JP7544505B2 - Polarizer protection film and polarizing plate - Google Patents

Description

The present invention relates to a polarizer protective film.

In liquid crystal display devices, two polarizing plates are usually placed on both sides of a liquid crystal cell. A polarizing plate is generally made by laminating a polarizer protective film on both sides of a polarizer, and the polarizer protective film is usually a film made of a cellulose-based material. In recent years, polarizer protective films made of (meth)acrylic materials have been proposed to improve durability.

Since (meth)acrylic films have low strength, research is being conducted into ways to improve their strength, such as by subjecting them to biaxial stretching or by incorporating crosslinked elastomers.

However, since (meth)acrylic films generally have poorer affinity with polarizers than cellulose-based films, it has been proposed to subject (meth)acrylic films to hydrophilic treatment (corona discharge treatment or plasma treatment) or to provide an easy-adhesion layer in order to improve adhesion (e.g., Patent Documents 1, 2, and 3).

As an easy-adhesion layer, one that contains a water-based urethane resin and a crosslinking agent is generally known, and it has been reported that oxazoline-based crosslinking agents are preferred (for example, Patent Document 4).

Also, an epoxy-based crosslinking agent has been reported as an easy-adhesion layer suitable for a (meth)acrylic resin film containing a crosslinked elastomer (for example, Patent Document 5).

JP 2007-127893 A JP 2008-216910 A Patent No. 5354733 Patent No. 5399082 Patent No. 6479362

However, it has been found that the epoxy-based crosslinking agent described in Patent Document 5 may not provide sufficient adhesion depending on the composition of the (meth)acrylic resin film. The object of the present invention is to provide a polarizing plate having excellent durability, excellent adhesion to the polarizer, and a polarizer protective film having excellent strength.

As a result of extensive research, the inventors discovered that the above problems could be solved by forming an easy-adhesion layer on a (meth)acrylic resin film, the easy-adhesion layer being made of an easy-adhesion composition containing a polyurethane having a carboxyl group and a multifunctional epoxy-based crosslinking agent having three or more functionalities, and thus completed the present invention.

That is, the present invention relates to a polarizer protective film having an easy-adhesion layer made of an easy-adhesion composition containing a polyurethane having a carboxyl group and a multifunctional epoxy-based crosslinking agent having tetrafunctionality or more, and a resin film containing a (meth)acrylic resin having a ring structure in its main chain.

In a preferred embodiment, the multifunctional epoxy crosslinker is polyglycerol polyglycidyl ether.

In a preferred embodiment, the adhesive composition contains colloidal silica or colloidal zircon.

In a preferred embodiment, the orientation birefringence of the resin film containing the (meth)acrylic resin having a ring structure in the main chain is −1.7×10 ⁻⁴ to 1.7×10 ⁻⁴ .

In a preferred embodiment, the resin film containing the (meth)acrylic resin having a ring structure in the main chain has an in-plane retardation Δnd of 5.0 nm or less and a thickness direction retardation Rth of 20.0 nm or less.

In a preferred embodiment, the photoelastic coefficient of the resin film containing the (meth)acrylic resin having a ring structure in the main chain is −10×10 ⁻¹² to 10×10 ⁻¹² Pa ⁻¹ .

In a preferred embodiment, the (meth)acrylic resin having a ring structure in the main chain has a unit represented by the following formula (1).

(In the formula, ^R1 and ^R2 are each independently hydrogen or an alkyl group having 1 to 8 carbon atoms, and ^R3 is hydrogen, an alkyl group having 1 to 18 carbon atoms, a cycloalkyl group having 3 to 12 carbon atoms, or an aryl group having 6 to 10 carbon atoms.)
In a preferred embodiment, the content of the unit represented by the above formula (1) is 2% by weight or more and 30% by weight or less.

In a preferred embodiment, the resin film containing the (meth)acrylic resin having a ring structure in the main chain has a crosslinked elastomer.

In a preferred embodiment, the crosslinked elastomer is a core-shell type elastomer having a core layer made of a rubber-like polymer and a shell layer made of a glassy polymer.

The polarizing plate of the present invention has a polarizer, an adhesive layer, and the polarizer protective film of the present invention.

In a preferred embodiment, the adhesive layer is made of an adhesive composition containing a polyvinyl alcohol resin.

The polarizer protective film of the present invention has excellent strength and adhesion to the polarizer. By using the polarizer protective film of the present invention, a polarizing plate with excellent durability can be obtained.

1 is a schematic cross-sectional view of a polarizing plate according to one preferred embodiment of the present invention.

Preferred embodiments of the present invention will be described below, but the present invention is not limited to these embodiments.
A. (Meth)acrylic Resin Film The (meth)acrylic resin film (hereinafter sometimes referred to as "the acrylic resin film of the present invention") is formed from an acrylic resin composition containing a (meth)acrylic resin.

The acrylic resin film of the present invention can be obtained, for example, by melt extruding a resin component containing a (meth)acrylic resin as a main component.

The (meth)acrylic resin preferably has a Tg (glass transition temperature) of 110°C or higher, more preferably 115°C or higher, and even more preferably 120°C or higher. By including a (meth)acrylic resin with a Tg of 110°C or higher as the main component, the composition has excellent heat resistance and durability.

Any suitable (meth)acrylic resin may be used as the (meth)acrylic resin. Examples include poly(meth)acrylic acid esters such as polymethyl methacrylate, methyl methacrylate-(meth)acrylic acid copolymers, methyl methacrylate-(meth)acrylic acid ester copolymers, methyl methacrylate-acrylic acid ester-(meth)acrylic acid copolymers, methyl (meth)acrylate-styrene copolymers (MS resins, etc.), and polymers having alicyclic hydrocarbon groups (for example, methyl methacrylate-cyclohexyl methacrylate copolymers, methyl methacrylate-norbornyl (meth)acrylate copolymers, etc.). Preferred are poly(meth)acrylic acid C1-6 alkyls ((meth)acrylic acid ester resins having an alkyl group with 1 to 6 carbon atoms) such as polymethyl (meth)acrylate. More preferred are methyl methacrylate resins having methyl methacrylate as the main component (50 to 100% by weight, preferably 70 to 100% by weight).

The (meth)acrylic resin used in the present invention preferably has a glass transition temperature of 110°C or higher. If the glass transition temperature of the (meth)acrylic resin is less than 110°C, the film will undergo significant dimensional change in a high-temperature environment. In practical use, the film is often laminated with other optical films, in which case warping may occur.

Here, as the (meth)acrylic resin having a glass transition temperature of 110°C or higher, a (meth)acrylic resin having a ring structure in the main chain can be suitably used. For example, the ring structure can be at least one ring structure selected from the group consisting of glutarimide ring, lactone ring, maleic anhydride, maleimide, and glutaric anhydride. These can provide heat resistance. Among them, it is particularly preferable that the ring structure is glutarimide from the viewpoints of ease of production, cost, and quality stability against moisture.

The ring structure content in a (meth)acrylic resin having a glass transition temperature of 110°C or higher is preferably in the range of 2% to 30% by weight. Within this range, a high ring structure content increases the glass transition temperature, while a low ring structure content reduces the phase difference, which is preferable.

Each ring structure is explained below.

The (meth)acrylic resin having a glutarimide ring structure is a resin containing glutarimide units and methyl methacrylate units, as shown in the following general formula (1), and is obtained by heating and melting polymethyl methacrylate, which contains less than 1% by weight of acrylic ester units, and treating the polymethyl methacrylate with an imidizing agent.

(wherein ^R1 and ^R2 each independently represent hydrogen or an alkyl group having 1 to 8 carbon atoms, and ^R3 represents hydrogen, an alkyl group having 1 to 18 carbon atoms, a cycloalkyl group having 3 to 12 carbon atoms, or an aryl group having 6 to 10 carbon atoms.)
The glutarimide ring content according to the present invention is a value that can be measured, for example, by the following method when ^R3 in general formula ( ¹ ) is a methyl group. ^1H -NMR measurement of the resin is performed using a BRUKER AvanceIII (400 MHz). Weight conversion is performed using the molar ratio calculated from the area A of the peak derived from the O- _CH3 proton of methyl methacrylate around 3.5 to 3.8 ppm and the area B of the peak derived from the N- _CH3 proton of glutarimide around 3.0 to 3.3 ppm.

If the glutarimide ring content is less than 2% by weight, it may not be possible to impart the desired heat resistance, and if it exceeds 30% by weight, the amount of imidizing agent added may increase, and odor may occur due to an increase in the amount of remaining volatile components, and the thickness direction retardation Rth may increase.

In this process, other than methyl methacrylate, for example, methyl acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, benzyl (meth)acrylate, cyclohexyl (meth)acrylate, etc. may be used in combination, but when using these in combination, it is preferable that the acrylic acid ester unit is less than 1 weight %. Furthermore, it is more preferable that the methyl acrylate unit is less than 0.5 weight %, and even more preferable that it is less than 0.3 weight %.

In addition to the above monomers, it is also possible to copolymerize nitrile monomers such as acrylonitrile and methacrylonitrile, and maleimide monomers such as maleimide, N-methylmaleimide, N-phenylmaleimide, and N-cyclohexylmaleimide.

The structure of the polymethyl methacrylate resin is not particularly limited, and may be any of a linear polymer, a block polymer, a core-shell polymer, a branched polymer, a ladder polymer, and a crosslinked polymer.

In the case of a block polymer, it may be an A-B type, an A-B-C type, an A-B-A type, or any other type of block polymer. In the case of a core-shell polymer, it may consist of only one layer of core and one layer of shell, or each may consist of multiple layers.

There are no particular limitations on the method for producing polymethyl methacrylate resin, and known methods such as emulsion polymerization, emulsion-suspension polymerization, suspension polymerization, bulk polymerization, and solution polymerization can be used. However, when used in the optical field, bulk polymerization and solution polymerization are particularly preferred from the viewpoint of low impurity content. For example, it can be produced in accordance with the methods described in JP-A-56-8404, JP-B-6-86492, JP-B-7-37482, or JP-B-52-32665.

The method for producing a (meth)acrylic resin containing glutarimide as a ring structure includes a step of heating and melting the polymethyl methacrylate resin and treating it with an imidizing agent (imidization step). This allows the production of a (meth)acrylic resin having glutarimide.

The imidizing agent is not particularly limited as long as it can generate the glutarimide ring represented by the above general formula (1), and examples thereof include those described in WO2005/054311. Specific examples include ammonia, aliphatic hydrocarbon group-containing amines such as methylamine, ethylamine, n-propylamine, i-propylamine, n-butylamine, i-butylamine, tert-butylamine, and n-hexylamine, aromatic hydrocarbon group-containing amines such as aniline, benzylamine, toluidine, and trichloroaniline, and alicyclic hydrocarbon group-containing amines such as cyclohexylamine.

It is also possible to use urea-based compounds that generate the above-listed amines upon heating, such as urea, 1,3-dimethylurea, 1,3-diethylurea, and 1,3-dipropylurea.

Among the above-listed imidizing agents, it is preferable to use methylamine, ammonia, and cyclohexylamine in terms of cost and physical properties, and it is particularly preferable to use methylamine.

Also, methylamine, which is gaseous at room temperature, may be used in a state dissolved in an alcohol such as methanol.

In this imidization process, the ratio of glutarimide units and (meth)acrylic acid ester units in the resulting (meth)acrylic resin can be adjusted by adjusting the ratio of the imidization agent added.

In addition, by adjusting the degree of imidization, it is possible to adjust the physical properties of the resulting (meth)acrylic resin and the optical properties of the optical film formed from the (meth)acrylic resin of the present invention.

The imidizing agent is preferably used in an amount of 0.5 to 20 parts by weight per 100 parts by weight of polymethyl methacrylate resin. If the amount of imidizing agent added exceeds the above range, the imidizing agent will remain in the resin, which may cause appearance defects and foaming after molding. If the amount of imidizing agent added falls below the above range, the glutarimide ring content of the final resin composition will be low, resulting in a significant decrease in heat resistance and possibly causing appearance defects after molding.

In addition to the imidization agent, a ring-closing promoter (catalyst) may be added in this imidization process, if necessary.

The method of heating and melting the resin and treating it with the imidizing agent is not particularly limited, and any conventionally known method can be used. For example, the polymethyl methacrylate resin can be imidized by a method using an extruder or a batch-type reaction tank (pressure vessel), etc.

When the resin is heated and melted using an extruder and treated with an imidizing agent, the extruder is not particularly limited, and various extruders can be used. Specifically, for example, a single screw extruder, a twin screw extruder, or a multi-screw extruder can be used.

Among these, it is preferable to use a twin-screw extruder. A twin-screw extruder can promote mixing of the imidization agent (and the ring-closing promoter, if used) with the polymethyl methacrylate resin.

Examples of twin-screw extruders include non-intermeshing co-rotating, intermeshing co-rotating, non-intermeshing counter-rotating, and intermeshing counter-rotating. Of these, it is preferable to use an intermeshing co-rotating twin-screw extruder. Intermeshing co-rotating twin-screw extruders can rotate at high speeds, which can further promote mixing of the imidization agent (if a ring-closing promoter is used, the imidization agent and the ring-closing promoter) with the raw material polymer.

The extruders exemplified above may be used alone or in combination in series. For example, the tandem-type reactive extruder described in JP-A-2008-273140 can be used.

When imidization is carried out in an extruder, for example, polymethyl methacrylate resin is fed into the raw material inlet of the extruder, the resin is melted, and the cylinder is filled with the resin. Then, an imidization agent is injected into the extruder using an additive pump, and the imidization reaction can proceed in the extruder.

In this case, the reaction zone temperature (resin temperature) in the extruder is preferably 180°C to 270°C, and more preferably 200°C to 250°C. If the reaction zone temperature (resin temperature) is less than 180°C, the imidization reaction hardly progresses and heat resistance tends to decrease. If the reaction zone temperature exceeds 270°C, the resin decomposes significantly, and the fold resistance of the film that can be formed from the resulting (meth)acrylic resin tends to decrease. Here, the reaction zone in the extruder refers to the region in the cylinder of the extruder between the injection position of the imidization agent and the resin discharge port (die section).

By increasing the reaction time in the reaction zone of the extruder, imidization can be made to proceed more rapidly. The reaction time in the reaction zone of the extruder is preferably longer than 10 seconds, and more preferably longer than 30 seconds. With a reaction time of 10 seconds or less, imidization may not proceed at all.

The resin pressure in the extruder is preferably in the range of atmospheric pressure to 50 MPa, and more preferably in the range of 1 MPa to 30 MPa. If it is less than 1 MPa, the solubility of the imidizing agent is low, and the reaction tends to be inhibited. Furthermore, if it is 50 MPa or more, it will exceed the mechanical pressure resistance limit of a normal extruder, and special equipment will be required, which is not preferable from the standpoint of cost.

When using an extruder, it is preferable to equip it with a vent hole that can reduce the pressure to below atmospheric pressure in order to remove unreacted imidizing agent and by-products. With this configuration, it is possible to remove unreacted imidizing agent, or by-products such as methanol and monomers.

In addition, in the production of the glutarimide ring-containing (meth)acrylic resin, a horizontal twin-screw reactor such as the Bivolac manufactured by Sumitomo Heavy Industries, Ltd. or a vertical twin-screw stirring tank such as Superblend, which is suitable for high viscosity reactions, can be suitably used instead of an extruder.

When the glutarimide ring-containing (meth)acrylic resin is produced using a batch reaction tank (pressure vessel), the structure of the batch reaction tank (pressure vessel) is not particularly limited.

Specifically, the polymethyl methacrylate resin needs to have a structure that allows it to be melted by heating and stirred, and to which an imidizing agent (and a ring-closing promoter, if one is used) can be added, but it is preferable that the structure has good stirring efficiency.

This type of batch reaction tank (pressure vessel) can prevent the polymer viscosity from increasing as the reaction progresses, resulting in insufficient mixing. An example of a batch reaction tank (pressure vessel) with this structure is the Max Blend mixing tank manufactured by Sumitomo Heavy Industries, Ltd.

Specific examples of imidization methods include known methods such as those described in JP-A-2008-273140 and JP-A-2008-274187.

In addition to the imidization step, the manufacturing method of the present invention can include a step of treating with an esterification agent. This esterification step allows the acid value of the imidized resin obtained in the imidization step to be adjusted to within a desired range.

Examples of esterifying agents include dimethyl carbonate, 2,2-dimethoxypropane, dimethyl sulfoxide, triethyl orthoformate, trimethyl orthoacetate, trimethyl orthoformate, diphenyl carbonate, dimethyl sulfate, methyl toluene sulfonate, methyl trifluoromethyl sulfonate, methyl acetate, methanol, ethanol, methyl isocyanate, p-chlorophenyl isocyanate, dimethylcarbodiimide, dimethyl-t-butylsilyl chloride, isopropenyl acetate, dimethylurea, tetramethylammonium hydroxide, dimethyldiethoxysilane, tetra-N-butoxysilane, dimethyl (trimethylsilane) phosphite, trimethyl phosphite, trimethyl phosphate, tricresyl phosphate, diazomethane, ethylene oxide, propylene oxide, cyclohexene oxide, 2-ethylhexyl glycidyl ether, phenyl glycidyl ether, and benzyl glycidyl ether. Among these, dimethyl carbonate and trimethyl orthoacetate are preferred from the viewpoints of cost, reactivity, etc., and dimethyl carbonate is preferred from the viewpoint of cost.

In this imidization process, the amount of the esterification agent is preferably 0 to 12 parts by weight, and more preferably 0 to 8 parts by weight, per 100 parts by weight of polymethyl methacrylate resin.

If the esterification agent is within the above range, the acid value can be adjusted to an appropriate range. On the other hand, if it is outside the above range, unreacted esterification agent may remain in the resin, which may cause foaming or odor generation when the resin is used for molding.

In addition to the above esterification agent, a catalyst can also be used in combination. The type of catalyst is not particularly limited, but examples include aliphatic tertiary amines such as trimethylamine, triethylamine, and tributylamine. Among these, triethylamine is preferred from the standpoint of cost, reactivity, etc.

In this esterification step, it is also possible to carry out only the heat treatment without treatment with an esterifying agent. If only the heat treatment (kneading/dispersion of the molten resin in the extruder) is carried out, some or all of the carboxylic acids can be converted to acid anhydride groups by dehydration reactions between carboxylic acids in the imide resin by-produced in the imidization step and/or dealcoholization reactions between carboxylic acids and alkyl ester groups. In this case, it is also possible to use a ring-closing promoter (catalyst).

Even when treating with an esterifying agent, it is possible to promote the conversion to an acid anhydride group by heat treatment.

The imide resin that has undergone the imidization and esterification processes contains unreacted imidization agents, unreacted esterification agents, volatile components produced as by-products during the reaction, and resin decomposition products, so it is possible to install a vent port that can reduce the pressure to below atmospheric pressure.

The (meth)acrylic resin having a lactone ring as a ring structure is not limited as long as it is a thermoplastic polymer having a lactone ring structure in the molecule (a thermoplastic polymer having a lactone ring structure introduced in the molecular chain), and there is also no limitation on the manufacturing method thereof, but it is preferably obtained by obtaining a polymer (a) having a hydroxyl group and an ester group in the molecular chain by polymerization (polymerization step), and then heat treating the obtained polymer (a) to introduce a lactone ring structure into the polymer (lactone cyclization condensation step).

In the polymerization process, a polymer having hydroxyl groups and ester groups in the molecular chain is obtained by carrying out a polymerization reaction of a monomer component containing an unsaturated monomer represented by the following general formula (2).

General formula (2)
(wherein R ⁴ and R ⁵ each independently represent a hydrogen atom or an alkyl group having 1 to 20 carbon atoms.)

Examples of unsaturated monomers represented by the above general formula (2) include methyl 2-(hydroxymethyl)acrylate, ethyl 2-(hydroxymethyl)acrylate, isopropyl 2-(hydroxymethyl)acrylate, normal butyl 2-(hydroxymethyl)acrylate, and tertiary butyl 2-(hydroxymethyl)acrylate. Among these, methyl 2-(hydroxymethyl)acrylate and ethyl 2-(hydroxymethyl)acrylate are preferred, and methyl 2-(hydroxymethyl)acrylate is particularly preferred because of its high effect of improving heat resistance. These unsaturated monomers may be used alone or in combination of two or more.

The content of the unsaturated monomer represented by the above general formula (2) in the monomer component is preferably 5 to 50% by weight, more preferably 10 to 40% by weight, and even more preferably 10 to 30% by weight. If the content is less than 5% by weight, the heat resistance, solvent resistance, and surface hardness of the resulting lactone ring-containing polymer may decrease. If the content is more than 50% by weight, a crosslinking reaction may occur when forming the lactone ring structure, making the polymer more likely to gel, reducing fluidity and making melt molding difficult. Unreacted hydroxyl groups may remain, which may cause further condensation reactions during molding, generating volatile substances and making it easier for silver streaks to appear, or the thickness direction retardation Rth may increase.

The monomer component may contain other monomers in addition to the unsaturated monomer represented by the general formula (2) above. The other monomers are not limited as long as they are selected within a range that does not impair the effects of the present invention, but preferred examples include (meth)acrylic acid esters, hydroxyl group-containing monomers, unsaturated carboxylic acids, and unsaturated monomers represented by the following general formula (3). The other monomers may be used alone or in combination of two or more.

General formula (3)
(wherein ^R6 represents a hydrogen atom or a methyl group, X represents a hydrogen atom, an alkyl group having 1 to 20 carbon atoms, an aryl group, an -OAc group, a -CN group, a -CO- ^R7 group, or a -C-O- ^R8 group, the Ac group represents an acetyl group, and ^R7 and ^R8 represent a hydrogen atom or an alkyl group having 1 to 20 carbon atoms.)

The (meth)acrylic acid ester is not limited as long as it is a (meth)acrylic acid ester other than the unsaturated monomer represented by the general formula (2) above. For example, acrylic acid esters such as methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, cyclohexyl acrylate, and benzyl acrylate; methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, cyclohexyl methacrylate, and benzyl methacrylate; and these may be used alone or in combination of two or more. Among them, methyl methacrylate is particularly preferred from the viewpoints of heat resistance and transparency.

When the above (meth)acrylic acid ester is used, its content in the monomer component is preferably 10 to 95% by weight, more preferably 10 to 90% by weight, even more preferably 40 to 90% by weight, and particularly preferably 50 to 90% by weight, in order to fully exert the effects of the present invention.

The acrylic resin film of the present invention may contain a crosslinked elastomer in order to improve the mechanical strength of the (meth)acrylic resin. The crosslinked elastomer can be produced by a known polymerization method such as suspension polymerization, dispersion polymerization, emulsion polymerization, solution polymerization, or bulk polymerization. In particular, to produce a crosslinked elastomer having a core-shell structure as described below, it is preferable to use a polymerization method such as suspension polymerization, dispersion polymerization, or emulsion polymerization.

As the crosslinked elastomer, a core-shell type elastomer having a core layer made of a rubber-like polymer and a shell layer made of a glassy polymer (hard polymer) is preferred. Furthermore, the core layer made of a rubber-like polymer may have one or more layers made of a glassy polymer as the innermost layer or intermediate layer.

The Tg of the rubbery polymer constituting the core layer is preferably 20°C or less, more preferably -60 to 20°C, and even more preferably -60 to 10°C. If the Tg of the rubbery polymer constituting the core layer exceeds 20°C, the mechanical strength of the (meth)acrylic resin may not be sufficiently improved. The Tg of the glassy polymer (hard polymer) constituting the shell layer is preferably 50°C or more, more preferably 50 to 140°C, and even more preferably 60 to 130°C. If the Tg of the glassy polymer constituting the shell layer is lower than 50°C, the heat resistance of the (meth)acrylic resin may be reduced.

In this application, the glass transition temperatures of "rubbery polymers" and "glassy polymers" are calculated using the values given in the Polymer Hand Book (J. Brandrup, Interscience 1989) using the Fox formula (for example, polymethyl methacrylate is 105°C, and polybutyl acrylate is -54°C).

The content of the core layer in the core-shell type elastomer is preferably 30 to 95% by weight, more preferably 50 to 90% by weight. The content of the glassy polymer layer in the core layer is 0 to 60% by weight, preferably 0 to 45% by weight, more preferably 10 to 40% by weight, based on 100% by weight of the total amount of the core layer. The content of the shell layer in the core-shell type elastomer is preferably 5 to 70% by weight, more preferably 10 to 50% by weight.

The core-shell type elastomer may contain any other appropriate components as long as they do not impair the effects of the present invention.

Any suitable polymerizable monomer may be used as the polymerizable monomer that forms the rubber-like polymer that constitutes the core layer.

The polymerizable monomer that forms the rubber-like polymer preferably contains an alkyl (meth)acrylate. Of 100% by weight of the polymerizable monomer that forms the rubber-like polymer, the alkyl (meth)acrylate is preferably contained in an amount of 50% by weight or more, more preferably 50 to 99.9% by weight, and even more preferably 60 to 99.9% by weight.

Examples of the alkyl (meth)acrylate include alkyl (meth)acrylates having an alkyl group with 2 to 20 carbon atoms, such as ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isononyl (meth)acrylate, lauroyl (meth)acrylate, and stearyl (meth)acrylate. These alkyl groups may have an alicyclic or aromatic cyclic substituent, a branched structure, or a functional group. Among these, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isononyl (meth)acrylate, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, and cyclohexyl (meth)acrylate are preferred, with butyl acrylate, 2-ethylhexyl acrylate, and isononyl acrylate being more preferred. These may be used alone or in combination of two or more.

The polymerizable monomer that forms the rubber-like polymer preferably contains a polyfunctional monomer having two or more vinyl groups in the molecule. The polymerizable monomer that forms the rubber-like polymer preferably contains 0.01 to 20% by weight of the polyfunctional monomer having two or more vinyl groups in the molecule, more preferably 0.1 to 20% by weight, even more preferably 0.1 to 10% by weight, and particularly preferably 0.2 to 5% by weight.

Examples of the polyfunctional monomer having two or more vinyl groups in the molecule include aromatic divinyl monomers such as divinylbenzene, alkane polyol poly(meth)acrylates such as ethylene glycol di(meth)acrylate, butylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, oligoethylene glycol di(meth)acrylate, trimethylolpropane di(meth)acrylate, and trimethylolpropane tri(meth)acrylate, as well as urethane di(meth)acrylate, epoxy di(meth)acrylate, and triallyl isocyanurate. Examples of the polyfunctional monomer having vinyl groups with different reactivity include allyl (meth)acrylate, diallyl maleate, diallyl fumarate, and diallyl itaconate. Among these, ethylene glycol dimethacrylate, butylene glycol diacrylate, and allyl methacrylate are preferred. These may be used alone or in combination of two or more.

The polymerizable monomers forming the rubber-like polymer may include other polymerizable monomers that are copolymerizable with the alkyl (meth)acrylate and the polyfunctional monomer having two or more vinyl groups in the molecule. The polymerizable monomers forming the rubber-like polymer preferably contain 0 to 49.9% by weight of the other polymerizable monomers, and more preferably 0 to 39.9% by weight.

Examples of the other polymerizable monomers include aromatic vinyls such as styrene, vinyl toluene, and α-methyl styrene, aromatic vinylidene, vinyl cyanides such as acrylonitrile and methacrylonitrile, vinylidene cyanide, methyl methacrylate, urethane acrylate, and urethane methacrylate. In addition, the other polymerizable monomers may be monomers having functional groups such as epoxy groups, carboxyl groups, hydroxyl groups, and amino groups. Specifically, examples of monomers having epoxy groups include glycidyl methacrylate, and examples of monomers having carboxyl groups include methacrylic acid, acrylic acid, maleic acid, and itaconic acid, and examples of monomers having hydroxyl groups include 2-hydroxyethyl methacrylate and 2-hydroxyethyl acrylate, and examples of monomers having amino groups include diethylaminoethyl methacrylate and diethylaminoethyl acrylate. These may be used alone or in combination of two or more.

The polymerizable monomers that form the rubber-like polymer may be used in combination with a small amount of a chain transfer agent. Examples of such chain transfer agents include alkyl mercaptans such as octyl mercaptan, dodecyl mercaptan, and t-dodecyl mercaptan, and thioglycolic acid derivatives.

Any suitable polymerizable monomer may be used as the polymerizable monomer that forms the glassy polymer that constitutes the shell layer and the glassy polymer layer in the core layer.

The polymerizable monomer that forms the glassy polymer preferably contains at least one monomer selected from alkyl (meth)acrylates and aromatic vinyl monomers. Of 100% by weight of the polymerizable monomer that forms the glassy polymer, it is preferable that at least one monomer selected from alkyl (meth)acrylates and aromatic vinyl monomers is contained in an amount of 50 to 100% by weight, and more preferably 60 to 100% by weight.

As the alkyl (meth)acrylate, those having 1 to 8 carbon atoms in the alkyl group are preferred. Furthermore, these alkyl groups may have an alicyclic or aromatic cyclic substituent, a branched structure, or a functional group. Examples of such alkyl (meth)acrylates include methyl (meth)acrylate, ethyl (meth)acrylate, benzyl (meth)acrylate, cyclohexyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. Of these, methyl methacrylate is particularly preferred. These may be used alone or in combination of two or more.

Examples of the aromatic vinyl monomer include styrene, vinyl toluene, and α-methyl styrene. Among these, styrene is preferred. These may be used alone or in combination of two or more.

The polymerizable monomer that forms the glassy polymer may contain a polyfunctional monomer having two or more vinyl groups in the molecule. In 100% by weight of the polymerizable monomer that forms the glassy polymer, the polyfunctional monomer having two or more vinyl groups in the molecule is preferably contained in an amount of 0 to 10% by weight, more preferably 0 to 8% by weight, and even more preferably 0 to 5% by weight.

Specific examples of the polyfunctional monomer having two or more vinyl groups in the molecule include those mentioned above.

The polymerizable monomers that form the glassy polymer may contain other polymerizable monomers that are copolymerizable with the alkyl (meth)acrylate and the polyfunctional monomer having two or more vinyl groups in the molecule. The other polymerizable monomers are preferably contained in an amount of 0 to 50% by weight, more preferably 0 to 40% by weight, out of 100% by weight of the polymerizable monomers that form the glassy polymer.

Examples of the other polymerizable monomers include vinyl cyanides such as acrylonitrile and methacrylonitrile, vinylidene cyanide, alkyl (meth)acrylates other than those mentioned above, urethane acrylates, and urethane methacrylates. In addition, the monomers may have functional groups such as epoxy groups, carboxyl groups, hydroxyl groups, and amino groups. Examples of monomers having epoxy groups include glycidyl methacrylate, and examples of monomers having carboxyl groups include methacrylic acid, acrylic acid, maleic acid, and itaconic acid. Examples of monomers having hydroxyl groups include 2-hydroxymethacrylate and 2-hydroxyacrylate, and examples of monomers having amino groups include diethylaminoethyl methacrylate and diethylaminoethyl acrylate. These may be used alone or in combination of two or more.

Furthermore, it is also preferable to use a small amount of a known chain transfer agent, similar to that used in the rubbery polymer layer, in combination with the polymerizable monomer that forms the glassy polymer.

As a method for producing the core-shell type elastomer in the present invention, any suitable method capable of producing core-shell type particles can be adopted.

For example, a method can be used in which a polymerizable monomer that forms a rubber-like polymer constituting the core layer is suspended or emulsion polymerized to produce a suspension or emulsion dispersion containing rubber-like polymer particles, and then a polymerizable monomer that forms a glassy polymer that constitutes the shell layer is added to the suspension or emulsion dispersion and radically polymerized to obtain a core-shell type elastomer having a multilayer structure in which the surfaces of the rubber-like polymer particles are coated with a glassy polymer. Here, the polymerizable monomer that forms the rubber-like polymer and the polymerizable monomer that forms the glassy polymer may be polymerized in one stage, or may be polymerized in two or more stages by changing the composition ratio.

In order to ensure a balance of physical properties of the acrylic resin film of the present invention, it is desirable to appropriately control the structure of the core-shell type elastomer.

Examples of preferred structures of the core-shell type elastic body include (a) a soft, rubber-like core layer and a hard, glassy shell layer, the core layer having a (meth)acrylic crosslinked elastic polymer layer, and (b) a multilayer structure in which the rubber-like core layer has one or more glassy layers inside and further has a glassy shell layer outside the core layer. By appropriately selecting the monomer type of each layer, the physical properties (mechanical properties, optical properties, particularly orientation birefringence and photoelastic coefficient) of the (meth)acrylic resin can be controlled as desired. The soft, rubbery layer preferably has a polymer glass transition temperature of less than 20°C, preferably less than 0°C, and the hard, glassy layer preferably has a polymer glass transition temperature of 0°C or higher, preferably 20°C or higher.

Specific examples of more preferred structures of the core-shell type elastomer include, for example, (i) a shell layer of the core-shell type elastomer that is a non-crosslinked methacrylic resin containing 3% by weight or more, more preferably 10% by weight or more, and even more preferably 15% by weight or more of alkyl acrylate; (ii) a shell layer of the core-shell type elastomer that is composed of two or more multilayers with different alkyl acrylate contents and is a non-crosslinked methacrylic resin that contains 10% by weight or more, more preferably 15% by weight or more of alkyl acrylate in total; and (iii) a core layer of the core-shell type elastomer that is composed of an alkyl methacrylate and a non-crosslinked methacrylic resin that contains 10% by weight or more, more preferably 15% by weight or more of alkyl acrylate in total. (iv) a multi-layer structure in which a rubber-like polymer layer is formed by polymerizing a mixture of acrylic acrylate, polyfunctional monomer, alkyl mercaptan, and other monomers in the presence of a glassy polymer layer polymerized using an organic peroxide as a redox-type polymerization initiator, and a rubber-like polymer layer is formed by polymerizing a peracid (persulfuric acid, perphosphate, etc.) as a thermal decomposition initiator in the presence of a glassy polymer layer polymerized using an organic peroxide as a redox-type polymerization initiator. Such a preferred core-shell type elastomer may have only one structural design element, or two or more design elements may be used in combination. By having such a structure, the core-shell type elastomer is easily dispersed in the (meth)acrylic resin of the present invention, and when a film is formed, there are few defects due to undispersion or aggregation, and the film has excellent strength, toughness, heat resistance, transparency, and appearance, and whitening due to temperature changes and stress is suppressed, making it possible to obtain a film of excellent quality.

When the core-shell type elastomer of the present invention is produced by emulsion polymerization, suspension polymerization, or the like, a known polymerization initiator can be used. Particularly preferred polymerization initiators include persulfates such as potassium persulfate, ammonium persulfate, and ammonium persulfate, perphosphates such as sodium perphosphate, organic azo compounds such as azobisisobutyronitrile, hydroperoxide compounds such as cumene hydroperoxide, tertiary butyl hydroperoxide, and 1,1 dimethyl-2 hydroxyethyl hydroperoxide, peresters such as tertiary butyl isopropyloxycarbonate and tertiary butyl peroxybutyrate, and organic peroxide compounds such as benzoyl peroxide, dibutyl peroxide, and lauryl peroxide. These may be used as thermal decomposition type polymerization initiators, or may be used as redox type polymerization initiators in the presence of a catalyst such as ferrous sulfate and a water-soluble reducing agent such as ascorbic acid or sodium formaldehyde sulfoxylate, and may be appropriately selected depending on the monomer composition to be polymerized, the layer structure, the polymerization temperature conditions, and the like.

When the core-shell type elastomer of the present invention is produced by emulsion polymerization, it can be produced by normal emulsion polymerization using a known emulsifier. Known emulsifiers include anionic surfactants such as sodium alkylsulfonate, sodium alkylbenzenesulfonate, sodium dioctyl sulfosuccinate, sodium lauryl sulfate, sodium fatty acid, and phosphate salts such as sodium polyoxyethylene lauryl ether phosphate, and nonionic surfactants such as reaction products of alkylphenols, aliphatic alcohols, and propylene oxide or ethylene oxide. These surfactants may be used alone or in combination of two or more. If necessary, a cationic surfactant such as an alkylamine salt may also be used. Of these, from the viewpoint of improving the thermal stability of the obtained core-shell type elastomer, it is preferable to polymerize using a phosphate ester salt (alkali metal or alkaline earth metal) such as sodium polyoxyethylene lauryl ether phosphate. The core-shell type elastomer latex obtained by emulsion polymerization is spray-dried, or, as is commonly known, the polymer content is coagulated by adding an electrolyte or an organic solvent as a coagulant to the latex, and the polymer content is dried by appropriate operations such as heating, washing, and separation of the aqueous phase, to obtain a core-shell type elastomer in a lump or powder form. As the coagulant, known substances such as water-soluble electrolytes and organic solvents can be used, but from the viewpoint of improving the thermal stability during molding of the obtained copolymer and from the viewpoint of productivity, it is preferable to use magnesium salts such as magnesium chloride or magnesium sulfate, or calcium salts such as calcium acetate or calcium chloride.

The acrylic resin film of the present invention preferably contains 1 to 40 parts by weight of a core-shell type elastomer per 100 parts by weight of (meth)acrylic resin, more preferably 2 to 35 parts by weight, and even more preferably 3 to 25 parts by weight. If the content of the core-shell type elastomer is less than 1 part by weight, the improvement in the mechanical strength of the (meth)acrylic resin is insufficient, and if it exceeds 40 parts by weight, the heat resistance of the (meth)acrylic resin may decrease.

The preferred particle size of the core-shell type elastic body is 1 to 500 nm for the soft core layer, more preferably 10 to 400 nm, even more preferably 50 to 300 nm, and particularly preferably 70 to 300 nm. If the particle size of the core layer of the core-shell type elastic body is less than 1 nm, the mechanical strength of the (meth)acrylic resin is not sufficiently improved, and if it is greater than 500 nm, the heat resistance and transparency of the (meth)acrylic resin may be impaired.

The particle diameter of the core layer of the core-shell type elastomer can be determined by molding a compound in which the core-shell crosslinked elastomer and Sumipex EX are blended in a weight ratio of 50:50, photographing the resulting film with a transmission electron microscope (JEM-1200EX, manufactured by JEOL Ltd.) at an acceleration voltage of 80 kV using the _RuO4 staining ultrathin section method, randomly selecting 100 rubber particle images from the resulting photograph, and determining the average particle diameter thereof.

The acrylic resin film of the present invention has a smaller orientation birefringence, and preferably has a birefringence of −1.7×10 ⁻⁴ to 1.7×10 ⁻⁴ . By using such a resin, the retardation value of the (meth)acrylic resin film can be reduced.

The acrylic resin film of the present invention has a smaller photoelastic coefficient, which is preferably from -10 x 10 ^-12 to 10 x 10 ^-12 Pa ^-1 , and more preferably from -4 x 10 ^-12 to 4 x 10 ^-12 Pa ^-1 .

The acrylic resin film of the present invention may contain other thermoplastic resins in addition to the (meth)acrylic resin. Examples of other thermoplastic resins include olefin polymers, vinyl halide polymers, styrene polymers, ester polymers, and amide polymers.

The content of the other thermoplastic resin in the acrylic resin film of the present invention is preferably 0 to 50% by weight, more preferably 0 to 30% by weight, based on 100% by weight of the (meth)acrylic resin composition.

The acrylic resin film of the present invention may contain any additives as necessary. Examples of additives include stabilizers such as antioxidants, light stabilizers, weather stabilizers, and heat stabilizers, UV absorbers, flame retardants, antistatic agents, fillers, plasticizers, and lubricants.

(Film manufacturing method)
One embodiment of the method for producing the acrylic resin film of the present invention will be described below, but the present invention is not limited thereto. In other words, any method known in the art can be used as long as it can produce a film by molding the (meth)acrylic resin of the present invention.

Specific examples of the molding methods include injection molding, melt extrusion molding, inflation molding, blow molding, and compression molding. In addition, the film according to the present invention can be produced by dissolving the (meth)acrylic resin according to the present invention in a solvent capable of dissolving the resin, and then molding the resulting solution by a solution casting method or a spin coating method.

Among these, it is preferable to use the melt extrusion method, which does not use solvents. The melt extrusion method can reduce manufacturing costs and the burden on the global environment and working environment caused by solvents.

As one embodiment of the method for producing a film according to the present invention, a method for producing a film by molding the (meth)acrylic resin according to the present invention by a melt extrusion method will be described in detail below. In the following description, the film obtained by the melt extrusion method will be referred to as a "melt extrusion film" to distinguish it from films obtained by other methods such as the solution casting method.

When the (meth)acrylic resin of the present invention is formed into a film by melt extrusion, the (meth)acrylic resin of the present invention is first fed to an extruder and heated to melt the (meth)acrylic resin.

It is preferable to pre-dry the (meth)acrylic resin before feeding it to the extruder. By performing such pre-drying, foaming of the resin extruded from the extruder can be prevented.

The method of pre-drying is not particularly limited, but for example, the raw material (i.e., the (meth)acrylic resin according to the present invention) can be made into a pellet form, and pre-drying can be performed using a hot air dryer, vacuum dryer, etc.

Next, the (meth)acrylic resin that has been heated and melted in the extruder is fed to a T-die through a gear pump and a filter. Using a gear pump at this time can improve the uniformity of the resin extrusion amount and reduce uneven thickness in the longitudinal direction of the film. On the other hand, using a filter can remove foreign matter from the (meth)acrylic resin, resulting in a film with excellent appearance and no defects.

Next, the (meth)acrylic resin supplied to the T-die is extruded from the T-die as a sheet-like molten resin. The sheet-like molten resin is then sandwiched between two cooling rolls and cooled to produce a film.

Of the two cooling rolls that sandwich the sheet-like molten resin, it is preferable that one is a rigid metal roll with a smooth surface, and the other is a flexible roll with a smooth surface and an elastic metallic outer cylinder that can be elastically deformed.

By sandwiching the sheet-like molten resin between such a rigid metal roll and a flexible roll equipped with an elastic metallic outer cylinder, and cooling it to form a film, minute surface irregularities and die lines can be corrected, resulting in a film with a smooth surface and thickness unevenness of 5 μm or less.

In this specification, the term "cooling roll" is used to include "touch roll" and "cooling roll".

Even when using the rigid metal roll and flexible roll described above, the surfaces of all the chill rolls are made of metal, so if the film being produced is thin, the surfaces of the chill rolls may come into contact with each other, causing scratches on the outer surface of the chill roll or damage to the chill roll itself.

Therefore, when forming a film by sandwiching a sheet-like molten resin between two cooling rolls as described above, the sheet-like molten resin is first sandwiched between the two cooling rolls and cooled to obtain a relatively thick raw film. It is then preferable to uniaxially or biaxially stretch the raw film to produce a film of a predetermined thickness.

To be more specific, when producing a film having a thickness of 40 μm, the sheet-shaped molten resin is sandwiched between the two cooling rolls and cooled to obtain a raw film having a thickness of 150 μm. The raw film is then stretched by biaxial stretching in the longitudinal and transverse directions to produce a film having a thickness of 40 μm.

In this way, when the film according to the present invention is a stretched film, the (meth)acrylic resin according to the present invention is first formed into an unstretched raw film, and then uniaxial or biaxial stretching is performed to produce a stretched film.

In order to improve the bending resistance of the optical film of the present invention in both the longitudinal direction (MD direction) and the transverse direction (TD direction), it is preferable to perform biaxial stretching.

For ease of explanation, in this specification, the film obtained by forming the (meth)acrylic resin according to the present invention into a film shape and before stretching it, i.e., the film in an unstretched state, is referred to as the "raw film."

When stretching the original film, the original film may be stretched continuously immediately after forming the original film, or the original film may be stored or moved once after forming and then stretched.

When the raw film is stretched immediately after being formed into a raw film, if the raw film is in a state of a very short time (sometimes even instantaneously) during the film manufacturing process, it is sufficient that the raw film maintains a film shape sufficient for stretching, and it does not need to be in a perfect film state. In addition, the raw film does not need to have the properties of a finished film.

(Film Stretching Method)
The method for stretching the raw 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 rolls, and sequential biaxial stretching which is a sequential combination of these methods may be used.

It is also possible to use a simultaneous biaxial stretching method in which the film is stretched lengthwise and widthwise at the same time, or a method in which the film is stretched lengthwise with a roll and then stretched widthwise with a tenter.

When stretching the raw film, it is preferable to first preheat the raw film to a temperature 0.5°C to 5°C higher than the stretching temperature, preferably 1°C to 3°C higher, and then cool it to the stretching temperature before stretching.

By preheating within the above range, the thickness of the original film in the width direction can be maintained with precision, and the thickness precision of the stretched film does not decrease and thickness unevenness does not occur. In addition, the original film does not stick to the roll or sag under its own weight.

On the other hand, if the preheating temperature of the raw film is too high, problems such as the raw film sticking to the roll or sagging under its own weight tend to occur. Also, if the difference between the preheating temperature of the raw film and the stretching temperature is small, it tends to be difficult to maintain the thickness precision of the raw film before stretching, thickness unevenness tends to increase, and thickness precision tends to decrease.

In addition, when the (meth)acrylic resin according to the present invention is stretched after being formed into a raw film, it is difficult to improve the thickness accuracy by utilizing the necking phenomenon. Therefore, in the present invention, controlling the preheating temperature is important in order to maintain or improve the thickness accuracy of the obtained film.

The stretching temperature when stretching the raw film is not particularly limited, and may be changed according to the mechanical strength, surface properties, thickness accuracy, etc. required for the stretched film to be produced. In general, when the glass transition temperature of the raw film ((meth)acrylic resin composition) determined by the DSC method is Tg, the temperature range is preferably (Tg-30°C) to (Tg+30°C), more preferably (Tg-20°C) to (Tg+30°C), even more preferably (Tg) to (Tg+30°C), and even more preferably (Tg+10°C) to (Tg+30°C). In other words, when the glass transition temperature of the (meth)acrylic resin composition is Tg, the stretching temperature for biaxial stretching of the optical film is preferably in the temperature range of Tg-30°C or more and Tg+30°C or less.

If the stretching temperature is within the above temperature range, the thickness unevenness of the resulting stretched film can be reduced, and the mechanical properties of elongation, tear propagation strength, and MIT flex resistance can be improved. In addition, problems such as the film sticking to the roll can be prevented.

On the other hand, if the stretching temperature is higher than the above temperature range, the thickness unevenness of the resulting stretched film tends to become large, and mechanical properties such as elongation, tear propagation strength, and fatigue resistance tend not to be sufficiently improved. Furthermore, problems such as the film sticking to the roll tend to occur more easily.

Furthermore, if the stretching temperature is lower than the above temperature range, the internal haze of the resulting stretched film will tend to increase, and in extreme cases, process problems such as film tearing or cracking will tend to occur.

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

If the stretching ratio is within the above range, the mechanical properties of the film, such as elongation, tear propagation strength, and fatigue resistance, can be significantly improved. Therefore, it is possible to produce a stretched film with thickness unevenness of 5 μm or less and internal haze of 1.0% or less.

When the (meth)acrylic resin of the present invention contains a crosslinked elastomer, the mechanical strength of the film is excellent, so that it can be suitably used in the form of any of an unstretched film, a uniaxially stretched film, and a biaxially stretched film.

The smaller the retardation value of the (meth)acrylic resin film of the present invention, the better. It is preferable that the in-plane retardation Δnd is 5.0 nm or less and the thickness direction retardation Rth is 20.0 nm or less, more preferably Δnd is 2.0 nm or less and Rth is 5.0 nm or less, and even more preferably Δnd is 1.0 nm or less and Rth is 3.0 nm or less. If the retardation value is greater than these values, the optical properties may be deteriorated.

The thickness of the (meth)acrylic resin film of the present invention is preferably 5 to 200 μm, and more preferably 10 to 100 μm. If the thickness is less than 5 μm, sufficient strength cannot be obtained, and if the thickness exceeds 200 μm, transparency may decrease, the drying time of the adhesive solvent (water, etc.) may become slow, and the thickness of the polarizing plate may become too thick.

The surface wet tension of the (meth)acrylic resin film of the present invention is preferably 40 mN/m or more, and more preferably 50 mN/m or more. If the surface wet tension is at least 40 mN/m or less, the adhesive strength between the (meth)acrylic resin film of the present invention and the polarizer may decrease.

To increase the wetting tension of the surface, corona discharge treatment, plasma treatment, ozone treatment, ultraviolet irradiation, flame treatment, chemical treatment, etc. may be performed. Of these, corona discharge treatment and plasma treatment are preferred.

B. Easy-adhesion layer The easy-adhesion layer is formed by applying an easy-adhesive composition (hereinafter sometimes referred to as the "easy-adhesive composition of the present invention") containing a polyurethane having a carboxyl group and a multifunctional epoxy-based crosslinking agent having three or more functionalities on the surface of the acrylic resin film of the present invention, and then drying the composition.

By applying the adhesive composition of the present invention to the adhesive surface of the acrylic resin film of the present invention that is to be bonded to the polarizer and then drying it, the adhesion to the polarizer is significantly improved.

The adhesive composition of the present invention may be aqueous or organic. From the viewpoints of the environment and workability, the aqueous adhesive composition is preferred. However, from the viewpoints of dispersibility and solubility, a small amount of organic solvent may be contained. As the organic solvent, alcohol, ketone, ester, and ether solvents are preferred from the viewpoints of compatibility with water and ease of handling.

Specific examples of alcohol-based solvents include methanol, ethanol, propanol, isopropanol, butanol, ethylene glycol, and propylene glycol.

Specific examples of ketone solvents include acetone, methyl ethyl ketone, and isobutyl ketone.

Examples of ester-based solvents include ethyl acetate and butyl acetate.

Ether solvents include dimethoxyethane, tetrahydrofuran, etc.

The above-mentioned polyurethane having a carboxyl group can be obtained, for example, by reacting a polyol, a polyisocyanate, and a chain extender having a free carboxyl group.

The polyol is not particularly limited as long as it has two or more hydroxyl groups in the molecule, and any appropriate polyol can be used. Examples include polyacrylic polyol, polyester polyol, polyether polyol, etc. These can be used alone or in combination of two or more kinds.

The polyisocyanate is not particularly limited as long as it has two or more isocyanate groups in the molecule, and any appropriate polyisocyanate can be used. Examples include aliphatic isocyanates and aromatic isocyanates. These can be used alone or in combination of two or more types.

Examples of chain lengtheners having a free carboxyl group include dihydroxycarboxylic acids and dihydroxysuccinic acid. Examples of dihydroxycarboxylic acids include dialkylolalkanoic acids such as dimethylolalkanoic acids (e.g., dimethylolacetic acid, dimethylolbutanoic acid, dimethylolpropionic acid, dimethylolbutyric acid, and dimethylolpentanoic acid). These can be used alone or in combination of two or more.

Any suitable urethane reaction catalyst may be used in producing the above-mentioned polyurethane having a carboxyl group.

The number average molecular weight of the polyurethane having a carboxyl group is preferably 5,000 to 600,000, and more preferably 10,000 to 400,000. The acid value of the polyurethane is preferably 10 or more, more preferably 10 to 50, and particularly preferably 20 to 45. When the acid value is within such a range, the adhesion between the polarizer and the (meth)acrylic resin film can be more excellent.

The adhesive composition of the present invention uses a multifunctional epoxy crosslinking agent having three or more functionalities.

The epoxy crosslinking agent used in the present invention preferably has at least three epoxy groups in the molecule, and more preferably has four or more epoxy groups.

The above-mentioned polyfunctional epoxy crosslinking agents include bisphenol-type epoxy compounds, novolac-type epoxy compounds, alicyclic epoxy compounds, aliphatic epoxy compounds, aromatic epoxy compounds, glycidylamine-type epoxy compounds, halogenated epoxy compounds, etc.

Specifically, novolac type epoxy compounds such as phenol novolac type epoxy compounds and cresol novolac type epoxy compounds;
Aliphatic epoxy compounds such as glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, and polyglycerol polyglycidyl ether;
Examples of the epoxy compounds include glycidylamine type epoxy compounds such as tetraglycidylaminophenylmethane.

Among these, polyglycerol polyglycidyl ether is preferred, and polyglycerol polyglycidyl ether having four or five epoxy groups is even more preferred. Such epoxy crosslinking agents are available, for example, from Nagase ChemteX Corporation under the trade names Denacol EX-512 and Denacol EX-521.

From the viewpoint of workability when forming the easy-adhesion layer, the solid content concentration of the easy-adhesion composition of the present invention is preferably 10 to 50% by weight, and more preferably 20 to 45% by weight.

The content of the crosslinking agent in the easy-adhesive composition is preferably 0.01 to 30 parts by weight, and more preferably 0.1 to 20 parts by weight, per 100 parts by weight of the polyurethane having a carboxyl group.

The thickness of the easy-adhesion layer can be set to any appropriate value. It is preferably 0.1 to 10 μm, more preferably 0.1 to 5 μm, and particularly preferably 0.2 to 1.5 μm. By setting it within such a range, excellent adhesion to the polarizer can be achieved.

Before applying the adhesive composition of the present invention, it is preferable to subject the surface of the acrylic resin film of the present invention to a corona discharge treatment or plasma treatment.

Any suitable method can be used to apply the adhesive composition of the present invention. Examples include bar coating, roll coating, gravure coating, rod coating, slot orifice coating, curtain coating, and fountain coating.

The drying temperature for the adhesive composition of the present invention is preferably 50°C or higher, and more preferably 80°C or higher.

The adhesive composition of the present invention may further contain any suitable additive. Examples of additives include antiblocking agents, dispersion stabilizers, thixotropic agents, antioxidants, UV absorbers, defoamers, thickeners, dispersants, surfactants, catalysts, fillers, lubricants, and antistatic agents.

As the blocking prevention agent, commercially available ones can be used as appropriate, but from the viewpoint of blocking prevention performance, colloidal silica, colloidal alumina, colloidal zircon, etc. can be mentioned, and are selected according to the composition of the easy adhesive composition. For example, in an easy adhesive composition containing a polyurethane having a carboxyl group and a multifunctional epoxy-based crosslinking agent having three or more functionalities, colloidal silica and colloidal zircon are preferred from the viewpoint of blocking prevention performance and dispersibility in the easy adhesive, and colloidal zircon is particularly preferred. Colloidal zircon is a zirconia sol in which zirconia particles are dispersed in water. There is no particular restriction on the particle size of the blocking prevention agent, but it is preferably 10-200 nm. If it is less than 10 nm, the blocking prevention effect is insufficient, and if it is more than 200 nm, it may have a negative effect on the appearance of the film.

Colloidal zircon is commercially available, for example, from Nissan Chemical Industries, Ltd. under the trade name Nanouse ZR.
C. Overall Configuration of Polarizing Plate Fig. 1 is a schematic cross-sectional view of a polarizing plate according to a preferred embodiment of the present invention.

The polarizing plate 5 is formed by laminating a polarizer 1, an adhesive layer 2, an easy-adhesion layer 3, and a (meth)acrylic resin film 4 in this order.
C-1. Polarizer As the polarizer 1, for example, a hydrophilic film such as a polyvinyl alcohol film, to which a dichroic substance such as iodine or a dichroic dye is adsorbed, and which is then uniaxially stretched, is suitably used. There is no particular limitation on the thickness of these polarizers, but it is about 1 to 80 μm.

A polarizer in which iodine is adsorbed onto a polyvinyl alcohol film and then uniaxially stretched can be produced, for example, by immersing the polyvinyl alcohol in an aqueous iodine solution and then stretching it.

If necessary, the polyvinyl alcohol film may be washed with water before dyeing. Washing the polyvinyl alcohol film with water can prevent uneven dyeing.

The stretching may be performed after dyeing with iodine, or the stretching may be performed while dyeing, or the stretching may be performed before dyeing with iodine.

C-2. Adhesive Layer Any appropriate adhesive can be used as the adhesive for forming the adhesive layer 2. In view of affinity with the polarizer, an adhesive composition containing a polyvinyl alcohol resin is preferred, and an acetoacetyl group-containing polyvinyl alcohol resin is particularly preferred. By using an adhesive composition containing an acetoacetyl group-containing polyvinyl alcohol resin, the adhesion between the polarizer and the acrylic resin film of the present invention is further improved.

There is no particular restriction on the average degree of polymerization of the polyvinyl alcohol resin, but it is preferably about 100 to 5,000, and more preferably 1,000 to 4,000.

The adhesive composition may contain a crosslinking agent as necessary. The crosslinking agent is preferably one having a functional group reactive with the polyvinyl alcohol resin.

Examples of functional groups that are reactive with polyvinyl alcohol resins include amine groups, isocyanate groups, epoxy groups, aldehyde groups, and methylol groups. Among these, compounds having methylol groups are preferred, and methylolmelamine is particularly preferred.

The amount of the crosslinking agent is not particularly limited, but is about 10 to 60 parts by weight, preferably 20 to 50 parts by weight, per 100 parts by weight of the polyvinyl alcohol resin.

To further improve adhesion, various coupling agents and tackifiers may be added to the adhesive composition. As a coupling agent, a silane coupling agent is preferred. In addition, ultraviolet absorbers, antioxidants, heat stabilizers, hydrolysis stabilizers, etc. may be added.

The adhesive composition is usually used as an aqueous solution. The resin concentration is preferably 0.1 to 15% by weight, more preferably 0.5 to 10% by weight, in order to balance application properties and stability.

The thickness of the adhesive layer formed from the adhesive composition is set according to the composition of the adhesive composition. It is preferably 10 to 300 nm, and from the viewpoint of adhesion, it is particularly preferably 20 to 150 nm.

C-3. Easy-Adhesion Layer The easy-adhesion layer 3 is formed by applying and drying the easy-adhesive composition of the present invention, as described above.

C-4 (Meth)acrylic Resin Film The (meth)acrylic resin film 4 is obtained by melt-extruding the (meth)acrylic resin composition as described above.

C-5. Others A second protective film may be formed on the side of the polarizer 1 opposite to the (meth)acrylic resin film 4. Any appropriate protective film may be used as the second protective film. Representative examples of materials for forming the second protective film include cellulose-based polymers such as triacetyl cellulose and cycloolefin-based polymers. The second protective film may be formed of the same material as the acrylic resin film of the present invention. The second protective film and the polarizer are attached to each other with any appropriate adhesive.

D. Manufacturing Method Any suitable method may be adopted as the manufacturing method of the polarizing plate of the present invention. Hereinafter, one embodiment will be described. The polarizer and the (meth)acrylic resin film are laminated via an easy-adhesion layer. For example, the easy-adhesion layer is formed in advance on one side of the (meth)acrylic resin film. The easy-adhesion layer is formed by applying an easy-adhesive composition to one side of the (meth)acrylic resin film and drying it. Any suitable method described above may be adopted as the method for applying the easy-adhesive composition.

As described above, at least one side of the (meth)acrylic resin film (the side on which the polarizer is disposed) may be subjected to a surface treatment. In this case, the above-mentioned surface treatment is performed before forming the easy-adhesion layer. The surface treatment is preferably a corona discharge treatment or a plasma treatment. By performing a corona discharge treatment, the adhesion and cohesion between the polarizer and the (meth)acrylic resin film may be further improved. The corona discharge treatment is performed under any appropriate conditions. For example, the amount of corona discharge electron irradiation is preferably 50 to 150 W/m ² /min, more preferably 70 to 100 W/m ² /min.

The polarizer and the (meth)acrylic resin film are laminated via an adhesive layer. Specifically, the above-mentioned adhesive composition is applied to one side of either the polarizer or the (meth)acrylic resin film, and then the polarizer and the (meth)acrylic resin film are laminated and dried. Examples of methods for applying the adhesive composition include a roll method, a spray method, and a dipping method. The drying temperature is typically 5 to 150°C, and preferably 30 to 120°C. The drying time is typically 120 seconds or more, and preferably 300 seconds or more.

The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples. The (meth)acrylic resin film was evaluated as follows. In the following, "parts" and "%" mean "parts by weight" and "% by weight" unless otherwise specified.

(Glass Transition Temperature)
Using 10 mg of the (meth)acrylic resin, a differential scanning calorimeter (DSC, manufactured by SII Corp., DSC7020) was used to measure in a nitrogen atmosphere at a temperature rise rate of 20° C./min, and the calorific value was determined by the midpoint method.

(Average refractive index)
The measurement was carried out using an Abbe refractometer 3T manufactured by Atago Co., Ltd.

(Calculation of Ring Structure Content)
The obtained (meth)acrylic resin was measured using ^1H -NMR BRUKER AvanceIII (400MHz). The molar ratio of the target ring structure portion and the other portions was converted into weight and calculated. Specifically, in the case of glutarimide in which R ³ in the above general formula (1) is a methyl group, the weight can be calculated by converting the molar ratio into weight using the area A of the peak derived from the O-CH ₃ proton of methyl methacrylate in the vicinity of 3.5 to 3.8 ppm and the area B of the peak derived from the N-CH ₃ proton of glutarimide in the vicinity of 3.0 to 3.3 ppm.

(Optical properties)
The in-plane retardation Δnd and the thickness direction retardation Rth were measured using a retardation measuring device KOBRA-WR manufactured by Oji Scientific Instruments Co., Ltd. The measurement was performed at a wavelength of 590 nm.

(Peel Strength)
A T-peel test was carried out using a vertical motorized test stand MX2 series (manufactured by Imada Co., Ltd.) in accordance with JIS K 6854. The test speed was 100 mm/min.
(Friction coefficient) Measured using a surface property tester (manufactured by Imada Co., Ltd., model number: MX2-500N) in accordance with JIS K7125.

(Production Example 1)
A production example of a (meth)acrylic resin is shown below.

The extruder used was a 40 mm diameter co-rotating twin screw extruder (L/D = 90). The temperature setting of each temperature control zone of the extruder was 250-280°C, and the screw speed was 85 rpm. Polymethyl methacrylate resin (Mw: 105,000) was supplied at 42.4 kg/hr, and the resin was melted and filled by the kneading block, and then 1.8 parts by weight of monomethylamine (manufactured by Mitsubishi Gas Chemical Co., Ltd.) was injected from the nozzle relative to the resin. A reverse flight was placed at the end of the reaction zone to fill it with resin. The by-products and excess methylamine after the reaction were removed by reducing the pressure at the vent port to -0.092 MPa. The resin that came out as strands from the die installed at the extruder outlet was cooled in a water tank and then pelletized with a pelletizer to obtain resin (I).

Next, a 40 mm diameter intermeshing co-rotating twin screw extruder was used, with the temperature setting for each temperature control zone set at 240-260°C and the screw speed set at 102 rpm. Resin (I) obtained from the hopper was fed at 41 kg/hr, and the resin was melted and filled by the kneading block, after which 0.56 parts by weight of dimethyl carbonate was injected into the resin from the nozzle to reduce the carboxyl groups in the resin. A reverse flight was placed at the end of the reaction zone to fill it with resin. The by-products and excess dimethyl carbonate after the reaction were removed by reducing the pressure at the vent port to -0.092 MPa. The resin that came out as strands from the die installed at the extruder outlet was cooled in a water tank and then pelletized with a pelletizer to obtain (meth)acrylic resin (A1) having glutarimide rings.

The (meth)acrylic resin (A1) had a glutarimide content of 6% by weight, a glass transition temperature of 125°C, and an average refractive index of 1.50.

(Production Example 2)
An example of the production of a (meth)acrylic film is shown below.

The (meth)acrylic resin pellets (A1) obtained in Production Example 1 were fed to a twin-screw extruder and melt-extruded into a sheet at approximately 280°C to obtain a film having a thickness of 100 μm. This unstretched film was stretched 1.6 times vertically and 1.6 times horizontally at a temperature of 130°C to obtain a (meth)acrylic resin film (thickness 40 μm, in-plane retardation Δnd 0.8 nm, thickness direction retardation Rth 1.5 nm).

(Corona discharge treatment)
One side of the (meth)acrylic resin film obtained in Production Example 2 was subjected to a corona discharge treatment (amount of corona discharge electron irradiation: 77 W/m2/min).

(Formation of easy-adhesion layer)
To 100 parts by weight of a water-based polyurethane having a carboxyl group (manufactured by Daiichi Kogyo Seiyaku, product name: Superflex 210, solid content: 33%), 2.02 parts by weight of a tetrafunctional epoxy-based crosslinking agent (manufactured by Nagase ChemteX Corporation, product name: Denacol EX-512, solid content: 100%, epoxy equivalent: 168 g/eq) and 250 parts by weight of pure water were added, and the mixture was applied to the corona discharge-treated surface of a (meth)acrylic resin film that had been subjected to a corona discharge treatment with a bar coater (#3) to form a coating film. The film was then placed in a hot air dryer (80°C) and the coating film was dried for about 5 minutes to form an easy-adhesion layer (0.2 to 0.4 μm), and a polarizer protective film was obtained.

(Preparation of Adhesive Composition)
20 parts by weight of methylolmelamine was dissolved in pure water at a temperature of 70° C. for 100 parts by weight of an acetoacetyl group-containing polyvinyl alcohol resin (average degree of polymerization: 1200, degree of saponification: 98.5 mol%, degree of acetoacetyl group modification: 5 mol%) to obtain an aqueous solution with a solid content of 1.0%. The obtained aqueous solution was used as an adhesive composition at a temperature of 25° C.

(Lamination of polyvinyl alcohol film and polarizer protective film)
The adhesive composition was applied to the easy-adhesion layer side of the polarizer protective film so that the thickness after drying would be 50 nm. Thereafter, a polyvinyl alcohol film, BOVLON-EX (film thickness 12 μm) manufactured by Nippon Synthetic Chemical Industry Co., Ltd., and the polarizer protective film were laminated through the coating film of the adhesive composition, and the laminate was placed in a hot air dryer (100° C.) and dried for 5 minutes to obtain a laminate.

Example 2
The polyvinyl alcohol film and the polarizer protective film were attached to each other in the same manner as in Example 1, except that the amount of the epoxy-based crosslinking agent added when forming the easy-adhesion layer was 4.87 parts by weight.

Example 3
The polyvinyl alcohol film and the polarizer protective film were attached to each other in the same manner as in Example 1, except that the amount of the epoxy-based crosslinking agent added when forming the easy-adhesion layer was 7.73 parts by weight.

(Comparative Example 1)
The polyvinyl alcohol film and the polarizer protective film were bonded to each other in the same manner as in Example 1, except that the epoxy-based crosslinking agent used in forming the easy-adhesion layer was 1.30 parts by weight of a bifunctional epoxy-based crosslinking agent (manufactured by Nagase ChemteX Corporation, product name: Denacol EX-810, solid content: 100%, epoxy equivalent: 113 g/eq).

(Comparative Example 2)
The polyvinyl alcohol film and the polarizer protective film were attached to each other in the same manner as in Comparative Example 1, except that the amount of the epoxy-based crosslinking agent added when forming the easy-adhesion layer was 3.30 parts by weight.

(Comparative Example 3)
The polyvinyl alcohol film and the polarizer protective film were attached to each other in the same manner as in Comparative Example 1, except that the amount of the epoxy-based crosslinking agent added when forming the easy-adhesion layer was 5.20 parts by weight.

(Comparative Example 4)
A polyvinyl alcohol film and a (meth)acrylic resin film were laminated together in the same manner as in Example 1, except that no easy-adhesion layer was formed.

A T-peel test was carried out on the laminates produced in Examples 1 to 3 and Comparative Examples 1 to 4 in accordance with JIS K 6854. The results are shown in Table 1.

As is clear from Table 1, the laminates produced in the examples had excellent adhesion. On the other hand, the laminates produced in Comparative Example 1 had poor adhesion. This shows that by forming the easy-adhesion layer from an easy-adhesive composition containing a polyurethane having a carboxyl group and a crosslinking agent having a multifunctional epoxy group with three or more functionalities, a polarizer protective film with excellent adhesion to the polarizer can be obtained.

Example 4
When forming the easy-adhesion layer, a polyvinyl alcohol film and a polarizer protective film were bonded to each other in the same manner as in Example 1, except that 1.25 parts by weight of colloidal zircon (manufactured by Nissan Chemical Industries, Ltd., product name: Nanouse ZR, solid content 40% by weight, particle size 90 nm, pH 9.6) was further added to 100 parts by weight of the aqueous polyurethane having a carboxyl group.

Example 5
A polyvinyl alcohol film and a polarizer protective film were laminated in the same manner as in Example 4, except that colloidal silica (Fuso Chemical Co., Ltd., Fortron PL-3, solid content 20 wt %, primary particle size 35 nm, pH 7.3) was used instead of colloidal zircon.

The coefficient of friction between the films produced in Examples 4, 5, and Comparative Example 4 and the (meth)acrylic film obtained in Production Example 2 was measured in accordance with JIS K7125. The adhesion of the resulting laminate was also evaluated. The results are shown in Table 2.

As is clear from Table 2, the polarizer protective films produced in Examples 4 and 5 have a low coefficient of friction, excellent blocking resistance, and excellent adhesion to the polarizer.

On the other hand, the polarizer protective film produced in Comparative Example 4 has a large friction coefficient, poor blocking resistance, and low adhesion to the polarizer.

These results indicate that the use of colloidal zircon or colloidal silica as an anti-blocking agent can produce a polarizer protective film with excellent blocking resistance and adhesion to the polarizer.

The polarizing plate of the present invention can be suitably used in image display devices such as liquid crystal display devices and self-luminous display devices.

1 Polarizer 2 Adhesive layer 3 Easy-adhesion layer 4 (Meth)acrylic resin film 5 Polarizing plate

Claims (12)

A polarizer protective film comprising an easy-adhesion layer made of an easy-adhesion composition containing a polyurethane having a carboxyl group and a multifunctional epoxy-based crosslinking agent having tetrafunctionality or higher, and a resin film containing a (meth)acrylic resin having a ring structure in its main chain. 2. The polarizer protective film according to claim 1, wherein the (meth)acrylic resin having a ring structure in the main chain is a (meth)acrylic resin having a unit represented by the following formula (1):

(In the formula, ^R1 and ^R2 are each independently hydrogen or an alkyl group having 1 to 8 carbon atoms, and ^R3 is hydrogen, an alkyl group having 1 to 18 carbon atoms, a cycloalkyl group having 3 to 12 carbon atoms, or an aryl group having 6 to 10 carbon atoms.) The polarizer protective film according to claim 2, characterized in that the content of the unit represented by the formula (1) in the (meth)acrylic resin is 2% by weight or more and 30% by weight or less. The polarizer protective film according to claim 1 , wherein the multifunctional epoxy-based crosslinking agent is polyglycerol polyglycidyl ether. The polarizer protective film according to any one of claims 1 to 4 , wherein the easy-adhesive composition contains colloidal silica or colloidal zircon. 6. The polarizer protective film according to claim 1 , wherein the orientation birefringence of the resin film containing the (meth)acrylic resin having a ring structure in the main chain is −1.7×10 ⁻⁴ to 1.7×10 ⁻⁴ . The polarizer protective film according to any one of claims 1 to 6 , wherein the resin film containing the (meth)acrylic resin having a ring structure in its main chain has an in-plane retardation Δnd of 5.0 nm or less and a thickness direction retardation Rth of 20.0 nm or less. 8. The polarizer protective film according to claim 1 , wherein the resin film containing the (meth)acrylic resin having a ring structure in its main chain has a photoelastic coefficient of -10×10 ^-12 to 10×10 ^-12 Pa ^-1 . 9. The polarizer protective film according to claim 1 , wherein the resin film containing a (meth)acrylic resin having a ring structure in its main chain contains a crosslinked elastomer. 10. The polarizer protective film according to claim 9 , wherein the crosslinked elastomer is a core-shell type elastomer having a core layer made of a rubber-like polymer and a shell layer made of a glassy polymer. A polarizing plate comprising a polarizer, an adhesive layer, and the polarizer protective film according to any one of claims 1 to 10 . The polarizing plate according to claim 11 , wherein the adhesive layer is made of an adhesive composition containing a polyvinyl alcohol resin.

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