Polyvinyl alcohol, preparation method thereof and optical film

Abstract

The invention relates to the technical field of polyvinyl alcohol preparation, and discloses a polyvinyl alcohol preparation method, which comprises: (1) in the presence of a first solvent and an initiator, carrying out a polymerization reaction on vinyl ester to obtain a polymerization liquid, and carrying out gel particle removal treatment on the polymerization liquid to obtain polyvinyl ester; (2) in the presence of a second solvent and first alkali, performing alcoholysis on polyvinyl ester to obtain a polyvinyl alcohol initial product; and (3) in the presence of second alkali, carrying out homogenization treatment on the polyvinyl alcohol initial product to obtain polyvinyl alcohol with a homogenized alcoholysis degree, and then carrying out purification treatment to obtain the polyvinyl alcohol. According to the method provided by the invention, the special polyvinyl alcohol product with narrow molecular weight distribution, high polymerization degree, high alcoholysis degree, concentrated alcoholysis degree distribution and high purity can be prepared, the processing performance is excellent, the special polyvinyl alcohol product can be directly used for preparing an optical material, and the transparency, the swelling degree, the dyeing performance and the tensile property of a PVA optical film can be improved.

Description

Technical Field

[0001] The invention relates to the technical field of polyvinyl alcohol preparation, and in particular to polyvinyl alcohol and a preparation method thereof and an optical film. Background Art

[0002] Polyvinyl alcohol (PVA) is a widely used water-soluble polymer that can be prepared into thin film products with excellent oil resistance, oxygen resistance, tear resistance, wear resistance, transparency, antistatic and corrosion resistance. Among them, PVA optical film is an important material for manufacturing polarizers for liquid crystal displays, which is used to achieve high brightness and high contrast functions of liquid crystal displays.

[0003] The PVA used in the field of optical materials is significantly different from ordinary PVA products in terms of structure, performance and purity. It is required that the alcoholysis degree of PVA reaches more than 95.5%, the purity reaches more than 93.5%, the intermolecular force is small, the crystallinity is small, and the swelling performance is excellent. Due to the difficulty of preparation and high technical requirements, there are currently no commercialized PVA products on the market that can be directly used for optical materials. The conventional PVA synthesis process does not strictly control the molecular weight and distribution, alcoholysis degree and distribution of the product. The resulting PVA is prone to poor processability during processing, resulting in a decrease in the optical performance of the processed optical material, which is difficult to meet the use requirements.

[0004] Therefore, it is urgent to develop an improved polyvinyl alcohol preparation process to improve the molecular weight distribution, alcoholysis degree distribution and processing performance of polyvinyl alcohol to meet the performance requirements of optical films for polyvinyl alcohol resin materials. Summary of the invention

[0005] The present invention aims to solve the problem that the polyvinyl alcohol product prepared by the existing preparation process has the disadvantages of wide molecular weight distribution, unconcentrated alcoholysis degree distribution, and contains more oligomers and impurities, which makes it difficult to meet the performance requirements of optical materials for polyvinyl alcohol. A polyvinyl alcohol, a preparation method thereof and an optical film are provided.

[0006] In order to achieve the above object, the present invention provides a method for preparing polyvinyl alcohol in a first aspect, comprising:

[0007] (1) subjecting a vinyl ester to a polymerization reaction in the presence of a first solvent and an initiator to obtain a polymerization solution, and subjecting the polymerization solution to a degelling particle treatment to obtain a polyvinyl ester;

[0008] (2) alcoholyzing the polyvinyl ester in the presence of a second solvent and a first base to obtain a primary polyvinyl alcohol product;

[0009] (3) In the presence of a second base, the polyvinyl alcohol primary product is homogenized to obtain polyvinyl alcohol with a uniform alcoholysis degree, and then purified to obtain polyvinyl alcohol.

[0010] The second aspect of the present invention provides polyvinyl alcohol prepared by the method described in the foregoing first aspect.

[0011] The third aspect of the present invention provides an optical film containing the polyvinyl alcohol described in the foregoing second aspect.

[0012] The preparation method of polyvinyl alcohol provided by the present invention removes agglomerated gel particles in the polymerization solution obtained by polymerizing raw material vinyl ester, optimizes the purity, molecular weight and molecular weight distribution of the polymerization product, and combines homogenization treatment of the alcoholysis product in the presence of alkali and subsequent purification treatment to prepare a special polyvinyl alcohol product with a narrow molecular weight distribution, high degree of polymerization, high degree of alcoholysis, concentrated alcoholysis degree distribution and high purity. It has excellent processing performance and can be directly used to prepare optical materials, enabling the prepared PVA optical film to have the characteristics of high transparency, high swelling degree, good dyeing performance and excellent tensile performance. Specific Embodiments

[0013] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values and individual point values of each range, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0014] The following details the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only for explaining and illustrating the present invention, and are not used to limit the present invention.

[0015] The first aspect of the present invention provides a method for preparing polyvinyl alcohol, comprising:

[0016] (1) In the presence of a first solvent and an initiator, polymerize vinyl ester to obtain a polymerization solution, and subject the polymerization solution to de-gel particle treatment to obtain polyvinyl ester;

[0017] (2) In the presence of a second solvent and a first alkali, subject the polyvinyl ester to alcoholysis to obtain a crude polyvinyl alcohol product;

[0018] (3) In the presence of a second alkali, subject the crude polyvinyl alcohol product to homogenization treatment to obtain polyvinyl alcohol with a homogenized degree of alcoholysis, and then perform purification treatment to obtain polyvinyl alcohol.

[0019] According to the present invention, in the preparation method, in step (1), the vinyl ester may be selected from at least one of vinyl formate, vinyl acetate, vinyl laurate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl versatate, vinyl stearate and vinyl benzoate, and preferably vinyl acetate.

[0020] According to the present invention, in the preparation method, in step (1), the vinyl acetate is further preferably synthesized by the natural gas acetylene route. Preferably, the purity of vinyl acetate is ≥99.9%, and the activity is 540 - 570 s.

[0021] According to the present invention, in the preparation method, in step (1), the first solvent may be selected from at least one of methanol, ethanol and methyl acetate, and preferably methanol.

[0022] According to the present invention, in the preparation method, in step (1), the initiator may be any initiator that can initiate the formation of free radicals of vinyl ester in solution and generate polyvinyl ester through solution polymerization reaction. Preferably, the initiator may be selected from at least one of tert-butyl peroxypivalate, tert-butyl peroxyneodecanoate, benzoyl peroxide, azobisisobutyronitrile and azobisisoheptonitrile.

[0023] According to the present invention, in the preparation method, in step (1), preferably, the weight ratio of the first solvent to the vinyl ester is (5 - 20):100, and more preferably (10 - 15):100. By controlling the ratio of the first solvent to the vinyl ester within the above range, it is beneficial to control the degree of polymerization of the polymerization product. If the weight ratio of the first solvent to the vinyl ester is too low, the degree of polymerization of the obtained polyvinyl ester is too low, which is not conducive to the optical properties of the product; if the weight ratio of the first solvent to the vinyl ester is too high, it will make the operation during subsequent product processing difficult.

[0024] According to the present invention, in the preparation method, in step (1), preferably, the weight ratio of the initiator to the vinyl ester is (0.1 - 1.5):100.

[0025] According to the present invention, in the preparation method, in step (1), there is no particular limitation on the feeding method of each raw material and the process of the polymerization reaction, and it can be carried out in a conventional manner in the art. Preferably, the conditions of the polymerization reaction include: the temperature is 58 - 62 °C, and the time is 3.5 - 4 h.

[0026] In the present invention, by controlling the ratio of the first solvent to the vinyl ester and the reaction conditions in the polymerization reaction, the conversion rate of the vinyl ester in the polymerization reaction is 35 - 37%, which is conducive to obtaining polyvinyl ester with a narrow molecular weight distribution. If the conversion rate of the vinyl ester is too low, the production efficiency decreases; if the conversion rate of the vinyl ester is too high, the molecular weight distribution of the polyvinyl ester becomes wider, which is not conducive to the optical properties of the product.

[0027] According to the present invention, polyvinyl ester gel is generated during the polymerization reaction and exists in the polymerization solution in the form of gel particles. These gel particles are difficult to be completely alcoholyzed during the alcoholysis process, resulting in an uneven distribution of the alcoholysis degree of the product, which in turn affects the swelling solubility and optical properties of the polyvinyl alcohol product. The present invention conducts a de-gel particle treatment on the polymerization solution, that is, separates and removes the gel particles from the polymerization solution, which is conducive to improving the purity of the polyvinyl ester obtained by the polymerization of raw material monomers and the uniformity of subsequent alcoholysis (concentrated alcoholysis degree distribution).

[0028] According to the present invention, in step (1) of the preparation method, the average particle size of the gel particles is 10 - 45 μm.

[0029] In the present invention, the average particle size (D50) of the gel particles is measured by a laser particle size analyzer.

[0030] According to the present invention, in step (1) of the preparation method, the de-gel particle treatment can be carried out by filtration separation. Preferably, a filter screen can be used to filter and remove the gel particles in the polymerization solution. Preferably, the average pore size of the filter screen is 15 - 35 μm.

[0031] According to the present invention, in step (1) of the preparation method, preferably, the weight average molecular weight of the polyvinyl ester is 150000 - 300000 g / mol, and the molecular weight distribution index ≤ 2.

[0032] According to the present invention, in step (1) of the preparation method, it further includes: after the de-gel particle treatment, separating the polymerization solution to remove unreacted vinyl ester monomers. Preferably, after the separation treatment, the content of unreacted monomers in the obtained polyvinyl ester is ≤ 0.5 wt%.

[0033] In the present invention, in step (1), there are no specific limitations on the method and conditions of the separation treatment, as long as the unreacted vinyl ester monomers can be separated out, for example, rectification or blowing can be used.

[0034] According to the present invention, in step (2) of the preparation method, preferably, the second solvent can be selected from at least one of methanol, ethanol, and methyl acetate, and preferably methanol.

[0035] According to the present invention, in the preparation method, in step (2), preferably, the first base may be selected from at least one of NaOH, KOH, and CH 3 ONa (sodium methoxide), and preferably NaOH.

[0036] According to the present invention, in the preparation method, in step (2), preferably, the weight ratio of the second solvent to the polyvinyl ester is 100:(15 - 25). By controlling the ratio of the second solvent to the polyvinyl ester within the above range, the polyvinyl ester can be fully dissolved in the second solvent, which is beneficial to obtaining a polyvinyl alcohol product with a more concentrated degree of alcoholysis distribution.

[0037] According to the present invention, in the preparation method, in step (2), preferably, the molar ratio of the first base to the polyvinyl ester is (0.001 - 0.015):1, and more preferably (0.005 - 0.01):1. By controlling the ratio of the first base to the polyvinyl ester within the above range, it is beneficial to control the target degree of alcoholysis. If the molar ratio of the first base to the polyvinyl ester is too high, the reaction rate is relatively fast, which is not conducive to obtaining a polyvinyl alcohol product with a concentrated degree of alcoholysis distribution; if the molar ratio of the first base to the polyvinyl ester is too low, the production efficiency decreases.

[0038] According to the present invention, in the preparation method, in step (2), preferably, the first base is fed in the form of an alkali solution. The alkali solution is preferably a solution with an alkali concentration of 20 - 45 g / L formed by the first base and a third solvent. Among them, the third solvent may be selected from at least one of methanol, ethanol, and methyl acetate. Preferably, the third solvent has the same type of substance as the second solvent. For example, they can both be methanol.

[0039] According to the present invention, in the preparation method, in step (2), preferably, the conditions for alcoholysis include: the temperature is 40 - 50 °C, and the time is 20 - 40 min.

[0040] According to the present invention, in the preparation method, in step (2), preferably, the degree of alcoholysis of the polyvinyl alcohol primary product is ≥99.5%; in the molecular chain of the polyvinyl alcohol primary product, the content of syndiotactic stereoisomers is 27 - 31 wt%.

[0041] In the present invention, the degree of alcoholysis is measured by the method specified in GB / T 12010.2 - 2010. In the molecular chain of the polyvinyl alcohol primary product, the content of syndiotactic stereoisomers is measured by 1H - NMR nuclear magnetic resonance hydrogen spectrum.

[0042] According to the present invention, in the preparation method, step (2) further includes pressing and crushing the initial polyvinyl alcohol product. The present invention has no particular limitation on the pressing and crushing, and conventional methods and parameters in the art can be used.

[0043] According to the present invention, in the preparation method, in step (3), preferably, the second base can be selected from at least one of NaOH, KOH, and CH 3 ONa, and more preferably NaOH.

[0044] According to the present invention, in the preparation method, in step (3), the homogenization treatment can be implemented by mixing the initial polyvinyl alcohol product with an aqueous solution of the second base. Through the homogenization treatment, the degree of alcoholysis of the initial polyvinyl alcohol product can be made higher and more concentrated, and a mixed slurry containing polyvinyl alcohol with a homogenized degree of alcoholysis is obtained.

[0045] According to the present invention, preferably, the conditions of the homogenization treatment include: pH value is 8 - 12, temperature is 20 - 40 °C, and time is 20 - 40 min.

[0046] According to the present invention, in the preparation method, in step (3), the purification treatment includes pickling treatment and water washing treatment carried out in sequence.

[0047] According to the present invention, the pickling treatment can be implemented by mixing the mixed slurry containing polyvinyl alcohol with a homogenized degree of alcoholysis with an acid. Through the pickling treatment, it is more conducive to removing the oligomers and alcoholysis by-products entrained in the polyvinyl alcohol with a homogenized degree of alcoholysis.

[0048] According to the present invention, preferably, the conditions of the pickling treatment include: pH value is 3 - 6, temperature is 20 - 40 °C, and time is 3 - 30 min.

[0049] In the present invention, the types of acids used in the pickling treatment are limited relatively widely. For example, it can be acetic acid or hydrochloric acid.

[0050] According to the present invention, through the water washing treatment, small molecule by-products, polyvinyl alcohol with low molecular weight and low degree of alcoholysis, and impurities such as salts remaining in the product can be effectively removed. Preferably, the conditions of the water washing include: temperature is 20 - 40 °C, and time is 30 - 90 min.

[0051] According to a preferred embodiment of the present invention, the water washing treatment is carried out under the condition of introducing nitrogen. The nitrogen gas flow can stir the material, which is conducive to sufficient washing and avoids the problem of difficult washing caused by the agglomeration of polyvinyl alcohol products. Preferably, the number of washing times is controlled to be 2 - 3 times.

[0052] The invention removes agglomerated gel particles in a polymer solution obtained by polymerizing a raw material vinyl ester, optimizes the purity, molecular weight and distribution of the polymer product, and combines homogenization treatment of the alcoholysis product with subsequent purification treatment to prepare a special polyvinyl alcohol product with narrow molecular weight distribution, high polymerization degree, high alcoholysis degree, concentrated alcoholysis degree distribution and high purity. Specifically, the molecular weight distribution index of the obtained polyvinyl alcohol is ≤1.9, the polymerization degree is 2000-3000, the alcoholysis degree is 99.85-99.95 mol%, the alcoholysis degree range is ≤0.1 mol%, the alcoholysis degree is high and the distribution is concentrated, and the sodium acetate content is ≤0.02%, which can well meet the performance requirements of optical materials, especially optical films for polarizers, for polyvinyl alcohol raw materials.

[0053] In the present invention, multiple samples are taken from the prepared polyvinyl alcohol product, and alcoholysis tests are performed respectively, wherein the difference between the highest alcoholysis value and the lowest alcoholysis value obtained by the test is defined as the alcoholysis range of the polyvinyl alcohol product. The specific test process of the alcoholysis range can be: 100 samples are randomly taken from the polyvinyl alcohol product, and they are pressed into 100 PVA test films respectively, and the above 100 PVA test films are subjected to infrared spectrum test to obtain infrared spectra, and the peak area integration method of the methyl and methylene combined frequency peak (about 1450nm) and the methyl absorption peak (about 1380nm) is set, and the method is used to process all the above measured infrared spectra to obtain multiple groups of two-peak ratio data, and multiple alcoholysis values ​​of the polyvinyl alcohol sample are obtained after correction with the alcoholysis degree of chemical analysis, and the alcoholysis distribution value and trend diagram are calculated by statistical processing, and the alcoholysis maximum value minus the alcoholysis minimum value in all alcoholysis test results of the above samples is used as the alcoholysis range of the polyvinyl alcohol product.

[0054] The second aspect of the present invention provides polyvinyl alcohol prepared by the method described in the first aspect.

[0055] According to the present invention, the polyvinyl alcohol prepared by the method of the present invention has the characteristics of narrow molecular weight distribution, high degree of polymerization, high alcoholysis degree, concentrated alcoholysis degree distribution, high purity, excellent processing performance, specifically, the molecular weight distribution index is ≤1.9, the degree of polymerization is 2000-3000, the alcoholysis degree is 99.85-99.95mol%, the alcoholysis degree range is ≤0.1mol%, the alcoholysis degree is high and the distribution is concentrated, and the sodium acetate content is ≤0.02%. The polyvinyl alcohol product can be directly used to prepare optical materials. Compared with the polyvinyl alcohol product prepared by the conventional method, the PVA optical film can have higher transparency, higher swelling degree, better dyeing performance and tensile performance.

[0056] The third aspect of the present invention provides an optical film comprising the polyvinyl alcohol described in the second aspect.

[0057] The optical film provided by the present invention can be obtained by preparing a film-forming stock solution from polyvinyl alcohol described in the second aspect of the present invention through a conventional dissolution method (or methods such as twin-screw plasticizing extrusion), casting, and drying processes. Compared with the optical film prepared using a conventional polyvinyl alcohol product, the crystal point or gel content is significantly reduced, and it has higher transparency, higher swelling degree, better dyeing performance, and tensile performance.

[0058] The present invention will be described in detail below through examples. In the following examples and comparative examples,

[0059] Vinyl acetate: Synthesized by the natural gas acetylene route, with a purity of 99.92% and an activity of 550 s.

[0060] Unless otherwise specified, the materials used are all ordinary commercially available products.

[0061] Example 1

[0062] (1) A methanol solution of vinyl acetate (where the weight ratio of methanol to vinyl acetate is 11:100) is subjected to a polymerization reaction in the presence of azobisisobutyronitrile (the weight ratio of azobisisobutyronitrile to vinyl acetate is 0.5:100) (the reaction temperature is 60 °C and the reaction time is 3.5 h) to obtain a polymerization solution containing polyvinyl acetate;

[0063] The gel particles (average particle size of 30 μm) in the above polymerization solution are filtered out using a filter screen (average pore size of 20 μm), and the unreacted vinyl acetate monomer in the filtered polymerization solution is blown off to obtain a polymerization solution containing polyvinyl acetate after monomer removal (the weight-average molecular weight of polyvinyl acetate is 202311 g / mol, the molecular weight distribution index is 1.9, and the content of unreacted monomer in polyvinyl acetate is 0.3%); in the above polymerization reaction, the conversion rate of vinyl acetate is 37%;

[0064] (2) The polymerization solution containing polyvinyl acetate after monomer removal is formulated with methanol into a polyvinyl acetate-methanol solution (where the weight ratio of methanol to polyvinyl acetate is 100:15), and is mixed at high speed with a NaOH-methanol solution with a NaOH concentration of 30 g / L (the molar ratio of NaOH to polyvinyl acetate is 0.006:1) for alcoholysis. The alcoholysis temperature is 45 °C and the alcoholysis time is 40 min. After the alcoholysis is completed, acetic acid with an equimolar amount to NaOH is added to terminate the reaction, obtaining a primary polyvinyl alcohol product; it is measured that the alcoholysis degree of the above primary polyvinyl alcohol product is 99.5 mol%, and the syndiotactic content is 29 wt%;

[0065] The above primary polyvinyl alcohol product is pressed and crushed to obtain a primary polyvinyl alcohol product with an average particle size of 0.75 mm;

[0066] (3) Mix the above-mentioned initial product of polyvinyl alcohol with an average particle size of 0.75 mm with an aqueous NaOH solution (pH value of 8.5), and use the aqueous NaOH solution to homogenize the initial product of polyvinyl alcohol (the temperature of the homogenization treatment is 40 °C, and the time of the homogenization treatment is 30 min) to obtain a mixed slurry containing polyvinyl alcohol with a homogenized degree of alcoholysis;

[0067] Add acetic acid to the above-mentioned mixed slurry containing polyvinyl alcohol with a homogenized degree of alcoholysis, adjust the pH value of the slurry to 5 for pickling treatment (the temperature of the pickling treatment is 35 °C, and the time of the pickling treatment is 25 min), and then perform centrifugal dehydration to obtain the pickled product; Under the condition of introducing nitrogen, wash the above-mentioned pickled product twice with pure water (the temperature of the first water wash is 30 °C and the time is 30 min; the temperature of the second water wash is 20 °C and the time is 20 min), use the nitrogen gas flow to stir the material for sufficient washing, and then perform centrifugal dehydration to obtain polyvinyl alcohol (denoted as P1);

[0068] The molecular weight distribution index, degree of polymerization, degree of alcoholysis, range of degree of alcoholysis and sodium acetate content of P1 are shown in Table 1.

[0069] Example 2

[0070] (1) Polymerize a methanol solution of vinyl acetate (where the weight ratio of methanol to vinyl acetate is 15:100) in the presence of azobisisobutyronitrile (the weight ratio of azobisisobutyronitrile to vinyl acetate is 0.7:100) (the reaction temperature is 60 °C, and the reaction time is 3.5 h) to obtain a polymerization solution containing polyvinyl acetate;

[0071] Filter out the gel particles (average particle size of 26 μm) in the above-mentioned polymerization solution using a filter screen (average pore size of 20 μm), and blow off the unreacted vinyl acetate monomer in the filtered polymerization solution to obtain a polymerization solution containing polyvinyl acetate after monomer removal (the weight-average molecular weight of polyvinyl acetate is 189398 g / mol, the molecular weight distribution index is 1.7, and the content of unreacted monomer in polyvinyl acetate is 0.2%); In the above-mentioned polymerization reaction, the conversion rate of vinyl acetate is 36%;

[0072] (2) The polymer solution containing polyvinyl acetate after the above-mentioned single removal is formulated with methanol into a polyvinyl acetate-methanol solution (where the weight ratio of methanol to polyvinyl acetate is 100:20), and is rapidly mixed with a NaOH-methanol solution with a NaOH concentration of 30 g / L (the molar ratio of NaOH to polyvinyl acetate is 0.01:1) for alcoholysis. The alcoholysis temperature is 45 °C, the alcoholysis time is 40 min. After the alcoholysis is completed, acetic acid with an equimolar amount to NaOH is added to terminate the reaction, and a crude polyvinyl alcohol product is obtained. After measurement, the alcoholysis degree of the above-mentioned crude polyvinyl alcohol product is 99.6 mol%, and the syndiotactic content is 28 wt%.

[0073] The above-mentioned crude polyvinyl alcohol product is pressed and pulverized to obtain a crude polyvinyl alcohol product with an average particle size of 0.75 mm.

[0074] (3) The above-mentioned crude polyvinyl alcohol product with an average particle size of 0.75 mm is mixed with an aqueous NaOH solution (pH value of 9.5), and the crude polyvinyl alcohol product is homogenized with the aqueous NaOH solution (the temperature of the homogenization treatment is 40 °C, the time of the homogenization treatment is 30 min), and a mixed slurry containing polyvinyl alcohol with a homogenized alcoholysis degree is obtained.

[0075] Acetic acid is added to the above-mentioned mixed slurry containing polyvinyl alcohol with a homogenized alcoholysis degree, and the pH value of the slurry is adjusted to 4 for pickling treatment (the temperature of the pickling treatment is 35 °C, the time of the pickling treatment is 25 min), and then centrifugal dehydration is carried out to obtain a pickled product. Under the condition of introducing nitrogen, the above-mentioned pickled product is washed twice with pure water (the temperature of the first water wash is 30 °C, the time is 30 min; the temperature of the second water wash is 20 °C, the time is 20 min), and the nitrogen gas flow is used to stir the material for sufficient washing, and then centrifugal dehydration is carried out to obtain polyvinyl alcohol (denoted as P2).

[0076] The molecular weight distribution index, degree of polymerization, alcoholysis degree, alcoholysis degree range and sodium acetate content of P2 are shown in Table 1.

[0077] Example 3

[0078] (1) A methanol solution of vinyl acetate (where the weight ratio of methanol to vinyl acetate is 5:100) is polymerized in the presence of azobisisobutyronitrile (the weight ratio of azobisisobutyronitrile to vinyl acetate is 0.5:100) (the reaction temperature is 58 °C, the reaction time is 4 h) to obtain a polymer solution containing polyvinyl acetate.

[0079] The gel particles (average particle size of 35 μm) in the above polymerization solution were filtered out using a filter screen (average pore size of 20 μm), and the unreacted vinyl acetate monomer in the filtered polymerization solution was blown off to obtain a polymerization solution containing polyvinyl acetate after monomer removal (the weight-average molecular weight of polyvinyl acetate was 235526 g / mol, the molecular weight distribution index was 1.8, and the content of unreacted monomer in polyvinyl acetate was 0.35%); in the above polymerization reaction, the conversion rate of vinyl acetate was 35%;

[0080] (2) The polymerization solution containing polyvinyl acetate after monomer removal was formulated with methanol into a polyvinyl acetate-methanol solution (where the weight ratio of methanol to polyvinyl acetate was 100:20), and was mixed at high speed with a NaOH-methanol solution with a NaOH concentration of 30 g / L (the molar ratio of NaOH to polyvinyl acetate was 0.001:1) for alcoholysis. The alcoholysis temperature was 50 °C, the alcoholysis time was 40 min, and after the alcoholysis was completed, acetic acid with an equimolar amount to NaOH was added to terminate the reaction, obtaining a primary product of polyvinyl alcohol; it was measured that the alcoholysis degree of the above primary product of polyvinyl alcohol was 99.7 mol%, and the syndiotactic content was 30 wt%;

[0081] The above primary product of polyvinyl alcohol was pressed and crushed to obtain a primary product of polyvinyl alcohol with an average particle size of 0.75 mm;

[0082] (3) The primary product of polyvinyl alcohol with an average particle size of 0.75 mm was mixed with an aqueous NaOH solution (pH value of 11), and the primary product of polyvinyl alcohol was homogenized using the aqueous NaOH solution (the temperature of the homogenization treatment was 30 °C, and the time of the homogenization treatment was 30 min) to obtain a mixed slurry containing polyvinyl alcohol with a homogenized alcoholysis degree;

[0083] Acetic acid was added to the above mixed slurry containing polyvinyl alcohol with a homogenized alcoholysis degree, and the pH value of the slurry was adjusted to 3 for pickling treatment (the temperature of the pickling treatment was 35 °C, and the time of the pickling treatment was 25 min), and then centrifugal dehydration was carried out to obtain the pickled product; under the condition of introducing nitrogen, the above pickled product was washed twice with pure water (the temperature of the first water wash was 30 °C, and the time was 30 min; the temperature of the second water wash was 20 °C, and the time was 20 min), and the nitrogen gas flow was used to stir the material for sufficient washing, and then centrifugal dehydration was carried out to obtain polyvinyl alcohol (denoted as P3).

[0084] The molecular weight distribution index, degree of polymerization, alcoholysis degree, alcoholysis degree range, and sodium acetate content of P3 are shown in Table 1.

[0085] Example 4

[0086] (1) A methanol solution of vinyl acetate (where the weight ratio of methanol to vinyl acetate is 20:100) is subjected to a polymerization reaction in the presence of azobisisobutyronitrile (where the weight ratio of azobisisobutyronitrile to vinyl acetate is 0.8:100) (the reaction temperature is 62 °C and the reaction time is 4 h) to obtain a polymerization solution containing polyvinyl acetate;

[0087] The gel particles (average particle size of 25 μm) in the above polymerization solution are filtered out using a filter screen (average pore size of 20 μm), and the unreacted vinyl acetate monomer in the filtered polymerization solution is blown off to obtain a polymerization solution containing polyvinyl acetate after monomer removal (the weight-average molecular weight of polyvinyl acetate is 189005 g / mol, the molecular weight distribution index is 1.8, and the content of unreacted monomer in polyvinyl acetate is 0.2%); in the above polymerization reaction, the conversion rate of vinyl acetate is 36%;

[0088] (2) The polymerization solution containing polyvinyl acetate after monomer removal is formulated with methanol into a polyvinyl acetate-methanol solution (where the weight ratio of methanol to polyvinyl acetate is 100:25), and is mixed at high speed with a NaOH-methanol solution with a NaOH concentration of 30 g / L (the molar ratio of NaOH to polyvinyl acetate is 0.015:1) for alcoholysis. The alcoholysis temperature is 50 °C and the alcoholysis time is 30 min. After the alcoholysis is completed, acetic acid with an equimolar amount to NaOH is added to terminate the reaction, obtaining a primary product of polyvinyl alcohol; it is measured that the alcoholysis degree of the above primary product of polyvinyl alcohol is 99.7 mol%, and the syndiotactic content is 48 wt%;

[0089] The above primary product of polyvinyl alcohol is pressed and pulverized to obtain a primary product of polyvinyl alcohol with an average particle size of 0.75 mm;

[0090] (3) The primary product of polyvinyl alcohol with an average particle size of 0.75 mm is mixed with an aqueous NaOH solution (pH value of 12), and the primary product of polyvinyl alcohol is homogenized using the aqueous NaOH solution (the temperature of the homogenization treatment is 40 °C and the time of the homogenization treatment is 20 min) to obtain a mixed slurry containing polyvinyl alcohol with a homogenized alcoholysis degree;

[0091] Acetic acid is added to the above mixed slurry containing polyvinyl alcohol with a homogenized alcoholysis degree, and the pH value of the slurry is adjusted to 5 for pickling treatment (the temperature of the pickling treatment is 35 °C and the time of the pickling treatment is 25 min), and then centrifugal dehydration is carried out to obtain a pickled product; under the condition of introducing nitrogen, the above pickled product is washed twice with pure water (the temperature of the first washing is 30 °C and the time is 30 min; the temperature of the second washing is 20 °C and the time is 20 min), and the nitrogen gas flow is used to stir the material for sufficient washing, and then centrifugal dehydration is carried out to obtain polyvinyl alcohol (denoted as P4);

[0092] The molecular weight distribution index, degree of polymerization, degree of alcoholysis, range of degree of alcoholysis, and sodium acetate content of P4 are shown in Table 1.

[0093] Example 5

[0094] According to the method of Example 4, the difference is that the temperature of the homogenization treatment in step (3) is adjusted to 25°C. Other steps and conditions are the same as those in Example 4, and polyvinyl alcohol (denoted as P4) is obtained.

[0095] The molecular weight distribution index, degree of polymerization, degree of alcoholysis, range of degree of alcoholysis, and sodium acetate content of P4 are shown in Table 1.

[0096] Comparative Example 1

[0097] According to the method of Example 1, the difference is that in step (3), the polyvinyl alcohol crude product is directly subjected to two water washing treatments (the water washing conditions are exactly the same as those in Example 1), and no homogenization treatment is performed. Other steps and conditions are the same as those in Example 1, and polyvinyl alcohol (denoted as D1) is obtained.

[0098] The molecular weight distribution index, degree of polymerization, degree of alcoholysis, range of degree of alcoholysis, and sodium acetate content of D1 are shown in Table 1.

[0099] Comparative Example 2

[0100] According to the method of Example 1, the difference is that in step (1), the step of filtering gel particles through a filter screen is omitted. Other steps and conditions are the same as those in Example 1, and polyvinyl alcohol (denoted as D2) is obtained.

[0101] The molecular weight distribution index, degree of polymerization, degree of alcoholysis, range of degree of alcoholysis, and sodium acetate content of D2 are shown in Table 1.

[0102] Comparative Example 3

[0103] (1) A methanol solution of vinyl acetate (where the weight ratio of methanol to vinyl acetate is 11:100) is subjected to a polymerization reaction in the presence of azobisisobutyronitrile (the weight ratio of azobisisobutyronitrile to vinyl acetate is 0.6:100) (the reaction temperature is 60°C, and the reaction time is 3.5 h) to obtain a polymerization solution containing polyvinyl acetate; the unreacted vinyl acetate monomer carried in the polyvinyl acetate is blown off to obtain polyvinyl acetate (the weight average molecular weight is 199850 g / mol, the molecular weight distribution index is 1.8, and the content of the unreacted monomer is 0.4%); in the above polymerization reaction, the conversion rate of vinyl acetate is 36%;

[0104] (2) Prepare a polyvinyl acetate - methanol solution by mixing the above polyvinyl acetate with methanol (where the weight ratio of methanol to polyvinyl acetate is 100:15), and mix it at high speed with a NaOH - methanol solution with a NaOH concentration of 30 g / L (the molar ratio of NaOH to polyvinyl acetate is 0.006:1) for alcoholysis. The alcoholysis temperature is 45 °C, the alcoholysis time is 40 min. After the alcoholysis is completed, add acetic acid with an equal molar amount to NaOH to terminate the reaction, and obtain the alcoholysis product;

[0105] Press and crush the above alcoholysis product to obtain a polyvinyl alcohol product (denoted as D3).

[0106] The molecular weight distribution index, degree of polymerization, degree of alcoholysis, range of degree of alcoholysis, and sodium acetate content of D3 are shown in Table 1.

[0107] Table 1

[0108]

[0109] Test Example

[0110] Perform viscosity tests on the polyvinyl alcohol samples P1 - P5 and D1 - D3 prepared in Examples 1 - 5 and Comparative Examples 1 - 3, and perform swelling degree, dyeing performance, and tensile performance tests on the optical films prepared using the above polyvinyl alcohol samples. The results are shown in Table 2.

[0111] Preparation of optical film: Prepare film - forming stock solutions by mixing the above polyvinyl alcohol samples P1 - P5 and D1 - D3 with water respectively (the PVA concentration is 25% by weight), and then cast films, peel, and dry on a forming roller to obtain PVA optical films (denoted as G 1- G 5 、DG 1 -DG 3 ).

[0112] Viscosity test: Prepare PVA aqueous solutions with a concentration of 25 wt% using the above polyvinyl alcohol samples P1 - P5 and D1 - D3 respectively, and use a high - pressure capillary rheometer to test the viscosity values of the above PVA aqueous solutions at 95 °C respectively, and evaluate the processability of polyvinyl alcohol using the viscosity values.

[0113] Swelling degree test: Weigh the above PVA optical films G 1- G 5 、DG 1 -DG 3 (record the weight as M 0 ) and soak them in a water bath at 30 °C for 30 min, then use a digital display centrifuge to spin - dry at a speed of 6000 revolutions / min for 6 min, and then weigh the weight of the PVA optical film after spin - drying respectively (recorded as M n ), then the swelling degree of the PVA optical film = (Mn -M 0 ) / M 0 × 100%).

[0114] Dyeing performance test: The above-mentioned PVA optical film G 1- G 5 , DG 1 -DG 3 was made into dyeing splines, and the transmittance of the above-mentioned dyeing splines was measured respectively by an ultraviolet spectrophotometer, and the dyeability of the PVA optical film was evaluated by the transmittance.

[0115] Tensile performance test: The above-mentioned PVA optical film G 1- G 5 , DG 1 -DG 3 was made into tensile splines, and the tensile properties of the above-mentioned tensile splines were measured respectively by a tensile tester at a tensile rate of 200 m / min to obtain a stress-strain curve, and the tensile performance of the film was evaluated by the tensile strength at break and the elongation at break.

[0116] Table 2

[0117]

[0118]

[0119] As can be seen from Table 1, the method provided by the present invention can produce special polyvinyl alcohol products with a narrow molecular weight distribution (low molecular weight distribution index), high degree of polymerization, high degree of alcoholysis, concentrated alcoholysis degree distribution, and high purity. In Comparative Examples 1-3, the method of the present invention was not adopted, and the polyvinyl alcohol products obtained could not have the comprehensive effects of narrow molecular weight distribution, concentrated alcoholysis degree distribution, and high purity.

[0120] As can be seen from Table 2, the aqueous solution viscosity of the polyvinyl alcohol prepared by the method provided by the present invention is appropriate, which can make the casting more uniform and the processing performance excellent. The swelling degrees of the PVA optical films G1-G5 obtained are relatively high, the transmittance (transparency) is high, the dyeing performance is good, and they have relatively high tensile strength at break and elongation at break. However, the aqueous solution viscosities of the polyvinyl alcohol products obtained in Comparative Examples 1-3 are relatively high, which is not conducive to processing, and the swelling degrees, transmittances and tensile properties of the PVA optical films DG1-DG3 obtained are all inferior to those of G1-G5.

[0121] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A preparation method of polyvinyl alcohol, characterized in that, comprising: (1) In the presence of a first solvent and an initiator, polymerize a vinyl ester to obtain a polymerization solution, and subject the polymerization solution to de-gel particle treatment to obtain polyvinyl ester; (2) In the presence of a second solvent and a first base, alcoholize the polyvinyl ester to obtain a crude polyvinyl alcohol product; (3) In the presence of a second base, homogenize the crude polyvinyl alcohol product to obtain polyvinyl alcohol with a homogenized alcoholysis degree, and then perform purification treatment to obtain polyvinyl alcohol.

2. The preparation method according to claim 1, wherein, in step (1), the vinyl ester is selected from at least one of vinyl formate, vinyl acetate, vinyl laurate, vinyl propionate, vinyl butyrate, trimethylacetic acid vinyl ester, vinyl versatate, vinyl stearate and vinyl benzoate, preferably vinyl acetate; and / or, the first solvent is selected from at least one of methanol, ethanol and methyl acetate, preferably methanol; and / or, the initiator is selected from at least one of tert-butyl peroxyneodecanoate, tert-butyl peroxyneopentanoate, benzoyl peroxide, azobisisobutyronitrile and azobisisoheptonitrile.

3. The preparation method according to claim 1 or 2, wherein, in step (1), the weight ratio of the first solvent to the vinyl ester is (5-25):100, preferably (10-15):100; and / or, the weight ratio of the initiator to the vinyl ester is (0.1-1.5):100; and / or, the conditions of the polymerization reaction include: temperature is 58-62 °C, time is 3.5-4 h.

4. The preparation method according to any one of claims 1-3, wherein, in step (1), the average particle size of the gel particles is 10-45 μm; and / or, the de-gel particle treatment adopts a filtration separation method; and / or, the weight-average molecular weight of the polyvinyl ester is 150000-300000 g / mol, and the molecular weight distribution index ≤ 2.

5. The preparation method according to any one of claims 1-4, wherein, in step (2), the second solvent is selected from at least one of methanol, ethanol and methyl acetate, preferably methanol; and / or, the first base is selected from at least one of NaOH, KOH, and CH 3 ONa, preferably NaOH.

6. The preparation method according to any one of claims 1-5, wherein, in step (2), the weight ratio of the second solvent to the polyvinyl ester is 100:(15-25); and / or, the molar ratio of the first base to the polyvinyl ester is (0.001-0.015):1, preferably (0.005-0.01):1; and / or, the conditions of the alcoholysis include: temperature is 40-50 °C, time is 20-40 min.

7. The preparation method according to any one of claims 1-6, wherein, In step (3), the second base is selected from at least one of NaOH, KOH, and CH 3 ONa, preferably NaOH; and / or, the conditions of the homogenization treatment include: pH value is 8-12, temperature is 20-40 °C, time is 20-40 min.

8. The preparation method according to any one of claims 1-7, wherein, in step (3), the purification treatment includes pickling treatment and water washing treatment carried out in sequence; Preferably, the conditions of the pickling treatment include: pH value of 3-6, temperature of 20-40°C, and time of 3-30min; Preferably, the water washing conditions include: temperature of 20-40°C and time of 30-90 min.

9. Polyvinyl alcohol obtained by the method according to any one of claims 1 to 8; preferably, the molecular weight distribution index of the polyvinyl alcohol is ≤1.9, the degree of polymerization is 2000-3000, the degree of alcoholysis is 99.85-99.95 mol%, the range of alcoholysis is ≤0.1 mol%, and the sodium acetate content is ≤0.02 wt%. 10 . An optical film comprising the polyvinyl alcohol according to claim 9 .

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