Ethylene-vinyl alcohol copolymer composition and its preparation method

Abstract

This invention provides an ethylene-vinyl alcohol copolymer composition and its preparation method. The composition contains an ethylene-vinyl alcohol copolymer, an alkali metal element, and a carboxyl-containing substance. The carboxyl-containing substance is characterized by containing a monobasic saturated carboxylic acid and a carboxylic acid containing a carbon-carbon double bond. This invention improves the yellowing resistance of the EVOH resin composition by simultaneously including acetic acid and a carboxylic acid containing a carbon-carbon double bond. After heat treatment at 210°C for 0.5 hours, the yellowing index YI does not exceed 20, an improvement of no more than 15.

Description

[0001] Cross-reference to related applications

[0002] This application claims the benefit of Chinese patent application CN202011103188.4, filed on October 15, 2020, the contents of which are incorporated herein by reference. Technical Field

[0003] This invention relates to an ethylene-vinyl alcohol copolymer composition and its preparation method. Background Technology

[0004] EVOH resin possesses excellent odor barrier properties, transparency, and gloss. It is also resistant to greases, chemicals, ultraviolet radiation, and other rays. Furthermore, it exhibits good mechanical properties, strength, and tensile modulus. It is used in composite packaging films for meat, oils, industrial solvents, and pesticides, as well as in foaming processes, hollow containers, and barrier layers. In addition, EVOH resin can be compounded with other materials to create fire-resistant, antibacterial, and antioxidant materials.

[0005] However, EVOH resin is prone to yellowing during melt processing, which affects product quality.

[0006] CN109651557A improves the degree of alcoholysis by adding an alkaline catalyst in two stages and controlling the amount of catalyst and reaction time in each stage, thereby reducing the color intensity of EVOH. The resulting ethylene-vinyl alcohol copolymer has an alcoholysis degree of 99.3-99.7% and low color intensity. Summary of the Invention

[0007] The purpose of this invention is to overcome the defect of high-temperature yellowing in existing EVOH resin compositions, and to provide an EVOH resin composition with high resistance to yellowing and its preparation method.

[0008] The first aspect of the present invention provides an ethylene-vinyl alcohol copolymer composition comprising an ethylene-vinyl alcohol copolymer, an alkali metal element and a carboxyl-containing substance, characterized in that the carboxyl-containing substance comprises a monobasic saturated carboxylic acid and a carboxylic acid containing a carbon-carbon double bond.

[0009] The second aspect of the present invention provides a method for preparing an ethylene-vinyl alcohol copolymer composition, the method comprising granulating an ethylene-vinyl acetate copolymer obtained by polymerization reaction by alcoholysis with an alkali metal hydroxide, followed by water washing and acid washing, characterized in that the acid used for acid washing includes monobasic saturated carboxylic acids and carboxylic acids containing carbon-carbon double bonds.

[0010] This invention improves the yellowing resistance of EVOH resin compositions by simultaneously including monobasic saturated carboxylic acids such as acetic acid and carboxylic acids containing carbon-carbon double bonds. After heat treatment at 210°C for 0.5 hours, the yellowing index YI does not exceed 20, representing an improvement of no more than 15.

[0011] The EVOH resin composition method provided by this invention is simple, easy to operate, and conducive to industrial production. Detailed Implementation

[0012] In ethylene-vinyl alcohol copolymer compositions, the ethylene-vinyl alcohol copolymer is the main component, obtained through alcoholysis following ethylene-vinyl acetate copolymerization. Sodium is a byproduct generated during the alcoholysis of the ethylene-vinyl acetate copolymer, and its content is typically reduced through subsequent water washing and acid washing. Currently, acetic acid is generally used for acid washing.

[0013] The inventors of this invention made an unexpected discovery that by using both monosaturated carboxylic acids and carboxylic acids containing carbon-carbon double bonds (also known as unsaturated carboxylic acids) simultaneously during the pickling process, the yellowing resistance of the composition can be improved, so that the YI of the product after melt processing will not increase significantly compared with that before processing, and thus is lower than the YI of the existing EVOH melt-processed product.

[0014] According to a preferred embodiment of the present invention, the weight ratio of the monosaturated carboxylic acid and the carboxylic acid containing carbon-carbon double bonds is 1:0.1-1, preferably 1:0.4-0.7.

[0015] In this invention, ion chromatography is used to detect the content of monosaturated carboxylic acids, phosphate groups, and carboxylic acids containing carbon-carbon double bonds. The specific testing method is as follows: EVOH sample is pulverized and passed through a 100-mesh sieve. 10g of sample is weighed and mixed with 50ml of deionized water, and extracted by stirring and reflux in a 95℃ water bath for 10 hours. The extract is diluted 5 times (by volume) with deionized water and tested using ion chromatography. The chromatographic column is Metrosep A Supp / 250,4, the mobile phase is a mixed solution of Na2CO3 and NaHCO3, and the calibration solution is an aqueous solution of sodium acetate.

[0016] Preferably, the content of the monobasic saturated carboxylic acid is 50-2000 ppm, more preferably 100-1500 ppm, and even more preferably 200-800 ppm, relative to the mass of the ethylene-vinyl alcohol copolymer, and the content of the carboxylic acid containing carbon-carbon double bonds is 20-2000 ppm, more preferably 50-1000 ppm, and even more preferably 150-250 ppm, relative to the mass of the ethylene-vinyl alcohol copolymer.

[0017] In this invention, the carboxylic acid containing a carbon-carbon double bond may contain one or more carboxyl groups, one or more carbon-carbon double bonds, and one or more hydroxyl groups. The number of carbon atoms in the carboxylic acid containing a carbon-carbon double bond may be 3-10, preferably one or more of sorbic acid, 2-hexenoic acid, 3-hexenoic acid, butenoic acid, vinylacetic acid, vinylpropionic acid, vinylglycolic acid, and cinnamic acid.

[0018] The monocarboxylic acid can be any substance containing a carboxyl group -COOH and an unsaturated carbon-carbon double bond, preferably a monocarboxylic acid with 1-8 carbon atoms, more preferably a monocarboxylic acid with 1-5 carbon atoms, such as one or more of formic acid, acetic acid, propionic acid, butyric acid, and valeric acid.

[0019] The inventors of this invention also unexpectedly discovered that by adding potassium after acid contact and controlling the mass ratio of sodium to potassium within a specific range of 0.1:1-1:1, preferably 0.2:1-0.8:1, and more preferably 0.2:1-0.5:1, the heat resistance of the resulting EVOH resin composition can be significantly improved. Experiments have shown that by controlling the potassium content within the above range, the initial decomposition temperature of the EVOH resin composition can be increased to 380-390°C, and the maximum decomposition temperature can be increased to over 420°C, enabling the ethylene-vinyl alcohol copolymer to adapt to processes requiring higher processing temperatures.

[0020] Controlling the sodium content within a certain range yields better results. Preferably, the total amount of potassium and sodium relative to the mass of the ethylene-vinyl alcohol copolymer is 100-3000 ppm, more preferably 200-2000 ppm, and even more preferably 800-1200 ppm.

[0021] In this invention, the contents of potassium and sodium elements were determined using inductively coupled plasma atomic emission spectrometry (ICP-OES). The specific testing method was as follows: EVOH sample was pulverized and passed through a 500-mesh sieve. 10g of sample was weighed and mixed with 50ml of ion-exchanged water, and the mixture was extracted by stirring and reflux in a 95℃ water bath for 10 hours. The extract was then diluted 5 times with ion-exchanged water and tested using ICP-OES.

[0022] Preferably, the potassium element exists in ionic form, more preferably in the form of a potassium salt, and even more preferably, the potassium salt is potassium carbonate and / or potassium bicarbonate. Compared to inorganic potassium salts such as potassium nitrate, potassium phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, potassium sulfate, and potassium chloride, as well as organic potassium salts such as potassium acetate and potassium formate, potassium carbonate and potassium bicarbonate can achieve better resistance to high-temperature yellowing.

[0023] According to a preferred embodiment of the present invention, the EVOH resin composition contains an ethylene-vinyl alcohol copolymer, a carboxyl-containing substance, a potassium salt, and a sodium salt. The carboxyl-containing substance includes acetic acid and a carboxylic acid containing a carbon-carbon double bond, wherein the carboxylic acid containing a carbon-carbon double bond includes one or more of sorbic acid, 2-hexenoic acid, 3-hexenoic acid, butenoic acid, vinylacetic acid, vinylpropionic acid, vinylglycolic acid, and cinnamic acid. The content of acetic acid is 50-2000 ppm relative to the mass of the ethylene-vinyl alcohol copolymer; the content of the carboxylic acid containing a carbon-carbon double bond is 20-2000 ppm relative to the mass of the ethylene-vinyl alcohol copolymer; the total amount of potassium salt and sodium salt is 200-3000 ppm relative to the mass of the ethylene-vinyl alcohol copolymer, and the mass ratio of sodium ions in the sodium salt to potassium ions in the potassium salt is 0.1:1-1:1.

[0024] To prevent coloring during melt molding of the EVOH resin composition, according to a preferred embodiment of the present invention, the EVOH resin composition may further contain boron and / or phosphorus. The boron element may be various boron compounds, such as boric acid, borate esters, borate salts, etc., preferably one or more of orthoboric acid (i.e., commonly known boric acid), metaboric acid, tetraboric acid, sodium borate flakes, potassium metaborate, sodium tetraborate, sodium pentaborate, lithium borate, borax, trimethyl borate, triethyl borate, etc., with boric acid being the most preferred.

[0025] Preferably, relative to the mass of the ethylene-vinyl alcohol copolymer, the boron content, calculated as boric acid (H3BO3), is 50-1000 ppm, preferably 300-500 ppm. Within this range, a significant improvement in heat resistance and resistance to yellowing can be achieved, while effectively avoiding adverse effects on the melt processing process.

[0026] More preferably, the EVOH resin composition further contains 200-1000 ppm, preferably 500-800 ppm, of a phosphorus compound based on H3PO4. The phosphorus compound can be various phosphates, such as alkali metal salts of phosphoric acid or alkaline earth metal salts of phosphoric acid. Specifically, it can be one or more of potassium phosphate, sodium phosphate, ammonium phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, dipotassium hydrogen phosphate, disodium hydrogen phosphate, potassium dihydrogen phosphate, and sodium dihydrogen phosphate. It should be noted that when the phosphorus compound is a potassium salt of phosphoric acid, the amount of potassium in it is included in the potassium content of the composition.

[0027] In this invention, the ethylene-vinyl acetate copolymer can be an EVOH copolymer obtained by various specifications and methods. Preferably, the melt index (190°C, 2160g) of the ethylene-vinyl acetate copolymer is 0.5-10g / 10min.

[0028] It can be any ethylene-vinyl acetate copolymer prepared by existing methods, for example, it can be obtained by polymerization reaction. The solvent used in the polymerization reaction is preferably an alcohol solvent, and the initiator used in the polymerization reaction is preferably an azo initiator or a peroxide initiator.

[0029] The second aspect of the present invention provides a method for preparing an ethylene-vinyl alcohol copolymer composition, the method comprising granulating an ethylene-vinyl acetate copolymer obtained by polymerization reaction by alcoholysis with an alkali metal hydroxide, followed by sequential water washing and acid washing, characterized in that the acid used for acid washing includes monobasic saturated carboxylic acids and carboxylic acids containing carbon-carbon double bonds.

[0030] The water washing and acid washing process reduces the sodium content to 50-2000 ppm.

[0031] Generally, acid is prepared into an aqueous solution of a certain concentration and then contacted with ethylene-vinyl acetate copolymer granules for pickling. The weight of the pickling solution is generally 2-5 times the weight of the ethylene-vinyl acetate copolymer granules.

[0032] Preferably, relative to 1 part by weight of the ethylene-vinyl alcohol copolymer, the amount of the monobasic saturated carboxylic acid is 0.001-0.05 parts by weight, preferably 0.005-0.03 parts by weight, and the content of the carboxylic acid containing carbon-carbon double bonds is 0.001-0.015 parts by weight, preferably 0.001-0.01 parts by weight.

[0033] When the alkali metal hydroxide used for alcoholysis is sodium hydroxide, the addition of a potassium source after washing can introduce potassium into the resulting EVOH resin composition, thereby improving the resin product's resistance to thermal decomposition. Preferably, the potassium source is a potassium salt, and more preferably, the potassium salt is one or more of potassium carbonate, potassium bicarbonate, potassium phosphate, potassium hydrogen phosphate, potassium dihydrogen phosphate, and dipotassium hydrogen phosphate.

[0034] Preferably, the amount of potassium source used is 0.001-0.003 times the mass of the ethylene-vinyl alcohol copolymer.

[0035] According to a preferred embodiment of the present invention, the method further includes adding a phosphorus-containing compound and / or a boron-containing compound to the acid-washed ethylene-vinyl acetate copolymer.

[0036] Preferably, the amount of boron-containing compound added is such that, relative to the mass of the ethylene-vinyl alcohol copolymer, the boron content, calculated as boric acid (H3BO3), is 50-1000 ppm, preferably 300-500 ppm.

[0037] Preferably, the amount of phosphorus-containing compound added is such that, relative to the mass of the ethylene-vinyl alcohol copolymer, the phosphorus content, calculated as H3PO4, is 200-1000 ppm, preferably 500-800 ppm.

[0038] Boron and / or phosphorus sources can be added together with or sequentially with potassium sources to obtain a composition containing potassium, boron, and phosphorus simultaneously. There are no particular restrictions on the method of adding potassium salts, boron-containing compounds, and phosphorus-containing compounds; preferably, the compounds are dissolved in water, and the solution is added to the ethylene-vinyl alcohol copolymer and mixed thoroughly. The concentration of the solution is not particularly limited and can be of various concentrations, as long as it allows the potassium salts, boron-containing compounds, and phosphorus-containing compounds to be loaded onto the ethylene-vinyl alcohol copolymer. Because ethylene-vinyl alcohol copolymers have a certain porous structure, a saturated impregnation method can be used to load the potassium salts, boron-containing compounds, and phosphorus-containing compounds onto the ethylene-vinyl alcohol copolymer, followed by drying to remove the solvent, thus obtaining the ethylene-vinyl alcohol copolymer composition.

[0039] The types of boron, phosphorus, and potassium sources are as described above.

[0040] The polymerization reaction can be used to prepare ethylene-vinyl acetate copolymers using various existing methods, such as solution polymerization, emulsion polymerization, or suspension polymerization. Solution polymerization is preferred, and the solvent used in the polymerization reaction is preferably an alcohol solvent. The alcohol solvent may include alcohols with 1-4 carbon atoms, such as methanol, ethanol, propanol, ethylene glycol, n-butanol, and tert-butanol, or it may be a mixture of two of the above alcohols, or a solvent with the above alcohols as the main component and other small components.

[0041] The initiator used in the polymerization reaction can be an azo initiator or a peroxide initiator.

[0042] Furthermore, azo initiators include oil-soluble initiators such as azobisisobutyronitrile, azobisisovalerate, azoisobutyronitrile, azobiscyclohexylformitrile, and dimethyl azobisisobutyrate.

[0043] Furthermore, peroxide initiators include organic peroxides (such as benzoyl peroxide, tert-butyl peroxide, methyl ethyl ketone peroxide, diisobutyryl peroxide, tert-amyl peroxyneodecanate, bis(4-tert-butylcyclohexyl peroxydicarbonate), tert-amyl peroxypentanoate, tert-butyl peroxyacetate, bisbutyl peroxydicarbonate, etc.) or inorganic peroxides (such as hydrogen peroxide, ammonium persulfate, potassium persulfate, etc.).

[0044] Furthermore, the initiator includes organic peroxide initiators.

[0045] Furthermore, the proportions by weight are as follows: 10-40 parts solvent, 60-200 parts vinyl acetate monomer, 0.01-0.3 parts initiator, and 5-60 parts ethylene monomer.

[0046] Furthermore, the proportions by weight are as follows: 10-35 parts solvent, 80-150 parts vinyl acetate monomer, 0.03-0.2 parts initiator, and 20-40 parts ethylene monomer.

[0047] The methods of alcoholysis and granulation are well known to those skilled in the art and will not be described in detail here.

[0048] The present invention will be further described below through embodiments.

[0049] The method for detecting sodium and potassium ion content is as follows: The EVOH sample is crushed and sieved. 10g of the sample is weighed and mixed with 50ml of ion-exchange water. The mixture is then extracted by stirring and reflux in a 95℃ water bath for 10 hours. The extract is diluted 5 times with ion-exchange water and tested using ICP-OES.

[0050] The method for detecting monocarboxylic acids, carboxylic acids containing carbon-carbon double bonds, and phosphate content is as follows: The EVOH sample is pulverized and passed through a 100-mesh sieve. 10g of the sample is weighed and mixed with 50ml of deionized water. The mixture is then extracted by stirring and reflux in a 95℃ water bath for 10 hours. The extract is diluted 5 times (by volume) with deionized water and tested using ion chromatography. The chromatographic column is Metrosep A Supp / 250,4, the mobile phase is a mixed solution of Na₂CO₃ and NaHCO₃, and the calibration solution is an aqueous solution of sodium acetate.

[0051] The method for testing boron is to weigh 100 grams of sample, aerate it in a muffle furnace, dissolve the ash in 200 ml of 0.01 equivalent nitric acid aqueous solution, and analyze the boron content by atomic absorption spectrometry. The boron content can then be converted into the weight of boric acid.

[0052] The method for detecting the initial decomposition temperature and the maximum decomposition temperature is as follows: using a thermogravimetric analyzer, under a nitrogen atmosphere, the temperature is increased at a rate of 10℃ / min from room temperature to 800℃. The temperature at which 5% (w) of the sample decomposes is the initial decomposition temperature T0 (℃), and the temperature at which 50% (w) of the sample decomposes is the maximum decomposition temperature T1 (℃).

[0053] The method for detecting the yellowness index YI is as follows: Weigh 10-15g of sample and use a colorimeter to test the initial yellowness index YI and the yellowness index YI after heat treatment at 210℃ for 0.5 hours. The smaller the YI value, the lower the chromaticity.

[0054] In the following embodiments, unless otherwise stated, all parts are parts by weight.

[0055] Example 1

[0056] A. Polymerization: 16 parts of methanol, 80 parts of vinyl acetate, and 0.01 parts of azobisisobutyronitrile were added to a polymerization reactor equipped with a stirrer. Ethylene was introduced to maintain the pressure inside the polymerization reactor at 3.7 MPa and the temperature at 65°C. The reaction was carried out for 5 hours to obtain an ethylene-vinyl acetate copolymer solution. The ethylene and vinyl acetate monomers were removed by depressurization and distillation to obtain the ethylene-vinyl acetate copolymer solution.

[0057] B. Alcohololysis: Adjust the mass fraction of the ethylene-vinyl acetate copolymer solution obtained in step A to 40%, and then add a sodium hydroxide-methanol solution with a concentration of 40 g / L (sodium hydroxide is the solute) for alcohololysis. The amount of sodium hydroxide-methanol solution used is such that the molar ratio of sodium hydroxide in the sodium hydroxide-methanol solution to the vinyl acetate groups contained in the ethylene-vinyl acetate copolymer is 0.05:1. React for 4 hours until alcohololysis is complete.

[0058] C. Granulation: After alcoholysis of ethylene-vinyl acetate copolymer, the ethylene-vinyl alcohol copolymer solution obtained by alcoholysis is extruded into an aqueous solution at 5°C through an extrusion device with a perforated plate. After precipitating into strips, it is cut into granules using a common cutting method.

[0059] D. Washing: Subsequently, the ethylene-vinyl alcohol copolymer particles were washed with 5 times the mass of water through a kettle equipped with a stirring device. Each wash lasted 2 hours and was repeated twice.

[0060] E. Pickling: Add water at a ratio of 5 times the mass of the ethylene-vinyl alcohol copolymer particles to a stirring vessel and wash the ethylene-vinyl alcohol copolymer particles. Add 0.015 parts of propionic acid and 0.007 parts of sorbic acid relative to the mass of EVOH (1 part by mass) to the washing solution and pickle for 2 hours. After pickling, centrifuge to dehydrate.

[0061] F. Conditioning: Add an aqueous solution of potassium carbonate (0.0015 parts by mass relative to EVOH, 0.001 parts by mass of boric acid, and 0.001 parts by mass of potassium dihydrogen phosphate) to the centrifuged and dehydrated EVOH particles, mix thoroughly and saturate impregnate, and then dry at 115°C for 24 hours to obtain the EVOH resin composition.

[0062] Examples 2-10

[0063] The EVOH resin composition was prepared according to the method of Example 1, except that the types and amounts of monocarboxylic acid, carboxylic acid containing unsaturated double bonds, phosphorus source, boron source and potassium source in steps E and F are shown in Table 1. The properties of the obtained EVOH resin composition are shown in Table 2.

[0064] Comparative Examples 1-3

[0065] The EVOH resin composition was prepared according to the method of Example 1, except that the types and amounts of monocarboxylic acid, carboxylic acid containing unsaturated carbon-carbon double bonds, and potassium source in steps E and F are shown in Table 1. The properties of the obtained EVOH resin composition are shown in Table 2.

[0066] Table 1

[0067]

[0068]

[0069] Table 2

[0070]

[0071]

[0072] As can be seen from the data in Table 2 above, the heat resistance and yellowing resistance of the EVOH resin composition can be improved by adding carboxylic acids containing carbon-carbon double bonds. The heat resistance and heat resistance and yellowing resistance of the EVOH resin composition can also be improved by simultaneously controlling the potassium salt and sodium salt within a certain range.

[0073] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. An ethylene-vinyl alcohol copolymer composition, comprising an ethylene-vinyl alcohol copolymer, an alkali metal element, and a carboxyl-containing substance, characterized in that, The carboxyl-containing substance contains a monobasic saturated carboxylic acid and a carboxylic acid containing a carbon-carbon double bond. The alkali metal element is potassium and sodium. The total amount of potassium and sodium relative to the mass of the ethylene-vinyl alcohol copolymer is 100-3000 ppm. The weight ratio of the monobasic saturated carboxylic acid to the carboxylic acid containing a carbon-carbon double bond is 1:0.1-1, and the mass ratio of sodium to potassium is 0.1:1-1:

1. The composition also contains boron and phosphorus. Relative to the mass of the ethylene-vinyl alcohol copolymer, the boron content is 50-1000 ppm (calculated as boric acid H3BO3), and the phosphorus content is 200-1000 ppm (calculated as H3PO4).

2. The composition according to claim 1, wherein, The weight ratio of the monosaturated carboxylic acid to the carboxylic acid containing carbon-carbon double bonds is 1:0.4-0.

7.

3. The composition according to claim 1 or 2, wherein, The content of the monobasic saturated carboxylic acid is 50-2000 ppm relative to the mass of the ethylene-vinyl alcohol copolymer, and the content of the carboxylic acid containing carbon-carbon double bonds is 20-2000 ppm relative to the mass of the ethylene-vinyl alcohol copolymer.

4. The composition according to claim 3, wherein, The content of the monobasic saturated carboxylic acid is 200-800 ppm relative to the mass of the ethylene-vinyl alcohol copolymer, and the content of the carboxylic acid containing carbon-carbon double bonds is 150-250 ppm relative to the mass of the ethylene-vinyl alcohol copolymer.

5. The composition according to claim 1 or 2, wherein, The carboxylic acid containing a carbon-carbon double bond contains one or two carboxyl groups.

6. The composition according to claim 1 or 2, wherein, Carboxylic acids containing carbon-carbon double bonds are one or more of the following: sorbic acid, 2-hexenoic acid, 3-hexenoic acid, butenoic acid, vinylacetic acid, vinylpropionic acid, vinylglycolic acid, and cinnamic acid.

7. The composition according to claim 1 or 2, wherein, The alkali metal elements are potassium and sodium, and the total amount of potassium and sodium is 800-1200 ppm relative to the mass of the ethylene-vinyl alcohol copolymer.

8. The composition according to claim 7, wherein, The mass ratio of sodium to potassium is 0.2:1 to 0.5:

1.

9. The composition according to claim 1 or 2, wherein, Relative to the mass of the ethylene-vinyl alcohol copolymer, the boron content, calculated as boric acid (H3BO3), is 300-500 ppm, and the phosphorus content, calculated as H3PO4, is 500-800 ppm.

10. A method for preparing an ethylene-vinyl alcohol copolymer composition, the method comprising alcoholystomizing an ethylene-vinyl acetate copolymer obtained by polymerization with an alkali metal hydroxide, followed by granulation, and then sequentially washing with water and acid, characterized in that, The acid used for pickling includes monobasic saturated carboxylic acids and carboxylic acids containing carbon-carbon double bonds. The alkali metal hydroxide used for alcoholysis is sodium hydroxide. The method further includes adding potassium salt to the pickled ethylene-vinyl acetate copolymer. The amount of potassium salt added is such that, relative to the mass of the ethylene-vinyl alcohol copolymer, the total amount of potassium and sodium is 100-3000 ppm, and the mass ratio of sodium to potassium is 0.1:1-1:

1. The weight ratio of the monobasic saturated carboxylic acid to the carboxylic acid containing carbon-carbon double bonds is 1:0.1-1. The method further includes adding phosphorus-containing compounds and boron-containing compounds to the pickled ethylene-vinyl acetate copolymer. The amount of phosphorus-containing compounds and boron-containing compounds added is such that, relative to the mass of the ethylene-vinyl alcohol copolymer, the boron content is 50-1000 ppm (based on boric acid H3BO3), and the phosphorus content is 500-800 ppm (based on H3PO4).

11. The preparation method according to claim 10, wherein, The weight ratio of the monobasic saturated carboxylic acid to the carboxylic acid containing carbon-carbon double bonds is 1:0.2-0.

8.

12. The preparation method according to claim 10 or 11, wherein, The amount of the monobasic saturated carboxylic acid used relative to 1 part by weight of ethylene-vinyl alcohol copolymer is 0.001-0.05 parts by weight, and the amount of the carboxylic acid containing carbon-carbon double bonds is 0.001-0.01 parts by weight.

13. The preparation method according to claim 10 or 11, wherein, The carboxylic acid containing a carbon-carbon double bond contains one or two carboxyl groups.

14. The preparation method according to claim 10 or 11, wherein, Carboxylic acids containing carbon-carbon double bonds are one or more of the following: sorbic acid, 2-hexenoic acid, 3-hexenoic acid, butenoic acid, vinylacetic acid, vinylpropionic acid, vinylglycolic acid, and cinnamic acid.

15. The preparation method according to claim 10 or 11, wherein, The amount of potassium salt added results in a total potassium and sodium content of 800-1200 ppm relative to the mass of the ethylene-vinyl alcohol copolymer.

16. The preparation method according to claim 15, wherein, The amount of potassium salt added makes the mass ratio of sodium to potassium 0.2:1 to 0.5:

1.

17. The preparation method according to claim 10 or 11, wherein, The addition of phosphorus-containing and boron-containing compounds results in a boron content of 300-500 ppm (based on boric acid H3BO3) and a phosphorus content of 300-500 ppm (based on H3PO4) relative to the mass of the ethylene-vinyl alcohol copolymer.

Images