Modified polyvinyl alcohol optical film as well as preparation method and application thereof

Abstract

The invention relates to the technical field of preparation of high polymer materials and optical films, in particular to a modified polyvinyl alcohol optical film as well as a preparation method and application thereof. The modified polyvinyl alcohol optical film is composed of a matrix and a modified monomer, the matrix is PVA resin, the alcoholysis degree of the PVA resin is greater than or equal to 99%, and the polymerization degree of the PVA resin is 1700-2000; the modified monomer is EVOH, and the ethylene unit content in the EVOH is 7-9 mol%; the thickness of the modified polyvinyl alcohol optical film is 10-60 [mu] m, the swelling degree is 90-115%, and the wet uniaxial stretching rate is 6.75-6.95 times. The modified PVA optical film has low swelling degree, high wet-state uniaxial stretching rate, low birefringence and excellent damp-heat stability, can realize high stretching rate under the wet-state uniaxial stretching condition without easy breakage, inhibits in-plane orientation unevenness in a drying film forming process, is beneficial to obtaining a PVA optical base film with good optical uniformity, and has a good application prospect. The invention is suitable for liquid crystal display and other optical purposes.

Description

Technical Field

[0001] This invention relates to the field of polymer materials and optical thin film preparation technology, specifically to a modified polyvinyl alcohol optical film, its preparation method, and its application. Background Technology

[0002] Polyvinyl alcohol (PVA) film occupies an important position in the field of optical materials due to its excellent high transparency and good optical uniformity. In particular, as the core material of polarizer base film for liquid crystal displays, it is widely used in display technology and precision optical devices.

[0003] However, traditional pure PVA optical films have significant performance shortcomings that limit their application in high-performance scenarios: PVA molecular chains have extremely strong hydrophilicity, and unmodified films are prone to severe swelling and dimensional deformation in water or high humidity environments, resulting in insufficient thermal stability and hydrothermal dimensional stability; at the same time, pure PVA films are brittle, have low elongation at break, and a narrow stretching processing window. When preparing high-quality films with a thickness of less than 60 μm, problems such as excessively rapid initial swelling, uneven crystal growth during drying, and premature pre-orientation of the wet film can easily occur, leading to excessively rapid transverse shrinkage, wrinkling, and breakage during subsequent uniaxial stretching, making it difficult to meet the requirements of high-ratio stretching processes.

[0004] To improve the water resistance and toughness of PVA films, the industry has undertaken various modification attempts: adding plasticizers such as glycerol and ethylene glycol to enhance flexibility, but this significantly weakens the film's damp heat durability; introducing crosslinking agents such as borides and aldehydes can improve structural stability, but increases process complexity and may reduce film transparency; when blended with vinyl acetate-based polymers, poor compatibility easily leads to phase separation, resulting in pore scattering, which severely affects optical haze and birefringence control. For applications such as polarizing film base films, which require extremely high optical uniformity, traditional modification methods often struggle to simultaneously improve film tensile properties and moisture resistance while maintaining film uniformity and orientation stability, easily introducing structural inhomogeneities or local defects, thus affecting the polarization efficiency, brightness consistency, and damp heat reliability of polarizers, limiting their application in high-performance display fields.

[0005] Kuraray Corporation of Japan, in its research on ethylene-modified polyvinyl alcohol (PVA) polymer films containing ethylene units at a content of 1–4 mol%, proposed a systematic method for determining in-plane birefringence. This method involves measuring the refractive indices nMD, nTD, and nz of the film in the mechanical flow direction (MD), width direction (TD), and thickness direction (z), defining the refractive index difference between the mechanical flow direction (MD) and the thickness direction (z) as... n(MD) = nMD The refractive index difference between nz and the width direction (TD) and the thickness direction (z) is n(TD) = nTD nz. And further employ thickness-direction averaging. n(MD) Ave and n(TD) Ave To evaluate the overall orientation state of the thin film. The birefringence parameters mentioned above are limited to a strict range (e.g., n(TD) Ave ≤2.5×10 -3 and n(MD) Ave -0.1×10 -3 ≤ n(TD) Ave ≤ n(MD) Ave +0.25×10 -3 This is used to distinguish whether uneven pre-orientation is formed during the film formation process. In the above formula, n(MD) Ave The birefringence of the ethylene-modified polyvinyl alcohol polymer film, expressed as an average value along the thickness direction of the film, represents the direction of mechanical flow. n(TD) Ave The value represents the birefringence in the width direction of the ethylene-modified polyvinyl alcohol (PVA) polymer film, averaged over the thickness direction. Simultaneously, by precisely controlling the volatile fraction interval, the drying roll speed ratio, and the winding speed ratio, undesirable orientation of the PVA film during the drying stage was effectively suppressed, enabling the ethylene-modified PVA film to achieve higher magnification and optical uniformity in subsequent wet stretching. These results indicate that modification with ethylene units in the range of 1–4 mol% not only improves wet toughness but also enhances the birefringence uniformity and stretching processing window of the final film.

[0006] In summary, the existing technology lacks a modification scheme that can significantly improve the film's resistance to moisture swelling, stretching processability, and dimensional stability. This technological gap limits the application of PVA optical films in more demanding environments and high-performance display fields. Therefore, developing a modified PVA optical film with multiple superior properties has become an urgent technical problem to be solved in this field. Summary of the Invention

[0007] The purpose of this invention is to overcome the defects of traditional PVA optical films, such as easy swelling, poor toughness, and insufficient stretching ratio, and to provide a modified PVA optical film with low swelling, high wet uniaxial stretching ratio, low birefringence, and excellent hygrothermal stability, as well as its preparation method. This modified PVA optical film can achieve a high stretching ratio under wet uniaxial stretching conditions without easily breaking, and suppresses in-plane orientation non-uniformity during the drying and film formation process, which is beneficial to obtaining PVA optical films with good optical uniformity and is suitable for optical applications such as liquid crystal displays.

[0008] To achieve the above objectives, in a first aspect, the present invention provides a modified polyvinyl alcohol optical film, wherein the modified polyvinyl alcohol optical film is composed of a matrix and a modified monomer; The matrix is ​​PVA resin, and the degree of alcoholysis of the PVA resin is ≥99%, and the degree of polymerization is 1700~2000; The modified monomer is EVOH, and the EVOH contains 7-9 mol% ethylene units. The modified polyvinyl alcohol optical film has a thickness of 10~60μm, a swelling degree of 90~115%, and a stretching ratio of 6.75~6.95 times.

[0009] Preferably, the vinyl content in the modified polyvinyl alcohol optical film is 2-5 mol%.

[0010] Secondly, the present invention provides a method for preparing a modified polyvinyl alcohol optical film as described in the present invention, the method comprising: S1. Mix the EVOH dispersion, plasticizer, and surfactant to obtain a pre-blended system; S2. Add PVA aqueous solution to the pre-blended system in step S1 and mix to form a modified blended casting solution; S3. The blended casting liquid from step S2 is used to form a film using a casting process.

[0011] Preferably, in step S1, the method for preparing the EVOH dispersion includes: adding EVOH resin to water and stirring at a stirring rate of 200-400 rpm for 0.5-3 hours at a temperature of 80-130°C.

[0012] Preferably, in step S1, the plasticizer is selected from one or more of glycerol, diglycerol, triethylene glycol, trimethylolpropane, and polyethylene glycol.

[0013] Preferably, in step S1, the surfactant is a nonionic surfactant with an HLB value of 11-17, preferably 12-15; it is preferably selected from one or more of fatty alcohol polyoxyethylene ethers, Tween surfactants, and nonylphenol polyoxyethylene ethers.

[0014] More preferably, the fatty alcohol polyoxyethylene ether is selected from one or more of AEO-7, AEO-9, AEO-10 ​​and AEO-12.

[0015] More preferably, the Tween type is selected from one or more of Tween-20, Tween-40, Tween-60 and Tween-80.

[0016] More preferably, the nonylphenol polyoxyethylene ether is selected from one or more of NP-8, NP-10, NP-12 and NP-15.

[0017] Preferably, in step S1, the mass ratio of the EVOH dispersion, plasticizer, and surfactant is 1:0.015~0.045:0.0003~0.006, wherein the EVOH dispersion is based on the mass of EVOH solids.

[0018] Preferably, the mass ratio of the EVOH dispersion to the surfactant is 1:0.0005~0.003, wherein the EVOH dispersion is based on the mass of EVOH solids. Preferably, in step S1, the mixing conditions include: a temperature of 80~130℃, a stirring rate of 200~400rpm, and a time of 0.5~3h.

[0019] Preferably, in step S2, the mass fraction of the PVA aqueous solution is 10~15wt%.

[0020] Preferably, in step S2, the mass ratio of the pre-blended system to the PVA aqueous solution is 0.3~0.5:1.

[0021] Preferably, in step S2, the mixing conditions include: gradually adding the pre-blended system to the PVA aqueous solution at a temperature of 80~130℃, with a stirring rate of 250~350rpm and a stirring time of 1~2h.

[0022] Preferably, in step S2, the modified blend casting solution needs to be degassed and filtered. The degassed solution is degassed using a vacuum degassed method, and the degassed time is 10-20 minutes.

[0023] Preferably, in step S3, the casting conditions include: uniformly coating the modified blend casting liquid onto the casting roller through a flat die, drying in sections, peeling off after drying to obtain a dried film, and winding it up to obtain a modified polyvinyl alcohol optical film.

[0024] More preferably, the material of the casting roller is mirror stainless steel or chrome-plated steel.

[0025] Preferably, the segmented drying adopts a multi-roller contact drying method, including pre-drying, heat setting and cooling.

[0026] More preferably, the pre-drying conditions include: a drying roller temperature of 80~100℃, preferably 80~90℃, and a drying roller speed ratio of 1~1.004.

[0027] More preferably, the heat setting conditions include: a drying roller temperature of 90~120℃, preferably 90~100℃, and a drying roller speed ratio of 1~1.003.

[0028] More preferably, the cooling conditions include: a drying roller temperature of 60~80℃ and a drying roller speed ratio of 0.992~0.998.

[0029] More preferably, the drying roller speed ratio is the ratio of the circumferential speed of each roller to the circumferential speed of the first roller, and the circumferential speed of the first roller is 7~8 m / min, preferably 7.4~7.8 m / min.

[0030] Thirdly, the present invention provides an application of the modified polyvinyl alcohol optical film as described in the present invention in the preparation of liquid crystal display devices, precision optical components and optical compensation devices.

[0031] In the above technical solution, the modified polyvinyl alcohol optical film of the present invention uses PVA with high degree of alcoholysis (≥99%) and high degree of polymerization (1700~2000) as the matrix to ensure the film strength and basic optical properties. By precisely controlling the EVOH ethylene unit content at 7~9 mol% and the blending ratio with PVA, a small amount of hydrophobic vinyl units are introduced to reduce the local crystallinity and hydrogen bond density in the modified polyvinyl alcohol optical film, thereby improving its overall ductility and providing a basis for high-ratio stretching. At the same time, the compatibility between PVA and EVOH is improved through the synergistic effect of plasticizer and surfactant, enhancing its wet ductility and stabilizing the wet uniaxial stretching ratio in the excellent range of 6.75~6.95 times.

[0032] Furthermore, the preparation method of the modified polyvinyl alcohol optical film of this invention involves selective pre-modification of EVOH before blending PVA and EVOH. The crystallinity of EVOH is initially reduced through water swelling dispersion, followed by the addition of a plasticizer and a surfactant. The plasticizer preferentially penetrates the EVOH chain segments, further softening the EVOH phase; the surfactant accumulates at the EVOH interface, reducing the interfacial tension between PVA and EVOH. The synergistic effect of these two agents solves the problems of poor compatibility and easy phase separation in traditional blending systems, ensuring the uniformity of the blend system and reducing optical scattering. Simultaneously, the pre-drying stage uses slow drying at 80-100℃ to avoid excessively rapid hardening of the film surface and internal stress concentration. During the heat-setting drying stage, the temperature is gradually increased and the tension is progressively increased to stabilize the film size and regulate the molecular orientation, controlling the refractive index difference between the mechanical and width directions of the film to within 10. -4 This achieves a synergy between low birefringence and high dimensional stability.

[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 modified polyvinyl alcohol optical film, wherein the modified polyvinyl alcohol optical film is composed of a matrix and a modified monomer; The matrix is ​​PVA resin, and the degree of alcoholysis of the PVA resin is ≥99%, and the degree of polymerization is 1700~2000, to ensure that the film has sufficient film strength and optical uniformity. The modified monomer is EVOH, and the ethylene unit content in EVOH is 7~9 mol%, which improves the toughness and moisture resistance of the film through synergistic effect with PVA; The modified polyvinyl alcohol optical film has a thickness of 10~60μm, a swelling degree of 90~115%, and a stretching ratio of 6.75~6.95 times.

[0037] The modified polyvinyl alcohol optical film of the present invention is modified by introducing EVOH, so that the prepared optical film has low swelling, high wet stretchability and excellent hygrothermal stability.

[0038] In this invention, the vinyl content in the modified polyvinyl alcohol optical film is 2-5 mol.

[0039] In this invention, the modified polyvinyl alcohol optical film also possesses the following preferred properties: refractive index difference in the mechanical direction (MD) and the width direction (TD). n is 10 -4 Quantity, and satisfy n(MD) Ave -0.1×10 -3 ≤ n(TD) Ave ≤ n(MD) Ave +0.25×10 -3 , n(TD) Ave ≤2.5×10 -3 The wet uniaxial stretching ratio is ≥6.5 times, and the mass swelling rate in water at room temperature (15~30℃) for 5 minutes is 110~120%. It has both stretching processability, wet heat stability and excellent optical isotropy.

[0040] Secondly, the present invention provides a method for preparing a modified polyvinyl alcohol optical film as described in the present invention, the method comprising: S1. Mix the EVOH dispersion, plasticizer, and surfactant to obtain a pre-blended system; S2. Add PVA aqueous solution to the pre-blended system in step S1 and mix to form a modified blended casting solution; S3. The blended casting liquid from step S2 is used to form a film using a casting process.

[0041] In this invention, in step S1, the method for preparing the EVOH dispersion includes: adding EVOH resin to water, stirring and swelling at a stirring rate of 200-400 rpm for 0.5-3 hours at a temperature of 80-130°C, so that EVOH can fully absorb water and swell to form a uniform aqueous dispersion, thereby reducing its crystallinity and hardness and facilitating subsequent blending.

[0042] In this invention, in step S1, the plasticizer can be a polyol that is compatible with both PVA and EVOH, such as one or more selected from glycerol, diglycerol, triethylene glycol, trimethylolpropane and polyethylene glycol. Low molecular weight polyethylene glycol such as PEG-400 is preferred, as it has good compatibility with both PVA and EVOH, can penetrate into the gaps between EVOH molecular chains, further soften the EVOH phase, and improve its ductility.

[0043] In this invention, in step S1, the surfactant is a nonionic surfactant with an HLB value of 11-17, preferably 12-15; preferably selected from one or more of fatty alcohol polyoxyethylene ethers, Tween compounds, and nonylphenol polyoxyethylene ethers. The fatty alcohol polyoxyethylene ether is selected from one or more of AEO-7, AEO-9, AEO-10, and AEO-12. The Tween compounds are selected from one or more of Tween-20, Tween-40, Tween-60, and Tween-80. The nonylphenol polyoxyethylene ether is selected from one or more of NP-8, NP-10, NP-12, and NP-15. The surfactant can adsorb and accumulate at the interface of EVOH particles, forming an interfacial wetting layer, effectively reducing the surface tension at the PVA-EVOH phase interface and preventing phase separation during blending.

[0044] In this invention, in step S1, the mass ratio of the EVOH dispersion, plasticizer and surfactant is 1:0.015~0.045:0.0003~0.006, wherein the EVOH dispersion is based on the mass of EVOH solids.

[0045] In a preferred embodiment of the present invention, the mass ratio of the EVOH dispersion to the surfactant is 1:0.0005~0.003, wherein the EVOH dispersion is based on the mass of EVOH solids.

[0046] In this invention, the mixing conditions in step S1 include: a temperature of 80~130℃, a stirring rate of 200~400rpm, and a time of 0.5~3h.

[0047] In this invention, the plasticizing and wetting pretreatment of EVOH in step S1 lowers the glass transition temperature and crystal domain strength of EVOH, making it easier to deform and flow; on the other hand, it increases the hydrophilicity of the EVOH particle surface, preventing large-scale phase separation during subsequent blending with PVA. After this pre-modification treatment, the EVOH in the resulting pre-blended system is in a softened microparticle or semi-dissolved state and is stably dispersed in the aqueous phase.

[0048] In this invention, in step S2, the mass fraction of the PVA aqueous solution is 10~15wt%, and its preparation method is as follows: PVA resin is added to deionized water at 80~130℃, stirred and dissolved until a homogeneous and transparent solution is formed, and the temperature is maintained at 80~130℃ to avoid cooling and precipitation. In a preferred embodiment of the present invention, in step S2, the method for preparing the PVA aqueous solution is as follows: PVA resin is added to deionized water at 80~95°C, stirred and dissolved until a homogeneous and transparent solution is formed, and the temperature is maintained at 80~95°C to prevent cooling and precipitation.

[0049] In this invention, in step S2, the mass ratio of the pre-blended system to the PVA aqueous solution is 0.3~0.5:1.

[0050] This invention adjusts the solid content of the EVOH dispersion so that, after mixing with the PVA aqueous solution, the EVOH solid accounts for 20-60 wt% of the total polymer in the mixed system, based on the solid mass of PVA and EVOH. This results in the molar fraction of ethylene units in the obtained modified blend casting solution being controlled at 2-5 mol.

[0051] In this invention, in step S2, the mixing conditions include: gradually adding the pre-blended system to a PVA aqueous solution at a temperature of 80~130℃, with a stirring rate of 250~350rpm and a stirring time of 1~2h. During this process, due to the presence of surfactants in the pre-blended system, EVOH particles can be well dispersed in the continuous PVA phase without flocculation or precipitation; the plasticizer is uniformly distributed between the PVA and EVOH segments, playing an internal plasticizing role.

[0052] In this invention, in step S2, if the modified blend casting solution contains suspended particles, it needs to undergo degassing and filtration to remove air bubbles and impurities, ensuring the uniformity and purity of the modified blend casting solution. The degassing is performed using a vacuum degassing method for 10-20 minutes to remove air bubbles from the system; filtration is performed using a filter screen to remove undissolved particles and impurities, ensuring the uniformity and purity of the casting solution.

[0053] In this invention, the casting conditions in step S3 include: uniformly coating the modified blend casting liquid onto the casting roller through a flat die, drying in sections, peeling off after drying to obtain a dried film, and winding to obtain a modified polyvinyl alcohol optical film.

[0054] In this invention, the material of the casting roller is mirror stainless steel or chrome-plated steel.

[0055] In this invention, the segmented drying adopts a multi-roller contact drying method, which includes pre-drying, heat setting and cooling.

[0056] First, a pre-drying process is performed to apply low-tension traction to the modified polyvinyl alcohol optical film to avoid wrinkles. The pre-drying conditions include: a drying roller temperature of 80~100℃, preferably 80~90℃, and a drying roller speed ratio of 1~1.004; the moisture content of the modified polyvinyl alcohol optical film is adjusted from 10~15wt% when it enters the first drying roller and stabilized in the range of 10~20wt%.

[0057] Secondly, heat setting is performed by appropriately increasing the traction speed and tension to slightly stretch the modified polyvinyl alcohol optical film longitudinally to compensate for drying shrinkage. The heat setting conditions include: drying roller temperature of 90~120℃, preferably 90~100℃, and drying roller speed ratio of 1~1.003. This step further dries and heat sets the modified polyvinyl alcohol optical film, reducing the moisture content of the modified polyvinyl alcohol optical film to <4.5wt%.

[0058] Finally, cooling is performed. The cooling conditions include: the temperature of the drying roller is 60~80℃, and the speed ratio of the drying roller is 0.992~0.998. This step of gradually cooling down and lowering the roller is used to cool down and relieve stress on the modified polyvinyl alcohol optical film, so as to stabilize the size and orientation of the modified polyvinyl alcohol optical film.

[0059] The drying roller speed ratio is the ratio of the circumferential speed of each roller to the circumferential speed of the first roller, and the circumferential speed of the first roller is 7~8 m / min, preferably 7.4~7.8 m / min.

[0060] In this invention, the conditions of the pre-drying stage include: a temperature of 60~80°C, drying until the residual moisture in the film is 10~20%, applying moderate tension during this stage to prevent the film surface from shrinking and curling, while allowing the film to release some stress.

[0061] In this invention, the conditions of the heat setting and drying stage include: a temperature of 90~120℃, gradual heating in multiple stages, drying until the film moisture content is <3%, and applying segmented increasing tension control in this stage, with the final tension being 1.2~1.5 times that of the pre-drying stage tension, in order to flatten the film, stabilize the lateral dimensions, and reduce optical anisotropy.

[0062] Thirdly, the present invention provides an application of the modified polyvinyl alcohol optical film as described in the present invention in the preparation of liquid crystal display devices, precision optical components and optical compensation devices.

[0063] 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.

[0064] Preparation Example 1 Preparation of EVOH dispersion: 144.30 kg of EVOH resin (ethylene unit content 8 mol%), 43.29 kg of glycerol and 2.40 kg of nonionic surfactant were added to 1443.00 kg of deionized water and stirred and dispersed at 98℃ for 2 h; then the temperature was maintained at 80℃ to obtain a uniform and stable EVOH dispersion with no obvious phase separation or precipitation, which was denoted as A1.

[0065] Preparation Example 2 The method of Preparation Example 1 was followed, except that "glycerol" was replaced with "a ternary mixture of glycerol, triethylene glycol and trimethylolpropane", resulting in a homogeneous and stable EVOH dispersion, denoted as A2.

[0066] Example 1 S1. Preparation of the pre-blended system: 336.75 kg of PVA resin (degree of polymerization 1800, degree of hydrolysis 99.9%) was added to 3030.75 kg of deionized water, heated to 100℃ and stirred for 2 h until completely dissolved to form a homogeneous transparent solution with a mass fraction of 10 wt%. After keeping warm at 80℃ for 2 h, a homogeneous and stable PVA aqueous solution with no obvious precipitation was obtained, denoted as B.

[0067] S2. Preparation of modified blend casting solution: The PVA aqueous solution B obtained in step S1 is slowly added to the EVOH dispersion A1 obtained in Preparation Example 1, and stirring is maintained for 1.5 h to form a transparent modified PVA stock solution; then the stock solution is degassed under reduced pressure for 15 min (the pressure range is recommended to be -0.05~-0.07 MPa) to remove the mixed air bubbles; then it is filtered through two-stage filters of 5 μm and 3 μm to ensure that there are no undissolved particles and impurities in the casting solution, so as to obtain a castable blend casting solution.

[0068] S3, Casting and casting film formation: The degassed and filtered blended casting liquid is injected into the T-die head and uniformly coated onto the surface of the clean chrome-plated steel casting roller. The coated film enters the multi-roller contact drying channel composed of 16 parallel rotating drying rollers, and undergoes pre-drying, heat setting and cooling stress relief in sequence. After drying, the film is automatically peeled off from the steel strip and wound up by the guide roller to obtain the modified polyvinyl alcohol optical film, denoted as B1.

[0069] The speed ratio is based on the circumferential speed S1 of the first roller, and the circumferential speeds S of each roller are... i The ratio of the circumferential speed to that of the first roller, the temperature is set by the roller surface, and the specific values ​​are shown in Table 1: Table 1 Note: S1 = 7.6 m / min.

[0070]

[0071] The modified polyvinyl alcohol optical film prepared in this embodiment has a smooth and transparent appearance with no visible spots or crystals. The dry film thickness was measured to be 60 μm with a thickness deviation of ±2%. The film surface has good uniformity and the conveyor belt is stable during continuous production without obvious deviation or film breakage.

[0072] Example 2 The method described in Example 1 was carried out, except that in step S2, "slowly add the PVA aqueous solution B obtained in step S1 to the EVOH dispersion A1 obtained in Preparation Example 1" was replaced with "slowly add the PVA aqueous solution B obtained in step S1 to the EVOH dispersion A2 obtained in Preparation Example 2". All other steps remained the same, and a modified polyvinyl alcohol optical film, denoted as B2, was obtained.

[0073] Comparative Example 1 The method of Example 1 was carried out, except that the EVOH dispersion A1 obtained in Preparation Example 1 was not added. Instead, PVA aqueous solution B was used as the casting liquid for casting film production. Other conditions remained unchanged, and a modified polyvinyl alcohol optical film, denoted as D1, was obtained.

[0074] Detection Example 1 The modified polyvinyl alcohol optical films prepared in Examples 1-2 and Comparative Example 1 were subjected to performance tests. All performance tests were conducted in a standard laboratory environment (temperature 25°C, relative humidity 50%), and the test standards and methods strictly followed industry specifications and technical solution verification requirements, as detailed below: (1) Mechanical property testing (tensile strength, Young's modulus, wet limit tensile strength): Refer to the national standard GB / T1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets".

[0075] Dry test: Cut a dumbbell-shaped sample with a length of 150 mm and a width of 15 mm, with a gauge length of 50 mm, and take the thickness as the average value of the test area; The specimen is clamped in the testing machine fixture, the equipment is started to perform tensile testing, and the tensile strength and Young's modulus at fracture are recorded (calculated by the initial slope of the tensile test).

[0076] Wet test: After cutting the sample to the same specification, immerse it in desalinated water at 30℃ for 30 minutes, take it out and blot the surface moisture with filter paper, and test it immediately. Similarly, clamp the specimen and stretch it continuously until the specimen breaks, and record the maximum stretch ratio (the ratio of the gauge length at break to the initial gauge length).

[0077] Instrument selection: Electronic universal testing machine was used, and the tensile speed was set to 50 mm / min.

[0078] Results determination: A dry tensile strength ≥110MPa, Young's modulus ≥2.9GPa, and wet ultimate tensile ratio ≥6.95 times are considered qualified.

[0079] (2) Moisture swelling resistance test (mass swelling rate): Immerse the sample completely in 30℃ desalinated water for 5 minutes, then remove it and quickly blot the surface moisture with clean filter paper. Immediately weigh the mass m1 after immersion (accurate to 0.001g).

[0080] Calculation results: Mass swelling rate = (m1-m0) / m0×100%, take the average value of 3 groups of parallel samples.

[0081] Result determination: A swelling rate within the range of 90% to 115% is considered acceptable.

[0082] (3) Thickness uniformity test: A high-precision contact thickness gauge (accuracy 0.1μm) was used to select 9 evenly distributed test points (3×3 matrix) on a 100mm×100mm sample, and the thickness values ​​were measured and the thickness deviation was calculated.

[0083] Thickness deviation = (maximum thickness) (Minimum thickness) / Average thickness × 100%.

[0084] The test results are shown in Table 2.

[0085] Table 2

[0086] As shown in Table 2, the swelling rates of the modified polyvinyl alcohol optical films prepared in Examples 1-2 of this invention are 115% and 98%, respectively. This indicates that the modified polyvinyl alcohol optical films of this invention have low swelling, small volume changes before and after water absorption, and high wet modulus, resulting in strong dimensional stability, small deformation under humid and hot conditions, good uniformity during stretching, and reduced probability of edge rot. In contrast, the modified polyvinyl alcohol optical film of Comparative Example 1 has a swelling rate of 125%, far exceeding the required 90-115%. It has a large volume change after water absorption, poor dimensional stability, and is prone to significant deformation under humid and hot conditions. The wet modulus decreases significantly, and the excessive swelling of molecular chains during stretching leads to uneven stress, greatly increasing the probability of edge rot and breakage of the optical film, making it difficult to adapt to high-ratio stretching processes. This directly confirms the necessity of the modification scheme of this invention. By introducing hydrophobic vinyl units through EVOH and synergistically optimizing compatibility with plasticizers and surfactants, the water absorption and swelling of PVA films can be effectively suppressed, significantly improving the core performance of low swelling and high dimensional stability.

[0087] Table 2 shows the thickness and thickness deviation data of the modified polyvinyl alcohol optical films prepared in Examples 1-2 of the present invention, which further confirms the advantage of the modified polyvinyl alcohol optical films prepared according to the technical solution of the present invention having strong dimensional stability.

[0088] The modified polyvinyl alcohol optical film of this invention solves the phase separation problem in the blended system through a precise blending and pre-modification process of EVOH and PVA, and reduces the refractive index difference between the mechanical and width directions of the film. n≤10 -4 With excellent optical isotropy, it effectively avoids polarization state changes or color shifts during light propagation, meeting the requirements of high-end optical devices. Furthermore, the modified polyvinyl alcohol optical film of this invention has a wet-state limiting tensile ratio of ≥6.5 times, reaching a maximum of 6.91 times, which is far superior to traditional pure PVA film (≤5 times). It can adapt to the high-ratio stretching process in polarizing film production, reducing breakage losses during the stretching process; at the same time, it maintains a room-temperature dry tensile strength of 110~125MPa, combining high strength and high ductility.

[0089] Furthermore, the modified polyvinyl alcohol optical film of the present invention, through the introduction of vinyl units and the interface regulation of surfactants, controls the mass swelling rate of the film in room temperature water for 5 minutes to be 90-115%, which is 10-20% lower than that of traditional pure PVA film. It does not exhibit bubbling or warping under humid heat cycling conditions, has stable optical performance, and can be well matched with polarizer substrates such as TAC film and COP film.

[0090] Furthermore, the process parameters of this invention, such as pre-modification, blending, segmented drying, and tension control, are clearly defined, making it easy to scale up for industrial production. Moreover, the raw materials are all commercially available products, resulting in low procurement costs and stable supply, thus demonstrating good prospects for industrial application.

[0091] The modified PVA optical film of this invention can be used as a high-performance optical base film and is widely used in liquid crystal display devices, precision optical components and optical compensation devices. It is especially suitable for high-brightness displays, precision optical instruments and other scenarios with stringent performance requirements, and has important industrial value.

[0092] Detection Example 2 The birefringence properties (refractive index difference) of the modified polyvinyl alcohol optical films prepared in Examples 1-2 were tested. (n), the specific operation is as follows.

[0093] Referencing the birefringence determination system proposed by Kuraray; A birefringence tester using polarized interferometry was employed, with a test wavelength of 589 nm. The refractive indices nMD, nTD, and nz of the thin film were measured in the mechanical flow direction (MD), width direction (TD), and thickness direction (nz), respectively; the birefringence parameters were calculated. n(MD) = nMD - nz n(TD) = nTD - nz, and then obtained by averaging in the thickness direction. n(MD) Ave and n(TD)Ave .

[0094] Result determination: Must meet the following conditions. n(TD) Ave ≤2.5×10 - ³, and n(MD) Ave -0.1×10 -3 ≤ n(TD)Ave≤ n(MD)Ave+0.25×10 -3 Refractive index difference between MD and TD directions n (i.e., |nMD-nTD|) is controlled within 10 -4 The magnitude is acceptable; see Table 3 for specific results.

[0095] Table 3

[0096] The refractive index difference in the MD and TD directions of the modified polyvinyl alcohol optical film prepared in Examples 1-2 of this invention n is 5 × 10 -5 It fully meets the refractive characterization requirements of the uniformly pre-oriented ethylene-modified PVA optical film proposed by Kuraray.

[0097] In summary, this invention successfully prepared a modified polyvinyl alcohol optical film with excellent comprehensive performance through optimized raw material ratios, innovative pre-modification processes, and precise film formation control. This overcomes many defects of traditional PVA films and provides a new high-performance solution for the field of optical thin films.

[0098] 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.

[0099] 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.

[0100] 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 modified polyvinyl alcohol optical film, characterized in that, The modified polyvinyl alcohol optical film is composed of a matrix and a modified monomer; The matrix is ​​PVA resin, and the degree of alcoholysis of the PVA resin is ≥99%, and the degree of polymerization is 1700~2000; The modified monomer is EVOH, and the EVOH contains 7-9 mol% ethylene units. The modified polyvinyl alcohol optical film has a thickness of 10~60μm, a swelling degree of 90~115%, and a stretching ratio of 6.75~6.95 times.

2. The modified polyvinyl alcohol optical film according to claim 1, characterized in that, The modified polyvinyl alcohol optical film contains 2-5 mol of vinyl content.

3. A method for preparing a modified polyvinyl alcohol optical film as described in claim 1 or 2, characterized in that, The preparation method includes: S1. Mix the EVOH dispersion, plasticizer, and surfactant to obtain a pre-blended system; S2. Add PVA aqueous solution to the pre-blended system in step S1 and mix to form a modified blended casting solution; S3. The blended casting liquid from step S2 is used to form a film using a casting process.

4. The preparation method according to claim 3, characterized in that, In step S1, the preparation method of the EVOH dispersion includes: adding EVOH resin to water and stirring at a stirring rate of 200-400 rpm for 0.5-3 hours at a temperature of 80-130℃.

5. The preparation method according to claim 3 or 4, characterized in that, In step S1, the plasticizer is selected from one or more of glycerol, diglycerol, triethylene glycol, trimethylolpropane, and polyethylene glycol; The surfactant is a nonionic surfactant with an HLB value of 11-17, preferably 12-15; it is preferably selected from one or more of fatty alcohol polyoxyethylene ethers, Tween surfactants, and nonylphenol polyoxyethylene ethers.

6. The preparation method according to claim 5, characterized in that, The fatty alcohol polyoxyethylene ether is selected from one or more of AEO-7, AEO-9, AEO-10 ​​and AEO-12; The Tween series is selected from one or more of Tween-20, Tween-40, Tween-60, and Tween-80; The nonylphenol polyoxyethylene ether is selected from one or more of NP-8, NP-10, NP-12 and NP-15.

7. The preparation method according to any one of claims 3-6, characterized in that, In step S1, the mass ratio of the EVOH dispersion, plasticizer, and surfactant is 1:0.015~0.045:0.0003~0.006, wherein the EVOH dispersion is based on the mass of EVOH solids. Preferably, the mass ratio of the EVOH dispersion to the surfactant is 1:0.0005~0.003, wherein the EVOH dispersion is based on the mass of EVOH solids. The mixing conditions include: a temperature of 80~130℃, a stirring rate of 200~400rpm, and a time of 0.5~3h.

8. The preparation method according to any one of claims 3-7, characterized in that, In step S2, the mass fraction of the PVA aqueous solution is 10~15 wt%. The mass ratio of the pre-blended system to the PVA aqueous solution is 0.3~0.5:1; The mixing conditions include: gradually adding the pre-blended system to the PVA aqueous solution at a temperature of 80~130℃, with a stirring rate of 250~350rpm and a stirring time of 1~2h. Preferably, the modified blend casting solution needs to be degassed and filtered. The degassed solution is performed using a vacuum degassed method, and the degassed time is 10-20 minutes.

9. The preparation method according to any one of claims 3-8, characterized in that, In step S3, the casting conditions include: uniformly coating the modified blend casting liquid onto the casting roller through a flat die, drying in sections, peeling off after drying to obtain a dried film, and winding it up to obtain a modified polyvinyl alcohol optical film. Preferably, the material of the casting roller is mirror stainless steel or chrome-plated steel; Preferably, the segmented drying adopts a multi-roller contact drying method, including pre-drying, heat setting and cooling; The pre-drying conditions include: the drying roller temperature is 80~100℃, preferably 80~90℃, and the drying roller speed ratio is 1~1.004; The heat setting conditions include: a drying roller temperature of 90~120℃, preferably 90~100℃, and a drying roller speed ratio of 1~1.003; The cooling conditions include: the temperature of the drying roller is 60~80℃, and the speed ratio of the drying roller is 0.992~0.998; The drying roller speed ratio is the ratio of the circumferential speed of each roller to the circumferential speed of the first roller. The circumferential speed of the first roller is 7~8 m / min, preferably 7.4~7.8 m / min.

10. The application of a modified polyvinyl alcohol optical film as described in claim 1 or 2 in the preparation of liquid crystal display devices, precision optical components, and optical compensation devices.

Images