CN111201458A

CN111201458A - Method for manufacturing optical laminate

Info

Publication number: CN111201458A
Application number: CN201880066179.7A
Authority: CN
Inventors: M.矢仙; 滨本大介; 池岛健太郎; 宫武稔
Original assignee: Nitto Denko Corp
Current assignee: Nitto Denko Corp
Priority date: 2017-10-18
Filing date: 2018-10-09
Publication date: 2020-05-26
Anticipated expiration: 2038-10-09
Also published as: KR102625781B1; JP7083612B2; CN111201458B; KR20200074098A; WO2019078050A1; JP2019074691A; TW201922504A

Abstract

The invention provides a method for obtaining an optical laminate which maintains the transparency of a thermoplastic resin substrate and suppresses curling. The method for manufacturing the optical laminate of the present invention includes: forming a polyvinyl alcohol resin layer on a thermoplastic resin substrate to produce a laminate; subjecting the polyvinyl alcohol resin layer to a stretching treatment and a dyeing treatment to produce a polarizing film; and subjecting the laminate to contact heating to subject the thermoplastic resin substrate to a crystallization treatment, the crystallization treatment comprising: a 1 st crystallization treatment stage for increasing the crystallization index of the thermoplastic resin substrate to a 1 st predetermined value; and a 2 nd crystallization treatment stage for making the crystallization index of the thermoplastic resin substrate a 2 nd predetermined value greater than the 1 st predetermined value.

Description

Method for manufacturing optical laminate

Technical Field

The present invention relates to a method for manufacturing an optical laminate.

Background

In a liquid crystal display device, which is a typical image display device, polarizing films are disposed on both sides of a liquid crystal cell due to its image forming system. As a method for producing a polarizing film, for example, a method has been proposed in which a laminate having a thermoplastic resin substrate and a polyvinyl alcohol (PVA) -based resin layer is stretched and then immersed in a dyeing solution to obtain a polarizing film (for example, patent document 1). Since a polarizing film having a small thickness can be obtained by this method, it has attracted attention to the thinning of liquid crystal display devices in recent years.

The polarizing film is usually produced by immersing the PVA-based resin film in a bath such as a dyeing bath or a crosslinking bath and then drying the PVA-based resin film. However, when a polarizing film is produced using a thermoplastic resin substrate as described above, curling (specifically, curling that protrudes toward the thermoplastic resin substrate) tends to occur during drying, and the problem is that the appearance of the resulting optical laminate (thermoplastic resin substrate and polarizing film) is poor. In contrast, as a method for producing an optical laminate excellent in appearance by suppressing the curl, a method including a drying treatment using a heating roller has been proposed (patent document 2). According to this production method, an optical laminate with suppressed curling can be obtained. On the other hand, the drying treatment using the heating roller may decrease the transparency of the thermoplastic resin substrate, and therefore, is not preferable from the viewpoint of being used as a protective film for a polarizing film without peeling off and removing the thermoplastic resin substrate.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2001-343521

Patent document 2: japanese patent laid-open publication No. 2013-122518

Disclosure of Invention

Problems to be solved by the invention

The present invention has been made to solve the above conventional problems, and a main object thereof is to provide a method for obtaining an optical laminate in which curling is suppressed while maintaining transparency of a thermoplastic resin substrate.

Means for solving the problems

According to the present invention, there is provided a method for manufacturing an optical laminate, the method comprising: forming a polyvinyl alcohol resin layer on a thermoplastic resin substrate to produce a laminate; subjecting the polyvinyl alcohol resin layer to a stretching treatment and a dyeing treatment to produce a polarizing film; and subjecting the laminate to contact heating to crystallize the thermoplastic resin substrate. The crystallization treatment includes: a 1 st crystallization treatment stage for increasing the crystallization index of the thermoplastic resin substrate to a 1 st predetermined value; and a 2 nd crystallization treatment stage for making the crystallization index of the thermoplastic resin substrate a 2 nd predetermined value greater than the 1 st predetermined value.

In one embodiment, the 1 st crystallization treatment stage includes a contact heating mechanism for contacting the laminate at 50 to 95 ℃, and the 1 st predetermined value is 0.4 or more.

In one embodiment, the 2 nd crystallization treatment stage includes a contact heating mechanism for contacting the stacked body at 96 to 120 ℃, and the 2 nd predetermined value is 0.55 or more.

In one embodiment, in the 1 st and 2 nd crystallization treatment stages, the contact time between any 1 point on the surface of the laminate and each of the contact heating means is 0.1 to 5 seconds.

In one embodiment, the thermoplastic resin substrate is made of a polyethylene terephthalate resin.

In one embodiment, the 1 st and 2 nd crystallization treatment stages are performed at an atmospheric temperature of 60 to 120 ℃.

In one embodiment, the shrinkage of the thermoplastic resin substrate due to the crystallization treatment is 3% or less.

According to another aspect of the present invention, there may be provided a method of manufacturing a polarizing plate, the method including: manufacturing an optical laminate by the method for manufacturing an optical laminate; and disposing an optical functional film on the polarizing film side of the obtained optical laminate.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, when the laminate is dried, the thermoplastic resin substrate is subjected to a multistage crystallization treatment including the 1 st crystallization treatment stage and the 2 nd crystallization treatment stage. Specifically, in the 1 st crystallization treatment stage, the thermoplastic resin substrate is crystallized to a given crystal index by the contact heating means of a relatively low temperature, and then in the 2 nd crystallization treatment stage, the crystallization is further advanced by the contact heating means of a relatively high temperature. Since the contact heating means having a high temperature may cause white turbidity when the thermoplastic resin substrate having a small crystal index is brought into contact with the contact heating means having a high temperature, the problem of white turbidity can be avoided by bringing the contact heating means into contact with the thermoplastic resin substrate having a low crystal index after the pre-crystallization treatment as described above, and efficient and sufficient crystallization can be achieved. As a result, an optical laminate in which the transparency of the thermoplastic resin substrate is maintained and curling is suppressed can be obtained.

Drawings

Fig. 1 is a schematic cross-sectional view of a laminate according to a preferred embodiment of the present invention.

Fig. 2 is a schematic diagram showing an example of crystallization treatment in the method for producing an optical laminate according to the present invention.

Fig. 3 is a schematic cross-sectional view of an optical stack according to a preferred embodiment of the present invention.

Fig. 4 is a graph showing the relationship between the crystal index and the durability of the thermoplastic resin base material.

Fig. 5 is a photograph showing the state of the laminate after the durability test.

Description of the symbols

10 laminated body

11 thermoplastic resin base Material

12 polyvinyl alcohol resin layer

100 optical stack

Detailed Description

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

The method for manufacturing the optical laminate of the present invention includes: forming a polyvinyl alcohol resin layer on a thermoplastic resin substrate to produce a laminate; subjecting the polyvinyl alcohol resin layer to a stretching treatment and a dyeing treatment to produce a polarizing film; and subjecting the laminate to contact heating to crystallize the thermoplastic resin substrate. The laminate is typically formed in a long shape.

A. Production of laminate

Fig. 1 is a schematic cross-sectional view of a laminate according to a preferred embodiment of the present invention. The laminate 10 has a thermoplastic resin substrate 11 and a PVA type resin layer 12, and can be produced by forming the PVA type resin layer 12 on the thermoplastic resin substrate. Any suitable method can be used for forming the PVA-based resin layer 12. The PVA-based resin layer 12 is preferably formed by applying a coating solution containing a PVA-based resin on the thermoplastic resin substrate 11 and drying the coating solution.

The thermoplastic resin substrate preferably has a crystal index (before crystallization treatment) of 0.16 or less, more preferably 0.12 or less. Such a thermoplastic resin substrate can promote crystallization and increase the crystallinity index during drying treatment. As a result, the thermoplastic resin substrate becomes rigid, and is in a state of being able to withstand shrinkage of the PVA type resin layer due to drying, and curling can be suppressed. By using such a thermoplastic resin substrate, the laminate can be satisfactorily stretched. Specifically, when the laminate is immersed in a stretching bath (for example, an aqueous boric acid solution) and stretched in an aqueous solution as described later, the stretching tension decreases and the stretchability increases.

The water absorption of the thermoplastic resin substrate is preferably 0.2% or more, and more preferably 0.3% or more. The thermoplastic resin substrate absorbs water, and the water can play a role of a plasticizer to plasticize. As a result, the tensile stress can be greatly reduced, and high-rate stretching can be achieved. On the other hand, the water absorption of the thermoplastic resin substrate is preferably 3.0% or less, and more preferably 1.0% or less. By using such a thermoplastic resin substrate, it is possible to prevent problems such as a significant decrease in dimensional stability of the thermoplastic resin substrate during production and deterioration in appearance of the obtained polarizing film. Further, the substrate can be prevented from being broken and the PVA based resin layer can be prevented from being peeled off from the thermoplastic resin substrate when stretched in an aqueous solution. The water absorption of the thermoplastic resin base material can be adjusted by, for example, introducing a modifying group into the constituent material. The water absorption is a value obtained according to JIS K7209.

The glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 170 ℃ or lower. By using such a thermoplastic resin substrate, the stretchability of the laminate can be sufficiently ensured while suppressing crystallization of the PVA-based resin layer. In addition, from the viewpoint of plasticizing the thermoplastic resin substrate with water and enabling good stretching in an aqueous solution, it is more preferably 120 ℃ or lower. On the other hand, the glass transition temperature of the thermoplastic resin substrate is preferably 60 ℃ or higher. By using such a thermoplastic resin substrate, it is possible to prevent troubles such as deformation (for example, generation of irregularities, slackness, wrinkles, and the like) of the thermoplastic resin substrate when the coating liquid containing the PVA-based resin is applied and dried, and to favorably produce a laminate. Further, the PVA-based resin layer can be favorably stretched at an appropriate temperature (for example, about 60 ℃). The glass transition temperature of the thermoplastic resin substrate can be adjusted by, for example, heating a crystallized material having a modifying group introduced into the constituent material. The glass transition temperature (Tg) is a value determined according to JIS K7121.

Any suitable material can be used as the constituent material of the thermoplastic resin base material. It is preferable to use a material that can give a resin base material having a desired crystallinity index. The crystallinity index can be adjusted, for example, by introducing a modifying group into the constituent material. The thermoplastic resin substrate is preferably made of an amorphous (noncrystalline) polyethylene terephthalate resin. Among them, amorphous (less likely to crystallize) polyethylene terephthalate resins are particularly preferably used. Specific examples of the amorphous polyethylene terephthalate resin include copolymers further containing isophthalic acid and/or cyclohexanedicarboxylic acid as dicarboxylic acids, and copolymers further containing cyclohexanedimethanol and diethylene glycol as ethylene glycol.

In a preferred embodiment, the thermoplastic resin substrate is composed of a polyethylene terephthalate-based resin having an isophthalic acid unit. This is because such a thermoplastic resin substrate is very excellent in stretchability and can be inhibited from crystallizing during stretching. This is considered to be caused by introduction of an isophthalic acid unit and large bending of the main chain. The polyethylene terephthalate resin has a terephthalic acid unit and an ethylene glycol unit. The content ratio of the isophthalic acid unit is preferably 0.1 mol% or more, more preferably 1.0 mol% or more, based on the total of all the repeating units. This is because a thermoplastic resin substrate having very excellent stretchability can be obtained. On the other hand, the content ratio of the isophthalic acid unit is preferably 20 mol% or less, more preferably 10 mol% or less, relative to the total of all the repeating units. By setting the content ratio as described above, the crystal index can be increased favorably in the crystallization treatment described later.

The thickness of the thermoplastic resin substrate before stretching is preferably 20 to 300. mu.m, more preferably 50 to 200. mu.m. If the thickness is less than 20 μm, the PVA based resin layer may be difficult to form. If it exceeds 300 μm, for example, in the stretching treatment in an aqueous solution described later, the thermoplastic resin base material may take a long time to absorb water and an excessive load may be required for stretching.

Any suitable PVA-based resin can be used. Examples thereof include polyvinyl alcohol and ethylene-vinyl alcohol copolymers. Polyvinyl alcohol can be obtained by saponifying polyvinyl acetate. The ethylene-vinyl alcohol copolymer can be obtained by saponifying an 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 degree of saponification was determined in accordance with JIS K6726-. By using the PVA-based resin having such a saponification degree, a polarizing film having excellent durability can be obtained. When the saponification degree is too high, gelation may occur.

The average polymerization degree of the PVA-based resin may be appropriately selected depending on the purpose. The average degree of polymerization is usually 1000 to 10000, preferably 1200 to 4500, and more preferably 1500 to 4300. The average polymerization degree can be determined in accordance with JIS K6726-.

The coating liquid is typically a solution obtained by dissolving the PVA-based resin in a solvent. Examples of the solvent include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols such as glycerol and trimethylolpropane, and amines such as ethylenediamine and diethylenetriamine. They may be used alone or two or more of them may be used in combination. Of these, water is preferred. The concentration of the PVA based resin in the solution is preferably 3 to 20 parts by weight based on 100 parts by weight of the solvent. When the resin concentration is such as described above, a uniform coating film can be formed in close contact with the thermoplastic resin substrate.

The coating liquid may contain an additive. Examples of the additives include plasticizers and surfactants. Examples of the plasticizer include polyhydric alcohols such as ethylene glycol and glycerin. Examples of the surfactant include nonionic surfactants. These can be used for further improving the uniformity, dyeing property and stretchability of the PVA-based resin layer obtained. Further, examples of the additive include an easily bondable component. By using the easily adhesive component, the adhesion between the resin base and the PVA-based resin layer can be improved. As a result, for example, the PVA-based resin layer can be prevented from being peeled off from the resin substrate, and dyeing and stretching in an aqueous solution described later can be performed satisfactorily. As the easy-adhesion component, for example, a modified PVA such as acetoacetyl-modified PVA can be used. Examples of the additive include halides such as potassium iodide, sodium iodide, lithium iodide, and sodium chloride, and urea. By adding these additives, optical characteristics (for example, monomer transmittance) can be improved. The amount of the additive to be blended may be appropriately set according to the purpose.

Any suitable method can be used for applying the coating liquid. Examples of the coating method include roll coating, spin coating, wire bar coating, dip coating, die coating, shower coating, spray coating, and blade coating (e.g., doctor blade coating).

The coating and drying temperature of the coating liquid is preferably 50 ℃ or higher.

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

Before the PVA-based resin layer is formed, the thermoplastic resin substrate may be subjected to a surface treatment (e.g., corona treatment), or an easy-adhesion layer may be formed on the thermoplastic resin substrate. By performing such treatment, the adhesion between the thermoplastic resin substrate and the PVA-based resin layer can be improved.

B. Production of polarizing film

The polarizing film is typically produced by subjecting a PVA-based resin layer formed on the thermoplastic resin substrate to a stretching treatment and a dyeing treatment. The PVA-based resin layer may be appropriately subjected to a process for forming the PVA-based resin layer into a polarizing film, in addition to the stretching process and the dyeing process. Examples of the treatment for forming the polarizing film include insolubilization treatment, crosslinking treatment, and washing treatment. These treatments may be selected according to purposes. In addition, processing conditions such as processing order, processing time, and the number of times of processing can be set as appropriate. The respective processes will be described below.

(dyeing treatment)

The dyeing treatment is typically performed by dyeing the PVA-based resin layer with a dichroic substance. Preferably, the dichroic material is adsorbed on the PVA-based resin layer. Examples of the adsorption method include: a method of immersing the PVA-based resin layer (laminate) in a dyeing solution containing a dichroic material; a method of applying the dyeing liquid to a PVA-based resin layer; and a method of spraying the dyeing solution onto the PVA-based resin layer. Preferably, the laminate is immersed in a dyeing solution. This is because the dichroic substance can be favorably adsorbed.

When iodine is used as the dichroic material, the dyeing liquid is preferably an aqueous iodine solution. The amount of iodine blended 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 an iodide to the aqueous iodine solution. The iodide may be exemplified by: potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, titanium iodide, and the like. Of these, potassium iodide is preferred. The amount of the 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.

In order to suppress dissolution of the PVA based resin, the dyeing liquid is preferably at a liquid temperature of 20 ℃ to 50 ℃ during dyeing. When the PVA-based resin layer is immersed in the dyeing liquid, the immersion time is preferably 5 seconds to 5 minutes in order to ensure the transmittance of the PVA-based resin layer. The dyeing conditions (concentration, liquid temperature, and immersion time) may be set so that the polarization degree or monomer transmittance of the polarizing film to be finally obtained falls within a predetermined range. In one embodiment, the immersion time is set so that the degree of polarization of the obtained polarizing film is 99.98% or more. In another embodiment, the immersion time is set so that the monomer transmittance of the obtained polarizing film is 40% to 45%.

(stretching treatment)

Any suitable method can be used for stretching the laminate. Specifically, the stretching may be performed by fixed-end stretching (for example, a method using a tenter) or by free-end stretching (for example, a method in which the laminate is uniaxially stretched by passing it between rolls having different peripheral speeds). Further, simultaneous biaxial stretching (for example, a method using a simultaneous biaxial stretcher) may be employed, or stepwise biaxial stretching may be employed. The stretching of the laminate may be performed in one stage or in multiple stages. When the stretching is performed in multiple stages, the stretching ratio (maximum stretching ratio) of the laminate described later is the product of the stretching ratios in the respective stages.

The stretching treatment may be a stretching method in an aqueous solution in which the laminate is immersed in a stretching bath, or a stretching method in a gas atmosphere. The stretching treatment in an aqueous solution is preferably performed at least 1 time, and more preferably the stretching treatment in an aqueous solution and the stretching treatment in a gas atmosphere are combined. The stretching in an aqueous solution allows stretching at a temperature lower than the glass transition temperature (typically, about 80 ℃) of the resin substrate or the PVA type resin layer, and allows stretching at a high magnification while suppressing crystallization of the PVA type resin layer. As a result, a polarizing film having excellent optical characteristics (e.g., degree of polarization) can be manufactured.

The stretching direction of the laminate may be any suitable direction. In one embodiment, the stretching is performed along the longitudinal direction of the elongated laminate. Specifically, the laminate is transported in the longitudinal direction, i.e., the transport direction (MD) thereof. In another embodiment, the stretching is performed along the width direction of the elongated laminate. Specifically, the laminate is transported in the longitudinal direction, i.e., in The Direction (TD) orthogonal to the transport direction (MD).

The stretching temperature of the laminate may be set to any appropriate value depending on the material for forming the resin base material, the stretching method, and the like. When the stretching method in a gas atmosphere is employed, the stretching temperature is preferably not less than the glass transition temperature (Tg) of the resin substrate, more preferably not less than the glass transition temperature (Tg) +10 ℃, and particularly preferably not less than Tg +15 ℃. The stretching temperature of the laminate is preferably 170 ℃ or lower. Stretching at such a temperature can suppress rapid progress of crystallization of the PVA type resin and can further suppress defects caused by the crystallization (for example, the PVA type resin layer is inhibited from being oriented by stretching).

When the drawing method in an aqueous solution is employed as the drawing method, the liquid temperature of the drawing bath is preferably 40 to 85 ℃, more preferably 50 to 85 ℃. At such a temperature, the PVA-based resin layer can be stretched at a high draw ratio while dissolution thereof is suppressed. Specifically, as described above, the glass transition temperature (Tg) of the resin substrate is preferably 60 ℃ or higher in view of the relationship with the formation of the PVA-based resin layer. In this case, if the stretching temperature is lower than 40 ℃, there is a possibility that the resin substrate cannot be stretched well in consideration of plasticization of the resin substrate by water. On the other hand, the higher the temperature of the stretching bath, the higher the solubility of the PVA-based resin layer, and the less excellent optical characteristics may be obtained.

When the stretching in an aqueous solution is employed, the laminate is preferably stretched by immersing it in an aqueous boric acid solution (stretching in an aqueous boric acid solution). By using an aqueous boric acid solution as a stretching bath, the PVA-based resin layer can be provided with rigidity capable of withstanding the tension applied during stretching and water resistance not dissolving in water. Specifically, boric acid generates tetrahydroxyborate anions in an aqueous solution, and crosslinks with the PVA-based resin through hydrogen bonds. As a result, rigidity and water resistance can be imparted to the PVA-based resin layer, and a polarizing film having excellent optical properties can be produced by stretching the PVA-based resin layer satisfactorily.

The aqueous boric acid solution is preferably obtained by dissolving boric acid and/or a borate in water as a solvent. The boric acid concentration is preferably 1 to 10 parts by weight with respect to 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 layer can be effectively suppressed, and a polarizing film having higher characteristics can be produced. In addition to boric acid or a borate, an aqueous solution obtained by dissolving a boron compound such as borax, glyoxal, glutaraldehyde, or the like in a solvent may be used.

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

The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

The stretch ratio (maximum stretch ratio) of the laminate is preferably 5.0 times or more of the original length of the laminate. Such a high draw ratio can be achieved, for example, by drawing in an aqueous solution (drawing in an aqueous boric acid solution). In the present specification, the "maximum stretching ratio" refers to the stretching ratio immediately before the laminate breaks, and the "maximum stretching ratio" refers to a value lower than this value by 0.2.

The stretching treatment in the aqueous solution is preferably performed after the dyeing treatment.

(insolubilization treatment)

The insolubilization is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. In particular, when the stretching method in an aqueous solution is employed, water resistance can be imparted to the PVA-based resin layer by performing insolubilization treatment. The concentration of the aqueous boric acid solution is preferably 1 to 4 parts by weight with respect to 100 parts by weight of water. The liquid temperature of the insolubilization bath (aqueous boric acid solution) is preferably 20 to 40 ℃. The insolubilization treatment is preferably performed after the laminate is produced or before the dyeing treatment or the stretching treatment in an aqueous solution.

(crosslinking treatment)

The crosslinking treatment is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. The PVA-based resin layer can be provided with water resistance by performing crosslinking treatment. The concentration of the aqueous boric acid solution is preferably 1 to 4 parts by weight with respect to 100 parts by weight of water. In addition, when the crosslinking treatment is performed after the dyeing treatment, it is preferable to further incorporate an iodide. The iodine compound can suppress elution of iodine adsorbed on the PVA-based resin layer. The amount of the iodide is preferably 1 to 5 parts by weight based on 100 parts by weight of water. Specific examples of the iodide are as described above. The liquid temperature of the crosslinking bath (aqueous boric acid solution) is preferably 20 ℃ to 50 ℃. The crosslinking treatment is preferably carried out before the stretching treatment in an aqueous solution. In a preferred embodiment, the dyeing treatment, the crosslinking treatment and the stretching treatment in an aqueous solution are performed in this order.

(cleaning treatment)

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

The polarizing film produced by the above various treatments is substantially a PVA-based resin film in which a dichroic material is adsorbed and oriented. The thickness of the polarizing film is typically 25 μm or less, preferably 15 μm or less, more preferably 10 μm or less, still more preferably 7 μm or less, and particularly preferably 5 μm or less. On the other hand, the thickness of the polarizing film is preferably 0.5 μm or more, more preferably 1.5 μm or more. The polarizing film preferably exhibits absorption dichroism at any wavelength of 380nm to 780 nm. The single transmittance of the polarizing film is preferably 40.0% or more, more preferably 41.0% or more, further preferably 42.0% or more, and particularly preferably 43.0% or more. The polarization degree of the polarizing film is preferably 99.8% or more, more preferably 99.9% or more, and further preferably 99.95% or more.

C. Crystallization treatment

The crystallization treatment is performed by bringing the laminate into contact with a contact heating mechanism. The laminate is brought into contact with a contact heating means, and the laminate is dried and the thermoplastic resin substrate is crystallized, whereby an optical laminate having excellent appearance can be produced while suppressing curling.

Examples of the contact heating mechanism include a heating roller (heated transport roller), a hot plate, and an electric heating wire. Among them, heated rolls are preferred. Drying the laminate while the laminate is being held along a heated roller (heated roller drying method) can effectively promote crystallization of the thermoplastic resin substrate and increase the crystal index. As a result, the rigidity of the thermoplastic resin substrate can be increased, and the PVA-based resin layer can be allowed to stand shrinkage due to drying, whereby curling can be suppressed. Further, since the laminate can be dried while being kept flat by using the heating roller, not only curling but also wrinkles can be suppressed. The contact surface of the laminate with the contact heating means is not particularly limited, and the contact heating means is preferably brought into contact with the thermoplastic resin base material surface of the laminate.

In the present invention, the crystallization treatment includes: a 1 st crystallization treatment stage for increasing the crystallization index of the thermoplastic resin substrate to a 1 st predetermined value; and a 2 nd crystallization treatment stage for making the crystallization index of the thermoplastic resin substrate a 2 nd predetermined value greater than the 1 st predetermined value. If necessary, the thermoplastic resin substrate may further include 1 or more additional crystallization stages for increasing the crystal index of the thermoplastic resin substrate. Hereinafter, the crystallization process using a hot roller as the contact heating means will be described, but the same description applies to the crystallization process using a contact heating means other than the hot roller.

The 1 st crystallization treatment stage typically includes contacting the laminate with at least 1 heated roller at 50 to 95 ℃, and preferably includes contacting the laminate with a plurality of heated rollers at 50 to 95 ℃. By using a plurality of heated rollers, the crystallinity can be precisely controlled.

The temperature of the heated rolls in the crystallization treatment stage 1 is preferably 60 to 93 ℃, more preferably 80 to 90 ℃. The number of heating rolls is not particularly limited, and is usually 2 to 20, preferably 3 to 15. The contact time of the laminate with each heating roller is preferably 0.1 to 5 seconds.

When a plurality of heating rollers are used, these heating rollers may be set to the same or different temperatures. When the temperatures are set to different temperatures, it is preferable to set the temperatures to gradually increase from the heating roller disposed on the upstream side of the process to the heating roller disposed on the downstream side. The laminate is contacted with the heating roller at a low temperature in this order, and shrinkage, curling of the end portion, and the like of the laminate in the width direction can be effectively suppressed.

The crystallization index of the thermoplastic resin substrate at the predetermined value 1, i.e., at the end of the 1 st crystallization treatment stage, is, for example, 0.35 or more, preferably 0.38 or more, and more preferably 0.40 or more (e.g., 0.43 or more, or 0.45 or more). By entering the 2 nd crystallization treatment stage after the crystallization index is set as described above, the thermoplastic resin substrate can be crystallized while preventing cloudiness even when it is brought into contact with a high-temperature (for example, 96 ℃ or higher) heating roll, and the durability can be improved. On the other hand, the upper limit of the above-mentioned given value 1 is appropriately set in consideration of the crystal index finally required (after the 2 nd crystallization treatment stage), and is not particularly limited, but may be, for example, 0.55 or less from the viewpoint of productivity.

The 2 nd crystallization treatment stage typically comprises contacting the laminate with at least 1 heated roll at 96 ℃ to 120 ℃. The temperature of the heated roll 2 is preferably from 100 ℃ to 115 ℃, more preferably from 105 ℃ to 115 ℃. The number of heating rolls is not particularly limited, and is usually 1 to 10, preferably 1 to 5. The contact time of the laminate with the heating roller (when there are a plurality of the laminates, the contact time with each heating roller) is preferably 0.1 to 5 seconds.

The crystallization index of the thermoplastic resin substrate at the predetermined value 2, i.e., at the end of the 2 nd crystallization treatment stage, is, for example, 0.52 or more, preferably 0.55 or more, and more preferably 0.60 or more (e.g., 0.65 or more, or 0.70 or more). By setting such a crystal index, sufficient durability can be obtained. On the other hand, the upper limit of the above-mentioned 2 nd predetermined value is not particularly limited, but may be set to 0.8 or less, for example, from the viewpoint of keeping the productivity and the appearance of the laminate in a good state.

In the 2 nd crystallization treatment stage, the crystal index of the thermoplastic resin substrate is preferably increased by 0.12 or more, more preferably by 0.15 to 0.30 from the 1 st value. Crystallization is efficiently performed by a high-temperature heating roller, and high productivity can be achieved.

The crystallization index of the thermoplastic resin substrate can be controlled by adjusting the temperature of the heating rolls, the number of heating rolls, the contact time with the heating rolls, and the like in the 1 st crystallization process and the 2 nd crystallization process. The total contact time (total contact time) of the laminate and the heating rolls in the 1 st crystallization treatment and the 2 nd crystallization treatment is preferably 3 seconds to 50 seconds, more preferably 4 seconds to 20 seconds, and still more preferably 5 seconds to 15 seconds.

Fig. 2 is a schematic diagram showing an example of the crystallization process. In the illustrated example, transfer rollers R1 to R4 are provided in oven 200a, and transfer rollers R5 to R6 are provided in oven 200 b. At least 2 of the transport rollers R1 to R4 were heated to 50 ℃ to 95 ℃ to form a 1 st heated roller, and at least 1 of the rollers R5 to R6 were heated to 96 ℃ to 120 ℃ to form a 2 nd heated roller. The 1 st crystallization treatment is performed by conveying the laminate 10 by conveying rollers R1 to R4 and guide rollers G1 to G4 heated to a given temperature while crystallizing the thermoplastic resin base material to a desired crystal index, and the 2 nd crystallization treatment is performed by further increasing the crystal index of the thermoplastic resin base material while conveying by conveying rollers R5 to R6 and guide rollers G5 to G6. The laminate 10 after the 1 st and 2 nd crystallization processes is sent to a straight path (straight path) by guide rollers G7 to G8. Unlike the illustrated example, the 1 st crystallization process and the 2 nd crystallization process may be performed in the same oven (i.e., at the same atmospheric temperature), or some of the 1 st crystallization process and the 2 nd crystallization process may be performed at different temperatures.

The heating roller may be installed in a heating furnace (for example, an oven) as in the illustrated example, or may be installed in a general manufacturing line (room temperature environment) differently from the illustrated example. Preferably in a furnace. The temperature of the atmosphere in the heating furnace is preferably 60 to 120 ℃ and more preferably 80 to 110 ℃. The heating furnace may be provided with an air blowing mechanism. By using both drying with a hot roller and hot air drying, a rapid temperature change between the hot rollers can be suppressed, and shrinkage in the width direction can be easily suppressed.

The shrinkage ratio of the thermoplastic resin substrate due to the crystallization treatment [ (width of the thermoplastic resin substrate before the 1 st crystallization treatment-width of the thermoplastic resin substrate after the 1 st crystallization treatment)/width of the thermoplastic resin substrate before the 1 st crystallization treatment × 100] is preferably 3% or less, and more preferably 2% or less.

The stretching treatment in the production of the polarizing film preferably includes stretching in an aqueous solution (stretching in an aqueous boric acid solution). According to such an embodiment, a high stretch ratio can be achieved as described above, and the orientation of the thermoplastic resin base material can be improved. The thermoplastic resin substrate is heated by crystallization treatment in a state of high orientation, so that crystallization rapidly proceeds and the crystal index is significantly increased. The crystal index of the thermoplastic resin substrate after the stretching in the aqueous solution (stretching in an aqueous boric acid solution) is preferably about 0.25 to 0.35.

D. Optical laminate

The optical laminate obtained by the production method of the present invention contains the polarizing film and the thermoplastic resin substrate having a predetermined crystal index. The optical laminate may contain other members as necessary.

Fig. 3(a) and (b) are schematic cross-sectional views of an optical laminate according to a preferred embodiment. The optical laminate 100a includes a thermoplastic resin substrate 11 ', a polarizing film 12', an adhesive layer 13, and a spacer 14 in this order. The optical laminate 100b includes a thermoplastic resin substrate 11 ', a polarizing film 12', an adhesive layer 15, an optical functional film 16, an adhesive layer 13, and a spacer 14 in this order. In the present embodiment, the thermoplastic resin substrate is used as an optical member without being peeled from the polarizing film 12' obtained. The thermoplastic resin substrate 11 'can function as a protective film for the polarizing film 12', for example.

The lamination of the layers constituting the optical laminate is not limited to the illustrated example, and any appropriate adhesive layer or adhesive layer may be used. The adhesive layer is typically formed of an acrylic adhesive. The adhesive layer is typically formed of a PVA-based adhesive. The optical functional film can function as a polarizing film protective film, a retardation film, or the like, for example. The optical laminate (polarizing plate) having the above-described optical functional film (e.g., polarizing film protective film) can be obtained by, for example, laminating an optical functional film on the polarizing film side of the optical laminate subjected to the crystallization treatment with an arbitrary appropriate pressure-sensitive adhesive layer or adhesive layer interposed therebetween.

Examples

The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. The measurement method of each characteristic is as follows.

1. Thickness of

Measured using a digital micrometer (product name "KC-351C" manufactured by Anritsu Co., Ltd.).

2. Crystallinity index of thermoplastic resin substrate

FT-IR (Fourier transform infrared spectrophotometer) manufactured by Perkinelmer corporation was used, and a multiple total reflection device (ATR) was attached thereto to have a resolution of 4cm^-132 cumulative counts of peaks 1340cm of the crystalline bands from the PET spectrum^-1And 1410cm of a non-dichroic band provided as a benzene ring^-1The crystal index of the PET film was measured using the peak intensities according to the following equation.

1340cm for crystallinity index^-1Peak intensity of (2)/1410 cm^-1Peak intensity of

3. Glass transition temperature (Tg) of thermoplastic resin substrate

Measured according to JIS K7121.

[ example 1]

As the thermoplastic resin substrate, amorphous polyethylene terephthalate (IPA copolymerized PET) having a thickness of 100 μm, a crystal index of 0 to 0.1 and 5 mol% of copolymerized isophthalic acid units having a Tg of 75 ℃ is used. The surface of the film was subjected to corona treatment (58W/m)²In/min). Preparing a mixed solution of 1: 9 (trade name: GOHSEFIMER Z200, average degree of polymerization: 1200, degree of saponification: 98.5 mol% or more, degree of acetoacetylation: 5%) and a PVA-based resin (average degree of polymerization: 4200, degree of saponification: 99.2 mol%) were contained in a ratio of acetoacetyl group-modified PVA (manufactured by Nippon synthetic chemical industries, Ltd.), and 13 parts by weight of potassium iodide was added to 100 parts by weight of the PVA-based resin to prepare an aqueous PVA-based resin solution (concentration of PVA-based resin: 5.5% by weight). The aqueous solution was applied to the corona-treated surface of the resin substrate so that the film thickness was 13 μm after drying, and dried by hot air at 60 ℃ for 10 minutes to prepare a laminate. The laminate obtained was first stretched to 2.4 times in air at 140 ℃ (stretching assisted in a gas atmosphere).

Next, the laminate was immersed in an insolubilization bath (an aqueous boric acid solution prepared by adding 3 parts by weight of boric acid to 100 parts by weight of water) at a liquid temperature of 30 ℃ for 30 seconds (insolubilization treatment).

Then, the polarizing film is dyed in a dyeing bath containing iodine and potassium iodide at a liquid temperature of 30 ℃ and is immersed so that the monomer transmittance (Ts) of the finally obtained polarizing film is 40 to 45%. The dyeing liquid is prepared by using water as a solvent, and adjusting the concentration of iodine to be in the range of 0.1-0.4 wt%, and the concentration of potassium iodide to be in the range of 0.7-2.8 wt%, and adjusting the concentration ratio of iodine to potassium iodide to be 1:7 (dyeing treatment).

Next, the laminate was immersed in an aqueous boric acid solution at a liquid temperature of 30 ℃ for 60 seconds to subject the iodine-adsorbed PVA resin layer to a crosslinking treatment. In the aqueous boric acid solution in the present step, the boric acid content was made 3 parts by weight with respect to 100 parts by weight of water, and the potassium iodide content was made 3 parts by weight with respect to 100 parts by weight of water (crosslinking treatment).

Then, while immersing the laminate in an aqueous boric acid solution (an aqueous solution obtained by adding 4 parts by weight of boric acid and 5 parts by weight of potassium iodide to 100 parts by weight of water) having a liquid temperature of 70 ℃, uniaxial stretching (stretching treatment in an aqueous solution) was performed in the longitudinal direction between rolls having different circumferential speeds. The stretch ratio at this time was set to 2.3 times (the final stretch ratio was 5.5 times).

Thereafter, the laminate was cleaned with a cleaning bath (aqueous solution prepared by adding 4 parts by weight of potassium iodide to 100 parts by weight of water) (cleaning step).

Then, as shown in fig. 2, the laminate was dried while being conveyed by a heated roller provided in an oven, and the thermoplastic resin substrate was crystallized. Here, the temperatures of R1 to R4 were set to 85 ℃, R5 was set to 100 ℃, and R6 was set to 50 ℃. The hot air was blown so that the atmosphere temperature in the 1 st crystallization oven became 90 ℃, and the hot air was blown so that the atmosphere temperature in the 2 nd crystallization oven became 95 ℃. The contact time of the laminate with the heating roller (contact time of any 1 point of the laminate with each heating roller) was 2.5 seconds (10 seconds in total) for each of the heating rollers R1 to R4, and 2.5 seconds for each of the heating rollers R5.

Thus, a polarizing film having a thickness of 5 μm (target transmittance 42.5%) was produced on the thermoplastic resin substrate. The crystal index of the thermoplastic resin substrate after the stretching treatment in the aqueous solution was about 0.28.

Next, a protective film (acrylic, 40 μm thick) was bonded to the polarizing film side of the obtained laminate by a roll-to-roll method using a UV curable adhesive.

[ example 2]

An optical layered body was produced in the same manner as in example 1, except that the temperature control of the 1 st roll R1 from the upstream side was turned off, the temperatures of the rolls R2 to R4 were set to 90 ℃, the contact times of the layered body with the heating rolls R2 to R4 were set to 3 seconds (total: 9 seconds), the contact times with the heating rolls R5 to R6 were set to 3 seconds (total: 6 seconds), and hot air was blown so that the atmospheric temperature in the 2 nd crystallization oven became 100 ℃.

[ example 3]

An optical laminate was produced in the same manner as in example 1, except that the temperature of R5 was set to 105 ℃, hot air was blown so that the atmospheric temperature in the 1 st crystallization oven became 95 ℃, and hot air was blown so that the atmospheric temperature in the 2 nd crystallization oven became 100 ℃.

[ example 4]

An optical laminate was produced in the same manner as in example 1, except that the temperatures of the heating rollers R1 to R4 were set to 65 ℃, the temperature of R5 was set to 110 ℃, and hot air was blown so that the atmospheric temperature in the 2 nd crystallization oven became 100 ℃.

[ example 5]

An optical laminate was produced in the same manner as in example 1, except that the temperature of R5 was set to 90 ℃, and hot air was blown so that the atmospheric temperature in the 2 nd crystallization oven became 90 ℃.

Comparative example 1

An optical laminate was produced in the same manner as in example 1, except that the laminate was brought into contact with 1 heating roller (110 ℃) for 3 seconds in an oven in which hot air was blown so that the atmospheric temperature became 60 ℃.

The evaluation results of the durability of the optical laminate and the transparency of the thermoplastic resin substrate obtained in each example and comparative example are shown in table 1 together with the crystal index and the treatment conditions after each crystallization treatment. The methods for evaluating the durability and the transparency of the thermoplastic resin substrate are as follows.

1. Durability

An acrylic pressure-sensitive adhesive was laminated on the acrylic protective film surface of the obtained optical laminate, and the optical laminate was bonded to glass to prepare a polarizing plate with glass, which was then placed in a constant-temperature constant-humidity chamber at 60 ℃ and 90% RH for 500 hours to observe whether or not the thermoplastic resin substrate was peeled from the end of the optical laminate. When the peeling was confirmed, the amount (mm) of peeling of the thermoplastic resin substrate from the end of the optical laminate was measured.

2. Transparency of thermoplastic resin base Material

The optical laminate obtained was evaluated by the following criteria, with the haze measured according to JIS K7136 as the haze of the thermoplastic resin substrate.

Haze is lower than 2% and is excellent

Haze of 2% or less and less than 3% is good

Haze of 3% or more

[ Table 1]

The optical laminates of examples 1 to 5 all had a haze of less than 3, and the transparency of the thermoplastic resin substrate was maintained. Among them, in examples 1 to 3, the thermoplastic resin substrate after the 1 st crystallization treatment had a crystal index of 0.40 or more, and the thermoplastic resin substrate after the 2 nd crystallization treatment had a crystal index of 0.55 or more, and the optical laminate was excellent in durability, and the thermoplastic resin substrate had a haze of 1.5% or less, and very good in appearance. The optical laminate of example 5 had a very excellent appearance with a haze of 1.0% or less for the thermoplastic resin substrate, but a small amount of peeling of the thermoplastic resin substrate from the end was confirmed in the durability test. On the other hand, in the optical laminate of comparative example 1 in which the stepwise crystallization treatment was not performed, the haze of the thermoplastic resin substrate was 3% or more, and white turbidity was generated.

[ reference example 1]

An optical laminate having a thermoplastic resin substrate with a crystal index of 0.35, 0.43, 0.51, 0.55, or 0.60 to 0.69 was obtained in the same manner as in example 1, except that the crystallization treatment was performed under various conditions. The optical laminate was put into a thermostatic bath at 80 ℃, a thermostatic bath at 85 ℃, a thermostatic and humidistatic bath at 60 ℃ and 90% RH, or a thermostatic and humidistatic bath at 60 ℃ and 95% RH, and the amount (mm) of peeling of the thermoplastic resin substrate from the end of the optical laminate after 500 hours had elapsed was measured. The results are shown in FIG. 4. Fig. 5(a) and (b) show photographs of an optical laminate without peeling and photographs of an optical laminate with peeling, respectively.

As shown in fig. 4, when the crystal index is 0.55 or more, the thermoplastic resin substrate is not peeled off.

Industrial applicability

The polarizing film of the present invention can be suitably used for liquid crystal panels of liquid crystal televisions, liquid crystal displays, cellular phones, digital cameras, video cameras, portable game machines, car navigations, copiers, printers, facsimile machines, clocks, microwave ovens, and the like.

Claims

1. A method of making an optical stack, the method comprising:

forming a polyvinyl alcohol resin layer on a thermoplastic resin substrate to produce a laminate;

subjecting the polyvinyl alcohol resin layer to a stretching treatment and a dyeing treatment to produce a polarizing film; and

subjecting the laminate to contact heating to crystallize the thermoplastic resin substrate,

the crystallization treatment comprises:

a 1 st crystallization treatment stage for increasing the crystallization index of the thermoplastic resin substrate to a 1 st predetermined value; and

a 2 nd crystallization treatment stage for making the crystallization index of the thermoplastic resin substrate a 2 nd predetermined value greater than the 1 st predetermined value.

2. The method for producing an optical laminate according to claim 1,

the 1 st crystallization treatment stage comprises a contact heating means for bringing the laminate into contact with 50 to 95 ℃,

the 1 st predetermined value is 0.4 or more.

3. The method for producing an optical laminate according to claim 1 or 2,

the 2 nd crystallization treatment stage comprises a contact heating mechanism for contacting the stacked body at 96 to 120 ℃,

the 2 nd given value is 0.55 or more.

4. The method for producing an optical laminate according to claim 2 or 3,

in the 1 st and 2 nd crystallization treatment stages, the contact time between any 1 point on the surface of the laminate and each of the contact heating means is 0.1 to 5 seconds, respectively.

5. The method for manufacturing an optical laminate according to any one of claims 1 to 4,

the thermoplastic resin substrate is composed of polyethylene terephthalate resin.

6. The method for manufacturing an optical laminate according to any one of claims 1 to 5,

the 1 st and 2 nd crystallization treatment stages are performed at an atmospheric temperature of 60 to 120 ℃.

7. The method for producing an optical laminate according to any one of claims 1 to 6, wherein the shrinkage of the thermoplastic resin substrate caused by the crystallization treatment is 3% or less.

8. A method of manufacturing a polarizing plate, comprising:

manufacturing an optical laminate by the manufacturing method according to any one of claims 1 to 7; and

an optical functional film is provided on the polarizing film side of the optical laminate obtained.