JP2014111747A - Acrylic copolymer, optical film, polarizing plate, and liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an acrylic copolymer having small orientation birefringence and small photoelastic birefringence, when molded into a film shape, and having excellent transparency and heat resistance, and also provide an optical film including the acrylic copolymer, and a polarizing plate and a liquid crystal display device comprising the optical film.SOLUTION: This acrylic copolymer includes as constituent units therefor: 9 mass% or more and 22 mass% or less of an N-alkylmaleimide unit; 75 mass% or more and 89 mass% or less of a (meth) acrylate alkyl unit; and 1 mass% or more and 10 mass% or less of a constituent unit indicated by general formula (1). In the formula, Rindicates a hydrogen atom or a methyl group, Rindicates an alkanediyl group, and Rindicates an aryl group.

Description

  The present invention relates to an acrylic copolymer, an optical film, a polarizing plate, and a liquid crystal display device.

  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.

  As an optical film used for a protective film of a polarizing plate, for example, Patent Document 1 includes a (meth) acrylic polymer having an N-substituted maleimide unit and a (meth) acrylic acid ester unit as constituent units. Small optical films are disclosed.

JP 2011-242754 A

  By the way, the birefringence exhibited by the optical film includes orientation birefringence whose main factor is the orientation of the main chain of the polymer constituting the optical film, and photoelastic birefringence caused by stress applied to the film.

  Oriented birefringence is birefringence that is generally manifested by the orientation of the main chain of a chain polymer, and this orientation of the main chain occurs, for example, in processes involving the flow of materials such as extrusion and stretching during film production. , It remains fixed to the film.

  On the other hand, photoelastic birefringence is birefringence caused by elastic deformation of the film. For example, volumetric shrinkage that occurs when the polymer is cooled from near the glass transition temperature (Tg) to lower temperatures causes elastic stress to remain in the film, which causes photoelastic birefringence. In addition, an external force received when the optical film is fixed to a device at a normal temperature also generates stress in the film and develops photoelastic birefringence.

  In addition to good transparency and heat resistance, an optical film applied to a polarizing plate, particularly a polarizing plate for IPS, is desired to have both sufficiently small orientation birefringence and photoelastic birefringence.

  Patent Document 1 discloses an optical film having a small phase difference, that is, a small orientation birefringence, but does not describe photoelastic birefringence. Patent Document 1 discloses heat resistance, flexibility, orientation birefringence. An optical film having good refraction and photoelastic birefringence has not been realized.

  An object of the present invention is to provide an acrylic copolymer that has both small orientation birefringence and photoelastic birefringence when molded into a film and is excellent in transparency and heat resistance. Another object of the present invention is to provide an optical film comprising the acrylic copolymer, a polarizing plate provided with the optical film, and a liquid crystal display device.

［式中、Ｒ^１は水素原子またはメチル基を示し、Ｒ^２はアルカンジイル基を示し、Ｒ^３はアリール基を示す。］ The acrylic copolymer according to the present invention has an N-alkylmaleimide unit of 9% by mass to 22% by mass, an alkyl (meth) acrylate unit of 75% by mass to 89% by mass, and the following general formula ( The structural unit represented by 1) includes 1% by mass or more and 10% by mass or less as structural units.

[Wherein, R ¹ represents a hydrogen atom or a methyl group, R ² represents an alkanediyl group, and R ³ represents an aryl group. ]

  The acrylic copolymer according to the present invention has both small orientation birefringence and photoelastic birefringence, and is excellent in properties such as transparency and heat resistance. Therefore, the acrylic copolymer according to the present invention can be suitably used as an optical film for use in optical equipment such as a liquid crystal display device, particularly as a protective film for a polarizing plate.

In the present invention, R ³ is preferably a phenyl group. Thereby, the effect of this invention can be acquired more notably.

In the present invention, R ² is preferably an alkanediyl group having 1 to 4 carbon atoms. Thereby, the effect of this invention can be acquired more notably.

In the present invention, the acrylic copolymer preferably has a weight average molecular weight of 0.5 × 10 ⁵ or more and 3.0 × 10 ⁵ or less. When the weight average molecular weight is 0.5 × 10 ⁵ or more, the obtained optical film has excellent flexibility. In addition, when the weight average molecular weight is 3.0 × 10 ⁵ or less, the flowability of the acrylic copolymer is good (for example, the melt flow rate is 5 g / 10 min or more), and film formation by melt extrusion becomes easy. The production efficiency of the film is improved.

  In the present invention, the alkyl group of the N-alkylmaleimide unit is preferably a chain or cyclic group having 1 to 10 carbon atoms. Thereby, the outstanding optical characteristic (birefringence) can be acquired more notably.

  In the present invention, the alkyl group of the alkyl (meth) acrylate unit is preferably a group having 1 to 6 carbon atoms. Thereby, the outstanding optical characteristic (birefringence) can be acquired more notably.

  In the present invention, it is preferable that the structural unit represented by the general formula (1) includes a phenoxyethyl acrylate unit. When the structural unit represented by the general formula (1) has a phenoxyethyl acrylate unit as a structural unit, excellent optical properties (birefringence) can be obtained more remarkably.

  In the present invention, the acrylic copolymer preferably has a glass transition temperature of 115 ° C. or higher. When a film is produced using such an acrylic copolymer, the heat resistance of the optical film is further improved, and as a result, the dimensional stability of the optical film against heat is further improved. Such an optical film with excellent dimensional stability is more effective in preventing warpage and peeling from the polarizing plate even when used in a temperature change environment or a high temperature environment as a protective film for a polarizing plate. It becomes a more suitable thing as a protective film for boards.

  In the present invention, the melt flow rate of the acrylic copolymer is preferably 1.0 g / 10 min or more. Since such an acrylic copolymer is excellent in fluidity, film formation by melt extrusion becomes easy, and the production efficiency of the film is improved.

  In the present invention, the residual monomer amount of the acrylic copolymer is preferably 5% by mass or less. According to such an acrylic copolymer, it is possible to obtain an optical film having a further improved hue while satisfying required characteristics relating to heat resistance, orientation birefringence and photoelastic birefringence.

  In the present invention, the 1% mass reduction temperature of the acrylic copolymer is preferably 280 ° C. or higher. Since such an acrylic copolymer is excellent in heat resistance, film formation by melt extrusion becomes easy, and the production efficiency of the film is improved.

  An optical film according to another aspect of the present invention comprises the above acrylic copolymer. Since such an optical film is excellent in optical characteristics, it can be suitably used as an optical film used in optical related equipment such as a liquid crystal display device, particularly as a protective film for a polarizing plate.

  In this invention, it is preferable that it is a film obtained by biaxially stretching the unstretched film which consists of a resin material containing the said acrylic copolymer. Such an optical film is excellent in flexibility, and is more suitable as an optical film used in an optical apparatus such as a liquid crystal display device, particularly as a protective film for a polarizing plate.

  In the present invention, the absolute value of the in-plane retardation Re and the absolute value of the thickness direction retardation Rth are both preferably 3.0 nm or less. A small absolute value of the in-plane retardation Re and a thickness direction retardation Rth mean that the orientation birefringence is small. These optical films having an absolute value of 3.0 nm or less are less likely to adversely affect image quality, and can be more suitably used as a protective film for a polarizing plate.

In the present invention, the absolute value of the photoelastic coefficient C is preferably 3.0 × 10 ⁻¹² / Pa or less. When the absolute value of the photoelastic coefficient C is small, the photoelastic birefringence becomes small. An optical film having an absolute value of the photoelastic coefficient C of 3.0 × 10 ⁻¹² / Pa or less can be more suitably used as a protective film for a polarizing plate.

  In the present invention, the number of MIT folding resistances measured in accordance with JIS P8115 is preferably 100 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.

  In this invention, a polarizing plate provided with the said optical film is also provided. In another aspect of the present invention, a liquid crystal display device including the polarizing plate is also provided. Since the optical film has small orientation birefringence and photoelastic birefringence, the adverse effect on the image quality of the liquid crystal display device can be sufficiently reduced. Therefore, according to the polarizing plate and the liquid crystal display device including such an optical film, good image quality is realized.

  According to the present invention, there is provided an acrylic copolymer which has both small orientation birefringence and photoelastic birefringence when molded into a film and is excellent in transparency and heat resistance. The present invention also provides an optical film comprising the acrylic copolymer, and a polarizing plate and a liquid crystal display device including the optical film.

  An embodiment of the present invention will be described.

<Acrylic copolymer>
The acrylic copolymer according to the present invention has an N-alkylmaleimide unit of 9% by mass to 22% by mass, an alkyl (meth) acrylate unit of 75% by mass to 89% by mass, and the following general formula (1). The structural unit represented by the formula 1 to 10% by mass is included as a structural unit. First, each monomer unit constituting the acrylic copolymer according to the present invention will be described.

In the 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.

(N-alkylmaleimide unit)
The N-alkylmaleimide unit is a structural unit obtained from an N-alkylmaleimide monomer. The N-alkylmaleimide unit is a structural unit in which an alkyl group is substituted on the nitrogen atom of the maleimide unit. The alkyl group may be a chain alkyl group or a cyclic alkyl group. preferable. 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 alkyl group in the N-alkylmaleimide unit is preferably 1 or more and 10 or less, more preferably 3 or more and 8 or less.

  Examples of the alkyl group in the N-alkylmaleimide unit 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-alkylmaleimide units, N-methylmaleimide units, N-ethylmaleimide units, Nn-propylmaleimide units, N-isopropylmaleimide units, Nn-butylmaleimide units, N-isobutylmaleimide units, Nt-butylmaleimide units, Nn-hexylmaleimide units, N-2-ethylhexylmaleimide units, N-dodecylmaleimide units, N-laurylmaleimide units, N-cyclohexylmaleimide units, and the like. Of these, N-methylmaleimide units, N-ethylmaleimide units, and N-cyclohexylmaleimide units are preferable, and N-cyclohexylmaleimide units are more preferable. The N-alkylmaleimide unit may be one of these or may contain two or more.

  The content of the N-alkylmaleimide unit in the acrylic copolymer is 9% by mass or more, preferably 11% by mass or more, more preferably 12% by mass or more, and further preferably 14% by mass or more. It is. When the content of the N-alkylmaleimide unit is 9% by mass or more, the absolute value of the thickness direction retardation Rth and the absolute value of the photoelastic coefficient C can be suppressed, and the glass transition of the acrylic copolymer can be suppressed. It can suppress that temperature falls. By suppressing the glass transition temperature from decreasing, it is possible to suppress thermal decomposition during melt extrusion.

  Moreover, content of the N-alkyl maleimide unit in an acrylic copolymer is 22 mass% or less, Preferably it is 20 mass% or less, More preferably, it is 18 mass% or less. If the content of the N-alkylmaleimide unit is 22% by mass or less, the absolute value of the thickness direction retardation Rth and the absolute value of the photoelastic coefficient C can be suppressed, and the film is yellowed. It can suppress becoming easy to do.

((Meth) alkyl acrylate units)
An alkyl (meth) acrylate unit is a structural unit obtained from an alkyl (meth) acrylate monomer. In the present invention, (meth) acrylic acid means acrylic acid or methacrylic acid. The alkyl group of the alkyl (meth) acrylate unit may be a chain alkyl group or a cyclic alkyl group, and is preferably a chain alkyl group.

  The number of carbon atoms of the alkyl group in the (meth) acrylic acid alkyl unit is preferably 1 or more and 6 or less, more preferably 1 or more and 4 or less.

  Examples of the alkyl group in the alkyl (meth) acrylate unit include methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, t-butyl group, n-hexyl group, and 2-ethyl. A xyl group etc. are mentioned, Among these, a methyl group, an ethyl group, a propyl group, and an isopropyl group are preferable, and a methyl group is more preferable.

  That is, as an alkyl (meth) acrylate unit, (meth) acrylic acid methyl unit, (meth) ethyl acrylate unit, (meth) acrylic acid n-propyl unit, (meth) acrylic acid isopropyl unit, (meth) acrylic Examples include an n-butyl acid unit, an isobutyl (meth) acrylate unit, a t-butyl (meth) acrylate unit, an n-hexyl (meth) acrylate unit, and a 2-ethylhexyl unit (meth) acrylate. Of these, methyl (meth) acrylate units, ethyl (meth) acrylate units, n-propyl (meth) acrylate units and isopropyl (meth) acrylate units are preferred, and methyl (meth) acrylate units are more preferred. . Further, as the alkyl (meth) acrylate unit, a methyl methacrylate unit is particularly preferable. In addition, the alkyl (meth) acrylate unit may be one of these, or may include two or more.

  The content of the alkyl (meth) acrylate unit in the acrylic copolymer is 75% by mass or more, preferably 77% by mass or more, more preferably 79% by mass or more, and further preferably 81% by mass. % Or more. If the content of the alkyl (meth) acrylate unit is 75% by mass or more, the absolute value of the thickness direction retardation Rth and the absolute value of the photoelastic coefficient C can be suppressed, and the film Yellowing can be suppressed.

  Moreover, content of the alkyl (meth) acrylate unit in an acrylic copolymer is 89 mass% or less, Preferably it is 87 mass% or less, More preferably, it is 85 mass% or less. If the content of the alkyl (meth) acrylate unit is 89% by mass or less, the absolute value of the thickness direction retardation Rth and the absolute value of the photoelastic coefficient C can be suppressed. It can suppress that the glass transition temperature of a polymer becomes low.

(Structural unit represented by general formula (1))
The structural unit represented by the general formula (1) is a structural unit obtained from a monomer represented by the following general formula (2). Incidentally, ^R 1, ^{R 2} and ^{R 3} in the formula (2) have the same meanings as in formula (1) ^R 1, ^{R 2} and ^{R 3} in.

In the general formula (2), R ¹ represents a hydrogen atom or a methyl group, preferably a hydrogen atom.

In the general formula (2), 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 (2), 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 structural unit represented by the general formula (1), a phenoxyethyl (meth) acrylate unit is preferable, and a phenoxyethyl acrylate unit is more preferable.

  The content of the structural unit represented by the general formula (1) in the acrylic copolymer according to the present invention is 1% by mass or more, preferably 2% by mass or more, more preferably 2.5% by mass. % Or more. By setting the content of the structural unit represented by the general formula (1) to 1% by mass or more, the amount of residual monomers in the acrylic copolymer after polymerization can be reduced, and as a result, optical characteristics Can be obtained.

  The content of the structural unit represented by the general formula (1) in the acrylic copolymer is 10% by mass or less, preferably 8% by mass or less, more preferably 6% by mass or less, More preferably, it is 4 mass% or less. By making content of the structural unit represented by the said General formula (1) into 10 mass% or less, it can suppress that the glass transition temperature of an acryl-type copolymer becomes low.

The weight average molecular weight (Mw) of the acrylic copolymer according to the present invention is preferably 0.5 × 10 ⁵ or more and 3.0 × 10 ⁵ or less, and 0.9 × 10 ⁵ or more and 2.6 ×. 10 ⁵ or less is more preferable, and 1.0 × 10 ⁵ or more and 2.5 × 10 ⁵ or less is more preferable. By setting the weight average molecular weight of the acrylic copolymer to 0.5 × 10 ⁵ or more, it is possible to improve the flexibility when the film is formed, and as a result, the number of MIT folding resistances of the film is improved. be able to. Moreover, by setting the weight average molecular weight of the acrylic copolymer to 3.0 × 10 ⁵ or less, it is possible to suppress a decrease in MFR of the acrylic copolymer and to suppress a decrease in film formability. Thus, the production efficiency can be improved. Therefore, if the weight average molecular weight is in the above range, the acrylic copolymer according to the present invention can improve the flexibility and film production efficiency when the film is formed.

  In addition, in this specification, the weight average molecular weight of an acrylic copolymer shows the value of standard polystyrene molecular weight conversion measured by Tosoh Corporation HLC-8220 GPC. In addition, Super-Multipore HZ-M manufactured by Tosoh Corporation is used as a column, and measurement conditions can be tetrahydrofuran for solvent HPLC (THF), a flow rate of 0.35 ml / min, and a column temperature of 40 ° C.

  The glass transition temperature (Tg) of the acrylic copolymer according to the present invention is preferably 115 ° C. or higher. Thereby, since the heat resistance of a film improves further and the dimensional stability of the film with respect to a heat | fever improves, it becomes a more suitable thing as a protective film for polarizing plates. The upper limit of the glass transition temperature is not particularly limited, but when used as an optical film, it may be 140 ° C. or lower and 130 ° C. or lower from the viewpoint of achieving sufficient heat resistance as an optical film. Also good.

  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 mass of the sample is 5 mg or more and 10 mg or less.

  The MFR of the acrylic copolymer according to the present invention is preferably 1.0 g / 10 min or more. Since such an acrylic copolymer is excellent in fluidity, film formation by melt extrusion becomes easy, and the production efficiency of the film is improved. Moreover, although there is no restriction | limiting in particular in the upper limit of MFR, it may be 40 g / 10min or less, and may be 30 g / 10min or less.

  In addition, in this specification, MFR shows the value measured using the Toyo Seiki Co., Ltd. melt indexer F-F01. The measurement method is based on JIS K7210, the load is 3.8 kgf, and the measurement temperature is 260 ° C.

  The acrylic copolymer according to the present invention has a 1% mass reduction thermal decomposition temperature (hereinafter also simply referred to as “thermal decomposition temperature”) of preferably 280 ° C. or higher, and more preferably 285 ° C. or higher. Thereby, the heat resistance of a film improves further and it becomes a more suitable thing as a protective film for polarizing plates. Moreover, although there is no restriction | limiting in particular in the upper limit of thermal decomposition temperature, 400 degreeC or less may be sufficient from the viewpoint from which sufficient heat resistance as an optical film is achieved, and 350 degrees C or less may be sufficient.

  In this specification, the thermal decomposition temperature is increased to 180 ° C. at a temperature increase temperature of 10 ° C./min using a differential thermothermal mass simultaneous measurement device TG / DTA7200 manufactured by SII Nano Technology, and held for 60 minutes. Then, the temperature is raised to 450 ° C. at a temperature rising rate of 10 ° C./min, and the temperature when the mass is reduced by 1% based on the mass of the sample at 250 ° C. is shown.

  The method for producing the acrylic copolymer according to the present invention is not particularly limited, and for example, it can be produced by a method such as bulk polymerization, suspension polymerization, emulsion polymerization, or solution polymerization. Among these, suspension polymerization is preferable from the viewpoint that treatment after polymerization is easy and heating for removing the organic solvent is unnecessary in the treatment after polymerization.

  The acrylic copolymer according to the present invention is particularly excellent in hue by being produced by suspension polymerization. Unlike the solution polymerization, the suspension polymerization does not require a step of removing the organic solvent from the polymerization system at a high temperature, so that an acrylic copolymer having an even better hue can be obtained.

  By the way, for example, when a copolymer prepared by suspension polymerization of methyl methacrylate and N-cyclohexylmaleimide described in Patent Document 1 is formed into a film, the hue of the film tends to deteriorate. The present inventors have found that the cause of the deterioration of the hue is due to a large amount of residual monomer in the acrylic copolymer after polymerization. And when the present inventors produce the acrylic copolymer according to the present invention by suspension polymerization, in addition to the N-alkylmaleimide monomer and the (meth) acrylic acid alkyl monomer as monomer units, By adopting the structural unit represented by formula (1) and setting a specific monomer ratio, the monomer conversion rate in suspension polymerization is improved, and the amount of residual monomer in the acrylic copolymer after polymerization is sufficiently reduced. I found out that It was also found that the amount of residual monomer after polymerization was further reduced by containing these specific monomers at a specific ratio. For this reason, the acrylic copolymer by this invention can be made into the acrylic copolymer which was further excellent in the hue by manufacturing by melt polymerization.

  The reason for such an effect is not necessarily clear, but is considered as follows. That is, the reactivity between the N-alkylmaleimide monomer and the alkyl (meth) acrylate monomer is not necessarily high, whereas the reactivity between the monomer represented by the general formula (2) and both monomers is high. It is considered that the polymerization reaction is accelerated by the monomer represented by the general formula (2), and a high monomer conversion rate is achieved. Even when the amount of residual monomer is large, the acrylic copolymer itself is not colored. According to the knowledge of the present inventors, when the amount of residual monomer is large, yellowing occurs due to heating or the like in the step of forming a resin material containing an acrylic copolymer into a film.

  In the present invention, the residual monomer amount of the acrylic copolymer is preferably 5% by mass or less, more preferably 4% by mass or less, and further preferably 3% by mass or less.

  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 monomers (N-alkylmaleimide, alkyl (meth) acrylate, and monomer represented by the general formula (2)) 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 dispersing agent are charged into the suspension polymerization apparatus and stirring is started. To do. Then, 1 part by mass of the weighed monomer, Nippon Oil & Fats Co., Ltd. Parroyl TCP as a polymerization initiator, and 0.22 part by mass of 1-octanethiol as a chain transfer agent are charged into a 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 reaction 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 5% by mass or less and a weight average molecular weight of 0.8 × 10 ⁵ or more and 2.7 × 10 ⁵ or less can be easily obtained. Can be obtained.

The types of polymerization initiator, chain transfer agent, and dispersant described above, and the input amount are merely examples, and the conditions for suspension polymerization are not limited to the above. In the suspension polymerization, the amount of the residual monomer is 5% by mass or less (if necessary, the weight average molecular weight is 0.8 × 10 ⁵ or more and 2.7 × 10 ⁵ or less). Can be changed as appropriate. 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. Moreover, the usage-amount of a polymerization initiator may be 0.05 mass part or more and 2.0 mass parts or less with respect to 100 mass parts of total monomers, for example, 0.1 mass part or more and 1.5 mass parts Or less.

  As the chain transfer agent, for example, thiols such as 1-octanethiol, 1-dodecanethiol, and tert-dodecanethiol can be used. The amount of the chain transfer agent used can be appropriately changed according to the desired weight average molecular weight. For example, it is 0.05 parts by mass or more and 0.6 parts by mass or less with respect to 100 parts by mass of the total amount of monomers. 0.07 mass part or more and 0.5 mass part or less may be sufficient.

  As the dispersant, for example, PVA such as Kuraraypa ball manufactured by Kuraray Co., Ltd., sodium polyacrylate, or the like can be used. Moreover, the usage-amount of a dispersing agent may be 0.01 mass part or more and 0.5 mass part or less with respect to 100 mass parts of monomer total amount, for example, 0.02 mass part or more, 0.3 mass part It may be the following.

  The conditions for suspension polymerization can be appropriately adjusted according to the types of polymerization initiator, chain transfer agent and dispersant, and the amount used. For example, the reaction temperature can be 50 ° C. or higher and 95 ° C. or lower, and preferably 60 ° C. or higher and 85 ° C. or lower. Moreover, reaction time should just ensure time for reaction to fully advance, for example, can be 2 hours or more and 10 hours or less, Preferably they are 3 hours or more and 8 hours or less. 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.

  The acrylic copolymer according to the present invention as described above can be suitably used as a resin material for an optical film. According to the acrylic copolymer of the present invention, an optical film having both small orientation birefringence and photoelastic birefringence and excellent transparency and heat resistance can be obtained.

<Optical film>
Next, the optical film according to the present invention will be described. The optical film according to the present invention contains the acrylic copolymer. The optical film according to the present invention is preferably a biaxially stretched film. Such a film can be obtained by biaxially stretching an unstretched film on which a resin containing the acrylic copolymer is formed. Since the biaxially stretched film according to the present invention has both small orientation birefringence and photoelastic birefringence and is excellent in properties such as transparency and heat resistance, it can be particularly suitably used as an optical film. Hereinafter, various properties of the optical film according to the present invention will be described in detail.

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

  The optical film according to the present invention may contain components other than the acrylic copolymer. As components other than the acrylic copolymer, additives used for the biaxially stretched film, such as an antioxidant, a lubricant, an ultraviolet absorber, and a stabilizer, 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 according to the present invention are both preferably 3.0 nm or less, more preferably 2.5 nm or less, and 2.0 nm or less. More preferred is 1.0 nm or less. When 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 as 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 1.5 × 10 ⁻¹² / Pa or less, and 1.0 × 10 ⁻¹² / ^Paper . Pa or less is more preferable. 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 the 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 n _{x is} the refractive index in one direction in the film plane, n _{y is} the refractive index in the direction perpendicular thereto, and dnm is the thickness of the film. .
_{Re = (n x -n y)} × d ... (1)

Rth (unit: nm) was the one direction of the refractive index in the film plane _{n x,} therewith _{n y} refractive index in a direction perpendicular, the refractive index in the thickness direction of the film _{n z,} the thickness of the film and dnm Is expressed by the following equation (2).
Rth = ((n _x + n _y ) / 2−n _z ) × 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 equation (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.

  As for an optical film, it is preferable that the number of times of MIT folding resistance measured based on JIS P8115 is 100 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.

  In this 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.

  If the number of MIT folding resistances is 100 times or more, it is possible to prevent breakage in a step of conveying and winding the optical film after the stretching step, or in a step of bonding to a polarizing plate or the like.

  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 via a paste, and raising and lowering the temperature in a range of −20 ° C. to 60 ° C. at intervals of 30 minutes for 500 cycles. 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 number of MIT folding resistances of the optical film is more preferably 120 times or more, and further preferably 150 times or more.

  The film thickness of the optical film can be 10 μm or more and 150 μm or less, and can also 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.

  In the present invention, the optical film according to the present invention may be a film obtained by biaxially stretching an unstretched film made of a resin material containing the acrylic copolymer according to the present invention. In the case of biaxial stretching, the stretching ratio can be adjusted as appropriate so that the above-mentioned number of MIT folding resistances can be achieved. For example, the draw ratio can be 1.3 times or more by area ratio, 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 (= pre-light irradiation MIT−post light irradiation MIT) before and after the light irradiation.

(Optical film manufacturing method)
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 by biaxially stretching an unstretched film made of a resin material containing an acrylic copolymer. That is, the method for producing a biaxially stretched film according to the present invention includes a step of melt-extruding a resin material containing the acrylic copolymer to obtain an unstretched film (melt-extrusion step), and a biaxially stretching the unstretched film. A step of drawing to obtain an optical film (stretching step).

  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, it is possible to prevent the molten resin film discharged from the T die lip from being rapidly cooled, and therefore 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 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 in line with the extrusion film former. 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, by using the method for producing an optical film according to the present invention, an optical film having both small orientation birefringence and photoelastic birefringence and excellent properties such as transparency and heat resistance can be obtained.

(Polarizer)
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, the polarizing plate according to the present invention as a protective film sufficiently suppresses deterioration of image quality due to the protective film when applied to a liquid crystal display device. can do.

  In the polarizing plate according to the present invention, the constituent elements other than the optical film are not particularly limited, and can 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. A polarizing plate may be provided with the structure which the said optical film, the polarizing layer, the polarizing layer protective film, and the adhesion layer laminated | stacked in this order, for example.

(Liquid crystal display device)
The liquid crystal display device by this invention is equipped with the said polarizing plate. As described above, since the polarizing plate according to the present invention includes the optical film as a protective film, deterioration of image quality due to the optical characteristics of the protective film can be sufficiently suppressed. Therefore, according to the liquid crystal display device of the present invention, good image quality is realized.

  In the liquid crystal display device according to the present invention, the constituent elements other than the polarizing plate are not particularly limited, and may have the same configuration 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.

  The preferred embodiment of the present invention has been described above, but the present invention is not limited to the above embodiment.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to an Example.

<Evaluation method of acrylic copolymer>
In the following examples and comparative examples, the weight average molecular weight (Mw), glass transition temperature (Tg), MFR, residual monomer amount, and 1% mass reduction temperature of the acrylic copolymer were measured as follows. .

  A weight average molecular weight shows the value of standard polystyrene molecular weight conversion measured using Tosoh Co., Ltd. HLC-8220 GPC. The column used was Super-Multipore HZ-M manufactured by Tosoh Corporation, and the measurement conditions were tetrahydrofuran for solvent HPLC (THF), a flow rate of 0.35 ml / min, and a column temperature of 40 ° C.

  The glass transition temperature was obtained from the onset temperature of the glass transition point when the temperature was increased at a rate of temperature increase of 10 ° C./min using a differential scanning calorimeter DSC7020 manufactured by SII Nanotechnology. The mass of the acrylic copolymer sample was 5 mg or more and 10 mg or less.

  MFR was measured using a melt indexer F-F01 manufactured by Toyo Seiki Co., Ltd.

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. In addition, the retention time and area / mass conversion factor of each component were as described in Table 1 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.

  The 1% mass reduction temperature was raised to 180 ° C. at a temperature increase temperature of 10 ° C./min using a differential thermothermal mass simultaneous measurement device TG / DTA7200 manufactured by SII Nano Technology, and held for 60 minutes. The temperature was raised to 450 ° C. at a rate of 10 ° C./min, and the temperature when the mass decreased by 1% based on the acrylic copolymer at 250 ° C. was determined.

<Synthesis of acrylic copolymer>
As described below, acrylic copolymers (a-1) to (a-9) and (b-1) to (b-6) were synthesized, and the weight average molecular weight of the obtained acrylic copolymer, glass The transition temperature, MFR, residual monomer amount, and 1% mass loss temperature were measured.

[Synthesis of Acrylic Copolymer (a-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.) as a dispersant are added. And stirring was started. Next, 19 parts by mass of N-cyclohexylmaleimide (hereinafter sometimes referred to as “CHMI”), 80 parts by mass of methyl methacrylate (hereinafter sometimes referred to as “MMA”), and phenoxyethyl acrylate (hereinafter referred to as “MMA”) In some cases, it is expressed as “PhOEA”.) 1 part by mass, 1 part by mass of Nippon Oil & Fats Co., Ltd. Parroyl TCP as a polymerization initiator and 0.22 part by mass of 1-octanethiol as a chain transfer agent are charged. The temperature was raised to 70 ° C. while passing nitrogen through the reaction kettle. After maintaining the state which reached 70 degreeC for 3 hours, it cooled, and the particulate acrylic copolymer (a-1) was obtained by filtration, washing | cleaning, and drying.

[Synthesis of acrylic copolymer (a-2)]
An acrylic copolymer (except that the amount of monomers charged was changed to 15 parts by mass of N-cyclohexylmaleimide (CHMI), 82 parts by mass of methyl methacrylate (MMA), and 3 parts by mass of phenoxyethyl acrylate (PhOEA). An acrylic copolymer was synthesized in the same manner as in a-1) to obtain an acrylic copolymer (a-2).

[Synthesis of acrylic copolymer (a-3)]
An acrylic copolymer (except that the amount of monomers charged was changed to 11 parts by mass of N-cyclohexylmaleimide (CHMI), 84 parts by mass of methyl methacrylate (MMA), and 5 parts by mass of phenoxyethyl acrylate (PhOEA). The acrylic copolymer was synthesized in the same manner as in a-1) to obtain an acrylic copolymer (a-3).

[Synthesis of acrylic copolymer (a-4)]
An acrylic copolymer (except that the amount of the monomer was changed to 9 parts by mass of N-cyclohexylmaleimide (CHMI), 88 parts by mass of methyl methacrylate (MMA), and 3 parts by mass of phenoxyethyl acrylate (PhOEA). An acrylic copolymer was synthesized in the same manner as in a-1) to obtain an acrylic copolymer (a-4).

[Synthesis of acrylic copolymer (a-5)]
The acrylic copolymer was synthesized in the same manner as the acrylic copolymer (a-2) except that the charge of the chain transfer agent (1-octanethiol) was changed to 0.46 parts by mass, An acrylic copolymer (a-5) was obtained.

[Synthesis of Acrylic Copolymer (a-6)]
The amount of the monomer charged was 13 parts by mass of N-cyclohexylmaleimide (CHMI), 85 parts by mass of methyl methacrylate (MMA), 2 parts by mass of phenoxyethyl acrylate (PhOEA), and chain transfer agent (1-octanethiol) 0. Except having changed to 09 mass parts, the acrylic copolymer was synthesize | combined like the acrylic copolymer (a-1), and the acrylic copolymer (a-6) was obtained.

[Synthesis of acrylic copolymer (a-7)]
The amount of the monomer charged was 15 parts by mass of N-cyclohexylmaleimide (CHMI), 82 parts by mass of methyl methacrylate (MMA), 3 parts by mass of phenoxyethyl acrylate (PhOEA), and chain transfer agent (1-octanethiol) 0. Except having changed to 49 mass parts, the acrylic copolymer was synthesize | combined like the acrylic copolymer (a-1), and the acrylic copolymer (a-7) was obtained.

[Synthesis of Acrylic Copolymer (a-8)]
The amount of monomers charged was 11 parts by mass of N-cyclohexylmaleimide (CHMI), 84 parts by mass of methyl methacrylate (MMA), 5 parts by mass of phenoxyethyl acrylate (PhOEA), and chain transfer agent (1-octanethiol) 0. Except having changed to 08 mass parts, the acrylic copolymer was synthesize | combined similarly to the acrylic copolymer (a-1), and the acrylic copolymer (a-8) was obtained.

[Synthesis of Acrylic Copolymer (a-9)]
An acrylic copolymer (except that the amount of monomer charged was changed to 22 parts by mass of N-cyclohexylmaleimide (CHMI), 77 parts by mass of methyl methacrylate (MMA), and 1 part by mass of phenoxyethyl acrylate (PhOEA)). In the same manner as in a-1), an acrylic copolymer was synthesized to obtain an acrylic copolymer (a-9).

[Synthesis of acrylic copolymer (b-1)]
An acrylic copolymer (except that the amount of monomers charged was changed to 10 parts by weight of N-cyclohexylmaleimide (CHMI), 90 parts by weight of methyl methacrylate (MMA), and 0 parts by weight of phenoxyethyl acrylate (PhOEA). The acrylic copolymer was synthesized in the same manner as in a-1) to obtain an acrylic copolymer (b-1).

[Synthesis of acrylic copolymer (b-2)]
An acrylic copolymer (except that the amount of monomers charged was changed to 20 parts by mass of N-cyclohexylmaleimide (CHMI), 80 parts by mass of methyl methacrylate (MMA), and 0 parts by mass of phenoxyethyl acrylate (PhOEA). An acrylic copolymer was synthesized in the same manner as in a-1) to obtain an acrylic copolymer (b-2).

[Synthesis of acrylic copolymer (b-3)]
An acrylic copolymer (except that the amount of monomers charged was changed to 24 parts by mass of N-cyclohexylmaleimide (CHMI), 75 parts by mass of methyl methacrylate (MMA), and 1 part by mass of phenoxyethyl acrylate (PhOEA)). Acrylic copolymer was synthesized in the same manner as in a-1) to obtain an acrylic copolymer (b-3).

[Synthesis of acrylic copolymer (b-4)]
An acrylic copolymer (except that the amount of monomers charged was changed to 8 parts by mass of N-cyclohexylmaleimide (CHMI), 90 parts by mass of methyl methacrylate (MMA), and 2 parts by mass of phenoxyethyl acrylate (PhOEA). An acrylic copolymer was synthesized in the same manner as in a-1) to obtain an acrylic copolymer (b-4).

[Synthesis of acrylic copolymer (b-5)]
An acrylic copolymer (except that the amount of monomers charged was changed to 15 parts by mass of N-cyclohexylmaleimide (CHMI), 73 parts by mass of methyl methacrylate (MMA), and 12 parts by mass of phenoxyethyl acrylate (PhOEA). The acrylic copolymer was synthesized in the same manner as in a-1) to obtain an acrylic copolymer (b-5).

[Synthesis of acrylic copolymer (b-6)]
Acrylic system, except that the monomer charge was changed to 25 parts by weight of N-cyclohexylmaleimide (CHMI), 74.6 parts by weight of methyl methacrylate (MMA), and 0.4 parts by weight of phenoxyethyl acrylate (PhOEA). The acrylic copolymer was synthesized in the same manner as the copolymer (a-1) to obtain an acrylic copolymer (b-6).

  The weight average molecular weight, glass transition temperature, MFR, residual monomer amount, and 1% mass of the obtained acrylic copolymers (a-1) to (a-9) and (b-1) to (b-6) The measurement result of the decrease temperature was as shown in Table 2 below.

<Evaluation method of optical film>
Next, using the acrylic copolymers (a-1) to (a-9) and (b-1) to (b-6) obtained above, optical films of the following Examples and Comparative Examples Manufactured. Thickness, in-plane retardation Re, thickness direction retardation Rth, photoelastic coefficient C, number of MIT folding resistances, b ^* value which is an index of yellowishness, and optical film thickness of each Example and Comparative Example obtained, and Light resistance was measured as follows.

The thickness of the optical film (A-1) was measured using a digital length measuring device (Digimicro MF501, manufactured by Nikon Corporation). In addition, the film thickness unevenness (unit:%) is t ₁ μm, the maximum thickness of the roll sample measured at 20 equal intervals in the width direction after tapping 10 mm each of the ears at both ends of the original film, and the minimum value When t ₂ μm and the average value is t ₃ μm, thickness unevenness = 100 × (t ₁ −t ₂ ) / t ₃ was calculated.

  The in-plane retardation Re and the thickness direction retardation Rth were measured using an Axoscan apparatus manufactured by Axometrics.

The photoelastic coefficient C is obtained by measuring the amount of change due to the stress applied to the retardation (Re) biaxially stretched film, which is the retardation value of the film, using an Axoscan apparatus manufactured by Axometrics. Specifically, it is as the following formula (3).
C = ΔRe / (Δσ × t) (3)
Δσ is the amount of change in stress applied to the film (unit: Pa), t is the film thickness (unit: m), and ΔRe is the amount of change in the in-plane retardation value corresponding to the amount of change in stress of Δσ ( Unit: m).

  The number of times of MIT folding endurance was measured using a BE-201 MIT folding endurance tester manufactured by Tester Sangyo Co., Ltd. according to JIS P8115. The measurement conditions were 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 ° left and right, and a film sample width of 15 mm. Then, the average value of the number of bendings when the biaxially stretched film is repeatedly bent in the conveyance direction (MD direction) and the number of bendings when the biaxially stretched film is repeatedly bent in the width direction (TD direction) is the MIT resistance. The number of bending tests was used.

The b ^* value, which is a yellowness index, was determined by measuring the spectral spectrum of the optical film using a Spectrophotometer SD6000 manufactured by Nippon Denshoku Industries Co., Ltd. Measurement conditions were as follows: Xenon weather meter [Toyo Seiki Seisakusho Atlas Ci4000] was used, and the optical film was irradiated with irradiance 60 W / m ² , black panel temperature 63 ± 3 ° C., humidity 50% RH, and light irradiation for 600 hours.

For evaluation of light resistance, a xenon weather meter [Toyo Seiki Seisakusho Atlas Ci4000] was used, a biaxially stretched film, irradiance 60 W / m ² , black panel temperature 63 ± 3 ° C., humidity 50% RH, 600 Performed by light irradiation for hours. 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 subtracted value [Delta] Re ( = 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), and subtraction value ΔC of photoelastic coefficient C before and after light irradiation. (= C before light irradiation−C after light irradiation) and a subtraction value ΔMIT (= MIT before light irradiation−MIT after light irradiation) of the number of MIT folding resistances before and after light irradiation were evaluated to evaluate light resistance.

<Manufacture of optical film>
[Example 1]
(Manufacture of optical film (A-1))
The particulate acrylic copolymer (a-1) was formed into a film using a twin screw extruder (KZW-30MG, manufactured by Technobel). The screw diameter of the twin screw extruder is 15 mm and the effective screw length (L / D) is 30, and the hanger coat type T-die is installed in the twin screw extruder through an adapter. Further, a polymer filter (manufactured by Nagase Sangyo Co., Ltd., leaf disk filter, aperture: 10 μm, 7 inches, 31 sheets unit) is provided between the melting zone of the twin screw extruder and the T die. The extrusion temperature Tp (° C.) was set to 241 ° C. because, in the case of an amorphous polymer having a glass transition temperature of Tg (° C.), the formula (5) is optimal.
Tp = 5 (Tg + 70) / 4 (5)
Moreover, the pressure difference (Mpa) before and behind the polymer filter in a twin screw type extruder was monitored, and the pressure applied to the polymer filter was examined. The results were as shown in Table 3 below.

Further, 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 128 ° C.

  The obtained film original (unstretched film) was stretched with a biaxial stretching machine manufactured by Imoto Seisakusho (stretching temperature: Tg + 9 ° C., stretching ratio: 1.5 × 1.5 times, simultaneous biaxial stretching), and optical film (A-1) was obtained.

[Example 2]
(Manufacture of optical film (A-2))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-2), the temperature T ₁ of the first roll except that a 125 ° C., an optical film in the same manner as in Example 1 The optical film (A-2) was obtained.

[Example 3]
(Manufacture of optical film (A-3))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-3), the temperature T ₁ of the first roll except that a 120 ° C., an optical film in the same manner as in Example 1 The optical film (A-3) was obtained.

[Example 4]
(Manufacture of optical film (A-4))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-4), except that the temperature T ₁ of the first roll and 121 ° C., an optical film in the same manner as in Example 1 The optical film (A-4) was obtained.

[Example 5]
(Manufacture of optical film (A-5))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-5), except that the temperature T ₁ of the first roll and 123 ° C., an optical film in the same manner as in Example 1 The optical film (A-3) was obtained.

[Example 6]
(Production of optical film (A-6))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-6), except that the temperature T ₁ of the first roll and 126 ° C., an optical film in the same manner as in Example 1 The optical film (A-3) was obtained.

[Example 7]
(Manufacture of optical film (A-7))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-7), except that the temperature T ₁ of the first roll and 121 ° C., an optical film in the same manner as in Example 1 Was produced to obtain an optical film (A-7).

[Example 8]
(Manufacture of optical film (A-8))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-8), except that the temperature T ₁ of the first roll and 124 ° C., an optical film in the same manner as in Example 1 Was produced to obtain an optical film (A-8).

[Example 9]
(Manufacture of optical film (A-9))
The optical film was changed in the same manner as in Example 1 except that the acrylic copolymer (a-1) was changed to the acrylic copolymer (a-2) and the temperature of the first roll was changed to 139 ° C. Manufacture was performed to obtain an optical film (A-9).

[Example 10]
(Manufacture of optical film (A-10))
The acrylic film (a-1) was changed to an acrylic copolymer (a-2), and the temperature of the first roll was changed to 99 ° C. in the same manner as in Example 1 except that the optical film was Manufacture was performed to obtain an optical film (A-10).

[Example 11]
(Manufacture of optical film (A-11))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-9), the temperature T ₁ of the first roll except that a 130 ° C., an optical film in the same manner as in Example 1 The optical film (A-11) was obtained.

[Comparative Example 1]
(Manufacture of optical film (B-1))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (b-1), except that the temperature T ₁ of the first roll and 122 ° C., an optical film in the same manner as in Example 1 The optical film (B-1) was obtained.

[Comparative Example 2]
(Manufacture of optical film (B-2))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (b-2), except that the temperature T ₁ of the first roll and 134 ° C., an optical film in the same manner as in Example 1 The optical film (B-2) was obtained.

[Comparative Example 3]
(Manufacture of optical film (B-3))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (b-3), except that the temperature T ₁ of the first roll and 132 ° C., an optical film in the same manner as in Example 1 The optical film (B-3) was obtained.

[Comparative Example 4]
(Manufacture of optical film (B-4))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (b-4), except that the temperature T ₁ of the first roll and 117 ° C., an optical film in the same manner as in Example 1 The optical film (B-4) was obtained.

[Comparative Example 5]
Production of an optical film in the same manner as in Example 1, except that the acrylic copolymer (a-1) was changed to the acrylic copolymer (b-5) and the temperature of the first roll was 132 ° C. However, the film adhered to the first roll and could not be formed.

[Comparative Example 6]
(Manufacture of optical film (B-5))
The optical film was changed in the same manner as in Example 1 except that the acrylic copolymer (a-1) was changed to the acrylic copolymer (b-5) and the temperature of the first roll was changed to 82 ° C. Manufacture was performed to obtain an optical film (B-5).

[Comparative Example 7]
(Manufacture of optical film (B-6))
The acrylic film (a-1) was changed to the acrylic copolymer (b-5), and the temperature of the first roll was changed to 112 ° C. in the same manner as in Example 1, except that the optical film was Manufacture was performed to obtain an optical film (B-6).

[Comparative Example 8]
(Manufacture of optical film (B-7))
The acrylic film (a-1) was changed to the acrylic copolymer (b-6), and the temperature of the first roll was changed to 134 ° C. in the same manner as in Example 1 except for the optical film. Manufacture was performed to obtain an optical film (B-7).

Thickness, thickness unevenness, in-plane retardation Re, and thickness direction retardation of optical films (A-1) to (A-11) and (B-1) to (B-7) obtained as described above. Rth, photoelastic coefficient C, number of MIT folding resistances, b ^* value, which is a yellowish index, and light resistance were measured. The measurement results were as shown in Table 3 below.

Claims (18)

9 mass% or more and 22 mass% or less of N-alkylmaleimide units;
75% by mass or more and 89% by mass or less of an alkyl (meth) acrylate unit;
The structural unit represented by the following general formula (1) is 1 mass% or more and 10 mass% or less,
An acrylic copolymer comprising as a structural unit.

[Wherein, R ¹ represents a hydrogen atom or a methyl group, R ² represents an alkanediyl group, and R ³ represents an aryl group. ] The acrylic copolymer according to claim 1, wherein R ³ is a phenyl group. The acrylic copolymer according to claim 1 or 2, wherein R ² is an alkanediyl group having 1 to 4 carbon atoms. 4. The acrylic copolymer according to claim 1, wherein a weight average molecular weight of the acrylic copolymer is 0.5 × 10 ⁵ or more and 3.0 × 10 ⁵ or less. ^5. .   The acrylic copolymer according to any one of claims 1 to 4, wherein the alkyl group of the N-alkylmaleimide unit is a chain or cyclic group having 1 to 10 carbon atoms.   The acrylic copolymer according to any one of claims 1 to 5, wherein the alkyl group of the alkyl (meth) acrylate unit is a group having 1 to 6 carbon atoms.   The acrylic copolymer according to any one of claims 1 to 6, wherein the structural unit represented by the general formula (1) comprises a phenoxyethyl (meth) acrylate unit.   The acrylic copolymer as described in any one of Claims 1-7 whose glass transition temperature of the said acrylic copolymer is 115 degreeC or more.   The acrylic copolymer as described in any one of Claims 1-8 whose melt flow rate of the said acrylic copolymer is 1.0 g / 10min or more.   The acrylic copolymer as described in any one of Claims 1-9 whose residual monomer amount of the said acrylic copolymer is 5 mass% or less.   The acrylic copolymer as described in any one of Claims 1-10 whose 1% mass reduction | decrease temperature of the said acrylic copolymer is 280 degreeC or more.   The optical film which comprises the acryl-type copolymer as described in any one of Claims 1-11.   The optical film according to claim 12, which is a film obtained by biaxially stretching an unstretched film made of a resin material containing the acrylic copolymer.   The optical film according to claim 12 or 13, wherein an absolute value of in-plane retardation Re and an absolute value of thickness direction retardation Rth are both 3.0 nm or less. The optical film as described in any one of Claims 12-14 whose absolute value of the photoelastic coefficient C is 3.0 * 10 < ^-12 > / Pa or less.   The optical film as described in any one of Claims 12-15 whose MIT folding-resistant frequency | count measured based on JISP8115 is 100 or more.   A polarizing plate provided with the optical film as described in any one of Claims 12-16.   A liquid crystal display device comprising the polarizing plate according to claim 17.