JP2012096461A - Method of manufacturing optical film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing an optical film which is formed of acrylic resin and hardly changes a phase difference even in a heat-applied environment where, for example, the film is incorporated in an image display device.SOLUTION: The method includes a process for stretching an acrylic resin film, and a process for thermally treating the stretched film while applying tensile force in an in-plane direction and in a direction vertical to an orientation direction of a polymer contained in the film, and the thermal treatment is carried out under conditions indicated by expressions (1) and (2): (1) A≥t≥18.7×e×e; and (2) 0.50≤F≤12.0. The letter t in the expression (1) is a thermal treatment time (sec.), T is a thermal treatment temperature (°C), Tg is a glass transition temperature (°C) of the acrylic resin. A is an upper limit of the thermal treatment time t, when T≤Tg, the time is 120 secs. and when T>Tg, the time is 30 secs. F in the expression (2) is tensile force (N/mm) applied on the stretched acrylic resin film in thermal treatment.

Description

  The present invention relates to a method for producing an optical film.

  A retardation film is incorporated in an image display device such as a liquid crystal display (LCD) for the purpose of color tone compensation and viewing angle compensation. The retardation film is generally formed by stretching a resin film. When the resin film is stretched, the polymer contained in the film is oriented to cause a phase difference. When the resin film contains two or more polymers, depending on the combination, an optical film having a retardation of substantially zero can be obtained by canceling out the retardation caused by the orientation of each polymer. Such an optical film is suitable, for example, as a polarizer protective film used for an LCD.

  When an optical film such as a retardation film is incorporated into an image display device, an appropriate optical film is selected according to the optical design matters of the device. In order to maintain the performance of the image display device, an optical film whose phase difference does not change even after being incorporated in the device is required. The problem here is the heat applied to the optical film when the image display device is used. The image display apparatus has a structure in which a large number of heating elements such as a backlight light source and a circuit board are integrated, and the inside of the apparatus becomes hot during use. Further, depending on the usage environment, the temperature inside the apparatus further increases. When the orientation of the polymer contained in the optical film is relaxed by these heats, the retardation of the film changes, and the performance of the image display device cannot be maintained.

Patent Document 1 (JP 2006-341393), while conveying the cellulose acetate resin film with 2N / cm ² or more 120 N / cm ² or less tension, the glass transition temperature Tg-30 ° C. or higher Tg + 20 of cellulose acetate resin There is disclosed a method for producing a cellulose acetate resin film, characterized by performing a heat treatment for 10 seconds or more and 600 seconds or less at a temperature of ℃ or less. Patent Document 1 describes that this manufacturing method suppresses thermal shrinkage that causes frame failure and color unevenness in the cellulose acetate resin film.

  Patent Document 2 (Japanese Patent Application Laid-Open No. 2010-26098) discloses a retardation film that includes two types of resins that are phase-separated from each other, and one resin is a thermoplastic elastomer. In Patent Document 2, the method of Patent Document 1 is not preferable because the retardation value of the retardation film is lowered by heat during heat treatment (paragraph 0012), whereas the method of Patent Document 2 is not suitable for such a method. It is described that a highly durable retardation film can be obtained without causing a decrease in retardation value and without deterioration of optical compensation performance against heat exposure in a liquid crystal display panel (paragraph 0088, etc.). .

JP 2006-341393 A JP 2010-26098 A

  One type of optical film is an optical film composed of an acrylic resin mainly composed of a (meth) acrylic polymer. The optical film has high transparency and surface strength derived from an acrylic resin and a low photoelastic coefficient, and is suitable for use in an image display device. However, since acrylic resin hardly develops a phase difference by stretching, it is necessary to stretch at higher magnification than other resins. And since it is necessary to stretch at a high magnification, a change in phase difference due to heat applied during use of the apparatus tends to become large after being incorporated into the image display apparatus.

The method for producing an optical film of the present invention is a method for producing an optical film composed of an acrylic resin containing a (meth) acrylic polymer as a main component, the step of stretching the acrylic resin film (stretching step), A step (heat treatment step) of heat-treating the stretched acrylic resin film while applying tension in a direction perpendicular to the orientation direction of the polymer contained in the film. In the heat treatment step, the heat treatment is performed under the conditions shown in the following formulas (1) and (2).
A ≧ t ≧ 18.7 × e ^0.0868Tg × e ^−0.09T (1)
0.50 ≦ F ≦ 12.0 (2)

In the formula (1), t is a heat treatment time (second), T is a heat treatment temperature (° C.), and Tg is a glass transition temperature (° C.) of the acrylic resin constituting the acrylic resin film. A is the upper limit of the heat treatment time t, which is 120 seconds when T ≦ Tg, and 30 seconds when T> Tg. F in Formula (2) is the tension (N / mm ² ) applied to the stretched acrylic resin film during the heat treatment.

  According to the present invention, it is possible to manufacture an optical film (for example, a retardation film) that hardly changes in retardation even in an environment where heat is applied, such as being incorporated in an image display device. Such an optical film realizes, for example, an image display device with excellent durability that hardly changes in performance over a long period of time.

  In the present specification, “resin” is a broader concept than “polymer”. The resin may be composed of, for example, one type or two or more types of polymers, and if necessary, materials other than the polymer, for example, additives such as ultraviolet absorbers, antioxidants, fillers, and compatibilizing agents. Further, it may contain a stabilizer and the like.

[Stretching process]
In the stretching step, the acrylic resin film is stretched. By stretching, the polymer contained in the acrylic resin film is oriented, and the film exhibits a retardation based on the orientation of the polymer. In addition, when the acrylic resin film contains two or more polymers, depending on the combination, the phase difference caused by the orientation of each polymer cancels out, so the phase difference indicated by the film is substantially zero (absolute value). 0 nm to 0.5 nm). The manufacturing method of this invention includes the form which manufactures such an optical film.

  The acrylic resin film may be stretched according to a known method. Stretching is, for example, uniaxial stretching or biaxial stretching. The uniaxial stretching is typically free end uniaxial stretching in which the change in the width direction of the acrylic resin film is free, but may be fixed end uniaxial stretching in which the change in the width direction of the acrylic resin film is fixed. The biaxial stretching is typically sequential biaxial stretching, but may be simultaneous biaxial stretching in which longitudinal and transverse stretching are simultaneously performed.

  A known stretching machine can be applied to the stretching of the acrylic resin film. The stretching machine includes a longitudinal stretching machine and a transverse stretching machine, both of which can be used. The longitudinal stretching machine is a device that stretches the film in the transport direction of the acrylic resin film, and typically includes transport rolls sequentially arranged in the direction of transporting the resin film. In the stretching machine, the resin film is stretched in the transport direction by giving a peripheral speed difference between the transport rolls. The transverse stretching machine is an apparatus that stretches the film in the in-plane direction of the acrylic resin film and is perpendicular to the transport direction, and is, for example, a tenter stretching machine. The tenter stretching machine may be a grip type or a pin type, but the grip type is preferable because the resin film is hard to tear. A grip-type tenter stretching machine generally includes a clip traveling device for transverse stretching. In the clip traveling device, the resin film is conveyed in a state where the lateral end of the resin film is sandwiched between the clips. At this time, the resin rail is stretched laterally by opening the guide rail of the clip traveling device and widening the distance between the left and right two rows of clips. In the grip-type tenter stretching machine, the resin film can be simultaneously biaxially stretched by providing a clip expansion / contraction function with respect to the transport direction of the resin film.

  The degree of stretching can be adjusted according to the composition of the acrylic resin constituting the acrylic resin film and the retardation desired to be finally obtained as the optical film.

  The stretching temperature is preferably in the vicinity of the glass transition temperature Tg of the acrylic resin constituting the acrylic resin film. Specifically, (Tg-30) ° C to (Tg + 100) ° C is preferable, and (Tg-20) ° C to (Tg + 80) ° C is more preferable. When the stretching temperature is lower than (Tg-30) ° C., the desired phase difference may not be obtained. If the stretching temperature exceeds (Tg + 100) ° C., the resin constituting the film may flow, and stable stretching may not be performed.

  The draw ratio is, for example, in the range of 1.1 to 25 times, preferably in the range of 1.3 to 10 times, depending on the composition of the acrylic resin constituting the acrylic resin film and the retardation desired to be finally obtained as an optical film. It can be adjusted with.

  The acrylic resin film stretched in the stretching step is typically an unstretched film, but an acrylic resin film that has been previously stretched may be stretched in the stretching step.

  The acrylic resin film stretched in the stretching step may be in the form of a band. In this case, the film can be continuously stretched by continuously supplying the acrylic resin film to a stretching machine. When the acrylic resin film is strip-shaped, stretching in the flow direction (MD direction) is longitudinal stretching, and stretching in the width direction (TD direction) is lateral stretching.

  The acrylic resin film is a film composed of an acrylic resin containing a (meth) acrylic polymer as a main component. The content of the (meth) acrylic polymer in the acrylic resin is usually 50% by weight or more, preferably 70% by weight or more, more preferably 90% by weight or more, and particularly preferably 95% by weight or more. The (meth) acrylic polymer has excellent optical properties such as high light transmittance and low wavelength dependency of refractive index, and is suitable for use in an optical film. The “main component” means a component having the largest content in the resin.

  The (meth) acrylic polymer is a polymer having a structural unit ((meth) acrylic acid ester unit) derived from a (meth) acrylic acid ester monomer. The content of the (meth) acrylic acid ester unit in the (meth) acrylic polymer is usually 50% by weight or more, preferably 70% by weight or more, more preferably 80% by weight or more. The (meth) acrylic polymer may have a ring structure in the main chain. The ring structure can be obtained by, for example, copolymerizing a (meth) acrylate monomer and a monomer having a ring structure, or polymerizing a monomer group including a (meth) acrylate monomer. Then, the cyclization reaction is allowed to proceed to introduce the main chain of the (meth) acrylic polymer. When the polymer has a ring structure in the main chain, the polymer is a (meth) acrylic polymer if the total content of the (meth) acrylic acid ester unit and the ring structure is 50% by weight or more.

  (Meth) acrylic acid ester units are, for example, methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, n-butyl (meth) acrylate, t- (meth) acrylic acid t- Butyl, n-hexyl (meth) acrylate, cyclohexyl (meth) acrylate, benzyl (meth) acrylate, chloromethyl (meth) acrylate, 2-chloroethyl (meth) acrylate, 2-hydroxy (meth) acrylate Each of ethyl, 3-hydroxypropyl (meth) acrylate, 2,3,4,5,6-pentahydroxyhexyl (meth) acrylate, and 2,3,4,5-tetrahydroxypentyl (meth) acrylate It is a structural unit derived from a monomer. The (meth) acrylic polymer preferably has methyl (meth) acrylate units. In this case, the optical properties and thermal stability of the finally obtained optical film are improved. The (meth) acrylic polymer may have two or more (meth) acrylic acid ester units.

  The (meth) acrylic polymer may have a structural unit other than the (meth) acrylic acid ester unit. Such structural units include, for example, styrene, vinyl toluene, α-methylstyrene, α-hydroxymethylstyrene, α-hydroxyethylstyrene, acrylonitrile, methacrylonitrile, ethylene, propylene, 4-methyl-1-pentene, acetic acid. It is a structural unit derived from each monomer of vinyl, 2-hydroxymethyl-1-butene, methyl vinyl ketone, N-vinyl pyrrolidone, and N-vinyl carbazole. The (meth) acrylic polymer may have two or more of these structural units. When the (meth) acrylic polymer has an N-vinylpyrrolidone unit or an N-vinylcarbazole unit, the degree of freedom in controlling the birefringence wavelength dispersibility in the finally obtained optical film is improved. For example, in the visible light region, a retardation film exhibiting wavelength dispersion (so-called reverse wavelength dispersion) having a smaller birefringence (a smaller phase difference) as the wavelength of light becomes shorter can be obtained.

  When a cyclic structure is introduced into the main chain by a cyclization reaction after polymerization, the (meth) acrylic polymer is formed by copolymerization of a monomer group including a monomer having a hydroxyl group and / or a carboxylic acid group. Is preferred. Examples of the monomer having a hydroxyl group include methyl 2- (hydroxymethyl) acrylate, ethyl 2- (hydroxymethyl) acrylate, isopropyl 2- (hydroxymethyl) acrylate, butyl 2- (hydroxymethyl) acrylate, These are methyl 2- (hydroxyethyl) acrylate, methallyl alcohol, and allyl alcohol. Examples of the monomer having a carboxylic acid group include acrylic acid, methacrylic acid, crotonic acid, 2- (hydroxymethyl) acrylic acid, and 2- (hydroxyethyl) acrylic acid. Two or more of these monomers may be used. These monomers become a ring structure located in the main chain of the (meth) acrylic polymer by a cyclization reaction, but it is not necessary that all of the monomers change into a ring structure during the cyclization reaction. The (meth) acrylic polymer after the cyclization reaction may have a structural unit derived from these monomers.

  The weight average molecular weight of the (meth) acrylic polymer is preferably 10,000 to 500,000, more preferably 50,000 to 300,000.

  The (meth) acrylic polymer preferably has a ring structure in the main chain. That is, the acrylic resin constituting the acrylic resin film preferably contains a (meth) acrylic polymer having a ring structure in the main chain, and more preferably as a main component. In this case, the heat resistance and hardness of the finally obtained optical film are improved. Moreover, although the ring structure of the main chain depends on the combination with other polymers contained in the acrylic resin, it contributes to the acrylic resin film exhibiting a large retardation by stretching.

  The ring structure that the (meth) acrylic polymer may have in the main chain is, for example, at least one selected from an N-substituted maleimide structure, a maleic anhydride structure, a glutarimide structure, a glutaric anhydride structure, and a lactone ring structure. It is a seed. The N-substituted maleimide structure is, for example, a cyclohexylmaleimide structure, a methylmaleimide structure, a phenylmaleimide structure, or a benzylmaleimide structure. From the viewpoint of the heat resistance of the finally obtained optical film, the ring structure is preferably a lactone ring structure, an N-alkyl-substituted maleimide structure, a glutarimide structure, a maleic anhydride structure, or a glutaric anhydride structure. From the viewpoint of imparting a positive retardation to the finally obtained optical film, the ring structure is preferably a lactone ring structure, a glutarimide structure, or a glutaric anhydride structure. From the viewpoint of improving the birefringence wavelength dispersibility in the finally obtained optical film, the structure is preferably a lactone ring structure.

  The lactone ring structure is usually a 4- to 8-membered ring, preferably a 5- to 6-membered ring, more preferably a 6-membered ring from the viewpoint of the stability of the ring structure. The lactone ring structure is, for example, a structure represented by the following formula (3).

In the formula (3), R ¹ , R ² and R ³ are each independently a hydrogen atom or an organic residue having 1 to 20 carbon atoms. The organic residue may contain an oxygen atom.

  The organic residue is, for example, an alkyl group having 1 to 20 carbon atoms such as a methyl group, an ethyl group, or a propyl group; an unsaturated aliphatic carbonization having 1 to 20 carbon atoms such as an ethenyl group or a propenyl group. A hydrogen group; an aromatic hydrocarbon group having 1 to 20 carbon atoms such as a phenyl group or a naphthyl group; one of the hydrogen atoms in the alkyl group, the unsaturated aliphatic hydrocarbon group or the aromatic hydrocarbon group; The above is a group substituted with at least one group selected from a hydroxyl group, a carboxyl group, an ether group and an ester group.

The lactone ring structure represented by formula (3) is obtained by, for example, copolymerizing a monomer group containing methyl methacrylate (MMA) and 2- (hydroxymethyl) methyl acrylate (MHMA), Adjacent MMA units and MHMA units in the coalescence can be formed by dealcoholization cyclocondensation. At this time, R ¹ is H, R ² is CH ₃ , and R ³ is CH ₃ .

  When the (meth) acrylic polymer has a ring structure in the main chain, the content of the ring structure in the polymer is not particularly limited, but is usually 5 to 90% by weight, preferably 10 to 70% by weight, and 10 -60 wt% is more preferable, and 10-50 wt% is more preferable. If the content of the ring structure is excessively large, the stretchability and handling properties of the acrylic resin film are lowered.

  The (meth) acrylic polymer having a ring structure in the main chain can be formed by a known method.

  Examples of the (meth) acrylic polymer having a lactone ring structure in the main chain include, for example, JP 2000-230016, JP 2001-151814, JP 2002-120326, JP 2002-254544, It is a polymer described in JP 2005-146084 A, and can be formed by the method described in the publication.

  The (meth) acrylic polymer having a glutaric anhydride structure in the main chain is, for example, a polymer described in JP 2006-283013 A, JP 2006-335902 A, or JP 2006-274118 A. Yes, it can be formed by the method described in the publication.

  Examples of the (meth) acrylic polymer having a glutarimide structure in the main chain include, for example, JP-A-2006-309033, JP-A-2006-317560, JP-A-2006-328329, JP-A-2006-328334, JP-A-2006-337491, JP-A-2006-337492, JP-A-2006-337493, JP-A-2006-337569, JP-A-2007-009182 It can be formed by the method described in the publication.

  The acrylic resin film may contain a thermoplastic polymer other than the (meth) acrylic polymer as long as the effect of the present invention is obtained. Other thermoplastic polymers include, for example, polyethylene, polypropylene, ethylene-propylene copolymers, olefin polymers such as poly (4-methyl-1-pentene); polyvinyl chloride, polyvinylidene chloride, polychlorinated vinyl, etc. Styrene polymers such as polystyrene, styrene-methyl methacrylate copolymer, styrene-acrylonitrile copolymer, acrylonitrile-butadiene-styrene block copolymer; polyethylene terephthalate, polybutylene terephthalate, polyethylene Polyester such as phthalate; Polyamide such as nylon 6, nylon 66, nylon 610; Polyacetal; Polycarbonate; Polyphenylene oxide; Polyphenylene sulfide; Polyether ether ketone; Sulfone; polyethersulfones; polyoxyethylene benzylidene alkylene; polyamideimide; polybutadiene rubber or ABS resin containing an acrylic rubber, rubber-like polymer such as ASA resin; a.

  When the acrylic resin film contains a styrene polymer, the styrene polymer is preferably a styrene-acrylonitrile copolymer from the viewpoint of compatibility with the (meth) acrylic polymer.

  The content of the other thermoplastic polymer in the acrylic resin film is preferably 0 to 50% by weight, more preferably 0 to 40% by weight, further preferably 0 to 30% by weight, particularly preferably 0 to 20% by weight. is there.

  The acrylic resin film may contain materials other than the polymer, for example, additives. Additives include, for example, hindered phenol-based, phosphorus-based, sulfur-based antioxidants; light-resistant stabilizers, weather-resistant stabilizers, thermal stabilizers, and other stabilizers; reinforcing materials such as glass fibers and carbon fibers; UV absorbers such as tyrates, (2,2′-hydroxy-5-methylphenyl) benzotriazole, 2-hydroxybenzophenone; near infrared absorbers; difficulties such as tris (dibromopropyl) phosphate, triallyl phosphate, antimony oxide Antistatic agent composed of anionic, cationic and nonionic surfactants; Colorants such as inorganic pigments, organic pigments and dyes; Organic fillers, inorganic fillers; Antiblocking agents; Resin modifiers; Organic Fillers, inorganic fillers; plasticizers; lubricants; flame retardants.

  The content of the additive in the acrylic resin film is preferably 0 to 5% by weight, more preferably 0 to 2% by weight, and still more preferably 0 to 0.5% by weight.

  The Tg (glass transition temperature) of the acrylic resin film is preferably 100 ° C. or higher, more preferably 110 ° C. or higher, still more preferably 115 ° C. or higher, and particularly preferably 120 ° C. or higher. Although the upper limit of Tg of an acrylic resin film is not specifically limited, From a viewpoint of the drawability of the said film, Preferably it is 170 degrees C or less.

  The (meth) acrylic polymer having a ring structure in the main chain increases the Tg of the acrylic resin film and the optical film obtained by stretching the film, and improves the heat resistance.

  The thickness of an acrylic resin film is not specifically limited, Preferably it is 5-200 micrometers, More preferably, it is 10-100 micrometers.

  The acrylic resin film can be formed by a known method (for example, melt extrusion, casting).

[Heat treatment process]
Next, the acrylic resin film stretched by the stretching process is heat-treated. The heat treatment is performed under the conditions shown in the following formulas (1) and (2) while applying tension in the in-plane direction of the acrylic resin film and perpendicular to the orientation direction of the polymer contained in the film. .
A ≧ t ≧ 18.7 × e ^0.0868Tg × e ^−0.09T (1)
0.50 ≦ F ≦ 12.0 (2)

In the formula (1), t is a heat treatment time (second), T is a heat treatment temperature (° C.), and Tg is a glass transition temperature (° C.) of the acrylic resin constituting the acrylic resin film. A is the upper limit of the heat treatment time t, which is 120 seconds when T ≦ Tg, and 30 seconds when T> Tg. F in Formula (2) is the tension (N / mm ² ) applied to the acrylic resin film during heat treatment.

  By performing such a heat treatment after the stretching step, an optical film (for example, a retardation film) that hardly changes in retardation even under an environment where heat is applied, such as being incorporated in an image display device, can be obtained.

  Formula (1) is the relationship between the heat treatment time t and temperature T, that is, the heat treatment conditions, and the Tg of the acrylic resin constituting the acrylic resin film that is the heat treatment object, that is, the thermal characteristics of the heat treatment object. Stipulate. When the heat treatment time t is less than this lower limit, the effect of the heat treatment becomes insufficient and the effect of the present invention cannot be obtained. On the other hand, if the heat treatment time t exceeds the upper limit A, the orientation of the polymer in the acrylic resin produced by stretching is lost, and an optical film showing the desired retardation cannot be obtained. The upper limit A of the heat treatment time varies greatly depending on whether the heat treatment temperature T is equal to or lower than Tg of the acrylic resin constituting the acrylic resin film or exceeds Tg. This is because the influence of the heat treatment temperature on the orientation of the polymer greatly changes at the Tg.

  The heat treatment can be carried out by storing or transporting the acrylic resin film in a heating furnace maintained at a predetermined temperature. The conveyance of the acrylic resin film to the heating furnace may be continuous or intermittent. When heat treatment is performed using a heating furnace, the heat treatment temperature is the holding temperature of the furnace. The heat treatment time is the time for housing the acrylic resin film in the furnace or the time for the acrylic resin film to pass from the entrance to the exit of the furnace.

  Formula (2) prescribes | regulates the tension | tensile_strength added to an acrylic resin film in the case of heat processing. When the conditions for the heat treatment are as in formula (1) and the tension applied to the acrylic resin film is within a specific direction and in the range specified in formula (2), the orientation of the polymer contained in the film is An optical film that is stabilized and has little change in retardation due to heat can be obtained.

  The direction in which the tension is applied in the heat treatment step is an in-plane direction of the acrylic resin film and a direction perpendicular to the orientation direction of the polymer contained in the film. This direction is a direction that relaxes the orientation of the polymer, but at the time after becoming an optical film by applying the tension in the direction without stretching under the condition of the predetermined heat treatment temperature / time. Relaxation of the orientation of the polymer due to heat is suppressed, and the change in retardation is reduced. In addition, "without extending | stretching" means that the dimension of an acrylic resin film does not change substantially before and after a heat treatment process. For example, when expressed by a draw ratio with respect to the direction in which the tension is applied, the draw ratio in the heat treatment step is in the range of 0.95 times to 1.05 times, preferably 0.98 times to 1.02 times. 99 times to 1.01 times are more preferable.

  The direction in which the tension is applied in the heat treatment step is a plane orthogonal to the slow axis direction of the film (the direction showing the maximum refractive index in the film plane) when the intrinsic birefringence exhibited by the stretched acrylic resin film is positive. Inward direction (fast axis direction). When the intrinsic birefringence exhibited by the stretched acrylic resin film is negative, it is the slow axis direction of the film. When the film is continuously transported in the heat treatment step, for example, the stretched acrylic resin film has a strip shape, the direction in which the tension is applied is not related to the transport direction of the film. Depending on the composition of the acrylic resin and the stretched state, tension is applied in a direction perpendicular to the transport direction. In the production method of the present invention, the direction in which tension is applied is determined by the orientation direction of the polymer contained in the stretched acrylic resin film.

  The positive or negative of the intrinsic birefringence of the resin film, that is, the resin constituting the resin film, is determined by the principal surface of the layer in a layer (for example, a film) in which the polymer contained in the resin is uniaxially oriented (the molecular chain of the polymer). The refractive index n2 of the layer for the vibration component perpendicular to the orientation axis is subtracted from the refractive index n1 of the vibration component perpendicular to the orientation axis of the light incident perpendicularly to the direction of the polymer in the layer (orientation axis). It can be determined based on the obtained value “n1-n2”. The sign of the intrinsic birefringence of the resin is determined by the balance of intrinsic birefringence of the polymer contained in the resin.

  The heat treatment method is not particularly limited as long as the conditions of the formulas (1) and (2) can be realized. For example, the above-described stretching machine can be used. When heat treatment is performed while applying tension to the belt-shaped acrylic resin film in the conveying direction, for example, using a stretching machine maintained at a heat treatment temperature, the heat treatment is performed by giving a peripheral speed difference between the conveying rolls. Can be implemented. The heat treatment for applying tension in the conveying direction can be performed even if the stretching machine is a transverse stretching machine. Specifically, the lateral end of the resin film is opened without being sandwiched between clips, or the distance between the clips that sandwich the lateral end of the resin film is narrowed so that no tension is applied in the width direction of the resin film. Heat treatment may be performed. When heat treatment is performed on a strip-shaped acrylic resin film while applying tension in the width direction (perpendicular to the conveying direction), for example, using a horizontal stretching machine maintained at the heat treatment temperature, the lateral end of the resin film The heat treatment can be carried out by sandwiching between the clips and controlling the distance between the clips.

  When the acrylic resin film has a strip shape, the stretching step and the heat treatment step can be continuously performed. For example, a belt-like acrylic resin film may be conveyed and passed continuously through a stretching device and a heat treatment device. The stretching apparatus and the heat treatment apparatus may be the same apparatus. In this case, for example, a zone for performing the stretching process and a zone for performing the heat treatment in the apparatus may be determined separately. As such an apparatus, for example, there is a stretching machine that includes a plurality of heating furnaces and can individually control the temperature of each heating furnace. The heat treatment temperature is the holding temperature of the zone where the heat treatment is performed. The heat treatment time is the time for the acrylic resin film to pass from the entrance to the exit of the zone where the heat treatment is performed.

  On the other hand, in the production method of the present invention, the stretching step and the heat treatment step may be performed separately. After the stretching step, the acrylic resin film may be once cooled before performing the heat treatment step.

  The production method of the present invention may include any step other than the stretching step and the heat treatment step as long as the effects of the present invention are obtained.

  The optical film obtained by the production method of the present invention is, for example, a retardation film. The in-plane retardation of the obtained retardation film is, for example, more than 0 nm and 1000 nm or less with respect to light having a wavelength of 589 nm, preferably 20 to 500 nm, and more preferably 50 to 300 nm. The retardation in the thickness direction shown by the obtained retardation film is, for example, 30 to 1000 nm, preferably 30 to 500 nm, more preferably 70 to 200 nm as an absolute value with respect to light having a wavelength of 589 nm. Such a retardation film is suitable for use in optical compensation in an image display device such as an LCD.

  Depending on the composition of the acrylic resin constituting the acrylic resin film, the optical film obtained by the production method of the present invention has a substantially zero phase difference (in absolute value of 0 to 0 in the in-plane direction and / or thickness direction). .5 nm). Such an optical film is suitable for use as a polarizer protective film in an image display device such as an LCD.

  The thickness of the optical film obtained by the production method of the present invention is, for example, 10 to 500 μm, preferably 20 to 300 μm, and more preferably 30 to 100 μm.

The optical film obtained by the production method of the present invention has little variation in retardation under a thermal atmosphere. When the film is allowed to stand at a temperature 22 ° C. lower than the Tg of the acrylic resin constituting the optical film (that is, (Tg−22) ° C.) for 20 hours, the phase difference before and after the standing (wavelength of 589 nm of the film with respect to the light) The change rate {(Re ₂ −Re ₁ ) / Re ₁ × 100 (%)} of the in-plane retardation is, for example, less than ± 5.0%, and the composition of the acrylic resin and the stretching conditions and heat treatment of the acrylic resin film Depending on conditions, it is less than ± 4.0%, and further less than ± 3.0%. In the above formula, Re ₁ is an in-plane phase difference before standing, and Re ₂ is an in-plane phase difference after standing.

  Hereinafter, the present invention will be described in more detail with reference to examples. The present invention is not limited to the examples shown below.

  Initially, the evaluation method of the polymer and optical film which were produced in the present Example is shown. The evaluation of the optical film was performed by obtaining an evaluation film piece from the center in the width direction of the film.

[Weight average molecular weight]
The weight average molecular weight of the acrylic resin produced in the production example was determined by gel permeation chromatography (GPC) under the following conditions.
System: GPC system HLC-8220 manufactured by Tosoh Corporation
Developing solvent: chloroform (made by Wako Pure Chemical Industries, special grade), flow rate 0.6 ml / min,
Standard sample: TSK standard polystyrene (manufactured by Tosoh Corporation, PS-oligomer kit),
Measurement side column configuration: Guard column (manufactured by Tosoh Corporation, TSKguardcolumn SuperHZ-L), separation column (manufactured by Tosoh Corporation, TSKgel SuperHZM-M) in series connection,
Reference side column configuration: Reference column (manufactured by Tosoh Corporation, TSKgel SuperH-RC)

[Glass-transition temperature]
The glass transition temperature (Tg) of the acrylic resin produced in the production example was determined in accordance with the provisions of JIS K7121. Specifically, it was obtained by using a differential scanning calorimeter (manufactured by Rigaku, DSC-8230) and heating a sample of about 10 mg from a normal temperature to 200 ° C. at a heating rate of 20 ° C./min in a nitrogen gas atmosphere. It calculated by the starting point method from the DSC curve. Α-alumina was used as a reference.

[thickness]
The thickness of the produced optical film was measured at 20 mm intervals using a Mitsutoyo Digimatic Micrometer (minimum display amount 0.001 mm), and the average value was obtained.

[In-plane retardation]
The in-plane retardation Re of the produced optical film was measured at a measurement wavelength of 589 nm using a retardation measuring device (manufactured by Oji Scientific Instruments, KOBRA-WR).

[In-plane retardation change rate]
The in-plane retardation change rate of the produced optical film was determined by allowing the produced optical film to stand at a temperature 22 ° C. lower than the Tg of the resin constituting the film, that is, (Tg−22) ° C. for 20 hours. The in-plane retardation Re ₁ (before standing) and Re ₂ (after standing) of the optical film were measured and determined as described above. The in-plane retardation change rate is a value obtained by the formula (Re ₂ −Re ₁ ) / Re ₁ × 100 (%). The 20-hour standing at a temperature 22 ° C. lower than the Tg of the resin corresponds to an acceleration test assuming an environment in which the optical film is incorporated into an image display device and used for 1000 hours.

[Intrinsic birefringence]
The intrinsic birefringence of the acrylic resin produced in the production example was determined as follows.

  Using the single-screw extruder (φ = 20 mm, L / D = 25) set at 280 ° C. and the coat hanger type T die (width 150 mm), the prepared resin was cooled from the T die at 110 ° C. The film was extruded and discharged onto a roll to produce an original film (unstretched film) having a thickness of 100 μm. Next, using the biaxial stretching machine (TYPE EX4, manufactured by Toyo Seiki Seisakusho), the produced unstretched film was stretched at a temperature 7 ° C. higher than the Tg of the resin and in the MD direction (extrusion from T die, ejection direction). The film was uniaxially stretched at a free end at a stretching ratio of 2.0 to produce a uniaxially stretched film having a thickness of 70 μm. Next, the orientation angle of the produced stretched film was determined by a fully automatic birefringence meter (manufactured by Oji Scientific Instruments, KOBRA-WR), and the positive / negative of the intrinsic birefringence of the resin was evaluated based on the value. When the measured orientation angle is near 0 ° with respect to the stretching direction, the intrinsic birefringence of the resin is positive, and when it is around 90 °, the intrinsic birefringence of the resin is negative.

(Production Example 1)
Into a reaction vessel having an internal volume of 1000 L equipped with a stirrer, a temperature sensor, a cooling pipe and a nitrogen introduction pipe, 40 parts by weight of methyl methacrylate (MMA), 10 parts by weight of 2- (hydroxymethyl) methyl acrylate (MHMA) Then, 50 parts by weight of toluene and 0.025 part by weight of an antioxidant (manufactured by Asahi Denka Kogyo Co., Ltd., Adeka Stub 2112) were charged as a polymerization solvent, and the temperature was raised to 105 ° C. while passing nitrogen. When refluxing with a rise in temperature started, 0.05 parts by weight of t-amyl peroxyisononanoate (manufactured by Arkema Yoshitomi, trade name: Luperox 570) was added as a polymerization initiator, and 0.10 parts by weight of While t-amyl peroxy isononanoate was added dropwise over 3 hours, solution polymerization was allowed to proceed under reflux at about 105 to 110 ° C., followed by further aging for 4 hours.

  Next, 0.05 parts by weight of 2-ethylhexyl phosphate (manufactured by Sakai Chemical Industry Co., Ltd., Phoslex A-8) is added to the resulting polymerization solution as a catalyst for the cyclization condensation reaction (cyclization catalyst). A cyclization condensation reaction (reaction in which the polymer is subjected to intramolecular dealcoholization reaction to form a lactone ring structure in the polymer molecule) is allowed to proceed for 2 hours under reflux at 110 ° C., and then the polymerization solution is obtained by an autoclave at 240 ° C. Was heated for 30 minutes to further proceed the cyclization condensation reaction.

Next, the obtained polymerization solution was subjected to a barrel temperature of 240 ° C., a rotation speed of 100 rpm, a degree of vacuum of 13.3 to 400 hPa (10 to 300 mmHg), a rear vent number of 1 and a forevent number of 4 (first and 2, a third and a fourth vent), a side feeder is provided between the third vent and the fourth vent, and a leaf disk type polymer filter (filtration accuracy 5 μm, filtration area 1.5 m at the tip) ² ) was introduced into a vent type screw twin screw extruder (φ = 50.0 mm, L / D = 30) at a processing rate of 45 kg / hr in terms of resin amount, and devolatilization was performed. At that time, a separately prepared mixed solution of antioxidant / cyclization catalyst deactivator was charged at a rate of 0.68 kg / hour from the back of the first vent, and ion-exchanged water was charged at 0.22 kg / hour. The speed was added from behind the second and third vents, respectively. In the mixed solution of the antioxidant / cyclization catalyst deactivator, 50 parts by weight of antioxidant (manufactured by Ciba Specialty Chemicals, Irganox 1010) and 35 parts by weight of zinc octylate (manufactured by Nippon Chemical Industry, Nikka octix zinc (3.6%) was dissolved in 200 parts by weight of toluene. In addition, pellets of a styrene-acrylonitrile copolymer (the ratio of styrene / acrylonitrile is 73% by weight / 27% by weight, weight average molecular weight 220,000) were charged from the side feeder at a charging rate of 19 kg / hour.

  After completion of devolatilization, the resin in the molten state remaining in the extruder is discharged from the tip of the extruder while filtering through a polymer filter, pelletized by a pelletizer, and a (meth) acryl having a lactone ring structure in the main chain A transparent acrylic resin (A) pellet containing 70% by weight of polymer and 30% by weight of styrene-acrylonitrile copolymer was obtained. The acrylic resin had a Tg of 122 ° C., a weight average molecular weight of 161,000, and an intrinsic birefringence of negative.

(Production Example 2)
MMA 150 kg, MHMA 75 kg, n-butyl methacrylate (BMA) 25 kg, and 250 kg of toluene as a polymerization solvent were charged in a reaction vessel having an internal volume of 1 m ² equipped with a stirrer, a temperature sensor, a cooling pipe, and a nitrogen introduction pipe. The temperature was raised to 105 ° C. When the reflux with the temperature rise started, 0.15 kg of t-amyl peroxyisononanoate (manufactured by Arkema Yoshitomi, trade name: Luperox 570) was added as a polymerization initiator, and 0.30 kg of t-amyl par While the polymerization initiator solution composed of oxyisononanoate and 3.5 kg of toluene was dropped over 6 hours, the solution polymerization was allowed to proceed under a reflux of about 105 to 111 ° C., and after the addition of the polymerization initiator solution, Aging was performed for 2 hours.

  Next, 0.25 kg of an octyl phosphate / dioctyl phosphate mixture (Phoslex A-8, manufactured by Sakai Chemical Industry Co., Ltd.) as a cyclization catalyst was added to the resulting polymerization solution, and 2 under a reflux of about 85 to 105 ° C. The cyclization condensation reaction was allowed to proceed over time.

  Next, after the obtained polymerization solution was heated to 220 ° C. through a heat exchanger, the barrel temperature was 250 ° C., the rotation speed was 170 rpm, the degree of vacuum was 13.3 to 400 hPa (10 to 300 mmHg), and the number of rear vents was one. In addition, in a vent type screw twin screw extruder (φ = 42 mm, L / D = 42) with four forevents (referred to as the first, second, third and fourth vents from the upstream side) It was introduced at a treatment rate of 15 kg / hour, and further cyclization condensation reaction and devolatilization treatment were performed. At that time, a separately prepared mixed solution of antioxidant / cyclization catalyst deactivator was added from between the first vent and the second vent at a treatment rate of 0.46 kg / hour. The antioxidant / cyclization catalyst deactivator mixed solution contained 0.8 parts by weight of Irganox 1010 manufactured by Ciba Specialty Chemicals and 0.8 parts by weight of Adeka Stub AO-412S manufactured by Asahi Denka Kogyo Co., Ltd. A solution prepared by dissolving 9.8 parts by weight of zinc octylate (manufactured by Nippon Kagaku Sangyo Co., Ltd., Nikka Octics Zinc 18%) in 88.6 parts by weight of toluene was used as an agent.

  After completion of devolatilization, the resin in the melted state remaining in the extruder is discharged from the tip of the extruder, pelletized by a pelletizer, and made of a transparent (meth) acrylic polymer having a lactone ring structure in the main chain. A pellet of acrylic resin (B) was obtained. The acrylic resin had a Tg of 133 ° C., a weight average molecular weight of 120,000, and an intrinsic birefringence of positive.

Example 1
The acrylic resin pellet (A) produced in Production Example 1 was melt-extruded to obtain an unstretched film having a thickness of 180 μm. Next, the obtained unstretched film is continuously stretched longitudinally and transversely and heat-treated using a longitudinal stretching apparatus and a lateral stretching apparatus to obtain an optical film composed of a biaxially stretched acrylic resin stretched film. Produced. Longitudinal stretching (stretching in the melt-extrusion direction) was performed using a difference in peripheral speed between a pair of nip rolls arranged in the longitudinal stretching apparatus held at the stretching temperature with a gap in the transport direction of the resin film. The transverse stretching was performed in a stretching zone in a transverse stretching machine provided with two zones (stretching zone and heat treatment zone) having different set temperatures (holding temperatures). Lateral stretching (stretching in the width direction) was performed using a tenter method, with the lateral end of the acrylic resin film sandwiched between clips. The stretching conditions were a stretching temperature of 140 ° C. and a stretching ratio of 1.9 times for longitudinal stretching, and a stretching temperature of 140 ° C. and a stretching ratio of 2.2 times for lateral stretching.

After the transverse stretching, the stretched film was conveyed to a heat treatment zone set at 125 ° C., and a heat treatment for 15 seconds was performed in the zone. The heat treatment time was defined as the time from when the conveyed stretched film entered the heat treatment zone until it exited the zone. In the heat treatment, a tension of 7.1 N / mm ² was applied to the flow direction of the stretched film (the slow axis direction, the direction perpendicular to the orientation direction of the polymer contained in the acrylic resin (A)). It added using the peripheral speed difference of a pair of conveyance roll arrange | positioned forward and backward. Before and after the heat treatment, there was almost no change in the dimensions of the acrylic resin film in both the flow direction and the width direction of the film, and the magnification was in the range of about 0.98 times to 1.02 times expressed by the draw ratio. . The state of the change of the dimension before and after the heat treatment was the same in each of the following Examples and Comparative Examples (excluding Comparative Example 4).

The stretchable optical film thus obtained has a thickness of 60 μm, Tg of 122 ° C., in-plane retardation Re ₁ of 107.0 nm, and retardation change rate of −2.43% (after standing in a heated atmosphere). The in-plane retardation Re ₂ was 104.4 nm.

(Comparative Example 1)
A stretchable optical film was obtained in the same manner as in Example 1 except that the heat treatment time was 40 seconds and the tension applied to the stretched film during the heat treatment was 7.0 N / mm ² . The thickness of the obtained optical film was 60 μm, Tg was 122 ° C., the in-plane retardation Re ₁ was 105.1 nm, and the retardation change rate was −5.14% (in-plane retardation Re after standing in a heated atmosphere). ₂ was 99.7 nm).

(Example 2)
Using the acrylic resin pellet (A) produced in Production Example 1, a biaxially stretchable optical film was produced in the same manner as in Example 1. The conditions for longitudinal stretching and lateral stretching were the same as in Example 1. After transverse stretching, the stretched film was transported to a heat treatment zone set at 120 ° C., and heat treatment was carried out for 20 seconds in the zone. In the heat treatment, a tension of 7.1 N / mm ² was applied to the flow direction of the stretched film (the slow axis direction, the direction perpendicular to the orientation direction of the polymer contained in the acrylic resin (A)). It added using the peripheral speed difference of a pair of conveyance roll arrange | positioned forward and backward.

The stretchable optical film thus obtained has a thickness of 60 μm, Tg of 122 ° C., in-plane retardation Re ₁ of 107.5 nm, and retardation change rate of −2.05% (after standing in a heated atmosphere). The in-plane retardation Re ₂ was 105.3 nm.

(Comparative Example 2)
Extensibility is the same as in Example 2 except that the heat treatment temperature is set to 112 ° C. by setting the temperature of the heat treatment zone to 112 ° C., and the tension applied to the stretched film during the heat treatment is 7.8 N / mm ² . An optical film was obtained. The thickness of the obtained optical film was 60 μm, Tg was 122 ° C., the in-plane retardation Re ₁ was 107.7 nm, and the retardation change rate was −5.76% (in-plane retardation Re after standing in a heated atmosphere). ₂ was 101.5 nm).

(Example 3)
The acrylic resin pellet (B) produced in Production Example 2 was melt-extruded to obtain an unstretched film having a thickness of 200 μm. Next, the obtained unstretched film is continuously stretched longitudinally and transversely and heat-treated using a longitudinal stretching apparatus and a lateral stretching apparatus to obtain an optical film composed of a biaxially stretched acrylic resin stretched film. Produced. Longitudinal stretching (stretching in the melt-extrusion direction) was performed using a difference in peripheral speed between a pair of nip rolls arranged in the longitudinal stretching apparatus held at the stretching temperature with a gap in the transport direction of the resin film. The transverse stretching was performed in a stretching zone in a transverse stretching machine provided with two zones (stretching zone and heat treatment zone) having different set temperatures (holding temperatures). Lateral stretching (stretching in the width direction) was performed using a tenter method, with the lateral end of the acrylic resin film sandwiched between clips. The stretching conditions were a stretching temperature of 155 ° C. and a stretching ratio of 1.8 times for longitudinal stretching, and a stretching temperature of 155 ° C. and a stretching ratio of 2.2 times for transverse stretching.

After the transverse stretching, the stretched film was conveyed to a heat treatment zone set at 135 ° C., and a heat treatment for 15 seconds was performed in the zone. The heat treatment time was defined as the time from when the conveyed stretched film entered the heat treatment zone until it exited the zone. During the heat treatment, a tension of 9.5 N / mm ² is applied in the width direction of the stretched film (the fast axis direction, the direction perpendicular to the orientation direction of the polymer contained in the acrylic resin (B)). It was added by adjusting the interval between clips that sandwich the end.

The stretchable optical film thus obtained has a thickness of 70 μm, Tg of 133 ° C., in-plane retardation Re ₁ of 82.2 nm, and retardation change rate of −2.43% (after standing in a heated atmosphere). The in-plane retardation Re ₂ was 80.2 nm.

(Comparative Example 3)
A stretchable optical film was obtained in the same manner as in Example 3 except that the tension applied to the stretched film during heat treatment was 0.2 N / mm ² . The obtained optical film had a thickness of 70 μm, Tg of 133 ° C., an in-plane retardation Re ₁ of 82.8 nm, and a retardation change rate of −6.52% (in-plane retardation Re after standing in a heated atmosphere). ₂ was 77.4 nm).

Example 4
A stretchable optical film was obtained in the same manner as in Example 3, except that the tension applied to the stretched film during heat treatment was 0.8 N / mm ² . The obtained optical film had a thickness of 70 μm, Tg of 133 ° C., an in-plane retardation Re ₁ of 82.5 nm, and a retardation change rate of −4.85% (in-plane retardation Re after standing in a heated atmosphere). ₂ was 78.5 nm).

(Example 5)
A stretchable optical film was obtained in the same manner as in Example 3 except that the tension applied to the stretched film during heat treatment was 11.0 N / mm ² . The thickness of the obtained optical film was 70 μm, Tg was 133 ° C., the in-plane retardation Re ₁ was 81.7 nm, the retardation change rate was −4.53% (in-plane retardation Re after standing in a heated atmosphere). ₂ was 78.0 nm).

(Comparative Example 4)
Except that the tension applied to the stretched film during heat treatment was 15.0 N / mm ² , an attempt was made to obtain a stretchable optical film in the same manner as in Example 3, but the film broke during the heat treatment. However, an optical film could not be obtained.

  The results of each Example and Comparative Example are shown in Table 1 below.

  As shown in Table 1, by performing heat treatment under the conditions shown in the formulas (1) and (2), the optical film has a small retardation change rate, that is, an optical film that hardly changes in retardation even in an environment where heat is applied. (Retardation film) was obtained.

  The optical film produced by the production method of the present invention can be suitably used for various applications in which an optical film produced by a conventional method is used, including an image display device such as an LCD.

Claims (2)

A method for producing an optical film comprising an acrylic resin containing a (meth) acrylic polymer as a main component,
Stretching the acrylic resin film;
Heat-treating the stretched acrylic resin film while applying tension in the in-plane direction of the film and in a direction perpendicular to the orientation direction of the polymer contained in the film,
The manufacturing method of an optical film which performs the said heat processing on the conditions shown to the following formula | equation (1) and (2).
A ≧ t ≧ 18.7 × e ^0.0868Tg × e ^−0.09T (1)
0.50 ≦ F ≦ 12.0 (2)
In the formula (1), t is a heat treatment time (second), T is a heat treatment temperature (° C.), and Tg is a glass transition temperature (° C.) of the acrylic resin constituting the acrylic resin film. A is the upper limit of the heat treatment time t, which is 120 seconds when T ≦ Tg, and 30 seconds when T> Tg. F in Formula (2) is the tension (N / mm ² ) applied to the stretched acrylic resin film during the heat treatment.   The manufacturing method of the optical film of Claim 1 in which the said acrylic resin contains the (meth) acrylic polymer which has a ring structure in a principal chain.