TWI624374B - Polyester multilayer film and manufacturing method thereof - Google Patents

Abstract

The present invention provides a polyester multilayer film having high transparency, excellent surface properties, and low shrinkage, and a process for the preparation thereof.

Description

Polyester multilayer film and preparation method thereof

The present application claims priority to Korean Patent Application No. 10-2016-0079242, filed on Jun.

The following disclosure relates to a polyester multilayer film having high transparency, excellent surface properties, and low shrinkage to enable use in an optical film and a method of preparing the same.

The optical film is used as an optical member of a display, and is used as an optical material for a liquid crystal display (LCD) backlight unit (BLU) or as a liquid crystal display or a plasma display (plasma display). Optical components on the surface of various displays such as panels, PDPs, and touch panels.

In order to make an optical film suitable for a touch panel, a mobile phone, etc., it is important to improve the film quality by achieving appropriate transparency, low shrinkage ratio, and excellent surface properties.

In order to achieve this, the polyester film may be aged and used at a high temperature, or a high heat resistant polymer such as polyethylene naphthalate (PEN) or polyimide (PI) may be used. However, when the polyester film is aged and used at a high temperature, there is a problem in that the polyester multilayer film is insufficient in production yield and deformed by moisture or the like, and the polyester multilayer film shrinks and deforms during aging. The manufacturing cost is much higher than the manufacturing cost of the polyester, and the post-treatment is difficult to carry out.

An embodiment of the present invention provides a polyester multilayer film which is excellent in transparency and which is easy to perform quality inspection of impurities and the like during coating treatment in post-treatment, and has excellent coating processability due to excellent surface properties. And the polyester multilayer film does not shrink during post-treatment due to low shrinkage.

In particular, embodiments of the present invention are directed to providing a polyester multilayer film that satisfies a specific range in a 45 degree direction and a 135 degree direction with respect to a machine direction (MD) of a polyester multilayer film. To prevent the occurrence of heat shrinkage in post-treatment.

In a general aspect, there is provided a polyester multilayer film having three or more layers by coextruding a polyester resin, wherein a transparency based on a thickness of 75 μm is 2.5% or less, relative to a polyester multilayer film The thermal contraction rate in the 45-degree direction and the 135-degree direction of the machine direction satisfies the following Equation 1, and when measured using a thermomechanical analyzer (TMA), the inflection point in the machine direction is 100 ° C. To 160 ° C, when the polyester multilayer film is heated to 180 ° C at a rate of 5 ° C per minute after being kept at 40 ° C for 3 minutes, the dimensional change satisfies the following Equation 2:

[Equation 1]

|S ₄₅ - S ₁₃₅ | ≤ 0.35

In the above Equation 1, S ₄₅ is a shrinkage ratio in a direction of 45 degrees with respect to the mechanical direction of the polyester multilayer film, and S ₁₃₅ is a shrinkage ratio in a direction of 135 degrees with respect to the mechanical direction of the polyester multilayer film, and

[Equation 2]

0.4 ≤ Dimensional change in the machine direction / Dimensional change in the transverse direction (TD) ≤ 2.0

In the above Equation 2, the dimensional change means the length at 120 ° C minus the initial length.

In another general aspect, a method of preparing a polyester multilayer film comprising: a) coextruding a sheet by melt extrusion of a first polyester resin composition and a second polyester resin composition For a stack of three or more layers, the first polyester resin composition is used for the core layer, which comprises a polyester resin having an intrinsic viscosity of 0.6 dl/g to 0.7 dl/g, and a second polyester resin composition for the skin a layer comprising a polyester resin and inorganic particles having an intrinsic viscosity of from 0.65 dl/g to 0.8 dl/g; b) biaxially stretch coextruded sheets in a range satisfying the following Equation 3, thereby preparing Polyester multilayer film;

[Equation 3]

E _MD × 1.1 ≤ E _TD ≤ E _MD × 1.5

In the above Equation 3, E _MD is a draw ratio in the machine direction, and E _TD is a draw ratio in the transverse direction,

c) Heat treatment is performed while simultaneously stretching the stretched film in the mechanical direction by 1.1% to 2.0% and in the transverse direction by 2% to 12%.

In the following, the invention will be described in detail by way of specific embodiments or examples of the accompanying drawings. In the meantime, the following exemplary embodiments or examples are provided as a reference for the detailed description of the present invention, and thus the invention is not limited thereto but may be embodied in various ways.

Further, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention belongs, unless otherwise defined. The terms used in the description of the present invention are intended to describe the specific exemplary embodiments more effectively, and are not intended to limit the invention.

In addition, the singular forms used in the claims and claims are intended to be

According to an exemplary embodiment of the present invention, there is provided a polyester multilayer film having three or more layers by coextruding a polyester resin, wherein a transparency based on a thickness of 75 μm is 2.5% or less, in comparison with a polyester multilayer The thermal contraction rate of the 45-degree direction and the 135-degree direction of the mechanical direction of the film satisfies the following Equation 1, and when measured using a thermomechanical analyzer (TMA), the reversal point in the mechanical direction is 100 ° C to 160 °C, and when the polyester multilayer film is heated to 180 ° C at a rate of 5 ° C per minute after being kept at 40 ° C for 3 minutes, the dimensional change satisfies the following Equation 2:

[Equation 1]

|S ₄₅ - S ₁₃₅ | ≤ 0.35

In the above Equation 1, S ₄₅ is a shrinkage ratio in a direction of 45 degrees with respect to the mechanical direction of the polyester multilayer film, and S ₁₃₅ is a shrinkage ratio in a direction of 135 degrees with respect to the mechanical direction of the polyester multilayer film, and

[Equation 2]

0.4 ≤ Dimensional change in the machine direction / Dimensional change in the lateral direction ≤ 2.0

In the above Equation 2, the dimensional change means the length at 120 ° C minus the initial length.

In an exemplary embodiment of the invention, the polyester multilayer film may have more than three layers including a core layer and at least one skin layer stacked on one or both sides of the core layer.

In an exemplary embodiment of the invention, the skin layer may comprise polyester resin and inorganic particles having an intrinsic viscosity of from 0.65 dl/g to 0.8 dl/g.

In an exemplary embodiment of the present invention, the inorganic particles may have an average particle diameter of 0.5 to 5 μm, and the content of particles in the skin layer may be 10 ppm to 100 ppm.

In an exemplary embodiment of the present invention, the polyester multilayer film may have a center line average roughness (Ra) of 6 nm to 25 nm and a ten point average roughness (Rz) of 80 nm to 400 nm.

In an exemplary embodiment of the present invention, the polyester multilayer film may have a total thickness of from 25 micrometers to 125 micrometers, and the core layer may be contained in an amount of from 70 wt% to 90 wt%, based on the total weight of the polyester multilayer film. The content of the layer may be from 10 wt% to 30 wt%.

According to another exemplary embodiment of the present invention, a method for preparing a polyester multilayer film is provided, comprising:

a) the coextruded sheet is a stack of three or more layers by melt-extruding the first polyester resin composition and the second polyester resin composition, and the first polyester resin composition is used for the core layer, Containing a polyester resin having an intrinsic viscosity of 0.6 dl/g to 0.7 dl/g, and a second polyester resin composition for the skin layer comprising a polycondensation having an intrinsic viscosity of 0.65 dl/g to 0.8 dl/g. Ester resin and inorganic particles;

b) biaxially stretching the coextruded sheet within a range satisfying the following Equation 3, thereby preparing a polyester multilayer film;

[Equation 3]

E _MD × 1.1 ≤ E _TD ≤ E _MD × 1.5

In the above Equation 3, E _MD is a draw ratio in the machine direction, and E _TD is a draw ratio in the transverse direction,

c) Heat treatment is performed while simultaneously stretching the stretched film in the mechanical direction by 1.1% to 2.0% and in the transverse direction by 2% to 12%.

In an exemplary embodiment of the present invention, the stretching ratio in the machine direction may be 2 to 4 times, and the stretching ratio in the transverse direction may be 2.2 to 6 times.

According to an exemplary embodiment of the present invention, in the polyester multilayer film, the transparency based on the thickness of 75 μm may be 2.5% or less, and the heat shrinkage ratio in the direction of 45 degrees and the direction of 135 degrees with respect to the mechanical direction of the polyester multilayer film may be The following Equation 1 is satisfied, and when measured using a thermomechanical analyzer (TMA), the reversal point in the mechanical direction may be 100 ° C to 160 ° C, and when the polyester multilayer film is kept at 40 ° C for 3 minutes After heating to 180 ° C at a rate of 5 ° C per minute, the dimensional change can satisfy the following Equation 2:

[Equation 1]

|S ₄₅ - S ₁₃₅ | ≤ 0.35

In the above Equation 1, S ₄₅ is a shrinkage ratio in a direction of 45 degrees with respect to the mechanical direction of the polyester multilayer film, and S ₁₃₅ is a shrinkage ratio in a direction of 135 degrees with respect to the mechanical direction of the polyester multilayer film,

[Equation 2]

0.4 ≤ Dimensional change in the machine direction / Dimensional change in the lateral direction ≤ 2.0

In the above Equation 2, the dimensional change means the length at 120 ° C minus the initial length.

In an exemplary embodiment of the present invention, the polyester multilayer film may have a center line average roughness (Ra) of 6 nm to 25 nm and a ten point average roughness (Rz) of 80 nm to 400 nm.

The various architectures of the present invention are described in more detail below.

In particular, the present invention relates to a polyester multilayer film having three or more layers including a core layer and at least one skin layer stacked on one or both sides of the core layer. More specifically, in an exemplary embodiment of the invention, the polyester multilayer film may include a core layer and one or more skin layers stacked on both sides of the core layer.

The polyester multilayer film may have a total thickness of from 25 μm to 125 μm, more preferably from 50 μm to 100 μm, and is applicable to a display which tends to be a film in the above range.

Further, when the content of the core layer is 70% by weight to 90% by weight and the content of the skin layer is 10% by weight to 30% by weight based on the total weight of the polyester multilayer film, it is preferable that the co-extrusion is performed. The interface is excellent in stability and can be easily formed into a film, and it is possible to prepare a film having a small surface roughness and a small shrinkage.

The core layer is preferably formed of a polyester resin, more specifically, a polyethylene terephthalate resin. The polyethylene terephthalate resin used herein preferably has an intrinsic viscosity of from 0.6 dl/g to 0.7 dl/g because it is excellent in heat resistance and does not cause interface instability during coextrusion.

The skin layers stacked on one or both sides of the core layer may be stacked with one layer or more than one layer and may be stacked by co-extrusion.

The skin layer may comprise a polyester resin and inorganic particles having an intrinsic viscosity of 0.65 dl/g to 0.8 dl/g, and can stably be stacked with the core layer without causing interface instability in a range in which the intrinsic viscosity satisfies the above range. Thereby, a multilayer film is prepared and can be easily processed.

The inorganic particles can be used without limitation as long as they are inorganic particles commonly used in the art. Specifically, for example, cerium oxide, zeolite, kaolin, or the like can be used, but the present invention is not limited thereto. These particles appear on the surface of the film through a stretching process to improve the slidability and windability of the film.

The size and content of the inorganic particles preferably satisfy a range in which the center line average roughness (Ra) is from 6 nm to 25 nm and the ten-point average roughness (Rz) is from 80 nm to 400 nm. As an example satisfying the above range, the inorganic particles may have an average particle diameter of 0.5 μm to 5 μm, and the content of the particles in the skin layer may be 10 ppm to 100 ppm. When the post-treatment is performed within a range in which the center line average roughness (Ra) and the ten-point average roughness (Rz) satisfy the above conditions, the coatability can be excellent and the coating stability required by the user can be satisfied, and The transparency of the film can be excellent, and is suitable for displays such as optical applications and touch panels.

When the average particle diameter of the particles is more than 5 μm, even if the particle content is less than 10 ppm, the transparency of the film may be drastically lowered, and scratches may occur when the coating process is performed. When the center line average roughness (Ra) is greater than 25 nm and the ten point average roughness (Rz) is greater than 400 nm, the protrusions on the surface may be transferred, etc., that is, the optical properties of the finished product may be affected. When the average particle diameter is less than 0.5 μm, even if the particle content is more than 100 ppm, the transparency is low, and when the coating process is performed, quality inspection is difficult. When the center line average roughness (Ra) is less than 6 nm and the ten point average roughness (Rz) is less than 80 nm, the smoothness is excellent, but the coating processability and product workability are degraded, and when the coating process is performed It may cause scratches or blockages, which may result in uneven coating.

The polyester multilayer film of the present invention is required to satisfy all of the following physical properties, wherein the transparency at a thickness of 75 μm is 2.5% or less, and the heat shrinkage ratio in a direction of 45 degrees and a direction of 135 degrees with respect to the mechanical direction of the polyester multilayer film. The following Equation 1 is satisfied, and when measured using a thermomechanical analyzer (TMA), the reversal point in the mechanical direction is 100 ° C to 160 ° C, and when the polyester multilayer film is held at 40 ° C for 3 minutes When heated to 180 ° C at a rate of 5 ° C per minute, the dimensional change satisfies the following Equation 2:

[Equation 1]

|S ₄₅ - S ₁₃₅ | ≤ 0.35

In the above Equation 1, S ₄₅ is a shrinkage ratio in a direction of 45 degrees with respect to the mechanical direction of the polyester multilayer film, and S ₁₃₅ is a shrinkage ratio in a direction of 135 degrees with respect to the mechanical direction of the polyester multilayer film, and

[Equation 2]

0.4 ≤ Dimensional change in the machine direction / Dimensional change in the lateral direction ≤ 2.0

In the above Equation 2, the dimensional change means the length at 120 ° C minus the initial length.

More specifically, the transparency means the haze measured according to JIS K 715, and based on the multilayer film having a total thickness of 75 μm, the transparency is 2.5% or less, more specifically 0.5% to 2.5%. When the transparency is more than 2.5%, the transparency is low. Therefore, when post-treatment is used to use the film as an optical film, quality inspection of impurities or the like is not easily performed, and thus it cannot be used as an optical film.

Further, the difference between the heat shrinkage ratio in the 45-degree direction with respect to the mechanical direction of the polyester multilayer film and the heat shrinkage ratio in the 135-degree direction is preferably 0.35 or less, and when the difference is more than 0.35, due to the high-temperature process (post-treatment) Shrinkage may occur in the use of the film as an optical film, and the twisted curl may occur in the 45-degree direction and the 135-degree direction, and thus is not preferable.

Further, when measured using a thermomechanical analyzer (TMA), the reversal point of the mechanical direction is preferably from 100 ° C to 160 ° C, and the process can be carried out within the above range. During normal post-treatment, film shrinkage does not occur, thus reducing thermal deformation of the film. Here, the inversion point refers to a change point of the amount measured by TMA when the film is expanded and contracted when the base film is sampled in the machine direction and after the temperature rises. By separately preparing three samples having the same position in the lateral direction of the polyester multilayer film and different in position from the machine direction, each sample was measured using TMA to measure the reversal point, and the average value of the reversal points was calculated to Calculate the reversal point of the machine direction.

Furthermore, when measured using a thermomechanical analyzer (TMA), it is preferred that when the film is held at 40 ° C for 3 minutes and then heated to 180 ° C at a rate of 5 ° C per minute, at 120 The dimensional change in the mechanical direction at ° C is preferably from 0.01% to less than 0.13%. When the dimensional change of the machine direction at 120 ° C is less than 0.01%, it is difficult to achieve the process. When the dimensional change of the machine direction is 0.13% or more, it is difficult to achieve the physical properties of the product due to the large deformation of the film in the post-treatment.

The film of the present invention is suitable as an optical film such as a thin display or a mobile phone within a range in which all physical properties are satisfied.

The preparation of the polyester multilayer film comprising the core layer and the skin layer of the present invention is not limited, but can be obtained by casting and biaxial stretching after melt extrusion in at least two melt extruders. More specifically, the polyester is extruded from one extruder while the polyester and additives of inorganic particles such as cerium oxide, zeolite, and kaolin are melt extruded in another extruder, and then Each melt is co-extruded in a feed block, cast, cooled, and sequentially subjected to biaxial stretching, heat treatment, and relaxation. The physical properties of the film can be adjusted by controlling the stretching, heat treatment and relaxation of the film.

More specifically, a method of preparing a polyester multilayer film of the present invention comprises:

a) the coextruded sheet is a stack of three or more layers by melt-extruding the first polyester resin composition and the second polyester resin composition, and the first polyester resin composition is used for the core layer, Containing a polyester resin having an intrinsic viscosity of 0.6 dl/g to 0.7 dl/g, and a second polyester resin composition for the skin layer comprising a polycondensation having an intrinsic viscosity of 0.65 dl/g to 0.8 dl/g. Ester resin and inorganic particles;

b) biaxially stretching the coextruded sheet within a range satisfying the following Equation 3, thereby preparing a polyester multilayer film;

[Equation 3]

E _MD × 1.1 ≤ E _TD ≤ E _MD × 1.5

In the above Equation 3, E _MD is a draw ratio in the machine direction, and E _TD is a draw ratio in the transverse direction,

c) Heat treatment is performed while simultaneously stretching the stretched film in the mechanical direction by 1.1% to 2.0% and in the transverse direction by 2% to 12%.

According to the present invention described above, the stretching can be carried out within a range in which the stretching ratio satisfies the above Equation 3, and then, when the film after stretching is simultaneously relaxed in the mechanical direction by 1.1% to 2.0% and in the transverse direction by 2% to 12 When % is used, heat treatment can be performed, and therefore, the film does not shrink at a high temperature, thereby preparing a film which is advantageous for post-treatment.

Specifically, the step a) is to co-extrude the polyester resin including the core layer and the skin layer, and then solidify it by a casting drum to prepare a polyester sheet, wherein the skin layer contains inorganic particles. The inorganic particles can be used without limitation as long as they are conventionally used in the art. Specifically, for example, cerium oxide, zeolite, kaolin or the like can be used, but the present invention is not limited thereto.

The inherent viscosity of the polyester resin used in the core layer is preferably from 0.6 dl/g to 0.7 dl/g, and the intrinsic viscosity of the polyester resin used in the skin layer is preferably from 0.65 dl/g to 0.8 dl/g.

The size and content of the inorganic particles preferably satisfy a range in which the center line average roughness (Ra) is from 6 nm to 25 nm and the ten-point average roughness (Rz) is from 80 nm to 400 nm. As an example satisfying the above range, the inorganic particles may have an average particle diameter of 0.5 μm to 5 μm, and the particle content in the skin layer may be 10 ppm to 100 ppm. When the post-treatment is performed within a range in which the center line average roughness (Ra) and the ten-point average roughness (Rz) satisfy the above conditions, the coatability can be excellent and the coating stability required by the user can be satisfied, and The transparency of the film can be excellent for use in displays such as optical applications and touch panels.

Next, step b) is to stretch the coextruded sheet to prepare a polyester multilayer film, which can be carried out by uniaxial stretching or biaxial stretching, and preferably by biaxial stretching.

More specifically, after co-extrusion may be performed to form three or more layers, it is cooled in a casting roll, and then the sheet may be stretched 2 to 4 times, more preferably 2 times in the machine direction. It is 3.7 times, more preferably 2.8 times to 3.7 times, and is stretched 2.2 times to 6 times, more preferably 3 times to 5.5 times, and even more preferably 3.4 times to 4.3 times in the transverse direction. When the stretching ratio in the machine direction is 2 to 4 times, the stretching in the transverse direction can be stably performed without reducing the mechanical strength of the film. Further, when the draw ratio in the transverse direction is 2.2 to 6 times, the mechanical strength of the film is not reduced, and the film may be prevented from being broken.

Step c) is a process for performing heat setting and relaxation, wherein the heat setting temperature may be 200 ° C to 245 ° C, and when the film is simultaneously relaxed in the mechanical direction by 1.1% to 2.0% and in the transverse direction by 2% to 12% At the time, heat treatment can be performed. Therefore, it is possible to provide a film having a low heat shrinkage rate at a high temperature.

The relaxation rate can be calculated as follows.

Relaxation rate (%) = (the maximum width of the film before the relaxation treatment zone is long - the minimum width of the film in the relaxation treatment zone is long) / the maximum width of the film before the relaxation treatment zone is long × 100

Here, it is preferable to simultaneously perform slack in the machine direction and slack in the lateral direction. When the relaxation in the machine direction is performed first and the relaxation in the lateral direction is performed, even if the relaxation rate is increased, it is difficult to reduce the shrinkage ratio in the lateral direction to 0.3% or less. When the relaxation in the transverse direction is performed first and then the mechanical direction is relaxed, it is difficult to reduce the shrinkage ratio in the machine direction to 0.4% or less. Further, when the relaxation rate in the machine direction is less than 1.1%, it is difficult to reduce the shrinkage ratio in the machine direction to 0.4% or less. When the relaxation rate of the machine direction is more than 2%, it is difficult to reduce the shrinkage rate in the machine direction to less than 2%, which may increase the burden on the apparatus and thus damage the apparatus. Further, when the relaxation ratio in the transverse direction is less than 2%, it is difficult to adjust the shrinkage ratio in the transverse direction to 0.4% or less. When the relaxation rate in the transverse direction is 12% or more, the film may be excessively slack, and the film may sag heavily in the device, which may cause scratches due to friction with the device, making the operation difficult.

Specifically, the polyester multilayer film prepared by the above-described production method can satisfy the following conditions: a transparency of not more than 2.5% based on a thickness of 75 μm, and a direction of 45 degrees and a direction of 135 degrees with respect to the mechanical direction of the polyester multilayer film. The heat shrinkage rate satisfies the following Equation 1, and the reversal point in the machine direction when measured using a thermomechanical analyzer (TMA) is 100 ° C to 160 ° C, while the polyester multilayer film is maintained at 40 ° C 3 After heating to 180 ° C at a rate of 5 ° C per minute after a minute, the dimensional change satisfies the following Equation 2:

[Equation 1]

|S ₄₅ - S ₁₃₅ | ≤ 0.35

In the above Equation 1, S ₄₅ is a shrinkage ratio in a direction of 45 degrees with respect to the mechanical direction of the polyester multilayer film, and S ₁₃₅ is a shrinkage ratio in a direction of 135 degrees with respect to the mechanical direction of the polyester multilayer film, and

[Equation 2]

0.4 ≤ Dimensional change in the machine direction / Dimensional change in the lateral direction ≤ 2.0

In the above Equation 2, the dimensional change means the length at 120 ° C minus the initial length.

Hereinafter, the present invention will be described in more detail based on examples and comparative examples. Meanwhile, the following examples and comparative examples are provided to exemplify the present invention in more detail, and thus, the present invention is not limited to these examples and comparative examples.

1) Intrinsic viscosity

0.4 g of polyethylene terephthalate (PET) particles (sample) was added to a mixture of phenol and 1,1,2,2-tetrachloroethanol in a weight ratio of 6:4. In a milliliter of reagent, the mixture was dissolved for 90 minutes, then transferred to a Ubbelohde viscometer and held in a 30 ° C thermostat for 10 minutes. Then, use a viscometer and an aspirator to calculate the number of seconds the solution falls. The solvent drop seconds were measured by the same method, and the relative viscosity (R.V.) value and the intrinsic viscosity (I.V.) value were calculated by calculating Equation 1 and Equation 2 below.

In the following calculation formula, C represents the concentration of the sample.

[Calculation 1]

[Calculation 2]

2) Transparency

The transparency of the film sample thus formed was measured in accordance with JIS K 715 and using a haze meter (model name: Nippon Denshoku, model NDH 5000).

3) Surface roughness

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

4) Thermal shrinkage rate

The heat shrinkage rate was determined as follows. The film was cut to a size of 200 mm × 200 mm, and the film length and film width were measured. The film was heat-treated at 140 ° C for 60 minutes in a hot air oven, and then the film length changed after heat treatment was measured and calculated as follows. The film is wide. Here, the dimensional change in the machine direction, the dimensional change in the 45-degree direction with respect to the machine direction and the 135-degree direction, and the dimensional change in the lateral direction are measured together.

Shrinkage (%) = (length measured before heat treatment - length after heat treatment) / length measured before heat treatment × 100

5) Measurement using a thermomechanical analyzer (TMA)

The film sample was cut to a size of 16 mm in the machine direction and 4.5 mm in the transverse direction, and the length (size change) of the thermal deformation was measured using TMA (TA Instruments, TMA Q400).

For measurement, the film was isothermally held at 40 ° C for 3 minutes, then heated to 180 ° C at a rate of 5 ° C per minute, and then the reversal point (ie, the point at which the expansion and contraction changed) and the dimensional change at 120 ° C were measured. .

[Example 1]

A polyethylene terephthalate (PET) having an intrinsic viscosity of 0.65 dl/g was used for the core layer, and the PET having an intrinsic viscosity of 0.64 dl/g and an average particle diameter of 70 ppm of 2.5 μm were 2.5 μm. The cerium oxide particles are used in the skin layer, each of which is melt extruded, and coextruded into a three-layer film in which a skin layer/core layer/skin layer is stacked, and then cast on a cooling roll to prepare without Stretched sheet. At this time, the content of the core layer was 80% by weight based on the total weight of the polyester multilayer film, and the content of the skin layer was 20% by weight. The film was sequentially stretched 3.8 times in the machine direction and 3.4 times in the transverse direction. When the film after stretching was simultaneously relaxed by 1.2% in the machine direction and 3.0% in the transverse direction, the film was heat-treated at a temperature of 230 ° C. A film having a total thickness of 75 μm was prepared.

The physical properties of the prepared film were measured and shown in Table 2 below.

[Example 2 and Example 3]

The film was prepared in the same manner as in Example 1 except that the intrinsic viscosity of the skin layer raw material was changed to the following Table 1.

The physical properties of the prepared film were measured and shown in Table 2 below.

[Example 4 and Example 5]

The film was prepared in the same manner as in Example 1 except that the weight of the skin layer was changed as shown in Table 1 below.

The physical properties of the prepared film were measured and shown in Table 2 below.

[Example 6 and Example 7]

The film was prepared in the same manner as in Example 1 except that the particle content of the skin layer was changed to the following Table 1.

The physical properties of the prepared film were measured and shown in Table 2 below.

[Example 8 and Example 9]

The film was prepared in the same manner as in Example 1 except that the particle size and content of the skin layer were changed as shown in Table 1 below.

The physical properties of the prepared film were measured and shown in Table 2 below.

[Example 10 to Example 13]

The film was prepared in the same manner as in Example 1 except that the relaxation rate of the machine direction (MD) and the transverse direction (TD) was changed as shown in Table 1 below.

The physical properties of the prepared film were measured and shown in Table 2 below.

[Example 14]

A film was prepared in the same manner as in Example 1 except that the draw ratio in the machine direction was 2.8 times and the draw ratio in the transverse direction was 4.3 times.

[Comparative Example 1 and Comparative Example 2]

The film was prepared in the same manner as in Example 1 except that the relaxation rate in the machine direction and the transverse direction was changed as shown in Table 1 below.

The physical properties of the prepared film were measured and shown in Table 2 below.

[Comparative Example 3 and Comparative Example 4]

The film was prepared in the same manner as in Example 1 except that the relaxation in the machine direction and the relaxation in the transverse direction were sequentially and reversely performed as shown in the following Table 1.

The physical properties of the prepared film were measured and shown in Table 2 below.

[Comparative Example 5 and Comparative Example 6]

The film was prepared in the same manner as in Example 1 except that the intrinsic viscosity of the skin layer was changed as shown in Table 1 below.

The physical properties of the prepared film were measured and shown in Table 2 below.

[Comparative Example 7 and Comparative Example 8]

The film was prepared in the same manner as in Example 1 except that the content of the skin layer was changed as shown in Table 1 below.

The physical properties of the prepared film were measured and shown in Table 2 below.

[Comparative Example 9 to Comparative Example 12]

The film was prepared in the same manner as in Example 1 except that the particle size and content of the skin layer were changed as shown in Table 1 below.

The physical properties of the prepared film were measured and shown in Table 2 below.

[Comparative Example 13 to Comparative Example 16]

The film was prepared in the same manner as in Example 1 except that the relaxation rate in the transverse direction of the machine direction was changed as shown in Table 1 below.

The physical properties of the prepared film were measured and shown in Table 2 below.

[Comparative Example 17]

A film was prepared in the same manner as in Example 1 except that the draw ratio in the machine direction was 2.7 times and the draw ratio in the transverse direction was 4.4 times.

The physical properties of the prepared film were measured and shown in Table 2 below.

[Comparative Example 18]

A film was prepared in the same manner as in Example 1 except that the draw ratio in the machine direction was 3.9 times and the draw ratio in the transverse direction was 3.4 times.

The physical properties of the prepared film were measured and shown in Table 2 below. [Table 1] *Coextruded to A/B/A. (The skin layer weight is the total amount of the two A layers.) [Table 2]

The polyester multilayer film according to the present invention has high transparency, and is easy to perform quality inspection of impurities and the like during coating treatment in post-treatment, and has excellent coating processability due to excellent surface properties, and polyester multilayer film The shrinkage is low and does not shrink during post-treatment. Therefore, it is possible to provide a polyester multilayer film which is less deformed.

Further, the polyester multilayer film according to the present invention can be suitably used as an optical film for a flat plate, a mobile phone, and the like, and is rarely applied to a field requiring post-treatment, a thin display, or the like because it is less thermally deformed in post-processing.

Claims (9)

A polyester multilayer film having three or more layers by co-extruding a polyester resin, wherein a transparency based on a thickness of 75 μm is 2.5% or less, in a direction of 45 degrees with respect to a mechanical direction of the polyester multilayer film The heat shrinkage ratio with the 135 degree direction satisfies the following Equation 1, and when measured using a thermomechanical analyzer, the reversal point in the mechanical direction is 100 ° C to 160 ° C, and when the polyester multilayer film is at 40 After heating at °C for 3 minutes and then heating to 180 °C at a rate of 5 ° C per minute, the dimensional change satisfies the following Equation 2: In the above Equation 1, S ₄₅ is a shrinkage ratio in a 45-degree direction with respect to the mechanical direction of the polyester multilayer film, and S ₁₃₅ is in the mechanical direction with respect to the polyester multilayer film Shrinkage in the 135 degree direction, and In the above Equation 2, the dimensional change means the length at 120 ° C minus the initial length. The polyester multilayer film of claim 1, wherein the polyester multilayer film has three or more layers including a core layer and at least one skin layer stacked on one or both sides of the core layer. . The polyester multilayer film of claim 2, wherein the watch The skin layer contains polyester resin and inorganic particles having an intrinsic viscosity of from 0.65 dl/g to 0.8 dl/g. The polyester multilayer film according to claim 3, wherein the inorganic particles have an average particle diameter of from 0.5 μm to 5 μm, and the inorganic particles in the skin layer are contained in an amount of from 10 ppm to 100 ppm. The polyester multilayer film of claim 4, wherein the polyester multilayer film has a center line average roughness (Ra) of 6 nm to 25 nm and a ten point of 80 nm to 400 nm. Average roughness (Rz). The polyester multilayer film of claim 2, wherein the polyester multilayer film has a total thickness of from 25 micrometers to 125 micrometers, and based on the total weight of the polyester multilayer film, the core layer The content is from 70% by weight to 90% by weight, and the content of the skin layer is from 10% by weight to 30% by weight. A method for preparing a polyester multilayer film, comprising: a) co-extruding a sheet into a stack of three or more layers by melt-extruding a first polyester resin composition and a second polyester resin composition, The first polyester resin composition is used for a core layer comprising a polyester resin having an intrinsic viscosity of 0.6 dl/g to 0.7 dl/g, and the second polyester resin composition is used for the skin layer, The skin layer comprises a polyester resin and inorganic particles having an intrinsic viscosity of 0.65 dl/g to 0.8 dl/g; b) the biaxially stretched coextruded sheet in a range satisfying the following Equation 3 , thereby preparing the polyester multilayer film; In the above Equation 3, E _MD is a draw ratio in the machine direction, E _TD is a draw ratio in the transverse direction, and c) when the polyester multilayer film after stretching is simultaneously relaxed in the mechanical direction by 1.1% The heat treatment was carried out at a rate of 2.0% and a relaxation of 2% to 12% in the transverse direction. The method for producing a polyester multilayer film according to claim 7, wherein the stretching ratio in the mechanical direction is 2 to 4 times, and the stretching ratio in the transverse direction is 2.2 to 6 times. The method for producing a polyester multilayer film according to claim 7, wherein in the polyester multilayer film, the transparency based on a thickness of 75 μm is 2.5% or less, relative to the polyester multilayer film. The thermal contraction rate of the 45-degree direction and the 135-degree direction of the mechanical direction satisfies the following Equation 1, and when measured using a thermomechanical analyzer, the reversal point in the mechanical direction is 100 ° C to 160 ° C, and When the polyester multilayer film was heated at 40 ° C for 3 minutes and then heated to 180 ° C at a rate of 5 ° C per minute, the dimensional change satisfies the following Equation 2: In the above Equation 1, S ₄₅ is a shrinkage ratio in a 45-degree direction with respect to the mechanical direction of the polyester multilayer film, and S ₁₃₅ is in the mechanical direction with respect to the polyester multilayer film Shrinkage in the 135 degree direction, and In the above Equation 2, the dimensional change means the length at 120 ° C minus the initial length.