CN111149028B

CN111149028B - Method for producing coating liquid, method for producing polarizer, and apparatus for producing coating liquid

Info

Publication number: CN111149028B
Application number: CN201880062912.8A
Authority: CN
Inventors: 上条卓史; 国方智; 松山裕纪
Original assignee: Nitto Denko Corp
Current assignee: Nitto Denko Corp
Priority date: 2017-09-28
Filing date: 2018-09-12
Publication date: 2022-04-05
Anticipated expiration: 2038-09-12
Also published as: JP7048238B2; JP2019061191A; KR20200062186A; WO2019065227A1; TW201919854A; CN111149028A; KR102615539B1

Abstract

The invention provides a method for producing a polyvinyl alcohol coating liquid with suppressed deterioration in transparency. The method for producing a coating liquid of the present invention is a method for producing a coating liquid containing a polyvinyl alcohol resin, the method comprising: keeping the temperature of the solution containing the polyvinyl alcohol resin above 80 ℃; cooling the solution with the temperature of above 80 ℃ to below 40 ℃; continuously supplying the cooled solution as a coating liquid; and heating the cooled solution to 80 ℃ or higher and maintaining the temperature thereof when the supply of the coating liquid is stopped.

Description

Method for producing coating liquid, method for producing polarizer, and apparatus for producing coating liquid

Technical Field

The present invention relates to a method for producing a coating liquid, a method for producing a polarizer, and an apparatus for producing a coating liquid.

Background

There is proposed a method for obtaining a polarizer having a small thickness as follows: a coating liquid containing polyvinyl alcohol (PVA) is applied to a thermoplastic resin film and dried, thereby forming a PVA-based resin layer on the thermoplastic resin film, and the laminate is stretched and dyed (for example, patent document 1). As a PVA coating solution for producing the polarizer described above, there is proposed a method in which: a PVA coating solution having excellent transparency and uniformity is obtained by dissolving PVA in water and then rapidly cooling the solution to a predetermined temperature of 10 to 40 ℃ (patent document 2).

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2000-338329

Patent document 2: japanese patent No. 5146091

Disclosure of Invention

Problems to be solved by the invention

When polarizers are mass-produced using the PVA coating liquid as described above, typically, PVA is dissolved at a high temperature to obtain a PVA coating liquid, and the PVA coating liquid is kept warm or slowly cooled in a tank, filtered through a filter, and then applied onto a thermoplastic resin film from a coating die. However, for example, when the filter is replaced, the supply of the PVA coating liquid must be intermittently stopped. As a result, the PVA coating liquid may be retained inside the piping or the like, and the transparency may be lowered due to gelation of the PVA coating liquid. When a polarizer is produced using a PVA coating liquid whose transparency is lowered by gelation, the optical characteristics of the obtained polarizer may be deteriorated. Further, when the PVA coating liquid is retained in the piping, the pump, the three-way valve, or the like, the PVA may precipitate to form a core, which may cause local gelation. Further, when the PVA coating liquid is retained for a long time in a portion where bubble accumulation is likely to occur, such as a joint portion in the pipe and a switching portion of the three-way valve, the PVA may be locally gelled. When the PVA coating liquid that is locally gelled is applied to a thermoplastic resin film to form a laminate, the locally gelled may cause uneven defects on the surface of the PVA-based resin layer. When a polarizer is manufactured using such a laminate, the obtained polarizer may have a concave-convex defect and/or a light leakage defect in the case of orthogonal display.

The present invention has been made to solve the above conventional problems, and a main object thereof is to provide a method for producing a polyvinyl alcohol coating liquid in which deterioration in transparency and generation and precipitation of local gel are suppressed; a process for producing a polarizer using the polyvinyl alcohol coating liquid; and a device for producing a polyvinyl alcohol coating liquid.

Means for solving the problems

The method for producing a coating liquid of the present invention is a method for producing a coating liquid containing a polyvinyl alcohol resin, the method comprising: keeping the temperature of the solution containing the polyvinyl alcohol resin above 80 ℃; cooling the solution at 80 ℃ or higher to 40 ℃ or lower; continuously supplying the cooled solution as a coating solution; and heating the cooled solution to 80 ℃ or higher and maintaining the temperature thereof, when the supply of the coating liquid is stopped.

In one embodiment, the cooling rate of the cooling is 1 ℃/min or more.

According to another aspect of the present invention, there is provided a method of manufacturing a polarizer. The manufacturing method of the polarizer comprises the following steps: the coating liquid is applied to a resin base material and dried, thereby forming a polyvinyl alcohol resin layer on the resin base material.

According to another aspect of the present invention, there is provided a manufacturing apparatus of a coating liquid. The apparatus for producing a coating liquid containing a polyvinyl alcohol resin comprises: a heat-retaining section for retaining the solution containing the polyvinyl alcohol resin at 80 ℃ or higher; a cooling unit for cooling the solution at 80 ℃ or higher to 40 ℃ or lower; a supply unit for continuously supplying the cooled solution as a coating liquid; and a heating circulation unit for heating the solution cooled by the cooling unit to 80 ℃ or higher and returning the solution to the heat retention unit when the supply of the coating liquid by the supply unit is stopped.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, when the supply of the coating liquid is stopped, the cooled solution is heated to 80 ℃ or higher and kept warm, whereby even when the supply of the solution (coating liquid) is stopped by, for example, replacing a filter or the like, the solution can be prevented from staying in a pipe or the like and gelling, and as a result, the transparency of the solution can be prevented from being lowered. Further, the occurrence of localized gel can be suppressed, and as a result, the occurrence of uneven defects can be suppressed when such a polyvinyl alcohol coating liquid is formed into a film.

Drawings

Fig. 1 is a schematic diagram showing a configuration of a coating liquid production apparatus according to an embodiment of the present invention.

Description of the symbols

10 heat preservation part

20 cooling part

30 supply part

40 heating cycle part

100 coating liquid manufacturing apparatus

Detailed Description

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

A. Method for producing coating liquid

A method for producing a coating liquid according to an embodiment of the present invention is a method for producing a coating liquid containing a polyvinyl alcohol (PVA) resin, the method including: keeping the temperature of the solution containing the PVA resin above 80 ℃; cooling the solution with the temperature of above 80 ℃ to below 40 ℃; continuously supplying the cooled solution as a coating liquid; and heating the cooled solution to 80 ℃ or higher and maintaining the temperature thereof when the supply of the coating liquid is stopped. The cooling rate of the cooling is preferably 1 ℃/min or more. The coating liquid is typically used for producing polarizers. The polarizer manufacturing method includes, as described in item B below: the coating liquid is applied to a resin base material and dried, thereby forming a polyvinyl alcohol resin layer on the resin base material.

Fig. 1 is a schematic diagram showing a configuration of a coating liquid production apparatus according to an embodiment of the present invention. The coating liquid production apparatus 100 can be suitably used for a method of producing a coating liquid. The coating liquid production apparatus 100 includes: a heat-retaining section 10 for retaining the solution containing the PVA based resin at 80 ℃ or higher; a cooling unit 20 for cooling the solution at 80 ℃ or higher to 40 ℃ or lower; a supply section 30 for continuously supplying the cooled solution as a coating liquid to a coating section (not shown); and a heating circulation unit 40 for heating the solution cooled by the cooling unit to 80 ℃ or higher and returning the solution to the heat retention unit 10 when the supply of the coating liquid by the supply unit 30 is stopped. The heat-retaining section 10, the cooling section 20, and the supply section 30 are typically arranged in series via a pipe P. In one embodiment, the pipe P branches between the cooling unit 20 and the supply unit 30, and a three-way valve 50 is disposed at the branching portion of the pipe. The pipe P branched from the branch portion is connected to the heat retaining portion 10 through the heating cycle portion 40. In one embodiment, a pump 60 for feeding the solution may be disposed between the heat-retaining unit 10 and the cooling unit 20. The supply 30 may have a filter for filtering the solution. With the above-described manufacturing apparatus 100, even when the supply unit 30 stops supplying the solution (coating liquid) due to, for example, replacement of a filter or the like, the solution can be prevented from staying in the pipe between the heat retention unit 10 and the branch unit, and as a result, gelation (including local gelation) of the solution can be prevented.

A-1 preparation of PVA-based resin-containing solution

Any suitable PVA type 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 polarizer having excellent durability can be obtained. When the degree of saponification 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 solution containing the PVA-based resin 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. These may be used alone, or two or more kinds 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, more preferably 5 to 15 parts by weight, and particularly preferably 7 to 12 parts by weight, based on 100 parts by weight of the solvent. When the PVA-based resin is applied to the resin substrate and dried, a uniform coating film can be provided so as to be closely adhered to the resin substrate.

Additives may be added to the solution. 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 resins can be used for the purpose of further improving the uniformity, dyeability, and stretchability of the PVA-based resin layer obtained when the PVA-based resin is applied to a resin substrate and dried to produce a PVA-based resin layer.

The viscosity of the solution is preferably 100 to 10000 mPas, more preferably 300 to 5000 mPas, and still more preferably 500 to 3000 mPas.

When the PVA-based resin is dissolved in a solvent, the PVA-based resin is typically mixed with the solvent at a temperature of 80 ℃. The temperature at the time of dissolution is preferably 80 ℃ or higher, more preferably 85 to 95 ℃. If the temperature at the time of dissolution is lower than 80 ℃, the PVA-based resin may not be sufficiently dissolved, and a uniform solution may not be obtained.

In one embodiment, the PVA-based resin is slowly added to a solvent at normal temperature, stirred to be sufficiently dispersed and swollen, and then heated to a temperature of 80 ℃ or higher while stirring is continued, and stirring is continued at that temperature for a predetermined time.

After the solution containing the PVA-based resin is prepared, the solution may be thermally insulated in a thermal insulation part.

A-2. Heat preservation of solution containing PVA-based resin

The solution containing the PVA based resin is incubated at 80 ℃ or higher as described above. In one embodiment, the solution is incubated at 80 ℃ or higher in an incubation portion. The temperature at which the solution is kept warm is preferably 80 to 95 ℃, more preferably 85 to 90 ℃. The PVA based resin can be inhibited from being precipitated and gelled by keeping the temperature at 80 ℃ or higher. The holding time is preferably 1 to 48 hours, more preferably 3 to 24 hours. The solution is preferably kept warm while being stirred. Thus, a solution having excellent transparency and uniformity can be obtained. The solution kept warm in the warm-keeping section can be continuously supplied to the cooling section by a pump. The capacity of the heat retaining portion is not particularly limited and may be appropriately set according to the purpose.

A-3 Cooling of the solution containing the PVA based resin

The solution containing the PVA based resin is kept at 80 ℃ or higher and then cooled to 40 ℃ or lower, as described above. In one embodiment, the solution is cooled to 40 ℃ or lower in a cooling section. The cooling temperature (temperature after cooling the solution) is preferably 10 to 40 ℃, and more preferably 20 to 30 ℃. If the cooling temperature is higher than 40 ℃, it is caused to slowly cool from the temperature to room temperature, and thus gelation and re-precipitation may occur. On the other hand, if the cooling temperature is less than 10 ℃, the solubility of the PVA-based resin in water becomes insufficient, and the PVA-based resin may be reprecipitated.

The cooling rate is preferably 1 ℃/min or more as described above. The cooling rate is preferably 1 to 10 ℃/min, more preferably 1 to 5 ℃/min. This can suppress precipitation and gelation of the PVA based resin. The cooling rate is expressed by the following equation.

Cooling rate (temperature of solution at the start of cooling-temperature of solution at the end of cooling)/cooling time

The cooling rate may be constant or may be changed from the start of cooling to the end of cooling, but it is preferable to cool the substrate so that there is no state where the temperature is substantially constant or no state where the temperature temporarily rises in the middle of cooling. That is, it is preferable to perform cooling so that the temperature continuously decreases with time.

The cooling portion may have any suitable configuration. Typically, a heat exchanger may be employed. Examples thereof include the following: a cooling pipe surrounding the tank containing the solution, and a refrigerant flowing into the pipe to forcibly cool the pipe; and a configuration in which a cooling pipe is disposed in the tank, and a refrigerant is flowed into the tank to forcibly cool the tank. Examples of the refrigerant include ice water and salt water (brine).

A-4. supply of coating liquid

The PVA-based resin-containing solution is continuously supplied as a coating solution after being cooled to 40 ℃ or lower, as described above. In one embodiment, the solution is continuously supplied from the supply section to the coating section. The feeding speed is not particularly limited, and may be appropriately set according to the coating width and the line speed (coating speed).

The coating section may be a coater that can perform coating by any suitable coating method. 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 machine may be a coating machine that can perform coating by the above-described coating method, typically a coating die. The coating amount of the coating section is not particularly limited, and may be appropriately set according to the coating width and the line speed (coating speed).

In one embodiment, the supply unit has a filter, and the solution may be filtered by the filter and supplied as the coating solution. Thereby removing bubbles and foreign substances from the solution. The filter may be any suitable filter. A depth filter may be preferably used. Specific examples of the depth filter include a wound type in which a wire is wound around a cylindrical core, a nonwoven fabric laminated type in which a nonwoven fabric is wound around a cylindrical core, and a resin molded type using a resin molded article such as a sponge, depending on the form of the filter material.

A-5 heating of the cooled solution

When the supply of the coating solution was stopped, the cooled solution was heated to 80 ℃ or higher and kept warm as described above. In one embodiment, the solution is heated by a heating cycle. The temperature for heating and holding the solution is preferably 80 to 95 ℃, more preferably 85 to 90 ℃. The solution heated to 80 ℃ or higher is again kept warm, and then cooled to 40 ℃ or lower, and then supplied as a coating liquid.

In one embodiment, the heating cycle unit heats the solution cooled by the cooling unit to 80 ℃ or higher, and returns the heated solution to the heat-retaining unit. That is, when the supply of the solution is stopped, the solution is circulated through the pipe while being heated again to 80 ℃. Thus, even when the supply of the coating liquid (solution) is stopped by, for example, replacing the filter, the solution can be prevented from staying in the piping, and as a result, gelation of the solution can be prevented.

Typically, the pipe for supplying the cooled solution to the supply unit is branched, a three-way valve is disposed in the branched portion, and the solution is sent to the supply unit or heated by switching the opening of the three-way valve.

The heating cycle may be of any suitable construction. Typically, a heat exchanger may be employed.

B. Use of coating liquid

The coating liquid obtained by the production method described in item A can be used for producing a polarizer. The method for manufacturing the polarizer can comprise the following steps: applying the coating liquid to a resin base material and drying the coating liquid to form a polyvinyl alcohol resin layer on the resin base material to prepare a laminate; and subjecting the laminate to a predetermined treatment for forming the PVA-based resin layer into a polarizer. Examples of the predetermined treatment include dyeing treatment, stretching treatment, insolubilization treatment, crosslinking treatment, washing treatment, and drying treatment. These treatments may be appropriately selected depending on the purpose. The processing sequence, processing time, number of times of processing, and the like may be set as appropriate. The respective processes will be described below.

B-1 preparation of resin base Material

The resin base material is typically formed of a thermoplastic resin. Any suitable thermoplastic resin may be used. Examples thereof include (meth) acrylic resins, olefin resins, norbornene resins, and polyester resins. Polyester-based resins are preferably used. Among them, amorphous (uncrystallized) polyethylene terephthalate-based resins are preferably used. Particularly, an amorphous (less-crystallizable) polyethylene terephthalate resin is preferably used. Specific examples of the amorphous polyethylene terephthalate resin include a copolymer further containing isophthalic acid as a dicarboxylic acid and a copolymer further containing cyclohexanedimethanol as a glycol.

The glass transition temperature (Tg) of the resin substrate is preferably 170 ℃ or lower. By using such a resin base material, 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 resin substrate with water and enabling good stretching in an aqueous solution, it is more preferably 120 ℃ or lower. In one embodiment, the glass transition temperature of the resin substrate is preferably 60 ℃ or higher. By using such a resin base material, it is possible to prevent the resin base material from being deformed (for example, irregular, loose, or wrinkled) when a coating liquid containing the PVA-based resin is applied and dried, and to produce a laminate satisfactorily. Further, the PVA-based resin layer can be favorably stretched at an appropriate temperature (for example, about 60 to 70 ℃). In another embodiment, the glass transition temperature may be lower than 60 ℃ as long as the resin base material is not deformed when the coating liquid containing the PVA-based resin is applied and dried. The glass transition temperature of the resin substrate can be adjusted by, for example, heating using a crystallized material capable of introducing a modifying group into the constituent material. The glass transition temperature (Tg) is a value determined in accordance with JIS K7121.

In one embodiment, the water absorption of the resin base material is preferably 0.2% or more, and more preferably 0.3% or more. Such a resin base material absorbs water, and the water can play a role of a plasticizer to plasticize. As a result, the tensile stress can be greatly reduced during stretching in an aqueous solution, and the stretchability is excellent. On the other hand, the water absorption of the resin base material is preferably 3.0% or less, and more preferably 1.0% or less. By using such a resin base material, it is possible to prevent a problem such as deterioration in appearance of the obtained laminate due to a significant decrease in dimensional stability of the resin base material during production. Further, it is possible to prevent the PVA-based resin layer from being broken when stretched in an aqueous solution or being peeled from the resin substrate. The water absorption is a value obtained according to JIS K7209.

The thickness of the resin base is preferably 20 to 300. mu.m, more preferably 30 to 200. mu.m.

The resin base material may be subjected to a surface treatment (e.g., corona treatment) in advance. This is because the adhesion between the resin base and the PVA-based resin layer can be improved.

B-2 formation of PVA-based resin layer

Any suitable method may be used for forming the PVA-based resin layer on the resin substrate. Preferably, the PVA-based resin layer is formed by applying the coating liquid obtained by the method described in the above item a to a resin substrate and drying the coating liquid.

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

The coating liquid is applied so that the thickness of the dried PVA based resin layer is preferably 3 to 40 μm, more preferably 3 to 20 μm. The coating and drying temperature of the coating liquid is preferably 50 ℃ or higher.

B-3. stretching treatment in gas atmosphere

The stretching method for assisting stretching in a gas atmosphere may be fixed-end stretching (for example, a method of stretching using a tenter) or free-end stretching (for example, a method of uniaxially stretching a laminate by passing the laminate between rolls having different peripheral speeds). In one embodiment, the stretching treatment in a gas atmosphere includes a heat roll stretching step of stretching the laminate by a difference in peripheral velocity between heat rolls while conveying the laminate in a longitudinal direction thereof. The stretching treatment in a gas atmosphere typically includes a zone (zone) stretching step and a hot roll stretching step. The order of the zone stretching step and the heat roll stretching step is not limited, and the zone stretching step may be performed first, or the heat roll stretching step may be performed first. The zone stretching step may also be omitted. In one embodiment, the zone stretching step and the hot roll stretching step are performed sequentially.

The stretching temperature of the laminate can be set to any appropriate value depending on the material for forming the resin base material, and the like. The stretching temperature at the time of stretching treatment in a gas atmosphere 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 ℃. On the other hand, the upper limit of the stretching temperature of the laminate is preferably 170 ℃. Stretching at such a temperature can suppress rapid progress of crystallization of the PVA-based resin, and can suppress defects caused by such crystallization (for example, the orientation of the PVA-based resin layer is hindered by stretching).

The stretch ratio of the laminate can be set to any appropriate value depending on the material for forming the resin base material, and the like. The stretching ratio in the stretching treatment in a gas atmosphere is preferably 1.5 times or more and 3.0 times or less.

B-4. insolubilization treatment

The insolubilization 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 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 50 ℃. The insolubilization treatment is preferably performed before the stretching and dyeing treatment in an aqueous solution.

B-5 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 liquid containing a dichroic material, a method of applying the dyeing liquid to the PVA-based resin layer, a method of spraying the dyeing liquid onto the PVA-based resin layer, and the like. Preferably, the laminate is immersed in a dyeing solution. Since the dichroic substance can be well adsorbed.

Examples of the dichroic substance include iodine and an organic dye. These may be used alone, or two or more kinds may be used in combination. The dichroic substance is preferably iodine. When iodine is used as the dichroic material, the dyeing liquid is preferably an aqueous iodine solution. The amount of iodine is preferably 0.05 to 5.0 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. Examples of the iodide include: 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. Among these, potassium iodide is more preferable. The amount of the iodide is preferably 0.3 to 15 parts by weight based on 100 parts by weight of water.

The liquid temperature of the dyeing liquid during dyeing is preferably 20 ℃ to 40 ℃. When the PVA-based resin layer is immersed in the dyeing liquid, the immersion time is preferably 10 seconds to 300 seconds. Under such conditions, the dichroic material can be sufficiently adsorbed to 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 polarizer finally obtained falls within a predetermined range. In one embodiment, the immersion time is set so that the degree of polarization of the polarizer obtained becomes 99.98% or more. In another embodiment, the immersion time is set so that the monomer transmittance of the polarizer obtained is about 40%.

B-6. Cross-linking treatment

The crosslinking treatment is typically performed by immersing the PVA-based resin layer (laminate) 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 5 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 60 ℃. The crosslinking treatment is preferably carried out before the stretching treatment in an aqueous solution. In a preferred embodiment, the stretching treatment, the dyeing treatment and the crosslinking treatment are sequentially performed in a gas atmosphere.

B-7 stretching treatment in aqueous solution

The stretching treatment in the aqueous solution may be any suitable method. Specifically, the stretching may be fixed-end stretching (for example, a method using a tenter) or free-end stretching (for example, a method of uniaxially stretching a laminate by passing the laminate between rolls having different peripheral speeds). Further, simultaneous biaxial stretching (for example, a method using a simultaneous biaxial stretcher) may be performed as well as stepwise biaxial stretching. 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 is the product of the stretching ratios in the respective stages.

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, that is, the transport direction (MD) thereof. In another embodiment, the stretching is performed along the width direction of the long laminate. Specifically, the laminate is transported in the longitudinal direction, that is, in The Direction (TD) orthogonal to the transport direction (MD).

The stretching temperature of the laminate can be set to any appropriate value depending on the material for forming the resin base material, the stretching method, and the like. The stretching temperature is preferably 40 to 85 ℃ and 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 there is a possibility that excellent optical characteristics cannot be obtained.

The stretching in an aqueous solution is preferably performed by immersing the laminate in an aqueous boric acid solution (stretching in an aqueous boric acid solution). By using an aqueous boric acid solution as a stretching bath, rigidity capable of withstanding the tension applied during stretching and water resistance not dissolving in water can be imparted to the PVA-based resin layer. Specifically, boric acid generates tetrahydroxyborate anions in an aqueous solution, and crosslinks with the PVA-based resin through hydrogen bonds. As a result, the PVA-based resin layer can be provided with rigidity and water resistance and can be stretched well, and a polarizer having excellent optical characteristics (for example, degree of polarization) can be produced.

The aqueous boric acid solution is preferably obtained by dissolving boric acid and/or a borate in a solvent, i.e., water. The boric acid concentration is preferably 1 to 10 parts by weight relative to 100 parts by weight of water. When the boric acid concentration is 1 part by weight or more, the dissolution of the PVA-based resin layer can be effectively suppressed, and a polarizer having higher characteristics can be produced. In addition to boric acid or borate, an aqueous solution obtained by dissolving a boron compound such as borax, glyoxal, glutaraldehyde, or the like in a solvent may be used.

When a dichroic material (represented by iodine) is adsorbed on the PVA-based resin layer in advance by dyeing treatment, it is preferable to add an iodide to the stretching bath (aqueous boric acid solution). The iodine compound can suppress elution of iodine adsorbed on the PVA-based resin layer. Examples of the iodide include: 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 concentration of the 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.

The stretch ratio (maximum stretch ratio) of the laminate is typically 4.0 times or more, preferably 5.0 times or more, with respect to the original length of the laminate. Such a high stretch ratio can be realized by, for example, stretching in an aqueous solution (stretching 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.

B-8. cleaning treatment

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

B-9. drying treatment

The drying temperature in the drying treatment is preferably 30 to 100 ℃.

The polarizer obtained is substantially a PVA-based resin film in which a dichroic material is adsorbed and oriented. The thickness of the polarizer is preferably 10 μm or less, more preferably 7 μm or less, and particularly preferably 5 μm or less. The polarizer preferably exhibits dichroism of absorption at any wavelength of 380nm to 780 nm. The polarizer preferably has a degree of polarization of 99.9% or more when the monomer transmittance is 42% or more. The polarizer may be used as a polarizing plate having a protective film disposed on at least one side. The protective film may be formed using the resin base material as it is, or may be formed using a film different from the resin base material. Examples of the material for forming the protective film include (meth) acrylic resins, cellulose resins such as cellulose diacetate and cellulose triacetate, cycloolefin resins, olefin resins such as polypropylene, ester resins such as polyethylene terephthalate resins, polyamide resins, polycarbonate resins, and copolymer resins thereof. The thickness of the protective film is preferably 10 μm to 100 μm.

Examples

The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

< production example 1>

1. Preparation of resin base Material

An amorphous polyethylene terephthalate (A-PET) resin was formed into a long film having a thickness of 200 μm to obtain an A-PET substrate.

A water-based urethane resin (product name: SUPERFLEX 210R, first Industrial pharmaceutical Co., Ltd., 35% in solid content) was added,

Oxazoline-based crosslinking agent (product name: EPOCROS WS700, product name: 25% by weight, product name: manufactured by Nippon catalyst Co., Ltd.), conductive material (product name: ORGACON LBS, product name: 1.2% by weight, product name: ORGACON LBS, product name: 1.2% by weight), 1% strength ammonia water and water in a weight ratio of 9.03: 1.00: 18.1: 0.060: 39.5 to prepare a mixed solution.

The obtained mixed solution was applied to one surface of an a-PET substrate and dried, thereby forming an antistatic layer on the surface of the a-PET substrate. The thickness of the antistatic layer was 1 μm.

Subsequently, the A-PET substrate having the antistatic layer formed thereon was stretched 2.3 times in the transverse direction at 115 ℃ while being transported in the longitudinal direction. Then, both ends were cut and removed, thereby obtaining a resin base material for coating.

2. Preparation of PVA-based resin-containing solution

PVA (polymerization degree: 4200, saponification degree: 99.2 mol%) and pure water were mixed at room temperature, and the mixture was heated to 95 ℃ while stirring and held, thereby dissolving PVA to prepare a PVA-containing solution.

< reference example 1>

The PVA-based resin layer was formed by supplying the solution of production example 1 as a coating liquid and applying the solution to the surface of the resin base opposite to the surface provided with the antistatic layer. The coating liquid was supplied as follows using an apparatus having the configuration shown in fig. 1.

First, the solution is sent to the heat-retaining section 10 and is held at 90 ℃ by the heat-retaining section. Next, the solution is cooled by a heat exchanger in the cooling unit 20. The temperature of the cooled solution was 25 ℃. Subsequently, the cooled solution is sent to the supply unit 30 through the three-way valve 50. In the supply section, the solution is supplied as a coating liquid to a slit die (coating section) through a filter, and is coated on a resin base material through the slit die. This was dried at 60 ℃ to obtain a laminate 1 in which a PVA based resin layer having a thickness of 10 μm was formed on a resin substrate.

< example 1>

A laminate was obtained in the same manner as in reference example 1, except that the solution of production example 1 was circulated and then supplied as a coating solution. Specifically, the following is described.

The three-way valve is switched to send the solution to the heating circulation portion 40 (at the same time, the supply portion stops supplying the coating liquid). The solution at 25 ℃ was heated to 90 ℃ in the heating and circulating section using a heat exchanger, and then sent to the heat-retaining section again, and the solution was continuously circulated for 10 hours in the order of (1) the heat-retaining section (2), the cooling section (3), and the heat-retaining section … in the circulating section (1). Then, the three-way valve was switched, the solution was sent to the supply portion, and the coating liquid was applied to the resin base material and dried in the same manner as in reference example 1, thereby obtaining a laminate 2. Further, while the solution was circulated for 10 hours, the piping between the supply section and the slit die, and the like were cleaned, and the filter was replaced with a new one.

< comparative example 1>

A laminate was obtained in the same manner as in reference example 1, except that the solution of production example 1 was retained and then supplied as a coating solution. Specifically, the following is described.

The three-way valve was closed, the pump was stopped, and the solution was left for 10 hours without being sent to either the heating circulation unit or the supply unit (even if the solution was retained). Then, the pump was restarted, the three-way valve was opened, and the solution was sent to the supply portion, and the coating liquid was applied to the resin base material and dried in the same manner as in reference example 1, to obtain a laminate 3. Further, while the solution was left for 10 hours, the piping between the supply section and the slit die, and the like were cleaned, and the filter was replaced with a new one.

< evaluation >

The number of uneven defects caused by PVA gels was evaluated for each laminate of reference example, and comparative example as follows.

First, the laminate was visually checked for irregularities and defects, and the defects were marked. The defect confirmed with the naked eye was designated as defect a.

Then, the defect a was observed with a microscope, and was classified into a defect in which air bubbles or foreign substances were observed in the PVA type resin layer and a defect in which these were not observed. Among the defects a, a defect in which no bubble or foreign matter was observed in the PVA type resin layer was defined as a defect B.

The PVA-based resin layer was peeled off, the surface of the resin base material was visually confirmed, and the defects B were classified into those in which the uneven defects were confirmed and those in which the uneven defects were not confirmed at the positions corresponding to the marks on the surface of the resin base material. Among the defects B, a defect in which an uneven defect is not observed at a position corresponding to the mark on the surface of the resin base material is set as a defect C.

The defect C is a concave-convex defect not derived from the surface shape of the resin substrate, bubbles in the PVA-based resin layer, or foreign matter, and is considered to be a defect caused by local gelation of the PVA. The number of defects C was defined as the number of uneven defects caused by PVA gel.

The number of uneven defects caused by the PVA gel in each laminate is as follows.

Laminate 1 (reference example 1). 2 pieces/m²

Laminate 2 (example 1). 3 pieces/m²

Laminate 3 (comparative example 1). 16 pieces/m²

From the above results, it was found that the laminate of comparative example 1, which was produced by supplying the coating liquid after retaining the solution, had many uneven defects due to the gel, as compared with the laminate of reference example 1, which was produced by circulating the solution without retaining the solution and supplying the solution as the coating liquid. In contrast, the number of uneven defects in the laminate of example 1 prepared by supplying the coating solution after circulating the solution was not much different from the number of uneven defects in the laminate of reference example 1.

Industrial applicability

The coating liquid obtained by the production method and the production apparatus of the present invention can be suitably used for the production of a polarizer, and the polarizer can be suitably used for an image display device.

Claims

1. A method for producing a coating liquid containing a polyvinyl alcohol resin, comprising:

keeping the temperature of the solution containing the polyvinyl alcohol resin above 80 ℃;

cooling the solution above 80 ℃ to below 40 ℃;

continuously supplying the cooled solution as a coating liquid;

heating the cooled solution to 80 ℃ or higher and maintaining the temperature while stopping the supply of the coating liquid; and

the solution is circulated when the supply of the coating liquid is stopped.

2. The method for producing a coating liquid according to claim 1, wherein,

the cooling speed of the cooling is more than 1 ℃/min.

3. A method of manufacturing a polarizer, the method comprising:

a polyvinyl alcohol resin layer is formed on a resin substrate by applying the coating liquid obtained by the production method according to claim 1 or 2 to the resin substrate and drying the coating liquid.

4. An apparatus for producing a coating liquid containing a polyvinyl alcohol resin, comprising:

a heat-retaining section for retaining the solution containing the polyvinyl alcohol resin at 80 ℃ or higher;

a cooling unit for cooling the solution at 80 ℃ or higher to 40 ℃ or lower;

a supply section for continuously supplying the cooled solution as a coating liquid; and

and a heating circulation unit for heating the solution cooled by the cooling unit to 80 ℃ or higher and returning the solution to the heat retaining unit when the supply of the coating liquid by the supply unit is stopped.