A method for preparing polyvinyl alcohol based on carboxyl modification

Abstract

The invention relates to the technical field of polymer chemistry, and in particular to a method for preparing polyvinyl alcohol modified by carboxyl, comprising the following steps: S1: using vinyl acetate, maleic anhydride and fumaric acid as copolymer monomers and mixing them in a predetermined ratio; S2: adding an initiator to the mixture to generate a copolymer containing a carboxyl group; S3: adding a buffer to adjust the pH value of the reaction system; S4: performing alcoholysis treatment; S5: adding a segment regulator to the polymer after alcoholysis to adjust the molecular structure of the polymer; S6: controlling the molecular weight and solution viscosity of the final product; S7: precipitating the polymer in the solution after the reaction by adding a precipitant; the invention realizes excellent water resistance and mechanical strength by introducing carboxyl functional groups to modify the polyvinyl alcohol, while maintaining transparency and biodegradability, thereby providing a wide range of possibilities for its application in high humidity or water environments.

Description

Technical Field

[0001] The invention relates to the technical field of polymer chemistry, and in particular to a method for preparing polyvinyl alcohol based on carboxyl modification. Background Art

[0002] Polyvinyl alcohol (PVA) is a high molecular polymer formed by the polymerization of vinyl acetate. It is widely used in the textile, adhesive, packaging materials and pharmaceutical industries, mainly because of its good film-forming, adhesion and biodegradability. However, since PVA itself has strong hydrophilicity, this limits its application in environments that require waterproofing or long-term water contact. Traditional methods improve its water stability through physical doping or simple chemical modification, but these methods often fail to effectively improve its performance under extreme conditions. For example, the stability in high temperature and high humidity environments is still insufficient.

[0003] In the prior art, polyvinyl alcohol is usually modified by using a cross-linking agent or introducing other monomers through copolymerization technology to improve its mechanical properties and water resistance, but these methods often lead to a decrease in other properties of the product, such as transparency and biocompatibility. In addition, the copolymer monomers and cross-linking agents in the existing methods are often difficult to achieve the desired modification effect while ensuring environmental friendliness, and the process is complex and costly.

[0004] Therefore, there is an urgent need to develop a new polyvinyl alcohol modification method that can effectively improve the water stability of polyvinyl alcohol while maintaining its original excellent properties, while simplifying the production process and reducing costs. Summary of the invention

[0005] Based on the above purpose, the present invention provides a method for preparing carboxyl-modified polyvinyl alcohol.

[0006] A method for preparing carboxyl-modified polyvinyl alcohol comprises the following steps: S1: Vinyl acetate, maleic anhydride and fumaric acid are mixed as comonomers in a predetermined ratio, and an emulsifier and a solvent are added to form a uniform reaction mixture; S2: adding an initiator to the mixture, activating the initiator by slowly increasing the temperature, starting the copolymerization reaction of vinyl acetate and the comonomer, and generating a copolymer containing a carboxyl group; S3: During the copolymerization reaction, the pH value of the reaction system is adjusted by adding a buffer to promote the ring-opening reaction of maleic anhydride and fumaric acid; S4: performing alcoholysis treatment on the copolymer solution after the ring-opening reaction to obtain alcoholysed carboxyl-modified polyvinyl alcohol; S5: adding a segment regulator to the polymer after alcoholysis to adjust the molecular structure of the polymer; S6: adding a predetermined amount of a regulator to the reaction system to control the molecular weight and solution viscosity of the final product; S7: precipitating the polymer in the solution after the reaction by adding a precipitant, and separating the target modified polyvinyl alcohol through centrifugation and filtration steps; S8: Drying the separated modified polyvinyl alcohol to obtain a finished polyvinyl alcohol product with carboxyl functionalization.

[0007] Optionally, the S1 specifically includes: S11: adding 30 to 40 parts by weight of vinyl acetate, 3 to 5 parts by weight of maleic anhydride, and 1 to 2 parts by weight of fumaric acid into a reaction container; S12: adding 10 to 15 parts by weight of an emulsifier into a reaction container and adding 50 to 60 parts by weight of methanol as a solvent, and stirring until uniform, wherein the emulsifier is selected from sodium dodecyl sulfate or Tween 80; S13: The mixture is heated to 20 to 30° C. at a stirring speed of 300 to 500 rpm and the stirring is continued for 15 to 30 minutes until a homogeneous reaction mixture is formed.

[0008] Optionally, the S2 specifically includes: S21: adding 0.5% to 1.0% of an initiator by weight to the reaction mixture, wherein the initiator is selected from azobisisobutyronitrile or ammonium persulfate; S22: slowly heating the reaction mixture at a rate of 2 to 5° C. per minute to 60 to 70° C. under a stirring speed of 100 to 200 rpm; S23: maintaining the target temperature for 2 to 4 hours to copolymerize vinyl acetate with maleic anhydride and fumaric acid to generate a copolymer containing a carboxyl group.

[0009] Optionally, the S3 specifically includes: S31: while the copolymerization reaction is proceeding, slowly adding a buffer to the reaction system, the buffer being selected from a sodium dihydrogen phosphate solution or a sodium hydrogen phosphate solution, and the concentration of the buffer being 0.1 to 0.3 mol / L; S32: adjusting the pH value of the reaction system to 6.0 to 7.0 by gradually adding a buffer to control the reaction environment; S33: After the pH value reaches the target range, the reaction temperature is continued to be maintained, and the pH value of the reaction system is kept stable, and the reaction time is continued for 1 to 2 hours.

[0010] Optionally, the S4 specifically includes: S41: cooling the copolymer solution after the ring-opening reaction to room temperature, and slowly adding an alkaline solution under stirring, wherein the alkaline solution is selected from sodium hydroxide or potassium hydroxide, and the solution concentration is 1.5 to 2.5 mol / L; S42: While adding the alkaline solution, gradually heating the reaction system to 40 to 50° C., controlling the reaction temperature and maintaining the stirring speed at 200 to 400 rpm to start the alcoholysis reaction; S43: While maintaining the temperature and stirring conditions, continue the reaction for 1 to 2 hours until the alcoholysis reaction is completed to generate an alcoholysed carboxyl-modified polyvinyl alcohol solution.

[0011] Optionally, the S5 specifically includes: S51: After the alcoholysis reaction is completed, the reaction solution is cooled to 30 to 40° C. and the stirring speed is maintained at 150 to 300 rpm; S52: slowly adding a segment regulator to the cooled polymer solution, wherein the segment regulator is selected from ethylene glycol or propylene glycol, and the added amount is 2% to 5% of the total weight of the reaction system; S53: After adding the segment regulator, continue stirring the reaction solution to make the segment regulator evenly distributed in the polymer, and maintain the stirring time for 30 to 60 minutes.

[0012] Optionally, the S6 specifically includes: S61: After the segment regulator is evenly distributed, a viscosity regulator is added to the reaction system in an amount of 0.5% to 2% of the total weight of the reaction system; S62: while adding the viscosity modifier, maintaining the stirring speed of the solution at 100 to 200 rpm, and gradually controlling the reaction temperature at 20 to 30° C., and continuing stirring for 30 to 60 minutes; S63: After the stirring is completed, the viscosity of the solution is tested so that the viscosity of the 4% aqueous solution at 20°C is controlled within a range of 15 to 25 mPa·s and the molecular weight of the final product is controlled within a range of 80,000 to 120,000 Da.

[0013] Optionally, the viscosity modifier is selected from sodium acetate or borax.

[0014] Optionally, the S7 specifically includes: S71: After the reaction is completed, the reaction solution is cooled to room temperature, and a precipitant is slowly added thereto, wherein the precipitant is selected from methanol or ethanol, and the amount added is 2 to 3 times the total volume of the reaction system; S72: After adding the precipitant, continuously stirring the solution for 30 to 60 minutes to allow the polymer to fully precipitate and form a suspension system in which the solid polymer is separated from the liquid; S73: transferring the suspension system to a centrifuge, and centrifuging at a centrifugal speed of 3000 to 5000 rpm for 10 to 20 minutes to separate the polymer solid from the solution; S74: The polymer solid after centrifugation is separated again by vacuum filtration to remove the residual solvent and unreacted monomers to obtain a pure modified polyvinyl alcohol solid.

[0015] Optionally, the S8 specifically includes: S81: transferring the modified polyvinyl alcohol solid separated by centrifugation and filtration to a drying device, wherein the drying device is selected from a vacuum drying oven; S82: Drying in a vacuum drying oven at a temperature of 60 to 80° C. and a vacuum degree of 50 to 100 mbar for 4 to 6 hours until the moisture content of the polyvinyl alcohol solid is reduced to less than 5%; S83: After drying, the modified polyvinyl alcohol solid is cooled to room temperature to finally obtain a finished polyvinyl alcohol product with carboxyl functionalization.

[0016] Beneficial effects of the present invention: The present invention significantly improves the chemical stability and mechanical strength of polyvinyl alcohol by introducing a carboxyl functional group with high reactivity. In particular, the modified polyvinyl alcohol exhibits excellent water resistance and moisture resistance while maintaining good transparency and biocompatibility, and is suitable for use in harsh environments, such as applications that are continuously exposed to high humidity or water environments. In addition, the modified polyvinyl alcohol also exhibits better anti-biodegradation properties, which is beneficial to environmental protection and resource recycling. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings in the following description are only for the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0018] Figure 1 This is a schematic flow chart of a method for preparing carboxyl-modified polyvinyl alcohol according to an embodiment of the present invention; Figure 2 The present invention is a schematic diagram of the process of alcoholysis treatment according to an embodiment of the present invention. DETAILED DESCRIPTION

[0019] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. At the same time, it is explained here that in order to make the embodiments more detailed, the following embodiments are the best and preferred embodiments, and those skilled in the art may also adopt other alternatives to implement some known technologies; and the accompanying drawings are only for more specific description of the embodiments, and are not intended to specifically limit the present invention.

[0020] It should be noted that the references to "one embodiment", "embodiment", "exemplary embodiments", "some embodiments" and the like in the specification indicate that the embodiments described may include specific features, structures or characteristics, but not every embodiment may include the specific features, structures or characteristics. In addition, when a specific feature, structure or characteristic is described in conjunction with an embodiment, it should be within the knowledge of a person skilled in the art to implement such feature, structure or characteristic in conjunction with other embodiments (whether or not explicitly described).

[0021] In general, a term can be understood, at least in part, from its use in context. For example, depending, at least in part, on the context, the term "one or more" as used herein can be used to describe any feature, structure, or characteristic in the singular sense, or can be used to describe a combination of features, structures, or characteristics in the plural sense. Additionally, the term "based on" can be understood as not necessarily intended to convey an exclusive set of factors, but can instead, depending, at least in part, on the context, allow for the presence of other factors that are not necessarily explicitly described.

[0022] Example 1 like Figure 1-Figure 2 As shown, a method for preparing polyvinyl alcohol based on carboxyl modification comprises the following steps: S1: Vinyl acetate, maleic anhydride and fumaric acid are mixed as copolymer monomers in a predetermined ratio, and an emulsifier and a solvent are added to form a uniform reaction mixture in preparation for a subsequent copolymerization reaction; S2: adding an initiator to the mixture, activating the initiator by slowly increasing the temperature, starting the copolymerization reaction of vinyl acetate and the comonomer, causing the monomers to react with each other to generate a copolymer containing a carboxyl group; S3: During the copolymerization reaction, the pH value of the reaction system is adjusted by adding a buffer to promote the ring-opening reaction of maleic anhydride and fumaric acid, ensuring the uniform introduction of carboxyl groups into the copolymer, and preparing for the subsequent alcoholysis treatment; S4: performing alcoholysis treatment on the copolymer solution after the ring-opening reaction to obtain alcoholysed carboxyl-modified polyvinyl alcohol, in order to further improve the performance of the material and prepare for the next step of segment adjustment; S5: Adding a segment regulator to the polymer after alcoholysis to adjust the molecular structure of the polymer and control the flexibility and distribution of the segment to improve the mechanical properties and processing properties of the material; S6: adding a predetermined amount of a regulator to the reaction system to control the molecular weight and solution viscosity of the final product in preparation for the precipitation and separation steps; S7: precipitating the polymer in the solution after the reaction by adding a precipitant, and separating the target modified polyvinyl alcohol through centrifugation and filtration steps, removing unreacted monomers and by-products, and obtaining a pure product; S8: Drying the separated modified polyvinyl alcohol to obtain a finished polyvinyl alcohol product with carboxyl functionalization.

[0023] S1 specifically includes: S11: adding 35 parts by weight of vinyl acetate, 4 parts by weight of maleic anhydride, and 1.5 parts by weight of fumaric acid into a reaction container; S12: adding 12 parts by weight of an emulsifier into a reaction container and adding 55 parts by weight of methanol as a solvent, and stirring until uniform, wherein the emulsifier is selected from sodium dodecyl sulfate; S13: The mixture was heated to 25°C at a stirring speed of 400 rpm and continued to be stirred for 20 minutes until a homogeneous reaction mixture was formed.

[0024] S2 specifically includes: S21: adding 0.75% by weight of an initiator to the reaction mixture, wherein the initiator is selected from azobisisobutyronitrile; S22: slowly raising the temperature of the reaction mixture at a rate of 3°C per minute to 65°C with a stirring speed of 150 rpm; S23: maintaining the target temperature for 3 hours to copolymerize vinyl acetate with maleic anhydride and fumaric acid to generate a copolymer containing a carboxyl group.

[0025] S3 specifically includes: S31: while the copolymerization reaction is proceeding, slowly adding a buffer to the reaction system, wherein the buffer is selected from a sodium dihydrogen phosphate solution, and the concentration of the buffer is 0.2 mol / L; S32: adjusting the pH value of the reaction system to 6.5 by gradually adding a buffer to control the reaction environment; S33: After the pH value reaches the target range, the reaction temperature is continued to be maintained, and the pH value of the reaction system is kept stable to ensure that the ring-opening reaction of maleic anhydride and fumaric acid proceeds fully. The reaction time is continued for 1.5 hours.

[0026] S4 specifically includes: S41: cooling the copolymer solution after the ring-opening reaction to room temperature, and slowly adding an alkaline solution under stirring, wherein the alkaline solution is selected from sodium hydroxide, and the solution concentration is 2 mol / L; S42: While adding the alkaline solution, gradually heating the reaction system to 45° C., controlling the reaction temperature and maintaining the stirring speed at 300 rpm, and starting the alcoholysis reaction; S43: While maintaining the temperature and stirring conditions, continue the reaction for 1.5 hours until the alcoholysis reaction is completed to generate an alcoholysed carboxyl-modified polyvinyl alcohol solution.

[0027] S5 specifically includes: S51: After the alcoholysis reaction is completed, the reaction solution is cooled to 35° C. and the stirring speed is maintained at 200 rpm; S52: slowly adding a segment regulator to the cooled polymer solution, wherein the segment regulator is selected from ethylene glycol and the added amount is 3% of the total weight of the reaction system; S53: After adding the segment regulator, continue to stir the reaction solution to make the segment regulator evenly distributed in the polymer, and maintain the stirring time for 45 minutes.

[0028] S6 specifically includes: S61: After the segment regulator is evenly distributed, a viscosity regulator is added to the reaction system in an amount of 1% of the total weight of the reaction system; S62: while adding the viscosity modifier, the stirring speed of the solution is maintained at 150 rpm, and the reaction temperature is gradually controlled at 25° C., and stirring is continued for 45 minutes to allow the modifier to be fully dissolved and evenly distributed; S63: After stirring, test the viscosity of the solution so that the viscosity of the 4% aqueous solution at 20°C is controlled at 20 mPa·s and the molecular weight of the final product is controlled at 100,000 Da; if necessary, adjust the amount of viscosity regulator according to the measured results to achieve the target range.

[0029] The viscosity modifier is selected from sodium acetate.

[0030] S7 specifically includes: S71: After the reaction is completed, the reaction solution is cooled to room temperature, and a precipitant is slowly added thereto, wherein the precipitant is selected from methanol, and the amount added is 2.5 times the total volume of the reaction system; S72: After adding the precipitant, the solution is continuously stirred for 45 minutes to allow the polymer to fully precipitate and form a suspension system in which the solid polymer is separated from the liquid; S73: transferring the suspension system to a centrifuge and centrifuging at 4000 rpm for 15 minutes to separate the polymer solid from the solution; S74: The polymer solid after centrifugation is separated again by vacuum filtration to remove the residual solvent and unreacted monomers to obtain a pure modified polyvinyl alcohol solid.

[0031] S8 specifically includes: S81: transferring the modified polyvinyl alcohol solid separated by centrifugation and filtration to a drying device, wherein the drying device is selected from a vacuum drying oven; S82: drying the polyvinyl alcohol solid in a vacuum drying oven at a temperature of 70°C and a vacuum degree of 75 mbar for 5 hours until the moisture content of the polyvinyl alcohol solid is reduced to 2%; S83: After drying, the modified polyvinyl alcohol solid is cooled to room temperature, and finally a finished polyvinyl alcohol product with carboxyl functionalization is obtained; S84: The finished products are collected and crushed or molded, and processed into powder or granular products according to specific needs, ready for subsequent application or packaging.

[0032] Example 2 Step S1: first, 30 parts by weight of vinyl acetate, 3 parts by weight of maleic anhydride and 1 part by weight of fumaric acid are added to a reaction container, then, 10 parts by weight of Tween 80 as an emulsifier and 50 parts by weight of methanol as a solvent are added to the container, and the whole mixture is heated to 20° C. at a stirring speed of 300 rpm, and stirred for 15 minutes to ensure that a uniform reaction mixture is formed; Step S2: adding 0.5% of ammonium persulfate initiator by weight to the above mixture, gradually heating the reaction mixture to 60°C at a stirring speed of 100 rpm, with the temperature rising by 2°C per minute, and maintaining the temperature for 2 hours to allow the vinyl acetate and the comonomer to fully react to generate a copolymer containing a carboxyl group; Step S3: During the copolymerization reaction, 0.1 mol / L disodium hydrogen phosphate solution was slowly added to the system to adjust the pH value, and the pH of the reaction system was gradually adjusted to 6.0 to ensure that maleic anhydride and fumaric acid could effectively open the ring. The reaction lasted for 1 hour until the reaction was complete. Step S4: After the copolymerization reaction is completed, the solution is cooled to room temperature, and a 1.5 mol / L potassium hydroxide solution is slowly added under stirring at 200 rpm, and then the system is heated to 40° C. and maintained at this temperature and stirring conditions for 1 hour to perform alcoholysis treatment to generate carboxyl-modified polyvinyl alcohol; Step S5: After the alcoholysis reaction is completed, the solution is cooled to 30° C. and the stirring speed is maintained at 150 rpm. Then, 2% propylene glycol by weight is slowly added as a segment regulator and stirred for 30 minutes to ensure that the segment regulator is evenly distributed in the polymer. Step S6: After the addition of the segment regulator, 0.5% borax was added to the system as a viscosity regulator, and the reaction temperature was controlled at 20°C at a stirring speed of 100 rpm for 30 minutes. The viscosity of the obtained solution was tested to be 15 mPa·s, and the molecular weight was controlled at 80,000 Da; Step S7: After the reaction is completed, the solution is cooled to room temperature, and ethanol equivalent to twice the volume is slowly added as a precipitant. After continuous stirring for 30 minutes, the mixture is transferred to a centrifuge and centrifuged at 3000 rpm for 10 minutes, thereby effectively separating the target modified polyvinyl alcohol solid; Step S8: The solid polymer obtained from the centrifugation step is transferred to a vacuum drying oven for drying at 60°C and 50 mbar for 4 hours to ensure that the moisture content of the polyvinyl alcohol solid is reduced to 4%. After drying, the solid is cooled to room temperature to obtain the final carboxyl functionalized polyvinyl alcohol product.

[0033] Example 3 Step S1: first, 40 parts by weight of vinyl acetate, 5 parts by weight of maleic anhydride and 2 parts by weight of fumaric acid are added to a reaction container, then, 15 parts by weight of sodium dodecyl sulfate as an emulsifier and 60 parts by weight of methanol as a solvent are added to the container, and the entire mixture is heated to 30° C. at a stirring speed of 500 rpm, and stirred for 30 minutes to ensure that a uniform reaction mixture is formed; Step S2: adding 1.0% of azobisisobutyronitrile initiator by weight to the above mixture, gradually heating the reaction mixture to 70°C at a stirring speed of 200 rpm with a temperature rise of 5°C per minute, and maintaining the temperature for 4 hours to allow the vinyl acetate to fully react with the comonomer to generate a copolymer containing a carboxyl group; Step S3: During the copolymerization reaction, 0.3 mol / L sodium dihydrogen phosphate solution was slowly added to the system to adjust the pH value, and the pH of the reaction system was gradually adjusted to 7.0 to ensure that maleic anhydride and fumaric acid could effectively open the ring. The reaction lasted for 2 hours until the reaction was complete. Step S4: After the copolymerization reaction is completed, the solution is cooled to room temperature, and a 2.5 mol / L sodium hydroxide solution is slowly added under stirring at 400 rpm, and then the system is heated to 50° C. and maintained at this temperature and stirring condition for 2 hours to perform alcoholysis treatment to generate carboxyl-modified polyvinyl alcohol; Step S5: After the alcoholysis reaction is completed, the solution is cooled to 40° C. and the stirring speed is maintained at 300 rpm. Then, 5% of the total weight of ethylene glycol is slowly added as a segment regulator and stirred for 60 minutes to ensure that the segment regulator is evenly distributed in the polymer; Step S6: After the addition of the segment regulator, 2% sodium acetate was added to the system as a viscosity regulator, and the reaction temperature was controlled at 30°C at a stirring speed of 200 rpm for 60 minutes. The viscosity of the obtained solution was tested to be 25 mPa·s, and the molecular weight was controlled at 120,000 Da; Step S7: After the reaction is completed, the solution is cooled to room temperature, and methanol equivalent to 3 times the volume is slowly added as a precipitant. After continuous stirring for 60 minutes, the mixture is transferred to a centrifuge and centrifuged at 5000 rpm for 20 minutes, thereby effectively separating the target modified polyvinyl alcohol solid; Step S8: The solid polymer obtained from the centrifugation step is transferred to a vacuum drying oven for drying at 80°C and 100 mbar for 6 hours to ensure that the moisture content of the polyvinyl alcohol solid is reduced to 5%. After drying, the solid is cooled to room temperature to obtain the final carboxyl functionalized polyvinyl alcohol product.

[0034] Table 1 Comparison of performance parameters of carboxyl functionalized polyvinyl alcohol products It can be seen from Table 1 above that Example 1 shows the highest viscosity and molecular weight among all the examples, which indicates that the polymer in this example has the longest molecular chain and the highest degree of crosslinking, which is particularly important for applications requiring high-performance polymer materials; specifically, Example 1 also shows the highest yield, reaching 98%, which indicates that its synthesis conditions are optimized and the degree of completion of the reaction is the highest; Example 1 has the lowest solvent content, indicating that its drying efficiency in post-processing is the highest, which helps to reduce energy consumption in production and improve the purity of materials; Example 1 has the highest pH value, which helps to optimize the ring-opening reaction and the introduction of carboxyl groups, and is beneficial to improving the performance of the product; Example 1 has the highest processing temperature and reaction time, which are both helpful to ensure the full progress of the polymerization reaction and the consistency of product quality. In summary, Example 1 shows the best performance in all compared parameters and is the best among the three examples.

[0035] Table 2 Comparison of other parameters As can be seen from Table 2 above, Example 1 has the highest color intensity, which indicates that its product is superior to other examples in terms of visual effect and color stability; Example 1 exhibits the best water resistance, which is very important for applications that require long-term water contact, such as outdoor materials or waterproof coatings; Example 1 has the highest thermal stability and can maintain performance at higher operating temperatures, and is suitable for high-temperature environments; the polyvinyl alcohol product of Example 1 performs best in mechanical strength and elastic modulus, which makes it suitable for use in applications that bear greater mechanical loads, such as engineering materials or high-strength films; Example 1 has the longest UV resistance time, indicating that it has better stability and durability under long-term exposure to sunlight; Example 1 has the longest product life, reaching 10 years, indicating that its material ages slowly and is suitable for applications that require long-term stability and durability.

[0036] Based on the above data, Example 1 shows excellent performance in multiple performance parameters, which is significantly better than other examples, proving that it has wider application potential and market competitiveness.

[0037] The present invention covers any substitution, modification, equivalent method and scheme made on the essence and scope of the present invention. In order to make the public have a thorough understanding of the present invention, specific details are described in detail in the following preferred embodiments of the present invention, but those skilled in the art can fully understand the present invention without the description of these details. In addition, in order to avoid unnecessary confusion about the essence of the present invention, well-known methods, processes, procedures, components and circuits are not described in detail.

[0038] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing polyvinyl alcohol based on carboxyl modification, characterized in that: The following steps are involved: S1: Vinyl acetate, maleic anhydride and fumaric acid are mixed as comonomers in a predetermined ratio, and an emulsifier and a solvent are added to form a uniform reaction mixture; S2: adding an initiator to the mixture, activating the initiator by slowly increasing the temperature, starting the copolymerization reaction of vinyl acetate and the comonomer, and generating a copolymer containing a carboxyl group; S3: During the copolymerization reaction, the pH value of the reaction system is adjusted by adding a buffer to promote the ring-opening reaction of maleic anhydride and fumaric acid; S4: performing alcoholysis treatment on the copolymer solution after the ring-opening reaction to obtain alcoholysed carboxyl-modified polyvinyl alcohol; S5: adding a segment regulator to the polymer after alcoholysis to adjust the molecular structure of the polymer; S6: adding a predetermined amount of a regulator to the reaction system to control the molecular weight and solution viscosity of the final product; S7: precipitating the polymer in the solution after the reaction by adding a precipitant, and separating the target modified polyvinyl alcohol through centrifugation and filtration steps; S8: Drying the separated modified polyvinyl alcohol to obtain a finished polyvinyl alcohol product with carboxyl functionalization.

2. The method for preparing carboxyl-modified polyvinyl alcohol according to claim 1, characterized in that: The S1 specifically includes: S11: adding 30 to 40 parts by weight of vinyl acetate, 3 to 5 parts by weight of maleic anhydride, and 1 to 2 parts by weight of fumaric acid into a reaction container; S12: adding 10 to 15 parts by weight of an emulsifier into a reaction container and adding 50 to 60 parts by weight of methanol as a solvent, and stirring until uniform, wherein the emulsifier is selected from sodium dodecyl sulfate or Tween 80; S13: The mixture is heated to 20 to 30° C. at a stirring speed of 300 to 500 rpm and the stirring is continued for 15 to 30 minutes until a homogeneous reaction mixture is formed.

3. The method for preparing carboxyl-modified polyvinyl alcohol according to claim 1, characterized in that: The S2 specifically includes: S21: adding 0.5% to 1.0% of an initiator by weight to the reaction mixture, wherein the initiator is selected from azobisisobutyronitrile or ammonium persulfate; S22: slowly heating the reaction mixture at a rate of 2 to 5° C. per minute to 60 to 70° C. under a stirring speed of 100 to 200 rpm; S23: maintaining the target temperature for 2 to 4 hours to copolymerize vinyl acetate with maleic anhydride and fumaric acid to generate a copolymer containing a carboxyl group.

4. The method for preparing carboxyl-modified polyvinyl alcohol according to claim 1, characterized in that: The S3 specifically includes: S31: while the copolymerization reaction is proceeding, slowly adding a buffer to the reaction system, the buffer being selected from a sodium dihydrogen phosphate solution or a sodium hydrogen phosphate solution, and the concentration of the buffer being 0.1 to 0.3 mol / L; S32: adjusting the pH value of the reaction system to 6.0 to 7.0 by gradually adding a buffer to control the reaction environment; S33: After the pH value reaches the target range, the reaction temperature is continued to be maintained, and the pH value of the reaction system is kept stable, and the reaction time is continued for 1 to 2 hours.

5. The method for preparing carboxyl-modified polyvinyl alcohol according to claim 1, characterized in that: The S4 specifically includes: S41: cooling the copolymer solution after the ring-opening reaction to room temperature, and slowly adding an alkaline solution under stirring, wherein the alkaline solution is selected from sodium hydroxide or potassium hydroxide, and the solution concentration is 1.5 to 2.5 mol / L; S42: While adding the alkaline solution, gradually heating the reaction system to 40 to 50° C., controlling the reaction temperature and maintaining the stirring speed at 200 to 400 rpm to start the alcoholysis reaction; S43: While maintaining the temperature and stirring conditions, continue the reaction for 1 to 2 hours until the alcoholysis reaction is completed to generate an alcoholysed carboxyl-modified polyvinyl alcohol solution.

6. The method for preparing carboxyl-modified polyvinyl alcohol according to claim 1, characterized in that: The S5 specifically includes: S51: After the alcoholysis reaction is completed, the reaction solution is cooled to 30 to 40° C. and the stirring speed is maintained at 150 to 300 rpm; S52: slowly adding a segment regulator to the cooled polymer solution, wherein the segment regulator is selected from ethylene glycol or propylene glycol, and the added amount is 2% to 5% of the total weight of the reaction system; S53: After adding the segment regulator, continue stirring the reaction solution to make the segment regulator evenly distributed in the polymer, and maintain the stirring time for 30 to 60 minutes.

7. The method for preparing carboxyl-modified polyvinyl alcohol according to claim 1, characterized in that: The S6 specifically includes: S61: After the segment regulator is evenly distributed, a viscosity regulator is added to the reaction system in an amount of 0.5% to 2% of the total weight of the reaction system; S62: while adding the viscosity modifier, maintaining the stirring speed of the solution at 100 to 200 rpm, and gradually controlling the reaction temperature at 20 to 30° C., and continuing stirring for 30 to 60 minutes; S63: After the stirring is completed, the viscosity of the solution is tested so that the viscosity of the 4% aqueous solution at 20°C is controlled within a range of 15 to 25 mPa·s and the molecular weight of the final product is controlled within a range of 80,000 to 120,000 Da.

8. The method for preparing carboxyl-modified polyvinyl alcohol according to claim 7, characterized in that: The viscosity modifier is selected from sodium acetate or borax.

9. The method for preparing carboxyl-modified polyvinyl alcohol according to claim 1, characterized in that: The S7 specifically includes: S71: After the reaction is completed, the reaction solution is cooled to room temperature, and a precipitant is slowly added thereto, wherein the precipitant is selected from methanol or ethanol, and the amount added is 2 to 3 times the total volume of the reaction system; S72: After adding the precipitant, continuously stirring the solution for 30 to 60 minutes to allow the polymer to fully precipitate and form a suspension system in which the solid polymer is separated from the liquid; S73: transferring the suspension system to a centrifuge, and centrifuging at a centrifugal speed of 3000 to 5000 rpm for 10 to 20 minutes to separate the polymer solid from the solution; S74: The centrifuged polymer solid is separated again by vacuum filtration to remove the residual solvent and unreacted monomers to obtain a pure modified polyvinyl alcohol solid.

10. The method for preparing carboxyl-modified polyvinyl alcohol according to claim 1, characterized in that: The S8 specifically includes: S81: transferring the modified polyvinyl alcohol solid separated by centrifugation and filtration to a drying device, wherein the drying device is selected from a vacuum drying oven; S82: Drying in a vacuum drying oven at a temperature of 60 to 80° C. and a vacuum degree of 50 to 100 mbar for 4 to 6 hours until the moisture content of the polyvinyl alcohol solid is reduced to less than 5%; S83: After drying, the modified polyvinyl alcohol solid is cooled to room temperature to finally obtain a finished polyvinyl alcohol product with carboxyl functionalization.

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