Alcoholysis method of polyvinyl ester and preparation method of polyvinyl alcohol

Abstract

The invention relates to the technical field of polyvinyl alcohol preparation, and discloses an alcoholysis method of polyvinyl ester and a preparation method of polyvinyl alcohol. The polyvinyl ester alcoholysis method comprises the following steps: in the presence of a first solvent and alkali, performing alcoholysis on polyvinyl ester, and measuring the turbidity value of polyvinyl alcohol obtained by alcoholysis to obtain polyvinyl alcohol with the alcoholysis degree of 35-60 mol%. The preparation method of the polyvinyl alcohol comprises the following steps: carrying out polymerization reaction on vinyl ester to obtain polyvinyl ester; and then carrying out alcoholysis on the polyvinyl ester by adopting the alcoholysis method provided by the invention to obtain polyvinyl alcohol. The alcoholysis degree of the product is calculated by using the turbidity value, so that the effects of short product analysis time and fast guide process adjustment can be realized, and the batch stability of the product is improved while the production efficiency and the qualification rate of the low-alcoholysis-degree polyvinyl alcohol product are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of polyvinyl alcohol preparation, and particularly relates to a method for alcoholysis of polyvinyl ester and a method for preparing polyvinyl alcohol. Background Art

[0002] Polyvinyl alcohol (PVA) is a water-soluble polymer. Depending on the raw material source, its production methods mainly include four routes: calcium carbide acetylene, petroleum ethylene, natural acetylene, and biomass alcohol. In all these methods, the raw material is first prepared into vinyl acetate monomer, and polyvinyl acetate is obtained through free radical solution polymerization reaction. Finally, polyvinyl alcohol resin is obtained through alcoholysis (transesterification or saponification) reaction. Usually, PVA (solid state) with an alcoholysis degree ≥ 70 mol% is pulverized and dried to obtain the polyvinyl alcohol finished product, while PVA (liquid state) with an alcoholysis degree ≤ 60 mol% is formulated into a solution finished product with a corresponding concentration according to product requirements.

[0003] Currently, there are the following deficiencies in the production of low-alcoholysis-degree polyvinyl alcohol: (1) It is difficult to control the reaction end point of low-alcoholysis-degree polyvinyl alcohol. Because during the process of preparing polyvinyl alcohol by alcoholysis of polyvinyl acetate, when the alcoholysis reaction proceeds to a certain extent, a phase change will occur. If the reaction is not terminated during the phase change, production accidents such as the stirring shaft being locked and large gels forming in the kettle will occur; (2) The analysis and inspection of the product low-alcoholysis-degree polyvinyl alcohol takes a long time, generally about 8 hours. Unlike high-alcoholysis-degree polyvinyl alcohol products (solids), it is impossible to quickly and accurately determine whether the prepared product (especially the product that needs to reach a specific alcoholysis degree range) is qualified at the production site, resulting in the inability to effectively and timely adjust the production process on site, leading to a decrease in product qualification rate and an extension of the time to produce qualified products. Summary of the Invention

[0004] Aiming at the problems in the existing production process of low-alcoholysis-degree polyvinyl alcohol, such as the long time-consuming inspection of the alcoholysis degree of the product and the low efficiency of judging the product qualification, resulting in the inability to promptly implement on-site rapid quality control, leading to a decrease in product qualification rate and an extension of the time to produce qualified products, the present invention provides a method for alcoholysis of polyvinyl ester and a method for preparing polyvinyl alcohol.

[0005] To achieve the above object, in a first aspect, the present invention provides a method for alcoholysis of polyvinyl ester, including: alcoholyzing polyvinyl ester in the presence of a first solvent and an alkali, and measuring the turbidity value of the polyvinyl alcohol obtained by alcoholysis to obtain polyvinyl alcohol with an alcoholysis degree of 35 mol% - 60 mol%.

[0006] In a second aspect, the present invention provides a method for preparing polyvinyl alcohol, including: polymerizing vinyl ester to obtain polyvinyl ester; and then alcoholyzing the polyvinyl ester by using the alcoholysis method described in the first aspect above to obtain polyvinyl alcohol.

[0007] Through the above technical solution, the alcoholysis method of polyvinyl ester and the preparation method of polyvinyl alcohol provided by the present invention can quickly determine whether the alcoholysis degree of the product is within the qualified range on the production site by measuring the turbidity value of the polyvinyl alcohol obtained by alcoholysis, which only takes 5 - 10 minutes. It can greatly improve the qualified judgment efficiency of low alcoholysis degree liquid polyvinyl alcohol products with an alcoholysis degree ≤ 60 mol%. Furthermore, it can effectively guide on-site production and timely adjust the production process. Compared with the existing preparation methods, it can quickly and accurately prepare polyvinyl alcohol products within a specific alcoholysis degree range of alcoholysis degree ≤ 60 mol%, achieving the improvement of production efficiency and product qualification rate, better ensuring the batch stability of products, and reducing the labor intensity and labor cost of on-site personnel. Detailed Embodiments

[0008] 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, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values 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.

[0009] The following details the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only for the purpose of illustration and explanation of the present invention, and are not used to limit the present invention.

[0010] The first aspect of the present invention provides an alcoholysis method of polyvinyl ester, including: alcoholyzing polyvinyl ester in the presence of a first solvent and a base, and measuring the turbidity value of the polyvinyl alcohol obtained by alcoholysis to obtain polyvinyl alcohol with an alcoholysis degree of 38 - 60 mol%.

[0011] In the existing process of alcoholysis of polyvinyl esters (e.g., polyvinyl acetate) to prepare polyvinyl alcohol with a low degree of alcoholysis (e.g., degree of alcoholysis ≤ 60 mol%), chemical analysis is usually used to determine the degree of alcoholysis of the product. Specifically, a product sample is dissolved in a mixed solvent of alcohol and water, a quantitative amount of sodium hydroxide is added to react with the residual acetate radicals in the product, and then a quantitative amount of sulfuric acid is added to neutralize the remaining sodium hydroxide. The excess sulfuric acid is titrated with a standard sodium hydroxide solution, and thus the content of residual acetate radicals in the product sample and the degree of alcoholysis of the product are calculated. This method takes a long time, and production personnel cannot quickly determine whether the degree of alcoholysis of the product is qualified, and thus cannot effectively and timely adjust the production process on site, resulting in an inevitable impact on the product qualification rate and a long time to produce qualified products. The inventors of the present invention recognized the above disadvantages and found in the research that there is a functional relationship between the turbidity value of polyvinyl alcohol and the degree of alcoholysis. It is proposed that by measuring the turbidity value of polyvinyl alcohol obtained by alcoholysis of polyvinyl esters and calculating the degree of alcoholysis of polyvinyl alcohol using the functional relationship, it is possible to quickly determine whether the degree of alcoholysis of the product meets the standard, which is conducive to quickly and timely implementing quality control on site and improving the production efficiency and product qualification rate of low-degree-of-alcoholysis polyvinyl alcohol products with a degree of alcoholysis ≤ 60 mol%.

[0012] According to the present invention, the degree of alcoholysis of polyvinyl alcohol is calculated using the functional relationship between the turbidity value of polyvinyl alcohol and the degree of alcoholysis, and the functional relationship is: Y = -A·ln(X) + B; where Y represents the degree of alcoholysis of polyvinyl alcohol, in mol%; X represents the turbidity value of a suspension prepared by mixing polyvinyl alcohol and water with a concentration of 0.1 - 2 wt%; 75 ≤ A ≤ 95; 330 ≤ B ≤ 370.

[0013] According to the present invention, in the functional relationship, preferably, X represents the turbidity value of a suspension prepared by mixing polyvinyl alcohol and water with a concentration of 0.1 - 1 wt%, and more preferably the turbidity value of a suspension prepared by mixing polyvinyl alcohol and water with a concentration of 0.2 wt%.

[0014] According to the present invention, in the functional relationship, preferably, 78 ≤ A ≤ 90; 340 ≤ B ≤ 365, which can make the degree of alcoholysis of polyvinyl alcohol calculated using the turbidity value and the functional relationship closer to the actual degree of alcoholysis.

[0015] According to a most preferred embodiment of the present invention, the functional relationship is: Y = -A·ln(X) + B; where Y represents the degree of alcoholysis of polyvinyl alcohol; X represents the turbidity value of a suspension prepared by mixing polyvinyl alcohol and water with a concentration of 0.2 wt%; A = 89.21; B = 364.96. Using the turbidity value and the functional relationship of this most preferred embodiment, the degree of alcoholysis of polyvinyl alcohol calculated can be made closer to the actual degree of alcoholysis.

[0016] In the present invention, the turbidity value of the suspension prepared from polyvinyl alcohol and water can be measured by a turbidimeter.

[0017] According to the present invention, in the method for alcoholysis of the polyvinyl ester, the polyvinyl ester may be selected from at least one of polyvinyl formate, polyvinyl acetate, polyvinyl laurate, polyvinyl propionate, polyvinyl butyrate, polyvinyl trimethylacetate, polyvinyl versatate, polyvinyl stearate, and polyvinyl benzoate, and is preferably polyvinyl acetate.

[0018] In the present invention, the polyvinyl ester can be prepared by oneself through any known processes and parameters, or commercial products can be used, and the present invention does not make special limitations thereto.

[0019] According to the present invention, in the method for alcoholysis of the polyvinyl ester, preferably, the first solvent is selected from at least one of methanol, ethanol, and methyl acetate, and is preferably methanol.

[0020] According to the present invention, in the method for alcoholysis of the polyvinyl ester, preferably, the base is selected from at least one of NaOH, KOH, and sodium methoxide, and is preferably NaOH.

[0021] According to the present invention, in the method for alcoholysis of the polyvinyl ester, preferably, the polyvinyl ester is fed in the form of a polymer solution. The polymer solution is preferably a solution with a polyvinyl ester concentration of 20 - 55 wt% formed by the polyvinyl ester and the first solvent.

[0022] According to the present invention, in the method for alcoholysis of the polyvinyl ester, preferably, the base is fed in the form of a base solution. The base solution is preferably a solution with a base concentration of 15 - 50 g / L formed by the base and the first solvent.

[0023] According to the present invention, in the method for alcoholysis of the polyvinyl ester, preferably, the molar ratio of the base to the polyvinyl ester is (0.001 - 0.01):1.

[0024] According to the present invention, in the method for alcoholysis of the polyvinyl ester, the conditions for alcoholysis include: the temperature is 40 - 50 °C, and the time is 20 - 50 min.

[0025] In the present invention, by controlling the base concentration in the solution formed by the base and the first solvent to be 15 - 50 g / L, and the molar ratio of the base to the polyvinyl ester to be (0.001 - 0.01):1, no phase change occurs in the alcoholysis product within the above-mentioned alcoholysis time range (when the degree of alcoholysis of the product is greater than 60 mol%, a phase change will occur, from liquid state to gel state).

[0026] The alcoholysis method of polyvinyl ester provided by the present invention involves sampling the polyvinyl alcohol product obtained by alcoholysis of polyvinyl ester, measuring the turbidity value of a suspension with a specific concentration prepared by mixing the sampled sample with water, and then calculating the alcoholysis degree of polyvinyl alcohol using a specific functional relationship between the turbidity value and the alcoholysis degree. If the calculated alcoholysis degree falls within the qualified range (in the art, when the difference between the actual alcoholysis degree and the target alcoholysis degree of the polyvinyl alcohol product after testing is ±0.85 mol / %, it can be considered qualified), the adopted alcoholysis process conditions are maintained to continue the production of subsequent products; if the calculated alcoholysis degree does not fall within the qualified range, the alcoholysis process conditions need to be adjusted until the alcoholysis degree of the sampled product falls within the qualified range. In the alcoholysis method of polyvinyl acetate of the present invention, by sampling and measuring the turbidity value of the product and combining the functional relationship between the turbidity value and the alcoholysis degree, the alcoholysis degree of the product can be quickly calculated, and the deviation rate between the calculated alcoholysis degree and the actual alcoholysis degree tested by chemical analysis method is less than 1.2%, preferably less than 1%, which is conducive to quickly and timely implementing quality control on-site, achieving the rapid and accurate preparation of low-alcoholysis polyvinyl alcohol products with an alcoholysis degree ≤ 60 mol%, and improving production efficiency and product qualification rate.

[0027] The second aspect of the present invention provides a method for preparing polyvinyl alcohol, comprising: polymerizing a vinyl ester to obtain a polyvinyl ester; and then subjecting the polyvinyl ester to alcoholysis using the alcoholysis method described in the first aspect above to obtain polyvinyl alcohol.

[0028] According to the present invention, 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, preferably vinyl acetate.

[0029] According to the present invention, in the method for preparing polyvinyl alcohol, the vinyl ester undergoes the polymerization reaction in the presence of a second solvent and an initiator.

[0030] According to the present invention, in the method for preparing polyvinyl alcohol, preferably, before the polymerization reaction, the vinyl ester is mixed with the second solvent and fed in the form of a mixed solution. Preferably, in the mixed solution, the concentration of the vinyl ester is 25 - 50 wt%.

[0031] According to the present invention, in the method for preparing polyvinyl alcohol, preferably, before the polymerization reaction, the mixed solution is preheated to 55 - 60 °C.

[0032] According to the present invention, in the method for preparing polyvinyl alcohol, preferably, the second solvent is selected from at least one of methanol, ethanol, and methyl acetate, preferably methanol.

[0033] According to the present invention, in the method for preparing polyvinyl alcohol, preferably, the initiator is selected from at least one of bis(3,5,5-trimethylhexanoyl) peroxide, bis(2-ethylhexyl) peroxydicarbonate, bis(4-tert-butylcyclohexyl) peroxydicarbonate, dipropyl peroxydicarbonate, and tert-amyl peroxypivalate.

[0034] According to the present invention, in the method for preparing polyvinyl alcohol, preferably, the weight ratio of the initiator to vinyl acetate is (0.027 - 0.2):100.

[0035] According to the present invention, in the method for preparing polyvinyl alcohol, preferably, the conditions for the polymerization reaction include: a temperature of 60 - 67 °C and a time of 5 - 7 h.

[0036] According to the present invention, the method for preparing polyvinyl alcohol further includes: separating the product system obtained from the polymerization reaction to separate unreacted vinyl ester, obtaining a mixture of polyvinyl ester and the second solvent. In the present invention, the mixture of polyvinyl ester and the second solvent can be further separated to obtain polyvinyl ester and subjected to alcoholysis, or separation can be not carried out. In a preferred embodiment of the present invention, the mixture of polyvinyl ester and the second solvent is directly subjected to alcoholysis by the alcoholysis method described in the first aspect of the present invention.

[0037] According to the present invention, in the method for preparing polyvinyl alcohol, the separation treatment is preferably carried out by rectification. The present invention does not make special limitations on the conditions for the rectification, as long as the unreacted vinyl ester can be distilled out and separated.

[0038] The method for preparing polyvinyl alcohol provided by the present invention polymerizes vinyl ester to obtain polyvinyl ester, and then the polyvinyl ester is subjected to alcoholysis by the alcoholysis method provided by the present invention. By using the specific functional relationship between the turbidity value of the suspension with a specific concentration prepared from the polyvinyl alcohol product obtained by alcoholysis and water and the alcoholysis degree of the polyvinyl alcohol product, the alcoholysis degree of the polyvinyl alcohol product can be quickly calculated, and thus quality control can be quickly and timely implemented on-site, realizing the rapid and accurate preparation of a low-alcoholysis-degree polyvinyl alcohol product with an alcoholysis degree ≤ 60 mol%. Compared with the conventional method for preparing low-alcoholysis-degree polyvinyl alcohol, the production efficiency and product qualification rate can be significantly improved.

[0039] The present invention will be described in detail below through examples. In the following examples and comparative examples,

[0040] Unless otherwise specified, the materials used are all ordinary commercially available products.

[0041] Example 1

[0042] A methanol solution of vinyl acetate with a concentration of 30 wt% is preheated to 55 °C by a preheater and then enters a polymerization kettle. Solution polymerization is carried out under the action of bis(3,5,5-trimethylhexanoyl) peroxide initiator at 0.14% (by weight percentage of vinyl acetate). The polymerization reaction temperature is stabilized at 66 °C, the polymerization time is 7.0 h, and the polymerization rate of vinyl acetate is 94.8%;

[0043] The polyvinyl acetate-methanol solution discharged from the polymerization kettle is introduced into the first distillation column (the temperature is controlled at 68 °C, and the reflux ratio of the distillation column is 0.1). Methanol vapor is blown in from the bottom of the column to distill out the unreacted vinyl acetate as an azeotrope of methanol. A polyvinyl acetate-methanol solution with a concentration of 25 wt% is obtained at the bottom of the column. The obtained polyvinyl acetate-methanol solution with a concentration of 25 wt% is sent to an alcoholysis device by a transfer pump and centrifugally mixed with a 30 g / L NaOH-methanol solution in an alcoholysis machine (the molar ratio of NaOH to polyvinyl acetate is 0.007:1). The alcoholysis temperature is controlled at 40 °C. Before the viscosity of the reaction system suddenly increases and is about to change from the liquid phase to the gel phase (also known as before the phase change), acetic acid with an equimolar amount to NaOH is added to terminate the reaction, and a low-degree-of-alcoholysis liquid polyvinyl alcohol is obtained. The alcoholysis time is controlled at 20 min;

[0044] The polyvinyl alcohol obtained by the above alcoholysis is taken with a sampling spoon and added to deionized water at 20 °C to prepare a suspension with a polyvinyl alcohol concentration of 0.2 wt%. After stirring evenly, it is added to a sample colorimetric bottle of a turbidimeter. The sample colorimetric bottle is inserted into a turbidimeter (model TU5200) for turbidity measurement, and the measured turbidity value is 36.04. Using the formula Y = -89.21ln(X) + 364.96 (Y represents the degree of alcoholysis of polyvinyl alcohol; X represents the turbidity value of the suspension with a concentration of 0.2 wt% prepared by polyvinyl alcohol and water), the degree of alcoholysis of polyvinyl alcohol is calculated to be 45.18 mol%, while the degree of alcoholysis measured by chemical analysis for the same sample is 45.61 mol%, and the deviation rate is 0.94% (deviation rate = (|calculated degree of alcoholysis - degree of alcoholysis measured by chemical analysis| / degree of alcoholysis measured by chemical analysis × 100%)).

[0045] Example 2

[0046] A methanol solution of vinyl acetate with a concentration of 38 wt% is preheated to 55 °C by a preheater and then enters a polymerization kettle. Solution polymerization is carried out under the action of bis(3,5,5-trimethylhexanoyl) peroxide initiator at 0.10% (by weight percentage of vinyl acetate). The polymerization reaction temperature is stabilized at 64 °C, the polymerization time is 7.0 h, and the polymerization rate of vinyl acetate is 94.1%;

[0047] The polyvinyl acetate - methanol solution discharged from the polymerization kettle is introduced into the first distillation column (the temperature is controlled at 68°C, and the reflux ratio of the distillation column is 0.25). Methanol vapor is blown in from the bottom of the column to distill out the unreacted vinyl acetate as an azeotrope with methanol, and a polyvinyl acetate - methanol solution with a concentration of 33 wt% is obtained at the bottom of the column. The obtained polyvinyl acetate - methanol solution with a concentration of 33% is sent to the alcoholysis equipment by a transfer pump and centrifugally mixed with a 30 g / L NaOH - methanol solution in an alcoholysis machine (the molar ratio of NaOH to polyvinyl acetate is 0.0076:1). The alcoholysis temperature is controlled at 40°C. Before the viscosity of the reaction system suddenly increases and is about to change from the liquid phase to the gel phase (also known as before the phase change), acetic acid with an equimolar amount to NaOH is added to terminate the reaction, and low - degree - of - alcoholysis liquid polyvinyl alcohol is obtained. The alcoholysis time is controlled at 27 min.

[0048] The polyvinyl alcohol obtained from the above alcoholysis is taken with a sampling spoon and added to deionized water at 20°C to prepare a suspension with a polyvinyl alcohol concentration of 0.2 wt%. After stirring evenly, it is added to the sample colorimetric bottle of a turbidimeter. The sample colorimetric bottle is inserted into the turbidimeter (model TU5200) for turbidity measurement, and the measured turbidity value is 31.90. Using the formula Y = - 89.21ln(X)+364.96 (Y represents the degree of alcoholysis of polyvinyl alcohol; X represents the turbidity value of the suspension prepared by polyvinyl alcohol and water with a concentration of 0.2 wt%), the degree of alcoholysis of polyvinyl alcohol is calculated to be 56.061 mol%. For the same sample, the degree of alcoholysis measured by chemical analysis is 55.99 mol%, and the deviation rate is 0.13%.

[0049] Example 3

[0050] The methanol solution of vinyl acetate with a concentration of 43 wt% is preheated to 56°C by a preheater and then enters the polymerization kettle. Solution polymerization is carried out under the action of 0.090% (by weight percentage of vinyl acetate) of bis(3,5,5 - trimethylhexanoyl) peroxide initiator. The polymerization reaction temperature is stabilized at 64°C, the polymerization time is 6.5 h, and the polymerization rate of vinyl acetate is 93.7%.

[0051] The polyvinyl acetate - methanol solution discharged from the polymerization kettle is introduced into the first distillation column (temperature controlled at 66 °C, reflux ratio of the distillation column is 0.5). Methanol vapor is blown into the bottom of the column to distill out the azeotrope of unreacted vinyl acetate and methanol, and a polyvinyl acetate - methanol solution with a concentration of 36 wt% is obtained at the bottom of the column. The obtained polyvinyl acetate - methanol solution with a concentration of 36 wt% is sent to the alcoholysis equipment by a transfer pump, and is centrifugally mixed with a 30 g / L NaOH - methanol solution in an alcoholysis machine (molar ratio of NaOH to polyvinyl acetate is 0.0078:1). The alcoholysis temperature is controlled at 41 °C. Before the viscosity of the reaction system suddenly increases and is about to change from the liquid phase to the gel phase (also known as before the phase change), acetic acid with an equimolar amount to NaOH is added to terminate the reaction, and a low - degree - of - alcoholysis liquid - state polyvinyl alcohol is obtained. The alcoholysis time is controlled at 38 min.

[0052] Take the polyvinyl alcohol obtained from the above - mentioned alcoholysis with a sampling spoon, add it to deionized water at 20 °C to prepare a suspension with a polyvinyl alcohol concentration of 0.2 wt%. After stirring evenly, add it to the sample colorimetric bottle of a turbidimeter. Insert the sample colorimetric bottle into the turbidimeter (model TU5200) for turbidity measurement, and the measured turbidity value is 38.45. Using the formula Y = - 89.21ln(X)+364.96 (Y represents the degree of alcoholysis of polyvinyl alcohol; X represents the turbidity value of the suspension with a concentration of 0.2 wt% prepared from polyvinyl alcohol and water), the degree of alcoholysis of polyvinyl alcohol is calculated to be 39.40 mol%. For the same sample, the degree of alcoholysis measured by chemical analysis is 39.67 mol%, and the deviation rate is 0.68%.

[0053] Example 4

[0054] The methanol solution of vinyl acetate with a concentration of 50 wt% is pre - heated to 60 °C by a pre - heater and then enters the polymerization kettle. Solution polymerization is carried out under the action of 0.078% (by weight percentage of vinyl acetate) bis(3,5,5 - trimethylhexanoyl) peroxide initiator. The polymerization reaction temperature is stabilized at 63 °C, the polymerization time is 5.0 h, and the polymerization rate of vinyl acetate is 91.9%.

[0055] The polyvinyl acetate - methanol solution discharged from the polymerization kettle is introduced into the first distillation column (the temperature is controlled at 68 °C, and the reflux ratio of the distillation column is 0.75). Methanol vapor is blown in from the bottom of the column to distill out the unreacted vinyl acetate as an azeotrope with methanol, and a polyvinyl acetate - methanol solution with a concentration of 42 wt% is obtained at the bottom of the column. The obtained polyvinyl acetate - methanol solution with a concentration of 42 wt% is pumped to the alcoholysis equipment, and is centrifugally mixed with a 30 g / L NaOH - methanol solution in the alcoholysis machine (the molar ratio of NaOH to polyvinyl acetate is 0.0085:1). The alcoholysis temperature is controlled at 40 °C. Before the viscosity of the reaction system suddenly increases and is about to change from the liquid phase to the gel phase (also known as before the phase change), acetic acid with an equimolar amount to NaOH is added to terminate the reaction, and low - degree - alcoholysis liquid polyvinyl alcohol is obtained. The alcoholysis time is controlled at 46 min.

[0056] The polyvinyl alcohol obtained from the above - mentioned alcoholysis is taken with a sampling spoon and added to deionized water at 20 °C to prepare a suspension with a polyvinyl alcohol concentration of 0.2 wt%. After stirring evenly, it is added to the sample colorimetric bottle of the turbidimeter. The sample colorimetric bottle is inserted into the turbidimeter (model TU5200) for turbidity measurement, and the measured turbidity value is 30.59. Using the formula Y = - 89.21ln(X)+364.96 (Y represents the degree of alcoholysis of polyvinyl alcohol; X represents the turbidity value of the suspension prepared with polyvinyl alcohol and water with a concentration of 0.2 wt%), the degree of alcoholysis of polyvinyl alcohol is calculated to be 59.8 mol%. For the same sample, the degree of alcoholysis measured by chemical analysis is 59.34 mol%, and the deviation rate is 0.78%.

[0057] Example 5

[0058] According to the method and parameters of Example 4, the difference is only that after alcoholysis, the polyvinyl alcohol obtained from alcoholysis is taken with a sampling spoon and added to deionized water at 20 °C to prepare a suspension with a polyvinyl alcohol concentration of 0.2 wt%. After stirring evenly, it is added to the sample colorimetric bottle of the turbidimeter. The sample colorimetric bottle is inserted into the turbidimeter (model TU5200) for turbidity measurement, and the measured turbidity value is 30.59. Using the formula Y = - 86.60ln(X)+356.2 (Y represents the degree of alcoholysis of polyvinyl alcohol; X represents the turbidity value of the suspension prepared with polyvinyl alcohol and water with a concentration of 0.2 wt%), the degree of alcoholysis of polyvinyl alcohol is calculated to be 60.01 mol%. For the same sample, the degree of alcoholysis measured by chemical analysis is 59.34 mol%, and the deviation rate is 1.13%.

[0059] The above embodiments show that for the alcoholysis method of polyvinyl acetate and the preparation method of polyvinyl alcohol provided by the present invention, by means of measuring the turbidity value of the polyvinyl alcohol obtained by alcoholysis, the alcoholysis degree of the product can be quickly determined on the production site, and the deviation rate between the calculated alcoholysis degree and the actual alcoholysis degree measured by chemical analysis method is less than 1.2%, preferably less than 1%. It has high accuracy, can improve the qualified judgment efficiency of the product, can quickly and accurately prepare polyvinyl alcohol products with an alcoholysis degree ≤ 60 mol%, realizes the improvement of production efficiency and product qualification rate, and better ensures the batch stability of the product.

[0060] 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 solution 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 method for alcoholysis of polyvinyl ester, characterized in that, it includes: In the presence of a first solvent and a base, alcoholyze the polyvinyl ester, and measure the turbidity value of the polyvinyl alcohol obtained by alcoholysis to obtain polyvinyl alcohol with an alcoholysis degree of 35-60 mol%.

2. The alcoholysis method according to claim 1, wherein, The alcoholysis degree of polyvinyl alcohol is calculated using the functional relationship between the turbidity value of polyvinyl alcohol and the alcoholysis degree. The functional relationship is: Y = -A·ln(X) + B; wherein, Y represents the alcoholysis degree of polyvinyl alcohol, in units of mol%; X represents the turbidity value of a suspension prepared by mixing polyvinyl alcohol and water with a concentration of 0.1-2 wt%; 75 ≤ A ≤ 95; 330 ≤ B ≤ 370.

3. The alcoholysis method according to claim 2, wherein, X represents the turbidity value of a suspension prepared by mixing polyvinyl alcohol and water with a concentration of 0.1-1 wt%, preferably the turbidity value of a suspension prepared by mixing polyvinyl alcohol and water with a concentration of 0.2 wt%; and / or, 78 ≤ A ≤ 90; 340 ≤ B ≤ 365.

4. The alcoholysis method according to any one of claims 1-3, wherein, The first solvent is selected from at least one of methanol, ethanol, and methyl acetate, preferably methanol; and / or, the base is selected from at least one of NaOH, KOH, and sodium methoxide, preferably NaOH.

5. The alcoholysis method according to any one of claims 1-4, wherein, The molar ratio of the base to polyvinyl ester is (0.001-0.01):

1.

6. The alcoholysis method according to any one of claims 1-5, wherein, The conditions for the alcoholysis include: temperature is 40-50 °C, and time is 20-50 min.

7. A method for preparing polyvinyl alcohol, including: Carry out a polymerization reaction on vinyl ester to obtain polyvinyl ester; Then alcoholyze the polyvinyl ester using the alcoholysis method according to any one of claims 1-6 to obtain polyvinyl alcohol.

8. The preparation method according to claim 7, wherein, The vinyl ester undergoes the polymerization reaction in the presence of a second solvent and an initiator; Preferably, the second solvent is selected from at least one of methanol, ethanol, and methyl acetate, preferably methanol; Preferably, the initiator is selected from at least one of bis(3,5,5-trimethylhexanoyl) peroxide, bis(2-ethylhexyl) peroxydicarbonate, bis(4-tert-butylcyclohexyl) peroxydicarbonate, dipropyl peroxydicarbonate, and tert-amyl peroxyneopentanoate.

9. The preparation method according to claim 8, wherein, The weight ratio of the initiator to vinyl ester is (0.027-0.2):

100.

10. The alcoholysis method according to any one of claims 7-9, wherein, The conditions for the polymerization reaction include: temperature is 60-67 °C, and time is 5-7 h.