PVA optical film and its preparation method, and high-transmittance, high-polarization polarizer

Abstract

This invention discloses a PVA optical film, its preparation method, and a high-transmittance, high-polarization polarizer, belonging to the field of PVA optical film manufacturing technology. This invention achieves this by controlling the in-plane refractive index difference ΔN of the PVA optical film. xy 1×10 ‑4 ~20×10 ‑4 The equilibrium swelling degree is 180% to 220%, and the average swelling rate is 90% to 110% / min. This gives the PVA optical film a pre-oriented molecular chain structure and increases the number of physical entanglement points in the crystal. Compared with the method of increasing the molecular weight of PVA optical film, the PVA optical film of the present invention can form more oriented polyiodide ions during the downstream stretching process of preparing polarizers, increasing the number of oriented polyiodide ions, reducing the content of random molecular chains, and reducing the orientation relaxation caused by molecular chain movement. This can simultaneously and greatly improve the polarization degree and transmittance of the polarizer.

Description

Technical Field

[0001] This invention relates to the field of PVA optical film manufacturing technology, and in particular to a PVA optical film, its preparation method, and a high-transmittance, high-polarization polarizer. Background Technology

[0002] PVA optical film, as a core raw material for new displays, imparts polarization properties to polarizers through iodine dyeing and stretching, enabling the optical display functions of new display panels. With the increasing demands for high image quality and energy efficiency in new displays, the performance requirements for high-transmittance, high-polarization polarizers are gradually rising. Specifically, the transmittance requirement for polarizers will increase to 44%, while the polarization degree requirement will reach ≥99.99%. Currently, PVA optical films on the market cannot meet the performance requirements of high-transmittance polarizers.

[0003] During the processing of PVA polarizers, the PVA optical film undergoes swelling, dyeing, and stretching, resulting in highly oriented molecular chains. These chains then complex with iodine to form polyiodide ions aligned along the PVA chain orientation, which are the key structures contributing to the polarization performance of the polarizer. Simultaneously, the polarizer also contains randomly arranged molecular chains and polyiodide ions. This structure cannot achieve polarization absorption of light and instead leads to a decrease in the polarizer's transmittance. Therefore, the key to improving the transmittance and polarization degree of the polarizer lies in increasing the content of oriented molecular chains (oriented polyiodide ions) and reducing the content of randomly arranged molecular chains.

[0004] Existing patents have proposed achieving the above-mentioned technical requirements by increasing the molecular weight. Although this method can improve polarization and transmittance to some extent, it is still a process of stretching the PVA optical film molecular chains from disorder to order. The number of oriented molecular chains formed is limited, and the improvement on the performance of the polarizer is limited. On the other hand, this method will greatly increase the tensile tension in the downstream stretching process, which can easily lead to film breakage. At the same time, the internal stress of the polarizer product will also be too high, and severe shrinkage will occur under high temperature and high humidity. Summary of the Invention

[0005] The main objective of this invention is to provide a PVA optical film, its preparation method, and a high-transmittance, high-polarization polarizer, thereby solving the technical problem of poor polarization degree and transmittance of polarizers prepared by PVA optical films.

[0006] To achieve the above objectives, the present invention provides a PVA optical film, wherein the in-plane refractive index difference ΔN of the PVA optical film is... xy 1×10 -4 ~20×10 -4 , where N x N is the refractive index of the PVA optical film along the casting direction. y The refractive index of the PVA optical film is perpendicular to the casting direction.

[0007] In some embodiments of the invention, the equilibrium swelling degree of the PVA optical film in water at 25°C is 180% to 220%, and the average swelling rate of the PVA optical film before swelling equilibrium in water at 25°C is 90% to 110% / min.

[0008] In some embodiments of the invention, the equilibrium swelling degree of the PVA optical film in water at 50°C is 250% to 300%.

[0009] The present invention also provides a method for preparing a PVA optical film, comprising the following steps: dissolving PVA resin and additives in a solvent to obtain a casting solution, wherein the casting solution is subjected to extrusion degassing, casting molding, pre-drying with a drying roller, and heat treatment for shaping to obtain a PVA optical film.

[0010] In some embodiments of the invention, the number of drying rollers in the pre-drying step of the drying rollers is 8 to 20.

[0011] In some embodiments of the invention, the pre-drying temperature of the drying roller is 30°C to 90°C during the pre-drying step.

[0012] In some embodiments of the invention, in the pre-drying step of the drying rollers, the number of drying rollers is set to m, m>n, n>0, m>0, satisfying: the roller speed ratio between the first n drying rollers is 100%-110%, 1≤n≤m / 2.

[0013] In some embodiments of the invention, the moisture content of the pre-dried PVA optical film obtained after the drying roller pre-drying is defined as the first moisture content, and the moisture content of the PVA optical film obtained after the heat treatment is defined as the second moisture content. The first moisture content is 8% to 15%, and the second moisture content is 0.1% to 5%.

[0014] In some embodiments of the invention, the heat treatment shaping process includes N heat treatment stages, where 2≤N≤20, and the heating temperature of each heat treatment stage first increases and then decreases along the film formation direction.

[0015] In some embodiments of the invention, the temperature of the high-temperature stage of each heat treatment stage is 100°C to 140°C, and the processing time corresponding to the high-temperature stage is 15% to 50% of the total heat treatment and shaping time Th.

[0016] In some embodiments of the invention, during the heat treatment shaping stage, the ratio of the linear velocity at the heat treatment outlet to the linear velocity at the heat treatment inlet is defined as the linear velocity ratio, which is 0.9 to 1.0.

[0017] The present invention also provides a high-transmittance, high-polarization polarizer, which is prepared by means of a PVA optical film as described above.

[0018] In some embodiments of the invention, the polarizer has a transmittance ≥44.0 and a polarization degree ≥99.990%.

[0019] The beneficial effects that this invention can achieve are:

[0020] This invention controls the in-plane refractive index difference ΔN of the PVA optical film. xy 1×10 -4 ~20×10 -4 This invention enables the PVA optical film to have a pre-oriented molecular chain structure and increases the number of physical entanglement points in the crystal. Compared with the method of increasing the molecular weight of the PVA optical film, the PVA optical film of the present invention can form more oriented polyiodide ions during the downstream stretching process of preparing the polarizer, thereby increasing the number of oriented polyiodide ions, reducing the content of random molecular chains, and reducing the orientation relaxation caused by molecular chain movement. This can simultaneously and greatly improve the polarization degree and transmittance of the polarizer. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the preparation process of a PVA optical film according to an embodiment of the present invention.

[0023] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0024] It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0026] In this invention, descriptions involving "first," "second," etc., are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. Furthermore, the technical solutions of the various embodiments can be combined with each other, but only on the basis of being achievable by those skilled in the art. If the combination of technical solutions is contradictory or impossible to implement, such a combination should be considered non-existent and not within the scope of protection claimed by this invention.

[0027] Existing technologies propose to prepare PVA polarizers with high transmittance and high polarization by increasing the molecular weight of PVA optical films. Although this method can improve polarization and transmittance to some extent, it is still a process of stretching the PVA optical film molecular chains from disorder to order, resulting in a limited number of oriented molecular chains and a limited improvement in polarizer performance. On the other hand, this method will greatly increase the tensile tension in the downstream stretching process, which can easily lead to film breakage. At the same time, the internal stress of the polarizer product will also be too high, and severe shrinkage will occur under high temperature and high humidity.

[0028] In view of this, the present invention provides a PVA optical film for high-transmittance, high-polarization polarizers, wherein the in-plane refractive index difference ΔN of the PVA optical film is... xy 1×10 -4 ~20×10 -4 , where N x N represents the refractive index of the PVA optical film along the casting direction. y is the refractive index of the PVA optical film perpendicular to the casting direction.

[0029] In this invention, ⊿N xy A larger value indicates a higher degree of pre-orientation of the PVA optical film, which is more conducive to the formation of oriented molecular chains during the stretching process of polarizer fabrication. However, ΔN xy Too high a value can increase downstream tensile stress, while too low a value will result in insignificant formation of oriented molecular chains during the stretching process. Therefore, this invention controls ΔN... xy 1×10 -4 ~20×10 -4 It can be 1×10 -4 2×10 -4 3×10 -4 4×10 -4 5×10 -4 6×10 -4 7×10 -4 8×10 -4 9×10 -4 10×10 -4 11×10 -4 12×10-4 13×10 -4 14×10 -4 15×10 -4 16×10 -4 17×10 -4 18×10 -4 19×10 -4 20×10 -4 wait.

[0030] It is understood that the PVA optical film used in polarizers typically lacks molecular chain orientation before polarizer processing. The orientation is formed during polarizer processing, such as iodine dyeing and stretching, and is the primary structural source of the polarization of the polarizer. The pre-orientation mentioned in this invention refers to the formation of the orientation structure during the PVA optical film production process, prior to iodine dyeing and stretching. This allows for the formation of more oriented polyiodide ions during the stretching of the PVA optical film to prepare the polarizer, increasing the number of oriented polyiodide ions, reducing the content of random molecular chains, and minimizing orientation relaxation caused by molecular chain movement. This simultaneously and significantly improves both the polarization degree and transmittance of the polarizer.

[0031] This invention controls the in-plane refractive index difference ΔN of the PVA optical film. xy 1×10 -4 ~20×10 -4 This invention enables the PVA optical film to have a pre-oriented molecular chain structure. Compared with methods that increase the molecular weight of PVA optical film, the PVA optical film of the present invention can form more oriented polyiodide ions during the downstream stretching process of preparing polarizers, thereby increasing the number of oriented polyiodide ions, reducing the content of random molecular chains, and reducing orientation relaxation caused by molecular chain movement. This allows the polarization degree and transmittance of the polarizer to be improved simultaneously and greatly.

[0032] During the swelling process of PVA optical film, water can only enter the amorphous region. The equilibrium swelling degree represents the crystallinity of PVA optical film. When the equilibrium swelling degree is too high, the molecular chains in the stretching process of PVA optical film are prone to slippage, leading to film breakage. When the equilibrium swelling degree is too low, the material modulus is too high, the tensile tension is large, and the film is also prone to breakage at the defect.

[0033] Therefore, in some embodiments, the equilibrium swelling degree of the PVA optical film in water at 25°C is 180% to 220%, which can be 180%, 190%, 200%, 210%, 220%, etc.

[0034] In some embodiments, the equilibrium swelling degree of the PVA optical film in water at 50°C is 250%–300%, which can be 250%, 260%, 270%, 280%, 290%, 300%, etc. This is to prevent excessively high equilibrium swelling in water at 50°C, which could lead to the destruction of too many crystal physical entanglement points during the downstream stretching process of polarizer fabrication, causing excessive relaxation of the orientation molecular chains and thus reducing the number of effective orientation molecular chains in the final polarizer. Simultaneously, it avoids excessively low equilibrium swelling in water at 50°C, which could cause a significant increase in the material modulus, increasing the tensile tension during downstream processing of the polarizer and easily leading to film breakage.

[0035] The swelling rate represents the number of physical entanglement points in the PVA optical film crystal. A lower swelling rate indicates more physical entanglement points, resulting in more oriented molecular chains formed during the stretching process. However, if the swelling rate is too low, the material modulus will increase significantly, leading to high downstream tensile tension and a higher risk of film breakage. Conversely, if the swelling rate is too high, it can cause structural inhomogeneity and uneven downstream dyeing. Therefore, in some embodiments, the average swelling rate before the PVA optical film reaches swelling equilibrium in water at 25°C is 90% / min to 110% / min, which can be 90% / min, 95% / min, 100% / min, 105% / min, 110% / min, etc.

[0036] This invention also provides a method for preparing a PVA optical film, referring to... Figure 1 This includes the following steps:

[0037] S10. Dissolve PVA resin and additives in a solvent to obtain the casting solution;

[0038] S20, the casting solution is degassed by extrusion, cast film forming, pre-dried by drying rollers, and heat-treated to obtain PVA optical film.

[0039] In some embodiments, the PVA resin includes at least one of polyvinyl alcohol homopolymer and polyvinyl alcohol copolymer modifier.

[0040] In some embodiments, the raw materials for preparing polyvinyl alcohol copolymer modifiers include modified monomers, thereby obtaining PVA resins with modified groups. The modified monomers include at least one of olefins, acrylates, methacrylates, methacrylamide derivatives, vinyl esters, and halogenated ethylene compounds.

[0041] In some embodiments, the olefinic substances include at least one of ethylene and propylene.

[0042] In some embodiments, acrylates include methyl acrylate.

[0043] In some embodiments, methacrylates include methyl methacrylate.

[0044] In some embodiments, the mass content of the modified groups in the PVA resin is controlled below 10% to avoid excessive content affecting the solubility of the PVA resin and the optical performance of the final PVA optical film.

[0045] In some embodiments, the degree of polymerization of PVA resin is 1200~4000, and more specifically 2000~3500. If the degree of polymerization is too low, the stability of the molecular chain entanglement network in the prepared PVA polarizer will decrease, and the number of effectively oriented molecular chains will also decrease. If the degree of polymerization is too high, it will easily cause excessive tensile tension during the processing of PVA polarizer, resulting in increased internal stress in the product, which may lead to serious shrinkage problems in high temperature and high humidity environments.

[0046] In some embodiments, the degree of alcoholysis of PVA resin is 98%~99.9%, and more specifically 99%~99.9%. If the degree of alcoholysis of PVA resin is too low, the hydrogen bond network stability of the prepared PVA polarizer will be worse, and the film shrinkage under high temperature and high humidity will be worse.

[0047] In some embodiments, the solvent includes at least one of water and dimethyl sulfoxide.

[0048] In some embodiments, the additives include plasticizers, surfactants, antioxidants, and ultraviolet absorbers.

[0049] In some embodiments, the plasticizer includes at least one of glycerol, diglycerol, polyglycerol, ethylene glycol, and propylene glycol.

[0050] In some embodiments, the total mass of PVA resin in the casting solution is 100%, the amount of plasticizer added is 8% to 15%, which can be 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, etc., and the amount of surfactant added is 1.0% to 5.0%, which can be 1%, 2%, 3%, 4%, 5%, etc., to avoid adding too much plasticizer and surfactant, which would cause the plasticizer and surfactant to precipitate and deteriorate the optical performance of the PVA optical film.

[0051] The amount of antioxidant and UV absorber added can be determined according to actual needs. In some embodiments, with the total mass of PVA resin as 100%, the amount of antioxidant added is 0.1% to 5%, which can be 0.1%, 0.5%, 1%, 1.5%, 2%, 3%, 4%, 5%, etc., and the amount of UV absorber added is 0.1% to 5%, which can be 0.1%, 0.5%, 1%, 1.5%, 2%, 3%, 4%, 5%, etc.

[0052] In some embodiments, the surfactant includes a mixture of at least two of nonionic and anionic surfactants.

[0053] In some embodiments, the anionic surfactant includes at least one of the carboxylic acid type such as potassium laurate and dodecylbenzene sulfonate.

[0054] In some embodiments, the nonionic surfactant includes at least one of alkyl esters, alkylamines, and alkylamides.

[0055] In some embodiments, the mass fraction of the solute in the casting solution is 20% to 40%, more specifically 25% to 35%.

[0056] In some embodiments, the dissolution temperature of the casting solution is 130°C to 170°C, more preferably 145°C to 160°C, and the dissolution time of the casting solution is 2h to 10h, more preferably 4h to 7h. This is to prevent excessively high dissolution temperature or time from causing side reactions such as degradation, oxidation, or cross-linking of the PVA resin, and to prevent insufficient dissolution due to excessively short temperature or time, resulting in residual crystal points and optical defects.

[0057] In some embodiments, alkyl esters include polyoxyethylene lauryl ester.

[0058] In some embodiments, alkylamines include polyoxyethylene lauryl amino ester.

[0059] In some embodiments, alkylamides include polyoxyethylene laurylamide.

[0060] In some embodiments, the casting solution is extruded and degassed by an extruder, extruded from a die, and cast into a PVA optical film by a casting roller. The cast PVA optical film is then pre-dried by a drying roller to obtain a pre-dried PVA optical film. After that, the pre-dried PVA optical film is heat-treated in an oven to obtain a PVA optical film.

[0061] In some embodiments, the number of drying rollers in the pre-drying step is 8 to 20.

[0062] In some embodiments, during the pre-drying process, the drying temperature of each drying roller is controlled at 30°C to 90°C, which can be 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, etc.

[0063] In some embodiments, during the pre-drying process of the drying rollers, the drying temperature distribution of the multiple drying rollers gradually decreases along the forward direction of the cast PVA optical film. For example, the drying temperature of the first drying roller that contacts the cast PVA optical film is the highest, and the drying temperature of subsequent drying rollers decreases sequentially. In this embodiment, the high temperature at the front end can quickly evaporate a large amount of moisture, while the low temperature at the rear end can gently reduce the moisture content of the film to a predetermined range and prevent the drastic migration of additives such as plasticizers.

[0064] In some embodiments, the number of drying rollers is m, and the roller speed ratio at the gap of the first n drying rollers is controlled within the range of 100% to 110%. The first n gaps include the gap between the casting roller and the first drying roller in the casting process, and the gaps between the 1st and 2nd drying rollers, ..., up to the (n-1)th and nth drying rollers, where n is an integer and satisfies 1 ≤ n ≤ m / 2. In this embodiment, the roller speed ratio is defined as the ratio of the linear velocity of the downstream roller to the linear velocity of the upstream roller. Furthermore, the first drying roller that comes into contact with the cast PVA optical film after preliminary drying by the casting roller is defined as the first drying roller. A higher roller speed ratio results in a higher pre-orientation of the PVA optical film; however, an excessively high roller speed ratio can lead to PVA liquid film breakage, while an excessively low roller speed ratio can result in insufficient pre-orientation and an in-plane refractive index difference ΔN. xy Too small a speed ratio is not ideal; a better balance can be achieved by controlling the roller speed ratio at 100%~110%.

[0065] In some embodiments, the water content of the pre-dried PVA optical film obtained after the drying roller pre-drying is defined as the first water content, which is 8% to 15%, and can be 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, etc.

[0066] In some embodiments, the heat treatment shaping process is carried out in multiple stages, which includes N heat treatment stages, where 2≤N≤20, and the heating temperature of each heat treatment stage first increases and then decreases along the film production direction.

[0067] In some embodiments, the process parameters for the high-temperature stage of each heat treatment stage, i.e., the rising stage after the aforementioned temperature increase, satisfy the following: the temperature range is 100℃~140℃, and its duration accounts for 15%~50% of the total heat treatment setting time Th, which can be 15%, 18%, 20%, 25%, 28%, 30%, 35%, 40%, 45%, 50%, etc. This embodiment avoids excessively high film modulus due to excessively high temperature or time, which makes the film prone to breakage. It also avoids excessively low temperature or time, which reduces the stability of the crystal physical entanglement points, causing the orientation structure formed by stretching to loosen and break down, ultimately leading to a decrease in the polarization degree and transmittance of the prepared polarizer.

[0068] To ensure the heat setting of the pre-oriented structure, in some embodiments, during the heat treatment setting stage, the ratio of the linear velocity at the heat treatment outlet to the linear velocity at the heat treatment inlet is defined as the linear velocity ratio. This ratio is controlled between 0.9 and 1.0, and can be values ​​such as 0.9, 0.92, 0.95, 0.96, 0.98, 0.99, or 1.0. A higher linear velocity ratio is beneficial for stabilizing more pre-oriented molecular chains, but an excessively high ratio can lead to excessive intrafilm tension, resulting in macroscopic defects such as tension lines, affecting the uniformity of subsequent dyeing. Conversely, a too-low linear velocity ratio causes the pre-oriented molecular chains to relax at high temperatures, leading to a decrease in the in-plane refractive index difference ΔNxy, resulting in a reduction in the pre-oriented structure in the final PVA optical film product.

[0069] It is understandable that the linear velocity at the heat treatment inlet refers to the linear velocity at which the film moves forward when it first enters the heat treatment setting stage, and the linear velocity at the heat treatment outlet refers to the linear velocity at which the film moves forward when it leaves the heat treatment setting stage, and the linear velocity at which the film moves forward when it leaves the heat treatment setting stage.

[0070] In some embodiments, the water content of the shaped PVA optical film obtained after heat treatment and shaping is set as the second water content. The second water content is 0.1% to 5%. This avoids the difficulty of dyeing during the polarizer processing due to excessively low water content, while also avoiding the increase in equilibrium swelling degree at 20°C, the decrease in the number of physical entanglement points of the crystal, and the decrease in the number of oriented molecular chains formed by stretching due to excessively high water content.

[0071] The present invention also provides a polarizer and a method for preparing the same. The preparation method includes the following steps: using the PVA optical film provided by the present invention as a material, the PVA optical film is swollen, dyed, and stretched to obtain a polarizer.

[0072] In some embodiments, the transmittance of the polarizer is ≥44.0%, which can be 44.0%, 45.0%, 48.0%, 49.0%, 50.0%, 51.0%, 52.0%, 55.0%, etc.

[0073] In some embodiments, the polarization degree of the polarizer is ≥99.990%.

[0074] The polarizer prepared by the PVA optical film provided by this invention has the advantages of high transmittance and high polarization.

[0075] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following specific embodiments are only used to explain the present invention and are not intended to limit the present invention.

[0076] Example 1

[0077] A mixture of 32 kg of PVA resin (degree of polymerization 2400, degree of hydrolysis 99.8%), plasticizer glycerin, surfactant sodium dodecylbenzenesulfonate, antioxidant, and UV absorber, along with 68 L of deionized water, was added to a reactor and stirred at 150 °C and 0.4 MPa for 6 hours to dissolve, yielding a PVA casting solution with a solute content of 32%. Specifically, based on a total PVA resin mass of 100%, the added amounts were: plasticizer 12%, surfactant 2.2%, antioxidant 2.5%, and UV absorber 2.5%.

[0078] The casting liquid is degassed by an extruder, extruded from a die, dried by a casting roller, and then pre-dried by 10 drying rollers. The speed ratio between the first drying roller and the casting roller, as well as between the first to fifth drying rollers, is 100%, thereby controlling the first moisture content to be 15%. After that, it enters an oven for heat treatment and shaping. The highest temperature of the high-temperature stage of the heat treatment is 100°C, and the ratio of the heat treatment outlet to the inlet linear speed is 0.9. Finally, it is wound up to obtain a PVA optical film, and the second moisture content is controlled to be 5%.

[0079] Example 2

[0080] This embodiment is prepared according to the method and conditions of Example 1, with the only difference being:

[0081] The speed ratio between the first drying roll and the casting roll, as well as between the first to fifth drying rolls, is 110%. The first moisture content is 8%. The highest temperature of the heat treatment section is 140°C. The ratio of the heat treatment outlet to the inlet linear speed is 1.0. Finally, the PVA optical film is obtained by winding. The second moisture content is 0.5%.

[0082] Comparative Example 1

[0083] This comparative example was prepared according to the method and conditions of Example 2, except that the speed ratio between the first drying roll and the casting roll, and between the first to the fifth drying rolls, was 99%.

[0084] Comparative Example 2

[0085] This comparative example was prepared according to the method and conditions of Example 2, with the only difference being:

[0086] The speed ratio between the first drying roll and the casting roll, as well as between the first to fifth drying rolls, is 110%. The highest temperature in the high-temperature stage of heat treatment is 135°C. The speed ratio between the heat treatment outlet and inlet is 1.1. Finally, the film is wound up to obtain a PVA optical film with a second moisture content of 1%.

[0087] Comparative Example 3

[0088] This comparative example was prepared according to the method and conditions of Example 2, with the only difference being:

[0089] The speed ratio between the first drying roll and the casting roll, as well as between the first to fifth drying rolls, is 105%. The first moisture content is 20%. The highest temperature in the high-temperature stage of heat treatment is 135°C. The linear speed ratio between the heat treatment outlet and the inlet is 0.9. Finally, the PVA optical film is obtained by winding. The second moisture content is 1.5%.

[0090] Comparative Example 4

[0091] This comparative example was prepared according to the method and conditions of Example 2, with the only difference being:

[0092] The speed ratio between the first drying roll and the casting roll, as well as between the first to fifth drying rolls, is 105%. The highest temperature in the high-temperature stage of heat treatment is 90°C. The linear speed ratio between the heat treatment outlet and the inlet is 0.9. Finally, the film is wound up to obtain a PVA optical film with a second water content of 1%.

[0093] Comparative Example 5

[0094] This comparative example was prepared according to the method and conditions of Example 2, with the only difference being:

[0095] The speed ratio between the first drying roll and the casting roll, as well as between the first to fifth drying rolls, is 105%. The linear speed ratio between the heat treatment outlet and the inlet is 0.9. Finally, the PVA optical film is obtained by winding, with a first moisture content of 16% and a second moisture content of 8%.

[0096] Performance testing

[0097] 1. Test methods for swelling degree, swelling rate, etc.

[0098] Cut film samples of 10mm × 200mm size and swell them in 500mL of water at a specific temperature (25℃ and 50℃ as required) for different times T. Centrifuge at 3000 rpm for 5 minutes and weigh the samples. T1 Dry in a 120℃ forced-air oven for 2 hours, then remove and weigh (m). T2 ,

[0099] Degree of swelling at different times = (m T1 / m T2 ) × 100%

[0100] Statistically analyze the degree of swelling at different times and record the time t for swelling equilibrium.

[0101] Equilibrium swelling degree = (m t1 / m t2 ) × 100%

[0102] Average swelling rate = equilibrium swelling degree / t

[0103] 2. In-plane refractive index difference test method

[0104] Cut a 50mm×50mm thin film sample, place it at the detection point of the phase difference meter, select 550nm wavelength light for detection, and directly obtain the in-plane refractive index difference ΔNxy result after instrument detection.

[0105] 3. Laboratory preparation of stretch-stained samples

[0106] Samples were prepared using a self-made solution stretching apparatus. The films to be tested were first treated in a constant temperature and humidity chamber for 24 hours (23±1℃, 55±5%Rh). After equilibration, 60mm×40mm pieces of the PVA optical film were cut along the stretching direction for later use. The solution ratios and stretching test parameters are shown in Table 1. Staining and washing solutions were prepared as needed, with 150mL used per film for single use only and discarded afterward. The stretching solution was prepared according to the actual volume of the solution stretching apparatus. The stretching process involved uniformly stretching to the specified stretch ratio within a specified time, then fixing the length and removing the film. The remaining steps involved immersing the film in a liquid container at the corresponding temperature for the specified time. After completion, the film was dried in a 65℃ forced-air oven for 5 minutes.

[0107] Table 1. Dyeing and Tensile Test Process Parameters

[0108]

[0109] 4. Test methods for optical performance

[0110] The optical properties of the self-made stretched and dyed film samples were tested using a phase difference meter. 550nm wavelength light was selected for detection, and the polarization degree and transmittance of the samples were obtained directly after the test.

[0111] Table 2. Comparison of process parameters and test results for different embodiments and comparative samples

[0112]

[0113] As can be seen from Tables 1 and 2, the PVA optical film ΔN in the embodiments xy 1×10 4 ~20×10 4 The equilibrium swelling degree at 25℃ is 180%~220%, and the swelling rate is 90% / min~110% / min. The equilibrium swelling degree at 50℃ is 250%~300%. The polarization degree and transmittance of PVA polarizers prepared using this PVA optical film are improved.

[0114] Furthermore, the polarizers prepared by dyeing and stretching the PVA optical films obtained from the casting solution in Examples 1 and 2 after casting, drying, and heat treatment exhibited relatively good transmittance and polarization degree. In Comparative Examples 1, 2, 3, 4, and 5, the processing parameters exceeded the scope of the present invention, making it impossible to prepare the desired PVA optical film with a pre-oriented molecular chain structure. The polarizers prepared from these PVA optical films had poor transmittance and polarization degree.

[0115] The above are merely preferred embodiments of the present invention and do not limit the scope of the patent. Any equivalent structural or procedural transformations made based on the description and drawings of the present invention, or direct or indirect applications in other related technical fields, are similarly included within the scope of patent protection of the present invention.

Claims

1. A method for preparing a PVA optical film, characterized in that, Includes the following steps: PVA resin and additives are dissolved in a solvent to obtain a casting solution. The casting solution is then subjected to extrusion degassing, casting molding, pre-drying with a drying roller, and heat treatment to obtain a PVA optical film. In the pre-drying step of the drying rollers, let the number of drying rollers be m, m>n, n>0, m>0, satisfying: the roller speed ratio between the first n drying rollers is 100%-110%, 1≤n≤m / 2; The moisture content of the pre-dried PVA optical film obtained after the drying roller pre-drying is defined as the first moisture content, and the moisture content of the PVA optical film obtained after the heat treatment and shaping is defined as the second moisture content. The first moisture content is 8%~15%, and the second moisture content is 0.1%~5%. The heat treatment shaping process includes N heat treatment stages, where 2≤N≤20, and the heating temperature of each heat treatment stage first increases and then decreases along the film formation direction. The high-temperature stage of each heat treatment stage has a heating temperature of 100℃~140℃, and the processing time corresponding to the high-temperature stage is 15%~50% of the total heat treatment and setting time Th. During the heat treatment shaping stage, the ratio of the linear velocity at the heat treatment outlet to the linear velocity at the heat treatment inlet is defined as the linear velocity ratio, which is 0.9 to 1.

0.

2. The method for preparing the PVA optical film according to claim 1, characterized in that, In the pre-drying step of the drying rollers, the number of drying rollers is 8 to 20; And / or, the pre-drying temperature of the drying roller is 30℃~90℃.

3. A PVA optical film, characterized in that, The PVA optical film is prepared by the method for preparing PVA optical film as described in claim 1 or 2; The in-plane refractive index difference ΔN of the PVA optical film xy 1×10 -4 ~20×10 -4 , where N x N is the refractive index of the PVA optical film along the casting direction. y The refractive index of the PVA optical film is perpendicular to the casting direction.

4. The PVA optical film according to claim 3, characterized in that, The equilibrium swelling degree of the PVA optical film in water at 25°C is 180% to 220%, and the average swelling rate of the PVA optical film before swelling equilibrium in water at 25°C is 90% to 110% / min. And / or, the equilibrium swelling degree of the PVA optical film in water at 50°C is 250% to 300%.

5. A high-transmittance, high-polarization polarizer, characterized in that, The high-transmittance, high-polarization polarizer is prepared using the PVA optical film described in claim 3 or 4.

6. The high-transmittance, high-polarization polarizer according to claim 5, characterized in that, The polarizer has a transmittance of ≥44.0 and a polarization degree of ≥99.990%.

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