A method for preparing highly stretchable polyvinyl alcohol optical film

Abstract

The present invention discloses a method for preparing a highly stretchable polyvinyl alcohol optical film. The method involves saponifying a PVA resin and adding an acid-reactive additive to the PVA casting solution. The acid-reactive additive reacts with hydroxyl groups on the PVA polymer molecular chain under high temperature and high pressure. The random nature of this reaction is utilized to reduce the alcoholysis degree of the PVA and uniformly distribute the alcoholysis units on the molecular chain, thereby obtaining a PVA optical film with small and uniform crystals and improving the stretchability of the PVA optical film. The method has the advantages of being simple to operate and requiring no additional process equipment, resulting in strong market competitiveness.

Description

Technical Field

[0001] The invention belongs to the field of polyvinyl alcohol optical films, and particularly relates to a method for preparing a highly stretchable polyvinyl alcohol optical film. Background Art

[0002] Polarizing film is a key component of liquid crystal displays (LCDs). It selectively transmits light and is a key component in determining LCD quality. Polarizing film is produced by iodine-dyeing, stretching, and drying PVA optical film. The stretchability and uniformity of the PVA optical film during the iodine-dyeing and stretching process determine the polarization properties and uniformity of the polarizing film. Therefore, producing highly stretchable PVA optical film is essential for achieving high-performance polarizers and is a key issue that must be addressed in achieving domestic production of PVA optical film.

[0003] PVA optical films are typically formed by casting a PVA aqueous solution through extrusion, casting, and drying. During the process, the solvent evaporates rapidly, the PVA solution crystallizes, and the solution-gel-film transition gradually occurs. The size and dispersion of the final film crystals determine the stretchability of the PVA optical film. However, the raw materials used in existing production processes are all almost completely alcoholyzed PVA, which easily forms hydrogen bonds between and within the PVA molecular chains. The processed PVA film tends to form larger crystals with higher content, resulting in problems such as film breakage and low stretch ratio during downstream stretching. On the other hand, although small crystals can be obtained by reducing the alcoholysis degree and inhibiting hydrogen bond formation, due to the characteristics of the alcoholysis reaction, the alcoholysis units tend to appear in a concentrated manner, resulting in serious non-uniformity in the distribution of alcoholysis units on the PVA molecular chain. Therefore, although the formed crystals are small, they have a wide size distribution. During the iodine dyeing and stretching process, they are also prone to uneven stretching and film breakage, which reduces the stretchability of the PVA film and makes it impossible to produce high-performance PVA polarizing film. Summary of the Invention

[0004] In order to solve the above problems, the purpose of the present invention is to provide a method for preparing a highly stretchable PVA optical film, aiming to prepare a PVA optical film with a high stretch ratio.

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

[0006] A method for preparing a highly stretchable PVA optical film is characterized by: saponifying the PVA resin pretreatment, adding an acid-reactive additive to the PVA casting solution, and reacting the acid-reactive additive with the hydroxyl groups on the PVA polymer molecular chain under high temperature and high pressure. Utilizing the randomness of this reaction, the alcoholysis degree of the PVA is reduced and the alcoholysis units are evenly distributed on the molecular chain, thereby obtaining a PVA optical film with small and uniform crystals and improving the stretchability of the PVA optical film. The method specifically comprises the following steps:

[0007] (1) PVA resin particles are added to an alkaline solution for saponification pretreatment to obtain completely alcoholyzed PVA resin (alcoholysis degree ≥ 99.9%, which is considered to be completely alcoholyzed);

[0008] (2) The completely alcoholyzed resin is transferred to a dissolving kettle to which a water solvent has been added, and a plasticizer, an acid reactive auxiliary agent, and a surfactant are added in sequence. The resin is dissolved and reacted at a high temperature to obtain a casting solution, which is then cast and dried to form a film product.

[0009] Furthermore, in step (1), the PVA resin particles are usually unmodified polyvinyl alcohol resin, modified polyvinyl alcohol resin, or a combination thereof. The copolymerization component of the modified polyvinyl alcohol resin can be an unsaturated carboxylic acid, a vinyl ether, an α-olefin, etc., preferably an α-olefin, and the content of the modified monomer (in terms of the molar ratio of the vinyl alcohol monomer) is preferably 0.5-8%.

[0010] Furthermore, the vinyl ester used in the synthesis of the above-mentioned PVA-based resin can be one or more combinations of vinyl acetate, vinyl formate, vinyl propionate, vinyl benzoate, etc. From the perspective of convenience in industrial production, vinyl acetate is preferred.

[0011] Furthermore, the average polymerization degree of the PVA resin particles in step (1) is 1500-5000, more preferably 2000-3000. If it is too low, the stretchability will be deteriorated, and if it is too high, hydrogen bonds between and within the molecular chains will remain after dissolution, and the particles cannot be completely dissolved.

[0012] Furthermore, the alcoholysis degree of the PVA resin particles in step (1) is greater than 98%, more preferably greater than 99%.

[0013] Furthermore, the alkaline solution in step (1) may be an aqueous solution of at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonium hydroxide and sodium acetate.

[0014] Furthermore, the concentration of the alkaline solution in step (1) is 0.1 g / L-50 g / L, preferably 0.5-30 g / L, more preferably 1-10 g / L, considering the reaction rate and reaction uniformity, and the molar ratio of the alkaline molecules to the PVA resin is 0.001-0.1.

[0015] Furthermore, the saponification pretreatment in step (1) is carried out at a temperature of 20-80° C. and for a time of 2-8 h.

[0016] Furthermore, in step (2), the acidic reactive auxiliary agent can be selected from one or more combinations of organic acids and inorganic acids, such as formic acid, acetic acid, propionic acid, butyric acid, phenylacetic acid, chloroacetic acid, iodoacetic acid, sulfuric acid, sulfurous acid, phosphoric acid, phosphorous acid and hypochlorous acid. Considering that inorganic oxygen-containing acids have strong corrosiveness and cause strong corrosion to equipment during the dissolution process, organic acid reactive auxiliary agents are preferred, which can be selected from formic acid, acetic acid, propionic acid, butyric acid, phenylacetic acid, chloroacetic acid and iodoacetic acid.

[0017] Furthermore, in step (2), the amount of the acid reactive auxiliary agent added is 0.1 wt% to 20 wt% of the mass of the PVA resin, preferably 0.5 wt% to 10 wt%.

[0018] Furthermore, there is no limitation on the method of adding the acid reactive auxiliary agent in step (2), and the acid reactive auxiliary agent can be added together with other components or added in steps, and finally mixed evenly to form a casting solution.

[0019] Furthermore, the plasticizer added in step (2) is used to improve the mechanical properties of the PVA film and can be a combination of one or more of glycerols (such as glycerol, diglycerol, triglycerol, etc.), ethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol, etc. To improve the stretchability of the film, glycerol is preferably used in the present invention, and the preferred content is 8-15wt% of the PVA mass;

[0020] Furthermore, the surfactant added in step (2) is used to improve the peeling effect of the PVA film on the roller, and can be one or more combinations of sulfonic acids such as dodecylbenzene sulfonate, alkyl ethers such as polyoxyethylene oleyl ether, alkyl phenyl ethers such as polyoxyethylene octylphenyl ether, alkyl esters such as polyoxyethylene laurate, and carboxylic acids such as potassium laurate. Considering the peeling effect of the PVA film on the roller, the content is preferably 0.01-1wt% of the PVA mass.

[0021] Furthermore, the solute concentration of the casting solution in step (2) is in the range of 5 wt % to 60 wt %, preferably 10 wt % to 50 wt %, more preferably 15 wt % to 40 wt %;

[0022] Furthermore, the dissolution and reaction temperature of the casting solution in step (2) is 100-180°C and the reaction time is 3-10 hours. If the temperature is too low or the reaction time is too short, it will be detrimental to the dissolution of PVA and the reaction between the reactive additive and PVA; if the temperature is too high or the reaction time is too long, it will easily cause oxidation, and the solution and product will easily turn yellow.

[0023] Furthermore, the dissolving kettle used in the casting solution dissolution reaction process in step (2) needs to be strictly sealed, and the pressure of the dissolving kettle remains unchanged in the range of 0.2-1 MPa. Too low a pressure is not conducive to the dissolution of PVA and the reaction between the reactive additive and PVA.

[0024] The highly stretchable polyvinyl alcohol optical film obtained by the present invention has a stretchable ratio of ≥700%, and the polarization degree of the polarizer prepared therefrom is ≥99.9%.

[0025] The beneficial effects of the present invention are embodied in:

[0026] (1) The present invention adds an acid-type reactive auxiliary agent and reacts with PVA under high temperature and high pressure, thereby reducing the alcoholysis degree of the PVA resin and film product and making the alcoholysis groups evenly distributed on the PVA molecular chain, thereby achieving a simultaneous reduction in grain size and size distribution, and ultimately obtaining a polyvinyl alcohol optical film with excellent stretchability;

[0027] (2) The present invention has the characteristics and advantages of being simple to operate and not requiring any additional process equipment, and has good market competitiveness. DETAILED DESCRIPTION

[0028] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention are described clearly and completely below. Obviously, the specific embodiments described are only part of the embodiments of the present invention, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the specific embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0029] Example 1

[0030] 1000 g of PVA polymer (degree of polymerization 1700, degree of alcoholysis 99%) was added to a 2 g / L sodium hydroxide aqueous solution and pretreated at 30° C. for 6 h to obtain a completely alcoholysed PVA resin, wherein the molar ratio of sodium hydroxide to PVA resin was 0.03.

[0031] The completely alcoholyzed resin was transferred to a dissolution kettle to which a water solvent had been added, and 100 g of glycerol, 1 g of sodium dodecylbenzenesulfonate surfactant, and 25 g of acetic acid reactive auxiliary (2.5% relative to the mass of PVA) were added. The mixture was dissolved in 4500 L of deionized water at 150 ° C and a pressure of 0.4 MPa in a sealed state. The dissolution reaction time was 10 h to prepare a PVA casting solution containing a reactive auxiliary.

[0032] The formed casting liquid is degassed and filtered by the extruder and then extruded from the die. After being dried by a casting roller at 90°C, it enters 11 sets of drying rollers (temperature range of 40-100°C) to repeatedly dry the front and back of the film. Then it enters an air flotation oven for heat treatment and is finally rolled up to obtain the PVA optical film.

[0033] Example 2

[0034] This example was prepared using the same method and conditions as in Example 1, with the only difference being that the mass of the acetic acid reactive auxiliary agent was 60 g, accounting for 6% of the mass of the PVA.

[0035] Example 3

[0036] This example was prepared by the same method and conditions as in Example 1, with the only difference being that the mass of the acetic acid reactive auxiliary agent was 60 g, which was 6% of the mass of the PVA, and the dissolution temperature was 160°C.

[0037] Comparative Example 1

[0038] This comparative example was prepared by the same method and conditions as Example 1, except that no reactive auxiliary agent was added.

[0039] The PVA optical films prepared in the above embodiments and comparative examples were subjected to a tensile performance test. To truly reflect the stretching ratio of the films during the polarizer processing, the PVA films were immersed in 30°C deionized water for 5 minutes, then dyed and stretched in a solution containing 0.5g / L iodine and 20g / L potassium iodide. The elongation at break when stretched was recorded, which was the stretching ratio. The results are shown in Table 1.

[0040] The PVA optical films prepared in the above examples and comparative examples were iodine-dyed and stretched, and the polarization degree of the polarizer was tested. The PVA film was immersed in 30°C deionized water for 5 minutes, then dyed and stretched in a solution containing 0.5g / L iodine and 20g / L potassium iodide, stretched to 5.5 times, and fixed in a solution containing boric acid and potassium iodide. Finally, it was dried at 40°C, and protective films were attached to both sides of the dried film. After drying, a polarizer was obtained. The polarization degree of the obtained polarizer was tested using a phase difference meter, and the results are shown in Table 1.

[0041] As shown in Table 1, the PVA optical film obtained by adding 6% acetic acid and dissolving at 160°C (Example 3) has the highest stretching ratio (reaching 700%) compared to the PVA film with lower acetic acid addition (Comparative Example 1 and Example 1) and a lower dissolution temperature of 150°C (Example 2). In addition, due to the low degree of alcoholysis and its uniform distribution, the PVA optical film has small and uniform crystals, and the polarizer processed therefrom also has the highest polarization degree of 99.9%.

[0042] Table 1. Characterization results of stretching ratio and polarization degree of polarizer of PVA optical film corresponding to each embodiment and comparative example

[0043]

[0044] The above description is merely an exemplary embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A method for preparing a highly stretchable polyvinyl alcohol optical film, characterized in that: The method comprises the following steps: performing saponification pretreatment on the PVA resin and adding an acid-reactive additive to the PVA casting solution; reacting the acid-reactive additive with the hydroxyl groups on the PVA polymer molecular chain under high temperature and high pressure; utilizing the randomness of the reaction to reduce the alcoholysis degree of the PVA and uniformly distribute the alcoholysis units on the molecular chain; thereby obtaining a PVA optical film with small and uniform crystals and improving the stretchability of the PVA optical film; and Step 1: 1000g of PVA polymer with a degree of polymerization of 1700 and a degree of alcoholysis of 99% was added to a 2g / L aqueous sodium hydroxide solution for 30 min. o C pretreatment for 6 h to obtain completely alcoholyzed PVA resin, wherein the molar ratio of sodium hydroxide to PVA resin was 0.03; Step 2: Transfer the completely alcoholyzed resin to a dissolution kettle to which a water solvent has been added, add 100 g of glycerol, 1 g of sodium dodecylbenzenesulfonate surfactant, and 60 g of acetic acid reactive auxiliary agent, and dissolve them in 4500 L of deionized water at 160° C. and a pressure of 0.4 MPa in a sealed state. The dissolution reaction time is 10 h to prepare a PVA casting solution containing reactive auxiliary agents; The formed casting liquid is degassed and filtered by the extruder and then extruded from the die. After being dried by the casting roller at 90℃, it enters the o Eleven sets of drying rollers within the C range repeatedly dry the front and back of the film, which then enters an air flotation oven for heat treatment and is finally rolled up to obtain the PVA optical film.

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