Method for tape casting and edge fixing of polyvinyl alcohol aqueous solution

Abstract

The invention relates to the field of preparation of polyvinyl alcohol optical films, and discloses a polyvinyl alcohol aqueous solution tape casting and edge fixing method, which comprises the following steps that: air knives are respectively and additionally arranged on two sides of a tape casting roller, and the air speed of the air knives is 0.2-5m / min. The air knives are additionally arranged on the two sides of the casting roller of the cast film respectively, so that the edge fixing effect can be achieved, the reduced width of a polyvinyl alcohol aqueous solution flowing out of the die lip is reduced, the edge film position is fixed, the thickness is smaller, and meanwhile the purpose that the edge film is attached to the roller better can be achieved; therefore, the problems that in the production process of the polyvinyl alcohol optical film, due to thickening caused by necking of the edge film and poor roller pasting, the edge film is poor in drying and stripping and prone to being adhered to the surface of a casting roller, and the film quality and the production stability are affected are solved.

Description

Technical Field

[0001] The present invention relates to a polyvinyl alcohol optical film, and more particularly to a method for casting and sizing the edges of an aqueous polyvinyl alcohol solution. Background Art

[0002] The PVA optical film is an optical film made mainly of special PVA, and can be processed by dyeing and stretching into a polarizing film with excellent polarization characteristics. It is a key material for manufacturing polarizing films used in liquid crystal displays (LCDs) and organic light-emitting diodes (OLEDs). In addition to being used in liquid crystal displays, polarizing films are widely used in sunglasses, anti-glare goggles, polarization filters in front of cameras and video cameras, anti-glare devices for automobile headlights, and various polarization microscopes or detection instruments.

[0003] The main production method of the polyvinyl alcohol optical film is to cast an aqueous polyvinyl alcohol solution onto the surface of a roller, and then dry it to form a film. Generally, the aqueous polyvinyl alcohol solution contains about 60 - 80% water. After casting and forming, the water content needs to be reduced to less than 30% to be smoothly peeled off from the surface of the roller, otherwise it cannot be peeled off from the surface of the roller.

[0004] During the process of casting an aqueous polyvinyl alcohol solution onto the roller, the necking effect of the nascent film curtain becomes a major constraint. When the aqueous polyvinyl alcohol solution flows out of the die lip, the two sides of its edge will contract inward, which is called necking. Necking will cause the edge thickness of the film to be greater than the middle thickness in the transverse direction of the film when the aqueous polyvinyl alcohol solution forms a film on the surface of the roller. Generally, the degree of necking has a greater relationship with the viscosity of the aqueous polyvinyl alcohol solution, the die lip height, etc. Usually, the film thickness at the outermost edges on both sides is about 2 - 3 times that of the middle thickness. Since the edge of the film is too thick, and the temperature at both sides of the roller is relatively lower than that in the middle, during the production process, the film in the middle is often dried well and can be peeled off, but the drying of the two sides is poor and it is difficult to peel off, and even it cannot be peeled off and adheres to the surface of the roller, seriously affecting the film quality and production stability. Summary of the Invention

[0005] The purpose of the present invention is to overcome the problem of poor edge peeling during the production of polyvinyl alcohol optical films existing in the prior art, and provide a method for casting and sizing the edges of an aqueous polyvinyl alcohol solution, which can improve the problem of edge warping on both sides of the polyvinyl alcohol edge film and improve the film quality.

[0006] To achieve the above purpose, the present invention discloses a method for casting and sizing the edges of an aqueous polyvinyl alcohol solution, the method including respectively installing air knives on both sides of the casting roller, and the wind speed of the air knives being 0.2 - 5 m / min.

[0007] In some embodiments of the present invention, the wind speed of the air knives is 0.3 - 3.0 m / min.

[0008] In some embodiments of the present invention, the air of the air knife is hot air at 30 to 97 °C.

[0009] In some embodiments of the present invention, the air of the air knife is hot air at 85 to 95 °C.

[0010] In some embodiments of the present invention, the difference D between the height of the air knife and the height of the film curtain is 5 to 300 mm.

[0011] In some embodiments of the present invention, the difference D between the height of the air knife and the height of the film curtain is 30 to 200 mm.

[0012] In some embodiments of the present invention, the angle between the air knife and the end face of the casting film roll is 5 to 45°.

[0013] In some embodiments of the present invention, the angle between the air knife and the end face of the casting film roll cylinder is 15 to 40°.

[0014] In some embodiments of the present invention, the humidity of the air knife is 50 to 95% RH.

[0015] In some embodiments of the present invention, the humidity of the air knife is 60 to 90% RH.

[0016] In some embodiments of the present invention, the concentration of the polyvinyl alcohol aqueous solution PVA is 10 to 30% wt.

[0017] Through the above technical solutions, air knives are respectively installed on both sides of the casting roll of the casting film, which can play a role in fixing the edges, reduce the shrinkage amplitude when the polyvinyl alcohol aqueous solution flows out from the die lip, make the position of the edge film fixed and the thickness thinner, and at the same time can also achieve the purpose of better sticking of the edge film to the roll, thereby solving the problems of thickening caused by necking of the edge film, poor drying of the edge film and poor peeling caused by poor sticking to the roll during the production of the polyvinyl alcohol optical film, and easy adhesion to the surface of the casting roll, which affects the film quality and production stability. Further, by controlling the environmental humidity on both sides of the die head, it is possible to avoid as much as possible the volatilization of water in the edge film, the increase in concentration, the deterioration of fluidity and the adhesion to both sides of the die head, thereby generating accumulated materials. Description of the Drawings

[0018] Figure 1 is a schematic structural diagram of a casting roll with an air knife added in a specific embodiment of the present invention.

[0019] Description of the Reference Numerals

[0020] 1 Die head; 2 Left air knife; 3 Right air knife; 4 Primary film curtain. Specific Embodiments

[0021] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only for the purpose of illustrating and explaining the present invention, and are not intended to limit the present invention.

[0022] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0023] In the present invention, unless otherwise stated, the orientation terms such as "upper, lower, left, right" generally refer to the upper, lower, left, and right shown in the reference drawings.

[0024] Generally, polyvinyl alcohol optical films are produced by the solution casting method. The polyvinyl alcohol aqueous solution is cast onto the surface of the casting roller, and after drying, the polyvinyl alcohol film is peeled off. During the casting and forming process of polyvinyl alcohol, the side films of the film curtain will shrink towards the middle, resulting in necking, causing the side films to become thicker, usually 2 - 3 times thicker or even higher than the middle part of the film. Due to the thicker side films, the drying on both sides of the casting roller is relatively poor, so side film drying differences often occur, and peeling is not good. In severe cases, film breakage occurs, affecting production stability and product quality.

[0025] To solve the above problems, the present invention discloses a method for fixing the edges of a polyvinyl alcohol aqueous solution during casting, and the method includes: installing air knives on both sides of the casting roller respectively, and the wind speed of the air knives is 0.2 - 5 m / min.

[0026] The present invention installs air knives on both sides of the casting film roller respectively, which can play a role in fixing the edges, reduce the shrinkage amplitude when the polyvinyl alcohol aqueous solution flows out from the die lip, make the position of the side film fixed and the thickness thinner, and at the same time can also achieve the purpose of better sticking of the side film to the roller, thereby solving the problems that often occur during the production of polyvinyl alcohol optical films, such as poor peeling of the side film, easy adhesion to the surface of the roller, affecting the film quality and production stability. It can be understood that the air source of the air knife is clean air, and its cleanliness needs to meet the requirements of optical products.

[0027] The inventors of the present application have found through research that if the wind speed is too high, it will cause the film curtain to shake and defects to occur. If the wind speed is too low, the improvement of the necking when the polyvinyl alcohol aqueous solution flows out from the die lip is not obvious, resulting in a large necking of the side film, too large a thickness, poor drying, and difficult peeling. Therefore, the wind speed of the air knife in the present invention is limited to 0.2 to 5 m / min. For example, 0.2 m / min, 0.5 m / min, 0.8 m / min, 1 m / min, 1.3 m / min, 1.6 m / min, 2.1 m / min, 2.5 m / min, 2.8 m / min, 3 m / min, 4 m / min, 5 m / min.

[0028] To further avoid the shaking of the film curtain and improve the necking problem, in some embodiments of the present invention, the wind speed of the air knife is 0.3 to 3.0 m / min.

[0029] On the basis of the foregoing disclosure, in order to have a better edge - fixing effect, in some embodiments of the present invention, the air knife is a hot air knife. Thus, by blowing hot air at a certain speed to the two - side films through the air knife, the shrinkage of the side films is reduced, making the shrinkage amplitude smaller, the edge - fixing better, and the film - roller adhesion better.

[0030] Through research, the applicant of the present application has found that during the production process of the polyvinyl alcohol optical film of the present application, if the temperature is too low, the drying efficiency of the film will be reduced, but if it is too high, the film will dry too fast, resulting in skin formation on the film surface, affecting the product quality. Preferably, the hot air of the air knife is at 30 to 97 °C, and more preferably, the hot air is at 85 to 95 °C. For example, 30 °C, 35 °C, 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, 65 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C, 95 °C.

[0031] Through research, the inventors of the present application have found that if the air knife is too high relative to the film, the edge - fixing effect is not good; conversely, if it is too low, when the hot air blows to the film surface, optical defects are likely to occur. In some embodiments of the present invention, the difference D between the height of the air knife and the height of the film curtain (the primary film curtain 4 formed when the solution flows out from the die slit) is 5 to 300 mm, preferably 30 to 200 mm. For example, 5 mm, 10 mm, 20 mm, 30 mm, 40 mm, 50 mm, 60 mm, 70 mm, 80 mm, 90 mm, 100 mm, 120 mm, 140 mm, 160 mm, 180 mm, 200 mm, 250 mm, 300 mm.

[0032] To further reduce the shrinkage of the side film, make the edge - fixing of the polyvinyl alcohol film better, and the film - roller adhesion better, in some embodiments of the present invention, the angle between the air knife and the end face of the casting film roller is 5 to 45 °, preferably 15 to 40 °. For example, 5 °, 10 °, 15 °, 20 °, 25 °, 30 °, 35 °, 40 °, 45 °.

[0033] It should be noted that in the process of polyvinyl alcohol casting, necking and die accumulation are two common problems. After research, the inventors of the present invention found that in the process of polyvinyl alcohol aqueous solution flowing out from the die slit to form a film curtain, the edge flow speed of the film curtain is relatively slow, and it is easy to adhere to the die due to water volatilization, high concentration, poor fluidity, and thus produce accumulation, affecting production stability. For this reason, in the present invention, on the basis of the above disclosure, further, in order to avoid the problem of die accumulation during the polyvinyl alcohol solution casting production process, in some embodiments of the present invention, the humidity of the air knife is 50-95% RH. After the solution flows out of the die slit, the edge part flows relatively slowly, so if the hot air humidity is too low, the water content of the solution is easy to volatilize, resulting in increased concentration, poor fluidity, and adhesion to both sides of the die, thereby producing accumulation; on the contrary, if the humidity is too high, it is easy to produce condensate on the die, and the condensate drips onto the surface of the film to produce defects.

[0034] To further avoid the problem of die accumulation during the polyvinyl alcohol solution casting production process, in some embodiments of the present invention, the humidity of the air knife is 60-90% RH.

[0035] In some embodiments of the present invention, steam is added to the hot air to adjust the humidity of the hot air to 50-95% RH (e.g., 50% RH, 60% RH, 70% RH, 80% RH, 90% RH, 95% RH), thereby controlling the ambient humidity on both sides of the die head and avoiding the evaporation of moisture from the edge film, the increase in concentration, the deterioration of fluidity, and the adhesion of moisture to both sides of the die head, thereby generating material accumulation.

[0036] In some embodiments of the present invention, the concentration of the polyvinyl alcohol aqueous solution PVA is 10-30%wt.

[0037] The advantages of the present invention are described below by way of examples, but the present invention is not limited thereto.

[0038] Example 1

[0039] According to Figure 1 As shown, a left wind knife 2 and a right wind knife 3 are arranged on both sides of the casting roller, and a polyvinyl alcohol aqueous solution is extruded through a die head 1 and cast onto the surface of the casting roller, and then peeled off to obtain a polyvinyl alcohol film after drying, wherein the polyvinyl alcohol aqueous solution PVA concentration is 15%wt, glycerol is 8%, surfactant lauric acid diethanolamide is 0.4%wt, sodium dodecyl sulfonate is 0.3%wt, and the rest is water. The PVA polymerization degree is 2200, and the alcoholysis degree is above 99.0%. The solution temperature is 90°C, the casting roller and die head temperature are 90°C, the wind speed of the wind knife is 1m / min, the hot air temperature is 90°C, the difference D between the height of the wind knife and the height of the film curtain is 100mm, the angle between the wind knife and the end face of the casting film roller is 30°, and the humidity of the wind knife is 80%RH.

[0040] After drying (using the same drying conditions for the examples and comparative examples), at the large roller peeling position, a meter stick was used to measure the film width to be 1480 mm;

[0041] A micrometer was used to measure the thickness of the edge film (the outermost part) of the film. Measurements were taken every 10 mm along the MD direction, at 5 points, and then the average value was taken. Moreover, both the left and right sides were measured separately. The average thickness of the left edge film was 81.1 μm, and the average thickness of the right edge film was 82.0 μm;

[0042] The full-width film was taken and hung flat in front of a white wall, at a distance of 80 - 100 cm from the wall. Then, a strong flashlight was used to irradiate the film at a distance of 40 - 50 cm directly in front of the film, so that its projection was onto the white wall. Then, the number of defects within a 10 cm * 10 cm area at the film edge (defects with a length or width greater than 1 cm) was observed. Three different points were selected, and the average value of the defect numbers taken three times was 0.

[0043] Regarding the die build-up, every 1 h, the die build-up situation was observed. If there was build-up, it was all removed and weighed (0.1 g), and then the average value over 24 h was taken as the average build-up amount under this working condition, which was 0.0 g / h.

[0044] Example 2

[0045] Differing from Example 1, the concentration of the polyvinyl alcohol aqueous solution PVA was 20% wt, the wind knife wind speed was 0.5 m / min, the hot air temperature was 85 °C, the difference D between the height of the wind knife and the film curtain height was 50 mm, the angle between the wind knife and the end face of the casting film roller was 15°, and the humidity of the wind knife was 70% RH.

[0046] Using the same method as in Example 1, the measured film width was 1475 mm; the average thickness of the left edge film was 82.8 μm, the average thickness of the right edge film was 82.4 μm; the average number of defects was 0; the average build-up amount was 0.4 g / h.

[0047] Example 3

[0048] Differing from Example 1, the concentration of the polyvinyl alcohol aqueous solution PVA was 28% wt, the wind knife wind speed was 2.0 m / min, the hot air temperature was 95 °C, the difference D between the height of the wind knife and the film curtain height was 200 mm, the angle between the wind knife and the end face of the casting film roller was 40°, and the humidity of the wind knife was 90% RH.

[0049] Using the same method as in Example 1, the measured film width was 1484 mm; the average thickness of the left edge film was 82.1 μm, the average thickness of the right edge film was 82.6 μm; the average number of defects was 0; the average build-up amount was 0 g / h.

[0050] Example 4

[0051] Different from Example 1, the wind speed of the air knife is 5.0 m / min.

[0052] Following the same method as in Example 1, the measured film width is 1489 mm; the average thickness of the left side film is 80.5 μm, and the average thickness of the right side film is 80.7 μm; the average number of defects is 2.0; the average amount of accumulated material is 0.5 g / h.

[0053] Example 5

[0054] Different from Example 1, the temperature of the air knife is 50 °C.

[0055] Result: Following the same method as in Example 1, the measured film width is 1483 mm; the average thickness of the left side film is 81.8 μm, and the average thickness of the right side film is 81.9 μm; the average number of defects is 0.8; the average amount of accumulated material is 10.0 g / h.

[0056] Example 6

[0057] Different from Example 1, the difference D between the height of the air knife and the height of the film curtain is 280 mm.

[0058] Result: Following the same method as in Example 1, the measured film width is 1473 mm; the average thickness of the left side film is 82.0 μm, and the average thickness of the right side film is 82.3 μm; the average number of defects is 0.3; the average amount of accumulated material is 1.0 g / h.

[0059] Example 7

[0060] Different from Example 1, the angle between the air knife and the end face of the casting film roller is 10°.

[0061] Result: Following the same method as in Example 1, the measured film width is 1476 mm; the average thickness of the left side film is 81.5 μm, and the average thickness of the right side film is 81.8 μm; the average number of defects is 0.3; the average amount of accumulated material is 0.3 g / h.

[0062] Example 8

[0063] Different from Example 1, the humidity of the air knife is 50% RH.

[0064] Result: Following the same method as in Example 1, the measured film width is 1484 mm; the average thickness of the left side film is 81.7 μm, and the average thickness of the right side film is 82.0 μm; the average number of defects is 0.3; the average amount of accumulated material is 3.4 g / h.

[0065] Comparative Example 1

[0066] Different from Example 1, air knives are not installed on both sides of the casting roller respectively.

[0067] Result: Using the same method as in Example 1, the measured film width was 1412 mm; the average thickness of the left side film was 98.5 μm, and the average thickness of the right side film was 91.1 μm; the average number of defects was 1.1; the average accumulated material amount was 19.0 g / h.

[0068] Comparative Example 2

[0069] Different from Example 1, the wind speed of the air knife was 9 m / min.

[0070] Result: Using the same method as in Example 1, the measured film width was 1490 mm; the average thickness of the left side film was 80.4 μm, and the average thickness of the right side film was 80.0 μm; the average number of defects was 3.0; the average accumulated material amount was 0.9 g / h.

[0071] Installing air knives on both sides of the die head can play a role in edge fixing, reduce the shrinkage of the polyvinyl alcohol aqueous solution when flowing out of the die lip, make the edge film thinner, improve film drying and peeling, and reduce defects. In addition, by controlling the humidity on both sides of the die head, the problem of die head material accumulation can be further solved.

[0072] The preferred embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited thereto. Within the technical concept scope of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any suitable combination of each specific technical feature. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods. But these simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for casting and sizing the edge of an aqueous polyvinyl alcohol solution, characterized in that, the method includes installing air knives on both sides of the casting roller, and the wind speed of the air knives is 0.2 - 5 m / min.

2. The method according to claim 1, wherein, the wind speed of the air knives is 0.3 - 3.0 m / min.

3. The method according to claim 1 or 2, wherein, the air of the air knives is hot air at 30 - 97 °C.

4. The method according to claim 3, wherein, the air of the air knives is hot air at 85 - 95 °C.

5. The method according to any one of claims 1 - 4, wherein, the difference D between the height of the air knives and the height of the film curtain is 5 - 300 mm.

6. The method according to claim 5, wherein, the difference D between the height of the air knives and the height of the film curtain is 30 - 200 mm.

7. The method according to any one of claims 1 - 6, wherein, the angle between the air knives and the end face of the casting film roller is 5 - 45°.

8. The method according to claim 7, wherein, the angle between the air knives and the end face of the casting film roller barrel is 15 - 40°.

9. The method according to any one of claims 1 - 8, wherein, the humidity of the air knives is 50 - 95% RH, preferably 60 - 90% RH.

10. The method according to any one of claims 1 - 9, wherein, the concentration of the aqueous polyvinyl alcohol solution PVA is 10 - 30% wt.

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