Ethylene-vinyl alcohol copolymers, processes for their preparation and use, and ethylene-vinyl alcohol cast films

Abstract

The present application relates to the technical field of preparation of ethylene-vinyl alcohol copolymer, and discloses an ethylene-vinyl alcohol copolymer, a preparation method and application thereof, and an ethylene-vinyl alcohol casting film.The preparation method of the ethylene-vinyl alcohol copolymer comprises the following steps: (1) first contacting a first solvent, an initiator, vinyl acetate and ethylene to generate a polymerization reaction; (2) second contacting the product obtained in the polymerization reaction with oxygen, and then third contacting the product with a terminating agent to obtain an ethylene-vinyl acetate copolymer solution; and (3) performing alcoholysis reaction on the ethylene-vinyl acetate copolymer solution to obtain an ethylene-vinyl alcohol copolymer; wherein the amount of the oxygen is 3-30 ppm based on the weight of the product obtained in step (1). The ethylene-vinyl alcohol copolymer prepared by the method has excellent melt processing performance, and the casting film prepared from the ethylene-vinyl alcohol copolymer has fewer crystal point defects.

Description

Technical Field

[0001] This invention relates to the field of ethylene-vinyl alcohol copolymer preparation technology, specifically to an ethylene-vinyl alcohol copolymer, its preparation method, and its application in ethylene-vinyl alcohol cast films. Background Technology

[0002] Ethylene-vinyl alcohol copolymers (EVOH), especially those with an ethylene content of 20-50 mol%, possess excellent gas barrier properties, approximately 10,000 times that of ordinary polyethylene. In addition, EVOH exhibits a variety of superior physical and chemical properties, such as high transparency, excellent processability, superior solvent resistance, and antistatic properties. These characteristics make EVOH widely applicable across various industries, including packaging materials, automotive fuel tank manufacturing, production of oxygen-barrier underfloor heating pipes, reinforcement of textile materials, and the development of medical materials.

[0003] Ethylene-vinyl alcohol copolymers are typically processed by melt processing. As a polymer with multiple hydroxyl structures in its molecular structure, EVOH is prone to intermolecular and intramolecular dehydration reactions under high temperature conditions, which leads to a decrease in processing stability. In particular, defects such as crystal points are generated during film formation, affecting the overall performance and market competitiveness of the product.

[0004] Therefore, there is an urgent need to develop an EVOH material with superior processing performance to further expand its market potential and application prospects. Summary of the Invention

[0005] The purpose of this invention is to overcome the problem of poor melt processing stability of ethylene-vinyl alcohol copolymers in the prior art, and to provide an ethylene-vinyl alcohol copolymer, its preparation method, and its application in ethylene-vinyl alcohol cast films. This method involves introducing oxygen into the product before the polymerization reaction of ethylene and vinyl acetate is complete. The ethylene-vinyl alcohol copolymer obtained after oxygen treatment exhibits superior melt processing properties.

[0006] To achieve the above objectives, a first aspect of the present invention provides a method for preparing an ethylene-vinyl alcohol copolymer, the method comprising:

[0007] (1) The first solvent, initiator, vinyl acetate and ethylene are brought into first contact to undergo a polymerization reaction;

[0008] (2) The product obtained in step (1) is subjected to a second contact with oxygen, and then to a third contact with a terminating agent to obtain an ethylene-vinyl acetate copolymer solution.

[0009] (3) The ethylene-vinyl acetate copolymer solution is subjected to alcoholysis to obtain ethylene-vinyl alcohol copolymer;

[0010] The oxygen content is 3-30 ppm, based on the weight of the product obtained in step (1).

[0011] A second aspect of the present invention provides an ethylene-vinyl alcohol copolymer prepared by the method described in the first aspect above.

[0012] A third aspect of the present invention provides an application of the ethylene-vinyl alcohol copolymer described in the second aspect above in at least one of packaging materials, automotive fuel tanks, oxygen-barrier floor heating pipes, textile materials, and medical materials.

[0013] A fourth aspect of the present invention provides an ethylene-vinyl alcohol cast film, wherein the cast film is prepared from the ethylene-vinyl alcohol copolymer described in the second aspect above.

[0014] Through the above technical solution, the present invention has the following beneficial effects:

[0015] The method for preparing ethylene-vinyl alcohol copolymer provided by this invention involves introducing a specific amount of oxygen into the product before adding a terminator to terminate the polymerization reaction. The oxygen reacts with macromolecular free radicals and initiator free radicals in the product, exhibiting low reactivity and blocking the growth of free radical polymer chains. This pre-terminates the polymerization reaction, preventing vinyl acetate self-polymerization and promoting better function of the terminator, eliminating active free radicals in the product. The resulting ethylene-vinyl alcohol copolymer exhibits excellent melt processing stability. When the ethylene-vinyl alcohol copolymer prepared by the method of this invention is used to prepare ethylene-vinyl alcohol cast films, the resulting cast films have fewer crystal point defects. Detailed Implementation

[0016] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0017] The first aspect of the present invention provides a method for preparing ethylene-vinyl alcohol copolymers, the method comprising:

[0018] (1) The first solvent, initiator, vinyl acetate and ethylene are brought into first contact to undergo a polymerization reaction;

[0019] (2) The product obtained in step (1) is subjected to a second contact with oxygen, and then to a third contact with a terminating agent to obtain an ethylene-vinyl acetate copolymer solution.

[0020] (3) The ethylene-vinyl acetate copolymer solution is subjected to alcoholysis to obtain ethylene-vinyl alcohol copolymer;

[0021] The oxygen content is 3-30 ppm, based on the weight of the product obtained in step (1).

[0022] In this invention, the ethylene-vinyl alcohol copolymer prepared by the method described above has excellent melt processing stability. When used to prepare ethylene-vinyl alcohol cast films, the resulting cast films have fewer crystal point defects.

[0023] In this invention, a specific amount of oxygen is introduced into the product before the polymerization reaction is terminated by adding a terminator. The oxygen reacts with the macromolecular free radicals and initiator free radicals in the product, thereby reducing the reactivity, blocking the growth of free radical polymerization chains, pre-terminating the polymerization reaction, preventing vinyl acetate self-polymerization, and promoting the better functioning of the terminator to eliminate active free radicals in the product.

[0024] In some embodiments of the present invention, preferably, the amount of oxygen used is 3-30 ppm, based on the weight of the product obtained in step (1). In the present invention, controlling the amount of oxygen within the above range is beneficial for pre-terminating the polymerization reaction in advance, preventing vinyl acetate self-polymerization, and promoting the better functioning of the terminator, so that the ethylene-vinyl acetate copolymer has a suitable and controllable weight-average molecular weight and molecular weight distribution, thereby having excellent melt processing stability.

[0025] In this invention, based on the weight of the product obtained in step (1), if the oxygen content is less than 3 ppm, then the amount of oxygen used is too small and cannot achieve the effect of pre-terminating the reaction; if based on the weight of the product obtained in step (1), if the oxygen content is greater than 30 ppm, then the amount of oxygen used is too large, which will have an adverse effect on the product structure of the polymer, and at the same time increase the safety risk of process control.

[0026] Preferably, the amount of oxygen used is 5-20 ppm, based on the weight of the product obtained in step (1). For example, it can be 5 ppm, 6 ppm, 8 ppm, 10 ppm, 12 ppm, 14 ppm, 15 ppm, 16 ppm, 18 ppm, 20 ppm, or any two of the above values ​​within a range or within the range, preferably 8-15 ppm.

[0027] In some embodiments of the present invention, preferably, the conditions for the second contact include: a temperature of 10-60°C and a time of 30-300 min. In the present invention, controlling the contact conditions between the product obtained in step (1) and oxygen within the above range is beneficial to pre-terminating the polymerization reaction, thereby obtaining an ethylene-vinyl acetate copolymer with excellent melt processing stability.

[0028] In this invention, if the product obtained in step (1) is in contact with oxygen for less than 30 minutes, the oxygen treatment time is too short and the pre-termination reaction effect is poor; if the product obtained in step (1) is in contact with oxygen for more than 300 minutes, the oxygen treatment time is too long, which has an adverse effect on the polymer structure and is also detrimental to the efficiency of process control.

[0029] Preferably, the conditions for the second contact include: a temperature of 20-40°C and a time of 60-200 min. In this invention, the pressure of the second contact is preferably 2-7 MPa.

[0030] In this invention, the amount of the initiator can be selected within a wide range. Preferably, based on the weight of the vinyl acetate, the amount of the initiator is 30-3000 ppm. In this invention, controlling the amount of the initiator within the above range is beneficial for controlling the intrinsic viscosity of the copolymer and improving the reaction efficiency.

[0031] Preferably, the amount of initiator is 50-2000 ppm based on the weight of the vinyl acetate.

[0032] In this invention, the type of initiator is not particularly limited, as long as it can initiate the polymerization reaction between vinyl acetate and ethylene. Preferably, the initiator is selected from azo initiators and / or peroxide initiators. More preferably, the azo initiator is at least one of azobisisobutyronitrile, azobisisovalerate, azoisobutylcyanoformamide, azobiscyclohexylformitrile, and dimethyl azobisisobutyrate; the peroxide initiator is an organic peroxide and / or an inorganic peroxide; more preferably, the organic peroxide is selected from at least one of benzoyl peroxide, benzoyl tert-butyl peroxide, methyl ethyl ketone peroxide, diisobutyryl peroxide, tert-pentyl peroxyneodecanate, bis(4-tert-butylcyclohexyl peroxydicarbonate), tert-pentyl peroxypentate, tert-butyl peroxyacetate, and dibutyl peroxydicarbonate; the inorganic peroxide is selected from at least one of hydrogen peroxide, ammonium persulfate, and potassium persulfate.

[0033] In this invention, the amount of the first solvent has a wide range of selection, as long as it can dissolve the initiator, vinyl acetate, and ethylene without affecting the polymerization reaction. Preferably, based on the weight of the vinyl acetate, the amount of the first solvent is 5-50% by weight, more preferably 10-40% by weight.

[0034] In some embodiments of the present invention, preferably, the amount of ethylene used is such that the polymerization pressure is 2-7 MPaG. In the present invention, by controlling the amount of ethylene used to keep the polymerization pressure within the above range, the content of structural units derived from ethylene in the ethylene-vinyl alcohol copolymer is controlled, resulting in a suitable content of ethylene structural units in the copolymer.

[0035] Preferably, the amount of ethylene used is such that the polymerization pressure is 3-6 MPaG.

[0036] In this invention, the conditions for the polymerization reaction are not particularly limited and can be those conventionally used in the art. Preferably, the polymerization reaction conditions include: a temperature of 40-70°C and a time of 2-9 hours. In this invention, controlling the polymerization conditions within the above range is beneficial for ensuring that the ethylene-vinyl acetate copolymer has a suitable intrinsic viscosity, facilitating monomer conversion, and simultaneously considering the reaction rate.

[0037] Preferably, the conditions for the polymerization reaction include: a temperature of 50-65°C and a time of 3-7 hours.

[0038] In this invention, preferably, the method in step (2) further includes: making a fourth contact between the product obtained in step (1) and a second solvent. In this invention, the product can be contacted with the second solvent before contacting oxygen, or the product can be contacted with the second solvent first and then contacted with oxygen. Preferably, the product obtained in step (1) is made into a fourth contact with the second solvent before contacting oxygen.

[0039] In this invention, the second solvent is beneficial for diluting free radicals, which can reduce the rate of polymerization reaction. It works synergistically with the introduced oxygen to further facilitate the initial pre-termination of polymerization reaction, reduce the occurrence of side reactions, and precisely control the length and distribution of ethylene-vinyl alcohol copolymer chains during polymerization to obtain ethylene-vinyl alcohol copolymers with excellent melt processing stability.

[0040] In this invention, the amount of the second solvent has a wide range of selection. Preferably, based on the weight of the vinyl acetate, the amount of the second solvent is 3-30% by weight. In this invention, controlling the amount of the second solvent within the above range is beneficial to reducing the reaction rate and inhibiting the self-polymerization of vinyl acetate into polyvinyl acetate.

[0041] Preferably, based on the weight of the vinyl acetate, the amount of the second solvent can be 3% by weight, 12% by weight, 14% by weight, 16% by weight, 18% by weight, 20% by weight, 22% by weight, 24% by weight, 26% by weight, 28% by weight, 30% by weight, or any two of the above values ​​within a range, preferably 5-20% by weight.

[0042] In this invention, the conditions for the fourth contact are not particularly limited. Preferably, the conditions for the fourth contact include: a temperature of 40-70°C and a time of 1-5 hours. In this invention, the pressure of the second contact is the same as that of the first contact. Controlling the contact conditions between the product obtained in step (1) and the second solvent within the above range has the characteristics of being consistent with the polymerization reaction conditions, eliminating the need to adjust process parameters, and resulting in convenient operation and uniform mixing.

[0043] In some embodiments of the present invention, preferably, the first solvent and the second solvent are each independently an alcohol solvent. In the present invention, the first solvent and the second solvent may be the same or different; preferably, the first solvent and the second solvent are the same.

[0044] In this invention, there is no particular limitation on the type of alcohol solvent. Preferably, the first solvent and the second solvent are each independently an alcohol solvent with 1-4 carbon atoms; more preferably, the first solvent and the second solvent are each independently at least one selected from methanol, ethanol, propanol, ethylene glycol, n-butanol, and tert-butanol; and even more preferably, both the first solvent and the second solvent are methanol.

[0045] In some embodiments of the present invention, preferably, the amount of the terminator is 100-5000 ppm of the amount of vinyl acetate. In the present invention, controlling the amount of the terminator within the above range effectively reduces the content of polyvinyl acetate in the ethylene-vinyl acetate copolymer solution, eliminates active free radicals in the polymerization system, and prevents vinyl acetate self-polymerization.

[0046] Preferably, the amount of the terminating agent is 300-3000 ppm of the amount of vinyl acetate.

[0047] In this invention, the type of terminating agent is not particularly limited. Preferably, the terminating agent is selected from at least one of 2,4-diphenyl-4-methyl-1-pentene, 2,2-diphenyl-1-trinitrophenylhydrazine, 1,1-diphenyl-2-trinitrophenylhydrazine, copper acetate, cuprous chloride, and ferric chloride. More preferably, the terminating agent is selected from at least one of 2,4-diphenyl-4-methyl-1-pentene, copper acetate, cuprous chloride, and ferric chloride.

[0048] In this invention, the conditions for the third contact with the terminating agent are not particularly limited. Preferably, the conditions for the third contact include: a temperature of 10-40°C and a time of 0.1-2 hours. In this invention, the third contact is carried out under normal pressure, and controlling the conditions of the third contact within the above range ensures that the terminating agent and the ethylene-vinyl acetate copolymer solution are mixed evenly, ensuring that the terminating agent can fully exert its terminating effect.

[0049] In this invention, preferably, the method in step (2) further includes: removing ethylene and vinyl acetate from the product after the third contact. More preferably, after the third contact, ethylene and vinyl acetate are removed from the product sequentially.

[0050] In this invention, the method for removing ethylene from the product is not particularly limited, as long as ethylene can be removed without introducing other impurities. Preferably, the method for removing ethylene is vacuum flash evaporation.

[0051] In some embodiments of the present invention, preferably, the method for removing vinyl acetate includes: a fifth contact with a third solvent after a third contact. The fifth contact of the present invention is preferably a vapor contact.

[0052] In this invention, preferably, the third solvent vapor is introduced via countercurrent contact. For example, if the removal of vinyl acetate is carried out in a distillation column, the ethylene-vinyl acetate copolymer solution after the addition of the terminator is introduced into the top of the distillation column, and the third solvent vapor is introduced into the bottom of the distillation column to remove unreacted vinyl acetate monomers from the top of the column by distillation.

[0053] In some embodiments of the present invention, preferably, the conditions for the fifth contact include: a temperature of 65-80°C, a pressure of 100-180 kPa, and a time of 0.5-3 h. In the present invention, controlling the conditions of the fifth contact within the above ranges allows for sufficient gas-liquid exchange between the solvent vapor and the ethylene-vinyl acetate copolymer solution, thereby removing unreacted vinyl acetate.

[0054] In this invention, the third solvent may be the same as or different from the first solvent and the second solvent. Preferably, it is the same as the first solvent and the second solvent.

[0055] In this invention, the type of the third solvent is not particularly limited, as long as it can remove vinyl acetate. Preferably, the third solvent is an alcohol solvent, preferably an alcohol solvent with 1-4 carbon atoms, more preferably at least one selected from methanol, ethanol, propanol, ethylene glycol, n-butanol, and tert-butanol, and even more preferably methanol.

[0056] In this invention, the method of the alcoholysis reaction is not particularly limited. Preferably, the method of the alcoholysis reaction includes contacting the ethylene-vinyl acetate copolymer solution with an alkali.

[0057] In this invention, the molar ratio of the alkali to the vinyl acetate groups in the ethylene-vinyl acetate copolymer solution has a wide range of selection. Preferably, the molar ratio of hydroxide ions in the alkali to vinyl acetate groups in the ethylene-vinyl acetate copolymer is 0.005-0.5:1. In this invention, controlling the molar ratio of hydroxide ions in the alkali to vinyl acetate groups in the ethylene-vinyl acetate copolymer within the above range is beneficial to improving reaction efficiency, reducing the generation of reaction by-products, and obtaining an ethylene-vinyl alcohol copolymer with high degree of alcoholysis and low by-product content.

[0058] Preferably, the molar ratio of hydroxide ions in the alkali to vinyl acetate groups in the ethylene-vinyl acetate copolymer is 0.01-0.3:1.

[0059] In some embodiments of the present invention, preferably, the content of ethylene-vinyl acetate copolymer in the ethylene-vinyl acetate copolymer solution in step (3) is 20% to 60% by weight. In the present invention, controlling the content of ethylene-vinyl acetate copolymer in the ethylene-vinyl acetate copolymer solution within the above range is beneficial to improving reaction efficiency.

[0060] Preferably, the content of ethylene-vinyl acetate copolymer in the ethylene-vinyl acetate copolymer solution in step (3) is 30%-50% by weight.

[0061] In some embodiments of the present invention, preferably, the conditions for contact with the alkali include: a temperature of 50-65°C and a time of 4-10 hours. Preferably, the pressure of the contact is atmospheric pressure. In the present invention, controlling the conditions for contact with the alkali within the above range and carrying out the reaction under atmospheric pressure is beneficial to giving the prepared ethylene-vinyl alcohol copolymer better melt processing stability.

[0062] In this invention, in actual operation, the alkali is an alkaline solution to facilitate better mixing. The type of solvent in the alkaline solution is not particularly limited, as long as it can dissolve the alkali without affecting the alcoholysis of ethylene-vinyl acetate. The solvent of the alkaline solution may be the same as or different from the first solvent; preferably, the solvent of the alkaline solution is the same as the first solvent. Preferably, the solvent of the alkaline solution is an alcohol solvent, preferably an alcohol solvent with 1-4 carbon atoms, more preferably at least one selected from methanol, ethanol, propanol, ethylene glycol, n-butanol, and tert-butanol, and even more preferably methanol.

[0063] In this invention, the concentration of the alkaline solution is not particularly limited, as long as the molar ratio of hydroxide ions in the alkaline solution to vinyl acetate groups in the ethylene-vinyl acetate copolymer meets the above-mentioned range. Preferably, the concentration of the alkaline solution is 2% by weight to 20% by weight.

[0064] In this invention, the type of alkali in the alkaline solution is not limited; various alkalis conventionally used in the art can be used, as long as an alkaline environment can be provided. Preferably, the alkali is sodium hydroxide and / or potassium hydroxide.

[0065] In this invention, preferably, the method further includes: mixing the alcoholyzed ethylene-vinyl alcohol copolymer solution with an acid solution. In this invention, the acid solution contains H... + The preferred dosage is 1 ppm to 2000 ppm of ethylene-vinyl alcohol copolymer. The concentration of the acid solution can be 5% to 50% by weight. There are no particular limitations on the type of acid used in this invention, as long as it can provide cationic hydrogen. The acid is preferably at least one of acetic acid, propionic acid, boric acid, and sodium dihydrogen phosphate. By mixing with the acid, the above-mentioned alkali is neutralized, thereby improving product quality.

[0066] In this invention, in order to obtain the final product, preferably, the method further includes: molding, granulating and washing the product of mixing ethylene-vinyl alcohol copolymer with acid solution.

[0067] In this invention, the molding and granulation method can be the extrusion method conventionally used in the art, which will not be described in detail here.

[0068] In this invention, the number of washing cycles is not particularly limited; the washing can be a single wash or multiple washes. The washing solution used in this invention is not particularly limited; for example, it can be a mixture of water and alcohol, with the weight ratio of water to alcohol preferably being 1-20:99-80. Preferably, the alcohol is an alcohol solvent with 1-4 carbon atoms, preferably at least one selected from methanol, ethanol, propanol, ethylene glycol, n-butanol, and tert-butanol, and more preferably ethanol.

[0069] A second aspect of the present invention provides an ethylene-vinyl alcohol copolymer prepared by the method described in the first aspect above.

[0070] In some embodiments of the present invention, preferably, the content of ethylene-derived structural units in the ethylene-vinyl alcohol copolymer is 20-50 mol%. The above content of ethylene-derived structural units is based on the total molar content of the copolymer. In the present invention, controlling the content of ethylene-derived structural units in the copolymer within the above range is beneficial to giving the copolymer better melt processability, resulting in better performance when processed into materials such as cast films or sheets.

[0071] Preferably, the content of structural units derived from ethylene in the ethylene-vinyl alcohol copolymer is 25-45 mol%.

[0072] In this invention, the ethylene structural unit content in the ethylene-vinyl alcohol copolymer is tested using the method specified in GB / T41877.2-2022.

[0073] In some embodiments of the present invention, preferably, the weight-average molecular weight of the copolymer is 40,000-150,000 g / mol, for example, it can be 40,000 g / mol, 50,000 g / mol, 70,000 g / mol, 90,000 g / mol, 110,000 g / mol, 130,000 g / mol, 150,000 g / mol, or any two of the above values ​​or values ​​within the range.

[0074] In some embodiments of the present invention, preferably, the molecular weight distribution of the copolymer is 1.5-3.5, for example, it can be 1.5, 2, 2.5, 3, 3.5, or any two of the above values ​​within a range or within a range. In the present invention, controlling the weight-average molecular weight and molecular weight distribution of the copolymer within the above range enables the copolymer to have good mechanical properties and melt processability.

[0075] In this invention, the weight-average molecular weight and molecular weight distribution of the ethylene-vinyl alcohol copolymer are measured by the GPC method.

[0076] In some embodiments of the present invention, preferably, the intrinsic viscosity of the copolymer at 30°C is 0.5-1.5 dl / g, for example, it can be 0.5 dl / g, 0.7 dl / g, 0.9 dl / g, 1.1 dl / g, 1.3 dl / g, 1.5 dl / g, or any value within a range formed by any two of the above values. In the present invention, the intrinsic viscosity of the copolymer is within the above range, so that the copolymer has both good mechanical properties and melt processability.

[0077] In this invention, the intrinsic viscosity test method for the ethylene-vinyl alcohol copolymer includes: accurately weighing approximately 1 g (accurate to 0.2 mg) of the dried ethylene-vinyl alcohol copolymer sample, placing it in a 250 mL ground glass joint Erlenmeyer flask, adding 50 mL of dimethyl sulfoxide, and refluxing to dissolve it in an 80 °C water bath. After complete dissolution, transfer the solution to a 100 mL volumetric flask and dilute to a final concentration, denoted as C. Filter the sample solution and place it in an Ubbelohde viscometer tube, maintaining the temperature at 30 ± 0.1 °C for 15 min. After calibrating the viscometer vertically, perform the test according to the Ubbelohde viscometer's operating instructions, allowing the sample solution to fall naturally within the viscometer tube. Accurately record the time taken by a stopwatch to allow the sample liquid surface to flow across the two measurement lines. Repeat the measurement several times and take the average value, denoted as t1. Blank test: Place the filtered dimethyl sulfoxide in the viscometer, maintain the temperature at 30 ± 0.1 °C, and accurately measure the time taken by the dimethyl sulfoxide to flow across the two measurement lines. Repeat the measurement several times and take the average value, denoted as t0. The intrinsic viscosity [η] (dL / g) of the ethylene-vinyl alcohol copolymer is calculated using the following formula:

[0078] Relative viscosity: η r =t / t0;

[0079] Specific viscosity: η sp =(t-t0) / t0=η r -1;

[0080] Intrinsic viscosity:

[0081] In some embodiments of the present invention, preferably, the melt index of the copolymer at 210°C and a load of 2.16 kg is 2-16 g / 10 min, for example, it can be 2 g / 10 min, 4 g / 10 min, 6 g / 10 min, 8 g / 10 min, 10 g / 10 min, 12 g / 10 min, 14 g / 10 min, 16 g / 10 min, or any range formed by any two of the above values ​​and values ​​within that range. In the present invention, the copolymer with a melt index within the above range has good processing fluidity.

[0082] In this invention, the melt index of the copolymer is tested according to GB / T3682.1-2018 standard.

[0083] In a preferred embodiment of the present invention, the method for preparing the ethylene-vinyl alcohol copolymer includes:

[0084] (1) The first solvent, initiator, vinyl acetate and ethylene are brought into first contact to undergo a polymerization reaction. The polymerization reaction temperature is 40-70℃ and the time is 2-9h. The amount of ethylene used makes the pressure of the polymerization system 2-6MPaG.

[0085] (2) The product obtained in step (1) is subjected to a fourth contact with a second solvent (temperature 40-70℃, pressure 2-6MPaG, time 1-5h), then subjected to a second contact with oxygen (temperature 10-60℃, pressure 2-6MPa, time 30-300min), and then subjected to a third contact with a terminator (temperature 10-40℃, time 0.1-2h, pressure at atmospheric pressure) to remove ethylene and vinyl acetate from the product, thereby obtaining an ethylene-vinyl acetate copolymer solution;

[0086] (3) The ethylene-vinyl acetate copolymer solution is contacted with an alkali to carry out an alcoholysis reaction (temperature 50-65℃, time 4-10h), wherein the molar ratio of hydroxide ions in the alkali to vinyl acetate groups in the ethylene-vinyl acetate copolymer is 0.005-0.5:1;

[0087] (4) The alcoholysis product is subjected to conditioning, extrusion and washing in sequence to obtain ethylene-vinyl alcohol copolymer;

[0088] The amount of oxygen used is 3-30 ppm, based on the weight of the product obtained in step (1).

[0089] A third aspect of the present invention provides an application of the ethylene-vinyl alcohol copolymer described in the second aspect above in at least one of packaging materials, automotive fuel tanks, oxygen-barrier floor heating pipes, textile materials, and medical materials.

[0090] A fourth aspect of the present invention provides an ethylene-vinyl alcohol cast film, wherein the cast film is prepared from the ethylene-vinyl alcohol copolymer described in the second aspect above.

[0091] In this invention, there are no limitations on the method for preparing ethylene-ethylene copolymer into ethylene-vinyl alcohol cast film. Conventional testing methods in the art can be used. For example, a blown film machine or a cast film machine can be used.

[0092] In some embodiments of the present invention, preferably, the number of crystal points smaller than 300 μm in the cast film is ≤9000 per m. 2 Preferably ≤6500 pieces / m 2 .

[0093] In this invention, the method for testing the number of crystal points in the cast film includes: obtaining a cast film using a casting film machine (ME20-MFA-FSA100V2) at 200-250℃ and a stretching speed of 2.5-3.5m / min, wherein the screw is a single screw with L / D≥25, the width of the cast film is controlled at 15mm, the film thickness is controlled at 20-40μm, and a line array camera and an LED light source are used to test the crystal point defects of the cast film.

[0094] In this invention, unless otherwise specified, "ppm" refers to mass concentration.

[0095] The present invention will be described in detail below through embodiments.

[0096] In the following examples and comparative examples, the test methods for the ethylene structural unit content, weight-average molecular weight and molecular weight distribution, intrinsic viscosity and melt index of the ethylene-vinyl alcohol copolymer are the same as those described in the specific embodiments, and will not be repeated here.

[0097] Example 1

[0098] (1) Add 100 parts by weight of vinyl acetate, 16 parts by weight of methanol, and 0.1 parts by weight of azobisisobutyronitrile to a pressure polymerization reactor equipped with a jacket, stirrer, raw material inlet, temperature and pressure gauge, and reaction product outlet. Raise the temperature to 63°C, maintain the ethylene pressure at 4.2 MPaG, and react for 5.5 hours with stirring.

[0099] (2) Add 12 parts of methanol to the product obtained in step (1) (keep the temperature and pressure constant for 3 hours), introduce 12 ppm of oxygen (based on the weight of the product obtained in step (1), temperature 35°C, pressure 3.6 MPa), react for 60 minutes under stirring, add 1500 ppm of 2,4-diphenyl-4-methyl-1-pentene (based on the amount of vinyl acetate used) (time 0.5 hours, temperature 25°C), stir evenly, release pressure and flash evaporate to remove unreacted ethylene, pressure at atmospheric pressure, then introduce methanol vapor (time 2 hours, temperature 70°C, pressure 140 kPa), distill to remove unreacted vinyl acetate, and obtain ethylene-vinyl acetate copolymer solution;

[0100] (3) Adjust the ethylene-vinyl acetate copolymer solution to a mass fraction of 40%, and then add a methanol solution of sodium hydroxide (concentration of 10wt%) at 65°C for alcoholysis (time of 5h, pressure of atmospheric pressure). The molar ratio of hydroxide ions in sodium hydroxide to vinyl acetate groups in ethylene-vinyl acetate copolymer is 0.15:1. After alcoholysis, an ethylene-vinyl alcohol copolymer solution is obtained.

[0101] (4) Add an acetic acid solution (25 wt%) to the ethylene-vinyl alcohol copolymer solution and stir until homogeneous. The acetic acid solution contains H... + The dosage is 1000 ppm relative to the solid parts of the ethylene-vinyl alcohol copolymer;

[0102] (5) The ethylene-vinyl alcohol copolymer solution obtained in step (4) 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 particles using a common cutting method. Then, water is added to a kettle with a stirring device to wash the ethylene-vinyl alcohol copolymer particles. Each wash lasts for 2 hours and is repeated twice. Then, a mixed solution of water and alcohol is added to a kettle with a stirring device. After washing at 50°C for 2 hours, it is washed with water once more. During the water washing process, acetic acid with a mass of 0.01 parts relative to the ethylene-vinyl alcohol copolymer (based on a mass of 100 parts) is added for acid washing and centrifugal dehydration. After drying, ethylene-vinyl alcohol copolymer particles are obtained.

[0103] The parameters of the obtained ethylene-vinyl alcohol copolymer are shown in Table 1.

[0104] Example 2

[0105] (1) Add 100 parts by weight of vinyl acetate, 28 parts by weight of methanol, and 0.15 parts by weight of azobisisobutyronitrile to a pressure polymerization reactor equipped with a jacket, stirrer, raw material inlet, temperature and pressure gauge, and reaction product outlet. Raise the temperature to 65°C, maintain the ethylene pressure at 5.5 MPaG, and react for 4.3 hours with stirring.

[0106] (2) Add 20 parts of methanol to the product obtained in step (1) (keep the temperature and pressure constant for 2 hours), introduce 15 ppm of oxygen (based on the weight of the product obtained in step (1), temperature 30°C, pressure 5.5 MPa), react for 100 minutes under stirring, add 2000 ppm of 2,4-diphenyl-4-methyl-1-pentene (based on the amount of vinyl acetate used) (time 0.1 hours, temperature 40°C), stir evenly, release pressure and flash evaporate to remove unreacted ethylene, pressure at atmospheric pressure, then introduce methanol vapor (time 3 hours, temperature 65°C, pressure 100 kPa), distill to remove unreacted vinyl acetate, and obtain ethylene-vinyl acetate copolymer solution;

[0107] (3) Adjust the ethylene-vinyl acetate copolymer solution to a mass fraction of 45%, and then add a methanol solution of sodium hydroxide (concentration of 18wt%) at 60°C for alcoholysis (time of 4h, pressure of atmospheric pressure). The molar ratio of hydroxide ions in sodium hydroxide to vinyl acetate groups in ethylene-vinyl acetate copolymer is 0.17:1. After alcoholysis, an ethylene-vinyl alcohol copolymer solution is obtained.

[0108] (4) Add an acetic acid solution (concentration of 32 wt%) to the ethylene-vinyl alcohol copolymer solution and stir until homogeneous. The acetic acid solution contains H... +The dosage is 1500 ppm relative to the weight of the ethylene-vinyl alcohol copolymer solids;

[0109] (5) The ethylene-vinyl alcohol copolymer solution obtained in step (4) is extruded into an aqueous solution at 5°C through an extrusion device with a perforated plate. After precipitating into strips, the strips are cut into particles using a common cutting method. Subsequently, the ethylene-vinyl alcohol copolymer particles are washed with water in a kettle with a stirring device for 2 hours each time, and the washing is repeated twice. Then, a mixed solution of water and alcohol is added to the kettle with a stirring device, and the mixture is washed at 55°C for 2 hours. After washing with water once more, acetic acid with a mass of 0.01 parts relative to the ethylene-vinyl alcohol copolymer (based on a mass of 100 parts) is added during the water washing process. The mixture is then acid washed and centrifuged to dehydrate. After drying, the ethylene-vinyl alcohol copolymer particles are obtained. The parameters of the obtained ethylene-vinyl alcohol copolymer are shown in Table 1.

[0110] Example 3

[0111] (1) Add 100 parts by weight of vinyl acetate, 9 parts by weight of methanol, and 0.07 parts by weight of azobisisobutyronitrile to a pressure polymerization reactor equipped with a jacket, stirrer, raw material inlet, temperature and pressure gauge, and reaction product outlet. Raise the temperature to 50°C, maintain the ethylene pressure at 3.3 MPaG, and react for 4.3 hours with stirring.

[0112] (2) Add 6 parts of methanol to the product obtained in step (1) (keep the temperature and pressure constant for 1 hour), introduce 8 ppm of oxygen (based on the weight of the product obtained in step (1), temperature 40°C, pressure 3.3 MPa), react for 80 minutes under stirring, add 500 ppm of copper acetate (based on the amount of vinyl acetate used) (time 1 hour, temperature 15°C), stir evenly, release pressure and flash evaporate to remove unreacted ethylene, pressure at atmospheric pressure, then introduce methanol vapor (time 0.5 hours, temperature 80°C, pressure 180 kPa), distill to remove unreacted vinyl acetate, and obtain ethylene-vinyl acetate copolymer solution;

[0113] (3) Adjust the ethylene-vinyl acetate copolymer solution to a mass fraction of 30%, and then add a methanol solution of sodium hydroxide (concentration of 5wt%) at 55°C for alcoholysis (time of 6h, pressure of atmospheric pressure). The molar ratio of hydroxide ions in sodium hydroxide to vinyl acetate groups in ethylene-vinyl acetate copolymer is 0.09:1. After alcoholysis, an ethylene-vinyl alcohol copolymer solution is obtained.

[0114] (4) Add an acetic acid solution (10 wt%) to the ethylene-vinyl alcohol copolymer solution and stir until homogeneous. The acetic acid solution contains H... +The dosage is 600 ppm relative to the weight of the ethylene-vinyl alcohol copolymer solids;

[0115] (5) The ethylene-vinyl alcohol copolymer solution obtained in step (4) is extruded into an aqueous solution at 5°C through an extrusion device with a perforated plate. After precipitating into strips, the strips are cut into particles using a common cutting method. Subsequently, the ethylene-vinyl alcohol copolymer particles are washed with water in a kettle with a stirring device for 2 hours each time, and the washing is repeated twice. Then, a mixed solution of water and alcohol is added to the kettle with a stirring device, and the mixture is washed at 55°C for 2 hours. After washing with water once more, acetic acid with a mass of 0.01 parts relative to the ethylene-vinyl alcohol copolymer (based on a mass of 100 parts) is added during the water washing process. The mixture is then acid washed and centrifuged to dehydrate. After drying, the ethylene-vinyl alcohol copolymer particles are obtained. The parameters of the obtained ethylene-vinyl alcohol copolymer are shown in Table 1.

[0116] Example 4

[0117] The method described in Example 1 differs in that, in step (2), 20 ppm of oxygen is introduced based on the weight of the product obtained in step (1).

[0118] The parameters of the obtained ethylene-vinyl alcohol copolymer are shown in Table 1.

[0119] Example 5

[0120] The method described in Example 1 differs in that, in step (2), 5 ppm of oxygen is introduced based on the weight of the product obtained in step (1).

[0121] The parameters of the obtained ethylene-vinyl alcohol copolymer are shown in Table 1.

[0122] Example 6

[0123] The method described in Example 1 differs in that, in step (2), 3 ppm of oxygen is introduced based on the weight of the product obtained in step (1).

[0124] The parameters of the obtained ethylene-vinyl alcohol copolymer are shown in Table 1.

[0125] Example 7

[0126] The method described in Example 1 is different except that in step (2), the reaction time under stirring is increased from 60 min to 350 min.

[0127] The parameters of the obtained ethylene-vinyl alcohol copolymer are shown in Table 1.

[0128] Example 8

[0129] The method described in Example 1 is different except that 12 parts of methanol are not added in step (2).

[0130] The parameters of the obtained ethylene-vinyl alcohol copolymer are shown in Table 1.

[0131] Example 9

[0132] The method is the same as in Example 1, except that in step (2), the terminating agent is replaced by an equal amount of copper acetate instead of 2,4-diphenyl-4-methyl-1-pentene.

[0133] The parameters of the obtained ethylene-vinyl alcohol copolymer are shown in Table 1.

[0134] Example 10

[0135] The method is the same as in Example 1, except that in step (2), 30 parts of methanol are added and 3 ppm of oxygen is introduced.

[0136] The parameters of the obtained ethylene-vinyl alcohol copolymer are shown in Table 1.

[0137] Example 11

[0138] The method is the same as in Example 1, except that in step (2), 30 ppm of oxygen is introduced; and the reaction time is 300 min under stirring.

[0139] The parameters of the obtained ethylene-vinyl alcohol copolymer are shown in Table 1.

[0140] Comparative Example 1

[0141] The method is the same as in Example 1, except that oxygen is not introduced in step (2).

[0142] The parameters of the obtained ethylene-vinyl alcohol copolymer are shown in Table 1.

[0143] Comparative Example 2

[0144] The method is the same as in Example 1, except that in step (2), 50 ppm of oxygen is introduced.

[0145] The parameters of the obtained ethylene-vinyl alcohol copolymer are shown in Table 1.

[0146] Comparative Example 3

[0147] The method is the same as in Example 1, except that no terminator is added in step (2).

[0148] The parameters of the obtained ethylene-vinyl alcohol copolymer are shown in Table 1.

[0149] Test case

[0150] The ethylene-vinyl alcohol copolymers prepared in the examples and comparative examples were used to prepare ethylene-vinyl alcohol cast films. Crystal point defects in the cast films were detected, and the results are shown in Table 1. A casting film machine (ME20-MFA-FSA100V2) and a gel detection system were used for testing. The screw was a single screw with an L / D ≥ 25, and the screw temperature was set in three zones: 190 / 220 / 220℃. The screw speed was 60 rpm, and the stretching speed was 3 m / s. The cast film was obtained with a width of 15 mm and a thickness of 20 μm. A line scan camera and an LED light source were used to test for crystal point defects in the cast films.

[0151] Table 1

[0152]

[0153]

[0154] As can be seen from the results in Table 1, the ethylene-vinyl alcohol copolymer prepared by the method provided in this invention has good melt processing stability, and the ethylene-vinyl alcohol cast film prepared from the copolymer has fewer crystal point defects.

[0155] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing ethylene-vinyl alcohol copolymer, characterized in that, The method includes: (1) The first solvent, initiator, vinyl acetate and ethylene are brought into first contact to undergo a polymerization reaction; (2) The product obtained in step (1) is subjected to a second contact with oxygen, and then to a third contact with a terminating agent to obtain an ethylene-vinyl acetate copolymer solution. (3) The ethylene-vinyl acetate copolymer solution is subjected to alcoholysis to obtain ethylene-vinyl alcohol copolymer; The amount of oxygen used is 3-30 ppm, based on the weight of the product obtained in step (1).

2. The method according to claim 1, wherein, Based on the weight of the product obtained in step (1), the amount of oxygen used is 5-20 ppm, preferably 8-15 ppm; Preferably, the conditions for the second contact include: a temperature of 10-60°C, more preferably 20-40°C; and a time of 30-300 min, more preferably 60-200 min.

3. The method according to claim 1 or 2, wherein, Based on the weight of the vinyl acetate, the amount of the initiator is 30-3000 ppm; Preferably, the amount of the first solvent is 5-50% by weight, based on the weight of the vinyl acetate. Preferably, the amount of ethylene used results in a polymerization pressure of 2-7 MPaG; Preferably, the conditions for the polymerization reaction include: a temperature of 40-70°C and a time of 2-9 hours.

4. The method according to claim 1, wherein, The method in step (2) further includes: making a fourth contact between the product obtained in step (1) and a second solvent before contacting oxygen; Preferably, based on the weight of the vinyl acetate, the amount of the second solvent is 3-30% by weight, preferably 5-20% by weight; Preferably, the conditions for the fourth contact include: a temperature of 40-70°C and a time of 1-5 hours; Preferably, the first solvent and the second solvent are each an alcohol solvent, preferably an alcohol solvent with 1-4 carbon atoms, and more preferably at least one of methanol, ethanol, propanol, ethylene glycol, n-butanol and tert-butanol; Preferably, the amount of the terminating agent is 100-5000 ppm of the amount of vinyl acetate, and more preferably 300-3000 ppm. Preferably, the terminating agent is selected from at least one of 2,4-diphenyl-4-methyl-1-pentene, 2,2-diphenyl-1-trinitrophenylhydrazine, 1,1-diphenyl-2-trinitrophenylhydrazine, 2,4-diphenyl-4-methyl-1-pentene, copper acetate, cuprous chloride, and ferric chloride; Preferably, the conditions for the third contact include: a temperature of 10-40°C and a time of 0.1-2 hours.

5. The method according to claim 1, wherein, The method in step (2) further includes: after the third contact, sequentially removing ethylene and vinyl acetate from the product; Preferably, the method for removing ethylene is vacuum flash evaporation; Preferably, the method for removing vinyl acetate includes: a fifth contact with a third solvent after the third contact; Preferably, the conditions for the fifth contact include: a temperature of 65-80℃, a pressure of 100-180kPa, and a time of 0.5-3h; Preferably, the third solvent is an alcohol solvent, more preferably an alcohol solvent with 1-4 carbon atoms, more preferably at least one of methanol, ethanol, propanol, ethylene glycol, n-butanol and tert-butanol, and even more preferably methanol.

6. The method according to claim 1, wherein, The alcoholysis method includes contacting the ethylene-vinyl acetate copolymer solution with an alkali. Preferably, the molar ratio of hydroxide ions in the alkali to vinyl acetate groups in the ethylene-vinyl acetate copolymer is 0.005-0.5:1, more preferably 0.01-0.3:1; Preferably, the content of ethylene-vinyl acetate copolymer in the ethylene-vinyl acetate copolymer solution is 20-60% by weight; Preferably, the conditions for contact with the alkali include: a temperature of 50-65°C and a time of 4-10 hours; Preferably, the alkali is sodium hydroxide and / or potassium hydroxide.

7. An ethylene-vinyl alcohol copolymer prepared by the method according to any one of claims 1-6.

8. The ethylene-vinyl alcohol copolymer according to claim 7, wherein, The copolymer contains 20-50 mol% structural units derived from ethylene, preferably 25-45 mol%. Preferably, the weight-average molecular weight of the copolymer is 40,000-150,000 g / mol; Preferably, the molecular weight distribution index of the copolymer is 1.5-3.5; Preferably, the intrinsic viscosity of the copolymer at 30°C is 0.5-1.5 dl / g; Preferably, the copolymer has a melt index of 2-16 g / 10 min at 210°C and a load of 2.16 kg.

9. The application of the ethylene-vinyl alcohol copolymer of claim 7 or 8 in at least one of packaging materials, automotive fuel tanks, oxygen-barrier floor heating pipes, textile materials, and medical materials.

10. An ethylene-vinyl alcohol cast film, characterized in that, The cast film is prepared from the ethylene-vinyl alcohol copolymer according to claim 7 or 8; Preferably, the cast film has ≤9000 crystal points / m with a diameter less than 300 μm. 2 .

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