TW201529317A - Multilayer stretch film - Google Patents

Abstract

Disclosed is a multilayer stretched film comprising (1) a first layer of 250 to 1,000 alternating layers, comprising a first polyester resin derived from a dicarboxylic acid as an acid component, and as an alcohol group a diol and a polyhydric alcohol having one or more hydroxyl groups; and (2) a second layer comprising a second polyester resin which is derived from the acid component ( i) a dicarboxylic acid and (ii) an ester-forming derivative of a terephthalic acid compound or terephthalic acid different from the dicarboxylic acid, and (i) a diol and (as an alcohol component) Ii) prepared with a polyhydric alcohol having one or more hydroxyl groups oriented by stretching in a single direction (x-direction), wherein the orientation (x-direction), the first layer The refractive index is greater than the refractive index of the second layer, and the thicknesses of the first layer and the second layers are continuously varied such that the maximum thickness of each layer is 1.2 to 1.5 times the minimum thickness of each layer. The multilayer stretched film selectively reflects and/or polarizes light in a specific wavelength range and produces uniform brightness in each region, and thus can be used as a reflective polarizing mode for one LCD.

Description

Multilayer stretch film Field of invention

The present invention relates to a multilayer stretched film, and more particularly to a multilayer stretched film that selectively reflects or polarizes light in a particular wavelength range and provides incident on each of the regions of the film. The light is evenly illuminated.

Background of the invention

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

Since an LCD lacks its own light source, it requires a backlight unit that illuminates the entire display device from the rear side of the LCD.

An LCD cannot completely 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 alternating layers of high refractive index and low refractive index causes optical interference, selectively reflecting or transmitting light having a specific wavelength range due to interference between the layers. It is known that when the thickness of an adjacent layer is gradually changed, a film having a sufficient number - for example, tens or hundreds (or even more) - of these alternately laminated layers is modified to a desired wavelength range. The reflection and/or polarization properties of light.

Optical principles relating 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 comprising two layers having different refractive indices can be reflected by light of a particular wavelength range depending on the refractive index difference of the two layers, the thickness of the layers, and the number of layers laminated. The wavelength of the reflected light is expressed by the following equation.

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

In Equation 1, the λ-based wavelength (nm), the m-based reflection level, the refractive indices of the individual layers of N ₁ and N ₂ are, and D ₁ and D ₂ are the thicknesses (nm) of the individual layers.

For this purpose, polyethylene-2,6-naphthalenedicarboxylate ("2,6-PEN") is generally used for one layer having a higher refractive index (see, for example, Korean Patent Laid-Open Publication No. 1997-0700585) No. and PCT Announcement No. WO95/17303). When a 2,6-PEN film is uniaxially stretched, the refractive index in the stretching direction (i.e., the x-direction) increases. On the other hand, the refractive index of the film perpendicular to the stretching direction (i.e., the y-direction) is hardly affected, and the refractive index of the film in the thickness direction (i.e., the z-direction) is remarkably lowered.

Therefore, when a reflective film is prepared for reflection in the direction of stretching (ie, x-direction) polarized light and polarized light transmitted in a direction perpendicular to the direction of stretching (ie, y-direction) due to a large difference in refractive index due to the thickness direction of the film (z-method) Partial reflection of light incident in the direction, an increase in the stretching ratio for improving the polarization property of the stretching direction causes irregular brightness and color change in each region of the film.

Accordingly, there is a need for a multilayer stretch film that provides uniform brightness and no brightness degradation and color change for incident light in each region.

Summary of invention

Accordingly, it is an object of the present invention to provide a multilayer stretched film which has excellent optical properties and which enables selective reflection and/or polarization of light in a specific wavelength range, and which provides uniform brightness and incidence for each region. The light has no brightness deterioration and color change.

According to an aspect of the invention, there is provided a multilayer film which is uniaxially stretched in the X-direction, the film comprising 250 to 1,000 layers, wherein a first layer and a second layer are alternately laminated, (1) The first layer comprises a first polyester resin derived from a dicarboxylic acid as one of the acid components, and a diol as one of the alcohol components and a polyhydric alcohol having one or more hydroxyl groups. Preparation; and (2) the second layer comprises a second polyester resin derived from the (i) dicarboxylic acid as the acid component and (ii) the terephthalic acid compound different from the dicarboxylic acid Or an ester-forming derivative of terephthalic acid, and (i) a monodiol as the alcohol component and (ii) a polyhydric alcohol having one or more hydroxyl groups, wherein x - direction, the refractive index of the first layer is greater than the refractive index of the second layer; and the thicknesses of the first layer and the second layers are continuously changed such that 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 in accordance with the present invention selectively reflects and/or polarizes light in a particular wavelength range and provides uniform brightness in each region. Therefore, it can be used as a reflective polarizing film for one of the LCDs.

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. The film is then uniaxially stretched in the x-direction.

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

In the multilayer stretched film of the present invention, the first layer comprises a polyester resin whose refractive index is changed upon stretching, and the second layer comprises a polyester resin whose refractive index hardly changes upon stretching. When the first layer is uniaxially stretched in the x-direction, it will have a higher reflectance in the x-direction. The difference in refractive index between the first layer and the second layer causes optical interference which imparts optical properties to the stretched film that selectively reflects or transmits light of a particular wavelength range.

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

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

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

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

The thicknesses of the first layer and the second layer are continuously varied such 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 first layer of a total of 145 and a second layer of a total of 146, the minimum thickness of the first and second layers individually refers to the first of the individual layers. And the minimum thickness of any of the second and second layers, and the maximum thickness of the respective first and second layers refers to the maximum thickness of any of the individual first and second layers.

a multilayer stretched film comprising a first layer and a second layer each having a fixed thickness reflects light having a specific wavelength, and a first layer and a second layer laminated with different thicknesses The stretched film can reflect light over a wide range of wavelengths. The thickness of the first layer and the second layer may be varied continuously or in a stepwise manner. Preferably, the thickness is continuously varied within the above range such that the film reflects light in the visible 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 such 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 layer And the second layer each has a thickness falling within the above range, and due to the optical interference between the layers, a film prepared therefrom can selectively and efficiently reflect light. It is not preferred if the thickness ratio between the first layer and the second layer exceeds 1.2 because the optical interference between the layers is lowered, causing the film to reduce reflectivity. The light leakage phenomenon can be limited and the degree of polarization can be maintained at 5% or lower by adjusting the thickness to 1.2 or lower.

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

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

The first layer comprises a first polyester resin derived from a dicarboxylic acid as one of the acid components, and a diol as one of the alcohol components and a polyhydric alcohol having one or more hydroxyl groups. preparation. The second layer comprises a second polyester resin which is derived from the (i) dicarboxylic acid as the acid component and (ii) one of the terephthalic acid compounds or terephthalic acid different from the dicarboxylic acid. An ester derivative is formed, and (i) a monodiol as the alcohol component and (ii) a polyhydric alcohol having one or more hydroxyl groups.

Specific examples of the dicarboxylic acid used to prepare the first layer and the second layer 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 Carboxylic acid, hydrazine dicarboxylic acid, α,β-bis(2-chlorophenoxy)ethane-4,4-dicarboxylic acid; monoaliphatic dicarboxylic acid, such as adipic acid, sebacic acid, hydrazine Diacid, decane dicarboxylic acid; monocyclic aliphatic dicarboxylic acid, such as cyclohexane dicarboxylic acid; and mixtures thereof.

Specific examples of the diol used to prepare the first layer and the second layer include ethylene glycol, propane-1,2-diol, propane-1,3-diol, butane-1,3-diol, and butyl 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, dibutyl glycol, and Polybutylene glycol.

The polyhydric alcohol having 3 or more hydroxyl groups for preparing the first layer and the second layer may be selected from the group consisting of trimethylolpropane, tris-2-hydroxyethyl isocyanate, and trishydroxyl Ethylethane, pentaerythritol, 3-hydroxy-2,2-bis(hydroxymethyl)propyl valerate of the following formula (I), and 3-hydroxy-2,2-double having the following 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 a diol, the use of a polyhydric alcohol having 3 or more hydroxyl groups improves the degree of polymerization by increasing the polymerization rate, avoids deterioration, and lowers the color-b value, thereby making the first layer and The color-b values of the second layer are similar to each other. That is, the use of a polyhydroxy alcohol having 3 or more hydroxyl groups can alleviate color mismatch attributable to color change due to refractive index difference between the first layer and the second layer in the y- and z-directions. problem.

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

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

The molar ratio of the (i) dicarboxylic acid to (ii) the terephthalic acid compound or the terephthalic acid ester forming derivative of the second layer for preparing the second layer may be from 30: 70 to 70:30, preferably in the range of 40:60 to 50:50. If (i) a dicarboxylic acid and (ii) a terephthalic acid compound other than the dicarboxylic acid or an ester-forming derivative of terephthalic acid are used in this range, the second layer is crystallized in the stretched invention. One of the layers can be minimized during stretch film stretching. Therefore, any change in the refractive index of the film can be minimized upon stretching. In other words, if the first The copolymerization ratio of the two layers is maintained within this range, and in the uniaxial or biaxial stretching of the film, in the less stretching direction (or y-direction) or the unstretching direction (or z-direction), The refractive index of the second layer can be maintained in an amount similar to that of the first layer (i.e., the layer having a high refractive index).

Further, a protective layer may be formed on each of the outermost layers of the multilayer stretched film. The protective layer may comprise a first polyester resin or a second polyester resin. The protective layer may have a thickness of 10 to 25%, preferably 13 to 20%, more preferably 15 to 18%, based on the total thickness of the multilayer stretched film. The protective film conforming to the above thickness range can prevent the laminated film including the first layer and the second layer from being damaged.

With respect to the multilayer stretched film of the present invention, the color change values represented by the following Equations 2 and 3, ^Δ x (0°) and ^Δ y (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 of 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 the above equations 2 to 5, xwo (0°) and ywo (0°) refer to 1931 CIE standard for the light incident at 0 ° angle (standard source C) measured on a spectral color x and y; xwo (60 °) and ywo (60 °) refers to the 1931 CIE standard for 60 Angle incident light (standard source C) is measured on a spectral color x and y; xw (0°) and yw (0°) refer to a multilayer at 0° angle of incidence at the 1931 CIE standard. The light of the stretched film is measured on a spectral color x and y; and xw (60°) and yw (60°) refer to a multilayer stretched film at an incident angle of 60° under the 1931 CIE standard. Light is measured on a spectrum of colors x and y.

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

Example 1

<Preparation of First Polyester Resin>

Into a reactor equipped with a stirrer and a distillation column, 100 moles of 2,6-naphthalene dicarboxylic acid (NDC) is injected as an acid component, and 100 moles of ethylene glycol and 0.075 Percent by weight of pentaerythritol (PEL) (based on total acid components) is used as the alcohol component. Based on the total weight of the 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, a previously prepared cerium oxide slurry is added, and the resulting mixture is subjected to a polycondensation reaction by adding a conventional polycondensation catalyst to obtain a first polyester resin having an intrinsic viscosity of 0.56 dL/g. .

<Preparation of second polyester resin>

Injecting 35 moles of NDC and 65 moles of dimethyl terephthalic acid (DMT) into a reactor equipped with a stirrer and a distillation column The acid component and 100 mole parts of ethylene glycol and 0.075% by weight of PEL (based on the total acid component) were used as the alcohol component. Based on the total weight of the 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, a previously prepared cerium oxide 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 intrinsic 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 of from 100 to 150 ppm (before use). Then, the resin is supplied to a first extruder and a second extruder, heated therebetween to 290 ° C, which is in a molten state from 20 to 40 ° C above its melting point (Tm). . It is then supplied to a multi-layer feedblock.

The resin supplied to the multilayer feed head was divided such that 160 first layers were formed from the first polyester resin and 161 second layers were formed from the second polyester resin. These first layers and the second layers are alternately laminated. The thicknesses of the first layer and the second layer are continuously varied such 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, the first polyester resin is extruded to each of the outermost sides of the laminate film at a thickness of 15% to 18% based on the total thickness of the laminate film to form a protective layer. Thus, an unstretched laminate film having a total of 323 layers 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 for the second polyester resin was changed in accordance with 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 First Polyester Resin>

In a reactor equipped with a stirrer and a distillation column, 100 moles of NDC was injected as an acid component and 100 moles of ethylene glycol as an alcohol component. Based on the total weight of the 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, a previously prepared cerium oxide slurry is added, and the resulting mixture is subjected to a polycondensation reaction by adding a conventional polycondensation catalyst to obtain a first polyester resin having an inherent value of 0.6 dL/g. Viscosity.

<Preparation of 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 an acid component and 100 moles of ethylene glycol as an alcohol component. Based on the total weight of the 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, a previously prepared cerium oxide 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 intrinsic 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 First Polyester Resin>

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

<Preparation of 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 an acid component and 100 moles of ethylene glycol as an alcohol component. Based on the total weight of the 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, a previously prepared cerium oxide slurry is added, and the formed mixture is subjected to a polycondensation reaction by adding a conventional polycondensation catalyst to obtain a second polyester resin. It has an intrinsic 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 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 the multilayer stretched film 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 subjected to differential scanning calorimetry (DSC, Perkin-Elmer) test, and one of the endothermic curves was obtained from a heating rate of 20 ° C /min. The Tg and Tm were measured.

(3) Refractive index

The resins prepared in Examples 1 to 4 and Comparative Examples 1 and 2 were individually extruded and drawn. The stretched film thus obtained was measured using an ABBE refractometer and a tantalum coupler (Sairon Tech.) to measure its 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 for thickness by a thickness measuring device (Beta ray, Yokogawa).

(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 film, and white was displayed on the LCD. Thereafter, the brightness of the center point was measured using a color analyzer (CA2000, Minolta). Brightness is measured before and after sample embedding. 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

The procedure of (5) above was repeated to evaluate the color shift between before and after each sample was embedded. Colors x and y are measured using a standard light source C under the 1931 CIE standard.

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

The x and y values were measured at 0° and 60° angles of incidence prior to embedding the multilayer stretch film of the present invention. After embedding the multilayer stretched film of the present invention, the x and y values of the incident angles of 0° and 60° were measured by the same procedure.

The color displacement of each incident angle is calculated according to Equations 2 to 5 below: [Equation 2] Δx (0°) = | xw (0°) - xwo (0°) |

[Equation 3] Δy(0°)=| yw(0°)-ywo(0°)|

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

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

In Equations 2 to 5 above, xwo (0°) and ywo (0°) refer to the color x measured on a spectrum of light incident at a 0° angle (standard source C) under the 1931 CIE standard. y;xwo(60°) and ywo(60°) are the colours x and y measured on a spectrum of light incident at 60° angle (standard source C) under the 1931 CIE standard; xw (0°) And yw(0°) means the color x and y of a spectrum measured by light passing through a multilayer stretch film at an incident angle of 0° under the 1931 CIE standard; and xw (60°) and yw (60°) is the color x and y measured on a spectrum measured by light passing through a multilayer stretch film at an incident angle of 60° under the 1931 CIE standard.

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

Claims (13)

a multilayer film uniaxially stretched in the x-direction, the film comprising 250 to 1,000 layers, wherein a first layer and a second layer are alternately laminated, (1) the first layer comprises a first polyester a resin prepared from a dicarboxylic acid as one of the acid components, and a diol as one of the alcohol components and a polyhydric alcohol having one or more hydroxyl groups; and (2) the second layer A second polyester resin comprising (i) a dicarboxylic acid as the acid component and (ii) an ester forming compound different from the terephthalic acid compound or terephthalic acid of the dicarboxylic acid a derivative, and as the alcohol component (i) monodiol and (ii) a polyhydric alcohol having one or more hydroxyl groups, wherein the first layer is refracted in the x-direction The rate is greater than the refractive indices of the second layers; and the thicknesses of the first layers and the second layers are continuously changed such that the maximum thickness of the first layers is 1.2 of the minimum thickness of the first layers Up 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. The multilayer stretched film of claim 1, wherein (i) the dicarboxylic acid pair (ii) the terephthalic acid compound or the ester-forming derivative of the terephthalic acid is used to prepare the second layer The molar ratio is in the range from 30:70 to 70:30. The multilayer stretched film of claim 1, wherein the ester-forming derivative of the terephthalic acid is dimethyl terephthalic acid. The multilayer stretched film of claim 1, wherein the polyhydric alcohol is selected from the group consisting of Trimethylolpropane, tris-2-hydroxyethyl isocyanate, trimethylolethane, pentaerythritol, 3-hydroxy-2,2-bis(hydroxymethyl)propylvalerate, and 3 At least one of the group consisting of -hydroxy-2,2-bis(hydroxymethyl)propyl 4-phenylvalerate. The multilayer stretched film of claim 1, wherein the amount of the polyhydric alcohol is from 0.01 to 0.2% by weight based on 100% by weight of all the acid components per layer. The multilayer stretched film of claim 1, wherein the polyhydric alcohol is 3-hydroxy-2,2-bis(hydroxymethyl)propylvalerate. The multilayer stretched film of 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 of claim 1, wherein the refractive index difference between the first layer and the second layer ranges from 0.1 to 0.5 in the x-direction. The multilayer stretched film of claim 1, wherein the first layer and the second layer are in a y-direction perpendicular to an x-direction on the surface of the film, and a z-direction in a thickness direction of the film 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 1.1 to 2 times in the y-direction. The multilayer stretched film of claim 1, wherein the color change values of ^Δ x (0°) and ^Δ y (0°) expressed by the following equations 2 and 3 are 0.1 or less, and the following equations 5 and 6 are used. The color change values of ^Δ x (60°) and ^Δ y (60°) are 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 031 CIE standard for 0° The angle incident light (standard source C) is measured on a spectral color x and y; xwo (60°) and ywo (60°) refer to light incident at 60° angle under the 1931 CIE standard (standard Source C) The color x and y measured on a spectrum; xw (0°) and yw (0°) refer to the light passing through a multilayer stretch film at an incident angle of 0° under the 1931 CIE standard. Measured on a spectral color x and y; and xw (60°) and yw (60°) are measured at 1931 CIE for light passing through a multilayer stretched film at an incident angle of 60°. The color x and y on a spectrum.