A method for preparing a polarizing film with high light transmittance and high polarization degree

Abstract

The invention discloses a method for preparing a polarizer with high transmittance and high polarization degree, which comprises washing and swelling a PVA optical film, dyeing, cleaning, uniaxially stretching, cleaning and coloring, and then drying to form a polarizing film, and then laminating protective layers on both sides of the polarizing film to obtain a polarizer, and performing transverse stretching during the washing and swelling process, and controlling the transverse swelling degree of the PVA film out of the swelling groove to 120%-160%. The method of the invention can significantly increase the speed at which iodine ions enter the PVA film, solve the problem of decreased polarization degree when the transmittance is increased, and the obtained polarizer has the advantages of high transmittance and high polarization degree, and can be applied to display fields such as OLED.

Description

Technical Field

[0001] The invention belongs to the technical field of optical components, and in particular relates to a method for preparing a polarizer. Background Art

[0002] Organic light-emitting displays (OLEDs) have now become standard for flagship mobile phone screens from major manufacturers. OLED screens can control colors at the pixel level, so the color accuracy is almost perfect. They have higher requirements for the quality of the polarizers they need, requiring the polarizers to be thinner, have high single-cell transmittance, and high polarization degree.

[0003] The main method for producing polarizers is to wash and swell the PVA optical film, dye it, uniaxially stretch it, and solidify it with boric acid to form a polarizing film, and then laminate protective layers on both sides of the polarizing film to make a polarizer. The ideal polarizer has a higher single-unit transmittance and a higher polarization degree, but the single-unit transmittance and polarization degree are contradictory. The higher the single-unit transmittance, the lower the polarization degree. Therefore, it is of great significance to study the preparation method of polarizers with both high single-unit transmittance and high polarization degree. Summary of the invention

[0004] In order to solve the problems existing in the above-mentioned prior art, the present invention provides a method for preparing a polarizer with high light transmittance and high polarization degree, aiming to obtain a polarizer with high single light transmittance and high polarization degree.

[0005] The present invention has conducted in-depth research to achieve the above purpose. The main method of producing polarizers is to wash and swell the PVA optical film, dye (the dyeing liquid is usually iodine solution), clean, uniaxially stretch, clean and color-correct the PVA optical film to form a polarizing film, and then adhere the protective layer on both sides to obtain the polarizer after drying. The key steps are dyeing and stretching of the PVA optical film. As shown in the attached figure: Figure 1 It is the state of PVA optical film after swelling; Figure 2 This is the PVA membrane after dyeing. PVA has adsorbed iodine. Figure 3 For the stretched PVA film, iodine is arranged along the direction of the PVA molecular chain to form a polarizing film.

[0006] In order to obtain a polarizer with a high degree of polarization, the total stretching multiple of the PVA optical film relative to the original length is as high as possible, and generally needs to be more than 5 times. In the actual production process, in order to ensure that the plasticizer in the PVA film is completely washed out and the crystal area in the PVA film is completely destroyed, a certain amount of water washing and swelling time is required. In order to ensure that the film does not have wrinkles, stripes and other conditions during the production process, the PVA film in each process is in a "tight" state, that is, the PVA film in each link has a certain degree of extension. When the PVA film is washed and swollen, it will absorb water and expand, and the length and width will increase. However, due to the existence of stretching, it will shrink in the width direction. If the shrinkage is too large, the PVA molecular chains in some areas will be arranged along the stretching direction. In the dyeing process, it is difficult for iodine ions to enter the PVA film, which will cause a decrease in polarization. If you want to increase the polarization, you need to increase the concentration of iodine in the dyeing tank, which will cause the transmittance of the polarizer to decrease.

[0007] Further in-depth research found that controlling the lateral swelling of the PVA optical film after water washing and swelling to above 120% can significantly increase the speed at which iodine ions enter the PVA film, greatly reduce the problem of decreased polarization or reduced transmittance, and the obtained polarizer has the advantages of high transmittance and high polarization. The lateral swelling refers to the ratio of the film width to the initial film width. Since the film needs to be tightened during the water washing and swelling process, it will have an extension effect during this process, which will make the lateral swelling at the swelling outlet lower than 120%.

[0008] In view of this, the present invention provides a method for preparing a polarizer with high transmittance and high polarization degree, which comprises the following steps: appropriately stretching the PVA optical film in the transverse direction during the washing and swelling process, and then dyeing, cleaning, uniaxially stretching, cleaning and coloring the film, and drying the film in sequence to form a polarizer, and then laminating transparent protective layers on both sides of the polarizer to obtain the polarizer.

[0009] As the key to the present invention, the lateral swelling degree of the PVA optical film after washing and swelling is controlled to be not less than 120%. The specific method is: washing and swelling the PVA optical film in process water at 20-40°C for 30s-600s, and appropriately stretching the PVA optical film in the lateral direction during this process.

[0010] Preferably, the tension of the transverse stretching is controlled at 10-80N, the time is controlled at 10s-300s, and finally the transverse swelling degree of the PVA optical film after washing and swelling is controlled at 120%-160%. The transverse swelling degree after washing and swelling is measured as follows: the original film width L0 and the width L1 after washing and swelling are measured, and the transverse swelling degree = (L1 / L0) × 100%.

[0011] More preferably, the transverse stretching tension is controlled at 20-50N, the time is controlled at 20s-120s, and the transverse swelling degree is controlled at 130%-150%. If the transverse swelling degree is too large, the PVA molecular chain will be stretched too much in the transverse direction, which is not conducive to the subsequent longitudinal stretching orientation, but will cause the polarization degree of the film to decrease.

[0012] As a follow-up to the present invention, a polarizer with high transmittance and high polarization degree is prepared according to the following method:

[0013] 1. Wash and swell the PVA optical film in process water at 20-40°C for 30s-600s, and appropriately stretch the PVA optical film transversely during this process.

[0014] 2. Immerse the PVA film treated in step 1 in a solution containing a dichroic substance for dyeing. For an iodine-based polarizer using iodine as a dichroic substance, the solution in the dyeing tank is a mixed aqueous solution of KI, I2 and boric acid, with a KI concentration of 1-3wt%, an I2 concentration of 0.01-0.5wt%, and a boric acid concentration of 0.5-3wt%, a solution temperature of 20-50°C, and a dyeing time of 50-300s.

[0015] 3. After dyeing, washing is performed to prevent or reduce the transfer of dichroic substances to subsequent processes. The washing solution is an aqueous solution containing a crosslinking agent, usually an aqueous solution of boric acid, with a boric acid concentration of 0.5 to 3 wt%. The temperature in the washing process is usually 20 to 40°C, and the washing time is 10 to 200 seconds.

[0016] 4. After cleaning, the PVA optical film is uniaxially stretched. In the actual production process, the PVA film is stretched from the beginning, which is equivalent to the original length, and the total stretching multiple is 4-8 times (preferably 5-6 times). The stretching liquid is an aqueous solution of boric acid and KI, with a boric acid concentration of 0.5-3wt% and a KI concentration of 1-3wt%.

[0017] 5. After stretching, it needs to be cleaned and color-corrected. The cleaning process is the same as that in step 3. Color-correction is to adjust the hue of the polarizer. The color-correction solution is an aqueous solution of boric acid and KI. The boric acid concentration is 0.5-3wt%, the KI concentration is 3-8wt%, and the color-correction time is 10s-100s.

[0018] 6. Dry the PVA film after color correction using a blower at a temperature of 30-60°C for 10s-100s.

[0019] 7. After drying, transparent protective layers are attached to both sides of the polarizing film to obtain a polarizer. The protective layer is generally a triacetyl cellulose film (TAC film). The operation and processing methods are conventional methods in the art. For the methods, refer to CN110172315A and CN108957616A, which will not be described in detail in the present invention.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] Under the premise of not substantially changing the existing polarizer manufacturing process, the present invention performs appropriate transverse stretching in the washing and swelling process of the PVA film, controls the transverse swelling degree of the PVA film out of the swelling tank within 120%-160%, can significantly increase the speed of iodine ions entering the PVA film, can solve the problem of decreased polarization degree when the transmittance is increased, and improves product quality; the polarizer obtained according to the method of the present invention has the advantages of high transmittance and high polarization degree, and can be applied to display fields such as OLED. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is the state of the PVA optical film after swelling in the production of polarizers;

[0023] Figure 2 It is the dyed PVA film in the production of polarizer;

[0024] Figure 3 It is the PVA film after stretching in the production of polarizer. DETAILED DESCRIPTION

[0025] In order to further explain the present invention, the embodiments of the present invention are described in detail below. This embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation method and a specific operation process are given, but the protection scope of the present invention is not limited to the following embodiments.

[0026] Example 1

[0027] In this embodiment, the polarizer is prepared according to the following steps:

[0028] The PVA optical film was washed and swollen in process water at 30°C for 180 seconds. During the washing and swelling process, a force of 30N was applied in the transverse direction perpendicular to the production line, i.e., the transverse direction, and the film was stretched in the transverse direction for 120 seconds.

[0029] The PVA film treated in step 1 is dyed in a dyeing solution containing 2wt% KI, 0.2wt% I2, 1wt% boric acid and 32°C for 60s; then it is washed in a solution with 1wt% boric acid and 32°C for 50s; then it is stretched in a stretching solution with 2wt% KI, 1wt% boric acid and 40°C to 5.5 times the length of the original film; after stretching, it is washed in a solution with 1wt% boric acid and 32°C for 20s, and it is recolored in a recoloring solution with 1wt% boric acid and 5wt% KI for 30s; after recoloring, it is dried and dried in hot air at 40°C for 80s to obtain a polarizing film. Then, the TAC film treated by lamination is bonded according to the conventional process to obtain a polarizing film sample, and the transmittance and polarization degree of the polarizing film are measured.

[0030] Example 2

[0031] In this embodiment, the polarizer is prepared according to the following steps:

[0032] The PVA optical film was washed and swollen in process water at 30°C for 180 seconds. During the washing and swelling process, a force of 5N was applied in the transverse direction perpendicular to the production line, i.e., the transverse direction, and the film was stretched in the transverse direction for 120 seconds.

[0033] The PVA film treated in step 1 is dyed in a dyeing solution containing 2wt% KI, 0.2wt% I2, 1wt% boric acid and 32°C for 60s; then it is washed in a solution with 1wt% boric acid and 32°C for 50s; then it is stretched in a stretching solution with 2wt% KI, 1wt% boric acid and 40°C to 5.5 times the length of the original film; after stretching, it is washed in a solution with 1wt% boric acid and 32°C for 20s, and it is recolored in a recoloring solution with 1wt% boric acid and 5wt% KI for 30s; after recoloring, it is dried and dried in hot air at 40°C for 80s to obtain a polarizing film. Then, the TAC film treated by lamination is bonded according to the conventional process to obtain a polarizing film sample, and the transmittance and polarization degree of the polarizing film are measured.

[0034] Example 3

[0035] The PVA optical film was washed and swollen in process water at 30°C for 180 seconds. During the washing and swelling process, a force of 100N was applied in the transverse direction perpendicular to the production line, i.e., the transverse direction, and the film was stretched transversely for 120 seconds.

[0036] The PVA film treated in step 1 is dyed in a dyeing solution containing 2wt% KI, 0.2wt% I2, 1wt% boric acid and 32°C for 60s; then it is washed in a solution with 1wt% boric acid and 32°C for 50s; then it is stretched in a stretching solution with 2wt% KI, 1wt% boric acid and 40°C to 5.5 times the length of the original film; after stretching, it is washed in a solution with 1wt% boric acid and 32°C for 20s, and it is recolored in a recoloring solution with 1wt% boric acid and 5wt% KI for 30s; after recoloring, it is dried and dried in hot air at 40°C for 80s to obtain a polarizing film. Then, the TAC film treated by lamination is bonded according to the conventional process to obtain a polarizing film sample, and the transmittance and polarization degree of the polarizing film are measured.

[0037] Example 4

[0038] The PVA optical film was washed and swollen in process water at 30°C for 180 seconds. During the washing and swelling process, a force of 30N was applied in the direction perpendicular to the production line, i.e., in the transverse direction, and the film was stretched in the transverse direction for 10 seconds.

[0039] The PVA film treated in step 1 is dyed in a dyeing solution containing 2wt% KI, 0.2wt% I2, 1wt% boric acid and 32°C for 60s; then it is washed in a solution with 1wt% boric acid and 32°C for 50s; then it is stretched in a stretching solution with 2wt% KI, 1wt% boric acid and 40°C to 5.5 times the length of the original film; after stretching, it is washed in a solution with 1wt% boric acid and 32°C for 20s, and it is recolored in a recoloring solution with 1wt% boric acid and 5wt% KI for 30s; after recoloring, it is dried and dried in hot air at 40°C for 80s to obtain a polarizing film. Then, the TAC film treated by lamination is bonded according to the conventional process to obtain a polarizing film sample, and the transmittance and polarization degree of the polarizing film are measured.

[0040] Example 5

[0041] The PVA optical film was washed and swollen in process water at 30°C for 180 seconds. During the washing and swelling process, a force of 30N was applied in the transverse direction perpendicular to the production line, i.e., the transverse direction, and the film was stretched in the transverse direction for 180 seconds.

[0042] The PVA film treated in step 1 is dyed in a dyeing solution containing 2wt% KI, 0.2wt% I2, 1wt% boric acid and 32°C for 60s; then it is washed in a solution with 1wt% boric acid and 32°C for 50s; then it is stretched in a stretching solution with 2wt% KI, 1wt% boric acid and 40°C to 5.5 times the length of the original film; after stretching, it is washed in a solution with 1wt% boric acid and 32°C for 20s, and it is recolored in a recoloring solution with 1wt% boric acid and 5wt% KI for 30s; after recoloring, it is dried and dried in hot air at 40°C for 80s to obtain a polarizing film. Then, the TAC film treated by lamination is bonded according to the conventional process to obtain a polarizing film sample, and the transmittance and polarization degree of the polarizing film are measured.

[0043] Comparative Example 1

[0044] The PVA optical film was washed and swelled in process water at 30° C. for 180 seconds, and no transverse stretching operation was performed during the washing and swelling process.

[0045] The PVA film treated in step 1 is dyed in a dyeing solution containing 2wt% KI, 0.2wt% I2, 1wt% boric acid and 32°C for 60s; then it is washed in a solution with 1wt% boric acid and 32°C for 50s; then it is stretched in a stretching solution with 2wt% KI, 1wt% boric acid and 40°C to 5.5 times the length of the original film; after stretching, it is washed in a solution with 1wt% boric acid and 32°C for 20s, and it is recolored in a recoloring solution with 1wt% boric acid and 5wt% KI for 30s; after recoloring, it is dried and dried in hot air at 40°C for 80s to obtain a polarizing film. Then, the TAC film treated by lamination is bonded according to the conventional process to obtain a polarizing film sample, and the transmittance and polarization degree of the polarizing film are measured.

[0046] In each embodiment and comparative example, the lateral swelling degree of the PVA optical film after washing and swelling, and the transmittance and polarization degree of the prepared polarizer are shown in Table 1.

[0047] Table 1

[0048] Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 1 Transverse swelling / % 135.6 116.9 170.5 117.1 168.8 115.6 Light transmittance / % 43.2 42.0 43.9 42.1 43.8 41.8 Polarization degree / % 99.92 99.93 98.90 99.94 99.05 99.94

[0049] Polarizers for high-end OLEDs generally require a polarization degree of 99.9% and a transmittance of more than 42.5%. It can be seen from the table that compared with the comparative example, the polarization degree of Example 1 has basically not decreased, but its transmittance reaches 43.2%, which is a significant improvement compared with the transmittance of Comparative Example 1. The tension applied in Example 2 is too low, and the lateral stretching time in Example 4 is too short. The lateral swelling of the two is basically not improved, and the transmittance is not improved; the tension applied in Example 3 is too large, and the lateral stretching time in Example 5 is too long. The lateral swelling of the two is too large. Although the transmittance is also significantly improved, its polarization degree is only 98.9%, which cannot meet the needs of high-end polarizers. It can be seen that in the water washing and swelling link, moderate lateral stretching is carried out to control the lateral swelling within an appropriate range, which can significantly improve the transmittance of the polarizer, and the polarizer still retains a high degree of polarization.

[0050] The above description is only 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 should be included in the protection scope of the present invention.

Claims

1. A method for preparing a polarizer with high light transmittance and high polarization degree, characterized in that: During the washing and swelling process of the PVA optical film in the polarizer production process, the film is appropriately stretched transversely to increase the speed at which iodine ions enter the PVA film, thereby solving the problem of decreased polarization degree while increasing transmittance; then the film is dyed, cleaned, uniaxially stretched, cleaned and color-corrected, and dried in sequence to form a polarizing film, and then a transparent protective layer is attached to both sides of the polarizing film to obtain a polarizer; The tension of the transverse stretching is controlled at 20-50N, the time is controlled at 20s-120s, and the transverse swelling degree of the PVA optical film after washing and swelling is controlled at 130%-150%.

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