JPH1176716A - Method of filtering polyester, production of polyester film, and polyester film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of filtering polyester for effectively obtaining a film of few faults and of high quality, a method of producing polyester, and a polyester film. SOLUTION: In this method of filtering polyester, soluble gas is infected into molten polyester between an extruder and a filtering process and this molten polyester in which the gas is dissolved is filtered. Resin pressure P1 (MPa) immediately before a filtering process when soluble gas is injected between the extruder and the filtering process and resin pressure P0 (MPa) immediately before when soluble gas is not injected preferably satisfies a formula, P1 <=P0 ×0.75.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for filtering a polyester, a method for producing a polyester film, and a polyester film.

More specifically, it has the effect of lowering the pressure of the molten polyester in the filtration step, prevents the thermal degradation of the molten resin, suppresses the generation of foreign substances, modified polymers, and the like due to the thermal degradation. A new polyester filtration method, which has the effect of improving the quality or yield of the film by reducing defects due to foreign matter, and further improving the productivity by preventing tearing during stretching of the film, A method for producing a polyester film,
Further, the present invention relates to a polyester film.

[0003]

2. Description of the Related Art Thermoplastic resin films are widely used for various industrial materials, such as packaging. Among them, polyester films, particularly polyethylene terephthalate and polyethylene-2,6-naphthalate, are widely used for applications that cannot be used with general-purpose resins such as polyethylene and polypropylene due to their excellent mechanical, thermal, and electrical properties. Demand is also increasing. However, demands for film characteristics and productivity have become more and more severe with the expansion of applications and the increase in production volume.

In particular, there is a tendency to increase the discharge rate per unit time in an extruder in order to improve productivity. As a result, the exothermic resin generates heat due to shearing heat in order to increase the number of rotations of a screw of the extruder. And the resin temperature increases. In the worst case, for the extruder set temperature,
There are cases where the resin temperature is higher than 20 ° C. or higher. Under such circumstances, thermal decomposition occurs depending on the type of resin, or thermal degradation occurs even if it can not be said to be decomposition, and the resulting foreign substances and modified modified resin are mixed into the extruded resin. There is such a problem. These deteriorated portions are detected as defects when finally formed into a film, and are rejected for inspection to lower the yield, or to lower the quality of the film even if not rejected. In a severe case, when the film is biaxially stretched, the portion serving as the foreign matter may be a trigger to cause the film to be torn, which may reduce the productivity.

[0005] Generally, a filter is provided in the extrusion system in order to collect such heat-deteriorated substances. However, if the filter is used for a certain period of time, the trapping ability will be reduced and will be mixed into the film. In particular, the worse the condition of thermal deterioration such as when the temperature is high, the shorter the period becomes, and there is a problem that the filter must be replaced in a short cycle. Conversely, if the temperature of the extruder is set low in order to keep the resin temperature low, the extruder is a process of melting the resin. It takes the same behavior as a thermally degraded product, and causes problems such as a decrease in productivity.

Therefore, a method has been proposed in which the material is once heated to a temperature higher than the melting temperature, then cooled to a temperature lower than the melting temperature and higher than the recrystallization temperature and extruded (Japanese Patent Laid-Open No. 4-347617). In this method, when laminating a heat-sensitive resin to a heat-resistant resin, in order to prevent thermal decomposition of the heat-sensitive resin side, the heat-resistant resin side is cooled and then laminated. is there. However, according to the study of the present inventors, these methods are effective when the discharge amount of the resin is small, but when applied to a high discharge amount such as an actual production line, heat conduction in the resin becomes rate-limiting. There is a problem that the resin cannot be cooled sufficiently. Furthermore, although this method cools immediately before lamination, the problem due to thermal degradation as described above is a problem that tends to occur in a filter portion having a relatively long residence time, and cooling is performed immediately before forming a sheet. Even if it did, a high effect could not be expected.

[0007] Further, it is possible to suppress shear heat generation in the extruder,
Compounding non-liquid crystalline polyester with liquid crystalline polyester for the purpose of lowering the filter filtration pressure
No. 245811) has been proposed. This method
It is to reduce the melt viscosity of a polyester having a high intrinsic viscosity by the presence of a liquid crystalline polyester. However, in this method, the effect of lowering the melt viscosity in the extruder is sufficient, but the effect of lowering the melt viscosity in the filter portion is insufficient. If the melt viscosity in the filter portion is too high, there is a problem that the filtration pressure becomes too high when the number of filters is reduced in order to reduce the residence time in order to suppress the generation of foreign matter. Conversely, when trying to lower the filtration pressure in the filter section, raising the temperature of the filter section or increasing the number of filters may increase the speed of thermal degradation, increase the residence time and increase the amount of foreign matter generated. Becomes

[0008]

As described above, there is a strong demand for preventing the resin from being thermally degraded in the extrusion system and for collecting the generated thermally degraded product or modified polymer without increasing the filtration pressure. Improvement methods have been proposed, but their effects are not sufficient. An object of the present invention is to provide a polyester filtration method, a polyester film production method, and a polyester film for solving the above-mentioned problems and obtaining a high-quality film with few defects with high yield.

[0009]

The present invention for achieving the above object is characterized in that a soluble gas is injected into a molten polyester from an extruder during a filtration step, and the molten polyester in which the gas is dissolved is filtered. This is a method for filtering polyester.

[0010] Further, the present invention is a method for producing a polyester film, characterized in that the polyester film is filtered by the polyester filtration method, and then the cast film obtained by cooling and solidifying is stretched in at least one direction.
A polyester film obtained by using the method for producing a polyester film, which is substantially free of a soluble gas.

[0011]

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

In the present invention, the polyester is a polymer obtained by condensation polymerization of a dicarboxylic acid and a diol. Examples of the dicarboxylic acid include terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, adipic acid, sebacic acid and the like. , And
The diol is represented by ethylene glycol, trimethylene glycol, tetramethylene glycol, cyclohexane dimethanol, polyalkylene glycol, bisphenol A ethylene oxide adduct and the like. Specifically, for example, polymethylene terephthalate, polyethylene terephthalate, polytetramethylene terephthalate, polyethylene-p-oxybenzoate, poly-1,4-cyclohexylene dimethylene terephthalate, polyethylene-2,6-naphthalate and the like can be used. . Of course, these polyesters may be either homopolymers or copolymers, and examples of the copolymerization components include diol components such as diethylene glycol, neopentyl glycol, polyalkylene glycol, and bisphenol A ethylene oxide adduct, Dicarboxylic acid components such as dimer acid, adipic acid, sebacic acid, phthalic acid, isophthalic acid, and 2,6-naphthalenedicarboxylic acid can be used. Also, as long as the effects of the present invention are not impaired,
Crystal nucleating agent, inorganic particles, flame retardant, heat stabilizer, antioxidant,
An ultraviolet absorber, an antistatic agent and the like may be blended.

The method of melt extrusion in the present invention includes:
Using a commercially available extruder, the polyester is supplied to the supply unit, the screw is rotated in a heated cylinder in the extruder, the resin is melted, and the molten resin sent out from the extruder is heated. Through the flow channel (polymer tube). If necessary, a gear pump may be provided to remove foreign substances and denatured polymer through a filter and to improve the quantitative supply. The polymer thus guided is widened to a required width inside the die, discharged from the die, and cooled and solidified in a sheet form on a casting drum.

Here, a single-screw or twin-screw extruder can be used as an extruder for melting the resin.
The extruder screw has the properties of the polyester to be applied,
An optimum gas may be used depending on the properties of the soluble gas to be injected. The setting of the temperature for heating the thermoplastic resin in the extruder, if the polyester shows crystallinity,
The melting point is set to be higher than the melting point so that no unmelted material remains. on the other hand,
When the polyester is amorphous, it does not show such a melting point, so the temperature of the extruder may be set to a temperature at which the resin has a melt viscosity that can withstand extrusion.

As a method of injecting the soluble gas into the polyester in the present invention, a method of injecting the soluble gas into the molten polyester during the filtration step from the extruder is preferable. Further, it is more preferable to inject the soluble gas while applying a pressure of several MPa or more at the vent portion of the extruder having a vent port. Such high pressures are advantageously obtained 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 that remains in a gaseous state without being dissolved, it becomes large bubbles and remains in the film, making it unusable for use. Examples of the extruder with a vent include a single-screw extruder, a twin-screw extruder, a tandem extruder in which extruders are connected in series, and the like, and may be selected according to the required use. Also,
In order to improve the degree of dispersion of the soluble gas in the molten polyester, a static mixer may be arranged at the tip of the extruder with a vent. If the dissolving gas is injected into the polyester before the extruder, it is difficult to uniformly dissolve the dissolving gas in the molten polyester. Further, even if a soluble gas is injected after the filtration step, the effect of the present invention cannot be obtained.

As a method for injecting such a soluble gas into the polyester, it is preferable to use a tandem extruder in which two or more extruders are connected in series. As mentioned above, it is important that the soluble gas be completely dissolved in the molten polyester. For this purpose, it is preferable to increase the pressure in the extruder after pressurizing the soluble gas into the extruder. The structure in which the pressure increases as it flows through the extruder completely dissolves the soluble gas. 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. A tandem extruder is a device that melts resin in a first extruder and supplies a fixed amount in a second extruder. The shear heat generation can be suppressed while increasing. The first extruder presses in the soluble gas and dissolves it.
It is preferable to take operating conditions that completely dissolve the gas remaining in the second extruder. Even when a tandem extruder is used, the screw shape 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 ring and a seal mechanism on the raw material supply side rather than the position where the carbon dioxide gas is injected, so as to prevent the gas from flowing backward. Without such a sealing mechanism,
Since the pressure is lower on the raw material supply side, a phenomenon in which the gas flows backward tends to occur.

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 made higher than that at the outlet of the first extruder and extruded. Is preferred. By taking such pressure conditions, complete dissolution becomes possible as described above.

Further, in the present invention, it is preferable that the second tandem extruder is cooled in an atmosphere at a temperature equal to or higher than the temperature at which the polyester resin starts to cool down and crystallize.

When the melting temperature in the first extruder is lowered to lower the temperature, there is a problem that unmelted material remains and becomes a defect. Further, when cooled in an atmosphere at a temperature equal to or lower than the temperature at which crystallization starts, the polyester resin crystallizes and cannot be extruded.

In general, the solubility of the soluble gas in the resin is advantageously higher when the temperature is lower. Further, when the temperature is equal to or higher than the temperature at which the crystallization starts, the material is in a so-called supercooled state and is in a liquid phase, so that it can be extruded.

In the present invention, the soluble gas is a gas that dissolves in polyester at a predetermined temperature, and carbon dioxide, carbon monoxide, nitrogen, chlorofluorocarbon, methane and the like can be used. In particular, carbon dioxide having a large amount of dissolution is preferable. The amount of the dissolved gas dissolved depends on the degree of the plasticizing effect of the dissolved gas on the polyester. Generally, the soluble gas contains 0.01% by weight or more and 10% by weight based on the cast film obtained by quenching and solidifying. The following is preferred. More preferably, the content is 0.1% by weight or more and 5% by weight or less. More preferably, the content is 1% by weight or more and 3% by weight or more. If the amount is less than 0.01% by weight, the plasticizing effect of carbon dioxide on the polyester is not sufficient, and there is no effect of sufficiently lowering the melt viscosity in the filtration step.
Conversely, it is difficult to make the dissolution amount exceed 10% by weight, which is very large in terms of equipment and undesirably increases the cost. Preferred examples of the combination of the polyester and the soluble gas include a combination of polyethylene terephthalate and carbon dioxide, and a combination of polyethylene naphthalate and carbon dioxide.

The soluble gas in the present invention is a supercritical fluid injected into the molten polyester, or the soluble gas injected into the molten polyester is converted from the extruder to the supercritical fluid during the filtration step. Preferably,
A supercritical fluid is a state in which a substance is at a temperature higher than the critical temperature and higher than the critical pressure, and exhibits physical properties intermediate between gas and liquid. If the dissolving gas is not a supercritical fluid, the solubility in polyester is low, which is not preferable.

Various conventional filters can be used as the filtration device in the present invention. Examples of the type of the filter include a cylindrical filter, a disk filter, and a leaf disk in which disk filters are stacked and used, and may be selected according to the application and purpose. Further, these may be used in combination. The filtration accuracy (95% cut diameter) of the filter varies as necessary,
It is preferable that it is not more than 80 μm cut. More preferably, it is 0.5 μm or more and 100 μm or less. When the filtration accuracy of the filter is larger than 80 μm, it is difficult to collect foreign substances and denatured modified resin.

In the present invention, the resin pressure P ₁ immediately before the filtration step when the soluble gas is injected from the extruder during the filtration step and the resin pressure P ₀ immediately before the filtration step when the soluble gas is not injected between the extruder and the filter step. Preferably satisfies P ₁ ≦ P ₀ × 0.75. More preferably, P ₁ ≦ P ₀ × 0.
7 is satisfied, and when P ₁ ≦ P ₀ × 0.75 is not satisfied, the effect obtained by the present invention may not be sufficient. This effect is a phenomenon that occurs because the melt viscosity of the polyester in which the dissolved gas is dissolved decreases due to the plasticizing effect of the soluble gas, but surprisingly,
The melt viscosity of the polyester into which the dissolving gas has been injected has a greater effect in a region where the shear rate is lower, and a remarkable viscosity-reducing effect in the filtration step as well as a viscosity-reducing effect in the extruder can be obtained.

In the present invention, when the polyester is polyethylene terephthalate, the melt viscosity ηm of the polyethylene terephthalate immediately before the filtration step preferably satisfies the following equation.

Ηm ≦ 0.75 × 1.55428 × 10 ⁴ × ｛[exp (-1
1.9755 + 6802.1 / T)] ηinh ^5.145 ηηinh: logarithmic viscosity Melt viscosity ηm and logarithmic viscosity η measured by the following evaluation methods
The fact that inh satisfies the above equation indicates that the melt viscosity of polyethylene terephthalate in the filtration step is lower than in the conventional case. If the above formula is not satisfied, the effect obtained by the present invention may not be sufficient, and the generation of foreign matter and modified resin may not be suppressed.

The viscosity-reducing effect obtained by the present invention suppresses shearing heat generation in the extruder, thereby reducing the generation of foreign substances and modified resin. Second, since the resin pressure in the filtration step can be reduced, a filter with higher filtration accuracy can be used, and smaller foreign substances and modified resin can be collected. Further, the number of leaf disk filters can be reduced by an amount corresponding to the decrease in the resin pressure in the filtration step, the residence time in the filtration step can be reduced, and the generation of foreign substances and modified resin can be suppressed. For this reason, it is possible to obtain a high-quality product with less defects compared to conventional products. In addition, since defects in the production of the polyester film cause tears, the number of tears in the production process is reduced, and the productivity is increased.

In the present invention, the resin cannot be extruded because the melt viscosity is high in the past, and the resin pressure in the filtration step exceeds the pressure resistance of a filter or the like in a predetermined application. It is possible to extrude. Examples of such a material include polyethylene terephthalate having a large molecular weight, that is, an intrinsic viscosity of 0.8 or more, and polyethylene naphthalate having an intrinsic viscosity of 0.7 or more. Further, the polyester resin in a supercooled state also has a high melt viscosity, and is difficult to extrude by the conventional technique, but can be made by using the present invention.

In the present invention, it is important that after the soluble gas is completely dissolved in the polyester, it is formed into a sheet from a die, and then rapidly cooled and solidified on a cooling drum. If not rapidly cooled after being released to the atmospheric pressure, the contained carbon dioxide gas precipitates as bubbles, and as the temperature is gradually cooled, the bubbles become larger and become unusable. In the present invention, it may be stretched in at least one direction. Further, it may be stretched in two directions. As a stretching method, a melt-extruded polyester is cooled on a casting drum, and a substantially amorphous and non-oriented film formed into a sheet is subjected to sequential biaxial stretching, a tubular method, a tenter method, and the like, which are known in the art. Is preferred. More preferably, sequential biaxial stretching is performed in which the film is stretched in the longitudinal direction by a group of rolls, stretched in the transverse direction by a clip running on a tenter, and then heat-treated while being held by the clip in the tenter. Here, the order of the longitudinal stretching and the transverse stretching is not particularly limited, but it is difficult to uniformly longitudinally stretch the film which has been widened by the transverse stretching. 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.

The cast film obtained by quenching and solidifying is cast and the glass transition temperature (T
ga) The film stretched in one direction at a temperature of (Tga + 30) ° C. and then stretched in a direction perpendicular to the stretching direction at a glass transition temperature (Tgb) ° C. to (Tgb + 40) ° C. of the film. Is preferred. If the stretching temperature is higher than (Tga + 30) ° C. in one direction, the soluble gas precipitates as large bubbles during the stretching, which may cause breakage, which is not preferable. Also,
If the stretching temperature is lower than Tga, the stretchability is poor and stretching may not be performed. Similarly, when a uniaxially stretched film is stretched in a direction perpendicular to the above direction, (Tg
When the film is stretched at a temperature higher than (b + 40) ° C., the soluble gas precipitates as large bubbles during the stretching, causing breakage,
If the stretching temperature is lower than Tgb, the stretchability is poor and stretching may not be performed, which is not preferable. It is preferable that the heat treatment be performed subsequently.

The polyester film of the present invention comprises:
It preferably does not contain a soluble gas. If a large amount of dissolved gas remains in the polyester film,
Since the glass transition point is lowered due to the plasticizing effect, heat resistance is deteriorated and mechanical properties are also lowered. However,
Although it depends on the solubility of the soluble gas, the polyester film containing the soluble gas is degassed by leaving it at normal pressure and room temperature within several hours without any influence. Accordingly, the obtained polyester film spontaneously exhibits almost the same performance as the conventional polyester film.

In the present invention, it is also preferable that fine bubbles are contained. After dissolving the soluble gas in the polyester, if the resin pressure is lowered at a pressure drop rate in an appropriate range, very fine bubbles of about 0.1 to 10 μm are obtained in the resin. By biaxially stretching the film containing such bubbles, conventionally obtained by adding white particles such as titanium dioxide and calcium carbonate, or by adding a resin incompatible with polyester,
A white film obtained by forming voids by stretching can be obtained. Compared with the conventional method, there is no need to add particles or other resins, so it is possible to reduce the loss due to contamination and switching of raw materials in the equipment, and to increase the elasticity by including a large amount of fine bubbles. It is suitable for printers, especially for image-receiving films of thermal transfer printers because of its high heat resistance.

In the present invention, it is also preferable not to contain fine bubbles. As described above, the use of fine films causes whitening of the film, which limits its use. For applications where transparency is required, it is better not to include microbubbles. A film containing no microbubbles can be obtained by dissolving a soluble gas in polyester and then stretching the cast film obtained by reducing the resin pressure at a rapid pressure drop rate under appropriate stretching conditions.

[0034]

[Physical property value method]

(1) Thermal characteristics: The differential scanning calorimeter uses a robot DSC “RDC220” manufactured by Seiko Denshi Kogyo Co., Ltd.
Using SC / 5200, about 5 mg of a sample was melted and held at 300 ° C. for about 5 minutes on an aluminum tray, quenched and solidified with liquid nitrogen, and then heated from room temperature at a rate of 20 ° C./min. 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 onset temperature of the exothermic peak of the temperature crystallization observed at this time was defined as the temperature crystallization onset temperature. The glass transition temperature (Tga) ° C. of the cast film obtained by quenching and solidification was determined by collecting a film formed by a casting drum, and then immediately measuring about 5 m.
g sample was heated from room temperature at a heating rate of 20 ° C./min, and the glass transition temperature was measured. In addition, the glass transition temperature Tgb of the film stretched in one direction is determined by collecting a film stretched longitudinally using a plurality of roll groups,
g sample was heated from room temperature at a heating rate of 20 ° C./min, and the glass transition temperature was measured.

(2) Dissolution amount of carbon dioxide gas: A film formed by a casting drum was collected, and immediately, about 10
A g sample is cut out and the weight (W ₀ ) is measured accurately. Thereafter, this sample was placed in a vacuum at room temperature (760 mm).
Hg) for 24 hours to release dissolved carbon dioxide. After that, the weight (W ₁ ) is accurately measured again.
Carbon dioxide dissolved amount (% by weight) = (W ₀ −W ₁ ) / W ₀ × 10
0 was set.

(3) Defects on the surface of the film: The film is illuminated with a plurality of lights while the surroundings are dark, and the film is observed 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.
If the number of surface defects is 10 or more, it is unacceptable for use; "x"; 3 to 9 is bad but usable but "使用";"O", and the case where no defect was observed were evaluated as "A".

(4) Intrinsic viscosity: a value measured at 25 ° C. at a concentration of 0.1 g / ml in orthochlorophenol.

(5) Logarithmic viscosity: Logarithmic viscosity is ASTM D
According to 2857-1983, polyethylene terephthalate was treated with o-chlorophenol as a solvent at 30 ° C.
It was measured at 0.005 g / ml.

(6) Melt viscosity: A tube from which a partially melted resin can be taken out from a short tube connecting an extruder and a filter pack (a filtering device containing a stack of several leaf disk filters) is provided. ) A plastic engineering laboratory online rheometer RH-1 was attached and the melt viscosity was measured. The measured shear rate is 10 to 100 (s-
1), and a capillary for low shear was used.

(7) Resin pressure: A pressure gauge was attached to the entrance of the filter pack, and the resin pressure was measured.

[0041]

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

Example 1 As a polyester, the intrinsic viscosity was 0.65 and the logarithmic viscosity was 0.1.
67, polyethylene terephthalate having a temperature-reducing crystallization start temperature of 203 ° C. was used. The polyethylene terephthalate pellets were vacuum-dried at 180 ° C. for 3 hours, and then supplied to a tandem extruder in which two extruders were connected in series. 2
It was supplied to the extruder 1 heated to 80 ° C. The first extruder has a screw length / diameter ratio of L / D = 3.
The 0th and 2nd units used L / D = 20. In the first extruder, a ring-shaped seal was applied to the screw at L / D = 15, and a soluble gas supply port was provided at L / D = 18. Carbon dioxide was used as the dissolving gas. The first extruder was brought into a molten state at 290 ° C., carbon dioxide gas was injected at a pressure of 5 MPa from a soluble gas supply port, and the extruder was extruded at a set temperature of 280 ° C. As a filtration device, a filter pack in which three leaf disk filters each having a filtration accuracy of 14 μm cut and a size of 8.3 / 4 inch were stacked and housed in a casing was used. The molten resin extruded from the extruder was filtered through a filter tube through a short tube, then formed into a sheet from a die and discharged. The film extruded from the base is
It is rapidly cooled and solidified on a casting drum maintained at a surface temperature of 25 ° C. while applying a positive electric charge, and then heated by rolls set at 70 ° C., stretched 3.3 times in the longitudinal direction, and then guided to a tenter. After preheating with hot air, the film was stretched 3.3 times in the lateral direction, heat-treated in a tenter with hot air at 200 ° C., gradually cooled to room temperature, and wound up. The thickness of the obtained film was 12 μm. Table 1 shows the obtained results.
Shown in

The amount of dissolved carbon dioxide in the film formed on the casting drum was 1.8% by weight. The glass transition temperature of the film formed on the casting drum was 60 ° C., and the glass transition temperature of the film stretched in the machine direction was 70 ° C. Further, the obtained film was transparent without containing bubbles and carbon dioxide gas.

The presence or absence of defects on the film surface was evaluated.
It was "O". The melt viscosity was 900 poise, and the resin pressure at the entrance of the filter pack was 8 MPa.

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 longitudinal stretching was performed using a group of rolls heated at 90 ° C. to obtain a film having a thickness of 12 μm. Obtained. Table 1 shows the obtained results.

Carbon dioxide was not dissolved in the film formed on the casting drum. The obtained film was transparent without containing bubbles.

The presence or absence of defects on the film surface was evaluated.
It was "x". The melt viscosity is 1550 poise
And the resin pressure at the filter pack entrance is 12Mp
a.

Example 2 In Example 1, longitudinal stretching was performed by heating with a group of rolls heated to 110 ° C.

The obtained film was almost the same as that obtained in Example 1, but the film was sometimes torn in the film forming step, which might be caused by the deposition of large bubbles of the soluble gas. It was determined that the film forming process was more difficult than in the case of Example 1.

The amount of dissolved carbon dioxide in the film formed on the casting drum was 1.8% by weight. The glass transition temperature of the film formed on the casting drum was 60 ° C. The melt viscosity is 90
0 poise, and the resin pressure at the entrance of the filter pack was 8 Mpa.

Example 3 The same apparatus and conditions as in Example 1 were used, except that the carbon dioxide gas injection pressure was 8 MPa and the set temperature of the second extruder was 2
40 ° C. In addition, longitudinal stretching was performed with a group of rolls heated to 60 ° C., and transverse stretching was performed at 75 ° C. with a tenter to obtain a film having a thickness of 12 μm. Table 1 shows the obtained results.

The amount of dissolved carbon dioxide in the film formed on the casting drum was 3.0% by weight. The glass transition temperature of the film formed on the casting drum was 50 ° C, and the glass transition temperature of the film stretched in the machine direction was 63 ° C. Further, the obtained film was transparent without containing bubbles and carbon dioxide gas.

The presence or absence of defects on the film surface was evaluated.
It was "◎". The melt viscosity is 1800 poise
And the resin pressure at the filter pack entrance is 15Mp
a.

Comparative Example 2 Film formation was carried out under the same apparatus and conditions as in Example 3 except that carbon dioxide gas was not injected, and longitudinal stretching was carried out by heating the rolls heated at 90 ° C. Table 1 shows the obtained results.

When extruded under these conditions, the film could not be formed because it exceeded 30 MPa, which is the pressure limit of the filter pack. The melt viscosity was 4000 poise.

Example 4 The same apparatus and conditions as in Example 1 were used, except that the number and size of the leaf disks remained the same, but the filtration accuracy was changed to 8 μm. Table 1 shows the obtained results. The amount of dissolved carbon dioxide in the film formed on the casting drum was 1.8% by weight. The glass transition temperature of the film formed on the casting drum is 60 ° C., and the glass transition temperature of the film stretched in the machine direction is 7 ° C.
It was 0 ° C. Further, the obtained film was transparent without containing bubbles and carbon dioxide gas.

The presence or absence of defects on the film surface was evaluated.
It was "◎". The melt viscosity was 900 poise, and the resin pressure at the inlet of the filter pack was 12 MPa.
Met.

Comparative Example 3 Film formation was carried out under the same apparatus and conditions as in Example 4, except that carbon dioxide gas was not injected, and longitudinal stretching was carried out by heating the rolls heated at 90 ° C. to a thickness of 12 μm. Was obtained. Table 1 shows the obtained results.

Carbon dioxide was not dissolved in the film formed on the casting drum. The obtained film was transparent without containing bubbles.

The presence or absence of defects on the film surface was evaluated.
It was "△". The melt viscosity is 1550 poise
And the resin pressure at the filter pack entrance is 18Mp
a.

Example 5 Polyethylene terephthalate having an intrinsic viscosity of 0.80, an logarithmic viscosity of 0.82, and a temperature-reducing crystallization onset temperature of 201 ° C. was used as the polyester under the same apparatus and conditions as in Example 1. Table 1 shows the obtained results.

The amount of dissolved carbon dioxide in the film formed on the casting drum was 1.9% by weight. The glass transition temperature of the film formed on the casting drum was 60 ° C., and the glass transition temperature of the film stretched in the machine direction was 70 ° C. Further, the obtained film was transparent without containing bubbles and carbon dioxide gas.

The presence or absence of defects on the film surface was evaluated.
It was "O". The melt viscosity is 3200 poise
And the resin pressure at the filter pack entrance is 29Mp
a.

Comparative Example 4 Film formation was carried out under the same apparatus and conditions as in Example 5, except that carbon dioxide gas was not injected, and longitudinal stretching was carried out by heating the roll group heated at 90 ° C. Table 1 shows the obtained results.

When extruded under these conditions, the film could not be formed because it exceeded the withstand pressure limit of the filter pack of 30 MPa. The melt viscosity was 6000 poise.

[0066]

[Table 1]

[0067]

According to the present invention, it is possible to reduce the amount of foreign substances and denatured denatured resin particularly generated in the extrusion system, and to obtain a high quality film as compared with the prior art. Since the defects caused by the resin are reduced, the effect of improving the productivity such as reducing the number of breaks can be obtained.

In addition, according to the present invention, a resin which could not be extruded because of its high melt viscosity, which made it difficult to filter because of its high melt viscosity, can be obtained according to the present invention. Is what you can do.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI // B29K 67:00 B29L 7:00 C08L 67:02

Claims (9)

[Claims] 1. A method for filtering a polyester, comprising injecting a soluble gas into a molten polyester from an extruder during a filtration step and filtering the molten polyester in which the gas is dissolved. 2. The method for filtering polyester according to claim 1, wherein the soluble gas is a supercritical fluid. 3. The method according to claim 1, wherein the soluble gas is carbon dioxide. 4. A resin pressure P ₁ (MP) immediately before the filtration step when a soluble gas is injected from the extruder during the filtration step.
The method for filtering a polyester according to any one of claims 1 to 3, wherein a) and the resin pressure P ₀ (MPa) immediately before the filtration step when no soluble gas is injected satisfy the following formula 1. . P ₁ ≦ P ₀ × 0.75 (Equation 1) 5. The method for filtering a polyester according to claim 1, wherein the polyester is polyethylene terephthalate, and the melt viscosity ηm of the polyethylene terephthalate immediately before the filtration step satisfies the following formula (2). ηm ≦ 0.75 × 1.55428 × 10 ⁴ × {[exp (-11.9755 + 6802.1 / T)] ηinh ^5.145 } (Equation 2) ηinh: logarithmic viscosity 6. The polyester filtration method according to claim 1, wherein a filter used in the filtration step has a filtration accuracy of not more than 80 μm. 7. A polyester film obtained by filtering a polyester by the method according to any one of claims 1 to 6, cooling and solidifying the cast film, and stretching the cast film in at least one direction. Production method. 8. The cast film obtained by cooling and solidifying is cast into a glass transition temperature (Tga) .degree.
a + 30) ° C., and subsequently unidirectionally stretched, the glass transition temperature (Tgb) ° C. to (T
The method for producing a polyester film according to claim 7, wherein the film is stretched at a temperature of (gb + 40) ° C in a direction perpendicular to the stretching direction and then heat-treated. 9. A polyester film obtained by using the method according to claim 7 or 8, substantially free of a soluble gas.