JP5697783B2 - Acrylic copolymer, biaxially stretched film, polarizing plate and liquid crystal display device - Google Patents

Description

  The present invention relates to an acrylic copolymer, a biaxially stretched 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. It is desired to use an optical film having a low refractive index.

  As an optical film used for the protective film of a polarizing plate, for example, Japanese Patent Application Laid-Open No. 2011-242754 (Patent Document 1) has an N-substituted maleimide unit and a (meth) acrylic acid ester unit as constituent units (meth). An optical film having a small retardation including an acrylic polymer is 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 around the glass transition temperature to below it 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 (Japanese Patent Laid-Open No. 2011-242754) discloses an optical film having a small phase difference, that is, a small orientation birefringence, but does not describe photoelastic birefringence. An optical film having good properties, heat resistance, orientation birefringence and photoelastic birefringence has not been realized.

  An object of the present invention is to provide an acrylic copolymer that has both low orientation birefringence and photoelastic birefringence when formed into a film and is excellent in transparency and heat resistance. Another object of the present invention is to provide a biaxially stretched film comprising the acrylic copolymer, a polarizing plate comprising the biaxially stretched 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は環状構造を有する炭化水素基を示す。］ The acrylic copolymer according to the present invention comprises an N-alkylmaleimide unit, a (meth) acrylic acid chain alkyl unit, and a third structural unit represented by the following general formula (1).

[Wherein, R ¹ represents a hydrogen atom or a methyl group, and R ² represents a hydrocarbon group having a cyclic structure. ]

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

  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 effect of this invention can be acquired more notably.

  In the present invention, the chain alkyl group in the (meth) acrylic acid chain alkyl unit preferably has 1 to 6 carbon atoms. Thereby, the effect of this invention can be acquired more notably.

  In the present invention, based on the total amount of the acrylic copolymer, the content of the N-alkylmaleimide unit is 5% by mass or more and 30% by mass or less, and the content of the (meth) acrylic acid chain alkyl unit is It is preferable that the content of the third structural unit is 40% by mass or more and 90% by mass or less and 1% by mass or more and 30% by mass or less. Thereby, the effect of this invention can be acquired more notably.

In the present invention, R ² is preferably a group having an alicyclic structure having 6 to 18 carbon atoms. Thereby, the effect of this invention can be acquired more notably.

  In the present invention, the content of the third structural unit is preferably 2% by mass or more and 23% by mass or less based on the total amount of the acrylic copolymer. Thereby, the effect of this invention can be acquired more notably and it becomes a film more suitable as a protective film for polarizing plates.

In the present invention, R ² is preferably a group having an aromatic ring having 6 to 16 carbon atoms. Thereby, the effect of this invention can be acquired more notably.

  In the present invention, the content of the third structural unit is preferably 1.5% by mass or more and 20% by mass or less based on the total amount of the acrylic copolymer. Thereby, the effect of this invention can be acquired more notably and if it uses for the protective film for polarizing plates, etc., it will become a more suitable film.

  In this invention, it is preferable that the glass transition temperature of the said acrylic copolymer is 120 degreeC or more. When a film is produced using such an acrylic copolymer, the heat resistance of the film is further improved, and as a result, the dimensional stability of the film against heat is further improved. Such a film having excellent dimensional stability can be more effectively prevented from being warped or peeled off from the polarizing plate even when used in a temperature change environment or a high temperature environment as a protective film for the polarizing plate. It becomes a more suitable thing as a protective film.

  In the present invention, the acrylic copolymer preferably has a melt flow rate of 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, the hue of the biaxially stretched film can be further improved while satisfying the required characteristics relating to orientation birefringence and photoelastic birefringence.

  In the present invention, the 1% mass reduction temperature of the acrylic copolymer is preferably 270 ° 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.

  A biaxially stretched film according to another aspect of the present invention comprises the above acrylic copolymer.

  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 biaxially stretched films having an absolute value of 3.0 nm or less are less likely to adversely affect image quality, and are more suitable as protective films for polarizing plates.

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. A biaxially stretched 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 150 or more. Such a biaxially stretched film sufficiently satisfies the flexibility required as a protective film for polarizing plates, and therefore can be more suitably used as a protective film for polarizing plates.

  In this invention, a polarizing plate provided with the said biaxially stretched film is also provided. Furthermore, in another aspect of the present invention, a liquid crystal display device including the polarizing plate is also provided. Since the biaxially stretched film is small in both 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, good image quality is realized.

  According to the present invention, there is provided an acrylic copolymer that has both low orientation birefringence and photoelastic birefringence when molded into a film and is excellent in transparency and heat resistance. Moreover, according to this invention, the polarizing plate and liquid crystal display device provided with the biaxially stretched film which comprises this acrylic copolymer, and this biaxially stretched film are provided.

  Embodiments of the present invention will be described below.

<Acrylic copolymer>
The acrylic copolymer according to the present invention comprises an N-alkylmaleimide unit, a (meth) acrylic acid chain alkyl unit, and a third structural unit represented by the following general formula (1). 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, and R ² represents a hydrocarbon group having a cyclic structure.

(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.

((Meth) acrylic acid chain alkyl unit)
The (meth) acrylic acid chain alkyl unit is a structural unit obtained from a (meth) acrylic acid chain alkyl monomer. In the present invention, (meth) acrylic acid means acrylic acid or methacrylic acid. The chain alkyl group in the (meth) acrylic acid chain alkyl unit may be linear or branched.

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

  As the chain alkyl group in the (meth) acrylic acid chain alkyl unit, methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, t-butyl group, n-hexyl group, 2-ethylhexyl 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 the (meth) acrylic acid chain alkyl unit, (meth) acrylic acid methyl unit, (meth) ethyl acrylate unit, (meth) acrylic acid n-propyl unit, (meth) acrylic acid isopropyl unit, (meth) ) N-butyl acrylate unit, isobutyl (meth) acrylate unit, t-butyl (meth) acrylate unit, n-hexyl (meth) acrylate unit, 2-ethylhexyl unit (meth) acrylate, etc. Of these, methyl (meth) acrylate units, ethyl (meth) acrylate units, propyl (meth) acrylate units, and isopropyl (meth) acrylate units are preferred, and methyl (meth) acrylate units are more preferred. preferable. The (meth) acrylic acid chain alkyl unit is particularly preferably a methyl methacrylate unit. In addition, the (meth) acrylic acid chain alkyl unit may be one of these, or may include two or more.

(Third component)
The third structural unit represented by the general formula (1) is obtained from a monomer represented by the following general formula (2).

In the general formula (2), R ¹ represents a hydrogen atom or a methyl group. R ² represents a hydrocarbon group having a cyclic structure. Examples of the hydrocarbon group having a cyclic structure include a hydrocarbon group having an alicyclic structure or an aromatic ring structure.

  The hydrocarbon group having an alicyclic structure is preferably a group having 6 to 18 carbon atoms, and more preferably a group having 8 to 12 carbon atoms.

  Examples of the hydrocarbon group having an alicyclic structure include a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, a dicyclopentanyl group, an isobornyl group, a 4-tert-butylcyclohexyl group, a dicyclopentanyloxyethyl group, 3,5-dihydroxy-1-adamantyl group, 3-hydroxy-1-adamantyl group, 2,2,5-trimethylcyclohexyl group, dicyclopentenyl group, 2-decahydronaphthyl group, dicyclopentenyloxyethyl group, etc. Can be mentioned. Among these, an alicyclic hydrocarbon group is preferable, and a dicyclopentanyl group, an isobornyl group, and a 4-tert-butylcyclohexyl group are more preferable.

  The hydrocarbon group having an aromatic ring is preferably a group having 6 to 16 carbon atoms, and more preferably a group having 7 to 10 carbon atoms. Further, the hydrocarbon group having an aromatic ring may have a substituent, and for example, a part of hydrogen of the aromatic ring may be substituted with a halogen atom.

  Examples of the hydrocarbon group having an aromatic ring include a phenyl group, a tribromophenyl group, a benzyl group, a phenoxyethyl group, and a phenoxyethylene glycol group. Of these, an aromatic hydrocarbon group is preferable, and a benzyl group and a phenoxyethyl group are more preferable.

  The content of each structural unit in the acrylic copolymer is based on the total amount of the acrylic copolymer, and the content of N-alkylmaleimide units is 5% by mass or more and 30% by mass or less, and a (meth) acrylic acid chain. It is preferable that the content of the alkyl unit is 40% by mass or more and 90% by mass or less and the content of the third structural unit is 1% by mass or more and 30% by mass or less.

Here, when R ² in the third structural unit is a hydrocarbon group having an alicyclic structure, the content of the third structural unit is preferably 2% by mass or more, more preferably 7% by mass or more. More preferably, it exceeds 13% by mass, and particularly preferably 13% by mass or more. When R ² in the third structural unit is a hydrocarbon group having an alicyclic structure, the content of the third structural unit is preferably 23% by mass or less, more preferably 21% by mass or less, and 20% by mass. % Or less is more preferable.

When R ² in the third structural unit is a hydrocarbon group having an alicyclic structure, the content of the N-alkylmaleimide unit is preferably 5% by mass or more, more preferably 15% by mass or more, and more preferably 20% by mass or more. Is more preferable. In this case, the content of the N-alkylmaleimide unit is preferably 30% by mass or less, more preferably 27% by mass or less, and further preferably 25% by mass or less.

Further, when R ² in the third structural unit is a hydrocarbon group having an alicyclic structure, the content of the (meth) acrylic acid chain alkyl unit is preferably 40% by mass or more, and more preferably 50% by mass or more. Preferably, 55% by mass or more is more preferable, and 57% by mass or more is particularly preferable. In this case, the content of the (meth) acrylic acid chain alkyl unit is preferably 90% by mass or less, more preferably 75% by mass or less, and further preferably 70% by mass or less.

  According to the acrylic copolymer according to the present invention comprising each structural unit in such a content range, the effects of the present invention can be more remarkably exhibited. As a result, the biaxially stretched film obtained using the acrylic copolymer according to the present invention can be suitably used for a protective film for a polarizing plate.

When R ² in the third structural unit is a hydrocarbon group having an aromatic ring, the content of the third structural unit is preferably 1.5% by mass or more, and more preferably 2% by mass or more. In this case, the content of the third structural unit is preferably 20% by mass or less, more preferably 15% by mass or less, further preferably 10% by mass or less, and may be 5% by mass or less. By setting the content of the third structural unit in the above range, the mechanical strength and heat resistance of the film can be maintained at a higher level while reducing the photoelastic coefficient in the case of the film.

Further, when R ² in the third structural unit is a hydrocarbon group having an aromatic ring, the content of the N-alkylmaleimide unit is preferably 7% by mass or more, and more preferably 10% by mass or more. When R ² in the third structural unit is a hydrocarbon group having an aromatic ring, the content of the N-alkylmaleimide unit is preferably 25% by mass or less, more preferably 20% by mass or less, and 17% by mass. The following is more preferable.

When R ² in the third structural unit is a hydrocarbon group having an aromatic ring, the content of the (meth) acrylic acid chain alkyl unit is preferably 65% by mass or more, and more preferably 70% by mass or more. 75 mass% or more is more preferable. When R ² in the third structural unit is a hydrocarbon group having an aromatic ring, the content of the (meth) acrylic acid chain alkyl unit is preferably 90% by mass or less, and more preferably 85% by mass or less. .

  According to the acrylic copolymer according to the present invention comprising each structural unit in such a content range, the effects of the present invention can be more remarkably exhibited. As a result, the biaxially stretched film obtained using the acrylic copolymer according to the present invention can be suitably used for a protective film for a polarizing plate.

The acrylic copolymer according to the present invention has a weight average molecular weight Mw of 0. 0 from the viewpoint of film production efficiency such as flexibility in film forming and melt flow rate (hereinafter also simply referred to as “MFR”). It is preferably in the range of 5 × 10 ⁵ or more and 2.6 × 10 ⁵ or less, more preferably in the range of 0.7 × 10 ⁵ or more and 2.4 × 10 ⁵ or less, and 0.9 × 10 5 A range of ⁵ or more and 2.2 × 10 ⁵ or less is particularly preferable.

  In addition, in this specification, the weight average molecular weight Mw shows the value of standard polystyrene molecular weight conversion measured by Tosoh Co., Ltd. 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 acrylic copolymer according to the present invention preferably has a glass transition temperature Tg of 120 ° 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. Further, the upper limit of the glass transition temperature Tg is not particularly limited, but when used as a biaxially stretched film, it may be 160 ° C or lower and 150 ° C from the viewpoint of achieving sufficient heat resistance of the biaxially stretched film. It may be the following.

  In the present specification, the glass transition temperature Tg is the onset temperature of the glass transition point when the temperature is raised at a rate of temperature rise of 10 ° C./min using a differential scanning calorimeter DSC7020 manufactured by SII Nanotechnology. The value obtained from The mass of the sample is 5 mg or more and 10 mg or less.

  The acrylic copolymer according to the present invention preferably has a melt flow rate (MFR) of 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 a melt flow rate (MFR), it may be 40 g / 10min or less, and may be 30 g / 10min or less.

  In the present specification, the melt flow rate (MFR) is measured using a melt indexer F-F01 manufactured by Toyo Seiki Co., Ltd. under a 3.8 kg heavy load and 260 ° C. conditions according to JIS K7020. Indicates the value.

  The 1% mass reduction thermal decomposition temperature (hereinafter also simply referred to as “thermal decomposition temperature”) of the acrylic copolymer according to the present invention is preferably 270 ° C. or higher, more preferably 280 ° C. or higher, More preferably, it is 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 acrylic copolymer according to the present invention can be obtained by copolymerizing the above three types of monomer units. The polymerization method is not particularly limited, and can be produced by, for example, bulk polymerization, suspension polymerization, emulsion polymerization, solution polymerization, or the like. Among these, suspension polymerization is preferable from the viewpoint that treatment after polymerization is easy and heating for removing the organic solvent is not necessary 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 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, as a monomer unit, the present inventors employ a third constitutional unit in addition to the N-alkylmaleimide monomer and the (meth) acrylic acid chain alkyl monomer, thereby improving the monomer conversion rate and polymerization. It was found that the residual monomer amount in the later acrylic copolymer was sufficiently reduced. It was also found that the amount of residual monomer after polymerization was further reduced by containing these specific monomers at a specific ratio.

  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 (meth) acrylic acid chain alkyl monomer is not necessarily high, whereas the reactivity between the monomer represented by the general formula (2) and both monomers is high. Therefore, 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, still more preferably 3% by mass or less, and still more preferably 2% by mass. % Or less.

  The conditions for suspension polymerization are not particularly limited, and known suspension polymerization conditions can be appropriately applied. Although an example of the manufacturing method of the acrylic copolymer by suspension polymerization is shown below, this invention is not limited to the following example.

  First, monomers (N-alkylmaleimide, (meth) acrylic acid chain alkyl, and monomer represented by the above general formula (2)) are weighed so as to obtain a desired mass ratio, and the total amount is 100 parts by mass. And With respect 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 as a dispersant (Kuraray Co., Ltd., Kuraray Co., Ltd.) are charged into the suspension polymerization apparatus and stirred. Start. 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 Mw in the range of 0.5 × 10 ⁵ or more and 2.6 × 10 ⁵ or less is easily obtained. Can get to.

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 conditions can be appropriately changed within a range where the residual monomer amount is 5% by mass or less and the weight average molecular weight Mw is 0.5 × 10 ⁵ or more and 2.6 × 10 ⁵ or less. it can. For example, the weight average molecular weight Mw 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. Moreover, although the usage-amount of a chain transfer agent can be suitably changed according to the desired weight average molecular weight Mw, it shall be 0.05 mass part or more and 0.6 mass part or less with respect to 100 mass parts of monomer total amounts, for example. 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 90 ° 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 a biaxially stretched film. According to the acrylic copolymer of the present invention, it is possible to obtain a biaxially stretched film that has both small orientation birefringence and photoelastic birefringence and is excellent in transparency and heat resistance.

<Biaxially stretched film>
Next, the biaxially stretched film according to the present invention will be described. The biaxially stretched film according to the present invention can be obtained by biaxially stretching an unstretched film obtained by forming a resin containing the above acrylic copolymer. The biaxially stretched film according to the present invention has a small orientation birefringence and a photoelastic birefringence, and is excellent in transparency and heat resistance. Hereinafter, various characteristics of the biaxially stretched film according to the present invention will be described in detail.

  The absolute value of the in-plane retardation Re and the thickness direction retardation Rth of the biaxially stretched film are preferably 3.0 nm or less, more preferably 2.5 nm or less, and further 2.0 nm or less. 1.0 nm or less is more preferable. 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 a biaxially stretched film, particularly a polarizing plate protective film.

The absolute value of the photoelastic coefficient C of the biaxially stretched film is preferably 3.0 × 10 ⁻¹² / Pa or less, more preferably 2.0 × 10 ⁻¹² / Pa or less, and 1.0 × 10 ^{− 12} / Pa or less is more preferable, 5.0 × 10 ⁻¹³ / Pa or less is more preferable, and 1.0 × 10 ⁻¹³ / Pa or less may be used. 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 a biaxially stretched film, particularly a polarizing plate protective film.

  The orientation birefringence of the biaxially stretched 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 d nm 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 Sometimes 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 biaxially stretched film is the same as the orientation birefringence, using the Axoscan device manufactured by Axometrics to measure the amount of change in the retardation Re, which is the retardation value of the film, due to the stress applied to the film. ^{Calculated as} C (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.

  The film thickness of the biaxially stretched 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, when the unstretched film obtained by forming the resin containing the acrylic copolymer according to the present invention is biaxially stretched, the stretch ratio can be appropriately adjusted. 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 biaxially stretched film preferably has a MIT folding resistance number of 150 or more as measured in accordance with JIS P8115. Such a biaxially stretched film sufficiently satisfies the flexibility required as a protective film for polarizing plates, and therefore can be more suitably used as a protective film for polarizing plates. Moreover, since such a biaxially stretched 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 biaxially stretched film is repeatedly bent in the conveyance direction and the number of bendings that are broken when the biaxially stretched film is repeatedly bent in the width direction is defined as the MIT folding resistance number.

  If the number of MIT folding resistances is 150 times or more, it is possible to prevent breakage in a process of transporting and winding the biaxially stretched film after the stretching process, or a process 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 150 times or more, the film can be prevented from cracking during the heat shock test.

  The number of MIT folding resistances of the biaxially stretched film is more preferably 160 times or more, and further preferably 170 times or more.

The b ^* value, which is a yellowness index of the biaxially stretched 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 a biaxially stretched film using Nippon Denshoku Industries Co., Ltd. Spectrophotometer SD6000.

The biaxially stretched 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 a xenon weather meter [Toyo Seiki Seisakusho Atlas Ci4000], the biaxially stretched 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.

  The biaxially stretched 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. That is, the content of the acrylic copolymer in the resin material 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. May be.

(Production method of biaxially stretched film)
One aspect of the method for producing a biaxially stretched film according to the present invention will be described in detail. In this embodiment, the biaxially stretched film according to the present invention can be obtained by biaxially stretching an unstretched film made of a resin material containing an acrylic copolymer as described above. 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 of the unstretched film. A step of stretching to obtain a biaxially stretched 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 is continuously discharged from a die lip, whereby an unstretched film can be obtained.

  The extrusion temperature at the time of 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 a biaxially stretched 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 and stretching by a tenter device are 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 higher and (Tg + 20) ° C. or lower, more preferably (Tg + 5) ° C. or higher and (Tg + 15) ° C. or lower, 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, a biaxially stretched film having both excellent optical properties and bending resistance can be obtained by using the resin material described above.

  As described above, by using the method for producing a biaxially stretched film according to the present invention, it is possible to obtain a biaxially stretched film having both small orientation birefringence and photoelastic birefringence and excellent transparency and heat resistance.

(Polarizer)
The polarizing plate by this invention equips the at least one surface of a polarizing film with the said biaxially stretched film as a protective film. Since the biaxially stretched film has small orientation birefringence and photoelastic birefringence, according to the polarizing plate comprising the biaxially stretched film as a protective film, the image quality of the protective film can be improved when applied to a liquid crystal display device. Deterioration can be sufficiently suppressed.

  In the polarizing plate according to the present invention, the components other than the biaxially stretched film are not particularly limited, and can have the same configuration as a known polarizing plate. For example, you may change at least one part of the protective film in a well-known polarizing plate to the said biaxially stretched film.

(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 biaxially stretched 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 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, the polarizing plate in a known liquid crystal display device may be changed to the polarizing plate.

  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, melt flow rate (MFR), residual monomer amount, and 1% mass reduction temperature of the acrylic copolymer are as follows. It was measured.

  The weight average molecular weight Mw shows the value of standard polystyrene molecular weight conversion measured using HLC-8220 GPC made by Tosoh Corporation. 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 Tg was determined 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 from 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 calculating the residual mass of each monomer in the precisely weighed acrylic copolymer particles by the above method and dividing the sum by the mass of the precisely weighed acrylic polymer particles, the remaining monomer amount% is calculated. did.

  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-23) and (b-1) to (b-5) were synthesized, and the weight-average molecular weight Mw of the obtained acrylic copolymer, The glass transition temperature Tg, 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, 15 parts by mass of N-cyclohexylmaleimide (hereinafter, sometimes referred to as “CHMI”), 81 parts by mass of methyl methacrylate (hereinafter, sometimes referred to as “MMA”), and benzyl methacrylate (hereinafter, referred to as “MMA”). In some cases, it is expressed as “BnMA”.) 4 parts by mass, 1 part by mass of Peroyl TCP manufactured by NOF Corporation as a polymerization initiator, and 0.22 parts by mass of 1-octanethiol as a chain transfer agent are prepared. While nitrogen was passed through the reaction kettle, the temperature was raised to 70 ° C. 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)]
Except for using 15 parts by mass of N-cyclohexylmaleimide (CHMI), 82 parts by mass of methyl methacrylate (MMA), and 3 parts by mass of phenoxyethyl acrylate (hereinafter sometimes referred to as “PhOEA”) as monomers. The acrylic copolymer was synthesized in the same manner as the acrylic copolymer (a-1) to obtain an acrylic polymer (a-2).

[Synthesis of acrylic copolymer (a-3)]
As the monomer, 19 parts by mass of N-cyclohexylmaleimide (CHMI), 70 parts by mass of methyl methacrylate (MMA), and 11 parts by mass of dicyclopentanyl acrylate (hereinafter sometimes referred to as “DCPA”) were used. Except for the above, an acrylic copolymer was synthesized in the same manner as the acrylic copolymer (a-1) to obtain an acrylic polymer (a-3).

[Synthesis of acrylic copolymer (a-4)]
As monomers, 23 parts by mass of N-cyclohexylmaleimide (CHMI), 58 parts by mass of methyl methacrylate (MMA), 9 parts by mass of isobornyl methacrylate (hereinafter sometimes referred to as “IBMA”), and dicyclopentanyl methacrylate (Hereinafter, it is expressed as “DCPMA” in some cases.) The acrylic copolymer was synthesized in the same manner as the acrylic copolymer (a-1) except that 10 parts by mass was used. The union (a-4) was obtained.

[Synthesis of acrylic copolymer (a-5)]
As the monomer, 21 parts by mass of N-cyclohexylmaleimide (CHMI), 60 parts by mass of methyl methacrylate (MMA), 9 parts by mass of isobornyl methacrylate (IBMA), and 10 parts by mass of dicyclopentanyl acrylate (DCPA) were used. Except for this, the acrylic copolymer was synthesized in the same manner as the acrylic copolymer (a-1) to obtain an acrylic polymer (a-5).

[Synthesis of Acrylic Copolymer (a-6)]
Except for using 21 parts by mass of N-cyclohexylmaleimide (CHMI), 69 parts by mass of methyl methacrylate (MMA), and 10 parts by mass of isobornyl acrylate (hereinafter sometimes referred to as “IBA”) as monomers, The acrylic copolymer was synthesized in the same manner as the acrylic copolymer (a-1) to obtain an acrylic polymer (a-6).

[Synthesis of acrylic copolymer (a-7)]
As monomers, 21 parts by mass of N-cyclohexylmaleimide (CHMI), 68 parts by mass of methyl methacrylate (MMA), and 11 parts by mass of 4-tert-butylcyclohexyl methacrylate (hereinafter sometimes referred to as “TBCHMA”) are used. Except for the above, an acrylic copolymer was synthesized in the same manner as the acrylic copolymer (a-1) to obtain an acrylic polymer (a-7).

[Synthesis of Acrylic Copolymer (a-8)]
The acrylic copolymer (a-1) was used except that 16 parts by mass of N-cyclohexylmaleimide (CHMI), 81 parts by mass of methyl methacrylate (MMA), and 3 parts by mass of benzyl methacrylate (BnMA) were used as monomers. ), An acrylic copolymer was synthesized to obtain an acrylic polymer (a-8).

[Synthesis of Acrylic Copolymer (a-9)]
An acrylic copolymer (a-1) except that 13 parts by mass of N-cyclohexylmaleimide (CHMI), 81 parts by mass of methyl methacrylate (MMA), and 6 parts by mass of benzyl methacrylate (BnMA) were used as monomers. ), An acrylic copolymer was synthesized to obtain an acrylic polymer (a-9).

[Synthesis of Acrylic Copolymer (a-10)]
An acrylic copolymer (a-1) except that 8 parts by mass of N-cyclohexylmaleimide (CHMI), 82 parts by mass of methyl methacrylate (MMA), and 10 parts by mass of benzyl methacrylate (BnMA) were used as monomers. ), An acrylic copolymer was synthesized to obtain an acrylic polymer (a-10).

[Synthesis of Acrylic Copolymer (a-11)]
An acrylic copolymer (except that 17 parts by mass of N-cyclohexylmaleimide (CHMI), 70 parts by mass of methyl methacrylate (MMA) and 13 parts by mass of dicyclopentanyl acrylate (DCPA) were used as monomers. The acrylic copolymer was synthesized in the same manner as in a-1) to obtain an acrylic polymer (a-11).

[Synthesis of acrylic copolymer (a-12)]
An acrylic copolymer (except that 18 parts by mass of N-cyclohexylmaleimide (CHMI), 74 parts by mass of methyl methacrylate (MMA), and 8 parts by mass of dicyclopentanyl acrylate (DCPA)) were used as monomers. An acrylic copolymer was synthesized in the same manner as in a-1) to obtain an acrylic polymer (a-12).

[Synthesis of Acrylic Copolymer (a-13)]
An acrylic copolymer (except that 22 parts by mass of N-cyclohexylmaleimide (CHMI), 66 parts by mass of methyl methacrylate (MMA), and 14 parts by mass of dicyclopentanyl methacrylate (DCPMA)) were used as monomers. An acrylic copolymer was synthesized in the same manner as in a-1) to obtain an acrylic polymer (a-13).

[Synthesis of acrylic copolymer (a-14)]
Acrylic copolymer (except that 20 parts by mass of N-cyclohexylmaleimide (CHMI), 62 parts by mass of methyl methacrylate (MMA), and 18 parts by mass of dicyclopentanyl methacrylate (DCPMA)) were used as monomers. The acrylic copolymer was synthesized in the same manner as in a-1) to obtain an acrylic polymer (a-14).

[Synthesis of Acrylic Copolymer (a-15)]
An acrylic copolymer (a-1) except that 20 parts by mass of N-cyclohexylmaleimide (CHMI), 77 parts by mass of methyl methacrylate (MMA), and 3 parts by mass of isobornyl acrylate (IBA) were used as monomers. ), An acrylic copolymer was synthesized to obtain an acrylic polymer (a-15).

[Synthesis of Acrylic Copolymer (a-16)]
The acrylic copolymer (a-1) was used except that 21 parts by mass of N-cyclohexylmaleimide (CHMI), 69 parts by mass of methyl methacrylate (MMA) and 10 parts by mass of isobornyl acrylate (IBA) were used as monomers. ), An acrylic copolymer was synthesized to obtain an acrylic polymer (a-16).

[Synthesis of Acrylic Copolymer (a-17)]
As monomers, 21 parts by mass of N-cyclohexylmaleimide (CHMI), 62 parts by mass of methyl methacrylate (MMA), 2 parts by mass of isobornyl acrylate (IBA), and 17 parts by mass of 4-tert-butylcyclohexyl methacrylate (TBCHMA) Except that it was used, the acrylic copolymer was synthesized in the same manner as the acrylic copolymer (a-1) to obtain an acrylic polymer (a-17).

[Synthesis of Acrylic Copolymer (a-18)]
Acrylic copolymer except that 17 parts by mass of N-cyclohexylmaleimide (CHMI), 80 parts by mass of methyl methacrylate (MMA), and 3 parts by mass of 4-tert-butylcyclohexyl methacrylate (TBCHMA) were used as monomers. An acrylic copolymer was synthesized in the same manner as in the polymer (a-1) to obtain an acrylic polymer (a-18).

[Synthesis of Acrylic Copolymer (a-19)]
As the monomer, 21 parts by mass of N-cyclohexylmaleimide (CHMI), 74 parts by mass of methyl methacrylate (MMA), 3 parts by mass of dicyclopentanyl methacrylate (DCPMA), and 2 parts by mass of isobornyl methacrylate (IBMA) were used. Except for this, the acrylic copolymer was synthesized in the same manner as the acrylic copolymer (a-1) to obtain an acrylic polymer (a-19).

[Synthesis of Acrylic Copolymer (a-20)]
As a monomer, 27 parts by mass of N-cyclohexylmaleimide (CHMI), 48 parts by mass of methyl methacrylate (MMA), 10 parts by mass of dicyclopentanyl methacrylate (DCPMA), and 15 parts by mass of isobornyl methacrylate (IBMA) were used. Except for this, the acrylic copolymer was synthesized in the same manner as the acrylic copolymer (a-1) to obtain an acrylic polymer (a-20).

[Synthesis of Acrylic Copolymer (a-21)]
The acrylic copolymer (a-1) was used except that 22 parts by mass of N-cyclohexylmaleimide (CHMI), 69 parts by mass of methyl methacrylate (MMA), and 9 parts by mass of isobornyl methacrylate (IBMA) were used as monomers. ), An acrylic copolymer was synthesized to obtain an acrylic polymer (a-21).

[Synthesis of Acrylic Copolymer (a-22)]
Except that 10 parts by mass of N-cyclohexylmaleimide (CHMI), 80 parts by mass of methyl methacrylate (MMA), and 10 parts by mass of 2,4,6-tribromophenyl acrylate (TBPhA) were used as monomers, acrylic was used. The acrylic copolymer was synthesized in the same manner as the acrylic copolymer (a-1) to obtain an acrylic polymer (a-22).

[Synthesis of Acrylic Copolymer (a-23)])
The acrylic copolymer (a-1) was used except that 15 parts by mass of N-cyclohexylmaleimide (CHMI), 81 parts by mass of methyl methacrylate (MMA), and 4 parts by mass of benzyl methacrylate (BnMA) were used as monomers. ), An acrylic copolymer was synthesized to obtain an acrylic polymer (a-23).

[Synthesis of acrylic copolymer (b-1)]
An acrylic copolymer was synthesized in the same manner as in Example 1 except that 10 parts by mass of N-cyclohexylmaleimide (CHMI) and 90 parts by mass of methyl methacrylate (MMA) were used as monomers. A copolymer (b-1) was obtained.

[Synthesis of acrylic copolymer (b-2)]
An acrylic copolymer was synthesized in the same manner as in Example 1 except that 20 parts by mass of N-cyclohexylmaleimide (CHMI) and 80 parts by mass of methyl methacrylate (MMA) were used as monomers. A copolymer (b-2) was obtained.

[Synthesis of acrylic copolymer (b-3)]
Except for using 15 parts by mass of N-cyclohexylmaleimide (CHMI), 75 parts by mass of methyl methacrylate (MMA), and 10 parts by mass of isobutyl methacrylate (hereinafter sometimes referred to as “IBuMA”) as monomers, The acrylic copolymer was synthesized in the same manner as in Example 1 to obtain an acrylic copolymer (b-3).

[Synthesis of acrylic copolymer (b-4)]
As the monomer, the acrylic type was changed to 10 parts by mass of N-cyclohexylmaleimide (CHMI) and 90 parts by mass of methyl methacrylate (MMA), and benzyl methacrylate (BnMA) was not included. The copolymer was synthesized to obtain an acrylic copolymer (b-4).

[Synthesis of acrylic copolymer (b-5)]
As the monomer, the acrylic type was changed to 20 parts by weight of N-cyclohexylmaleimide (CHMI) and 80 parts by weight of methyl methacrylate (MMA) and benzyl methacrylate (BnMA) was not included. The copolymer was synthesized to obtain an acrylic copolymer (b-5).

  Mass average molecules Mw, glass transition temperature Tg, residual monomer amount, MFR, and 1 of the obtained acrylic copolymers (a-1) to (a-23) and (b-1) to (b-5) The measurement result of the% mass reduction temperature was as shown in Table 2 below.

<Evaluation method of biaxially stretched film>
Next, using the acrylic copolymers (a-1) to (a-23) and (b-1) to (b-5) obtained above, the following examples and comparative examples were biaxial. A stretched film was produced. Thickness, thickness unevenness, in-plane retardation Re, thickness direction retardation Rth, photoelastic coefficient C, number of MIT folding resistances, b yellowness index of the obtained biaxially stretched films of Examples and Comparative Examples ^* Values and light resistance were measured as follows.

The thickness of the biaxially stretched 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 spectrum of a biaxially stretched film using a Spectrophotometer SD6000 manufactured by Nippon Denshoku Industries Co., Ltd. The measurement conditions were as follows: Xenon weather meter [Toyo Seiki Seisakusho Atlas Ci4000], biaxially stretched film, irradiance 60 W / m ² , black panel temperature 63 ± 3 ° C., humidity 50% RH, light irradiation for 600 hours. It was.

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 biaxially stretched film>
[Example 1]
(Production of biaxially stretched film (A-1))
The particulate acrylic copolymer (a-1) was made into a film using a twin screw type extruder KZW-30MG manufactured by Technobel. 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. In the case of an amorphous polymer having a glass transition temperature of Tg ° C., the extrusion temperature Tp ° C. is set to 238 ° C. because the following formula (5) is optimal.
Tp = 5 (Tg + 70) / 4 (5)

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 130 ° 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 biaxial A stretched film (A-1) was obtained.

[Example 2]
(Production of biaxially stretched 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., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-2).

[Example 3]
(Production of biaxially stretched film (A-3))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-3), except that the temperature T ₁ of the first roll and 129 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-3).

[Example 4]
(Production of biaxially stretched 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 147 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-4).

[Example 5]
(Production of biaxially stretched 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 138 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-5).

[Example 6]
(Production of biaxially stretched 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 134 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-6).

[Example 7]
(Production of biaxially stretched 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 141 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-7).

[Example 8]
(Production of biaxially stretched 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 126 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-8).

[Example 9]
(Production of biaxially stretched film (A-9))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-9), the temperature T ₁ of the first roll except that a 125 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-9).

[Example 10]
(Production of biaxially stretched film (A-10))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-10), the temperature T ₁ of the first roll except that a 125 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-10).

[Example 11]
(Production of biaxially stretched film (A-11))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-11), the temperature T ₁ of the first roll except that a 125 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-11).

[Example 12]
(Production of biaxially stretched film (A-12))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-12), the temperature T ₁ of the first roll except that a 125 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-12).

[Example 13]
(Production of biaxially stretched film (A-13))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-13), except that the temperature T ₁ of the first roll and 136 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-13).

[Example 14]
(Production of biaxially stretched film (A-14))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-14), except that the temperature T ₁ of the first roll and 137 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-14).

[Example 15]
(Production of biaxially stretched film (A-15))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-15), except that the temperature T ₁ of the first roll and 127 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-15).

[Example 16]
(Production of biaxially stretched film (A-16))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-16), except that the temperature T ₁ of the first roll and 127 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-16).

[Example 17]
(Production of biaxially stretched film (A-17))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-17), except that the temperature T ₁ of the first roll and 137 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-17).

[Example 18]
(Production of biaxially stretched film (A-18))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-18), except that the temperature T ₁ of the first roll and 129 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-18).

[Example 19]
(Production of biaxially stretched film (A-19))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-19), except that the temperature T ₁ of the first roll and 136 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-19).

[Example 20]
(Production of biaxially stretched film (A-20))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-20), the temperature T ₁ of the first roll except that a 0.99 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-20).

[Example 21]
(Production of biaxially stretched film (A-21))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-21), except that the temperature T ₁ of the first roll and 138 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-21).

[Example 22]
(Production of biaxially stretched film (A-22))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-22), except that the temperature T ₁ of the first roll and 132 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-22).

[Example 23]
(Production of biaxially stretched film (A-23))
Acrylic copolymer (a-1) was changed to the acrylic copolymer (a-23), except that the temperature T ₁ of the first roll and 145 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (A-23).

[Example 24]
(Production of biaxially stretched film (A-24))
Except for changing the temperature T ₁ of the first roll 105 ° C., subjected to production of biaxially oriented film in the same manner as in Example 1 to obtain a biaxially oriented film (A-24).

[Comparative Example 1]
(Production of biaxially stretched 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., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (B-1).

[Comparative Example 2]
(Manufacture of biaxially stretched 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., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (B-2).

[Comparative Example 3]
(Production of biaxially stretched 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 113 ° C., biaxially in the same manner as in Example 1 A stretched film was produced to obtain a biaxially stretched film (B-3).

[Comparative Example 4]
Acrylic copolymer (a-1) was changed to the acrylic copolymer (b-4), except that the temperature T ₁ of the first roll and 142 ° C., biaxially in the same manner as in Example 1 Although a stretched film was produced, the film was stuck to the first roll and could not be formed.

[Comparative Example 5]
(Production of biaxially stretched film (B-4))
Except for changing the temperature T ₁ of the first roll 92 ° C., subjected to production of biaxially oriented film in the same manner as in Example 1 to obtain a biaxially oriented film (B-4).

[Comparative Example 6]
Acrylic copolymer (a-1) was changed to the acrylic copolymer (b-5), except that the temperature T ₁ of the first roll and 154 ° C., biaxially in the same manner as in Example 1 Although a stretched film was produced, the film was stuck to the first roll and could not be formed.

[Comparative Example 7]
(Production of biaxially stretched film (B-5))
Except for changing the temperature T ₁ of the first roll 104 ° C., subjected to production of biaxially oriented film in the same manner as in Example 1 to obtain a biaxially oriented film (B-5).

The thickness, thickness unevenness, in-plane retardation Re, thickness direction position of the biaxially stretched films (A-1) to (A-24) and (B-1) to (B-7) obtained as described above. The phase difference Rth, the photoelastic coefficient C, the number of MIT folding resistances, the b ^* value that is a yellowish index, and the light resistance were measured. The measurement results were as shown in Table 3 below.

Claims (17)

An N-alkylmaleimide unit, a (meth) acrylic acid chain alkyl unit, a third structural unit represented by the following general formula (1),

[Wherein, R ¹ represents a hydrogen atom or a methyl group, and R ² represents a hydrocarbon group having a cyclic structure. ]
An acrylic copolymer comprising: an optical film comprising:
A resin material comprising the acrylic copolymer is melt-extruded to form a film, and is manufactured by a melt film-forming method using a T-die extrusion device. temperature _{T 1} ° C. of the first roll, when the glass transition temperature of the molten resin was Tg ℃, (Tg +5) meets _{≦ T 1 ≦ (Tg + 24} ), the melt film method, unstretched formed as a film An optical film comprising a step of biaxially stretching a film. The optical film according to claim 1, wherein the alkyl group of the N-alkylmaleimide unit is a chain or cyclic group having 1 to 10 carbon atoms. The optical film according to claim 1 or 2 , wherein the chain alkyl group in the (meth) acrylic acid chain alkyl unit has 1 to 6 carbon atoms. Based on the total amount of the acrylic copolymer, the content of the N-alkylmaleimide unit is 5% by mass or more and 30% by mass or less, and the content of the (meth) acrylic acid chain alkyl unit is 40% by mass or more and 90% by mass. The optical film according to any one of claims 1 to 3 , wherein the content of the third structural unit is 1% by mass or more and 30% by mass or less. Wherein R ² is a hydrocarbon group having an alicyclic structure having 6 to 18 carbon atoms, an optical film according to any one of claims 1-4. The optical film according to claim 5 , wherein the content of the third structural unit is 2% by mass or more and 23% by mass or less based on the total amount of the acrylic copolymer. Wherein R ² is a hydrocarbon group having an aromatic ring having 6 to 16 carbon atoms, an optical film according to any one of claims 1-4. The optical film according to claim 7 , wherein the content of the third structural unit is 1.5% by mass or more and 20% by mass or less based on the total amount of the acrylic copolymer. The optical film as described in any one of Claims 1-8 whose glass transition temperature of the said acrylic copolymer is 120 degreeC or more. The optical film according to any one of claims 1 to 9 , wherein the acrylic copolymer has a melt flow rate of 1.0 g / 10 min or more. The optical film as described in any one of Claims 1-10 whose residual monomer amount of the said acrylic copolymer is 5 mass% or less. The 1% weight loss temperature of the acrylic copolymer is 270 ° C. or higher, the optical film according to any one of claims 1 to 11. The optical film according to any one of claims 1 to 12 , wherein an absolute value of the in-plane retardation Re and an absolute value of the thickness direction retardation Rth are both 3.0 nm or less. The absolute value of the photoelastic coefficient C is less 3.0 × ¹⁰ -12 / Pa, the optical film according to any one of claims 1 to 13. MIT folding endurance number which is measured according to JIS P8115 is 150 or more, the optical film according to any one of claims 1-14. A polarizing plate provided with the optical film as described in any one of Claims 1-15 . A liquid crystal display device comprising the polarizing plate according to claim 16 .