Polyvinyl alcohol film, and polarizing film and polarizing plate using the same

Abstract

Provided are a PVA film in which stretching breakage is less likely to occur in uniaxial stretching when manufacturing an optical film such as a polarizing film, and a polarizing film and a polarizing plate using the same. A polyvinyl alcohol film, wherein, in at least one face of the polyvinyl alcohol film, the average of the detection intensity of positive silicon fragment ions obtained by positive ion analysis based on time-of-flight secondary ion mass spectrometry is 0.001 to 0.01, the polyvinyl alcohol film being a raw material film for optical film manufacturing. [The average of the detection intensity of positive silicon fragment ions refers to the average of the detection intensity of positive silicon fragment ions obtained by positive ion analysis based on a time-of-flight secondary ion mass spectrometer at five points on an arbitrary straight line parallel to the TD direction of the polyvinyl alcohol film and equally dividing the polyvinyl alcohol film into six along the TD direction].

Description

Technical Field

[0001] This invention relates to polyvinyl alcohol films and polarizing films and polarizing plates using the same. Background Technology

[0002] A polarizing plate, which has both light-transmitting and light-blocking functions, and a liquid crystal, which has light-switching functions, are both fundamental components of a liquid crystal display (LCD). In recent years, the application areas of LCDs have expanded from small devices such as calculators and watches at the beginning of their development to various fields such as laptops, LCD monitors, LCD color projectors, LCD TVs, in-vehicle navigation systems, mobile phones, and measuring devices used indoors and outdoors.

[0003] Polarizing plates are manufactured by attaching protective films such as cellulose triacetate (TAC) or cellulose acetate butyrate (CAB) films to the surface of a polarizing film. The polarizing film is typically manufactured as follows: a polyvinyl alcohol film (hereinafter sometimes referred to as "PVA" or "polyvinyl alcohol film" as "PVA film") is dyed and then uniaxially stretched; or, uniaxially stretched while dyeing; or, dyed after uniaxial stretching, resulting in a dyed uniaxially stretched film, which is then immobilized using a boron compound. Uniaxial stretching is usually controlled by adjusting the rotational speed of a set of drive rollers (made of NBR rubber). It should be noted that the boron compound-based immobilization treatment is sometimes performed simultaneously with uniaxial stretching or dyeing.

[0004] For products with large LCD screens, such as LCD monitors and LCD TVs, high contrast and clear images are required. Consequently, high-performance polarizing films are also required, specifically, to increase the polarization degree of the polarizing film. To increase the polarization degree, the stretching ratio during uniaxial stretching of the PVA film needs to be increased. However, increasing the stretching ratio can easily lead to tensile breakage of the PVA film. As a result, the production efficiency and yield of polarizing films tend to decrease, and the cost tends to increase.

[0005] As a method to reduce the tensile fracture of PVA film under uniaxial stretching, for example, it is known in Patent Documents 1 and 2: a method of forming a PVA layer on a plastic film by coating, and then performing stretching treatment, dyeing treatment, etc. on the laminate to process the PVA layer into a polarizing film.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2012-133303

[0009] Patent Document 2: Japanese Patent Application Publication No. 2012-073570 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] However, the following problems exist in the method of using laminates formed by coating a PVA layer on a plastic film.

[0012] (i) The coating operation and subsequent drying operation are complicated.

[0013] (ii) Since heat treatment for the PVA layer is required to be performed in a laminated state without melting, the plastic film used is limited to be stretchable after heat treatment, which increases the cost.

[0014] Therefore, there is a need for PVA films that are less prone to tensile fracture during uniaxial stretching in the manufacture of optical films such as polarizing films, and which can help reduce the cost of optical films such as polarizing films.

[0015] Therefore, the object of the present invention is to provide a PVA film that is not prone to tensile fracture during uniaxial stretching in the manufacture of optical films such as polarizing films, as well as polarizing films and polarizing plates using the same.

[0016] Methods for solving problems

[0017] To achieve the aforementioned objective, the inventors conducted repeated and in-depth research, and discovered that the above-mentioned problem could be solved by adjusting the average detection intensity of silicon fragment ions obtained from positive ion analysis based on time-of-flight secondary ion mass spectrometry on at least one side of the PVA film to a specific range. The detailed rationale is not yet clear, but it can be speculated that by moderately presenting silicon ions on the surface of the PVA film, the friction between the PVA film and the rolls becomes moderate during uniaxial stretching in the manufacture of optical films such as polarizing films, thus suppressing tensile fracture of the PVA film. Based on these findings, the inventors conducted further repeated research, thereby completing this invention.

[0018] That is, the present invention relates to:

[0019] [1] A polyvinyl alcohol film, wherein the average value of the detection intensity of positive silicon fragment ions obtained by positive ion analysis based on time-of-flight secondary ion mass spectrometry in at least one side of the polyvinyl alcohol film is 0.001 to 0.01, and the polyvinyl alcohol film is a raw material film for optical film manufacturing.

[0020] [The average detection intensity of silicon fragment ions refers to the average detection intensity of silicon fragment ions obtained by positive ion analysis using a time-of-flight secondary ion mass spectrometer at five points located on any straight line parallel to the TD direction of the polyvinyl alcohol film and dividing the polyvinyl alcohol film into six equal parts along the TD direction.]

[0021] [2] According to the polyvinyl alcohol film described above [1], the difference between the maximum and minimum values ​​of the detection intensity of the positive silicon fragment ions at 5 points where the polyvinyl alcohol film is divided into six equal parts along the aforementioned TD direction is 0.0005 to 0.002.

[0022] [3] The polyvinyl alcohol film according to [1] or [2] above, wherein the average value of the coefficient of variation of the thickness of the polyvinyl alcohol film is 0.01 to 0.03.

[0023] [The average value of the coefficient of variation of the thickness refers to the average value of the coefficient of variation of the polyvinyl alcohol film along a straight line of 1.2m length parallel to the MD direction of the polyvinyl alcohol film, divided into six equal parts by five points along the aforementioned TD direction.]

[0024] [4] The polyvinyl alcohol film according to any one of [1] to [3] above has a swelling degree of 180 to 240% when immersed in water at 30°C for 30 minutes;

[0025] [5] The polyvinyl alcohol film according to any one of [1] to [4] above, wherein the length in the TD direction is 1.5 m or more;

[0026] [6] The polyvinyl alcohol film according to any one of [1] to [5] above, wherein the length in the MD direction is 3,000 m or more;

[0027] [7] The polyvinyl alcohol film according to any one of [1] to [6] above has a thickness of 10 to 40 μm;

[0028] [8] A polarizing film, which is made of any one of the polyvinyl alcohol films described in [1] to [7] above;

[0029] [9] A polarizing plate, which is obtained by attaching a protective film to at least one side of the polarizing film described in [8].

[0030] Invention Effects

[0031] According to the present invention, a PVA film that is not prone to tensile fracture during uniaxial stretching in the manufacture of optical films such as polarizing films is provided, as well as a polarizing film and a polarizing plate using the same. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the measurement site when the average detection intensity of silicon fragment ions in a PVA film is determined by positive ion analysis based on time-of-flight secondary ion mass spectrometry.

[0033] Figure 2 This is a schematic diagram of the measurement site when determining the coefficient of variation of the PVA film thickness.

[0034] Figure 3 It is a schematic diagram of the measurement site when the average detection intensity of positive silicon fragment ions in the PVA film is obtained by positive ion analysis based on time-of-flight secondary ion mass spectrometry in an example or a comparative example.

[0035] Figure 4 It is a schematic diagram of the measurement site when the coefficient of variation of the thickness of the PVA film is obtained in an example or a comparative example. Detailed implementation mode

[0036] Hereinafter, the present invention will be described in detail.

[0037] <Time-of-flight secondary ion mass spectrometry>

[0038] As a method for analyzing the components present on the film surface and their distribution states, time-of-flight secondary ion mass spectrometry (hereinafter sometimes referred to as TOF-SIMS) is known. In this analysis method, by identifying the fragment ions derived from various additives contained in the film, it is possible to grasp to what extent and how the components such as various additives derived from these fragments are distributed on the film surface.

[0039] For example, if a PVA film is measured using TOF-SIMS, a variety of fragment ions are detected. Among these fragment ions, by analyzing the signals of the fragment ions derived from the plasticizer and surfactant contained in the PVA film and comparing the signal intensities, it is possible to know the distribution state and segregation state of the surface part of these films.

[0040] In the present invention, attention is paid to the positive fragment ions derived from silicon (hereinafter sometimes referred to as positive silicon fragment ions) detected by positive ion analysis of the PVA film using TOF-SIMS. As a result of intensive research by the present inventors, by setting the average detection intensity of the positive silicon fragment ions in at least one surface of the PVA film within a specific range, it is possible to obtain a PVA film that is not easily broken during uniaxial stretching when manufacturing an optical film such as a polarizing film. Here, the detection intensity of the positive silicon fragment ions is a value obtained by dividing the count of the positive silicon fragment ions detected by positive ion analysis based on TOF-SIMS by the count of all fragment ions (T.c: total count intensity).

[0041] <PVA film>

[0042] Regarding the PVA film of the present invention, in at least one surface of the PVA film, the average detection intensity of the positive silicon fragment ions obtained by positive ion analysis based on TOF-SIMS is 0.001 to 0.01. Figure 1The diagram shows the measurement site used to calculate the average detection intensity of silicon fragment ions. The average detection intensity refers to the average detection intensity of silicon fragment ions at five points (P1, P2, P3, P4, and P5) located on any straight line A parallel to the TD direction of the PVA film and dividing the PVA film into six equal parts along the TD direction. In this invention, the average detection intensity of silicon fragment ions at these five measurement points is 0.001 to 0.01.

[0043] In this invention, the average detection intensity of silicon fragment ions on at least one side of the PVA film is 0.001 to 0.01, or the average detection intensity of silicon fragment ions on both sides of the PVA film is 0.001 to 0.01. If the average detection intensity is less than 0.001, excessive tension may be applied to the PVA film during uniaxial stretching in the manufacture of optical films such as polarizing films, leading to tensile fracture. This is likely because if the amount of silicon fragment ions present on the surface of the PVA film is too small, the friction between the PVA film and the rollers will increase. On the other hand, if the average detection intensity exceeds 0.01, the PVA film may not be stretched during uniaxial stretching in the manufacture of optical films such as polarizing films, and may instead dissolve and break in the stretching solution, resulting in tensile fracture. This is likely because if the amount of silicon fragment ions present on the surface of the PVA film is too large, slippage may occur between the rollers and the film. The average detection intensity is preferably 0.002 or more, more preferably 0.003 or more, and even more preferably 0.004 or more. The average value of the aforementioned detection intensity is preferably 0.01 or less, more preferably 0.009 or less, even more preferably 0.008 or less, and most preferably 0.007 or less.

[0044] There are no particular limitations on the method for setting the average value of the aforementioned detection intensity to 0.001 to 0.01. For example, a method of containing a silicon-containing compound in the PVA film can be cited. Among the silicon-containing compounds, an organosilicon surfactant is preferred. In this case, by appropriately adjusting the content of the organosilicon surfactant, the volatile fraction of the PVA aqueous sheet or film-forming solution, the screw speed of the extruder during the melt mixing of PVA, the surface temperature of the support for casting the film-forming solution, the contact time between the PVA film and the support, the hot air temperature for blowing the PVA film, and the temperature of the drying roller or drying oven, the average value of the detection intensity can be set to 0.001 to 0.01.

[0045] In this invention, the difference between the maximum and minimum detection intensities of silicon fragment ions at five points (points P1 to P5) along an arbitrary straight line A parallel to the TD direction of the PVA film and dividing the PVA film into six equal parts along the TD direction is preferably 0.0005 or more, more preferably 0.0007 or more, and even more preferably 0.0008 or more. The difference between the maximum and minimum detection intensities of silicon fragment ions is preferably 0.002 or less, more preferably 0.0018 or less, and even more preferably 0.0016 or less. By making the aforementioned difference between the maximum and minimum values ​​0.0005 to 0.002, uniform in-plane stretching of the PVA film is achieved during uniaxial stretching in the manufacture of optical films such as polarizing films, thus suppressing the occurrence of tensile fracture.

[0046] There are no particular limitations on the method for setting the difference between the maximum and minimum values ​​of the aforementioned detection intensity to 0.0005 to 0.002. For example, methods involving the inclusion of silicon-containing compounds such as organosilicon surfactants in the PVA film can be employed. In this case, by appropriately adjusting the content of the organosilicon surfactant, the volatile fraction of the PVA aqueous sheet or film-forming solution, the screw speed of the extruder during PVA melt mixing, the surface temperature of the support for which the film-forming solution is to be cast, the contact time between the PVA film and the support, the hot air temperature for blowing the PVA film, and the temperature of the drying roller or drying oven, the difference between the maximum and minimum values ​​of the aforementioned detection intensity can be set to 0.0005 to 0.002.

[0047] In this invention, the average value of the coefficient of variation of the thickness of the PVA film is preferably 0.01 to 0.03. Figure 2 The measurement site is shown when determining the coefficient of variation of the PVA film thickness. In this invention, as... Figure 2 As shown, the thickness of the PVA film is measured at points on a 1.2m long straight line B1 to B5, which divides the PVA film into six equal parts along the TD direction (points P1 to P5) and is parallel to the MD direction of the PVA film. The coefficient of variation of the PVA film thickness is then calculated. The aforementioned 1.2m long straight line B1 to B5 can be, for example, a straight line centered at the five points (P1 to P5) that divide the PVA film into six equal parts along the TD direction. When measuring the thickness at multiple points on the straight line B1 to B5, the measurement interval can be appropriately set, for example, at intervals of 0.5mm.

[0048] The average value of the aforementioned coefficient of variation is preferably 0.01 or more, more preferably 0.011 or more, even more preferably 0.012 or more, and most preferably 0.013 or more. The average value of the aforementioned coefficient of variation is preferably 0.03 or less, more preferably 0.025 or less, even more preferably 0.022 or less, and most preferably 0.018 or less. By making the average value of the coefficient of variation 0.01 to 0.03, the uniaxial stretching during the manufacture of optical films such as polarizing films can be uniformly stretched in the plane of the PVA film, thereby suppressing the occurrence of tensile fracture.

[0049] There are no particular limitations on the method for setting the average value of the aforementioned coefficient of variation to 0.01 to 0.03. Examples include increasing the volatile fraction of the PVA aqueous sheet or film-forming solution, decreasing the temperature of the hot air blown onto the PVA film, and the temperature of the drying roller or drying oven. In this case, by appropriately adjusting the content of the silicone surfactant, the volatile fraction of the PVA aqueous sheet or film-forming solution, the screw speed of the extruder during PVA melt mixing, the surface temperature of the support for casting the film-forming solution, the contact time between the PVA film and the support, the temperature of the hot air blown onto the PVA film, and the temperature of the drying roller or drying oven, the difference between the maximum and minimum values ​​of the aforementioned detection intensity can be set to 0.0005 to 0.002.

[0050] In this invention, the degree of swelling of the PVA film after immersion in water at 30°C for 30 minutes is preferably 180% or more, more preferably 190% or more, and even more preferably 195% or more. The degree of swelling of the PVA film after immersion in water at 30°C for 30 minutes is preferably 240% or less, more preferably 210% or less, and even more preferably 205% or less. By achieving a swelling degree of 180% to 240%, the PVA film becomes moderately soft during water immersion in swelling treatments such as those used in the manufacture of optical films such as polarizing films. This prevents excessive tension during uniaxial stretching of the PVA film, thereby suppressing tensile breakage.

[0051] In this invention, the MD direction of the PVA film refers to the length direction of the PVA film, which is consistent with the mechanical flow direction during PVA film manufacturing. On the other hand, the TD direction of the PVA film refers to the width direction of the PVA film, which is orthogonal to the mechanical flow direction during PVA film manufacturing. In the PVA film of this invention, whether a certain direction is the MD direction or the TD direction can also be determined retrospectively after PVA film manufacturing by measuring the phase difference unevenness of the PVA film. That is, it is usually difficult to make the thickness unevenness of the PVA film completely uniform during manufacturing; therefore, the direction with a large phase difference unevenness of the PVA film can be identified as the TD direction. On the other hand, the direction with a small phase difference unevenness of the PVA film can be identified as the MD direction.

[0052] (PVA)

[0053] In the PVA film of the present invention, the PVA can be a polymer manufactured by saponifying a vinyl ester polymer obtained by polymerizing vinyl ester monomers. Examples of vinyl ester monomers include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl neovalerate, and vinyl tert-carboxylate. Among these, vinyl acetate is preferred as the vinyl ester monomer.

[0054] Vinyl ester polymers are preferably polymers obtained using only one or more vinyl ester monomers as monomers, and more preferably polymers obtained using only one vinyl ester monomer as monomer. It should be noted that vinyl ester polymers can be copolymers of one or more vinyl ester monomers with other monomers capable of copolymerizing with them.

[0055] Other monomers include, for example, ethylene; olefins with 3 to 30 carbon atoms such as propylene, 1-butene, and isobutene; acrylic acid or its salts; acrylates such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, and octadecyl acrylate; methacrylic acid or its salts; methacrylates such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, and octadecyl methacrylate; acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetone acrylamide, acrylamide propanesulfonic acid or its salts, acrylamide propyl dimethylamine or its salts, and N-hydroxymethylacrylamide or its derivatives. Bio-based acrylamide derivatives; methacrylamide derivatives such as methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamide propanesulfonic acid or its salts, methacrylamide propyl dimethylamine or its salts, N-hydroxymethylmethacrylamide or its derivatives; N-vinyl amides such as N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, tert-butyl vinyl ether, dodecyl vinyl ether, stearyl vinyl ether; cyanide such as acrylonitrile and methacrylonitrile; halogenated vinylides such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl acetate, allyl chloride, and other allyl compounds; maleic acid or its salts, esters, or anhydrides; itaconic acid or its salts, esters, or anhydrides; vinyl silyl compounds such as vinyltrimethoxysilane; isopropyl acetate, etc. It should be noted that vinyl ester polymers can have one or more structural units derived from these other monomers.

[0056] The proportion of structural units derived from other monomers in the vinyl ester polymer is not necessarily limited as long as it does not impede the effect of the present invention. Depending on the total number of moles of all structural units constituting the vinyl ester polymer, it is sometimes preferred to be 15 mol% or less, more preferably 5 mol% or less, further preferably 1 mol% or less, and particularly preferably 0.1 mol% or less.

[0057] The degree of polymerization of PVA is not particularly limited. The degree of polymerization of PVA is preferably 1,000 or more, and more preferably 8,000 or less. From the viewpoint of improving the optical properties and resistance to damp heat of the obtained optical film, the lower limit of the degree of polymerization of PVA is more preferably 1,500 or more, and even more preferably 2,000 or more. On the other hand, from the viewpoint of improving the productivity of PVA, the upper limit of the degree of polymerization of PVA is more preferably 5,000 or less, and even more preferably 4,000 or less.

[0058] Here, the degree of polymerization refers to the average degree of polymerization measured according to JIS K 6726-1994. That is, in this invention, the degree of polymerization (Po) is determined as follows: after resaponification and purification of the residual acetic acid groups of PVA, it is measured in water at 30°C, and the intrinsic viscosity [η] (decL / g) obtained therefrom is calculated using the following formula.

[0059] Degree of polymerization Po = ([η] × 10) 4 / 8.29) (1 / 0.62)

[0060] In this invention, the lower limit of the degree of saponification of PVA is 98.7 mol%, preferably 99.0 mol%, more preferably 99.5 mol%, even more preferably 99.8 mol%, and particularly preferably 99.9 mol%. By setting the degree of saponification to the above-mentioned lower limit or above, there is a tendency to obtain an optical film with excellent optical properties and resistance to damp heat. On the other hand, the upper limit of the degree of saponification is not particularly limited, but from the viewpoint of PVA productivity, it is preferably 99.99 mol% or less.

[0061] Here, the degree of saponification of PVA refers to the proportion (mol%) of the number of moles of vinyl alcohol units relative to the total number of moles of structural units (typically vinyl ester monomer units) and vinyl alcohol units that can be converted into vinyl alcohol units through saponification. The degree of saponification of PVA can be determined according to JIS K 6726-1994.

[0062] The PVA film of the present invention may contain only one type of PVA, or it may contain two or more types of PVA with different degrees of polymerization, saponification, and modification.

[0063] There is no particular upper limit to the PVA content ratio in the PVA film. On the other hand, the lower limit of the PVA content ratio is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more.

[0064] (Plasticizer)

[0065] The PVA film of the present invention preferably contains a plasticizer. By including a plasticizer in the PVA film, the stretchability of the PVA film can be improved during the stretching process in the manufacture of the optical film. Polyols are preferred as plasticizers. Examples of polyols include ethylene glycol, glycerol, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol, and trimethylolpropane. Among these, glycerol is preferred from the viewpoint of improving stretchability. One type of plasticizer may be used alone or in combination of two or more.

[0066] The plasticizer content in the PVA film of the present invention is preferably 1 part by weight or more, more preferably 3 parts by weight or more, and even more preferably 5 parts by weight or more, relative to 100 parts by weight of PVA. On the other hand, the plasticizer content is preferably 40 parts by weight or less, more preferably 30 parts by weight or less, and even more preferably 20 parts by weight or less, relative to 100 parts by weight of PVA. If the plasticizer content is within the above range, it is possible to prevent the PVA film from becoming too soft, which would reduce its processability or prevent the plasticizer from seeping to the surface of the PVA film.

[0067] (Organosilicon surfactant)

[0068] The PVA film of the present invention preferably contains a silicon-containing compound. More preferably, it contains an organosilicon-type surfactant. By containing an organosilicon-type surfactant, the average detection intensity of the silicon fragment ions can be easily adjusted to the aforementioned range. Specific examples of silicone-based surfactants include silicone surfactants with a polyether structure at a single end of the silicone (e.g., "SN Uotsuto 125" and "SN Uotsuto 126" manufactured by Sunopco Co., Ltd.), silicone surfactants with a polyether structure at both ends of the silicone (e.g., "X-22-4952", "X-22-4272", and "X-22-6266" manufactured by Shin-Etsu Chemical Industry Co., Ltd.), and silicone surfactants with a polyether structure in the side chain of the silicone (e.g., "KF-351A", "KF-352A", "KF-353", "KF-354L", "KF-355A", "KF-615A", and "KF-94" manufactured by Shin-Etsu Chemical Industry Co., Ltd.). 5, “KF-640”, “KF-642”, “KF-643”, “KF-6020”, “KS-604”, “X-50-1039A”, “X-50-1105G”, “X-22-6191”, “X-22-4515”, “KF-6011”, “KF-6012”, “KF-6015” and “KF-6017”), and surfactants with polyether structures at both ends of organosilicon (“KF-6004”, “KF-889”, “X-22-4741”, “KF-1002”, “X-22-4952”, “X-22-4272” and “X-22-6266” manufactured by Shin-Etsu Chemical Industry Co., Ltd.), etc.

[0069] In the PVA film of the present invention, the content of the silicone surfactant relative to 100 parts by weight of PVA is preferably 0.02 parts by weight or more, more preferably 0.04 parts by weight or more, and even more preferably 0.06 parts by weight or more. On the other hand, the content of the silicone surfactant is preferably 0.14 parts by weight or less, more preferably 0.12 parts by weight or less, and even more preferably 0.10 parts by weight or less. By setting the content of the silicone surfactant within the above range, the average value of the detection intensity of silicon fragment ions can be easily adjusted to the aforementioned range.

[0070] (Other surfactants)

[0071] The PVA film of the present invention preferably contains a surfactant other than a silicone-based surfactant. By including such a surfactant, the processability of the PVA film and the peelability of the PVA film during manufacturing can be improved. There are no particular limitations on the surfactant other than a silicone-based surfactant, but anionic surfactants and nonionic surfactants are preferred.

[0072] 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 dodecylbenzene sulfonate.

[0073] As nonionic surfactants, examples 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; alkylamine surfactants such as polyoxyethylene lauryl amino ether; alkylamide surfactants such as polyoxyethylene lauryl 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.

[0074] Surfactants other than silicone-type surfactants can be used alone or in combination of two or more. As surfactants other than silicone-type surfactants, nonionic surfactants are preferred due to their excellent effect in reducing surface abnormalities during PVA film manufacturing; alkanolamide surfactants are more preferred; and dialkylolamides (e.g., diethanolamides) of aliphatic carboxylic acids (e.g., saturated or unsaturated aliphatic carboxylic acids with 8 to 30 carbon atoms) are even more preferred.

[0075] The content of surfactants other than silicone-type surfactants 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 content of surfactants other than silicone-type surfactants 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. If the content of surfactants other than silicone-type surfactants is within the above range, the peelability of the PVA film self-made film device during manufacturing becomes good, and adhesion between PVA films (hereinafter sometimes referred to as "adhesion") can be prevented. In addition, it is possible to prevent surfactants other than silicone-type surfactants from seeping onto the surface of the PVA film or from causing deterioration of the appearance of the PVA film due to surfactant aggregation.

[0076] (Other ingredients)

[0077] In addition to PVA, the PVA film of the present invention may also contain water-soluble polymers, water, antioxidants, ultraviolet absorbers, lubricants, crosslinking agents, colorants, fillers, preservatives, mildew inhibitors, and other polymeric compounds, within a range that does not impair the effects of the present invention. The total mass of the components other than PVA, surfactants, plasticizers, and PVA constitutes a proportion of 60-100% by mass of the total mass of the PVA film, more preferably 80-100% by mass, and even more preferably 90-100% by mass.

[0078] (physical properties)

[0079] The PVA film of this invention is water-insoluble. By making the PVA film water-insoluble, even at high maximum stretching speeds, uniaxial stretching during the manufacture of optical films such as polarizing films in aqueous solutions can be performed without causing the PVA film to break. Here, in this invention, water-insoluble means: according to the following steps... <1> ~ <4> When the PVA membrane is immersed in water (deionized water) at 30°C, the PVA membrane does not completely dissolve; even if only a portion dissolves, some residue remains.

[0080] <1> The PVA film is placed in a constant temperature and humidity chamber set to 20℃-65%RH for more than 16 hours to adjust the humidity.

[0081] <2> After cutting a rectangular sample with a length of 40mm and a width of 35mm from the conditioned PVA film, sandwich the sample between two 50mm × 50mm plastic plates with an opening of 35mm × 23mm in length and fix it in place, with the length of the sample parallel to the length of the window and the sample located approximately in the center of the width of the window.

[0082] <3> Add 300mL of deionized water to a 500mL beaker and stir with a magnetic stirrer with a 3cm rod at 280rpm to adjust the water temperature to 30℃.

[0083] <4> Regarding the above <2> The sample, fixed to a plastic plate, is immersed in deionized water in a beaker for 1000 seconds while being careful not to let it come into contact with the rod of the rotating magnetic stirrer.

[0084] (shape)

[0085] In the present invention, the thickness of the PVA film is preferably 10 μm or more, more preferably 15 μm or more, still more preferably 18 μm or more, and further preferably 20 μm or more. In addition, the thickness of the PVA film is preferably 40 μm or less, more preferably 38 μm or less, still more preferably 36 μm or less, particularly preferably 34 μm or less, and still further preferably 32 μm or less. By making the thickness within the above range, the occurrence of tensile fracture can be suppressed during uniaxial stretching in the production of optical films such as polarizing films. It should be noted that the "thickness" refers to the average value of the thickness measured at any five points.

[0086] In the present invention, the length of the PVA film in the TD direction is preferably 1.5 m or more, more preferably 3 m or more. In recent years, from the perspective of the increasing size of liquid crystal TVs and liquid crystal monitors, if the length of the PVA film in the TD direction is preset to 1.5 m or more, it is suitable for use as a final product. On the other hand, the length of the PVA film in the TD direction is preferably 7 m or less, more preferably 6 m or less. By setting the length in the TD direction to 7 m or less, uniaxial stretching treatment and the like can be effectively carried out when manufacturing optical films using commercially available devices.

[0087] The shape of the PVA film of the present invention is not particularly limited. From the viewpoints of being able to continuously and smoothly manufacture a more uniform PVA film and being continuously used in the production of optical films, etc., a long strip film is preferred. The length (flow direction length) of the long strip film is not particularly limited and can be appropriately set. The length of the film is preferably 3,000 m or more, more preferably 5,000 m or more. On the other hand, the length of the film is preferably 30,000 m or less. The long strip film is preferably wound around a core or the like to form a film roll.

[0088] (Use)

[0089] The PVA film of the present invention is used as a raw material film in the production of optical films. Examples of the optical film of the present invention include polarizing films, viewing angle improvement films, retardation films, brightness enhancement films, etc., and it can be suitably used for polarizing films.

[0090] <Manufacturing method of PVA film>

[0091] The manufacturing method of the PVA film of the present invention is not particularly limited, and any method such as the following can be used. Examples of such methods include: a film-forming method using a casting method, a wet film-forming method (a method of spraying into a poor solvent), a dry-wet film-forming method, a gel film-forming method (a method of temporarily cooling and gelling the film-forming solution and then extracting and removing the solvent), or a combination thereof, to form a film-forming solution obtained by adding solvent, additives, etc. to PVA and homogenizing it; a melt extrusion film-forming method, in which the obtained film-forming solution is extruded from a T-die or the like using an extruder; and a blow molding method, etc. Among these, casting and melt extrusion are preferred methods for manufacturing PVA films. Using these methods, a uniform PVA film can be obtained with good productivity. The following describes the case of manufacturing PVA films using casting or melt extrusion.

[0092] When manufacturing the PVA film of the present invention using a casting method or a melt extrusion method, firstly, a film-forming stock solution containing PVA, a solvent, and additives such as plasticizers as needed is prepared. Next, the film-forming stock solution is cast (supplyed) into a film shape on a rotating support such as a metal roller or metal strip. This forms a liquid coating of the film-forming stock solution on the support. The liquid coating is cured by heating it on the support to remove the solvent. Examples of methods for heating the liquid coating include: heating the support itself to a high temperature using a heat medium, or blowing hot air onto the side of the liquid coating opposite to the side in contact with the support. The cured strip film (PVA film) is peeled off from the support, dried as needed using a drying roller or drying oven, and then heat-treated and wound into a roll as needed.

[0093] Typically, PVA film drying is achieved by gradually evaporating volatile components from the open film surface that is not in contact with the support, drying rollers, etc. Silicone-based surfactants migrate to the surface along with the evaporating water; therefore, during the drying process, real-time temperature and stretching conditions affect the detection intensity of silicon fragment ions. This detection intensity can be adjusted by modifying the content of the silicone-based surfactant, the volatile fraction of the PVA aqueous sheet or film-forming solution, the screw speed of the extruder during PVA melt mixing, the surface temperature of the support for which the film-forming solution is to be cast, the contact time between the PVA film and the support, the temperature of the hot air blown onto the PVA film, and the temperature of the drying rollers or drying oven.

[0094] The volatile fraction (concentration of volatile components such as solvents removed by evaporation or volatilization during film formation) of the film-forming solution is preferably in the range of 50-80% by mass. If the volatile fraction is within this range, the viscosity of the film-forming solution can be adjusted to a suitable range, thus improving the film-forming properties of the liquid coating cast on the support and easily obtaining a PVA film with a uniform thickness. The film-forming solution may contain dichroic dyes as needed. Furthermore, the volatile fraction of the film-forming solution refers to the value calculated using the following formula.

[0095] The volatile fraction (mass%) of the film-forming solution = {(Wa-Wb) / Wa} × 100

[0096] In the above formula, Wa represents the mass (g) of the film-forming stock solution, and Wb represents the mass (g) of Wa(g) film-forming stock solution after drying in an electric dryer at 105°C for 16 hours.

[0097] There are no particular limitations on the method of adjusting the film-forming solution. Examples include dissolving PVA and additives such as plasticizers and surfactants in a solvent in a dissolving tank; or using a single-screw extruder or a twin-screw extruder to melt and mix aqueous PVA with additives such as plasticizers and surfactants.

[0098] The film-forming solution is typically cast into a film on a support such as a metal roller or metal strip through the die lip of a die such as a T-die. On the support, the solvent gradually evaporates from the side of the cast film that is not in contact with the support (hereinafter sometimes referred to as the free side), while it does not evaporate from the side that is substantially in contact with the support (hereinafter sometimes referred to as the contact side). Therefore, relative to the thickness direction of the film, a distribution of low solvent concentration on the free side and high solvent concentration on the contact side is produced. Consequently, the curing of PVA also begins from the free side.

[0099] PVA crystallization occurs simultaneously with the curing of PVA. PVA crystallization is difficult to achieve regardless of whether the solvent concentration is too high or too low, and it also depends on the primary structure of the PVA molecule. It is easier to crystallize when the volatile fraction of the cast film-forming solution is in the range of 20-60% by mass. Furthermore, higher temperatures result in faster PVA crystallization, but also faster solvent evaporation.

[0100] <Methods for manufacturing optical films>

[0101] The PVA film of the present invention can be used as a raw material film in the manufacture of optical films. Examples of optical films include polarizing films, field-of-view improvement films, phase difference films, and brightness enhancement films, with polarizing films being preferred. Hereinafter, as an example of a method for manufacturing an optical film, a method for manufacturing a polarizing film will be described in detail.

[0102] The polarizing film of the present invention can be manufactured from the PVA film of the present invention. By using the PVA film of the present invention, tensile breakage is less likely to occur during uniaxial stretching during the manufacture of the polarizing film, thus resulting in the manufacture of the polarizing film with a good yield. The polarizing film is generally manufactured by using a PVA film as the raw material film and undergoing processing steps such as swelling, dyeing, crosslinking, stretching, and fixing. Specific examples of the processing solutions used in each step include: a swelling treatment solution used in the swelling process, a dyeing treatment solution (dyeing solution) used in the dyeing process, a crosslinking treatment solution used in the crosslinking process, a stretching treatment solution used in the stretching process, a fixing treatment solution used in the fixing process, and a cleaning treatment solution (cleaning solution) used in the cleaning process.

[0103] The following details the various processing steps that can be used in the manufacturing method for polarizing films. It should be noted that in the manufacturing method for polarizing films, one or more of the following processes can be omitted, the same process can be performed multiple times, and other processes can be performed simultaneously.

[0104] (Cleaning treatment before swelling)

[0105] Before performing swelling treatment on the PVA film, it is preferable to clean the PVA film. This pre-swelling cleaning removes anti-adhesion agents and other contaminants adhering to the PVA film, preventing contamination of the processing solutions in the polarizing film manufacturing process by these agents. Cleaning is preferably performed by immersing the PVA film in a cleaning solution, or by blowing the cleaning solution onto the PVA film. Water, for example, can be used as the cleaning solution. The temperature of the cleaning solution is preferably 20°C or higher. Maintaining a temperature of 20°C or higher facilitates the removal of anti-adhesion agents and other contaminants adhering to the PVA film. Furthermore, the temperature of the cleaning solution is preferably 40°C or lower. Maintaining a temperature of 40°C or lower prevents partial dissolution of the PVA film surface, which could lead to film adhesion and reduced processability.

[0106] (Swelling treatment)

[0107] Swelling treatment can be performed by immersing the PVA membrane in a swelling treatment solution such as water. The temperature of the swelling treatment solution is preferably 20°C or higher, and more preferably 40°C or lower. It should be noted that the water used as the swelling treatment solution is not limited to pure water; it can be an aqueous solution containing various components such as boron compounds, or a mixture of water and an aqueous medium. The type of boron compound is not particularly limited, but from a processability point of view, boric acid or borax is preferred. When the swelling treatment solution contains a boron compound, from the viewpoint of improving the tensile strength of the PVA membrane, its concentration is preferably 6% by mass or lower.

[0108] (Staining treatment)

[0109] The dyeing process can be performed using iodine-based dyes as dichroic dyes. The dyeing stage can be any period before, during, or after the stretching treatment. Preferably, the dyeing process is performed by immersing the PVA membrane in a solution containing iodine and potassium iodide (suitably an aqueous solution). The concentration of iodine in the dyeing solution is preferably in the range of 0.005 to 0.2% by mass, and the potassium iodide / iodine (by mass) ratio is preferably in the range of 20 to 100. The temperature of the dyeing solution is preferably above 20°C and preferably below 50°C. The dyeing solution may contain boron-containing compounds such as boric acid as crosslinking agents. It should be noted that if the PVA membrane used as a raw material already contains a dichroic dye, the dyeing process can be omitted. Alternatively, the PVA membrane used as a raw material may also contain boron-containing compounds such as boric acid or borax.

[0110] (Cross-linking treatment)

[0111] In manufacturing polarizing films, for the purpose of firmly adsorbing dichroic dyes onto the PVA film, it is preferable to perform a crosslinking treatment after dyeing. The crosslinking treatment can be performed by immersing the PVA film in a solution containing a crosslinking agent (suitably an aqueous solution). One or more boron-containing compounds, such as boric acid and borax, can be used as the crosslinking agent. If the concentration of the crosslinking agent in the crosslinking treatment solution is too high, the crosslinking reaction tends to over-progress, making it difficult to achieve sufficient stretching in the subsequent stretching process; conversely, if the concentration is too low, the effectiveness of the crosslinking treatment tends to decrease. The concentration of the crosslinking agent in the crosslinking treatment solution is preferably 1% by mass or more, more preferably 1.5% by mass or more, and most preferably 6% by mass or less.

[0112] To suppress the dissolution of dichroic dyes from the dyed PVA film, the crosslinking treatment solution may contain iodine-containing compounds such as potassium iodide. If the concentration of the iodine-containing compound in the crosslinking treatment solution is too high, the heat resistance of the resulting polarized film tends to decrease, although the reason is unclear. Conversely, if the concentration of the iodine-containing compound in the crosslinking treatment solution is too low, the effect of suppressing the dissolution of dichroic dyes tends to decrease. The concentration of the iodine-containing compound in the crosslinking treatment solution is preferably 1% by mass or more, and more preferably 6% by mass or less.

[0113] If the temperature of the crosslinking treatment solution is too high, dichroic dyes may dissolve, and the resulting polarizing film may exhibit uneven dyeing. Conversely, if the temperature is too low, the effectiveness of the crosslinking treatment may be reduced. The temperature of the crosslinking treatment solution is preferably 20°C or higher, and more preferably 45°C or lower.

[0114] In addition to the stretching treatment described later, the PVA film can also be stretched separately during or between the above-mentioned treatments. This stretching (pre-stretching) prevents wrinkles from forming on the surface of the PVA film. From the viewpoint of the polarization performance of the resulting polarizing film, the total stretching ratio of the pre-stretching (the ratio obtained by multiplying the stretching ratios in each treatment) is preferably 4 times or less based on the original length of the PVA film before stretching. From the viewpoint of the polarization performance of the resulting polarizing film, the total stretching ratio of the pre-stretching is more preferably 1.5 times or more based on the original length of the PVA film before stretching. The stretching ratio in the swelling treatment is preferably 1.1 times or more based on the original length of the PVA film. The stretching ratio in the swelling treatment is preferably 3 times or less based on the original length of the PVA film. The stretching ratio in the dyeing treatment is preferably 2 times or less based on the original length of the PVA film. The stretching ratio in the dyeing treatment is further preferably 1.1 times or more based on the original length of the PVA film. The stretching ratio in the crosslinking treatment is preferably 2 times or less based on the original length of the PVA film. The stretching ratio during the crosslinking process is further preferably 1.05 times or more based on the original length of the PVA film.

[0115] (Stretching treatment)

[0116] The stretching treatment can be performed by either a wet stretching method or a dry stretching method. In the case of a wet stretching method, a solution containing a boron-containing compound such as boric acid (suitably an aqueous solution) can be used as the stretching treatment solution, and the stretching treatment can be performed in this solution, or in a dyeing treatment solution or a fixation treatment solution described later. Alternatively, in the case of a dry stretching method, a water-absorbing PVA film can be used to perform the stretching treatment in air. Of these, a wet stretching method is preferred, and uniaxial stretching in an aqueous solution containing boric acid is more preferred. When the stretching treatment solution contains a boron-containing compound, the concentration of the boron-containing compound in the stretching treatment solution is preferably 1.5% by mass or more, from the perspective of improving the stretchability of the PVA film. From the perspective of improving the stretchability of the PVA film, the concentration of the boron-containing compound in the stretching treatment solution is preferably 7% by mass or less.

[0117] The stretching treatment solution preferably contains iodine-containing compounds such as potassium iodide. If the concentration of the iodine-containing compound in the stretching treatment solution is too high, the resulting polarized film tends to have a noticeably bluish hue. Conversely, if the concentration of the iodine-containing compound in the stretching treatment solution is too low, although the reason is unclear, the heat resistance of the resulting polarized film tends to decrease. The concentration of the iodine-containing compound in the stretching treatment solution is preferably 2% by mass or more. The concentration of the iodine-containing compound in the stretching treatment solution is preferably 8% by mass or less.

[0118] If the temperature of the stretching treatment solution is too high, the PVA film tends to dissolve, soften, and easily break. Conversely, if the temperature is too low, the tensile properties tend to decrease. The temperature of the stretching treatment solution is preferably 50°C or higher, and more preferably 67.5°C or lower. It should be noted that the preferred range of stretching temperature for dry stretching is also as described above.

[0119] A higher stretching ratio during the stretching process results in a polarizing film with superior polarization properties; therefore, a ratio of 1.2 times or more is preferred, more preferably 1.5 times or more, and even more preferably 2 times or more. Furthermore, from the viewpoint of the polarization properties of the resulting polarizing film, the total stretching ratio (the ratio obtained by multiplying the stretching ratios in each process), including the aforementioned stretching ratio before stretching, is preferably 5.5 times or more, more preferably 5.7 times or more, and even more preferably 5.9 times or more, depending on the original length of the PVA film of the raw material before stretching. There is no particular upper limit to the stretching ratio; however, if it is too high, stretching breakage is likely to occur, so a ratio of 8 times or less is preferred.

[0120] There is no particular limitation on the method of stretching by uniaxial stretching; uniaxial stretching in the length direction or transverse uniaxial stretching in the width direction can be used. In the case of manufacturing polarizing films, from the viewpoint of obtaining materials with excellent polarization properties, uniaxial stretching in the length direction is preferred. Uniaxial stretching in the length direction can be performed by using a stretching device with multiple parallel rollers and varying the circumferential speed between each roller.

[0121] The maximum stretching speed (% / min) during uniaxial stretching is not particularly limited, but is preferably 200% / min or higher, more preferably 300% / min or higher, and even more preferably 400% / min or higher. Here, maximum stretching speed refers to the fastest stretching speed in a given stage when stretching the PVA film using three or more rollers with different circumferential speeds in two or more stages. It should be noted that when stretching the PVA film in one stage without dividing it into two or more stages, the stretching speed in that stage is considered the maximum stretching speed. Furthermore, stretching speed refers to the increase in the length of the PVA film per unit time, relative to its original length before stretching. For example, a stretching speed of 100% / min means the speed at which the PVA film doubles in length within one minute. A higher maximum stretching speed allows for faster PVA film stretching (uniaxial stretching), resulting in increased productivity of the polarizing film, which is therefore preferable. On the other hand, if the maximum stretching speed becomes too high, excessive local tension may occur in the PVA film during the stretching process (uniaxial stretching), which can easily lead to tensile fracture. From this perspective, the maximum stretching speed is preferably no more than 900% / min.

[0122] (Fixed treatment)

[0123] In manufacturing polarizing films, a fixation treatment is preferably performed to ensure that dichroic dyes are firmly adsorbed onto the PVA film. The fixation treatment can be performed by immersing the PVA film (suitably a stretched PVA film) in a solution containing one or more boron-containing compounds such as boric acid and borax. Additionally, the fixation solution may contain iodine-containing compounds or metal compounds, as needed. The concentration of the boron-containing compound in the fixation solution is preferably 2% by mass or more. The concentration of the boron-containing compound in the fixation solution is preferably 15% by mass or less. The temperature of the fixation solution is preferably 15°C or more. The temperature of the fixation solution is preferably 60°C or less.

[0124] (Cleaning process after dyeing)

[0125] After dyeing, it is preferable to clean the stretched PVA film. Cleaning is preferably performed by immersing the PVA film in a cleaning solution, or by blowing the PVA film with the cleaning solution. Water, for example, can be used as the cleaning solution. The water is not limited to pure water and may contain iodine-containing compounds such as potassium iodide. It should be noted that the cleaning solution may contain boron-containing compounds; in this case, the concentration of the boron-containing compound is preferably 2.0% by mass or less.

[0126] The temperature of the cleaning solution is preferably in the range of 5 to 40°C. Maintaining a temperature of 5°C or higher helps to suppress PVA film breakage caused by water freezing. Furthermore, maintaining a temperature of 40°C or lower improves the optical properties of the resulting polarizing film. The temperature of the cleaning solution is preferably 5°C or higher. Alternatively, the temperature of the cleaning solution is preferably 40°C or lower.

[0127] Specific methods for manufacturing polarizing films include dyeing, stretching, crosslinking, and / or fixing treatments on a PVA film. As a preferred example, a method may be described as sequentially performing swelling, dyeing, crosslinking, stretching (especially uniaxial stretching), and cleaning treatments on the PVA film. Furthermore, the stretching treatment can be performed through any of the preceding processing steps, or through multiple stages of two or more steps.

[0128] By drying the PVA film after the aforementioned treatments, a polarizing film can be obtained. The drying method is not particularly limited; examples include contact drying (where the film is brought into contact with heated rollers), drying in a hot air dryer, and floating drying (where the film is dried using hot air while it is floating).

[0129] <Polarizing plate>

[0130] To compensate for mechanical strength, the polarizing film of the present invention is manufactured by adhering a protective film to at least one side. The polarizing film of the present invention is generally preferably used as a polarizing plate by adhering an optically transparent and mechanically strong protective film to one or both sides. As the protective film, cellulose triacetate (TAC) film, cyclic olefin polymer (COP) film, cellulose acetate butyrate (CAB) film, acrylic film, polyester film, etc., can be used. Furthermore, as the adhesive used for adhering, PVA-based adhesives, urethane-based adhesives, etc., are examples, with PVA-based adhesives being preferred.

[0131] The polarizing plate obtained by the above operation can be laminated onto a glass substrate after being coated with acrylic adhesives, and used as a component of an LCD. It can also be used to attach phase retardation films, viewing angle improvement films, brightness enhancement films, etc.

[0132] Example

[0133] The present invention will be specifically described below through examples, but the present invention is not limited to any of the following examples.

[0134] <Positive Ion Analysis Based on TOF-SIMS>

[0135] The PVA film roll obtained in the following Examples or Comparative Examples was taken out, and a PVA film with a thickness of 30 μm, a width (length in the TD direction) of 1.65 m, and a length (length in the MD direction) of 1 m was cut out. For 5 points on a straight line parallel to the width direction (TD direction) of the PVA film and equally dividing the PVA film into six parts along the TD direction, positive ion analysis was performed using TOF-SIMS. Specifically, as Figure 3 shown, on a straight line passing through the center of the MD direction of the PVA film and parallel to the TD direction, at positions 0.275 m, 0.55 m, 0.825 m, 1.1 m, and 1.375 m from one end of the PVA film (points P1, P2, P3, P4, and P5), the PVA film was cut into 5 mm × 5 mm sizes as measurement specimens. Each of these measurement specimens was placed on the base of the TOF-SIMS measurement device using conductive double-sided tape, and positive ion analysis was performed using TOF-SIMS under the following measurement conditions.

[0136] <Measurement Conditions>

[0137] Measurement device: TOF-SIMS 5 (manufactured by ION-TOF Co., Ltd.)

[0138] Analysis software: Surface Lab 6 (manufactured by ION-TOF Co., Ltd.)

[0139] Primary ion source: Bi3 ++

[0140] Measurement current: 0.2 pA at 25 keV (10 kHz)

[0141] Measurement range: 200 μm × 200 μm

[0142] Number of measurement pixels: 128 Pix × 128 Pix

[0143] Charge neutralization condition: No neutral electron gun was used

[0144] Measurement of counting: Number of fragments supplemented by the detector (detector intensity)

[0145] <Method for Measuring Detection Intensity of Positive Silicon Fragment Ions>

[0146] For the above 5 measurement specimens, positive ion analysis was performed using TOF-SIMS respectively. The detection intensity of positive silicon fragment ions was obtained by dividing the count of detected positive silicon fragment ions by the count of all fragment ions, and the average value of these detection intensities was calculated. In addition, the difference between the maximum value and the minimum value of these detection intensities was calculated.

[0147] <Measurement of PVA Film Thickness>

[0148] The PVA film roll obtained in the following Examples or Comparative Examples was taken out, and a PVA film with a thickness of 30 μm, a width (length in the TD direction) of 1.65 m, and a length (length in the MD direction) of 1.5 m was cut out. For the points on the straight line parallel to the width direction (TD direction) of the PVA film and the points on the straight line parallel to the MD direction at five points that divide the PVA film into six equal parts along the TD direction, the coefficient of variation of the thickness of the PVA film was determined. Specifically, as Figure 4 shown, on the straight line A passing through the center in the MD direction of the PVA film and parallel to the TD direction, straight lines B (straight lines B1, B2, B3, B4, and B5) with a length of 1.2 m and parallel to the MD direction were set in such a way that they passed through the positions (points P1, P2, P3, P4, and P5) at 0.275 m, 0.55 m, 0.825 m, 1.1 m, and 1.375 m from one end of the PVA film. The thickness of the PVA film was measured at multiple points on each straight line B. At this time, the thickness of the PVA film was measured at an interval of 0.5 mm, and as the measuring device, a contact thickness gauge "Continuous Thickness Measuring Film Tester S2246" (manufactured by Fujiwa Co., Ltd.) was used. Additionally, as Figure 4 shown, the straight lines with a length of 1.2 m and parallel to the MD direction were set in such a way that their midpoints coincided with the five points that divide the PVA film into six equal parts along the width direction (TD direction). By operating in this way, the thickness of the PVA film was measured on the straight lines parallel to the MD direction and with a length of 1.2 m, and based on the average value and standard deviation of the obtained PVA film thickness, the coefficient of variation of the PVA film thickness (standard deviation / average value) was determined. And for the straight lines parallel to the MD direction and with a length of 1.2 m, the coefficient of variation of the thickness of the PVA film was determined respectively, and the average value of these coefficients of variation was calculated.

[0149] <Measurement of the swelling degree of the PVA film>

[0150] The PVA film roll obtained in the following Examples or Comparative Examples was taken out, and a PVA film with a thickness of 30 μm, a width (length in the TD direction) of 1.65 m, and a length (length in the MD direction) of 1 m was cut out. A test piece of about 1.5 g was cut out from this PVA film and immersed in 1000 g of distilled water at 30°C. After 30 minutes of immersion, the test piece was taken out, the water on the surface was blotted with filter paper, and its mass (We) was measured. Then, the test piece was put into a hot air dryer and dried at 105°C for 16 hours, and its mass (Wf) was measured. Based on the obtained masses We and Wf, the swelling degree of the PVA film was calculated using the following formula.

[0151] Swelling degree (%) = (We / Wf) × 100

[0152] <Evaluation of the tensile fracture property of the PVA film>

[0153] A PVA film is drawn from a roll of PVA film obtained from the following Examples or Comparative Examples, and the tensile fracture property of the PVA film is evaluated according to the following procedure. First, while immersing the PVA film in water (swelling treatment liquid) at a temperature of 30°C for 1 minute, the PVA film is uniaxially stretched along the length direction to 1.6 times its original length. Next, while immersing the PVA film in an iodine / potassium iodide aqueous solution (staining treatment liquid) at a temperature of 30°C (iodine is 0.053% by mass, potassium iodide is 5.3% by mass) for 1 minute, the PVA film is uniaxially stretched (second-stage stretching) along the length direction to 2.7 times its original length. Then, while immersing the PVA film in a boric acid / potassium iodide aqueous solution (crosslinking treatment liquid) at a temperature of 30°C (boric acid is 3% by mass, potassium iodide is 3% by mass) for 2 minutes, the PVA film is uniaxially stretched (third-stage stretching) along the length direction to 3 times its original length. Next, while immersing the PVA film in a boric acid / potassium iodide aqueous solution (crosslinking treatment liquid) at a temperature of 62°C (boric acid is 4.5% by mass, potassium iodide is 6% by mass), the PVA film is uniaxially stretched (fourth-stage stretching) along the length direction to 6 times its original length. And, after immersing the PVA film in a boric acid / potassium iodide aqueous solution (cleaning treatment liquid) at a temperature of 30°C (boric acid is 1.5% by mass, potassium iodide is 3% by mass) for 5 seconds, it is dried with a dryer at 60°C for 240 seconds, thereby continuously manufacturing a polarizing film with a thickness of 13 μm. Here, the uniaxial stretching is controlled by separately adjusting the rotation speeds of a set of rollers with a driving function (the material is NBR rubber). In this test, when the number of fractures of the PVA film is 0 times within 60 minutes, it is evaluated as A, when it is 1 time, it is evaluated as B, when there are 2 times, it is evaluated as C, and when there are 3 or more times, it is evaluated as D.

[0154] <Example 1>

[0155] <Manufacture and Evaluation of PVA Film>

[0156] 100 parts by weight of PVA (degree of polymerization 2400, degree of saponification 99.9 mol%) flakes were immersed in 2500 parts by weight of distilled water at 70°C for 24 hours, followed by centrifugation to dehydrate, yielding PVA aqueous flakes with a volatile fraction of 70% by weight. 10 parts by weight of glycerol as a plasticizer and 0.08 parts by weight of "SN-wet126" (manufactured by Sanopco), a surfactant with polyether structures at both ends of polydimethylsiloxane, were mixed with 333 parts by weight of this PVA aqueous flakes (100 parts by weight of dried PVA) as an organosilicon surfactant. The resulting mixture was then heated and melted using a vented twin-screw extruder (maximum temperature 130°C, screw rotation speed 20 rpm) to prepare a film-forming stock solution. The film-forming solution was cooled to 100°C using a heat exchanger and then extruded through a 180cm wide coat hanger die onto a support with a surface temperature of 90°C. A hot air drying device was then used to blow hot air at 114°C onto the side of the PVA film not in contact with the support to dry it. The contact time between the PVA film and the support was set to 100 seconds to allow the PVA film to peel off from the support. Next, by cutting off the ends of the film that had thickened due to necking during film formation, PVA films with a thickness of 30μm, a width (TD direction length) of 1.65m, and a swelling degree of 200% were continuously manufactured. A 4000m length (MD direction length) of this PVA film was wound onto a cylindrical core to form a PVA film roll. Using the obtained PVA film roll, the detection intensity of silicon fragment ions was determined according to the above method, and the average value and the difference between the maximum and minimum values ​​of these detection intensities were calculated. In addition, using the obtained PVA film rolls, the average value of the coefficient of variation of the PVA film thickness was calculated according to the above method to further evaluate the tensile fracture resistance. The results are shown in Table 1.

[0157] <Examples 2-4, Comparative Examples 1-4>

[0158] The manufacturing conditions of the PVA film were changed as shown in Table 1, but otherwise the manufacturing and evaluation of the PVA film were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0159] [Table 1]

[0160]

[0161] As shown in Table 1, when the polarizing film was manufactured using the PVA films of Examples 1-4, the film fracture occurred 0-2 times every 60 minutes. Therefore, it can be said that the PVA films of Examples 1-4 exhibited suppressed fracture during stretching (uniaxial stretching). On the other hand, when the polarizing film was manufactured using the PVA films of Comparative Examples 1-4, the film fracture occurred more than 3 times every 60 minutes. It should be noted that Table 1 lists the average value of the detection intensity of silicon fragment ions, the difference between the maximum and minimum values, and the average value of the coefficient of variation of the thickness for the side of the PVA film in contact with the support. However, the same evaluation results are also presented for the side of the PVA film not in contact with the support.

Claims

1. Polyvinyl alcohol film, wherein, In at least one side of the polyvinyl alcohol film, the average detection intensity of positive silicon fragment ions, obtained by positive ion analysis based on time-of-flight secondary ion mass spectrometry, is 0.001 to 0.

01. The polyvinyl alcohol film is a raw material film for optical film manufacturing. The average detection intensity of silicon fragment ions refers to the average detection intensity of silicon fragment ions obtained by positive ion analysis using a time-of-flight secondary ion mass spectrometer at five points located on any straight line parallel to the TD direction of the polyvinyl alcohol film and dividing the polyvinyl alcohol film into six equal parts along the TD direction. The method for determining the detection intensity of silicon fragment ions is as follows: For the polyvinyl alcohol film test sample, TOF-SIMS was used for positive ion analysis. The detection intensity of the positive silicon fragment ions was obtained by dividing the count of the detected positive silicon fragment ions by the total count of all fragment ions, and the average value of these detection intensities was calculated.

2. The polyvinyl alcohol film according to claim 1, wherein, The difference between the maximum and minimum detection intensities of silicon fragment ions at five points where the polyvinyl alcohol film is divided into six equal parts along the TD direction is 0.0005~0.

002.

3. The polyvinyl alcohol film according to claim 1 or 2, wherein, The average coefficient of variation for the thickness of polyvinyl alcohol films ranged from 0.01 to 0.

03. The average value of the coefficient of variation of the thickness refers to the average value of the coefficient of variation of the thickness of the polyvinyl alcohol film along a straight line of 1.2m length parallel to the MD direction of the polyvinyl alcohol film, which is divided into six equal parts by five points along the TD direction.

4. The polyvinyl alcohol film according to claim 1 or 2, wherein the swelling degree when immersed in water at 30°C for 30 minutes is 180~240%.

5. The polyvinyl alcohol film according to claim 1 or 2, wherein, The length in the TD direction is 1.5m or more.

6. The polyvinyl alcohol film according to claim 1 or 2, wherein, The length of the polyvinyl alcohol film in the MD direction is more than 3,000 m.

7. The polyvinyl alcohol film according to claim 1 or 2, wherein the thickness is 10~40 μm.

8. A polarizing film, which is made from the polyvinyl alcohol film according to any one of claims 1 to 7.

9. A polarizing plate obtained by attaching a protective film to at least one side of the polarizing film of claim 8.

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