Thin low-curl high-stretching-rate PVA optical film and preparation method thereof

Abstract

The invention belongs to the technical field of optical film production, and particularly relates to a thin low-curl high-stretching-ratio PVA optical film and a preparation method thereof. The preparation method comprises the following steps: pre-treating PVA resin, dissolving the pre-treated PVA resin in a solvent to prepare a PVA solution, filtering, defoaming and homogenizing, carrying out curtain coating to obtain a PVA film, drying, carrying out heat treatment, slitting and rolling, wherein the weight-average molecular weight Mw of the PVA resin is 130-190 KDa, the polydispersity PDI is smaller than or equal to 1.5, the polymerization degree is 2000-3000, the alcoholysis degree is larger than or equal to 99.9%, the content of macromolecular substances with the particle size of 20-25 mesh and the molecular weight larger than or equal to 500 KDa is smaller than or equal to 0.1 wt%, the content of iron ions is smaller than or equal to 1 ppm, the content of calcium ions is smaller than or equal to 1 ppm, the content of syndiotactic structure PVA is 40-70 wt%, and the content of 1, 2-diol is smaller than or equal to 0.2 wt%. According to the preparation method, four technical means of raw material super purification, two-side drying rate matching, gradient temperature control and thin film crystallinity control are synergistically integrated, and on the premise that the production line speed is guaranteed, synchronous achievement that the stretching rate is larger than or equal to 5.9 times and swelling is not curled is achieved for the first time.

Description

Technical Field

[0001] This invention belongs to the field of optical film production technology, specifically relating to a thin, low-curvature, high-stretch-ratio PVA optical film and its preparation method. Background Technology

[0002] Polyvinyl alcohol (PVA) optical films are the core substrate for preparing polarizers for liquid crystal displays and are widely used in high-end optical fields such as display panels, automotive displays, and flexible displays. As display devices develop towards thinner, lighter, higher resolution, and higher reliability, more stringent requirements are being placed on the thickness uniformity, moisture content uniformity, crystallinity uniformity, high tensile strength, and low curling characteristics after swelling of PVA optical films.

[0003] PVA optical films are manufactured by washing, swelling, dyeing, stretching, and cross-linking with boric acid, followed by lamination with triacetic acid cellulose (TAC) films. The dyeing process causes dichroic iodide ions to attach to the hydroxyl groups of PVA, forming a complex. During stretching, the PVA molecular chains guide the iodide ions to orient themselves, thus achieving polarization. The higher the stretching ratio, the more uniform the iodide ion orientation and the higher the degree of polarization.

[0004] Currently, the stretching ratio of PVA optical films on the market is generally around 5 to 5.8 times. In China, many PVA optical films are prone to problems such as edge curling, wrinkles, and film breakage during the washing, swelling, and dyeing processes in polarizer production. Edge curling leads to uneven edge dyeing and significant product waste; film breakage requires re-cleaning the equipment and re-threading the film, which is time-consuming and results in low output.

[0005] The root cause of the above problems lies in the non-uniform structure and inconsistent internal stress distribution of the PVA optical film itself. Specifically, this manifests as: asynchronous drying rates along the thickness and width directions leading to stress gradients on the film surface; the wide molecular weight distribution, high content of macromolecular impurities and metal cations, and insoluble substances in the PVA raw material forming gel crystal points, disrupting the consistency of molecular chain orientation; and sudden changes in drying temperature causing excessive differences in crystallinity within the film, resulting in uneven stress release during swelling. Existing technologies often focus on localized optimization of single process parameters or raw material purity, failing to systematically address the problems caused by the coupling of these multiple factors.

[0006] Patent CN119526666A discloses a low-curl, thin PVA optical film for polarizers and its preparation method. By strictly controlling the temperature of the casting roller and drying roller and setting a suitable temperature gradient, the thin PVA optical film dries uniformly on both sides without surface skinning, reducing the difference in crystallinity and moisture content between the two sides, resulting in a uniform structure on both sides. The swelling and curling of the film in water is significantly improved. The overall process is simple and highly feasible for practical applications, significantly improving the quality of the thin PVA optical film. Although it proposes a way to improve drying uniformity, it is only a qualitative description and does not disclose quantitative process parameters that can be implemented in engineering. In particular, it does not disclose the temperature matching relationship between the casting roller and the fan shroud, making it impossible to achieve precise matching of drying rates on both sides of the film and effective control of internal stress. It is difficult to simultaneously meet the comprehensive performance requirements of thinness, high uniformity, high stretch ratio, and low curling.

[0007] Therefore, developing a preparation method that can solve the above-mentioned technical pain points and achieve precise control of film thickness, water content, crystallinity, tensile properties and curling degree has important engineering application value and market prospects, and is also the technical problem to be solved by this invention. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of existing technologies in PVA optical film, such as unsystematic purification of raw materials, poor synergy in film formation processes, difficulty in precisely controlling drying uniformity, and the lack of publicly disclosed quantitative process parameters, which prevents stable engineering production. This invention provides a thin, low-curl, high-stretch-ratio PVA optical film and its preparation method that effectively solves problems such as uneven film moisture content, large differences in crystallinity, and easy curling through synergistic optimization of the entire process. This preparation method integrates four technical means: ultra-purification of raw materials, matching of drying rates on both sides, gradient temperature control, and control of film crystallinity. For the first time, while ensuring production line speed, it achieves a stretch ratio ≥ 5.9 times and swelling without curling simultaneously.

[0009] The inventors of this invention discovered that the curling of PVA films is caused by several factors: asynchronous evaporation rates of moisture on both sides during film formation and drying, resulting in a stress gradient along the film thickness direction; the achievement of high stretch ratios depends on the highly regular arrangement of PVA molecular chains, while macromolecular impurities and metal cations can act as crystallization defects, disrupting orientation consistency; and a sudden drop in drying temperature leads to an increase in the difference in crystallinity between the surface and interior of the PVA film, resulting in uneven stress release during swelling. Based on these findings, this invention proposes a four-in-one technical solution integrating two-sided drying rate matching, low-temperature gradient drying, raw material ultra-purification, and crystallinity control.

[0010] To achieve the above objectives, in a first aspect, the present invention provides a method for preparing a thin, low-curvature, high-stretch-ratio PVA optical film, the method comprising: pretreating PVA resin, dissolving the pretreated PVA resin in a solvent to prepare a PVA solution, filtering, defoaming and homogenizing, casting to obtain a PVA film, drying, heat treatment, slitting and winding. The PVA resin has a weight-average molecular weight (Mw) of 130-190 kDa, a polydispersity index (PDI) of ≤1.5, a degree of polymerization of 2000-3000, a degree of hydrolysis of ≥99.9%, a particle size of 20-25 mesh, a content of macromolecules with a molecular weight of ≥500 kDa of ≤0.1 wt%, an iron ion content of ≤1 ppm, a calcium ion content of ≤1 ppm, a syndiotactic PVA content of 40-70 wt%, and a 1,2-diol content of ≤0.2 wt%.

[0011] Preferably, the pretreatment conditions include: cleaning the PVA resin to make the sodium acetate content of the PVA resin ≤0.01wt%.

[0012] Preferably, the solvent is deionized water with a conductivity ≤0.5μS / cm and a pH of 7~7.5.

[0013] More preferably, the solid content of the PVA solution is 25~35wt%.

[0014] More preferably, the preparation conditions of the PVA solution include: a dissolution temperature of 130~150℃, and nitrogen gas being used to purge oxygen during the dissolution process.

[0015] Preferably, the PVA solution also contains plasticizers and antioxidants.

[0016] More preferably, the plasticizer is selected from one or more of glycerol, sorbitol, pentaerythritol, xylitol, polyethylene glycol, and caprolactam.

[0017] More preferably, the antioxidant is a hindered phenolic antioxidant and / or a benzene ring-containing phosphite antioxidant.

[0018] More preferably, the hindered phenolic antioxidant is selected from one or more of antioxidant 1010, antioxidant 1076, antioxidant 3114, antioxidant 245 and antioxidant 1330.

[0019] More preferably, the benzene ring-containing phosphite antioxidant is selected from one or more of antioxidant 168, antioxidant 626, antioxidant TPP, and antioxidant TNPP.

[0020] Preferably, the filtration adopts a four-stage series filtration with filtration accuracies of 20μm, 10μm, 5μm and 2μm respectively.

[0021] Preferably, the defoaming and homogenization conditions include: circulating and homogenizing the PVA solution between at least three defoaming vessels, with a defoaming negative pressure of -0.1 to -0.06 MPa.

[0022] More preferably, the PVA solution is conveyed by an extruder before defoaming and homogenization after filtration, with the inlet pressure of the GP pump of the extruder ≥2.5MPa and the negative pressure of the exhaust port of the extruder being -0.09 ~ -0.07MPa.

[0023] Preferably, the conditions for casting film formation include: casting the defoamed PVA solution through a die to a casting roller, the temperature of the casting roller being 80~95℃, a fan hood being installed above the casting roller, the temperature of the fan hood being 5~10℃ lower than the temperature of the casting roller, the surface roughness Ra of the casting roller being ≤0.005, and the concentricity of the outer surface being <0.02mm.

[0024] Preferably, the drying conditions include: passing the cast PVA film sequentially through an even number of drying rollers, with the drying rollers arranged in pairs and each pair having the same temperature; the temperature difference between adjacent pairs of drying rollers being ≤5℃; the overall temperature of the drying rollers decreasing gradually from 95℃ to 40℃; the wrap angle of the PVA film entering each drying roller being the same; the ratio of the linear velocity of the first drying roller to the linear velocity of the casting roller being 100.1~103%; and the linear velocity of subsequent drying rollers being adjusted step by step according to the thermal shrinkage rate of the PVA film, wherein the linear velocity of the current drying roller = the linear velocity of the previous drying roller × (1 - the thermal shrinkage rate of the current segment of PVA film). If, when the film reaches roller A, the film surface shrinkage rate is 0.5% relative to the previous roller, that is, the speed of roller A is 99.5% of that of the previous roller.

[0025] Preferably, the drying roller is made of carbon fiber.

[0026] Preferably, the heat treatment conditions include: sending the dried PVA film into an air-floating oven, with the temperature of each section of the oven not exceeding 80°C and showing a trend of first increasing and then decreasing, and the moisture content of the PVA film after heat treatment being controlled at 3~8%.

[0027] Preferably, the conditions for slitting and winding include: slitting the heat-treated PVA film, and ensuring that the inner and outer winding directions of the slitting roll are opposite to the winding direction of the semi-finished roll.

[0028] Preferably, the guide rollers used in the PVA film transport process in the preparation method are made of carbon fiber.

[0029] Secondly, the present invention provides a PVA optical film prepared by the preparation method described in the present invention, wherein the thickness of the PVA optical film is 30~45μm, the thickness tolerance is ≤1μm; the difference in water content between any two points on the film surface is ≤1%, and the difference in water content between the front and back sides is ≤0.8%; the crystallinity difference is ≤3%; the stretching ratio is 5.9~6 times; and the height h of the four corners after swelling is ≤1mm.

[0030] In the above technical solution, the preparation method of the thin, low-curl, high-stretch-ratio PVA optical film of the present invention firstly involves multi-dimensional precise limitation of PVA resin raw materials, strictly controlling molecular weight distribution, macromolecular impurities, metal ions, syndiotactic structures, and 1,2-diol content, effectively reducing the size and number of gel crystal points, improving the consistency of molecular chain orientation, and ensuring a stable wet stretch ratio of ≥5.9 times, thus guaranteeing film uniformity, crystallization stability, and high stretch performance from the source. Secondly, ultrapure water dissolution, nitrogen protection, four-stage precision filtration, and multi-stage circulating negative pressure defoaming are used to significantly reduce bubbles, gels, and impurity defects, improving solution quality. Furthermore, during the casting process, the fan temperature is 5-10°C lower than the surface temperature of the casting roller, achieving precise matching of the drying rates on both sides of the film, reducing internal stress and curling tendency.

[0031] In the preparation method of the PVA optical film of this invention, the drying method uses an even number of drying rollers, which are paired and heated at the same temperature, with a gradual gradient descent, consistent wrap angle, and linear velocity that is matched to the thermal shrinkage rate. Combined with the low deformation characteristics of the carbon fiber roller, the method avoids the difference in crystallinity between the surface and the interior caused by sudden temperature changes on the film surface. The crystallinity difference is ≤3%, and the stress release during swelling is stable. At the same time, it ensures that the film is subjected to uniform stress during the drying process, the moisture content transitions smoothly, and the crystallization is uniform.

[0032] Furthermore, the method for preparing the PVA optical film of the present invention employs air-float heat treatment to gently control the moisture content, and reverse winding to further release internal stress, ultimately obtaining an ultra-thin, high-precision, highly uniform, high-stretch ratio PVA optical film with extremely low curling after swelling, which fully meets the requirements for high-end polarizers.

[0033] Other features and advantages of the present invention will be described in detail in the following detailed description section. Detailed Implementation

[0034] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0035] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0036] In a first aspect, the present invention provides a method for preparing a thin, low-curvature, high-stretch-ratio PVA optical film, the method comprising: pretreating PVA resin, dissolving the pretreated PVA resin in a solvent to prepare a PVA solution, filtering, defoaming and homogenizing, casting to obtain a PVA film, drying, heat treatment, slitting and winding. The PVA resin has a weight-average molecular weight (Mw) of 130-190 kDa, which is moderate to ensure film toughness and stretching orientation ability, avoiding excessively high molecular weight leading to poor solubility and excessively low molecular weight leading to insufficient strength. Its polydispersity index (PDI) is ≤1.5, indicating a narrow molecular weight distribution and synchronous molecular chain orientation, improving stretching uniformity and ratio. The degree of polymerization is 2000-3000, balancing solubility and film strength, suitable for high-ratio stretching. The degree of alcoholysis is ≥99.9%, and the particle size is within a 20-25 mesh sieve, ensuring uniform dissolution rate and avoiding localized over-dissolution or insolubility, thus reducing the defects in the resulting PVA optical film. Intramembrane gel; the content of macromolecules with a molecular weight ≥500KDa is ≤0.1wt%, eliminating crystal points and stress concentration points caused by ultra-large molecules; the iron ion content is ≤1ppm and the calcium ion content is ≤1ppm, avoiding the catalytic degradation of metal ions, the generation of crystal points, and the impact on the optical transmittance of PVA optical films; the content of syndiotactic PVA is 40~70wt%, making the prepared PVA optical film crystallize uniformly, swell steadily, and improve tensile stability; the 1,2-diol content is ≤0.2wt%, reducing molecular defects in PVA optical films and improving structural regularity.

[0037] PDI is the molecular weight polydispersity index, defined as the ratio of weight-average molecular weight Mw to number-average molecular weight Mn. The closer the PDI is to 1, the more uniform the length of the PVA molecular chain and the better the film-forming performance.

[0038] The method for preparing thin, low-curl, high-stretch-ratio PVA optical films of this invention employs ultra-purified raw material screening and deep impurity removal, precisely matching the drying rates on both sides of the film. It utilizes a synergistic process involving paired, temperature-gradient slow-drying, air-float-type low-temperature controllable heat treatment, and reverse winding to relieve stress. This eliminates gel crystal points and molecular defects at the source, effectively balancing moisture evaporation and crystallization uniformity along the film thickness direction, significantly reducing internal stress. Furthermore, it boasts advantages such as stable and controllable process, suitability for continuous production, and no risk of high-temperature damage or oxidative degradation. This method can stably prepare PVA optical films with both high stretch ratio and low curl characteristics, solving problems such as uneven drying, stress concentration, and high raw material impurities leading to film breakage, curling, and quality fluctuations in traditional processes.

[0039] In this invention, the pretreatment conditions include: cleaning the PVA resin to ensure that the sodium acetate content of the PVA resin is ≤0.01wt%, removing residual sodium salts from the PVA resin, avoiding ion contamination, and preventing surface defects and processing abnormalities in the obtained PVA optical film.

[0040] In this invention, in order to ensure that the surface of the obtained PVA optical film is pure and free of spots, the solvent is deionized water with a conductivity ≤0.5μS / cm and a pH of 7~7.5.

[0041] In this invention, the solid content of the PVA solution is 25~35wt%. Such a PVA solution has moderate viscosity, uniform casting, and stable thickness of the PVA optical film.

[0042] In this invention, in order to ensure that the PVA resin is fully dissolved without gel, the preparation conditions of the PVA solution include: a dissolution temperature of 130~150℃, and nitrogen gas being purged and oxygen being vented during the dissolution process to isolate oxygen and prevent high-temperature oxidative degradation of the PVA resin.

[0043] In this invention, the PVA solution also contains plasticizers and antioxidants.

[0044] In this invention, the plasticizer is selected from one or more of glycerol, sorbitol, pentaerythritol, xylitol, polyethylene glycol and caprolactam. The addition of the plasticizer can improve the uniformity of plasticization, enhance the tensile strength of the PVA optical film, and reduce brittleness.

[0045] In this invention, in order to suppress high-temperature thermo-oxidative aging of PVA optical films, reduce haze, and maintain molecular chain stability, the antioxidant is a hindered phenolic antioxidant and / or a benzene ring-containing phosphite antioxidant.

[0046] In this invention, the hindered phenolic antioxidant is selected from one or more of antioxidant 1010, antioxidant 1076, antioxidant 3114, antioxidant 245 and antioxidant 1330.

[0047] In this invention, the benzene ring-containing phosphite antioxidant is selected from one or more of antioxidant 168, antioxidant 626, antioxidant TPP and antioxidant TNPP.

[0048] In this invention, the filtration adopts a four-stage series filtration with filtration accuracies of 20μm, 10μm, 5μm and 2μm respectively, which can remove impurities, gels and insoluble substances step by step, ensuring that the membrane surface is free of crystal points and defects.

[0049] In this invention, in order to eliminate air bubbles in the PVA solution, eliminate batch differences, and make the PVA solution uniform, the defoaming and homogenization conditions include: circulating the PVA solution among at least three defoaming vessels for homogenization, with a defoaming negative pressure of -0.1 to -0.06 MPa.

[0050] In this invention, the PVA solution is conveyed by an extruder before defoaming and homogenization after filtration. The inlet pressure of the GP pump of the extruder is ≥2.5MPa, and the negative pressure of the exhaust port of the extruder is -0.09 ~ -0.07MPa, which ensures the stable delivery of the PVA solution, pre-defoaming, and avoids the introduction of air bubbles into the casting die.

[0051] In this invention, the conditions for casting film formation include: casting the defoamed PVA solution through a die onto a casting roller at a temperature of 80-95°C; setting a fan above the casting roller at a temperature 5-10°C lower than the casting roller temperature; the fan cooling can force a matching of the drying rates on both sides, eliminate the stress gradient in the thickness direction, and suppress curling from the source; the surface roughness Ra of the casting roller is ≤0.005, and the concentricity of the outer surface is <0.02mm, ensuring that the obtained PVA optical film has a smooth surface, uniform thickness, and no transverse stripes.

[0052] In this invention, the drying conditions include: passing the cast PVA film sequentially through an even number of drying rollers, with the drying rollers arranged in pairs and each pair having the same temperature; the temperature difference between adjacent pairs of drying rollers being ≤5℃; and the overall temperature of the drying rollers gradually decreasing from 95℃ to 40℃, ensuring a smooth temperature transition and avoiding sudden cooling or heating. The PVA film enters each drying roller with the same wrap angle. The ratio of the linear velocity of the first drying roller to the linear velocity of the casting roller is 100.1~103%, ensuring uniform crystallization inside and outside the film, with a crystallinity difference ≤3%. The linear velocity of subsequent drying rollers is adjusted step by step according to the thermal shrinkage rate of the PVA film. Specifically, the linear velocity of the current drying roller = the linear velocity of the previous drying roller × (1 - the thermal shrinkage rate of the current segment of the PVA film). For example, if the PVA film surface shrinkage rate is 0.5% relative to the previous roller when the film reaches roller A, then the speed of roller A is 99.5% of that of the previous roller. This ensures that the PVA film has no localized shrinkage, no internal stress accumulation, and can be smoothly peeled without stretching.

[0053] In this invention, the drying roller is made of carbon fiber.

[0054] In this invention, the heat treatment conditions include: sending the dried PVA film into an air-floating oven, with the temperature of each section of the oven not exceeding 80°C and showing a trend of first increasing and then decreasing. Temperature ≤80°C can avoid plasticizer precipitation, haze increase, and excessive crystallization; the first increase and then decrease allows for full release of internal stress; the moisture content of the PVA film after heat treatment is controlled at 3~8%, which is suitable for subsequent stretching and lamination.

[0055] In this invention, the conditions for slitting and winding include: slitting the heat-treated PVA film and making the inner and outer winding directions of the slitting roll opposite to the winding direction of the semi-finished roll; releasing the internal stress during winding to further reduce the tendency to curl during use.

[0056] In this invention, the guide rollers used in the PVA film transport process during the preparation method are made of carbon fiber.

[0057] Secondly, the present invention provides a PVA optical film prepared by the preparation method described in the present invention, wherein the thickness of the PVA optical film is 30~45μm, the thickness tolerance is ≤1μm; the difference in water content between any two points on the film surface is ≤1%, and the difference in water content between the front and back sides is ≤0.8%; the crystallinity difference is ≤3%; the stretching ratio is 5.9~6 times; and the height h of the four corners after swelling is ≤1mm.

[0058] The PVA optical film of this invention features thinness, high precision, excellent thickness uniformity, and highly uniform water content and crystallinity. It can achieve a stable high stretch ratio of ≥5.9 times, and the height of the four corners after swelling does not exceed 1mm. There is no obvious curling or edge warping, few film surface defects, and excellent optical performance. It maintains a stable morphology during the washing, swelling, dyeing, and stretching processes of polarizer processing, and is not prone to breakage or wrinkles. It can significantly improve polarization performance and production yield, and perfectly meet the requirements of high-end display polarizers.

[0059] The present invention will be described in detail below through examples. In the following examples, the pharmaceuticals and agents are all conventional commercially available products.

[0060] Example 1 (1) Raw material pretreatment: Select PVA resin powder that meets the requirements, with a weight-average molecular weight Mw=150KDa, polydispersity index PDI=1.3, degree of polymerization 2500, degree of alcoholysis 99.9%, particle size within 20~25 mesh sieve, content of macromolecules with molecular weight ≥500KDa of 0.08%, iron ion content of 0.5ppm, calcium ion content of 0.5ppm, syndiotactic PVA content of 65wt%, and 1,2-diol content of 0.1wt%; The PVA resin powder was cleaned with deionized water with a conductivity ≤0.5μS / cm and a pH of 7~7.5 until the sodium acetate content in the PVA resin was ≤0.01wt%, thus completing the raw material pretreatment.

[0061] (2) Solution preparation: Using the pretreated PVA resin as raw material and the above-mentioned deionized water as solvent, a PVA solution with a solid content of 30wt% is prepared; plasticizer and antioxidant are added to the PVA solution, wherein the plasticizer is glycerol (injection grade) and the amount added is 10wt% of the dry basis of PVA, and the antioxidant is hindered phenolic antioxidant 1010 and the amount added is 0.2wt% of the dry basis of PVA; the prepared PVA solution is sent to the dissolving kettle, the temperature of the dissolving kettle is controlled at 130℃, nitrogen is purged and oxygen is purged throughout the process, and the mixture is stirred for 12h to fully dissolve the PVA resin and obtain a uniform PVA solution.

[0062] (3) Filtration and defoaming homogenization: The fully dissolved PVA solution is filtered in a four-stage series filtration method with filtration precision of 20μm, 10μm, 5μm and 2μm respectively, in order to thoroughly remove gel particles, impurities and insoluble matter in the solution and ensure the cleanliness of the PVA solution. The filtered PVA solution was sent to a defoaming and homogenization system, and was processed using a three-stage defoaming vessel circulation homogenization method. The defoaming negative pressure was controlled at -0.08 MPa, and the circulation homogenization time was 12 hours to ensure that there were no residual bubbles in the PVA solution and that the solution concentration was uniform.

[0063] (4) Casting film: The defoamed and homogenized PVA solution is uniformly cast onto the surface of the casting roller through the die head. The temperature of the casting roller is controlled at 85℃, the temperature of the air hood set above the casting roller is 80℃, and the casting speed is 8m / min. When the PVA film is peeled off from the surface of the casting roller, the water content of the PVA film is controlled at 6% to obtain a preliminary PVA film sheet. The surface roughness Ra of the casting roller is ≤0.005, and the concentricity of the outer surface is <0.02mm to ensure the flatness of the cast film surface.

[0064] (5) Drying: The PVA film sheets peeled off after casting are sequentially dried by passing them through 20 drying rollers. The 20 drying rollers are set in pairs, and each pair of drying rollers has the same temperature. The temperature distribution of each pair of drying rollers is as follows: 1~2#: 85℃, 3~4#: 85℃, 5~6#: 85℃, 7~8#: 80℃, 9~10#: 75℃, 11~12#: 70℃, 13~14#: 65℃, 15~16#: 60℃, 17~18#: 55℃, 19~20#: 50℃; The ratio of the linear speed of the first drying roller to the linear speed of the casting roller is controlled to be 101.5%. The linear speed of each subsequent drying roller is finely adjusted step by step according to the thermal shrinkage rate of the PVA film. The linear speed of the current drying roller = the linear speed of the previous drying roller × (1 - the thermal shrinkage rate of the current PVA film). At the same time, it is ensured that the wrap angle of the PVA film entering each drying roller is the same.

[0065] (6) Heat treatment: The dried PVA film is sent into a five-section air flotation oven for heat treatment. The temperature of each section of the oven is set to 55℃, 65℃, 75℃, 65℃ and 50℃ respectively.

[0066] (7) Slitting and winding: The heat-treated PVA film is slitting by swapping the inner and outer layers. During the slitting process, the inner and outer layers of the roll are swapped so that the inner and outer winding directions of the slitting roll are opposite to the winding direction of the semi-finished roll, thus producing a PVA optical film, denoted as B1.

[0067] Example 2 The method described in Example 1 was carried out, except that in step (1), the polydispersity index (PDI) of the PVA resin powder was 1.5, the content of macromolecules with a molecular weight ≥ 500 kDa was 0.1%, the iron ion content was 1 ppm, the calcium ion content was 1 ppm, and other conditions remained unchanged, and a PVA optical film was obtained, which was denoted as B2.

[0068] Example 3 The method described in Example 1 is implemented, except that in step (4), "controlling the temperature of the casting roller to 85°C and the temperature of the hood set above the casting roller to 80°C" is replaced with "controlling the temperature of the casting roller to 85°C and the temperature of the hood set above the casting roller to 75°C". In step (5), the statement “The temperature distribution of each pair of drying rollers is as follows: 1~2#: 85℃, 3~4#: 85℃, 5~6#: 85℃, 7~8#: 80℃, 9~10#: 75℃, 11~12#: 70℃, 13~14#: 65℃, 15~16#: 60℃, 17~18#: 55℃, 19~20#: 50℃” is replaced with “The temperature distribution of each pair of drying rollers is as follows: 1~2#: 95℃, 3~4#: 90℃, 5~6#: 85℃, 7~8#: 80℃, 9~10#: 75℃, 11~12#: 70℃, 13~14#: 65℃, 15~16#: 60℃, 17~18#: 50℃, 19~20#: 40℃”; In step (6), the phrase “the temperature of each section of the oven is set to 55℃, 65℃, 75℃, 65℃ and 50℃ in sequence” is replaced with “the temperature of each section of the oven is set to 55℃, 65℃, 80℃, 65℃ and 50℃ in sequence”. Other conditions remain unchanged, and a PVA optical film is obtained, which is denoted as B3.

[0069] Comparative Example 1 The method described in Example 1 was implemented, except that in step (4), "the temperature of the casting roller is controlled at 85°C and the temperature of the hood set above the casting roller is 80°C" was replaced with "the temperature of the casting roller is controlled at 85°C and the temperature of the hood set above the casting roller is 115°C". Other conditions remained unchanged, and a PVA optical film was obtained, which was denoted as D1.

[0070] Comparative Example 2 The method described in Example 1 is implemented, except that in step (5), "the 20 drying rollers are arranged in pairs, and each pair of drying rollers has the same temperature. The temperature distribution of each pair of drying rollers is as follows: 1~2#: 85℃, 3~4#: 85℃, 5~6#: 85℃, 7~8#: 80℃, 9~10#: 75℃, 11~12#: 70℃, 13~14#: 65℃, 15~16#: 60℃, 17~18#: 55℃, 19~20#: 50℃". Replace "The drying rollers are not paired, and the temperature distribution of each drying roller is as follows: 1~3#: 85℃, 4#: 90℃, 5#: 88℃, 6#: 70℃, 7#: 85℃, 8#: 60℃, 9#: 70℃, 10#: 60℃, 11~12#: 60℃, 13~14#: 60℃, 15~16#: 70℃, 17#: 50℃, 18#: 45℃, 19~20#: 45℃" with other conditions unchanged, and a PVA optical film is obtained, denoted as D2.

[0071] Comparative Example 3 The method described in Example 1 was carried out, except that in step (1), the weight-average molecular weight of the PVA resin powder was Mw=3500KDa, the polydispersity index PDI=2.3, the content of macromolecules with a molecular weight ≥500KDa was 1%, the iron ion content was 6ppm, the calcium ion content was 6ppm, and other conditions remained unchanged, and a PVA optical film was obtained, which was denoted as D3.

[0072] Detection Example 1 The performance of the PVA optical films prepared in Examples 1-3 and Comparative Examples 1-3 was tested, and the specific methods are as follows: (1) Film thickness and tolerance: Use a micrometer to take no less than 50 measurement points evenly in the transverse and longitudinal directions of the film surface, take the arithmetic mean as the film thickness, calculate the deviation of the maximum and minimum values ​​relative to the average value, and take the larger absolute value as the thickness tolerance.

[0073] (2) Moisture content and moisture content difference between the two sides: A halogen moisture analyzer was used to heat the membrane to constant weight at 105℃. The moisture content of the PVA membrane was tested on both sides (at least 5 points on each side). The average value was calculated. The absolute value of the difference between the average values ​​of the two sides was the moisture content difference between the two sides. The moisture content difference between any two points on the membrane was the difference between the maximum and minimum values ​​among all test points.

[0074] (3) Crystallinity difference: The crystallinity of the PVA film on both sides was determined by X-ray diffraction (XRD), and the absolute value of the difference between the crystallinity of the two sides was calculated.

[0075] (4) Stretching ratio: The wet stretching ratio is measured on the polarizer production line, which is the limit value of the longitudinal stretching ratio of the PVA film in the dyeing and stretching unit (based on the criterion that the film does not break within 3 hours).

[0076] (5) Gel size: The longest diameter of the largest gel particle in the field of view was taken by observing with a polarizing microscope.

[0077] (6) Evaluation method for curling degree: Cut a 100mm×100mm PVA optical film sample, immerse it in deionized water at 25±1℃ for 5 minutes, take it out and gently place it on a horizontal glass plate surface. Let it stand for 10 seconds until the film surface is stable, and use a feeler gauge or height gauge to measure the maximum vertical height h of the edge of the four corners that is raised.

[0078] h≤1mm: judged as no curling; 1mm<h≤5mm: judged as curling; h>5mm: judged as obvious curling.

[0079] The specific results are shown in Table 1.

[0080] Table 1

[0081] As can be seen from the data in Table 1, Examples 1-3 of this invention, through precise screening of PVA raw materials, purification, temperature difference control of the casting hood, paired gradient drying, air flotation heat treatment, and reverse winding to eliminate internal stress, produced PVA optical films with uniform thickness, uniform moisture content distribution, small crystallinity differences, and a stretching ratio of 5.9-6 times. After swelling, the corner warping height was ≤1mm, with no curling, demonstrating excellent overall performance. In contrast, Comparative Example 1 suffered from crude raw material control, Comparative Example 2 from an unreasonable drying process, and Comparative Example 3 from a lack of hood temperature difference control and internal stress elimination methods, resulting in significantly deteriorated PVA optical films in terms of film surface uniformity, crystallinity uniformity, stretching properties, and curling degree.

[0082] In summary, this invention effectively solves the technical pain points of existing PVA optical films, such as uneven moisture content, large crystallinity differences, easy curling, and insufficient tensile properties, through the systematic synergy of raw material ultra-purification, casting hood temperature difference matching, paired gradient drying, air flotation heat treatment, and reverse winding. It can stably prepare PVA optical films with a thickness of 30~45μm, thickness tolerance ≤1μm, moisture content difference between any two points on the film surface ≤1%, moisture content difference between the front and back sides ≤0.8%, crystallinity difference ≤3%, stretching ratio of 5.9~6 times, and corner warping height ≤1mm after swelling. The overall performance is significantly better than traditional processes and can meet the requirements of high-end polarizer industrial production.

[0083] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0084] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0085] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for preparing a thin, low-curvature, high-stretch-ratio PVA optical film, characterized in that, The preparation method includes: pretreating PVA resin, dissolving the pretreated PVA resin in a solvent to prepare a PVA solution, filtering, defoaming and homogenizing, casting to obtain a PVA film, drying, heat treatment, slitting and winding. The PVA resin has a weight-average molecular weight (Mw) of 130-190 kDa, a polydispersity index (PDI) of ≤1.5, a degree of polymerization of 2000-3000, a degree of hydrolysis of ≥99.9%, a particle size of 20-25 mesh, a content of macromolecules with a molecular weight of ≥500 kDa of ≤0.1 wt%, an iron ion content of ≤1 ppm, a calcium ion content of ≤1 ppm, a syndiotactic PVA content of 40-70 wt%, and a 1,2-diol content of ≤0.2 wt%.

2. The preparation method according to claim 1, characterized in that, The pretreatment conditions include: cleaning the PVA resin to ensure that the sodium acetate content of the PVA resin is ≤0.01wt%.

3. The preparation method according to claim 1 or 2, characterized in that, The solvent is deionized water with a conductivity ≤0.5μS / cm and a pH of 7~7.5; The solid content of the PVA solution is 25-35 wt%. The preparation conditions for the PVA solution include: a dissolution temperature of 130~150℃, and nitrogen gas being used to purge oxygen during the dissolution process.

4. The preparation method according to any one of claims 1-3, characterized in that, The PVA solution also contains plasticizers and antioxidants; Preferably, the plasticizer is selected from one or more of glycerol, sorbitol, pentaerythritol, xylitol, polyethylene glycol, and caprolactam; The antioxidant is a hindered phenolic antioxidant and / or a benzene ring-containing phosphite antioxidant; The hindered phenolic antioxidant is selected from one or more of antioxidant 1010, antioxidant 1076, antioxidant 3114, antioxidant 245 and antioxidant 1330; The benzene ring-containing phosphite antioxidant is selected from one or more of antioxidant 168, antioxidant 626, antioxidant TPP, and antioxidant TNPP.

5. The preparation method according to any one of claims 1-4, characterized in that, The filtration system employs a four-stage cascade filtration process with filtration accuracies of 20μm, 10μm, 5μm, and 2μm, respectively. The defoaming and homogenization conditions include: circulating the PVA solution among at least three defoaming vessels for homogenization, with a defoaming negative pressure of -0.1 ~ -0.06 MPa; Preferably, the PVA solution is conveyed by an extruder before defoaming and homogenization after filtration. The inlet pressure of the GP pump of the extruder is ≥2.5MPa, and the negative pressure of the exhaust port of the extruder is -0.09 ~ -0.07MPa.

6. The preparation method according to any one of claims 1-5, characterized in that, The conditions for casting film formation include: casting the defoamed PVA solution through a die to a casting roller, the temperature of the casting roller being 80~95℃, a fan hood being installed above the casting roller, the temperature of the fan hood being 5~10℃ lower than the temperature of the casting roller, the surface roughness Ra of the casting roller being ≤0.005, and the concentricity of the outer surface being <0.02mm.

7. The preparation method according to any one of claims 1-6, characterized in that, The drying conditions include: passing the cast PVA film sequentially through an even number of drying rollers, with the drying rollers arranged in pairs and each pair having the same temperature, the temperature difference between adjacent pairs of drying rollers ≤ 5℃, the overall temperature of the drying rollers decreasing gradually from 95℃ to 40℃, the wrap angle of the PVA film entering each drying roller being the same, the ratio of the linear velocity of the first drying roller to the linear velocity of the casting roller being 100.1~103%, and the linear velocity of subsequent drying rollers being adjusted step by step according to the thermal shrinkage rate of the PVA film, wherein the linear velocity of the current drying roller = the linear velocity of the previous drying roller × (1 - the thermal shrinkage rate of the current segment of PVA film); Preferably, the drying roller is made of carbon fiber.

8. The preparation method according to any one of claims 1-7, characterized in that, The heat treatment conditions include: sending the dried PVA film into an air-floating oven, with the temperature of each section of the oven not exceeding 80°C and showing a trend of first increasing and then decreasing, and the moisture content of the PVA film after heat treatment being controlled at 3~8%.

9. The preparation method according to any one of claims 1-8, characterized in that, The conditions for slitting and winding include: slitting the heat-treated PVA film, and making the inner and outer winding directions of the slitting roll opposite to the winding direction of the semi-finished roll. Preferably, the guide rollers used in the PVA film transport process in the preparation method are made of carbon fiber.

10. A PVA optical film prepared by the method according to any one of claims 1-9, characterized in that, The thickness of the PVA optical film is 30~45μm, with a thickness tolerance of ≤1μm; the difference in water content between any two points on the film surface is ≤1%, and the difference in water content between the front and back sides is ≤0.8%; the crystallinity difference is ≤3%; the stretching ratio is 5.9~6 times; and the height h of the four corners after swelling is ≤1mm.

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