JP7280014B2 - laminated polyester film - Google Patents

Description

TECHNICAL FIELD The present invention relates to a laminated polyester film that can be used as an optical film such as a protective film that constitutes a polarizing plate.

In liquid crystal display devices such as televisions, personal computers, digital cameras, and mobile phones, a light source, a back-side polarizing plate, a liquid crystal layer, and a front-side polarizing plate are often laminated in this order from the light source toward the viewing side. Of these, the polarizing plate is generally laminated with a protective film such as a biaxially stretched polyester film on its surface for scratch prevention. For example, a polarizing plate having a structure such as protective film/polarizing film/protective film or protective film/polarizing film/retardation film has been known.

Conventionally, a triacetyl cellulose (TAC) film has been used as a protective film constituting a polarizing plate. However, when the thickness of the TAC film is reduced, sufficient mechanical strength cannot be obtained and moisture permeability is deteriorated. Therefore, in recent years, it has been proposed to use a polyester film instead of a TAC film in order to make the polarizing plate thinner (see, for example, Patent Document 1).

Polyester films, especially biaxially stretched polyester films typified by polyethylene terephthalate films, are excellent in electrical properties, mechanical properties, thermal properties, workability and chemical resistance. It is used as a protective film for parts.

As a film used for optical applications such as a protective film constituting a polarizing plate, it is generally preferable to have less optical bias, and therefore it is required to reduce optical anisotropy (retardation).
However, since the biaxially stretched polyester film has birefringence, it has a problem that it is difficult to lower the optical anisotropy (retardation).
As a countermeasure therefor, for example, a method of reducing the retardation by using a copolyester as the polyester has been proposed (see Patent Document 2, etc.).

Furthermore, biaxially stretched polyester films used for optical applications, for example, have good transparency and slipperiness, are less likely to be scratched during film formation, and have excellent transparency and light interference. As a polyester film that can suppress color development and has high durability against ultraviolet rays, a laminated structure of three or more layers in which a copolyester is used as an intermediate layer and both outermost layers have a thickness of 2.5 μm or more. It has an in-plane retardation of 600 nm or less, an internal haze of 0.5% or less, a surface roughness Ra of 9.0 nm or more, and layers other than both outermost layers A laminated polyester film has been proposed, which contains an ultraviolet absorber in the film and has a light transmittance of 5.0% or less at 380 nm (Patent Document 3).

Japanese Patent Application Laid-Open No. 2004-219620 WO2011-162198 JP 2014-205275 A

In order to further reduce the retardation of a laminated polyester film having a laminated structure of three or more layers as disclosed in Patent Document 3, it is necessary to reduce the retardation of not only the intermediate layer but also the surface layer.
In such a structure, since the retardation of the surface layer is higher than that of the intermediate layer, one method for reducing the retardation is to make the thickness of the surface layer thinner. However, it was found that when the thickness of the surface layer becomes thin, new problems such as rupture in the slitting process occur.

Therefore, the present invention relates to a laminated polyester film having a laminated structure of three or more layers, a new lamination that can reduce the retardation in the thickness direction of the film and can ensure productivity in the slitting process. The object is to provide a polyester film.

The present invention is a laminated polyester film comprising at least three layers, an intermediate layer containing a polyester copolymer as a main component resin and two surface layers containing a polyester polymer as a main component resin, wherein at least one of the surface layers A laminated polyester film is proposed, characterized by an intrinsic viscosity of 0.70 to 1.20 dl/g.

In the laminated polyester film proposed by the present invention, retardation can be reduced by using a polyester copolymer in the intermediate layer. Furthermore, by increasing the intrinsic viscosity of the surface layer polyester, the retardation in the thickness direction of the film can be further reduced while ensuring the productivity in the slitting process.

Next, the present invention will be described based on examples of modes for carrying out the present invention. However, the present invention is not limited to the embodiments described below.

<This laminated polyester film>
A laminated polyester film (referred to as "this laminated polyester film") as an example of an embodiment of the present invention is a laminated polyester film having a laminated structure of three or more layers having at least an intermediate layer and two surface layers.

<Middle layer>
The intermediate layer is preferably a layer containing a polyester copolymer as a main component resin.
Here, the "main component resin" indicates the resin having the highest content among the resins constituting the intermediate layer.
Moreover, in the present invention, the "polyester copolymer" means a polyester having two or more types of repeating units based on an ester bond.
In the present invention, the term "polyester copolymer" refers to a polyester-based resin containing 5 mol % or more of copolymer components.

(polyester copolymer)
The polyester copolymer, which is the main component resin of the intermediate layer, has low crystallinity. It is preferably polymeric, in other words amorphous. Specifically, a polyester copolymer having a heat of fusion of 0 or 15 J/g or less obtained by differential scanning calorimetry (based on the method described in JIS K7121 and JIS K7122) for analyzing thermal properties is preferred. .

Polyester copolymers include polyesters formed by condensation of a dicarboxylic acid component and a diol component, polyesters formed by condensation of a hydroxycarboxylic acid component such as lactic acid, and ring-opening condensation of a lactone component such as ε-caprolactone. Formed polyesters and the like can be mentioned. Furthermore, polyesters in which these different condensation modes are used in combination may also be used. Among these, a polyester formed by condensation of a dicarboxylic acid component and a diol component is preferred.
In the case of polyester formed by condensation of hydroxycarboxylic acid or lactone, a polyester copolymer can be obtained by using two or more raw materials in combination. In addition, if the polyester is formed by using different condensation modes in combination, it naturally becomes a polyester copolymer.

The content of the copolymer component in the polyester copolymer is preferably 5 mol % or more. Above all, the content of the copolymer component is more preferably 12 mol % or more or 90 mol % or less, more preferably 20 mol % or more or 80 mol % or less.

Here, the content ratio of the copolymer component means the total content ratio of the components other than the main component, and the "main component" means the component with the highest content. For example, in the case of a polyester copolymer composed of component A: 40 mol%, component B: 30 mol%, and component C: 30 mol%, the content of the copolymer component is 60 mol%.
In the case of a polyester copolymer formed from a dicarboxylic acid component and a diol component, it means the sum of the ratio of components other than the main component in the dicarboxylic acid component and the ratio of components other than the main component in the diol component. For example, if it is a polyester copolymer composed of dicarboxylic acid component A: 60 mol%, dicarboxylic acid component B: 40 mol%, diol component A: 60 mol%, and diol component B: 40 mol%, copolymerization The content ratio of the component is 80 mol %.

The above polyester copolymer is a polyester copolymer containing 5 mol % or more of a copolymer component in the dicarboxylic acid component, the diol component, or both components from the viewpoint of lowering the optical anisotropy (retardation). is preferred.
Among them, the above polyester copolymer is a polyester copolymer containing 12 mol% or more and 50 mol% or less, especially 20 mol% or more and 30 mol% or less of a dicarboxylic acid component or a diol component or both components. More preferably it is a polymer.

Examples of the dicarboxylic acid component of the polyester copolymer include terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, cyclohexanedicarboxylic acid, oxalic acid, malonic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, neopentylic acid, and diphenyl ether dicarboxylic acid. acid, p-oxybenzoic acid, and the like. These may be of one type or two or more types. A polyester copolymer can be obtained by using two or more of the above dicarboxylic acid components. Moreover, when using only one of the above dicarboxylic acid components, a polyester copolymer can be obtained by using two or more diol components described later.
The main component of the dicarboxylic acid component is preferably terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, cyclohexanedicarboxylic acid, etc. Among them, terephthalic acid or naphthalenedicarboxylic acid is preferable.

Examples of the diol component of the polyester copolymer include ethylene glycol, diethylene glycol, triethylene glycol, cyclohexanedimethanol, propylene glycol, trimethylene glycol, 1,4-butanediol, hexamethylene glycol, neopentyl glycol, and polyalkylene. Glycol, methoxypolyalkylene glycol, spiroglycol, isosorbide, 2,2,4,4-tetramethyl-1,3-cyclobutanediol and the like can be mentioned. These may be of one type or two or more types. A polyester copolymer can be obtained by using two or more of the above diol components. Moreover, when only one of the diol components is used as the diol component, a polyester copolymer can be obtained by using two or more of the above-described dicarboxylic acid components.
As the main component in the diol component, ethylene glycol, cyclohexanedimethanol, 1,4-butanediol and the like are preferable, and ethylene glycol or cyclohexanedimethanol is particularly preferable.

Examples of other copolymerization components of the polyester copolymer include acid components such as phthalic acid, trimellitic acid, and pyromellitic acid.
In addition, as the alcohol component as the copolymerization component other than the above, trifunctional or higher functional components such as glycenrin, pentaerythritol, and trimethylol may be used.

An example of the polyester copolymer is a polyester copolymer containing a dicarboxylic acid component containing terephthalic acid or isophthalic acid or both, and a diol component containing ethylene glycol or 1,4-cyclohexanedimethanol or both. be able to.
For example, polyester copolymers having isophthalic acid as an acid component, typified by polyethylene terephthalate/isophthalate copolymers and polycyclohexanedimethylene terephthalate/isophthalate copolymers (PCTA), and 1,4-cyclohexanedimethanol copolymers Polymerized polyethylene terephthalate (PETG), polyester copolymer having 1,4-cyclohexanedimethanol as a glycol component represented by polycyclohexane dimethylene terephthalate copolymer (PCTG), etc., furthermore, the glycol component has an alicyclic structure. polyester copolymers having
As a more specific example, a dicarboxylic acid component containing terephthalic acid as the main component and 20-40 1,4-cyclohexanedimethanol (“CHDM”) to 80-60 (mole ratio) of ethylene glycol as the main component. A polyester copolymer obtained by polycondensation with a glycol component containing (molar ratio, total 100), so-called "PETG" can be mentioned.

Incidentally, even homopolyethylene terephthalate usually contains several mol % or more of diethylene glycol component due to its reaction mode. In the present invention, even if it is produced as homopolyethylene terephthalate, it is treated as a polyester copolymer when the content of the diethylene glycol component is within the above range.

The glass transition temperature (Tg) of the polyester copolymer is preferably 20 to 150° C. from the viewpoint of heat resistance and productivity, especially 40° C. or higher or 120° C. or lower, especially 60° C. or higher or 90° C. The following are particularly preferred.
The glass transition temperature (Tg) was measured by differential scanning calorimetry (according to JIS K7121 and JIS K7122) by heating from 30°C to 300°C at a rate of 10°C/min, holding the temperature for 1 minute, and then from 300°C to 30°C. After lowering the temperature at 10 ° C./min to ° C., holding it for 1 minute, and further increasing the temperature from 30 ° C. to 300 ° C. at 10 ° C./min. Let the temperature be the point where the straight line intersects the curve of the step change portion of the glass transition.

The intrinsic viscosity (IV value) of the polyester copolymer is preferably 0.50 to 0.77 dl / g from the viewpoint of improving productivity, and among them, 0.54 dl / g or more or 0.72 dl / g or less , more preferably 0.58 dl/g or more or 0.68 dl/g or less.
The intrinsic viscosity (IV) can be measured under the measurement conditions described in Examples below.

A resin other than the main component resin may be added to the intermediate layer, if necessary. The type of the resin is not particularly limited, and specific examples include polycarbonate resins, acrylic resins, polyester resins other than the main component resin, and the like. Among them, a resin having compatibility with the main component resin is preferable.

(other ingredients)
Preferably, the intermediate layer is substantially free of particles. The term "substantially free" as used herein specifically refers to a particle content of 150 ppm or less. In order to stably wind the film into a roll, it is sufficient to add particles to the surface layer, and depending on the stretching conditions, voids may be formed around the particles, reducing the light transmittance. Because there is

The intermediate layer can also contain an ultraviolet absorber. Its content is preferably in the range of 0.10 to 10.0% by mass.
If the content of the ultraviolet absorber in the intermediate layer is 0.10% by mass or more, it is possible to suppress deterioration of the polyester film due to ultraviolet rays, and on the other hand, if the content is 10.0% by mass or less , it is possible to suppress the ultraviolet absorber from bleeding out on the surface, and to prevent the deterioration of the adhesiveness and the deterioration of the surface functionality.

Examples of the ultraviolet absorber include benzophenone compounds, 1,3,5-triazine compounds, benzoxazinone compounds, and the like, and these can be used alone or in combination of two or more. Considering the color tone, benzoxazinone-based compounds that are less yellowish are suitable.

(orientation)
Preferably, the intermediate layer is a biaxially oriented layer and yet has no orientation.
Strength and dimensional stability can be enhanced by biaxially stretching the intermediate layer containing a polyester copolymer as a main component resin. The retardation of the laminated polyester film can be reduced by making the intermediate layer oriented by biaxial stretching non-oriented again by heat treatment.

(thickness of intermediate layer)
The thickness of the intermediate layer is preferably 7-120 μm.
The thickness of the intermediate layer of 7 μm or more is preferable from the standpoint of handleability, and the thickness of 120 μm or less is preferable because it is easy to incorporate into the polarizing plate.
From this point of view, the intermediate layer preferably has a thickness of 7 to 120 μm, more preferably 20 μm or more or 95 μm or less, and more preferably 33 μm or more or 70 μm or less.

<Surface layer>
The surface layer is preferably a layer containing a polyester polymer as a main component resin.
The main component resins of the two surface layers may be the same resin or different resins.
Here, the "main component resin" indicates a resin having the highest content among the resins forming the surface layer.

(polyester polymer)
The polyester polymer that forms the main component resin of the surface layer is determined by differential scanning calorimetry (based on the method described in JIS K7121 and JIS K7122) for analyzing thermal properties. Those having coalescence, in other words, crystallinity are preferred.
The heat of fusion was measured by differential scanning calorimetry, where the temperature was raised from 30°C to 300°C at a rate of 10°C/min and held for 1 minute, then the temperature was lowered from 300°C to 30°C at a rate of 10°C/min and held for 1 minute. Further, it can be calculated from the melting peak area in the reheating process when the temperature is reheated from 30° C. to 300° C. at 10° C./min.

The polyester polymer forming the main component resin of the surface layer is a polyester formed by condensation of a dicarboxylic acid component and a diol component, a polyester formed by condensation of a hydroxycarboxylic acid component such as lactic acid, and a lactone component such as ε-caprolactone. and polyesters formed by ring-opening condensation of. Among these, a polyester formed by condensation of a dicarboxylic acid component and a diol component is preferred. In particular, it is preferable that the dicarboxylic acid component, the diol component, or both of them contain a polyester resin containing less than 5 mol % of the copolymer component. Among them, homopolyester is preferable because it has a relatively high melting point.

Examples of the polyester polymer include polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polytrimethylene terephthalate, poly-1,4-cyclohexylene dimethylene terephthalate, polyethylene succinate, polybutylene succinate, polylactic acid, poly Polyester-based resins such as -ε-caprolactone can be used.

If necessary, a resin other than the main component resin may be added to the surface layer. The type of the resin is not particularly limited, and specific examples include polycarbonate resins, acrylic resins, polyester resins other than the main component resin, and the like. Among them, a resin having compatibility with the main component resin is preferable.

(Intrinsic viscosity)
At least one surface layer preferably has an intrinsic viscosity of 0.70 to 1.20 dl/g.
If the intrinsic viscosity of the surface layer is 0.70 dl/g or more, the thickness of the surface layer can be reduced while ensuring the productivity in the slitting process, and the retardation of the laminated polyester film can be further reduced. On the other hand, if the intrinsic viscosity of the surface layer is 1.20 dl/g or less, deterioration of the resin due to shear heat generation can be suppressed, which is preferable.
From this point of view, the intrinsic viscosity of the surface layer is preferably 0.70 to 1.20 dl/g, especially 0.73 dl/g or more or 1.08 dl/g or less, especially 0.77 dl/g or more or 1.08 dl/g or less. It is more preferably 00 dl/g or less.

At least one of the surface layers preferably has the above intrinsic viscosity, and more preferably both surface layers. In addition, one surface layer and the other surface layer may have the same or different intrinsic viscosities, and the same applies to combinations within suitable ranges. Above all, it is preferable that both surface layers have the same intrinsic viscosity because the productivity in the slitting step is good and co-extrusion molding with two kinds and three layers is possible.
Here, "both surface layers have the same intrinsic viscosity" means that the difference in intrinsic viscosity is 0.10 dl/g or less, preferably 0.05 dl/g or less.

From the viewpoint of adjusting the intrinsic viscosity of the surface layer to the above range, the intrinsic viscosity (IV value) of the polyester polymer, which is the main component resin of the surface layer, is preferably 0.70 to 1.35 dl / g, especially 0.80 dl. /g or more or 1.20 dl/g or less, more preferably 0.85 dl/g or more or 1.10 dl/g or less.

In addition, the intrinsic viscosity of the surface layer can be measured by the method shown in the examples described later when the compounding ratio of the raw materials and the like are known. On the other hand, when the compounding ratio of raw materials is unknown, it can be calculated by the following method.
First, the cross section of the laminated polyester film is observed with an optical microscope, an electron microscope, or the like to measure the thickness of the surface layer and the intermediate layer. Next, the intrinsic viscosities of the entire laminated polyester film and the intermediate layer of the laminated polyester film are measured by the measuring method described in the examples below.
At this time, the intrinsic viscosity of the entire laminated polyester film may be measured by the method shown in Examples described later. On the other hand, the intrinsic viscosity of the intermediate layer of the laminated polyester film may be measured using a sample obtained by completely removing the surface layer of the laminated polyester film with a microtome or the like.
Then, from the thickness of each layer of the laminated polyester film and the intrinsic viscosity of each sample, the intrinsic viscosity η ₁ of the surface layer of the laminated polyester film can be calculated from the following formula.

η ₁ = {(a+b)/a}×[η-{b/(a+b)}×η ₂ ]
η: Intrinsic viscosity of the entire laminated polyester film η ₁ : Intrinsic viscosity of surface layer η ₂ : Intrinsic viscosity of intermediate layer a: Total thickness of surface layer b: Thickness of intermediate layer

(Relationship between polyester polymer in surface layer and polyester copolymer in intermediate layer)
The melting point of the polyester polymer in the surface layer is preferably 150 to 250° C. higher than the glass transition temperature of the polyester copolymer in the intermediate layer.
If the melting point (Tm) of the polyester polymer is sufficiently higher than the glass transition temperature (Tg) of the polyester copolymer, heat treatment at a high temperature is performed to render the intermediate layer non-oriented, as described later. Even when the surface layer is melted or fluidized, the sheet shape can be maintained.
From this point of view, the melting point of the polyester polymer is preferably 150 to 250° C. higher than the glass transition temperature of the polyester copolymer, more preferably 160 to 230° C. higher, especially 170 to 210° C. higher. is more preferred.

From this point of view, the melting point (Tm) of the polyester polymer is preferably in the range of 200° C. to 300° C., more preferably 230° C. or higher or 290° C. or lower, especially 250° C. or higher or 270° C. or lower. preferable.

(other ingredients)
In order to facilitate handling, the surface layer may contain particles under the condition that the transparency is not impaired.
Examples of particles in this case include inorganic particles such as calcium carbonate, calcium phosphate, silica, kaolin, talc, titanium dioxide, alumina, barium sulfate, calcium fluoride, lithium fluoride, zeolite, molybdenum sulfide, and crosslinked polymer particles. and organic particles such as calcium oxalate.
If the particle size is too large, the haze of the film may increase and the transparency may decrease. , preferably 0.05 μm to 5.0 μm, more preferably 0.1 μm or more or 4.0 μm or less, and more preferably 0.3 μm or more or 3.5 μm or less.
If the particle content of the surface layer is too large, the haze may increase, and if it is too small, the film may become difficult to handle. Among them, it is more preferably 0.01% by mass or more and 10.0% by mass or less, more preferably 0.1% by mass or more and 5.0% by mass or less.

(orientation)
The surface layer is preferably a biaxially stretched layer and has orientation.
Strength and dimensional stability can be enhanced by biaxially stretching the surface layer containing a polyester polymer as a main component resin. Even if the oriented surface layer is heat-treated by biaxial stretching, it does not become non-oriented.

(Thickness of surface layer)
The thickness of each surface layer is preferably 0.5 to 10% of the total thickness of the laminated polyester film.
If the thickness of each surface layer is sufficiently small, the retardation of the laminated polyester film can be made smaller.
From this point of view, the thickness of each surface layer is preferably 0.5 to 10% of the total laminated polyester film, especially 0.8% or more or 8.0% or less, especially 1.0% or more or 5% 0% or less is more preferable.
From the same point of view, the thickness of each surface layer is preferably less than 2.5 μm, especially 0.3 μm or more or 2.0 μm or less, especially 0.5 μm or more or 1.5 μm or less. is more preferred.

<Physical properties of this laminated polyester film>
This laminated polyester film is preferably a biaxially stretched film from the viewpoint of high strength and dimensional stability.

The thickness of the laminated polyester film is preferably 12 to 125 μm.
When the thickness of the present laminated polyester film is 12 μm or more, it is preferable from the viewpoint of handleability, and when it is 125 μm or less, it is preferable because it is easily incorporated into a polarizing plate.
From this point of view, the thickness of the laminated polyester film is preferably 12 to 125 μm, more preferably 25 μm or more or 100 μm or less, and more preferably 38 μm or more or 75 μm or less.

The intrinsic viscosity of the present laminated polyester film as a whole is preferably 0.55 to 0.75 dl/g from the viewpoint of film forming stability, especially 0.57 dl/g or more or 0.70 dl/g or less. Above all, it is more preferably 0.59 dl/g or more or 0.63 dl/g or less.

The thickness direction retardation (Rth) of the present laminated polyester film is preferably 900 nm or less.
If the thickness direction retardation (Rth) of the present laminated polyester film is 900 nm or less, it can be used for optical applications such as a protective film constituting a polarizing plate.
From this point of view, the thickness direction retardation (Rth) of the laminated polyester film is preferably 900 nm or less, more preferably 1 nm or more or 500 nm or less, and more preferably 10 nm or more or 300 nm or less.
If the thickness direction retardation (Rth) of the present laminated polyester film is 900 nm or less, it can be used for optical applications such as a protective film constituting a polarizing plate.
From this point of view, the thickness direction retardation (Rth) of the laminated polyester film is preferably 900 nm or less, more preferably 1 nm or more or 500 nm or less, and more preferably 10 nm or more or 300 nm or less.
The thickness direction retardation (Rth) can be measured under the measurement conditions described in Examples below.

The in-plane retardation (R ₀ ) of the laminated polyester film is preferably 100 nm or less, more preferably 1 nm or more or 80 nm or less, more preferably 5 nm or more or 60 nm or less.
If the in-plane retardation (R ₀ ) of the present laminated polyester film is 100 nm or less, it can be used for optical applications such as a protective film constituting a polarizing plate.
The in-plane retardation (R ₀ ) can be measured under the measurement conditions described in Examples below.

The edge tear resistance value of the laminated polyester film is preferably 60 N or more.
If the value of the edge tear resistance of the laminated polyester film is 60 N or more, the productivity in the slitting step is improved, which is preferable.
From this point of view, the tear resistance value of the present laminated polyester film is preferably 60N or more, more preferably 70N or more or 500N or less, more preferably 75N or more or 300N or less.
The end tear resistance can be measured under the measurement conditions described in Examples below.

<Method for producing the present laminated polyester film>
Next, an example of the method for producing the present laminated polyester film will be described. However, the manufacturing method of this laminated polyester film is not limited to the manufacturing method described below.

An intermediate layer forming raw material containing the above polyester copolymer as a main component resin, a surface side surface layer forming raw material containing the above polyester polymer as a main component resin, and a back side surface layer forming containing the above polyester polymer as a main component resin. The raw materials for use in the present invention can be co-extruded into three layers by a co-extrusion method, then stretched and heat-treated to produce the present laminated polyester film.

By using the above polyester copolymer as a raw material for forming an intermediate layer, orientation caused by stretching can be made non-orientated again by heat treatment, and retardation can be reduced.
At this time, in order to make the intermediate layer non-oriented, it is necessary to perform a heat treatment at such a high temperature that the intermediate layer can be brought into a state close to being melted or fluidized. However, since it is difficult to manufacture a single intermediate layer, it is necessary to provide both the front and back surfaces with surface layers that do not melt or flow even when subjected to such heat treatment. For that purpose, it is preferable to use the above polyester polymer for the surface layer.
Since a polyester polymer is used as the raw material for forming the surface layer, the surface layer does not become non-oriented even after the heat treatment. Therefore, in order to further reduce the retardation, it is necessary to reduce the thickness of the surface layer. However, when the surface layer is thinned in the conventional technique, problems such as rupture in the slitting process occur.
Therefore, by using polyester with a high intrinsic viscosity as the main component resin of the surface layer to increase the intrinsic viscosity of the surface layer, it is possible to reduce the thickness of the surface layer while ensuring productivity in the slitting process. can be made smaller.

An intermediate layer forming raw material containing the above polyester copolymer as a main component resin, a surface side surface layer forming raw material containing the above polyester polymer as a main component resin, and a back side surface layer forming containing the above polyester polymer as a main component resin. By co-extrusion of the raw material into three layers by a co-extrusion method, it is possible to suppress the occurrence of interfacial peeling at the lamination interface.
Specifically, for example, each of the above raw materials is supplied to a melt extruder, heated and melted to a temperature higher than the melting point of each polymer, and then the melted polymer is extruded into three layers from a die and placed on a rotating cooling drum. to obtain a non-oriented laminate sheet by quenching and solidifying to a temperature below the glass transition point.

Next, the unstretched sheet thus obtained may be stretched in two axial directions.
At this time, the unstretched sheet is preferably stretched 1.3 to 6 times in the machine direction (longitudinal direction) at 80 to 130 ° C. to form a longitudinally uniaxially stretched film, and then stretched in the width direction (lateral direction) by 90 to 6 times. It is preferable to draw the film 1.3 to 6 times at 160°C.

Then, it is preferable to heat-treat the intermediate layer at a high temperature so that the intermediate layer is in a state close to a molten or fluid state.
At this time, the heat treatment temperature is preferably in a temperature range that is 100° C. or more higher than the glass transition temperature of the polyester copolymer in the intermediate layer and not higher than the melting point of the polyester polymer in the surface layer.
Furthermore, it is preferable to relax 0.1 to 20% longitudinally and/or transversely in the highest temperature zone of the heat treatment and/or in the cooling zone at the exit of the heat treatment.

<Uses of this laminated polyester film>
This laminated polyester film can be effectively used as an optical film for optical applications such as a protective film constituting a polarizing plate.

<Explanation of terms>
In this specification, when expressed as "X to Y" (X and Y are arbitrary numbers), unless otherwise specified, "X or more and Y or less" and "preferably larger than X" or "preferably Y It also includes the meaning of "less than".
In addition, when expressed as "X or more" (X is an arbitrary number) or "Y or less" (Y is an arbitrary number), "preferably larger than X" or "preferably less than Y" It also includes intent.

Hereinafter, the present invention will be described in further detail based on the following examples and comparative examples.

(1) Measurement of haze and total light transmittance According to JIS-K7136, the haze and total light transmittance of the film were measured using a turbidity meter NDH-300A manufactured by Nippon Denshoku Industries.

(2) Measurement of retardation A phase difference measuring device (KOBRA-21ADH) manufactured by Oji Scientific Instruments Co., Ltd. was used. Cut out a sample of 3.5 cm × 3.5 cm from the center in the film width direction, set it in the device so that the angle defined by this measurement device in the film width direction is 0 °, and set the incident angle to 0 ° In-plane retardation (R ₀ ) at a wavelength of 590 nm was measured.
As for the retardation in the thickness direction (Rth), the retardation at a wavelength of 590 nm at an incident angle of 50° was measured in refractive index mode.

(3) Measurement of intrinsic viscosity (dl/g) of surface layer of laminated film The flow-down time of the solution with a concentration of 1.0 g/dl at 30° C. and the flow-down time of the solvent alone were measured, and the intrinsic viscosity was calculated from the time ratio using the Huggins formula. At that time, the Huggins constant was assumed to be 0.33.
Next, the intrinsic viscosity of the entire surface layer before extrusion molding was calculated from the compounding ratio of the raw materials used for the surface layer of each laminated polyester film.

For both the surface layer and the intermediate layer, the polyester (intrinsic viscosity 0.96 dl / g) of the surface layer portion of Example 1 described later was used as a raw material, and the same molding machine and molding conditions as in Example 1 described later were used. A stretched film was produced. As a result of measuring the intrinsic viscosity of the obtained biaxially stretched film in the same manner as described above, it was found to be about 10% lower than that of the raw material.
Therefore, the intrinsic viscosity of the surface layer of the polyester film in the examples was calculated to be 0.9 times the value of the intrinsic viscosity of the entire surface layer portion before extrusion molding, and the surface layer of the actual laminated polyester film was regarded as the intrinsic viscosity of , and the values are listed in the table.

(4) Measurement of intrinsic viscosity (dl/g) of the entire laminated polyester film The laminated polyester film is dissolved in a mixed solvent of phenol/tetrachloroethane = 50/50 (mass ratio), The flow-down time of a solution with a concentration of 1.0 (g/dl) at °C and the flow-down time of the solvent alone were measured. From these time ratios, the intrinsic viscosity was calculated using the Huggins formula. At that time, the Huggins constant was assumed to be 0.33.

(5) Measurement of end tear resistance According to JIS C2318-1975, MD (machine direction) and TD (width direction) were measured at three measurement points on the right end, center, and left end of the roll sample, and cut out from each position. The average value of the three points was taken as the edge tear resistance value.

(6) Evaluation of Productivity in Slitting Process Based on the measurement results of edge tear resistance in MD (machine direction) and TD (transverse direction), productivity in the slitting process was evaluated according to the following criteria.
◎ (very good): Both MD and TD edge tear resistance are 75 N or more ○ (good): Either MD or TD edge tear resistance is 60 N or more and less than 75 N × (poor): MD or TD edge tear Any of the resistances is less than 60N

(7) Evaluation of rainbow unevenness Based on the measurement result of retardation (Rth), the rainbow unevenness of the film under polarized light was evaluated according to the following criteria.
◎ (very good): retardation (Rth) is 500 nm or less ○ (good): retardation (Rth) is greater than 500 nm and less than or equal to 900 nm × (poor): retardation (Rth) is greater than 900 nm

[material]
<Polyester polymer>
・PET-A: homopolyethylene terephthalate, heat of fusion (JIS K7121) 29.6 J/g, intrinsic viscosity 1.1 dl/g, melting point 245°C, glass transition temperature 79°C
・PET-B: homopolyethylene terephthalate, heat of fusion (JIS K7121) 31.6 J/g, intrinsic viscosity 0.82 dl/g, melting point 248°C, glass transition temperature 78°C
・PET-C: homopolyethylene terephthalate, heat of fusion (JIS K7121) 35.7 J/g, intrinsic viscosity 0.64 dl/g, melting point 250°C, glass transition temperature 78°C
・PET-D: A masterbatch (intrinsic viscosity of 0.62 dl / g) containing 0.6% by mass of silica particles with an average particle size of 3 μm in homopolyethylene terephthalate

<Polyester copolymer>
Copolyester X: Polyester copolymer containing a dicarboxylic acid component consisting of 78 mol% terephthalic acid and 22 mol% isophthalic acid and a diol component consisting of ethylene glycol, heat of fusion (JIS K7121) 0 J/g, intrinsic viscosity 0.70 dl/g, glass transition temperature 64°C
Copolyester Y: a polyester copolymer containing a dicarboxylic acid component composed of terephthalic acid and a glycol component composed of 68 mol% ethylene glycol and 32 mol% 1,4-cyclohexanedimethanol, heat of fusion (JIS K7121) 0J /g, intrinsic viscosity 0.80dl/g, glass transition temperature 70°C

[Example 1]
As the surface layer, a raw material obtained by mixing PET-A at a ratio of 70% by mass and PET-D at a ratio of 30% by mass was used.
For the intermediate layer, a raw material obtained by mixing 60% by mass of copolyester X and 40% by mass of PET-C was used.

The raw materials for the surface layer and the intermediate layer are co-extruded by separate melt extruders at an extrusion temperature of 280 ° C., and cooled and solidified on a casting drum cooled to 25 ° C. to form two types of three layers (surface layer /intermediate layer/surface layer) to obtain a non-oriented sheet.
Then, the film was stretched 3.2 times in the machine direction at 85°C with a roll stretching machine, further preheated at 90°C in a tenter, and then stretched 4.0 times in the width direction at 110°C. Finally, heat treatment was performed at 240° C. to obtain a laminated polyester film (sample) having a thickness of 50 μm (each surface layer: 0.75 μm, intermediate layer: 48.5 μm). Table 1 shows the evaluation results.

[Example 2, Reference Example 1 and Comparative Examples 1 to 3]
The procedure of Example 1 was repeated except that the composition and production conditions shown in Table 1 below were used. Table 1 shows the evaluation results.

(Discussion)
Measurement of the refractive index in the film in-plane direction and thickness direction using an Abbe refractometer manufactured by Atago Co., Ltd. revealed that the intermediate layers of Examples 1 and 2 and Reference Example 1 had no orientation.
In Comparative Example 3, as a result of using a polyester polymer for the surface layer and the intermediate layer, the productivity in the slitting process was good, but a film with low retardation (Rth) was not obtained.
In Comparative Example 1, unlike Comparative Example 3, a polyester copolymer was used in the intermediate layer, but a film with low retardation (Rth) was not obtained. This is because the intrinsic viscosity of the surface layer is low, so the surface layer cannot be made thin in order to ensure the productivity in the slitting process, and the orientation of the surface layer has an effect.
In Comparative Example 2, the surface layer was made thinner than in Comparative Example 1, and the heat setting temperature was slightly raised. It got worse.

In Examples 1 and 2 and Reference Example 1 , a polyester copolymer was used in the intermediate layer and the intrinsic viscosity of the surface layer was set to a predetermined value or higher, resulting in a significant reduction in retardation (Rth). It is considered that this is because the thickness of the oriented surface layer can be reduced and the heat setting temperature can be increased. Surprisingly, unlike Comparative Example 2, it was found that the productivity in the slitting process could be maintained extremely well even if the surface layer was made thin.

Claims (10)

A laminated polyester film comprising at least three layers of an intermediate layer containing a polyester copolymer as a main resin and two surface layers containing a polyester polymer as a main resin,
At least one surface layer has an intrinsic viscosity of 0.70 to 1.20 dl/g,
A laminated polyester film, wherein the thickness of each of the surface layers is 0.5% or more and 1.5% or less of the total thickness of the laminated polyester film and is less than 2.5 µm. 2. The laminated polyester film according to claim 1 , wherein the melting point of said polyester polymer in said surface layer is 150 to 250° C. higher than the glass transition temperature of said polyester copolymer in said intermediate layer. A laminated polyester film comprising at least three layers of an intermediate layer containing a polyester copolymer as a main resin and two surface layers containing a polyester polymer as a main resin,
At least one surface layer has an intrinsic viscosity of 0.70 to 1.20 dl/g,
The thickness of each surface layer is 0.5% or more and 1.5% or less of the entire laminated polyester film,
A laminated polyester film, wherein the melting point of the polyester polymer in the surface layer is 150 to 250° C. higher than the glass transition temperature of the polyester copolymer in the intermediate layer. 4. The laminated polyester film according to claim 3 , wherein each surface layer has a thickness of less than 2.5 [mu]m. The polyester copolymer in the intermediate layer is a polyester copolymer containing a dicarboxylic acid component containing terephthalic acid or isophthalic acid or both, and a diol component containing ethylene glycol or 1,4-cyclohexanedimethanol or both. The laminated polyester film according to any one of claims 1 to 4, characterized in that: The laminated polyester film according to any one of claims 1 to 5, wherein the polyester copolymer in the intermediate layer contains 5 mol% or more of the copolymer component. The laminated polyester film according to any one of claims 1 to 6, wherein the laminated polyester film is a biaxially stretched film and the intermediate layer has no orientation. The laminated polyester film according to any one of claims 1 to 7, characterized in that the intrinsic viscosity of the laminated film as a whole is 0.55 to 0.75 dl/g. The laminated polyester film according to any one of claims 1 to 8, wherein the thickness direction retardation (Rth) of the laminated film is 500 nm or less . An optical film using the laminated polyester film according to any one of claims 1 to 9.