JP5731054B2 - 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. 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 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, heat resistance and flexibility, an optical film applied to a polarizing plate, particularly a polarizing plate for IPS, has both sufficiently small orientation birefringence and photoelastic birefringence. Is desired.

  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, flexibility, orientation birefringence 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 formed into a film and is excellent in transparency, heat resistance, and flexibility. Another object of the present invention is to provide a biaxially stretched film comprising the acrylic copolymer, and a polarizing plate and a liquid crystal display device including the biaxially stretched film.

The acrylic copolymer according to the present invention comprises an N-alkylmaleimide unit of 3% by mass to 22% by mass, an alkyl (meth) acrylate unit of 74% by mass to 87% by mass, and a (meth) acrylic. It comprises 1% by mass or more and 11% by mass or less of acid benzyl unit as a structural unit, and the weight average molecular weight is 0.5 × 10 ⁵ or more and 2.6 × 10 ⁵ or less.

  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 properties such as transparency, heat resistance and flexibility. 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 outstanding optical characteristic (low birefringence) can be acquired more notably.

  In the present invention, the chain alkyl group in the (meth) acrylic acid chain alkyl unit preferably has 1 or more and 6 or less carbon atoms. Thereby, the outstanding optical characteristic (low birefringence) can be acquired more notably.

  In the present invention, the acrylic copolymer preferably has a glass transition temperature of 120 ° C. or higher. 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 regarding heat resistance, flexibility, orientation birefringence and photoelastic birefringence.

  In the present invention, the acrylic copolymer preferably has a 1% mass reduction temperature of 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.

  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 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. 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 100 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 which has both small orientational birefringence and photoelastic birefringence when formed into a film, and is excellent in transparency, heat resistance and flexibility. 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 has an N-alkylmaleimide unit of 3% by mass to 22% by mass, an alkyl (meth) acrylate unit of 74% by mass to 87% by mass, and a benzyl (meth) acrylate unit 1 It has mass% or more and 11 mass% or less as a structural unit. In the present invention, the acrylic copolymer has a weight average molecular weight Mw of 0.5 × 10 ⁵ or more and 2.6 × 10 ⁵ or less. Hereinafter, the acrylic copolymer will be described in detail.

(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 indicates an alkyl group having no ring structure, and the cyclic alkyl group indicates an alkyl group having a ring 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 3% by mass or more, preferably 5% by mass or more, more preferably 8% by mass or more, and further preferably 12% by mass or more. It is. If the content of the N-alkylmaleimide unit is too small, the absolute value of the thickness direction retardation Rth and the absolute value of the photoelastic coefficient C tend to increase, and the glass transition temperature Tg of the acrylic copolymer is low. Tend to be. When the glass transition temperature Tg is lowered, there may be a problem that it is likely to be thermally decomposed during melt extrusion.

  The content of N-alkylmaleimide units in the acrylic copolymer is 22% by mass or less, preferably 21% by mass or less, more preferably 19% by mass or less, and further preferably 17% by mass. % Or less. When the content of the N-alkylmaleimide unit is excessive, the absolute value of the thickness direction retardation Rth and the absolute value of the photoelastic coefficient C tend to increase, and there is a problem that the film is easily yellowed.

((Meth) acrylic acid chain alkyl unit)
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, 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 74% by mass or more, preferably 75% by mass or more, more preferably 78% by mass or more, and further preferably 80% by mass. % Or more. If the content of the alkyl (meth) acrylate unit is too small, the absolute value of the thickness direction retardation Rth and the absolute value of the photoelastic coefficient C tend to increase, and there is a problem that the film is easily yellowed. is there.

  Moreover, content of the alkyl (meth) acrylate unit in an acrylic copolymer is 87 mass% or less, Preferably it is 86 mass% or less, More preferably, it is 84 mass% or less. If the content of the alkyl (meth) acrylate unit is excessive, the absolute value of the thickness direction retardation Rth and the absolute value of the photoelastic coefficient C tend to increase, and the glass transition temperature Tg of the acrylic copolymer is increased. Tend to be lower.

(Benzyl (meth) acrylate unit)
The (meth) acrylic acid benzyl unit is a structural unit obtained from a (meth) acrylic acid benzyl monomer. Examples of the benzyl (meth) acrylate unit include a benzyl acrylate unit and a benzyl methacrylate unit, and among them, a benzyl methacrylate unit is preferable.

  Content of the benzyl (meth) acrylate unit in an acrylic copolymer is 1 mass% or more, Preferably it is 2 mass% or more, More preferably, it is 3 mass% or more. If the content of the benzyl (meth) acrylate unit is too small, it is difficult to sufficiently reduce the absolute value of the in-plane retardation Re, the absolute value of the thickness direction retardation Rth, and the absolute value of the photoelastic coefficient C at the same time. In addition, even if this can be achieved, the hue of the film tends to deteriorate. In addition, the melt flow rate (hereinafter, also simply referred to as “MFR”) of the acrylic copolymer is lowered, and problems such as a decrease in moldability in extrusion molding and a decrease in film production efficiency may occur.

  The content of the benzyl (meth) acrylate unit in the acrylic copolymer is 11% by mass or less, preferably 9% by mass or less, more preferably 7% by mass or less, and further preferably 6% by mass. % Or less. When the content of the benzyl (meth) acrylate unit is excessive, the absolute value of the thickness direction retardation Rth and the absolute value of the photoelastic coefficient C tend to increase, and the glass transition temperature Tg of the acrylic copolymer is increased. Tend to be lower.

The acrylic copolymer according to the present invention has a weight average molecular weight Mw of 0.5 × 10 ⁵ or more from the viewpoint of film production efficiency such as flexibility and melt flow rate (MFR) when the film is formed. 6 × 10 ⁵ or less, preferably 0.7 × 10 ⁵ or more, and 2.4 × 10 ⁵ or less, preferably 0.8 × 10 ⁵ or more and 2.2 × 10 ⁵ or less. It is more preferable that By setting the weight average molecular weight Mw of the acrylic copolymer to 0.5 × 10 ⁵ or more, the flexibility when the film is formed can be improved. As a result, the MIT folding resistance frequency of the film is improved. Can be made. On the other hand, when the weight average molecular weight Mw of the acrylic copolymer exceeds 2.6 × 10 ⁵ , the viscosity at the time of melting of the acrylic copolymer becomes too high, and the production efficiency of the film may deteriorate. For example, when a film is formed by an extruder using an acrylic copolymer, the extruder is equipped with a filter for removing foreign substances in the resin, but the resin has a high melt viscosity. When it becomes too much, the pressure applied to the filter increases, and the filter performance may be deteriorated or the filter may be damaged in some cases. According to the present invention, by setting the weight average molecular weight Mw of the acrylic copolymer to 2.6 × 10 ⁵ or less, the melt flow rate (MFR) can be set to 1.0 g / 10 min or more. The production efficiency can be improved by suppressing the deterioration of the filter performance during film formation. Therefore, since the acrylic copolymer according to the present invention has a weight average molecular weight Mw within the above range, it is possible to improve flexibility and film production efficiency when the film is formed.

  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, 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. Moreover, although there is no restriction | limiting in particular in the upper limit of glass transition temperature Tg, when used as a biaxially stretched film, from the viewpoint of achieving sufficient heat resistance of a biaxially stretched film, it may be 140 degrees C or less, and 130 degrees C 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 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 a biaxially stretched film is achieved, and 350 degreeC or less may be sufficient.

  In the present specification, the thermal decomposition temperature is increased to 180 ° C. at a heating rate 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, in addition to an N-alkylmaleimide unit and an alkyl (meth) acrylate unit, the present inventors improve the monomer conversion rate by employing a benzyl (meth) acrylate unit, It was found that the amount of residual monomer in the acrylic copolymer after polymerization 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, while the reactivity between the N-alkylmaleimide monomer and the alkyl (meth) acrylate monomer is not necessarily high, the reactivity between the (meth) acrylic acid benzyl monomer and both monomers is high. It is considered that the polymerization reaction is promoted by benzyl acrylate 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.

  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, the monomers (N-alkylmaleimide, alkyl (meth) acrylate, and benzyl (meth) acrylate) are weighed so that the desired mass ratio is obtained, and the total amount is 100 parts by mass. 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 95 ° C. or lower, and preferably 60 ° C. or lower 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 having small orientation birefringence and photoelastic birefringence and excellent in transparency, heat resistance and flexibility.

<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. Since the biaxially stretched film according to the present invention has both small orientation birefringence and photoelastic birefringence and is excellent in transparency, heat resistance and flexibility, it can be suitably used particularly as an optical film. 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, measuring the amount of change in the retardation Re, which is the retardation value of the film, due to the stress applied to the film, and the photoelastic coefficient C It is determined as (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.

  In the biaxially stretched film, the number of MIT folding resistances measured in accordance with JIS P8115 is preferably 100 times 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. 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 100 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 attaching 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 biaxially stretched film is more preferably 120 times or more, and further preferably 150 times or more.

  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 material containing the acrylic copolymer according to the present invention is biaxially stretched, the stretching ratio of the biaxial stretching is the above-mentioned MIT folding resistance. It is possible to adjust appropriately so that the number of times 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 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 manufacturing method of the biaxially stretched film manufacturing method according to the present invention includes a step of melt-extruding the resin material containing the acrylic copolymer to obtain an unstretched film (melt-extrusion step), And biaxially 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, 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 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, heat resistance, and flexibility. Can do.

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

  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.

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 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.

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

  The 1% mass decrease temperature was increased to 180 ° C. at a temperature increase rate 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, the acrylic copolymers (a-1) to (a-9) and (b-1) to (b-7) 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)]
The acrylic copolymer was synthesized in the same manner as the acrylic copolymer (a-1) except that the amount of the chain transfer agent was changed to 0.48 parts by mass, and the acrylic copolymer (a -2) was obtained.

[Synthesis of acrylic copolymer (a-3)]
The acrylic copolymer was synthesized in the same manner as the acrylic copolymer (a-1) except that the amount of the chain transfer agent was changed to 0.11 part by mass, and the acrylic copolymer (a -3) was obtained.

[Synthesis of acrylic copolymer (a-4)]
Acrylic copolymer (a) except that the amount of monomers charged was changed to 20 parts by mass of N-cyclohexylmaleimide (CHMI), 75 parts by mass of methyl methacrylate (MMA), and 5 parts by mass of benzyl methacrylate (BnMA). In the same manner as in -1), an acrylic copolymer was synthesized to obtain an acrylic copolymer (a-4).

[Synthesis of acrylic copolymer (a-5)]
Acrylic copolymer (a) except that the monomer charge was changed to 12 parts by mass of N-cyclohexylmaleimide (CHMI), 86 parts by mass of methyl methacrylate (MMA), and 2 parts by mass of benzyl methacrylate (BnMA). Acrylic copolymer was synthesized in the same manner as in -1) to obtain an acrylic copolymer (a-5).

[Synthesis of Acrylic Copolymer (a-6)]
Acrylic copolymer (a) except that the amount of monomer charged was changed to 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). In the same manner as in -1), an acrylic copolymer was synthesized to obtain an acrylic copolymer (a-6).

[Synthesis of acrylic copolymer (a-7)]
Acrylic copolymer (a) except that the amount of monomer charged was changed to 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). The acrylic copolymer was synthesized in the same manner as in -1) to obtain an acrylic copolymer (a-7).

[Synthesis of Acrylic Copolymer (a-8)]
Acrylic copolymer (a) except that the monomer charge was changed to 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). The acrylic copolymer was synthesized in the same manner as in -1) to obtain an acrylic copolymer (a-8).

[Synthesis of Acrylic Copolymer (a-9)]
Acrylic copolymer (a) except that the monomer charge was changed to 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). The acrylic copolymer was synthesized in the same manner as in -1) 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 mass of N-cyclohexylmaleimide (CHMI), 90 parts by mass of methyl methacrylate (MMA), and 0 parts by mass of benzyl methacrylate (BnMA) ( 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 benzyl methacrylate (BnMA)). 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 23 parts by mass of N-cyclohexylmaleimide (CHMI), 73 parts by mass of methyl methacrylate (MMA), and 4 parts by mass of benzyl methacrylate (BnMA)). 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), 88 parts by mass of methyl methacrylate (MMA), and 4 parts by mass of benzyl methacrylate (BnMA)). 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 7 parts by mass of N-cyclohexylmaleimide (CHMI), 81 parts by mass of methyl methacrylate (MMA), and 12 parts by mass of benzyl methacrylate (BnMA)). 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)]
The acrylic copolymer was synthesized in the same manner as the acrylic copolymer (a-1) except that the amount of the chain transfer agent was changed to 0.52 parts by mass, and the acrylic copolymer (b -6) was obtained.

[Synthesis of acrylic copolymer (b-7)]
The acrylic copolymer was synthesized in the same manner as the acrylic copolymer (a-1) except that the amount of the chain transfer agent was changed to 0.09 parts by mass, and the acrylic copolymer (b -7) was obtained.

  The resulting acrylic polymer acrylic copolymers (a-1) to (a-9), (b-1) to (b-7) of mass average molecular Mw, glass transition temperature Tg, MFR, residual The measurement results of the monomer amount and the 1% mass reduction temperature were as shown in Table 2 below.

<Evaluation method of biaxially stretched film>
Next, using the acrylic copolymers (a-1) to (a-9) and (b-1) to (b-7) 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 formed into a film using a twin screw extruder (KZW-30MG, manufactured by Technobel). The screw diameter of the biaxial extruder is 15 mm, the effective screw length (L / D) is 30, and a hanger coat type T-die is installed in the extruder via 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 extruder and the T die. In the case of an amorphous polymer having a glass transition temperature of Tg ° C., the extrusion temperature Tp ° C. was set to 238 ° C. because 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 T1 (° C.) of the first roll with which the molten resin discharged from the T die lip first contacts is (Tg−24) ≦ T1 ≦ (Tg (° C.) when the glass transition temperature of the molten resin is Tg (° C.). Since the range of Tg + 24) is preferable, the temperature T1 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))
A biaxially stretched film was produced in the same manner as in Example 1 except that the stretch ratio was changed to 1.2 × 1.2 and the thickness was 40 μm. 2) was obtained.

[Example 3]
(Production of biaxially stretched film (A-3))
A biaxially stretched film was produced in the same manner as in Example 1 except that the acrylic copolymer (a-1) was changed to the acrylic copolymer (a-2). -3) was obtained.

[Example 4]
(Production of biaxially stretched film (A-4))
A biaxially stretched film was produced in the same manner as in Example 1 except that the acrylic copolymer (a-1) was changed to the acrylic copolymer (a-3). -4) was obtained.

[Example 5]
(Production of biaxially stretched film (A-5))
A biaxially stretched film was produced in the same manner as in Example 1 except that the stretch ratio was changed to 2.0 × 2.0 times and the thickness was 40 μm. 5) was obtained.

[Example 6]
(Production of biaxially stretched film (A-6))
A biaxially stretched film was produced in the same manner as in Example 1 except that the acrylic copolymer (a-1) was changed to the acrylic copolymer (a-4). -6) was obtained.

[Example 7]
(Production of biaxially stretched film (A-7))
A biaxially stretched film was produced in the same manner as in Example 1 except that the acrylic copolymer (a-1) was changed to the acrylic copolymer (a-5), and the biaxially stretched film (A -7) was obtained.

[Example 8]
(Production of biaxially stretched film (A-8))
A biaxially stretched film was produced in the same manner as in Example 1 except that the acrylic copolymer (a-1) was changed to the acrylic copolymer (a-6). -8) was obtained.

[Example 9]
(Production of biaxially stretched film (A-9))
A biaxially stretched film was produced in the same manner as in Example 1 except that the acrylic copolymer (a-1) was changed to the acrylic copolymer (a-7). -9) was obtained.

[Example 10]
(Production of biaxially stretched film (A-10))
A biaxially stretched film was produced in the same manner as in Example 1 except that the acrylic copolymer (a-1) was changed to the acrylic copolymer (a-8). -10) was obtained.

[Example 11]
(Production of biaxially stretched film (A-11))
Biaxially stretched film in the same manner as in Example 1 except that the acrylic copolymer (a-1) was changed to the acrylic copolymer (a-9) and the temperature of the first roll was 145 ° C. The biaxially stretched film (A-11) was obtained.

[Example 12]
(Production of biaxially stretched film (A-12))
The biaxially stretched film was produced in the same manner as in Example 11 except that the temperature of the first roll was changed to 105 ° C. and the thickness was 40 μm, and the biaxially stretched film (A-12) was obtained. Obtained.

[Example 13]
(Production of biaxially stretched film (A-13))
A biaxially stretched film was produced in the same manner as in Example 11 except that the temperature of the first roll was changed to 140 ° C. and the thickness was 40 μm, and the biaxially stretched film (A-13) was obtained. Obtained.

[Comparative Example 1]
(Production of biaxially stretched film (B-1))
A biaxially stretched film in the same manner as in Example 1 except that the acrylic copolymer (a-1) was changed to the acrylic copolymer (b-1) and the temperature of the first roll was 122 ° C. The biaxially stretched film (B-1) was obtained.

[Comparative Example 2]
(Manufacture of biaxially stretched film (B-2))
Biaxially stretched film in the same manner as in Example 1 except that the acrylic copolymer (a-1) is changed to the acrylic copolymer (b-2) and the temperature of the first roll is set to 134 ° C. The biaxially stretched film (B-2) was obtained.

[Comparative Example 3]
(Production of biaxially stretched film (B-3))
Biaxially stretched film in the same manner as in Example 1 except that the acrylic copolymer (a-1) was changed to the acrylic copolymer (b-3) and the temperature of the first roll was 135 ° C. The biaxially stretched film (B-3) was obtained.

[Comparative Example 4]
(Production of biaxially stretched film (B-4))
A biaxially stretched film in the same manner as in Example 1 except that the acrylic copolymer (a-1) is changed to the acrylic copolymer (b-4) and the temperature of the first roll is 120 ° C. The biaxially stretched film (B-4) was obtained.

[Comparative Example 5]
(Production of biaxially stretched film (B-5))
Biaxially stretched 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 119 ° C. The biaxially stretched film (B-5) was obtained.

[Comparative Example 6]
(Production of biaxially stretched film (B-6))
A biaxially stretched film was produced in the same manner as in Example 1 except that the acrylic copolymer (a-1) was changed to the acrylic copolymer (b-6), and the biaxially stretched film (B -6) was obtained.

[Comparative Example 7]
(Production of biaxially stretched film (B-7))
A biaxially stretched film was produced in the same manner as in Example 1 except that the acrylic copolymer (a-1) was changed to the acrylic copolymer (b-7). -7) was obtained.

[Comparative Example 8]
Biaxially stretched film in the same manner as in Example 1 except that the acrylic copolymer (a-1) is changed to the acrylic copolymer (b-1) and the temperature of the first roll is 142 ° C. However, the film was stuck to the first roll and could not be formed.

[Comparative Example 9]
(Production of biaxially stretched film (B-8))
A biaxially stretched film was produced in the same manner as in Example 1 except that the temperature of the first roll was changed to 92 ° C. to obtain a biaxially stretched film (B-8).

[Comparative Example 10]
Biaxially stretched film in the same manner as in Example 1 except that the acrylic copolymer (a-1) is changed to the acrylic copolymer (b-2) and the temperature of the first roll is 154 ° C. However, the film was stuck to the first roll and could not be formed.

[Comparative Example 11]
(Production of biaxially stretched film (B-9))
A biaxially stretched film was produced in the same manner as in Example 1 except that the temperature of the first roll was changed to 104 ° C. to obtain a biaxially stretched film (B-9).

Biaxially stretched films (A-1) to (A-13) and (B-1) to (B-9) obtained as described above, thickness unevenness, in-plane retardation Re, thickness direction position 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 (7)

N-alkylmaleimide units in an amount of 3% by mass to 22% by mass;
74% by mass or more and 87% by mass or less of an alkyl (meth) acrylate unit,
1% by mass or more and 11% by mass or less of benzyl (meth) acrylate unit,
An acrylic copolymer comprising as a structural unit,
The weight average molecular weight is 0.5 × 10 ⁵ or more and 2.6 × 10 ⁵ or less,
B ^* value which is an index of yellowishness after irradiation with light at an irradiance of 60 W / m ² for 600 hours is 0.30 or less,
A light-resistant resin material having a subtraction value ΔMIT of the number of MIT folding resistances before and after irradiation with light at an irradiance of 60 W / m ² for 600 hours .   The light-resistant resin material 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 light-resistant resin material 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.   The light-resistant resin material as described in any one of Claims 1-3 whose glass transition temperature of the said acrylic copolymer is 120 degreeC or more.   The light-resistant resin material according to any one of claims 1 to 4, wherein a melt flow rate of the acrylic copolymer is 1.0 g / 10 min or more.   The light-resistant resin material as described in any one of Claims 1-5 whose residual monomer amount of the said acrylic copolymer is 5 mass% or less.   The light-resistant resin material according to claim 1, wherein the acrylic copolymer has a 1% mass reduction temperature of 280 ° C. or higher.