CN110028687B

CN110028687B - Polyester film and method for producing same

Info

Publication number: CN110028687B
Application number: CN201811653488.2A
Authority: CN
Inventors: 郑斗焕; 白尚铉; 崔城蘭; 曹铉; 宋起相; 金时敏
Original assignee: Kolon Industries Inc
Current assignee: Kolon Industries Inc
Priority date: 2013-06-27
Filing date: 2014-06-27
Publication date: 2022-03-18
Anticipated expiration: 2034-06-27
Also published as: CN105473649A; CN105473649B; TWI562890B; JP2016525465A; CN110028687A; TW201500210A

Abstract

The invention provides a polyester film and a preparation method thereof. More particularly, the present invention provides a polyester film having excellent optical properties, having a controlled thermal shrinkage rate, retarding the migration of oligomers during heating, and having a low haze change rate after heating, thereby being useful for optical applications, and a method for preparing the same.

Description

Polyester film and method for producing same

The invention relates to a polyester film and a preparation method thereof, wherein the application number of the original application is 201480046297.3, the application date is 2014, 6 and 27.

Technical Field

The present invention relates to a polyester film and a method for producing the same. More particularly, the present invention relates to a polyester film having excellent optical properties, having a controlled thermal shrinkage rate, retarding the migration of oligomers during heating, and having a low haze change rate after heating, thereby being useful for optical applications, and a method for preparing the same.

Background

The optical film is a film used as an optical member of a display, which is used as an optical material of an LCD BLU, or as an optical member for surface protection of various displays (e.g., LCD, PDP, touch screen, etc.).

Such an optical film requires excellent transparency and visibility, and a biaxially stretched polyester film having excellent mechanical and electrical properties is used as a base film.

However, since such a biaxially stretched polyester film has low surface hardness and lacks abrasion resistance or scratch resistance, when used as an optical member of various displays, its surface is easily damaged due to friction with an object or contact therewith. In order to avoid such damage, a hard coat layer is laminated on the surface of the film to be used, and in order to improve the adhesion between the polyester film as the base film and the hard coat layer, a primer layer may be formed as an intermediate layer.

Polyester films used for displays have problems in terms of quality due to oligomer migration. Therefore, in a post-treatment process after coating an adhesive thereon, such as a curing process, an aging process, etc., when the polyester film is exposed to a high temperature, oligomers migrate within the polyester film, thereby causing the polyester film to turn white or curl due to thermal deformation. In addition, diamond traces may be generated in which diamond patterns are formed under the pressure of a diamond pattern roll used in a cutting process after the preparation of the polyester film. When such whitening phenomenon and rhomboid trace occur, the film roll is contaminated in the process, and the optical properties of the final product are deteriorated.

Attempts have been made to prevent migration of oligomers in polyester films. As a method for controlling oligomerization by forming a laminated film on a polyester filmPatent document for substance migration, Japanese patent laid-open publication No.2007-253511 (10.4.2007) discloses a polyester film having a laminated film on at least one surface thereof, wherein when the film is heated at 150 ℃ for 60 minutes, the average size of oligomer particles precipitated on the laminated film is 10 μm in terms of area²And the number of particles in a field of view of 100. mu. m.times.100. mu.m is 100 or less. This invention is intended to control oligomer migration, however, oligomer migration cannot be completely retarded. Also, a polyester film is aged at a high temperature and used, or a high heat-resistant polymer such as polyethylene Phthalate (PEN) or Polyimide (PI) is used. However, when a polyester film aged at high temperature is used, the production yield of the film is insufficient, deformation due to moisture or the like occurs, and when a high heat-resistant polymer is used, although oligomer migration does not occur, the production cost is significantly higher than that of a polyester, and post-treatment is difficult.

Also, in addition to retarding oligomer migration, the touch screen panel product is essentially required to ensure processability in post-processing. In addition to the ITO film, the touch screen panel product is used by laminating three or more films such as a heat-resistant film for ITO protection and a polyester film for a hard coat layer (coating a base layer for reducing doming on both surfaces thereof). Therefore, in the case where the heat shrinkage rates of three or more films are different, problems such as curling, wrinkling, etc. occur due to the difference in heat shrinkage rates during post-treatment at high temperatures, and thus, the quality of products may be reduced.

Therefore, it is also essential to improve the process conditions to ensure different heat shrinkage rates (high heat shrinkage rate and low heat shrinkage rate) matching the process conditions of the customer enterprises.

Disclosure of Invention

Technical problem

It is an object of the present invention to provide a polyester film that completely retards oligomer migration.

Further, another object of the present invention is to provide a polyester film having high thermal properties and excellent post-treatment properties, and capable of significantly reducing thermal shrinkage in post-treatment.

Further, another object of the present invention is to provide an optical film including the polyester film.

Further, another object of the present invention is to provide a method for manufacturing a polyester film capable of retarding oligomer migration while controlling heat shrinkage rate after heating.

Further, it is still another object of the present invention to provide a polyester multilayer film suitable for an ITO process, a base film for ITO, and a base film for a hard coating layer of a touch screen.

Technical scheme

In one general aspect, a polyester film includes: a polyester-based film formed of a polyester resin, and a primer layer formed by coating a water-dispersed resin composition on one or both surfaces of the polyester-based film, wherein the polyester film satisfies the following formulas 1 and 2:

0-Smd-1.5 [ equation 1]

0-Std 1.0 (formula 2)

Wherein Smd and Std represent the thermal shrinkage (%) of the film measured according to JIS C-2318 after a polyester film having a size of 10cm width and 10cm length is maintained at 150 ℃ for 30 minutes, wherein the thermal shrinkage (%) is (length of film before heat treatment-length of film after maintaining at 150 ℃ for 30 minutes)/length of film before heat treatment x 100, Smd represents the shrinkage (%) of the film in the longitudinal direction (MD) and Std represents the shrinkage (%) of the film in the Transverse Direction (TD).

A polyester film according to an exemplary embodiment of the present invention may satisfy the following formulas 3 and 4:

0.2 or less Vmd or less 0.2[ equation 3]

Vtd is 0.2 or more and 0.2 or less [ equation 4]

Where Vmd and Vtd represent heat shrinkage (%) of a film measured according to JIS C-2318 after a polyester film having a size of 10cm wide and 10cm long is maintained at 150 ℃ for 30 minutes, where the heat shrinkage (%) is (length of film before heat treatment-length of film after maintaining at 150 ℃ for 30 minutes)/length of film before heat treatment x 100, Vmd represents deviation (%) of heat shrinkage in the longitudinal direction of 10 samples selected at intervals of 50cm based on the full width of the film, and Vtd represents deviation (%) of heat shrinkage in the transverse direction of 10 samples selected at intervals of 50cm based on the full width of the film.

A polyester film according to an exemplary embodiment of the present invention may satisfy the following formulas 5 to 7:

s (45) is more than or equal to 0 and less than or equal to 1.0 (formula 5)

S (135) is more than or equal to 0 and less than or equal to 1.0 (formula 6)

I S (135) -S (45) I is less than or equal to 0.2 (formula 7)

Wherein S (45) and S (135) represent the thermal shrinkage (%) of the film measured according to JIS C-2318 after a polyester film having a size of 10cm wide and 10cm long is maintained at 150 ℃ for 30 minutes, wherein the thermal shrinkage (%) is (the length of the film before heat treatment-the length of the film after being maintained at 150 ℃ for 30 minutes)/the length of the film before heat treatment x 100, S (45) represents the shrinkage (%) in the diagonal direction at an angle of 45 ° clockwise based on the Transverse Direction (TD) of the film, and S (135) represents the shrinkage (%) in the diagonal direction at an angle of 135 ° clockwise based on the Transverse Direction (TD) of the film.

A polyester film according to an exemplary embodiment of the present invention may satisfy the following formulas 8 and 9:

0.1590. ltoreq. ns [ formula 8]

Hf is not more than Hi x 2.5 [ formula 9]

Where ns is a plane orientation coefficient, where ns { (refractive index in the longitudinal direction + refractive index in the lateral direction)/2 } - { (refractive index of thickness in the longitudinal direction + refractive index of thickness in the lateral direction)/2 }; and is

Hf represents the haze of the film after 30 minutes at 150 ℃; hi represents the haze of the film before heating.

In the polyester film according to an exemplary embodiment of the present invention, the polyester-based film may include: and at least two skin layers laminated on both surfaces of the substrate layer, wherein the polyester resin forming the skin layers may have an oligomer content of 0.3 to 0.6 wt% and a diethylene glycol content of 0.1 to 1.2 wt%.

In the polyester film according to one exemplary embodiment of the present invention, the polyester base film may be formed by coextrusion of the base layer and the skin layer, and has an intrinsic viscosity satisfying the following formula 10:

1 < Ns/Nc.ltoreq.1.2 [ equation 10]

Wherein Ns represents the intrinsic viscosity of the polyester resin forming the skin layer; nc represents the intrinsic viscosity of the polyester resin forming the base layer.

In the polyester film according to an exemplary embodiment of the present invention, in the polyester-based film, the intrinsic viscosity of the polyester resin forming the base layer is 0.5 to 1.0, and the intrinsic viscosity of the polyester resin forming the skin layer is 0.6 to 1.0.

In the polyester film according to an exemplary embodiment of the present invention, the primer layer may have a Tg of 60 ℃ or more, a swelling ratio of 30% or less, a gel fraction of 95% or more, and a density of 1.3 to 1.4.

The haze change rate (Δ H) of the polyester film according to one exemplary embodiment of the present invention may be 0.1% or less according to the following formula 11:

Δ H (%) ═ Hf-Hi [ equation 11]

Wherein Hf represents the haze of the film after being maintained at 150 ℃ for 60 minutes; hi represents the haze of the film before heating.

In the polyester film according to an exemplary embodiment of the present invention, the water-dispersible resin composition may include (a) an acrylic resin formed by copolymerizing a glycidyl group-containing radical-polymerizable unsaturated monomer and (B) a binder resin formed of a water-dispersible polyester resin. The weight ratio of the solid content between the acrylic resin (a) formed by copolymerizing a glycidyl group-containing radical polymerizable unsaturated monomer and the water dispersible polyester-based resin (B) may be (a)/(B) of 20 to 80/80 to 20.

In the polyester film according to one exemplary embodiment of the present invention, the binder resin of the water dispersion resin composition may have a solid content of 0.5 to 10 wt%.

In the polyester film according to an exemplary embodiment of the present invention, the water-dispersed resin composition may further include a silicone-based wetting agent.

In the polyester film according to an exemplary embodiment of the present invention, the water-dispersible polyester-based resin may be formed by copolymerizing a dicarboxylic acid component including a sulfonic acid alkali metal salt compound and a diol component including diethylene glycol.

In the polyester film according to an exemplary embodiment of the present invention, the water-dispersible polyester-based resin may include diethylene glycol in an amount of 20 to 80 mol% of the entire glycol component.

In the polyester film according to an exemplary embodiment of the present invention, the water-dispersible polyester-based resin may include a sulfonic acid alkali metal salt compound in an amount of 6 to 20 mol% of the total acid components.

In the polyester film according to one exemplary embodiment of the present invention, the acrylic resin may include a glycidyl group-containing radical-polymerizable unsaturated monomer as a comonomer in an amount of 20 to 80 mol% of the total monomer components.

In the polyester film according to an exemplary embodiment of the present invention, the polyester base film may be a polyethylene terephthalate film.

In the polyester film according to one exemplary embodiment of the present invention, the thickness of the polyester-based film may be 25 to 250 μm.

In the polyester film according to one exemplary embodiment of the present invention, the dry coating thickness of the primer layer may be 20 to 150 nm.

In the polyester film according to an exemplary embodiment of the present invention, the polyester-based film may include 70 to 90 wt% of the base layer, and 10 to 30 wt% of the skin layer.

The polyester film according to an exemplary embodiment of the present invention may have a surface roughness (Ra) of 10nm or less.

In the polyester film according to one exemplary embodiment of the present invention, the surface layer may include 100ppm or less of inorganic particles.

In the polyester film according to one exemplary embodiment of the present invention, the inorganic particles may have an average particle diameter of less than 3 μm.

In the polyester film according to one exemplary embodiment of the present invention, the inorganic particles may be any one or a mixture of two or more selected from the group consisting of silica, zeolite, and kaolin.

In another general aspect, an optical film has any one or more functional coating layers selected from a hard coating layer, an adhesive layer, a light diffusion layer, an ITO layer, and a printed layer formed on the above polyester film.

In another general aspect, a method of preparing a polyester film comprises: a) preparing a polyester-based film uniaxially stretched in a Machine Direction (MD);

b) coating a water-dispersed resin composition having oligomer blocking properties on one or both surfaces of the uniaxially stretched polyester base film, thereby forming a primer layer;

c) biaxially stretching the uniaxially stretched polyester base film having the undercoat layer formed thereon in a Transverse Direction (TD); and

d) heat-setting the biaxially stretched film, and relaxing the film in the MD within a range satisfying the following formula 12:

a relaxation rate of 1.1 or more (%). or less than 2.5 [ formula 12]

Where the relaxation rate (%) (moving speed of the film before the relaxation treatment stage — moving speed of the film before the relaxation treatment stage)/moving speed of the film before the relaxation treatment stage × 100.

In the method of manufacturing a polyester film according to an exemplary embodiment of the present invention, the relaxation in the MD in the step d) may be performed in a temperature range satisfying the following formula 13:

a stretching temperature (DEG C) of not more than a relaxation temperature (DEG C) of not more than a heat-setting temperature (DEG C) (formula 13)

In the method of manufacturing a polyester film according to an exemplary embodiment of the present invention, the water-dispersible resin composition may include a binder resin formed of (a) an acrylic resin formed by copolymerizing a glycidyl group-containing radical-polymerizable unsaturated monomer and (B) a water-dispersible polyester-based resin, and the solid content weight ratio between the (a) acrylic resin formed by copolymerizing a glycidyl group-containing radical-polymerizable unsaturated monomer and the (B) water-dispersible polyester-based resin is (a)/(B) of 20 to 80/80 to 20.

In the method of manufacturing a polyester film according to an exemplary embodiment of the present invention, the polyester film may have a thermal shrinkage (%) satisfying the following formulas 1 to 4:

0-Smd-1.0 [ equation 1]

0-0 Std-0.5 (formula 2)

0.2 or less Vmd or less 0.2[ equation 3]

Vtd is 0.2 or more and 0.2 or less [ equation 4]

Wherein Smd, Std, Vmd, and Vtd represent heat shrinkage (%) of the film measured according to JIS C-2318 after a polyester film having a size of 10cm wide and 10cm long is maintained at 150 ℃ for 30 minutes, wherein the heat shrinkage (%) is (length of the film before heat treatment-length of the film after being maintained at 150 ℃ for 30 minutes)/length of the film before heat treatment x 100, Smd represents shrinkage (%) of the film in the Machine Direction (MD), Std represents shrinkage (%) of the film in the Transverse Direction (TD), Vmd represents deviation (%) of heat shrinkage in the MD of 10 samples selected at intervals of 50cm based on the full width of the film, and Vtd represents deviation (%) of heat shrinkage in the TD of 10 samples selected at intervals of 50cm based on the full width of the film.

In the method of manufacturing a polyester film according to an exemplary embodiment of the present invention, the polyester film may satisfy the following formulas 5 to 7:

s (45) is more than or equal to 0 and less than or equal to 1.0 (formula 5)

S (135) is more than or equal to 0 and less than or equal to 1.0 (formula 6)

I S (135) -S (45) I is less than or equal to 0.2 (formula 7)

Wherein S (45) and S (135) represent the thermal shrinkage (%) of the film measured according to JIS C-2318 after a polyester film having a size of 10cm wide and 10cm long is maintained at 150 ℃ for 30 minutes, wherein the thermal shrinkage (%) is (length of film before heat treatment-length of film after being maintained at 150 ℃ for 30 minutes)/length of film before heat treatment × 100; s (45) represents a shrinkage (%) in a diagonal direction at an angle of 45 ° clockwise based on a Transverse Direction (TD) of the film; s (135) represents a shrinkage (%) in a diagonal direction at an angle of 135 ° clockwise based on a Transverse Direction (TD) of the film.

In the method of manufacturing a polyester film according to an exemplary embodiment of the present invention, the primer layer may have a Tg of 60 ℃ or more, a swelling ratio of 30% or less, a gel fraction of 95% or more, and a density of 1.3 to 1.4.

In the method of manufacturing a polyester film according to one exemplary embodiment of the present invention, the haze change rate (Δ H) of the polyester film according to the following formula 11 may be 0.1% or less:

AH (%) ═ Hf-Hi [ equation 11]

In another general aspect, a method of preparing a polyester film comprises:

a) melt-extruding and co-extruding a skin layer composition comprising a first polyester resin comprising 0.3 to 0.6 wt% of an oligomer and 0.1 to 1.2 wt% of diethylene glycol of the polyester resin, and a second polyester resin for a base layer, having an intrinsic viscosity satisfying the following formula 10:

1 < Ns/Nc.ltoreq.1.2 [ equation 10]

Wherein Ns represents the intrinsic viscosity of the polyester resin forming the skin layer; nc represents the intrinsic viscosity of the polyester resin forming the base layer;

b) subjecting the coextruded sheet to unidirectional or bidirectional stretching to produce a film; and

c) heat-setting the stretched film, and relaxing the film in TD within a range satisfying the following formula 14:

TDr (%) less than or equal to 2 and less than or equal to 11.5 (formula 14)

Where TDr represents a relaxation rate in TD, where { (maximum width length of the film in TD before the relaxation treatment stage — minimum width length of the film in TD before the relaxation treatment stage)/maximum width length of the film before the relaxation treatment stage } × 100.

The top layer composition in step a) may contain less than 100ppm of inorganic particles.

Further, the inorganic particles may have an average particle diameter of less than 3 μm.

The polyester film may have a surface roughness (Ra) of 10nm or less, a thermal shrinkage rate that may satisfy the following equations 1 and 2, a plane orientation coefficient (ns) that may satisfy the following equation 8, and a haze after maintaining the film at 150 ℃ for 30 minutes that may satisfy the following equation 9:

0-Smd-1.5 [ equation 1]

0-Std 1.0 (formula 2)

0.1590. ltoreq. ns [ formula 8]

Hf is not more than Hi x 2.5 [ formula 9]

Wherein Smd and Std represent the thermal shrinkage (%) of the film measured according to JIS C-2318 after a polyester film having a size of 10cm width and 10cm length is maintained at 150 ℃ for 30 minutes, wherein the thermal shrinkage (%) is (length of film before heat treatment-length of film after being maintained at 150 ℃ for 30 minutes)/length of film before heat treatment x 100, Smd represents the shrinkage (%) of the film in MD, and Std represents the shrinkage (%) of the film in TD;

ns is a plane orientation coefficient, where ns { (refractive index in the longitudinal direction + refractive index in the lateral direction)/2 } - { (refractive index of thickness in the longitudinal direction + refractive index of thickness in the lateral direction)/2 }; and

hf represents the haze of the film after being held at 150 ℃ for 30 minutes, Hi represents the haze of the film before heating.

Further, when the relaxation is performed in step c), the relaxation in MD may be performed in a range satisfying the following formula 15 while performing the relaxation in TD:

0.3-MDr (%) -2.5 (formula 15)

Where MDr denotes a relaxation rate in MD, where the relaxation rate (%) (moving speed of the film before the relaxation treatment stage — moving speed of the film before the relaxation treatment stage)/moving speed of the film before the relaxation treatment stage × 100.

Advantageous effects

The polyester film according to the present invention has the effect of completely retarding oligomer migration under high temperature conditions.

Further, the polyester film according to the present invention has optical physical properties suitable for a heat-resistant film for ITO protection or the like in a touch screen film, and by controlling the heat shrinkage rate over the full width of the film, post-processability, particularly processability during lamination of three or more layers of films such as an ITO film, an ITO heat-resistant protective film, a low-interference pattern polyester film or the like, is easily ensured.

Detailed Description

Hereinafter, embodiments of the present invention will be described in detail.

Here, the oligomer means a dimer, trimer, tetramer, etc. having a weight average molecular weight of about 500-10000.

A polyester film according to an embodiment of the present invention includes: a polyester-based film formed of a polyester resin, and a primer layer formed by coating a water-dispersed resin composition having oligomer blocking properties on one or both surfaces of the polyester-based film and having a heat shrinkage (%) satisfying the following formulas 1 to 4:

0-Smd-1.0 [ equation 1]

0-0 Std-0.5 (formula 2)

0.2 or less Vmd or less 0.2[ equation 3]

Vtd is 0.2 or more and 0.2 or less [ equation 4]

Wherein Smd, Std, Vmd, and Vtd represent heat shrinkage (%) of the film measured according to JIS C-2318 after a polyester film having a size of 10cm wide and 10cm long is maintained at 150 ℃ for 30 minutes, wherein the heat shrinkage (%) is (length of the film before heat treatment-length of the film after being maintained at 150 ℃ for 30 minutes)/length of the film before heat treatment x 100, Smd represents shrinkage (%) of the film in a longitudinal direction (MD), Std represents shrinkage (%) of the film in a Transverse Direction (TD), Vmd represents deviation (%) of heat shrinkage in MD of 10 samples selected at intervals of 50cm based on the full width of the film, and Vtd represents deviation (%) of heat shrinkage in TD of 10 samples selected at intervals of 50cm based on the full width of the film.

The polyester film according to an exemplary embodiment of the present invention may satisfy formulas 1 and 2, or formulas 3 and 4, or all formulas 1 to 4 of the above formulas 1 to 4. The physical property condition may be combined with other conditions.

The heat shrinkage (%) of the film in the MD may be 0 to 1.5%, preferably 0.2 to 1.5%, more preferably 0 to 1.0%. When the heat shrinkage (%) of the film in the MD is less than 0%, the film may expand to increase the possibility of causing curling in post-treatment, and when it is more than 1.5%, the shrinkage property in the MD in post-treatment may be improved to also increase the possibility of causing curling. More preferably, the heat shrinkage in MD may be 0-0.9%.

Further, the heat shrinkage (%) in the TD may be 0 to 1.0%, preferably 0 to 0.5%. When the thermal shrinkage (%) in TD is less than 0%, expansion of the film in TD is caused, and when it is more than 1.0%, shrinkage property in TD in post-treatment is improved, so that it is difficult to control the curl. More preferably, the thermal shrinkage in TD is 0-0.4%.

Further, the polyester film may include those satisfying the following formulas 5 to 7:

s (45) is more than or equal to 0 and less than or equal to 1.0 (formula 5)

S (135) is more than or equal to 0 and less than or equal to 1.0 (formula 6)

I S (135) -S (45) I is less than or equal to 0.2 (formula 7)

Wherein S (45) and S (135) represent the thermal shrinkage (%) of the film measured according to JIS C-2318 after a polyester film having a size of 10cm wide and 10cm long was maintained at 150 ℃ for 30 minutes, wherein the thermal shrinkage (%) is (length of film before heat treatment-length of film after being maintained at 150 ℃ for 30 minutes)/length of film before heat treatment × 100. Further, S (45) represents a shrinkage (%) in a diagonal direction at an angle of 45 ° clockwise based on the Transverse Direction (TD) of the film, and S (135) represents a shrinkage (%) in a diagonal direction at an angle of 135 ° clockwise based on the Transverse Direction (TD) of the film.

In the present invention, the polyester film may adjust the thermal shrinkage in the diagonal direction within the above range, thereby maximizing the migration retarding property of the oligomer under high temperature conditions. In addition, an effect of improving the conventional physical properties of the polyester film (including its optical properties) can be achieved. The thermal shrinkage of the film at 45 ° clockwise based on TD and 135 ° clockwise based on TD (these angles are in the diagonal direction of the polyester film) may preferably be 0 to 1.0%. Meanwhile, the absolute value of the difference between the heat shrinkage rates in the two diagonal directions may preferably be 0.2% or less. When the absolute value of the difference between the heat shrinkages is more than 0.2%, the shrinkage balance in the diagonal direction is broken, thereby causing curling in the form of twisting.

Further, in the range where the variation of the heat shrinkage rate over the full width of the film is ± 0.2%, the uniformity of the heat shrinkage rate can be secured, and the curl can be easily controlled.

0.1590. ltoreq. ns [ formula 8]

Hf is not more than Hi x 2.5 [ formula 9]

Where ns is a plane orientation coefficient, where ns { (refractive index in the longitudinal direction + refractive index in the lateral direction)/2 } - { (refractive index of thickness in the longitudinal direction + refractive index of thickness in the lateral direction)/2 }; and

The polyester film according to an exemplary embodiment of the present invention preferably has a plane orientation coefficient of 0.1590, more preferably 0.1590 to 0.1610. When the plane orientation coefficient is less than 0.1590, the surface structure of the film is not dense, and thus surface migration of oligomers may easily occur.

Haze is used to determine the migration of the oligomer under high temperature conditions, and when it is out of the range of the above formula 9, it means that the oligomer is severely migrated, and thus the haze is reduced. Within the above haze range, post-processability is not significantly affected, and thus, the polyester film has physical properties suitable for use as an optical film.

A polyester film according to an exemplary embodiment of the present invention may include a polyester-based film including: a substrate layer, and at least two skin layers laminated on both surfaces of the substrate layer. Here, the polyester resin forming the skin layer may have an oligomer content of 0.3 to 0.6 wt% and a diethylene glycol content of 0.1 to 1.2 wt%.

The polyester base film may be composed of three or more layers including a base layer, and two or more surface layers laminated on both surfaces of the base layer, and may be formed by co-extrusion.

In order to improve the processability when the base layer and the skin layer are coextruded, it is preferable that the following formula 10 is satisfied:

1 < Ns/Nc.ltoreq.1.2 [ equation 10]

When the intrinsic viscosity ratio between the surface layer and the base layer is more than 1.2, a problem of interfacial instability occurs during the co-extrusion process so that a multi-layer structure is not formed, and therefore, the above range is preferably satisfied, and more preferably, 1.0 to 1.05 to effectively improve processability.

The total thickness of the polyester-based film is preferably 25 to 250 μm, more preferably 50 to 188 μm more effectively. When the thickness is less than 25 μm, mechanical physical properties suitable for the optical film cannot be realized, and when the thickness is more than 250 μm, the thickness of the film becomes too thick to be suitable for thinning the display.

Further, it is preferable that the content of the base layer is 70 to 90 wt% and the content of the surface layer is 10 to 30 wt% based on the entire film, and more preferably, the content of the base layer is 70 to 80 wt% and the content of the surface layer is 20 to 30 wt% based on the entire film, in order to have excellent interface stability during co-extrusion, and excellent oligomer retardation effect, thereby functioning.

Preferably, the base layer formed of a polyester resin is composed of only a polyethylene terephthalate (PET) resin. Here, the intrinsic viscosity of the polyethylene terephthalate resin used is preferably 0.5 to 1.0, more preferably, 0.60 to 0.80, thereby functioning. When the intrinsic viscosity of the polyethylene terephthalate resin of the base layer is less than 0.5, its heat resistance is lowered, and when it exceeds 1.0, it is not easy to perform raw material treatment, thereby deteriorating processability.

The surface layer formed by coextruding at least two layers on both surfaces of the polyester substrate layer may have an oligomer content of 0.3 to 0.6 wt%, more preferably 0.4 to 0.6 wt%, and a diethylene glycol (DEG) content of 0.1 to 1.1 wt%, more preferably 0.7 to 1.1 wt%, based on the total film weight. When the contents of the oligomer and diethylene glycol of the polyester resin of the surface layer are higher than the above ranges, the haze value of the initial film increases, and the haze change rate sharply increases when heat treatment processing is performed, thereby causing a problem that optical properties usable for the optical film cannot be realized.

Further, in order to make the contents of the oligomer and diethylene glycol of the polyester resin of the surface layer within the above ranges, the resin may be prepared by a synthetic method known in the art, however, in particular, the preparation of the resin by solid-phase polymerization may be effective in reducing the contents of the oligomer and diethylene glycol.

Further, it is preferable that the intrinsic viscosity of the polyester resin of the skin layer is 0.6 to 1.0, more preferably, 0.65 to 0.85 is more effective. When the intrinsic viscosity of the polyethylene terephthalate resin of the surface layer is less than 0.6, its heat resistance may be reduced, and when it exceeds 1.0, raw material processing may not be easily performed, thereby reducing processability.

A polyester film according to an exemplary embodiment of the present invention may include a primer layer having Tg of 60 ℃ or more, a swelling ratio of 30% or less, a gel fraction of 95% or more, and a density of 1.3 to 1.4. The primer layer can control migration of oligomers while reducing thermal shrinkage when manufacturing a polyester film.

Further, in the heat setting step when the film is produced, relaxation is performed under predetermined conditions so as to satisfy physical properties that the haze change rate before and after heating at 150 ℃ for 60 minutes is 0.1% or less, and satisfy the heat shrinkage rate of the film to be achieved in the present invention.

Further, it can be confirmed that the film having the oligomer mobility satisfying the above range does not show rhombohedral marks and whitening.

That is, the undercoat layer may satisfy physical properties such as a haze change rate (Δ H) of 0.1% or less according to the following formula 11 in a range where the undercoat layer satisfies the physical properties such as Tg of 60 ℃ or more, a swelling ratio of 30% or less, a gel fraction of 95% or more, and a density of 1.3 or more:

AH (%) ═ Hf-Hi [ equation 11]

Specifically, in the range where the undercoat layer satisfies physical properties such as Tg of 60 ℃ or more, more specifically 60 ℃ or more and no upper limit, the swelling ratio of 30% or less, more specifically 0% to 30%, the gel fraction of 95% or more, more specifically 95 to 100%, and the density of 1.3 or more, more specifically 1.3 to 1.4, it can be confirmed that the oligomer in the polyester film does not migrate to the surface thereof even in the case where the structural compactness of the coating layer and the mobility of the undercoat layer are reduced and higher temperature and pressure are applied thereto.

In the polyester film according to one exemplary embodiment of the present invention, the primer layer may be formed by coating a water-dispersed resin composition having oligomer blocking properties.

As the water-dispersed resin composition for forming the undercoat layer, a water-dispersed resin composition containing an acrylic resin formed by copolymerizing a glycidyl group-containing radical-polymerizable unsaturated monomer and a water-dispersed polyester resin can be used.

In one embodiment, in the water-dispersible resin composition, the solid content weight ratio between the (a) acrylic resin formed by copolymerizing a glycidyl group-containing radical-polymerizable unsaturated monomer and the (B) water-dispersible polyester-based resin may be (a)/(B) of 20 to 80/80 to 20. More specifically, a weight ratio of 40-60/60-40 may be used. When (B) the solid content of the water-dispersible polyester-based resin is less than 20 wt% and (a) the solid content of the acrylic resin formed by copolymerizing the glycidyl group-containing radical-polymerizable unsaturated monomer is more than 80 wt%, the particle size of the emulsion is increased, and thus, color unevenness occurs at the time of inline coating, and the adhesiveness and transparency with the polyester-based film are lowered. When (B) the solid content of the water-dispersible polyester-based resin is more than 80% by weight and (a) the solid content of the acrylic resin formed by copolymerizing the glycidyl group-containing radical-polymerizable unsaturated monomer is less than 20% by weight, a sufficient oligomer blocking effect cannot be achieved.

The water-dispersible resin composition of the present invention can be prepared by mixing (B) a water-dispersible polyester-based resin and (a) a binder resin formed by mixing an acrylic resin formed by copolymerizing glycidyl group-containing radically polymerizable unsaturated monomers, with water, or can be prepared by polymerizing glycidyl group-containing radically polymerizable unsaturated monomers alone or with glycidyl group-containing radically polymerizable unsaturated monomers copolymerizable with glycidyl group-containing radically polymerizable unsaturated monomers in (B) a water-dispersible polyester-based resin aqueous dispersion. Here, a surfactant or a polymerization initiator may be used. The surfactant and the polymerization initiator may be used without limitation as long as they are generally used in emulsion polymerization. Specifically, for example, as the surfactant, an anionic surfactant, a nonionic surfactant, or a non-reactive surfactant may be used, or a combination thereof may be used. As the polymerization initiator, a nitrogen compound (which is a radical polymerization initiator) such as a peroxide-based initiator or azobisisobutyronitrile may be used.

The water-dispersed composition of the present invention may further contain a defoaming agent, a wetting agent, a surfactant, a thickener, a plasticizer, an antioxidant, a UV absorber, a preservative, a crosslinking agent, and the like, as necessary.

In the aqueous dispersion composition of the present invention, (B) the aqueous dispersion polyester-based resin may be formed by copolymerizing a dicarboxylic acid component including a sulfonic acid alkali metal salt compound with a diol component including diethylene glycol.

More specifically, as the dicarboxylic acid component, aromatic dicarboxylic acids and alkali metal sulfonate compounds may be used, wherein the alkali metal sulfonate compounds may be contained in an amount of 6 to 20 mol% based on the total acid component.

As the dicarboxylic acid component, aromatic dicarboxylic acids such as phthalic acid, terephthalic acid, dimethyl terephthalic acid, isophthalic acid, dimethyl isophthalic acid, 2, 5-dimethyl terephthalic acid, 2, 6-naphthalenedicarboxylic acid or biphenyldicarboxylic acid; aliphatic dicarboxylic acids such as adipic acid or sebacic acid; alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid and the like.

As the sulfonic acid alkali metal salt compound, specifically, for example, alkali metal salts of sulfoterephthalic acid, 5-sulfoisophthalic acid, 4-sulfonaphthoic acid-2, 7-dicarboxylic acid or the like can be used, and 6 to 20 mol% is preferably used. When less than 6 mol% is used, the dispersion time of the resin in water is prolonged and the dispersibility is reduced, and when more than 20 mol% is used, the water resistance is deteriorated.

As the diol component, diethylene glycol, alicyclic diols having 2 to 8 carbon atoms or 6 to 12 carbon atoms, and the like can be used. Specifically, for example, ethylene glycol, 1, 3-propanediol, 1, 2-propanediol, neopentyl glycol, 1, 4-butanediol, 1, 4-cyclohexanedimethanol, 1, 3-cyclohexanedimethanol, 1, 2-cyclohexanedimethanol, 1, 6-hexanediol, P-xylylene glycol (P-xylylene glycol), triethylene glycol, and the like can be used. Here, preferably, diethylene glycol is contained in an amount of 20 to 80 mol% based on the entire glycol component.

Preferably, the number average molecular weight of the (B) water-dispersible polyester-based resin is 1000-50000, more preferably 2000-30000. When the number average molecular weight is less than 1000, the oligomer blocking effect may be insignificant, and when the number average molecular weight is more than 50000, water dispersion is difficult.

For the water-dispersible polyester-based resin using (B), it can be uniformly dispersed in water or a liquid solvent containing water by heating to 50 to 90 ℃ while stirring. Preferably, the aqueous dispersion thus prepared has a solid content concentration of 30 wt% or less, more preferably 10 to 30 wt%, for uniform dispersion. The aqueous solvent may be an alcohol such as methanol, ethanol or propanol, a polyhydric alcohol such as ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol or glycerol, or the like.

Next, the acrylic resin formed by copolymerizing (a) a glycidyl group-containing radical polymerizable unsaturated monomer will be described.

(A) The acrylic resin formed by copolymerizing a glycidyl group-containing radically polymerizable unsaturated monomer may be a resin formed by copolymerizing a glycidyl group-containing radically polymerizable unsaturated monomer or a homopolymer of another radically polymerizable unsaturated monomer copolymerizable with the glycidyl group-containing radically polymerizable unsaturated monomer.

The acrylic resin may contain, as a comonomer, a glycidyl group-containing radical polymerizable unsaturated monomer in an amount of 20 to 80 mol% based on the total monomer components. The glycidyl group-containing radical polymerizable unsaturated monomer improves the coating strength of the undercoat layer through a crosslinking reaction and increases the crosslinking density, thereby retarding oligomer migration. Specifically, for example, glycidyl ethers such as glycidyl acrylate, glycidyl methacrylate, allyl glycidyl ether, or the like can be used.

The radical polymerizable unsaturated monomer copolymerizable with the glycidyl group-containing radical polymerizable unsaturated monomer may be a vinyl ester, an unsaturated carboxylic acid amide, an unsaturated nitrile, an unsaturated carboxylic acid, an allyl compound, a nitrogen-containing vinyl monomer, a hydrocarbon-based vinyl monomer, a vinyl silane compound, or the like. As the vinyl ester, vinyl propionate, vinyl stearate, vinyl chloride, or the like can be used. Examples of the unsaturated carboxylic acid ester include methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, ethyl methacrylate, butyl maleate, octyl maleate, butyl fumarate, octyl fumarate, hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxypropyl methacrylate, and hydroxypropyl acrylate. As the unsaturated carboxylic acid amide, acrylamide, methacrylamide, methylolacrylamide, butoxymethylolacrylamide, and the like can be used. As the unsaturated nitrile, acrylonitrile or the like can be used. As the unsaturated carboxylic acid, acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, maleic acid ester, fumaric acid ester, itaconic acid ester, and the like can be used. As the allyl compound, allyl acetate, allyl methacrylate, allyl acrylate, allyl itaconate, diallyl itaconate, or the like can be used. As the nitrogen-containing vinyl monomer, vinylpyridine, vinylimidazole, or the like can be used. As the hydrocarbon-based vinyl monomer, ethylene, propylene, hexene, octene, styrene, vinyl toluene, butadiene, and the like can be used. Examples of the vinyl silane compound that can be used include dimethylvinylmethoxysilane, dimethylvinylethoxysilane, methylvinyldimethoxysilane, methylvinyldiethoxysilane, γ -methacryloxypropyltrimethoxysilane, and γ -methacryloxypropyldimethoxysilane.

Preferably, the water-dispersible resin composition according to one embodiment of the present invention is a water-dispersible or aqueous composition having a solid content of (a) an acrylic resin formed by copolymerizing a glycidyl group-containing radical-polymerizable unsaturated monomer and (B) a water-dispersible polyester-based resin of 0.5 to 10 wt%. More specifically, the water-dispersible resin composition may include (a) an acrylic resin formed by copolymerizing a glycidyl group-containing radically polymerizable unsaturated monomer and (B) a water-dispersible polyester-based resin in a solid content of 0.5 to 10 wt%, and the balance of water, and further include an additive such as a wetting agent or a dispersant, as needed. Wetting agents are used to improve coatability, and specifically, for example, modified silicone-based wetting agents such as Q2-5212 of Dow Corning, TEGO WET 250 of ENBODIC, BYK 348 of BYK CHEMIE, and the like can be used, but are not limited thereto. Preferably, 0.1 to 0.5 wt% of wetting agent is used, within which range the desired improvement in coating properties can be achieved.

The primer layer in the present invention may be applied in a dry thickness of 20 to 150 nm. When the dry coating thickness is less than 20nm, the oligomer retardation property cannot be sufficiently achieved, and when it is more than 150nm, coating mottle occurs and clogging, i.e., the primer layers adhere to each other, is highly likely to be caused after the film is wound.

The water-dispersed resin composition of the present invention can be coated by an in-line coating method in a polyester film production process. That is, when the polyester-based film is prepared, the water-dispersed resin composition may be coated by an in-line coating method before stretching, or before secondary stretching after primary stretching, and then stretched, and the primer layer is formed by heating to evaporate water during the secondary stretching and heat-setting processes. The coating method is not limited as long as it is any coating method known in the art.

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

a) preparing a polyester-based film uniaxially stretched in a Machine Direction (MD);

a relaxation rate of 1.1 or more (%). or less than 2.5 [ formula 12]

In step d) of the present invention, relaxation in MD is performed in a temperature range satisfying the following formula 13:

In the present invention, after the stretching as described above, the heat-setting and the relaxation are performed in the MD, the relaxation is performed under the condition satisfying formulas 9 and 10, and thus the oligomer does not migrate under the high temperature condition and the film does not shrink, thereby preparing the film advantageous for the post-treatment.

More specifically, in the method of manufacturing a polyester film of the present invention, the step a) is to feed polyester chips into an extruder and melt-extrude, and then quench and solidify them with a casting drum (casting drum) to prepare polyester flakes, after which the flakes are uniaxially stretched in MD at 80-100 ℃. Here, the stretching ratio is preferably 2 to 4 times.

Step b) is a process of coating the water-dispersible resin composition on the uniaxially stretched polyester-based film using a method known to those skilled in the art.

Step c) is to biaxially stretch the polyester base film with the primer layer formed thereon in the TD, preferably by stretching the film at 110-4 times at 150 ℃.

Step d) is heat-setting and relaxing, and may be performed in a tenter. The heat-setting temperature may be 200-240 deg.c, and the temperature during the relaxation is preferably performed within a range satisfying the following formula 13:

The relaxation may be performed in the Machine Direction (MD) using MD relaxation equipment, and in the cross direction (TD) in different paths on the sheet. The MD relaxation apparatus can control subsequent shrinkage performance in the MD by making a speed differential of about 1.1-2.5% between all 9 rails after the heat treatment stage. The speed difference may preferably be 1.2-2.0%, more preferably 1.25-2.0%. The heat shrinkage in MD under the condition of being maintained at 150 ℃ for 30 minutes is in the range of 0 to 1.0%, more preferably in the range of 0.3 to 0.9%, adjusted according to the MD relaxation ratio. Further, the thermal shrinkage in TD under the above conditions is in the range of 0 to 0.5%, more preferably 0.0 to 0.4%. Further, it is preferable that the deviation of the heat shrinkage rate based on the full width of the main roll is within ± 0.2% in MD/TD.

The surface layer in the present invention may include inorganic particles, and preferably, the initial haze of the film satisfies a range of less than 1.5%. Further, the surface roughness of the film is preferably 10nm or less. When the surface roughness is more than 10nm, a problem of smoothness of the final product after hard coating may occur.

More specifically, it is preferable to use particles having an average particle diameter of less than 3 μm at 100ppm or less. As the inorganic particles, any particles used in the film such as silica, zeolite, or kaolin may be used. Such inorganic particles are present on the surface of the film through a stretching process, thereby improving sliding properties and winding properties of the film.

If the particle size is larger than 3 μm, the transparency of the film is deteriorated even in the case where the particle content is 100ppm or less, and further, the surface roughness (Ra) is 10nm or more, that is, the smoothness is deteriorated, and therefore, it is difficult to use the film for optical use, particularly for touch panels.

Further, if the content of the particles is 100ppm or more, the transparency of the film is lowered, and thus, the film is not suitable for use in a touch screen. Further, if the haze is 1.5% or more, the transparency is rapidly reduced when used for optical use or touch panels, the light transmittance is greatly deteriorated, and it is difficult to determine defects by naked eyes in BLU evaluation, thereby being difficult to use for optical use.

Although the preparation of the polyester multilayer film comprising the substrate layer and the surface layer of the present invention is not limited, the film may be obtained by melt-extruding in at least two melt-extruders, followed by casting, and biaxial stretching. More specifically, the polyester is extruded in one extruder while the polyester and additives such as inorganic particles such as silica, kaolin or zeolite are melt extruded in another extruder, after which the various melts are combined in a feed block for sequential co-extrusion, casting, cooling, and then biaxial stretching.

In one embodiment, a method of preparing a polyester film may comprise:

a) the surface layer composition comprising a first polyester resin containing 0.3 to 0.6 wt% of an oligomer and 0.1 to 1.2 wt% of diethylene glycol of the polyester resin, and a second polyester resin for a base layer are melt-extruded and co-extruded, having an intrinsic viscosity satisfying the following formula 10:

1 < Ns/Nc.ltoreq.1.2 [ equation 10]

TDr (%) less than or equal to 2 and less than or equal to 11.5 (formula 14)

In the present invention, after the stretching as described above, the heat-setting and the relaxation in the MD are performed, the relaxation is performed under the condition satisfying the formula 6, and thus the oligomer does not migrate under the high temperature condition and the film does not shrink, thereby preparing the film advantageous for the post-treatment.

More specifically, in the method for preparing polyester of the present invention, step a) is to co-extrude polyester resins forming the base layer and the skin layer, and then quench and cure them using a casting drum to prepare a polyester sheet, preferably, the polyester resins used in the skin layer and the base layer have intrinsic viscosities satisfying the following formula 10:

1 < Ns/Nc.ltoreq.1.2 [ equation 10]

Next, step b) is to stretch the coextruded sheet to produce a film, the stretching being carried out unidirectionally or biaxially, preferably biaxially. In the case of biaxial stretching, uniaxial stretching is carried out in the MD at 80 to 100 ℃ with the stretching ratio preferably being 2 to 4 times. Next, as a process of biaxial stretching in TD, the film is preferably stretched at 110-150 ℃ in a ratio of 2-4 times.

Step c) is heat-set and relaxed, while step c) may be performed in a tenter frame. The heat-setting temperature may be 200-240 deg.c, and the relaxation rate in TD satisfies the following formula 14, thereby controlling the heat shrinkage rate:

TDr (%) less than or equal to 2 and less than or equal to 11.5 (formula 14)

Further, if necessary, while the relaxation is performed in TD, the relaxation in MD may be performed within a range satisfying the following formula 15:

0.3-MDr (%) -2.5 (formula 15)

By performing the relaxation in the range satisfying equations 14 and 15, the shrinkage rate after the film is maintained at 150 ℃ for 30 minutes can satisfy equations 1 and 2.

A hard coat layer, an adhesive layer, a light diffusion layer, an ITO layer, a printed layer, and the like may be formed on the upper portion of the polyester film of the present invention, and even in the case of heating after forming such a functional coating layer, oligomer migration may be blocked to maintain optical properties, and therefore, the polyester film of the present invention is suitably used for an optical film.

Hereinafter, a more detailed description of the present invention will be provided by way of examples, however, the present invention is not limited to the following examples.

The physical properties were measured by the following measurement methods.

1) Thermal shrinkage rate

The film was cut into a standard size of 10cm in width and 10cm in length, and then the dimensional change after being left in a hot air oven maintained at 150 ℃ for 30 minutes was measured according to JIS C-2318. However, full width rolls of film were measured at 50cm intervals, measuring the dimensional change in MD, TD, and TD in the 45 ° and 135 ° clockwise directions, respectively.

Heat shrinkage (%) — the length of the film before heat treatment-the length of the film after holding at 150 ℃ for 30 minutes)/the length of the film before heat treatment × 100.

2) Intrinsic viscosity

0.4g of PET particles (sample) was added to 100ml of a reagent in which phenol and 1, 1, 2, 2-tetrachloroethanol were mixed in a weight ratio of 6: 4, and it took 90 minutes to dissolve therein, and thereafter, it was transferred to a Uberode viscometer, maintained in a constant temperature bath at 30 ℃ for 10 minutes, and then the number of seconds of fall of the solution was obtained using a viscometer and an aspirator. After the number of seconds of fall of the solvent was obtained in the same manner, r.v. and i.v. values were calculated using the following equations 1 and 2.

[ equation 1]

[ equation 2]

Wherein C represents the concentration of the sample.

3) Oligomer content (%)

Chloroform was added to a sample solvent HFIP (1, 1, 1, 3, 3, 3-hexafluoro-2-propanol) in an oligomer quantitative manner, dissolved at room temperature, and then acetonitrile was precipitated as a polymer. Thereafter, a calibration curve of a standard substance (cyclic oligomer) was obtained using an LC analysis apparatus, and then the purity of the cyclic oligomer was determined by sample analysis. As the analytical device, LC (liquid chromatography) and Agilent 1100 series were used.

4) DEG (diethylene glycol) content (%)

DEG content was measured as follows: to a 50ml vessel, 1g of the sample was added, followed by 3ml of monoethanolamine, heated using a hot plate to completely dissolve the sample, then the solution was cooled to 100 ℃, a solution of 0.005g of 1, 6-hexanediol dissolved in 20ml of methanol was added, and 10g of terephthalic acid was added to neutralize the solution. The neutralized solution thus obtained was filtered using a funnel and filter paper, and then the DEG content (wt%) of the filtrate was measured by gas chromatography. GC analysis was performed using a Shimadzu GC analyzer, and measurements were performed according to the Shimadzu GC manual.

5) Haze degree

The haze of the film-form sample was measured according to JIS K715 using a haze meter (Model name: Nipon denshoku, Model NDH 5000).

6) Haze Change Rate (Δ H)

To measure the surface migration of oligomers in the film, the film was placed in an open-topped box having dimensions of 3cm height, 21cm width and 27cm length, heat treated at 150 ℃ for 60 minutes to migrate the oligomers to the surface of the film, then held for 5 minutes, and then measured for haze value according to JIS K715 using a haze meter (nippon denshook, Model NDH 5000).

The haze change rate is calculated according to the following equation 11:

Δ H (%) ═ Hf-Hi [ equation 11]

Wherein Hf represents the haze of the film after being maintained at 150 ℃ for 60 minutes, and Hi represents the haze of the film before heating.

7) Dry coating thickness of undercoat

On the full width of the base film coated with the coating composition, 5 points in the vertical direction (TD) of the longitudinal direction were designated at intervals of 1m, the portion of the film was measured using SEM (Hitachi S-4300), and 30 points within the interval were measured with a magnification of 50,000 × to calculate an average value.

8) Surface roughness (Ra)

The use equipment comprises the following steps: 3D non-contact surface roughness tester (NT 2000, WYCO)

Ra (center line average roughness) was measured using the above apparatus

9) Coefficient of plane orientation

The refractive indices in the longitudinal, transverse and thickness directions were measured using an abbe refractometer (ATOGO), and the following orientation coefficients were calculated:

the plane orientation coefficient (ns) { (refractive index in the longitudinal direction + refractive index in the lateral direction)/2 } - { (refractive index in thickness in the longitudinal direction + refractive index in thickness in the lateral direction)/2 }.

10) Measurement of swelling ratio, gel fraction and Tg

15g of the water-dispersed resin compositions prepared in examples and comparative examples were charged into a round bowl having a size of 80mm in diameter and 15mm in height, dried at 80 ℃ for 24 hours, dried at 120 ℃ for 3 hours, and aged at 180 ℃ for 1 hour. After 1g of the dried coating film was taken out therefrom, Tg was measured. Further, the dried coating film was immersed in 50g of distilled water and then left at 70 ℃ for 24 hours. The coating film after the standing was taken out therefrom for measurement of the swelling ratio. After the retained coated film was dried at 120 ℃ for 3 hours, its weight was recorded to measure the gel fraction.

(1) Swelling ratio

About 1g of the dried coating film was immersed in 50g of distilled water, left at 70 ℃ for 24 hours, and the left coating film was taken out therefrom to record its weight.

Swelling ratio (weight after standing-initial weight)/initial weight × 100

(2) Fraction of gel

The coating film after standing was dried at 120 ℃ for 3 hours and its weight was recorded.

Gel fraction (weight after drying/initial weight) × 100

(3) Tg measurement

The Tg was measured in a two-pass mode using a DSC apparatus (Perkin Elmer DSC 7). Measurements were made with a Perkin Elmer DSC 7 using 10-11mg of dried coating film.

The first round is raised at a temperature of 0-200 deg.C at a rate of 200 deg.C/min,

the temperature was maintained at 200 ℃ for a holding time of 3 minutes, and then

Cooling at 200-40 deg.c at 200 deg.c/min,

the temperature was maintained at-40 ℃ for a holding time of 5 minutes.

The second round was measured at a rate of 20 ℃/min at-40-200 ℃.

11) Occurrence of curling

The film after the hard coat treatment and the film to be evaluated were laminated at 150 ℃ at a rate of 3mpm, and the laminate was cut in the TD to A4 size standard (29.7cm wide and 21.0cm long). Then, the dimensional change after being placed in a hot air oven maintained at 80 ℃ for 12 hours was measured. The change in each dimension from the bottom of the a4 film to the height of the 4 edges was measured.

The determination of whether curling has occurred is made by the height from the bottom to the edge of the film: when the height was 3mm or less, it was determined that no curling occurred.

The binder resins used in the following examples and comparative examples are as follows:

1) KLX-007 adhesive

(A) An acrylic resin formed by copolymerizing a glycidyl group-containing radically polymerizable unsaturated monomer, wherein the acrylic resin contains a glycidyl group-containing radically polymerizable unsaturated monomer in an amount of 50 mol% of the total monomer components, and (B) a water-dispersible polyester-based resin containing diethylene glycol in an amount of 50 mol% of the total glycol components as comonomers, and a sulfonic acid alkali metal salt compound in an amount of 10 mol% of the total acid components, in a solid content weight ratio of (a)/(B) 50/50.

2) P3208 adhesive

The product of Rohm & Haas, comprising 40 wt% methyl methacrylate, 40 wt% ethyl acrylate and 20 wt% melamine.

[ example 1]

1) Preparation of Water-Dispersion resin composition (1)

A binder having a solid content weight ratio of (a)/(B) 40/60 between (a) an acrylic resin formed by copolymerizing a glycidyl group-containing radical polymerizable unsaturated monomer and (B) a water-dispersible polyester-based resin was used as the binder.

(A) The acrylic resin was a resin formed by copolymerizing 60 mol% of glycidyl acrylate and 40 mol% of vinyl propionate, and had a weight average molecular weight of 35000.

(B) The water-dispersible polyester-based resin was a resin obtained by copolymerizing 50 mol% of an acid component (15 mol% of sulfoterephthalic acid and 85 mol% of terephthalic acid) with 50 mol% of a diol component (50 mol% of diethylene glycol and 50 mol% of ethylene glycol) and had a weight average molecular weight of 14000.

A binder having a solid content of 0.5% by weight and a silicone-based wetting agent (BYK 348 of BYK CHEMIE) having a solid content of 0.3% by weight were added to water and stirred for 2 hours, thereby preparing a water-dispersible resin composition (1) having a total solid content of 0.8% by weight.

The swelling ratio, gel fraction and Tg as described above were measured using the water-dispersed resin composition thus prepared, and the results are shown in table 1 below.

2) Preparation of oligomer-retarded polyester films

As the base layer (B), polyethylene terephthalate having an intrinsic viscosity of 0.65, a diethylene glycol content of 1.2 wt%, and an oligomer content of 1.4 wt% was fed to an extruder to be melt-extruded, and as the surface layer (a), polyethylene terephthalate sheets prepared by solid-phase polymerization and having an intrinsic viscosity of 0.67, a diethylene glycol content of 0.8 wt%, and an oligomer content of 0.5 wt%, and silica particles having a diameter of 0.7 μm in an amount of 50ppm relative to the total weight of polyethylene terephthalate were used, thereby preparing sheet extrudates cast into a/B/A3 layers. Thereafter, the water-dispersible resin composition (1) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 230 ℃ in a 5-column tenter, 10% relaxation was performed in MD and TD at 200 ℃, and heat setting was performed, thereby preparing a 188 μm biaxially oriented film having coating layers on both surfaces.

The polyester multilayer film thus prepared had a base layer of 80 wt% of the total film weight and a skin layer of 20 wt% of the total film weight, and the primer layer of the composition had a dry coating thickness of 20 nm. The physical properties of the obtained polyester film are shown in table 2 below.

[ example 2]

1) Preparation of Water-Dispersion resin composition (2)

A binder having a solid content weight ratio of (a)/(B) 70/30 between (a) an acrylic resin formed by copolymerizing a glycidyl group-containing radical polymerizable unsaturated monomer and (B) a water-dispersible polyester-based resin was used as the binder.

(A) The acrylic resin was obtained by copolymerizing 60 mol% of glycidyl acrylate and 40 mol% of vinyl propionate, and had a weight average molecular weight of 30000.

(B) The water-dispersible polyester-based resin was a resin obtained by copolymerizing 50 mol% of an acid component (15 mol% of sulfoterephthalic acid and 85 mol% of terephthalic acid) with respect to 50 mol% of a glycol component (50 mol% of diethylene glycol and 50 mol% of ethylene glycol) and had a weight average molecular weight of 12000.

A binder having a solid content of 5 wt% and a silicone-based wetting agent (BYK 348 of BYK CHEMIE) having a solid content of 0.3 wt% were added to water and stirred for 2 hours, thereby preparing a water-dispersible resin composition (2) having a total solid content of 5.3 wt%. The swelling ratio, gel fraction and Tg as described above were measured using the water-dispersed resin composition thus prepared, and the results are shown in table 1 below.

Using the prepared water-dispersible resin composition (2), a 188 μm biaxially oriented film having a coating layer on both surfaces was prepared in the same manner as in example 1. The primer layer of the composition had a dry coating thickness of 110 nm. The physical properties of the obtained polyester film are shown in table 2 below.

[ example 3]

As the base layer (B), polyethylene terephthalate having an intrinsic viscosity of 0.65, a diethylene glycol content of 1.2 wt%, and an oligomer content of 1.4 wt% was fed to an extruder to be melt-extruded, and as the surface layer (a), polyethylene terephthalate sheets prepared by solid-phase polymerization and having an intrinsic viscosity of 0.67, a diethylene glycol content of 0.7 wt%, and an oligomer content of 0.5 wt%, and silica particles having a diameter of 0.7 μm in an amount of 50ppm relative to the total weight of polyethylene terephthalate were used, thereby preparing sheet extrudates cast into a/B/A3 layers. Thereafter, the water-dispersible resin composition (1) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 230 ℃ in a 5-column tenter, 10% relaxation was performed in MD and TD at 200 ℃, and heat setting was performed, thereby preparing a 188 μm biaxially oriented film having coating layers on both surfaces.

The polyester multilayer film thus prepared had a base layer of 80 wt% of the total film weight and a surface layer of 20 wt% of the total film weight, and the primer layer of the composition had a dry coating thickness of 20 nm. The physical properties of the obtained polyester film are shown in table 2 below.

[ example 4]

As the base layer (B), polyethylene terephthalate having an intrinsic viscosity of 0.65, a diethylene glycol content of 1.2 wt%, and an oligomer content of 1.4 wt% was fed to an extruder to be melt-extruded, and as the surface layer (a), polyethylene terephthalate sheets prepared by solid-phase polymerization and having an intrinsic viscosity of 0.67, a diethylene glycol content of 0.7 wt%, and an oligomer content of 0.5 wt%, and silica particles having a diameter of 0.7 μm in an amount of 50ppm relative to the total weight of polyethylene terephthalate were used, thereby preparing sheet extrudates cast into a/B/A3 layers. Thereafter, the water-dispersible resin composition (2) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 230 ℃ in a 5-column tenter, 10% relaxation was performed in MD and TD at 200 ℃, and heat setting was performed, thereby preparing a 188 μm biaxially oriented film having coating layers on both surfaces.

The polyester multilayer film thus prepared had a base layer of 80 wt% of the total film weight and a surface layer of 20 wt% of the total film weight, and the primer layer of the composition had a dry coating thickness of 110 nm. The physical properties of the obtained polyester film are shown in table 2 below.

[ example 5]

As the base layer (B), polyethylene terephthalate having an intrinsic viscosity of 0.65, a diethylene glycol content of 1.2 wt%, and an oligomer content of 1.4 wt% was fed to an extruder to be melt-extruded, and as the surface layer (a), polyethylene terephthalate sheets prepared by solid-phase polymerization and having an intrinsic viscosity of 0.67, a diethylene glycol content of 0.8 wt%, and an oligomer content of 0.4 wt%, and silica particles having a diameter of 0.7 μm in an amount of 50ppm relative to the total weight of polyethylene terephthalate were used, thereby preparing sheet extrudates cast into a/B/A3 layers. Thereafter, the water-dispersible resin composition (1) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 230 ℃ in a 5-column tenter, 10% relaxation was performed in MD and TD at 200 ℃, and heat setting was performed, thereby preparing a 188 μm biaxially oriented film having coating layers on both surfaces.

[ example 6]

As the base layer (B), polyethylene terephthalate having an intrinsic viscosity of 0.65, a diethylene glycol content of 1.2 wt%, and an oligomer content of 1.4 wt% was fed to an extruder to be melt-extruded, and as the surface layer (a), polyethylene terephthalate sheets prepared by solid-phase polymerization and having an intrinsic viscosity of 0.67, a diethylene glycol content of 0.8 wt%, and an oligomer content of 0.4 wt%, and silica particles having a diameter of 0.7 μm in an amount of 50ppm relative to the total weight of polyethylene terephthalate were used, thereby preparing sheet extrudates cast into a/B/A3 layers. Thereafter, the water-dispersible resin composition (2) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 230 ℃ in a 5-column tenter, 10% relaxation was performed in MD and TD at 200 ℃, and heat setting was performed, thereby preparing a 188 μm biaxially oriented film having coating layers on both surfaces.

[ example 7]

The polyester multilayer film thus prepared had a base layer of 70 wt% of the total film weight and a surface layer of 30 wt% of the total film weight, and the primer layer of the composition had a dry coating thickness of 20 nm. The physical properties of the obtained polyester film are shown in table 2 below.

[ example 8]

As the base layer (B), polyethylene terephthalate having an intrinsic viscosity of 0.65, a diethylene glycol content of 1.2 wt%, and an oligomer content of 1.4 wt% was fed to an extruder to be melt-extruded, and as the surface layer (a), polyethylene terephthalate sheets prepared by solid-phase polymerization and having an intrinsic viscosity of 0.67, a diethylene glycol content of 0.8 wt%, and an oligomer content of 0.5 wt%, and silica particles having a diameter of 0.7 μm in an amount of 50ppm relative to the total weight of polyethylene terephthalate were used, thereby preparing sheet extrudates cast into a/B/A3 layers. Thereafter, the water-dispersible resin composition (2) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 230 ℃ in a 5-column tenter, 10% relaxation was performed in MD and TD at 200 ℃, and heat setting was performed, thereby preparing a 188 μm biaxially oriented film having coating layers on both surfaces.

The polyester multilayer film thus prepared had a base layer of 70 wt% of the total film weight and a skin layer of 30 wt% of the total film weight, and the primer layer of the composition had a dry coating thickness of 110 nm. The physical properties of the obtained polyester film are shown in table 2 below.

[ comparative example 1]

As the base layer (B), polyethylene terephthalate having an intrinsic viscosity of 0.65, a diethylene glycol content of 1.2 wt%, and an oligomer content of 1.4 wt% was fed to an extruder to be melt-extruded, and as the surface layer (a), polyethylene terephthalate sheets prepared by solid-phase polymerization and having an intrinsic viscosity of 0.67, a diethylene glycol content of 0.8 wt%, and an oligomer content of 0.5 wt%, and silica particles having a diameter of 0.7 μm in an amount of 50ppm relative to the total weight of polyethylene terephthalate were used, thereby preparing sheet extrudates cast into a/B/A3 layers. Thereafter, the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was carried out in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 230 ℃ in a 5-column tenter, 10% relaxation was performed in MD and TD at 200 ℃, and heat setting was performed, thereby preparing a 188 μm biaxially oriented film having coating layers on both surfaces.

The polyester multilayer film was prepared with a substrate of 80 wt% of the total film weight and a skin layer of 20 wt% of the total film weight. The physical properties of the obtained polyester film are shown in table 2 below.

[ comparative example 2]

As the base layer (B), polyethylene terephthalate having an intrinsic viscosity of 0.65, a diethylene glycol content of 1.2 wt%, and an oligomer content of 1.4 wt% was fed to an extruder to be melt-extruded, and as the surface layer (a), polyethylene terephthalate sheets prepared by solid-phase polymerization and having an intrinsic viscosity of 0.67, a diethylene glycol content of 0.8 wt%, and an oligomer content of 1.4 wt%, and silica particles having a diameter of 0.7 μm in an amount of 50ppm relative to the total weight of polyethylene terephthalate were used, thereby preparing sheet extrudates cast into a/B/A3 layers. Thereafter, the water-dispersible resin composition (1) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 230 ℃ in a 5-column tenter, 10% relaxation was performed in MD and TD at 200 ℃, and heat setting was performed, thereby preparing a 188 μm biaxially oriented film having coating layers on both surfaces.

[ comparative example 3]

An adhesive was used which is a product of Rohm & Haas comprising 40 wt% of methyl methacrylate, 40 wt% of ethyl acrylate and 20 wt% of melamine.

A binder having a solid content of 2 wt% and a silicone-based wetting agent (BYK 348 of BYK CHEMIE) having a solid content of 0.3 wt% were added to water and stirred for 2 hours, thereby preparing a water-dispersible resin composition (3) having a total solid content of 2.3 wt%. The swelling ratio, gel fraction and Tg as described above were measured using the water-dispersed resin composition thus prepared, and the results are shown in table 1 below.

Using the prepared water-dispersible resin composition (3), 188 μm biaxially oriented films having coating layers on both surfaces were prepared in the same manner as in example 1. The primer layer of the composition had a dry coating thickness of 80 nm. The physical properties of the obtained polyester film are shown in table 2 below.

[ comparative example 4]

An aqueous polyurethane adhesive having a solid content of 20 wt% was prepared by reacting 9 wt% of polyester-based polyol (polyethylene adipate diol) having a weight average molecular weight of 1000), 10 wt% of hexamethylene diisocyanate, 1 wt% of a reactive emulsifier having an ionic group (Asahi Denka, Adekaria Soap SETM, sulfonic acid ester of allyl glycidyl nonylphenyl polyoxyethylene ether (sulfoacid ester of polyoxylethylene allyl nonylphenyl ether)), and 80 wt% of water.

A binder having a solid content of 4 wt% and a silicone-based wetting agent (BYK 348 of BYK CHEMIE) having a solid content of 0.3 wt% were added to water and stirred for 2 hours, thereby preparing a water-dispersible resin composition (4) having a total solid content of 4.3 wt%. The swelling ratio, gel fraction and Tg as described above were measured using the water-dispersed resin composition thus prepared, and the results are shown in table 1 below.

Using the prepared water-dispersible resin composition (4), 188 μm biaxially oriented films having coating layers on both surfaces were prepared in the same manner as in example 1. The primer layer of the composition had a dry coating thickness of 80 nm. The physical properties of the obtained polyester film are shown in table 2 below.

[ comparative example 5]

40mol (26 mol%) of 2, 6-naphthalenedicarboxylic acid, 5mol (3.3 mol%) of sodium 2, 5-dicarboxylate benzenesulfonate, 5mol (3.3 mol%) of dimethylterephthalic acid, and 100mol (66.66 mol%) of a 1: 1 mixture of ethylene glycol and 1, 4-butanediol were mixed in the absence of a solvent, placed in a reactor, and reacted by raising the temperature from 170 ℃ to 250 ℃ per minute at 1 ℃ to thereby conduct an emulsification reaction while removing water or methanol as a by-product, raising the temperature to 260 ℃ while reducing the pressure in the reactor to 1mmHg to thereby conduct a polycondensation reaction while collecting the diol as a by-product, thereby preparing a polyester resin having an intrinsic viscosity of 0.4.

75 wt% of water was added to 25 wt% of the polyester resin thus prepared, and emulsification was performed to prepare 25 wt% of an aqueous polyester binder.

A binder having a solid content of 4 wt% and a silicone-based wetting agent (BYK 348 of BYK CHEMIE) having a solid content of 0.3 wt% were added to water and stirred for 2 hours, thereby preparing a water-dispersible resin composition (5) having a total solid content of 4.3 wt%. The swelling ratio, gel fraction and Tg as described above were measured using the water-dispersed resin composition thus prepared, and the results are shown in table 1 below.

Using the prepared water-dispersible resin composition (5), 188 μm biaxially oriented films having coating layers on both surfaces were prepared in the same manner as in example 1. The primer layer of the composition had a dry coating thickness of 70 nm. The physical properties of the obtained polyester film are shown in table 2 below.

[ Table 1]

[ Table 2]

As shown in tables 1 and 2 above, it can be seen that the polyester multilayer film according to the present invention has a low haze change rate before and after the heat treatment, thereby exhibiting characteristics suitable for use as an optical film.

However, as seen from comparative example 1, in the case where the primer treatment was not performed, only the polymerized sheet of the base film was improved, the haze change rate was high, and a large amount of oligomers occurred during lamination with other films in the post-treatment, thereby failing to satisfy the physical properties of the present invention. Further, it can be determined from comparative example 2 that when the oligomer content of the surface layer is 1.4%, the film is out of the desired physical property range for haze. As can be seen from comparative examples 3, 4 and 5, the haze change rate varies with the composition of the undercoat layer.

[ example 9]

1) Preparation of Water-Dispersion resin composition (6)

A water-dispersible resin composition (6) having a total solid content of 4.6 wt% was prepared by adding 16 wt% of KLX-007 binder (aqueous dispersion composition having a solid content of 25%), 0.3 wt% of silicone-based wetting agent (Dow Coming, polyester siloxane copolymer, Q2-5212), and 0.3 wt% of colloidal silica particles having an average particle diameter of 140nm to water and stirring for 2 hours.

2) Preparation of oligomer-retarded polyester films with controlled thermal shrinkage

The water-removed polyethylene terephthalate sheet was fed into an extruder, melt-extruded therefrom, and quenched and solidified by a casting drum having a surface temperature of 20 c, thereby preparing a polyethylene terephthalate sheet having a thickness of 1500 μm. The produced polyethylene terephthalate sheet was stretched in the MD at 80 ℃ in a ratio of 3.5 times, and then cooled to room temperature. Thereafter, the water-dispersible resin composition (6) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 235 ℃ in a 5-column tenter, relaxation was performed at 200 ℃ in TD by 10%, heat setting was performed, and the relaxation rate of the MD relaxation device was adjusted to 1.25%, thereby producing 125 μm biaxially oriented film having coating layers on both surfaces.

The primer layer of the composition had a dry coating thickness of 80 nm. The physical properties of the polyester film thus obtained are shown in tables 3 and 4 below.

[ example 10]

1) Preparation of Water-Dispersion resin composition (7)

8 wt% of KLX-007 adhesive (aqueous dispersion composition having a solid content of 25%), 0.3 wt% of silicone-based wetting agent (Dow Coming, polyester siloxane copolymer, Q2-5212), and 0.3 wt% of colloidal silica particles having an average particle diameter of 140nm were added to water and stirred for 2 hours, thereby preparing aqueous dispersion resin composition (7) having a total solid content of 2.6 wt%.

The water-removed polyethylene terephthalate sheet was fed into an extruder, melt-extruded therefrom, and quenched and solidified by a casting drum having a surface temperature of 20 c, thereby preparing a polyethylene terephthalate sheet having a thickness of 1500 μm. The produced polyethylene terephthalate sheet was stretched in the MD at 80 ℃ in a ratio of 3.5 times, and then cooled to room temperature. Thereafter, the water-dispersible resin composition (7) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 235 ℃ in a 5-column tenter, relaxation was performed at 200 ℃ in TD by 10%, heat setting was performed, and the relaxation rate of the MD relaxation device was adjusted to 1.25%, thereby producing 125 μm biaxially oriented film having coating layers on both surfaces.

The primer layer of the composition had a dry coating thickness of 40 nm. The physical properties of the polyester film thus obtained are shown in tables 3 and 4 below.

[ example 11]

1) Preparation of Water-Dispersion resin composition (8)

A water-dispersible resin composition (8) having a total solid content of 6.6 wt% was prepared by adding 24 wt% of KLX-007 binder (aqueous dispersion composition having a solid content of 25%), 0.3 wt% of silicone-based wetting agent (Dow Coming, polyester siloxane copolymer, Q2-5212), and 0.3 wt% of colloidal silica particles having an average particle diameter of 140nm to water and stirring for 2 hours.

The water-removed polyethylene terephthalate sheet was fed into an extruder, melt-extruded therefrom, and quenched and solidified by a casting drum having a surface temperature of 20 c, thereby preparing a polyethylene terephthalate sheet having a thickness of 1500 μm. The produced polyethylene terephthalate sheet was stretched in the MD at 80 ℃ in a ratio of 3.5 times, and then cooled to room temperature. Thereafter, the water-dispersible resin composition (8) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 235 ℃ in a 5-column tenter, relaxation was performed at 200 ℃ in TD by 10%, heat setting was performed, and the relaxation rate of the MD relaxation device was adjusted to 1.25%, thereby producing 125 μm biaxially oriented film having coating layers on both surfaces.

The primer layer of the composition had a dry coating thickness of 160 nm. The physical properties of the polyester film thus obtained are shown in tables 3 and 4 below.

[ example 12]

1) Preparation of oligomer-retarded polyester films with controlled thermal shrinkage

The water-removed polyethylene terephthalate sheet was fed into an extruder, melt-extruded therefrom, and quenched and solidified by a casting drum having a surface temperature of 20 c, thereby preparing a polyethylene terephthalate sheet having a thickness of 1500 μm. The produced polyethylene terephthalate sheet was stretched in the MD at 80 ℃ in a ratio of 3.5 times, and then cooled to room temperature. Thereafter, the water-dispersible resin composition (6) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 245 ℃ in a 5-column tenter, relaxation was performed at 200 ℃ in TD by 10%, heat setting was performed, and the relaxation rate of the MD relaxation device was adjusted to 1.25%, thereby producing 125 μm biaxially oriented film having coating layers on both surfaces.

[ example 13]

The water-removed polyethylene terephthalate sheet was fed into an extruder, melt-extruded therefrom, and quenched and solidified by a casting drum having a surface temperature of 20 c, thereby preparing a polyethylene terephthalate sheet having a thickness of 1500 μm. The produced polyethylene terephthalate sheet was stretched in the MD at 80 ℃ in a ratio of 3.5 times, and then cooled to room temperature. Thereafter, the water-dispersible resin composition (6) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 237 ℃ in a 5-column tenter, relaxation was performed at 200 ℃ in TD by 10%, heat setting was performed, and the relaxation rate of the MD relaxation device was adjusted to 2.0%, thereby producing 125 μm biaxially oriented film having coating layers on both surfaces.

[ example 14]

The water-removed polyethylene terephthalate sheet was fed into an extruder, melt-extruded therefrom, and quenched and solidified by a casting drum having a surface temperature of 20 c, thereby preparing a polyethylene terephthalate sheet having a thickness of 1500 μm. The produced polyethylene terephthalate sheet was stretched in the MD at 80 ℃ in a ratio of 3.5 times, and then cooled to room temperature. Thereafter, the water-dispersible resin composition (6) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 244 ℃ in a 5-column tenter, relaxation was performed at 200 ℃ in TD by 10%, heat setting was performed, and the relaxation rate of the MD relaxation device was adjusted to 2.0%, thereby producing 125 μm biaxially oriented film having coating layers on both surfaces.

[ comparative example 6]

1) Preparation of Water-Dispersion resin composition (9)

9.1% by weight of P-3208 binder (a water dispersion composition having a solid content of 44%), 0.3% by weight of a silicone-based wetting agent (Dow Corning, polyester siloxane copolymer, Q2-5212), and 0.3% by weight of colloidal silica particles having an average particle diameter of 140nm were added to water, and stirred for 2 hours, thereby preparing a water dispersion resin composition (9) having a total solid content of 4.6% by weight.

The water-removed polyethylene terephthalate sheet was fed into an extruder, melt-extruded therefrom, and quenched and solidified by a casting drum having a surface temperature of 20 c, thereby preparing a polyethylene terephthalate sheet having a thickness of 1500 μm. The produced polyethylene terephthalate sheet was stretched in the MD at 80 ℃ in a ratio of 3.5 times, and then cooled to room temperature. Thereafter, the water-dispersible resin composition (9) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 235 ℃ in a 5-column tenter, relaxation was performed at 200 ℃ in TD by 10%, heat setting was performed, and the relaxation rate of the MD relaxation device was adjusted to 1.25%, thereby producing 125 μm biaxially oriented film having coating layers on both surfaces.

[ comparative example 7]

1) Preparation of oligomer-retarded polyester films with uncontrolled thermal shrinkage

The water-removed polyethylene terephthalate sheet was fed into an extruder, melt-extruded therefrom, and quenched and solidified by a casting drum having a surface temperature of 20 c, thereby preparing a polyethylene terephthalate sheet having a thickness of 1500 μm. The produced polyethylene terephthalate sheet was stretched in the MD at 80 ℃ in a ratio of 3.5 times, and then cooled to room temperature. Thereafter, the water-dispersible resin composition (6) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 235 ℃ in a 5-column tenter, relaxation was performed at 200 ℃ in TD by 10%, heat setting was performed, and the relaxation rate of the MD relaxation device was adjusted to 1.00%, thereby producing 125 μm biaxially oriented film having coating layers on both surfaces.

[ comparative example 8]

The water-removed polyethylene terephthalate sheet was fed into an extruder, melt-extruded therefrom, and quenched and solidified by a casting drum having a surface temperature of 20 c, thereby preparing a polyethylene terephthalate sheet having a thickness of 1500 μm. The produced polyethylene terephthalate sheet was stretched in the MD at 80 ℃ in a ratio of 3.5 times, and then cooled to room temperature. Thereafter, the water-dispersible resin composition (6) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 235 ℃ in a 5-column tenter, relaxation was performed at 200 ℃ in TD by 10%, heat setting was performed, and the relaxation rate of the MD relaxation device was adjusted to 3.00%, thereby producing 125 μm biaxially oriented film having coating layers on both surfaces.

[ comparative example 9]

The water-removed polyethylene terephthalate sheet was fed into an extruder, melt-extruded therefrom, and quenched and solidified by a casting drum having a surface temperature of 20 c, thereby preparing a polyethylene terephthalate sheet having a thickness of 1500 μm. The produced polyethylene terephthalate sheet was stretched in the MD at 80 ℃ in a ratio of 3.5 times, and then cooled to room temperature. Thereafter, the water-dispersible resin composition (6) thus prepared was coated on both surfaces by a bar coating method, and then the temperature was raised to 110-150 ℃ per second at 1 ℃ and, after preheating and drying, stretching was performed in TD at a ratio of 3.5 times. Thereafter, heat treatment was performed at 235 ℃ in a 5-column tenter, relaxation was performed at 200 ℃ in TD by 10%, heat setting was performed, and the relaxation rate of the MD relaxation device was adjusted to 0%, thereby producing 125 μm biaxially oriented film having coating layers on both surfaces.

[ Table 3]

[ Table 4] MD/TD Heat shrinkage (%)

[ Table 5] diagonal heat shrinkage (%)

As shown in table 3 above, it can be seen that the polyester film according to the present invention has a low haze change rate before and after the heat treatment, thereby exhibiting excellent characteristics as an optical film. However, as can be confirmed from comparative example 6, the haze change rate varies depending on the composition of the undercoat layer. That is, it was determined that the difference in the degree of oligomer retardation properties was due to the choice of the composition of the coating.

Further, as shown in table 4 above, by the heat treatment temperature and the relaxation rate in MD (MD relaxation rate), the target heat shrinkage rate can be uniformly ensured on the full width basis. This determines that the curl problem of the product can be controlled. However, as can be seen from comparative example 7, it is difficult to ensure the heat shrinkage to a desired level only with conventional equipment when the MD relaxation rate of 1.0% is used, and the uniformity is also lowered. Further, when the MD relaxation rate was set to 3.0%, a film could not be formed in comparative example 8.

From comparative example 9, it was confirmed that although the MD heat shrinkage rate was good, in the case where the deviation of the heat shrinkage rate between MD and TD was 0.2%, the curling of 3.5mm occurred, which was problematic. In this case, the shape of the generated curl is a twisted shape (twisted curl) in which the curl occurs only on both sides in the diagonal direction among the four edges, which is caused by the difference in the diagonal heat shrinkage rate. As shown in table 3, it was determined that the difference in the diagonal heat shrinkage rate was caused by the difference in the conditions of the film forming process, and this difference caused the occurrence of the twisting curl in the lamination process. That is, in the case of comparative example 9, it was confirmed that although the deviation of the heat shrinkage rates in MD and TD was good in the full width range of the film, the difference of the diagonal heat shrinkage rates was not uniform, and therefore, the curl control could be performed in the central portion without a large difference, but the curl control could not be performed in the both side portions having a difference of 0.2% or more, and the twist curl occurred.

[ example 15]

Separately, a PET sheet having an intrinsic viscosity of 0.65, a diethylene glycol (DEG) content of 1.2%, and an oligomer content of 1.4% was used in the base layer, a PET sheet having an intrinsic viscosity of 0.67, a DEG content of 0.8 wt%, and an oligomer content of 0.5% was used in the surface layer, and co-extrusion casting was performed using 30ppm of particles having a particle diameter of 0.7. mu.m. Thereafter, stretching was performed in MD and TD in order of 3.2 times and 3.2 times, and heat treatment was performed at 230 ℃, with 3% relaxation being applied to TD to prepare a 125 μm multilayer film. Here, the relaxation in TD sequentially provides relaxation of three stages of the heat treatment region in the tenter within the maximum width length after stretching, however, the width length is reduced by a length of 3% in the maximum width direction.

Here, the particle composition and content of the surface layer are shown in table 6.

The base layer of the multilayer film accounted for 80% and the skin layer accounted for 20% of the total film weight, so that after preparation of the film, the film was measured for oligomer surface migration, surface roughness, haze, plane orientation coefficient, and shrinkage.

[ examples 16 and 17]

As shown in table 6 below, the procedure was performed in the same manner as in example 15 except for the DEG content of the raw material of the surface layer.

The obtained film was measured for oligomer surface migration, surface roughness, haze, plane orientation coefficient and shrinkage.

[ examples 18 and 19]

As shown in table 6 below, the process was performed in the same manner as in example 15 except for the oligomer content of the raw material of the surface layer.

[ examples 20 and 21]

This procedure was carried out in the same manner as in example 15, except that the weight of the skin layer was as in table 6 below.

[ examples 22 and 23]

This procedure was carried out in the same manner as in example 15 except that the particle content of the surface layer was as in table 6 below.

[ example 24]

This procedure was carried out in the same manner as in example 15, except that the relaxation was performed in TD while applying 1.5% relaxation in MD.

[ comparative example 10]

This process was performed in the same manner as in example 15, except that the intrinsic viscosity of the skin layer was 0.65, and PET having an oligomer content of 1.4% was used alone, and only the particles of the skin layer were as in example 16. After the film was prepared, oligomer surface migration, surface roughness, haze, plane orientation coefficient and shrinkage thereof were measured.

[ comparative examples 11 and 12]

This procedure was carried out in the same manner as in example 15, except that the DEG content of the surface layer was as described in table 6.

[ comparative examples 13 and 14]

This procedure was carried out in the same manner as in example 15, except that the oligomer content of the skin layer was as described in table 6. The obtained film was measured for oligomer surface migration, surface roughness, haze, plane orientation coefficient and shrinkage.

[ comparative examples 15 and 16]

[ comparative examples 17 and 18]

Comparative examples 19 and 20

This process was performed in the same manner as in example 16, except that a film was formed at heat treatment temperatures of 200 ℃ and 210 ℃, and the relaxation in TD was 1% and 1.5%.

[ Table 6]

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

[ Table 7]

As shown in table 7 above, in the examples of the present invention, it was determined that the film had a lower heat shrinkage rate in the range satisfying formulas 1 and 2 after being maintained at 150 ℃ for 30 minutes, the plane orientation coefficient was 0.1590 or more, the surface roughness was 10nm or less, the haze (Hi) before heating the film was less than 1.5%, and the haze after maintaining the film at 150 ℃ for 30 minutes all satisfied formula 9.

In the foregoing, preferred exemplary embodiments of the present invention have been described, however, various modifications and equivalents may be used in the present invention, and it is apparent that the above embodiments may be appropriately modified and identically applied. Accordingly, the above description should not limit the scope of the invention, which is defined by the appended claims.

Claims

1. A polyester film comprising:

a substrate layer, and at least two skin layers laminated on both surfaces of the substrate layer,

wherein the polyester resin forming the surface layer has an oligomer content of 0.3 to 0.6 wt% and a diethylene glycol content of 0.1 to 1.1 wt%,

the polyester film has a heat shrinkage rate satisfying the following formulas 1 and 2, a plane orientation coefficient (ns) satisfying the following formula 8, a haze of the film before heating of less than 1.5%, and a haze of the film after being maintained at 150 ℃ for 30 minutes satisfying the following formula 9:

0.2 ≦ Smd ≦ 1.5 [ equation 1]

0 ≦ Std ≦ 1.0 [ equation 2]

0.1590 ≦ ns [ equation 8]

Hf ≦ Hi × 2.5 [ equation 9]

hf represents the haze of the film after being maintained at 150 ℃ for 30 minutes, Hi represents the haze of the film before heating,

wherein the polyester film is formed by co-extrusion of the base layer and the skin layer, and has an intrinsic viscosity satisfying the following formula 10:

1 < Ns/Nc.ltoreq.1.2 [ equation 10]

Wherein Ns denotes an intrinsic viscosity of the polyester resin forming the skin layer, and Nc denotes an intrinsic viscosity of the polyester resin forming the base layer;

wherein the polyester film has 70 wt% to 90 wt% of the base layer and 10 wt% to 30 wt% of the skin layer;

wherein the surface layer contains 100ppm or less of inorganic particles, and the average particle diameter of the inorganic particles is less than 3 [ mu ] m.

2. A polyester film according to claim 1 wherein the surface roughness Ra of the polyester film is 10nm or less.

3. A polyester film according to claim 2, wherein the intrinsic viscosity of the polyester resin forming the base layer is 0.5 to 1.0, and the intrinsic viscosity of the polyester resin forming the skin layer is 0.6 to 1.0.

4. A polyester film according to claim 1 wherein the total thickness of the polyester film is from 25 to 250 μm.

5. A polyester film according to claim 1, wherein the inorganic particles are any one or a mixture of two or more selected from silica, zeolite and kaolin.

6. An optical film having a functional coating layer selected from any one or more of a hard coating layer, an adhesive layer, a light diffusion layer, an ITO layer and a printed layer formed on the polyester film of any one of claims 1 to 5.

7. A method of making a polyester film comprising:

a) melt-extruding and co-extruding a skin layer composition comprising a first polyester resin comprising 0.3 wt% to 0.6 wt% of an oligomer and 0.1 wt% to 1.2 wt% of diethylene glycol of the polyester resin, and a second polyester resin for a base layer, having an intrinsic viscosity satisfying the following formula 10:

1 < Ns/Nc.ltoreq.1.2 [ equation 10]

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

TDr (%) less than or equal to 2 and less than or equal to 11.5 (formula 14)

Where TDr represents a relaxation rate in TD, where { (maximum width length of the film in TD before the relaxation treatment stage-minimum width length of the film in TD before the relaxation treatment stage)/maximum width length of the film before the relaxation treatment stage } × 100,

wherein the surface layer composition described in a) contains 100ppm or less of inorganic particles, and the average particle diameter of the inorganic particles is less than 3 μm.

8. The method as claimed in claim 7, wherein the polyester film has a surface roughness Ra of 10nm or less, a heat shrinkage rate satisfying the following formulas 1 and 2, a plane orientation coefficient (ns) satisfying the following formula 8, and a haze after the film is maintained at 150 ℃ for 30 minutes satisfying the following formula 9:

0.2 ≦ Smd ≦ 1.5 [ equation 1]

0 ≦ Std ≦ 1.0 [ equation 2]

0.1590 ≦ ns [ equation 8]

Hf ≦ Hi × 2.5 [ equation 9]

9. The method as claimed in claim 7, wherein the polyester film is formed by co-extrusion of the base layer and the skin layer, and has an intrinsic viscosity satisfying the following formula 10:

1 < Ns/Nc.ltoreq.1.2 [ equation 10]

10. The method according to claim 7, wherein, when the relaxation is performed in c), the relaxation in the Machine Direction (MD) is performed while the relaxation in the Transverse Direction (TD) is performed within a range satisfying the following formula 15:

0.3-MDr (%) -2.5 (formula 15)