Polyvinyl acetal resin solution, method for producing the same, and use thereof

Abstract

The present application relates to the technical field of functional polymer material preparation, and particularly relates to a polyvinyl acetal resin solution, a preparation method and application thereof.The polyvinyl acetal resin solution prepared by the present application can effectively meet the performance requirements of a polarizing plate.The composite solvent of the present application can significantly improve the solubility of aromatic aldehyde compounds, ensure that the acetalization reaction is fully and uniformly, and ensure the stability of the polyvinyl acetal resin performance;The present application uses a composite catalyst to reduce salt formation, adjusts the pH value of the system, and uses ceramic membrane filtration instead of water washing, so that the yield of the polyvinyl acetal resin is improved, and the white turbidity problem of the resin film is solved;The present application balances the hydrophilicity and water resistance of the polyvinyl acetal resin by controlling the temperature in sections and monitoring the acid value and acetal degree;The present application uses a polyether compound containing an epoxy group and an organic silane coupling agent to synergistically modify the resin film, so that the mechanical properties, high and low temperature resistance and anti-aging performance of the resin film are significantly improved.

Description

Technical Field

[0001] This invention relates to the field of functional polymer material preparation technology, and in particular to a polyvinyl alcohol acetal resin solution, its preparation method and application. Background Technology

[0002] Polyvinyl acetal resin is an important polymer material obtained by acetalization of polyvinyl alcohol (PVA) and aldehyde compounds. Due to its excellent film-forming properties, transparency, and mechanical properties, it is widely used in coatings, adhesives, and liquid crystal displays (such as polarizing films). When used as a polarizing film, polyvinyl acetal resin requires even higher performance. It must not only possess excellent transparency but also be insoluble in both water and alcohol, while also exhibiting good hydrophilicity and water resistance to meet the needs of polarizing film preparation and use.

[0003] Currently, when preparing polyvinyl acetal resin solutions, technicians face several technical bottlenecks when using aromatic aldehydes as acetalizing agents: First, acid catalysts are typically required during the reaction, resulting in salt formation in the system after the reaction. To ensure the purity of the polyvinyl acetal resin product, multiple water washes are necessary to remove the salts. However, some polyvinyl acetal resin is lost with the washing solution during these washes, significantly reducing the yield. Second, if the salts are not completely removed, the residual salts can cause turbidity in the subsequently prepared resin membrane. First, the solid content of the prepared polyvinyl acetal resin solution is less than 10%, and when a resin film with a thickness of more than 30 μm is prepared using this polyvinyl acetal resin solution, uneven evaporation rate during solvent evaporation can easily cause ripples on the surface of the resin film, which also leads to a decrease in the gloss of the resin film. Second, aromatic aldehydes have extremely poor solubility in commonly used organic solvents, which makes it difficult for the acetalization reaction to proceed fully, resulting in low reaction efficiency and uneven performance of polyvinyl acetal resin.

[0004] Researchers have not yet proposed effective solutions to the above problems. For example, some researchers have tried to improve the salt removal rate by increasing the number of water washes, but this further reduces the yield of polyvinyl acetal resin; other researchers have improved the solubility of aromatic aldehydes by changing the solvent, but the new solvent may introduce new impurities or affect the performance of polyvinyl acetal resin; still others have improved the surface state of the resin film by adjusting the film-forming process parameters, but this cannot achieve a balance between solid content, resin film thickness, and resin film performance.

[0005] Therefore, how to prepare a polyvinyl acetal resin solution that meets the performance requirements of polarizers has been a long-standing problem in this field. Summary of the Invention

[0006] In view of this, the present invention provides a polyvinyl acetal resin solution, its preparation method and application. The preparation method provided by the present invention has a high yield and produces a resin film with high transparency. The resin film is neither soluble in water nor alcohol alone and has both hydrophilicity and water resistance, thus meeting the performance requirements of polarizers.

[0007] This invention provides a method for preparing a polyvinyl acetal resin solution, comprising the following steps: (1) Polyvinyl alcohol and a composite solvent are mixed to obtain a polyvinyl alcohol solution; the composite solvent includes a main solvent and a co-solvent; the main solvent includes N,N-dimethylformamide (DMF); the co-solvent includes at least one of ethylene glycol monomethyl ether and ethylene glycol monoethyl ether; (2) The polyvinyl alcohol solution is mixed with aromatic aldehyde compounds and a composite catalyst to carry out a segmented temperature-controlled acetalization reaction to obtain a first resin solution; the composite catalyst includes organic acids and organic amine salts; (3) The first resin solution and the synergistic modifier are mixed for synergistic modification. After adjusting the pH value of the system to 6.5~7.5, the mixture is filtered through a ceramic membrane to obtain the polyvinyl acetal resin solution. The synergistic modifier includes a polyether compound containing an epoxy group and an organosilane coupling agent.

[0008] Preferably, the volume ratio of the main solvent to the co-solvent is 7~8:2~3; the mass ratio of the polyvinyl alcohol to the volume ratio of the composite solvent is (10~15)g:100mL.

[0009] Preferably, the aromatic aldehyde compound includes at least one of benzaldehyde, p-hydroxybenzaldehyde, o-methoxybenzaldehyde, m-hydroxybenzaldehyde, and p-methoxybenzaldehyde; the molar ratio of the aromatic aldehyde compound to polyvinyl alcohol is 0.3~0.5:1.

[0010] Preferably, the organic acid includes at least one of formic acid, acetic acid, and propionic acid; the organic amine salt includes at least one of triethanolamine hydrochloride, diethanolamine hydrochloride, and ethanolamine hydrochloride; the mass ratio of the organic acid to the organic amine salt is 1:0.5~1; and the mass ratio of the composite catalyst to polyvinyl alcohol is 2~5:100.

[0011] Preferably, the segmented temperature-controlled acetalization reaction is carried out under stirring conditions; the stirring speed is 200~300 rpm; the segmented temperature-controlled acetalization reaction includes a first acetalization stage and a second acetalization stage carried out sequentially; the temperature of the first acetalization stage is 50~55℃, and the holding time is 1.5~2h; the temperature of the second acetalization stage is 60~70℃; the second acetalization stage is stopped when the acid value of the reaction system stabilizes at 5~10mgKOH / g and the degree of acetalization stabilizes at 30~45mol%.

[0012] Preferably, the first acetalization stage is preceded by a first heating; the rate of the first heating is 2~3℃ / min; the second acetalization stage is preceded by a second heating; the rate of the second heating is 2~3℃ / min.

[0013] Preferably, the epoxy-containing polyether compound includes at least one of glycidyl ether polyoxyethylene ether and glycidyl ether polyoxypropylene ether; the number average molecular weight of the epoxy-containing polyether compound is 500-1000, and the number average molecular weight deviation is ≤±10%; the organosilane coupling agent includes at least one of γ-glycidyl ether propyltrimethoxysilane, γ-glycidyl ether propyltriethoxysilane and γ-methacryloyloxypropyltrimethoxysilane; the mass ratio of the epoxy-containing polyether compound to the organosilane coupling agent is 3-4:1; and the mass ratio of the synergistic modifier to polyvinyl alcohol is 4-10:100.

[0014] Preferably, the temperature for synergistic modification is 75~85℃, and the time for synergistic modification is 1~2h; the synergistic modification is carried out under stirring conditions.

[0015] The present invention also provides a polyvinyl alcohol acetal resin solution prepared by the above-described method, with a solid content of 12-18% and a viscosity (25°C) of 500-1500 mPa·s.

[0016] The present invention also provides the application of the polyvinyl acetal resin solution described above in the field of liquid crystal displays.

[0017] Compared with the prior art, the present invention has achieved the following beneficial effects: This invention provides a method for preparing a polyvinyl acetal resin solution. The method provides a high yield, and the resulting resin film from the polyvinyl acetal resin solution exhibits high transparency, no turbidity, no surface ripples, and good gloss. It is neither soluble in water nor alcohol alone, possessing both hydrophilicity and water resistance, effectively meeting the performance requirements of polarizers in the liquid crystal display field. Specifically: 1) Efficiently solves the solubility problem of aromatic aldehydes: The composite solvent constructed in this invention can significantly improve the solubility of aromatic aldehydes, ensure that the acetalization reaction is fully and uniform, improve the reaction efficiency by more than 30%, and ensure the stable performance of polyvinyl acetal resin products. 2) Avoiding the defects of water washing and improving the yield of polyvinyl acetal resin and the transparency of resin membrane: This invention uses a composite catalyst to reduce the salt generation during the preparation process, combined with adjusting the pH value of the system and ceramic membrane filtration, to replace the multiple water washing in the traditional process. The yield of polyvinyl acetal resin is increased to more than 90%, and the problem of resin membrane turbidity caused by residual salt is well solved. 3) Precisely control the degree of acetalization and balance the core performance: This invention precisely controls the degree of acetalization within the optimal range of 30~45mol% by segmented temperature control and dual monitoring of acid value and degree of acetalization, effectively balancing the hydrophilicity and water resistance of polyvinyl alcohol acetal resin and solving the performance fluctuation problem of traditional processes. 4) Synergistic modification to enhance the comprehensive performance of resin membrane: This invention utilizes polyether compounds containing epoxy groups and organosilane coupling agents for synergistic modification to form a stable cross-linking network, which significantly improves the mechanical properties, high and low temperature resistance, and anti-aging properties of the resin membrane. 5) The preparation method provided by this invention has simple steps, mild reaction conditions, is green and friendly, does not generate a large amount of washing wastewater in the post-treatment, has a simple post-treatment process, does not require complex equipment, takes into account both environmental protection and the needs of large-scale production, and is suitable for industrial production.

[0018] This invention also provides a polyvinyl acetal resin solution obtained by the preparation method described above. The polyvinyl acetal resin solution provided by this invention produces a resin film with a tensile strength of 15-20 MPa and an elongation at break of 120-150%. It exhibits excellent resistance to high and low temperatures and damp heat aging, making it suitable for the extremely high-performance requirements of polarizers in the complex operating environment of liquid crystal displays.

[0019] This invention also provides the application of the polyvinyl acetal resin solution described above in the field of liquid crystal displays. The polyvinyl acetal resin solution prepared by this invention is suitable for use in the field of liquid crystal displays, and is particularly suitable as a polarizer material in the field of liquid crystal displays. Detailed Implementation

[0020] This invention provides a method for preparing a polyvinyl acetal resin solution, comprising the following steps: (1) Mix polyvinyl alcohol and a composite solvent to obtain a polyvinyl alcohol solution; (2) The polyvinyl alcohol solution is mixed with aromatic aldehyde compounds and a composite catalyst to carry out a segmented temperature-controlled acetalization reaction to obtain a first resin solution; (3) The first resin solution and the synergistic modifier are mixed for synergistic modification. After adjusting the pH value of the system to 6.5~7.5, the mixture is filtered through a ceramic membrane to obtain the polyvinyl acetal resin solution.

[0021] This invention involves mixing polyvinyl alcohol and a composite solvent (referred to as the first mixture) to obtain a polyvinyl alcohol solution. In this invention, the degree of polymerization of the polyvinyl alcohol is preferably 1800-3500, more preferably 2000-3000, even more preferably 2400-2700, the degree of polymerization deviation is preferably ≤±5%, and the degree of alcoholysis is preferably 98-99 mol%, more preferably 98.5 mol%.

[0022] In this invention, the composite solvent preferably includes a main solvent and a co-solvent; the volume ratio of the main solvent and the co-solvent is preferably 7~8:2~3, more preferably 7.5:2.5.

[0023] In this invention, the main solvent preferably includes N,N-dimethylformamide (DMF); the co-solvent preferably includes at least one of ethylene glycol monomethyl ether and ethylene glycol monoethyl ether; and the purity of the co-solvent is preferably analytical grade.

[0024] In this invention, the mass ratio of the polyvinyl alcohol to the volume ratio of the composite solvent is preferably (10~15) g:100 mL, more preferably (11~14) g:100 mL, and even more preferably (12~13) g:100 mL.

[0025] In this invention, the first mixing is preferably carried out in a protective atmosphere; the protective atmosphere is preferably nitrogen; the nitrogen introduction rate is preferably 0.5~1 L / min; the temperature of the first mixing is preferably 80~90℃, more preferably 85℃, and the mixing time is preferably 2~3 h, more preferably 2.5 h; the first mixing is preferably stirred, and the stirring speed is preferably 300~500 rpm, more preferably 400 rpm. This invention ensures that the reaction system is in an inert atmosphere by using a protective atmosphere, thus preventing the oxidation of polyvinyl alcohol.

[0026] After obtaining a polyvinyl alcohol solution, the present invention mixes the polyvinyl alcohol solution with an aromatic aldehyde compound and a composite catalyst to carry out a segmented temperature-controlled acetalization reaction to obtain a first resin solution. In the present invention, the aromatic aldehyde compound preferably includes at least one selected from benzaldehyde, p-hydroxybenzaldehyde, o-methoxybenzaldehyde, m-hydroxybenzaldehyde, and p-methoxybenzaldehyde; the purity of the aromatic aldehyde compound is preferably ≥98%. The present invention uses an aromatic aldehyde compound as an acetalizing agent.

[0027] In this invention, the molar ratio of the aromatic aldehyde compound to polyvinyl alcohol is preferably 0.3 to 0.5:1, more preferably 0.4:1.

[0028] In this invention, the composite catalyst preferably comprises an organic acid and an organic amine salt; the mass ratio of the organic acid to the organic amine salt is preferably 1:0.5~1, more preferably 1:0.6~0.9, and even more preferably 1:0.7~0.8.

[0029] In this invention, the organic acid preferably includes at least one of formic acid, acetic acid and propionic acid; the purity of the organic acid is preferably ≥99%.

[0030] In this invention, the organic amine salt preferably includes at least one of triethanolamine hydrochloride, diethanolamine hydrochloride, and ethanolamine hydrochloride; the purity of the organic amine salt is preferably ≥98%.

[0031] In this invention, the mass ratio of the composite catalyst to polyvinyl alcohol is preferably 2~5:100, more preferably 3~4:100, and even more preferably 3.5:100.

[0032] In this invention, the segmented temperature-controlled acetalization reaction is preferably carried out under stirring conditions; the stirring speed is preferably 200~300 rpm, more preferably 230~270 rpm; the segmented temperature-controlled acetalization reaction preferably includes a first acetalization stage and a second acetalization stage carried out sequentially.

[0033] In this invention, the temperature of the first acetalization stage is preferably 50-55°C, more preferably 52°C, and the holding time is preferably 1.5-2 hours, more preferably 1.8 hours. In the first acetalization stage, this invention promotes the uniform dispersion of aromatic aldehyde compounds and facilitates the initial acetalization reaction.

[0034] In this invention, a first heating step is preferably included before the first acetalization stage; the rate of the first heating step is preferably 2~3℃ / min, more preferably 2.5℃ / min. By using the above heating rate, this invention can avoid excessively rapid heating that could lead to overly violent local reactions.

[0035] In this invention, the temperature of the second acetalization stage is preferably 60-70°C, more preferably 65°C; the holding time of the second acetalization stage is preferably stopped when the acid value of the reaction system stabilizes at 5-10 mg KOH / g and the degree of acetalization stabilizes at 30-45 mol%, specifically preferably 2.5-4 h, more preferably 3-4 h. This invention enhances the degree of acetalization reaction in the second acetalization stage. During the second acetalization stage, samples are taken every 1 hour to detect the acid value and degree of acetalization of the reaction system.

[0036] In this invention, a second heating step is preferably included before the second acetalization stage; the rate of the second heating step is preferably 2~3℃ / min, more preferably 2.5℃ / min. By using the above heating rate, this invention can avoid excessively rapid heating that could lead to overly intense local reactions.

[0037] After obtaining the first resin solution, the present invention mixes the first resin solution with a synergistic modifier for synergistic modification, adjusts the pH of the system to 6.5-7.5, and then filters through a ceramic membrane to obtain the polyvinyl acetal resin solution. In the present invention, the synergistic modifier preferably comprises a polyether compound containing epoxy groups and an organosilane coupling agent; the mass ratio of the polyether compound containing epoxy groups to the organosilane coupling agent is preferably 3-4:1, more preferably 3.5:1.

[0038] In this invention, the epoxy group-containing polyether compound preferably includes at least one of glycidyl ether polyoxyethylene ether (polyethylene glycol glycidyl ether) and glycidyl ether polyoxypropylene ether (polypropylene glycol glycidyl ether); the number average molecular weight of the epoxy group-containing polyether compound is preferably 500~1000, and the number average molecular weight deviation is preferably ≤±10%.

[0039] In this invention, the organosilane coupling agent preferably includes at least one selected from γ-glycidyl etherpropyltrimethoxysilane (KH560), γ-glycidyl etherpropyltriethoxysilane (KH561), and γ-methacryloyloxypropyltrimethoxysilane (KH570); the purity of the organosilane coupling agent is preferably ≥98%. This invention uses a synergistic modifier of the above specifications, which ensures a synergistic modification effect.

[0040] In this invention, the mass ratio of the synergistic modifier to polyvinyl alcohol is preferably 4~10:100, more preferably 6~8:100.

[0041] In this invention, the temperature for synergistic modification is preferably 75-85℃, more preferably 80℃, and the time for synergistic modification is preferably 1-2 hours, more preferably 1.5 hours. The synergistic modification is preferably carried out under stirring conditions; the stirring speed is preferably 200-350 rpm, more preferably 250-300 rpm. In the synergistic modification process of this invention, a cross-linked network is formed through the ring-opening reaction of epoxy groups and hydroxyl groups, and the hydrolysis and condensation reaction of the silane coupling agent, thereby completing the modification.

[0042] In this invention, the synergistic modification preferably further includes cooling the resulting reaction solution; the final cooling temperature is preferably room temperature.

[0043] In this invention, the reagent used to adjust the pH value of the system to 6.5-7.5 is preferably an alkaline solution; the alkaline solution is preferably a sodium bicarbonate solution; the mass fraction of the alkaline solution is preferably 5-10%, more preferably 8%.

[0044] In this invention, the preferred filtration accuracy of the ceramic membrane is 0.22 μm; the preferred filtration method is cross-flow filtration; and the preferred filtration pressure is 0.2~0.3 MPa, more preferably 0.25 MPa. This invention removes small amounts of insoluble impurities through ceramic membrane filtration without requiring multiple water washes, and improves filtration efficiency through pressurization.

[0045] The present invention also provides a polyvinyl alcohol acetal resin solution prepared by the above-described method, with a solid content of 12-18% and a viscosity (25°C) of 500-1500 mPa·s.

[0046] The present invention also provides the application of the polyvinyl acetal resin solution described above in the field of liquid crystal displays.

[0047] The polyvinyl acetal resin solution provided by this invention can be used in the field of liquid crystal displays, for example, in the preparation of polarizers.

[0048] In this invention, the preferred method of application includes the following steps: casting a polyvinyl acetal resin solution into a film to obtain a resin film. The resin film prepared by this invention can be used as a polarizer in the field of liquid crystal displays.

[0049] In this invention, the casting temperature is preferably 80~100℃, more preferably 90℃, and the holding time is preferably 30~60min, more preferably 40~50min.

[0050] In this invention, the thickness of the resin film is preferably 30-50 μm, more preferably 40 μm.

[0051] The resin film prepared by this invention has a tensile strength of 15~20MPa, an elongation at break of 120~150%, and a light transmittance decrease rate of ≤2% after high and low temperature cycling tests at -40~80℃.

[0052] To further illustrate the present invention, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments thereof.

[0053] In specific embodiments of the present invention, the sources and specifications of the reagents and materials used are as follows: Polyvinyl alcohol: degree of polymerization 1800~3500, degree of alcoholysis 98~99 mol%, degree of polymerization deviation ≤±5%; Aromatic aldehydes (benzaldehyde, p-hydroxybenzaldehyde, o-methoxybenzaldehyde, m-hydroxybenzaldehyde, p-methoxybenzaldehyde): purity ≥ 98%; Organic solvents (N,N-dimethylformamide, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether): analytical grade; Organic acids (formic acid, acetic acid, propionic acid): purity ≥ 99%; Organic amine salts (triethanolamine hydrochloride, diethanolamine hydrochloride, ethanolamine hydrochloride): purity ≥ 98%; Polyether compounds containing epoxy groups (glycidyl ether polyoxyethylene ether, glycidyl ether polyoxypropylene ether): number average molecular weight 500~1000, number average molecular weight deviation ≤±10%; Organosilane coupling agents (γ-glycidyl ether propyltrimethoxysilane, γ-glycidyl ether propyltriethoxysilane, γ-methacryloyloxypropyltrimethoxysilane): purity ≥98%; Ceramic membrane: filtration accuracy 0.22μm.

[0054] Example 1: 1) Raw material preparation: Polyvinyl alcohol (PVA-2599, degree of polymerization deviation ≤ ±5%) with a degree of polymerization of 2500 and a degree of alcoholysis of 98.5 mol%; benzaldehyde as acetalizing agent (purity ≥ 98%); composite solvent (DMF and ethylene glycol monomethyl ether in a volume ratio of 7.5:2.5, both analytical grade); composite catalyst (formic acid and triethanolamine hydrochloride in a mass ratio of 1:0.7, formic acid purity ≥ 99%, triethanolamine hydrochloride purity ≥ 98%); synergistic modifier (glycidyl ether polyoxyethylene ether, number average molecular weight of 800, mass ratio of glycidyl ether polyoxyethylene ether to KH560 of 3.5:1). 2) Dissolution of polyvinyl alcohol: 12g of polyvinyl alcohol was added to 100mL of composite solvent, nitrogen gas was introduced at a rate of 0.8L / min, and the mixture was stirred at 85℃ and 400rpm for 2.5h to obtain a polyvinyl alcohol solution. 3) Segmented temperature-controlled acetalization reaction: The molar ratio of benzaldehyde to polyvinyl alcohol was 0.4:1, and the mass ratio of composite catalyst to polyvinyl alcohol was 3:100. The polyvinyl alcohol solution was mixed with benzaldehyde and composite catalyst, kept at 52℃ for 1.8h, and then heated to 65℃ for 3.2h. The mixture was stirred at 250rpm. The final acid value was 7mgKOH / g and the degree of acetalization was 38mol%. The reaction solution was obtained. 4) Synergistic modification treatment: The synergistic modifier and the reaction solution were mixed at a mass ratio of 7:100 and stirred at 80℃ and 200rpm for 1.5h. 5) Post-treatment: The pH of the system was adjusted to 7.0 using an 8% sodium bicarbonate solution, and then cross-flow filtration was performed using a 0.22μm ceramic membrane (pressure 0.25MPa) to obtain a polyvinyl acetal resin solution.

[0055] Example 2: 1) Raw material preparation: Polyvinyl alcohol with a degree of polymerization of 2000 and a degree of alcoholysis of 98 mol%; p-hydroxybenzaldehyde as an acetalizing agent; composite solvent (DMF and ethylene glycol monomethyl ether, volume ratio 7:3); composite catalyst (formic acid and triethanolamine hydrochloride, mass ratio 1:0.5); single modifier (glycidyl ether polyoxyethylene ether, number average molecular weight of 500). 2) Dissolution of polyvinyl alcohol: Add 10g of polyvinyl alcohol to 100mL of composite solvent, purge with nitrogen gas at a rate of 0.5L / min, and stir at 80℃ and 300rpm for 2h to dissolve and obtain a polyvinyl alcohol solution. 3) Acetalization reaction: The molar ratio of p-hydroxybenzaldehyde to polyvinyl alcohol is 0.3:1, and the mass ratio of composite catalyst to polyvinyl alcohol is 2:100. The polyvinyl alcohol solution is mixed with p-hydroxybenzaldehyde and composite catalyst, and the temperature is increased to 60℃ at a programmed rate of 2℃ / min. The temperature is maintained for 4h, and the mixture is stirred at 200rpm. The final acid value is 5mgKOH / g to obtain the reaction solution. 4) Modification treatment: The modifier and reaction solution were stirred at 75°C and 350 rpm for 1 hour according to the mass ratio of modifier to polyvinyl alcohol of 3:100. 5) Post-treatment: The pH of the system was adjusted to 6.5 using a 5% sodium bicarbonate solution and cross-flow filtration was performed using a 0.22μm ceramic membrane (pressure 0.2MPa) to obtain a polyvinyl acetal resin solution.

[0056] Example 3: 1) Raw material preparation: Polyvinyl alcohol with a degree of polymerization of 3000 and a degree of alcoholysis of 99 mol%; o-methoxybenzaldehyde as an acetalizing agent; composite solvent (DMF and ethylene glycol monomethyl ether, volume ratio 8:2); composite catalyst (formic acid and triethanolamine hydrochloride, mass ratio 1:1); single modifier (glycidyl ether polyoxyethylene ether, number average molecular weight of 1000). 2) Dissolution of polyvinyl alcohol: Add 15g of polyvinyl alcohol to 100mL of composite solvent, purge with nitrogen gas at a rate of 1L / min, and stir at 90℃ and 500rpm for 3h to dissolve and obtain polyvinyl alcohol solution. 3) Acetalization reaction: The molar ratio of o-methoxybenzaldehyde to polyvinyl alcohol is 0.5:1, and the mass ratio of composite catalyst to polyvinyl alcohol is 5:100. The polyvinyl alcohol solution is mixed with o-methoxybenzaldehyde and composite catalyst, and the temperature is increased to 70℃ at a programmed rate of 3℃ / min. The mixture is kept at this temperature for 6 hours and stirred at 300 rpm. The final acid value is 10 mg KOH / g, and the reaction solution is obtained. 4) Modification treatment: The reaction solution and the modifier were mixed at a mass ratio of 8:100 and stirred at 85℃ and 250 rpm for 2 hours. 5) Post-treatment: The pH of the system was adjusted to 7.5 using a 10% sodium bicarbonate solution and cross-flow filtration was performed using a 0.22μm ceramic membrane (pressure 0.3MPa) to obtain a polyvinyl acetal resin solution.

[0057] Comparative Example 1: The preparation method of this comparative example is the same as that of Example 1, except that: ethanol is used instead of composite solvent and hydrochloric acid is used instead of composite catalyst; the acetalization reaction conditions are changed to react at 50°C for 6 hours; after the acetalization reaction is completed, the product is washed with water 4 times to remove salt, filtered, and dried to obtain polyvinyl alcohol acetal resin, and then the polyvinyl alcohol acetal resin is dissolved in ethanol to obtain a polyvinyl alcohol acetal resin solution with a solid content of 10%.

[0058] Comparative Example 2: The preparation method of this comparative example is the same as that of Example 1, except that a single glycidyl ether polyoxyethylene ether (number average molecular weight of 800) is used instead of the synergistic modifier.

[0059] Test Example 1: Performance tests were conducted on Examples 1-3 and Comparative Examples 1-2, and the test indicators were all performed in accordance with the relevant national standards, as detailed below: Solid content of resin solution: Refer to GB / T 1725-2007 "Determination of nonvolatile matter content in paints, varnishes and plastics"; Viscosity of resin solution (25℃): Refer to GB / T 2794-2013 "Determination of viscosity of adhesives"; Resin film thickness: Refer to GB / T 6672-2001 "Mechanical Measurement Method for Determination of Thickness of Plastic Films and Sheets"; Light transmittance of resin film: Refer to GB / T 2410-2008 "Determination of light transmittance and haze of transparent plastics"; Gloss at 60° angle of resin film: Refer to GB / T 9754-2007 "Determination of specular gloss at 20°, 60° and 85° of paint film without metallic pigments"; Water absorption rate of resin film: Refer to GB / T 1034-2008 "Determination of water absorption of plastics"; Tensile strength and elongation at break of resin film: Refer to GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets"; High and low temperature cycling test of resin film: Refer to GB / T 2423.1-2008 "Environmental testing of electrical and electronic products - Part 2: Test methods - Test A: Low temperature" and GB / T 2423.2-2008 "Environmental testing of electrical and electronic products - Part 2: Test methods - Test B: High temperature"; The resin film underwent damp heat aging test according to GB / T 2423.3-2016 "Environmental Testing Part 2: Test Methods, Cab: Constant Damp Heat Test". The test results are shown in Table 1. Table 1. Test results of Examples 1-3 and Comparative Examples 1-2:

[0060] Note: In the column on the environmental stability of the resin film, the data before the semicolon is the percentage decrease in light transmittance of the resin film after 10 cycles at -40℃ to 80℃; the data after the semicolon is the water absorption rate and swelling of the resin film after aging at 60℃ and 90% humidity for 72 hours.

[0061] As shown in Table 1, Examples 1-3 all employed the composite solvent, composite catalyst, and post-treatment method of the present invention, achieving a polyvinyl acetal resin yield of over 90%, significantly higher than Comparative Example 1 (65%). This indicates that the present invention, by adjusting the pH of the system with sodium bicarbonate solution combined with ceramic membrane filtration, replaces the traditional process of multiple water washing, effectively preventing the loss of polyvinyl acetal resin and effectively solving the problem of resin membrane turbidity caused by residual salt. In contrast, Comparative Example 1, due to incomplete salt removal through water washing, exhibited slight turbidity and obvious surface ripples in its resin membrane. The composite solvent of the present invention significantly enhances the solubility of aromatic aldehyde compounds, maintaining the light transmittance of the resin membranes in Examples 1-3 at 92-94% and the gloss at 90-93%, both superior to Comparative Example 1, demonstrating a more complete and uniform acetalization reaction in Examples 1-3.

[0062] Regarding the modification system: Example 1 uses epoxy polyether and organosilane coupling agent for synergistic modification, and the tensile strength of the resin film is 18 MPa, the elongation at break is 135%, and the resistance to high and low temperature and damp heat aging is excellent; while Comparative Example 2 uses a single modifier to replace the synergistic modifier, and the mechanical properties and environmental stability are significantly reduced, with a tensile strength of only 11 MPa and a light transmittance decrease rate of 4.3% after high and low temperature cycling. This shows that the synergistic modifier of the present invention enhances the comprehensive performance of the resin film by forming a stable cross-linking network, and is suitable for the complex application scenarios of polarizers in the field of liquid crystal display.

[0063] Process parameter adaptation: Example 2 uses low-polymerization-degree polyvinyl alcohol, with less reagent dosage and simplified synergistic modifier; Example 3 uses high-polymerization-degree polyvinyl alcohol, with more reagent dosage and higher process parameters. Both examples yielded polyvinyl alcohol acetal resin products with satisfactory performance, and the resin film thickness covered the range of 30-50 μm. This indicates that the preparation method provided by this invention has good adaptability to parameter fluctuations and can adjust the process and its parameters according to actual needs, balancing low-cost production with high-performance requirements. Example 1, as the example of the optimal parameter combination, shows that all performance aspects are at a balanced and excellent level.

[0064] Effectiveness of performance balance: The 24-hour water absorption rate of the resin films in Examples 1-3 was controlled at 5-8%, which ensured good hydrophilicity to adapt to the preparation method of polarizers and also had excellent water resistance. In contrast, the water absorption rate of Comparative Example 1 reached 15%, and the water absorption rate of Comparative Example 2 after aging reached 13%, neither of which could achieve a balance between hydrophilicity and water resistance. This shows the necessity of the present invention to precisely control the acetal degree (30-45 mol%) through segmented temperature control.

[0065] As can be seen from the above embodiments, this invention effectively overcomes technical bottlenecks such as poor solubility of aromatic aldehydes, low resin yield due to water washing and desalting, resin film turbidity, and performance imbalance by using composite solvents, composite catalysts, segmented temperature-controlled acetalization, and synergistic modifiers. The polyvinyl alcohol acetal resin solution prepared by this invention has a solid content of 12-18% and a viscosity of 500-1500 mPa·s (25℃), and can prepare resin films with a thickness of 30-50 μm. The resin film has a light transmittance ≥92% and a tensile strength ≥15 MPa (synergistic modification). It also has excellent hydrophilicity, water resistance, and environmental stability, and is not soluble in water or alcohol alone, fully meeting the performance requirements of polarizers in the liquid crystal display field.

[0066] In terms of process, this invention does not require multiple water washings, generates no large amounts of wastewater, has mild reaction conditions, and a simple process, making it suitable for industrial-scale production. At the same time, the process parameters are highly adaptable, allowing for adjustments to raw material specifications and reagent dosages based on cost and performance requirements, thus broadening the application scenarios.

[0067] In summary, the preparation method provided by this invention combines high product performance with industrial feasibility, offering an efficient and green approach to the preparation of polarizers in the field of liquid crystal displays, and has significant practical value and promotional potential.

[0068] The embodiments of the present invention have been described above; however, these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. All other embodiments obtained by those skilled in the art based on the above embodiments of the present invention without inventive effort are within the protection scope of the present invention.

Claims

1. A method for preparing a polyvinyl acetal resin solution, characterized in that, Includes the following steps: (1) Mix polyvinyl alcohol and a composite solvent to obtain a polyvinyl alcohol solution; The composite solvent includes a main solvent and a co-solvent; The main solvent includes N,N-dimethylformamide; The co-solvent includes at least one of ethylene glycol monomethyl ether and ethylene glycol monoethyl ether; (2) The polyvinyl alcohol solution is mixed with aromatic aldehyde compounds and a composite catalyst to carry out a segmented temperature-controlled acetalization reaction to obtain a first resin solution; The composite catalyst comprises organic acids and organic amine salts; (3) The first resin solution and the synergistic modifier are mixed for synergistic modification. After adjusting the pH value of the system to 6.5~7.5, the mixture is filtered through a ceramic membrane to obtain the polyvinyl acetal resin solution. The synergistic modifier comprises a polyether compound containing an epoxy group and an organosilane coupling agent.

2. The preparation method according to claim 1, characterized in that, The volume ratio of the main solvent to the co-solvent is 7~8:2~3; The mass ratio of the polyvinyl alcohol to the volume ratio of the composite solvent is (10~15) g: 100 mL.

3. The preparation method according to claim 1, characterized in that, The aromatic aldehyde compounds include at least one of benzaldehyde, p-hydroxybenzaldehyde, o-methoxybenzaldehyde, m-hydroxybenzaldehyde, and p-methoxybenzaldehyde; The molar ratio of the aromatic aldehyde compound to polyvinyl alcohol is 0.3~0.5:

1.

4. The preparation method according to claim 1, characterized in that, The organic acid includes at least one of formic acid, acetic acid and propionic acid; The organic amine salt includes at least one of triethanolamine hydrochloride, diethanolamine hydrochloride, and ethanolamine hydrochloride; The mass ratio of the organic acid to the organic amine salt is 1:0.5~1; The mass ratio of the composite catalyst to polyvinyl alcohol is 2~5:

100.

5. The preparation method according to claim 1, characterized in that, The segmented temperature-controlled acetalization reaction was carried out under stirring conditions; The stirring speed is 200~300 rpm; The segmented temperature-controlled acetalization reaction includes a first acetalization stage and a second acetalization stage performed sequentially. The temperature of the first acetalization stage is 50~55℃, and the holding time is 1.5~2h; The temperature for the second acetalization stage is 60~70℃; The second acetalization stage is stopped when the acid value of the reaction system stabilizes at 5~10 mgKOH / g and the degree of acetalization stabilizes at 30~45 mol%.

6. The preparation method according to claim 5, characterized in that, The process includes a first heating phase preceding the first acetalization stage; The first heating rate is 2~3℃ / min; The second acetalization stage is preceded by a second heating process; The second heating rate is 2~3℃ / min.

7. The preparation method according to claim 1, characterized in that, The epoxy group-containing polyether compound includes at least one of glycidyl ether polyoxyethylene ether and glycidyl ether polyoxypropylene ether. The number-average molecular weight of the epoxy-containing polyether compound is 500~1000, and the number-average molecular weight deviation is ≤±10%; The organosilane coupling agent includes at least one of γ-glycidyl ether propyltrimethoxysilane, γ-glycidyl ether propyltriethoxysilane and γ-methacryloyloxypropyltrimethoxysilane; The mass ratio of the epoxy-containing polyether compound to the organosilane coupling agent is 3~4:1; The mass ratio of the synergistic modifier to polyvinyl alcohol is 4~10:

100.

8. The preparation method according to claim 1, characterized in that, The temperature for synergistic modification is 75~85℃, and the time for synergistic modification is 1~2h; The synergistic modification is carried out under stirring conditions.

9. The polyvinyl acetal resin solution obtained by the preparation method according to any one of claims 1 to 8, characterized in that, The solid content is 12-18%, and the viscosity at 25℃ is 500-1500 mPa·s.

10. The application of the polyvinyl acetal resin solution according to claim 9 in the field of liquid crystal displays.

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