A polarizing plate using a polyvinyl alcohol solution and a method for preparing the same

Abstract

This invention discloses a polyvinyl alcohol (PVA) solution for polarizers and its preparation method, belonging to the field of optical thin film manufacturing technology. The method includes: washing, swelling, and centrifuging to remove surface water from PVA resin particles, then mixing them with additives and pure water and adding them to a dissolving vessel; initially introducing nitrogen gas to pressurize to a first pressure and heating to allow the PVA particles to fully swell and initially dissolve; then slowly depressurizing to atmospheric pressure, continuously introducing nitrogen gas during the process to remove volatile impurities and dissolved oxygen; after depressurization, sealing the system again, introducing nitrogen gas to pressurize to a second pressure and heating to the dissolution temperature, and maintaining the temperature to dissolve to obtain a uniform and transparent PVA solution. This invention, through repeated pressurization and depressurization physical purification, effectively removes residual volatile impurities inside the PVA particles and microbubbles in the solution, significantly improving the transmittance and uniformity of the solution, thereby improving the optical and mechanical properties of PVA optical films and increasing production yield.

Description

Technical Field

[0001] This invention belongs to the field of optical thin film manufacturing technology, specifically relating to a polyvinyl alcohol solution for polarizers and its preparation method. Background Technology

[0002] Polyvinyl alcohol (PVA) optical films are the core layer material for polarizers used in liquid crystal displays (LCDs) and organic light-emitting diodes (OLEDs), and their quality directly determines the polarization performance and optical durability of the polarizer. PVA optical films are typically made from PVA resin through processes such as dissolution, extrusion, casting, drying, and heat treatment. Among these processes, the preparation of the PVA solution is the fundamental and crucial step that determines the final film performance.

[0003] Current technologies typically employ heating and stirring to dissolve PVA solutions. However, trace amounts of volatile impurities, such as methanol and methyl ester, may remain in the PVA resin during polymerization and post-processing. Furthermore, air can easily be incorporated during dissolution, forming microbubbles. These volatile impurities and bubbles can vaporize upon heating during subsequent film formation, drying, or heat treatment, creating micron- or even nanometer-sized vacancies within the film with refractive indices different from the PVA matrix. These vacancies cause strong scattering of incident light, leading to a significant increase in film haze and a sharp decrease in light transmittance, severely impacting its application in high-end displays. Simultaneously, the inhomogeneity of the solution increases the risk of film breakage and fracture during subsequent stretching processes, reducing production yield.

[0004] Patent application CN102816333A discloses a method for preparing a high-concentration PVA solution. This method involves adding a dissolving agent and preparing the PVA solution under constant pressure and continuous heating in an inert gas atmosphere. The main aim of this method is to shorten the dissolution time of high-polymerization-degree PVA and increase the dissolution concentration, but it does not specifically address the optical quality issues caused by volatile impurities and bubbles. Patent application CN117510897A discloses a method for preparing a high-transparency PVA solution. This method prevents yellowing of the solution and improves transparency through saponification, water washing, and multi-stage deoxygenation. While this method addresses transparency, its core lies in preventing oxidative degradation through chemical treatment and multiple deoxygenation processes. The process is relatively complex and does not involve physically removing dissolved or entrained volatile impurities and bubbles through pressure changes.

[0005] Therefore, developing a preparation method that can efficiently remove volatile impurities and bubbles from PVA solutions to obtain solutions with high transmittance and high homogeneity, and significantly improve the optical properties of thin films, has important practical significance and industrial value. Summary of the Invention

[0006] One of the objectives of this invention is to provide a method for preparing a polyvinyl alcohol solution for polarizing films, in order to solve the technical problems of high haze, low transmittance, and poor processing performance of existing PVA solutions due to residual volatile impurities and microbubbles.

[0007] The second objective of this invention is to provide a polyvinyl alcohol solution for polarizing films, which is prepared by the above-described method for preparing polyvinyl alcohol solution for polarizing films.

[0008] The objective of this invention can be achieved through the following technical solutions: In a first aspect, a method for preparing a polyvinyl alcohol solution for polarizing films includes the following steps: S1. Pretreatment: Polyvinyl alcohol resin particles are washed and swollen in ultrapure water, and then the surface moisture is removed by centrifugation to obtain pretreated PVA material; S2. First dissolution and pressurization: The pretreated PVA material, additives and pure water are mixed and put into a dissolution vessel. Nitrogen gas is introduced into the dissolution vessel to remove air and pressurize to a first pressure. At the same time, it is heated to a first temperature and kept at that temperature for a first time to allow the PVA particles to fully swell and partially dissolve. S3. Slow depressurization and degassing: Under the condition of continuous nitrogen gas supply, open the pressure relief valve and slowly release the pressure in the dissolving vessel to atmospheric pressure at a controlled rate. The pressure relief process lasts for a second duration to remove volatile impurities and dissolved oxygen. S4. Secondary Dissolution and Pressurization: After depressurization, reseal the dissolution vessel, introduce nitrogen gas into the vessel again and pressurize it to the second pressure, then heat it to the second temperature and keep it at that temperature for a third time to completely dissolve the PVA and obtain a uniform and transparent polyvinyl alcohol solution.

[0009] A cyclical process of "first-stage pressurization dissolution – slow depressurization defoaming – second-stage pressurization dissolution" physically removes volatile impurities and microbubbles from PVA solutions. In the first pressurization stage, nitrogen gas is used to increase the dissolution temperature, allowing the PVA particles to fully swell and promoting the migration of residual solvent from the interior to the surface. Subsequently, under continuous nitrogen protection, the pressure is slowly released, utilizing the driving force generated by the sudden pressure drop to rapidly expand and escape the dissolved impurities and bubble nuclei, which are then carried away by the nitrogen flow. The second pressurization stage further increases the temperature to completely dissolve the PVA and maintain a homogeneous and stable solution. This method does not rely on chemical additives and solves the long-standing haze problem of PVA solutions used in optical films through purely physical means.

[0010] Furthermore, in S2, the first pressure is a gauge pressure of 80~120 kPa; the first temperature is 80~100℃; and the first duration is 1-3h.

[0011] A temperature of 80-100℃ is the suitable range for PVA to swell but not completely dissolve. Too low a temperature will result in insufficient swelling, while too high a temperature will cause premature formation of colloids that hinder the release of internal impurities. A time of 1-3 hours ensures sufficient swelling, balancing production efficiency with the prevention of thermal degradation.

[0012] Furthermore, in S3, the second duration of the depressurization process is 0.5-2 hours, and the final pressure of the depressurization process is atmospheric pressure (101.3 kPa).

[0013] Depressurization that is too rapid can lead to explosive boiling and prevent bubbles from escaping completely; depressurization that is too slow increases the risk of thermal degradation. The endpoint pressure is atmospheric pressure (101.3 kPa), which can maintain a sufficient pressure difference to allow bubbles to escape completely and prevent backflow of outside air into the solution.

[0014] Furthermore, in S3, the temperature remains constant during the depressurization process.

[0015] Furthermore, in step S4, the second pressure is 400 kPa; the second temperature is 100-150°C; and the third duration is 1-8 hours. The temperature, pressure, and time for the secondary dissolution must ensure complete molecular dissolution of PVA.

[0016] Furthermore, the degree of polymerization of the polyvinyl alcohol resin is 1500~3000, and the degree of hydrolysis is 98~99.9%.

[0017] Furthermore, the additive includes at least one of plasticizer, surfactant, antioxidant and ultraviolet absorber; the plasticizer includes at least one of glycerol, diglycerol, polyglycerol, ethylene glycol and propylene glycol, and its content is 8 to 15% of the total mass of PVA solution; the surfactant is a mixture of nonionic surfactant and anionic surfactant, and its content is 1.5 to 5.0‰ of the total mass of PVA solution.

[0018] Furthermore, the final polyvinyl alcohol solution obtained has a light transmittance of ≥80%.

[0019] Secondly, a PVA optical film is prepared by extruding, casting, drying and heat treatment of the above-mentioned polarizer with a polyvinyl alcohol solution.

[0020] A polarizer comprising the aforementioned PVA optical film.

[0021] The beneficial effects of this invention are: (1) This invention employs a process of "one-time pressure increase dissolution - slow pressure release degassing - secondary pressure increase dissolution". Compared to traditional constant pressure dissolution or simple stirring, this method utilizes drastic pressure changes (slow pressure release process) as a physical purification method, which can penetrate deep into the interior of PVA particles and the solution bulk, effectively removing trace amounts of volatile organic compounds and dissolved oxygen that are difficult to remove by conventional methods, and eliminating micron / nano-scale bubbles in the solution. This fundamentally eliminates the light scattering sources caused by impurities and bubbles.

[0022] (2) The PVA solution prepared by the above method has extremely high uniformity and transparency. The optical film made from it has extremely low haze and high transmittance, which can meet the stringent requirements of high-end polarizers for the optical quality of the base film. It is especially suitable for the automotive display field where durability and optical consistency requirements are extremely high.

[0023] (3) The uniform and impurity-free solution prepared by the present invention is less likely to break or break the film due to local defects in the subsequent casting and stretching process, which significantly improves the stability of production and product yield and reduces production costs.

[0024] (4) The method of the present invention only requires program control optimization of the existing dissolving vessel, without the need to introduce expensive new equipment. It is simple to operate, easy to control, and has good industrial feasibility and market competitiveness. Detailed Implementation

[0025] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below.

[0026] In some embodiments, a method for preparing a polyvinyl alcohol solution for polarizing films includes the following steps: S1. Pretreatment: Polyvinyl alcohol resin particles are washed and swollen in ultrapure water to remove surface impurities and ensure sufficient swelling. Then, they are centrifuged to remove surface moisture, yielding pretreated PVA material. This step helps ensure uniformity in the subsequent dissolution process.

[0027] S2. Primary Dissolution and Pressurization: The pretreated PVA material, additives, and pure water are mixed and then added to a dissolution vessel. First, nitrogen gas is introduced into the dissolution vessel to completely remove air. Then, nitrogen gas is continued to be introduced and the pressure is increased to a first pressure (gauge pressure 80~120 kPa). Simultaneously, the system is heated to a first temperature (80~100℃) using a jacket heating system and held at this temperature for a first duration (1-3 hours). The purpose of this stage is to allow the PVA particles to fully swell and initially dissolve, while simultaneously using nitrogen pressure and temperature to promote the escape of some volatile impurities from inside the particles.

[0028] S3. Slow Pressure Relief and Degassing: After the first dissolution is complete, under the protection of continuous nitrogen flow into the vessel, open the pressure relief valve to slowly release the pressure in the dissolution vessel to atmospheric pressure (101.3 kPa) at a controlled rate. The entire pressure relief process should be carried out smoothly and last for a second duration (0.5-2 hours). During this process, the temperature should be kept constant. The slow pressure reduction causes volatile impurities dissolved in the PVA solution (such as methanol and methyl ester) and tiny bubble nuclei to rapidly expand and escape from the solution under the pressure difference, and are carried away by the continuously supplied nitrogen, thereby achieving efficient physical purification and degassing.

[0029] S4. Secondary Dissolution and Pressurization: After depressurization, reseal the dissolution vessel. Introduce nitrogen gas into the vessel again and pressurize to a second pressure (preferably 400 kPa). Then, heat the system to a higher second temperature (100-150°C) and maintain this temperature and pressure for a third time (1-8 hours) to completely dissolve the PVA, forming a fully extended, uniform, and transparent polyvinyl alcohol solution. Secondary pressurization and heating further increases the dissolution temperature, promoting the complete disintegration of the PVA molecular chains. Simultaneously, the high temperature and high pressure environment helps maintain the stability and homogeneity of the solution.

[0030] In some embodiments, the degree of polymerization of the polyvinyl alcohol resin is 1500-3000 and the degree of hydrolysis is 98-99.9% to ensure good solubility and optical properties and water resistance after film formation.

[0031] In some embodiments, the additives include plasticizers, surfactants, antioxidants, and ultraviolet absorbers. The plasticizer is selected from at least one of glycerol, diglycerol, polyglycerol, ethylene glycol, and propylene glycol, and its content is controlled at 8-15% of the total mass of the PVA solution. The surfactant is a mixture of nonionic and anionic surfactants, and its content is controlled at 1.5-5.0‰ of the total mass of the PVA solution.

[0032] In some embodiments, the final polyvinyl alcohol solution has a transmittance of ≥80%, which provides a basis for the preparation of high-performance optical thin films.

[0033] The specific embodiments of the present invention will be described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0034] Example 1 This embodiment provides a polyvinyl alcohol solution for polarizing films, which is prepared through the following steps: S1. Pretreatment: Weigh 3500 kg of polyvinyl alcohol resin particles with a degree of polymerization of 2200 and a degree of alcoholysis of 99%, add them to ultrapure water and stir and wash for 30 minutes, then soak them in ultrapure water at 25℃ for 2 hours to swell, and then centrifuge to remove surface moisture to obtain pretreated PVA material. S2. First Dissolution and Pressurization: Add the pretreated PVA material, 12 kg of sodium dodecylbenzenesulfonate (surfactant), 350 kg of glycerin (plasticizer), and a suitable amount of antioxidant mixture to a dissolving vessel along with 6500 L of deionized water (controlling the PVA concentration to 25 wt%). Purge the dissolving vessel with nitrogen gas and pressurize to a gauge pressure of 100 kPa. Start stirring and heat the vessel through a jacket to 90°C, maintaining this temperature and pressure for 2 hours. S3. Slow pressure relief and degassing: After the heat preservation is completed, under the condition of continuous nitrogen gas supply (the flow rate is to maintain a slight positive pressure inside the vessel), slowly open the pressure relief valve, control the pressure drop rate, and slowly release the pressure inside the vessel to atmospheric pressure (101.3 kPa) over a period of 1 hour.

[0035] S4. Secondary Dissolution and Pressurization: After depressurization, reseal the dissolution vessel. Introduce nitrogen gas into the vessel again to pressurize to 400 kPa. Then, continue heating to 120°C and maintain this temperature and pressure with stirring for 4 hours to completely dissolve the PVA. Afterward, stop heating and stirring, allow to stand to remove large surface bubbles, and obtain a uniform and transparent polyvinyl alcohol aqueous solution.

[0036] Example 2 This embodiment is basically the same as Embodiment 1, except that in step S2, the maximum pressure for the first pressurization is increased to 120 kPa gauge pressure. The remaining steps and parameters are the same as in Embodiment 1.

[0037] Example 3 This embodiment is basically the same as Embodiment 1, except that in step S4, the temperature for the secondary dissolution is increased to 150°C. The remaining steps and parameters are the same as in Embodiment 1.

[0038] Example 4 This embodiment is basically the same as Embodiment 1, except that in step S4, the heat preservation time for the second dissolution is extended to 8 hours. The remaining steps and parameters are the same as in Embodiment 1.

[0039] Example 5 This embodiment incorporates several preferred conditions: the primary pressure in step S2 is 95 kPa, the depressurization time in step S3 is 1.5 h, and the secondary temperature in step S4 is 125 °C and the secondary duration is 5 h. The remaining steps and parameters are the same as in embodiment 1.

[0040] Comparative Example 1 This comparative example is basically the same as Example 1, except that in step S2, the maximum pressure of the first pressurization is reduced to 85 kPa gauge pressure. The remaining steps and parameters are the same as in Example 1.

[0041] Comparative Example 2 This comparative example is basically the same as Example 1, except that in step S4, the temperature for the secondary dissolution is reduced to 85°C. The remaining steps and parameters are the same as in Example 1.

[0042] Comparative Example 3 This comparative example is basically the same as Example 1, except that in step S4, the heat preservation time for the second dissolution is shortened to 1 hour. The remaining steps and parameters are the same as in Example 1.

[0043] Comparative Example 4 This comparative example is basically the same as Example 1, except that in step S3, the pressure at the end of the depressurization is controlled at 120 kPa, meaning that the depressurization is not complete. The remaining steps and parameters are the same as in Example 1.

[0044] Comparative Example 5 This comparative example is basically the same as Example 1, except that in step S3, nitrogen is not continuously introduced during the depressurization process; instead, natural venting is used. The remaining steps and parameters are the same as in Example 1.

[0045] Performance testing The polyvinyl alcohol solutions prepared in the above embodiments and comparative examples were subjected to performance tests, and then further cast into films, dried, and subjected to film performance tests.

[0046] 1. Solution transmittance test: The transmittance (%) of the PVA solution was determined by spectrophotometry at a wavelength of 550 nm. The cooled PVA solution gel was weighed and redissolved in 35℃ warm water to prepare a certain concentration for testing.

[0047] 2. Thin film haze test: The solution is cast into a thin film with a thickness of 60±5μm, and the haze (%) of the film is tested using a haze meter in accordance with GB / T2410-2008 standard.

[0048] 3. Thin film tensile property test: Cut the film into standard strips and perform uniaxial tensile testing on a universal testing machine to test its breaking strength (MPa).

[0049] The results are shown in Table 1: Table 1

[0050] This invention utilizes a cyclical process of "first-stage pressure increase and dissolution—slow pressure release and degassing—second-stage pressure increase and dissolution" to remove volatile impurities and microbubbles from PVA solutions using the physical driving force generated by pressure changes. Test results show that the solution prepared by this method achieves a transmittance of 91.8% (Example 5) and a film haze as low as 0.12%, significantly better than the comparative example. The principle is as follows: the first-stage pressure increase allows the PVA to fully swell under pressure, promoting the migration of residual impurities from the interior to the surface; during the subsequent slow pressure release, the sudden pressure drop causes the dissolved impurities and bubble nuclei to rapidly expand and escape, and are carried away by continuously introduced nitrogen gas; the second-stage pressure increase ensures complete dissolution of the PVA and maintains a homogeneous and stable solution.

[0051] Depressurization and degassing are key steps in this invention. In Comparative Example 4 (depressurization endpoint 0.12 MPa), degassing was incomplete due to insufficient pressure reduction, resulting in a light transmittance of 83.5%. In Comparative Example 5 (without nitrogen protection), oxidation led to a light transmittance of 79.8%. These comparisons clearly demonstrate that achieving atmospheric pressure at the depressurization endpoint and maintaining nitrogen protection throughout the process are necessary conditions for ensuring effective degassing.

[0052] Comparative Example 1 (primary pressure 85 kPa) had a transmittance of 80.5% due to insufficient swelling; Comparative Examples 2 (secondary temperature 85°C) and 3 (secondary duration 1 h) had transmittances as low as 78.3% and 77.4%, respectively, due to insufficient dissolution. Example 5 achieved the best performance by combining all the preferred parameters, proving that the parameter range defined in the claims has clear physical significance and technical support, and deviations from this range all lead to significant performance degradation.

[0053] The above-disclosed embodiments are merely a few specific examples of the present invention. However, the embodiments of the present invention are not limited thereto, and any variations that can be conceived by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A method for preparing a polyvinyl alcohol solution for polarizing films, characterized in that, S1. Pretreatment: Polyvinyl alcohol resin particles are washed and swollen in ultrapure water, and then the surface moisture is removed by centrifugation to obtain pretreated PVA material; S2. First dissolution and pressurization: The pretreated PVA material, additives and pure water are mixed and put into a dissolution vessel. Nitrogen gas is introduced into the dissolution vessel to remove air and pressurize to a first pressure. At the same time, it is heated to a first temperature and kept at that temperature for a first time to allow the PVA particles to fully swell and partially dissolve. S3. Slow depressurization and degassing: Under the condition of continuous nitrogen gas supply, open the pressure relief valve and slowly release the pressure in the dissolving vessel to atmospheric pressure at a controlled rate. The pressure relief process lasts for a second duration to remove volatile impurities and dissolved oxygen. S4. Secondary Dissolution and Pressurization: After depressurization, reseal the dissolution vessel, introduce nitrogen gas into the vessel again and pressurize it to the second pressure, then heat it to the second temperature and keep it at that temperature for a third time to completely dissolve the PVA and obtain a uniform and transparent polyvinyl alcohol solution.

2. The method for preparing a polyvinyl alcohol solution for polarizing film according to claim 1, characterized in that, In S2, the first pressure is a gauge pressure of 80~120 kPa; the first temperature is 80~100℃; and the first duration is 1-3h.

3. The method for preparing a polyvinyl alcohol solution for polarizing film according to claim 1, characterized in that, In S3, the second duration of the depressurization process is 0.5-2 hours, and the final pressure of the depressurization process is atmospheric pressure.

4. The method for preparing a polyvinyl alcohol solution for polarizing film according to claim 1, characterized in that, In step S3, the temperature remains constant during the depressurization process.

5. The method for preparing a polyvinyl alcohol solution for polarizing film according to claim 1, characterized in that, In step S4, the second pressure is 400 kPa; the second temperature is 100~150℃; and the third duration is 1-8h.

6. The method for preparing a polyvinyl alcohol solution for polarizing film according to claim 1, characterized in that, The degree of polymerization of the polyvinyl alcohol resin is 1500~3000, and the degree of hydrolysis is 98~99.9%.

7. The method for preparing a polyvinyl alcohol solution for polarizing film according to claim 1, characterized in that, The additives include at least one of plasticizers, surfactants, antioxidants, and ultraviolet absorbers; the plasticizers include at least one of glycerol, diglycerol, polyglycerol, ethylene glycol, and propylene glycol, with a content of 8-15% of the total mass of the PVA solution; the surfactants are a mixture of nonionic and anionic surfactants, with a content of 1.5-5.0‰ of the total mass of the PVA solution.

8. A polyvinyl alcohol solution for polarizing films, characterized in that, The polarizer is prepared by the method for preparing a polyvinyl alcohol solution according to any one of claims 1-7, wherein the transmittance of the polyvinyl alcohol solution is ≥80%.

9. A PVA optical thin film, characterized in that, The polarizer as described in claim 8 is prepared by extrusion, casting, drying and heat treatment of a polyvinyl alcohol solution.

10. A polarizer, characterized in that, It includes the PVA optical film as described in claim 9.

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