JPWO2019012968A1 - Manufacturing method of laminate and polarizing film - Google Patents

Abstract

The present invention provides a manufacturing method capable of achieving both high-level removal of air bubbles and removal of foreign substances, and as a result, a laminate in which appearance defects are suppressed. The method for producing a laminate of the present invention is a coating solution containing a polyvinyl alcohol-based resin, which is passed through a plurality of depth type filters to remove air bubbles and foreign matters, and the coating which is passed through the plurality of depth type filters. Applying the liquid to one side of the resin substrate and drying the liquid to form a polyvinyl alcohol-based resin layer. The plurality of depth type filters include at least three types of depth type filters having different filtration precisions from each other, and the final depth type filter through which the coating liquid passes last has the lowest filtration precision. The filtration accuracy of the depth type filter is 50 μm to 100 μm.

Description

  The present invention relates to a method for producing a laminate and a polarizing film.

  A liquid crystal display device, which is a typical image display device, has polarizing films disposed on both sides of a liquid crystal cell due to the image forming method. As a method for manufacturing a polarizing film, for example, a method has been proposed in which a laminate having a resin base material and a polyvinyl alcohol (PVA) -based resin layer is stretched, and then dipped in a staining solution to obtain a polarizing film (for example, a method for obtaining a polarizing film). , Patent Document 1). According to such a method, a polarizing film having a small thickness can be obtained, and thus, it is noted that it can contribute to a reduction in thickness of a liquid crystal display device in recent years. The PVA-based resin layer is formed by applying and drying a coating solution containing a PVA-based resin. If bubbles are present in the coating solution, a streak-like or dot-like appearance defect occurs in the formed PVA-based resin layer. Therefore, it is desirable to remove bubbles from the coating solution. Techniques for removing bubbles in the coating liquid include a technique of passing a coating liquid containing a PVA resin through a filter in the production of a PVA-based resin film (for example, Patent Document 2), and a coating method containing a PVA-based resin. A technique has been proposed in which a liquid is passed through a depth-type filter having a predetermined filtration accuracy and the pressure applied to a coating liquid supplied to the filter is varied (for example, Patent Document 3).

  However, a thin PVA-based resin film for use as a polarizing film is required to have a further improved appearance, and there is a demand for achieving a high degree of compatibility between the removal of air bubbles and the removal of foreign matter.

JP 2001-343521 A JP-A-2002-144419 JP-A-2015-013242

  The present invention has been made to solve the above-mentioned conventional problems, and the main object of the present invention is to remove air bubbles and remove foreign substances at a high level, and as a result, to obtain a laminate in which appearance defects are suppressed. It is to provide a manufacturing method that can be performed.

  In the production of a thin PVA-based resin film for use as a polarizing film, the number of times of passing through a filter is reduced in order to prevent appearance defects caused by the gel, since the PVA-based resin solution generally tends to form a gel due to shearing. Tend to. When a plurality of filters are used, it is general that the filters are sequentially passed from a filter having a low filtration accuracy to a filter having a high filtration accuracy from the viewpoint of filtration efficiency. The present inventors, contrary to these common general knowledge, pass a coating liquid for forming a PVA-based resin layer through a plurality of depth type filters having relatively high filtration accuracy, and then have the lowest filtration accuracy. It has been found that by passing through a depth type filter having a predetermined filtration accuracy, it is possible to achieve both a high level of removal of air bubbles and a high level of removal of foreign substances, thereby completing the present invention.

That is, according to the present invention, a method for manufacturing a laminate is provided. The method for producing the laminate, the coating liquid containing a polyvinyl alcohol-based resin, by passing a plurality of depth type filters to remove bubbles and foreign matter, and the coating liquid passed through the plurality of depth type filters And coating and drying on one side of the resin substrate to form a polyvinyl alcohol-based resin layer. The plurality of depth type filters include at least three types of depth type filters having different filtration accuracy. The final depth type filter through which the coating solution passes last has the lowest filtration accuracy, and the filtration accuracy of the final depth type filter is 50 μm to 100 μm.
In one embodiment, in the at least three types of depth type filters, the first depth type filter through which the application liquid passes first has the second lowest filtration accuracy.
In one embodiment, the first depth type filter has a filtration accuracy of 5 μm to 20 μm.
In one embodiment, the pressure applied to the application liquid supplied to the plurality of depth type filters is changed to remove bubbles inside the plurality of depth type filters.
In one embodiment, the viscosity of the coating liquid that passes through the plurality of depth type filters is 100 mPa · s to 10,000 mPa · s.
According to another aspect of the present invention, a method for manufacturing a polarizing film is provided. The method for producing the polarizing film includes obtaining a laminate having a resin substrate and a polyvinyl alcohol-based resin layer formed on the resin substrate by the above-described method for producing a laminate. Dyeing and stretching.

  ADVANTAGE OF THE INVENTION According to this invention, the removal method of a bubble and the removal of a foreign material can be made compatible at a high level, and as a result, the manufacturing method which can obtain the laminated body which suppressed the appearance defect was provided.

5 is a graph showing an example of a pressure fluctuation profile at the time of removing bubbles and foreign matter in the production method of the present invention. FIG. 3 is a schematic diagram showing an example of a mode of removing bubbles and foreign matter of a coating solution in the production method of the present invention. 1 is a schematic sectional view of a laminate according to a preferred embodiment of the present invention. 4 is a graph showing a pressure fluctuation profile at the time of removing bubbles and foreign substances used in the example.

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

[A. Manufacturing method of laminate]
The method for manufacturing a laminate according to one embodiment of the present invention includes the steps of: passing a coating solution containing a PVA-based resin through a plurality of depth type filters to remove bubbles and foreign substances; Applying the passed coating liquid to one side of the resin substrate and drying to form a PVA-based resin layer. Hereinafter, a typical embodiment of the manufacturing method will be described. In addition, in this specification, the "foreign matter" includes not only a main component different from the coating liquid contained due to external factors but also a PVA gel.

[A-1. Coating liquid]
Any appropriate resin can be adopted as the PVA-based resin contained in the coating solution. For example, polyvinyl alcohol and an ethylene-vinyl alcohol copolymer are mentioned. Polyvinyl alcohol is obtained by saponifying polyvinyl acetate. The ethylene-vinyl alcohol copolymer is obtained by saponifying the ethylene-vinyl acetate copolymer. The saponification degree of the PVA-based resin is usually 85 mol% to 100 mol%, preferably 95.0 mol% to 99.95 mol%, and more preferably 99.0 mol% to 99.93 mol%. . The saponification degree can be determined according to JIS K 6726-1994. By using a PVA-based resin having such a saponification degree, a polarizing film having excellent durability can be obtained. If the degree of saponification is too high, gelation may occur.

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

  The coating liquid is typically a solution obtained by dissolving the PVA-based resin 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 preferable. The concentration of the PVA-based resin in the solution is preferably 3 to 20 parts by weight (for example, 3 to 15 parts by weight, or 4 to 12 parts by weight, for example) based on 100 parts by weight of the solvent. With such a resin concentration, a uniform coating film adhered to the resin substrate can be provided.

  An additive may be added to the coating solution. Examples of the additive include a plasticizer and a surfactant. Examples of the plasticizer include polyhydric alcohols such as ethylene glycol and glycerin. Examples of the surfactant include a nonionic surfactant. These can be used for the purpose of further improving the uniformity, dyeability and stretchability of the obtained PVA-based resin layer.

  The viscosity of the coating liquid (the viscosity of the coating liquid passing through a plurality of depth type filters) is preferably 100 mPa · s to 10,000 mPa · s, more preferably 300 mPa · s to 5000 mPa · s, and further preferably 500 mPa · s. s to 3000 mPa · s.

[A-2. Removal of air bubbles and foreign matter]
The coating liquid is passed through a plurality of depth type filters to remove bubbles and foreign substances. In the present invention, at least three types of depth type filters having different filtration accuracy are used as the plurality of depth type filters. The types of depth type filters having mutually different filtration accuracy may be, for example, three, four, five, six or more. Optionally, a plurality of depth type filters having the same filtration accuracy may be arranged in series or in parallel to allow the application liquid to pass.

  Among the at least three types of depth type filters, a filter having the lowest (coarse) filtration accuracy is used as the last filter through which the coating solution passes last. Preferably, the first filter through which the coating liquid first passes has the second lowest filtration accuracy, and the other filters (intermediate filters) have the highest (precision) filtration accuracy. By arranging the filters in this manner, the first filter and the intermediate filter can precisely remove foreign matter, and the final filter can suitably remove foreign matter and air bubbles.

  The filtration accuracy of the final filter is 50 μm to 100 μm, preferably 60 μm to 100 μm, and more preferably 70 μm to 100 μm.

  The filtration accuracy of the first filter is preferably 5 μm to 20 μm, more preferably 6 μm to 15 μm, and more preferably 8 μm to 12 μm.

  The filtration accuracy of the intermediate filter is preferably 1 μm to 10 μm, more preferably 1.5 μm to 8 μm, and more preferably 2 μm to 6 μm. As the intermediate filter, two or more types of depth type filters having different filtration accuracy can be used.

  In the present specification, the filtration accuracy refers to the minimum particle that can be separated by 99.9% or more when a liquid obtained by dispersing 0.3 ppm of the test powder 1 specified in JIS Z 8901 in pure water is filtered. Refers to the diameter. Therefore, the larger the value of the filtration accuracy, the larger the particle size that can be filtered (that is, the lower the filtration accuracy and the coarser the filter).

  In one embodiment, the coating liquid is passed in order from the filter with the second lowest filtration accuracy to the filter with the highest filtration accuracy, and finally through the filter with the lowest filtration accuracy. By passing through the filter in such an order, it is possible to achieve both high-level removal of foreign substances such as gel and removal of bubbles even when the number of shears caused by the passage of the filter increases.

  Preferably, when filling the coating liquid into the plurality of depth type filters, the pressure applied to the coating liquid supplied to the filters is changed. Thereby, the air remaining in the space inside the filter can be expanded and united. As a result, the air in the filter is easily evacuated, the filter can be quickly filled with the coating liquid, and the productivity can be improved. For example, when the pressure of 0.2 MPa applied to the coating solution is released to atmospheric pressure, the average diameter of the bubbles, which was about 100 μm when the pressure was applied, can be increased to about 125 μm. The pressure fluctuation may be performed for all of the plurality of depth type filters, or may be performed for some of the depth type filters (for example, a final filter). Preferably, the pressure fluctuation may be performed for all of the plurality of depth type filters.

  The fluctuation of the pressure can be performed by changing the discharge amount of the application liquid by the pump, the output of the pump, and the like. Fluctuation of the pressure can be performed with any appropriate profile depending on the purpose and the type of the coating liquid. For example, the pressure may fluctuate in a sine curve shape as shown in FIG. 1A, or may fluctuate in a pulse shape as shown in FIG. You may. In one embodiment, the fluctuation of the pressure may be a profile in which the pressure applied to the coating solution is reduced for at least a certain period of time as shown in FIG. 1 (c). As shown in the figure, any profile may be used as long as the pressure applied to the coating liquid is zero for at least a certain period of time (the coating liquid is kept in an atmospheric pressure state). The pressure fluctuation profile as shown in FIG. 1D can be realized, for example, by intermittently stopping the pump. The difference between the maximum pressure and the minimum pressure in the pressure fluctuation is preferably 0.10 MPa to 0.25 MPa, more preferably 0.15 MPa to 0.22 MPa. By varying the pressure in a predetermined profile and / or by setting the difference between the maximum pressure and the minimum pressure in the pressure variation in the above range, the average diameter of the bubbles inside the filter increases as described above. In such a case, air bubbles inside the filter are easily removed, and the filter can be quickly filled with the coating liquid. As a result, air bubbles in the coating liquid can be satisfactorily removed.

  The depth type filter is a depth filtration type filter. The depth type filter may have a filtration accuracy gradient in the thickness direction of the filter medium. Any appropriate configuration can be adopted as the configuration of the depth type filter. Specific examples include a thread winding type in which a yarn is wound around a cylindrical core, a nonwoven fabric lamination type in which a nonwoven fabric is wound around a cylindrical core, and a resin molding type using a resin molded product such as a sponge, depending on the form of the filter medium. Examples of a constituent material of the filter medium include a polyolefin-based composite fiber and a heat-adhesive polyester fiber. The depth type filter is typically attached to a pressure vessel (housing), and pressurizes a liquid to be filtered (in the present invention, a PVA-based coating liquid) to flow from the outside of the filter in the housing to the inside. Thus, air bubbles and / or foreign substances in the liquid are removed at the thickness of the filter medium. As a filter, a surface type (surface filtration type) filter (for example, a pleated type filter) is known in addition to the depth type filter. However, in the present invention, a depth type filter is used in view of air bubble removal, filtration ability, durability, and the like. A filter is used.

  Depth type filters are commercially available as cartridge type filters. In the present invention, such a commercially available depth type filter can also be suitably used. Specific examples of commercially available products include HDCII, Profile, Profile II, Ultipleated Profile, Profile II Plus, and Petrosorp as those manufactured by Pall Corporation; CP filters, BM filters, Porous Fine, and Superfine as those manufactured by Chisso Corporation. Wind filter, stem filter, GF filter; Lokitechno's SL filter, micro serial filter, Dia II type filter, micropure filter; Fujifilm's Astropore PPE.

  FIG. 2 is a schematic diagram showing an example of a system for removing bubbles and foreign matter in a coating solution in the production method of the present invention. As shown in FIG. 2, in the system for removing bubbles and foreign matter from the application liquid, the application liquid is prepared in the preparation tank 11 and supplied to the charge tank 12 via the piping system 1 including the liquid feed pump P1. The piping system 2 includes a liquid feed pump P2 and depth type filters F1a, F2a, F3a, and is connected to the charge tank 12. The coating liquid is supplied from the charge tank 12 to the depth type filters F1a, F2a, and F3a by the liquid sending pump P2, and is filtered by these filters to remove bubbles and foreign matters. The coating liquid from which bubbles and foreign matter have been removed is sent to the coating die 20 via the piping system 3 by returning the three-way valve V1 to open or returned to the charge tank 12 via the circulation piping system 4. It is. When the coating liquid is sent to the coating die 20 (when the three-way valve is opened to the coating die side), it is preferable that the liquid sending system (particularly, inside the filter) is sufficiently filled with the coating liquid. That is, it is preferable that the coating liquid passes through the filter in a state where the liquid sending system is sufficiently filled. When such filling is insufficient, air bubbles escape from the inside of the filter in the coating liquid, and air bubbles often exist in the coating liquid. As a result, a defect occurs in the obtained PVA-based resin layer, which may lead to quality deterioration.

  In the system shown in FIG. 2, the depth type filters F1a, F2a, and F3a have different filtration accuracy from each other, and the filter F3a has the lowest (coarse) filtration accuracy. Preferably, the filtration accuracy of the filter F2a is the highest. Further, preferably, the pressure applied to the coating liquid supplied to the filters F1a, F2a, F3a is changed through adjustment of the discharge amount of the coating liquid by the liquid sending pump P2 or the output of the pump.

  The coating liquid may pass through each filter a plurality of times through the circulation piping system, or may pass through each filter only once without passing through the circulation piping system. The number of times the coating solution passes through the filter (in the case of a circulation system, the circulation time) can be appropriately set depending on the purpose, the use of the laminate, the state of the coating solution, and the like. When the coating liquid is supplied to the circulation piping system, the order of the above-described filters (first filter, intermediate filter, and final filter) is determined based on the order in which the coating liquid first passes through each filter. .

[A-3. Formation of polyvinyl alcohol-based resin layer]
The coating solution from which bubbles and foreign matter have been removed as described above is applied to a resin substrate.

  The resin substrate is typically formed of a thermoplastic resin. Any appropriate resin is used as the thermoplastic resin. For example, (meth) acrylic resin, olefin resin, norbornene resin, polyester resin and the like can be mentioned. Preferably, a polyester resin is used. Among them, an amorphous (non-crystallized) polyethylene terephthalate resin is preferably used. In particular, an amorphous (hard to crystallize) polyethylene terephthalate resin is particularly preferably used. Specific examples of the amorphous polyethylene terephthalate-based resin include a copolymer further containing isophthalic acid as a dicarboxylic acid and a copolymer further containing cyclohexane dimethanol as a glycol.

  When the underwater stretching method is employed in the stretching process described below, the above-mentioned preferable resin base material can absorb water, and the water can act as a plasticizer to be plasticized. As a result, the stretching stress can be greatly reduced, stretching can be performed at a high magnification, and the stretchability of the resin base material can be superior to that in air stretching. As a result, a polarizing film having excellent optical characteristics can be manufactured. In one embodiment, the resin substrate preferably has a water absorption of 0.2% or more, more preferably 0.3% or more. On the other hand, the water absorption of the resin substrate is preferably 3.0% or less, more preferably 1.0% or less. By using such a resin base material, it is possible to prevent problems such as dimensional stability of the resin base material being remarkably reduced at the time of production and deterioration in appearance of the obtained polarizing film. Further, it is possible to prevent the base material from being broken during the stretching in water and the PVA-based resin layer from peeling off from the resin base material. The water absorption of the resin substrate can be adjusted, for example, by introducing a modifying group into the constituent material. The water absorption is a value determined according to JIS K 7209.

  The glass transition temperature (Tg) of the resin substrate is preferably 170 ° C. or lower. By using such a resin base material, it is possible to sufficiently secure the stretchability of the laminate while suppressing crystallization of the PVA-based resin layer. Furthermore, the temperature is more preferably 120 ° C. or less in consideration of plasticization of the resin base material with water and good stretching in water. In one embodiment, the glass transition temperature of the resin substrate is preferably 60 ° C or higher. By using such a resin base material, problems such as deformation of the resin base material (for example, generation of irregularities, bulges, wrinkles, etc.) can be prevented when the coating liquid containing the PVA-based resin is applied and dried. Thus, a laminate can be favorably manufactured. Further, the stretching of the PVA-based resin layer can be favorably performed at a suitable temperature (for example, about 60 ° C.). In another embodiment, the glass transition temperature may be lower than 60 ° C. if the resin base material is not deformed when applying and drying the coating liquid containing the PVA-based resin. The glass transition temperature of the resin substrate can be adjusted, for example, by heating using a crystallization material that introduces a modifying group into the constituent material. The glass transition temperature (Tg) is a value determined according to JIS K7121.

  The thickness of the resin substrate is preferably 20 μm to 300 μm, more preferably 30 μm to 200 μm.

  The resin substrate may be previously subjected to a surface treatment (for example, a corona treatment or the like). This is because the adhesion between the resin base material and the PVA-based resin layer can be improved.

  Any appropriate method can be adopted as a method of applying the coating liquid. For example, a roll coating method, a spin coating method, a wire bar coating method, a dip coating method, a die coating method, a curtain coating method, a spray coating method, a knife coating method (such as a comma coating method) and the like can be mentioned.

  The coating liquid is applied so that the thickness of the PVA-based resin layer after drying is preferably 3 μm to 40 μm, more preferably 3 μm to 20 μm. The coating / drying temperature of the coating solution is preferably 50 ° C. or higher.

As described above, a PVA-based resin layer is formed on a resin substrate, and a laminate can be obtained. FIG. 3 is a schematic sectional view of a laminate according to a preferred embodiment of the present invention. Preferably, the laminate 100 includes a long resin substrate 110 and a polyvinyl alcohol (PVA) -based resin layer 120 provided on one side of the resin substrate 110. As a result obtained by the production method of the present invention, the laminate has very few defects caused by bubbles or foreign matters in the PVA-based resin layer. More specifically, the number of defects having a maximum diameter of 100 μm or more in the PVA-based resin layer is, for example, 0.18 / m ² or less, preferably 0.09 / m ² or less, and more preferably 0/9 or less. .018 / m ² or less. As described above, a thin polarizing film excellent in quality and optical characteristics can be obtained by using a laminate in which the defects of the PVA-based resin layer are significantly reduced.

[B. Method for producing polarizing film]
The method for producing a polarizing film of the present invention, according to the method for producing a laminate described in the above section A, to obtain a laminate having a resin substrate and a PVA-based resin layer formed on the resin substrate, Dyeing and stretching the PVA-based resin layer.

[B-1. Production of laminated body]
The production of the laminate is performed according to the method for producing a laminate described in the section A. Therefore, the details are omitted.

[B-2. staining]
The PVA-based resin layer may be subjected to a dyeing treatment. As a method of dyeing, for example, a method of immersing a PVA-based resin layer (laminate) in a dyeing solution containing a dichroic substance, a method of coating the PVA-based resin layer with the dyeing solution, and a method of applying the dyeing solution to a PVA-based resin layer A method of spraying the resin layer can be used. Preferably, a method of immersing the PVA-based resin layer (laminate) in the dyeing solution is used.

  Specific examples of the dichroic substance include iodine and organic dyes. These can be used alone or in combination of two or more. Preferably, iodine is used as the dichroic substance.

  The staining solution is preferably an iodine aqueous solution. The amount of iodine is preferably 0.1 to 0.5 parts by weight based on 100 parts by weight of water. In order to increase the solubility of iodine in water, it is preferable to add iodide to an aqueous iodine solution. Examples of iodides include potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, and titanium iodide. And the like. Among these, potassium iodide is preferable. The amount of iodide is preferably 0.02 to 20 parts by weight, more preferably 0.1 to 10 parts by weight, based on 100 parts by weight of water.

  The temperature of the dyeing solution at the time of dyeing is preferably 20 ° C to 50 ° C. The immersion time is preferably 5 seconds to 5 minutes. The staining conditions (concentration, solution temperature, immersion time) can be set so that the degree of polarization or the single transmittance of the finally obtained polarizing film is within a predetermined range.

[B-3. Stretching]
The PVA-based resin layer can be stretched. Typically, the PVA-based resin layer can be stretched integrally with the resin substrate, that is, as a laminate. Any appropriate method can be adopted as the stretching method. Specifically, fixed-end stretching (for example, a method using a tenter stretching machine) or free-end stretching (for example, uniaxial stretching through a laminate between rolls having different peripheral speeds) may be used. Simultaneous biaxial stretching (for example, a method using a simultaneous biaxial stretching machine) or sequential biaxial stretching may be used. The stretching of the laminate may be performed in one stage or may be performed in multiple stages. In the case of performing in multiple stages, the stretching ratio of the laminate described later is a product of the stretching ratios in each stage.

  Any appropriate direction can be selected as the stretching direction of the laminate. In one embodiment, the elongate laminate is stretched in the longitudinal direction. Specifically, the laminate is transported in the longitudinal direction, and is in the transport direction (MD). In another embodiment, the elongate laminate is stretched in the width direction. Specifically, it is a direction (TD) orthogonal to the transport direction (MD) of transporting the laminate in the longitudinal direction.

  The laminate is preferably stretched 4.0 times or more from the original length, more preferably 5.0 times or more.

  The stretching treatment may be an underwater stretching method performed while the laminate is immersed in a stretching bath, or may be an air stretching method. Preferably, the underwater stretching treatment is performed at least once, and more preferably, the underwater stretching treatment and the air stretching treatment are combined. According to the in-water stretching, the thermoplastic resin substrate or 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 is crystallized. Stretching can be performed at a high magnification while suppressing it. As a result, a polarizing film having excellent optical characteristics (for example, a degree of polarization) can be manufactured.

  The stretching temperature of the laminate can be set to any appropriate value according to the forming material of the resin substrate, the stretching method, and the like. When the aerial stretching method is employed, the stretching temperature is preferably equal to or higher than the glass transition temperature (Tg) of the resin substrate, more preferably equal to or higher than the glass transition temperature (Tg) of the resin substrate + 10 ° C., and particularly preferably Tg + 15 ° C. That is all. On the other hand, the stretching temperature of the laminate is preferably 170 ° C. or less. By stretching at such a temperature, rapid progress of crystallization of the PVA-based resin can be suppressed, and problems caused by the crystallization (for example, hindering the orientation of the PVA-based resin by stretching) can be suppressed. .

  When employing the underwater stretching method as the stretching method, the liquid temperature of the stretching bath is preferably from 40 ° C to 85 ° C, more preferably from 50 ° C to 85 ° C. At such a temperature, stretching can be performed at a high magnification while suppressing dissolution of the PVA-based resin. Specifically, as described above, the glass transition temperature (Tg) of the 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., there is a possibility that the stretching may not be performed well even in consideration of the plasticization of the resin substrate by water. On the other hand, the higher the temperature of the stretching bath is, the higher the solubility of the PVA-based resin is, and there is a possibility that excellent polarization characteristics cannot be obtained. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

  When the underwater stretching method is employed, the laminate is preferably immersed in a boric acid aqueous solution and stretched (boric acid in water stretching). By using a boric acid aqueous solution as the stretching bath, it is possible to give the PVA-based resin rigidity to withstand the tension applied during stretching and water resistance that does not dissolve in water. The boric acid aqueous solution is preferably obtained by dissolving boric acid and / or borate in water as a solvent. The concentration of boric acid is preferably 1 to 10 parts by weight based on 100 parts by weight of water. By setting the boric acid concentration to 1 part by weight or more, the dissolution of the PVA-based resin can be effectively suppressed.

  Preferably, the in-water stretching is performed after the dyeing of the PVA-based resin layer. This is because it can be more excellent in stretchability. In this case, it is preferable to mix iodide in the boric acid aqueous solution. This is because elution of iodine contained in the PVA-based resin layer can be suppressed. The concentration of iodide is preferably 0.05 to 15 parts by weight, more preferably 0.5 to 8 parts by weight, based on 100 parts by weight of water.

  Preferably, the underwater stretching is performed at least once. By adopting underwater stretching, the dyeability can be ensured while the PVA-based resin contained in the PVA-based resin layer has a high degree of saponification (for example, 99.0 mol% or more). Specifically, when a PVA-based resin having a high degree of saponification is stretched at a high temperature (for example, 120 ° C. or higher), sufficient dyeing properties may not be ensured after stretching. In one embodiment, the laminate is subjected to a dyeing step after being stretched in the air at, for example, 95 ° C. to 150 ° C., and then stretched by underwater stretching. In this case, the stretch ratio of the laminate by aerial stretching is, for example, 1.5 to 3.5 times, and preferably 2.0 to 3.0 times. The stretch ratio of the laminate by underwater stretching is preferably 2.0 times or more.

[B-4. Other processing]
The PVA-based resin layer (laminate) may be subjected to any appropriate treatment other than the above. For example, an insolubilization treatment, a cross-linking treatment, a washing treatment, and a drying treatment are included.

(Insolubilization treatment)
The insolubilization treatment is typically performed by immersing a PVA-based resin layer (laminate) in a boric acid aqueous solution. In particular, when the underwater stretching method is adopted, water resistance can be imparted to the PVA-based resin by performing the insolubilization treatment. The concentration of the boric acid aqueous solution is preferably 1 part by weight to 4 parts by weight based on 100 parts by weight of water. The liquid temperature of the insolubilizing bath (boric acid aqueous solution) is preferably 20C to 40C. Preferably, the insolubilization treatment is performed before dyeing or stretching in water.

(Cross-linking treatment)
The crosslinking treatment is typically performed by immersing a PVA-based resin layer (laminate) in an aqueous boric acid solution. By performing the cross-linking treatment, water resistance can be imparted to the PVA-based resin. The concentration of the boric acid aqueous solution is preferably 1 part by weight to 4 parts by weight based on 100 parts by weight of water. When a crosslinking treatment is performed after the above-mentioned dyeing, it is preferable to further mix an iodide. By blending iodide, elution of iodine adsorbed on the PVA-based resin layer can be suppressed. The amount of iodide is preferably 1 to 5 parts by weight based on 100 parts by weight of water. Specific examples of iodide are as described above. The liquid temperature of the crosslinking bath (boric acid aqueous solution) is preferably from 20C to 50C. Preferably, the crosslinking treatment is performed before stretching in water. In one embodiment, dyeing, crosslinking and stretching in water are performed in this order.

(Washing treatment)
The above-described cleaning treatment is typically performed by immersing the PVA-based resin layer in an aqueous potassium iodide solution.

(Drying)
The drying temperature in the above drying treatment is, for example, 30C to 100C.

[B-5. Polarizing film]
The polarizing film obtained by the production method of the present invention 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 42.0% or more, further preferably 42.5% or more, and particularly preferably 43.0% or more. The degree of polarization of the polarizing film is preferably 99.8% or more, more preferably 99.9% or more, and further preferably 99.95% or more. The degree of polarization (P) is calculated by the following equation by measuring the single transmittance (Ts), the parallel transmittance (Tp), and the orthogonal transmittance (Tc). Here, Ts, Tp, and Tc are Y values measured with a 2-degree visual field (C light source) according to JIS Z8701 and subjected to visibility correction.
Degree of polarization (P) (%) = {(Tp−Tc) / (Tp + Tc)} ^1/2 × 100

  The thickness of the polarizing film (resin film) is 8 μm or less, preferably 5 μm or less. On the other hand, the thickness of the polarizing film is preferably at least 1.0 μm, more preferably at least 2.0 μm.

According to the production method of the present invention, the number of appearance defects having a maximum diameter of 100 μm or more is, for example, 0.12 / m ² , due to the use of a laminate having a PVA-based resin layer having significantly less appearance defects. A polarizing film having a thickness of at most 0.06 / m ² or less, more preferably at most 0.012 / m ² can be obtained.

Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited to these Examples. In addition, the measuring method of each characteristic is as follows.
1. The thickness was measured using a digital micrometer (manufactured by Anritsu Corporation, product name “KC-351C”).
2. Glass transition temperature (Tg)
It measured according to JISK7121.

[Example 1]
A 7% aqueous solution of PVA was prepared by dissolving PVA powder having a polymerization degree of 4200 and a saponification degree of 99.2% in water. The viscosity of this PVA aqueous solution was measured at 23 ° C. and a rotor rotation speed of 20 rpm using a B-type viscometer (manufactured by Toki Sangyo Co., Ltd.). The viscosity of the PVA aqueous solution was 2000 mPa · s. On the other hand, a system as shown in FIG. 2 for removing bubbles and foreign matters of the coating solution was constructed. In this system, a charge tank, a liquid sending pump, and three types of filters F1a, F2a, and F3a are provided in series, and the PVA aqueous solution is supplied from the tank to the filter by the liquid sending pump, and is passed through the filter to the tank. It is possible to cycle back and forth. As the filter F1a, a depth type cartridge filter (manufactured by Loki Techno Co., Ltd., model number "500L-MAP-150EF", filtration accuracy: 10 μm) is used. As the filter F2a, a depth type cartridge filter (manufactured by Loki Techno Co., Ltd., model number) “500L-SHP-100EF”, filtration accuracy 5 μm) was used, and a depth type cartridge filter (manufactured by Loki Techno Co., model number “500L-SRL-750PZF”, filtration accuracy 100 μm) was used as the filter F3a.

  The PVA aqueous solution prepared above was circulated in this system for 1 hour. More specifically, a PVA aqueous solution is charged into a charge tank, supplied to a filter by a liquid sending pump (liquid sending amount: 10 L / min), returned to the tank after passing through the filter, and this cycle is defined as one cycle. Repeated. At that time, the pressure was fluctuated by a profile as shown in FIG. 4 by intermittently stopping the liquid feed pump. Here, the difference in pressure applied to the coating liquid between when the pump was operating and when the pump was stopped was set to be 0.18 MPa. FIG. 4 shows a pressure profile for 20 minutes, and in this embodiment, this was repeated three cycles.

  A commercially available polyethylene terephthalate film (manufactured by Mitsubishi Plastics, Inc., trade name “SH046”, Tg: 70 ° C., thickness: 200 μm) was used as it was as a resin substrate. On one surface of the resin substrate, a PVA aqueous solution from which bubbles and foreign matters were removed as described above was applied by a slot die coater, and dried at a temperature of 60 ° C. to form a 10 μm thick PVA-based resin layer.

The formed PVA-based resin layer was observed under a microscope, and evaluated based on the number of appearance defects (bubble defects and foreign matter defects) having a maximum diameter of 100 μm or more. Table 1 shows the evaluation results. In the evaluation of appearance defects, from the viewpoint of being applicable to a 55-inch size image display device when the PVA-based resin layer is used as a polarizing film, 0.553 m ² of PVA is used for each of the bubble defect and the foreign matter defect. When one or more defects having a maximum diameter of 100 μm or more per system resin layer were judged as “poor”, when less than one defect was judged as “good”.

[Example 2]
Removal of bubbles and foreign matter from the PVA aqueous solution in the same manner as in Example 1 except that a depth type cartridge filter (manufactured by Loki Techno Co., Ltd., model number “500L-MAP-050EF”, filtration accuracy: 3 μm) was used as the filter F2a. Then, a PVA-based resin layer was formed to obtain a laminate. Further, the same evaluation as in Example 1 was performed. Table 1 shows the results.

[Example 3]
Removal of air bubbles and foreign matter in the PVA aqueous solution in the same manner as in Example 1 except that a depth type cartridge filter (manufactured by Loki Techno Co., Ltd., model number "500L-SRL-100EF", filtration accuracy: 50 m) is used as the filter F3a. Then, a PVA-based resin layer was formed to obtain a laminate. Further, the same evaluation as in Example 1 was performed. Table 1 shows the results.

[Comparative Example 1]
Except that the filters F1a and F2a were not used (only the filter F3a was passed through the aqueous PVA solution), bubbles and foreign matters in the PVA aqueous solution were removed in the same manner as in Example 1, and then a PVA-based resin layer was formed. Thus, a laminate was obtained. Further, the same evaluation as in Example 1 was performed. Table 1 shows the results.

[Comparative Example 2]
As the filter F1a, a depth type cartridge filter (manufactured by Loki Techno Co., Ltd., model number “500L-SRL-750EF”, filtration accuracy: 100 μm) is used. As the filter F2a, a depth type cartridge filter (manufactured by Loki Techno Co., Ltd., model number) "500L-SRL-100EF", filtration accuracy 50 m), except that a depth type cartridge filter (manufactured by Loki Techno Co., model number "500L-SHP-100EF", filtration accuracy 5 m) is used as the filter F3a. In the same manner as in Example 1, air bubbles and foreign substances in the PVA aqueous solution were removed, and then a PVA-based resin layer was formed to obtain a laminate. Further, the same evaluation as in Example 1 was performed. Table 1 shows the results.

[Comparative Example 3]
Except that the filter F3a was not used (only the filters F1a and F2a were passed through the PVA aqueous solution), bubbles and foreign substances of the PVA aqueous solution were removed in the same manner as in Example 1, and then a PVA-based resin layer was formed. Thus, a laminate was obtained. Further, the same evaluation as in Example 1 was performed. Table 1 shows the results.

  As is clear from Table 1, according to the embodiment of the present invention, both the removal of air bubbles and the removal of foreign matter are preferably compatible. On the other hand, according to the comparative example, compatibility between the removal of air bubbles and the removal of foreign matter is insufficient.

  The laminate obtained by the production method of the present invention is suitably used for producing a polarizing film.

DESCRIPTION OF SYMBOLS 11 Preparation tank 12 Charge tank 20 Coating die F1a-F3a Depth type filter P1, P2 Liquid sending pump V1 Three-way valve 100 Laminated body 110 Resin base material 120 Polyvinyl alcohol (PVA) resin layer

Claims (6)

The coating solution containing a polyvinyl alcohol resin, through a plurality of depth type filters, to remove bubbles and foreign matter,
A method for producing a laminate, comprising: applying the coating solution passed through the plurality of depth type filters to one side of a resin substrate and drying to form a polyvinyl alcohol-based resin layer,
The plurality of depth type filters include at least three types of depth type filters having different filtration accuracy from each other,
The final depth type filter through which the coating liquid finally passes has the lowest filtration accuracy,
The filtration accuracy of the final depth type filter is 50 μm to 100 μm,
A method for manufacturing a laminate.   2. The method according to claim 1, wherein, of the at least three types of depth type filters, the first depth type filter through which the coating liquid first passes has the second lowest filtration accuracy. 3.   The method for producing a laminate according to claim 2, wherein the filtration accuracy of the first depth type filter is 5 m to 20 m.   The method for manufacturing a laminate according to any one of claims 1 to 3, wherein a pressure applied to the coating liquid supplied to the plurality of depth type filters is changed to remove bubbles inside the plurality of depth type filters. .   The method for producing a laminate according to any one of claims 1 to 4, wherein a viscosity of the coating liquid that passes through the plurality of depth type filters is 100 mPa · s to 10,000 mPa · s. A laminate having a resin substrate and a polyvinyl alcohol-based resin layer formed on the resin substrate by the method for producing a laminate according to any one of claims 1 to 5,
Dyeing and stretching the polyvinyl alcohol-based resin layer,
A method for producing a polarizing film, comprising:

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