CN107031160B

CN107031160B - Optical polyester film

Info

Publication number: CN107031160B
Application number: CN201610849574.5A
Authority: CN
Inventors: 曹铉
Original assignee: Kolon Corp
Current assignee: Kolon Corp
Priority date: 2015-09-23
Filing date: 2016-09-23
Publication date: 2020-01-31
Anticipated expiration: 2036-09-23
Also published as: KR20170035715A; CN107031160A; KR102267596B1

Abstract

The present invention relates to kinds of polyester films including a core layer and surface layers on both surfaces thereof, and more particularly, to kinds of optical polyester multilayer films that achieve high transparency and brightness and have excellent dimensional stability.

Description

Optical polyester film

Technical Field

The present invention relates to optical films, and more particularly, to optical polyester multilayer films, which include a core layer and skin layers on both surfaces thereof, achieve high transparency and brightness, and have excellent dimensional stability.

Background

The optical film is a film used as an optical material for a display. It is used as an optical material for surface protection of various displays such as LCD BLU (Back light unit) or Touch Panel (Touch Panel).

Such an optical film is required to have high transparency, excellent surface properties, low shrinkage, and other physical properties, and is therefore suitable for touch applications such as Tablet computers (tablets) and smart phones. If the transparency is low, the brightness is reduced, and when the surface characteristics are poor or the shrinkage of the film is high, problems such as curling (curl) or deformation of the final product may occur after coating.

In view of these physical properties, an optical film is formed by high-temperature curing of a polyester film or is made of a high heat-resistant polymer such as polyethylene naphthalate (PEN), Polyimide (PI), however, the high-temperature curing process of a polyester film may reduce productivity or generate deformation due to moisture or the like, in addition, the use of PEN or PI is advantageous in terms of heat resistance and dimensional stability but causes an increase in manufacturing cost, and subsequent processing is difficult compared to polyester, and , a multilayer polyester film which is excellent in dimensional stability and heat resistance against humidity change and can suppress curling is disclosed in japanese laid-open patent No. 2009 and 279923 (patent document 1), which is intended to achieve the object by using a copolymer component, but is developed to curl (curl) due to the increase in shrinkage rate in the subsequent processing process, and is considerably expensive in manufacturing cost.

Prior art documents

Patent document

Patent document 1: japanese laid-open patent No. 2009-279923 (2009.12.03)

Disclosure of Invention

Technical problem to be solved

The present invention has been made to solve the above problems, and an object of the present invention is to provide kinds of optical polyester multilayer films having low heat shrinkage and capable of realizing high transparency and brightness.

Further, it is an object of the present invention to provide kinds of optical polyester multilayer films having excellent surface characteristics and dimensional stability, excellent coating processability, less thermal deformation, and excellent manufacturability.

Another object of the present invention is to provide optical films which have excellent physical properties and manufacturability when applied to a display, particularly, a touch panel or a backlight unit (BLU).

Technical scheme

In order to achieve the above object, the present invention provides polyester multilayer films, which are formed by co-extrusion and stretching of a polyester resin into three or more layers, comprising a core layer in an amount of 70 to 90 wt% and a surface layer of 10 to 30 wt% of at least two layers laminated on both surfaces of the core layer, based on the total weight of the film, wherein the difference in intrinsic viscosity between the polyester resin for forming the surface layer and the polyester resin for forming the core layer is less than 0.15, and the surface layer contains 20 to 100ppm of particles having an average particle diameter of 0.5 to 5 μm.

In this case, the polyester multilayer film may be a three-layer film including a core layer and surface layers formed on both surfaces thereof, but is not limited thereto, and may be a multilayer film formed so that two or more surface layers are formed on both surfaces of the core layer.

Specifically, the polyester multilayer film of the present invention can be produced by extrusion-melting the polyester film in two or more melt extruders, casting and biaxial stretching the melt, and then subjecting the melt to heat treatment. In addition, the heat treatment can adjust the shrinkage rate by relaxing in the longitudinal direction and the width direction, but is not necessarily limited thereto.

The present invention also provides optical films, which comprise any or more functional coating layers selected from a hard coat layer, an adhesive layer, a light diffusion layer, an ITO layer and a printed layer on the top of the polyester multilayer film.

In addition, the present invention provides a method for manufacturing kinds of polyester multilayer films, comprising the steps of:

(a) melt-extruding the respective polyester resin compositions for forming the core layer and the skin layer, and performing coextrusion;

(b) uniaxially or biaxially stretching the coextruded sheet to produce a film; and

(c) the stretched film is subjected to a heat treatment,

wherein, when the total weight of the film is taken as a reference, the content of the core layer is 70-90 wt%, and the content of the surface layer is 10-30 wt%;

the polyester resin composition for forming the surface layer comprises a polyester resin having an inherent viscosity difference of less than 0.15 from that of the polyester resin for forming the core layer, and 20 to 100ppm of particles having an average particle diameter of 0.5 to 5 μm.

Advantageous effects

The optical polyester multilayer film according to the present invention has advantages of high transparency and brightness, and excellent dimensional stability due to low heat shrinkage.

In addition, the optical film provided by the invention has excellent surface characteristics and coating processability, and less thermal deformation, so that the manufacturability is extremely excellent.

The polyester multilayer film for an optical film according to the present invention has advantages in that it can realize excellent physical properties and improve productivity when applied to a display such as a touch panel or BLU.

Drawings

Fig. 1 is a graph showing a thermal expansion coefficient and an inflection point using a thermomechanical analyzer (TMA).

Detailed Description

The optical polyester film of the present invention will be described in detail below with reference to preferred embodiments. However, this is not intended to limit the scope of protection defined by the claims. In addition, unless defined otherwise, all technical and scientific terms used in the description of the present invention have the meanings commonly understood by those skilled in the art.

In the present invention, "%" means "% by weight" unless otherwise specified.

In the present invention, "longitudinal direction" means "mechanical direction" unless otherwise mentioned.

The inventors of the present invention have found the following, and thus have completed the present invention: a multilayer film comprising three or more layers formed by co-extruding and stretching a polyester resin, comprising a core layer and skin layers formed on both surfaces of the core layer, wherein the film has high transparency and brightness and is effectively improved in dimensional stability by utilizing the characteristic of low heat shrinkage by adjusting the weight ranges of the core layer and the skin layers based on the total weight of the film, the difference between the intrinsic viscosities of the polyester resins of the skin layers and the core layer, and the average particle diameter and content of particles contained in the skin layers.

Next, an example of the present invention will be described in more detail.

The polyester multilayer film according to the present invention is a multilayer film of three or more layers formed by co-extruding and stretching a polyester resin,

based on the total weight of the film, the film comprises 70-90 wt% of a core layer, 10-30 wt% of at least two surface layers laminated on both surfaces of the core layer,

the difference between the intrinsic viscosity of the polyester resin forming the surface layer and the intrinsic viscosity of the polyester resin forming the core layer is less than 0.15, and the polyester resin forming the core layer contains 20 to 100ppm of particles having an average particle diameter of 0.5 to 5 μm.

That is, the present invention is a polyester multilayer film comprising a core layer and a surface layer, which can achieve the desired effects by using a specific polyester resin for forming each layer and combining a composition in which the weight ratio of the core layer and the surface layer and the average particle size and content of particles contained in the core layer are adjusted, and when out of the specific resin, the weight ratio of each layer, and the size and content of particles in the surface layer are not within the range, it is difficult to achieve the effects of improving dimensional stability, achieving high transparency and brightness, improving surface characteristics, and improving coating processability by a low shrinkage ratio, which are desired in the present invention.

Specifically, the polyester multilayer film according to the present invention may be a three-layer film formed of a core layer and surface layers on both surfaces thereof, but is not limited thereto, and may be a multilayer film formed so that two or more surface layers are formed on both surfaces of the core layer.

The core layer and the surface layer are formed of polyester resin. The polyester resin is not particularly limited, but polyethylene terephthalate (PET) alone may be preferred. In this case, the polyester resin has specific physical properties when used in the core layer and each of the skin layers, and is characterized in that the difference between the intrinsic viscosity of the polyester resin used in the skin layer and the intrinsic viscosity of the polyester resin used in the core layer is preferably less than 0.15. The difference between the intrinsic viscosities of the polyester resins that may be used to form the core layer and the skin layers is preferably less than 0.09, more preferably less than 0.05. When the ratio is within the above range, the effect of improving the dimensional stability can be achieved by a low shrinkage ratio by combining the constitution of adjusting the weight ratio of the core layer and the surface layer and the particles in the surface layer. When not in the above range, breakage continues to occur, and the interface is unstable, so that there is a possibility that processing is difficult in terms of process.

As embodiments, the intrinsic viscosity of the polyester resin used to form the core layer may be 0.6 to 0.8, preferably 0.62 to 0.78, and more preferably 0.65 to 0.72. when the intrinsic viscosity of the polyester resin of the core layer is less than 0.6, heat resistance is reduced, interfacial instability may occur during co-extrusion, and when it exceeds 0.8, extrusion processing is not easily performed, and thus workability may be reduced.

In addition, the polyester resin forming two or more skin layers on both surfaces of the core layer has an intrinsic viscosity of 0.6 to 0.8, preferably 0.64 to 0.75, by co-extrusion. When the intrinsic viscosity of the polyester resin of the skin layer is less than 0.6, interfacial instability may occur during coextrusion, and since the viscosity is too low, there may be difficulty in an extrusion process, and when it exceeds 0.8, since extrusion processing in a general extruder is difficult, expensive equipment is required, and when extrusion is performed at high temperature, there is a possibility that the film properties are significantly reduced.

The surface layer of the present invention comprises particles. The particles are not particularly limited, but inorganic particles may be preferable. In this case, it is important to adjust the average particle diameter and the content of the particles by combination with other constituent components.

The particles according to the present invention are characterized in that the average particle diameter is 0.5 to 5 μm. If the average particle diameter of the particles is not within the above range, the transmittance is greatly reduced, and thus, it is difficult to determine defects with the naked eye in BLU evaluation, and thus it is difficult to use the particles for optical applications, and particularly, the thermal shrinkage rate characteristics are poor, and thus the dimensional stability may be significantly reduced.

Further, the composition in which the content range and the average particle size of the particles are adjusted in terms of weight ratio in combination with the specific resin forming the core layer and the surface layer can improve dimensional stability by a remarkable margin by virtue of excellent heat shrinkage characteristics, whereas if not in the above content range, transparency or smoothness is lowered, and it may be difficult to use the composition when applied to a touch panel or the like, and as , if the content of the particles exceeds 100ppm, even if particles of 0.5 μm are used, quality inspection is difficult to perform due to the lowering of transparency of the film in a coating process, and it is difficult to use the composition for protection, and if the content of the particles is less than 10ppm, even if particles of 5 μm are used, adhesion may occur in a film forming process or a scratch may occur in a subsequent process.

In the present invention, the kind of the particles is not particularly limited, but kinds or a mixture of two or more kinds selected from inorganic particles consisting of silica, zeolite and kaolin can be preferably used.

In addition, in the polyester multilayer film, the content of the core layer is 70 to 90% by weight, the content of the skin layer is 10 to 30% by weight, preferably the content of the core layer is 70 to 80% by weight, and the content of the skin layer is 20 to 30% by weight, relative to the content of the entire film, which is advantageous for interface stabilization at the time of coextrusion, and at the same time, by combining with other constituent components, an excellent effect of improving the heat shrinkage characteristics and dimensional stability can be achieved.

The polyester multilayer film according to the present invention is not particularly limited, but the total thickness of the film is preferably 50 to 300. mu.m, preferably 75 to 250. mu.m.

Specifically, embodiments of the present invention are produced by extruding polyester in extruders, melt-extruding polyester and particles simultaneously in extruders, subjecting the respective melts to coextrusion on feed plates, then sequentially biaxially stretching the melt after the transition from casting to cooling, and then winding the melt.

As described above, the polyester multilayer film of the present invention has a center line average roughness Ra of 6 to 20nm, a ten-point average roughness Rz of 80 to 400nm, and a heat shrinkage ratio after heat treatment at 85 ℃ for three days can be 0.3% or less.

In view of the above, the polyester multilayer film according to the present invention is characterized in that the center line average roughness Ra is 6 to 20nm and the ten point average roughness Rz is 80 to 400nm, and when one of the Ra and Rz is not in the above range, the quality of the final product may be deteriorated due to the coating manufacturability and coating unevenness.

The polyester multilayer film according to the present invention is characterized in that the heat shrinkage rate after heat treatment at 85 ℃ for three days is 0.3% or less. In this case, the film satisfies the heat shrinkage rate in both the longitudinal direction and the width direction, and if the heat shrinkage rate is not within the above range, the film is difficult to use for a prism film or the like due to thermal deformation.

The polyester multilayer film of the present invention has an inflection point of 70 to 100 ℃, preferably 80 to 90 ℃ as measured along the longitudinal direction of the film by a thermomechanical analyzer (TMA). Further, the thermal expansion coefficient between inflection points is 0.1 to 0.4 μm/DEG C at 40 ℃, and the thermal deformation length at 90 ℃ after the heat treatment is 10 to 30 μm. In this case, the heat treatment method is as follows: a sample (1 cm wide and 10cm long) was subjected to heat treatment at 120 ℃ for 3 minutes under a 200g load, cooled at room temperature for 5 minutes, subjected to heat treatment at 140 ℃ for 3 minutes under a 200g load again, and then cooled at room temperature for 5 minutes.

In this case, when the film is sampled in the longitudinal direction and measured by TMA, the film expands and contracts with increasing temperature as shown in fig. 1, and the inflection point is the temperature at the position where the film changes from expansion to contraction. In addition, the thermal expansion coefficient means a change in length according to a temperature change from 40 ℃ to an inflection point.

In addition, when TMA measurement was performed after the heat treatment, the length change rate (%) at 90 ℃ was (length change/initial length 16 mm). times.100.

When the range of the thermal expansion coefficient and the change rate in the longitudinal direction after the heat treatment exceeds the above range, severe thermal deformation occurs in the high-temperature post-processing step, and thus it is difficult to realize the product after the post-processing step, and when the hard coating or the ITO sputtering treatment is performed, curling or thermal wrinkle occurs, an additional offline curing treatment is required, additional cost and time are incurred, and it is necessary to pass through steps more, and thus there are problems in terms of scratches, foreign matters, and the like.

example is obtained by extruding polyester in extruders, simultaneously melt-extruding polyester and additives such as inorganic particles of silica, kaolin, zeolite in another extruders, and then subjecting the respective melts to a feeding plate, co-extrusion casting, cooling, and then sequentially biaxially stretching, and heat treatment and relaxation.

The embodiment of the present invention relates to a method for making a polyester multilayer film comprising the steps of:

(a) melt-extruding the respective polyester resin compositions for forming the core layer and the skin layer, thereby performing co-extrusion;

(b) uniaxially or biaxially stretching the sheet which has been coextruded, to produce a film; and

(c) the film which has been stretched is subjected to a heat treatment,

wherein the core layer and the surface layer are 70 to 90 wt% and 10 to 30 wt%, respectively, based on the total weight of the film,

In the step of heat-treating the stretched film, the shrinkage rate may be controlled by adjusting the heat-treatment temperature or simultaneously performing relaxation in the longitudinal direction and the width direction.

Specifically, the heat treatment step is more preferably performed to relax in the width direction and the length direction, thereby reducing the shrinkage rate of the produced film, the width direction relaxation rate may be performed such that 2 to 4 heat treatment regions are divided in the heat treatment step and relaxation is performed at a similar rate, if the relaxation rate is too high, the film sags, and if the relaxation rate is too low, there is a possibility that the shrinkage rate in the width direction and the length direction of the film increases, therefore, it is preferable to divide the heat treatment regions at the time of relaxation and then perform different relaxation rates for the respective regions, and after dividing the heat treatment region into two regions, the relaxation rates of a certain region and another region may be different from each other as a concrete example, in this case, it is more preferable that the relational expression of the relaxation rate at the th time of relaxation and the relaxation rate at the second time of relaxation satisfies the following expression 1, which is more advantageous for improving the shrinkage characteristics of the film.

Formula 1: step relaxation ratio/second step relaxation ratio ≧ 1.05

In addition, in order to reduce the shrinkage rate in the longitudinal direction, relaxation is performed in a heat treatment region by a longitudinal relaxation device, and at this time, the heat treatment temperature is characterized by 80 to 240 ℃. If the heat treatment temperature exceeds 240 ℃, the film may be crystallized and broken, making handling difficult, and if the temperature is less than 80 ℃, the film may not be relaxed in the longitudinal direction, and the shrinkage rate may not be reduced.

The present invention provides an optical film comprising the above polyester multilayer film.

The present invention will be described in more detail with reference to examples, but the present invention is not limited to the examples described below.

The following physical properties were measured by the following measurement methods.

(1) Intrinsic viscosity

0.4g of PET pellets (sample) was added to 100ml of a 6:4 weight ratio reagent for mixing phenol and 1,1,2, 2-tetrachloroethanol, dissolved for 90 minutes, transferred into an Ubbelohde viscometer, and kept at 30 ℃ for 10 minutes, and then the number of seconds of the solution falling was determined by a viscometer and an air extractor (aspirator). After the number of seconds of solvent drop was determined in the same manner, R.V values and i.v. values were calculated by mathematical formulae 1 and 2.

In the following numerical expression, C means the concentration of the sample.

[ mathematical formula 1]

R.V-seconds of sample/seconds of solvent drop

[ mathematical formula 2]

(2) Transparency (haze,%)

A film sample was measured by using a haze meter (company name: Nipon denshoku, model NDH 5000) in accordance with JIS K7105. A test piece having a length of 6cm and a width of 6cm was produced and used.

(3) Surface roughness (Ra, Rz)

Ra (center line average roughness) and Rz (ten-point average roughness) were measured by a two-dimensional contact surface roughness meter (Kosaka Co., Ltd., SE 3300).

(4) Shrinkage rate

After a film sample was cut into a size of 20cm in length and 20cm in width and heat-treated at 85 ℃ for three days, the shrinkage was measured by the following formula 1.

Formula 1: percent shrinkage (%) (length before heat treatment-length after heat treatment)/length before heat treatment × 100

(5) Thermomechanical analyzer (TMA) measurement

After the film was cut into pieces of 16mm and 4.5mm in the longitudinal direction and the width direction, the temperature was raised under an initial load of 0.05N by TMA (TA Q400) under a temperature range of 30 to 180 ℃ and a temperature rise rate of 5 ℃/min, and then the thermal deformation length (dimensional change), the inflection point, and the thermal expansion coefficient were measured.

[ example 1]

PET having an inherent viscosity of 0.65 was used for the core layer, PET having an inherent viscosity of 0.64 and 80ppm of silica particles having an average particle diameter of 1.6 μm were used for the skin layer, and they were respectively co-extruded and cast. Thereafter, the film was stretched 3 times and 3.4 times in the machine direction and the width direction, respectively, and then heat-treated at 230 ℃ to relax 1% and 2.5% in the longitudinal direction and the width direction, respectively, by a thermal shrinkage controller in the longitudinal direction, thereby obtaining a 188 μm multilayer film. The multilayer film is prepared into 80 wt% of a core layer and 20 wt% of a surface layer based on the total weight of the film. The physical properties of the produced film were evaluated and shown in table 2 below.

[ example 2]

The procedure of example 1 was repeated, except that PET having an intrinsic viscosity of 0.64 was used in place of PET having an intrinsic viscosity of 0.7 for the skin layer. The thickness of the resulting multilayer film was 188 μm, and the physical properties thereof were evaluated and shown in Table 2 below.

[ example 3]

The procedure of example 1 was repeated, except that PET having an intrinsic viscosity of 0.64 was used in place of PET having an intrinsic viscosity of 0.6 for the skin layer. The thickness of the resulting multilayer film was 188 μm, and the physical properties thereof were evaluated and shown in Table 2 below.

[ example 4]

The procedure of example 1 was repeated, except that the core layer was prepared in an amount of 70 wt% and the skin layer was prepared in an amount of 30 wt%, based on the total weight of the film. The thickness of the resulting multilayer film was 188 μm, and the physical properties thereof were evaluated and shown in Table 2 below.

[ example 5]

The procedure of example 1 was repeated, except that the core layer was 90 wt% and the skin layer was 10 wt%, based on the total weight of the film. The thickness of the resulting multilayer film was 188 μm, and the physical properties thereof were evaluated and shown in Table 2 below.

[ example 6]

The procedure of example 1 was repeated, except that the amount of silica particles used in the surface layer was changed from 80ppm to 100 ppm. The thickness of the resulting multilayer film was 188 μm, and the physical properties thereof were evaluated and shown in Table 2 below.

[ example 7]

The procedure of example 1 was repeated, except that the amount of silica particles used in the surface layer was changed from 80ppm to 5 ppm. The thickness of the resulting multilayer film was 188 μm, and the physical properties thereof were evaluated and shown in Table 2 below.

[ example 8]

The procedure of example 1 was repeated, except that the silica particles having an average particle size of 1.6 μm used in the surface layer were changed to those having an average particle size of 5 μm. The thickness of the resulting multilayer film was 188 μm, and the physical properties thereof were evaluated and shown in Table 2 below.

[ example 9]

The procedure of example 1 was repeated, except that the silica particles having an average particle size of 1.6 μm used in the surface layer were changed to those having an average particle size of 0.5. mu.m. The thickness of the resulting multilayer film was 188 μm, and the physical properties thereof were evaluated and shown in Table 2 below.

Comparative example 1

The same procedure as in example 1 was repeated, except that PET having an intrinsic viscosity of 0.64 was used in place of PET having an intrinsic viscosity of 0.55 for the skin layer. The thickness of the resulting multilayer film was 188 μm, and the physical properties thereof were evaluated and shown in Table 2 below.

Comparative example 2

The same procedure as in example 1 was repeated, except that PET having an intrinsic viscosity of 0.64 was used in place of PET having an intrinsic viscosity of 0.85 for the skin layer. The thickness of the resulting multilayer film was 188 μm, and the physical properties thereof were evaluated and shown in Table 2 below.

Comparative example 3

The procedure of example 1 was repeated, except that the core layer was 60 wt% and the skin layer was 40 wt%, based on the total weight of the film. The thickness of the resulting multilayer film was 188 μm, and the physical properties thereof were evaluated and shown in Table 2 below.

Comparative example 4

The procedure of example 1 was repeated, except that 95 wt% of the core layer and 5 wt% of the skin layer were prepared based on the total weight of the film. The thickness of the resulting multilayer film was 188 μm, and the physical properties thereof were evaluated and shown in Table 2 below.

Comparative example 5

The procedure of example 1 was repeated, except that the silica particles used in the surface layer were changed from 80ppm to 110 ppm. The thickness of the resulting multilayer film was 188 μm, and the physical properties thereof were evaluated and shown in Table 2 below.

Comparative example 6

The procedure of example 1 was repeated, except that the silica particles used in the surface layer were changed from 80ppm to 10 ppm. The thickness of the resulting multilayer film was 188 μm, and the physical properties thereof were evaluated and shown in Table 2 below.

Comparative example 7

The obtained steel sheet was produced in the same manner as in example 1, and was relaxed by 2.5% only in the width direction without relaxing in the longitudinal direction. The thickness of the resulting multilayer film was 188 μm, and the physical properties thereof were evaluated and shown in Table 2 below.

[ Table 1]

□ are co-extruded in an A/B/A manner. (the thickness of the surface layer is the total of the two A layers)

[ Table 2]

As shown in table 2, the examples according to the present invention satisfy the physical properties of surface roughness, transparency, and dimensional stability by thermal shrinkage, and also satisfy the physical properties of inflection point, thermal expansion coefficient, and length change rate by TMA measurement, and it was confirmed that high transparency, low shrinkage, and excellent surface characteristics can be achieved. This proves that it is easily applicable to an optical film capable of realizing excellent physical properties and manufacturability when used for a display, particularly a touch panel or BLU. In contrast, in comparative examples 1,2 and 4, processing was difficult due to fracture and interfacial instability, the surface properties and transparency were significantly reduced in comparative examples 3 and 5, and the rate of change in length after heat treatment was also reduced compared to examples. In addition, the transparency, shrinkage rate, thermal expansion coefficient, and length change rate of comparative examples 6 and 7 were significantly reduced.

As described above, the present invention is described with respect to a limited number of embodiments, but this is merely for facilitating a more comprehensive understanding of the present invention, and the present invention is not limited to the above-described embodiments, and those skilled in the art can make various modifications and variations to the above description.

Therefore, the idea of the present invention is not limited to the embodiments described, and not only the claims but also equivalent or equivalent variations to the claims fall within the scope of the idea of the present invention.

Claims

A multilayer polyester film of kinds formed by co-extrusion and stretching of a polyester resin into three layers, characterized in that:

the total thickness of the polyester multilayer film is 75-250 μm,

the film comprises a core layer in an amount of 70 to 90 wt% and surface layers laminated on both surfaces of the core layer in an amount of 10 to 30 wt% based on the total weight of the film,

the difference in intrinsic viscosity between the polyester resin forming the surface layer and the polyester resin forming the core layer is less than 0.09, the surface layer contains 20 to 100ppm particles having an average particle diameter of 0.5 to 5 μm, the center line average roughness Ra of the polyester multilayer film is 6 to 20nm, the ten point average roughness Rz is 80 to 400nm, and the heat shrinkage ratio after heat treatment of a film sample of 20cm x 20cm at 85 ℃ for three days is 0.3% or less.
2. The polyester multilayer film according to claim 1,

the polyester multilayer film has a coefficient of thermal expansion in the longitudinal direction up to the inflection point of 0.2 to 0.4 [ mu ] m/DEG C as measured by a thermomechanical analyzer TMA, and is heat-treated at 120 ℃ for 3 minutes under a 200g load, cooled at room temperature for 5 minutes, heat-treated again at 140 ℃ for 3 minutes under a 200g load, and cooled at room temperature for 5 minutes, and then changes in the longitudinal direction at 90 ℃ to 10 to 30 [ mu ] m.
3. The polyester multilayer film according to claim 1,

the multilayer film has a haze of 0.8 to 2.5% measured according to JIS K7105.
4. The polyester multilayer film according to claim 1,

the particles are or mixture of more than two selected from silica, zeolite and kaolin.
5. The polyester multilayer film according to claim 1,

the inherent viscosity of the polyester resin forming the surface layer is 0.6-0.8.
An optical film of , wherein,

forming any or more functional coatings selected from a hard coat layer, an adhesive layer, a light diffusion layer, an ITO layer and a printed layer on the upper portion of the polyester multilayer film of in any one of claims 1 to 5.
7, A method for manufacturing a polyester multilayer film, comprising the steps of:

(a) melt-extruding each of the polyester resin compositions for forming the core layer and the skin layers, and performing coextrusion to obtain a three-layered coextruded sheet in which the skin layers are laminated on both surfaces of the core layer;

(b) uniaxially or biaxially stretching the coextruded sheet, thereby producing a film; and

(c) the stretched film is subjected to a heat treatment,

wherein the total thickness of the polyester multilayer film is 75-250 μm,

based on the total weight of the film, the content of the core layer is 70-90 wt%, the content of the surface layer is 10-30 wt%,

the polyester resin composition for forming the skin layer comprises: a polyester resin having an inherent viscosity difference of less than 0.09 from that of the polyester resin forming the core layer, and 20 to 100ppm of particles having an average particle diameter of 0.5 to 5 μm, wherein the polyester multilayer film has a center line average roughness Ra of 6 to 20nm and a ten point average roughness Rz of 80 to 400nm, and a heat shrinkage ratio of 0.3% or less after heat treatment of a film sample of 20cm x 20cm at 85 ℃ for three days.
8. The process for producing a polyester multilayer film according to claim 7,

the heat treatment step includes a step of performing relaxation in the machine direction and the width direction.
9. The process for producing a polyester multilayer film according to claim 8,

when the relaxation is performed, the relaxation rate is adjusted so as to satisfy the following formula 1,

the relation 1 is that the relaxation rate of the th step/the relaxation rate of the second step is ≧ 1.05.