JP7695918B2 - Polyvinyl alcohol film, optical film produced therefrom and method for producing same - Google Patents

Description

The present invention relates to a polyvinyl alcohol (PVA) film that can be used in the manufacture of optical films, particularly polarizing films.

Polyvinyl alcohol (PVA) film is a type of hydrophilic polymer that has properties such as transparency, mechanical strength, water solubility, and good processability, and is widely used in packaging materials and optical films for electronic products, especially polarizing films.

The polarizing film obtained by processing the polyvinyl alcohol film in the subsequent polarization process can control the brightness of the light that passes through it, and is one of the essential components in display devices. The process of manufacturing a polarizing film from a polyvinyl alcohol film includes swelling, stretching, and dyeing. Specifically, the PVA film is placed in a solution and the above-mentioned process is carried out, and the dye molecules are diffused and inserted between the polyvinyl alcohol molecules of the polyvinyl alcohol film to arrange them in a regular manner, allowing the polarizing film to absorb the light components parallel to the arrangement direction and transmit the light components perpendicular to the arrangement direction, resulting in the property of having polarized light.

An ideal polarizing film should have properties such as uniform color, few color spots, no wrinkles, and good hue effect, and should be able to provide excellent optical properties. In order to improve the optical properties of polarizing films, conventional techniques have generally used methods such as changing the structure of polyvinyl alcohol or adding functional groups (e.g., cationic groups) to change the viscosity or degree of saponification to improve the optical properties.

However, in the prior art, when an optical film is manufactured from a polyvinyl alcohol film, a problem of variation in hue, such as the hue of the finished product being generally redder or bluer, often occurs. The present inventors have found that the cause of the above problem is probably that, when an additive is to be mixed into a polyvinyl alcohol film for use in manufacturing an optical film, the additive precipitates after the swelling process of the polyvinyl alcohol film and some of the additive remains on the film. For example, if the amount of the additive remaining is too much, it occupies too much of the amorphous region on the film, making it difficult to crosslink the pentaiodo anion (I ₅ ^- ) for absorbing long wavelength light with the polyvinyl alcohol, and the hue of the finished optical film becomes redder. On the other hand, if the amount of the additive remaining is too little, the amount of crosslinking between I ₅ ^- and the amorphous region of the polyvinyl alcohol becomes too large, and in this case, the absorption of the polarizing film for red light becomes too large, and the hue of the finished optical film becomes bluer.

Furthermore, without being limited to a particular theory, the inventors have realized that the above-mentioned situation is also related to a number of factors, such as the temperature of the heating roller that contacts the preformed film after it is formed during the manufacturing process of the polyvinyl alcohol film, the temperature of the dryer that performs the subsequent drying, and the temperature difference between the heating roller and the dryer, all of which at least partially affect the amount of additive that precipitates and remains after the film body has swelled.

Therefore, in order to solve the above problems, the present invention provides a polyvinyl alcohol film by adjusting the residual amount of additives to a certain range, which can be used to manufacture optical films with excellent color expression.

Specifically, the polyvinyl alcohol film provided in one embodiment of the present invention has an additive residual amount of 0.50 to 3.00 wt % after swelling and stretching. In the above swelling and stretching, the polyvinyl alcohol film was stretched in water at a speed of 4.3 cm/min to double its length. The additive residual amount was calculated using the formula: (W3-W4)/W3 x 100%, where W3 is the weight of the polyvinyl alcohol film after swelling, stretching, and drying, and W4 is the weight of the polyvinyl alcohol film after swelling, stretching, drying, placing in pure water, stirring for 5 minutes, and then drying again.

In a preferred embodiment, when the polyvinyl alcohol film after swelling, stretching and drying is analyzed by Fourier transform infrared spectroscopy (FTIR), the intensity ratio at wavelengths of 3200 cm ^-1 and 1425 cm ^-1 is 0.70 to 1.00.

In a preferred embodiment, when the polyvinyl alcohol film after swelling, stretching and drying is analyzed by Fourier transform infrared spectroscopy, the intensity ratio at wavelengths of 1140 cm ⁻¹ and 1425 cm ⁻¹ is between 1.20 and 1.48. More preferably, the intensity ratio is between 1.20 and 1.45.

In a preferred embodiment, the polyvinyl alcohol film has an initial additive content between 6 and 15 wt %.

In a preferred embodiment, the degree of polymerization of the polyvinyl alcohol film is between 1800 and 3000.

In a preferred embodiment, the moisture content of the polyvinyl alcohol film is between 1.0 and 5.0 wt %.

An optical film provided in another embodiment of the present invention is made from the polyvinyl alcohol film described above.

In a preferred embodiment, the optical film is a polarizing film. Preferably, the polarizing film has a polarization degree of 99.8% or more.

In a preferred embodiment, the optical film has a ratio of absorbance at a wavelength of 480 nm to absorbance at a wavelength of 700 nm under perpendicular conditions of 1.40 to 1.60.

In yet another embodiment of the present invention, a method for producing a polyvinyl alcohol film is provided, comprising the steps of:
(a) Polyvinyl alcohol based resin, plasticizer, surfactant and water are stirred and heated to a dissolution temperature above 100° C. to form a polyvinyl alcohol casting solution.
(b) A polyvinyl alcohol casting solution is cast onto a casting drum and dried to prepare a preformed film.
(c) exposing the preformed film to n heated rollers whose temperatures are gradually decreased from high to low, and then entering a multi-section dryer for drying;
Wherein, the temperature of the first heating roller is 84-92°C, the maximum temperature of the dryer is not more than 115°C, and the temperature difference between the nth heating roller and the first section dryer is not more than 30°C, where n is between 10 and 20.

In a preferred embodiment, the dissolution temperature in step (a) is 120 to 140°C.

In a preferred embodiment, the polyvinyl alcohol resin concentration in the polyvinyl alcohol casting solution in step (b) is 20.0 to 40.0%.

In a preferred embodiment, the plasticizer is 3 to 30 parts by weight per 100 parts by weight of the polyvinyl alcohol resin.

The optical film manufactured using the polyvinyl alcohol film provided based on the above-mentioned specifications of the present invention has excellent color expression and polarization degree. In addition, the specifications of the present invention can be used to more precisely manufacture and adjust the above-mentioned polyvinyl alcohol film.

FIG. 2 is a schematic diagram of FTIR peaks and corresponding chemical structures according to one embodiment of the present invention. FIG. 1 is a schematic diagram of a process for producing a polyvinyl alcohol film according to one embodiment of the present invention. 1 is a schematic diagram of a polyvinyl alcohol film manufacturing apparatus according to one embodiment of the present invention.

To more clearly illustrate the above and other objects, features, advantages and embodiments of the present invention, the drawings are described as follows.

In this specification, the proportions of various features and components in the drawings are not actual proportions, but are drawn in a drawing method based on a conventional work method in order to show the specific features and components related to the present invention in an optimal manner. In addition, in different drawings, the same or similar component symbols refer to similar components or parts.

In order to describe the present invention in more detail and without omission, the following description is provided with reference to the embodiments and specific examples of the present invention, which are not the only forms of specific examples for carrying out or applying the present invention. In this specification and the appended claims, the terms "one" and "the" may be interpreted as plural unless otherwise specified in the context. In this specification and the appended claims, unless otherwise specified, "installed on an object" may be considered to be attached to or in other ways contacted with the surface of an object, directly or indirectly, and the definition of the surface shall be determined by the implication of the preceding/following paragraphs/paragraphs of the contents of the specification and the common knowledge in the field to which this specification pertains.

All numerical ranges and parameters specifying the present invention are approximate, but the relevant numerical values in the specific examples are as precise as possible. However, it is inherently inevitable that any numerical value will include standard deviations resulting from individual testing methods. In this context, "about" generally means that the actual numerical value is within ±10%, 5%, 1% or 0.5% of the specified numerical value or range. Alternatively, the term "about" means that the actual numerical value is within an acceptable standard error of the mean value, as considered and determined by a person skilled in the art to which the present invention pertains. Thus, unless otherwise stated, all numerical parameters disclosed in this specification and the appended claims are approximate and may be considered to vary as necessary. At the very least, such numerical parameters should be interpreted as being the numerical values obtained by applying the indicated number of significant digits and ordinary carryover techniques.

The present invention provides a polyvinyl alcohol film, which is stretched in water at a speed of 4.3 cm/min, doubled in length, and then dried. The weight of the stretched and dried polyvinyl alcohol film is W3, and the weight of the stretched and dried polyvinyl alcohol film is W4 after stirring in pure water for 5 minutes and then completely drying again, and the value of (W3-W4)/W3 x 100% is 0.50 to 3.00 wt%.

The present invention solves the problem of the polarizing film becoming redder or bluer by adjusting the amount of additives remaining in the polyvinyl alcohol film after swelling and stretching, i.e., by simulating the conditions before dyeing the polarizing plate and controlling the amount of additives remaining before it enters the dyeing tank. The inventors realized that when actually manufacturing a polarizing plate, the polyvinyl alcohol film goes through swelling and stretching treatment, but additives and other impurities in the polyvinyl alcohol are precipitated during the swelling process, and the additives in each part of the polyvinyl alcohol film that is finally manufactured into a polarizing film change somewhat, so that if only the initial amount of additives in the polyvinyl alcohol film is measured, it is not possible to represent the residual state of additives when the polarizing plate is actually manufactured, and the problem of the polarizing film becoming redder or bluer cannot be solved. Specifically, the above parameters are specified based on the process of preparing the polyvinyl alcohol film into an optical film, particularly a polarizing film. The method of preparing a polyvinyl alcohol film into a polarizing film is not particularly limited, but preferably includes a swelling treatment (also referred to as swelling and stretching in this specification) in which the polyvinyl alcohol film is placed in water and stretched, a dyeing treatment in which the film is dyed with a dichroic dye, and a stretching treatment in which the film is uniaxially stretched. Among these, methods such as boric acid crosslinking treatment, fixing treatment, washing treatment, and heating treatment may be further performed as necessary. In this case, the order of each treatment is not particularly limited, but preferably, the swelling treatment, dyeing treatment, and stretching treatment are performed in that order. In the industry, it is common to stretch the film to twice its length in the swelling tank, which is the approximate stretching ratio.

The amount of residual additives described above was determined by stirring the swollen, stretched, and dried polyvinyl alcohol film in pure water for 5 minutes to precipitate the additives in the film. The amount of residual additives was calculated using the formula: (W3-W4)/W3 x 100%, and can be the average value of multiple films (e.g., 5 or more sheets) cut evenly in the width direction.

As used herein, "additives" include, but are not limited to, plasticizers, surfactants, opacifiers, emulsifiers, or foaming agents. Among them, "plasticizers" can increase the flexibility of materials or liquefy materials, and are, for example, but not limited to, phthalate esters (Phthalates), bis(2-ethylhexyl) phthalate (DEHP), glycerin, dibutyl phthalate (DBP), diisononyl phthalate (DINP), diisodecyl phthalate (DIDP), butyl benzyl phthalate (BBP), dioctyl phthalate (DOP), ethylene glycol, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, or trimethylolpropane. Preferably, the plasticizer is glycerin, ethylene glycol, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, or trimethylolpropane. The "surfactant" is not limited to cationic, anionic or nonionic surfactants, such as, but not limited to, carboxylate type such as potassium laurate, sulfate ester type such as sodium laureth sulfate, sulfonate type such as dodecylbenzenesulfonate, alkylphenyl ether type such as polyoxyethylene octylphenyl ether, alcohol-based phenyl ether type such as polyethylene glycol monooctylphenyl ether, alkyl ester type such as polyoxyethylene laurate, alkylamine type such as polyoxyethylene laurylamine, alkylamide type such as polyoxyethylene lauric acid amide, polypropylene glycol ether type such as polyoxyethylene polyoxypropylene ether, alkanolamide type such as lauric acid diethanolamide, oleyl diethanolamide, allyl phenyl ether type such as polyoxyethylene allyl phenyl ether, or sodium polyoxyethylene lauryl ether sulfate. According to a preferred embodiment of the present invention, the polyvinyl alcohol film has an additive initial content between 6 and 15 wt%, for example, 7 to 14, 8 to 13, 9 to 12, or 10 to 11 wt%.

According to a preferred embodiment of the present invention, the additives contained in the polyvinyl alcohol film are mainly plasticizers and surfactants. Plasticizers and surfactants are low molecular weight compounds, which increase the free volume of the material when blended with a polymer. During the swelling process of the polyvinyl alcohol film, some of the additives remain in the polyvinyl alcohol film. If the amount of the additives remaining is too large, the free cavities will be occupied by the additives. When the polyvinyl alcohol film is placed in the dyeing tank, the iodide ions cannot form complexes (e.g., I ₅ ^- and PVA complexes) with the polyvinyl alcohol in the free cavities, which reduces the ability of the polarizing film to absorb light in the long wavelength range, resulting in a problem of color variation. Among them, the I ₅ ^-complex absorbs long wavelength light, and the I ₃ ^-complex absorbs short wavelength light, so if the amount of the complexes is too small, the polarizing film may become reddish or blueish.

According to at least one embodiment of the present invention, when the polyvinyl alcohol film after swelling, stretching and drying is analyzed by Fourier transform infrared spectroscopy, the value of the intensity ratio at wavelengths of 3200 cm ⁻¹ and 1425 cm ⁻¹ is 0.70 to 1.00, for example, 0.71, 0.75, 0.77, 0.79, 0.81, 0.89, 0.90, 0.91, 0.93, 0.95, 0.98 or 0.99. In addition, the value of the intensity ratio at wavelengths of 1140 cm ⁻¹ and 1425 cm ⁻¹ is 1.48 or less, for example, 1.20, 1.26, 1.28, 1.31, 1.32, 1.33, 1.38, 1.42, 1.45 or 1.48, but may be within a range consisting of any two of the above values. According to a preferred embodiment of the invention, the value of the intensity ratio at wavelengths 1140 cm ⁻¹ and 1425 cm ⁻¹ is between 1.20 and 1.48, preferably between 1.20 and 1.45.

Fourier transform infrared spectroscopy (FTIR) is a technique for obtaining infrared absorption and emission spectra of solids, liquids, or gases. In the present application, as can be seen by referring to the structural conceptual diagram of the polyvinyl alcohol film shown in FIG. 1, the absorption peak at a wavelength of 3200 cm ⁻¹ can correspond to OH in the polyvinyl alcohol film skeleton, and corresponds to the alkalinity in the polyvinyl alcohol film, so that when measuring the polyvinyl alcohol film after swelling and stretching, the absorption peak at a wavelength of 3200 cm ⁻¹ can represent the plasticizer (e.g., glycerin) in the film body. The absorption peak at a wavelength of 1140 cm ⁻¹ can correspond to the main structure in the polyvinyl alcohol film skeleton, and indicates the polyvinyl alcohol crystallinity characteristic peak (ν(C-C) stretching vibrations). The absorption peak at a wavelength of 1425 cm ⁻¹ corresponds to C-H in the polyvinyl alcohol film skeleton, and since its content is fixed, it can be considered as a definite peak. Thereby, the value of the intensity ratio of the wavelength 3200 cm ^-1 to the wavelength 1425 cm ^-1 shows a positive correlation with the amount of plasticizer remaining in the polyvinyl alcohol film, and the higher the FT-IR ratio of OH (3200 cm ^{-1 )} /CH ₂ (1425 cm ^-1 ) is, the higher the amount of plasticizer remaining in the polyvinyl alcohol film. The value of the intensity ratio of the wavelength 1140 cm -1 to the wavelength 1425 cm ^-1 is, the higher the FT-IR ratio of C-C (1140 cm ^-1 )/C-H (1425 cm ^-1 ) is, the higher the crystallinity of the polyvinyl alcohol film after swelling and stretching. In the present invention, the residual plasticizer amount and the crystallinity of the polyvinyl alcohol film after swelling, stretching, and drying are adjusted by utilizing the above-mentioned FT-IR intensity ratio, so that the residual plasticizer amount is kept within an appropriate range and the crystallinity is reduced, thereby obtaining a finished polyvinyl alcohol film with uniform dyeing and good hue.

According to at least one embodiment of the present invention, the degree of polymerization of the polyvinyl alcohol film is between 1800 and 3000, such as, but not limited to, 1800, 1900, 2000, 2100, 2200, 2300, 2400, 2500, 2600, 2700, 2800, 2900, or 3000. According to at least one embodiment of the present invention, the moisture content of the polyvinyl alcohol film is between 1.0 and 5.0 wt%, such as, but not limited to, 1, 1.1, 1.3, 1.5, 1.7, 1.9, 2.0, 2.1, 2.3, 2.5, 2.7, 2.9, 3.0, 3.1, 3.3, 3.5, 3.7, 3.9, 4.0, 4.1, 4.3, 4.5, 4.7, 4.9, or 5.0 wt%.

The thickness of the polyvinyl alcohol film of the present invention may be between 20 and 100 μm, preferably between 60 and 75 μm, for example, but not limited to, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74 or 75 μm.

The present invention provides an optical film, which is made from the polyvinyl alcohol film described above. The "optical film" described in this specification may refer to a polarizing film, a blue light cut film, a filter lens, etc., but the present invention is not limited thereto. Preferably, the polyvinyl alcohol film of the present invention is a polarizing film. Furthermore, the polarization degree of the polarizing film of the preferred embodiment provided by the present invention is 99.8% or more. In addition, the optical film provided by the present invention has a ratio of the absorbance at a wavelength of 480 nm to the absorbance at a wavelength of 700 nm under perpendicular conditions of 1.40 to 1.60, for example, 1.40, 1.42, 1.44, 1.46, 1.48, 1.50, 1.52, 1.54, 1.56, 1.58 or 1.60, but is not limited thereto. Specifically, the measurement of the ratio of the absorbance at a wavelength of 480 nm to the absorbance at a wavelength of 700 nm can correspond to the hue expression of the optical film. Without being limited to any particular theory, the above data range of 1.4 to 1.6 can be considered to be a good, consistent representation of hue.

In another embodiment, the present invention also provides a method for producing a polyvinyl alcohol film, and the steps and the production apparatus thereof can be referred to in the contents shown in FIG. 2 and FIG. 3. The production method includes the following: In step S100, a polyvinyl alcohol resin, a surfactant, a plasticizer, and water are stirred and heated to form a polyvinyl alcohol casting solution. In step S102, the polyvinyl alcohol casting solution is cast into a casting drum and dried to prepare a preformed film. In step S104, the preformed film is brought into contact with n heating rollers whose temperature is gradually decreased from high to low, and then is fed into a dryer with multiple sections for drying.

Specifically, in step S100, polyvinyl alcohol resin, plasticizer, surfactant and water are placed in a dissolution tank 110, stirred, and heated to a dissolution temperature of more than 100°C for at least 180 minutes to form a polyvinyl alcohol solution. The dissolution temperature is preferably 120 to 140°C, for example, 120, 125, 130, 135 or 140°C. The amount of plasticizer added is usually between 3 and 30 parts by weight, preferably between 7 and 20 parts by weight, for example, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 or 30 parts by weight, based on 100 parts by weight of polyvinyl alcohol resin. If the plasticizer content is insufficient, the polyvinyl alcohol film formed will be prone to crystallization, which will affect the dyeing effect in subsequent processing. On the other hand, if the plasticizer content is too high, the mechanical properties of the polyvinyl alcohol film will be impaired.

The polyvinyl alcohol resin concentration when preparing the polyvinyl alcohol casting solution is 10.0 to 60.0% by weight, preferably 15.0 to 40.0% by weight, more preferably 20.0 to 40.0% by weight, for example, 10.0, 15.0, 20.0, 25.0, 30.0, 35.0, 40.0, 45.0, 50.0, 55.0 or 60.0% by weight. The above-mentioned polyvinyl alcohol resin concentration is calculated as polyvinyl alcohol resin/(polyvinyl alcohol resin + water + plasticizer + surfactant). If the content of polyvinyl alcohol resin is insufficient, the viscosity of the polyvinyl alcohol casting solution becomes too low, the drying load becomes excessively large, and the film formation efficiency in the preparation of the polyvinyl alcohol film becomes poor. On the other hand, if the content of polyvinyl alcohol resin is too high, the polyvinyl alcohol resin becomes difficult to dissolve uniformly overall, and clusters tend to remain.

Specifically, in step S102, the polyvinyl alcohol casting solution is selectively filtered through a filter, and the polyvinyl alcohol casting solution is introduced into a T-shaped slit die in a quantitative manner, and then discharged and cast onto the casting drum 120 to prepare the preformed film M. The rotation speed of the casting drum 120 is about 3 to 12 m/min, and preferably 5 to 10 m/min. If the speed of the casting drum 120 is too slow, productivity may decrease. On the other hand, if the speed of the casting drum 120 is too fast, the drying of the casting solution may be insufficient, resulting in poor peelability. In a preferred embodiment, the temperature of the casting drum 120 is set to 85 to 95°C, specifically, for example, 85, 87, 89, 91, 93, 95°C, or between any two of the above values. If the temperature of the casting drum 120 is too high, the casting solution is prone to foaming.

Specifically, in step S104, the preformed film M is peeled off from the casting drum 120 and then brought into contact with a plurality of heating rollers 130 to dry both the upper and lower surfaces of the film body. Here, the first of the plurality of heating rollers 130 is the hottest of all the heating rollers 130, and the temperature of the subsequent heating rollers 130 is adjusted to gradually decrease. Next, drying is performed in a dryer 140 having multiple sections. Furthermore, the temperature of the first heating roller 130 is 84 to 92°C, for example, 84, 85, 86, 87, 88, 89, 90, 91, or 92°C. The maximum temperature of the dryer 140 is 115°C or less, for example, 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, or 115°C. The temperature difference between the nth heating roller 130 and the first section of the dryer 140 is 30° C. or less, for example, 1° C., 5° C., 10° C., 15° C., 20° C., 25° C., or 30° C. n is between 10 and 20, for example, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20. The multi-section dryer may be a 2-10 section dryer, preferably 3-8 sections, for example, 3, 4, 5, 6, 7, or 8.

The polyvinyl alcohol aqueous solution and the additives are mixed uniformly before being discharged into the casting drum. The amount of additives precipitated in the polyvinyl alcohol film can be controlled within a certain range by designing the drying conditions. If the amount of additives remaining in the polyvinyl alcohol film is large during the swelling, stretching, and dyeing processes of the polarizing film, the polarizing film will become redder, and if the amount of additives remaining in the polyvinyl alcohol film is small during the stretching and dyeing processes of the polarizing film, the polarizing film will become bluer. The inventor of the present application noticed that the higher the maximum temperature of the dryer 140 and the temperature of the first heating roller 130, or the larger the temperature difference between the rearmost heating roller 130 and the first section of the dryer 140, the higher the average amount of additives remaining. Therefore, by further adjusting the above parameters, it is possible to control the amount of additives precipitated after swelling and stretching within a specific range, and a polyvinyl alcohol film with a better hue can be obtained.

The polyvinyl alcohol resin is obtained by forming a polyvinyl ester resin by polymerization of a vinyl ester resin monomer, followed by a saponification reaction. The vinyl ester resin monomer includes vinyl esters such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl pentanoate, and vinyl octanoate, but the present invention is not limited thereto, and is preferably vinyl acetate. Copolymers formed by copolymerization of olefin compounds or acrylate derivatives with the vinyl ester resin monomers can also be used. Olefin compounds include ethylene, propylene, and butylene, but the present invention is not limited thereto. Acrylate derivatives include acrylic acid, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, and n-butyl acrylate, but the present invention is not limited thereto.

The saponification degree/alkalinization degree of the polyvinyl alcohol resin disclosed above is preferably 99.00% or more, which provides good optical properties, but specifically, for example, 99.00% to 100.00%, 99.00% to 99.99%, 99.00% to 99.95%, 99.00% to 99.90%, 99.00% to 99.85%, 99.00% to 99.80%, 99.00% to 99.75%, 99.00% to 99.70%, 99.00% to 99.65%, 99.00% to 99.60%, 99.00% to 99.55%, 99.00% to 99.50%, 99.0 0% to 99.45%, 99.00% to 99.40%, 99.00% to 99.35%, 99.00% to 99.30%, 99.00% to 99.25%, 99.00% to 99.20%, 99.00% to 99.15%, 99.00% to 99.10%, 99.00% to 99.05%, 99 . 20% to 100.00%, 99.20% to 99.99%, 99.20% to 99.95%, 99.20% to 99.90%, 99.20% to 99.85%, 99.20% to 99.80%, 99.20% to 99.75%, 99.20% to 99.70%, 99.20% to 99.65%, 99.20% to 99.60%, 99.20% to 99.55%, 99.20% to 99.50%, 99.20% to 99.45%, 99.20% to 99.40%, 99.20% to 99.35%, 99.20% to 99.30%, 99.20% to 99.25%, 99.40% to 100.0 0%, 99.40% to 99.99%, 99.40% to 99.95%, 99.40% to 99.90%, 99.40% to 99.85%, 99.40% to 99.80%, 99.40% to 99.75%, 99.40% to 99.70%, 99.40% to 99.65%, 99.40% to 99. 60%, 99.40% to 99.55%, 99.40% to 99.50%, 99.40% to 99.45%, 99.60% to 100.00%, 99.60% to 99.99%, 99.60% to 99.95%, 99.60% to 99.90%, 99.60% to 99.85%, 99.60% to 9 9.80%, 99.60% to 99.75%, 99.60% to 99.70%, 99.60% to 99.65%, 99.80% to 100.00%, 99.80% to 99.99%, 99.80% to 99.95%, 99.80% to 99.90%, or 99.80% to 99.85%, etc.

The present invention will be described in more detail below with reference to examples. However, it should be understood that these examples are intended to help the present invention be more easily understood and are not intended to limit the scope of the present invention.
Working Example

The non-limiting examples of each aspect of the present invention provided below are intended primarily to clarify each aspect of the present invention and the advantages achieved thereby.

1. Preparation of Polyvinyl Alcohol Films Non-limiting methods for preparing polyvinyl alcohol films are provided below. Eleven non-limiting example polyvinyl alcohol films (Examples 1-11) and four comparative polyvinyl alcohol films (Comparative Examples 1-4) were prepared based on methods similar to those disclosed below. However, the specific methods for preparing Examples 1-11 and Comparative Examples 1-4 generally differ from the methods disclosed below in one or more aspects.

Specifically, the method for preparing the polyvinyl alcohol film includes the following steps: 1800 kg of polyvinyl alcohol resin with an alkalinity of >99.9% and a polymerization degree of about 2400, 4000 kg of water, and 207 kg of glycerin plasticizer were added, and a surfactant was added so that the final content was 0.15 wt%, and the mixture was heated to 140°C while stirring, and dissolved for 180 minutes while maintaining the temperature at 140°C. Water was added to the polyvinyl alcohol resin solution after uniform dissolution until the resin concentration was 30.0%, to obtain a polyvinyl alcohol casting solution. Among them, the surfactant used in Examples 1 to 6 and Comparative Examples 1 and 2 was polyoxyethylene lauryl ether, and the surfactant used in Examples 7 to 11 and Comparative Examples 3 and 4 was sodium polyoxyethylene lauryl ether sulfate. After the polyvinyl alcohol casting solution was defoamed, it was discharged from a T-shaped slit die, curtain-coated on a rotating high-temperature casting drum, and dried to prepare a preformed film. After the preformed film was peeled off from the casting drum, it was dried on both the top and bottom sides by contacting it with multiple heated rollers, and then dried in a dryer. The maximum temperature of the dryer was 115°C, and a final polyvinyl alcohol film with a moisture content of 2.3 wt% was obtained.

2. Analysis and Measurement Methods The method for preparing the samples was as follows: first, the polyvinyl alcohol film was divided into 5 equal pieces along the transverse direction (TD, also called the width direction), and the center of the divided polyvinyl alcohol film was cut. The cut area of each piece was MD 5 cm x TD 10 cm (MD stands for machine direction, i.e., the machine direction or longitudinal direction). Next, the samples were left in a thermo-hygrostat at 23°C and 50% relative humidity (RH) for 24 hours.

The measurement conditions were as follows: first, the polyvinyl alcohol film was dried and dehydrated at 105°C for 10 minutes, and weighed after drying (W1). Next, the polyvinyl alcohol film was stirred in 2000 ml of pure water at 30°C with a stirrer (115-120 rpm) for 5 minutes to precipitate the initial additives, and after completion, the water on the surface of the polyvinyl alcohol film was dehydrated, placed in a dryer and completely dried at 105°C for 1 hour, and the weight was measured (W2). The initial additive content is then (W1-W2)/W1 x 100%. The initial additive content is calculated by adding the initial surfactant content to the initial plasticizer content.

2-2. Analysis of residual amount of additive after swelling The method of preparing the sample was as follows: first, the polyvinyl alcohol film was divided into 5 equal parts along the transverse direction, and the center of the polyvinyl alcohol film after the division was cut. The cut area of each piece was 20 cm in MD x 15 cm in TD. Then, the film was left in a thermohygrostat for 24 hours at 23°C and 50% RH. Next, the polyvinyl alcohol film (5 cm in MD x 15 cm in TD) was fixed, and the polyvinyl alcohol film was stretched to twice its length at 4.3 cm/min in pure water at 30°C. After completion, the moisture on the surface of the polyvinyl alcohol film was sucked out, and the film was left in a thermohygrostat for 24 hours at 23°C and 50% RH. Finally, a polyvinyl alcohol film of 5 cm in MD x 10 cm in TD was cut out from the stretched part.

Measurement conditions:
First, the stretched polyvinyl alcohol film was dried and dehydrated at 105°C for 10 minutes, and weighed after drying (W3). Next, the polyvinyl alcohol film was stirred in 30°C/2000 ml pure water with a stirrer (115-120 rpm) for 5 minutes to precipitate glycerin and surfactant, and after completion, the water on the surface of the polyvinyl alcohol film was dehydrated, placed in a dryer and completely dried at 105°C for 1 hour, and weighed (W4). Then, the residual amount of plasticizer after swelling is (W3-W4)/W3 x 100%. The residual amount of the surfactant after swelling/stretching is added to the residual amount of the plasticizer after swelling/stretching to obtain the residual amount of additive after swelling/stretching.

2-3. FTIR peak ratio analysis (3200 cm ^-1 /1425 cm ^-1 )
The method of preparing the sample was as follows: first, the polyvinyl alcohol film was divided into 5 equal parts along the transverse direction, and the center of the polyvinyl alcohol film after the division was cut. The cut area of each piece was 20 cm in MD x 15 cm in TD. Then, the film was left in a thermohygrostat for 24 hours at 23°C and 50% RH. Next, the polyvinyl alcohol film (5 cm in MD x 15 cm in TD) was fixed, and the polyvinyl alcohol film was stretched to twice its length at 4.3 cm/min in pure water at 30°C. After completion, the moisture on the surface of the polyvinyl alcohol film was sucked out, and the film was left in a thermohygrostat for 24 hours at 23°C and 50% RH. Finally, a polyvinyl alcohol film of 5 cm in MD x 10 cm in TD was cut out from the stretched part.

The analytical instrument used here was a Perkin Elmer Spectrum 100. The analytical conditions were that the sample was left in a thermo-hygrostat and then FTIR analysis was performed in ATR transmission mode with 4 integration times in the 650-4000 cm ^-1 range. The FTIR peak intensity ratio was measured as [100-(3200 cm ^-1 transmission value)]/[100-(1425 cm ^-1 transmission value)], and the average value was calculated for the same FTIR peak ratio in the TD direction.

2-4. FTIR peak ratio analysis (1140 cm ^-1 /1425 cm ^-1 )
The method of preparing the sample was as follows: first, the polyvinyl alcohol film was divided into 5 equal parts along the transverse direction, and the center of the polyvinyl alcohol film after the division was cut. The cut area of each piece was 20 cm in MD x 15 cm in TD. Then, the film was left in a thermohygrostat for 24 hours at 23°C and 50% RH. Next, the polyvinyl alcohol film (5 cm in MD x 15 cm in TD) was fixed, and the polyvinyl alcohol film was stretched to twice its length at 4.3 cm/min in pure water at 30°C. After completion, the moisture on the surface of the polyvinyl alcohol film was sucked out, and the film was left in a thermohygrostat for 24 hours at 23°C and 50% RH. Finally, a polyvinyl alcohol film of 5 cm in MD x 10 cm in TD was cut out from the stretched part.

The analytical instrument used here was a Perkin Elmer Spectrum 100. The analytical conditions were that the sample was left in a thermo-hygrostat and then FTIR analysis was performed in ATR transmission mode with 4 integration times in the 650-4000 cm ^-1 range. The FTIR peak intensity ratio was measured as [100-(1140 cm ^-1 transmission value)]/[100-(1425 cm ^-1 transmission value)], and the average value was further calculated for the same FTIR peak ratio in the TD direction.

2-5. Measurement of color expression The sample preparation method is, first, a process of preparing a polyvinyl alcohol film into a polarizing film. The method of preparing a polyvinyl alcohol film into a polarizing film is not particularly limited, but preferably includes a swelling treatment in which the polyvinyl alcohol film is put in water and stretched, a dyeing treatment in which the film is dyed with a dichroic dye, and a stretching treatment in which the film body is uniaxially stretched. Among these, methods such as boric acid crosslinking treatment, fixing treatment, washing treatment, and heating treatment may be further performed as necessary. In this case, the order of each treatment is not particularly limited, but preferably, the swelling treatment, dyeing treatment, and stretching treatment are performed in this order.

The measuring instrument used here was a Perkin Elmer Lambda 365 (equipped with an integrating sphere). The measurement conditions were that with an integrating sphere, absorbance (A) at a wavelength of 480 nm and absorbance (B) at a wavelength of 700 nm were measured under orthogonal conditions. Finally, the ratio of A/B was taken as the hue ratio data.

2-6. Measurement of Polarization Expression The sample preparation method is a process of preparing a polyvinyl alcohol film into a polarizing film, and the process is generally as follows. First, the polyvinyl alcohol film prepared according to the above-mentioned method was prepared into a test piece of 9 cm in the longitudinal direction x 10 cm in the width direction. Next, both ends of the test piece along the longitudinal direction were fixed to a stretching jig so that the size of the stretched part was 5 cm in the longitudinal direction x 10 cm in the width direction, and the test piece was immersed in water at a temperature of 30°C for 70 seconds or less, and uniaxially stretched along the longitudinal direction at a stretching speed of 4.3 cm/min until it was twice its original length. The test piece was immersed in an iodine/potassium iodide aqueous solution containing 0.03 mass percent iodine and 3 mass percent potassium iodide and having a temperature of 30°C, and uniaxially stretched along the longitudinal direction (second stage stretching) at a stretching speed of 24 cm/min within 60 seconds until it was 3.3 times its original length. The test piece was then immersed in a boric acid/potassium iodide aqueous solution containing 3 mass percent boric acid and 3 mass percent potassium iodide at a temperature of 30° C., and uniaxially stretched along the longitudinal direction at a stretching speed of 24 cm/min to 3.6 times the original length within about 20 seconds (third-stage stretching). The test piece was then immersed in a boric acid/potassium iodide aqueous solution containing 4 mass percent boric acid and about 5 mass percent potassium iodide at a temperature of 58° C., and uniaxially stretched along the longitudinal direction at a stretching speed of 24 cm/min to 5.5 times the original length (fourth-stage stretching). Finally, the test piece was immersed in a potassium iodide aqueous solution containing 1.5 mass percent boric acid and 3 mass percent potassium iodide for 10 seconds to perform a fixing treatment, and then dried in a dryer at 60° C. for 4 minutes to obtain a polarizing film.

The measuring instrument used here is Perkin Elmer Lambda 365. The measurement conditions are based on the standard method of JIS Z 8722, and a luminosity correction is performed in the visible light region at 2° using a C light source. Then, two polarizing films are stacked with the same orientation direction, and the light transmittance (H ₀ ) is measured under a wavelength. Separately, two polarizing films are stacked with the orientation direction perpendicular to the film, and the light transmittance (H ₉₀ ) is measured under a wavelength. Finally, the polarization degree data is calculated using the formula: polarization degree=[(H ₀ -H ₉₀ )/(H ₀ +H ₉₀ )] ^1/2 .

3. Data Contents of Examples and Comparative Examples First, the variables were set for three items during the polyvinyl alcohol film preparation process of the examples and comparative examples, namely, the first heating roller, the maximum temperature of the dryer, and the temperature difference between the rearmost heating roller and the first section dryer, and related analysis or measurement data were further obtained. Please refer to Table 1 for details.

As can be seen from the results, when the residual amount of additives after swelling and stretching of the polyvinyl alcohol film of Examples 1 to 11 was adjusted to a certain range, the prepared polarizing film had good color expression. In contrast, the residual amount of additives after swelling and stretching of the polyvinyl alcohol film of Comparative Examples 1 to 4 was greater than 3.00 wt% or less than 0.50 wt%, and the prepared polarizing film had poor color expression. In addition, the FTIR intensity ratio value of OH (3200 cm ^-1 ) / CH ₂ (1425 cm ^-1 ) of Comparative Examples 1 to 3 was greater than 1.00, indicating that the greater the residual amount of plasticizer in the polyvinyl alcohol film, the worse the color expression. The FTIR intensity ratio value of OH (3200 cm ^-1 ) / CH ₂ (1425 cm ^-1 ) of Comparative Example 4 was less than 0.70, indicating that the color expression was also poor when the residual amount of plasticizer in the polyvinyl alcohol film was too low.

Next, during the polyvinyl alcohol film preparation process of the examples and comparative examples, variables were controlled for two items, namely the polyvinyl alcohol dissolution time and the polyvinyl alcohol resin concentration after dissolution, and further related analytical or measurement data was obtained. Please refer to Table 2 for details.

As shown in Table 2, the inventors have noticed that the higher the crystallinity of the polyvinyl alcohol film, the more likely it is that some undissolved polyvinyl alcohol crystalline regions will be present during preparation of the polarizing film, and the polyvinyl alcohol crystalline regions will cause iodine to swell and not enter the film, resulting in a decrease in the degree of polarization. For example, Examples 8 and 9 and Comparative Examples 1 to 4 have high FTIR intensity ratios at 1140 cm ^-1 /1425 cm ^-1 (greater than 1.45), indicating that the crystallinity of the polyvinyl alcohol is high, and the degree of polarization does not reach 99.9%. In contrast, Examples 1 to 7, 10, and 11 all have FTIR intensity ratios at 1140 cm ^-1 /1425 cm ^-1 of 1.45 or less, indicating that the degree of polarization of the prepared polarizing films all reached 99.9%.

According to Examples 1 to 11 in Table 2, the polyvinyl alcohol dissolution time in Example 9 and Comparative Examples 2 to 4 is 150 minutes, which is outside the range of 180 to 300 minutes in the other Examples. The polyvinyl alcohol resin concentration after dissolution in Example 8 and Comparative Examples 1 and 3 is 35%, which is outside the range of 25 to 30%. The inventors have noticed that the control of the above variables causes the FTIR peak intensity ratio of 1140 cm ^-1 /1425 cm ^-1 to deviate from the ideal range.

In view of the above results, the inventors have found that if the residual amount of additives after swelling and stretching of the polyvinyl alcohol film is controlled within a certain range, the polarizing film prepared thereby will have good color expression. In the preparation process of the polyvinyl alcohol film, three variables, namely, the temperature of the first heating roller, the maximum temperature of the dryer, and the temperature difference between the rearmost heating roller and the first section dryer during the preparation process of the polyvinyl alcohol film, can affect the residual amount of additives in the polyvinyl alcohol film and can also affect the optical properties of the optical film prepared therefrom. In addition, by controlling the division result of the average residual amount of additives in the polyvinyl alcohol film and the initial content of additives, the FTIR peak ratio (3200 cm ^-1 /1425 cm ^-1 ), and the FTIR peak ratio (1140 cm ^-1 /1425 cm ^-1 ), it can also be used to prepare polyvinyl alcohol films with other ideal properties. Furthermore, the present inventors have also found that the polyvinyl alcohol dissolving time and the polyvinyl alcohol-based resin concentration after dissolution affect the crystallinity of the polyvinyl alcohol film and the polarization degree of the optical film prepared therefrom.

In summary, the polyvinyl alcohol film provided by the present invention based on the above-mentioned specifications has a low degree of crystallinity, and the optical film prepared thereby has excellent color expression and polarization degree. Furthermore, the definition of the present invention can be used to more accurately manufacture, identify and select the above-mentioned polyvinyl alcohol film.

All ranges provided herein are meant to include each specific range within the assigned range and any combination of subranges between the assigned ranges. Additionally, unless otherwise stated, all ranges provided herein include the endpoints of the range. Thus, the range 1-5 specifically includes 1, 2, 3, 4, and 5, as well as subranges such as 2-5, 3-5, 2-3, 2-4, 1-4, etc.

All publications and patent applications referenced in this specification are hereby incorporated by reference, and each and every publication or patent application is expressly and individually indicated to be incorporated by reference into this specification for all purposes. In the event of a conflict between this specification and any publication or patent application incorporated by reference into this specification, this specification controls.

110 Dissolution tank 120 Casting drum 130 Heating roller 140 Dryer M Preformed film S100 to S104 Processes

Claims (15)

A polyvinyl alcohol film having an additive residual amount of 0.50 to 3.00 wt % and a water content of 1.0 to 5.0 wt % after swelling and stretching as defined below:
The swelling and stretching was carried out by leaving the polyvinyl alcohol film at 23° C. and 50% relative humidity (RH) for 24 hours, stretching the film to twice its original length at a speed of 4.3 cm/min in pure water at 30° C., and then leaving the film at 23° C. and 50% relative humidity (RH) for 24 hours .
The residual amount of additives was calculated by the formula: (W3-W4)/W3x100% (W3 is the weight of the polyvinyl alcohol film after it is swollen and stretched, and dried at 105°C for 10 minutes , and W4 is the weight of the polyvinyl alcohol film after it is swollen and stretched, stirred in 30°C/2000 ml of pure water at 115 to 120 rpm for 5 minutes, and dried at 105°C for 1 hour ). The polyvinyl alcohol film according to claim 1, wherein, when the polyvinyl alcohol film is swollen, stretched, and dried and then analyzed by Fourier transform infrared spectroscopy, the ratio of intensity at a wavelength of 3200 cm ^- 1 to intensity at a wavelength of 1425 cm ^-1 is 0.70 to 1.00. The polyvinyl alcohol film according to claim 1, wherein, when the polyvinyl alcohol film is swollen, stretched, and dried and then analyzed by Fourier transform infrared spectroscopy, the ratio of intensity at a wavelength of 1140 cm ⁻¹ to intensity at a wavelength of 1425 cm ⁻¹ is 1.20 to 1.48. The polyvinyl alcohol film according to claim 1, wherein, when the polyvinyl alcohol film is swollen, stretched, and dried and then analyzed by Fourier transform infrared spectroscopy, the ratio of intensity at a wavelength of 1140 cm ⁻¹ to intensity at a wavelength of 1425 cm ⁻¹ is 1.20 to 1.45. The polyvinyl alcohol film of claim 1, wherein the additives include a plasticizer and a surfactant. The polyvinyl alcohol film of claim 1, wherein the polyvinyl alcohol film has an initial additive content of 6 to 15 wt %. The polyvinyl alcohol film according to claim 1, wherein the degree of polymerization of the polyvinyl alcohol film is 1800 to 3000. An optical film produced from the polyvinyl alcohol film according to any one of claims 1 to 7. The optical film according to claim 8, which is a polarizing film. The optical film according to claim 8, wherein the film has a polarization degree of 99.8% or more. The optical film according to claim 8, wherein the ratio of the absorbance at a wavelength of 480 nm to the absorbance at a wavelength of 700 nm under orthogonal conditions is 1.40 to 1.60. A method for producing the polyvinyl alcohol film according to any one of claims 1 to 7, comprising the steps of:
(a) stirring and heating a polyvinyl alcohol based resin, a plasticizer, a surfactant, and water to a dissolution temperature of greater than 100° C. to form a polyvinyl alcohol casting solution;
(b) casting the polyvinyl alcohol casting solution onto a casting drum and drying to form a preformed film;
(c) contacting the preformed film with n heated rollers whose temperatures are gradually decreased from high to low, and then entering a multi-section dryer for drying;
Including,
A method for producing a polyvinyl alcohol film, wherein the temperature of the first heating roller is 84-92°C, the maximum temperature of the dryer is not more than 115°C, and the temperature difference between the nth heating roller and the first section dryer is not more than 30°C, and n is between 10 and 20. The method of claim 12, wherein the dissolution temperature in step (a) is 120 to 140°C. The manufacturing method according to claim 12, wherein the polyvinyl alcohol resin concentration in the polyvinyl alcohol casting solution in step (b) is 20.0 to 40.0%. The manufacturing method according to claim 12, wherein the plasticizer is 3 to 30 parts by weight per 100 parts by weight of the polyvinyl alcohol resin.