JP6760066B2 - Biaxially oriented polyester film - Google Patents

Description

The present invention relates to a biaxially oriented polyester film having excellent mechanical properties and workability.

Polyester resins, especially polyethylene terephthalate (hereinafter sometimes abbreviated as PET) and polyethylene 2,6-naphthalene carboxylate (hereinafter sometimes abbreviated as PEN) have mechanical properties, thermal properties, chemical resistance, electrical properties, etc. It has excellent moldability and is used for various purposes. A polyester film obtained by converting the polyester into a film, particularly a biaxially oriented polyester film, is used as a process film for protecting a transparent electrode substrate from scratches during the processing process due to its excellent mechanical properties and workability.

In general, a transparent conductive film-forming substrate (such as an ITO (Indium Tin Oxide) vapor-deposited substrate) used in a display or the like requires a substrate curing process at a constant temperature in order to increase the conductivity of the ITO film. Become. In this step, heat is applied to the protective film on the substrate at the same time. Therefore, if there is a difference in the thermal characteristics between the transparent conductive film-forming substrate and the protective film, particularly the dimensional change rate (linear expansion coefficient (CTE)) when the temperature drops from 150 ° C. to 50 ° C., the transparent conductive film-forming substrate It is preferable that the dimensional change rates of the transparent conductive film-forming substrate and the protective film are close to each other because the flatness of the film may be deteriorated or the protective film may be peeled off to deteriorate the protective function. Therefore, conventionally, a PET film oriented biaxially has been used for the film-forming substrate of the transparent conductive film (Patent Document 1), so that the PET film is often used as the protective film.

However, in recent years, from the viewpoint of improving the performance of the display and thinning the film, it has been studied to use a film made of an amorphous resin such as a cycloolefin polymer (COP) as a film-forming substrate of a transparent conductive film (Patent Documents). 2, 3).

Japanese Unexamined Patent Publication No. 2013-093310 Japanese Unexamined Patent Publication No. 2013-114344 JP-A-2014-112510

When a biaxially oriented PET film is used for the film-forming substrate of the transparent conductive film, biaxially oriented PET can be preferably used as the protective film of the substrate. However, when a sheet made of COP is used for the film-forming substrate of the transparent conductive film, the COP is generally an amorphous resin, and the dimensional change rate is about 2 to 3 times larger than that of the biaxially oriented PET. When a biaxially oriented PET film is used as a protective film, there arises a problem that the flatness of the film-forming substrate of the transparent conductive film is deteriorated and the protective film is peeled off.

On the other hand, considering reducing the difference in the dimensional change rate between the transparent conductive film-forming substrate and the protective film, it is conceivable to use a film made of COP as the protective film. However, when a film made of COP is used as a protective film, since COP is an amorphous resin, it has lower toughness and inferior flexibility as compared with a biaxially oriented PET film, so that cracks occur during the processing process. There is a problem of doing.

An object of the present invention is to provide a biaxially oriented polyester film that is suitably used as a protective film for COP used in applications such as a film-forming substrate for a transparent conductive film, in view of the background of the prior art.

In order to solve the above problems, the present invention has the following configuration. That is,
[I] The dimensional change rate when the temperature is lowered from 150 ° C. to 50 ° C. in the film width direction (TD direction) and the direction perpendicular to the film width direction (MD direction) is 50 ppm / ° C. or higher and 130 ppm / ° C. or lower, respectively. A biaxially oriented polyester film having a plane orientation coefficient (fn) of 0.111 or more and 0.145 or less.
[II] The dimensional change rate when the temperature drops from 150 ° C. to 50 ° C. in the film width direction (TD direction) and the direction perpendicular to the film width direction (MD direction) is 50 ppm / ° C. or higher and 130 ppm / ° C. or lower, respectively. A biaxially oriented polyester film having a heat shrinkage rate of 1.0% or less at 130 ° C. for 30 minutes in each of the film width direction (TD direction) and the direction perpendicular to the film width direction (MD direction).
[III] The biaxially oriented polyester film according to [I] or [II], wherein the plane orientation coefficient (fn) is 0.120 or more and 0.140 or less.
[IV] The heat shrinkage at 130 ° C. for 30 minutes in the film width direction (TD direction), the direction perpendicular to it (MD direction), and the direction 45 ° from the film width direction was compared in each direction. When, the absolute value of their difference is 0% or more and 0.5% or less, and their average value is 0.5% or less.
In addition, the dimensional change rate during temperature reduction from 150 ° C. to 50 ° C. in the film width direction (TD direction), the direction perpendicular to it (MD direction), and the direction 45 ° from the film width direction is measured in each direction. The biaxially oriented polyester film according to any one of [I] to [III], wherein the absolute values of the differences are 0 ppm / ° C. or higher and 10 ppm / ° C. or lower when compared. [V] The biaxially oriented polyester film according to any one of [I] to [IV], wherein the heat of crystal melting of the polyester resin constituting the polyester film is 30 J / g or less.
[VI] The polyester film is a laminated polyester film composed of at least three layers, and the amount of heat of crystal fusion (ΔHmA) of the polyester resin constituting the surface layers on both sides of the film is 30 J / g or more, and both sides of the film. The biaxially oriented polyester film according to any one of [I] to [V], wherein the heat of crystal fusion (ΔHmB) of the polyester resin constituting a layer other than the surface layer of the above is 30 J / g or less.
[VII] The polyester resin constituting the layers other than the surface layer on both sides of the film is a resin containing terephthalic acid and ethylene glycol as main constituents, and any of isophthalic acid and cyclohexylene dimethane as the other constituent units. The biaxially oriented polyester film according to [VI], which contains only one type or only two types.
[VIII] The polyester film is a laminated polyester film composed of at least three layers, and the melting point TmA of the polyester resin constituting the surface layers on both sides of the film is 250 ° C. or higher and 280 ° C. or lower. The biaxially oriented polyester film according to any one of I] to [V].
[IX] The ratio of the sum of the thicknesses of the surface layers on both sides of the polyester film to the sum of the thicknesses of the layers other than the surface layer (sum of the thicknesses of the surface layers on both sides / sum of the thicknesses of the layers other than the surface layer) is 1/9 to 1. The biaxially oriented polyester film according to [IV] to [VIII], which is / 2.
[X] The biaxially oriented polyester film according to any one of [I] to [IX] used for bonding to a film made of an amorphous resin.
[XI] The biaxially oriented polyester film according to any one of [I] to [IX] used for bonding to a film made of a cycloolefin polymer (COP).
[XII] The biaxially oriented polyester film according to any one of [I] to [XI] used for protecting a film made of a cycloolefin polymer (COP).

According to the present invention, a biaxially oriented polyester film having a dimensional change rate close to that of a film made of an amorphous resin such as COP or PC (polycarbonate), excellent mechanical properties, and good workability can be obtained.

The present invention will be described in detail below with reference to specific examples.
The polyester film of the present invention needs to be a biaxially oriented polyester film from the viewpoint of mechanical properties. The polyester referred to here has a dicarboxylic acid component and a diol component. In addition, in this specification, a constituent component means the smallest unit which can be obtained by hydrolyzing polyester. From the viewpoint of mechanical properties, the polyester film of the present invention is preferably made of polyethylene terephthalate or a copolymer of polyethylene terephthalate.

In one aspect of the present invention, the dimensional change rate when the temperature is lowered from 150 ° C. to 50 ° C. in each of the film width direction (TD direction) and the direction perpendicular to the film width direction (MD direction) is 50 ppm / ° C. or more and 130 ppm / A biaxially oriented polyester film having a surface orientation coefficient (fn) of 0.111 or more and 0.145 or less at ° C. or lower.

In general, the transparent conductive film is formed on a substrate at a temperature higher than room temperature, then cured at a temperature higher than room temperature, and then cooled to room temperature. That is, it is important to maintain the flatness of the substrate after the transparent conductive film is formed in order to prevent the transparent conductive film from being lost and the conductivity being impaired. The protective film on the substrate also goes through the process. That is, in order to maintain good flatness of the substrate, it is important to set the dimensional change rate of the protective film of the substrate when the temperature is lowered to a value close to that of the substrate.

In recent years, a film made of COP, which is an amorphous resin, has been widely used as a film-forming substrate for a transparent conductive film. The dimensional change rate of the film made of COP when the temperature is lowered from 150 ° C. to 50 ° C. is 50 ppm / ° C. or higher and 150 ppm / ° C. or lower, although it depends on the molecular skeleton of COP. By setting the dimensional change rate of the polyester film of the present invention when the temperature is lowered from 150 ° C. to 50 ° C. to a range close to the dimensional change rate of the film made of COP, the film-forming substrate of the transparent conductive film is formed after the film-forming process of the conductive film. The conductivity can be kept good without impairing the flatness of the film. It is particularly preferably 60 ppm / ° C. or higher and 110 ppm / ° C. or lower, and more preferably 80 ppm / ° C. or higher and 100 ppm / ° C. or lower.

The dimensional change rate of the biaxially oriented polyester film is determined by the orientation of the molecular chains of the polyester constituting the film. That is, when the molecular chains are oriented, the molecular chains cannot move freely due to heat, and as a result, the value of the dimensional change rate becomes low. That is, in the case of a general biaxially oriented polyester film, when the degree of orientation of the molecular chains having an fn of more than 0.145 is high, the value of the dimensional change rate is low (less than 50 ppm / ° C.). On the other hand, if the orientation of the film such that fn is less than 0.111 is not sufficient, the mechanical properties, particularly the elongation at break, are inferior, resulting in inferior workability. In addition, since the orientation of the film is not sufficient and the molecular chains are close to an amorphous state, random coarse crystals are generated when the film is heated, which not only impairs the transparency of the film but also randomly coarses the film. As a result of the crystal fixing the molecular chains around it, the value of the dimensional change rate is also low. Therefore, the biaxially oriented polyester film of the present invention has a dimensional change rate of 50 ppm / 50 ° C. when the temperature is lowered from 150 ° C. to 50 ° C. in each of the film width direction (TD direction) and the direction perpendicular to the film width direction (MD direction). By using a biaxially oriented polyester film having a temperature of ° C. or higher and 130 ppm / ° C. or lower and a plane orientation coefficient (fn) of 0.111 or higher and 0.145 or lower, the mechanical properties and workability are excellent, and the film is heated. Can be a film that maintains high transparency. The fn is more preferably 0.120 or more and 0.140 or less.

In another aspect of the present invention, the dimensional change rate when the temperature is lowered from 150 ° C. to 50 ° C. in the film width direction (TD direction) and the direction perpendicular to the film width direction (MD direction) is 50 ppm / ° C. or more and 130 ppm, respectively. Biaxially oriented polyester having a temperature of / ° C or less and a heat shrinkage rate of 1.0% or less at 130 ° C. for 30 minutes in each of the film width direction (TD direction) and the direction perpendicular to the film width direction (MD direction). It is a film. As described above, the film forming step of the transparent conductive film includes a step of forming the transparent conductive film on the substrate at a temperature higher than room temperature. Therefore, when the heat shrinkage rate of the protective film of the substrate is large, the flatness of the substrate may be impaired. When a polyester film having a dimensional change rate at a temperature drop of 150 ° C. to 50 ° C. in the above range and a heat shrinkage rate at 130 ° C. for 30 minutes in the above range is used as a protective film for the COP film, COP The smoothness of the film becomes good, and the bonding with the COP becomes good. More preferably, the heat shrinkage rate in the MD direction and the TD direction at 130 ° C. for 30 minutes is 0.5% or less, respectively. The lower limit of the heat shrinkage rate is preferably −0.2%. -Means to expand. If it expands by more than -0.2%, the base material and the protective film may peel off.

The dimensional change rate and heat shrinkage rate of the polyester film of the present invention preferably satisfy the following requirements. That is, when the heat shrinkage rate at 130 ° C. for 30 minutes in the film width direction (TD direction), the direction perpendicular to it (MD direction), and the direction 45 ° from the film width direction is compared in each direction. , The absolute value of the difference is (the absolute value of the difference in heat shrinkage between the TD direction and the MD direction, the absolute value of the difference in heat shrinkage between the TD direction and the direction 45 ° from the TD direction, and the MD direction. Absolute value of the difference in heat shrinkage in the direction 45 ° from the TD direction) 0% or more and 0.5% or less, and their average value is 0.5% or less, and 150 ° C to 50 ° C When the dimensional change rates during temperature decrease are compared in each direction, the absolute values of the differences are all (absolute value of the difference between the dimensional change rates in the TD direction and the MD direction, 45 ° from the TD direction and the TD direction. The absolute value of the difference in the dimensional change rate in the direction, the absolute value of the difference in the dimensional change rate in the direction 45 ° from the MD direction and the TD direction) is preferably 0 ppm / ° C. or higher and 10 ppm / ° C. or lower.

When a sheet made of an amorphous resin is used for a film-forming substrate of a transparent conductive film, it is usually used in a non-stretched state, so that the heat responsiveness (dimension change rate, heat shrinkage rate) of the sheet made of an amorphous resin is high. It is isotropic. On the other hand, in the biaxially oriented polyester film, between the film width direction (TD direction), which is the stretching direction, the direction perpendicular to it (MD direction), and the direction forming 45 ° from the film width direction in between. There is a difference in the responsiveness to heat (dimensional change rate, heat shrinkage rate). When a polyester film having a large difference is used as a laminated body by laminating a sheet made of an amorphous resin and a film, the laminated body may be curled. By using a biaxially oriented polyester film that satisfies the above range, even if the laminated body is laminated with a sheet made of an amorphous resin, the response to heat is close and curl of the laminated body is suppressed, which is preferable. ..

Further, the heat shrinkage rate of the biaxially oriented polyester film of the present invention at 150 ° C. for 30 minutes is preferably 1.5% or less in both the MD and TD directions. More preferably, the heat shrinkage rate at 150 ° C. for 30 minutes is −0.2% or more and 0.5% or less in both the MD and TD directions.

In order to keep the dimensional change rate, the plane orientation coefficient (fn), and the heat shrinkage rate of the biaxially oriented polyester film within the above ranges, for example, the following method (a) can be taken.

(A) A method in which the amount of heat of crystal melting of the polyester resin constituting the biaxially oriented polyester film of the present invention is set to 30 J / g or less, and biaxial stretching is performed by the method described later.

The biaxial stretching method can be as follows.

First, the polyester resin is heated and melted in an extruder and then discharged from a mouthpiece to obtain an unstretched sheet.
(1) When the molten polyester is discharged from the mouthpiece to produce an unstretched sheet, the unstretched sheet is produced by closely cooling and solidifying by static electricity on a drum cooled to a surface temperature of 10 ° C. or higher and 40 ° C. or lower.
(2) Area magnification of the unstretched sheet obtained in (1) in the longitudinal direction (MD) of the film and the width direction (TD) of the film at a temperature T1n (° C.) satisfying the following equation (i). Biaxial stretching is performed 5 times or more and 16.0 times or less.
(I) Tg (° C.) ≤ T1n (° C.) ≤ Tg + 40 (° C.)
Tg: Glass transition temperature (° C) of the resin constituting the polyester film
(3) The biaxially stretched film obtained in (2) is heat-fixed for 1 second to 30 seconds at a temperature (Th0 (° C.)) satisfying the following formula (ii), and then slowly cooled uniformly. Then, by cooling to room temperature, a polyester film is obtained.
(Ii) Tm-60 (° C.) ≤ Th0 (° C.) ≤ Tm-20 (° C.)
Tm: Melting point (° C) of the resin constituting the film
By obtaining an unstretched sheet under the conditions satisfying (1), a substantially amorphous polyester film can be obtained, and it is easy to give orientation to the film in the steps after (2), and the film has good mechanical properties. Can be easily obtained.
By obtaining a biaxially stretched film under the conditions satisfying (2), it is possible to impart an appropriate orientation to the film and obtain a film having good mechanical properties.
By completing the crystal orientation under the conditions satisfying (3), the structure of the oriented polyester molecular chain is stable, and a film having good mechanical properties and heat shrinkage can be obtained.

In (2), as a method of biaxial stretching, the longitudinal direction (MD) of the film and the width direction of the film (direction perpendicular to the longitudinal direction of the film, TD) are separated and sequentially biaxially stretched. Either the stretching method or the simultaneous biaxial stretching method in which stretching in the longitudinal direction and the width direction is performed at the same time may be used. Further, when the stretching temperature (T1n) (° C.) is less than Tg (° C.), it is difficult to stretch. When T1n (° C.) exceeds Tg + 40 (° C.), film tearing occurs frequently, and a film may not be obtained by stretching. More preferably, Tg + 10 (° C.) ≤ T1n (° C.) ≤ Tg + 30 (° C.).

In the step (3), when Th0 exceeds Tm-20 ° C., the orientation of the film applied by stretching may be lost and the heat shrinkage rate may increase. When Th0 is lower than Tm-60 ° C., the structure of the molecular chain is not stable, and the flatness is deteriorated or the film forming property is deteriorated. Further, by setting the value of Th0 to Tm-30 (° C.) ≤ Th0 (° C.) ≤ Tm-20 (° C.), the tension of the molecular chain during film stretching is relaxed, and the heat shrinkage rate is set within a preferable range. be able to. In the step (3), a relaxing process for reducing the distance in the film width direction by 2% to 10% can also be performed.

The amount of heat of crystal melting of the polyester resin constituting the biaxially oriented polyester film of the present invention is preferably 30 J / g or less. When the amount of heat of crystal fusion exceeds 30 J / g, the crystallinity of the resin is high, the orientation of the molecular chains is strong even when biaxially stretched by the above method, the fn is large, and the dimensional change rate is low. It becomes a tendency to become. The lower limit of the amount of heat of crystal melting is preferably 2 J / g or more. If it is less than 2 J / g, the film-forming property may be poor or fn may be below the lower limit of the preferable range. As a method for reducing the heat of crystal melting of the polyester resin to 30 J / g or less, when the polyester resin is PET, a method of copolymerizing isophthalic acid as a dicarboxylic acid component, a method of copolymerizing cyclohexanedimethanol as a diol component, etc. Can be mentioned. These may be copolymerized alone or a plurality of types may be copolymerized. It is preferable to copolymerize alone because it is easy to control the crystallinity. In particular, when isophthalic acid is used as a copolymerization component, it is particularly preferable because it has a structure similar to that of terephthalic acid and it is easy to impart orientation by stretching to bring fn into a preferable range. As for the amount of copolymerization, it is preferable that the total amount of the copolymerization components is 7 mol% or more and 20 mol% or less with respect to the total amount of the constituent components of the polyester.

As a method of a more preferable embodiment, there is a method in which the film has the following constitution (b).

(B) The polyester film is a laminated polyester film composed of at least three layers, and the amount of heat of crystal fusion (ΔHmA) of the polyester resin constituting the surface layers on both sides of the film is 30 J / g or more, and other than the surface layers on both sides of the film. The amount of heat of crystal fusion (ΔHmB) of the polyester resin constituting the layer shall be 30 J / g or less.

With this configuration, the surface layers on both sides of the film have higher crystallinity than the other layers and are more easily oriented. Therefore, the orientation of the other layers is increased following the layer, and as a result of improving the orientation of the entire film, the mechanical properties are improved and the workability is improved, which is preferable. Further, as a result of improving the orientation, whitening when the film is heated is suppressed, which is preferable. ΔHmA is preferably 31 J / g or more and 60 J / g or less, and ΔHmB is preferably 2 J / g or more and less than 30 J / g.
When this configuration is adopted, the stretching temperature preferably satisfies the following formula (iii) in order to enhance the orientation of the resins constituting the surface layers on both sides.
(Iii) TgA (° C.) ≤ T1n (° C.) ≤ TgA + 40 (° C.)
TgA represents the glass transition temperature of the polyester resin constituting the surface layers on both sides of the film. When the surface layers on both sides of the polyester film of the present invention are made of polyester resins having different compositions (for example, A / B / C), the higher temperature of the Tg of the polyester resins constituting the surface layers on both sides. Satisfies the equation (iii).
Further, when the polyester film of the present invention is a laminated polyester film composed of at least three layers, it is also preferable that the melting point TmA of the polyester resin constituting the surface layers on both sides of the film is 250 ° C. or higher and 280 ° C. or lower. Is.

When the value of TmA is 250 ° C. or lower, the flatness may be deteriorated or the film forming property may be deteriorated when heat is received by heat treatment during film forming. A three-layer laminated polyester film having a melting point TmA of 250 ° C. or higher and 280 ° C. or lower, which is a polyester resin constituting the surface layers on both sides of the film, is particularly preferable because it has good flatness and film-forming property.

When the film of the present invention is a laminated polyester film having three or more layers, the ratio of the sum of the thicknesses of the surface layers on both sides of the polyester film to the sum of the thicknesses of the layers other than the surface layer (sum of the thicknesses of the surface layers on both sides / other than the surface layer). The sum of the thicknesses of the layers) is preferably 1/9 to 1/2.

If the thickness of the surface layer is thin and the ratio of the sum of the thicknesses of the surface layers on both sides to the sum of the thicknesses of the layers other than the surface layer is less than 1/9, the effect of improving the film forming property and the mechanical properties by laminating may not be obtained. is there. On the other hand, when the surface layer is thick and the ratio of the sum of the thicknesses of the surface layers on both sides to the sum of the thicknesses of the layers other than the surface layer exceeds 1/2, the inner layer is forcibly stretched due to the strong influence of the orientation of the surface layer. As a result, the film-forming property of the film may deteriorate.

In order to set the heat shrinkage ratio in a more preferable range, it is also a preferable embodiment to go through the following steps (c).

(C) The film obtained by the method (a) or (b) is annealed at a heat treatment temperature Th1 (° C.) satisfying the following formula (iv) for a time of 70 seconds or more and 600 seconds or less. Examples of the method for performing the annealing treatment include a method of heat-treating the film (off-annealing) in an oven installed between the film winding roll and the film winding roll.
(Iv) 120 ° C. ≤ Th1 (° C.) ≤ Th0 (heat fixing temperature) (° C.)
When Th1 (° C.) exceeds Th0 (heat fixation temperature) (° C.), in the step (4), the structure of the molecular chain in the film immobilized in the step (3) is destroyed, and as a result, the film becomes It will shrink significantly, and the flatness may deteriorate. On the other hand, when Th1 (° C.) is lower than 120 ° C., the heat shrinkage rate at 130 ° C. may not be within a preferable range.

By going through the steps (c), the stress remaining in the molecular chains constituting the film can be removed in the steps (a) and (b), and the heat shrinkage rate is reduced, the isotropic property, and the dimensional change. It is a preferred embodiment of the present invention because the isotropic rate is obtained. In particular, in the case of the film obtained by the method (b), since the resin having a TmA of 250 ° C. or higher and 280 ° C. or lower is arranged on both surface layers, when heat is applied in the step (c), the film is subjected to. This is preferable because only the heat shrinkage rate can be reduced without breaking the orientation of the film.

The thickness of the biaxially oriented polyester film of the present invention is preferably 30 μm or more and 150 μm or less. If it is less than 30 μm, tearing may easily occur when used as a protective film, and if it exceeds 150 μm, the handleability may be inferior. More preferably, it is 50 μm or more and 125 μm or less.

The biaxially oriented polyester film of the present invention preferably has a haze change amount (Δ haze) of 2% or less before and after the treatment at 100 ° C. for 12 hr. By setting Δhaze within the above range, even when the film of the present invention and the sheet made of the amorphous resin are bonded together, the sheet made of the amorphous resin can be visually recognized through the film of the present invention. preferable. It is considered that the cause of the increase in Δhaze is that the amorphous part of the film becomes coarse crystals by heating and the oligomer is precipitated on the film surface by heating. In the former, a method of imparting orientation to the film to have fn 0.111 or more is used, and in the latter, a resin having a laminated structure and a resin constituting the outermost layer is solid-phase polymerized to reduce the oligomer content. As a result, the Δ haze can be reduced by reducing the oligomer precipitation.

The biaxially oriented polyester film of the present invention obtained as described above is excellent in mechanical properties and workability, and the value of the dimensional change rate when the temperature is lowered from 150 ° C. to 50 ° C. is close to that of the film made of amorphous resin. Therefore, it is preferably used for bonding to a film made of an amorphous resin. In particular, it can be suitably used as a protective film for a film made of COP. Further, since it is excellent in transparency even when heated, it can be suitably used as a protective film for a COP film used for forming a transparent conductive film.

[Characteristic evaluation method]
A. Melting point (Tm, TmA, TmB) (° C) of the film and the resin constituting each layer
The sample was prepared by a method based on JIS K 7121 (1999), using a differential scanning calorimetry device "Robot DSC-RDC220" manufactured by Seiko Electronics Co., Ltd., and a disk session "SSC / 5200" for data analysis. Perform the measurement as follows.
Weigh 5 mg of the sample into a sample pan, heat the sample from 25 ° C to 300 ° C at a heating rate of 20 ° C / min (1st RUN), hold it in that state for 5 minutes, and then quench it to 25 ° C or lower. .. Immediately thereafter, the temperature is raised again from 25 ° C. to 20 ° C./min to 300 ° C., and the measurement is performed. The differential scanning calorimetry chart of 2nd RUN (vertical axis is thermal energy, horizontal axis is temperature). To get. In the differential scanning calorimetry chart of the 2nd RUN, the temperature of the peak top at the crystal melting peak, which is the endothermic peak, is determined and used as the melting point (° C.). When two or more crystal melting peaks are observed, the temperature of the peak top having the largest peak area is taken as the melting point.
When measuring the melting point of the resin constituting each layer of the laminated polyester film, only the resin constituting each layer is scraped from the laminated polyester film using a microtome and used for the measurement.

B. The amount of heat of crystal fusion (ΔHm, ΔHmA, ΔHmB) (J / g) of the film and the resin constituting each layer
The sample was prepared by a method based on JIS K 7121 (1999), using a differential scanning calorimetry device "Robot DSC-RDC220" manufactured by Seiko Electronics Co., Ltd., and a disk session "SSC / 5200" for data analysis. Perform the measurement as follows.
Weigh 5 mg of the sample into a sample pan, heat the sample from 25 ° C to 300 ° C at a heating rate of 20 ° C / min (1st RUN), hold it in that state for 5 minutes, and then quench it to 25 ° C or lower. .. Immediately thereafter, the temperature is raised again from 25 ° C. to 20 ° C./min to 300 ° C., and the measurement is performed. The differential scanning calorimetry chart of 2nd RUN (vertical axis is thermal energy, horizontal axis is temperature). To get. In the differential scanning calorimetry chart of the 2nd RUN, the peak area of the endothermic peak is obtained and used as the heat of crystal melting. When two or more crystal melting peaks are observed, the area of the peak with the highest temperature is defined as the amount of heat of crystal melting, and when two or more peaks cannot be separated, the two peaks are combined to obtain the peak area.
When measuring the amount of heat of crystal melting of the resin constituting each layer of the laminated polyester film, only the resin constituting each layer is scraped from the laminated polyester film using a microtome and used for the measurement.

C. Glass transition temperature (Tg, TgA) ((° C.)) of the resin constituting the film and the outermost layer
According to JIS K 7121 (1999), use the differential scanning calorimetry device "Robot DSC-RDC220" manufactured by Seiko Electronics Co., Ltd., and use the disk session "SSC / 5200" for data analysis, as described below. , Carry out the measurement.

5 mg of the sample is weighed in a sample pan, the sample is heated from 25 ° C. to 300 ° C. at a heating rate of 20 ° C./min (1stRUN), held in that state for 5 minutes, and then rapidly cooled to 25 ° C. or lower. Immediately thereafter, the temperature is raised again from 25 ° C. to 20 ° C./min to 300 ° C., and the measurement is performed. The differential scanning calorimetry chart of 2nd RUN (vertical axis is thermal energy, horizontal axis is temperature). To get. In the differential scanning calorimetry chart of the 2nd RUN, in the stepwise change portion of the glass transition, the straight line equidistant from the extended straight line of each baseline in the vertical direction and the curve of the stepwise change portion of the glass transition are Find from the point of intersection. When two or more stepwise changes in the glass transition are observed, the glass transition temperature is obtained for each, and the average value of these temperatures is taken as the glass transition temperature (Tg) (° C.) of the sample.
When measuring the glass transition temperature of the resin constituting the outermost surface layer of the laminated polyester film, only the resin constituting the outermost surface layer is scraped from the laminated polyester film using a microtome and used for the measurement.

D. Film plane orientation coefficient (fn)
According to JIS K 7105 (1999), the refractive index at 20 ° C. is determined using an Abbe type refractive index meter manufactured by Atago Co., Ltd. The refractive index in the longitudinal direction (Nmd), the refractive index in the width direction (Nd), and the refractive index in the thickness direction (Nz) of the surface of the film are measured, and the plane orientation coefficient (fn) is calculated. The measurement is carried out at n = 5 and calculated as the average value thereof.
(V) fn = (Nmd + Ntd) /2-Nz
E. Film shrinkage (%)
The heat shrinkage of the film is measured according to JIS C 2318 (1997). Cut the film into strips with a width of 10 mm and a length of 150 mm. A marked line is attached to the film so that the length-measured portion is approximately 100 mm, and the length of the marked line is measured under the condition of 23 ° C., and the value is L0. Then, the film is hung with a weight of 2 g in a hot air oven heated to a predetermined temperature (200 ° C. or 220 ° C.), and left for 30 minutes. After removing the film from the oven and cooling it to 23 ° C., the length of the marked line is measured and used as L1. The shrinkage rate of the film is calculated by the following formula (vi). The measurement is performed by randomly cutting out five points so that the length in the film longitudinal direction or the film width direction is 150 mm. Calculate the average value in each of the longitudinal direction and the width direction, and use this as the heat shrinkage rate of the film.
(Vi) (Film heat shrinkage rate) = (L0-L1) / L0 × 100
F. Film thickness (μm)
The film thickness was measured at any five locations with 10 films stacked in accordance with the JIS K7130 (1992) A-2 method using a dial gauge. The average value was divided by 10 to obtain the film thickness.

G. Thickness of each layer of laminated polyester film (μm)
When the film was a laminated film, the thickness of each layer was determined by the following method. A cross section of the film is cut out with a microtome in a direction parallel to the film width direction. The cross section is observed with a scanning electron microscope at a magnification of 5000 times, and the thickness ratio of each laminated layer is determined. The thickness of each layer is calculated from the obtained lamination ratio and the above-mentioned film thickness.

H. Film-forming property The number of times the film is torn in one hour during film-forming is counted, and the film is evaluated as A if it is less than 1 time, B if it is 1 time or more and less than 5 times, and C if it is 5 times or more. A has the best film-forming property, and C has the worst film-forming property.

I. Dimensional change rate (ppm / ° C) when the temperature drops from 150 ° C to 50 ° C
According to JIS K7197 (1991), a thermomechanical measuring device TMA / SS6000 (manufactured by Seiko Instruments) is used to load a sample with a sample width of 4 mm and a sample length (distance between chucks) of 20 mm with a load of 3 g. The temperature is raised from room temperature to 160 ° C. at a heating rate of 10 ° C./min, held for 10 minutes, and then lowered to 20 ° C. at 10 ° C./min to obtain sample size values at each temperature (° C.). It is calculated from the following formula (vii) from the sample size L (150 ° C.) (mm) at 150 ° C. and the sample size L (50 ° C.) (mm) at 50 ° C. The dimensional change rate is calculated at n = 5 in each of the film width direction (TD) and the direction orthogonal to the film width direction (TD), and is calculated as an average value thereof.
(Vii) the dimensional change ^{(ppm / ℃) = 10 6} × (L (150 ℃) -L (50 ℃))) / {20 × (150-50)}
J. Δ Haze (amount of change in haze before and after treatment at 100 ° C. for 12 hr) (%)
The film was cut into a square shape with a side of 10 cm, and the haze meter NDH-5000 manufactured by Nippon Denshoku Co., Ltd. was used to randomly measure the haze at three locations to calculate the average value, and the haze H0 (%) before the test. And. The sample is heat-treated for 12 hours under dry heat conditions of 100 ° C. and 10% RH or less by fixing the four sides of the sample in an oven manufactured by Tabie Spec Co., Ltd., which is left in a room maintained at 23 ° C. and 65% RH. .. The haze of the film after the heat treatment is measured in the same manner to determine H1 (%). The Δ haze (ΔH) is calculated by the following formula (viii).
(Viii) Δ Haze (%) = H1-H0
The value of Δhaze is judged as follows.
A; Δ haze 1.5% or less B; Δ haze more than 1.5% and 2.0% or less C; Δ haze more than 2.0% A is the best and C is the worst.

K. Workability The workability is determined by determining the breaking elongation (%) in each of the width direction (TD) of the film and the direction (MD) perpendicular to the width direction (MD) in n5, and determining the average value thereof as follows.
A; Breaking elongation 120% or more B; Breaking elongation 105% or more and less than 120% C; Breaking elongation 90% or more and less than 105% D; Breaking elongation 75% or more and less than 90% E; Breaking elongation less than 75% A Is the best and E is the worst.

The elongation at break retention rate is measured as follows. The film was cut out to a size of 1 cm × 15 cm so that the long sides were parallel to the MD and TD of the film, and pulled at a chuck distance of 5 cm and a pulling speed of 300 mm / min based on ASTM-D882 (1997). Measure the breaking elongation at the time. The number of samples is set to n = 5, and after measuring in each of the longitudinal direction and the width direction of the film, the average value thereof is calculated and used as the breaking elongation of the film.
L. Evaluation of bonding with COP film The film of the present invention was cut into a size of 20 cm × 20 cm, bonded with COP film to prepare a laminate, and then placed in an oven at 120 ° C. and allowed to stand for 1 hour. The oven was then cooled to room temperature at a rate of 20 ° C./min. Then, the number of wrinkles having a length of 3 cm or more in the laminated body obtained by laminating the film of the present invention and the COP film is measured, and the determination is made as follows. The evaluation was carried out at n = 5, and the evaluation was performed using the average value thereof.

Less than 4; S
4 or more and less than 10; A
10 or more and less than 16; B
16 or more; C
S is the best and C is the worst.
As the COP film, a film of "Zeonor ZF14" manufactured by Zeon Corporation and a thickness of 40 μm is used. For bonding, "Leocoat" R5000 manufactured by Toray Coatex Co., Ltd. was added as an adhesive to a toluene solution adjusted so that the adhesive content was 15%, and Toluene Coatex Co., Ltd. was applied to 100 parts by mass of the toluene solution. A cross-linking agent "Coronate L" added in an amount of 3 parts by mass is applied so that the coating thickness after drying is 10 μm.

M. Curling property of the laminate with the COP film L. The laminate prepared in the above section was placed in an oven at 120 ° C. and allowed to stand for 1 hour. Then, the oven was cooled to room temperature at a rate of 20 ° C./min and left for 1 hour. After that, the film is placed on a horizontal surface so that the COP film is on the upper side, the amount of floating from the horizontal surface at the four corners of the laminate is measured, the average value is calculated, and the curl amount (mm) is obtained. Judgment is made as follows. If the four corners of the laminate do not float from the horizontal surface by the above method, the curl amount is set to 0 mm.

0 mm or more and less than 10 mm; A
10 mm or more and less than 25 mm; B
25 mm or more and less than 40 mm; C
40 mm or more and less than 55 mm; D
55 mm or more; E.

In the above measurement, when the longitudinal direction and the width direction of the film to be measured are not known, the direction having the maximum refractive index in the film is regarded as the longitudinal direction, and the direction orthogonal to the longitudinal direction is regarded as the width direction. Further, the direction of the maximum refractive index in the film may be obtained by measuring the refractive index in all directions of the film with a refractive index meter, and the slow axis direction is determined by a phase difference measuring device (birefringence measuring device) or the like. It may be obtained by deciding.

Hereinafter, the present invention will be described with reference to examples, but the present invention is not necessarily limited thereto.
[Production of PET-1] Polymerization of terephthalic acid and ethylene glycol was carried out by a conventional method using antimony trioxide as a catalyst to obtain melt-polymerized PET. The obtained melt-polymerized PET had a glass transition temperature of 80 ° C., a melting point of 255 ° C., and an intrinsic viscosity of 0.62.
[Production of PET-2] PET-1 was subjected to solid phase polymerization by a conventional method to obtain PET-A. The obtained PET-A had a glass transition temperature of 82 ° C., a melting point of 255 ° C., and an intrinsic viscosity of 0.85.
[Production of PET-A] Polymerization of terephthalic acid, isophthalic acid and ethylene glycol was carried out by a conventional method using antimony trioxide as a catalyst so that the amount of isophthalic acid copolymerization was 7 mol% with respect to the total amount of digalbonic acid components. Copolymerized PET was obtained. The obtained copolymerized PET had a glass transition temperature of 77 ° C., a melting point of 243 ° C., and an intrinsic viscosity of 0.62.
[Production of PET-B] Polymerization of terephthalic acid, isophthalic acid and ethylene glycol was carried out by a conventional method using an antimony trioxide as a catalyst so that the amount of isophthalic acid copolymerization was 10 mol% with respect to the total amount of digalbonic acid components. Copolymerized PET was obtained. The obtained copolymerized PET had a glass transition temperature of 76 ° C., a melting point of 235 ° C., and an intrinsic viscosity of 0.62.
[Production of PET-C] Polymerization of terephthalic acid, isophthalic acid and ethylene glycol was carried out by a conventional method using antimony trioxide as a catalyst so that the amount of isophthalic acid copolymerization was 15 mol% with respect to the total amount of digalbonic acid components. Copolymerized PET was obtained. The obtained copolymerized PET had a glass transition temperature of 74 ° C., a melting point of 230 ° C., and an intrinsic viscosity of 0.62.
[Production of PET-D] Polymerization of terephthalic acid, isophthalic acid and ethylene glycol was carried out by a conventional method using antimony trioxide as a catalyst so that the amount of isophthalic acid copolymerization was 20 mol% with respect to the total amount of digalbonic acid components. Copolymerized PET was obtained. The obtained copolymerized PET had a glass transition temperature of 73 ° C., a melting point of 220 ° C., and an intrinsic viscosity of 0.62.
[Production of PET-E] Polymerization of terephthalic acid, isophthalic acid and ethylene glycol was carried out by a conventional method using antimony trioxide as a catalyst so that the amount of isophthalic acid copolymerization was 25 mol% with respect to the total amount of digalbonic acid components. Copolymerized PET was obtained. The glass transition temperature of the obtained copolymerized PET was 70 ° C., and no melting point was observed. The intrinsic viscosity was 0.62. [Production of PET-F] From terephthalic acid, cyclohexanedimethanol (CHDM) and ethylene glycol, using antimony trioxide as a catalyst, the amount of cyclohexanedimethanol copolymerization is 10 mol% with respect to the total amount of the diol component by a conventional method. Polymerization was carried out to obtain copolymerized PET. The obtained copolymerized PET had a glass transition temperature of 72 ° C., a melting point of 235 ° C., and an intrinsic viscosity of 0.62.
[Production of PET-G] From terephthalic acid, cyclohexanedimethanol (CHDM) and ethylene glycol, using antimony trioxide as a catalyst, the amount of cyclohexanedimethanol copolymerization is 20 mol% with respect to the total amount of the diol component by a conventional method. Polymerization was carried out to obtain copolymerized PET. The obtained copolymerized PET had a glass transition temperature of 70 ° C., a melting point of 221 ° C., and an intrinsic viscosity of 0.62.

(Example 1)
PET-A was vacuum dried at 160 ° C. for 2 hours, then put into an extruder, melted in the extruder, and extruded onto a casting drum having a surface temperature of 25 ° C. to prepare an unstretched sheet. Subsequently, the sheet was preheated with a heated roll group, stretched 3.1 times in a direction perpendicular to the width direction (MD direction) at a temperature of 90 ° C., and then cooled with a roll group having a temperature of 25 ° C. A uniaxially stretched film was obtained. While gripping both ends of the obtained uniaxially stretched film with clips, the film was stretched 3.6 times in the film width direction (TD direction) in a heating zone at a temperature of 100 ° C. in the tenter. Subsequently, heat fixation was performed in the heat treatment zone in the tenter at a temperature of 210 ° C. for 10 seconds. In the heat fixing step, a 2% relaxation treatment was applied in the film width direction. Then, after slowly cooling uniformly in the cooling zone, the film was wound to obtain a biaxially oriented polyester film. The characteristics of the obtained biaxially oriented polyester film are shown in the table. The dimensional change rate was 50 ppm / ° C. or higher and 130 ppm / ° C. or lower in both the MD direction and the TD direction, and the film was excellent in bonding with COP. In addition, the film was excellent in processability and had a small change in haze due to heating.

(Examples 2-5, Comparative Examples 1 and 2)
A biaxially oriented polyester film was obtained in the same manner as in Example 1 except that the resins constituting the film were changed as shown in the table. The characteristics of the biaxially oriented polyester film are shown in the table. In Examples 2 to 5, the films had fn and a dimensional change rate in a preferable range, were excellent in workability, and had a small haze change due to heating. In Comparative Example 1, as a result of the high crystallinity of the resin and the large ΔHm, the fn of the film was large and the dimensional change rate was small. In Comparative Example 2, since the crystallinity of the resin was so low that ΔHm was not observed, the fn was small and the dimensional change rate was small, and the film was inferior in bonding with COP. Further, since the fn is small, the film is inferior in processability and has a large haze change due to heating.

(Example 6)
It had a three-layer structure of A / B / A, and PET-2 was 100 parts by mass as a resin constituting the surface layer, vacuum-dried at 160 ° C. for 2 hours, and then charged into the extruder 1. Further, 100 parts by mass of PET-A as a resin constituting the inner layer was vacuum-dried at 160 ° C. for 2 hours, and then charged into the extruder 2. Each raw material is melted in the extruder, the resin put into the extruder 1 by the merging device is merged so as to be both surface layers of the film, and extruded onto a casting drum having a surface temperature of 25 ° C., a laminated sheet having a three-layer structure. Was produced. Subsequently, the sheet was preheated by a heated roll group, then stretched 3.1 times in the longitudinal direction (MD direction) at a temperature of 90 ° C., and then cooled by a roll group at a temperature of 25 ° C. to form a uniaxially stretched film. Got While gripping both ends of the obtained uniaxially stretched film with clips, the film was stretched 3.6 times in the width direction (TD direction) perpendicular to the longitudinal direction in a heating zone at a temperature of 100 ° C. in the tenter. Subsequently, heat fixation was performed in the heat treatment zone in the tenter at a temperature of 210 ° C. for 10 seconds. Then, after slowly cooling uniformly in the cooling zone, it was wound up to obtain a laminated polyester film. Each characteristic of the film is shown in the table. The dimensional change rate was 50 ppm / ° C. or higher and 130 ppm / ° C. or lower in both the MD direction and the TD direction, and the film was excellent in bonding with COP. In addition, the film was excellent in processability and had a small change in haze due to heating. It was found that by using PET-2 for the surface layer, it is possible to obtain a film having more excellent workability and a small change in haze due to heating.

(Example 7-21)
The film was formed in the same manner as in Example 6 except that the composition of the resin and the film forming conditions were changed as shown in the table. The characteristics of the film are shown in the table. The dimensional change rate was 50 ppm / ° C. or higher and 130 ppm / ° C. or lower in both the MD direction and the TD direction, and the film was excellent in bonding with COP. In addition, the film was excellent in processability and had a small change in haze due to heating.

(Example 22-24)
In Example 22, the film obtained in Example 6, the film obtained in Example 7 in Example 23, and the film obtained in Example 8 were used in Example 24, and the obtained films were wound into a film. In a hot air oven installed between the dashi roll and the film take-up roll, the film was annealed at a temperature of 140 ° C. so that the film was heat-treated for 5 minutes to obtain a film having a thickness of 125 μm. .. Each characteristic of the film is shown in the table. The dimensional change rate was 50 ppm / ° C. or higher and 130 ppm / ° C. or lower in both the MD direction and the TD direction, and the heat shrinkage rate at 130 ° C. for 30 minutes was small, and the film was particularly excellent for bonding with COP. In addition, the film was excellent in processability and had a small change in haze due to heating. Further, the average value of the heat shrinkage rate in the MD direction, the TD direction, and the 45 ° direction is 0.5% or less, the absolute value of the difference in the heat shrinkage rate is 0.5% or less, and the difference in the dimensional change rate in each direction. The absolute value of was 10 or less, and the curl property of the laminate with COP was also good.

(Comparative Example 3-7)
The film was formed in the same manner as in Example 6 except that the composition of the resin and the film forming conditions were changed as shown in the table. The characteristics of the film are shown in the table. In Comparative Examples 3 and 7, as a result of the high crystallinity of the resin and the large ΔHmB, the fn of the film was high and the dimensional change rate was also small. In Comparative Example 4, since the crystallinity was so low that ΔHm was not observed, the fn was small and the dimensional change rate was small. Further, since the fn is small, the film is inferior in processability and has a large haze change due to heating. In Comparative Examples 5 and 6, the heat treatment temperature during film formation was high, and as a result of the orientation of the film being disturbed, fn became extremely low, the elongation at break was lowered, and not only was the workability inferior, but also Δ haze due to heating. It was a large film.

(Example 26)
The film was formed in the same manner as in Example 6 except that the composition of the resin and the film forming conditions were changed as shown in the table. The film characteristics are shown in the table. The dimensional change rate was 50 ppm / ° C. or higher and 130 ppm / ° C. or lower in both the MD direction and the TD direction, and the film was excellent in bonding with COP. In addition, the film was excellent in processability and had a small change in haze due to heating.

(Examples 25, 27-36)
Example 25 is a film of Example 2, Example 27 is a film of Example 26, Example 28 is a film of Example 9, Example 29 is a film of Example 11, and Example 30 is an example. Twelve films, Example 31 was a film of Example 14, Example 32 was a film of Example 15, Example 33 was a film of Example 16, and Example 34 was a film of Example 18. Example 35 uses the film of Example 19, and the film of Example 36 uses the film of Example 20, and the obtained films are used in a hot air oven installed between the film winding roll and the film winding roll. The film was annealed at a temperature of 140 ° C. so that the heat treatment time was 5 minutes.

In Examples 29, 30, 31, 34 to 36, the average value of the heat shrinkage rate in the MD direction, the TD direction, and the 45 ° direction is 0.5% or less, and the absolute value of the difference between the heat shrinkage rates is also 0.5%. Hereinafter, the absolute value of the difference in the dimensional change rate in each direction was also 10 or less, and the curl property of the laminated body with COP was also good.

In Example 25, since it was a single film, the heat shrinkage rate could not be reduced, and the curl property was slightly inferior, but there was no problem in actual use. In Example 27, since there were a plurality of types of copolymerization components, the heat shrinkage rate could not be reduced, and the curl property was slightly inferior, but there was no problem in actual use. In Example 28, since the fn was small and the amorphous property was strong, the heat shrinkage rate could not be reduced, and the curl property was slightly inferior, but there was no problem in actual use. In Examples 32 and 33, the difference in the dimensional change rate in each direction was large, and the curl property was slightly inferior, but there was no problem in actual use.

The polyester film of the present invention has excellent mechanical properties and workability, and the dimensional change rate when the temperature is lowered from 150 ° C. to 50 ° C. is close to that of a film made of an amorphous resin. Therefore, the polyester film is made of an amorphous resin. It can be suitably used for bonding applications. Further, since it is excellent in transparency even when heated, it can be suitably used as a protective film for a COP film used for forming a transparent conductive film.

Claims (8)

The dimensional change rate when the temperature drops from 150 ° C. to 50 ° C. in the film width direction (TD direction) and the direction perpendicular to the film width direction (MD direction) is 50 ppm / ° C. or higher and 130 ppm / ° C. or lower, respectively, and the plane orientation. A biaxially oriented polyester film having a coefficient (fn) of 0.111 or more and 0.145 or less and used for bonding to a film made of an amorphous resin. The biaxially oriented polyester film according to claim 1 , wherein the plane orientation coefficient (fn) is 0.120 or more and 0.140 or less. The biaxially oriented polyester film according to claim 1 or 2 crystal melting heat quantity of the polyester resin constituting the front Symbol polyester film is not more than 30 J / g. The polyester film is a laminated polyester film composed of at least three layers, and the amount of heat of crystal fusion (ΔHmA) of the polyester resin constituting the surface layers on both sides of the film is 30 J / g or more, and other than the surface layers on both sides of the film. The biaxially oriented polyester film according to any one of claims 1 to 3 , wherein the heat of crystal fusion (ΔHmB) of the polyester resin constituting the layer is 30 J / g or less. The polyester resin that constitutes the layers other than the surface layer on both sides of the film is a resin whose main constituents are terephthalic acid and ethylene glycol, and one of isophthalic acid and cyclohexylene dimethane is used as the other constituent unit. The biaxially oriented polyester film according to claim 4 , which contains only or only two types. Wherein the polyester film is a laminated polyester film comprising at least three layers, from claim 1, the melting point TmA of the polyester resin constituting the surface layer on both sides of the film, characterized in that both at 250 ° C. or higher 280 ° C. or less The biaxially oriented polyester film according to any one of 3 . The ratio of the sum of the thicknesses of the surface layers on both sides of the polyester film to the sum of the thicknesses of the layers other than the surface layer (sum of the thicknesses of the surface layers on both sides / sum of the thicknesses of the layers other than the surface layer) is 1/9 to 1/2. The biaxially oriented polyester film according to any one of claims 4 to 6 . The biaxially oriented polyester film according to any one of claims 1 to 7 , which is used for bonding to a film made of a cycloolefin polymer (COP).