Polyvinyl alcohol film and method for producing same

Abstract

The present invention provides a polyvinyl alcohol film which satisfies formula (1) and formula (2).　(1): 1.50075 ≤ Nx ≤ 1.50105 (2): 9.0 ≤ (Nx - Nz) / (Nx - Ny) ≤ 20.0 In the formulae, Nx represents the refractive index in the length direction (MD direction) of the film, Ny represents the refractive index in the width direction (TD direction) of the film, and Nz represents the refractive index in the film thickness direction of the film.　As a result, there is provided a polyvinyl alcohol film which has excellent stretchability while maintaining a high degree of polarization when formed into a polarizing film.

Description

Polyvinyl alcohol film and its manufacturing method

[0001] The present invention relates to a polyvinyl alcohol film and a method for producing the same.

[0002] Polarizing plates, which have the function of transmitting and blocking light, are the basic components of liquid crystal displays (LCDs), along with liquid crystals, which change the polarization state of light. LCDs are widely used in a wide range of applications, including small devices such as calculators and watches, laptops, LCD monitors, LCD color projectors, LCD televisions, car navigation systems, mobile phones, and measuring instruments used indoors and outdoors.

[0003] Polarizing plates are generally produced by producing a polarizing film by swelling, dyeing, crosslinking, uniaxially stretching a polyvinyl alcohol film (hereinafter, "polyvinyl alcohol" may be abbreviated as "PVA"), and optionally further subjecting it to a fixing treatment using a boron compound or the like, and then laminating a protective film such as a triacetate cellulose (TAC) film onto the surface of the polarizing film. Because the optical performance of an LCD, such as its contrast ratio, is highly dependent on the polarization performance, such as the polarization degree, of the polarizing film, PVA films with high stretchability have been proposed to improve polarization performance.

[0004] Generally, to increase the degree of polarization, a PVA film needs to be stretched in the length direction or machine direction (hereinafter, "length direction" or "machine direction" may be abbreviated as "MD direction") to orient it in the MD direction. Further stretching a PVA film oriented in the MD direction is difficult, resulting in the problem of breakage during polarizing film production. Patent Documents 1 to 3 disclose that a PVA film for polarizing film production, in which the birefringence in the length direction, the width direction (hereinafter, "width direction" may be abbreviated as "TD direction") perpendicular to the length direction, and the birefringence in the width direction and film thickness direction have specific relationships, exhibits excellent swellability, stretchability, and polarization performance during polarizing film production. Specifically, the film is obtained by stretching the width direction using a stretching machine while conveying the film in the machine direction. However, since the degree of polarization decreases as the orientation in the TD direction increases, it remains a challenge to improve the stretchability of a polarizing film while maintaining a high degree of polarization.

[0005] JP 2017-102439 A JP 2017-102437 A JP 2017-102436 A

[0006] In view of the above circumstances, an object of the present invention is to provide a PVA film that, when made into a polarizing film, maintains a high degree of polarization and has excellent stretchability.

[0007] As a result of extensive research, the present inventors have found that the above-mentioned problems can be solved by adjusting the balance between the MD refractive index Nx, the TD refractive index Ny, and the thickness direction refractive index Nz, and using a PVA film in which Nx and (Nx-Nz) / (Nx-Ny) fall within specific ranges. Furthermore, they have found that the above-mentioned PVA film can be particularly easily obtained by employing a production method in which an orientation adjustment step is performed after a heat treatment step and the cumulative stretch ratio of the film during these steps falls within a specific range. Based on this finding, further research led to the completion of the present invention.

[0008] That is, the present invention relates to [1] to [8]. [1] A PVA film satisfying the following formulas (1) and (2): 1.50075≦Nx≦1.50105 (1) 9.0≦(Nx−Nz) / (Nx−Ny)≦20.0 (2) Nx represents the refractive index in the length direction (MD) of the film, Ny represents the refractive index in the width direction (TD) of the film, and Nz represents the refractive index in the thickness direction of the film. [2] A PVA film according to [1], which satisfies the following formula (3): 1.50050≦Ny≦1.50080 (3) [3] A PVA film according to [1] or [2], in which the degree of polymerization of the PVA is 1,000 or more and 8,000 or less. [4] A PVA film according to [1] to [3], in which the degree of saponification of the PVA is 98.0 mol % or more. [5] The PVA film according to any one of [1] to [4], having a width of 3 m or more. [6] The PVA film according to any one of [1] to [5], having a film thickness of 20 to 80 μm. [7] A method for producing a PVA film, comprising: a casting step of casting a film-forming solution containing PVA onto a support; a drying step of removing the solvent from the film peeled from the support in the casting step while transporting the film using drying rolls; a heat treatment step of heat-treating the film while transporting it using heat treatment rolls; and an orientation adjustment step of adjusting the orientation using orientation adjustment rolls, wherein the casting step, drying step, heat treatment step, and orientation adjustment step are performed in this order, and the cumulative draw ratio in the heat treatment step is 0.970 to 0.994, the cumulative draw ratio in the orientation adjustment step is 1.005 to 1.020, and the total cumulative draw ratio in the heat treatment step and the orientation adjustment step is 0.995 to 1.005. [8] The method for producing a PVA film according to [7], wherein the temperature of the heat treatment roll in the heat treatment step is 95 to 115°C.

[0009] According to the present invention, there is provided a PVA film that, when made into a polarizing film, maintains a high polarization degree and has excellent stretchability. Also provided is a method for producing the polyvinyl alcohol film.

[0010] [PVA Film] The present invention relates to a PVA film that satisfies the following formulas (1) and (2). The PVA film of the present invention contains PVA as a main component and may contain other components as long as the effects of the present invention are not impaired. The main component means more than 50% by mass when the weight of the PVA film is taken as 100% by mass. 1.50075≦Nx≦1.50105 (1) 9.0≦(Nx−Nz) / (Nx−Ny)≦20.0 (2) Nx represents the refractive index in the length direction (MD direction) of the PVA film, Ny represents the refractive index in the width direction (TD direction) of the film, and Nz represents the refractive index in the thickness direction of the film.

[0011] The above formula (1) represents the range of Nx, i.e., the refractive index in the length direction (MD direction) of the PVA film. By satisfying this range, a high degree of polarization can be obtained. If Nx is less than 1.50075, the tension decreases when the film is stretched, preventing the orientation of the PVA, and resulting in a low degree of polarization of the resulting polarized film. On the other hand, if Nx exceeds 1.50105, the tension increases when the film is stretched, increasing the load on production equipment and the frequency of breakage of the polarized film.

[0012] The above formula (2) is an index of the symmetry of the orientation axis. In other words, when (Nx-Nz) / (Nx-Ny) approaches 1.0, the film becomes uniaxially oriented, and the MD orientation becomes high, making it prone to breakage. On the other hand, when the value is large, the film becomes biaxially oriented, and breakage does not occur, but the degree of polarization decreases. Note that (Nx-Nz) / (Nx-Ny) is synonymous with the Nz coefficient.

[0013] In the PVA film of the present invention, the lower limit of Nx is preferably 1.50085 or more, more preferably 1.50095 or more. On the other hand, the upper limit of Nx is preferably 1.50103 or less, more preferably 1.50101 or less. When Nx is equal to or more than the above lower limit or equal to or less than the above upper limit, the degree of polarization of the resulting polarized film is high.

[0014] In the PVA film of the present invention, Ny is not limited as long as it satisfies formula (2), but the lower limit of Ny is preferably 1.50050 or more, more preferably 1.50060 or more, and even more preferably 1.50070 or more. On the other hand, the upper limit of Ny is preferably 1.50080 or less, more preferably 1.50078 or less, and even more preferably 1.50075 or less. When Ny is equal to or greater than the above lower limit or equal to or less than the above upper limit, the degree of polarization of the resulting polarized film is increased.

[0015] In the PVA film of the present invention, Nz is not limited as long as it satisfies formula (2), but the lower limit of Nz is preferably 1.49810 or more, more preferably 1.49815 or more, and even more preferably 1.49820 or more. On the other hand, the upper limit of Nz is preferably 1.49870 or less, more preferably 1.49850 or less, and even more preferably 1.49840 or less. When Nz is equal to or greater than the above lower limit or equal to or less than the above upper limit, the MD orientation is not excessively impaired, and the degree of polarization of the resulting polarized film is high.

[0016] In the PVA film of the present invention, from the viewpoint of achieving both the degree of polarization and the stretchability of the polarizing film, the lower limit of (Nx-Nz) / (Nx-Ny) in the above formula (2) is preferably 9.5 or more, more preferably 10.0 or more, and particularly preferably more than 10.0, and the upper limit is preferably 15.0 or less, more preferably 13.0 or less.

[0017] (PVA) In the PVA film of the present invention, a polymer produced by saponifying a vinyl ester polymer obtained by polymerizing a vinyl ester monomer can be used as the PVA. Examples of the vinyl ester monomer include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, and vinyl versatate. Among these, vinyl acetate is preferred as the vinyl ester monomer.

[0018] The vinyl ester polymer is preferably a polymer obtained using only one or more vinyl ester monomers as the monomer, more preferably a polymer obtained using only one vinyl ester monomer as the monomer, and may also be a copolymer of one or more vinyl ester monomers with other monomers copolymerizable therewith.

[0019] Examples of other monomers include ethylene; olefins having 3 to 30 carbon atoms such as propylene, 1-butene, and isobutene; acrylic acid or salts thereof; acrylic acid esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, i-propyl acrylate, n-butyl acrylate, i-butyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, and octadecyl acrylate; methacrylic acid or salts thereof; methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, methacrylic acid, and methacrylic acid. methacrylic acid esters such as i-propyl methacrylate, n-butyl methacrylate, i-butyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, and octadecyl methacrylate; acrylamides such as acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetone acrylamide, acrylamidopropanesulfonic acid or a salt thereof, acrylamidopropyldimethylamine or a salt thereof, and N-methylolacrylamide or a derivative thereof; methacrylamide derivatives such as methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidepropanesulfonic acid or a salt thereof, methacrylamidepropyldimethylamine or a salt thereof, N-methylolmethacrylamide or a derivative thereof; N-vinylamides such as N-vinylformamide, N-vinylacetamide, and N-vinylpyrrolidone; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; vinyl cyanides such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid or a salt, ester, or acid anhydride thereof; itaconic acid or a salt, ester, or acid anhydride thereof; vinylsilyl compounds such as vinyltrimethoxysilane; and isopropenyl acetate.The vinyl ester polymer may have structural units derived from one or more of these other monomers.

[0020] The proportion of structural units derived from other monomers in the vinyl ester polymer is preferably 15 mol% or less, more preferably 8 mol% or less, based on the number of moles of all structural units constituting the vinyl ester polymer. Generally, the crystallization of PVA tends to proceed more slowly as the proportion of structural units derived from other monomers in the vinyl ester polymer increases. Therefore, by appropriately copolymerizing these other monomers in the above-mentioned proportions, each refractive index can be adjusted.

[0021] The degree of polymerization of the PVA is preferably 1000 or more, more preferably 1500 or more, and even more preferably 1700 or more. By making the degree of polymerization of the PVA equal to or greater than the above-mentioned lower limit, the optical properties of the polarized film obtained by stretching the PVA film can be improved. On the other hand, the degree of polymerization of the PVA is preferably 8000 or less, more preferably 4000 or less, and even more preferably 3500 or less. By making the degree of polymerization of the PVA equal to or less than the above-mentioned upper limit, it is possible to suppress an increase in the PVA production cost and the occurrence of defects during film formation. The degree of polymerization of the PVA means the average degree of polymerization measured in accordance with the description of JIS K6726-1994.

[0022] The saponification degree of PVA is preferably 98 mol% or higher, more preferably 99 mol% or higher, and even more preferably 99.3 mol% or higher, because this improves the wet heat resistance of the resulting polarized film. Furthermore, the higher the saponification degree of PVA, the lower the water solubility of the PVA film, making it suitable for use in the manufacture of polarized films, including a stretching step in water. The saponification degree of PVA refers to the ratio (mol%) of the number of moles of vinyl alcohol units to the total number of moles of structural units (typically vinyl ester units) that can be converted to vinyl alcohol units by saponification and vinyl alcohol units. The saponification degree can be measured in accordance with the description of JIS K6726-1994.

[0023] The PVA film of the present invention may contain one type of PVA alone, or may contain two or more types of PVAs that differ from each other in degree of polymerization, degree of saponification, degree of modification, etc.

[0024] The content of PVA in the PVA film of the present invention is not necessarily limited, but is preferably 60% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more.

[0025] (Plasticizer) The PVA film of the present invention preferably contains a plasticizer. By containing a plasticizer, the handleability and stretchability of the PVA film can be improved. Examples of plasticizers include polyhydric alcohols such as ethylene glycol, glycerin, diglycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, and sorbitol. These plasticizers may be used alone or in combination of two or more. Among these, ethylene glycol or glycerin is preferred as the plasticizer, and glycerin is more preferred, because it is less likely to bleed out onto the surface of the PVA film.

[0026] The content of the plasticizer in the PVA film of the present invention is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and even more preferably 4 parts by mass or more, per 100 parts by mass of PVA. By setting the content of the plasticizer to be equal to or greater than the above-mentioned lower limit, the stretchability of the PVA film is improved. On the other hand, the content of the plasticizer is preferably 40 parts by mass or less, more preferably 30 parts by mass or less, and even more preferably 20 parts by mass or less, per 100 parts by mass of PVA. By setting the content of the plasticizer to be equal to or less than the above-mentioned upper limit, it is possible to prevent the plasticizer from bleeding out onto the surface of the PVA film, which would reduce the handleability of the PVA film.

[0027] (Surfactant) The PVA film of the present invention preferably contains a surfactant. By including a surfactant, the handleability of the PVA film and the releasability of the PVA film from a film-forming device during production can be improved. The surfactant is not particularly limited, and for example, anionic surfactants and nonionic surfactants are preferably used.

[0028] Examples of anionic surfactants include carboxylic acid surfactants such as potassium laurate; sulfate ester surfactants such as octyl sulfate; and sulfonic acid surfactants such as dodecylbenzenesulfonate.

[0029] Examples of nonionic surfactants include alkyl ether surfactants such as polyoxyethylene lauryl ether and polyoxyethylene oleyl ether; alkyl phenyl ether surfactants such as polyoxyethylene octylphenyl ether; alkyl ester surfactants such as polyoxyethylene laurate; alkyl amine surfactants such as polyoxyethylene lauryl amino ether; alkyl amide surfactants such as polyoxyethylene lauric acid amide; polypropylene glycol ether surfactants such as polyoxyethylene polyoxypropylene ether; alkanolamide surfactants such as lauric acid diethanolamide and oleic acid diethanolamide; and allyl phenyl ether surfactants such as polyoxyalkylene allyl phenyl ether.

[0030] Such surfactants may be used alone or in combination of two or more. As the surfactant, nonionic surfactants are preferred, alkanolamide surfactants are more preferred, and dialkanolamides (e.g., diethanolamides) of aliphatic carboxylic acids (e.g., saturated or unsaturated aliphatic carboxylic acids having 8 to 30 carbon atoms) are even more preferred, due to their excellent effect of reducing surface abnormalities during film formation of the PVA film.

[0031] The surfactant content in the PVA film of the present invention is preferably 0.01 parts by mass or more, more preferably 0.02 parts by mass or more, and even more preferably 0.05 parts by mass or more, relative to 100 parts by mass of PVA. On the other hand, the surfactant content is preferably 10 parts by mass or less, more preferably 1 part by mass or less, even more preferably 0.5 parts by mass or less, and particularly preferably 0.3 parts by mass or less, relative to 100 parts by mass of PVA. A surfactant content within the above range improves the releasability of the PVA film from the film-forming apparatus during production and prevents adhesion between PVA films (hereinafter sometimes referred to as "blocking"). It also prevents the surfactant from bleeding out onto the surface of the PVA film and deterioration of the appearance of the PVA film due to surfactant aggregation.

[0032] (Other Components) In addition to PVA, surfactants, and plasticizers, the PVA film of the present invention may contain components such as water-soluble polymers, moisture, antioxidants, UV absorbers, lubricants, crosslinking agents, colorants, fillers, preservatives, antifungal agents, and other polymer compounds, to the extent that the effects of the present invention are not impaired. The proportion of the total mass of the components other than PVA, surfactants, and plasticizers to the total mass of the PVA film is preferably 60 to 100 mass%, more preferably 80 to 100 mass%, and even more preferably 90 to 100 mass%.

[0033] (Thickness and Shape of PVA Film) The upper limit of the average thickness of the PVA film of the present invention is not particularly limited, but is preferably 100 μm, more preferably 80 μm, even more preferably 60 μm, and particularly preferably 50 μm. On the other hand, the lower limit of this average thickness is preferably 10 μm, more preferably 15 μm, and even more preferably 20 μm, from the viewpoint of obtaining a PVA film that satisfies the above formulas (1) to (3).

[0034] The shape of the PVA film of the present invention is not particularly limited, but a long film is preferred because it allows for continuous production of polarized films with good productivity. The length of the long film is not particularly limited and can be set appropriately depending on the application of the polarized film to be produced, and can be, for example, within a range of 5 m or more and 20,000 m or less. The width of the long film is not particularly limited and can be, for example, 50 cm or more. However, given the recent demand for wider polarized films, it is preferably 1 m or more, more preferably 3 m or more, and even more preferably 4 m or more. There is no particular upper limit to the width of the long film, but if the width is too wide, it tends to be difficult to uniformly stretch the film when producing polarized films using commercially available equipment, so the width of the PVA film is preferably 7 m or less.

[0035] The shape of the PVA film of the present invention is not particularly limited, and it may be a single-layer film or a multi-layer film (laminate). However, from the viewpoint of the complexity and cost of lamination (coating, etc.) work, a single-layer film is preferred.

[0036] The PVA film of the present invention has excellent stretchability while maintaining a high degree of polarization when made into a polarizing film, and therefore can be suitably used as a film for producing optical films such as polarizing films. Note that a stretched PVA film is also within the scope of the present invention.

[0037] The PVA film of the present invention may be produced by any method, including, for example, the following. Examples of such methods include a method in which a film-forming solution obtained by adding a solvent, additives, etc. to PVA and homogenizing it is subjected to a casting film-forming method, a wet film-forming method (a method in which the film-forming solution is extruded into a poor solvent), a dry-wet film-forming method, a gel film-forming method (a method in which the film-forming solution is cooled to gel, and then the solvent is extracted and removed), or a combination of these methods; a melt-extrusion film-forming method or an inflation molding method in which a film-forming solution obtained using an extruder or the like is extruded through a T-die or the like to form a film; and the like. Among these, the casting film-forming method and the melt-extrusion film-forming method are preferred.

[0038] In particular, by using a casting film-forming method and controlling the cumulative stretch ratio in the heat treatment step and the orientation adjustment step within a specific range, the PVA film of the present invention can be produced more efficiently.

[0039] [PVA Film Manufacturing Method] That is, the PVA film manufacturing method of the present invention is as follows: A PVA film manufacturing method including a casting step of casting a film-forming dope solution containing PVA onto a support, a drying step of removing the solvent from the film peeled from the casting step while transporting the film using drying rolls, a heat treatment step of heat treating the film while transporting it using heat treatment rolls, and an orientation adjusting step of adjusting the orientation using orientation adjusting rolls, wherein the casting step, drying step, heat treatment step, and orientation adjusting step are performed in this order, wherein the cumulative stretch ratio of the heat treatment step is 0.970 or more and 0.994 or less, the cumulative stretch ratio of the orientation adjusting step is 1.005 or more and 1.020 or less, and the total cumulative stretch ratio of the heat treatment step and the orientation adjusting step is 0.995 or more and 1.005 or less.

[0040] In the present invention, the cumulative draw ratio in the heat treatment step is the product of the draw ratios between the supports used in the heat treatment step, and can be controlled by changing the rotation speed of the support. From the viewpoint of controlling the processability and the refractive index of the PVA film, the cumulative draw ratio in the heat treatment step is preferably 0.980 to 0.996, and particularly preferably 0.982 to 0.994. The draw ratio means the ratio of the winding speed (peripheral speed) of the support, such as a drum or roll (peripheral speed of the support in the latter stage / peripheral speed of the support in the former stage), and is also used as a value representing the stretch ratio of the film.

[0041] In the present invention, the cumulative stretch ratio in the orientation adjustment step is the product of the draw ratios between the supports used in the orientation adjustment step, and is controlled by changing the rotation speed of the rolls. From the viewpoint of processability and controlling the refractive index of the PVA film, the cumulative stretch ratio in the orientation adjustment step is preferably 1.001 to 1.025, and particularly preferably 1.005 to 1.020. If the cumulative stretch ratio is too low, the refractive index Nx of the PVA film in the MD direction decreases, making it difficult to maintain the polarization degree of the polarized film. On the other hand, if the cumulative stretch ratio is too high, the refractive index Nx of the PVA film in the MD direction increases, resulting in poor stretchability of the PVA film.

[0042] In the present invention, the total cumulative stretch ratio in the heat treatment step and the orientation adjustment step is a value obtained by multiplying the cumulative stretch ratio in the heat treatment step by the cumulative stretch ratio in the orientation adjustment step. From the viewpoint of processability and control of the refractive index of the PVA film, the total cumulative stretch ratio is preferably 0.998 to 1.005, and particularly preferably 0.998 to 1.001.

[0043] In the method for producing the PVA film of the present invention, when the film is produced by a casting film-forming method or a melt-extrusion film-forming method, the production steps are divided into a casting step, a drying step, a heat treatment step, an orientation adjusting step, and a winding step, which are carried out in this order. Each step will be described below.

[0044] (Casting process) In the casting process, first, a film-forming solution containing PVA, a solvent, and, if necessary, additives such as a plasticizer is prepared. Next, this film-forming solution is cast (supplied) in the form of a film onto a rotating support such as a metal roll or a metal belt. This forms a film of the film-forming solution on the support.

[0045] The film cast on the support can be subjected to subsequent drying, heat treatment, and orientation adjustment processes to control the draw ratio between the supports, thereby adjusting the crystallization rate and degree of orientation of the PVA. The crystallization rate and degree of orientation are affected by the proportion of structural units derived from other monomers in the PVA, the polymerization degree of the PVA, the saponification degree of the PVA, and the plasticizer content, as well as the moisture content and temperature in the PVA.

[0046] Typically, drying of a PVA film proceeds by volatilization of volatiles from the open film surface that is not in contact with the support or drying rolls. Therefore, during the drying process, a concentration distribution of volatiles such as water occurs across the thickness of the PVA film. This causes changes in the crystal orientation of the PVA film in-plane and across the thickness depending on the temperature and drawing conditions, resulting in changes in the refractive index in the MD, TD, and thickness directions. These three refractive indices can be adjusted by the support temperature, hot air temperature, the volatile content of the PVA film when peeled from the support, the heat treatment method, the volatile content of the PVA film during heat treatment, the heat treatment temperature, etc. Therefore, by appropriately controlling the above factors, the crystallization and orientation of the PVA can be appropriately promoted, and the refractive index of the PVA film in the three directions can be adjusted. The PVA film of the present invention can be easily obtained by adjusting the cumulative stretch ratio in the heat treatment and orientation adjustment steps within appropriate ranges.

[0047] The volatile content of the film-forming solution (the concentration of volatile components such as solvents removed by volatilization or evaporation during film formation) is preferably 60% by mass or more, more preferably 65% ​​by mass or more. The volatile content of the film-forming solution is preferably 90% by mass or less, more preferably 80% by mass or less. When the volatile content is within the above range, the viscosity of the film-forming solution can be adjusted to a suitable range, improving the film-forming properties of the film cast on the support and making it easier to obtain a PVA film with a uniform thickness. The film-forming solution may contain a dichroic dye as necessary.

[0048] Here, the volatile content of the membrane-forming solution in this specification refers to a value calculated by the following formula (4): Volatile content of membrane-forming solution (mass%) = {(Wa - Wb) / Wa} × 100 (4) In the above formula (4), Wa represents the mass (g) of the membrane-forming solution, and Wb represents the mass (g) of the membrane-forming solution Wa (g) after drying it in an electric dryer at 105°C for 16 hours.

[0049] The method for preparing the film-forming solution is not particularly limited, and examples thereof include a method in which PVA and additives such as a plasticizer and a surfactant are dissolved in a solvent in a dissolution tank or the like, and a method in which a water-containing PVA is melt-kneaded together with additives such as a plasticizer and a surfactant using a single-screw or twin-screw extruder.

[0050] The film-forming solution generally passes through the die lip of a die such as a T-die and is cast in the form of a film onto a support such as a metal roll or a metal belt. On the support, the solvent evaporates from the surface of the cast film that is not in contact with the support (hereinafter sometimes referred to as the "free surface"), while it does not substantially evaporate from the surface that is in contact with the support (hereinafter sometimes referred to as the "touch surface"). This results in a distribution of the solvent concentration in the film thickness direction, where the solvent concentration is low on the free surface side and high on the touch surface side. Therefore, solidification of the PVA also proceeds first from the free surface.

[0051] Crystallization of PVA proceeds in parallel with the solidification of PVA. Crystallization of PVA does not proceed easily if the solvent concentration is too high or too low. Although it depends on the primary structure of the PVA molecule, it proceeds easily when the volatile fraction of the drained film-forming solution is 10% by mass or more and 70% by mass or less. If the volatile fraction is too high, the formed crystals will re-dissolve. On the other hand, if the volatile fraction is too low, the mobility of the PVA molecules will be reduced, making it difficult for crystallization to proceed.

[0052] To obtain the PVA film of the present invention, the temperature of the support onto which the film-forming solution is poured is preferably 65°C or higher, more preferably 70°C or higher, and even more preferably 75°C or higher. To obtain the PVA film of the present invention, the temperature of the support onto which the film-forming solution is poured is preferably 120°C or lower, more preferably 105°C or lower, and even more preferably 99°C or lower. If the temperature is too high, bubbles will be generated, resulting in deterioration of the film surface. On the other hand, if the temperature is too low, the film will not dry sufficiently.

[0053] While the film is heated on the support, hot air may be blown uniformly onto the entire area of ​​the non-contact side of the film at a speed of 1 to 10 m / sec. The temperature of the hot air blown onto the non-contact side is preferably 50°C or higher, more preferably 70°C or higher. The temperature of the hot air blown onto the non-contact side is preferably 150°C or lower, more preferably 120°C or lower. When the temperature of the hot air blown onto the non-contact side is within the above range, it becomes easier to adjust the refractive index of the PVA film in each direction.

[0054] To obtain the PVA film of the present invention, it is preferable to adjust the volatile content of the PVA film when peeled from the support. The value obtained by dividing the volatile content (mass%) when peeled from the support by the thickness (μm) of the PVA film after drying is preferably 0.1 mass% / μm or more, more preferably 0.2 mass% / μm or more, and even more preferably 0.3 mass% / μm or more. The value obtained by dividing the volatile content (mass%) when peeled from the support by the thickness (μm) of the PVA film after drying is preferably 1.0 mass% / μm or less, more preferably 0.8 mass% / μm or less, and even more preferably 0.5 mass% / μm or less. If the volatile content of the PVA film when peeled from the support is too low, the film will not adhere well to the support, making it impossible to convey. On the other hand, if the volatile content of the PVA film is too high, peeling from the roll will become unstable, resulting in uneven thickness and retardation, which is undesirable.

[0055] Here, the volatile content of a film in this specification refers to a value calculated by the following formula (5): Volatile content of film (mass%)={(Wc-Wd) / Wc}×100 (5) In the above formula (5), Wc represents the mass (g) of a sample taken from the film, and Wd represents the mass (g) of the sample Wc (g) when it is placed in a vacuum dryer at a temperature of 50°C and a pressure of 0.1 kPa or less and dried for 4 hours.

[0056] The volatile content of the PVA film when peeled from the support can also be adjusted by changing the peripheral speed of the support, such as a metal roll or metal belt. For example, when the peripheral speed of the support is slow, the film is in contact with the support for a longer period of time, thereby reducing the volatile content of the PVA film when peeled from the support. The peripheral speed of the support is preferably 8 m / min or more, more preferably 11 m / min or more, and even more preferably 14 m / min or more. The peripheral speed of the support is preferably 30 m / min or less, more preferably 27 m / min or less, and even more preferably 24 m / min or less. When the peripheral speed of the support is within the above range, a PVA film with excellent in-plane uniformity and minimal thickness unevenness can be produced while maintaining the productivity of the PVA film.

[0057] In the drying step, the film peeled from the support such as the metal roll or metal belt in the casting step is transported to a support such as a drying roll and heated on the support to remove the solvent contained in the film. Examples of methods for heating the film include a method of raising the temperature of the support itself with a heat medium or a method of blowing hot air onto the surface of the film opposite to the surface in contact with the support.

[0058] The temperature of the drying roll in the drying step is preferably 50°C or higher, more preferably 60°C or higher. When the temperature of the drying roll is equal to or higher than the above lower limit, the film is dried well, and the transportability of the film is improved. Furthermore, the temperature of the drying roll is preferably 105°C or lower, more preferably 95°C or lower. When the temperature of the drying roll is equal to or lower than the above upper limit, the film is dried well, and the film can be prevented from becoming hard and breaking.

[0059] (Heat Treatment Step) In the heat treatment step, the film peeled from the drying roll or metal belt in the drying step is transported to a support such as a heated roll and heat-treated on the support. The heat treatment method is not particularly limited, and examples include a method of blowing hot air using a floating dryer, a method of irradiating near-infrared rays using an infrared lamp, and a method of contacting the film with a heat treatment roll. However, to obtain the PVA film of the present invention, a method of directly contacting the surface of the PVA film with a heat treatment roll is preferred. Furthermore, when drying the PVA film using multiple heat treatment rolls, it is preferable to alternately contact one surface and the other surface of the PVA film with the heat treatment roll. This allows the refractive index in each direction on both sides of the PVA film to be adjusted. In this case, the number of heat treatment rolls is preferably two or more, and more preferably four or more.

[0060] The volatile content of the PVA film when brought into contact with the first heat treatment roll is preferably 2% by mass or more, more preferably 4% by mass or more. The volatile content of the PVA film when brought into contact with the first heat treatment roll is preferably 15% by mass or less, more preferably 12% by mass or less. By keeping the volatile content of the PVA film when brought into contact with the first heat treatment roll within the above range, it becomes easier to adjust the refractive index of the PVA film in each direction.

[0061] The temperature of the heat treatment roll in the heat treatment step is preferably 80° C. or higher, more preferably 90° C. or higher. The temperature of the heat treatment roll for performing the heat treatment is preferably 140° C. or lower, more preferably 130° C. or lower. By setting the temperature of the heat treatment roll within the above range, it becomes easier to adjust the refractive index in each direction of the PVA film.

[0062] The PVA film that has been subjected to the heat treatment step can be subjected to an additional heat treatment, if necessary.

[0063] In the orientation adjustment step, the film peeled from the heat roll or metal belt in the heat treatment step is transported to a support such as a metal roll, and if necessary, the roll speed is adjusted to perform appropriate stretching and adjust the orientation. The PVA film undergoes the orientation adjustment step and is wound into a roll.

[0064] The temperature of the PVA film in the orientation adjustment step is not particularly limited, but is preferably 20 to 80°C. When the temperature of the PVA film is 80°C or less, it becomes easier to achieve both flexibility and a uniform in-plane refractive index of the film. Furthermore, when the temperature of the PVA film is 20°C or more, elastic deformation of the film is prevented, and it becomes easier to adjust the refractive index of the PVA film.

[0065] The volatile content of the PVA film when brought into contact with the orientation adjustment roll is not necessarily limited, but is preferably 1 to 5% by mass. A volatile content of 5% by mass or less prevents the film from becoming soft and makes it easier to uniformly adjust the orientation of the film. Furthermore, a volatile content of 1% by mass or more prevents the film from elastically deforming and makes it easier to adjust the refractive index of the film.

[0066] The temperature of the orientation adjusting roll in the orientation adjusting step is preferably 15°C or higher, more preferably 20°C or higher. When the temperature of the orientation adjusting roll is equal to or higher than the above lower limit, it becomes easier to achieve both flexibility of the film and a uniform refractive index in the plane. Furthermore, the temperature of the drying roll is preferably 60°C or lower, more preferably 50°C or lower. When the temperature of the drying roll is equal to or lower than the above upper limit, elastic deformation of the film is prevented, and it becomes easier to adjust the refractive index.

[0067] The volatile content of the PVA film finally obtained by the series of treatments is not necessarily limited, but is preferably 1% by mass or more, more preferably 2% by mass or more. The volatile content of the PVA film finally obtained is not necessarily limited, but is preferably 5% by mass or less, more preferably 4% by mass or less.

[0068] (Winding process) The PVA film produced in this manner may be further subjected to humidity conditioning treatment, cutting of both ends (edges) of the film, etc., as necessary, and then wound into a roll on a cylindrical core and moisture-proof packaged to form a product.

[0069] [Optical Film] By using the PVA film of the present invention as a raw sheet of an optical film, an optical film having excellent polarization performance and moist heat resistance can be produced, and therefore the PVA film is suitable as a film for producing an optical film. Examples of optical films include polarizing films, viewing angle improving films, retardation films, and brightness improving films, but polarizing films are preferred. Below, a method for producing a polarizing film will be specifically described as an example of a method for producing an optical film.

[0070] The method for producing a polarized film is not particularly limited, and any conventionally used method may be employed. For example, a polarized film can be produced by subjecting the PVA film of the present invention to a swelling treatment, a dyeing treatment, a uniaxial stretching treatment, and, if necessary, a crosslinking treatment, a fixing treatment, a drying treatment, a heat treatment, or the like. In this case, the order of each treatment, such as the swelling treatment, the dyeing treatment, the uniaxial stretching treatment, and the fixing treatment, is not particularly limited, and one or more treatments may be performed simultaneously. Furthermore, one or more treatments may be performed twice or more times.

[0071] The swelling treatment can be carried out by immersing the PVA film in water. The lower limit of the water temperature during immersion is preferably 20°C, more preferably 22°C, and even more preferably 25°C. Meanwhile, the upper limit is preferably 40°C, more preferably 38°C, and even more preferably 35°C. The immersion time in water is preferably, for example, 0.1 minutes or more and 5 minutes or less. The water used for immersion in water is not limited to pure water, and may be an aqueous solution in which various components are dissolved, or a mixture of water and an aqueous medium.

[0072] The dyeing treatment is carried out using a dichroic dye such as an iodine-based pigment or dye, and the dyeing may be carried out at any stage before, during, or after uniaxial stretching. Dyeing is generally carried out by immersing the PVA film in a solution (particularly an aqueous solution) containing iodine and potassium iodide as a dye bath. The iodine concentration in the dye bath is preferably 0.01% by mass or more. The iodine concentration in the dye bath is preferably 0.5% by mass or less. The potassium iodide concentration is preferably 0.01% by mass or more. The potassium iodide concentration is preferably 10% by mass or less. The dye bath temperature is preferably 20°C or more. The dye bath temperature is preferably 60°C or less.

[0073] The crosslinking treatment can be carried out by immersing the PVA film in an aqueous solution containing a crosslinking agent. Crosslinking treatment introduces crosslinks into the PVA film, allowing uniaxial stretching at a relatively high temperature. The crosslinking agent used can be one or more boron compounds, such as boric acid and borax or other borate salts. The concentration of the crosslinking agent in the aqueous solution containing the crosslinking agent is preferably 1% by mass or more, more preferably 2% by mass or more. The concentration of the crosslinking agent in the aqueous solution containing the crosslinking agent is preferably 15% by mass or less, more preferably 7% by mass or less. The temperature of the aqueous solution containing the crosslinking agent is preferably 20°C or higher. The temperature of the aqueous solution containing the crosslinking agent is preferably 60°C or lower.

[0074] The uniaxial stretching may be performed by either a wet stretching method or a dry stretching method. In the case of the wet stretching method, it can be performed in an aqueous solution containing boric acid, or in the dye bath described above or the fixing treatment bath described below. In the case of the dry stretching method, it can be performed in air. Among these, the wet stretching method is preferred, and uniaxial stretching in an aqueous solution containing boric acid is more preferred. The concentration of boric acid in the aqueous boric acid solution is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, and even more preferably 1.5% by mass or more. The concentration of boric acid in the aqueous boric acid solution is preferably 6.0% by mass or less, more preferably 5.0% by mass or less, and even more preferably 4.0% by mass or less. The aqueous boric acid solution may also contain potassium iodide, and the concentration thereof is preferably 0.01% by mass or more. The concentration of potassium iodide is preferably 10% by mass or less.

[0075] In the wet stretching method, the stretching temperature in uniaxial stretching is preferably 30° C. or higher, more preferably 40° C. or higher, and even more preferably 50° C. or higher. The stretching temperature in uniaxial stretching is preferably 90° C. or lower, more preferably 80° C. or lower, and even more preferably 70° C. or lower.

[0076] The stretching ratio in uniaxial stretching is preferably 5 times or more, more preferably 5.5 times or more, from the viewpoint of the polarizing performance of the resulting polarizing film. There is no particular upper limit to the stretching ratio, but the stretching ratio is preferably 8 times or less.

[0077] When producing a polarizing film, a fixation treatment is preferably performed to strengthen the adsorption of the dichroic dye onto the PVA film. The fixation bath used for the fixation treatment can be an aqueous solution containing one or more boron compounds, such as boric acid and borax. If necessary, an iodine compound or a metal compound may be added to the fixation bath. The concentration of the boron compound in the fixation bath is generally preferably 2% by mass or more, more preferably 3% by mass or more. The concentration of the boron compound in the fixation bath is generally preferably 15% by mass or less, more preferably 10% by mass or less. The temperature of the fixation bath is preferably 15°C or more, more preferably 25°C or more. The temperature of the fixation bath is preferably 60°C or less, more preferably 40°C or less.

[0078] The drying treatment is preferably carried out at 30° C. or higher, and more preferably at 50° C. or higher. The drying treatment is preferably carried out at 150° C. or lower, and more preferably at 130° C. or lower. Drying at a temperature within the above range makes it easier to obtain a polarizing film with excellent dimensional stability.

[0079] There are no particular limitations on the film thickness of the polarizing film, and it can be, for example, 30 μm or less, or even 25 μm or less. From the viewpoints of reducing the shrinkage stress of the polarizing film and even the polarizing plate using the polarizing film and preventing warping of the thin glass to be laminated, the film thickness of the polarizing film is preferably 20 μm or less, more preferably 15 μm or less. On the other hand, since a polarizing film that is too thin tends to be difficult to manufacture and handle, the film thickness of the polarizing film is preferably 3 μm or more.

[0080] [Polarizing Plate] The polarizing film obtained as described above is preferably used as a polarizing plate by laminating an optically transparent and mechanically strong protective film on one or both sides thereof. Examples of the protective film include cellulose triacetate (TAC) film, cycloolefin polymer (COP) film, cellulose acetate butyrate (CAB) film, acrylic film, and polyester film. Examples of adhesives used for lamination include PVA-based adhesives and urethane-based adhesives, with PVA-based adhesives being preferred.

[0081] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. The evaluation methods used in the following examples and comparative examples are shown below.

[0082] [Refractive Index (Nx, Ny, Nz) and Nz Coefficient] The three-dimensional refractive index (Nx, Ny, Nz) of the central portion in the width direction of the PVA film was measured using a retardation measurement device ("RETS-100nx" manufactured by Otsuka Electronics Co., Ltd., measurement wavelength: 520 nm). Here, x: machine direction, y: width direction, z: film thickness direction. The Nz coefficient was calculated from the obtained refractive index. The average refractive index was set to 1.5000. The tilt direction of the tilt retardation was the fast axis direction, the tilt angle was 45°, and the measurement wavelength was 520 nm. The retardation calculation conditions were the rotating analyzer method, and a fourth-order equation was used as the retardation dispersion equation. The thickness was set to a representative value of 45 μm.

[0083] [Polarization Degree of Polarizing Film at a Transmittance of 43.2%] In each of the following Examples and Comparative Examples, five types of polarizing films with different dye concentrations were produced by changing the iodine concentration in the iodine / potassium iodide aqueous solution used in the second-stage stretching step of producing the polarizing film from a PVA film. The single-piece transmittance Y (%) and polarization degree V (%) of each of these five types of polarizing film were determined using the following method. An approximation curve was then created by plotting the data for the five types of polarizing film with the single-piece transmittance Y on the horizontal axis and the polarization degree V on the vertical axis. From this approximation curve, the value of polarization degree V when the single-piece transmittance Y was 43.2% was determined, and this was designated the "polarization degree (%) at a transmittance of 43.2%."

[0084] A rectangular sample measuring 4 cm in the MD direction and 2 cm in the TD direction was collected from the center of the polarizing film in the TD direction, and the parallel transmittance and crossed Nicol transmittance of the polarizing film were measured using a V-7100 spectrophotometer with an integrating sphere (manufactured by JASCO Corporation) and an automatic polarizing film measuring device VAP-7070S (manufactured by JASCO Corporation) equipped with a Glan-Taylor polarizer. The measurement wavelength range was set to 380 to 780 nm, and the transmittance when the vibration direction of polarized light incident on the polarizing film through the Glan-Taylor polarizer was parallel to the transmission axis of the polarizing film was defined as the parallel transmittance, and the transmittance when the vibration direction was perpendicular to the transmission axis of the polarizing film was defined as the crossed Nicol transmittance. Thereafter, using a "Polarizing Film Evaluation Program" (manufactured by JASCO Corporation), luminosity correction was performed in the visible light region using the parallel transmittance and crossed Nicol transmittance described above in accordance with JIS Z 8722 (Method for measuring object color), with a C light source and a 2° field of view, to calculate the single transmittance and degree of polarization of the polarizing film, and these two values ​​were obtained as the optical properties of the polarizing film.

[0085] [Break Ratio] (i) Preparation of Measurement Films Three rectangular films measuring 50 mm in width and 90 mm in length were cut from the PVA films obtained in the following Examples or Comparative Examples along an arbitrary line parallel to the width direction, from one end of the film in the width direction to a position 50% of the entire width direction. The rectangular films were cut so that the center of gravity of the film coincided with the above position. Next, a reference line was drawn 20 mm inward from both ends of the 90 mm length direction of each of the three films. (ii) Preparation of Calibration Curve The portions outside the reference lines on both ends of the three films obtained in "(i) Preparation of Measurement Films" above were clamped with a chuck, and a stretching test was performed using a laboratory stretching machine under the following conditions. A calibration curve was created from the relationship between the obtained stretch ratio and stretching time. First, while immersed in water at a temperature of 30°C, the films were uniaxially stretched in the length direction to twice their original length (swelling treatment). Next, the film after the swelling treatment was immersed in an aqueous solution containing 0.05% by mass of iodine and 1.2% by mass of potassium iodide (temperature 32 ° C.), while being uniaxially stretched in the longitudinal direction to 2.5 times its original length (dyeing treatment). Next, the film after the dyeing treatment was immersed in an aqueous solution containing 2.6% by mass of boric acid (temperature 32 ° C.), while being uniaxially stretched in the longitudinal direction to 3 times its original length (crosslinking treatment). Next, the film after the crosslinking treatment was immersed in an aqueous solution containing 2.8% by mass of boric acid and 5% by mass of potassium iodide (58 ° C.), while being uniaxially stretched in the longitudinal direction to any ratio (4 times in total) between 6 times its original length and the ratio at which it broke (uniaxial stretching treatment), and the gauge length and stretching time after the stretching test were measured. Then, the gauge length after the stretching test was divided by the gauge length before the stretching test (50 mm) to determine the stretch ratio, and a calibration curve was created from the relationship between the obtained stretch ratio and stretching time. (iii) Breaking ratio: The three films obtained in "(i) Preparation of film for measurement" above were subjected to a stretching test until the films broke, in the same manner as in "(ii) Preparation of calibration curve" above, and the stretching break time for each of the three films was measured. Next, using the stretching break time obtained, the stretching ratio at which the film broke was calculated from the calibration curve obtained in "(ii) Preparation of calibration curve" above. The same measurement was performed 20 times, and the average value was calculated as the breaking ratio of the PVA film.

[0086] Example 1: 100 parts by weight of PVA (saponification degree 99.9 mol%, polymerization degree 2,800), 10 parts by weight of glycerin as a plasticizer, 0.1 parts by weight of lauric acid diethanolamide as a surfactant, and 233 parts by weight of water were melt-mixed in a melt extruder to prepare a film-forming solution with a volatile content of 70% by weight. The resulting film-forming solution was then extruded from a T-die onto the first drying roll (temperature 93°C) of a film-forming apparatus equipped with multiple drying rolls whose rotation axes are parallel to each other to form a PVA film, which was then dried until the volatile content of the PVA film reached 18% by weight. In this example, the first drying roll served as a support in the casting process. The PVA film was then peeled from the first drying roll, and the PVA film was subjected to the subsequent drying, heat treatment, and orientation adjustment steps, with the contact surface and non-contact surface alternately facing the drying or heat treatment rolls, to obtain a PVA film (film thickness 45 μm, width 440 cm, length 200 m). The roll temperatures and cumulative stretch ratios for the drying, heat treatment, and orientation adjustment steps are shown in Table 1. The three-dimensional refractive indices Nx, Ny, Nz, and Nz coefficients, as well as the polarization performance and break ratio at each transmittance, were determined for the resulting PVA film using the methods described above. The results are shown in Table 2.

[0087] [Examples 2 and 3, Comparative Examples 1 to 3] PVA films were obtained in the same manner as in Example 1, except that the surface temperatures of the drying roll and heat treatment roll, the cumulative stretch ratio in the heat treatment step, and the cumulative stretch ratio in the orientation adjustment step were changed as shown in Table 1. The three-dimensional refractive indices Nx, Ny, Nz, and Nz coefficients, as well as the polarization performance and break ratio at each transmittance, were determined using the methods described above. These results are shown in Table 2.

[0088]

[0089]

[0090] As shown in Table 2, the PVA films of Examples 1 to 3 had high Nz coefficients, which improved the breaking ratio, and it was confirmed that the PVA films were less likely to break in the latter half of the stretching process during production of the polarized film. Furthermore, it was found that the polarized films obtained from the PVA films of Examples 1 to 3 had excellent stretchability while maintaining a high level of polarization degree.

Claims

1. A polyvinyl alcohol film satisfying the following formulas (1) and (2): 1.50075≦Nx≦1.50105 (1) 9.0≦(Nx-Nz) / (Nx-Ny)≦20.0 (2) Nx represents the refractive index in the length direction (MD) of the film, Ny represents the refractive index in the width direction (TD) of the film, and Nz represents the refractive index in the thickness direction of the film.

2. The polyvinyl alcohol film according to claim 1, which satisfies the following formula (3): 1.50050≦Ny≦1.50080 (3) 3. The polyvinyl alcohol film according to claim 1 or 2, wherein the polymerization degree of the polyvinyl alcohol is 1,000 or more and 8,000 or less.

4. The polyvinyl alcohol film according to claim 1 or 2, wherein the degree of saponification of the polyvinyl alcohol is 98.0 mol % or more.

5. The polyvinyl alcohol film according to claim 1 or 2, which has a width of 3 m or more.

6. The polyvinyl alcohol film according to claim 1 or 2, having a thickness of 20 to 80 μm.

7. A method for producing a polyvinyl alcohol film, comprising: a casting step of casting a film-forming solution containing polyvinyl alcohol onto a support; a drying step of transporting the film peeled off from the support in the casting step using a drying roll to remove the solvent; a heat treatment step of performing heat treatment while transporting the film using a heat treatment roll; and an orientation adjustment step of adjusting the orientation using an orientation adjustment roll, wherein the steps are carried out in the order of the casting step, drying step, heat treatment step, and orientation adjustment step, the cumulative stretch ratio of the heat treatment step is 0.970 or more and 0.994 or less, the cumulative stretch ratio of the orientation adjustment step is 1.005 or more and 1.020 or less, and the total cumulative stretch ratio of the heat treatment step and the orientation adjustment step is 0.995 or more and 1.005 or less.

8. The method for producing a polyvinyl alcohol film according to claim 7, wherein the heat treatment roll in the heat treatment step is at 95 to 115°C.

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