JP7165813B2 - 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 as described above has a problem that color loss tends to occur at the edges (particularly at the edges in the width direction) in a high-humidity environment.

JP-A-2001-343521

SUMMARY OF THE INVENTION The present invention has been made to solve the conventional problems described above, and a main object thereof is to provide a polarizing film that suppresses color loss at the edges in a high-humidity environment.

According to one aspect of the present invention, the following formula (1 ) is provided.
y≧−0.06x+2.88 (1)
In one embodiment, the polarizing film has a thickness of 8 μm or less.
In one embodiment, the single transmittance of the polarizing film is 41% to 43%, and the orientation function of the polyvinyl alcohol resin film is 0.30 to 0.45.
In one embodiment, the single transmittance of the polarizing film is more than 43% and 45% or less, and the orientation function of the polyvinyl alcohol resin film is 0.20 to 0.35.
In one embodiment, the single transmittance of the polarizing film is more than 45% and 47% or less, and the orientation function of the polyvinyl alcohol resin film is 0.15 to 0.25.
According to another aspect of the present invention, there is provided a polarizing plate including the 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, a method for manufacturing a polarizing film is provided. A method for producing a polarizing film includes forming a laminate by forming a polyvinyl alcohol-based resin layer containing a halide and a polyvinyl alcohol-based resin on one side of a long thermoplastic resin substrate, and forming the laminate. 2, aerial auxiliary stretching treatment, dyeing treatment, underwater stretching treatment, and drying shrinkage treatment to shrink 2% or more in the width direction by heating while conveying in the longitudinal direction, in this order. The stretching ratio of the auxiliary in-air stretching treatment is 2.6 to 4.0 times the original length of the laminate, and the ratio of the stretching ratio of the auxiliary in-air stretching treatment to the stretching ratio of the underwater stretching treatment is , from 120% to 300%.
In one embodiment, the total stretching ratio of the auxiliary in-air stretching treatment and the underwater stretching treatment is 5.0 times or more with respect to the original length of the laminate.

According to the polarizing film of the present invention, by controlling the orientation of the polyvinyl alcohol-based resin layer, it is possible to suppress color loss at the edges in a high-humidity environment.

1 is a schematic cross-sectional view of a polarizer according to one embodiment of the invention; FIG. 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. It is a figure which shows the relationship between the single transmittance|permeability, the orientation function of a PVA-type resin layer, and Formula (1). It is a graph which shows the result of color loss evaluation.

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. It satisfies the following formula (1). A polarizing film that satisfies the formula (1) has excellent single transmittance and can suppress color loss at the edges in a high-humidity environment. From the viewpoint of securing a practically sufficient single transmittance, it is preferable that the above y further satisfies the following formula (2).
y≧−0.06x+2.88 (1)
y≦−0.06x+3.40 (2)

The polarizing film is composed of a PVA-based resin film containing iodine, as described above. Examples of PVA-based resins 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 having 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.

The PVA-based resin preferably contains an acetoacetyl-modified PVA-based resin. With such a configuration, a polarizing film having desired mechanical strength can be obtained. The amount of the acetoacetyl-modified PVA-based resin is preferably 5-20% by weight, more preferably 8-12% by weight, when the total PVA-based resin is 100% by weight. . If the blending amount is within such a range, a polarizing film having better mechanical strength can be obtained.

The thickness of the polarizing film is preferably 8 μm or less, more preferably 7 μm or less, even more preferably 5 μm or less, and particularly 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. Such a thickness can be achieved, for example, by producing a polarizing film using a laminate of a thermoplastic resin substrate and a PVA-based resin layer formed by coating on the thermoplastic resin substrate, as described later. obtain.

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 40.0% or more, more preferably 41.0% or more. The upper limit of single transmittance can be, for example, 49.0%. The single transmittance of the polarizing film is 40.0% to 47.0% in one embodiment. The polarization degree of the polarizing film is preferably 99.0% or more, more preferably 99.4% or more. The upper limit of the degree of polarization can be, for example, 99.999%. The polarization degree of the polarizing film is 99.0% to 99.9% in one embodiment. 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 determined by the following formula based on the parallel transmittance Tp and the orthogonal transmittance Tc, which are measured using an ultraviolet-visible spectrophotometer and subjected to visibility correction.
Degree of polarization (%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 × 100

In the PVA-based resin film that constitutes the polarizing film, the PVA-based resin is oriented along the absorption axis direction. In general, there is a negative correlation between the single transmittance of a polarizing film and the orientation of a PVA-based resin film constituting the polarizing film, and the single transmittance tends to decrease as the orientation of the PVA-based resin film increases. . In contrast, the polarizing film of the present invention has an improved balance between the single transmittance and the orientation of the PVA-based resin film. The orientation of the PVA-based resin film is improved. By improving the orientation of the PVA-based resin film that constitutes the polarizing film, the elution of iodine from the edges is suppressed, and as a result, color loss can be suitably suppressed.

In one embodiment, the single transmittance of the polarizing film is 41% to 43%, and the orientation function of the PVA-based resin film constituting the polarizing film is preferably 0.30 to 0.45, more preferably 0.35 to 0.45.

In another embodiment, the single transmittance of the polarizing film is more than 43% and 45% or less, and the orientation function of the PVA-based resin film that constitutes the polarizing film is preferably 0.20 to 0.35, more than It is preferably 0.25 to 0.35.

In still another embodiment, the single transmittance of the polarizing film is more than 45% and 47% or less, and the orientation function of the PVA-based resin film constituting the polarizing film is preferably 0.15 to 0.25, It is more preferably 0.18 to 0.25.

The orientation function (y) is obtained by attenuated total reflection (ATR) measurement using, for example, a Fourier transform infrared spectrophotometer (FT-IR) and polarized light as measurement light. Specifically, the measurement is performed with the stretching direction of the polarizing film parallel and perpendicular to the polarization direction of the measurement light, and the intensity at 2941 cm ^-1 of the obtained absorbance spectrum is used to calculate according to the following formula. be done. Here, the intensity I is a value of 2941 cm ^-1 /3330 cm ^-1 with 3330 cm ^-1 as a reference peak. When y=1, the orientation is perfect, and when y=0, the orientation is random. The peak at 2941 cm ⁻¹ is considered to be absorption due to vibration of the PVA main chain (—CH ₂ —) in the polarizing film.
y=(3<cos ² θ>−1)/2
=(1−D)/[c(2D+1)]
=-2 x (1-D)/(2D+1)
however,
For c=(3cos ² β−1)/2 and a vibration of 2941 cm ⁻¹ β=90°.
θ: Molecular chain angle with respect to the stretching direction β: Transition dipole moment angle with respect to the molecular chain axis D=(I _⊥ )/(I _// ) (In this case, D increases as the PVA molecules are oriented)
I _⊥ : Absorption intensity when the polarization direction of the measurement light is perpendicular to the stretching direction of the polarizing film I _// : Absorption intensity when the polarization direction of the measurement light is parallel to the stretching direction of the polarizing film

B. Polarizers FIGS. 1A and 1B are each schematic cross-sectional views of polarizers according to one embodiment of the present invention. The polarizing plate 100 a shown in FIG. 1A includes a polarizing film 10 and a first protective layer 20 disposed on one side of the polarizing film 10 . Polarizing plate 100b shown in FIG. 30 and The polarizing film 10 is the polarizing film described in Section A. One of the first protective layer and the second protective layer may be a thermoplastic resin substrate used in the manufacture of polarizing films. A method for manufacturing the polarizing film will be described in detail in Section C.

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 is applied to an image display device, the protective layer (outer protective layer) disposed on the side opposite to the display panel typically has a thickness of 300 µm or less, preferably 100 µm or less, and more preferably 5 µm. ~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.

The thickness of the protective layer (inner protective layer) disposed on the display panel side when the polarizing plate is applied to an image display device is preferably 5 μm to 200 μm, more preferably 10 μm to 100 μm, and still more preferably 10 μm to 60 μm. . 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 A method for producing a polarizing film according to one embodiment of the present invention includes forming a laminate by forming a PVA-based resin layer on one side of a long thermoplastic resin substrate, and It includes subjecting the body to an in-air auxiliary stretching treatment, a dyeing treatment, and an underwater stretching treatment in this order. Preferably, the method for producing a polarizing film includes forming a PVA-based resin layer on one side of a long thermoplastic resin base material to form a laminate, and subjecting the laminate to an in-air auxiliary stretching treatment and dyeing. treatment, underwater stretching treatment, and drying shrinkage treatment for shrinking in the width direction by 2% or more by heating while conveying in the longitudinal direction, in this order. In the production method of the present embodiment, the stretching ratio of the auxiliary stretching treatment in the air is 2.6 to 4.0 times the original length of the laminate. Also, the ratio of the draw ratio of the auxiliary drawing treatment in air to the draw ratio of the drawing treatment in water is 120% to 300%. Preferably, the total draw ratio of the in-air auxiliary drawing treatment and the underwater drawing treatment (the product of the draw ratio of the in-air auxiliary drawing and the draw ratio of the underwater drawing, hereinafter also referred to as "total draw ratio") is the same as that of the laminate. It is at least 5.0 times the original length. According to the production method of the present embodiment, the polarizing film described in section A can be suitably obtained.

In the above manufacturing method, the proportion of the auxiliary in-air stretching treatment in the combination of the auxiliary in-air stretching treatment and the underwater stretching treatment is increased while ensuring a sufficient total stretching ratio for imparting good optical properties to the polarizing film. can be mentioned as one of the characteristics. During the underwater stretching process, the degree of orientation tends to vary in the thickness direction of the PVA-based resin layer. For example, the degree of orientation on the thermoplastic resin substrate side of the PVA-based resin layer tends to be lower than that on the exposed surface side. There is On the other hand, by increasing the draw ratio of the in-air auxiliary drawing, the laminate can be subjected to the underwater drawing treatment while the PVA-based resin is highly and uniformly oriented. As a result, even when the stretching ratio in the underwater stretching treatment is reduced, a sufficient total stretching ratio can be secured, and as a result, the variation in the degree of orientation in the thickness direction is suppressed without impairing the optical properties, and the PVA system A polarizing film in which the orientation of the resin layer as a whole is improved 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 PVA-based resin layer is formed on the thermoplastic resin substrate by applying a coating liquid containing the PVA-based resin to the surface of the thermoplastic resin substrate and drying the coating liquid.

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.

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. Polyester-based resins are preferably used, and polyethylene terephthalate-based resins are more preferably used.

The coating liquid is typically a solution in which a PVA-based resin is dissolved in a solvent. The PVA-based resin is as described in Section A. 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, the solvent is preferably water.

The concentration of the PVA-based resin in the coating liquid 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 coating liquid preferably further contains a halide. Any suitable 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 compounded in the coating solution 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. weight part. If the amount of the halide compounded 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.

The coating liquid may further contain additives. 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.

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-stage stretching, it is possible to stretch while suppressing crystallization of the thermoplastic resin base material. 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 auxiliary in-air stretching process includes a heated roll stretching step in which the laminate is stretched by a peripheral speed difference between heated rolls while being conveyed in the longitudinal direction. The auxiliary aerial stretching process typically includes a zone stretching process and a hot roll stretching 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, for example, 2.6 to 4.0 times, preferably 2.8 to 3.8 times, more preferably 3.0 to 3.6 times. If the stretch ratio of the in-air auxiliary stretching is within such a range, the PVA-based resin layer can be highly and uniformly oriented, and the orientation of the PVA-based resin layer after underwater stretching can be improved. .

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 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 while suppressing 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. In multistage stretching, the total stretching ratio is the product of the stretching ratios in each stage.

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 in the underwater drawing is preferably 1.3 times to 2.3 times, more preferably 1.4 times to 2.1 times, still more preferably 1.5 times to 1.9 times. . If the stretch ratio in the underwater stretching is within such a range, the orientation of the entire PVA-based resin layer can be improved while setting the total stretch ratio within a desired range when combined with the auxiliary stretching in the air. As a result, it is possible to obtain a polarizing film having excellent optical properties and suppressing color loss at the edges.

Typically, the draw ratio for the auxiliary drawing in the air is higher than the draw ratio for the underwater drawing. The ratio of the stretch ratio of auxiliary in-air stretching to the stretch ratio of underwater stretching (stretch ratio of auxiliary in-air stretching/stretch ratio by underwater stretching) is preferably 120% to 300%, more preferably 140% to 260%. , more preferably 160% to 230%. By setting it as such a ratio, the orientation of the whole PVA-type resin layer can be improved, setting a total draw ratio in a desired range. As a result, it is possible to obtain a polarizing film having excellent optical properties and suppressing color loss at the edges.

As described above, the total draw ratio of the auxiliary in-air drawing treatment and the water drawing treatment is preferably 5.0 times or more, more preferably 5.2 times to 7.0 times, relative to the original length of the laminate. Yes, more preferably 5.5 to 6.5 times. By achieving such a high draw ratio, a polarizing film with extremely excellent optical properties can be produced.

C-5. Drying Shrinkage Treatment The drying shrinkage treatment may be performed by zone heating performed by heating the entire zone, or by heating the transport roll (using a so-called heating roll) (heated 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 increases in its rigidity and becomes in a state capable of withstanding shrinkage of the PVA-based resin layer due to drying, thereby suppressing curling (warping). 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 plastic resin substrate surface).

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. Moreover, 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-6. Washing Treatment Preferably, washing treatment is performed after the underwater stretching treatment and before the drying shrinkage treatment. The cleaning treatment is typically performed by immersing the PVA-based resin layer in an aqueous solution of potassium iodide.

As described above, a laminate of a thermoplastic resin substrate and a polarizing film (PVA-based resin layer) can be obtained. The laminate can be used as a polarizing plate as it is without peeling off the thermoplastic resin substrate. In this case, the thermoplastic resin substrate functions as a protective layer for the polarizing film. Alternatively, the first protective layer (first protective layer (second 1 resin film)/polarizing film] can be produced and used as a polarizing plate. In addition, a second resin film is attached to the surface of the polarizing film of these laminates via an arbitrary appropriate adhesive layer, [first protective layer (thermoplastic resin substrate or first resin film) / polarizing film A polarizing plate having a structure of film/second protective layer (second resin film)] may also be produced.

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) Transmittance of a Single Unit Transmittance of a single unit of the polarizing film/protective layer laminate (polarizing plate) obtained in Examples and Comparative Examples was measured using an ultraviolet-visible spectrophotometer ("V-7100" manufactured by JASCO Corporation). The ratio (Ts) was measured and used as the Ts of the polarizing film. Single transmittance is a Y value measured with a 2-degree field of view (C light source) according to JIS Z8701 and subjected to visibility correction.
(3) Orientation function (y) of PVA-based resin layer
For the polarizing films obtained in Examples and Comparative Examples, a Fourier transform infrared spectrophotometer (FT-IR) (manufactured by Perkin Elmer, trade name: "Frontier") was used to measure polarized infrared light. As such, attenuated total reflection (ATR) was measured on the surface of the polarizing film. Germanium was used for the crystallites for adhering the polarizing film, and the incident angle of the measurement light was set at 45°. The orientation function was calculated according to the following procedure. The incident polarized infrared light (measurement light) is polarized light (s-polarized light) that oscillates parallel to the surface of the germanium crystal sample that is adhered, and the stretching direction of the polarizing film is set perpendicular to the polarization direction of the measurement light ( ⊥) and parallel (//), and each absorbance spectrum was measured. From the obtained absorbance spectrum, (2941 cm ^-1 intensity) I was calculated with reference to (3330 cm ^-1 intensity). I _⊥ is (2941 cm ⁻¹ intensity)/(3330 cm ⁻¹ intensity) obtained from the absorbance spectrum obtained when the stretching direction of the polarizing film is arranged perpendicular (⊥) to the polarization direction of the measurement light. Further, I _// is obtained from the absorbance spectrum obtained when the stretching direction of the polarizing film is arranged parallel (//) to the polarization direction of the measurement light (2941 cm ⁻¹ intensity)/(3330 cm ⁻¹ intensity). is. Here, (2941 cm ⁻¹ intensity) is the absorbance at 2941 cm ⁻¹ when 2770 cm ⁻¹ and 2990 cm ⁻¹ are the bottoms of the absorbance spectrum, and (3330 cm ⁻¹ intensity) is 2990 cm ⁻ Absorbance at 3330 cm ⁻¹ with ¹ and 3650 cm ⁻¹ as baseline. Using the obtained I _⊥ and I _// , the orientation function y was calculated according to Equation 1. When y=1, the orientation is perfect, and when y=0, the orientation is random. The peak at 2941 cm ⁻¹ is said to be absorption due to vibration of the PVA main chain (—CH ₂ —) in the polarizing film. The peak at 3330 cm ⁻¹ is said to be absorption due to vibration of hydroxyl groups of PVA.
(Formula 1) y=(3<cos2θ>−1)/2
=(1−D)/[c(2D+1)]
However, c=(3cos ² β−1)/2
Then, when 2941 cm ⁻¹ is used as described above, β=90°⇒y=−2×(1−D)/(2D+1).
θ: the angle of the molecular chain with respect to the stretching direction β: the angle of the transition dipole moment with respect to the molecular chain axis D=( _{I⊥)/(I// )}
I _⊥ : Absorption intensity when the polarization direction of the measurement light is perpendicular to the stretching direction of the polarizing film I _// : Absorption intensity when the polarization direction of the measurement light is parallel to the stretching direction of the polarizing film

[Example 1]
A long amorphous isophthalic copolymerized polyethylene terephthalate film (thickness: 100 μm) having a Tg of about 75° C. was used as the thermoplastic resin substrate, and one side of the resin substrate was subjected to corona treatment.
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") were mixed at a ratio of 9:1, and 100 parts by weight of PVA-based resin. was added with 13 parts by weight of potassium iodide and dissolved in water to prepare an aqueous PVA solution (coating solution).
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 resulting laminate was uniaxially stretched 3.0 times in the machine direction (longitudinal direction) 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 45.1% (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% by weight, potassium iodide concentration: 5% by weight) at a liquid temperature of 70° C., the laminate was moved vertically (longitudinally) between rolls with different peripheral speeds. The film was uniaxially stretched so that the stretch ratio was 5.5 times (underwater stretching treatment: the stretching ratio in the underwater stretching treatment was 1.83 times).
After that, the laminate was immersed in a washing bath (aqueous solution obtained by blending 4 parts by weight of potassium iodide with 100 parts by weight of water) at a liquid temperature of 20° C. (washing treatment).
Thereafter, while drying in an oven maintained at about 90° C., it was brought into contact with a heating roll made of SUS whose surface temperature was maintained at about 75° C. (dry shrinkage treatment). The shrinkage rate in the width direction of the laminate due to the drying shrinkage treatment was 2%.
In this manner, a polarizing film having a thickness of about 5 μm was formed on the resin base material to obtain a polarizing plate having a structure of resin base material/polarizing film.
Furthermore, on the surface of the obtained polarizing film (the surface opposite to the resin substrate), a cycloolefin film (ZF-12, 23 μm, manufactured by Nippon Zeon Co., Ltd.) is coated with an ultraviolet curable adhesive as a protective substrate. pasted together through Specifically, the curable adhesive was applied so as to have a total thickness of about 1.0 μm, and was bonded using a roll machine. After that, UV rays were irradiated from the cycloolefin film side to cure the adhesive. Then, the resin substrate was peeled off to obtain a polarizing plate having a structure of cycloolefin film (protective substrate)/polarizing film.

[Example 2]
A polarizing plate was produced in the same manner as in Example 1, except that the concentration of the dyeing bath was adjusted so that the single transmittance of the polarizing film was 41.9%.

[Example 3]
A polarizing plate was produced in the same manner as in Example 1, except that the concentration of the dyeing bath was adjusted so that the single transmittance of the polarizing film was 45.3%.

[Example 4]
A polarizing plate was produced in the same manner as in Example 1, except that the concentration of the dyeing bath was adjusted so that the single transmittance of the polarizing film was 43.8%.

[Comparative Example 1]
The concentration of the dyeing bath was adjusted so that the single transmittance of the polarizing film was 41.8%, the stretching ratio in the air auxiliary stretching treatment was set to 2.4 times, and the stretching ratio in the underwater stretching treatment was set to A polarizing plate was produced in the same manner as in Example 1, except that the stretch ratio was 2.3 times (the total draw ratio was 5.5 times).

[Comparative Example 2]
A polarizing plate was produced in the same manner as in Comparative Example 1, except that the concentration of the dyeing bath was adjusted so that the single transmittance of the polarizing film was 42.7%.

[Comparative Example 3]
A polarizing plate was produced in the same manner as in Comparative Example 1, except that the concentration of the dyeing bath was adjusted so that the single transmittance of the polarizing film was 43.8%.

[Comparative Example 4]
A polarizing plate was produced in the same manner as in Comparative Example 1, except that the concentration of the dyeing bath was adjusted so that the single transmittance of the polarizing film was 45.0%.

Table 1 and FIG. 3 show the single transmittance of the polarizing plate (polarizing film) obtained in the above Examples and Comparative Examples, the orientation function of the PVA-based resin layer, and the relationship between these values and formula (1). .

≪Evaluation of color loss at the edges in the width direction≫
The polarizing plates obtained in Examples and Comparative Examples were cut into rectangular shapes, and glycerin was applied to the entire periphery. Then, the polarizing plate was placed under conditions of 65° C. and 90% RH for 72 hours. Next, at both ends of the polarizing film in the width direction (the direction perpendicular to the MD direction), the presence or absence of color loss was confirmed with an optical microscope, and the length of the color loss portion was measured. In addition, the length of the longest portion of color loss was defined as the color loss length, and the average value of the color loss lengths at both ends was determined as the color loss amount (μm). FIG. 4 shows the relationship between the single transmittance (Ts) and the amount of color loss for each polarizing plate.

As is clear from FIG. 4, when the polarizing plate (polarizing film) of the example and the polarizing plate (polarizing film) of the comparative example having the same single transmittance are compared, it is found that the polarizing plate (polarizing film) of the example has a higher It can be seen that color loss is suppressed. As shown in Table 1, in the polarizing plates of Examples, the orientation of the PVA-based resin layer (polarizing film) is higher than that of the polarizing plates of Comparative Examples having the same single transmittance. , it is presumed that the elution of iodine became difficult to occur.

The polarizing film of the present invention is 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 (7)

Consists of a polyvinyl alcohol resin film containing iodine,
When the single transmittance is x% and the orientation function of the polyvinyl alcohol resin film is y, the following formula (1) is satisfied,
y≧−0.06x+2.88 (1)
i) Single transmittance is more than 43% and 45% or less, and the orientation function of the polyvinyl alcohol resin film is 0.20 to 0.35, or
ii) the single transmittance is more than 45% and 47% or less, and the orientation function of the polyvinyl alcohol resin film is 0.15 to 0.25;
polarizing film. 2. The polarizing film according to claim 1, which has a thickness of 8 μm or less. A polarizing plate comprising the polarizing film according to claim 1 or 2 and a protective layer disposed on at least one side of the polarizing film. Consists of a polyvinyl alcohol resin film containing iodine,
A method for producing a polarizing film that satisfies the following formula (1), where x% is the single transmittance and y is the orientation function of the polyvinyl alcohol resin film,
y≧−0.06x+2.88 (1)
Forming a polyvinyl alcohol-based resin layer containing a halide and a polyvinyl alcohol-based resin on one side of a long thermoplastic resin substrate to form a laminate, and performing an in-air auxiliary stretching treatment on the laminate, A dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment that shrinks by 2% or more in the width direction by heating while conveying in the longitudinal direction, in this order.
The stretch ratio in the air auxiliary stretching treatment is 2.8 to 3.8 times the original length of the laminate,
The production method, wherein the ratio of the draw ratio of the air auxiliary drawing treatment to the draw ratio of the underwater drawing treatment is 120% to 300%. 5. The manufacturing method according to claim 4 , wherein the total stretching ratio of the auxiliary in-air stretching treatment and the underwater stretching treatment is 5.0 times or more with respect to the original length of the laminate. 6. The method for producing a polarizing film according to claim 4, which is a method for producing a polarizing film having a thickness of 8 [mu]m or less. A polarizing film having a single transmittance of 41% to 43% and an orientation function of the polyvinyl alcohol resin film of 0.30 to 0.45;
A polarizing film having a single transmittance of more than 43% and 45% or less, and an orientation function of the polyvinyl alcohol resin film of 0.20 to 0.35; or
7. The method for producing a polarizing film according to any one of claims 4 to 6, wherein the single transmittance is more than 45% and 47% or less, and the polyvinyl alcohol resin film has an orientation function of 0.15 to 0.25. The method for producing the polarizing film according to 1.

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