JPH10138331A - Biaxially oriented polyester film and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent process contamination and to eliminate a surface stain at a time of film formation by biaxially stretching film from such a state that the glass transition temp. of the film immediately before first. stretching is lowered to a specific temp. range as compared with a polyester resin being a raw material. SOLUTION: A biaxially stretched and oriented polyester film is obtained by cooling polyester extruded in a molten state on a casting drum and longitudinally and laterally stretching an amporphous non-oriented film formed into a sheet shape in succession or stretching the same longitudinally or laterally at the same time. The film is biaxially oriented from such a state that the glass transition temp. of the film immediately before first. stretching is lowered to a specific temp. range as compared with a polyester resin being a raw material by 5-30 deg.C. When the glass transition temp. is below 5 deg.C, effect is hard to obtain and, when the glass transition temp. is lowered so as to exceed 30 deg.C, crystallization is easy to advance until first stretching is started after the film is cast and there is possibility obstructing stretchability.

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 and a method for producing the same.
More specifically, by incorporating carbon dioxide into the polyester, the stretchability of the film is improved to improve the productivity, the surface properties of the film are improved, and the functions such as adhesion are improved. To improve the quality such as heat shrinkage, or to achieve an effect such as obtaining a whitened film by finely foaming the contained carbon dioxide gas.

[0002]

2. Description of the Related Art Polyester films, especially polyethylene terephthalate films, are widely used in industrial applications due to their excellent mechanical, thermal and electrical properties, and their demand is increasing. However, demands for film characteristics and productivity have become more and more severe with the expansion of applications and production volume.

Above all, it is a major problem that productivity is not improved due to contamination of the apparatus during film formation, defects on the film surface, and the like. Low molecular weight substances (hereinafter referred to as oligomers) on the film surface which cause this problem. There is an increasing demand for films with less.

In particular, in polyethylene terephthalate, oligomers including ethylene terephthalate cyclic trimer (hereinafter referred to as cyclic trimer) are usually 0.8 to 0.8%.
About 1.3% by weight is contained. Such oligomers are deposited on the surface by applying heat to the film, adhere to the film forming apparatus, and cause, in particular, roll contamination in the longitudinal stretching step, and the contamination scratches the surface of the running film, It is a surface defect of the film. For this reason, it is necessary to periodically perform roll cleaning, and productivity is reduced, which is a problem.

[0005] Therefore, as a method for preventing the precipitation of oligomers, a method of laminating a low oligomer resin on a polyester resin (Japanese Patent Application Laid-Open No. 63-197643, Japanese Patent Application Laid-Open No. 2-27
No. 2713) has been studied, but these require a composite extrusion device or a coating device, and cost increases are inevitable. In addition, it is not preferable because the recoverability of the raw material is deteriorated. Although a method of extracting and removing surface oligomers before stretching has also been proposed, it has not been put to practical use because extraction takes time and productivity is significantly reduced. As means for reducing the amount of oligomers contained,
Although a method of performing solid phase polymerization on a polyester resin chip as a raw material has been proposed, an increase in cost due to an increase in the number of solid phase polymerization steps is unavoidable.

On the other hand, in order to impart various properties to the polyester film, studies have been made to include carbon dioxide in the polyester. A method in which a polyester pellet is melted in a gas atmosphere containing a carbon dioxide gas to contain a carbon dioxide gas in the polyester (JP-A-63-19762).
No. 9, JP-A-63-197630, etc.), but this method does not apply a high pressure, so that the content of carbon dioxide is low and the effect of carbon dioxide content is small. Points. On the other hand, a method of injecting carbon dioxide gas into a molten resin in an extruder (Japanese Patent Publication No. 50-6026) is disclosed. But,
In this method, the carbon dioxide gas is not completely dissolved in the molten resin, and the carbon dioxide gas present in the resin in a gaseous state is dispersed and mixed by a shearing force in the extruder. The obtained foam cannot be obtained, and the obtained foam has a disadvantage that the cell diameter is large.

[0007]

As described above, there is a strong demand for the prevention of process contamination, the elimination of surface defects, and the enhancement of productivity during the production of a biaxially stretched polyester film. Methods have been proposed, but their effectiveness is not yet sufficient. The present invention solves these problems and provides a method for extruding a thermoplastic resin for obtaining a high-quality biaxially stretched film with few defects and high yield.

[0008]

That is, according to the present invention, the glass transition temperature of the film immediately before the start of the first stretching is 5 ° C. or more compared to the polyester resin as the raw material.
The present invention relates to a biaxially oriented polyester film which is biaxially stretched from a temperature of 0 ° C. or lower.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.

The polyester in the present invention is a polymer having an ester group in the main chain obtained by condensation polymerization of a diol and a dicarboxylic acid.
Terephthalic acid, isophthalic acid, diphenic acid, phthalic acid,
Naphthalenedicarboxylic acid, adipic acid, sebacic acid, dimer acid, eicoic acid, dodecanedioic acid and the like, and diols are represented by ethylene glycol, trimethylene glycol, tetramethylene glycol, bisphenol and the like. Things. Specific examples include polyethylene terephthalate, polytetramethylene terephthalate, polyethylene-p-oxybenzoate, poly-1,4-cyclohexylene dimethylene terephthalate, and polyethylene-2,6-naphthalate. Of course, these polyesters may be a homopolymer or a copolymer, and as the copolymerization component, for example, diol components such as diethylene glycol, neopentyl glycol, and polyalkylene glycol, adipic acid, sebacic acid, phthalic acid, And dicarboxylic acid components such as isophthalic acid and 2,6-naphthalenedicarboxylic acid. In the present invention, especially polyethylene terephthalate, polyethylene-
It is preferable because the use of a biaxially stretched film of 2,6-naphthalate is strongly required and highly effective.

The polyester film of the present invention contains various known additives such as an antioxidant, an antistatic agent, a crystal nucleating agent, as long as the effects of the present invention are not impaired.
Inorganic particles and the like may be added.

In the present invention, the film needs to be biaxially stretched. Biaxial stretching means longitudinal (or machine) and transverse (or width) directions of the film.
Stretching. Further, a film in which molecular orientation is imparted in the vertical and horizontal biaxial directions by such biaxial stretching is referred to as a biaxially oriented film. In general, it is obtained by cooling a melt-extruded polyester on a casting drum and sequentially or simultaneously performing longitudinal stretching and transverse stretching on an amorphous non-oriented film formed into a sheet. The order of the longitudinal stretching and the transverse stretching is not particularly limited, but it is difficult to uniformly longitudinally stretch the film having a wide width by the transverse stretching. Therefore, it is preferable to perform the transverse stretching after performing the longitudinal stretching.
The effect of the present invention is exhibited by biaxial stretching, and is meaningless for a film that is not stretched.

[0013] It is also within the scope of the present invention to perform longitudinal stretching and / or transverse stretching again after longitudinal stretching and transverse stretching. In addition, after biaxial stretching and re-stretching, heat treatment is performed,
It is preferable to provide thermal dimensional stability. The heat treatment is generally performed in a state where the clips are held in a tenter, following the transverse stretching or the simultaneous stretching. It is also preferable to perform a heat treatment by providing a tenter for heat treatment after the longitudinal re-stretching by the roll.

In the present invention, it is preferable to perform longitudinal stretching or re-longitudinal stretching using a roll. As one of the effects obtained by the present invention, there is a point that the contamination of the roll is small as described later, and by applying to the longitudinal stretching by the roll which is generally regarded as having the highest productivity, the effect is exerted highly. You.

In the present invention, the film is biaxially stretched from a state in which the glass transition temperature of the film immediately before the start of the first stretching is 5 ° C. or more and 30 ° C. or less as compared with the polyester resin as a raw material. The film immediately before the start of the first stretching is an amorphous non-oriented film formed into a sheet by cooling on a casting drum, and is limited to the first stretching (longitudinal stretching, horizontal stretching, and simultaneous stretching). Rather, the film immediately before it is subjected to the stretching step first introduced). In the present invention, the productivity of a biaxially stretched film is enhanced by stretching while lowering the glass transition temperature. When the decrease in glass transition temperature is less than 5 ° C,
When the effect of the present invention is hardly obtained, and conversely, when the temperature is lowered to more than 30 ° C., crystallization is apt to proceed from the casting of the film to the start of the first stretching, which may hinder stretchability. There is.

In the present invention, it is preferable that the film immediately before the start of the first stretching contains 0.01 to 5% by weight of carbon dioxide gas. More preferably, the content is 0.1% by weight or more and 3% by weight or less. More preferably, the content is 1% by weight or more and 3% by weight or less.
In the present invention, it is preferable to use the phenomenon that the glass transition temperature is lowered by adding carbon dioxide to the polyester, but the film immediately before the first stretching starts contains carbon dioxide in an amount of 0.01% by weight or more. If it is not contained, it is difficult to obtain the effect of the present invention, and it is difficult to contain more than 5% by weight of carbon dioxide gas.

In the present invention, it is also preferable that fine bubbles are contained. After injecting carbon dioxide gas in the extruder, if the resin pressure is reduced at a pressure falling rate within an appropriate range, very fine bubbles having a bubble diameter of about 0.1 to 10 μm are obtained in the resin. In this case, it is important that the carbon dioxide gas is completely dissolved in the resin in the extruder to eliminate the carbon dioxide gas remaining in the gas state. If the carbon dioxide gas remains in a gaseous state, it becomes bubbles having a large diameter of several tens to several hundreds μm, and cannot be used as a film. Very fine bubbles are obtained by complete dissolution in the extruder. By biaxially stretching the film containing air bubbles in this way, conventionally obtained by adding white particles such as titanium dioxide or calcium carbonate, or by adding a resin incompatible with polyester, and stretching. Thus, a white film obtained by forming voids can be obtained. Compared to the conventional method, it is not necessary to add particles, other resins, etc., so it is possible to reduce the loss due to contamination and material switching in the apparatus, and furthermore, by including a large amount of fine bubbles, Since it has high elasticity and high heat insulation, it is most suitable for applications such as image receiving films of printers, especially thermal transfer printers.

Further, in the present invention, it is preferable that fine bubbles are not contained. As described above, since the film is whitened by containing the fine bubbles, the use is limited. Generally, the polyester film must be transparent. Therefore, carbon dioxide gas is injected with an extruder and completely dissolved, and then the pressure is rapidly released with a die, formed into a sheet, and quenched and solidified on a casting drum. As a result, a transparent unstretched film solidified while being dissolved without being precipitated is obtained. Even when this film is biaxially stretched, no bubbles are generated and a transparent biaxially stretched film is obtained.

In the present invention, it is preferable that the film surface has a wetting tension of 45 dyn / cm or more. More preferably, it is 50 dyn / cm or more. When the wetting tension is 45 dyn / cm or more, the adhesiveness is enhanced when the film surface is subjected to post-treatment of coating or bonding. In order to obtain this wetting tension, the carbon dioxide gas interacts with the end groups of the polyester by biaxially stretching while containing the carbon dioxide gas,
It utilizes the phenomenon that the film surface is activated and the wetting tension increases.

As a method of obtaining a film as described above, a polyester resin is supplied to an extruder, carbon dioxide gas is injected into the extruder, and the resin is formed into a sheet from a die and extruded. It is preferable to solidify rapidly on a cooling drum to obtain an unstretched sheet, and then to perform biaxial stretching. In order for the film to contain only the amount of carbon dioxide gas in the present invention, it is necessary to inject at a pressure of several MPa or more, and it is advantageous to obtain such a high pressure in an extruder. It is important that the carbon dioxide gas is completely dissolved in the resin in the extruder. If there is a portion remaining in a gaseous state without being dissolved, as described above, large bubbles remain in the film and become unusable. Also, immediately after forming into a sheet from the die,
It is important to quench and solidify on the cooling drum. If not rapidly cooled after being released to the atmospheric pressure, the contained carbon dioxide gas precipitates as bubbles, and the slower the cooling, the larger the bubbles become, and the more unusable the gas becomes. Even when fine bubbles are contained in the film, rapid cooling is important.

Further, in the present invention, it is preferable to use, as the extruder, a tandem extruder in which two or more extruders are connected in series. As described above, it is important to completely dissolve the carbon dioxide gas in the molten resin.
For this purpose, it is preferable to increase the pressure in the extruder after press-fitting carbon dioxide gas into the extruder. As the gas flows through the extruder, the pressure increases, so that the gas is completely dissolved. In order to obtain such a pressure distribution structure, it is preferable to use a tandem extruder in which two or more extruders are connected in series. The tandem extruder increases the discharge rate by distributing the basic function of the extruder to two units, that is, the first extruder melts the resin and the second extruder supplies a fixed amount. However, it is possible to suppress the heat generated by shearing. With this tandem extruder, the first and second
Since there is a high degree of freedom in the operating conditions of the unit, conditions such as the pressure structure can be taken relatively freely. It is preferable to adopt operating conditions in which carbon dioxide gas is injected and dissolved in the first extruder and the gas remaining in the second extruder is completely dissolved. Even when a tandem extruder is used, the shape of the screw is different from a normal screw shape, and is preferably a screw shape having a sealing mechanism for preventing backflow of carbon dioxide gas. That is, in the first extruder, it is preferable to provide a seal mechanism such as a ring that prevents the gas from flowing backward on the raw material supply side rather than the position where the carbon dioxide gas is injected. Without such a sealing mechanism, the gas is likely to flow backward because the pressure on the raw material supply side is lower.

The operating conditions of such a tandem extruder are as follows: carbon dioxide gas is injected into the first extruder, and the pressure at the outlet of the second extruder is higher than that at the outlet of the first extruder and extruded. Is preferred. By taking such pressure conditions, the gas can be completely dissolved as described above.

Further, in the present invention, it is preferable to cool the second tandem extruder in an atmosphere at a temperature equal to or higher than the temperature at which the temperature of crystallization of the polyester resin falls. Generally, the lower the temperature, the higher the solubility of the gas in the resin. Therefore, in order to completely dissolve the gas, it is preferable that the resin temperature is low. Therefore, it is preferable to lower the temperature from the first extruder,
When the melting temperature in the first extruder is lowered to lower the temperature, an unmelted substance remains, which causes a problem that a defect occurs. Therefore, in the first unit, the resin is completely dissolved, and the gas is dispersed and mixed in the resin. Thereafter, the resin is preferably cooled by a second extruder, and the gas is preferably completely dissolved in the resin. Here, it is preferable that the resin is cooled in an atmosphere at a temperature equal to or higher than the temperature at which the resin starts to be cooled. In the case of a polymer resin, even if the resin in a molten state is cooled to a temperature lower than the melting end temperature of the resin, the resin does not solidify in a short time and can maintain a so-called supercooled liquid phase state. At lower temperatures, the resin begins to crystallize and solidifies over time, making extrusion impossible. In the case of performing cooling in the present invention, the portion where the temperature of the resin is reduced most is the portion of the resin that is in contact with the cylinder wall surface of the cooled second extruder, and this temperature is maintained at or above the temperature at which the temperature starts to decrease. That is, by cooling the resin in an atmosphere at a temperature equal to or higher than the temperature drop crystallization start temperature, while the resin is cooled,
In all parts, the temperature is kept at the temperature lower than the temperature at which the crystallization starts, and solidification does not occur. That is, the set temperature of the second tandem extruder may be set to such a condition. In the case of a second extruder having only a heating mechanism using a heater, cooling only depends on natural heat radiation, and the cooling capacity is low. It is preferable to use an extruder having a cooling mechanism such as using a heater and water together or circulating a heat medium having a forced cooling mechanism. The temperature of the cooled resin may be lower than the melting end temperature of the resin. Even if the resin temperature is equal to or lower than the melting end temperature, as described above, the liquid state is maintained due to the supercooling state, so there is no problem, and the lower the temperature, the higher the gas solubility.

In the present invention, the initial stretching temperature is equal to or lower than the glass transition temperature of the polyester resin as a raw material.
It is preferable to carry out the reaction at a temperature 25 ° C. lower than the glass transition temperature. By including carbon dioxide in the polyester, the glass transition temperature is lowered and the mobility of the molecules is increased. Therefore, even if the stretching temperature is lowered, the stretchability is maintained. Therefore, what cannot be stretched at a temperature lower than the glass transition temperature in a state where ordinary carbon dioxide gas is not contained, can be stretched even at this temperature or lower by including carbon dioxide gas. Therefore, by lowering the stretching temperature,
Since the oligomer contained in the film is less likely to precipitate on the surface, the amount of the oligomer attached to the stretching roll and the like is reduced, and the contamination of the process is reduced. Longer, and productivity is improved. In addition, scratches and defects on the film surface caused by oligomer stains adhered to rolls and the like are reduced, and effects such as an improvement in inspection yield and an improvement in film quality are obtained. Here, when the stretching temperature is higher than the glass transition temperature in a state where carbon dioxide is not contained, these effects cannot be obtained without changing from the normal stretching temperature. On the other hand, when the film is stretched at a temperature lower by 25 ° C. than the glass transition temperature, the stretchability is deteriorated, and the film is easily broken. In addition, in general stretching, lowering the stretching temperature reduces stains due to oligomers and the like.However, as in the present invention, performing at a temperature equal to or lower than the glass transition temperature is performed by containing carbon dioxide gas. Although the apparent glass transition temperature is lowered, since the temperature is lower than the glass transition point of the original molecular chain, the precipitation of oligomers on the surface is critically reduced, and a great effect can be obtained. .

[0025]

[Evaluation method of physical properties]

1. Thermal characteristics A robot DSC “RDC220” manufactured by Seiko Electronics Industry Co., Ltd. was used as a differential scanning calorimeter, and a disk station “SSC / 520” manufactured by the company was used as a data analyzer.
Using “0”, about 5 mg of a sample was melted and held at 300 ° C. for 5 minutes on an aluminum tray, quenched and solidified with liquid nitrogen, and then heated from room temperature at a heating rate of 20 ° C./min. The glass transition temperature observed at this time was defined as “the glass transition temperature of the resin as a raw material”, and the end temperature of the endothermic peak of melting was defined as the melting end temperature. After the temperature was raised to 300 ° C., it was melted and held for 5 minutes, and the temperature was lowered at a rate of 20 ° C./min. The starting temperature of the exothermic peak of the cooling crystallization observed at this time was defined as the cooling crystallization start temperature. In addition, the glass transition temperature of the film immediately before the start of the first stretching was determined by taking a film formed by a casting drum, and immediately raising a sample of about 5 mg from room temperature at a heating rate of 20 ° C./min. Then, the glass transition temperature was measured.

2. The carbon dioxide content in the film immediately before the first stretching starts The film formed on the casting drum is collected,
Immediately, a sample of about 10 g was cut out and weighed (W0).
Is measured accurately. Thereafter, the sample is heated to 280 ° C. under a nitrogen stream, melted, and left for 30 minutes to release the captured carbon dioxide gas. Thereafter, the mixture is cooled to room temperature, and the weight (W1) is accurately measured again in a solidified state. From the difference in weight before and after heating, the carbon dioxide content (% by weight)
= (W0−W1) / W0 × 100.

3. Film surface defects Darken the surroundings, illuminate the film with multiple lights, and observe the film with transmitted light. At this time, the surface of the film having a length of 20 m was observed, and the number of surface defects that could be visually confirmed was counted. When the number of surface defects is 10 or more, "x" indicates that it cannot withstand use, 3-9, and "△" indicates that it can be used even in a bad state, 1-2, and there are almost no defects. The evaluation was evaluated as “○” for good and “、” for no defect.

4. Wet tension of film surface Using a commercially available wetting tension measuring solution, apply it to the film surface with a cotton swab, observe the repelling of the measuring solution, and determine the wetting tension of the measuring solution that is critical so that the measuring solution will not repel. Wet tension was used.

5. Resin pressure A hole was provided at the outlet of the first extruder and the outlet of the second extruder, and a pressure gauge was screwed in to measure the resin without leakage.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described based on embodiments.

Example 1 As a polyester, polyethylene terephthalate having an intrinsic viscosity of 0.65 was used. When the thermal characteristics were measured using DSC, the glass transition temperature was 78 ° C and the melting end temperature was 26.
The temperature was 8 ° C., and the temperature at which crystallization started was 203 ° C. The polyethylene terephthalate pellets were vacuum dried at 180 ° C. for 3 hours and supplied to a tandem extruder in which two extruders were connected in series. The first extruder has a screw length / diameter ratio of L / D = 30 for the first unit and L / D = 20 for the second unit.
Was used. In the first extruder, L / D =
Apply a ring-shaped seal to the screw at the point 15
A supply port for carbon dioxide gas was provided at a point of / D = 18. In the first extruder, the mixture was melted at 290 ° C., and carbon dioxide gas was injected at a pressure of 5 MPa from a carbon dioxide gas supply port. Thereafter, the second extruder was extruded at a set temperature of 280 ° C. The screw speeds of the two extruders were adjusted, and the pressure regulating valve provided in front of the die was adjusted to increase the pressure at the outlet of the first extruder to 8 MPa and the pressure at the outlet of the second extruder to 12 MPa.
And The extruded resin was formed into a sheet from a die and discharged. The film extruded from the die is quenched and solidified on a casting drum maintained at a surface temperature of 25 ° C. while applying static electricity, and then heated by a group of rolls set at 70 ° C. and stretched 3.3 times in the longitudinal direction. Thereafter, it was guided to a tenter, preheated with hot air at 90 ° C., stretched 3.3 times in the horizontal direction, heat-treated in the tenter with hot air at 220 ° C., gradually cooled to room temperature, and wound up. The thickness of the obtained film was 12 μm. Table 1 shows the obtained results.

The carbon dioxide content of the film formed by the casting drum was 1.8% by weight, and the glass transition temperature was 64 ° C. In addition, the obtained film did not contain air bubbles and was transparent.

Using this method, the film was continuously formed, the degree of contamination of the casting drum and the longitudinal stretching roll was observed, and the time required for cleaning was determined to be 60 hours. Further, the surface defect was evaluated, and the result was “「 ”. By this method, a high quality biaxially oriented film having few surface defects was obtained, and the process was less contaminated and good productivity was obtained.

Comparative Example 1 A film was formed using the same apparatus and under the same conditions as in Example 1 except that no carbon dioxide gas was injected, and the film was stretched longitudinally by heating with a group of rolls heated to 90 ° C. to obtain a film having a thickness of 12 μm. Was obtained. Table 1 shows the obtained results.

Carbon dioxide was not contained in the film formed by the casting drum, and the glass transition temperature was 78 ° C., the same as that of the raw material. In addition, the obtained film did not contain air bubbles and was transparent.

Using this method, the film was continuously formed, the degree of contamination of the casting drum and the longitudinal stretching roll was observed, and the time required for cleaning was determined to be 30 hours. Further, the surface defect was evaluated, and the result was "x". There were many surface defects, and the process was easy to become dirty.

Comparative Example 2 In Comparative Example 1, longitudinal stretching was performed at 70 ° C. in the same manner as in Example 1.
The heating was carried out with a group of rolls heated to a temperature. However, film tearing occurred frequently in the tenter, and no film was obtained.

Example 2 The same apparatus and conditions as in Example 1 were used, except that the injection pressure of carbon dioxide gas was 8 MPa and the set temperature of the second extruder was 24.
Performed at 0 ° C. At this time, the pressure at the outlet of the first extruder was 10 MPa, and the pressure at the outlet of the second extruder was 15 MPa.
a. Further, longitudinal stretching was performed by heating with a group of rolls heated to 60 ° C. to obtain a film having a thickness of 12 μm. Table 1 shows the obtained results.

The film formed by the casting drum had a carbon dioxide content of 2.5% by weight and a glass transition temperature of 56 ° C. In addition, the obtained film did not contain air bubbles and was transparent.

Using this method, the film was continuously formed, the degree of contamination of the casting drum and the longitudinal stretching roll was observed, and the time required for cleaning was determined to be 72 hours. Further, the surface defect was evaluated, and the result was “◎”. By this method, a high quality biaxially oriented film having less surface defects than in Example 1 was obtained.

Example 3 The same apparatus and conditions as in Example 1 were used, except that a pressure regulating valve was also provided where the second extruder was exited, and the pressure from the second extruder to the mouthpiece In the meantime, while the pressure was gradually lowered, the film was further subjected to longitudinal stretching by heating with a group of rolls heated to 75 ° C. to obtain a film having a thickness of 20 μm. Table 1 shows the obtained results.

The content of carbon dioxide in the film formed by the casting drum was 0.8% by weight, and the glass transition temperature was 72 ° C. Further, the obtained film contained fine bubbles and was white.

Using this method, the film was continuously formed, the degree of contamination of the casting drum and the longitudinal stretching roll was observed, and the time required for cleaning was 48 hours. Further, the surface defect was evaluated, and the result was “「 ”. A biaxially oriented film of good quality with few surface defects was obtained, and a white film was obtained under conditions of low process contamination and good productivity.

Example 4 A film having a thickness of 20 μm was prepared in the same manner as in Example 3 except that the pressure at the outlet of the first extruder and the pressure at the outlet of the second extruder were the same at 8 MPa. Obtained. Table 1 shows the obtained results.

The film formed by the casting drum had a carbon dioxide content of 0.6% by weight and a glass transition temperature of 74 ° C. The resulting film was white, containing fine bubbles, but also contained a considerable amount of coarse bubbles.

Using this method, the film was continuously formed, the degree of contamination of the casting drum and the longitudinal stretching roll was observed, and the time required for cleaning was 48 hours. Further, the surface defect was evaluated, and the result was “「 ”. A biaxially oriented film of good quality with few surface defects was obtained, and a white film was obtained under conditions of low process contamination and good productivity. However, although the quality as a white film was at a practical level, it was inferior to that of Example 3 due to many coarse bubbles.

[0047]

[Table 1]

[0048]

According to the present invention, in particular, roll contamination such as a longitudinal stretching system can be suppressed, and the cycle of roll cleaning can be lengthened, so that the effect of improving productivity can be obtained. By suppressing defects on the film surface, mainly due to oligomer contamination, on the roll, the inspection yield was improved and the effect of forming a high quality film was obtained.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI B29L 7:00 C08L 67:02

Claims (10)

[Claims] 1. A biaxially oriented polyester which is biaxially stretched from a state in which the glass transition temperature of the film immediately before the start of the first stretching is lowered by 5 ° C. or more and 30 ° C. or less as compared with the polyester resin as a raw material. the film. 2. The biaxially oriented polyester film according to claim 1, wherein the film immediately before the start of the first stretching contains carbon dioxide gas in an amount of 0.01% by weight or more and 5% by weight or less. 3. The biaxially oriented polyester film according to claim 2, wherein the film immediately before the first stretching is started contains fine bubbles. 4. The biaxially oriented polyester film according to claim 2, wherein the film immediately before the start of the first stretching does not contain fine bubbles. 5. The wet tension of the film surface immediately before the start of the first stretching is 45 dyn / cm or more.
The biaxially oriented polyester film according to any one of claims 1 to 4. 6. A polyester resin is supplied to an extruder, carbon dioxide gas is injected into the extruder, the resin is formed into a sheet from a die, extruded, and quenched and solidified on a cooling drum to form an unstretched sheet. The method for producing a biaxially oriented polyester film according to any one of claims 2 to 5, wherein the obtained film is biaxially stretched. 7. The method for producing a biaxially oriented polyester film according to claim 6, wherein a tandem extruder in which two or more extruders are connected in series is used. 8. The tandem extruder according to claim 7, wherein carbon dioxide gas is injected into the first extruder, and the extruder is extruded by increasing the pressure at the second extruder outlet from the pressure at the first extruder outlet. A method for producing a biaxially oriented polyester film. 9. The method for producing a biaxially oriented polyester film according to claim 7, wherein the second tandem extruder is cooled in an atmosphere at a temperature equal to or higher than the temperature at which crystallization of the polyester resin begins to cool. 10. The biaxially oriented polyester film according to claim 6, wherein the initial stretching temperature is not higher than the glass transition temperature of the polyester resin as a raw material and not lower than 25 ° C. lower than the glass transition temperature. Production method.