JP7585879B2 - Polyester film, laminated polyester film, method for producing polyester film, method for producing laminated polyester film, and method for producing biaxially oriented polyester film - Google Patents

Description

The present invention relates to a polyester film, a laminated polyester film, a method for producing a polyester film, a method for producing a laminated polyester film, and a method for producing a biaxially oriented polyester film.

While plastics are used in a variety of fields, microplastics and other substances are considered to be a cause of marine pollution, and there is an urgent need to reduce the environmental burden caused by plastics. In addition, in recent years, the evolution of the Internet of Things (IoT) has led to a rapid increase in electronic devices such as CPUs and liquid crystal displays installed in computers and smartphones, and accordingly, the number of multilayer ceramic capacitors (MLCCs), which are important for driving electronic devices, and polarizing films used in liquid crystal display devices have also increased explosively. A large amount of process film is used to manufacture MLCCs and these films, but the process film after use is generally discarded. In other words, the burden on the environment caused by the increase in discarded process film is becoming an issue. In response to this issue, technologies for reusing used process films have been disclosed (Patent Documents 1, 2, and 3).

JP 2012-171276 A Patent No. 4284936 JP 2019-066841 A

Both films undergo a process of transporting the film in order to recover the polyester film used in the process film. However, there is room for further improvement in the process of producing polyester film, particularly in terms of transportability, which is the decrease in productivity and decrease in recoverability due to film breakage during the production process.

The object of the present invention is to provide a polyester film, a laminated polyester film, a method for producing a polyester film, a method for producing a laminated polyester film, and a method for producing a biaxially oriented polyester film, all of which have excellent transportability.

A preferred embodiment of the present invention has the following configuration.
[I] A polyester film, which, when measured by the following Measurement Method A and Measurement Method B, exhibits a peak at a wave number of 3300 cm ^-1 to 3400 cm ^-1 in Measurement Method A and/or Measurement Method B, and the peak intensity is 1.0 x 10 ^-3 or more.
(Measurement Method A) A method of measuring using a microscopic infrared spectrometer.
Apparatus: Microscopic infrared spectroscopic analyzer; LUMOS (manufactured by Bruker)
Measurement conditions:
Light source: Silicon carbide rod heating element (glow bar)
Detector: Narrow MCT (HgCdTe)
Detection wave number range: 4000 to 650 cm ^-1
Purge; Nitrogen gas Measurement mode; Transmission method Resolution; 8 cm ^-1
Number of integrations: 256. A spectrum is obtained by plotting the absorption intensity against each wave number.

In the cross section of the polyester film, measurements are made at 10 points evenly spaced in the thickness direction, at least 5 μm inside (hereinafter referred to as the inner layer portion) measured in the direction perpendicular to the outermost surface of the polyester film, using the above-mentioned device and conditions to obtain a spectrum. A PET spectrum is then subtracted from the above spectrum (a PET spectrum (measured under the same conditions using #50T60 manufactured by Toray Industries, Inc.) is used, and the four arithmetic operations are performed so that the peak shapes between 1000 cm ^-1 and 1500 cm ^-1 match as closely as possible, and this is then divided from the spectrum obtained by measurement).

Here, if a peak is detected at wavenumbers of 3300 cm ^-1 to 3400 cm ^-1 in the divided spectrum at at least one measurement point out of the 10 measurement points, it is deemed that a peak is detected at wavenumbers of 3300 cm ^-1 to 3400 cm ^-1 when measured by Measurement Method A. Furthermore, if the peak intensity detected at at least one measurement point out of the 10 measurement points in the divided spectrum is 1.0 x 10 ^-3 or more, it is deemed that the peak intensity is 1.0 x 10 ^-3 or more when measured by Measurement Method A.

Furthermore, in the case where two or more peaks are observed in the wavenumber range from 3300 cm ^-1 to 3400 cm ^-1 in the divided spectrum at a given measurement point, if the maximum peak intensity among them is 1.0 x ^10-3 or more, the peak intensity detected in the divided spectrum at that measurement point is deemed to be 1.0 x ^10-3 or more.
(Measurement Method B) A method of measuring using an infrared spectrometer.
Apparatus: Infrared spectroscopic analyzer; "Nicolet" (registered trademark) 6700 (manufactured by Thermofisher Scientific)
Measurement conditions:
Light source: Silicon carbide rod heating element (glow bar)
Detector: DTGS (triglycine sulfate)
Detection wave number range: 4000 to 680 cm ^-1
Purge; Nitrogen gas measurement mode; ATR method (attenuated total reflection measurement method)
Single reflection type ATR measurement accessory device (OMNI sampler), ATR crystal: Ge, incident angle: 45°
Resolution: 4 ^cm
Number of integrations: 256. A spectrum is obtained by plotting the absorption intensity against each wave number.

First, 50 pieces of 4 cm square polyester film are each scraped off 5 μm from one side of the surface using a microtome while observing under a microscope. Then, 5 μm is scraped off from the surface of the opposite side in the same way to obtain the inner layer of the polyester film.

The obtained inner layer is immersed in hexafluoro-2-propanol (HFIP) and dissolved for 30 minutes while stirring at 50°C under a nitrogen atmosphere. The liquid is then cooled to 25°C, and insoluble matter is collected using a PTFE filter (manufactured by ADVANTEC, model number: 13010802). The collected matter is measured using the above-mentioned device and conditions to obtain a spectrum.

Here, if a peak is detected in the wavenumber range of 3300 cm ^-1 to 3400 cm ^-1 in the obtained spectrum, it is deemed that a peak is detected in the wavenumber range of 3300 cm ^-1 to 3400 cm ^-1 when measured by Measurement Method B. Furthermore, if the peak intensity detected in the obtained spectrum is 1.0 x 10 ^-3 or more, it is deemed that the peak intensity is 1.0 x 10 ^-3 or more when measured by Measurement Method B.

Furthermore, in the case where two or more peaks are observed in the wavenumber range from 3300 ^cm to 3400 ^cm in the obtained spectrum, when the maximum peak intensity among them is 1.0×10 ^or more, the peak intensity detected in the obtained spectrum is deemed to be 1.0×10 ^or more.
[II] The polyester film according to [I], in which a peak is detected at a wave number of 3300 cm ^-1 to 3400 cm ^-1 in the measurement method B and the peak intensity is 1.0 x ^10-3 or more, and the polyester film contains an organic component insoluble in HFIP in an inner layer portion of the polyester film in an amount of 10 ppm to 1000 ppm by mass.
[III] The polyester film according to [I] or [II], in which a peak is detected at a wave number of 3300 cm ^-1 to 3400 cm ^-1 and the peak intensity is 1.0 x 10 ^-3 or more in the measurement method B, and the HFIP-insoluble organic component has a polyethylene terephthalate skeleton.
[IV] The polyester film according to any one of [I] to [III], wherein the outermost layer of the polyester film contains a component having a polyvinyl alcohol skeleton.
[V] A laminated polyester film having a layer X mainly composed of a resin having a polyvinyl alcohol skeleton on at least one surface of the polyester film according to any one of [I] to [IV].
[VI] The laminated polyester film according to [V], which has the polyester film, the layer X, and a release layer Y in this order.
[VII] The laminated polyester film according to [VI], having the polyester film, the layer X, the release layer Y, and the functional layer Z in this order.
[VIII] A method for producing a laminated polyester film, comprising a step of peeling off the functional layer Z from the laminated polyester film according to [VII].
[IX] A method for producing a polyester film, comprising a step of simultaneously removing the layer X and the release layer Y from the laminated polyester film according to [VI].
[X] A method for producing a polyester film, comprising the steps of peeling off the functional layer Z from the laminated polyester film described in [VII] and simultaneously removing the layer X and the release layer Y, in this order.
[XI] A method for producing a polyester film, comprising laminating a release layer Y on layer X of the laminated polyester film described in [V], providing a functional layer Z on the release layer Y, peeling off the functional layer Z, and then removing the layer X and the release layer Y.
[XII] A method for producing a polyester film, comprising the step of melting a polyester film obtained by removing a part of layer X from the laminated polyester film according to any one of [V] to [VII], and molding the polyester film using a composition obtained by melting the polyester film.
[XIII] A method for producing a biaxially oriented polyester film, comprising the steps of melting a polyester and a component having a polyvinyl alcohol skeleton and forming the polyester into a film.

The present invention can provide a polyester film, a laminated polyester film, a method for producing a polyester film, a method for producing a laminated polyester film, and a method for producing a biaxially oriented polyester film, all of which have excellent transportability.

The present invention will be described in detail below with specific examples.

The present invention relates to a polyester film. The polyester referred to in the present invention is one that has a dicarboxylic acid component and a diol component. In this specification, the term "component" refers to the smallest unit that can be obtained by hydrolyzing a polyester. Examples of the dicarboxylic acid component that constitutes such a polyester include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, and 4,4'-diphenyletherdicarboxylic acid, or ester derivatives thereof.

The diol components constituting such polyesters include aliphatic diols such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,2-butanediol, and 1,3-butanediol, alicyclic diols such as cyclohexanedimethanol and spiroglycol, and diols in which multiple units of the above-mentioned diols are linked together. Among these, from the viewpoint of mechanical properties and transparency, polyesters in which polyethylene terephthalate (PET), polyethylene-2,6-naphthalenedicarboxylate (PEN), and PET dicarboxylic acid components are copolymerized with isophthalic acid or naphthalenedicarboxylic acid, and PET diol components are copolymerized with cyclohexanedimethanol, spiroglycol, or diethylene glycol are copolymerized with a portion of the PET diol components are preferably used.

From the viewpoint of mechanical strength and chemical resistance, the polyester of the present invention is preferably composed mainly of polyethylene terephthalate (PET) consisting of terephthalic acid and ethylene glycol. The term "main component" means that polyethylene terephthalate accounts for 90 mol % or more of 100 mol % of the polyester constituents.

A preferred embodiment of the polyester film of the present invention is a polyester film which, when measured by the following Measurement Method A and Measurement Method B, exhibits a peak at a wave number of 3300 ^cm to 3400 ^cm in Measurement Method A and/or Measurement Method B, and the peak intensity is 1.0 × 10 ^or more.
(Measurement Method A) A method of measuring using a microscopic infrared spectrometer.
Apparatus: Microscopic infrared spectroscopic analyzer; LUMOS (manufactured by Bruker)
Measurement conditions:
Light source: Silicon carbide rod heating element (glow bar)
Detector: Narrow MCT (HgCdTe)
Detection wave number range: 4000 to 650 cm ^-1
Purge; Nitrogen gas Measurement mode; Transmission method Resolution; 8 cm ^-1
Number of integrations: 256. A spectrum is obtained by plotting the absorption intensity against each wave number.

In the cross section of the polyester film, measurements are made at 10 points evenly spaced in the thickness direction, at least 5 μm inside (hereinafter referred to as the inner layer portion) measured in the direction perpendicular to the outermost surface of the polyester film, using the above-mentioned device and conditions to obtain a spectrum. A PET spectrum is then subtracted from the above spectrum (a PET spectrum (measured under the same conditions using #50T60 manufactured by Toray Industries, Inc.) is used, and the four arithmetic operations are performed so that the peak shapes between 1000 cm ^-1 and 1500 cm ^-1 match as closely as possible, and this is then divided from the spectrum obtained by measurement).

Here, if a peak is detected at wavenumbers of 3300 cm ^-1 to 3400 cm ^-1 in the divided spectrum at at least one measurement point out of the 10 measurement points, it is deemed that a peak is detected at wavenumbers of 3300 cm ^-1 to 3400 cm ^-1 when measured by Measurement Method A. Furthermore, if the peak intensity detected at at least one measurement point out of the 10 measurement points in the divided spectrum is 1.0 x 10 ^-3 or more, it is deemed that the peak intensity is 1.0 x 10 ^-3 or more when measured by Measurement Method A.

Furthermore, in the case where two or more peaks are observed in the wavenumber range from 3300 cm ^-1 to 3400 cm ^-1 in the divided spectrum at a given measurement point, if the maximum peak intensity among them is 1.0 x ^10-3 or more, the peak intensity detected in the divided spectrum at that measurement point is deemed to be 1.0 x ^10-3 or more.
(Measurement Method B) A method of measuring using an infrared spectrometer.
Apparatus: Infrared spectroscopic analyzer; "Nicolet" (registered trademark) 6700 (manufactured by Thermofisher Scientific)
Measurement conditions:
Light source: Silicon carbide rod heating element (glow bar)
Detector: DTGS (triglycine sulfate)
Detection wave number range: 4000 to 680 cm ^-1
Purge; Nitrogen gas measurement mode; ATR method (attenuated total reflection measurement method)
Single reflection type ATR measurement accessory device (OMNI sampler), ATR crystal: Ge, incident angle: 45°
Resolution: 4 ^cm
Number of integrations: 256. A spectrum is obtained by plotting the absorption intensity against each wave number.

First, 50 pieces of 4 cm square polyester film are each scraped off 5 μm from one side of the surface using a microtome while observing under a microscope. Then, 5 μm is scraped off from the surface of the opposite side in the same way to obtain the inner layer of the polyester film.

The obtained inner layer is immersed in hexafluoro-2-propanol (HFIP) and dissolved for 30 minutes while stirring at 50°C under a nitrogen atmosphere. The liquid is then cooled to 25°C, and insoluble matter is collected using a PTFE filter (manufactured by ADVANTEC, model number: 13010802). The collected matter is measured using the above-mentioned device and conditions to obtain a spectrum.

Here, if a peak is detected in the wavenumber range of 3300 cm ^-1 to 3400 cm ^-1 in the obtained spectrum, it is deemed that a peak is detected in the wavenumber range of 3300 cm ^-1 to 3400 cm ^-1 when measured by Measurement Method B. Furthermore, if the peak intensity detected in the obtained spectrum is 1.0 x 10 ^-3 or more, it is deemed that the peak intensity is 1.0 x 10 ^-3 or more when measured by Measurement Method B.

Furthermore, in the case where two or more peaks are observed in the wavenumber range from 3300 ^cm to 3400 ^cm in the obtained spectrum, when the maximum peak intensity among them is 1.0×10 ^or more, the peak intensity detected in the obtained spectrum is deemed to be 1.0×10 ^or more.

If there is a large amount of collected material and the entire amount of the collected material cannot be measured in one measurement, measurements are performed for every 0.1 mg of the collected material, and if a peak is detected at wavenumbers of 3300 cm ^-1 to 3400 cm ^-1 in the spectrum obtained in at least one of the measurements, it is deemed that a peak is detected at wavenumbers of 3300 cm ^-1 to 3400 cm ^-1 when measured by Measurement Method B. Furthermore, if the peak intensity detected in the spectrum obtained in the at least one measurement is 1.0 x 10 ^-3 or more, it is deemed that the peak intensity is 1.0 x 10 ^-3 or more when measured by Measurement Method B.

In addition, in measurement method A, a peak is detected at wave numbers from 3300 cm ^-1 to 3400 cm ^-1 and the peak intensity is 1.0 x 10 ^-3 or more, which is said to meet condition A; in measurement method B, a peak is detected at wave numbers from 3300 cm ^-1 to 3400 cm ^-1 and the peak intensity is 1.0 x 10 ^-3 or more, which is said to meet condition B; and so forth.

Since the peaks detected at wave numbers of 3300 cm ^-1 to 3400 cm ^-1 are mainly attributable to components having hydroxyl groups, when the condition A and/or condition B is satisfied, the components interact with the polyester molecular chains, and as a result, when the film is transported under tension at a temperature near the glass transition point of the polyester film, for example, at a temperature of about 80° C. to 150° C., or when the film is transported under tension in contact with a liquid (water or an organic solvent) heated to the temperature, contraction or expansion of the film is suppressed, improving transportability. Also, by the same mechanism, film breakage is suppressed during biaxial stretching, improving productivity.

The reason for the improved transportability is believed to be as follows: Hydroxyl groups generally have electric polarization due to differences in the electronegativity of elements. Polyesters also have carboxyl groups, and therefore generally have electric polarization due to differences in the electronegativity of elements. Even between polyester molecular chains, there is interaction due to polarization caused by differences in electronegativity, but by further containing a component with a hydroxyl group, the interaction between the molecular chains is strengthened via that component, and it is believed that the effect can be obtained at the specific temperature and conditions mentioned above.

A preferred method for achieving condition A or B is to add a component having a polyvinyl alcohol skeleton to the inner layer of the polyester film.

A component having a polyvinyl alcohol skeleton has a molecular structure represented by chemical formula (1).

Here, a is 1 or more and b is 0 or more.

Methods for obtaining a polyester film that satisfies condition A and/or condition B include a method of melting polyester and a component having a polyvinyl alcohol skeleton and forming a film. Specifically, when obtaining a polyester film by melt film formation, a method of adding a component having a polyvinyl alcohol skeleton when melting raw material polyester pellets, a method of obtaining a polyester film using a raw material obtained by melting a laminated polyester film in which a component having a polyvinyl alcohol skeleton is applied to the surface of the polyester film in advance, and a method of mixing a film in which a component having a polyvinyl alcohol skeleton is applied to the surface of a polyester film with polyester pellets and then melting the mixture to obtain a polyester film, but are not particularly limited thereto.

As the component having the polyvinyl alcohol skeleton, polyvinyl alcohol or a polyvinyl alcohol-based resin such as a copolymerized polyvinyl alcohol can be used. Preferred examples of copolymerized polyvinyl alcohol include vinyl alcohol-vinyl acetate copolymer, ethylene-vinyl alcohol copolymer, vinyl alcohol-(meth)acrylic acid-alkyl (meth)acrylate copolymer, vinyl alcohol-isoprene sulfonic acid copolymer, vinyl alcohol-vinyl pyrrolidone copolymer, and polyvinyl alcohol-based resin having a specific modifying group in the side chain as shown in the chemical formula (2) described below.

From the viewpoint of heat resistance and dispersibility, the degree of polymerization of the polyvinyl alcohol resin is preferably 1000 or less, more preferably 500 or less. In order to efficiently obtain the above-mentioned interaction, the degree of saponification reflecting the amount of hydroxyl groups in the polyvinyl alcohol resin is preferably 80 or more, particularly preferably 85 or more. As a method for obtaining a polyester film that satisfies condition A and/or condition B, among the above-mentioned methods, a method in which at least 0.001% by mass or more of polyvinyl alcohol having a degree of saponification of 80 or more is contained in 100% by mass of the inner layer portion can be mentioned as an example.

In addition, from the viewpoint of strengthening the interaction between the component having a polyvinyl alcohol skeleton and the polyester molecular chain and further improving the transportability of the film, it is more preferable to use a polyvinyl alcohol resin having a specific modifying group on the side chain of polyvinyl alcohol, as shown in chemical formula (2).

Here, Q is one or more selected from a 1,2-ethanediol group, an acetoacetyl group, a sulfonic acid group, a sulfonate group, a carboxyl group, a carboxyl group, an amino group, and an ammonium group. Also, c and e are 1 or more, and d is 0 or more.

This embodiment is preferable because a chemical bond is formed between the component having a polyvinyl alcohol skeleton and the polyester molecular chain during melt extrusion, further improving transportability. The mechanism is presumed to be as follows. It is believed that the action of highly polar carboxylate bases, sulfonate bases, ammonium bases, etc. causes depolymerization of the ester bonds in the polyester molecular chain, resulting in acid ends and alcohol ends in the polyester, which in turn causes polycondensation of the acid ends of the polyester and the hydroxyl groups of the polyvinyl alcohol, resulting in chemical bonds.

In addition, from the viewpoint of manufacturability, when the total amount of hydroxyl groups, acetoxy groups, and functional group Q in the polyvinyl alcohol resin is 100 mol%, the content of functional group Q is preferably 1 mol% or more and 10 mol% or less.

In order to efficiently disperse the component having a polyvinyl alcohol skeleton in the polyester film, the shear rate in the melting step is preferably 10 sec ^-1 or more and 250 sec ^-1 or less, more preferably 25 sec ^-1 or more and 180 sec ^-1 or less. From the viewpoint of generating chemical bonds between the component having a polyvinyl alcohol skeleton and the polyester molecular chain during melt extrusion, the extrusion temperature is preferably 260° C. or more and 300° C. or less, more preferably 270° C. or more and 290° C. or less. The melting time is preferably 30 seconds or more and 600 seconds or less, more preferably 60 seconds or more and 300 seconds or less.

As described above, a preferred embodiment of the polyester film of the present invention is a polyester film that satisfies the following condition A and/or condition B. When the polyester film satisfies condition A, it is more preferred that the spectrum obtained under condition A has a peak at wave numbers of 2800 cm ^-1 to 3000 cm ^-1 , and the ratio of the peak intensity at wave numbers of 2800 cm ^-1 to 3000 cm ^-1 /peak intensity at wave numbers of 3000 cm ^-1 to 3400 cm ^-1 is 0.05 to 2.0. The peak at wave numbers of 2800 cm ^-1 to 3000 cm ^-1 represents the stretching vibration of an aliphatic hydrocarbon, and when the polyester film of the present invention is obtained, the peak intensity at wave numbers of 2800 cm ^-1 to 3000 cm ^-1 can be set in a preferred range by using a polyvinyl alcohol resin having an aliphatic hydrocarbon.

When a polyester film containing a component having a polyvinyl alcohol skeleton is obtained by melt film formation, in order to efficiently express the interaction between the component having a polyvinyl alcohol skeleton and the polyester molecular chain, it is preferable that the content of a compound having an isocyanate group or an oxazoline group, which crosslinks compounds via hydroxyl groups by reacting with the hydroxyl groups, that is, a so-called crosslinking agent, is 0.01 mass% or less in 100 mass% of the raw material for the polyester film. By making the content of the crosslinking agent 0.01 mass% or less, it is possible to reduce the weakening of the interaction between the component having a polyvinyl alcohol skeleton and the polyester molecular chain due to crosslinking between the components having a polyvinyl alcohol skeleton. Note that the crosslinking agent does not include the dicarboxylic acid component of polyester.

It is more preferable that the polyester film of the present invention meets the above condition B. Organic components that are insoluble in HFIP, which is a parent solvent for polyester films such as PET, exist in the film with a certain shape, and therefore can not only interact with the polyester molecular chains but also function as a lubricant.

As a method that satisfies the above-mentioned condition B, there is a method in which a component having a polyvinyl alcohol skeleton is chemically bonded to a polyester molecular chain during melt extrusion, and a preferred example of such a method is the method of kneading at the above-mentioned temperature and time.

The content of HFIP-insoluble organic components in the inner layer of the polyester film of the present invention is preferably 10 ppm or more and 1000 ppm or less by mass. A content of 10 ppm or more allows sufficient interaction with the polyester molecular chains to be obtained. A content of 1000 ppm or less prevents the organic components from precipitating as foreign matter in the film and becoming the starting point for film breakage.

It is more preferable that the HFIP-insoluble organic component in the present invention has a polyethylene terephthalate (PET) skeleton. It is believed that when a component having a polyvinyl alcohol skeleton is bonded to a PET molecular chain, it becomes insoluble in HFIP, and the HFIP-insoluble component has a polyethylene terephthalate skeleton. When the HFIP-insoluble organic component has a PET skeleton, the HFIP-insoluble organic component interacts more with the PET molecular chain in the polyester film, making it easier to obtain the effects of the present invention.

The fact that the organic components insoluble in HFIP have a PET skeleton can be confirmed by the method described in the Examples.

In the polyester film of the present invention, it is also preferable that the outermost layer of the polyester film contains a component having a polyvinyl alcohol skeleton. In this embodiment, when coating is used as a means for bringing the layer X or other layers described below into contact with the polyester film of the present invention, the transportability is improved in the step of drying the solvent used in forming the layer X or other layers, so that the occurrence of defects during the formation of the layer X or other layers can be suppressed.

The outermost layer of a polyester film has a component having a polyvinyl alcohol skeleton means that the outermost layer of the polyester film is measured according to conditions A and B and is found to meet condition A and/or condition B. The outermost layer refers to the area from at least one surface of the polyester film to a depth of 5 μm in the thickness direction.

From the viewpoints of productivity, transportability, and handleability, it is preferable that the polyester film of the present invention is a biaxially oriented polyester film. Biaxial orientation can be confirmed by the Young's modulus of the film, and means that the Young's modulus (Emd) in the flow direction (MD) of the film and the Young's modulus (Etd) in the direction perpendicular thereto are measured, and Emd/Etd is 0.6 or more and 1.7 or less. If the direction of the film is unknown, the direction of the film is defined as 0°, and the Young's modulus is measured while shifting the angle by 30°, and the maximum/minimum value is 1.0 or more and 1.7 or less.

Next, a preferred embodiment of the method for producing the polyester film of the present invention will be described below.

In the method for producing the polyester film of the present invention, a method (melt casting method) can be used in which dried raw materials are heated and melted in an extruder as necessary, and extruded from a die onto a cooled casting drum to process into a sheet. When extruding onto the casting drum, the raw materials are cooled to a surface temperature of 20°C to 60°C and then cooled and solidified by static electricity to produce an unstretched sheet. The temperature of the casting drum is more preferably 20°C to 40°C, and even more preferably 20°C to 30°C.

The raw materials used here can be polyester pellets to which a component having a polyvinyl alcohol skeleton has been added, a laminated polyester film in which a component having a polyvinyl alcohol skeleton has been applied to the surface of the polyester film and then melted again, or a mixture of a polyester film in which a component having a polyvinyl alcohol skeleton has been applied to the surface of the polyester film and polyester pellets.

Next, the unstretched sheet is preferably biaxially stretched at a temperature T1n (°C) that satisfies the following formula (i), by 3.6 times or more in the longitudinal direction (MD) of the film, by 3.9 times or more in the width direction (TD) of the film, and by an area stretching ratio of 14.0 times to 20.0 times.
(i) Tg (℃)≦T1n (℃)≦Tg+40 (℃)
Tg: glass transition temperature of polyester film (°C)
The method of stretching the film in the longitudinal direction is preferably a method using a speed difference between rolls. In this case, it is also a preferred embodiment to stretch the film in a plurality of sections while fixing the film with nip rolls to prevent the film from slipping.

The polyester film of the present invention is biaxially oriented by biaxial stretching, which allows the components having a polyvinyl alcohol skeleton to move within the film and interact most stably with the polyester molecular chains.

Next, the biaxially stretched film or biaxially oriented film is heat-set for 1 second to 30 seconds at a temperature (Th0 (°C)) that satisfies the following formula (ii), and then uniformly cooled slowly to room temperature to obtain a polyester film.
(ii) Tmf-35 (℃)≦Th0 (℃)≦Tmf (℃)
Tmf: melting point of the film (°C)
By obtaining a biaxially oriented film under the condition that satisfies (ii), it is possible to impart an appropriate orientation to the film and improve the handling properties of the film.

Next, a preferred embodiment of the laminated polyester film of the present invention will be described below.

The laminated polyester film of the present invention preferably has a layer X, the main component of which is a resin having a polyvinyl alcohol skeleton, on at least one surface of the polyester film of the present invention.

A resin having a polyvinyl alcohol skeleton is a resin having a molecular structure represented by chemical formula (3).

Here, f is 1 or greater and g is 0 or greater.

The polyvinyl alcohol resins described above can be suitably used as the resin having a polyvinyl alcohol skeleton.

The layer X mainly composed of a resin having a polyvinyl alcohol skeleton means that the layer X contains 90% by mass or more of the resin having a polyvinyl alcohol skeleton out of 100% by mass of all components of the layer X. More preferably, it contains 95% by mass or more.

Since resins having a polyvinyl alcohol skeleton are hydrophilic, this embodiment makes it possible to remove layer X, which is mainly composed of a resin having a polyvinyl alcohol skeleton, and other layers laminated in contact with layer X using water. In addition, since layer X has a skeleton similar to that of the component having a polyvinyl alcohol skeleton contained in the polyester film of the present invention, the polyester film of the present invention can be manufactured by removing layer X from the laminated polyester film as part of the raw material, as described below, and this is easy to recycle.

The method for coating layer X can be a general coating method such as gravure coating, Mayer bar coating, air knife coating, doctor knife coating, etc. In particular, from the viewpoint of the film-forming properties of layer X, an in-line coating method is preferably used in which the raw material of layer X is coated on the surface of the polyester film after uniaxial stretching in the longitudinal direction, and layer X is formed at the same time as the polyester film is stretched in the width direction.

The thickness of layer X is preferably 50 nm or more and 1000 nm or less. If it is less than 50 nm, the water absorbency of layer X may not be fully expressed, and removability may be poor. If it exceeds 1000 nm, blocking may occur, and handling properties may be reduced.

In the laminated polyester film of the present invention, it is preferable to have a polyester film, the layer X, and the release layer Y in this order. This embodiment allows the film to be used as a release film, and after use as a release film, the layer X and the release layer Y are removed to obtain a polyester film, which can be recycled and used as part of the raw material.

The laminated polyester film described above can be prepared by applying a coating solution in which the raw material components of the release layer Y are dissolved to the laminated polyester film containing the layer X obtained by the above-mentioned method, using a general coating method such as gravure coating, Mayer bar coating, air knife coating, or doctor knife coating.

Here, the release layer Y is preferably a layer whose main component is one or more components selected from a silicone compound, a compound having a long-chain alkyl group (having 10 or more carbon atoms), and a compound having fluorine. Among them, from the viewpoint of coatability onto the layer X, a silicone having a dimethylsiloxane main skeleton can be preferably used.

The thickness of the release layer Y is preferably 10 nm or more and 1000 nm or less. If it is less than 10 nm, the release function of the release layer Y may not be exhibited, and if it exceeds 1000 nm, the film-forming properties may be poor.

Furthermore, a functional layer Z can be laminated on the release layer Y side of the laminated polyester film in which layer X and release layer Y are laminated on the polyester film of the present invention. The functional layer Z is not particularly limited, but preferred examples include layers that are peeled off from the release layer Y, such as ceramic green sheets used in the manufacture of MLCCs, and adhesives for transferring to an adherend. When a ceramic green sheet is used as the functional layer Z, silicone having dimethylsiloxane as the main skeleton is preferably used as the release layer Y from the viewpoint of peelability. By using a laminated polyester film in which layer X and release layer Y are laminated on the polyester film of the present invention as a release film for the MLCC manufacturing process, the ceramic green sheet can be peeled off without affecting the physical properties of the ceramic green sheet, and the sheet can be used for ceramic manufacturing, and the release film can be easily recycled.

A laminated polyester film of the present invention can be obtained by peeling off the functional layer Z from a film in which layer X, release layer Y, and functional layer Z are laminated in this order on the polyester film of the present invention. By using this method, a laminated polyester film in which layer X and release layer Y are laminated on the polyester film of the present invention can be obtained without using the energy required for melt film formation or the energy required for coating and drying. Methods for peeling off the functional layer Z include, but are not limited to, a method of peeling off using an adhesive roll or a method of peeling off only the functional layer Z by suction.

Here, we will explain another preferred embodiment of the method for producing the polyester film of the present invention.

As a method for producing the polyester film of the present invention, a preferred embodiment can be carried out in which layer X and release layer Y are simultaneously removed from the laminated polyester in which layer X and release layer Y are laminated on the polyester film of the present invention, and the polyester film of the present invention is obtained again. By adopting this embodiment, it is possible to save the energy required to obtain a polyester film by performing a new melt film formation.

Next, the process of removing layer X and release layer Y will be described.

The resin having a polyvinyl alcohol skeleton, which is the main component of layer X, is generally water-absorbent, and therefore washing with water is a preferred embodiment. In order to increase the water-absorbency of the resin having a polyvinyl alcohol skeleton, which is the main component of layer X, it is preferable to include a highly hydrophilic modified group in the side chain of the resin. Examples of highly hydrophilic modified groups include those with strong polarity such as carboxylate groups, carboxylate groups, sulfonate groups, and sulfonic acid groups. In addition, when the total amount of hydroxyl groups and acetoxy groups and the amount of modified groups in the resin having a polyvinyl alcohol skeleton is 100 mol%, it is preferable that the content of modified groups is 1 mol% or more and 10 mol% or less.

The method of washing with water preferably includes the steps of unwinding the laminated polyester film, supplying hot water to the surface of the unwound laminated polyester film to peel off the surface laminated portion such as layer X from the laminated polyester film, and winding up the polyester film after peeling. The temperature of the hot water is preferably 50°C or higher and 150°C or lower. If the temperature is less than 50°C, sufficient cleaning properties may not be obtained. If the temperature exceeds 150°C, the glass transition temperature of the polyester film may be exceeded, and the film may not be transportable. The time that the water is in contact with the surface of the laminated polyester film is 5 seconds or more, preferably 10 seconds or more, and more preferably 30 seconds or more. From the viewpoint of productivity, the time is preferably 600 seconds or less. The step of supplying hot water to the surface of the unwound laminated polyester film is preferably performed in a water tank to cover the entire laminated polyester film, or heated water is pressurized and sprayed onto the film.

The film is conveyed at a speed of preferably 5 m/min or more, more preferably 10 m/min or more, and even more preferably 20 m/min to 100 m/min. When conveying the laminated polyester film provided with layer X and release layer Y in the process of removing layer X and release layer Y, it is also important to apply tension to the laminated polyester film. By applying tension, the surface of the laminated polyester film is stretched, improving the mobility of layer X and release layer Y, and as a result, the washability can be improved. The tension is 5 N/m to 100 N/m, more preferably 20 N/m to 80 N/m, and even more preferably 30 N/m to 50 N/m. If the tension is less than 5 N/m, the surface of the laminated polyester film may not be stretched, resulting in poor washability. If the tension exceeds 100 N/m, the film may wrinkle, the surface may not be stretched, and the washability may be poor.

When the polyester film of the present invention is used and the film is washed by the method described above, the transportability of the film is improved, and as a result, it becomes possible to transport the film while applying a certain tension and removing layer X and release layer Y, which is preferable because it improves the washability.

In addition, by obtaining a polyester film through the following cycle, the polyester film can be recycled and used. Specifically, this method involves laminating layer X on at least one side of the polyester film of the present invention by the above-mentioned method, further laminating release layer Y on layer X, providing functional layer Z on release layer Y, and then simultaneously removing layer X and release layer Y from the laminated polyester film obtained by peeling functional layer Z from release layer Y. In addition, the polyester film after simultaneously removing layer X and release layer Y can be melted as it is or together with other PET raw materials or resins having a polyvinyl alcohol skeleton, and molded into a film to obtain a polyester film. By melting once and forming a film, a polyester film with good quality such as grade can be obtained while being recycled.

As a recycling method, a polyester film can be preferably obtained by melting a polyester film obtained by removing a part of layer X from the laminated polyester film, and molding the polyester film using the composition obtained. By leaving a part of layer X, a component having a polyvinyl alcohol skeleton can be introduced into the polyester film without going through a step of adding a component having a polyvinyl alcohol skeleton to the melt-cast polyester film, and productivity is good. The amount of layer X to be removed is preferably 90% or more by mass, more preferably 95% or more, from the viewpoint of setting the amount of the component having a polyvinyl alcohol skeleton to be contained in an appropriate range.

[Method of evaluating characteristics]
A. (Measurement Method A) A method of measuring using a micro infrared spectrometer.
Apparatus: Microscopic infrared spectroscopic analyzer; LUMOS (manufactured by Bruker)
Measurement conditions:
Light source: Silicon carbide rod heating element (glow bar)
Detector: Narrow MCT (HgCdTe)
Detection wave number range: 4000 to 650 cm ^-1
Purge; Nitrogen gas Measurement mode; Transmission method Resolution; 8 cm ^-1
Number of integrations: 256. A spectrum is obtained by plotting the absorption intensity against each wave number.

In the cross section of the polyester film, measurements are made at 10 points evenly spaced in the thickness direction, at least 5 μm inside (hereinafter referred to as the inner layer portion) measured in the direction perpendicular to the outermost surface of the polyester film, using the above-mentioned device and conditions to obtain a spectrum. A PET spectrum is then subtracted from the above spectrum (a PET spectrum (measured under the same conditions using #50T60 manufactured by Toray Industries, Inc.) is used, and the four arithmetic operations are performed so that the peak shapes between 1000 cm ^-1 and 1500 cm ^-1 match as closely as possible, and this is then divided from the spectrum obtained by measurement).

Here, the minimum value among the ten measurements of the peak intensity detected at wave numbers from 3300 cm ^-1 to 3400 cm ^-1 obtained by the measurement at each point is calculated.

B. (Measurement Method B) A method of measuring using an infrared spectrometer.
Apparatus: Infrared spectroscopic analyzer; "Nicolet" (registered trademark) 6700 (manufactured by Thermofisher Scientific)
Measurement conditions:
Light source: Silicon carbide rod heating element (glow bar)
Detector: DTGS (triglycine sulfate)
Detection wave number range: 4000 to 680 cm ^-1
Purge; Nitrogen gas measurement mode; ATR method (attenuated total reflection measurement method)
Single reflection type ATR measurement accessory device (OMNI sampler), ATR crystal: Ge, incident angle: 45°
Resolution: 4 ^cm
Number of integrations: 256. A spectrum is obtained by plotting the absorption intensity against each wave number.

First, 50 pieces of 4 cm square polyester film are each scraped off 5 μm from one side of the surface using a microtome while observing under a microscope. Then, 5 μm is scraped off from the surface of the opposite side in the same way to obtain the inner layer of the polyester film.

The obtained inner layer is immersed in hexafluoro-2-propanol (HFIP) and dissolved for 30 minutes while stirring at 50°C under a nitrogen atmosphere. The liquid is then cooled to 25°C, and insoluble matter is collected using a PTFE filter (manufactured by ADVANTEC, model number: 13010802). The collected matter is measured using the above-mentioned device and conditions to obtain a spectrum.

Here, for the spectrum obtained, the peak intensity is determined at wave numbers from 3300 cm ^-1 to 3400 cm ^-1 .

C. Thickness of each layer The thickness of each layer of the polyester film and laminated polyester film is measured by the following method. The cross section of the film is cut out in a direction parallel to the width direction of the film using a microtome. The cross section is observed under a scanning electron microscope at a magnification of 5000 times to measure the thickness of each layer.

D. Method for identifying components having a PET skeleton (infrared spectroscopy)
Apparatus: Infrared spectroscopic analyzer; "Nicolet" (registered trademark) 6700 (manufactured by Thermofisher Scientific)
・Conditions:
Light source: Silicon carbide rod heating element (glow bar)
Detector: DTGS (triglycine sulfate)
Detection wave number range: 4000 to 680 cm ^-1
Purge; Nitrogen gas measurement mode; ATR method (attenuated total reflection measurement method)
Single reflection type ATR measurement accessory device (OMNI sampler), ATR crystal: Ge, incident angle: 45°
Resolution: 4 cm ^-1
Number of integrations: 256. A spectrum is obtained by plotting the absorption intensity against each wave number.

First, a microtome is used to remove 5 μm from one surface of the polyester film while observing under a microscope. Then, 5 μm is removed from the surface of the opposite surface in the same manner to obtain the inner layer of the polyester film.

The obtained inner layer portion is immersed in hexafluoro-2-propanol (HFIP) and dissolved for 30 minutes while stirring at 50°C under a nitrogen atmosphere. The liquid is then cooled to 25°C, and insoluble matter is collected using a PTFE filter (manufactured by ADVANTEC, model number: 13010802). The collected matter is measured using the above-mentioned device and conditions to obtain a spectrum. In the obtained spectrum, a peak is detected at wave numbers of 1090 cm ^-1 to 1110 cm ^-1 , and the peak intensity is 1.0 x ^10-3 or more.

If two or more peaks are observed, use the intensity of the strongest peak.

E. Amount of HFIP insoluble organic components in the inner layer (ppm)
First, 5 μm is scraped off from one surface of the polyester film using a microtome while observing under a microscope. Similarly, 5 μm is scraped off from the surface of the other side to obtain an inner layer part of the polyester film. The mass of the obtained inner layer part is weighed and designated as m0.

The obtained inner layer is immersed in hexafluoro-2-propanol (HFIP) and dissolved for 30 minutes under nitrogen atmosphere at 50°C while stirring. The liquid is then cooled to 25°C, and insoluble matter is collected using a PTFE filter (manufactured by ADVANTEC, model number: 13010802). The collected matter is dried under vacuum at 80°C, cooled to 25°C, and the mass is measured and designated as m1. Next, the sample is placed on a sample boat made of quartz glass with a known mass, and heated for 45 minutes under a nitrogen atmosphere in a cylindrical electric furnace previously heated to 320°C, to incinerate the desired composition. The sample boat is allowed to cool for 10 minutes under a nitrogen atmosphere, then transferred to a desiccator and allowed to cool to room temperature, after which the mass is weighed and designated as m2. After subtracting m2 from m1, the result is divided by m0 and expressed as ppm.

F. Amount of HFIP-insoluble organic components in the outermost layer (ppm)
First, 5 μm is scraped off from one surface of the polyester film using a microtome while observing under a microscope. In the same manner, 5 μm is scraped off from the surface of the opposite surface, and the resultant is weighed and the mass of the outermost surface portion is designated as m3.

The obtained inner layer is immersed in hexafluoro-2-propanol (HFIP) and dissolved for 30 minutes while stirring at 50°C under a nitrogen atmosphere. The liquid is then cooled to 25°C, and insoluble matter is collected using a PTFE filter (manufactured by ADVANTEC, model number: 13010802). The collected matter is dried under vacuum at 80°C, cooled to 25°C, and the mass is measured and designated as m4. Next, the sample is placed on a sample boat made of quartz glass with a known mass, and heated for 45 minutes under a nitrogen atmosphere in a cylindrical electric furnace previously heated to 320°C, to incinerate the desired composition. The sample boat is allowed to cool for 10 minutes under a nitrogen atmosphere, then transferred to a desiccator and allowed to cool to room temperature, after which the mass is weighed and designated as m5. After subtracting m5 from m4, the result is divided by m3 and expressed as ppm.

G. Water contact angle (°)
The contact angle is measured using a contact angle meter DM500 manufactured by Kyowa Interface Science Co., Ltd. and the attached analysis software FAMAS in the following manner. In an atmosphere of 23°C and 65% RH, the time when the water droplet contacts the sample surface is set as 0 seconds, and an image of the droplet is taken 1 second later, and the contact angle is measured from the image. The measurement is performed 5 times at different locations, and the average value is calculated.

H. Film Transportability The transportability of the film is judged by the shape of the roll after transport. The film is 300 mm wide and 500 m long, and the rolled up state is observed and judged. After unwinding the film, it is passed through a water bath at 80° C. for 10 seconds, then dried in a drying oven heated to 150° C. for 10 seconds, and then wound up. The judgment method is to measure the occurrence of lateral wrinkles on the wound roll. The lateral wrinkles are taken from the roll by taking one revolution of the roll, and the occurrence ratio of lateral wrinkles on the film is obtained. The occurrence ratio (%) of lateral wrinkles is calculated by [(total length of lateral wrinkles connected in the width direction of the film in the length direction of the film, i.e., total width of lateral wrinkles)/(length of one revolution of the roll)]×100. To observe the lateral wrinkles, toner is sprayed on the unwound film. The evaluation is made based on the average value per roll when five or more rolls are wound, and is judged as follows.

A: Horizontal wrinkle occurrence rate is less than 5%. B: Horizontal wrinkle occurrence rate is 5% or more but less than 10%. C: Horizontal wrinkle occurrence rate is 10% or more.

I. Residual amount (%) of layer X on polyester film after layer X and release layer Y are peeled off
The laminated polyester film as a sample is cut into 20 pieces of 200 mm square, dried at 50°C for 30 minutes, cooled to 25°C, and the mass is measured and designated as B(0). The film is immersed entirely in a water bath at 80°C, and layer X is dissolved in water for 3 hours while being vibrated with ultrasonic waves. The film is then removed and washed in another water bath at 80°C in the same manner for 30 minutes. The film is then wiped to remove moisture, dried at 50°C for 30 minutes, cooled to 25°C, and the mass is measured and designated as B(1).

Calculate the remaining amount (%) of layer X relative to the polyester film by (B(0) - B(1))/B(0) x 100.

The present invention will be described below with reference to examples, but the present invention is not necessarily limited to these.

[Production of PET-1] Terephthalic acid and ethylene glycol were polymerized in a conventional manner using antimony trioxide and magnesium acetate tetrahydrate as catalysts to obtain melt-polymerized PET. The resulting melt-polymerized PET had a glass transition temperature of 81°C, a melting point of 255°C, an intrinsic viscosity of 0.65, and a terminal carboxyl group content of 20 eq./t.
[PVA-1] "GOHSENOL" (registered trademark) GL-05 manufactured by Mitsubishi Chemical Corporation was used.
[PVA-2] PVA having a degree of saponification of 88, an average degree of polymerization of 500, and 1 mol % of sodium sulfonate in the side chain was prepared with reference to JP-A-9-227627.
[PVA-3] With reference to Patent Document JP-A-2008-291120, PVA having a degree of saponification of 88, an average degree of polymerization of 1000, and 1 mol % of sodium carboxylate in the side chain was prepared.
[Preparation of Coating Agent A] 100 parts by weight of an addition reaction type silicone resin release agent (manufactured by Dow Corning Toray Silicone Co., Ltd., under the product name "LTC" (registered trademark) 750A) and 2 parts by weight of a platinum catalyst (manufactured by Dow Corning Toray Silicone Co., Ltd., under the product name SRX212) were mixed in toluene as a solvent to give a solids content of 5% by weight, thereby obtaining Coating Agent A.

[Preparation of Coating B] PVA-2 was dissolved in water to a concentration of 4% by mass to obtain Coating B.

[Preparation of dielectric paste] 100 parts by weight of barium titanate (manufactured by Fuji Titanium Industries Co., Ltd. under the trade name HPBT-1), 10 parts by weight of polyvinyl butyral (manufactured by Sekisui Chemical Co., Ltd. under the trade name BL-1), 5 parts by weight of dibutyl phthalate, and 60 parts by weight of toluene-ethanol (mass ratio 30:30) were mixed with glass beads having a number average particle size of 2 mm, mixed and dispersed in a jet mill for 20 hours, and then filtered to prepare a paste-like dielectric paste.

Example 1
99.999 parts by mass of PET-1 and 0.001 parts by mass of PVA-1 were mixed and fed into an extruder with a vent hole while reducing the pressure to 100 torr or less, melted at 280 ° C. and a shear rate of 20 sec ^-1 for 100 seconds, and extruded through a die onto a casting drum with a surface temperature of 25 ° C. to produce an unstretched sheet. The sheet was then preheated with a heated roll group, stretched 3.8 times in the longitudinal direction (MD direction) at a temperature of 90 ° C., and then cooled with a roll group at a temperature of 25 ° C. to obtain a uniaxially stretched film. Then, while holding both ends of the film with clips, the film was stretched 4.3 times in the width direction (TD direction) perpendicular to the longitudinal direction in a heating zone at a temperature of 100 ° C. in a tenter. Further, the film was heat-set for 10 seconds at a temperature of 235 ° C. in a heat treatment zone in the tenter. Then, the film was uniformly cooled slowly in a cooling zone, and then wound up to obtain a polyester film. The properties of the obtained polyester film are shown in Table 1. In addition, in the method for determining the component having a polyvinyl alcohol skeleton (microscopic infrared spectroscopy), the measured value was less than 1×10 ⁻³ , so it is shown as "-" in the table.

The obtained polyester film contained a component having a polyvinyl alcohol skeleton in the inner layer of the film and met condition B, so it was a film with excellent transportability.

(Examples 2-4 and 2-6)
A polyester film was obtained in the same manner as in Example 1, except that the amount and type of polyvinyl alcohol added were changed as shown in the table. The properties of the obtained polyester film are shown in the table. As the amount of the component having a polyvinyl alcohol skeleton increased, the transportability improved. Example 4, which used PVA having sodium sulfonate in the side chain, showed particularly good transportability.

Example 5
Two extruders, A and B, were prepared, and the same raw material as in Example 4 was melted in the extruder B in the same manner as in Example 4. Only PET-1 was fed into the other extruder A while reducing the pressure to 10 torr, and melted at 280°C and a shear rate of 20 sec ^-1 for 100 seconds. The resin melted in the two extruders was introduced into a lamination device so that the thickness of the A layer was 5 μm after stretching, and the two were merged to form an A/B/A structure, and then a polyester film was obtained in the same manner as in Example 4. The properties of the obtained polyester film are shown in the table. The obtained polyester film contained a component having a polyvinyl alcohol skeleton in the inner layer portion, and fell under conditions A and B, and was therefore a film with excellent transportability.

(Reference Example 1)
99.95 parts by weight of PET-1 and 0.05 parts by weight of PVA-2 were mixed and fed into an extruder with a vent hole while reducing the pressure to 10 torr, melted at 280 ° C. and a shear rate of 20 sec ^-1 for 100 seconds, and extruded through a die onto a casting drum with a surface temperature of 25 ° C. to produce an unstretched sheet. The sheet was then preheated with a heated roll group, stretched 3.8 times in the longitudinal direction (MD direction) at a temperature of 90 ° C., and then cooled with a roll group at a temperature of 25 ° C. to obtain a uniaxially stretched film. The obtained uniaxially stretched film was coated with coating agent B by a bar coat method so that the coating thickness after drying and stretching was 100 nm, and then stretched 4.3 times in the width direction (TD direction) perpendicular to the longitudinal direction in a heating zone at a temperature of 100 ° C. in a tenter while holding both ends of the film with clips. The film was then heat-set for 10 seconds at 230° C. in a heat treatment zone in the tenter, and then uniformly cooled slowly in a cooling zone and taken up to obtain a laminated polyester film having a layer X of 100 nm laminated thereon.

On the layer X of the obtained laminated polyester film, coating material A was applied by gravure coating to form a release layer Y so that the thickness after drying would be 100 nm, and a laminated polyester film was obtained.

A dielectric paste was applied to the obtained laminated polyester film as functional layer Z by a die coating method so that the thickness after drying would be 1.0 μm. Then, functional layer Z was released from the obtained laminate, and a film roll of laminated polyester film from which functional layer Z had been peeled was obtained. The film roll was introduced into a water washing device equipped with an unwinding and winding device, and washed with water at 100°C for 2 minutes under a tension of 30 N/m, and the polyester film from which layer X and release layer Y had been removed was collected.

The water contact angle on the film surface of the polyester film of Example 4 was 72°, while the water contact angle on the layer X side surface of the recovered polyester film was 55°, indicating hydrophilicity, suggesting that some layer X remained in the recovered polyester film. When the remaining amount was measured, the mass of layer X/mass of the recovered polyester film was 0.07%.

(Example 7)
The polyester film recovered in Reference Example 1 was cut into flakes, and 20 parts by mass of the flakes were mixed with 80 parts by mass of PET-1 to obtain a polyester film in the same manner as in Example 1, except that no PVA-1 was added. The properties of the obtained film are shown in Table 1. Since the film contained a component having a polyvinyl alcohol skeleton and met Condition A and Condition B, the film had excellent transportability.

(Example 8)
The polyester film recovered in Reference Example 1 was cut into flakes, and 20 parts by mass of the flakes were mixed with 79.97 parts by mass of PET-1 and 0.03 parts by mass of PVA-2 to obtain a polyester film in the same manner as in Example 1, except that PVA-1 was not added. The properties of the obtained film are shown in Table 1. Since the film contained a component having a polyvinyl alcohol skeleton and met Condition A and Condition B, the film had excellent transportability.

(Comparative Example 1)
A polyester film was obtained in the same manner as in Example 1, except that only PET-1 was used and PVA-1 was not added. Since the film did not have a polyvinyl alcohol skeleton, it did not satisfy conditions A and B and had poor transportability.

(Comparative Example 2)
A uniaxially stretched film was obtained in the same manner as in Example 1, except that the raw materials and stretching conditions were changed as shown in the table. The film did not satisfy conditions A and B and had poor transportability.

The polyester film of the present invention has excellent transportability, making it suitable for a variety of industrial material applications and improving the recyclability of the film after use.

Claims (13)

A biaxially oriented polyester film containing a component having a polyvinyl alcohol skeleton, in which, when measured by the following Measurement Method A and Measurement Method B, a peak is detected in the wave number range of 3300 cm ^-1 to 3400 cm ^-1 in Measurement Method A and / or Measurement Method B, and the peak intensity is 1.0 x ^10-3 or more.
(Measurement Method A) A method of measuring using a microscopic infrared spectrometer.
Apparatus: Microscopic infrared spectroscopic analyzer; LUMOS (manufactured by Bruker)
Measurement conditions:
Light source: Silicon carbide rod heating element (glow bar)
Detector: Narrow MCT (HgCdTe)
Detection wave number range: 4000 to 650 cm ^-1
Purge; Nitrogen gas Measurement mode; Transmission method Resolution; 8 cm ^-1
Number of integrations: 256. A spectrum is obtained by plotting the absorption intensity against each wave number.
In the cross section of the polyester film, measurements are made at 10 points evenly spaced in the thickness direction, at least 5 μm inside (hereinafter referred to as the inner layer portion) measured in the direction perpendicular to the outermost surface of the polyester film, using the above-mentioned device and conditions to obtain a spectrum. A PET spectrum is then subtracted from the above spectrum (a PET spectrum (measured under the same conditions using #50T60 manufactured by Toray Industries, Inc.) is used, and the four arithmetic operations are performed so that the peak shapes between 1000 cm ^-1 and 1500 cm ^-1 match as closely as possible, and this is then divided from the spectrum obtained by measurement).
Here, if a peak is detected at wavenumbers of 3300 cm ^-1 to 3400 cm ^-1 in the divided spectrum at at least one measurement point out of the 10 measurement points, it is deemed that a peak is detected at wavenumbers of 3300 cm ^-1 to 3400 cm ^-1 when measured by Measurement Method A. Furthermore, if the peak intensity detected at at least one measurement point out of the 10 measurement points in the divided spectrum is 1.0 x 10 ^-3 or more, it is deemed that the peak intensity is 1.0 x 10 ^-3 or more when measured by Measurement Method A.
Furthermore, in the case where two or more peaks are observed in the wavenumber range from 3300 cm ^-1 to 3400 cm ^-1 in the divided spectrum at a given measurement point, if the maximum peak intensity among them is 1.0 x ^10-3 or more, the peak intensity detected in the divided spectrum at that measurement point is deemed to be 1.0 x ^10-3 or more.
(Measurement Method B) A method of measuring using an infrared spectrometer.
Apparatus: Infrared spectroscopic analyzer; "Nicolet" (registered trademark) 6700 (manufactured by Thermofisher Scientific)
Measurement conditions:
Light source: Silicon carbide rod heating element (glow bar)
Detector: DTGS (triglycine sulfate)
Detection wave number range: 4000 to 680 cm ^-1
Purge; Nitrogen gas measurement mode; ATR method (attenuated total reflection measurement method)
Single reflection type ATR measurement accessory device (OMNI sampler), ATR crystal: Ge, incident angle: 45°
Resolution: 4 ^cm
Number of integrations: 256. A spectrum is obtained by plotting the absorption intensity against each wave number.
First, 50 pieces of 4 cm square polyester film are each scraped off 5 μm from one surface of the polyester film using a microtome while observing under a microscope, and then 5 μm is scraped off from the surface of the opposite surface in the same manner to obtain the inner layer of the polyester film.
The obtained inner layer portion is immersed in hexafluoro-2-propanol (HFIP) and dissolved for 30 minutes with stirring at 50° C. in a nitrogen atmosphere. The liquid is then cooled to 25° C., and insoluble matter is collected with a PTFE filter (manufactured by ADVANTEC Corporation, model number: 13010802). The collected matter is measured with the above-mentioned device and conditions to obtain a spectrum.
Here, if a peak is detected in the wavenumber range of 3300 cm ^-1 to 3400 cm ^-1 in the obtained spectrum, it is deemed that a peak is detected in the wavenumber range of 3300 cm ^-1 to 3400 cm ^-1 when measured by Measurement Method B. Furthermore, if the peak intensity detected in the obtained spectrum is 1.0 x 10 ^-3 or more, it is deemed that the peak intensity is 1.0 x 10 ^-3 or more when measured by Measurement Method B.
Furthermore, in the case where two or more peaks are observed in the wavenumber range from 3300 ^cm to 3400 ^cm in the obtained spectrum, when the maximum peak intensity among them is 1.0×10 ^or more, the peak intensity detected in the obtained spectrum is deemed to be 1.0×10 ^or more. The biaxially oriented polyester film according to claim 1, wherein the content of the component having a polyvinyl alcohol skeleton is 0.001 to 0.05% by mass. 3. The biaxially oriented polyester film according to claim 1, wherein a peak is detected at a wave number of 3300 cm ^-1 to 3400 cm ^- 1 in the measurement method B and the peak intensity is 1.0 x ^10-3 or more, and the inner layer portion of the polyester film contains an organic component insoluble in HFIP in an amount of 10 ppm to 1000 ppm by mass. The biaxially oriented polyester film according to any one of claims 1 to 3 , wherein a peak is detected at a wave number of 3300 cm ^-1 to 3400 cm ^-1 and the peak intensity is 1.0 x 10 ^-3 or more in the measurement method B, and the organic component insoluble in HFIP has a polyethylene terephthalate skeleton. The biaxially oriented polyester film according to any one of claims 1 to 4 , wherein the outermost layer of the polyester film contains a component having a polyvinyl alcohol skeleton. 6. A laminated polyester film having a layer X mainly composed of a resin having a polyvinyl alcohol skeleton on at least one surface of the biaxially oriented polyester film according to claim 1. The laminated polyester film according to claim 6 , comprising the polyester film, the layer X, and a release layer Y in this order. The laminated polyester film according to claim 7 , comprising the polyester film, the layer X, the release layer Y, and the functional layer Z in this order. A method for producing a laminated polyester film, comprising a step of peeling off the functional layer Z from the laminated polyester film according to claim 8 . A method for producing a polyester film, comprising the step of simultaneously removing the layer X and the release layer Y from the laminated polyester film according to claim 7 . A method for producing a polyester film, comprising the steps of peeling off the functional layer Z and simultaneously removing the layer X and the release layer Y from the laminated polyester film according to claim 8 , in that order. A method for producing a polyester film, comprising laminating a release layer Y on the layer X of the laminated polyester film according to claim 6 , providing a functional layer Z on the release layer Y, peeling off the functional layer Z, and then removing the layer X and the release layer Y. A method for producing a polyester film, comprising a step of melting a polyester film obtained by removing a part of the layer X from the laminated polyester film according to any one of claims 6 to 8 , and molding the polyester film using a composition obtained by the melting a polyester film.