JP2021030555A - Biaxially stretched polyester film and method for producing the same - Google Patents

Abstract

To provide a polyester film which is excellent in gas barrier properties and easiness of tearing and is suitable as a packaging material of medicines, medical products, food products and beverages.SOLUTION: There is provided a biaxially stretched polyester film in which three or more layers of thermoplastic resin layers (A layers) and thermoplastic resin layers (B layers) are alternately layered, in which the thermoplastic resin layers (A layers) contain at least 60-95 mass% of a polyester resin and 5-40 mass% of a liquid crystal polymer having a crystal melting temperature of 250°C or lower, and the thermoplastic resin layers (B layers) contain at least 80-100 mass% of a polyester resin and 0-20 mass% of a liquid crystal polymer having a crystal melting temperature of 250°C or lower.SELECTED DRAWING: None

Description

The present invention relates to a biaxially stretched polyester film used in the packaging field of foods, pharmaceuticals, industrial products, etc., and a method for producing the same. More specifically, the present invention relates to a biaxially stretched polyester film which is excellent in gas barrier property and easily tearable and can be suitably used as a packaging material for pharmaceuticals, medical products, foods, beverages and the like.

Packaging materials used in foods, pharmaceuticals, etc. must have the property of blocking gases such as oxygen and water vapor, that is, gas barrier properties, in order to suppress the oxidation of proteins and fats and oils, maintain the taste and freshness, and maintain the efficacy of pharmaceuticals. It has been demanded. Further, gas barrier materials used for electronic devices such as solar cells and organic EL, electronic parts, etc. require higher gas barrier properties than packaging materials such as foods.

Conventionally, in food applications that require blocking of various gases such as water vapor and oxygen, a metal thin film made of aluminum or the like and an inorganic oxide such as silicon oxide or aluminum oxide are used on the surface of a base film layer made of plastic. A gas barrier laminated film on which an inorganic thin film is formed, or a film coated with a layer having a barrier property such as PVDC is generally used.

However, since the processing process such as metal vapor deposition is complicated, the cost is high, and there is a problem that disposal is also restricted from the viewpoint of environmental problems.

On the other hand, the liquid crystal polymer has an excellent gas barrier property, and a laminate having the liquid crystal polymer as a gas barrier layer is known. For example, a composite film composed of a liquid crystal polymer layer and a thermoplastic resin layer laminated on at least one surface thereof has been proposed (Patent Document 1).

However, the liquid crystal polymer has low adhesiveness to other resins, and it is necessary to functionalize the surface or interpose an adhesive layer in order to form a laminate. Further, when processing into a packaging container or the like, the liquid crystal polymer layer is easily damaged, so that a certain thickness is required, which makes it difficult to reduce the wall thickness, and there is a problem that the processing process becomes complicated and the cost increases. ..
Patent Document 2 discloses a film composed of a polypropylene resin composition containing 1 to 100 parts by mass of a liquid crystal polymer having a crystal melting temperature of 250 ° C. or lower with respect to 100 parts by mass of the polypropylene resin. According to such a technique, a film having excellent gas barrier properties against water vapor and oxygen gas and excellent processability can be obtained. However, since polypropylene is inferior in heat resistance and mechanical strength, there is a problem that the applications that can be used are limited.

On the other hand, Patent Document 3 describes films, sheets, preforms, containers and other products having at least one total aromatic, amorphous, stretchable liquid crystal polymer layer and at least one non-liquid crystal thermoplastic polyester layer. A method for producing and stretching a multilayer laminate and a multilayer product to be included is disclosed. Further, in Patent Document 4, a film made of thermotropic liquid crystal polyester is reheated and melted in the range of the melting temperature to the temperature-decreasing crystallization temperature, and the stretching ratio in the longitudinal direction and the horizontal direction is stretched by 1.5 to 20 times. A method for producing a liquid crystal polymer film is disclosed. The simultaneous biaxial stretching method and the inflation method used here have a problem that the stretching ratio cannot be increased and the mechanical strength is inferior.

In Patent Document 5, polyethylene naphthalate (hereinafter abbreviated as PEN) having a weight of 99.9% by weight or less and exceeding 90.0% by weight and a condensation type liquid crystal polymer having a weight of 0.1% by weight or more and less than 10.0% by weight are used. A biaxially oriented polyester film for packaging, which is made of a blended polymer and has excellent weather resistance and gas blocking property, is disclosed. According to such a technique, the biaxially oriented heat-fixed polyethylene terephthalate (hereinafter abbreviated as PET) film generally has excellent mechanical properties, heat resistance, dimensional stability at high temperature, and gas blocking property. It is said that a highly transparent biaxially oriented polyester film for packaging with excellent film resistance can be obtained, but since PEN resin is expensive, there is a problem that raw material costs are high for use in packaging. ..

Japanese Unexamined Patent Publication No. 4-135750 JP-A-2019-43980 Japanese Unexamined Patent Publication No. 11-268191 Heisei 4-166322 Gazette Special Fair 6-2870

The present invention has been made against the background of the problems of the prior art.
That is, an object of the present invention is to provide a biaxially stretched polyester film which is excellent in gas barrier property and easy to tear property and can be suitably used as a packaging material for pharmaceuticals, medical products, foods, beverages and the like. is there.

As a result of diligent studies to solve such a problem, the present inventors have excellent gas barrier properties by biaxially stretching a polyester resin composition containing a liquid crystal polymer having a specific crystal melting temperature with respect to the polyester resin. It was found that a film having easy tearability can be obtained. Furthermore, the present inventor has made it possible to stably perform biaxial stretching by alternately arranging layers having a low liquid crystal polymer concentration and easy stretching even when the liquid crystal polymer is contained in a high concentration in the film. It was found that this was the case, and the present invention was completed.

That is, the present invention has the following configuration.
(1) A biaxially stretched polyester film in which three or more layers of a thermoplastic resin layer (A layer) and a thermoplastic resin layer (B layer) are alternately laminated, and the thermoplastic resin layer (A layer) is at least a polyester resin. 60 to 95% by mass and 5 to 40% by mass of a liquid crystal polymer having a crystal melting temperature of 250 ° C. or lower, the thermoplastic resin layer (B layer) contains at least 80 to 100% by mass of a polyester resin, and the crystal melting temperature is 250 ° C. A biaxially stretched polyester film containing 0 to 20% by mass of the following liquid crystal polymer.
(2) The biaxially stretched polyester film according to (1), wherein the oxygen permeability measured in an environment of 23 ° C. and a relative humidity of 65% is 100 cc / m ^{2 / day / atm or less.}
(3) The biaxially stretched polyester film according to (1) or (2), wherein the tear strength in the longitudinal direction is 7.5 mJ / μm or less.
(4) The biaxial according to any one of (1) to (3), wherein the thermoplastic resin layer (A layer) and / or the thermoplastic resin layer (B layer) contains a compatibilizer. Stretched polyester film.
(5) The method for producing a biaxially stretched polyester film according to any one of (1) to (4), wherein the thermoplastic resin (A layer) and the thermoplastic resin (B layer) are alternately formed into 60 or more layers. A method for producing a biaxially stretched polyester film, which comprises biaxially stretching an unstretched sheet cast after multi-layering.

According to the present invention, there is provided a biaxially stretched polyester film which can impart better gas barrier properties than a conventional PET film and has easy tearability without laminating an inorganic thin film layer or a PVDC coat layer. can do.

Hereinafter, the present invention will be described in detail.
The biaxially stretched polyester film according to the embodiment of the present invention contains a polyester resin and a liquid crystal polymer as main constituents.

(Polyester resin)
The polyester resin used in the present invention is a polyester resin containing an ethylene terephthalate component as a main component. The "main constituent" is 80 mol% or more of terephthalic acid in 100 mol% of all dicarboxylic acids constituting the polyester resin, and 80 mol% or more of ethylene glycol in 100 mol% of all glycol components.

Other dicarboxylic acid components and glycol components may be copolymerized as long as the object of the present invention is not impaired. The copolymerization amount of the other dicarboxylic acid component and the glycol component is less than 20 mol%, preferably 10 mol% or less, and 5 mol% or less, respectively, with respect to the total dicarboxylic acid component or the total glycol component. Is particularly preferred.
Examples of the above other dicarboxylic acid components include aromatic dicarboxylic acids such as isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, 4,4'-dicarboxybiphenyl, and 5-sodium sulfoisophthalic acid, and 1,4-cyclohexanedicarboxylic acid. Alicyclic dicarboxylic acids such as 1,3-cyclohexanedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid, 2,5-norbornenedicarboxylic acid, and tetrahydrophthalic acid, oxalic acid, malonic acid, succinic acid, adipic acid, and azelaic acid. , Sebacic acid, undecanedioic acid, dodecanedioic acid, octadecanedioic acid, fumaric acid, maleic acid, itaconic acid, mesaconic acid, citraconic acid, dimeric acid and other aliphatic dicarboxylic acids.

Other glycol components mentioned above include 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2-methyl-1, 3-Propanediol, 2-amino-2-ethyl-1,3-propanediol, 2-amino-2-methyl-1,3-propanediol, 1,10-decanediol, dimethyloltricyclodecane, diethylene glycol, Polyethylene glycols such as triethylene glycol, bisphenol A, bisphenol S, bisphenol C, bisphenol Z, bisphenol AP, ethylene oxide adducts or propylene oxide adducts of 4,4'-biphenol, 1,2-cyclohexanedimethanol, 1 , 3-Cyclohexanedimethanol, alicyclic glycols such as 1,4-cyclohexanedimethanol, polyethylene glycol, polypropylene glycol and the like.

The polymerization method of the polyester resin used in the present invention includes a direct polymerization method in which terephthalic acid and ethylene glycol, and if necessary, other dicarboxylic acid components and glycol components are directly reacted, and a dimethyl ester of terephthalic acid (if necessary). A production method such as a transesterification method in which a transesterification reaction is carried out after a transesterification reaction of ethylene glycol (containing another glycol component if necessary) with a methyl ester of another dicarboxylic acid can be used. ..

As the polyester resin used in the present invention, a recycled resin obtained by reusing a PET bottle or a polyester resin containing a monomer component derived from biomass can also be used.

The intrinsic viscosity of the polyester resin used in the present invention is preferably in the range of 0.57 to 1.10 dl / g, more preferably 0.60 to 0.9 dl / g, and further preferably 0.63 to 0. It is 7 dl / g. If the intrinsic viscosity is lower than 0.57 dl / g, the polyester film is easily torn in the manufacturing process (so-called breakage occurs). If it is higher than 1.10 dl / g, the increase in filter pressure becomes large, and it tends to be difficult to extrude the resin through the filter.

The intrinsic viscosity of the biaxially stretched polyester film according to the embodiment of the present invention is preferably in the range of 0.51 to 0.90 dl / g, more preferably 0.56 to 0.80 dl / g, and further preferably. Is 0.59 to 0.7 dl / g. When the intrinsic viscosity is lower than 0.51 dl / g, the polyester film is easily torn in the processing process such as printing, and when the intrinsic viscosity is higher than 0.9 dl / g, the effect of improving the mechanical properties becomes saturated. Tends to be.

(Liquid crystal polymer)
The liquid crystal polymer constituting the biaxially stretched polyester film according to the embodiment of the present invention is a liquid crystal polyester or a liquid crystal polyester amide forming an anisotropic molten phase called a thermotropic liquid crystal polymer.

The liquid crystal polymer used in the present invention has an anisotropic molten phase. The anisotropic molten phase can be confirmed by a normal polarization inspection method using an orthogonal polarizer (a method of placing a sample on a hot stage and observing the molten state in a nitrogen atmosphere).

Examples of the repeating unit constituting the liquid crystal polymer used in the present invention include an aromatic oxycarbonyl repeating unit, an aromatic dicarbonyl repeating unit, an aromatic dioxy repeating unit, an aromatic oxydicarbonyl repeating unit, and an aromatic aminooxy repeating unit. Examples thereof include aromatic diamino repeating units, aromatic aminocarbonyl repeating units and combinations thereof.

Specific examples of the monomer giving the aromatic oxycarbonyl repeating unit include, for example, 4-hydroxybenzoic acid, 3-hydroxybenzoic acid, 2-hydroxybenzoic acid, and 6-hydroxy-2-, which are aromatic hydroxycarboxylic acids. Naftoeic acid, 5-hydroxy-2-naphthoic acid, 3-hydroxy-2-naphthoic acid, 4'-hydroxyphenyl-4-benzoic acid, 3'-hydroxyphenyl-4-benzoic acid, 4'-hydroxyphenyl-3 -Includes benzoic acid and the like and their alkyl, alkoxy or halogen substituents, as well as ester-forming derivatives such as these acylated products, ester derivatives and acid halides. Of these, 4-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid are preferable because they have characteristics such as gas barrier properties of the obtained liquid crystal polymer and the crystal melting temperature can be easily adjusted.

Specific examples of the monomer giving an aromatic dicarbonyl repeating unit include, for example, terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 1,6-naphthalenedicarboxylic acid, and 2,7 which are aromatic dicarboxylic acids. -Naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 4,4'-dicarboxybiphenyl, etc., and their alkyl, alkoxy or halogen substituents, and ester-forming derivatives such as their ester derivatives, acid halides, etc. Can be mentioned. Among these, terephthalic acid and 2,6-naphthalenedicarboxylic acid are preferable because the mechanical properties, heat resistance, crystal melting temperature, and moldability of the obtained liquid crystal polymer can be easily adjusted to appropriate levels.

Specific examples of the monomer giving an aromatic dioxy repeating unit include, for example, aromatic diols hydroquinone, resorcin, 2,6-dihydroxynaphthalene, 2,7-dihydroxynaphthalene, 1,6-dihydroxynaphthalene, 1, 4-Dihydroxynaphthalene, 4,4'-dihydroxybiphenyl, 3,3'-dihydroxybiphenyl, 3,4'-dihydroxybiphenyl, 4,4'-dihydroxybiphenyl ether and the like, and their alkyl, alkoxy or halogen substituents, In addition, ester-forming derivatives such as these acylated products can be mentioned. Among these, hydroquinone and 4,4'-dihydroxybiphenyl are preferable from the viewpoints of properties such as reactivity at the time of polymerization and gas barrier property of the obtained liquid crystal polymer.

Specific examples of the monomer giving an aromatic oxydicarbonyl repeating unit include, for example, 3-hydroxy-2,7-naphthalenedicarboxylic acid, 4-hydroxyisophthalic acid, and 5-hydroxyisophthalic acid, which are aromatic hydroxydicarboxylic acids. Examples thereof include acids and their alkyl, alkoxy or halogen substituents, and ester-forming derivatives such as these acylated products, ester derivatives and acid halides.

Examples of the monomer giving the aromatic aminooxy repeating unit, the aromatic diamino repeating unit and the aromatic aminocarbonyl repeating unit include aromatic hydroxyamine, aromatic diamine and aromatic aminocarboxylic acid.

The liquid crystal polymer used in the present invention may be a blend of two or more kinds of liquid crystal polymers.

The crystal melting temperature measured by the differential scanning calorimeter of the liquid crystal polymer used in the present invention is 250 ° C. or lower, preferably 235 ° C. or lower, and more preferably 220 ° C. or lower.
The lower limit of the crystal melting temperature of the liquid crystal polymer is preferably 150 ° C. or higher, more preferably 170 ° C. or higher, still more preferably 180 ° C. or higher.

When the crystal melting temperature of the liquid crystal polymer is 250 ° C. or lower, the liquid crystal polymer is easily dispersed in the polyester resin as a matrix in a fibrous state, and the excellent gas barrier property and easy tearing property, which are the objects of the present invention, can be obtained. It has the effect of obtaining a film to have. When a liquid crystal polymer having a crystal melting temperature of more than 250 ° C. is used, it is necessary to perform melt-kneading with the polyester resin at a high temperature, the polyester resin deteriorates, and a film having physical strength cannot be obtained.

The "crystal melting temperature" is obtained from the crystal melting peak temperature when measured at a heating rate of 20 ° C./min by a differential scanning calorimeter (hereinafter abbreviated as DSC). More specifically, after observing the heat absorption peak temperature (Tm1) observed when the liquid crystal polymer sample is measured at a temperature rising condition of 20 ° C./min from room temperature, the temperature is 20 to 50 ° C. higher than Tm1. After holding for 1 minute and then cooling the sample to room temperature under the temperature lowering condition of 20 ° C./min, the heat absorption peak when measured again under the temperature rising condition of 20 ° C./min was observed, and the temperature indicating the peak top was set to the liquid crystal. The crystal melting temperature of the polymer.

As the liquid crystal polymer according to the present invention, an all-aromatic liquid crystal polyester in which the repeating unit is composed of an aromatic oxycarbonyl repeating unit, an aromatic dicarbonyl repeating unit and an aromatic dioxy repeating unit is particularly suitable.

When used with a semi-aromatic liquid crystal polyester containing an aliphatic group such as ethylene glycol in the repeating unit, the desired gas barrier property tends not to be obtained.

As the liquid crystal polymer having a crystal melting temperature of 250 ° C. or lower used in the present invention, a total aromatic liquid crystal polyester containing a repeating unit represented by the formulas (I) to (IV) is more suitable.

[In the formula, Ar ₁ and Ar ₂ represent one or more divalent aromatic groups, respectively, and p, q, r and s are the composition ratios of each repeating unit in the liquid crystal polymer, respectively. It is (mol%) and satisfies the following conditions. 0.5 ≦ p / q ≦ 2.5, 2 ≦ r ≦ 15, and 2 ≦ s ≦ 15]
The molar ratio (p / q) of the composition ratio p (mol%) according to the above formula (I) and the composition ratio q (mol%) according to the formula (II) is more preferably 0.6 to 1.8, and is 0. .8 to 1.6 is more preferable.
With respect to the above-mentioned preferably used total aromatic liquid crystal polyester, the total composition ratio of p and q is preferably 70 to 96 mol%, more preferably 76 to 90 mol%.

With respect to the above-mentioned preferably used total aromatic liquid crystal polyester, the composition ratio p according to the formula (I) and the composition ratio q according to the formula (II) are preferably 32 to 54 mol%, respectively, and 35 to 48 mol%. Is more preferable, and 38 to 45 mol% is further preferable.

In the total aromatic liquid crystal polyester preferably used in the present invention, the repeating unit represented by the formulas (I) and (II) is at least the above-mentioned molar ratio (p / q) and, in some cases, the above-mentioned p. The crystal melting temperature of 250 ° C. or lower is indicated by including the total composition ratio of q and / or the respective composition ratios of p and q (mol%).

Further, with respect to the total aromatic liquid crystal polyester preferably used in the present invention, the composition ratio r according to the formula (III) and the composition ratio s according to the formula (IV) are preferably 2 to 15 mol%, respectively, and 5 to 5 13 mol% is more preferable. r and s are preferably equimolar quantities.

In the above repeating unit, for example, when Ar ₁ (or Ar ₂ ) represents two or more divalent aromatic groups, the repeating unit represented by the formula (III) (or (IV)) is a total aromatic group. It means that two or more kinds are contained in the liquid crystal polyester depending on the kind of divalent aromatic group. In this case, the composition ratio r according to the formula (III) (or the composition ratio s according to the formula (IV)) represents the composition ratio obtained by summing up two or more repeating units.

Specific examples of the monomer giving the repeating unit represented by the formula (I) include 4-hydroxybenzoic acid and ester-forming derivatives such as acylates, ester derivatives and acid halides thereof.

Specific examples of the monomer giving the repeating unit represented by the formula (II) include 6-hydroxy-2-naphthoic acid and an ester-forming derivative such as an acylated product, an ester derivative, or an acid halide. Be done.

Specific examples of the monomer giving the repeating unit represented by the formula (III) include terephthalic acid and isophthalic acid, which are aromatic dicarboxylic acids, 2,6-naphthalenedicarboxylic acid, and 1,6-naphthalenedicarboxylic acid. , 2,7-Naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 4,4'-dicarboxybiphenyl and the like, and their alkyl, alkoxy or halogen substituents, and esters of these ester derivatives, acid halides and the like. Examples include forming derivatives.

Specific examples of the monomer giving the repeating unit represented by the formula (IV) include, for example, aromatic diols hydroquinone, resorcin, 2,6-dihydroxynaphthalene, 2,7-dihydroxynaphthalene, and 1,6-. Dihydroxynaphthalene, 1,4-dihydroxynaphthalene, 4,4'-dihydroxybiphenyl, 3,3'-dihydroxybiphenyl, 3,4'-dihydroxybiphenyl, 4,4'-dihydroxybiphenyl ether, etc., and their alkyl, alkoxy Alternatively, halogen-substituted products and ester-forming derivatives such as these acylated products can be mentioned.

The content of the liquid crystal polymer with respect to all the layers in the biaxially stretched polyester film of the present invention is preferably 10% by mass or more, more preferably 20% by mass or more. If it is less than 10% by mass, the effect of improving the gas barrier property is lowered, and the film characteristics are not sufficient.
The content of the liquid crystal polymer is preferably 40% by mass or less. If the content of the liquid crystal polymer exceeds 40%, the amount of oriented liquid crystal polymer becomes too large, and as a result, continuous film formation becomes difficult as a result of easy tearing during stretching.

(Compatible agent)
The biaxially stretched polyester film according to the embodiment of the present invention can further contain a compatibilizer.
The compatibilizer refers to one that is localized at the interface of each resin phase constituting the blended resin composition, has a function of reducing the interfacial tension between the phases, and improving the compatibility.

The compatibilizer used in the present invention is not particularly limited as long as it can reduce the interfacial tension between the polyester resin and the liquid crystal polymer, but particularly preferable ones are shown in, for example, Japanese Patent Application Laid-Open No. 2017-214460. The diglycidyl ether ester compound represented by the formula (V) is preferable.
The diglycidyl ether ester compound is produced as a mixture of the compound having n = 0 and the compound having n = 1 to 10 in the formula (V). The compound having n = 0 is preferably contained in an amount of 80% by mass or more, more preferably 90% by mass or more, in that a compatibilizing effect can be exhibited in a small amount.

The content of the compatibilizer is preferably 0.5 to 10% by mass, more preferably 1 to 5% by mass, based on the biaxially stretched polyester film.
If the amount of the compatibilizer added is less than 0.5% by mass, the effect of improving compatibility is difficult to obtain, and if it exceeds 10% by mass, the gas barrier property of the film and the thermal stability of the raw material resin during melt-kneading tend to decrease. There is.

The biaxially stretched polyester film according to the embodiment of the present invention may contain a polyester resin (other polyester resin) other than the above polyester resin for the purpose of adjusting mechanical properties and the like.
Examples of the other polyester resin include polyethylene naphthalate, polybutylene naphthalate, polypropylene terephthalate, and copolymer polybutylene terephthalate (for example, isophthalic acid, orthophthalic acid, naphthalenedicarboxylic acid, biphenyldicarboxylic acid, cyclohexanedicarboxylic acid, adipic acid, and azeline. Polybutylene terephthalate resin copolymerized with at least one dicarboxylic acid selected from the group consisting of acid and sebacic acid, or ethylene glycol, 1,3-propylene glycol, 1,2-propylene glycol, neopentyl glycol, 1, (PBT resin in which at least one diol component selected from the group consisting of 5-pentanediol, 1,6-hexanediol, diethylene glycol, cyclohexanediol, polyethylene glycol, polytetramethylene glycol and polycarbonate diol is copolymerized) and the like can be mentioned. Be done.

The upper limit of the amount of these other polyester resins added is preferably less than 30% by mass, more preferably 25% by mass or less. If the amount of other polyester resin added exceeds 30% by mass, the mechanical properties of the film will be impaired, impact strength, pinhole resistance, or bag breakage resistance will be insufficient, and transparency and gas barrier properties will deteriorate. Etc. may occur.

The biaxially stretched polyester film according to the embodiment of the present invention may contain other resins such as polyamide, polystyrene, polyolefin, and polyesters other than the above. However, in terms of mechanical properties and heat resistance of the biaxially stretched polyester film, the content of other resins is 30% by mass or less, further 20% by mass or less, and further 10% by mass with respect to the biaxially stretched polyester film. Below, it is particularly preferable that it is 5% by mass or less.

The biaxially stretched polyester film according to the embodiment of the present invention may contain conventionally known additives such as lubricant, stabilizer, colorant, antistatic agent, ultraviolet absorber and the like, if necessary. Good.
As the lubricant type, in addition to inorganic lubricants such as silica, calcium carbonate and alumina, organic lubricants are preferable, silica and calcium carbonate are more preferable, and silica is particularly preferable in that haze is reduced. These can be expressed as transparent and slippery.
The lower limit of the lubricant concentration is preferably 100 ppm, more preferably 500 ppm, and even more preferably 800 ppm. If it is less than the above, the slipperiness of the base film layer may decrease. The upper limit of the lubricant concentration is preferably 20000 ppm, more preferably 10000 ppm, still more preferably 1800 ppm. If it exceeds the above, transparency may decrease.

As a method for stably producing a biaxially stretched polyester film according to an embodiment of the present invention, a thermoplastic resin layer (A layer) containing a large amount of liquid crystal polymer and a ratio of 20% by mass or less containing no liquid crystal polymer are used. It is preferable to alternately stack three or more layers of the thermoplastic resin layer (B layer) containing the resin.
When a resin composition containing a large amount of liquid crystal polymer is formed into a single layer, the liquid crystal polymer is strongly oriented and easily torn during stretching, and as a result, stable film formation is difficult to obtain, whereas the content of liquid crystal polymer is low. By laminating layers that can be stretched relatively easily, a biaxially stretched polyester film containing a liquid crystal polymer having excellent gas barrier properties can be stably produced.
In the biaxially stretched polyester film of the present invention, the thermoplastic resin layer (A layer) containing a large amount of liquid crystal polymer and the thermoplastic resin layer (B layer) containing no liquid crystal polymer or containing 20% by mass or less are alternately arranged. As a configuration in which three or more layers are laminated, a three-layer laminate formed by laminating in the order of A / B / A or B / A / B, A / B / A / B / A or B / A / B / A / In the case of 3 layers (A / B / A), such as a 5-layer laminate formed in the order of B, the thickness of each layer is A layer (0.5 to 33.3 μm) / B layer (1 to 1). 33.3 μm) / A layer (0.5 to 33.3 μm) is preferable, and more preferably A layer (0.5 to 25 μm) / B layer (4 to 50 μm) / A layer (0.5 to 25 μm). Is.

In the case of 5 layers (A / B / A / B / A), the thickness of each layer is A layer (0.5 to 29.5 μm) / B layer (1 to 49 μm) / A layer (0.5 to 20 μm). / B layer (1 to 49 μm) / A layer (0.5 to 29.5 μm) is preferable, and more preferably A layer (0.5 to 19.5 μm) / B layer (1 to 27.5 μm) / A. It is a layer (0.5 to 15 μm) / a layer B (1 to 27.5 μm) / a layer A (0.5 to 19.5 μm).

When the polyester film of the present invention is laminated with the above structure, for example, the raw material resin composition for forming the A layer and the B layer is, for example, A / B / A, A / B / A / B / A, or the like. Examples thereof include a manufacturing method in which a flat polyester film is prepared by co-extruding from a T-die onto a chill roll in which cooling water circulates.

As a second method for stably producing the biaxially stretched polyester film of the present invention, a thermoplastic resin layer (A layer) containing a large amount of liquid crystal polymer and a liquid crystal polymer not contained or contained in a proportion of 20% by mass or less. It is mentioned that the thermoplastic resin layer (B layer) is alternately made into 60 or more layers.
As a method of alternately layering the thermoplastic resin layer (A layer) and the thermoplastic resin layer (B layer) in 60 layers or more, a general multi-layer device (multi-layer feed block, static mixer, multi-layer multi-manifold, etc.) For example, a method of laminating thermoplastic resins fed from different flow paths using two or more extruders in multiple layers using a field block, a static mixer, a multi-manifold die, or the like is used. be able to. In the case of multi-layering with the same composition, it is also possible to achieve the object of the present invention by introducing the above-mentioned multi-layering device into the melt line from the extruder to the die using only one extruder. Is.

The lower limit of the thickness of the biaxially stretched polyester film according to the embodiment of the present invention is preferably 3 μm, more preferably 5 μm, and further preferably 8 μm. When it is 3 μm or more, the strength and gas barrier property of the base film layer are sufficient.
The upper limit of the thickness of the biaxially stretched polyester film according to the embodiment of the present invention is preferably 100 μm, more preferably 75 μm, and further preferably 50 μm. When it is 100 μm or less, the processing for the purpose of the present invention becomes easier.

The upper limit of the heat shrinkage rate after heating the biaxially stretched polyester film according to the embodiment of the present invention at 150 ° C. in the longitudinal direction (sometimes abbreviated as MD) for 15 minutes is preferably 4.0%, more preferably. Is 3.0%, more preferably 2%. If the upper limit is exceeded, not only pitch deviation may occur due to dimensional changes in high temperature treatment such as laminating process with other films or printing process, but also wrinkles occur in heating process such as retort sterilization process, and the bag It may spoil the appearance.

The upper limit of the heat shrinkage rate after heating the biaxially stretched polyester film according to the embodiment of the present invention at 150 ° C. in the lateral direction (sometimes abbreviated as TD) for 15 minutes is preferably 3.0%, more preferably. It is 2.0%, more preferably 1%. If the upper limit is exceeded, not only pitch deviation may occur due to dimensional changes in high temperature treatment such as laminating process with other films or printing process, but also wrinkles occur in heating process such as retort sterilization process, and the bag It may spoil the appearance.

The lower limit of the heat shrinkage rate after heating the polyester film according to the embodiment of the present invention at 150 ° C. in the longitudinal direction for 15 minutes is preferably 0%. Even if it is less than the above, the effect of improvement cannot be obtained any more (saturation). In addition, it may become mechanically brittle.
The lower limit of the heat shrinkage rate after heating the biaxially stretched polyester film according to the embodiment of the present invention at 150 ° C. in the lateral direction for 15 minutes is preferably 1.0%. Even if it is less than the above, the effect of improvement cannot be obtained any more (saturation). It may also become mechanically brittle.

The lower limit of the tear strength of the biaxially stretched polyester film according to the embodiment of the present invention is preferably 0.8 mJ / μm. If it is 0.8 mJ / μm or more, a stable film can be formed and the strength of the film is sufficient.
The upper limit of the tear strength of the biaxially stretched polyester film according to the embodiment of the present invention is preferably 7.5 mJ / μm. If it is 7.5 mJ / μm or less, it shows good tearability.

The lower limit of the impact strength of the biaxially stretched polyester film according to the embodiment of the present invention is preferably 0.012 J / μm. If it is 0.012 J / μm or more, the strength becomes insufficient when used as a bag. The upper limit of the impact strength is not particularly limited, but it is difficult to make it stronger than 0.05 J / μm.

The oxygen permeability of the biaxially stretched polyester film according to the embodiment of the present invention measured in an environment of 23 ° C. and a relative humidity of 65% is preferably 100 cc / m ² / day / atm or less.
When the oxygen permeability is 100 cc / m ² / day / atm or less, the effect of preventing oxidation of the contents can be obtained when the bag is made.

The biaxially stretched polyester film according to the embodiment of the present invention may be subjected to a corona discharge treatment, a glow discharge treatment, a flame treatment, and a surface roughening treatment as long as the object of the present invention is not impaired, and is known. Anchor coating treatment, printing, decoration, etc. may be applied.

Next, a manufacturing method for obtaining a biaxially stretched polyester film according to an embodiment of the present invention will be specifically described. It is not limited to these.
In order to obtain the biaxially stretched polyester film according to the embodiment of the present invention, in the production method, the raw materials of the A layer and the B layer are laminated in a molten state using separate extruders, extruded into a sheet, and placed on a casting drum. A step of cooling to form an unstretched laminated sheet, a longitudinal stretching step of stretching the molded unstretched sheet in the longitudinal direction (also referred to as a longitudinal direction), and then a lateral direction (also referred to as a width direction) orthogonal to the longitudinal direction. It consists of a transverse stretching step of stretching to, a heat fixing step of heating and crystallizing the film after performing longitudinal stretching and transverse stretching, and a heat relaxation step of removing residual strain of the heat-fixed film.

[Unstretched sheet molding process]
First, the film raw material is dried or hot air dried. Next, the raw materials are weighed, mixed, supplied to an extruder, heated and melted, and melt-casted in the form of a sheet.
Further, the molten resin sheet is brought into close contact with a cooling roll (casting roll) by an electrostatic application method to be cooled and solidified to obtain an unstretched sheet. The electrostatic application method is a method in which a voltage is applied to an electrode installed in the vicinity of a molten resin sheet in contact with a rotating metal roll and in the vicinity of a surface opposite to the surface of the resin sheet in contact with the rotating metal roll. This is a method in which the resin sheet is charged and the resin sheet and the rotary cooling roll are brought into close contact with each other.

The lower limit of the heating and melting temperature of the resin is preferably 200 ° C., more preferably 250 ° C., and even more preferably 260 ° C. If it is less than the above, the discharge may become unstable. The upper limit of the resin melting temperature is preferably 300 ° C, more preferably 280 ° C. If it exceeds the above, the resin is decomposed and the film becomes brittle.

The method of extruding and casting the molten raw material resin is specifically a step of melting a polyester resin and a liquid crystal polymer resin composition to form a molten fluid, and discharging the formed molten fluid from a die to bring it into contact with a cooling roll. It has at least a step of solidifying and forming an unstretched sheet.

The method of melting the raw material resin to form a molten fluid is not particularly limited, but a preferred method includes a method of heating and melting using a single-screw extruder or a twin-screw extruder.

The molten resin is discharged from the die and brought into contact with a cooling roll to solidify.
The lower limit of the cooling roll temperature is preferably −10 ° C. If it is less than the above, the effect of suppressing crystallization may be saturated. The upper limit of the cooling roll temperature is preferably 40 ° C. If it exceeds the above, the crystallinity may become too high and stretching may become difficult. The upper limit of the cooling roll temperature is preferably 25 ° C. When the temperature of the cooling roll is within the above range, it is preferable to lower the humidity of the environment near the cooling roll in order to prevent dew condensation. It is preferable to reduce the temperature unevenness on the surface of the cooling roll. At this time, the thickness of the unstretched sheet is preferably in the range of 15 to 2500 μm.

[Vertical stretching and horizontal stretching steps]
Next, the stretching method will be described.
The biaxially stretched polyester film according to the embodiment of the present invention needs to be biaxially stretched in the longitudinal direction (also referred to as the longitudinal direction and the mechanical direction) and the lateral direction (also referred to as the width direction). As a biaxial stretching method, a simultaneous biaxial stretching method or a sequential biaxial stretching method can be adopted. Sequential biaxial stretching is preferable in terms of high film forming speed and high productivity. In order to increase the degree of surface orientation of the film by biaxial stretching and increase the impact strength, it is necessary to keep the film.

The lower limit of the stretching temperature in the longitudinal direction is preferably 55 ° C., more preferably 60 ° C. Breakage is unlikely to occur at 55 ° C. or higher. Further, since the vertical orientation of the film does not become too strong, the shrinkage stress during the heat fixing treatment can be suppressed, and a film with less distortion of the molecular orientation in the width direction can be obtained. The upper limit of the stretching temperature in the longitudinal direction is preferably 100 ° C., more preferably 95 ° C. When the temperature is 100 ° C. or lower, the orientation of the film is not too weak and the mechanical properties of the film are not deteriorated.

The lower limit of the stretching ratio in the longitudinal direction is preferably 2.8 times, particularly preferably 3.0 times. When it is 2.8 times or more, the degree of surface orientation is increased, the mechanical strength of the film is improved, and the thickness accuracy of the film is improved.
The upper limit of the stretching ratio in the longitudinal direction is preferably 4.3 times, more preferably 4.0 times, and particularly preferably 3.8 times. When it is 4.3 times or less, the degree of orientation of the film in the lateral direction does not become too strong, the shrinkage stress during the heat fixing process does not become too large, and the distortion of the molecular orientation in the lateral direction of the film becomes small, resulting in As a result, the vertical tearability is improved. Moreover, the effect of improving the mechanical strength and the thickness unevenness is saturated in this range.

The lower limit of the stretching temperature in the lateral direction is preferably 60 ° C., and if it is 60 ° C. or higher, fracture may be less likely to occur. The upper limit of the stretching temperature in the lateral direction is preferably 130 ° C., and when the temperature is 130 ° C. or lower, the degree of orientation in the lateral direction increases and the mechanical properties are improved.

The lower limit of the stretching ratio in the lateral direction is preferably 3.5 times, more preferably 3.6 times, and particularly preferably 3.7 times. If it is 3.5 times or more, the degree of orientation in the lateral direction is not too weak, and the mechanical properties and thickness unevenness are improved. The upper limit of the stretching ratio in the lateral direction is preferably 5 times, more preferably 4.5 times, and particularly preferably 4.0 times. If it is 5.0 times or less, the effect of improving the mechanical strength and thickness unevenness is maximized (saturated) even in this range.

[Heat fixing process]
The lower limit of the heat fixing temperature in the heat fixing step is preferably 195 ° C., more preferably 200 ° C. When the temperature is 195 ° C. or higher, the heat shrinkage rate of the film is reduced. The upper limit of the heat fixing temperature is preferably 250 ° C., and if it is 250 ° C. or lower, the base film layer does not melt and is less likely to become brittle.

[Heat relaxation process]
The lower limit of the relaxation rate in the heat relaxation step is preferably 0.5%. If it is 0.5% or more, breakage may be less likely to occur during heat fixing. The upper limit of the relaxation rate is preferably 10%. When it is 10% or less, the shrinkage in the vertical direction at the time of heat fixing becomes small, and as a result, the distortion of the molecular orientation at the edge of the film becomes small, and the straight tearability is improved. In addition, the film is less likely to sag and uneven thickness is less likely to occur.

[Packaging material]
When the biaxially stretched polyester film of the present invention is used as a packaging material, it is preferable to form a thermosetting resin layer called a sealant. The heat-sealing resin layer is usually provided on the inorganic thin film layer, but may be provided on the outside of the base film layer (the surface opposite to the adhesive layer forming surface). The heat-sealing resin layer is usually formed by an extrusion laminating method or a dry laminating method. The thermoplastic polymer that forms the heat-sealable resin layer may be any as long as it can sufficiently exhibit sealant adhesiveness, and is a polyethylene resin such as HDPE, LDPE, LLDPE, a polypropylene resin, or an ethylene-vinyl acetate copolymer. , Ethethylene-α-olefin random copolymer, ionomer resin and the like can be used.

Further, at least one or more layers of a printing layer, another plastic base material and / or a paper base material may be laminated on the biaxially stretched polyester film of the present invention.

As the printing ink forming the printing layer, water-based and solvent-based resin-containing printing inks can be preferably used. Examples of the resin used for the printing ink include an acrylic resin, a urethane resin, a polyester resin, a vinyl chloride resin, a vinyl acetate copolymer resin, and a mixture thereof. Known printing inks include antistatic agents, light blocking agents, UV absorbers, plasticizers, lubricants, fillers, colorants, stabilizers, lubricants, defoamers, cross-linking agents, blocking agents, antioxidants and the like. Additives may be included. The printing method for providing the print layer is not particularly limited, and known printing methods such as an offset printing method, a gravure printing method, and a screen printing method can be used. For drying the solvent after printing, known drying methods such as hot air drying, hot roll drying, and infrared drying can be used.

Next, the present invention will be described in more detail by way of examples, but the present invention is not limited to the following examples. The film was evaluated by the following measurement method.

[Continuous film forming property]
The film-forming property of the biaxially stretched polyester film was determined as follows from the continuous film-forming time in which film tear did not occur during continuous film-forming.
⊚: No film tear for 2 hours 〇: One film tear occurred during 2 hours of film formation Δ: 3 times of film tear occurred during 2 hours of film formation ×: 4 during 2 hours of film formation Film tear occurs more than once

[Thickness]
It was measured using a dial gauge in accordance with JIS K7130-199 A method.

[Haze]
The sample was measured at three different points using a haze meter (Nippon Denshoku Co., Ltd., NDH2000) by a method according to JIS-K-7105, and the average value was taken as haze.

[Impact strength]
Using an impact tester manufactured by Toyo Seiki Seisakusho Co., Ltd., the strength of the film against impact punching in an atmosphere of 23 ° C. was measured. The impact spherical surface used was one with a diameter of 1/2 inch. The unit is J / μm.

[Tear strength]
The tear strength was measured according to JIS K7128-1: 1998. The average value was calculated by measuring three times in each of the vertical direction and the horizontal direction.

[Coefficient of friction]
The dynamic friction coefficient was measured in accordance with JIS-K-7125, and the static friction coefficient μs and the dynamic friction coefficient μd were measured in an environment of a temperature of 23 ° C. and a relative humidity of 65%. The measured surface was measured by sliding the F surface (the surface that contacts the cooling roll in the unstretched sheet molding step) and the B surface (the surface that does not contact the cooling roll in the unstretched sheet molding step).

[Heat shrinkage rate]
The heat shrinkage was measured in the vertical and horizontal directions by the dimensional change test method described in JIS-C-2151-2006.21, except that the test temperature was 150 ° C. and the heating time was 15 minutes. The value was calculated. The test piece was used as described in 21.1 (a).

[Oxygen permeability]
According to the JIS K7126-2A method, the measurement was performed in an environment of 23 ° C. and 65% relative humidity using an oxygen permeability measuring device (“OX-TRAN 2/21” manufactured by MOCON).

The details of the raw material resins used in this example and the comparative example are described below.
1) Polyester resin (I): PET resin (terephthalic acid // ethylene glycol = 100 // 100 (mol%), manufactured by Toyobo Co., Ltd., intrinsic viscosity 0.62 dl / g) was used.

2) Liquid crystal polymer (II): 6-Hydroxy-2-naphthoic acid: 376.34 g (40 mol), 4-hydroxybenzoic acid: 276.23 g (40 mol) in a reaction vessel equipped with a stirrer and a distillate. Parts), hydroquinone: 55.05 g (10 mol parts) and terephthalic acid: 83.06 g (10 mol parts), followed by 0.05 g of potassium acetate (67 mol ppm with respect to all monomers) and the amount of hydroxyl groups of all monomers (parts). 1.025 times the molar amount of acetic anhydride was charged with respect to the molar amount), and deacetic acid polymerization was carried out under the following conditions.
The polymerization was carried out in a nitrogen gas atmosphere from room temperature to 150 ° C. in 1 hour and maintained at the same temperature for 30 minutes. Next, the temperature was rapidly raised to 210 ° C. while distilling off acetic acid produced as a by-product, and the temperature was maintained at the same temperature for 30 minutes. Then, after raising the temperature to 335 ° C. over 3 hours, the pressure was reduced to 20 mmHg over 30 minutes, the polymerization reaction was terminated when a predetermined torque was exhibited, the contents of the reaction vessel were taken out, and pulverized. Pellets of a liquid crystal polymer having an anisotropic molten phase were obtained. The amount of distillate acetic acid at the time of polymerization was almost the same as the theoretical value. The crystal melting temperature of the obtained liquid crystal polymer measured by DSC was 221 ° C.

3) Competitive agent (III): 95.0 g of 6-hydroxy-2-naphthoic acid and 467 g of epichlorohydrin were added to 1 L of 4-port corben, and the temperature was raised to 80 ° C. under a nitrogen stream. Then, a 50% aqueous solution of tetramethylammonium chloride was added dropwise at 80 ° C. over 2 hours, and the mixture was stirred at the same temperature for 1 hour. Further, 87.1 g of a 48% aqueous sodium hydroxide solution was added dropwise at 80 ° C. over 3 hours, and the mixture was stirred at the same temperature for 30 minutes, and then epichlorohydrin was removed by distillation under reduced pressure. After adding 560 g of toluene to the residue and stirring for 10 minutes, the precipitate was filtered. The filtrate was washed with 250 g of water, 19 g of a 48% NaOH aqueous solution was added, and the mixture was refluxed for 1 hour. Further, it was washed with 250 g of water, then with 250 g of a 5% phosphorus aqueous solution, and again with 250 g of water. Toluene was removed by distillation under reduced pressure to obtain 105 g of a compatibilizer. The epoxy equivalent of the resulting compatibilizer was 168.

The method for producing the biaxially stretched polyester film used in each Example and Comparative Example is described below. Table 1 shows the physical characteristics of the following biaxially stretched polyester film.
<Example 1>
Two single-screw extruders are used, one extruder uses 100% by mass of polyester resin (I) as layer A melted at 290 ° C. and introduced into the melt line, and the other extruder uses polyester as layer B. A mixture of 80% by mass of the resin (I) and 20% by mass of the liquid crystal polymer (II) was melted at 290 ° C. and then introduced into a melt line. Next, they were stacked with a feed block in the order of A / B / A, cast from a T-die at 280 ° C., and adhered to a cooling roll at 20 ° C. by an electrostatic adhesion method. A stretched sheet was obtained.
Then, it was rolled 3.0 times in the vertical direction at 120 ° C., then passed through a tenter and stretched 4.0 times in the horizontal direction at 120 ° C., and tension heat fixing treatment at 228 ° C. for 3 seconds and 5% heat. After the relaxation treatment, a cooling treatment was performed at 50 ° C. for 2 seconds, and then the grips at both ends were cut and removed by 10% to obtain a biaxially stretched polyester film having a thickness of 12 μm. Table 1 shows the film forming conditions, physical properties, and evaluation results of the obtained film.

<Example 2>
In the film forming step of the biaxially stretched polyester film, the same procedure as in Example 1 was carried out except that the content of the liquid crystal polymer (II), the content of the compatibilizer, the vertical and horizontal stretching ratio, and the relaxation rate were shown in Table 1. ..

<Example 3>
Two uniaxial extruders were used, one of which melted 100% by mass of the polyester resin (I) at 290 ° C. and then introduced into the melt line. In the other extruder, a mixture of 80% by mass of polyester resin (I) and 20% by mass of liquid crystal polymer (II) was melted at 290 ° C. and then introduced into a melt line. Next, they are stacked with a feed block in the order of A / B / A / B / A, cast from a T-die at 280 ° C., and adhered to a cooling roll at 20 ° C. by an electrostatic adhesion method. An unstretched sheet having an A / B / A composition was obtained. Next, longitudinal stretching, transverse stretching, heat fixing treatment, and heat relaxation treatment were carried out in the same manner as in Example 1 to obtain a biaxially stretched polyester film having a thickness of 12 μm. Table 1 shows the film forming conditions, physical properties, and evaluation results of the obtained film.

<Example 4>
Two uniaxial extruders were used, one of which melted 100% by mass of the polyester resin (I) at 290 ° C. and then introduced into the melt line. In the other extruder, a mixture of 80% by mass of polyester resin (I) and 20% by mass of liquid crystal polymer (II) was melted at 290 ° C. and then introduced into a melt line, and then these were introduced. The molten resin of No. 1 was introduced into a 12-element static mixer. As a result, the resin composition was divided and laminated to obtain a multilayer melt having 4096 theoretical layers. It was cast from a T-die at 280 ° C. and adhered to a cooling roll at 15 ° C. by an electrostatic adhesion method to obtain an unstretched sheet. Next, in the same manner as in Examples 1 to 3, longitudinal stretching, transverse stretching, heat fixing treatment, and heat relaxation treatment were carried out under the film forming conditions shown in Table 1 to obtain a biaxially stretched polyester film having a thickness of 12 μm. Table 1 shows the film forming conditions, physical properties, and evaluation results of the obtained film.

<Examples 5 to 8>
In the film forming process of the biaxially stretched polyester film, the theoretical number of layers is 4096, as in Example 4, except that the content of the liquid crystal polymer (II), the content of the compatibilizer, and the film forming conditions are shown in Table 1. The multilayer unstretched sheet of No. 1 was subjected to longitudinal stretching, transverse stretching, heat fixing treatment, and heat relaxation treatment to obtain a biaxially stretched polyester film having a thickness of 12 μm. Table 1 shows the film forming conditions, physical properties, and evaluation results of the obtained film.

<Comparative example 1>
Using one single-screw extruder, 100% by mass of the polyester resin (I) was melted at 290 ° C. and then introduced into a melt line. Then, it was cast from a T-die at 280 ° C. and adhered to a cooling roll at 20 ° C. by an electrostatic adhesion method to obtain an unstretched sheet having a single layer structure. Next, longitudinal stretching, transverse stretching, heat fixing treatment, and heat relaxation treatment were carried out in the same manner as in Example 1 to obtain a biaxially stretched polyester film having a thickness of 12 μm. Table 1 shows the film forming conditions, physical properties, and evaluation results of the obtained film.

<Comparative Examples 2-3>
A biaxially stretched polyester film having a thickness of 12 μm was obtained in the same manner as in Comparative Example 1 except that the content of the liquid crystal polymer (II) was set to the value shown in Table 1 in the film forming step of the biaxially stretched polyester film. It was. Table 1 shows the film forming conditions, physical properties, and evaluation results of the obtained film.

As shown in Examples 1 to 8 in Table 1, by adding the liquid crystal polymer ratio within the range of the present invention, a biaxially stretched polyester film having excellent gas barrier property and easy tearing property could be obtained. Further, as seen in Example 2, when the compatibilizer was added, excellent gas barrier properties could be imparted even with a smaller ratio of the liquid crystal polymer, and the transparency of the film could be further improved.
Further, as shown in Comparative Examples 2 and 3, a layer containing a large amount of liquid crystal polymer (layer A) and a layer containing no liquid crystal polymer or containing 20% by mass or less, as compared with the case of forming a film with a single layer. The continuous film-forming property was superior when the (B layer) was alternately laminated in multiple layers.

Since Comparative Example 1 did not contain a liquid crystal polymer, it was inferior in gas barrier property and easy tearing property.
In Comparative Examples 2 and 3, although the physical characteristics of the obtained film are good, the liquid crystal polymer is too strongly oriented during stretching because it does not have a laminated structure, and as a result, the film is easily torn during stretching, resulting in stable film formation. It was difficult to do.

Since the biaxially stretched polyester film of the present invention has excellent gas barrier properties and easy tearability, it can be suitably used as a packaging material for pharmaceuticals, medical products, foods, beverages and the like. In addition to packaging applications, it can also be widely used in industrial applications such as solar cells, electronic paper, organic EL elements, and semiconductor elements.

Claims (5)

A biaxially stretched polyester film in which three or more layers of a thermoplastic resin layer (A layer) and a thermoplastic resin layer (B layer) are alternately laminated, and the thermoplastic resin layer (A layer) contains at least 60 to polyester resin. A liquid crystal containing 95% by mass and a liquid crystal polymer having a crystal melting temperature of 250 ° C. or less, having a thermoplastic resin layer (B layer) of at least 80 to 100% by mass of a polyester resin and a crystal melting temperature of 250 ° C. or less. A biaxially stretched polyester film containing 0 to 20% by mass of a polymer. The biaxially stretched polyester film according to claim 1, wherein the oxygen permeability measured in an environment of 23 ° C. and a relative humidity of 65% is 100 cc / m ^{2 / day / atm or less.} The biaxially stretched polyester film according to claim 1 or 2, wherein the tear strength in the longitudinal direction is 7.5 mJ / μm or less. The biaxially stretched polyester film according to any one of claims 1 to 3, wherein the thermoplastic resin layer (A layer) and / or the thermoplastic resin layer (B layer) contains a compatibilizer. The method for producing a biaxially stretched polyester film according to any one of claims 1 to 4, after the thermoplastic resin (A layer) and the thermoplastic resin (B layer) are alternately laminated into 60 or more layers. A method for producing a biaxially stretched polyester film, which comprises biaxially stretching a cast unstretched sheet.