PVA (Polyvinyl Alcohol) optical film, preparation method thereof and high-transmittance and high-polarization polaroid

Abstract

The invention discloses a PVA optical film, a preparation method thereof and a high-transmittance and high-polarization polaroid, and belongs to the technical field of PVA optical film manufacturing. By controlling the in-plane refractive index difference delta Nxy of the PVA optical film to be 1 * 10 <-4 >-20 * 10 <-4 >, the equilibrium swelling degree to be 180%-220% and the average swelling rate to be 90% / min-110% / min, the PVA optical film has a pre-oriented molecular chain structure, crystal physical entanglement points are increased, and compared with a method for improving the molecular weight of the PVA optical film, the PVA optical film has the advantages that in the downstream stretching process of preparing a polaroid, the tensile strength of the PVA optical film is greatly improved, and the tensile strength of the PVA optical film is greatly improved. More oriented polyiodide ions can be formed, the number of the oriented polyiodide ions is increased, the content of random molecular chains is reduced, and orientation relaxation caused by molecular chain movement is reduced, so that the polarization degree and transmittance of the polarizer can be greatly improved at the same time.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of PVA optical film manufacturing, and particularly relates to a PVA optical film, a preparation method thereof and a high-transmittance high-polarization polarizing plate. BACKGROUND

[0002] As a new type of display core raw material, the PVA optical film is stretched by iodine dyeing to endow the polarizing plate with polarization performance and realize the optical display function of the new type of display panel. With the demand for high image quality and energy saving of the new type of display, the performance demand of the high-transmittance high-polarization polarizing plate is gradually increasing, in which the transmittance requirement of the polarizing plate is increased to 44%, and the polarization degree requirement is increased to 99.99%. The PVA optical film on the market cannot meet the performance demand of the high-transmittance polarizing plate.

[0003] In the processing of the PVA polarizing plate, the PVA optical film is swelled, dyed and stretched, the molecular chains are highly oriented, and the polyiodide ions arranged along the orientation direction of the PVA chains are formed by complexing with iodine, which is the key structure contributing to the polarization performance of the polarizing plate. At the same time, the polarizing plate also contains randomly arranged molecular chains and polyiodide ions. This structure cannot realize the polarization absorption of light, but will cause the decrease of the transmittance of the polarizing plate. Therefore, the key to improving the transmittance and polarization degree of the polarizing plate lies in increasing the content of the oriented molecular chains (oriented polyiodide ions) and reducing the content of the randomly arranged molecular chains.

[0004] The existing patent proposes to realize the above technical demand by increasing the molecular weight. This method can improve the polarization and transmittance to a certain extent, but it is still a process of stretching the PVA optical film molecular chains from disorder to order, and the formed oriented molecular chains are limited, which has limited effect on the improvement of the performance of the polarizing plate. On the other hand, this method greatly increases the stretching tension in the downstream stretching process, which easily leads to film breakage, and the internal stress of the polarizing plate product is also too large, which will cause serious shrinkage under high temperature and high humidity. SUMMARY

[0005] The main purpose of the present application is to provide a PVA optical film, a preparation method thereof and a high-transmittance high-polarization polarizing plate, which solve the technical problem of poor polarization degree and transmittance of the polarizing plate prepared by the PVA optical film.

[0006] To achieve the above purpose, the present application provides a PVA optical film, the in-plane refractive index difference of the PVA optical film is xy 1x10 -4 ~20x10 -4 , wherein N x is the refractive index of the PVA optical film along the casting direction, and N y is the refractive index of the PVA optical film perpendicular to the casting direction.

[0007] In some embodiments of the application, the PVA optical film has an equilibrium swelling degree of 180% to 220% in water at 25℃, and an average swelling rate of 90% to 110% per minute before the PVA optical film reaches the equilibrium swelling degree in water at 25℃.

[0008] In some embodiments of the application, the PVA optical film has an equilibrium swelling degree of 250% to 300% in water at 50℃.

[0009] The application 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, and subjecting the casting solution to extrusion degassing, casting, pre-drying by drying rollers, and heat treatment to obtain the PVA optical film.

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

[0011] In some embodiments of the application, the pre-drying temperature of the drying rollers in the step of pre-drying by drying rollers is 30℃ to 90℃.

[0012] In some embodiments of the application, in the step of pre-drying by drying rollers, the number of drying rollers is m, m > n, n > 0, m > 0, and the speed ratio between the first n drying rollers satisfies 100% to 110%, and 1 ≤ n ≤ m / 2.

[0013] In some embodiments of the application, the water content of the pre-dried PVA optical film obtained after the step of pre-drying by drying rollers is defined as a first water content, the water content of the PVA optical film obtained after the step of heat treatment is defined as a second water content, the first water content is 8% to 15%, and the second water content is 0.1% to 5%.

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

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

[0016] In some embodiments of the application, in the step of heat treatment, the ratio of the linear speed at the heat treatment outlet to the linear speed at the heat treatment inlet is defined as a linear speed ratio, and the linear speed ratio is 0.9 to 1.0.

[0017] The application also provides a high-transmittance and high-polarization polarizing plate, which is prepared by the PVA optical film as described above.

[0018] In some embodiments of the application, the light transmittance of the polarizing plate is greater than or equal to 44.0, and the polarization degree is greater than or equal to 99.990%.

[0019] The application can achieve the following beneficial effects: The application controls the in-plane refractive index difference of the PVA optical film xy to 1x10 -4 ~20x10 -4 The PVA optical film has a pre-oriented molecular chain structure, and the number of physical entanglement points of the crystal is increased, compared with the method of increasing the molecular weight of the PVA optical film, the PVA optical film of the application can form more oriented polyiodide ions in the downstream stretching process of preparing the polarizing plate, increase the number of oriented polyiodide ions, reduce the content of random molecular chains, and reduce the orientation relaxation caused by the movement of the molecular chains, so as to greatly improve the polarization degree and transmittance of the polarizing plate. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following briefly describes the drawings needed in the embodiments or prior art description. Obviously, the drawings described below are only some embodiments of the present application, and for those skilled in the art, other drawings can be obtained without creative labor based on the drawings shown.

[0021] Figure 1 It is an embodiment of the application PVA optical film preparation process schematic diagram.

[0022] The implementation, functional features and advantages of the application will be further described with reference to the embodiments and the accompanying drawings. DETAILED DESCRIPTION

[0023] It should be understood that the specific embodiments described herein are only used to explain the present application, and are not used to limit the present application.

[0024] The technical solutions in the embodiments of the present application will be described clearly and completely with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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 x 10 -4 , 14 x 10 -4 , 15 x 10 -4 , 16 x 10 -4 , 17 x 10 -4 , 18 x 10 -4 , 19 x 10 -4 , 20 x 10 -4 , etc.

[0029] It can be understood that generally the polarizing sheet uses the PVA optical film without molecular chain orientation before the polarizing sheet processing, and the orientation is formed in the polarizing sheet processing such as iodine dyeing and stretching process, which is the main structural source of the polarizing property of the polarizing sheet. The pre-orientation mentioned in the present application refers to forming the orientation structure in advance in the PVA optical film production process before the iodine dyeing and stretching of the PVA optical film, so that more oriented polyiodine ions can be formed when the PVA optical film is stretched to prepare the polarizing sheet, the number of the oriented polyiodine ions is increased, the content of the random molecular chain is reduced, the orientation relaxation caused by the molecular chain movement is reduced, and thus the degree of polarization and the transmittance of the polarizing sheet can be greatly improved at the same time.

[0030] The present application controls the in-plane refractive index difference of the PVA optical film xy for 1 x 10 -4 ~ 20 x 10 -4 , so that the PVA optical film has a pre-oriented molecular chain structure. Compared with the method of increasing the molecular weight of the PVA optical film, the PVA optical film of the present application can form more oriented polyiodine ions, increase the number of the oriented polyiodine ions, reduce the content of the random molecular chain, and reduce the orientation relaxation caused by the molecular chain movement in the downstream stretching process of preparing the polarizing sheet, so that the degree of polarization and the transmittance of the polarizing sheet can be greatly improved at the same time.

[0031] In the swelling process of the PVA optical film, water can only enter the amorphous region, and the equilibrium swelling degree represents the size of the crystallinity of the PVA optical film. When the equilibrium swelling degree is too high, the molecular chain is easy to slip during the stretching process of the PVA optical film, resulting in film breakage, and when the equilibrium swelling degree is too low, the modulus of the material is too high, the stretching tension is large, and the film is also easy to break at defects.

[0032] Therefore, in some embodiments, the equilibrium swelling degree of the PVA optical film in water at 25°C is 180%~220%, which can be 180%, 190%, 200%, 210%, 220%, etc.

[0033] In some embodiments, the PVA optical film has an equilibrium swelling degree in 50℃ water of 250% to 300%, which can be 250%, 260%, 270%, 280%, 290%, 300%, or the like. Preventing the equilibrium swelling degree in 50℃ water from being too high can prevent too many crystal physical entanglements from being destroyed during downstream stretching for preparing a polarizing sheet, causing excessive relaxation of oriented molecular chains, thereby reducing the number of effective oriented molecular chains in the final formed polarizing sheet. Meanwhile, avoiding the equilibrium swelling degree in 50℃ water from being too low can prevent the modulus of the material from increasing significantly, thereby increasing the stretching tension during downstream processing for preparing a polarizing sheet, which can easily cause the film to break.

[0034] The swelling rate represents the number of crystal physical entanglements of the PVA optical film. The lower the swelling rate, the more physical entanglements, and the more oriented molecular chains formed during stretching. However, if the swelling rate is too low, the modulus of the material will also increase significantly, and the downstream stretching tension will be large, which can easily cause the film to break. If the swelling rate is too high, the structure will be non-uniform, and problems such as uneven dyeing during downstream processing can occur. Therefore, in some embodiments, the average swelling rate of the PVA optical film before swelling equilibrium in 25℃ water is 90% / min to 110% / min, which can be 90% / min, 95% / min, 100% / min, 105% / min, 110% / min, or the like.

[0035] The present application also provides a preparation method of a PVA optical film, which refers to Figure 1 and includes the following steps: S10, dissolving the PVA resin and the additive into a solvent to obtain a casting solution; S20, performing extrusion degassing, casting molding, pre-drying by a drying roller, and heat treatment shaping on the casting solution to obtain the PVA optical film.

[0036] In some embodiments, the PVA resin includes at least one of a polyvinyl alcohol homopolymer and a polyvinyl alcohol copolymer.

[0037] In some embodiments, the raw material for preparing the polyvinyl alcohol copolymer includes a modified monomer, so that a PVA resin with a modified group can be prepared. The modified monomer includes at least one of an olefinic substance, an acrylic ester substance, a methacrylic ester substance, a methacrylic amide derivative, a vinyl ester substance, and a halogenated ethylene substance.

[0038] In some embodiments, the olefinic substance includes at least one of ethylene and propylene.

[0039] In some embodiments, the acrylic ester substance includes methyl acrylate.

[0040] In some embodiments, the methacrylic ester substance includes methyl methacrylate.

[0041] In some embodiments, the mass content of the modified groups in the PVA resin is controlled to be less than 10%, so as to avoid excessive content affecting the solubility of the PVA resin and the optical performance of the final PVA optical film.

[0042] In some embodiments, the degree of polymerization of the PVA resin is 1200-4000, further 2000-3500. A too low degree of polymerization can cause the stability of the molecular chain entanglement network in the prepared PVA polarizer to decrease, and the number of effectively oriented molecular chains to also decrease. A too high degree of polymerization can easily cause the stretching tension in the PVA polarizer processing process to be too large, causing the internal stress of the product to increase, and the film to shrink seriously under a high temperature and high humidity environment.

[0043] In some embodiments, the alcoholysis degree of the PVA resin is 98%-99.9%, further 99%-99.9%. A too low alcoholysis degree of the PVA resin can cause the hydrogen bond network stability of the prepared PVA polarizer to be worse, and the film shrinkage under a high temperature and high humidity to be worse.

[0044] In some embodiments, the solvent includes at least one of water and dimethyl sulfoxide.

[0045] In some embodiments, the additive includes a plasticizer, a surfactant, an antioxidant, and an ultraviolet absorber.

[0046] In some embodiments, the plasticizer includes at least one of glycerol, diglycerol, polyglycerol, ethylene glycol, and propylene glycol.

[0047] In some embodiments, in the casting solution, the addition amount of the plasticizer is 8%-15% based on the total mass of the PVA resin, which can be 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, etc., and the addition amount of the surfactant is 1.0%-5.0%, which can be 1%, 2%, 3%, 4%, 5%, etc., so as to avoid excessive addition causing the plasticizer and the surfactant to precipitate, and causing the optical performance of the PVA optical film to deteriorate.

[0048] The addition amount of the antioxidant and the ultraviolet absorber can be determined according to actual needs. In some embodiments, the addition amount of the antioxidant is 0.1%-5% based on the total mass of the PVA resin, which can be 0.1%, 0.5%, 1%, 1.5%, 2%, 3%, 4%, 5%, etc., and the addition amount of the ultraviolet absorber is 0.1%-5%, which can be 0.1%, 0.5%, 1%, 1.5%, 2%, 3%, 4%, 5%, etc.

[0049] In some embodiments, the surfactant includes a mixture of at least two of a non-ionic surfactant and an anionic surfactant.

[0050] In some embodiments, the anionic surfactant includes at least one of carboxylic acid type such as potassium laurate, etc., and dodecyl benzene sulfonate.

[0051] In some embodiments, the non-ionic surfactant includes at least one of alkyl ester, alkyl amine, and alkyl amide.

[0052] In some embodiments, the mass fraction of the solute in the casting solution is 20% to 40%, and further 25% to 35%.

[0053] In some embodiments, the dissolving temperature of the casting solution is 130°C to 170°C, and further 145°C to 160°C, and the dissolving time of the casting solution is 2h to 10h, and further 4h to 7h, so as to prevent the PVA resin from degradation, oxidation or cross-linking and other side reactions caused by too high temperature and too long time, and to prevent insufficient dissolution caused by too short temperature and too long time, and optical defects caused by residual crystal points.

[0054] In some embodiments, the alkyl ester includes polyoxyethylene lauryl ester.

[0055] In some embodiments, the alkyl amine includes polyoxyethylene lauryl amine ester.

[0056] In some embodiments, the alkyl amide includes polyoxyethylene lauryl amide.

[0057] In some embodiments, the casting solution is extruded and defoamed by an extruder, extruded from a die, and formed into a cast molding PVA optical film by a casting roll, and then the cast molding PVA optical film is pre-dried by a drying roll to obtain a pre-dried PVA optical film, and then the pre-dried PVA optical film is heated and treated in an oven to obtain a PVA optical film.

[0058] In some embodiments, the number of drying rolls in the step of pre-drying by the drying roll is 8 to 20.

[0059] In some embodiments, the drying temperature of each drying roll is controlled to be 30°C to 90°C, which can be 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, etc.

[0060] In some embodiments, the drying temperature of the plurality of drying rolls in the pre-drying process by the drying roll gradually decreases along the advancing direction of the cast molding PVA optical film, for example, the drying temperature of the first drying roll contacting the cast molding PVA optical film is the highest, and the drying temperature of the subsequent drying rolls decreases in turn. In this embodiment, the high temperature at the front end can quickly evaporate a large amount of water, and the low temperature at the rear end can gently reduce the water content of the film to a predetermined range and prevent the additives such as plasticizers from migrating sharply.

[0061] In some embodiments, the number of drying rollers is m, and the roller speed ratio at the n front roller gaps is controlled in the range of 100% to 110%. The n front roller gaps include the gap between the casting roller and the first drying roller in the casting process, and the gaps between the first and second drying rollers, and so on, until the n-1 and n 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 speed of the downstream roller to the linear speed of the upstream roller. In addition, the first drying roller is defined as the drying roller that first contacts the PVA optical film after the preliminary drying of the casting PVA optical film by the casting roller. The higher the roller speed ratio, the higher the pre-orientation degree of the PVA optical film. However, too high a roller speed ratio can cause the PVA liquid film to break, and too low a roller speed ratio can result in insufficient pre-orientation, and the in-plane refractive index difference ΔN xy is too small. Controlling the roller speed ratio to be 100% to 110% can achieve a good balance.

[0062] In some embodiments, the water content of the pre-dried PVA optical film obtained after the preliminary drying of the drying rollers is defined as the first water content, and the first water content is 8% to 15%, which can be 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, etc.

[0063] In some embodiments, the heat treatment and shaping process is carried out in multiple stages, and the process includes N heat treatment stages, 2≤N≤20, and the heating temperature of each heat treatment stage increases first and then decreases along the film production direction.

[0064] In some embodiments, the process parameters of the high temperature stage of the heating temperature of each heat treatment stage, i.e., the increasing stage of the increasing and then decreasing of the heating temperature, satisfy: the temperature range is 100°C to 140°C, and the duration time accounts for 15% to 50% of the total heat treatment and shaping time Th, which can be 15%, 18%, 20%, 25%, 28%, 30%, 35%, 40%, 45%, 50%, etc. This embodiment avoids the film modulus being too high due to too high temperature or too long time, which can easily cause the film to break, and also avoids the stability of the crystal physical entanglement points being reduced due to too low temperature or too short time, which can cause the orientation structure formed by stretching to relax and damage, and finally cause the polarization degree and transmittance of the prepared polarizing sheet to decrease.

[0065] To ensure the heat setting of the pre-orientation structure, in some embodiments, the ratio of the linear speed at the heat treatment outlet to the linear speed at the heat treatment inlet is defined as the linear speed ratio, and the linear speed ratio is controlled between 0.9 and 1.0, which can be 0.9, 0.92, 0.95, 0.96, 0.98, 0.99, 1.0, etc. A higher linear speed ratio is conducive to stabilizing more pre-orientation molecular chains, but if the linear speed ratio is too high, it will cause excessive tension in the film, resulting in macro defects such as tension marks, and affecting the uniformity of subsequent dyeing. On the contrary, if the linear speed ratio is too low, the pre-orientation molecular chains will relax at high temperature, resulting in a decrease in the in-plane refractive index difference Nxy, and causing a decrease in the pre-orientation structure in the final PVA optical film product.

[0066] It can be understood that the linear speed at the heat treatment inlet refers to the linear speed at which the film is forwarded by the traction roller or clamping roller and the like when the film just enters the heat treatment setting stage, and the linear speed at the heat treatment outlet refers to the linear speed at which the film is forwarded by the traction roller or clamping roller and the like when the film leaves the heat treatment setting stage.

[0067] In some embodiments, the second water content of the set PVA optical film obtained after the heat treatment setting process is 0.1% to 5%, which avoids the difficulty in dyeing during the processing of the polarizer caused by too low water content, and avoids the increase in the equilibrium swelling degree at 20℃ and the decrease in the number of crystal physical entanglement points and the number of orientation molecular chains formed by stretching caused by too high water content.

[0068] The application also provides a polarizer and a preparation method thereof, the preparation method comprising the following steps: using the PVA optical film provided by the application as a material, and obtaining the polarizer through swelling, dyeing and stretching of the PVA optical film.

[0069] In some embodiments, the light 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.

[0070] In some embodiments, the polarization degree of the polarizer is ≥99.990%.

[0071] The polarizer prepared from the PVA optical film provided by the application has the advantages of high transmittance and high polarization.

[0072] The technical solutions of the application are further described in detail below in combination with specific embodiments, and it should be understood that the specific embodiments below are only used to explain the application and do not limit the application.

[0073] Embodiment 1 A PVA resin with a degree of polymerization of 2400 and an alcoholysis degree of 99.8%, a plasticizer glycerol, a surfactant sodium dodecylbenzenesulfonate, an antioxidant, and a UV absorber mixture were mixed with 68 L of deionized water in a reaction kettle, and then dissolved at 150°C and 0.4 MPa for 6 h with stirring to obtain a PVA casting solution with a solute content of 32%. In this case, the total mass of the PVA resin was taken as 100%, the plasticizer was added in an amount of 12%, the surfactant was added in an amount of 2.2%, the antioxidant was added in an amount of 2.5%, and the UV absorber was added in an amount of 2.5%.

[0074] The casting solution was deaerated by an extruder, extruded from a die, dried by a casting roll, and then pre-dried by 10 drying rolls. The roll speed ratio between the first drying roll and the casting roll and between the first and fifth drying rolls was 100%, so that the first water content was controlled to be 15%. Then, the PVA optical film was obtained by heat treatment and shaping in an oven, the maximum temperature of the high-temperature stage of the heating temperature was 100°C, the outlet-to-inlet linear velocity ratio of the heat treatment was 0.9, and the second water content was controlled to be 5%.

[0075] Example 2 This example was prepared according to the method and conditions of Example 1, except that: The roll speed ratio between the first drying roll and the casting roll and between the first and fifth drying rolls was 110%, the first water content was 8%, the maximum temperature of the high-temperature stage of the heating temperature was 140°C, the outlet-to-inlet linear velocity ratio of the heat treatment was 1.0, and the second water content was 0.5%.

[0076] Comparative Example 1 This comparative example was prepared according to the method and conditions of Example 2, except that:

[0077] Comparative Example 2 This comparative example was prepared according to the method and conditions of Example 2, except that: The roll speed ratio between the first drying roll and the casting roll and between the first and fifth drying rolls was 110%, the maximum temperature of the high-temperature stage of the heating temperature was 135°C, the outlet-to-inlet linear velocity ratio of the heat treatment was 1.1, and the second water content was 1%.

[0078] Comparative Example 3 This comparative example was prepared according to the method and conditions of Example 2, except that: The first drying roller and the casting roller, and the first to fifth drying rollers have a speed ratio of 105%, the first moisture content is 20%, the highest temperature of the heat treatment high-temperature stage is 135°C, the linear speed ratio of the heat treatment outlet to the inlet is 0.9, and the PVA optical film is obtained by finally winding, and the second moisture content is 1.5%.

[0079] Comparative Example 4 This comparative example is prepared according to the method and conditions of Example 2, and the only difference is that: The first drying roller and the casting roller, and the first to fifth drying rollers have a speed ratio of 105%, the highest temperature of the heat treatment high-temperature stage is 90°C, the linear speed ratio of the heat treatment outlet to the inlet is 0.9, and the PVA optical film is obtained by finally winding, and the second moisture content is 1%.

[0080] Comparative Example 5 This comparative example is prepared according to the method and conditions of Example 2, and the only difference is that: The first drying roller and the casting roller, and the first to fifth drying rollers have a speed ratio of 105%, the linear speed ratio of the heat treatment outlet to the inlet is 0.9, and the PVA optical film is obtained by finally winding, and the first moisture content is 16%, and the second moisture content is 8%.

[0081] Performance test 1. Test method of swelling degree, swelling rate, etc. Cut the film sample with a size of 10mm x 200mm, and swell in 500mL water at a certain temperature (25°C and 50°C as required) for different times T, and use a 3000rpm centrifuge to centrifuge for 5min, and weigh m T1 . Dry in a 120°C air oven for 2h, and take out and weigh m T2 , Swelling degree at different times = (m T1 / m T2 ) x 100% Statistical swelling degree at different swelling times, record the swelling equilibrium time t, Equilibrium swelling degree = (m t1 / m t2 ) x 100% Average swelling rate = equilibrium swelling degree / t 2. Test method of in-plane refractive index difference Cut the film sample with a size of 50mm x 50mm, and place it on the detection point of the phase difference instrument, select 550nm wavelength light for detection, and directly obtain the in-plane refractive index difference of the sample Nxyresult by the instrument detection.

[0082] 3. Laboratory preparation of tensile dyeing sample 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.

[0083] Table 1. Dyeing and Tensile Test Process Parameters

[0084] 4. Test methods for optical performance The optical properties of the self-made stretched dyed film sample were tested using a phase difference meter. 550nm wavelength light was selected for detection, and the polarization degree and transmittance of the sample were obtained directly after the test.

[0085] Table 2. Comparison of process parameters and test results for different embodiments and comparative examples

[0086] 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.

[0087] 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.

[0088] 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 PVA optical film characterized by, 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.

2. The PVA optical film according to claim 1, wherein, The PVA optical film has an equilibrium swelling degree of 180% to 220% in water at 25℃, and an average swelling rate of 90% to 110% per minute before the PVA optical film reaches the swelling equilibrium in water at 25℃. And / or, the PVA optical film has an equilibrium swelling degree of 250% to 300% in water at 50℃.

3. A method of producing the PVA optical film according to claim 1 or 2, characterized by, The method comprises the following steps: The PVA resin and the additives are dissolved in a solvent to obtain a casting solution, and the casting solution is subjected to extrusion degassing, casting molding, pre-drying by a drying roller, and heat treatment to obtain the PVA optical film.

4. The method of claim 3, wherein the PVA optical film is prepared by the steps of: In the pre-drying step by the drying roller, the number of the drying rollers is 8 to 20. And / or, the pre-drying temperature of the drying roller is 30℃ to 90℃.

5. The method of claim 3, wherein the PVA optical film is prepared by the steps of: In the pre-drying step by the drying roller, the number of the drying rollers is m, m > n, n > 0, m > 0, and the speed ratio between the first n drying rollers is 100% to 110%, 1 ≤ n ≤ m / 2. And / or, the water content of the pre-drying PVA optical film obtained after the pre-drying step is defined as a first water content, and the water content of the PVA optical film obtained after the heat treatment step is defined as a second water content, the first water content is 8% to 15%, and the second water content is 0.1% to 5%.

6. The method of claim 3, wherein the PVA optical film is prepared by the steps of: The heat treatment process comprises N heat treatment stages, 2 ≤ N ≤ 20, and the heating temperature of each heat treatment stage increases first and then decreases along the film generation direction.

7. The method of claim 6, wherein the PVA optical film is prepared by the steps of: The temperature of the high-temperature stage of the heating temperature of each heat treatment stage is 100℃ to 140℃, and the corresponding processing time of the high-temperature stage is 15% to 50% of the total heat treatment time Th.

8. The method of claim 3, wherein the PVA optical film is prepared by the steps of: In the heat treatment step, the ratio of the linear speed at the heat treatment outlet to the linear speed at the heat treatment inlet is defined as a linear speed ratio, and the linear speed ratio is 0.9 to 1.

0.

9. A highly transparent and highly polarized polarizing plate, characterized by comprising a transparent protective film according to any one of claims 1 to 8. The high-transmission and high-polarization polarizing plate is prepared by the PVA optical film of claim 1 or 2.

10. The high transmission high polarization polarizing plate of claim 9, wherein, The transmittance of the polarizing plate is ≥44.0, and the polarization degree is ≥99.990%.

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