CN107031160A - Optical Polyester Film - Google Patents

Abstract

The present invention relates to a kind of polyester film on the top layer including sandwich layer and its two surface.In further detail, being related to a kind of optics polyester multilayer film realized the high grade of transparency and brightness and there is outstanding dimensional stability.

Description

Optical Mylar

technical field

The invention relates to an optical film. More specifically, it relates to an optical polyester multilayer film that includes a core layer and skin layers on both surfaces thereof, realizes high transparency and brightness, and has excellent dimensional stability.

Background technique

Optical films are films used as optical materials for displays. It is used as an optical material for surface protection of various displays such as LCD BLU (Back Light Unit) and touch panel (Touch Panel).

This optical film has high transparency, and requires excellent surface properties and low shrinkage, so it is suitable for touch applications such as tablets and smartphones. If the transparency is low, the brightness decreases, and when the surface properties are poor or the shrinkage rate of the film is high, problems such as curl after coating or deformation of the final product may occur.

Considering these physical properties, optical films are made of polyester film cured at high temperature, or made of high heat-resistant polymers such as polyethylene naphthalate (PEN) and polyimide (PI). However, the high-temperature aging process of polyester film may lower productivity or cause 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 it leads to an increase in production cost, and post-processing is difficult compared with polyester. On the other hand, a multilayer polyester film excellent in dimensional stability and heat resistance against humidity changes and capable of suppressing curl is disclosed in Japanese Laid-Open Patent No. 2009-279923 (Patent Document 1). It intends to use copolymerized components to achieve the purpose, but curls due to increased shrinkage in subsequent processing steps, and is relatively expensive to manufacture.

prior art literature

patent documents

Patent Document 1: Japanese Laid-Open Patent No. 2009-279923 (2009.12.03)

Contents of the invention

technical problem to be solved

The present invention was made in order to solve the above-mentioned problems, and an object of the present invention is to provide an optical polyester multilayer film which has low heat shrinkage and can achieve high transparency and brightness.

Another object of the present invention is to provide an optical polyester multilayer film which has excellent surface properties and dimensional stability, and which is excellent in coating processability and less thermally deformed, so that it is excellent in manufacturability.

In addition, the purpose of the present invention is to provide an optical film that can achieve excellent physical properties and manufacturing processability when applied to a display, especially a touch panel or a backlight unit (BLU).

Technical solutions

In order to achieve the above object, the present invention provides a polyester multilayer film, which is made of polyester resin through co-extrusion and stretching to form a multilayer film with more than three layers. % of the core layer and 10 to 30% by weight of at least two skin layers laminated on both surfaces of the core layer, the intrinsic viscosity of the polyester resin used to form the skin layer and the polyester resin used to form the core layer The difference is less than 0.15, and the surface layer contains 20-100 ppm of particles with an average particle size of 0.5-5 μm.

In this case, the polyester multilayer film may be a three-layer film composed of a core layer and surface layers formed on both surfaces, but is not limited thereto, and may be formed by forming two or more surface layers on both surfaces of the core layer. formed multilayer films.

Specifically, the polyester multilayer film of the present invention can be produced by extruding and melting through two or more melt extruders, casting and biaxially stretching, and then performing heat treatment. In addition, during the heat treatment, the shrinkage ratio can be adjusted by performing relaxation in the longitudinal direction and the width direction, but it is not necessarily limited to this.

In addition, the present invention provides an optical film comprising any one or more functions selected from a hard coat layer, an adhesive layer, a light diffusion layer, an ITO layer, and a printing layer on the upper portion of the polyester multilayer film. permanent coating.

In addition, the present invention provides a kind of manufacture method of polyester multilayer film, it comprises the following steps:

(a) melt extrusion is used to form each polyester resin composition of core layer and surface layer, and carry out co-extrusion;

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

(c) heat-treating the stretched film,

Wherein, based on the total weight of the film, the content of the core layer is 70-90% by weight, and the content of the surface layer is 10-30% by weight;

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

Beneficial effect

The optical polyester multilayer film according to the present invention is advantageous in that it can achieve high transparency and brightness, has low heat shrinkage, and thus has excellent dimensional stability.

In addition, the optical film provided by the present invention has excellent surface properties and coating processability, and has little thermal deformation, so it is extremely excellent in manufacturability.

In addition, the polyester multilayer film for optical films according to the present invention is advantageous in that when it is applied to a display such as a touch panel or a BLU, it can realize excellent physical properties and improve productivity.

Description of drawings

FIG. 1 is a graph showing the coefficient of thermal expansion and the point of inflection using a thermomechanical analyzer (TMA).

detailed description

Next, the optical polyester film of the present invention will be described in detail through preferred embodiments. However, this is not intended to limit the scope of protection defined by the claims. In addition, unless otherwise defined, all technical terms 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 mentioned.

In addition, the "longitudinal direction" in this invention means a "machine direction" unless it mentions especially.

The inventors of the present invention have completed the present invention by discovering that a multilayer film having three or more layers formed by coextruding and stretching polyester resin includes a core layer and a film formed on both sides of the core layer. The skin layer on the surface, by including the weight range of the core layer and the skin layer based on the total weight of the film, the difference in the intrinsic viscosity of the polyester resin of the skin layer and the core layer, and the average particle size and content of the particles contained in the skin layer By adjusting it, not only high transparency and brightness can be achieved, but also the dimensional stability can be effectively improved by utilizing the characteristics of low heat shrinkage.

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

The polyester multilayer film involved in the present invention is a multilayer film with more than three layers formed by co-extrusion and stretching of polyester resin,

Based on the total weight of the film, it contains 70-90% by weight of the core layer and 10-30% by weight of at least two surface layers laminated on both surfaces of the core layer,

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

That is, the present invention is a polyester multilayer film composed of a core layer and a surface layer, by using a specific polyester resin for forming each layer, combining the weight ratio of the core layer and the surface layer, and the weight ratio contained in the core layer. The composition in which the average particle size and content of the particles are adjusted can achieve the desired effect. When the specific resin, the weight ratio of each layer, and the particle size and content in the surface layer are not within the range, it is difficult to achieve the expected improvement of dimensional stability through low shrinkage, high transparency and brightness, Effect of improving surface properties and coating workability.

Specifically, the polyester multilayer film involved in the present invention may be a three-layer film formed of a core layer and skin layers on both surfaces thereof, but it is not limited thereto, and may also be a film having two or more layers formed on both surfaces of the core layer. A multilayer film formed in the form of a surface 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. At this time, when the polyester resin is used for the core layer and each layer of the surface layer, it has specific physical properties, and it is characterized in that the intrinsic viscosity of the polyester resin used for the surface layer and the intrinsic viscosity of the polyester resin used for the core layer are preferable. The difference in viscosity is less than 0.15. It may be preferred that the difference between the intrinsic viscosities of the polyester resins used to form the core layer and the skin layer is less than 0.09, more preferably less than 0.05. When it is in the above-mentioned range, the effect of improving dimensional stability can be achieved through low shrinkage by combining the composition of adjusting the weight ratio of the core layer and the surface layer and the particles in the surface layer. If it is not within the above range, fractures will continue to occur and the interface will be unstable, so processing may be difficult in terms of process.

As an embodiment, 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, more preferably 0.65 to 0.72. When the intrinsic viscosity of the polyester resin in the core layer is less than 0.6, the heat resistance may decrease and the interface may become unstable during co-extrusion. When it exceeds 0.8, the extrusion process may be difficult to perform, and thus the handleability may decrease.

In addition, the intrinsic viscosity of the polyester resin forming two or more skin layers on both surfaces of the core layer by co-extrusion is 0.6 to 0.8, preferably 0.64 to 0.75. When the intrinsic viscosity of the polyester resin of the surface layer is less than 0.6, the interface instability may occur during co-extrusion, and because the viscosity is too low, there may be difficulties in the extrusion process, and when it exceeds 0.8, due to It is difficult to perform extrusion processing in an ordinary extruder, expensive equipment is required, and when extruded at a high temperature, the physical properties of the film may be significantly reduced.

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

The particles according to the present invention are characterized by having an average particle diameter of 0.5 to 5 μm. When the average particle size of the particles is not within the above range, the light transmittance is greatly reduced, and it is difficult to judge the defects with the naked eye when performing BLU evaluation, so it is difficult to be used for optical applications, especially the thermal shrinkage rate characteristics are poor, so the size Stability may be significantly reduced.

In addition, the content of the particles in the surface layer is 20-100 ppm. When the content is in the above range, the optical performance can be effectively improved, which can be adjusted in combination with the average particle diameter of the particles, so as to achieve a synergistic effect. Furthermore, by adjusting the content range and average particle size of the particles, by combining specific resins that form the core layer and the surface layer according to the weight ratio, the dimensional stability can be greatly improved by virtue of excellent heat shrinkage characteristics. On the contrary, when it is not in the said content range, transparency and smoothness will fall, and when it applies to a touch panel etc., it may become difficult to use. In addition, as an example, when the particle content exceeds 100ppm, even if 0.5μm particles are used, the quality inspection will be difficult due to the decrease in the transparency of the film during the coating process, and it is difficult to be used for protection purposes. When the particle content is less than 10ppm, Even if 5 μm particles are used, sticking and scratches may occur during the film forming process, or scratches may occur in subsequent processes.

In the present invention, the type of particles is not particularly limited, but one or a mixture of two or more inorganic particles selected from silica, zeolite, and kaolin can be preferably used. Such inorganic particles can improve the slidability and windability of the film through the stretching process.

In addition, in the polyester multilayer film, relative to the content of the whole film, the content of the core layer is 70 to 90% by weight, the content of the surface layer is 10 to 30% by weight, preferably the content of the core layer is 70 to 80% by weight, The content of the surface layer is 20 to 30% by weight, which is beneficial to the stabilization of the interface during co-extrusion. At the same time, by combining with other constituent components, it can achieve excellent heat shrinkage characteristics and the effect of improving dimensional stability.

The polyester multilayer film involved in the present invention is not particularly limited, but the total thickness of the film is preferably 50 to 300 µm, preferably 75 to 250 µm.

As a specific embodiment, the polyester multilayer film is extruded in one extruder, and melted and extruded the polyester and particles in another extruder, so that each melt is processed After the material sheets meet, they are made by co-extrusion. Thereafter, after casting is transferred to cooling, after being biaxially stretched in turn, it is coiled. At this time, the biaxial stretching may be performed in the longitudinal direction first, and then stretched in the width direction, but it is not necessarily limited to this. Thereafter, the stretched polyester multilayer film was subjected to a heat treatment process. At this time, in order to control the shrinkage, increase the heat treatment temperature or perform relaxation in the longitudinal direction and the width direction, rather than control the heat treatment temperature, it is preferable to perform relaxation in the longitudinal direction and the width direction to manufacture.

As mentioned above, the centerline average roughness Ra of the polyester multilayer film involved in the present invention is 6-20nm, and the ten-point average roughness Rz is 80-400nm, and the thermal shrinkage after heat treatment at 85°C for three days can be Less than or equal to 0.3%.

When the centerline average roughness Ra exceeds 20 nm and the ten-point average roughness Rz exceeds 400 nm, the surface of the base film may be transferred to the adhesive layer, and thus the quality of the final product may decrease. In addition, when Ra is less than 6nm and Rz is less than 80nm, the smoothness is excellent, but the coating process and product handling properties are reduced, scratches or embossing may occur during the coating process, and coating unevenness may occur. In view of this, the polyester multilayer film according to the present invention is characterized in that the centerline average roughness Ra is 6 to 20 nm, and the ten-point average roughness is 80 to 400 nm. When any one of the aforementioned Ra and Rz is out of the aforementioned range, the quality of the final product may deteriorate due to coating manufacturability and uneven coating.

In addition, the polyester multilayer film according to the present invention is characterized in that its heat shrinkage rate after heat treatment at 85° C. for three days is 0.3% or less. In this case, it is necessary to satisfy the thermal contraction rate in the longitudinal direction and the width direction of the film, and if it is not in the above range, it is difficult to use it for a prism film or the like due to thermal deformation.

In addition, the polyester multilayer film according to the present invention has an inflection point of 70 to 100°C, preferably 80 to 90°C, when measured along the length direction of the film by a thermomechanical analyzer (TMA). In addition, it is characterized in that the coefficient of thermal expansion between inflection points is 0.1 to 0.4 μm/°C at 40°C, and the thermal deformation length change at 90°C after heat treatment is 10 to 30 μm. At this time, the heat treatment method is as follows: heat-treat the sample (width 1cm, length 10cm) at 120°C for 3 minutes under a load of 200g, cool at room temperature for 5 minutes, heat-treat again at 140°C for 3 minutes under a load of 200g, and then cool at room temperature 5 minutes.

At this time, after sampling the film along the length direction, when measured by TMA, as shown in Figure 1, it will expand and then shrink as the temperature rises, and the inflection point refers to the temperature at the position where the expansion changes to contraction. In addition, the coefficient of thermal expansion means a change in length according to a temperature change from 40° C. to an inflection point.

In addition, when performing TMA measurement after a heat treatment, the length change rate (%) at 90 degreeC means (length change/initial length 16mm)x100.

When the range of the above-mentioned coefficient of thermal expansion and the rate of change in the longitudinal direction after heat treatment exceeds the above-mentioned range, severe thermal deformation will occur in the high-temperature post-processing process, so it is difficult to realize productization after the post-processing process. During the injection treatment, due to curling or heat wrinkles, additional off-line aging treatment is required, which will incur additional costs, time, and an additional process, so there are problems such as scratches or foreign objects in the process. In addition, when the range of the thermal expansion coefficient and the rate of change in the longitudinal direction after heat treatment is smaller than the above range, there is no thermal deformation, but it is difficult to achieve the effect due to the decrease in physical properties.

The method of manufacturing the polyester multilayer film including the core layer and the skin layer of the present invention is not particularly limited, but it can be obtained by extruding and melting in at least two melt extruders, followed by casting, and by biaxial stretching. As an example, after polyester is extruded in one extruder, polyester and additives such as silica, kaolin, zeolite, etc. After meeting, co-extrusion casting, after cooling, biaxial stretching in turn, heat treatment and relaxation. At this time, by adjusting stretching, heat treatment, and relaxation of the film, the refractive index and shrinkage rate of the film and the thermal change of the film can be controlled.

The manufacture method of the polyester multilayer film that an embodiment of the present invention relates to comprises the following steps:

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

(b) uniaxially or biaxially stretch the coextruded sheet to make a film; and

(c) heat-treating the stretched film,

Wherein, based on the total weight of the film, the core layer and the surface layer are respectively 70-90% by weight and 10-30% by weight,

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

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

In this case, the relaxation is characterized in that the relaxation is performed simultaneously in the machine direction and the width direction. Specifically, in the heat treatment step, it is more preferable to perform relaxation along the width direction and the longitudinal direction, so that the shrinkage rate of the produced film can be reduced. The relaxation rate in the width direction can be implemented by dividing 2 to 4 heat treatment regions in the heat treatment process and performing relaxation at a similar rate. If the relaxation rate is too high, the film will sag, and if the relaxation rate is too low, the shrinkage rate in the width direction and the longitudinal direction of the film may increase. Therefore, it is preferable to divide the heat treatment area during relaxation and implement different relaxation rates for each area. As a specific example, by dividing the heat treatment region into two, the relaxation rate of one region and the other region can be made different from each other. At this time, it is more preferable that the relationship between the relaxation rate during the first relaxation and the relaxation rate during the second relaxation satisfies the following formula 1, which is more conducive to improving the shrinkage characteristics of the film.

Formula 1: Relaxation rate of the first step/relaxation rate of the second step≧1.05

In addition, in order to reduce the shrinkage rate in the longitudinal direction, relaxation is performed in the heat treatment region using a relaxation device in the longitudinal direction. At this time, the characteristic of the heat treatment temperature is 80 to 240°C. If the heat treatment temperature exceeds 240° C., the film may crystallize and may break, making handling difficult. If it is lower than 80° C., there will be no relaxation along the longitudinal direction, and the shrinkage rate cannot be reduced.

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

Hereinafter, in order to demonstrate this invention more concretely, an Example is given and demonstrated, However, this invention is not limited to the Example mentioned below.

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

(1) Intrinsic viscosity

Add 0.4g of PET particles (sample) to 100ml of the reagent mixed with phenol and 1,1,2,2-tetrachloroethanol in a weight ratio of 6:4, dissolve for 90 minutes, and then transfer it into the Ubbelohde viscometer. After maintaining for 10 minutes in a 30° C. thermostat, the number of seconds for the solution to fall was measured using a viscometer and an aspirator. The R.V value and I.V value were calculated by Mathematical Formulas 1 and 2 after calculating the falling seconds of the solvent in the same manner.

In the following mathematical formula, C refers to the concentration of the sample.

[mathematical formula 1]

R.V = the number of seconds of falling of the sample/the number of seconds of falling of the solvent

[mathematical formula 2]

(2) Transparency (haze, %)

The film sample was measured in accordance with JIS K 7105 using a haze meter (company name: Nipon denshoku, model NDH 5000). A test piece having a length of 6 cm and a width of 6 cm was produced and used.

(3) Surface roughness (Ra, Rz)

Ra (center line average roughness) and Rz (ten-point average roughness) were measured with a two-dimensional contact surface roughness meter (Kosaka SE3300).

(4) Shrinkage

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

Formula 1: Shrinkage rate (%) = (length before heat treatment - length after heat treatment) / length before heat treatment × 100

(5) Determination by thermomechanical analyzer (TMA)

After cutting the film into 16mm and 4.5mm along the length direction and the width direction respectively, use TMA (TA company, TMA Q400), under the initial load of 0.05N, according to the temperature range of 30-180°C and the temperature rise of 5°C/min Conditions of Speed After heating up, the thermal deformation length (Dimension change: dimensional change), inflection point, and thermal expansion coefficient were measured.

[Example 1]

PET with an intrinsic viscosity of 0.65 was used for the core layer, and PET with an intrinsic viscosity of 0.64 and 80 ppm of silica particles with an average particle diameter of 1.6 μm were used for the surface layer, respectively, and co-extruded to perform casting. Thereafter, after stretching to 3 times and 3.4 times along the machine direction and width direction respectively, heat treatment is carried out at 230°C, and the thermal shrinkage controller in the length direction is used to relax 1% along the length direction and the width direction respectively. 2.5%, made into a 188μm multilayer film. Based on the total weight of the film, the multi-layer film is made into a core layer of 80% by weight and a surface layer of 20% by weight. The physical properties of the produced films were evaluated and shown in Table 2 below.

[Example 2]

The same method as in Example 1 was carried out except that the PET used for the surface layer with an intrinsic viscosity of 0.64 was replaced with a PET with an intrinsic viscosity of 0.7. The produced multilayer film had a thickness of 188 μm, and its physical properties were evaluated and are shown in Table 2 below.

[Example 3]

The same method as in Example 1 was carried out except that the PET used for the surface layer with an intrinsic viscosity of 0.64 was replaced with PET with an intrinsic viscosity of 0.6. The produced multilayer film had a thickness of 188 μm, and its physical properties were evaluated and are shown in Table 2 below.

[Example 4]

The same method as in Example 1 was carried out except that a core layer of 70% by weight and a surface layer of 30% by weight were made on the basis of the total weight of the film. The produced multilayer film had a thickness of 188 μm, and its physical properties were evaluated and are shown in Table 2 below.

[Example 5]

Except that the core layer of 90% by weight and the surface layer of 10% by weight were made on the basis of the total weight of the film, the same method as in Example 1 was carried out. The produced multilayer film had a thickness of 188 μm, and its physical properties were evaluated and are shown in Table 2 below.

[Example 6]

It carried out in the same manner as in Example 1 except that the silica particles used in the surface layer were changed from 80 ppm to 100 ppm. The produced multilayer film had a thickness of 188 μm, and its physical properties were evaluated and are shown in Table 2 below.

[Example 7]

It carried out in the same manner as in Example 1 except that the silica particles used in the surface layer were changed from 80 ppm to 5 ppm. The produced multilayer film had a thickness of 188 μm, and its physical properties were evaluated and are shown in Table 2 below.

[Example 8]

It carried out in the same manner as in Example 1, except that the silica particles with an average particle diameter of 1.6 μm used in the surface layer were changed to those with an average particle diameter of 5 μm. The produced multilayer film had a thickness of 188 μm, and its physical properties were evaluated and are shown in Table 2 below.

[Example 9]

It carried out in the same manner as in Example 1 except that the silica particles with an average particle diameter of 1.6 μm used in the surface layer were changed to those with an average particle diameter of 0.5 μm. The produced multilayer film had a thickness of 188 μm, and its physical properties were evaluated and are shown in Table 2 below.

[Comparative example 1]

The same method as in Example 1 was carried out except that the PET used for the surface layer with an intrinsic viscosity of 0.64 was replaced with PET with an intrinsic viscosity of 0.55. The produced multilayer film had a thickness of 188 μm, and its physical properties were evaluated and are shown in Table 2 below.

[Comparative example 2]

The same method as in Example 1 was carried out except that the PET used for the surface layer with an intrinsic viscosity of 0.64 was replaced with PET with an intrinsic viscosity of 0.85. The produced multilayer film had a thickness of 188 μm, and its physical properties were evaluated and are shown in Table 2 below.

[Comparative example 3]

Except that the core layer of 60% by weight and the surface layer of 40% by weight were made on the basis of the total weight of the film, it was carried out in the same manner as in Example 1. The produced multilayer film had a thickness of 188 μm, and its physical properties were evaluated and are shown in Table 2 below.

[Comparative example 4]

Except that the core layer of 95% by weight and the surface layer of 5% by weight were made on the basis of the total weight of the film, the same method as in Example 1 was carried out. The produced multilayer film had a thickness of 188 μm, and its physical properties were evaluated and are shown in Table 2 below.

[Comparative Example 5]

It carried out in the same manner as in Example 1 except that the silica particles used in the surface layer were changed from 80 ppm to 110 ppm. The produced multilayer film had a thickness of 188 μm, and its physical properties were evaluated and are shown in Table 2 below.

[Comparative Example 6]

It carried out in the same manner as in Example 1 except that the silica particles used in the surface layer were changed from 80 ppm to 10 ppm. The produced multilayer film had a thickness of 188 μm, and its physical properties were evaluated and are shown in Table 2 below.

[Comparative Example 7]

It was carried out in the same manner as in Example 1, and was produced by only relaxing 2.5% in the width direction without performing relaxation in the longitudinal direction during manufacture. The produced multilayer film had a thickness of 188 μm, and its physical properties were evaluated and are shown in Table 2 below.

[Table 1]

□Co-extrusion in A/B/A mode. (The thickness of the surface layer is the total amount of the two A layers)

[Table 2]

As shown in the above Table 2, multiple embodiments of the present invention satisfy the physical properties of surface roughness, transparency, and dimensional stability based on thermal shrinkage rate, and at the same time satisfy the physical properties of inflection point, thermal expansion coefficient, and length change rate based on TMA, It was confirmed that high transparency, low shrinkage, and excellent surface properties can be achieved. This proves that it is easy to apply to an optical film that can realize excellent physical properties and manufacturing processability when used for a display, especially a touch panel or a BLU. On the contrary, in Comparative Examples 1, 2 and 4, it is difficult to process due to fracture and interface instability, and the surface properties and transparency of Comparative Examples 3 and 5 are significantly reduced, and the length change rate after heat treatment is also lower than that of Examples. In addition, the transparency, shrinkage rate, thermal expansion coefficient, and length change rate of Comparative Examples 6 and 7 were significantly reduced.

As mentioned above, the present invention is described through limited embodiments, but this is only to help to understand the present invention more comprehensively, the present invention is not limited to the above-mentioned embodiments, and those skilled in the art can make various modifications to the above-mentioned descriptions. modification and deformation.

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

Claims (11)

1. a polyester multilayer film is a multilayer film more than three layers formed by carrying out co-extrusion and stretching to polyester resin, characterized in that: When based on the total weight of the film, it includes 70-90% by weight of the core layer and 10-30% by weight of at least two skin layers laminated on both surfaces of the core layer, A difference in intrinsic viscosity between the polyester resin forming the surface layer and the polyester resin forming the core layer is less than 0.15, and the surface layer contains 20˜100 ppm of particles having an average particle diameter of 0.5˜5 μm. 2. polyester multilayer film according to claim 1, is characterized in that, The centerline average roughness (Ra) of the polyester multilayer film is 6-20nm, and the ten-point average roughness (Rz) is 80-400nm, and the sample of 20cm×20cm is heat-treated at 85°C for three days. The thermal shrinkage rate is less than or equal to 0.3%. 3. polyester multilayer film according to claim 1, is characterized in that, When using a thermomechanical analyzer (TMA) to measure, the thermal expansion coefficient of the polyester multilayer film in the longitudinal direction up to the inflection point is 0.2 to 0.4 μm/°C. After heat treatment at 120°C for 3 minutes under a load of 200g, the polyester multilayer film can be heated at room temperature. After cooling for 5 minutes, heat treatment was performed again at 140° C. for 3 minutes under a load of 200 g, and after cooling at room temperature for 5 minutes, the change in the longitudinal direction was 10 to 30 μm at 90° C. 4. polyester multilayer film according to claim 1, is characterized in that, The transparency of the multi-layer film is 0.8-2.5%. 5. polyester multilayer film according to claim 1, is characterized in that, The total thickness of the multilayer film is 50-300 μm. 6. polyester multilayer film according to claim 1, is characterized in that, The particles are one or a mixture of two or more selected from silica, zeolite and kaolin. 7. polyester multilayer film according to claim 1, is characterized in that, The polyester resin forming the surface layer has an intrinsic viscosity of 0.6 to 0.8. 8. An optical film, characterized in that, On the upper part of the polyester multilayer film described in any one of claims 1 to 7, any one or more functional components selected from a hard coat layer, an adhesive layer, a light diffusion layer, an ITO layer, and a printing layer are formed. coating. 9. A method for producing a polyester multilayer film, comprising the following steps: (a) melt extrusion is used to form each polyester resin composition of core layer and surface layer, and carry out co-extrusion; (b) uniaxially or biaxially stretching the coextruded sheet to produce a film; and (c) heat-treating the stretched film, Wherein, based on the total weight of the film, the content of the core layer is 70-90% by weight, and the content of the surface layer is 10-30% by weight, The polyester resin composition forming the surface layer includes a polyester resin having a difference in intrinsic viscosity from the polyester resin forming the core layer of less than 0.15, and 20-100 ppm of particles having an average particle diameter of 0.5-5 μm. 10. the manufacture method of polyester multilayer film according to claim 9, is characterized in that, The heat treatment step includes the step of performing relaxation along the machine direction and the width direction. 11. the manufacture method of polyester multilayer film according to claim 9, is characterized in that, When performing the relaxation, the relaxation rate is adjusted by dividing regions so as to satisfy the following formula 1, Formula 1: Relaxation rate of the first step/relaxation rate of the second step≧1.05.