Modified polyvinyl alcohol and preparation method thereof

Abstract

The invention belongs to the technical field of polyvinyl alcohol, and particularly relates to modified polyvinyl alcohol, polyvinyl alcohol is modified by a structural monomer unit shown in a formula (I): # imgabs0 # (I), in the formula (I), R1 is a hydrogen atom or a chain alkyl group, and n and m are respectively and independently positive integers. The modified polyvinyl alcohol prepared in the invention has good solubility in water, and the aqueous solution has less foaming and fast defoaming; the film-forming property is good, and the film strength is high.

Description

Technical Field

[0001] The present invention relates to a polyvinyl alcohol, and particularly to a modified polyvinyl alcohol and a preparation method thereof. Background Art

[0002] Polyvinyl alcohol (PVA) is a colorless, non-toxic, non-corrosive, biodegradable water-soluble organic polymer ("Research Progress in Modification of Polyvinyl Alcohol", Zhang Jing et al., Synthetic Fiber Industry, Vol. 28, No. 1, 2005, page 57, left column, paragraph 1, lines 1-2, publication date February 28, 2005). Polyvinyl alcohol is prepared by alcoholysis of polyvinyl acetate, has a planar zigzag conformation similar to that of PE, its molecular main chain is a C-C bond, the chemical structure is regular, the bond energy is high, and each repeating unit has a hydroxyl group, with small size and strong polarity, which is easy to form hydrogen bonds, so that polyvinyl alcohol has high crystallinity and high barrier properties. At the same time, it also has oil resistance, solvent resistance, and unique adhesive properties, and can be miscible with plastics, starches, synthetic resins, etc. Therefore, polyvinyl alcohol is widely used in industries such as textiles, coatings, adhesives, emulsifiers, paper processing aids, fibers, films, food packaging, building materials, and medicine ("Research on Plasticization Modification of Polyvinyl Alcohol", Ma Cong, Master's Thesis of Donghua University, page 1, paragraph 1, lines 1-5 and page 2, paragraph 2, lines 1-4, publication date December 31, 2017).

[0003] In the industrial application process of polyvinyl alcohol, such as in production processes like textile sizing, paper coating, and water-soluble film manufacturing, a large amount of foam will be generated due to operations such as dissolution stirring and roll coating, which brings many inconveniences to the production process: such as a decrease in the dissolution filling amount, an extension of the solution defoaming time, inability to be applied to automatic metering devices, and it will also affect the quality of the finished product. Therefore, it is necessary to effectively inhibit foam generation during the production process.

[0004] Chinese Patent Document CN106029713A discloses a modified polyvinyl alcohol and a manufacturing method thereof, including: a modified polyvinyl alcohol containing 0.1-10 mol% of monomer units represented by the following formula (1) and having a degree of polymerization exceeding 2000; wherein R 1 represents a hydrogen atom or a methyl group. This document claims to provide a modified polyvinyl alcohol with excellent solubility in water, less foaming of the aqueous solution, and excellent gas barrier properties; in addition, the film containing this modified polyvinyl alcohol has high strength even at high humidity and also has excellent gas barrier properties.

[0005] However, the existing technology has the following deficiencies: The typical structure of the monomer used in this technology is methallyl alcohol. This monomer has relatively low polymerization activity, high toxicity, and the defoaming effect of the PVA aqueous solution prepared with this monomer is limited, and it cannot fundamentally solve the problems of poor water solubility, easy foaming, and low film strength of PVA at the same time.

[0006] Chinese Patent Document CN 110291120A discloses a multilayer film, which comprises at least one layer comprising or consisting of a polymer composition obtainable by free-radical polymerization of a monomer composition, said monomer composition comprising at least one monomer A) selected from α,β-ethylenically unsaturated mono- and dicarboxylic acids, salts of α,β-ethylenically unsaturated mono- and dicarboxylic acids, anhydrides of α,β-ethylenically unsaturated mono- and dicarboxylic acids, and mixtures thereof, wherein said free-radical polymerization is carried out in the presence of at least one polyether component. The invention also relates to a method for producing such a multilayer film, to various uses of such a multilayer film, and in particular to an outer skin or coating comprising or consisting of such a multilayer film for the portioning of detergent compositions, cleaning compositions or dishwashing compositions.

[0007] However, the modification itself in this document cannot solve the foam problem, and the solutions to the foam problem disclosed therein are still through traditional defoamers and foam inhibitors: "In principle, all known foam inhibitors or defoamers can be used. For example, there should be mentioned here (1) oil-based systems based on mineral oil or vegetable oil, which may additionally contain wax or silica particles, (2) water-based systems in which oil and wax are dispersed, (3) silicone-based systems (polysiloxanes), for example in water-soluble form, as oil-based or water-based emulsions, (4) EO / PO-based polyalkoxylates, (5) alkyl polyacrylates, (6) fatty acids and fatty acid esters, especially monoglycerides and diglycerides of fatty acids, (8) fatty alcohol alkoxylates, (9) defoamers from phosphoric acid esters and their salts, such as sodium (C6-C20-alkyl) phosphates, for example sodium octyl phosphate or tris(C1-C20-alkyl) phosphates, for example tributyl phosphate, and (10) metal soaps, such as aluminum stearate or calcium oleate." Chinese Patent Document CN 114426621A discloses a modified polyvinyl alcohol, which is characterized by comprising a structural unit shown in formula (1): In formula (1), R1 is a hydrogen atom or a linear or branched alkyl group with 1-3 carbon atoms, and n = 1-10. This document is another modification technology of the applicant's unit. This modified polyvinyl alcohol can significantly improve the ink absorption of inkjet printing substrates and can significantly improve the clarity of inkjet printing substrates.

[0008] Similarly, the modification technology disclosed in this document still cannot solve the foam problem. Summary of the Invention

[0009] In order to fundamentally solve the technical problems of poor water solubility, easy foaming and low film strength of PVA simultaneously, the present invention provides a modified polyvinyl alcohol, in which the polyvinyl alcohol is modified by the structural monomer unit shown in formula (I): (I) In formula (I), R 1 is a hydrogen atom or a chain alkyl group, and n and m are each independently a positive integer.

[0010] Further, the R 1 is hydrogen or a linear or branched alkyl group with 1-3 carbon atoms, n = 1-40 and m = 1-30.

[0011] Further, the content of the structural monomer unit shown in formula (I) is 0.1-10 mol%.

[0012] Further, the content of the structural monomer unit shown in formula (I) is 0.3-6.0 mol%.

[0013] Further, the degree of polymerization of the modified polyvinyl alcohol is 300-3000, and the degree of alcoholysis is 80 mol%-99.9 mol%.

[0014] Further, the degree of alcoholysis of the modified polyvinyl alcohol is 85 mol%-99 mol%.

[0015] In the second aspect, the present invention provides a method for preparing the modified polyvinyl alcohol of the present invention, including the following steps: copolymerizing a vinyl ester monomer with a compound shown in formula (II) or formula (III), and then performing alcoholysis: (II) (III) Among them, R 2 is a chain alkyl group; R 1 is a hydrogen atom or a chain alkyl group, and n and m are each independently a positive integer.

[0016] In the formula (II), R 2 is an alkyl group with 1-5 carbon atoms, R 1 is a hydrogen atom or a linear or branched alkyl group with 1-3 carbon atoms, n = 1-40 and m = 1-30.

[0017] Preferably, the compound shown in formula (II) or formula (III) is selected from one of the following compounds: CH 2 =C(CH 3 )CH 2 CH 2 O(CH 2 CH2 O) 3 (CHCH 3 CH 2 O) 2 H;CH 2 =C(CH 3 )CH 2 CH 2 O(CH 2 CH 2 O) 10 (CHCH 3 CH 2 O) 20 H;CH 2 =C(CH 3 )CH 2 CH 2 O(CH 2 CH 2 O) 15 (CHCH 3 CH 2 O) 10 H;CH 2 =C(CH 3 )CH 2 CH 2 O(CH 2 CH 2 O) 2 (CHCH 3 CH 2 O) 3 COCH 3 。

[0018] The application of the modified polyvinyl alcohol of the present invention in the water-soluble film and adhesive industries is also provided.

[0019] The present invention has the following beneficial effects: 1. The modified polyvinyl alcohol prepared by the present invention has good solubility in water, less foaming and fast defoaming in the aqueous solution; at the same time, the modified polyvinyl alcohol has good film-forming property and high film strength.

[0020] 2. The modified polyvinyl alcohol prepared by the present invention can be used alone or as an additive to be added to other components by utilizing its characteristics, especially as a water-soluble composition.

[0021] 3. The modified polyvinyl alcohol prepared by the present invention can be used in various applications where polyvinyl alcohol is usually used; for example, it can be used in surfactants for various applications, various coating agents, sizing agents for paper, pigment binders, coatings, textile mortars, fiber processing agents, various films, sheets, bottles, tackifiers, soil conditioners, ion exchange resins, ion exchange membranes, etc.; Detailed implementation mode

[0022] The following is a further detailed description through specific embodiments.

[0023] The degree of alcoholysis of the present invention refers to the molar fraction of hydroxyl groups relative to the sum of hydroxyl groups and ester groups in the modified polyvinyl alcohol.

[0024] The preparation method of the modified polyvinyl alcohol of the present invention specifically includes the following steps: (1) Polymerization Using a polymerization tank equipped with a reflux condenser, a raw material feeding system, a reaction liquid discharging system, a thermometer, a nitrogen adding system, and stirring blades, adding vinyl esters and alcohols as comonomers of the monomers shown in formula (II) or (III), while stirring, replacing with nitrogen, heating the reactor, adding an initiator to start polymerization, after polymerizing for a period of time, cooling and stopping the polymerization; then removing the unreacted monomers to obtain an alcohol solution of modified polyvinyl acetate (PVAc); (2) Alcoholysis and cleaning Adding an alcohol solution of an alkali to the alcohol solution of the PVAc copolymer for alcoholysis. After adding the sodium hydroxide methanol solution, a jelly-like substance will be formed, which is crushed with a crushing device, and after alcoholysis, adding an acid to neutralize the remaining alkali, and filtering after neutralization to obtain a white solid; (3) Cleaning Washing the obtained white solid with alcohol, centrifuging to remove the liquid, and drying the white solid obtained by centrifuging to remove the liquid in a vacuum dryer to obtain a modified polyvinyl alcohol containing the monomer unit of structural formula (I).

[0025] The vinyl ester monomers of the present invention are not particularly limited and may be vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, isobutyrate vinyl, trimethyl acetate vinyl, vinyl caproate, vinyl octanoate, vinyl laurate, vinyl stearate, vinyl oleate, vinyl benzoate; further preferably, vinyl acetate.

[0026] The method of copolymerizing the monomers shown in formula (II) or (III) with vinyl ester monomers of the present invention can be any one of batch polymerization and continuous polymerization; among them, the polymerization method can be known methods such as bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, etc.

[0027] When carrying out bulk polymerization or solution polymerization in the present invention, alcohol is usually used as a solvent; among them, in the solution polymerization method, lower alcohols such as methanol, ethanol, propanol, and butanol are more suitable.

[0028] The initiator used in the copolymerization of the present invention can be a well-known initiator selected according to the polymerization method, generally an azo initiator, a peroxide initiator, or a redox initiator; among them, the azo initiator can be 2,2-azobisisobutyronitrile; the peroxide initiator can be diisopropyl peroxydicarbonate; the redox initiator can be a composition of diisopropyl peroxydicarbonate and sodium bisulfite.

[0029] The temperature for the monomer copolymerization of the present invention is 40 - 70 °C, preferably 55 - 65 °C.

[0030] The alcoholysis reaction of the vinyl ester copolymer of the present invention can use a well-known method; for example, it can be carried out in a state where the polymer of the present invention is dissolved in an alcohol or a solution containing water and alcohol.

[0031] The alcohol used in the alcoholysis can be a lower alcohol such as methanol or ethanol, preferably methanol.

[0032] The base used in the alcoholysis can be potassium hydroxide, sodium hydroxide, etc.

[0033] The alcoholysis temperature is 30 - 50 °C.

[0034] There is no particular limitation on the arrangement order among the structural units, vinyl alcohol units, and vinyl ester units represented by formula (I) in the modified polyvinyl alcohol of the present invention, and it can be any one or more of random, block, and graft.

[0035] The detection method of the modified polyvinyl alcohol of the present invention is as follows: In the present invention, the viscosity-average degree of polymerization of polyvinyl alcohol is determined according to "GB / T 12010.9 - 1989 Determination Method for Average Degree of Polymerization of Polyvinyl Alcohol Resin".

[0036] In the present invention, the degree of alcoholysis of polyvinyl alcohol is determined according to "GB / T 12010.5 - 2010 Determination Method for Residual Acetic Acid Radical (or Degree of Alcoholysis) of Polyvinyl Alcohol Resin".

[0037] In the present invention, the content of the monomer unit represented by formula (I) in the modified polyvinyl alcohol is determined by 1H-NMR measurement of the modified polyvinyl alcohol (A). Specifically, the method can be carried out by dissolving the modified polyvinyl alcohol (A) in D 2 O, and using a 600 MHz 1H-NMR measurement device to carry out the measurement at 80 °C for determination.

[0038] In the present invention, the test method for the defoaming performance of the polyvinyl alcohol solution is as follows: Prepare an aqueous solution of PVA with a concentration of 4 wt%. Weigh 350 g of the PVA aqueous solution and add it to a 500 ml beaker. Heat the PVA aqueous solution to 60 °C, place it on a magnetic stirrer, stir at a speed of 1500 rpm for 1 minute and 30 seconds, then stop stirring, place it in a 60 °C water bath for heat preservation. At the same time, record the time, observe the state of the surface foam and the foam height, and record the time required for defoaming. A: The foam height is less than 4 cm, and the defoaming time is less than 10 minutes. B: The foam height is 4 - 8 cm, and the defoaming time is less than 30 minutes. C: The foam height is greater than 8 cm, and the defoaming time is greater than 30 minutes.

[0039] The evaluation method for the solubility of PVA in the present invention is as follows: Add 95 g of water at 20 °C to a 250 mL flask with stirring placed in a water bath, and stir at 300 rpm. Add 5 g of PVA particles passing through 15 - mesh to the water in the flask, and heat up to 80 °C. After 30 minutes, pass the aqueous solution through a 200 - mesh filter screen, and observe the amount of undissolved part. A: There is no dissolution residue. B: There is a slight dissolution residue. C: There is a relatively large amount of dissolution residue.

[0040] The evaluation method for the film strength under high humidity in the present invention is as follows: Prepare an aqueous solution of PVA with a concentration of 6%. Cast it on a glass plate and dry it at 20 °C for 1 week to obtain a film with a thickness of about 40 μm. Cut the prepared film into strips with a size of 10 mm × 80 mm. After conditioning in an atmosphere of 20 °C and 65% RH for 1 week, conduct a tensile test with a tensile testing machine; perform strength measurement under the conditions that the clamp spacing is set to 50 mm and the tensile speed is set to 500 mm / min. Measure 5 times, calculate the average value, and evaluate according to the following criteria: A: Above 300 kgf / mm. B: Above 260 kgf / mm and less than 300 kgf / mm. C: Above 220 kgf / mm and less than 260 kgf / mm.

[0041] Example 1 Raw material preparation: 810 parts by mass of vinyl acetate (VAC), 190 parts by mass of methanol (MeOH), isopentenyl polyoxyethylene polyoxypropylene ether (CH 2 =C(CH 3 )CH 2 CH 2 O(CH 2 CH 2 O) n (CHCH3 CH 2 O) m H, n = 3, m = 2) 8.5 parts by mass.

[0042] (1) Polymerization Using a polymerization tank equipped with a reflux condenser, a raw material feeding system, a reaction liquid discharging system, a thermometer, a nitrogen adding system, and stirring blades, vinyl acetate, methanol, and isopentenyl polyoxyethylene polyoxypropylene ether as a comonomer are added therein, and nitrogen is used to displace for 30 minutes while stirring. When the temperature in the reactor is raised to 64 °C, an initiator is added to start polymerization. After polymerizing at 64 °C for 280 minutes, cooling is carried out and polymerization is stopped. The polymerization rate at the end of polymerization is 60%. Then, unreacted monomers are removed to obtain a methanol solution of vinyl acetate-isopentenyl polyoxyethylene ether copolymer (modified PVAc).

[0043] (2) Alcoholysis Take 150 parts by mass of this methanol solution (the PVAc concentration in the solution is 32%), add a sodium hydroxide methanol solution (sodium hydroxide concentration 5.0%) thereto, and carry out alcoholysis at 40 °C (the molar ratio of sodium hydroxide in the sodium hydroxide methanol solution to the vinyl acetate unit in PVAc is 0.009). A jelly-like substance is formed about 8 minutes after the addition of the sodium hydroxide methanol solution, which is pulverized with a pulverizing device, and after standing for 7 minutes for alcoholysis at 40 °C, methyl acetate is added to neutralize the remaining alkali (confirmed using a phenolphthalein indicator). After the neutralization is completed, filtration is carried out to obtain a white solid.

[0044] (3) Washing The obtained white solid is washed with methanol 3 times, centrifuged to remove the liquid, and the white solid obtained by centrifuging and removing the liquid is dried in a vacuum dryer at 85 °C for 3 hours to obtain modified polyvinyl alcohol containing monomer units of structural formula (Ⅰ). The degree of polymerization is 1700, the degree of alcoholysis is 88.1 mol%, and the modification amount (the percentage content of monomer units of structural formula (Ⅰ) in the modified polyvinyl alcohol) is 0.45 mol%.

[0045] Example 2 Raw material preparation: 700 parts by mass of vinyl acetate (VAC), 300 parts by mass of methanol (MeOH), 18.2 parts by mass of comonomer (CH 2 =C(CH 3 )CH 2 CH 2 O(CH 2 CH 2 O)n(CHCH 3 CH 2 O)mH, n = 10, m = 20).

[0046] (1) Polymerization Using a polymerization tank equipped with a reflux condenser, a raw material feeding system, a reaction liquid discharging system, a thermometer, a nitrogen adding system, and stirring blades, vinyl acetate, methanol, and isopentenyl polyoxyethylene polypropylene ether as a comonomer are added therein, and nitrogen is purged for 30 minutes while stirring. When the temperature in the reactor is raised to 64 °C, an initiator is added to start the polymerization. After polymerizing at 64 °C for 280 minutes, cooling is carried out and the polymerization is stopped. The polymerization rate at the end of polymerization is 70%. Then, unreacted monomers are removed to obtain a methanol solution of vinyl acetate-isopentenyl polyoxyethylene ether copolymer (modified PVAc).

[0047] (2) Alcoholysis Take 150 parts by mass of this methanol solution (the PVAc concentration in the solution is 40%), add a sodium hydroxide methanol solution (sodium hydroxide concentration 5.0%) thereto, and carry out alcoholysis at 40 °C (the molar ratio of sodium hydroxide in the sodium hydroxide methanol solution to the vinyl acetate unit in PVAc is 0.009). A gel-like substance is formed about 8 minutes after adding the sodium hydroxide methanol solution, which is crushed with a crushing device. After standing for 7 minutes for alcoholysis at 40 °C, methyl acetate is added to neutralize the remaining alkali (confirmed using a phenolphthalein indicator). After the neutralization is completed, filtration is carried out to obtain a white solid.

[0048] (3) Washing The obtained white solid is washed 3 times with methanol, centrifuged to remove the liquid, and the white solid obtained by centrifuging and removing the liquid is dried in a vacuum dryer at 85 °C for 3 hours to obtain modified polyvinyl alcohol containing monomer units of structural formula (Ⅰ). The degree of polymerization is 1300, the degree of alcoholysis is 88.3 mol%, and the modification amount (the percentage content of monomer units of structural formula (Ⅰ) in the modified polyvinyl alcohol) is 0.20 mol%.

[0049] Example 3 Raw material preparation: 550 parts by mass of vinyl acetate (VAC), 450 parts by mass of methanol (MeOH), 33.9 parts by mass of comonomer (CH 2 =C(CH 3 )CH 2 CH 2 O(CH 2 CH 2 O) n (CHCH 3 CH 2 O) m H, n = 15, m = 10).

[0050] (1) Polymerization Use a polymerization tank equipped with a reflux condenser, a raw material feeding system, a reaction liquid discharging system, a thermometer, a nitrogen adding system, and stirring blades. Add vinyl acetate, methanol, and isopentenyl polyoxyethylene polyoxypropylene ether as a comonomer into it, and displace with nitrogen for 30 minutes while stirring. When the temperature in the reactor is raised to 64 °C, add an initiator to start polymerization. After polymerizing at 64 °C for 280 minutes, cool and stop polymerization. The polymerization rate at the end of polymerization is 80%. Then, remove the unreacted monomers to obtain a methanol solution of vinyl acetate-isopentenyl polyoxyethylene ether copolymer (modified PVAc).

[0051] (2)Alcoholysis Take 150 parts by mass of this methanol solution (the PVAc concentration in the solution is 45%), add a sodium hydroxide methanol solution (sodium hydroxide concentration 5.0%) to it, and perform alcoholysis at 40 °C (the molar ratio of sodium hydroxide in the sodium hydroxide methanol solution to the vinyl acetate unit in PVAc is 0.009). A jelly-like substance is formed about 8 minutes after adding the sodium hydroxide methanol solution. Crush it with a crushing device, place it at 40 °C for 7 minutes for alcoholysis, then add methyl acetate to neutralize the remaining alkali (confirmed using phenolphthalein indicator), and after the neutralization is completed, filter to obtain a white solid.

[0052] (3)Washing Wash the obtained white solid with methanol 3 times, centrifuge to remove the liquid, and dry the white solid obtained by centrifuging and removing the liquid in a vacuum dryer at 85 °C for 3 hours to obtain modified polyvinyl alcohol containing monomer units of structural formula (Ⅰ). The degree of polymerization is 800, the degree of alcoholysis is 88.2 mol%, and the modification amount (the percentage content of monomer units of structural formula (Ⅰ) in the modified polyvinyl alcohol) is 0.50 mol%.

[0053] Example 4 Raw material preparation: 500 parts by mass of vinyl acetate (VAC), 500 parts by mass of methanol (MeOH), 55.0 parts by mass of comonomer (CH 2 =C(CH 3 )CH 2 CH 2 O(CH 2 CH 2 O) n (CHCH 3 CH 2 O) m COCH 3 , n = 2, m = 3).

[0054] (1)Polymerization Using a polymerization tank equipped with a reflux condenser, a raw material feeding system, a reaction liquid discharging system, a thermometer, a nitrogen adding system, and stirring blades, vinyl acetate, methanol, and isopentenyl polyoxyethylene polyoxypropylene ether as a comonomer were added thereto, and nitrogen was purged for 30 minutes while stirring. When the temperature inside the reactor was raised to 64 °C, an initiator was added to start the polymerization. After polymerizing at 64 °C for 280 minutes, cooling was carried out and the polymerization was stopped. The polymerization rate at the end of polymerization was 85%. Then, the unreacted monomers were removed to obtain a methanol solution of vinyl acetate-isopentenyl polyoxyethylene ether copolymer (modified PVAc).

[0055] (2)Alcoholysis 150 parts by mass of this methanol solution (the PVAc concentration in the solution was 50%) was taken, and a sodium hydroxide methanol solution (sodium hydroxide concentration 5.0%) was added thereto, and alcoholysis was carried out at 40 °C (the molar ratio of sodium hydroxide in the sodium hydroxide methanol solution to the vinyl acetate unit in PVAc was 0.010). A jelly-like substance was formed about 8 minutes after the addition of the sodium hydroxide methanol solution, which was pulverized with a pulverizing device, and after standing at 40 °C for 7 minutes for alcoholysis, methyl acetate was added to neutralize the remaining alkali (confirmed using a phenolphthalein indicator). After the neutralization was completed, filtration was carried out to obtain a white solid.

[0056] (3)Washing The obtained white solid was washed 3 times with methanol, centrifuged to remove the liquid, and the white solid obtained by centrifuging and removing the liquid was dried in a vacuum dryer at 85 °C for 3 hours to obtain modified polyvinyl alcohol containing monomer units of structural formula (Ⅰ). The degree of polymerization was 500, the degree of alcoholysis was 88.4 mol%, and the modification amount (the percentage content of monomer units of structural formula (Ⅰ) in the modified polyvinyl alcohol) was 3.2 mol%.

[0057] Comparative Example 1 Raw material preparation: 500 parts by mass of vinyl acetate (VAC), 500 parts by mass of methanol (MeOH).

[0058] (1)Polymerization Using a polymerization tank equipped with a reflux condenser, a raw material feeding system, a reaction liquid discharging system, a thermometer, a nitrogen adding system, and stirring blades, vinyl acetate and methanol were added thereto, and nitrogen was purged for 30 minutes while stirring. When the temperature inside the reactor was raised to 64 °C, an initiator was added to start the polymerization. After polymerizing at 64 °C for 280 minutes, cooling was carried out and the polymerization was stopped. The polymerization rate at the end of polymerization was 86%. Then, the unreacted monomers were removed to obtain a methanol solution of polyvinyl acetate.

[0059] (2)Alcoholysis Take 150 parts by mass of this methanol solution (the PVAc concentration in the solution is 50%), add a sodium hydroxide methanol solution (sodium hydroxide concentration 5.0%) thereto, and carry out alcoholysis at 40 °C (the molar ratio of sodium hydroxide in the sodium hydroxide methanol solution to the vinyl acetate unit in PVAc is 0.008). A jelly-like substance is formed about 8 minutes after the addition of the sodium hydroxide methanol solution, which is pulverized with a pulverizing device. After alcoholysis at 40 °C for 7 minutes, methyl acetate is added to neutralize the remaining alkali (confirmed using a phenolphthalein indicator). After the neutralization is completed, filtration is carried out to obtain a white solid.

[0060] (3) Washing The obtained white solid is washed 3 times with methanol, centrifuged to remove the liquid, and the white solid obtained by centrifuging and removing the liquid is dried in a vacuum dryer at 85 °C for 3 hours to obtain polyvinyl alcohol. The degree of polymerization is 500, the degree of alcoholysis is 88.3 mol%, and there is no modification.

[0061] Comparative Example 2 Raw material preparation: 550 parts by mass of vinyl acetate (VAC), 450 parts by mass of methanol (MeOH), and 18 parts by mass of 2-methyl-2-propenyl acetate.

[0062] (1) Polymerization Using a polymerization tank equipped with a reflux condenser, a raw material feeding system, a reaction liquid discharging system, a thermometer, a nitrogen adding system, and stirring blades, vinyl acetate, methanol, and 2-methyl-2-propenyl acetate are added therein, and nitrogen replacement is carried out for 30 minutes while stirring. When the temperature in the reactor is raised to 64 °C, an initiator is added to start polymerization. After polymerization at 64 °C for 280 minutes, cooling is carried out and polymerization is stopped. The polymerization rate at the end of polymerization is 59%. Then, the unreacted monomer is removed to obtain a methanol solution of polyvinyl acetate.

[0063] (2) Alcoholysis Take 150 parts by mass of this methanol solution (the PVAc concentration in the solution is 45%), add a sodium hydroxide methanol solution (concentration 5.0%) thereto, and carry out alcoholysis at 40 °C, (the molar ratio of sodium hydroxide in the sodium hydroxide methanol solution to the vinyl acetate unit in PVAc is 0.009). A jelly-like substance is formed about 8 minutes after the addition of the sodium hydroxide methanol solution, which is pulverized with a pulverizing device. After alcoholysis at 40 °C for 7 minutes, methyl acetate is added to neutralize the remaining alkali (confirmed using a phenolphthalein indicator). After the neutralization is completed, filtration is carried out to obtain a white solid.

[0064] (3) Washing The obtained white solid is washed 3 times with methanol, centrifuged to remove the liquid, and the white solid obtained by centrifuging and removing the liquid is dried in a vacuum dryer at 85 °C for 3 hours to obtain polyvinyl alcohol. The degree of polymerization is 800, the degree of alcoholysis is 88.4 mol%, and the modification amount is 2.2 mol%.

[0065] Table 1 Process parameters used in the examples and comparative examples Table 2 Performance parameters of each polyvinyl alcohol product As can be seen from Table 2 above, the solution defoaming performance, dissolution performance and film strength effect of the polyvinyl alcohol prepared in Examples 1-4 of the present invention are better than those of Comparative Examples 1-2; it can thus be proved that the modified polyvinyl alcohol prepared in the present invention has good solubility in water, less foaming in aqueous solution and fast defoaming; at the same time, the modified polyvinyl alcohol has good film-forming property and high film strength.

[0066] The above are only the embodiments of the present invention. Specific structures and common knowledge such as characteristics that are well known in the art are not described in detail herein. Those of ordinary skill in the art know all the common general technical knowledge in the technical field to which the invention pertains before the filing date or the priority date, can know all the prior art in this field, and have the ability to apply the conventional experimental means before this date. Those of ordinary skill in the art can, under the inspiration given in this application, complete and implement this solution in combination with their own abilities. Some typical well-known structures or well-known methods should not be an obstacle for those of ordinary skill in the art to implement this application. It should be noted that for those skilled in the art, without departing from the structure of the present invention, several modifications and improvements can still be made, and these should also be regarded as the protection scope of the present invention, and these will not affect the implementation effect of the present invention and the practicality of the patent. The protection scope required by this application should be subject to the content of its claims, and the specific implementation manners described in the specification can be used to interpret the content of the claims.

Claims

1. Modified polyvinyl alcohol, Characterized in that: The polyvinyl alcohol is modified with the structural monomer unit shown in formula (I): （I） In formula (I), R 1 is a hydrogen atom or an alkyl chain, and n and m are each independently a positive integer.

2. The modified polyvinyl alcohol according to claim 1, Characterized in that: Said R 1 is hydrogen or a linear or branched alkyl group having 1 to 3 carbon atoms, n = 1 to 40 and m = 1 to 30.

3. The modified polyvinyl alcohol according to claim 1 or 2, Characterized in that: The content of the structural monomer unit shown in formula (I) is 0.1-10 mol% of the modified polyvinyl alcohol.

4. The modified polyvinyl alcohol according to any one of claims 1-3, Characterized in that: The content of the structural monomer unit shown in formula (I) is 0.3-6.0 mol% of the modified polyvinyl alcohol.

5. The modified polyvinyl alcohol according to any one of claims 1-4, Characterized in that: The degree of polymerization of the modified polyvinyl alcohol is 300-3000, and the degree of alcoholysis is 80 mol%-99.9 mol%.

6. The modified polyvinyl alcohol according to claim 5, Characterized in that: The degree of alcoholysis of the modified polyvinyl alcohol is 85 mol%-99 mol%.

7. The preparation method of the modified polyvinyl alcohol according to any one of claims 1-6, Characterized in that: Comprises the following steps: Copolymerize a vinyl ester monomer with the compound shown in formula (II) or formula (III), and then carry out alcoholysis: (Ⅱ) （Ⅲ） wherein, R 2 is an alkyl chain; R 1 is a hydrogen atom or an alkyl chain, and n and m are each independently a positive integer.

8. The preparation method of the modified polyvinyl alcohol according to claim 7, Characterized in that: Wherein R 2 is an alkyl group having 1 to 5 carbon atoms, R 1 is a hydrogen atom or a linear or branched alkyl group having 1 to 3 carbon atoms, n = 1 to 40 and m = 1 to 30.

9. The preparation method of the modified polyvinyl alcohol according to claim 8, wherein the compound shown in formula (II) or formula (III) is selected from one of the following compounds: CH 2 =C(CH 3 )CH 2 CH 2 O(CH 2 CH 2 O) 3 (CHCH 3 CH 2 O) 2 H; CH 2 =C(CH 3 )CH 2 CH 2 O(CH 2 CH 2 O) 10 (CHCH 3 CH 2 O) 20 H; CH 2 =C(CH 3 )CH 2 CH 2 O(CH 2 CH 2 O) 15 (CHCH 3 CH 2 O) 10 H; CH 2 =C(CH 3 )CH 2 CH 2 O(CH 2 CH 2 O) 2 (CHCH 3 CH 2 O) 3 COCH 3 。 10. The application of the modified polyvinyl alcohol according to any one of claims 1-6 in the water-soluble film and adhesive industries.

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