Polarizing plate

Abstract

To provide a polarizing plate capable of suppressing appearance defects due to intermittent cracks.SOLUTION: A polarizing plate 100 includes a first protective film, a polyvinyl alcohol-based polarizer 20, and a second protective film in this order. One surface side 20a of the polyvinyl alcohol-based polarizing film 20 is provided with a corrugated structure side 15mm having a crest side 20M and a trough side 20V respectively extending in the MD direction over a length equal to or more than the side 20MV and alternately arranged in the TD direction, and a plurality of cleavage side 20V extending in the trough side 10mm of the corrugated structure along the MD direction over a length equal to or less than the side 20CR. The arithmetic average height Sa of the top side 20a and the difference Δ Z between the height of the bottom side 20CRB of each notch side 20a and the height of the higher peak side 20CR of the two peak side 20CRB adjacent to each notch side 20CR satisfy the following formula: 20MP 20CR 20M. Δ Z / Sa ≤ 5.5 SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a polarizing plate. [Background technology]

[0002] BACKGROUND ART Polarizing plates including polyvinyl alcohol-based polarizers (hereinafter also referred to as "polarizers") have been known. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6166431 Summary of the Invention [Problem to be solved by the invention]

[0004] However, polarizing plates containing polyvinyl alcohol-based polarizers have a problem in that minute cracks formed intermittently in the polarizers are visually recognized as a defect in appearance.

[0005] The present invention has been made in view of the above-mentioned problems, and has an object to provide a polarizing plate that can suppress poor appearance due to intermittent cracks. [Means for solving the problem]

[0006] [1] A polarizing plate comprising a first protective film, a polyvinyl alcohol-based polarizer, and a second protective film in this order, One surface of the polyvinyl alcohol-based polarizer is a wave-shaped structure having peaks and valleys extending along a first direction over a length of 15 mm or more and alternately arranged in a second direction perpendicular to the first direction; a plurality of fissures extending in the valleys of the corrugated structure over a length of 10 mm or less along the first direction; the arithmetic mean height Sa of the surface, and A polarizing plate in which, in a surface shape profile of a cross section along a second direction perpendicular to the first direction and along the thickness direction of the polyvinyl alcohol-based polarizer, and including the bottoms of the cracks on the surface of the polyvinyl alcohol-based polarizer, the difference ΔZ between the height of the bottom of each crack and the height of the higher of the peaks of the two mountain portions adjacent to the crack satisfies the following formula:

[0007] ΔZ / Sa≦5.5

[0008] [2] The polarizing plate according to [1], which satisfies 2.0≦ΔZ / Sa.

[0009] [3] The polarizing plate according to [1] or [2], wherein the polyvinyl alcohol polarizer has a thickness of 5 to 30 μm.

[0010] [4] The polarizing plate according to any one of [1] to [3], wherein Sa is 0.02 to 0.10 μm. [Effects of the Invention]

[0011] Defective appearance due to intermittent cracks is suppressed. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic cross-sectional view showing the layer structure of a polarizing plate. [Figure 2] FIG. 2 is an enlarged perspective schematic diagram of a polyvinyl alcohol-based polarizer. [Figure 3] FIG. 3 is a schematic diagram of a surface shape profile of a cross section along the TD direction (second direction) perpendicular to the MD direction (first direction) in FIG. 2 and along the thickness direction, the cross section including the bottom of a crack in the surface of a polyvinyl alcohol-based polarizer. [Figure 4] 1A is a schematic top view of an expander roll used in a method for producing a polarizing plate according to one embodiment of the present invention; FIG. 1B is a schematic cross-sectional view of the expander roll; and FIG. 1C is a schematic view showing the action of the force applied to a film by the expander roll. DETAILED DESCRIPTION OF THE INVENTION

[0013] (polarizing plate) A polarizing plate according to an embodiment will be described with reference to the drawings.

[0014] As shown in FIG. 1, a polarizing plate 100 according to this embodiment includes a first protective film 10, a polyvinyl alcohol-based polarizer 20, and a second protective film 30 in this order.

[0015] (First protective film 10 and second protective film 30) There are no particular limitations on the first protective film 10 and the second protective film 30 as long as they are transparent optical films. For example, the protective films may be films of polyolefin-based resins such as polyethylene, polypropylene, and cyclic polyolefin; cellulose acetate-based resins such as triacetyl cellulose and diacetyl cellulose; polyester-based resins such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate; polycarbonate-based resins; (meth)acrylic resins; and polypropylene-based resins.

[0016] From the viewpoint of thinning, the thickness of the protective film is usually 100 μm or less, preferably 80 μm or less, and more preferably 65 μm or less, and is usually 15 μm or more, and preferably 20 μm or more. The protective film may have a retardation.

[0017] The first protective film and the second protective film may be the same or different from each other.

[0018] (Polyvinyl alcohol polarizer 20) (Basic properties of polyvinyl alcohol polarizers) A polyvinyl alcohol polarizer is a uniaxially stretched polyvinyl alcohol resin film to which a dichroic pigment (iodine or a dichroic dye) is adsorbed and aligned, and which is crosslinked with a boron compound such as boric acid or borax.

[0019] The boron content in the polarizer may be 5.0% by mass or less, 4.2% by mass or less, and preferably 4.0% by mass or less, and may be 0.5% by mass or more, 1.0% by mass or more, or 1.5% by mass or more, and preferably 2.0% by mass or more.

[0020] The boron content (mass %) of the polarizer can be calculated from the amount of sodium hydroxide solution (1 mol / L) added dropwise to a measurement sample solution prepared by dissolving 0.2 g of the polarizer in 200 g of a 1.9 wt % mannitol aqueous solution until the measurement sample solution reaches the neutralization point.

[0021] Examples of polyvinyl alcohol resins constituting the polyvinyl alcohol resin film include saponified polyvinyl acetate resins, such as polyvinyl acetate, which is a homopolymer of vinyl acetate, and copolymers of vinyl acetate with other monomers copolymerizable therewith.

[0022] Examples of other monomers copolymerizable with vinyl acetate include unsaturated carboxylic acids, olefins, vinyl ethers, unsaturated sulfonic acids, and (meth)acrylamides having an ammonium group.

[0023] The degree of saponification of the polyvinyl alcohol resin is usually about 85 mol % or more, preferably about 90 mol % or more, and more preferably about 99 mol % or more.

[0024] The polyvinyl alcohol resin may be modified, and for example, polyvinyl formal, polyvinyl acetal, polyvinyl butyral, etc. modified with aldehydes may also be used.

[0025] The average degree of polymerization of the polyvinyl alcohol resin is preferably from 100 to 10,000, more preferably from 1,500 to 8,000, and even more preferably from 2,000 to 5,000. The average degree of polymerization of the polyvinyl alcohol resin can be determined in accordance with JIS K 6726 (1994). When the average degree of polymerization is within the above range, the polarizing performance and film processability tend to be excellent.

[0026] The thickness of the polarizer is usually 65 μm or less, preferably 50 μm or less, more preferably 35 μm or less, even more preferably 30 μm or less, and particularly preferably 20 μm or less. The thickness of the polarizer is usually 2 μm or more, preferably 5 μm or more, and more preferably 10 μm or more. The thickness of the polarizer can be controlled, for example, by selecting a polyvinyl alcohol-based resin film, adjusting the stretching ratio, etc.

[0027] (Surface structure of polyvinyl alcohol polarizer) Figure 2 shows an enlarged perspective view of the polyvinyl alcohol polarizer 20 in the polarizing plate 100, and Figure 3 shows a schematic diagram of the surface shape profile of a cross section along the TD direction (second direction) perpendicular to the MD direction (first direction) in Figure 2 and along the thickness direction, which includes the bottom of a crack on the surface of the polyvinyl alcohol polarizer.

[0028] One surface 20a of the polyvinyl alcohol-based polarizer 20 has a corrugated structure 20MV. The corrugated structure 20MV extends along the MD direction (first direction) over a length of 15 mm or more and has peaks 20M and valleys 20V alternately arranged in the TD direction (second direction) perpendicular to the MD direction (first direction). The MD direction (first direction) and the TD direction (second direction) are both in-plane directions of the polarizer (directions perpendicular to the thickness direction) and are perpendicular to each other.

[0029] In this specification, the MD direction refers to the machine flow direction of the polarizer, which is the same as the stretching direction of the polarizer and the same as the absorption axis direction of the polarizer.

[0030] The other surface 20b of the polyvinyl alcohol-based polarizer 20 similarly has a corrugated structure 20MV having peaks 20M and valleys 20V, and cracks 20CR (not shown). As shown in Fig. 2, the peaks 20M of one surface 20a face the valleys 20V of the other surface 20b, and the valleys 20V of one surface 20a face the peaks 20M of the other surface 20b, so that the entire polyvinyl alcohol-based polarizer 20 has a corrugated shape.

[0031] The corrugated structure 20MV typically extends over a length in the MD of 20 mm or more, and may extend over a length of 50 mm or more.

[0032] The wave period of the wave structure 20MV, that is, the interval T between the crests 20P of the peaks 20M in the TD direction, is not limited, but is typically 500 μm or more and 1500 μm or less.

[0033] The arithmetic mean height Sa of the surface 20a is not particularly limited, and may be 0.01 μm or more, 0.02 μm or more, 0.03 μm or more, 0.04 μm or more, or 0.05 μm or more. From the viewpoint of reducing the visibility of the corrugated structure 20MV itself, the arithmetic mean height Sa of the surface 20a may be 0.50 μm or less, 0.40 μm or less, 0.30 μm or less, 0.20 μm or less, 0.10 μm or less, or 0.08 μm or less.

[0034] The arithmetic mean height Sa of the surface 20a is defined by the following formula: Sa=(1 / A)∫∫ A |Z(x, y)|dxdy

[0035] where x is the coordinate in the MD direction, y is the coordinate in the TD direction, Z(x, y) is the height of the surface 20a at the coordinates x and y relative to the average plane of the surface 20a in the integration range, and A indicates the area to be integrated. Sa is defined for the three-dimensional surface shape.

[0036] Z(x,y) can be optically acquired using, for example, a microscope equipped with a white light interferometer. Z(x,y) can be acquired in accordance with ISO 25178. The integral range of Z(x,y) is 2.8 mm or more in the MD direction and 3.8 mm or more in the TD direction. The upper limit of the size in the MD direction may be 5.5 mm, and the upper limit of the size in the TD direction may be 10.7 mm.

[0037] A plurality of fissures 20CR extending over a length of 10 mm or less along the MD direction (first direction) are formed in the valleys 20V of the corrugated structure 20MV.

[0038] 3 shows a conceptual diagram of a surface shape profile of a cross section of the surface 20a of the polyvinyl alcohol-based polarizer 20 along the TD direction (second direction) and the thickness direction, the cross section including the bottom 20CRB of the crevice 20CR on the surface 20a of the polyvinyl alcohol-based polarizer 20. The bottom of the crevice 20CR is the point of the lowest height in the crevice 20CR.

[0039] In this embodiment, the difference ΔZ between the above-mentioned arithmetic mean height Sa and the height of the bottom 20CRB of each crevice 20CR and the height of the higher of the peaks 20MP of the two ridges 20M adjacent to the crevice 20CR in the surface shape profile of a cross section along the TD direction (second direction) and the thickness direction and including the bottom 20CRB of the crevice 20CR on the surface 20a of the polyvinyl alcohol-based polarizer 20 satisfies the following formula: Hereinafter, ΔZ may be referred to as the height difference related to intermittent crevice on the surface of the polarizer. ΔZ / Sa≦5.5

[0040] The clefts that are the subject of the calculation of ΔZ are each cleft that is recognized in the integral range for calculating the above Sa.

[0041] ΔZ / Sa may be 5.3 or less, preferably 5.0 or less, more preferably 4.5 or less, and even more preferably 4.0 or less.

[0042] ΔZ / Sa may be 2.0 or more, 3.0 or more, 3.5 or more, or 4.0 or more.

[0043] ΔZ may be 1.0 μm or less, 0.7 μm or less, or 0.5 μm or less.

[0044] (Method for producing polyvinyl alcohol polarizer) A polyvinyl alcohol-based polarizer can be produced by a step of bringing a raw material polyvinyl alcohol-based resin film (raw film) into contact with a treatment liquid (hereinafter also referred to as a "treatment step"). The method of bringing the film into contact with the treatment liquid may be a method of immersing the film in a treatment liquid (hereinafter also referred to as a "treatment bath") contained in a treatment tank, or a method of treating the film by applying the treatment liquid to the film surface by spraying, pouring, dropping, or the like. When the treatment step is performed by a method of immersing the film in a treatment bath, the number of treatment baths used in one treatment step is not limited to one, and one treatment step may be completed by sequentially immersing the film in two or more treatment baths.

[0045] Examples of the treatment liquid include a swelling liquid, a dyeing liquid, a crosslinking liquid, and a cleaning liquid. Examples of the treatment process include a swelling process in which a swelling liquid is brought into contact with the raw film to perform a swelling treatment, a dyeing process in which a dyeing liquid is brought into contact with the film after the swelling treatment to perform a dyeing treatment, a crosslinking process in which a crosslinking liquid is brought into contact with the film after the dyeing treatment to perform a crosslinking treatment, and a cleaning process in which a cleaning liquid is brought into contact with the film after the crosslinking treatment to perform a cleaning treatment. Between these series of treatment processes (i.e., before, after, and / or during any one or more treatment processes), a wet or dry uniaxial stretching treatment is performed. Other treatment processes may be added as necessary.

[0046] The swelling step is carried out for the purposes of removing foreign matter from the surface of the raw film, removing plasticizers in the raw film, imparting ease of dyeing, plasticizing the raw film, etc. As the swelling liquid, a medium containing water as the main component, such as water, distilled water, or pure water, is usually used.

[0047] The crosslinking process is a treatment performed for purposes such as water resistance and color adjustment through crosslinking. The crosslinking liquid can be a solution in which a crosslinking agent is dissolved in a solvent. Examples of crosslinking agents include boron compounds such as boric acid and borax. These may be used alone or in combination. The solvent can be, for example, water, but may also contain a water-compatible organic solvent. The concentration of the crosslinking agent in the crosslinking liquid, the temperature of the crosslinking bath, the immersion time of the film, and the number of crosslinking baths in which the film is immersed are not particularly limited. By appropriately selecting these, a polarized film with a boron content of 5.0% by mass or less can be obtained. When the dichroic dye used in the dyeing process is iodine, the crosslinking liquid preferably contains an iodide in addition to boric acid. The amount of iodide can be, for example, 1 to 30 parts by weight per 100 parts by weight of water. Examples of iodides include potassium iodide and zinc iodide. In addition, compounds other than iodides, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, and sodium sulfate, may also be present. The crosslinking treatment may be carried out multiple times, usually 2 to 5 times. In this case, the composition and temperature of each crosslinking bath used may be the same or different.

[0048] The cleaning step is carried out for the purpose of removing excess chemicals such as boric acid and iodine adhering to the polyvinyl alcohol-based resin film. The cleaning step is carried out, for example, by immersing the crosslinked polyvinyl alcohol-based resin film in a cleaning bath containing a cleaning solution. Note that the cleaning step can also be carried out by spraying the cleaning solution onto the film as a shower instead of immersing the film in the cleaning bath, or by combining immersion in the cleaning bath with spraying the cleaning solution. The boron content of the final polarizer can be adjusted by adjusting the cleaning treatment conditions (e.g., the time of contact with the cleaning solution and the temperature of the cleaning solution).

[0049] As described above, the raw film is subjected to a wet or dry uniaxial stretching treatment during the series of treatment steps (i.e., before, after, and / or during any one or more treatment steps). The uniaxial stretching step can be performed multiple times until a polarizer is obtained from the raw film.

[0050] (glue) An adhesive layer (not shown) may be provided between the polyvinyl alcohol polarizer 20 and the first protective film 10 and / or between the polyvinyl alcohol polarizer 20 and the second protective film 30 to bond them together.

[0051] The adhesive composition forming the adhesive layer may be a water-based adhesive or an active energy ray-curable adhesive. The adhesive compositions forming the adhesive layer on the first protective film 10 side and the adhesive layer on the second protective film 30 side may be the same or different.

[0052] Examples of aqueous adhesives include conventionally known adhesive compositions that use a polyvinyl alcohol resin or a urethane resin as a main component. Active energy ray-curable adhesives are adhesives that are cured by irradiation with active energy rays such as ultraviolet light, visible light, electron beams, and X-rays. When an active energy ray-curable adhesive is used, the adhesive layer is a cured product layer of the adhesive.

[0053] The active energy ray-curable adhesive may be an adhesive containing an epoxy compound that cures by cationic polymerization as a curable component, and is preferably an ultraviolet-curable adhesive containing such an epoxy compound as a curable component. The epoxy compound refers to a compound having an average of one or more, preferably two or more, epoxy groups in the molecule. Only one type of epoxy compound may be used, or two or more types may be used in combination.

[0054] Examples of epoxy compounds include hydrogenated epoxy compounds (glycidyl ethers of polyols having alicyclic rings) obtained by reacting epichlorohydrin with an alicyclic polyol obtained by hydrogenating the aromatic rings of an aromatic polyol; aliphatic epoxy compounds such as polyglycidyl ethers of aliphatic polyhydric alcohols or their alkylene oxide adducts; and alicyclic epoxy compounds, which are epoxy compounds having one or more epoxy groups bonded to an alicyclic ring in the molecule.

[0055] The active energy ray-curable adhesive may contain a radically polymerizable (meth)acrylic compound as a curable component, instead of or in addition to the epoxy compound. Examples of the (meth)acrylic compound include (meth)acryloyloxy group-containing compounds such as (meth)acrylate monomers having one or more (meth)acryloyloxy groups in the molecule, and (meth)acrylate oligomers obtained by reacting two or more functional group-containing compounds and having at least two (meth)acryloyloxy groups in the molecule.

[0056] When the active energy ray-curable adhesive contains an epoxy compound that cures by cationic polymerization as a curable component, it preferably contains a photocationic polymerization initiator. Examples of the photocationic polymerization initiator include aromatic diazonium salts, onium salts such as aromatic iodonium salts and aromatic sulfonium salts, and iron-allene complexes.

[0057] When the active energy ray-curable adhesive contains a radically polymerizable component such as a (meth)acrylic compound, it preferably contains a photoradical polymerization initiator. Examples of the photoradical polymerization initiator include acetophenone-based initiators, benzophenone-based initiators, benzoin ether-based initiators, thioxanthone-based initiators, xanthone, fluorenone, camphorquinone, benzaldehyde, and anthraquinone.

[0058] (Mechanism of action) The polarizing plate according to this embodiment has a good appearance when viewed visually. The reasons for this are considered to be as follows.

[0059] When ΔZ / Sa is smaller than a predetermined value, it means that the depth of the valley fissures 20CR is equal to or smaller than a predetermined ratio of the arithmetic mean height Sa, which is the degree of amplitude of the corrugated structure.

[0060] This is thought to cause intermittent cracks to be hidden by diffuse reflection from the surface of the corrugated structure with a certain amplitude or more, making them difficult to see from the outside.

[0061] On the other hand, the corrugated structure itself is unlikely to deteriorate the appearance, probably because it is a periodic structure.

[0062] (Polarizer manufacturing method) First, a polyvinyl alcohol-based film is stretched in the MD direction while immersed in a swelling liquid, a dyeing liquid, a crosslinking liquid, or the like by a known method, and then dried to produce a polyvinyl alcohol-based polarizer. When the polarizer is dried, cracks are formed on the surface of the polarizer along the MD direction.

[0063] An adhesive is applied to both sides of the obtained polarizer, and protective films are attached to both sides to obtain a laminate in which the adhesive is not dried or cured.

[0064] Next, this laminate is transported by a transport roll system and passed through a drying oven or a light irradiation device, where it is heated or irradiated with light, thereby drying any undried adhesive or curing any uncured adhesive.

[0065] Here, an expander roll is inserted in the transport roll system immediately before drying and light irradiation, and the laminate is brought into contact with the expander roll in the opposite direction to the normal direction, thereby causing the undried laminate to shrink in the TD direction. This allows the polarizer to shrink in the TD direction to form a corrugated shape while being bonded to the protective film, thereby enabling the arithmetic mean height Sa of the surface of the polyvinyl alcohol-based polarizer to be made higher than usual.

[0066] The arithmetic mean height Sa can be controlled by the amount of curvature of the expander roll, the contact angle with the expander roll, the angle between the bend direction of the expander roll and the contact start point with the film, and the like.

[0067] An example of the arrangement of the expander roll 12 is shown in FIG. 4. As shown in FIG. 4(a), the expander roll has multiple spools, each with a built-in ball bearing, arranged on a curved shaft 120. An expander roll in which the spools are covered with a rubber tube is called a rubber expander roll, and an expander roll in which the spools are not covered with a rubber tube is called a metal expander roll. The expander roll 12 may be either a rubber expander roll or a metal expander roll. Typically, the vertical cross section of the expander roll 12 with respect to the shaft 120 is circular, and the area is the same in all vertical cross sections. The diameter of the vertical cross section of the expander roll 12 with respect to the shaft 120 is, for example, 50 mm to 400 mm, and may be approximately 100 mm.

[0068] A cross section perpendicular to the axis 120 at the center of the expander roll 12 (cross section aa in FIG. 4(a)) is shown as cross section 122 in FIG. 4(b), and its center is C'. A cross section perpendicular to the axis 120 at the portion where the expander roll 12 contacts the end of the undried or uncured laminate 100' is shown as cross section 121 in FIG. 4(b), and its center is C. When the center C' of the cross section 122 of the central portion is positioned upstream in the conveying direction from the center C of the cross section 121 of the end, that is, when the expander roll is positioned so that the center in the width direction of the undried or uncured laminate 100' contacts the expander roll 12 before the end, a contraction force can be applied from the outside to the inside in the width direction to the undried or uncured laminate 100'.

[0069] 4(c) shows the action of the force applied to the film by the expander roll. When the expander roll 12 is positioned so that the widthwise center (indicated by region A2) of the undried or uncured laminate 100' comes into contact with the expander roll 12 before the widthwise edges, and then the widthwise edges come into contact with the expander roll 12, an inward contraction force (indicated by arrow b2) is applied to the film, which can promote the generation of wrinkles.

[0070] In FIG. 4(b), if the arrow x indicates the direction of incidence of the end of the undried or uncured laminate 100' entering the expander roll 12, and the arrow y indicates the direction from center C toward center C' in the expander roll 12, then from the perspective of appropriately shrinking the undried or uncured laminate 100', the angle θ (clockwise is positive) of arrow x with respect to arrow y is preferably in the range of 200° to 360°, and may be 225° to 300°. The arc height of the expander roll 12 is, for example, 1 mm to 20 mm, and may be approximately 10 mm. As used herein, the "arc height of the expander roll" refers to the distance between the center C and the center C', and the "arrangement angle of the expander roll" refers to the angle θ of arrow y with respect to arrow x.

[0071] The magnitude of the external force applied to the film can also be adjusted by the contact angle α of the undried or uncured laminate 100′ on the expander roll 12. The contact angle α can be adjusted by raising or lowering the position of the expander roll 12 or the rolls before and after it. Since this makes it easy to adjust the magnitude of the stretching force applied by the expander roll 12, the contact angle α is preferably 0° to 30°, and may be about 10°. [Example]

[0072] The present invention will be described in more detail below with reference to examples. In the examples, "%" and "parts" are by mass % and mass parts unless otherwise specified.

[0073] <Measurement and evaluation methods> (1) Thickness of polarizer The thickness of the polarizers obtained in each of the Examples and Comparative Examples was measured using a digital micrometer MH-15M manufactured by Nikon Corporation.

[0074] (2) The height difference ΔZ for intermittent cracks and the arithmetic mean height Sa of the surface Using a white light interferometer "VertScan R5500" manufactured by Ryoka Systems Co., Ltd., the surface shape of the polyvinyl alcohol polarizer in the polarizing plate was measured three-dimensionally, and the height difference ΔZ and arithmetic mean height Sa were calculated from the results.

[0075] (3) Appearance evaluation of polarizing plates The polarizing plate was attached to a blackboard and the reflected light of a fluorescent lamp was visually observed to evaluate the degree of visibility of intermittent cracks. A polarizing plate with almost no visible intermittent cracks was marked with ⊚, a polarizing plate with very slight visible intermittent cracks was marked with ◯, and a polarizing plate with some visible intermittent cracks and widespread visible intermittent cracks was marked with ×. The appearance of the polarizing plate was visually observed using reflected light from a fluorescent lamp to evaluate the degree of visibility of the periodic streaks: ⊚: barely visible periodic streaks, ◯: slightly visible periodic streaks, and ×: slightly visible periodic streaks and widely visible periodic streaks.

[0076] The surface of the polyvinyl alcohol-based polarizer included in the polarizing plates manufactured in Examples and Comparative Examples had a wave-like structure extending over a length of 15 mm or more along the first direction (absorption axis direction) and having peaks and valleys alternately arranged in the second direction (transmission axis direction).The surface of the polyvinyl alcohol-based polarizer included in the polarizing plates manufactured in Examples and Comparative Examples had a plurality of cracks extending over a length of 10 mm or less along the first direction in the valleys of the wave-like structure.

[0077] <Preparation of water-based adhesive> The polyvinyl alcohol adhesive was prepared by dissolving 2 parts by weight of acetoacetyl-modified polyvinyl alcohol (trade name "GOHSEFFIMER (registered trademark) Z-200" manufactured by Nippon Synthetic Chemical Industry Co., Ltd.) and 2 parts by weight of sodium glyoxylate (trade name "SPM-01" manufactured by Nippon Synthetic Chemical Industry Co., Ltd.) in 100 parts by weight of water.

[0078] <Preparing the protective film> Protective film A: a saponified triacetyl cellulose (TAC) film with a hard coat layer (Toppan Printing Co., Ltd.'s "25KCHCN-TC", thickness 32 μm). Protective film B: Triacetyl cellulose (TAC) film [FUJIFILM Corporation's "TD40", thickness 40 μm] saponified

[0079] Example 1 (Preparation of polyvinyl alcohol-based polarizer A) A long polyvinyl alcohol film (average polymerization degree: approximately 2400, saponification degree: 99.9 mol% or more, thickness: 30 μm) was continuously transported and immersed in a swelling bath of pure water at 20°C for a residence time of 80 seconds (swelling process).

[0080] Next, the film was pulled out of the swelling bath and immersed in a dye bath containing iodine and having a potassium iodide / water ratio of 2 / 100 (mass ratio) at 30° C. for a residence time of 60 seconds (dyeing step).

[0081] Next, the film taken out of the dye bath was immersed in a first crosslinking bath containing potassium iodide / boric acid / water in a mass ratio of 12 / 3.0 / 100 at 56° C. for a residence time of 40 seconds (first crosslinking step).

[0082] Subsequently, the substrate was immersed in a second crosslinking bath containing potassium iodide / boric acid / water in a mass ratio of 8.9 / 2.49 / 100 at 40° C. for a residence time of 6 seconds (second crosslinking step). In the dyeing step, the first crosslinking step, and the second crosslinking step, the film was stretched between rolls in a bath to a longitudinal uniaxial stretching ratio of 6 times the original film.

[0083] Next, the film was pulled out from the crosslinking bath and immersed in a washing bath of pure water at 5°C for a residence time of 1 second (washing step).

[0084] The film was placed in a drying oven at 78°C for a residence time of 100 seconds and dried to obtain a polarizer A having a thickness of 12 µm (drying step).

[0085] (Preparation of Polarizing Plate A) Protective film A was laminated to one side of the polyvinyl alcohol-based polarizer A thus prepared, and protective film B was laminated to the other side via an aqueous adhesive, with the triacetyl cellulose surface facing the polarizer A side, to prepare a laminate A of protective film A / aqueous adhesive / polarizer A / aqueous adhesive / protective film B.

[0086] The laminate A thus prepared was continuously conveyed through an expander roll (diameter: 100 mm, arc height: 5 mm) to apply a shrinkage force in the TD direction, and then passed through a hot air dryer at 80°C for 300 seconds to dry the aqueous adhesive, yielding a polarizing plate A. The contact angle α of the polarizing plate on the expander roll was 37.5°, and the expander roll was positioned immediately after the entrance of the hot air drying oven. The angle θ of the expander roll was 230°. The height difference ΔZ of the intermittent cracks on the polarizer surface of the obtained polarizing plate was 0.164 μm, the arithmetic mean height Sa of the polarizer was 0.036 μm, and the value of the height difference ΔZ / arithmetic mean height Sa of the intermittent cracks on the polarizer surface was 4.6, indicating that the visibility of the intermittent cracks was not a problem.

[0087] <Example 2> (Preparation of Polarizing Plate B) Polarizing plate B was produced in the same manner as in Example 1, except that the arrangement angle θ of the expander roll was changed to 240°. The polarizing plate thus obtained had a height difference ΔZ related to intermittent cracks on the surface of the polarizer of 0.271 μm, an arithmetic mean height Sa of the polarizer of 0.051 μm, and a value of height difference ΔZ / arithmetic mean height Sa related to intermittent cracks on the surface of the polarizer of 5.3, which meant that the visibility of the cracks was at an acceptable level.

[0088] Example 3 (Preparation of Polarizing Plate C) Polarizing plate C was produced in the same manner as in Example 1, except that the arrangement angle θ of the expander roll was changed to 255°. The polarizing plate thus obtained had a height difference ΔZ related to intermittent cracks on the surface of the polarizer of 0.362 μm, an arithmetic mean height Sa of the polarizer of 0.100 μm, and a value of height difference ΔZ / arithmetic mean height Sa related to the intermittent cracks on the surface of the polarizer of 3.6, which meant that the visibility of the cracks was at an acceptable level.

[0089] <Comparative Example 1> (Preparation of Polarizing Plate D) Polarizing plate D was produced in the same manner as in Example 1, except that an expander roll was not used. The height difference ΔZ of the intermittent cracks on the surface of the polarizer of the polarizer thus obtained was 0.176 μm, the arithmetic mean height Sa of the polarizer was 0.027 μm, and the value of the height difference ΔZ / arithmetic mean height Sa of the intermittent cracks on the surface of the polarizer was 6.5, which was a problematic level of high crack visibility.

[0090] [Table 1] [Explanation of symbols]

[0091] 10...first protective film, 20...polyvinyl alcohol polarizer, 30...second protective film, 20a, 20b...surface, 20CR...crack, 20CRB...bottom, 20M...ridge, 20MV...wave structure, 20MP...peak, 20V...valley, 100...polarizing plate.

Claims

1. A polarizing plate comprising a first protective film, a polyvinyl alcohol-based polarizer, and a second protective film in this order, One surface of the polyvinyl alcohol-based polarizer is a wave-shaped structure having peaks and valleys extending along a first direction over a length of 15 mm or more and alternately arranged in a second direction perpendicular to the first direction; a plurality of fissures extending in the valleys of the corrugated structure over a length of 10 mm or less along the first direction; the arithmetic mean height Sa of the surface, and A polarizing plate in which, in a surface shape profile of a cross section along a second direction perpendicular to the first direction and along the thickness direction of the polyvinyl alcohol-based polarizer, and including the bottoms of the cracks on the surface of the polyvinyl alcohol-based polarizer, the difference ΔZ between the height of the bottom of each crack and the height of the higher of the peaks of the two mountain portions adjacent to each crack satisfies the following formula: ΔZ / Sa≦5.5

2. The polarizing plate according to claim 1 , wherein ΔZ / Sa satisfies 2.0≦ΔZ / Sa.

3. 3. The polarizing plate according to claim 1, wherein the polyvinyl alcohol-based polarizer has a thickness of 5 to 30 μm.

4. 3. The polarizing plate according to claim 1, wherein the Sa is 0.02 to 0.10 μm.

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