A polyvinyl acetal resin and its preparation method based on supercritical carbon dioxide medium.

Abstract

This invention relates to the field of polymer synthesis technology, and more particularly to a polyvinyl alcohol acetal resin and its preparation method based on supercritical carbon dioxide medium. The invention places polyvinyl alcohol resin and aldehyde compounds in a supercritical carbon dioxide medium, and performs an acetalization reaction under specific conditions without adding any external acid catalyst. Subsequently, online extraction and purification are performed using supercritical carbon dioxide, and the polyvinyl alcohol acetal resin product in a specific form is directly obtained by controlling the decompression parameters, simultaneously achieving product purification and morphology control. The preparation method provided by this invention requires no acid catalyst, no cumbersome post-treatment such as neutralization and washing, is simple, green, and environmentally friendly, and produces polyvinyl alcohol acetal resin products with extremely high purity and controllable morphology. It overcomes the inherent defects of existing methods, such as the need for external acid catalysts, complex post-treatment, limited product purity, and significant environmental pollution.

Description

Technical Field

[0001] This invention relates to the field of polymer material synthesis technology, and in particular to a polyvinyl alcohol acetal resin and its preparation method based on supercritical carbon dioxide medium. Background Technology

[0002] Polyvinyl acetal resins, especially polyvinyl butyral (PVB), are widely used in fields such as interlayers for safety glass, coatings, and adhesives due to their excellent film-forming properties, adhesion to substrates (such as glass), transparency, and flexibility.

[0003] Currently, the industrial method for large-scale production of polyvinyl acetal resin is the aqueous-phase acid-catalyzed acetalization method. A typical preparation process involves: first, dissolving polyvinyl alcohol (PVA) resin in water to form a resin solution of a certain concentration; then adding an aldehyde compound (e.g., butyraldehyde) to the resin solution; next, adding a strong inorganic acid (e.g., hydrochloric acid or sulfuric acid) as a catalyst; and finally, carrying out the acetalization reaction under heating conditions (usually 60–100°C). During the acetalization reaction, the hydroxyl groups on the PVA chain undergo dehydration condensation with the aldehyde compound to form an acetal structure. As the acetalization reaction proceeds, the polyvinyl acetal resin gradually precipitates from the solution. However, this method has several inherent drawbacks: First, a strong acid catalyst must be used: the essence of the acetalization reaction in this method is an acid-catalyzed nucleophilic addition-dehydration process. Therefore, the addition of acid is a necessary condition for the start and advancement of the reaction, which leads to the introduction of acidic substances into the reaction system and the product.

[0004] Secondly, the post-processing steps are cumbersome and complex: after the acetalization reaction, the resulting product contains residual acidic substances such as unreacted acid catalysts. To remove these acidic substances and obtain a neutral product, neutralization treatment is necessary, typically using alkalis (such as sodium hydroxide or sodium carbonate) to neutralize the residual acid. The neutralization reaction generates corresponding salts (such as sodium chloride or sodium sulfate), and these salts, along with excess alkali, can then mix into the polyvinyl acetal resin, forming new impurities.

[0005] Third, product purification is difficult, and high levels of impurities remain: To remove salts and other water-soluble impurities generated during neutralization, polyvinyl acetal resin needs to be washed, filtered, or centrifuged multiple times. This process consumes a large amount of water and energy, and it is difficult to completely remove impurities trapped inside the polyvinyl acetal resin. Therefore, polyvinyl acetal resin products typically contain hundreds of ppm levels of metal ions (derived from saponified alkali metals in PVA raw materials and neutralizing agents) and trace amounts of acid, affecting the thermal stability, transparency, and long-term weather resistance of polyvinyl acetal resin.

[0006] Fourth, the environmental and energy pressure is high: the entire preparation process generates a large amount of washing wastewater containing acid, alkali and salt, which has high treatment costs, heavy environmental burden, long process flow and high energy consumption. From reaction, neutralization, washing to drying, there are many steps and long cycle, resulting in high overall energy consumption.

[0007] To address these issues, researchers attempted to reduce the amount of acid used or improve the washing method, but none of these approaches deviated from the basic framework of "external acid catalysis" and "aqueous phase post-treatment," failing to fundamentally overcome their inherent defects.

[0008] Therefore, developing a method for preparing polyvinyl acetal resin that does not require external acid catalysts, has simple post-processing, generates less waste, and produces high-purity products is of great significance to the development of green chemistry. Summary of the Invention

[0009] In view of this, the present invention provides a polyvinyl acetal resin and a preparation method based on supercritical carbon dioxide medium. The preparation method provided by the present invention does not require the addition of an external acid catalyst, the post-processing is simple, the waste generated is small, and the product has high purity.

[0010] This invention provides a method for preparing polyvinyl acetal resin based on supercritical carbon dioxide medium, comprising the following steps: Polyvinyl alcohol resin, aldehyde compounds, and supercritical carbon dioxide medium are mixed and subjected to acetalization reaction at 100-150℃ and 12-25MPa to obtain crude polyvinyl alcohol acetal resin. The crude polyvinyl alcohol acetal resin is then purified by in-situ extraction using supercritical carbon dioxide medium and then subjected to reduced pressure to obtain the polyvinyl alcohol acetal resin.

[0011] Preferably, the polyvinyl alcohol resin has a degree of polymerization of 300-2500, a degree of alcoholysis of 85-99.9 mol%, a residual sodium acetate content of 0.55-0.65 wt%, and a particle size of 100-150 micrometers. The polyvinyl alcohol resin can be an industrial or analytical grade product commonly used in the art, and its particle size, residual sodium acetate content, and other parameters are not considered essential technical features of the present invention. The aldehyde compound is an aliphatic aldehyde with 2-8 carbon atoms.

[0012] Preferably, the molar ratio of the aldehyde compound to the hydroxyl group in the structural unit of the polyvinyl alcohol resin is 0.25~0.55:1; and the mass ratio of the supercritical carbon dioxide medium to the polyvinyl alcohol resin is 5~50:1.

[0013] Preferably, the supercritical carbon dioxide medium further includes a co-solvent; the co-solvent includes one or more of ethanol and acetone; and the mass ratio of the co-solvent to the polyvinyl alcohol resin is 0.01~0.2:1.

[0014] Preferably, the holding time for the acetalization reaction is 150-200 minutes.

[0015] Preferably, the in-situ extraction and purification process is carried out at a pressure of 8-25 MPa, a temperature of 40-120°C, and a time of 60-120 minutes; during the in-situ extraction and purification process, the total volume of supercritical carbon dioxide fluid introduced is 5-50 times the effective volume of the reactor.

[0016] Preferably, the decompression rate is 0.1~10 MPa / s.

[0017] Preferably, the process after depressurization further includes drying the resulting product; the drying temperature is 45~55℃, the vacuum degree is -0.096~-0.095MPa, and the holding time is 5~7 hours.

[0018] The present invention also provides a polyvinyl acetal resin prepared by the above-described method, wherein the total content of alkali metal ions is less than 20 ppm and the residual amount of aldehyde compounds is less than 100 ppm.

[0019] Preferably, the polyvinyl acetal resin is in the form of polyvinyl acetal resin microspheres; the average particle size of the polyvinyl acetal resin microspheres is 1~100μm, and the particle size of the polyvinyl acetal resin microspheres can be precisely controlled by adjusting the decompression rate.

[0020] This invention provides a method for preparing polyvinyl acetal resin using supercritical carbon dioxide as a medium. The method involves placing polyvinyl alcohol resin and aldehyde compounds in a supercritical carbon dioxide medium and conducting an acetalization reaction at 100-150°C and 12-25 MPa without adding any external acid catalyst. This invention hypothesizes that under high temperature and pressure, the supercritical carbon dioxide medium enhances the diffusion ability of aldehyde compounds in polyvinyl alcohol resin. Simultaneously, its weak Lewis acidity may have a synergistic effect with trace amounts of residual sodium acetate in the polyvinyl alcohol resin, thereby inducing the acetalization reaction without the need for additional acid catalysts. Subsequently, online extraction and purification are performed using supercritical carbon dioxide, and specific forms of polyvinyl acetal resin products are directly obtained by adjusting the decompression parameters, simultaneously achieving product purification and morphology control. The preparation method provided by this invention requires no acid catalyst, no cumbersome post-treatment such as neutralization and washing, is simple, green, and environmentally friendly, and produces polyvinyl acetal resin products with extremely high purity and controllable morphology. This overcomes the inherent defects of existing methods, such as the need for external acid catalysts, complex post-treatment, limited product purity, and significant environmental pollution.

[0021] This invention also provides a polyvinyl acetal resin obtained by the preparation method described above. The polyvinyl acetal resin provided by this invention has high purity and low impurity content, making it particularly suitable for high-end applications with stringent impurity requirements, effectively expanding the application range of polyvinyl acetal resin.

[0022] Overall, compared with the prior art, the present invention achieves the following beneficial effects: 1) Novel catalytic mechanism: This invention is the first to achieve the preparation of polyvinyl acetal resin in supercritical carbon dioxide medium without adding any form of external acid catalyst, breaking the traditional understanding that acetalization reaction must rely on acid catalysis, which is a fundamental process revolution.

[0023] 2) Significantly simplified process: Since no acid catalyst is used, the subsequent cumbersome and energy-intensive steps such as neutralization, washing, and dehydration are omitted. The acetalization reaction and purification process are both completed continuously or in one step in a supercritical carbon dioxide system, significantly shortening the process flow and improving production efficiency.

[0024] 3) Significantly improved product purity: By avoiding the introduction of acid and the generation of salt during neutralization, and by effectively removing small molecule impurities through the excellent extraction capability of supercritical carbon dioxide, the purity of polyvinyl acetal resin has reached an unprecedented high level, making it particularly suitable for high-end applications that are sensitive to impurities.

[0025] 4) Environmentally friendly: Using recyclable carbon dioxide as the main medium, the production process requires no process water, does not generate acidic or saline wastewater, and the carbon dioxide can be recycled, meeting the requirements of green chemistry and clean production.

[0026] 5) Controllable product morphology: Supercritical fluids facilitate the precipitation and molding of polyvinyl acetal resin at the molecular level. By controlling the decompression rate (0.1~10MPa / s), polyvinyl acetal resin microspheres with an average particle size of 1~100μm and controllable particle size can be prepared, which are suitable for different application scenarios and provide new possibilities for expanding the application of polyvinyl acetal resin. Detailed Implementation

[0027] This invention provides a method for preparing polyvinyl acetal resin based on supercritical carbon dioxide medium, comprising the following steps: Polyvinyl alcohol resin, aldehyde compounds, and supercritical carbon dioxide medium are mixed and subjected to acetalization reaction at 100-150℃ and 12-25MPa to obtain crude polyvinyl alcohol acetal resin. The crude polyvinyl alcohol acetal resin is then purified by in-situ extraction using supercritical carbon dioxide medium and then subjected to reduced pressure to obtain the polyvinyl alcohol acetal resin.

[0028] This invention involves mixing polyvinyl alcohol resin, aldehyde compounds, and supercritical carbon dioxide medium, and carrying out an acetalization reaction at 80-180°C and 8-30 MPa to obtain crude polyvinyl alcohol acetal resin.

[0029] In this invention, the degree of polymerization of the polyvinyl alcohol resin is preferably 300-2500, more preferably 1000-1500, the degree of alcoholysis is preferably 85-99.9 mol%, more preferably 88-99 mol%, and even more preferably 93-98 mol%, the residual sodium acetate content is preferably 0.55-0.65 wt%, and the particle size is preferably 100-150 micrometers.

[0030] In this invention, the aldehyde compound is preferably an aliphatic aldehyde with 2 to 8 carbon atoms, more preferably one or more of acetaldehyde, propionaldehyde, n-butyraldehyde, n-pentanaldehyde, n-hexanaldehyde, n-heptanaldehyde and n-octanaldehyde, and even more preferably n-butyraldehyde.

[0031] In this invention, the molar ratio of the aldehyde compound to the hydroxyl groups in the structural units of the polyvinyl alcohol resin is preferably 0.25~0.55:1, more preferably 0.3~0.5:1, further preferably 0.35~0.45:1, and even more preferably 0.4:1. Using the above-mentioned ratio range helps to obtain an appropriate degree of acetalization and avoids excessive aldehydes leading to increased side reactions or product coloring.

[0032] In this invention, the supercritical carbon dioxide medium preferably further includes a co-solvent; the co-solvent preferably includes one or more of ethanol and acetone; the ethanol is preferably anhydrous ethanol; the mass ratio of the co-solvent to the polyvinyl alcohol resin is preferably 0.01~0.2:1, more preferably 0.05~0.15:1. By adding a co-solvent, this invention helps improve the dispersion and swelling of polyvinyl alcohol resin in the supercritical carbon dioxide medium, promoting the acetalization reaction.

[0033] In this invention, the mass ratio of the supercritical carbon dioxide medium to the polyvinyl alcohol resin is preferably 5~50:1, more preferably 10~40:1, and even more preferably 20~30:1.

[0034] In this invention, the temperature of the acetalization reaction is preferably 110-140°C, more preferably 120°C; the pressure is preferably 13-22 MPa, more preferably 15-20 MPa, and even more preferably 17 MPa; and the holding time is preferably 150-200 minutes, more preferably 180 minutes. The above conditions are sufficient to effectively advance the acetalization reaction without the addition of external acid, while avoiding excessively high temperatures that could lead to product degradation or exacerbate side reactions.

[0035] After obtaining crude polyvinyl acetal resin, the present invention uses supercritical carbon dioxide medium to perform in-situ extraction and purification of crude polyvinyl acetal resin, followed by depressurization, to obtain the polyvinyl acetal resin.

[0036] In this invention, the pressure of the in-situ extraction and purification is preferably 8~25MPa, more preferably 12~20MPa, and even more preferably 15MPa; the temperature is preferably 40~120℃, more preferably 60~100℃, and even more preferably 80℃; and the time is preferably 60~120 minutes, more preferably 80~100 minutes, and even more preferably 90 minutes.

[0037] In this invention, during the in-situ extraction and purification process, the total volume of supercritical carbon dioxide fluid introduced is preferably 5 to 50 times the effective volume of the reactor, more preferably 15 to 30 times. This invention utilizes the excellent dissolving and penetrating power of supercritical carbon dioxide to efficiently extract unreacted aldehydes, co-solvents, and possible oligomers and other small molecule impurities from polyvinyl acetal resin, achieving in-situ purification.

[0038] In this invention, the decompression rate is preferably 0.1~10 MPa / s, more preferably 0.5~6 MPa / s, and even more preferably 1~3 MPa / s. After online extraction and purification, this invention uses the above-mentioned rate to decompress the supercritical carbon dioxide fluid containing the target product, causing polyvinyl acetal resin to precipitate in the form of microspheres, thereby achieving precise control over the particle size and morphology of the product and directly obtaining a microsphere product with high fluidity and large specific surface area.

[0039] In this invention, the process of reducing pressure preferably further includes drying the resulting product; the drying temperature is preferably 45~55℃, more preferably 50℃, the vacuum degree is preferably -0.096~-0.095MPa, more preferably -0.095MPa, and the heat preservation time is preferably 5~7 hours, more preferably 6 hours.

[0040] In this invention, the apparatus used in the method is preferably an integrated apparatus for continuous operation. The method of this invention is performed in an integrated apparatus for continuous operation, enabling continuous production throughout the entire process from feeding, reaction, purification to molding, resulting in high efficiency and ease of scalability.

[0041] The present invention also provides a polyvinyl acetal resin prepared by the above-described method, wherein the total content of alkali metal ions is less than 20 ppm and the residual amount of aldehyde compounds is less than 100 ppm.

[0042] In this invention, the alkali metal ion preferably includes Na. + and K + .

[0043] In this invention, the polyvinyl acetal resin is preferably in the form of polyvinyl acetal resin microspheres; the average particle size of the polyvinyl acetal resin microspheres is preferably 1~100μm, more preferably 10~70μm, and even more preferably 30~50μm.

[0044] The polyvinyl acetal resin prepared by this invention does not contain anions (e.g., Cl-) from the added acid catalyst. - or SO4 2- ) Residue.

[0045] In this invention, the added acid catalyst specifically refers to acidic substances such as hydrochloric acid, sulfuric acid, or p-toluenesulfonic acid added additionally to achieve the acetalization reaction. This invention relies entirely on the unique physicochemical environment of the supercritical carbon dioxide medium and the reaction temperature to drive the acetalization reaction.

[0046] To further illustrate the present invention, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments thereof.

[0047] Example 1: 1) Raw materials and specifications: Polyvinyl alcohol resin: degree of polymerization is 1700±50, degree of alcoholysis is 98.0±0.5mol%, sodium acetate residue content is 0.60±0.05wt% (this residual sodium acetate may play a weak alkaline catalytic or interface regulation role under supercritical CO2 conditions, promoting the reaction), and powder particle size is 100~150 micrometers.

[0048] Aldehyde compounds: n-Butyraldehyde, analytical grade, purity ≥99.0%, moisture ≤0.10%.

[0049] Co-solvent: Anhydrous ethanol, chromatographic grade, purity ≥99.9%, moisture ≤0.03%.

[0050] Reaction medium: food-grade carbon dioxide, purity ≥99.9%.

[0051] 2) The specific steps of the preparation method are as follows: (1) Raw material premixing: In a 2-liter premixing tank equipped with a stirrer, add 500 g of polyvinyl alcohol resin powder and 75 g of anhydrous ethanol. Stir mechanically at 200 rpm for 60 minutes at room temperature (25±5℃) to obtain a homogeneous, flowable slurry.

[0052] (2) Feeding and reaction: The slurry was transferred to a 5-liter high-pressure reactor (made of 316L stainless steel, equipped with electromagnetic stirring and an electric heating jacket). The reactor was sealed, and the inlet valve was opened to purge the air inside the reactor three times with carbon dioxide gas. Subsequently, the stirring (speed controlled at 300 rpm) and heating system were started to raise the temperature of the reaction system to 120°C. Carbon dioxide gas was continuously introduced through a high-pressure metering pump until the pressure inside the high-pressure reactor stabilized at 15.0 ± 0.2 MPa, reaching the supercritical state.

[0053] 42.5 g of n-butyraldehyde (the molar ratio of aldehyde compound to hydroxyl groups in the structural units of polyvinyl alcohol resin is 0.40:1) was injected into the high-pressure reactor using a high-pressure injection pump. During this process, the system temperature was maintained at 120°C and the pressure at 15.0 MPa. The acetalization reaction continued for 180 minutes.

[0054] (3) Purification: After the acetalization reaction is complete, stirring is stopped. The system temperature is adjusted to 90℃, and the system pressure is precisely controlled at 12.0±0.2MPa using a back pressure valve. Under these conditions, pure supercritical carbon dioxide is dynamically extracted into the high-pressure reactor at a flow rate of 1.5 L / min, with a cumulative injection volume of 15 times the effective volume of the high-pressure reactor, for a duration of 90 minutes. This process dissolves and removes unreacted n-butyraldehyde, ethanol, and possible low-molecular-weight byproducts.

[0055] (4) Product shaping and separation: After purification, the bottom outlet valve of the high-pressure reactor is connected to a capillary throttling valve (0.3 mm inner diameter) preheated to 80°C. The outlet valve is quickly opened, allowing the supercritical carbon dioxide fluid containing polyvinyl butyral resin products in the high-pressure reactor to expand to atmospheric pressure (0.101 MPa) within 0.05 seconds through the throttling valve. The supercritical carbon dioxide fluid then enters a connected cyclone separator (made of glass), where a white, fluffy solid powder is collected at the bottom.

[0056] The gas phase flowing out from the top of the separator (mainly carbon dioxide and trace amounts of solvent) passes through a condensation recovery system (-10°C condenser) and a gas purification device. The carbon dioxide is liquefied, recovered, and recompressed for reuse in the next batch of reaction.

[0057] (5) Drying: The solid powder collected in the cyclone separator was transferred to a vacuum drying oven. It was dried at 50°C and -0.095 MPa for 6 hours to completely remove trace amounts of solvent adsorbed on the powder surface, yielding well-defined microspheres of polyvinyl butyral resin.

[0058] Comparative Example 1: 1) Raw materials and specifications: Same as in Example 1.

[0059] 2) The specific steps of the preparation method are as follows: (1) Add 500g of PVA powder and 4500g of deionized water to the reactor and stir and dissolve at 95°C for 3 hours to obtain a 10wt% PVA aqueous solution. Cool to 40°C.

[0060] (2) Add 42.5 g of n-butyraldehyde and 10 g of concentrated hydrochloric acid (concentration of 37 wt%) to the PVA aqueous solution, stir evenly to obtain a mixture.

[0061] (3) The mixture was transferred to an autoclave and reacted at 100°C for 120 minutes (autogenous pressure 0.3 MPa). After the reaction was completed, resin blocks were precipitated.

[0062] (4) Pour off the upper layer of acid water and wash the resin block repeatedly with deionized water. Each time, the volume of deionized water used is 5 times the volume of the resin block. After washing 5 times, the collected washing water is tested with precision pH test paper (measurement accuracy ±0.3). Its pH value is stable between 6.0 and 8.0, and wet resin is obtained.

[0063] (5) Place the washed wet resin in a 70℃ forced-air drying oven and dry for 24 hours.

[0064] (6) The dried hard block was crushed by a pulverizer and passed through a 180-micron sieve to obtain irregular granular polyvinyl butyral resin.

[0065] Comparative Example 2: This comparative example involves adding p-toluenesulfonic acid to a supercritical carbon dioxide medium to catalyze an acetalization reaction, aiming to compare the effect of added acid on product performance. However, adding acid may trigger side reactions, leading to product discoloration, as detailed below: 1) Raw materials and specifications: Based on the raw materials in Example 1, a solid acid catalyst, p-toluenesulfonic acid monohydrate (PTSA), of analytical grade was added.

[0066] 2) The specific steps of the preparation method are as follows: The preparation method is the same as in Example 1, except that in step (1) when the raw materials are premixed, 5.0 g (1.0% of the mass of PVA powder) of p-toluenesulfonic acid monohydrate is added to the mixture of PVA powder and anhydrous ethanol.

[0067] Test Example 1: The polyvinyl butyral resins prepared in Example 1 and Comparative Examples 1-2 were subjected to the following tests: 1) Residual Na +(ppm): Measured by atomic absorption spectrometry (AAS) or inductively coupled plasma optical emission spectrometry (ICP-OES). Reference standard: GB / T 11904 or ISO 11885.

[0068] 2) Residual Cl - (ppm): Determined by ion chromatography (IC). Reference standards: GB / T 15453 or ISO10304-1.

[0069] 3) Residual n-butyraldehyde (ppm): Determined by headspace gas chromatography-mass spectrometry (HS-GC / MS). Reference standard: USP <467> Or ISO 16000-6 standard.

[0070] 4) Butyral degree (mol%): determined by proton nuclear magnetic resonance spectroscopy (NMR). 1 H-NMR determination. Based on industry-standard methods, similar in principle to ASTM D4273.

[0071] 5) Transmittance (%) and haze (%): measured using an integrating sphere haze meter according to ASTM D1003 or GB / T 2410 standard methods. The results are shown in Table 1.

[0072] Table 1. Performance comparison between Example 1 and Comparative Examples 1-2:

[0073] As can be seen from Table 1, in terms of purity, the polyvinyl butyral resin prepared in Example 1 is superior to Comparative Examples 1 and 2 in all key impurity indicators; its sodium ion residue (18 ppm) is only 8.6% of that of the traditional method (210 ppm), and it is completely free of chloride ions and p-toluenesulfonate ions, proving that "no external acid catalysis" eliminates the introduction of ionic impurities from the source; supercritical carbon dioxide in-situ extraction and purification also resulted in the lowest residual n-butyraldehyde content.

[0074] Regarding product performance: The film made of polyvinyl butyral resin in Example 1 exhibited the highest light transmittance (91.8%) and the lowest haze (1.5%), demonstrating excellent optical performance, which is directly attributed to its extremely high purity; while in Comparative Example 2, the addition of organic acid caused a side reaction, resulting in slight coloring of the product and a decrease in light transmittance.

[0075] In terms of process and environmental protection: Example 1 has a simple process flow, requires no water washing, does not generate wastewater, and carbon dioxide can be recycled, which meets the requirements of green manufacturing process; Comparative Example 1 has many environmental problems such as high water consumption and difficulty in wastewater treatment.

[0076] Regarding technical feasibility: Example 1 successfully synthesized a polyvinyl butyral resin with a degree of acetalization of 65.8 mol% without adding any acid catalyst. This degree of acetalization is at the same level as Comparative Example 1 (64.5 mol%) and Comparative Example 2 (69.2 mol%), proving that the method of the present invention can effectively drive the acetalization reaction. More importantly, thanks to its extremely high purity, the optical properties (transmittance 91.8%, haze 1.5%) of the polyvinyl butyral resin in Example 1 are significantly better than those of the two comparative examples. This fully demonstrates that the method of the present invention achieves green preparation while obtaining products with excellent application performance, possessing significant technical feasibility and advancement.

[0077] In summary, the present invention, based on the preparation of polyvinyl butyral resin using supercritical carbon dioxide medium, has significant advantages in improving product purity, simplifying process flow, and achieving green manufacturing.

[0078] The embodiments of the present invention have been described above. However, these embodiments are for illustrative purposes only and are not intended to limit the scope of the present invention. All other embodiments obtained by those skilled in the art based on the above embodiments of the present invention without inventive effort are within the protection scope of the present invention.

Claims

1. A method for preparing polyvinyl acetal resin based on supercritical carbon dioxide medium, characterized in that, Includes the following steps: Polyvinyl alcohol resin, aldehyde compounds, and supercritical carbon dioxide medium are mixed and subjected to acetalization reaction at 100-150℃ and 12-25MPa to obtain crude polyvinyl alcohol acetal resin. The crude polyvinyl alcohol acetal resin is then purified by in-situ extraction using supercritical carbon dioxide medium and then subjected to reduced pressure to obtain the polyvinyl alcohol acetal resin.

2. The preparation method according to claim 1, characterized in that, The polyvinyl alcohol resin has a degree of polymerization of 300-2500, a degree of alcoholysis of 85-99.9 mol%, a sodium acetate residue content of 0.55-0.65 wt%, and a particle size of 100-150 micrometers. The aldehydes are aliphatic aldehydes with 2 to 8 carbon atoms.

3. The preparation method according to claim 1, characterized in that, The molar ratio of the aldehyde compound to the hydroxyl group in the structural unit of the polyvinyl alcohol resin is 0.25~0.55:1; The mass ratio of the supercritical carbon dioxide medium to the polyvinyl alcohol resin is 5~50:

1.

4. The preparation method according to claim 1, characterized in that, The supercritical carbon dioxide medium also includes a co-solvent; the co-solvent includes one or more of ethanol and acetone; the mass ratio of the co-solvent to the polyvinyl alcohol resin is 0.01~0.2:

1.

5. The preparation method according to claim 1, characterized in that, The holding time for the acetalization reaction is 150-200 minutes.

6. The preparation method according to claim 1, characterized in that, The in-situ extraction and purification process is carried out at a pressure of 8-25 MPa, a temperature of 40-120°C, and a time of 60-120 minutes. During the in-situ extraction and purification process, the total volume of supercritical carbon dioxide fluid introduced is 5 to 50 times the effective volume of the reactor.

7. The preparation method according to claim 1, characterized in that, The decompression rate is 0.1~10 MPa / s.

8. The preparation method according to claim 1, characterized in that, The process of reducing pressure also includes drying the resulting product. The drying temperature is 45~55℃, the vacuum degree is -0.096~-0.095MPa, and the holding time is 5~7 hours.

9. The polyvinyl acetal resin obtained by the preparation method according to any one of claims 1 to 8, characterized in that, The total content of alkali metal ions is less than 20 ppm, and the residual amount of aldehyde compounds is less than 100 ppm.

10. The polyvinyl acetal resin according to claim 9, characterized in that, The polyvinyl acetal resin is in the form of polyvinyl acetal resin microspheres. The average particle size of the polyvinyl acetal resin microspheres is 1~100μm.

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