TWI649367B - Protective film, polarizer and display device therewith - Google Patents

Abstract

The present invention relates to a protective film, a polarizer, and a display device including the same. The protective film for polarizing plates according to the present invention includes a mixed resin obtained by mixing a fluorine-based resin and an acrylic resin in a certain ratio, which not only improves the breakage when preparing the sheet and the film due to the improved elongation (i.e. , Machinability, also due to low in-plane retardation (R _o ) and low thickness direction retardation (R _th ) does not generate color spots.

Description

Protective film, polarizer and display device containing the same

TECHNICAL FIELD A specific example relates to a protective film, a polarizer, and a display device including the same.

BACKGROUND OF THE INVENTION The advent of the information age has prompted the development and commercialization of various display devices including liquid crystal displays (LCD), plasma display panels (PDP), electrophoretic displays (ELD), and so on. Display devices for indoor use become larger in size and thinner in thickness, while portable display devices for outdoor use become smaller in size and lighter in weight. Various optical films have been adopted to further enhance the functions of such displays.

Depending on the type of display, the materials used for such optical films generally need to satisfy requirements such as high transmittance, optical isotropy, defect-free surface, high heat resistance, high moisture resistance, high flexibility, high surface hardness, and low shrinkage Rate, good workability (ie, low breakage), etc.

In the manufacture of polarizers, triethoxylated cellulose (TAC), acrylic acid, polyethylene terephthalate (PET), etc. with properties such as high transmittance, optical isotropy, defect-free surface, etc. are commonly used The film made serves as a protective film on one or both sides of a polarizer made of polyvinyl alcohol to protect the polarizer. However, their disadvantages are that TAC is expensive, does not have many sources, and is susceptible to moisture; because acrylic acid has high breakage, so it is not easy to make a sheet or film; and because of its high retardation, PET tends to appear stained (mura ) (Polarization unevenness).

Accordingly, in recent years, many studies have been performed to improve the above-mentioned disadvantages. For example, Japanese Patent No. 4962661 discloses a protective film using polyester resin. However, it has the problem of uneven stretching in the transverse/longitudinal direction, and the reduction in thickness is limited. In addition, there is no disclosure of improvement in breakability or increase in elongation.

Therefore, the present inventors are committed to solving the aforementioned problems related to the prior art, and have discovered a protective film for polarizing plates made of a mixed resin in which a fluorine-based resin and an acrylic resin are mixed in a certain ratio With the completion of the present invention, the protective film for polarizing plates has improved breakage (ie, workability) and has a low in-plane retardation (R _o ) and a low thickness direction retardation (R _th ), so that no color spots appear. Prior Technical Document Patent Document (Patent Document 1) Japanese Patent No. 4962661

Technical Problem According to this, a specific example aims to provide a protective film for polarizing plates that improves breakage (ie, workability) without reducing hardness and has low in-plane retardation (R _o ) and low thickness The directional delay (R _th ) means that no color spots appear; the polarizing plate containing it; and the display device containing it.

Solution to Problem According to a specific example, there is provided a protective film for polarizing plates, which contains a fluorine-based resin and an acrylic resin and has an in-plane retardation (R _o ) of 10 nm or less and 50 nm or less The thickness direction delay (R _th ).

Furthermore, according to a specific example, there is provided a method for preparing a protective film for polarizing plates, which comprises (1) melt-extrusion of a mixed resin composition containing a fluorine-based resin and an acrylic resin to produce a An unstretched sheet; (2) stretch the unstretched sheet 1.5 to 4 times in the longitudinal direction and 1.5 to 4 times in the transverse direction to produce a stretched film; and (3) heat-set the stretched film.

Furthermore, according to an embodiment, there is provided a polarizing plate including a polarizer layer; and a protective film for the polarizing plate adjacent to at least one of an upper side and a lower side of the polarizer layer.

Furthermore, according to an embodiment, there is provided a display device including a display panel; the polarizing plate disposed on at least one of an upper side and a lower side of the display panel.

Effect of the Invention The protective film for polarizing plates according to specific examples not only has improved elongation without reducing hardness, but also has improved processability (ie, breakability) when preparing sheets and films, and has low The in-plane retardation (R _o ) and the low thickness-direction retardation (R _th ) do not cause color spots. Due to its low thermal shrinkage, it also shows excellent reliability. Therefore, the protective film can be advantageously used for polarizing plates.

Detailed description for carrying out the invention Hereinafter, the invention will be explained in more detail.

A specific example provides a protective film for a polarizing plate, which includes a fluorine-based resin and an acrylic resin, and has an in-plane retardation (R _o ) of 10 nm or less and a thickness direction retardation (R of 50 nm or less) _th ).

Specifically, a specific example provides a protective film for a polarizing plate, which contains a fluorine-based resin and an acrylic resin. Since the protective film includes a mixed resin obtained by mixing the fluorine-based resin and the acrylic resin, it is possible to compensate for the breakage of the acrylic resin and the low hardness of the fluorine-based resin. The protective film made of the mixed resin may have an appropriate level of elongation and hardness.

In one example, the protective film may include the fluorine-based resin and the acrylic resin in a weight ratio of 1:1.5 to 1:19, especially a weight ratio of 1:4 to 1:9. Within this range, the elongation can be improved while maintaining the hardness at an appropriate level.

In another example, the protective film may include a weight ratio of 1:1.5 to 1:99, or a weight ratio of 1:9 to 1:99, especially a weight ratio of 1:10 to 1:30, especially weight The fluorine-based resin and the acrylic resin in a ratio of 1:9 to 1:20, especially in a weight ratio of 1:10 to 1:20, in particular in a weight ratio of 1:15 to 1:20. If the blending ratio of the fluorine-based resin to the acrylic resin falls within the above range, it is possible to prevent the deterioration of the workability due to the brittle nature of the acrylic resin, and it is suitable as a protective film for polarizing plates Delayed.

The fluorine-based resin may be a fluorine-containing polymer.

Examples of the fluorine-based resin may include ethylene-tetrafluoroethylene (ETFE) copolymer, polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene (FEP) copolymer, tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride Vinyl difluoride (THV) copolymer, polychlorotrifluoroethylene (PCTFE or PTFCE) copolymer, copolymers thereof, and mixtures thereof. Among them, the fluorine-based resin can be selected from the group consisting of: ethylene-tetrafluoroethylene (ETFE) copolymer, polyvinylidene fluoride (PVDF), and tetrafluoroethylene-hexafluoropropylene (FEP) Copolymer.

More specifically, the fluorine-based resin may be a homopolymerized polyvinylidene fluoride (PVDF) resin or a copolymerized polyvinylidene fluoride (PVDF) resin.

In the case where the fluorine-based resin is a copolymerized polyvinylidene fluoride (PVDF) resin, vinylidene fluoride (VF2) and a comonomer can be copolymerized in a weight ratio of 50:50 to 99:1.

The comonomer can be a fluorinated monomer. For example, the comonomer can be at least one selected from the group consisting of vinyl fluoride; trifluoroethylene (VF3); chlorotrifluoroethylene (CTFE); 1,2-difluoroethylene; tetrafluoro Ethylene (TFE); hexafluoropropylene (HFP); perfluoro (alkyl vinyl) ethers, such as perfluoro (methyl vinyl) ether (PMVE), perfluoro (ethyl vinyl) ether (PEVE), and Perfluoro(propyl vinyl) ether (PPVE); perfluoro (1,3-dioxolene); and perfluoro (2,2-dimethyl-1,3-dioxolene ) (PDD). Preferred among these are chlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP), trifluoroethylene (VF3), and tetrafluoroethylene (TFE).

By measuring with a capillary flowmeter at a shear rate of 100 s ^-1 and 230°C, the fluorine-based resin may have a viscosity of 100 to 2,500 Pa·s, or 500 to 2,000 Pa·s to be suitable for extrusion And injection molding.

The fluorine-based resin may have a weight average molecular weight of 50,000 to 250,000 g/mole, or 100,000 to 200,000 g/mole.

Furthermore, the fluorine-based resin may have a melt index (MI) of 5 to 20 g/10 min or 10 to 15 g/min at 230°C.

Based on the total weight of the mixed resin of the fluorine-based resin and the acrylic resin, the content of the fluorine-based resin is 1 to 40% by weight, 1 to 20% by weight, 1 to 10% by weight, 2 to 8% by weight, Or 3 to 7% by weight.

The acrylic resin may be a homopolymer or copolymer of alkyl (meth)acrylate having an alkyl group having 1 to 20 carbon atoms.

For example, the acrylic resin may be a homopolymer or copolymer selected from the group consisting of the polymerization of an acrylate monomer: methyl (meth)acrylate, ethyl (meth)acrylate, ( N-propyl meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, tertiary butyl (meth)acrylate, secondary butyl (meth)acrylate, (meth) Amyl acrylate, and combinations thereof.

For example, the acrylic resin may be a copolymer of the acrylate monomer and a comonomer. Here, the comonomer may include one selected from the group consisting of acrylonitrile, butadiene, styrene, isoprene, and combinations thereof.

In one example, the acrylic resin may include a homopolymer of methyl (meth)acrylate or a copolymer of methyl (meth)acrylate and a comonomer.

In the case where the acrylic resin is a copolymer resin of methyl (meth)acrylate and a comonomer, the acrylic resin may be methyl (meth)acrylate and the comonomer at 50:50 to 99 :1 weight ratio of copolymerized resin. More specifically, based on the weight of all monomers constituting the acrylic resin, the methyl acrylate and/or ethyl acrylate comonomer may be 1 to 20% by weight, especially 5 to 15% by weight. Copolymerize.

The acrylic resin may be a substituted or unsubstituted homopolymer. In the case of substitution, the substituent is not particularly limited.

In one example, the acrylic resin may be substituted or unsubstituted poly(methyl methacrylate) (PMMA).

The acrylic resin may have a melt index (MI) of 1 to 20 g/10 min, 1 to 10 g/10 min, or 1 to 5 g/10 min at 230°C.

The acrylic resin may have a softening point of 100°C or higher. Specifically, the acrylic resin may have a softening point of 100°C to 120°C, 105°C to 115°C, or 110°C to 120°C.

Based on the total weight of the mixed resin of the fluorine-based resin and the acrylic resin, the content of the acrylic resin is 60 to 99% by weight, 80 to 99% by weight, 90 to 99% by weight, 92 to 98% by weight, or 93 to 97% by weight.

The thickness of the protective film may be 100 μm or less. Specifically, the thickness of the protective film may be 10 μm to 100 μm, 20 μm to 80 μm, or 20 μm to 60 μm.

The protective film may have an in-plane retardation (R _o ) of 10 nm or less and a thickness direction retardation (R _th ) of 50 nm or less. Furthermore, the thickness direction retardation (R _th ) and the in-plane retardation (R _o ) of the protective film may be 0 or more, respectively; and R _th /R _o may be 1 to 50.

Specifically, the protective film may have an in-plane retardation (R _o ) of 0.1 nm to 10 nm, a thickness direction retardation (R _th ) of 1 nm to 40 nm, and an R _th /R _{o of} 1 to 40.

More specifically, the protective film may have an in-plane retardation (R _o ) of 0.5 nm to 5 nm, or 0.5 nm to 4 nm, and 1 nm to 30 nm, 1 nm to 10 nm, or 1 nm to 5 nm The thickness direction delay (R _th ).

The in-plane retardation (R _o ) is the product of the refractive index anisotropy (ΔN _xy = | N _x -N _y |) and the film thickness d (nm) of two mutually perpendicular axes on the film (ΔN _xy × d) The defined parameter, which is a measure of the degree of optical isotropy and anisotropy. The in-plane retardation (R _o ) is the in-plane retardation value at a wavelength of 550 nm and is represented by the following formula 1: [Formula 1] R _o = (n _x -n _y )× d

In this formula, n _x is the axial refractive index in the film plane, n _y is the axial refractive index orthogonal to the x axis of the film plane, and d is the thickness (nm) of the film.

If the in-plane retardation and the thickness direction retardation fall within the above range, the protective film will not generate color spots.

Under the condition of a light incident angle of -50° to 50°, the protective film may have an in-plane retardation (R _o ) of 3 nm or less. Specifically, the protective film may have an in-plane retardation (R _o ) of 0 nm to 3 nm under a light incident angle of -50° to 50°.

Furthermore, under the condition of a light incident angle of -50° to 50°, the difference between the maximum value and the minimum value of the in-plane retardation (R _o ) of the protective film may be 1.5 or less. Specifically, under the condition that the light incident angle is -50° to 50°, the protective film may have an in-plane retardation (R _o ) where the difference between the maximum value and the minimum value is 1.1 or less.

The thickness direction retardation represents the average of two birefringences Δ N _xz (= | N _x -N _z |) and ΔN _yz (= | N _y -N _z |) The value obtained by multiplying the film thickness d by the delay average parameter. The thickness direction retardation (R _th ) is the thickness direction retardation value at a wavelength of 550 nm and is represented by the following formula 2: [Formula 2] R _th = {(n _x + n _y )/2-n _z } × d

In this formula, n _x is the refractive index in the axial direction (x direction) of the film plane, n _y is the axial refractive index perpendicular to the x axis of the film plane, and n _z is the refractive index in the thickness direction of the film, d is the thickness (nm) of the film.

The protective film may have a degree of planar orientation (R _th /thickness) of 0 to 0.005. Specifically, the protective film may have a degree of planar orientation (R _th /thickness) of 0 to 0.001.

The degree of plane orientation (R _th /thickness) is the difference between the refractive index in the thickness direction (n _z ) at a wavelength of 550 nm and the average value of the plane refractive index ((n _x + n _y )/2).

The protective film having a size of 5 cm × 5 cm × 42 µm (width × length × thickness) may have a haze of 0.1 to 1%, or 0.1 to 0.8% after heat treatment at 150°C for 3 hours.

Moreover, the protective film may have an elongation of 28% to 61% and a pencil hardness of HB to 2H. Within the above range, breakage does not occur when the sheet or film is prepared, and the film can have suitable elongation and hardness values.

In addition, the protective film may have a longitudinal shrinkage rate of 1% or less and a transverse shrinkage rate of 1% or less after treatment at 85°C for 24 hours. Specifically, the respective shrinkage ratios in the longitudinal direction and the transverse direction after treatment at 85°C for 24 hours may be 0.5% or less, 0.01 to 1%, 0.01 to 0.9%, or 0.01 to 0.5%. If the shrinkage rate after the heat treatment falls within the above range, it is possible to prevent light leakage or the like by preventing battery warpage that occurs when the battery is manufactured after the polarizing plate has been prepared.

Another specific example provides a method of preparing a protective film for polarizing plates.

Specifically, the method for preparing a protective film for polarizing plates may include (1) melt-extrusion of a mixed resin composition containing a fluorine-based resin and an acrylic resin to produce an unstretched sheet; (2) The unstretched sheet is stretched 1.5 to 4 times in the longitudinal direction and 1.5 to 4 times in the transverse direction to produce a stretched film; and (3) heat-setting the stretched film.

The details of the fluorine-based resin, acrylic resin, and mixed resin thereof are as described above.

Specifically, in the above step (1), the mixed resin in which the fluorine-based resin and the acrylic resin are blended can be melt-extruded by an extruder and then cooled to manufacture an unstretched sheet. In such an event, depending on the type of extruder, the acrylic resin may be subjected to a pretreatment process, for example, a drying process at 80°C to 150°C for 4 to 10 hours. However, in the case of using a vented extruder or a twin screw extruder, the melt extrusion can be performed without such pretreatment.

The mixed resin composition has amorphous (ie, amorphous) characteristics, and the melt extrusion can be performed at about 200°C to about 240°C. If the temperature of the melt extrusion falls within the above range, the mixed resin can be easily melted, and the viscosity of the mixed resin composition melt-extruded accordingly can be appropriately maintained.

Moreover, in the above step (2), the unstretched sheet can be stretched in the transverse direction by 1.5 to 4 times and can be stretched, for example, by 3 to 4 times. Furthermore, the unstretched sheet can be stretched 1.5 to 4 times in the longitudinal direction and can be stretched, for example, 2 to 3 times.

Stretching can be performed so that the ratio of transverse stretching to longitudinal stretching is 1.2:1 to 1.5:1. Specifically, stretching can be performed so that the ratio of transverse stretching to longitudinal stretching is 1.2:1 to 1.45:1.

In one example, the stretching may include (2-1) preheating the unstretched sheet at 100°C to 110°C while the sheet moves at a speed of 5 m/min to 10 m/min; (2 -2) At 100°C to 120°C, stretch the preheated unstretched sheet in the longitudinal direction while the sheet moves at a speed of 10 m/min to 20 m/min; and (2-3) at 120 At 140°C, the longitudinally stretched sheet was stretched in the transverse direction while the sheet was moving at a speed of 10 m/min to 20 m/min.

More specifically, the stretching may include (2-1) preheating the unstretched sheet at 100°C to 110°C while the sheet moves at a speed of 5 m/min to 10 m/min; (2 -2) At 100°C to 120°C, stretch the preheated unstretched sheet in the longitudinal direction by 2.3 to 3.0 times, and heat treat the sheet at 600°C to 650°C with a far-infrared heater (R/H), while the The sheet system moves at a speed of 10 m/min to 20 m/min; and (2-3) at 120 to 140°C, the longitudinally stretched sheet is stretched 3.0 to 3.8 times in the transverse direction while the sheet system is moved at 10 m /min to 20 m/min.

Furthermore, in the above step (3), the heat setting may be performed at 150°C to 250°C, for example, 150°C to 170°C, and may be performed for about 1 to 2 minutes. More specifically, the heat fixing may be performed at 150°C to 165°C for 1 to 2 minutes. Once heat setting begins, the film relaxes in the machine direction and/or the transverse direction. If heat setting is performed under the above conditions, the retardation property of the protective film within a desired range can be advantageously achieved.

The thickness of the heat-set film is 100 μm or less and may be, for example, 20 μm to 80 μm, or 20 μm to 60 μm.

According to the above-mentioned production method, a protective film having an appropriate thickness, low in-plane retardation, and low thickness-direction retardation can be prepared so that color spots do not appear and the elongation is improved.

In addition, the protective film of the present invention may contain various additives, such as common static charge agents, antistatic agents, anti-sticking agents, and other inorganic lubricants, as long as the effects of this specific example are not impaired.

Yet another specific example provides a polarizing plate including a polarizer layer; and a protective film for polarizing plate adjacent to at least one of upper and lower sides of the polarizer layer. The polarizer can have improved optical, mechanical and thermal properties.

FIG. 1 schematically shows a cross section of a polarizing plate according to an embodiment of the present invention. 1, the polarizing plate (100) has a polarizer layer (10) and a protective film (20) in a structure, wherein the protective film (20) is disposed on the upper side of the polarizer layer (10) and Underside.

The polarizer layer (10) may be, for example, a layer of polyvinyl alcohol (PVA) that has been dyed with iodine or the like. In the event of such events, the polyvinyl alcohol molecules contained in the polyvinyl alcohol layer can be aligned in one direction.

Yet another specific example provides a display device including a display panel; the polarizing plate disposed on at least one of the upper side and the lower side of the display panel.

FIG. 2 schematically shows a cross section of a display device according to an embodiment of the present invention. The display device (A) has a display panel (200) and a polarizing plate (100) in a structure, wherein the polarizing plate (100) is disposed on the upper and lower sides of the display panel (200). In this case, the polarizing plate (100) has a polarizer layer (10) and a protective film (20) in a structure, wherein the protective film (20) is disposed on the upper side of the polarizer layer (10) And on the lower side, as described above.

Here, the display device (A) may be a liquid crystal display device or an organic electroluminescence display device. The display device is not limited to the above-mentioned structure and various modifications can be made as necessary.

Referring to FIG. 2, in the case where the above protective film (20) is applied to the display device (A), the protective film (20) may be applied to both of the upper and lower sides arranged on the display panel (200) Polarizers (100). The protective film (20) including these members and prepared by the above-described preparation method does not generate stains even when a plurality of protective films are stacked. As a result, even when a plurality of protective films are applied to a display device, it is possible to achieve excellent optical characteristics.

Hereinafter, the present invention will be explained in detail by examples. The following examples are intended to further illustrate the invention without limiting its scope. Examples and Comparative Examples

The raw materials used in the examples and comparative examples are as follows. -PMMA I: melt index (MI) at 230°C is 12 g/10 min, refractive index (n) is 1.49, LGMMA, IF850 -PMMA II: unsubstituted PMMA, melt index (MI) at 230°C ) Is 2.3 g/10 min, softening point is 109 °C, LGMMA, IH830 -PVDF I: melt index (MI) at 230 °C is 5-11 g/10 min, refractive index (n) is 1.425, Solvay, MP9009 -PVDF II: vinylidene fluoride, 100 mol%, melt index (MI) at 230°C is 5.9 g/10 min, glass transition temperature is -32°C, weight average molecular weight (Mw) is 224,000 g /Mohr, Solvay, solef1008 Example 1

Poly(methyl methacrylate) (PMMA I) and polyvinylidene fluoride (PVDF I) were prepared, and PMMA I was dried at 90°C for 6 hours. 5% by weight of PVDF I was added to 95% by weight of dry PMMA I, and these were thoroughly mixed in a belt mixer to obtain a mixed resin of PMMA I and PVDF I.

The mixed resin was melt extruded through an extruder at about 230°C, and then cooled on a casting roll at about 25°C to prepare an unstretched sheet. At a stretching temperature of 110°C, the unstretched sheet was sequentially stretched biaxially 2.2 times in the longitudinal direction and biaxially stretched 2.6 times in the transverse direction. The stretched sheet was heat set at about 210°C for about 20 seconds to prepare a protective film having a thickness of 30 μm. Examples 2 to 5

The protective film was prepared in the same manner as in Example 1, except that the weight ratio of PMMA I: PVDF I and/or the thickness of the final protective film were changed as shown in Table 1 below. Example 6

Poly(methyl methacrylate) (PMMA II) and polyvinylidene fluoride (PVDF II) were prepared, and PMMA II was dried at 90°C for 6 hours. 5% by weight of PVDF II was added to 95% by weight of dry PMMA II, and these were thoroughly mixed in a belt mixer to obtain a mixed resin of PMMA II and PVDF II. The mixed resin was melt extruded through an extruder at about 230°C, and then cooled on a casting roll at about 25°C to prepare an unstretched sheet. The unstretched sheet was preheated to 60°C while moving at a speed of 5 m/min, and stretched 2.5 times (ie, 250%) in the longitudinal direction at 110°C, using a radiant heater (R/H) at The upper part was heat-treated at 650°C, and the lower part was heat-treated at 600°C, while it was moving at a speed of 13 m/min. Subsequently, at 127°C, it was stretched in the transverse direction by 3.0 times (ie, 300%) while moving at a speed of 13 m/min. At about 150° C., the stretched sheet was heat set for about 80 seconds to prepare a protective film for polarizing plates having a thickness of 42 μm. Examples 7 to 13

In addition to changing the mixing ratio of PMMA II to PVDF II, stretching ratio in longitudinal and transverse directions, stretching temperature, stretching speed, heat setting temperature and/or thickness as shown in Tables 2 and 3, the protective film system Prepared in the same manner as Example 6. Comparative Examples 1 to 3

In addition to the use of acryl (IF850, LG Chemical), triethyl cellulose (TAC) (FujiTAC, Fuji), or polyethylene terephthalate (PET) (PET, SKC), the protective film system is implemented Example 1 was prepared in the same manner. Comparative Examples 4 to 8

In addition to changing the mixing ratio of PMMA II to PVDF II, stretching ratio in longitudinal and transverse directions, stretching temperature, stretching speed, heat setting temperature and/or thickness as shown in Tables 2 and 3, the protective film system Prepared in the same manner as Example 6. Test Example (1) Measurement of glass transition temperature (Tg)

The glass transition temperature (°C) of the mixed resin of PMMA and PVDF used in Examples 1 to 5 and the mixed resin of resins (acrylic acid, TAC, PET) used in Comparative Examples 1 to 3 is by differential scanning calorimetry (DSC-Q100, TA Instrument) was measured at a heating rate of 10°C/min. The results are shown in Table 1 below. (2) Measurement of elongation

The longitudinal elongation of the protective films prepared in Examples 1 to 5 and the longitudinal elongation of the protective films prepared in Comparative Examples 1 to 3 were measured with a UTM measuring instrument (5566A, Instron). The results are shown in Table 1 below. (3) Delay measurement

(i) The in-plane retardation (R _o ) and thickness direction retardation (R _th ) of the protective films prepared in Examples 1 to 5 and the protective films prepared in Comparative Examples 1 to 3 were measured using a phase difference measuring instrument (RETS- 100, OTSUKA Electronics) measurement. The results are shown in Table 1 below.

(ii) The protective films prepared in Examples 6 to 13 and the protective films prepared in Comparative Examples 4 to 8 were each cut into a rectangular shape with a size of 4 cm × 2 cm in the lateral direction of the film, regardless of the orientation of the film The axis direction, which is used as a measurement sample. The refractive index (N _x and N _y ) and the refractive index in the thickness direction (N _z ) of the sample were measured with Abbe refractometer (NAR-4T, obtained from Atago Co., Ltd.; the measurement wavelength was 589 nm ). The absolute value of the difference in refractive index between two orthogonal directions (|N _x -N _y |) is called the anisotropy of refractive index (ΔN _xy ). The thickness d (nm) of the film was measured with an electronic micrometer (Millitron 1245D, available from Feinpruf GmbH), which was converted into nm units. The in-plane retardation (R _o ) is measured as the product of the refractive index anisotropy (ΔN _xy ) and the film thickness d (nm) (ΔN _xy × d). The results are shown in Table 4 below.

(iii) The refractive indexes N _x , N _y and N _z of the films prepared in Examples 6 to 13 and the films prepared in Comparative Examples 4 to 8 were measured in the same manner as in paragraph (ii) above to measure in-plane retardation Thickness d (nm). Calculate the average of (△N _xz × d) and (△N _yz × d) to determine the thickness direction retardation (R _th ). The results are shown in Table 4 below. (4) Measurement of total transmittance, haze and total light transmittance

(i) The total transmittance and haze of the protective films prepared in Examples 1 to 5 and the protective films prepared in Comparative Examples 1 to 3 are using a permeability measuring device (NDH-5000W, NIPPON Denkoku Kogyo ))measuring. The results are shown in Table 1 below.

(ii) The protective films prepared in Examples 6 to 13 and the protective films prepared in Comparative Examples 4 to 8 were each cut into a size of 10 cm×10 cm, and heat-treated in an oven at 150° C. for 3 hours. The haze after heat treatment was measured using a haze measuring instrument (NDH-5000W, Nippon Electric Industries). The results are shown in Table 5 below.

(iii) The total light transmittance of the protective films prepared in Examples 6 to 13 and the protective films prepared in Comparative Examples 4 to 9 was measured using the measurement device of paragraph (i) above. The results are shown in Table 5 below. (5) Hardness measurement

The hardness of the protective films prepared in Examples 1 to 13 and the protective films prepared in Comparative Examples 1 to 8 was measured using a pencil hardness tester (Kyushu Cis Co., Ltd.). The results are shown in Tables 1 and 4 below. (6) In-plane retardation changes with light incident angle

The in-plane retardation of the protective film prepared in Example 6 was measured in the same manner as in paragraph (3) (ii) above except that the incident angle of the irradiated light was adjusted. As a control group, TAC film (manufacturer: Fuji, product name: n-tac, thickness: 40 μm) and COP film (manufacturer: Zeon, product name: zeoner, thickness: 25 μm) were used. The results are shown in Figure 3. (7) Color spots

The protective film of Example 6 and the protective film of Comparative Example 4 (manufacturer: Samsung SDI) were laminated on both sides of the polarizing plate, and the color was evaluated. Thereafter, two protective films were inserted between the polarizing plates, and then the color was evaluated. The measurement results are shown in Figures 4 and 5.

In FIG. 4, (A) shows the result when laminating one film of Comparative Example 4, and (B) shows the result when laminating two films of Example 6.

In FIG. 5, (A) shows the results when laminating two films in Example 6, and (B) shows the results when laminating two films in Comparative Example 4. (8) Shrink

The protective films prepared in Examples 6 to 13 and the protective films prepared in Comparative Examples 4 to 8 were subjected to heat treatment in an oven at 85° C. for 24 hours, and the shrinkage in the longitudinal direction (MD; machine direction) was calculated according to the following formula 3 ( %) and transverse direction (TD; tenter direction) shrinkage (%). The results are shown in Table 5 below. [Formula 3] Shrinkage (%) = (initial length-length after heat treatment)/initial length × 100 [Table 1] [Table 2] [table 3] [Table 4] [table 5]

The results given in Table 1 show that the protective films prepared in Examples 1 to 5 have an MD elongation of greater than 25% and a hardness of not less than HB, due to the protection made by the acrylic resin of Comparative Example 1 The membranes are flexible (unbreakable), so their breakability (ie, workability) is improved. They also have an in-plane retardation (R _o ) of about 5 nm or less and a thickness-direction retardation (R _th ) of about 26 nm, which does not cause color spots. In addition, in terms of total transmittance and haze, the protective film prepared in the example is equivalent to or better than the protective film in Comparative Example 2.

On the contrary, the protective films prepared in Comparative Examples 1 to 3, especially the protective film of Comparative Example 3, have high in-plane retardation and high thickness direction retardation, which in turn leads to color spots.

As shown in Tables 4 and 5, the protective films prepared in Examples 6 to 13 have low in-plane retardation (R _o ) and low thickness direction retardation (R _th ), and N _z (R _th /R greater than 0) _o ), low haze, shrinkage in the machine direction (MD) and shrinkage in the transverse direction (TD) are 1% or less, thereby ensuring a high degree of reliability.

On the contrary, the protective films prepared in Comparative Examples 4 to 8 have high in-plane retardation, high thickness-direction retardation, or high shrinkage after heat treatment, which in turn leads to color spots or low reliability.

10‧‧‧ Polarizer layer

20‧‧‧ Polarizer protective film

100‧‧‧ Polarizer

200‧‧‧Display panel

A‧‧‧Display device

FIG. 1 schematically shows a cross section of a polarizing plate according to an embodiment of the present invention. FIG. 2 schematically shows a cross section of a display device according to an embodiment of the present invention. FIG. 3 is a graph showing the in-plane retardation change of the protective film for polarizing plates with respect to the change of the light incident angle according to an embodiment of the present invention. 4 and 5 are photographs showing the results of evaluating the color spots of the protective film for polarizing plates according to an embodiment of the present invention.

Claims (20)

A protective film for polarizing plates, which contains a fluorine-based resin and an acrylic resin and has an in-plane retardation (R _o ) of 10 nm or less and a thickness direction retardation (R _th ) of 50 nm or less; and the protective film After treatment at 85°C for 24 hours, it has a longitudinal shrinkage of 1% or less and a transverse shrinkage of 1% or less. The protective film for polarizing plates according to claim 1, which contains the fluorine-based resin and the acrylic resin in a weight ratio of 1:1.5 to 1:99. The protective film for polarizing plates according to claim 2, which contains the fluorine-based resin and the acrylic resin in a weight ratio of 1:9 to 1:99. The protective film for polarizing plates according to claim 2, which contains the fluorine-based resin and the acrylic resin in a weight ratio of 1:10 to 1:30. The protective film for polarizing plates according to claim 1, wherein the acrylic resin is a substituted or unsubstituted homopolymer. The protective film for polarizing plates according to claim 1, wherein the acrylic resin has a softening point of 100°C or higher. The protective film for polarizing plates according to claim 1, wherein the acrylic resin has a melt index (MI) of 1 to 20 g/10 min at 230°C. The protective film for polarizing plates according to claim 1, wherein the acrylic resin is substituted or unsubstituted poly(methyl methacrylate) (PMMA). The protective film for polarizing plates according to claim 1, wherein the fluorine-based resin is a homopolymerized polyvinylidene fluoride (PVDF) resin or a copolymerized polyvinylidene fluoride (PVDF) resin. The protective film for polarizing plates according to claim 1, wherein the fluorine-based resin has a weight average molecular weight of 50,000 to 250,000 g/mole. The protective film for polarizing plates according to claim 1, wherein the fluorine-based resin has a melt index (MI) of 5 to 20 g/10 min at 230°C. The protective film for polarizing plates according to claim 1, which has an in-plane retardation (R _o ) of 0.1 nm to 10 nm, a thickness direction retardation (R _th ) of 1 nm to 40 nm, and an R _th /R _{o of} 1 to 40. The protective film for polarizing plates according to claim 1 has an elongation of 28% to 61%. The protective film for polarizing plates according to claim 1, which has a pencil hardness of HB to 2H. The protective film for polarizing plates according to claim 1, which has an in-plane retardation (R _o ) of 3 nm or less under the condition of a light incident angle of -50° to 50°. The protective film for a polarizing plate according to claim 1, wherein the difference between the maximum value and the minimum value of the in-plane retardation (R _o ) may be 1.5 or more under the condition of a light incident angle of -50° to 50° small. The protective film for polarizing plates according to claim 1, which has a degree of planar orientation (R _th /thickness) of 0 to 0.005. A method for preparing a protective film for polarizing plates, comprising: (1) melt-extrusion of a mixed resin composition containing a fluorine-based resin and an acrylic resin to produce an unstretched sheet; (2) The unstretched sheet is stretched 1.5 to 4 times in the longitudinal direction and 1.5 to 4 times in the transverse direction to produce a stretched film; and (3) The stretched film is heat-set and the protective film is treated at 85°C for 24 hours , With a longitudinal shrinkage of 1% or less and a transverse shrinkage of 1% or less. A polarizing plate comprising: a polarizer layer; and a protective film for the polarizing plate according to claim 1 adjacent to at least one of an upper side and a lower side of the polarizer layer. A display device includes: a display panel; and a polarizing plate according to claim 19 disposed on at least one of an upper side and a lower side of the display panel.