TWI747761B - Polyvinyl alcohol film, its manufacturing method and optical film thereof - Google Patents

Abstract

The present invention relates to a polyvinyl alcohol (PVA) film, its manufacturing method and an optical film thereof. After boric acid treatment, the PVA film has a melting point (Tm) ＞210˚C and ΔTm ＜ 3.2 ˚C of any two sites of the PVA film. The PVA film having such characteristics is uniformly colored during stretching and dyeing with no uneven dyeing or color patches or spots, addressing the defects of manufacturing optical film (made by polyvinyl alcohol film) and improving the yield of manufacture of optical film.

Description

Polyvinyl alcohol film, its preparation method and optical film using it

The present invention relates to a polyvinyl alcohol (PVA) film, which can be used as an optical film, especially a polarizing film.

Polyvinyl alcohol (PVA) film is a hydrophilic polymer with properties such as transparency, mechanical strength, water solubility, and good processability. It has been widely used in packaging materials or optical films for electronic products, especially polarized light. membrane.

When making a polarizing film, the PVA film is stretched and dyed in a boric acid solution. The dye molecules diffuse into the PVA molecules of the PVA film and arrange them regularly, so that the polarizing film can absorb parallel to its arrangement direction. Light component, and allow the light component in the vertical direction to pass through and produce a polarization characteristic.

A good polarizing film has the characteristics of uniform color, less color spots and no wrinkles, and can provide better optical properties. In order to improve the optical properties of the polarizing film, the prior art will change the polyvinyl alcohol structure or add functional groups (such as cationic groups) to change the viscosity or the degree of saponification, thereby improving the optical properties.

Although the prior art has provided a way to improve the optical properties of the PVA film when it is made into a polarizing film, when the PVA film is used to prepare a large-size optical film in the prior art, uneven dyeing, color patches and stains, or The phenomenon of uneven color during stretching increases the difficulty of preparing optical films.

Therefore, in order to solve the above-mentioned problems, the object of the present invention is to provide a polyvinyl alcohol (PVA) film, which is treated with 0.2 wt% boric acid and has a differential scanning calorimetry (DSC) measurement The measured melting point (Tm)>210˚C, and the melting point difference (ΔTm) between any two places of the PVA film<3.2˚C.

In a preferred embodiment, the PVA film treated with 0.2 wt% boric acid aqueous solution has a crystallization temperature (T _c ) measured by DSC<153.5˚C.

_{In a preferred embodiment, the crystallization temperature difference (ΔT c} ) between any two places of the PVA film treated with 0.2 wt% boric acid aqueous solution is less than 3.85˚C.

In a preferred embodiment, the PVA film has a hysteresis standard deviation of less than 3 nm.

In a preferred embodiment, 0.12 g of the PVA film is treated with a 0.2 wt% boric acid aqueous solution and dissolved in water, and the boron content of the aqueous solution is 16 to 19 ppm.

Another object of the present invention is to provide an optical film made from the PVA film of the present invention.

In a preferred embodiment, the optical film is a polarizing film.

In a preferred embodiment, the polarizing film has a degree of polarization> 99.8.

Another object of the present invention is to provide a method for manufacturing the PVA film of the present invention, which comprises: (a) stirring a polyvinyl alcohol resin at a heating rate of 4-20˚C/hour, and the dissolution temperature is >100˚C After that, the temperature is maintained for 2 to 4 hours, and the stirring direction is reversed at least twice per hour to form a polyvinyl alcohol casting solution; (b) the casting solution is cast on the casting drum; and (c) the PVA film is formed after drying .

In a preferred embodiment, the rotation rate of the casting drum is 3-7 m/min.

In a preferred embodiment, the drying in step (c) is processed by a hot roller or a floating dryer.

In a preferred embodiment, the maximum/minimum air flow rate received by the film along the width direction of the floating dryer is less than or equal to 3.0.

In a preferred embodiment, the polyvinyl alcohol casting solution further includes a plasticizer, and the plasticizer is 3 to 30 parts by weight relative to 100 parts by weight of the polyvinyl alcohol resin.

In a preferred embodiment, the content of the polyvinyl alcohol-based resin in the polyvinyl alcohol casting solution is 10 to 60% by weight.

In a preferred embodiment, the degree of polymerization of the polyvinyl alcohol-based resin is between 800 and 10,000.

Compared with the prior art, the PVA film of the present invention has uniform color after dyeing, does not produce uneven color problems such as color patches or stains, effectively improves the defects of PVA manufacturing optical film, and improves the yield rate of manufacturing optical film , Especially can be used to make large-size polarizing film.

The following embodiments should not be regarded as excessively limiting the present invention. Those with ordinary knowledge in the technical field to which the present invention belongs can modify and change the embodiments discussed herein without departing from the spirit or scope of the present invention, and still fall within the scope of the present invention.

The terms "one" and "one" in this article represent one or more than one (ie at least one) grammatical objects in this article.

In the present invention, the MD is the machine direction, that is, the longitudinal direction of the PVA film; the TD is the transverse direction, that is, the transverse direction of the PVA film.

The method for manufacturing polyvinyl alcohol (PVA) film of the present invention includes the following steps: after preparing a polyvinyl alcohol casting solution, casting the polyvinyl alcohol casting solution onto a casting drum, and drying to form a polyvinyl alcohol polymer物膜。 The film.

Specifically, the manufacturing method of the PVA film includes the following steps: in a dissolving tank, dissolving the polyvinyl alcohol resin in a solution (for example, water) to form the polyvinyl alcohol casting solution; optionally using a filter to filter The polyvinyl alcohol casting solution; subsequently, using a gear pump and a coater (for example, a T-die coater), the polyvinyl alcohol casting solution is cast onto the casting drum; finally, the drum is After the filmed PVA film is peeled off, the PVA film is obtained through a series of thermal roller and/or drying heat treatments.

The polyvinyl alcohol casting solution is configured to dissolve the polyvinyl alcohol resin in a dissolution tank. The dissolution temperature in the dissolution tank is >100˚C, preferably>110˚C, and even more>120˚C, for example, 105˚ C, 110˚C, 115˚C, 120˚C, 125˚C, 130˚C, 135˚C or 140˚C, and the present invention is not limited to these. The dissolving tank is preferably heated at a rate of 4-20˚C per hour, preferably 5-15˚C, more preferably 6-9˚C, for example 4.0˚C/hr, 5.0˚C/hr , 6.0˚C/hr, 7.0˚C/hr, 8.0˚C/hr, 9.0˚C/hr, 10˚C/hr, 11˚C/hr, 12˚C/hr, 13˚C/hr, 14 ˚C/hr, 15˚C/hr, 16˚C/hr, 17˚C/hr, 18˚C/hr, 19˚C/hr or 20˚C/hr, etc. If the heating rate is too fast, polyethylene Alcohol resins are prone to agglomeration, leading to incomplete dissolution. After heating to the desired dissolution temperature, the polyvinyl alcohol casting solution is continuously stirred for 2 to 4 hours, preferably 3 hours, and the stirring direction is changed at least 2 times, preferably 3 times every 1 hour, for example clockwise After 20 minutes of rotation, it is changed to counterclockwise rotation for 20 minutes. Reversing the direction in the above-mentioned stirring process can increase the dissolution effect and avoid clusters remaining in the polyvinyl alcohol casting solution.

The polyvinyl alcohol casting solution is configured, and the content of the polyvinyl alcohol resin is 10 to 60% by weight, preferably 15 to 40% by weight, more preferably 20 to 30% by weight, for example, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60% by weight, etc. If the content of the polyvinyl alcohol resin is insufficient, the viscosity of the polyvinyl alcohol casting solution will be too low, and the drying load will be too large, resulting in poor film-forming efficiency for preparing the PVA film. On the contrary, if the content of the polyvinyl alcohol resin is too high, the polyvinyl alcohol resin is not easy to dissolve, and clusters are likely to remain, resulting in deterioration of the uniformity of the phase difference of the PVA film and affecting the dyeing uniformity of the subsequent process.

The above-mentioned polyvinyl alcohol resin is obtained by polymerizing vinyl ester resin monomers to form a polyvinyl ester resin and then performing saponification reaction; wherein, the vinyl ester resin monomers include vinyl formate and vinyl acetate Vinyl esters such as vinyl propionate, vinyl butyrate, vinyl valerate or vinyl octanoate, and the present invention is not limited to these, preferably vinyl acetate. In addition, olefin compounds or acrylate derivatives, and copolymers formed by copolymerization of the above-mentioned vinyl ester resin monomers can also be used; the olefin compounds include ethylene, propylene, butene, etc., and the present invention is not limited thereto. Wait. The acrylate derivative includes acrylic acid, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, or n-butyl acrylate, etc., and the present invention is not limited to these.

The degree of saponification of the polyvinyl alcohol resin is 90% or more, preferably 99% or more, to obtain better optical properties, such as 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 99.1, 99.2, 99.3, 99.4, 99.5, 99.6, 99.7, 99.8, 99.9%, etc. The degree of polymerization of the polyvinyl alcohol is between 800 and 10,000, and preferably between 2200 and 10,000, such as 800, 900, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, 10000 and so on, the degree of polymerization is higher than 800 has better processing characteristics, but if the degree of polymerization is higher than 10000, it is not conducive to dissolution.

In addition to polyvinyl alcohol resins, the casting solution can also contain plasticizers to improve the processability of film formation. Plasticizers that can be used include polyols, such as ethylene glycol, diethylene glycol, and triethylene glycol. , Tetraethylene glycol, propylene glycol or glycerol, etc., and the present invention is not limited to these, preferably ethylene glycol and glycerol. The added amount of the plasticizer relative to 100 parts by weight of the polyvinyl alcohol resin is usually 3-30 parts by weight, preferably 7-20 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, 30 parts by weight, etc. If the content of the plasticizer is insufficient, the formed PVA film will easily crystallize and affect the dyeing performance of subsequent processing. On the contrary, if the content of the plasticizer is too high, the mechanical properties of the PVA film will be damaged.

The equipment used in the manufacturing method of the PVA film includes a dissolving tank, a filter, a coating machine, and a conveying pipeline connected from the dissolving tank to the front of the coating machine. The heat preservation device can be a metal electric heating wire, or a jacket containing a liquid inside, such as oil or water. By heating the liquid in the metal wire or the jacket, the equipment can maintain a uniformly heated heat preservation state, especially for equipment And the surface of the pipeline to prevent the polyvinyl alcohol in the polyvinyl alcohol casting solution from forming gels or clusters due to the loss of temperature on the surface of the equipment or pipeline. In addition, the holding temperature should not be too high, otherwise part of the polyvinyl alcohol casting solution will be dehydrated or gelled, forming a burnt yellow or black gel, which will affect the surface quality and uniformity of the PVA film after the back end is coated and formed. . The heat preservation temperature of the polyvinyl alcohol casting solution at the coating and forming part is 80~120˚C, preferably 90~110˚C, more preferably 90~100˚C, for example 80, 85, 90, 95, 100, 105, 110, 115, 120 ˚C, etc.

When the polyvinyl alcohol casting solution is cast on the casting drum, the rotation speed of the casting drum is about 3-7 m/min, preferably 4-6 m/min. When the drum speed is too slow, the casting solution is too dry, and there is a tendency for the phase difference and melting point distribution to be uneven. Conversely, when the drum speed is too fast, the casting solution is not sufficiently dried and the peelability is reduced. In addition, in a preferred embodiment, the temperature of the drum is set at 85~90 ˚C, specifically such as 85, 86, 88, 87, 88, 89, 90 ˚C, etc. If the temperature of the drum is too high, the temperature on the drum The casting solution is prone to foaming.

After the PVA film initially formed on the casting drum is peeled from the drum, it is dried to form a PVA film. The drying process can be carried out on a hot roll or a floating dryer. The number of heat rollers and floating dryers is not particularly limited, and can be adjusted as needed. However, in a preferred embodiment, the highest/lowest temperature ratio of the dry compartment (i.e. oven) is 2.0~2.4; when the compartment temperature ratio is too large, it is easy to cause uneven crystallinity, which will cause problems when the PVA film is used to make optical films. It reacts unevenly with boric acid. In addition, the temperature difference between adjacent cars is preferably 65˚C or less, 60˚C or less is better, and 50˚C is the best. If the temperature difference between adjacent cars is too large, it is easy to cause uneven phase difference distribution. In addition, there is no specific limit on the air flow rate received by the PVA film formed along the width direction (that is, perpendicular to the machine direction) in the floating dryer, but the maximum/minimum air flow rate ratio must be controlled below 3.0. For example, 0, 0.5, 1.0, 1.5, 2.0, 2.5, or 3.0. Because the place with large air volume is easy to take away the moisture, the degree of dryness is large, and the air flow ratio range can control the air volume of each position of the PVA film to be the same to achieve uniform drying. In addition, if the gas flow rate ratio is too large, the PVA film will react unevenly with boric acid and the phase difference will be uneven when the PVA film is used to make an optical film.

The "treatment with 0.2wt% boric acid aqueous solution" in the present invention refers to (a) placing about 0.1g of PVA film in 20g of pure water, so that the PVA film is completely immersed below the level of pure water, and stirring at a stirring speed of 250rpm 1 hour; (b) Take the PVA film out of pure water and place it in a 0.2wt% boric acid aqueous solution, then let it stand for 1.0 hour; and (c) After taking the PVA film out of the 0.2wt% boric acid aqueous solution, place the PVA film in The boric acid treatment was completed by standing and aging in a closed container for 16.0 hours. In the present invention, a lower concentration of boric acid is used, because if a higher concentration of boric acid is used, the areas that are originally difficult to react in the PVA film will also be reactive because of the high concentration of boric acid, so it is difficult to observe the uniformity of the film.

The PVA film of the present invention, after being treated with boric acid, has a melting point (Tm)> 210˚C measured by a differential scanning calorimetry (DSC), and the difference in melting point (ΔTm) between any two places of the PVA film <3.2 ˚C. For example, the melting point (Tm) is 211˚C, 212˚C, 215˚C, 220˚C, 225˚C or 230˚C, etc., and the present invention is not limited to these; the ΔTm at any two places of the PVA film Specific examples are 1˚C, 1.5˚C, 2˚C, 2.5˚C, 3˚C or 3.1˚C, etc., and the present invention is not limited to these.

PVA film at the time of producing the optical film, will be stretched and dyed to a polarizing film, for example, because the manufacturing process uses a polarizing film containing I ₃ ^-, I ₅ ^- boric acid solution of iodide staining PVA film, PVA with boric acid After cross-linking occurs in the amorphous region, the iodide ions are fixed to prevent the iodide ions from dissolving out. After However, PVA crosslinking with boric acid molecules, will reduce the crystallinity, the melting point will drop; when the crystallinity distribution unevenness, unevenness causes the PVA film to react with boric acid, so that I ₃ ^-, I ₅ ^- iodide Uneven fixation, which in turn leads to uneven color. Therefore, in the present invention, the PVA film is treated with boric acid to simulate the boric acid immersion used in the polarizer manufacturing process, and the use of a lower boric acid concentration (in this paper, 0.2wt%) is easier to analyze the difference of different positions, and to indicate the difference. The melting point (Tm) measured by Differential scanning calorimetry (DSC)>210˚C, and the ΔTm of any two places of the PVA film <3.2˚C, the PVA film has suitable and uniform crystallinity. When stretched and dyed, a polarizing film with uniform color can be obtained. The PVA film of the present invention can also be used to prepare other stretched and dyed optical films, and is not limited to polarizing films.

Furthermore, the PVA film treated with boric acid has a crystallization temperature (T _c ) measured by DSC<153.5˚C. For example, the crystallization temperature (T _c ) of the PVA film after boric acid treatment is 110˚C, 115˚C, 120˚C, 125˚C, 130˚C, 135˚C, 140˚C, 150˚C or 155 ˚C, etc., and the present invention is not limited to these.

_{Furthermore, the ΔT c of} any two places of the PVA film treated with boric acid is less than 3.85˚C. _{For example, the ΔT c} of any two places of the boric acid-treated PVA film is 0˚C, 0.5˚C, 1˚C, 1.5˚C, 2˚C, 2.5 C, 3˚C or 3.19˚C, etc., and The present invention is not limited to these.

In the present invention, the method steps of the DSC measurement include: as shown in Fig. 1, one of the MD positions of the PVA film is cut out along the TD direction within 40 cm from the end of the TD direction. Three test pieces with an area of 10cm*10cm in the middle and right. Next, weigh 0.1 g of the PVA film from each test piece, put it into a glass bottle and add 20 g of pure water to make the PVA film completely below the level of pure water, add a stirring stone, stir at 250 rpm, and stir at room temperature for 1.0 Hour. Subsequently, the pure water in the glass bottle was poured out, 20 g of 0.2 wt% boric acid aqueous solution was added, and after standing for 1.0 hour, the boric acid aqueous solution was poured out and the boric acid-treated PVA film was sealed in the bottle and allowed to stand and matured for 16.0 hours. Finally, take out the boric acid-treated PVA film, spread it flat on an iron plate, dry it at 105˚C for 1.0 h, take out the boric acid-treated PVA film, and analyze the DSC; DSC experiment temperature range: 30˚C~250˚C, Maintain the temperature at 30˚C and 250˚C each for 1 minute, and record the melting point (T _m ) at the first temperature _{rise, the crystallization temperature (T c} ) at the first temperature drop, and the glass transition temperature at the second temperature rise (T _g ). The DSC measuring instrument can be a general DSC instrument, such as DSC: TA Instruments DSC 25.

Further, the PVA film of the present invention has a standard deviation of hysteresis <3 nm. For example, the standard deviation of the retardation of the PVA film is 0 nm, 0.5 nm, 1.0 nm, 1.5 nm, 2.0 nm, 2.5 nm, or 2.9 nm, etc., and the present invention is not limited to these.

The retardation in the present invention refers to the amount of phase change of incident polarized light when light penetrates through the film, that is, the amount of phase retardation, in nm; the uniformity of the retardation value involves molecules Orientation uniformity and thickness uniformity have a huge impact on the subsequent optical film manufacturing process. As shown in Figure 1, the measurement of hysteresis is to cut one of the MD positions of the PVA film treated with boric acid within 40 cm from the end in the TD direction, and cut out the left, center, and right three along the TD direction. A test piece with an area of 10cm*10cm can be measured to obtain the hysteresis value and statistical data (maximum value, standard deviation, etc.) of all points in the surface.

Further, the 0.12 g of the PVA film of the present invention is treated with a 0.2 wt% boric acid aqueous solution and dissolved in water, and the boron content of the aqueous solution is 16 to 19 ppm. Specifically, for example, 16 ppm, 16.5 ppm, 17 ppm, 17.5 ppm, 18 ppm, 18.5 ppm, or 19 ppm, etc., and the present invention is not limited to these. If it is lower than the above-mentioned boron content, the fixing ability of the PVA film during subsequent dyeing is insufficient and poor; if it is higher than the above-mentioned boron content, the degree of cross-linking between boric acid and PVA is too high, which is disadvantageous to the subsequent stretching process and is poor .

In the present invention, the steps of the method for measuring the boron content include: taking out the PVA film treated with boric acid, putting it in a 30 ml glass sampling bottle after passing pure water, adding 20 g of pure water, and dissolving it fully and then using ICP- OES analyzes the concentration of boron in the solution.

The optical film of the present invention is completed by using the PVA film of the present invention. The optical film includes a polarizing film, an anti-blue film, a filter, etc., and the present invention is not limited to these. Preferably, the optical film is a polarizing film, and the polarizing film has a degree of polarization> 99.8. [ Specific Examples ]

Hereinafter, the present invention will be further described with detailed description and examples. However, it should be understood that these examples are only used to help make the present invention easier to understand and not to limit the scope of the present invention.

Example 1 to 4

Manufacturing PVA film : PVA resins with melting points (Tm) of 233.21˚C, 233.24 ˚C, 233.01 ˚C and 233.08˚C are used as the main components of the polyvinyl alcohol casting solution, and the PVA film of the present invention is used for manufacturing Methods The PVA films of Examples 1 to 4 were prepared, and the operating conditions of the manufacturing method are shown in Table 1.

Comparative example 1 to 3

Production of PVA film : The polyvinyl alcohol resins with melting points (Tm) of 233.15 ˚C and 233.17 ˚C are used as the main components of the polyvinyl alcohol casting solution. The manufacturing method of the PVA film of the present invention is used but the operating conditions are different. The PVA films of Comparative Examples 1 to 3 have the operating conditions of the manufacturing method as shown in Table 1.

Experimental example

Analysis of the properties of the PVA film: The PVA films of Examples 1 to 4 and Comparative Examples 1 to 3 were measured for the standard deviation of the hysteresis, and further treated with a 0.2wt% boric acid aqueous solution, and then the melting point (Tm) and melting point difference were measured (ΔTm), crystallization temperature (T _c ), crystallization temperature difference (ΔT _c ), boron content, the measurement results are shown in Table 2.

Preparation of polarizing film: the PVA films of Examples 1 to 4 and Comparative Examples 1 to 3 were immersed in water at about 30˚C and swelled, and then subjected to the first uniaxial stretching, stretching in the MD direction, and stretching The back length is 2.0 times the original length of the PVA film. Next, the PVA film after the first stretch was immersed in a 30˚C aqueous solution containing 0.03% by mass of iodine and 3% by mass of potassium iodide, and then uniaxially stretched for the second time, and stretched in the MD direction. The length is 3.3 times the original length of the PVA film. Next, the PVA film after the second stretching was immersed in a 30˚C aqueous solution containing 3% by mass of potassium iodide and 3% by mass of boric acid, and then uniaxially stretched for the third time, and stretched in the MD direction. The length of is 3.6 times the original length. Subsequently, the PVA film after the third stretching was immersed in a 60˚C aqueous solution containing 5 mass% potassium iodide and 4 mass% boric acid, and the fourth uniaxial stretching was performed to stretch in the MD direction. The length is 6.0 times the length of the PVA film after the third stretch. Finally, the fourth stretched PVA film was immersed in an aqueous solution containing 3% by mass of potassium iodide for 15 seconds, and then dried at 60˚C for 4 minutes to obtain a polarized film. The polarization degree and color uniformity of the polarizing films prepared in Examples 1 to 4 and Comparative Examples 1 to 3 were measured, and the measurement results are shown in Table 2.

Polarization measurement method : overlap two identical polarizing films with the same orientation direction, and measure the light transmittance (H ₀ ) at the wavelength, and when the orientation direction of the two identical polarizing films is perpendicular Measure the light transmittance (H ₉₀ ) at the wavelength, and calculate the polarization degree by the following formula: [(H ₀ -H ₉₀ )/ (H ₀ +H ₉₀ )] ^1/2 .

Color uniformity measurement method : overlap two identical polarizing films in an orthogonal manner, use a light box with light output = 14000 lx to illuminate one side, observe the color uniformity of the other side, and evaluate according to the following rules: O : No color unevenness, Δ: Faint color unevenness, X: Obvious color unevenness.

Table 1 PVA raw materials PVA film manufacturing Tm(˚C) Heating rate of dissolving tank Dissolution temperature Frequency of changing the stirring direction (times/hour) T-die temperature (˚C) Vehicle speed (m/min.) Maximum temperature difference between adjacent compartments (˚C) Maximum/minimum air flow (˚C/hour) (˚C) Example 1 233.21 6 140 3 90 4 50 1.5 Example 2 233.24 7 140 3 92 5 60 2 Example 3 233.01 8 130 3 94 6 65 2.5 Example 4 233.08 9 120 3 96 6 65 3 Comparative example 1 233.15 10 100 2 88 7 65 3.5 Comparative example 2 233.15 11 100 0 86 7 70 4 Comparative example 3 233.17 12 90 0 84 7 75 5

Table 2 PVA film Tm(˚C) Tc(˚C) Tg(˚C) ΔTm(˚C) ΔTc(˚C) ΔTg(˚C) Left middle right Left middle right Left middle right Example 1 221.19 220.07 219.81 152 149.53 148.81 91.28 92.08 92.34 1.38 3.19 1.06 Example 2 218.15 219.79 220.04 148.16 148.70 149.74 93.07 91.83 90.46 1.89 1.58 2.61 Example 3 221.41 220.21 220.71 152.01 149.33 150.11 92.16 91.71 94.66 1.2 2.68 2.95 Example 4 212.35 211.08 213.55 145.91 145.88 147.10 90.24 89.61 94.77 2.47 1.22 5.16 Comparative example 1 208.05 207.00 210.20 141.77 141.30 144.25 90.00 89.55 90.31 3.2 2.95 0.76 Comparative example 2 224.71 223.58 221.51 155.3 153.5 151.11 88.61 90.27 94.05 3.21 4.19 5.44 Comparative example 3 219.05 221.37 223.91 148.62 151.33 153.54 93.15 95.47 88.11 4.86 4.92 7.36 (Continued from Table 2) PVA film Polarizing film Boron content (ppm) Hysteresis standard deviation (nm) Polarization Color uniformity performance Left middle right Left middle right Example 1 16 18 19 1.733 1.704 1.698 99.9 99.9 99.9 O Example 2 19 19 17 1.087 1.812 1.092 99.9 99.9 99.9 O Example 3 17 18 18 2.961 2.338 2.817 99.9 99.9 99.9 O Example 4 19 19 19 1.387 1.885 1.946 99.9 99.9 99.9 O Comparative example 1 20 20 20 1.331 1.328 1.240 99.9 99.9 99.9 Δ Comparative example 2 14 15 16 2.987 3.065 3.115 99.8 99.8 99.9 X Comparative example 3 17 17 15 2.741 1.448 3.245 99.8 99.8 99.8 X (O: no color unevenness, Δ: slight color unevenness, X: obvious color unevenness)

As shown in Table 2, the melting points (Tm) of the PVA films of Examples 1 to 4 are all> 210˚C, and the ΔTm of any two places are both <3.2˚C. The color uniformity of the polarizing films made by them is The performance is good (no color unevenness). In contrast, the PVA films of Comparative Examples 1 to 3 have a melting point (Tm) greater than 210˚C, or any two ΔTm greater than 3.2˚C, and the polarizing film made by using them has the performance of color uniformity It is not good, there are slight color unevenness, obvious color unevenness, especially in Comparative Examples 2 and 3, where the crystallization temperature (T _c ) is greater than 153.5˚C, the boron content is not 16 to 19 ppm or the standard deviation of the hysteresis If it is larger than 3 nm, the color uniformity of its polarizing film is obviously poor (obvious color unevenness). Therefore, the melting point (Tm) of the PVA film in this case (Tm)>210˚C, and any two ΔTm<3.2˚C, can have color uniformity when used in the preparation of polarizing film, and it is not easy to produce color patches and stains.

In summary, the PVA film of the present invention has a melting point (Tm)>210˚C, and any two places ΔTm<3.2˚C. The color uniformity after stretching and dyeing will not produce color patches or stains. Such as the problem of uneven color, effectively improve the defects of PVA film manufacturing optical film, and improve the yield of manufacturing optical film, especially large-size polarizing film.

The present invention has been described in detail above, but what has been described above is only a preferred embodiment of the present invention, and should not be used to limit the scope of implementation of the present invention, that is, everything made in accordance with the scope of the patent application of the present invention is equal Changes and modifications should still fall within the scope of the patent of the present invention.

Figure 1 is a schematic diagram of PVA film cut out three test pieces with an area of 10cm*10cm on the left, middle and right.

Claims (15)

A polyvinyl alcohol (PVA) film, which is treated with a 0.2 wt% boric acid aqueous solution _{, and the melting point (T m} ) measured by a differential scanning calorimetry (DSC) is> 210˚C, and the _{The melting point difference (ΔT m} ) between any two places of the PVA film is less than 3.2˚C. For example, the PVA film of claim 1, after being treated with 0.2 wt% boric acid aqueous solution, has a crystallization temperature (T _c ) measured by DSC <153.5˚C. For example, the PVA film of claim 2, after being treated with 0.2 wt% boric acid aqueous solution, the crystallization temperature difference (ΔT _c ) of any two places of the PVA film is less than 3.85˚C. For example, the PVA film of any one of claims 1 to 3 has a standard deviation of hysteresis <3 nm. Such as the PVA film of claim 4, wherein 0.12 g of the PVA film is treated with a 0.2 wt% boric acid aqueous solution and dissolved in water, and the boron content of the aqueous solution is 16 to 19 ppm. An optical film made from the PVA film according to any one of claims 1 to 7. Such as the optical film of claim 6, which is a polarizing film. For the optical film of claim 7, the polarizing film has a degree of polarization> 99.8. A method for manufacturing a PVA film according to any one of claims 1 to 4, comprising: (a) Stir a polyvinyl alcohol resin at a heating rate of 4-20˚C/hour. After the dissolution temperature is> 100˚C, hold the temperature for 2 to 4 hours, and reverse the stirring direction at least twice per hour. Form polyvinyl alcohol casting solution; (b) Cast the casting solution onto the casting drum; and (c) The PVA film is formed after drying. Such as the manufacturing method of claim 9, wherein the rotation rate of the casting drum is 3~7m/min. Such as the manufacturing method of claim 10, wherein the drying in the step (c) is processed by a hot roller or a floating dryer. The manufacturing method of claim 11, wherein the maximum/minimum air flow rate received by the film along the width direction of the floating dryer is less than or equal to 3.0. The manufacturing method according to any one of claims 9 to 12, wherein the polyvinyl alcohol casting solution further contains a plasticizer, and the plasticizer is 3 to 30 parts by weight relative to 100 parts by weight of the polyvinyl alcohol-based resin. For example, please request the manufacturing method of item 13, wherein the polyvinyl alcohol-based resin content in the polyvinyl alcohol casting solution is 10 to 60% by weight. According to the manufacturing method of claim 14, wherein the degree of polymerization of the polyvinyl alcohol-based resin is between 800 and 10,000.

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