CN113429614A

CN113429614A - Polyester film and method for producing same

Info

Publication number: CN113429614A
Application number: CN202110054454.7A
Authority: CN
Inventors: 金泰勇; 金志赫; 金吉中
Original assignee: Toray Advanced Materials Korea Inc
Current assignee: Toray Advanced Materials Korea Inc
Priority date: 2020-03-23
Filing date: 2021-01-15
Publication date: 2021-09-24
Anticipated expiration: 2041-01-15
Also published as: CN113429614B; KR20210118512A; KR102358413B1

Abstract

A polyester film according to an embodiment of the present invention includes: a film formed of a copolymerized polyester resin; and a primer layer formed on at least one surface of the film; wherein the optical loss at a wavelength of 550nm in the longitudinal direction (MD) and the Transverse Direction (TD) is 10% or less, and the deviation of the optical axis angle is + -5 deg. or less. Therefore, when used as a protective film for a curved display, the polyester film according to an embodiment of the present invention may prevent a curved edge from rising, and may improve a fingerprint recognition rate in an optical fingerprint recognition display.

Description

Polyester film and method for producing same

Technical Field

The following description relates to a polyester film for protecting a display of a smart phone, and more particularly, to a polyester film and a method of manufacturing the same, wherein the polyester film may provide a high fingerprint recognition rate and may completely cover the display, even a curved surface, by optimizing film components and an optical axis of the film for a smart phone having a curved display having an optical fingerprint recognition function.

Background

Recently, smart phones provide users with various functions and usages, so that the market thereof is rapidly growing. To meet the demand for both design and functionality, smartphones increasingly employ curved displays, and at the same time, more smartphones provide fingerprint recognition functionality for information security. In addition to existing smartphones having a separate part for fingerprint recognition, smartphones having a display with their own fingerprint recognition function have been released.

Such a smartphone having a display with a fingerprint recognition function is advantageous in design because a separate fingerprint recognition sensor is not exposed to the outside of the smartphone, and can provide user convenience.

In particular, fingerprint recognition technology using a display may be classified into a capacitance type, an ultrasonic type, and an optical type, wherein an ultrasonic type and an optical type sensor may be installed in the display itself. Although the ultrasonic fingerprint sensor has an advantage in that errors caused by foreign substances are less likely to occur, it is expensive. For this reason, most smart phones having a display with fingerprint recognition function employ an optical fingerprint sensor. In a smartphone having a display with an optical fingerprint recognition function, a polarizing plate is located at the outermost portion of the display, wherein if the optical axes of the protective film and the polarizing plate do not coincide, optical loss occurs so that fingerprint recognition may not be performed correctly.

For example, korean laid-open patent publication No. 10-2018-0080747 relates to a liquid crystal protective film for a smart phone, in which a PET film material is used as a film for protecting a liquid crystal display panel. However, the protective film has a problem in that, when a general PET film having poor flexibility is used, a bent edge portion of the display is lifted and deviation of an optical axis occurs, thereby reducing a fingerprint recognition rate during fingerprint recognition of the optical display.

Therefore, the PET film generally used as the protective film has poor flexibility such that when the PET film is attached to the curved edge portion of the display, the edge is lifted, and deviation of the optical axis occurs due to biaxial stretching, making it unsuitable for an optical fingerprint recognition display. In order to replace the PET film having such a problem, glass or TPU material (urethane) is used. However, glass is expensive and can easily break under impact, TPU is difficult to attach, is prone to scratching, and can easily tear.

Documents of the prior art

Patent document

(patent document 0001) Korean patent laid-open publication No. 10-2018-0080747

Disclosure of Invention

Technical problem

An object of the present invention is to provide a polyester film that can suppress lifting at a portion having a curvature (e.g., an edge) when the polyester film is used for a curved display.

Further, another object of the present invention is to provide a polyester film in which the optical axis of the polyester film can be controlled by optimizing the manufacturing method of the polyester film, thereby enabling a fingerprint recognition function.

The above and other objects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments.

Technical scheme

According to one aspect of the present invention, the above object is achieved by providing a polyester film comprising: a film formed of a copolymerized polyester resin; and a primer layer formed on at least one surface of the film; wherein the optical loss at a wavelength of 550nm in the longitudinal direction (MD) and the Transverse Direction (TD) is 10% or less, and the deviation of the optical axis angle is + -5 deg. or less.

Preferably, the copolymerized polyester resin may be copolymerized with ethylene glycol from a diol component comprising a dicarboxylic acid or a derivative for forming an ester thereof.

Preferably, the dicarboxylic acid may be selected from terephthalic acid, isophthalic acid, 2, 6-naphthalenedicarboxylic acid and 1, 4-cyclohexanedicarboxylic acid.

Preferably, the glycol component comprising ethylene glycol may comprise 70 to 99 mol% ethylene glycol and 1 to 30 mol% of at least one selected from the group consisting of neopentyl glycol, 1, 4-cyclohexanedimethanol, diethylene glycol, propylene glycol and butanediol.

Preferably, a stretch ratio between a longitudinal direction (MD) and a Transverse Direction (TD) of the polyester film may satisfy formulas 1 and 2, wherein:

(formula 1)

0.3 or less of a stretch ratio in the Machine Direction (MD)/a stretch ratio in the transverse direction of 0.6 or less; and

(formula 2)

2 times or less and 4 times or less of a stretching ratio in the Machine Direction (MD).

Preferably, the polyester film may have a fingerprint recognition rate of 98% or more in the optical fingerprint recognition type.

Preferably, the flexibility of the polyester film in the longitudinal direction (MD) and the Transverse Direction (TD) may be 15gr or less.

Preferably, the thickness of the polyester film may be 25 μm to 100 μm.

Preferably, the polyester film may be a biaxially stretched film.

Preferably, the undercoat layer may be formed by using a polyurethane-based binder or an acrylic binder as a main component. Further, the thickness of the undercoat layer may be 5nm to 200 nm.

Preferably, the polyester film may have a haze of 1.5% or less and a transmittance of 90% or more.

Preferably, the copolymerized polyester resin may further include 2 to 20% by weight of particles, based on 100% by weight of the copolymerized polyester resin. Furthermore, the particles may be silica particles having an average diameter of 1 μm to 10 μm.

According to another aspect of the present invention, the above object is also achieved by providing a method for manufacturing a polyester film, the method comprising: preparing bis (hydroxyethylene) terephthalate or an oligomer thereof by heating a dicarboxylic acid or a derivative thereof for forming an ester thereof and a glycol component comprising ethylene glycol; preparing a copolymerized polyester resin through a polycondensation reaction of the prepared bis (hydroxyethylidene) terephthalate or an oligomer thereof by adding thereto a polycondensation catalyst and a phosphate-based heat stabilizer; producing a uniaxially stretched film by extruding a copolymerized polyester resin and then uniaxially stretching the copolymerized polyester resin in a longitudinal direction; applying a base coat layer on the uniaxially stretched film; producing a biaxially stretched film by stretching the uniaxially stretched film having the undercoat layer coated thereon in a transverse direction; and heat-treating the biaxially stretched film.

Preferably, the heat treatment may include performing the heat treatment at 190 ℃ to 220 ℃.

Preferably, in the uniaxial stretching and the biaxial stretching, the stretching ratio between the longitudinal direction (MD) and the Transverse Direction (TD) of the polyester film may satisfy the above formulas 1 and 2.

According to still another aspect of the present invention, the above object is also achieved by providing a polyester film in which the aforementioned polyester film is used as a protective film for a curved display having an optical fingerprint recognition function.

Advantageous effects

As described above, since the polyester film according to the present invention is produced by using the copolymerized polyester resin prepared according to the present invention instead of using the existing PET resin, the flexibility of the polyester film may be improved so that the entire curved display of the optical fingerprint recognition smartphone may be protected and the curved edge of the curved display may be prevented from being lifted.

Further, the optical axis can be controlled by optimizing the manufacturing process of the polyester film according to the present invention, so that the fingerprint recognition rate of the optical fingerprint recognition type using the curved display can be improved.

However, the effects of the present invention are not limited to the foregoing, and other objects not described herein will be clearly understood by those skilled in the art from the following description.

Drawings

Fig. 1 is a flowchart illustrating a method of manufacturing a polyester film according to an embodiment of the present invention.

Detailed Description

Hereinafter, the configuration and effect of the present invention will be described in further detail with reference to examples and drawings. It should be understood, however, that these examples are provided to more particularly illustrate the invention and are not to be construed as limiting the scope of the invention.

In the drawings, the thickness of layers, regions, etc. may be exaggerated for clarity. Like reference numerals generally refer to like elements throughout the specification. It will be understood that when an element such as a layer, film, region or substrate is referred to as being "on" another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being "directly on" another element, there are no intervening elements present.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present specification, including any definitions herein, will control. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of embodiments of the present invention, the preferred methods and materials are described herein.

Hereinafter, the present invention will be described in further detail.

A polyester film according to one embodiment of the present invention includes a film formed of a copolymerized polyester resin and a primer layer formed on at least one surface of the film, wherein an optical loss at a wavelength of 550nm in a longitudinal direction (MD) and a Transverse Direction (TD) is 10% or less, and a deviation of an optical axis angle is ± 5 ° or less.

In the polyester film according to one embodiment of the present invention, a copolymerized polyester resin is used as a main material of the film. More specifically, the copolymerized polyester resin used for producing the polyester film is a resin copolymerized with a dicarboxylic acid or a derivative for forming an ester thereof and a glycol component containing ethylene glycol. In this case, the glycol component containing ethylene glycol contains ethylene glycol (first glycol) and at least one glycol (second glycol) selected from the group consisting of neopentyl glycol, 1, 4-cyclohexanedimethanol, diethylene glycol, propylene glycol and butanediol. In this case, the component other than ethylene glycol in the diol component is preferably neopentyl glycol.

Further, the dicarboxylic acid is preferably selected from terephthalic acid, isophthalic acid, 2, 6-naphthalenedicarboxylic acid and 1, 4-cyclohexanedicarboxylic acid. In this case, the dicarboxylic acid is preferably terephthalic acid.

By adopting the above configuration and changing the conditions of the manufacturing process, it is possible to provide a polyester film which is suitable for optical fingerprint recognition and can prevent the edge of the curved display from lifting.

More specifically, a copolymerized polyester resin is prepared by a polycondensation reaction of terephthalic acid or a derivative for forming an ester thereof with a glycol component comprising ethylene glycol in the presence of a polycondensation catalyst and a heat stabilizer.

In this case, in the glycol component containing ethylene glycol, at least one glycol (second glycol) selected from the group consisting of neopentyl glycol, 1, 4-cyclohexanedimethanol, diethylene glycol, propylene glycol and butanediol is preferably contained in an amount of 1 to 30 mol% in addition to ethylene glycol (first glycol). That is, it is preferable that the glycol component contains 70 to 99 mol% of ethylene glycol (first glycol) and 1 to 30 mol% of other glycol component (second glycol).

Hereinafter, the present invention will be described in further detail with reference to fig. 1 showing a flowchart of a manufacturing method of a polyester film according to an embodiment of the present invention.

Referring to fig. 1, a method of manufacturing a polyester film according to an embodiment of the present invention includes: a step (S101) of producing bis (hydroxyethylene) terephthalate or an oligomer thereof by heating; a step (S102) of preparing a copolymerized polyester resin by polycondensation; a step (S103) of uniaxially stretching the copolymerized polyester resin in the longitudinal direction; a step (S104) of forming an undercoat layer on one surface of the uniaxially stretched film; a step (S105) of biaxially stretching the film in the transverse direction; and a step (S106) of performing a heat treatment.

First, in the step (S101) of preparing bis (hydroxyethylene) terephthalate or an oligomer thereof by heating, a dicarboxylic acid or a derivative for forming an ester thereof and a glycol component comprising ethylene glycol are heated at 260 ℃ to 300 ℃ to prepare bis (hydroxyethylene) terephthalate or an oligomer thereof.

In this case, the glycol component contains 70 to 99 mol% of ethylene glycol (first glycol) and 1 to 30 mol% of at least one glycol (second glycol) selected from the group consisting of neopentyl glycol, 1, 4-cyclohexanedimethanol, diethylene glycol, propylene glycol and butanediol. If the amount of the second glycol is less than 1 mol%, the flexibility of the film increases, so that there is a concern that the edge of the film may lift when the film is attached to a smartphone having a curved display; whereas if the amount of the second glycol is more than 30 mol%, the surface hardness decreases so that the scratch resistance decreases.

Then, in the step of preparing a copolymerized polyester resin by polycondensation (S102), the copolymerized polyester resin is prepared by a polycondensation reaction of bis (hydroxyethylidene) terephthalate or an oligomer thereof prepared in S101 in the presence of various additives, such as a polycondensation catalyst, a phosphate-based heat stabilizer, etc., and particles dispersed in ethylene glycol, at 280 to 310 ℃.

Further, the polycondensation catalyst used in S102 is preferably at least one selected from the group consisting of antimony compounds, titanium compounds and germanium compounds. Among these compounds, glycol-soluble antimony compounds such as antimony oxide, antimony acetate and the like are further preferred.

In this case, the amount of the polycondensation catalyst is preferably in the range of 150ppm to 350ppm with respect to the copolymerized polyester resin. If the amount of the polycondensation catalyst is less than 150ppm, a longer polymerization time is required and the speed of increasing the Intrinsic Viscosity (IV) is significantly reduced, making it difficult to obtain a polymer having a desired molecular weight. In addition, in order to overcome this disadvantage, the polycondensation temperature should be maintained at a high level, so that a coloring phenomenon occurs due to the side reaction product. In this case, the haze during film production is improved, but the friction coefficient with the roller during film production is reduced, thereby deteriorating the workability of the film. Further, if the amount of the polycondensation catalyst is more than 350ppm, the polycondensation reaction time is reduced, but the molecular weight is not uniform and a colored polymer is obtained, haze during film production is reduced, and coarse particles are caused to be formed.

Further, the heat stabilizer used in S102 is preferably a phosphate ester-based heat stabilizer, and may be, for example, trimethyl phosphate, triethyl phosphonoacetate, or phosphoric acid.

In this case, the amount of the heat stabilizer is preferably 100ppm to 300ppm with respect to the copolymerized polyester resin. If the amount of the heat stabilizer is less than 100ppm, it is not desirable because side reaction products increase to cause colored polymers and decrease heat resistance; in contrast, if the amount of the heat stabilizer is more than 300ppm, it is not desirable because the polycondensation reaction is delayed and side reaction products increase, thereby causing coloration during film production and reducing the life cycle of the polymer filter and the cycle of LIP washing.

Further, particles may be added in S102 to achieve the operability of the membrane. In this case, the particles may preferably be silica particles. The amount of the particles is preferably 2 to 20% by weight based on 100% by weight of the copolymerized polyester resin. If the amount of particles is less than 2 wt%, sufficient operability of the film may not be achieved; in contrast, if the amount of the particles is more than 20 wt%, roughness on the surface of the film may be caused, thereby deteriorating the appearance of the molded product, and the film may be broken during the stretching of the film due to the particles.

Further, the average diameter of the particles is preferably 1 μm to 10 μm. If the average diameter of the particles is less than 1 μm, agglomeration of the particles may occur, and it is difficult to obtain uniform particles and operability of the film; in contrast, if the average diameter of the particles is greater than 10 μm, the surface roughness of the film increases, thus deteriorating the appearance of the molded product.

In addition, the method for manufacturing a polyester film according to one embodiment of the present invention may further include a step of sufficiently drying the prepared copolymerized polyester resin at 150 to 170 ℃ for 6 to 9 hours by using a vacuum dryer. In this case, the pre-crystallization may be performed as needed.

Subsequently, in the step (S103) of uniaxially stretching the copolymerized polyester resin in the longitudinal direction, the copolymerized polyester resin after vacuum drying prepared in S102 is melted by an extruder to be extruded into a sheet shape through a feed block (feed block) by using a T-die, then pressed on a casting drum by an electrostatic application method, and cooled and solidified by a cooling roller to obtain an unstretched sheet. Thereafter, the unstretched sheet is uniaxially stretched in the longitudinal direction (MD direction) with a difference in tip speed ratio between rollers heated to a temperature equal to or higher than the glass transition temperature of the unstretched sheet to produce a uniaxially stretched film.

Then, in the step of forming an undercoat layer on one surface of the uniaxially stretched film (S104), the undercoat layer may be formed by various coating methods, such as bar coating, gravure coating, slit die coating, and the like.

In this case, the undercoat layer is formed by using a polyurethane-based binder or an acrylic binder as a main component, and the undercoat layer may contain a material selected from the group consisting of

One or more crosslinking agents of an oxazoline compound, a carbodiimide compound, and a melamine compound.

Further, the thickness of the undercoat layer is preferably in the range of 5nm to 200 nm. If the thickness of the undercoat layer is less than 5nm, the adhesion to the coating layer coated on the undercoat layer is reduced; whereas if the thickness of the undercoat layer is greater than 200nm, a rainbow phenomenon is visible due to the difference in refractive index. Therefore, the thickness of the undercoat layer is preferably within the above range.

Subsequently, in the step (S105) of biaxially stretching the film in the transverse direction, the uniaxially stretched film with a primer layer obtained in S104 is re-stretched to produce a biaxially stretched film. In this case, stretching is performed in a direction (TD) perpendicular to the uniaxial stretching direction; a stretching temperature in a stretching region of the tenter at an initial stage is 100 ℃ to 130 ℃; and the stretching temperature in the stretching zone of the tenter at the final stage is 130 ℃ to 160 ℃.

According to one embodiment of the present disclosure, in steps S103 and S105, the stretch ratio in the longitudinal direction (MD) and the Transverse Direction (TD) satisfies the following formulas 1 and 2.

(formula 1)

0.3 or less of a stretch ratio in the Machine Direction (MD)/0.6 or less of a stretch ratio in the transverse direction

(formula 2)

A stretching ratio in the Machine Direction (MD) of 2 times or less and 4 times or less

With regard to equation 1, if the stretch ratio in the Transverse Direction (TD) is reduced such that the value of equation 1 exceeds 0.6, the bending phenomenon increases, resulting in deviation of the optical axis; whereas if the stretching ratio is increased to be greater than a specific value such that the value of formula 1 is less than 0.3, the risk of film rupture increases and poor flexibility of the film is caused due to an increase in strain-induced crystallization caused by excessive stretching, thereby causing a spring back phenomenon and restricting the device.

Further, if the stretching ratio in the Machine Direction (MD) is less than 2 times, stretching unevenness occurs due to non-uniform stretching; whereas if the stretch ratio in the longitudinal direction (MD) is more than 4 times, the deviation of the optical axis due to the bending phenomenon increases.

Thereafter, in the step of heat-treating (S106) the biaxially stretched film, the biaxially stretched film is heat-treated at 190 to 220 ℃ to produce a biaxially stretched polyester film. In particular, if the heat treatment is performed at a temperature lower than 190 ℃, large heat shrinkage occurs, so that curling may occur during processing in a customer company; whereas if the heat treatment is performed at a temperature higher than 220 deg.c, the bending phenomenon increases, resulting in an increase in deviation of the optical axis, making it unsuitable for optical fingerprint recognition.

In the polyester film according to one embodiment of the present invention, the deviation of the optical axis is preferably ± 5 ° or less. As described above, the polyester film of the present invention may be used as a protective film for a display having its own optical fingerprint recognition function without using a separate fingerprint recognition module, so that if the deviation of the optical axis exceeds ± 5 °, a fingerprint may not be recognized or it takes longer time for fingerprint recognition.

Further, in the polyester film according to one embodiment of the present invention, the optical loss at a wavelength of 550nm in the longitudinal direction (MD) and the Transverse Direction (TD) is preferably 10% or less. If the light loss rate exceeds 10%, the fingerprint on the display may not be correctly recognized, so that fingerprint recognition takes a longer time.

Further, the flexibility of the polyester film according to an embodiment of the present invention in the longitudinal direction (MD) and the Transverse Direction (TD) is preferably 15gr or less. If the compliance exceeds 15gr, a rebound phenomenon occurs.

Further, the thickness of the polyester film according to an embodiment of the present invention is preferably 25 μm to 100 μm. If the thickness of the polyester film is less than 25 μm, the protective characteristics are degraded; whereas if the thickness of the polyester film exceeds 100 μm, moldability is deteriorated.

In addition, the haze of the polyester film according to an embodiment of the present invention is preferably 1.5% or less. If the haze exceeds 1.5%, the fingerprint may not be correctly recognized during fingerprint recognition on the optical fingerprint recognition type due to a high haze level.

Further, the transmittance of the polyester film according to an embodiment of the present invention is preferably 90% or more. If the transmittance is less than 90%, the fingerprint on the display may not be correctly recognized.

Further, the polyester film according to one embodiment of the present invention preferably has a fingerprint recognition rate of 98% or more in the optical fingerprint recognition type. If the fingerprint recognition rate is less than 98%, the fingerprint of the user may not be recognized or may be erroneously recognized, and the fingerprint recognition takes a longer time.

Hereinafter, the present invention will be described in further detail with reference to the following examples. It should be understood, however, that these examples are provided to more specifically illustrate the present invention and are not to be construed as limiting the scope of the present invention.

[ examples ]

[ example 1]

The copolymerized polyester resin is produced by: a first step of preparing bis (hydroxyethylene) terephthalate or an oligomer thereof by heating terephthalic acid and a diol component comprising ethylene glycol at 270 ℃; and a second step of preparing liquid PET by a conventional polycondensation reaction of bis (hydroxyethylidene) terephthalate or an oligomer thereof prepared in the presence of various additives such as an antimony compound catalyst, a phosphate-based heat stabilizer, etc., and particles dispersed in ethylene glycol at 290 ℃. In this case, the glycol component comprising ethylene glycol may be prepared by adding 99 mol% of ethylene glycol and 1 mol% of neopentyl glycol.

The produced copolymerized polyester resin was dried at 160 ℃ for 7 hours by using a vacuum dryer. Thereafter, the dried copolymerized polyester resin was melted by an extruder, and then pressed against a casting drum (cooling drum) by electrostatic application pressure via a feed section and a T-die to obtain an amorphous unstretched sheet. Subsequently, the unstretched sheet was heated again and stretched to 2.5 times in the longitudinal direction (film running direction) at 95 ℃, and then one surface of the polyester resin layer of the sheet was coated to a thickness of 100nm by gravure coating with an undercoat layer coating solution having a refractive index of 1.5. In this case, 30% by weight of a urethane-based binder, 15% by weight of

An oxazoline curing agent, 1 wt% silica particles, 5 wt% fluorine-based surfactant, and 49 wt% water were used to prepare a coating solution for undercoat coating. Then, the resulting sheet was stretched to 5 times at 130 ℃ in a direction transverse to the running direction of the film (vertical direction) and heat-treated at 200 ℃ to produce a film. In this case, the film was produced to have a thickness of 50 μm.

[ example 2]

A film was produced in the same manner as in example 1 except that 70 mol% of ethylene glycol and 30 mol% of neopentyl glycol were added to the glycol component.

[ example 3]

A film was produced in the same manner as in example 1 except that the film was stretched 2 times in the longitudinal direction and 6.5 times in the transverse direction.

[ example 4]

A film was produced in the same manner as in example 1 except that the film was stretched 2 times in the longitudinal direction and 3.4 times in the transverse direction.

[ example 5]

A film was produced in the same manner as in example 1 except that the film was stretched 4 times in the longitudinal direction and 13 times in the transverse direction.

[ example 6]

A film was produced in the same manner as in example 1 except that the film was stretched 4 times in the longitudinal direction and 6.8 times in the transverse direction.

[ example 7]

A film was produced in the same manner as in example 1 except that after stretching in the transverse direction, heat treatment was performed at 190 ℃.

[ example 8]

A film was produced in the same manner as in example 1 except that after stretching in the transverse direction, heat treatment was performed at 220 ℃.

Comparative example 1

A film was produced in the same manner as in example 1 except that neopentyl glycol was not added during the production of the polyester resin.

Comparative example 2

A film was produced in the same manner as in example 1 except that 99.5 mol% of ethylene glycol and 0.5 mol% of neopentyl glycol were in the glycol component containing ethylene glycol.

Comparative example 3

A film was produced in the same manner as in example 1 except that 68 mol% of ethylene glycol and 32 mol% of neopentyl glycol were in the glycol component containing ethylene glycol.

Comparative example 4

A film was produced in the same manner as in example 1 except that the film was stretched 1.5 times in the longitudinal direction and 3 times in the transverse direction.

Comparative example 5

A film was produced in the same manner as in example 1 except that the film was stretched 4.5 times in the longitudinal direction and 7.5 times in the transverse direction.

Comparative example 6

A film was produced in the same manner as in example 1 except that the film was stretched 2.5 times in the longitudinal direction and 3.9 times in the transverse direction.

Comparative example 7

A film was produced in the same manner as in example 1 except that the film was stretched 2.5 times in the longitudinal direction and 9 times in the transverse direction.

Comparative example 8

A film was produced in the same manner as in example 1 except that after stretching in the transverse direction, heat treatment was performed at 180 ℃.

Comparative example 9

A film was produced in the same manner as in example 1 except that after stretching in the transverse direction, heat treatment was performed at 230 ℃.

The physical properties of the films produced in examples 1 to 8 and comparative examples 1 to 9 were evaluated based on the following experimental examples, and the results are shown in table 1 below.

[ Experimental example ]

(1) Rebound characteristics

In the a 4-sized film, after one surface of the urethane coating was coated with a silicone adhesive to a thickness of about 20 μm, the resulting layer was attached to a glass having a curved surface of 10R, and then separation or blistering was observed.

Good: without separation or foaming

Difference: separation or foaming takes place

(2) Rate of light loss

After the film was cut into pieces 5cm wide and 5cm long, two polarizing films were made into a 100% transmittance state (non-crossed nicol), and the film was placed between the two polarizing films while rotating to a direction of 45 degrees with respect to the Transverse Direction (TD). Then, light of 550nm wavelength was transmitted from below, and the amount of the passed light was measured using a light transmission meter on the opposite side. Then, the light loss rate is calculated by using the following formula 3 based on the measured amount of light.

(formula 3)

Light loss rate (amount of light before placing film-amount of light after placing film)/amount of light before placing film × 100%

In the measurement of the light loss rate, the light loss rate in the lateral direction and the longitudinal direction is equal to the light loss rate at angles of 45 degrees, 90 degrees, 180 degrees, 270 degrees, and 360 degrees in consideration of the light loss characteristics.

(3) Measurement of optical axis

During the measurement of the light loss rate in experimental example 2, when the film was rotated between two polarizing films, an angle at which the light loss rate was the smallest was measured as the optical axis. In this case, the longitudinal direction (TD) is defined as 0 degree.

(4) Non-uniformity of stretching

After cutting to a4 size, the film was placed between two polarizing films and observed with the naked eye using a white LED light source. The stretching unevenness was evaluated based on the following criteria.

Severe: film deformation was observed

Medium: no film deformation was observed, but color difference was observed

Slight: no color difference was observed

(5) Hardness of pencil

The Pencil Hardness was measured at room temperature by using pencils having Hardness ranging from 6B to 6H among pencils for Hardness test certification manufactured by Mitsubishi using a test apparatus manufactured by Pencil Hardness Tester (Yuyu Corp.) according to JIS K5400, 5600. After coating the film with an acrylic hard coat layer to a thickness of 2 μm, pencil hardness was measured 5 times, and indentation and scratch of the film were observed. The tested film failed when three of the five impressions and scratches occurred, and was determined to be good when two impressions and scratches occurred. When it is difficult to determine the rupture with the naked eye, the film is examined with a microscope.

(6) Rate of fingerprint identification

After laminating the film on OCA having a thickness of 50 μm, a smartphone size (apo co., ltd.r17 model, smartphone with optical fingerprint recognition function) was cut at an angle of 45 degrees with respect to the Transverse Direction (TD) and attached to the smartphone. Then, after performing fingerprint recognition 100 times, the fingerprint recognition rate is calculated by using the following equation 4.

(formula 4)

The fingerprint identification rate is the successful times of fingerprint identification/100 times multiplied by 100 percent

(7) Occurrence of curling

After one surface of the film was coated to a thickness of 2 μm with an acrylic hard coat layer, the film was dried and UV-cured at 120 ℃ for 1 minute, and then it was checked whether curling occurred at the time of film manufacturing.

(8) Flexibility

After the film was cut into pieces 100mm long and 3.81mm wide, the flexibility of the film was measured using a flexibility meter (T5000, Toyo Seiki co., Ltd.).

[ Table 1]

As shown in table 1 above, it can be seen that in examples 1 to 8, when copolymerization is performed with 1 to 30 mol% of neopentyl glycol and the stretch ratios in the longitudinal and transverse directions satisfy formulas 1 and 2, films all excellent in the spring back characteristics, deviation of optical axis, optical loss rate, fingerprint recognition rate, and the like can be provided.

In contrast, with comparative example 1, the flexibility of the film was increased, resulting in poor spring back characteristics. Further, with comparative example 2, as in comparative example 1, the flexibility of the film was increased, resulting in poor spring back characteristics.

Further, with comparative example 3, the addition of an excessive amount of neopentyl glycol causes a decrease in pencil hardness, and a decrease in scratch resistance.

Further, with comparative example 4, the stretch ratio in the Machine Direction (MD) was low, thereby causing stretching unevenness.

Further, with comparative example 5, the flexibility of the film increased with strain-induced crystallization due to excessive stretching in the transverse and longitudinal directions, resulting in poor spring back characteristics; meanwhile, the deviation of the optical axis is significantly increased, so that the fingerprint recognition rate is lowered.

Further, with comparative example 6, the stretch ratio of the Transverse Direction (TD) to the longitudinal direction is low so that the value of expression 1 exceeds 0.6, thereby causing an increase in the bending phenomenon of the film, resulting in deviation of the optical axis, an increase in the light loss rate, and a decrease in the fingerprint recognition rate.

Further, with comparative example 7, the stretch ratio in the Transverse Direction (TD) to the longitudinal direction was too high, so that the value of formula 1 was less than 0.3, resulting in poor spring back characteristics. The reason for this is an increase in strain-induced crystallization due to excessive stretching in the Transverse Direction (TD), resulting in poor flexibility of the film.

Further, with comparative example 8, heat treatment was performed at a low temperature, so that curling occurred due to large heat shrinkage; in contrast, with comparative example 9 in which the heat treatment was performed at a high temperature, the bending phenomenon increased, thereby increasing the deviation of the optical axis and the optical loss, and decreasing the fingerprint recognition rate.

As described above, the polyester film according to one embodiment of the present disclosure may be used as a protective film for a smart phone having a display with a fingerprint recognition function, in which separation (springback) of a bent edge portion of a bent display may be prevented in the case of using a copolymerized polyester resin, and at the same time, a fingerprint recognition rate of optical fingerprint recognition may be improved by satisfying the above equations 1 and 2 to control deviation of an optical axis.

Although preferred embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as defined in the accompanying claims.

Claims

1. A polyester film comprising:

a film formed of a copolymerized polyester resin; and

a primer layer formed on at least one surface of the film;

wherein:

an optical loss at a wavelength of 550nm in a longitudinal direction (MD) and a Transverse Direction (TD) of 10% or less, and

the deviation of the optical axis angle is ± 5 ° or less.

2. A polyester film according to claim 1 wherein the copolymerised polyester resin is copolymerised from a dicarboxylic acid or derivative thereof to form an ester thereof and a diol component comprising ethylene glycol.

3. A polyester film according to claim 2 wherein the dicarboxylic acid is selected from terephthalic acid, isophthalic acid, 2, 6-naphthalenedicarboxylic acid and 1, 4-cyclohexanedicarboxylic acid.

4. A polyester film according to claim 2 wherein the glycol component comprising ethylene glycol comprises from 70 to 99 mol% ethylene glycol and from 1 to 30 mol% of at least one selected from neopentyl glycol, 1, 4-cyclohexanedimethanol, diethylene glycol, propylene glycol and butanediol.

5. A polyester film according to claim 1 wherein the draw ratio between the longitudinal direction (MD) and the Transverse Direction (TD) of the polyester film satisfies formulae 1 and 2,

wherein:

formula 1

formula 2

6. A polyester film according to claim 1, wherein the polyester film has a fingerprint recognition rate of 98% or more in an optical fingerprint recognition type.

7. A polyester film according to claim 1 wherein the flexibility of the polyester film in the longitudinal direction (MD) and the Transverse Direction (TD) is 15gr or less.

8. A polyester film according to claim 1 wherein the thickness of the polyester film is from 25 to 100 μm.

9. A polyester film according to claim 1 wherein the polyester film is a biaxially stretched film.

10. A polyester film according to claim 1, wherein the undercoat layer is formed by using a polyurethane-based binder or an acrylic binder as a main component.

11. A polyester film according to claim 1 wherein the thickness of the primer layer is from 5nm to 200 nm.

12. A polyester film according to claim 1 wherein the polyester film has a haze of 1.5% or less and a transmission of 90% or more.

13. A polyester film according to claim 1 wherein the copolymerized polyester resin further comprises 2 to 20 wt% of particles, based on 100 wt% of the copolymerized polyester resin.

14. A polyester film according to claim 13 wherein the particles are silica particles having an average diameter of from 1 to 10 μm.

15. A method of manufacturing a polyester film, the method comprising:

preparing bis (hydroxyethylene) terephthalate or an oligomer thereof by heating a dicarboxylic acid or a derivative thereof for forming an ester thereof and a glycol component comprising ethylene glycol;

preparing a copolymerized polyester resin from a polycondensation reaction of the prepared bis (hydroxyethylidene) terephthalate or an oligomer thereof by adding thereto a polycondensation catalyst and a phosphate-based heat stabilizer;

producing a uniaxially stretched film by extruding the copolymerized polyester resin and then uniaxially stretching the copolymerized polyester resin in a longitudinal direction;

applying a base coat layer on the uniaxially stretched film;

producing a biaxially stretched film by stretching the uniaxially stretched film having the undercoat layer coated thereon in a transverse direction; and

heat treating the biaxially stretched film.

16. The method of claim 15, wherein the glycol component comprising ethylene glycol comprises from 70 to 99 mole% ethylene glycol and from 1 to 30 mole% of at least one selected from the group consisting of neopentyl glycol, 1, 4-cyclohexanedimethanol, diethylene glycol, propylene glycol, and butanediol.

17. The method of claim 15, wherein the heat treating comprises heat treating at 190 ℃ to 220 ℃.

18. The method according to claim 15, wherein in the monoaxial stretching and biaxial stretching, a stretching ratio between the longitudinal direction (MD) and the Transverse Direction (TD) of the polyester film satisfies formulas 1 and 2,

wherein:

formula 1

formula 2

19. A polyester film, wherein the polyester film according to any one of claims 1 to 14 is used as a protective film for a curved display having an optical fingerprint recognition function.