KR19990049686A - Manufacturing method of biaxially oriented polyester film - Google Patents

Abstract

1. TECHNICAL FIELD OF THE INVENTION

The present invention relates to a biaxially oriented polyester film method which is excellent in uniformity in thickness and capable of preventing breakage failure.

2. The technical problem to be solved by the invention

The present invention stretches the polyester film through the multi-step longitudinal stretching, transverse stretching, and re-stretching process, but at the time of longitudinal stretching, the stretching temperature, the stretching ratio and the crystallization energy of the film are controlled to have excellent thickness uniformity and subsequent transverse stretching process. It is an object of the present invention to provide a method for producing a biaxially oriented polyester film to prevent the risk of width shrinkage and non-uniform thickness of both ends of the film.

3. Summary of Solution to Invention

The present invention for achieving the above object is "a method for producing a biaxially oriented polyester film comprising multi-end stretching, transverse stretching, and graded stretching, the third longitudinal stretching of 3.5 times or more the total longitudinal stretching ratio Parallel stretching between nip rolls is carried out in the longitudinal drawing step, so that the crystallization energy of the first longitudinal drawing sheet is 20 J / g or more, the crystallization energy of the second longitudinal drawing sheet is 15 J / g or more, and the stretching temperature is 110 to After the third stage longitudinal stretching at a draw ratio of 1.5 to 2.5 times at 140 ° C, and after transverse stretching and heat setting, a method for producing a biaxially oriented polyester film of the "extended drawing at 1.2 times or more".

4. Important uses of the invention

The present invention has a use such as a base film of a magnetic recording medium.

Description

Manufacturing method of biaxially oriented polyester film

The present invention is directed to a method for producing a biaxially oriented polyester film, and more particularly, to a biaxially oriented polyester film method which is excellent in thickness uniformity and can prevent breakage failure.

Polyester film has excellent mechanical strength, heat resistance, electrical insulation, and chemical resistance, and its use has been increased for magnetic recording media, food packaging, and electrical insulators.

In particular, the polyester film has excellent mechanical strength and heat resistance, and thus has high utility value as a base film in manufacturing tapes for magnetic recording media such as video tapes, audio tapes, and computer tapes.

In order to meet the technical level of the current magnetic recording and reproducing apparatus which requires high magnetic recording density and high smoothness in the production of such a polyester film, the thickness uniformity and mechanical strength of the polyester film are most important. Of course, it must be considered.

In Japanese Patent Application No. 30-5639, a conventional method for producing a polyester film, polyester monomers are polymerized under a predetermined reaction condition such as pressure, temperature, and catalyst to obtain a polyester polymer intermediate product (chip or granule), After drying the content to a certain level or less, and then melt-extruded in an extruder to obtain an unstretched sheet in the die, the method of producing a biaxially stretched polyester film that stretches the unstretched suit in the longitudinal direction and stretches laterally again There is a bar.

In addition, Japanese Patent Application Laid-Open No. 54-8672 discloses a method of improving the strength and thickness uniformity of a film by performing uniaxial or biaxial re-stretch again after performing the biaxial stretching process as described above.

On the other hand, it is well known that the longitudinal stretching method of first stretching an unstretched sheet in the stretching process significantly affects the thickness uniformity of the final biaxially oriented film.

However, in the conventional drawing process, the longitudinal drawing sheet could not be stretched at a high enough magnification so that the thickness of the longitudinal drawing sheet was uniform, and even if excessive high magnification longitudinal drawing was made, the film was contracted by excessive orientation crystallization, resulting in an increase in the width shrinkage of the film. Not only the thickness is poor, but also the breaking and non-uniform stretching occurs in the lateral stretching process has a limit in producing a polyester film of a good thickness.

In this regard, Japanese Patent Laid-Open Nos. 48-43772, 50-75, 50-139872, 49-42277, 54-56674, 58-78729, 58-16023, Nos. 60-61233 and 57-18377, 57-49377, and 59-36851 provide a method for producing a polyester film comprising a longitudinal drawing process in multiple stages. The repetition of the cooling and heating processes in the middle of the stretching step not only reduces the thickness uniformity but also implies the inefficiency of the longitudinal stretching device.

In addition, the technique known by U.S. Pat.Nos. 4370291, 4497865 and Japanese Patent Laid-Open No. 58-118220 also has a longitudinal multistage drawing method as a main component, which is the birefringence of each longitudinal drawing step. It is not sufficient to realize the amorphous high magnification longitudinal stretching, rapid cooling and film surface defects, which are the core technologies of longitudinal multi-stage stretching by not only limiting and limiting the stretching temperature and stretching ratio, but also performing diagonal stretching in the final longitudinal stretching stage. There was one disadvantage.

The present invention is to solve the drawbacks of the prior art described above is to stretch the polyester film through a multi-stage longitudinal stretching, transverse stretching or longitudinal stretching process to adjust the stretching temperature, the stretching ratio and the crystallization energy of the film during longitudinal stretching It is possible to stretch at high magnification while suppressing the orientation determination, so that the thickness uniformity is excellent and the risk of width shrinkage and thickness non-uniformity at both ends of the film in the subsequent transverse stretching process can be prevented. It is an object of the present invention to provide a method for producing a biaxially oriented polyester film having a large improvement in mechanical properties by adjusting the draw ratio.

1 is a three-stage longitudinal drawing device showing an embodiment of the present invention

Figure 2 is a three-stage longitudinal drawing device showing another embodiment of the present invention

3 is a three-stage longitudinal drawing apparatus of the comparative example.

In order to achieve the above object, the present invention provides a method for producing a biaxially oriented polyester film including multi-end longitudinal stretching, transverse stretching and longitudinal stretching, wherein the total longitudinal stretching ratio is 3.5 times or more, and the third longitudinal stretching is performed. Parallel stretching between nip rolls is carried out in the step so that the crystallization energy of the first longitudinal drawing sheet is 20 J / g or more, the crystallization energy of the second longitudinal drawing sheet is 15 J / g or more, and the stretching temperature is 110 to 140 ° C. It provides a method for producing a biaxially oriented polyester film of the method of longitudinal stretching at a ratio of 1.5 to 2.5 times the stretch ratio, then undergoes transverse stretching and heat setting, and then stretched to a stretch ratio of 1.2 times or more.

The present invention also provides a method for producing a biaxially oriented polyester film in a method of transverse stretching at a stretching ratio of 3.5 times to 4.5 times at the time of transverse stretching of the polyester film that has been finished in the longitudinal stretching process.

According to the method of the present invention, the thickness uniformity of the longitudinal drawing sheet is improved by high magnification stretching of 3.5 times or more the total longitudinal drawing ratio, and the longitudinal stretching of the film surface is prevented by parallel stretching between the nip rolls in the final longitudinal drawing step. The third stage longitudinal stretching may be performed at a stretching ratio of 1.5 to 2.5 times at a stretching temperature of 110 to 140 ° C., so that the crystallization energy of the longitudinal drawing sheets of the first and second stages is 20 J / g and 15 J / g or more, respectively. This reduces the shrinkage of the longitudinal stretch sheet and promotes less orientation determination, and reduces the breakage and nonuniformity in the subsequent transverse stretching process, and when it is renewed at a draw ratio of more than 1.2 times, the mechanical properties such as the tensile strength in the longitudinal direction are reduced. Physical properties are improved.

In addition, when the stretching ratio in the stretching ratio in the range of 3.5 to 4.5 times during lateral stretching, the fracture and non-uniform stretching in the lateral stretching process after longitudinal stretching is further reduced, and ultimately the thickness of the polyester film is uniform in the width direction and the longitudinal direction.

The term "parallel stretching between nip rolls" as used herein means stretching by a speed difference between two stretching rolls between nip rolls positioned on the vertices of two stretching rolls having the same rotation direction, and "crystallization energy of longitudinal drawing sheet". It means that the energy generated during the crystallization process is above a certain level, and the large crystallization energy implies that there are few orientation crystals in the longitudinally stretched sheet. The stretching temperature of the third stage refers to the surface temperature of the third stage stretching roll.

In the present invention, the first stage and the second stage longitudinal drawing so that the crystallization energy of the first stage and the second stage longitudinal drawing sheet are 20 J / g and 15 J / g or more, respectively, may be obtained by a combination of the draw temperatures in each step and the draw ratio in each step. And is not limited to any specific stretching temperature and stretching ratio in each stage of the first stage and the second stage longitudinal stretching.

In the longitudinal drawing process, the circumferential speed ratio of each drawing stage determines the longitudinal drawing ratio of each stage.The product of the longitudinal drawing ratio of each stage is defined as the total longitudinal drawing ratio.If the total longitudinal drawing ratio is less than 3.5 times, the thickness uniformity of the longitudinal drawing sheet is insufficient. Can not do it.

In addition, if the parallel stretching between the nip rolls is not performed in the final longitudinal stretching step, the breaking of the stretching stress is poor, resulting in longitudinal scratches of the sheet surface due to the speed difference between the stretching roll surface and the sheet surface, and the first stage longitudinal stretching. If the crystallization energy of the sheet is less than 20 J / g, since the orientation crystals in the first stage longitudinal stretching process are large, the longitudinal shrinkage of the longitudinally stretched sheet becomes large, and if the crystallization energy of the second stage longitudinally stretched sheet is 15 J / g or less, Even if the crystallization energy of the end-stretch sheet is 20J / g or more, many orientation crystals have occurred in the second-end longitudinal drawing process, so that the width shrinkage of the second- and third-end longitudinal drawing sheets increases, so that breakage and non-uniform stretching in the subsequent transverse drawing process This is accompanied by.

In addition, the specific gravity of the first stage and the second stage longitudinal drawing sheet is 1.345 or less and 1.355 or less, respectively, when larger than this, there is a problem that the draw failure and fracture occurs in the subsequent third stage longitudinal stretching process and the lateral stretching process.

On the other hand, if the longitudinal drawing conditions of the three stages are not satisfied, there is a problem that the total longitudinal draw ratio of 3.5 times and the crystallization energy of the specified range of the first and second stages cannot be reached. When the stretching temperature of 110 to 140 ° C. and the stretching ratio of 1.5 to 2.5 times are not satisfied, not only does the shrinkage of the longitudinal drawing sheet increase but also the thickness of the longitudinal drawing sheet becomes poor, resulting in breakage and uneven stretching in the transverse stretching process. .

In addition, when the longitudinal stretching of the biaxially oriented polyester film subjected to the longitudinal stretching and transverse stretching process is less than 1.2 times the draw ratio there is a problem that the mechanical properties such as the desired longitudinal tensile strength is not sufficiently achieved.

At the same time, when the stretching ratio during the longitudinal stretching after the longitudinal stretching is less than 3.5 times, the thickness uniformity of the film may be lowered. When the stretching ratio is larger than 4.5 times, the breakage occurs frequently, resulting in deterioration of the film forming productivity.

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

1 is a longitudinal drawing device used for description of the embodiment, but the present invention is not limited thereto. Therefore, as an example in which the present invention can be implemented, there is a schematic diagram of the three-end drawing apparatus of FIG.

The rolls 1-3 of FIG. 1 are the 1st preheating roll, the 4th roll is the 1st draw roll, the 5th roll is the 2nd preheat roll, the 6-7 rolls are the 2nd and 3rd draw rolls, and the 8th roll is the cooling roll 4 ′, 5 ′, 6 ′, 7 ′, 8 ′ are nip rolls.

In drawing a polyester film by the longitudinal drawing apparatus comprised in this way, in FIG. 1, 1st longitudinal drawing is performed by the circumferential speed difference of a roll between the 4th and 5th rolls of polyester unstretched sheet F, Two-stage longitudinal stretching is carried out between rolls 6 and 7, followed by three-stage longitudinal stretching between rolls 7 and 8, followed by rapid cooling between rolls 8 and 9 to form the longitudinal stretching sheet (F '). Manufacture.

Also, in Fig. 2, rolls 11-14 are preheating rolls, rolls 15, 16 and 17 are one-stage, two-stage and three-stage stretching rolls in turn, roll 18 is a cooling roll, and 15 'and 16' Rolls 17 'and 18' are nip rolls.

In the case of using the apparatus shown in FIG. 2, the polyester unstretched sheet (F) is preheated by passing through 11 and 14 rolls, and then between 15 and 16, between 16 and 17 rolls, and 17 The first stage longitudinal stretching, the second stage longitudinal stretching and the third stage longitudinal stretching are performed by the circumferential speed difference between the rolls and the 18th rolls, and then rapidly cooled between the 8th and 9th rolls to obtain the longitudinal drawing sheet (F '). To prepare.

The biaxially oriented polyester film subjected to the longitudinal stretching process may be laterally stretched and longitudinally stretched at the above-described stretching ratio to prepare the biaxially oriented polyester film of the present invention.

FIG. 3 is a schematic view of the comparative example, which is the same as the roll configuration of FIG. 1, but the roll rotation direction after roll 8 is opposite to that of FIG. 1, and diagonally diagonally crosses between rolls 7 and 8 in a tangential direction. .

Hereinafter, examples and comparative examples of the present invention will be described based on the drawings described above. However, the following examples are provided only to assist in understanding the present invention, and the present invention is not limited thereto.

(Example 1)

A polyester chip with an intrinsic viscosity of 0.63 dl / g was melt-extruded through a die at a sheet forming speed of 60 m / min at a temperature of 280 ° C. to obtain an unstretched sheet, and then roll temperatures 1-3 in the longitudinal drawing apparatus of FIG. 100 ° C, roll number 4 at 110 ° C, roll number 5 at 110 ° C, rolls 6-7 at 120 ° C, roll 8 at 20 ° C, and the unstretched sheet at roll 4 2.0 times first stage longitudinal stretching between rolls 5 and 5, 1.5 times second stage longitudinal stretching between rolls 6 and 7, and 1.5 times 3rd stage longitudinal stretching between rolls 7 and 8 After stretching and heat-setting the stretched sheet by a conventional method, a stretch ratio of 1.5 times the grade was obtained to obtain a biaxially oriented polyester film having a thickness of 14 μm. The crystallization energy, specific gravity, The number of breaks, thickness uniformity, and tensile strength were measured and shown in Table 1 below.

Experimental Example

Crystallization Energy

The crystallization energy of the longitudinally stretched sheet was measured at a temperature increase rate of 20 ° C./min using a differential calorimeter (DSC, manufactured by Perkin-Elmer).

- importance -

Specific gravity of longitudinally stretched sheets was measured according to ASTM D1505 using a density gradient tube.

-Number of breaks-

The number of breaks that occurred during the 72 hours of the transverse stretching step of the polyester film was measured.

-Thickness uniformity-

The thickness uniformity of the film was measured by thickness measurement at an interval of 20 mm using a thickness gauge (manufactured by Annitsu Co., Ltd.), and the difference between the maximum value and the minimum value was expressed as thickness deviation.

- The tensile strength -

The tensile strength of the film was measured by the method defined in ASTM D882.

(Example 2)

In Example 1, the sheet forming speed was 55.6 m / min, the 4-5 roll temperature was 115 deg. C in the longitudinal drawing apparatus of FIG. 1, the first longitudinal drawing ratio was 1.5 times and the second longitudinal drawing ratio was It carried out similarly to Example 1 except having set 1.8 times and 3rd stage longitudinal stretch ratio to 1.8 times.

(Example 3)

In Example 1, the sheet forming speed was 50 m / min, in the longitudinal drawing apparatus of FIG. 1, the roll temperature of 4-5 was 115 ° C, the roll temperature of 7 was 130 ° C, and the first longitudinal draw ratio was 1.5. The same procedure as in Example 1 was carried out except that the vessel, the second stage longitudinal draw ratio was 1.8 times, and the third stage longitudinal draw ratio was 2.0 times.

(Comparative Example 1)

In Example 1, in the longitudinal drawing apparatus of FIG. 1, the roll temperature of 4-5 is 100 ° C, the roll temperature of 6 is 110 ° C, and the first stage longitudinal draw ratio is 4.5 times between rolls 7 and 8. Was carried out in the same manner as in Example 1.

(Comparative Example 2)

In Example 1, in the longitudinal drawing apparatus of FIG. 1, the roll temperature of 4-5 is set to 100 ° C and the roll temperature of 6-7 to 110 ° C, between rolls 6 and 7, and between rolls 7 and 8, respectively. The same procedure was followed as in Example 1 except that the first stage longitudinal draw ratio was 3.0 times and the second stage longitudinal draw ratio was 1.5 times.

(Comparative Example 3)

In Example 1, the sheet forming speed was 82.3 m / min, the roll stretching apparatus of FIG. 1 was 4-5 roll temperature at 100 ° C, 6 roll temperature was 105 ° C, 7 roll temperature was 110 ° C, The same procedure was followed as in Example 1 except that the first stage longitudinal draw ratio was 1.3 times, the second stage longitudinal draw ratio 1.4 times, and the third stage longitudinal draw ratio 1.8 times.

(Comparative Example 4)

In Example 1, in the longitudinal drawing apparatus of FIG. 1, the fourth roll temperature is 100 ° C, the sixth roll temperature is 110 ° C, the seventh roll temperature is 115 ° C, the first stage longitudinal draw ratio is 2.0 times, and the second stage longitudinal edge. It carried out similarly to Example 1 except having set to 1.5 times the mystery and 1.5 times the 3rd stage longitudinal stretch ratio.

(Comparative Example 5)

In Example 1, sheet forming speed was 50 m / min, and it carried out similarly to Example 1 except having made longitudinal drawing ratio 1.5 time of FIG. 1, 2nd longitudinal drawing ratio 1.2 times, and 3rd longitudinal drawing ratio 3.0 times. .

(Comparative Example 6)

In Example 1, the sheet forming speed was 50 m / min, the roll temperature of No. 6 was 120 ° C. and the roll temperature No. 7 was 120 ° C. in the longitudinal drawing apparatus of FIG. The same procedure as in Example 1 was repeated except that the longitudinal draw ratio was 1.5 times and the third stage longitudinal draw ratio was 2.4 times.

(Comparative Example 7)

In Example 1, the sheet forming speed was 50 m / min, the roll temperature of No. 7 was set to 120 ° C., the roll temperature of No. 8 was set to 150 ° C., and the first stage longitudinal draw ratio was 1.5 times. The same procedure as in Example 1 was conducted except that the second stage longitudinal draw ratio was 1.5 times and the third stage longitudinal draw ratio was 2.4 times.

(Comparative Example 8)

The sheet forming speed is 50 m / min in Example 1, except that diagonal longitudinal stretching of the third stage longitudinal stretching between rolls 7 and 8 is tangentially staggered in the longitudinal stretching apparatus of FIG. It carried out similarly to Example 1.

(Comparative Example 9)

In Example 1, the sheet forming speed was 81.8 m / min, and the drawing extension ratio was 1.1 times the same as that in Example 1 except that the drawing was stretched.

TABLE 1

※ In Table 1, tensile strength means longitudinal tensile strength.

As shown in Table 1, the polyester films of Examples 1 to 3 of the present invention have better thickness uniformity and longitudinal tensile strength than the polyester films of Comparative Examples 1 to 8, which are not based on the production method of the present invention. In addition, it can be seen that the productivity is good because the number of breaks during the transverse stretching process is small.

(Example 4)

In the same manner as in Example 1, the longitudinally stretched polyester sheet was transversely stretched at a draw ratio of 4.0 times, and then heat-set by a conventional method, and stretched at a stretch ratio of 1.5 times, and the test determined in the above Experimental Example. Crystallization energy, specific gravity, number of breaks, thickness uniformity, and tensile strength were measured through the method, and are shown in Table 2 below.

(Example 5)

In Example 4, the sheet forming speed was 61.7 m / min, the 4-5 roll temperature was 115 deg. C in the longitudinal drawing apparatus of FIG. 1, the first longitudinal drawing ratio was 1.5 times and the second longitudinal drawing ratio was The same procedure as in Example 4 was conducted except that 1.8 times and the third stage longitudinal draw ratio were 1.8 times and the transverse draw ratio was 3.5 times.

(Example 6)

In Example 4, the sheet forming speed was 45.5 m / min, the roll stretching apparatuses of Fig. 1 were 115 deg. C and 115 roll temperature was made 130 deg. It carried out similarly to Example 4 except having made 1.5 times, 2nd stage longitudinal draw ratio 1.8 times, 3rd stage longitudinal draw ratio 2.0 times, and lateral stretch ratio 4.4 times.

(Comparative Example 9)

In Example 4, in the longitudinal drawing apparatus of FIG. 1, the roll temperature of 4-5 is 100 ° C, the roll temperature of 6 is 110 ° C, and the first stage longitudinal draw ratio is 4.5 times between the rolls 7 and 8. Was carried out in the same manner as in Example 4.

(Comparative Example 10)

In Example 4, in the longitudinal drawing apparatus of FIG. 1, the roll temperature of 4-5 is set to 100 ° C and the roll temperature of 6-7 to 110 ° C, between rolls 6 and 7, and between rolls 7 and 8, respectively. The same procedure as in Example 4 was conducted except that the first stage longitudinal draw ratio was 3.0 times and the second stage longitudinal draw ratio was 1.5 times.

(Comparative Example 11)

In Example 4, the sheet forming speed was 82 m / min, the roll stretching apparatus of FIG. 1 was 4-5 roll temperature at 100 ° C, 6 roll temperature was 105 ° C, 7 roll temperature was 110 ° C, and It carried out similarly to Example 4 except having set 1.3 times of 1st stage longitudinal drawing ratio, 1.4 times of 2nd stage longitudinal drawing ratio, and 1.8 times of 3rd stage longitudinal drawing ratio.

(Comparative Example 12)

In Example 4, in the longitudinal stretching apparatus of FIG. 1, the fourth roll temperature is 100 ° C, the sixth roll temperature is 110 ° C, the seventh roll temperature is 115 ° C, the first stage longitudinal draw ratio is 2.0 times, and the second stage longitudinal edge It carried out similarly to Example 4 except having set to 1.5 times the mystery and 1.5 times the 3rd stage longitudinal stretch ratio.

(Comparative Example 13)

In Example 4, the sheet forming speed was 50 m / min, the 6-7 roll temperature was 125 ° C in the longitudinal drawing apparatus of FIG. 1, the first stage longitudinal drawing ratio 2.5 times, the second stage longitudinal drawing ratio 1.2 times, The same procedure as in Example 4 was repeated except that the three-stage longitudinal draw ratio was 3.0 times.

(Comparative Example 14)

In Example 4, the sheet forming speed was 50 m / min, the roll temperature No. 7 was 100 ° C. in the longitudinal drawing apparatus of FIG. 1, the first stage longitudinal drawing ratio 1.5 times, the second stage longitudinal drawing ratio 1.5 times, and the third stage. The same procedure as in Example 4 was repeated except that the longitudinal draw ratio was 2.4 times.

(Comparative Example 15)

In Example 4, the sheet forming speed was 50 m / min, the roll temperature at No. 7 was 150 ° C. in the longitudinal drawing apparatus of FIG. 1, the first stage longitudinal drawing ratio 1.5 times, the second stage longitudinal drawing ratio 1.5 times, and the third stage. The same procedure as in Example 4 was repeated except that the longitudinal draw ratio was 2.4 times.

(Comparative Example 16)

In Example 4, it carried out similarly to Example 4 except having set sheet forming speed to 70.6 m / min, and lateral stretch ratio to 3.4 times.

(Comparative Example 17)

In Example 4, it carried out similarly to Example 4 except having set sheet forming speed to 51.6 m / min, and lateral stretch ratio to 4.7 times.

(Comparative Example 18)

In Example 4, it carried out similarly to Example 4 except having diagonally extended longitudinal stretch between roll 7 and roll 8 in the longitudinal drawing apparatus of FIG.

(Comparative Example 19)

In Example 4, it carried out similarly to Example 4 except having set sheet forming speed to 81.8 m / min, and grade extension ratio to 1.1 times.

TABLE 2

※ In Table 2, tensile strength means longitudinal tensile strength.

From the above Table 2, the polyester films of Examples 4 to 6 of the present invention have better thickness uniformity and longitudinal tensile strength than the polyester films of Comparative Examples 9 to 19, which are not based on the production method of the present invention, and also have transverse stretching. It can be seen that the productivity is good due to the low number of breaks during the process.

As described above, the present invention can be suitably used for the base film of video recording, audio recording, and computer magnetic recording media requiring high magnetic recording density, high smoothness, and high running stability. It is a useful invention to provide a method for producing a biaxially oriented polyester film.

Claims (3)

In the method for producing a biaxially oriented polyester film including multi-species stretching, transverse stretching, and graded stretching, a three-stage longitudinal stretching of 3.5 times or more is performed for the total longitudinal stretching ratio, and parallel stretching between nip rolls is performed in the third longitudinal stretching stage. The crystallization energy of the first stage longitudinal drawing sheet is 20 J / g or more, and the crystallization energy of the second stage longitudinal drawing sheet is 15 J / g or more, and the third stage species with the stretching ratio 1.5 to 2.5 times at the stretching temperature of 110 to 140 ° C. After drawing, after stretching and heat setting, the method of producing a biaxially oriented polyester film, characterized in that the grade is drawn at a draw ratio of 1.2 times or more. The method for producing a biaxially oriented polyester film according to claim 1, wherein the stretching ratio during the lateral stretching is 3.5 to 4.5 times. The method for producing a biaxially oriented polyester film according to claim 1 or 2, wherein a specific gravity of the longitudinally stretched sheet is 1.345 or less and a specific gravity of the second longitudinally stretched sheet is 1.355 or less.