Process for the preparation of evo h with removal of alkaline metal impurities

Abstract

The application discloses an EVOH preparation method for removing alkaline metal impurities and relates to the technical field of EVOH. The application uses carbon dioxide gas neutralization instead of traditional acetic acid liquid phase neutralization, solves the gas-liquid mass transfer bottleneck of a complex system, speeds up the neutralization process and accurately controls the pH of the system. Meanwhile, a unique decompression and temperature rising post-processing step is designed, sodium ions entangled by polymer chains are effectively driven to migrate to the water phase, EVOH is deeply purified, the efficient removal of alkaline metal impurities is finally realized, the number of subsequent water washing times is reduced, and the water consumption is saved.

Description

Technical Field

[0001] This invention relates to the field of EVOH technology, and more specifically to a method for preparing EVOH by removing alkaline metal impurities. Background Technology

[0002] Ethylene-vinyl alcohol copolymer (EVOH) is an important barrier resin, widely used in packaging materials and many other fields due to its excellent barrier properties, non-toxicity, good stability, and processability. EVOH is typically produced from ethylene-vinyl acetate copolymer (EVA) via alcoholysis. This reaction generally uses an alkaline metal catalyst, resulting in a large amount of alkaline metal impurities in the reaction system after alcoholysis. If these impurities are not completely removed, they will cause product performance degradation. Therefore, removing these alkaline metal impurities is a crucial and technically challenging purification step in the EVOH production process.

[0003] Currently, the most classic and widely used method for removing alkali metal impurities in industrial production is the acetic acid neutralization method. This method first adds acetic acid to neutralize the alkali metal impurities, forming salts, which are then removed by water washing. However, this method introduces acetic acid to form difficult-to-elute organic salts. These organic salts react strongly with the polymer matrix, requiring massive amounts of water for multi-stage washing, and resulting in high residue levels. Furthermore, to ensure complete neutralization, an excess of acetic acid must be added in practice. Excess acetic acid is difficult to remove completely and can catalyze the degradation of EVOH molecular chains in subsequent processing, leading to yellowing and gel formation in the product.

[0004] Chinese invention patent CN115490787A discloses a method for removing salt impurities from ethylene-vinyl alcohol copolymers. This patent describes a method for removing salt impurities after acetic acid neutralization. The ethylene-vinyl alcohol copolymer solution obtained from alcoholysis is passed through an adsorption tower containing anion and cationic adsorbents under certain temperature conditions to adsorb acetate and sodium ions. While this method achieves efficient removal of salt impurities without generating industrial wastewater and is environmentally friendly, the ethylene-vinyl alcohol copolymer solution is a high-viscosity polymer solution, leading to problems such as uneven fluid distribution, excessive pressure drop, and a sharp decrease in mass transfer efficiency, posing significant technical challenges. Therefore, there is an urgent need to develop an easy-to-operate, environmentally friendly, and efficient method for removing alkaline metal impurities during EVOH production. Summary of the Invention

[0005] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and provide a method for preparing EVOH to remove alkaline metal impurities. This method uses carbon dioxide gas neutralization instead of traditional acetic acid liquid phase neutralization, which solves the gas-liquid mass transfer bottleneck in complex systems, accelerates the neutralization process, and precisely controls the pH of the system. At the same time, a unique depressurization and heating post-treatment step is designed to effectively drive sodium ions entangled by polymer chains to migrate to the aqueous phase, so that EVOH can be deeply purified, and finally achieve efficient removal of alkaline metal impurities, reduce the number of subsequent water washings, and save water consumption.

[0006] The technical solution of this invention is as follows:

[0007] A method for preparing EVOH with alkaline metal impurities removed includes the following steps:

[0008] S1 alcoholysis stage: EVA methanol solution and alkaline catalyst are added to the alcoholysis reactor to carry out the EVA alcoholysis reaction;

[0009] S2 Controlled Gas-Liquid Enhanced Neutralization Stage: After the reaction is complete, the reaction mixture is transferred to a neutralization vessel equipped with an online pH meter, water is added, and the mixture is stirred. When the system temperature naturally drops to 30-40℃, carbon dioxide is introduced in stages for neutralization.

[0010] S2-1 Rapid Mass Transfer Neutralization Stage: Control the carbon dioxide flow rate at 0.5-1 L / min and neutralize at 0.3-0.8 MPa to reduce the pH of the system;

[0011] S2-2 Precise Balancing and Stabilization Stage: When the pH drops to 8.3-8.7, switch the carbon dioxide flow rate to 0.1-0.3 L / min and mix in 5-10 vol.% nitrogen to adjust the pH of the system to 7, and then stop the introduction of carbon dioxide and nitrogen.

[0012] S3 Deep Purification and Residual Removal: Reduce the reaction pressure to 0.11-0.13 MPa and maintain it for 10 min, then reduce it to atmospheric pressure. Heat the system to 60-90℃ to allow residual carbon dioxide to volatilize and neutralize.

[0013] After S4 neutralization, the discharged material is washed with water, vacuum dried, and then fed into an extrusion device. After pelleting and washing, EVOH is obtained.

[0014] Preferably, in step S1, the concentration of the EVA methanol solution is 10-30 wt.%.

[0015] Preferably, in step S1, the alkaline catalyst is sodium hydroxide.

[0016] Preferably, in step S1, the mass ratio of alkaline catalyst to EVA is (0.02-0.03):1.

[0017] Preferably, in step S1, the alcoholysis reaction temperature is 60-80℃ and the alcoholysis reaction time is 2-5h.

[0018] Preferably, the mass ratio of water in step S2 to EVA methanol solution in step S1 is (0.1-0.5):1.

[0019] This invention involves adding water and introducing carbon dioxide into a neutralization vessel after the alcoholysis of EVA. The alcoholyzed EVA is then transferred to the neutralization vessel for neutralization. Water reacts with carbon dioxide to produce carbonic acid, neutralizing the alkaline substances. During the transfer, the temperature decreases, and carbon dioxide is first rapidly introduced. Under high pressure, the carbon dioxide dissolves quickly, generating carbonic acid, which reacts with alkaline metal ions in the system, rapidly lowering the pH. Subsequently, the carbon dioxide flow rate is switched to a low flow rate, and an inert gas is introduced to slow the reaction rate and prevent localized over-acidity. Real-time feedback from an online pH meter ensures the system pH is precisely adjusted to the endpoint value of 7, eliminating the risk of polymer chain degradation. After neutralization, a gradient depressurization strategy is employed to volatilize most of the free carbon dioxide. The pressure is then reduced to ambient level and the temperature is increased, driving the carbonates entangled by polymer chains to migrate to the aqueous phase, while simultaneously removing residual carbon dioxide completely.

[0020] This invention addresses the technical challenges in removing alkaline metal impurities during existing EVOH production by proposing an innovative solution combining gas neutralization and gradient post-treatment. Compared to existing technologies, this solution offers the following significant advantages:

[0021] 1. This invention employs carbon dioxide gas neutralization instead of traditional acetic acid liquid-phase neutralization. Carbon dioxide reacts with alkaline metal impurities to form water-soluble inorganic carbonates (such as sodium carbonate). Compared to the difficult-to-elute organic salts generated by acetic acid neutralization, these carbonates have weaker interactions with the polymer matrix and are easier to separate by water washing. Simultaneously, a staged aeration strategy of "rapid mass transfer neutralization + precise leveling and stabilization," coupled with real-time control using an online pH meter, avoids localized over-acidity or over-alkalinity in the system. This solves the gas-liquid mass transfer bottleneck in complex polymer systems and achieves precise pH control (final pH=7), eliminating the risk of EVOH molecular chain degradation caused by excessive carbon dioxide neutralizer. Furthermore, the unique "gradient depressurization + heating" post-treatment step utilizes the mass transfer driving force generated by pressure changes and the regulatory effect of temperature increases on the polymer chain entanglement state, effectively promoting the migration of sodium ions encapsulated by EVOH chains to the aqueous phase, achieving deep removal of alkaline metal impurities. Experimental data show that the sodium content of the EVOH product prepared by this invention is as low as 522-781 ppm, which is far superior to the traditional acetic acid neutralization method (sodium content 1730 ppm). The product purity is greatly improved, avoiding the deterioration of product performance caused by impurities from the source.

[0022] 2. Because the inorganic carbonates generated by this invention are easily eluted, and the deep purification step further reduces impurity residue, compared to the traditional acetic acid neutralization method which requires multiple water washes, this invention only requires one water wash to achieve the ideal purification effect. The overall water consumption is only 250-296g, greatly reducing water consumption and wastewater treatment costs in the production process. Simultaneously, carbon dioxide, as a neutralizing agent, can be used to recover industrial waste gas, realizing the resource utilization of waste. Compared to acetic acid as a neutralizing agent, it not only has lower raw material costs but also reduces the carbon footprint, aligning with the concept of green production.

[0023] 3. This invention employs a pressure of 0.3-0.8 MPa during the rapid mass transfer neutralization stage, significantly improving the solubility of carbon dioxide in the reaction system and accelerating the neutralization reaction rate. Comparative experiments show that the neutralization time of this invention is only 28-45 min, far shorter than the atmospheric pressure single-gas neutralization process. Furthermore, the gradient depressurization step promotes the volatilization of residual carbon dioxide while further facilitating the separation of salt impurities, avoiding the impact of residual carbon dioxide on product performance in subsequent processes. It eliminates the need for an additional degassing step, simplifying the production process and improving overall production efficiency.

[0024] 4. In traditional acetic acid neutralization methods, excess residual acetic acid can catalyze molecular chain degradation during subsequent EVOH processing, leading to yellowing and gelation, affecting product appearance and performance. This invention uses carbon dioxide gas for neutralization, introducing no additional organic impurities. Furthermore, residual carbon dioxide can be removed by heating and depressurizing, effectively avoiding the negative impact of neutralizing agent residue on the product. Experimental verification shows that the EVOH product prepared by this invention has a lower yellow index and exhibits no yellowing or gelation, meeting the requirements of high-end packaging materials, electronic device encapsulation, and other applications with higher purity requirements, thus broadening the market application scope of EVOH products. Attached Figure Description

[0025] Figure 1 It is the EVOH prepared in Example 1 of this invention. 1 HNMR spectrum. Detailed Implementation

[0026] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention.

[0027] Example 1

[0028] The method for preparing EVOH with alkaline metal impurities removed in this embodiment includes the following steps:

[0029] S1 alcoholysis stage: Add 50g EVA and 200g methanol to the alcoholysis vessel to prepare an EVA methanol solution with a concentration of 20wt.%, add 1.25g sodium hydroxide, and carry out alcoholysis reaction at 70℃ for 3.5h;

[0030] S2 Controlled Gas-Liquid Enhanced Neutralization Stage: After the reaction is complete, the reaction mixture is transferred to a neutralization vessel equipped with an online pH meter, 75g of water is added, and the mixture is stirred at 1000r / min. When the system temperature naturally drops to 35℃, carbon dioxide is introduced in stages for neutralization.

[0031] S2-1 Rapid Mass Transfer Neutralization Stage: The carbon dioxide flow rate is controlled at 0.75 L / min, and neutralization is carried out at 0.5 MPa to reduce the pH of the system;

[0032] S2-2 Precise Balancing and Stabilization Stage: When the pH drops to 8.5, switch the carbon dioxide flow rate to 0.2 L / min and mix in 7.5 vol.% nitrogen to adjust the pH of the system to 7;

[0033] S3 Deep Purification and Residual Removal: The reaction pressure was reduced to 0.12 MPa and maintained for 10 min, then reduced to atmospheric pressure, and the system was heated to 75°C to allow residual carbon dioxide to volatilize and neutralization to be completed.

[0034] After S4 neutralization, the discharged material is washed with water, vacuum dried, and then fed into an extrusion device. After pelleting and washing, EVOH is obtained.

[0035] The EVOH prepared in this embodiment 1 HNMR spectrum as follows Figure 1 As shown, the peaks with chemical shifts of 0.8-0.9 ppm represent hydrogen atoms at the terminal -CH3; the peaks with chemical shifts of 1.2-1.7 ppm represent hydrogen atoms at the -CH2 of ethylene and vinyl alcohol; the peaks with chemical shifts of 3.3-4.1 ppm represent hydrogen atoms at the -CH of vinyl alcohol; and the peaks with chemical shifts of 4.1-4.6 ppm represent hydrogen atoms at the -OH of vinyl alcohol. The peaks at 2.5 ppm and 3.2 ppm are the elution positions of DMSO and H2O, respectively. In summary, this example demonstrates the successful preparation of EVOH.

[0036] Example 2

[0037] The method for preparing EVOH with alkaline metal impurities removed in this embodiment includes the following steps:

[0038] S1 alcoholysis stage: Add 50g EVA and 450g methanol to the alcoholysis vessel to prepare an EVA methanol solution with a concentration of 10wt.%, add 1g sodium hydroxide, and carry out alcoholysis reaction at 60℃ for 5h.

[0039] S2 Controllable Gas-Liquid Enhanced Neutralization Stage: After the reaction is complete, the reaction mixture is transferred to a neutralization vessel equipped with an online pH meter, 50g of water is added, and the mixture is stirred at 800r / min. When the system temperature naturally drops to 30℃, carbon dioxide is introduced in stages for neutralization.

[0040] S2-1 Rapid Mass Transfer Neutralization Stage: The carbon dioxide flow rate is controlled at 0.5 L / min, and neutralization is carried out at 0.3 MPa to reduce the pH of the system;

[0041] S2-2 Precise Balancing and Stabilization Stage: When the pH drops to 8.3, switch the carbon dioxide flow rate to 0.1 L / min and mix in 5 vol.% nitrogen to adjust the pH of the system to 7;

[0042] S3 Deep Purification and Residual Removal: The reaction pressure was reduced to 0.11 MPa and maintained for 10 min, then reduced to atmospheric pressure, and the system was heated to 60 °C to allow residual carbon dioxide to volatilize and neutralization to be completed;

[0043] After S4 neutralization, the discharged material is washed with water, vacuum dried, and then fed into an extrusion device. After pelleting and washing, EVOH is obtained.

[0044] Example 3

[0045] The method for preparing EVOH with alkaline metal impurities removed in this embodiment includes the following steps:

[0046] S1 alcoholysis stage: Add 50g EVA and 117g methanol to the alcoholysis vessel to prepare an EVA methanol solution with a concentration of 30wt.%, add 1.5g sodium hydroxide, and carry out alcoholysis reaction at 80℃ for 2h.

[0047] S2 Controlled Gas-Liquid Enhanced Neutralization Stage: After the reaction is complete, the reaction mixture is transferred to a neutralization vessel equipped with an online pH meter, 83.5g of water is added, and the mixture is stirred at 1200r / min. When the system temperature naturally drops to 40℃, carbon dioxide is introduced in stages for neutralization.

[0048] S2-1 Rapid Mass Transfer Neutralization Stage: The carbon dioxide flow rate is controlled at 1 L / min, and neutralization is carried out at 0.8 MPa to reduce the pH of the system;

[0049] S2-2 Precise Balancing and Stabilization Stage: When the pH drops to 8.7, switch the carbon dioxide flow rate to 0.3 L / min and mix in 10 vol.% nitrogen to adjust the pH of the system to 7.

[0050] S3 Deep Purification and Residual Removal: The reaction pressure was reduced to 0.13 MPa and maintained for 10 min, then reduced to atmospheric pressure, and the system was heated to 90 °C to allow residual carbon dioxide to volatilize and neutralization to be completed.

[0051] After S4 neutralization, the discharged material is washed with water, vacuum dried, and then fed into an extrusion device. After pelleting and washing, EVOH is obtained.

[0052] Comparative Example 1

[0053] Step S1 is the same as in Example 1, with the following subsequent operations: After the reaction is complete, the reaction mixture is transferred to a neutralization vessel equipped with an online pH meter. The system temperature is allowed to drop naturally. Acetic acid is added for acid washing and neutralization, wherein the molar ratio of acetic acid to sodium hydroxide in step S1 is 1.2:1. The mixture is then washed three times with pure water, filtered, and a precipitate is obtained. The precipitate is vacuum dried and then fed into an extrusion device. After pelleting and washing, EVOH is obtained.

[0054] Comparative Example 2

[0055] Step S1 is the same as in Example 1, with the following subsequent operations: After the reaction is complete, the reaction mixture is transferred to a neutralization vessel equipped with an online pH meter, 75g of water is added, and the mixture is stirred at 1000 rpm, allowing the system temperature to drop naturally. Carbon dioxide is then introduced, and the pressure is controlled at atmospheric pressure, with real-time monitoring of the pH value. When the system pH drops to 7, the carbon dioxide introduction is stopped, and the system temperature is raised to 50°C, completing neutralization. The discharged material is washed once with water, vacuum dried, and then fed into an extrusion device. After pelleting and washing, EVOH is obtained.

[0056] Comparative Example 3

[0057] The difference from Example 1 is that step S3 is not performed.

[0058] The sodium content and yellowness index (YI) of the EVOH prepared in Examples 1-3 and Comparative Examples 1-3 were tested. The test methods were as follows: sodium content was tested according to GB / T43574-2023 "Determination of Heavy Metal Content in Chemical Fibers - Inductively Coupled Plasma Atomic Emission Spectrometry and Inductively Coupled Plasma Mass Spectrometry"; yellowness index (YI) was tested according to HG / T3862 "Test Method for Yellowness Index of Plastics". The total water consumption during the experiment and cleaning process was recorded. The test results are shown in Table 1.

[0059] Table 1. Results of sodium content, yellowness index YI, and water consumption of EVOH in Examples 1-3 and Comparative Examples 1-3.

[0060]

[0061] As shown in Table 1, compared to Example 1, Comparative Example 1 has a higher sodium content, higher water consumption, and a higher yellowness index (YI) in EVOH. This is because Comparative Example 1 uses the traditional acetic acid neutralization method, where acetic acid reacts with alkaline metal impurities (sodium ions) to generate organic salts (such as sodium acetate). The organic salts interact strongly with the EVOH polymer matrix, are easily adsorbed and encapsulated by the polymer chains, and are difficult to separate by water washing, resulting in a significant increase in sodium residue. In contrast, this invention uses carbon dioxide neutralization to generate inorganic carbonates (such as sodium carbonate). Inorganic salts have a weaker interaction with the polymer matrix and are easier to elute. Furthermore, because organic salts are difficult to elute, multiple stages of water washing are required to achieve basic purification, resulting in a water consumption as high as 1500g; while the inorganic salts generated by this invention only require one water wash, consuming only 275g of water, significantly saving water resources. In addition, the acetic acid neutralization method requires the addition of excess acetic acid to ensure thorough neutralization, but excess acetic acid cannot be completely removed. Residual acetic acid can catalyze the degradation of EVOH molecular chains during subsequent processing, leading to yellowing of the product (YI=10.1) and gel formation. In contrast, this invention uses carbon dioxide for neutralization, leaving no organic impurities. Furthermore, residual carbon dioxide can be removed by heating and depressurizing, avoiding the risk of degradation, resulting in a YI of only 3.7.

[0062] Compared to Example 1, Comparative Example 2 had a longer neutralization time. This is because Comparative Example 2 introduced carbon dioxide at atmospheric pressure, while Example 1 used a high pressure of 0.5 MPa in step S2-1. According to the gas dissolution law, high pressure can significantly increase the solubility of carbon dioxide in the reaction system, accelerate the carbonic acid formation rate, and thus shorten the neutralization time; at atmospheric pressure, the solubility of carbon dioxide is low, and carbonic acid formation is slow, resulting in a neutralization time extended to 83 min. Comparative Example 2 did not use the staged aeration method of "rapid mass transfer neutralization + precise leveling and stabilization," but only a single flow rate aeration, which could not quickly reduce the initial high pH, ​​nor could it avoid local over-acidity through low flow rate + nitrogen dilution. Although the final pH reached the target, the reaction efficiency during the neutralization process was low, indirectly resulting in a slightly higher sodium content than in Example 1.

[0063] Compared to Example 1, Comparative Example 3 showed a higher sodium content, longer neutralization time, and slightly higher yellowness index in EVOH. This is because Comparative Example 3 omitted the unique step S3, "gradient depressurization + heating," for deep purification. During the reaction, the EVOH polymer chains become entangled and encapsulate some sodium ions. Example 1, through a strategy of "first reducing to 0.12 MPa and holding for 10 min → atmospheric pressure → heating to 75°C," utilizes the mass transfer driving force generated by pressure changes and the disruption of the entangled state of the polymer chains by temperature increases to drive the encapsulated sodium ions to migrate to the aqueous phase, achieving deep removal. However, Comparative Example 3 lacked this step, and the entangled sodium ions could not be effectively released, resulting in a sodium content increase to 857 ppm. Furthermore, the gradient depressurization + heating can quickly volatilize the residual carbon dioxide in the system, preventing it from continuously reacting with water to form carbonic acid and causing local over-acidity. Comparative Example 3 only heated to 50°C, so the residual carbon dioxide could not be completely removed, and some residue slightly catalyzed the degradation of the polymer chains, resulting in a slightly higher yellowness index and a prolonged stabilization time after neutralization.

Claims

1. A method for preparing EVOH with alkaline metal impurities removed, characterized in that, Includes the following steps: S1 alcoholysis stage: EVA methanol solution and alkaline catalyst are added to the alcoholysis reactor to carry out the EVA alcoholysis reaction; S2 Controlled Gas-Liquid Enhanced Neutralization Stage: After the reaction is complete, the reaction mixture is transferred to a neutralization vessel equipped with an online pH meter, water is added, and the mixture is stirred. When the system temperature naturally drops to 30-40℃, carbon dioxide is introduced in stages for neutralization. S2-1 Rapid Mass Transfer Neutralization Stage: Control the carbon dioxide flow rate at 0.5-1 L / min and neutralize at 0.3-0.8 MPa to reduce the pH of the system; S2-2 Precise Balancing and Stabilization Stage: When the pH drops to 8.3-8.7, switch the carbon dioxide flow rate to 0.1-0.3 L / min and mix in 5-10 vol.% nitrogen to adjust the pH of the system to 7, and then stop the introduction of carbon dioxide and nitrogen. S3 Deep Purification and Residual Removal: Reduce the reaction pressure to 0.11-0.13 MPa and maintain it for 10 min, then reduce it to atmospheric pressure. Heat the system to 60-90℃ to allow residual carbon dioxide to volatilize and neutralize. After S4 neutralization, the discharged material is washed with water, vacuum dried, and then fed into an extrusion device. After pelleting and washing, EVOH is obtained.

2. The method for preparing EVOH with alkaline metal impurities as described in claim 1, characterized in that, In step S1, the concentration of the EVA methanol solution is 10-30 wt.%.

3. The method for preparing EVOH with alkaline metal impurities as described in claim 1, characterized in that, In step S1, the alkaline catalyst is sodium hydroxide.

4. The method for preparing EVOH with alkaline metal impurities as described in claim 1, characterized in that, In step S1, the mass ratio of alkaline catalyst to EVA is (0.02-0.03):

1.

5. The method for preparing EVOH with alkaline metal impurities as described in claim 1, characterized in that, In step S1, the alcoholysis reaction temperature is 60-80℃ and the alcoholysis reaction time is 2-5h.

6. The method for preparing EVOH with alkaline metal impurities as described in claim 1, characterized in that, The mass ratio of water in step S2 to EVA methanol solution in step S1 is (0.1-0.5):1.

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