JP6272048B2 - Polyester film - Google Patents

Description

  The present invention relates to a polyester film in which generation of wrinkles and curling are suppressed. In particular, a polyester film that can be suitably used as a base film for a transparent electrode or a protective film for a transparent electrode, and suppresses the occurrence of wrinkles in the transport process and curling when bonded to other members. About.

  In recent years, polyester films (usually oriented polyester films for the purpose of obtaining preferred strength, etc.) have been developed for transparent electrode base films having transparent conductive films such as ITO films because of their transparency and heat-resistant dimensional stability. It is often used as a base film of a protective film for protecting such a transparent electrode.

  The base film used for the transparent electrode is generally used with a hard coat layer provided for the purpose of preventing appearance scratches and the like in the processing step of laminating the ITO film. In addition, the transparent electrode particularly used for the capacitive touch panel needs to pattern the transparent conductive film in a line shape or a diamond shape in order to detect the touch position. In order to prevent the pattern from being visually recognized due to the difference in reflectance and transmittance depending on the presence or absence of the transparent conductive film, an optical compensation undercoat layer is provided between the ITO film and the hard coat layer. It is common.

  When forming the hard coat layer or the optical compensation undercoat layer, heat treatment is performed at a temperature of, for example, 80 ° C. or more in order to dry the solvent or cure the hard coat layer or the undercoat layer. When continuously processing while being conveyed by a roll, there are problems that wrinkles and sagging occur in the base film, greatly reducing production efficiency, and it is difficult to keep the thickness of the hard coat layer and undercoat layer uniform. is there.

  In addition, the transparent electrode is used as a laminate in which a protective film is bonded for the purpose of protecting the base film and / or the ITO film. For example, the transparent electrode remains in the form of such a laminate for crystallization of the ITO film. There is a case where heat treatment is performed, and there is a problem that the laminated body curls in such heat treatment.

With respect to the problem relating to the wrinkles at the time of processing, there is a report as described in Patent Documents 1 and 2, in which the heat shrinkage stress in the 45 ° direction of the film is reduced to suppress this.
In addition, regarding curling suppression of the laminated body, as in Patent Documents 3 and 4, it is reported that the thermal shrinkage rate in the surface direction of each layer constituting the laminated body is reduced or the thermal shrinkage rate in the surface direction of each layer is adjusted. There is.

JP 2005-186555 A JP 2006-181996 A Japanese Patent Laid-Open No. 11-254576 JP 2002-73282 A

  However, in the above Patent Documents 1 and 2, no consideration is given to curling at the time of bonding. In addition, in the methods of Patent Documents 3 and 4, it is necessary to select a base film or to match the direction of bonding in consideration of the heat shrinkage rate in the surface direction of each layer of the laminate. Is not realistic.

  In the oriented film, usually, in the film width direction (direction perpendicular to the film forming machine axis direction in the film plane), variations in characteristics such as orientation and heat shrinkage occur due to the bowing phenomenon. Is. Then, for example, in the center in the width direction of the film, even if it is easy to manufacture a film having favorable characteristics against wrinkles and curls, the end (closer to the portion gripped with the clip by the stenter than the center of the film) (Part) is actually difficult.

  Therefore, the present invention may be a film corresponding to an end portion in film production, for example, even if it undergoes a heat treatment applied when forming a hard coat layer or an undercoat layer on a base film in the production of a transparent electrode. Can suppress wrinkles and sagging during transportation, and easily suppress curl even after heat treatment such as in the crystallization process of ITO film after being laminated with a transparent electrode It aims at providing the polyester film which can be performed.

  With regard to the problem of wrinkles and curls described above, the present inventors set both the heat shrinkage rate in the oblique direction of the film and the linear expansion coefficient in the direction of the maximum orientation axis of the film and in the direction perpendicular thereto as a specific range. The inventors have found that this can be solved and have reached the present invention.

That is, the present invention employs the following configuration in order to solve the above problems.
1. A polyester film made of a polyethylene terephthalate resin, wherein the polyester film has a region in which the maximum orientation axis direction is 20 ° or more and 55 ° or less when the width direction is set to 0 °. A polyester film whose shrinkage rate satisfies the following (A) and whose linear expansion rate satisfies the following (B).
(A) In the above region, the 150 ° C. 30 minute heat shrinkage ratio SD1 in the 45 ° direction and the 150 ° C. 30 minute heat shrinkage ratio SD2 in the 135 ° direction are expressed by the following formula 1 with respect to 0 ° in the width direction of the film. Meet.
| SD1-SD2 | ≦ 0.20% (Formula 1)
(B) The linear expansion coefficient αtmin in the maximum orientation axis direction in the region and the linear expansion coefficient αtmax in the direction orthogonal to the film plane satisfy the following expressions 2 and 3.
αtmax <42 ppm / ° C. (Formula 2)
αtmax−αtmin <10 ppm / ° C. (Formula 3)
2. In the region where the maximum orientation axis direction is 20 ° or more and 55 ° or less, the heat shrinkage rate at 150 ° C. for 30 minutes in the longitudinal direction is 1.00% or less and the heat shrinkage rate at 150 ° C. for 30 minutes in the width direction. 2. The polyester film according to 1 above, wherein the content is 0.50% or less.
3. 3. The polyester film as described in 1 or 2 above, which is formed by simultaneous biaxial stretching.
4). The polyester film according to any one of 1 to 3 above, which is for a transparent electrode.
5). 5. The polyester film as described in 4 above, which is for a transparent electrode substrate.
6). 5. The polyester film as described in 4 above, which is for a transparent electrode protective film.

  According to the present invention, even if the film corresponds to an edge in film production, for example, it has undergone a heat treatment that is performed when a hard coat layer or an undercoat layer is formed on a base film in the production of a transparent electrode. However, curling is easy even if it is subjected to heat treatment that is performed in the crystallization process of the ITO film, for example, after suppressing wrinkles and sagging during conveyance and forming a laminate by laminating with a transparent electrode. A polyester film that can be suppressed can be provided.

  The polyester film of the present invention exhibiting such effects can be suitably used as a film for transparent electrodes, such as a base film for transparent electrodes and a base film for protective films for transparent electrodes.

[Polyester film]
The polyester film of the present invention is preferably a film made of polyethylene terephthalate resin.

(Polyethylene terephthalate resin)
In the present invention, the polyethylene terephthalate resin constituting the polyester film is a polyester in which 90 mol% or more of all repeating units are composed of an ethylene terephthalate component. According to such an embodiment, all the properties of transparency, heat resistance, and mechanical properties required as an optical use film, particularly a touch panel film, a transparent electrode, and a protective film used for the transparent electrode can be obtained simultaneously.

  This polyester may be a homopolymer or a copolymer. In the case of a copolymer, the copolymer component may be copolymerized at a ratio of 10 mol% or less, preferably 5 mol% or less, more preferably 3 mol% or less, based on the total dicarboxylic acid component. However, the copolymer is preferably a homopolymer from the viewpoint of hydrolysis resistance, since the water tends to diffuse due to disorder of the molecular arrangement in the film and promotes hydrolysis. In the case of a copolymer, examples of copolymer components include aromatic dicarboxylic acids such as isophthalic acid and 2,6-naphthalenedicarboxylic acid; aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid, and 1,10-decanedicarboxylic acid. In addition, aliphatic diols such as 1,4-butanediol, 1,6-hexanediol, and neopentyl glycol; and alicyclic diols such as 1,4-cyclohexanedimethanol can be used. These copolymer components may be used alone or in combination of two or more.

[Characteristics of polyester film]
(Maximum orientation axis direction)
In the polyester film of the present invention, the width direction (in the present invention, the width direction refers to a direction perpendicular to the film forming machine axis direction and the thickness direction. Hereinafter, it may be referred to as a lateral direction or TD) is 0. It has a region where the maximum orientation axis direction is 20 ° or more and 55 ° or less. The maximum orientation axis direction is preferably 30 ° or more, more preferably 40 ° or more, still more preferably 45 ° or more, and preferably 50 ° or less. For example, since the film in the parent roll obtained by the stretch film-forming machine has the region as described above between the end portion and the center portion of the film, this is the case. Moreover, the film in the child roll obtained by slitting the vicinity of the central portion from the end portion of the parent roll also has the region as described above and corresponds to this.

  In the present invention, the maximum orientation axis direction means a direction in which the refractive index is maximized by measuring the refractive index up to 180 ° every 5 ° with TD being 0 °. When the angle exceeds 90 ° and is equal to or less than 180 °, the TD in the 180 ° direction opposite to the TD initially set to 0 ° is defined as 0 ° and expressed in a range of 0 to 90 °.

(Heat shrinkage and linear expansion coefficient)
The polyester film of the present invention is required to satisfy the following (A) and (B) in terms of thermal shrinkage and linear expansion coefficient in the above-mentioned region.
(A) In the above region, the 150 ° C. 30 minute heat shrinkage ratio SD1 in the 45 ° direction and the 150 ° C. 30 minute heat shrinkage ratio SD2 in the 135 ° direction are expressed by the following formula 1 with respect to 0 ° in the width direction of the film. Meet.
| SD1-SD2 | ≦ 0.20% (Formula 1)
(B) The linear expansion coefficient αtmin in the maximum orientation axis direction in the region and the linear expansion coefficient αtmax in the direction orthogonal to the film plane satisfy the following expressions 2 and 3.
αtmax <42 ppm / ° C. (Formula 2)
αtmax−αtmin <10 ppm / ° C. (Formula 3)

By satisfying the above aspects (A) and (B) at the same time, wrinkles can be suppressed during heat treatment and curling can be suppressed in a laminate using the same.
In the above formula (1), if the difference (absolute value) between SD1 and SD2 is larger than the value on the right side, wrinkle suppression and curl suppression tend to be inferior. From this viewpoint, the difference between SD1 and SD2 is preferably 0.15% or less, more preferably 0.12% or less, further preferably 0.10% or less, and particularly preferably 0.08% or less. The smaller the difference, the better. The lower limit is preferably 0.

  In addition, in the said area | region, it is preferable that the thermal contraction rate of a polyester film is small, and it exists in the tendency for the improvement effect of a wrinkle suppression and the improvement effect of curl suppression to become high by this. Moreover, when the thermal contraction rate is large, the above formula 1 tends to be difficult to be satisfied. From this viewpoint, the heat yield at 150 ° C. for 30 minutes in the longitudinal direction MD is preferably 1.00% or less, more preferably 0.60% or less, and further preferably 0.45% or less. Further, the heat yield at 150 ° C. for 30 minutes in the width direction TD is preferably 0.50% or less, more preferably 0.30% or less, and further preferably 0.15% or less. It is more preferable that the thermal shrinkage ratios of MD and TD are simultaneously preferable.

Further, in the above equation (2), even if αtmax is equal to or greater than the value on the right side, and in the above equation (3), even if the value of αtmax−αtmin is equal to or greater than the value on the right side, It tends to be inferior to the suppression. From this viewpoint, αtmax is more preferably 35 ppm / ° C. or less, and further preferably 25 ppm / ° C. or less. The αtmax is preferably as small as possible from the viewpoint of the above effects. However, in a polyester film made of a polyethylene terephthalate resin, the lower limit is practically 10 ppm / ° C. The shrinkage rate tends to increase, and as a result, it tends to be difficult to balance the thermal shrinkage rate and the linear expansion rate.
Further, the value of αtmax−αtmin is more preferably 8 ppm / ° C. or less, and further preferably 5 ppm / ° C. or less. The difference between αtmax and αtmin is preferably as small as possible from the viewpoint of the above effects, and most preferably 0 ppm / ° C.

The present invention comprises the above-described aspect that could not be obtained conventionally, that is, an aspect in which the difference in the direction of thermal shrinkage is small and at the same time the linear expansion coefficient and the direction difference are small. And by setting it as such a special aspect, the problem concerning a wrinkle and a curl is suppressed simultaneously.
The heat shrinkage of the film is derived from the effect of relieving the stretching strain caused by stretching, that is, the effect of stretching is large. Generally, when the orientation is increased by increasing the stretching ratio, etc., the heat shrinkage in the orientation direction. The rate tends to increase. When the film is stretched in a certain direction, it also acts on the orientation in a direction perpendicular to the one direction, and this effect becomes greater as the stretch ratio increases. For this reason, if the thermal shrinkage rate increases due to a draw ratio that is too high, it tends to be difficult to reduce the direction difference of the thermal shrinkage rate.

  On the other hand, the linear expansion of the film is derived from the orientation of the molecular chain in the stretching direction by stretching, that is, this is also greatly influenced by stretching, but generally when the orientation is increased by increasing the stretching ratio, etc. The linear expansion coefficient in the orientation direction tends to be small. Therefore, usually, under the same draw ratio conditions, there is a contradictory relationship between reducing the thermal shrinkage rate (direction difference in thermal shrinkage rate) and reducing the linear expansion rate, and while maintaining a low linear expansion rate, It is difficult to reduce the shrinkage rate. In order to make these approaches close to each other, it is common to perform heat treatment (heat setting) after completion of stretching, or to perform relaxation treatment after heat setting, but although it can be adjusted to some extent, the above formula (1 ) To (3) are not satisfied at the same time.

  In sequential biaxial stretching, the second-stage stretching direction (usually, the first-stage stretching is often the longitudinal direction and the second-stage stretching is often the transverse direction) is dominant. Therefore, the orientation of the first stage and the orientation of the second stage are balanced, and the orientation and the diagonal orientation are balanced. In other words, it is very difficult to satisfy all of the above formulas (1) to (3) at the same time, which has not been achieved so far.

  For this reason, in the present invention, simultaneous biaxial stretching is preferred because it is relatively easy to balance the orientation in the plane direction. However, the longitudinal stretching in simultaneous biaxial stretching is a stretching method in which the distance between adjacent clips is widened while holding both sides of the film with clips as viewed from the film forming machine axis direction of the tenter. Stretching stress spreads, and longitudinal stretching in the vicinity of the center of the film tends to slow down the longitudinal stretching speed in comparison with both sides. At this time, since the transverse stretching also proceeds at the same time, as a result, both sides of the film are selectively stretched in an oblique direction with respect to the film forming machine axis direction, and the refractive index ellipsoid of the film is distorted. The linear expansion coefficient in the direction orthogonal to the maximum orientation axis direction in the plane increases, and the difference between the linear expansion coefficient in the direction of the maximum alignment axis direction and the linear expansion coefficient in the direction orthogonal to that in the film plane also increases. Become. In addition, the difference between the thermal shrinkage rate in the 45 ° direction and the thermal shrinkage rate in the 135 ° direction with respect to 0 ° in the width direction of the film, which is important when forming a laminated body by bonding, tends to increase. is there.

  Therefore, it is necessary not only to simultaneously stretch the biaxial direction but also to adopt a special stretching method that corrects the orientation distortion as described above. That is, in the present invention, after stretching by simultaneous biaxial stretching up to a predetermined stretching ratio, then preferably continuously, further by adopting a stretching method that performs lateral uniaxial stretching at a predetermined stretching ratio, A polyester film satisfying the above formulas (1) to (3) at the same time is obtained by satisfying both the thermal contraction rate and the linear expansion rate which are in a contradictory relationship. Furthermore, it is preferable that the polyester film satisfying the above formulas (1) to (3) is easily obtained by performing heat treatment after stretching and then performing a relaxation treatment at a predetermined relaxation rate while gradually cooling.

(Other ingredients)
In the polyester film of the present invention, other thermoplastic resins such as polyethylene naphthalate and polytrimethylene terephthalate can be mixed as long as the effects of the present invention are not impaired. Moreover, you may mix | blend coloring agents, such as a ultraviolet absorber, antioxidant, surfactant, a fluorescent whitening agent, a lubricant agent, a light-shielding agent, a matting agent, and a dye, a pigment.
In addition, when using the polyester film of this invention for the optical film, especially the protective film for protecting the base film and transparent electrode of a transparent electrode, it is preferable that it is more transparent. Therefore, the polyester film of the present invention preferably has a particle content of 500 nm or more, more preferably 300 nm or more, and preferably 150 ppm or less, more preferably 100 ppm or less.

[Production method of polyester film]
Hereinafter, although the manufacturing method of the polyester film of this invention is demonstrated concretely, as long as the summary of this invention is satisfied, this invention is not specifically limited to the following illustrations.
First, a polyester chip (polyethylene terephthalate chip) dried by a known method is supplied to a melt extrusion apparatus and heated to a temperature equal to or higher than the melting point of each polymer and melted. Next, the molten polymer is extruded from a die and rapidly cooled and solidified on a rotary cooling drum so that the temperature is equal to or lower than the glass transition temperature to obtain a substantially amorphous unoriented sheet. In this case, in order to improve the flatness of the sheet, it is preferable to improve the adhesion between the sheet and the rotary cooling drum. In the present invention, an electrostatic application adhesion method and / or a liquid application adhesion method is preferably employed.

In the present invention, the sheet thus obtained is stretched biaxially to form a film. At this time, it is important to adopt the stretching conditions using the simultaneous biaxial stretching and the lateral uniaxial stretching as described above in order to satisfy the expressions (1) to (3) defined by the present invention.
Specifically describing stretching conditions, the unstretched sheet is preferably biaxial at 80 ° C. to 120 ° C. with a longitudinal stretching ratio of 2.5 to 4.0 times and a lateral stretching ratio of 1.5 to 2.5 times. The film is stretched, and then preferably continuously and uniaxially stretched at 80 to 120 ° C. in the transverse direction by 1.5 to 2.5 times. Under such stretching conditions, the longitudinal stretching ratio in the simultaneous biaxial stretching in the previous stage is preferably 3.0 to 4.0 times, more preferably 3.2 to 4.0 times, and still more preferably 3.4 to 3.8. Is double. Moreover, the transverse draw ratio in the simultaneous biaxial stretching in the previous stage is preferably 1.7 to 2.5 times, more preferably 1.7 to 1.9 times. Furthermore, the transverse draw ratio in the subsequent transverse uniaxial drawing is preferably 1.9 to 2.3 times, more preferably 2.0 to 2.2 times.

  Following the stretching, heat treatment (heat setting) is performed at a temperature of 200 to 240 ° C. for 1 to 300 seconds. Further, at this time, it is preferable to perform a relaxation treatment with a relaxation rate of 0.1 to 10% in the longitudinal direction and / or the transverse direction in the maximum temperature zone for heat treatment and / or the cooling zone at the exit of the heat treatment. In the belt, it is preferable for the purpose of improving the flatness of the film that the longitudinal relaxation is gradually relaxed over 10 seconds.

  The total thickness of the polyester film of the present invention is usually 23 to 200 μm, preferably 38 to 188 μm. If the thickness is too thin, the mechanical strength and heat resistance of the film may be insufficient, and problems such as wrinkles may occur in subsequent processing steps. On the other hand, if the thickness of the film is too thick, the film may be too stiff and handleability in the subsequent process may be poor.

[Coating layer]
In transparent electrode base film, optical film is secured while ensuring adhesion between hard coat film and polyester film for the purpose of preventing appearance scratches in the process of laminating ITO film on polyester film. It is preferable to provide a coating layer on the surface of the polyester film in order to prevent a typical interference pattern and improve the appearance and visibility of the laminate in which the ITO film is laminated. In the formation of such a coating layer, the method of performing a film manufacturing process, particularly before stretching can form a very thin coating layer, and the drying and curing reaction of the coating solution can be carried out in the film forming process. This is preferable. The coating layer is preferably composed of a combination of a crosslinking agent and various binder resins, and as the binder resin, at least one polymer selected from polyester, acrylic polymer and polyurethane is used from the viewpoint of adhesiveness. . Each of the above polymers shall also include derivatives thereof. The derivative here refers to a polymer obtained by reacting a reactive compound with a copolymer or a functional group with another polymer. As the cross-linking agent, melamine-based, epoxy-based, and oxazoline-based resins are generally used, but oxazoline-based resins are particularly preferable in terms of coating properties and durable adhesiveness. Furthermore, particles such as organic particles, inorganic particles, and organic-inorganic composite particles may be added for the purpose of adjusting the refractive index and imparting slipperiness. The content ratio is preferably 40 to 98% by mass of the binder resin, 1 to 30% by mass of the crosslinking agent, and 1 to 30% of the particles.

[Usage]
The polyester film of the present invention can be suitably used for a transparent electrode. Examples of the transparent electrode application include a substrate film application for a transparent electrode and a substrate film application for a protective film used by being attached to a transparent electrode.

(Transparent electrode substrate film)
The polyester film of the present invention can be used as a transparent electrode substrate film (transparent electrode substrate), and a transparent conductive film such as an ITO film can be provided thereon to form a transparent electrode. In such applications, generally, a hard coat layer is provided in order to prevent scratches in the step of laminating the transparent electrode film. Further, when the transparent conductive film is patterned, an optical compensation undercoat layer is provided in order to suppress the appearance of the bone. Since the polyester film of the present invention has the above-described embodiment, it is applied when other layers such as a hard coat layer and an optical compensation undercoat layer are provided. The occurrence of wrinkles and sagging can be suitably suppressed.

(Transparent electrode protective film)
The polyester film of the present invention can be suitably used as a base film for a protective film (transparent electrode protective film) for protecting a transparent electrode. This protective film is the structure which provided the adhesive layer on the base film of a protective film. And it is affixed and used for a transparent electrode in order to protect the base film and transparent conductive film of a transparent electrode. At this time, heat treatment is performed for the purpose of crystallizing the transparent conductive film, for example, when the transparent conductive film is an ITO film in the form of a laminate in which a protective film is attached to the transparent electrode. In the case of an ITO film, the temperature of the heat treatment is about 150 ° C. Moreover, it is often heat-treated after being cut into sheets. Since the polyester film of the present invention has the above-described aspect, curling can be suitably suppressed even when such heat treatment is performed. The base film of the transparent electrode in the laminate is not particularly limited as long as it is made of polyester, preferably polyethylene terephthalate, and generally used as a base film of the transparent electrode. Although it can show | play, a particularly preferable aspect is the aspect which employ | adopted the polyester film of this invention mentioned above as a base material of this transparent electrode.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a following example, unless the summary is exceeded. In the examples and comparative examples, “part” means “part by mass”. The measuring method used in the present invention is as follows.

(1) Maximum orientation axis direction A 10 cm × 10 cm square sample is prepared, and the refractive index at the central portion and the four corner portions thereof is set using an Abbe refractometer, the sodium D line (589 nm) is used as a light source, and TD is set to 0 °. The refractive index was measured up to 180 ° every 5 °, and the direction in which the refractive index was the maximum was taken as the maximum orientation axis direction. In the case of exceeding 90 ° and not more than 180 °, the TD in the 180 ° direction opposite to the TD initially set to 0 ° is set to 0 ° and expressed in the range of 0 to 90 °.

(2) Thermal contraction rate Samples having a length of 120 mm and a width of 15 mm in the 45 ° direction and the 135 ° direction with respect to the longitudinal direction and the width direction of the film and the width direction of 0 °, respectively. The sample is marked with 100 mm intervals in the vicinity of both ends of the sample, one end is fixed to an oven adjusted to a temperature of 150 ° C., and the other end is left free for 30 minutes. This was taken out and allowed to cool to room temperature, the distance between the gauge points was measured, and the thermal shrinkage rate was determined by the following formula. In addition, the dimension was measured using the Olympus digital small microscope (STM5).
Thermal shrinkage (%)
= {(Distance between the pre-heat treatment gauge points -Distance between the heat treatment gauge points) / Distance between the pre-heat treatment gauge points} × 100
The above measured 10 points | pieces and calculated | required the average value.

(3) Linear expansion coefficient About the sample used in the above (1), in the maximum orientation axis direction of the film and the minimum orientation axis direction orthogonal thereto, the length is 15 mm and the width is 5 mm so that each direction becomes the measurement direction. Cut out and set in TMA3000 manufactured by Vacuum Riko Co., Ltd., pretreated at 60 ° C. for 30 minutes in a nitrogen atmosphere (0% RH), and then cooled to room temperature. Thereafter, the temperature is raised from 25 ° C. to 70 ° C. at 2 ° C./min, the sample length at each temperature is measured, and the coefficient of thermal expansion (αt, unit ppm / ° C.) is calculated from the following formula. In addition, it measured 5 times and used the average value.
αt = {(L60−L40)} / (L40 × ΔT)} + 0.5
Here, L40 in the above formula is a sample length (mm) at 40 ° C., L60 is a sample length (mm) at 60 ° C., ΔT is 20 (= 60-40) ° C., and 0.5 is quartz. It is a coefficient of thermal expansion (ppm / ° C.) of glass.

(4) Curl evaluation, wrinkle evaluation The polyester film obtained in the example has a heat shrinkage of 0.20% in the width direction at 150 ° C for 30 minutes and a heat shrinkage ratio of 150 ° C in the longitudinal direction for 30 minutes. Another PET film having a thickness of 60 μm and a thickness of 60% was attached using the following adhesive as a medium, and then cut into 30 cm square sheets to obtain a laminate. The laminated body obtained here is placed in an oven adjusted to 150 ° C. so that the polyester film side obtained in the example is on the upper side and is horizontal with the bottom surface, and left for 60 minutes. After that, the laminate is taken out from the oven while keeping the polyester film obtained in the example on the upper side, and after sufficiently cooling it to room temperature (25 ° C.), it is placed on a horizontal plane. The curl degree was measured with a metal scale, and the curl was evaluated according to the following criteria. In the curl amount, the curl when the upper side is a concave shape is represented by (−), and the curl amount is represented by (+).
A: Curling amount is -5 mm to +5 mm
○: Curling amount is less than −5 mm to −10 mm or more than +5 mm to +10 mm
X: The curl amount is less than −10 mm or exceeds +10 mm. Further, during the process of sufficiently cooling the film taken out from the oven to room temperature (25 ° C.), wrinkles were evaluated according to the following criteria.
○: No wrinkle △: Slightly wrinkled ×: Many wrinkles

<Adhesive>
As an adhesive, 20 parts of an isocyanate curing agent (manufactured by Nippon Polyurethane Co., Ltd., Coronate HL) with respect to 100 parts of an acrylic adhesive (Teikoku Chemical Co., Ltd., SG-800) (solid part by weight). ) Was applied using a gravure coater, and the coating film was dried and cured at 100 ° C. for 2 minutes to provide an adhesive layer having a thickness of 20 μm.

<Manufacture of unoriented polyester sheet A>
A polyester chip (polyethylene terephthalate chip) dried by a known method is supplied to a melt extrusion apparatus and melted at 285 ° C. Next, the molten polymer was extruded from a die and rapidly cooled and solidified on a rotary cooling drum cooled to 30 ° C. so that the temperature was equal to or lower than the glass transition temperature to obtain a substantially amorphous unoriented polyester sheet A. . The intrinsic viscosity of the obtained unoriented polyester sheet A was 0.62 dl / g. The unoriented polyester sheet A was controlled in thickness so that the thickness after stretching was 125 μm in each of the following Examples and Comparative Examples.

Example 1
Using the simultaneous biaxial stretching machine, the unoriented polyester sheet A obtained above was 3.4 times in the vertical direction (longitudinal magnification) at the stretching temperature of 100 ° C. and 1.7 times in the horizontal direction at the same time (lateral magnification). 1) After stretching, the film was stretched 2.0 times in the transverse direction at a stretching temperature of 110 ° C. (transverse magnification 2), heat treated at 230 ° C. for 10 seconds, and then 2.0% in the longitudinal direction (longitudinal relaxation rate). The polyester film relaxed by 2.0% (lateral relaxation rate) in the transverse direction to obtain a polyester film having a thickness of 125 μm.

[Examples 2 to 6]
A polyester film having a thickness of 125 μm was obtained in the same manner as in Example 1 except that the film forming conditions were as shown in Table 1.

[Comparative Example 1]
The polyester sheet A was stretched 3.6 times in the machine direction at a film temperature of 85 ° C. using the roll peripheral speed difference, led to a tenter, and stretched 4.0 times at 120 ° C. in the transverse direction. After heat treatment at 0 ° C. for 10 seconds, the film was relaxed by 2.0% in the vertical direction and 2.0% in the horizontal direction to obtain a polyester film having a thickness of 125 μm.

[Comparative Examples 2 to 4]
A polyester film having a thickness of 125 μm was obtained in the same manner as in Example 1 except that the film forming conditions were as shown in Table 1.
The evaluation results of the obtained film are shown in Table 1. As shown in Table 1, the polyester film of the present invention had excellent curling characteristics.

According to the present invention, even a film corresponding to an end portion in film production has been subjected to a heat treatment that is performed, for example, when a hard coat layer or an undercoat layer is formed on a base film in the production of a transparent electrode. However, curling can be easily suppressed even if heat treatment that is performed, for example, in the crystallization process of the ITO film is obtained after forming wrinkles and sagging during conveyance and forming a laminate. Can be provided.
The polyester film of the present invention having such an effect can be suitably used as a transparent electrode polyester film such as a transparent electrode base film or a transparent electrode protective film, and its industrial application. The possibility is high.

Claims (5)

A polyester film made of a polyethylene terephthalate resin, the polyester film having a region where the maximum orientation axis direction is 20 ° or more and 55 ° or less when the width direction is 0 ° , The heat shrinkage rate at 150 ° C. for 30 minutes in the longitudinal direction is 1.00% or less, the heat shrinkage rate at 150 ° C. for 30 minutes in the width direction is 0.50% or less, and the heat shrinkage rate is the following (A) A polyester film satisfying and having a linear expansion coefficient satisfying the following (B).
(A) In the above region, the 150 ° C. 30 minute heat shrinkage ratio SD1 in the 45 ° direction and the 150 ° C. 30 minute heat shrinkage ratio SD2 in the 135 ° direction are expressed by the following formula 1 Meet.
| SD1-SD2 | ≦ 0.20% (Formula 1)
(B) The linear expansion coefficient αtmin in the maximum orientation axis direction in the region and the linear expansion coefficient αtmax in the direction orthogonal to the film plane satisfy the following expressions 2 and 3.
αtmax <42 ppm / ° C. (Formula 2)
αtmax−αtmin <10 ppm / ° C. (Formula 3) The polyester film according to claim 1, wherein the polyester film is formed by simultaneous biaxial stretching. A transparent electrode, a polyester film according to any one of claims 1 or 2. The polyester film according to claim 3 , which is for a transparent electrode substrate. The polyester film according to claim 3 , which is for a transparent electrode protective film.