JP2012082304A - Optical film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical film in which there is a little film defect resulting from thermal deterioration at the time of molding processing, and which has outstanding transparency in view of subjects, such as a film defect in a prolonged production, contamination of a mold or a forming roll, deterioration of productivity, and decomposition of a resin in a manufacturing process.SOLUTION: The optical film comprises a homopolymer of methyl methacrylate, wherein the content of an acrylate unit contained as an impurity in a homopolymer main chain of methyl methacrylate is less than 1.0% by a thermal decomposition GC/MS analysis.

Description

  The present invention relates to an optical film, and more particularly to an optical film with few film defects caused by thermal deterioration during film formation.

  2. Description of the Related Art In recent years, electronic devices have become increasingly smaller, and liquid crystal display devices that take advantage of the features of light weight and compactness, such as notebook personal computers, mobile phones, and portable information terminals, have been increasingly used. These liquid crystal display devices start with a polarizing film, and various films are used to maintain the display quality. Also, a plastic liquid crystal display device using a resin film instead of a glass substrate has been put into practical use for the purpose of further reducing the weight of the liquid crystal display device for portable information terminals and mobile phones.

  When handling polarized light as in a liquid crystal display device, the resin film used is optically transparent, optically homogeneous, less colored or discolored, and appearance defects such as punctiform or streak-like It is required that there is little. Further, as in the case of a film substrate for a plastic liquid crystal display device in which the glass substrate is replaced with a resin film, it may be required that the phase difference represented by the product of birefringence and thickness is small. Furthermore, it may be required that the phase difference of the film hardly changes due to external stress or the like.

  Similarly to liquid crystal display devices, conventional glass used in various photographing devices such as cameras, film-integrated cameras, video cameras, optical pickup devices such as CDs and DVDs, OA equipment such as projectors, etc. has been used. Lenses are also being replaced with resin to reduce weight. Such plastic lenses are easily affected by phase difference due to focal length shift due to distortion due to temperature, humidity, and other usage environments, and stress during processing such as injection molding. It is demanded that it is difficult to change as well as film. As a resin composition that satisfies these requirements, the use of a (meth) acrylic resin having high transparency and high heat resistance in place of conventional triacetyl cellulose has been studied. A typical (meth) acrylic resin is polymethylmethacrylate (PMMA), but commercially available PMMA is used to prevent resin depolymerization and improve flowability during polymerization and processing. For this purpose, methyl acrylate is copolymerized.

  Because optical films with few appearance defects are required, if a resin material mainly composed of (meth) acrylic resin is used as the optical film material, foreign substances in the resin material that cause appearance defects are removed. Therefore, when extruding and molding an optical film by molding a molding material containing the resin material, it is necessary to remove foreign substances through a polymer filter. However, when the melt film formation method is used, the heat applied during this filtering causes thermal degradation of the (meth) acrylic resin, resulting in foaming due to decomposed monomers and optical defects in the molded film. was there. It is known to add an antioxidant such as a phosphorus compound, a hindered phenol compound, a lactone compound, a hindered amine compound, and a thioether compound in order to prevent such thermal deterioration of the (meth) acrylic resin. (Patent Document 1). Moreover, the resin composition which introduce | transduced the monomer unit which has a cyclic structure into the (meth) acrylic resin used as a raw material and improved heat resistance is disclosed (patent document 2).

WO 2006/054410 JP 2006-328334 A

  In the method of Patent Document 1, there is room for improvement with respect to film defects in long-time production. Furthermore, since the resin becomes high temperature during processing, a part of the additive volatilizes and contaminates the mold and the molding roll, so there is room for improvement. In addition, the method of Patent Document 2 requires a step of introducing a monomer unit having a cyclic structure into the raw material (meth) acrylic resin, resulting in a decrease in productivity or the progress of decomposition of the resin during the manufacturing process. Therefore, there was room for improvement.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide an optical film having excellent transparency with few film defects due to thermal deterioration during molding.

As a result of intensive studies in view of the above problems, the present inventors have found that an optical film using a specific (meth) acrylic thermoplastic resin is excellent as an optical film. That is, the present invention relates to the following.
(I) An optical film composed of a homopolymer of methyl methacrylate, and the content of acrylic ester units contained as impurities in the homopolymer main chain of methyl methacrylate is 1.0% by pyrolysis GC / MS analysis An optical film characterized by being less than.
(Ii) The optical film as described in (i), which is a stretched film.
(Iii) A polarizer protective film comprising the optical film according to (i) or (ii).
(Iv) A polarizing plate comprising at least one polarizer protective film according to (iii).
(V) A polarizing plate using at least one polarizer protective film according to (iii) on the liquid crystal side of the polarizing plate.

  According to the present invention, it is possible to obtain an optical film that has few film defects due to thermal degradation during molding and has an excellent appearance.

  An embodiment of the present invention will be described below.

  The optical film of the present invention comprises a homopolymer of methyl methacrylate.

(Methyl methacrylate)
The methyl methacrylate contained in the optical film of the present invention is a homopolymer, and the content of acrylate units contained as impurities in the methyl methacrylate homopolymer main chain is 1.0 by pyrolysis GC / MS analysis. It is characterized by being less than%. In pyrolysis GC / MS analysis measurement, the ratio of monomers constituting the resin was calculated from the area ratio of the obtained monomer components.

  Usually, polymethyl methacrylate that is generally commercially available is copolymerized with a few percent of an acrylic ester such as methyl acrylate to prevent depolymerization of the resin, but the homopolymer of methyl methacrylate of the present invention is It is characterized by not containing an acrylate ester. However, in the case of synthesis by a continuous bulk polymerization method or the like, it is a case where it is unintentionally mixed as an impurity for manufacturing reasons, and the content of acrylate units must be less than 1.0%. Less than 1% is preferable, and it is more preferable not to contain.

  Usually, when the acrylate unit is less than 1.0%, the above-described depolymerization occurs, the heat resistance is not sufficient, and a problem may occur during molding. However, on the contrary, it has been found that the appearance of the formed film is improved by making the acrylate unit less than 1.0%.

  When the acrylic ester unit is contained, the glass transition temperature of the resin tends to be low. Furthermore, when an acrylate unit is contained, the appearance of the molded film tends to deteriorate. This is presumed to be due to the following mechanism. Since the resin becomes high temperature during melt film formation, not only depolymerization but also cleavage of the polymer main chain occurs. When an acrylate unit is contained, depolymerization can be prevented, but cleavage of the main chain is more likely to occur compared to a methacrylic ester unit. Compared with the monomer component produced by depolymerization, the low molecular weight component produced by cleavage of the seed chain is likely to cause defects in the film, so the appearance of the film is poor when acrylate units are contained. Tend to be.

  The polymethyl methacrylate has a glass transition temperature of 110 ° C. or higher, preferably 115 ° C. or higher, and more preferably 120 ° C. or higher. Below this range, the heat resistance of the film is inferior, resulting in a large change in physical properties at high temperatures and a narrow application range. In particular, when used in optical applications, if the glass transition temperature is lower than the above range, the film tends to be distorted in a high temperature environment, and stable optical characteristics tend not to be obtained.

While the polymethyl weight average molecular weight of methacrylate is not particularly limited, it is preferably ^{1 × 10 4 ~5 × 10 5} , and more preferably a ^{5 × 10 4 ~3 × 10 5} . If it is in the said range, moldability will not fall or the mechanical strength at the time of film processing will not be insufficient.

  On the other hand, if the weight average molecular weight is smaller than the above range, the mechanical strength of the film tends to be insufficient. Moreover, when larger than the said range, the viscosity at the time of melt-extrusion is high, there exists a tendency for the moldability to fall and for the productivity of a molded article to fall.

(Method for producing methyl methacrylate)
Here, although one embodiment of the method for producing the homopolymer of methyl methacrylate will be described, the present invention is not limited to this.

  The homopolymer of methyl methacrylate in the present invention can be obtained by polymerizing methyl methacrylate. The production method is not particularly limited, and known emulsion polymerization methods, emulsion-suspension polymerization methods, suspension polymerization methods, bulk polymerization methods, solution polymerization methods, and the like can be applied. From the viewpoint of being small, the bulk polymerization method and the solution polymerization method are particularly preferable. For example, it can be produced according to the methods described in JP-A-56-8404, JP-B-6-86492, JP-B-52-32665, and the like.

(Additive)
In the optical film of the present invention, when forming into a film, if necessary, it contains a heat stabilizer, a UV absorber, a processability improver such as a lubricant, or a known additive such as a filler and other polymers. You can do it. The other additives are optional components, and the methyl methacrylate homopolymer according to the present invention may not contain these other additives.

  Specific examples of the UV absorber include triazine UV absorbers, benzotriazole UV absorbers, benzophenone UV absorbers, cyanoacrylate UV absorbers, benzoxazine UV absorbers, and oxadiazoles. An ultraviolet absorber can be mentioned. These ultraviolet absorbers may be used alone or in combination. Of these, triazine-based ultraviolet absorbers are preferred from the viewpoints of ultraviolet absorption ability and volatility. Specific examples of preferable triazine-based ultraviolet absorbers include the following compounds described in WO2001 / 047900, WO2005 / 109052, WO2007 / 114112, and JP2007-217667. Commercially available products include trade names Tinuvin 400, Tinuvin 460, Tinuvin 477 and the like manufactured by Ciba Specialty Chemicals. These may be used alone or in combination of two or more.

  The heat stabilizer is not particularly limited, but phosphorus compounds, hindered phenol compounds, lactone compounds, hindered amine compounds, and thioether compounds can be suitably used. Among these, a phosphorus-based heat stabilizer can be preferably used.

  The plasticizer includes, in addition to a so-called plasticizer, a polymer having flexibility (flexible polymer). That is, in this specification, a plasticizer, a flexible polymer, and the like are collectively referred to as a plasticizer.

  The plasticizer is not particularly limited, and any conventionally known plasticizer can be used. Specific examples include aliphatic dibasic acid plasticizers such as di-n-decyl adipate and phosphate ester plasticizers such as tributyl phosphate.

  By including the plasticizer in the homopolymer of methyl methacrylate according to the present invention, the mechanical properties can be improved.

  On the other hand, the addition of the plasticizer may cause a problem that the glass transition temperature of the resulting film is lowered and the heat resistance is impaired or the transparency is impaired.

  Therefore, when the plasticizer is contained in the methyl methacrylate homopolymer according to the present invention, it is preferably added within a range that does not hinder the performance of the film.

  Specifically, the content of the plasticizer in the homopolymer of methyl methacrylate is preferably 20% by weight or less, preferably 10% by weight or less, and more preferably 5% by weight or less. .

  When the above-mentioned lubricant is contained in the methyl methacrylate homopolymer according to the present invention, when the methyl methacrylate homopolymer is molded, the releasability between the molding roll and the methyl methacrylate homopolymer can be improved. The lubricant to be used is not particularly limited, and any known lubricant can be used. For example, higher alcohols such as stearyl alcohol, fatty acids such as stearic acid, fatty acid amides such as ethylene bis stearic acid amide and stearyl amide, fatty acid esters such as stearyl stearate, zinc stearate, calcium 12-hydroxystearate, etc. Metal soap etc. can be used. Among these, fatty acid amides are preferable from the viewpoint of compatibility with the resin.

  When a lubricant is added, the content of methyl methacrylate in the homopolymer is preferably 1% by weight or less, more preferably 0.1% by weight or less, and further preferably 0.01% by weight or less. preferable.

  In addition, resin contained as another additive may be used individually by 1 type, and may be used in combination of multiple types of resin.

  When the above resin is added as another additive, the content of methyl methacrylate in the homopolymer is preferably 1% by weight to 30% by weight, more preferably 2% by weight to 20% by weight, More preferably, it is 3 to 10% by weight.

  On the other hand, when the content of the resin is larger than the above range, the above-described performance of the methyl methacrylate homopolymer tends not to be sufficiently exhibited. On the other hand, if the content of the resin is less than the above range, the effect of adding the resin may be difficult to obtain.

  The filler is not particularly limited, and any conventionally known filler used for a film can be used. The filler may be inorganic fine particles or organic fine particles.

  Examples of fillers that are inorganic fine particles include metal oxide fine particles such as silicon dioxide, titanium dioxide, aluminum oxide, and zirconium oxide, and calcined calcium silicate, hydrated calcium silicate, aluminum silicate, and magnesium silicate. Examples include acid salt fine particles, calcium carbonate, talc, clay, calcined kaolin, and calcium phosphate.

  Examples of the filler that is an organic fine particle include resin fine particles such as a silicon resin, a fluorine resin, an acrylic resin, and a crosslinked styrene resin.

  By adding such a filler, the slipperiness of the film can be improved.

  Although the addition amount of the filler is not particularly limited, it is preferable that the filler is in a range in which the optical properties of the optical film according to the present invention are not significantly impaired.

  Generally, in the methyl methacrylate homopolymer according to the present invention, the filler content is preferably 10% by weight or less.

(Optical film and manufacturing method thereof)
Examples of the optical film according to the present invention include a retardation film, a polarizer protective film, a polarizer protective film having a retardation development function, and a base film of a coating material imparting an optical compensation function. The optical film according to the present invention may be a film that satisfies the conditions described above, but is preferably a stretched film, that is, a stretched film. According to the stretched film, the mechanical properties can be improved.

  In addition, when the optical film concerning this invention is a stretched film, the uniaxially stretched film uniaxially stretched may be sufficient, and the biaxially stretched film obtained by combining a extending process may be sufficient.

  When the optical film according to the present invention is a stretched film, the thickness is not particularly limited, but is preferably 10 μm to 200 μm, more preferably 20 μm to 150 μm, and more preferably 30 μm to 100 μm. More preferably it is.

  When the thickness of the film is within the above range, an optical film having uniform optical characteristics and good haze can be obtained.

  On the other hand, if the thickness of the film exceeds the above range, the cooling of the film becomes non-uniform and the optical characteristics tend to be non-uniform. Moreover, when the thickness of the film is less than the above range, the draw ratio becomes excessive, and the haze tends to deteriorate.

  The optical film according to the present invention preferably has a haze of 1% or less, more preferably 0.7% or less, and even more preferably 0.5% or less.

  If the haze of the optical film according to the present invention is within the above range, the transparency of the film can be increased. Therefore, the optical film according to the present invention can be suitably used for applications requiring transparency.

  The optical film according to the present invention preferably has a total light transmittance of 85% or more, and more preferably 88% or more.

  If the total light transmittance is within the above range, the transparency of the film can be increased. Therefore, the optical film according to the present invention can be suitably used for applications requiring transparency.

  The optical film according to the present invention preferably has a small optical anisotropy when used as a polarizer protective film. In particular, it is preferable that not only the optical anisotropy in the in-plane direction (length direction, width direction) of the film but also the optical anisotropy in the thickness direction is small. In other words, it is preferable that both the in-plane retardation and the thickness direction retardation are small.

  More specifically, the in-plane retardation is preferably 20 nm or less, more preferably 10 nm or less, and even more preferably 5 nm or less with respect to the raw material film. The in-plane retardation of the stretched film is preferably 20 nm or less, more preferably 10 nm or less, and further preferably 5 nm or less.

  The thickness direction retardation is preferably 50 nm or less, more preferably 20 nm or less, and further preferably 5 nm or less with respect to the raw material film. Moreover, regarding the stretched film, the thickness direction retardation is preferably 50 nm or less, more preferably 20 nm or less, and further preferably 10 nm or less.

  If it is set as the structure which has such an optical characteristic, the optical film concerning this invention can be used suitably as a polarizer protective film with which the polarizing plate of a liquid crystal display device is equipped.

  When the in-plane retardation of the film exceeds 10 nm or the thickness direction retardation exceeds 50 nm, the polarizer protective film using the optical film according to the present invention is used as a polarizing plate of a liquid crystal display device. Problems such as a decrease in contrast may occur in the display device.

  The in-plane retardation (Re) and the thickness direction retardation (Rth) can be calculated by the following equations, respectively.

Re = (nx−ny) × d Rth = | (nx + ny) / 2−nz | × d
In the above formula, nx, ny, and nz are respectively the direction in which the in-plane refractive index is the maximum as the X axis, the direction perpendicular to the X axis as the Y axis, and the thickness direction of the film as the Z axis. Represents the refractive index in each axial direction. D represents the thickness of the film, and || represents an absolute value.

Moreover, when the optical film concerning this invention is used as a polarizer protective film, it is preferable that the value of orientation birefringence is 0-0.1 * 10 < ^-3 >, and 0-0.01 * 10 < ^-3 >. It is more preferable that

  If the orientation birefringence is within the above range, stable optical characteristics can be obtained without causing birefringence during molding even when the environment changes.

On the other hand, when the optical film according to the present invention is used for a retardation film or a polarizer protective film having a retardation development function, the value of orientation birefringence is 1.0 × 10 ⁻³ or more. It is preferable that it is 2.0 × 10 ⁻³ or more.

  In the present specification, unless otherwise specified, “orientation birefringence” is intended to mean birefringence that develops when stretched 100% at a temperature 5 ° C. higher than the glass transition temperature of a thermoplastic resin. The orientation birefringence (Δn) is defined by Δn = nx−ny = Re / d and can be measured by a phase difference meter, using nx and ny described above.

In the optical film according to the present invention, the absolute value of the photoelastic coefficient is preferably 20 × 10 ⁻¹² m ² / N or less, more preferably 10 × 10 ⁻¹² m ² / N or less, More preferably, it is 5 × 10 ⁻¹² m ² / N or less.

  If the photoelastic coefficient is within the above range, even if the optical film according to the present invention is used in a liquid crystal display device, phase difference unevenness occurs, contrast at the periphery of the display screen decreases, or light leakage occurs. There is nothing to do.

On the other hand, when the absolute value of the photoelastic coefficient is larger than 20 × 10 ⁻¹² m ² / N, when the optical film according to the present invention is used for a liquid crystal display device, unevenness in phase difference occurs, or the peripheral portion of the display screen There is a tendency that the contrast of the light is reduced or light leakage is likely to occur. This tendency becomes particularly remarkable in a high temperature and high humidity environment.

  In addition, when an external force is applied to an isotropic solid to generate stress (ΔF), it temporarily exhibits optical anisotropy and exhibits birefringence (Δn). By “photoelastic coefficient” is intended the ratio of stress to birefringence. That is, the photoelastic coefficient (c) is calculated by the following equation.

c = △ n / △ F
However, in the present invention, the photoelastic coefficient is a value measured at 23 ° C. and 50% RH at a wavelength of 515 nm by the Senarmon method.

  The optical film according to the present invention may be subjected to surface treatment as necessary. Specifically, for example, when the optical film according to the present invention is used by applying surface processing such as coating processing to the surface or laminating another film on the surface, the optical film according to the present invention is used. It is preferable to perform a surface treatment.

  By performing such a surface treatment, mutual adhesion between the optical film according to the present invention and another film to be coated or laminated can be improved.

  In addition, the objective of the surface treatment with respect to the optical film concerning this invention is not limited to what aims at an effect. That is, the optical film according to the present invention may be subjected to a surface treatment regardless of its use.

  The surface treatment is not particularly limited, and examples thereof include corona treatment, plasma treatment, ultraviolet irradiation, and alkali treatment. Among these, corona treatment is preferable.

  Since the optical film according to the present invention has the characteristics as described above, it can be used as it is as a final product for various applications. Moreover, the range of uses can be expanded by performing various processes as described above.

  Although the use of the optical film according to the present invention is not particularly limited, specifically, for example, imaging fields such as cameras, VTRs, projectors, viewfinders, filters, prisms, and Fresnel lenses, CDs, etc. Optical field for optical discs such as optical discs for CD players, DVD players, MD players, liquid crystal light guide plates, polarizer protective films, retardation films, etc. Information equipment field such as LCD display film and surface protection film, optical communication field such as optical fiber, optical switch and optical connector, automotive field such as automobile headlight, tail lamp lens, inner lens, instrument cover, sunroof, glasses and contact Len , Endoscopic lenses, medical devices such as medical supplies that require sterilization, road translucent plates, pair glass lenses, lighting windows and carports, lighting lenses and covers, and building material sizing It can be suitably used in the field of building materials, microwave cooking containers (tableware) and the like.

  As described above, the optical film according to the present invention is excellent in optical characteristics such as optical homogeneity and transparency. Therefore, it can use especially suitably for well-known optical uses, such as a liquid crystal display periphery, such as an optically isotropic film, a polarizer protective film, and a transparent conductive film, using these optical characteristics.

  The optical film of the present invention can be used as a polarizing plate by being attached to a polarizer. That is, the optical film according to the present invention can be used as a polarizer protective film for a polarizing plate. The polarizer is not particularly limited, and any conventionally known polarizer can be used. Specific examples include a polarizer obtained by containing iodine in stretched polyvinyl alcohol.

  The optical film of the present invention can be used for a polarizing plate in which at least one polarizer protective film is used on the liquid crystal side of the polarizing plate. Although it can be used as a polarizer protective film as described above, when it does not contain an ultraviolet absorber as another additive, it is preferable to use it in a site where the exposure amount of ultraviolet rays is small, since it is more effective. That is, when a liquid crystal display device including a liquid crystal and two polarizing plates is assumed, it is preferably used for a polarizer protective film used on the liquid crystal side with a small amount of exposure to ultraviolet rays.

  Here, although one Embodiment of the method to manufacture the optical film concerning this invention is described, this invention is not limited to this. In other words, any conventionally known method can be used as long as it can produce the optical film according to the present invention.

  Specific examples include injection molding, melt extrusion film molding, inflation molding, blow molding, compression molding, spinning molding, solution casting molding, spin coating molding, and the like.

  Among them, it is preferable to use a melt extrusion method that does not use a solvent. According to the melt extrusion method, it is possible to reduce the burden on the global environment and the working environment due to manufacturing costs and solvents.

  Hereinafter, as an embodiment of the method for producing a film according to the present invention, a method for producing an optical film according to the present invention by a melt extrusion method will be described in detail. In the following description, a film formed by the melt extrusion method is distinguished from a film formed by another method such as a solution casting method and is referred to as a “melt extrusion film”.

  When the optical film according to the present invention is formed by a melt extrusion method, first, the above-mentioned methyl methacrylate homopolymer is supplied to an extruder and heated and melted.

  The methyl methacrylate homopolymer is preferably pre-dried before being fed to the extruder. By performing such preliminary drying, foaming of the resin extruded from the extruder can be prevented.

  Although the method of preliminary drying is not particularly limited, for example, the raw material (that is, the homopolymer of methyl methacrylate according to the present invention) can be formed into pellets or the like, and can be performed using a hot air dryer or the like.

  Next, the homopolymer of methyl methacrylate heated and melted in the extruder is supplied to the T die through a gear pump and a filter. At this time, if a gear pump is used, the uniformity of the extrusion amount of the resin can be improved and thickness unevenness can be reduced. On the other hand, if a filter is used, the foreign material in the homopolymer of methyl methacrylate can be removed, and a film excellent in appearance without defects can be obtained.

  Next, the methyl methacrylate homopolymer supplied to the T die is extruded from the T die as a sheet-like molten resin. Then, the sheet-like molten resin is sandwiched between two cooling rolls and cooled to form a film.

  One of the two cooling rolls sandwiching the sheet-like molten resin is a rigid metal roll having a smooth surface, and the other has a metal elastic outer cylinder having a smooth surface and capable of elastic deformation. A flexible roll is preferred.

  With such a rigid metal roll and a flexible roll having a metal elastic outer cylinder, the sheet-like molten resin is sandwiched and cooled to form a film, thereby correcting minute surface irregularities and die lines. Thus, a film having a smooth surface and thickness unevenness of 5 μm or less can be obtained.

  In this specification, “cooling roll” is used to mean “touch roll” and “cooling roll”.

  Even when the rigid metal roll and the flexible roll are used, since the surface of each cooling roll is metal, when the film to be formed is thin, the surfaces of the cooling roll come into contact with each other. The outer surface may be scratched, or the cooling roll itself may be damaged.

  Therefore, when forming a film by sandwiching the sheet-like molten resin between the two cooling rolls as described above, first, the sheet-like molten resin is sandwiched and cooled by the two cooling rolls, and the film is relatively thick. Obtain raw material film once. Thereafter, the raw film is preferably uniaxially or biaxially stretched to produce a film having a predetermined thickness.

  More specifically, when an optical film having a thickness of 40 μm is produced, the sheet-like molten resin is sandwiched and cooled by the two cooling rolls, and a raw material film having a thickness of 150 μm is once obtained. Thereafter, the raw material film may be stretched by biaxial stretching in the vertical and horizontal directions to produce a film having a thickness of 40 μm.

  Thus, when the optical film according to the present invention is a stretched film, the homopolymer of methyl methacrylate according to the present invention is once formed into an unstretched raw material film, and then uniaxially stretched or biaxially stretched. Thus, a stretched film can be produced.

  In this specification, for convenience of explanation, a film after the methyl methacrylate homopolymer according to the present invention is formed into a film and before being stretched, that is, an unstretched film is referred to as a “raw film”. It should be noted that the raw film is also an embodiment of the optical film according to the present invention.

  When stretching the raw material film, the raw material film may be stretched immediately after the raw material film is formed, or after the raw material film is molded, the raw material film is temporarily stored or moved to stretch the raw material film. You may go.

  In addition, when the raw material film is stretched immediately after being formed into a raw material film, the state of the raw material film may exist only for a very short time (in some cases, instantaneously) in the film manufacturing process.

  Moreover, when the said raw material film is extended | stretched after that, what is necessary is just to maintain the film form of a grade sufficient to be extended | stretched, and does not need to be the state of a perfect film. Moreover, the said raw material film does not need to have the performance as an optical film which is a finished product.

  The method for stretching the raw material film is not particularly limited, and any conventionally known stretching method may be used. Specifically, for example, transverse stretching using a tenter, longitudinal stretching using a roll, sequential biaxial stretching in which these are sequentially combined, and the like can be used.

  Moreover, the simultaneous biaxial stretching method which extends | stretches the length and the width | variety simultaneously can be used, or the method of performing the horizontal extending | stretching by a tenter can be used after performing roll vertical stretching.

  When the raw material film is stretched, it is preferable that the raw material film is once preheated to a temperature 0.5 to 5 ° C., preferably 1 to 3 ° C. higher than the stretching temperature, and then cooled and stretched to the stretching temperature.

  By preheating within the above range, the thickness of the raw material film can be maintained with high accuracy, and the thickness accuracy of the stretched film does not decrease and thickness unevenness does not occur. Further, the raw material film does not stick to the roll and does not loosen due to its own weight.

  On the other hand, if the preheating temperature of the raw material film is too high, there is a tendency that the raw material film sticks to the roll or is loosened by its own weight. In addition, if the difference between the preheating temperature and the stretching temperature of the raw material film is small, it tends to be difficult to maintain the thickness accuracy of the raw material film before stretching, the thickness unevenness increases, or the thickness accuracy tends to decrease.

  In addition, when the methyl methacrylate homopolymer according to the present invention is stretched after being formed into a raw material film, it is difficult to improve the thickness accuracy by utilizing a necking phenomenon. Therefore, in the present invention, it is important to manage the preheating temperature in order to maintain or improve the thickness accuracy of the obtained optical film.

  The stretching temperature when stretching the raw material film is not particularly limited, and may be changed according to the mechanical strength, surface property, thickness accuracy, and the like required for the stretched film to be produced.

  In general, when the glass transition temperature of the raw material film obtained by the DSC method is defined as Tg, the temperature range is preferably (Tg-30 ° C) to (Tg + 30 ° C), and (Tg-20 ° C) to ( Tg + 20 ° C. is more preferable, and a temperature range of (Tg) to (Tg + 20 ° C.) is more preferable.

  When the stretching temperature is within the above temperature range, thickness unevenness of the obtained stretched film can be reduced, and further, mechanical properties such as elongation, tear propagation strength, and fatigue resistance can be improved. Moreover, generation | occurrence | production of the trouble that a film adheres to a roll can be prevented.

  On the other hand, when the stretching temperature is higher than the above temperature range, the thickness unevenness of the stretched film obtained tends to be large, and the mechanical properties such as elongation, tear propagation strength, and fatigue resistance cannot be improved sufficiently. There is. Furthermore, there is a tendency that troubles such as the film sticking to the roll tend to occur.

  In addition, when the stretching temperature is lower than the above temperature range, the haze of the stretched film to be obtained becomes large, or in extreme cases, there is a tendency that process problems such as tearing or cracking of the film occur. is there.

  When the raw material film is stretched, the stretch ratio is not particularly limited, and may be determined according to the mechanical strength, surface properties, thickness accuracy, and the like of the stretched film to be produced. Although it depends on the stretching temperature, the stretching ratio is generally preferably selected in the range of 1.1 to 3 times, more preferably in the range of 1.3 to 2.5 times. Preferably, it is more preferable to select in the range of 1.5 times to 2.3 times.

  If the draw ratio is within the above range, the mechanical properties such as the elongation rate of the film, tear propagation strength, and fatigue resistance can be greatly improved. Therefore, a stretched film having a thickness unevenness of 5 μm or less and a haze of 1% or less can be produced.

  The optical film according to the present invention may be used by laminating another film with an adhesive or the like, or forming a hard coat layer or a coating layer of a compound having an optical compensation function on the surface, as necessary. Can do.

  When surface processing such as coating is applied to the surface of the optical film according to the present invention or when another film is laminated on the surface, the stretched film produced by the above method (the raw film is the optical film according to the present invention) In this case, it is preferable to subject the raw material film) to a surface treatment.

  The types of surface treatment are as described above. In the film according to the present invention, when the surface treatment is performed, the degree of the surface treatment is not particularly limited, but is preferably 50 dyn / cm or more, and is 50 dyn / cm to 80 dyn / cm or less. It is more preferable.

  With such a surface treatment, the surface treatment can be performed using a conventionally known surface treatment facility.

  The present invention is not limited to the configurations described above, and various modifications can be made within the scope of the claims, and the technical means disclosed in different embodiments are appropriately combined. The obtained embodiment is also included in the technical scope of the present invention.

  The present invention will be described more specifically based on examples and comparative examples, but the present invention is not limited to this. Those skilled in the art can make various changes, modifications, and alterations without departing from the scope of the present invention.

〔Glass-transition temperature〕
Using an acrylic resin 10 mg, a differential scanning calorimeter (DSC, DSC-50 manufactured by Shimadzu Corporation) was used to measure under a nitrogen atmosphere at a heating rate of 20 ° C./min, and determined by the midpoint method. .

[Quantification of acrylate units]
By the pyrolysis GC / MS analysis of polymethylmethacrylate, the monomer ratio constituting the resin was calculated from the area ratio of the obtained monomer components. For pyrolysis GC / MS, GC / MS-5993N manufactured by Agilent Technologies was used, and DB-5MS (0.25 mmΦ × 30 m) manufactured by J & W was used as the column. The column temperature was maintained at 35 ° C. for 5 minutes, then heated to 290 ° C. at 10 ° C./minute, and maintained for 19.5 minutes. Helium (1 mL / min) was used as the carrier gas. The inlet temperature was 290 ° C., and the interface temperature was 290 ° C. The thermal decomposition apparatus used JCI-22 made by Nippon Analysis Industry Co., Ltd., and the thermal decomposition temperature was 590 ° C. × 5 seconds. [Measurement of heat loss]
Heat loss (weight%) when 60 minutes passed after heating to 270 ° C. in an air atmosphere with a thermogravimetric measuring device (TGA, TGA-50 manufactured by Shimadzu Corporation) using 15 mg of a homopolymer of methyl methacrylate ) Was measured. The greater the heat loss, the greater the decomposition of the resin due to heating, and the greater the number of defects when producing a film.

(Thickness measurement)
The thickness of the retardation film was measured using a Digimatic Indicator (manufactured by Mitutoyo Corporation). [In-plane retardation Re and thickness direction retardation Rth measurement]
A 40 mm × 40 mm test piece was cut out from the film. Using an automatic birefringence meter (KOBRA-WR manufactured by Oji Scientific Co., Ltd.), the in-plane retardation Re was measured at a temperature of 23 ± 2 ° C. and a humidity of 50 ± 5% at a wavelength of 590 nm and an incident angle of 0 °. It was measured.

  The thickness d of the test piece measured using a digimatic indicator (manufactured by Mitutoyo Corporation), the refractive index n measured by an Abbe refractometer (manufactured by Atago Co., Ltd. 3T), the wavelength 590 nm measured by an automatic birefringence meter, and the surface The three-dimensional refractive index nx, ny, nz is obtained from the internal phase difference Re and the phase difference value in the 40 ° tilt direction, and the thickness direction phase difference Rth = | (nx + ny) / 2−nz | × d (|| is the absolute value) Represents).

[Film appearance evaluation]
A test piece of 210 mm length and 300 mm width was cut out from the obtained film, and the periphery of the film defect observed visually was irradiated with light from a desk stand (National SQ948H, fluorescent lamp 27W) in a dark room. I checked with a pen. Subsequently, the defects checked with a transmission optical microscope (digital microscope (VH-Z75), manufactured by Keyence Corporation) with a magnification of 50 times were observed. The above observations were made on both sides of the film, and the total number and type of defects were measured. A transmission optical microscope with a magnification of 50 times or more is used to distinguish between defects derived from resin caused by thermal degradation of resin, defects derived from contamination of rolls due to additive bleed-out, and defects caused by general foreign matter. Judgment was made based on the color of the defect when observed with. Defects resulting from thermal degradation are yellow and colorless and transparent. The case where the number of defects was very large was rated as x, and the case where there were almost no defects was rated as ◯.

[Production Example 1]
In 8L polymerization machine with a stirrer, water 120 parts, methyl methacrylate 100 parts, dilauroyl peroxide 0.28 parts, 1,1 di (t-butylperoxy) cyclohexane (concentration 80%) 0.2 parts, t -1.4 parts of dodecyl mercaptan, 0.008 part of sodium α-olefin sulfonate, and 0.24 part of calcium phosphate were charged. After deoxygenation, while appropriately adding calcium phosphate, the internal temperature was raised to 80 ° C. and heated for 2.5 hours, and the internal temperature was raised to 90 ° C. and heated for 1.5 hours to carry out polymerization. The obtained latex was removed using hydrochloric acid having a pH of 3 to 4, and further washed thoroughly with water. The molecular weight (Mw) of the obtained methyl methacrylate homopolymer was 106,000. As a result of pyrolysis GC / MS, the monomer unit constituting the resin was 100% methyl methacrylate.

  Example 1 The methyl methacrylate homopolymer obtained in Production Example 1 was pelletized using a single screw extruder to obtain a pellet-shaped resin composition. The glass transition temperature and heat loss of the pellet were measured.

  After drying this pellet-shaped resin composition at 100 ° C. for 5 hours, the sheet-shaped molten resin obtained by extruding at 270 ° C. using a 40 mmφ single screw extruder and a 400 mm wide T-die is cooled with a cooling roll. Upon cooling, a film having a width of 300 mm and a thickness of 130 μm was obtained. About this film, film external appearance evaluation was performed according to said method (Table 1).

  This film was subjected to simultaneous biaxial stretching (biaxial stretching apparatus X4HD manufactured by Toyo Seiki Co., Ltd.) at a stretching ratio of 2 times (longitudinal and lateral) and a temperature 10 ° C. higher than the glass transition temperature to prepare a biaxially stretched film. About this biaxially stretched film, R0 and Rth were measured according to said method (Table 1).

[Comparative Example 1]
A film was prepared in the same manner as in Example 1, except that Kuraray Parapet (grade name HR-S) was used instead of the methyl methacrylate homopolymer. As a result of pyrolysis GC / MS, the monomer units constituting the resin were 98.9% methyl methacrylate and 1.1% methyl acrylate.

Claims (5)

An optical film comprising a homopolymer of methyl methacrylate, wherein the content of acrylate units contained as impurities in the homopolymer main chain of methyl methacrylate is less than 1.0% by pyrolysis GC / MS analysis An optical film characterized by the above. The optical film according to claim 1, which is a stretched film. A polarizer protective film comprising the optical film according to claim 1. A polarizing plate comprising at least one polarizer protective film according to claim 3. A polarizing plate using at least one polarizer protective film according to claim 3 on the liquid crystal side of the polarizing plate.