阅读说明：本技术 原材膜、拉伸光学膜的制造方法和拉伸光学膜 (Raw material film, method for producing stretched optical film, and stretched optical film ) 是由 川崎绘美 川崎雅洋 大桥亘 稻富敦 矶崎孝德 久保敬次 于 2018-05-28 设计创作，主要内容包括：本发明提供能够比较容易地获得薄型且不易开裂的拉伸光学膜的原材膜、薄型且不易开裂的拉伸光学膜和能够比较容易地获得这种拉伸光学膜的拉伸光学膜的制造方法。本发明为拉伸光学膜制造用的原材膜,其平均厚度为45μm以下,含有作为主成分的乙烯醇系聚合物、和玻璃化转变温度为30℃以下的树脂颗粒,上述树脂颗粒的含量相对于上述乙烯醇系聚合物100质量份为1质量份以上且50质量份以下。(The invention provides a raw material film capable of easily obtaining a thin stretched optical film which is not easy to crack, a thin stretched optical film which is not easy to crack and a manufacturing method of the stretched optical film capable of easily obtaining the stretched optical film. The present invention is a raw material film for producing a stretched optical film, which has an average thickness of 45 [ mu ] m or less, contains a vinyl alcohol polymer as a main component, and resin particles having a glass transition temperature of 30 ℃ or less, and the content of the resin particles is 1 part by mass or more and 50 parts by mass or less with respect to 100 parts by mass of the vinyl alcohol polymer.)

1. A raw material film for producing a stretched optical film, having an average thickness of 45 μm or less,

comprising a vinyl alcohol polymer as a main component and resin particles having a glass transition temperature of 30 ℃ or lower,

the content of the resin particles is 1 part by mass or more and 50 parts by mass or less with respect to 100 parts by mass of the vinyl alcohol polymer.

2. The raw material film according to claim 1, wherein an average particle diameter of the resin particles is 1nm or more and 300nm or less.

3. A method for producing a stretched optical film, comprising:

a process for stretching the raw material film according to claim 1 or claim 2.

4. A stretched optical film having an average thickness of 20 μm or less,

comprising a vinyl alcohol polymer as a main component and resin particles having a glass transition temperature of 30 ℃ or lower,

the content of the resin particles is 1 part by mass or more and 50 parts by mass or less with respect to 100 parts by mass of the vinyl alcohol polymer.

5. The stretched optical film according to claim 4, wherein the length of the resin particles in the stretching direction observed in a transmission electron microscope image of a cut surface parallel to the stretching direction is longer than the length in a direction perpendicular to the stretching direction.

Technical Field

The present invention relates to a raw material film (original reverse フィルム), a method for producing a stretched optical film, and a stretched optical film.

Background

A polarizing plate having light transmitting and light blocking functions and a liquid crystal that changes the polarization state of light are both basic components of a Liquid Crystal Display (LCD). Many polarizing plates have a structure in which a protective film such as a Triacetylcellulose (TAC) film is bonded to the surface of a polarizing film. As a polarizing film constituting a polarizing plate, a stretched film in which a polyvinyl alcohol film (hereinafter, the polyvinyl alcohol film may be abbreviated as "PVA") is uniaxially stretched and oriented is adsorbed with an iodine-based dye (I)₃ ^-、I₅ ^-Etc.), dichroic dyes such as dichroic organic dyes, have become the mainstream of polarizing films. The polarizing film is obtained by uniaxially stretching a PVA film containing a dichroic dye in advance, or by allowing the PVA film to adsorb the dichroic dye while uniaxially stretching the PVA film, orThe PVA film is uniaxially stretched and then made to adsorb a dichroic dye or the like.

LCDs are increasingly used in a wide range of small devices such as calculators and wristwatches, smartphones, notebook computers, liquid crystal monitors, liquid crystal color projectors, liquid crystal televisions, car navigation systems, cellular phones, and metering devices used indoors and outdoors. In recent years, there has been a strong demand for thinner polarizing plates, particularly in the field of mobile applications such as small-sized notebook personal computers and cellular phones. Further, since the range of use places is increased due to portability, improvement of durability is also required.

One method for making a polarizing plate thin is to make a polarizing film or a protective film thin. For this reason, it is necessary to make a raw material film (PVA film) which is a raw material of the polarizing film thin. However, a thin raw material film is likely to be cracked in the stretching direction due to a drying step in the production of a polarizing film, a step of bonding the obtained polarizing film and a protective film, and the like. When the polarizing film is thin, the polarizing film is likely to be cracked in the stretching direction or to be cracked finely at the end face of the polarizing film when the polarizing film or the polarizing plate is subjected to a process such as punching or cutting. Therefore, when a thin raw material film is used, productivity and yield of the polarizing plate and the LCD are reduced, and the cost is likely to increase. As described above, thinning of the raw material film or the polarizing film tends to reduce productivity and yield, and increase in cost. Further, a stretched optical film other than the polarizing film, such as a retardation film, is expected to be thin, but similarly, there is a problem that cracking or the like is likely to occur.

As a technique for producing a thin polarizing film with good yield, a method is known in which a thin PVA film is formed on a plastic film by a coating method, and a laminate thereof is stretched and dried (see patent documents 1 and 2). In addition, in order to improve the processability such as punching property of the obtained polarizing film, there have been proposed: a polarizing plate in which a polarizing film is produced under specific conditions and a urethane resin layer is laminated on at least one surface of the polarizing film (see patent document 3); and a composition capable of forming a cured resin layer having excellent flexibility (see patent document 4).

Disclosure of Invention

Problems to be solved by the invention

However, the methods described in patent documents 1 and 2 have the following disadvantages.

(1) The coating operation and the subsequent drying operation are complicated.

(2) Since it is necessary to perform heat treatment for insolubilization of the PVA film in a state of a laminate, the plastic film used is limited to a plastic film that can be stretched even after heat treatment, which increases the cost.

(3) In a laminate in which a PVA film is formed on a plastic film by a coating method, the adhesion strength between the plastic film and the PVA film is high. Therefore, when such a laminate having high adhesive strength is stretched, appropriate necking of the PVA film is inhibited, and it is difficult to obtain a polarizing film having excellent polarizing performance.

In addition, in the methods described in patent documents 3 and 4, the number of steps in the production of the polarizing plate is increased, which causes problems such as an increase in cost and a decrease in yield.

The present invention has been made in view of the above-described circumstances, and an object thereof is to provide a raw material film from which a thin stretched optical film that is less likely to crack can be relatively easily obtained, a thin stretched optical film that is less likely to crack, and a method for producing a stretched optical film from which such a stretched optical film can be relatively easily obtained.

Means for solving the problems

The present inventors have made extensive studies to achieve the above object, and as a result, have found that: even in the case of thinning the film thickness, by adding resin particles having a relatively low glass transition temperature to the film in advance, a stretched optical film which is not easily cracked can be obtained, and further studies have been repeated based on these findings, thereby completing the present invention.

That is, the present invention made to solve the above problems is as follows.

[1] A raw material film for producing a stretched optical film, which has an average thickness of 45 [ mu ] m or less, contains a vinyl alcohol polymer as a main component and resin particles having a glass transition temperature of 30 ℃ or less, and the content of the resin particles is 1 part by mass or more and 50 parts by mass or less relative to 100 parts by mass of the vinyl alcohol polymer.

[2] The base film according to [1], wherein the resin particles have an average particle diameter of 1nm to 300 nm.

[3] A method for producing a stretched optical film, comprising a step of stretching the raw material film of [1] or [2 ].

[4] A stretched optical film having an average thickness of 20 [ mu ] m or less and containing a vinyl alcohol polymer as a main component and resin particles having a glass transition temperature of 30 ℃ or less, wherein the content of the resin particles is 1 part by mass or more and 50 parts by mass or less relative to 100 parts by mass of the vinyl alcohol polymer.

[5] The stretched optical film according to [4], wherein the length of the resin particles in the stretching direction, which is observed in a transmission electron microscope image of a cut surface parallel to the stretching direction, is longer than the length in a direction perpendicular to the stretching direction.

Effects of the invention

According to the present invention, a raw material film from which a thin stretched optical film that is less likely to crack can be relatively easily obtained, a thin stretched optical film that is less likely to crack, and a method for producing a stretched optical film from which such a stretched optical film can be relatively easily obtained can be provided.

Drawings

Fig. 1 is a schematic view showing a cross section parallel to a stretching direction of a stretched optical film according to an embodiment of the present invention.

Detailed Description

< raw Material film >

The raw material film according to one embodiment of the present invention is a film for producing a stretched optical film. That is, the raw material film is a film to be used as a material for a stretched optical film such as a polarizing film or a retardation film. The raw material film is stretched to obtain a stretched optical film.

The raw material film may be a single layer film or a multilayer film (laminate). Examples of the form of the multilayer film include a film having a PVA layer formed on a thermoplastic resin film by a coating method or the like. From the viewpoint of further remarkably exhibiting the effect of the present invention, the complexity of the lamination (coating, etc.) operation, the cost of the thermoplastic resin film, and the like, the raw material film is preferably a single-layer film.

(average thickness)

The upper limit of the average thickness of the raw material film is 45 μm, preferably 40 μm, more preferably 35 μm, and still more preferably 30 μm. By setting the average thickness of the raw material film to the upper limit or less, a thin stretched optical film can be obtained. On the other hand, the lower limit of the average thickness is preferably 1 μm, more preferably 3 μm, still more preferably 10 μm, and yet more preferably 20 μm. When the average thickness of the raw material film is not less than the lower limit, the crack resistance of the obtained stretched optical film can be further improved.

(PVA)

The raw material film contains PVA (polyvinyl alcohol) as a main component. The main component is a component having the largest content by mass (the same applies hereinafter). PVA is a polymer having a vinyl alcohol unit (-CH)₂-CH (OH) -) as a structural unit. PVA may have vinyl alcohol units, as well as vinyl ester units, among other units.

As the PVA, PVA obtained by saponifying polyvinyl ester obtained by polymerizing 1 or 2 or more kinds of vinyl esters such as vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl versatate, vinyl laurate, vinyl stearate, vinyl benzoate, and isopropenyl acetate can be used. Among the vinyl esters, PVA can be easily produced and obtainedFrom the viewpoints of easiness, cost and the like, it is preferable that the compound has an ethyleneoxycarbonyl group (H) in the molecule₂A compound of C ═ CH-O-CO —), and more preferably vinyl acetate.

The polyvinyl ester is preferably a polyvinyl ester obtained by using only 1 or 2 or more vinyl esters as monomers, and more preferably a polyvinyl ester obtained by using only 1 vinyl ester as a monomer. The copolymer may be a copolymer of 1 or 2 or more vinyl esters and other monomers copolymerizable therewith, as long as the effects of the present invention are not significantly impaired.

Examples of the other monomers copolymerizable with the vinyl ester include:

α -olefins having 2 to 30 carbon atoms such as ethylene, propylene, 1-butene, and isobutylene;

(meth) acrylic acid or a salt thereof;

(meth) acrylates such as methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, isopropyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth) acrylate, t-butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, dodecyl (meth) acrylate, and octadecyl (meth) acrylate;

(meth) acrylamide;

(meth) acrylamide derivatives such as N-methyl (meth) acrylamide, N-ethyl (meth) acrylamide, N-dimethyl (meth) acrylamide, diacetone (meth) acrylamide, (meth) acrylamidopropanesulfonic acid or a salt thereof, (meth) acrylamidopropyldimethylamine or a salt thereof, and N-methylol (meth) acrylamide or a derivative thereof;

n-vinylamides such as N-vinylformamide, N-vinylacetamide, and N-vinylpyrrolidone;

vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, tert-butyl vinyl ether, dodecyl vinyl ether and stearyl vinyl ether;

ethylene cyanide such as (meth) acrylonitrile;

vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride and vinylidene fluoride;

allyl compounds such as allyl acetate and allyl chloride;

maleic acid or a salt, ester or anhydride thereof;

itaconic acid or a salt, ester or anhydride thereof;

vinyl silyl compounds such as vinyltrimethoxysilane;

unsaturated sulfonic acids or salts thereof, and the like.

The above polyvinyl ester may have a structural unit derived from 1 or 2 or more of the above monomers.

The upper limit of the proportion of the structural unit derived from the other monomer in the polyvinyl ester is preferably 15 mol%, more preferably 10 mol%, further preferably 5 mol%, and further preferably 1 mol%, based on the number of moles of the total structural units constituting the polyvinyl ester.

The PVA may be preferably one which has not been subjected to graft copolymerization, but the PVA may be one which has been modified with 1 or 2 or more kinds of graft-copolymerizable monomers, as long as the effects of the present invention are not significantly impaired, or at least one of a polyvinyl ester and a PVA obtained by saponifying the polyvinyl ester may be subjected to graft copolymerization, examples of the above-mentioned graft-copolymerizable monomers include unsaturated carboxylic acids or derivatives thereof, unsaturated sulfonic acids or derivatives thereof, α -olefins having 2 to 30 carbon atoms, and the like, and the proportion of the structural units derived from the graft-copolymerizable monomers in the polyvinyl ester or PVA is preferably 5 mol% or less based on the number of moles of the total structural units constituting the polyvinyl ester or PVA.

A part of the hydroxyl groups in the PVA may be crosslinked or uncrosslinked. In addition, a part of the hydroxyl groups in the PVA may be reacted with an aldehyde compound such as acetaldehyde or butylaldehyde to form an acetal structure, or may be not reacted with these compounds and not subjected to an acetal structure.

The lower limit of the polymerization degree of the PVA is preferably 1,000, more preferably 1,500, and still more preferably 2,000. When the polymerization degree of PVA is not less than the lower limit, the optical properties of the obtained stretched optical film can be improved. On the other hand, the upper limit of the polymerization degree is preferably 10,000, more preferably 8,000, and still more preferably 5,000. When the polymerization degree of the PVA is not more than the upper limit, an increase in the production cost of the PVA and defects in film formation can be suppressed. The polymerization degree of PVA is an average polymerization degree measured according to JIS K6726-1994.

The lower limit of the saponification degree of PVA is preferably 95 mol%, more preferably 98 mol%, still more preferably 99 mol%, and particularly preferably 99.5 mol%, from the viewpoint that the wet heat resistance of the obtained stretched optical film is good. On the other hand, the upper limit of the saponification degree may be substantially 100 mol%. The saponification degree of PVA means: the ratio (mol%) of the number of moles of the vinyl alcohol unit to the total number of moles of the structural unit (typically, a vinyl ester unit) that can be converted into the vinyl alcohol unit by saponification and the vinyl alcohol unit. The degree of saponification can be measured according to JIS K6726-1994.

The lower limit of the PVA content in the raw material film is preferably 60 mass%, more preferably 70 mass%, and still more preferably 75 mass%. When the PVA content is not less than the lower limit, the obtained stretched optical film can exhibit optical properties such as polarization performance more favorably. On the other hand, the upper limit of the content is preferably 95% by mass, more preferably 90% by mass, and still more preferably 85% by mass in some cases. When the content of PVA is not more than the upper limit, the obtained stretched optical film becomes less likely to crack.

(resin particles)

The raw material film contains resin particles having a glass transition temperature of 30 ℃ or lower. By containing such resin particles, the raw material film can be a thin stretched optical film which is less likely to crack. The reason for this effect is not clear, but it is presumed that when a cross section of a stretched optical film produced from the raw material film at which cracking occurs is observed, the cross section becomes rough: since the resin particles dispersed in the film inhibit propagation of cracks, it becomes less likely to crack. In particular, when the glass transition temperature of the resin particles is 30 ℃ or lower, the resin particles are deformed in the stretching direction during the stretching step, even at a temperature not higher than the ordinary stretching treatment temperature in the production of a stretched optical film from a raw material film. This makes it possible to sufficiently orient the PVA while maintaining the adhesion between the PVA and the resin particles. Therefore, the stretched optical film obtained from the base film is less likely to crack, and it is presumed that: by adjusting the content and average particle diameter of the resin particles, optical characteristics such as polarization performance can be improved.

The resin particles are particles whose main component is a polymer (resin). The lower limit of the content of the polymer in the resin particles is, for example, 50 mass%, preferably 80 mass%, and more preferably 95 mass%. The resin particles may be formed substantially only of the resin.

The upper limit of the glass transition temperature (Tg) (Tg of the polymer as the main component of the resin particles) of the resin particles is 30 ℃, preferably 25 ℃, more preferably 20 ℃, still more preferably 15 ℃, and still more preferably 10 ℃. By setting the glass transition temperature to the upper limit or lower, a stretched optical film which is less likely to crack can be obtained. Further, by setting the glass transition temperature to the above upper limit or less and adjusting the content and average particle diameter of the resin particles, optical characteristics such as polarization performance can be improved.

The lower limit of the glass transition temperature of the resin particles is not particularly limited, and is, for example, preferably-100 ℃, more preferably-80 ℃, and still more preferably-60 ℃. When the glass transition temperature is not lower than the lower limit, aggregation of the resin particles at the time of temperature rise in the film forming step of producing the raw material film is suppressed, and white turbidity of the obtained raw material film and the stretched optical film can be suppressed. In addition, the optical properties of the resulting stretched optical film can be improved.

The glass transition temperature of the resin particles is: the resin particles were used for film formation, and the obtained resin film was measured by DSC (differential scanning calorimetry). When the resin particles contain a plurality of different resins, the glass transition temperature of the resin having the lowest glass transition temperature is set as the glass transition temperature of the resin particles.

The lower limit of the content of the resin particles in the raw material film is 1 part by mass, but may be preferably 3 parts by mass, more preferably 5 parts by mass, and still more preferably 7 parts by mass with respect to 100 parts by mass of PVA. By setting the content of the resin particles to the lower limit or more, the obtained stretched optical film is less likely to crack, and the handling property and the like can be improved. On the other hand, the upper limit of the content is 50 parts by mass, preferably 30 parts by mass, more preferably 20 parts by mass, and still more preferably 15 parts by mass. When the content of the resin particles is not more than the upper limit, the light transmittance of the obtained stretched optical film can be made good, and optical characteristics such as polarization performance can be improved.

The lower limit of the average particle diameter of the resin particles in the raw material film is preferably 1nm, more preferably 5nm, still more preferably 10nm, yet more preferably 20nm, and yet more preferably 30 nm. When the average particle diameter of the resin particles is not less than the lower limit, the obtained stretched optical film is less likely to crack and the handling property and the like are improved. On the other hand, the upper limit of the average particle size may be, for example, 500nm, preferably 300nm, more preferably 200nm, and still more preferably 100 nm. By setting the average particle diameter of the resin particles to the upper limit or less, particularly 300nm or less, the light transmittance of the obtained stretched optical film can be made good, and the optical characteristics such as polarization performance can be improved.

The average particle diameter of the resin particles in the base film is a measured value based on a TEM (transmission electron microscope) image of a cross section of the film. By measuring the TEM of a cross section perpendicular to the in-plane direction of the base film, the presence or absence and the dispersed state of the resin particles can be observed as a sea-island structure. The sea-island structure means: in a mixture having two physical properties, a discontinuous portion (island portion) is mixed in a continuous visible portion (sea portion). Since the resin particles are different in dyeing property from PVA, the color of the continuously visible portion becomes darker and the color of the discontinuously visible portion becomes lighter, or the color of the continuously visible portion becomes lighter and the color of the discontinuously visible portion becomes darker. In the TEM image, the resin particles were observed as islands. The TEM image of the cut film surface was extracted mechanically by using image analysis software to obtain resin particles, and the average value of the diameters of the resin particles was calculated. The calculated value was defined as the average particle diameter of the resin particles. The specific method for measuring the average particle diameter of the resin particles is the method described in examples.

The resin particles contain a polymer. The polymer is not particularly limited as long as it has a glass transition temperature of 30 ℃ or lower, and examples thereof include polyolefins, polyurethanes, acrylic resins, and the like, with acrylic resins being preferred. The acrylic resin refers to a polymer containing a structural unit derived from a (meth) acrylate ester.

Examples of the (meth) acrylic acid ester include:

alkyl (meth) acrylates such as methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, butyl (meth) acrylate, hexyl (meth) acrylate, dodecyl (meth) acrylate, and octadecyl (meth) acrylate;

(meth) acrylates having an alicyclic group such as dicyclopentanyl (meth) acrylate and isobornyl (meth) acrylate;

aryl (meth) acrylates such as phenyl (meth) acrylate, and the like.

Among the acrylic resins, the resin particles preferably contain: an acrylic resin containing a structural unit derived from an alkyl (meth) acrylate (an alkyl (meth) acrylate unit). The lower limit of the number of carbon atoms of the alkyl group of the alkyl (meth) acrylate is 1, preferably 2, more preferably 3, and still more preferably 4. On the other hand, the upper limit of the number of carbon atoms of the alkyl group is, for example, 10, preferably 8, more preferably 6, and still more preferably 4. Further, an alkyl acrylate unit is also preferable. That is, among the alkyl (meth) acrylate units, the butyl acrylate unit is most preferable. The resin particles obtained using an acrylic resin containing such an alkyl (meth) acrylate unit have a low glass transition temperature, and can further improve the cracking difficulty and optical characteristics. The reason is not clear, but it can be presumed that: the resin particles have improved flexibility, and the resin particles are likely to be deformed in the stretching direction during the stretching treatment.

Further, as the acrylic resin, a resin containing a (meth) acrylic acid unit (-CH) can also be suitably used₂-CHCOOH-and-CH₂-C(CH₃) COOH-) and an acrylic resin. Resin particles using such a polymer exhibit good dispersibility in a PVA matrix. In this case, for example, a block copolymer of (meth) acrylic acid units and alkyl (meth) acrylate units is preferable. When the acrylic resin is a block copolymer, the acrylic resin may be any of a diblock copolymer, a triblock copolymer, and the like.

The resin particles may be particles formed of one polymer, or particles having a so-called core-shell structure in which the material of the inner side and the material of the outer side are different from each other. When the core-shell type particles are used, the material of the core preferably contains an acrylic resin containing alkyl (meth) acrylate units. When the core-shell particles are used, the material of the shell is preferably an acrylic resin containing a (meth) acrylate unit containing an alicyclic group and a (meth) acrylic acid unit.

The resin particles can be produced by a known method. Further, commercially available resin particles can be used. The method for incorporating the resin particles into the raw material film is not particularly limited. For example, the resin particles may be added to the PVA chips, or may be added to a film-forming stock solution used for film formation.

(plasticizer)

The stock film may further comprise a plasticizer. By including a plasticizer in the base film, the handleability, stretchability, and the like can be improved. Examples of the preferred plasticizer include polyhydric alcohols, and specific examples thereof include ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, trimethylolpropane, and the like. These plasticizers may be used in an amount of 1 or 2 or more. Among them, glycerin is preferable from the viewpoint of the effect of improving stretchability.

The lower limit of the content of the plasticizer in the raw material film is preferably 2 parts by mass, more preferably 3 parts by mass, still more preferably 4 parts by mass, and yet more preferably 6 parts by mass, based on 100 parts by mass of PVA. When the content of the plasticizer is not less than the lower limit, the stretchability is further improved. On the other hand, the upper limit of the content is preferably 20 parts by mass, more preferably 17 parts by mass, and still more preferably 14 parts by mass. When the content of the plasticizer is not more than the upper limit, it is possible to suppress the deterioration of the handling property due to the material film becoming too soft or the plasticizer bleeding out to the surface.

(other additives, etc.)

The raw material film may further contain, in addition to the PVA, the resin particles and the plasticizer, other additives such as a filler, a processing stabilizer such as a copper compound, a weather resistance stabilizer, a coloring agent, an ultraviolet absorber, a light stabilizer, an antioxidant, an antistatic agent, a flame retardant, other thermoplastic resins, a lubricant, a fragrance, a defoaming agent, a deodorizing agent, an extender, a releasing agent, a reinforcing agent, a crosslinking agent, a mildewproofing agent, a preservative, a crystallization rate retarder, and the like, as necessary.

The upper limit of the content of the additives other than PVA, resin particles, and plasticizer in the raw material film may be preferably 1% by mass, and more preferably 0.2% by mass. When the content of the other additive exceeds the upper limit, the crack strength and optical properties of the resulting stretched optical film may be affected.

The lower limit of the swelling degree of the raw material film is preferably 160%, more preferably 170%, and still more preferably 180%. When the swelling degree is not less than the lower limit, the progress of crystallization can be suppressed to a high degree and the drawing can be stably performed at a high magnification. On the other hand, the upper limit of the swelling degree is preferably 240%, more preferably 230%, and still more preferably 220%. When the swelling degree is not more than the upper limit, the dissolution during stretching is suppressed, and stretching can be performed even under higher temperature conditions. The swelling degree of the raw material film is: the percentage of the mass of the raw material film obtained by immersing the raw material film in distilled water at 30 ℃ for 15 minutes was divided by the mass of the raw material film obtained by immersing the raw material film in distilled water at 30 ℃ for 15 minutes and then drying the raw material film at 105 ℃ for 16 hours.

The shape of the raw material film is not particularly limited, and a long film is preferable in that a stretched optical film can be continuously produced with good productivity. The length of the long raw material film is not particularly limited, and may be appropriately set according to the application of the stretched optical film to be produced, and may be, for example, in the range of 5m to 20,000 m. The width of the material film is not particularly limited, and for example, the lower limit may be set to 50cm, and from the viewpoint of a recent demand for a wide polarizing film, the lower limit is preferably 1m, more preferably 2m, and still more preferably 4 m. The upper limit of the width of the raw material film is not particularly limited, and may be, for example, 7 m. If the width is too wide, it tends to be difficult to uniformly stretch the stretched optical film when the stretched optical film is produced by a practical apparatus.

The raw material film can be relatively easily produced into a stretched optical film which is not easily cracked during production and handling. Therefore, the composition can be suitably used as a material for stretched optical films such as polarizing films and retardation films. Among them, the raw material film is particularly preferably used as a raw material film for producing a polarizing film, from the viewpoint that a polarizing film having good polarizing performance can be easily produced.

(method for producing Material film)

The method for producing the raw material film of the present invention is not particularly limited, and a production method in which the thickness and width of the raw material film after film formation are more uniform can be preferably employed. For example, the film can be obtained by using a film-forming stock solution in which 1 or 2 or more kinds of the PVA and the resin particles constituting the stock film, and further, if necessary, a plasticizer, other additives, a surfactant described later, and the like are dissolved in a liquid medium to form a film. Further, the film can be produced by using a film-forming stock solution in which PVA is melted, as necessary. In the film-forming dope, resin particles are preferably uniformly mixed. When the film-forming dope contains at least 1 of the plasticizer, other additives, and the surfactant, it is preferable to uniformly mix these components.

Examples of the liquid medium include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, glycerol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, ethylenediamine, and diethylenetriamine. These liquid media may be used in 1 kind or 2 or more kinds. Among these, water is preferable from the viewpoint of a small load on the environment and recyclability.

The evaporation fraction of the film-forming dope (the content ratio of volatile components such as a liquid medium that are removed by evaporation or evaporation during film formation) varies depending on the film-forming method, the film-forming conditions, and the like, and generally, the lower limit is preferably 50 mass%, more preferably 55 mass%, and still more preferably 60 mass%. By setting the evaporation fraction of the film-forming dope to the lower limit or more, the viscosity of the film-forming dope does not become excessively high, and filtration and deaeration are smoothly performed during preparation of the film-forming dope, and a raw material film with less impurities and defects can be easily produced. On the other hand, the upper limit of the volatilization fraction is preferably 95% by mass, more preferably 90% by mass, and still more preferably 85% by mass. By setting the evaporation fraction of the film-forming dope to the upper limit or less, the concentration of the film-forming dope does not become too low, and an industrial raw material film can be easily produced.

The film-forming dope preferably contains a surfactant. By including the surfactant, the film forming property is improved, the occurrence of thickness unevenness in the raw material film is suppressed, and the film is easily peeled from the metal roll or belt used for film formation. When a material film is produced from a film-forming dope containing a surfactant, the material film may contain a surfactant. The type of the surfactant is not particularly limited, but from the viewpoint of releasability from a metal roll or a belt, an anionic surfactant and a nonionic surfactant are preferable.

As the anionic surfactant, for example, a carboxylic acid type such as potassium laurate is preferable; sulfuric acid ester types such as polyoxyethylene lauryl ether sulfate and octyl sulfate; sulfonic acid types such as dodecylbenzene sulfonate, and the like.

The nonionic surfactant is preferably an alkyl ether type such as polyoxyethylene oleyl ether; alkylphenyl ether type such as polyoxyethylene octylphenyl ether; alkyl ester types such as polyoxyethylene laurate; alkylamine type such as polyoxyethylene lauryl amino ether; alkylamide types such as polyoxyethylene laurylamide; polypropylene glycol ether type such as polyoxyethylene polyoxypropylene ether; alkanolamide types such as lauric acid diethanolamide and oleic acid diethanolamide; and an allylphenyl ether type such as polyoxyalkylene allylphenyl ether.

These surfactants may be used alone in 1 kind or in combination of 2 or more kinds.

When the film-forming stock solution or the obtained stock film contains the surfactant, the lower limit of the content thereof is preferably 0.01 part by mass, and more preferably 0.02 part by mass, relative to 100 parts by mass of PVA contained in the film-forming stock solution or the stock film. When the content of the surfactant is not less than the lower limit, the film forming property and the peeling property are further improved. On the other hand, the upper limit of the content is preferably 0.5 parts by mass, more preferably 0.3 parts by mass, and still more preferably 0.1 parts by mass. When the content of the surfactant is not more than the upper limit, the surfactant can be prevented from bleeding out to the surface of the base film, blocking can be prevented, and the handleability can be prevented from being lowered.

Examples of the film forming method in forming a raw material film using the film forming dope include a casting film forming method, an extrusion film forming method, a wet film forming method, a gel film forming method, and the like. These film-forming methods may be used alone in 1 kind, or 2 or more kinds may be used in combination. Among these film forming methods, a casting film forming method and an extrusion film forming method are preferable in that a raw material film having a uniform thickness and width and excellent physical properties can be obtained. The raw material film to be formed may be dried and heat-treated as necessary.

The heat treatment temperature is not particularly limited, and may be appropriately adjusted. If the heat treatment temperature is too high, discoloration and deterioration of the base material film are observed. Therefore, the upper limit of the heat treatment temperature is preferably 210 ℃, more preferably 180 ℃, and still more preferably 150 ℃. On the other hand, the lower limit of the heat treatment temperature is, for example, 60 ℃ and preferably 90 ℃.

The heat treatment time is not particularly limited, and may be appropriately adjusted, and from the viewpoint of efficiently producing the raw material film, the upper limit is preferably 30 minutes, and more preferably 15 minutes. On the other hand, the lower limit thereof is preferably, for example, 1 minute, and more preferably 3 minutes.

< stretched optical film >

The stretched optical film according to one embodiment of the present invention is an optical film including PVA oriented in a specific direction, such as a polarizing film or a retardation film. The stretched optical film may be uniaxially stretched or biaxially stretched, and preferably uniaxially stretched. The stretched optical film subjected to uniaxial stretching can be suitably used as a polarizing film or the like. The stretched optical film may be a single-layer film or a multilayer film, and is preferably a single-layer film.

(average thickness)

The upper limit of the average thickness of the stretched optical film is 20 μm, preferably 18 μm, more preferably 16 μm, and still more preferably 14 μm. By setting the average thickness of the stretched optical film to the upper limit or less, sufficient thinning can be achieved. On the other hand, the lower limit of the average thickness is preferably 5 μm, more preferably 8 μm, and still more preferably 10 μm. When the average thickness of the stretched optical film is not less than the lower limit, the film is less likely to crack and can be improved in handling properties and the like.

(Components, etc.)

The stretched optical film contains resin particles and PVA as a main component.

The upper limit of the glass transition temperature (Tg) of the resin particles contained in the stretched optical film (Tg of the polymer that is the main component of the resin particles) is 30 ℃, preferably 25 ℃, more preferably 20 ℃, even more preferably 15 ℃, and even more preferably 10 ℃. When the glass transition temperature is not higher than the upper limit, the stretched optical film is less likely to crack and is excellent in handling properties and the like. Further, by setting the glass transition temperature to the above upper limit or less and adjusting the content and average particle diameter of the resin particles, optical characteristics such as polarization performance can be improved. The lower limit of the glass transition temperature of the resin particles is not particularly limited, and is, for example, preferably-100 ℃, more preferably-80 ℃, and still more preferably-60 ℃. When the glass transition temperature is not lower than the lower limit, aggregation of the resin particles is suppressed, and white turbidity of the stretched optical film can be suppressed. In addition, the optical characteristics of the stretched optical film can be improved.

The glass transition temperature of the resin particles contained in the stretched optical film is: the resin particles are used for film formation in the same manner as the resin particles contained in the raw material film, and the obtained resin film is subjected to DSC (differential scanning calorimetry) measurement.

The lower limit of the content of the resin particles in the stretched optical film is 1 part by mass, and may be preferably 3 parts by mass, more preferably 5 parts by mass, and still more preferably 7 parts by mass with respect to 100 parts by mass of PVA. When the content of the resin particles is not less than the lower limit, the stretched optical film is less likely to crack and the handling property and the like can be improved. On the other hand, the upper limit of the content is 50 parts by mass, preferably 30 parts by mass, more preferably 20 parts by mass, and still more preferably 15 parts by mass. When the content of the resin particles is not more than the upper limit, the light transmittance of the stretched optical film can be improved, and the optical characteristics such as polarization performance can be improved.

In the stretched optical film, it is preferable that the length of the resin particles in the stretching direction (diameter in the stretching direction) observed in a Transmission Electron Microscope (TEM) image of a cross section parallel to the stretching direction is longer than the length of the resin particles in the direction perpendicular to the stretching direction (diameter in the direction perpendicular to the stretching direction). That is, the resin particles in the stretched optical film are preferably elliptical having a major axis along the stretching direction. In this case, the PVA can be brought into a sufficiently oriented state while maintaining the adhesion between the PVA and the resin particles. Therefore, it can be presumed that: the stretched optical film is less likely to crack, and optical properties such as polarization performance can be improved by adjusting the content and average particle size of the resin particles. Such elliptical resin particles can be formed by deforming resin particles in the stretching direction when a raw material film containing resin particles having a glass transition temperature of 30 ℃ or lower is subjected to a stretching treatment. The stretching direction in which the optical film is stretched is usually the orientation direction of the crystals of PVA.

Specifically, as shown in fig. 1, in a TEM image of a cross section of the stretched optical film 1 parallel to the stretching direction X, the length a of the resin particles 2 (island portions) in the stretching direction X is preferably longer than the length B in the direction perpendicular to the stretching direction X. The lower limit of the ratio (a/B) of the length a of the resin particle 2 in the stretching direction X to the length B in the direction perpendicular to the stretching direction X is preferably 1.2, more preferably 1.6, and still more preferably 2.0. The upper limit of the ratio (a/B) may be, for example, 3 or 2.6.

The lower limit of the length a of the resin particle in the stretching direction is preferably 1nm, more preferably 10nm, still more preferably 30nm, yet more preferably 50nm, and yet more preferably 70 nm. On the other hand, the upper limit of the length A may be, for example, 800nm, preferably 300nm, more preferably 200nm, and still more preferably 100 nm. The lower limit of the length B in the direction perpendicular to the stretching direction of the resin particles is preferably 1nm, more preferably 10nm, still more preferably 20nm, and yet more preferably 30 nm. On the other hand, the upper limit of the length B may be, for example, 500nm, preferably 200nm, more preferably 100nm, and still more preferably 50 nm. When the length a and the length B are in the above ranges, cracks are less likely to occur, and optical characteristics such as polarization performance can be improved by adjusting the content and the average particle diameter of the resin particles.

The length a of the resin particles in the stretching direction and the length B of the resin particles in the direction perpendicular to the stretching direction are measured by the following method. The resin particles were mechanically extracted from the TEM image of the section of the film parallel to the stretching direction by using image analysis software. The average length of the extracted resin particles in the major axis direction is referred to as length a, and the average length in the minor axis direction is referred to as length B. A length ratio (A/B) is also determined from the length A and the length B. The specific measurement methods are described in examples.

The preferred forms of PVA and resin particles contained in the stretched optical film are the same as those of PVA and resin particles contained in the raw material film. Other components optionally contained in the stretched optical film are also the same as those of the raw material film. In the case where the stretched optical film is a polarizing film, the stretched optical film has a dichroic dye adsorbed on the front and back surfaces. As the dichroic dye, an iodine-based dye is generally used.

When the stretched optical film is a polarizing film, the polarizing performance may be, for example, 70%, preferably 99.0%, more preferably 99.8%, and still more preferably 99.9% in terms of the lower limit of the degree of polarization at a transmittance of 44.0%. When the polarization degree is less than the lower limit, the contrast of the LCD may be reduced when the LCD is used in a smartphone, a notebook computer, a liquid crystal television, a car navigation system, or the like.

When the stretched optical film is a polarizing film, the polarizing film is generally used by laminating optically transparent protective films having mechanical strength on both or one side thereof to form a polarizing plate. As the protective film, a cellulose Triacetate (TAC) film, a Cellulose Acetate Butyrate (CAB) film, an acrylic film, a polyester film, or the like is used. The adhesive used for bonding may be a PVA adhesive, an ultraviolet-curable adhesive, or the like, and is preferably a PVA adhesive.

The polarizing plate obtained in the above manner may further be bonded with a retardation film, a viewing angle improving film, a brightness enhancing film, or the like. As the retardation film, the stretched optical film of the present invention can be used. The polarizing plate can be used by coating an adhesive such as acrylic resin and then bonding the resultant to a glass substrate to form a member of an LCD.

< method for producing stretched optical film >

The stretched optical film according to one embodiment of the present invention can be obtained by a production method including a step of stretching the raw material film. That is, the stretched optical film can be produced by the same method as the conventional method except that the raw material film is used. That is, according to this production method, a thin stretched optical film which is less likely to crack can be relatively easily obtained without going through a special step. Hereinafter, a specific production method in the case where the stretched optical film is a polarizing film will be described.

Specific methods for producing the polarizing film include: the raw material film is subjected to swelling treatment, dyeing treatment, uniaxial stretching treatment, and if necessary, further subjected to crosslinking treatment, fixing treatment, washing treatment, drying treatment, heat treatment, and the like. In this case, the order of the respective treatments such as the swelling treatment, the dyeing treatment, the crosslinking treatment, the uniaxial stretching, and the fixing treatment is not particularly limited, and 2 or more treatments may be simultaneously performed. Further, 1 or 2 or more of the respective treatments may be performed 2 times or 2 or more.

The swelling treatment can be performed by immersing the raw material film in water. The lower limit of the water temperature when immersed in water is preferably 20 ℃, more preferably 22 ℃, and still more preferably 25 ℃. On the other hand, the upper limit of the temperature is preferably 40 ℃, more preferably 38 ℃, and still more preferably 35 ℃. The lower limit of the time for immersion in water is preferably 0.1 minute, and more preferably 0.5 minute. On the other hand, the upper limit of the time is preferably 5 minutes, and more preferably 3 minutes. The water to be immersed in water is not limited to pure water, and may be an aqueous solution in which various components are dissolved, or may be a mixture of water and an aqueous medium.

The dyeing treatment may be performed by contacting a dichroic dye with the raw material film. As the dichroic dye, an iodine-based dye is generally used. The dyeing treatment may be carried out at any stage of before the uniaxial stretching treatment, during the uniaxial stretching treatment, or after the uniaxial stretching treatment. The dyeing treatment is generally performed by immersing the raw material film in a solution (particularly, an aqueous solution) containing iodine-potassium iodide as a dyeing bath. The iodine concentration in the dyeing bath is preferably 0.01 mass% or more and 0.5 mass% or less, and the potassium iodide concentration is preferably 0.01 mass% or more and 10 mass% or less. The lower limit of the temperature of the dyeing bath is preferably 20 ℃ and more preferably 25 ℃. On the other hand, the upper limit of the temperature is preferably 50 ℃ and more preferably 40 ℃.

By subjecting the raw material film to the crosslinking treatment, it is possible to effectively prevent the dissolution of PVA into water when wet-stretched at high temperature. From this viewpoint, the crosslinking treatment is preferably performed before the uniaxial stretching treatment. The crosslinking treatment may be performed by immersing the raw material film in an aqueous solution containing a crosslinking agent. As the crosslinking agent, 1 or 2 or more kinds of boron inorganic compounds such as boric acid and boric acid salt such as borax can be used. The lower limit of the concentration of the crosslinking agent in the aqueous solution containing the crosslinking agent is preferably 1% by mass, more preferably 2% by mass, and still more preferably 3% by mass. On the other hand, the upper limit of the concentration is preferably 15% by mass, more preferably 7% by mass, and still more preferably 6% by mass. When the concentration of the crosslinking agent is in the above range, sufficient stretchability can be maintained. The aqueous solution containing the crosslinking agent may contain an auxiliary such as potassium iodide. The lower temperature limit of the aqueous solution containing the crosslinking agent is preferably 20 ℃ and more preferably 25 ℃. On the other hand, the upper limit of the temperature is preferably 50 ℃ and more preferably 40 ℃. When the temperature is in the above range, crosslinking can be efficiently performed.

The uniaxial stretching treatment may be performed by either a wet stretching method or a dry stretching method. In the case of the wet stretching method, the stretching may be performed in an aqueous boric acid solution, or may be performed in the above-mentioned dyeing bath or in a fixing treatment bath described later. In the case of the dry stretching method, the uniaxial stretching treatment may be performed directly at room temperature, may be performed while heating, or may be performed in the air using a raw material film after water absorption. Among these, wet stretching is preferable, and uniaxial stretching treatment in an aqueous solution of boric acid is more preferable. The lower limit of the boric acid concentration of the aqueous boric acid solution is preferably 0.5% by mass, more preferably 1.0% by mass, and still more preferably 1.5% by mass. On the other hand, the upper limit of the boric acid concentration is preferably 6.0 mass%, more preferably 5.0 mass%, and still more preferably 4.0 mass%. The aqueous boric acid solution may contain potassium iodide, and the concentration thereof is preferably 0.01 mass% or more and 10 mass% or less.

The lower limit of the stretching temperature in the uniaxial stretching treatment is preferably 30 ℃, more preferably 40 ℃, and still more preferably 50 ℃. By setting the lower limit of the stretching temperature to the upper limit of the glass transition temperature of the resin particles, that is, 30 ℃ or more, the resin particles are favorably deformed so as to be along the stretching direction during stretching. Thus, a stretched optical film which is less likely to crack and has excellent handling properties can be obtained relatively easily.

The lower limit of the stretching ratio in the uniaxial stretching treatment is preferably 5 times, more preferably 5.5 times, and still more preferably 6 times, from the viewpoint of the polarizing performance of the obtained polarizing film. The upper limit of the stretch ratio is not particularly limited, and for example, it may be 10 times, more preferably 8 times.

In order to make the adsorption of the dichroic dye (iodine-based dye, etc.) to the raw material film strong in the production of the polarizing film, it is preferable to perform a fixing treatment after the uniaxial stretching treatment. As the fixing treatment bath used for the fixing treatment, an aqueous solution containing 1 or 2 or more kinds of boron inorganic compounds such as boric acid and borax may be used. Further, an iodine compound or a metal compound may be added to the fixing treatment bath as needed. The lower limit of the concentration of the boron inorganic compound in the fixing treatment bath is preferably 0.5 mass%, and more preferably 1 mass%. On the other hand, the upper limit of the concentration is preferably 15% by mass, and more preferably 10% by mass. When the concentration is within the above range, the adsorption of the dichroic dye can be more firmly performed. The lower limit of the temperature of the fixing treatment bath is preferably 15 ℃. On the other hand, the upper limit of the temperature is preferably 60 ℃ and more preferably 40 ℃.

The cleaning treatment is usually performed by immersing the raw material film in water or the like. In this case, the water or the like used for the cleaning treatment preferably contains an auxiliary agent such as potassium iodide from the viewpoint of improving the polarization performance. In this case, the concentration of an iodide such as potassium iodide is preferably 0.5 mass% or more and 10 mass% or less. The lower limit of the temperature of water or the like used for the washing treatment is usually 5 ℃, preferably 10 ℃, and more preferably 15 ℃. On the other hand, the upper limit of the temperature is usually 50 ℃, preferably 45 ℃, and more preferably 40 ℃. From the viewpoint of economy, the temperature of water or the like is not preferably too low. On the other hand, if the temperature of water or the like is too high, the polarization performance may be degraded.

The conditions of the drying treatment are not particularly limited, and the lower limit of the drying temperature is preferably 30 ℃ and more preferably 50 ℃. On the other hand, the upper limit of the drying temperature is preferably 150 ℃ and more preferably 130 ℃. By drying at a temperature within the above range, a polarizing film excellent in dimensional stability is easily obtained.

By performing the heat treatment after the drying treatment, a polarizing film having more excellent dimensional stability can be obtained. Here, the heat treatment means: and (3) a process of further heating the polarizing film having a moisture content of 5% or less after the drying process to improve the dimensional stability of the polarizing film. The conditions of the heat treatment are not particularly limited, but the heat treatment is preferably performed in the range of 60 ℃ to 150 ℃. If the heat treatment is performed at a temperature lower than 60 ℃, the effect of stabilizing the dimension by the heat treatment is insufficient. On the other hand, if the heat treatment is performed at a temperature higher than 150 ℃, the polarizing film may be severely yellowed.

< other embodiment >

The raw material film, the stretched optical film, and the method for producing the stretched optical film of the present invention are not limited to the above embodiments. For example, as the stretched optical film and the method for producing the same, the case where the stretched optical film is a polarizing film has been mainly described, but the stretched optical film is not limited to the polarizing film. For example, a stretched optical film such as a retardation film other than a polarizing film is also within the scope of the present invention, and can be produced by a production method including a step of stretching the raw material film of the present invention. The method for producing the retardation film according to the embodiment of the present invention can be carried out by a conventionally known method, in addition to stretching the raw film of the present invention.