Drying treatment method of polyvinyl alcohol optical film

Abstract

The invention relates to the technical field of optical film processing, and discloses a drying treatment method of a polyvinyl alcohol optical film, which comprises the following steps: carrying out low-temperature drying molding on a polyvinyl alcohol aqueous solution through a casting roller, and then carrying out microwave drying. The thin film is dried in a microwave drying mode, microwaves can drive water molecules in the thin film to resonate, a remarkable heat effect is generated, the temperature in the thin film and the temperature on the outer surface are increased at the same time, and the non-uniform phenomena that the thin film surface is dry and crystallized and the water content in the thin film is high are avoided; the problems of longitudinal shrinkage and transverse shrinkage from the two sides of the edge of the thin film to the center in the drying and water loss process of the thin film are fundamentally solved. And meanwhile, the film surface is uniformly dried, the thickness range and the standard deviation of the film are reduced, and the optical consistency of the film is ensured.

Description

Technical Field

[0001] The invention relates to the technical field of optical film processing, and in particular to a drying treatment method for a polyvinyl alcohol-based optical film. Background Art

[0002] Polyvinyl alcohol (PVA) film is an emerging high-tech functional material with a wide range of uses. It can be used as packaging film, water-soluble film, agricultural film, and can also be used as the base film of liquid crystal polarizer. Liquid crystal polarizer is currently the most advanced application field of PVA film. Liquid crystal polarizer is made of polyvinyl alcohol optical film after stretching, dyeing, and laminating. It can be widely used in twisted nematic (TN), super twisted nematic (STN) and thin film transistor (TFT) liquid crystal display technology. In addition, polarizer is also a key raw material for polarized products such as sunglasses, anti-glare goggles, and polarizing filters in front of cameras and video cameras. Therefore, PVA optical film has a good market development prospect.

[0003] At present, the preparation method of PVA optical film is basically to form a film by the aqueous solution casting method, and then dry it to make a film product; the drying method usually includes roller drying and oven drying. During the roller drying process, the inner surface of the film close to the roller is heated more intensely than the outer surface, and longitudinal shrinkage and lateral shrinkage from both sides of the film edge to the center are prone to occur during the drying and water loss process, which will lead to problems such as ribs, longitudinal stripes, and warping when winding. When drying in an oven, it is necessary not only to control the oven temperature, air volume, wind speed and wind pressure, but also to control process parameters such as wind field uniformity. The control precision is very high, and any slight fluctuation in the process will cause wrinkles, and the wrinkle problem is particularly obvious at the winding point. Therefore, it is urgent to develop a new PVA optical film drying method to solve the above problems. Summary of the invention

[0004] The present invention aims to provide a drying treatment method for a polyvinyl alcohol-based optical film, so as to solve the problems of uneven drying and high precision requirements for drying condition control when the polyvinyl alcohol-based optical film is dried in the prior art.

[0005] To achieve the above object, the present invention adopts the following technical scheme: a method for drying a polyvinyl alcohol-based optical film, wherein a polyvinyl alcohol aqueous solution is formed by low-temperature drying with a casting roller and then subjected to microwave drying.

[0006] The principle and advantages of this solution are: in actual application, in this technical solution, in order to solve the problems of ribs, longitudinal stripes and warping when rolling up due to uneven heating of the inner and outer surfaces of the film when the film is dried by rollers in the prior art, a two-stage heating method is used to dry the polyvinyl alcohol optical film. The first stage of heating adopts the low-temperature drying method of the casting roller. In this process, the roller plays the role of film shaping rather than the traditional drying and dehydration. In this stage, due to the low drying temperature, the temperature difference between the inner surface (the side close to the roller) and the outer surface (the side away from the roller) of the film is not large, which avoids the longitudinal and lateral shrinkage of the polyvinyl alcohol optical film during the high-temperature drying process. After the initial shaping and finalization, the second stage of heating is carried out. The second stage of heating adopts the microwave drying method. The microwave will drive the resonance of the water molecules in the film, produce a significant thermal effect, and has the characteristics of heating all the water molecules in the film inside and outside at the same time, so that the temperature inside and outside the film rises at the same time, avoiding the uneven phenomenon of drying and crystallization of the film surface and the high water content in the film, and fundamentally solves the problem of longitudinal shrinkage and lateral shrinkage from the edges of the film to the center during the drying and water loss process. At the same time, the film surface is dried evenly, and the thickness range and standard deviation of the film are reduced, ensuring the optical consistency of the film. In addition, when the film prepared by this scheme is quality inspected, it is found that the film dried at low temperature has low crystallinity, a larger stretching multiple, and better tensile strength. In addition, microwave heating is fast, and the water in the film can be removed in a short time, which is more energy-saving than the film prepared by the traditional method.

[0007] Preferably, as an improvement, the drying temperature of the casting roller low-temperature drying stage is 60-95° C., and the drying time is 21-90 s.

[0008] In this technical solution, if the casting roller drying temperature is too high, the polyvinyl alcohol film will lose water too quickly, resulting in an uneven film surface and affecting the product quality of the polyvinyl alcohol film; and if the temperature is too high, it will also lead to uneven crystallization and unsatisfactory tensile strength of the prepared optical film. If the temperature is too low, it will be difficult to form the film. The temperature and time of the casting roller forming stage are the optimal combination verified by experiments.

[0009] Preferably, as an improvement, the drying temperature of the casting roller low-temperature drying stage is 60-75° C., and the drying time is 27-60 s.

[0010] In this technical solution, during the actual research and development stage, the drying temperature and drying time of the casting roller are further optimized, and the optical film performance obtained under the above drying and shaping conditions is better.

[0011] Preferably, as an improvement, after the casting roller is dried at low temperature, the moisture content of the polyvinyl alcohol film is 5-60%.

[0012] In this technical solution, it was found during the research process that the water content of the film after the casting roller is dried and formed is related to the crystallinity and swelling degree of the finished optical film; controlling the water content of the film within the range of 5-60% during the forming stage can ensure the optimal crystallinity and swelling degree, thereby ensuring the tensile properties of the optical film.

[0013] Preferably, as an improvement, after the casting roller is dried at low temperature, the moisture content of the polyvinyl alcohol film is 5-20%.

[0014] In this technical solution, in the actual research and development stage, the water content of the film after the casting roller drying and shaping is further optimized, and the optical film performance obtained when the water content of the film after shaping is in the range of 5-20% is better.

[0015] Preferably, as an improvement, the microwave drying stage is completed using a microwave dryer.

[0016] In the technical solution, a microwave dryer is used in the microwave drying stage, which is easy to operate.

[0017] Preferably, as an improvement, the power of the microwave dryer is 30-150kW and the air volume is 500-1500m 3 / h, drying temperature is 60-140℃.

[0018] In the present technical solution, the power and air volume of the microwave dryer have a key influence on the effect of microwave drying. With the coordination of the above-mentioned temperature and air volume, the temperature field in the microwave dryer can be guaranteed to be uniform, so that the polyvinyl alcohol film entering the microwave dryer is heated uniformly, eliminating lateral and longitudinal shrinkage, reducing the standard deviation and range of the film thickness, and also increasing the crystallization uniformity of the film.

[0019] Preferably, as an improvement, the power of the microwave dryer is 30-100 kW and the air volume is 500-1000 m 3 / h, drying temperature is 80-110℃.

[0020] In this technical solution, the microwave drying conditions were further optimized during the actual research and development stage. The power of the microwave dryer was 30-100kW and the air volume was 500-1000m 3 / h and a drying temperature of 80-110°C, the performance of the prepared optical film can be further guaranteed.

[0021] Preferably, as an improvement, after being dried in a microwave dryer, the moisture content of the polyvinyl alcohol film is 1-5%.

[0022] In this technical solution, during the microwave drying stage, the water content of the polyvinyl alcohol film after drying directly affects the performance of the optical film. If the water content is too high, the film roll will produce adhesion, and if the water content is too low, the film will absorb water, thereby producing wrinkles.

[0023] Preferably, as an improvement, after being dried in a microwave dryer, the moisture content of the polyvinyl alcohol film is 2-4%.

[0024] In this technical file, during the actual research and development stage, the moisture content of the dried polyvinyl alcohol film was further optimized. When the moisture content is within the range of 2-4%, the flatness of the film can be guaranteed, thereby ensuring the performance of the optical film. DETAILED DESCRIPTION

[0025] The following is further described in detail through specific implementations, but the implementation of the present invention is not limited thereto. Unless otherwise specified, the technical means used in the following implementations are conventional means well known to those skilled in the art; the experimental methods used are all conventional methods; the materials, reagents, etc. used can all be obtained from commercial channels.

[0026] Program Overview:

[0027] A drying method for a polyvinyl alcohol-based optical film comprises the following steps:

[0028] Step 1, low temperature drying: the polyvinyl alcohol aqueous solution is low temperature dried on a casting roller, the drying temperature is 60-95°C, the drying time is 21-90s, and the polyvinyl alcohol optical film is formed after drying, forming and peeling, and the water content of the polyvinyl alcohol optical film is 5-60% at this time;

[0029] Step 2: Microwave drying: Place the polyvinyl alcohol optical film formed in step 1 in a microwave dryer for drying. The power of the microwave dryer is 30-150kW and the air volume is 500-1500m 3 / h, drying temperature is 60-140°C, and the moisture content of the polyvinyl alcohol optical film is 1-5%.

[0030] Example 1

[0031] A drying method for a polyvinyl alcohol-based optical film comprises the following steps:

[0032] Step 1, low temperature drying: the polyvinyl alcohol aqueous solution is low temperature dried on a casting roller at a drying temperature of 65°C, and a polyvinyl alcohol optical film is formed after drying, forming and peeling. At this time, the water content of the polyvinyl alcohol optical film is 10%;

[0033] Step 2: Microwave drying: Place the polyvinyl alcohol optical film formed in step 1 in a microwave dryer for drying. The power of the microwave dryer is 100 kW and the air volume is 1000 m 3 / h, drying temperature is 95°C, and the moisture content of the polyvinyl alcohol optical film is 2-4%.

[0034] Comparative Example 1

[0035] The difference between this comparative example 1 and the example 1 is that after the casting roller is dried at low temperature, the polyvinyl alcohol optical film is formed after drying and forming, and then it is put into the oven for drying. The oven power is adjusted to 100kw and the air volume is 1000m 3 / h, control the internal temperature to 95℃ for drying, and control the film speed to ensure that the moisture content of the film is 2-4% when it leaves the microwave dryer.

[0036] Comparative Example 2

[0037] The difference between this comparative example and Example 1 is that after low-temperature drying on the casting roller, the polyvinyl alcohol optical film is formed after drying, forming and peeling, and then secondary drying is carried out on the roller. The drying temperature is controlled at 95°C, and the film speed is controlled to ensure that the moisture content of the film is 2-4% when it comes out of the microwave dryer.

[0038] The difference between Examples 2 to 6 and Comparative Examples 3 to 1 is only the setting of some operating parameters, which are detailed in Table 1. Wherein: Drying temperature 1 refers to the temperature when the casting roller is dried at low temperature; Drying temperature 2 refers to the drying temperature when microwave drying is performed. Average moisture content 1 refers to the moisture content of the polyvinyl alcohol optical film after the casting roller is dried at low temperature; Average moisture content 2 refers to the moisture content of the polyvinyl alcohol optical film after microwave drying.

[0039] Table 1

[0040]

[0041]

[0042] Experimental Example 1: Tensile Properties Test

[0043] The performance of the polyvinyl alcohol optical film after secondary drying in the above embodiments and comparative examples was tested, and the test indicators included tensile strength and stretch ratio, and the range and standard deviation were calculated. The results are shown in Tables 2-4. The data in the table show that the PVA optical film treated by the drying method of the present invention has excellent indicators, uniform film thickness, and ideal tensile strength and stretch ratio, which fully meets the use requirements for preparing high-end products. Comparative Example 1 adopts the traditional method of combining casting rollers with oven drying, and Comparative Example 2 adopts the method of casting roller drying, which both lead to a significant decrease in tensile strength and stretch ratio, while the range and standard deviation of film thickness increase, and the film uniformity is poor. In addition, the study of this technical solution shows that if the drying temperature of the casting roller is too high, the polyvinyl alcohol film will lose water too quickly, resulting in an uneven film surface, and will affect the tensile strength and stretch ratio of the film; if the drying temperature of the casting roller is too low, it will cause molding difficulties and extend the drying time. In addition, in the microwave drying stage, microwave power, air volume and drying temperature will have a certain impact on the uniformity and tensile properties of the prepared optical film.

[0044] Table 2

[0045]

[0046] Table 3

[0047]

[0048] Table 4

[0049]

[0050] Experimental Example 2: Detection of moisture content at any three points on the film surface

[0051] The moisture content of the polyvinyl alcohol optical film after secondary drying in the above embodiments and comparative examples was tested at any three points on the film surface, and the test results are shown in Tables 5 to 7. From the data in the table, it can be seen that the moisture content of the optical film after the drying treatment method of the technical solution is relatively uniform, indicating that the film is dried uniformly. The traditional casting roller drying method or the casting roller combined with oven drying method will lead to large differences in moisture content at each point of the film, and the uniformity of film drying is poor.

[0052] Table 5

[0053]

[0054] Table 6

[0055]

[0056] Table 7

[0057]

[0058] The above is only an embodiment of the present invention, and the common knowledge such as the known specific technical solutions and / or characteristics in the solution is not described in detail here. It should be pointed out that for those skilled in the art, without departing from the technical solution of the present invention, several modifications and improvements can be made, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A drying method for a polyvinyl alcohol-based optical film, characterized in that: The polyvinyl alcohol aqueous solution is dried at low temperature by a casting roller and then dried by microwave.

2. The drying method of a polyvinyl alcohol-based optical film according to claim 1, characterized in that: The drying temperature of the casting roller low-temperature drying stage is 60-95° C., and the drying time is 21-90 seconds.

3. The drying method of a polyvinyl alcohol-based optical film according to claim 2, characterized in that: The drying temperature of the casting roller low-temperature drying stage is 60-75° C., and the drying time is 27-60 seconds.

4. The drying method of a polyvinyl alcohol-based optical film according to claim 3, characterized in that: After the casting roller is dried at low temperature, the water content of the polyvinyl alcohol film is 5-60%.

5. The drying method of a polyvinyl alcohol-based optical film according to claim 4, characterized in that: After the casting roller is dried at low temperature, the water content of the polyvinyl alcohol film is 5-20%.

6. The drying method of a polyvinyl alcohol-based optical film according to claim 5, characterized in that: The microwave drying stage is completed using a microwave dryer.

7. The drying method of a polyvinyl alcohol-based optical film according to claim 6, characterized in that: The power of the microwave dryer is 30-150kW, and the air volume is 500-1500m 3 / h, drying temperature is 60-140℃.

8. The drying method of a polyvinyl alcohol-based optical film according to claim 7, characterized in that: The power of the microwave dryer is 30-100kW and the air volume is 500-1000m 3 / h, drying temperature is 80-110℃.

9. The drying method of a polyvinyl alcohol-based optical film according to claim 8, characterized in that: After being dried by a microwave dryer, the moisture content of the polyvinyl alcohol film is 1-5%.

10. The drying method of a polyvinyl alcohol-based optical film according to claim 9, characterized in that: After being dried by a microwave dryer, the moisture content of the polyvinyl alcohol film is 2-4%.

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