CN113248645B - Low-melting point polyvinyl alcohol and preparation method thereof - Google Patents
Low-melting point polyvinyl alcohol and preparation method thereof Download PDFInfo
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- CN113248645B CN113248645B CN202110694966.XA CN202110694966A CN113248645B CN 113248645 B CN113248645 B CN 113248645B CN 202110694966 A CN202110694966 A CN 202110694966A CN 113248645 B CN113248645 B CN 113248645B
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F218/00—Copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by an acyloxy radical of a saturated carboxylic acid, of carbonic acid or of a haloformic acid
- C08F218/02—Esters of monocarboxylic acids
- C08F218/04—Vinyl esters
- C08F218/08—Vinyl acetate
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F8/00—Chemical modification by after-treatment
- C08F8/12—Hydrolysis
Abstract
The invention belongs to the field of macromolecular compounds obtained by only using carbon-carbon unsaturated bond reaction, and particularly relates to low-melting-point polyvinyl alcohol and a preparation method thereof. The method comprises the following steps: 1) introducing a second monomer, a third monomer and vinyl acetate for copolymerization to prepare a polyvinyl acetate copolymer; 2) performing two-stage alcoholysis; 3) and (5) drying. According to the preparation method, the second monomer and the third monomer are introduced to change the main chain structure of the polyvinyl acetate copolymer, so that the polyvinyl alcohol obtained after alcoholysis of the polyvinyl acetate copolymer has the performance of low melting point, and the production requirement is met.
Description
Technical Field
The invention belongs to the field of macromolecular compounds obtained by only using carbon-carbon unsaturated bond reaction, and particularly relates to low-melting-point polyvinyl alcohol and a preparation method thereof.
Background
Polyvinyl alcohol (PVA) is a water-soluble polymer, and its special properties make it widely used. For example: textile industry, construction industry, coating field, adhesives field, electronic ceramics, etc.
Polyvinyl alcohol is used as a completely degradable material and is a water-soluble high polymer, and the traditional processing of materials such as polyethylene, polypropylene and polyvinyl chloride is far from being realized in the field of plastic processing, because a large amount of hydroxyl groups on a polyvinyl alcohol molecular chain enable a large amount of hydrogen bonds to be formed in molecules and among molecules, and the melting temperature (220 + 240 ︒ C) and the decomposition temperature (200 + 250 ︒ C) of polyvinyl alcohol (PVA) are very close, so that the polyvinyl alcohol can not be extruded and processed by a traditional hot melting method.
A process for physically modifying PVA to make its melting point lower than decomposition temperature to realize thermoplastic processing of PVA into film is provided in the market, the process is to modify materials by adding plasticizer auxiliary agent, the selectable plasticizer is mainly polyhydroxy micromolecule substance, alcohol amine substance or phthalein amine substance, these substances and hydroxyl on PVA molecular chain form hydrogen bond, break the hydrogen bond between PVA and reduce the melting point of PVA to realize thermoplastic processing of PVA.
But the existence of a large amount of auxiliary agents reduces the biodegradability of the PVA product, reduces the environmental protection performance and does not meet the market demand.
Disclosure of Invention
The invention aims to overcome the defects in the prior art and provides polyvinyl alcohol with low melting point and a preparation method thereof.
In order to achieve the purpose, the invention adopts the technical scheme that:
a preparation method of low-melting-point polyvinyl alcohol comprises the following steps:
1) introducing a second monomer, a third monomer and vinyl acetate for copolymerization to prepare a polyvinyl acetate copolymer; wherein, the second monomer is an alcohol ether monomer containing alkenyl; the addition amount is 0.0003 to 1.15 weight percent of the vinyl acetate; the third monomer is an alcohol monomer containing alkenyl, and the addition amount of the third monomer is 0.0002 to 2.55 weight percent of the vinyl acetate;
2) performing two-stage alcoholysis; diluting the polyvinyl acetate copolymer obtained in the step 1), adding a catalyst, and simultaneously adding a suspension solvent which is 40-150 wt% of the polymerization solution for secondary alcoholysis to obtain low-melting-point polyvinyl alcohol;
3) and (3) a drying process: and (3) putting the polyvinyl alcohol subjected to alcoholysis and solvent removal into an aluminum tray, putting the tray into a drying oven with vacuumizing for drying, and stirring the tray at the drying temperature of 85-150 ℃ for 1.5-5.5 hours.
Preferably, the second monomer in step 1) is one of vinyl diethylene glycol ether, diethylene glycol monovinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether or a mixture thereof, and the third monomer is one of dibromobutenediol, trimethylbutenediol, 1, 4-butenediol, 1, 4-butynediol or 1, 4-butenediol diacetate (ethacrylate), 1, 2-ethanediol diacetate or a mixture thereof.
Preferably, the first and second liquid crystal materials are,
the adding amount of the second monomer in the step 1) is 0.02-0.05 wt%; the third monomer is added in an amount of 0.12 to 0.3 wt%.
Preferably, the specific steps of step 1) are as follows:
1a) the method comprises the following steps Adding a certain amount of vinyl acetate monomer into a normal-pressure stirring polymerization kettle, and then adding a certain amount of second monomer to carry out bulk polymerization, or adding a solution with the concentration of the vinyl acetate monomer not more than 10% to carry out solution polymerization;
1b) the method comprises the following steps During the polymerization, a mixed solution of a solvent containing a fixed amount of a third monomer in an amount equal to the weight of the vinyl acetate monomer is injected by an injection pump; the injection time is half of the polymerization time, the whole polymerization time is 8-16 hours, when the conversion rate exceeds 65%, the polymerization is finished, the temperature is raised to 90 ︒ C, the residual monomer is evaporated and recovered, and the polyvinyl acetate copolymer is obtained.
Preferably, the solvent in step 1 b) is one or a mixture of methanol, ethanol, isopropanol, methyl acetate or butyl acetate.
Preferably, the suspending solvent in the step 2) is one or a mixture of isododecane, n-heptane, liquid paraffin, white oil, or n-alkane with more than C15.
Preferably, the specific steps of step 2) are as follows:
2a) the method comprises the following steps Adding a polyvinyl acetate copolymer solution diluted by a solvent into a stirring alcoholysis kettle with good heat exchange, wherein the diluted concentration is 15-55 wt%, adding 0.25-5.5% millimole of alkali solution or acid solution as a catalyst under the stirring condition, and simultaneously adding 40-150 wt% of suspension solvent which is equivalent to the polymerization solution, and carrying out alcoholysis reaction for 0.35-3.2 hours at the reaction temperature of 30 ︒ -50 ℃; after the set reaction time is reached and solid materials are separated out, centrifuging;
preferably, in a stirring alcoholysis kettle with good heat exchange, adding a polyvinyl acetate copolymer solution diluted by a solvent, wherein the concentration of the diluted polyvinyl acetate copolymer solution is 25wt% -40wt%, adding 0.22% -0.7% millimole of alkali solution or acid solution as a catalyst under the stirring condition, and simultaneously adding 100wt% of suspension solvent which is equivalent to the polymerization solution, and carrying out alcoholysis reaction for 1.5-3 hours at the reaction temperature of 35-70 ℃; after the set reaction time is reached and solid materials are separated out, centrifuging; 2b) the method comprises the following steps Adding the solid material obtained by centrifugation into another stirring alcoholysis kettle, and mixing the solid material with the stirring alcoholysis kettle according to the weight ratio of 1: 1.0-1.3, adding a solvent, continuously adding 0.25-5.5% millimole of alkali solution or acid solution as a catalyst under the stirring condition for alcoholysis reaction for 0.35-3.2 hours at the reaction temperature of 30 ︒ -50 ℃ until the reaction is finished at the set time, and centrifuging and washing the solid to obtain the material.
Preferably, the solid material obtained by centrifugation is added into another stirring alcoholysis kettle, and the weight ratio of the solid material to the solid material is 1: 1.0-1.3, adding a solvent, continuously adding 0.22-0.7% millimole of alkali solution or acid solution as a catalyst under the stirring condition for alcoholysis reaction for 0.35-3.2 hours at the reaction temperature of 35 ︒ -70 ℃ until the reaction is finished within a set time, and centrifuging and washing the solid to obtain the material.
Preferably, the alkali is potassium hydroxide or sodium hydroxide, and the diluting solvent is methanol, ethanol, isopropanol or n-butanol or a mixture thereof; the acid is inorganic acid or organic acid; the inorganic acid is hydrochloric acid or sulfuric acid; the organic acid is benzene sulfonic acid or p-methyl benzene sulfonic acid.
The invention also comprises the low-melting-point polyvinyl alcohol obtained by the preparation method of the low-melting-point polyvinyl alcohol, and the appearance of the low-melting-point polyvinyl alcohol is non-crushed white spherical small particles;
the polymerization degree is: 860 and 1530; the alcoholysis degree is as follows: 86.9-99.2 mol%; viscosity of 4% solution: 8.6-16.8 mPa.s; melting point: 170 ℃ and 204 ℃.
Compared with the prior art, the invention has the beneficial effects that:
according to the preparation method, the second monomer and the third monomer are introduced to change the main chain structure of the polyvinyl acetate copolymer, so that the polyvinyl alcohol obtained after alcoholysis of the polyvinyl acetate copolymer has the performance of low melting point, and the production requirement is met.
Meanwhile, the polymerization reaction is discontinuous batch polymerization, and because the dosage of the initiator in the early stage is small, the possibility that the bulk polymerization mode of the VAM, the second monomer, the third monomer and the initiator in the traditional process can generate implosion and even explosion because heat is not transferred out is avoided.
Compared with the traditional belt continuous process, the adopted batch alcoholysis method does not need the subsequent crushing, and has low ash content, high purity and short flow. Meanwhile, the reason of the two-step alcoholysis is that methyl acetate is generated by one-step alcoholysis in the kettle-type alcoholysis process and consumes the alcoholysis catalyst, so that the alcoholysis degree in one step cannot reach more than 95.0 mol%, and the alcoholysis degree in the second step can reach high alcoholysis degree even more than 99.0 mol% after methyl acetate is removed by centrifugation in the alcoholysis.
The low-melting-point polyvinyl alcohol produced by chemical modification through copolymerization can be blown or injected by using traditional plastic processing equipment, so that the processability of the polyvinyl alcohol is greatly improved.
Detailed Description
In order to make those skilled in the art better understand the technical solution of the present invention, the following provides a detailed description of the present invention with reference to the embodiments.
Example 1A 1500ml stirred glass reaction flask equipped with a glass condenser tube and cooled with water below 20C 20 ︒ C was placed in a temperature-controlled jacket, 100g of vinyl acetate, 0.05g (0.05%) of ethylene glycol ether and 0.04g of azobisisoheptonitrile as initiator were added, stirring was started, nitrogen was introduced to displace air, the flask was heated to 56 ℃ and polymerization was started, 100g of a methanol solution containing 0.14g of 1, 4-butynediol was injected by a syringe pump at 80 minutes for 5.5 hours and the reaction was terminated after 11 hours of total reaction time. After the polymerization was completed, 50g of methanol was added and the temperature was raised to 90 ︒ C to evaporate for 2.5 hours, thereby recovering the residual monomer.
Testing the solid content of the polymer, adding methanol to adjust the content to 25wt%, weighing 200g of polymerization solution, adding the polymerization solution into a 1500ml glass reaction bottle with a stirrer, then adding 12ml of methanol solution of 0.45% millimole sodium hydroxide, stirring uniformly, adding 210 g of white oil, carrying out alcoholysis at 35 ℃ for 1.5 hours, and observing the precipitation of particles;
and adding the filtered solid material into another stirring glass alcoholysis bottle, adding 50g of methanol, continuously adding 14ml of 0.45% millimole sodium hydroxide methanol solution, carrying out alcoholysis at 35 ℃ for 1.5 hours to terminate the reaction, and filtering to remove the methanol to obtain the solid material.
The material was placed in an aluminum pan and dried in a desiccator with vacuum at 125 ℃ for 1 hour to give white solid spherical small particles of the present invention.
The test results were as follows:
total solid content: 98.7 percent
Polymerization degree: 1530
Degree of alcoholysis (mole%): 98.9 percent
Viscosity 20 ︒ C, 4% aqueous solution: 16.8mPa.s
Melting point (23 ℃, 65% RH) ︒ C: 190
Example 2A 1500ml glass reaction flask with stirring was equipped with a glass condenser and cooled with water below 20 ︒ C, placed in a controlled temperature rise jacket, 100g vinyl acetate, 0.05g triethylene glycol divinyl ether, 0.1g 97.0% t-butyl peroxyneodecanoate as initiator, stirred, purged with nitrogen to displace air, heated to 60 ℃ and polymerization started, 100g of a methanol solution containing 0.12g trimethylbutylene glycol was injected by an injection pump when the reaction reached 50 minutes, the injection time was 4.5 hours, and the reaction was terminated after 9 hours of total reaction time. After the polymerization, 50g of methanol was added and the temperature was raised to 80-90 ︒ C to evaporate for 2.5 hours and recover the residual monomer.
Testing the solid content of the polymer, adding methanol to adjust the content to 30wt%, weighing 200g of diluted polymerization solution, adding the polymerization solution into a 1500ml glass reaction bottle with a stirrer, adding 15ml of methanol solution of 0.3% millimole sodium hydroxide, stirring uniformly, adding 200g of liquefied paraffin, carrying out alcoholysis at 38 ℃ for 1.5 hours, observing that particles are separated out,
and adding the filtered solid material into another stirring glass alcoholysis bottle, adding 50g of methanol, continuously adding 15ml of 0.3% millimole sodium hydroxide methanol solution, performing alcoholysis at 38 ℃ for 1.5 hours to terminate the reaction, filtering, washing and removing the methanol to obtain the solid material.
The material was placed in an aluminum pan and dried in a desiccator with vacuum at 125 ℃ for 1 hour to give white solid spherical small particles of the present invention.
The test results were as follows:
total solid content: 98.0 percent
Polymerization degree: 1300
Degree of alcoholysis (mole%): 95.2 percent
Viscosity 20 ︒ C, 4% aqueous solution: 13.2mPa.s
Melting point (23 ℃, 65% RH) ︒ C: 184
Example 3A 1500ml stirred three-neck glass reaction flask was equipped with a glass condenser and cooled with water below 20 ︒ C, placed in a jacket with controlled temperature rise, 100g of vinyl acetate monomer, 8g of methanol, 0.05g of triethylene glycol divinyl ether, 0.095g of 97.0% t-butyl peroxyneodecanoate as initiator were added, stirring was started, nitrogen was introduced to displace air, the flask was heated to 58 ℃ and the polymerization was started and timed, 100g of a methanol solution containing 0.25g of 1, 4-butenediol was injected by an injection pump when the reaction reached 50 minutes, the injection time was 4.5 hours, and the total reaction time was terminated after 9 hours. After the polymerization was completed, 50g of methanol was added and the temperature was raised to 90 ︒ C to evaporate for 2.5 hours, thereby recovering the residual monomer.
Testing the solid content of the polymer, adding methanol to adjust the content to 35wt%, weighing 200g of diluted polymerization solution, adding the polymerization solution into a 1500ml glass reaction bottle with a stirrer, adding 15ml of methanol solution of 0.32% millimole sodium hydroxide, stirring uniformly, adding 200g of liquefied paraffin corresponding to the reaction mixture, carrying out alcoholysis at 40 ℃ for 1.5 hours, observing the precipitation of particles,
and adding the filtered solid material into another stirring glass alcoholysis bottle, adding 50g of methanol, continuously adding 25ml of 0.35% millimole sodium hydroxide methanol solution, performing alcoholysis at 40 ℃ for 1.5 hours to terminate the reaction, filtering, washing and removing the methanol to obtain the solid material.
This material was placed in an aluminum pan and dried in a desiccator with vacuum at 110 ℃ for 1.5 hours to give white solid spherical small particles of the present invention.
The test results were as follows:
total solid content: 98.0 percent
Polymerization degree: 995
Degree of alcoholysis (mole%): 99.2 percent
Viscosity 20 ︒ C, 4% aqueous solution: 10.5mPa.s
Melting point (23 ℃, 65% RH) ︒ C: 192
Example 4: adding a glass condenser tube on a 1500ml stirring three-mouth glass reaction bottle, cooling with water below 20 ︒ ℃, placing the glass reaction bottle in a jacket with controllable temperature rise, adding 100g vinyl acetate monomer, 8g ethanol, 0.05g vinyl diglycol ether and 0.195g 95.0% di (3, 5, 5-trimethylhexanoyl) peroxide as an initiator, starting stirring, introducing nitrogen to replace air, heating the reaction bottle, raising the temperature to 60 ℃, starting a polymerization reaction timing, injecting 100g methanol solution containing 0.2g 1, 4-butynediol by an injection pump when the reaction reaches 40 minutes, wherein the injection time is 4.5 hours, and ending after the total reaction time is 9 hours.
After the polymerization was completed, 50g of methanol was added and the temperature was raised to 90 ︒ C to evaporate for 2.5 hours, thereby recovering the residual monomer.
Testing the solid content of the polymer, adding methanol to adjust the content to 40wt%, weighing 200g of diluted polymerization solution, adding the polymerization solution into a 1500ml glass reaction bottle with a stirrer, adding 15ml of methanol solution of 0.22% millimole sodium hydroxide, stirring uniformly, adding 200g of liquefied paraffin corresponding to the reaction mixture, carrying out alcoholysis at 40 ℃ for 1.5 hours, observing the precipitation of particles,
and adding the filtered solid material into another stirring glass alcoholysis bottle, adding 50g of methanol, continuously adding 30ml of 0.3% millimole sodium hydroxide methanol solution, performing alcoholysis at 40 ℃ for 1.5 hours to terminate the reaction, filtering, washing and removing the methanol to obtain the solid material.
This material was placed in an aluminum pan and dried in a desiccator with vacuum at 120 ℃ for 1.5 hours to give white solid spherical small particles of the present invention.
The test results were as follows:
total solid content: 99.0 percent
Polymerization degree: 910
Degree of alcoholysis (mole%): 99.2 percent
Viscosity 20 ︒ C, 4% aqueous solution: 9.3mPa.s
Melting point (23 ℃, 65% RH) ︒ C: 204
Example 5: the polymerization solution of the above example 4 was taken, the content of the test polymer was adjusted to 35% by weight by adding methanol, 200g of the diluted polymerization solution was weighed and charged into a 1500ml glass reaction flask with stirring, then 40ml of a 0.7% mmol solution of benzenesulfonic acid in methanol was added, 200g of liquefied paraffin corresponding to the reaction mixture was added with stirring, alcoholysis was carried out at 60 ℃ for 3.5 hours to observe the precipitation of particles,
and adding the filtered solid material into another stirring glass alcoholysis bottle, adding 50g of methanol, continuously adding 40ml of 0.7% millimole benzenesulfonic acid methanol solution, carrying out alcoholysis for 2.0 hours at 70 ℃ to terminate the reaction, filtering, washing and removing the methanol to obtain the solid material.
This material was placed in an aluminum pan and dried in a desiccator with vacuum at 120 ℃ for 1.5 hours to give white solid spherical small particles of the present invention.
The test results were as follows:
total solid content: 96.7 percent
Polymerization degree: 910
Degree of alcoholysis (mole%): 95.1 percent
Viscosity 20 ︒ C, 4% aqueous solution: 9.1mPa.s
Melting point (23 ℃, 65% RH) ︒ C: 194
Example 6: adding a glass condenser tube on a 1500ml glass reaction bottle with a stirring three-opening, cooling with water below 20 ︒ ℃, placing the bottle in a jacket with controllable temperature rise, adding 100g vinyl acetate monomer, 5g methanol, 5g ethanol, 0.02g vinyl diglycol ether and 0.15g95.0% bis (3, 5, 5-trimethylhexanoyl) peroxide as an initiator, starting stirring, introducing nitrogen to replace air, heating the reaction bottle, raising the temperature to 60 ℃, starting a polymerization reaction, injecting 100g methanol solution containing 0.3g 1, 4-butynediol by an injection pump when the reaction reaches 40 minutes, wherein the injection time is 4.5 hours, and ending after the total reaction time is 9 hours.
After the polymerization was completed, 50g of methanol was added and the temperature was raised to 90 ︒ C to evaporate for 2.5 hours, thereby recovering the residual monomer.
Testing the solid content of the polymer, adding methanol to adjust the content to 40wt%, weighing 200g of diluted polymerization solution, adding the polymerization solution into a 1500ml reaction bottle with stirring glass, then adding 15ml of methanol solution of 0.25% millimole of sodium hydroxide, stirring uniformly, then adding 200g of liquefied paraffin corresponding to the reaction mixture, observing particle precipitation at 35 ℃ for alcoholysis for 1.5 hours, carrying out suction filtration to obtain a solid material, adding the solid material into another bottle with stirring glass for alcoholysis, adding 50g of methanol, continuously adding 30ml of methanol solution of 0.3% millimole of sodium hydroxide, terminating the reaction at 35 ℃ for alcoholysis for 2.5 hours, carrying out suction filtration, washing and removing the methanol to obtain the solid material.
This material was placed in an aluminum pan and dried in a desiccator with vacuum at 120 ℃ for 1.5 hours to give white solid spherical small particles of the present invention.
The test results were as follows:
total solid content: 99.0 percent
Polymerization degree: 860
Degree of alcoholysis (mole%): 86.9 percent
Viscosity 20 ︒ C, 4% aqueous solution: 8.6mPa.s
Melting point (23 ℃, 65% RH) ︒ C: 170
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (5)
1. A preparation method of polyvinyl alcohol with low melting point is characterized by comprising the following steps:
1) introducing a second monomer, a third monomer and vinyl acetate for copolymerization to prepare a polyvinyl acetate copolymer; wherein, the second monomer is an alcohol ether monomer containing alkenyl; the addition amount is 0.02-0.05wt% of vinyl acetate; the third monomer is an alcohol monomer containing alkenyl, and the addition amount of the third monomer is 0.12 to 0.3 weight percent of the vinyl acetate;
the method comprises the following specific steps:
1a) the method comprises the following steps Adding a certain amount of vinyl acetate monomer into a normal-pressure stirring polymerization kettle, then adding a certain amount of second monomer for bulk polymerization, or adding a solution with the concentration of the vinyl acetate monomer not more than 10% for solution polymerization; the second monomer is one of vinyl diethylene glycol ether and diethylene glycol monovinyl ether or a mixture of the two;
1b) the method comprises the following steps During the polymerization, a fixed amount of a mixed solution of a third monomer and a solvent of the same weight as that of the vinyl acetate monomer is injected by an injection pump; the injection time is half of the polymerization time, the whole polymerization time is 8-16 hours, when the conversion rate exceeds 65%, the polymerization is finished, the temperature is raised to 90 ︒ C, the residual monomer is evaporated and recovered, and the polyvinyl acetate copolymer is obtained; the third monomer is one or a mixture of dibromobutenediol, trimethylbutenediol, 1, 4-butenediol, 1, 4-butynediol or 1, 4-butenediol diacetate;
2) performing two-stage alcoholysis; diluting the polyvinyl acetate copolymer obtained in the step 1), adding a catalyst, and simultaneously adding a suspension solvent which is 40-150 wt% of the polymerization solution for secondary alcoholysis to obtain low-melting-point polyvinyl alcohol;
3) and (3) a drying process: putting the polyvinyl alcohol with the solvent removed after alcoholysis into an aluminum tray, putting the tray into a drying box with a vacuumizing function for drying, and stirring; drying at 85-150 deg.C for 1.5-5.5 hr to obtain low-melting-point polyvinyl alcohol in the form of non-pulverized white spherical small particles; the polymerization degree is: 860 and 1530; the alcoholysis degree is as follows: 86.9-99.2 mol%; viscosity of 4% solution: 8.6-16.8 mPa.s; melting point: 170 ℃ and 204 ℃.
2. The method for preparing polyvinyl alcohol with low melting point according to claim 1, wherein the solvent in step 1 b) is one or a mixture of methanol, ethanol, isopropanol, methyl acetate or butyl acetate.
3. The method for producing a low melting point polyvinyl alcohol according to claim 1, wherein the suspending solvent in the step 2) is one or a mixture of isododecane, n-heptane, liquid paraffin, white oil, or n-alkane having at least C15.
4. The method for preparing polyvinyl alcohol with low melting point according to claim 1, wherein the specific steps of step 2) are as follows:
2a) the method comprises the following steps Adding a polyvinyl acetate copolymer solution diluted by a solvent into a stirring alcoholysis kettle with good heat exchange, wherein the diluted concentration is 15-55 wt%, adding 0.25-5.5% millimole of alkali solution or acid solution as a catalyst under the stirring condition, and simultaneously adding 40-150 wt% of suspension solvent which is equivalent to the polymerization solution, and carrying out alcoholysis reaction for 0.35-3.2 hours at the reaction temperature of 30 ︒ -50 ℃; after the set reaction time is reached and solid materials are separated out, centrifuging;
2b) the method comprises the following steps Adding the solid material obtained by centrifugation into another stirring alcoholysis kettle, and mixing the solid material with the stirring alcoholysis kettle according to the weight ratio of 1: 1.0-1.3, adding a solvent, continuously adding 0.25-5.5% millimole of alkali solution or acid solution as a catalyst under the stirring condition for alcoholysis reaction for 0.35-3.2 hours at the reaction temperature of 30 ︒ -50 ℃ until the reaction is finished for a set time, and centrifuging and washing the solid to obtain the material.
5. The method for preparing polyvinyl alcohol having low melting point according to claim 4, wherein the alkali in the alkali solution is potassium hydroxide or sodium hydroxide, and the diluting solvent is methanol, ethanol, isopropanol, n-butanol or a mixture thereof; the acid in the acid solution is inorganic acid or organic acid; the inorganic acid is hydrochloric acid or sulfuric acid; the organic acid is benzene sulfonic acid or p-methyl benzene sulfonic acid.
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