JP2589547B2 - Method for producing biaxially oriented polyester film for magnetic recording medium - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a biaxially oriented polyester film for a magnetic recording medium by a tenter method, and has both strength against external force and heat and dimensional stability. An object of the present invention is to provide a biaxially oriented polyester film for a magnetic recording medium.

(Prior Art) Biaxially oriented polyester films are used for magnetic tapes and a wide range of other applications because of their excellent mechanical properties, heat resistance and chemical resistance. However, among magnetic tapes, especially for magnetic tapes for video, when the recorded tape is stored at a high temperature, or when the tape is repeatedly subjected to heat during running due to repeated playback, the tape undergoes thermal contraction. There is a problem that image distortion (skew distortion) occurs due to this, and dimensional stability of the tape is further required for solving the problem.

In order to obtain a magnetic recording medium having a small heat shrinkage rate when exposed to a high temperature in this way, while running a biaxially oriented polyester film obtained by performing longitudinal stretching and transverse stretching, a heating roller having a different peripheral speed is used. A method of relaxing with a cooling roller is known. Also, JP-A-51-46372 discloses that a vertically stretched polyester film is supplied to a tenter, horizontally stretched, and immediately after heat fixing, right and left sides of a biaxially oriented polyester film gripped by a tenter clip. A method is disclosed in which the ear end is cut off, and only the central portion other than the ear end is released from the restraint of the tenter clip and relaxed.

(Problems to be Solved by the Invention) A method of relaxing a biaxially oriented polyester film by passing it through a heating roller and a cooling roller is to stretch the film in the longitudinal and lateral directions, heat-fixed, cooled, and then leave the tenter. Since the film is reheated and relaxed, it is uneconomical in terms of energy, and the number of steps increases, and the film surface is damaged by contact with the heating roller and the cooling roller. In addition, the method of performing the heat relaxation treatment in the tenter is such that the film is located in the heat fixing zone of the tenter,
Since the edge of the ear is cut at a high temperature where the film surface temperature is almost equal to the heat setting temperature, there is a drawback that the cut portion sharply shrinks in the width direction, causing the film to break and the value of the maximum thermal stress to decrease. Was.

The present invention cuts the edge of the biaxially oriented film near the exit of the tenter rail to relax the central body at a relatively low temperature, thereby solving the above-mentioned conventional problems, and The present invention provides a method for producing a biaxially oriented polyester film for a magnetic recording medium, which does not require any treatment for lowering the heat shrinkage in the process, does not reduce the strength of the film, and has excellent strength and dimensional stability. is there.

(Means for Solving the Problems) Production of a biaxially oriented polyester film which is biaxially oriented by a tenter method, heat-set in a heat setting zone of a tenter, and then separated from a tenter clip and taken out by a take-off roller. In the method, the surface temperature of the biaxially oriented polyester film gripped by the tenter clip at the exit side of the tenter rail is Tg to Tg + 80 ° C.
(However, Tg is the glass transition temperature), preferably Tg + 15 ° C
The left and right ear ends of the biaxially oriented polyester film are cut in a state where the temperature is in the range of ~ Tg + 55 ° C, and the film tension between the cut portion and the take-up roller is 0.48 to 1.90 kg / mm ^2.
And taking the above biaxially oriented polyester film.

The polyester used in the film of the present invention is a crystalline polyester such as polyethylene terephthalate and polyalkylene naphthalate, and is not particularly limited. Further, a polyester film biaxially oriented by a tenter method is obtained by melt-extruding the above polyester by a known method, forming a sheet, and forming a sheet at a glass transition temperature Tg (° C.) or higher.
Although it is stretched in the transverse biaxial direction, it may be a so-called strengthened type which is further stretched in the uniaxial or biaxial direction.

In this invention, the stretched polyester film is heat-set on a tenter. The heat setting temperature is 150 to 220 ° C. in the case of polyethylene terephthalate, and the processing time is 1 to 10 seconds.

(Action) The edge of the film held by the tenter clip at the tenter outlet side is cut at a surface temperature of Tg to Tg + 80 ° C, which is lower than the heat setting temperature and higher than room temperature, and is 0.48 to 1.90 kg.
By引取Ru in tension / mm ^2, preferably relaxation is obtained, as a result, the obtained film longitudinal thermal shrinkage when left in an atmosphere of 70 ° C. maximum temperature during use as a video tape and When the speed becomes 0.020% / hour or less, the heat shrinkage for magnetic tapes decreases to such an extent that practically no problem occurs, and the maximum heat of the thermal stress curve when the temperature is raised at a heating rate of 4 ° C./min. The stress value becomes 0.3 kg / mm ² or more, and sufficient strength is obtained as a film for magnetic tape.
Elongation during the magnetic coating process is eliminated.

On the other hand, if the surface temperature when cutting the ear end is lower than the glass transition temperature, the relaxation becomes insufficient and the purpose of lowering the shrinkage rate cannot be achieved. Conversely, if the temperature exceeds Tg + 80 ° C, When cutting the end of the ear, the width of the cut portion is sharply shrunk to be easily broken, and the value of the maximum thermal stress cannot be satisfied, resulting in insufficient strength for magnetic tape. In the case the film tension between the film cutting position and the take-up roller is less than 0.48 kg / mm ^2, wrinkles are generated in the film, the surface resistance is remarkably deteriorated, 1.90 kg in the opposite / mm ²
When the temperature exceeds 70 ° C., the relaxation becomes insufficient, and the heat shrinkage rate in the atmosphere at 70 ° C. becomes larger than 0.020% / hour.

(Embodiment) The drawings show an example of a tenter used in the present invention,
The pair of left and right tenter chains 1 and 1 are wound around the front and rear sprockets 2 and 3 and rotate in the direction of arrow a, and are vertically attached to clips (not shown) attached to the tenter chains 1 and 1 at the entrance side. The stretched film Fa is gripped and conveyed in the direction of the arrow b, and sequentially passes through the preheating zone A, the transverse stretching zone B, the heat setting zone C, and the thermostatic zone D to form a biaxially oriented film Fb, near the exit of the tenter chain 1. The edge of the biaxially oriented film Fb is cut by the cutting blade 4 provided in the above, and the main body Fc at the center of the film is taken off by the take-off roller 5.

Polyethylene terephthalate is melt-extruded by a conventional method, formed into a sheet, stretched 3.6 times in the longitudinal direction at 95 ° C, and the obtained longitudinally stretched film Fa is supplied to a tenter shown in
After preheating in the preheating zone A in the horizontal stretching zone B at 90 ° C
It is stretched 4.0 times to obtain a biaxially oriented polyester film Fb, which is heat-treated in a heat-setting zone C at 200 ° C. while running at a speed of 150 m / min, and subjected to a 3% relaxation treatment in the lateral direction. Then, heat is passed through a thermostat zone D of 135 ° C., and then the edge of the biaxially oriented film Fb is heated to 100 ° C. whose surface temperature is about 30 ° C. higher than the glass transition temperature (Tg = 70 ° C.). Under the condition, and the film tension is 0.9
The biaxially oriented polyester film Fc of Example 1 having a thickness of 14 μm was taken up by the take-up roller 5 at 6 kg / mm ² .

In the first embodiment, the temperature of the constant temperature zone was
200 ° C, and the surface temperature when cutting the film is 160 ° C (Tg
+ 90 ° C.), and a biaxially oriented polyester film of Comparative Example 1 was obtained in the same manner as in Example 1.

Further, the biaxially oriented polyester of Example 2 was prepared in the same manner as in Example 1 except that the temperature of the thermostat zone of Example 1 was set at 160 ° C. and the surface temperature at the time of cutting the film was set at 120 ° C. (Tg + 50 ° C.). A film was obtained.

In addition, the same procedure as in Example 1 was carried out except that the temperature of the constant temperature zone in Example 1 was set to 80 ° C. and the surface temperature at the time of cutting the film was set to 60 ° C. (Tg−10 ° C.). An oriented polyester film was obtained.

Further, the film tension on the exit side of the tenter of Example 1 was set to 1.
A biaxially oriented polyester film of Example 3 was obtained in the same manner as in Example 1, except that the setting was 8 kg / mm ² .

Further, the film tension at the tenter outlet side in Example 1 was set to 2.
A biaxially oriented polyester film of Comparative Example 3 was obtained in the same manner as in Example 1 except that the weight was set to 0 kg / mm ² .

Further, the cutting of the film in Example 1 was omitted, and the edge of the ear was cut and removed at room temperature immediately before winding the film as usual, to obtain a biaxially oriented polyester film of Comparative Example 4.

The characteristics of the biaxially oriented polyester films of Examples 1 to 3 and Comparative Examples 1 to 4 and the characteristics as a magnetic recording medium coated with a magnetic layer are shown in the table.

 However, the above characteristics were measured as follows.

(1) Thermal shrinkage rate at 70 ° C From the biaxially oriented polyester film after cutting the ear end,
A sample having a length of 50 mm and a width of 4 mm was sampled so that the length direction of the film coincided with the length direction of the sample, and this sample was attached to a TMA-3000 thermomechanical tester manufactured by Vacuum Riko Co., Ltd.
In the deformation mode, measure the deformation curve of the film at 70 ° C under a constant load of 9.4g, calculate the amount of deformation that occurred within 1 hour after the sample atmosphere reached 70 ° C, and calculate the amount of deformation. And the heat shrinkage per hour, that is, the heat shrinkage rate.

(2) Thermal stress curve When a sample taken in the same manner as in (1) above is attached to the same thermomechanical tester as in (1) above, and the temperature is raised from room temperature at a rate of 4 ° C./min. Is measured, and the maximum thermal stress (kg / mm ² ) reached at 150 to 220 ° C. is calculated.

(3) Heat shrinkage rate at 70 ° C. after application of the magnetic layer Measured according to the method for measuring the heat shrinkage rate of the film in (1) above.

As is clear from the above table, Examples 1, 2, and 3 in which both the surface temperature at the time of cutting the film and the film tension on the tenter outlet side (between the cutting portion and the take-up roller) satisfy the requirements of the present invention are as follows. In both cases, the heat shrinkage rate before and after the application of the magnetic layer is a very small heat shrinkage rate of 0.02% / hour or less under a temperature atmosphere of 70 ° C, and the maximum thermal stress is 0.3 kg / hour.
It has a large strength of mm ² or more. On the other hand, in Comparative Example 1 in which the film surface temperature at the time of cutting was too high, the maximum thermal stress was small and strength was insufficient, and Comparative Example 2 and Comparative Example 4 in which the film surface temperature at the time of cutting was lower than the glass transition temperature.
In Comparative Example 3 in which the heat shrink rate before and after coating the magnetic layer was too high, the dimensional stability was poor, and the film tension on the exit side of the tenter was set too high, the heat shrink rate before and after applying the magnetic layer was both high. Both were excessive and had extremely poor dimensional stability.

(Effects of the Invention) In the present invention, when a biaxially oriented polyester film is produced by a tenter method, heat setting is performed in a tenter,
Immediately before the biaxially oriented polyester film separates from the clip of the tenter chain, the ear end is cut and thermally relaxed, and the surface temperature of the film at that time is set to Tg to Tg + 80 ° C., which is slightly higher than the glass transition temperature Tg. and from the take-up roller side tension of the cut is to set 0.48~1.90kg / mm ^2, particularly preferred biaxially oriented polyester film is obtained as a magnetic recording medium having excellent dimensional stability and strength Post-processing for reducing the heat shrinkage is not required even after the application of the magnetic layer.

[Brief description of the drawings]

The drawing is a plan view of a tenter suitable for use in the present invention. Fa: longitudinally stretched polyester film, Fb: biaxially oriented polyester film, Fc: biaxially oriented polyester film Fb main body after cutting the edge, A: preheating zone, B: horizontal stretching zone,
C: heat fixing zone, D: constant temperature zone, 1: tenter chain, 2, 3: chain sprocket, 4: cutting blade, 5: take-up roller.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masashi Oki 10-24 Toyocho, Tsuruga-shi, Fukui Prefecture Toyobo Co., Ltd. No.Toyobo Co., Ltd.Tsuruga Branch Chamber of Research Institute Co., Ltd. (72) Inventor Katsuro Kuze 10-24 Toyocho, Tsuruga City, Fukui Prefecture JP, A) JP-B-57-54290 (JP, B2) JP-B 4-57181 (JP, B2)

Claims (1)

(57) [Claims] 1. A method for producing a biaxially oriented polyester film, wherein the polyester film biaxially oriented by a tenter method is heat-set in a heat fixing zone of the tenter, and then separated from a tenter clip and taken out by a take-off roller. The surface temperature of the biaxially oriented polyester film gripped by the tenter clip on the exit side of the rail is Tg to Tg + 80
° C (where Tg is the glass transition temperature), cut the left and right edge portions of the biaxially oriented polyester film, and set the film tension between the cut portion and the take-off roller to 0.48 to 1.90. A method for producing a biaxially oriented polyester film for a magnetic recording medium, wherein the biaxially oriented polyester film is set to kg / mm ² and the above biaxially oriented polyester film is taken off.