KR101838498B1 - Polarizer protective film with excellent thickness uniformity - Google Patents

Abstract

The present invention relates to a polarizing plate protection film, and a resin composition for manufacturing the film of the present invention comprises: unit 70-95 parts by weight of methyl methacrylate; unit 1-15 parts by weight of maleimide; and unit 1-15 parts by weight of methyl acrylate with respect to 100 parts by weight of the overall resin composition, and a molecular weight is 700,000 to 1,200,000 g/mol. The protection film of the present invention has an improved thickness uniformity, and, under an environment of 23 °C and 55 %RH, Ro, a phase difference value in a film surface, is in a range of 20 nm or less and Rth, a phase difference value in a thickness direction of the film, is 30 nm or less. Under an environment of 40 °C and 90 %RH, a moisture permeability is 150 g/m^2*24h or less after leaving for 24 hours. Therefore, a phase difference value and moisture resistance are excellent.

Description

[0001] The present invention relates to a polarizer protective film having excellent thickness uniformity,

The present invention relates to a polarizing plate protective film, and more particularly to an acrylic protective film having excellent thickness uniformity.

2. Description of the Related Art In recent years, a liquid crystal display device that consumes a small amount of power and operates at a low voltage and is lightweight and thin has been widely used in information display devices such as a mobile phone, a monitor for a portable information terminal computer, and a television. Such an information display device is required to have reliability under a severe environment depending on its use. For example, in a car navigation system liquid crystal display device, the temperature and humidity inside the vehicle in which it is placed may become very high, and the temperature and humidity conditions required are more stringent than those of ordinary monitors for a television or a personal computer. In a liquid crystal display device, a polarizing plate is used to enable the display. In a liquid crystal display device requiring such a strict temperature and / or humidity condition, a polarizing plate constituting it is required to have high durability.

The polarizing plate usually has a structure in which a transparent protective film is laminated on both sides or one side of a polarizing film made of a polyvinyl alcohol resin in which a dichroic dye is adsorbed and oriented. Conventionally, triacetyl cellulose (TAC) is widely used for this protective film, and this protective film is bonded to the polarizing film through an adhesive composed of an aqueous solution of a polyvinyl alcohol resin. However, the polarizing plate in which the protective film made of triacetylcellulose is laminated has a high moisture permeability of triacetyl cellulose, so that when the polarizing plate is used for a long time under a high humid environment, the polarization performance deteriorates or the protective film and the polarizing film peel off have. In order to solve these problems, attempts have been made to use a (meth) acrylic resin film having a lower moisture permeability as a protective film of a polarizing plate than a triacetylcellulose film.

In recent years, there has been a demand for a thin film in accordance with the increasingly slimmer thickness of the LCD. However, since the conventional acrylic protective film has a reduced thickness smoothness, it has been difficult to thin the film.

Japanese Patent Application Laid-Open No. 2009-078940 Japanese Unexamined Patent Application Publication No. 2006-061910 Korean Patent Publication No. 2015-0093111

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and it is an object of the present invention to provide a polarizing plate protective film which is thin and has excellent thickness uniformity.

In order to achieve the above object, the present invention provides a polarizing plate protective film produced using the following acrylic resin composition and acrylic resin composition.

The acrylic resin composition of the present invention is a resin composition comprising a methyl methacrylate unit, a maleimide unit and a methyl acrylate unit, wherein 100 parts by weight of the total resin composition is mixed with methyl methacrylate Wherein the acrylic resin composition comprises 70 to 95 parts by weight of a Methyl Methacrylate unit, 1 to 15 parts by weight of a maleimide unit and 1 to 15 parts by weight of a Methyl Acrylate unit, To 1,200,000 g / mol.

The polarizing plate protective film of the present invention is obtained by forming a film of a dope containing the acrylic resin composition by a solvent casting method (also referred to as a "softener film method"). The thickness uniformity of the film is 1,000 or less, And a thickness of 10 to 60 탆.

Further, the polarizing plate protective film of the present invention is characterized in that, under an environment of a temperature of 23 ° C and a humidity of 55% RH, the retardation value Ro in the film represented by the following formula (i) is in the range of 20 nm or less, And the water vapor permeability after allowing to stand for 24 hours in an environment of a temperature of 40 ° C and 90% RH is 150 g / m 2 * 24 h or less.

≪ EMI ID =

Ro = (nx-ny) xd

≪ EMI ID =

Rth = {(nx + ny) / 2-nz} xd

(Where nx is refractive index in the slow axis direction in the film plane, ny is refractive index in the direction perpendicular to the slow axis, nz is refractive index in the film thickness direction, and d is the film thickness (nm) .

The polarizing plate protective film of the present invention is thin and has an excellent thickness uniformity.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a photograph of a moisture-permeable cup used for measuring the moisture resistance of each protective film produced according to Examples and Comparative Examples of the present invention. FIG.

Hereinafter, the best mode for carrying out the present invention will be described in detail, but the present invention is not limited to these.

The present inventors have conducted extensive research to develop an acrylic protective film having excellent thickness uniformity of a thin film. As a result, it has been found that a resin composition containing a specific component in an average molecular weight of 700,000 to 1,200,000 g / mol of a resin composition, It has been found that a protective film having a low hygroscopicity, an excellent optical performance and a remarkably improved thickness uniformity even when being a thin film can be obtained.

The protective film of the present invention is a polarizing plate protective film formed of an acrylic resin composition comprising a methyl methacrylate unit, a maleimide unit and a methyl acrylate unit, And a film thickness of 10 to 60 탆. If the thickness uniformity exceeds 1,000 over the entire width, it is not suitable for use as a protective film of a polarizing plate in terms of thickness uniformity. When the thickness of the protective film is less than 10 mu m, sufficient retardation characteristics are not exhibited. When the thickness of the protective film exceeds 60 mu m, it is not suitable for use in a thin polarizer.

At this time, it is preferable that the acrylic resin composition of the present invention is a copolymer resin in which each of the monomer units is contained in the form of a repeating unit, and each of the unit components is contained in the form of a repeating unit.

In consideration of optical characteristics, transparency, compatibility, processability and productivity, the content of methyl methacrylate units in the resin composition of the present invention is preferably about 70 to 95 parts by weight based on 100 parts by weight of the total resin composition . When the content of methyl methacrylate units is in the above range, excellent retardation and optical characteristics can be obtained. When the methylmethacrylate unit content is less than 70 parts by weight, the optical performance of the protective film deteriorates. When the methylmethacrylate unit content exceeds 95 parts by weight, the uniformity of the thickness of the protective film is lowered.

In the resin composition, the maleimide unit may have a role of imparting an appropriate retardation value and a compatibility between methyl methacrylate and methyl acrylate, and the content of the maleimide unit may vary depending on the total resin composition Is preferably about 1 to 15 parts by weight, relative to 100 parts by weight. When the content of the maleimide unit is within the above range, desired retardation characteristics can be obtained.

The content of the methyl acrylate unit is preferably about 1 to 15 parts by weight based on the whole resin composition. When the content of methyl acrylate units is in the above range, preferable thickness uniformity characteristics can be obtained.

The resin composition used in the polarizing plate protective film of the present invention has a molecular weight of 700,000 to 1,200,000 g / mol, in particular, from the viewpoint of improvement of optical performance and thickness uniformity. When the molecular weight of the resin composition is less than 700,000, the improvement of the uniformity of the film thickness becomes insufficient. When the molecular weight exceeds 1,200,000, the resin composition may not be easily formed into a film.

Hereinafter, a preferable film-forming method of the present invention will be described.

Containing an acrylic polymer resin Dof  Produce

To prepare the protective film of the present invention, first, a copolymer resin solution of a methyl methacrylate monomer, a maleimide monomer and a methyl acrylate monomer is prepared. There is no particular limitation on the method for producing the copolymer of these monomers, and the copolymer can be produced according to a method for producing a copolymer resin well known in the art such as suspension polymerization, emulsion polymerization, bulk polymerization or solution polymerization.

In the present invention, a polarizing plate protective film is produced by a solvent casting method. In the solvent casting method, a solution (dope) obtained by dissolving an acrylic copolymer in a casting solvent is cast on a support, and the solvent is evaporated to form a film. The preparation of the dope is carried out by mixing the acrylic copolymer resin solution and the auxiliary additive in the casting solvent.

At this time, the casting solvent is preferably selected from the group consisting of methylene chloride (MC), methanol, ethanol, and a mixture of two or more thereof. As the casting solvent, it is more preferable to use a mixed solvent of methylene chloride and methanol. In this case, a mixed solvent of methylene chloride and methanol mixed at a weight ratio within a range of 80:20 to 95: 5 is preferable.

In the production of the acrylic protective film, auxiliary additives may further be used. To the acrylic copolymer solution (dope), auxiliary additives such as ultraviolet ray inhibitor, fine particles, infrared absorbing agent and exfoliating agent may be added in various manufacturing processes depending on the application. The specific kind of such additives can be used without limitation as long as they are commonly used in the field, and the content thereof is preferably used within a range that does not deteriorate the physical properties of the film. The timing of adding the additives depends on the type of additive. A step of adding an additive to the end of the doping treatment may be performed.

Film forming process

In the solvent casting method of the present invention, a polymer solution (dope) is poured on a metal support from a nozzle of a pressurizing die and allowed to stand for a predetermined time to form a semi-dry film. Thereafter, the semi-dried film is peeled from the metal support, transferred to a drying system, and dried to remove the solvent. Then, a uniaxial stretching process or a biaxial stretching process is performed on the dried film. By this stretching process, it is possible to improve the film uniformity and the retardation value of the protective film.

Here, the support serves to evaporate the solvent present in the casting solution while casting the casting stock solution on the sheet extruded from the die to form an acrylic protective film. The support or the surface thereof is preferably made of a metal and has a mirror finished surface, and a steel belt such as a stainless steel belt is preferably used as the support. The surface temperature of the metal support is in the range of 30 to 35 ° C. The higher the temperature, the better the evaporation of the solvent present in the casting solution can be accelerated. However, if the metal support is too high, the casting solution may be foamed or the planarity may deteriorate And is preferably 0 to 75 캜, more preferably 5 to 45 캜. As the support, a metal support in the form of a planar conveyor belt may be used. The casting stock solution applied to the support moves along with the support for a sufficient time and distance to be formed into a film, and is then peeled off from the support. The peeling roller for peeling the film is a conventional guide roller for guiding the film on the support to the outside of the support and peeling off the support from the support.

After peeling, the web is dried using a tenter stretching device for clipping and conveying both ends of the web with a clip, or a drying device for alternately passing the web through a plurality of rolls arranged in the drying device for conveying. The drying means generally blows hot air on both sides of the web, but there is also a means of heating the micro web in place of the wind. Too rapid drying tends to impair the planarity of the finished film. The drying at a high temperature is preferably carried out at about 8 parts by mass or less of the residual solvent.

In the case of using the tenter stretching device, it is preferable to use a device capable of controlling the holding length of the film from the left and right by the left and right gripping means of the tenter.

The stretching operation may be carried out by dividing into a plurality of steps or biaxially stretching in the machine direction and the width direction. When biaxial stretching is carried out, simultaneous biaxial stretching may be carried out or may be performed stepwise.

In this case, the stepwise means that the stretching in different stretching directions can be performed sequentially, the stretching in the same direction can be divided into multiple steps, and the stretching in the other direction can be added to any one of them.

The simultaneous biaxial stretching may include stretching in one direction and relaxation of the other stretch by shrinking. The preferred stretching magnification of the simultaneous biaxial stretching can be in the range of 1.01 to 2.0 times in both the width direction and the longitudinal direction.

The amount of residual solvent in the web in the case of performing the tenter is preferably 20 to 100 parts by mass at the beginning of the tenter, and it is preferable to dry the web by hanging the tenter until the amount of residual solvent in the web becomes 10 parts by mass or less, Preferably 5 parts by mass or less.

The drying temperature in the case of performing the tenter is preferably 50 to 150 ° C, more preferably 90 to 140 ° C, and most preferably 110 to 130 ° C.

Thereafter, when the amount of the residual solvent in the web becomes 3 parts by mass or less, a step of winding by a winder as a protective film is carried out. At this time, when the residual solvent amount is 0.5 part by mass or less, a film excellent in dimensional stability can be obtained. The winding method is not particularly limited, and can be wound by any known method.

The polarizing plate protective film of the present invention produced according to the above method has an in-plane retardation value Ro in the range of 20 nm or less, expressed by the following equation (i), at a temperature of 23 ° C and a humidity of 55% The film thickness direction retardation value Rth expressed by the following formula (ii) satisfies 30 nm or less under an environment of? C and 55% RH, and has a good retardation value.

≪ EMI ID =

Ro = (nx-ny) xd

≪ EMI ID =

Rth = {(nx + ny) / 2-nz} xd

(Where nx is refractive index in the slow axis direction in the film plane, ny is refractive index in the direction perpendicular to the slow axis, nz is refractive index in the film thickness direction, and d is the film thickness (nm) .

Further, the polarizing plate protective film of the present invention is characterized by having excellent moisture resistance, with a moisture permeability of 150 g / m 2 * 24 h or less after standing for 24 hours in an environment of 40 ° C and 90% RH.

Hereinafter, the present invention will be described in more detail based on examples. However, the present invention is not limited by the following examples.

≪ Example 1 >

82 parts by weight of methyl methacrylate monomer, 8 parts by weight of maleimide monomer and 10 parts by weight of methyl acrylate monomer were mixed in a polymerization solvent (mixed solvent of methylene chloride and methanol 9: 1). 20.0 parts by weight of the acrylic polymer solution, 1.5 parts by weight of silica (SiO2), and 78.5 parts by weight of a mixed solvent obtained by mixing 9: 1 (by weight) of methylene chloride and methanol were mixed to prepare a diluted acrylic polymer solution (initiator solution) . 100 parts by mass of the initiator solution and 5 parts by mass of the fine particle addition liquid were added to prepare a dope. At this time, the composition of the dope is 64.17 parts by weight of methylene chloride, 7.13 parts by weight of methanol, 27.94 parts by weight of acrylic polymer, 0.72 part by weight of Tinvuin 928 (manufactured by BASF) and 0.04 part by weight of silica.

Thereafter, the dope liquid was sufficiently mixed with an inline mixer, and then uniformly plied to a stainless steel band support having a width of 600 mm by using a belt softener. The solvent was evaporated on the stainless steel band support and peeled off from the stainless band support. Then, both ends of the web were held with a tenter and stretched so that the stretching magnification in the TD direction was 1.5 times in a temperature environment of 130 ° C. After the stretching, the stretching was maintained for a few seconds, the tensile force in the width direction was relaxed, the stretching in the width direction was carried out, the stretching was carried out for 35 minutes in the drying section set at 90 ° C, An acrylic polymer film having a thickness of 30 m and a knurling width of 10 mm and a height of 8 m was formed at the end.

≪ Example 2 >

A film was prepared in the same manner as in Example 1 except that the film had a thickness of 60 mu m.

≪ Comparative Example 1 &

The composition of the dope liquid was 72.63 parts by weight of methylene chloride, 8.07 parts by weight of methanol, 16.99 parts by weight of TAC Flake, 1.93 parts by weight of a TPP plasticizer, 1.25 parts by weight of an ultraviolet absorber Tinvuin 928 (manufactured by BASF), 0.03 part by weight of silica, and a film thickness of 30 μm.

≪ Comparative Example 2 &

A film was prepared in the same manner as in Comparative Example 1 except that the film had a thickness of 60 mu m.

≪ Comparative Example 3 &

96 parts by weight of a methyl methacrylate monomer and 4 parts by weight of a methyl acrylate monomer were copolymerized to prepare an acrylic resin pellet. As the acrylic elastomer particles, the inner layer is composed of butyl acrylate as a main component and styrene and a small amount of aralkyl methacrylate are polymerized, and the outer layer is a hard polymer obtained by polymerizing methyl methacrylate with a small amount of ethyl acrylate As the elastic particles of the two-layer structure constituted, elastic particles having a number average particle diameter of 75 nm were prepared. Then, the acrylic resin pellets and the acrylic elastomer particles were mixed in a super mixer at a ratio of 7: 3 and melt-kneaded with an extruder to prepare acrylic resin composition pellets. The pellets of the acrylic resin composition were poured into a 65 mm single screw extruder, extruded through a T-shaped die having a set temperature of 275 ° C, and both sides of the film were sandwiched between two polishing rolls having a mirror surface temperature set at 45 ° C And cooled to obtain an acrylic melt extruded film having a thickness of 30 탆.

≪ Comparative Example 4 &

A film was prepared in the same manner as in Comparative Example 3 except that the film had a thickness of 60 mu m.

The thus obtained films of Examples 1 to 2 and Comparative Examples 1 to 4 were measured for the retardation value Ro at a wavelength of 590 nm under an environment of 23 ° C and 55% RH using an optical measuring instrument of AxoScan (OPMF-1, Axometrics) Rth was measured.

Also, the moisture permeability was measured using a moisture-permeable cup (Model: KP-M6680) shown in Fig.

Finally, the thickness uniformity in the width direction was measured using a thickness meter, and the thickness, the moisture permeability, and the retardation value of the film were shown in Table 1.

Resin Film thickness
(탆) Moisture permeability
(g / m 2 * 24 hr) Ro / Rth
(nm) Thickness uniformity
(Standard Deviation) Example 1 Acrylic 1 30 95 2/3 0.812 Example 2 Acrylic 1 60 50 3/5 0.552 Comparative Example 1 TAC 30 620 1/24 0.417 Comparative Example 2 TAC 60 450 3/52 0.334 Comparative Example 3 Acrylic 2 30 130 0/5 1.411 Comparative Example 4 Acrylic 2 60 65 2/7 0.915

As shown in Table 1, the polarizing plate protective film produced according to the present invention showed excellent characteristics in terms of retardation, moisture permeability and thickness uniformity even in a thin state.

On the other hand, the TAC protective film has good thickness uniformity and good phase difference, but it is vulnerable to moisture because of its high moisture permeability. When the melt extruded acrylic film is thinned, the thickness uniformity is reduced.

Claims (7)

Based resin composition comprising a methyl methacrylate unit, a maleimide unit, and a methyl acrylate unit,
The acrylic resin composition preferably comprises 70 to 95 parts by weight of a methyl methacrylate unit, 1 to 15 parts by weight of a maleimide unit, 1 to 15 parts by weight of a methyl acrylate ) Unit of 1 to 15 parts by weight.
The polarizing plate protective film according to claim 1, wherein the thickness uniformity of the protective film is 1,000 or less with respect to the entire width. The polarizing plate protective film according to claim 1, wherein the protective film has a thickness of 10 to 60 탆. The polarizing plate protective film according to claim 1, wherein the acrylic resin composition has a molecular weight of 700,000 to 1,200,000 g / mol. delete The film according to claim 1, wherein the protective film has an in-plane retardation value Ro in the range of 20 nm or less and expressed by the following formula (ii) under the environment of a temperature of 23 ° C and a humidity of 55% Wherein the retardation value Rth in the film thickness direction of the polarizing plate protective film is 30 nm or less.
≪ EMI ID =
Ro = (nx-ny) xd
≪ EMI ID =
Rth = {(nx + ny) / 2-nz} xd
(Where nx is refractive index in the slow axis direction in the film plane, ny is refractive index in the direction perpendicular to the slow axis, nz is refractive index in the film thickness direction, and d is the film thickness (nm) . The polarizing plate protective film according to claim 1, wherein the protective film has a moisture permeability of 150 g / m 2 * 24 h or less after standing for 24 hours in an environment of 40 ° C and 90% RH.

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