JP7513419B2 - Method for manufacturing retardation film - Google Patents

Description

The present invention relates to a method for manufacturing a retardation film.

In recent years, image display devices in which a retardation film is provided on the viewing side have been widely used. In the manufacture of such retardation films, typically, a resin film is formed by extruding or coating a resin, and the obtained resin film is stretched to obtain a retardation film. However, conventional methods for manufacturing retardation films have problems such as an insufficient appearance of the obtained retardation film and insufficient retardation expression.

Japanese Patent No. 3325560

The present invention has been made to solve the above-mentioned problems in the past, and its main objective is to provide a method for manufacturing a retardation film that has excellent appearance and retardation expression.

The method for producing a retardation film of the present invention includes a first step of heating a resin film to obtain an intermediate film, and a second step of subjecting the intermediate film to a stretching treatment.
In one embodiment, the method further includes a step of winding up the intermediate film between the first step and the second step.
In one embodiment, the heating temperature in the first step is (Tg+25° C.)/2 or more.
In one embodiment, the heating temperature in the first step is equal to or higher than Tg, and an increase in Re(550) of the intermediate film relative to the resin film before heating is 10 nm or less.
In one embodiment, the heating time in the first step is from 10 seconds to 180 seconds.
In one embodiment, the heating time in the first step is from 30 seconds to 120 seconds.
In one embodiment, the heating temperature in the first step is higher than the stretching temperature in the second step.
In one embodiment, the method includes a step of adhering a shrinkable film to form a laminate between the first step and the second step.
In another embodiment, the method includes, prior to the first step, a step of adhering the resin film to a shrinkable film to form a laminate, or a step of applying a coating liquid in which a resin is dissolved or dispersed in a solvent to the shrinkable film, and the increase in Re(550) of the intermediate film relative to the resin film before heating is 10 nm or less.

According to an embodiment of the present invention, a method for producing a retardation film includes a first step of heating a resin film to obtain an intermediate film, and a second step of subjecting the intermediate film to a stretching process, thereby making it possible to realize a retardation film with excellent appearance and retardation expression.

The following describes embodiments of the present invention, but the present invention is not limited to these embodiments.

(Definition of terms and symbols)
The definitions of terms and symbols used in this specification are as follows.
(1) Refractive index (nx, ny, nz)
"nx" is the refractive index in the direction in which the in-plane refractive index is maximum (i.e., the slow axis direction), "ny" is the refractive index in the direction perpendicular to the slow axis in the plane (i.e., the fast axis direction), and "nz" is the refractive index in the thickness direction.
(2) In-plane retardation (Re)
"Re(λ)" is the in-plane retardation measured with light having a wavelength of λ nm at 23° C. For example, "Re(550)" is the in-plane retardation measured with light having a wavelength of 550 nm at 23° C. Re(λ) is calculated by the formula: Re(λ)=(nx−ny)×d, where d (nm) is the thickness of the layer (film).
(3) Retardation in the thickness direction (Rth)
"Rth(λ)" is the retardation in the thickness direction measured with light having a wavelength of λ nm at 23° C. For example, "Rth(550)" is the retardation in the thickness direction measured with light having a wavelength of 550 nm at 23° C. Rth(λ) is calculated by the formula: Rth(λ)=(nx-nz)×d, where d (nm) is the thickness of the layer (film).
(4) Nz Coefficient The Nz coefficient is calculated by Nz=Rth/Re.

A. Method for Producing Retardation Film The method for producing a retardation film in the present invention includes a step of producing a resin film, a step of heating the resin film to obtain an intermediate film (first step), and a step of stretching the intermediate film (second step).

The method for producing a retardation film may further include a winding step of an intermediate film between the first and second steps, if necessary.

In one embodiment, the method for producing a retardation film may further include a step of laminating an intermediate film and a shrinkable film between the first step and the second step. In another embodiment, the method for producing a retardation film may include forming a laminate by bonding a resin film to a shrinkable film, and then heating the laminate to obtain an intermediate film. In yet another embodiment, the method for producing a retardation film may include forming a laminate by applying a coating liquid in which a resin is dissolved or dispersed in a solvent to a shrinkable film, and then heating the laminate to obtain an intermediate film. Each step of the method for producing a retardation film will be described in detail below.

B. Resin film preparation process The resin film can be formed by any suitable resin. Examples of the resin that forms the resin film include polycarbonate-based resins, cyclic olefin-based resins, cellulose-based resins, polyester-based resins, polyvinyl alcohol-based resins, polyamide-based resins, polyimide-based resins, polyether-based resins, polystyrene-based resins, acrylic resins, and polyester carbonate resins. Among these, polycarbonate-based resins or cyclic olefin-based resins can be preferably used.

As the polycarbonate resin, any suitable polycarbonate resin can be used as long as the effects of the present invention can be obtained. Preferably, the polycarbonate resin contains a structural unit derived from an isosorbide-based dihydroxy compound and a structural unit derived from at least one dihydroxy compound selected from the group consisting of an alicyclic diol, an alicyclic dimethanol, a di-, tri- or polyethylene glycol, and an alkylene glycol or a spiro glycol. More preferably, the polycarbonate resin contains a structural unit derived from an isosorbide-based dihydroxy compound and a structural unit derived from an alicyclic dimethanol and/or a structural unit derived from a di-, tri- or polyethylene glycol. The polycarbonate resin may contain a structural unit derived from another dihydroxy compound as necessary. Details of the polycarbonate resin and the method for producing the retardation film that can be suitably used in the present invention are described, for example, in International Publication No. 2011/062239, and the description is incorporated herein by reference.

Cyclic olefin resin is a general term for resins polymerized with cyclic olefins as polymerization units, and examples of such resins include those described in JP-A-1-240517, JP-A-3-14882, and JP-A-3-122137. Specific examples include ring-opening (co)polymers of cyclic olefins, addition polymers of cyclic olefins, copolymers (typically random copolymers) of cyclic olefins with α-olefins such as ethylene and propylene, and graft modified products obtained by modifying these with unsaturated carboxylic acids and their derivatives, as well as hydrogenated products thereof. Specific examples of cyclic olefins include norbornene monomers. Examples of norbornene monomers include those described in JP-A-2015-210459. Various products of the above cyclic olefin resins are commercially available. Specific examples include Zeon Corporation's Zeonex and Zeonor products, JSR Corporation's Arton product, TICONA's Topas product, and Mitsui Chemicals' APEL product.

Any suitable method may be used to form the resin film. Examples include melt extrusion (e.g., T-die molding), cast coating (e.g., casting), calendar molding, heat pressing, co-extrusion, co-melting, multi-layer extrusion, and inflation molding. Preferably, the T-die molding, casting, and inflation molding methods are used.

The thickness of the resin film can be set to any appropriate value depending on the desired optical properties, the stretching conditions described below, etc. It is preferably 30 μm to 300 μm, and more preferably 40 μm to 250 μm.

C. Step of heating the resin film to obtain an intermediate film (first step)
The resin film obtained in the above B. can be heated to obtain an intermediate film. According to an embodiment of the present invention, a retardation film having excellent appearance and retardation expression can be obtained by heating the resin film under predetermined conditions as a separate step before stretching (i.e., not preheating for stretching).

The heating temperature of the resin film in this step is preferably (Tg + 25°C)/2 or higher, more preferably Tg or higher. If the heating temperature is in this range, an intermediate film having the desired orientation state and/or Re(550) can be obtained. In one embodiment, the heating temperature in the first step is higher than the stretching temperature of the intermediate film.

The heating time for the resin film is preferably 10 to 180 seconds, and more preferably 30 to 120 seconds. If the heating time is within this range, an intermediate film having the desired orientation state and/or Re(550) can be obtained.

The Re(550) of the intermediate film obtained by this process is preferably 0 nm to 30 nm, more preferably 0 nm to 20 nm, and even more preferably 0 nm to 10 nm. According to the present invention, an intermediate film having such an Re(550) can be obtained by heating the resin film under the heating conditions described above. If the heating temperature is equal to or higher than Tg, the Re(550) can be smaller.

The increase in Re(550) of the intermediate film relative to the resin film before heating in this process is preferably 10 nm or less, more preferably 5 nm or less, and even more preferably 3 nm or less. The smaller the increase in Re(550) of the intermediate film, the more preferable, and the lower limit is preferably substantially 0 nm. According to the present invention, by heating the resin film under the heating conditions as described above, the increase in Re(550) before and after heating can be reduced. If the heating temperature is equal to or higher than Tg, the increase in Re(550) can be smaller.

This process is preferably carried out under tension-free and low wind conditions. Tension-free processes include lining the resin film with a process protection film and transporting the film on a tenter belt conveyor. Low wind processes include processes using IR heaters, etc. This is to prevent the resin film from being blown around by the air currents from the fan.

D. Winding step of intermediate film The intermediate film obtained in C above is wound around an axis perpendicular to the conveying direction as necessary to form a roll body. The roll body may be directly subjected to the stretching step or may be stored for a predetermined time. When the roll body is stored, the storage time is preferably 12 hours to 96 hours, more preferably 24 hours to 48 hours.

E. Stretching process of intermediate film (second process)
In one embodiment, the retardation film is produced by uniaxially stretching or fixed-end uniaxially stretching the intermediate film. A specific example of fixed-end uniaxial stretching is a method in which the intermediate film is stretched in the width direction (transverse direction) while traveling in the longitudinal direction. The stretching ratio is preferably 1.1 to 3.5 times, more preferably 1.3 to 2.0 times.

The stretching temperature of the intermediate film is preferably Tg-30°C to Tg+30°C, more preferably Tg-20°C to Tg+20°C, and even more preferably Tg-15°C to Tg+15°C. By stretching at such a temperature, a retardation film having suitable properties in the present invention can be obtained. Tg is the glass transition temperature of the constituent material of the film.

In this process, the intermediate film heated in C. above is stretched to obtain a so-called positive A plate (nx>ny=nz) or negative B plate (nx>ny>nz).

In another embodiment, the retardation film is produced by continuously obliquely stretching the intermediate film in a direction at an angle θ to the longitudinal direction. Examples of stretching machines used for oblique stretching include tenter-type stretching machines that can apply a feed force, a pulling force, or a take-up force at different speeds in the transverse and/or longitudinal directions. Tenter-type stretching machines include transverse uniaxial stretching machines and simultaneous biaxial stretching machines, but any appropriate stretching machine can be used as long as it can continuously obliquely stretch the intermediate film.

F. An embodiment using a shrinkable film F-1. An embodiment using a shrinkable film after the first step In one embodiment, between the above C. (first step) and the above E. (second step), a step of bonding an intermediate film and a shrinkable film to form a laminate is included. The intermediate film and the shrinkable film may be bonded before or after the intermediate film of the above D. is wound up. By bonding the intermediate film and the shrinkable film and stretching the resulting laminate, a retardation film having a refractive index characteristic of nx>nz>ny can be obtained.

In the intermediate film stretching step (second step), the shrinkage ratio of the shrinkable film in the direction perpendicular to the stretching direction is preferably in the range of 0.50 to 0.99, more preferably 0.60 to 0.98, and even more preferably 0.75 to 0.95.

The material for forming the shrinkable film is not particularly limited, but thermoplastic resins are preferred because they are suitable for stretching. Specifically, for example, acrylic resins, polyolefin resins such as polyethylene and polypropylene (PP), polyester resins such as polyethylene terephthalate (PET), polyamides, polycarbonate resins, norbornene resins, cellulose resins such as polystyrene, polyvinyl chloride, polyvinylidene chloride, and triacetyl cellulose, polyethersulfone, polysulfone, polyimide, polyacrylic, acetate resins, polyarylate, polyvinyl alcohol, and mixtures thereof can be mentioned. Liquid crystal polymers and the like can also be used. The shrinkable film is preferably a uniaxially or biaxially stretched film formed from one or more of the above-mentioned forming materials. For example, a commercially available product may be used as the shrinkable film. Examples of commercially available products include "Space Clean" manufactured by Toyobo Co., Ltd., "Fancy Wrap" manufactured by Gunze Ltd., "Trefan" manufactured by Toray Industries, Inc., "Lumirror" manufactured by Toray Industries, Inc., "Arton" manufactured by JSR Corporation, "Zeonoa" manufactured by Zeon Corporation, and "Suntech" manufactured by Asahi Kasei Corporation.

The thickness of the shrinkable film is not particularly limited, but is, for example, in the range of 10 μm to 300 μm, preferably in the range of 20 μm to 200 μm, and more preferably in the range of 40 μm to 150 μm. The surface of the shrinkable film may be subjected to a surface treatment for the purpose of improving adhesion to the intermediate film. Examples of the surface treatment include chemical or physical treatments such as chromate treatment, ozone exposure, flame exposure, high-voltage shock exposure, and ionizing radiation treatment. In addition, a primer layer may be formed on the surface of the shrinkable film by applying an undercoat (for example, an adhesive substance).

There are no particular limitations on the method for bonding the shrinkable film to one or both sides of the intermediate film, but a method of providing an acrylic adhesive layer having a (meth)acrylic polymer as a base polymer between the intermediate film and the shrinkable film is preferred from the viewpoints of workability and economy.

The intermediate film obtained by heating in C. above is laminated with the shrinkable film in F. above, and the resulting laminate is stretched as described in E. above, to obtain a retardation film having a refractive index characteristic of nx>nz>ny. The Nz coefficient of the obtained retardation film is preferably 0.3 to 0.9, and more preferably 0.4 to 0.8.

F-2. An embodiment in which a shrinkable film is used before the first step F-2-1. Step of bonding a resin film and a shrinkable film In another embodiment, the resin film may be bonded to a shrinkable film to form a laminate, and then the laminate may be heated to obtain an intermediate film. Details of the shrinkable film are as described in F-1. The step of bonding a resin film to a shrinkable film to form a laminate may be performed in the same manner as in F-1.

F-2-2. Step of applying a resin solution to a shrinkable film In another embodiment, a coating solution in which any suitable resin is dissolved or dispersed in a solvent is applied to a shrinkable film to form a laminate, and then the laminate may be heated to obtain an intermediate film. Details of the shrinkable film are as described in F-1. above.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. The methods for measuring each characteristic are as follows.

(1) Appearance The appearance of the retardation films obtained in the examples and comparative examples described below was evaluated by visual inspection. The evaluation was based on the absence of wrinkles and unevenness due to reflection and transmission, and the absence of wrinkles and unevenness when sandwiched between orthogonal polarizing plates and observed from the front and oblique directions. The evaluation criteria were 1 to 5 as follows.
1: No wrinkles or unevenness in the reflection, transmission and polarizing plate evaluations.
2: No wrinkles or unevenness in the reflection and transmission evaluations, but slight wrinkles and unevenness in the polarizing plate evaluation.
3: No wrinkles or unevenness in the reflection and transmission evaluations, but significant wrinkles and unevenness in the polarizing plate evaluation.
4: There are slight wrinkles and unevenness in the reflection and transmission evaluation.
5: There are significant wrinkles and unevenness in the reflection and transmission evaluation.
(2) Retardation Value Re(550) and Nz Coefficient A sample of 50 mm x 50 mm was cut out from the retardation film obtained in the examples and comparative examples described later to obtain a measurement sample, and the in-plane retardation Re(550) was measured using an Axoscan manufactured by Axometrics. The measurement temperature was 23°C. Furthermore, the retardation Rth(550) in the thickness direction was measured, and the Nz coefficient was calculated.

The following is an example of resin film manufacturing.

[Production Example 1]
81.98 parts by mass of isosorbide (hereinafter sometimes abbreviated as "ISB"), 47.19 parts by mass of tricyclodecane dimethanol (hereinafter sometimes abbreviated as "TCDDM"), 175.1 parts by mass of diphenyl carbonate (hereinafter sometimes abbreviated as "DPC"), and 0.979 parts by mass of a 0.2% by mass aqueous solution of cesium carbonate as a catalyst were charged into a reaction vessel, and in a nitrogen atmosphere, as the first step of the reaction, the temperature of the heating vessel was heated to 150°C, and the raw materials were dissolved while stirring as necessary (about 15 minutes). Next, the pressure was changed from normal pressure to 13.3 kPa, and the temperature of the heating vessel was increased to 190°C over one hour, while the generated phenol was extracted from the reaction vessel. After the entire reaction vessel was kept at 190°C for 15 minutes, the pressure in the reaction vessel was set to 6.67 kPa, the heating tank temperature was raised to 230°C in 15 minutes, and the generated phenol was extracted outside the reaction vessel as the second step. As the stirring torque of the stirrer increased, the temperature was raised to 250°C in 8 minutes, and the pressure in the reaction vessel was allowed to reach 0.200 kPa or less in order to remove the further generated phenol. After reaching a predetermined stirring torque, the reaction was terminated, and the reaction product was extruded into water to obtain polycarbonate resin pellets. The obtained polycarbonate resin was vacuum-dried at 80°C for 5 hours, and then a polycarbonate resin film (1) having a thickness of 90 μm was produced using a film-forming device equipped with a single-screw extruder (manufactured by Toshiba Machine Co., Ltd., cylinder setting temperature: 250°C), a T-die (width 300 mm, setting temperature: 250°C), a chill roll (setting temperature: 120 to 130°C) and a winder. The Tg was 130°C.

[Production Example 2]
Polymerization was carried out using a batch polymerization apparatus consisting of two vertical reactors equipped with stirring blades and a reflux condenser controlled at 100°C. 29.60 parts by mass (0.046 mol) of bis[9-(2-phenoxycarbonylethyl)fluoren-9-yl]methane, 29.21 parts by mass (0.200 mol) of isosorbide (ISB), 42.28 parts by mass (0.139 mol) of spiroglycol (SPG), 63.77 parts by mass (0.298 mol) of diphenyl carbonate (DPC), and 1.19 x 10 ^-2 parts by mass (6.78 x 10 ^-5 mol) of calcium acetate monohydrate as a catalyst were charged. After the inside of the reactor was replaced with nitrogen under reduced pressure, it was heated with a heat medium, and stirring was started when the inside temperature reached 100°C. The internal temperature was allowed to reach 220°C 40 minutes after the start of the temperature rise, and the pressure was controlled to maintain this temperature while simultaneously starting decompression, and the pressure was reduced to 13.3 kPa in 90 minutes after reaching 220°C. Phenol vapor by-produced during the polymerization reaction was led to a reflux condenser at 100°C, a small amount of monomer components contained in the phenol vapor were returned to the reactor, and uncondensed phenol vapor was led to a condenser at 45°C and recovered. Nitrogen was introduced into the first reactor to restore the pressure to atmospheric pressure once, and the oligomerized reaction liquid in the first reactor was transferred to the second reactor. Next, the temperature rise and decompression in the second reactor were started, and the internal temperature was set to 240°C and the pressure to 0.2 kPa in 50 minutes. Thereafter, polymerization was allowed to proceed until a predetermined stirring power was reached. Nitrogen was introduced into the reactor at the time the predetermined power was reached to restore the pressure, and the polyester carbonate resin produced was extruded into water, and the strands were cut to obtain pellets.
The obtained polyester carbonate-based resin (pellets) was vacuum-dried at 80°C for 5 hours, and then a film-forming device equipped with a single-screw extruder (manufactured by Toshiba Machine Co., Ltd., cylinder set temperature: 250°C), a T-die (width 200 mm, set temperature: 250°C), a chill roll (set temperature: 120 to 130°C) and a winder was used to produce a long reverse dispersion polycarbonate-based resin film (2) having a thickness of 130 μm. The Tg was 140°C.

[Production Example 3]
A commercially available cyclic olefin resin film (manufactured by Zeon Corporation under the trade name "ZEONOR ZF14") was used as the resin film (3). The thickness was 40 μm and the Tg was 136° C.

[Production Example 4]
A commercially available cyclic olefin resin film (manufactured by JSR Corporation, product name "Arton (R5000)") was used as the resin film (4). The thickness was 130 μm and the Tg was 137° C.

[Example 1]
1. Step of heating a resin film to obtain an intermediate film (first step)
The polycarbonate resin film (1) obtained in Production Example 1 was heated at 110°C for 60 seconds using a tenter and an IR heater to obtain an intermediate film. The intermediate film obtained was subjected to the evaluation of (2) above. The Re(550) value of the intermediate film after heating was 1 nm, and the increase in Re(550) was 0 nm.

2. Winding process of intermediate film The intermediate film obtained in the above 1. was wound around an axis perpendicular to the conveying direction to form a roll body. The roll body was stored for 48 hours.

3. Stretching process of intermediate film (second process)
The intermediate film wound up in the above 2. was subjected to uniaxial stretching to obtain a retardation film. The stretching temperature was 145° C., and the stretching ratio was 1.5 times. The obtained retardation film was subjected to the evaluations of (1) and (2) above. The results are shown in Table 1.

[Examples 2 to 8 and 17 to 30, and Comparative Examples 2 to 4 and 9 to 10]
A retardation film was produced in the same manner as in Example 1, except that the resin films having the numbers (1) to (4) shown in Table 1 were used, and the heating conditions in the first step and the stretching conditions in the second step shown in Table 1 were adopted. The obtained retardation film was subjected to the evaluations of (1) and (2) above. The results are shown in Table 1.

[Examples 9 to 16 and 31 to 44, and Comparative Examples 5 to 8 and 11 to 12]
A 60 μm-thick shrinkable film (manufactured by Toray Industries, Inc., product name "Torayfan BO2873") was attached to one side of the intermediate film wound up in the above 3. via an acrylic adhesive layer (thickness 15 μm). A retardation film was produced in the same manner as in Example 1, except that the intermediate film and the shrinkable film were attached to each other, that the resin films numbered (1) to (4) in Table 1 were used, and that the heating conditions in the first step and the stretching conditions in the second step in Table 1 were adopted.

<Evaluation>
It can be seen that the retardation film subjected to the heating in the first step is superior in appearance and retardation expression compared to the retardation film not subjected to the heating in the first step (Comparison between Examples 1-2 and Comparative Example 1, Examples 3-4 and Comparative Example 2, Examples 5-6 and Comparative Example 3, Examples 7-8 and Comparative Example 4, and Comparison between Examples 17-23 and Comparative Example 9, and Examples 24-30 and Comparative Example 10). It can also be seen that similar results are obtained when a bonding step with a shrinkable film is performed (Comparison between Examples 9-10 and Comparative Example 5, Examples 11-12 and Comparative Example 6, Examples 13-14 and Comparative Example 7, Examples 15-16 and Comparative Example 8, and Comparison between Examples 31-37 and Comparative Example 11, and Examples 38-44 and Comparative Example 12).
Furthermore, even if the heating temperature in the first step is a temperature below the Tg of the resin film, by setting the heating time to 30 seconds or more, it is found that excellent appearance and retardation expression can be obtained (Examples 19, 26, 33 and 40). Furthermore, if the heating temperature in the first step is a temperature above the Tg of the resin film, by setting the heating time to 10 seconds or more, it is found that excellent appearance and retardation expression can be obtained (Examples 23, 30, 37 and 44).

The retardation film according to the embodiment of the present invention can be suitably used in an image display device.

Claims (9)

The method includes a step of preparing a resin film, a first step of heating the resin film to obtain an intermediate film, and a second step of stretching the intermediate film,
The increase in Re(550) of the intermediate film relative to the resin film before heating is 0 nm or more and 10 nm or less,
The material forming the resin film includes a polycarbonate-based resin or a cyclic olefin-based resin.
A method for manufacturing a retardation film. The method further includes a winding step of the intermediate film between the first step and the second step.
The method for producing the retardation film according to claim 1 . The heating temperature in the first step is (Tg + 25 ° C.) / 2 or more;
The method for producing the retardation film according to claim 1 or 2. The heating temperature in the first step is Tg or higher.
The method for producing the retardation film according to claim 3 . The heating time in the first step is 10 seconds to 180 seconds.
The method for producing the retardation film according to claim 3 or 4. The heating time in the first step is 30 seconds to 120 seconds.
The method for producing the retardation film according to claim 5 . The heating temperature in the first step is higher than the stretching temperature in the second step.
The method for producing the retardation film according to claim 3 . A step of bonding a shrinkable film to form a laminate is included between the first step and the second step.
A method for producing the retardation film according to claim 1 . The resin film preparation step includes, before the first step, a step of adhering the resin film to a shrinkable film to form a laminate, or a step of applying a coating liquid in which a resin is dissolved or dispersed in a solvent to a shrinkable film to form the resin film on the shrinkable film,
The increase in Re (550) of the intermediate film relative to the resin film before heating is 10 nm or less;
A method for producing the retardation film according to claim 1 .