JP2022111165A - Polarizing film, polarizing plate, and method for producing the polarizing film - Google Patents

Abstract

A polarizing film in which breakage along the absorption axis direction is suppressed is provided. A polarizing film of the present invention comprises a polyvinyl alcohol resin film containing a dichroic substance, and has an orientation function of 0.30 or less. In one embodiment, the polarizing film has a thickness of 8 μm or less. The polarizing plate of the present invention has the above polarizing film and a protective layer arranged on at least one side of the polarizing film. [Selection drawing] Fig. 1

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 it is easily torn (easily broken) along the direction of the absorption axis.

JP-A-2001-343521

The present invention has been made to solve the above-described conventional problems, and a main object thereof is to provide a polarizing film in which breakage along the absorption axis direction is suppressed.

A polarizing film according to an embodiment of the present invention is composed of a polyvinyl alcohol-based resin film containing a dichroic material, and has an orientation function of 0.30 or less.
In one embodiment, the polarizing film has a thickness of 8 μm or less.
In one embodiment, the polarizing film has a single transmittance of 40.0% or more and a degree of polarization of 99.0% or more.
In one embodiment, the polarizing film has a puncture strength of 30 gf/μm or more.
A polarizing film according to another embodiment of the present invention is composed of a polyvinyl alcohol-based resin film containing a dichroic material, and has a puncture strength of 30 gf/μm or more.
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 another aspect of the present invention, there is provided a method for manufacturing the above polarizing film. This manufacturing method comprises forming a polyvinyl alcohol-based resin layer containing iodide or sodium chloride and a polyvinyl alcohol-based resin on one side of a long thermoplastic resin substrate to form a laminate; and the laminate. subjecting the body to an in-air auxiliary stretching process, a dyeing process, an underwater stretching process, and a drying shrinkage process in which the body shrinks by 2% or more in the width direction by heating while being transported in the longitudinal direction, in this order; . The total stretching ratio of the auxiliary in-air stretching treatment and the underwater stretching treatment is 3.0 to 4.5 times the original length of the laminate; It is larger than the draw ratio of the drawing process.

According to the present invention, by setting the orientation function to 0.30 or less, or by setting the puncture strength to 30 gf/μm or more, it is possible to realize a polarizing film in which breakage along the absorption axis direction is suppressed. can. Conventionally, it has been difficult to obtain acceptable optical properties (typically single transmittance and degree of polarization) with a polarizing film having such a small orientation function. A trade-off between orientation function and acceptable optical properties can be achieved. Furthermore, according to the present invention, it is possible to achieve both such a high puncture resistance and acceptable optical properties.

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.

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

A. Polarizing film A polarizing film according to an embodiment of the present invention is composed of a polyvinyl alcohol (PVA) resin film containing a dichroic substance (typically iodine or a dichroic dye), and has an orientation function of 0.30 or less. is. With such a configuration, it is possible to remarkably suppress the polarizing film from tearing (breaking) along the absorption axis direction. As a result, a polarizing film (and, as a result, a polarizing plate) having extremely excellent flexibility can be obtained. Such a polarizing film (and consequently a polarizing plate) can be preferably applied to a curved image display device, more preferably a foldable image display device, and even more preferably a foldable image display device. The orientation function is, for example, 0.25 or less, preferably 0.22 or less, more preferably 0.20 or less, still more preferably 0.18 or less, and particularly preferably 0.15 or less. . A lower bound on the orientation function can be, for example, 0.05. An orientation function that is too small may not provide acceptable unitary transmission and/or degree of polarization.

The orientation function (f) is determined by attenuated total reflection (ATR) measurement using 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 f=1, the orientation is perfect, and when f=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.
f=(3<cos ² θ>−1)/2
=(1−D)/[c(2D+1)]
=-2 x (1-D)/(2D+1)
however,
At c=(3cos ² β−1)/2, β=90° for a vibration of 2941 cm ⁻¹ .
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 measuring light and the stretching direction of the polarizing film are perpendicular I _// : Absorption intensity when the polarization direction of the measuring light and the stretching direction of the polarizing film are parallel

The thickness of the polarizing film is preferably 8 µm or less, more preferably 7 µm or less, still more preferably 5 µm or less, particularly preferably 3 µm or less, and particularly preferably 2 µm or less. The lower limit of the thickness of the polarizing film can be, for example, 1 μm. The thickness of the polarizing film is 2 μm to 6 μm in one embodiment, 2 μm to 4 μm in another embodiment, 2 μm to 3 μm in still another embodiment, and 5.5 μm to 7.5 μm in still another embodiment. It may be 5 μm, and in yet another embodiment between 6 μm and 7.2 μm. By making the thickness of the polarizing film very thin in this way, thermal shrinkage can be made very small. It is speculated that such a configuration can also contribute to suppression of breakage in the direction of the absorption axis.

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 45.0% 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 degree of polarization of the polarizing film is 99.0% to 99.99% in one embodiment. According to the present invention, although the orientation function is very small as described above, such practically acceptable single transmittance and degree of polarization can be realized. This is presumed to be due to the manufacturing method described later. The single transmittance is typically a Y value measured using an ultraviolet-visible spectrophotometer and subjected to visibility correction. 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

The polarizing film has a puncture strength of 30 gf/μm or more, preferably 35 gf/μm or more, more preferably 40 gf/μm or more, still more preferably 45 gf/μm or more, and particularly preferably 50 gf/μm or more. is. The upper limit of puncture strength can be, for example, 80 gf/μm. By setting the puncture strength of the polarizing film within such a range, it is possible to remarkably prevent the polarizing film from tearing along the absorption axis direction. As a result, a polarizing film (and, as a result, a polarizing plate) having extremely excellent flexibility can be obtained. The piercing strength indicates the cracking resistance of the polarizing film when the polarizing film is pierced with a predetermined strength. The piercing strength can be expressed, for example, as the strength (breaking strength) at which the polarizing film is broken when a predetermined needle is attached to a compression tester and the needle is pierced into the polarizing film at a predetermined speed. As is clear from the unit, the piercing strength means the piercing strength per unit thickness (1 μm) of the polarizing film.

The polarizing film is composed of a PVA-based resin film containing iodine, as described above. Preferably, the PVA-based resin constituting the PVA-based resin film (substantially, the polarizing film) contains an acetoacetyl-modified PVA-based resin. With such a configuration, a polarizing film having a desired puncture 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 in such a range, the puncture strength can be made in a more suitable range.

A polarizing film can typically 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 this embodiment, 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, the stretching preferably further includes in-air stretching of the laminate at a high temperature (eg, 95° C. or higher) before stretching in an aqueous boric acid solution. In the embodiment of the present invention, the total draw ratio is, for example, 3.0 times to 4.5 times, which is significantly smaller than usual. Even with such a total draw ratio, a polarizing film having acceptable optical properties can be obtained by combining the halide addition and drying shrinkage treatment. Furthermore, in the embodiment of the present invention, the draw ratio of the in-air auxiliary drawing is higher than the draw ratio of the boric acid aqueous drawing. With such a configuration, it is possible to obtain a polarizing film having acceptable optical properties even if the total draw ratio is small. In addition, the laminate is preferably subjected to drying shrinkage treatment to shrink the laminate by 2% or more in the width direction by heating while conveying in the longitudinal direction. In one embodiment, a method for manufacturing a polarizing film includes subjecting a laminate to an auxiliary air-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 having 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 (i.e., slow axis direction), and “ny” is the in-plane direction perpendicular to the slow axis (i.e., 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 comprises polyvinyl alcohol containing a halide and a polyvinyl alcohol resin (PVA resin) on one side of a long thermoplastic resin substrate. By forming a system resin layer (PVA system resin layer) to form a laminate, and by subjecting the laminate to an auxiliary stretching process in the air, a dyeing process, an underwater stretching process, and heating while conveying in the longitudinal direction. and drying shrinkage treatment for shrinking by 2% or more in the width direction. 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. 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 2 μm to 30 μm, more preferably 2 μm to 20 μm. By making the thickness of the PVA-based resin layer before stretching very thin as described above and by reducing the total stretching ratio as described later, an allowable single transmittance and A polarizing film having a degree of polarization can be obtained.

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. As described above, the PVA-based resin preferably contains an acetoacetyl-modified PVA-based resin.

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-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 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 1.0 to 4.0 times, more preferably 1.5 to 3.5 times, still more preferably 2.0 to 3.0 times. be. If the stretch ratio of the auxiliary in-air stretching is within such a range, the total stretching ratio can be set within a desired range when combined with the underwater stretching, and a desired orientation function can be achieved. As a result, it is possible to obtain a polarizing film in which breakage along the absorption axis direction is suppressed. Furthermore, as described above, the draw ratio of the in-air auxiliary drawing is larger than the draw ratio of the boric acid aqueous drawing. With such a configuration, it is possible to obtain a polarizing film having acceptable optical properties even if the total draw ratio is small.

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 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 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 1.0 to 3.0 times, more preferably 1.0 to 2.0 times, still more preferably 1.0 to 1.5 times. . If the draw ratio in the underwater drawing is within such a range, the total draw ratio can be set within a desired range, and a desired orientation function can be achieved. As a result, it is possible to obtain a polarizing film in which breakage along the absorption axis direction is suppressed. The total draw ratio (sum of draw ratios in the case of combining auxiliary in-air drawing and underwater drawing) is, as described above, for example 3.0 to 4.5 times the original length of the laminate. It is preferably 3.0 to 4.0 times, more preferably 3.0 to 3.5 times. By appropriately combining the addition of a halide to the coating liquid, the adjustment of the stretching ratios of the auxiliary stretching in the air and the stretching in water, and the drying shrinkage treatment, it is possible to obtain acceptable optical properties even at such a total stretching ratio. A polarizing film can be obtained.

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 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. 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. Other Treatments Preferably, a 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.

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"). The calculation wavelength range used for thickness calculation was 400 nm to 500 nm, and the refractive index was 1.53.
(2) Orientation function The polarizing films obtained in Examples and Comparative Examples were measured using a Fourier transform infrared spectrophotometer (FT-IR) (manufactured by Perkin Elmer, trade name: “Frontier”). Attenuated total reflection (ATR) of the surface of the polarizing film was measured using outside light as measurement light. 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 f was calculated according to Equation 1. When f=1, the orientation is perfect, and when f=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) f = (3<cos2θ>-1)/2
=(1−D)/[c(2D+1)]
However, c=(3cos ² β−1)/2
β=90°⇒f=−2×(1−D)/(2D+1) when 2941 cm ⁻¹ is used as described above.
θ: Angle of molecular chain with respect to stretching direction β: Angle of transition dipole moment with respect to 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 (3) Single transmittance and Degree of polarization The polarizing film is peeled off from the polarizing film/thermoplastic resin substrate laminate used in the examples and comparative examples, and the polarizing film is measured with an ultraviolet-visible spectrophotometer ("V-7100" manufactured by JASCO Corporation). The single transmittance Ts, the parallel transmittance Tp, and the orthogonal transmittance Tc measured using the polarizer were defined as Ts, Tp, and Tc of the polarizing film, respectively. These Ts, Tp and Tc are Y values measured with a 2-degree field of view (C light source) according to JIS Z8701 and subjected to visibility correction.
From the obtained Tp and Tc, the degree of polarization P was determined by the following formula.
Degree of polarization P (%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 × 100
Equivalent measurement can be performed with a spectrophotometer such as "LPF-200" manufactured by Otsuka Electronics Co., Ltd., and equivalent measurement results can be obtained using any spectrophotometer. has been confirmed.
(4) Breaking strength The polarizing film was peeled off from the polarizing film/thermoplastic resin substrate laminate used in the examples and comparative examples, and a compression tester equipped with a needle (manufactured by Kato Tech Co., Ltd., product name “NDG5” needle) Penetration force measurement specification), pierced at a piercing speed of 0.33 cm/sec in a room temperature (23°C ± 3°C) environment, and the strength when the polarizing film cracked was defined as breaking strength (piercing strength). As the evaluation value, the breaking strength of 10 test pieces was measured and the average value was used. The needle used had a tip diameter of 1 mmφ and 0.5R. For the polarizing film to be measured, a jig having a circular opening with a diameter of about 11 mm was fixed on both sides of the polarizing film, and a needle was inserted into the center of the opening to perform the test. Breaking strength per unit thickness was used as an index of resistance to tearing, and was evaluated according to the following criteria.
○: breaking strength exceeds 30 gf / μm ×: breaking strength is 30 gf / μm or less

[Example 1]
As a thermoplastic resin substrate, a long amorphous isophthalic copolymerized polyethylene terephthalate film (thickness: 100 μm) having a water absorption rate of 0.75% and a Tg of about 75° C. was used. Corona treatment (treatment condition: 55 W·min/m ² ) was applied to one side of the resin substrate.
Polyvinyl alcohol (degree of polymerization: 4200, 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 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 41.6% (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, 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 62° C., and is moved between rolls having different peripheral speeds in the longitudinal direction (longitudinal direction). ) was uniaxially stretched so that the total stretch ratio was 3.0 times (underwater stretching treatment: the stretching ratio in the underwater stretching treatment was 1.25 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).
After that, while drying in an oven kept at 90° C., it was brought into contact with a heating roll made of SUS whose surface temperature was kept at 75° C. for about 2 seconds (drying shrinkage treatment). The shrinkage rate in the width direction of the laminate due to the drying shrinkage treatment was 2%.
Thus, a polarizing film having a thickness of 7.1 μm was formed on the resin substrate.

Table 1 shows the orientation function, single transmittance, degree of polarization and breaking strength of the obtained polarizing film.

[Example 2]
A polarizing film having a thickness of 6.6 μm was produced in the same manner as in Example 1, except that the stretching ratio in the underwater stretching treatment was 1.45 times and the total stretching ratio was 3.5 times. The obtained polarizing film was subjected to the same evaluation as in Example 1. Table 1 shows the results.

[Example 3]
A polarizing film having a thickness of 6.1 μm was produced in the same manner as in Example 1, except that the stretching ratio in the underwater stretching treatment was 1.67 times and the total stretching ratio was 4.0 times. The obtained polarizing film was subjected to the same evaluation as in Example 1. Table 1 shows the results.

[Example 4]
A polarizing film having a thickness of 5.6 μm was produced in the same manner as in Example 1, except that the stretching ratio in the underwater stretching treatment was 1.87 times and the total stretching ratio was 4.5 times. The obtained polarizing film was subjected to the same evaluation as in Example 1. Table 1 shows the results.

[Comparative Example 1]
In the same manner as in Example 1, except that the stretching ratio in the underwater stretching treatment was 2.4 times, the total stretching ratio was 5.5 times, and the liquid temperature of the stretching bath was 70° C., a thickness of 5.5 mm was obtained. A polarizing film of 0 μm was produced. The width retention rate of the obtained polarizing film was 48% (the shrinkage rate in the width direction was 52%). The obtained polarizing film was subjected to the same evaluation as in Example 1. Table 1 shows the results.

[Comparative Example 2]
A polarizing film with a thickness of 5.5 μm was produced in the same manner as in Comparative Example 1, except that the width residual rate was 43% (the shrinkage rate in the width direction was 57%). The obtained polarizing film was subjected to the same evaluation as in Example 1. Table 1 shows the results.

[Comparative Example 3]
A long roll of PVA-based resin film with a thickness of 30 μm (manufactured by Kuraray, product name “PE3000”) is uniaxially stretched in the longitudinal direction by a roll stretching machine so that the total draw ratio is 6.0 times, and at the same time A polarizer having a thickness of 12 μm was produced by performing swelling, dyeing, cross-linking, washing, and finally drying. The obtained polarizer was subjected to the same evaluation as in Example 1. Table 1 shows the results.

As is clear from Table 1, the polarizing films of the examples of the present invention have practically acceptable single transmittance and degree of polarization, and very high breaking strength along the direction of the absorption axis. Such breaking strength means that the polarizing film is difficult to tear along the absorption axis direction.

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 (7)

A polarizing film comprising a polyvinyl alcohol resin film containing a dichroic substance and having an orientation function of 0.30 or less. 2. The polarizing film according to claim 1, having a thickness of 8 μm or less. 3. The polarizing film according to claim 1, which has a single transmittance of 40.0% or more and a degree of polarization of 99.0% or more. The polarizing film according to any one of claims 1 to 3, having a puncture strength of 30 gf/μm or more. A polarizing film comprising a polyvinyl alcohol resin film containing a dichroic substance and having a puncture strength of 30 gf/μm or more. A polarizing plate comprising the polarizing film according to claim 1 and a protective layer disposed on at least one side of the polarizing film. A method for manufacturing a polarizing film according to any one of claims 1 to 5,
Forming a polyvinyl alcohol-based resin layer containing iodide or sodium chloride and a polyvinyl alcohol-based resin on one side of a long thermoplastic resin substrate to form a laminate, and treatment, dyeing treatment, underwater stretching treatment, and drying shrinkage treatment for shrinking by 2% or more in the width direction by heating while conveying in the longitudinal direction, and applying in this order,
The total stretching ratio of the air auxiliary stretching treatment and the underwater stretching treatment is 3.0 to 4.5 times the original length of the laminate,
The stretch ratio of the in-air auxiliary stretching treatment is greater than the stretch ratio of the underwater stretching treatment,
Production method.

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