Method for manufacturing an optical film

Abstract

To provide an optical film which contains polyethylene terephthalate and suppresses rainbow spots, and to provide a method of manufacturing the same.SOLUTION: An optical film provided herein is made of a resin containing polyethylene terephthalate, has an NZ coefficient of 1.10 or less and an in-plane retardation of 2000 nm or greater, and contains 1 wt.% or more of an organic solvent.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an optical film and a method for manufacturing the same.

Background Art

[0002] Conventionally, film manufacturing techniques using resins have been proposed (Patent Documents 1 to 4).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0004] Films containing polyethylene terephthalate have been widely used in various fields conventionally, and in recent years, for example, studies have been underway for use as optical films such as protective films for polarizing plates. Films containing polyethylene terephthalate are likely to exhibit color unevenness (rainbow spots) like a rainbow due to the influence of birefringence, and for example, when placed on the viewing side of a display device, it becomes a factor in image quality degradation, and thus improvement is demanded.

[0005] The present invention has been made in view of the above circumstances, and an object thereof is to provide an optical film containing polyethylene terephthalate and having suppressed rainbow spots, and a manufacturing method capable of easily manufacturing the above optical film.

Means for Solving the Problems

[0006] The present inventors have intensively studied to solve the above problems. As a result, it has been found that by bringing an organic solvent into contact with a resin film containing polyethylene terephthalate, the birefringence in the in-plane direction can be changed, and by changing the birefringence in the in-plane direction, the NZ coefficient and the in-plane retardation can be adjusted, and an optical film with suppressed iridescence can be obtained by a simple manufacturing method, thereby completing the present invention. The present invention includes the following contents.

[0007] [1] An optical film formed of a resin containing polyethylene terephthalate, having an NZ coefficient of 1.10 or less, an in-plane retardation of 2000 nm or more, and containing 1% by weight or more of an organic solvent. [2] The optical film according to [1], wherein the organic solvent is one or more selected from the group consisting of chloroform, methylene chloride, and toluene. [3] The optical film according to [1] or [2], which is used as a protective film for a polarizing plate. [4] A method for manufacturing an optical film, comprising: step (a) of preparing a resin film formed of a resin containing polyethylene terephthalate; and step (b) of bringing the resin film into contact with an organic solvent to change the birefringence in the in-plane direction. [5] The method for manufacturing an optical film according to [4], wherein the resin film is a stretched film. [6] When the in-plane retardation of the stretched film prepared in step (a) is Re1, and the in-plane retardation of the stretched film in which the birefringence in the in-plane direction is changed in step (b) is Re2, the increase rate of the in-plane retardation represented by Re2 / Re1 is 1.5 times or more. The method for manufacturing an optical film according to [5]. [7] The method for manufacturing an optical film according to any one of [4] to [6], wherein the organic solvent is one or more selected from the group consisting of chloroform, methylene chloride, and toluene. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide an optical film containing polyethylene terephthalate and having suppressed iridescence, and a manufacturing method capable of easily manufacturing the above-described optical film.

Embodiments for Carrying Out the Invention

[0009] Hereinafter, the present invention will be described in detail with reference to embodiments and examples. However, the present invention is not limited to the embodiments and examples shown below, and can be arbitrarily modified and implemented without departing from the scope of the claims of the present invention and its equivalent scope.

[0010] In the following description, unless otherwise specified, the in-plane retardation Re of the film is a value represented by Re = (nx - ny) × d. Further, the birefringence in the in-plane direction of the film is a value represented by (nx - ny) unless otherwise specified, and thus is represented by Re / d. Furthermore, the retardation Rth in the thickness direction of the film is a value represented by Rth = [((nx + ny) / 2) - nz] × d unless otherwise specified. Also, the birefringence in the thickness direction of the film is a value represented by [((nx + ny) / 2) - nz] unless otherwise specified, and thus is represented by Rth / d. Furthermore, the NZ coefficient of the film is a value represented by (nx - nz) / (nx - ny) unless otherwise specified. Here, nx represents the refractive index in the direction perpendicular to the thickness direction (in-plane direction) of the film and giving the maximum refractive index. ny represents the refractive index in the in-plane direction of the film and perpendicular to the direction of nx. nz represents the refractive index in the thickness direction of the film. d represents the thickness of the film. Unless otherwise specified, the measurement wavelength is 590 nm.

[0011] In the following description, the shape of "long" means a shape having a length of 5 times or more with respect to the width, preferably having a length of 10 times or more, and specifically means the shape of a film having a length such that it can be wound up and stored or transported in a roll shape. There is no particular limitation on the upper limit of the length, but it is usually 100,000 times or less with respect to the width.

[0012] In the following description, unless otherwise specified, the directions of elements may include errors within a range that does not impair the effects of the present invention, for example, within a range of ±5°, even if the directions of the elements are "parallel", "perpendicular", and "orthogonal".

[0013] Unless otherwise specified, the "polarizing plate" includes not only a rigid member but also a flexible member such as a resin film.

[0014] [1. Outline of the optical film according to the first embodiment] The optical film according to the first embodiment of the present invention is formed of a resin containing polyethylene terephthalate, has an NZ coefficient of 1.10 or less, an in-plane retardation of 2000 nm or more, and contains 1% by weight or more of an organic solvent.

[0015] According to the first embodiment, since the NZ coefficient and the in-plane retardation are within a predetermined range, it is possible to obtain an optical film in which iridescence is suppressed. Therefore, when the optical film is disposed on the viewing side of the display device, it is possible to suppress the observation of iridescence, and thus it is possible to suppress a deterioration in the image quality of the display device.

[0016] Further, according to the first embodiment, by changing the birefringence in the in-plane direction by a simple method such as bringing an organic solvent into contact with the resin film, the above-described NZ coefficient and in-plane retardation can be imparted, and it is possible to obtain an optical film with high productivity and suppressed manufacturing cost. The optical film of the first embodiment obtained by such a manufacturing method usually contains an organic solvent.

[0017] [2. Resin containing polyethylene terephthalate] The optical film according to the first embodiment is formed of a resin containing polyethylene terephthalate (PET). The polyethylene terephthalate contained in the optical film may or may not have crystallinity. That the polyethylene terephthalate has crystallinity means that it has a melting point Tm, and specifically means that the melting point can be observed with a differential scanning calorimeter (DSC).

[0018] Polyethylene terephthalate can usually observe a melting point at around 260°C with a differential scanning calorimeter (DSC). Also, polyethylene terephthalate can usually observe a glass transition temperature at around 75°C with a differential scanning calorimeter (DSC). The melting point and glass transition point of polyethylene terephthalate (PET) can be measured, for example, by the following method. PET is melted by heating, and the melted PET is quenched with dry ice. Subsequently, using this PET as a specimen, the glass transition temperature Tg and melting point Tm of PET can be measured at a heating rate of 10°C / min (heating mode) using a differential scanning calorimeter (DSC).

[0019] The proportion of polyethylene terephthalate in the resin constituting the optical film is preferably 50% by weight or more, more preferably 70% by weight or more, and particularly preferably 90% by weight or more. When the proportion of polyethylene terephthalate is at or above the lower limit of the above range, the expression of birefringence of the optical film can be enhanced. The upper limit of the proportion of polyethylene terephthalate can be 100% by weight.

[0020] The resin constituting the optical film may contain optional components in addition to polyethylene terephthalate. Examples of the optional components include antioxidants such as phenolic antioxidants, phosphorus-based antioxidants, and sulfur-based antioxidants; light stabilizers such as hindered amine light stabilizers; waxes such as petroleum waxes, Fischer-Tropsch waxes, and polyalkylene waxes; nucleating agents such as sorbitol-based compounds, metal salts of organic phosphoric acids, metal salts of organic carboxylic acids, kaolin, and talc; fluorescent brighteners such as diaminostilbene derivatives, coumarin derivatives, azole-based derivatives (e.g., benzoxazole derivatives, benzotriazole derivatives, benzimidazole derivatives, and benzothiazole derivatives), carbazole derivatives, pyridine derivatives, naphthalic acid derivatives, and imidazolone derivatives; ultraviolet absorbers such as benzophenone-based ultraviolet absorbers, salicylic acid-based ultraviolet absorbers, and benzotriazole-based ultraviolet absorbers; inorganic fillers such as talc, silica, calcium carbonate, and glass fibers; colorants; flame retardants; flame retardant aids; antistatic agents; plasticizers; near-infrared absorbers; lubricants; fillers; and any polymers other than polyethylene terephthalate such as soft polymers; and the like. The optional components may be used alone or in combination of two or more in any ratio. The amount of the optional components can be appropriately determined within a range that does not significantly impair the effects of the present invention. The amount of the optional components can be, for example, within a range that can maintain the total light transmittance of the optical film at 85% or more.

[0021] [3. NZ Coefficient and Retardation of Optical Film] The optical film according to the first embodiment has an NZ coefficient of 1.10 or less and an in-plane retardation of 2000 nm or more.

[0022] The NZ coefficient of the optical film according to the first embodiment is usually 1.10 or less, preferably less than 1.10, and more preferably 1.05 or less. The lower limit value of the NZ coefficient of the optical film is arbitrary and can be, for example, 1.0 or more.

[0023] The NZ coefficient of the optical film can be obtained by calculation from the in-plane retardation Re and the retardation Rth in the thickness direction of the film.

[0024] The in-plane retardation Re of the optical film according to the first embodiment is usually 2000 nm or more, preferably 2500 nm or more, more preferably 3000 nm or more, preferably 4000 nm or less, and more preferably 3600 nm or less. In an optical film containing polyethylene terephthalate, when the in-plane retardation Re is within the above-described range, particularly, iridescence tends to occur easily. By adjusting the NZ coefficient to 1.10 or less, iridescence can be effectively suppressed.

[0025] The retardation Rth in the thickness direction of the optical film according to the first embodiment is, for example, 1000 nm or more, preferably 1200 nm or more, more preferably 1500 nm or more, preferably 2400 nm or less, and more preferably 2200 nm or less.

[0026] The retardation of the film can be measured by a retardation meter ("AxoScan OPMF-1" manufactured by AXOMETRICS).

[0027] [4. Organic Solvent Contained in the Optical Film] The optical film according to the first embodiment contains an organic solvent. This organic solvent is usually incorporated into the film in step (b) of the method for manufacturing the optical film described later.

[0028] Specifically, all or part of the organic solvent incorporated into the film in step (b) can enter the interior of polyethylene terephthalate. Therefore, even if drying is performed, it is difficult to completely remove the organic solvent easily. Thus, the optical film according to the first embodiment contains an organic solvent.

[0029] As the organic solvent described above, one that does not dissolve polyethylene terephthalate can be used. Preferred organic solvents include, for example, chloroform, methylene chloride, and toluene. The type of organic solvent may be one type or two or more types.

[0030] The solvent content ratio of the organic solvent contained in the optical film with respect to 100% by weight of the optical film according to the first embodiment is usually 1% by weight or more, and can be 3% by weight or more. Further, the solvent content ratio can be, for example, 20% by weight or less, and can be 16% by weight or less.

[0031] The solvent content of the optical film can be measured by the measurement method described in the examples.

[0032] [5. Other characteristics of the optical film] The optical film according to the first embodiment preferably has an in-plane birefringence Re / d within an appropriate range according to its use. For example, the specific in-plane birefringence Re / d of the optical film is preferably 60.0×10 -3 or more, more preferably 65.0×10 -3 or more, preferably 80.0×10 -3 or less, more preferably 75.0×10 -3 or less.

[0033] The optical film according to the first embodiment preferably has a thickness-direction birefringence Rth / d within an appropriate range according to its use. For example, the specific thickness-direction birefringence Rth / d of the optical film is preferably 35.0×10 -3 or more, more preferably 40.0×10 -3 or more, preferably 50.0×10 -3 or less, more preferably 45.0×10 -3 or less.

[0034] The optical film preferably has high transparency. The specific total light transmittance of the optical film is preferably 80% or more, more preferably 85% or more, and particularly preferably 88% or more. The total light transmittance of the film can be measured in the wavelength range of 400 nm to 700 nm using an ultraviolet-visible spectrometer.

[0035] The optical film preferably has a low haze. The haze of the optical film is preferably less than 1.0%, more preferably less than 0.8%, particularly preferably less than 0.5%, and ideally 0.0%. Such an optical film with a low haze can enhance the sharpness of the image displayed on the display device when provided in the display device. The haze of the film can be measured using a haze meter (for example, "NDH5000" manufactured by Nippon Denshoku Industries Co., Ltd.).

[0036] The optical film may be a film that has not been subjected to a stretching process, but is preferably an optically stretched film that has been subjected to a stretching process. When the optical film is an optically stretched film, the optically stretched film is preferably a uniaxially stretched film. A uniaxially stretched film refers to a film in which the stretching process is actively performed only in one direction, and no active stretching process is performed in other directions. Since a uniaxially stretched film can be manufactured by stretching only in one direction, the manufacturing process can be simplified, and thus simple manufacturing can be realized.

[0037] The optical film may be a sheet film or a long film having a long shape.

[0038] The thickness of the optical film can be appropriately set according to the use of the optical film. The specific thickness of the optical film is preferably 5 μm or more, more preferably 10 μm or more, particularly preferably 30 μm or more, preferably 400 μm or less, more preferably 300 μm or less, and particularly preferably 200 μm or less.

[0039] [6. Manufacturing Method of the Optical Film According to the Second Embodiment] The method for manufacturing an optical film according to the second embodiment of the present invention includes: a step (a) of preparing a resin film formed of a resin containing polyethylene terephthalate; and a step (b) of bringing the resin film into contact with an organic solvent to change the birefringence in the in-plane direction.

[0040] According to the second embodiment, by bringing the resin film into contact with the organic solvent in step (b), the birefringence in the in-plane direction can be changed, so that the NZ coefficient and the in-plane retardation can be adjusted. Therefore, an optical film with suppressed iridescence can be manufactured by a simple method of bringing the organic solvent into contact with the resin film.

[0041] The inventor speculates that the mechanism by which the optical film is obtained by the above manufacturing method is as follows. However, the technical scope of the present invention is not limited by the following mechanism.

[0042] When an organic solvent is brought into contact with a resin film formed of a resin containing polyethylene terephthalate, the organic solvent penetrates into the resin film. Due to the action of the penetrated solvent, Brownian motion occurs in the molecules of polyethylene terephthalate in the film, and the molecules are oriented. According to the study of the inventor, it is considered that the solvent-induced crystallization phenomenon of polyethylene terephthalate may progress during the orientation of the molecular chain.

[0043] [7. Step (a): Preparation of resin film] Step (a) is a step of preparing a resin film formed of a resin containing polyethylene terephthalate. In the following description, the resin film before contact with the organic solvent in step (b) may be appropriately referred to as the "original film" for explanation.

[0044] The original film prepared in step (a) is formed of polyethylene terephthalate. The resin constituting the original film can be the same as the content described in the item of [2. Resin containing polyethylene terephthalate] described above.

[0045] The original film may be a resin film having optical isotropy or a resin film having optical anisotropy, but the latter is preferred. Further, as the resin film having optical anisotropy, a stretched film is preferably used. This is because in step (b) described later, it is possible to easily change the birefringence in the in-plane direction. The present inventor speculates as follows about the reason. However, the technical scope of the present invention is not limited by the following mechanism.

[0046] The molecules of polyethylene terephthalate contained in a stretched film formed of a resin containing polyethylene terephthalate are oriented to an extent according to the stretching conditions. Usually, stretching is performed in the in-plane direction perpendicular to the thickness direction of the film, so most of the molecules of polyethylene terephthalate can be oriented in the in-plane direction of the stretched film. When an organic solvent penetrates into the stretched film containing the thus-oriented molecules of polyethylene terephthalate, the orientation of the molecules in the film further progresses due to the microscopic Brownian motion of the molecules by the action of the penetrated solvent, and the degree of orientation increases. When the degree of molecular orientation increases in this way, the birefringence of the film changes, and thus the retardation may also change. Usually, the change in retardation proceeds such that the in-plane retardation Re increases.

[0047] When a stretched film is used as the original film, the in-plane birefringence Re / d of the stretched film is 50.0×10 -3 or more and 60.0×10 -3 or less, and the birefringence Rth / d in the thickness direction can be 40.0×10 -3 or more and 50.0×10 -3 or less. Further, the in-plane retardation Re of the stretched film can be, for example, 2000 nm or more and 2500 nm or less. Also, the retardation Rth in the thickness direction of the stretched film can be, for example, 1500 nm or more and 2000 nm or less. The NZ coefficient of the resin film can be, for example, 1.20 or more and 1.5 or less.

[0048] On the other hand, when the original anti-film has optical isotropy, the in-plane birefringence Re / d is usually less than 1.0×10 -3 and the absolute value of the birefringence in the thickness direction |Rth / d| can usually be less than 1.0×10 -3 .

[0049] The original anti-film preferably has a low content of organic solvent, and more preferably does not contain an organic solvent. The solvent content ratio, as the ratio of the organic solvent contained in the original anti-film to 100% by weight of the original anti-film, is preferably 1% by weight or less, more preferably 0.5% by weight or less, particularly preferably 0.1% by weight or less, and ideally 0% by weight.

[0050] The solvent content of the original anti-film can be measured by density.

[0051] The thickness of the original anti-film is preferably set according to the thickness of the optical film to be manufactured. Also, the original anti-film may be a single-sheet film, but is preferably a long film. By using a long film, continuous production of the optical film by the roll-to-roll method is possible, so that the productivity of the optical film can be effectively increased.

[0052] As a method for manufacturing the original anti-film, resin molding methods such as injection molding, extrusion molding, press molding, inflation molding, blow molding, calendar molding, casting molding, and compression molding are preferable because an original anti-film that does not contain an organic solvent can be obtained. Among these, the extrusion molding method is preferable because it is easy to control the thickness.

[0053] The manufacturing conditions in the extrusion molding method are preferably as follows. The cylinder temperature (molten resin temperature) is preferably at least Tm, more preferably at least "Tm + 20°C", preferably at most "Tm + 100°C", and more preferably at most "Tm + 50°C". Also, the cooling body that the molten resin extruded in film form first contacts is not particularly limited, but usually a casting roll is used. The temperature of this casting roll is preferably at least "Tg - 50°C", preferably at most "Tg + 70°C", and more preferably at most "Tg + 40°C". Further, the cooling roll temperature is preferably at least "Tg - 70°C", more preferably at least "Tg - 50°C", preferably at most "Tg + 60°C", and more preferably at most "Tg + 30°C". When manufacturing a raw film under such conditions, a raw film with a thickness of 1 μm to 1 mm can be easily manufactured. Here, "Tm" represents the melting point of polyethylene terephthalate, and "Tg" represents the glass transition temperature of polyethylene terephthalate.

[0054] When the raw film is a stretched film, the method for manufacturing the raw film may usually include preparing a pre-stretched film, performing a pre-treatment if necessary, and then stretching the pre-stretched film. Usually, by stretching, the molecules of polyethylene terephthalate contained in the pre-stretched film can be oriented in a direction corresponding to the stretching direction to obtain a stretched film.

[0055] The pre-stretched film can be obtained, for example, by the resin molding method described above. In this embodiment, after preparing the pre-stretched film, a step of performing a pre-treatment for heating the pre-stretched film to the stretching temperature may be included before stretching the pre-stretched film. Usually, the pre-heating temperature and the stretching temperature are the same, but they may be different. The pre-heating temperature is preferably at least T1 - 10°C, more preferably at least T1 - 5°C, and preferably at most T1 + 5°C, more preferably at most T1 + 2°C, with respect to the stretching temperature T1. The pre-heating time is arbitrary, preferably at least 1 second, more preferably at least 5 seconds, and preferably at most 60 seconds, more preferably at most 30 seconds.

[0056] There is no limitation in the stretching direction, and examples include the longitudinal direction, the width direction, the diagonal direction, etc. Here, the diagonal direction refers to a direction perpendicular to the thickness direction and not parallel or perpendicular to the width direction. Also, the stretching direction may be one direction or two or more directions. Therefore, as stretching methods, for example, a uniaxial stretching method such as a method of uniaxially stretching the pre-stretched film in the longitudinal direction (longitudinal uniaxial stretching method), a method of uniaxially stretching the pre-stretched film in the width direction (transverse uniaxial stretching method), etc.; a biaxial stretching method such as a simultaneous biaxial stretching method of stretching the pre-stretched film in the longitudinal direction and simultaneously in the width direction, a sequential biaxial stretching method of stretching the pre-stretched film in one of the longitudinal direction and the width direction and then stretching in the other direction; a method of stretching the pre-stretched film in the diagonal direction (diagonal stretching method); and so on.

[0057] The stretching ratio is preferably 1.1 times or more, more preferably 1.2 times or more, preferably 20.0 times or less, more preferably 10.0 times or less, still more preferably 5.0 times or less, and particularly preferably 2.0 times or less. It is desirable to appropriately set the specific stretching ratio according to factors such as the optical properties, thickness, and strength of the optical film to be manufactured. When the stretching ratio is equal to or higher than the lower limit value of the above range, birefringence can be greatly changed by stretching. Also, when the stretching ratio is equal to or lower than the upper limit value of the above range, the direction of the slow axis can be easily controlled, and film breakage can be effectively suppressed.

[0058] The stretching temperature is preferably "Tg + 5°C" or higher, more preferably "Tg + 10°C" or higher, preferably "Tg + 100°C" or lower, and more preferably "Tg + 90°C" or lower. Here, "Tg" represents the glass transition temperature of polyethylene terephthalate. When the stretching temperature is equal to or higher than the lower limit value of the above range, the resin containing polyethylene terephthalate can be sufficiently softened to perform stretching uniformly. Also, when the stretching temperature is equal to or lower than the upper limit value of the above range, the haze of the obtained stretched film can be reduced to enhance transparency.

[0059] In step (a), the raw film may be prepared by manufacturing the raw film by the above manufacturing method by oneself, or may be prepared by purchasing the raw film from a third party.

[0060] [8. Step (b): Contact between the resin film and the organic solvent] Step (b) is a step (b) in which the resin film as the raw film prepared in step (a) is brought into contact with an organic solvent to change the birefringence in the in-plane direction. As the organic solvent, a solvent that can penetrate into the resin film without dissolving the polyethylene terephthalate contained in the resin film can be used. Examples of the organic solvent include chloroform, methylene chloride, and toluene. The type of the organic solvent may be one type or two or more types.

[0061] The method of bringing the resin film into contact with the organic solvent is arbitrary. Examples of the contact method include a spray method of spraying the organic solvent on the resin film; a coating method of coating the organic solvent on the resin film; an immersion method of immersing the resin film in the organic solvent; and the like. Among them, the immersion method is preferable because continuous contact can be easily performed.

[0062] The temperature of the organic solvent brought into contact with the resin film is arbitrary within the range in which the organic solvent can maintain a liquid state, and thus can be set within the range from the melting point to the boiling point of the organic solvent.

[0063] The time for bringing the resin film into contact with the organic solvent is not particularly specified and can be appropriately adjusted according to the type of the organic solvent. The contact time is, for example, 5.0 seconds or more, preferably 10 seconds or more, and also for example 24 hours or less, preferably 1 hour or less, more preferably 120 seconds or less, particularly preferably 80 seconds or less, and still more preferably 60 seconds or less. When the contact time is equal to or greater than the lower limit value of the above range, the adjustment of the NZ coefficient by contact with the organic solvent can be effectively performed. On the other hand, even if the immersion time is lengthened, the adjustment amount of the NZ coefficient tends not to change significantly. Therefore, when the contact time is equal to or less than the upper limit value of the above range, the productivity can be increased without impairing the quality of the optical film.

[0064] In step (b), by being brought into contact with an organic solvent, the in-plane birefringence Re / d of the resin film changes. Thereby, the NZ coefficient and the in-plane retardation Re are adjusted. For example, an in-plane retardation Re of 2000 nm or more with an NZ coefficient of 1.10 or less can be obtained.

[0065] In the second embodiment, since it is preferable to use a stretched film as the raw film, in step (b), it is preferable to adjust so that the birefringence of the resin film after solvent contact with respect to the birefringence of the raw film which is a stretched film falls within a predetermined range. When the in-plane retardation of the stretched film as the raw film is represented as Re1 and its thickness is represented as d1, and the in-plane retardation of the stretched film whose in-plane birefringence is changed in step (b) is represented as Re2 and its thickness is represented as d2, the amount of change in the in-plane birefringence Re / d is defined as the amount represented by the absolute value of the change in the in-plane birefringence Re / d (|(Re2 / d2) - (Re1 / d1)|), which is the amount of change in the in-plane birefringence Re / d of the stretched film. The amount of change in the in-plane birefringence Re / d of the stretched film in step (b) is preferably 10.0×10 -3 or more, more preferably 15.0×10 -3 or more, and preferably 25.0×10 -3 or less, more preferably 20.0×10 -3 or less.

[0066] The thickness-direction birefringence Rth / d of the resin film may or may not change due to contact with the organic solvent. From the viewpoint of simplifying the control of the retardation Rth in the thickness direction of the optical film, it is preferable that the change in the thickness-direction birefringence Rth / d generated in the resin film due to contact with the organic solvent is small, and it is more preferable that no change occurs.

[0067] The retardation in the thickness direction of the stretched film as the original film is represented by Rth1, its thickness is represented by d1, the retardation in the thickness direction of the stretched film with the in-plane birefringence changed in step (b) is represented by Rth2, and its thickness is represented by d2. When the change amount of the birefringence Rth / d in the thickness direction is represented by the absolute value of the change in the birefringence Rth / d in the thickness direction (|(Rth2 / d2) - (Rth1 / d1)|), this amount is taken as the change amount of the in-plane birefringence Re / d of the stretched film. The change amount of the in-plane birefringence Re / d of the stretched film in step (b) is preferably 0.00×10 -3 or more and 5.00×10 -3 or less.

[0068] In step (b), when brought into contact with an organic solvent, the in-plane retardation of the resin film usually increases. When the in-plane retardation of the stretched film as the original film is represented by Re1 and the in-plane retardation of the stretched film with the in-plane birefringence changed is represented by Re2, the amount represented by Re2 / Re1 is taken as the increase rate of the in-plane retardation. The increase rate Re2 / Re1 of the in-plane retardation in step (b) is preferably 1.50 times or more, more preferably 1.53 times or more, preferably 2.0 times or less, and more preferably 1.8 times or less. In step (b), usually, since the orientation of the polyethylene terephthalate molecules in the in-plane direction is promoted and the in-plane retardation increases, the increase rate of the in-plane retardation represented by Re2 / Re1 can also be regarded as the orientation promotion rate of the polyethylene terephthalate molecules.

[0069] In step (b), when brought into contact with an organic solvent, the retardation Rth in the thickness direction of the resin film may or may not change. When the retardation in the thickness direction of the stretched film as the original film is represented by Rth1 and the retardation in the thickness direction of the stretched film with the in-plane birefringence changed in step (b) is represented by Rth2, the change rate of the retardation in the thickness direction represented by Rth2 / Rth1 is preferably 0.9 times or more and 1.2 times or less.

[0070] In the manufacturing method according to the second embodiment, an arbitrary process may be further performed on the resin film after the process (b).

[0071] [9. Arbitrary process] According to the manufacturing method described above, a long optical film can be manufactured using a long resin film. The manufacturing method of the optical film may include a step of winding the long optical film thus manufactured in a roll shape. Further, the manufacturing method of the optical film may include a step of cutting out the long optical film into a desired shape.

[0072] [10. Manufactured optical film] According to the manufacturing method of the optical film according to the second embodiment of the present invention, since the in-plane birefringence can be adjusted by a simple step of bringing the raw film into contact with an organic solvent, an optical film having a desired NZ coefficient and in-plane retardation can be easily manufactured. Therefore, for example, an optical film according to the first embodiment, such as having an NZ coefficient of 1.10 or less, a retardation Re in the thickness direction of 2000 nm or more, and containing 1% by weight or more of an organic solvent, can be easily manufactured.

[0073] Specifically, the NZ coefficient of the optical film manufactured by the manufacturing method according to the second embodiment may be the same as the NZ coefficient of the optical film according to the first embodiment. Further, the optical film manufactured by the manufacturing method according to the second embodiment may be the same as the optical film according to the first embodiment also in characteristics other than the NZ coefficient. Therefore, the optical film manufactured by the manufacturing method according to the second embodiment has characteristics such as the resin contained in the optical film; the haze of the optical film; the amount of the organic solvent contained in the optical film; the retardations Re and Rth of the optical film; the birefringences Re / d and Rth / d of the optical film; the total light transmittance of the optical film; the thickness of the optical film; etc., may be the same as those of the optical film according to the first embodiment.

[0074] [11. Use of optical film] There are no restrictions on the uses of the optical film according to the above-described first embodiment and the optical film manufactured by the manufacturing method according to the second embodiment. These optical films can be used alone or in combination with other members for a wide range of applications in the optical field. Examples of the uses of the optical film include a base film for forming an arbitrary layer on the base film; a protective film for a polarizing plate; and the like. Among these, it is preferable that the optical film be a protective film for a polarizing plate. This is because by using the optical film as a protective film for a polarizing plate disposed on the viewing side of a display device, it is possible to suppress a decrease in the image quality of the display device due to iridescence.

Examples

[0075] Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to the examples shown below, and can be arbitrarily modified and implemented without departing from the scope of the claims of the present invention and its equivalent scope. In the following description, “%” and “parts” representing amounts are based on weight unless otherwise specified. Also, the operations described below were performed under conditions of normal temperature and pressure (23°C and 1 atm) in the atmosphere unless otherwise specified.

[0076] [Evaluation Method] (Measurement Method for Film Retardation and NZ Coefficient) The in-plane retardation Re, the retardation Rth in the thickness direction, the NZ coefficient, and the slow axis direction of the film were measured using a phase difference meter (“AxoScan OPMF-1” manufactured by AXOMETRICS). The measurement wavelength was 590 nm.

[0077] (Measurement Method for Film Thickness) The thickness of the film was measured using a contact thickness gauge (Code No. 543-390 manufactured by MITUTOYO).

[0078] (Measurement Method for Solvent Content of Optical Film) An amorphous orientation film (a stretched film before solvent immersion, hereinafter referred to as "original film") used for manufacturing an optical film as a sample was weighed by thermogravimetric analysis (TGA: in a nitrogen atmosphere, heating rate of 10 °C / min, 30 °C to 300 °C). The weight loss amount ΔWO of the original film at 300 °C was obtained by subtracting the weight WO(300 °C) of the original film at 300 °C from the weight WO(30 °C) of the original film at 30 °C. Since the original films used in the examples and comparative examples described later were manufactured by the melt extrusion method, they did not contain a solvent. Therefore, the weight loss amount ΔWO of this original film was adopted as a reference in formula (X) described later.

[0079] Also, for the optical film as a sample, its weight was measured by the same thermogravimetric analysis (TGA: in a nitrogen atmosphere, heating rate of 10 °C / min, 30 °C to 300 °C). The weight loss amount ΔWR of the optical film at 300 °C was obtained by subtracting the weight WR(300 °C) of the optical film at 300 °C from the weight WR(30 °C) of the optical film at 30 °C.

[0080] From the weight loss amount ΔWO of the original film at 300 °C and the weight loss amount ΔWR of the optical film at 300 °C described above, the solvent content of the optical film was calculated by the following formula (X). Solvent content (%) = {(ΔWR - ΔWO) / WR(30 °C)} × 100 (X)

[0081] (Iris spots) A polarizing film was placed on the backlight, and a surface light source emitting linearly polarized light was prepared. An optical film to be evaluated was placed so that the vibration direction of the linearly polarized light was aligned with the slow axis of the optical film. The room was darkened to allow only the light of the backlight, and the optical film was observed from all angles. Those in which no iris spots were observed were rated A, those in which a slight iris spot was observed were rated B, and those in which a strong iris spot was observed were rated C.

[0082] [Production Example 1: Production of an unstretched original film] A crystalline polyethylene terephthalate (PET) resin film (manufactured by Toray Industries, Inc., "Lumirror T60", thickness 100 μm, glass transition temperature Tg: 76°C, melting point 263°C) was prepared. The resin obtained by finely pulverizing the PET resin film with a pulverizer was melt-extruded using a hot melt extrusion film forming machine equipped with a T-die ("Measuring Extruder Type Me-20 / 2800V3" manufactured by Optical Control Systems), and a long resin film with a width of approximately 120 mm was obtained. The operating conditions of the film forming machine are listed below in bullet points. · Barrel temperature setting = 280°C to 300°C · Die temperature = 300°C · Screw rotation speed = 60 rpm · Cast roll temperature = 70°C

[0083] [Production Example 2: Production of Stretched Film (Base Film)] A stretching device ("SDR-562Z" manufactured by Etou Co., Ltd.) was prepared. This stretching device was equipped with clips capable of gripping the ends of a rectangular resin film and an oven. Five clips were provided per side of the resin film, and a total of 24 clips were provided at each vertex of the resin film. By moving these clips, the resin film could be stretched. In addition, it was equipped with an oven and the stretching temperature could be set.

[0084] Using the above stretching device, the film obtained in Production Example 1 was stretched at a stretching temperature of 105°C and a stretching draw ratio of 5.0 times to obtain a stretched film.

[0085] [Example 1] The stretched film obtained in Production Example 2 was cut into a size of 50 mm × 50 mm. The bath was filled with chloroform, which is a treatment solvent, and the stretched film was immersed in the treatment solvent for 10 seconds (10 s). Then, the stretched film was taken out from the treatment solvent and the surface was wiped with gauze. The obtained stretched film was evaluated as an optical film by the method described above.

[0086] [Example 2] An optical film was obtained in the same manner as in Example 1, except that the processing solvent was methylene chloride.

[0087] [Example 3] An optical film was obtained in the same manner as in Example 1, except that the processing solvent was toluene and the immersion time in the processing solvent was 24 hours (24 hrs).

[0088] [Comparative Example 1] An optical film was obtained in the same manner as in Example 1, except that the processing solvent was toluene and the immersion time in the processing solvent was 10 seconds (10 s).

[0089] [Comparative Example 2] The stretched film obtained in Production Example 2 was transferred to an oven for heat treatment while being gripped with clips, and heat treatment was performed at 170 °C for 60 seconds (60 s). The stretched film after this heat treatment was evaluated as an optical film by the method described above.

[0090] [Results] The results of the above-described Examples and Comparative Examples are shown in the following table. In the following table, the meanings of the abbreviations are as follows. PET: polyethylene terephthalate. d: thickness Re: in-plane retardation Rth: retardation in the thickness direction

[0091] [Table 1]

[0092] [Consideration] As shown in Examples 1 to 3, by bringing an organic solvent into contact with a resin film to change the birefringence in the in-plane direction, an optical film having an NZ coefficient of 1.10 or less and an in-plane retardation of 2000 nm or more could be produced. Further, it was confirmed that the obtained optical film could suppress iridescence as compared with the optical film in which the NZ coefficient and the in-plane retardation were adjusted by heat treatment in Comparative Example 2.

[0093] As shown in Comparative Example 3, when the in-plane retardation of the optical film was 2000 nm or more and the NZ coefficient exceeded 1.10, iridescence was strongly observed.

Claims

1. Formed of a resin containing polyethylene terephthalate, having an NZ coefficient of 1.10 or less, an in-plane retardation of 2000 nm or more, and a retardation in the thickness direction of 2200 nm or less, and containing 1% by weight or more of an organic solvent, a method for manufacturing an optical film, comprising: a step (a) of preparing a resin film formed of a resin containing polyethylene terephthalate; a step (b) of bringing the resin film into contact with an organic solvent to change the birefringence in the in-plane direction, wherein the resin film is a stretched film, when the in-plane retardation of the stretched film prepared in step (a) is Re1 and the in-plane retardation of the stretched film in which the birefringence in the in-plane direction is changed in step (b) is Re2, the increase rate of the in-plane retardation represented by Re2 / Re1 is 1.5 times or more. A method for manufacturing an optical film.

2. The method for manufacturing an optical film according to claim 1, wherein the organic solvent is at least one selected from the group consisting of chloroform, methylene chloride, and toluene.

3. The method for manufacturing an optical film according to claim 1 or 2, wherein the optical film is used as a protective film for a polarizing plate.

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