Ethylene-vinyl alcohol copolymer and method for producing the same

Abstract

The application discloses ethylene-vinyl alcohol copolymer and a preparation method thereof, and relates to the technical field of EVOH. The technical scheme is as follows: the ethylene-vinyl alcohol copolymer is prepared by alcoholysis of ethylene-vinyl acetate copolymer under the synergistic action of a sodium alcoholate solution and a phase transfer catalyst; the content of vinyl acetate oligomers in the ethylene-vinyl acetate copolymer is controlled to be below 100 ppm, and the residual amount of sodium hydroxide in the sodium alcoholate solution is controlled to be below 0.3 wt.%. The application effectively controls the content of residual vinyl acetate oligomers in the EVA alcoholysis precursor of EVOH and the content of residual sodium hydroxide in the sodium alcoholate solution, and assists the phase transfer catalyst to improve the uniformity of the sodium alcoholate alcoholysis process and prevent the generation of local 'hot spots', thereby reducing the alcoholysis catalyst consumption, improving the alcoholysis efficiency and alcoholysis degree, further reducing the yellowing phenomenon of the EVOH product, and improving the yellow index of the product.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of EVOH, in particular to an ethylene-vinyl alcohol copolymer and a preparation method thereof. BACKGROUND

[0002] Ethylene-vinyl alcohol copolymer (EVOH) has the properties of polyethylene and polyvinyl alcohol, and is a widely used high-barrier and solvent-resistant resin material, and is therefore widely used in food, medical and cosmetic packaging, automobile fuel tanks, industrial pipelines and other fields. The hydrogen bond interaction and crystallinity in EVOH are the key to its excellent gas barrier property and solvent resistance. EVOH cannot be obtained by direct copolymerization of ethylene and vinyl alcohol, because vinyl alcohol is easily isomerized into acetaldehyde. The production process of EVOH is similar to that of PVA, which can be regarded as a modified product of PVA. The preparation process includes polymerization and alcoholysis. First, ethylene-vinyl acetate copolymer (EVA) is generated by copolymerization of ethylene and vinyl acetate, and then alcoholysis reaction is performed on EVA to convert the ester group therein into a hydroxyl group, thereby obtaining EVOH.

[0003] The formation of hydrogen bonds and crystallinity in EVOH is closely related to the alcoholysis process. A simple and efficient alcoholysis process is crucially important for the alcoholysis degree and the yellow index of the obtained EVOH. Chinese invention patent CN116529269A discloses a preparation method of ethylene-vinyl alcohol copolymer, which improves the alcoholysis efficiency and reduces impurity residues by controlling the addition or stepwise addition of catalyst. The catalyst used is sodium hydroxide. Chinese invention patent CN119775458A discloses a method and system for producing ethylene-vinyl alcohol copolymer, which can ensure that the process is in a dissolved state by using an alcohol / water mixture for the hydrolysis of EVA instead of a transesterification reaction, thereby improving the alcoholysis degree of EVOH. Chinese invention patent CN104098728A discloses an alcoholysis method of ethylene-vinyl acetate copolymer, which improves the reaction control stability and the alcoholysis efficiency by removing the by-product methyl acetate in the alcoholysis process with hot low-carbon alcohol vapor. Chinese invention patent CN108431054A discloses an ethylene-vinyl alcohol copolymer and a manufacturing method thereof, which effectively reduces the melting point of the product and improves the processing stability by using a sodium alcoholate solution as a catalyst to control the presence of carboxylate groups in the end of the obtained EVOH.

[0004] The residual monomer, water content and the concentration of the catalyst used in EVA have a great influence on the alcoholysis process, but few people pay attention to the influence of the content of residual vinyl acetate oligomers in EVA and the purity of sodium alcoholate on alcoholysis. In addition, due to the use of organic metal base as the alcoholysis catalyst, although it is convenient and efficient, there is a problem of uneven distribution of metal ions and local "hot spots" in the process. Although there are patents (such as Chinese invention patent CN113861313B discloses an ethylene-vinyl alcohol copolymer and its preparation method and application and Chinese invention patent CN113736004B discloses an ethylene-vinyl alcohol copolymer and its preparation method and application) report the use of basic or acidic ionic liquid as catalyst, avoiding the introduction of metal ions, reducing the difficulty of washing and being able to realize recycling and reuse, but the cost and operation difficulty are extremely great.

[0005] Therefore, how to control the content of residual vinyl acetate oligomers in EVA and residual sodium hydroxide in sodium alcoholate, and improve the uniform diffusion of sodium alcoholate in the alcoholysis process, to improve the alcoholysis efficiency, alcoholysis degree and improve the color of the product of EVOH is a very worthwhile topic to explore. SUMMARY

[0006] The technical problem to be solved by the present application is to overcome the shortcomings of the prior art, provide an ethylene-vinyl alcohol copolymer and a preparation method thereof, by effectively and synergistically controlling the content of residual vinyl acetate oligomers in EVOH alcoholysis precursor EVA and residual sodium hydroxide in sodium alcoholate solution, and assisting the phase transfer catalyst to improve the uniformity of the alcoholysis process of sodium alcoholate, to prevent the generation of local "hot spots", reduce the amount of alcoholysis catalyst, improve the alcoholysis efficiency and alcoholysis degree, and further reduce the yellowing phenomenon of EVOH products and improve the yellow index of the products.

[0007] The technical scheme of the present application is:

[0008] On the one hand, the present application provides a preparation method of ethylene-vinyl alcohol copolymer, which is prepared by alcoholysis of ethylene-vinyl acetate copolymer under the synergistic action of sodium alcoholate solution and phase transfer catalyst; the phase transfer catalyst is tetrabutylammonium bromide, tetrabutylammonium chloride, benzyltriethylammonium chloride or trioctylmethylammonium chloride, the molar ratio of the phase transfer catalyst to sodium alcoholate in the sodium alcoholate 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 amount of sodium hydroxide in the sodium alcoholate solution is controlled to be below 0.3wt.%. Among them, the vinyl acetate oligomers refer to polymers formed by polymerization of vinyl acetate, including dimers, trimers and tetramers, etc.; the sodium hydroxide in the sodium alcoholate solution is the purity control of industrial grade sample of sodium alcoholate or the water or carbon dioxide in the air absorbed by the sodium alcoholate solution during use.

[0009] Preferably, the solvent used in the alcoholysis is methanol, ethanol, isopropanol or n-butanol.

[0010] Preferably, the concentration of the ethylene-vinyl acetate copolymer in the alcoholysis is 10-30 wt.%.

[0011] Preferably, the alcoholysis temperature is 60-100℃, and the alcoholysis time is 4-8h.

[0012] Preferably, the sodium alcoholate in the sodium alcoholate solution is sodium methoxide, sodium ethoxide, sodium isopropoxide or sodium n-butoxide.

[0013] Preferably, the molar ratio of the sodium alcoholate in the sodium alcoholate solution to the vinyl acetate groups in the ethylene-vinyl acetate copolymer is (0.02-0.2):1.

[0014] Preferably, the ethylene-vinyl alcohol copolymer is obtained by alcoholysis, acid washing, water washing and drying of the ethylene-vinyl alcohol copolymer crude product.

[0015] In another aspect, the present application provides an ethylene-vinyl alcohol copolymer prepared by the above method for preparing an ethylene-vinyl alcohol copolymer.

[0016] Compared with the prior art, the present application has the following advantages:

[0017] 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

[0018] Figure 1 It is the EVOH prepared in Example 1 of this invention. 1 HNMR spectrum. Detailed Implementation

[0019] 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.

[0020] Example 1

[0021] The preparation method of the ethylene-vinyl alcohol copolymer in this embodiment includes the following steps:

[0022] 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 vinyl acetate oligomer 50 ppm) methanol solution 240 g, the oil bath was warmed and maintained at 60 °C after the reaction system, the addition of sodium methoxide solution (mass concentration 30 wt.%, containing sodium hydroxide 0.2 wt.%) and tetrabutylammonium chloride, the molar ratio of sodium methoxide and vinyl acetate groups contained in EVA was 0.1:1, the molar ratio of tetrabutylammonium chloride and sodium methoxide was 0.2:1, the reaction was 6 h, the alcoholysis process was completed.

[0023] S2 washing: the above reaction liquid was poured into 1000 mL of deionized water containing acetic acid, the molar ratio of acetic acid to sodium methoxide was 1:1, stirred for 6 h, then centrifuged, the lower layer of the solid was washed with 1000 mL of deionized water for 3 times, finally centrifuged and filtered to obtain EVOH crude product.

[0024] S3 drying: the above EVOH crude product was placed in a vacuum oven at 80 °C, dried for 16 h to obtain EVOH product.

[0025] The EVOH product prepared in this example has the following properties: 1 HNMR spectrum is shown in Figure 1 , it can be seen that the peak at 0.7-0.9 ppm represents the hydrogen on the -CH3 of the branched chain; the peak at 1.4-1.5 ppm represents the hydrogen on the -CH2 of the vinyl and vinyl alcohol; the peak at 3.4-3.9 ppm represents the hydrogen on the -CH of the vinyl alcohol; the peak at 3.9-4.5 ppm represents the hydrogen on the -OH of the vinyl alcohol; among them, 2.5 ppm and 3.4 ppm are the peak positions of H2O and DMSO respectively. In summary, it is proved that the EVOH product is successfully prepared in this example.

[0026] Example 2

[0027] The preparation method of the ethylene-vinyl alcohol copolymer of this example comprises the following steps:

[0028] S1 alcoholysis: 10 wt.% EVA (M n =5.2×10 4 g / mol, M w / M n =2.4, containing vinyl acetate oligomer residue 10 ppm) methanol solution 240 g, the oil bath was warmed and maintained at 60 °C after the reaction system, the addition of sodium methoxide solution (mass concentration 30 wt.%, containing sodium hydroxide 0.3 wt.%) and tetrabutylammonium chloride, the molar ratio of sodium methoxide and vinyl acetate groups contained in EVA was 0.02:1, the molar ratio of tetrabutylammonium chloride and sodium methoxide was 0.1:1, the reaction was 8 h, the alcoholysis process was completed.

[0029] S2 washing and S3 drying process are the same as example 1.

[0030] Example 3

[0031] The preparation method of the ethylene-vinyl alcohol copolymer of this example comprises the following steps:

[0032] S1 alcoholysis: 240 g of 30 wt.% EVA (M n = 5.2 x 10 4 g / mol, M w / M n = 2.4, vinyl acetate oligomer residue 100 ppm) methanol solution was added to a 500 mL three-necked flask, the oil bath was warmed and the reaction system was maintained at 60°C, then sodium methoxide solution (mass concentration 30 wt.%, sodium hydroxide content 0.1 wt.%) and tetrabutylammonium chloride were added, the molar ratio of sodium methoxide to vinyl acetate groups contained in EVA was 0.2:1, the molar ratio of tetrabutylammonium chloride to sodium methoxide was 0.3:1, and the reaction was carried out for 4 h to complete the alcoholysis process.

[0033] S2 washing and S3 drying processes are the same as in Example 1.

[0034] Example 4

[0035] The difference from Example 1 is that sodium methoxide in step S1 is replaced by sodium ethoxide, methanol is replaced by ethanol, tetrabutylammonium chloride is replaced by tetrabutylammonium bromide, and the alcoholysis temperature is 70°C.

[0036] Example 5

[0037] The difference from Example 1 is that sodium methoxide in step S1 is replaced by sodium isopropoxide, methanol is replaced by isopropanol, tetrabutylammonium chloride is replaced by benzyltriethylammonium chloride, and the alcoholysis temperature is 80°C.

[0038] Example 6

[0039] The difference from Example 1 is that sodium methoxide in step S1 is replaced by sodium n-butylate, methanol is replaced by n-butanol, tetrabutylammonium chloride is replaced by tricaprylylmethylammonium chloride, and the alcoholysis temperature is 100°C.

[0040] Comparative Example 1

[0041] The difference from Example 1 is that the content of vinyl acetate oligomer in EVA in step S1 is 150 ppm.

[0042] Comparative Example 2

[0043] The difference from Example 1 is that in step S1, a common commercially available sodium methoxide solution is used, and the content of sodium hydroxide is 0.4 wt.%.

[0044] Comparative Example 3

[0045] The difference from Example 1 is that in step S1, no tetrabutylammonium chloride is added.

[0046] Comparative Example 4

[0047] 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 8h.

[0048] Comparative Example 5

[0049] The difference from Example 1 is that in step S1, the molar ratio of sodium methoxide to the vinyl acetate groups contained in EVA is 0.2:1, and the molar ratio of tetrabutylammonium chloride to sodium methoxide is 0.5:1.

[0050] The process conditions of Examples 1-6 and Comparative Examples 1-5 and the performance test results of EVOH products are shown in Table 1. Among them, the test of residual vinyl acetate oligomer content in EVA refers to “ASTM D4415-05 Standard Test Method for Determining Dimers in Acrylic Acid”, and the residual vinyl acetate oligomer therein is separated by headspace or immersion method and then tested by HPLC-MS; the test of EVOH alcoholysis degree refers to “GB12010.5-89 Method for Determining Residual Acetate (or Alcoholysis Degree) of Polyvinyl Alcohol Resin”; and the test of color value refers to “HG / T 3862-2006 Test Method for Yellow Index of Plastics”.

[0051] Table 1 Process conditions of Examples 1-6 and Comparative Examples 1-5 and performance test results of EVOH products

[0052]

[0053] From Table 1, the content of vinyl acetate oligomer in the alcoholysis substrate EVA of Examples 1-6 is less than 100 ppm, and the residual amount of sodium hydroxide in the alcoholysis catalyst sodium alcoholate solution is less than 0.3 wt.%, which can improve the alcoholysis efficiency and alcoholysis degree, and improve the yellow index of the obtained EVOH product. In Comparative Examples 1-3, whether the content of vinyl acetate oligomer in EVA is too high, or the residual amount of sodium hydroxide in the sodium alcoholate solution is too high, or no phase transfer catalyst is added, the alcoholysis degree of the obtained EVOH product is less than 99.5%, and the yellow index YI is greater than or equal to 5. Even in Comparative Example 4, the alcoholysis time is further extended, or in Comparative Example 5, the amount of sodium alcoholate is further increased, and the alcoholysis degree and yellow index of the obtained EVOH product also fail to reach the target level of Example 1. It is speculated that the possible reasons are as follows: first, the residual vinyl acetate oligomer in EVA consumes a large amount of added sodium alcoholate catalyst, and generates unstable low molecular weight EVOH or depolymerizes into monomers (monomers may isomerize into acetaldehyde), which not only sacrifices the alcoholysis efficiency but also increases the possibility of product yellowing; second, the reaction of residual sodium hydroxide in sodium methoxide with methanol produces water, which further sacrifices the alcoholysis efficiency. Under the above two conditions, the residual sodium acetate in the washed EVOH increases, which also has an adverse effect on the thermal stability. The presence of the phase transfer catalyst can increase the diffusion and migration efficiency of the organometallic base between the solvent and the EVA body, and generate an appropriate amount of quaternary ammonium base, which increases the solubility of sodium alcoholate and accelerates the removal of metal salts in the subsequent washing process, thus improving the alcoholysis uniformity and product color value to some extent. However, when the phase transfer catalyst is added too much, it will cause problems such as emulsification or self-decomposition of the phase transfer catalyst, affecting the normal catalytic effect, and excessive residual phase transfer catalyst will also cause problems such as increased difficulty in post-treatment and separation.

Claims

1. A process for the preparation of an ethylene-vinyl alcohol copolymer, characterized in that, The ethylene-vinyl alcohol copolymer is prepared by alcoholysis of ethylene-vinyl acetate copolymer in the presence of sodium alcoholate solution and phase transfer catalyst; the phase transfer catalyst is tetrabutylammonium bromide, tetrabutylammonium chloride, benzyltriethylammonium chloride or trioctylmethylammonium chloride, the molar ratio of the phase transfer catalyst to sodium alcoholate in the sodium alcoholate solution is (0.1-0.3):1; the content of vinyl acetate oligomer in the ethylene-vinyl acetate copolymer is controlled to be less than 100 ppm, and the residual amount of sodium hydroxide in the sodium alcoholate solution is controlled to be less than 0.3 wt.%; wherein the vinyl acetate oligomer includes dimer, trimer and tetramer.

2. The method for producing an ethylene-vinyl alcohol copolymer according to claim 1, wherein The solvent used in the alcoholysis is methanol, ethanol, isopropanol or n-butanol.

3. The method for producing an ethylene-vinyl alcohol copolymer according to claim 1, wherein The concentration of the ethylene-vinyl acetate copolymer in the alcoholysis is 10-30 wt.%.

4. The method for producing an ethylene-vinyl alcohol copolymer according to claim 1, wherein The alcoholysis temperature is 60-100℃, and the alcoholysis time is 4-8h.

5. The method for producing an ethylene-vinyl alcohol copolymer according to claim 1, wherein The sodium alcoholate in the sodium alcoholate solution is sodium methoxide, sodium ethoxide, sodium isopropoxide or sodium n-butoxide.

6. The method for producing an ethylene-vinyl alcohol copolymer according to claim 1, wherein The molar ratio of the sodium alcoholate in the sodium alcoholate solution to the vinyl acetate group contained in the ethylene-vinyl acetate copolymer is (0.02-0.2):

1.

7. The method of producing an ethylene-vinyl alcohol copolymer according to claim 1, wherein After the alcoholysis, the ethylene-vinyl alcohol copolymer crude product is obtained, and then the ethylene-vinyl alcohol copolymer is obtained after acid washing, water washing and drying.

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