Polyvinyl alcohol film, method for preparing the same, and use thereof

Abstract

The application discloses a polyvinyl alcohol film and a preparation method and application thereof. The material of the polyvinyl alcohol film comprises a main material and an auxiliary material. The main material is polyvinyl alcohol, and the auxiliary material is a water-soluble high polymer material. The average polymerization degree of the water-soluble high polymer material is 2-10 times of the average polymerization degree of the polyvinyl alcohol. The polyvinyl alcohol film is modified by forming a high polymer double network. The water-soluble high polymer material with a high polymerization degree, whose average polymerization degree is 2-10 times of the average polymerization degree of the polyvinyl alcohol, is added into the polyvinyl alcohol, so that a high polymer double network structure with the polyvinyl alcohol as the main body is obtained. The tensile property of the polyvinyl alcohol film is greatly improved, and the thinness of a polarizing film with the polyvinyl alcohol film as an optical base film and the optical property of the polarizing film are improved.

Description

Technical Field

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

[0002] With the development of communication technology, mobile phones, as carriers of information, have become an indispensable external organ for modern people. Driven by requirements for aesthetics, portability, space saving, and reduced physical effort, the thinning of mobile phones has become a key research and development focus for manufacturers. On the one hand, thinner phones meet these increasingly demanding requirements; on the other hand, the thinning of components has also spurred the development of new display devices such as larger screens, flexible screens, and foldable screens.

[0003] Current thinning efforts primarily focus on two aspects: first, ensuring the optical performance of each component does not deteriorate or even needs improvement during the thinning process; second, ensuring the mechanical properties of the display module meet the thinning requirements. Among these, the thinning of the polarizing film, which plays a crucial role in both polarization initiation and detection, is particularly important in the overall display module's optical film thinning process. This is because the production of polarizing films typically involves stretching a polyvinyl alcohol (PVA) film to achieve high orientation, thereby inducing dichroic substances to align along the stretching direction to form a PVA polarizing film. The orientation obtained during film stretching determines the polarization degree and transmittance of the polarizing film (in addition, protective films can be added to the polarizing film). The high orientation during stretching is fixed by removing moisture and cross-linking with boric acid. However, the inherent shrinkage force generated by the high stretching is released under high temperature or high humidity conditions, causing the display module to bend and resulting in light leakage. Only by thinning the polarizing film and reducing the shrinkage force can the other parts of the display module be fixed to the polarizing film using optical adhesive. For new types of displays such as foldable and flexible displays, a thinner polarizing film also ensures stable performance during deformation.

[0004] Improving the optical performance of polarizing films and achieving thinner profiles by increasing the stretch ratio and orientation is a straightforward approach. However, due to the limitations of the film's own stretching properties, the degree of thinning achieved by stretching methods for polyvinyl alcohol-based films prepared using traditional formulations and processes is limited. Summary of the Invention

[0005] The main objective of this invention is to provide a polyvinyl alcohol film, its preparation method, and its application, with the aim of improving the tensile properties of polyvinyl alcohol films.

[0006] To achieve the above objectives, the present invention proposes a polyvinyl alcohol film, wherein the polyvinyl alcohol film comprises a main material and an auxiliary material, wherein the main material is polyvinyl alcohol, the auxiliary material is a water-soluble polymer material, and the average degree of polymerization of the water-soluble polymer material is 2 to 10 times that of the average degree of polymerization of the polyvinyl alcohol.

[0007] Optionally, the average degree of polymerization of the polyvinyl alcohol is 1000~5000; and / or,

[0008] The water-soluble polymeric material includes at least one of the following: high-polymer polyvinyl alcohol, cationic polyacrylamide, anionic polyacrylamide, nonionic polyacrylamide and its derivatives, acrylic acid and methacrylic acid polymers, polyethylene glycol and polyethylene oxide polymers, polymaleic acid, water-soluble polyurethane, cellulose ether, chitosan, xanthan gum, starch derivatives and polyvinylpyrrolidone; and / or,

[0009] The mass of the auxiliary material is 2 to 20% of the mass of the main material.

[0010] Optionally, the auxiliary material includes polyvinyl alcohol with a high degree of polymerization, and the degree of alcoholysis of the polyvinyl alcohol with a high degree of polymerization is 70-99%.

[0011] Optionally, the average thickness of the polyvinyl alcohol film is not higher than 60 μm; and / or,

[0012] The stretch ratio of the polyvinyl alcohol film is not less than 7; and / or,

[0013] The water content of the polyvinyl alcohol film is 2-4%.

[0014] Optionally, the material of the polyvinyl alcohol film may also include at least one of plasticizers, surfactants, antioxidants, ultraviolet absorbers, lubricants, colorants, preservatives, and mildew inhibitors.

[0015] Optionally, the polyvinyl alcohol film further includes a plasticizer, wherein the plasticizer includes at least one selected from glycerol, diglycerol, and ethylene glycol, and / or, the mass of the plasticizer is 5-15% of the mass of the main material; and / or,

[0016] The polyvinyl alcohol film also includes a surfactant, wherein the surfactant includes nonionic surfactants and anionic surfactants, and / or the mass of the surfactant is 0.005~0.015% of the mass of the main material.

[0017] Furthermore, the present invention also proposes a method for preparing a polyvinyl alcohol film, comprising the following steps:

[0018] The main material and auxiliary materials are dissolved in a solvent to form a mixed solution;

[0019] A polyvinyl alcohol film is prepared by using the mixed solution as the casting solution in a solution casting process.

[0020] The main material is polyvinyl alcohol, the auxiliary material is a water-soluble polymer, and the average degree of polymerization of the water-soluble polymer is 2 to 10 times that of the average degree of polymerization of the polyvinyl alcohol.

[0021] Furthermore, the present invention also proposes a method for preparing a polyvinyl alcohol polarizing film, comprising the following steps:

[0022] A polyvinyl alcohol film is dyed and stretched to produce a polyvinyl alcohol polarizing film, wherein the polyvinyl alcohol film is the polyvinyl alcohol film as described above.

[0023] Optionally, the average thickness of the polyvinyl alcohol polarizing film is not higher than 25 μm.

[0024] Furthermore, the present invention also proposes a polarizing film, comprising a polyvinyl alcohol polarizing film, wherein the polyvinyl alcohol polarizing film is prepared by the method described above for preparing a polyvinyl alcohol polarizing film.

[0025] In the technical solution provided by this invention, polyvinyl alcohol film is modified by forming a polymeric double network. By adding a water-soluble polymeric material with an average degree of polymerization 2 to 10 times that of polyvinyl alcohol, a polymeric double network structure with polyvinyl alcohol as the main body is obtained, which greatly improves the tensile properties of polyvinyl alcohol film. When polyvinyl alcohol film is dyed and stretched to prepare polyvinyl alcohol polarizing film, it can be made thinner, thereby realizing the thinning of polarizing film used in display modules and the improvement of its optical performance. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially. Furthermore, the meaning of "and / or" throughout the text includes three parallel solutions; for example, "A and / or B" includes solution A, or solution B, or a solution where both A and B are satisfied simultaneously. In addition, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] Improving the optical performance of polarizing films and achieving thinner profiles by increasing the stretch ratio and orientation is a readily conceived method. However, due to the inherent limitations of the film's stretchability, the degree of thinning achievable through stretching methods is limited for polyvinyl alcohol (PVA) films prepared using traditional formulations and processes. Therefore, after in-depth research, the inventors proposed a PVA film that modifies the film by forming a polymeric double network, significantly enhancing its stretchability.

[0028] In a specific embodiment of the polyvinyl alcohol film provided by the present invention, the material of the polyvinyl alcohol film includes a main material and an auxiliary material, wherein the main material is polyvinyl alcohol, the auxiliary material is a water-soluble polymer material, and the average degree of polymerization of the water-soluble polymer material is 2 to 10 times that of the average degree of polymerization of the polyvinyl alcohol.

[0029] In the technical solution provided by this invention, polyvinyl alcohol (PVA) films are modified by forming a polymeric double network. By adding a water-soluble polymeric material with an average degree of polymerization 2 to 10 times that of PVA, a polymeric double network structure with PVA as the main component is obtained. The long-chain molecules can effectively transfer stress, thereby greatly improving the tensile properties of the PVA film. This allows for the preparation of thinner PVA polarizing films, thus achieving the thinning of polarizing films made from the PVA polarizing films and improving their optical properties.

[0030] The polyvinyl alcohol (PVA) is used as the main material to form the main network in the double-network structure. When selecting PVA, a low average degree of polymerization will affect the tensile properties of the PVA film and its weather resistance during subsequent use, while a high average degree of polymerization will increase production costs and reduce the uniformity of the PVA film. In the embodiments provided by this invention, the PVA can specifically be selected with an average degree of polymerization of 1000-5000 (the average degree of polymerization of PVA in this paper is determined according to the method provided in JIS K6726-1994; the test sample needs to be further saponified and purified before testing the intrinsic viscosity in water at 30°C) to ensure that the prepared PVA film has better mechanical properties. Further, the lower limit of the average degree of polymerization of the PVA is preferably 2000, and the upper limit is preferably 3000, resulting in PVA films with better tensile properties, weather resistance, and uniformity.

[0031] Furthermore, the polyvinyl alcohol can be either unmodified polyvinyl alcohol, i.e., a polyvinyl alcohol polymer obtained by polymerizing vinyl ester monomers to obtain polyvinyl ester and then alcoholystolytically, or a modified polyvinyl alcohol polymer obtained by copolymerizing vinyl ester monomers with other copolymerizing monomers containing double bonds and then alcoholystolytically, or a blend obtained by blending the above-mentioned polyvinyl alcohol polymers and modified polyvinyl alcohol polymers. In the modified polyvinyl alcohol polymer, the molar amount of the modifying monomer is within 5% of the molar amount of the vinyl ester monomer, and the modifying monomer is generally ethylene.

[0032] The water-soluble polymer material is used as an auxiliary material to form the second network in the double network structure. Specifically, as long as the average degree of polymerization of the water-soluble polymer material is 2 to 10 times that of the average degree of polymerization of the polyvinyl alcohol, a polymer double network structure can be obtained. More preferably, the average degree of polymerization of the water-soluble polymer material is 2 to 5 times that of the average degree of polymerization of the polyvinyl alcohol. In this way, while ensuring the formation of an interpenetrating double network structure, it is also beneficial to control the raw material and production costs.

[0033] In specific embodiments provided by this invention, the water-soluble polymeric material may be selected from at least one of the following: high-polymerization-degree polyvinyl alcohol, cationic polyacrylamide, anionic polyacrylamide, nonionic polyacrylamide and its derivatives, acrylic acid and methacrylic acid polymers, polyethylene glycol and polyethylene oxide polymers, polymaleic acid, water-soluble polyurethane, cellulose ether, chitosan, xanthan gum, starch derivatives, and polyvinylpyrrolidone. It should be noted that "high-polymerization-degree polyvinyl alcohol" as used herein refers to polyvinyl alcohol whose average degree of polymerization is higher than the average polymer of polyvinyl alcohol used as the main material, provided that its average degree of polymerization satisfies the aforementioned relationship between the average degree of polymerization of the main material and auxiliary materials.

[0034] Furthermore, the water-soluble polymeric material is preferably a high-polymerization-degree polyvinyl alcohol (PVA), which facilitates the formation of a bimolecular network structure with PVA, the main material, and results in relatively low raw material and production costs. Even further, when using a high-polymerization-degree PVA as the auxiliary material, the degree of hydrolysis of the PVA is preferably 70-99% (the degree of hydrolysis refers to the ratio of the number of structural units converted to ethylene alcohol units through saponification to the total number of molar ethylene alcohol units, which can be determined by the method for determining the degree of hydrolysis provided in JIS K6726-1994), and even more preferably 88-98%. Within this range of hydrolysis degree, the high-polymerization-degree PVA can be guaranteed to have good solubility and weak crystallization properties, thereby ensuring effective control of the crystallization kinetics during the drying process of the PVA film in subsequent processing, resulting in a film with a uniform crystal structure. In addition, the degree of alcoholysis of polyvinyl alcohol, the main material, is preferably 99% or more, more preferably 99.5% or more, and even more preferably 99.9% or more. It has good solubility and weak crystallization properties, which helps to ensure control of the subsequent drying process and obtain a film with a uniform crystal structure.

[0035] The specific amount of the auxiliary material added is as follows: the mass of the auxiliary material is 2-20% of the mass of the main material, and more preferably 5-10%. Within this range, a stable double-network structure can be ensured, while diluting the high degree of alcoholysis and low average degree of polymerization of polyvinyl alcohol, which is the main material, to prevent it from crystallizing too quickly and forming large crystals that would affect the subsequent dyeing process used to prepare the polarizing film.

[0036] The polyvinyl alcohol (PVA) film provided by this invention exhibits excellent stretchability due to its dual-network structure. When used as an optical base film for preparing polarizing films, it enables the thinning of polarizing films and improves their optical performance. Specifically, when used as an optical base film for preparing polarizing films, the PVA film preferably has an average thickness of no more than 60 μm; and / or, the stretch ratio of the PVA film is no less than 7. Thus, in the process of preparing polarizing films using the PVA film, the PVA film can be stretched to obtain a PVA polarizing film with an average thickness of less than 25 μm while maintaining its optical performance. Furthermore, the PVA film has a water content of 2-4 wt%, thus giving it good appearance and mechanical properties.

[0037] In some embodiments of the present invention, to improve the mechanical properties of the polyvinyl alcohol film, such as impact strength, tensile properties during subsequent stretching, and solvent resistance, a certain amount of plasticizer is added to the polyvinyl alcohol film. Specifically, the type of plasticizer is not limited, but preferably includes at least one of glycerol, diglycerol, and ethylene glycol. It can be any one of these substances, or a mixture of two or three, all of which fall within the scope of protection of the present invention. Furthermore, the mass of the plasticizer is preferably 5-15% of the mass of the main material, thus effectively improving the performance of the polyvinyl alcohol film while preventing the plasticizer from precipitating out.

[0038] To improve the processability of polyvinyl alcohol (PVA) films and facilitate their easy peeling from production equipment after molding, as well as to prevent film adhesion, a certain amount of surfactant is added to the PVA film in some embodiments of this invention. Specifically, the surfactant includes nonionic and anionic surfactants, which are added to the solution system in the order of nonionic surfactant, anionic surfactant, and then another nonionic surfactant. The purpose of the anionic surfactant is to enhance the efficiency of the nonionic surfactant, reduce the CMC value, and improve the surfactant's efficiency. The anionic surfactant is generally a carboxylic acid type such as potassium laurate, a sulfate type such as octyl sulfate, or a sulfate type such as dodecylbenzene sulfonate. The nonionic surfactant is generally a dialkylamide of an aliphatic carboxylic acid. Furthermore, the preferred mass of the surfactant is 0.01~0.05% of the mass of the main material. Within this range, the processability and peelability of the PVA film can be effectively improved, and film adhesion can be prevented.

[0039] In other embodiments provided by this invention, at least one of the following additives can be added, such as antioxidants, ultraviolet absorbers, lubricants, colorants, preservatives, and fungicides, according to the actual performance requirements of the polyvinyl alcohol film. This will impart better antioxidant properties, aging resistance, and different color appearances to the polyvinyl alcohol film, thereby enhancing its market competitiveness. The specific substances that can be used for the various additives mentioned above are not limited here; commonly used additives in the art can be selected.

[0040] Based on the polyvinyl alcohol film provided above, the present invention also proposes a method for preparing a polyvinyl alcohol film, comprising the following steps:

[0041] Step S10: Dissolve the main material and auxiliary materials in a solvent to form a mixed solution;

[0042] Step S20: Using solution casting to prepare a film, the mixed solution is used as the casting solution to prepare a film, thus obtaining a polyvinyl alcohol film.

[0043] Polyvinyl alcohol (PVA) is selected as the main material, and a water-soluble polymer with an average degree of polymerization 2 to 10 times that of PVA is selected as the auxiliary material. The main material and auxiliary material are dissolved to form a mixed solution, which is then used to prepare a film. This yields a PVA film with a double-network structure, significantly improving the tensile properties of the PVA film. The preparation method is simple and easy to implement. Since both the main material and auxiliary material are water-soluble, water is preferred as the solvent in step S10. Water is inexpensive and readily available. During the dissolution of the main material and auxiliary material, heating or pressurization can be used to promote rapid and uniform dissolution of the resin raw materials. The concentration of the solute in the mixed solution can be selected according to the solubility of the resin raw materials and the required film thickness of the PVA film. After dissolving to form the mixed solution, degassing treatment can be performed to prevent the presence of bubbles from affecting its film-forming properties, appearance, and performance. In addition, when it is necessary to add additives such as plasticizers and surfactants, during the preparation of the mixed solution in step S10, they can be added directly to the solvent in the predetermined order and amount and mixed evenly.

[0044] The following describes the preparation method of the polyvinyl alcohol film in more detail, taking polyvinyl alcohol with a high degree of polymerization as the water-soluble polymer material.

[0045] First, the main materials, auxiliary materials, surfactants, and other raw materials used to prepare the casting solution are added to a dissolving tank equipped with a stirring impeller that generates an upward and downward circulating flow. Steam is introduced into the tank, and stirring begins when the temperature reaches 40-80°C. More steam is then introduced, and when the temperature reaches 90-100°C, the pressure inside the tank is increased to 0.1-1 MPa. Dissolution is then performed under pressure at a temperature of 120-150°C. After the surfactant is fully dissolved, the concentration of the polyvinyl alcohol aqueous solution is controlled by steam removal and water addition, generally selecting a solute concentration of 20-50%.

[0046] Then, the prepared polyvinyl alcohol aqueous solution is allowed to stand for 1~20 hours to complete the first degassing. Then, a twin-screw extruder with ventilation holes is used to carry out a foaming process at 100~115℃ and a steam removal process at 92~98℃, with a screw inlet pressure of 0.1~10MPa, for the second degassing. After the bubbles are discharged, the insoluble matter is filtered through multiple mesh filters with different mesh sizes. Generally, a disc filter with coarse mesh (10~100μm), medium mesh (1~50μm), and small mesh (0.1~10μm) is selected. After filtration, the casting solution is obtained.

[0047] Next, the prepared casting solution (with a water content of not less than 60 wt%) is discharged through a T-shaped outlet to a casting roller (the roller roughness needs to be below 0.5S). The extrusion temperature is between 50 and 160°C. After extrusion, the film is dried sequentially through multiple drying rollers. The temperature of the first drying roller is between 85 and 95°C, and the circumferential speed is 15 to 26 m / min. Preferably, the water content of the film is between 8 and 15 wt% when it leaves the first drying roller, which can avoid excessive thickness deviation and prevent curling. Then, the film is dried sequentially through multiple subsequent drying rollers, with the temperature of the subsequent drying rollers being 50 to 80°C. To achieve a uniform drying process and increase the drying speed, hot air is blown onto the non-contact surface of the first drying roller while it is being heated. This heats and dries the film from both sides. If the air velocity is too low, water vapor will condense and drip during drying, causing defects. If the air velocity is too high, it will easily cause thickness deviations. Therefore, the air velocity is preferably controlled at 3~8 m / s. In addition, the air temperature is controlled at 80~95℃, the dew point temperature of the blown hot air is 10~15℃, and the blowing method is nozzle method, rectifier plate method, or a combination thereof. The blowing direction is the opposite direction of the non-contact surface or along the circumference of the non-contact surface. The volatile components generated by the film during the drying process are exhausted along with the blown hot air.

[0048] Finally, the prepared film is subjected to subsequent heat treatment at a temperature of 110~130℃ for 5~30s to obtain a polyvinyl alcohol film with a water content of 2~4wt%.

[0049] Based on the polyvinyl alcohol film provided above, the present invention also proposes a method for preparing a polyvinyl alcohol polarizing film, wherein the polyvinyl alcohol film is prepared into an optical base film for preparing a polarizing film in a display module. Specifically, the method for preparing the polyvinyl alcohol polarizing film includes the following steps:

[0050] A polyvinyl alcohol film is dyed and stretched to produce a polyvinyl alcohol polarizing film, wherein the polyvinyl alcohol film is the polyvinyl alcohol film as described above.

[0051] The polyvinyl alcohol (PVA) film with a dual-network structure provided by the present invention, after dyeing and stretching, becomes a PVA polarizing film, which can be used as an optical film for preparing polarizing films in display modules. This enables the thinning of polarizing films while improving their optical performance. The dyeing can be achieved by dyeing the PVA film with dichroic iodine or a dichroic dye, and the stretching can be achieved by uniaxially stretching and orienteding the PVA film. Iodine molecules or dichroic dyes absorb light vibrating in the stretching direction of the polarizing film while allowing light vibrating in the perpendicular direction to pass through, thus obtaining polarization with a specific vibration direction.

[0052] The polyvinyl alcohol film provided by the present invention has excellent tensile properties. The average thickness of the polyvinyl alcohol polarizing film formed after stretching can be controlled below 25 μm, which helps to achieve the thinning of the polarizing film.

[0053] The following describes the preparation process of the polyvinyl alcohol polarizing film in detail, taking the polyvinyl alcohol polarizing film as an iodine-based polarizing film as an example (that is, the dyeing is dyeing the polyvinyl alcohol film with dichroic iodine).

[0054] First, the prepared polyvinyl alcohol (PVA) film is swollen and washed in a first bath to remove dirt and other contaminants from the film surface. This swelling step extends the molecular chains and removes internal plasticizers, creating vacancies for the adsorption of dichroic substances in subsequent processing steps. Water is typically used as the treatment solution for this swelling step, but appropriate amounts of iodides, surfactants, or other additives may be added. The temperature is 10–45°C, and the time is 0.1–10 min. Alternatively, the PVA molecular chains can be pre-oriented during the swelling step, or the film can be fully swollen through multiple steps.

[0055] Then, the swollen polyvinyl alcohol film is placed in a dyeing bath, usually with water as the solvent. An appropriate amount of the same organic solvent as water, such as alcohol or DMSO, can also be added. The temperature of the dyeing bath is maintained between 25 and 40°C. Below 25°C, the degree of iodine staining increases, but the degree of swelling of the polyvinyl alcohol film decreases, causing severe film shrinkage. Above 40°C, iodine sublimation occurs, increasing the iodine consumption during the dyeing process. When iodine is used as a dichroic material, it is added at 0.06-0.25% of the solvent mass. Additionally, since elemental iodine has very low solubility in water at the dyeing temperature, auxiliaries are usually added to promote its dissolution. These auxiliaries include, but are not limited to, at least one of potassium iodide, lithium iodide, zinc iodide, and calcium iodide. The most commonly used auxiliary is potassium iodide, added at a ratio of potassium iodide:iodine of 1:5 to 1:10. The immersion time in the dyeing bath is 30-120 seconds.

[0056] Subsequently, the dyed polyvinyl alcohol (PVA) film is placed in a first crosslinking bath and a second crosslinking bath (or, alternatively, a solution containing a crosslinking agent can be sprayed onto the PVA film), and stretched within them. The crosslinking agent is typically a boron compound, such as boric acid, and its content is 1-6% of the solvent mass in the crosslinking bath. Iodine compounds from the dyeing bath can also be added appropriately to further adjust polarization characteristics and color; the amount added is 1-10% of the solvent mass in the crosslinking bath. Water is used as the solvent in both crosslinking baths, and the temperature is typically 30-60°C. The temperature selection is based on the following: as the crosslinking agent content increases, the crosslinking bath temperature is adjusted to a higher temperature to improve the chain mobility of the PVA film; as the crosslinking agent content decreases, the crosslinking bath temperature is adjusted to a relatively lower temperature. The temperature and solvent of the two crosslinking baths can be adjusted according to actual conditions. The immersion time in the crosslinking bath is 30-120 seconds to ensure uniform crosslinking. The stretching temperature of the PVA film in the crosslinking step is the same as the crosslinking bath temperature, also 30-60°C, and the stretching rate is 4.5-8 times. Alternatively, suction or adhesives can be used to adhere the polyvinyl alcohol film to a base film (such as high-density polyethylene film, polyurethane film, etc.) to form a film laminate, which is then stretched together to prevent the polyvinyl alcohol film from cracking during the stretching process. This produces a polyvinyl alcohol polarizing film with a thickness of no more than 10 μm. However, it should be noted that in this method, the removal of the base film after stretching may damage the surface of the polyvinyl alcohol film. Therefore, in actual processing, an appropriate method should be chosen to remove the base film or to stretch the polyvinyl alcohol film.

[0057] Next, the cross-linked and stretched polyvinyl alcohol film is cleaned using water or an aqueous solution of iodides such as potassium iodide to remove any precipitates from the film interior and any residual dyes on the surface. The washing bath temperature is 10–45°C, and the time is 10–24 seconds. Then, the cleaned film is dried in air at 40–80°C for 1–10 minutes to remove residual solvents and adjust for any deformations caused during processing, thus obtaining the polyvinyl alcohol polarizing film.

[0058] Based on the preparation method of the polyvinyl alcohol polarizing film provided by the present invention, the present invention also proposes a polarizing film, wherein the polarizing film comprises a polyvinyl alcohol polarizing film, and the polyvinyl alcohol polarizing film is prepared by the preparation method of the polyvinyl alcohol polarizing film provided above. Using the polyvinyl alcohol polarizing film provided above as the optical film of the polarizing film can reduce the thickness of the polarizing film to below 25 μm, or even below 10 μm, which helps to achieve thinner polarizing films while ensuring the optical performance of the polarizing film.

[0059] Furthermore, the polarizing film may also include protective films disposed on both sides of the polyvinyl alcohol polarizing film. In some embodiments of the polarizing film provided by the present invention, a first triacetate cellulose film and a second triacetate cellulose film are respectively disposed on both sides of the polyvinyl alcohol polarizing film, and a protective film is disposed on the side of the first triacetate cellulose film facing away from the polyvinyl alcohol polarizing film, and a release film is disposed on the side of the second triacetate cellulose film facing away from the polyvinyl alcohol polarizing film. In this way, it can be used directly as an optical film in devices such as mobile phones.

[0060] Correspondingly, in the preparation of the polarizing film, a pre-prepared polyvinyl alcohol polarizing film is used as the base film. After the dried film is washed, a TAC film (triacetate cellulose film) is laminated to both sides to form an intermediate roll. Subsequently, a protective film is laminated onto the surface of one TAC film, and a release film is glued to the surface of the other TAC film with an adhesive, thus obtaining the polarizing film. Alternatively, the polarizing film roll can be cut into polarizing film products of the required size, and then inspected and packaged.

[0061] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are only used to explain the present invention and are not intended to limit the present invention.

[0062] Example 1

[0063] (1) Preparation of polyvinyl alcohol film: 100 parts by mass of polyvinyl alcohol (average degree of polymerization of 2400, degree of hydrolysis of 99.95 mol%), 12 parts by mass of glycerol, 0.03 parts by mass of surfactant, 5 parts by mass of high degree of polymerization polyvinyl alcohol (average degree of polymerization of 8000, degree of hydrolysis of 98 mol%) were mixed with water to obtain a polyvinyl alcohol aqueous solution, which was then degassed by heat preservation; then, the degassed polyvinyl alcohol aqueous solution was poured onto a drying roller and dried to obtain a polyvinyl alcohol film, which was then heat-treated to obtain a polyvinyl alcohol film with an average thickness of 30 μm;

[0064] (2) Preparation of polyvinyl alcohol polarizing film: The polyvinyl alcohol film obtained in step (1) is cut into rectangular films with a length of 9cm and a width of 5cm. The two ends of the film in the length direction are fixed to the stretching fixture with the stretching part having a length of 5cm and a width of 5cm. After swelling, iodine staining, stretching, washing and drying, except for the stretching step, a portion of the polyvinyl alcohol film is stretched in each step until the ultimate stretching ratio (4 films are installed, and the stretching ratio of 2 films that break is set as the ultimate stretching ratio) is obtained, resulting in a polyvinyl alcohol polarizing film with an average thickness of about 11μm.

[0065] Example 2

[0066] Similar to Example 1, the difference lies in the addition amount of high-polymerization-degree polyvinyl alcohol, average degree of polymerization, degree of alcoholysis, average thickness of the polyvinyl alcohol film prepared in step (1), and average thickness of the polyvinyl alcohol polarizing film prepared in step (2), as shown in Table 1.

[0067] Example 3

[0068] Similar to Example 1, the difference lies in the addition amount of high-polymerization-degree polyvinyl alcohol, average degree of polymerization, degree of alcoholysis, average thickness of the polyvinyl alcohol film prepared in step (1), and average thickness of the polyvinyl alcohol polarizing film prepared in step (2), as shown in Table 1.

[0069] Example 4

[0070] Similar to Example 1, the difference lies in the addition amount of high-polymerization-degree polyvinyl alcohol, average degree of polymerization, degree of alcoholysis, average thickness of the polyvinyl alcohol film prepared in step (1), and average thickness of the polyvinyl alcohol polarizing film prepared in step (2), as shown in Table 1.

[0071] Example 5

[0072] (1) Preparation of polyvinyl alcohol film: 100 parts by mass of polyvinyl alcohol (average degree of polymerization of 1000, degree of hydrolysis of 99.5 mol%), 5 parts by mass of glycerol, 0.01 parts by mass of surfactant, 2 parts by mass of high degree of polymerization polyvinyl alcohol (average degree of polymerization of 5000, degree of hydrolysis of 99 mol%) and water were mixed to obtain a polyvinyl alcohol aqueous solution, and then heat-preserved and degassed; then, the degassed polyvinyl alcohol aqueous solution was poured onto a drying roller and dried to obtain a polyvinyl alcohol film, and then heat-treated to obtain a polyvinyl alcohol film with an average thickness of 60 μm;

[0073] (2) Preparation of polyvinyl alcohol polarizing film: The polyvinyl alcohol film obtained in step (1) is cut into rectangular films with a length of 9cm and a width of 5cm. The two ends of the film in the length direction are fixed to the stretching fixture with the stretching part having a length of 5cm and a width of 5cm. After swelling, iodine staining, stretching, washing and drying, except for the stretching step, a portion of the polyvinyl alcohol film is stretched in each step until the ultimate stretching ratio is reached (4 films are installed, and the stretching ratio of 2 films that break is set as the ultimate stretching ratio), a polyvinyl alcohol polarizing film with an average thickness of about 20μm is obtained.

[0074] Example 6

[0075] (1) Preparation of polyvinyl alcohol film: 100 parts by mass of polyvinyl alcohol (average degree of polymerization of 2000, degree of hydrolysis of 99.5 mol%), 15 parts by mass of glycerol, 0.05 parts by mass of surfactant, 10 parts by mass of high degree of polymerization polyvinyl alcohol (average degree of polymerization of 8000, degree of hydrolysis of 88 mol%) and water were mixed to obtain a polyvinyl alcohol aqueous solution, and then heat-preserved and degassed; then, the degassed polyvinyl alcohol aqueous solution was poured onto a drying roller and dried to obtain a polyvinyl alcohol film, and then heat-treated to obtain a polyvinyl alcohol film with an average thickness of 50 μm;

[0076] (2) Preparation of polyvinyl alcohol polarizing film: The polyvinyl alcohol film obtained in step (1) is cut into rectangular films with a length of 9cm and a width of 5cm. The two ends of the film in the length direction are fixed to the stretching fixture with the stretching part having a length of 5cm and a width of 5cm. After swelling, iodine staining, stretching, washing and drying, except for the stretching step, a portion of the polyvinyl alcohol film is stretched in each step until the ultimate stretching ratio is reached (4 films are installed, and the stretching ratio of 2 films that break is set as the ultimate stretching ratio), a polyvinyl alcohol polarizing film with an average thickness of about 18μm is obtained.

[0077] Example 7

[0078] (1) Preparation of polyvinyl alcohol film: 100 parts by mass of polyvinyl alcohol (average degree of polymerization of 3000, degree of hydrolysis of 99.95 mol%), 8 parts by mass of glycerol, 0.02 parts by mass of surfactant, 15 parts by mass of high degree of polymerization polyvinyl alcohol (average degree of polymerization of 9000, degree of hydrolysis of 70 mol%) and water were mixed to obtain a polyvinyl alcohol aqueous solution, and then heat-preserved and degassed; then, the degassed polyvinyl alcohol aqueous solution was poured onto a drying roller and dried to obtain a polyvinyl alcohol film, and then heat-treated to obtain a polyvinyl alcohol film with an average thickness of 40 μm;

[0079] (2) Preparation of polyvinyl alcohol polarizing film: The polyvinyl alcohol film obtained in step (1) is cut into rectangular films with a length of 9cm and a width of 5cm. The two ends of the film in the length direction are fixed to the stretching fixture with the stretching part having a length of 5cm and a width of 5cm. After swelling, iodine staining, stretching, washing and drying, except for the stretching step, a portion of the polyvinyl alcohol film is stretched in each step until the ultimate stretching ratio (4 films are installed, and the stretching ratio of 2 films that break is set as the ultimate stretching ratio) is obtained, resulting in a polyvinyl alcohol polarizing film with an average thickness of about 15μm.

[0080] Example 8

[0081] (1) Preparation of polyvinyl alcohol film: 100 parts by mass of polyvinyl alcohol (average degree of polymerization of 5000, degree of hydrolysis of 99.95 mol%), 10 parts by mass of glycerol, 0.04 parts by mass of surfactant, 20 parts by mass of high degree of polymerization polyvinyl alcohol (average degree of polymerization of 10000, degree of hydrolysis of 90 mol%) and water were mixed to obtain a polyvinyl alcohol aqueous solution, and then heat-preserved to remove bubbles; then, the degassed polyvinyl alcohol aqueous solution was poured onto a drying roller and dried to obtain a polyvinyl alcohol film, and then heat-treated to obtain a polyvinyl alcohol film with an average thickness of 30 μm;

[0082] (2) Preparation of polyvinyl alcohol polarizing film: The polyvinyl alcohol film obtained in step (1) is cut into rectangular films with a length of 9cm and a width of 5cm. The two ends of the film in the length direction are fixed to the stretching fixture with the stretching part being 5cm long and 5cm wide. After swelling, iodine staining, stretching, washing and drying, except for the stretching step, a portion of the polyvinyl alcohol film is stretched in each step until the ultimate stretching ratio is reached (4 films are installed, and the stretching ratio of 2 films that break is set as the ultimate stretching ratio), a polyvinyl alcohol polarizing film with an average thickness of about 10μm is obtained.

[0083] Comparative Example 1

[0084] Similar to Example 1, the difference lies in the addition amount of high-polymerization-degree polyvinyl alcohol, average degree of polymerization, degree of alcoholysis, average thickness of the polyvinyl alcohol film prepared in step (1), and average thickness of the polyvinyl alcohol polarizing film prepared in step (2), as shown in Table 1.

[0085] Comparative Example 2

[0086] Similar to Example 1, the difference lies in the addition amount of high-polymerization-degree polyvinyl alcohol, average degree of polymerization, degree of alcoholysis, average thickness of the polyvinyl alcohol film prepared in step (1), and average thickness of the polyvinyl alcohol polarizing film prepared in step (2), as shown in Table 1.

[0087] Comparative Example 3

[0088] Similar to Example 1, the difference lies in the addition amount of high-polymerization-degree polyvinyl alcohol, average degree of polymerization, degree of alcoholysis, average thickness of the polyvinyl alcohol film prepared in step (1), and average thickness of the polyvinyl alcohol polarizing film prepared in step (2), as shown in Table 1.

[0089] Comparative Example 4

[0090] Similar to Example 1, except that in step (2), the polyvinyl alcohol polarizing film obtained in step (1) is replaced with a commercially available polyvinyl alcohol film.

[0091] The optical and tensile properties of the polyvinyl alcohol polarizing films prepared in the above embodiments and comparative examples were tested. The test methods and results are as follows:

[0092] (1) Optical performance testing:

[0093] Using the stretching direction of the polyvinyl alcohol polarizing film as the reference edge, cut the sample into two 40mm × 30mm original polarizing sheets along the 45° direction. The two sides of the sample should be flat and parallel, free from visible defects such as dust, bubbles, top scratches, marks, and streaks. At the same time, the pressure-sensitive layer should be removed, and the individual original polarizing sheets should be kept for testing. The film surface should not be contaminated and should be kept clean.

[0094] Step A: Open the preheated spectrophotometer sample chamber lid, place the single sample onto the sample test holder, close the sample chamber lid, and perform a spectral scan between 400 and 700 nm using air as a reference and a scanning interval of 1 nm to obtain the corresponding monomer transmittance at each wavelength within this range. .

[0095] Step B: After aligning the optical axes of the two samples, place them on the sample testing holder and repeat Step A to obtain the corresponding parallel transmittance at each wavelength within this range. .

[0096] Step C: After aligning the optical axes of the two samples perpendicularly, place them on the sample testing fixture to obtain the corresponding vertical transmittance at each wavelength within this range. .

[0097] Transmittance calculation formula

[0098]

[0099] Where λ is the wavelength. Transmittance (%) at each wavelength. T The average transmittance is denoted by n, which is the wavelength number and is typically set to 5 nm.

[0100] Formula for calculating degree of polarization

[0101]

[0102] P The degree of polarization (%) Parallel transmittance (%) Vertical transmittance (%).

[0103] (2) Tensile property test:

[0104] A rectangular test piece measuring 9 cm in length and 5 cm in width was used. Both ends of the test piece along its length were fixed to a tensile fixture with the tensile portion measuring 5 cm in length and 5 cm in width. The sample was immersed in an iodine / potassium iodide / boric acid aqueous solution at 30°C for 60 seconds, containing 0.03 wt% iodine, 3 wt% potassium iodide, and 3 wt% boric acid. The sample was then stretched to its ultimate tensile strength (the median value obtained by stretching four samples to breakage). A and B were considered good (ultimate tensile strength ratio greater than 6 times), and C was considered poor (ultimate tensile strength ratio less than 6 times).

[0105] Table 1 compares the performance of auxiliary materials and the prepared polyvinyl alcohol polarizing films in each embodiment and comparative example.

[0106] As shown in Table 1, the thickness of commercially available polyvinyl alcohol (PVA) films after stretching is significantly greater than that of the PVA polarizing films prepared in this invention, and their tensile and optical properties are also poorer. Furthermore, when auxiliary materials are added to form a double-network structure, if the average degree of polymerization of the auxiliary materials is less than twice the average degree of polymerization of the PVA substrate, the degree of hydrolysis is higher than 99%, or the amount added is less than 2% of the PVA substrate, the tensile properties of the resulting PVA polarizing film are worse than those in this invention, and its optical properties are also poorer. This indicates that by selecting water-soluble polymeric materials with an average degree of polymerization 2-10 times that of the PVA substrate, a degree of hydrolysis of 70-99%, and an addition amount of 2-20% of the mass of the PVA substrate as auxiliary materials, and forming a double-network structure with the PVA substrate, this invention can limit and improve the tensile properties of the PVA film, achieving thinner PVA polarizing films while also improving their optical properties.

[0107] The above are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the patent protection scope of the present invention.

Claims

1. A method for preparing a polyvinyl alcohol polarizing film, characterized in that, Includes the following steps: Polyvinyl alcohol (PVA) films are dyed and stretched to produce PVA polarizing films, wherein... The method for preparing the polyvinyl alcohol film includes the following steps: dissolving the main material and auxiliary material in a solvent to form a mixed solution; and using the mixed solution as a casting solution to prepare a film, thereby obtaining a polyvinyl alcohol film. The main material is polyvinyl alcohol, and the auxiliary material is a water-soluble polymer, wherein the average degree of polymerization of the water-soluble polymer is 2 to 10 times that of the average degree of polymerization of the polyvinyl alcohol; wherein, The average degree of polymerization of the polyvinyl alcohol is 1000~5000; The mass of the auxiliary material is 2-20% of the mass of the main material; The auxiliary material includes high-polymer polyvinyl alcohol, and the degree of alcoholysis of the high-polymer polyvinyl alcohol is 70-99%. The stretch ratio of the polyvinyl alcohol film is not less than 7; The average thickness of the polyvinyl alcohol film is not higher than 60 μm; The average thickness of the polyvinyl alcohol polarizing film is no higher than 25 μm.

2. The method for preparing the polyvinyl alcohol polarizing film as described in claim 1, characterized in that, The water content of the polyvinyl alcohol film is 2-4%.

3. The method for preparing the polyvinyl alcohol polarizing film as described in claim 1, characterized in that, The polyvinyl alcohol film also includes at least one of the following: plasticizer, surfactant, antioxidant, ultraviolet absorber, lubricant, colorant, preservative and mildew inhibitor.

4. The method for preparing the polyvinyl alcohol polarizing film as described in claim 1, characterized in that, The polyvinyl alcohol film also includes a plasticizer, wherein the plasticizer comprises at least one of glycerol, diglycerol, and ethylene glycol, and / or the mass of the plasticizer is 5-15% of the mass of the main material; and / or, The polyvinyl alcohol film also includes a surfactant, wherein the surfactant includes nonionic surfactants and anionic surfactants, and / or the mass of the surfactant is 0.005~0.015% of the mass of the main material.

5. A polarizing film, characterized in that, It is prepared by the method for preparing polyvinyl alcohol polarizing film as described in any one of claims 1-4.

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