Ethylene-vinyl alcohol copolymer resin and preparation method thereof

Abstract

The invention provides a preparation method of ethylene-vinyl alcohol copolymer resin capable of simultaneously improving bending resistance (flexibility), secondary processability and secondary processing yellowing resistance of a product, which comprises the following steps: taking a solvent, a vinyl acetate monomer, an initiator and an ethylene monomer as raw materials; the ethylene-vinyl alcohol copolymer is prepared through the steps of initial feeding, polymerization reaction, residual monomer blowing-out and alcoholysis. The initial charging stage comprises the following steps: simultaneously adding a vinyl acetate monomer and a solvent, and then respectively introducing nitrogen and ethylene to enable the pressure in a kettle to be 5-30bar and the temperature to be 55-64 DEG C for replacement for multiple times; and adding an initiator after the replacement is completed, filling ethylene to enable the pressure in the polymerization kettle to be 25-50 bar, mixing for 10-20 minutes at normal temperature, and then heating to start polymerization reaction. The ethylene-vinyl alcohol copolymer product disclosed by the invention is excellent in bending resistance (flexibility), better in secondary processing yellowing resistance and fewer in crystal points, and can better meet higher requirements of industrial production and specific fields on the ethylene-vinyl alcohol copolymer product.

Description

Technical Field

[0001] The invention relates to an ethylene-vinyl alcohol copolymer, in particular to an ethylene-vinyl alcohol copolymer resin and a preparation method thereof. Background Art

[0002] Ethylene-vinyl alcohol copolymer resin (EVOH for short) has excellent gas barrier properties compared to general polymer materials. Its transparency, processability, solvent resistance and antistatic properties make it widely used in packaging materials, automobile fuel tanks, oxygen-barrier floor heating pipes, textile materials and medical materials. The preparation of ethylene-vinyl alcohol copolymers is difficult, and it is difficult to achieve industrialization. However, it is an urgent need for ethylene-vinyl alcohol copolymers to have good flexibility, bending resistance, secondary processability and light transmittance in various application scenarios, especially in automobile fuel tanks, oxygen-barrier floor heating pipes and other fields.

[0003] The preparation process of ethylene-vinyl alcohol copolymer mainly includes two stages: polymerization reaction and alcoholysis reaction. In the polymerization reaction stage, vinyl acetate and ethylene monomers are used as reaction raw materials, and a polymerization reaction is carried out in the presence of an initiator and a solvent (for example, through emulsion polymerization, solution polymerization or suspension polymerization) to obtain a polyethylene-vinyl acetate copolymer solution; the intermediate product is used as a reactant in the second stage alcoholysis reaction stage, and an alcoholysis reaction (also called saponification) is carried out by adding an alkaline solution to obtain an ethylene-vinyl alcohol copolymer; the obtained ethylene-vinyl alcohol copolymer can be further formed into packaging materials such as films, barrier materials, etc. through an extrusion process for application.

[0004] Due to the different pre-polymerization processes, the composition and structure of the final product are also different, which affects the final performance of the product, including gas barrier properties, mechanical strength, elasticity, wear resistance, cold resistance, transparency, gloss, surface strength, processability and other performance indicators. In the existing technology, optimization is achieved by controlling the raw material ratio of ethylene monomer and vinyl acetate monomer, selecting suitable solvents and initiators, selecting suitable alkaline solutions, and adjusting and optimizing various parameters (time, temperature, pressure, feeding sequence, feeding amount, etc.) during the polymerization reaction and alcoholysis reaction to ensure a certain product conversion rate while meeting specific performance requirements.

[0005] Chinese patent document CN106146719A discloses a method for preparing EVOH, which is characterized in that: 10 to 60 parts of solvent, 40 to 90 parts of vinyl acetate monomer, 0.002 to 1 part of initiator and ethylene monomer are used as raw materials, and the preparation is carried out through the above-mentioned raw material deoxygenation, in-kettle replacement, initial charging, polymerization reaction, blowing out residual monomer and saponification steps respectively; the solvent is a mixed solvent composed of more than two alcohol solvents with carbon atoms of 1 to 4, the polymerization reaction is to introduce ethylene monomer, control the pressure to 10 to 60 bar, start the timing reaction when the temperature in the kettle is 40 to 80°C, and close the ethylene inlet valve after the reaction is completed after 2.5 to 8.0 hours, reduce the temperature to below 35°C, and release the pressure to 5.0 to 8.0 bar to obtain the polymerization reaction product. The preparation method has high polymerization reaction efficiency, a polymerization reaction conversion rate of more than 60%, which can be as high as 85%, and a controllable reaction rate; the obtained EVOH product has an alcoholysis degree of 94.0-99.9% (mol), a melt index of 3.0-10.0 (g / 10min), and good quality; the chromaticity is 1.0-3.0 degrees, the transparency is good, and it is crystal clear, with high barrier properties and an oxygen permeability of 1.0-1.2cm 3 / (m 2 ·24h·0.1Mpa), water vapor permeability is 3.0~3.2g / (m 2 24h), good processing performance.

[0006] However, this patented technology mainly optimizes the polymerization reaction efficiency and conversion rate. By adding a deoxygenation step before the in-vessel replacement before the polymerization reaction, the inhibitory effect of dissolved oxygen on the polymerization reaction is solved, thereby improving the polymerization reaction efficiency and conversion rate. However, no targeted optimization was performed on the specific polymerization reaction conditions and alcoholysis reaction conditions, and they can be implemented within the conventional acceptable parameter range; no specific optimization was performed on the product performance, and it was shown that the reaction conversion rate was improved, and the obtained product met the requirements of conventional products. It can be seen that this technology has not made corresponding optimization and control for the specific performance requirements of these products.

[0007] Chinese patent document CN102942649A discloses a method for preparing ethylene vinyl alcohol copolymer, the steps are: dissolving vinyl acetate and oil-soluble initiator into monohydric alcohol with 1 to 5 carbon atoms, then introducing ethylene gas to keep it under a reaction pressure of 5 to 50 atmospheres, stirring and heating to 45 to 75°C, stirring at a speed of 25 to 500 rpm, maintaining the temperature for reaction for 0.5 to 10 hours, and obtaining EVA solution; then adding alkali solution with a mass concentration of 1 to 40%, stirring and heating to 50 to 85°C, maintaining the temperature for reaction for 0.5 to 12 hours, cooling to room temperature, washing with water, and drying at 30 to 200°C to obtain EVOH. The present invention can obtain the contents of three kinds of vinyl acetate and ethylene sequence structures, thereby obtaining EVOH resins for different purposes.

[0008] However, the patent technology mainly focuses on controlling the ethylene content of the product and the corresponding parameters such as molecular weight and melt index through parameter adjustment, and does not make corresponding optimization and control for the specific performance requirements of these products. It is clearly mentioned that the resin is easier to process when the ethylene content is high, but the gas barrier performance decreases; on the contrary, the resin is not easy to process when the ethylene content is low, but the gas barrier performance of the polymer is improved. This performance contradiction means that it is impossible to obtain a product that is both easy to process and has improved gas barrier performance.

[0009] Chinese patent document CN113924322A discloses a method for producing an ethylene-vinyl alcohol copolymer, which comprises the following steps: a copolymerization step of copolymerizing ethylene and vinyl ester in a solution using an alkoxy-containing azonitrile compound as a polymerization initiator to obtain an ethylene-vinyl ester copolymer solution; and a saponification step of saponifying the ethylene-vinyl ester copolymer in the solution in the presence of an alkali catalyst to obtain an ethylene-vinyl alcohol copolymer, wherein in the saponification step, 1.2 mol or more of the alkali catalyst is used relative to 1 mol of the vinyl ester component contained in the ethylene-vinyl ester copolymer, and the solution is irradiated with ultrasonic waves or microwaves.

[0010] However, the patent technology mainly focuses on improving the long-term stability of EVOH's melt viscosity, providing EVOH with excellent viscosity stability at the beginning of melting, which can stabilize the melt forming process and has excellent color resistance under high temperature (e.g. 80°C) and alkaline conditions. The invention contribution claimed by the inventor of this patent is that the problem is that the rapid increase in viscosity at the beginning of melting is obviously related to the terminal structure of the molecules constituting EVOH derived from the polymerization initiator, thereby providing EVOH containing molecules with specific terminal structures. It can be seen that the other specific performance requirements of these products have not been optimized and controlled accordingly.

[0011] It can be seen from the above existing technologies that the basic process for preparing ethylene-vinyl alcohol copolymers is mature and fixed. However, due to the different requirements for specific performance indicators of ethylene-vinyl alcohol copolymer products in different fields, there is still a need for ethylene-vinyl alcohol copolymer preparation processes that meet specific needs, especially ethylene-vinyl alcohol copolymer products that can be used in the fields of automobile fuel tanks, oxygen-barrier floor heating pipes, etc., while improving flexibility, bending resistance, secondary processability and secondary processing anti-yellowing ability. Summary of the invention

[0012] In order to solve the above technical problems, the present invention provides a method for preparing an ethylene-vinyl alcohol copolymer resin that simultaneously improves the product's anti-bending property (flexibility), secondary processing property, and secondary processing anti-yellowing ability. The ethylene-vinyl alcohol copolymer is prepared by using a solvent, vinyl acetate monomer, an initiator, and ethylene monomer as raw materials through the steps of initial feeding, polymerization reaction, blowing out residual monomers, and alcoholysis; the characteristic is that: the initial feeding stage includes introducing nitrogen and ethylene respectively to make the pressure in the kettle at 5-30 bar and the temperature at 55-64°C for multiple replacements; after the replacement is completed, an initiator is added, ethylene is charged to make the pressure in the polymerization kettle reach 25-50 bar, and the temperature is raised after mixing at room temperature for 10-20 minutes to start the polymerization reaction. The ethylene-vinyl alcohol copolymer product of the present invention has a low yellowness index, better secondary processing anti-yellowing ability, and fewer crystal points, and can better meet the higher demand for ethylene-vinyl alcohol copolymer products in industrial production and specific fields.

[0013] Preferably, the polymerization step maintains an ethylene pressure between 30-45 bar.

[0014] Furthermore, the solvent used in the polymerization step and the alcoholysis step of the present invention is the same, and the same solvent is used to wash the product obtained in the alcoholysis step. It is particularly preferred that the solvent is a monohydric alcohol or dihydric alcohol with 1 to 5 carbon atoms and a mixture thereof. These alcohols are most beneficial for dissolving the relevant reaction raw materials and ensuring the smooth progress of the reaction system.

[0015] Preferably, the initiator of the present invention is an azo initiator, which can better ensure the smooth progress of the polymerization reaction and help improve the performance indicators of the final product.

[0016] Particularly preferably, the preparation method of the present invention specifically comprises the steps of: adding 40-90 parts of vinyl acetate and 10-60 parts of methanol into a reactor, and then introducing nitrogen and ethylene respectively, so that the pressure in the reactor is 5-30 bar and the temperature is 55-64°C for replacement 3 times; after the replacement is completed, adding 0.001-1 parts of azobisisobutyronitrile as an initiator, charging ethylene to make the pressure in the polymerization reactor reach 25-50 bar, and then starting stirring to dissolve ethylene and then start heating; when the solution temperature reaches the polymerization temperature of 50-70°C, performing a polymerization reaction for 5-7h, and maintaining the temperature in the reactor at 50-70°C and the pressure at 25-50°C during the polymerization process. bar remains unchanged; after the polymerization is completed, the polymerization liquid is cooled to below 35°C and then the pressure is released to remove the unreacted ethylene gas to obtain a polyethylene-vinyl acetate copolymer methanol solution; by heating, the unreacted vinyl acetate monomer in the polymerization liquid is blown out cleanly, and methanol is added to keep the polymerization liquid concentration at 10-20%wt; the above copolymer methanol solution is kept at 60-70°C for 10-20min, and then a sodium hydroxide methanol solution with a concentration of 30-50g / L is slowly added for alcoholysis, the alcoholysis temperature is 60-70°C, and the alcoholysis time is 6-8h; after the alcoholysis product is centrifuged, it is washed with methanol for many times, the amount of methanol is 3-5 times that of the solid, the washing temperature is 30-45°C, and the washing time is 40-50min each time; the washed solid is placed in an oven at 90-100°C for 6-10h to obtain a dry ethylene-vinyl alcohol copolymer resin. This preferred scheme is the most preferred range for all step parameters, and can particularly reflect the improvement of the performance parameters of the obtained product.

[0017] The present invention also provides an ethylene-vinyl alcohol copolymer resin prepared according to the preparation method.

[0018] The present invention has the following beneficial effects: In the existing preparation process of ethylene-vinyl alcohol copolymer, it usually includes the steps of initial feeding, polymerization reaction, blowing out residual monomers and alcoholysis. In the initial feeding process, ethyl acetate monomer and methanol solvent are first added to the kettle for mixing, and then nitrogen and ethylene are introduced respectively for displacement; after the displacement is completed, an initiator is added to carry out polymerization reaction. That is to say, before the polymerization reaction, generally only nitrogen or ethylene gas is used for displacement to remove oxygen that may affect the polymerization, and the reaction raw materials, especially the monomers (vinyl acetate monomer and ethylene monomer), are heated immediately after addition to start the polymerization reaction.

[0019] The inventor team found that by introducing nitrogen and ethylene gas respectively in the initial feeding stage, and making the pressure in the reactor at 5-30 bar and the temperature at 55-64°C for multiple replacements; after the replacement is completed, adding initiator, charging ethylene to make the pressure in the polymerization reactor reach 25-50 bar, and mixing at room temperature for 10-20 minutes before heating to start the polymerization reaction. Adding the reaction raw materials in advance and premixing for a period of time before starting the polymerization reaction can help improve the various performance indicators of the final product.

[0020] Specifically, the final product of the ethylene-vinyl alcohol copolymer obtained by the preparation method of the present invention can be improved simultaneously in terms of performance indicators such as bending resistance (flexibility), secondary processability and secondary processing anti-yellowing ability. DETAILED DESCRIPTION

[0021] The following is further described in detail through specific implementation methods. However, it should be pointed out that the following embodiments of the present invention are only for better illustrating the content of the present invention, but do not mean that the content of the present invention is limited to the examples. Therefore, those skilled in the art can make non-essential improvements and adjustments to the implementation scheme according to the above invention content, which still belongs to the protection scope of the present invention and is subject to the protection scope of the attached claims.

[0022] The ethylene-vinyl alcohol copolymer of the present invention is formed by copolymerization of ethylene units and vinyl alcohol units, and generally includes 20-40% of ethylene units and 60-80% of vinyl alcohol units. However, since vinyl alcohol cannot exist independently in the form of a monomer, the ethylene-vinyl acetate copolymer is first obtained by copolymerizing ethylene monomer and vinyl acetate monomer as an intermediate, and then saponifying and alcoholyzing the intermediate to obtain the ethylene-vinyl alcohol copolymer.

[0023] For ethylene-vinyl acetate copolymer, it is usually divided into three forms: resin, rubber and emulsion according to the different vinyl acetate contents: products with vinyl acetate content less than 40% are resins, products with vinyl acetate content of 40%-70% are rubbers, and products with vinyl acetate content in the range of 70%-95% are emulsions. Ethylene-vinyl alcohol copolymer is usually used in the form of resin, also known as EVOH resin, so the intermediate ethylene-vinyl acetate copolymer described in the present invention is usually in the form of resin. Therefore, the terms ethylene-vinyl alcohol copolymer, ethylene-vinyl alcohol copolymer resin, EVOH, EVOH resin, etc. in the present invention have the same meaning and can be used interchangeably.

[0024] The existing preparation process of the ethylene-vinyl alcohol copolymer described in the present invention generally includes the steps of initial feeding, polymerization reaction, blowing out residual monomers and saponification.

[0025] Usually, an inert gas (such as nitrogen) is introduced into the reactor to remove the oxygen in the reactor. The monomer gas ethylene gas of the present invention can also be used simultaneously or alone to remove the oxygen. Ethylene gas is one of the reaction raw materials and its residue in the reactor will not affect the reaction.

[0026] Since the raw materials are in different forms such as solid, liquid and gas, the order of feeding is usually to add vinyl acetate monomer into liquid solvent (such as methanol), then introduce gaseous monomer ethylene, and then add initiator, and after heating to the polymerization reaction temperature (such as 60°C), the polymerization reaction can be started. The feeding order known in the prior art can be to add the above raw materials in appropriate proportions simultaneously or successively. There are also studies that try to add a certain reaction raw material in several stages to obtain products with different properties, but the polymerization reaction must be started quickly after the monomer is added after the initial addition.

[0027] The inventor team found that after adding vinyl acetate monomer and solvent in the initial feeding stage, nitrogen and ethylene gas were introduced respectively, and the pressure in the reactor was kept at 5-30 bar and the temperature was kept at 55-64°C for multiple replacements; after the replacement was completed, the initiator was added, ethylene was filled to make the pressure in the polymerization reactor reach 25-50 bar, and the temperature was raised after mixing at room temperature for 10-20 minutes to start the polymerization reaction. The ethylene-vinyl alcohol copolymer product of the present invention has excellent bending resistance (flexibility), better anti-yellowing ability in secondary processing, fewer crystal points, and can better meet the higher demand for ethylene-vinyl alcohol copolymer products in industrial production and specific fields.

[0028] Since no initiator is added during the initial feeding process, the polymerization reaction cannot be directly initiated. However, the displacement process, especially the displacement and premixing at a specific pressure and temperature, can have a significant impact on the performance indicators of the final product.

[0029] The solvent described in the present invention is well known to those skilled in the art, especially monohydric alcohol or dihydric alcohol with 1 to 5 carbon atoms and mixtures thereof, such as one or more mixed alcohols of methanol, ethanol, propanol, n-butanol, isobutanol, tert-butanol, n-pentanol, isopentanol, sec-pentanol, ethylene glycol, propylene glycol, etc. In the polymerization reaction stage, the amount of the solvent used is 1-100% relative to the weight of the vinyl acetate monomer, preferably 10-30%, more preferably 15-25%, such as 10%, 15%, 20%, 25%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, etc., and other proportions can also be selected according to actual needs.

[0030] The initiator described in the present invention, also known as a free radical initiator, refers to a class of compounds that are easily decomposed into free radicals by heat, which can be used to initiate free radical polymerization and copolymerization reactions of olefin and diene monomers, and is essential in ethylene-vinyl acetate copolymerization reactions. According to the molecular structure of the initiator, the initiator can be divided into azo, peroxide and redox initiators; it can also be divided into water-soluble initiators (soluble in aqueous solvents) and oil-soluble (soluble in organic solvents) according to its solubility; it can also be divided into high temperature (above 100°C) initiators, medium temperature (40-100°C) initiators and low temperature (0-40°C) initiators according to the use temperature range of the initiator. Since the solvent in the present invention is methanol and the polymerization reaction temperature is 60°C, oil-soluble azo initiators (belonging to medium temperature initiators) are preferred, and such initiators include azobisisobutyronitrile (AIBN), azobisisoheptanenitrile (ABVN), etc. Of course, the polymerization reaction of the present invention can also select oil-soluble peroxide initiators, such as dibenzoyl peroxide, cumene peroxide, tert-butyl benzoyl peroxide, methyl ethyl ketone peroxide, etc. Those skilled in the art can select any suitable initiator according to the system and conditions of the polymerization reaction. The amount of the initiator can be 0.005-5% of the weight of the vinyl acetate monomer, preferably 0.01-1%, more preferably 0.05-0.1%, for example, 0.005%, 0.01%, 0.05%, 0.1%, 1%, 5% can be selected, and those skilled in the art can select a suitable initiator and its amount according to the specific circumstances of the reaction.

[0031] The specific conditions for the polymerization of the ethylene-vinyl acetate copolymer of the present invention are well known to those skilled in the art. The polymerization temperature is 40-80°C, preferably 50-70°C, for example, 50°C, 55°C, 60°C, 65°C, etc. The polymerization time is 1-10 hours, preferably 5-7 hours, for example, 5 hours, 6 hours, 7 hours, etc. The ethylene gas pressure is maintained at 5-50 bar, preferably 10-30 bar, for example, 10 bar, 15 bar, 20 bar, 25 bar, etc. during the polymerization.

[0032] The alkali solution used in the alcoholysis or saponification stage of the present invention is any alkaline substance, such as sodium hydroxide, potassium hydroxide or its alkaline salt, including sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, sodium methoxide, sodium ethoxide, potassium methoxide, potassium ethoxide, etc., or a mixture thereof, dissolved in the above-mentioned solvent of the present invention for alcoholysis reaction. Its dosage is in the concentration range of 20-60 g / L relative to the total volume of the alcoholysis system, preferably 30-50 g / L, for example, 30 g / L, 40 g / L or 50 g / L of alkaline substance dosage can be selected. The alcoholysis temperature and time can refer to the temperature and time of the polymerization reaction stage, preferably the alcoholysis temperature is 60-70 ° C, and the alcoholysis time is 6-8h. Those skilled in the art can select the appropriate alcoholysis temperature and time according to specific needs (such as alcoholysis degree, etc.).

[0033] The ethylene-ethylene final product of the present invention is further subjected to separation, washing, purification and drying processes after alcoholysis. Separation can be carried out by conventional separation means such as centrifugation; washing can be carried out by alcohol washing or water washing, preferably by using the same solvent (such as methanol) used in the entire process of the present invention, the washing amount is 3-5 times that of the solid, the washing temperature is 30-45°C, and the washing time each time is 40-50 minutes; drying can be carried out by using conventional means such as a drying oven at about 100°C.

[0034] The processing performance test method of the ethylene-vinyl alcohol copolymer of the present invention is as follows: using a 15 mm diameter extruder (L / D=20, compression ratio=3) manufactured by Thermo Fisher, the dried ethylene-vinyl alcohol resin sample is cast into film, and the film making conditions are: extrusion temperature: C1 / C2 / C3 / C4 / C5 / die head=185 / 210 / 220 / 220 / 220 / 220; screw speed: 50rpm; extrusion volume: 1.2kg / h; film thickness: 30um. After about 2 hours of film stabilization, the film is rolled up. The film is manually cut into 120mm*120mm samples, and the number of film crystal points (diameter greater than 0.15mm) is tested using a defect meter (MVT-SIS, Wuxi Dongfuda Technology Co., Ltd.). At the same time, a colorimeter (ultrascan VIS, HunterLab, USA) is used to test the yellowness index of the sample.

[0035] The light transmittance test method of the ethylene-vinyl alcohol copolymer film of the present invention is as follows: a film sample (30 um) is prepared by a casting method, and the light transmittance of the film sample is tested by a transparency / haze meter (WGT-S, Shanghai Shenguang Instrument Co., Ltd.).

[0036] The testing method for the bending resistance of the ethylene-vinyl alcohol copolymer of the present invention is as follows: the dried ethylene-vinyl alcohol resin is prepared into a dumbbell-shaped specimen (25 mm wide at both ends, 10 mm wide in the middle, 150 mm long, and 0.3 mm thick) using an injection molding machine. The number of folding times of the specimen is tested using a folding tester, with a folding angle of 90° and a folding speed of 150 times / min. The folding resistance and flexibility of the specimen are graded according to the number of folding times, and the specific grading standards are shown in Table 1 below.

[0037] Table 1 Classification standard of bending resistance of ethylene-vinyl alcohol copolymer Example 1 87 parts of vinyl acetate and 13 parts of methanol were added to a 3L high-pressure reactor, and then nitrogen and ethylene were introduced respectively to make the pressure in the reactor 10bar and the temperature 55℃ for replacement (3 times). After the replacement was completed, 1 part of AIBN methanol solution (concentration of 5%) was added, and ethylene was charged to make the pressure in the polymerization reactor reach 35bar, and then stirring was started to dissolve the ethylene. After dissolving for 15 minutes, the temperature began to rise. When the solution temperature reached the polymerization temperature of 60℃, the reaction started timing, and the reaction time was 6h. During the polymerization process, the temperature in the reactor was kept constant at 60℃ and the pressure at 34bar. After the polymerization was completed, the polymerization liquid was cooled to 25℃, and then the pressure was released to remove the unreacted ethylene gas to obtain a polyethylene-vinyl acetate copolymer methanol solution. The polymerization liquid was added to a reactor with a cooling device, and the unreacted vinyl acetate monomer in the polymerization liquid was blown out cleanly. At the same time, methanol was added to keep the polymerization liquid concentration at about 15%wt, and finally a polyethylene-vinyl acetate copolymer methanol solution with low residual monomers was obtained, and the resin concentration was adjusted to about 15%wt.

[0038] The copolymer methanol solution is added to a 3L reaction bottle, placed in a 65℃ water bath for 15 minutes, and then slowly added with a 40g / L sodium hydroxide methanol solution for alcoholysis. The alcoholysis temperature is 65℃, and the alcoholysis time is 6-8h. During the alcoholysis process, the liquid level in the reaction bottle is kept constant by dripping. After centrifugation, the alcoholysis product is washed with methanol 5-6 times, with the amount of methanol being 3-5 times that of the solid, the washing temperature being 30-45℃, and the washing time being about 45 minutes each time. After washing, the solid is placed in an oven at 95℃ and dried for 8 hours to obtain a dry polyethylene-vinyl alcohol resin.

[0039] The sample film has 11 crystal points, a folding resistance of level 2, a film transmittance of 91.3%, and a secondary processing yellowness index of 9.8.

[0040] Example 2 The same as in Example 1, except that the nitrogen and ethylene replacement pressure in the initial feeding stage in Example 1 was changed to 20 bar, and the replacement temperature was changed to 60°C. After the replacement was completed, the pressure in the polymerization reactor was changed to 43 bar when ethylene was charged, and then the stirring was started to dissolve the ethylene. After dissolving for 15 minutes, the temperature was started to rise. After the polymerization was completed, the dried polyethylene-vinyl alcohol resin was obtained according to the same steps.

[0041] The sample film has 7 crystal points, folding resistance of level 2, film transmittance of 91.5%, and secondary processing yellowness index of 8.7.

[0042] Example 3 The same as in Example 1, except that the nitrogen and ethylene replacement pressure in the initial feeding stage in Example 1 was changed to 30 bar, and the replacement temperature was changed to 64°C. After the replacement was completed, the pressure in the polymerization reactor was changed to 45 bar when ethylene was charged, and then the stirring was started to dissolve the ethylene. After dissolving for 15 minutes, the temperature was started to rise. After the polymerization was completed, the dried polyethylene-vinyl alcohol resin was obtained according to the same steps.

[0043] The sample film has 3 crystal points, folding resistance of level 3, light transmittance of 91.6%, and secondary processing yellowness index of 7.4.

[0044] Comparative Example 1 The same as Example 1, except that the nitrogen and ethylene replacement pressure in the initial feeding stage in Example 1 was changed to 3 bar, and the replacement temperature was changed to 25°C. After the replacement was completed, the pressure in the polymerization reactor was changed to 5 bar when ethylene was charged, and then the stirring was started to dissolve the ethylene. After dissolving for 15 minutes, the temperature was started to rise. After the polymerization was completed, the dried polyethylene-vinyl alcohol resin was obtained according to the same steps.

[0045] The sample film has 22 crystal points, a folding resistance of level 1, a film transmittance of 90.0%, and a secondary processing yellowness index of 13.7.

[0046] Comparative Example 2 The same as in Example 1, except that the nitrogen and ethylene replacement pressure in the initial feeding stage in Example 1 was changed to 40 bar, and the replacement temperature was changed to 70°C. After the replacement was completed, the pressure in the polymerization reactor was changed to 55 bar when ethylene was charged, and then the stirring was started to dissolve the ethylene. After dissolving for 15 minutes, the temperature was started to rise. After the polymerization was completed, the dried polyethylene-vinyl alcohol resin was obtained according to the same steps.

[0047] The sample film has 15 crystal points, a folding resistance of level 2, a film transmittance of 90.5%, and a secondary processing yellowness index of 11.2.

[0048] The test results of the products obtained in Examples 1-3 and Comparative Examples 1-2 are shown in Table 2 below: Table 2 Performance indicators of ethylene vinyl alcohol products project Flexibility and bending resistance Film transmittance% Ethylene vinyl alcohol processability / crystallinity Yellow Index Example 1 Level 2 91.3 11 9.8 Example 2 Level 2 91.5 7 8.7 Example 3 Level 3 91.6 3 7.4 Comparative Example 1 Level 1 90.0 22 13.7 Comparative Example 2 Level 2 90.5 15 11.2 It can be seen from the above examples 1-3 and comparative examples 1-2 that, in the feeding stage, the control of the nitrogen and ethylene replacement pressure and the pressure of ethylene after the replacement is completed is one of the key innovations of the present invention. Too low or too high pressures cannot achieve the excellent technical effects of the present invention. The anti-bending performance of the ethylene-vinyl alcohol copolymers obtained in Examples 1-3 of the present invention listed in Table 2 above is graded as above 2, and the flexibility is better than that of the ethylene-vinyl alcohol copolymers obtained in Comparative Examples 1-2; the number of crystal points of the ethylene-vinyl alcohol copolymers obtained in Examples 1-3 reflecting the processability is 3-11, which is better than 15-22 of Comparative Examples 1-2; the yellow index of the ethylene-vinyl alcohol copolymers obtained in Examples 1-3 reflecting the anti-yellowing ability of secondary processing is 7.4-9.8, which is better than 11.2-13.7 of Comparative Examples 1-2. It can be seen that the final product of the ethylene-vinyl alcohol copolymer obtained by the preparation method of the present invention can be improved simultaneously in terms of performance indicators such as anti-bending (flexibility), secondary processability, and secondary processing anti-yellowing.

[0049] The above is only an embodiment of the present invention. The commonly known technical common sense in the scheme is not described in detail here. The ordinary technicians in the relevant field are aware of all the common technical knowledge in the technical field to which the invention belongs before the application date, can obtain all the existing technologies in the field, and have the ability to apply the conventional experimental means before that date. The ordinary technicians in the relevant field can improve and implement this scheme in combination with their own abilities under the enlightenment given by this application. Some typical commonly known technologies should not become obstacles for ordinary technicians in the relevant field to implement this application. It should be pointed out that for those skilled in the art, without departing from the structure of the present invention, they can also make some adjustments and improvements, which should also be regarded as the scope of protection of the present invention, which will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection claimed by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A method for preparing an ethylene-vinyl alcohol copolymer resin, which uses a solvent, a vinyl acetate monomer, an initiator and an ethylene monomer as raw materials, and prepares the ethylene-vinyl alcohol copolymer through the steps of initial feeding, polymerization reaction, blowing out residual monomers and alcoholysis; characterized in that: The initial feeding stage includes introducing nitrogen and ethylene respectively to make the pressure in the reactor at 5-30 bar and the temperature at 55-64° C. for multiple replacements; after the replacement is completed, an initiator is added, ethylene is charged to make the pressure in the polymerization reactor reach 25-50 bar, and after mixing at room temperature for 10-20 minutes, the temperature is raised to start the polymerization reaction.

2. The method for preparing the ethylene-vinyl alcohol copolymer resin according to claim 1, characterized in that: The polymerization step maintains an ethylene pressure between 30 and 45 bar.

3. The method for preparing the ethylene-vinyl alcohol copolymer resin according to claim 1 or 2, characterized in that: The solvent used in the polymerization step and the alcoholysis step is the same, and the same solvent is used to wash the product obtained in the alcoholysis step.

4. The method for preparing the ethylene-vinyl alcohol copolymer resin according to any one of claims 1 to 3, characterized in that: The solvent is a monohydric alcohol or dihydric alcohol having 1 to 5 carbon atoms and a mixture thereof.

5. The method for preparing the ethylene-vinyl alcohol copolymer resin according to any one of claims 1 to 4, characterized in that: The initiator is an azo initiator.

6. The method for preparing the ethylene-vinyl alcohol copolymer resin according to any one of claims 1 to 5, characterized in that: 40-90 parts of vinyl acetate and 10-60 parts of methanol are put into a reactor, and then nitrogen and ethylene are introduced respectively to make the pressure in the reactor 5-30 bar and the temperature 55-64°C for replacement 3 times; after the replacement is completed, 0.001-1 parts of azobisisobutyronitrile as an initiator is added, and ethylene is introduced to make the pressure in the polymerization reactor reach 25-50 bar, and then stirring is started to dissolve the ethylene and then start to heat up; when the solution temperature reaches the polymerization temperature of 50-70°C, the polymerization reaction is carried out for 5-7h, and the temperature in the reactor is kept at 50-70°C and the pressure is kept at 25-50bar during the polymerization process; after the polymerization is completed, the polymerization liquid is cooled to below 35°C and then the pressure is released to remove the unreacted ethylene gas to obtain a polyethylene-vinyl acetate copolymer methanol solution; by heating, the unreacted vinyl acetate monomer in the polymerization liquid is blown out cleanly, and methanol is added to keep the concentration of the polymerization liquid at 10-20%wt ; The copolymer methanol solution is placed at a constant temperature of 60-70°C for 10-20 minutes, and then a sodium hydroxide methanol solution with a concentration of 30-50g / L is slowly added for alcoholysis, the alcoholysis temperature is 60-70°C, and the alcoholysis time is 6-8h; after the alcoholysis product is centrifuged, it is washed with methanol for many times, the amount of methanol is 3-5 times that of the solid, the washing temperature is 30-45°C, and the washing time is 40-50min each time; the washed solid is placed in an oven at 90-100°C and dried for 6-10h to obtain a dry ethylene-vinyl alcohol copolymer resin.

7. The ethylene-vinyl alcohol copolymer resin prepared by the method for preparing the ethylene-vinyl alcohol copolymer resin according to any one of claims 1 to 6.

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