CN109789683B

CN109789683B - Polyester multilayer film and process for producing the same

Info

Publication number: CN109789683B
Application number: CN201780059748.0A
Authority: CN
Inventors: 曹铉
Original assignee: Kolon Industries Inc
Current assignee: Kolon Industries Inc
Priority date: 2016-09-29
Filing date: 2017-09-26
Publication date: 2021-08-13
Anticipated expiration: 2037-09-26
Also published as: TW201813802A; CN109789683A; KR102466413B9; TWI707763B; KR20180035341A; KR102466413B1; WO2018062816A1

Abstract

The present invention relates to an optical polyester multilayer film having high transparency and excellent surface properties and a method for manufacturing the same, and more particularly, to a polyester multilayer film having a low shrinkage rate and a small difference in shrinkage rate between a central portion and an edge portion in a wide film having a full width of 5000mm or more, thereby reducing a bending phenomenon and a sag (baggy) phenomenon occurring in a subsequent process step, and a method for manufacturing the same.

Description

Polyester multilayer film and process for producing the same

Technical Field

The present invention relates to an optical polyester multilayer film having high transparency and excellent surface properties and a method for manufacturing the same, and more particularly, to a polyester multilayer film having a low shrinkage rate and a small difference in shrinkage rate between a central portion and an edge portion in a wide film having a full width of 5000mm or more, thereby improving a bending phenomenon and reducing a sag (baggy) phenomenon occurring in a post-treatment process such as prism coating or diffusion coating, and a method for manufacturing the same.

Background

The optical film is a film used as an optical member for a display, and is used as an optical material for an LCD BLU, or as an optical member for surface protection of various displays such as an LCD, a PDP, and a touch panel (touch panel).

In order to adapt such optical films to applications such as touch panels or mobile phones, important factors for improving the quality of the films are to exhibit appropriate transparency, low shrinkage and excellent surface characteristics.

In addition, when an optical film is applied to a display product after a post-treatment process such as prism coating or diffusion coating, the heat resistance is more excellent when two or three films having a thickness of 125 μm or less are laminated than when one film having a thickness of 250 μm is used in order to improve the heat resistance as the display product area is increased in recent years. However, such a laminated film has a problem that, when films having different physical properties such as lamination shrinkage, orientation, and thickness are laminated, the film may curl or be difficult to laminate.

Therefore, in a master roll (master roll) having a film full width of 5000mm or more, a difference in shrinkage rate occurs between the edge portion and the central portion of the film due to a bending phenomenon in the width direction which occurs during the stretching process and the relaxation process. In order to improve such a bending phenomenon, a method of finely stretching in a heat-set region is proposed, but in stretching in the heat-set region, uniformity of physical properties in a width direction is deteriorated due to stretching non-uniformity, or since stretching is performed at a high temperature, crystallization occurs to cause breakage, and the like, so that workability is deteriorated. In addition, when the relaxation in the width direction is generally performed, the relaxation is performed in two regions or one region to control the shrinkage rate, and the relaxation method causes the bending more.

Further, when the wide film is laminated as described above to be applied to a display, only the central portion is used or the edge portions are laminated to each other in order to laminate films having the same physical properties.

When only the central portion is used, the edge portions need to be cut and discarded, which reduces the yield of the product, and when the edge portions are used while being laminated, an additional inspection step of measuring the orientation angle for lamination is required due to the difference in physical properties of the edge portions, which makes management and production difficult and also reduces the production yield.

Disclosure of Invention

Technical problem

The present invention provides a polyester multilayer film, which has a small difference in shrinkage rate between a central portion and an edge portion based on the entire width of the film, thereby having a uniform shrinkage rate and a uniform thickness, and thus preventing curling of the film during lamination, and preventing sagging (baggy) during a subsequent coating process such as prism coating and diffusion coating.

Further, it is an object of the present invention to provide a polyester multilayer film which is high in transparency, easy to inspect the quality of foreign substances and the like in the subsequent coating step, excellent in surface properties, good in coating workability, and low in shrinkage rate, and thus does not shrink in the subsequent step.

Technical scheme

As a result of studies to achieve the above object, the present inventors have found that the object can be achieved by performing multi-stage relaxation in 3 to 5 zones (zones) in the machine direction during relaxation, performing relaxation by providing a gradient in relaxation rate, and performing relaxation and thermal fixation under specific conditions, and have completed the present invention.

The invention relates to a polyester multilayer film, which comprises a core layer and surface layers, wherein the surface layers are respectively laminated on two surfaces of the core layer by more than one layer, the inherent viscosity of polyester resin forming the surface layers is 0.6 to 0.7,

when portions from both ends to 1/3 are defined as edge portions and portions other than the edge portions are defined as a central portion based on the full width of the film, the shrinkage rates of the edge portions and the central portion after holding at 95 ℃ for 60 minutes satisfy the following formula 1 and the difference in shrinkage rate based on the following formula 2 is 0.1 or less,

the curvature of the edge portion is 0.3% or less based on the following formula 3,

formula 1:

|S₄₅-S₁₃₅|≤0.15

in said formula 1, S₄₅Is the shrinkage in the direction at 45 degrees to the machine direction of the film, and S₁₃₅Is the shrinkage in the direction 135 degrees from the machine direction of the film,

formula 2:

the shrinkage rate in the width direction of the edge part-the shrinkage rate in the width direction of the central part is less than or equal to 0.10

Formula 3:

further, the present invention relates to a method for manufacturing a polyester multilayer film, comprising the steps of:

a) melt-extruding a first polyester resin composition for a core layer and a second polyester resin composition for a skin layer, and co-extruding the first polyester resin composition for a core layer and the second polyester resin composition for a skin layer so as to laminate three or more layers, wherein the first polyester resin composition for a core layer comprises a polyester resin, and the second polyester resin composition for a skin layer comprises a polyester resin having an inherent viscosity of 0.6 to 0.7dl/g and inorganic particles;

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

c) the stretched film is subjected to heat treatment while being relaxed, and the stretched film is divided into 3 to 5 zones (zones) along the machine direction, a gradient in relaxation rate is provided in the width direction, the difference in relaxation rate between the first zone and the last zone is 0.1 to 0.5%, and the relaxation and heat treatment are performed at a temperature of 210 ℃ or lower.

Advantageous effects

The polyester multilayer film according to the present invention is produced from a main-roll film having a total film width of 5000mm or more, and is cut into a wide width of 1500mm or more to produce a product, and since the difference in shrinkage in the width direction between the central portion and the edge portion is small, the central portion and the edge portion can be used in laminating the film in the subsequent step, the production yield is improved, the film is not curled after lamination, and the relaxation phenomenon can be remarkably reduced.

Further, the present invention can provide a polyester multilayer film which has high transparency, is easy to perform quality inspection for foreign substances and the like in a subsequent coating process, has excellent coating workability due to excellent surface characteristics, and has less deformation due to no shrinkage during the subsequent process due to low shrinkage.

The polyester multilayer film according to the present invention is suitable for use as an optical film for a tablet personal computer, a cellular phone, or the like, and is suitable for use in a field requiring a subsequent process, a thin display, or the like because of small thermal deformation in the subsequent process.

Drawings

FIG. 1 shows an embodiment of the edge portion and the central portion of the film of the present invention.

Fig. 2 shows a method for measuring a bending ratio according to the present invention.

Fig. 3 shows the direction in which the shrinkage rate was measured in the present invention.

Reference numerals:

a: full width of film

a1, a 2: edge part

b: center part

Detailed Description

The present invention will be described in more detail below with reference to specific examples or embodiments including the drawings. However, the following specific examples or embodiments are merely one reference for describing the present invention in detail, and the present invention is not limited thereto but may be implemented in various forms.

In addition, unless defined otherwise, all technical and scientific terms have the same meaning as commonly understood by one of ordinary skill in the art. The terms used in the description of the present invention are only for effectively describing specific examples, and are not intended to limit the present invention.

As used in this specification and the appended claims, the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise.

One embodiment of the present invention is a polyester multilayer film comprising a core layer and at least one surface layer laminated on each of both surfaces of the core layer, wherein the intrinsic viscosity of a polyester resin forming the surface layer is 0.6 to 0.7,

formula 1:

|S₄₅-S₁₃₅|≤0.15

formula 2:

the shrinkage rate in the width direction of the edge part-the shrinkage rate in the width direction of the central part is less than or equal to 0.1

Formula 3:

in one embodiment of the present invention, the change in length at 95 ℃ of the polyester multilayer film measured by a thermomechanical analyzer (TMA) may satisfy the following formula 4,

formula 4:

length change in machine direction/length change in width direction is less than or equal to 1.5

In the above formula 4, the machine direction length variation and the width direction length variation are 40 μm or less, respectively, with the length variation being 95 ℃ — the initial length.

In one embodiment of the present invention, the difference in the curvature ratio between the edge portion and the central portion of the polyester multilayer film may satisfy the following formula 5,

formula 5:

the ratio of curvature of the edge portion to that of the central portion is not more than 0.2.

In one embodiment of the present invention, the inherent viscosity of the polyester resin of the polyester multilayer film may satisfy the following formula 6,

formula 6:

0≤|Ns-Nc|<0.1

in the formula 6, Ns is the intrinsic viscosity of the polyester resin forming the skin layer, and Nc is the intrinsic viscosity of the polyester resin forming the core layer.

In one embodiment of the present invention, the center line average roughness Ra of the polyester multilayer film may be 10 to 25nm, and the ten-point average roughness Rz may be 100 to 400 nm.

In one embodiment of the present invention, the total thickness of the polyester multilayer film may be 25 to 125 μm, and the core layer may account for 70 to 90 wt% and the surface layer may account for 10 to 30 wt% of the entire film.

In one embodiment of the present invention, the surface layer may include 10 to 100ppm of inorganic particles.

In one embodiment of the present invention, the haze of the polyester multilayer film may be 0.5% to 2.5%.

In one embodiment of the present invention, the inorganic particles may have an average particle diameter of 0.5 to 5 μm.

One embodiment of the method for producing a polyester multilayer film of the present invention comprises the steps of:

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

In one embodiment of the method for manufacturing a polyester multilayer film of the present invention, in the step c), the total relaxation rate may be 2% to 7%, and the relaxation rate of the first region may be 1.0% to 1.5%.

In one embodiment of the method for producing a polyester multilayer film of the present invention, in the step b), the stretching ratio in the mechanical direction and the width direction may be 2 to 6 times when the biaxial stretching is performed.

In one embodiment of the method for manufacturing a polyester multilayer film of the present invention, in the step a), the intrinsic viscosity of the polyester resin may satisfy the following formula 6,

formula 6:

0≤|Ns-Nc|<0.1

In one embodiment of the method for manufacturing a polyester multilayer film of the present invention, in the step a), the core layer may account for 70 to 90 wt% of the entire film, and the skin layer may account for 10 to 30 wt%.

In one embodiment of the method for manufacturing a polyester multilayer film of the present invention, in the step a), the inorganic particles may have an average particle diameter of 0.5 to 5 μm and may be contained in an amount of 10 to 100 ppm.

The structure of the present invention will be described in more detail below.

Specifically, one embodiment of the present invention is a polyester multilayer film comprising three or more layers, including a core layer and one or more surface layers laminated on one surface or both surfaces of the core layer. More specifically, in one embodiment of the present invention, the laminate may include a core layer and one or more surface layers laminated on both surfaces of the core layer.

The total thickness of the polyester multilayer film may be 25 to 125 μm, and more preferably 38 to 100 μm, and in the above range, it can be suitably used in a display device that tends to be manufactured from the film, but is not limited thereto.

Further, if the core layer accounts for 70 to 90 wt% and the surface layer accounts for 10 to 30 wt% of the entire film, it is preferable that the interface stability is excellent at the time of coextrusion, film formation is easy, and a film having low haze and small surface roughness and shrinkage can be produced, but the present invention is not limited thereto.

The core layer may be formed of a polyester resin, and more particularly, may be formed of a polyethylene terephthalate resin alone. Preferably, the polyethylene terephthalate resin having an intrinsic viscosity of 0.6 to 0.7dl/g is used because it has excellent heat resistance and does not cause interfacial instability during coextrusion, but is not limited thereto.

The skin layers laminated on one surface or both surfaces of the core layer may be laminated by one layer or two or more layers, and may be laminated by coextrusion.

The surface layer may include polyester resin having an intrinsic viscosity of 0.6 to 0.7dl/g and inorganic particles, and may be stably laminated with the core layer without interfacial instability within a range in which the intrinsic viscosity satisfies the above range, so that a multi-layer film may be manufactured and may be easily processed.

More preferably, the intrinsic viscosity of the polyester resin for the core layer and the skin layer satisfies the following formula 6,

formula 6:

0≤|Ns-Nc|<0.1

In formula 6, when the value is 0.1 or more, the interface may be unstable and break, and it is difficult to form a film by coextrusion using a dispenser, and therefore, the above range is preferably satisfied, but not limited thereto.

The surface layer may contain inorganic particles, which may be used without limitation as long as they are inorganic particles generally used in this field. Specifically, for example, silica, zeolite, kaolin, titanium dioxide, and the like can be used, but are not limited thereto. These inorganic particles may be used alone or in combination of two or more. Such inorganic particles are present on the surface of the film through the stretching process, thereby improving the smoothness and windability of the film. The size and content range of the inorganic particles can meet the requirements that the average roughness Ra of a central line is 10-25 nm, and the average roughness Rz of ten points is 100-400 nm. As an example satisfying the above range, the average particle diameter of the inorganic particles may be 0.5 to 5 μm, and the content of the particles in the surface layer may be 10 to 100 ppm.

In the range where the center line average roughness Ra and the ten-point average roughness Rz satisfy the above range, the coating property is excellent in the subsequent process, the coating stability required by the user can be satisfied, and the transparency of the entire film is excellent, and the film is suitable for displays such as optical and touch panels, but the invention is not limited thereto.

When the average particle diameter of the particles is more than 5 μm, the transparency of the film is greatly reduced and there is a possibility that scratches may occur in the coating process even if the content of the particles is less than 10 ppm. When the center line average roughness Ra is more than 25nm and the ten-point average roughness Rz is more than 400nm, the protrusions of the surface are transferred and the like, which may affect the optical characteristics of the final product. When the average particle size is less than 0.5. mu.m, the transparency is lowered even if the content of the particles used is more than 100ppm, and the quality inspection is not easily performed in the coating step. When the center line average roughness Ra is less than 6nm and the ten-point average roughness Rz is less than 80nm, although the smoothness is excellent, the coating workability and the product handling property are deteriorated, so that there is a possibility that scratches or clogging occurs when the coating process is performed, resulting in coating unevenness.

As shown in FIG. 1, one embodiment of the present invention is characterized in that, when the portions from both ends to 1/3 are defined as edge portions a1 and a2 and the portions other than the edge portions are defined as a central portion b based on the full width A of the film, the shrinkage rates of the edge portions and the central portion satisfy the following formula 1 after being held at 95 ℃ for 60 minutes, the difference in shrinkage rate based on the following formula 2 is 0.1 or less,

formula 1:

|S₄₅-S₁₃₅|≤0.15

in said formula 1, S₄₅Is the shrinkage in the direction at 45 degrees to the machine direction of the film, and S₁₃₅Is a film ofIs a shrinkage rate in a direction of 135 degrees,

formula 2:

Formula 3:

in one embodiment of the present invention, when the full width a of the film is 6000mm, after cutting both sides by cutting 100mm, the edge portion a1, the center portion b and the edge portion a2 of the product are 1800 mm.

In an embodiment of the present invention, after the main roll of the film shown in fig. 1 is divided into three equal parts in the width direction a of the film to cut the edge portions a1, a2 and the central portion b, the cut roll is unwound by a predetermined length as shown in fig. 2, and the lengths of the left side and the right side are measured in a state where the same load is applied to the left side and the right side of the unwound film, more specifically, in a state where the same load is applied by a weight of 1kg, and equation 3 is calculated, thereby obtaining the bending ratios of the central portion and the edge portions.

In the above equation 2, the average length of the left and right sides means a value obtained by dividing the sum of the length of the left side and the length of the right side by 2.

The present invention satisfies the above-mentioned physical properties at the same time, and therefore, the difference in physical properties between the edge portion and the central portion is small, and the occurrence of curling or sagging (baggy) phenomenon can be prevented when the optical film is laminated. The sag phenomenon is a phenomenon in which one side of the film sags down and swells like a backpack when a coating process such as prism coating is performed.

Preferably, in the formula 1, the difference between the shrinkage rate in the direction at 45 degrees to the machine direction of the film and the shrinkage rate in the direction at 135 degrees is 0.15 or less, more specifically, 0.01 to 0.15. If it is more than 0.15, shrinkage may occur during a subsequent high-temperature process, thereby causing distortion in the 45 degree and 135 degree directions. Further, the edge portion may be severely bent, so that a severe difference in physical properties occurs between the central portion and the edge portion.

Preferably, in the formula 2, a difference between the widthwise shrinkage rate of the edge portion and the widthwise shrinkage rate of the central portion is 0.1 or less, more specifically, 0 to 0.1. When it is more than 0.1, there is a possibility that the edge portion is severely bent, so that curling occurs at the time of lamination, and a relaxation phenomenon occurs. Further, a severe difference in physical properties between the central portion and the edge portion may occur due to severe bending of the edge portion.

The bending ratio based on the formula 3 is preferably 0.3% or less, more specifically, 0 to 0.3%. In particular, in the range satisfying the physical properties that the bending ratio of the edge portion is 0.3% or less, the curling and sagging phenomenon at the time of lamination can be prevented, and further, in the range satisfying the bending ratios of the edge portion and the central portion of 0.3% or less, the curling phenomenon and the sagging phenomenon at the time of lamination can be further reduced. More preferably, the difference in the curvature between the edge portion and the central portion satisfies the following formula 5, and still more preferably, is in the range of 0 to 0.2, in which case a film having a small variation in curvature can be produced,

formula 5:

Further, when the temperature is raised to 180 ℃ at a rate of 5 ℃/min after keeping the temperature at 40 ℃ for 3 minutes as measured by a thermomechanical analyzer (TMA), the length change (dimension change) at 95 ℃ can satisfy the following formula 4,

formula 4:

In the above formula 4, more specifically, it may be 0.01 to 1.5, and it is confirmed that the bending phenomenon is improved when the amount is within the above range, and when the amount is more than 1.5, the deformation of the thin film in the subsequent processing step is large, and it may be difficult to realize the physical properties of the product.

In one embodiment of the present invention, the composition is suitable for use as an optical film such as a prism film in a range satisfying all of the physical properties of the above formulas 1 to 3.

In one embodiment of the present invention, the polyester multilayer film including the core layer and the skin layer is produced without limitation, but may be obtained by melt-extruding from two or more melt extruders, casting, and biaxial stretching. More specifically, polyester is extruded from one extruder and simultaneously melt-extruded with additives such as inorganic particles of silica, kaolin, zeolite, titanium dioxide, etc. from another extruder, and then the respective melts are polymerized in a feeding zone, subjected to co-extrusion, cast, cooled, and then sequentially subjected to biaxial stretching, heat treatment, and relaxation. By adjusting the stretching, heat treatment, and relaxation of such films, the physical properties of the films can be controlled.

More specifically, one embodiment of the method for producing a polyester multilayer film of the present invention comprises the steps of:

b) biaxially stretching the coextruded sheet to produce a film;

Specifically, the step a) is a step of manufacturing a polyester sheet by quenching and curing through a casting drum (casting drum) after co-extruding a polyester resin for forming a core layer and a skin layer, which contains inorganic particles and may be used without limitation as long as it is inorganic particles generally used in this field. Specifically, for example, silica, zeolite, kaolin, titanium dioxide, and the like can be used, but are not limited thereto.

Preferably, the intrinsic viscosity of the polyester resin used in the core layer is 0.6 to 0.7dl/g, and the intrinsic viscosity of the polyester resin used in the surface layer is 0.6 to 0.7 dl/g. More preferably, the following formula 6 is satisfied, in which case co-extrusion can be performed without occurrence of cracks or the like,

formula 6:

0≤|Ns–Nc|<0.1

Preferably, the size and content range of the inorganic particles satisfy the range of the center line average roughness Ra of 10-25 nm and the ten-point average roughness Rz of 100-400 nm. As an example satisfying the above range, the average particle diameter of the inorganic particles may be 0.5 to 5 μm, and the content of the particles in the surface layer may be 10 to 100 ppm. In the range where the center line average roughness Ra and the ten-point average roughness Rz satisfy the above range, the coating property is excellent in the subsequent process, the coating stability required by the user can be satisfied, and the transparency of the entire film is excellent, and the film is suitable for displays such as optical and touch panels, but the invention is not limited thereto.

Next, step b) is a step of stretching the coextruded sheet to produce a film, and may be monoaxially or biaxially stretched, more specifically, biaxially stretched, and may be produced by multi-stage stretching in which stretching in the machine direction is performed first and then stretching in the width direction is performed, or simultaneous stretching in both the machine direction and the width direction is performed.

More specifically, three or more layers may be formed by coextrusion, cooled on a casting roll, and then stretched 2 to 6 times, more preferably 2 to 3.7 times, and more preferably 2.8 to 3.7 times in the machine direction, and stretched 2 to 6 times, more preferably 3 to 5.5 times, and more preferably 3.4 to 4.3 times in the width direction. The stretching ratio in the machine direction is in the range of 2 to 4 times, and the stretching in the width direction can be stably performed without lowering the mechanical strength of the film, but the stretching ratio is not limited thereto. In addition, the film is prevented from breaking without decreasing the mechanical strength of the film in the range of 2 to 6 times of the stretching ratio in the width direction, but the invention is not limited thereto.

The step c) is a process of performing heat-setting and relaxation, and is characterized in that the heat-setting temperature is 210 ℃ or less, more specifically, it can be performed at a temperature of 200 to 210 ℃, and as described above, a gradient of relaxation rate in the width direction is imparted when the relaxation is performed, and the difference between the relaxation rates of the first region and the final region is 0.1 to 0.5%, and the heat treatment is performed while the relaxation is performed at a temperature of 210 ℃ or less, thereby manufacturing a film satisfying all the desired physical properties.

The relaxation rate can be calculated as follows.

The relaxation rate (%) (maximum width of the film before the relaxation processing interval-minimum width of the film in the relaxation processing interval)/maximum width of the film before the relaxation processing interval × 100.

In addition, in an embodiment of the present invention, the total relaxation rate may be 2% to 7%, and the relaxation rate of the first region may be 1.0% to 1.5%.

The relaxation and heat treatment steps are characterized in that the temperature and the relaxation rate are controlled to be within the above ranges, and when the temperature is higher than 210 ℃, the shrinkage rate in the machine direction can be reduced, but the shrinkage rate in the width direction is increased, and the bending phenomenon is accelerated.

Further, when the difference in the relaxation rates of the first region and the last region to which the relaxation rate is given is less than 0.1, it is difficult to control the shrinkage rate in the width direction, and when the relaxation rate is increased to control the shrinkage rate, the film sags inside a tenter (tenter). When the difference in relaxation rate is greater than 0.5, a more severe bowing phenomenon occurs, and the difference in shrinkage rate between the edge portion and the central portion may become large.

The present invention will be described in more detail below based on examples and comparative examples. However, the following examples and comparative examples are merely illustrative examples for describing the present invention in more detail, and the present invention is not limited to the following examples and comparative examples.

1) Intrinsic viscosity

To 100ml of a reagent prepared by mixing phenol and 1,1,2, 2-tetrachloroethanol at a weight ratio of 6:4, 0.4g of PET particles (sample) was added, and after dissolving for 90 minutes, the mixture was transferred to an Ubbelohde viscometer and kept in a 30 ℃ incubator for 10 minutes, and then the number of seconds of fall of the solution was determined using the viscometer and a pipette (asparator). After the number of seconds of solvent drop was measured by the same method, R.V values and i.v. values were calculated by the following equations 1 and 2.

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

Mathematical formula 1:

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

Mathematical formula 2:

2) haze degree

The test pieces of the produced film were measured in accordance with JIS K715 using a HAZE METER (Model: Nipon 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 corporation, SE 3300).

4) Shrinkage rate

After the film was cut into a size of 200mm × 200mm, the length and width of the film were measured, and then heat-treated in a hot air oven at 95 ℃ for 60 minutes, after which the length and width after the change were measured and calculated by the following formulas. At this time, changes in the machine direction and the direction at 45 degrees to the machine direction, and in the direction at 135 degrees to the machine direction and the width direction were measured together. Directions at 45 degrees and 135 degrees to the machine direction are shown in fig. 3.

Percent shrinkage (%) (length measured before heat treatment-length measured after heat treatment)/length measured before heat treatment x 100

5) Thermomechanical analyzer (TMA) measurement

The film test piece was cut into a size of 16mm in the machine direction and 4.5mm in the width direction, and then the heat distortion length (dimensional change) was measured using TMA (TA corporation, TMA Q400).

The measurement was carried out by holding the temperature at 40 ℃ for 3 minutes at a constant temperature (isothermal), then raising the temperature at a rate of 5 ℃/min to 180 ℃, and then measuring the change in length at 95 ℃. Length change-length at 95 ℃ -initial length.

6) Bending Rate (%)

The rolls having a width of 2000mm were cut into three equal parts from both ends of a main roll having a film full width of 6000mm, and as shown in fig. 2, the lengths of the left side and the right side of the product roll (main roll) at the edge portion and the central portion were measured in a state where the same load of 1kg was applied to the left side and the right side of the film, and the bending ratio was calculated according to the following equation.

[ example 1]

Polyethylene terephthalate (PET) having an inherent viscosity of 0.65dl/g was used for the core layer, PET having an inherent viscosity of 0.64dl/g and silica particles having an average particle size of 2.5 μm at 70ppm were used for the skin layers, which were respectively co-extruded to form a three-layer film in which the skin layer/core layer/skin layer were laminated, and then cast on a chill roll to produce an unstretched sheet. In this case, the core layer accounts for 80 wt% of the total weight of the film, and the skin layer accounts for 20 wt% of the total weight of the film. The film was stretched 3 times in the machine direction and 3.4 times in the width direction in this order, and heat-treated at 210 ℃ to relax 3.5% in total in the four regions as shown in table 1, and relaxation rate gradients of 1.2%, 1.0%, 0.9%, and 0.8% were applied to the first region, the second region, the third region, and the fourth region, respectively, to produce a film having a total thickness of 75 μm.

The physical properties of the produced films were measured and are shown in the following tables 2 and 3.

[ examples 2 and 3]

As shown in table 1 below, films were produced in the same manner as in example 1, except that the relaxation rate in the width direction was changed.

[ examples 4 and 5]

As shown in table 1 below, films were produced in the same manner as in example 1, except that the intrinsic viscosity of the skin layer material and the relaxation rate in the width direction were changed.

[ examples 6 and 7]

As shown in table 1 below, films were produced in the same manner as in example 1, except that the weight of the surface layer was changed.

[ examples 8 to 11]

As shown in table 1 below, films were produced in the same manner as in example 1, except that the particle content and particle size of the surface layer were changed.

[ examples 12 to 16]

As shown in table 1 below, the conditions were changed, and a thin film was produced in the same manner as in example 1.

Comparative example 1

The same procedure as in example 1 was conducted, except that after heat treatment was conducted at 200 ℃ and relaxation was conducted only in the first region along the width direction as shown in table 1 below, a film having a thickness of 75 μm was obtained.

Comparative examples 2 and 3

As shown in table 1 below, a film was produced in the same manner as in example 1, except that the relaxation rate in the width direction was changed.

Comparative examples 4 and 5

As shown in table 1, the procedure was performed in the same manner as in example 1, except that the inherent viscosity and the relaxation rate of the skin layer were changed.

Comparative example 6

As shown in table 1 below, the procedure was performed in the same manner as in example 1, except that the weight and the relaxation rate of the skin layer were changed.

Comparative example 7

As shown in Table 1 below, the same procedure as in example 1 was repeated except that the heat treatment was carried out at 220 ℃.

[ Table 1]

Co-extrusion into A/B/A (the thickness of the surface layer is the total of the two A layers)

[ Table 2]

[ Table 3]

Claims

1. A polyester multilayer film characterized in that,

comprises a core layer and a surface layer, wherein the surface layer is laminated with more than one layer on two surfaces of the core layer respectively, the intrinsic viscosity of polyester resin forming the surface layer is 0.6 to 0.7dl/g,

formula 1:

|S₄₅-S₁₃₅|≤0.15

formula 2:

Formula 3:

the bending ratio was calculated in equation 3 by cutting a main roll film having a total width of 6000mm into three parts, i.e., two edge portions and one central portion, such that each width from both ends is 2000mm, and measuring the lengths of the right side and the left side of the main roll film of the edge portions and the central portion in a state where the same load of 1kg is applied to the left side and the right side of the main roll film.

2. The polyester multilayer film according to claim 1,

the change in length at 95 ℃ as measured by a thermomechanical analyzer (TMA) satisfies the following formula 4,

formula 4:

3. The polyester multilayer film according to claim 1,

the difference in the bending ratios between the edge portion and the central portion satisfies the following formula 5,

formula 5:

4. The polyester multilayer film according to claim 1,

the intrinsic viscosity of the polyester resin satisfies the following formula 6,

formula 6:

0≤|Ns-Nc|<0.1

5. The polyester multilayer film according to claim 1,

the center line average roughness Ra is 10-25 nm, and the ten-point average roughness Rz is 100-400 nm.

6. The polyester multilayer film according to claim 1,

the total thickness of the polyester multilayer film is 25-125 μm,

in the whole film, the core layer accounts for 70-90 wt%, and the surface layer accounts for 10-30 wt%.

7. The polyester multilayer film according to claim 1,

the surface layer contains 10 to 100ppm of inorganic particles.

8. The polyester multilayer film according to claim 1,

the haze of the polyester multilayer film is 0.5-2.5%.

9. The polyester multilayer film according to claim 7,

the inorganic particles have an average particle diameter of 0.5 to 5 μm.

10. A method for manufacturing a polyester multilayer film according to claim 1, comprising the steps of:

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

c) the stretched film is subjected to heat treatment while being relaxed, and the stretched film is divided into 3 to 5 zones in the machine direction, and a gradient in relaxation rate is provided in the width direction, and the difference in relaxation rate between the first zone and the last zone is 0.1 to 0.5%, and the relaxation and heat treatment are performed at a temperature of 210 ℃ or lower.

11. The process for producing a polyester multilayer film according to claim 10,

in the step c), the total relaxation rate is 2% to 7%, and the relaxation rate of the first region is 1.0% to 1.5%.

12. The process for producing a polyester multilayer film according to claim 10,

in the step b), the biaxial stretching is performed with a stretching ratio of 2 to 6 times in the machine direction and the width direction.