KR101790108B1 - Polarizing plate - Google Patents

Abstract

More particularly, the present invention relates to a polarizing plate, and more particularly to a polarizing plate in which a cellulose-based protective film and a low-moisture-permeable protective film having a moisture permeability (40 ° C., 90% RH, 24 hr) of 1 to 200 g / By limiting the tensile strength in the TD direction and the tensile strength in the MD direction of the cellulose-based protective film to a certain range, the curling of the polarizing plate can be controlled and the degradation of the polarization degree due to the increase in light leakage and transmittance can be prevented To a polarizing plate.

Description

POLARIZING PLATE

The present invention relates to a polarizing plate having an asymmetric structure in which one side capable of controlling curling is composed of a cellulose-based protective film.

The polarizing plate is widely used as a polarizing light supplying element and a polarizing light detecting element in a liquid crystal display.

Generally, a polarizing plate is formed by applying a transparent protective film such as triacetylcellulose (TAC) on both sides of a polarizer made of a polyvinyl alcohol (PVA) based resin in which a dichroic dye is adsorbed and oriented, And the film is laminated via an adhesive.

In recent years, polarizers for arranging different kinds of protective films on both sides of a polarizer have been increasing for optical properties. However, if different types of protective films are placed on both sides of the polarizer, the forces applied to both sides of the polarizer become asymmetric. As a result, there is a problem that the curl, which is a phenomenon in which the polarizing plate is bent, is more severe than before. Particularly, when a protective film having a different moisture permeability is disposed on both sides of the polarizer, curl is further exacerbated.

The polarizing plate is required to suppress curling because handling is required along with improvement of LCD function, improvement of luminance and enlargement of size. In addition, there is a problem that the polarizing plate on which the curl is formed contains bubbles when bonded to the liquid crystal cell.

In consideration of this point, Korean Patent Laid-Open No. 2007-96826 discloses that a cycloolefin-based resin film is laminated on one side of a polarizer with an aqueous adhesive, and then an acetic acid cellulose-based film is laminated on the other side, Is adjusted to 0.5% or more before a polarizing plate is produced. Although this polarizer can suppress curling, there is a problem that the process becomes complicated because the moisture content in a state where the polarizer and the cycloolefin-based resin film are laminated should be 0.5%. Therefore, it is required to develop a polarizer capable of suppressing curling while simplifying the process.

The present invention provides an asymmetric polarizer capable of controlling curling caused by an asymmetric structure in which a cellulose-based protective film is bonded to one side of a polarizer and a low moisture-permeable protective film is bonded to another side of a polarizer.

Another object of the present invention is to provide a polarizing plate capable of preventing light leakage due to curling and lowering of polarization due to an increase in transmittance.

The inventors of the present invention found that the curling of an asymmetric polarizing plate caused by the difference in the moisture permeability of two protective films (cellulose-based protective film and low moisture-permeable protective film) bonded to both sides of the polarizer is caused by the tensile force in the TD direction and the MD direction of the cellulose- It has been found that control is possible by adjusting the intensity to a certain range.

Therefore, in the present invention, a cellulose-based protective film is bonded to one side of a polarizer and a low moisture-permeable protective film having a moisture permeability (40 DEG C, 90% RH, 24 hr) of 1 to 200 g / Based protective film has a tensile strength in the TD direction (Transverse Direction) of 110 to 160 MPa, and a tensile strength in the TD direction is larger than a tensile strength in the MD direction (Machine Direction) by 20 MPa or more.

The tensile strength in the TD direction may be 20 to 40 MPa larger than the tensile strength in the MD direction.

The moisture permeability (40 ° C, 90% RH, 24 hours) of the cellulose-based protective film may be higher than the moisture permeability of the low moisture-permeable protective film by 200 g / m 2 · day or more.

The low moisture-permeable protective film may be a film selected from the group consisting of a cycloolefin-based film, a polynorbornene-based film, a polyolefin-based film, an acrylic film, a polyester-based film and a polyimide-based film.

The cellulose-based protective film may have a moisture permeability (40 ° C, 90% RH, 24 hours) of 200 to 2000 g / m 2 · day.

The cellulose-based protective film may be a triacetylcellulose film.

The bonding may be made of an aqueous adhesive.

The polarizing plate of the present invention is effective in controlling curling of an asymmetric polarizing plate whose one side is made of a cellulose-based protective film.

In addition, the polarizing plate of the present invention can prevent a phenomenon such as a decrease in polarization degree due to light leakage due to curling and an increase in transmittance.

Further, the polarizing plate of the present invention is advantageous for producing thinned and enlarged polarizing plates by controlling the generation of curl.

The present invention relates to an asymmetric polarizer plate having a cellulose-based protective film whose one side is capable of controlling curling.

Hereinafter, the present invention will be described in detail.

In the polarizing plate of the present invention, a cellulose-based protective film is bonded to one side of the polarizer and a low moisture-permeable protective film having a moisture permeability (40 ° C, 90% RH, 24 hr) of 1 to 200 g / m 2 · day is bonded to the other side.

The cellulose protective film has a tensile strength in the TD direction (Transverse Direction) of 110 to 160 MPa, and a tensile strength in the TD direction is larger than a tensile strength in the MD direction (Machine Direction) by 20 MPa or more.

A polarizer is an optical film that serves to convert incident natural light into a desired single polarization state (linearly polarized light state), and a film made of a polyvinyl alcohol resin and having a dichroic dye adsorbed and oriented can be used.

The polyvinyl alcohol resin constituting the polarizer can be produced by saponifying a polyvinyl acetate resin. Examples of the polyvinyl acetate resin include copolymers of vinyl acetate and other monomers copolymerizable with vinyl acetate in addition to polyvinyl acetate, which is a homopolymer of vinyl acetate. Specific examples of other monomers copolymerizable with vinyl acetate include unsaturated carboxylic acids, unsaturated sulfonic acids, olefins, vinyl ethers, acrylamides having an ammonium group, and the like. In addition, the polyvinyl alcohol resin may be modified. For example, polyvinyl formal or polyvinyl acetal modified with aldehydes may be used. The degree of saponification of the polyvinyl alcohol resin may be generally 85 to 100 mol%, preferably 98 mol% or more. The polymerization degree of the polyvinyl alcohol resin is usually 1,000 to 10,000, preferably 1,500 to 5,000.

Such a polyvinyl alcohol-based resin is formed into a film and used as a polarizer. The film-forming method of the polyvinyl alcohol-based resin is not particularly limited, and various known methods can be used. The film thickness of the polyvinyl alcohol-based resin is not particularly limited, and may be, for example, 10 to 150 mu m.

The polarizer is usually produced by a process of uniaxially stretching a polyvinyl alcohol film as described above, dyeing with a dichroic dye and adsorbing, treating with an aqueous boric acid solution, and washing and drying.

The step of uniaxially stretching the polyvinyl alcohol film may be performed before dyeing, concurrently with dyeing, or after dyeing. If uniaxial stretching is carried out after dyeing, it may be carried out before the boric acid treatment or during the boric acid treatment. Of course, it is also possible to perform uniaxial stretching by a plurality of steps each of which is combined. As the uniaxial stretching, other rolls or rolls having different circumferences may be used, and may be dry stretching by stretching in air or wet stretching by stretching in the state of being swollen by a solvent. The stretching ratio is usually 3 to 8 times.

In the step of dyeing a stretched polyvinyl alcohol film with a dichroic dye, for example, a method of immersing a polyvinyl alcohol film in an aqueous solution containing a dichroic dye may be used. A specific example of the dichroic dye is iodine or a dichroic organic dye. It is preferable that the polyvinyl alcohol film is pre-immersed in water before dyeing to swell.

When iodine is used as the dichroic dye, a method in which a polyvinyl alcohol-based film is dipped in an aqueous solution for dyeing usually containing iodine and potassium iodide may be used. Usually, the content of iodine in an aqueous solution for dyeing is 0.01 to 1 part by weight with respect to 100 parts by weight of water (distilled water), and the content of potassium iodide is 0.5 to 20 parts by weight with respect to 100 parts by weight of water. The temperature of the aqueous solution for dyeing is usually 20 to 40 占 폚, and the immersion time, for example, the dyeing time is usually 20 to 1800 seconds.

On the other hand, when a dichroic organic dye is used as the dichroic dye, a method in which a polyvinyl alcohol film is dipped in an aqueous solution for dyeing usually containing a water-soluble dichroic organic dye is used. The content of the dichroic organic dye in the dyeing aqueous solution is usually 1 x ^10-4 to 10 parts by weight, preferably 1 x ^10-3 to 1 part by weight based on 100 parts by weight of water. The aqueous solution for dyeing may further contain an inorganic salt such as sodium sulfate as a dyeing aid. The temperature of the aqueous solution for dyeing is usually 20 to 80 캜, and the immersion time, for example, the dyeing time is usually 10 to 1,800 seconds.

The step of treating the dyed polyvinyl alcohol film with boric acid can be carried out by immersing it in an aqueous solution containing boric acid. The content of boric acid in an aqueous solution containing boric acid is usually 2 to 15 parts by weight, preferably 5 to 12 parts by weight based on 100 parts by weight of water. The aqueous solution containing boric acid when iodine is used as the dichroic dye is preferably contained in an amount of 0.1 to 15 parts by weight, preferably 5 to 12 parts by weight, based on 100 parts by weight of water. The boric acid-containing aqueous solution preferably has a temperature of 50 DEG C or higher, preferably 50 to 85 DEG C, more preferably 60 to 80 DEG C, and the immersing time is 60 to 1,200 seconds, preferably 150 to 600 seconds, Preferably 200 to 400 seconds.

After the boric acid treatment, the polyvinyl alcohol film is washed with water and dried. The water treatment can be carried out by immersing the boric acid-treated polyvinyl alcohol-based film in water. The water temperature during the water treatment is 5 to 40 ° C, and the immersion time is 1 to 120 seconds. After washing with water, the polarizer can be obtained by drying. The drying treatment is usually carried out using a hot air dryer or a far infrared ray heater. The drying treatment temperature is usually 30 to 100 ° C, preferably 50 to 80 ° C, and the drying time is usually 60 to 600 seconds, 600 seconds is good.

The thickness of the polarizer may be from 5 to 40 mu m.

The polarizing plate of the present invention is an asymmetric structure in which a cellulose-based protective film is bonded to one surface of a polarizer and a low moisture-permeable protective film is bonded to the other surface of a polarizer. In this polarizing plate of asymmetric structure, curling occurs in the difference in the moisture permeability of the two protective films.

In order to control the generation of curl, the present invention controls the tensile strength in the TD and MD directions of the cellulose-based protective film.

The protective film refers to a film for protecting the polarizer because the polarizer is mechanically weak. When the retardation film serves as a polarizer protective film, the retardation film is also considered to belong to the protective film of the present invention.

The protective film has a different moisture permeability depending on the type of resin, and can be selectively used depending on transparency, mechanical strength, thermal stability, moisture barrier properties, and isotropic properties.

The cellulose-based protective film to be bonded to one side of the polarizer has a moisture permeability (40 DEG C, 90% RH, 24 hours) in the range of 200 to 2000 g / m2 day, preferably 400 to 800 g / It is known as moisture permeable protective film.

Preferably, the cellulose-based protective film has a moisture permeability (40 ° C, 90% RH, 24 hr) of 200 g / m 2 · day or more in order to efficiently control the generation of curl.

Specifically, a film made of a cellulose-based resin such as diacetylcellulose or triacetylcellulose may be used.

The cellulose-based protective film has a TD (Transverse Direction) tensile strength of 110 to 160 MPa. When the tensile strength in the TD direction is less than 110 MPa, curling occurs in the direction of the low moisture-permeable protective film. When the tensile strength in the TD direction is more than 160 MPa, high elongation and wave curl of 30 mm or more in the direction of the cellulose-

The TD direction is a direction perpendicular to the machine direction (MD), and the MD direction is a longitudinal direction corresponding to a progress direction of the continuous production of the film.

The tensile strength in the TD direction is 20 MPa or more, preferably 20 to 40 MPa larger than the tensile strength in the MD direction.

If the tensile strength in the TD direction is smaller than 20 MPa than the tensile strength in the MD direction, there is a problem that a counter-current is generated.

The low moisture permeability protective film bonded to the other side of the polarizer has a moisture permeability (40 DEG C, 90% RH, 24 hours) of 1 to 200 g / m < 2 >

The low moisture permeation protective film may be a film selected from the group consisting of a cycloolefin film, a polynorbornene film, a polyolefin film, an acrylic film, a polyester film and a polyimide film. Specific examples thereof include polyester resins such as polyethylene terephthalate, polyethylene isophthalate, polyethylene naphthalate and polybutylene terephthalate; Polycarbonate resin; Acrylic resins such as polymethyl (meth) acrylate and polyethyl (meth) acrylate; Styrene resins such as polystyrene and acrylonitrile-styrene copolymer; Polyolefin resins such as polyethylene, polypropylene, cycloolefins or polyolefins having a norbornene structure, and ethylene-propylene copolymers; Vinyl chloride resin; Amide resins such as nylon and aromatic polyamide; Polyimide resin; Polyether sulfone type resin; Sulfone based resin; Polyether sulfone type resin; Polyether ether ketone resin; A sulfided polyphenylene resin; Vinyl alcohol-based resin; Vinylidene chloride resins; Vinyl butyral resin; Allylate series resin; Polyoxymethylene type resin; Epoxy resin, and the like, and a film composed of the blend of the thermoplastic resin may also be used. Further, a film made of a thermosetting resin such as (meth) acrylic, urethane, acrylic urethane, epoxy, or silicone or a film made of an ultraviolet curable resin may also be used.

The protective film may contain one or more suitable additives. Specific examples of the additive include an ultraviolet absorber, an antioxidant, a lubricant, a plasticizer, a mold release agent, a coloring inhibitor, a flame retardant, a nucleating agent, an antistatic agent, a pigment and a colorant.

In the present invention, the polarizer and the protective film are bonded with an aqueous adhesive.

Specific examples of the water-based adhesive include an isocyanate-based adhesive, a polyvinyl alcohol-based adhesive, a gelatin-based adhesive, a vinyl-based latex-based, a water-based polyurethane, and a water-based polyester. Among them, a polyvinyl alcohol-based adhesive is preferable. The water-based adhesive may suitably contain a crosslinking agent.

For example, a polyvinyl alcohol-based adhesive containing a crosslinking agent is usually used as an aqueous solution. The concentration of the aqueous solution is not particularly limited, but it is 0.1 to 15% by weight, preferably 0.5 to 10% by weight, and more preferably 0.5 to 5% by weight, in consideration of the coating property or the standing stability.

Further, the water-based adhesive may be further mixed with a coupling agent such as a silane coupling agent or a titanium coupling agent, various tackifiers, ultraviolet absorbers, antioxidants, heat stabilizers, stabilizers such as hydrolysis stabilizers.

The protective film may be subjected to a surface treatment to improve adhesion with the polarizer. Specifically, a saponification treatment, a plasma treatment, a corona treatment, an ultraviolet irradiation treatment, a frame (flame) treatment, and the like can be performed. Considering the adhesion efficiency and easiness of the process, the cellulose-based film is preferably subjected to a saponification treatment, and the low-moisture-permeable film is preferably subjected to plasma treatment, corona treatment and the like.

An adhesive for bonding the polarizer and the protective film is applied to either side of the polarizer or the protective film or both. The coating operation is not particularly limited, and various methods such as a roll method, a spray method, and a dipping method can be used.

It is preferable that the adhesive layer is applied so that the thickness of the adhesive layer formed after drying is 10 to 1000 nm, preferably 40 to 600 nm, considering the solid content. If the thickness is less than 10 nm, the adhesive force may be poor. If the thickness exceeds 1000 nm, uniform application of the adhesive may be difficult or the thickness may be uneven, resulting in poor appearance.

After the adhesive is applied, the polarizer and the protective film are bonded together by a roll laminator or the like. In addition, a tension can be applied to the protective film when the polarizer and the protective film are bonded, but the tension is preferably as low as possible because the protective film deforms. After bonding the polarizer and the protective film, a drying process is performed to form an adhesive layer comprising a coating and drying layer.

The polarizing plate to which the polarizer and the protective film are bonded is subjected to a drying process. Examples of the drying process include hot air drying using hot air at a drying temperature of 40 to 100 占 폚, preferably 60 to 100 占 폚. The drying time is 20 to 1200 seconds, but it is preferable that the drying time is as short as possible in order to improve the productivity. After hot air drying, it is preferable to cure at room temperature or higher than room temperature, for example, at 20 to 50 DEG C for 12 to 600 hours.

The polarizing plate of the present invention may further include a surface treatment layer such as a hard coating layer, an antireflection layer, an antiglare layer, and an antistatic layer on one side of the protective film. Further, the optical functional film may be further laminated by the pressure-sensitive adhesive layer. Specific examples of the optical functional film include an optical compensation film in which a liquid crystalline compound or a polymer compound thereof is oriented on the surface of a substrate, Polarized light having a property of reflecting on the surface, a reflection type polarized light separation film to be reflected, a retardation film including a polycarbonate resin, a retardation film including a cyclic polyolefin resin, an anti- An additional film, a reflecting film having a reflecting function on the surface, and a semi-transmitting film having both a reflecting function and a transmitting function.

The polarizing plate of the present invention can be used in a liquid crystal display device. The liquid crystal display device of the present invention can be manufactured by peeling off the peelable release film of the polarizing plate with a pressure-sensitive adhesive and then attaching the pressure-sensitive adhesive layer to the liquid crystal panel.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the present invention. Such variations and modifications are intended to be within the scope of the appended claims.

Example

Production Example 1: Production of Polarizer

A polyvinyl alcohol film (VF-PS, KURARAY Co.) having a thickness of 75 탆 and an average degree of polymerization of 2,400 and a degree of saponification of 99.9 mol% or more was uniaxially stretched by 6.0 times in a dry state, ) For 1 minute, and then immersed in an aqueous solution at 28 캜 in which the weight ratio of iodine / potassium iodide / water was 0.5 / 5/100 for 60 seconds. Thereafter, the substrate was immersed in an aqueous solution of 72 ° C having a weight ratio of potassium iodide / boric acid / water of 8.5 / 8.5 / 100 for 300 seconds. Subsequently, the film was washed with water at 26 DEG C for 20 seconds, and then dried at 65 DEG C to obtain a polarizer in which iodine was adsorbed and oriented on the polyvinyl alcohol resin.

Production Example 2: Preparation of adhesive

To 100 parts by weight of water were added 3 parts by weight of a carboxyl group-modified polyvinyl alcohol (Kurarupofal KL318, Kuraray Co., Ltd.) and a water-soluble polyamide epoxy resin having a solid content of 30% (Sumireze Resin 650, Sumitomo Chemtex Co., Ltd.) 1.5 By weight was added to prepare a polyvinyl alcohol-based adhesive.

Example 1

On one side of the polarizer (thickness: 30 탆) obtained in Production Example 1, a triacetyl cellulose film (TD60UL, Fuji Co., Ltd.) having a thickness of 50 탆 and a moisture permeability (40 캜, 90% RH, 24 hours) of 520 g / Film Co., Ltd.). The triacetylcellulose film had a tensile strength in the TD direction of 115 MPa and a tensile strength in the MD direction of 80 MPa.

On the other side, a cycloolefin-based film (ZB12, Zeon) having a thickness of 50 μm and a moisture permeability (40 ° C., 90% RH, 24 hours) of 12 g / m 2 · day and a corona treatment was placed.

Thereafter, the PVA adhesive prepared in Production Example 2 was coated and then bonded using a nip roll. The bonded polarizing plate was placed in a hot air oven and dried at 70 DEG C for 10 minutes to prepare a polarizing plate.

Example 2 and Comparative Examples 1 to 4

The polarizing plates were prepared in the same manner as in Example 1 except that the tensile strength in the TD direction and the tensile strength in the MD direction of the triacetylcellulose film were changed as shown in Table 1 below.

division Tensile strength in the TD direction (Mpa) Tensile Strength in MD Direction (Mpa) TD-MD Example 2-1 112 90 22 Example 2-2 141 90 51 Example 2-3 157 90 67 Examples 2-4 150 110 40 Comparative Example 1 105 80 25 Comparative Example 2 88 90 -2 Comparative Example 3 105 90 15 Comparative Example 4 165 90 75

Example 3

A polarizing plate was prepared in the same manner as in Example 1 except that various low moisture permeable films were used in place of the cycloolefin-based films as shown in Table 2 below.

division Low humidity film Kinds Moisture permeability
(40 ° C, 90% RH, 24 hr, g / m 2 · day) Example 3-1 Norbornene-based film
(Aton, Okura Industries) 84 Example 3-2 Polyester film
(Toyobo, T100) 13

Test Example

The physical properties of the polarizers prepared in the above Examples and Comparative Examples were measured by the following methods, and the results are shown in Table 3 below.

1. Optical characteristics (polarization degree, transmittance)

The prepared polarizing plate was cut into a size of 4 cm x 4 cm and then measured using an external visible ray spectrometer (V-7100, manufactured by JASCO). At this time, the polarization degree is defined by the following equation (1).

(Wherein T ₁ is the parallel transmittance obtained when the pair of polarizers are arranged in parallel with the absorption axis, and T ₂ is the orthogonal transmittance obtained when the pair of polarizers are arranged so that the absorption axes are perpendicular to each other) .

2. Curl measurement

The prepared polarizing plate was punched at a size of 100 mm × 100 mm in a direction of 45 ° with respect to the absorption axis of the polarizer to prepare a specimen. When the prepared specimen was placed on a flat surface, the space distance And evaluated based on the following criteria. At this time, the positive is indicated by (+) and the negative by (-).

<Evaluation Criteria>

○: No backlash occurs, and the amount of positive (+) is 30 mm or less.

×: A backlash occurs or the amount of the graphic is more than (+) 30 mm.

division Optical characteristic Curl Remarks Transmittance (%) Polarization degree (%) Example 1 40.5 99.9933 4 ○ Example 2-1 40.3 99.9935 5 ○ Example 2-2 40.2 99.9932 13 ○ Example 2-3 40.2 99.9930 22 ○ Examples 2-4 40.4 99.9937 20 ○ Example 3-1 40.1 99.9934 5 ○ Example 3-2 40.3 99.9935 4 ○ Comparative Example 1 40.2 99.9932 -3 × Comparative Example 2 40.3 99.9936 -15 × Comparative Example 3 40.1 99.9932 -9 × Comparative Example 4 40.2 99.9935 32 × (wave curl occurrence)

As shown in the table, Examples 1 to 3, which are asymmetric polarizing plates in which a cellulose protective film having a tensile strength in the TD direction and a tensile strength in the MD direction are bonded to one side of the polarizer according to the present invention in a specific range, It was confirmed that the transmittance and the degree of polarization were excellent.

On the other hand, when the range of the tensile strength in the TD direction and the tensile strength in the MD direction deviates from the range of the present invention, it is confirmed that a counter-current is generated or an amount of the generated crystal is increased.

Claims (7)

A cellulose-based protective film is bonded to one side of the polarizer, and a low moisture-permeable protective film having a moisture permeability (40 ° C, 90% RH, 24 hr) of 1 to 200 g /
The cellulose-based protective film has a tensile strength in the TD direction (Transverse Direction) of 110 to 160 MPa,
Wherein the tensile strength in the TD direction is greater than the machine direction tensile strength by at least 20 MPa.
The polarizing plate according to claim 1, wherein the tensile strength in the TD direction is 20 to 40 MPa larger than tensile strength in the MD direction.
The polarizing plate according to claim 1, wherein the moisture permeability (40 ° C, 90% RH, 24 hours) of the cellulose-based protective film is larger than the moisture permeability of the low moisture-permeable protective film by 200 g / m 2 · day or more.
[4] The polarizer of claim 3, wherein the low moisture permeation protective film is a film selected from the group consisting of a cycloolefin film, a polynorbornene film, a polyolefin film, an acrylic film, a polyester film and a polyimide film.
The polarizing plate according to claim 3, wherein the cellulose-based protective film has a moisture permeability (40 ° C, 90% RH, 24 hours) of 200 to 2000 g / m 2 · day.
The polarizing plate according to claim 5, wherein the cellulose-based protective film is a triacetylcellulose film.
The polarizing plate according to claim 1, wherein the bonding comprises an aqueous adhesive.