Ethylene-vinyl acetate copolymer solution as well as preparation method and application thereof

Abstract

The invention relates to the field of resin, and discloses an ethylene-vinyl acetate copolymer solution as well as a preparation method and application thereof. The method comprises the following steps: (1) in the presence of a first solvent and an initiator, polymerizing vinyl acetate and ethylene; and (2) sequentially mixing the product obtained in the step (1) with a second solvent and a terminator to obtain an ethylene-vinyl acetate copolymer solution. The ethylene-vinyl acetate alcohol solution prepared by the method provided by the invention has good viscosity stability, and the prepared ethylene-vinyl alcohol copolymer has good thermal stability, few surface defects and long processing operation cycle.

Description

Technical Field

[0001] The present invention relates to the field of resins, and particularly to an ethylene-vinyl acetate copolymer solution, a preparation method thereof, and an application thereof. Background Art

[0002] Ethylene-vinyl alcohol copolymer (EVOH), especially ethylene-vinyl alcohol copolymer with an ethylene content of 20-50 mol%, has excellent gas barrier properties, approximately 10,000 times that of ordinary polyethylene. Coupled with its transparency, processability, solvent resistance, and antistatic properties, it is widely used in fields such as packaging materials, automotive fuel tanks, oxygen barrier floor heating pipes, textile materials, and medical materials. The synthesis of ethylene-vinyl alcohol copolymer is usually carried out by copolymerizing ethylene and vinyl acetate to obtain an ethylene-vinyl acetate copolymer, and then preparing an alcohol solution with a certain concentration and carrying out an alcoholysis reaction under the catalysis of an alkali to generate an ethylene-vinyl alcohol copolymer. The higher the concentration of the ethylene-vinyl acetate copolymer solution, the more favorable the alcoholysis reaction, the higher the alcoholysis efficiency, and the higher the degree of alcoholysis. However, when the concentration of the ethylene-vinyl acetate copolymer solution increases, the viscosity of the solution will increase sharply, and the fluidity will decrease, resulting in difficult material transportation. At the same time, it also limits the mixing effect of the alkali catalyst and the resin solution in the subsequent alcoholysis reaction, reducing the uniformity of the alcoholysis reaction. The present invention provides an ethylene-vinyl acetate copolymer solution with improved solution viscosity stability and a preparation method thereof. Summary of the Invention

[0003] The purpose of the present invention is to overcome the problem of poor viscosity stability of ethylene-vinyl acetate copolymer in the prior art, and provide an ethylene-vinyl acetate copolymer solution, a preparation method thereof, and an application thereof.

[0004] To achieve the above purpose, the first aspect of the present invention provides a method for preparing an ethylene-vinyl acetate copolymer solution, the method comprising:

[0005] (1) Polymerizing vinyl acetate and ethylene in the presence of a first solvent and an initiator;

[0006] (2) Mixing the product of step (1) with a second solvent and a terminator in sequence to obtain an ethylene-vinyl acetate copolymer solution.

[0007] The second aspect of the present invention provides an ethylene-vinyl acetate copolymer solution prepared by the above method.

[0008] The third aspect of the present invention provides an application of the above ethylene-vinyl acetate copolymer solution in the preparation of ethylene-vinyl alcohol.

[0009] Through the above technical solution, the present invention first mixes the second solvent and then mixes the terminator. On the one hand, the second solvent can be used for preliminary termination to prevent the self-polymerization of vinyl acetate. On the other hand, the product after mixing the second solvent is better mixed with the terminator, enabling the terminator to play a better role, so that the ethylene-vinyl acetate copolymer has a suitable and controllable weight-average molecular weight and molecular weight distribution, and thus has a stable viscosity.

[0010] Further, for the alcohol solution of the ethylene-vinyl acetate copolymer prepared by the method provided by the present invention at a temperature of 20 °C, the concentration and viscosity of the copolymer alcohol solution satisfy the following relationship: Y = ae bX , where a is 1.5 - 2.2 and b is 0.15 - 0.2. Here, Y is the viscosity of the ethylene-vinyl acetate copolymer alcohol solution, mPa·s; X is the concentration of the ethylene-vinyl acetate copolymer alcohol solution, wt%, X ≤ 70, and to ensure that the copolymer has a suitable weight-average molecular weight. At the same temperature, as the concentration increases, the change in viscosity is small, and the values of a and b are small, having good viscosity stability. Detailed implementation mode

[0011] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0012] The first aspect of the present invention provides a method for preparing a solution of an ethylene-vinyl acetate copolymer, the method comprising:

[0013] (1) Polymerize vinyl acetate and ethylene in the presence of a first solvent and an initiator;

[0014] (2) Sequentially mix with a second solvent and a terminator in the product of step (1) to obtain a solution of an ethylene-vinyl acetate copolymer.

[0015] In the present invention, through the above method, the viscosity stability of the ethylene-vinyl acetate copolymer solution is better.

[0016] In the present invention, first mixing the second solvent and then mixing the terminator can, on the one hand, use the second solvent for preliminary termination to prevent the self-polymerization of vinyl acetate, and on the other hand, the product after mixing the second solvent is better mixed with the terminator, enabling the terminator to play a better role, so that the ethylene-vinyl acetate copolymer has a suitable and controllable weight-average molecular weight and molecular weight distribution, and thus has a stable viscosity.

[0017] Preferably, based on the weight of the vinyl acetate, the amount of the initiator is 0.001% to 0.5% by weight, and can be 0.001% by weight, 0.005% by weight, 0.01% by weight, 0.015% by weight, 0.02% by weight, 0.025% by weight, 0.03% by weight, 0.035% by weight, 0.04% by weight, 0.045% by weight, 0.05% by weight, 0.1% by weight, 0.15% by weight, 0.2% by weight, 0.25% by weight, 0.3% by weight, 0.35% by weight, 0.4% by weight, 0.45% by weight, 0.5% by weight or any range formed by any two of the above values and the values within the range.

[0018] In the present invention, controlling the amount of the initiator within the above range can control the weight-average molecular weight of the copolymer and improve the reaction efficiency.

[0019] In the present invention, the type of the initiator is not limited as long as it can make vinyl acetate and ethylene polymerize better. The initiator is an azo initiator and / or a peroxide initiator. Preferably, the azo initiator is at least one of azobisisobutyronitrile, azobisisovaleronitrile, azoisobutyronitrile formamide, azodicyclohexylcarbonitrile and dimethyl azobisisobutyrate; preferably, the peroxide initiator is an organic peroxide and / or an inorganic peroxide; more preferably, the organic peroxide is at least one of benzoyl peroxide, tert-butyl peroxybenzoate, methyl ethyl ketone peroxide, diisobutyryl peroxide, tert-amyl peroxyneodecanoate, bis(4-tert-butylcyclohexyl) peroxydicarbonate, tert-amyl peroxy pivalate, tert-butyl peroxyacetate and dibutyl peroxydicarbonate; further preferably, the inorganic peroxide is at least one of hydrogen peroxide, ammonium persulfate and potassium persulfate.

[0020] In the present invention, the amount of the first solvent is not limited as long as it does not affect the polymerization of vinyl acetate and ethylene. Preferably, based on the weight of the vinyl acetate, the amount of the first solvent is 5% to 50% by weight, and can be 5% by weight, 10% by weight, 15% by weight, 20% by weight, 25% by weight, 30% by weight, 35% by weight, 40% by weight, 45% by weight, 50% by weight or any range formed by any two of the above values and the values within the range.

[0021] In the present invention, controlling the amount of the first solvent within the above range can ensure that the copolymer has an appropriate weight-average molecular weight and is easy to control the monomer conversion rate.

[0022] Preferably, the amount of the ethylene is such that the pressure of the polymerization system is 2 - 6 MPaG, and can be 2 MPaG, 3 MPaG, 4 MPaG, 5 MPaG, 6 MPaG or any range formed by any two of the above values and the values within the range.

[0023] In the present invention, when the amount of ethylene is controlled such that the polymerization pressure is within the above range, it is possible to control the content of structural units derived from ethylene in the ethylene-vinyl acetate copolymer, so that the copolymer has an appropriate content of ethylene structural units.

[0024] Preferably, the polymerization conditions include: the temperature is 40 - 70 °C, which can be 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, 65 °C, 70 °C or any range formed by any two of the above values and the values within the range, and the time is 2 - 9 h, which can be 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, 5 h, 5.5 h, 6 h, 6.5 h, 7 h, 7.5 h, 8 h, 8.5 h, 9 h or any range formed by any two of the above values and the values within the range.

[0025] In the present invention, controlling the polymerization conditions within the above range can ensure that the copolymer has an appropriate weight-average molecular weight and is easy to control the monomer conversion rate, while taking into account the reaction rate, and has the prospect of industrial production.

[0026] 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 can be the same or different.

[0027] In the present invention, the type of the alcohol solvent is not limited. The first solvent only needs to enable the smooth polymerization of ethylene and vinyl acetate, and the second solvent only needs to inhibit the self-polymerization of vinyl acetate to form polyvinyl acetate. The first solvent and the second solvent are each independently preferably an alcohol solvent having 1 - 4 carbon atoms, more preferably at least one of methanol, ethanol, propanol, ethylene glycol, n-butanol and tert-butanol. The first solvent is more preferably methanol, and the second solvent is more preferably ethanol.

[0028] Preferably, based on the weight of the vinyl acetate, the amount of the second solvent is 1 wt% - 30 wt%, which can be 1 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt% or any range formed by any two of the above values and the values within the range.

[0029] In the present invention, controlling the amount of the second solvent within the above range can reduce the reaction rate, inhibit the self-polymerization of vinyl acetate to form polyvinyl acetate, and keep the viscosity of the prepared ethylene-vinyl acetate copolymer solution stable.

[0030] Preferably, the dosage of the terminator is 100 - 5000 ppm of the dosage of vinyl acetate in step (1), and can be 100 ppm, 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 1100 ppm, 1200 ppm, 1300 ppm, 1400 ppm, 1500 ppm, 1700 ppm, 2000 ppm, 2200 ppm, 2500 ppm, 2800 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm or any value within the range formed by any two of the above values, and more preferably 300 - 3000 ppm.

[0031] In the present invention, controlling the dosage of the terminator within the above range can effectively reduce the content of polyvinyl acetate in the ethylene-vinyl acetate copolymer solution.

[0032] In the present invention, the type of the terminator is not limited, as long as it can prevent vinyl acetate from self-polymerizing, and is preferably 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.

[0033] Preferably, the method further includes: removing ethylene from the product of step (1) in step (2).

[0034] Preferably, the ethylene removal is carried out after mixing with the second solvent.

[0035] In the present invention, carrying out the ethylene removal step after mixing with the second solvent can reduce the reaction activity, and at the same time reduce the concentration and viscosity of the ethylene-vinyl acetate copolymer solution, which is beneficial to the flash evaporation and removal of dissolved ethylene in the solution.

[0036] Preferably, the way of removing ethylene is vacuum flash evaporation.

[0037] More preferably, the pressure when the product of step (1) is mixed with the second solvent is 2 - 6 MPaG, and can be 2 MPaG, 3 MPaG, 4 MPaG, 5 MPaG, 6 MPaG or any value within the range formed by any two of the above values, and the pressure when mixing the terminator is atmospheric pressure.

[0038] In the present invention, controlling the pressure when mixing the terminator within the above range can mix at atmospheric pressure without a high-pressure feeding pump, and the operation is convenient.

[0039] In the present invention, the removal of ethylene can be carried out before mixing the termination agent, and at this time the pressure during mixing the termination agent is atmospheric pressure, or it can be carried out after mixing the termination agent, and at this time the pressure during mixing the termination agent is 2 - 6 MPaG.

[0040] In the present invention, when the pressure for mixing the product of step (1) with the termination agent is atmospheric pressure, better viscosity stability can be obtained, and there is no need for a high-pressure feeding pump, which is convenient for operation.

[0041] Preferably, the conditions for mixing the product of step (1) with the second solvent include: the temperature is 40 - 70 °C, which can be 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, 65 °C, 70 °C or any range formed by any two of the above values and the values within the range, and the time is 1 - 5 h, which can be 1 h, 2 h, 3 h, 4 h, 5 h or any range formed by any two of the above values and the values within the range.

[0042] In the present invention, controlling the conditions for mixing the second solvent within the above range is consistent with the polymerization reaction conditions, without the need to adjust process parameters, bringing the effects of convenient operation and uniform mixing.

[0043] Preferably, the conditions for mixing the product of step (1) with the termination agent include: the temperature is 10 - 40 °C, which can be 10 °C, 15 °C, 20 °C, 25 °C, 30 °C, 35 °C, 40 °C or any range formed by any two of the above values and the values within the range, and the time is 0.1 - 2 h, which can be 0.1 h, 0.2 h, 0.4 h, 0.6 h, 0.8 h, 1 h, 1.2 h, 1.4 h, 1.6 h, 1.8 h, 2 h or any range formed by any two of the above values and the values within the range.

[0044] In the present invention, controlling the conditions for adding the termination agent within the above range has the advantages of simple operation, enabling the termination agent to be uniformly mixed with the ethylene-vinyl acetate copolymer solution, and ensuring that the termination agent can fully play its termination role.

[0045] In the present invention, in order to remove vinyl acetate, the method further includes: contacting with the vapor of the third solvent after adding the termination agent.

[0046] In the present invention, during the actual operation process, the vapor of the third solvent can be introduced in a countercurrent contact manner. For example, if the removal of vinyl acetate is carried out in a distillation column, the ethylene-vinyl acetate copolymer solution after adding the termination agent is introduced into the top of the distillation column, and the vapor of the third solvent is introduced into the bottom of the distillation column, and the unreacted vinyl acetate monomer is removed by distillation from the top of the column.

[0047] Preferably, the conditions for contacting with the third solvent vapor include: the temperature is 65 - 80°C, which can be 65°C, 68°C, 70°C, 75°C, 74°C, 76°C, 78°C, 80°C or any range formed by any two values above and the values within the range; the pressure is 100 - 180 kPa, which can be 100 kPa, 110 kPa, 120 kPa, 130 kPa, 140 kPa, 150 kPa, 160 kPa, 170 kPa, 180 kPa or any range formed by any two values above and the values within the range; the time is 0.5 - 3 h, which can be 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, 3 h or any range formed by any two values above and the values within the range.

[0048] In the present invention, controlling the conditions for contacting with the third solvent vapor within the above ranges can enable sufficient gas - liquid exchange between the solvent vapor and the ethylene - vinyl acetate copolymer solution, thereby removing unreacted vinyl acetate.

[0049] In the present invention, there is no limitation on the third solvent, as long as it can carry away vinyl acetate. Preferably, the third solvent is an alcohol solvent, 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 more preferably methanol.

[0050] In the present invention, the third solvent can be the same as or different from the first solvent and the second solvent.

[0051] The second aspect of the present invention provides an ethylene - vinyl acetate copolymer solution prepared by the above - mentioned method.

[0052] According to the present invention, the content of the structural unit derived from ethylene in the copolymer's structural units is 20 - 50 mol%, which can be 20 mol%, 25 mol%, 30 mol%, 35 mol%, 40 mol%, 45 mol%, 50 mol% or any range formed by any two values above and the values within the range, and the above content of the ethylene - derived structural unit is based on the total molar content of the copolymer.

[0053] According to the present invention, the weight - average molecular weight of the copolymer is 110,000 - 400,000 g / mol, which can be 110,000 g / mol, 150,000 g / mol, 200,000 g / mol, 250,000 g / mol, 300,000 g / mol, 350,000 g / mol, 400,000 g / mol or any range formed by any two values above and the values within the range; the molecular weight distribution is 2 - 3.5, which can be 1, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3, 3.1, 3.2, 3.3, 3.4, 3.5 or any range formed by any two values above and the values within the range.

[0054] In the present invention, the weight-average molecular weight of the copolymer is controlled within the above range, so that the ethylene-vinyl acetate solution has good viscosity stability and its alcoholysis product has good mechanical properties, thereby obtaining good application properties.

[0055] According to the present invention, the solvent of the copolymer solution is an alcohol solvent, preferably an alcohol solvent having 1 to 4 carbon atoms, more preferably at least one of methanol, ethanol, propanol, ethylene glycol, n-butanol and tert-butanol, and more preferably methanol. At a temperature of 20 °C, the concentration and viscosity of the copolymer alcohol solution satisfy the following relationship: Y = ae bX , where a is 1.5 - 2.2 and b is 0.15 - 0.2. Here, Y is the viscosity of the ethylene-vinyl acetate copolymer alcohol solution, mPa·s; X is the concentration of the ethylene-vinyl acetate copolymer alcohol solution, wt%, and X ≤ 70.

[0056] The third aspect of the present invention provides the application of the above-mentioned ethylene-vinyl acetate copolymer solution in the preparation of ethylene-vinyl alcohol.

[0057] In the present invention, when the ethylene-vinyl acetate copolymer solution is used to prepare an ethylene-vinyl alcohol copolymer, the obtained ethylene-vinyl acetate copolymer solution is mixed with a fourth solvent and a base for further alcoholysis to obtain a product containing an ethylene-vinyl alcohol copolymer. After neutralizing the product containing the ethylene-vinyl alcohol copolymer with an acid, it is cooled, washed and dried to obtain an ethylene-vinyl alcohol copolymer product. The fourth solvent is an alcohol solvent, preferably an alcohol solvent having 1 to 4 carbon atoms, more preferably at least one of methanol, ethanol, propanol, ethylene glycol, n-butanol and tert-butanol, and more preferably methanol. The fourth solvent may be the same as or different from the first solvent.

[0058] A more preferred embodiment of the present invention is as follows:

[0059] (1) In the presence of 5 - 50 parts by weight of a first solvent and 0.001 - 0.5 parts by weight of an initiator, 100 parts by weight of vinyl acetate is polymerized with ethylene (temperature is 40 - 70 °C, time is 2 - 9 h), and the amount of ethylene used is such that the pressure of the polymerization system is 2 - 6 MPaG;

[0060] (2) Keeping the pressure unchanged, 1 - 30 parts by weight of a second solvent is mixed in the product of step (1) (temperature is 40 - 70 °C, time is 1 - 5 h), ethylene is removed by vacuum flashing to make the pressure normal pressure, and then 100 - 1500 ppm of a terminator relative to the ethylene-vinyl acetate copolymer is mixed in the product (temperature is 10 - 40 °C, time is 0.1 - 2 h);

[0061] (3) Contact the product of step (2) with the third solvent vapor (temperature: 65 - 80 °C, pressure: 100 - 180 kPa, time: 0.5 - 3 h).

[0062] The present invention will be described in detail below through examples.

[0063] Example 1

[0064] (1) Add 100 parts by weight of vinyl acetate, 12 parts by weight of methanol, and 0.1 part of azobisisobutyronitrile into a pressure polymerization kettle equipped with a jacket, a stirrer, a raw material inlet, a temperature and pressure detector, and a reaction product discharge port. Raise the temperature to 60 °C, introduce ethylene, and maintain the pressure at 3.8 MPaG. React under stirring for 4 hours;

[0065] (2) Add 20 parts by weight of ethanol (while keeping the temperature and pressure unchanged for 3 h). After stirring evenly, remove ethylene by vacuum flashing. The pressure is normal pressure. Mix 500 ppm of 2,4 - diphenyl - 4 - methyl - 1 - pentene relative to vinyl acetate into the product (time: 0.5 h, temperature: 25 °C);

[0066] (3) Introduce methanol vapor (time: 2 h, temperature: 70 °C, pressure: 140 kPa), and distill to remove unreacted vinyl acetate.

[0067] The parameters of the obtained ethylene - vinyl acetate copolymer solution are shown in Tables 1 and 2.

[0068] Example 2

[0069] (1) Add 100 parts by weight of vinyl acetate, 5 parts by weight of methanol, and 0.1 part of azobisisobutyronitrile into a pressure polymerization kettle equipped with a jacket, a stirrer, a raw material inlet, a temperature and pressure detector, and a reaction product discharge port. Raise the temperature to 70 °C, introduce ethylene, and maintain the pressure at 5 MPaG. React under stirring for 2 hours;

[0070] (2) Add 5 parts by weight of ethanol (while keeping the temperature and pressure unchanged for 5 h). After stirring evenly, remove ethylene by vacuum flashing. The pressure is normal pressure. Mix 100 ppm of 2,4 - diphenyl - 4 - methyl - 1 - pentene relative to vinyl acetate into the product (time: 0.1 h, temperature: 40 °C);

[0071] (3) Introduce methanol vapor (time: 3 h, temperature: 65 °C, pressure: 100 kPa), and distill to remove unreacted vinyl acetate.

[0072] The parameters of the obtained ethylene - vinyl acetate copolymer solution are shown in Tables 1 and 2.

[0073] Example 3

[0074] (1) In a pressure polymerization kettle equipped with a jacket, a stirrer, a raw material inlet, a temperature and pressure detector, and a reaction product discharge port, add 100 parts by weight of vinyl acetate, 50 parts by weight of methanol, and 0.1 part of azobisisobutyronitrile. Raise the temperature to 40 °C, introduce ethylene, and maintain the pressure at 2 MPaG. React for 2 hours under stirring;

[0075] (2) Add 30 parts by weight of ethanol (while keeping the temperature and pressure unchanged for 1 h), stir evenly, and then flash distill under reduced pressure to remove ethylene. The pressure is normal pressure, and 1000 ppm of 2,2-diphenyl-1-trinitrophenylhydrazine relative to vinyl acetate is mixed in the product (for 1 h at a temperature of 10 °C);

[0076] (3) Introduce methanol vapor (for 0.5 h at a temperature of 80 °C and a pressure of 180 kPa), and distill to remove unreacted vinyl acetate.

[0077] The parameters of the obtained ethylene-vinyl acetate copolymer solution are shown in Tables 1 and 2.

[0078] Example 4

[0079] According to the method of Example 1, the difference is that in step (2), the terminator is changed to copper acetate.

[0080] The parameters of the obtained ethylene-vinyl acetate copolymer solution are shown in Tables 1 and 2.

[0081] Example 5

[0082] According to the method of Example 1, the difference is that in step (2), ethanol is changed to methanol.

[0083] The parameters of the obtained ethylene-vinyl acetate copolymer solution are shown in Tables 1 and 2.

[0084] Example 6

[0085] According to the method of Example 1, the difference is that in step (2), ethanol is changed to tert-butanol.

[0086] The parameters of the obtained ethylene-vinyl acetate copolymer solution are shown in Tables 1 and 2.

[0087] Example 7

[0088] According to the method of Example 1, the difference is that in step (2), first flash distill under reduced pressure to remove ethylene. After the pressure is normal pressure, add ethanol and 2,4-diphenyl-4-methyl-1-pentene.

[0089] The parameters of the obtained ethylene-vinyl acetate copolymer solution are shown in Tables 1 and 2.

[0090] Example 8

[0091] According to the method of Example 1, the difference is that in step (2), ethanol and 2,4-diphenyl-4-methyl-1-pentene are added first, and then ethylene is removed by vacuum flashing to make the pressure normal pressure.

[0092] The parameters of the obtained ethylene-vinyl acetate copolymer solution are shown in Tables 1 and 2.

[0093] Comparative Example 1

[0094] According to the method of Example 1, the difference is that in step (2), ethanol and the terminator are not added.

[0095] The parameters of the obtained ethylene-vinyl acetate copolymer solution are shown in Tables 1 and 2.

[0096] Comparative Example 2

[0097] According to the method of Example 1, the difference is that in step (2), ethanol is not added.

[0098] The parameters of the obtained ethylene-vinyl acetate copolymer solution are shown in Tables 1 and 2.

[0099] Comparative Example 3

[0100] According to the method of Example 1, the difference is that in step (2), the terminator is not added.

[0101] The parameters of the obtained ethylene-vinyl acetate copolymer solution are shown in Tables 1 and 2.

[0102] Comparative Example 4

[0103] According to the method of Example 1, the difference is that in step (1), 12 parts of methanol and 20 parts of ethanol are added together, and in step (2), ethanol is not added.

[0104] The parameters of the obtained ethylene-vinyl acetate copolymer solution are shown in Tables 1 and 2.

[0105] Comparative Example 5

[0106] According to the method of Example 1, the difference is that in step (2), the addition order of methanol and the terminator is exchanged.

[0107] The parameters of the obtained ethylene-vinyl acetate copolymer solution are shown in Tables 1 and 2.

[0108] Test Example:

[0109] Method for determining the content of ethylene structural units in ethylene-vinyl acetate: GBT 41877.2-2022;

[0110] Testing and calculation methods for weight-average molecular weight and molecular weight distribution: Measured by GPC method;

[0111] Testing method for viscosity: GB-T 2794-2022.

[0112] Table 1

[0113]

[0114] Note: The correlation coefficient R of the curves obtained by fitting the above examples and comparative examples 2 > 0.99.

[0115] From the above results, it can be seen that the ethylene-vinyl acetate copolymer solution prepared by the method of the present invention has good viscosity stability. While ensuring a sufficiently high weight-average molecular weight, at the same temperature and concentration, it has a lower viscosity, and at the same temperature, the values of a and b are smaller, having good viscosity stability.

[0116] 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 technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for preparing an ethylene-vinyl acetate copolymer solution, characterized in that, the method comprises: (1) Polymerizing vinyl acetate and ethylene in the presence of a first solvent and an initiator; (2) Mixing the product of step (1) with a second solvent and a terminator in sequence to obtain an ethylene-vinyl acetate copolymer solution.

2. The method according to claim 1, wherein, based on the weight of the vinyl acetate, the amount of the initiator is 0.001 wt%-0.5 wt%; and / or, based on the weight of the vinyl acetate, the amount of the first solvent is 5 wt%-50 wt%; and / or, the amount of the ethylene is such that the pressure of the polymerization system is 2-6 MPaG; and / or, the conditions of the polymerization include: temperature is 40-70 °C, time is 2-9 h; and / or, the first solvent and the second solvent are each independently an alcohol solvent, preferably an alcohol solvent having 1-4 carbon atoms, more preferably at least one of methanol, ethanol, propanol, ethylene glycol, n-butanol and tert-butanol, the first solvent is more preferably methanol, and the second solvent is more preferably ethanol.

3. The method according to claim 1, wherein, based on the weight of the vinyl acetate, the amount of the second solvent is 1 wt%-30 wt%; and / or, the amount of the terminator is 100-5000 ppm based on the amount of vinyl acetate used in step (1), preferably 300-3000 ppm; and / or, the terminator 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.

4. The method according to any one of claims 1-3, wherein, the method further comprises: removing ethylene from the product of step (1) in step (2); preferably, the method for removing ethylene is vacuum flashing; preferably, the ethylene is removed after mixing with the second solvent.

5. The method according to any one of claims 1-4, wherein, the conditions for mixing the product of step (1) with the second solvent include: temperature is 40-70 °C, time is 1-5 h; and / or, the conditions for mixing the product of step (1) with the terminator include: temperature is 10-40 °C, time is 0.1-2 h; and / or, the pressure for mixing the product of step (1) with the second solvent is 2-6 MPaG; and / or, the pressure for mixing the product of step (1) with the terminator is atmospheric pressure.

6. The method according to any one of claims 1-5, wherein, the method further comprises: contacting with the vapor of a third solvent after mixing with the terminator.

7. The method according to claim 6, wherein, the conditions for contacting with the vapor of the third solvent include: temperature is 65-80 °C, pressure is 100-180 kPa, time is 0.5-3 h; Preferably, the third solvent is an alcohol solvent, preferably an alcohol solvent having 1 to 4 carbon atoms, more preferably at least one of methanol, ethanol, propanol, ethylene glycol, n-butanol and tert-butanol, and still more preferably methanol.

8. An ethylene-vinyl acetate copolymer solution prepared by the method according to any one of claims 1 to 7.

9. The ethylene-vinyl acetate copolymer solution according to claim 8, wherein, the content of the structural unit derived from ethylene in the structural unit of the copolymer is 20-50 mol%; and / or, the weight average molecular weight of the copolymer in the solution is 110,000-400,000 g / mol, and the molecular weight distribution is 2-3.5; And / or, the solvent of the copolymer solution is an alcohol solvent. At a temperature of 20 °C, the concentration and viscosity of the copolymer alcohol solution satisfy the following relationship: Y = ae bX , where a is 1.5 - 2.2 and b is 0.15 - 0.

2. Here, Y is the viscosity of the ethylene-vinyl acetate copolymer alcohol solution, mPa·s; X is the concentration of the ethylene-vinyl acetate copolymer alcohol solution, wt%, and X ≤ 70.

10. Use of the ethylene-vinyl acetate copolymer solution according to claim 8 or 9 in the preparation of ethylene-vinyl alcohol copolymer.

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