JP7382974B2 - Polarizing film, polarizing plate, and method for manufacturing the polarizing film - Google Patents

Description

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

In a liquid crystal display device, which is a typical image display device, polarizing films are 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, a method of stretching a laminate having a resin base material and a polyvinyl alcohol (PVA) resin layer and then subjecting it to a dyeing treatment to obtain a polarizing film on the resin base material is available. It has been proposed (for example, Patent Document 1). According to such a method, since a thin polarizing film can be obtained, it is attracting attention as it can contribute to the slimming down of recent image display devices. However, the above-mentioned thin polarizing film has a problem in that it is easily torn (easily torn) along the absorption axis direction.

Japanese Patent Application Publication No. 2001-343521

The present invention has been made to solve the above-mentioned conventional problems, and its main purpose 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 made of 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.
In one embodiment, the polarizing film has a single transmittance of 40.0% or more and a polarization degree 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 made of a polyvinyl alcohol resin film containing a dichroic substance, 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, a method for manufacturing the above polarizing film is provided. This manufacturing method includes forming a polyvinyl alcohol resin layer containing iodide or sodium chloride and a polyvinyl alcohol resin on one side of a long thermoplastic resin base material to form a laminate; Subjecting the body to, in this order, an auxiliary stretching treatment in the air, a dyeing treatment, a stretching treatment in water, and a drying shrinkage treatment in which the body is heated while conveying in the longitudinal direction to shrink by 2% or more in the width direction; . The total stretching ratio of the aerial auxiliary stretching treatment and the underwater stretching treatment is 3.0 to 4.5 times with respect to the original length of the laminate; It is larger than the stretching ratio of the stretching 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 was difficult to obtain acceptable optical properties (typically, single transmittance and degree of polarization) with a polarizing film having such a small orientation function. Both orientation function and acceptable optical properties can be achieved. Furthermore, according to the present invention, it is possible to achieve both such high puncture strength and acceptable optical properties.

1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. FIG. 2 is a schematic diagram showing an example of dry 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 The polarizing film according to the embodiment of the present invention is composed of a polyvinyl alcohol (PVA) resin film containing a dichroic substance (typically iodine, dichroic dye), and has an orientation function of 0.30 or less. It is. With such a configuration, tearing (breaking) of the polarizing film along the absorption axis direction can be significantly suppressed. As a result, a polarizing film (as a result, a polarizing plate) with very excellent flexibility can be obtained. Such a polarizing film (as a result, a polarizing plate) can be applied to preferably 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, particularly preferably 0.15 or less. . The lower limit of the orientation function may be, for example, 0.05. If the orientation function is too small, acceptable single transmittance and/or degree of polarization may not be obtained.

The orientation function (f) is determined by, for example, attenuated total reflection spectroscopy (ATR) using a Fourier transform infrared spectrophotometer (FT-IR) using polarized light as measurement light. Specifically, the measurement was carried out 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 was used to calculate according to the following formula. be done. Here, the intensity I has a value of 2941 cm ⁻¹ /3330 cm ⁻¹ with 3330 cm ⁻¹ as a reference peak. Note that when f=1, the orientation is perfect, and when f=0, it is random. Furthermore, the peak at 2941 cm ⁻¹ is considered to be absorption caused by the vibration of the main chain (—CH ₂ −) of PVA in the polarizing film.
f=(3<cos ² θ>-1)/2
=(1-D)/[c(2D+1)]
=-2×(1-D)/(2D+1)
however,
c=(3cos ² β-1)/2, and for a vibration of 2941 cm ^-1 , β=90°.
θ: Angle of the molecular chain with respect to the stretching direction β: Angle of the transition dipole moment with respect to the molecular chain axis D = (I _⊥ )/(I _// ) (In this case, the more the PVA molecules are oriented, the larger D becomes)
I _⊥ : Absorption intensity when the polarization direction of the measurement light and the stretching direction of the polarizing film are perpendicular I _// : Absorption intensity when the polarization direction of the measurement 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, even more preferably 5 μm or less, particularly preferably 3 μm or less, particularly preferably 2 μm or less. The lower limit of the thickness of the polarizing film may 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 yet another embodiment, and 5.5 μm to 7.0 μm in yet another embodiment. 5 μm, and in further embodiments may be between 6 μm and 7.2 μm. By making the polarizing film very thin in this way, thermal shrinkage can be made very small. It is presumed that such a configuration can also contribute to suppressing breakage in the absorption axis direction.

The polarizing film preferably exhibits absorption dichroism at a 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 the single transmittance may be, for example, 49.0%. In one embodiment, the polarizing film has a single transmittance of 40.0% to 45.0%. The degree of polarization 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%. In one embodiment, the degree of polarization of the polarizing film is 99.0% to 99.99%. According to the present invention, although the orientation function is extremely small as described above, it is possible to achieve such a practically acceptable single transmittance and degree of polarization. This is presumably due to the manufacturing method described below. Note that the single transmittance is typically a Y value measured using an ultraviolet-visible spectrophotometer and corrected for visibility. The degree of polarization is typically determined by the following formula based on parallel transmittance Tp and cross transmittance Tc measured using an ultraviolet-visible spectrophotometer and corrected for visibility.
Degree of polarization (%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 ×100

The puncture strength of the polarizing film is 30 gf/μm or more, preferably 35 gf/μm or more, more preferably 40 gf/μm or more, even more preferably 45 gf/μm or more, particularly preferably 50 gf/μm or more. It is. The upper limit of the puncture strength may be, for example, 80 gf/μm. By setting the puncture strength of the polarizing film within such a range, tearing of the polarizing film along the absorption axis direction can be significantly suppressed. As a result, a polarizing film (as a result, a polarizing plate) with very excellent flexibility can be obtained. Puncture strength indicates the cracking resistance of a polarizing film when the polarizing film is punctured with a predetermined strength. Puncture strength can be expressed as, for example, the strength at which a polarizing film breaks when a predetermined needle is attached to a compression testing machine and the needle is pierced into the polarizing film at a predetermined speed (breaking strength). Note that, as is clear from the unit, the puncture strength means the puncture strength per unit thickness (1 μm) of the polarizing film.

As described above, the polarizing film is composed of a PVA-based resin film containing iodine. Preferably, the PVA-based resin constituting the PVA-based resin film (substantially, the polarizing film) includes an acetoacetyl-modified PVA-based resin. With such a configuration, a polarizing film having desired puncture strength can be obtained. The blending amount of the acetoacetyl-modified PVA resin is preferably 5% to 20% by weight, more preferably 8% to 12% by weight when the entire PVA resin is 100% by weight. . If the blending amount is within this range, the puncture strength can be made into 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 includes a polarizing film obtained using a laminate of a resin base material and a PVA-based resin layer coated on the resin base material. A polarizing film obtained using a laminate of a resin base material and a PVA resin layer coated on the resin base material can be obtained by, for example, applying a PVA resin solution to the resin base material and drying it. Forming a PVA-based resin layer thereon to obtain a laminate of a resin base material 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 resin layer containing a halide and a polyvinyl alcohol resin is formed on one side of the resin base material. Stretching typically includes immersing the laminate in an aqueous boric acid solution and stretching. Furthermore, the stretching preferably further includes stretching the laminate in air at a high temperature (eg, 95° C. or higher) before stretching in the aqueous boric acid solution. In the embodiment of the present invention, the total stretching magnification is, for example, 3.0 to 4.5 times, which is significantly smaller than usual. Even at such a total stretching ratio, a polarizing film with acceptable optical properties can be obtained by combining the addition of a halide and a drying shrinkage treatment. Furthermore, in the embodiment of the present invention, the stretching ratio of the in-air auxiliary stretching is larger than the stretching ratio of the stretching in boric acid water. With such a configuration, it is possible to obtain a polarizing film having acceptable optical properties even if the total stretching ratio is small. In addition, the laminate is preferably subjected to a drying shrinkage treatment in which it is heated while being conveyed in the longitudinal direction to shrink by 2% or more in the width direction. In one embodiment, the method for manufacturing a polarizing film includes subjecting the laminate to an in-air auxiliary stretching process, a dyeing process, an underwater stretching process, and a dry shrinkage process in this order. By introducing auxiliary stretching, even when PVA is applied onto a thermoplastic resin, it becomes possible to improve the crystallinity of PVA and achieve high optical properties. 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 it is immersed in water during the subsequent dyeing and stretching processes, resulting in high optical properties. becomes possible to achieve. Furthermore, when the PVA-based resin layer is immersed in a liquid, disturbance in the orientation of polyvinyl alcohol molecules and deterioration of orientation can be suppressed compared to when the PVA-based resin layer does not contain a halide. This can improve the optical properties of a polarizing film obtained through a treatment process performed by immersing the laminate in a liquid, such as dyeing treatment and underwater stretching treatment. Furthermore, optical properties can be improved by shrinking the laminate in the width direction by drying shrinkage treatment. The obtained resin base material/polarizing film laminate may be used as is (that is, the resin base material may be used as a protective layer for the polarizing film), or the resin base material may be peeled from the resin base material/polarizing film laminate. However, any appropriate protective layer may be laminated on the peeled surface depending on the purpose. Details of the method for manufacturing 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 includes a polarizing film 10, a first protective layer 20 disposed on one side of the polarizing film 10, and a second protective layer 30 disposed 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. Note that, as described above, one of the first protective layer and the second protective layer may be a resin base material used for manufacturing the above-mentioned polarizing film.

The first and second protective layers are formed of any suitable film that can be used as a protective layer for a polarizing film. Specific examples of materials that are the main components of the film include cellulose resins such as triacetylcellulose (TAC), polyesters, polyvinyl alcohols, polycarbonates, polyamides, polyimides, polyethersulfones, and polysulfones. Examples include transparent resins such as polystyrene, polynorbornene, polyolefin, (meth)acrylic, and acetate. Further, thermosetting resins or ultraviolet curable resins such as (meth)acrylic, urethane, (meth)acrylic urethane, epoxy, and silicone resins may also be mentioned. Other examples include glassy polymers such as siloxane polymers. Furthermore, the polymer film 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 its side chain, and a thermoplastic resin having a substituted or unsubstituted phenyl group and nitrile group in its side chain. For example, a resin composition containing an alternating copolymer of isobutene and N-methylmaleimide and an acrylonitrile/styrene copolymer can be used. The polymer film may be, for example, an extrusion molded product 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 It is 5 μm to 80 μm, more preferably 10 μm to 60 μm. In addition, when 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 100 is applied to an image display device is preferably 5 μm to 200 μm, more preferably 10 μm to 100 μm, and even more preferably 10 μm to 60 μm. be. In one embodiment, the inner protective layer is a retardation layer having 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 determined by the formula: Re=(nx−ny)×d. Here, "nx" is the refractive index in the direction in which the in-plane refractive index is maximum (i.e., slow axis direction), and "ny" is the refractive index in the in-plane direction perpendicular to the slow axis (i.e., fast phase direction). "nz" is the refractive index in the thickness direction (axial direction), "d" is the thickness (nm) of the layer (film).

C. Method for manufacturing a polarizing film A method for manufacturing a polarizing film according to an embodiment of the present invention includes a method for manufacturing a polarizing film according to an embodiment of the present invention, in which a polyvinyl alcohol containing a halide and a polyvinyl alcohol resin (PVA resin) is added to one side of a long thermoplastic resin base material. By forming a laminate by forming a PVA-based resin layer (PVA-based resin layer), and subjecting the laminate to aerial auxiliary stretching treatment, dyeing treatment, underwater stretching treatment, and heating while conveying in the longitudinal direction. This includes performing a drying shrinkage treatment in which the material is shrunk by 2% or more in the width direction in this order. The content of the halide in the PVA resin layer is preferably 5 parts by weight to 20 parts by weight based on 100 parts by weight of the PVA resin. The drying shrinkage treatment is preferably carried out using a heating roll, and the temperature of the heating roll is preferably 60°C to 120°C. The shrinkage rate of the laminate in the width direction due to the 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, by producing a laminate including a PVA resin layer containing a halide, stretching the laminate in multiple stages including auxiliary stretching in the air and stretching in water, and heating the laminate after stretching with a heating roll. , a polarizing film having excellent optical properties (typically, single transmittance and unit absorbance) can be obtained.

C-1. Preparation of Laminate Any suitable method may be adopted as a method for producing a laminate of a thermoplastic resin base material and a PVA-based resin layer. Preferably, a coating liquid containing a halide and a PVA resin is applied to the surface of the thermoplastic resin base material and dried to form a PVA resin layer on the thermoplastic resin base material. As mentioned above, the content of the halide in the PVA resin layer is preferably 5 parts by weight to 20 parts by weight based on 100 parts by weight of the PVA resin.

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

The thickness of the PVA resin layer is preferably 2 μm to 30 μm, more preferably 2 μm to 20 μm. By making the thickness of the PVA resin layer before stretching extremely thin and reducing the total stretching ratio as described below, we can achieve acceptable single transmittance and A polarizing film having a degree of polarization can be obtained.

Before forming the PVA-based resin layer, the thermoplastic resin base material may be subjected to surface treatment (for example, corona treatment, etc.), or an easily adhesive layer may be formed on the thermoplastic resin base material. By performing such a treatment, the adhesion between the thermoplastic resin base material and the PVA resin layer can be improved.

C-1-1. Thermoplastic resin base material Any suitable thermoplastic resin film may be employed as the thermoplastic resin base material. Details of the thermoplastic resin base material are described in, for example, JP-A No. 2012-73580. The entire description of this publication is incorporated herein by reference.

C-1-2. Coating Liquid The coating liquid contains a halide and a PVA resin as described above. The coating liquid is typically a solution in which the halide and the PVA resin are dissolved in a solvent. Examples of the solvent 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 resin in the solution is preferably 3 to 20 parts by weight based on 100 parts by weight of the solvent. With such a resin concentration, a uniform coating film that adheres to the thermoplastic resin base material can be formed. The content of the halide in the coating liquid is preferably 5 parts by weight to 20 parts by weight based on 100 parts by weight of the PVA resin.

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

Any suitable resin may be employed as the PVA resin. Examples include polyvinyl alcohol and ethylene-vinyl alcohol copolymers. Polyvinyl alcohol is obtained by saponifying polyvinyl acetate. Ethylene-vinyl alcohol copolymer can be obtained by saponifying ethylene-vinyl acetate copolymer. The degree of saponification of the PVA 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 resin having such a degree of saponification, a polarizing film with excellent durability can be obtained. If the degree of saponification is too high, there is a risk of gelation. As mentioned above, the PVA-based resin preferably includes 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, and more preferably 1,500 to 4,300. Note that the average degree of polymerization can be determined according to JIS K 6726-1994.

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

Generally, by stretching the PVA resin layer, the orientation of the polyvinyl alcohol molecules in the PVA resin increases. However, when the stretched PVA resin layer is immersed in a liquid containing water, the orientation of the polyvinyl alcohol molecules increases. The orientation may be disturbed and the orientation may be deteriorated. In particular, when stretching a laminate of a thermoplastic resin and a PVA resin layer in boric acid water, when stretching the laminate in boric acid water at a relatively high temperature to stabilize the stretching of the thermoplastic resin, The tendency of the degree of orientation to decrease is remarkable. For example, while it is common for a PVA film alone to be stretched in boric acid water at 60°C, a laminate of A-PET (thermoplastic resin base material) and a PVA resin layer is stretched. It is carried out at a high temperature of around 70°C, and in this case, the orientation of PVA at the initial stage of stretching may decrease before it increases due to underwater stretching. In contrast, by producing a laminate of a PVA resin layer containing a halide and a thermoplastic resin base material, and stretching the laminate at high temperature in air (auxiliary stretching) before stretching it in boric acid water. , crystallization of the PVA resin in the PVA 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, disturbance in the orientation of polyvinyl alcohol molecules and deterioration of orientation can be suppressed compared to when the PVA-based resin layer does not contain a halide. This can improve the optical properties of a polarizing film obtained through a treatment process performed by immersing the laminate in a liquid, such as dyeing treatment and underwater stretching treatment.

C-2. Auxiliary Stretching Treatment in Air In particular, in order to obtain high optical properties, a two-stage stretching method is selected that combines dry stretching (auxiliary stretching) and stretching in boric acid water. By introducing auxiliary stretching like two-stage stretching, stretching can be performed while suppressing crystallization of the thermoplastic resin base material. Furthermore, in order to suppress the influence of the glass transition temperature of the thermoplastic resin base material, when applying PVA resin on a thermoplastic resin base material, compared to the case where PVA resin is applied 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 PVA-based resin even when applying PVA-based resin onto thermoplastic resin, making it possible to achieve high optical properties. Become. At the same time, by increasing the orientation of the PVA resin in advance, it is possible to prevent problems such as a decrease in orientation and dissolution of the PVA resin when it is immersed in water during the subsequent dyeing and stretching processes. , it becomes possible to achieve high optical properties.

The stretching method for aerial auxiliary stretching may be fixed end stretching (for example, stretching using a tenter stretching machine) or free end stretching (for example, uniaxial stretching by passing the laminate between rolls with different circumferential speeds). However, in order to obtain high optical properties, free end stretching may be actively employed. In one embodiment, the aerial stretching process includes a heating roll stretching step of stretching the laminate using a difference in peripheral speed between heating rolls while conveying the laminate in its longitudinal direction. The aerial stretching process typically includes a zone stretching process and a heated roll stretching process. Note that the order of the zone stretching step and the heating roll stretching step is not limited, and the zone stretching step may be performed first, or the heating roll stretching step may be performed first. The zone stretching step may be omitted. In one embodiment, the zone stretching step and the heated roll stretching step are performed in this order. In another embodiment, the film is stretched in a tenter stretching machine by gripping the end portion of the film and widening the distance between the tenters in the machine direction (the widening of the distance between the tenters becomes the stretching ratio). At this time, the distance of the tenter in the width direction (direction perpendicular to the flow direction) is set to be arbitrarily close. Preferably, the stretching ratio in the machine direction can be set to be closer to free end stretching. In the case of free end stretching, the shrinkage rate in the width direction = (1/stretching ratio) is calculated as ^1/2 .

Aerial assisted stretching may be performed in one step or in multiple steps. In the case of multi-stage stretching, the stretching ratio is the product of the stretching ratios of each stage. The stretching direction in the aerial auxiliary stretching is preferably substantially the same as the stretching direction in the underwater stretching.

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

The stretching temperature of the in-air auxiliary stretching can be set to any appropriate value depending on the forming material of the thermoplastic resin base material, the stretching method, etc. The stretching temperature is preferably at least the glass transition temperature (Tg) of the thermoplastic resin base material, more preferably at least the glass transition temperature (Tg) of the thermoplastic resin base material +10°C, particularly preferably at least Tg +15°C. 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 the rapid progress of crystallization of the PVA-based resin and to suppress defects caused by the crystallization (for example, preventing the orientation of the PVA-based resin layer due to stretching). can.

C-3. Insolubilization treatment, dyeing treatment, and crosslinking treatment If necessary, insolubilization treatment is performed after the aerial auxiliary stretching treatment and before the underwater stretching treatment and dyeing treatment. The above-mentioned insolubilization treatment is typically performed by immersing the PVA resin layer in a boric acid aqueous solution. The dyeing process is typically performed by dyeing the PVA resin layer with a dichroic substance (typically iodine). If necessary, a crosslinking treatment is performed after the dyeing treatment and before the underwater stretching treatment. The above crosslinking treatment is typically performed by immersing the PVA resin layer in a boric acid aqueous solution. Details of the insolubilization treatment, dyeing treatment, and crosslinking treatment are described in, for example, JP 2012-73580A (mentioned above).

C-4. Underwater Stretching Treatment The underwater stretching treatment is performed by immersing the laminate in a stretching bath. According to the underwater stretching treatment, stretching can be performed at a temperature lower than the glass transition temperature (typically about 80° C.) of the thermoplastic resin base material or the PVA resin layer, and the PVA resin layer can be It can be stretched while suppressing the As a result, a polarizing film having excellent optical properties can be manufactured.

Any suitable method can be used to stretch the laminate. Specifically, fixed-end stretching or free-end stretching (for example, a method in which the laminate is passed between rolls having different circumferential speeds and uniaxially stretched) may be used. Preferably, free end stretching is chosen. The laminate may be stretched in one step or in multiple steps. When carried out in multiple stages, the total stretching ratio is the product of the stretching ratios of each stage.

Stretching in water is preferably performed by immersing the laminate in an aqueous boric acid solution (stretching in boric acid water). By using a boric acid aqueous solution as a stretching bath, it is possible to impart rigidity to the PVA-based resin layer to withstand tension applied during stretching, and water resistance that does not dissolve in water. Specifically, boric acid can generate a tetrahydroxyborate anion in an aqueous solution and crosslink with the 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 stretched well, and a polarizing film having excellent optical properties can be manufactured.

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

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

The stretching temperature (the liquid temperature of the stretching bath) is preferably 40°C to 85°C, more preferably 60°C to 75°C. At such a temperature, it is possible to stretch to 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 base material 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., there is a possibility that the stretching cannot be performed satisfactorily even if the plasticization of the thermoplastic resin base material by water is taken into account. On the other hand, as the temperature of the stretching bath increases, the solubility of the PVA-based resin layer increases, and there is a possibility that excellent optical properties may not be obtained. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

The stretching ratio by underwater stretching is preferably 1.0 times to 3.0 times, more preferably 1.0 times to 2.0 times, and even more preferably 1.0 times to 1.5 times. . If the stretching ratio in underwater stretching is within such a range, the total stretching ratio can be set within a desired range, and a desired orientation function can be achieved. As a result, a polarizing film in which breakage along the absorption axis direction is suppressed can be obtained. The total stretching ratio (total stretching ratio when aerial auxiliary stretching and underwater stretching are combined) is, for example, 3.0 to 4.5 times the original length of the laminate, as described above. Preferably it is 3.0 times to 4.0 times, more preferably 3.0 times to 3.5 times. By appropriately combining the addition of halides to the coating solution, adjustment of the stretch ratios for aerial auxiliary stretching and underwater stretching, and drying shrinkage treatment, acceptable optical properties can be achieved even at such total stretching ratios. A polarizing film can be obtained.

C-5. Drying Shrinkage Treatment The drying shrinkage treatment described above may be performed by zone heating performed by heating the entire zone, or may be performed by heating a conveyance roll (using a so-called heating roll) (heated roll drying method). Preferably, both are used. By drying using a heating roll, heating curl of the laminate can be efficiently suppressed, and a polarizing film with excellent appearance can be manufactured. Specifically, by drying the laminate along a heating roll, it is possible to efficiently promote crystallization of the thermoplastic resin base material and increase the degree of crystallinity, which is relatively low. Even at drying temperatures, the degree of crystallinity of the thermoplastic resin base material can be increased favorably. As a result, the thermoplastic resin base material has increased rigidity and is in a state where it can withstand shrinkage of the PVA resin layer due to drying, thereby suppressing curling. Furthermore, by using a heating roll, the laminate can be dried while maintaining it in a flat state, so that not only curling but also wrinkles can be suppressed. At this time, the optical properties of the laminate can be improved by shrinking the laminate in the width direction by drying shrinkage treatment. This is because the orientation of PVA and PVA/iodine complex can be effectively improved. The shrinkage rate of the laminate in the width direction due to the drying shrinkage treatment is preferably 1% to 10%, more preferably 2% to 8%, 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, 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 base material. The transport rolls R1 to R6 may be arranged so as to continuously heat only the resin base material surface).

The drying conditions can be controlled by adjusting the heating temperature of the conveying roll (temperature of the heating roll), the number of heating rolls, the contact time with the heating roll, etc. The temperature of the heating roll is preferably 60°C to 120°C, more preferably 65°C to 100°C, particularly preferably 70°C to 80°C. It is possible to satisfactorily increase the degree of crystallinity of the thermoplastic resin, to satisfactorily suppress curling, and to produce an optical laminate having extremely excellent durability. Note that the temperature of the heating roll can be measured with a contact thermometer. In the illustrated example, six conveyance rolls are provided, but there is no particular restriction on the number of conveyance rolls as long as there is a plurality of conveyance rolls. Usually 2 to 40 conveyance rolls, preferably 4 to 30 conveyance rolls are provided. The contact time (total contact time) between the laminate and the heating roll is preferably 1 second to 300 seconds, more preferably 1 to 20 seconds, and even more preferably 1 to 10 seconds.

The heating roll may be provided within a heating furnace (for example, an oven) or may be provided on a normal production line (at room temperature). Preferably, it is provided in a heating furnace equipped with air blowing means. By using both heating roll drying and hot air drying, a sharp temperature change between the heating rolls can be suppressed, and shrinkage in the width direction can be easily controlled. The temperature of hot air drying is preferably 30°C to 100°C. Further, the hot air drying time is preferably 1 second to 300 seconds. The wind speed of the hot air is preferably about 10 m/s to 30 m/s. Note that the wind speed is the wind speed within 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 above cleaning treatment is typically performed by immersing the PVA resin layer in an aqueous potassium iodide solution.

EXAMPLES Hereinafter, the present invention will be specifically explained with reference to Examples, but the present invention is not limited to these Examples. The method for measuring each characteristic is as follows. Note that unless otherwise specified, "parts" and "%" in Examples and Comparative Examples are based on weight.
(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 the Examples and Comparative Examples were examined using a Fourier transform infrared spectrophotometer (FT-IR) (manufactured by Perkin Elmer, trade name: "Frontier"). Attenuated total reflection spectroscopy (ATR) on the surface of the polarizing film was performed using external light as the measurement light. Germanium was used as the crystallite to which the polarizing film was brought into close contact, and the incident angle of the measurement light was 45°. The orientation function was calculated using the following procedure. The incident polarized infrared light (measurement light) is polarized light (s-polarized light) that vibrates parallel to the surface of the germanium crystal that is brought into close contact with the sample, and the stretching direction of the polarizing film is perpendicular to the polarization direction of the measurement light ( The absorbance spectra of each were measured in parallel (⊥) and parallel (//) positions. From the obtained absorbance spectrum, (2941 cm ⁻¹ intensity) I was calculated using (3330 cm ⁻¹ intensity) as a reference. _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 perpendicularly (⊥) to the polarization direction of the measurement light. In addition, I _// is obtained from the absorbance spectrum obtained when the stretching direction of the polarizing film is placed parallel (//) to the polarization direction of the measurement light (2941 cm ^-1 intensity) / (3330 cm ^-1 intensity) It is. Here, (2941 cm ^-1 intensity) is the absorbance at 2941 cm ^-1 when 2770 cm ^-1 and 2990 cm ^-1 , which are the bottom of the absorbance spectrum, are the baseline, and (3330 cm ^-1 intensity) is 2990 cm ^-1 This is the absorbance at 3330 cm ⁻¹ when ¹ and 3650 cm ⁻¹ are used as the baseline. Using the obtained I _⊥ and I _// , the orientation function f was calculated according to Equation 1. Note that when f=1, the orientation is perfect, and when f=0, it is random. Furthermore, the peak at 2941 cm ⁻¹ is said to be absorption caused by vibration of the PVA main chain (—CH ₂ −) in the polarizing film. Furthermore, the peak at 3330 cm ^-1 is said to be absorption caused by vibration of the hydroxyl group of PVA.
(Formula 1) f=(3<cos2θ>-1)/2
=(1-D)/[c(2D+1)]
However, c=(3cos ² β-1)/2
When 2941 cm ⁻¹ is used as described above, β=90°⇒f=−2×(1−D)/(2D+1).
θ: Angle of the molecular chain with respect to the stretching direction β: Angle of transition dipole moment with respect to the molecular chain axis D=(I _⊥ )/(I _// )
I _⊥ : Absorption intensity when the polarization direction of the measurement light and the stretching direction of the polarizing film are perpendicular I _// : Absorption intensity when the polarization direction of the measurement light and the stretching direction of the polarizing film are parallel (3) Single transmittance and Degree of Polarization The polarizing film was peeled off from the polarizing film/thermoplastic resin base laminate used in the Examples and Comparative Examples, and the polarizing film was measured using an ultraviolet-visible spectrophotometer (“V-7100” manufactured by JASCO Corporation). The single transmittance Ts, parallel transmittance Tp, and orthogonal transmittance Tc measured using the polarizing film were defined as Ts, Tp, and Tc of the polarizing film, respectively. These Ts, Tp, and Tc are Y values measured using a 2-degree visual field (C light source) according to JIS Z8701 and subjected to visibility correction.
The degree of polarization P was determined from the obtained Tp and Tc using the following formula.
Polarization degree P (%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 ×100
Note that it is also possible to perform equivalent measurements with a spectrophotometer such as Otsuka Electronics' LPF-200, and equivalent measurement results can be obtained no matter which spectrophotometer is used. has been confirmed.
(4) Breaking strength The polarizing film was peeled from the polarizing film/thermoplastic resin base laminate used in the Examples and Comparative Examples, and a compression tester equipped with a needle (manufactured by Kato Tech, product name "NDG5" needle) was used. The polarizing film was placed on a tube (penetration force measurement specifications) and punctured at a puncture speed of 0.33 cm/sec in a room temperature (23°C ± 3°C) environment, and the strength when the polarizing film was broken was defined as the breaking strength (piercing strength). For the evaluation value, the breaking strength of 10 sample pieces was measured and the average value was used. Note that the needle used had a tip diameter of 1 mmφ and a diameter of 0.5 R. For the polarizing film to be measured, a jig having a circular opening with a diameter of about 11 mm was fixed between both sides of the polarizing film, and a needle was inserted into the center of the opening to perform the test. The breaking strength per unit thickness was used as an index of resistance to tearing, and evaluation was made based on the following criteria.
○: Breaking strength exceeds 30 gf/μm ×: Breaking strength is 30 gf/μm or less

[Example 1]
As the thermoplastic resin base material, 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 conditions: 55 W·min/m ² ) was performed on one side of the resin base material.
100 weight PVA resin prepared by mixing polyvinyl alcohol (degree of polymerization 4200, degree of saponification 99.2 mol%) and acetoacetyl-modified PVA (manufactured by Nippon Gosei Kagaku Kogyo Co., Ltd., trade name "Gosefaimer Z410") at a ratio of 9:1. 13 parts by weight of potassium iodide was added to 13 parts by weight of potassium iodide to prepare a PVA aqueous solution (coating liquid).
The 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 with a thickness of 13 μm, thereby producing a laminate.
The obtained laminate was uniaxially stretched free end to 2.4 times in the longitudinal direction (longitudinal direction) between rolls having different circumferential speeds in an oven at 130° C. (in-air auxiliary stretching treatment).
Next, the laminate was immersed for 30 seconds in an insolubilization bath (boric acid aqueous solution obtained by blending 4 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 40° C. (insolubilization treatment).
Next, the final polarizing film was added to a dyeing bath (an aqueous iodine solution obtained by mixing iodine and potassium iodide in a weight ratio of 1:7 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% (staining treatment).
Next, it was immersed for 30 seconds in a crosslinking bath (an aqueous boric acid solution 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) at a liquid temperature of 40°C. (Crosslinking treatment).
Thereafter, while immersing the laminate in a boric acid aqueous solution (boric acid concentration 4.0% by weight, potassium iodide 5.0% by weight) at a liquid temperature of 62°C, it was passed between rolls having different circumferential speeds in the longitudinal direction (longitudinal direction). ) was uniaxially stretched so that the total stretching ratio was 3.0 times (underwater stretching treatment: the stretching ratio in the underwater stretching treatment was 1.25 times).
Thereafter, the laminate was immersed in a cleaning bath (an 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. (cleaning treatment).
Thereafter, while drying in an oven maintained at 90°C, it was brought into contact with a SUS heating roll whose surface temperature was maintained at 75°C for about 2 seconds (drying shrinkage treatment). The shrinkage rate of the laminate in the width direction due to the drying shrinkage treatment was 2%.
In this way, a polarizing film with a thickness of 7.1 μm was formed on the resin base material.

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

[Example 2]
A polarizing film with 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. The results are shown in Table 1.

[Example 3]
A polarizing film with 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. The results are shown in Table 1.

[ Reference example 4 ]
A polarizing film with 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. The results are shown in Table 1.

[Comparative example 1]
The procedure was the same 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 temperature of the stretching bath was 70°C. A 0 μm polarizing film was produced. The width remaining ratio of the obtained polarizing film was 48% (the shrinkage ratio in the width direction was 52%). The obtained polarizing film was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[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 remaining 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. The results are shown in Table 1.

[Comparative example 3]
A long roll of PVA resin film (manufactured by Kuraray, product name "PE3000") with a thickness of 30 μm was simultaneously stretched uniaxially in the longitudinal direction using a roll stretching machine at a total stretching ratio of 6.0 times. A polarizer with a thickness of 12 μm was produced by performing swelling, dyeing, crosslinking, and washing treatments, and finally by drying. The obtained polarizer was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

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 have very high breaking strength along the absorption axis direction. 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 suitably used in liquid crystal display devices.

10 Polarizing film 20 First protective layer 30 Second protective layer 100 Polarizing plate

Claims (6)

A polarizing film composed of a polyvinyl alcohol resin film containing a dichroic substance,
The single transmittance is 40.0% or more, and the degree of polarization is 99.97% or more,
the orientation function is 0.05 or more and 0.25 or less,
The orientation function is determined by performing attenuated total reflection spectroscopy using a Fourier transform infrared spectrophotometer, using polarized light as the measurement light, with the stretching direction of the polarizing film parallel and perpendicular to the polarization direction of the measurement light. Polarizing film, calculated according to the following (Formula 1) using the intensity of 2941 cm ^-1 of the obtained absorbance spectrum:
(Formula 1) f=(3<cos ² θ>-1)/2
=(1-D)/[c(2D+1)]
=-2×(1-D)/(2D+1)
In formula 1, θ represents the angle of the molecular chain with respect to the stretching direction; c represents (3cos ² β-1)/2; β represents the angle of the transition dipole moment with respect to the molecular chain axis, 2941 cm ^-1 _is ⁹⁰ ° for the _vibration ^of Absorption intensity indicates the value of (intensity at 2941 cm ^-1 )/(intensity at 3330 cm ^-1 ); I _⊥ is the absorption intensity when the stretching direction of the polarizing film is arranged perpendicular to the polarization direction of the measurement light. I indicates the above I; I _// indicates the above I when the stretching direction of the polarizing film is arranged parallel to the polarization direction of the measurement light. The polarizing film according to claim 1, having a thickness of 8 μm or less. The polarizing film according to claim 1 or 2 , having a puncture strength of 35 gf/μm or more. A polarizing film composed of a polyvinyl alcohol resin film containing a dichroic substance,
The single transmittance is 40.0% or more, and the degree of polarization is 99.97% or more,
The orientation function is 0.05 or more, the puncture strength is 35 gf/μm or more,
The orientation function is determined by performing attenuated total reflection spectroscopy using a Fourier transform infrared spectrophotometer, using polarized light as the measurement light, with the stretching direction of the polarizing film parallel and perpendicular to the polarization direction of the measurement light. Polarizing film, calculated according to the following (Formula 1) using the intensity of 2941 cm ^-1 of the obtained absorbance spectrum:
(Formula 1) f=(3<cos ² θ>-1)/2
=(1-D)/[c(2D+1)]
=-2×(1-D)/(2D+1)
In formula 1, θ represents the angle of the molecular chain with respect to the stretching direction; c represents (3cos ² β-1)/2; β represents the angle of the transition dipole moment with respect to the molecular chain axis, 2941 cm ^-1 _is ⁹⁰ ° for the _vibration ^of Absorption intensity indicates the value of (intensity at 2941 cm ^-1 )/(intensity at 3330 cm ^-1 ); I _⊥ is the absorption intensity when the stretching direction of the polarizing film is arranged perpendicular to the polarization direction of the measurement light. I indicates the above I; I _// indicates the above I when the stretching direction of the polarizing film is arranged parallel to the polarization direction of the measurement light. 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 4 , comprising:
A polyvinyl alcohol resin layer containing iodide or sodium chloride and a polyvinyl alcohol resin is formed on one side of a long thermoplastic resin base material to form a laminate, and the laminate is subjected to aerial assisted stretching. treatment, dyeing treatment, underwater stretching treatment, and drying shrinkage treatment to shrink by 2% or more in the width direction by heating while conveying in the longitudinal direction,
The total stretching ratio of the aerial auxiliary stretching treatment and the underwater stretching treatment is 3.0 to 4.0 times the original length of the laminate;
The stretching ratio of the aerial auxiliary stretching treatment is larger than the stretching ratio of the underwater stretching treatment.
Production method.

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