CN113429614B

CN113429614B - Polyester film and method for producing same

Info

Publication number: CN113429614B
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: 2023-07-25
Anticipated expiration: 2041-01-15
Also published as: KR20210118512A; CN113429614A; KR102358413B1

Abstract

A polyester film according to an embodiment of the present invention includes: films formed from copolymerized polyester resins; and a primer layer formed on at least one surface of the film; wherein the light loss ratio at a wavelength of 550nm in the longitudinal direction (MD) and the Transverse Direction (TD) is 10% or less, and the deviation of the angle of the optical axis 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 can prevent the curved edge from being lifted up, and can improve the 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 smart phone display, and more particularly, to a polyester film and a method of manufacturing the same, in which for a smart phone having a curved display having an optical fingerprint recognition function, the polyester film can provide a high fingerprint recognition rate and can entirely cover the display, even a curved surface, by optimizing the film composition and the optical axis of the film.

Background

Recently, smartphones provide users with various functions and uses, so that their markets are rapidly growing. To meet the demands for both design and functionality, smartphones increasingly employ curved displays, and at the same time, more smartphones provide fingerprint identification functionality for information security. In addition to existing smartphones having separate parts for fingerprinting, smartphones having displays with their own fingerprinting functionality have also been published.

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

In particular, fingerprint recognition technologies using a display can be classified into a capacitive type, an ultrasonic type, and an optical type, wherein ultrasonic type and optical type sensors can be installed in the display itself. Although the ultrasonic fingerprint sensor has an advantage in that an error caused by foreign matter rarely occurs, it is expensive. For this reason, most smartphones having a display with a fingerprint recognition function employ an optical fingerprint sensor. In a smart phone having a display with an optical fingerprint recognition function, a polarizing plate is positioned at the outermost portion of the display, wherein if a protective film is not coincident with an optical axis of the polarizing plate, optical loss occurs so that fingerprint recognition may not be performed correctly.

For example, korean patent laid-open 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. However, the protective film has a problem in that when a general PET film having poor flexibility is used, a curved edge portion of the display is lifted up 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 a curved edge portion of the display, the edge is lifted up, 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 may be prone to breakage under impact, TPU is difficult to attach, is prone to scratch and may be prone to tearing.

Prior art literature

Patent literature

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

Disclosure of Invention

Technical problem

It is an object of the present invention to provide a polyester film which can suppress lifting at a portion (e.g., edge) having curvature 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 the 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 proposal

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

Preferably, the copolymerized polyester resin may be copolymerized with ethylene glycol from a glycol component including 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-naphthalene dicarboxylic acid and 1, 4-cyclohexane dicarboxylic acid.

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

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

(1)

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

(2)

The stretch ratio in the Machine Direction (MD) is 2 times or less and 4 times or less.

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

Preferably, the polyester film may have a flexibility (flexability) in the Machine Direction (MD) and the Transverse Direction (TD) of 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 primer 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 200nm.

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. Further, 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 producing a polyester film, the method comprising: 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; preparing a copolymerized polyester resin by a polycondensation reaction of the prepared bis (hydroxyethylene) 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 primer layer over the uniaxially stretched film; producing a biaxially stretched film by stretching the uniaxially stretched film having the primer layer coated thereon in the transverse direction; and heat treating the biaxially stretched film.

Preferably, the heat treatment may include 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 foregoing 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 can be improved so that the entire curved display of the optical fingerprint recognition smart phone can be protected and the curved edge of the curved display can be prevented from being lifted.

Furthermore, 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 showing a method of manufacturing a polyester film according to an embodiment of the present invention.

Detailed Description

Hereinafter, the configuration and effects 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 specifically illustrate the present 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.

The polyester film according to an embodiment of the present invention includes a film formed of a copolymerized polyester resin and an undercoat layer formed on at least one surface of the film, wherein a light loss rate 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 glycol component containing ethylene glycol from a dicarboxylic acid or a derivative for forming an ester thereof. In this case, the glycol component including ethylene glycol includes 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.

Furthermore, the dicarboxylic acid is preferably selected from terephthalic acid, isophthalic acid, 2, 6-naphthalene dicarboxylic acid and 1, 4-cyclohexane dicarboxylic acid. In this case, the dicarboxylic acid is preferably terephthalic acid.

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

More specifically, the copolymerized polyester resin is prepared by polycondensation reaction of terephthalic acid or a derivative for forming an ester thereof with a glycol component including 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 30mol% in addition to ethylene glycol (first glycol). That is, it is preferable that the glycol component contains 70mol% to 99mol% of ethylene glycol (first glycol) and 1mol% to 30mol% of other glycol component (second glycol).

Hereinafter, the present invention will be described in further detail with reference to fig. 1, which shows a flowchart of a method of manufacturing 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 preparing 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 heat treatment.

First, in the step of preparing bis (hydroxyethylene) terephthalate or an oligomer thereof by heating (S101), a dicarboxylic acid or a derivative for forming an ester thereof and a glycol component including 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 99mol% of ethylene glycol (first glycol) and 1 to 30mol% 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 1mol%, the flexibility of the film increases, so that there is a concern that the edge of the film may be lifted when the film is attached to a smart phone having a curved display; and if the amount of the second glycol is more than 30mol%, the surface hardness decreases so that scratch resistance decreases.

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

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 rate of increasing the Intrinsic Viscosity (IV) is significantly reduced, making it difficult to obtain a polymer having a desired molecular weight. Furthermore, in order to overcome this disadvantage, the polycondensation temperature should be maintained at a high level so that coloration occurs due to side reaction products. In this case, haze during film production is improved, but a friction coefficient with the roller during film production is reduced, thereby deteriorating operability of the film. Further, if the amount of the polycondensation catalyst is more than 350ppm, the polycondensation reaction time decreases, 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-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, colored polymers are caused and heat resistance is lowered because of an increase in side reaction products, so that it is undesirable; in contrast, if the amount of the heat stabilizer is more than 300ppm, this is undesirable because the polycondensation reaction is delayed and the side reaction products increase, resulting in coloration during film production and reducing the life cycle of the polymer filter and the period of LIP cleaning.

Further, particles may be added in S102 to achieve membrane operability. 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 the 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 causing film breakage during film stretching 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 workability of the film; in contrast, if the average diameter of the particles is more than 10 μm, the surface roughness of the film increases, thus deteriorating the appearance of the molded product.

In addition, the method of manufacturing a polyester film according to an 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 section (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 roll to obtain an unstretched sheet. Thereafter, the unstretched sheet is uniaxially stretched in the machine direction (MD direction) using the 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, slot 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 ofOne or more crosslinking agents of the oxazoline compound, the carbodiimide compound and the melamine compound.

Furthermore, the thickness of the undercoat layer is preferably in the range of 5nm to 200nm. If the thickness of the primer layer is less than 5nm, the adhesion with the coating layer coated on the primer layer is lowered; 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 of biaxially stretching the film in the transverse direction (S105), the uniaxially stretched film with the 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; the stretching temperature in the stretching region of the tenter at the initial stage is 100 ℃ to 130 ℃; and the stretching temperature in the stretching region 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) to the Transverse Direction (TD) satisfies the following equations 1 and 2.

(1)

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

(2)

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

Regarding 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 breakage increases, and poor flexibility of the film is caused due to an increase in strain-induced crystallization caused by excessive stretching, thereby causing a rebound phenomenon (spring back phenomenon) and restricting the apparatus.

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

Thereafter, in the step of heat-treating the biaxially stretched film (S106), 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 client company; and 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 an 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 for fingerprint recognition.

Further, in the polyester film according to an embodiment of the present invention, the light loss rate at a wavelength of 550nm in the Machine 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 polyester film according to an embodiment of the present invention preferably has a compliance in the Machine Direction (MD) and the Transverse Direction (TD) of 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 properties are lowered; and if the thickness of the polyester film exceeds 100 μm, moldability is deteriorated.

Further, 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 due to a high haze level during the fingerprint recognition on the optical fingerprint recognition type.

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 an 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 user's fingerprint may not be recognized or may be erroneously recognized, and 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)

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 glycol component comprising ethylene glycol at 270 ℃; and a second step of preparing liquid PET by a conventional polycondensation reaction of the bis (hydroxyethylene) terephthalate or 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 including ethylene glycol may be prepared by adding 99mol% of ethylene glycol and 1mol% 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 is melted by an extruder and then passed through a feed section anda T-die pressed on a casting drum (cooling drum) by an electrostatic application method 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 coating solution having a refractive index of 1.5. In this case, 30% by weight of a urethane-based binder, 15% by weight ofAn oxazoline curing agent, 1 wt% silica particles, 5 wt% fluorine-based surfactant, and 49 wt% water were prepared to prepare a coating solution for primer coating. Then, the resulting sheet was stretched to 5 times at 130 ℃ in the transverse direction (vertical direction) of the film running 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 70mol% of ethylene glycol and 30mol% of neopentyl glycol were added to the diol component.

Example 3

A film was produced in the same manner as in example 1 except that the film was stretched to 2 times in the longitudinal direction and to 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 to 2 times in the longitudinal direction and to 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 99.5mol% of ethylene glycol and 0.5mol% of neopentyl glycol were in the diol component comprising ethylene glycol.

Comparative example 3

A film was produced in the same manner as in example 1 except that 68mol% of ethylene glycol and 32mol% of neopentyl glycol were in the diol component comprising 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 to 4.5 times in the longitudinal direction and to 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 to 2.5 times in the longitudinal direction and to 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 to 2.5 times in the longitudinal direction and to 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 Property

In the A4-sized film, after one surface of the urethane coating layer was coated to a thickness of about 20 μm with a silicone adhesive, the resulting layer was attached to glass having a curved surface of 10R, and then separation or foaming was observed.

Good: no separation or foaming occurs

The difference is: separation or foaming occurs

(2) Light loss rate

After the film was cut into pieces 5cm wide and 5cm long, two polarizing films were made into a 100% transmittance state (non-orthogonal nicols), and the film was placed between the two polarizing films while being rotated 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 passing light was measured using a light transmission meter on the opposite side. Then, the light loss rate is calculated by using the following equation 3 based on the measured amount of light.

(3)

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

In the measurement of the light loss rate, the light loss rates in the lateral direction and the longitudinal direction are equal to the light loss rates at the 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, the angle at which the light loss rate was minimum was measured as the optical axis. In this case, the longitudinal direction (TD) is defined as 0 degrees.

(4) Non-uniformity of stretching

After cutting to A4 size, the film was placed between two sheets of polarizing film and observed with naked eyes using a white LED light source. The non-uniformity of stretching was evaluated based on the following criteria.

Serious: film deformation was observed

And (3) moderately: no film deformation but color difference was observed

Slightly: no color difference was observed

(5) Hardness of pencil

The pencil hardness was measured at room temperature by using a pencil having a hardness range of 6B to 6H among pencils for hardness test certification manufactured by Mitsubishi by using a test device manufactured by Pencil Hardness Tester (yuyuyuu corp.) according to JIS K5400, 5600. After the film was coated with the 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. When three out of five times the film tested failed and when two times the film was determined to be good. When it is difficult to determine the rupture with the naked eye, the film is examined with a microscope.

(6) Fingerprint recognition rate

After laminating the film on an OCA having a thickness of 50 μm, a smartphone size (model No. l.td.r17, a smartphone having an 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.

(4)

Fingerprint identification rate = number of successful fingerprint identifications/100 times x 100%

(7) Curl generation

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

(8) Compliance of

After cutting the film into pieces 100mm long and 3.81mm wide, the compliance of the film was measured using a compliance 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 was performed with 1 to 30mol% of neopentyl glycol and the stretch ratio in the longitudinal direction and the transverse direction satisfied formulas 1 and 2, films excellent in all of rebound characteristics, deviation of optical axis, light loss rate, fingerprint recognition rate, and the like could be provided.

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

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

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

Further, with comparative example 5, due to excessive stretching in the transverse direction and the longitudinal direction, the flexibility of the film increases with strain-induced crystallization, resulting in poor rebound characteristics; meanwhile, the deviation of the optical axis is remarkably increased, so that the fingerprint recognition rate is reduced.

Further, with comparative example 6, the lower stretch ratio of the Transverse Direction (TD) to the longitudinal direction makes the value of equation 1 exceed 0.6, so that the film bending phenomenon increases, resulting in deviation of the optical axis, increasing the light loss rate, and decreasing the fingerprint recognition rate.

Further, with comparative example 7, the stretch ratio of 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 rebound characteristics. The reason for this result is that strain-induced crystallization increases 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 having a fingerprint recognition function, wherein in the case of using a copolymerized polyester resin, separation (rebound) of a curved edge portion of a curved display may be prevented, and at the same time, by satisfying the above formulas 1 and 2 to control deviation of an optical axis, a fingerprint recognition rate of optical fingerprint recognition may be improved.

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

Claims

1. A polyester film comprising:

films formed from copolymerized polyester resins; and

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

wherein:

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

deviation of the angle of the optical axis is + -5 DEG or less

Wherein the polyester film has a fingerprint recognition rate of 98% or more in the optical fingerprint recognition type,

wherein the copolymerized polyester resin further comprises 2 to 20 wt% of particles based on 100 wt% of the copolymerized polyester resin,

wherein the particles are silica particles having an average diameter of 1 μm to 10 μm,

wherein the primer layer is formed by using a polyurethane-based binder or an acrylic binder as a main component, and the primer layer comprises a material selected from the group consisting ofOne or more crosslinking agents of the oxazoline compound, the carbodiimide compound and the melamine compound.

2. The polyester film according to claim 1, wherein the copolymerized polyester resin is copolymerized with a glycol component comprising ethylene glycol from a dicarboxylic acid or a derivative for forming an ester thereof.

3. The polyester film of claim 2 wherein the dicarboxylic acid is selected from the group consisting of terephthalic acid, isophthalic acid, 2, 6-naphthalene dicarboxylic acid, and 1, 4-cyclohexane dicarboxylic acid.

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

5. The polyester film according to claim 1, wherein a stretch ratio between the Machine Direction (MD) and the Transverse Direction (TD) of the polyester film satisfies formulas 1 and 2,

wherein:

1 (1)

2, 2

6. The polyester film according to claim 1, wherein the polyester film has a compliance in the Machine Direction (MD) and the Transverse Direction (TD) of 15gr or less.

7. The polyester film of claim 1, wherein the polyester film has a thickness of 25 μιη to 100 μιη.

8. The polyester film of claim 1 wherein the polyester film is a biaxially stretched film.

9. The polyester film of claim 1 wherein the primer layer has a thickness of 5nm to 200nm.

10. The polyester film of claim 1, wherein the polyester film has a haze of 1.5% or less and a transmittance of 90% or more.

11. A method of making a polyester film, the method comprising:

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;

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

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 primer layer over the uniaxially stretched film;

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

heat treating the biaxially stretched film,

wherein the deviation of the angle of the optical axis of the polyester film is + -5 DEG or less, and

wherein the copolymerized polyester resin comprises 2 to 20% by weight of the particles based on 100% by weight of the copolymerized polyester resin,

12. The method of claim 11, wherein the glycol component comprising ethylene glycol comprises 70 to 99 mole% ethylene glycol and 1 to 30 mole% of at least one selected from the group consisting of neopentyl glycol, 1, 4-cyclohexanedimethanol, diethylene glycol, propylene glycol, and butylene glycol.

13. The method of claim 11, wherein the heat treating comprises heat treating at 190 ℃ to 220 ℃.

14. The method according to claim 11, wherein in the uniaxial stretching and biaxial stretching, a stretching ratio between the Machine Direction (MD) and the Transverse Direction (TD) of the polyester film satisfies the formulas 1 and 2,

wherein:

1 (1)

2, 2

15. A polyester film, wherein the polyester film according to any one of claims 1 to 10 is used as a protective film for a curved display having an optical fingerprint recognition function.