JP7593151B2 - Optical film manufacturing method - Google Patents

Description

The present invention relates to a method for producing an optical film.

Resin films with specific optical properties have been used for optical purposes. For example, a film with an NZ coefficient that satisfies 0<NZ<1 is called a three-dimensional retardation film. It is known that when a three-dimensional retardation film is provided in a display device such as a liquid crystal display device, it can exert an effect of reducing coloring of the display surface when viewed from an oblique direction.

Three-dimensional retardation films have a greater retardation in the z-axis direction (i.e., thickness direction) than in the y-axis direction (i.e., in-plane direction perpendicular to the in-plane slow axis direction). Therefore, they cannot be manufactured by a normal retardation film manufacturing method, such as simply stretching a resin for optical films with a normal positive intrinsic birefringence. For this reason, it has been proposed to manufacture three-dimensional retardation films or similar films by combining multiple types of resins or performing a complex stretching process (for example, Patent Documents 1 and 2).

International Publication No. 2019/188205 International Publication No. 2020/137409

With the manufacturing methods for three-dimensional retardation films proposed so far, it has been difficult to obtain a large birefringence in the thickness direction (large absolute value of Rth), and it has been difficult to easily manufacture a film that can exhibit good effects as a three-dimensional retardation film.

The object of the present invention is therefore to provide a method for producing an optical film that can easily produce an optical film that can exhibit good effects as a three-dimensional retardation film.

The present inventors have conducted studies to solve the above problems. In the course of their studies, the present inventors have investigated the production of a three-dimensional retardation film by contacting a film of a resin containing a crystalline polymer with a solvent and impregnating the resin with the solvent, thereby changing the birefringence in the thickness direction of the film. However, in the course of their studies, it has become clear that it is difficult to easily achieve a sufficient value of birefringence in the thickness direction in such a production method.

The present inventors have further investigated this point and found that a film capable of exhibiting good effects as a three-dimensional retardation film can be easily produced by contacting the film with a solvent and then drying the film using a specific drying method. The present invention was completed based on this finding.
That is, the present invention includes the following.

[1] A step (I) of preparing a film (pA) having a layer made of a crystalline resin (a);
a step (II) of contacting at least one surface of the film (pA) with a solvent to obtain a film (qA);
and (III) drying the film (qA) at a drying speed of 0.1 g/sec ^m2 or more for a drying time of 0.1 seconds or more and 60 seconds or less to reduce the solvent content to 6 wt% or less, thereby obtaining a film (rA).
[2] The method for producing an optical film according to [1], further comprising a step of stretching the film (rA).
[3] The boiling point of the solvent is BpS (°C),
The method for producing an optical film according to [1] or [2], wherein the step (II) includes immersing the film (pA) in the solvent at 20° C. or higher and (BpS+10)° C. or lower for 0.5 seconds or longer and 300 seconds or shorter.
[4] The method for producing an optical film according to any one of [1] to [3], wherein the optical film has an NZ coefficient greater than 0 and less than 1.
[5] The method for producing an optical film according to any one of [1] to [4], wherein the optical film is a single-layer film.
[6] The method for producing an optical film according to any one of [1] to [5], wherein the crystalline resin (a) has a positive intrinsic birefringence value.
[7] The method for producing an optical film according to any one of [1] to [6], wherein the crystalline resin (a) is a resin containing an alicyclic structure-containing polymer.

The present invention provides a method for producing an optical film that can easily produce an optical film that can exhibit good effects as a three-dimensional retardation film.

The present invention will be described in detail below with reference to embodiments and examples. However, the present invention is not limited to the embodiments and examples shown below, and may be modified and implemented as desired without departing from the scope of the claims of the present invention and the scope of equivalents thereto.

In the following description, the in-plane retardation Re of a layered structure such as a film is a value expressed by Re = (nx - ny) x d unless otherwise specified. The thickness direction retardation Rth of a layered structure is a value expressed by Rth = [{(nx + ny)/2} - nz] x d unless otherwise specified. The NZ coefficient of a layered structure is a value expressed by (nx - nz)/(nx - ny) unless otherwise specified.

nx represents the refractive index in a direction perpendicular to the thickness direction of the layered structure (in-plane direction) that gives the maximum refractive index. ny represents the refractive index in the in-plane direction of the layered structure that is perpendicular to the nx direction. nz represents the refractive index in the thickness direction of the layered structure. d represents the thickness of the layered structure. The measurement wavelength is 590 nm unless otherwise specified.

In the following description, unless otherwise specified, a material with positive intrinsic birefringence means a material whose refractive index in the stretching direction is greater than the refractive index in the direction perpendicular to the stretching direction. Furthermore, a material with negative intrinsic birefringence means a material whose refractive index in the stretching direction is smaller than the refractive index in the direction perpendicular to the stretching direction, unless otherwise specified. The value of intrinsic birefringence can be calculated from the dielectric constant distribution.

In the following description, a "long" film refers to a film that is 5 times or more longer than its width, preferably 10 times or more longer, specifically a film that is long enough to be wound into a roll for storage or transportation. There is no particular upper limit on the length, but it is usually 100,000 times or less longer than the width.

In the following description, the slow axis of a layered structure is the in-plane slow axis unless otherwise specified.

[Optical film manufacturing method: overview]
The method for producing an optical film of the present invention includes the following steps (I) to (III). The method for producing an optical film of the present invention may further include the following step (IV) in addition to the steps (I) to (III). The method for producing an optical film of the present invention may be performed under normal temperature and pressure conditions, unless otherwise specified regarding temperature and pressure (for example, regarding matters where there is no explicit description regarding heating, cooling, pressurization, decompression, etc.).

Step (I): A step of preparing a film (pA) having a layer made of a crystalline resin (a).
Step (II): A step of contacting at least one surface of the film (pA) with a solvent to form a film (qA).
Step (III): A step of drying the film (qA) at a drying speed of 0.1 g/sec· ^m2 or more for a drying time of 0.1 seconds to 60 seconds to reduce the solvent content to 6 wt% or less to obtain a film (rA).
Step (IV): A step of stretching the film (rA).

[Step (I)]
In step (I), a film (pA) having a layer made of a crystalline resin (a) is prepared. The film (pA) may be prepared by simply purchasing a commercially available product, or may be prepared by forming the film (pA) using the crystalline resin (a) as a material.

The film (pA) may have a layer other than the layer made of crystalline resin (a), but may be a film made of only crystalline resin (a). When the film (pA) has a layer other than the layer made of crystalline resin (a), at least one of the front surface and the back surface of the film (pA) is a layer made of crystalline resin (a). The film (pA) may have two or more layers made of crystalline resin (a), but is usually a single-layer film made of only one layer made of crystalline resin (a). Therefore, the optical film obtained by the manufacturing method of the present invention may also have a layer other than the layer made of crystalline resin (a), but may be a film made of only crystalline resin (a), and may have two or more layers made of crystalline resin (a), but is usually a single-layer film made of only one layer made of crystalline resin (a).

In the manufacturing method of the present invention, when the film (pA) is formed as a long film in step (I), the film (pA) can be transported in the longitudinal direction in the manufacturing line, and the subsequent steps can be performed continuously. By carrying out the manufacturing method in this manner, efficient manufacturing can be performed. However, the manufacturing method of the present invention is not limited to this. For example, after the film (pA) is formed as a long film in step (I), the film (pA) can be cut into a sheet-like film having a rectangular shape or the like, and the manufacturing method can be carried out by sequentially subjecting the film to the subsequent steps.

When the step (I) is carried out by forming the film (pA) using the crystalline resin (a) as a material, the film can be formed by various known film forming methods. In particular, from the viewpoint of production efficiency, it is preferable to form the film by melt extrusion molding. The film forming conditions can be appropriately adjusted according to the properties of the crystalline resin (a). The thickness of the film (pA) formed in the step (I) is not particularly limited, and can be appropriately adjusted so that the thickness of the optical film as a product is the desired value. The film (pA) may be a film having optical anisotropy, but even if it does not have optical anisotropy, the optical film of the present invention can be easily produced by subjecting it to the subsequent steps.

[Resin (a)]
The resin (a) may be a crystalline polymer, i.e., a resin containing a polymer having crystallinity. A polymer having crystallinity refers to a polymer having a melting point Tm. That is, a polymer having crystallinity refers to a polymer whose melting point can be observed by a differential scanning calorimeter (DSC). A resin containing a crystalline polymer as a main component may exhibit properties based on the crystalline polymer. Such a resin may be called a crystalline resin. The crystalline resin is preferably a thermoplastic resin.

It is preferred that the crystalline polymer has a positive intrinsic birefringence, so that the crystalline resin has a positive intrinsic birefringence value. By using a resin that is both crystalline and has a positive intrinsic birefringence, it is particularly easy to manufacture an optical film that has particularly useful properties regarding the NZ coefficient.

The crystalline polymer may be, for example, polyesters such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN); polyolefins such as polyethylene (PE) and polypropylene (PP); and the like, but is not particularly limited, and preferably contains an alicyclic structure. By using a crystalline polymer containing an alicyclic structure, the mechanical properties, heat resistance, transparency, low moisture absorption, dimensional stability, and light weight of the film can be improved. A polymer containing an alicyclic structure refers to a polymer having an alicyclic structure in the molecule. Such a polymer containing an alicyclic structure may be, for example, a polymer obtained by a polymerization reaction using a cyclic olefin as a monomer, or a hydrogenated product thereof.

Examples of alicyclic structures include a cycloalkane structure and a cycloalkene structure. Among these, a cycloalkane structure is preferred because it is easy to obtain a retardation film having excellent properties such as thermal stability. The number of carbon atoms contained in one alicyclic structure is preferably 4 or more, more preferably 5 or more, and preferably 30 or less, more preferably 20 or less, and particularly preferably 15 or less. When the number of carbon atoms contained in one alicyclic structure is within the above range, mechanical strength, heat resistance, and moldability are highly balanced.

In a crystalline polymer containing an alicyclic structure, the ratio of structural units having an alicyclic structure to all structural units is preferably 30% by weight or more, more preferably 50% by weight or more, and particularly preferably 70% by weight or more. By increasing the ratio of structural units having an alicyclic structure as described above, heat resistance can be improved. The ratio of structural units having an alicyclic structure to all structural units can be 100% by weight or less. In addition, in a crystalline polymer containing an alicyclic structure, the remainder other than the structural units having an alicyclic structure is not particularly limited and can be appropriately selected depending on the purpose of use.

Examples of the crystalline polymer containing an alicyclic structure include the following polymer (α) to polymer (δ). Among these, polymer (β) is preferred because it is easy to obtain a retardation film having excellent heat resistance.
Polymer (α): A ring-opening polymer of a cyclic olefin monomer, which has crystallinity.
Polymer (β): A hydrogenated product of polymer (α) and has crystallinity.
Polymer (γ): An addition polymer of a cyclic olefin monomer, which has crystallinity.
Polymer (δ): A hydrogenated polymer (γ) having crystallinity.

Specifically, as the crystalline polymer containing an alicyclic structure, a ring-opening polymer of dicyclopentadiene having crystallinity and a hydrogenated ring-opening polymer of dicyclopentadiene having crystallinity are more preferable. Among them, a hydrogenated ring-opening polymer of dicyclopentadiene having crystallinity is particularly preferable. Here, a ring-opening polymer of dicyclopentadiene refers to a polymer in which the ratio of structural units derived from dicyclopentadiene to all structural units is usually 50% by weight or more, preferably 70% by weight or more, more preferably 90% by weight or more, and even more preferably 100% by weight.

The hydrogenated ring-opening polymer of dicyclopentadiene preferably has a high ratio of racemo dyads. Specifically, the ratio of racemo dyads in the repeating units of the hydrogenated ring-opening polymer of dicyclopentadiene is preferably 51% or more, more preferably 70% or more, and particularly preferably 85% or more. A high ratio of racemo dyads indicates high syndiotactic stereoregularity. Therefore, the higher the ratio of racemo dyads, the higher the melting point of the hydrogenated ring-opening polymer of dicyclopentadiene tends to be.
The ratio of the racemo dyads can be determined based on the ¹³ C-NMR spectrum analysis described in the Examples below.

As the above polymer (α) to polymer (δ), polymers obtained by the manufacturing method disclosed in WO 2018/062067 can be used.

The melting point Tm of the crystalline polymer is preferably 200°C or higher, more preferably 230°C or higher, and preferably 290°C or lower. By using a crystalline polymer having such a melting point Tm, an optical film with an even better balance between moldability and heat resistance can be obtained.

Crystalline polymers usually have a glass transition temperature, and therefore, for crystalline resins that contain a crystalline polymer as the main component, a glass transition temperature based on the glass transition temperature of the crystalline polymer can be observed. The glass transition temperature Tg of a crystalline polymer is usually 85°C or higher and usually 170°C or lower.

The glass transition temperature Tg and melting point Tm can be measured by the following method. First, the sample is melted by heating, and the melted sample is rapidly cooled with dry ice. The glass transition temperature Tg and melting point Tm of this sample can then be measured using a differential scanning calorimeter (DSC) at a heating rate of 10°C/min (heating mode).

The weight average molecular weight (Mw) of the crystalline polymer is preferably 1,000 or more, more preferably 2,000 or more, and preferably 1,000,000 or less, more preferably 500,000 or less. A crystalline polymer having such a weight average molecular weight has an excellent balance between moldability and heat resistance.

The molecular weight distribution (Mw/Mn) of the crystalline polymer is preferably 1.0 or more, more preferably 1.5 or more, and preferably 4.0 or less, more preferably 3.5 or less. Here, Mn represents the number average molecular weight. A crystalline polymer having such a molecular weight distribution has excellent moldability.

The weight average molecular weight (Mw) and molecular weight distribution (Mw/Mn) of the polymer can be measured in polystyrene equivalent values by gel permeation chromatography (GPC) using tetrahydrofuran as the developing solvent.

The degree of crystallinity of the crystalline polymer contained in the crystalline resin (a) at the time of forming an optical film by the manufacturing method of the present invention is not particularly limited, but is usually high to a certain degree. When the crystallinity of a resin containing a crystalline polymer is measured, the specific range of the crystallinity is preferably 5% or more, more preferably 10% or more. The crystallinity can be measured by X-ray diffraction.

The crystalline polymer may be used alone or in any combination of two or more types in any ratio.

The proportion of the crystalline polymer in the crystalline resin (a) 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 the crystalline polymer is equal to or more than the lower limit, the birefringence expression and heat resistance of the resulting optical film can be improved. The upper limit of the proportion of the crystalline polymer can be 100% by weight or less, or 99.999% by weight or less.

The crystalline resin (a) may contain optional components in addition to the crystalline polymer and the solvent. Optional components include, for example, antioxidants such as phenol-based antioxidants, phosphorus-based antioxidants, and sulfur-based antioxidants; light stabilizers such as hindered amine-based light stabilizers; waxes such as petroleum wax, Fischer-Tropsch wax, and polyalkylene wax; nucleating agents such as sorbitol-based compounds, metal salts of organic phosphoric acids, metal salts of organic carboxylic acids, kaolin, and talc; diaminostilbene derivatives, coumarin derivatives, and azole derivatives (e.g., benzoxazole derivatives, benzotriazole derivatives, benzoimide derivatives, and benzoyl ester derivatives). Examples of the optional components include fluorescent brighteners such as 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 fiber; colorants; flame retardants; flame retardant assistants; antistatic agents; plasticizers; near-infrared absorbers; lubricants; fillers; and any polymer other than a crystalline polymer, such as a soft polymer. One type of optional component may be used alone, or two or more types may be used in any ratio.

[Step (II)]
In step (II), the film (pA) is contacted with a solvent. As the solvent, a solvent that can penetrate into the crystalline resin (a) without dissolving the resin can be appropriately selected. As the solvent, an organic solvent is usually used. Examples of preferred organic solvents include hydrocarbon solvents such as toluene, decahydronaphthalene, and limonene; tetrahydrofuran; chlorobenzene; and carbon disulfide. The type of solvent may be one type or two or more types.

The contact in step (II) can be achieved by any operation. Examples of the contact operation include a spray method in which the solvent is sprayed onto the surface of the film (pA); a coating method in which the solvent is applied to the surface of the film (pA); and an immersion method in which the film (pA) is immersed in the solvent. From the viewpoint of facilitating continuous contact, the immersion method is preferred. However, when it is necessary to control the amount of the solvent to be contacted by the coating thickness, etc., the spray method and the coating method are preferably used.

When the film (pA) is brought into contact with the solvent by immersion, the contact time is preferably 0.5 seconds or more, more preferably 1.0 seconds or more, and particularly preferably 5.0 seconds or more, and is preferably 300 seconds or less, more preferably 80 seconds or less, and particularly preferably 60 seconds or less.

The temperature of the solvent during contact in step (II) is arbitrary within a range in which the solvent can maintain a liquid state, and therefore can be set in a range from the melting point to the boiling point of the solvent. In particular, the temperature of the solvent can be set relative to the boiling point BpS (°C) of the solvent. Specifically, the lower limit of the solvent temperature can be room temperature or higher, for example, 20°C or higher. The upper limit of the solvent temperature can be preferably (BpS+10)°C or lower, more preferably BpS°C or lower, and even more preferably (BpS-10)°C or lower. By contacting with a solvent in such a temperature range, a good change in thickness direction birefringence can be achieved. In particular, when the film (pA) and the solvent are contacted by immersion, it is preferable to contact with the solvent in the above temperature range. The boiling point BpS (°C) of the solvent is the boiling point at 1 atmospheric pressure, and when contacting at a temperature above the boiling point, the contact can be performed, for example, under pressurized conditions.

When the film (pA) and the solvent are brought into contact by coating the solvent, the coating thickness calculated from the coating area and the amount of solvent supplied can be adjusted appropriately. The coating thickness is preferably 5 μm or more, more preferably 10 μm or more, while it is preferably 100 μm or less.

When the contact time or coating thickness is equal to or greater than the lower limit, the NZ coefficient of the optical film can be effectively adjusted by contact with the solvent. On the other hand, even if the contact time is longer than the upper limit or the coating thickness is greater than the upper limit, the amount of adjustment of the NZ coefficient tends not to change significantly. Therefore, when the contact time or coating thickness is equal to or less than the upper limit, productivity can be increased without compromising the quality of the optical film.

As a result of contact with the solvent in step (II), film (pA) becomes film (qA). Film (qA) may have a changed thickness and birefringence in the thickness direction compared to film (pA). Such changes brought about by contact with the solvent are difficult to obtain by a normal retardation film manufacturing method, such as simply stretching a resin for optical films. Therefore, as a result of such changes, it becomes possible to easily manufacture useful optical films such as three-dimensional retardation films.

The resin (a) before being subjected to step (II) may or may not contain an organic solvent. In the film (qA) obtained as a result of step (II), the resin (a) is in a state of being impregnated with the solvent as a result of contact, and contains a larger amount of organic solvent than the resin (a) before being subjected to step (II). The solvent content of the film (qA), i.e., the amount of solvent contained in a unit amount of the film (qA), is preferably 6% by weight or more, more preferably 10% by weight or more, and is preferably 50% by weight or less, more preferably 40% by weight or less, by weight ratio. By having the solvent content within the above range, a good change in the refractive index in the thickness direction can be achieved. The content of the solvent in the film can be measured by thermogravimetric analysis.

[Step (III)]
In step (III), the film (qA) is dried at a predetermined drying rate for a predetermined time, thereby reducing the solvent content of the film (qA) to a predetermined value to obtain a film (rA).

Drying can be performed by heating the film (qA) in some way and volatilizing the solvent. Specific drying procedures include heating in an oven, heating by contact with a heated roller, heating by irradiation with an electron beam (e.g., infrared rays and microwaves), and combinations of these. When heating a film by contacting a heated roller with the film, it is possible to transfer the surface shape of the roller to the film at the same time as heating. For example, when a roller with a high surface smoothness is used, it is possible to increase the surface smoothness of the resulting film (rA).

The drying time in step (III) is from 0.1 seconds to 60 seconds. The drying time is preferably 1 second or more, and more preferably 30 seconds or less.

In step (III), drying is performed at a drying rate of 0.1 g/sec·m ² or more during the drying time, thereby reducing the solvent content of the film to 6 wt % or less. The drying rate is a value calculated from the solvent content of the film immediately before the start of the drying, the solvent content of the film immediately after the drying is completed, and the length of the drying time. The drying rate is preferably 0.2 g/sec·m ² or more, more preferably 0.3 g/sec·m ² or more, while preferably 10 g/sec·m ² or less, more preferably 5 g/sec·m ² or less. The solvent content reduced by step (III) can be preferably 6 wt %, more preferably 5.8 wt %. The lower limit of the solvent content can be, for example, 0.5 wt % or more. According to the inventor's findings, by performing drying that satisfies the above-mentioned drying rate, drying time, and solvent content after drying, a good change in the thickness direction refractive index can be achieved, and as a result, an optical film with a large absolute value of Rth can be easily manufactured.

The film (rA) obtained as a result of step (III) can be used as it is, or can be made into an optical film product by going through further steps as necessary. As a result of the processing in steps (I) to (III), the NZ coefficient NZ(rA) of the film (rA) can usually be less than 1. More specifically, NZ(rA) can satisfy 0<NZ(rA)<1 or NZ(rA)<0. The former can be usefully used as a so-called three-dimensional retardation film. The latter can be usefully used as a material for producing a three-dimensional retardation film. That is, in the latter case, it can be easily converted into a film that satisfies 0<NZ<1 by a simple process such as uniaxial stretching in step (IV). When NZ(rA)<0, the upper limit is preferably -1 or less, more preferably -10 or less. The lower limit of NZ(rA) is not particularly limited, but can be -1000 or more. When 0<NZ(rA)<1, the preferred range can be the same as the range for NZ(sA) described below.

When the film (rA) obtained as a result of step (III) is subjected to step (IV), the film (rA) may be subjected to step (IV) immediately after the completion of step (III), or may be subjected to step (IV) after undergoing any step after the completion of step (III). For example, additional drying may be performed.

[Step (IV)]
In step (IV), after step (III), the film (rA) is stretched to obtain a film (sA). By this stretching, the molecules of the polymer contained in the film (rA) are oriented in a direction according to the stretching direction. Since the film (rA) has undergone steps (II) and (III), it is possible to easily obtain a film (sA) as an optical film having optical properties that are difficult to obtain by a normal retardation film manufacturing method such as simply stretching a resin for an optical film.

The stretching in step (IV) may be uniaxial stretching or biaxial or more stretching. The number of stretchings may be one or more. Preferably, one uniaxial stretching or one biaxial stretching in one direction and one stretching in the other direction are performed simultaneously or sequentially. Since film (rA) has been through steps (II) and (III), such simple stretching can easily produce film (sA) as an optical film with optical properties that are difficult to obtain by a normal retardation film manufacturing method.

When uniaxial stretching is performed, the stretching may be free-end uniaxial stretching or fixed-end uniaxial stretching. Free-end uniaxial stretching of a film is uniaxial stretching performed in a manner that allows shrinkage in the in-plane direction perpendicular to the stretching direction. In contrast, fixed-end uniaxial stretching is uniaxial stretching performed in a manner that fixes the dimension in the direction perpendicular to the stretching direction and does not allow shrinkage in that direction (i.e., stretching in which the stretch ratio in the direction perpendicular to the stretching direction is set to 1).

There are no limitations on the stretching direction in step (IV), and examples include the longitudinal direction, width direction, and diagonal direction of the rectangular film. Here, the diagonal direction refers to a direction perpendicular to the thickness direction, which forms an angle with the width direction that is neither 0° nor 90° (i.e., a direction which forms an angle with the width direction that is greater than 0° and less than 90°).

When attempting to manufacture a three-dimensional retardation film using a manufacturing method that does not involve steps (II) and (III), a more complicated stretching process and a more complicated resin film structure are usually required, which is disadvantageous in terms of manufacturing efficiency. In contrast, the manufacturing method of the present invention makes it possible to obtain an optical film that can be used as a three-dimensional retardation film using a simpler process.

The stretching ratio is preferably 1.1 times or more, more preferably 1.2 times or more, and is preferably 20.0 times or less, more preferably 10.0 times or less, even more preferably 5.0 times or less, and particularly preferably 2.0 times or less. It is desirable to set the specific stretching ratio appropriately depending on factors such as the optical properties, thickness, and strength of the optical film product. When the stretching ratio is equal to or greater than the lower limit, the birefringence can be significantly changed by stretching. Also, when the stretching ratio is equal to or less than the upper limit, the direction of the slow axis can be easily controlled and breakage of the film can be effectively suppressed.

The stretching temperature can be defined relative to the glass transition temperature Tg of the crystalline polymer. The stretching temperature is preferably "Tg+5"°C or higher, more preferably "Tg+10"°C or higher, and preferably "Tg+100"°C or lower, more preferably "Tg+90"°C or lower. When the stretching temperature is equal to or higher than the lower limit, the film can be sufficiently softened and stretched uniformly. When the stretching temperature is equal to or lower than the upper limit, the film can be stretched smoothly, since the hardening of the film caused by the progress of crystallization of the crystalline polymer can be suppressed, and large birefringence can be expressed by stretching. Furthermore, the haze of the optical film obtained can usually be reduced to increase the transparency. In addition, by performing stretching at such a temperature, the crystallinity of the crystalline polymer is increased, and the optical properties of the optical film obtained as a result can be easily adjusted to the desired range.

Since the birefringence can be changed by step (IV), the NZ coefficient can be adjusted. Thus, the film (sA) is obtained as an optical film having the desired optical properties by the stretching in step (IV). The film (sA) obtained as a result of step (IV) can be used as is as an optical film product. Alternatively, the obtained film can be further subjected to optional processing to become an optical film product. Examples of optional processing include adjustment of birefringence by heat treatment while maintaining the stretched dimensions or relaxation treatment by reducing the stretched dimensions.

The NZ coefficient of the optical film obtained by the manufacturing method of the present invention is preferably greater than 0, more preferably greater than 0.2, and preferably less than 1, more preferably less than 0.8. A film having such an NZ coefficient is called a three-dimensional retardation film. When a three-dimensional retardation film is provided in a display device such as a liquid crystal display device, it can exert an effect of reducing coloring of the display surface when viewed from an inclined direction.

The in-plane retardation Re of the optical film can be adjusted to a value suitable for the application of the optical film. In one example, the preferred range of the in-plane retardation Re(590) at a wavelength of 590 nm is 137.5 nm or a value close to it, specifically, preferably 127.5 to 147.5 nm, and more preferably 130.5 to 144.5 nm. By setting the value of Re(590) within this range, the optical film can be used as a λ/4 waveplate. In another example, the preferred range of Re(590) is 275 nm or a value close to it, specifically, preferably 265 to 285 nm, and more preferably 268 to 282 nm. By setting the value of Re(590) within this range, the optical film can be used as a λ/2 waveplate.

In general, optical films used in devices such as display devices need to be at least a certain thickness to exhibit optical properties, but are also required to be thin in order to make devices thinner. The thickness of the optical film obtained by the manufacturing method of the present invention is not particularly limited, but it is possible to obtain a film that satisfies the desired optical properties even if it is thin. Specifically, the thickness of the optical film is preferably 150 μm or less, more preferably 120 μm or less, and even more preferably 100 μm or less. The lower limit of the thickness of the optical film is not particularly limited, but can be, for example, 20 μm or more.

In the optical film obtained by the manufacturing method of the present invention, the resin (a) may contain an organic solvent. This organic solvent is usually the organic solvent used in step (II) of the manufacturing method of the present invention that is incorporated into the film and remains even in the subsequent drying step without being removed. All or a part of the organic solvent incorporated into the film in step (II) may penetrate into the inside of the polymer. Therefore, even if a subsequent drying step is performed, it is difficult to easily remove the solvent completely. Therefore, the optical film obtained by the manufacturing method of the present invention usually contains an organic solvent.

The ratio of the organic solvent contained in the optical film obtained by the manufacturing method of the present invention (solvent content) to 100% by weight is preferably 6% by weight or less, more preferably 5.8% by weight or less, and particularly preferably 5.5% by weight or less. The lower limit of the solvent content is not particularly limited, but may be 0.001% by weight or more, or 0.01% by weight or more. The solvent content in the film may be measured by thermogravimetric analysis.

[Application]
The optical film of the present invention can be processed into a desired shape such as a rectangle as necessary and used as a component of an optical device such as a display device. When the optical film of the present invention is used as a component of a display device, the display quality, such as the viewing angle, contrast, and image quality, of an image displayed on the display device can be improved.

The present invention will be described in detail below with reference to examples. However, the present invention is not limited to the examples shown below, and can be modified and carried out as desired without departing from the scope of the claims of the present invention and the scope of equivalents thereto.
In the following description, the units "%" and "parts" are by weight unless otherwise specified. Furthermore, the operations described below were carried out at room temperature and pressure unless otherwise specified.

[Evaluation method]
(Method of measuring weight average molecular weight Mw and number average molecular weight Mn of polymer)
The weight average molecular weight Mw and number average molecular weight Mn of the polymer were measured as polystyrene equivalent values using a gel permeation chromatography (GPC) system ("HLC-8320" manufactured by Tosoh Corporation). In the measurement, an H-type column (manufactured by Tosoh Corporation) was used as the column, and tetrahydrofuran was used as the solvent. The temperature during the measurement was 40°C.

(Method of measuring hydrogenation rate of polymer)
The hydrogenation rate of the polymer was measured by ¹ H-NMR measurement at 145° C. using orthodichlorobenzene-d4 as a solvent.

(Method of measuring glass transition temperature Tg and melting point Tm)
The glass transition temperature Tg and melting point Tm of the polymer were measured as follows. First, the polymer was melted by heating, and the melted polymer was quenched with dry ice. Then, the glass transition temperature Tg and melting point Tm of the polymer were measured using a differential scanning calorimeter (DSC) at a heating rate of 10° C./min (heating mode) using the polymer as a test specimen.

(Method for measuring the ratio of racemo-dyads in polymers)
The ratio of the racemo-dyad in the polymer was measured as follows. Using orthodichlorobenzene-d4 as a solvent, the polymer was subjected to 13C-NMR measurement by applying the inverse-gated decoupling method at 200°C. In the results of this ^13C -NMR measurement, a signal at 43.35 ppm derived from the meso-dyad and a signal at 43.43 ppm derived from the racemo-dyad were identified, with the peak at 127.5 ppm of orthodichlorobenzene-d4 as the reference shift. The ratio of the racemo-dyad in the polymer was calculated based on the intensity ratio of these signals.

(Method of measuring optical properties such as retardation Re and Rth of film)
The optical properties of the film, such as the in-plane retardation Re and the retardation in the thickness direction Rth, were measured using a retardation meter (AxoScan OPMF-1 manufactured by AXOMETRICS) at a wavelength of 590 nm.

(Method of measuring film thickness)
The thickness of the film was measured using a contact thickness meter (manufactured by MITUTOYO Corporation, Code No. 543-390).

(Method of measuring solvent content)
The weight of the film (pA) was measured by thermogravimetric analysis (TGA: under nitrogen atmosphere, heating rate 10°C/min, 30°C to 300°C). The weight of the film (pA) at 300°C WO (300°C) was subtracted from the weight of the film (pA) at 30°C WO (30°C) to obtain the weight loss amount ΔWO of the film at 300°C. The film (pA) used in the examples and comparative examples described below was produced by the melt extrusion method and does not contain a solvent. Therefore, the weight loss amount ΔWO of this film (pA) was adopted as a reference in the formula (X) described below.

The weight of the film to be measured was measured by thermogravimetric analysis (TGA: in a nitrogen atmosphere, heating rate 10°C/min, 30°C to 300°C) as described above. The weight loss of the film at 300°C, ΔWR, was calculated by subtracting the weight of the film at 300°C, WR(300°C), from the weight of the film at 30°C, WR(30°C).

The solvent content in the film was calculated from the weight loss ΔWO of the film (pA) at 300° C. and the weight loss ΔWR of the measurement target film at 300° C. according to the following formula (X).
Solvent content (%)={(ΔWR-ΔWO)/WR(30° C.)}×100 (X)

[Production Example 1. Production of crystalline resin containing hydrogenated product of ring-opened polymer of dicyclopentadiene]
A metallic pressure-resistant reactor was thoroughly dried and then purged with nitrogen. 154.5 parts of cyclohexane, 42.8 parts of a 70% cyclohexane solution of dicyclopentadiene (endo isomer content of 99% or more) (30 parts as the amount of dicyclopentadiene), and 1.9 parts of 1-hexene were added to the metallic pressure-resistant reactor, and the reactor was heated to 53°C.

0.014 parts of tetrachlorotungsten phenylimide (tetrahydrofuran) complex was dissolved in 0.70 parts of toluene to prepare a solution. 0.061 parts of a 19% diethylaluminum ethoxide/n-hexane solution was added to this solution and stirred for 10 minutes to prepare a catalyst solution. This catalyst solution was added to a pressure-resistant reactor to start the ring-opening polymerization reaction. The reaction was then continued for 4 hours while maintaining the temperature at 53°C to obtain a solution of a ring-opened polymer of dicyclopentadiene. The number-average molecular weight (Mn) and weight-average molecular weight (Mw) of the obtained ring-opened polymer of dicyclopentadiene were 8,750 and 28,100, respectively, and the molecular weight distribution (Mw/Mn) calculated from these was 3.21.

0.037 parts of 1,2-ethanediol was added as a terminator to 200 parts of the resulting solution of ring-opened dicyclopentadiene polymer, and the mixture was heated to 60°C and stirred for 1 hour to terminate the polymerization reaction. 1 part of a hydrotalcite-like compound (Kyowa Chemical Industry Co., Ltd.'s "Kyoward (registered trademark) 2000") was added to the mixture, which was heated to 60°C and stirred for 1 hour. 0.4 parts of a filter aid (Showa Chemical Industry Co., Ltd.'s "Radiolite (registered trademark) #1500") was then added, and the adsorbent and solution were filtered using a PP pleated cartridge filter (Advantec Toyo Co., Ltd.'s "TCP-HX").

100 parts of cyclohexane were added to 200 parts of the filtered ring-opened polymer solution (polymer amount 30 parts), and 0.0043 parts of chlorohydridocarbonyltris(triphenylphosphine)ruthenium was added, and a hydrogenation reaction was carried out at 6 MPa hydrogen pressure and 180°C for 4 hours. This produced a reaction liquid containing a hydride of the ring-opened polymer of dicyclopentadiene. The hydride precipitated from this reaction liquid, forming a slurry solution.

The hydride and the solution contained in the reaction liquid were separated using a centrifuge and dried under reduced pressure at 60°C for 24 hours to obtain 28.5 parts of a crystalline ring-opening polymer of dicyclopentadiene. The hydrogenation rate of this hydride was 99% or more, the glass transition temperature (Tg) was 93°C, the melting point (Tm) was 262°C, and the racemo-dyad ratio was 89%.

100 parts of the obtained hydrogenated ring-opening polymer of dicyclopentadiene was mixed with 1.1 parts of an antioxidant (tetrakis[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane;"Irganox (registered trademark) 1010" manufactured by BASF Japan Ltd.), and then the mixture was charged into a twin-screw extruder (product name "TEM-37B", manufactured by Toshiba Machine Co., Ltd.) equipped with four die holes having an inner diameter of 3 mmΦ. The mixture of the hydrogenated ring-opening polymer of dicyclopentadiene and the antioxidant was molded into a strand shape by hot melt extrusion molding, and then chopped with a strand cutter to obtain a pellet-shaped crystalline resin (a). The operating conditions of the twin-screw extruder were as follows.
・Barrel temperature setting = 270-280℃
Die set temperature = 250°C
Screw rotation speed = 145 rpm

Example 1
(1-1. Step (I): Production of film (pA))
The crystalline resin (a) produced in Production Example 1 was molded using a hot melt extrusion film molding machine equipped with a T-die (Optical Control Systems'"Measuring Extruder Type Me-20/2800V3") and wound on a roll at a speed of 1.5 m/min to obtain a long film (pA) with a width of approximately 120 mm (thickness of 50 μm). The operating conditions of the film molding machine were as follows.
・Barrel setting temperature = 280℃ to 300℃
Die temperature = 270°C
Screw rotation speed = 30 rpm
Cast roll temperature = 80°C

The film (pA) was cut into 100 mm x 100 mm pieces of rectangular film (pA). The optical properties of the film (pA) were measured. The in-plane retardation Re of the film (pA) was 5 nm, and the retardation Rth in the thickness direction was 6 nm. As described above, this resin film was produced by hot melt extrusion at high temperatures (280°C to 300°C), and therefore the resin film is considered to contain no solvent, and therefore the solvent content is 0.0%.

(1-2. Step (II): Contact of film (pA) with solvent)
The tray was filled with toluene. The temperature of the toluene was adjusted to 23° C. A rectangular film (pA) was immersed in toluene for 10 seconds. The film was then taken out of the toluene to obtain a film (qA). A portion of the film (qA) was immediately subjected to measurement of the solvent content after taking out, and the solvent content was found to be 32%.

(1-3. Step (III): Drying of film (qA))
Another part of the film (qA) was taken out and immediately dried in a drying oven at 90° C. for 60 seconds to obtain an optical film (rA).

The optical characteristics and physical properties of the optical film (rA) were evaluated. The in-plane retardation Re of the optical film (rA) was 16 nm, the thickness direction retardation Rth was -560 nm, the thickness was 64 μm, the crystallinity was 12%, and the solvent content was 5.6%.

The amount of solvent evaporated (unit: g/ ^m2 ) was calculated from the difference between the solvent content of the film (qA) and the solvent content of the optical film (rA). The amount of solvent evaporated was divided by the drying time (60 seconds in this example) to obtain a drying rate of 0.3 g/sec· ^m2 .

Example 2
An optical film (rA) was obtained and evaluated by the same procedure as in Example 1, except that the drying temperature of the film (qA) was 160° C. and the drying time was 10 seconds. The in-plane retardation Re of the optical film (rA) was 14 nm, the thickness direction retardation Rth was −636 nm, the thickness was 64 μm, the crystallinity was 12%, and the solvent content was 5.3%. The drying speed was 1.8 g/sec ^m2 .

Example 3
Except for the immersion time of the film (pA) being 60 seconds, the same procedure as in Example 1 was used to obtain and evaluate an optical film (rA). The in-plane retardation Re of the optical film (rA) was 15 nm, the thickness direction retardation Rth was −602 nm, the thickness was 64 μm, the crystallinity was 12%, and the solvent content was 5.8%. The drying speed was 0.46 g/sec ^m2 .

Example 4
Toluene was applied to the rectangular film (pA) obtained in Example 1 (1-1) to a thickness of 20 μm using a bar coater to obtain a film (qA). The solvent content was determined to be 23%. Immediately after coating, the film (qA) was dried in a drying oven at 90° C. for 60 seconds to obtain an optical film (rA). The optical film (rA) had an in-plane retardation Re of 13 nm, a thickness direction retardation Rth of −460 nm, a thickness of 64 μm, a crystallinity of 11%, and a solvent content of 5.7%. The drying speed was 0.18 g/sec·m ² .

Comparative Example 1
An optical film (rA) was obtained and evaluated by the same procedure as in Example 1, except that the drying temperature of the film (qA) was 60° C. and the drying time was 600 seconds. The in-plane retardation Re of the optical film (rA) was 9 nm, the thickness direction retardation Rth was −258 nm, the thickness was 64 μm, the crystallinity was 9%, and the solvent content was 5.8%. The drying speed was 0.03 g/sec ^m2 .

Comparative Example 2
Except for the drying temperature of the film (qA) being 60° C. and the drying time being 60 seconds, the same operation as in Example 1 was used to obtain and evaluate an optical film (rA). The thickness of the optical film (rA) was 69 μm, and the solvent content was 25%. The drying speed was 0.09 g/sec·m ^2. Immediately after the drying was completed, the film contained a large amount of solvent and was in an unstable state, so the Re, Rth, and crystallinity of the optical film (rA) could not be measured.

The overview and results of Examples 1 to 4 and the Comparative Example are shown in Table 1.

Example 5
A stretching device ("SDR-562Z" manufactured by Etoh Corporation) was prepared. This stretching device was equipped with clips capable of gripping the ends of a rectangular resin film, and an oven. A total of 24 clips were provided, five per side of the resin film and one at each vertex of the resin film, and the resin film could be stretched by moving these clips. In addition, two ovens were provided, and each could be set to a stretching temperature and a heat treatment temperature. Furthermore, in this stretching device, the resin film could be transferred from one oven to the other oven while being gripped by the clips.

The optical film (rA) obtained in Example 1 was attached to a stretching device, and the optical film (rA) was treated at a preheating temperature of 110°C for 10 seconds. Thereafter, the optical film (rA) was stretched at a stretching temperature of 110°C with a longitudinal stretching ratio of 1, a transverse stretching ratio of 1.5, and a stretching speed of 1.5/10 seconds. The "longitudinal stretching ratio" refers to the stretching ratio in the direction corresponding to the longitudinal direction of the long original film, and the "transverse stretching ratio" refers to the stretching ratio in the direction corresponding to the width direction of the long original film. In this way, the optical film (rA) was stretched to obtain the optical film (sA).

The optical characteristics and physical properties of the optical film (sA) were evaluated. The in-plane retardation Re of the optical film (sA) was 362 nm, the thickness direction retardation Rth was -15 nm, the thickness was 47 μm, and the crystallinity was 14%.

As is clear from the above results, in the examples in which a drying process that satisfies the requirements of the present invention was carried out, it is easy to obtain a large birefringence in the thickness direction (large absolute value of Rth), and it is easy to manufacture a film that can exhibit good effects as a three-dimensional retardation film.

Claims (11)

A step (I) of preparing a film (pA) having a layer made of a crystalline resin (a);
a step (II) of contacting at least one surface of the film (pA) with a solvent to obtain a film (qA);
and (III) drying the film (qA) at a drying speed of 0.1 g/sec ^m2 or more for a drying time of 0.1 seconds or more and 60 seconds or less to reduce the solvent content to 6 wt% or less, thereby obtaining a film (rA). The method for producing an optical film according to claim 1, further comprising a step of stretching the film (rA). the boiling point of the solvent is BpS (° C.);
3. The method for producing an optical film according to claim 1, wherein the step (II) comprises immersing the film (pA) in the solvent at 20°C or higher and (BpS+10)°C or lower for 0.5 seconds or higher and 300 seconds or lower. The method for producing an optical film according to any one of claims 1 to 3, wherein the NZ coefficient of the optical film is greater than 0 and less than 1. The method for producing an optical film according to any one of claims 1 to 4, wherein the optical film is a single-layer film. The method for producing an optical film according to any one of claims 1 to 5, wherein the crystalline resin (a) has a positive intrinsic birefringence value. The method for producing an optical film according to any one of claims 1 to 6, wherein the crystalline resin (a) is a resin containing an alicyclic structure-containing polymer. The method for producing an optical film according to any one of claims 1 to 7, wherein the NZ coefficient NZ(rA) of the film (rA) is less than 0. The method for producing an optical film according to claim 8, wherein the NZ coefficient NZ(rA) of the film (rA) is −1 or less. The method for producing an optical film according to claim 9, wherein the NZ coefficient NZ(rA) of the film (rA) is −10 or less. The method for producing an optical film according to any one of claims 1 to 10, wherein the film (rA) has a thickness direction retardation Rth(rA) of -460 nm or less.