JP2007197703A - Acrylic film and polarizing plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an acrylic film suitable as an optical film having a simple production process and having a small retardation suitable for protecting a polarizer, and a polarizing plate. <P>SOLUTION: This acrylic film consisting of a mixture of (a) an acrylic polymer having a ring structure and (b) a polymer containing a styrene-based polymer having ≤10,000 weight-average molecular weight is provided by satisfying the following conditions (i) and (ii) simultaneously. (i) On taking 100 pts.mass as total acrylic resin, the pts.mass of the (a) as (m), and the pts.mass of the (b) as (n), the following formulae (1) to (3) are satisfied, provided that; 80.0≤m≤99.9 (1); 0.1≤n≤20.0 (2); and 95≤m+n≤100 (3), (ii) The total light transmittance is ≥85%, The polarlizing plate constituted by using a polarizer-protecting film consisting of such the acrylic film is also provided. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an acrylic film and a polarizing plate used as an optical film having a small retardation suitable for use for protecting a polarizer and having a simple manufacturing process.

  Polyacrylic acid, polymethyl methacrylate, and the like are widely used as optical materials because they have high transparency. However, when used in applications such as a film substrate for a liquid crystal display device or a polarizer protective film, there is a problem that it cannot be passed through a process involving heat treatment because of poor heat resistance. In addition, since birefringence is caused by stress, a display device that handles polarized light has a problem in that the beauty of display is impaired, so that it cannot be used.

  On the other hand, there is disclosed a technique (Patent Document 1) for improving heat resistance and reducing birefringence change due to stress by generating a glutaric anhydride unit in an acrylic polymer copolymer. However, although heat resistance is improved, there is a demand for further reduction of birefringence change due to stress when used in large screen liquid crystal televisions, high definition liquid crystal monitors, and the like.

  Also disclosed is a method (Patent Document 2) in which a styrene monomer is also copolymerized in an acrylic polymer in which a glutaric anhydride unit is formed. In this method, it is difficult to control the copolymer composition ratio and the formation of glutaric anhydride units, and it is difficult to reduce the change in birefringence due to stress, that is, to obtain a polymer having zero birefringence.

  Furthermore, a film (Patent Document 3) using a polymer in which a rubbery polymer is contained in an acrylic polymer in which a glutaric anhydride unit is generated has also been proposed. However, this method has a drawback that a kneading process is required to uniformly disperse the rubber-containing polymer, which complicates the production process and leads to an increase in cost.

As the polarizer protective film, cellulose triacetate (TAC) is generally used. TAC is excellent in that it has a high light transmittance and good adhesion to a polarizer. However, in reality, improvement is required to further reduce the amount of change in birefringence with respect to stress.
JP 2004-291302 A Japanese Unexamined Patent Publication No. 3-705 JP 2004-292812 A

  In view of the background of the above-described prior art, the present invention has a maximum refractive index within the film plane of 0.0001 or less and a glass transition temperature (Tg) of -15 ° C. when stretched by 50% while maintaining heat resistance. In-plane maximum refractive index is 0.0015 or less. In other words, we do not provide acrylic film and polarizer protective film that are suitable for polarizer protection applications and are suitable as optical films with a simple manufacturing process. It is what.

The present invention employs the following means in order to solve such problems. That is, the acrylic film of the present invention is an acrylic film made of a resin comprising a mixture of an acrylic polymer (A) having a ring structure and a polymer (I) containing a styrene polymer having a weight average molecular weight of 10,000 or less. And an acrylic film that satisfies the following conditions (i) and (ii) simultaneously.
(I) When the whole resin is 100 parts by mass, the mass part of (A) is m, and the mass part of (I) is n, the following expressions (1) to (3) are satisfied.
80.0 ≦ m ≦ 99.9 (1)
0.1 ≦ n ≦ 20.0 (2)
95 ≦ m + n ≦ 100 (3)
(Ii) Total light transmittance of 85% or more.

  In addition, the polarizing plate of the present invention is characterized by using a polarizer protective film made of such an acrylic film.

  According to the present invention, it is possible to provide an acrylic film which is excellent in transparency and hardly generates birefringence, and can be used for, for example, various covers, cameras, VTRs, projection TVs, etc., filters, etc. Furthermore, it can be used very suitably for films for liquid crystal display devices such as polarizer protective films, various optical disk substrate protective films, and the like.

  The present invention is a specific acrylic type having a ring structure as a result of intensive investigations to solve the above-mentioned problem of obtaining a film suitable for use in protecting a polarizer having heat resistance and low birefringence change by a simple production process. When a polymer and a polymer containing a styrene polymer having a weight average molecular weight of 10,000 or less were mixed under specific conditions, it was found that such problems could be solved all at once.

  According to the present invention, the mixing method does not require a large-scale apparatus such as a melt kneader, and in the case of solution casting, it may be formed into a film after simultaneously and / or sequentially dissolving in a good solvent. In some cases, it has been found that a film can be formed after mixing with a blender or the like, and can be produced by a simple manufacturing process.

  In the present invention, it is important to use a mixture of an acrylic polymer (A) having a ring structure and a polymer (I) containing a styrene polymer having a weight average molecular weight of 10,000 or less.

  The acrylic polymer (a) having a ring structure used in the present invention is not particularly limited as long as it satisfies the properties required for optical applications such as transparency, heat resistance and mechanical properties, but the following structural formula ( An acrylic polymer containing at least one of the structural units represented by a) to (c) is preferable because of its excellent heat resistance.

(In the above formulas, R ¹ and R ² represent the same or different hydrogen atoms, an alkyl group having 1 to 5 carbon atoms, a carboxyl group, or a carboxyalkyl group having 2 to 5 carbon atoms. , X ¹ and X ² represent the same or different CH ₂ or C═O, X ³ represents O or NR ³ , R ³ represents a hydrogen atom, an alkyl group having 1 to 5 carbon atoms or a carboxyl group Represents a group or a carboxyalkyl group having 2 to 5 carbon atoms.)
In particular, from the viewpoint of heat resistance, R ¹ and R ² are preferably hydrogen, a methyl group or a carboxymethyl group, particularly preferably a methyl group, and X ¹ and X ² are preferably C═O. From the viewpoint of transparency, X ³ is preferably O.

(In the above formula, R ^{4 represents} a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, a carboxyl group, or a carboxyalkyl group having 2 to 5 carbon atoms.)
In particular, from the viewpoint of heat resistance, R ⁴ is preferably a methyl group.

(In the above formula, R ⁵ represents a substituent containing an alicyclic structure having 6 to 15 carbon atoms.)
In particular, from the viewpoint of low hygroscopicity, R ⁵ is preferably a substituent represented by the following structural formulas (d) and (e).

Among the structural formulas (a) to (c), when an acrylic polymer having a cyclized structure represented by the structural formula (f) is used, the transparency, heat resistance and productivity are excellent, and optical isotropy is achieved. It is preferable because an excellent film can be obtained.
In particular, from the viewpoint of heat resistance, R ¹ and R ² are preferably hydrogen or a methyl group, and particularly preferably a methyl group.

(In said formula, R < ⁶ >, R < ⁷ > represents the same or different hydrogen atom or a C1-C5 alkyl group.).

  From the viewpoint of the strength of the film, the acrylic polymer (A) having a ring structure comprises an unsaturated carboxylic acid alkyl ester unit and a glutaric anhydride unit. When it is made into a mass part, it is desirable that they are 50-90 mass parts of unsaturated carboxylic-acid alkylester units and 10-50 mass parts of glutaric anhydride units.

  The polymer containing a styrene polymer having a weight average molecular weight of 10,000 or less is not particularly limited as long as it is a styrene polymer obtained by polymerizing a styrene monomer by a known method. Even commercially available products may be used. For example, when it is obtained by polymerization, a conventionally known method such as radical polymerization or ionic polymerization may be used. The initiator used is a peroxide such as benzoyl peroxide, an azo compound such as azobisisobutyronitrile, organolithium, etc. A compound or the like may be selected according to the polymerization method, but an anionic polymerization method is desirable from the viewpoint of reducing the molecular weight distribution. Specific examples of such styrene monomers include styrene, α-substituted styrene (α-methylstyrene, etc.), styrene having a substituent on the benzene ring (o-methylstyrene, p-methylstyrene, p-tert-butylstyrene, p -Chlorostyrene, p-fluorostyrene, p-methoxystyrene, p-tert-butoxystyrene, etc.). These may be used alone or in combination of two or more. Among these monomers, styrene and α-methylstyrene are preferable from the viewpoint of availability, and α-methylstyrene is particularly preferable from the viewpoint of heat resistance. Examples of commercially available polymers containing a styrene polymer having a weight average molecular weight of 10,000 or less include FTR series manufactured by Mitsui Chemicals, Hymer series manufactured by Sanyo Chemical Industries, Ltd., and Kristalex series manufactured by EASTMAN CHEMICAL COMPANY. It is done. In particular, FTR-0120 and FTR-0140 are preferable in that they are α-methylstyrene single polymerization systems.

In addition, when the acrylic polymer having a ring structure (A) used in the present invention and a polymer (I) containing a styrene polymer having a weight average molecular weight of 10,000 or less are mixed, phase separation occurs and haze increases. Since it is easy, as a polymer (ii) containing a styrene-type polymer, it is preferable that the weight average molecular weights measured by GPC method are 300-10000. From the two viewpoints of reducing the birefringence change due to stress and suppressing the haze increase of the film, the weight average molecular weight of the polymer (ii) containing a more preferable styrenic polymer is 300 or more and less than 7000, more preferably It is 300 or more and less than 5000. Further, the polymer containing a styrene polymer having a weight average molecular weight of 10,000 or less that can be used in the present invention contains other vinyl monomer units that can be copolymerized. Also good. Other vinyl monomers that can be copolymerized include vinyl cyanides such as acrylonitrile and methacrylonitrile, acrylic acid, methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, acrylic acid t -Acrylic esters such as butyl, cyclohexyl acrylate, benzyl acrylate, methacrylic acid, methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, methacrylic acid And methacrylic acid esters such as cyclohexyl and benzyl methacrylate. These may be used alone or in combination of two or more. Styrene polymers such as styrene-methyl methacrylate copolymer, styrene-acrylonitrile copolymer, and acrylonitrile-butadiene-styrene block copolymer may also be used. In particular, when the weight average molecular weight of a polymer (ii) containing a styrene polymer having a weight average molecular weight of 10,000 or less is increased, the compatibility with an acrylic polymer (a) having a ring structure is increased or refraction is performed. From the viewpoint of ensuring transparency by bringing the rate values closer, it is preferable to use vinyl cyanides or copolymers of (meth) acrylic esters.

  Acrylic polymer (A) having a ring structure and a polymer (I) containing a styrene polymer having a weight average molecular weight of 10,000 or less may be used in a predetermined ratio when filmed by the solution casting method. What is necessary is just to stir-mix in a solvent and to use as a uniform liquid mixture. In this case, both may be dissolved simultaneously and / or sequentially in accordance with the shape of the dissolution tank and the stirring blade, and there is no particular restriction on the order of charging into the good solvent. In the case of forming a film by the melt extrusion method, both polymers may be used by mixing them at a predetermined ratio with a blender or the like. In the conventional method disclosed in Japanese Patent Application Laid-Open No. 2004-291302, a kneading step is required to obtain an acrylic polymer containing a rubber-containing polymer, which increases the process cost. According to the method of the present invention, both can be mixed by a simple method, leading to cost reduction.

  Specifically, the acrylic film of the present invention comprises a mixture of an acrylic polymer having a ring structure (A) and a polymer (I) containing a styrene polymer having a weight average molecular weight of 10,000 or less, and The following conditions (i) and (ii) are satisfied at the same time.

(I) When the whole acrylic resin is 100 parts by mass, the mass part of (A) is m, and the mass part of (I) is n, the following expressions (1) to (3) are satisfied.
80.0 ≦ m ≦ 99.9 (1)
0.1 ≦ n ≦ 20.0 (2)
95 ≦ m + n ≦ 100 (3).

  When the mass part (n) of the styrene polymer having a weight average molecular weight of 10,000 or less is less than 0.1, there is a problem that birefringence change due to stress cannot be reduced. Moreover, when n exceeds 20, there exists a problem that the glass transition temperature (Tg) of a film will fall. When 0.1 ≦ n ≦ 20, it is preferable because the film maintains heat resistance with a small change in birefringence due to stress. From the two viewpoints of reducing the birefringence change due to stress and suppressing the glass transition temperature (Tg) drop of the film, 0.5 ≦ n ≦ 15 is more preferable, and 1 ≦ n ≦ 10 is most preferable. .

(Ii) Total light transmittance of 85% or more When the total light transmittance is 85% or less, there is a problem that the transparency is poor and it cannot be used for polarizer protection. The total light transmittance is preferably 88% or more, and more preferably 91% or more.

In particular, the film defines the maximum refractive index direction in the film plane as the x direction, the orthogonal direction to x in the plane as the y direction, and the film thickness direction as the z direction, and the refractive index in each direction is defined as nx, ny, When nz, the in-plane birefringence N1 represented by the following formula (4) is 0.0001 or less, and the thickness birefringence N2 represented by the following formula (5) satisfies 0.0001 or less. It is preferable.
N1 = | nx−ny | (4)
N2 = | (nx + ny) / 2−nz | (5)
In addition, when exceeding the said range, there exists a problem that the retardation value which is a product of birefringence and thickness becomes large. For example, when a 50 μm film is used, the retardation value exceeds 5 nm. In this case, when used for protecting a polarizer, there is a problem that correct color display cannot be performed.

Further, when the film is stretched 50% at a glass transition temperature (Tg) of −15 ° C., the in-plane maximum refractive index direction is the x ′ direction, the orthogonal direction to the in-plane x ′ is the y ′ direction, and the film thickness direction. Is defined as the z ′ direction, and the refractive index in each direction is nx ′, ny ′, nz ′, the in-plane birefringence N3 represented by the following formula (6) is 0.0015 or less, and It is more preferable that the thickness birefringence N4 represented by the formula (7) is a film satisfying 0.0015 or less.
N3 = | nx′−ny ′ | (6)
N4 = | (nx ′ + ny ′) / 2−nz ′ | (7)
In addition, when the above range is exceeded, the retardation value increases significantly when stretched for film strength improvement or thinning, etc., and when used for polarizer protection, correct color display cannot be performed. It becomes a problem. In this case, N3 is more preferably 0.0008 or less and N4 is 0.0008 or less, further preferably N3 is 0.0002 or less and N4 is 0.0002 or less.

  Although the example of the manufacturing method of the acrylic polymer (A) which has a ring structure containing the glutaric anhydride unit represented by the said structural formula (f) is demonstrated, this invention is not limited to this.

  That is, the unsaturated carboxylic acid monomer (A) and the unsaturated carboxylic acid alkyl ester monomer (B) that give the glutaric anhydride unit represented by the structural formula (f) in the subsequent heating step, and others When the vinyl monomer unit is included, the vinyl monomer (C) giving the unit is polymerized to form a copolymer (a), and then the copolymer (a) is converted into an appropriate catalyst. It can be produced by heating in the presence or absence of and causing an intramolecular cyclization reaction by dealcoholization and / or dehydration. In this case, typically, the carboxyl group of the unsaturated carboxylic acid unit of 2 units is dehydrated by heating the copolymer (a), or the adjacent unsaturated carboxylic acid unit and unsaturated carboxylic acid alkyl ester unit. 1 unit of the glutaric anhydride unit is generated by the elimination of the alcohol. The unsaturated carboxylic acid monomer (A) used in this case is not particularly limited, and can be copolymerized with another vinyl compound (C), and the unsaturated carboxylic acid monomer having the structural formula (g). The body can be used.

(In the above formula, R ⁸ represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms).

  In particular, acrylic acid and methacrylic acid are preferable from the viewpoint of excellent thermal stability, and methacrylic acid is more preferable. These can be used alone or in combination of two or more thereof. The unsaturated carboxylic acid monomer (A) represented by the structural formula (g) gives an unsaturated carboxylic acid unit having the structure represented by the structural formula (f) when copolymerized.

  The unsaturated carboxylic acid alkyl ester monomer (B) is not particularly limited, but preferred examples include those represented by the following structural formula (h).

(In the above formula, R ⁹ represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms. R ¹⁰ represents a hydrogen atom or an aliphatic or alicyclic hydrocarbon group having 1 to 6 carbon atoms).

  Of these, acrylates and / or methacrylates having an aliphatic or alicyclic hydrocarbon group having 1 to 6 carbon atoms or a hydrocarbon group having a substituent are particularly preferred in terms of excellent thermal stability. is there. The unsaturated carboxylic acid alkyl ester monomer represented by the structural formula (h) gives an unsaturated carboxylic acid alkyl ester unit having the structure represented by the structural formula (f) when copolymerized.

  Preferable specific examples of the unsaturated carboxylic acid alkyl ester monomer (B) include methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, and n-butyl (meth) acrylate. T-butyl (meth) acrylate, n-hexyl (meth) acrylate, cyclohexyl (meth) acrylate, chloromethyl (meth) acrylate, 2-chloroethyl (meth) acrylate, 2- (meth) acrylic acid 2- Hydroxyethyl, 3-hydroxypropyl (meth) acrylate, 2,3,4,5,6-pentahydroxyhexyl (meth) acrylate and 2,3,4,5-tetrahydroxypentyl (meth) acrylate Among them, methyl methacrylate is most preferably used. These can be used alone or in combination of two or more thereof.

  Further, in the production of the acrylic polymer (a) having a ring structure used in the present invention, other vinyl monomers (C) such as styrene, acrylamide, methacrylamide and the like are used as long as the effects of the present invention are not impaired. Although it may be used, a monomer that does not contain an aromatic ring can be more preferably used in terms of transparency, birefringence, and chemical resistance. These may be used alone or in combination of two or more.

  As for the polymerization method of the acrylic polymer (a) having a ring structure, polymerization methods such as bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, etc., which are basically radical polymerization, can be used. Solution polymerization, bulk polymerization, and suspension polymerization are particularly preferable in terms of few points.

  The polymerization temperature is not particularly limited, but from the viewpoint of color tone, a monomer mixture containing an unsaturated carboxylic acid monomer and an unsaturated carboxylic acid alkyl ester monomer is polymerized at a polymerization temperature of 95 ° C. or less. Is preferred. The lower limit of the polymerization temperature is not particularly limited as long as the polymerization proceeds, but is usually 50 ° C. or higher from the viewpoint of productivity considering the polymerization rate. For the purpose of improving the polymerization yield or polymerization rate, it is possible to raise the polymerization temperature as the polymerization proceeds. The polymerization time is not particularly limited as long as it is a sufficient time to obtain the required degree of polymerization, but is preferably in the range of 60 to 360 minutes from the viewpoint of production efficiency.

  In the present invention, a preferable ratio of the monomer mixture used in the production of the acrylic polymer (A) having a ring structure is 100 parts by mass of the monomer mixture, and the unsaturated carboxylic acid monomer (A ) Is 5 to 50 parts by mass, more preferably 9 to 33 parts by mass, and the unsaturated carboxylic acid alkyl ester monomer (B) is preferably 50 to 95 parts by mass, more preferably 67 to 91 parts by mass. When using the other polymerizable vinyl monomer (C), the preferable ratio is 0 to 5 parts by mass, and the more preferable ratio is 0 to 3 parts by mass.

  When the unsaturated carboxylic acid monomer amount (A) is less than 5 parts by mass, the amount of glutaric anhydride unit represented by the structural formula (f) generated by heating the copolymer (a) is There is a tendency that the effect of improving the heat resistance of the acrylic film of the present invention is reduced. On the other hand, when the amount of unsaturated carboxylic acid monomer (A) exceeds 50 parts by mass, a large amount of unsaturated carboxylic acid units tend to remain after the cyclization reaction by heating the copolymer (a). , Colorless transparency and retention stability tend to decrease.

  Moreover, it is preferable that the acrylic polymer (a) having a ring structure used in the acrylic film of the present invention has a mass average molecular weight of 50,000 to 150,000. The acrylic polymer (a) having a ring structure having such a molecular weight is obtained by converting the copolymer (a) to a desired molecular weight, that is, a mass average molecular weight of 50,000 to 150,000 at the time of producing the copolymer (a). This can be achieved by controlling in advance. When the mass average molecular weight exceeds 150,000, there is a tendency of coloring during cyclization in the subsequent step. On the other hand, when the mass average molecular weight is less than 50,000, the mechanical strength of the acrylic film tends to decrease.

  The copolymer (a) used in the production of the acrylic polymer (a) having a ring structure preferably used in the present invention is heated, and (i) dehydration and / or (b) dealcoholization reaction is carried out by dealcoholization. . A method for producing a thermoplastic polymer containing a glutaric anhydride unit is not particularly limited, but a method of producing a thermoplastic polymer through a heated extruder having a vent, or degassing under an inert gas atmosphere or vacuum. A method of manufacturing in an apparatus that can be used is preferable from the viewpoint of productivity. Among them, when an intramolecular cyclization reaction is carried out by heating in the presence of oxygen, the yellowness tends to deteriorate, so it is preferable to sufficiently substitute the inside of the system with an inert gas such as nitrogen. Moreover, it is more preferable that the apparatus has a structure capable of introducing an inert gas such as nitrogen into them. For example, as a method for introducing an inert gas such as nitrogen into a twin-screw extruder, a method of connecting a pipe of an inert gas stream of about 10 to 100 liters / minute from the upper part and / or the lower part of the hopper can be mentioned. .

  The temperature during cyclization is not particularly limited as long as (i) dehydration and / or (b) dealcoholization causes an intramolecular cyclization reaction, but is preferably in the range of 180 to 300 ° C, particularly 200 to 200 ° C. A range of 280 ° C is preferred.

  Further, the cyclization time at this time is not particularly limited and can be appropriately set according to the desired copolymer composition, but is usually 1 minute to 60 minutes, preferably 2 minutes to 30 minutes, especially 3 to 20 minutes. The range of is preferable. In particular, the length / diameter ratio (L / D) of the extruder screw is preferably 40 or more in order to secure a heating time for allowing sufficient intramolecular cyclization reaction to proceed using an extruder. When an extruder with a short L / D is used, a large amount of unreacted unsaturated carboxylic acid units remain, so that the reaction proceeds again during the heat molding process, and there is a tendency for silver and bubbles to be seen in the molded product and molding retention. Sometimes the color tends to deteriorate significantly.

  Furthermore, in the present invention, when the copolymer (a) is heated by the above method or the like, one or more of acid, alkali and salt compounds may be added as a catalyst for promoting the cyclization reaction to glutaric anhydride. it can. The addition amount is not particularly limited, and is preferably about 0.01 to 1 part by mass with respect to 100 parts by mass of the copolymer (a).

  The acrylic polymer (A) having a ring structure preferably has a glass transition temperature (Tg) of 120 ° C. or higher in terms of heat resistance. The method for raising the glass transition temperature is not particularly limited, but the content of the cyclized structural unit represented by, for example, the structural formula (f) in the acrylic polymer (A) having a ring structure is increased. It is effective. It is also effective to orient the obtained film by stretching.

  As the acrylic polymer (A) having a ring structure of the present invention, a copolymer comprising a glutaric anhydride unit represented by the structural formula (f) and an unsaturated carboxylic acid alkyl ester unit is preferably used. it can. The content of the unsaturated carboxylic acid alkyl ester unit and the glutaric anhydride unit is not particularly limited. However, since heat resistance is improved, the total of the unsaturated carboxylic acid alkyl ester unit and the glutaric anhydride unit is 100 masses. Parts, preferably 50 to 90 parts by weight of unsaturated carboxylic acid alkyl ester units and 10 to 50 parts by weight of glutaric anhydride units, more preferably 55 to 80 parts by weight of unsaturated carboxylic acid alkyl ester units. And 20 to 45 parts by mass of glutaric anhydride units. When the glutaric anhydride unit is less than 10 parts by mass, not only the heat resistance improving effect is reduced, but also sufficient low birefringence (optical isotropy) and chemical resistance tend not to be obtained.

In addition, an infrared spectrophotometer or a proton nuclear magnetic resonance (1H-NMR) measuring instrument is generally used for quantification of each component unit in the acrylic polymer (A) having a ring structure of the present invention. In infrared spectroscopy, glutaric anhydride units, absorption of 1800 cm ^-1 and 1760 cm ^-1 are characteristic can be distinguished from unsaturated carboxylic acid units and unsaturated carboxylic acid alkyl ester unit. In the 1H-NMR method, for example, in the case of a copolymer consisting of a glutaric anhydride unit, methacrylic acid, and methyl methacrylate, the attribution of the spectrum in dimethyl sulfoxide heavy solvent is 0.5 to 1.5 ppm. The peak is methacrylic acid, methyl methacrylate and α-methyl group hydrogen of glutaric anhydride ring compound, the peak of 1.6 to 2.1 ppm is hydrogen of the methylene group of the polymer main chain, and the peak of 3.5 ppm is methacrylic acid The hydrogen of methyl carboxylic acid ester (—COOCH ₃ ), the peak at 12.4 ppm can determine the copolymer composition from the hydrogen of carboxylic acid of methacrylic acid and the integral ratio of the spectrum. In addition to the above, in the case of a copolymer containing styrene as another copolymer component, hydrogen of an aromatic ring of styrene is seen at 6.5 to 7.5 ppm, and the copolymer is similarly obtained from the spectral ratio. The composition can be determined.

  In addition, the acrylic polymer (A) having a ring structure of the present invention includes other unsaturated carboxylic acid units and / or other vinyl monomers capable of copolymerization in the acrylic polymer (A) having a ring structure. It can contain body units.

  The amount of other unsaturated carboxylic acid units contained in 100 parts by mass of the thermoplastic polymer is preferably 10 parts by mass or less, that is, 0 to 10 parts by mass, more preferably 0 to 5 parts by mass, most preferably Preferably it is 0-1 mass part. When the unsaturated carboxylic acid unit exceeds 10 parts by mass, the colorless transparency and the residence stability tend to decrease.

  Further, the amount of other copolymerizable vinyl monomer units is preferably 5 parts by mass or less, that is, in the range of 0 to 5 parts by mass, more preferably 0 to 5 parts by mass, in 100 parts by mass of the thermoplastic polymer. 3 parts by mass. In particular, when an aromatic vinyl monomer unit such as styrene is contained and the content exceeds the above range, colorless transparency, optical isotropy, and chemical resistance tend to decrease.

  As a method of forming the film, any conventionally known method can be used. Examples thereof include a solution casting method and a melt extrusion method, and any of them can be employed. For example, when a film is to be obtained by using the solution casting method, a predetermined ratio of an acrylic polymer (A) having a ring structure and a polymer (I) containing a styrene polymer having a weight average molecular weight of 10,000 or less. In a good solvent, the mixture may be stirred and mixed to be used as a uniform mixed solution. When a melt extrusion method is used to obtain a film, a predetermined ratio of both may be mixed with a blender or the like. In addition, you may mix a plasticizer, a ultraviolet absorber, etc. as needed. In this case, from the viewpoint of preventing the glass transition temperature (Tg) from being lowered and preventing haze rise of the film, the whole resin is 100 parts by mass, the mass part of the acrylic polymer (A) having a ring structure is l, and the weight average molecular weight is 10,000. It is preferable that 95 ≦ m + n ≦ 100, where m is a mass part of a polymer (i) containing the following styrenic polymer.

  The method for confirming the mixing amount of the acrylic polymer (A) having a ring structure and the polymer (ii) containing a styrene polymer having a weight average molecular weight of 10,000 or less is not particularly limited as long as it is an existing measurement method. The composition ratio of the polymer fractionated by GPC (gel permeation chromatography) may be measured, and it contains an acrylic polymer (A) having a ring structure and a styrene polymer having a weight average molecular weight of 10,000 or less. When the polymerization average molecular weight of each polymer (I) is known, it may be measured with a proton nuclear magnetic resonance (1H-NMR) measuring machine or a carbon nuclear magnetic resonance (13C-NMR) measuring machine.

  In optical applications, a high degree of uniformity is often required, and in such a case, it is preferable to use a solution casting method. The solvent to be used is not particularly limited, but haloalkanes such as dichloromethane, chloroform, 1,2-dichloroethane; cyclic ethers such as tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane; acetone, methyl ethyl ketone, methyl Ketones such as isobutyl ketone and cyclohexanone, and aromatic solvents such as chlorobenzene are used. You may use these individually or in mixture of 2 or more types. The concentration of the solution in the case of using the solution casting method is not particularly limited, and is usually about 15 to 30% by mass from the viewpoint of facilitating control of the film thickness and thickness accuracy according to the application. As a preferred embodiment, the resin composition to be used may be pre-dried before film formation. Preliminary drying is very useful because defects such as foaming of the film can be prevented.

  The thickness of the film of the present invention is preferably 100 μm or less from the viewpoint of reducing the in-plane retardation value and the thickness retardation value of the obtained film. More preferably, it is 50 μm or less.

  The film of the present invention may be unstretched as a final product in terms of simplification of the manufacturing process, cost reduction, etc., but using a known stretching method for improving the strength of the film or reducing the film thickness. Further, at least one axis may be stretched.

  In addition, the film of the present invention can be used for various uses as it is or after being subjected to various processing. In particular, it can be suitably used for known optical applications such as the periphery of liquid crystal display devices such as an optically isotropic film, a polarizer protective film and a transparent conductive film by utilizing excellent transparency and low birefringence.

  Furthermore, the film of the present invention can be subjected to surface treatment on one side or both sides of the film, if necessary. Examples of the surface treatment method include corona treatment, plasma treatment, and ultraviolet irradiation. In particular, when surface processing such as coating is applied to the film surface, or when another film is laminated with an adhesive, surface treatment of the film is performed as a means for improving mutual adhesion. Is preferred.

  In addition, a coating layer such as a hard coat layer can be formed on the surface of the stretched film of the present invention as necessary. Moreover, the film of this invention may form a transparent conductive layer by sputtering method etc. through or without a coating layer.

  The stretched film of the present invention can be used as a polarizer protective film to be bonded to a polarizer and can be used as a polarizing plate. Here, as a polarizer, conventionally well-known arbitrary polarizers, such as what made iodine contain in the stretched polyvinyl alcohol, can be used, for example. The polarizer protective film is laminated on one side or both sides of the polarizer. Generally, a polarizer protective film is laminated on both sides of the polarizer.

  EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, this invention is not limited to this Example. Each characteristic value was measured as follows.

1. Film thickness Five points were measured using a micro thickness gauge (manufactured by Anritsu), and an average value was obtained.

2. Residual solvent The film immediately after film formation was sampled in a 20 cm square, and the film mass w ₂ was weighed. Next, after heat-treating this film for 10 minutes in a hot air oven at a temperature of 200 ° C., the film mass w ₃ was weighed, and the residual solvent in the film was determined from the following formula. In addition, the measurement was performed twice and the average value was calculated | required.
· Residual solvent _{(%) = (w 2 -w} 3) / w 3 × 100.

3. Glass transition temperature (Tg)
Using a differential scanning calorimeter (DSC-7 manufactured by Perkin Elmer), the measurement was performed at a temperature increase rate of 20 ° C./min in a nitrogen atmosphere. The sample amount was 5 mg.

The glass transition temperature here was measured at a rate of temperature increase of 20 ° C./min using a differential scanning calorimeter (DSC-7 manufactured by Perkin Elmer) and determined according to JIS K7121 (1987). The midpoint glass transition temperature (T _mg ).

4). Transparency (total light transmittance, haze value)
Using a direct reading haze meter manufactured by Toyo Seiki Co., Ltd., the total light transmittance (%) and haze value (%) at 23 ° C. were measured three times, and transparency was evaluated with an average value. A halogen lamp (12V50W) was used as the light source, the total light transmittance was measured according to JIS-K7361-1997, and the haze was measured according to JIS-K7136-2000.

5). In-plane birefringence (N1, N3), thickness birefringence (N2, N4)
Retardation (R (589)) at a wavelength of 589 nm is measured using the following measuring device, the in-plane maximum refractive index direction is the x direction, the in-plane orthogonal direction to x is the y direction, and the film thickness direction is The z direction was defined (stretched films were similarly defined as the nx ′, ny ′, and nz ′ directions) and calculated using the following formula. Measurement was performed at 5 points, and an average value was obtained.
・ Device: Phase difference measuring device KOBRA-21ADH (manufactured by Oji Scientific Instruments)
・ Measurement diameter: φ5mm
-Constant wavelength: 589nm
In-plane birefringence: N1 = | nx−ny |
N3 = | nx′−ny ′ |
Thickness birefringence: N2 = | (nx + ny) / 2−nz |
N4 = | (nx ′ + ny ′) / 2−nz ′ |.

6). Photoelastic coefficient Retardation at a wavelength of 589 nm (R (589)) when no load is applied to the test piece and when a load of 4.9 N is applied in the slow axis direction of the test piece, respectively ) And the photoelastic coefficient was calculated according to the following equation. One measurement was performed.
-Retardation measuring device: Polarization microscope 5892 (manufactured by Nikon Corporation)
Photoelastic coefficient [m ² / N] = (phase difference under load [nm] −phase difference under no load [nm]) × test specimen width [mm] × 10 ⁻¹² / load [N].

[Example 1]
(1) Preparation of acrylic polymer (A-1) First, a methyl methacrylate / acrylamide copolymer suspension was prepared as follows.

  While charging 20 parts by mass of methyl methacrylate, 80 parts by mass of acrylamide, 0.3 part by mass of potassium persulfate, and 1500 parts by mass of ion-exchanged water into the reactor, and replacing the reactor with nitrogen gas, The reaction was allowed to proceed at 70 ° C. until it was completely converted to a polymer. The obtained aqueous solution was used as a suspending agent. A solution in which 0.05 part by mass of the above suspending agent is dissolved in 165 parts by mass of ion-exchanged water is supplied to a stainless steel autoclave having a capacity of 5 liters and equipped with a baffle and a foudra-type stirring blade, and the system is filled with nitrogen gas. It stirred at 400 rpm, replacing.

Next, a mixed substance having the following charge composition was added while stirring the reaction system.
Methacrylic acid: 27 parts by mass Methyl methacrylate: 73 parts by mass t-dodecyl mercaptan: 1.2 parts by mass 2,2′-azobisisobutyronitrile: 0.4 parts by mass

  After the addition, the temperature was raised to 70 ° C., and the time when the internal temperature reached 70 ° C. was set as the polymerization start time, and the polymerization was continued for 180 minutes.

Thereafter, the reaction system was cooled, the polymer was separated, washed and dried in accordance with a normal method to obtain a bead-shaped copolymer. The polymerization rate of this copolymer was 97%, and the mass average molecular weight was 130,000. 0.2% by mass of additive (NaOCH ₃ ) is blended in the above copolymer, and 10L of nitrogen is added from the hopper part using a twin screw extruder (TEX30 (manufactured by Nippon Steel Co., Ltd., L / D = 44.5)). An intramolecular cyclization reaction was carried out at a screw rotation speed of 100 rpm, a raw material supply rate of 5 kg / h, and a cylinder temperature of 290 ° C. while purging at an amount of / min. To obtain a pellet-like acrylic polymer (A-1). The molecular weight of the polymer (A-1) was 130,000, and the glass transition temperature (Tg) was 140 ° C.

(2) Film formation Acrylic polymer (A-1) With respect to 95 parts by mass, a styrene-based polymer having a weight average molecular weight of 2700 as a polymer (I-1) containing a styrene polymer having a weight average molecular weight of 10,000 or less. 5 parts by mass of coalescence (FTR0140 manufactured by Mitsui Chemicals) was added to 300 parts by mass of methyl ethyl ketone, followed by stirring and mixing at room temperature for 4 hours to obtain a transparent solution. This solution is cast on a PET film, 5 minutes at 60 ° C, 5 minutes at 70 ° C, 5 minutes at 90 ° C, 5 minutes at 100 ° C, 5 minutes at 120 ° C, 15 minutes at 130 ° C, and 170 ° C. After drying for 20 minutes, the film was peeled off from the PET film. The thickness of the film was 30 μm, the residual solvent was 0.7%, and the glass transition temperature (Tg) was 123 ° C.

The obtained film had a light transmittance of 92.5% and a haze value of 0.6%, and it was confirmed that the film had high transparency. The in-plane birefringence (N1) of this film was 0.000020, the thickness birefringence (N2) was 0.000027, and the photoelastic coefficient was 2.0 × 10 ⁻¹² m ² / N. Further, when this film was stretched 50% at 108 ° C., a homogeneous stretch-oriented film was obtained, and the in-plane birefringence (N3) was 0.001090 and the thickness birefringence (N4) was 0.000460.

[Comparative Example 1]
In Example 1, a film was formed in the same manner as in Example 1 except that a polymer (i) containing a styrene polymer having a weight average molecular weight of 10,000 or less was not added.

  The obtained film had a thickness of 30 μm, a residual solvent of 0.7%, and a glass transition temperature (Tg) of 130 ° C.

Moreover, the light transmittance of the obtained film was 93.1%, the haze value was 0.2%, and it was confirmed that the transparency was high. The in-plane birefringence (N1) of this film was 0.000001, the thickness birefringence (N2) was 0.000010, and the photoelastic coefficient was 1.0 × 10 ⁻¹² m ² / N. Further, when this film was stretched 50% at 115 ° C., a homogeneous stretch-oriented film was obtained, and the in-plane birefringence (N3) was 0.001580 and the thickness birefringence (N4) was 0.001520.

[Comparative Example 2]
Example 1 is the same as Example 1 except that 25 parts by mass of a polymer containing a styrene polymer having a weight average molecular weight of 10,000 or less (I-1: Mitsui Chemicals FTR0140, weight average molecular weight 2700) is added. The film was formed by the method. The obtained film had a thickness of 30 μm, a residual solvent of 0.7%, and a glass transition temperature (Tg) of 112 ° C.

Moreover, the light transmittance of the obtained film was 80.1%, the haze value was 88%, and transparency was impaired. The in-plane birefringence (N1) of this film was 0.000010, the thickness birefringence (N2) was 0.000010, and the photoelastic coefficient was 1.0 × 10 ⁻¹² m ² / N. Furthermore, when this film was stretched 50% at 97 ° C., a homogeneous stretch-oriented film was obtained, and its in-plane birefringence (N3) was 0.000600 and thickness birefringence (N4) was 0.000410.

[Comparative Example 3]
Example 1 is the same as Example 1 except that a styrene polymer having a weight average molecular weight of 50000 (Piccolastic D125 manufactured by EASTMAN) is used instead of the polymer (ii) containing a styrene polymer having a weight average molecular weight of 10,000 or less. The film was formed by the method. The thickness of the obtained film was 30 μm, the residual solvent was 0.7%, and the glass transition temperature (Tg) was 125 ° C.

Further, the obtained film had a light transmittance of 83% and a haze value of 23%, and the transparency was impaired. The in-plane birefringence (N1) of this film was 0.000010, the thickness birefringence (N2) was 0.000010, and the photoelastic coefficient was 2.0 × 10 ⁻¹² m ² / N. Further, when this film was stretched 50% at 110 ° C., a homogeneous stretch-oriented film was obtained, and its in-plane birefringence (N3) was 0.000880 and thickness birefringence (N4) was 0.000430.

  Since the film of the present invention is excellent in transparency, it can be used, for example, in various covers, cameras, VTRs, projection TVs and other finders, filters, and the like. Further, since almost no birefringence is generated, it is extremely useful for a film for a liquid crystal display device such as a polarizer protective film, various optical disk substrate protective films and the like.

Claims (8)

An acrylic film made of a resin comprising a mixture of an acrylic polymer (A) having a ring structure and a polymer (I) containing a styrene polymer having a weight average molecular weight of 10,000 or less, and the following condition (i) An acrylic film that satisfies (ii) at the same time.
(I) When the whole resin is 100 parts by mass, the mass part of (A) is m, and the mass part of (I) is n, the following expressions (1) to (3) are satisfied.
80.0 ≦ m ≦ 99.9 (1)
0.1 ≦ n ≦ 20.0 (2)
95 ≦ m + n ≦ 100 (3)
(Ii) Total light transmittance of 85% or more In the acrylic film, the maximum refractive index direction in the film plane is defined as the x direction, the orthogonal direction to the x direction in the film plane is defined as the y direction, and the film thickness direction is defined as the z direction. When nx, ny, and nz are set, the in-plane birefringence N1 represented by the following formula (4) is 0.0001 or less, and the thickness birefringence N2 represented by the following formula (5) is 0.0001 or less. The acrylic film according to claim 1.
N1 = | nx−ny | (4)
N2 = | (nx + ny) / 2−nz | (5) When the acrylic film is stretched by 50% at a glass transition temperature (Tg) of −15 ° C., the in-plane maximum refractive index direction is defined as the x ′ direction, and the orthogonal direction to the in-plane x ′ direction is defined as y ′. Direction, the film thickness direction is defined as z ′ direction, and the refractive index in each direction is nx ′, ny ′, nz ′, the in-plane birefringence N3 represented by the following formula (6) is 0. The acrylic film according to claim 1 or 2, wherein the thickness birefringence N2 represented by the following formula (7) is 0.0015 or less.
N3 = | nx′−ny ′ | (6)
N4 = | (nx ′ + ny ′) / 2−nz ′ | (7) The acrylic polymer (A) having the ring structure is composed of an acrylic polymer containing at least one of structural units represented by the following structural formulas (a) to (c). An acrylic film according to any one of the above.

(In the above formulas, R ¹ and R ² represent the same or different hydrogen atoms, an alkyl group having 1 to 5 carbon atoms, a carboxyl group, or a carboxyalkyl group having 2 to 5 carbon atoms. , X ¹ and X ² represent the same or different CH ₂ or C═O, X ³ represents O or NR ³ , R ³ represents a hydrogen atom, an alkyl group having 1 to 5 carbon atoms or a carboxyl group Represents a group or a carboxyalkyl group having 2 to 5 carbon atoms.)

(In the above formula, R ^{4 represents} a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, a carboxyl group, or a carboxyalkyl group having 2 to 5 carbon atoms.)

(In the above formula, R ⁵ represents a substituent containing an alicyclic structure having 6 to 15 carbon atoms.) The acrylic polymer according to any one of claims 1 to 4, wherein the acrylic polymer (A) having a ring structure contains a glutaric anhydride unit represented by the following structural formula (f). the film.

(In said formula, R < ⁶ >, R < ⁷ > represents the same or different hydrogen atom or a C1-C5 alkyl group.)   When the acrylic polymer (A) having the ring structure includes an unsaturated carboxylic acid alkyl ester unit and a glutaric anhydride unit, and the acrylic polymer (A) having the ring structure is 100 parts by mass. The acrylic film according to claim 1, comprising 50 to 90 parts by mass of an unsaturated carboxylic acid alkyl ester unit and 10 to 50 parts by mass of a glutaric anhydride unit.   The acrylic film according to claim 1, wherein the acrylic film is used as a polarizer protective film.   A polarizing plate comprising a polarizer protective film comprising the acrylic film according to claim 1.