Ethylene-vinyl alcohol copolymer and preparation method thereof

Abstract

The invention discloses an ethylene-vinyl alcohol copolymer and a preparation method thereof, and relates to the technical field of EVOH. According to the technical scheme, the ethylene-vinyl alcohol copolymer is prepared by alcoholysis of an ethylene-vinyl acetate copolymer under the synergistic effect of a sodium alcoholate solution and a phase transfer catalyst; the content of vinyl acetate oligomer in the ethylene-vinyl acetate copolymer is controlled to be 100 ppm or below, and the residual amount of sodium hydroxide in the sodium alcoholate solution is controlled to be 0.3 wt.% or below. The content of residual vinyl acetate oligomer in the EVOH alcoholysis precursor EVA and the content of residual sodium hydroxide in the sodium alkoxide solution are effectively and cooperatively controlled, the phase transfer catalyst is assisted to improve the uniformity of the alcoholysis process of the sodium alkoxide, local hot spots are prevented from being generated, the consumption of the alcoholysis catalyst is reduced, the alcoholysis efficiency and the alcoholysis degree are improved, and meanwhile, the alcoholysis time is shortened. The yellowing phenomenon of the EVOH product is further reduced, and the yellow index of the product is improved.

Description

Technical Field

[0001] This invention relates to the field of EVOH technology, specifically to ethylene-vinyl alcohol copolymers and their preparation methods. Background Technology

[0002] Ethylene-vinyl alcohol copolymer (EVOH) combines the properties of both polyethylene and polyvinyl alcohol, making it a widely used high-barrier and solvent-resistant resin material. Therefore, it is extensively used in food, medical, and cosmetic packaging, as well as automotive fuel tanks and industrial pipelines. The hydrogen bonding interactions and crystallinity of EVOH are key to its excellent gas barrier properties and solvent resistance. EVOH cannot be obtained by direct copolymerization of ethylene and vinyl alcohol because vinyl alcohol readily isoforms into acetaldehyde. The production process of EVOH is similar to that of PVA; it can be considered a modified PVA. The preparation process includes polymerization and alcoholysis. First, ethylene-vinyl acetate copolymer (EVA) is generated through copolymerization of ethylene and vinyl acetate. Then, the EVA undergoes alcoholysis, converting the ester groups into hydroxyl groups to obtain EVOH.

[0003] The formation of hydrogen bonds and crystals in EVOH is inseparable from the alcoholysis process. A simple and efficient alcoholysis process has a crucial impact on the degree of alcoholysis and the yellow index of the resulting EVOH. Chinese invention patent CN116529269A discloses a method for preparing ethylene-vinyl alcohol copolymers, which improves alcoholysis efficiency and reduces impurity residues by controlling the dropwise or stepwise addition of a catalyst; the catalyst used is sodium hydroxide. Chinese invention patent CN119775458A discloses a method and system for producing ethylene-vinyl alcohol copolymers. By using an alcohol / water mixture for EVA hydrolysis instead of transesterification, the process is ensured to be in a dissolved state, thus improving the degree of alcoholysis of EVOH. Chinese invention patent CN104098728A discloses an alcoholysis method for ethylene-vinyl acetate copolymers, which uses hot low-carbon alcohol vapor to remove methyl acetate, a byproduct of the alcoholysis process, improving reaction control stability and increasing alcoholysis efficiency. Chinese invention patent CN108431054A discloses an ethylene-vinyl alcohol copolymer and a method for manufacturing the ethylene-vinyl alcohol copolymer. By using a sodium alkoxide solution as a catalyst, the resulting EVOH ends are controlled to contain carboxylic acid ester groups, which effectively reduces the melting point of the product and improves the stability of processing.

[0004] The residual monomers, moisture content, and catalyst concentration in EVA significantly affect the alcoholysis process. However, few studies have focused on the impact of the residual vinyl acetate oligomer content and the purity of sodium alkoxide on alcoholysis. Furthermore, while organometallic bases are convenient and efficient catalysts for alcoholysis, uneven metal ion distribution can lead to localized "hot spots." Although some patents (such as Chinese invention patent CN113861313B disclosing an ethylene-vinyl alcohol copolymer and its preparation method and application, and Chinese invention patent CN113736004B disclosing an ethylene-vinyl alcohol copolymer and its preparation method and application) report the use of alkaline or acidic ionic liquids as catalysts to avoid introducing metal ions, reduce washing difficulty, and enable recycling, these methods are extremely costly and complex to operate.

[0005] Therefore, how to control the content of residual vinyl acetate oligomers in EVA and residual sodium hydroxide in sodium alkoxide, and improve the uniform diffusion of sodium alkoxide during the alcoholysis process, so as to improve the alcoholysis efficiency, degree of alcoholysis and product color, is a topic worthy of investigation. Summary of the Invention

[0006] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and provide an ethylene-vinyl alcohol copolymer and its preparation method. By effectively and synergistically controlling the content of residual vinyl acetate oligomers in the EVOH alcoholysis precursor EVA and residual sodium hydroxide in the sodium alkoxide solution, and by using a phase transfer catalyst to improve the uniformity of the sodium alkoxide alcoholysis process and prevent the generation of local "hot spots", the invention further reduces the yellowing phenomenon of EVOH products and improves the yellow index of the products while reducing the amount of alcoholysis catalyst, improving the alcoholysis efficiency and degree of alcoholysis.

[0007] The technical solution of this invention is as follows: On one hand, this invention provides a method for preparing an ethylene-vinyl alcohol copolymer, wherein the ethylene-vinyl alcohol copolymer is obtained by alcoholysis of an ethylene-vinyl acetate copolymer under the synergistic action of a sodium alkoxide solution and a phase transfer catalyst; the phase transfer catalyst is tetrabutylammonium bromide, tetrabutylammonium chloride, benzyltriethylammonium chloride, or trioctylmethylammonium chloride, and the molar ratio of the phase transfer catalyst to the sodium alkoxide in the sodium alkoxide solution is (0.1-0.3):1; the content of vinyl acetate oligomers in the ethylene-vinyl acetate copolymer is controlled to be below 100 ppm, and the residual sodium hydroxide in the sodium alkoxide solution is controlled to be below 0.3 wt.%. Herein, vinyl acetate oligomers refer to polymers formed by the polymerization of vinyl acetate, including dimers, trimers, and tetramers; the sodium hydroxide in the sodium alkoxide solution is formed by controlling the purity of the industrial-grade sodium alkoxide sample or by the sodium alkoxide solution absorbing water or carbon dioxide from the air during use.

[0008] Preferably, the solvent used for alcoholysis is methanol, ethanol, isopropanol or n-butanol.

[0009] Preferably, during alcoholysis, the concentration of the ethylene-vinyl acetate copolymer is 10-30 wt.%.

[0010] Preferably, the alcoholysis temperature is 60-100℃ and the alcoholysis time is 4-8h.

[0011] Preferably, the sodium alkoxide in the sodium alkoxide solution is sodium methoxide, sodium ethoxide, sodium isopropoxide, or sodium n-butoxide.

[0012] Preferably, the molar ratio of sodium alkoxide in the sodium alkoxide solution to vinyl acetate groups contained in the ethylene-vinyl acetate copolymer is (0.02-0.2):1.

[0013] Preferably, after alcoholysis, crude ethylene-vinyl alcohol copolymer is obtained, which is then subjected to acid washing, water washing and drying to obtain ethylene-vinyl alcohol copolymer.

[0014] On the other hand, the present invention provides an ethylene-vinyl alcohol copolymer, which is prepared by the above-described method for preparing ethylene-vinyl alcohol copolymer.

[0015] Compared with the prior art, the present invention has the following advantages: Since EVOH is generally produced through continuous solution polymerization, during the storage and recycling of vinyl acetate, the monomer and residual vinyl acetate oligomers in the resulting EVA gradually accumulate due to the influence of temperature and oxygen. This leads to a higher catalyst consumption during alcoholysis and the formation of unstable low-molecular-weight polymers, affecting the product's color. Sodium alkoxide solution is a highly efficient alcoholysis catalyst generated by the reaction of lower alcohols and sodium hydroxide, which can improve alcoholysis efficiency and reduce the formation of byproducts. However, industrial-grade sodium alkoxide solution inevitably contains a small amount of sodium hydroxide, and contact with water or carbon dioxide during use increases its impurity concentration, reducing alcoholysis efficiency and generating more sodium acetate byproducts. Simultaneously, organometallic alkalis exhibit uneven distribution between the solvent and the EVA bulk during alcoholysis, easily forming "hot spots" and accelerating the breaking of weak bonds during the drying of EVOH products. The method for preparing the ethylene-vinyl alcohol copolymer of the present invention, by controlling the content of residual vinyl acetate oligomers in the substrate EVA before alcoholysis and the content of residual sodium hydroxide in the sodium alkoxide solution of the alcoholysis catalyst, and in conjunction with a highly efficient phase transfer catalyst, on the one hand, the control of oligomer content and impurities in the sodium alkoxide solution can improve alcoholysis efficiency, reduce the amount of alcoholysis catalyst used, and shorten the alcoholysis time and the time for washing to remove residual sodium acetate; on the other hand, the introduction of the phase transfer catalyst can cause some organometallic bases to form some organic bases (quaternary ammonium bases), which improves their solubility and the exchange and diffusion rate between the solvent phase and the EVA bulk phase, making the alcoholysis reaction more uniform, significantly reducing the difference in the degree of alcoholysis at different locations, and improving the overall degree of alcoholysis of the obtained EVOH; finally, it also reduces the yellowing phenomenon of EVOH products and improves the product color value. Attached Figure Description

[0016] Figure 1 It is the EVOH prepared in Example 1 of this invention. 1 HNMR spectrum. Detailed Implementation

[0017] 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.

[0018] Example 1 The preparation method of the ethylene-vinyl alcohol copolymer in this embodiment includes the following steps: S1 alcoholysis: Add 20 wt.% EVA (M) to a 500 mL three-necked flask. n =5.2×10 4 g / mol, M w / M n=2.4, containing 50 ppm vinyl acetate oligomer) 240 g methanol solution, heat the oil bath and maintain the reaction system at 60℃, then add sodium methoxide solution (mass concentration 30 wt.%, containing 0.2 wt.% sodium hydroxide) and tetrabutylammonium chloride. The molar ratio of sodium methoxide to vinyl acetate groups in EVA is 0.1:1, and the molar ratio of tetrabutylammonium chloride to sodium methoxide is 0.2:1. React for 6 h to complete the alcoholysis process.

[0019] S2 Washing: Pour the above reaction solution into 1000 mL of deionized water containing acetic acid, wherein the molar ratio of acetic acid to sodium methoxide is 1:1, stir for 6 h, then centrifuge, wash the solid precipitated in the lower layer with 1000 mL of deionized water 3 times, and finally centrifuge and filter to obtain crude EVOH.

[0020] S3 Drying: Place the above crude EVOH in a vacuum oven at 80°C and dry for 16 hours to obtain the EVOH product.

[0021] The EVOH product prepared in this embodiment 1 HNMR spectrum as follows Figure 1 As shown, the peaks with chemical shifts of 0.7-0.9 ppm represent hydrogen atoms on the -CH3 side of the branched chain; the peaks with chemical shifts of 1.4-1.5 ppm represent hydrogen atoms on the -CH2 side of ethylene and vinyl alcohol; the peaks with chemical shifts of 3.4-3.9 ppm represent hydrogen atoms on the -CH side of vinyl alcohol; and the peaks with chemical shifts of 3.9-4.5 ppm represent hydrogen atoms on the -OH side of vinyl alcohol. The elution positions of 2.5 ppm and 3.4 ppm are those of H2O and DMSO, respectively. In summary, this example demonstrates the successful preparation of the EVOH product.

[0022] Example 2 The preparation method of the ethylene-vinyl alcohol copolymer in this embodiment includes the following steps: S1 alcoholysis: Add 10 wt.% EVA (M) to a 500 mL three-necked flask. n =5.2×10 4 g / mol, M w / M n =2.4, containing 10 ppm of vinyl acetate oligomer residue) 240 g of methanol solution, after heating the oil bath and maintaining the reaction system at 60°C, add sodium methoxide solution (mass concentration 30 wt.%, containing 0.3 wt.% sodium hydroxide) and tetrabutylammonium chloride. The molar ratio of sodium methoxide to vinyl acetate groups in EVA is 0.02:1, and the molar ratio of tetrabutylammonium chloride to sodium methoxide is 0.1:1. The reaction is carried out for 8 h to complete the alcoholysis process.

[0023] The S2 washing and S3 drying processes are the same as in Example 1.

[0024] Example 3 The preparation method of the ethylene-vinyl alcohol copolymer in this embodiment includes the following steps: S1 alcoholysis: Add 30 wt.% EVA (M) to a 500 mL three-necked flask. n =5.2×10 4 g / mol, M w / M n =2.4, containing 100ppm of vinyl acetate oligomer residue) 240g of methanol solution, after heating the oil bath and maintaining the reaction system at 60℃, add sodium methoxide solution (mass concentration 30wt.%, containing 0.1wt.% sodium hydroxide) and tetrabutylammonium chloride. The molar ratio of sodium methoxide to vinyl acetate groups in EVA is 0.2:1, and the molar ratio of tetrabutylammonium chloride to sodium methoxide is 0.3:1. React for 4h to complete the alcoholysis process.

[0025] The S2 washing and S3 drying processes are the same as in Example 1.

[0026] Example 4 The difference from Example 1 is that sodium methoxide in step S1 is replaced with sodium ethoxide, methanol is replaced with ethanol, tetrabutylammonium chloride is replaced with tetrabutylammonium bromide, and the alcoholysis temperature is 70°C.

[0027] Example 5 The difference from Example 1 is that sodium methoxide in step S1 is replaced with sodium isopropoxide, methanol is replaced with isopropanol, tetrabutylammonium chloride is replaced with benzyltriethylammonium chloride, and the alcoholysis temperature is 80°C.

[0028] Example 6 The difference from Example 1 is that sodium methoxide in step S1 is replaced with sodium n-butoxide, methanol is replaced with n-butanol, tetrabutylammonium chloride is replaced with trioctylmethylammonium chloride, and the alcoholysis temperature is 100°C.

[0029] Comparative Example 1 The difference from Example 1 is that in step S1, the content of vinyl acetate oligomers in EVA is 150 ppm.

[0030] Comparative Example 2 The difference from Example 1 is that in step S1, a commercially available sodium methoxide solution with a sodium hydroxide content of 0.4 wt.% is used.

[0031] Comparative Example 3 The difference from Example 1 is that tetrabutylammonium chloride is not added in step S1.

[0032] Comparative Example 4 The difference from Example 1 is that in step S1, the molar ratio of tetrabutylammonium chloride to sodium methoxide is 0.05:1, and the reaction time is 8 hours.

[0033] Comparative Example 5 The difference from Example 1 is that in step S1, the molar ratio of sodium methoxide to vinyl acetate groups contained in EVA is 0.2:1, and the molar ratio of tetrabutylammonium chloride to sodium methoxide is 0.5:1.

[0034] The process conditions and performance test results of EVOH products for Examples 1-6 and Comparative Examples 1-5 are shown in Table 1. The test for residual vinyl acetate oligomer content in EVA was conducted according to ASTM D4415-05, "Standard Test Method for Determination of Dimers in Acrylic Acids." The residual vinyl acetate oligomers were separated by headspace or immersion methods and then tested using HPLC-MS. The degree of hydrolysis of EVOH was tested according to GB12010.5-89, "Determination Method for Residual Acetate (or Degree of Hydrolysis) in Polyvinyl Alcohol Resin." The color value was tested according to HG / T 3862-2006, "Test Method for Yellow Index of Plastics."

[0035] Table 1. Process conditions and performance test results of EVOH products in Examples 1-6 and Comparative Examples 1-5.

[0036] As shown in Table 1, Examples 1-6 of this invention synergistically control the content of vinyl acetate oligomers in the EVA substrate (alcohololysis substrate) to be below 100 ppm and the residual sodium hydroxide in the sodium alkoxide solution (alcohololysis catalyst) to be below 0.3 wt.%. This improves both the alcoholysis efficiency and degree of alcoholysis, and also enhances the yellow index of the resulting EVOH product. In contrast, in Comparative Examples 1-3, regardless of whether the content of vinyl acetate oligomers in EVA was too high, the residual sodium hydroxide in the sodium alkoxide solution was too high, or no phase transfer catalyst was added, the degree of alcoholysis of the obtained EVOH product was <99.5%, and the yellow index YI was ≥5. Even with further extending the alcoholysis time in Comparative Example 4, or further increasing the amount of sodium alkoxide in Comparative Example 5, the degree of alcoholysis and yellow index of the obtained EVOH product still did not reach the target levels of Example 1. The possible reasons are as follows: First, the residual vinyl acetate oligomers in EVA consume a large amount of the added sodium alkoxide catalyst, generating unstable low-molecular-weight EVOH or depolymerizing into monomers (which may isomerize into acetaldehyde), sacrificing both alcoholysis efficiency and increasing the likelihood of product yellowing. Second, the residual sodium hydroxide in sodium methoxide reacts with methanol to produce water, further sacrificing alcoholysis efficiency. Under both conditions, the amount of residual sodium acetate in the washed EVOH increases, which also adversely affects thermal stability. The presence of a phase transfer catalyst can increase the diffusion and migration efficiency of organometallic bases between the solvent and the EVA bulk, and generates an appropriate amount of quaternary ammonium base, increasing the solubility of sodium alkoxides. Furthermore, it accelerates the removal of metal salts during subsequent washing, thus improving the uniformity of alcoholysis and the color value of the product to some extent. However, excessive addition of phase transfer catalyst can lead to problems such as emulsification or self-decomposition of the catalyst, affecting normal catalytic performance. Excessive residual phase transfer catalyst can also increase the difficulty of post-processing and separation.

Claims

1. A method for preparing ethylene-vinyl alcohol copolymer, characterized in that, The ethylene-vinyl alcohol copolymer is prepared by alcoholysis of ethylene-vinyl acetate copolymer under the synergistic action of sodium alkoxide solution and phase transfer catalyst; the phase transfer catalyst is tetrabutylammonium bromide, tetrabutylammonium chloride, benzyltriethylammonium chloride or trioctylmethylammonium chloride, and the molar ratio of phase transfer catalyst to sodium alkoxide in sodium alkoxide solution is (0.1-0.3):1; the content of vinyl acetate oligomer in ethylene-vinyl acetate copolymer is controlled to be below 100 ppm, and the residual sodium hydroxide in sodium alkoxide solution is controlled to be below 0.3 wt.%.

2. The method for preparing the ethylene-vinyl alcohol copolymer as described in claim 1, characterized in that, The solvents used for alcoholysis are methanol, ethanol, isopropanol, or n-butanol.

3. The method for preparing the ethylene-vinyl alcohol copolymer as described in claim 1, characterized in that, During alcoholysis, the concentration of the ethylene-vinyl acetate copolymer is 10-30 wt.%.

4. The method for preparing the ethylene-vinyl alcohol copolymer as described in claim 1, characterized in that, The alcoholysis temperature is 60-100℃, and the alcoholysis time is 4-8h.

5. The method for preparing the ethylene-vinyl alcohol copolymer as described in claim 1, characterized in that, The sodium alkoxide in the sodium alkoxide solution is sodium methoxide, sodium ethoxide, sodium isopropoxide, or sodium n-butoxide.

6. The method for preparing the ethylene-vinyl alcohol copolymer as described in claim 1, characterized in that, The molar ratio of sodium alkoxide in the sodium alkoxide solution to vinyl acetate groups in the ethylene-vinyl acetate copolymer is (0.02-0.2):

1.

7. The method for preparing the ethylene-vinyl alcohol copolymer as described in claim 1, characterized in that, After alcoholysis, crude ethylene-vinyl alcohol copolymer is obtained, which is then subjected to acid washing, water washing and drying to obtain ethylene-vinyl alcohol copolymer.

8. An ethylene-vinyl alcohol copolymer, characterized in that, It is prepared by the method for preparing ethylene-vinyl alcohol copolymer as described in any one of claims 1-7.

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