TWI649201B - Multilayer stretch film - Google Patents

Abstract

A multilayer stretched film is disclosed, which comprises 250 to 1,000 alternating layers (1) a first layer, which comprises a first polyester resin, which is a dicarboxylic acid as an acid component and an alcohol group Prepared by one part diol and one polyhydric alcohol having 3 or more hydroxyl groups; and (2) a second layer, which contains a second polyester resin, which is derived from the acid component ( i) a dicarboxylic acid and (ii) a terephthalic acid compound or an ester-forming derivative of terephthalic acid which is different from this dicarboxylic acid, and (i) a diol and ( ii) Prepared by a polyhydric alcohol having one or more hydroxyl groups, the film is oriented by stretching in a single direction (x-direction), where, in this orientation (x-direction), the first layer The refractive index is greater than the refractive index of the second layer, and the thickness of the first layer and the second layer is continuously changed so that the maximum thickness of each layer is 1.2 to 1.5 times the minimum thickness of each layer. The multi-layer stretched film can selectively reflect and/or polarize light in a specific wavelength range, and produce uniform brightness in each area. Therefore, it can be used as a reflective polarizing mode for an LCD.

Description

Multi-layer stretched film Field of invention

The present invention relates to a multi-layer stretched film, and more particularly, a multi-layer stretched film, which can selectively reflect or polarize light in a specific wavelength range and provide incidence on each area of the film The light shines evenly.

Background of the invention

A liquid crystal display (LCD) is one of the most widely used flat panel displays today. Generally, an LCD is a display device that uses one of the dimming properties of liquid crystal. In an LCD, an electric field is applied to liquid crystal with a specific molecular arrangement to change its molecular arrangement. Changes in the molecular arrangement cause changes in the optical properties of the liquid crystal, such as birefringence, optical activity, dichroism, and light scattering properties, which translate into visual changes.

Because an LCD lacks its own light source, it requires a backlight unit, which illuminates the entire display device from behind the LCD.

An LCD cannot fully penetrate the light generated by its backlight unit. Therefore, the brightness of a panel is important. Various films have been used to improve the brightness of a panel. A typical example is a reflective polarizing film.

A film comprising a plurality of layers with high refractive index and low refractive index laminated alternately causes optical interference. Due to the interference between these layers, light with a specific wavelength range is selectively reflected or transmitted. It is known that when the thickness of adjacent layers changes gradually, a film with a sufficient number-for example, tens or hundreds (or even more)-of these alternately laminated layers will be improved in a desired wavelength range The reflection and/or polarization properties of light.

The optical principles related to this have been disclosed, for example, in Vasicek, Optics of Thin Film (1960), pages 100 to 139 and pages 69 and 70.

A multilayer film containing two layers with different refractive indexes can reflect light in a specific wavelength range according to the difference in refractive index of the two layers, the thickness of these layers, and the number of laminated layers. The wavelength of reflected light is expressed by the following equation.

[Equation 1] λm=(2/m)(N ₁ D ₁ +N ₂ D ₂ )

In Equation 1, λ is the wavelength (nm), m is the reflection level, N ₁ and N ₂ are the refractive index of the individual layers, and D ₁ and D ₂ are the thickness (nm) of the individual layers.

For this purpose, polyethylene-2,6-naphthalene dicarboxylate ("2,6-PEN") is generally used for a layer having a higher refractive index (see, for example, Korean Patent Early Publication No. 1997-0700585 Case and PCT Announcement No. WO95/17303 case). When a 2,6-PEN film is uniaxially stretched, the refractive index in the stretching direction (ie, x-direction) increases. On the other hand, the refractive index of the method perpendicular to the stretching direction on the film surface (ie, y-direction) is hardly affected, and the refractive index in the thickness direction of the film (ie, z-direction) is significantly reduced.

Therefore, when a reflective film is prepared for reflection in the stretching direction (Ie, x-direction) polarized light and polarized light transmitted in a direction perpendicular to the stretching direction (ie, y-direction), due to a large refractive index difference in the thickness direction of the film (z-method) caused by a tilt The light incident in the direction is partially reflected, and the increase in the stretching ratio used to improve the polarizing properties of the stretching direction will cause irregular brightness and color changes in each area of the film.

Therefore, there is a need for a multi-layer stretched film that can provide uniform brightness with no deterioration in brightness and color change for incident light in each region.

Summary of the invention

Therefore, an object of the present invention is to provide a multilayer stretched film that has excellent optical properties and can selectively reflect and/or polarize light in a specific wavelength range, and can provide uniform brightness, and for each region of incidence The light does not deteriorate and the color changes.

According to one aspect of the present invention, there is provided a multilayer film which is uniaxially stretched in the X-direction. The film comprises 250 to 1,000 layers, wherein a first layer and a second layer are alternately laminated, (1) This first layer contains a first polyester resin derived from a dicarboxylic acid as an acid component, a diol as an alcohol component and a polyhydric alcohol having 3 or more hydroxyl groups Preparation; and (2) This second layer contains a second polyester resin which is derived from (i) a dicarboxylic acid and (ii) a terephthalic acid compound different from this dicarboxylic acid as an acid component Or an ester-forming derivative of terephthalic acid, and (i) a glycol and (ii) a polyhydric alcohol having one or more hydroxyl groups as the alcohol component, wherein, in x -Direction, the refractive index of the first layer is greater than the refractive index of the second layer; and the thickness of these first layers and these second layers is continuously changed, making the first layer The maximum thickness is 1.2 to 1.5 times the minimum thickness of the first layer, and the maximum thickness of the second layer is 1.2 to 1.5 times the minimum thickness of the second layer.

The multilayer stretched film according to the present invention can selectively reflect and/or polarize light in a specific wavelength range and provide uniform brightness in each area. Therefore, it can be used as a reflective polarizing film for an LCD.

Detailed description of the invention

A multilayer stretched film of the present invention is formed by alternately laminating a first layer and a second layer. Then, the film was uniaxially stretched in the x-direction.

The first layer and the second layer are alternately laminated to form a laminate film, which may have 250 to 1,000 layers, preferably 300 to 500 layers.

In the multilayer stretched film of the present invention, the first layer contains a polyester resin whose refractive index changes during stretching, and the second layer contains a polyester resin whose refractive index hardly changes during stretching. When the first layer is uniaxially stretched in the x-direction, it will have a higher reflectivity in the x-direction. The difference in refractive index between the first layer and the second layer causes optical interference, which gives the stretched film optical properties to selectively reflect or transmit light in a specific wavelength range.

Conversely, the film does not have the ability to reflect light in a direction perpendicular to the stretching direction on the film (ie, the y-direction) because there is no substantial difference in the refractive index of the first layer and the second layer. Therefore, the film will have the polarizing property of reflecting light in only one direction.

The film can be stretched 3 to 7 times in the x-direction, preferably 4 to 6 times.

The difference in refractive index in the x-direction between the first layer and the second layer attributable to stretching may range from 0.1 to 0.5, preferably 0.2 to 0.4. If the refractive index difference falls within this range, the reflective properties of the film will be effectively improved.

Each difference in refractive index between the first layer and the second layer in the y and z directions may be 0.05 or less, preferably 0.04 or less, and more preferably 0.03 or less. If the refractive index difference in the y and z directions is 0.05 or less, when light is incident in an oblique direction, color change can be avoided.

The thicknesses of the first and second layers are continuously changed so that the maximum thickness of the individual first and second layers is 1.2 to 1.5 times the minimum thickness of the individual first and second layers.

If a multilayer stretched film is formed by, for example, alternately laminating a total of 145 first layers and a total of 146 second layers, the minimum thickness of the individual first and second layers refers to the individual first And the minimum thickness of any of the second and second layers, and the maximum thickness of the individual first and second layers refers to the maximum thickness of any of the individual first and second layers.

A multi-layer stretched film including a first layer and a second layer each having a fixed thickness reflects light having a specific wavelength, and a multi-layer laminated with a first layer and a second layer having different thicknesses The stretched film can reflect light in a wide range of wavelengths. The thickness of the first layer and the second layer can be changed continuously or in a stepwise manner. Preferably, the thickness can be continuously changed within the above range, so that the film reflects light in the visible light wavelength range.

Each of the first layer and the second layer may have a thickness of 0.05 to 0.5 μm, preferably 0.05 to 0.3 μm. The thickness is adjusted so that the ratio of the average thickness of the first layer to the average thickness of the second layer is preferably from 0.8 to 1.2. When the first floor Each of the second layer and the second layer has a thickness that falls within the above range. Due to the optical interference between these layers, a film prepared therefrom can selectively and efficiently reflect light. It is not preferable if the thickness ratio between the first layer and the second layer exceeds 1.2, because the optical interference between these layers is reduced, causing the film to reduce reflectivity. By adjusting the thickness to 1.2 or lower, the light leakage phenomenon can be limited and the degree of polarization can be maintained at 5% or lower.

The multilayer stretched film of the present invention can also be stretched by 1.1 to 2 times in a direction perpendicular to the x-direction (or y-direction) on the film surface, preferably 1.1 to 1.6 times.

The stretching in the y-direction is performed to compensate for the reduced refractive index in the y-direction caused by the stretching in the x-direction. The adjustment of this stretching ratio makes the difference in refractive index between two adjacent layers larger in the direction of greater stretching, and makes the difference in refractive index between two adjacent layers smaller in the direction of less stretching, thus limiting the transmission of light Color changes.

The first layer contains a first polyester resin derived from a dicarboxylic acid as an acid component, a diol as an alcohol component and a polyhydric alcohol having 3 or more hydroxyl groups preparation. The second layer contains a second polyester resin which is derived from (i) a dicarboxylic acid and (ii) a terephthalic acid compound or one of terephthalic acid different from this dicarboxylic acid as an acid component An ester-forming derivative, and (i) a monodiol and (ii) a polyhydric alcohol having one or more hydroxyl groups as an alcohol component.

Particular examples of dicarboxylic acids used to prepare the first and second layers include an aromatic dicarboxylic acid, such as terephthalic acid, dimethyl terephthalic acid, isophthalic acid, dimethyl-2, 5-Naphthalene dicarboxylic acid, naphthalene dicarboxylic acid, cyclohexane dicarboxylic acid, diphenoxyethane dicarboxylic acid, diphenyl dicarboxylic acid, diphenyl ether dicarboxylic acid Carboxylic acid, anthracene dicarboxylic acid, α,β-bis(2-chlorophenoxy)ethane-4,4-dicarboxylic acid; an aliphatic dicarboxylic acid, such as adipic acid, azelaic acid, decane Diacid, decane dicarboxylic acid; monocyclic aliphatic dicarboxylic acid, such as cyclohexane dicarboxylic acid; and mixtures of these.

Particular examples of diols used to prepare the first and second layers include ethylene glycol, propane-1,2-diol, propane-1,3-diol, butane-1,3-diol, butane Alkane-1,4-diol, butyne-1,4-diol, pentane-1,5-diol, neopentyl glycol, 1,4-bis(hydroxymethyl)cyclohexane, 2- Methylpropane-1,3-diol, methylpentanediol, diethylene glycol, diethylene glycol, tetraethylene glycol, polyethylene glycol, dipropylene glycol, polypropylene glycol, dibutylene glycol, and Polybutylene glycol.

The polyhydric alcohol having 3 or more hydroxyl groups used for preparing the first layer and the second layer may be selected from trimethylolpropane, tri-2-hydroxyethyl isocyanate, trimethylol Ethane, pentaerythritol, 3-hydroxy-2,2-bis(hydroxymethyl)propyl valerate with the following chemical formula (I), and 3-hydroxy-2,2-bis with the following chemical formula (II) At least one of the groups consisting of (hydroxymethyl)propyl 4-phenylvalerate:

The 3-hydroxy-2,2-bis(hydroxymethyl)propylvalerate compound of formula (I) can be prepared by the following reaction scheme I.

Compared with diols, the use of polyhydroxy alcohols with 3 or more hydroxyl groups improves the degree of polymerization by increasing the polymerization rate, avoids deterioration, and reduces the color-b value. Therefore, the first layer and The color-b values of the second layer are similar to each other. That is, the use of polyhydric alcohols having 3 or more hydroxyl groups can reduce the color mismatch that can be attributed to the color change caused by the difference in refractive index between the first and second layers in the y- and z-directions problem.

In the preparation of the first layer and the second layer, the amount of polyhydroxy alcohol having 3 or more hydroxyl groups may be 0.01 to 0.2% by weight based on 100% by weight of all acid components in each layer. It is preferably 0.05 to 0.1% by weight.

In order to prepare the second polyester resin, a terephthalic acid compound different from one of dicarboxylic acids or an ester-forming derivative of one of terephthalic acids is also used together with this dicarboxylic acid. The ester-forming derivative of terephthalic acid may be dimethyl terephthalic acid.

The molar ratio of (i) dicarboxylic acid used to prepare the second layer to (ii) a terephthalic acid compound or an ester-forming derivative of terephthalic acid different from this dicarboxylic acid can be from 30: 70 to 70:30, preferably 40:60 to 50:50. If (i) dicarboxylic acid and (ii) a terephthalic acid compound different from this dicarboxylic acid or an ester-forming derivative of terephthalic acid are used in this range, the crystallization of the second layer will stretch the invention One of the multi-layer stretched film can reach the minimum during. Therefore, during stretching, any change in the refractive index of the film can be minimized. In other words, if The copolymerization ratio of the two layers is maintained within this range. When the film is uniaxially or biaxially stretched, in the less stretched direction (or y-direction) or unstretched direction (or z-direction), the first The refractive index of the second layer can be maintained in an amount similar to that of the first layer (ie, the layer with a high refractive index).

Furthermore, a protective layer may be formed on each outermost layer of the multilayer stretched film. The protective layer may include the first polyester resin or the second polyester resin. Based on the total thickness of the multilayer stretched film, the thickness of the protective layer may be 10 to 25%, preferably 13 to 20%, and more preferably 15 to 18%. The protective film conforming to the above thickness range can avoid damage to the laminated film including the first layer and the second layer.

For the multilayer stretched film of the present invention, the color change values represented by the following equations 2 and 3, ^Δx (0°) and ^Δy (0°), may be 0.1 or less, preferably 0.09 or less. The color change values represented by the following equations 4 and 5, ^Δ x (60°) and ^Δ y (60°), may be 0.2 or less, preferably 0.15 or less. If the color change value falls outside these ranges, the color mismatch problem occurs when light is incident in an oblique direction.

[Equation 2] ^△ x(0°)=| xw(0°)-xwo(0°)|

[Equation 3] ^△ y(0°)=| yw(0°)-ywo(0°)|

[Equation 4] ^△ x(60°)=| xw(60°)-xwo(60°)|

[Equation 5] ^△ y(60°)=| yw(60°)-ywo(60°)|

In equations 2 to 5 above, xwo(0°) and ywo(0°) refer to The colors x and y in a spectrum measured under the 1931 CIE standard for light incident at 0° angle (standard source C); xwo (60°) and ywo (60°) refer to the 1931 CIE standard for the 60 ° Angles of incident light (standard source C) measure the colors x and y in a spectrum; xw (0°) and yw (0°) refer to the 1931 CIE standard for passing through a multi-layer at an incident angle of 0° The colors x and y of a spectrum measured by the light of the stretched film; and xw (60°) and yw (60°) refer to the 1931 CIE standard for passing through a multilayer stretched film at an incident angle of 60° Light measures the colors x and y on a spectrum.

In the following, the present invention is further illustrated by the following examples, but these examples are provided for illustrative purposes only, and the present invention is not limited to these examples.

Example 1

<Preparation of the first polyester resin>

Into a reactor equipped with a stirrer and a distillation column, inject 100 molar parts of 2,6-naphthalene dicarboxylic acid (NDC) as an acid component, and 100 molar parts of ethylene glycol and 0.075 Weight percent pentaerythritol (PEL) (based on the total acid component) is used as the alcohol component. Based on the total weight of NDC, 0.030% by weight of manganese acetate was added to the reactor as a transesterification catalyst. The reactor was heated to 250°C to carry out a transesterification reaction. Thereafter, one of the previously prepared silica slurry was added, and the resulting mixture was subjected to a polycondensation reaction by adding a conventional polycondensation catalyst to obtain a first polyester resin with an inherent viscosity of 0.56 dL/g .

<Preparation of the second polyester resin>

In a reactor equipped with a stirrer and a distillation tower, inject 35 mol of NDC and 65 mol of dimethyl terephthalic acid (DMT) as It is an acid component and 100 mol ethylene glycol and 0.075% by weight of PEL (based on the total acid component) as the alcohol component. Based on the total weight of NDC, 0.030% by weight of manganese acetate was added to the reactor as a transesterification catalyst. The reactor was heated to 250°C to carry out a transesterification reaction. Thereafter, one of the previously prepared silica slurry was added, and the resulting mixture was subjected to a polycondensation reaction by adding a conventional polycondensation catalyst to obtain a second polyester resin with an inherent viscosity of 0.62 dL/g .

<Preparation of multilayer laminate film>

Each of the first polyester resin and the second polyester resin prepared as above was dried using a dehumidifying dryer and a paddle dryer so that the water content of the resin was in the range from 100 to 150 ppm (before use). Then, the resin is supplied to a first extruder and a second extruder, and heated to 290°C in between, which falls within a range from 20 to 40°C above its melting point (Tm) in a molten state . Then, it is supplied to a multi-layer feeding head.

The resin supplied to the multi-layer feed head is divided so that 160 first layers are formed from the first polyester resin and 161 second layers are formed from the second polyester resin. These first layers and these second layers are alternately laminated. The thicknesses of the first layer and the second layer continuously change so that the maximum thickness of the individual first and second layers is 1.5 times the minimum thickness of the individual first and second layers. A laminate film having a total of 321 layers was formed.

Thereafter, based on the total thickness of the laminate film, the first polyester resin is extruded on each outermost side of the laminate film with a thickness of 15% to 18% to form a protective layer. Thus, an unstretched laminate film having one of 323 layers in total was obtained.

The unstretched laminate film thus obtained was uniaxially stretched 5.2 times in the x-direction at 140°C, and then thermally cured at 120°C to obtain a multilayer stretched film having a thickness of 34 μm.

Examples 2 to 4

The procedure of Example 1 was repeated, except that the ratio of the alcohol component used in the second polyester resin was changed according to Table 1 below to obtain a uniaxially stretched multilayer film.

Examples 5 to 8

Each of the multilayer stretched films obtained in Examples 1 to 4 was stretched 1.2 times in the y-axis direction to obtain a biaxially stretched multilayer film.

Comparative example 1

<Preparation of the first polyester resin>

In a reactor equipped with a stirrer and a distillation column, 100 moles of NDC were injected as an acid component and 100 moles of ethylene glycol as a monoalcohol component. Based on the total weight of NDC, 0.030% by weight of manganese acetate was added to the reactor as a transesterification catalyst. The reactor was heated to 250°C to carry out a transesterification reaction. Thereafter, one of the previously prepared silica slurry is added, and the resulting mixture undergoes a polycondensation reaction by adding a conventional polycondensation catalyst to obtain a first polyester resin, which has an inherent property of 0.6 dL/g Viscosity.

<Preparation of the second polyester resin>

In a reactor equipped with a stirrer and a distillation column, 48 moles of NDC and 52 moles of DMT were injected as the acid component and 100 moles of ethylene glycol as the monool component. Based on the total weight of NDC, 0.030% by weight of manganese acetate was added to the reactor as a transesterification catalyst. The reactor was heated to 250°C to carry out a transesterification reaction. Thereafter, one of the previously prepared silica slurry is added, and the resulting mixture is subjected to a polycondensation reaction by adding a conventional polycondensation catalyst to obtain a second polyester resin having an inherent viscosity of 0.74 dL/g .

<Preparation of multilayer laminate film>

The procedure of Example 1 was repeated, except that the first polyester resin and the second polyester resin prepared as above were used to obtain a uniaxially stretched multilayer film.

Comparative example 2

<Preparation of the first polyester resin>

The procedure of Comparative Example 1 was repeated to obtain a first polyester resin.

<Preparation of the second polyester resin>

In a reactor equipped with a stirrer and a distillation column, 48 moles of NDC and 52 moles of DMT were injected as the acid component and 100 moles of ethylene glycol as the monool component. Based on the total weight of NDC, 0.030% by weight of manganese acetate was added to the reactor as a transesterification catalyst. The reactor was heated to 250°C to carry out a transesterification reaction. Thereafter, one of the previously prepared silica slurry is added, and the resulting mixture undergoes a polycondensation reaction by adding a conventional polycondensation catalyst to obtain a second polyester resin, which has It has an inherent viscosity of 0.74dL/g.

<Preparation of multilayer laminate film>

The procedure of Example 1 was repeated, except that the first polyester resin and the second polyester resin prepared as above were used to obtain a uniaxially stretched multilayer film.

Comparative Examples 3 and 4

Each of the multilayer stretched films obtained in Comparative Examples 1 to 2 was stretched 1.2 times in the y-direction to obtain a biaxially stretched multilayer film.

Experimental example

The physical properties of the first and second polyester resins and multilayer stretched films prepared in Examples 1 to 8 and those prepared in Comparative Examples 1 to 4 were evaluated as follows. The results are shown in Table 2.

(1) Copolymerization ratio

The polyester resin prepared as above was evaluated for the content of NDC and DMT using ¹ H-NMR.

(2) Glass transition temperature (Tg) and melting point (Tm)

Each of the polyester resins prepared in Example 1 and Comparative Example 1 was tested using differential scanning calorimetry (DSC, Perkin-Elmer), and a peak of an endothermic curve was obtained from a heating rate of 20°C/min Measure its Tg and Tm.

(3) Refractive index

The resins prepared in Examples 1 to 4 and Comparative Examples 1 and 2 were individually extruded and stretched. The stretched film thus obtained was tested using an ABBE refractometer and a Lei coupler (Sairon Tech.) to measure the refractive index equal to the uniaxial stretching direction and the direction perpendicular thereto.

(4) Thickness

The multilayer stretched films prepared in Examples 1 to 8 and Comparative Examples 1 to 4 were tested using a thickness measuring device (Beta ray, Yokogawa) to measure their equivalent thickness.

(5) Brightness enhancement rate

Each laminate film sample was embedded in an LCD panel (Samsung UN46D 6900, prepared in June 2011) between a polarizer and a thin film, and white was displayed on the LCD. Thereafter, the brightness of the central point is measured using a color analyzer (CA2000, Minolta). The brightness is measured before and after embedding the sample. The brightness enhancement is calculated using Equation 6 below: [Equation 6] Brightness enhancement rate = brightness after embedding (L ₁ )/brightness before embedding (L _o )×100

(6) Color shift

Repeat the procedure of (5) above to evaluate the color shift between each sample before and after embedding. The colors x and y were measured using a standard light source C under the 1931 CIE standard.

The color displacement of the multilayer stretched film of the present invention is measured as follows.

Before embedding the multilayer stretched film of the present invention, the x and y values were measured at 0° and 60° incidence angles. After embedding the multilayer stretched film of the present invention, the same procedure was repeated to measure the x and y values at 0° and 60° incidence angles.

The color displacement at each angle of incidence is calculated according to the following equations 2 to 5: [Equation 2]△x(0°)=| xw(0°)-xwo(0°)|

[Equation 3]△y(0°)=| yw(0°)-ywo(0°)|

[Equation 4]△x(60°)=| xw(60°)-xwo(60°)|

[Equation 5]△y(60°)=| yw(60°)-ywo(60°)|

In equations 2 to 5 above, xwo (0°) and ywo (0°) refer to the color x and x in a spectrum measured under the 1931 CIE standard for light incident at 0° (standard source C) y; xwo (60°) and ywo (60°) refer to the colors x and y in a spectrum measured under the 1931 CIE standard for light incident at a 60° angle (standard source C); xw (0° ) And yw (0°) refer to the colors x and y in a spectrum measured under the 1931 CIE standard for light passing through a multilayer stretched film at an incidence angle of 0°; and xw (60°) and yw (60°) refers to the colors x and y in a spectrum measured under the 1931 CIE standard for light passing through a multilayer stretched film at an incident angle of 60°.

According to the method outlined above, for a value measured at an incident angle of 0° of 0.1 or less and for a value measured at an incident angle of 60° of 0.2 or less, the color displacement system is evaluated as good.

Claims (12)

A multilayer film uniaxially stretched in the x-direction. The film includes 250 to 1,000 layers, wherein a first layer and a second layer are alternately laminated. (1) The first layer includes a first polyester The resin is prepared from dicarboxylic acid which is one of the acid components, and diol which is one of the alcohol components and one polyhydric alcohol having 3 or more hydroxyl groups; and (2) the second layer Contains a second polyester resin derived from (i) a dicarboxylic acid and (ii) a terephthalic acid compound different from the dicarboxylic acid or an ester-forming terephthalic acid as an acid component Derivatives, and (i) a glycol and (ii) a polyhydric alcohol having one or more hydroxyl groups as alcohol components are prepared, wherein, in the x-direction, the refraction of these first layers The rate is greater than the refractive index of the second layers; and the thicknesses of the first layers and the second layers are continuously changed so that the maximum thickness of the first layers is 1.2 of the minimum thickness of the first layers To 1.5 times, and the maximum thickness of the second layers is 1.2 to 1.5 times the minimum thickness of the second layer, and wherein the polyhydric alcohol is selected from trimethylolpropane, triisocyanate tri- 2-hydroxyethyl, trimethylolethane, pentaerythritol, 3-hydroxy-2,2-bis(hydroxymethyl)propyl valerate, and 3-hydroxy-2,2-bis(hydroxymethyl) At least one of the groups consisting of propyl 4-phenylvalerate. The multilayer stretched film according to claim 1, wherein (i) the dicarboxylic acid pair (ii) the terephthalic acid compound or the ester-forming derivative of the terephthalic acid used to prepare the second layer The molar ratio is in the range from 30:70 to 70:30. The multilayer stretched film according to claim 1, wherein the ester-forming derivative of terephthalic acid is dimethyl terephthalic acid. The multilayer stretched film according to claim 1, wherein the amount of the polyhydric alcohol is 0.01 to 0.2% by weight based on 100% by weight of all acid components in each layer. The multilayer stretched film according to claim 1, wherein the polyhydric alcohol is 3-hydroxy-2,2-bis(hydroxymethyl)propylvalerate. The multilayer stretched film according to claim 1, wherein the polyhydric alcohol is 3-hydroxy-2,2-bis(hydroxymethyl)propyl 4-phenylvalerate. The multilayer stretched film of claim 1, wherein the ratio of the average thickness of the first layers to the average thickness of the second layers ranges from 0.8 to 1.2. The multilayer stretched film according to claim 1, wherein, in the x-direction, the refractive index difference between the first layer and the second layer ranges from 0.1 to 0.5. The multilayer stretched film of claim 1, wherein in the y-direction perpendicular to the x-direction on the film surface and in the z-direction in the thickness direction of the film, the first layers and the second Each refractive index difference of the layers is 0.05 or less. The multilayer stretched film of claim 1, wherein the film is stretched 3 to 7 times in the x-direction. The multilayer stretched film of claim 1, wherein the film is stretched by 1.1 to 2 times in the y-direction. The multilayer stretched film according to claim 1, wherein the color change values of △x(0°) and △y(0°) expressed by the following equations 2 and 3 are 0.1 or less, and the following equations 5 and 6 The color change value of expressed △x(60°) and △y(60°) is 0.2 or less: [Equation 2]△x(0°)=|xw(0°)-xwo(0°)|[ Equation 3]△y(0°)=|yw(0°)-ywo(0°)|[Equation 4]△x(60°)=|xw(60°)-xwo(60°)|[Equation 5 ]△y(60°)=|yw(60°)-ywo(60°)|In the above equations 2 to 5, xwo(0°) and ywo(0°) refer to the 0° under 1931 CIE standard The colors x and y in a spectrum measured by angle-incident light (standard source C); xwo (60°) and ywo (60°) refer to the light incident at 60° angle under the 1931 CIE standard (standard Source C) measured colors x and y on a spectrum; xw (0°) and yw (0°) refer to the light source that passes through a multilayer stretched film at an incidence angle of 0° under the 1931 CIE standard The colors x and y measured on a spectrum; and xw (60°) and yw (60°) refer to the measurement of light passing through a multilayer stretched film at an incidence angle of 60° under the 1931 CIE standard The colors x and y in a spectrum.