JPH06289225A - Production of polarizing film - Google Patents

Abstract

PURPOSE:To improve polarization performance of a polarizing film obtd. by low temp. and uniaxial. orientation, by uniaxially orienting a film produced by mixing a specified weight ratio of plasticizer to polyvinylalcohol and forming a film from this soln., and uniaxially orienting. CONSTITUTION:This film is obtd. from a soln. containing >=15wt.% plasticizer to polyvinylalcohol (PVA), forming a film, and uniaxially orienting the film. The upper limit of the plasticizer to the PVA is not limited, but preferably <=60wt.% considering the practical strength of the film. If the mixing ratio of the plasticizer is <15wt.%, the effect to improve orienting property of the PVA film is not enough. The stretching rate of the oriented film is >=3 times, preferably >=4 times. If the rate is less than three times, a polarizing film of high polarization property is not obtd. Thereby, the polarization performance of the polarizing film by low temp. orientation can be significantly improved, and the obtd. film has const. transmittance and excellent polarization performance.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a polyvinyl alcohol type polarizing film.

[0002]

2. Description of the Related Art A polarizing plate having a light transmitting / shielding function is
Together with the liquid crystal having a light switching function, it is a basic component of a liquid crystal display (LCD). This L
The range of applications for CDs has expanded from small devices such as calculators and watches in the early days to applications requiring high functionality such as laptop computers, word processors, liquid crystal color projectors, in-vehicle navigation systems, and liquid crystal televisions. There is a demand for a polarizing plate having a constant optical property and a high polarization property more than that of the polarizing film. The conventional polarizing film is
Generally, polyvinyl alcohol film made of polyvinyl alcohol (hereinafter, polyvinyl alcohol is referred to as PV
A, a polyvinyl alcohol film may be abbreviated as PVA film) as a raw material film, dyed with iodine and an iodine compound as a dye, and then uniaxially stretched at 30 to 40 ° C. in a water bath containing an iodine compound and a boron compound. Then, it is manufactured by carrying out a fixing treatment in a water bath containing an iodine compound and a boron compound. In the production of such a polarizing film, the polarizing performance of the polarizing film is greatly influenced by the uniaxial stretching ratio, so that a stretching ratio as high as possible is adopted to enhance the polarizing performance. As a means for obtaining a high stretching ratio, it has been proposed to uniaxially stretch at a high temperature of 50 ° C to 70 ° C, but if the uniaxial stretching temperature is increased in this way, the sublimation of iodine in the film will become violent and the final product As a result, fluctuations in the transmittance of the polarizing film due to fluctuations in the iodine concentration in the polarizing film cannot be prevented, making it difficult to collect products with constant optical characteristics. Further, it has been proposed to use a high polymerization degree PVA film to enhance the polarization property, but the high polymerization degree PVA film has a lower stretchability than a normal polymerization degree PVA film, and has a high stretching temperature at a conventional low stretching temperature. Polarization is not obtained, and in order to exhibit high polarization, it is necessary to raise the uniaxial stretching temperature to a level higher than that of a PVA film having a normal degree of polymerization. Therefore, there is a problem of variation in transmittance of the polarizing film due to sublimation of iodine. Is left. As described above, in the conventional technique, it was not possible to achieve high polarization without causing a change in the transmittance of the polarizing film.

[0003]

An object of the present invention is to improve the polarization performance of a polarizing film. More specifically, the present invention is to improve the polarization performance of the obtained polarizing film even when uniaxially stretched at a low temperature.

[0004]

The inventors of the present invention have found that the raw material PVA
As a result of diligent studies on improvement of uniaxial stretchability at low temperature of a film, in particular, a PVA film having a high degree of polymerization, it was found that a film formed from a solution containing 15% by weight or more of a plasticizer mixed with polyvinyl alcohol is uniaxially stretched. The present invention has been completed by finding a method for producing a polarizing film that is a feature.

As the polyvinyl alcohol used in the polarizing film of the present invention, a general PVA produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate is generally used. From the viewpoint of maintaining the strength of the film, the polymerization degree of PVA is preferably 1000 or more, more preferably 2400 or more, and even more preferably 4000 or more in terms of obtaining a polarizing film having high polarization performance and high durability performance. The upper limit of the polymerization of PVA is not particularly limited, but a polymerization degree of 10,000 or less is preferable, and a polymerization degree of 8,000 or less is more practical from the viewpoint of film forming property of the film. The degree of saponification of PVA may be appropriately selected depending on the situation, but is usually preferably 90 mol% or more, more preferably 95 mol% or more, and 99 mol% or more in order to obtain a polarizing film having more excellent durability. , Even more preferred. In addition to these ordinary PVA, unsaturated carboxylic acids or their derivatives, unsaturated sulfonic acids or their derivatives, unsaturated amino group-containing compounds, unsaturated ammonium group-containing compounds, α-olefins having 2 to 30 carbon atoms, etc. Modified polyvinyl alcohol copolymerized with less than about 15 mol%; polyvinyl acetals such as polyvinyl formal, polyvinyl acetoacetal, and polyvinyl butyral;
Examples thereof include an ethylene-vinyl alcohol copolymer having an ethylene unit content of 20 mol% or more.

The plasticizer used in the present invention includes various polyols and their condensates, and among these, glycerin, diglycerin, triglycerin, 1,
Triols such as 3,5-pentanetriol, ethylene glycol, diethylene glycol, propylene glycol, butanediol, and glycols such as polyethylene glycol having a molecular weight of 2000 or less are preferable. Among them, glycerin, diglycerin, diethylene glycol, polyethylene having a molecular weight of 400 or less are preferable. It is more preferable to use glycols alone or as a mixture. In the present invention, the mixing ratio of the plasticizer to PVA is 15% by weight or more, preferably 20% by weight or more, which is significantly higher than the conventional technical range (13% by weight or less). The upper limit of the plasticizer mixing ratio is not particularly limited, but is preferably 60% by weight or less from the viewpoint of practical strength of the film. Mixing ratio of plasticizer is 1
If it is less than 5% by weight, the effect of improving stretchability is not exhibited.

In order to produce a PVA film, for example, a PVA solution obtained by dissolving a mixture of a raw material PVA and a plasticizer in water, an organic solvent or a mixed solution of water and an organic solvent is dried on a resin film. It may be formed on the above or on a drying belt by a casting method or a die casting method, dried, and optionally heat-treated. PVA here
If necessary, a surfactant, iodine, iodine compound, dichroic dye, or the like can be added to the solution. The thickness of the raw material PVA film can be 5 to 150 μm, and is preferably 30 to 100 μm.

Next, the raw material PVA thus obtained
In order to produce a polarizing film from a film, a known method such as a wet stretching method or a dry heat stretching method is used, and the polarizing film is subjected to preliminary swelling, dyeing, uniaxial stretching and fixing treatment, and if necessary, drying treatment. Manufactured. In the production method of the present invention, it is preferable to pre-swell the PVA film before dyeing and stretching. PV during this preliminary swelling process
Part or all of the plasticizer in the A film is eluted. Dyeing can be performed before stretching, during stretching, and after stretching. As the dye used for this dyeing, iodine-potassium iodide or a dichroic dye can be used. Uniaxial stretching can be carried out in warm water or in air using a film after absorbing water. The stretching temperature is not particularly limited, but when the film is stretched in warm water, it is preferably 15 to 80 ° C and 20 to 5 ° C.
0 ° C. is more preferable, and 50 to 150 in the case of dry heat stretching.
C is preferred. Boric acid and a boron compound are usually added to the treatment bath in the fixing treatment. An iodine compound may be mixed in the fixing treatment bath.

The stretching ratio for uniaxial stretching is 3 times or more, preferably 4 times or more. The upper limit of the draw ratio is not particularly limited, but is preferably 8.0 times or less from the viewpoint that uniform drawing can be obtained. Here, if the stretching ratio is less than 3 times, the desired highly polarizing polarizing film cannot be obtained. The thus obtained polarizing film of the present invention is usually used by laminating a supporting film such as a cellulose triacetate film on both outer surfaces of the polarizing film. The reason why the PVA film of the present invention exhibits high stretchability is presumed as follows. That is, generally PVA
The amount of plasticizer that can be stably contained in the film is 13% by weight or less, and if more than this amount of plasticizer is added, excess plasticizer leaches out to the film surface. Considering this point, the amount of the plasticizer added to the conventional raw material PVA film for the polarizing film was 13% by weight or less. On the other hand, in the present invention, PVA, which originally cannot be stably held, is 15% by weight.
It is presumed that by forming the film by adding the above plasticizer, the generation of crystals in the film is suppressed, and as a result, the uniaxial stretchability is improved and high polarization is obtained.

[0010]

The present invention will be described in more detail with reference to the following examples. The dichroic ratio used in the following examples is an index for evaluating the polarization performance of the polarizing film, and this dichroic ratio is
Regarding the obtained polarizing film, the Japan Electronic Machinery Manufacturers Association Standard (E
In accordance with IAJ) LD-201-1983, using a spectrophotometer, the transmittance (Ts (%) and the polarization degree P (%) obtained by measuring and calculating in a C light source and a 2 degree visual field are used to calculate the formula (I ). Dichroic ratio = log (Ts / 100−Ts × P / 10000) / log (Ts / 100 + Ts × P / 10000) …… (I)

Example 1 100 parts by weight of PVA having a degree of polymerization of 1750 and a degree of saponification of 99.9 mol% and glycerin 25 were used in a drum dry film forming machine.
A solution prepared by dissolving 1000 parts by weight of water in 1000 parts by weight of water is cast on the drum to form a film, which is dried at 70 ° C. to form a P film having a film thickness of 75 μm.
A VA film was created. Next, this film was processed in the order of pre-swelling, dyeing, uniaxial stretching, fixing treatment, and drying to form a polarizing film. At this time, the preliminary swelling bath was water alone, the iodine concentration in the dyeing bath water was 0.4 g / liter, the potassium iodide concentration was 40 g / liter, the boric acid concentration in the stretching bath water was 4% by weight, and the fixing treatment bath water was The concentration of boric acid was 4% by weight and the concentration of potassium iodide was 4% by weight. The water temperatures of the preliminary swelling bath, the dyeing bath, the stretching bath, the uniaxial stretching bath and the fixing treatment bath were all 30 ° C. The uniaxial stretching ratio was 5.8, which is a ratio immediately before the cutting limit. The polarizing film was dried with hot air at 50 ° C. The polarizing film obtained has a thickness of 25 μm.
m, transmittance 42.8%, polarization degree 99.7%, dichroic ratio 4
2.4 showed excellent polarization performance.

Comparative Example 1 Using the same apparatus as in Example 1, 100 parts by weight of PVA having a polymerization degree of 1750 and a saponification degree of 99.9 mol% and glycerin 1 were used.
Using a solution prepared by dissolving 0 parts by weight in 1000 parts by weight of water, a film was formed in the same manner as in Example 1 to form a PVA film having a film thickness of 75 μm. Next, except that the uniaxial stretching ratio of this film was 4.2 times, which is the ratio immediately before the cutting limit,
Preliminary swelling, dyeing, uniaxial stretching, fixing treatment and drying were carried out under the same conditions as in Example 1 to prepare a polarizing film. The obtained polarizing film had a thickness of 28 μm, a transmittance of 42.4% and a polarization degree of 97.
3, the dichroic ratio was 25.1.

Example 2 Using the same apparatus as in Example 1, 70 parts by weight of PVA having a polymerization degree of 4400 and a saponification degree of 99.7 mol% and 30 parts by weight of polyethylene glycol having a molecular weight of 200 are dissolved in 1000 parts by weight of water. The solution was dried under the same conditions as in Example 1 to prepare a PVA film having a thickness of 70 μm. Next, this film was processed in the order of pre-swelling, dyeing, uniaxial stretching, fixing treatment, and drying to form a polarizing film. The composition of each bath at this time was the same as in Example 1. The water temperature of the preliminary swelling bath, the dyeing bath, the stretching bath, and the fixing treatment bath was 40 ° C. The uniaxial stretching ratio was 5.6, which is the ratio immediately before the cutting limit. Drying was performed at 50 ° C. The obtained polarizing film had a thickness of 24 μm, a transmittance of 43.8%, a polarization degree of 99.8%, and a dichroic ratio of 48.7, and had excellent polarization performance.

Comparative Example 2 Using the same apparatus as in Example 1, a solution prepared by dissolving 90 parts by weight of a polymerization degree of 4400 and a saponification degree of 99.7 mol% and 10 parts by weight of polyethylene glycol having a molecular weight of 200 in 1000 parts by weight of water. And dried under the same conditions as in Example 1 to obtain a film thickness of 7
A 0 μm PVA film was made. Next, pre-swelling and dyeing were conducted under the same conditions as in Example 2 except that the uniaxial stretching ratio of this film was set to 4.0, which is the ratio immediately before the cutting limit.
Uniaxial stretching, fixing treatment and drying were performed to prepare a polarizing film. The obtained polarizing film had a thickness of 29 μm and a transmittance of 42.7.
%, The degree of polarization was 98.4%, and the dichroic ratio was 30.1.

[0015]

According to the present invention, the polarizing performance of the polarizing film can be remarkably improved by uniaxial stretching at a low temperature, so that a polarizing film having a constant transmittance and excellent polarizing performance can be obtained. The polarizing film obtained by the present invention is useful in fields requiring high-quality polarizing plates such as word processors, personal computers, and liquid crystal car televisions.

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Claims (1)

[Claims] 1. A method for producing a polarizing film, which comprises uniaxially stretching a film formed from a solution in which 15% by weight or more of a plasticizer is mixed with polyvinyl alcohol.