Low-curl thin PVA optical film for polaroid and preparation method thereof

Abstract

The invention discloses a PVA optical film for a low-curl thin polarizer and a preparation method of the PVA optical film, and belongs to the technical field of optical film production.The PVA optical film is obtained by subjecting a film casting stock solution prepared from PVA resin to defoaming and film casting and then sequentially passing through a casting roller, a drying roller and heat treatment. By strictly controlling the temperature of a casting roller and a drying roller and setting a proper temperature gradient, under the condition that the surface of the thin PVA optical film is not crust, the front surface and the back surface are uniformly dried, the crystallinity and the moisture content difference of the front surface and the back surface are reduced, the structures of the two surfaces are uniform, the swelling and curling conditions of the film in water are obviously improved, the whole process is simple, and the production cost is low. The practical application feasibility is high, and the quality of the thin PVA optical film is remarkably improved.

Description

Technical Field

[0001] The invention belongs to the technical field of optical film production, and in particular relates to a low-curl thin PVA optical film for polarizers and a preparation method thereof. Background Art

[0002] Polyvinyl alcohol optical film is mainly used in the production of polarizers, and the end products are various types of liquid crystal displays, such as display panels for televisions, computer monitors, mobile phones, car navigation and wearable electronic devices. PVA polarizers are made by bonding triacetate cellulose TAC film after PVA optical film is moisturized, dyed, uniaxially stretched, and cross-linked with boride. As display panels develop towards thinner, more durable, and more transparent, polyvinyl alcohol polarizers are also gradually developing towards thinner and more transparent directions.

[0003] PVA optical film is made of PVA resin through dissolution, extrusion, casting, drying and other processes. The casting and drying process affects the changes in the microstructure of the PVA film, thus determining the processing performance of the PVA film in the polarizer manufacturing process. Since the PVA film needs to be dried on the front and back sides by multiple sections of metal drying rollers after being dried and formed by the casting roller, the uniformity of the film's drying degree affects the uniformity of the film's internal structure. For PVA films with small thickness, the uniformity of the drying process has a more obvious effect on the uniformity of the film's front and back structures. Thin PVA optical films with uneven internal structures are prone to curling during the downstream swelling process, resulting in serious folding problems. In response to this problem, some literature points out that by making the heat received by the front and back sides of the PVA film consistent during the cast drying process, the same drying effect can be achieved and the curling of the PVA optical film during the swelling process can be improved. However, due to the influence of the crusting on the film surface during the drying process, the evaporation rate of water on the front and back sides of the film is different. Therefore, even if the heat received by the front and back sides is consistent, the water content of the front and back sides of the film cannot be maintained consistent, and the goal of uniform structure on both sides of the PVA film cannot be achieved. Summary of the invention

[0004] The object of the present invention is to provide a low-curl thin PVA optical film for polarizer and a preparation method thereof, so as to achieve uniform structure on both sides of the PVA optical film and solve the curling problem during the swelling process.

[0005] The purpose of the present invention can be achieved through the following technical solutions:

[0006] The present invention provides a low-curl thin PVA optical film for polarizer and a preparation method thereof, comprising the following steps:

[0007] (1) dissolving PVA resin and additives in a solvent to prepare a film casting stock solution;

[0008] (2) After the film casting stock solution is extruded and degassed, it is cast through a die to form a film;

[0009] (3) The film is passed through a casting roller and a drying roller in sequence. After the film is pre-dried by the drying roller, it enters an oven for heat treatment, drying and shaping, and is rolled up to obtain a PVA optical film.

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

[0011] Furthermore, the content of the modified groups in the PVA resin is less than 10%. Too high a content will affect the solubility of the PVA resin and the optical properties of the film.

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

[0013] The lower the polymerization degree, the worse the stability of the molecular chain entanglement network of the prepared PVA polarizer, and the higher the stretching ratio will cause the cross-linking network to be destroyed, and the optical properties of the prepared polarizer will deteriorate; the higher the polymerization degree, the higher the viscosity of the stock solution will be, the more severe the pre-stretching will be, the film will have large anisotropy after cast molding, and it will be easy to break.

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

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

[0016] If the alcoholysis degree is too low, the content of polyiodide ions formed in the PVA film during the iodine dyeing and stretching process is small, which affects the optical properties of the polarizer; if the alcoholysis degree is too low, the solubility becomes worse, resulting in unstable cast film production and reduced qualified rate of finished products.

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

[0018] Too much additive content will cause precipitation, affecting the film's surface and optical properties and contaminating the downstream iodine dyeing and stretching tank.

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

[0020] Furthermore, the surfactant is at least one of dodecylbenzene sulfonate, potassium laurate, polyoxyethylene lauryl ester and polyoxyethylene lauric acid amide.

[0021] Furthermore, the antioxidant is sodium bisulfite.

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

[0023] Due to the low film thickness and fast evaporation rate of surface moisture, if the temperature of the casting roller is too high, the evaporation rate of the film moisture is fast, the viscosity of the system increases, and surface crusting will lead to uneven drying and structural differences in the film casting and drying process; if the temperature of the casting roller is too low, it will lead to uneven film peeling and easy to produce water ripples due to external disturbances. The initial PVA casting solution has a high water content, and the film does not crystallize on the casting drying roller. Its crystallization process mainly occurs in the subsequent drying process.

[0024] Further, 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;

[0025] The temperature range of the drying roller is divided into two drying processes, the temperature of the drying roller in the front drying process is 55°C-75°C, and the temperature of the drying roller in the back drying process is 35°C-50°C.

[0026] The front drying process is to change the crystallization rate of the front and back sides of the film by setting a temperature gradient at high temperature, thereby achieving the technical effect of a small difference in crystallinity on both sides of the PVA film; the back drying process is to set a temperature gradient at low temperature to meet the requirement of a small difference in water content between the front and back sides of the film.

[0027] Furthermore, the temperatures of the odd-numbered rollers in the drying rollers in the front drying process gradually decrease from the first drying roller onwards according to the difference, and the difference range is 4° C.-10° C.

[0028] Furthermore, the temperature of the odd-numbered rollers in the drying rollers in the latter drying process gradually decreases from the front to the back according to the difference, and the difference ranges from 4°C to 10°C.

[0029] Furthermore, the temperatures of the even-numbered rollers in the drying rollers in the front drying process gradually decrease from the second drying roller onwards according to the difference, and the difference range is 0°C-10°C.

[0030] Furthermore, the temperature of the even-numbered rollers in the drying rollers in the latter drying process gradually decreases from the front to the back according to the difference, and the difference ranges from 4°C to 10°C.

[0031] Furthermore, the number of drying rollers in the front drying process is 6-8, and the number of drying rollers in the back drying process is 4-6.

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

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

[0034] Furthermore, the heat treatment temperature is 80° C.-130° C. 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 the polarizer is prepared; if the heat treatment temperature is too low, the cross-linking network of the PVA film is incomplete, which easily leads to film breakage.

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

[0036] Furthermore, the difference in crystallinity between the front and back surfaces of the PVA optical film is ≤5%; the comprehensive difference in crystallinity can achieve the technical effect that the film does not curl or the curling degree of the film does not affect the processing process of the polarizer.

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

[0038] In order to improve the crystal consistency and drying uniformity of the front and back sides of the film, the crystallinity and moisture content of the front and back sides of the film must be strictly controlled during the casting and drying process. The crystallinity of a single side of the film can be obtained by ATR reflection infrared spectroscopy. Due to the different water content on the surface of the film, the transmission intensity of light is different. Based on this principle, the changes in the water content on the surface of the film can be collected online.

[0039] Beneficial effects of the present invention:

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

[0041] (2) The process of the present invention is simple to operate, does not require the addition of other process equipment, avoids cost increase, and has high feasibility for practical application. DETAILED DESCRIPTION

[0042] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work 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, polymerization degree 2200, alcoholysis degree 99%), sodium dodecylbenzene sulfonate, glycerol and sodium bisulfite are mixed, and the total mass is 3500g, and the reactor is put into the reactor, wherein the mass of sodium dodecylbenzene sulfonate is 2% of the mass of PVA resin, the mass of glycerol is 10% of the mass of PVA resin, and the mass of sodium bisulfite is 3% of the mass of PVA resin. Then 6500L of deionized water is added to the reactor, and the reaction temperature of the reactor is set to 150°C, the pressure is 0.4MPa, and the stirring is continued for 6h to dissolve, and a casting film stock solution with a solute mass content of 35% is obtained. The casting film stock solution is extruded from the die after degassing and filtering by the extruder, and after casting by the casting roller, it enters the drying roller for drying, and the first drying roller after the casting roller is the first drying roller, and the subsequent drying rollers are arranged in order, wherein the odd-numbered rollers dry the front of the film, and the even-numbered rollers dry the reverse side of the film, and the water content of the film after casting and drying is 10%. Afterwards, heat treatment is performed, and the heat treatment temperature is set at 80°C-130°C. Finally, the film is rolled up to obtain a 20-40 μm PVA optical film.

[0045] The temperature of the casting roller is set to 80°C, the line speed is set to 12m / min, and the diameter is 2m. The drying section in the drying roller is divided into two sections. During the first drying section, the temperature of the drying rollers from the first drying roller to the sixth drying roller is set to 75, 60, 65, 55, 60, 55. During the second drying section, the temperature of the drying rollers from the seventh drying roller to the tenth drying roller is set to 50, 45, 40, 35. The line speed and diameter of all drying rollers are the same, the line speed is 12m / min, the diameter is 1m, and the residence time of the film on each drying roller is the same.

[0046] Example 2

[0047] This example was prepared by the same method and conditions as 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 example was prepared by the same method and conditions as Example 1, except that the temperature of the casting roller was lowered to 75° C. and the moisture content of the film after casting and drying was 11%.

[0050] Example 4

[0051] This embodiment is prepared by the same method and conditions as in embodiment 1, except that the temperatures of the first drying roller to the sixth drying roller in the front drying process are set to 72, 67, 62, 60, 57, 55. The moisture content of the film after cast drying is 9%.

[0052] Example 5

[0053] This example was prepared in the same manner and under the same conditions as in Example 1, except that the temperatures of the seventh drying roller to the tenth drying roller in the subsequent drying process were set to 45, 40, 35, and 35, respectively. The moisture content of the film after cast drying was 11%.

[0054] Example 6

[0055] This embodiment is prepared by the same method and conditions as embodiment 1, with the only difference that: the number of drying roller temperatures in the front drying process is 8, and the temperatures are set to 75, 72, 70, 67, 65, 62, 60, 58 in sequence; the number of drying rollers in the rear drying process is 6, and the temperatures are set to 50, 48, 46, 44, 42, 40 in sequence; the moisture content of the film after cast drying is 8%.

[0056] Example 7

[0057] This embodiment is prepared by the same method and conditions as embodiment 1, with the only difference that: the number of drying roller temperatures in the front drying process is 7, and the temperatures are set to 76, 74, 72, 70, 65, 60, 55 in sequence; the number of drying rollers in the rear drying process is 5, and the temperatures are set to 50, 45, 40, 38, 35 in sequence; the moisture content of the film after cast drying is 8%.

[0058] Comparative Example 1

[0059] Compared with Example 1, the preparation method of this comparative example is different 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 those of Example 1.

[0060] Comparative Example 2

[0061] Compared with Example 1, the preparation method of this comparative example is different in that: in the front drying process, the temperatures of the first drying roller to the sixth drying roller are set to 83, 80, 73, 70, 63, 60 in sequence. 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 with Example 1, the preparation method of this comparative example is different in that: the temperature of the casting roller is increased to 90°C, and the temperatures of the first drying roller to the sixth drying roller in the front drying process are set to 85, 80, 75, 70, 65, and 60, respectively. 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 with Example 1, the preparation method of this comparative example is different in that: in the latter drying process, the temperatures of the seventh drying roller to the tenth drying roller are set to 60, 55, 50, and 45, respectively. The moisture content of the film after casting and drying is 9%. Other steps and conditions are the same as those of Example 1.

[0066] Comparative Example 5

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

[0068] Comparative Example 6

[0069] Compared with Example 1, the preparation method of this comparative example is different in that: the number of the front-stage drying rollers is 2, the temperature is set to 75°C and 70°C, the number of the rear-stage drying rollers is also 2, the temperature is set to 65 and 60, and 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 performed on Examples 1 to 7 and Comparative Examples 1 to 6, and the results are shown in Table 1:

[0071] PVA film curling evaluation method: The PVA optical film prepared in the above embodiments and comparative examples was subjected to a curling test. The PVA film was cut into a 60*60mm rectangular sheet, and the upper and lower ends were clamped with dovetail clamps along the casting direction to ensure that the dovetail clamps at both ends were aligned. The PVA sheet with the dovetail clamp was immersed in deionized water at 30°C for 30s, and the curling degree of the film was observed. Among them, ○ represents a good degree of curling that can be accepted by downstream processing, and × represents severe curling. The results of all embodiments and comparative examples are shown in Table 1 below.

[0072] Table 1

[0073]

[0074]

[0075] As can be seen from Table 1, the difference in crystallinity between the two sides of the PVA optical film prepared in Examples 1-5 is controlled within 5%, and the difference in moisture content is controlled within 2%. After testing, the curling of the PVA optical film is significantly improved. In Examples 6 and 7, increasing the number of drying rollers can completely achieve the regulation of curling and moisture content by adjusting the temperature. In Comparative Examples 1-5, due to the high temperature, the difference in crystallinity and moisture content between the two sides of the film is large, and the moisture content is low after cast drying, resulting in severe curling of the film. Although Comparative Example 6 can achieve the regulation of film curling with fewer drying rollers, its high moisture content makes it unable to meet downstream application requirements.

[0076] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

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

Claims

1. A method for preparing a low-curl thin PVA optical film for polarizer, characterized in that: The following steps are involved: (1) dissolving PVA resin and additives in water to prepare a film casting stock solution; (2) After the film casting stock solution is extruded and degassed, it is cast through a die to form a film; (3) After the film passes through a casting roller and a drying roller, it is heat-treated, dried and shaped, and rolled up to obtain a PVA optical film.

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

3. The method for preparing a low-curl thin PVA optical film for polarizer according to claim 1, characterized in that: The temperature of the casting roller is 75°C-85°C.

4. The method for preparing a low-curl thin PVA optical film for polarizer according to claim 1, characterized in that: 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 front drying process is 55°C-75°C, and the temperature of the drying roller in the back drying process is 35°C-50°C.

5. The method for preparing a low-curl thin PVA optical film for polarizer according to claim 4, characterized in that: The temperatures of the odd-numbered rollers in the drying rollers of the front drying process gradually decrease from the first drying roller to the next according to the difference, and the difference range is 4° C.-10° C.

6. The method for preparing a low-curl thin PVA optical film for polarizer according to claim 4, characterized in that: The temperature of the odd-numbered rollers in the drying rollers in the latter drying process gradually decreases from the front to the back according to the difference, and the difference ranges from 4°C to 10°C.

7. The method for preparing a low-curl thin PVA optical film for polarizer according to claim 4, characterized in that: The temperatures of the even-numbered rollers in the drying rollers of the front drying process gradually decrease from the second drying roller onwards according to the difference, and the difference range is 0° C.-10° C.

8. The method for preparing a low-curl thin PVA optical film for polarizer according to claim 4, characterized in that: The temperature of the even-numbered rollers in the drying rollers in the latter drying process gradually decreases from the front to the back according to the difference, and the difference ranges from 4°C to 10°C.

9. The method for preparing a low-curl thin PVA optical film for polarizer according to claim 5, characterized in that: The number of drying rollers in the front drying process is 6-8, and the number of drying rollers in the back drying process is 4-6.

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

11. The method for preparing a low-curl thin PVA optical film for polarizer according to claim 1, characterized in that: The temperature of the heat treatment is 80°C-130°C.

12. A low curl thin PVA optical film for polarizer, characterized in that: The PVA optical film is prepared by the preparation method according to any one of claims 1 to 11.

13. The low curl thin PVA optical film for polarizer according to claim 12, characterized in that: The difference in crystallinity between the front and back surfaces of the PVA optical film is ≤5%.

14. The low curl thin PVA optical film for polarizer according to claim 12, characterized in that: The difference in moisture content between the front and back surfaces of the PVA optical film is ≤2%.

15. The low curl thin PVA optical film for polarizer according to claim 12, characterized in that: The thickness of the PVA optical film is 20 μm-40 μm.

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