JP7468368B2 - Optical films and polarizing plates - Google Patents

Description

The present invention relates to an optical film, a polarizing plate, and a method for manufacturing an optical film.

In displays such as liquid crystal displays and organic EL displays, optical films such as polarizing plate protective films are used. As such optical films, methacrylic resin films, whose main component is a methacrylic resin such as polymethyl methacrylate, are sometimes used because of their excellent transparency, dimensional stability, and low moisture absorption.

Methacrylic resin films are generally brittle, so rubber particles are sometimes added to impart toughness. For example, Patent Document 1 discloses an acrylic resin film containing acrylic resin and rubber elastomer particles as an outer protective film for a polarizing plate (a protective film disposed on the side opposite to the liquid crystal cell). Patent Document 2 discloses an optical film obtained by stretching a thermoplastic resin composition containing acrylic resin and rubber particles.

Furthermore, Patent Document 3 discloses an optical film containing an acrylic copolymer of methyl methacrylate and a polymerizable monomer having a cyclic partial structure.

JP 2012-13850 A International Publication No. WO 2016/158968 JP 2014-38180 A

However, in recent years, with increasing demands for more flexible display devices and higher performance polarizing plates, optical films are desired to have even higher toughness. Therefore, the optical films shown in Patent Documents 1 to 3 are also desired to have even higher toughness than before.

Methacrylic resin films are sometimes manufactured by a solution casting method (casting method) because a relatively high molecular weight resin can be used and a film with good toughness can be easily obtained. In the solution casting method, a dope in which a resin is dissolved in an organic solvent (hereinafter referred to as "solvent") is cast onto a support, and then a process of removing the solvent is carried out to obtain a film. However, since methacrylic resins are highly hydrophobic, they have a high affinity with the solvent, and it takes time to remove the solvent, i.e., they have a problem of poor drying properties.

In contrast, in Patent Document 3, the drying property can be improved by using an acrylic copolymer of methyl methacrylate and a polymerizable monomer having a cyclic partial structure. On the other hand, from the viewpoint of easily obtaining a film having higher toughness (flexibility), it is desired to be able to improve the drying property without using a large amount of a polymerizable monomer having a cyclic partial structure.

The present invention has been made in consideration of the above circumstances, and aims to provide an optical film, a polarizing plate, and a method for manufacturing an optical film that can be obtained with high drying efficiency and have sufficient flexibility.

The above problem can be solved by the following configuration.

The optical film of the present invention comprises a methacrylic resin including a structural unit derived from methyl methacrylate and a structural unit derived from a copolymerizable monomer other than methyl methacrylate that is copolymerizable therewith, and rubber particles, wherein the structural unit derived from the copolymerizable monomer includes a structural unit derived from a specific copolymerizable monomer having a mobile volume of 0.5 to 3.6, as represented by the following formula (1):
Equation (1): Mobile volume = hard sphere volume of monomer / molecular weight of monomer (In equation (1), the hard sphere volume indicates the volume of a hard sphere having a radius equal to the radius of gyration obtained by analyzing the structure of the monomer).

The polarizing plate of the present invention includes the optical film of the present invention.

The method for producing an optical film of the present invention includes the steps of obtaining a dope containing a methacrylic resin containing a structural unit derived from methyl methacrylate and a structural unit derived from a copolymerizable monomer other than methyl methacrylate that is copolymerizable therewith, the structural unit derived from the copolymerizable monomer containing a structural unit derived from a specific copolymerizable monomer having a mobile volume of 0.5 to 3.6 and represented by the following formula (1), rubber particles, and a solvent; casting the dope on a support and peeling it off to obtain a film-like material; and drying the film-like material.
Equation (1): Mobile volume = hard sphere volume of monomer / molecular weight of monomer (In equation (1), the hard sphere volume indicates the volume of a hard sphere having a radius equal to the radius of gyration obtained by analyzing the structure of the monomer).

According to the present invention, it is possible to provide an optical film, a polarizing plate and a method for manufacturing an optical film that can be obtained with high drying efficiency and has sufficient flexibility.

FIG. 1 is a graph showing an example of the relationship between the number of carbon atoms in a linear alkylene moiety in a copolymerization monomer constituting a methacrylic resin and the oxygen permeability of a film.

As a result of extensive investigations, the inventors have found that by using a methacrylic resin containing structural units derived from specific copolymerizable monomers whose mobile volume falls within a specific range, it is possible to significantly improve the removability of the solvent, i.e., the drying ability, when producing a film by a solution casting method.

The mobile volume of a copolymerization monomer indicates the range of movement of the group (specifically, the group containing a linear alkylene moiety) that will become the side chain of the methacrylic resin in the copolymerization monomer per unit molecular weight of the copolymerization monomer; the larger this value is, the wider the range of movement of the side chain group. The mobile volume of a copolymerization monomer correlates with the number of carbon atoms in the linear alkylene moiety. In other words, the more carbon atoms in the linear alkylene moiety, the larger the mobile volume of the copolymerization monomer. The drying property of a film also correlates to a certain extent with the oxygen permeability of the film. In other words, the higher the oxygen permeability of the film, the higher the drying property of the film tends to be.

Figure 1 is a graph showing an example of the relationship between the number of carbon atoms in the linear alkylene moiety in a copolymerization monomer and the oxygen permeability of the film.

As shown in Figure 1, the oxygen permeability of the film increases as the number of carbon atoms in the linear alkylene moiety in the copolymerization monomer increases; however, once the number of carbon atoms reaches a certain level, the oxygen permeability of the film decreases. In other words, the inventors have discovered that by adjusting the number of carbon atoms in the linear alkylene moiety in the copolymerization monomer, and thus the mobile volume of the copolymerization monomer, to a specific range (0.5 to 3.6, preferably 1.2 to 3.2), the oxygen permeability of the film, and thus the drying properties, can be significantly increased.

The reason for this is unclear, but is speculated to be as follows. If the movable volume of the copolymerization monomer is equal to or greater than the lower limit, the side chains of the methacrylic resin can move over a wide range, which can suppress excessive entanglement of the main chains of the methacrylic resin. As a result, when the solvent molecules move in the resin matrix while pushing aside the resin chains of the methacrylic resin, relatively large voids (spaces) through which the solvent molecules can move are easily formed, which increases the mobility of the solvent molecules and improves drying properties. On the other hand, if the movable volume of the copolymerization monomer is equal to or less than the upper limit, excessive entanglement of the side chains of the methacrylic resin can be suppressed, which can suppress the voids (spaces) formed when the solvent molecules move.

It was also found that a film containing a methacrylic resin containing structural units derived from a specific copolymerization monomer with an adjusted mobile volume has high toughness. It is presumed that the increased mobile volume increases the range over which the side chains of the methacrylic resin can move, thereby improving toughness. The present invention was made based on these findings.

1. Optical Film The optical film of the present invention contains a methacrylic resin and rubber particles.

1-1. Methacrylic resin Methacrylic resin is a copolymer containing a structural unit derived from methyl methacrylate and a structural unit derived from a copolymerizable monomer copolymerizable therewith. The structural unit derived from the copolymerizable monomer includes a structural unit derived from a specific copolymerizable monomer having a mobile volume of 0.5 to 3.6, as represented by the following formula (1).
Equation (1): Mobile volume = volume of a hard sphere of a monomer / molecular weight of a monomer

The volume of the hard sphere in formula (1) indicates the volume of a hard sphere whose radius is the radius of gyration obtained by analyzing the monomer structure using radius of gyration analysis software.

When the movable volume of the specific copolymerization monomer is 0.5 or more, excessive entanglement between the main chains of the methacrylic resin can be suppressed, so that when the solvent molecules move in the resin matrix while pushing aside the resin chains of the methacrylic resin, a relatively large space derived from the specific copolymerization monomer is easily formed. Since the solvent molecules are easy to move in such a space, the drying property during solution film formation can be improved. On the other hand, when the movable volume of the specific copolymerization monomer is 3.6 or less, excessive entanglement between the side chains of the methacrylic resin can be suppressed, so that the space formed when the solvent molecules move can be suppressed from being blocked. As a result, the space in which the solvent molecules can move is less likely to be lost, so the drying property during solution film formation can be improved. From the above viewpoint, it is more preferable that the movable volume of the specific copolymerization monomer is 1.2 to 3.2.

The mobile volume of a particular copolymerization monomer can be measured by the following procedure.
1) First, a Quench calculation is performed for 500 ps at room temperature using the NVT ensemble for the structure of a specific copolymerization monomer, and energy evaluation is performed every 1 ps. Dreiding is used as the force field. The results of the Quench calculation are then analyzed using a Radius of Gyration analysis module, and the average value of the radius of gyration distribution (average radius of gyration value) is calculated as the "radius of gyration". For the analysis, Materials Studio 2017R2 Forcite module (Dassault Systèmes Biovia Co., Ltd.) can be used as a simulation software. The volume of a hard sphere having a radius equal to the radius of gyration of the obtained copolymerization monomer is defined as the "hard sphere volume".
2) The hard sphere volume calculated in 1) above and the molecular weight of the specific copolymerizable monomer are substituted into the above formula (1) to calculate the movable volume.

From the viewpoint of easily adjusting the mobile volume within the above range, it is preferable that the specific copolymerization monomer has a linear alkylene portion. In other words, the mobile volume of the specific copolymerization monomer can be adjusted by the length (number of carbon atoms) of the linear alkylene portion of the specific copolymerization monomer and the presence or absence of branching. In order to increase the mobile volume of the specific copolymerization monomer, it is preferable to increase the number of carbon atoms of the linear alkylene portion of the specific copolymerization monomer, and it is even more preferable that the specific copolymerization monomer does not have a branched structure.

That is, the specific copolymerization monomer preferably has a linear alkylene moiety having 4 to 20 carbon atoms, preferably 5 to 15 carbon atoms, and more preferably 8 to 15 carbon atoms. "Having a linear alkylene moiety having 4 to 20 carbon atoms" includes not only cases in which a linear alkylene group having 4 to 20 carbon atoms is present, but also cases in which a branched alkylene group having a linear portion having 4 to 20 carbon atoms is present.

Examples of such specific copolymerization monomers include n-butyl (meth)acrylate (number of carbon atoms in the linear alkylene portion: 4), pentyl (meth)acrylate (number of carbon atoms in the linear alkylene portion: 5), hexyl (meth)acrylate (number of carbon atoms in the linear alkylene portion: 6), 2-ethylhexyl (meth)acrylate (number of carbon atoms in the linear alkylene portion: 6), n-octyl (meth)acrylate (number of carbon atoms in the linear alkylene portion: 8), 2-propylnonyl (meth)acrylate (number of carbon atoms in the linear alkylene portion: 9), dodecyl (meth)acrylate ( (Meth)acrylic acid alkyl esters having a linear alkylene moiety having 4 to 20 carbon atoms, such as (meth)acrylic acid alkyl esters having linear alkylene moieties having 4 to 20 carbon atoms, such as (meth)acrylic acid alkyl esters having linear alkylene moieties having 4 to 20 carbon atoms, such as (meth)acrylic acid alkyl esters having linear alkylene moieties having 4 to 20 carbon atoms, such as (meth)acrylic acid alkyl esters having linear alkylene moieties having 4 to 20 carbon atoms, such as (meth)acrylic acid alkyl esters having linear alkylene moieties having 4 to 20 carbon atoms, such as (meth)acrylic acid alkyl esters having linear alkylene moieties having 4 to 20 carbon atoms, such as (meth)acrylic acid alkyl esters having linear alkylene moieties having 4 to 20 carbon atoms, such as (meth)acrylic acid alkyl esters having linear alkylene moieties having 8 to 15 ...

In addition, the specific copolymerizable monomer may further have a ring structure (aromatic ring or aliphatic ring) as long as the movable volume falls within the above range.

The methacrylic resin may further contain structural units derived from other copolymerizable monomers other than the specific copolymerizable monomer, as necessary. It is preferable that the other copolymerizable monomers do not have a linear alkylene moiety having 4 to 20 carbon atoms.

Examples of other copolymerizable monomers include:
(meth)acrylic acid alkyl esters in which the linear alkylene moiety has less than 4 carbon atoms, such as methyl acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, t-butyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, phenyl (meth)acrylate, benzyl (meth)acrylate, 2-phenoxyethyl (meth)acrylate, dicyclopentanyl (meth)acrylate, isobornyl (meth)acrylate, adamantyl (meth)acrylate, and cyclohexyl (meth)acrylate;
Aromatic vinyls such as styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, and α-methylstyrene;
Alicyclic vinyls such as vinylcyclohexane;
Unsaturated nitriles such as (meth)acrylonitrile and (meth)acrylonitrile-styrene copolymers;
Unsaturated carboxylic acids such as (meth)acrylic acid, crotonic acid, (meth)acrylic acid, itaconic acid, itaconic acid monoesters, maleic acid, and maleic acid monoesters;
Vinyl acetate, olefins such as ethylene and propylene;
Vinyl halides such as vinyl chloride, vinylidene chloride, and vinylidene fluoride;
(meth)acrylamides such as (meth)acrylamide, methyl(meth)acrylamide, ethyl(meth)acrylamide, propyl(meth)acrylamide, butyl(meth)acrylamide, tert-butyl(meth)acrylamide, and phenyl(meth)acrylamide;
Unsaturated glycidyls such as glycidyl (meth)acrylate;
These include maleimides such as N-phenylmaleimide, N-ethylmaleimide, N-propylmaleimide, N-cyclohexylmaleimide, N-o-chlorophenylmaleimide, etc. These may be used alone or in combination of two or more kinds.

In particular, from the viewpoint of increasing the glass transition temperature and further improving the drying properties during solution casting, the other copolymerizable monomer may be a monomer having a ring structure.

Examples of monomers having a ring structure include:
(meth)acrylic acid esters having an aliphatic ring, such as dicyclopentanyl (meth)acrylate, isobornyl (meth)acrylate, adamantyl (meth)acrylate, cyclohexyl (meth)acrylate, and six-membered ring lactone (meth)acrylates;
Aromatic vinyls such as styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, and α-methylstyrene;
Alicyclic vinyls such as vinylcyclohexane;
Included are maleimides such as N-phenylmaleimide, N-ethylmaleimide, N-propylmaleimide, N-cyclohexylmaleimide, and No-chlorophenylmaleimide.

The total amount of structural units derived from copolymerization monomers constituting the methacrylic resin is preferably 1 to 50% by mass, more preferably 5 to 40% by mass, and even more preferably 10 to 30% by mass, based on all structural units constituting the methacrylic resin. Of these, the content of structural units derived from specific copolymerization monomers whose mobile volume is within the above range is preferably 100% by mass based on the total amount of structural units derived from copolymerization monomers, from the viewpoint of, for example, making it easier to improve the drying properties and toughness of the film.

That is, the content of the structural unit derived from the specific copolymerization monomer is preferably 1 to 50% by mass relative to the total structural units constituting the methacrylic resin. When the content of the structural unit derived from the specific copolymerization monomer is 1% by mass or more, many spaces in which the solvent molecules can move easily can be formed when the solvent molecules move while pushing aside the resin chains of the methacrylic resin in a film-like material containing the methacrylic resin, so that the drying property can be sufficiently improved during solution film formation. When the content of the structural unit derived from the specific copolymerization monomer is 50% by mass or less, the glass transition temperature (Tg) of the methacrylic resin is not too low, so that not only the heat resistance is not easily impaired, but also the decrease in drying property due to the decrease in drying temperature can be suppressed. From the above viewpoint, the content of the structural unit derived from the specific copolymerization monomer is more preferably 5 to 40% by mass, and even more preferably 10 to 30% by mass relative to the total structural units constituting the methacrylic resin.

Alternatively, from the viewpoint of making it easier to increase the drying temperature and heat resistance of the film, the methacrylic resin may further contain structural units derived from, for example, a monomer having a ring structure (another copolymerizable monomer). In this case, the content ratio of the structural units derived from the specific copolymerizable monomer and the structural units derived from the monomer having a ring structure may be, for example, 50/50 to 90/10 (mass ratio).

The type and composition of the monomers in the methacrylic resin can be identified by ¹ H-NMR.

From the viewpoints of handling and post-processing (stretchability, etc.), the glass transition temperature (Tg) of the methacrylic resin is preferably 80°C or higher, and more preferably 100°C or higher. In addition, from the viewpoint of preventing the toughness of the optical film from being impaired, the upper limit of the glass transition temperature (Tg) of the methacrylic resin can be, for example, 160°C.

The glass transition temperature (Tg) of methacrylic resins can be measured in accordance with JIS K 7121-2012 using DSC (Differential Scanning Calorimetry).

The glass transition temperature (Tg) of the methacrylic resin can be adjusted by the type and composition of the monomer. In order to increase the glass transition temperature (Tg) of the methacrylic resin, for example, it is preferable to set the content of structural units derived from a specific copolymerization monomer to a certain level or less, or to set the number of carbon atoms of the linear alkylene moiety contained in the specific copolymerization monomer to a certain level or less.

The weight-average molecular weight (Mw) of the methacrylic resin is preferably 600,000 to 3,000,000. When the weight-average molecular weight of the methacrylic resin is in the above range, the film is provided with sufficient mechanical strength (toughness) while the film-forming property and drying property are not easily impaired. From the above viewpoint, the weight-average molecular weight of the methacrylic resin is more preferably 1,000,000 to 3,000,000.

The weight average molecular weight (Mw) of the methacrylic resin can be measured in polystyrene equivalent terms by gel permeation chromatography (GPC). Specifically, it can be measured using a column (Tosoh Corporation's HLC8220GPC) and a column (Tosoh Corporation's TSK-GEL G6000HXL-G5000HXL-G5000HXL-G4000HXL-G3000HXL in series). The measurement conditions can be the same as those in the examples described below.

The content of methacrylic resin in the optical film is preferably 70% by mass or more, and more preferably 80% by mass or more.

1-2. Rubber particles Rubber particles can facilitate the movement of molecular chains of methacrylic resins in the drying process during solution casting of optical films, thereby improving the drying properties during solution casting. In addition, rubber particles can impart flexibility and toughness to the optical film, while forming irregularities on the surface of the optical film, thereby imparting slipperiness.

The rubber particles are preferably graft copolymers containing a rubber-like polymer (crosslinked polymer), i.e., core-shell type rubber particles having a core made of a rubber-like polymer (crosslinked polymer) and a shell covering the core.

It is preferable that the glass transition temperature (Tg) of the rubber particles is -10°C or lower. If the glass transition temperature (Tg) of the rubber particles is -10°C or lower, it is easy to impart sufficient toughness to the film. It is more preferable that the glass transition temperature (Tg) of the rubber particles is -15°C or lower, and even more preferable that it is -20°C or lower. The glass transition temperature (Tg) of the rubber particles is measured in the same manner as described above.

The glass transition temperature (Tg) of the rubber particles can be adjusted, for example, by the monomer composition constituting the core and shell parts, the mass ratio (graft ratio) of the core and shell parts, and the mass ratio of the soft layer and hard layer as described below. In order to lower the glass transition temperature (Tg) of the rubber particles, it is preferable to increase the mass ratio of the acrylic acid ester having an alkyl group with 4 or more carbon atoms/copolymerizable monomer in the monomer mixture (a') constituting the acrylic rubber-like polymer (a) of the core part (for example, 3 or more, preferably 4 to 10).

Examples of rubber-like polymers include butadiene-based crosslinked polymers, (meth)acrylic crosslinked polymers, and organosiloxane crosslinked polymers. Among these, (meth)acrylic crosslinked polymers are preferred, and acrylic crosslinked polymers (acrylic rubber-like polymers) are more preferred, from the viewpoint of having a small refractive index difference with methacrylic resins and being less likely to impair the transparency of the optical film.

That is, the rubber particles are preferably acrylic graft copolymers containing acrylic rubber-like polymer (a). The acrylic graft copolymers containing acrylic rubber-like polymer (a) are preferably core-shell type particles having a core portion containing acrylic rubber-like polymer (a) and a shell portion covering the core portion. Such core-shell type particles are multi-stage polymers obtained by polymerizing a monomer mixture (b) mainly composed of methacrylic acid esters in at least one stage in the presence of acrylic rubber-like polymer (a). The polymerization can be carried out by emulsion polymerization.

(About the core part)
The acrylic rubber-like polymer (a) constituting the core portion is a crosslinked polymer mainly composed of an acrylic acid ester. The acrylic rubber-like polymer (a) is a crosslinked polymer obtained by polymerizing a monomer mixture (a') containing an acrylic acid ester and any monomer copolymerizable therewith, and a polyfunctional monomer having two or more non-conjugated reactive double bonds (radical polymerizable groups) per molecule. The acrylic rubber-like polymer (a) may be obtained by mixing and polymerizing all of these monomers, or by polymerizing two or more times with different monomer compositions.

The acrylic ester is preferably an alkyl acrylate ester having an alkyl group of 1 to 12 carbon atoms, such as methyl acrylate or butyl acrylate. The acrylic ester may be one type or two or more types. From the viewpoint of setting the glass transition temperature of the rubber particles to -15°C or lower, the acrylic ester preferably contains at least an alkyl acrylate ester having 4 to 10 carbon atoms.

The content of the acrylic acid ester is preferably 50 to 100% by mass, more preferably 60 to 99% by mass, and even more preferably 70 to 99% by mass, relative to 100% by mass of the monomer mixture (a'). When the content of the acrylic acid ester is 50% by weight or more, it is easy to impart sufficient toughness to the film.

In addition, from the viewpoint of easily adjusting the glass transition temperature of the rubber particles to -10°C or lower, the mass ratio of the acrylic acid alkyl ester having an alkyl group with 4 or more carbon atoms to the total of other copolymerizable monomers in the monomer mixture (a') is preferably 3 or more, and more preferably 4 or more and 10 or less.

Examples of copolymerizable monomers include methacrylic acid esters such as methyl methacrylate; styrenes such as styrene and methylstyrene; unsaturated nitriles such as acrylonitrile and methacrylonitrile.

Examples of polyfunctional monomers include allyl (meth)acrylate, triallyl cyanurate, triallyl isocyanurate, diallyl phthalate, diallyl malate, divinyl adipate, divinylbenzene, ethylene glycol di(meth)acrylate, diethylene glycol (meth)acrylate, triethylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, tetramethylolmethane tetra(meth)acrylate, dipropylene glycol di(meth)acrylate, and polyethylene glycol di(meth)acrylate.

The content of the polyfunctional monomer is preferably 0.05 to 10% by mass, and more preferably 0.1 to 5% by mass, relative to 100% by mass of the total monomer mixture (a'). If the content of the polyfunctional monomer is 0.05% by mass or more, the degree of crosslinking of the resulting acrylic rubber-like polymer (a) is easily increased, so that the hardness and rigidity of the resulting film are not excessively impaired, and if the content is 10% by mass or less, the toughness of the film is not easily impaired.

(About the shell part)
The polymer of the monomer mixture (b) constituting the shell portion is a graft component for the acrylic rubber-like polymer (a). The monomer mixture (b) contains a methacrylic acid ester as a main component.

The methacrylic acid ester is preferably an alkyl methacrylic acid ester having an alkyl group of 1 to 12 carbon atoms, such as methyl methacrylate. The methacrylic acid ester may be one type or two or more types.

The content of the methacrylic acid ester is preferably 50% by mass or more relative to 100% by mass of the monomer mixture (b). If the content of the methacrylic acid ester is 50% by mass or more, the hardness and rigidity of the resulting film can be prevented from decreasing. In addition, from the viewpoint of increasing the affinity with solvents such as methylene chloride, the content of the methacrylic acid ester is more preferably 70% by mass or more relative to 100% by mass of the monomer mixture (b), and even more preferably 80% by mass or more.

The monomer mixture (b) may further contain other monomers as necessary. Examples of other monomers include acrylic esters such as methyl acrylate, ethyl acrylate, and n-butyl acrylate; (meth)acrylic monomers having an alicyclic structure, a heterocyclic structure, or an aromatic group (ring structure-containing (meth)acrylic monomers) such as benzyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and phenoxyethyl (meth)acrylate.

(Regarding rubber particles (acrylic graft copolymer))
Examples of the acrylic graft copolymer include a polymer obtained by polymerizing, in at least one stage, 95 to 25 parts by mass of a monomer mixture (b) mainly composed of a methacrylic acid ester in the presence of 5 to 75 parts by mass of an acrylic rubber-like polymer (a).

The acrylic graft copolymer may further contain a hard polymer inside the acrylic rubber-like polymer (a) as necessary. Such an acrylic graft copolymer can be obtained through the following polymerization steps (I) to (III).
(I) A step of polymerizing a monomer mixture (c1) consisting of 40 to 100% by mass of a methacrylic acid ester and 60 to 0% by mass of other monomers copolymerizable therewith, and 0.01 to 10 parts by mass of a polyfunctional monomer (relative to a total of 100 parts by mass of the monomer mixture (c1)) to obtain a rigid polymer. (II) A step of polymerizing a monomer mixture (a1) consisting of 60 to 100% by mass of an acrylic acid ester and 0 to 40% by mass of other monomers copolymerizable therewith, and 0.1 to 5 parts by mass of a polyfunctional monomer (relative to a total of 100 parts by mass of the monomer mixture (a1)) to obtain a flexible polymer. (III) A step of polymerizing a monomer mixture (b1) consisting of 60 to 100% by mass of a methacrylic acid ester and 40 to 0% by mass of other monomers copolymerizable therewith, and 0 to 10 parts by mass of a polyfunctional monomer (relative to a total of 100 parts by mass of the monomer mixture (b1)) to obtain a rigid polymer.

Other polymerization steps may be further included between each of the polymerization steps (I) to (III).

The acrylic graft copolymer may be obtained by further passing through a polymerization step (IV).
(IV) A monomer mixture (b2) consisting of 40 to 100% by mass of a methacrylic acid ester, 0 to 60% by mass of an acrylic acid ester, and 0 to 5% by mass of another copolymerizable monomer, and 0 to 10 parts by mass of a polyfunctional monomer (based on 100 parts by mass of the monomer mixture (b2)) are polymerized to obtain a rigid polymer.

The methacrylic acid esters, acrylic acid esters, other copolymerizable monomers, and polyfunctional monomers used in each step can be the same as those described above.

The soft layer can impart shock absorption to the optical film. Examples of the soft layer include a layer made of an acrylic rubber-like polymer (a) whose main component is an acrylic acid ester. The hard layer can prevent the toughness of the optical film from being damaged, and can suppress the rubber particles from becoming coarse or clumpy during production. Examples of the hard layer include a layer made of a polymer whose main component is a methacrylic acid ester.

The graft ratio (mass ratio of the graft component to the acrylic rubber-like polymer (a)) in the acrylic graft copolymer is preferably 10 to 250%, more preferably 25 to 200%, more preferably 40 to 200%, and even more preferably 60 to 150%. When the graft ratio is 10% or more, the proportion of the shell portion is not too small, so the hardness and rigidity of the film are not easily impaired. When the graft ratio of the acrylic graft copolymer is 250% or less, the proportion of the acrylic rubber-like polymer (a) is not too small, so the toughness and brittleness improvement effect of the film are not easily impaired.

The graft ratio of the acrylic graft copolymer is measured by the following method.
1) 2 g of an acrylic graft copolymer was dissolved in 50 ml of methyl ethyl ketone, and the solution was centrifuged at 30,000 rpm and 12° C. for 1 hour using a centrifuge (Hitachi Koki Co., Ltd., CP60E) to separate the solution into an insoluble fraction and a soluble fraction (the centrifugation process was repeated a total of three times).
2) The weight of the insoluble matter thus obtained is applied to the following formula to calculate the graft ratio.
Graft ratio (%)=[(weight of methyl ethyl ketone insoluble matter)−(weight of acrylic rubber-like polymer (a))/(weight of acrylic rubber-like polymer (a)]×100

The average particle size of the rubber particles is preferably 100 to 400 nm, and more preferably 150 to 300 nm. If the average particle size is 100 nm or more, it is easy to impart sufficient toughness to the film, and if it is 400 nm or less, the transparency of the film is less likely to decrease.

The average particle size of rubber particles is determined as the average of the circle-equivalent diameters of 100 particles obtained by SEM or TEM photography of the film surface and slices. The circle-equivalent diameter can be determined by converting the projected area of the particles obtained by photography into the diameter of a circle with the same area. In this case, the rubber particles (acrylic graft copolymer) observed by SEM observation and/or TEM observation at a magnification of 5000 times are used to calculate the average particle size. The average particle size of the rubber particles (acrylic graft copolymer) in the dispersion can be measured using a zeta potential/particle size measurement system (ELSZ-2000ZS, manufactured by Otsuka Electronics Co., Ltd.).

The rubber particle content is preferably 2 to 30% by mass relative to the methacrylic resin. If the rubber particle content is 2% by mass or more, not only can the drying property during solution casting be sufficiently improved, but also sufficient toughness can be imparted to the resulting film. If the content is 30% by mass or less, the haze does not increase too much. From the above viewpoint, it is more preferable that the rubber particle content is 2 to 20% by mass relative to the methacrylic resin.

In addition, the methacrylic resin used in the present invention can impart good toughness derived from a specific copolymerization monomer to the film, and therefore the content of rubber particles can be reduced. From this perspective, the content of rubber particles may be 5 to 10% by mass relative to the methacrylic resin.

1-3. Other Components The optical film may further contain other components in addition to those described above, as long as the effects of the present invention are not impaired. Examples of other components include fine particles, residual solvents, ultraviolet absorbers, antioxidants, etc.

(Microparticles)
From the viewpoint of further enhancing the slipperiness, the optical film may further contain inorganic fine particles or organic fine particles other than rubber particles as a matting agent.

Examples of inorganic materials constituting the inorganic fine particles include silicon dioxide (SiO ₂ ), titanium dioxide, aluminum oxide, zirconium oxide, calcium carbonate, calcium carbonate, talc, clay, calcined kaolin, calcined calcium silicate, hydrated calcium silicate, aluminum silicate, magnesium silicate, and calcium phosphate, and from the viewpoint of reducing an increase in haze, silicon dioxide is preferred. The organic fine particles are resin particles having a glass transition temperature (Tg) of preferably 80° C. or higher. Among them, organic fine particles are preferred from the viewpoint of easily increasing the toughness of the film.

(Organic solvent)
Since the optical film is produced by a solution casting method as described later, the optical film may contain a residual solvent derived from the solvent of the dope used in the solution casting method.

The amount of residual solvent is preferably 700 ppm or less, and more preferably 30 to 700 ppm, relative to the optical film. The content of residual solvent can be adjusted by the drying conditions of the dope cast onto the support in the optical film manufacturing process described below.

The amount of residual solvent in optical films can be measured by headspace gas chromatography. In headspace gas chromatography, a sample is sealed in a container and heated, and when the container is filled with volatile components, the gas in the container is quickly injected into a gas chromatograph, where mass analysis is performed to identify the compounds and quantify the volatile components. The headspace method makes it possible to observe all peaks of volatile components using a gas chromatograph, and by using an analytical method that utilizes electromagnetic interactions, it is also possible to quantify volatile substances, monomers, etc. with high accuracy.

It is preferable that the total content of other components is 10 mass% or less of the optical film.

1-4. Physical properties of optical films (drying coefficient)
As described above, the optical film can be produced with high drying efficiency. Specifically, the drying coefficient (D) of the optical film is preferably 0.02 or more, more preferably 0.05 or more, and even more preferably 0.1 or more.

The drying coefficient (D) of the optical film can be measured by the following method.
1) First, cut the optical film into a predetermined size to prepare a sample. After drying the sample at 90° C. for 60 minutes, measure the mass and record it as the “mass before heat treatment.”
2) Next, the sample is heat-treated at 140° C. for 15 minutes, and then the mass is measured and this mass is taken as the "mass after heat treatment."
3) The values obtained in 1) and 2) above are substituted into the following formula to calculate the amount of residual solvent after heat treatment, Z.
Residual solvent amount Z (%)=(mass of sample before heat treatment−mass of sample after heat treatment)/(mass of sample after heat treatment)×100
4) The residual solvent amount Z (%) obtained in 3) above, the initial value Zo (%), and the heating time t (min) are substituted into the following formula to calculate the drying coefficient (D). The initial value Zo (%) is set to 5 (%).
Formula (II): D = (-1 / t) x ln (Z / Zo)

The drying coefficient (D) of the optical film can be adjusted by the monomer composition of the methacrylic resin. In order to increase the drying coefficient (D) of the optical film, it is preferable to increase the content of structural units derived from a specific copolymerization monomer in the methacrylic resin, or to set the number of carbon atoms in the linear alkylene moiety in the specific copolymerization monomer to within the range of 4 to 20.

(Oxygen permeability)
The oxygen permeability of the optical film measured in accordance with JIS K 7126-2 (2006) is preferably 200 to 700 (cc/ ^m2 ·day). When the oxygen permeability is 200 (cc/ ^m2 ·day) or more, the drying property is high, and when it is 700 (cc/ ^m2 ·day) or less, the flexibility of the optical film is not too high, and the handling property is not easily impaired. The oxygen permeability is more preferably 250 to 700 (cc/ ^m2 ·day), and further preferably 350 to 700 (cc/ ^m2 ·day).

The oxygen permeability of the optical film can be adjusted by the monomer composition of the methacrylic resin. In order to set the oxygen permeability of the optical film within the above range, it is preferable to increase the content of structural units derived from a specific copolymerization monomer in the methacrylic resin, or to set the number of carbon atoms in the linear alkylene moiety in the specific copolymerization monomer to within the range of 4 to 20.

(Hayes)
The optical film is preferably highly transparent. The haze of the optical film is preferably 4.0% or less, more preferably 2.0% or less, and even more preferably 1.0% or less. The haze can be measured on a 40 mm x 80 mm sample at 25°C and 60% RH using a haze meter (HGM-2DP, Suga Test Instruments) in accordance with JIS K-6714.

(Phase Differences Ro and Rt)
From the viewpoint of using the optical film as a retardation film for an IPS mode, for example, the in-plane retardation Ro measured at a measurement wavelength of 550 nm under an environment of 23° C. and 55% RH is preferably 0 to 10 nm, more preferably 0 to 5 nm, and the thickness direction retardation Rt of the optical film is preferably −20 to 20 nm, more preferably −10 to 10 nm.

Ro and Rt are defined by the following formulas, respectively.
Formula (2a): Ro = (nx - ny) x d
Formula (2b): Rt = ((nx + ny) / 2 - nz) x d
(Wherein,
nx represents the refractive index in the in-plane slow axis direction of the film (the direction in which the refractive index is maximum);
ny represents the refractive index in a direction perpendicular to the in-plane slow axis of the film,
nz represents the refractive index in the thickness direction of the film,
d represents the thickness of the film (nm).

The in-plane slow axis of an optical film is the axis along which the refractive index is maximum on the film surface. The in-plane slow axis of an optical film can be confirmed using an automatic birefringence meter, Axoscan (Axo Scan Mueller Matrix Polarimeter, manufactured by Axometrics).

Ro and Rt can be measured by the following method.
1) The optical film is conditioned for 24 hours in an environment of 23° C. and 55% RH. The average refractive index of this film is measured with an Abbe refractometer, and the thickness d is measured with a commercially available micrometer.
2) The retardation Ro and Rt of the film after humidity conditioning at a measurement wavelength of 550 nm are measured in an environment of 23° C. and 55% RH using an automatic birefringence meter Axoscan (Axo Scan Mueller Matrix Polarimeter, manufactured by Axometrics).

The phase difference Ro and Rt of the optical film can be adjusted, for example, by the type of methacrylic resin. In order to reduce the phase difference Ro and Rt of the optical film, it is preferable to use a methacrylic resin that is less likely to produce phase difference when stretched (for example, a monomer ratio such that the phase difference can be offset between a structural unit derived from a monomer having negative birefringence and a structural unit derived from a monomer having positive birefringence).

(Thickness)
The thickness of the optical film may be, for example, 5 to 100 μm, preferably 5 to 40 μm.

2. Method for Producing Optical Film The method for producing the optical film of the present invention is not particularly limited, but a solution casting method (casting method) is preferred from the viewpoint of fewer limitations on usable materials, such as the ability to use high molecular weight resins.

That is, the optical film of the present invention can be manufactured through the steps of 1) obtaining a dope containing at least a methacrylic resin, rubber particles, and a solvent, 2) casting the obtained dope onto a support, drying and peeling it off to obtain a film-like material, and 3) further drying the obtained film-like material.

Regarding the step 1), in this step, for example, a methacrylic resin and rubber particles are dissolved or dispersed in a solvent to obtain a dope. The methacrylic resin and rubber particles are the same as those described above.

The solvent used in the dope includes an organic solvent (good solvent) capable of dissolving at least the methacrylic resin. Examples of good solvents include chlorine-based organic solvents such as methylene chloride; and non-chlorine-based organic solvents such as methyl acetate, ethyl acetate, acetone, and tetrahydrofuran. Among these, methylene chloride is preferred.

The solvent used in the dope may further contain a poor solvent. Examples of poor solvents include linear or branched aliphatic alcohols having 1 to 4 carbon atoms. When the ratio of alcohol in the dope is high, the film-like material is more likely to gel and is more likely to be easily peeled off from the metal support. Examples of linear or branched aliphatic alcohols having 1 to 4 carbon atoms include methanol, ethanol, n-propanol, iso-propanol, n-butanol, sec-butanol, and tert-butanol. Of these, ethanol is preferred because of the stability of the dope, its relatively low boiling point, and its good drying properties.

The dope may be prepared by directly adding the methacrylic resin and rubber particles to the solvent and mixing them; or it may be prepared by first preparing a resin solution in which the methacrylic resin is dissolved in the solvent, and then preparing a rubber particle dispersion in which rubber particles are dispersed in the solvent, and then mixing them.

Regarding the step 2), in this step, the obtained dope is cast onto a support. The dope can be cast by being discharged from a casting die.

Next, the solvent in the dope cast onto the support is evaporated to an appropriate extent (drying), and the dope is peeled off from the support to obtain a film-like material.

The amount of residual solvent in the dope when peeled off from the support (the amount of residual solvent in the film-like material when peeled off) is, for example, preferably 25% by mass or more, more preferably 30 to 37% by mass, and even more preferably 30 to 35% by mass. If the amount of residual solvent when peeled off is 25% by mass or more, the solvent can be easily evaporated all at once from the film-like material after peeling. Furthermore, if the amount of residual solvent when peeled off is 37% by mass or less, excessive stretching of the film-like material due to peeling can be suppressed.

The amount of the remaining solvent in the dope at the time of peeling is defined by the following formula: The same applies hereinafter.
Residual solvent amount of dope (mass %)=(mass of dope before heat treatment−mass of dope after heat treatment)/mass of dope after heat treatment×100
The heat treatment in measuring the amount of residual solvent refers to a heat treatment at 140° C. for 15 minutes.

The amount of residual solvent at the time of peeling can be adjusted by the drying temperature and drying time of the dope on the support, the temperature of the support, etc.

Regarding step 3), in this step, the obtained film-like material is dried.

Drying may be performed in one step or multiple steps. Drying may also be performed while stretching the film, if necessary.

Stretching may be performed according to the desired optical properties, and it is preferable to stretch in at least one direction, but stretching in two mutually perpendicular directions (for example, biaxial stretching in the width direction (TD) of the film and the transport direction (MD) perpendicular thereto) may also be performed.

When the optical film is to be used as, for example, a retardation film for IPS, the stretching ratio can be 1.01 to 2 times. The stretching ratio is defined as (size of film in the stretching direction after stretching) / (size of film in the stretching direction before stretching). When performing biaxial stretching, it is preferable to set the above stretching ratios for both the TD and MD directions.

In addition, the in-plane slow axis direction of the optical film (the direction in which the refractive index is maximum in the plane) is usually the direction in which the stretching ratio is maximum.

The drying temperature (stretching temperature) during stretching is preferably Tg (°C) or higher, and more preferably (Tg + 10) to (Tg + 50)°C, where Tg is the glass transition temperature of the methacrylic resin. If the stretching temperature is Tg°C or higher, the solvent is easily volatilized appropriately, making it easy to adjust the stretching tension to an appropriate range, and if the temperature is (Tg + 50)°C or lower, the solvent does not volatilize too much, making it difficult for stretchability to be impaired. The stretching temperature can be, for example, 115°C or higher.

The stretching temperature can be measured as either (a) the temperature inside the stretching machine or the hot air temperature, when drying is performed using a non-contact heating method such as a tenter stretching machine, (b) the temperature of the contact heating part, when drying is performed using a contact heating method such as a hot roller, or (c) the surface temperature of the film-like material (surface to be dried). Among these, it is preferable to measure (a) the temperature inside the stretching machine or the hot air temperature, or the like.

It is preferable that the amount of residual solvent in the film-like material at the start of stretching is approximately the same as the amount of residual solvent in the film-like material at the time of peeling, for example, 20 to 30% by mass, and more preferably 25 to 30% by mass.

The film can be stretched in the TD direction (width direction) by, for example, fixing both ends of the film with clips or pins and widening the distance between the clips or pins in the direction of travel (tenter method). The film can be stretched in the MD direction by, for example, applying different peripheral speeds to multiple rolls and utilizing the difference in roll peripheral speeds between them (roll method).

From the viewpoint of further reducing the amount of residual solvent, it is preferable to further dry (post-dry) the film obtained after stretching. For example, it is preferable to further dry the film obtained after stretching while transporting it with a roll or the like.

The drying temperature at this time (the drying temperature without stretching or the drying temperature after stretching) is preferably (Tg-40) to (Tg+30)°C, and more preferably (Tg-30) to Tg°C, where Tg is the glass transition temperature of the methacrylic resin. If the drying temperature is (Tg-40)°C or higher, preferably (Tg-30)°C or higher, the solvent can be easily and sufficiently removed from the film-like material after stretching, and if the drying temperature is (Tg+30)°C or lower, preferably Tg°C or lower, deformation of the film-like material can be highly suppressed. As described above, it is preferable to measure the drying temperature by measuring the ambient temperature, such as (a) the temperature inside the stretching machine or the hot air temperature, as the drying temperature.

In the present invention, the film-like material contains the above-mentioned methacrylic resin. Such a film-like material can form a relatively large space in which the solvent molecules can move when they move while pushing aside the resin chains of the methacrylic resin, making it easier to volatilize and remove the solvent from the film-like material in the drying process, especially in the drying process after stretching. This makes it possible to increase the drying speed compared to conventional methods, or to achieve a drying speed equal to or greater than conventional methods even at a low drying temperature.

The optical film of the present invention is used as an optical component in a display device such as a liquid crystal display device or an organic EL display device. Examples of optical components include polarizing plate protective films (including retardation films and brightness enhancement films), transparent substrates, and light diffusion films. In particular, the optical film of the present invention is preferably used as a polarizing plate protective film.

3. Polarizing Plate The polarizing plate of the present invention has a polarizer, the optical film of the present invention, and an adhesive layer disposed therebetween.

A polarizer is an element that transmits only light polarized in a certain direction, and is a polyvinyl alcohol polarizing film. There are two types of polyvinyl alcohol polarizing film: one dyed with iodine and one dyed with a dichroic dye.

The polyvinyl alcohol polarizing film may be a film obtained by uniaxially stretching a polyvinyl alcohol film and then dyeing it with iodine or a dichroic dye (preferably further treating it with a boron compound for durability); or a film obtained by uniaxially stretching a polyvinyl alcohol film and then dyeing it with iodine or a dichroic dye (preferably further treating it with a boron compound for durability). The absorption axis of the polarizer is usually parallel to the maximum stretching direction.

For example, ethylene-modified polyvinyl alcohol having an ethylene unit content of 1 to 4 mol%, a polymerization degree of 2000 to 4000, and a saponification degree of 99.0 to 99.99 mol%, as described in JP-A-2003-248123 and JP-A-2003-342322, etc., is used.

The thickness of the polarizer is preferably 5 to 30 μm, and more preferably 5 to 20 μm in order to make the polarizing plate thinner.

3-2. Optical Film The optical film of the present invention is disposed on at least one surface of the polarizer (at least the surface facing the liquid crystal cell), and can function as a protective film for the polarizing plate.

When the optical film of the present invention is disposed on only one side of the polarizer, another optical film may be disposed on the other side of the polarizer. Examples of other optical films include commercially available cellulose ester films (e.g., Konica Minolta TAC KC8UX, KC5UX, KC4UX, KC8UCR3, KC4SR, KC4BR, KC4CR, KC4DR, KC4FR, KC4KR, KC8UY, KC6UY, KC4UY, KC4UE, KC8UE, KC8UY-HA, KC2UA, KC4UA , KC6UA, KC8UA, KC2UAH, KC4UAH, KC6UAH, all manufactured by Konica Minolta, Inc., FUJITAC T40UZ, FUJITAC T60UZ, FUJITAC T80UZ, FUJITAC TD80UL, FUJITAC TD60UL, FUJITAC TD40UL, FUJITAC R02, FUJITAC R06, all manufactured by Fuji Film Corporation.

The thickness of the other optical films may be, for example, 5 to 100 μm, preferably 40 to 80 μm.

The adhesive layer is disposed between the optical film (or other optical film) and the polarizer. The thickness of the adhesive layer may be, for example, about 0.01 to 10 μm, and preferably about 0.03 to 5 μm.

3-4. Manufacturing method of polarizing plate The polarizing plate of the present invention can be obtained by bonding a polarizer and the optical film of the present invention via an adhesive. The adhesive can be a fully saponified polyvinyl alcohol aqueous solution (water paste) or an active energy ray curable adhesive. The active energy ray curable adhesive can be any of a photoradical polymerization type composition utilizing photoradical polymerization, a photocationic polymerization type composition utilizing photocationic polymerization, or a combination thereof.

4. Liquid Crystal Display Device The liquid crystal display device of the present invention includes a liquid crystal cell, a first polarizing plate disposed on one surface of the liquid crystal cell, and a second polarizing plate disposed on the other surface of the liquid crystal cell.

The display mode of the liquid crystal cell can be, for example, STN (Super-Twisted Nematic), TN (Twisted Nematic), OCB (Optically Compensated Bend), HAN (Hybridaligned Nematic), VA (Vertical Alignment, MVA (Multi-domain Vertical Alignment), PVA (Patterned Vertical Alignment)), IPS (In-Plane-Switching), etc. Among them, VA (MVA, PVA) mode and IPS mode are preferred.

One or both of the first and second polarizing plates are polarizing plates of the present invention. It is preferable that the polarizing plate of the present invention is arranged so that the optical film of the present invention is on the liquid crystal cell side.

The present invention will be explained in detail below with reference to examples, but the present invention is not limited to these.

1. Materials for Optical Film (1) Preparation of Methacrylic Resins Methacrylic resins 1 to 13 shown in Table 1 were prepared.

The mobile volume, glass transition temperature (Tg) and weight average molecular weight (Mw) of the copolymerized monomers of methacrylic resins 1 to 13 were measured using the following methods.

[Movable volume]
1) First, the structure of the copolymerization monomer was calculated at room temperature by Quench calculation in the NVT ensemble for 500 ps, and energy evaluation was performed every 1 ps. Dreiding was used as the force field. The results of the Quench calculation were then analyzed using a Radius of Gyration analysis module, and the average value of the radius of gyration distribution (average radius of gyration value) was calculated as the "radius of gyration". For the analysis, Materials Studio 2017 R2 Forcite module (Dassault Systèmes Biovia Co., Ltd.) was used as a simulation software. The volume of a hard sphere having a radius equal to the radius of gyration of the obtained copolymerization monomer was calculated as the "hard sphere volume".
2) The hard sphere volume obtained in 1) above and the molecular weight of the copolymer monomer were applied to the following formula (1) to calculate the mobile volume.
Equation (1): Movable volume = volume of hard sphere of copolymerization monomer / molecular weight of copolymerization monomer

[Glass transition temperature (Tg)]
The glass transition temperature of the methacrylic resin was measured in accordance with JIS K 7121-2012 using DSC (Differential Scanning Colorimetry).

[Weight average molecular weight (Mw)]
The weight average molecular weight (Mw) of the methacrylic resin was measured using gel permeation chromatography (HLC8220GPC manufactured by Tosoh Corporation) and a column (TSK-GEL G6000HXL-G5000HXL-G5000HXL-G4000HXL-G3000HXL in series manufactured by Tosoh Corporation). 20±0.5 mg of the sample was dissolved in 10 ml of tetrahydrofuran and filtered with a 0.45 mm filter. 100 ml of this solution was injected into the column (temperature 40°C), measured at a detector RI temperature of 40°C, and the weight average molecular weight was calculated in terms of styrene.

(2) Rubber particles <Rubber particles R1>
Acrylic rubber particles M-210 (core-shell rubber particles with core: acrylic rubber polymer, shell: methacrylic acid ester polymer mainly composed of methyl methacrylate, Tg: about -10°C, average particle size: 220 nm)

<Rubber particles R2>
The following compounds were charged into an 8 L polymerization apparatus equipped with a stirrer.
Deionized water: 175 parts by weight Polyoxyethylene lauryl ether phosphate: 0.104 parts by weight Boric acid: 0.4725 parts by weight Sodium carbonate: 0.04725 parts by weight

After the atmosphere inside the polymerization reactor was thoroughly replaced with nitrogen gas, the internal temperature was raised to 80° C., and 26 mass% of a mixture consisting of 27 parts by mass of monomer mixture (c1) (97 mass% methyl methacrylate, 3 mass% butyl acrylate) and 0.135 parts by mass of allyl methacrylate was added all at once to the polymerization reactor, followed by the addition of 0.0645 parts by mass of sodium formaldehyde sulfoxylate, 0.0056 parts by mass of 2-sodium ethylenediaminetetraacetate, 0.0014 parts by mass of ferrous sulfate, and 0.0207 parts by mass of t-butyl hydroperoxide, and 15 minutes later, 0.0345 parts by mass of t-butyl hydroperoxide was added, and the polymerization was continued for a further 15 minutes.
Next, 0.0098 parts by mass of sodium hydroxide in the form of a 2% by mass aqueous solution and 0.0852 parts by mass of polyoxyethylene lauryl ether phosphate were added as they were, and the remaining 74% by mass of the mixture was continuously added over 60 minutes. 30 minutes after the end of the addition, 0.069 parts by mass of t-butyl hydroperoxide was added, and polymerization was continued for another 30 minutes to obtain a polymer. The polymerization conversion rate was 100.0%.

Then, 0.0267 parts by mass of sodium hydroxide was added in the form of a 2% by mass aqueous solution, and 0.08 parts by mass of potassium persulfate was added in the form of a 2% by mass aqueous solution, and then a mixture consisting of 50 parts by mass of monomer mixture (a1) (butyl acrylate 82% by mass, methyl methacrylate 18% by mass) and 0.75 parts by mass of allyl methacrylate was added continuously over 150 minutes. After the addition was completed, 0.015 parts by mass of potassium persulfate was added in the form of a 2% by mass aqueous solution, and polymerization was continued for 120 minutes to obtain a polymer. The polymerization conversion rate was 99.0%, and the average particle size was 225 nm.

Then, 0.023 parts by mass of potassium persulfate was added in the form of a 2% by mass aqueous solution, and 15 parts by mass of monomer mixture (b1) (95% by mass of methyl methacrylate, 5% by mass of butyl acrylate) was continuously added over 45 minutes, and polymerization was continued for a further 30 minutes.

Then, 8 parts by mass of monomer mixture (b2) (52% by mass of methyl methacrylate, 48% by mass of butyl acrylate) was added continuously over 25 minutes, and polymerization was continued for another 60 minutes to obtain a graft copolymer latex. The polymerization conversion rate was 100.0%.

The obtained latex was salted out with magnesium chloride, coagulated, washed with water, and dried to obtain a white powdery graft copolymer (rubber particles C1). The graft ratio of rubber particles C1 was 24.2%, the glass transition temperature (Tg) was -30°C, and the average particle diameter was 250 nm.

(3) Fine particles <Fine particles A1>
As the fine particles A1, organic fine particles prepared by the following method were used.

(Preparation of seed particles)
A polymerization vessel equipped with a stirrer and a thermometer was charged with 1000 g of deionized water, to which 50 g of methyl methacrylate and 6 g of t-dodecyl mercaptan were then added, and the mixture was heated to 70°C while being purged with nitrogen and stirred. The internal temperature was kept at 70°C, and 20 g of deionized water in which 1 g of potassium persulfate had been dissolved as a polymerization initiator was added, followed by polymerization for 10 hours. The average particle size of the seed particles in the resulting emulsion was 0.05 μm.

(Preparation of organic fine particles)
In a polymerization vessel equipped with a stirrer and a thermometer, 800 g of deionized water in which 2.4 g of sodium lauryl sulfate was dissolved as a gelation inhibitor was placed, and a mixture of 66 g of methyl methacrylate, 20 g of styrene, and 64 g of ethylene glycol dimethacrylate as a monomer mixture, and 1 g of azobisisobutyronitrile as a polymerization initiator was placed therein. The mixture was then stirred with a T.K Homomixer (manufactured by Tokushu Kika Kogyo Co., Ltd.) to obtain a dispersion.

60 g of the emulsion containing the seed particles was added to the resulting dispersion and stirred at 30°C for 1 hour to allow the seed particles to absorb the monomer mixture. The absorbed monomer mixture was then heated at 50°C for 5 hours under a nitrogen stream to polymerize, and then cooled to room temperature (approximately 25°C) to obtain a slurry of polymer fine particles (organic fine particles). The average particle size of the resulting organic fine particles was 0.14 μm, and the glass transition temperature (Tg) was 280°C.

(Preparation of Aggregates of Organic Fine Particles)
This emulsion was spray-dried under the following conditions using a spray dryer manufactured by Sakamoto Giken Co., Ltd. (type: atomizer take-up type, model number: TRS-3WK) to obtain an aggregate of composite 1. The average particle size of the aggregate of polymer particles was 30 μm.
Supply rate: 25 ml/min
Atomizer rotation speed: 11,000 rpm
Air volume: 2 ^m3 /min
Slurry inlet temperature of spray dryer: 100°C
Polymer particle aggregate outlet temperature: 50°C

<Fine particle A2>
As the fine particles A2, inorganic fine particles (Aerosil (registered trademark) R812, manufactured by Nippon Aerosil Co., Ltd.) were used.

The average particle size of rubber particles and microparticles was measured using the following method.

(Average particle size)
The particle size of the fine particles in the obtained dispersion was measured by a zeta potential/particle size measurement system (ELSZ-2000ZS, manufactured by Otsuka Electronics Co., Ltd.). The average particle size of the fine particles measured by using the zeta potential/particle size measurement system (ELSZ-2000ZS, manufactured by Otsuka Electronics Co., Ltd.) is almost the same as the average particle size of the fine particles measured by observing the film with a TEM.

2. Preparation and Evaluation of Optical Films <Preparation of Optical Film 101>
(Preparation of Rubber Particle Dispersion)
20 parts by mass of rubber particles R1 and 380 parts by mass of methylene chloride were mixed and stirred in a dissolver for 50 minutes, and then dispersed at 1500 rpm using a Milder disperser (manufactured by Pacific Machinery Works, Ltd.) to obtain a rubber particle dispersion.

(Preparation of Dope)
Next, a dope having the following composition was prepared. First, methylene chloride and ethanol were added to a pressurized dissolution tank. Next, methacrylic resin 1 was added to the pressurized dissolution tank while stirring. Next, the fine particle dispersion liquid prepared above was added, heated to 60°C, and completely dissolved while stirring. The heating temperature was increased from room temperature at 5°C/min, dissolved for 30 minutes, and then decreased at 3°C/min. The obtained solution was filtered to obtain a dope.
Methacrylic resin 1: 100 parts by weight Methylene chloride: 318 parts by weight Ethanol: 61 parts by weight Rubber particle dispersion: 400 parts by weight

(Film formation)
Next, using an endless belt casting apparatus, the dope was uniformly cast onto a stainless steel belt support having a width of 1800 mm at a temperature of 31° C. The temperature of the stainless steel belt was controlled at 28° C. The conveying speed of the stainless steel belt was 20 m/min.
The solvent was evaporated until the residual solvent content in the cast film was 30% on the stainless steel belt support. Then, the film was peeled off from the stainless steel belt support at a peeling tension of 128 N/m. While the peeled film was transported by multiple rolls, the obtained film-like material was stretched 1.2 times in the width direction by a tenter under the condition of (Tg+10)°C (Tg indicates the Tg of the methacrylic resin). Thereafter, while transporting by rolls, the film was further dried at (Tg-30)°C (Tg indicates the Tg of the methacrylic resin), and the end portion held by the tenter clip was slit by a laser cutter and wound up to obtain an optical film with a thickness of 40 μm.

<Preparation of Optical Films 102 to 103, 107 to 108, 110 to 113, and 115 to 117>
Optical films were produced in the same manner as in Optical Film 101, except that the type of methacrylic resin was changed as shown in Table 2.

<Preparation of Optical Film 104>
(Preparation of Microparticle Dispersion 1)
12 parts by weight of the above fine particles A1 and 388 parts by weight of methylene chloride were mixed and stirred in a dissolver for 50 minutes, and then dispersed at 1500 rpm using a Milder disperser (manufactured by Pacific Machinery Works, Ltd.) to obtain fine particle dispersion 1.

An optical film was obtained in the same manner as for the optical film 102, except that the dope composition was changed as follows.
Methacrylic resin 2: 100 parts by weight Methylene chloride: 296 parts by weight Ethanol: 61 parts by weight Rubber particle dispersion: 400 parts by weight Microparticle dispersion 1: 23 parts by weight

<Preparation of Optical Film 105>
(Preparation of Microparticle Dispersion 2)
11.3 parts by mass of the fine particles A2 and 84 parts by mass of ethanol were mixed and stirred for 50 minutes in a dissolver, and then dispersed with a Manton-Gaulin mixer.
On the other hand, 5 parts by mass of the above obtained solution was slowly added to methylene chloride (100 parts by mass) in a dissolution tank that was being thoroughly stirred. Further, the mixture was dispersed in an attritor so that the secondary particles had a predetermined particle size. The mixture was filtered through Finemet NF manufactured by Nippon Seisen Co., Ltd. to obtain microparticle dispersion 2.

Then, an optical film was obtained in the same manner as optical film 104, except that microparticle dispersion 1 was changed to microparticle dispersion 2 and the amount of microparticle dispersion 2 was adjusted so that the microparticle content in the film was the value shown in Table 2.

<Preparation of Optical Film 106>
Optical films were obtained in the same manner as for Optical Film 102, except that the type of rubber particles was changed as shown in Table 2.

<Preparation of Optical Films 109 and 114>
An optical film was obtained in the same manner as in Optical Film 102, except that the content of the rubber particles was changed as shown in Table 2.

The oxygen permeability, drying coefficient, and MIT flexibility of the obtained optical films 101 to 117 were evaluated by the following methods.

[Oxygen permeability]
The oxygen permeability of the optical film was measured in accordance with JIS K 7126-2 2006. Specifically, a 100 mm square film was prepared as a test piece. The oxygen permeability of this film was measured using a gas permeation tester (OX-TRAN1_50 oxygen permeability tester manufactured by MOCON) in accordance with JIS K 7126-2 2006. The measurement temperature was 23°C.

[Drying coefficient]
1) First, the optical film was cut to a predetermined size to prepare a sample. This sample was dried at 90° C. for 60 minutes, and then the mass was measured and recorded as the “mass before heat treatment.”
2) Next, this sample was heat-treated at 140° C. for 15 minutes, and then the mass was measured and recorded as the "mass after heat treatment."
3) The values obtained in 1) and 2) above were substituted into the following formula to calculate the amount of residual solvent Z after heat treatment.
Residual solvent amount Z (%)=(mass of sample before heat treatment−mass of sample after heat treatment)/(mass of sample after heat treatment)×100
4) Next, the drying coefficient (D) was calculated by applying the obtained residual solvent amount Z (%), initial value Zo (%), and heating time t (min) to the following formula. The initial value Zo (%) was set to 5 (%).
Formula (II): D = (-1 / t) x ln (Z / Zo)

The drying property was evaluated based on the following criteria.
⊚: Drying coefficient (D) is 0.1 or more ◯: Drying coefficient (D) is 0.05 or more and less than 0.1 △: Drying coefficient (D) is 0.02 or more and less than 0.05 ×: Drying coefficient (D) is less than 0.02 The larger the value of D, the better the solvent removal (drying property) during production by the solution casting method. If it was △ or higher, it was judged to be good.

[MIT Flexibility]
The obtained optical film was cut to a width of 15 mm and a length of 150 mm to prepare a test specimen. The test specimen was left to stand for 1 hour or more under conditions of a temperature of 25° C. and a relative humidity of 65% RH. Thereafter, using a folding endurance tester (MIT, BE-201 type, bending curvature radius 0.38 mm, manufactured by Tester Sangyo Co., Ltd.), the number of times the test specimen was folded until it broke was measured under a load of 500 g in accordance with JIS P8115:2001. The MIT bending property of the optical film was then evaluated according to the following criteria.
⊚: 20,000 times or more ◯: 15,000 to 19,999 times Δ: 5,000 to 14,999 times ×: 4,999 times or less The more the number of times of bending until breakage, the more excellent the flexibility and the more excellent the resistance to repeated bending.
If it was △ or better, it was judged to be good.

The evaluation results of optical films 101 to 117 are shown in Table 2.

In addition, when the residual solvent amount of the obtained optical films 101 to 117 was measured by the headspace gas chromatography described above, the residual solvent amount was in the range of 30 to 600 ppm by mass for all of them.

As shown in Table 2, optical films 101 to 111 and 115 to 117 containing a methacrylic resin whose movable volume of the copolymerized monomer is within the range of formula (1) and rubber particles all have a high drying coefficient (drying property) and high flexibility.

In particular, it can be seen that the drying property is improved by setting the movable volume of the copolymerization monomer to 1.2 or more (comparison between optical films 103 and 107). This is thought to be because, when the movable volume of the copolymerization monomer is larger, the space in which the solvent can move, which is formed when the solvent molecules move while pushing aside the resin chains of the methacrylic resin, can be made larger in the resin matrix. In addition, it can be seen that the drying property is improved by setting the movable volume of the copolymerization monomer to 3.2 or less (comparison between films 101 and 108). This is thought to be because, if the movable volume of the copolymerization monomer is not too large, entanglement of the side chains derived from the copolymerization monomer is unlikely to occur, which makes it possible to prevent the space in which the solvent molecules can move, which is formed when the solvent molecules move, from becoming smaller, and the drying property is unlikely to be impaired.

It can also be seen that when the copolymerization ratio of the copolymerizable monomer is approximately 10 to 30 mass %, the drying properties of the resulting optical film are further improved (comparison of optical films 102, 110 and 111).

It can also be seen that even with a small content of rubber particles, the film has good drying and flexibility (comparison between optical films 102 and 109).

It can also be seen that the inclusion of organic microparticles can further improve the flexibility of the resulting optical film (comparison of optical films 102 and 104).

Furthermore, as shown in Table 1, it is suggested that the Tg of the resin is further increased and the heat resistance of the film is improved by using a methacrylic resin that further contains a structural unit derived from a monomer having a ring structure as another copolymerizable monomer, or by using a methacrylic resin with an even higher molecular weight (comparison of optical film 102 with 116 or 117).

In contrast, it can be seen that optical films 112 and 113, which contain a methacrylic resin in which the movable volume of the copolymerization monomer is outside the range of formula (1), both have low drying coefficients (drying properties). In optical film 112, the movable volume of the copolymerization monomer is small, so the space through which the solvent can move is likely to be small; in optical film 113, the movable volume of the copolymerization monomer is too large, so the copolymerization monomer itself becomes entangled, and the space through which the solvent can move is likely to be small, which is thought to be why both have low drying properties.

It can also be seen that the optical film 114 that does not contain rubber particles has poor drying and bending properties. The decrease in bending properties is believed to be due to the optical film becoming more brittle due to the absence of rubber particles; the decrease in drying properties is believed to be due to the absence of rubber particles, which makes it difficult for the resin to move during drying and for the solvent molecules to migrate.

This application claims priority from Japanese Patent Application No. 2019-025927, filed February 15, 2019. The entire contents of said application are incorporated herein by reference.

According to the present invention, it is possible to provide an optical film that can be obtained with high drying efficiency and has sufficient flexibility.

Claims (8)

a methacrylic resin including a structural unit derived from methyl methacrylate and a structural unit derived from a specific copolymerizable monomer copolymerizable therewith, the specific copolymerizable monomer being represented by the following formula (1) and having a mobile volume of 0.5 to 3.6 Å3 ^;
and rubber particles,
The specific copolymerizable monomer includes a (meth)acrylic acid alkyl ester having a linear alkylene moiety having 5 to 15 carbon atoms and no branched structure.
Optical film.
Equation (1): Mobile volume = hard sphere volume of monomer / molecular weight of monomer (In equation (1), the hard sphere volume indicates the volume of a hard sphere having a radius equal to the radius of gyration obtained by analyzing the structure of the monomer). a methacrylic resin including a structural unit derived from methyl methacrylate and a structural unit derived from a specific copolymerizable monomer copolymerizable therewith and represented by the following formula (1) and having a mobile volume of 0.5 to 3.6 Å3 ^;
and rubber particles,
The specific copolymerizable monomer includes an alkyl (meth)acrylate ester having a linear alkylene moiety having 4 to 20 carbon atoms and no branched structure,
The content of the structural unit derived from the specific copolymerization monomer is 10 to 30 mass% based on the total structural units constituting the methacrylic resin.
Optical film.
Equation (1): Mobile volume = hard sphere volume of monomer / molecular weight of monomer (In equation (1), the hard sphere volume indicates the volume of a hard sphere having a radius equal to the radius of gyration obtained by analyzing the structure of the monomer). a methacrylic resin including a structural unit derived from methyl methacrylate and a structural unit derived from a specific copolymerizable monomer copolymerizable therewith and represented by the following formula (1) and having a mobile volume of 0.5 to 3.6 Å3 ^;
and rubber particles,
The specific copolymerizable monomer includes an alkyl (meth)acrylate ester having a linear alkylene moiety having 4 to 20 carbon atoms and no branched structure,
The weight average molecular weight of the methacrylic resin is 600,000 to 3,000,000.
Optical film.
Equation (1): Mobile volume = hard sphere volume of monomer / molecular weight of monomer (In equation (1), the hard sphere volume indicates the volume of a hard sphere having a radius equal to the radius of gyration obtained by analyzing the structure of the monomer). The mobile volume of the specific copolymerization monomer is 1.2 to ^3.2 Å3 ;
The optical film according to any one of claims 1 to 3. The oxygen permeability measured in accordance with JIS K 7126-2 (2006) is 250 to 700 (cc/ ^m2 ·day).
The optical film according to any one of claims 1 to 4. The content of the rubber particles is 5 to 10% by mass relative to the methacrylic resin.
The optical film according to any one of claims 1 to 5. The content of the methacrylic resin is 80% by mass or more based on the optical film.
The optical film according to any one of claims 1 to 6. The optical film according to any one of claims 1 to 7,
Polarizer.