JPH11151751A - Biaxially oriented polyester film and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance thermal dimensional stability without lowering the modulus of elasticity and productivity by biaxially stretching an unstretched film based on polyethylene terephthalate and specified in the relaxing time of carbonyl carbon in structural analysis by solid high resolving power NMR to heat-treat the stretched film. SOLUTION: A molten extruded film comprising polyester based on polyethylene terephthalate is cooled on the surface of a cooling metal roll by a static electricity applying casting method to obtain an unstretched film. The melt extrusion temp. at this time is pref. higher than the m.p. of polyester by 25-60 deg.C. Herein, by biaxially stretching the unstretched film characterized by that the relaxing time of carbonyl carbon in structural analysis by solid high resolving power NMR is 55 msec or less, further pref., 45 msec or less and amorphous and high in amorphous motility is biaxially stretched and heat-treated to obtain a film very small in heat shrinkage factor.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a biaxially oriented polyester film, particularly to a polyester film having a high strength in the longitudinal direction and a small heat shrinkage, and a method for producing the same.

[0002]

2. Description of the Related Art Polyester films, particularly polyester films containing polyethylene terephthalate as a main component, have excellent mechanical properties, thermal properties, electrical properties, surface properties, optical properties, heat resistance, chemical resistance and the like. Utilizing its properties, it is widely used in various applications such as for magnetic recording media, capacitors, and packaging.

[0003] On the other hand, in accordance with the needs for high quality and compactness of these application products, the required characteristics of polyester films are becoming more and more severe.
Among the above applications, particularly for magnetic recording media,
Along with high quality, long-term recording and thinning of the base film due to compactness, further higher elastic modulus and higher strength are desired. In addition to higher recording density, higher thermal dimensional stability Is desired.

In general, a polyester film has a property of shrinking with temperature. As means for reducing the heat shrinkage of the polyester film, heat treatment at a high temperature, relaxation treatment after stretching, or in a state where the film is wound into a roll in an atmosphere of 110 ° C to 130 ° C for 6 hours The above-mentioned treatment method (for example, JP-A-60-1)
No. 03517) and a method in which a film is relatively loosely wound and heat-treated in a heating furnace as it is (for example, Japanese Unexamined Patent Publication No.
JP-A-98219) and a method of reducing the intrinsic viscosity of a film (JP-A-9-16952).

[0005]

However, the polyester film produced by the above-mentioned conventional technique has the following problems, for example.

When a heat treatment is performed at a high temperature or a relaxation treatment is performed after stretching, mechanical properties such as an elastic modulus are reduced accordingly. When the film is wound into a roll and left in an atmosphere at 110 to 130 ° C. for 6 hours or more, the surface of the roll and the core of the roll have uneven processing. As a result, not only the appearance of the roll film deteriorates, but also the heat treatment time is long, and the productivity is not good. In addition, it is difficult to heat the film by winding the film relatively loosely, and it is difficult to wind a wide and long film loosely.In addition, since an air layer exists between the films, heat conduction between the films is slow. In addition, it takes time to raise the temperature of the entire roll film. Further, the method using heat treatment after winding can reduce the heat shrinkage near the glass transition point of the polyester, but cannot reduce the heat shrinkage near 100 ° C. In addition, a film having a small intrinsic viscosity tends to be broken during stretching because of a decrease in tear resistance, and not only does productivity decrease,
In film processing steps such as cutting, coating of a magnetic layer on a magnetic recording medium, and printing on a packaging material, breakage is liable to occur, and workability is reduced.

An object of the present invention is to reduce the mechanical properties such as the elastic modulus, to require a long processing time, to install special equipment, and to reduce productivity and workability. An object of the present invention is to provide a biaxially oriented polyester film which is excellent in thermal dimensional stability and mechanical properties without any problem.

[0008]

The biaxially oriented polyester film for this purpose can be obtained by the following production method. That is, a biaxially oriented polyester film comprising polyethylene terephthalate as a main component, biaxially stretching an unstretched film having a carbonyl carbon relaxation time T1ρ of 55 msec or less in structural analysis by solid high-resolution NMR, and further heat-treating the unstretched film. It is a manufacturing method.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS That is, the relaxation time T1ρ of carbonyl carbon in the structural analysis by solid-state high-resolution NMR.
A film having an extremely small heat shrinkage ratio is obtained by biaxially stretching and heat-treating an unstretched, amorphous and highly motile unstretched film of 55 msec or less, preferably 50 msec or less, more preferably 45 msec or less. Can be obtained.

Generally, in order to obtain a biaxially oriented polyester film containing polyethylene terephthalate as a main component, a sheet extruded at a melting point or higher is cooled and solidified on a metal casting roll, and then biaxially stretched. , And a method of performing a heat treatment to manufacture.
As a method for increasing the cooling efficiency after extrusion, a method of applying an electrostatic charge to increase the adhesion to a casting roll is effective. The unstretched film after extrusion formed in this manner is generally in an amorphous state having a very small crystallinity, but a fine regular structure that did not completely melt when melting is left, or when extruded. Is caused by reorientation in the cooling process and the orientation due to the shearing force of the GaN. Therefore, the amorphous state is not completely obtained, and the relaxation time T1ρ of carbonyl carbon in the structural analysis by solid-state high-resolution NMR is 55 ms.
larger than c.

In order to release the ordered structure of the unstretched film, the melt-cast unstretched film is heated at a temperature 30 to 80 ° C. higher than the glass transition point measured by a differential scanning calorimeter (DSC). It is preferable to perform the treatment for 1 to 10 seconds. If the temperature of the heat treatment is too low or the treatment time is shorter than this, the ordered structure cannot be released.If the treatment temperature is too high or the treatment time is too long, crystallization starts. Carbon relaxation time T1ρ is 5
A film of 5 msec or less cannot be obtained, and a film having a small heat shrinkage cannot be obtained. A more preferred heat treatment temperature is a temperature 40 to 60 ° C. higher than the glass transition point, and a more preferred heat treatment time is 0.1 to 7 seconds.

[0012] After performing such a treatment, when performing biaxial stretching, crystallization occurs if held for a long time at or above the glass transition point, or even if crystallization does not occur, it may be measured in minute units. In some cases, ordering occurs, and the relaxation time T1ρ of carbonyl carbon of the unstretched film becomes longer than 55 msec. In order to avoid this, it is effective to perform biaxial stretching by cooling once to a temperature lower than the glass transition point by 5 ° C. or more before stretching and then heating again. The heat shrinkage reduction effect of the production method of the present invention is such that the intrinsic viscosity of the polyester used is 0.55 to 0.70 d.
1 / g, preferably in the range of 0.58 to 0.65;
The stretching ratio in the longitudinal direction is 4.5 times or more as a whole, preferably 5.0 times or more, and the elastic modulus in the longitudinal direction of the obtained film is 6.0 GPa or more, preferably 7.0 GPa.
The heat shrinkage in the longitudinal direction at 80 ° C. for 30 minutes is 0.15% or less, the heat shrinkage in 100 ° C. for 30 minutes is 0.5% or less, and the tear propagation resistance in the longitudinal direction is 5000 N. / M or more.

The polyester containing polyethylene terephthalate as a main component used in the present invention refers to a polyester containing ethylene terephthalate as a repeating unit in an amount of 90% by weight or more, and 10% by weight or less depending on the purpose of use. A polymerization component and an additive may be used, or a mixture with a small amount of various polymers may be used. In order to give the film a slipperiness, inert particles may be added in a range of 5.0% by weight or less.

The type of particles to be added is not particularly limited, but in order to achieve both good friction characteristics and abrasion resistance, substantially spherical silica particles derived from colloidal silica,
Spherical crosslinked polymer particles (such as crosslinked polystyrene and crosslinked divinylbenzene) are preferred. As a method of including particles in the polyester constituting the film, particles are dispersed in a predetermined ratio in the form of a slurry in ethylene glycol, which is a diol component, and the ethylene glycol is polymerized with a dicarboxylic acid component (mainly terephthalic acid). The method is preferred. When the particles are added, for example, if the water sol or alcohol sol obtained during the synthesis is added without drying, the particles have good dispersibility and can have good output characteristics. It is also effective to directly mix the water slurry of the particles with predetermined polyester pellets and knead the polyester with a vented twin-screw extruder. As a method of adjusting the content and the number of particles, a master of high-concentration particles is prepared by the above method,
It is effective to dilute it with a polyester substantially not contained in the particles at the time of film formation and adjust the content of the particles.

The intrinsic viscosity of the polyester used is from 0.55 to 0.70 dl / g, preferably from 0.58 to 0.70 dl / g, from the viewpoints of stability at the time of extrusion, stability in film formation such as tearing, and workability in a subsequent step. It is preferably in the range of 0.65.

The polyester film of the present invention may be a single-layer film or a laminated film of two or more layers. In addition, as long as the proportion of polyethylene terephthalate in the entire film is 90% by weight or more, the content of the particles, additives, and copolymer components added to the laminated portion may be increased, or the content other than polyethylene terephthalate may be increased. Polyester, for example polybutylene terephthalate, polypropylene terephthalate, polyethylene
2,6-naphthalate, polybutylene 2,6-naphthalate and the like may be used. In particular, when used as a magnetic tape, a laminated film having a two-layer or three-layer structure and separately designing the front and back surface morphologies can provide friction characteristics,
It is preferable to achieve both abrasion resistance and output characteristics as a magnetic tape.

Next, a method for producing the film of the present invention will be described. A melt extruded film made of a polyester containing polyethylene terephthalate as a main component is cooled on a cooling metal roll surface by using an electrostatic application casting method to obtain an unstretched film. The melt extrusion temperature at this time is preferably a temperature 25 to 60 ° C. higher than the melting point of the polyester. If the extrusion temperature is too low, a film having high amorphousness cannot be obtained, and if it is too high, a decomposition product is generated and a film having good physical properties cannot be obtained. In order to perform high-speed film formation, the temperature of the cooling roll is 5 to 50 ° C., preferably 10 to 3 ° C.
The temperature is preferably in the range of 0 ° C.

Since the unstretched film thus obtained is rapidly cooled from the molten state, it is generally an amorphous film having a very small crystallinity, and the crystallinity determined by the density method is generally low. Is 5% or less, but in the present invention, the crystallinity is reduced to 2% or less by lowering the temperature of the cooling roll and using a metal cooling roll having a smooth surface. Preferred for obtaining a film. In the unstretched film immediately after the extrusion, even if the crystallinity by the density method or the X-ray diffraction method is 0%, a fine ordered structure that did not completely melt at the time of melting remains, Since the orientation by the force and the re-ordering in the cooling process occur, the amorphous state is not completely obtained, and the relaxation time T1ρ of the carbonyl carbon in the structural analysis by solid-state high-resolution NMR is larger than 55 msec.

Next, the unstretched film is treated to release the ordered structure. As a method of releasing the ordered structure, there is a method of moving the molecular chain by using an organic solvent or irradiating electromagnetic waves, but it is very difficult to control the order until re-ordering occurs by crystallization. Therefore, a method of performing heat treatment at a temperature equal to or higher than the glass transition point is most preferable. As a method of heat-treating the unstretched film,
There are (1) a method of heating a film on a heating roll, (2) a method of using an infrared heater, and (3) a method of hot air treatment in an oven. The method of using a roll together is most preferable. Here, the unstretched film is in the same state as the film at the time of extrusion, or the stretch ratio is 1.5 times or less,
A film having a crystallinity of 10% or less. The temperature of the heat treatment is preferably 30 to 80 ° C higher than the glass transition point of the polyester, more preferably 40 to 60 ° C higher than the glass transition point, and the treatment time is 0.1 to 10 seconds, more preferably 0 to 10 seconds. A range of 0.1 to 7 seconds is preferred. If the temperature of the heat treatment is lower than this, or if the treatment time is short, the ordered structure cannot be released,
If the treatment temperature is too high or the treatment time is too long, crystallization starts conversely, and the carbonyl carbon relaxation time T1ρ
Cannot be obtained.

After performing such a treatment, when biaxial stretching is performed, crystallization may occur if the crystallization is maintained for a long time at or above the glass transition point. In some cases, ordering occurs, and the relaxation time T1ρ of carbonyl carbon of the unstretched film becomes longer than 55 msec. In order to avoid this, it is effective to perform biaxial stretching by cooling once to a temperature lower than the glass transition point by 5 ° C. or more before stretching and then heating again. As a cooling method, there are a cooling method using cold air and a method using a cooling roll, and a method using a cooling roll is preferable to obtain a good flat film.

Further, the unstretched film is biaxially stretched and heat-treated by a known method to obtain a biaxially oriented polyester film. As the stretching method, it is effective to use a longitudinal method first, and then a sequential biaxial stretching in which the film is stretched in the width direction. In order to obtain a film with a large elastic modulus in the longitudinal direction,
After stretching in the width direction, a method of further stretching in the longitudinal direction may be adopted.

The stretching in the longitudinal direction is performed at a temperature higher than the glass transition temperature Tg of the polyester by 10 ° C. or more and lower than the heat treatment temperature before stretching by using a heating roll at 5000 to 5 ° C.
It is effective to carry out the stretching at a stretching rate of 0000% / min at once or in several times in a range of 4.5 to 7.0 times, preferably 5.0 to 6.5 times. When stretching in the longitudinal direction after stretching in the width direction, the stretching temperature in the longitudinal direction in the previous step is performed at a temperature higher by 10 to 40 ° C. than the glass transition point of the polyester. It is preferable that the heating is performed at a temperature 30 to 90 ° C. higher than the point and in a range of 1.1 to 2.0 times so that the overall longitudinal magnification is within the above-described preferable range. The stretching in the width direction is
Using a known tenter, at a temperature 5 to 40 ° C. higher than the glass transition point, at a stretching speed of 1000 to 20000% / min,
It is preferable to carry out in a range of 3 to 7 times.

The heat treatment after the stretching is performed at 170 to 240 ° C.
It is preferably performed for 60 seconds. In order to increase the thermal dimensional stability in the width direction, it is preferable to perform a 2-15% relaxation heat treatment in the width direction during the heat treatment. After the heat treatment, the temperature is further reduced by 1 to 60 at a temperature 20 to 160 ° C. lower than the heat treatment temperature.
By performing a two-stage heat treatment in seconds, the effect of the present invention is further improved, which is preferable.

The film obtained in this manner has a heat shrinkage smaller than that of a polyester film produced by an ordinary production method, and has an elastic modulus in the longitudinal direction of 6.0 GPa or more.
A biaxially oriented polyester film having a heat shrinkage in the longitudinal direction of 0.15% or less when held at 0 ° C. for 30 minutes can be obtained. Furthermore, by taking preferable conditions and setting the relaxation time T1ρ of carbonyl carbon in the structural analysis of the unstretched film by solid high-resolution NMR to 50 msec or less, more preferably 45 msec or less, the elastic modulus in the longitudinal direction is 7.0 GPa or more. Even with this, a biaxially oriented polyester film having a heat shrinkage of 0.20% or less, and even 0.15% or less can be obtained. Further, a film excellent in workability with not only a small number of tears during stretching but also excellent productivity but also a tear propagation resistance of 500 N / m or more can be obtained.

The use of the polyester film obtained by the present invention is not particularly limited, but it is required to use a high-density magnetic tape, a high-density magnetic disk, a capacitor, an electrical insulation, an ink ribbon, and the like, which require a large strength and a small heat shrinkage.
Suitable for image receiving sheets, packaging materials, etc.

[Method of measuring physical properties and method of evaluating effects]
The method for measuring characteristic values and the method for evaluating effects according to the present invention are as follows.

(1) Relaxation time T1ρ of carbonyl carbon A high-resolution solid-state NMR measuring device is a JEOL spectrometer JNM-GX270, a JEOL solid-state amplifier,
MAS controller NM-GSH27MU, JEOL probe NM-GSH27T VT. W was used. For the measurement, T1ρ (longitudinal relaxation in rotational coordinates) of ¹³ C nucleus was measured.

The measurement was conducted at a temperature of 24.5 ° C. and a humidity of 50 RH.
%, Under static magnetic field strength of 6.34 T (tesla), ¹ H, ¹³ C
Have resonance frequencies of 270.2 MHz and 67.9 M, respectively.
Hz. In order to eliminate the influence of the anisotropy of the chemical shift, a MAS (magic angle rotation) method was adopted. The rotation speed was 3.5 to 3.7 kHz. The condition of the pulse sequence is
90 ° C. against ¹ H, pulse width 4 .mu.sec, and the locking field strength 62.5 kHz, the contact time is 1 CP transferring polarization of the ¹ H to ¹³ C (cross polarimetry internalization).
5 msec. The holding time τ is 0.00
1,0.5,0.7,1,3,7,10,20,30,
40, 50 msec was used. The free induction decay (FID) of the ¹³ C magnetization vector after the retention time τ was measured (F
High power decoupling was performed to eliminate the influence of dipole interaction due to ¹ H during ID measurement. Note that 512 integrations were performed to improve the S / N ratio.) Further, the pulse repetition time was set between 5 seconds and 15 seconds. The peak of PET carbonyl carbon is 165 ppm.
Observed.

In general, T1ρ is represented by a polynomial I (t) = Σ (Ai) exp (−t / T1ρi) (T1ρi: the i-th component of T1ρ, Ai: the ratio of the component to T1ρi). However, an unstretched film having a small crystallinity can be approximated by a one-component system.
The T1ρ value was determined from the fitting result in the component system.
That is, I (t) = exp (−t / T1ρ), and T1ρ was determined from the slope of the semilogarithmic plot.

(2) Modulus of elasticity According to the method specified in JIS K-7127,
Using a tensile tester manufactured by Toyo Sokki, 25 ° C, 65% RH
Was measured. The sample is 200mm long in the measurement direction,
A 10 mm wide strip was cut out, the initial distance between the tension chucks was 100 mm, and the tension speed was 300 mm / min.

(3) Heat Shrinkage The heat shrinkage in the longitudinal direction of the film was measured according to the method specified in JIS C-2318. However, the temperature and holding time of the oven are 80 ° C, 30 minutes and 100 ° C,
The sample was changed to 30 minutes, and each sample was changed, and the average of the results of 20 measurements was used. The smaller the heat shrinkage, the more preferable. However, in the field of magnetic recording materials such as high-density metal tape, the elastic modulus in the film longitudinal direction is 6.0 GPa or more,
The heat shrinkage at 80 ° C. for 30 minutes is 0.15% or less,
The heat shrinkage at 100 ° C. for 30 minutes is preferably 0.5% or less.

(4) Glass transition point and melting point Measured using a DSC (differential scanning calorimeter) type II manufactured by Parking Elmer. A sample (10 mg) is set in a DSC device, melted at a temperature of 300 ° C. for 5 minutes, and then rapidly cooled in liquid nitrogen. This sample is heated at a rate of 10 ° C./min, and the glass transition point is detected. The temperature was further increased, and the crystallization exothermic peak temperature from the glassy state was defined as the cold crystallization temperature, and the endothermic peak temperature based on crystal melting was defined as the melting point.

(5) Intrinsic viscosity [η] of polyester (unit: dl / g) A value calculated from the following equation from the solution viscosity measured at 25 ° C. in orthochlorophenol is used. η _sp / C = [η] + K [η] ² · C, where η _sp = (solution viscosity / solvent viscosity) −1 C is the weight of dissolved polymer per 100 ml of solvent (g / 10
0 ml, usually 1.2) K is the Haggins constant (0.343), and the solution viscosity and the solvent viscosity were measured using an Ostwald viscometer.

(6) Tear Propagation Resistance AST was measured using a light load type tear tester (Toyo Seiki Co., Ltd.).
It was measured according to MD-1922. The sample size was 51 × 54 mm, and a cut of 1R3 mm was made in the longitudinal direction of the film, and the indicated value when the remaining 51 mm was torn was read.

The tearing resistance is reduced by strengthening the orientation by strengthening the stretching in the longitudinal direction or the like. If the orientation is the same, the intrinsic viscosity of the polymer tends to decrease as the intrinsic viscosity decreases. 4 tear propagation resistance
If it is 000 N / m or less, tearing is likely to occur in the process of processing the film, and the workability is significantly deteriorated.

The present invention will be specifically described below with reference to examples.

Example 1 According to a conventional method, an intrinsic viscosity of 0.60 d containing 0.5% by weight of crosslinked polystyrene particles having an average particle diameter of 0.45 μm.
1 / g of polyethylene terephthalate was polymerized to obtain pellets. The particles were dispersed in ethylene glycol using glass beads having a diameter of 50 μm as a medium. After removing the glass beads, the particles were polymerized with terephthalic acid. 0.06% by weight of magnesium acetate and 0.008% by weight of antimony trioxide were used as a polymerization catalyst, and 0.02% by weight of trimethyl phosphate was used as a phosphorus compound.

The polyester pellet was heated at 180 ° C.
After drying for 3 hours, using a known extruder, melt-extruded at 290 ° C., and wrapped around a metal casting drum having a surface temperature of 20 ° C. using an electrostatic application casting method.
After cooling and solidification, an unstretched film was obtained. At this time, T1ρ of the carbonyl carbon of the film was 62 as shown in Table 1.
msec. The unstretched film was heated for 0.1 second with a known infrared heater, and then treated on a known silicone rubber roll having a surface temperature of 130 ° C. for 3.0 seconds.
Cooled on metal roll at 0 ° C. for 1.5 seconds. At this time, T1ρ of the carbonyl carbon of the film was 44 msec. The unstretched film was heated on a known Teflon roll at 100 ° C. for 3 seconds, and then heated at 100 ° C. in the longitudinal direction.
3 times, 3.0 in the width direction in a tenter with hot air temperature of 95 ° C
Times, and 1.7 in the longitudinal direction on a heating roll at 140 ° C.
The film was stretched 0 times. Further, the biaxially stretched film is fixed in the width direction with a clip, and the tension is set to 200 in a tenter under tension.
After heat treatment at 6 ° C. for 6 seconds, heat treatment at 80 ° C. for 1 second was performed to obtain a biaxially oriented polyester film. As shown in Table 2, the obtained film had a large elastic modulus, a small heat shrinkage and a small tear propagation resistance.

Example 2 In Example 1, the heat treatment by the infrared heater was not performed before the heat treatment on the roll, the surface temperature of the heated roll before stretching was 130 ° C., and the contact time with the roll was 4.0 seconds. In addition, the cooling process is not performed on the metal roll after the heating process, and
A film was formed under exactly the same conditions as in Example 1 except that the stretching ratio in the longitudinal direction at the first time was 3.6 times and the stretching ratio at the second longitudinal direction after the stretching in the width direction was 1.40 times. As shown in Table 1, T1ρ of the carbonyl carbon of the film before stretching was 48.
msec, which was within the range of the present application, and the obtained film properties were also good as shown in Table 2.

Example 3 In Example 2, the surface temperature of the heating roll before stretching was set to 1
10 ° C., except that the contact time with the roll was 4.0 seconds, the first stretching ratio in the longitudinal direction was 3.5 times, and the second stretching ratio in the longitudinal direction after stretching in the width direction was 1.50 times. Produced a film under exactly the same conditions as in Example 1. As shown in Table 1, T1ρ of the carbonyl carbon of the film before stretching was 52.
msec, which was within the range of the present application, and the obtained film properties were also good as shown in Table 2.

Example 4 In Example 2, before stretching in the width direction, firstly 11
A film was produced under exactly the same conditions as in Example 2 except that the film was stretched 3.0 times at 0 ° C. and 2.0 times at 90 ° C. in the longitudinal direction, and the longitudinal stretching was not performed after the stretching in the width direction. As shown in Table 1, T1ρ of the carbonyl carbon of the film before stretching was 48 msec, which was within the range of the present application, and the obtained film properties were good as shown in Table 2.

Comparative Example 1 A film was produced under exactly the same conditions as in Example 1 except that the heat treatment using an infrared heater, the heat treatment on a roll, and the cooling treatment on a metal roll were not performed. As shown in Table 1, T1ρ of the carbonyl carbon of the film before stretching was 62 m
sec and out of the scope of the present application, the obtained film is also shown in Table 2.
As shown in the figure, the heat shrinkage was significantly inferior.

Comparative Example 2 In Example 1, the surface temperature of the heating roll before stretching was set to 1
A film was produced under exactly the same conditions except that the contact time with the roll was 11.0 seconds at 50 ° C. and the cooling treatment was not performed on the metal roll after the heat treatment. Since the heating temperature was high and the time was long, crystallization occurred and T1ρ of the carbonyl carbon of the film before stretching was 105 msec as shown in Table 1.
As shown in Table 2, the heat shrinkage of the obtained film was also inferior.

Comparative Example 3 In Comparative Example 1, the polymerization time was shortened and the intrinsic viscosity was 0.5
An attempt was made to form a film under exactly the same conditions except that 2 dl / g was used, but the film was broken at the time of re-stretching in the longitudinal direction after stretching in the width direction,
No film was obtained. Then, when the magnification of the longitudinal re-stretching was reduced to 1.40 times, the film could not be formed stably, but the frequency of tearing decreased, and a film was obtained for about 30% of the total film forming time. I was able to. As shown in Table 1, T1ρ of the carbonyl carbon of the film before stretching was 63 msec, which is out of the range of the present application. As shown in Table 2, although the heat shrinkage was small, the tear propagation resistance was small and the workability was poor. there were.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

As described above, it is possible to obtain a biaxially oriented polyester film containing polyethylene terephthalate as a main component, which has good productivity and workability, and has a small heat shrinkage despite its large elastic modulus in the longitudinal direction.

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Claims (6)

[Claims] 1. A method for producing a biaxially oriented polyester film containing polyethylene terephthalate as a main component, comprising: biaxially stretching an unstretched film having a carbonyl carbon relaxation time T1ρ of 55 msec or less in structural analysis by solid high-resolution NMR. And a method for producing a biaxially oriented polyester film, which further comprises a heat treatment. 2. An unstretched film that has been melt-cast is taken from a glass transition point measured by a differential scanning calorimeter (DSC) at 3 points.
After performing the treatment at a high temperature of 0 to 80 ° C. for 0.1 to 10 seconds, 2
The method for producing a biaxially oriented polyester film according to claim 1, wherein the film is subjected to axial stretching and further heat treatment. 3. An unstretched film that has been melt-cast is taken from a glass transition point measured by a differential scanning calorimeter (DSC) at 3 points.
After performing the treatment at a high temperature of 0 to 80 ° C. for 0.1 to 10 seconds,
Once cooled to a temperature 5 ° C or more lower than the glass transition point,
It is heated to a temperature higher than the glass transition point again and biaxially stretched.
The method for producing a biaxially oriented polyester film according to claim 1, further comprising performing a heat treatment. 4. The method for producing a biaxially oriented polyester film according to claim 1, wherein the stretching ratio in the longitudinal direction is 4.5 times or more as a whole. 5. A biaxially oriented polyester film produced by the production method according to claim 1, wherein the elastic modulus in the longitudinal direction of the film is 6.0 GPa or more. 6. The intrinsic viscosity of the whole film is 0.55 to
The biaxially oriented polyester film according to claim 5, wherein the range is 0.75.