TWI861000B - Polyvinyl alcohol film - Google Patents

Abstract

A polyvinyl alcohol film, wherein the width direction of the PVA film is taken as the TD direction and the machine flow direction is taken as the MD direction, the number of stripe-like defects with a height difference of 0.15 μm or more extending linearly and continuously for more than 1.5 m in the MD direction is less than 5 in the entire width of the film, and the number of stripe-like defects with a height difference of 0.50 μm or more extending linearly and continuously for more than 1.5 m in the MD direction does not exist in the entire width of the film; and when the thickness of the central part in the TD direction is measured at intervals of 0.5 mm every 1.0 m in the MD direction, the maximum value is taken as t _MAX , the minimum value is taken as t _MIN , and the average value is taken as t _AVE , t _AVE is less than 35 μm, and the thickness unevenness rate specified by the following formula (1) is less than 5.5%. The film is thin but has few stripe defects, and the thickness unevenness in the MD direction is reduced, so the generation of wrinkles can be suppressed during stretching.

Thickness unevenness = (t _MAX -t _MIN )/t _AVE × 100(%) (1)

Description

Polyvinyl alcohol film

The present invention relates to a polyvinyl alcohol film with few film surface defects and uniform thickness and a method for manufacturing the same.

Polarizing plates that have the ability to transmit and block light, together with liquid crystals that change the polarization state of light, are the basic components of liquid crystal displays (LCDs). LCDs are used in a wide range of applications, including small machines such as computers and watches, laptops, LCD screens, LCD color projectors, LCD TVs, car navigation systems, mobile phones, and measuring instruments for indoor and outdoor use. Among these LCD application areas, LCD TVs and LCD screens are becoming thinner, and the demand for thin films is increasing.

A typical polarizing plate has a structure in which a protective film such as a triacetate cellulose (TAC) film is attached to a polarizing film made by uniaxially stretching and dyeing a polyvinyl alcohol film.

As a method for manufacturing a polyvinyl alcohol film with uniform thickness, for example, Patent Document 1 describes a manufacturing method, which is a manufacturing method in which polyvinyl alcohol is dissolved in a solvent such as water to prepare a stock solution, the stock solution is cast on a cylinder (roller) to form a film, and the film on the cylinder is heated and dried, wherein the speed ratio of the cylinder speed to the stock solution discharge speed is set to 1 to 5. Patent Document 1 describes that by setting such a speed ratio, a polyvinyl alcohol film with uniform thickness and excellent smoothness can be manufactured over a large area.

Patent document 2 discloses a polyvinyl alcohol film having a thickness of 5 to 60 μm, a width of more than 2 m, and a length of more than 2 km, wherein the coefficient of variation of the thickness of the film is less than 1%. Patent document 2 discloses a method of stabilizing the touch line when the polyvinyl alcohol aqueous solution lands on a casting cylinder (roller) by using an air knife to reduce the uneven thickness in the MD direction (the flow direction of the film) in order to set the coefficient of variation to less than 1%.

Furthermore, Patent Document 3 discloses that a polyvinyl alcohol film containing a non-ionic surfactant in polyvinyl alcohol will not produce die lines or foreign matter, and has excellent long-term film-forming properties. Even when a polyvinyl alcohol film having a thickness of 120 μm is subjected to a biaxial stretching film-forming step for 60 consecutive days, die lines or gel will not be produced.

[Prior Art Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Application Publication No. 2002-79530

[Patent Document 2] Japanese Patent Application Publication No. 2016-172851

[Patent Document 3] Japanese Patent Application Publication No. 2001-253993

Patent document 1 records that the difference between the maximum thickness and the minimum thickness of the film obtained by adjusting the speed ratio of the drum speed to the stock solution discharge speed is more than 1.6μm. The thickness of the polyvinyl alcohol film (hereinafter sometimes referred to as PVA) used in the polarizing plate in the past (for example, 45μm~70μm), the thickness unevenness in the mechanical flow direction of the PVA film (hereinafter sometimes referred to as the mechanical flow direction as the MD direction) has not become a big problem. However, due to the thinning of PVA film in recent years, the problem of deterioration of the thickness unevenness (the ratio of the thickness unevenness relative to the average thickness) will be manifested, especially when the thickness is less than 35μm.

The method described in Patent Document 2 is to use an air knife to physically press the polyvinyl alcohol aqueous solution (film-forming stock solution) discharged from the T-die against the cylindrical roller. Therefore, the front end of the T-die lip contacts the discharged film-forming stock solution, causing stripe-like defects to be continuously generated in the MD direction.

Patent Document 3 does not describe that the thickness non-uniformity is particularly deteriorated when the thickness of the polyvinyl alcohol film is less than 35 μm, nor does it describe or suggest a solution.

The object of the present invention is to provide: a PVA film having a thickness of 35 μm or less, in which the thickness unevenness in the MD direction is reduced and the number of stripe-like defects continuously generated in the MD direction is small; and a method for manufacturing the film.

The above-mentioned object can be achieved by the present invention described below.

That is, the present invention is:

A polyvinyl alcohol film, wherein the width direction of the PVA film is taken as the TD direction, the machine flow direction is taken as the MD direction, the number of stripe-like defects with a height difference of 0.15 μm or more extending linearly and continuously for more than 1.5 m in the MD direction is less than 5 in the entire width of the film, and the number of stripe-like defects with a height difference of 0.50 μm or more extending linearly and continuously for more than 1.5 m in the MD direction does not exist in the entire width of the film; and when the maximum value of the thickness of the central part in the TD direction is measured at intervals of 0.5 mm in the MD direction every 1.0 m is taken as t _MAX , the minimum value is taken as t _MIN , and the average value is taken as t _AVE , t _AVE is less than 35 μm, and the thickness unevenness rate specified by the following formula (1) is less than 5.5%; thickness unevenness rate = (t _MAX -t _MIN )/t _AVE × 100(%) (1)

The PVA film as described in [1] above, wherein the width is not less than 2.0 m and not more than 7.0 m;

A method for producing a polyvinyl alcohol film as described in [1] or [2] above, wherein the film is formed by casting a film-forming stock solution containing polyvinyl alcohol at 94-98°C on a roll at 88-92°C.

According to the present invention, a PVA film having a film thickness of less than 35 μm but having few stripe defects continuously generated in the MD direction and reduced thickness unevenness in the MD direction can be provided, and a method for producing the PVA film can be provided. In addition, by using the PVA film of the present invention, a polarizing film with suppressed wrinkles can be obtained. Furthermore, since the wrinkles of the polarizing film are suppressed, when a protective film is attached to the polarizing film to be processed into a polarizing plate, bubbles will not be mixed between the polarizing film and the protective film, and the quality of the obtained polarizing plate is improved.

The PVA film of the present invention is characterized in that the average thickness t _AVE is less than 35 μm and the thickness unevenness in the MD direction is within 5.5%. In addition, when the width direction of the film is taken as the TD direction, the mechanical flow direction of the film is taken as the MD direction, the maximum value of the film thickness when the thickness of the central part in the TD direction is measured at intervals of 0.5 mm every 1.0 m in the MD direction of the film is taken as t _MAX , the minimum value of the film thickness is taken as t _MIN , and the average value of the film thickness is taken as t _AVE , the thickness unevenness in the present invention is defined by the following formula (1).

Thickness unevenness = (t _MAX -t _MIN )/t _AVE × 100(%) (1)

PVA films thicker than 35 μm rarely show the problem of continuous stripe defects in the MD direction regardless of the above-mentioned thickness unevenness. The lower limit of the average value of thickness t _AVE is not particularly limited, but if it is too thin, it becomes easy to cause stretching interruption during uniaxial stretching for manufacturing polarizing film. Therefore, the average value t _AVE is preferably 5 μm or more, more preferably 10 μm or more, and particularly preferably 15 μm or more.

As the PVA constituting the PVA film of the present invention, a vinyl ester polymer obtained by saponifying a vinyl ester monomer and using a vinyl ester unit as a vinyl alcohol unit can be used. Examples of the vinyl ester monomer include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl trimethylacetate, and vinyl viscidate, among which vinyl acetate is preferably used.

When the vinyl ester monomer is polymerized, a monomer copolymerizable with the vinyl ester monomer (preferably 15 mol% or less, more preferably 5 mol% or less, relative to the total monomer units in PVA) may be copolymerized as needed within the scope of the present invention. Examples of such monomers include: olefins having 3 to 30 carbon atoms such as ethylene, propylene, 1-butene, and isobutylene; acrylic acid and its salts; acrylic acid esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tertiary butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, and octadecyl acrylate; methacrylic acid and its salts; methyl methacrylate, ethyl methacrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tertiary butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, and octadecyl acrylate; Methacrylates such as propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tertiary butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, octadecyl methacrylate, etc.; acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetoneacrylamide, acrylamidepropanesulfonic acid and its salts, acrylamidepropyldimethylamine and its salts, N-hydroxymethylacrylamide and its salts acrylamide derivatives such as methacrylamide, N-methyl methacrylamide, N-ethyl methacrylamide, methacrylamide propanesulfonic acid and its salts, methacrylamide propyl dimethylamine and its salts, N-hydroxymethyl methacrylamide and its derivatives; N-vinylamides such as N-vinylformamide, N-vinylacetamide, and N-vinylpyrrolidone; methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl ethyl Vinyl ethers such as butyl vinyl ether, tertiary butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; nitriles such as acrylonitrile and methacrylonitrile; halogenated vinyls such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid and its salts or esters; itaconic acid and its salts or esters; vinylsilyl compounds such as vinyltrimethoxysilane; N-vinylamides such as isopropylene acetate, N-vinylformamide, N-vinylacetamide, and N-vinylpyrrolidone, etc.

The average degree of polymerization of PVA constituting the PVA film of the present invention is preferably 500 or more from the viewpoint of the strength of the PVA film, more preferably 1000 or more from the viewpoint of the polarization performance, further preferably 2000 or more, and particularly preferably 3500 or more. On the other hand, the upper limit of the degree of polymerization of PVA is preferably 10000 or less from the viewpoint of the film forming properties of the PVA film of the present invention.

From the viewpoint of water resistance of the polarizing film obtained by uniaxially stretching the film, the saponification degree of PVA constituting the PVA film of the present invention is preferably 95 mol% or more, more preferably 96 mol% or more, and even more preferably 98 mol% or more. In addition, the saponification degree of PVA in this specification refers to the ratio (mol%) of the molar number of the vinyl alcohol unit relative to the total molar number of the structural unit (typically vinyl ester unit) and the vinyl alcohol unit that can be converted into the vinyl alcohol unit ( _-CH2 -CH(OH)-) by saponification of the PVA.

When manufacturing the PVA film of the present invention, it is preferred to add polyols to PVA as plasticizers. Examples of polyols include ethylene glycol, glycerol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trihydroxymethyl propane, etc., and one of these can be used alone or in combination of two or more. Among them, ethylene glycol or glycerol can be preferably used in terms of the effect of improving the elongation.

The amount of polyol added is preferably 1-30 parts by mass, 3-25 parts by mass, and 5-20 parts by mass relative to 100 parts by mass of PVA. If the amount added is less than 1 part by mass, the dyeability and elongation may be reduced, and if it is greater than 30 parts by mass, the PVA film may be too soft and the handling property may be reduced.

Furthermore, when manufacturing the PVA film of the present invention, it is preferred to add a surfactant. The surfactant is not particularly limited, but anionic or nonionic surfactants are preferred. Examples of anionic surfactants include carboxylic acid type surfactants such as potassium laurate, sulfate type surfactants such as octyl sulfate, and sulfonic acid type surfactants such as dodecylbenzenesulfonate. Examples of nonionic surfactants include alkyl ethers such as polyoxyethylene oleyl ether, alkyl phenyl ethers such as polyoxyethylene octyl phenyl ether, alkyl esters such as polyoxyethylene laurate, alkyl amines such as polyoxyethylene lauryl amino ether, alkyl amides such as polyoxyethylene laurate amide, polypropylene glycol ethers such as polyoxyethylene polyoxypropylene ether, alkanolamides such as lauric acid diethanolamide and oleic acid diethanolamide, and allyl phenyl ethers such as polyoxyalkylene allyl phenyl ether. As nonionic surfactants, alkanolamides are preferred, and lauric acid diethanolamide is more preferred. These surfactants may be used alone or in combination of two or more.

When adding a surfactant, the amount added is preferably 0.01 to 1 part by weight relative to 100 parts by weight of PVA, more preferably 0.02 to 0.5 parts by weight, and particularly preferably 0.05 to 0.3 parts by weight. If the amount added is less than 0.01 parts by weight, the effects of improving the elongation and dyeing properties are difficult to be seen. If the amount added is greater than 1 part by weight, the surfactant is dissolved on the surface of the PVA film, causing the PVA films to stick to each other, and the handling properties are reduced.

In the PVA film of the present invention, it is important that the thickness unevenness specified by the above formula (1) is 5.5% or less. If the thickness unevenness of the PVA film is greater than this, the resulting polarizing film will have wrinkles. The thickness unevenness is preferably 4% or less. In order to make the thickness unevenness less than 3%, the drying roller temperature must be increased, but if the drying roller temperature is set to, for example, 96°C or above, it becomes easy to produce cracks in the coating layer on the roller surface. In addition, even if the thickness unevenness is set to less than 3%, the stripe defects are not significantly improved, so it is better to set the thickness unevenness to 3% or more.

In the present invention, it is necessary that: the number of stripe-like defects with a height difference of 0.15 μm or more extending in a straight line in the MD direction for more than 1.5 m is less than 5 in the full width of the PVA film, and the number of stripe-like defects with a height difference of 0.50 μm or more extending in a straight line in the MD direction for more than 1.5 m does not exist in the full width of the PVA film. Stripe-like defects are defects that extend in the MD direction and can be regarded as a continuous line of more than 1.5 m, and are caused by the height difference with the surrounding thickness. If the height difference becomes more than 0.15 μm, the color unevenness after dyeing becomes more intense. If such stripe-like defects with height difference become more than 5 in the full width of the film, problems will arise in practice. If the height difference is 0.5 μm or more, the color unevenness after dyeing becomes more serious, and even if there is only one streak defect of such a height difference, it will cause practical problems. The streak defect is evaluated by the method described in the embodiment.

The width of the PVA film of the present invention is not particularly limited, and can be set to be, for example, 0.5 m or more. In recent years, due to the pursuit of a wide range of polarizing films, the width is preferably 2.0 m or more, 2.5 m or more is more preferred, and 3.5 m or more is further preferred. On the other hand, a PVA film with too wide a width will increase the manufacturing cost of the film-making device, and when using a practical manufacturing device to manufacture an optical film, it becomes difficult to stretch it uniformly. Therefore, the width of the PVA film is preferably 7.0 m or less, and 6.5 m or less is more preferred.

In the present invention, in order to manufacture a PVA film, a roll film-making machine equipped with one or more rotating rolls, a drying device, a humidifying device, a winding device, etc. is used. As for the drive of each device, a motor, a speed changer, etc. can be used to adjust the conveying speed. The drying temperature of the aforementioned PVA film is usually 50~150°C. As a film-making method, for example: a casting film-making method using a film-making solution in which PVA is dissolved in a solvent in a solution state; a roll film-making method in which a film-making solution containing aqueous polyvinyl alcohol is extruded and cast on a roll, etc., among which the latter is preferred. The aforementioned film-making solution containing aqueous polyvinyl alcohol may contain an organic solvent.

When manufacturing the PVA film of the present invention, the film-forming solution containing PVA supplied to the roll is preferably 94-98°C at the die inlet, and more preferably 94.5-96.5°C. If the temperature of the film-forming solution is less than 94°C, the leveling property of the film-forming solution supplied to the roll is reduced and the thickness unevenness is likely to increase. On the other hand, when the temperature is greater than 98°C, the viscosity of the film-forming solution is reduced and the front end of the die lip and the discharged film-forming solution interfere with each other, so that it becomes easy to generate stripe defects in the MD direction.

The volatile fraction of the film-forming solution containing PVA used in the production of the aforementioned PVA film is preferably 50-90 mass %, and more preferably 55-80 mass %. If the volatile fraction is less than 50 mass %, the viscosity may be increased, making it difficult to form a film. If the volatile fraction is greater than 90 mass %, the viscosity may be too low, thereby compromising the thickness uniformity of the PVA film.

When manufacturing PVA film, it is preferred to use a mold including a flexible die lip to supply the film-making solution from the mold to the roll of the roll film-making machine. Examples of the above-mentioned mold include: T-die, I-die, die lip coating die, etc. Next, the volatile components such as water and organic solvent contained in the film-making solution are evaporated on the above-mentioned roll.

In order to manufacture the aforementioned PVA film, the temperature of the roll supplied with the film-making stock solution is preferably 88°C to 92°C, and more preferably 89°C to 90°C. If it is less than 88°C, the peeling property from the roll may deteriorate due to poor drying and the thickness unevenness may increase. In addition, when it is greater than 92°C, the PVA will crystallize on the surface of the film that contacts the roll, and there may be a difference in crystallinity from the other side. If there is a difference in crystallinity between one side and the other side of the film, the water absorption characteristics will be different, so in the processing step of immersion in water, the film becomes easy to curl, and the step pass performance deteriorates.

The circumferential speed of the roller is preferably in the range of 10.0~50.0m/min. When the circumferential speed is less than the lower limit, productivity is poor. When the circumferential speed is greater than the upper limit, the contact time with the roller becomes shorter, so the drying of the PVA film is likely to become insufficient.

After drying and moistening the PVA film peeled off from the roll to adjust it into a suitable PVA film, the PVA film is wound up.

In order to manufacture a polarizing film from the PVA film of the present invention, the PVA film is dyed, uniaxially stretched, fixed, dried, and further heat-treated as needed. The order of the steps is not particularly limited, and it is also possible to perform two steps, such as dyeing and uniaxial stretching, at the same time. Moreover, each step can be repeated multiple times. In addition, dyeing can be performed before, during, or after uniaxial stretching. Since PVA is easily crystallized due to uniaxial stretching, dyeability is sometimes reduced. Therefore, it is better to perform it at any step before uniaxial stretching or during uniaxial stretching.

As dyes used for dyeing, the following dyes can be used alone or in a mixture of two or more: iodine-potassium iodide; Direct Black 17, 19, 154; Direct Brown 44, 106, 195, 210, 223; Direct Red 2, 23, 28, 31, 37, 39, 79, 81, 240, 242, 247; Direct Blue 1, 15, 22, 78, 90, 98, 151, 168, 202, 236, 249, 270; Direct Violet 9, 12, 51, 98; Direct Green 1, 85; Direct Yellow 8, 12, 44, 86, 87; Direct Orange 26, 39, 106, 107 and other dichroic dyes. Dyeing is usually performed by immersing the PVA film in a solution containing the aforementioned dye. When the aforementioned dye is mixed with a film-forming stock solution containing PVA and the film is formed, the treatment conditions and the treatment method are not particularly limited.

As a uniaxial stretching method for the length direction of the aforementioned PVA film, there can be adopted: a wet stretching method of stretching by immersing in a solution; or a dry heat stretching method of stretching in the air after the PVA film absorbs water. The uniaxial stretching of the PVA film can be carried out in a solution containing the aforementioned dye or in a fixing treatment bath described later. It is better to stretch the PVA film at the highest possible stretching ratio without cutting the PVA film. Specifically, 4 times or more is better, 5 times or more is more better, and 6 times or more is particularly better. If the stretching ratio is less than 4 times, it is difficult to obtain practically sufficient polarization performance and durability. The stretching temperature is not particularly limited, but when the PVA film is stretched in water (wet stretching), it is preferably 30~90°C, and when dry heat stretching is performed, it is preferably 50~180°C. The thickness of the stretched film is preferably 2-25 μm, more preferably 5-20 μm.

The fixing treatment can be performed for the purpose of strengthening the adsorption of the dye to the PVA film. Boric acid and boron compounds are usually added to the treatment bath used for the fixing treatment. In addition, iodine compounds can be added to the treatment bath as needed.

The drying treatment (heat treatment) of PVA film is preferably carried out at 30~150℃, and more preferably at 50~150℃.

The polarizing film obtained as described above is usually used as a polarizing plate by attaching an optically transparent and mechanically strong protective film to both sides or one side thereof. As the protective film, cellulose acetate film, acrylic film, polyester film, etc. can be usually used.

[Implementation example]

The present invention is specifically described below by way of examples, but the present invention is not limited to these examples. The analysis, measurement and evaluation in the examples and comparative examples are performed using the methods shown below.

(Evaluation of streak defects)

Stripe defects are defects that extend in the machine flow direction (MD direction) during film formation and can be seen as a continuous line of more than 1.5 m. When a light source is used to illuminate the film and the light that penetrates the film is projected onto a white wall, it can be confirmed as a continuous bright or dark pattern extending in the MD direction. Films with stripe defects have a height difference of more than 0.15 μm between the concave or convex parts of the surface, and the above-mentioned bright or dark patterns can be observed. Specifically, the PVA film obtained from the following embodiments or comparative examples is cut into a test piece of 1.5 m in the MD direction and the full width, and is hung upside down in the MD direction. A halogen lamp light source with a luminosity of about 550 Lux manufactured by S-1 Co., Ltd. is set at a position 350 cm away from the film surface to project light perpendicularly to the film surface. Next, when the light penetrating the film is projected onto a white wall 10 cm away from the film, the observed stripe-like bright or dark lines are judged as stripe-like defects. Then, the contact thickness gauge "KG601A" manufactured by Anritsu Co., Ltd. is used to measure the height difference in the film thickness direction between the defective part and the surrounding part for the total number of stripe-like defects at each location of the film corresponding to the stripe-like defects. In this way, the number of stripe-like defects with a height difference of 0.15 to 0.50 μm and extending straight and continuously for more than 1.5 m in the MD direction in the full width of the PVA film and the number of stripe-like defects with a height difference of 0.5 μm or more and extending straight and continuously for more than 1.5 m in the MD direction were determined, and the following criteria were used for evaluation. In addition, when the height difference is greater than 0.50 μm, even if there is only one stripe-like defect, it becomes a practical problem.

A: There are less than 5 stripe defects with a height difference of 0.15~0.50μm in the full width of the PVA film, and there are no stripe defects with a height difference of more than 0.5μm.

B: There are more than 5 stripe defects with a height difference of 0.15~0.50μm in the full width of the PVA film, or there are stripe defects with a height difference of more than 0.5μm

(Measurement of PVA film thickness and calculation of thickness unevenness)

The thickness of the central part of the PVA film in the TD direction (width direction) is measured at a pitch of 0.5 mm every 1.0 m in the MD direction. The measurement is performed using a contact thickness meter "KG601A" manufactured by Anritsu Co., Ltd. The maximum value of the above-mentioned measured values is t _MAX (μm), the minimum value is t _MIN (μm), and the average value is t _AVE (μm). The obtained values are used to calculate the thickness unevenness using the following formula (1).

Thickness unevenness = (t _MAX -t _MIN )/t _AVE × 100(%)． ． ． (1)

(Evaluation of wrinkles on polarizing films)

After the PVA film was stretched 2.6 times in the MD direction using a continuous stretching machine, the presence of wrinkles arranged in the TD direction was visually observed and evaluated based on the following criteria.

A: No wrinkles or very slight wrinkles.

B: Prominent wrinkles are produced.

[Implementation Example 1]

Using a roll film machine, a film-making stock solution containing 100 parts by mass of PVA with a saponification degree of 99.9 mol% and an average degree of polymerization of 2400, 12 parts by mass of glycerin, 0.1 parts by mass of lauric acid diethanolamide, and a volatile fraction of water of 70% by mass was discharged from a T-die at 96°C to the first drying roll (surface temperature of 91.7°C, peripheral speed of 24.8 m/min), and then stripped by the second drying roll, and then dried by the second drying roll at an average temperature of 85°C. Finally, the film was wound by a winding device to obtain a PVA film (thickness t _AVE 21.1 μm, width 3000 mm). The obtained film had a thickness unevenness of 3.79%, and the number of stripe-shaped defects with a height difference of 0.15-0.50 μm was 2 in the entire width, and there was no stripe-shaped defect with a height difference of more than 0.5 μm. In addition, after stretching 2.6 times in the MD direction, no wrinkles arranged in the TD direction were generated.

[Reference Example 2]

Using a roll film machine, a film-making stock solution containing 100 parts by mass of PVA with a saponification degree of 99.9 mol% and an average degree of polymerization of 2400, 12 parts by mass of glycerol, 0.1 parts by mass of lauric acid diethanolamide, and a volatile fraction of water of 70% by mass was discharged from a T-die to the first drying roll (surface temperature of 89.7°C, peripheral speed of 24.8 m/min) at 94°C, and then stripped on the second drying roll, and then dried at an average temperature of 85°C by the second drying roll. Finally, a PVA film (thickness t _AVE 20.7 μm, film width 3000 mm) was obtained by winding it using a winding device. The obtained film had a thickness unevenness of 4.55%, and the number of stripe-shaped defects with a height difference of 0.15-0.50μm was 1 in the entire width, and there was no stripe-shaped defect with a height difference of more than 0.5μm. In addition, after stretching 2.6 times in the MD direction, very slight wrinkles were generated in the TD direction.

[Reference Example 3]

Using a roll film machine, a film-making stock solution containing 100 parts by mass of PVA with a saponification degree of 99.9 mol% and an average degree of polymerization of 2400, 12 parts by mass of glycerol, 0.1 parts by mass of lauric acid diethanolamide, and a volatile fraction of water of 70% by mass was discharged from a T-die to the first drying roll (surface temperature of 89.7°C, peripheral speed of 24.8 m/min) at 97°C, and then stripped on the second drying roll, and then dried at an average temperature of 85°C by the second drying roll. Finally, a PVA film (thickness t _AVE 20.4 μm, film width 3000 mm) was obtained by winding using a winding device. The obtained film had a thickness unevenness of 5.11% and no streak defects were generated in the entire width. Furthermore, after being stretched 2.6 times in the MD direction, very slight wrinkles were generated in the TD direction.

[Reference Example 4]

Using a roll film machine, a film-making stock solution containing 100 parts by mass of PVA with a saponification degree of 99.9 mol% and an average degree of polymerization of 2400, 12 parts by mass of glycerol, 0.1 parts by mass of lauric acid diethanolamide, and a volatile fraction of water of 70% by mass was discharged from a T-die at 96°C to the first drying roll (surface temperature of 88.0°C, peripheral speed of 24.8 m/min), and then stripped on the second drying roll, and then dried at an average temperature of 85°C by the second drying roll. Finally, a PVA film (thickness t _AVE 20.1 μm, film width 3000 mm) was obtained by winding using a winding device. The obtained film had a thickness unevenness of 5.22% and no streak defects were generated in the entire width. Furthermore, after being stretched 2.6 times in the MD direction, very slight wrinkles were generated in the TD direction.

[Comparison Example 1]

Using a roll film machine, a film-making stock solution containing 100 parts by mass of PVA with a saponification degree of 99.9 mol% and an average degree of polymerization of 2400, 12 parts by mass of glycerin, 0.1 parts by mass of lauric acid diethanolamide, and a volatile fraction of water of 70% by mass was discharged from a T-die to the first drying roll (surface temperature of 92.3°C, peripheral speed of 24.8 m/min) at 96.0°C, and then stripped on the second drying roll, and then dried at an average temperature of 85°C by the second drying roll. Finally, the film was wound using a winding device to obtain a PVA film (thickness t _AVE 20.1 μm, film width 3000 mm). The obtained film had a thickness unevenness of 4.13%, and the number of stripe-shaped defects with a height difference ranging from 0.15 to 0.50 μm was 12 in the entire width. In addition, after being stretched 2.6 times in the MD direction, significant wrinkles were generated in the TD direction.

[Comparison Example 2]

Using a roll film machine, a film-making stock solution containing 100 parts by mass of PVA with a saponification degree of 99.9 mol% and an average degree of polymerization of 2400, 12 parts by mass of glycerol, 0.1 parts by mass of lauric acid diethanolamide, and a volatile fraction of water of 70% by mass was discharged from a T-die to the first drying roll (surface temperature of 89.7°C, peripheral speed of 24.8 m/min) at 98.5°C, and then stripped on the second drying roll, and then dried at an average temperature of 85°C on the second drying roll. Finally, the film was wound using a winding device to obtain a PVA film (thickness t _AVE 20.4 μm, film width 3000 mm). The obtained film had a thickness unevenness of 7.13%, and the number of stripe-shaped defects with a height difference of 0.15-0.50 μm was more than 50 in the entire width. In addition, after being stretched 2.6 times in the MD direction, significant wrinkles were generated in the TD direction.

[Comparison Example 3]

Using a roll film machine, a film-making stock solution containing 100 parts by mass of PVA with a saponification degree of 99.9 mol% and an average degree of polymerization of 2400, 12 parts by mass of glycerol, 0.1 parts by mass of lauric acid diethanolamide, and a volatile fraction of water of 70% by mass was discharged from a T-die to the first drying roll (surface temperature of 88.0°C, peripheral speed of 24.8 m/min) at 93.5°C, and then stripped on the second drying roll, and then dried at an average temperature of 85°C by the second drying roll. Finally, the film was wound using a winding device to obtain a PVA film (thickness t _AVE 20.3 μm, film width 3000 mm). The obtained film had a thickness unevenness of 5.95%, and the number of stripe-shaped defects with a height difference of 0.15-0.50μm was 2 in the entire width, and there was no stripe-shaped defect with a height difference of more than 0.5μm. In addition, after stretching 2.6 times in the MD direction, significant wrinkles were generated in the TD direction.

[Comparison Example 4]

Using a roll film machine, a film-making stock solution containing 100 parts by mass of PVA with a saponification degree of 99.9 mol% and an average degree of polymerization of 2400, 12 parts by mass of glycerin, 0.1 parts by mass of lauric acid diethanolamide, and a volatile fraction of water of 70% by mass was discharged from a T-die to the first drying roll (surface temperature of 86.7°C, peripheral speed of 24.8 m/min) at 96.0°C, and then stripped on the second drying roll, and then dried at an average temperature of 85°C by the second drying roll. Finally, the film was wound using a winding device to obtain a PVA film (thickness t _AVE 20.6 μm, film width 3000 mm). The obtained film had a thickness unevenness of 7.22%, and the number of stripe-shaped defects with a height difference of 0.15-0.50μm was 1 in the entire width, and there was no stripe-shaped defect with a height difference of more than 0.5μm. In addition, after stretching 2.6 times in the MD direction, significant wrinkles were generated in the TD direction.

[Reference Example 1]

Using a roll film machine, a film-making stock solution containing 100 parts by mass of PVA with a saponification degree of 99.9 mol% and an average degree of polymerization of 2400, 12 parts by mass of glycerol, 0.1 parts by mass of lauric acid diethanolamide, and a volatile fraction of water of 67% by mass was discharged from a T-die to the first drying roll (surface temperature of 90.0°C, peripheral speed of 17.5 m/min) at 99.6°C, and then stripped on the second drying roll, and then dried at an average temperature of 85°C by the second drying roll. Finally, the film was wound using a winding device to obtain a PVA film (thickness t _AVE 44.7 μm, film width 3000 mm). The obtained film had a thickness unevenness of 6.71%, and the number of stripe-shaped defects with a height difference of 0.15-0.50μm was 1 in the entire width, and there was no stripe-shaped defect with a height difference of more than 0.5μm. In addition, after stretching 2.6 times in the MD direction, no wrinkles arranged in the TD direction were generated.

The manufacturing conditions of the PVA films in the embodiments, comparative examples, and reference examples and the evaluation results of the obtained PVA films are shown in Table 1.

1)高低差為0.15~0.50μm的條紋狀缺陷之條數

1) Number of stripe defects with a height difference of 0.15~0.50μm

According to the present invention, a polyvinyl alcohol film can be manufactured which is thin but can suppress stripe defects and does not produce wrinkles when stretched.

Claims (2)

A polyvinyl alcohol film, wherein the width direction of the polyvinyl alcohol film is taken as the TD direction, the machine flow direction is taken as the MD direction, the number of stripe-like defects with a height difference of 0.15 μm or more extending linearly and continuously for more than 1.5 m in the MD direction is less than 5 in the entire width of the film, and the number of stripe-like defects with a height difference of 0.50 μm or more extending linearly and continuously for more than 1.5 m in the MD direction does not exist in the entire width of the film; and when the maximum value of the thickness in the central part of the TD direction at intervals of 0.5 mm every 1.0 m in the MD direction is taken as t _MAX , the minimum value is taken as t _MIN , and the average value is taken as t _AVE , t _AVE is less than 35 μm, and the thickness unevenness rate specified by the following formula (1) is less than 4%; thickness unevenness rate = (t _MAX -t _MIN )/t _AVE × 100(%) (1). For example, the polyvinyl alcohol film in claim 1, where the width is greater than 2.0m and less than 7.0m.