Preparation method of PVA raw material for a highly stretchable PVA optical film

Abstract

The invention discloses a preparation method of a PVA raw material for a highly stretchable PVA optical film. The method is to separately prepare a polyvinyl acetate with an ultra-low degree of polymerization and a polyvinyl acetate with a high degree of polymerization by a stepwise polymerization method, and then mix the two and carry out alcoholysis to obtain a PVA raw material with a bimodal molecular weight distribution. According to the preparation method of the invention, a PVA optical film with a uniform structure can be obtained, and the stretchability of the PVA optical film can be improved.

Description

Technical Field

[0001] The present invention relates to a preparation method of raw materials for a highly stretchable polyvinyl alcohol optical film. Background Art

[0002] As one of the key components determining the quality of liquid crystal displays, a polarizing film is prepared by iodine dyeing, stretching and drying a PVA optical film. The key performance indicators, polarization degree and polarization uniformity, are affected by the stretchability and stretching uniformity of the PVA optical film during the iodine dyeing and stretching process. Therefore, a high-performance polarizing film requires a PVA optical film with high stretchability. Improving the stretchability of the PVA optical base film is one of the key problems to be solved for replacing imported PVA optical films.

[0003] The PVA optical film is formed by dissolving, extruding, casting and drying a mixed solution of PVA, water, plasticizer, etc. The uniformity of the final crystal morphology structure of the film determines the stretchability of the PVA optical film. The purpose of adding a plasticizer is to disrupt the order between PVA molecular chains through hydrogen bond interaction with the PVA molecular chain, so as to inhibit excessive crystal growth and improve uniformity. Therefore, the magnitude of the interaction between the plasticizer and the PVA molecular chain and its distribution in the system determine the uniformity of the final crystal morphology structure of the PVA film. Currently reported plasticizers are mostly glycerol, polyglycerol, etc. However, due to the difference in the molecular structure between these plasticizers and the PVA molecular chain, the interaction between these plasticizers and the PVA molecular chain is poor. On the one hand, it leads to a weakened effect of disrupting the order between PVA molecular chains. On the other hand, it results in uneven distribution of the plasticizer in the system, and the obtained film crystals are also uneven, with poor stretchability and insufficient polarization degree of the obtained polarizing film. Moreover, due to the poor interaction between the plasticizer and the PVA molecular chain, during the processing and heat treatment of the PVA optical film, especially in the processing of thin PVA optical base films, problems such as glycerol volatilization polluting the oven and the film occur, seriously affecting the processing stability. For this reason, although there are methods to weaken the influence by reducing the line speed and reducing the drying load, this method not only has poor effects but also increases the production cost. Summary of the Invention

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

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

[0006] A preparation method of PVA raw material for a highly stretchable PVA optical film, characterized in that: ultra-low degree of polymerization polyvinyl acetate and high degree of polymerization polyvinyl acetate are respectively prepared by a stepwise polymerization method, and then the two are mixed and subjected to alcoholysis to obtain a PVA raw material with a bimodal molecular weight distribution. The specific steps are as follows:

[0007] Step 1: Prepare ultra-low degree of polymerization polyvinyl acetate

[0008] Using vinyl acetate as a reaction monomer and a reagent with a high chain transfer constant as a solvent, after the two are mixed, polymerization reaction is carried out under the action of an initiator, and then after removing monomers, a solution of ultra-low degree of polymerization polyvinyl acetate is obtained;

[0009] Step 2: Prepare high degree of polymerization polyvinyl acetate

[0010] Using vinyl acetate as a reaction monomer and methanol as a solvent, after the two are mixed, polymerization reaction is carried out under the action of an initiator, and then after removing monomers, a methanol solution of high degree of polymerization polyvinyl acetate is obtained;

[0011] Step 3: Mix and alcoholyze

[0012] Mix the solutions of polyvinyl acetate with two degrees of polymerization, add an alkali for alcoholysis, and then after washing and drying, a PVA raw material with a bimodal molecular weight distribution is obtained.

[0013] Further, in step 1, the solvent with a high chain transfer constant is at least one of toluene, ethanol, acetone and carbon tetrachloride.

[0014] Further, in step 1, the mass ratio of the solvent to the reaction monomer is preferably 5-100:1, more preferably 20-80:1.

[0015] Further: The degree of polymerization of the ultra-low degree of polymerization polyvinyl acetate obtained in step 1 is preferably 3-25, more preferably 5-15. If it is too high or too low, when alcoholyzed into PVA, due to its own crystallization or too small molecules, it will not play a role in promoting the structural uniformity. The degree of polymerization of the high degree of polymerization polyvinyl acetate obtained in step 2 is preferably 1500-5000, more preferably 2000-3000.

[0016] Further, in step 3, the mass ratio of the ultra-low degree of polymerization polyvinyl acetate to the high degree of polymerization polyvinyl acetate when mixing is preferably 0.1-20:100, more preferably 1-15:100. If the proportion of the ultra-low degree of polymerization polyvinyl acetate is too low, it will not play a role in promoting the structural uniformity after alcoholysis into PVA; if the proportion of the ultra-low degree of polymerization polyvinyl acetate is too high, the PVA optical film processed after alcoholysis will have poor mechanical properties.

[0017] Furthermore, the alcoholysis degree of the polyvinyl acetate after alcoholysis in step 3 is preferably above 99%, more preferably above 99.5%.

[0018] Furthermore, the sodium acetate impurity content in the polyvinyl alcohol obtained in step 3 is preferably below 0.5%, more preferably below 0.1%. Too high an impurity content may cause yellowing of the raw material.

[0019] Furthermore, in step 1, the polymerization reaction temperature is 60-120° C. and the time is 6-9 hours; in step 2, the polymerization reaction temperature is 40-60° C. and the time is 2-6 hours.

[0020] Furthermore, the drying temperature in step 3 is preferably not higher than 100° C., more preferably not higher than 80° C. Too high a temperature will cause the ultra-low polymerization degree component to degrade and turn yellow, making it unable to promote structural homogenization, and causing problems such as deterioration of the optical properties of the PVA raw material and the optical film.

[0021] Compared with the existing process, the beneficial effects of the present invention are embodied in:

[0022] (1) The present invention obtains a PVA raw material with a bimodal molecular weight distribution by a step-by-step polymerization method. The low molecular weight component is used to play a role similar to that of a plasticizer, destroying the order between molecular chains and improving the uniformity of the crystal morphology structure of the high molecular weight component of PVA. Since the molecular structures of the low molecular weight component and the high molecular weight component are completely consistent, the two interact strongly and have good compatibility. The plasticizing effect and dispersion uniformity in the system are much higher than those of the additionally added plasticizer reported so far; moreover, when an additional plasticizer is added to the raw material during the film processing process, the low molecular weight component can also play the role of a compatibilizer between the raw material and the externally added plasticizer, thereby improving the effect of the externally added plasticizer in destroying the order between PVA molecular chains and promoting its uniformity of dispersion in the system, thereby obtaining a PVA optical film with uniform structure and improving the stretchability of the PVA optical film.

[0023] (2) Since the low molecular weight components of the present invention interact strongly with the high molecular weight components, during the heat treatment process of the PVA optical film processing, there is no problem of glycerol volatilization contaminating the oven and the film as in the current process.

[0024] (3) The mixing of the high and low molecular weight polyvinyl acetate components of the present invention is carried out before alcoholysis, and the problem of uneven distribution due to different alcoholysis degrees, incomplete dissolution and mixing that occurs when alcoholysis is performed to form PVA and then mixed will not occur. The low molecular weight components of the present invention are evenly distributed in the system, and the obtained film structure is uniform.

[0025] (4) Compared with other raw materials, the PVA optical film processed from the PVA raw material obtained in the present invention has more uniform crystallization and excellent stretchability when processed into a polarizer. The polarization degree of the obtained polarizer reaches more than 99.9%. Detailed implementation manners

[0026] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below. Obviously, the described specific implementation manners are only a part of the implementation manners of the present invention, rather than all of them. Based on the specific implementation manners of the present invention, all other implementation manners obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0027] Example 1

[0028] Step 1: Prepare ultra-low degree of polymerization polyvinyl acetate

[0029] Add vinyl acetate and an ethanol solvent with a high chain transfer constant into a reaction flask at a mass ratio of 1:10, and add an AIBN initiator solution with a mass concentration of 1.2% accounting for 0.5% of the total volume thereto. React at 90 °C for 7 hours to obtain a mixed solution containing polyvinyl acetate, vinyl acetate, ethanol, etc. After removing monomers by a rotary evaporator, an ethanol solution of polyvinyl acetate with a degree of polymerization of 20 is obtained.

[0030] Step 2: Prepare high degree of polymerization polyvinyl acetate

[0031] Mix 800 g of monomer vinyl acetate, 200 g of methanol and an AIBN initiator solution with a concentration of 1.2% and a total mass of 10 g and add them into a reaction flask. After reacting at 50 °C for 4 hours, a mixed solution containing polyvinyl acetate, vinyl acetate, methanol, etc. is obtained. After removing monomers by a rotary evaporator, a methanol solution of polyvinyl acetate with a degree of polymerization of 2000 is obtained.

[0032] Step 3: Mixed alcoholysis

[0033] Mix the solutions of polyvinyl acetate with two degrees of polymerization at a mass ratio of low degree of polymerization PVAc: high degree of polymerization PVAc of 5:100, add a sodium hydroxide solution at a molar ratio of sodium hydroxide:PVAc of 0.02:100, react in a constant temperature water bath at 38 °C for 1 hour, and then dry in vacuum at 80 °C for 24 h to obtain a PVA raw material with a bimodal molecular weight distribution having an alcoholysis degree of more than 99.5% and a sodium acetate content of less than 0.1%.

[0034] Step 4: Dissolve PVA to prepare an optical film

[0035] Dissolve 1000 g of the above PVA raw material and 1 g of sodium dodecylbenzenesulfonate surfactant in 4000 g of deionized water under sealed conditions at 150 °C to prepare a PVA casting solution, and the dissolution reaction time is 10 h. The formed casting solution is degassed by an extruder, filtered, and then extruded from a die. After drying by a casting roller at 90 °C, it enters 11 drying rollers (the temperature ranges from 40 to 100 °C) to repeatedly dry the front and back sides of the film, and then enters a pneumatic floating oven for heat treatment, and finally is wound up to obtain a PVA optical film.

[0036] Example 2

[0037] This example is prepared by the same method and conditions as in Example 1, except that: in step 1, the mass ratio of vinyl acetate to ethanol is 1:20, and the degree of polymerization of the obtained low-degree-of-polymerization polyvinyl acetate is 10.

[0038] Example 3

[0039] This example is prepared by the same method and conditions as in Example 2, except that: in step 3, the mass ratio of low-degree-of-polymerization PVAc to high-degree-of-polymerization PVAc is 10:100.

[0040] Comparative Example 1

[0041] This comparative example directly uses the methanol solution of polyvinyl acetate with a degree of polymerization of 2000 prepared in step 2 of Example 1 to carry out alcoholysis under the same conditions to prepare a PVA raw material with a single-peak molecular weight distribution and an optical film based on it.

[0042] Comparative Example 2:

[0043] This comparative example prepares a PVA raw material with a single-peak molecular weight distribution by the same method as in Comparative Example 1, and prepares an optical film based on it by the following method:

[0044] Dissolve 909 g of the PVA raw material, 90.9 g of glycerol, and 1 g of sodium dodecylbenzenesulfonate surfactant in 4000 g of deionized water under sealed conditions at 150 °C to prepare a PVA casting solution, and the dissolution reaction time is 10 h. The formed casting solution is degassed by an extruder, filtered, and then extruded from a die. After drying by a casting roller at 90 °C, it enters 11 drying rollers (the temperature ranges from 40 to 100 °C) to repeatedly dry the front and back sides of the film, and then enters a pneumatic floating oven for heat treatment, and finally is wound up to obtain a PVA optical film.

[0045] Perform DSC tests on the PVA optical films prepared in the above examples and comparative examples to obtain the melting points of the optical films to reflect the structural uniformity of the optical films, and the results are shown in Table 1.

[0046] The stretchability of the PVA optical films prepared in the above examples and comparative examples was tested: First, a solution containing 0.5 g / L iodine and 20 g / L potassium iodide was prepared; Second, the PVA optical film was soaked in deionized water at 30 °C for 5 min, and then put into the above solution for iodine staining and stretching. The elongation at break when stretched to failure was recorded and denoted as the stretching ratio. The results are shown in Table 1.

[0047] The PVA optical films prepared in the above examples and comparative examples were subjected to iodine staining and stretching, and the polarization degree of the polarizer was tested: First, a solution containing 0.5 g / L iodine and 20 g / L potassium iodide was prepared; Second, the PVA film was soaked in deionized water at 30 °C for 5 min and then put into the above solution for iodine staining and stretching. After stretching to the maximum ratio, it was fixed in a solution containing boric acid and potassium iodide, and finally dried in an oven (40 °C). Protective films were attached to both sides of the dried film, and a polarizer was obtained after drying. The polarization degree of the obtained polarizer was tested using a spectrophotometer. The results are shown in Table 1.

[0048] As can be seen from Table 1, compared with Comparative Example 1, the melting points of the optical films obtained in all examples were lower, and the film stretching ratio and the polarization degree of the polarizer were higher. This shows that the bimodal PVA obtained in the present invention can effectively improve the structural uniformity, the stretchability of the film and the polarization degree of the polarizer. Among the several examples, the melting points of the samples in Examples 1, 2, and 3 decreased in turn, and the film stretching ratio and the polarization degree of the polarizer increased in turn. This shows that reducing the degree of polymerization of the low-polymerization-degree component or increasing the content of the low-polymerization-degree component is beneficial to the homogenization of the structure, and thus a film with higher stretchability and a polarizer with higher polarization degree can be obtained. Compared with Comparative Example 2, Example 3 had a lower film melting point, higher stretching ratio, and higher polarization degree of the polarizer. This shows that compared with the PVA system with the same proportion of glycerol added, the two-component PVA of the present invention has a better structural homogenization effect, the obtained film has better stretchability, and the polarization degree of the processed polarizer is higher, reaching more than 99.9%.

[0049] Table 1. Performance characterization results of the melting point, stretching ratio, and polarization degree of the PVA optical films corresponding to each example and comparative example

[0050]

[0051] The above are only exemplary embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. Preparation method of PVA raw material for highly stretchable PVA optical film, characterized in that: ultra-low degree of polymerization polyvinyl acetate and high degree of polymerization polyvinyl acetate are respectively prepared by a stepwise polymerization method, and then the two are mixed and subjected to alcoholysis to obtain a PVA raw material with a bimodal molecular weight distribution; the degree of polymerization of the ultra-low degree of polymerization polyvinyl acetate is 10, and the degree of polymerization of the high degree of polymerization polyvinyl acetate is 2000; the mass ratio of the ultra-low degree of polymerization polyvinyl acetate to the high degree of polymerization polyvinyl acetate when mixed is 5-10:

100.

2. The preparation method according to claim 1, characterized in that, comprises the following steps: Step 1, prepare ultra-low degree of polymerization polyvinyl acetate Using vinyl acetate as a reaction monomer and a reagent with a high chain transfer constant as a solvent, the two are mixed and subjected to a polymerization reaction under the action of an initiator, and then de-singled to obtain a solution of ultra-low degree of polymerization polyvinyl acetate; Step 2, prepare high degree of polymerization polyvinyl acetate Using vinyl acetate as a reaction monomer and methanol as a solvent, the two are mixed and subjected to a polymerization reaction under the action of an initiator, and then de-singled to obtain a methanol solution of high degree of polymerization polyvinyl acetate; Step 3, mixed alcoholysis Mix the solutions of polyvinyl acetate with two degrees of polymerization, add an alkali for alcoholysis, and then after washing and drying, obtain a PVA raw material with a bimodal molecular weight distribution.

3. The preparation method according to claim 2, characterized in that: in Step 1, the solvent with a high chain transfer constant is at least one of toluene, ethanol, acetone and carbon tetrachloride.

4. The preparation method according to claim 2, characterized in that: in Step 1, the mass ratio of the solvent to the reaction monomer is 5-100:

1.

5. The preparation method according to claim 2, characterized in that: in Step 1, the temperature of the polymerization reaction is 60-120 °C and the time is 6-9 h; in Step 2, the temperature of the polymerization reaction is 40-60 °C and the time is 2-6 h.

6. The preparation method according to claim 2, characterized in that: in Step 3, the drying temperature is not higher than 100 °C.

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