A thin, low-curvature PVA optical film for polarizers and its preparation method.

Abstract

This invention discloses a low-curl, thin PVA optical film for polarizers and its preparation method, belonging to the field of optical film production technology. The invention involves preparing a casting solution from PVA resin, which is then degassed and cast into a film. The film is then sequentially passed through casting rollers, drying rollers, and undergoes heat treatment to obtain the PVA optical film. By strictly controlling the temperatures of the casting rollers and drying rollers and setting a suitable temperature gradient, the thin PVA optical film dries uniformly on both sides without surface skinning, reducing the difference in crystallinity and moisture content between the two sides. This results in a uniform structure on both sides and significantly improves the swelling and curling of the film in water. The overall process is simple, has high practical application feasibility, and significantly improves the quality of the thin PVA optical film.

Description

Technical Field

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

[0002] Polyvinyl alcohol (PVA) optical films are mainly used in the production of polarizers, with end products being various liquid crystal displays, such as display panels for televisions, computer monitors, mobile phones, car navigation systems, and wearable electronic devices. PVA polarizers are made by laminating a cellulose triacetate (TAC) film onto a PVA optical film after swelling, dyeing, uniaxial stretching, and boride crosslinking. As display panels develop towards thinner, more durable, and higher transmittance, PVA polarizers are also gradually developing towards thinner profiles and higher transmittance.

[0003] PVA optical films are manufactured from PVA resin through processes such as dissolution, extrusion, casting, and drying. The casting and drying process affects the microstructure of the PVA film, thus determining its processing performance in polarizer manufacturing. Since the PVA film, after being dried on the casting rollers, needs to pass through multiple metal drying rollers for front and back drying, the uniformity of the drying process affects the uniformity of the film's internal structure. For thin PVA films, the uniformity of the drying process has a more significant impact on the uniformity of the film's front and back structure. Thin PVA optical films with uneven internal structures are prone to curling and severe edge folding during downstream swelling. To address this issue, some literature suggests that by ensuring the PVA film receives the same amount of heat on both sides during the casting and drying process, the same drying effect can be achieved, thus improving the edge curling of the PVA optical film during the swelling process. However, due to the skin formation on the film surface during the drying process, the evaporation rate of moisture differs between the front and back sides of the film. Therefore, even if the heat received on both sides is the same, it is impossible to maintain a consistent water content on both sides of the film, and the goal of achieving a uniform structure on both sides of the PVA film cannot be achieved. Summary of the Invention

[0004] The purpose of this invention is to provide a low-curl, thin PVA optical film for polarizers and its preparation method, thereby achieving uniform structure on both sides of the PVA optical film and solving the curling problem during the swelling process.

[0005] The objective of this invention can be achieved through the following technical solutions:

[0006] This invention provides a low-curvature, thin PVA optical film for polarizers and its preparation method, comprising the following steps:

[0007] (1) PVA resin and additives are dissolved in a solvent to prepare a casting solution;

[0008] (2) After the casting solution is extruded and degassed, it is cast into a thin film through a die head;

[0009] (3) The film is passed through the casting roller and the drying roller in sequence. After the drying roller pre-dries the film, it is put into the oven for heat treatment, drying and shaping, and then wound up to obtain PVA optical film.

[0010] Furthermore, the PVA resin includes at least one of polyvinyl alcohol homopolymer and polyvinyl alcohol copolymer; the modifying monomer of the polyvinyl alcohol copolymer includes at least one of olefins, acrylates, methacrylates, methacrylamide derivatives, vinyl esters, and halogenated vinyl groups.

[0011] Furthermore, the content of modified groups in the PVA resin is <10%, as an excessively high content will affect the solubility and optical properties of the PVA resin and the thin film.

[0012] Furthermore, the degree of polymerization of the PVA resin is 1500-3000.

[0013] If the polymerization ratio is too low, the stability of the molecular chain entanglement network of the prepared PVA polarizer will be worse. A higher stretching ratio will cause the cross-linking network to be destroyed, resulting in a deterioration in the optical performance of the prepared polarizer. If the polymerization ratio is too high, the viscosity of the raw solution will be high, the pre-stretching will be severe, the anisotropy of the film after casting will be large, and the film will be easy to break.

[0014] Furthermore, the degree of alcoholysis of the PVA resin is 98%-99.9%.

[0015] Furthermore, the degree of alcoholysis of the PVA resin is 99%-99.9%.

[0016] If the hydrolysis is too low, the amount of polyiodide ions formed in the PVA film during the iodine dyeing and stretching process will be low, which will affect the optical performance of the polarizer. If the hydrolysis is too low, the solubility will be poor, which will lead to instability in the casting production and a decrease in the yield of finished products.

[0017] Furthermore, the additives are plasticizers, surfactants, and antioxidants, with the plasticizer accounting for 8%-15% of the mass of the PVA resin, the surfactant accounting for 1.5%-5.0% of the mass of the PVA resin, and the antioxidant accounting for 1%-3% of the mass of the PVA resin.

[0018] Excessive additive content can lead to precipitation, affecting the appearance and optical properties of the film and contaminating the downstream iodine dyeing stretching tank.

[0019] Furthermore, the plasticizer includes at least one of glycerol, diglycerol, polyglycerol, ethylene glycol, and propylene glycol.

[0020] Further, the surfactant is at least one selected from dodecylbenzene sulfonate, potassium lauryl ester, polyoxyethylene lauryl ester, and polyoxyethylene lauryl amide.

[0021] Furthermore, the antioxidant is sodium bisulfite.

[0022] Furthermore, the temperature of the casting roller is 75℃-85℃.

[0023] Due to the low film thickness and rapid surface moisture evaporation rate, if the casting roller temperature is too high, the film moisture evaporation rate will be fast, the system viscosity will increase, and surface skin formation will lead to uneven drying and structural differences during the film casting and drying process. If the casting roller temperature is too low, the film will peel off unevenly and be susceptible to water ripples caused by external disturbances. The initial PVA casting solution has a high water content, and the film will not crystallize on the casting and drying roller. Its crystallization process mainly occurs in the subsequent drying process.

[0024] Furthermore, the first drying roller after the casting roller is the first drying roller, and the subsequent drying rollers are arranged in sequence, wherein odd-numbered rollers dry the front side of the film and even-numbered rollers dry the back side of the film;

[0025] The temperature range of the drying roller is divided into two drying processes: the temperature of the drying roller in the first drying process is 55℃-75℃, and the temperature of the drying roller in the second drying process is 35℃-50℃.

[0026] The initial drying process involves setting a temperature gradient at high temperature to change the crystallization rate of the two sides of the film, thereby achieving a smaller difference in crystallinity between the two sides of the PVA film. The subsequent drying process involves setting a temperature gradient at low temperature to meet the requirement of a small difference in water content between the two sides of the film.

[0027] Furthermore, in the drying process of the first stage, the temperature of the odd-numbered rollers gradually decreases from the first drying roller to the next according to the difference, with the difference ranging from 4℃ to 10℃.

[0028] Furthermore, in the subsequent drying process, the temperature of the odd-numbered rollers gradually decreases from front to back according to the difference, with the difference ranging from 4℃ to 10℃.

[0029] Furthermore, in the drying process of the first stage, the temperature of the even-numbered rollers gradually decreases from the second drying roller onwards according to the difference, with the difference ranging from 0℃ to 10℃.

[0030] Furthermore, in the subsequent drying process, the temperature of the even-numbered rollers gradually decreases from front to back according to the difference, with the difference ranging from 4℃ to 10℃.

[0031] Furthermore, the number of drying rollers in the first stage of drying process is 6-8, and the number of drying rollers in the second stage of drying process is 4-6.

[0032] Furthermore, the moisture content of the film after passing through the casting roller and drying roller is 7%-12%.

[0033] Furthermore, the moisture content of the film after passing through the casting roller and drying roller is 8%-11%. Too low a moisture content leads to high internal stress in the film, which can easily cause film breakage.

[0034] Furthermore, the heat treatment temperature is 80℃-130℃. If the heat treatment temperature is too high, the crystallinity is high, the internal stress of the film is large, and the shrinkage is high after it is made into a polarizer; if the heat treatment temperature is too low, the cross-linking network of the PVA film will be incomplete, which will easily lead to film breakage.

[0035] The present invention also provides a low-curvature thin PVA optical film for polarizers, wherein the PVA optical film is prepared by the above-described preparation method.

[0036] Furthermore, the crystallinity difference between the front and back sides of the PVA optical film is ≤5%; the overall crystallinity difference value can achieve the technical effect of the film not curling or the degree of film curling not affecting the processing of the polarizer.

[0037] Furthermore, the difference in water content between the front and back sides of the PVA optical film is ≤2%.

[0038] To improve the crystallinity consistency and drying uniformity of the film on both the front and back sides, it is necessary to strictly control the crystallinity and moisture content of the film on both sides during the casting and drying process. The degree of crystallinity on one side of the film can be obtained by ATR reflectance infrared spectroscopy. Since the light transmission intensity varies depending on the moisture content of the film surface, changes in the surface moisture content can be collected online based on this principle.

[0039] The beneficial effects of this invention are:

[0040] (1) Based on the structural changes of the film during the casting and drying process and the evaporation rate of water, this invention adjusts the temperature of the casting roller and the drying roller so that the film is dried evenly on the front and back sides without forming a skin on the surface, thereby reducing the difference in crystallinity and water content between the front and back sides. The prepared PVA optical film has a uniform structure on both sides, and the swelling and curling of the film in water is significantly improved.

[0041] (2) The process of this invention is simple to operate, requires no additional process equipment, avoids increased costs, and has high feasibility in practical application. Detailed Implementation

[0042] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0043] Example 1

[0044] PVA resin (Anhui Wanwei High-Tech Materials Co., Ltd. 24-99L, degree of polymerization 2200, degree of alcoholysis 99%), sodium dodecylbenzenesulfonate, glycerol, and sodium bisulfite were mixed to a total mass of 3500g and added to a reactor. The sodium dodecylbenzenesulfonate accounted for 2% of the PVA resin mass, the glycerol for 10%, and the sodium bisulfite for 3%. 6500L of deionized water was then added to the reactor. The reactor was set to a reaction temperature of 150℃ and a pressure of 0.4MPa, and stirred continuously for 6 hours to dissolve the resin, obtaining a casting solution with a solute content of 35%. The casting solution was degassed and filtered by an extruder before being extruded from a die. After casting on a casting roller, the film was dried on a drying roller. The first drying roller after the casting roller was the first drying roller, and subsequent drying rollers were arranged sequentially. Odd-numbered rollers dried the front side of the film, and even-numbered rollers dried the back side. The moisture content of the film after casting and drying was 10%. Then, heat treatment is performed at a temperature of 80℃-130℃, and finally, the film is wound up to obtain a 20-40μm PVA optical film.

[0045] The casting roller temperature was set to 80℃, the linear speed to 12m / min, and the diameter to 2m. The drying rollers were divided into two sections. In the first drying section, the drying roller temperatures from the first to the sixth drying roller were set to 75℃, 60℃, 65℃, 55℃, 60℃, and 55℃ respectively. In the second drying section, the drying roller temperatures from the seventh to the tenth drying roller were set to 50℃, 45℃, 40℃, and 35℃ respectively. All drying rollers had the same linear speed and diameter (12m / min), and the film residence time on each drying roller was the same.

[0046] Example 2

[0047] This embodiment was prepared using the same method and conditions as in Example 1, except that the temperature of the casting roller was increased to 85°C and the moisture content of the film after casting and drying was 9 wt%.

[0048] Example 3

[0049] This embodiment was prepared using the same method and conditions as in Example 1, with the only difference being that the temperature of the casting roller was reduced to 75°C and the moisture content of the film after casting and drying was 11%.

[0050] Example 4

[0051] This embodiment was prepared using the same method and conditions as Example 1, with the only difference being that the temperatures of the first to sixth drying rollers during the initial drying process were set sequentially to 72, 67, 62, 60, 57, and 55 degrees Celsius. The moisture content of the film after casting and drying was 9%.

[0052] Example 5

[0053] This embodiment was prepared using the same method and conditions as Example 1, with the only difference being that the temperatures of the seventh to tenth drying rollers during the subsequent drying process were set to 45, 40, 35, and 35 degrees Celsius, respectively. The moisture content of the film after casting and drying was 11%.

[0054] Example 6

[0055] This embodiment was prepared using the same method and conditions as in Example 1, with the only difference being that: during the initial drying process, the number of drying roller temperatures was 8, and the temperatures were set sequentially as follows: 75, 72, 70, 67, 65, 62, 60, 58; during the subsequent drying process, the number of drying roller temperatures was 6, and the temperatures were set sequentially as follows: 50, 48, 46, 44, 42, 40. The moisture content of the film after casting and drying was 8%.

[0056] Example 7

[0057] This embodiment was prepared using the same method and conditions as in Example 1, with the only difference being that: the number of drying roller temperatures in the first stage of drying was 7, and the temperatures were set sequentially to 76, 74, 72, 70, 65, 60, and 55; the number of drying roller temperatures in the second stage of drying was 5, and the temperatures were set sequentially to 50, 45, 40, 38, and 35; and the moisture content of the film after casting and drying was 8%.

[0058] Comparative Example 1

[0059] Compared to Example 1, the preparation method of this comparative example differs in that the temperature of the casting roller is increased to 90°C, and the moisture content of the film after casting and drying is 8%. Other steps and conditions are the same as in Example 1.

[0060] Comparative Example 2

[0061] Compared to Example 1, the preparation method of this comparative example differs in that the temperatures of the first to sixth drying rollers during the initial drying process are set sequentially to 83, 80, 73, 70, 63, and 60 degrees Celsius. The moisture content of the film after casting and drying is 8%. Other steps and conditions are the same as in Example 1.

[0062] Comparative Example 3

[0063] Compared to Example 1, the preparation method of this comparative example differs in that the casting roller temperature is increased to 90°C, and the temperatures of the first to sixth drying rollers during the initial drying process are set sequentially as follows: 85°C, 80°C, 75°C, 70°C, 65°C, and 60°C. The moisture content of the film after casting and drying is 7%. Other steps and conditions are the same as in Example 1.

[0064] Comparative Example 4

[0065] Compared to Example 1, the preparation method of this comparative example differs in that the temperatures of the seventh to tenth drying rollers during the subsequent drying process are set sequentially to 60, 55, 50, and 45 degrees Celsius. The moisture content of the film after casting and drying is 9%. Other steps and conditions are the same as in Example 1.

[0066] Comparative Example 5

[0067] Compared to Example 1, the preparation method of this comparative example differs in that the casting roller temperature is increased to 90°C, and the temperatures of the seventh to tenth drying rollers during the subsequent drying process are set sequentially to 60, 55, 50, and 45°C. The moisture content of the film after casting and drying is 7%. Other steps and conditions are the same as in Example 1.

[0068] Comparative Example 6

[0069] Compared to Example 1, the preparation method of this comparative example differs in that: the number of front drying rollers is 2, and the temperature is set to 75°C and 70°C; the number of rear drying rollers is also 2, and the temperature is set to 65°C and 60°C; the moisture content of the film after casting and drying is 16%. Other steps and conditions are the same as in Example 1.

[0070] Performance tests were conducted on Examples 1-7 and Comparative Examples 1-6, and the results are shown in Table 1:

[0071] PVA film curling evaluation method: A curling test was performed on the PVA optical films prepared in the above examples and comparative examples. The PVA film was cut into 60*60mm rectangular sheets, and dovetail clips were used to clamp the top and bottom ends along the casting direction, ensuring the clips were aligned. The PVA sheets with dovetail clips were immersed in deionized water at 30℃ for 30 seconds, and the degree of film curling was observed. ○ indicates good curling, acceptable for downstream processing, while × indicates severe curling. The results of all examples and comparative examples are shown in Table 1 below.

[0072] Table 1

[0073]

[0074]

[0075] As shown in Table 1, the crystallinity difference between the two sides of the PVA optical films prepared in Examples 1-5 was controlled within 5%, and the moisture content difference was controlled within 2%. Testing revealed a significant improvement in the curling of the PVA optical films. In Examples 6 and 7, increasing the number of drying rollers allowed for complete control of curling and moisture content by adjusting the temperature. However, in Comparative Examples 1-5, the higher temperature resulted in a large difference in crystallinity and moisture content between the two sides of the film, and the low moisture content after casting and drying led to severe film curling. Although Comparative Example 6 achieved control over film curling with fewer drying rollers, its excessively high moisture content prevented it from meeting the requirements of downstream applications.

[0076] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0077] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a thin, low-curvature PVA optical film for polarizers, characterized in that, Includes the following steps: (1) The casting solution is prepared by dissolving PVA resin and additives in water; (2) After the casting solution is extruded and degassed, it is cast into a thin film through a die head; (3) After passing the film through casting rollers and drying rollers, heat treatment is performed to dry and shape it, and then it is wound up to obtain a PVA optical film; the temperature of the casting rollers is 75℃-85℃; The first drying roller after the casting roller is the first drying roller, and the subsequent drying rollers are arranged in sequence, wherein the odd-numbered rollers dry the front side of the film and the even-numbered rollers dry the back side of the film. The temperature range of the drying roller is divided into two drying processes: the temperature of the drying roller in the first drying process is 55℃-75℃, and the temperature of the drying roller in the second drying process is 35℃-50℃. In the drying process of the first stage, the temperature of the odd-numbered drying rollers gradually decreases from the first drying roller to the next according to the difference, with the difference ranging from 4℃ to 10℃. In the subsequent drying process, the temperature of the odd-numbered rollers in the drying rollers gradually decreases from front to back according to the difference, with the difference ranging from 4℃ to 10℃. In the preceding drying process, the temperature of the even-numbered rollers in the drying rollers gradually decreases from the second drying roller onwards according to the difference, with the difference ranging from 0℃ to 10℃. In the subsequent drying process, the temperature of the even-numbered rollers gradually decreases from front to back according to the difference, which ranges from 4℃ to 10℃.

2. The method for preparing a low-curvature, thin PVA optical film for polarizers according to claim 1, characterized in that, The additives include plasticizers, surfactants, and antioxidants.

3. The method for preparing a low-curvature, thin PVA optical film for polarizers according to claim 1, characterized in that, The number of drying rollers in the first stage of drying process is 6-8, and the number of drying rollers in the second stage of drying process is 4-6.

4. The method for preparing a low-curvature, thin PVA optical film for polarizers according to claim 1, characterized in that, The moisture content of the film after passing through the casting roller and drying roller is 8%-11%.

5. The method for preparing a low-curvature, thin PVA optical film for polarizers according to claim 1, characterized in that, The heat treatment temperature is 80℃-130℃.

6. A thin, low-curvature PVA optical film for polarizers, characterized in that, The PVA optical film is prepared using the preparation method described in any one of claims 1-5.

7. The low-curvature, thin PVA optical film for polarizers according to claim 6, characterized in that, The difference in crystallinity between the front and back sides of the PVA optical film is ≤5%.

8. The low-curvature, thin PVA optical film for polarizers according to claim 6, characterized in that, The difference in water content between the front and back sides of the PVA optical film is ≤2%.

9. A low-curvature, thin PVA optical film for polarizers according to claim 6, characterized in that, The thickness of the PVA optical film is 20μm-40μm.

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