JPH07120617A - Polarizing plate - Google Patents

Abstract

PURPOSE:To provide a polarizing plate excellent in wet heat resistance by controlling the moisture content of a polarizing film to the specified value at the time of bonding a cellulose acetate-based protective film to the polarizing film, and using an urethane as adhesive. CONSTITUTION:The polarizing plate is formed by bonding a cellulose acetate- based protective film to a polarizing film of 8wt.% or more in moisture content using an urethane prepolymer as adhesive formed by the reaction of a compound with two isocyanate radicals or more in molecules and a compound having active hydrogen, reacting on the isocyanate radical, in one molecule. It is so prepared that the moisture content of the polarizing film is 8wt.% or more in the end, desirably 8-50wt.%, further desirably 10-30wt.%. With the moisture content being less than 8wt.%, the reaction to the urethane prepolymer is lowered, and it takes time to harden. With the moisture content exceeding 50wt.%, on the other hand, the contraction of base material is easily generated, and as a result, the adhesion to the urethane prepolymer is lowered.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarizing plate which is excellent in adhesion between a polarizing film and a protective film and resistance to moist heat.

[0002]

2. Description of the Related Art In recent years, liquid crystal display devices have been used in desk-top electronic computers, electronic timepieces, word processors, measuring instruments such as automobiles and machines, and the demand for polarizing plates has increased accordingly. The polarizing plate is generally formed by forming a protective film on both surfaces of a polarizing film having a polarizing ability through an adhesive layer. Currently, as a typical polyvinyl alcohol-based polarizing film, there are a polyvinyl alcohol-based film dyed with iodine and a dichroic dye, which are used to form an aqueous solution of polyvinyl alcohol. , Which is uniaxially stretched to be dyed, or after dyeing and then uniaxially stretched, preferably subjected to a durability treatment with a boron compound, and as the protective film, a cellulose acetate film is used as a surface protective film. It is widely used because of its excellent optical transparency and non-orientation.

Since both the polyvinyl alcohol film and the cellulose acetate film are hydrophilic, it is common to bond them by using a hydrophilic adhesive. However, the resulting polarizing plate has a large water absorption rate in a high-humidity atmosphere for a long time, the adhesive force decreases as it absorbs water, and the physical properties of the polarizing plate deteriorate, causing problems in so-called moisture resistance and dimensional stability. Or, if it is placed at a high temperature, the degree of polarization may be lowered and the heat resistance may be impaired.

In order to solve these problems, for example, in JP-A-56-50301, the surface of the cellulose acetate film of the protective film is saponified to strengthen the adhesive force between the adhesive and the cellulose acetate. , Moisture resistance, heat resistance and dimensional stability have been proposed.

Further, in JP-A-61-245107, after the surface of a polarizing film is once treated with an aqueous solution or adhesive of a hydrophilic polymer such as polyvinyl alcohol, carboxymethyl cellulose, urethane type, acrylic type, epoxy type, etc. It has been proposed to provide a polarizing plate with improved adhesive strength by adhering a protective film again via an adhesive.

[0006]

However, in JP-A-56-50301, although the adhesive force between the protective film and the adhesive layer is improved, the adhesive force between the polarizing film and the adhesive layer has been conventionally improved. As it is, no consideration is given, and it cannot be said that sufficient durability is obtained in a high temperature and high humidity state. That is, the surface of the polarizing film has more adsorbed iodine and the like than the inside, and in a high temperature and high humidity state, iodine and the like partially migrate to the surface of the polarizing film, which causes adhesiveness. Make it worse. It is possible to increase the thickness of the adhesive layer in order to prevent these, but it is difficult to increase the thickness of the adhesive layer for industrial uniform coating, and it is also disadvantageous in terms of optical properties. Absent.

In Japanese Patent Laid-Open No. 61-245107, the aim is to improve the adhesiveness between the polarizing film and the adhesive layer, and between the adhesive layer and the protective film. However, it cannot be said that the resistance to moisture and heat is still insufficient, and it is necessary to use the adhesive again after treating the polarizing film with the adhesive, which complicates the work process, and when using the urethane adhesive, It is indispensable to use a compound having active hydrogen such as polyisocyanate and polyhydric alcohol, and since a two-component curing type urethane adhesive mixed and stirred with these is applied, the pot life of the adhesive However, the process management becomes complicated and problems still remain.

The applicant of the present invention has also disclosed in Japanese Patent Laid-Open No. 2-135402 that in order to improve resistance to moist heat and dimensional stability,
It has been proposed to use an aqueous solution of a boron compound as an adhesive for the polyvinyl alcohol-based polarizing film and the cellulose acetate-based protective film, but in this method, a polarizing plate having significantly improved adhesiveness is obtained. However, the polarizing plate is not so satisfactory as a medium-durability one, and it is actually desired to develop a highly durable and highly polarized type polarizing plate having a heat and humidity resistance higher than that of conventional polarizing plates. Is.

[0009]

However, as a result of intensive studies by the present inventors in order to solve such a problem, the compound (A) having two or more isocyanate groups in the molecule and the isocyanate in one molecule. Using a urethane prepolymer (C) obtained by reacting a compound (B) having active hydrogen that reacts with a group as an adhesive, a polarizing film having a water content of 8% by weight or more and a cellulose acetate-based protective film are adhered to each other. The present invention has been completed, and it was found that the resulting polarizing plate becomes a polarizing plate that adheres very firmly, has improved wet heat resistance, and is useful as a highly durable and highly polarized type polarizing plate.

That is, in the present invention, since the urethane prepolymer is applied as it is to the surface of the polarizing film, there is no problem of pot life as in the application of urethane resin, and the protective film is adhered as it is. It is possible to exert a very excellent effect on processing suitability, but in coating such urethane prepolymer, by controlling the water content of the polarizing film of the base material within a specific range, high durability and high polarization property can be obtained. Is achieved. Hereinafter, the present invention will be specifically described.

The polarizing film of the present invention is a uniaxially stretched film of a polyvinyl alcohol film. Polyvinyl alcohol is generally produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate, but the present invention is not necessarily limited to this, and a small amount of unsaturated carboxylic acid (salt, ester, amide, nitrile, etc. , Etc.), olefins, vinyl ethers, unsaturated sulfonates and the like, and may contain a component copolymerizable with vinyl acetate. The average degree of saponification in polyvinyl alcohol is 85 to 10
0 mol%, preferably 98 to 100 mol% is practical. As the average degree of polymerization of the polyvinyl alcohol of the present invention, any can be used, but 1500 or more,
Preferably 2600 or more, more preferably 3500-5
000 is advantageous.

The method for producing the polarizing film of the present invention includes:
A stock solution of polyvinyl alcohol dissolved in water or an organic solvent is cast to form a film, and the film is stretched and dipped in an aqueous solution of iodine or a dichroic dye for dyeing, or stretching and dyeing are performed simultaneously, or iodine or A method of treating with a boron compound after dyeing with a dichroic dye and stretching. There is also a method of dyeing and then stretching in a solution of a boron compound, which can be appropriately selected and used. Examples of the solvent used for preparing the undiluted solution include not only water but also dimethyl sulfoxide (DMSO), N-methylpyrrolidone, glycerin, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, and the like. Polyhydric alcohol, ethylenediamine,
Amines such as diethylenetriamine and mixtures thereof are used.

A small amount, for example, 5 to 30 is contained in the above organic solvent.
It is permissible to include water in a weight percentage. The concentration of polyvinyl alcohol in the stock solution is practically 5 to 20% by weight. The polyvinyl alcohol film-forming stock solution obtained using the solvent is formed into a film by an arbitrary method such as a casting method or an extrusion method. The film forming method is a dry / wet film forming method, that is, the solution is once discharged from the mouth slit into the air or into an inert atmosphere such as nitrogen, helium, or argon, and then introduced into a coagulation bath for unstretching. Form a film. Alternatively, the film-forming solution discharged from the die may be introduced into the coagulation bath after the solvent is partially dried on a roller, a belt conveyor or the like.

The solvent used in the coagulation bath is miscible with the solvent of polyvinyl alcohol, and examples thereof include alcohols such as methanol, ethanol, propanol and butanol, acetone, benzene and toluene. As a method for obtaining a polyvinyl alcohol-based film, a so-called gel film forming method in which a solution of polyvinyl alcohol is introduced into a coagulation bath to form a film may be used.

The thickness of the raw film is 30 to 1
00 μm, preferably 50 to 90 μm is required. Three
When the thickness is 0 μm or less, stretching becomes impossible, and when the thickness is 100 μm or more, the film thickness accuracy is lowered, which is unsuitable. The polyvinyl alcohol unstretched film obtained as described above is then stretched, dyed, and treated with a boron compound. The stretching and dyeing and the boron compound treatment may be carried out separately or simultaneously,
In the present invention, it is desirable to carry out uniaxial stretching during at least one of the dyeing step and the boron compound treatment step.

It is desirable that the film is stretched uniaxially by 3.5 to 10 times, preferably 4.5 to 7 times. At this time, some stretching (stretching to a degree that prevents shrinkage in the width direction or more) may be performed in the direction perpendicular to the above. The temperature condition during stretching is preferably selected from 40 to 130 ° C. Furthermore, the stretching ratio may be finally set within the above range, and the stretching operation may be carried out not only in one stage but also in any stage of the manufacturing process.

The dyeing of the film, that is, the adsorption of the polarizing element is performed by bringing the liquid containing the polarizing element into contact with the film. Usually, an aqueous solution of iodine-potassium iodide is used, the concentration of iodine is 0.1 to 2 g / l, the concentration of potassium iodide is 10 to 50 g / l, and the weight ratio of iodine / potassium iodide is 20 to 100. Appropriate. Staining time is 30-5
Practical time is about 00 seconds. The temperature of the treatment bath is 30-80
C is preferred. A small amount of an organic solvent compatible with water may be contained in addition to the water solvent. As the contact means, any means such as dipping, coating and spraying can be applied.

The dyed film is then treated with a boron compound. Boric acid and borax are practical as the boron compound. The boron compound is used in the form of an aqueous solution or a water-organic solvent mixture at a concentration of about 0.5 to 2 mol / l, and it is practically desirable to allow a small amount of potassium iodide to coexist in the solution. The treatment method is preferably a dipping method, but of course, a coating method and a spraying method can also be carried out. The temperature during processing is 50-
The treatment time is preferably about 70 ° C. and the treatment time is about 5 to 20 minutes, and if necessary, a stretching operation may be performed during the treatment.

The polyvinyl alcohol polarizing film obtained by the method as described above has a very high water content, and the present invention is characterized in that the water content of the film is controlled within a specific range. In order to obtain the above, the film is obtained by drying at 30 to 120 ° C. for 30 to 500 seconds. That is, the water content of the polarizing film is finally adjusted to 8% by weight or more, preferably 8 to 50% by weight, more preferably 10 to 30% by weight. Further, a polarizing film having a water content of less than 8% by weight can be adjusted to have the water content in the above range by a method such as humidity control or rewetting, and then used for the polarizing film of the present invention. 8
If the water content is less than 5% by weight, the reactivity with the urethane prepolymer will decrease and it will take a long time to cure.
If the amount exceeds 5% by weight, the base material is likely to shrink, and the adhesiveness with the urethane prepolymer is lowered, so that the effect of the present invention is difficult to obtain.

Examples of the cellulose acetate-based protective film of the present invention include films of cellulose diacetate, cellulose triacetate and the like. Further, it is more effective to saponify the surface of the protective film with an alkali, plasma treatment, glow discharge treatment, corona discharge treatment, high frequency treatment, electron beam treatment or the like.

In the present invention, the water content of 8 is obtained as described above.
A urethane prepolymer (C) is used as an adhesive at the time of adhering the weight percentage of the polarizing film and the cellulose acetate-based protective film.

The urethane prepolymer (C) is
An isocyanate group-containing compound formed by reacting a compound (A) having two or more isocyanate groups in the molecule with a compound (B) having active hydrogen that reacts with an isocyanate group in one molecule. Examples of the compound (A) having two or more isocyanate groups therein include aliphatic diisocyanates such as ethylene diisocyanate, 1,4-butane diisocyanate, and 1,6-hexane diisocyanate; dicyclohexylmethane diisocyanate, cyclohexane diisocyanate, and methylcyclohexane diisocyanate. Alicyclic diisocyanates such as 2,4 and 2,6-tolylene diisocyanate,
There are aromatic diisocyanates such as methylene diphenyl diisocyanate, diphenylmethane-4,4 diisocyanate and m-xylylene diisocyanate; aromatic triisocyanates such as triphenylmethane triisocyanate.

As the compound (B) having active hydrogen which reacts with an isocyanate group in one molecule, mono-di- or tri-ethylene- or -propylene-glycol, 1,4-butanediol, 2,2- A diol-based polyol such as bis- (4-hydroxylhexyl) propane, or a triol-based polyol such as glycerin, hexanetriol, trimethylolpropane, trimethylolethane, trimethylolbenzene; adipic acid, succinic acid, phthalic acid, terephthalic acid or Polyester polyol which is a condensation product of a polycarboxylic acid such as a copolymer of acrylic acid and maleic acid and the above polyol; castor oil or other glyceride ester of hydroxylated fatty acid; polyalkylene glycol such as polyethylene glycol and polypropylene glycol. There is Lumpur and the like.

When the urethane prepolymer (C) is produced by reacting the compound (A) with the compound (B), the ratio of isocyanate groups to hydroxyl groups is 1.5 / 1 or more, preferably 2/1. It is necessary to control the free isocyanate group content in the urethane prepolymer (C) to 3% by weight or more, preferably 7 to 30% by weight, and the free isocyanate group is the polarizing film. A strong adhesive property can be obtained by causing a crosslinking reaction with the water content in the air or the water content in the air, and further with a hydroxyl group contained in cellulose triacetate used as a protective film. If the amount of free isocyanate groups is less than 3% by weight, the adhesiveness between the adhesive layer and the polarizing film or protective film cannot be expected sufficiently, which is not preferable. Furthermore, the molecular weight of the urethane prepolymer (C) used in the present invention is 500 to 2
000, preferably 600 to 1500, is desirable, and the effect of the present invention is remarkably exhibited.

Thus, the above urethane prepolymer (C)
Is a solvent that does not impair the polarization of the polarizing film, such as ethyl acetate, butyl acetate, cellosolve acetate, methyl glycol acetate, ethylene glycol acetate, methoxybutyl acetate, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, methylene chloride, toluene, xylene, mineram. It is diluted to a desired viscosity with spirit, cresol, xylenol, furfural, naphtha, etc., and applied as an adhesive between the polarizing film and the protective film as described above.

Usually, the adhesive is advantageously applied so as to form a uniform film on the polarizing film or the protective film, and the thickness after drying is 1 to 2 when applied.
It is practical that the thickness is 0 μm, preferably 5 to 15 μm. If it is less than 1 μm, the adhesive strength is insufficient,
On the other hand, when it exceeds 20 μm, the effect is not increased for the amount used and the appearance is deteriorated, which is not practical. The coating operation is not necessarily limited to coating means using a roll or the like, and means such as spraying method and dipping method are also included. Further, a water-soluble polymer such as polyvinyl alcohol may be used in combination with such an adhesive.

The urethane prepolymer is applied to at least one surface of the polarizing film, and the coating is applied at 30 to 90 ° C. for 1 hour.
~ 2 minutes, then, the above cellulose acetate-based protective film is laminated, further 20 ~ 30 ℃, 50 ~ 80%
It is cured under RH conditions for 1 to 14 days, whereby a polarizing plate in which the polarizing film surface and the protective film surface are firmly adhered to each other can be obtained. In addition, the urethane prepolymer is applied to the polarizing film (or cellulose acetate-based protective film),
After bonding with a cellulose acetate-based protective film (or a polarizing film), it is dried at 30 to 90 ° C for 1 to 2 minutes, and further 1 to 14 under the conditions of 20 to 30 ° C and 50 to 80% RH.
It may be cured over a period of days.

If necessary, a transparent pressure-sensitive adhesive layer can be provided on the above polarizing plate by a commonly known method. Examples of the pressure-sensitive adhesive layer include acrylic acid esters such as butyl acrylate, ethyl acrylate, methyl acrylate, 2-ethylhexyl acrylate, and α-monoolefin carboxylic acids such as acrylic acid and maleic acid.
Those mainly composed of copolymers with itaconic acid, methacrylic acid, crotonic acid, etc. (including those added with vinyl monomers such as acrylonitrile, vinyl acetate and styrene) impair the polarization properties of the polarizing film. However, the pressure-sensitive adhesive is not limited to this, and any pressure-sensitive adhesive having transparency can be used. For example, polyvinyl ether-based or rubber-based adhesive may be used.

The thus obtained polarizing plate has excellent adhesiveness between the polarizing film and the cellulose acetate-based protective film, so that the durability is improved, and the polarizing property is lowered even when it is left for a long period of time at high temperature and high humidity. It has the property of not being used, and is used for liquid crystal display applications by utilizing such properties, and is particularly useful for vehicle applications, various industrial instruments, and display of household appliances.

[0030]

According to the present invention, when the cellulose acetate protective film is adhered to the polarizing film, the water content of the polarizing film is set to 8%.
It is characterized by using a urethane prepolymer as an adhesive, with the content controlled to be more than weight%. The moisture in the polarizing film or the humidity in the air, and further the hydroxyl groups in the protective film react with the urethane prepolymer to cure. Thus, a strong adhesive property can be obtained, and thus a polarizing plate excellent in wet heat resistance can be obtained.

[0031]

EXAMPLES The present invention will be specifically described below with reference to examples. In the examples, "part" and "%" are based on weight unless otherwise specified. In addition, the degree of polarization in the present invention is

[Equation 1] H ₁₁ is a transmittance (%) measured by using a spectrophotometer in a state where the two polarizing film samples are superposed so that the orientation directions of the polarizing films are the same, when the two polarizing film samples are superposed. H ₁ is a transmittance (%) measured in a state where the two polarizing plates are superposed so that the orientation directions of the polarizing films are orthogonal to each other.

Example 1 Average degree of polymerization 3800, degree of saponification 99.5 mol%, thickness 8
A polyvinyl alcohol-based film having a thickness of 0 μm was treated with iodine.
3 in an aqueous solution consisting of 2 g / l and 60 g / l potassium iodide
Immerse at 0 ° C. for 240 seconds, then soak in an aqueous solution having a composition of boric acid 70 g / l and potassium iodide 30 g / l,
At the same time, boric acid treatment was carried out for 5 minutes while uniaxially stretching 6 times. Then, it was dried at 30 ° C. for 24 hours to finally obtain a polarizing film having a water content of 10%. On the film,
Component (A): 2,4-tolylene diisocyanate (80
%), 2,6-tolylene diisocyanate (20%),
Component (B): urethane prepolymer consisting of trimethylolpropane (free isocyanate group content: 19.2
%) 60% ethyl acetate solution has a thickness of 10 μm after drying.
And then dried at 80 ° C. for 2 minutes, bonded with cellulose triacetate surface-treated with alkali, and further cured at 20 ° C. and 75% RH for 7 days,
A polarizing plate was obtained.

The polarizing plate thus obtained was dipped in water at 90 ° C. for 30 minutes, and allowed to stand at 20 ° C. and 75% RH for 4 hours, and then the change in polarization degree (%) of the polarizing plate was measured.
Regarding the adhesiveness between the polarizing film and cellulose triacetate, the underwater peeling test, that is, a laminated sample of the polarizing film and the protective film was cut into a width of 25 mm and a length of 90 mm, and one end was peeled off to remove one film end. Load on the part (1
00g) and hang the other end of the film
The adhesive strength was evaluated by measuring the time (seconds) from the time of immersion in hot water at 70 ° C. until the adhesive surface was completely peeled off. Naturally, the longer the peeling time, the better. The evaluation criteria are as follows. ○ Peeling time 900 seconds or more △ Peeling time 700 to 900 seconds × Peeling time 700 seconds or less

Example 2 Average degree of polymerization 1700, degree of saponification 99.9 mol%, thickness 8
Using a polyvinyl alcohol-based film having a thickness of 0 μm and a water content of 10%, the urethane prepolymer (C) was mixed with the component (A): methylenebis- (4-phenylisocyanate),
Component (B): urethane prepolymer consisting of trimethylolethane (content of free isocyanate group: 14.0
%) Was carried out in the same manner as in Example 1 to obtain a polarizing plate. The polarizing plate thus obtained was measured in the same manner as in Example 1.

Example 3 Average degree of polymerization 4500, degree of saponification 99.3 mol%, thickness 8
A urethane prepolymer (C) was used as a component (A): 2,4-tolylene diisocyanate (80%), using a polyvinyl alcohol-based film having a thickness of 0 μm and a water content of 15%.
2,6-Tolylene diisocyanate (20%), (B)
Component: A polarizing plate was obtained in the same manner as in Example 1 except that the urethane prepolymer composed of diethylene glycol (free isocyanate group content: 20.0%) was used. The polarizing plate thus obtained was measured in the same manner as in Example 1.

Comparative Example 1 In Example 1, after the water content was adjusted to 5%, the urethane prepolymer (C) was replaced with a urethane prepolymer consisting of (A) component: hexamethylene diisocyanate and (B) component: trimethylolpropane. A polarizing plate was obtained in the same manner except that the polymer (content of free isocyanate group: 19.1%) was used. The polarizing plate thus obtained was measured in the same manner as in Example 1.

Comparative Example 2 A composition prepared by blending a urethane prepolymer similar to that of Example 1 with polyester polyol as the urethane prepolymer (C) in Example 1 to adjust the free isocyanate group content to 1.9%. A polarizing plate was obtained in the same manner except that was used. The polarizing plate thus obtained was used in Example 1.
It measured similarly to.

Comparative Example 3 A polarizing plate was obtained in the same manner as in Example 1, except that a 4% boric acid aqueous solution was used instead of the urethane prepolymer (C). The polarizing plate thus obtained was measured in the same manner as in Example 1. The results of Examples and Comparative Examples are summarized in Table 1.

[0039]

[Table 1]

[0040]

EFFECTS OF THE INVENTION The polarizing plate of the present invention has excellent adhesiveness between the polarizing film and the cellulose acetate-based protective film, so that the durability is improved and its polarizing property is maintained even when left for a long period of time at high temperature and high humidity. It has the effect of not decreasing.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] February 7, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0019

[Correction method] Change

[Correction content]

The polarizing film of polyvinyl alcohol obtained in such previously described methods a water content is very high, the
In the invention, the film is dried at 30 to 120 ° C.
It is special to control the moisture content of the film within a specific range.
It is a sign. That is, the water content of the polarizing film is finally adjusted to 8% by weight or more, preferably 8 to 50% by weight, more preferably 10 to 30% by weight. Further, a polarizing film having a water content of less than 8% by weight can be adjusted to have the water content in the above range by a method such as humidity control or rewetting, and then used for the polarizing film of the present invention. If the water content is less than 8% by weight, the reactivity with the urethane prepolymer decreases,
Further, it takes a long time to cure, and when it exceeds 50% by weight, the substrate is likely to shrink, and the adhesiveness with the urethane prepolymer is lowered, so that the effect of the present invention is difficult to obtain.

Claims (3)

[Claims] 1. A urethane prepolymer (C) obtained by reacting a compound (A) having two or more isocyanate groups in the molecule with a compound (B) having active hydrogen which reacts with an isocyanate group in one molecule. Using as an adhesive
A polarizing plate comprising a polarizing film having a water content of 8% by weight or more and a cellulose acetate-based protective film adhered to each other. 2. The polarizing plate according to claim 1, wherein the content of free isocyanate groups in the urethane prepolymer (C) is 3% by weight or more. 3. The polarizing plate according to claim 1, wherein a polarizing film having a water content of 8 to 50% by weight is used.