JP2010274430A - Method for heat-treating biaxially oriented polyester film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a slackening heat treatment method which enables the efficient obtaining of a biaxially oriented thermoplastic resin film improved in flatness and excellent in thermal dimensional stability. <P>SOLUTION: In this method for heat-treating a biaxially oriented polyester film, a long biaxially oriented polyester film is guided into a heating zone by a pair of sending-out rolls to be heated while being suspended in the heating zone to be allowed to run downward and the biaxially oriented polyester film is reversed in its running direction under the outside area of the heating zone to be guided to a cooling zone and cooled while being allowed to run upward by a pair of the take-off rolls arranged outside the area above the cooling zone. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a heat treatment method for a biaxially stretched polyester film. More specifically, the present invention relates to a relaxation heat treatment method for a biaxially stretched polyester film having good flatness and excellent thermal dimensional stability.

  Conventionally, a biaxially stretched thermoplastic resin film, for example, a biaxially stretched polyester film exhibits excellent performance in heat resistance, mechanical properties, chemical resistance, and the like, and thus has been used in many applications. Due to the reduction in size, weight, and accuracy of products that apply these oil films, higher performance has been required for films. In particular, a resin film having a low heat shrinkage is required in applications where a customer heats up at the stage of processing into an object, for example, membrane switches, FPCs, and heat-developable photographic photosensitive materials. Specifically, in the conventional film for touch panel, the heat shrinkage rate is about 0.5% or less when heated at a temperature of 150 ° C. for 30 minutes, but the performance of 0.1% or less is required. became.

  In response to these demands, on-line processing of the biaxially stretched polyester film involves a method in which the crystallization temperature is increased by a method of setting a high heat setting temperature, or relaxation heat treatment is performed in the longitudinal and lateral directions. ing. However, since the film cannot be sufficiently heat-shrinked by on-line processing alone, a method of heat relaxation treatment in a separate process (offline) after film production has been proposed and put into practical use.

  A number of thermal relaxation treatment methods for biaxially stretched polyester films have been proposed offline. For example, there is a method in which the thermal shrinkage rate is reduced to 0.1% or less by relaxing a film continuously run in a suspended state under a specific temperature condition (Patent Document 1). Furthermore, a method in which a resin film continuously run in a suspended state from a preheating roll installed above is heated in a heat treatment oven under a specific temperature condition and then cooled in multiple stages by a cooling roll (Patent Document 2, Patent) Document 3 and Patent Document 4). In the methods described in these patent documents, the weight of the suspended film itself (self-weight) is loaded in a state in which the film reaches the maximum temperature and becomes easy to extend, and further, the film is transferred by a cooling roll. It is inevitable that a tension is applied, and the heat relaxation effect is greatly reduced.

  As another method, there is a method in which the film travels upward from below while being heated in a heating oven, reverses the traveling direction in a state of being lifted by air pressure above and cooled in the course of traveling downward (Patent Document) 5). In this method, the film is heated up to the maximum temperature in the upper part of the heating oven while being moved from the bottom to the top, and the film reaches its maximum temperature by the air pressure and floats by the air from the highest position to the cooling roll. Therefore, it acts to stretch the resin film at the part that floats by air pressure. In addition, since the film heated to the maximum temperature in the heating oven is stretched by the air to float, it becomes difficult to maintain the flatness (no wrinkles or sagging) of the resin film at this time, The heat relaxation treatment effect is reduced.

Japanese Examined Patent Publication No. 6-17065 JP 2001-158053 A JP 2001-158054 A JP 2001-322166 A JP 2002-144421 A

  Under such circumstances, the present inventor has intensively studied to provide a heat treatment technique that eliminates various disadvantages existing in the prior art, and as a result, has completed the present invention. That is, an object of the present invention is to provide a heat treatment method capable of obtaining a biaxially stretched polyester film free from wrinkles and sagging and having good flatness and excellent thermal dimensional stability.

  In order to solve the above problems, in the present invention, in a method of heat treating a biaxially stretched polyester film, a long biaxially stretched polyester film is guided into a heating zone by a pair of film feed rolls and suspended in the heating zone. Heating in the process of running downward, reversing the running direction of the biaxially stretched polyester film outside the heating zone, guiding it into the cooling zone, and running up with a pair of film take-up rolls arranged outside the cooling zone And a method for heat-treating the biaxially stretched polyester film, characterized in that the method is cooled in the process.

The present invention is as described in detail below, has the following effects, and its industrial utility value is extremely large.
1. According to the heat treatment method of the present invention, a biaxially stretched polyester film having good flatness without wrinkles and sagging can be obtained.
2. According to the heat treatment method of the present invention, a biaxially stretched polyester film having excellent thermal dimensional stability can be obtained.

FIG. 1 is a schematic side view of an example illustrating a heat treatment method according to the present invention.

  Hereinafter, the present invention will be described in detail. In the present invention, the polyester resin is a linear saturated polyester synthesized from an aromatic dibasic acid or an ester-forming derivative thereof and a diol or an ester-forming derivative thereof. Specific examples of the polyester resin include polyethylene terephthalate, polyethylene isophthalate, polybutylene terephthalate, poly (1,4-cyclohexylenedimethylene terephthalate), polyethylene-2,6-naphthalene dicarboxylate, and the like. Also included are polymers or mixtures of these with small proportions of other resins. Among these polyesters, those having an intrinsic viscosity in the range of 0.35 to 0.9 dl / g used for molding are suitable, and those having a smaller content of residual monomers and oligomers are preferred.

  The thermoplastic resin may contain (blend) a commonly used resin additive. Examples of the resin additive to be included include a film slipperiness improver, a pigment, a heat stabilizer, a light stabilizer, and an ultraviolet absorber. Examples of the slip property improving agent for improving the slip property of the film surface include calcium carbonate, kaolin, silica, titanium oxide, alumina, crosslinked polystyrene particles, and silicon resin particles.

  The biaxially stretched polyester film to which the method of the present invention can be applied is a polyester film produced by a conventionally known method to be a biaxially stretched film and heat-set online. After the raw material polyester is dried, a die (for example, T-die) is melted in an extruder set at a temperature range in which the lower limit is the melting point (Tm ° C.) of the resin and the upper limit is (Tm + 70 ° C.). , I-die, etc.) onto a cooling drum and quenched to obtain an unstretched film. Next, the unstretched film is stretched in the longitudinal direction at a magnification of 2.5 to 5.0 times in a temperature range in which the glass transition temperature (Tg ° C.) of the resin is the lower limit and the upper limit is (Tg + 70) ° C. Furthermore, it can be manufactured through a process of heat setting in a temperature range where the lower limit is (Tg + 70) ° C. and the upper limit is Tm ° C. When the film is heat-set online, the film is subjected to a tension of 0.04 to 0.60 MPa, heat-set in a range of 190 to 240 ° C. and a processing time of 1 to 60 seconds. The time is preferably short when the temperature is high and long when the temperature is low. The thickness of the film varies depending on the end use, and is usually selected in the range of 50 μm to 500 μm. A continuously produced film is usually handled by being wound into a roll.

  The polyester film may be subjected to a surface treatment. Examples of the surface treatment of the film include an antistatic treatment, an ultraviolet ray prevention treatment, a surface hardness improvement treatment, a slip property improvement treatment, and the like, and any of them can be performed by a conventionally known treatment method.

  When the method of the present invention is used, the film is heat-treated while traveling in the heating zone and the cooling zone offline. The film wound in a roll shape is guided into the heating zone by a pair of film feed rolls. At this time, the temperature of the pair of film delivery rolls is preferably adjusted to Tg or less in advance. In the heating zone, the film is heated from the front and back in a state where the film is suspended. The hanging direction of the film is not directly below the vertical direction, but is preferably an angle that forms a curve (catenary curve) similar to a parabola when the reversal position described later and the edge running in the cooling zone are observed from the side surface (described later). See FIG. 1). By performing the heat treatment while the film is suspended, the film is sufficiently relaxed (heat-shrinked), eliminating the “wrinkles” and “sagging” that are visually observed on the film surface before heating and improving the flatness. Residual strain can be reduced. In heating in this heating zone, relaxation heat shrinkage is performed so that the heat shrinkage rate after the heat treatment is 1.0% or less, preferably 0.5% or less of the original dimension of the film. The size (width, length, volume, etc.) of the heating zone depends on the type and width of the raw resin film to be heat-treated, and can be selected in the range of 20 cm to 5 m in width and 20 cm to 2.5 m in length. The volume of the heating zone may be a volume where heating means is installed and the film can run.

  Examples of the heating means in the heating zone include (1) a method of blowing hot air, (2) a method using an infrared heater or a ceramic heater, and (3) a method of combining hot air blowing with a heater. In any method, by making the temperature distribution in the width direction uniform, the width direction distribution of the heat shrinkage rate can be reduced. In the case of the method of blowing hot air, although depending on the length of the heating zone, it is preferable that a plurality of hot air nozzles are arranged uniformly in the width direction and the length direction and blown upward on both sides of the film. In addition, it is preferable to install a blow amount adjusting valve inside each nozzle so that the wind speed of the hot air nozzle can be adjusted. In the case of the method of blowing hot air, it is further preferable to set the wind speed to minimize the fluttering of the film traveling in the heating zone. In the case of a method using a heater, it is preferable that a plurality of small heaters are arranged uniformly in the width direction and the length direction and the temperature is controlled for each heater. The heater is a far infrared radiation heater and preferably has a small heat capacity.

  The setting temperature of the heating zone is preferably selected in the temperature range in which the lower limit is (Tg + 20) ° C. and the upper limit is (Tg + 150) ° C. with respect to the Tg ° C. (secondary transition temperature) of the resin constituting the film. If the lower limit of the set temperature is (Tg + 20) ° C. or less, the film cannot be sufficiently heat relaxed (heat shrinkage) in a suspended state, and if the upper limit exceeds (Tg + 150) ° C., the temperature is too high and the shrinkage increases. Thus, an excellent flat film cannot be obtained. The residence time of the film in the heating zone varies depending on the type of film, the length of the heating zone, the set temperature of the heating zone, the heating means, the running speed of the film, etc., and can be selected in the range of 1 to 30 seconds. it can. The running speed of the film varies depending on the type of film, the length of the heating zone, the set temperature of the heating zone, the mechanism for running the film, and the like, and can be selected in the range of 5 to 30 m / min.

  The film reaches the maximum temperature {(Tg + 70) ° C. or lower} at the lower end of the heating zone. Immediately after the film has passed the lower end of the heating zone, the dead weight loaded on the heated film is minimal at this position. However, since the direction of travel is reversed immediately after passing through the lower end of the heating zone and guided into the cooling zone, the weight of the suspended film itself (self-weight) is loaded on the film when the maximum temperature is reached. Since it is not loaded for a long time, the heat relaxation effect is not reduced. In the heating zone, hot air is discharged from the heating zone at the upper end of the heating zone while flowing upward from the lower end of the heating zone. At this time, it is preferable to provide a shielding plate so that hot air does not blow directly to the pair of film delivery rolls.

  In the present invention, reversal means that a film that has been running downward in a suspended state is made to form a curve (catenary curve) similar to a parabola (see FIG. 1 to be described later) and is turned upward to enter the cooling zone. It refers to guiding. The direction in which the film travels upward is not directly above the vertical, but an angle that forms a curve (catenary curve) similar to a parabola when the heating zone, the reversal position, and the end running in the cooling zone are observed from the side. It is preferable to do this. In the cooling zone, a pair of take-up rolls arranged above and below the cooling zone are arranged, the film is cooled while running at a controlled speed, and the tensioned state suspended in the heating zone is fixed. And “sagging” can be eliminated, and a product with low heat shrinkage rate and improved flatness can be obtained.

  Examples of the cooling means in the cooling zone include (1) a method in which water-cooled plates are arranged on both sides of the film surface, and (2) a method in which cold air is blown between the water-cooled plates of the film in addition to arranging the water-cooled plates. It is done. The water-cooled plate has a sufficient surface area and is a method of cooling the film by radiant heat transfer. The water-cooled plate preferably has a far-infrared radiation coating on the surface facing the film to enhance the heat-absorbing effect, and has a water-cooled circulation pipe for cooling the copy heat-absorbing plate on the back surface. The circulating cooling water is preferably maintained at a constant water temperature by controlling the temperature to the lowest water temperature at which the cooling plate does not condense. The cold air is preferably arranged by arranging a plurality of cold air nozzles uniformly in the width direction and the length direction of the film and blowing upward. In addition, it is preferable to install a blowing amount adjusting valve inside each nozzle so that the wind speed of the cold air nozzle can be adjusted. The cold air is preferably air at room temperature or dry air having a dew point lowered to below that. It is preferable to provide a shielding plate so that the cold air is not blown directly onto the pair of film take-up rolls from above the cooling zone.

  The film cooled in the vicinity of the glass transition point (Tg) in the cooling zone is continuously taken up and wound into a roll while being run upward by a pair of film take-up rolls arranged outside the cooling zone. The At this time, it is preferable to adjust the surface temperature of the pair of film take-up rolls so as to be (Tg-10) ° C. or lower. The traveling speed of the film is adjusted by the rotational speed of a pair of film feed rolls arranged outside the heating zone and a pair of film take-up rolls arranged outside the cooling zone. The length of the film staying in the portion that reverses the traveling direction can be adjusted by a pair of film delivery rolls and a pair of film take-up rolls. If the lower limit point of the film staying in the part that reverses the traveling direction is detected by the lower limit point detection sensor and the film traveling speed is adjusted in conjunction with a pair of film take-up rolls arranged outside the cooling zone, The continuous operation is preferable.

  Hereinafter, the present invention will be described with reference to the drawings. FIG. 1 is a schematic side view of an example illustrating a heat treatment method according to the present invention. In the figure, 1 is a biaxially stretched polyester film, 2, 3 is a film feed roll, 4 is a heating zone, 5 is a position where the maximum temperature is reached, 6 is a lower limit point of the film, 7 is a film traveling direction reversal zone, and 8 is a lower limit. The point detection sensors 9, 9a, 9b are radiation thermometers, 9a measures the maximum temperature of the film at the heating zone outlet, and 9b measures the film temperature at the cooling zone outlet. 10 is a catenary curve, 11 is a cooling zone, 12 and 13 are film take-up rolls, and 14 is a central axis of the catenary curve.

  The film to which the heat treatment method according to the present invention can be applied is a long biaxially stretched polyester film. Since short films can be processed on a laboratory scale, they are not suitable for applying the heat treatment method according to the present invention. A long biaxially stretched polyester film (not shown) wound in a roll shape not shown is usually guided into the heating zone 4 by a pair of film feed rolls 2 and 3, Suspend and drive down. As shown in FIG. 1, the heating zone 4 and the cooling zone 11 are arranged such that the letter “K” of the katakana is reversed in a side view. The traveling speed of the film can be adjusted by interlocking the pair of film feed rolls 2 and 3 disposed above and outside the heating zone 4 with the pair of film take-up rolls 12 and 13 disposed above and outside the cooling zone 11.

  When the treatment is specifically performed by the heat treatment method according to the present invention, the following procedure can be used. While performing heat treatment of the film, the film surface temperature is measured at 9a and 9b, and the film transfer speed, the heating zone setting conditions, and the cooling zone setting conditions are adjusted. At this time, the rotation speed of the pair of film feed rolls 2 and 3 and the rotation speed of the pair of film take-up rolls 12 and 13 are set so that the lower limit point of the film is held at a certain position by the signal of the detection sensor 8 of the lower limit point. At the same time, the radiation thermometer can be controlled so that the measured value by 9b becomes the glass transition point (Tg), and the operation can be shifted to the steady operation. By adopting such a series of control methods, it is possible to always give a certain thermal relaxation effect even with various films having different shrinkage rates.

  In the heat treatment apparatus used in the heat treatment method according to the present invention, the film delivery rolls 2 and 3 and the film take-up rolls 12 and 13 have a central axis in the same plane and parallel to each other, and a catenary curve (parabolic curve). Is arranged at the target position around the central axis of the curve), but the heights are not necessarily the same. The delivery roll 2 and the take-up roll 10 are preferably made of the same material, the same diameter, and the same length. When the diameter of the delivery roll 2 is x, the minimum distance between the facing surfaces of the delivery roll 2 and the take-up roll 12 is preferably 2x, and the minimum distance from the delivery roll 2 to the lower limit point 6 is preferably 2x or more. When the minimum distance between the facing surfaces of the delivery roll 2 and the take-up roll 12 and the minimum distance from the delivery roll 2 to the lower limit point 6 are less than 2x, the heating zone 4 and the cooling zone are between curves similar to a parabola. 9 is not preferable. The upper limit of the distance from the delivery roll 2 to the lower limit point 6 is not limited from the apparatus side, and can be determined according to restrictions on the installation location of equipment, a preferable processing speed, and the like.

  EXAMPLES Hereinafter, although this invention is demonstrated based on an Example, this invention is not limited to the following description examples, unless the summary is exceeded. In addition, the characteristic value of polyester and the characteristic value of the obtained film were measured with the following method.

(1) Glass transition temperature (Tg):
10 mg of a sample was set in a DSC (differential scanning calorimeter) manufactured by PerkinElmer, and the sample was melted at a temperature of 300 ° C. for 5 minutes and then rapidly cooled in liquid nitrogen. The rapidly cooled sample was cooled at 10 ° C./min. The glass transition temperature (Tg) was measured.

(2) Film temperature (° C) in the heating zone:
A small hole of about 0.5 mm was formed at a 30 cm pitch at a position that is not easily affected by the heater of the box frame in the heating zone, and the film temperature during running from the small hole was measured with a radiation thermometer. The small holes were covered when the temperature was not measured so that the temperature in the heating zone did not cool.

(3) Film temperature (° C) in the cooling zone:
A small hole of about 0.5 mm was formed at a 30 cm pitch at a position that is not easily affected by the hot air blowing nozzle of the box frame in each cold zone, and the film temperature during traveling was measured from this small hole with a radiation thermometer. The small holes were covered when the temperature was not measured so that the temperature in the cooling zone did not cool.

(4) 150 ° C. heat shrinkage (%):
A sample having a size of 350 mm in the running (longitudinal) direction and 50 mm in the width direction is cut out from the heat-treated film, marked at 300 mm intervals in the vicinity of both ends in the longitudinal direction of the sample, and placed in an oven adjusted to a temperature of 150 ° C. Leave the end unconstrained (as a free end) for 30 minutes. After leaving, it was taken out and left at room temperature to reach room temperature, and the distance between the gauge points was measured. The length at this time was taken as Lmm, and the following formula, 100 × (300−L) / 300, was calculated.

(5) Film breaking strength (MPa):
It measured based on JIS C2151.
(6) Film flatness:
A sample having a size of 250 cm in the running (longitudinal) direction and 100 cm in the width direction was cut out from the heat-treated film, this sample was spread on a mirror-like flat plate, and the state of undulation and swelling was visually observed and evaluated.

[Example 1]
<Biaxially stretched polyethylene terephthalate film>
A commercially available highly transparent biaxially stretched polyethylene terephthalate film (manufactured by Teijin DuPont Films Ltd., Teijin Tetron Film, Brand: HS), with a thickness of 188 μm and a break in the running (longitudinal) direction (MD) The strength is 190 MPa, the breaking strength in the width direction (TD) direction is 220 MPa, and the 150 ° C. heat shrinkage ratio is 0.9% in the MD direction and 0.3% in the TD direction.

<Heat treatment equipment for biaxially stretched polyethylene terephthalate film>
1 heater panel with 1000 mm in length and 1600 mm in width, 18 far-infrared radiant heaters with a heating capacity of 600 W, arranged and equipped at equal intervals in the length and width, and the surface side that is 1000 mm in length and 1600 mm in width and faces the heater A far-infrared reflector having a far-infrared radiation coating applied thereto was prepared. A heater panel and a far-infrared reflective panel were installed facing each other, a gap for hanging the film was provided between the two panels, and fixed to a cover (not shown) to form a heating zone 4. Small holes of about 5 mm were drilled at a 30 cm pitch at positions that are not easily affected by the heater on the far-infrared reflective panel side of the heating zone, and the film temperature during traveling from these small holes could be measured with a radiation thermometer. A pair of film delivery rolls 2 (diameter 200 mm, length 1500 mm) and 3 (diameter 100 mm, length 1500 mm) were disposed above and outside the heating zone 4. A radiation thermometer 9a is arranged at the outlet 5 below the heating zone 4 so that the maximum surface temperature of the running film can be measured, and the maximum surface temperature of the film is always constant even if the running speed of the film changes. So that the output of the heater can be controlled. In the film traveling direction reversal zone 7, a lower limit detection sensor 8 for detecting the lower limit point 6 of the film is disposed, and the film is linked to a pair of film delivery rolls 2 and 3 disposed above and outside the heating zone. The running speed can be adjusted. By doing in this way, even if the shrinkage rate of the film changes greatly, the lower limit point 6 of the film running direction reversal zone 7 can always keep a constant position.

  The cooling zone 11 has a structure in which two water-cooled radiant heat transfer heat-absorbing plates, whose dimensions are larger than those of the heating zone, are 1200 mm long and 1600 mm wide, and the cooling water maintained at a water temperature of 25 ° C. is circulated. A far-infrared radiation paint was applied to the surface of the two radiant heat transfer heat absorbing plates facing each other. These two cooling plates were installed facing each other, and a gap for running the film was provided between them and fixed to a cover (not shown) to form a cooling zone 11. Small holes of about 5 mm were drilled at a 30 cm pitch at positions that were not easily affected by cooling of the cooling radiation heat transfer surface in the cooling zone, and the film temperature during traveling could be measured with a radiation thermometer from these small holes. The film reversed at the portion 7 for reversing the traveling direction of the film below the heating zone 4 was allowed to travel upward by a pair of film take-up rolls 12 and 13 disposed outside the cooling zone 11. At the exit of the cooling zone 11, the temperature of the film after cooling was measured with a radiation thermometer 9b, and the speeds of the film take-up rolls 12 and 13 were controlled so that the temperature would be the Tg of the film.

<Heat treatment method of biaxially stretched polyester film>
A roll-shaped biaxially stretched polyester film having a width of 1500 mm was suspended from the pair of film delivery rolls 2 and 3 in FIG. In the heating zone, the temperature was set to 120 ° C. on the upper side with an infrared heater and 150 ° C. on the lower side, and the film running speed was set to 20 m / min.

  The shortest distance between the film delivery roll and the film take-up roll is 320 mm, and the length from the height of the film delivery roll to the lower limit point 6 of the film is 1300 mm. The film was drawn to film take-up rolls 12 and 13 and taken at a running speed of 5 m / min. The set temperature of the cooling water in the cooling zone 11 was 25 ° C. The film temperature when reaching the take-up rolls 12 and 13 was 70 ° C. After that, the film was cooled by the take-up roll and then ran between a plurality of rolls (not shown) and wound into a roll.

<Characteristics of film after heat treatment>
About the obtained heat-treated film, the 150 ° C. thermal shrinkage measured by the method described in the evaluation method described above is 0.06% in the vertical direction and 0.04% in the horizontal direction, and the flatness visually observed is extremely high. It was good.

[Example 2]
<Biaxially stretched polyethylene terephthalate film>
A commercially available highly transparent biaxially stretched polyethylene terephthalate film (manufactured by Teijin DuPont Films Ltd., trade name: Teijin Tetron Film, brand: HSL) with a thickness of 188 μm and a running (longitudinal) direction (MD ) Has a breaking strength of 190 MPa, a breaking strength in the width direction (TD) direction of 200 MPa, and a 150 ° C. heat shrinkage ratio is 0.4% in the MD direction and −0.1% in the TD direction.

<The heat processing method of a biaxially stretched polyester film, the characteristic of the film after heat processing>
About the said film, the same heat processing apparatus as used in Example 1 was used, and it heat-processed on the conditions similar to the same example. About the obtained heat-treated film, the 150 ° C. heat shrinkage ratio measured by the method described in the above evaluation method is 0.04% in the vertical direction and 0.03% in the horizontal direction, and the flatness visually observed is extremely high. It was good.

[Example 3]
<Biaxially stretched polyethylene terephthalate film>
A commercially available biaxially stretched polyethylene terephthalate film (Toyobo Co., Ltd., trade name: Cosmo Shine, brand name: A4100) having a thickness of 188 μm, a tensile strength in the MD direction of 179 MPa, and a TD direction The tensile strength is 197 MPa, and the thermal shrinkage at 150 ° C. is 0.90% in the MD direction and 0.70% in the TD direction.

<The heat processing method of a biaxially stretched polyester film, the characteristic of the film after heat processing>
About the said film, the same heat processing apparatus as used in Example 1 was used, and it heat-processed on the conditions similar to the same example. About the obtained heat-treated film, the 150 ° C. heat shrinkage ratio measured by the method described in the above evaluation method is 0.07% in the vertical direction and 0.06% in the horizontal direction, and the flatness visually observed is extremely high. It was good.

  Since the biaxially stretched polyester film heat-treated by the method according to the present invention has good flatness and excellent thermal dimensional stability, it is suitable for use in membrane switches, touch panel switches, FPCs, and heat-developable photographic photosensitive materials. Is preferred.

1: Biaxially stretched polyester film 2, 3: Film feed roll 4: Heating zone 5: Maximum temperature position 6: Film lower limit point 7: Film running direction reversal zone 8: Lower limit detection sensor 9: Radiation thermometer 9a: Radiation thermometer for maximum film temperature measurement 9b: Radiation thermometer for film cooling temperature measurement 10: Catenary curve 11: Cooling zone 12, 13: Film take-up roll 14: Central axis of catenary curve

Claims (5)

  In the method of heat-treating a biaxially stretched polyester film, a long biaxially stretched polyester film is introduced into a heating zone with a pair of film feed rolls, heated in the process of being suspended in the heating zone and traveling downward, outside the heating zone The direction of the biaxially stretched polyester film is reversed in the lower direction of the film, guided into the cooling zone, and cooled in the process of traveling upward with a pair of film take-up rolls arranged outside the cooling zone. A heat treatment method for an axially stretched polyester film.   The biaxially stretched polyester film according to claim 1, wherein the running direction is reversed immediately after the film has passed the lower end of the heating zone and at a position where the dead weight applied to the film heated to the maximum temperature is minimized. Heat treatment method.   The heat processing method of the biaxially stretched polyester film according to claim 1 or 2, wherein the biaxially stretched polyester film is caused to travel so as to form a catenary curve when an end portion where the film travels is observed from a side surface.   The method for heat-treating a biaxially stretched polyester film according to claim 3, wherein the film feeding roll and the film take-up roll are arranged so that the central axes thereof are parallel to each other and the central axis of the catenary curve is the center. .   The lower limit point for reversing the traveling direction of the biaxially stretched polyester film is detected by a detection sensor for the lower limit point, and the film traveling speed is adjusted in conjunction with a pair of take-up rolls arranged on the upper outside. 5. A heat treatment method for a biaxially stretched polyester film according to claim 4.

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