JP2825727B2 - Method for producing biaxially oriented polyester film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a biaxially oriented polyester film, and more particularly to a method for suppressing a bowing phenomenon which occurs during a heat treatment after a successive biaxial stretching, so as to obtain uniform physical and chemical properties in the longitudinal and transverse directions. And a method for producing a biaxially oriented polyester film having physicochemical properties.

[0002]

2. Description of the Related Art Biaxially oriented polyester films have excellent properties such as mechanical properties and thermal stability.
For example, it is used as a base film for magnetic recording media such as video tapes, audio tapes, floppy disks, and the like, and because of its excellent electrical insulation properties, it is used for various industrial uses such as film capacitors and other electrical discharge applications. In recent years, with the increase in recording density of magnetic recording disks, isotropic properties such as a coefficient of thermal expansion and mechanical properties of a base film have been further required. Further, it is desirable that these characteristics be uniform in any part in the width direction.

A biaxially oriented polyester film is generally manufactured as follows. The polyester resin is supplied to an extruder, melt-extruded into a film from a die (die slit), and quenched to produce an unstretched film. Next, this unstretched film is rolled using a plurality of rolls (Tg-
10) The film is stretched in the longitudinal direction at a constant temperature of from (Cg) to (Tg + 50) C (where Tg is the glass transition temperature of the polyester), and then the end of the film is gripped by a clip, and (Tg + 10) C. The stretching is performed in the transverse direction at a temperature of (Tg + 40) ° C. Then, while holding the end portion with the clip, a tension heat treatment of a certain width is performed at a temperature of (Tg + 10) ° C. to (Tm−10) ° C., and then cooled to room temperature to obtain a biaxially oriented polyester film. This polyester film is slit into a predetermined width as required and wound up.

However, it is extremely difficult to obtain a biaxially oriented polyester film having excellent isotropy such as a coefficient of thermal expansion in the width direction and mechanical properties by the conventional stretching and heat treatment methods. This is due to the anisotropy of the film deformation due to the heat shrinkage stress generated in the heat treatment step. That is, since the end of the film is gripped by the clip, the deformation of the film due to the heat shrinkage stress generated during the heat treatment is large at the center of the film and small at the end, and as a result, the characteristics in the width direction can be distributed. Becomes If a straight line is drawn along the lateral direction on the surface of the film before the heat treatment step, the straight line on the surface of the film that has gone through the heat treatment step has an arcuate shape in which the center portion is concave toward the downstream. This phenomenon is called the Boeing phenomenon. This is a cause of disturbing the isotropy and uniformity in the width direction of the film. This is because the bowing phenomenon causes a distribution in the degree of orientation and orientation angle of the main chain axis in the width direction, and as a result, the vertical and horizontal thermal contraction rates, thermal expansion rates, humidity expansion rates, mechanical properties, etc. This is because anisotropy occurs in the direction. Due to this anisotropy, a problem such as track deviation occurs in a high-density magnetic recording disk, and a reduction in recording density and an access error occur.

[0005] Many proposals have been made for reducing the bowing phenomenon. For example, Tokuhei 4-45336
Japanese Patent Laid-Open Publication No. H11-15064 proposes a method for producing a film in which the film is stretched in the horizontal direction while sequentially increasing the temperature from the initial temperature, and relaxed in both the vertical and horizontal directions while being held by the clip. However, in this method, the user relaxes in the vertical direction while holding the end of the film with the clip,
The equipment burden is very large and requires special equipment for manufacturing. Even if the film is relaxed in the vertical and horizontal directions, if the stress generated during the heat treatment is extremely reduced, bowing occurs and the anisotropy occurs in the characteristics in the width direction.

In Japanese Patent Application Laid-Open No. 4-142917, a region having a temperature lower than the temperature of the transverse stretching is provided between the transverse stretching and the heat treatment, and the film is relaxed in the transverse direction in the process and then subjected to the heat treatment. Manufacturing methods have been proposed. This method is effective in slightly reducing the amount of bowing, but essentially does not show a significant improvement effect. The reason for this is that, in this method, not only the cooling temperature before the heat treatment is insufficient, but also there is no measure against the contraction force generated in the heat treatment step.

[0007]

SUMMARY OF THE INVENTION The present inventors have carefully observed the change in the bowing line in the heat treatment process from the lateral stretching, measured the stress generated in each process, and clarified the process of the occurrence of the bowing phenomenon. As a result of studying means for reducing this bowing, the present invention has been reached.

Accordingly, an object of the present invention is to provide a method for producing a polyester film having improved isotropy such as a coefficient of thermal expansion and mechanical properties in a width direction of the polyester film.

[0009]

SUMMARY OF THE INVENTION The object of the present invention is as follows.
According to the present invention, a method for producing a biaxially oriented polyester film by sequentially stretching an unstretched film in the longitudinal and transverse directions, and then heat-treating the film, wherein a cooling step is provided between the transverse stretching step and the heat treatment step. The biaxially oriented film is once cooled to a temperature below the glass transition temperature (Tg) of the polyester, then the heat treatment step is divided into two or more zones, and the temperature of each zone is (Tg + 10) ° C. or more and the melting point of the polyester (Tg)
m) a temperature lower by at least 10 ° C., but at least the temperature of the final zone is higher than the transverse stretching temperature, and the heat treatment is performed so that the stress of the film in the second and subsequent zones satisfies the following equation. This is achieved by a method for producing a biaxially oriented polyester film.

[0010]

(Σm−σn) ≧ −1.0 (kg / mm ² ) (where, σm and σn indicate stress generated in each zone, and m and n are zone numbers assigned to the heat treatment zone) And m = n + 1.)

The polyester in the present invention is a linear saturated polyester synthesized from an aromatic dibasic acid or an ester-forming derivative thereof and a diol or an ester-forming derivative thereof. Preferred specific examples of such a polyester include polyethylene terephthalate, polyethylene isophthalate, polytetramethylene terephthalate, poly (1,4-cyclohexylene dimethylene terephthalate), polyethylene 2,6-naphthalenedicarboxylate, and the like. Or a blend of these with a small proportion of another resin.
Among these, polyethylene terephthalate and polyethylene-2,6-naphthalenedicarboxylate are particularly preferred.

In the present invention, the polyester is heated and melted to a temperature not lower than the melting point (Tm), melt-extruded into a film from an extruding means including a slit die, brought into close contact with a cooling drum, quenched, and rapidly cooled to form an unstretched film. And This unstretched film is subjected to (Tg-1
0) to (Tg + 50) ° C., preferably (Tg + 10) to
The film is stretched in the machine direction at a temperature of (Tg + 40) ° C. (where Tg is the glass transition temperature of the polyester). The stretching ratio in the machine direction is 3 times or more, more preferably 3 to 4.5 times, especially 3.
Two to four times is preferred. Next, the longitudinally stretched film is gripped at its end with a stenter clip to obtain (Tg + 10)-
The film is stretched in the transverse direction at a temperature of (Tg + 40) ° C. The stretching ratio in the transverse direction is preferably substantially balanced with the stretching ratio in the longitudinal direction, specifically 3 times or more, more preferably 3 to 4.5 times.
Especially 3.2 to 4 times is preferable. Thereafter, a biaxially oriented film is cooled to Tg (° C.) or less, preferably
Cool below 0 ° C. At this time, the end of the film may be released while being held by the clip. further,
There is no restriction on the length of the cooling step.

The film which has been biaxially oriented by the above stretching process is then guided to a heat treatment process by gripping its ends with clips. This step is divided into two or more zones, and the temperature of each zone is at least (Tg + 10) ° C. and at least 10 ° C. lower than the melting point (Tm) of the polyester, provided that at least the final zone is transversely stretched. Heated above temperature. Preferably, these zones have the lowest temperature at the beginning, successively higher temperatures downstream, and the highest temperature in the last zone. Then, the heat treatment is performed so that the stress (the stress applied to the clip) of the film in the second (m = 2) and subsequent zones satisfies the following expression.

[0014]

(Σm−σn) ≧ −1.0 (kg / mm ² ) (where, σm and σn indicate stress generated in each zone, and m and n are zone numbers assigned to the heat treatment zone) And m = n + 1.)

If this expression is not satisfied, bowing is suppressed little and it is difficult to make the physical properties in the film width direction (lateral direction) uniform.

In this heat treatment, it is preferable to relax 0 to 20% in the width direction at the beginning of the heat treatment in order to relax a large stress generated at the beginning. Further, in the subsequent heat treatment step, it is preferable to give an elongation (tension) of 1% or more, and more preferably 3% or more, so that the width of the final product is 90 to 120% of that before the heat treatment.
By performing such a process, it is possible to obtain a film in which the occurrence of bowing is very small and the isotropy such as the coefficient of thermal expansion in the width direction and mechanical properties is very excellent.

After passing through this heat treatment step, the polyester film is cooled to room temperature to obtain a polyester film. This polyester film is slit into a predetermined width and wound as required, as in the conventional case. In the final step of the tension heat treatment, a relaxation treatment can be performed in the lateral direction for the purpose of improving the heat shrinkage.

The biaxially oriented polyester film thus obtained has excellent uniformity of properties in the in-plane direction. In particular, the difference in properties between the center and both ends (about 80% of the entire width) of the film is small. The refraction (Δn) is 5 × 10 ⁻³ or less, and the difference (ΔαT) between the maximum value and the minimum value of the thermal expansion coefficient is 5 × 10 ^−3.
-6 or less.

[0019]

The present invention will be further described below with reference to examples. The characteristics in the examples were measured by the following methods.

(1) Stress at the time of heat treatment A tension meter is attached to the jaw portion of the clip for gripping the end of the film, and a signal transmitted from the telemeter transmitter is received by a receiver outside the tenter, so that each zone can receive a signal. Find the generated stress.

(2) Amount of bowing A straight line is drawn on the substantially unstretched film surface before the longitudinal stretching, and the bow shape obtained by the finally obtained film is

[0022]

(Equation 4) Is calculated by

(3) Difference in thermal expansion coefficient (ΔαT) A length of 15 m along the optical orientation direction (long axis) and a direction perpendicular to the optical orientation direction (short axis) determined by a polarizing microscope in advance.
m, a test piece having a width of 5 mm was sampled, and the amount of change from room temperature to 80 ° C. was measured with a TMA tester at a rate of temperature increase of 2 ° C./min. The thermal expansion coefficient (αT: mm / mm / ° C.) in each direction is determined.

[0024]

(Equation 5)

It is known that the thermal expansion coefficient varies depending on the in-plane direction, but is substantially minimum in the long axis direction and substantially maximum in the single axis direction. The difference (ΔαT) between the thermal expansion coefficients is obtained by the following equation.

[0026]

## EQU6 ## ΔαT = αT (short axis) −αT (long axis)

(4) Birefringence The Abbe-type refractometer reads the refractive index in the optical orientation direction (major axis) and the direction perpendicular to the major axis (minor axis) obtained by a polarizing microscope in advance, and determines the difference between the refractive indices as birefringence. .

(5) Degree of Molecular Orientation and Orientation Angle The degree of molecular orientation (MOR) and the orientation angle of the long axis of the molecular chain are measured using a MOA-2001A type microwave molecular orientation meter manufactured by Kanzaki Paper Co., Ltd. The degree of molecular orientation indicates the ratio between the vertical and horizontal molecular orientations, and the closer to 1, the higher the isotropy.

[0029]

Example 1 Polyethylene terephthalate was supplied to an extruder, extruded from a die into a film, wound around a cooling drum, and cooled and solidified to obtain a substantially non-oriented unstretched film. This unstretched sheet is
The film was stretched 3.6 times in the machine direction between a roll maintained at 25 ° C and a cooling roll maintained at 25 ° C. Next, grip the end with a clip, and in an oven kept at 100 ° C. in the lateral direction.
The film was stretched 6 times. After this transverse stretching, it was cooled to room temperature (25 ° C.). Thereafter, the film was guided to a tenter for performing heat treatment, and heat treatment was performed. This tenter consists of four zones, each of which can be independently temperature controlled. Table 1 shows the temperature of each zone and the film width. The heat-treated film was passed through a 120 ° C. temperature atmosphere and then cooled to room temperature to obtain a biaxially oriented polyester film having a width of 2 m.

[0030]

Example 2 Table 1 shows the temperature and film width of each zone.
The biaxially oriented polyester film having a width of 2 m was obtained in the same manner as in Example 1 except that the film was changed as shown in (1).

[0031]

[Comparative Example 1] Table 1 shows the temperature and film width of each zone.
The biaxially oriented polyester film having a width of 2 m was obtained in the same manner as in Example 1 except that the film was changed as shown in (1).

[0032]

Comparative Example 2 Table 1 shows the temperature and film width of each zone.
The biaxially oriented polyester film having a width of 2 m was obtained in the same manner as in Example 1 except that the film was changed as shown in (1).

The film forming conditions, the stress during the heat treatment, the bowing amount, and the difference (ΔαT) between the center part and the edge part of the film, and the coefficient of thermal expansion between the center part and the middle part thereof in the example and the comparative example were measured. Furthermore, the dimensional change rate, birefringence,
In addition, the degree of molecular orientation by measuring the anisotropy of microwave transmittance and the orientation angle of the main chain axis were evaluated. Table 2 shows the measurement results.

[0034]

[Table 1]

[0035]

[Table 2]

From Table 2, it can be seen that the polyester films obtained in Examples 1 and 2 have a small bowing, a small difference in properties between the end and the center of the film, and have excellent uniformity in the width direction. Recognize. On the other hand, it can be seen that the polyester films obtained in Comparative Examples 1 and 2 have large bowing, and the difference in characteristics between the end and the center of the film is very large.

[0037]

According to the present invention, it is possible to obtain a biaxially oriented polyester film which can be heat-treated while significantly suppressing the occurrence of bowing, and which is excellent in isotropy such as a coefficient of thermal expansion in the width direction and mechanical properties. Can be.

Continuation of the front page (56) References JP-A-3-161319 (JP, A) JP-A-1-165423 (JP, A) JP-A-58-24418 (JP, A) JP-A-1-204722 (JP, A) , A) JP-A-57-57629 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B29C 55/02-55/12 B29C 71/02 C08J 5/18 CFD

Claims (5)

(57) [Claims] 1. A method for producing a biaxially oriented polyester film by sequentially stretching an unstretched film in a machine direction and a transverse direction, and then heat-treating the film, wherein a cooling step is provided between the transverse stretching step and the heat treatment step. The axially oriented film is once cooled to a temperature lower than the glass transition temperature (Tg) of the polyester, and then the heat treatment step is divided into two or more zones, and the temperature of each zone is higher than (Tg + 10) ° C and the melting point of the polyester (Tg).
m) a temperature lower by at least 10 ° C., but at least the temperature of the final zone is higher than the transverse stretching temperature, and the heat treatment is performed so that the stress of the film in the second and subsequent zones satisfies the following equation. A method for producing a biaxially oriented polyester film. (Σm−σn) ≧ −1.0 (kg / mm ² ) (where, σm and σn indicate stresses generated in each zone, and m and n are zone numbers assigned to the heat treatment zone) And m = n + 1.) 2. At the beginning of the heat treatment, the film is provided with a relaxation of 0 to 20% in the lateral direction of the film, and thereafter, by 1% or more so as to have a width of 90 to 120% with respect to the film width before the heat treatment by the final zone. The method for producing a biaxially oriented polyester film according to claim 1, which provides elongation. 3. The method for producing a biaxially oriented polyester film according to claim 1, wherein the heat treatment is performed by sequentially increasing the temperature of the heat treatment zone. 4. A biaxially oriented polyester film produced by the method according to claim 1, 2 or 3. 5. The film has a refractive index (Δn) of 5 × 10 ^−3.
5. The biaxially oriented polyester film according to claim 4, wherein the difference (ΔαT) between the maximum value and the minimum value of the coefficient of thermal expansion in the in-plane direction of the film is 5 × 10 ⁻⁶ or less.