JP7578033B2 - Laminated Polyester Film - Google Patents

Description

The present invention relates to a laminated polyester film.

Polyester film has excellent properties such as mechanical strength, dimensional stability, flatness, heat resistance, chemical resistance, and optical properties, and is cost-effective, so it is used as a base film for packaging materials, plate-making materials, display materials, transfer materials, etc.

In these applications, a functional layer such as a hard coat layer is often formed on the surface of the polyester film, which is the base film, in order to improve scratch resistance and surface hardness and to prevent curling.
However, most substrate films are non-reactive, and the adhesion between the substrate film and the functional layer may be insufficient.
Therefore, in order to improve the adhesion between the substrate film and the functional layer, a highly adhesive coating layer is generally provided as an intermediate layer.

For example, Patent Documents 1 and 2 disclose a method of coating a base film with a water-soluble or water-dispersible coating liquid of polyurethane resin, polyester resin, acrylic resin, or the like.
However, under conditions of high temperature and high humidity, the adhesion between the highly adhesive coating layer and the base film and/or functional layer tends to be insufficient.

In view of the above circumstances, methods are known in which the adhesiveness under high temperature and high humidity conditions is improved by crosslinking the resin, as disclosed in Patent Documents 3 and 4.
Specifically, by adding a crosslinking agent such as a melamine compound, an epoxy compound, or an oxazoline compound to the polyurethane resin, polyester resin, or acrylic resin that forms the coating layer, the adhesion to the base film and/or the functional layer can be improved.

Japanese Unexamined Patent Publication No. 55-15825 Japanese Unexamined Patent Publication No. 58-78761 Japanese Patent Application Publication No. 8-281890 Japanese Patent Application Publication No. 11-286092

However, when the coating layer is produced at a high temperature in order to promote the curing reaction of the crosslinking agent in the coating layer, oligomers (mainly cyclic ester trimers) may precipitate from the surface of the coating layer.
On the other hand, when the coating layer was produced at a low temperature in order to suppress the amount of oligomers precipitating from the surface of the coating layer, depending on the composition in the coating layer, the curing reaction did not proceed sufficiently, and the adhesion between the coating layer and the functional layer did not reach the desired level.
Thus, it may be difficult to simultaneously suppress oligomer precipitation and improve adhesion.
In view of the above circumstances, an object of the present invention is to provide a laminated polyester film which inhibits precipitation of oligomers and has excellent adhesion to a functional layer.

As a result of intensive research, the inventors have found that by setting the surface hardness of a coating layer containing a melamine compound to a specific value or less, not only can the precipitation of oligomers be suppressed, but also the adhesion between the coating layer and the functional layer can be improved, thereby solving the above-mentioned problems.
The present invention has been completed based on these findings and has the following aspects.
[1] A laminated polyester film having a coating layer on at least one surface of a polyester film, the coating layer being formed from a coating liquid containing a crosslinking agent (A) that contains a melamine compound (A1), and the surface hardness of the coating layer being 640 MPa or less.
[2] The laminated polyester film according to the above [1], wherein the coating layer has a surface elastic modulus of 4.0 GPa or more and 9.0 GPa or less.
[3] The laminated polyester film according to the above [1] or [2], wherein the melamine compound (A1) is at least one selected from the group consisting of hexamethoxymethylolmelamine, hexamethoxymethylmelamine, N,N',N"-trimethylolmelamine, and pentamethoxymethylmelamine.
[4] The laminated polyester film according to any one of the above [1] to [3], wherein the crosslinking agent (A) further contains an epoxy compound (A2).
[5] The laminated polyester film according to any one of the above [1] to [4], wherein the crosslinking agent (A) further contains an epoxy compound (A2) and an oxazoline compound (A3), the epoxy compound (A2) is present in an amount of 10 to 300 parts by mass relative to 100 parts by mass of the melamine compound (A1), and the oxazoline compound (A3) is present in an amount of 30 to 200 parts by mass relative to 100 parts by mass of the melamine compound (A1).
[6] The laminated polyester film according to any one of the above [1] to [5], wherein the coating liquid further contains an acid catalyst (C).
[7] The laminated polyester film according to the above [6], wherein the acid catalyst (C) has a sulfonic acid group or a phosphoric acid group.
[8] The laminated polyester film according to the above [6] or [7], wherein the acid catalyst (C) is at least one selected from the group consisting of p-toluenesulfonic acid, dinonylnaphthalene disulfonic acid, dinonylnaphthalene (mono)sulfonic acid, and dodecylbenzenesulfonic acid.
[9] The laminated polyester film according to any one of the above [1] to [8], wherein the coating liquid further contains a binder resin (B), the binder resin (B) contains a polyester resin (B1) and an acrylic resin (B2), and the content ratio of the polyester resin (B1) to the acrylic resin (B2), (B1)/(B2), is 40/60 to 60/40.
[10] The laminated polyester film according to any one of the above [1] to [9], having a micro-melting heat peak temperature of 160° C. or more and 210° C. or less.
[11] A laminated polyester film having a functional layer, comprising the laminated polyester film according to any one of [1] to [10] above and a functional layer on the coating layer.
[12] The laminated polyester film with a functional layer according to the above [11], wherein the functional layer is a hard coat layer.

The present invention provides a laminated polyester film that suppresses oligomer precipitation and has excellent adhesion to the functional layer, and is highly valuable in industrial applications.

An example of an embodiment of the present invention will be described in detail below.
However, the present invention is not limited to the embodiment described below, and can be modified in any manner without departing from the gist of the present invention.

The laminated polyester film of the present invention has a coating layer on at least one surface of a polyester film, the coating layer being formed from a coating liquid containing a melamine compound, and the coating layer has a surface hardness of 640 MPa or less.
Each of the constituent elements will be described in detail below.

<Polyester film>
The polyester film constituting the laminated polyester film in the present invention may have a single-layer structure or a multi-layer structure.
When the polyester film has a multi-layer structure, the polyester film may have a two-layer structure, a three-layer structure, or a four-layer structure or more without departing from the spirit of the present invention, and the number of layers is not particularly limited.
The polyester film may be a non-stretched film (sheet) or a stretched film. Of these, a uniaxially or biaxially stretched film is preferred. Of these, a biaxially stretched film is more preferred from the viewpoints of balance of mechanical properties, flatness, and thinning.

The polyester used as the raw material for the polyester film may be either a homopolyester or a copolymer polyester. In the case of a homopolyester, it is preferable that the homopolyester is obtained by polycondensation of an aromatic dicarboxylic acid and an aliphatic glycol.
Examples of aromatic dicarboxylic acids include terephthalic acid, 2,6-naphthalenedicarboxylic acid, and esters thereof (e.g., dimethyl terephthalate), and examples of aliphatic glycols include ethylene glycol, diethylene glycol, and 1,4-cyclohexanedimethanol. A representative example of homopolyesters is polyethylene terephthalate.
On the other hand, examples of the dicarboxylic acid component of the copolymer polyester include one or more of isophthalic acid, phthalic acid, terephthalic acid, 2,6-naphthalenedicarboxylic acid, adipic acid, sebacic acid, oxycarboxylic acid, etc., and examples of the glycol component include one or more of ethylene glycol, diethylene glycol, propylene glycol, butanediol, 4-cyclohexanedimethanol, neopentyl glycol, etc., with the proviso that two or more of at least one of the dicarboxylic acid component and the glycol component are used.

There are no particular limitations on the polyester polymerization catalyst, and any conventionally known compound can be used, such as titanium compounds, germanium compounds, antimony compounds, manganese compounds, aluminum compounds, magnesium compounds, and calcium compounds.

In order to suppress the amount of precipitation of oligomer components, the film may be produced using a polyester having a low content of oligomer components as the raw material. As a method for producing a polyester having a low content of oligomer components, various known methods can be used, such as a method of performing solid phase polymerization after the production of a polyester.
In addition, the amount of precipitation of oligomer components may be suppressed by forming the polyester film into a three-layer or more layer structure and using a polyester raw material having a low content of oligomer components as the outermost layer of the polyester film.
The polyester may be obtained by carrying out the esterification or transesterification reaction, followed by melt polycondensation at a higher reaction temperature under reduced pressure.

The polyester film may contain an ultraviolet absorbing agent to improve the weather resistance of the film and to prevent deterioration of the adherend (e.g., liquid crystal). There are no particular limitations on the ultraviolet absorbing agent, so long as it is a compound that absorbs ultraviolet light and can withstand the heat applied during the polyester film manufacturing process.

The ultraviolet absorbing agent includes organic ultraviolet absorbing agents and inorganic ultraviolet absorbing agents, and from the viewpoint of transparency, organic ultraviolet absorbing agents are preferred.The organic ultraviolet absorbing agent is not particularly limited, but for example, cyclic imino ester type, benzotriazole type, benzophenone type, etc. are included.From the viewpoint of durability, cyclic imino ester type and benzotriazole type are more preferred.
Two or more kinds of ultraviolet absorbents can be used in combination.

Particles may be blended into the polyester film for the main purpose of imparting lubricity and preventing scratches during each process. The type of particles to be blended is not particularly limited as long as it is a particle capable of imparting lubricity, and specific examples include inorganic particles such as silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, magnesium phosphate, kaolin, aluminum oxide, and titanium oxide, and organic particles such as acrylic resin, styrene resin, urea resin, phenol resin, epoxy resin, and benzoguanamine resin.
Furthermore, precipitated particles obtained by precipitating and finely dispersing a part of a metal compound such as a catalyst during the polyester production process can also be used.

The shape of the particles to be used is not particularly limited, and any of spherical, block, rod-like, flat, etc. may be used.
There is also no particular restriction on the hardness, specific gravity, color, etc. These particles may be used in combination of two or more kinds, if necessary.

The average particle size is usually 5 μm or less, and preferably in the range of 0.01 to 3 μm. If it is 5 μm or less, the surface roughness of the film can be made appropriate, and it is preferable because it does not affect the formation of various functional layers in later processes.

Furthermore, the content of particles in the polyester film is usually 5% by mass or less, preferably in the range of 0.0003 to 3% by mass. When there are no particles or only a small amount of particles, the film has high transparency and is of good quality, but the slipperiness may be insufficient, so that it may be necessary to take measures such as adding particles to the coating layer to improve the slipperiness.
Furthermore, when the particle content is 5% by mass or less, the transparency of the film can be sufficiently ensured.
When particles are contained, for example, it is preferable to provide a surface layer and an intermediate layer and to contain particles in the surface layer. In this case, it is more preferable to form a multilayer structure having a surface layer containing particles, an intermediate layer, and a surface layer containing particles in this order.

The method of adding particles to a polyester film is not particularly limited, and any conventionally known method can be used. For example, in the case of a multi-layer polyester film, the particles can be added at any stage in the production of the polyester that constitutes each layer, but it is preferable to add the particles after the esterification or transesterification reaction is completed.

In addition to the above-mentioned particles, conventionally known antioxidants, antistatic agents, heat stabilizers, lubricants, dyes, pigments, etc. can be added to the polyester film as necessary.

The thickness of the polyester film is not particularly limited as long as it is within the range in which it can be formed into a film, but from the viewpoints of mechanical strength, handling properties, productivity, etc., it is usually in the range of 10 to 350 μm, preferably 25 to 250 μm, and more preferably 38 to 125 μm.

Next, a specific example of the production of a polyester film will be described, but the production method is not limited to the following production example. For example, when producing a biaxially stretched film, a method is preferred in which the dried pellets of the polyester raw material described above are extruded as a molten sheet from a die using an extruder, and then cooled and solidified with a cooling roll to obtain an unstretched sheet. In this case, it is preferred to increase the adhesion between the sheet and the rotating cooling drum in order to improve the flatness of the sheet, and an electrostatic application adhesion method and/or a liquid application adhesion method are preferably used.
The unstretched sheet is stretched in a biaxial direction. In this case, the unstretched sheet is first stretched in one direction by a roll or tenter type stretching machine. The stretching temperature is usually 70 to 120°C, preferably 80 to 110°C, and the stretching ratio is usually 2.5 to 7 times, preferably 3.0 to 6 times.
Then, the film is stretched in a direction perpendicular to the first-stage stretching direction. In this case, the stretching temperature is usually 70 to 170° C., and the stretch ratio is usually 3.0 to 7 times, preferably 3.5 to 6 times.
The film is then heat-treated at a temperature of 180 to 270° C. under tension or relaxation of 30% or less to obtain a biaxially stretched film. In the above stretching, a method of stretching in one direction in two or more stages can also be adopted. In that case, it is preferable to perform the stretching so that the final stretch ratios in both directions are each within the above range.

Also, a simultaneous biaxial stretching method can be adopted.
The simultaneous biaxial stretching method is a method in which the unstretched sheet is simultaneously stretched and oriented in the longitudinal and transverse directions under a temperature controlled condition usually at 70 to 120°C, preferably 80 to 110°C, and the stretching ratio is 4 to 50 times, preferably 7 to 35 times, more preferably 10 to 25 times in terms of area ratio.
The film is then subsequently heat-treated under tension or relaxation of 30% or less at a temperature of 170 to 250° C. to obtain a stretched and oriented film. Regarding the simultaneous biaxial stretching device employing the above-mentioned stretching method, any conventionally known stretching method such as a screw method, a pantograph method, or a linear drive method can be employed.

<Coating Layer>
The laminated polyester film of the present invention has a coating layer on at least one surface of the polyester film.
The coating layer according to the present invention is an easily adhesive layer provided as an intermediate layer between a polyester film and a functional layer such as a hard coat layer in order to improve adhesion therebetween, and has particularly excellent adhesion to the functional layer.

[Crosslinking agent (A)]
The coating layer according to the present invention is formed from a coating liquid containing a melamine compound (A1) as a crosslinking agent (A).
The crosslinking agent has the effect of crosslinking the binder resin described below, thereby improving the adhesion between the coating layer and the functional layer even under high temperature and high humidity conditions, and can also improve the adhesion between the coating layer and the polyester film.
As the crosslinking agent other than the melamine compound, various known crosslinking agents can be used, such as an epoxy compound, a carbodiimide compound, an oxazoline compound, an isocyanate compound, and a silane coupling compound.

(Melamine compound (A1))
The melamine compound refers to a compound having a melamine skeleton in the compound, and for example, an alkylolated melamine derivative, a compound obtained by reacting an alkylolated melamine derivative with an alcohol to partially or completely etherify it, and a mixture of these can be used.
As the alcohol used for the etherification, methyl alcohol, ethyl alcohol, isopropyl alcohol, n-butanol, isobutanol, etc. are preferably used.
The melamine compound may be either a monomer or a dimer or higher polymer, or a mixture of these may be used.
Furthermore, melamine partially co-condensed with urea or the like can also be used.
Examples of the melamine compound include hexamethoxymethylolmelamine, pentamethoxymethylmelamine, hexamethoxymethylmelamine, pentamethoxymethylmelamine, hexaethoxymethylmelamine, hexakis-(methoxymethyl)melamine, N,N',N"-trimethyl-N,N',N"-trimethylolmelamine, N,N',N"-trimethylolmelamine, N-methylolmelamine, N,N'-(methoxymethyl)melamine, N,N',N"-tributyl-N,N',N"-trimethylolmelamine, and the like. Among these, one or more types selected from hexamethoxymethylolmelamine, hexamethoxymethylmelamine, N,N',N"-trimethylolmelamine, and pentamethoxymethylmelamine are preferable.

The mechanism by which the coating solution that forms the coating layer contains a melamine compound but has a surface hardness of a specific value or less can achieve both inhibition of oligomer precipitation and adhesion to the functional layer, is believed to be as follows.
Normally, the crosslinking reaction of melamine compounds proceeds by homopolymerization in the later stage (high temperature region) of the coating layer formation process due to its high activation energy and reaction temperature. However, it has been found that if the coating layer is not cured at a sufficiently high temperature region due to concerns about oligomer precipitation, poor adhesion between the coating layer and the functional layer can occur.
According to the present invention, it has been found that by setting the surface hardness of the coating layer to a specific value or less, the wettability of the coating layer in contact with the functional layer and its conformity with the functional layer are improved, and the effect of improving the adhesion between the coating layer and the functional layer is obtained. Note that, when the surface hardness of the coating layer is greater than a specific value, it is considered that a melamine compound that homopolymerizes in a high temperature region is unevenly distributed in large amounts on the surface of the coating layer.
Therefore, one of the methods for making the surface hardness of the coating layer below a specific value is to contain an acid catalyst described later in the coating solution for forming the coating layer, thereby lowering the activation energy and reaction temperature during the crosslinking reaction of the melamine compound and randomly proceeding the reaction with the reactive group of the binder resin and other crosslinking agents. According to this method, the melamine compound is dispersed in the coating layer and the molecular weight between crosslinking points increases, so that the surface hardness of the coating layer becomes below a specific value, and the adhesiveness between the coating layer and the functional layer can be secured, and the precipitation of oligomers can be suppressed by proceeding the crosslinking reaction of the melamine compound at a low temperature.

(Epoxy compound (A2))
The epoxy compound is a compound having an epoxy group in the molecule, and examples thereof include condensations of epichlorohydrin, ethylene glycol, polyethylene glycol, glycerin, polyglycerin, tris(2-hydroxyethyl) isocyanurate, bisphenol A, etc. with a hydroxyl group or an amino group, polyepoxy compounds, diepoxy compounds, monoepoxy compounds, and glycidylamine compounds.
Examples of the polyepoxy compound include sorbitol polyglycidyl ether, polyglycidyl ether, pentaerythritol polyglycidyl ether, diglycerol polyglycidyl ether, triglycidyl tris(2-hydroxyethyl)isocyanate, glycerol polyglycidyl ether, and trimethylolpropane polyglycidyl ether.
Examples of diepoxy compounds include neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, resorcinol diglycidyl ether, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, and polytetramethylene glycol diglycidyl ether.
Examples of the monoepoxy compound include allyl glycidyl ether, 2-ethylhexyl glycidyl ether, and phenyl glycidyl ether.
Examples of the glycidylamine compound include N,N,N',N'-tetraglycidyl-m-xylylenediamine, 1,3-bis(N,N-diglycidylamino)cyclohexane, and the like.
As the epoxy compound, from the viewpoint of improving adhesion, a polyether-based epoxy compound is preferred, and the amount of epoxy groups contained in the polyether-based epoxy compound is preferably bifunctional, and more preferably trifunctional or more. Among these, at least one selected from the group consisting of sorbitol polyglycidyl ether, polyglycidyl ether, pentaerythritol polyglycidyl ether, and diglycerol polyglycidyl ether is preferred.

(Carbodiimide Compound)
A carbodiimide compound is a compound having a carbodiimide structure, and is a compound having one or more carbodiimide structures in its molecule. From the viewpoint of improving adhesion, a polycarbodiimide compound having two or more carbodiimide structures in its molecule is more preferred.

The carbodiimide compound can be synthesized by a conventionally known technique, and generally, a condensation reaction of a diisocyanate compound is used.
The diisocyanate compound is not particularly limited, and either aromatic or aliphatic diisocyanates can be used. Specific examples of the diisocyanate compound include tolylene diisocyanate, xylylene diisocyanate, diphenylmethane diisocyanate, phenylene diisocyanate, naphthalene diisocyanate, hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, dicyclohexyl diisocyanate, and dicyclohexylmethane diisocyanate.

The content of carbodiimide groups contained in the carbodiimide compound is, in terms of carbodiimide equivalent (weight [g] of the carbodiimide compound required to give 1 mol of carbodiimide groups), usually in the range of 100 to 1000, preferably 250 to 800, and more preferably 300 to 700. By using within the above range, easy adhesion is improved.

Furthermore, within the scope of the present invention, in order to improve the water solubility or water dispersibility of the polycarbodiimide compound, a surfactant may be added, or a hydrophilic monomer such as a polyalkylene oxide, a quaternary ammonium salt of a dialkylamino alcohol, or a hydroxyalkylsulfonate may be added.

(Oxazoline compound (A3))
The oxazoline compound is a compound having an oxazoline group in the molecule, and is preferably a polymer containing an oxazoline group. The oxazoline compound can be prepared by polymerization of an addition-polymerizable oxazoline group-containing monomer alone or with other monomers.
Examples of the addition-polymerizable oxazoline group-containing monomer include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, and 2-isopropenyl-5-ethyl-2-oxazoline, and these can be used alone or in mixtures of two or more. Among these, 2-isopropenyl-2-oxazoline is preferred because it is easily available industrially.
The other monomer is not limited as long as it is a monomer copolymerizable with the addition-polymerizable oxazoline group-containing monomer, and examples thereof include (meth)acrylic acid esters such as alkyl(meth)acrylates (alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, 2-ethylhexyl, and cyclohexyl groups); unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, styrenesulfonic acid, and salts thereof (sodium salts, potassium salts, ammonium salts, tertiary amine salts, etc.); unsaturated nitriles such as acrylonitrile and methacrylonitrile; (meth)acrylamide, N-alkyl(meth)acrylate, etc. Examples of the monomer include unsaturated amides such as (t)acrylamide and N,N-dialkyl(meth)acrylamide (the alkyl group can be, for example, a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a t-butyl group, a 2-ethylhexyl group, a cyclohexyl group, etc.); vinyl esters such as vinyl acetate and vinyl propionate; vinyl ethers such as methyl vinyl ether and ethyl vinyl ether; α-olefins such as ethylene and propylene; halogen-containing α,β-unsaturated monomers such as vinyl chloride and vinylidene chloride; and α,β-unsaturated aromatic monomers such as styrene and α-methylstyrene. One or more of these monomers can be used.
The oxazoline compound may have a polyalkylene oxide chain such as a polyethylene oxide chain, and for example, a (meth)acrylate having a polyalkylene oxide chain may be used as another monomer.
From the viewpoint of improving adhesion, the amount of oxazoline groups in the oxazoline compound is preferably in the range of 0.5 to 10 mmol/g, more preferably 1 to 9 mmol/g, further preferably 3 to 8 mmol/g, and particularly preferably 4 to 6 mmol/g.

(Isocyanate Compound)
The isocyanate compound is a compound having an isocyanate derivative structure, typically an isocyanate or a blocked isocyanate.
Examples of the isocyanate include aromatic isocyanates such as tolylene diisocyanate, xylylene diisocyanate, methylene diphenyl diisocyanate, phenylene diisocyanate, and naphthalene diisocyanate; aliphatic isocyanates having an aromatic ring such as α,α,α',α'-tetramethylxylylene diisocyanate; aliphatic isocyanates such as methylene diisocyanate, propylene diisocyanate, lysine diisocyanate, trimethylhexamethylene diisocyanate, and hexamethylene diisocyanate; and alicyclic isocyanates such as cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, methylene bis(4-cyclohexyl isocyanate), and isopropylidenedicyclohexyl diisocyanate.
Further, polymers and derivatives of these isocyanates such as biuretized products, isocyanurate products, uretdione products, and carbodiimide modified products are also included. These may be used alone or in combination of two or more kinds.
Among the above-mentioned isocyanates, aliphatic isocyanates or alicyclic isocyanates are more preferable than aromatic isocyanates in order to prevent yellowing due to ultraviolet rays.

When used in the form of a blocked isocyanate, examples of the blocking agent include bisulfites, phenolic compounds such as phenol, cresol, and ethylphenol; alcoholic compounds such as propylene glycol monomethyl ether, ethylene glycol, benzyl alcohol, methanol, and ethanol; active methylene compounds such as isobutanoyl methyl acetate, dimethyl malonate, diethyl malonate, methyl acetoacetate, ethyl acetoacetate, and acetylacetone; mercaptan compounds such as butyl mercaptan and dodecyl mercaptan; lactam compounds such as ε-caprolactam and δ-valerolactam; amine compounds such as diphenylaniline, aniline, and ethyleneimine; acid amide compounds such as acetanilide and acetic acid amide; and oxime compounds such as formaldehyde, acetaldoxime, acetoneoxime, methyl ethyl ketoneoxime, and cyclohexanoneoxime, which may be used alone or in combination of two or more.

The isocyanate compound may be used alone or as a mixture or bond with various polymers. In order to improve the dispersibility and crosslinking properties of the isocyanate compound, it is preferable to use a mixture and/or bond with a polyester resin or a urethane resin.

(Silane coupling compound)
The silane coupling compound is an organosilicon compound having an organic functional group and a hydrolyzable group such as an alkoxy group in one molecule.
Examples of the silane coupling compound include epoxy group-containing compounds such as 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; vinyl group-containing compounds such as vinyltrimethoxysilane and vinyltriethoxysilane; styryl group-containing compounds such as p-styryltrimethoxysilane and p-styryltriethoxysilane; (meth)acrylic group-containing compounds such as 3-(meth)acryloxypropyltrimethoxysilane, 3-(meth)acryloxypropyltriethoxysilane, 3-(meth)acryloxypropylmethyldimethoxysilane, and 3-(meth)acryloxypropylmethyldiethoxysilane; 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-2-( amino group-containing compounds such as N-(1,3-dimethylbutylidene)propylamine, N-phenyl-3-aminopropyltrimethoxysilane, N-phenyl-3-aminopropyltriethoxysilane, N-phenyl-3-aminopropyltriethoxysilane; isocyanurate group-containing compounds such as tris(trimethoxysilylpropyl)isocyanurate and tris(triethoxysilylpropyl)isocyanurate; mercapto group-containing compounds such as 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 3-mercaptopropylmethyldimethoxysilane and 3-mercaptopropylmethyldiethoxysilane.

When the coating layer is formed from a coating liquid, the content of the crosslinking agent in the total non-volatile components in the coating liquid is usually in the range of 0.1 to 95 mass%, preferably 1 to 80 mass%, more preferably 10 to 60 mass%, and even more preferably 20 to 50 mass%.
When the content of the crosslinking agent is within the above range, the adhesion between the coating layer and the functional layer is good, and the surface hardness and surface elasticity of the coating layer can be set within the desired range.

The content of the melamine compound (A1) in the total crosslinking agent is preferably from 1 to 100 mass%, more preferably from 5 to 80 mass%, even more preferably from 10 to 70 mass%, and even more preferably from 20 to 50 mass%.
When the content of the melamine compound is within the above range, the surface hardness and surface elasticity of the coating layer can be set within the desired range, the adhesion between the coating layer and the functional layer can be improved, and the adhesion between the coating layer and the polyester film can also be improved.
In addition, within such a range, the molecular weight between crosslinking points becomes appropriate due to the acid catalytic action described below, and the surface hardness of the coating layer can be made to a specific value or less.

In addition, the crosslinking agent (A) containing a melamine compound (A1) preferably contains an epoxy compound (A2), more preferably contains one or more selected from the epoxy compound (A2) and the oxazoline compound (A3), and further preferably contains an epoxy compound (A2) and an oxazoline compound (A3).
When the crosslinking agent (A) containing a melamine compound (A1) contains an epoxy compound (A2) and an oxazoline compound (A3), the epoxy compound (A2) is preferably 1 to 500 parts by mass, more preferably 10 to 300 parts by mass, even more preferably 30 to 200 parts by mass, even more preferably 50 to 150 parts by mass, and even more preferably 75 to 125 parts by mass, relative to 100 parts by mass of the melamine compound (A1). The oxazoline compound (A3) is preferably 10 to 500 parts by mass, more preferably 30 to 200 parts by mass, even more preferably 50 to 150 parts by mass, and even more preferably 75 to 125 parts by mass, relative to 100 parts by mass of the melamine compound (A1).

[Binder resin (B)]
The resin composition for forming the coating layer according to the present invention preferably contains a binder resin (B).
As a suitable example of the binder resin, one or more compounds selected from the group consisting of polyurethane resins, polyester resins, and acrylic resins that impart easy adhesion properties can be mentioned.

(Polyurethane resin)
The polyurethane resin is a polymeric compound having urethane bonds in the molecule, and is preferably water-dispersible or water-soluble.
The polyurethane resin may be used alone or in combination of two or more kinds.

In order to impart water dispersibility or water solubility, it is common and preferable to introduce hydrophilic groups such as hydroxyl groups, carboxyl groups, sulfonic acid groups, sulfonyl groups, phosphate groups, and ether groups into the urethane resin. Among the hydrophilic groups, carboxyl groups and sulfonic acid groups are particularly preferable from the viewpoint of improving adhesion.

One method for producing polyurethane resin is by reacting a hydroxyl group-containing compound with an isocyanate compound. As the hydroxyl group-containing compound used as a raw material, polyols are preferably used, such as polyether polyols, polyester polyols, polycarbonate polyols, polyolefin polyols, and acrylic polyols. These compounds may be used alone or in combination.

Examples of polyether polyols include polyethylene glycol, polypropylene glycol, polyethylene propylene glycol, polytetramethylene ether glycol, polyhexamethylene ether glycol, etc.

Examples of polyester polyols include polycarboxylic acids (malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, sebacic acid, fumaric acid, maleic acid, terephthalic acid, isophthalic acid, etc.) or their acid anhydrides and polyhydric alcohols (ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 2-methyl-1,3-propanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, 2- methyl-2,4-pentanediol, 2-methyl-2-propyl-1,3-propanediol, 1,8-octanediol, 2,2,4-trimethyl-1,3-pentanediol, 2-ethyl-1,3-hexanediol, 2,5-dimethyl-2,5-hexanediol, 1,9-nonanediol, 2-methyl-1,8-octanediol, 2-butyl-2-ethyl-1,3-propanediol, 2-butyl-2-hexyl-1,3-propanediol, cyclohexanediol, bishydroxymethylcyclohexane, dimethanolbenzene, bishydroxyethoxybenzene, alkyldialkanolamine, lactonediol, etc.)

Examples of polycarbonate polyols include polycarbonate diols obtained by dealcoholization reaction of polyhydric alcohols with dimethyl carbonate, diethyl carbonate, diphenyl carbonate, ethylene carbonate, etc., such as poly(1,6-hexylene) carbonate and poly(3-methyl-1,5-pentylene) carbonate.

Among the above-mentioned hydroxyl group-containing compounds used to obtain polyurethane resins, polyester polyols are preferred.

Examples of polyisocyanate compounds used to obtain polyurethane resins include aromatic diisocyanates such as tolylene diisocyanate, xylylene diisocyanate, methylene diphenyl diisocyanate, phenylene diisocyanate, naphthalene diisocyanate, and tolidine diisocyanate; aliphatic diisocyanates having an aromatic ring such as α,α,α',α'-tetramethyl xylylene diisocyanate; aliphatic diisocyanates such as methylene diisocyanate, propylene diisocyanate, lysine diisocyanate, trimethylhexamethylene diisocyanate, and hexamethylene diisocyanate; and alicyclic diisocyanates such as cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, dicyclohexylmethane diisocyanate, and isopropylidenedicyclohexyl diisocyanate. These may be used alone or in combination.

A chain extender may be used when synthesizing the polyurethane resin. There are no particular limitations on the chain extender as long as it has two or more active groups that react with isocyanate groups, and generally, chain extenders with two hydroxyl groups or two amino groups can be used.

Examples of chain extenders having two hydroxyl groups include glycols such as aliphatic glycols, such as ethylene glycol, propylene glycol, and butanediol; aromatic glycols, such as xylylene glycol and bishydroxyethoxybenzene; and ester glycols, such as neopentyl glycol hydroxypivalate.

Examples of chain extenders having two amino groups include aromatic diamines such as tolylenediamine, xylylenediamine, and diphenylmethanediamine; aliphatic diamines such as ethylenediamine, propylenediamine, hexanediamine, 2,2-dimethyl-1,3-propanediamine, 2-methyl-1,5-pentanediamine, trimethylhexanediamine, 2-butyl-2-ethyl-1,5-pentanediamine, 1,8-octanediamine, 1,9-nonanediamine, and 1,10-decanediamine; and alicyclic diamines such as 1-amino-3-aminomethyl-3,5,5-trimethylcyclohexane, dicyclohexylmethanediamine, 1,4-diaminocyclohexane, and 1,3-bisaminomethylcyclohexane.

In order to impart water dispersibility or water solubility, a method is also preferably used in which carboxyl groups are introduced into the urethane skeleton using dimethylolpropionic acid, dimethylolbutanoic acid, etc., and then the urethane resin is made hydrophilic by neutralizing it with a basic compound.

(Polyester resin)
The polyester resin may be composed mainly of polyvalent carboxylic acids and polyvalent hydroxy compounds such as those shown below.
That is, examples of polyvalent carboxylic acids that can be used include terephthalic acid, isophthalic acid, orthophthalic acid, phthalic acid, 4,4'-diphenyldicarboxylic acid, 2,5-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 2-potassium sulfoterephthalic acid, 5-sodium sulfoisophthalic acid, adipic acid, azelaic acid, sebacic acid, dodecanedicarboxylic acid, glutaric acid, succinic acid, trimellitic acid, trimesic acid, pyromellitic acid, trimellitic anhydride, phthalic anhydride, p-hydroxybenzoic acid, trimellitic acid monopotassium salt, and ester-forming derivatives thereof.
Examples of the polyhydric hydroxy compound include ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 2-methyl-1,5-pentanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, p-xylylene glycol, bisphenol A-ethylene glycol adduct, diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, polytetramethylene oxide glycol, dimethylolpropionic acid, glycerin, trimethylolpropane, sodium dimethylolethylsulfonate, potassium dimethylolpropionate, etc. One or more of these polyhydric carboxylic acids and polyhydric hydroxy compounds may be appropriately selected, and a polyester resin may be synthesized by a conventional polycondensation reaction.

In order to impart water dispersibility or water solubility, a method is also preferably used in which sulfoisophthalic acid is used as a copolymerization component as part of the above-mentioned polycarboxylic acid to introduce a sulfonic acid group into the polyester skeleton, and then neutralized with a basic compound to hydrophilize the polyester resin.
The amount of the copolymerized polycarboxylic acid is usually 1 to 10 mol %, preferably 2 to 8 mol %, based on the total amount of the polycarboxylic acid. By introducing an appropriate amount of sulfonic acid groups, the water dispersion stability can be further improved.

(Acrylic resin)
The acrylic resin is a polymer made of polymerizable monomers including acrylic and methacrylic monomers. These may be homopolymers or copolymers, or copolymers with polymerizable monomers other than acrylic and methacrylic monomers.
Also included are copolymers of these polymers with other polymers (such as polyesters and polyurethanes), such as block copolymers and graft copolymers. That is, the acrylic resin may be an acrylic-modified polyester resin or an acrylic-modified polyurethane resin.
In addition, the term also includes a polymer (or a mixture of polymers in some cases) obtained by polymerizing a polymerizable monomer having a carbon-carbon double bond in a polyester solution or polyester dispersion. Similarly, the term also includes a polymer (or a mixture of polymers in some cases) obtained by polymerizing a polymerizable monomer having a carbon-carbon double bond in a polyurethane solution or polyurethane dispersion. Similarly, the term also includes a polymer (or a mixture of polymers in some cases) obtained by polymerizing a polymerizable monomer having a carbon-carbon double bond in another polymer solution or dispersion.
In order to further improve adhesion to polyester films, the resin may contain a hydroxyl group or an amino group.

The polymerizable monomer having a carbon-carbon double bond is not particularly limited, but particularly representative compounds include, for example, various carboxyl group-containing monomers such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid, maleic acid, and citraconic acid, and their salts; various hydroxyl group-containing monomers such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, monobutylhydroxyfumarate, and monobutylhydroxyitaconate; methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, lauryl (meth)acrylate, These include various alkyl (meth)acrylic acid esters such as acrylate; various nitrogen-containing compounds such as (meth)acrylamide, diacetone acrylamide, or (meth)acrylonitrile; nitrogen-containing compounds containing hydroxyl groups such as N-methylol (meth)acrylamide; various styrene derivatives such as styrene, α-methylstyrene, divinylbenzene, and vinyltoluene; various vinyl esters such as vinyl propionate; various silicon-containing polymerizable monomers such as γ-methacryloxypropyltrimethoxysilane and vinyltrimethoxysilane; phosphorus-containing vinyl monomers; various vinyl halides such as vinyl chloride and vinylidene chloride; and various conjugated dienes such as butadiene.

Among the above, polymers obtained by polymerizing polymerizable monomers including acrylic and methacrylic monomers are preferred, and it is more preferred that the polymerizable monomers include alkyl (meth)acrylic esters.
In addition, when the coating liquid is an aqueous system, from the viewpoint of making it easy to dissolve or disperse the binder resin, it is preferable that the polymerizable monomer has a hydrophilic group such as a hydroxyl group or a carboxyl group. Therefore, the acrylic resin is also preferably a polymer obtained by polymerizing a polymerizable monomer including an alkyl (meth)acrylic acid ester and a hydrophilic group-containing monomer such as a hydroxyl group-containing monomer or a carboxyl group-containing monomer.
The acrylic resin may also be an emulsion polymer obtained by polymerizing a polymerizable monomer in the presence of a surfactant.

As the binder resin, from the viewpoints of adhesion between the coating layer and the polyester film and between the coating layer and the functional layer, and the surface hardness and surface elasticity of the coating layer, it is preferable to use one or more resins selected from polyester resin (B1) and acrylic resin (B2), and it is more preferable to use polyester resin (B1) and acrylic resin (B2).
When a polyester resin (B1) and an acrylic resin (B2) are used as the binder resin (B), the content ratio of the polyester resin (B1) to the acrylic resin (B2), (B1)/(B2), is preferably 20/80 to 95/5, more preferably 30/70 to 70/30, and even more preferably 40/60 to 60/40.

When a coating layer is formed using a coating liquid for forming the coating layer, the content of the binder resin (B) in the total non-volatile components in the coating liquid is usually in the range of 1 to 99.9 mass%, preferably 10 to 90 mass%, more preferably 30 to 80 mass%, and even more preferably 45 to 75 mass%.
When the content of the binder resin (B) is within the above range, the surface hardness and surface elasticity of the coating layer fall within the desired range, and the adhesion between the coating layer and the functional layer becomes good.

[Acid catalyst (C)]
In the present invention, one method for making the surface hardness of the coating layer 640 MPa or less is to add an acid catalyst (C) to the coating liquid for forming the coating layer.

The acid catalyst is not particularly limited, and examples thereof include inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid; oxalic acid, acetic acid, formic acid, phosphoric acid, methanesulfonic acid, trifluoromethanesulfonic acid, isoprene sulfonic acid, camphorsulfonic acid, hexanesulfonic acid, octane sulfonic acid, nonanesulfonic acid, decane sulfonic acid, hexadecanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, cumenesulfonic acid, dodecylbenzenesulfonic acid, naphthalenesulfonic acid, dinonylnaphthalene disulfonic acid, dinonylnaphthalene (mono)sulfonic acid, methyl acid phosphate, ethyl acid phosphate, propyl acid phosphate, isopropyl acid phosphate, butyl acid phosphate, butoxyethyl acid phosphate, octyl acid phosphate, 2-ethylhexyl acid phosphate, decyl acid phosphate, lauryl acid phosphate, Examples of suitable organic acids include stearyl acid phosphate, oleyl acid phosphate, behenyl acid phosphate, phenyl acid phosphate, nonylphenyl acid phosphate, cyclohexyl acid phosphate, phenoxyethyl acid phosphate, alkoxypolyethylene glycol acid phosphate, bisphenol A acid phosphate, dimethyl acid phosphate, diethyl acid phosphate, dipropyl acid phosphate, diisopropyl acid phosphate, dibutyl acid phosphate, dioctyl acid phosphate, di-2-ethylhexyl acid phosphate, dilauryl acid phosphate, distearyl acid phosphate, diphenyl acid phosphate, and bisnonylphenyl acid phosphate; sulfonium salts, benzothiazolium salts, ammonium salts, and phosphonium salts. These may be used alone or in combination of two or more.

From the viewpoint of compatibility with the crosslinking agent, it is preferable to use organic acids as the acid catalyst. Among them, from the viewpoint of catalytic activity and cost, an acid catalyst having a sulfonic acid group or a phosphoric acid group is more preferable, and specifically, one or more selected from p-toluenesulfonic acid, dinonylnaphthalene disulfonic acid, dinonylnaphthalene (mono)sulfonic acid, and dodecylbenzenesulfonic acid are more preferable. These acid catalysts may be hydrates or neutralized products of the hydrates.

When the coating layer is formed from a coating liquid, the content of the acid catalyst in the total non-volatile components in the coating liquid is usually in the range of 0.01 to 10 mass%, preferably 0.1 to 5 mass%, more preferably 0.3 to 3 mass%, and even more preferably 0.5 to 1.5 mass%.
When the content of the acid catalyst is within the above range, the compatibility with the crosslinking agent containing a melamine compound is good, and the catalytic activity is sufficient.
In addition, within this range, the molecular weight between crosslinking points becomes appropriate by the reaction with the crosslinking agent containing a melamine compound, and the surface hardness of the coating layer can be made to a specific value or less.

[Other components (D)]
In the coating solution for forming the coating layer according to the present invention, particles can be used in combination for the purpose of improving blocking resistance, slippage, etc., within the scope of the present invention.
The content of particles in the total non-volatile components in the coating liquid is preferably 0.1 to 20% by mass, more preferably 0.5 to 10% by mass, and further preferably 1 to 7% by mass.
In addition to the above components, additives such as reaction regulators, adhesion enhancers, surfactants, and antistatic agents may be appropriately added.

[solvent]
The coating solution for forming the coating layer is used as a liquid coating solution by incorporating a solvent therein, and the coating solution is applied to the polyester film, and then dried and cured as necessary to form the coating layer.
The coating liquid for forming the coating layer may be prepared by dissolving or dispersing a resin composition containing a crosslinking agent containing a melamine compound, a binder resin, an acid catalyst, etc. in a solvent to form a solution or dispersion. The coating liquid for forming the coating layer may be prepared by dissolving or dispersing a binder resin in a solvent in advance, and mixing this binder resin solution or dispersion with a resin composition containing a crosslinking agent containing a melamine compound, a binder resin, an acid catalyst, etc. to form a solution or dispersion.
The solvent is not particularly limited, and either water or an organic solvent may be used, but from the viewpoint of environmental protection, it is preferable to use an aqueous coating liquid containing water as the main solvent (50% by mass or more). The water content is preferably 60% by mass or more, more preferably 70% by mass or more. The aqueous coating liquid may contain a small amount of organic solvent. The specific amount of organic solvent is preferably less than water on a mass basis, and is, for example, less than 30% by mass, preferably less than 20% by mass, more preferably less than 10% by mass in the solvent.
Examples of organic solvents used in combination with water include alcohols such as ethanol, isopropanol, ethylene glycol, and glycerin, ethers such as ethyl cellosolve, t-butyl cellosolve, propylene glycol monomethyl ether, and tetrahydrofuran, ketones such as acetone and methyl ethyl ketone, esters such as ethyl acetate, and amines such as dimethylethanolamine. These can be used alone or in combination. By appropriately selecting and adding these organic solvents to the aqueous coating liquid as necessary, the stability and coatability of the coating liquid can be improved in some cases.

When an organic solvent is used alone as the above-mentioned solvent, examples of the organic solvent include aromatic hydrocarbons such as toluene, aliphatic hydrocarbons such as hexane, heptane, isooctane, esters such as ethyl acetate, butyl acetate, ketones such as ethyl methyl ketone (MEK), isobutyl methyl ketone, alcohols such as ethanol, 2-propanol, ethers such as diisopropyl ether, dibutyl ether, etc. These may be used alone or in combination, taking into consideration the solubility, coatability, boiling point, etc.

[Method of forming coating layer]
The method for forming a coating layer according to the present invention will be described below.
The method for forming the coating layer is not particularly limited, and any conventionally known coating method such as reverse gravure coating, direct gravure coating, roll coating, die coating, bar coating, curtain coating, etc. can be used.
Alternatively, the coating layer may be formed by in-line coating or off-line coating.
The drying and curing conditions of the coating layer are not particularly limited. For example, when a coating layer is provided on a polyester film by off-line coating, the heat treatment is usually performed at 80 to 200° C. for 3 to 40 seconds, preferably at 100 to 180° C. for 3 to 40 seconds.
On the other hand, when a coating layer is formed on a polyester film by in-line coating, it is usually preferable to carry out heat treatment at 70 to 280°C for 3 to 200 seconds as a guideline. From the viewpoints of suppressing precipitation of oligomers from the polyester film and promoting the progress of the curing reaction of the crosslinking agent, it is more preferable to carry out heat treatment at 120 to 250°C, even more preferably 150 to 220°C, and particularly preferably 170 to 200°C.

In the present invention, it is preferable to form a coating layer on a polyester film by in-line coating during the film-forming process of the polyester film.
In-line coating is a method of coating a coating solution within the polyester film production process; specifically, it is a method of coating a coating solution at any stage from melt extrusion of polyester to stretching, heat setting, and winding up.
Usually, the coating is carried out on any of the following: an unstretched sheet obtained by melting and quenching, a uniaxially stretched film that has been stretched, a biaxially stretched film before heat setting, or a film after heat setting and before being wound up.
Although not limited to the following, for example, in the case of sequential biaxial stretching, a method in which a uniaxially stretched polyester film stretched in the longitudinal direction (machine direction) is coated with a coating liquid and then stretched in the width direction (transverse direction) to obtain a biaxially stretched polyester film is particularly excellent. According to such a method, film formation and coating layer formation can be carried out simultaneously, which is advantageous in terms of production costs, and since stretching is carried out after coating with the coating liquid, the thickness of the coating layer can be changed by the stretching ratio, and thin film coating can be carried out more easily than in the case of off-line coating films.
Furthermore, by providing a coating layer on the uniaxially stretched polyester film before the biaxial stretching, the coating layer can be stretched together with the polyester film, thereby allowing the coating layer to adhere firmly to the polyester film.
Furthermore, in the production of biaxially stretched polyester film, the film can be stretched while holding the ends of the film with clips or the like, thereby restraining the film in the longitudinal and transverse directions, and in the heat setting process, the film can be heat-treated without wrinkles or the like and while maintaining its flatness.
Therefore, the crosslinking reaction of the coating layer progresses sufficiently, improving film-forming properties, and enabling the coating layer to adhere more firmly to the polyester film.
Furthermore, the surface hardness of the coating layer can be set to a specific value or less, and performance such as adhesion to various functional layers that may be formed on the coating layer and resistance to moist heat can be improved.

When forming a coating layer by in-line coating, it is preferable to manufacture a laminated polyester film by coating a solution or dispersion (coating liquid) containing the above-mentioned resin composition, the solids concentration of which is adjusted to approximately 0.1 to 50% by mass, onto a polyester film.

Regardless of whether off-line coating or in-line coating is used, heat treatment and irradiation with active energy rays such as ultraviolet light may be used in combination as necessary. The polyester film constituting the laminated polyester film of the present invention may be previously subjected to a surface treatment such as a corona treatment or a plasma treatment.

The thickness of the coating layer formed on the polyester film is preferably 0.002 μm or more and 1.0 μm or less, more preferably 0.005 μm or more and 0.25 μm or less, and further preferably 0.02 μm or more and 0.10 μm or less.
When the thickness of the coating layer is within the above range, the adhesion between the coating layer and the polyester film and between the coating layer and the functional layer is good.

<Physical properties of laminated polyester film>
In the present invention, the surface hardness (MPa) of the coating layer of the laminated polyester film is 640 MPa or less. By having the surface hardness of 640 MPa or less, not only can the precipitation of oligomers be suppressed, but also when a functional layer is provided on the coating layer, the wettability of the coating layer in contact with the functional layer and the conformity with the functional layer are improved, thereby improving the adhesion between the coating layer and the functional layer.
From the above viewpoints, the surface hardness (MPa) of the coating layer of the laminated polyester film is preferably 620 MPa or less, more preferably 600 MPa or less, and even more preferably 550 MPa or less. The lower limit of the surface hardness (MPa) of the coating layer is not particularly limited, but from the viewpoint of preventing blocking of the film during processing, it may be, for example, 50 MPa or more, 100 MPa or more, 200 MPa or more, 300 MPa or more, or 350 MPa or more.
Methods for adjusting the surface hardness of the coating layer to 640 MPa or less include a method of adjusting the composition of the coating layer by adding an acid catalyst to the coating liquid that forms the coating layer, and a method of adjusting the heat treatment temperature during production.
The surface hardness of the coating layer can be determined by measuring the surface hardness using the method described in the Examples.

In the present invention, the surface elastic modulus (GPa) of the coating layer is preferably from 4.0 GPa to 9.0 GPa, more preferably from 5.0 GPa to 8.0 GPa, and even more preferably from 5.0 GPa to 7.0 GPa.
If the thickness falls within the above range, defects due to scratches or scraping of the coating layer during the manufacturing and processing processes can be suppressed, and adhesion to the functional layer can be improved.
The surface elastic modulus of the coating layer can be determined specifically by measuring using the method described in the Examples.

The amount (mg/m ² ) of oligomer (ester cyclic trimer) precipitated on the coating layer surface of the laminated polyester film is preferably 8.0×10 ⁻⁵ mg/m ² or less, more preferably 7.5×10 ⁻⁵ mg/m ² or less, and even more preferably 7.0×10 ⁻⁵ mg/m ² or less.
The lower limit is not particularly limited, but the lower the better, and is 0 mg/m2 ^or more.
The amount of oligomer (cyclic ester trimer) precipitated on the surface of the coating layer can be determined by measurement using the method described in the Examples.

The laminated polyester film of the present invention has a micro heat of fusion peak temperature of preferably 160°C or more and 210°C or less, more preferably 170°C or more and 200°C or less, and even more preferably 175°C or more and 190°C or less.
Here, the micro-fusion heat peak temperature is correlated with the heat treatment temperature during production; the higher the heat treatment temperature, the higher the micro-fusion heat peak temperature; and the lower the heat treatment temperature, the lower the micro-fusion heat peak temperature.
Within the above range, it is possible to suppress the precipitation of oligomers and to achieve good adhesion at the same time.
The minute heat of fusion peak temperature can be specifically determined by measurement using the method described in the Examples.

Analysis of the components in the coating layer can be performed, for example, by TOF-SIMS, ESCA, or X-ray fluorescence.

The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples without departing from the gist of the present invention.
The measurement and evaluation methods used in the present invention are as follows.

<Measurement and evaluation methods>
(1) Intrinsic Viscosity of Polyester 1 g of polyester from which components incompatible with the polyester had been removed was precisely weighed, dissolved in 100 mL of a mixed solvent of phenol/tetrachloroethane = 50/50 (mass ratio), and measured at 30°C.

(2) Average particle size of particles The coating layer was observed using a transmission electron microscope (TEM) (Hitachi High-Tech H-7650, accelerating voltage 100 kV), and the average particle size of 10 particles was recorded as the average particle size.

(3) Film Thickness of Coating Layer The surface of the coating layer was stained with _RuO4 and embedded in epoxy resin. Then, sections prepared by ultrathin sectioning were stained with _RuO4 , and the cross section of the coating layer was measured using a transmission electron microscope (TEM) (Hitachi High-Technologies Corporation, H-7650, accelerating voltage 100 kV).

(4) Surface hardness and surface elasticity of the coating layer A nanoindenter "Triboindenter TI980" manufactured by Bruker was used for the measurement. A drop of "Aron Alpha" (registered trademark) Professional impact resistance manufactured by Toagosei Co., Ltd. was applied to a glass slide, and the polyester film side, which is the base material of the laminated polyester film, was fixed to the glass slide side via an instant adhesive. In order to fix the glass slide and the mounting stage, correction fluid was applied to the back of the glass slide, and the glass slide was placed on the device stage and fixed. The surface hardness (MPa) and surface elasticity (GPa) were measured with the coating layer side as the measurement surface.
Measurement mode: Load control Maximum load: 10 μN
Holding time when the maximum load is reached: 2 seconds Loading speed, unloading speed: 2 μN/sec Measurement temperature: 23° C.
Nanoindenter indenter: Berkovich indenter with a triangular pyramid shape and a tip angle of 142° (manufactured by Bruker, model number: TI-0039)

(5) Formation of hard coat layer On the coating layer surface of the laminated polyester film obtained in each example, a mixed coating solution of 80 parts by mass of KAYARAD DPHA (manufactured by Nippon Kayaku Co., Ltd.), 20 parts by mass of KAYARAD R-128H (manufactured by Nippon Kayaku Co., Ltd.), 5 parts by mass of a photopolymerization initiator (product name: Irgacure 651, manufactured by BASF Corporation), and 230 parts by mass of toluene was applied to a dry film thickness of 5 μm, and the mixture was dried at 80° C. for 1 minute to remove the solvent, and then irradiated with 250 mJ/cm ² of ultraviolet light to harden the coating layer, forming a hard coat layer on the coating layer.

(6) Adhesion to hard coat layer Cross cuts were made in the laminated polyester film with a hard coat layer obtained in (5) above so as to form 100 grids per inch, and an 18 mm wide tape (Cellotape (registered trademark) CT-18, manufactured by Nichiban Co., Ltd.) was applied on the cross cuts. After the tape was rapidly peeled off at a peel angle of 180°, the peeled surface was observed and evaluated based on the peeled area. The criteria for evaluation were as follows. If the peeled area was less than 50%, there was no problem in practical use.
◎ (Excellent): Peeled area less than 10% 〇 (Good): Peeled area 10% to less than 50% × (Poor): Peeled area 50% or more

(7) Amount of Oligomer (Ester Cyclic Trimer) Precipitated on the Coating Layer Surface The laminated polyester film obtained in each example was cut into a box shape having an open top, length and width of 10 cm, and a height of 3 cm, with the measurement surface (coating layer side) facing the inner surface.
Next, 4 mL of DMF (dimethylformamide) was placed in the box prepared by the above method and allowed to stand for 3 minutes, after which the DMF was recovered and fed to a liquid chromatograph (Shimadzu Corporation: LC-7A, mobile phase A: acetonitrile, mobile phase B: 2% aqueous acetic acid solution, column: Mitsubishi Chemical Corporation's "MCI GEL ODS 1HU", column temperature: 40°C, flow rate: 1 mL/min, detection wavelength: 254 nm) to determine the amount of ester cyclic trimer in the DMF, and this value was divided by the area of the film contacted with the DMF to determine the amount of oligomer (ester cyclic trimer) (mg/m ² ) on the coating layer surface of the laminated polyester film. The amount of ester cyclic trimer in the DMF was determined from the peak area ratio between the standard sample peak area and the measured sample peak area (absolute calibration curve method).
The standard sample was prepared by accurately weighing a previously separated ester cyclic trimer and dissolving it in an accurately weighed amount of DMF.

(8) Micro-melting heat peak temperature A measurement sample (5 mg) was prepared using the laminated polyester film obtained in each example, and the micro-melting heat peak was measured using a differential scanning calorimeter (PerkinElmer, model DSC8500) according to JIS K7121 (1999) by the following method.
The sample was measured at a temperature range of 25° C. to 300° C. at a heating rate of 20° C./min. The apex of a small peak observed at this time, which is lower than the apex temperature of the endothermic peak attributed to the melting of the sample and exists near the endothermic peak, or the apex of a small shoulder observed in the endothermic peak, was determined as the small heat of fusion peak (° C.).

<Materials used>
The method for producing the polyester used in the present invention is described below.

[Production method of polyester (1)]
100 parts by mass of dimethyl terephthalate and 55 parts by mass of ethylene glycol were used as starting materials, and 0.04 parts by mass of magnesium acetate tetrahydrate was added as a catalyst to a reactor. The reaction was started at 150°C, and the reaction temperature was gradually increased with the distillation of methanol, reaching 230°C after 3 hours. After 4 hours, the transesterification reaction was essentially completed. 0.02 parts by mass of ethyl acid phosphate was added to this reaction mixture, and then 0.04 parts by mass of antimony trioxide was added, and the polycondensation reaction was carried out for 4 hours. That is, the temperature was gradually increased from 230°C to 280°C.
Meanwhile, the pressure was gradually reduced from normal pressure to 0.3 mmHg. After the start of the reaction, the reaction was stopped at a point corresponding to an intrinsic viscosity of 0.65 dL/g due to a change in the stirring power of the reaction vessel, and the polymer was discharged under nitrogen pressure to obtain polyester (1) having an intrinsic viscosity of 0.65 dL/g.

[Production method of polyester (2)]
100 parts by mass of dimethyl terephthalate and 45 parts by mass of ethylene glycol were used as starting materials, and 0.06 parts by mass of magnesium acetate tetrahydrate was taken in a reactor as a catalyst. The reaction was started at 150°C, and the reaction temperature was gradually increased with the distillation of methanol, and reached 230°C after 3 hours. After 4 hours, the transesterification reaction was essentially completed. After adding 0.03 parts by mass of ethyl acid phosphate to this reaction mixture, 0.3 parts by mass of silica particles having an average particle size of 2.7 μm dispersed in ethylene glycol and 0.03 parts by mass of antimony trioxide were added, and a polycondensation reaction was carried out for 4 hours. That is, the temperature was gradually increased from 230°C to 280°C. Meanwhile, the pressure was gradually reduced from normal pressure, and finally reached 0.3 mmHg. After the start of the reaction, the reaction was stopped at a point corresponding to an intrinsic viscosity of 0.65 dL/g due to a change in the stirring power of the reaction tank, and the polymer was discharged under nitrogen pressure to obtain a polyester (2) having an intrinsic viscosity of 0.65 dL/g.

The following coating liquid was used to form the coating layer.
[(A) Crosslinking agent]
(A1): Melamine compound Hexamethoxymethylolmelamine (A2): Epoxy compound Water-soluble polyglycerol polyglycidyl ether (A3): Oxazoline compound Acrylic polymer having an oxazoline group and a polyalkylene oxide chain EPOCROS (registered trademark) (oxazoline group amount = 4.5 mmol/g, manufactured by Nippon Shokubai Co., Ltd.)

[(B) Binder resin]
(B1): Aqueous dispersion of polyester resin copolymerized with the following composition (dicarboxylic acid component) 2,6-naphthalenedicarboxylic acid/5-sodium sulfoisophthalic acid=92/8 (mol%)
(Diol component) Ethylene glycol/diethylene glycol=80/20 (mol%)
(B2): Aqueous dispersion of acrylic resin polymerized with the following composition: Emulsion polymer of ethyl acrylate/n-butyl acrylate/methyl methacrylate/N-methylolacrylamide/acrylic acid = 65/21/10/2/2 (mass%) (emulsifier: anionic surfactant)

[(C) Acid catalyst]
(C1): p-Toluenesulfonic acid monohydrate (p-TSA) (manufactured by Nacalai Tesque, Inc.)
(C2): p-toluenesulfonic acid ammonia neutralized product (p-TSA neutralized product)
(C3): Dinonylnaphthalene (mono)sulfonic acid (DNNSA)

[(D) Other Components]
(D1): Particles Silica sol with an average particle size of 0.07 μm

Example 1
A blend of polyester (1) and polyester (2) in a mass ratio of 82:18 was used as the raw material for layer A, and polyester (1) alone was used as the raw material for layer B. These were respectively supplied to an extruder and heated and melted at 285°C. Layer A was divided into two to form an outermost layer (surface layer) and layer B as the middle layer, resulting in a layer structure of two types of three layers (A/B/A) with a thickness composition ratio of A/B/A = 5/90/5 under extrusion conditions. The layer was then cooled and solidified while being in close contact with a mirror-finished cooling drum with a surface temperature of 40 to 50°C, to produce an unstretched polyethylene terephthalate film.
This film was stretched 3.7 times in the longitudinal direction while passing through a group of heated rolls at 85° C., to obtain a uniaxially stretched polyester film.
Coating solution 1 shown in Table 1 below was applied to one side of this uniaxially stretched polyester film, and then this film was introduced into a tenter stretching machine, stretched 4.3 times in the width direction at 100°C, and further subjected to heat treatment at 200°C, and then subjected to a 2% relaxation treatment in the width direction to obtain a biaxially stretched polyester film (laminated polyester film) having a thickness of 50 μm and a coating layer having a thickness (after drying) of 0.06 μm.

(Examples 2 to 8)
A laminated polyester film was obtained in the same manner as in Example 1, except that the coating layer had a composition shown in Table 1.

(Comparative Example 1)
A laminated polyester film was obtained in the same manner as in Example 1, except that the coating layer had a composition shown in Table 1.

The evaluation results of the laminated polyester films obtained in Examples 1 to 8 and Comparative Example 1 are shown in Table 2 below.

As shown in Table 2, the laminated polyester film of the present invention has a surface hardness of 640 MPa or less of the coating layer formed from a coating solution containing a melamine compound, thereby suppressing the precipitation of oligomers and providing excellent adhesion to the functional layer.

Claims (14)

A coating layer is provided on at least one surface of the polyester film,
The coating layer is formed from a coating liquid containing a crosslinking agent (A) containing a melamine compound (A1) , an acid catalyst (C), and particles ,
the surface hardness of the coating layer is 640 MPa or less, and the surface elasticity of the coating layer is 4.0 GPa or more and 9.0 GPa or less;
The content of particles in the total non-volatile components in the coating solution is 0.1 to 20% by mass . A coating layer is provided on at least one surface of the polyester film,
The coating layer is formed from a coating liquid containing a crosslinking agent (A) containing a melamine compound (A1) , an epoxy compound (A2) and an oxazoline compound (A3) , and an acid catalyst (C) ;
The surface hardness of the coating layer is 640 MPa or less,
the epoxy compound (A2) is present in an amount of 10 to 300 parts by mass relative to 100 parts by mass of the melamine compound (A1);
The oxazoline compound (A3) is contained in an amount of 30 to 200 parts by mass per 100 parts by mass of the melamine compound (A1) . A coating layer is provided on at least one surface of the polyester film,
The coating layer is formed from a coating liquid containing a crosslinking agent (A) containing a melamine compound (A1) , an acid catalyst (C), and a binder resin (B) ,
The surface hardness of the coating layer is 640 MPa or less,
The binder resin (B) contains a polyester resin (B1) and an acrylic resin (B2),
The polyester resin (B1) and the acrylic resin (B2) have a content ratio (B1)/(B2) of 40/60 to 60/40 . A coating layer is provided on at least one surface of the polyester film,
The coating layer is formed from a coating liquid containing a crosslinking agent (A) containing a melamine compound (A1) and an acid catalyst (C) ,
The surface hardness of the coating layer is 640 MPa or less,
A laminated polyester film having a micro heat of fusion peak temperature of 160°C or higher and 210°C or lower . The laminated polyester film according to any one of claims 2 to 4 , wherein the coating layer has a surface elastic modulus of 4.0 GPa or more and 9.0 GPa or less. The laminated polyester film according to any one of claims 1 to 5, wherein the melamine compound (A1) is at least one selected from the group consisting of hexamethoxymethylolmelamine, hexamethoxymethylmelamine, N,N',N"-trimethylolmelamine, and pentamethoxymethylmelamine. The laminated polyester film according to any one of claims 1 and 3 to 6 , wherein the crosslinking agent (A) further contains an epoxy compound (A2). The crosslinking agent (A) further contains an epoxy compound (A2) and an oxazoline compound (A3),
the epoxy compound (A2) is present in an amount of 10 to 300 parts by mass relative to 100 parts by mass of the melamine compound (A1);
The laminated polyester film according to any one of claims 1 to 7 , wherein the oxazoline compound (A3) is present in an amount of 30 to 200 parts by mass based on 100 parts by mass of the melamine compound (A1). The laminated polyester film according to any one of claims 1 to 8 , wherein the acid catalyst (C) has a sulfonic acid group or a phosphoric acid group. The laminated polyester film according to any one of claims 1 to 9, wherein the acid catalyst (C) is at least one selected from the group consisting of p-toluenesulfonic acid, dinonylnaphthalene disulfonic acid, dinonylnaphthalene (mono)sulfonic acid, and dodecylbenzenesulfonic acid. the coating liquid further contains a binder resin (B), the binder resin (B) contains a polyester resin (B1) and an acrylic resin (B2);
The laminated polyester film according to any one of claims 1, 2, and 4 to 10 , wherein a content ratio (B1)/(B2) of the polyester resin (B1) to the acrylic resin (B2) is from 40/60 to 60/40. The laminated polyester film according to any one of claims 1 to 3 and 5 to 11 , which has a micro fusion heat peak temperature of 160°C or higher and 210°C or lower. A laminated polyester film having a functional layer, comprising the laminated polyester film according to any one of claims 1 to 12 and a functional layer on the coating layer. The laminated polyester film with a functional layer according to claim 13 , wherein the functional layer is a hard coat layer.