JPS6243857B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a biaxially oriented polyester film, and more specifically, to a method for producing a film with excellent uniformity that has excellent balance of physical properties in the longitudinal and lateral directions, with almost no difference in physical properties along the width direction of the film. Involved. Biaxially oriented polyester films are used for industrial purposes, but especially for applications such as photography, drafting, and magnetic disks, physical properties in both the vertical and horizontal directions, especially thermal expansion coefficient, humidity expansion coefficient, and thermal contraction coefficient, are well balanced. It is hoped that there will be. It is also desirable that the properties be the same in all parts of the product film. However, it has been extremely difficult to make the physical properties of the product film uniform in the width direction using the usual biaxial stretching method, that is, a method in which longitudinal stretching is followed by transverse stretching. The reason for this is that both ends of the film are held in the tenter, and the shrinkage stress of the film in the longitudinal direction due to lateral stretching is restrained by the holding means such as clips. The restraining force becomes relatively weak, and the expansion of the central portion of the film is delayed due to the shrinkage stress.
If a straight line is drawn along the lateral direction on the film surface before lateral stretching, this straight line will be deformed and curved into a concave shape in the film traveling direction. This phenomenon is called boeing. This bowing phenomenon causes the physical properties of the film in the width direction, particularly the humidity expansion coefficient and the temperature expansion coefficient, to become non-uniform. Due to the bowing phenomenon, a main axis of orientation is created at both end portions of the film that is further inclined vertically with respect to the bowing line, and the coefficient of humidity expansion and temperature expansion in the direction of this main axis is low, and the coefficient of expansion perpendicular to the direction of the main axis is lower. The physical property values tend to be higher in the direction. Several methods have been proposed to eliminate differences in physical properties in the width direction. For example, Japanese Patent Publication No. 37-1588 discloses a transverse-longitudinal stretching method, but it is not an essential countermeasure. JP-A-50-73978 proposes a technique of using nip rolls between the lateral stretching process and the heat treatment process, but this is impractical due to the risk of surface scratches. Also, JP-A-51-80372 and JP-A-Sho.
No. 54-137076 both show measures to reduce bowing, but these are related to simultaneous biaxial stretching and cannot be applied to sequential stretching. The inventors elucidated the process by which Boeing occurs, and
As a result of various studies on ways to reduce the bowing phenomenon, the present invention was arrived at. That is, in the present invention, when a thermoplastic polyester film that has been uniaxially stretched in the longitudinal direction is stretched in the transverse direction at a temperature higher than the glass transition temperature and then heat-treated, (a) the temperature of the film after the transverse stretching is once lowered to below the glass transition temperature; b) Heat-treat the film while holding both ends in a temperature range of [melting point -160℃] to [melting point -80℃], (c)
Give the film 0.1% to 10% relaxation in the longitudinal direction without gripping both ends, and then (d) contact the film with a smooth roll having a surface temperature of [the above heat treatment temperature] to [the above heat treatment temperature -20°C]. A method for producing a biaxially oriented polyester film is characterized in that: To explain the present invention, the polyester used in the present invention is selected from linear polyesters such as polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, etc., which are provided by biaxial stretching, and these may contain inorganic additives. , a partially modified copolymer, or a mixture containing small amounts of other polymers. The film forming method and the uniaxial stretching method are not particularly specified. (a) It is desirable that the transverse stretching is carried out by 3.0 to 4.5 times at a temperature higher than the glass transition temperature. After the stretching is completed, the film is cooled to below the glass transition temperature as quickly as possible. In reality, the neutral zone of the tenter is cooled with air at about room temperature, but a cooling roll may also be used. This cooling is necessary to prevent bowing during lateral stretching. (b) Next, heat treatment is performed while holding both ends, but at this time, the temperature is considerably lower than that of normal heat treatment [melting point -160°C] to [melting point -80°C].
℃] is important. At temperatures higher than this, bowing occurs in this process,
Differences in physical properties occur along the width direction. Further, if a temperature lower than the above-mentioned lower limit temperature is selected, bowing will not occur, but the flatness of the film will be impaired. However, as it is, heat fixation is insufficient and the heat shrinkage rate is large. Therefore, (c) after removing the clip, it is essential to perform some relaxation treatment in the longitudinal direction. When the magnitude of relaxation is expressed as a percentage difference between the circumferential speed of the delivery roll to the relaxation zone and the circumferential speed of the take-up roll, it is preferable to slow down the take-up roll side within a range of 0.1 to 10%. (d) A certain degree of heating is required for relaxation, but this temperature is 20°C higher than the heat setting temperature.
A range between lower temperatures is desirable. If the temperature is too high, physical properties tend to deteriorate and production efficiency decreases. As for the heating method, it is most suitable to use a roll because it is industrially simple and provides the desired result. During the relaxation treatment, a nip roll for the heating roll and a dancer roll for stabilizing the tension can be used. Improving lateral heat shrinkage can be achieved by toe-in in which the rail width is fitted first in the heat fixing in the tenter. Next, further explanation will be given by giving examples. Example 1 Polyethylene terephthalate [intrinsic viscosity 0.62]
was melted and extruded through a T-die, formed into a film on a quenching drum, and then stretched 3.4 times in the longitudinal direction.
Then, it was stretched 3.6 times in the transverse direction using a tenter.
A biaxially stretched film of 73 μm in diameter was passed through a 1 m section (cooling section) held at 55°C while holding both ends of the film, and then passed through a 1.5 m long section held at 150°C at a speed of 20 m/min. The train passed through three sections and applied 2% toe-in (adjustment of the rail spacing) in the third section. After that, it is removed from the clip and 3%
The film was brought into contact with a smooth roll having a surface temperature of 140°C while being vertically relaxed, and then rolled up. At this time, a nip roll was used to nip the film at the position where it started to come into contact with the heating roll. Comparative Example 1 A biaxially stretched film was obtained in exactly the same manner as in Example 1 except that the longitudinal relaxation treatment was omitted. Comparative Example 2 In Example 1, the heat fixing temperature inside the tenter was set to 230
75μ by the same manufacturing method except that the temperature was changed to ℃.
It was made into a biaxially stretched film. Comparative Example 3 In Example 1, the heat fixing temperature inside the tenter was set to 80°C.
A biaxially stretched film was produced in exactly the same manner except that the film had poor flatness and it was not possible to obtain a film with a good appearance. Comparative Example 4 The steps up to the heat treatment in the tenter were exactly the same as in Example 1, and the roll temperature during longitudinal relaxation was changed.
When the temperature was set at 180°C, the film became wavy and it was not possible to obtain a film with good appearance. Among the above, the physical properties of the film of Example 1 and the films of Comparative Examples 1 and 2, which produced films with good appearance, were compared as follows.

[Table] Although Example 1 obtained satisfactory values for all items, Comparative Example 1 had a large longitudinal heat shrinkage rate. In Comparative Example 2, the center of the film is good, but at the edges, the linear expansion coefficient differs greatly depending on the direction, so only the center can be used. Comparative Example 5 The same procedure as in Example 1 was carried out except that the temperature in the cooling section of the biaxially stretched film was changed from 55°C to 80°C. The properties of the obtained biaxially stretched film are as follows. Bowing amount: 60mm Heat shrinkage rate at 140℃ for 30 minutes Longitudinal direction: 0.3% Lateral direction: 0.2% Difference between maximum and minimum thermal expansion coefficient at the center of the film: 0.40×10 ^-5 /℃ Temperature at the edge of the film Difference between maximum and minimum expansion rate: 1.00×10 ^-5 /℃

Claims (1)

[Claims] 1. When a thermoplastic polyester film uniaxially stretched in the longitudinal direction is stretched in the transverse direction at a temperature higher than the glass transition temperature and then subjected to heat treatment, the film immediately after the horizontal stretching is cooled to a temperature lower than the glass transition temperature, and then After holding both ends of the film, heat treatment is performed at a temperature T ₁ in the range of Tm - 160℃ to Tm - 80℃ (where Tm is the melting point of the film),
Release the gripping means on both sides of the film and twist it vertically by 0.1
Give relaxation in the range of ~10%, T ₁ ≧T ₂ ≧T ₁ -20℃
A method for producing a biaxially stretched polyester film, which comprises bringing the film into contact with a smooth roll whose surface temperature is maintained at a temperature _T2 that satisfies the following conditions.