this time, the electrostatic pinning conditions were as described in Example 1.
Using the same electrode, voltage: 7.2Kv, current: 0.040m
A/cm, and the distance between the electrode and the film was 5.0 m/m. At this time, the P value is 31.1. At this time, no curling occurred and no air bubbles were observed.

Example 4 The same polyester resin as in Example 3 was extruded into a film under the same conditions as in Example 3. The film forming conditions were: air gap: 45 m/m, network speed: 42 m/m, take-off speed: 60 m/min, draft ratio: 10.9, film thickness: 75 μ, take-off angle: 55°. The electrostatic pinning conditions at this time were that the same electrodes as in Example 1 were used, voltage: 7.0 Kv, current: 0.046 mA/cm, and distance between electrode and film: 4.0 m/m. At this time, the P value is
It is 28.4 (cm ² /sec).

At this time, no curling occurred and no air bubbles were observed.

Comparative Example 1 As a product corresponding to Example 1, a film was formed by extrusion using the same resin and at the same temperature as in Example 1. The film forming conditions were a take-up speed of 42 m/min, draft ratio: 6.7, film thickness: 120 μ, take-off angle: 30°, and when the air gap was set to 50 m/m, the net-in was 42 m/m. The P value at this time is 17.5 (cm ² /sec).
At this time, curling occurred, and the ends of the film came close to the electrodes by about 3 m/m compared to the center of the film.
Films were formed under various pinning conditions, but a film free from air bubble entrainment could not be obtained.

Comparative Example 2 Corresponding to Example 2, the same resin as in Example 2 was extruded to form a film at the same temperature. The film forming conditions were a take-up speed of 45 m/min, a draft ratio of 8.4, a film thickness of 120μ, and a take-off angle of 40°.
When the speed was 50m/m, the net speed was 46m/m. The P value at this time is 24.0 (cm ² /sec). At this time, curling occurred, and the ends of the film came close to the electrodes by about 3 m/m compared to the center of the film.
Films were formed under various pinning conditions, but a film free from air bubble entrainment could not be obtained.

Comparative Example 3 Corresponding to Example 3, the same resin as in Example 3 was extruded to form a film at the same temperature. Film forming conditions were: take-off speed: 50 m/min, draft ratio: 9.1, film thickness: 90 μ, take-off angle: 35°.
When the speed was set at 45m/m, the net speed was 44m/m. At this time, the P value is 22.0 (cm ² /sec). At this time, curling occurred, and the ends of the film were brought closer to the electrode by about 2 m/m than the center of the film. We sought pinning conditions that would not entrain air bubbles, but were unable to find them.

Example 5 The angle between the discharge direction of the extrusion die and the flow direction of the molten resin at the tip of the die lip was set to 40°, and Example 1
Same resin at 290℃, draft ratio 6.7, take-up speed 40
A film of 120μ was obtained by extrusion at a speed of m/min.
At this time, the film forming time per lip cleaning is 180
It took about an hour.

Example 6 A film was formed by extrusion under the same conditions as in Example 5, except that the angle between the discharge direction of the extrusion die and the flow direction of the molten resin at the tip of the die lip was 55°. At this time, the film forming time per lip cleaning was about 160 hours.

Comparative Example 4 Corresponding to Examples 5 and 6, a film was formed by extrusion under the same conditions as in Example 5 except that the angle between the extrusion die discharge direction and the flow direction of the molten resin at the tip of the die lip was set at 65°. At this time, the film forming time per lip cleaning was about 40 hours.

Example 7 The same resin as in Example 3 was extruded into a film under the same conditions as in Example 5. As a result, the film forming time per lip cleaning was about 180 hours.

Example 8 The same resin as in Example 7 was extruded into a film under the same conditions as in Example 6. As a result, the film forming time per lip cleaning was about 160 hours.

Comparative Example 5 Corresponding to Examples 7 and 8, extrusion film formation was carried out under the same conditions as Example 7, except that the angle between the extrusion die discharge direction and the flow direction of the molten resin at the tip of the die lip was set at 65°. As a result, the film forming time per lip cleaning was about 32 hours.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing whether or not electrostatic pinning is possible with respect to P value and take-up speed. FIG. 2 is a front view during film formation, and FIG. 3 is a side view thereof. Fig. 4 shows the case where the take-off angle is increased in the normal T-die arrangement, and Fig. 5 shows the case where the extrusion die is used to reduce the angle between the discharging direction and the take-off direction of the molten resin at the tip of the extrusion die lip. FIG. 6 is an enlarged view of the extrusion die lip shown in FIG. 5, and FIG. 7 is an enlarged view of the extrusion die lip shown in FIG. 4. The numbers and letters shown in the figure are as follows. 1...
...Die, 2...Rotating cooling body, 3...Film, 4
...Film end, 5, 6...Die lip. 7...
...molten resin pool.

Claims (1)

[Claims] 1. Molten polyester resin is extruded from an extrusion die for 40 minutes.
It is extruded in the form of a film onto a rotating cooling surface that rotates at a circumferential speed of m/min or more, and an electrode is placed near the contact point between the film and the rotating cooling surface to deposit electrostatic charges on the film surface. In the process of forming a film by adhering to the rotating cooling surface and rapidly cooling it due to electrostatic attraction acting between the film and the cooling surface, the density of the molten film is ρ (g/cm ³ ), and the film is first melted from the extrusion die lip onto the rotating cooling surface. The air gear up S (cm) is given by the length measured along the molten film up to the point where the film contacts, and is reduced at each edge of the molten film extruded from the extrusion die before it first contacts the rotating cooling surface. net-in δ (cm), which is expressed as the transverse amount (1/2 of the difference between the die lip width and the film width at the contact point), is the value obtained by dividing the rotational cooling surface velocity by the average discharge velocity in the transverse direction at the exit of the extrusion die. The temperature of the resin passing through the extrusion die lip is given by the draft ratio λ(-), the melt viscosity μ (g sec/cm ² ) corresponding to the shear rate, and the temperature between the extrusion die and the first contact point with the rotating cooling surface. An index P regarding the amount of deflection of the film edge, which is expressed by the following formula, is 25 ( 1. A method for producing a polyester resin film, which comprises adjusting the positional relationship between an extrusion die lip and a rotating cooling roll so that the extrusion die lip is at least 2 cm ² /sec). 2. Production of a polyester resin film according to claim 1, wherein the angle between the discharge direction of the extrusion die lip and the vertical direction is |α-θ|≦60°, where θ (degrees) Method.