WO2015098759A1 - Acrylic copolymer and method for producing same - Google Patents

Abstract

[Problem] To provide a method for producing an acrylic copolymer, which is capable of reducing the amount of monomers remaining in a polymer that is produced by a suspension polymerization method. [Solution] A method for producing an acrylic copolymer according to the present invention is a method for producing an acrylic copolymer by suspension polymerization from a reaction system which is composed of a dispersion medium and a dispersed phase that contains a monomer mixture containing an alkyl (meth)acrylate monomer, which serves as a first monomer, and a monomer other than alkyl (meth)acrylate monomers, which serves as a second monomer. The dispersed phase contains a saturated hydrocarbon-based solvent that is a good solvent for the monomer mixture but a poor solvent for the acrylic copolymer.

Description

Acrylic copolymer and process for producing the same

The present invention relates to a method for producing an acrylic copolymer, and more specifically, an acrylic copolymer is produced by suspension polymerization from a reaction system comprising a dispersed phase containing a monomer mixture and an additive solvent, and a dispersion medium. It relates to a method of manufacturing. The present invention also relates to an acrylic copolymer obtained by the production method.

A film-like optical member (for example, a film used in a liquid crystal display device or a prism sheet substrate) used in various optical-related devices is generally called an “optical film”. One of the important optical properties of this optical film is birefringence. That is, it may not be preferable that the optical film has a large birefringence. In particular, in an IPS mode liquid crystal display device, the presence of a film having a large birefringence may adversely affect the image quality. Therefore, the protective film for the polarizing plate used in the liquid crystal display device has a birefringence. It is desired to use an optical film with low properties.

Conventionally, polymers for forming such optical films have been produced by methods such as bulk polymerization, suspension polymerization, emulsion polymerization, and solution polymerization. In particular, the suspension polymerization method is a suitable polymerization method because it is safe and easy to operate. However, in the suspension polymerization method, in the latter half of the reaction, the viscosity of the oil phase (system) increases, the monomer diffusion rate decreases, and it is inevitable that about several percent of the monomer remains. If the polymer remains in the polymer, heat resistance decreases due to the plasticizing effect, the polymer is colored, and when the polymer is melted at a high temperature during processing, the monomer volatilizes and significantly adversely affects work safety. Challenges existed.

In order to solve the above technical problem, it has been proposed to add an oil-soluble azo polymerization catalyst as a polymerization initiator and an organic solvent having a boiling point of 75 to 130 ° C. that does not contain a halogen in the suspension polymerization method. (See Patent Document 1). In addition, using a monomer and a radical polymerization initiator in an amount 2 to 10 times the amount required for polymerization, after polymerizing to a polymerization rate of 50 to 99% at a temperature 5 to 40 ° C. lower than the 10-hour half-life temperature of the radical initiator, It has been proposed to further polymerize 99 mol% of the radical polymerization initiator at a temperature ± 5 ° C. at which pyrolysis takes 2-5 hours (see Patent Document 2).

Japanese Patent Laid-Open No. 5-5015 JP 2008-266555 A

However, the inventors have found in Patent Document 1 a technical problem that methyl ethyl ketone or toluene used as an organic solvent in the suspension polymerization method dissolves the produced polymer. Further, in Patent Document 2, since the amount of radical polymerization initiator added is excessive, there is a possibility of gelation or discoloration of the produced polymer, and the amount of heat generated during the polymerization reaction is large and the risk is high. I knew the problem.

The present invention has been made in view of the above technical problems, and its purpose is to produce an acrylic copolymer that can reduce the amount of monomer remaining in the polymer produced by the suspension polymerization method. It is to provide a method.

As a result of intensive studies to solve the above technical problem, the present inventors have found that the above technical problem can be solved by adding a specific saturated hydrocarbon solvent in the suspension polymerization method. . The present invention has been completed based on such findings.

That is, according to one aspect of the present invention,
Suspended from a reaction system comprising a dispersion phase containing a monomer mixture containing a (meth) acrylic acid alkyl monomer as a first monomer and a monomer other than the (meth) acrylic acid alkyl monomer as a second monomer, and a dispersion medium. A method for producing an acrylic copolymer by polymerization,
Production of an acrylic copolymer, wherein the dispersed phase comprises a saturated hydrocarbon solvent that is a good solvent for the monomer mixture and a poor solvent for the acrylic copolymer. A method is provided.

In the embodiment of the present invention, it is preferable that the reactivity ratios r ₁ and r _{2 of} the first monomer and the second monomer satisfy 1 ≦ r ₁ <10 and 0 <r ₂ <1.

In the embodiment of the present invention, the boiling point of the saturated hydrocarbon solvent is preferably 60 to 180 ° C.

In the embodiment of the present invention, the saturated hydrocarbon solvent is preferably a C6 to C10 alkane.

In the embodiment of the present invention, the content of the saturated hydrocarbon solvent in the dispersed phase is preferably 1 to 50% by mass with respect to the total amount of the monomer mixture.

In the embodiment of the present invention, the dispersed phase preferably contains N-substituted maleimide as the second monomer.

In the aspect of the present invention, the dispersed phase further includes a monomer other than the first monomer and the second monomer as the third monomer, and the third monomer is benzyl (meth) acrylate and the following general formula (1) It is preferable that it comprises at least one selected from the group consisting of monomers represented by

[In General Formula (1), R ¹ represents a hydrogen atom or a methyl group, R ² represents an alkanediyl group, and R ³ represents an aryl group. ]

In the embodiment of the present invention, the content of the first monomer in the dispersed phase is preferably 30 to 99% by mass with respect to the total amount of the monomer mixture.

In the embodiment of the present invention, the content of the second monomer in the dispersed phase is preferably 1 to 50% by mass with respect to the total amount of the monomer mixture.

In the embodiment of the present invention, the content of the third monomer in the dispersed phase is preferably 0 to 30% by mass with respect to the total amount of the monomer mixture.

In another aspect of the present invention, there is provided an acrylic copolymer obtained by the above production method, wherein the acrylic monomer has a residual monomer amount of 2.0% by mass or less.

In another aspect of the present invention, there is provided an acrylic copolymer obtained by the above production method, wherein the acrylic copolymer has a glass transition temperature of 110 ° C. or higher.

In another aspect of the present invention, an acrylic copolymer obtained by the above production method, wherein the optical film containing the acrylic copolymer has a longitudinal shrinkage of 1.0% or less. Provided.

In another aspect of the present invention, an acrylic copolymer obtained by the above production method, wherein the optical film containing the acrylic copolymer has a lateral shrinkage of 1.0% or less. Provided.

According to the present invention, there is provided a method for producing an acrylic copolymer that can reduce the amount of monomer remaining in a polymer produced by a suspension polymerization method. Thereby, the hue of the acrylic copolymer obtained by the present invention is suppressed, and the shrinkage rate of the optical film containing the acrylic copolymer is reduced.

<Method for producing acrylic copolymer>
The method for producing an acrylic copolymer according to the present invention comprises a reaction comprising a monomer mixture containing a first monomer and a second monomer, a dispersed phase containing a specific saturated hydrocarbon solvent as an additive solvent, and a dispersion medium. From the system, an acrylic copolymer is produced by suspension polymerization. Unlike suspension polymerization, suspension polymerization does not require a step of removing the organic solvent from the reaction system at a high temperature. Therefore, by producing an acrylic copolymer by suspension polymerization, an acrylic copolymer excellent in hue can be obtained. A polymer can be obtained.

(Dispersed phase)
In the method for producing an acrylic copolymer according to the present invention, the dispersed phase in the reaction system in suspension polymerization includes a monomer mixture and an additive solvent, and the monomer mixture comprises a first monomer and a second monomer. Comprising. Hereinafter, the first monomer, the second monomer, and the additive solvent will be described in detail.

(First monomer)
As the first monomer, an alkyl (meth) acrylate monomer is used. In the present invention, (meth) acrylic acid means acrylic acid or methacrylic acid.

The alkyl group in the (meth) acrylic acid alkyl monomer may be a chain alkyl group or a cyclic alkyl group. The chain alkyl group refers to an alkyl group having no cyclic structure, and the cyclic alkyl group refers to an alkyl group having a cyclic structure.

The number of carbon atoms of the chain alkyl group in the (meth) acrylic acid alkyl monomer is preferably 1 to 6, more preferably 1 to 4, and the number of carbon atoms of the cyclic alkyl group in the (meth) acrylic acid alkyl monomer. Is preferably 5 to 18, more preferably 6 to 16. Examples of such (meth) acrylic acid chain alkyl monomers include methyl (meth) acrylate, ethyl (meth) acrylate, and n-butyl (meth) acrylate, and cyclic alkyl (meth) acrylate. Examples of the monomer include isobornyl (meth) acrylate, dicyclopentanyl (meth) acrylate, ethyl adamantyl (meth) acrylate, methyl adamantyl (meth) acrylate, cyclohexyl (meth) acrylate, and the like. Of these, (meth) acrylic acid chain alkyl monomers are preferred, methyl acrylate and methyl methacrylate are more preferred, and methyl methacrylate is more preferred. As the first monomer, one or more alkyl (meth) acrylate monomers may be used.

The content of the first monomer in the dispersed phase is preferably 30 to 99% by mass, more preferably 40 to 95% by mass, still more preferably 50 to 90% by mass, based on the total amount of the monomer mixture. Even more preferably, it is 65 to 90% by mass. If the content of the first monomer is about the above numerical range, the monomer reacts sufficiently and the amount of monomer remaining in the produced polymer can be further reduced.

(Second monomer)
As the second monomer, a monomer other than the (meth) acrylic acid alkyl monomer is used.

As the second monomer, those in which the reactivity ratios r ₁ and r _{2 of} the first monomer and the second monomer satisfy 1 ≦ r ₁ <10 and 0 <r ₂ <1 are preferably used. The reactivity ratios r ₁ and r ₂ of the first monomer and the second monomer are more preferably 1 <r ₁ <5, 0 <r ₂ <0.8, and 1 <r ₁ <4, 0 <r. More preferably, ₂ <0.5. An example of such reactivity ratios r ₁ and r ₂ is shown in Table 1.

Here, the reactivity ratios r ₁ and r _{2 of} the first monomer and the second monomer can be expressed as follows. First, a first monomer and a second monomer displayed as M ₁ and M _2, respectively, and displays as follows kinetics of M ₁ and M _2.
The reaction rate at which M ₁ binds to the polymer chain at the M ₁ end is expressed as k ₁₁ .
The reaction rate at which M ₂ binds to the polymer chain at the M ₁ end is expressed as k ₁₂ .
The reaction rate at which M ₁ binds to the polymer chain at the M ₂ end is expressed as k ₂₁ .
· _{M 2} end _{M 2} in the polymer chain of the displays the kinetics of binding to _{k 22.}
At this time, r ₁ = k ₁₁ / k ₁₂ and r ₂ = k ₂₂ / k ₂₁ can be shown.

It is preferable to use N-substituted maleimide as the second monomer. Examples of the N-substituted maleimide include N-aromatic substituted maleimide, N-alkyl substituted maleimide, and N-aromatic alkyl substituted maleimide.

N-aromatic substituted maleimide is a compound in which an aromatic group is substituted on the nitrogen atom of maleimide. Here, the aromatic group may be a monocyclic aromatic group or a polycyclic aromatic group.

The number of carbon atoms of the aromatic group in the N-aromatic substituted maleimide is preferably 6-18, more preferably 6-14.

Examples of the aromatic group in the N-aromatic substituted maleimide include a phenyl group, a naphthyl group, an anthryl group, and a phenanthryl group. Among these, a phenyl group and a naphthyl group are preferable, and a phenyl group is more preferable.

That is, examples of the N-aromatic substituted maleimide include N-phenylmaleimide, N-naphthylmaleimide, N-anthrylmaleimide, N-phenanthrylmaleimide and the like, and among these, N-phenylmaleimide and N-naphthylmaleimide N-phenylmaleimide is more preferable. The acrylic copolymer may have one or more N-aromatic substituted maleimides.

N-alkyl-substituted maleimide is a compound in which an alkyl group is substituted on the nitrogen atom of maleimide. Here, the alkyl group may be a chain alkyl group or a cyclic alkyl group, and a cyclic alkyl group is preferred. The chain alkyl group represents an alkyl group having no ring structure, and the cyclic alkyl group represents an alkyl group having an alicyclic structure.

The number of carbon atoms of the alkyl group in the N-alkyl-substituted maleimide is preferably 1 to 10, more preferably 3 to 8.

Examples of the alkyl group in the N-alkyl-substituted maleimide include methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, t-butyl group, n-hexyl group, and 2-ethylhexyl group. , A dodecyl group, a lauryl group, a cyclohexyl group, and the like. Among these, a methyl group, an ethyl group, and a cyclohexyl group are preferable, and a cyclohexyl group is more preferable.

That is, as N-alkyl-substituted maleimide, N-methylmaleimide, N-ethylmaleimide, Nn-propylmaleimide, N-isopropylmaleimide, Nn-butylmaleimide, N-isobutylmaleimide, Nt-butylmaleimide Nn-hexylmaleimide, N-2-ethylhexylmaleimide, N-dodecylmaleimide, N-laurylmaleimide, N-cyclohexylmaleimide and the like, among which N-methylmaleimide, N-ethylmaleimide, N-cyclohexylmaleimide is preferred, and N-cyclohexylmaleimide is more preferred. The N-alkyl-substituted maleimide may be one of these or may contain two or more.

N-aromatic alkyl-substituted maleimide is a compound in which an aromatic alkyl group is substituted on the nitrogen atom of maleimide. Here, the aromatic group may be a monocyclic aromatic group or a polycyclic aromatic group. The alkyl group may be a chain alkyl group or a cyclic alkyl group. As the aromatic alkyl, a benzyl group is preferable. That is, as the N-aromatic alkyl-substituted maleimide, N-benzylmaleimide is preferable.

The content of the second monomer in the dispersed phase is preferably 1 to 50% by mass, more preferably 5 to 40% by mass, and further preferably 10 to 30% by mass with respect to the total amount of the monomer mixture. is there. If the content of the second monomer is about the above numerical range, the monomer reacts sufficiently and the amount of monomer remaining in the produced polymer can be further reduced.

(Third monomer)
The monomer mixture in the dispersed phase may further contain a monomer other than the first monomer and the second monomer as the third monomer. As the third monomer, it is preferable to further use at least one selected from the group consisting of benzyl (meth) acrylate, a monomer represented by the following general formula (1), and styrene.

[In General Formula (1), R ¹ represents a hydrogen atom or a methyl group, R ² represents an alkanediyl group, and R ³ represents an aryl group. ]

As a monomer represented by the general formula (1), R ¹ in the general formula (1) represents a hydrogen atom or a methyl group, preferably a hydrogen atom.

In the general formula (1), R ² represents an alkanediyl group, preferably an alkanediyl group having 1 to 4 carbon atoms, more preferably an alkanediyl group having 1 to 3 carbon atoms. It is. The alkanediyl group may be linear or branched, but is preferably linear (that is, a linear alkanediyl group).

Examples of the alkanediyl group include a methylene group, an ethylene group, a propane-1,3-diyl group, and a butane-1,4-diyl group. Among these, an ethylene group and a methylene group are preferable, and an ethylene group is particularly preferable.

In the general formula (1), R ³ represents an aryl group, preferably an aryl group having 6 to 15 carbon atoms, and more preferably an aryl group having 6 to 10 carbon atoms.

Examples of the aryl group include a phenyl group, a methylphenyl group, a benzyl group, a xylyl group, and a cumenyl group. Among these, a phenyl group, a methylphenyl group, and a benzyl group are preferable, and a phenyl group is particularly preferable.

As the monomer represented by the general formula (1), phenoxyethyl (meth) acrylate is preferable, and phenoxyethyl acrylate is more preferable.

Benzyl (meth) acrylate includes benzyl acrylate and benzyl methacrylate, among which benzyl methacrylate is preferred.

The content of the third monomer in the dispersed phase is preferably 0 to 30% by mass, more preferably 0.1 to 20% by mass, and further preferably 1 to 15% by mass with respect to the total amount of the monomer mixture. %. If the content of the third monomer is about the above numerical range, the monomer reacts sufficiently and the amount of the monomer remaining in the produced polymer can be further reduced.

(Addition solvent)
In the method for producing an acrylic copolymer according to the present invention, a specific saturated hydrocarbon solvent is used as a solvent to be added to the dispersion phase of suspension polymerization. As the saturated hydrocarbon solvent, a solvent that is a good solvent for the monomer mixture and a poor solvent for the acrylic copolymer to be produced is used. By adding such a saturated hydrocarbon solvent to the dispersed phase of suspension polymerization, the amount of monomer remaining in the produced polymer can be reduced.

Here, the saturated hydrocarbon solvent is a good solvent for the monomer mixture. After mixing 100 parts by mass of the saturated hydrocarbon solvent and 100 parts by mass of the monomer mixture, the mixture is at 25 ° C. and 100 rpm for 1 minute. No two-phase separation or the like is observed even when the stirring operation is performed. A saturated hydrocarbon solvent is a poor solvent for an acrylic copolymer when solid precipitation is observed when a 10% solution of the acrylic copolymer is dropped into the saturated hydrocarbon solvent. is there.

The boiling point of the saturated hydrocarbon solvent is preferably 60 to 180 ° C, more preferably 70 to 170 ° C, and further preferably 80 to 160 ° C. If the boiling point of the saturated hydrocarbon solvent is 60 ° C. or higher, volatilization during the polymerization reaction can be suppressed. Moreover, if the boiling point of the saturated hydrocarbon solvent is 180 ° C. or less, it is easy to remove the saturated hydrocarbon solvent in a later step.

The saturated hydrocarbon solvent is preferably at least one selected from the group consisting of C6 to C10 alkanes, more preferably at least one selected from the group consisting of C7 to C9 alkanes. N-heptane and / or n-octane is more preferable. By using such a saturated hydrocarbon solvent as the additive solvent, the amount of monomer remaining in the produced polymer can be further reduced.

The content of the saturated hydrocarbon solvent in the dispersed phase is preferably 1 to 50% by mass, more preferably 3 to 40% by mass, and further preferably 5 to 30% by mass with respect to the total amount of the monomer mixture. %. When the content of the saturated hydrocarbon solvent is 50% by mass or less, it is easy to remove the saturated hydrocarbon solvent in a later step.

(Dispersion medium)
In the method for producing an acrylic copolymer according to the present invention, a conventionally known dispersion medium for suspension polymerization can be used as the dispersion medium in the suspension polymerization reaction system. As a dispersion medium for suspension polymerization, water is usually used.

(Suspension polymerization)
The conditions for suspension polymerization are not particularly limited, and known suspension polymerization conditions can be appropriately applied. Hereinafter, one embodiment of a method for producing an acrylic copolymer by suspension polymerization is shown, but the present invention is not limited to the following example.

First, the first monomer and the second monomer are weighed so that a desired mass ratio is obtained, and the total amount is 100 parts by mass. To 100 parts by mass of the total amount of monomers, 300 parts by mass of deionized water and 0.6 parts by mass of polyvinyl alcohol (Kuraray Co., Ltd., Kuraray Co., Ltd.) as a dispersant are charged into the suspension polymerization apparatus and stirring is started. To do. Subsequently, the weighed monomer, 5 parts by mass of n-heptane as an additive solvent, 1 part by mass of perloyl TCP (manufactured by NOF Corporation) as a polymerization initiator, and 0.22 parts by mass of 1-octanethiol as a chain transfer agent To the suspension polymerization apparatus.

Thereafter, the temperature of the reaction system is raised to 70 ° C. while passing nitrogen through the suspension polymerization apparatus, and then the reaction is carried out by maintaining at 70 ° C. for 3 hours. After the reaction, the mixture is cooled to room temperature, and if necessary, operations such as filtration, washing and drying can be performed to obtain a particulate acrylic copolymer. According to such a method, an acrylic copolymer having a residual monomer amount of 2% by mass or less can be easily obtained.

The types and input amounts of the polymerization initiator, chain transfer agent, and dispersant described above are examples, and the conditions for suspension polymerization are not limited to the above. In the suspension polymerization, the conditions can be appropriately changed within a range in which the residual monomer amount of 2% by mass or less can be achieved. For example, the weight average molecular weight of the acrylic copolymer can be appropriately adjusted by changing the input amount of the chain transfer agent.

As the polymerization initiator, for example, Parroyl TCP, Perocta O, Niper BW, etc. manufactured by Nippon Oil & Fats Co., Ltd. can be used. The amount of the polymerization initiator used may be, for example, 0.05 to 2.0 parts by mass, or 0.1 to 1.5 parts by mass with respect to 100 parts by mass of the total amount of the monomer mixture. Good.

As the chain transfer agent, for example, thiols such as 1-octanethiol, 1-dodecanethiol, and tert-dodecanethiol can be used. Further, the amount of the chain transfer agent used can be appropriately changed according to the desired weight average molecular weight. For example, it can be 0.05 to 0.6 parts by mass with respect to 100 parts by mass of the total amount of the monomer mixture. 0.07 to 0.5 parts by mass.

As the dispersant, for example, PVA such as Kuraray Poval manufactured by Kuraray Co., Ltd., sodium polyacrylate, or the like can be used. The amount of the dispersant used may be, for example, 0.01 to 0.5 parts by mass, or 0.02 to 0.3 parts by mass with respect to 100 parts by mass of the total amount of the monomer mixture. .

The conditions for suspension polymerization can be appropriately adjusted according to the types and amounts of polymerization initiators, chain transfer agents and dispersants. For example, the reaction temperature can be 50 to 95 ° C., preferably 60 to 85 ° C. The reaction time may be sufficient if the reaction proceeds sufficiently. For example, the reaction time may be 2 to 10 hours, and preferably 3 to 8 hours. Since the monomer conversion rate is determined by the lifetime of the reactive species, the reactivity of the monomer, etc., the monomer conversion rate does not necessarily improve even if the reaction time is extended.

<Acrylic copolymer>
The acrylic copolymer according to the present invention is obtained by the above production method, and includes a structural unit obtained from the first monomer and a structural unit obtained from the second monomer.

The acrylic copolymer has a constitutional unit obtained from the second monomer, preferably 30 to 99% by mass, more preferably 40 to 95% by mass, and still more preferably 50 to 85% by mass. The unit preferably comprises 1 to 50% by mass, more preferably 5 to 40% by mass, and still more preferably 10 to 30% by mass. The acrylic copolymer may further contain a constituent unit obtained from the third monomer, preferably 0 to 30% by mass, more preferably 0.1 to 20% by mass, and further preferably 1 to 10% by mass.

The glass transition temperature of the acrylic copolymer is preferably 110 ° C. or higher, more preferably 115 ° C. or higher, and further preferably 120 ° C. or higher. If the glass transition temperature is 110 ° C. or higher, the heat resistance of the film formed using the acrylic copolymer is further improved, and the dimensional stability of the film against heat is further improved. It will be something. Moreover, although there is no restriction | limiting in particular in the upper limit of glass transition temperature, from a viewpoint with which sufficient heat resistance as an optical film is achieved, it may be 140 degrees C or less, and may be 135 degrees C or less. For example, it is preferably 110 ° C. to 140 ° C., more preferably 115 ° C. to 135 ° C.

In the present specification, the glass transition temperature is determined from the onset temperature of the glass transition point when the differential scanning calorimeter DSC7020 manufactured by SII Nanotechnology is used and the temperature is raised at a rate of temperature increase of 10 ° C./min. Indicates the obtained value. The sample mass is 5 mg to 10 mg.

The residual monomer amount of the acrylic copolymer is preferably 2% by mass or less, more preferably 1.5% by mass or less, and further preferably 1% by mass or less. If the amount of residual monomers is 2% or less, deterioration of the hue of the produced copolymer can be suppressed.

<Optical film>
The optical film according to the present invention comprises the acrylic copolymer according to the present invention. The optical film is preferably a biaxially stretched film, and such a film can be obtained by biaxially stretching an unstretched film obtained by forming a resin containing the above acrylic copolymer. The optical film according to the present invention can be suitably used as an optical film because both the orientation birefringence and the photoelastic birefringence are small and the properties such as transparency and heat resistance are excellent. Furthermore, in the present invention, since the amount of monomer remaining in the acrylic polymer produced by the suspension polymerization method is reduced, the hue of the optical film containing the acrylic polymer is suppressed, and the optical film shrinks. The rate is reduced. Hereinafter, various properties of the optical film according to the present invention will be described in detail.

In the resin material constituting the optical film, the content of the acrylic copolymer is preferably 90% by mass or more, more preferably 95% by mass or more, and 99% by mass or more based on the total amount of the resin material. It may be.

The optical film may contain a component other than the acrylic copolymer. That is, when the optical film is obtained by biaxially stretching an unstretched film made of a resin material containing an acrylic copolymer, the resin material contains a component other than the acrylic copolymer. May be.

As components other than the acrylic copolymer, additives used for optical films such as antioxidants, lubricants, ultraviolet absorbers, stabilizers and the like can be used as necessary. The blending amount of these components is not particularly limited as long as the effect of the present invention is effectively exhibited, but it is preferably 10% by mass or less, based on the total amount of the resin material, and is 5% by mass or less. It is more preferable.

The absolute value of the in-plane retardation Re and the absolute value of the thickness direction retardation Rth of the optical film are both preferably 3.5 nm or less, more preferably 3.0 nm or less, and 2.5 nm or less. More preferably, 2.0 nm or less is even more preferable, and 1.0 nm or less is even more preferable. If the absolute value of the in-plane retardation Re and the absolute value of the thickness direction retardation Rth are small, the orientation birefringence becomes small, so that it can be more suitably used as an optical film, particularly a protective film for a polarizing plate.

The absolute value of the photoelastic coefficient C of the optical film is preferably 3.0 × 10 ⁻¹² (/ Pa) or less, more preferably 2.0 × 10 ⁻¹² (/ Pa) or less, and 1.0 × More preferably, it is 10 ⁻¹² (/ Pa) or less. When the absolute value of the photoelastic coefficient C is small, the photoelastic birefringence becomes small, so that it can be more suitably used as an optical film, particularly as a protective film for a polarizing plate.

The orientation birefringence of the optical film can be evaluated by measuring retardation (Re) which is an in-plane retardation value of the film and Rth which is a thickness direction retardation value using an Axoscan apparatus manufactured by Axometrics.

Re (unit: nm) is expressed by the following formula (1), where nx is the refractive index in one direction in the film plane, ny is the refractive index in the direction perpendicular thereto, and d (nm) is the thickness of the film. Is done.
Re = (nx−ny) × d (1)

Rth (unit: nm) is nx as the refractive index in one direction in the film plane, ny as the refractive index in the direction perpendicular thereto, nz as the refractive index in the thickness direction of the film, and d (nm) as the thickness of the film. Is represented by the following mathematical formula (2).
Rth = ((nx + ny) / 2−nz) × d (2)

The sign of the retardation value of the film is positive when the refractive index is large in the orientation direction of the polymer main chain, and negative when the refractive index is large in the direction perpendicular to the stretching direction.

The photoelastic birefringence of the optical film is measured by measuring the amount of change due to the stress applied to the film of retardation (Re), which is the retardation value of the film, with an Axoscan apparatus manufactured by Axometrics, as with the orientation birefringence. (Unit: 10 ⁻¹² / Pa). The specific calculation method of the photoelastic coefficient C is as the following mathematical formula (3).
C = ΔRe / (Δσ × t) (3)

Δσ is the amount of change in stress applied to the film in units of [Pa], t is the film thickness in units of [m], and ΔRe is the amount of change in the in-plane retardation corresponding to the amount of change in stress of Δσ. The unit is [m]. The sign of the photoelastic coefficient C is positive when the refractive index increases in the stressed direction, and negative when the refractive index increases in the direction perpendicular to the stressed direction.

In the optical film, the number of MIT folding resistances measured in accordance with JIS P8115 is preferably 100 times or more, more preferably 120 times or more, and further preferably 150 times or more. Since such an optical film sufficiently satisfies the flexibility required as a protective film for polarizing plates, it can be more suitably used as a protective film for polarizing plates. Moreover, since such an optical film is excellent in bending resistance, it can be used more suitably for applications that require a large area. Furthermore, if the MIT folding endurance number is 100 times or more, the optical film after the stretching process is prevented from being broken in the process of transporting and winding, or being bonded to a polarizing plate or the like. be able to.

In the present specification, the MIT folding resistance test can be performed using a BE-201 MIT bending resistance tester manufactured by Tester Sangyo Co., Ltd. The BE-201 MIT bending resistance tester manufactured by Tester Sangyo Co., Ltd. is also called an MIT folding resistance tester. The measurement conditions are a load of 200 g, a bending point tip R of 0.38, a bending speed of 175 times / minute, a bending angle of 135 ° on the left and right, and a width of the film sample of 15 mm. The average value of the number of bendings that are broken when the optical film is repeatedly bent in the conveyance direction and the number of bendings that are broken when the optical film is repeatedly bent in the width direction is defined as the MIT folding resistance number.

In addition, as a test method for heat shock resistance of a protective film for polarizing plate, heat shock is repeated by laminating a film on a glass substrate through a paste, raising the temperature in the range of −20 ° C. to 60 ° C., and lowering the temperature for 500 cycles at 30 minute intervals. Although the test is known, if the above-mentioned MIT folding resistance number is 100 times or more, the film can be prevented from cracking during the heat shock test.

The film thickness of the optical film can be 10 μm or more and 150 μm or less, and can be 15 μm or more and 120 μm or less. When the film thickness is 10 μm or more, the handleability of the film is improved, and when it is 150 μm or less, problems such as an increase in haze and an increase in material cost per unit area are less likely to occur.

The optical film according to the present invention may be a film obtained by stretching an unstretched film made of a resin material containing the acrylic copolymer according to the present invention in at least one direction, and a film obtained by stretching in two directions. (Biaxially stretched film) is preferable. For example, the draw ratio can be adjusted as appropriate so that the above-mentioned number of MIT folding resistances can be achieved. The area ratio can be 1.3 times or more, and can also be 1.5 times or more. Moreover, the draw ratio may be 6.0 times or less in area ratio, and may be 4.0 times or less.

The b ^* value, which is an index of yellowness of the optical film, is preferably 1.00 or less, more preferably 0.50 or less, and even more preferably 0.30 or less. In addition, b ^* value which is a yellowish parameter | index can be calculated | required by measuring the spectral spectrum of an optical film using Nippon Denshoku Industries Co., Ltd. Spectrophotometer SD6000.

The optical film according to the present invention has excellent light resistance. Light resistance can be evaluated by the amount of change in film property values before and after light irradiation. As a film physical property value, b ^* value which is a yellowish index, in-plane retardation Re, thickness direction retardation Rth, photoelastic coefficient C, MIT folding resistance frequency, and the like are used. For example, using an xenon weather meter [Toyo Seiki Seisakusho Atlas Ci4000], the optical film is irradiated with light, and the light resistance can be evaluated as follows.

Light fastness, the ^{b *} value after light irradiation before the light irradiation ^{b *} value ^{(b * 1)} subtracted from the value ^{Δb * (= b * 1 -b} *), in-plane retardation Re before and after light irradiation Subtraction value ΔRe (= Re before light irradiation−Re after light irradiation), Subtraction value ΔRth of thickness direction retardation Rth before and after light irradiation (= Rth before light irradiation−Rth after light irradiation), Photoelastic coefficient C before and after light irradiation The subtraction value ΔC (= C before light irradiation−C after light irradiation) and the subtraction value ΔMIT (= MIT before light irradiation−MIT after light irradiation) of the number of MIT folding resistances before and after light irradiation can be evaluated.

The shrinkage ratio of the optical film according to the present invention can be calculated as follows. First, an unstretched film (raw film) obtained in the melt extrusion step described below is stretched twice in the longitudinal direction and then stretched twice in the transverse direction to obtain a biaxially stretched film of 15 cm × 15 cm. The length in the machine direction and the transverse direction of the obtained biaxially stretched film is measured with a vernier caliper capable of measuring up to 100 μm effective figures, and put into a 100 ° C. thermostat. The length in the vertical direction obtained at this time is _LMD1 , and the length in the horizontal direction is _LTD1 . The film which has passed 240 hours after being put into the thermostatic chamber is allowed to cool to room temperature, and the lengths in the vertical and horizontal directions are measured. The length in the vertical direction obtained at this time is _LMD2 , and the length in the horizontal direction is _LTD2 . Using these, the shrinkage rate is expressed by the following mathematical formulas (5) and (6):
Longitudinal shrinkage rate (%) = (L _MD 1−L _MD 2) ÷ L _MD 1 × 100 (5)
Lateral shrinkage (%) = (L _TD 1−L _TD 2) ÷ L _TD 1 × 100 (6)
Can be calculated.

The shrinkage rate of the optical film according to the present invention is preferably 1.0% or less, and more preferably 0.7% or less, in both the vertical direction shrinkage rate and the horizontal direction shrinkage rate. When the shrinkage rate is 1.0% or less, peeling or cracking due to shrinkage can be prevented when the optical film is used as a member of an optical display by being laminated with another film, a glass substrate or the like.

<Method for producing optical film>
One embodiment of the method for producing an optical film according to the present invention will be described in detail.

As described above, the optical film according to the present invention can be obtained from the resin material containing the acrylic copolymer according to the present invention. Furthermore, it is preferable to obtain an unstretched film made of the resin material by biaxial stretching. That is, the method for producing an optical film according to the present invention includes a step (melt-extrusion step) for obtaining an unstretched film by melt-extruding a resin material comprising the acrylic copolymer according to the present invention. It is preferable to further include a step (stretching step) of biaxially stretching the film to obtain an optical film. *

The melt extrusion process can be performed by, for example, an extrusion film forming machine including a die lip. At this time, the resin material is heated and melted in an extrusion film forming machine and continuously discharged from a die lip, whereby a film-like unstretched film can be obtained.

The extrusion temperature during melt extrusion is preferably 130 ° C. or higher and 300 ° C. or lower, and more preferably 150 ° C. or higher and 280 ° C. or lower. When the extrusion temperature is 130 ° C. or higher, the acrylic copolymer in the resin material is sufficiently melted and kneaded, so that the unmelted product is sufficiently prevented from remaining in the film. Further, when the temperature is 300 ° C. or lower, problems such as coloring of the film due to thermal decomposition and adhesion of the decomposition product to the die lip are sufficiently prevented. Therefore, if the extrusion temperature of the melt extrusion is within the above range, the acrylic copolymer in the resin material is sufficiently melted and kneaded, so that it is possible to sufficiently prevent the unmelted product from remaining in the film. At the same time, it is possible to suppress film coloring due to thermal decomposition, adhesion of decomposition products to the die lip, and the like.

In the melt film-forming method using the T-die extrusion device, the temperature T ₁ ° C of the first roll with which the molten resin discharged from the T-die lip first comes into contact when the glass transition temperature of the molten resin is Tg ° C. The range of Tg−24) ≦ T ₁ ≦ (Tg + 24) is preferable, and the range of (Tg−20) ≦ T ₁ ≦ (Tg + 20) is more preferable. If the temperature of T ₁ is (Tg−24) ° C. or higher, the molten resin film discharged from the T die lip can be prevented from being rapidly cooled, so the thickness accuracy of the film formed due to shrinkage unevenness deteriorates. This can be suppressed. If the temperature of T ₁ is (Tg + 24) ℃ or less, the molten resin discharged from the T die lip can be suppressed that would stick to the first roller.

In addition, the film thickness unevenness (unit:%) is the maximum value of the thickness measured by measuring 20 roll samples at equal intervals in the width direction after cutting 10 mm each of the ears at both ends of the unstretched film (raw film) at t _1. When μm, the minimum value is t ₂ μm, and the average value is t ₃ μm, the following formula (4):
Thickness variation (%) = 100 × (t ₁ −t ₂ ) / t ₃ (4)
It means the value calculated from

In the stretching process, the unstretched film (raw film) obtained in the melt extrusion process is stretched to obtain an optical film. As the stretching method, a conventionally known uniaxial stretching method or biaxial stretching method can be appropriately selected. As the biaxial stretching device, for example, in the tenter stretching device, a simultaneous biaxial stretching device in which the clip interval for gripping the film end portion also extends in the film transport direction can be used. Further, in the stretching step, a sequential biaxial stretching method in which stretching between rolls utilizing a peripheral speed difference, stretching by a tenter device, or the like is combined can also be applied.

The stretching device may be an integrated line with the extrusion film forming machine. Further, the stretching step may be performed by a method in which a raw film wound up by an extrusion film forming machine is sent off-line to a stretching apparatus and stretched.

The stretching temperature is preferably (Tg + 2) ° C. or more and (Tg + 20) ° C. or less, more preferably (Tg + 5) ° C. or more and (Tg + 15) ° C. or less when the glass transition temperature of the raw film is Tg ° C. When the stretching temperature is (Tg + 2) ° C. or higher, problems such as breakage of the film during stretching and an increase in haze of the film can be sufficiently prevented. Further, when it is (Tg + 20) ° C. or lower, the polymer main chain is easily oriented, and a better degree of polymer main chain orientation tends to be obtained.

A film made of a polymer material having a low birefringence while the polymer main chain is oriented to improve the bending resistance of the film by stretching the raw film formed by the melt film formation method. Otherwise, the retardation value of the film increases, and the image quality deteriorates when incorporated in a liquid crystal display device. In this embodiment, an optical film having both excellent optical properties and flex resistance can be obtained by using the resin material described above.

As described above, when the method for producing an optical film according to the present invention is used, an optical film having both small orientation birefringence and photoelastic birefringence and excellent properties such as transparency and heat resistance can be obtained.

<Polarizing plate>
The polarizing plate according to the present invention comprises the optical film as a protective film on at least one surface of the polarizing film. Since the optical film has both small orientation birefringence and photoelastic birefringence, according to the polarizing plate according to the present invention having the optical film as a protective film, image quality deteriorates due to the protective film when applied to a liquid crystal display device. Can be sufficiently suppressed.

In the polarizing plate according to the present invention, the constituent elements other than the optical film are not particularly limited, and may have the same configuration as a known polarizing plate. For example, the polarizing plate according to the present invention may be obtained by changing at least a part of a protective film in a known polarizing plate to the optical film. For example, the polarizing plate may have a configuration in which the optical film, the polarizing layer, the polarizing layer protective film, and the adhesive layer are laminated in this order.

<Liquid crystal display device>
The liquid crystal display device by this invention is equipped with the said polarizing plate. Since the polarizing plate according to the present invention includes the optical film as a protective film, it is possible to sufficiently suppress deterioration in image quality due to the optical characteristics of the protective film. Therefore, according to the liquid crystal display device of the present embodiment, good image quality is realized.

In the liquid crystal display device according to the present invention, the components other than the polarizing plate are not particularly limited, and can be configured in the same manner as a known liquid crystal display device. For example, in the liquid crystal display device according to the present invention, the polarizing plate in the known liquid crystal display device may be changed to the polarizing plate.

The liquid crystal display device may have, for example, a configuration in which the polarizing plate, the backlight, the color filter, the liquid crystal layer, the transparent electrode, and the glass substrate are laminated in this order.

Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples, but the present invention is not construed as being limited to the following examples.

<Evaluation method of acrylic copolymer>
In the evaluation of the acrylic copolymer below, the residual monomer amount and glass transition temperature (Tg) of the acrylic copolymer were measured as follows.

The glass transition temperature of the acrylic copolymer is obtained from the onset temperature of the glass transition point when the differential scanning calorimeter DSC7020 manufactured by SII Nanotechnology is used and the temperature is raised at a rate of temperature increase of 10 ° C./min. It was. The mass of the acrylic copolymer sample was 5 mg to 10 mg.

The residual monomer amount of the acrylic copolymer was measured by the following apparatus and method.
(apparatus)
Gas chromatography device: GC 6850 manufactured by Agilent Technologies
Column: HP-5 30m
Oven temperature condition: held at 50 ° C. for 5 minutes, then heated to 250 ° C. at 10 ° C./minute, and held for 10 minutes.
Injection volume: 0.5 μl
Mode: Split method Split ratio: 80/1
Carrier: Pure nitrogen detector: FID

(Method)
About 1 g of acrylic copolymer particles were precisely weighed, about 10 ml of acetone was added and stirred, and the particles were completely dissolved to obtain an acetone solution. About 90 ml of methanol was weighed into a 100 ml container containing a stirrer, and the acetone solution was added dropwise to precipitate a polymer to obtain a slurry solution. Next, about 0.1 ml of chlorobenzene was precisely weighed as an internal standard substance, added to the slurry, and mixed well by shaking vigorously. This solution was allowed to stand, and each monomer was detected by GC (gas chromatography) using a filtrate obtained by filtering about 1.5 ml of the supernatant. The retention time and area / mass conversion factor of each component were as shown in Table 2 below.

The GC area value of each monomer was multiplied by an area / mass conversion factor, and the mass of each monomer was calculated by the following proportional expression.
Formula: Internal standard substance mass: Mass of each monomer = (Internal standard substance GC area value × Area / mass conversion coefficient): (Each monomer GC area value × Area / mass conversion coefficient)
By the above method, the residual mass of each monomer in the precisely weighed acrylic copolymer particles was determined, and the total amount was divided by the mass of the accurately weighed acrylic copolymer particles to obtain the residual monomer amount (% ) Was calculated.

<Synthesis of acrylic copolymer>
[Example 1]
Into a reaction kettle equipped with a stirrer, temperature sensor, cooling pipe and nitrogen introduction pipe, 300 parts by mass of deionized water and 0.6 parts by mass of polyvinyl alcohol (Kuraray Co., Ltd., Kuraray Co., Ltd.) are added as a dispersant. And stirring was started. Next, 80 parts by mass of methyl methacrylate (hereinafter, sometimes referred to as “MMA”) as monomers and 20 parts by mass of N-cyclohexylmaleimide (hereinafter, sometimes referred to as “CHMI”), n-heptane as an additive solvent 5 parts by mass, 1 part by mass of Perocta O (manufactured by NOF Corporation) as a polymerization initiator, and 0.22 parts by mass of 1-octanethiol as a chain transfer agent were added, and the temperature was raised to 70 ° C. while passing nitrogen through the reaction kettle I let you. After maintaining the state which reached 70 degreeC for 2 hours, it cooled, and the particulate acrylic copolymer was obtained by filtration, washing | cleaning, and drying. The obtained acrylic copolymer was colorless in visual inspection.

[Example 2]
The monomer was changed to 70 parts by weight of methyl methacrylate (MMA), 20 parts by weight of N-cyclohexylmaleimide (CHMI), and 10 parts by weight of N-phenylmaleimide (hereinafter sometimes referred to as “PhMI”) and added. Suspension polymerization was performed in the same manner as in Example 1 except that the solvent was changed to 10 parts by mass of n-heptane to obtain an acrylic copolymer. The obtained acrylic copolymer was colorless in visual inspection.

[Example 3]
The monomer was changed to 80 parts by mass of methyl methacrylate (MMA), 10 parts by mass of N-cyclohexylmaleimide (CHMI), and 10 parts by mass of N-phenylmaleimide (PhMI), and the additive solvent was changed to 30 parts by mass of n-heptane. Suspension polymerization was performed in the same manner as in Example 1 except for changing to obtain an acrylic copolymer. The obtained acrylic copolymer was colorless in visual inspection.

[Example 4]
Suspension polymerization was performed in the same manner as in Example 1 except that the additive solvent was changed to 5 parts by mass of n-octane to obtain an acrylic copolymer. The obtained acrylic copolymer was colorless in visual inspection.

[Example 5]
The monomer was changed to 70 parts by weight of methyl methacrylate (MMA), 20 parts by weight of N-cyclohexylmaleimide (CHMI), and 10 parts by weight of N-phenylmaleimide (PhMI), and the additive solvent was changed to 10 parts by weight of n-octane. Suspension polymerization was performed in the same manner as in Example 1 except for changing to obtain an acrylic copolymer. The obtained acrylic copolymer was colorless in visual inspection.

[Example 6]
Suspension polymerization was performed in the same manner as in Example 1 except that the additive solvent was changed to 30 parts by mass of n-octane to obtain an acrylic copolymer. The obtained acrylic copolymer was colorless in visual inspection.

[Example 7]
The monomer was changed to 81 parts by mass of methyl methacrylate (MMA), 15 parts by mass of N-cyclohexylmaleimide (CHMI), and 4 parts by mass of benzyl methacrylate (hereinafter, sometimes referred to as “BnMA”), and an additive solvent Was changed to 10 parts by mass of n-heptane, and suspension polymerization was performed in the same manner as in Example 1 to obtain an acrylic copolymer. The obtained acrylic copolymer was colorless in visual inspection.

[Example 8]
The monomer was changed to 82 parts by weight of methyl methacrylate (MMA), 15 parts by weight of N-cyclohexylmaleimide (CHMI), and 3 parts by weight of phenoxyethyl acrylate (hereinafter sometimes referred to as “PhOEA”) and added. Suspension polymerization was performed in the same manner as in Example 1 except that the solvent was changed to 10 parts by mass of n-heptane to obtain an acrylic copolymer. The obtained acrylic copolymer was colorless in visual inspection.

[Example 9]
The monomer was changed to 65 parts by weight of methyl methacrylate (MMA), 20 parts by weight of N-cyclohexylmaleimide (CHMI), and 15 parts by weight of styrene (hereinafter sometimes referred to as “St”), and the additive solvent was changed to n -Suspension polymerization was carried out in the same manner as in Example 1 except that the amount was changed to 10 parts by mass of heptane to obtain an acrylic copolymer. The obtained acrylic copolymer was colorless in visual inspection.

[Example 10]
The monomer was changed to 81 parts by weight of methyl methacrylate (MMA), 12 parts by weight of N-cyclohexylmaleimide (CHMI), 2 parts by weight of N-phenylmaleimide (PhMI), and 5 parts by weight of benzyl methacrylate (BnMA), and Suspension polymerization was performed in the same manner as in Example 1 except that the additive solvent was changed to 10 parts by mass of n-heptane to obtain an acrylic copolymer. The obtained acrylic copolymer was colorless in visual inspection.

[Example 11]
The monomer was changed to 80 parts by mass of methyl methacrylate (MMA), 13 parts by mass of N-cyclohexylmaleimide (CHMI), and 7 parts by mass of N-phenylmaleimide (PhMI), and the additive solvent was changed to 15 parts by mass of n-heptane. Suspension polymerization was performed in the same manner as in Example 1 except for changing to obtain an acrylic copolymer. The obtained acrylic copolymer was colorless in visual inspection.

[Example 12]
The monomer was changed to 76 parts by weight of methyl methacrylate (MMA), 23 parts by weight of N-cyclohexylmaleimide (CHMI), and 1 part by weight of styrene (St), and the additive solvent was changed to 15 parts by weight of n-heptane. Was suspension-polymerized in the same manner as in Example 1 to obtain an acrylic copolymer. The obtained acrylic copolymer was colorless in visual inspection.

[Example 13]
The monomer was changed to 80 parts by mass of methyl methacrylate (MMA) and 20 parts by mass of N-cyclohexylmaleimide (CHMI), and the additive solvent was changed to 10 parts by mass of n-heptane and 10 parts by mass of n-octane. Suspension polymerization was performed in the same manner as in Example 1 to obtain an acrylic copolymer. The obtained acrylic copolymer was colorless in visual inspection.

[Example 14]
The monomer was changed to 90 parts by weight of methyl methacrylate (MMA) and 10 parts by weight of N-benzylmaleimide (hereinafter referred to as “BnMI”), and the additive solvent was changed to 10 parts by weight of n-heptane. Suspension polymerization was performed in the same manner as in Example 1 to obtain an acrylic copolymer. The obtained acrylic copolymer was colorless in visual inspection.

[Comparative Example 1]
Example except that the monomer was changed to 80 parts by weight of methyl methacrylate (MMA), 10 parts by weight of N-cyclohexylmaleimide (CHMI), and 10 parts by weight of N-phenylmaleimide (PhMI), and no n-heptane was added. Suspension polymerization was performed in the same manner as in Example 1 to obtain an acrylic copolymer. The obtained acrylic copolymer was light yellow in visual inspection.

[Comparative Example 2]
Suspension polymerization in the same manner as in Comparative Example 1 except that the monomer was changed to 70 parts by weight of methyl methacrylate (MMA), 20 parts by weight of N-cyclohexylmaleimide (CHMI), and 10 parts by weight of N-phenylmaleimide (PhMI). And an acrylic copolymer was obtained. The obtained acrylic copolymer was yellow in visual inspection.

[Comparative Example 3]
Suspension polymerization was performed in the same manner as in Comparative Example 1 except that the monomers were changed to 90 parts by mass of methyl methacrylate (MMA) and 10 parts by mass of N-cyclohexylmaleimide (CHMI) to obtain an acrylic copolymer. The obtained acrylic copolymer was light yellow in visual inspection.

[Comparative Example 4]
Suspension polymerization was performed in the same manner as in Comparative Example 1 except that the monomers were changed to 80 parts by mass of methyl methacrylate (MMA) and 20 parts by mass of N-phenylmaleimide (PhMI) to obtain an acrylic copolymer. The obtained acrylic copolymer was yellow in visual inspection.

[Comparative Example 5]
Suspension polymerization was carried out in the same manner as in Comparative Example 1 except that the monomers were changed to 81 parts by mass of methyl methacrylate (MMA), 15 parts by mass of N-cyclohexylmaleimide (CHMI), and 4 parts by mass of benzyl methacrylate (BnMA). And an acrylic copolymer was obtained. The obtained acrylic copolymer was light yellow in visual inspection.

[Comparative Example 6]
Comparison except that the monomers were changed to 82 parts by mass of methyl methacrylate (MMA), 15 parts by mass of N-cyclohexylmaleimide (CHMI), and 3 parts by mass of phenoxyethyl acrylate (hereinafter sometimes referred to as “PhOEA”). Suspension polymerization was performed in the same manner as in Example 1 to obtain an acrylic copolymer. The obtained acrylic copolymer was light yellow in visual inspection.

[Comparative Example 7]
Comparative Example 1 except that the monomers were changed to 65 parts by weight of methyl methacrylate (MMA), 20 parts by weight of N-cyclohexylmaleimide (CHMI), and 15 parts by weight of styrene (hereinafter, sometimes referred to as “St”). Similarly, suspension polymerization was performed to obtain an acrylic copolymer. The obtained acrylic copolymer was yellow in visual inspection.

[Comparative Example 8]
The monomer was changed to 81 parts by mass of methyl methacrylate (MMA), 12 parts by mass of N-cyclohexylmaleimide (CHMI), 2 parts by mass of N-phenylmaleimide (PhMI), and 5 parts by mass of benzyl methacrylate (BnMA). Suspension polymerization was performed in the same manner as in Comparative Example 1 to obtain an acrylic copolymer. The obtained acrylic copolymer was light yellow in visual inspection.

[Comparative Example 9]
Suspension polymerization was carried out in the same manner as in Comparative Example 1 except that the monomer was changed to 80 parts by mass of methyl methacrylate (MMA) and 20 parts by mass of N-phenylmaleimide (PhMI) and 10 mass of toluene was added as an additive solvent. And an acrylic copolymer was obtained. The obtained acrylic copolymer contains toluene and has jelly-like elasticity, and aggregation of the particles was observed in the drying step. Further, the acrylic copolymer was light yellow in visual inspection.

The measurement results of the residual monomer amount and glass transition temperature (Tg) of the acrylic copolymer obtained as described above were as shown in Table 3 below.

<Manufacture of optical film>
The acrylic copolymers obtained in the above Examples and Comparative Examples were melt-extruded using a twin screw extruder KZW-30MG manufactured by Technobel to form an unstretched film. The screw diameter of the biaxial extruder is 15 mm and the effective screw length (L / D) is 30, and a hanger coat type T-die is installed in the extruder via an adapter. The extrusion temperature Tp ° C. is the temperature calculated by the equation (7) since the equation (7) is optimal in the case of an amorphous polymer having a glass transition temperature of Tg ° C.
Tp = 5 (Tg + 70) / 4 (7)

The temperature T ₁ of the first roll with which the molten resin discharged from the T die lip first contacts is in the range of (Tg−24) ≦ T ₁ ≦ (Tg + 24), where Tg is the glass transition temperature of the molten resin. Therefore, the temperature T ₁ of the first roll was set to 130 ° C.

<Evaluation method of optical film>
The unstretched film (raw film) obtained in the melt extrusion step was stretched twice in the machine direction and then stretched twice in the transverse direction to obtain a 15 cm × 15 cm biaxially stretched film. The length in the machine direction and the transverse direction of the obtained biaxially stretched film was measured with a vernier caliper capable of measuring up to 100 μm effective figures, and placed in a thermostat at 100 ° C. The longitudinal length obtained at this time was L _{MD 1,} the lateral length and L _{TD 1.} The film which passed 240 hours after putting into the thermostat was allowed to cool to room temperature, and the lengths in the vertical and horizontal directions were measured. The longitudinal length obtained at this time was L _{MD 2,} of the lateral length and L _{TD 2.} Using these, the shrinkage rate is expressed by the following formulas (5) and (6)
Longitudinal shrinkage rate (%) = (L _MD 1−L _MD 2) ÷ L _MD 1 × 100 (5)
Lateral shrinkage (%) = (L _TD 1−L _TD 2) ÷ L _TD 1 × 100 (6)
Calculated by The shrinkage rate of the optical film thus obtained was as shown in Table 3 below.

Claims (14)

1. Suspended from a reaction system comprising a dispersion phase containing a monomer mixture containing a (meth) acrylic acid alkyl monomer as a first monomer and a monomer other than the (meth) acrylic acid alkyl monomer as a second monomer, and a dispersion medium. A method for producing an acrylic copolymer by polymerization,
   Production of an acrylic copolymer, wherein the dispersed phase comprises a saturated hydrocarbon solvent that is a good solvent for the monomer mixture and a poor solvent for the acrylic copolymer. Method.
2. The method for producing an acrylic copolymer according to claim 1, wherein the reactivity ratios r ₁ and r _{2 of} the first monomer and the second monomer satisfy 1 ≦ r ₁ <10, 0 <r ₂ <1.
3. The method for producing an acrylic copolymer according to claim 1 or 2, wherein the boiling point of the saturated hydrocarbon solvent is 60 to 180 ° C.
4. The method for producing an acrylic copolymer according to any one of claims 1 to 3, wherein the saturated hydrocarbon solvent is at least one selected from the group consisting of C6 to C10 alkanes.
5. The acrylic copolymer according to any one of claims 1 to 4, wherein a content of the saturated hydrocarbon solvent in the dispersed phase is 1 to 50 mass% with respect to a total amount of the monomer mixture. Manufacturing method of coalescence.
6. The method for producing an acrylic copolymer according to any one of claims 1 to 5, wherein the dispersed phase comprises N-substituted maleimide as the second monomer.
7. The dispersed phase further includes a monomer other than the first monomer and the second monomer as the third monomer, and the third monomer is composed of benzyl methacrylate, a monomer represented by the following general formula (1), and styrene. The method for producing an acrylic copolymer according to any one of claims 1 to 6, comprising at least one selected from the group consisting of:
   

   

   [In General Formula (1), R ¹ represents a hydrogen atom or a methyl group, R ² represents an alkanediyl group, and R ³ represents an aryl group. ]
8. The method for producing an acrylic copolymer according to any one of claims 1 to 7, wherein the content of the first monomer in the dispersed phase is 30 to 99 mass% with respect to the total amount of the monomer mixture.
9. The method for producing an acrylic copolymer according to any one of claims 1 to 8, wherein the content of the second monomer in the dispersed phase is 1 to 50 mass% with respect to the total amount of the monomer mixture.
10. 10. The method for producing an acrylic copolymer according to claim 1, wherein the content of the third monomer in the dispersed phase is 0 to 30% by mass with respect to the total amount of the monomer mixture.
11. The acrylic copolymer obtained by the production method according to any one of claims 1 to 10, wherein the residual monomer amount of the acrylic copolymer is 2.0% by mass or less. Polymer.
12. 11. An acrylic copolymer obtained by the production method according to any one of claims 1 to 10, wherein the acrylic copolymer has a glass transition temperature of 110 ° C. or higher.
13. The acrylic copolymer obtained by the production method according to any one of claims 1 to 10, wherein an optical film containing the acrylic copolymer has a longitudinal shrinkage of 1.0% or less. , Acrylic copolymer.
14. An acrylic copolymer obtained by the production method according to any one of claims 1 to 10, wherein the optical film containing the acrylic copolymer has a lateral shrinkage of 1.0% or less. , Acrylic copolymer.