High-barrier moisture-absorption-resistant ethylene-vinyl alcohol copolymer and preparation method thereof

Abstract

The invention provides a high-barrier moisture-absorption-resistant ethylene-vinyl alcohol copolymer which is composed of the following repetitive units: an ethylene unit, a vinyl alcohol unit and an enediol unit with a general formula structure as shown in a formula I. In the formula I, R is selected from C1-C4 alkylene; the molar ratio of the enediol unit to the vinyl alcohol unit is 1: 1-70, and the molar fraction of the ethylene unit is 20-40 mol% with respect to all repeating units constituting the copolymer. The preparation method comprises the following steps: carrying out a free radical polymerization reaction on a pre-polymerized monomer, vinyl acetate and ethylene under the action of an initiator to obtain a modified EVA elastomer, and then carrying out a saponification reaction. The high-barrier moisture-absorption-resistant ethylene-vinyl alcohol copolymer has excellent barrier property, moisture absorption resistance and mechanical property, halogen is not introduced into the structure, and the high-barrier moisture-absorption-resistant ethylene-vinyl alcohol copolymer is more friendly to human bodies and environment and can be safely used for food and medicine packaging. # imgabs0 #

Description

Technical Field

[0001] The invention relates to the technical field of ethylene-vinyl alcohol copolymer preparation, and in particular to a high-barrier, moisture-absorption-resistant ethylene-vinyl alcohol copolymer and a preparation method thereof. Background Art

[0002] Ethylene-Vinyl Alcohol Copolymer (EVOH) is a crystalline polymer with a chain molecular structure, which is composed of 20-50% ethylene and 50-80% vinyl alcohol by mole fraction. EVOH has the barrier properties of polyvinyl alcohol (PVA) and the processing properties of polyethylene (PE), and is known as the three major barrier resins together with polyvinylidene chloride (PVDC) and polyamide (PA). EVOH has many advantages such as gas barrier properties, oil resistance, chemical corrosion resistance, weather resistance, processability, excellent transparency, non-toxicity and tastelessness, and complies with the European Community Food Packaging Guidelines and the requirements of the US Food and Drug Administration. It is widely used in packaging materials, structural materials, textile materials, medical materials and other fields.

[0003] Since there are a large number of hydroxyl groups in the EVOH structure, hydrogen bonds are formed between the hydroxyl groups, which enhances the intermolecular forces, makes the molecular chains stacked tightly, and improves the crystallinity, thus having an excellent barrier effect. However, as a hydrophilic group, the hydroxyl group itself will cause it to be easy to absorb moisture and reduce its own barrier properties. The existing technical means for modifying EVOH mainly include the following: blending and compounding, multi-layer composite, and surface modification. These methods can increase the barrier properties of the material to water vapor to a certain extent, but will cause the EVOH film to become thicker and cause other properties to decrease. The best way is to adjust the EVOH structure itself. For example, Chinese patent CN117945844A modifies the structure by designing a third monomer, increasing hydrogen bonds in the structure, reducing the generation of side chains, and improving the utilization rate of VAc at the same time, but the problem is that there are halogens in its structure, which may be toxic to the human body when used for food and / or medical packaging.

[0004] How to design and synthesize an ethylene-vinyl alcohol copolymer that has both high barrier properties and low hygroscopicity, as well as excellent mechanical properties, still requires further research. Summary of the invention

[0005] The present invention aims to provide a high barrier and moisture absorption resistant ethylene-vinyl alcohol copolymer and a method for preparing the ethylene-vinyl alcohol copolymer, so as to solve the above-mentioned defects existing in the background technology.

[0006] The present invention is achieved by adopting the following technical solutions:

[0007] The first aspect of the present invention relates to a high barrier and moisture absorption resistant ethylene-vinyl alcohol copolymer, which is composed of the following repeating units: an ethylene unit, a vinyl alcohol unit and an ethylene glycol unit having a general structure shown in Formula I, wherein R is selected from C 1 -C 4 Alkylene; the molar ratio of the ethylene glycol unit to the vinyl alcohol unit is 1:1-70, and the molar fraction of the ethylene unit relative to the total repeating units constituting the copolymer is 20-40 mol%.

[0008]

[0009] The C 1 -C 4 The alkylene group includes a linear or branched alkylene group, such as but not limited to methylene (-CH 2 -), 1,2-ethylene (-CH 2 CH 2 -), ethylene ( ), 1,3-propylene (-CH 2 CH 2 CH 2 -), 1,2-propylene (-CHCH 3 CH 2 -), isopropylidene ( )、1,4-butylene(-CH 2 CH 2 CH 2 CH 2 -) is preferably a methylene group, a 1,2-ethylene group, a 1,2-propylene group or a 1,3-propylene group.

[0010] Preferably, the molar ratio of the enediol unit to the vinyl alcohol unit is 1:5-20.

[0011] The inventors have found that the introduction of a repeating unit with a certain carbon chain length and containing dihydroxyl groups at the end into the structure of ethylene-vinyl alcohol copolymer unexpectedly improves the barrier properties of the product while increasing moisture resistance. The specific mechanism is not very clear yet, and it may be due to the following reasons: on the one hand, due to the introduction of repeating units containing dihydroxyl groups, the present invention contains more hydroxyl groups than the conventional EVOH structure, thereby significantly improving the barrier properties of the product; on the other hand, since the introduced repeating units contain long carbon chains with hydrophobicity, the hygroscopicity problem caused by the increase in the number of hydroxyl groups can be overcome; in addition, the introduced units do not contain halogens and are more friendly to the human body and the environment.

[0012] The inventors further discovered that if the introduced repeating unit contains only one hydroxyl group, or its carbon chain exceeds a certain length, the improvement in the barrier properties and moisture resistance of the product is very limited, further illustrating the unpredictability of the above-mentioned improvements made to the ethylene-vinyl alcohol copolymer structure of the present invention.

[0013] The second aspect of the present invention relates to a method for preparing the above-mentioned high barrier and moisture absorption resistant ethylene-vinyl alcohol copolymer, comprising subjecting a prepolymer monomer of the general structure shown in Formula II, vinyl acetate (VAc) and ethylene to a free radical polymerization reaction under the action of an initiator to obtain a modified EVA elastomer (i.e., a modified ethylene-vinyl acetate copolymer), and then subjecting the modified EVA elastomer to a saponification reaction to obtain the high barrier and moisture absorption resistant ethylene-vinyl alcohol copolymer; wherein R is selected from C 1 -C 4 Alkylene.

[0014]

[0015] As a preferred technical solution, the prepolymer monomer is selected from 4-pentene-1,1-dimethyl diacetate, 5-hexene-1,1-dimethyl diacetate, 4-methyl-5-hexene-1,1-dimethyl diacetate or 6-heptene-1,1-dimethyl diacetate.

[0016] Furthermore, the prepolymer monomer is obtained by dissolving an aldehyde substance and acetic anhydride in an organic solvent and reacting them under the action of a Lewis acid catalyst. The specific reaction formula is as follows:

[0017]

[0018] Wherein, the alkenal substance is C 5 -C 8 The aldehydes include 4-pentenal, 5-hexenal, hept-6-enal, 7-octenal, 8-nonenal, 4-methyl-4-pentenal, 4-methyl-5-hexenal, 2-ethyl-4-pentenal, 2-methyl-5-hexenal, 3-methyl-5-hexenal, and 4-methyl-4-hexenal; preferably 4-pentenal, 5-hexenal, 4-methyl-5-hexenal, and hept-6-enal.

[0019] The olefinic aldehydes and acetic anhydride can be reacted in a stoichiometric ratio. In order to increase the yield, the acetic anhydride can be appropriately excessive, for example, 1-2 times the stoichiometric ratio, that is, the molar ratio of acetic anhydride to olefinic aldehydes is 1:1-2.

[0020] The Lewis acid catalyst can be a Lewis acid commonly used in the art, such as but not limited to indium trichloride, aluminum chloride, gallium chloride, preferably indium trichloride. The amount of catalyst added is preferably to ensure smooth reaction, for example, a suitable amount of catalyst added is 1-8% of the molar amount of acetic anhydride.

[0021] As a preferred technical solution, in terms of molar ratio, the usage ratio of acetic anhydride, olefinic aldehyde and Lewis acid catalyst is (30-40):(20-30):(1-2), and the usage ratio of the three is more preferably (30-35):(20-25):(1-1.5).

[0022] The organic solvent is an organic solvent that can dissolve olefinic aldehydes, acetic anhydride, and Lewis acid catalysts, such as but not limited to dichloromethane, chloroform, and ether.

[0023] The reaction temperature is 0-50°C, preferably room temperature; the reaction time is 0.1-2h, preferably 0.1-1h.

[0024] As a further preferred technical solution, after the reaction of the olefinic aldehyde substance and acetic anhydride is completed, a post-treatment step is further performed to obtain the prepolymer monomer, specifically comprising: treating the reaction mixture with an aqueous sodium acetate solution, washing the organic layer with brine for multiple times, removing the aqueous phase through a separatory funnel, collecting the organic phase, adding anhydrous sodium sulfate to dry, and removing the solvent by reduced pressure distillation to obtain a liquid product, and then separating and purifying by thin layer chromatography to obtain the prepolymer monomer.

[0025] As a preferred technical solution, the method for preparing the high barrier and moisture absorption resistant ethylene-vinyl alcohol copolymer specifically comprises the following steps:

[0026] S1. In a reactor, add the prepolymer monomer, vinyl acetate, initiator and an appropriate amount of solvent and mix them evenly, introduce ethylene gas, maintain the pressure in the reactor at 1-5PMa, and carry out free radical polymerization at a reaction temperature of 55-90°C to obtain a modified EVA elastomer after sufficient reaction; the molar fractions of the prepolymer monomer, vinyl acetate and initiator are: 10-100 parts of the prepolymer monomer, 100-700 parts of vinyl acetate, and 1-15 parts of the initiator;

[0027] S2, mixing the modified EVA elastomer, a strong base and an alcohol solvent, performing alcoholysis at 50-70° C. for 1-4 hours, and obtaining the high-barrier and moisture-resistant ethylene-vinyl alcohol copolymer through post-treatment.

[0028] The initiator may be an initiator commonly used in free radical polymerization, including peroxides, bifunctional peroxides, azo compound initiators, etc., such as but not limited to dibenzoyl peroxide, diisopropylbenzene peroxide, di-tert-butyl peroxide, tert-butyl peroxide isopropylbenzene, tert-butyl peroxy-2-ethylhexanoate, azobisisobutyronitrile, 2,2'-azobisisobutyronitrile, etc. Azobisisobutyronitrile (AIBN) is preferably used.

[0029] The reaction temperature of step S1 is preferably 60-80° C., and the reaction time is 1-6 h.

[0030] The strong base can be an alkali substance commonly used in saponification reaction, such as but not limited to NaOH, KOH, sodium methoxide, sodium ethoxide; the alcohol solvent can be an alcohol substance commonly used in the art, such as but not limited to methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol.

[0031] Preferably, the mass ratio of the modified EVA elastomer to the strong base is 3-4:1.

[0032] The post-treatment step in step S2 includes: adjusting the pH value to neutral, transferring the reactants into water and washing them repeatedly, then washing them repeatedly with anhydrous ethanol, and finally drying to obtain the high barrier and moisture absorption resistant ethylene-vinyl alcohol copolymer.

[0033] Specifically, in the post-treatment, an organic acid alcohol solution is generally used to adjust the pH value, for example, a 1 mol / L acetic acid methanol solution is used to adjust the pH value to about 7.0. The purpose of washing the reactants with water is to remove water-soluble impurities, the purpose of washing with alcohol is to remove excess water, and drying is to further remove water from the product.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] The present invention introduces a prepolymer monomer with a long carbon chain structure having two hydroxyl groups, thereby introducing a greater number of hydroxyl groups, so that the EVOH structure contains more hydroxyl groups, thereby improving the barrier properties of the product. At the same time, due to the hydrophobic long carbon chain, the hygroscopicity problem caused by the hydrophilic hydroxyl groups can be alleviated, thereby ensuring its barrier to polar solvents and increasing moisture resistance. In addition, the high barrier and moisture-resistant ethylene-vinyl alcohol copolymer of the present invention does not introduce halogens into its structure, is more friendly to the human body and the environment, and can be safely used for food and medicine packaging. DETAILED DESCRIPTION

[0036] The following description is used to disclose the present invention so that those skilled in the art can implement the present invention. The preferred embodiments described below are only used as examples, but are not used to limit the scope of the present invention. Those skilled in the art can think of other obvious variations. The basic principles of the present invention defined in the following description can be applied to other embodiments, variations, improvements, equivalent solutions, and other technical solutions that do not deviate from the spirit and scope of the present invention.

[0037] The instruments or reagents in the embodiments of the present invention without indicating the manufacturer are all conventional commercial instruments or reagents.

[0038] Example 1

[0039] 1) Prepare prepolymer monomer 4-pentene-1,1-dimethyl diacetate, the structural formula of which is as shown in Formula IIA. The specific preparation process is as follows:

[0040]

[0041] In a clean and dry three-necked flask, add acetic anhydride, 4-pentenal, and indium trichloride to the flask at room temperature, add dichloromethane as a solvent, the molar ratio of acetic anhydride, 4-pentenal, and indium trichloride is 30:25:1, and the amount of solvent added is 10 times the total weight of the raw materials. Stir the reaction at room temperature for about 25 minutes. End the reaction, treat the reaction mixture with 25% sodium acetate aqueous solution, wash the organic layer with brine 4 times, remove the aqueous phase through a separatory funnel, collect the organic phase, and remove the solvent by reduced pressure distillation to obtain a liquid product, and then separate and purify by thin layer chromatography to obtain 4-pentene-1,1-dimethyl diacetate, which is stored below 10°C and kept in a dry environment for later use.

[0042] 2) Preparation of modified EVA elastomer: high purity N 2 Under protection, AIBN, VAc, 4-pentene-1,1-dimethyl diacetate and anhydrous methanol were mixed in a mixing container at a molar ratio of 1:600:60:150, and the clarified solution was transferred into a reactor with a polytetrafluoroethylene liner, and 3MPa of ethylene was charged. After the pressure was stabilized, the circulation pump was turned on for heating, and the polymerization temperature was controlled at about 80°C. After 2 hours of polymerization, the material was taken out for distillation while hot, and vacuum dried at 60°C for 24 hours to remove volatiles to obtain a transparent modified EVA elastomer;

[0043] 3) Preparation of EVOH by saponification of modified EVA elastomer: Add modified EVA elastomer, NaOH and methanol in a ratio of 5g:1.5g:5mL into a three-necked flask, stir and mix, and after alcoholysis at 65°C for 2h, add 1mol / L methanol solution of acetic acid to the mixed solution to neutralize the reactants until pH = 7, and finally transfer the reactants to cold water, wash them repeatedly with water three times to remove water-soluble impurities in the solution, and then wash them repeatedly with anhydrous ethanol three times to remove excess water, and dry them at 100°C overnight to obtain the final product, high barrier and moisture-resistant ethylene-vinyl alcohol copolymer (hereinafter referred to as EVOH).

[0044] Example 2

[0045] 1) Prepare prepolymer monomer 5-hexene-1,1-dimethyl diacetate, the structural formula of which is as shown in Formula IIB. The specific preparation process is as follows:

[0046]

[0047] In a clean and dry three-necked flask, acetic anhydride, 5-hexenal, and indium chloride are added to the flask at room temperature, and dichloromethane is added as a solvent. The molar ratio of acetic anhydride, 5-hexenal, and indium chloride is 30:25:1, and the amount of solvent added is 10 times the total weight of the raw materials. Stir the reaction at room temperature for about 30 minutes. After the reaction is terminated, the reaction mixture is treated with a 25% sodium acetate aqueous solution, the organic layer is washed with brine 4 times, the aqueous phase is removed by a separatory funnel, the organic phase is collected, and the solvent is removed by reduced pressure distillation to obtain a liquid product, and then 5-hexene-1,1-dimethyl diacetate is obtained after thin layer chromatography separation and purification, which is stored below 10°C and kept in a dry environment for later use.

[0048] 2) Preparation of modified EVA elastomer: high purity N 2 Under protection, AIBN, VAc, 5-hexene-1,1-dimethyl diacetate and anhydrous methanol were mixed in a mixing container at a molar ratio of 1:600:60:150, and the clarified solution was transferred into a reactor with a polytetrafluoroethylene liner, and filled with 3 MPa of ethylene. After the pressure stabilized, the circulating pump was turned on for heating, the polymerization temperature was 60°C, and after the polymerization for 2 hours, the material was taken out for distillation while hot, and vacuum dried at 60°C for 24 hours to remove volatiles to obtain a transparent modified EVA elastomer.

[0049] Preparation of EVOH by saponification of modified EVA elastomer: Add modified EVA elastomer, NaOH and methanol in a ratio of 5g:1.5g:5mL into a three-necked flask, stir and mix, and after alcoholysis at 65°C for 2h, add 1mol / L acetic acid methanol solution to the mixed solution to neutralize the reactants until pH = 7. Finally, transfer the reactants to cold water, wash them repeatedly with water three times to remove water-soluble impurities in the solution, and then wash them repeatedly with anhydrous ethanol three times to remove excess water. Dry at 100°C overnight to obtain the final product EVOH.

[0050] Example 3

[0051] 1) Prepare prepolymer monomer 4-methyl-5-hexene-1,1-dimethyl diacetate, the structural formula of which is as shown in Formula IIC. The specific preparation process is as follows:

[0052]

[0053] In a clean and dry three-necked flask, acetic anhydride, 4-methyl-5-hexenal, and indium trichloride are added to the flask at room temperature, and dichloromethane is added as a solvent. The molar ratio of acetic anhydride, 4-methyl-5-hexenal, and indium trichloride is 25:32.5:1, and the amount of solvent added is 12 times the total weight of the raw materials. Stir the reaction at room temperature for about 15 minutes. End the reaction, treat the reaction mixture with 25% sodium acetate aqueous solution, wash the organic layer with brine 4 times, remove the aqueous phase through a separatory funnel, collect the organic phase, and remove the solvent by vacuum distillation to obtain a liquid product, and then separate and purify by thin layer chromatography to obtain 4-methyl-5-hexene-1,1-dimethyl diacetate, which is stored below 10°C and kept in a dry environment for later use.

[0054] 2) Preparation of modified EVA elastomer: high purity N 2 Under protection, AIBN, VAc, 4-methyl-5-hexene-1,1-dimethyldiacetate and anhydrous methanol were mixed in a molar ratio of 1:600:60:150 in a mixing container, and the clarified solution was transferred into a reactor with a polytetrafluoroethylene liner, and filled with 3MPa of ethylene. After the pressure stabilized, the circulating pump was turned on for heating, the polymerization temperature was 60°C, and after the polymerization for 2h, the material was taken out while hot, and vacuum dried at 60°C for 24h to remove volatiles to obtain a transparent modified EVA elastomer.

[0055] 3) Preparation of EVOH by saponification of modified EVA elastomer: Add modified EVA elastomer, NaOH and methanol in a three-necked flask at a ratio of 5g:1.5g:5mL, stir and mix, and after alcoholysis at 65°C for 2h, add 1mol / L methanol solution of acetic acid to the mixed solution to neutralize the reactants until pH = 7. Finally, transfer the reactants to cold water, wash them repeatedly with water three times to remove water-soluble impurities in the solution, and then wash them repeatedly with anhydrous ethanol three times to remove excess water. Dry at 100°C overnight to obtain the final product EVOH.

[0056] Example 4

[0057] 1) Prepare prepolymer monomer 6-heptene-1,1-dimethyl diacetate, the structural formula of which is as shown in Formula IID. The specific preparation process is as follows:

[0058]

[0059] In a clean and dry three-necked flask, acetic anhydride, 6-heptenal, and indium trichloride are added to the flask at room temperature, and dichloromethane is added as a solvent. The molar ratio of acetic anhydride, 6-heptenal, and indium trichloride is 25:32.5:1, and the amount of solvent added is 10 times the total weight of the raw materials. Stir the reaction at room temperature for about 18 minutes. End the reaction, treat the reaction mixture with a 25% sodium acetate aqueous solution, wash the organic layer with brine 4 times, remove the aqueous phase through a separatory funnel, collect the organic phase, and remove the solvent by vacuum distillation to obtain a liquid product, and then separate and purify by thin layer chromatography to obtain 6-heptene-1,1-dimethyl diacetate, which is stored below 10°C and kept in a dry environment for later use.

[0060] 2) Preparation of modified EVA elastomer: high purity N 2 Under protection, AIBN, VAc, 6-heptene-1,1-dimethyl diacetate and anhydrous methanol were mixed in a molar ratio of 1:600:60:150 in a mixing container, and the clarified solution was transferred into a reactor with a polytetrafluoroethylene liner, and filled with 3MPa of ethylene. After the pressure stabilized, the circulating pump was turned on for heating, the polymerization temperature was 60°C, and after the polymerization for 2h, the material was taken out for distillation while hot, and vacuum dried at 60°C for 24h to remove volatiles to obtain a transparent modified EVA elastomer.

[0061] 3) Preparation of EVOH by saponification of modified EVA elastomer: Add modified EVA elastomer, NaOH and methanol in a ratio of 5g:1.5g:5mL into a three-necked flask, stir and mix, and after alcoholysis at 65°C for 2h, add 1mol / L methanol solution of acetic acid to the mixed solution to neutralize the reactants until pH = 7. Finally, transfer the reactants to cold water, wash them three times with water to remove water-soluble impurities in the solution, and then wash them three times with anhydrous ethanol to remove excess water. Dry at 100°C overnight to obtain the final product EVOH.

[0062] Comparative Example 1 (without prepolymer monomer)

[0063] Compared with Example 1, the step of preparing the prepolymer monomer is omitted, and no prepolymer monomer is added when preparing the EVA elastomer. AIBN, VAc, and anhydrous methanol are mixed in a mixing container at a molar ratio of 1:600:150, and the remaining steps are the same as in Example 1.

[0064] Comparative Example 2 (monomer containing only one ester group)

[0065] Compared with Example 1, the prepolymer monomer pent-4-ene-1,1-dimethyl diacetate is replaced by 4-pentene-1-acetate (CAS No. 1576-85-8) containing only one ester group, and the remaining steps are the same as Example 1.

[0066] Comparative Example 3 (increasing the proportion of alkali during saponification)

[0067] Compared with Example 1, the amount of NaOH is increased when the modified EVA elastomer is saponified to prepare EVOH. The modified EVA elastomer, NaOH and methanol are added to a three-necked flask at a ratio of 5g:3g:5mL and stirred to mix. The remaining steps are the same as in Example 1.

[0068] Comparative Example 4 (Long carbon chain monoester monomer)

[0069] Compared with Example 1, the prepolymer monomer pent-4-ene-1,1-dimethyl diacetate is replaced by 9-decene-1,1-dimethyl diacetate with a longer carbon chain, and the remaining steps are the same as those in Example 1.

[0070] The structural formula of 9-decene-1,1-dimethyl diacetate is as shown in Formula D:

[0071]

[0072] The preparation method is as follows: in a clean and dry three-necked flask, acetic anhydride, 9-decenal, and indium trichloride are added to the flask at room temperature, dichloromethane is added as a solvent, the molar ratio of acetic anhydride, 9-decenal, and indium trichloride is 30:25:1, the amount of solvent added is 10 times the total weight of the raw materials, and the reaction is stirred at room temperature for about 30 minutes. After the reaction is terminated, the reaction mixture is treated with a 25% sodium acetate aqueous solution, the organic layer is washed with brine 4 times, the aqueous phase is removed by a separatory funnel, the organic phase is collected, anhydrous sodium sulfate is added to dry, and the solvent is removed by reduced pressure distillation to obtain a liquid product, and then 9-decene-1,1-dimethyl diacetate is obtained after thin layer chromatography separation and purification, and stored below 10°C and kept in a dry environment for later use.

[0073] The performance tests of the EVOH obtained in each embodiment and comparative example were carried out, specifically including:

[0074] (1) Test the ethylene mole fraction of EVOH according to GBT41877.2-2022;

[0075] (2) Test EVOH O according to GBT41877.2-2022 2 Transmittance (GTR);

[0076] (3) Test the impact strength and tensile strength of EVOH according to GBT 1043.2-2018;

[0077] (4) Moisture resistance: Place the test sample in a constant temperature and humidity chamber at 60°C and 90% humidity. Take it out after 12 hours and place it at room temperature for 2 hours. Test the moisture resistance of the sample according to GBT41877.2-2022. 2 Transmittance. Specific test results are shown in Table 1.

[0078] Table 1 Performance test results

[0079]

[0080] As shown in Table 1, in Example 1-4, adding a prepolymer monomer with a specific structure during the preparation process of EVA helps to reduce the molar fraction of ethylene in the product and increase the proportion of vinyl alcohol. The obtained EVOH product has excellent O 2 The barrier rate is high, and it also has excellent impact strength and tensile strength. In addition, due to the longer carbon chain structure, the hydrophobicity of the product is improved and the moisture resistance is increased. Compared with comparative example 1, after moisture absorption, 2 The transmittance did not increase significantly, but the moisture absorption resistance was significantly improved. Comparative Example 2 used a monomer containing an ester group, and the product improvement was not obvious; Comparative Example 3 showed that too much alkali may damage the structure of the product, causing the barrier properties and mechanical properties to decrease; Comparative Example 4 showed that the monomer with too long carbon chain structure would affect the insertion of the reaction, making it difficult to participate in the reaction, and the product improvement was not obvious.

[0081] It should be understood by those skilled in the art that the above embodiments are only examples and do not limit the present invention. The purpose of the present invention has been fully and effectively achieved. The functions and principles of the present invention have been demonstrated and explained in the embodiments, and the embodiments of the present invention may be modified or altered in any way without departing from the principles.

Claims

1. A high barrier and moisture absorption resistant ethylene-vinyl alcohol copolymer, characterized in that: It is composed of the following repeating units: ethylene units, vinyl alcohol units and ethylene glycol units having the general structure shown in Formula I; Wherein R is selected from C1-C4 alkylene; the molar ratio of the ethylene glycol unit to the vinyl alcohol unit is 1:1-70, and the molar fraction of the ethylene unit is 20-40 mol% relative to all repeating units constituting the copolymer.

2. The high barrier and moisture absorption resistant ethylene-vinyl alcohol copolymer according to claim 1, characterized in that: The C1-C4 alkylene group is a methylene group, a 1,2-ethylene group, a 1,2-propylene group or a 1,3-propylene group.

3. The high barrier and moisture absorption resistant ethylene-vinyl alcohol copolymer according to claim 1, characterized in that: The molar ratio of the enediol unit to the vinyl alcohol unit is 1:5-20.

4. A method for preparing a high barrier and moisture absorption resistant ethylene-vinyl alcohol copolymer, characterized in that: The method comprises subjecting a prepolymer monomer of the general structure shown in formula II, vinyl acetate and ethylene to a free radical polymerization reaction under the action of an initiator to obtain a modified EVA elastomer, and then subjecting the modified EVA elastomer to a saponification reaction to obtain the high barrier and moisture absorption resistant ethylene-vinyl alcohol copolymer; wherein R is selected from C1-C4 alkylene.

5. The method according to claim 4, characterized in that The prepolymer monomer is selected from 4-pentene-1,1-dimethyl diacetate, 5-hexene-1,1-dimethyl diacetate, 4-methyl-5-hexene-1,1-dimethyl diacetate or 6-heptene-1,1-dimethyl diacetate.

6. The method according to claim 4, characterized in that The prepolymer monomer is obtained by dissolving an aldehyde substance and acetic anhydride in an organic solvent and reacting them under the action of a Lewis acid catalyst. The aldehyde substance is a C5-C8 aldehyde.

7. The method according to claim 6, characterized in that The olefinic aldehyde substance is selected from 4-pentenal, 5-hexenal, 4-methyl-5-hexenal or hept-6-enal.

8. The method according to claim 6, characterized in that The molar ratio of acetic anhydride to olefinic aldehydes is 1:1-2.

9. The method according to claim 4, characterized in that The specific steps include: S1. In a reactor, add the prepolymer monomer, vinyl acetate, initiator and an appropriate amount of solvent and mix them evenly, introduce ethylene gas, maintain the pressure in the reactor at 1-5PMa, and carry out free radical polymerization at a reaction temperature of 55-90°C to obtain a modified EVA elastomer after sufficient reaction; the molar fractions of the prepolymer monomer, vinyl acetate and initiator are: 10-100 parts of the prepolymer monomer, 100-700 parts of vinyl acetate, and 1-15 parts of the initiator; S2, mixing the modified EVA elastomer, a strong base and an alcohol solvent, performing alcoholysis at 50-70° C. for 1-4 hours, and obtaining the high-barrier and moisture-resistant ethylene-vinyl alcohol copolymer through post-treatment.

10. The method according to claim 9, characterized in that The mass ratio of the modified EVA elastomer to the strong base is 3-4:1.

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