Polyvinyl alcohol optical film and preparation method thereof

Abstract

The invention provides a polyvinyl alcohol optical film and a preparation method thereof, and relates to the field of optical films. The preparation method of the polyvinyl alcohol optical film comprises the following steps: providing a film casting solution containing polyvinyl alcohol; coating a casting roller with the film casting solution for first drying to obtain a film; the thin film passes through a roller set drying area to be subjected to second drying; and carrying out third drying on the film subjected to the second drying, and rolling to obtain the polyvinyl alcohol optical film. Wherein the roller group drying area comprises a nucleation area and a non-nucleation area, and the second drying comprises the following steps: controlling the temperature of a drying roller of the nucleation area to be 90-150 DEG C, and applying airflow above the drying roller of the nucleation area to remove a volatilized solvent; and the third drying comprises a preheating stage, a main drying stage and an annealing stage which are sequentially carried out. The method can be used for preparing the polyvinyl alcohol optical film with high light transmittance.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical films, in particular to a polyvinyl alcohol optical film and a preparation method thereof. BACKGROUND

[0002] The polyvinyl alcohol optical film is a core component of a polarizer and is widely used in LCD, OLED display panels such as mobile phones, computers, vehicle-mounted screens and televisions. With the rapid evolution of display panels towards large size, high image quality, low energy consumption and thinness, the requirements for the polyvinyl alcohol optical film in core indicators such as light transmittance are becoming increasingly stringent. How to improve the light transmittance of the polyvinyl alcohol optical film has become an important target for technical breakthrough.

[0003] Polyvinyl alcohol is a semi-crystalline material crystallized by hydrogen bonds, and the crystallinity, crystalline uniformity and crystal size of polyvinyl alcohol determine the light transmittance of the film. Therefore, precisely controlling the crystallinity, crystalline uniformity and crystal size of polyvinyl alcohol is of great significance for optimizing the light transmittance of the film. Under the existing preparation process conditions, the light transmittance of the prepared polyvinyl alcohol optical film is low, which cannot meet the needs of high-performance display panels. SUMMARY

[0004] The purpose of the present application is to provide a polyvinyl alcohol optical film and a preparation method thereof to solve the above problems.

[0005] To achieve the above purpose, the following technical solutions are adopted in the present application: A preparation method of a polyvinyl alcohol optical film, comprising: providing a casting solution, the casting solution comprising polyvinyl alcohol; coating the casting solution to a casting roll to perform first drying to obtain a film; drying the film through a roller group drying zone to perform second drying; performing third drying on the film dried in the second drying to obtain a polyvinyl alcohol optical film by winding; wherein the roller group drying zone comprises a nucleation zone and a non-nucleation zone, and the second drying comprises controlling the temperature of the drying rollers in the nucleation zone at 90-150℃ and applying air flow above the drying rollers in the nucleation zone to remove volatile solvents; the third drying comprises a preheating stage, a main drying stage and an annealing stage performed in sequence, the temperature of the preheating stage is different from the temperature of the main drying stage, and the temperature of the main drying stage is different from the temperature of the annealing stage.

[0006] According to the embodiments of the present application, the nucleation zone comprises 1-5 drying rollers in series in the film advancing direction and an air extraction hood above the drying rollers, and a negative pressure zone is formed between the drying rollers and the air extraction hood; And / or, the nucleation zone is provided with a plurality of blow nozzles, the blow nozzles are located between the drying roller and the exhaust hood, and the wind speed of the blow nozzles is 1-50 m / s. And / or, the wind speed uniformity in the transverse direction of the film is less than 5%. And / or, the moisture content of the film when entering the nucleation zone is 15-25 wt%.

[0007] According to the embodiments of the present application, the roller group drying zone comprises 2-30 drying rollers. And / or, the surface temperature of the drying roller in the non-nucleation zone is 30-150℃. And / or, when the second drying is performed, the surface temperature difference between adjacent drying rollers is 0-15℃. And / or, the time of the second drying is 80-200 seconds.

[0008] According to the embodiments of the present application, the temperature of the preheating stage is 30-140℃, and the time of the preheating stage is 10-40 seconds. And / or, the temperature of the main drying stage is 100-160℃, and the time of the main drying stage is 50-180 seconds. And / or, the temperature of the annealing stage is 30-110℃, and the time of the annealing stage is 10-90 seconds.

[0009] According to the embodiments of the present application, the surface temperature of the casting roller is 60-130℃. And / or, the surface roughness Ra of the casting roller is 0.001-0.1 μm. An infrared lamp tube is additionally provided on the casting roller, the temperature of the wind hood is 30-180℃, and the wind speed of the wind hood is 1-50 m / s.

[0010] According to the embodiments of the present application, the polymerization degree of the polyvinyl alcohol is 1000 to 5000. And / or, the content of the polyvinyl alcohol in the casting solution is 10-40 wt%.

[0011] According to the embodiments of the present application, the casting solution further comprises a plasticizer, and the plasticizer comprises at least one of methanol, ethanol, propanol, butanol, ethylene glycol, polyethylene glycol, propylene glycol, glycerol, and starch. And / or, the content of the plasticizer in the casting solution is 4-20 wt%. And / or, the thickness of the polyvinyl alcohol optical film is 20-75 μm.

[0012] The present application also provides a polyvinyl alcohol optical film, which is prepared by the preparation method of the polyvinyl alcohol optical film described above.

[0013] According to embodiments of the present application, the polyvinyl alcohol optical film has a crystallinity of 10-50%; And / or, the polyvinyl alcohol optical film has a light transmittance of greater than 88%; And / or, the polyvinyl alcohol optical film has a draw ratio of greater than 445%; And / or, the polyvinyl alcohol optical film has a grain size of 1-45 nm.

[0014] According to embodiments of the present application, the polyvinyl alcohol optical film has a range of grain spacing of 0.5-10 nm; And / or, the polyvinyl alcohol optical film has a variance of grain spacing of 0.01-10.

[0015] Compared with the prior art, the present application has the following beneficial effects: The present application can form a large number of fine and uniformly distributed crystal nuclei by the second drying treatment, which can quickly and uniformly dehydrate the film under the combined action of high drying roller temperature and air flow, thereby improving the light transmittance of the optical film. The present application can prepare a polyvinyl alcohol optical film with high light transmittance. Moreover, the present application has the advantages of simple preparation method, easy operation, etc., and is suitable for large-scale industrial production. DETAILED DESCRIPTION

[0016] As used herein: "Made from" is synonymous with "comprising". The terms "comprising", "including", "having" or "with" or any other variation thereof, as used herein, are intended to cover a non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but can include other elements not expressly listed or inherent to such composition, step, method, article, or apparatus.

[0017] The conjunction "consisting of" excludes any unrecited elements, steps, or components. If used in the claims, this phrase shall be construed to mean that the claims encompass only those materials specifically recited in the claims, to the exclusion of anything not specified. When the phrase "consisting of" appears in a clause of the body of the claims, it limits the clause in which it appears to only the elements specified in that clause; other elements not specified in that clause are not excluded from the claim as a whole.

[0018] When expressing amounts, concentrations, or other values or parameters of a range, preferably a range, or a series of upper preferred values and lower preferred values, it should be understood that the disclosure specifically encompasses all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether that range is expressly disclosed. For example, where a range "1-5" is disclosed, the disclosure is to be interpreted to include ranges such as "1-4," "1-3," "1-2," "1-2 and 4-5," "1-3 and 5," etc. When numerical ranges are disclosed, unless otherwise stated, the range is intended to include both the upper and lower values and all intervening values of the range, as well as the integers within the range.

[0019] In these embodiments, the parts and percentages described are by mass, unless otherwise indicated.

[0020] "Mass parts" refers to a basic unit of measurement that represents the proportional relationship of the mass of multiple components, 1 part can represent any unit mass, such as 1 g, 2.689 g, etc. If we say that the mass parts of component A is a parts, and the mass parts of component B is b parts, it means that the mass ratio of component A to component B is a:b. Or, it means that the mass of component A is aK, and the mass of component B is bK (K is an arbitrary number, indicating a multiple factor). It should not be misunderstood that, unlike mass parts, the sum of the mass parts of all components is not limited to 100 parts.

[0021] "and / or" is used to indicate that one or both of the described conditions can occur, for example, A and / or B includes (A and B) and (A or B).

[0022] A method for preparing a polyvinyl alcohol optical film, comprising: providing a casting solution, the casting solution comprising polyvinyl alcohol; coating the casting solution to a casting roll, performing first drying to obtain a film; drying the film through a roller group drying zone to perform second drying; performing third drying on the film after the second drying, and winding to obtain a polyvinyl alcohol optical film; wherein the roller group drying zone comprises a nucleation zone and a non-nucleation zone, and the second drying comprises: controlling the temperature of the drying roller in the nucleation zone at 90-150°C, and applying air flow above the drying roller in the nucleation zone to remove volatile solvent; the third drying comprises a preheating stage, a main drying stage and an annealing stage performed in sequence, the temperature of the preheating stage is different from the temperature of the main drying stage, and the temperature of the main drying stage is different from the temperature of the annealing stage.

[0023] The first drying mainly plays a role of shaping the optical film, the second drying mainly plays a role of forming a crystal nucleus and controlling the uniformity of in-plane crystallization, and the third drying mainly adjusts the crystallinity and the crystal size. The first drying, the second drying and the third drying cooperate with each other, so that the crystallinity, the crystalline uniformity and the crystal size of the polyvinyl alcohol optical film can be better adjusted, and the comprehensive performance of the polyvinyl alcohol optical film can be effectively improved.

[0024] The polyvinyl alcohol optical film prepared in the application has a suitable crystal size and a suitable crystallinity, which is beneficial to manufacturing a polarizer with high transmittance, high polarization degree and low haze. At the same time, the suitable crystal size and the suitable crystallinity ensure that the polyvinyl alcohol optical film can withstand higher strength and has good tensile performance.

[0025] If the crystal size and the crystallinity of the polyvinyl alcohol optical film are too large, the light transmittance of the polyvinyl alcohol optical film will be reduced, the haze will be increased, the brittleness will be increased, and the tensile ratio will be reduced; if the crystal size and the crystallinity of the polyvinyl alcohol optical film are too small, the tensile strength of the polyvinyl alcohol optical film will be reduced, the tensile ratio will also be affected, and the polarization degree will also be reduced, which affects the performance of the polarizer.

[0026] In some embodiments, the temperature of the drying roller in the nucleation zone is 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, 120℃, 125℃, 130℃, 135℃, 140℃, 145℃, 150℃ or any value between 90-150℃.

[0027] In some embodiments, the third drying is performed in an oven.

[0028] In some embodiments, the temperature of the drying roller in the nucleation zone is preferably 100-145℃, and most preferably 105-140℃.

[0029] According to the embodiments of the application, the nucleation zone comprises 1-5 (1, 2, 3, 4, 5 or any value between 1-5) drying rollers in series in the direction of the film advancing, and an air suction hood above the drying rollers, a negative pressure zone being formed between the drying rollers and the air suction hood; And / or, the nucleation zone is provided with a plurality of air nozzles, the air nozzles being located between the drying rollers and the air suction hood, the air speed of the air nozzles being 1-50m / s; For example, the air speed of the air nozzles is 1m / s, 5m / s, 10m / s, 15m / s, 20m / s, 25m / s, 30m / s, 35m / s, 40m / s, 45m / s, 50m / s or any value between 1-50m / s.

[0030] And / or, the air speed uniformity in the transverse direction of the film is less than 5%; In some embodiments, the wind speed uniformity = (wind speed maximum - wind speed minimum) / wind speed average x 100%.

[0031] And / or, the moisture content of the film when entering the nucleation zone is 15-25 wt%.

[0032] For example, the moisture content of the film when entering the nucleation zone is 15 wt%, 16 wt%, 17 wt%, 18 wt%, 19 wt%, 20 wt%, 21 wt%, 22 wt%, 23 wt%, 24 wt%, 25 wt%, or any value between 15-25 wt%.

[0033] Through the treatment of the nucleation zone, the film can be quickly dried, and a large number of crystal nuclei can be formed in a short time.

[0034] According to embodiments of the present application, the roller group drying zone comprises 2-30 drying rollers; For example, the number of drying rollers in the roller group drying zone is 2, 5, 8, 10, 13, 15, 18, 20, 23, 25, 28, 30, or any value between 2-30.

[0035] In some embodiments, in the direction of the film travel, the plurality of drying rollers in the roller group drying zone are arranged alternately up and down.

[0036] And / or, the surface temperature of the drying roller in the non-nucleation zone is 30-150℃; For example, the surface temperature of the drying roller in the non-nucleation zone is 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, 120℃, 125℃, 130℃, 135℃, 140℃, 145℃, 150℃, or any value between 30-150℃.

[0037] And / or, when performing the second drying, the surface temperature difference between adjacent drying rollers is 0-15℃; For example, when performing the second drying, the surface temperature difference between adjacent drying rollers is 0, 1℃, 3℃, 5℃, 7℃, 10℃, 12℃, 15℃, or any value between 0-15℃.

[0038] And / or, the time of the second drying is 80-200 seconds.

[0039] For example, the time of the second drying is 80 seconds, 90 seconds, 100 seconds, 110 seconds, 120 seconds, 130 seconds, 140 seconds, 150 seconds, 160 seconds, 170 seconds, 180 seconds, 190 seconds, 200 seconds, or any value between 80-200 seconds.

[0040] The second drying time refers to the total residence time of the film in the roller group drying zone.

[0041] According to the embodiments of the present application, the temperature of the preheating stage is 30-140℃, and the time of the preheating stage is 10-40 seconds; For example, the temperature of the preheating stage is 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, 120℃, 125℃, 130℃, 135℃, 140℃, or any value between 30-140℃, and the time of the preheating stage is 10 seconds, 15 seconds, 20 seconds, 25 seconds, 30 seconds, 35 seconds, 40 seconds, or any value between 10-40 seconds. In some embodiments, the temperature of the preheating stage is preferably 40-120℃, more preferably 50-110℃, and most preferably 60-105℃. And / or, the temperature of the main drying stage is 100-160℃, and the time of the main drying stage is 50-180 seconds. For example, the temperature of the main drying stage is 100℃, 105℃, 110℃, 115℃, 120℃, 125℃, 130℃, 135℃, 140℃, 145℃, 150℃, 155℃, 160℃, or any value between 100-160℃, and the time of the main drying stage is 50 seconds, 60 seconds, 70 seconds, 80 seconds, 90 seconds, 100 seconds, 110 seconds, 120 seconds, 130 seconds, 140 seconds, 150 seconds, 160 seconds, 170 seconds, 180 seconds, or any value between 50-180 seconds.

[0042] In some embodiments, the temperature of the main drying stage is preferably 105-150℃, more preferably 110-140℃, and most preferably 115-135℃.

[0043] And / or, the temperature of the annealing stage is 30-110℃, and the time of the annealing stage is 10-90 seconds. After the annealing stage, the molecular chain order arrangement can be adjusted, the crystallization can be made more perfect, the crystal size can be further controlled, and the mechanical strength and light transmittance of the polyvinyl alcohol film can be further improved.

[0044] For example, the temperature of the annealing stage is 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, 95℃, 100℃, 105℃, 110℃, or any value between 30-110℃, and the time of the annealing stage is 10 seconds, 20 seconds, 30 seconds, 40 seconds, 50 seconds, 60 seconds, 70 seconds, 80 seconds, 90 seconds, or any value between 10-90 seconds.

[0045] In some embodiments, the temperature of the annealing stage is preferably 40-100℃, more preferably 50-95℃, and most preferably 60-95℃.

[0046] According to embodiments of the present application, the surface temperature of the casting roller is 60-130℃. For example, the surface temperature of the casting roller is 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, 120℃, 125℃, 130℃, or any value between 60-130℃.

[0047] If the surface temperature of the casting roller is too high, the phenomenon of skin shrinkage is prone to occur, which inhibits the volatilization of the internal moisture of the casting solution; if the surface temperature of the casting roller is too low, the drying effect cannot reach the film peeling condition.

[0048] In some embodiments, the surface temperature of the casting roller is preferably 70-120℃, more preferably 75-110℃, and most preferably 80-100℃. In some embodiments, the surface temperature uniformity of the casting roller is required to be ±0.5℃. If the surface temperature of the casting roller is not uniform, the uniformity of the crystallization of the film surface will be affected during the drying process, and finally the light transmittance will be affected.

[0049] And / or, the surface roughness Ra of the casting roller is 0.001-0.1μm; thereby the film surface can be ensured to be flat and flawless.

[0050] For example, the surface roughness Ra of the casting roller is 0.001μm, 0.01μm, 0.03μm, 0.05μm, 0.08μm, 0.1μm, or any value between 0.001-0.1μm.

[0051] And / or, an air cover with an infrared lamp tube is arranged above the casting roller, the temperature of the air cover is 30-180℃, and the air speed of the air cover is 1-50m / s. The infrared has penetrating property, and the arrangement of the infrared lamp tube can make the inside and outside of the film be heated uniformly, and the film after drying by the casting roller and the air cover can reach the film peeling condition, at this time the moisture content of the polyvinyl alcohol film is between 10-35%.

[0052] For example, the temperature of the air hood above the casting roll is 30℃, 40℃, 50℃, 60℃, 70℃, 80℃, 90℃, 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, or any value between 30-180℃, and the air speed of the air hood above the casting roll is 1 m / s, 5 m / s, 10 m / s, 15 m / s, 20 m / s, 25 m / s, 30 m / s, 35 m / s, 40 m / s, 45 m / s, 50 m / s, or any value between 1-50 m / s.

[0053] In some embodiments, the temperature of the air hood above the casting roll is preferably 50-150℃; more preferably 60-140℃, and most preferably 80-130℃.

[0054] In some embodiments, the air speed of the air hood above the casting roll is preferably 2-40 m / s; more preferably 3-45 m / s, and most preferably 5-30 m / s, with an air speed uniformity of less than 5%.

[0055] According to embodiments of the present application, the polyvinyl alcohol has a degree of polymerization of 1000 to 5000; For example, the polyvinyl alcohol has a degree of polymerization of 1000, 2000, 3000, 4000, 5000, or any value between 1000-5000.

[0056] And / or, the content of the polyvinyl alcohol in the casting solution is 10-40 wt%. For example, the content of the polyvinyl alcohol in the casting solution is 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, or any value between 10-40 wt%.

[0057] In some embodiments, the content of the polyvinyl alcohol in the casting solution is preferably 15-35%, more preferably 20-30%, and most preferably 22-28%; According to embodiments of the present application, the casting solution further comprises a plasticizer, and the plasticizer comprises at least one of methanol, ethanol, propanol, butanol, ethylene glycol, polyethylene glycol, propylene glycol, glycerol, and starch; And / or, the content of the plasticizer in the casting solution is 4-20 wt%; For example, the content of the plasticizer in the casting solution is 4 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, or any value between 4-20 wt%.

[0058] In some embodiments, the content of the plasticizer in the casting solution is preferably 5-15%, more preferably 6-12%, and most preferably 7-11%; and / or, the thickness of the polyvinyl alcohol optical film is 20-75 μm.

[0059] For example, the thickness of the polyvinyl alcohol optical film is 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, or any value between 20-75 μm.

[0060] The present application also provides a polyvinyl alcohol optical film prepared by the method for preparing a polyvinyl alcohol optical film described above.

[0061] According to embodiments of the present application, the crystallinity of the polyvinyl alcohol optical film is 10-50%, preferably 27-44%; and / or, the light transmittance of the polyvinyl alcohol optical film is greater than 88%, preferably 88.4-93.6%; and / or, the draw ratio of the polyvinyl alcohol optical film is greater than 445%, preferably 448-686%; and / or, the grain size of the polyvinyl alcohol optical film is 1-45 nm, preferably 16-45 nm.

[0062] In some embodiments, the haze of the polyvinyl alcohol optical film is 0.56-0.99%, and the polyvinyl alcohol optical film has a lower haze.

[0063] According to embodiments of the present application, the range of the inter-grain distance in the polyvinyl alcohol optical film is 0.5-10 nm, preferably 0.8-3.8 nm; In some embodiments, the range of the inter-grain distance in the polyvinyl alcohol optical film is preferably 0.5-8 nm, further preferably 0.5-6 nm, and more preferably 0.8-3.8 nm.

[0064] and / or, the variance of the inter-grain distance in the polyvinyl alcohol optical film is 0.01-10.

[0065] In some embodiments, the variance of the inter-grain distance in the polyvinyl alcohol optical film is preferably 0.01-1, further preferably 0.01-0.5, and more preferably 0.014-0.089.

[0066] The embodiments of the present application will be described in detail below with specific examples, but those skilled in the art will understand that the following examples are only for illustration of the present application and should not be regarded as limiting the scope of the present application. If the specific conditions are not specified in the examples, the conventional conditions or the conditions recommended by the manufacturer are used. If the reagents or instruments used are not specified by the manufacturer, they are all conventional products that can be purchased on the market.

[0067] Example 1 Example 1 provides a polyvinyl alcohol optical film, a method for preparing the same comprising: (1) 100 Kg of polyvinyl alcohol resin with a polymerization degree of 2000 and an alcoholysis degree of 99%, 3 Kg of glycerol, 0.2 kg of sodium dodecyl benzene sulfonate, and 396.8 Kg of deionized water are stirred uniformly at 90°C to obtain a casting solution containing 25 wt% of polyvinyl alcohol.

[0068] (2) The casting solution is deaerated by standing, and then coated onto a casting roll using a twin-screw extruder through a die, and dried on the casting roll, wherein the die lip opening is 0.7 mm, the extrusion temperature is 85°C, and the temperature of the casting roll is 88°C. The temperature difference of the roll surface of the casting roll is within ±0.5°C to ensure uniform heating of the film surface. An air hood equipped with infrared lamp tubes is provided above the casting roll, and the air hood is divided into three sections, each with different air speed and temperature settings. The first section has an air speed of 5 m / s and a temperature of 110°C; the second section has an air speed of 25 m / s and a temperature of 130°C; and the third section has an air speed of 25 m / s and a temperature of 120°C. Each section of the air hood covers 90° of the casting roll, and the air hood is equipped with infrared heating lamp tubes with a temperature uniformity of ±1°C and an air speed uniformity in the transverse direction of the film of less than 5%.

[0069] (3) The film obtained after drying on the casting roll is transported to the roller group drying area for roller group drying. The roller group drying is equipped with 20 heating rollers (roller group numbers R1, R2, R3, R4, R5…R 20 ), arranged alternately in an upper and lower arrangement, and the temperature difference of the roll surface of each roller is within ±0.5°C. The roller group drying area includes a nucleation zone and a non-nucleation zone. An air extraction hood is provided above the drying rollers in the nucleation zone, and the moisture content of the film entering the nucleation zone is 25%. The air extraction hood is arranged at the first 5 rollers, and 5 air nozzles are provided between the drying rollers and the air extraction hood. The air speed uniformity in the transverse direction of the film is less than 5%, the air speed of the air nozzles is 15 m / s, and the air extraction area of the air extraction hood covers the transverse direction of the film. The temperature of the first 5 rollers is 130°C, and the temperature of the last 15 rollers decreases by 3°C in turn (R6 is 127°C, R7 is 124°C, R8 is 121°C, and so on). The drying time of the heating roller group is 140 seconds.

[0070] (4) The film after roller group drying is subjected to oven drying, which includes a preheating stage, a main drying stage, and an annealing stage in sequence. The temperature of the preheating stage is 90°C, the time of the preheating stage is 20 seconds, the temperature of the main drying stage is 120°C, the time of the main drying stage is 75 seconds, the temperature of the annealing stage is 80°C, and the time of the annealing stage is 20 seconds.

[0071] (5) After oven drying is completed, a polyvinyl alcohol optical film is obtained by winding.

[0072] Example 2 Example 2 differs from Example 1 in that in step (3), the air speed of the air nozzle is 5 m / s when the roll group is drying. The rest is the same as Example 1.

[0073] Example 3 Example 3 differs from Example 1 in that in step (3), the air speed of the air nozzle is 25 m / s when the roll group is drying. The rest is the same as Example 1.

[0074] Example 4 Example 4 differs from Example 1 in that in step (3), when the roll group is drying, 20 heating rolls are set, the temperature of the first 5 rolls is 100℃, and the temperature of the last 15 rolls is increased by 1℃ in turn (R6 is 101℃, R7 is 102℃, R8 is 103℃, and so on). The rest is the same as Example 1.

[0075] Example 5 Example 5 differs from Example 1 in that in step (3), when the roll group is drying, 20 heating rolls are set, the temperature of the first 5 rolls is 150℃, and the temperature of the last 15 rolls is decreased by 5℃ in turn (R6 is 145℃, R7 is 140℃, R8 is 135℃, and so on). The rest is the same as Example 1.

[0076] Example 6 Example 6 differs from Example 1 in that in step (3), the time for drying by the heating roll group is 100 seconds. In step (4), the time for the preheating stage is 14 seconds, the time for the main drying stage is 54 seconds, and the time for the annealing stage is 14 seconds. The rest is the same as Example 1.

[0077] Example 7 Example 7 differs from Example 1 in that in step (3), the time for drying by the heating roll group is 175 seconds. In step (4), the time for the preheating stage is 25 seconds, the time for the main drying stage is 94 seconds, and the time for the annealing stage is 25 seconds. The rest is the same as Example 1.

[0078] Example 8 Example 8 differs from Example 1 in that in step (3), when the roll group is drying, the temperature of all the rolls is 120℃. The rest is the same as Example 1.

[0079] Comparative Example 1 Comparative Example 1 differs from Example 1 in that in step (3), when the roll group is drying, no exhaust hood is set. The rest is the same as Example 1.

[0080] Comparative Example 2 Comparative Example 2 differs from Example 1 in that in Step (4), the oven drying is not segmented, the oven drying temperature is 120°C, and the oven drying time is 115 seconds. The rest is the same as Example 1.

[0081] The polyvinyl alcohol films prepared in Examples 1-8 and Comparative Examples 1-2 were tested for tensile properties, light transmittance, and crystal size, including: (1) Tensile ratio test: A 10 cm long and 8 cm wide optical film was cut, and the two ends of the optical film were fixed on a tensile fixture. The optical film was swelled in water at 30°C for 10 minutes, and then stretched along the length direction at a stretching speed of 2 m / min until it was broken. The tensile ratio was calculated according to the comparison between the length at the time of breaking and the original length.

[0082] (2) Light transmittance and haze: A 50 mm diameter disc was cut, and a haze meter or a spectrophotometer was used to test the light transmittance of the sample. The light transmittance was calculated according to the total projected light flux passing through the sample and the incident light flux. The haze of the material was calculated by measuring the difference between the light transmitted through the material and the scattered light.

[0083] (3) Average crystal size test: X-ray diffraction (XRD) was used to characterize the crystal size and the distance between the crystal grains of the polyvinyl alcohol optical film. The Scherrer formula was used to calculate the size of the crystal grains in a specific crystal plane direction according to the half-height width of the diffraction peak: D = Kλ / (βcosθ), where D is the grain size, K is the Scherrer constant (~0.9), λ is the X-ray wavelength, β is the half-height width of the diffraction peak, and θ is the Bragg angle. The size of 20 samples was tested and the average value was calculated.

[0084] (4) Crystal grain spacing test: Small-angle X-ray scattering was used to detect the difference in electron density between the crystal and amorphous regions in the polyvinyl alcohol. The crystal grain spacing was calculated from the scattering curve obtained by small-angle X-ray scattering measurement. The finished film was randomly cut into 20 samples of 2 cm x 1 cm, and the average value of the crystal grain spacing of each sample was tested. The range and variance of the average values of the crystal grain spacing of the 20 samples were calculated.

[0085] (4) Crystallinity test Differential scanning calorimetry was used to measure the enthalpy required for crystal melting and compare it with the theoretical melting enthalpy of 100% crystalline polyvinyl alcohol to calculate the crystallinity: Crystallinity (%) = (ΔHf / ΔHf°) x 100%. ΔHf° is 138.6 J / g.

[0086] The test results of the polyvinyl alcohol optical films of the examples and comparative examples are shown in Table 1.

[0087] Table 1 Comparison of test results of polyvinyl alcohol optical films of examples and comparative examples

[0088] As can be seen from Table 1, when the second drying is performed, by controlling the temperature of the drying roller in the nucleation zone at 90-150°C and providing an air extraction hood above the drying roller in the nucleation zone, the crystallization uniformity can be increased and a smaller crystal grain size can be obtained. When the oven drying is performed, by controlling the temperature of the oven drying in stages, the crystallinity and the crystal grain size of the polyvinyl alcohol optical film can be adjusted, and the light transmittance and the tensile properties of the polyvinyl alcohol optical film are further effectively improved.

[0089] It should be finally pointed out that the above examples are only used to illustrate the technical solutions of the present application, but not to limit the same; although the present application has been described in detail with reference to the above examples, those skilled in the art should understand that the technical solutions recorded in the above examples can still be modified, or some or all of the technical features thereof can be replaced by equivalents; and such modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the examples of the present application.

[0090] In addition, those skilled in the art can understand that although some of the embodiments herein include certain features rather than others included in other embodiments, the combination of the features of different embodiments means to be within the scope of the present application and forms different embodiments. For example, in the above claims, any one of the claimed embodiments can be used in any combination. The information disclosed in the BACKGROUND section is only intended to deepen the understanding of the general background of the present application, and should not be regarded as acknowledging or implying in any form that the information constitutes the prior art known to those skilled in the art.

Claims

1. A method for preparing a polyvinyl alcohol optical film, characterized in that, include: A casting solution is provided, the casting solution comprising polyvinyl alcohol; The casting solution is coated onto a casting roll and subjected to a first drying process to obtain a film. The film is passed through the roller drying zone for a second drying process; The film that has undergone the second drying process is then subjected to a third drying process and wound up to obtain a polyvinyl alcohol optical film. The roller drying zone includes a nucleation zone and a non-nucleation zone. The second drying process includes: controlling the temperature of the drying roller in the nucleation zone at 90-150°C, and applying an airflow above the drying roller in the nucleation zone to remove the volatile solvent. The third drying process includes a preheating stage, a main drying stage, and an annealing stage performed sequentially. The temperature of the preheating stage is different from the temperature of the main drying stage, and the temperature of the main drying stage is different from the temperature of the annealing stage.

2. The method for preparing the polyvinyl alcohol optical film according to claim 1, characterized in that, The nucleation zone includes 1-5 drying rollers continuously arranged in the film travel direction and an exhaust hood located above the drying rollers, with a negative pressure zone formed between the drying rollers and the exhaust hood. And / or, the nucleation zone is provided with multiple air nozzles, the air nozzles are located between the drying roller and the exhaust hood, and the air velocity of the air nozzles is 1-50m / s; And / or, the wind speed uniformity in the transverse direction of the membrane is less than 5%; And / or, the moisture content of the film when it enters the nucleation region is 15-25 wt%.

3. The method for preparing the polyvinyl alcohol optical film according to claim 1, characterized in that, The roller drying zone includes 2-30 drying rollers; And / or, the surface temperature of the drying roller in the non-nucleation zone is 30-150°C; And / or, during the second drying process, the surface temperature difference between adjacent drying rollers is 0-15°C; And / or, the second drying time is 80-200 seconds.

4. The method for preparing the polyvinyl alcohol optical film according to claim 1, characterized in that, The temperature of the preheating stage is 30-140℃, and the preheating stage lasts for 10-40 seconds; And / or, the temperature of the main drying stage is 100-160°C, and the time of the main drying stage is 50-180 seconds; And / or, the temperature of the annealing stage is 30-110°C, and the duration of the annealing stage is 10-90 seconds.

5. The method for preparing the polyvinyl alcohol optical film according to claim 1, characterized in that, The surface temperature of the casting roll is 60-130℃; And / or, the surface roughness Ra of the casting roller is 0.001-0.1 μm; And / or, an air hood equipped with infrared lamps is provided above the casting roller, the temperature of the air hood is 30-180℃, and the wind speed of the air hood is 1-50m / s.

6. The method for preparing a polyvinyl alcohol optical film according to any one of claims 1-5, characterized in that, The degree of polymerization of the polyvinyl alcohol is 1000 to 5000; And / or, the polyvinyl alcohol content in the casting solution is 10-40 wt%.

7. The method for preparing the polyvinyl alcohol optical film according to claim 6, characterized in that, The casting solution also includes a plasticizer, which includes at least one of methanol, ethanol, propanol, butanol, ethylene glycol, polyethylene glycol, propylene glycol, glycerol, and starch. And / or, the plasticizer content in the casting solution is 4-20 wt%; And / or, the thickness of the polyvinyl alcohol optical film is 20-75 μm.

8. A polyvinyl alcohol optical film, characterized in that, The polyvinyl alcohol optical film is prepared by the method for preparing polyvinyl alcohol optical film according to any one of claims 1-7.

9. The polyvinyl alcohol optical film according to claim 8, characterized in that, The crystallinity of the polyvinyl alcohol optical film is 10-50%; And / or, the transmittance of the polyvinyl alcohol optical film is greater than 88%; And / or, the stretching ratio of the polyvinyl alcohol optical film is greater than 445%; And / or, the grain size of the polyvinyl alcohol optical film is 1-45 nm.

10. The polyvinyl alcohol optical film according to claim 9, characterized in that, The range of intergranular spacing within the polyvinyl alcohol optical film is 0.5-10 nm; And / or, the variance of the intergranular spacing within the polyvinyl alcohol optical film is 0.01-10.

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