JP7267396B2 - Polarizing film, polarizing plate, and method for producing the polarizing film - Google Patents

Description

The present invention relates to a polarizing film, a polarizing plate, and a method for producing the polarizing film.

A liquid crystal display device, which is a typical image display device, has polarizing films arranged on both sides of a liquid crystal cell due to its image forming method. As a method for producing a polarizing film, for example, there is a method in which a laminate having a resin substrate and a polyvinyl alcohol (PVA) resin layer is stretched and then dyed to obtain a polarizing film on the resin substrate. It has been proposed (for example, Patent Document 1). According to such a method, since a thin polarizing film can be obtained, it has attracted attention as a potential contribution to the thinning of image display devices in recent years. However, the thin polarizing film has the problem that it shrinks significantly in a high-temperature environment and cracks are likely to occur.

JP-A-2001-343521

The present invention has been made to solve the above conventional problems, and its main object is to provide a polarizing film and a polarizing plate in which shrinkage and cracking are suppressed in a high-temperature environment, and a method for producing such a polarizing film. to provide.

The polarizing film of the present invention is composed of a polyvinyl alcohol resin film containing iodine and contains 0.1% to 1.0% by weight of an alcohol having a boiling point of 100° C. or higher.
In one embodiment, the alcohol having a boiling point of 100° C. or higher is at least one selected from the group consisting of glycerin and ethylene glycol.
In one embodiment, the polarizing film has a thickness of 8 μm or less.
According to another aspect of the invention, a polarizing plate is provided. This polarizing plate has the above polarizing film and a protective layer disposed on at least one side of the polarizing film.
According to still another aspect of the present invention, there is provided a method for manufacturing the polarizing film. In this method, a polyvinyl alcohol-based resin layer is formed on one side of a long thermoplastic resin substrate to form a laminate; the laminate is stretched and dyed to convert the polyvinyl alcohol-based resin layer into a polarizing film. and introducing an alcohol having a boiling point of 100° C. or higher into the polarizing film.
In one embodiment, the manufacturing method includes immersing the polarizing film in a treatment liquid containing alcohol having a boiling point of 100° C. or higher.
In one embodiment, the manufacturing method further includes heating the laminate after introducing the alcohol having a boiling point of 100° C. or higher into the polarizing film.
In one embodiment, the stretching comprises underwater stretching.

According to the present invention, by introducing an alcohol having a boiling point of 100° C. or higher into the polarizing film, it is possible to obtain a polarizing film in which shrinkage and cracking are suppressed in a high-temperature environment.

1 is a schematic cross-sectional view of a polarizer according to one embodiment of the invention; FIG. FIG. 4 is a schematic diagram showing an example of drying shrinkage treatment using a heating roll. (a) is a conceptual diagram illustrating test samples for the contractility tests of Examples 1 and 2 and Comparative Example 1, and (b) is the results of the contractility tests of Examples 3 and 6 and Comparative Examples 2 and 3. It is a conceptual diagram explaining a test sample. It is a conceptual diagram explaining the measuring method of a guitar pick test.

Embodiments of the present invention will be described below, but the present invention is not limited to these embodiments.

A. Polarizing Film The polarizing film according to the embodiment of the present invention is composed of a polyvinyl alcohol (PVA) resin film containing iodine, and has an alcohol with a boiling point of 100° C. or higher (hereinafter sometimes referred to as a high boiling point alcohol) of 0.1 % to 1.0% by weight. By including a predetermined amount of such a high boiling point alcohol in the polarizing film, it is possible to obtain a polarizing film in which shrinkage and cracking are suppressed in a high-temperature environment. Such high-boiling alcohols can be introduced into the polarizing film, typically between the stretching process in water and the drying-shrinking process, as described later in Section C regarding the manufacturing method. By introducing such a high-boiling-point alcohol, the high-boiling-point alcohol functions as a plasticizer, improving the flexibility of the finally obtained polarizing film, and suppressing shrinkage in a high-temperature environment. It is estimated that cracks can also be suppressed as The content of the high-boiling-point alcohol in the polarizing film is preferably 0.1 wt % to 0.9 wt %, more preferably 0.1 wt % to 0.8 wt %, still more preferably 0.1 wt % to 0.8 wt %. 2% to 0.7% by weight, particularly preferably 0.2% to 0.6% by weight. If the content is too small, the effect of the high boiling point alcohol may not be obtained. If the content is too large, the degree of polarization may decrease significantly in a high-temperature, high-humidity environment.

As described above, the boiling point of the high-boiling alcohol is 100°C or higher, preferably 150°C or higher, more preferably 180°C or higher, and still more preferably 250°C or higher. The upper boiling point limit can be, for example, 310°C.

Representative examples of high-boiling alcohols include higher alcohols, alcohols having a ring structure (eg, aromatic alcohols, alicyclic alcohols), and polyhydric alcohols. Specific examples include glycerin, ethylene glycol, butanol, phenol and pentanol. High boiling point alcohols may be used alone or in combination of two or more. Glycerin and ethylene glycol are preferred. These impart good flexibility to the resulting polarizing film and, as a result, can suppress shrinkage and cracking in high-temperature environments.

The thickness of the polarizing film is, for example, 8 μm or less, preferably 7 μm or less, more preferably 5 μm or less, and still more preferably 3 μm or less. The lower limit of the thickness of the polarizing film may be 1 μm in one embodiment and 2 μm in another embodiment.

The polarizing film preferably exhibits absorption dichroism at any wavelength of 380 nm to 780 nm. The single transmittance of the polarizing film is preferably 42.0% or more, more preferably 42.5% or more, and still more preferably 43.0% or more. On the other hand, the single transmittance is preferably 47.0% or less, more preferably 46.0% or less. The polarization degree of the polarizing film is preferably 99.90% or more, more preferably 99.95% or more. On the other hand, the degree of polarization is preferably 99.998% or less. According to the embodiment of the present invention, in this way, it is possible to achieve both high single transmittance and high degree of polarization, and to achieve excellent durability in a high-temperature and high-humidity environment as described above. can. The single transmittance is typically a Y value measured using an ultraviolet-visible spectrophotometer and subjected to visibility correction. The single transmittance is a value when the refractive index of one surface of the polarizing plate is converted to 1.50 and the refractive index of the other surface is converted to 1.53. The degree of polarization is typically obtained by the following formula based on the parallel transmittance Tp and the orthogonal transmittance Tc measured using an ultraviolet-visible spectrophotometer and subjected to visibility correction.
Degree of polarization (%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 × 100

The polarizing film preferably has a shrinkage rate in the absorption axis direction after heating at 85° C. for 120 hours, more preferably 0.18% or less, and still more preferably 0.16% or less. and particularly preferably 0.14% or less. According to the embodiment of the present invention, it is possible to obtain a polarizing film with a small shrinkage factor in such a high temperature environment. The shrinkage rate is the shrinkage rate for a sample having a size of 10 cm in the absorption axis direction×10 cm in the transmission axis direction.

The polarizing film may be produced using a single resin film, or may be produced using a laminate of two or more layers. A specific example of a polarizing film obtained using a laminate is a polarizing film obtained using a laminate of a resin substrate and a PVA-based resin layer formed by coating on the resin substrate. A polarizing film obtained by using a laminate of a resin base material and a PVA-based resin layer formed by coating on the resin base material can be obtained, for example, by applying a PVA-based resin solution to the resin base material and drying the resin base material. forming a PVA-based resin layer thereon to obtain a laminate of a resin substrate and a PVA-based resin layer; stretching and dyeing the laminate to make the PVA-based resin layer a polarizing film; obtain. In an embodiment of the present invention, a high boiling point alcohol is introduced into the polarizing film. Thereby, it is possible to obtain a polarizing film in which shrinkage and cracking in a high-temperature environment as described above are suppressed. Preferably, a polyvinyl alcohol-based resin layer containing a halide and a polyvinyl alcohol-based resin is formed on one side of the resin substrate. Stretching typically includes immersing the laminate in an aqueous boric acid solution for stretching. Furthermore, stretching may further include stretching the laminate in air at a high temperature (eg, 95° C. or higher) before stretching in an aqueous boric acid solution, if necessary. In addition, in the present embodiment, the laminate is preferably subjected to drying shrinkage treatment in which the laminate is heated while being conveyed in the longitudinal direction to shrink the laminate by 2% or more in the width direction. Typically, the manufacturing method of the present embodiment includes subjecting the laminate to an in-air auxiliary stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment in this order. By introducing auxiliary stretching, it is possible to improve the crystallinity of PVA and achieve high optical properties even when PVA is coated on a thermoplastic resin. At the same time, by increasing the orientation of PVA in advance, it is possible to prevent problems such as deterioration of orientation and dissolution of PVA when immersed in water in the subsequent dyeing process or stretching process, resulting in high optical properties. can be achieved. Furthermore, when the PVA-based resin layer is immersed in a liquid, disturbance of the orientation of the polyvinyl alcohol molecules and deterioration of the orientation can be suppressed as compared with the case where the PVA-based resin layer does not contain a halide. This can improve the optical properties of the polarizing film obtained through treatment steps such as dyeing treatment and underwater stretching treatment in which the laminate is immersed in a liquid. Furthermore, the optical properties can be improved by shrinking the laminate in the width direction by drying shrinkage treatment. The obtained resin substrate/polarizing film laminate may be used as it is (that is, the resin substrate may be used as a protective layer for the polarizing film), or the resin substrate may be peeled off from the resin substrate/polarizing film laminate. Then, any appropriate protective layer may be laminated on the release surface according to the purpose. Details of the manufacturing method of the polarizing film will be described later in section C.

B. Polarizing Plate FIG. 1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. The polarizing plate 100 has a polarizing film 10, a first protective layer 20 arranged on one side of the polarizing film 10, and a second protective layer 30 arranged on the other side of the polarizing film 10. The polarizing film 10 is the polarizing film of the present invention described in section A above. One of the first protective layer 20 and the second protective layer 30 may be omitted. As described above, one of the first protective layer and the second protective layer may be the resin base material used for manufacturing the polarizing film.

The first and second protective layers are formed of any suitable film that can be used as protective layers for polarizing films. Specific examples of the material that is the main component of the film include cellulose-based resins such as triacetyl cellulose (TAC), polyester-based, polyvinyl alcohol-based, polycarbonate-based, polyamide-based, polyimide-based, polyethersulfone-based, and polysulfone-based resins. , polystyrene-based, polynorbornene-based, polyolefin-based, (meth)acrylic-based, and acetate-based transparent resins. Thermosetting resins such as (meth)acrylic, urethane, (meth)acrylic urethane, epoxy, and silicone, or ultraviolet curable resins may also be used. In addition, for example, a glassy polymer such as a siloxane-based polymer can also be used. Further, polymer films described in JP-A-2001-343529 (WO01/37007) can also be used. Materials for this film include, for example, a resin composition containing a thermoplastic resin having a substituted or unsubstituted imide group in a side chain and a thermoplastic resin having a substituted or unsubstituted phenyl group and nitrile group in a side chain. can be used, for example, a resin composition comprising an alternating copolymer of isobutene and N-methylmaleimide and an acrylonitrile-styrene copolymer. The polymer film can be, for example, an extrudate of the resin composition.

When the polarizing plate 100 is applied to an image display device, the thickness of the protective layer (outer protective layer) disposed on the side opposite to the display panel is typically 300 μm or less, preferably 100 μm or less, more preferably 100 μm or less. 5 μm to 80 μm, more preferably 10 μm to 60 μm. In addition, when the surface treatment is performed, the thickness of the outer protective layer is the thickness including the thickness of the surface treatment layer.

When the polarizing plate 100 is applied to an image display device, the protective layer (inner protective layer) disposed on the display panel side preferably has a thickness of 5 μm to 200 μm, more preferably 10 μm to 100 μm, and still more preferably 10 μm to 60 μm. be. In one embodiment, the inner protective layer is a retardation layer with any suitable retardation value. In this case, the in-plane retardation Re(550) of the retardation layer is, for example, 110 nm to 150 nm. “Re(550)” is an in-plane retardation measured with light having a wavelength of 550 nm at 23° C., and is obtained by the formula: Re=(nx−ny)×d. Here, “nx” is the refractive index in the direction in which the in-plane refractive index is maximized (that is, the slow axis direction), and “ny” is the in-plane direction orthogonal to the slow axis (that is, the fast is the refractive index in the axial direction), 'nz' is the refractive index in the thickness direction, and 'd' is the layer (film) thickness (nm).

C. Method for Producing Polarizing Film In a method for producing a polarizing film according to one embodiment of the present invention, a PVA-based resin solution is applied to one side of a long thermoplastic resin substrate and dried to form a PVA-based resin layer. forming a laminate; stretching and dyeing the laminate to form a PVA-based resin layer into a polarizing film; and introducing a high-boiling-point alcohol into the polarizing film. By introducing a high-boiling-point alcohol, it is possible to realize a polarizing film in which shrinkage and cracking in a high-temperature environment are suppressed. Preferably, the PVA-based resin solution further contains a halide. Preferably, the above-described manufacturing method includes applying an auxiliary aerial stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment to shrink the laminate by 2% or more in the width direction by heating while conveying the laminate in the longitudinal direction, in that order. In this case, the introduction of the high-boiling alcohol can preferably be carried out between the underwater stretching treatment and the drying shrinkage treatment. The content of the halide in the PVA-based resin solution (resulting in the PVA-based resin layer) is preferably 5 to 20 parts by weight with respect to 100 parts by weight of the PVA-based resin. The drying shrinkage treatment is preferably performed using a heating roll, and the temperature of the heating roll is preferably 60°C to 120°C. The shrinkage ratio in the width direction of the laminate due to drying shrinkage treatment is preferably 2% or more. According to such a manufacturing method, the polarizing film described in section A above can be obtained. In particular, a laminate containing a PVA-based resin layer containing a halide is produced, the laminate is stretched in multiple stages including auxiliary stretching in the air and stretching in water, and the laminate after stretching is heated with a heating roll. , a polarizing film having excellent optical properties (typically, unit transmittance and unit absorbance) can be obtained.

C-1. Production of Laminate Any appropriate method can be adopted as a method for producing a laminate of a thermoplastic resin substrate and a PVA-based resin layer. Preferably, a coating liquid containing a halide and a PVA-based resin is applied to the surface of the thermoplastic resin substrate and dried to form a PVA-based resin layer on the thermoplastic resin substrate. As described above, the content of the halide in the PVA-based resin layer is preferably 5 to 20 parts by weight with respect to 100 parts by weight of the PVA-based resin.

Any appropriate method can be adopted as a method for applying the coating liquid. Examples thereof include roll coating, spin coating, wire bar coating, dip coating, die coating, curtain coating, spray coating, and knife coating (comma coating, etc.). The coating/drying temperature of the coating liquid is preferably 50° C. or higher.

The thickness of the PVA-based resin layer is preferably 3 μm to 40 μm, more preferably 3 μm to 20 μm.

Before forming the PVA-based resin layer, the thermoplastic resin substrate may be surface-treated (for example, corona treatment), or an easy-adhesion layer may be formed on the thermoplastic resin substrate. By performing such treatment, the adhesion between the thermoplastic resin substrate and the PVA-based resin layer can be improved.

C-1-1. Thermoplastic Resin Substrate Any appropriate thermoplastic resin film can be employed as the thermoplastic resin substrate. Details of the thermoplastic resin substrate are described, for example, in JP-A-2012-73580. The publication is incorporated herein by reference in its entirety.

C-1-2. Coating Liquid The coating liquid contains a halide and a PVA-based resin as described above. The coating liquid is typically a solution in which the halide and the PVA-based resin are dissolved in a solvent. Examples of solvents include water, dimethylsulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyhydric alcohols such as trimethylolpropane, and amines such as ethylenediamine and diethylenetriamine. These can be used alone or in combination of two or more. Among these, water is preferred. The concentration of the PVA-based resin in the solution is preferably 3 to 20 parts by weight with respect to 100 parts by weight of the solvent. With such a resin concentration, it is possible to form a uniform coating film in close contact with the thermoplastic resin substrate. The content of the halide in the coating liquid is preferably 5 to 20 parts by weight with respect to 100 parts by weight of the PVA-based resin.

Additives may be added to the coating liquid. Examples of additives include plasticizers and surfactants. Examples of plasticizers include polyhydric alcohols such as ethylene glycol and glycerin. Surfactants include, for example, nonionic surfactants. These can be used for the purpose of further improving the uniformity, dyeability and stretchability of the obtained PVA-based resin layer.

Any appropriate resin can be adopted as the PVA-based resin. Examples include polyvinyl alcohol and ethylene-vinyl alcohol copolymers. Polyvinyl alcohol is obtained by saponifying polyvinyl acetate. An ethylene-vinyl alcohol copolymer is obtained by saponifying an ethylene-vinyl acetate copolymer. The degree of saponification of the PVA-based resin is usually 85 mol% to 100 mol%, preferably 95.0 mol% to 99.95 mol%, more preferably 99.0 mol% to 99.93 mol%. . The degree of saponification can be determined according to JIS K 6726-1994. By using a PVA-based resin having such a degree of saponification, a polarizing film with excellent durability can be obtained. If the degree of saponification is too high, gelation may occur.

The average degree of polymerization of the PVA-based resin can be appropriately selected depending on the purpose. The average degree of polymerization is usually 1,000 to 10,000, preferably 1,200 to 4,500, more preferably 1,500 to 4,300. The average degree of polymerization can be determined according to JIS K 6726-1994.

Any appropriate halide can be employed as the halide. Examples include iodide and sodium chloride. Iodides include, for example, potassium iodide, sodium iodide, and lithium iodide. Among these, potassium iodide is preferred.

The amount of the halide in the coating liquid is preferably 5 parts by weight to 20 parts by weight with respect to 100 parts by weight of the PVA resin, and more preferably 10 parts by weight to 15 parts by weight with respect to 100 parts by weight of the PVA resin. Department. If the amount of the halide exceeds 20 parts by weight with respect to 100 parts by weight of the PVA-based resin, the halide may bleed out and the finally obtained polarizing film may become cloudy.

In general, when the PVA-based resin layer is stretched, the orientation of the polyvinyl alcohol molecules in the PVA-based resin increases. Orientation may be disturbed and the orientation may be lowered. In particular, when stretching a laminate of a thermoplastic resin and a PVA-based resin layer in boric acid water, the laminate is stretched in boric acid water at a relatively high temperature in order to stabilize the stretching of the thermoplastic resin. The tendency for the degree of orientation to decrease is remarkable. For example, the stretching of a single PVA film in boric acid water is generally carried out at 60° C., whereas the stretching of a laminate of A-PET (thermoplastic resin substrate) and a PVA-based resin layer is It is carried out at a high temperature of about 70° C., and in this case, the orientation of PVA at the initial stage of stretching may be lowered before it is increased by stretching in water. On the other hand, by preparing a laminate of a PVA-based resin layer containing a halide and a thermoplastic resin substrate and stretching the laminate at a high temperature (auxiliary stretching) in air before stretching in boric acid water, , the crystallization of the PVA-based resin in the PVA-based resin layer of the laminate after auxiliary stretching can be promoted. As a result, when the PVA-based resin layer is immersed in a liquid, the disturbance of the orientation of the polyvinyl alcohol molecules and the deterioration of the orientation can be suppressed compared to the case where the PVA-based resin layer does not contain a halide. This can improve the optical properties of the polarizing film obtained through treatment steps such as dyeing treatment and underwater stretching treatment in which the laminate is immersed in a liquid.

C-2. Aerial Auxiliary Stretching In order to obtain particularly high optical properties, a two-stage stretching method combining dry stretching (auxiliary stretching) and stretching in boric acid solution is selected. By introducing auxiliary stretching, such as two-step stretching, it is possible to stretch while suppressing crystallization of the thermoplastic resin substrate, and excessive crystallization of the thermoplastic resin substrate in the subsequent stretching in boric acid water. It is possible to solve the problem that stretchability is reduced by stretching, and stretch the laminate at a higher magnification. Furthermore, when applying the PVA-based resin on the thermoplastic resin substrate, in order to suppress the influence of the glass transition temperature of the thermoplastic resin substrate, compared to the case of applying the PVA-based resin on a normal metal drum As a result, the crystallization of the PVA-based resin becomes relatively low, which may cause a problem that sufficient optical properties cannot be obtained. On the other hand, by introducing auxiliary stretching, it is possible to increase the crystallinity of the PVA-based resin even when the PVA-based resin is applied on the thermoplastic resin, and it is possible to achieve high optical properties. Become. At the same time, by increasing the orientation of the PVA-based resin in advance, it is possible to prevent problems such as deterioration of the orientation and dissolution of the PVA-based resin when it is immersed in water in the subsequent dyeing process or stretching process. , making it possible to achieve high optical properties.

The stretching method of the in-air auxiliary stretching may be fixed edge stretching (e.g., a method of stretching using a tenter stretching machine) or free edge stretching (e.g., a method of uniaxially stretching the laminate through rolls having different peripheral speeds). Although good, free-end drawing may be positively employed in order to obtain high optical properties. In one embodiment, the in-air stretching process includes a heating roll stretching step in which the laminate is stretched by a peripheral speed difference between heating rolls while being conveyed in the longitudinal direction. The air drawing process typically includes a zone drawing process and a hot roll drawing process. The order of the zone stretching process and the heating roll stretching process is not limited, and the zone stretching process may be carried out first, or the heating roll stretching process may be carried out first. The zone drawing step may be omitted. In one embodiment, the zone drawing step and the heated roll drawing step are performed in this order. In another embodiment, in a tenter stretching machine, the film is stretched by gripping the ends of the film and increasing the distance between the tenters in the machine direction (the extension of the distance between the tenters is the stretching ratio). At this time, the distance between the tenters in the width direction (perpendicular to the machine direction) is set to be arbitrarily close. Preferably, the draw ratio in the machine direction can be set to be closer to the free end draw. In the case of free end stretching, the shrinkage ratio in the width direction is calculated by (1/stretching ratio) ^1/2 .

The in-air auxiliary stretching may be performed in one step or in multiple steps. When it is carried out in multiple stages, the draw ratio is the product of the draw ratios in each step. The stretching direction in the in-air auxiliary stretching is preferably substantially the same as the stretching direction in the underwater stretching.

The draw ratio in the in-air auxiliary drawing is preferably 2.0 to 3.5 times. The maximum draw ratio when the auxiliary drawing in the air and the drawing in water are combined is preferably 5.0 times or more, more preferably 5.5 times or more, and still more preferably 6.0 times the original length of the laminate. That's it. As used herein, the term "maximum draw ratio" refers to the draw ratio immediately before the laminate breaks, and is 0.2 lower than the draw ratio at which the laminate breaks.

The stretching temperature for the in-air auxiliary stretching can be set to any appropriate value depending on the material for forming the thermoplastic resin base material, the stretching method, and the like. The stretching temperature is preferably the glass transition temperature (Tg) of the thermoplastic resin substrate or higher, more preferably the glass transition temperature (Tg) of the thermoplastic resin substrate + 10°C or higher, and particularly preferably Tg + 15°C or higher. On the other hand, the upper limit of the stretching temperature is preferably 170°C. By stretching at such a temperature, it is possible to suppress rapid crystallization of the PVA-based resin and suppress problems caused by the crystallization (for example, hindrance of orientation of the PVA-based resin layer due to stretching). can.

C-3. Insolubilization Treatment, Dyeing Treatment, and Crosslinking Treatment If necessary, an insolubilization treatment is performed after the auxiliary stretching treatment in the air and before the stretching treatment in water or the dyeing treatment. The insolubilization treatment is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. The dyeing treatment is typically performed by dyeing the PVA-based resin layer with a dichroic substance (typically iodine). If necessary, a cross-linking treatment is performed after the dyeing treatment and before the underwater stretching treatment. The cross-linking treatment is typically performed by immersing the PVA-based resin layer in an aqueous solution of boric acid. Details of the insolubilization treatment, dyeing treatment and cross-linking treatment are described, for example, in JP-A-2012-73580 (above).

C-4. Underwater Stretching Treatment Underwater stretching treatment is performed by immersing the laminate in a stretching bath. According to the underwater stretching treatment, the thermoplastic resin substrate and the PVA-based resin layer can be stretched at a temperature lower than the glass transition temperature (typically, about 80° C.), and the PVA-based resin layer undergoes its crystallization. can be stretched at a high magnification while suppressing the As a result, a polarizing film having excellent optical properties can be produced.

Any appropriate method can be adopted as a method for stretching the laminate. Specifically, fixed-end stretching or free-end stretching (for example, a method of uniaxially stretching a laminate by passing it between rolls having different peripheral speeds) may be used. Free-end drawing is preferably chosen. The laminate may be stretched in one step or in multiple steps. When the stretching is performed in multiple stages, the draw ratio (maximum draw ratio) of the laminate described below is the product of the draw ratios in each step.

The stretching in water is preferably carried out by immersing the laminate in an aqueous boric acid solution (stretching in boric acid water). By using an aqueous boric acid solution as the stretching bath, the PVA-based resin layer can be imparted with rigidity to withstand tension applied during stretching and water resistance that does not dissolve in water. Specifically, boric acid can form a tetrahydroxyborate anion in an aqueous solution and crosslink with a PVA-based resin through hydrogen bonding. As a result, rigidity and water resistance can be imparted to the PVA-based resin layer, which can be satisfactorily stretched, and a polarizing film having excellent optical properties can be produced.

The boric acid aqueous solution is preferably obtained by dissolving boric acid and/or a borate salt in water as a solvent. The boric acid concentration is preferably 1 part by weight to 10 parts by weight, more preferably 2.5 parts by weight to 6 parts by weight, and particularly preferably 3 parts by weight to 5 parts by weight with respect to 100 parts by weight of water. is. By setting the boric acid concentration to 1 part by weight or more, the dissolution of the PVA-based resin layer can be effectively suppressed, and a polarizing film with higher properties can be produced. In addition to boric acid or borate salts, an aqueous solution obtained by dissolving a boron compound such as borax, glyoxal, glutaraldehyde, or the like in a solvent can also be used.

Preferably, an iodide is added to the drawing bath (boric acid aqueous solution). By blending iodide, elution of iodine adsorbed on the PVA-based resin layer can be suppressed. Specific examples of iodides are as described above. The concentration of iodide is preferably 0.05 to 15 parts by weight, more preferably 0.5 to 8 parts by weight, per 100 parts by weight of water.

The stretching temperature (liquid temperature of the stretching bath) is preferably 40°C to 85°C, more preferably 60°C to 75°C. At such a temperature, the film can be stretched at a high magnification while suppressing dissolution of the PVA-based resin layer. Specifically, as described above, the glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 60° C. or higher in relation to the formation of the PVA-based resin layer. In this case, if the stretching temperature is lower than 40° C., it may not be possible to stretch well even if the plasticization of the thermoplastic resin base material by water is considered. On the other hand, the higher the temperature of the stretching bath, the higher the solubility of the PVA-based resin layer, which may make it impossible to obtain excellent optical properties. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

The draw ratio by underwater drawing is preferably 1.5 times or more, more preferably 3.0 times or more. The total draw ratio of the laminate is preferably 5.0 times or more, more preferably 5.5 times or more, relative to the original length of the laminate. By achieving such a high draw ratio, a polarizing film with extremely excellent optical properties can be produced. Such a high draw ratio can be achieved by adopting an underwater drawing method (boric acid solution drawing).

C-5. Introduction of High Boiling Alcohol In an embodiment of the present invention, a high boiling alcohol is introduced after the underwater stretching treatment (and typically before the drying shrinkage treatment described below). Introduction of the high boiling alcohol can be done in any suitable manner. For example, the laminate may be immersed in a treatment liquid containing a high boiling point alcohol, or the surface of the polarizing film of the laminate may be coated with a treatment liquid containing a high boiling point alcohol. Typically, the introduction of the high boiling point alcohol can be done by immersion. Immersion can be done in any suitable manner. For example, a high-boiling point alcohol may be added to the washing bath for the cleaning treatment to serve as the treatment liquid bath, the treatment liquid bath may be used instead of the washing bath, or the treatment liquid bath may be provided separately from the washing bath. good too. Typically, a high boiling point alcohol may be added to the cleaning bath (cleaning liquid) of the cleaning process. The high boiling point alcohol concentration of the treatment liquid (cleaning liquid) is preferably 0.03% by weight to 1.0% by weight.

C-6. Drying Shrinkage Treatment The drying shrinkage treatment may preferably be performed after the introduction of the high boiling point alcohol. By performing the drying shrinkage treatment after introducing the high boiling point alcohol, the high boiling point alcohol functions as a plasticizer during the drying shrinkage treatment, and the flexibility of the finally obtained polarizing film can be improved.

The drying shrinkage treatment may be performed by zone heating performed by heating the entire zone, or may be performed by heating the transport roll (using a so-called heating roll) (heating roll drying method). Preferably both are used. By drying using a heating roll, it is possible to efficiently suppress heat curling of the laminate and produce a polarizing film having an excellent appearance. Specifically, by drying the laminated body along the heating roll, the crystallization of the thermoplastic resin substrate can be efficiently promoted to increase the degree of crystallinity, which is relatively low. Even at the drying temperature, the degree of crystallinity of the thermoplastic resin substrate can be favorably increased. As a result, the thermoplastic resin base material has increased rigidity and is in a state capable of withstanding shrinkage of the PVA-based resin layer due to drying, thereby suppressing curling. Moreover, by using a heating roll, the layered product can be dried while being maintained in a flat state, so that not only curling but also wrinkling can be suppressed. At this time, the laminate can be shrunk in the width direction by drying shrinkage treatment, thereby improving the optical properties. This is because the orientation of PVA and PVA/iodine complex can be effectively enhanced. The shrinkage ratio of the laminate in the width direction due to drying shrinkage treatment is preferably 1% to 10%, more preferably 2% to 8%, and particularly preferably 4% to 6%.

FIG. 2 is a schematic diagram showing an example of drying shrinkage treatment. In the drying shrinkage process, the laminate 200 is dried while being transported by transport rolls R1 to R6 heated to a predetermined temperature and guide rolls G1 to G4. In the illustrated example, the transport rolls R1 to R6 are arranged so as to alternately and continuously heat the surface of the PVA resin layer and the surface of the thermoplastic resin substrate. The transport rolls R1 to R6 may be arranged so as to continuously heat only the surface of the resin base material.

The drying conditions can be controlled by adjusting the heating temperature of the transport rolls (the temperature of the heating rolls), the number of heating rolls, the contact time with the heating rolls, and the like. The temperature of the heating roll is preferably 60°C to 120°C, more preferably 65°C to 100°C, and particularly preferably 70°C to 80°C. The degree of crystallinity of the thermoplastic resin can be favorably increased, curling can be favorably suppressed, and an optical laminate having extremely excellent durability can be produced. The temperature of the heating roll can be measured with a contact thermometer. In the illustrated example, six transport rolls are provided, but there is no particular limitation as long as the number of transport rolls is plural. Conveying rolls are usually 2 to 40, preferably 4 to 30 in number. The contact time (total contact time) between the laminate and the heating roll is preferably 1 to 300 seconds, more preferably 1 to 20 seconds, still more preferably 1 to 10 seconds.

The heating roll may be provided in a heating furnace (for example, oven), or may be provided in a normal production line (under room temperature environment). Preferably, it is provided in a heating furnace equipped with air blowing means. By using both heating roll drying and hot air drying, abrupt temperature changes between the heating rolls can be suppressed, and shrinkage in the width direction can be easily controlled. The temperature for hot air drying is preferably 30°C to 100°C. The hot air drying time is preferably 1 second to 300 seconds. The wind speed of the hot air is preferably about 10m/s to 30m/s. The wind speed is the wind speed in the heating furnace and can be measured with a mini-vane digital anemometer.

C-7. Others The thermoplastic resin substrate/polarizing film laminate obtained as described above may be used as it is as a polarizing plate (the thermoplastic resin substrate may be used as a protective layer); After laminating the protective layer on the surface of the film, the thermoplastic resin substrate may be peeled off and used as a polarizing plate having a structure of protective layer/polarizing film; It may be used as a polarizing plate having a structure of protective layer/polarizing film/protective layer by laminating layers.

EXAMPLES The present invention will be specifically described below with reference to Examples, but the present invention is not limited to these Examples. The measurement method of each characteristic is as follows. "Parts" and "%" in Examples and Comparative Examples are by weight unless otherwise specified.

(1) Thickness Measured using an interference film thickness meter (manufactured by Otsuka Electronics Co., Ltd., product name: "MCPD-3000").

(2) Alcohol Concentration of Polarizing Film About 0.02 g of the polarizing films obtained in Examples and Comparative Examples were freeze-ground and collected in a screw tube, and 0.5 ml of methanol was added for permeation extraction overnight or more, followed by extraction. The liquid was filtered with a 0.45 μm membrane filter, and 1 μL of the filtrate was injected into a gas chromatograph (manufactured by Agilent Technologies, product name “6890N”), and the following calibration curve was used from the peak area corresponding to each alcohol type. The alcohol content was calculated.
Glycerin calibration curve y=5.666E ⁻⁰¹ x + 1.833E ⁻⁰⁰
Ethylene glycol calibration curve y = 2.478E ^-01 x + 1.315E ^-00

(3) Single Transmittance For the polarizing plates (protective film/polarizing film) of Examples and Comparative Examples, the single transmittance Ts measured using an ultraviolet-visible spectrophotometer (LPF-200 manufactured by Otsuka Electronics) was Transmittance.

(4) Shrinkability A test sample having a size of 10 cm in the direction of the absorption axis and 10 cm in the direction of the transmission axis was cut out from the polarizing plate (protective film/polarizing film) of Examples and Comparative Examples. This test sample is attached to a glass plate via an adhesive, and a CNC image measuring machine (Mitutoyo QickVision (QV606)) is used to measure 8 points including the film edge as shown in FIG. and accurately measured dimensions. After that, it was placed in a heating oven (85 ° C.) for 120 hours, removed from the oven, and the dimensions were accurately measured again. evaluated.
◎: The shrinkage rate is smaller than that of Comparative Example 1 (the absolute value in the negative direction is smaller)
△: Shrinkage ratio equivalent to that of Comparative Example 1 ×: Shrinkage ratio larger than that of Comparative Example 1 (larger absolute value in the negative direction)

(5) Cracks (nano slits): guitar pick test The polarizing plates (protective film/polarizing film) of Examples and Comparative Examples were cut into a size of 50 mm × 150 mm (absorption axis direction is 50 mm) and placed on the side of the protective film. , a surface protective film prepared by the following method was laminated.
(Surface protection film for test samples)
In a four-neck flask equipped with a stirring blade, a thermometer, a nitrogen gas inlet tube, and a condenser, 94 parts by mass of 2-ethylhexyl acrylate (2EHA), 1 part by mass of N,N-diethylacrylamide (DEAA), ethoxydiethylene glycol acrylate ( 1 part by mass of EDE), 4 parts by mass of 4-hydroxybutyl acrylate (HBA), 0.2 parts by mass of 2,2'-azobisisobutyronitrile as a polymerization initiator, and 150 parts by mass of ethyl acetate were charged and gently stirred. Nitrogen gas was introduced while maintaining the liquid temperature in the flask at about 60° C., and the polymerization reaction was carried out for 5 hours to prepare an acrylic polymer solution (40 mass %). The acrylic polymer had a weight average molecular weight of 570,000 and a glass transition temperature (Tg) of -68°C. The acrylic polymer solution (40% by mass) was diluted with ethyl acetate to 20% by mass, and 500 parts by mass of this solution (solid content: 100 parts by mass) was added to an isocyanurate form of hexamethylene diisocyanate (manufactured by Nippon Polyurethane Industry Co., Ltd., Coronate HX: C/HX) 2 parts by mass (solid content 2 parts by mass) and 2 parts by mass of dibutyltin dilaurate (1% by mass ethyl acetate solution) as a cross-linking catalyst (solid content 0.02 parts by mass) were added and mixed and stirred. to prepare an acrylic pressure-sensitive adhesive solution. The acrylic adhesive solution is applied to a 38 μm thick transparent polyethylene terephthalate (PET) film (polyester film) and heated at 130° C. for 1 minute to form a 15 μm thick adhesive layer to protect the surface. A film was produced.

Next, the polarizing plate to which the above surface protection film was attached was attached on a glass plate via an adhesive to prepare a test sample. A load of 200 g was applied to the central portion of this test sample (on the side of the surface protective film) using a guitar pick (manufactured by HISTORY, model number “HP2H (HARD)”) as shown in FIG. The load was applied 50 times reciprocatingly at a distance of 100 mm in the direction to the left. The load was applied at one point. Moreover, the load application was performed at a high speed (5 m/min) and a low speed (1 m/min), respectively. Then, after leaving the test sample in an environment of 80° C. for 1 hour, the number of cracks generated was counted using a differential interference contrast microscope. Using Comparative Example 1 as a reference, evaluation was performed as follows.
◎: The number is smaller than that of Comparative Example 1 △: The number is the same as that of Comparative Example 1 ×: The number is larger than that of Comparative Example 1

[Example 1]
As a thermoplastic resin substrate, a long amorphous isophthalic copolymerized polyethylene terephthalate film (thickness: 100 μm) having a Tg of about 75° C. was used. Corona treatment was applied to one side of the resin substrate.
Polyvinyl alcohol (degree of polymerization: 4,200, degree of saponification: 99.2 mol%) and acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOSEFIMER Z410") mixed at 9:1: 100 weight of PVA-based resin 13 parts by weight of potassium iodide was added to 10 parts by weight to prepare a PVA aqueous solution (coating liquid).
The above PVA aqueous solution was applied to the corona-treated surface of the resin base material and dried at 60° C. to form a PVA-based resin layer having a thickness of 13 μm, thereby producing a laminate.
The obtained laminate was uniaxially stretched 2.4 times at the free end in the machine direction (longitudinal direction) between rolls with different peripheral speeds in an oven at 130° C. (in-air auxiliary stretching treatment).
Next, the laminate was immersed in an insolubilizing bath (an aqueous boric acid solution obtained by mixing 4 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 40° C. for 30 seconds (insolubilizing treatment).
Next, the finally obtained polarizing film is placed in a dyeing bath (iodine aqueous solution obtained by blending iodine and potassium iodide at a weight ratio of 1:7 with respect to 100 parts by weight of water) at a liquid temperature of 30 ° C. It was immersed for 60 seconds while adjusting the concentration so that the single transmittance (Ts) was 45.0%±0.2% (dyeing treatment).
Next, it was immersed for 30 seconds in a cross-linking bath at a liquid temperature of 40°C (an aqueous solution of boric acid obtained by blending 3 parts by weight of potassium iodide and 5 parts by weight of boric acid with respect to 100 parts by weight of water). (crosslinking treatment).
After that, while immersing the laminate in an aqueous solution of boric acid (boric acid concentration: 4.0% by weight, potassium iodide: 5.0% by weight) at a liquid temperature of 70°C, the laminate is moved between rolls with different peripheral speeds in the longitudinal direction (longitudinal direction). ) was uniaxially stretched so that the total draw ratio was 5.5 times (underwater stretching treatment).
After that, the laminate was immersed in a treatment bath (aqueous solution of 4% by weight of potassium iodide and 0.23% by weight of glycerin) at a liquid temperature of 20°C to wash the laminate and apply glycerin to the PVA-based resin layer (polarizing film). introduced (wash treatment and introduction of glycerin).
After that, it was dried for 4 minutes in an oven kept at 90°C (drying treatment).
Thus, a polarizing film having a thickness of 5.0 μm was formed on the resin substrate. A cycloolefin film (manufactured by ZEON, product name “G-Film”) as a protective layer (protective film) is attached to the surface of the polarizing film with a UV curable adhesive (thickness 1.0 μm), and then a resin base is attached. A polarizing plate having a structure of protective layer/polarizing film was obtained by peeling off the material. The glycerin concentration in the polarizing film of the obtained polarizing plate was 0.30% by weight.

Table 1 shows the single transmittance of the obtained polarizing plate (substantially, polarizing film). Furthermore, Table 1 also shows the evaluation results of (4) and (5) above.

[Example 2]
A polarizing plate was produced in the same manner as in Example 1, except that the concentration of glycerin in the treatment bath was 0.68% by weight. The obtained polarizing plate (or polarizing film) was subjected to the same evaluation as in Example 1. Table 1 shows the results.

[Example 3]
A polarizing plate (protective film/polarizing film) was produced in the same manner as in Example 1. This polarizing plate was cut into a size of 10 cm in the direction of the absorption axis×10 cm in the direction of the transmission axis, and the cut piece was used as a test sample as it was (that is, without being attached to a glass plate via an adhesive). For this test sample, a CNC image measuring machine (QickVision (QV606) manufactured by Mitutoyo Co.) was used to measure four points without film edges as shown in FIG. After that, it was placed in a heating oven (85° C.) for 120 hours, then taken out of the oven and measured again for accurate dimensions. The shrinkage rate in the absorption axis direction was calculated from the initial length, and evaluated as follows based on Comparative Example 2 (described later). Table 1 shows the results.
◎: The shrinkage rate is smaller than that of Comparative Example 2 (the absolute value in the negative direction is smaller)
△: Shrinkage ratio equivalent to that of Comparative Example 2 ×: Shrinkage ratio larger than that of Comparative Example 2 (larger absolute value in the negative direction)

[Example 4]
A polarizing plate was produced in the same manner as in Example 2. The obtained polarizing plate was evaluated in the same manner as in Example 3. Table 1 shows the results.

[Example 5]
A laminate of resin base material/PVA-based resin layer was produced in the same manner as in Example 1, and the laminate was subjected to an in-air auxiliary stretching treatment in the same manner as in Example 1.
Next, the laminate was cut into 15 cm × 10 cm in the auxiliary stretching axis direction, and the short side of the sample was fixed with a special stretching jig. It was immersed in an aqueous boric acid solution obtained by blending 3 parts by weight for 30 seconds (insolubilization treatment).
Next, the finally obtained polarizing film is placed in a dyeing bath (iodine aqueous solution obtained by blending iodine and potassium iodide at a weight ratio of 1:7 with respect to 100 parts by weight of water) at a liquid temperature of 30 ° C. It was immersed for 60 seconds while adjusting the concentration so that the single transmittance (Ts) was 42.0%±0.2% (dyeing treatment).
Next, it was immersed for 30 seconds in a cross-linking bath at a liquid temperature of 30°C (an aqueous solution of boric acid obtained by blending 3 parts by weight of potassium iodide and 5 parts by weight of boric acid with respect to 100 parts by weight of water). (crosslinking treatment).
After that, the laminate is immersed in an aqueous solution of boric acid (boric acid concentration: 4.0% by weight, potassium iodide: 5.0% by weight) at a liquid temperature of 70°C, and the total stretching ratio is 5 in the longitudinal direction (longitudinal direction). The film was uniaxially stretched to 0.5 times (stretching treatment in water).
After that, the laminate is immersed in a treatment bath (aqueous solution of 3% by weight of potassium iodide and 1% by weight of ethylene glycol) at a liquid temperature of 20° C. for 3 seconds to wash the laminate and to wash the PVA-based resin layer (polarizing film) with ethylene. Glycol was introduced (wash treatment and introduction of ethylene glycol).
After that, it was dried for 4 minutes in an oven kept at 60°C (drying treatment).
Thus, a polarizing film having a thickness of 5.0 μm was formed on the resin substrate. A cycloolefin film (manufactured by ZEON, product name “G-Film”) as a protective layer (protective film) is attached to the surface of the polarizing film with a UV curable adhesive (thickness 1.0 μm), and then a resin base is attached. A polarizing plate having a structure of protective layer/polarizing film was obtained by peeling off the material.

A test sample having a size of 4 cm in the direction of the absorption axis and 4 cm in the direction of the transmission axis was cut out from the obtained polarizing plate. This test sample was attached to a glass plate via an adhesive, and four points without film edges were measured using a CNC image measuring machine (QickVision (QV606) manufactured by Mitutoyo Co., Ltd.) as shown in FIG. 3(b). length and accurately measured dimensions. After that, it was placed in a heating oven (85 ° C.) for 120 hours, removed from the oven, and the dimensions were accurately measured again. evaluated in this way. Table 1 shows the results.
◎: The shrinkage rate is smaller than that of Comparative Example 3 (the absolute value in the negative direction is smaller)
△: Shrinkage ratio equivalent to that of Comparative Example 3 ×: Shrinkage ratio larger than that of Comparative Example 3 (larger absolute value in the negative direction)
In addition, evaluation of (5) was not performed.

[Example 6]
A polarizing plate was produced in the same manner as in Example 5, except that the ethylene glycol concentration in the treatment bath was 3% by weight. The obtained polarizing plate (or polarizing film) was evaluated in the same manner as in Example 5. Table 1 shows the results.

[Comparative Example 1]
A polarizing plate was produced in the same manner as in Example 1, except that the high boiling point alcohol was not added to the cleaning bath (cleaning liquid). The obtained polarizing plate (or polarizing film) was subjected to the same evaluation as in Example 1. Table 1 shows the results.

[Comparative Example 2]
A polarizing plate was produced in the same manner as in Example 3, except that the high boiling point alcohol was not added to the cleaning bath (cleaning liquid). The obtained polarizing plate (or polarizing film) was evaluated in the same manner as in Example 3. Table 1 shows the results.

[Comparative Example 3]
A polarizing plate was produced in the same manner as in Example 5, except that the high boiling point alcohol was not added to the cleaning bath (cleaning liquid). The obtained polarizing plate (or polarizing film) was evaluated in the same manner as in Example 5. Table 1 shows the results.

As is clear from Table 1, the polarizing plates (polarizing films) of the examples of the present invention contain a predetermined amount of high-boiling-point alcohol, thereby suppressing shrinkage in a high-temperature environment. Furthermore, the polarizing plates (polarizing films) of Examples 1 and 2 are remarkably suppressed in cracking as compared with Comparative Example 1.

The polarizing film and polarizing plate of the present invention are suitable for use in liquid crystal display devices.

REFERENCE SIGNS LIST 10 polarizing film 20 first protective layer 30 second protective layer 100 polarizing plate

Claims (4)

A method for producing a polarizing film comprising a polyvinyl alcohol resin film containing iodine and containing 0.30 % to 1.0% by weight of an alcohol having a boiling point of 100° C. or higher,
forming a polyvinyl alcohol-based resin layer on one side of a long thermoplastic resin substrate to form a laminate;
The laminate is subjected in this order to an in-air auxiliary stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment for shrinking the laminate by 2% or more in the width direction by heating while being transported in the longitudinal direction, After the polyvinyl alcohol-based resin layer is used as a polarizing film, and after the underwater stretching treatment and before the drying shrinkage treatment, the polyvinyl alcohol-based resin layer is brought into contact with a treatment liquid containing alcohol having a boiling point of 100° C. or higher. introducing an alcohol having a boiling point of 100° C. or higher into the polyvinyl alcohol-based resin layer;
including
the concentration of the alcohol having a boiling point of 100° C. or higher in the treatment liquid is 0.23 % by weight to 3.0 % by weight ;
the alcohol having a boiling point of 100° C. or higher is glycerin or ethylene glycol ;
Production method. 2. The manufacturing method according to claim 1, comprising immersing the polyvinyl alcohol-based resin layer in the treatment liquid. 3. The manufacturing method according to claim 1, further comprising heating the laminate after introducing the alcohol having a boiling point of 100[deg.] C. or higher into the polyvinyl alcohol-based resin layer. A coating liquid containing 100 parts by weight of polyvinyl alcohol resin and 5 to 20 parts by weight of iodide or sodium chloride is applied to one side of the elongated thermoplastic resin substrate and dried to obtain iodide or The manufacturing method according to any one of claims 1 to 3, comprising forming a polyvinyl alcohol-based resin layer containing sodium chloride and a polyvinyl alcohol-based resin to form the laminate.

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