Ethylene-vinyl alcohol copolymer and preparation method thereof

Abstract

The invention provides an ethylene-vinyl alcohol copolymer and a preparation method thereof. The ethylene-vinyl alcohol copolymer is obtained by saponifying a copolymer of ethylene and a vinyl ester, and is characterized in that the molecular weight measured using a gel permeation chromatograph having a differential refractive index detector and an ultraviolet-visible absorbance detector satisfies the following conditions: Formula (1): SUV360 / SRIDlt; 5, formula (2): SUV220 / SRIDlt; 5. By controlling the unsaturated bond content and carbonyl / aldehyde group content in the main chain, the oxidation, crosslinking and dehydration reaction of the resin in a high-temperature molten state can be avoided, and the problems of crystal point, fisheye, yellowing, barrier property reduction and the like in the processing process are avoided.

Description

Technical Field

[0001] The present invention relates to the field of ethylene-vinyl alcohol copolymers, and in particular to an ethylene-vinyl alcohol copolymer with excellent processing performance and high barrier properties. Background Art

[0002] Ethylene-vinyl alcohol copolymer (EVOH) is a polymer material with excellent gas barrier properties, transparency, oil resistance, non-static properties, and mechanical strength. It is widely used in various packaging materials such as films, sheets, and containers. EVOH is typically produced by saponifying a copolymer of ethylene and vinyl ester. However, due to the presence of active hydroxyl groups within the EVOH molecule, it is susceptible to oxidation and cross-linking reactions in the high-temperature molten state, producing thermal degradation products. Especially during long-term continuous operation, these thermal degradation products accumulate inside the molding machine, easily forming gel-like particles, resulting in defects such as fisheyes in the molded product, affecting long-term operability. Furthermore, EVOH is a highly crystalline polymer, and during melt processing, the hydroxyl groups are prone to dehydration reactions, which deteriorates the polymer quality.

[0003] In the preparation process of ethylene-vinyl alcohol copolymer, the following method can improve the thermal stability of the polymer to a certain extent, but it cannot completely avoid the thermal degradation and yellowing problems of the copolymer. The thermal stability and processing performance of the copolymer still need to be further improved.

[0004] 1) Remove aldehyde impurities and polymerization inhibitors from vinyl acetate before polymerization.

[0005] 2) Continuously remove the aldehydes produced during the polymerization process.

[0006] 3) Adding organic acid during the polymerization process to inhibit the transesterification reaction. Examples of organic acids include

[0007] Acetic acid, citric acid, lactic acid, tartaric acid, malonic acid, maleic acid, oxalic acid, etc.

[0008] 4) After the polymerization is completed, the residual vinyl acetate monomer and residual initiator are completely removed.

[0009] 5) After saponification, wash thoroughly to remove impurities.

[0010] While the addition of additives can improve EVOH's processing stability, their use often leads to issues such as polymer coloration and a decrease in appearance. Therefore, improving EVOH's melt processing stability, long-term performance, and appearance while maintaining its excellent gas barrier properties remains a pressing technical challenge.

[0011] Patent CN109661428B discloses an ethylene-vinyl alcohol resin composition that improves yellowing and manufacturing stability by adding hindered amine compounds and hindered phenol compounds with ester or amide bonds. Patent CN118946627A adds a specific trace amount of titanium compound to EVOH resin, combined with alkaline earth metal compounds, alkali metal compounds, lubricants, antioxidants, and other agents, to control the composition's moisture content and component ratios to suppress color changes during melt molding. However, adding additives to improve resin yellowing and processing performance often leads to problems such as reduced appearance and limited application. Summary of the Invention

[0012] To address the above technical issues, the present invention provides an ethylene-vinyl alcohol copolymer and a method for preparing the same. By controlling the unsaturated bond content and carbonyl / aldehyde group content in the main chain, oxidation, crosslinking, and dehydration reactions can be avoided in the high-temperature molten state, thereby preventing problems such as crystallization, fisheyes, yellowing, and decreased barrier properties during processing.

[0013] The present invention provides an ethylene-vinyl alcohol copolymer. The molecular weight measured by a gel permeation chromatograph equipped with a differential refractive index detector and an ultraviolet-visible absorbance detector satisfies the conditions represented by the following formulas (1) and (2):

[0014] Formula (1): S UV360 / S RID <5,

[0015] Formula (2): S UV220 / S RID <5;

[0016] Among them, S UV360 The copolymer is dissolved in a solvent and then derivatized with 2,4-dinitrophenylhydrazine. The peak area of ​​the derivative on the copolymer main chain is measured at a UV-visible detector wavelength of 360 nm. UV220 is the peak area of ​​the copolymer main chain at a UV-visible detector wavelength of 220 nm; S RID is the peak area of ​​the copolymer main chain under the differential refractive index detector.

[0017] As a preferred solution, for formula (1), S UV360 / S RID Preferably it is less than 4, more preferably less than 3.

[0018] During the ethylene-vinyl acetate polymerization process, vinyl acetate generates acetaldehyde and / or other unsaturated aldehydes with conjugated structures due to transesterification reactions. These aldehydes may be attached to the polymer backbone through chain transfer reactions, which is one of the main causes of yellowing in ethylene-vinyl alcohol copolymer resins. These groups, after being derivatized with 2,4-dinitrophenylhydrazine, produce ultraviolet absorption at a wavelength of 360 nm within the molecule. When the polymer molecular weight test results satisfy equation (1), it indicates that fewer conjugated carbonyl and aldehyde groups are formed during the polymer preparation process, which helps prevent yellowing of the resin.

[0019] As a preferred solution, for formula (2), S UV200 / S RID Preferably it is less than 4, more preferably less than 3.

[0020] For ethylene-vinyl alcohol copolymers, thermal degradation such as dehydration at high temperatures creates carbon-carbon double bonds and carbonyl groups within the molecule that absorb ultraviolet light at a wavelength of 220 nm. This can lead to resin gelation, resulting in defects such as crystal points or fisheyes. When the molecular weight test results for ethylene-vinyl alcohol copolymers satisfy equation (2), it indicates that the polymer contains low levels of carbon-carbon double bonds and carbonyl groups, which inhibits dehydration degradation of the resin.

[0021] A method for preparing an ethylene-vinyl alcohol copolymer satisfying the conditions represented by formula (1) and formula (2) comprises the following steps:

[0022] First, ethylene and vinyl acetate are polymerized to prepare ethylene-vinyl acetate copolymer; then, in the presence of a catalyst, alcoholysis reaction is carried out to prepare ethylene-vinyl alcohol copolymer.

[0023] As a preferred embodiment, in the steps of preparing the ethylene-vinyl alcohol copolymer of the present invention, the reactions are all carried out in an alcohol solvent, preferably the same alcohol solvent. The alcohol solvent is an aliphatic alcohol having 4 or fewer carbon atoms, preferably methanol.

[0024] Currently, the synthesis gas (mainly CO, CO2, and H2) method is widely used in industry to produce methanol. When the synthesis gas contains trace amounts of amines (NH3), an amination reaction will occur during the methanol synthesis process, generating byproducts such as monomethylamine, dimethylamine, and trimethylamine. These methylamines promote the decomposition of vinyl acetate monomer into acetaldehyde and / or other unsaturated aldehydes with conjugated structures during the polymerization process. These aldehydes may also be attached to the polymer backbone due to chain transfer reactions, causing the resin to yellow. In the present invention, when the alkali content (calculated as NH3) in the methanol is ≤15ppm, the prepared ethylene-vinyl alcohol copolymer has excellent yellowing resistance, preferably ≤8ppm, and more preferably ≤2ppm.

[0025] By controlling the alkali content in the alcohol solvent, an ethylene-vinyl alcohol copolymer that satisfies the conditions represented by formula (1) and formula (2) can be prepared.

[0026] As a preferred solution, the present invention provides a method for preparing an ethylene-vinyl alcohol copolymer with excellent processing performance and high barrier properties, comprising the following steps:

[0027] 1) Solution polymerization: Add initiator, vinyl acetate and ethylene to an alcohol solvent to carry out free radical polymerization to prepare ethylene-vinyl acetate copolymer solution;

[0028] 2) Adding a polymerization inhibitor to remove ethylene and residual vinyl acetate;

[0029] 3) adding a catalyst to carry out alcoholysis reaction to prepare an ethylene-vinyl alcohol copolymer solution;

[0030] 4) concentrating the ethylene-vinyl alcohol copolymer solution and replacing the alcohol solvent in the solution with water vapor;

[0031] 5) Using desalted water to wash the salt and residual alcohol solvent in the resin;

[0032] 6) The copolymer is melted, extruded into granules, and dried.

[0033] In some specific embodiments of the present invention, the alcohol solvent in step 1) is an aliphatic alcohol having a carbon number of 4 or less, preferably methanol.

[0034] In some specific embodiments of the present invention, in step 1), the alkali content (calculated as NH3) in the alcohol solvent is ≤15ppm, preferably ≤8ppm, more preferably ≤2ppm.

[0035] In some specific embodiments of the present invention, the amount of alcohol solvent added is 5-50 wt %, preferably 10-30 wt %, of the polymerization reaction solution in step 1).

[0036] As a preferred solution, in the solution polymerization process of step 1), the molar content of ethylene in the copolymer prepared is 20-60%, preferably 25-45%.

[0037] The initiator of the present invention is one or more azo or organic peroxides, preferably one or more of azobisisobutyronitrile, azobisisovaleronitrile, azobisisoheptanoonitrile, dilauroyl peroxide, benzoyl peroxide, n-propyl peroxydicarbonate, bis(4-tert-butylcyclohexyl)peroxydicarbonate, etc., and the added amount thereof is 0.01-1.5wt%, preferably 0.1-1wt% of the polymerization reaction solution.

[0038] As a preferred solution, in step 1), the polymerization temperature is 40-70° C., preferably 40-60° C., and the polymerization time is 2-10 h, preferably 3-6 h.

[0039] In some embodiments of the present invention, the removal of ethylene and residual vinyl acetate in step 2) can be performed by flash evaporation, vacuum distillation, or the like. Preferably, the organic solvent vapor is introduced from the bottom of a packed tower, and a mixed vapor of the organic solvent and unreacted vinyl acetate is distilled from the top of the tower. The organic solvent may be the same as or different from the solvent used in the polymerization process, but is preferably the same.

[0040] As a preferred solution, in step 2), the temperature during the removal process is 50-150°C, preferably 80-120°C.

[0041] As a preferred embodiment, in step 2), before removing ethylene and vinyl acetate, a polymerization inhibitor is preferably added to prevent further polymerization of residual monomers. The polymerization inhibitor is one or more of phenols, quinones, aromatic amines, conjugated polyenes, and inorganic polymerization inhibitors, preferably one or more of conjugated polyenes and inorganic polymerization inhibitors, such as sorbic acid and / or cupric acetate. The amount of the polymerization inhibitor added is 0.01-1.5 wt %, preferably 0.05-1.3 wt %, of the mass of the product in step 1).

[0042] In some specific embodiments of the present invention, the catalyst in step 3) is a basic catalyst such as sodium hydroxide, potassium hydroxide, or sodium methoxide, or an acidic catalyst such as sulfuric acid, hydrochloric acid, or p-toluenesulfonic acid, preferably sodium hydroxide. The amount of the catalyst used is in a molar ratio of 0.001 to 1:1, preferably 0.01 to 0.5:1, relative to the vinyl acetate monomer units in the ethylene-vinyl acetate copolymer.

[0043] In some specific embodiments of the present invention, the alcoholysis process described in step 3) is carried out in an alcohol solvent. The alcohol solvent can be the same as or different from the solvent in the polymerization process, preferably the same. The alcoholysis process can be carried out in a kettle apparatus or a tower apparatus, preferably a tower apparatus.

[0044] In some specific embodiments of the present invention, in step 3), the alkali content (calculated as NH3) in the alcohol solvent is ≤15ppm, preferably ≤8ppm, more preferably ≤2ppm.

[0045] In some embodiments of the present invention, the reaction temperature of the alcoholysis process in step 3) is 30-150° C., preferably 60-120° C., more preferably 80-110° C. The alcoholysis rate of the polymer is above 90%, preferably above 95%.

[0046] In some embodiments of the present invention, the polymer in step 4) is first concentrated to a solid content of 30-50 wt%, and then the alcohol solvent is replaced by steam in a tower device. The temperature of the steam is 110-130°C, preferably 110-120°C.

[0047] In some embodiments of the present invention, the washing process in step 5) can be carried out in an autoclave, centrifugal, filtration, kneader, extruder, or the like, preferably a kneader and / or extruder. The washing process temperature is 70-150°C, preferably 80-130°C.

[0048] In some specific embodiments of the present invention, in step 5), in the washing process, after the salt substances are washed away, a functional additive is added according to performance requirements; alternatively, the functional additive is added together with the desalted water during the washing process, and the functional additive is fused with the ethylene vinyl alcohol resin during the washing process.

[0049] In some specific embodiments of the present invention, the functional aid includes one or more of a boron compound, a conjugated polyene compound, an organometallic salt and a phosphorus compound. Wherein the boron compound is one or more of boric acid, borate and boric ester, preferably boric acid, and its content is more than 50ppm and less than 1000ppm, preferably more than 100ppm and less than 800ppm. The conjugated polyene compound is one or more of sorbic acid and sorbate, preferably sorbic acid, and its content is more than 0.001ppm and less than 500ppm, preferably more than 1ppm and less than 400ppm. The organometallic salt is one or more of the carboxylates of metals such as lithium, sodium and potassium, preferably one or more of the carboxylates of sodium and potassium, more preferably sodium acetate and potassium acetate, and its content is more than 10ppm and less than 2000ppm, preferably more than 50ppm and less than 1000ppm. The phosphorus compound is one or more of phosphoric acid, phosphorous acid and salts thereof, preferably one or more of disodium hydrogen phosphate, sodium dihydrogen phosphate, dipotassium hydrogen phosphate and potassium dihydrogen phosphate, and its content is 0.01 ppm or more and 1000 ppm or less, preferably 1 ppm or more and 800 ppm or less.

[0050] In some specific embodiments of the present invention, other functional additives may also be added, such as one or more of a lubricant, an antioxidant, a UV absorber, a plasticizer, a heat stabilizer, etc. These may be used alone or in combination of two or more, with the content being from 0.01 ppm to 1000 ppm of the ethylene-vinyl alcohol copolymer.

[0051] In some specific embodiments of the present invention, the melting, extrusion, and granulation processes described in step 6) can be carried out in equipment such as a screw extruder and a kneader, preferably a screw extruder, and more preferably a twin-screw extruder. During the melting and extrusion processes, the resin is preferably in a hydrated state, with a water content of 10-70wt%, preferably 30-50wt%. The processing temperature during the melt extrusion process is 50-150°C, preferably 80-130°C. The granulation process can be carried out by strand pelletizing, water ring cutting, and underwater pelletizing.

[0052] In some embodiments of the present invention, the drying process described in step 6) can be performed using a static dryer and / or a dynamic dryer, such as a belt dryer, tower dryer, vacuum rake dryer, fluidized bed dryer, or fluidized bed dryer. A static dryer and a dynamic dryer can also be used in combination, preferably using the dynamic dryer first and then the static dryer. The drying temperature is 40-180°C, preferably 60-150°C, and more preferably 70-120°C. The resin is finally dried to a moisture content of less than 0.3 wt%.

[0053] Compared with the existing technology, the beneficial effect of the present invention is that by controlling the unsaturated bond content and carbonyl / aldehyde group content in its main chain, it is possible to avoid oxidation, cross-linking, and dehydration reactions of the resin in a high-temperature molten state, and to avoid problems such as crystal points, fish eyes, yellowing, and decreased barrier properties during processing. DETAILED DESCRIPTION

[0054] The present invention will be further described below through specific examples. The examples of the present invention are only for illustration of the present invention and do not limit the scope of the present invention.

[0055] 1. Sources of main raw materials for the examples and comparative examples of the present invention:

[0056] Vinyl acetate: Sinopec Great Wall Energy Chemical (Ningxia) Co., Ltd.

[0057] Ethylene: Wanhua Chemical Group Co., Ltd.

[0058] Methanol: Shanxi Guangda Coking Gas Source Co., Ltd.;

[0059] Azobisisobutyronitrile, lauroyl peroxide and other initiators: Arkema Chemical Co., Ltd.;

[0060] Polymerization inhibitor: Suqian Liansheng Technology Co., Ltd.

[0061] Other materials, unless otherwise specified, are common raw materials purchased from the market.

[0062] 2. Main testing instruments and methods used in the embodiments and comparative examples of the present invention:

[0063] Resin molecular weight: GPC test, model Optilab T-Rex / ViscoStar-Ⅱ / HELEOS-Ⅱ.

[0064] Resin ethylene content: GBT41877.2-2022.

[0065] Resin gas barrier properties: GBT41877.2-2022, equipment model MOCON Model 2 / 22.

[0066] Resin processing properties: Film blowing machine, model Labtech LF-400.

[0067] Alkali content in alcohol solvents: GB 338-2011.

[0068] Example 1

[0069] (1) In a 10 L stainless steel reactor equipped with a stirrer and a feed port, 500 g of methanol, 4900 g of vinyl acetate (removing impurities and inhibitors), and 15 g of azobisisobutyronitrile were added in sequence and stirred to dissolve evenly. The air in the reactor was replaced three times with 1 MPa nitrogen, and then ethylene was introduced to a pressure of 3.5 MPa. The temperature was raised to 60°C and the reaction was continued for 4 h. The conversion of vinyl acetate was 47%.

[0070] (2) After the reaction is completed, 500 g of copper acetate methanol solution (1 wt%) is added and stirred evenly.

[0071] The reaction solution was transferred to a packed tower, and methanol vapor at 110°C was blown into the tower from the bottom, so that the mixed vapor of methanol and unreacted vinyl acetate was distilled out from the top of the tower.

[0072] (3) The polymer solution was transferred to an alcoholysis reactor, 5000 g of sodium hydroxide methanol solution (10 wt%) was added, and the reaction was carried out at 65° C. for 4 h. The final alcoholysis rate of the polymer was 99.8%.

[0073] (4) The polymer solution is concentrated to 40 wt% solid content, and methanol is displaced using steam in a tower.

[0074] (5) Transfer the polymer to a twin-screw extruder and heat the polymer to 115°C. Inject 5000 g of desalted water containing 0.1 wt% boric acid, 0.05 wt% sorbic acid, 0.5 wt% sodium acetate, and 0.1 wt% sodium dihydrogen phosphate from the extruder water inlet, and drain the water from the side drain port. Repeat this operation several times.

[0075] (6) The washed resin with a water content of 35 wt% was transferred to another twin-screw extruder, heated to 110° C. for melting, extrusion, stranding and pelletizing to obtain polymer pellets.

[0076] The polymer particles were dried using a fluidized bed dryer at 110° C. for 24 h to prepare the final EVOH-1 resin, in which the boric acid content was 500 ppm, the sorbic acid content was 100 ppm, the sodium acetate content was 1000 ppm, the sodium dihydrogen phosphate content was 600 ppm, and the water content of the resin was 0.26 wt%.

[0077] After testing, the alkali content in the methanol solvent used in the reaction process was 1ppm, and the S UV360 / S RID =2.3, formula (2): S UV220 / S RID =2.7.

[0078] Example 2

[0079] The preparation was carried out in the same manner as in Example 1, except that the alkali content in the methanol solvent used in the reaction was 15 ppm. The final EVOH-2 resin was obtained. UV360 / S RID =4.8, S UV220 / S RID =4.5.

[0080] Example 3

[0081] The preparation was carried out in the same manner as in Example 1, except that the alkali content in the methanol solvent used in the reaction was 8 ppm. The final EVOH-3 resin was obtained. UV360 / S RID =3.5, S UV220 / S RID =2.8.

[0082] Example 4

[0083] (1) In a 10 L stainless steel reactor equipped with a stirrer and a feed port, 150 g of methanol, 4100 g of vinyl acetate (removing impurities and inhibitors), and 35 g of di(4-tert-butyl)cyclohexyl peroxydicarbonate were added in sequence and stirred to dissolve uniformly. The air in the reactor was replaced three times with 1 MPa nitrogen, and then ethylene was introduced to a pressure of 6.5 MPa. The temperature was raised to 55°C and the reaction was continued for 8 h. The conversion of vinyl acetate was 45%.

[0084] (2) After the reaction is completed, 500 g of sorbic acid methanol solution (7.5 wt%) is added and stirred evenly.

[0085] The reaction solution was transferred to a packed tower, and methanol vapor at 110°C was blown into the tower from the bottom, so that the mixed vapor of methanol and unreacted vinyl acetate was distilled out from the top of the tower.

[0086] (3) The polymer solution was transferred to an alcoholysis reactor, 860 g of a 10 wt % sodium hydroxide methanol solution was added, and the mixture was reacted at 110° C. for 1 h. The final alcoholysis rate of the polymer was 99.8%.

[0087] (4) The polymer solution is concentrated to 30 wt% solid content, and methanol is displaced using steam in a tower.

[0088] (5) Transfer the polymer to a twin-screw extruder and heat the polymer to 120°C. Inject 5000 g of desalted water containing 0.02 wt% boric acid, 0.1 wt% sorbic acid, 0.05 wt% sodium acetate, and 0.01 wt% disodium hydrogen phosphate from the extruder water inlet, and drain the water from the side drain port. Repeat this operation several times.

[0089] (6) The washed resin with a water content of 35 wt% was transferred to another twin-screw extruder, heated to 110° C. for melting, extrusion, stranding and pelletizing to obtain polymer pellets.

[0090] The polymer particles were dried using a fluidized bed dryer at 110° C. for 24 h to prepare the final EVOH-4 resin, in which the boric acid content was 110 ppm, the sorbic acid content was 450 ppm, the sodium acetate content was 100 ppm, the disodium hydrogen phosphate content was 10 ppm, and the resin water content was 0.23 wt%.

[0091] After testing, the alkali content in the methanol solvent used in the reaction process is 2ppm, and the S UV360 / S RID =1.9, formula (2): S UV220 / S RID =2.4.

[0092] Example 5

[0093] (1) In a 10 L stainless steel reactor equipped with a stirrer and a feed port, 650 g of methanol, 5300 g of vinyl acetate (removing impurities and inhibitors), and 5 g of azobisisobutyronitrile were added in sequence and stirred to dissolve uniformly. The air in the reactor was replaced three times with 1 MPa nitrogen, and then ethylene was introduced to a pressure of 2 MPa. The temperature was raised to 40°C and the reaction was continued for 10 h. The conversion of vinyl acetate was 40%.

[0094] (2) After the reaction is completed, 500 g of sorbic acid methanol solution (4 wt%) is added and stirred evenly.

[0095] The reaction solution was transferred to a packed tower, and methanol vapor at 120°C was blown into the tower from the bottom, so that the mixed vapor of methanol and unreacted vinyl acetate was distilled out from the top of the tower.

[0096] (3) The polymer solution was transferred to an alcoholysis reactor, 2465 g of a 10 wt% sodium hydroxide methanol solution was added, and the mixture was reacted at 130° C. for 0.5 h. The final alcoholysis rate of the polymer was 99.9%.

[0097] (4) The polymer solution is concentrated to 50 wt% solid content, and methanol is displaced using steam in a tower.

[0098] (5) Transfer the polymer to a twin-screw extruder and heat the polymer to 130°C. Inject 5000 g of desalted water containing 0.2 wt% boric acid, 0.05 wt% sorbic acid, 0.11 wt% sodium acetate, and 0.2 wt% potassium dihydrogen phosphate from the extruder water inlet, and drain the water from the side drain port. Repeat this operation several times.

[0099] (6) The washed resin with a water content of 35 wt% was transferred to another twin-screw extruder, heated to 110° C. for melting, extrusion, stranding and pelletizing to obtain polymer pellets.

[0100] The polymer particles were dried using a fluidized bed dryer at 110° C. for 24 h to prepare the final EVOH-5 resin, in which the boric acid content was 900 ppm, the sorbic acid content was 10 ppm, the sodium acetate content was 2000 ppm, the potassium dihydrogen phosphate content was 950 ppm, and the resin water content was 0.25 wt%.

[0101] After testing, the alkali content in the methanol solvent used in the reaction process is 3ppm, and the S UV360 / S RID =2.8, formula (2): S UV220 / S RID =2.1.

[0102] Example 6

[0103] (1) In a 10 L stainless steel reactor equipped with a stirrer and a feed port, 1500 g of methanol, 4125 g of vinyl acetate (removing impurities and inhibitors), and 50 g of azobisisoheptanonitrile were added in sequence and stirred to dissolve uniformly. The air in the reactor was replaced three times with 1 MPa nitrogen, and then ethylene was introduced to a pressure of 2.3 MPa. The temperature was raised to 45°C and the reaction was continued for 6 h. The conversion of vinyl acetate was 40%.

[0104] (2) After the reaction is completed, 80 g of copper acetate methanol solution (1 wt%) is added and stirred evenly.

[0105] The reaction solution was transferred to a packed tower, and methanol vapor at 120°C was blown into the tower from the bottom, so that the mixed vapor of methanol and unreacted vinyl acetate was distilled out from the top of the tower.

[0106] (3) The polymer solution was transferred to an alcoholysis reactor, 6140 g of a 10 wt % sodium hydroxide methanol solution was added, and the mixture was reacted at 120° C. for 0.5 h. The final alcoholysis rate of the polymer was 99.9%.

[0107] (4) The polymer solution is concentrated to 40 wt% solid content, and methanol is displaced using steam in a tower.

[0108] (5) Transfer the polymer to a twin-screw extruder, heat the polymer to 100°C, and inject 5000 g of desalted water containing 0.05 wt% boric acid, 0.18 wt% sorbic acid, 0.26 wt% potassium acetate, and 0.06 wt% dipotassium hydrogen phosphate through the extruder water inlet. Drain the water through the side drain port. Repeat this operation several times.

[0109] (6) The washed resin with a water content of 35 wt% was transferred to another twin-screw extruder, heated to 110° C. for melting, extrusion, stranding and pelletizing to obtain polymer pellets.

[0110] The polymer particles were dried using a fluidized bed dryer at 110° C. for 24 h to prepare the final EVOH-6 resin, in which the boric acid content was 200 ppm, the sorbic acid content was 300 ppm, the potassium acetate content was 500 ppm, the dipotassium hydrogen phosphate content was 300 ppm, and the water content of the resin was 0.25 wt%.

[0111] After testing, the alkali content in the methanol solvent used in the reaction process is 5ppm, and the S UV360 / S RID =3.1, formula (2): S UV220 / S RID =3.3.

[0112] Comparative Example 1

[0113] The preparation was carried out in the same manner as in Example 1, except that the alkali content in the methanol solvent used in the reaction was 67 ppm. The final EVOH-7 resin was obtained. UV360 / S RID =7.1, S UV220 / S RID =7.5.

[0114] Comparative Example 2

[0115] The preparation was carried out in the same manner as in Example 4, except that the alkali content in the methanol solvent used in the reaction was 35 ppm. The final EVOH-8 resin was obtained. UV360 / S RID =6.1, SUV220 / S RID =6.9.

[0116] Comparative Example 3

[0117] The preparation was carried out in the same manner as in Example 5, except that the alkali content in the methanol solvent used in the reaction was 20 ppm. The final EVOH-9 resin was obtained. UV360 / S RID =5.6, S UV220 / S RID =6.5.

[0118] Comparative Example 4

[0119] The preparation was carried out in the same manner as in Example 6, except that the alkali content in the methanol solvent used in the reaction was 30 ppm. The final EVOH-10 resin was obtained. UV360 / S RID =6.6, S UV220 / S RID =6.2.

[0120] Application test case

[0121] The molecular weight of the EVOH resin was measured using a gel permeation chromatograph equipped with a differential refractive index detector and a UV-visible absorbance detector. The oxygen transmission rate of the prepared EVOH resin was tested at 20°C and 65% humidity according to GBT 41877.2-2022. The prepared EVOH resin was blown using a film blowing machine. Films prepared at 230°C were examined for crystallinity, fisheyes, and yellowing, and their processing properties were also evaluated. The test results are shown in Table 1.

[0122] Table 1

[0123]

[0124] Note: The lower the oxygen permeability value, the better the oxygen barrier property is; the higher the processing performance value, the better the processing performance is.

Claims

1. An ethylene-vinyl alcohol copolymer, wherein the molecular weight measured by gel permeation chromatography with a differential refractive index detector and an ultraviolet-visible absorbance detector satisfies the conditions represented by the following formulas (1) and (2): Formula (1): S UV360 / S RID <5, Formula (2): S UV220 / S RID <5; in, S UV360 The copolymer is dissolved in a solvent and then derivatized with 2,4-dinitrophenylhydrazine. The peak area of ​​the derivative on the copolymer main chain is measured at a UV-visible detector wavelength of 360 nm. UV220 is the peak area of ​​the copolymer main chain at a UV-visible detector wavelength of 220 nm; S RID is the peak area of ​​the copolymer main chain under the differential refractive index detector.

2. The copolymer according to claim 1, characterized in that Formula (1), S UV360 / S RID Preferably it is less than 4, more preferably less than 3.

3. The copolymer according to claim 1, characterized in that Formula (2), S UV200 / S RID Preferably it is less than 4, more preferably less than 3.

4. A method for preparing the copolymer according to any one of claims 1 to 3, comprising the following steps: First, ethylene and vinyl acetate are polymerized to prepare ethylene-vinyl acetate copolymer; then, in the presence of a catalyst, alcoholysis reaction is carried out to prepare ethylene-vinyl alcohol copolymer.

5. The method according to claim 4, characterized in that The method comprises the following steps: 1) Solution polymerization: Add initiator, vinyl acetate and ethylene to an alcohol solvent to carry out free radical polymerization to prepare ethylene-vinyl acetate copolymer solution; 2) Adding a polymerization inhibitor to remove ethylene and residual vinyl acetate; 3) adding a catalyst to carry out alcoholysis reaction to prepare an ethylene-vinyl alcohol copolymer solution; 4) concentrating the ethylene-vinyl alcohol copolymer solution and replacing the alcohol solvent in the solution with water vapor; 5) Using desalted water to wash the salt and residual alcohol solvent in the resin; 6) The copolymer is melted, extruded into granules, and dried.

6. The method according to claim 5, characterized in that In the solution polymerization process of step 1), the alcohol solvent is an aliphatic alcohol having 4 or less carbon atoms, preferably methanol.

7. The method according to claim 6, characterized in that The alkali content (calculated as NH3) in the alcohol solvent is ≤15ppm, preferably ≤8ppm, more preferably ≤2ppm.

8. The method according to claim 1, characterized in that The amount of the alcohol solvent added is 5-50 wt %, preferably 10-30 wt %, of the polymerization reaction solution in step 1).

9. The method according to claim 1, characterized in that In the alcoholysis process of step 3), the reaction is carried out in an alcohol solvent, which is the same as or different from the solvent in step 1), preferably the same.

10. The method according to claim 1, characterized in that In step 3), the alkali content (calculated as NH3) in the alcohol solvent is ≤15ppm, preferably ≤8ppm, more preferably ≤2ppm.

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