KR100369441B1 - Manufacturing method of biaxially oriented polyester film - Google Patents

Abstract

A method for producing a biaxially oriented polyester film comprising melt extruding a polyester resin and longitudinally stretching an unstretched sheet into two or more stages of multistage, and then transversely stretching the polyester resin. In order to maintain amorphous high magnification longitudinal uniformity and uniformity of physical properties of the upper and lower surfaces, "the longitudinal stretching is performed at 3.5 times or more relative to the total stretching ratio in the stretching step, but the stretching is performed in the longitudinal stretching process of the final stretching step. It is intended to provide a method for producing a biaxially oriented polyester film, wherein the sheet on a roll is longitudinally stretched while heated with an infrared heater, and then transversely stretched.

Description

The method of the processing Bi-oriented polyster film

The present invention relates to a method for producing a biaxially oriented polyester film having excellent uniformity of thickness.

Polyester films are excellent in mechanical strength, heat resistance, electrical insulation, and chemical resistance, and their usage is increasing day by day for magnetic recording media, food packaging, and electrical insulators.

In particular, the polyester film is excellent in mechanical strength and heat resistance, and has a very high value as a base film in the manufacture of video tapes, audio tapes, floppy disks, and the like.

Therefore, in the technical level of the current magnetic recording and reproducing apparatus requiring high magnetic recording density and high smoothness, the uniformity of the thickness of the polyester film occupies a very important position.

In the conventional method for producing a polyester film, a polyester monomer is polymerized by a predetermined reaction such as a constant temperature, pressure, and catalyst to obtain an intermediate product made of a polymer such as a polyester chip or granule, and the moisture content is below a certain level. After drying until melted in an extruder to obtain an unstretched sheet from the extrusion die, a method of producing a polyester film through a biaxial stretching process of stretching in the longitudinal direction and transverse direction It is disclosed in Japanese Patent Publication No. 30-5639, and a method for improving the strength and thickness uniformity of a film by re-stretching again after the biaxial stretching process and stretching in the transverse direction is provided. It is known from Japanese Patent Laid-Open No. 54-5672.

However, the fact that the longitudinal stretching process of primary stretching of the unstretched sheet has a significant influence on the uniformity of the thickness of the final biaxially oriented film is publicly known in the art.

However, in the conventional drawing process, it was not possible to draw at a high magnification sufficiently to make the thickness of the longitudinal drawing sheet uniform in the longitudinal drawing process. In addition to the poor thickness, the breakage and non-uniform stretching occurs in the lateral stretching process has a limit in producing a polyester film of a uniform thickness.

Therefore, in view of not only the uniformity of the thickness of the biaxially oriented film, but also the stability of the lateral stretching step, a method of longitudinally stretching at the highest possible magnification while suppressing the orientation determination as much as possible has been desired.

Recently, as a method for securing film thickness uniformity as compared to the conventional stretching method, a method of stretching in a longitudinal direction in multiple stages has been disclosed. For example, Japanese Patent Application Laid-Open No. 48-43772, No. 50 No. 75, No. 50-139872, No. 49-42277, No. 54-56674, No. 58-78729, No. 58-160123, No. 60-61233, Japanese Patent Publication The method of extending | stretching in multiple directions in the longitudinal direction is disclosed by So 57-48377, So 57-49377, Sho 59-36851, etc.

However, the method disclosed in the above-described Japanese Patent Laid-Open Publications and Patent Publications not only decreases the uniformity of thickness by repeating the cooling process and the temperature raising process in the middle of the stepwise stretching process, but also improves the inefficiency of the longitudinal stretching device. There is a problem that results.

Further, the longitudinal multi-step stretching method as described in US Pat. Nos. 4,370,291, 4,497,865, and Japanese Patent Application Laid-open No. 58-118220 specifies the birefringence of each step or draw temperature and draw ratio. It is limited as an element, and it is difficult to maintain amorphous high magnification longitudinal stretching, which can be called a longitudinal multi-stage stretching technique, and maintain uniformity of physical properties of upper and lower surfaces.

The present invention aims to provide a method for producing a polyester film having a biaxial orientation that improves the uniformity of the thickness in manufacturing a polyester film to solve the above disadvantages of the prior art.

1 is a three-stage longitudinal stretch of a polyester film according to one embodiment of the present invention

Figure 2 is a two-stage longitudinal stretch of the polyester film according to another embodiment of the present invention

The present invention for achieving the above object is a method for producing a biaxially oriented polyester film comprising a melt-extruded polyester resin and a non-stretched sheet in two or more longitudinal stretching and transverse stretching step, the stretching step In the longitudinal stretching process, the longitudinal stretching is at least 3.5 times the total stretching ratio, but in the longitudinal stretching process of the final stretching stage, the longitudinal stretching is performed while the sheet on the stretching roll is heated with an infrared heater, and then the lateral stretching is performed. It can be achieved by providing a method for producing an oriented polyester film.

According to the method of the present invention as described above, the uniformity of the thickness of the longitudinal stretching sheet is improved by high magnification stretching of 3.5 times or more the total stretching ratio, and the infrared heater is installed on the longitudinal stretching roll of the final stage to emit from the infrared heater. Infrared rays can penetrate into the surface as well as the inside of the sheet to resolve the calorie imbalance between the roll contact surface and the air contact surface of the sheet.

In general, in the production of the film, longitudinal stretching is performed in a heated state, and the single stage stretching results in the inability to stretch at high magnification due to severe crystallization, so that at least two longitudinal stretching is performed. It includes a rapid cooling process for cooling the produced film after stretching, usually two-stage or three-stage longitudinal stretching.

At this time, the temperature of the film in each stretching step is made in the state of rising to the softening point, but is generated by the difference between the temperature of the surface in contact with the stretching roll that supplies the film inside and heat and the temperature of the surface in contact with the atmosphere Infrared heater, which is installed by installing infrared heater in the longitudinal drawing step to solve the unevenness of the heat by dissipating the unevenness of the heat, has a rated power in the range of 5 to 15 Kw and a surface temperature in the range of 500 ° C to 1,000 ° C. It is appropriate to have a distance of 10 mm to 30 mm from the surface of the stretching roll.

The film heated by the infrared heater is stable at 95 ° C to 140 ° C in the case of two-stage stretching, and the stretching ratio in the first stage, which is the previous stage, is 1.5 times to 3.0 times, which leads to less orientation determination, and in the case of three-stage stretching. Longitudinal stretching is performed 1.5 to 2.5 times in the stretching temperature range of 110 ° C to 140 ° C, so that the progression of the orientation crystal can be reduced, so that the shrinkage in the width direction of the longitudinal drawing sheet can be reduced.

In addition, when the crystallization energy is 10 J / g or more in the longitudinal drawing step, more effective results can be obtained. In the case of the two-stage drawing, it is preferable that the crystallization energy in the first step, which is the previous step, is 20 J / g, In the case of the third longitudinal drawing, it is more preferable to perform longitudinal stretching so that the crystallization energy of the longitudinal drawing sheet of the first stage is 20 J / g or more and the crystallization energy of the second longitudinal drawing sheet is 15 J / g or more. Do.

Lateral stretching of the stretching ratio of 3.5 times to 4.5 times reduces breakage and uneven stretching in the lateral stretching process, and ultimately the quality of the polyester film becomes uniform in the width direction, the longitudinal direction and the thickness direction.

In the present invention, the term "crystallization energy of longitudinally stretched sheet" means energy that is generated during crystallization, and the large crystallization energy implies that the orientation crystallization of longitudinally stretched sheet is small. "Refers to the surface temperature of the stretching roll.

In the case of the three-stage stretching in the present invention, the longitudinal stretching is performed at each stage so that the crystallization energy of the first-stretch longitudinal sheet is 20 J / g or more, and the crystallization energy of the second-stage longitudinal stretch sheet is 15 J / g or more. It is performed by the combination of and draw ratio, and is not limited to the combination of specific draw temperature and specific draw ratio.

In the longitudinal drawing process, the roll speed ratio for each drawing step is the longitudinal drawing ratio for each step, and the product of the longitudinal drawing ratios for each step is defined as the total longitudinal drawing ratio.If the total longitudinal drawing ratio is 3.5 times or less, it is sufficient to ensure the uniformity of the thickness of the longitudinally drawn sheet. I can't.

If the crystallization energy of the longitudinal drawing sheet of the first stage is less than 20 J / g in any of the two longitudinal drawing and the three longitudinal drawing, the second stage and the third stage because the crystallization energy is generated in the first stage longitudinal drawing process. In the longitudinal drawing process, the shrinkage in the width direction is increased, and in the case of the third longitudinal drawing, when the crystallization energy of the second longitudinal drawing sheet is less than 15 J / g, the crystallization energy of the first longitudinal drawing sheet is 20 J / g. Even if it is more than g, since the orientation crystal | crystallization generate | occur | produces in the longitudinal stretch process of a 2nd stage | paragraph, the width direction shrinkage of a sheet | seat increases in the longitudinal stretch process of a 2nd stage and a 3rd stage.

In addition, when the transverse stretching ratio is 3.5 times or less in the transverse stretching process, it is difficult to ensure the uniformity of the thickness of the film to be produced. When longitudinal stretching is 4.5 times or more, the film breaks frequently during the transverse stretching process. Cause problems that occur.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

1 is a schematic diagram of a three-stage longitudinal drawing apparatus of a polyester film according to an embodiment of the present invention, wherein numerals 11, 12, and 13 are preheating rolls, and numeral 21 is a first edge. The new roll, 31 is the first stage cooling roll, numerals 41 and 42 are the second stage preheating rolls, the numeral 51 is the second stage stretching roll, and the numeral 61 is the third stage stretching roll, Reference numerals 71 and 72 denote the third stage cooling roll, reference numerals 21 ', 31', 51 ', 61', and 71 ', which are roll rolls, and reference numeral 80 denotes infrared rays. Means open.

When producing a biaxially oriented polyester film using the three-stage longitudinal drawing device having the above configuration, the unstretched polyester sheet (F) is used by using the preheat rolls (11), (12) and (13). After preheating, the first stage stretching roll 21 and the first stage cooling roll 31 are cooled by the first stage cooling roll 31 while performing the first stage longitudinal stretching by the main speed difference of the rolls. . The sheet longitudinally stretched first is preheated again in the second stage preheating rolls 41 and 42, and the second stage is stretched by the difference in the peripheral speed between the second stage stretching rolls 51 and the third stage stretching rolls 61. The longitudinal stretching is performed, and then the main speed difference between the third stage stretching roll 61 and the third stage cooling roll 71 under the heating condition by the infrared heater 80 provided in proximity to the third stage stretching roll 61. The third stage longitudinal stretching using to prepare a longitudinal stretch sheet (F ').

Figure 2 is a schematic diagram of a two-stage longitudinal drawing apparatus of a polyester film according to another embodiment of the present invention, the reference numerals used are as defined in FIG.

In the case of using the apparatus shown in FIG. 2 having the above configuration, the unstretched polyester sheet F is preheated by passing through the preheat rolls 11, 12 and 13, and the first end draw roll 21 and The first stage longitudinal stretching is performed between the first stage cooling rolls 31 by the circumferential speed difference, and is heated by an infrared heater 80 provided in proximity to the second stage stretching roll 61 which is the final longitudinal stretching. A longitudinal drawing sheet F 'is produced by performing two-stage longitudinal stretching between the two-stage stretching roll 61 and the second stage cooling roll 71.

Hereinafter, the production of the biaxially oriented polyester film using the apparatus shown in FIGS. 1 and 2 will be described in more detail with reference to Examples. However, the present invention is not limited to the following Examples.

Example 1

A polyester chip having an intrinsic viscosity of 0.63 dl / g was melt-extruded at a sheet forming speed of 60 m / min at 280 ° C. using an extruder to obtain an unstretched sheet, and a preheat roll was used in a longitudinal drawing apparatus as shown in FIG. 11), (12), (13) the temperature of 100 ℃, the temperature of the first stage draw rolls 21 120 ℃, the temperature of the first stage cooling roll 31 is 30 ℃ second stage preheat roll (41) Longitudinal stretching at a temperature of 42 ° C at 110 ° C, a temperature of the second step draw roll 51 and a third step draw roll 61 at 125 ° C, and a temperature of the third step cool rolls 71, 72 at 20 ° C. The unstretched sheet is supplied to the apparatus, the first stage longitudinal stretching is performed by 2.0 times between the first stage stretching roll 21 and the first stage cooling roll 31, and the second stage stretching roll 51 and the third stage stretching roll ( 61) The second stage longitudinal stretch 1.5 times between 61, the inclined direction 45 ° from the vertical direction of the third stage stretching roll (61) and installed 20mm apart from the third stage stretching roll (61) 10 ㎾, heated by an infrared heater with a surface temperature of 800 After the third longitudinal stretching is performed 1.5 times between the third stage stretching rolls 61 and the third stage cooling rolls 71, the longitudinally stretched sheet is transversely stretched at 4.0 times, and heat-set in the usual manner, and the thickness is 14 µm. A phosphorus biaxially oriented polyester film was obtained, and the crystallization energy, the number of breaks, the uniformity of thickness, and the uniformity of physical properties in the thickness direction were measured and shown as Table 2 below.

Example 2

Using the same apparatus and method as in Example 1, the sheet forming speed was 61.7 m / min, the temperature of the first end drawing roll 21 was 115 ° C., the first end drawing ratio was 1.5 times, and the second end type was finished. A biaxially oriented polyester film was obtained with a draw ratio of 1.8 times, a third end drawing ratio of 1.8 times, and a lateral draw ratio of 3.6 times, and the crystallization energy, the number of breaks, the uniformity of thickness, and the uniformity of physical properties in the thickness direction were measured. In addition to Example 1 it is shown as Table 1 and Table 2 below.

Example 3

Using the same apparatus and method as in Example 1, the sheet forming speed was 45.5 m / min, the temperature of the first single stretching roll 21 was 115 deg. C, and the temperature of the third single stretching roll 61 was 130 deg. The biaxially oriented polyester film was obtained by 1.5 times the first end draw ratio, 1.8 times the second end draw ratio, 1.8 times the third end draw ratio, and 2.0 times the transverse draw ratio, and the crystallization energy, the number of breaks, and the uniformity of the thickness were obtained. The uniformity of the physical properties in the properties and thickness direction was measured and shown in Table 1 and Table 2 together with Example 1.

Example 4

A polyester chip having an intrinsic viscosity of 0.63 dl / g was melt extruded through a die at a sheet forming rate of 70 m / min at 280 ° C. to obtain an unstretched sheet, and using a device as shown in FIG. 11) The temperature of 12 is 100 ° C., the temperature of the preheat roll 13 is 120 ° C., and the temperature of the first step stretching roll 21 and the second step stretching roll 61 is 125 ° C., and the cooling roll 71 is used. The temperature of 72 is set to 20 ° C., and the first stage stretching roll 21 and the cooling roll 31 are subjected to 2.0 times of one stage longitudinal stretching, and the second stage stretching roll 61 is inclined in the direction of 45 degrees from the vertical direction. Between the second stage drawing roll 61 and the cooling roll 71 under an infrared heater with a rated power of 10 ㎾ and a surface temperature of 800 ° C., spaced 20 mm from the surface of the second stage drawing roll 61. After the second stage longitudinal drawing, a biaxially oriented polyester film having a thickness of 14 µm is obtained by lateral stretching and heat setting in a conventional manner, and the crystallization energy, the number of breaks, the uniformity of thickness, The uniformity of the physical properties in the thickness direction was measured and shown in Table 1 and Table 2 together with Example 1.

Example 5

Using the same apparatus and method as in Example 4, the sheet forming speed was 66.7 m / min, the temperature of the first end drawing roll 21 was 120 ° C., the first end drawing ratio was 1.8 times, and the second end type was finished. A biaxially oriented polyester film was obtained with the draw ratio 2.1 times, and the crystallization energy, the number of breaks, the uniformity of thickness, and the uniformity of physical properties in the thickness direction were measured, and the results are shown in Table 1 and Table 2 together with Example 1. .

Example 6

Using the same apparatus and method as in Example 4, the sheet forming speed was 52.5 m / min, the temperature of the second end drawing roll 61 was 120 ° C., the first end drawing ratio was 2.0 times, and the second end type was finished. A biaxially oriented polyester film was obtained with the draw ratio 2.4 times, and the crystallization energy, the number of breaks, the uniformity of thickness, and the uniformity of physical properties in the thickness direction were measured, and the results are shown in Table 1 and Table 2 together with Example 1. .

Comparative Example 1

Using the same apparatus and method as in Example 1, the temperature of the first stage drawing rolls 21 and the second stage preheating rolls 41 and 42 is set to 100 ° C, and the temperature of the second stage drawing rolls 51 is set to 110 ° C. The temperature of the third stage cooling rolls 71 and 72 is set to 20 ° C., and the draw ratio of the second stage stretching process is set to 4.5 times to obtain a biaxially oriented polyester film, and the crystallization energy, the number of breaks, and the thickness are uniform. The uniformity of the physical properties in the properties and thickness direction was measured and shown in Table 1 and Table 2 together with Example 1.

Comparative Example 2

Using the same apparatus and method as in Example 1, the temperature of the first stage drawing rolls 21 and the second stage preheating rolls 41 and 42 is set to 100 ° C, and the temperature of the second stage drawing rolls 51 is set to 110 ° C. The temperature of the third stage cooling rolls 71 and 72 is set to 20 ° C., the draw ratio in the second stage drawing process is 3.0 times, and the draw ratio in the third stage drawing process is set to 1.5 times. An ester film was obtained, and the crystallization energy, the number of breaks, the uniformity of thickness, and the uniformity of physical properties in the thickness direction were measured, and the results are shown in Table 1 and Table 2 together with Example 1.

Comparative Example 3

Using the same apparatus and method as Example 1, the sheet forming speed was 82.3 m / min, the temperature of the first single drawing roll 21 was 100 ° C., and the temperature of the second single drawing roll 51 was 105 ° C. The temperature of the third end drawing roll 61 is 110 ° C, the first end drawing ratio is 1.3 times, the second end drawing ratio is 1.4 times, and the third end drawing ratio is set to 1.8 times to obtain a biaxially oriented polyester film. The crystallization energy, the number of breaks, the uniformity of the thickness, and the uniformity of the physical properties in the thickness direction were measured and shown in Table 1 and Table 2 together with Example 1.

Comparative Example 4

Using the same apparatus and method as in Example 1, the temperature of the first single draw roll 21 is 100 ° C., the temperature of the second single draw roll 51 is 110 ° C., and the temperature of the third single draw roll 61 is adjusted. 115 ° C., the first end draw ratio is 2.0 times, the second end draw ratio is 1.5 times, and the third end draw ratio is set to 1.5 times to obtain a biaxially oriented polyester film, the crystallization energy, the number of breaks, the uniformity of thickness, The uniformity of the physical properties in the thickness direction was measured and shown in Table 1 and Table 2 together with Example 1.

Comparative Example 5

Using the same apparatus and method as in Example 1, the sheet forming speed was 50 m / min, the temperature of the second end drawing roll 51 and the third end drawing roll 61 was 125 ° C, and the first end drawing ratio was 1.5 times, the second end draw ratio is 1.2 times, the third end draw ratio is set to 3.0 times to obtain a biaxially oriented polyester film, by measuring the crystallization energy, the number of breaks, the uniformity of thickness, the uniformity of the physical properties in the thickness direction In addition to Example 1 it is shown as Table 1 and Table 2 below.

Comparative Example 6

Using the same apparatus and method as in Example 1, the sheet forming speed was 50 m / min, the temperature of the second single drawing roll 51 was 120 ° C., and the temperature of the third single drawing roll 61 was 100 ° C. The biaxially oriented polyester film was obtained by setting the first end-to-end draw ratio 1.5 times, the second end-to-end draw ratio 1.5 times and the third end-to-end draw ratio 2.4 times. The crystallization energy, the number of breaks, the uniformity of thickness, and the physical properties in the thickness direction The uniformity of was measured and shown in Table 1 and Table 2 together with Example 1.

Comparative Example 7

Using the same apparatus and method as in Example 1, the sheet forming speed was 50 m / min, the temperature of the second single drawing roll 51 was 120 ° C., and the temperature of the third single drawing roll 61 was 150 ° C. The biaxially oriented polyester film is obtained by setting the first end draw ratio to 4.5 times, the second end draw ratio to 1.5 times, and the third end draw ratio to 2.4 times, and the crystallization energy, the number of breaks, the uniformity of thickness, and the physical properties in the thickness direction. The uniformity of was measured and shown in Table 1 and Table 2 together with Example 1.

Comparative Example 8

Using the same apparatus and method as in Example 1, a biaxially oriented polyester film was obtained with the infrared heater 80 removed from the third stage drawing roll 61, and the crystallization energy, the number of breaks, the uniformity of the thickness, and the thickness were obtained. Uniformity of the physical properties of the direction was measured and shown in Table 1 and Table 2 together with Example 1.

Comparative Example 9

Using the same apparatus and method as in Example 1, the sheet forming speed was 58.8 m / min and the transverse draw ratio was 3.4 times to obtain a biaxially oriented polyester film, and crystallization energy, the number of breaks, the uniformity of the thickness, and the thickness were obtained. Uniformity of the physical properties of the direction was measured and shown in Table 1 and Table 2 together with Example 1.

Comparative Example 10

Using the same apparatus and method as in Example 1, the biaxially oriented polyester film was obtained with the sheet forming speed of 42.6 m / min and the lateral draw ratio of 4.7 times, and the crystallization energy, the number of breaks, the uniformity of the thickness, and the thickness were obtained. Uniformity of the physical properties of the direction was measured and shown in Table 1 and Table 2 together with Example 1.

Comparative Example 11

Using the same apparatus and method as in Example 4, the temperature of the preheat rolls 11, 12 and 13 is 90 ° C, the temperature of the first end drawing roll 21 is 110 ° C, and the first end drawing ratio is 4.0. The double-axis orientation polyester film was obtained by doubling, and the crystallization energy, the number of breaks, the uniformity of thickness, and the uniformity of physical properties in the thickness direction were measured, and the results are shown in Table 1 and Table 2 together with Example 1.

Comparative Example 12

Using the same apparatus and method as in Example 4, the sheet forming speed was set to 77.8 m / min, and the temperature of the preheat rolls 11, 12, 13 was set at 100 ° C. A biaxially oriented polyester film was obtained by setting the temperature of the 105 ° C. and the second end drawing roll 61 to 110 ° C., the first end drawing ratio to 1.8 times, and the second end drawing ratio to 1.8 times to obtain a crystallization energy, the number of breaks, and the thickness. The uniformity of and the uniformity of physical properties in the thickness direction were measured and shown in Table 1 and Table 2 together with Example 1.

Comparative Example 13

Using the same apparatus and method as in Example 4, the sheet forming speed was 63 m / min, the temperature of the preheat rolls 11, 12, 13 was set to 90 ° C., and the temperature of the first single drawing roll 21 was 100. The biaxially oriented polyester film was obtained by setting the temperature of the second end drawing roll 61 to 110 ° C. and the first end drawing ratio to 2.0 times and the second end drawing ratio to 2.0 times to obtain a biaxially oriented polyester film. Uniformity and the uniformity of the physical properties in the thickness direction were measured and shown in Table 1 and Table 2 together with Example 1.

Comparative Example 14

Using the same apparatus and method as in Example 4, the sheet forming speed was 48 m / min, the preheat rolls 11, 12 and 13 were heated at 110 ° C., and the temperature of the first single drawing roll 21 was 120. The biaxially oriented polyester film was obtained by setting the first end-to-end draw ratio to 1.5 times and the second end-to-end draw ratio to 3.5 times, and the crystallization energy, the number of breaks, the uniformity of thickness, and the uniformity of physical properties in the thickness direction were measured. In addition to Example 1 it is shown as Table 1 and Table 2 below.

Comparative Example 15

Using the same apparatus and method as in Example 4, the sheet forming speed was 63 m / min, and a biaxially oriented polyester film was obtained without an infrared heater installed on the second single drawing roll 61, and crystallization was performed. The energy, the number of breaks, the uniformity of the thickness, and the uniformity of the physical properties in the thickness direction were measured and shown as Tables 1 and 2 below with Example 1.

Table 1

Elongation ratio Temperature of longitudinal drawing roll Crystallization Energy of the First End-stretched Sheet (J / g) Crystallization Energy of Second Ended Drawing Sheet (J / g) First stage 2nd stage 3rd stage Total extension First stage 2nd stage 3rd stage Example 1 2.0 1.5 1.5 4.5 120 125 125 21.3 15.5 Example 2 1.5 1.8 1.8 4.86 115 125 125 21.7 15.4 Example 3 1.5 1.8 2.0 5.4 115 125 130 21.7 15.3 Example 4 2.0 1.8 - 3.6 125 125 - 21.7 - Example 5 1.8 2.1 - 3.78 120 125 - 20.4 - Example 6 2.0 2.4 - 4.8 125 130 - 20.1 - Comparative Example 1 1.0 4.5 1.0 4.5 100 110 20 12.3 5.7 Comparative Example 2 1.0 3.0 1.5 4.5 100 110 20 14.0 9.0 Comparative Example 3 1.3 1.4 1.8 3.28 100 105 110 21.5 16.1 Comparative Example 4 2.0 1.5 1.5 4.5 100 110 115 13.7 7.6 Comparative Example 5 1.5 1.2 3.0 5.4 120 125 125 21.5 16.1 Comparative Example 6 1.5 1.5 2.4 5.4 120 120 100 20.5 15.7 Comparative Example 7 1.5 1.5 2.4 5.4 120 120 150 20.5 15.7 Comparative Example 8 2.0 1.5 1.5 4.5 120 125 125 21.3 15.5 Comparative Example 9 2.0 1.5 1.5 4.5 120 125 125 21.3 15.5 Comparative Example 10 2.0 1.5 1.5 4.5 120 125 125 21.3 15.3 Comparative Example 11 4.0 1.0 - 4.0 110 20 - 10.7 - Comparative Example 12 1.8 1.8 - 3.24 105 110 - 21.1 - Comparative Example 13 2.0 2.0 - 4.0 100 110 - 18.2 - Comparative Example 14 1.5 3.5 - 5.25 120 125 - 18.3 - Comparative Example 15 2.0 1.8 - 3.6 125 125 - 21.2 -

Table 2

Infrared heater Number of breaks Thickness deviation (㎛) Uniformity of physical properties Example 1 U One 0.2 Good Example 2 U One 0.2 Good Example 3 U One 0.2 Good Example 4 U One 0.3 Good Example 5 U One 0.3 Good Example 6 U One 0.3 Good Comparative Example 1 U 10 0.7 Good Comparative Example 2 U 5 0.5 Good Comparative Example 3 U One 0.4 Good Comparative Example 4 U 6 0.5 Good Comparative Example 5 U 5 0.6 Good Comparative Example 6 U 8 0.4 Good Comparative Example 7 U 8 0.6 Good Comparative Example 8 radish One 0.5 Bad Comparative Example 9 U One 0.5 Good Comparative Example 10 U 6 0.2 Good Comparative Example 11 U 8 0.8 Good Comparative Example 12 U One 0.6 Good Comparative Example 13 U 5 0.5 Good Comparative Example 14 U 7 0.6 Good Comparative Example 15 radish One 0.4 Bad

<Evaluation Method>

Crystallization Energy

Using a differential calorimeter (DSC, manufactured by Perkin-Elmer), the temperature was measured at a rate of 20 ° C./min.

Breaking frequency

The number of breaks during the course of operation for 72 hours in the transverse stretching process of the polyester film was measured.

Thickness uniformity

The thickness was measured at intervals of 20 mm in the lateral direction using a thickness gauge (manufactured by Annitsu Co., Ltd.), and the difference between the maximum value and the minimum value was taken as thickness deviation.

Uniformity of Properties

The case where the edge was curled by the shape in which the edge of the square film cut | disconnected so that a longitudinal direction and the width direction became 200 mm, respectively, was treated as defect.

The reaction conditions prepared in the same manner as in Example 1 to Example 6 and Comparative Example 1 and Comparative Example 15 and the results measured by the test method are the same as the contents of Table 1 and Table 2, In Examples 1 to 3, which are three-stage longitudinal stretching examples prepared by the method, and Examples 4 to 6, which are two-stage longitudinal stretching examples, the number of breaks, thickness deviation, and uniformity of physical properties in the thickness direction In addition to the excellent results compared to the results of Comparative Examples 1 to 15, Comparative Example 1, Comparative Example 6, Comparative Example 7, Comparative Example 11, and Comparative Example 13 were markedly outstanding. In the case of Comparative Example 8 and Comparative Example 15, in which the number of breaks was high, and the infrared heater was not installed, a phenomenon in which the edge was curled due to poor physical properties in the thickness direction occurred. In case of classroom example 5, comparative example 7, and comparative example 11 to comparative example 15, the thickness deviation is prominent, and in the case of the majority of the comparative examples, the deviation is compared with Examples 1 to 6, which are examples of the present invention. Was measured to fly a lot.

Therefore, the manufacturing method of the present invention is particularly suitable for the production of polyester films suitable as base films for electronic recording media such as video, audio, and computer floppy disks, which require high magnetic recording density, high smoothness and high driving stability. It is a suitable method.

Claims (9)

A method for producing a biaxially oriented polyester film comprising melt extruding a polyester resin and stretching the unstretched sheet in at least three stages of longitudinal stretching and transverse stretching, wherein the stretching step is performed at 3.5 times or more relative to the total stretching ratio in the stretching step. Stretching, but in the longitudinal stretching step of the final stretching step, longitudinal stretching is carried out while the sheet on the stretching roll is heated by an infrared heater, and then transverse stretching is carried out, characterized in that the biaxially oriented polyester film production method. The biaxially oriented polyester film according to claim 1, wherein the rated power of the infrared heater is in the range of 5 kW to 15 kW, and the surface temperature of the nuclear heater is 500 ° C to 1000 ° C. Manufacturing method. The method of manufacturing a biaxially oriented polyester film according to claim 1, wherein an interval between the infrared heater and the third longitudinal drawing roll is 10 mm to 30 mm. The method for producing a biaxially oriented polyester film according to any one of claims 1 to 3, wherein the multistage stretching is two-stage stretching. The method for producing a biaxially oriented polyester film according to claim 4, wherein the crystallization energy of the longitudinal drawing sheet is 20 J / g in the longitudinal drawing of the first step. The method for producing a biaxially oriented polyester film according to claim 5, wherein the stretching temperature heated by the infrared heater in the two stages of longitudinal stretching is 95 ° C to 140 ° C, and the draw ratio is 2.0 to 3.0 times. The method for producing a biaxially oriented polyester film according to any one of claims 1 to 3, wherein the multistage stretching is three-stage stretching. The biaxially oriented polyester according to claim 7, characterized in that the crystallization energy of the longitudinally stretched sheet is 20 J / g or more in the first stage longitudinal drawing and the crystallization energy of the longitudinally stretched sheet is 15 J / g in the second stage longitudinal drawing. Method for producing a film. The method for producing a biaxially oriented polyester film according to claim 8, wherein the stretching temperature heated by the infrared heater in the three stages of longitudinal stretching is 110 ° C to 140 ° C and the draw ratio is 1.5 to 2.5 times.