Method for preparing polyvinyl alcohol optical film without horizontal stripes

Abstract

The present invention discloses a method for preparing a non-stripe polyvinyl alcohol optical film. The method optimizes the die angle of a die used in the PVA casting process, the die lip angle between the die and the casting roll, and the distance between the die and the casting roll, as well as their combination, thereby optimizing the shape and length of the PVA casting solution between the die and the roll. This method avoids the formation of horizontal striations in the PVA optical film without affecting the stable processing and production efficiency of the PVA casting solution, thereby obtaining a non-stripe PVA optical film that can be used to produce high-quality polarizers. The method is simple and easy to implement, does not require additional process equipment, and has good market competitiveness.

Description

Technical Field

[0001] The invention relates to a method for preparing a polyvinyl alcohol optical film, in particular to a method for preparing a polyvinyl alcohol optical film without horizontal stripes. Background Art

[0002] Polyvinyl alcohol (PVA) optical films are a key component of polarizers and are widely used in display panels for televisions, computer monitors, mobile phones, in-car navigation systems, and wearable electronic devices. As display panels become thinner, the quality requirements for PVA optical films are becoming increasingly stringent. Improving the quality of PVA optical films, reducing optical defects, and increasing the yield rate of finished products have become paramount technological advancements within the industry.

[0003] The current production process is designed only for the stability of production and the drying efficiency of the PVA casting liquid, and the parameter of the relative position between the die and the casting roller is designed. This results in poor stability of the PVA casting liquid between the die and the casting roller, which is easily affected by the stretching during the casting process and the disturbance caused by the wind field. The liquid film therefore produces periodic fluctuations, resulting in horizontal stripes, which are the most obvious defects of PVA optical films. Specifically, when the film is placed under parallel light, its projection can be observed to have relatively obvious light and dark stripes perpendicular to the casting direction. These stripes are defined as horizontal stripes, and the degree of difference in light and dark between the stripes is defined as the intensity of the horizontal stripes. The horizontal stripe phenomenon seriously affects the performance of polyvinyl alcohol optical films. Polaroids prepared by the dyeing and stretching method using polyvinyl alcohol optical films with horizontal stripes also have obvious dark and light stripes. The optical uniformity of the polaroid products is seriously reduced, which limits their application in the field of high-end displays. Summary of the Invention

[0004] In order to solve the above-mentioned problem of horizontal stripes, the present invention provides a method for preparing a PVA optical film, aiming to obtain a PVA optical film without horizontal stripes and with optical uniformity, which can be used to produce higher quality polarizers.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for preparing a PVA optical film without horizontal stripes is characterized by defining the die head angle of a die used for PVA casting liquid casting processing, the die lip angle between the die and the casting roller, and the distance between the die and the casting roller, and designing these three parameters and their combination, thereby optimizing the shape and length of the PVA casting liquid between the die and the roller, improving the stability of the PVA casting liquid between the die and the roller, avoiding periodic fluctuations of the casting liquid caused by stretching and wind field disturbances, and thus avoiding the formation of horizontal stripes in the PVA optical film.

[0007] Furthermore, the die angle refers to the angle between the centerline of the die and the vertical direction, and is preferably 15-45°, more preferably 20-40°. If the die angle is too small, the casting solution will easily adhere to the die, resulting in longitudinal grain defects in the film. If the die angle is too large, the casting solution will be less stable and more susceptible to disturbances caused by stretching, significantly increasing the strength of the transverse grains.

[0008] Furthermore, the die lip angle refers to the angle between the line connecting the die lip and the origin and the vertical direction, with the center of the casting roll as the origin. It is preferably 15-45°, more preferably 20-40°. If the die lip angle is too small, the film may not dry completely, affecting the subsequent film formation stability and film quality. If the die lip angle is too large, the casting liquid is more susceptible to the disturbance caused by stretching, the strength of the horizontal stripes is increased, and the casting liquid is more likely to adhere to the die, resulting in defects such as longitudinal stripes in the film.

[0009] Furthermore, the distance between the die and the casting roll refers to the shortest distance from the die lip to the roll surface during the casting process, and is preferably 0.5-10 mm, more preferably 1-5 mm. A too large distance between the die and the roll will cause the PVA casting solution to be too long, resulting in significant vibration due to wind disturbances and increased horizontal streaking. A too small distance increases the risk of collision between the two devices.

[0010] Furthermore, the absolute value of the angle difference between the die lip angle and the die head angle is not greater than 20°, more preferably not greater than 10°. If the difference is too large, the casting solution is easily affected by the disturbance caused by stretching, and the horizontal strength of the stripes is enhanced.

[0011] Furthermore, the PVA used in the casting solution has a degree of polymerization of 1200-4000, more preferably 1500-3000. If the degree of polymerization is too low, the mechanical properties of the PVA optical film will be poor; if the degree of polymerization is too high, the casting solution fluctuation caused by stretching will be amplified, resulting in enhanced horizontal stripes.

[0012] Furthermore, the PVA includes at least one of a polyvinyl alcohol homopolymer and a polyvinyl alcohol copolymer modified product, and the modified group includes at least one of ethylene, propylene, acrylic acid, methyl acrylate, etc.

[0013] Furthermore, the solvent used in the casting solution includes at least one of water, dimethyl sulfoxide, N,N-dimethylformamide, N-methylpyrrolidone and triethylamine.

[0014] Furthermore, the PVA casting solution further comprises at least one of a plasticizer, a surfactant, an antioxidant and an ultraviolet absorber.

[0015] Furthermore, the plasticizer includes at least one of glycerol, diglycerol, polyglycerol, ethylene glycol and propylene glycol.

[0016] Furthermore, the surfactant is a mixed surfactant of a nonionic surfactant and an anionic surfactant, wherein: the non-surfactant includes at least one of alkyl esters such as polyoxyethylene lauryl ester, alkylamines such as polyoxyethylene lauryl amino ester, and alkylamides such as polyoxyethylene lauric acid amide; the anionic surfactant includes at least one of carboxylic acid type anionic surfactants such as potassium laurate and sulfonate type anionic surfactants such as dodecylbenzene sulfonate.

[0017] Furthermore, the mass fraction of solute in the casting solution is 20-40%, more preferably 25-35%. If the mass fraction is too small, it will not be conducive to the drying of the casting solution, affecting the processing performance of the casting solution and the performance of the final optical film product; if it is too large, it will also amplify the casting solution fluctuation caused by stretching, resulting in enhanced horizontal stripes.

[0018] Compared with the prior art, the beneficial effects of the present invention are embodied in:

[0019] (1) The present invention optimizes the shape and length of the PVA casting liquid between the die and the roller by designing the die head angle of the die for PVA casting liquid casting processing, the die lip angle between the die and the casting roller, the distance between the die and the casting roller, and the combination of the three parameters. Without affecting the stable processing and production efficiency of the PVA casting liquid, the generation of horizontal stripes in the PVA optical film is avoided, and a PVA optical film without horizontal stripes is obtained, which can be used to process high-quality polarizers.

[0020] (2) The method of the present invention has the characteristics and advantages of being simple and easy to implement and not requiring the addition of other process equipment, and has good market competitiveness. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of the die, casting roller, the casting solution between them, and the distance between the die and casting roller during the PVA optical film casting process; Figure 2 Schematic diagram of die head angle and die lip angle;

[0022] Markings in the figure: 1 is the die, 2 is the PVA casting liquid between the die and the casting roller, 3 is the casting roller, 4 is the distance between the die and the casting roller, the angle shown in 5 is the die head angle, and the angle shown in 6 is the die lip angle. DETAILED DESCRIPTION

[0023] The following is a detailed description of an embodiment of the present invention. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process. However, the protection scope of the present invention is not limited to the following embodiment.

[0024] The method for testing the horizontal stripe strength of the PVA optical film in the following embodiments is as follows:

[0025] Photograph the projection of the sample film under the illumination of a point light source, use software to perform tilt correction on the image, convert it into a frontal projection, and use a blank background photo to subtract it to obtain the transmittance matrix of the selected area of ​​the film. Convert the transmittance of each point into absorbance per unit thickness, perform Fourier transform in the column direction, and then average the results of each column to obtain the Fourier frequency spectrum of the fluctuation signal. A more obvious peak can be seen in the graph of the sample with horizontal stripes after processing. The horizontal axis is the frequency value, that is, the number of times the horizontal stripes are repeated in the length of the selected area; the vertical axis is the absorbance value per millimeter, which is defined here as the horizontal stripe intensity, and the unit is mm -1 The sample has no obvious peak and the sample's horizontal stripe strength value is 0.02mm. -1 The following is called weak, and the sample's transverse grain strength is 0.02~0.05mm -1 Generally, 0.05~0.1mm -1 Stronger, 0.1mm -1 The above are very strong.

[0026] Example 1

[0027] A total weight of 3500g of PVA polymer (degree of polymerization 1700), sodium dodecylbenzenesulfonate, glycerol, and an antioxidant mixture were placed in a reactor with 6500L of deionized water and stirred at 150°C and 0.4MPa for 6 hours to dissolve, obtaining a PVA film casting solution with a solute content of 35%. The resulting casting solution was deaerated and filtered in an extruder, then extruded from a die. After drying on a casting roller, it passed through 8 sets of drying rollers to repeatedly dry the front and back of the film. The film was then placed in an air flotation oven for heat treatment and finally wound to obtain a PVA optical film. The relative position relationship between the die and the casting roller was as follows: die head angle 35°, die lip angle 35°, and a distance of 2mm between the die and the casting roller. The prepared PVA optical film was tested for transverse stripe strength, and the results are shown in Table 1.

[0028] Example 2

[0029] The same processing method as in Example 1 was used, except that the relative positions of the die and casting roll were changed to: the die head angle and die lip angle were both 50°, and the distance between the die and casting roll was 2 mm. The resulting PVA optical film was tested for transverse striation strength. The results are shown in Table 1.

[0030] Example 3

[0031] The same processing method as in Example 1 was used, except that the relative positions of the die and casting roll were changed to: a die head angle of 35°, a die lip angle of 15°, and a distance of 2 mm between the die and casting roll. The resulting PVA optical film was tested for transverse striation strength. The results are shown in Table 1.

[0032] Example 4

[0033] The same processing method as in Example 1 was used, except that the relative positions of the die and casting roll were changed to: a die head angle of 35°, a die lip angle of 35°, and a distance of 4.0 mm between the die and casting roll. The resulting PVA optical film was tested for transverse striation strength. The results are shown in Table 1.

[0034] Example 5

[0035] The same processing method as in Example 1 was used, except that the relative positions of the die and casting roll were changed to: a die head angle of 35°, a die lip angle of 35°, and a distance of 6.0 mm between the die and casting roll. The resulting PVA optical film was tested for transverse striation strength. The results are shown in Table 1.

[0036] Example 6

[0037] The same processing method as in Example 1 was used, except that the relative positions of the die and casting roll were changed to: a die head angle of 35°, a die lip angle of 10°, and a distance of 2 mm between the die and casting roll. The resulting PVA optical film was tested for transverse striation strength. The results are shown in Table 1.

[0038] Table 1 Relative position relationship between the die and the casting roll in each embodiment and the horizontal stripe strength of the obtained film

[0039]

[0040] As can be seen from Table 1, under the premise that the equipment conditions and other processes remain unchanged, by adjusting the three parameters of the relative position of the die and the casting roll, namely the die head angle, the die lip angle, and the distance between the die and the casting roll, the horizontal stripe strength of the finished PVA optical film can be significantly affected. Specifically, in Examples 1, 3, and 4, the die lip angle and the die head angle are both between 15-45°, the distance between the die and the casting roll is 1-5 mm, and the difference between the die lip angle and the die head angle does not exceed 20°, no horizontal stripes are observed on the PVA optical film; while in Examples 2, 5, and 6, obvious horizontal stripes are observed on the PVA optical film because the die lip angle and the die head angle exceed 45°, the distance between the die and the casting roll exceeds 5 mm, and the difference between the die lip angle and the die head angle exceeds 20°.

[0041] The above description is merely an exemplary embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A method for preparing a non-stripe polyvinyl alcohol optical film, characterized in that: By setting the three parameters of the die angle, the lip angle between the die and the casting roll, and the distance between the die and the casting roll, the formation of horizontal stripes in the PVA optical film can be avoided. The die angle refers to the angle between the center line of the die and the vertical direction, and the die angle is 15-45°; The die lip angle refers to the angle between the line connecting the die lip and the origin and the vertical direction with the center of the casting roll as the origin; the die lip angle is 15-45°; The distance between the die and the casting roller refers to the shortest distance from the die lip to the roller surface during the casting process; the distance between the die and the casting roller is 0.5-10 mm; The absolute value of the angle difference between the die lip angle and the die head angle is not greater than 20°.

2. The method for preparing a streak-free polyvinyl alcohol optical film according to claim 1, wherein: The die angle is 20-40°.

3. The method for preparing a streak-free polyvinyl alcohol optical film according to claim 1, wherein: The die lip angle is 20-40°.

4. The method for preparing a streak-free polyvinyl alcohol optical film according to claim 1, wherein: The distance between the die and the casting roller is 1-5 mm.

5. The method for preparing a streak-free polyvinyl alcohol optical film according to claim 1, wherein: By defining the die head angle of the die used for PVA casting liquid casting processing, the die lip angle between the die and the casting roller, and the distance between the die and the casting roller, and designing the three parameters and the parameter combination between the three, the shape and length of the PVA casting liquid between the die and the roller are optimized, the stability of the PVA casting liquid between the die and the roller is improved, and the periodic fluctuation of the casting liquid caused by stretching and wind field disturbance is avoided, thereby avoiding the formation of horizontal stripes in PVA optical films.

Images