A method for preparing a thin polyvinyl alcohol optical film

Abstract

The application discloses a preparation method of a thin polyvinyl alcohol optical film. The method comprises the following steps: designing a formula of raw materials and a polyvinyl alcohol casting solution, controlling a starting crystallization temperature of the casting solution to be within a certain temperature range when the casting solution is peeled off from a casting roller, and designing the temperature of the casting roller and a drying roller in a film processing process according to the starting crystallization temperature, so as to realize the refinement of the crystal size of the film and obtain the thin PVA optical film. The method is simple and efficient, and does not cause the problems of the decline of the optical performance of a base film and the weather resistance of a downstream polarizing sheet product, and has good market competitiveness.

Description

TECHNICAL FIELD

[0001] The present application relates to a preparation method of a polyvinyl alcohol optical film, in particular to a preparation method of a thin polyvinyl alcohol optical film, which can solve the film breaking problem of the thin PVA optical base film in the downstream processing process. BACKGROUND

[0002] In the information society, display is ubiquitous, and is a window for information exchange and a command and control platform, and is an indispensable high-tech for the development of modern economy and society and national security. The upstream raw material PVA optical base film is one of the important components of the polarizer in display. The PVA polarizer is obtained by combining the upper and lower TAC after the PVA optical base film is processed by iodine dyeing, stretching and drying. With the development of new display towards thinner direction, higher requirements are put forward for the thickness of the PVA polarizer and the PVA optical base film (20-30 μm). However, compared with the thicker PVA optical base film, the thin PVA optical base film is prone to film breaking and other problems in the downstream processing process.

[0003] The processing of the PVA optical film adopts a casting processing technology. After the PVA solution system (PVA, water, plasticizer, etc.) is dissolved in a dissolving kettle, it is extruded through an extruder and a die, gradually dried on a casting roller and a drying roller, and finally heat treated to form a product. The crystal size and uniformity of the thin film crystal structure determine the stretchability and film breaking of the thin film in the downstream stretching process. Therefore, how to reduce the crystal size and improve the uniformity of the crystal size is the key to solving the film breaking problem of the thin PVA optical base film in the downstream processing process. In view of the film breaking problem, the current patents mainly improve the stretchability by modifying the molecular chain of the raw material. However, since the key to solving the problem lies in the refinement of the crystal morphology of the PVA film, the adjustment of the crystal morphology is only realized by modifying the raw material, which has poor effect and also causes problems such as poor solubility of PVA and poor optical performance of the film. There are also patents that adjust the heat treatment process to adjust the crystal morphology of the PVA film. However, the skeleton of the film crystal and the size of the final crystal, i.e. the stretchability of the film, are determined in the previous casting and drying stage. Therefore, the effect of improving the film breaking performance by adjusting the heat treatment process is very small. SUMMARY

[0004] To solve the above-mentioned film breaking problem of the thin PVA optical base film, the present application provides a preparation method of a thin PVA optical film, which aims to obtain a thin PVA optical film, solve the film breaking problem in the production process of the polarizer, and be capable of being used for stably producing a thin polarizer.

[0005] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0006] The preparation method of a thin PVA optical film is characterized in that: the starting crystallization temperature of the casting solution is controlled within a certain temperature range when the casting roll is peeled off through the design of raw materials and formula composition, and the temperature of the casting roll and drying roll in the film processing process is designed according to the starting crystallization temperature, so as to realize the refinement of the crystal size of the film and obtain the thin PVA optical film which does not break in the downstream processing process. The above preparation method is essentially a method for solving the film breaking problem of the thin PVA optical base film in the downstream processing process.

[0007] Further, the polyvinyl alcohol casting solution includes PVA, solvent, plasticizer, surfactant and antioxidant.

[0008] Further, the PVA in the polyvinyl alcohol casting solution includes at least one of polyvinyl alcohol homopolymer and polyvinyl alcohol copolymer modifier, and the modification group includes at least one of ethylene, propylene, acrylic acid and methyl acrylate.

[0009] Further, the content of the PVA modification group in the casting solution is controlled between 0-10%, and too high will affect the PVA solubility and optical performance of the film.

[0010] Further, the polydispersity of the PVA used in the casting solution is 1200-4000, and more preferably 1500-3000, and too low PVA optical film weather resistance will be poor.

[0011] Further, the alcoholysis degree of the PVA used in the casting solution is 98-99.9%, and too low solubility and weather resistance will be poor.

[0012] Further, the plasticizer in the casting solution includes at least one of glycerol, diglycerol, polyglycerol, ethylene glycol and propylene glycol.

[0013] Further, the content of the plasticizer in the casting solution is controlled between 8-15%, and too much will cause the plasticizer to precipitate, resulting in poor surface performance of the film.

[0014] Further, the surfactant in the casting solution is a mixed surfactant of non-ionic surfactant and anionic surfactant, wherein the non-surfactant includes at least one of alkyl esters such as polyoxyethylene lauryl ester, alkyl amines such as polyoxyethylene lauryl amino ester, and alkyl amides such as polyoxyethylene lauryl amide, and the anionic surfactant includes at least one of carboxylic acid type such as potassium laurate, and sulfonate type anionic surfactant such as dodecyl benzene sulfonate.

[0015] Further, the mass fraction of the solute in the casting solution is 20-40%, and more preferably 25-35%.

[0016] Further, the casting solution is extruded from the extruder die, and then dried by a casting roller and a drying roller, and heat treated to form a film.

[0017] Further, the solid content of the casting solution after being peeled off from the casting roller is controlled at 60-85%, and too high solid content will cause serious curling of the film, and too low solid content will cause incomplete drying of the film, which is not conducive to stable production.

[0018] Further, by adjusting the raw materials and formula of the casting solution, the initial crystallization temperature of the casting solution is controlled at 60-90°C when the casting solution is peeled off from the casting roller (i.e., the solid content is 60-85%).

[0019] Further, based on the design of the initial crystallization temperature of the casting solution, the temperature of the casting roller in the drying stage of the casting solution needs to be designed to be higher than the initial crystallization temperature when the casting solution is peeled off from the casting roller and lower than 100°C, and too low temperature will cause large crystals due to crystallization under high water content, and too high temperature will cause the casting solution to boil, resulting in bubbles in the film.

[0020] Further, based on the design of the initial crystallization temperature of the casting solution, the temperature of the drying roller in the drying stage of the casting solution needs to be designed to be lower than the initial crystallization temperature when the casting solution is peeled off from the casting roller and higher than 35°C, and too high temperature will cause rapid crystal growth of the film, resulting in large crystals, and too low temperature will cause incomplete drying of the film, which is not conducive to stable production.

[0021] Compared with the prior art, the beneficial effects of the present application are embodied in:

[0022] (1) The present application directly targets the key influencing factor of film breakage, i.e., crystal structure, and designs raw materials, formula and process, controls the initial crystallization temperature of the casting solution within a certain temperature range through the design of raw materials and formula composition, and designs the temperatures of the casting roller and the drying roller in the film processing process according to the initial crystallization temperature, thereby realizing the refinement of the crystal size of the film and achieving the purpose of improving the stretchability and avoiding film breakage. Compared with the existing patent reporting technology, the present application is more efficient and does not cause problems such as the decline of the optical performance of the base film and the weather resistance of the downstream polarizing sheet product.

[0023] (2) The present application has the characteristics and advantages of being simple and easy to implement, without the need to add other process equipment, and has good market competitiveness. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical solutions and advantages of the present application clearer, the technical solutions of the present application will be described clearly and completely below. Obviously, the described specific embodiments are only part of the embodiments of the present application, rather than all. Based on the specific embodiments of the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present application.

[0025] Example 1

[0026] A mixture of 3500 g of PVA polymer (degree of polymerization 1700, alcoholysis degree 99%), sodium dodecyl benzene sulfonate, glycerol and antioxidant ammonium sulfate (mass ratio of each component 100:1:20:1) and 6500 L of deionized water were put into a reaction kettle, dissolved at 150°C and 0.4 MPa for 6 h under stirring to obtain a PVA casting solution with a solid content of about 35%. The formed casting solution was defoamed by an extruder, filtered and then extruded from a die, dried by a casting roller at 75°C, and the solid content was increased to 60%. The initial crystallization at this solid content was measured to be 100°C. Then the casting solution was dried by a drying roller at 85°C, and then heat treated in an oven. Finally, a 20 μm thick PVA optical film was obtained by winding.

[0027] Example 2

[0028] This example was prepared by the same method and conditions as in Example 1, except that the drying roller temperature was adjusted to 75°C.

[0029] Example 3

[0030] This example was prepared by the same method and conditions as in Example 1, except that the casting roller temperature was adjusted to 82°C.

[0031] Example 4

[0032] This example was prepared by the same method and conditions as in Example 1, except that the casting roller temperature was adjusted to 82°C and the drying roller temperature was adjusted to 75°C.

[0033] Example 5

[0034] This example was prepared by the same method and conditions as in Example 1, except that the mass ratio of PVA to glycerol was adjusted to 100:15, and the initial crystallization at the time of casting roller peeling was adjusted to 80°C.

[0035] Example 6

[0036] This example was prepared by the same method and conditions as in Example 1, except that the mass ratio of PVA to glycerol was adjusted to 100:15, and the initial crystallization at the time of casting roller peeling was adjusted to 80°C, and the drying roller temperature was adjusted to 75°C.

[0037] Example 7

[0038] This example was prepared by the same method and conditions as in Example 1, except that the mass ratio of PVA to glycerol was adjusted to 100:15, and the initial crystallization at the time of casting roller peeling was adjusted to 80°C, and the casting roller temperature was adjusted to 82°C.

[0039] Example 8

[0040] This example was prepared by the same method and conditions as example 1, except that the mass ratio of PVA to glycerol was adjusted to 100:15, the initial crystallization at the time of stripping the casting roll was adjusted to 80°C, the temperature of the casting roll was adjusted to 82°C, and the temperature of the drying roll was adjusted to 75°C.

[0041] The method for testing the initial crystallization temperature of a casting solution with a certain solid content is as follows: the PVA casting solution with a certain solid content and formulation is placed in a self-made sealed sample chamber, which can ensure that the water content of the casting solution does not change during the heating process and can allow X-rays and sample signals to pass through. The sample is gradually reduced from 200°C to 0°C, and the sample crystal signal is detected synchronously using X-rays. The temperature point at which the sample crystal signal begins to appear is recorded as the initial crystallization temperature of the casting solution with the solid content.

[0042] The method for evaluating the film breaking condition is as follows: the PVA optical film prepared in the above examples is subjected to tensile property testing. In order to truly reflect the film breaking condition during the processing of a polarizing sheet, the PVA film is immersed in 30°C deionized water for 5 min, then stretched in a solution containing 0.5 g / L iodine and 20 g / L potassium iodide while being dyed, the stretching ratio is 5.8 times, and the stretching film breaking condition is recorded. If the film breaks, it is recorded as X, otherwise it is recorded as O.

[0043] The experimental results of different examples are shown in Table 1.

[0044] Table 1: Initial crystallization temperature, processing technology and film breaking condition of samples in different examples

[0045] Onset of crystallization at peel (°C) Cast roll temperature (°C) Dryer roll temperature (°C) Film breakage Example 1 100 75 85 × Example 2 100 75 75 × Example 3 100 82 85 × Example 4 100 82 75 × Example 5 80 75 85 × Example 6 80 75 75 × Example 7 80 82 85 × Example 8 80 82 75 O

[0046] As can be seen from Table 1, in addition to example 8, i.e., the initial crystallization temperature at the stripping point is 80°C, the temperature of the casting roll is 82°C, and the temperature of the drying roll is 75°C, the films prepared in the other examples all have the problem of film breaking. This indicates that: (1) when the initial crystallization temperature at the stripping point is too high, the film is prone to breaking, which may be related to the fact that the casting solution crystallizes too easily at this time; (2) when the temperature of the casting roll is lower than the initial crystallization temperature, the film is prone to breaking, which may be related to the fact that the crystals obtained by the crystallization of the casting solution at high temperature are too large; (3) when the temperature of the drying roll is higher than the initial crystallization temperature, the film is also prone to breaking, which may be related to the fact that the crystal growth is accelerated and the film crystals are too large; (4) only when the initial crystallization temperature of the casting solution at the stripping point is in the range of 60-90°C, the temperature of the casting roll is higher than the initial crystallization temperature, and the temperature of the drying roll is lower than the initial crystallization temperature, can a stable and unbroken PVA optical base film be obtained.

Claims

1. A method for preparing a thin polyvinyl alcohol optical film, characterized in that: By designing the raw materials and polyvinyl alcohol casting solution formulation, the initial crystallization temperature of the casting solution during the peeling of the casting roll is controlled within a certain temperature range. Based on the initial crystallization temperature, the temperatures of the casting roll and drying roll in the film processing process are designed, thereby achieving the refinement of the film crystal size and obtaining a thin PVA optical film. The initial crystallization temperature of the casting solution during the peeling process of the casting roll is controlled at 60-90℃; the temperature of the casting roll is controlled above the initial crystallization temperature and below 100℃; the temperature of the drying roll is controlled below the initial crystallization temperature and above 35℃.

2. The preparation method according to claim 1, characterized in that: The polyvinyl alcohol casting solution includes PVA, solvent, plasticizer, surfactant, and antioxidant.

3. The preparation method according to claim 1, characterized in that: The mass fraction of solute in the polyvinyl alcohol casting solution is 20-40%.

4. The preparation method according to claim 1, characterized in that: The solid content of the polyvinyl alcohol casting solution after being peeled off from the casting roll is controlled at 60-85%.