Method for improving leveling property of vinyl acetate-ethylene copolymer emulsion

Abstract

The invention discloses a method for improving the leveling property of vinyl acetate-ethylene copolymer emulsion. In the emulsion polymerization reaction process, a vinyl acetate monomer and an emulsifier are uniformly added for multiple times. The vinyl acetate-ethylene copolymer emulsion prepared by the method disclosed by the invention is good in leveling property, the thicknesses of adhesive films of all parts are uniform when the adhesive films are measured by a thickness gauge after film formation, and the average error of the thicknesses of the adhesive films does not exceed 2%, so that the vinyl acetate-ethylene copolymer emulsion is uniformly spread during film formation.

Description

Technical Field

[0001] The present invention relates to a vinyl acetate-ethylene copolymer emulsion, and particularly to a method for improving the leveling property of a vinyl acetate-ethylene copolymer emulsion. Background Art

[0002] Vinyl acetate-ethylene copolymer emulsion (VAE emulsion) is a kind of polymer emulsion copolymerized by emulsion polymerization method. Because of its excellent properties, it has been widely used in the fields of adhesives, coatings, paper processing, fabric processing, packaging, construction, etc., and has functions such as permanent flexibility, good acid and alkali resistance, ultraviolet aging resistance, good film-forming property, good adhesion, etc.

[0003] Leveling property is one of the important properties affecting the practical application of VAE emulsion, and it is a characterization of the ability of a wet coating film to flow and eliminate coating marks after the action of the external environment.

[0004] Chinese Patent Document CN112979862A discloses a styrene-acrylic emulsion and its preparation method, including: the components of the styrene-acrylic emulsion include: acrylate monomers, styrene, special monomers; it includes five steps, and the first to third steps are: Step 1: After fully stirring and mixing the emulsifier and water, add acrylate monomers, styrene, and special monomers and stir to emulsify to obtain an emulsion; Step 2: Prepare the materials in the reaction kettle, stir the emulsifier and water evenly, add part of the emulsion prepared in Step 1, and then dropwise add part of the initiator and reducing agent solution to prepare a seed emulsion; Step 3: Dropwise add the remaining emulsion, initiator, and reducing agent solution in Step 1. This invention claims that by blending or copolymerizing a small amount of functional monomers, the modification of the styrene-acrylic emulsion is realized, endowing the styrene-acrylic emulsion with better leveling property; adding an initiator to the styrene-acrylic emulsion can promote the polymerization reaction of acrylate monomers, styrene, and special monomers, and the polymerization efficiency is higher; controlling the dosage of the initiator at 0.12% - 0.7% of the monomer amount can not only control the reaction speed moderately but also ensure the normal progress of the reaction without affecting the performance of the polymer.

[0005] Chinese Patent Document CN 115991842A discloses a vinyl acetate-ethylene copolymer emulsion with excellent adhesion performance to difficult-to-bond materials, and its contact angle is 55-65°. It also discloses a preparation method of a vinyl acetate-ethylene copolymer emulsion, which is prepared from raw materials including an emulsifier, and the dosage of the emulsifier is 10‰-15‰ of the total mass of the raw materials; the emulsifier includes a non-ionic emulsifier with an ethylene oxide addition number EO≥40; the raw materials also include polyvinyl alcohol, and the polyvinyl alcohol includes one or more of polyvinyl alcohols with a polymerization degree of 500-1800 and a degree of alcoholysis of 86mol%-92mol%. This document claims that its beneficial effects are as follows: significantly improving the adhesion performance of the emulsion to the adhesive material; significantly increasing the ethylene content of the vinyl acetate-ethylene copolymer emulsion; the ethylene content of the emulsion is high, and when used in redispersible latex powder, it can significantly improve the flexibility and freeze-thaw cracking resistance of the mortar.

[0006] Chinese Patent Document CN 113493536A discloses a vinyl acetate-ethylene copolymer emulsion. The content of rubber particles with a particle size ≤0.5μm is 15%-20% by volume percentage. The solid content of the vinyl acetate-ethylene copolymer emulsion is 58%-62% by mass percentage. The viscosity of the vinyl acetate-ethylene copolymer emulsion at 25°C is 1000-1500mPa·s. The average particle size of the vinyl acetate-ethylene copolymer emulsion is 0.8-1.3μm. It also discloses a preparation method of the vinyl acetate-ethylene copolymer emulsion, which is prepared from raw materials including monomers, protective colloids, initiator systems, and water. The protective colloid is polyvinyl alcohol with a polymerization degree ≤500 and a degree of alcoholysis of 86%-90%. This document discloses that when the protective colloid is added in the manner described in the present invention, the prepared vinyl acetate-ethylene copolymer emulsion has good stability; the vinyl acetate-ethylene copolymer emulsion of the present invention has good stability, and its dilution stability ≤1.6%; the vinyl acetate-ethylene copolymer emulsion of the present invention has a high solid content, which can effectively reduce energy consumption when used in the production of redispersible latex powder; the vinyl acetate-ethylene copolymer emulsion of the present invention can reduce the addition amount of the dispersant in the later stage; the vinyl acetate-ethylene copolymer emulsion of the present invention can be well mixed with cement, and the obtained rubber powder has good dispersibility; the preparation method of the present invention has a stable reaction.

[0007] Although the existing methods disclosed in the above-mentioned prior art disclose that a small amount of functional monomers are introduced into styrene-acrylic emulsion by blending or copolymerization to make its leveling property good, they do not disclose how to solve the problem of uneven spreading during film formation of the existing vinyl acetate-ethylene copolymer emulsion. None of the other prior arts mention this technical problem and its solution. Summary of the Invention

[0008] On the one hand, in order to solve the problem that the existing vinyl acetate-ethylene copolymer emulsion spreads unevenly during film formation, the present invention provides a method for improving the leveling property of the vinyl acetate-ethylene copolymer emulsion, wherein the vinyl acetate monomer and the emulsifier are added in several equal portions during the emulsion polymerization reaction process.

[0009] Further, the emulsifier includes a non-ionic emulsifier and an anionic emulsifier.

[0010] Further, the initially added emulsifier is a non-ionic emulsifier; the subsequently added emulsifier is an anionic emulsifier.

[0011] Further, the time for subsequently adding the emulsifier is: when there are 20 - 30 minutes remaining in the time for subsequently adding the vinyl acetate monomer.

[0012] Further, the subsequent addition of the emulsifier is continuous addition, and the time for controlling its complete addition is 40 - 55 minutes.

[0013] Further, the emulsion is also prepared from raw materials including ethylene, a protective colloid, an initiator system, and a pH regulator; the protective colloid is composed of partially saponified polyvinyl alcohol and fully saponified polyvinyl alcohol, and the ratio of partially saponified polyvinyl alcohol to fully saponified polyvinyl alcohol is 3 - 4:1.

[0014] Further, the partially saponified polyvinyl alcohol is polyvinyl alcohol with a polymerization degree of 500 - 1700 and a saponification degree of 86 - 92%, and the fully saponified polyvinyl alcohol is polyvinyl alcohol with a polymerization degree of 1300 - 1700 and a saponification degree of 97 - 99%.

[0015] Further, it includes the following steps: Add a protective colloid, a non-ionic emulsifier, a pH regulator, 50% - 70% of the total amount of vinyl acetate and ethylene, continuously add an oxidizing agent and a reducing agent. When the temperature is controlled to 78 - 82°C and the pressure rises to 5.0 - 6.0 MPa, add the remaining vinyl acetate, and control the addition time of the remaining vinyl acetate to be 80 - 100 minutes; when there are 20 - 30 minutes remaining in the addition time of the remaining vinyl acetate, add the anionic emulsifier, and control its addition to stop 20 - 25 minutes after the vinyl acetate is added.

[0016] On the second hand, the present invention provides a vinyl acetate-ethylene copolymer emulsion prepared by the method of the present invention.

[0017] On the third hand, the present invention provides the application of the vinyl acetate-ethylene copolymer emulsion in paper and packaging.

[0018] The present invention has the following beneficial effects: 1) The vinyl acetate-ethylene copolymer emulsion prepared by the method of the present invention has good leveling property. When measured by a thickness gauge after film formation, the thickness of the rubber film in each part is uniform, and the error does not exceed 2%. The vinyl acetate-ethylene copolymer emulsion spreads evenly during film formation.

[0019] 2) The vinyl acetate-ethylene copolymer emulsion prepared by the method of the present invention has good leveling property, can reduce the usage amount of the emulsion and improve the bonding strength in practical applications.

[0020] 3) The preparation method of the present invention is simple, which is beneficial to realizing industrial production. Specific embodiments

[0021] The following is a further detailed description through specific embodiments. It should be noted that the embodiments cited below are only for better explaining the content of the present invention, but do not represent that the content of the present invention is limited to the cited embodiments. Therefore, those skilled in the art who make non-essential improvements and adjustments to the implementation solutions according to the above invention content still fall within the protection scope of the present invention, subject to the protection scope of the appended claims.

[0022] In the present invention, the amounts of each component are generally expressed in parts by mass. In some contents expressed in absolute mass, 1 part and 1 kg are corresponding and interchangeable expressions.

[0023] For vinyl acetate-ethylene copolymers, they are usually classified into three forms: resin, rubber and emulsion according to the different vinyl acetate contents: products with a vinyl acetate content of less than 40 wt% are resins, products with a vinyl acetate content of 40-70 wt% are rubbers, and products with a vinyl acetate content in the range of 70-95 wt% are emulsions. The vinyl acetate-ethylene copolymer described in the present invention specifically refers to the emulsion form. Therefore, the content of the ethylene unit is less than 30 wt%, generally mostly in the range of 15-20 wt%. In the present invention, terms such as vinyl acetate-ethylene copolymer, vinyl acetate-ethylene copolymer emulsion, VAE, and VAE emulsion have the same meaning and can be used interchangeably.

[0024] The existing preparation processes of the vinyl acetate-ethylene copolymer described in the present invention generally include steps such as in-kettle replacement and deoxygenation, initial feeding, polymerization reaction, and post-treatment.

[0025] The in-kettle replacement and deoxygenation step usually excludes the oxygen present in the reaction kettle by introducing an inert gas (such as nitrogen) into the reaction kettle, or the monomer gas ethylene gas of the present invention can also be used alone or simultaneously to exclude the oxygen, and the ethylene gas is one of the reaction raw materials, and the residue in the kettle will not affect the reaction.

[0026] Due to the involvement of raw materials in different forms such as solid, liquid, and gas, the feeding sequence is usually to add vinyl acetate monomer to water containing emulsifier, then introduce gaseous monomer ethylene, and then add initiator (if it is an initiator of redox system, the oxidant or reductant can be added to the initial feeding first, and the other initiator is added after the polymerization reaction starts). After heating to the polymerization reaction temperature (such as 60 °C), the polymerization reaction can start. The feeding sequences known in the prior art can add the above raw materials in appropriate proportions simultaneously or successively. There are also studies attempting to add a certain reaction raw material in several stages successively to obtain products with different properties.

[0027] The product of the present invention is in the form of emulsion, so emulsion polymerization is preferably used. Emulsion polymerization refers to the process of dispersing polymerization monomer raw materials in water to form an emulsion with the help of emulsifier and mechanical stirring, etc., and then initiating the polymerization of monomers through an initiator to obtain a product in the form of emulsion.

[0028] Emulsion polymerization methods include continuous emulsion polymerization and semi - continuous emulsion polymerization. Continuous emulsion polymerization means continuously adding monomer raw materials during the polymerization process and continuously taking out the reaction products through the emulsion polymerization reaction process. Semi - continuous emulsion polymerization means slowly and continuously adding monomer raw materials to the aqueous solution of emulsifier in the polymerization reaction kettle, while dropping the aqueous solution of initiator, and controlling the polymerization reaction temperature by the dropping rate; there is also a method of first adding part of the monomer and other raw materials (initiator, emulsifier, etc.) to the polymerization reaction kettle, and then completing the entire polymerization process by continuously adding the remaining monomer and other raw materials after polymerization to a certain extent. The present invention adopts the latter semi - continuous emulsion polymerization method, first adding part of vinyl acetate monomer as the initial monomer together with raw materials such as protective colloid, non - ionic emulsifier, reductant, etc., and then continuously feeding the remaining vinyl acetate monomer and anionic emulsifier.

[0029] The initiator described in the present invention, also known as free radical initiator, refers to a class of compounds that are easily decomposed into free radicals by heat and can be used to initiate the free radical polymerization and copolymerization reactions of olefin and diene monomers, and is essential for the ethylene - vinyl acetate copolymerization reaction. According to the molecular structure of the initiator, the initiator can be divided into azo - type, peroxide - type, and redox - type 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 temperature range of use of the initiator.

[0030] The present invention preferably uses hydrogen peroxide as the initiator and sodium erythorbate or zinc formaldehyde sulfoxylate as the reducing agent. Of course, other peroxy initiators can also be selected for the polymerization reaction of the present invention, such as persulfates, dibenzoyl peroxide, cumene hydroperoxide, tert-butyl benzoyl peroxide, methyl ethyl ketone peroxide, etc.; other reducing agents with free radical initiation effects can also be used as the reducing agent. Those skilled in the art can select any suitable initiator according to the polymerization reaction system and conditions. The dosage 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, dosages of 0.005%, 0.01%, 0.05%, 0.1%, 1%, and 5% can be selected. Those skilled in the art can select a suitable initiator and its dosage according to the specific situation of the reaction.

[0031] The specific conditions of the polymerization reaction of the vinyl acetate-ethylene copolymer of the present invention are well-known to those skilled in the art. The polymerization reaction temperature is 40-90°C, preferably 60-80°C, more preferably 78-82°C. For example, polymerization reaction temperatures such as 60°C, 70°C, 75°C, and 80°C can be selected. During the polymerization reaction, the ethylene gas pressure is maintained at 1-10 MPa, preferably 5-7 MPa, and most preferably 5-6 MPa.

[0032] The post-treatment process of the present invention includes pH adjustment, filtration, addition of bactericides, etc. These steps are all carried out using conventional operating methods known in the art.

[0033] The vinyl acetate-ethylene copolymer emulsion used in the present invention is prepared from raw materials including an appropriate amount of monomers, 2.6-3.4 parts of a protective colloid, 0.4-0.6 parts of an emulsifier, 0.3-0.4 parts of an initiation system, 0.1-0.2 parts of a pH regulator, and 40-50 parts of water, by mass.

[0034] The mass ratio of vinyl acetate to ethylene of the monomers used in the present invention is 4-5:1.

[0035] The ratio of the non-ionic emulsifier to the anionic emulsifier of the emulsifier used in the present invention is 1-1.5:1-1.5; among them, the non-ionic emulsifier is nonylphenol polyoxyethylene ether or fatty alcohol polyoxyethylene ether with an ethylene oxide addition number EO of 9-35; the anionic emulsifier is sodium dodecylbenzenesulfonate (SDBS) or fatty alcohol polyoxyethylene ether sulfate.

[0036] The initiation system used in the present invention includes an oxidizing agent and a reducing agent, and the mass ratio of the oxidizing agent to the reducing agent is 0.15-0.25:0.1-0.15; among them, the oxidizing agent is hydrogen peroxide, and the reducing agent is zinc formaldehyde sulfoxylate or sodium erythorbate.

[0037] The pH regulator used in the present invention is acetate.

[0038] The steps for preparing the vinyl acetate-ethylene copolymer emulsion in the present invention include: A. Raw material preparation Prepare the initiator system solution: Prepare an oxidant solution with a concentration of 1.2% - 1.8% in tank A; prepare a reductant solution with a concentration of 1.9% - 3.4% in tank B; Prepare the anionic emulsifier solution: Prepare an anionic emulsifier solution with a concentration of 7.2% - 15% in tank C; B. Feeding and production Add 2.6 - 3.4 parts of protective colloid and 18 - 20 parts of water, start stirring, heat up after full mixing until it is completely dissolved; After complete dissolution, add 0.2 - 0.3 parts of non-ionic emulsifier, pH regulator, and vinyl acetate accounting for 50% - 70% of the total amount of vinyl acetate, stir evenly and then add ethylene to boost the pressure to 3.0 - 3.9 MPa; When the temperature rises to 65 - 70 °C, add the solutions in tank A and tank B at a rate of 0.1 - 0.3 kg / min. When the temperature in the reaction kettle reaches 78 - 82 °C, boost the pressure to 5.0 - 6.0 MPa, add the remaining vinyl acetate, and control the addition time to be 80 - 100 min; when there are still 20 - 30 min left for the addition time of the remaining vinyl acetate, start adding the solution in tank C at a rate of 0.05 - 0.1 part / min, and stop adding the solution in tank C 20 - 25 min after the vinyl acetate is completely added; After adding the solution in tank C, cut off the ethylene, continue to add the solution in tank A for 30 - 40 min at a rate of 0.1 - 0.3 kg / min, and add the solution in tank B at a rate of 0.1 - 0.3 kg / min. Among them, the addition of the solution in tank B stops 10 - 15 min earlier than that in tank A, keep warm for 10 - 20 min, cool down to 40 - 50 °C, and then carry out defoaming, filtration, and cooling, and discharge the product.

[0039] The detection method used in the present invention: The following is the method for testing the average thickness of the rubber film: Use a 500 μm film applicator to coat the emulsion on a smooth glass plate, gently remove it after natural drying for 72 hours to form a film, and use a CHY-C2 film thickness gauge to test the thickness of each part of the rubber film, and take the average value by continuously testing ten points.

[0040] The following is the calculation formula for the thickness error of the rubber film: Rubber film thickness error = (rubber film thickness - average rubber film thickness) / average rubber film thickness * 100% Calculate the thickness errors of ten points and take the average value to obtain the average thickness error of the rubber film.

[0041] Example 1 Vinyl acetate-ethylene copolymer emulsion is prepared from the following raw materials according to the following steps: A. Raw material preparation Prepare the initiator system solution: Prepare a 1.8% hydrogen peroxide solution in tank A; Prepare a 2.90% zinc formaldehyde sulfoxylate solution in tank B; Add the emulsifier solution after preparation: Prepare a 7.5% fatty alcohol polyoxyethylene ether sulfate M70 solution in tank C; B. Feeding and production Add 1.0 part of polyvinyl alcohol 0588, 1.4 parts of polyvinyl alcohol 1788, 0.6 part of polyvinyl alcohol 1399 and 20 parts of water into the reaction kettle, start stirring, heat up after full mixing until it is completely dissolved; After the bottom materials in the kettle are completely dissolved, add 0.2 part of isomeric alcohol polyoxyethylene ether EH-9, 0.2 part of sodium acetate, and 35 parts of vinyl acetate. After stirring evenly, add ethylene to increase the pressure to the set pressure of 3.9 MPa; When the temperature rises to 68 °C, pump the solutions in tank A and tank B into the reaction kettle at a rate of 0.3 kg / min. When the temperature in the reaction kettle reaches 78 °C, increase the pressure to 6.0 MPa, and drop the remaining 15 parts of vinyl acetate, controlling the dropping time to be 80 min; When there are still 20 min left for the dropping time of the remaining vinyl acetate, start dropping the solution in tank C, and the solution in tank C is dropped within 20 min after the vinyl acetate is completely dropped; After the solution in tank C is added, cut off the ethylene, and continue to pump the solutions in tank A and tank B into the reaction kettle at a rate of 0.3 kg / min. The solution in tank A stops being added after 30 min, and the solution in tank B stops being added 10 min earlier than the solution in tank A. Keep the temperature for 10 min, cool down to 50 °C, and after defoaming, filtering and cooling, discharge the product.

[0042] Example 2 Vinyl acetate-ethylene copolymer emulsion is prepared from the following raw materials according to the following steps: A. Raw material preparation Prepare the initiator system solution: Prepare a 1.5% hydrogen peroxide solution in tank A; Prepare a 1.90% zinc formaldehyde sulfoxylate solution in tank B; Add the emulsifier solution after preparation: Prepare a 10% sodium dodecylbenzenesulfonate SDBS solution in tank C; B. Feeding and production Add 1.55 parts of polyvinyl alcohol 0888, 1 part of polyvinyl alcohol 1388, 0.85 part of polyvinyl alcohol 1399 and 20 parts of water into the reaction kettle, start stirring, heat up after full mixing until it is completely dissolved; After the bottom material of the kettle is completely dissolved, add 0.2 parts of secondary alcohol polyoxyethylene ether 15S-30, 0.15 parts of sodium acetate, and 24 parts of vinyl acetate, stir evenly, then add ethylene to increase the pressure to the set pressure of 3.5 MPa; When the temperature rises to 66°C, pump the solution in tank A and tank B at a rate of 0.1kg / min. When the temperature in the reactor reaches 80°C, increase the pressure to 5.6MPa and dropwise add the remaining 16 parts of vinyl acetate. Control the dropping time to 90min. When there are 25 minutes left for the addition of vinyl acetate, start adding the solution in tank C. The solution in tank C should be added within 20 minutes after the addition of vinyl acetate. After the solution in tank C is added, cut off ethylene and continue to pump the solutions in tanks A and B at a rate of 0.1 kg / min. Stop adding the solution in tank A after 30 minutes, and stop adding the solution in tank B 15 minutes earlier than the solution in tank A. Keep warm for 15 minutes, cool to 40°C, degas, filter, cool and discharge.

[0043] Example 3 The vinyl acetate-ethylene copolymer emulsion is prepared by using the following raw materials in accordance with the following steps: A. Raw material preparation Prepare the initiation system solution: prepare a 1.2% hydrogen peroxide solution in tank A; prepare a 3.2% sodium isoascorbate solution in tank B; Add emulsifier solution after preparation: prepare 15% sodium dodecylbenzene sulfonate SDBS solution in tank C; B. Feed production Add 1.0 part of polyvinyl alcohol 0588, 1.0 part of polyvinyl alcohol 0888, 0.8 part of polyvinyl alcohol 1797 and 18 parts of water into the reaction kettle, start stirring, mix thoroughly and then heat up to completely dissolve; After the bottom material of the kettle is completely dissolved, add 0.3 parts of secondary alcohol polyoxyethylene ether 15S-9, 0.1 parts of zinc acetate, and 34 parts of vinyl acetate, stir evenly, then add ethylene to increase the pressure to the set pressure of 3.5 MPa; When the temperature rises to 66°C, pump the solution in tank A and tank B at a rate of 0.1 kg / min. When the temperature in the reactor reaches 80°C, increase the pressure to 5.8 MPa and drop the remaining 16 parts of vinyl acetate. Control the dropwise addition time to 90 min. When there are 25 minutes left for the addition of vinyl acetate, start adding the solution in tank C. The solution in tank C should be added within 22 minutes after the addition of vinyl acetate. After the solution in tank C is completely added, cut off the ethylene, and continue to pump the solutions in tanks A and B at a rate of 0.1 kg / min. The addition of the solution in tank A stops after 40 min, and the addition of the solution in tank B stops 12 min earlier than that in tank A. Keep warm for 15 min, then cool down to 40 °C, and after degassing, filtering, and cooling, discharge the product.

[0044] Comparative Example 1 The vinyl acetate-ethylene copolymer emulsion is prepared from the following raw materials according to the following steps: A. Raw material preparation Prepare the initiator system solution: Prepare a 1.8% hydrogen peroxide solution in tank A; prepare a 2.90% zinc formaldehyde sulfoxylate solution in tank B. Add the emulsifier solution after preparation: Prepare a 7.5% fatty alcohol polyoxyethylene ether sulfate M70 solution in tank C. B. Feeding and production Add 1.6 parts of polyvinyl alcohol 0588, 1.4 parts of polyvinyl alcohol 1788, and 20 parts of water into the reaction kettle, start stirring, heat up after full mixing until it is completely dissolved. After the bottom material in the kettle is completely dissolved, add all the solution in tank C, 0.2 part of sodium acetate, and 35 parts of vinyl acetate, stir evenly, then add ethylene to increase the pressure to the set pressure of 3.9 MPa. When the temperature rises to 68 °C, pump the solutions in tanks A and B at a rate of 0.3 kg / min. When the temperature in the reaction kettle reaches 78 °C, increase the pressure to 6.0 MPa, and dropwise add the remaining 15 parts of vinyl acetate, controlling the dropping time to be 80 min; cut off the ethylene, and continue to pump the solutions in tanks A and B at a rate of 0.3 kg / min. The addition of the solution in tank A stops after 30 min, and the addition of the solution in tank B stops 10 min earlier than that in tank A. Keep warm for 10 min, then cool down to 50 °C, and after degassing, filtering, and cooling, discharge the product.

[0045] Comparative Example 2 The vinyl acetate-ethylene copolymer emulsion is prepared from the following raw materials according to the following steps: A. Raw material preparation Prepare the initiator system solution: Prepare a 1.8% hydrogen peroxide solution in tank A; prepare a 2.90% zinc formaldehyde sulfoxylate solution in tank B. Add the emulsifier solution after preparation: Prepare a 7.5% fatty alcohol polyoxyethylene ether sulfate M70 solution in tank C. B. Feeding and production Add 1.0 part of polyvinyl alcohol 0588, 1.4 parts of polyvinyl alcohol 1788, 0.6 part of polyvinyl alcohol 1399, and 20 parts of water into the reaction kettle, start stirring, heat up after full mixing until it is completely dissolved; After the bottom material of the kettle is completely dissolved, add all the solution in tank C, 0.2 parts of sodium acetate, and 35 parts of vinyl acetate. After stirring evenly, add ethylene to increase the pressure to the set pressure of 3.9 MPa. When the temperature rises to 68°C, pump the solutions in tank A and tank B at a rate of 0.3kg / min. When the temperature in the reactor reaches 78°C, increase the pressure to 6.0MPa, add the remaining 15 parts of vinyl acetate dropwise, and control the dropping time to 80min. Cut off the ethylene, and continue to pump the solutions in tank A and tank B at a rate of 0.3kg / min. Stop adding the solution in tank A after 30min, and stop adding the solution in tank B 10min earlier than the solution in tank A. Keep warm for 10min, cool to 50°C, degas, filter, cool, and discharge.

[0046] Performance Testing The vinyl acetate-ethylene copolymer emulsions prepared in Example 1-2 and Comparative Example 1-2 were film-formed using a film applicator, and the film thickness was tested after natural drying. The results are shown in Table 1.

[0047] Table 1 Performance test results Test items Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Average thickness of the adhesive film μm 393 388 375 356 374 Average error of the adhesive film thickness % 1 2 1.8 5 4 As can be seen from Table 1, in Examples 1-3 of the present invention, a protective colloid composed of partially alcoholyzed polyvinyl alcohol and completely alcoholyzed polyvinyl alcohol, and a nonionic emulsifier are added at the beginning of the reaction, and an anionic emulsifier is added after a period of time after the second addition of vinyl acetate, while Comparative Example 1-2 does not adopt this method, but rather adds a nonionic emulsifier and anionic emulsifier at the beginning of the reaction, and / or only uses a method of partially alcoholyzed polyvinyl alcohol. The results obtained thereby show that the average thickness of the film of Examples 1-3 is 375-393 μm, with an average error within 2%; the average thickness of the film of Comparative Example 1-2 is 356-374 μm, with an average error of 4%-5%. It can be seen that the technical solution of using a specific protective colloid and adding the emulsifier twice in the preparation of the emulsion, namely, first adding a nonionic emulsifier, reacting for a period of time, and then adding an anionic emulsifier, can make the obtained vinyl acetate-ethylene copolymer emulsion have good leveling properties and can spread evenly during film formation.

[0048] The above are only embodiments of the present invention. Well-known technical common sense in the solution is not described in detail here. Those of ordinary skill in the art know all the ordinary technical knowledge in the technical field to which the invention belongs before the filing date, are able to know all the existing technologies in this field, and have the ability to apply conventional experimental means before this date. Those of ordinary skill in the art can, under the inspiration given in this application, combine their own abilities to improve and implement this solution. Some typical well-known technologies should not become obstacles for those of ordinary skill in the art to implement this application. It should be noted that for those skilled in the art, without departing from the structure of the present invention, several adjustments and improvements can still be made, and these should also be regarded as the protection scope of the present invention, and these will not affect the implementation effect of the present invention and the practicality of the patent. The protection scope required by this application should be based on the content of its claims, and the specific implementation manners and the like recorded in the specification can be used to interpret the content of the claims.

Claims

1. A method for improving the leveling property of vinyl acetate-ethylene copolymer emulsion, wherein vinyl acetate monomer and emulsifier are added in several portions during the emulsion polymerization reaction.

2. The method for improving the leveling property of vinyl acetate-ethylene copolymer emulsion according to claim 1, characterized in that: the emulsifier includes non-ionic emulsifier and anionic emulsifier.

3. The method for improving the leveling property of vinyl acetate-ethylene copolymer emulsion according to claim 1 or 2, characterized in that: the initially added emulsifier is a non-ionic emulsifier; the subsequently added emulsifier is an anionic emulsifier.

4. The method for improving the leveling property of vinyl acetate-ethylene copolymer emulsion according to claim 3, characterized in that: the time for subsequently adding the emulsifier is when there are 20 - 30 minutes left for the time of subsequently adding vinyl acetate monomer.

5. The method for improving the leveling property of vinyl acetate-ethylene copolymer emulsion according to claim 3 or 4, characterized in that: the subsequent addition of the emulsifier is continuous addition, and the time for controlling all of it to be added is 40 - 55 minutes.

6. The method for improving the leveling property of vinyl acetate-ethylene copolymer emulsion according to any one of claims 1 - 5, characterized in that: the emulsion is also prepared from raw materials including ethylene, protective colloid, initiation system and pH regulator; the protective colloid is composed of partially saponified polyvinyl alcohol and fully saponified polyvinyl alcohol, and the ratio of partially saponified polyvinyl alcohol to fully saponified polyvinyl alcohol is 3 - 4:

1.

7. The method for improving the leveling property of vinyl acetate-ethylene copolymer emulsion according to claim 6, characterized in that: the partially saponified polyvinyl alcohol is polyvinyl alcohol with a polymerization degree of 500 - 1700 and a saponification degree of 86 - 92%, and the fully saponified polyvinyl alcohol is polyvinyl alcohol with a polymerization degree of 1300 - 1700 and a saponification degree of 97 - 99%.

8. The method for improving the leveling property of vinyl acetate-ethylene copolymer emulsion according to claim 1, characterized in that: it includes the following steps: adding protective colloid, non-ionic emulsifier, pH regulator, 50% - 70% of the total amount of vinyl acetate and ethylene of vinyl acetate, continuously adding oxidant and reductant, when the temperature is controlled to 78 - 82 °C and the pressure rises to 5.0 - 6.0 MPa, adding the remaining vinyl acetate, and controlling the adding time of the remaining vinyl acetate to be 80 - 100 minutes; when there are 20 - 30 minutes left for the adding time of the remaining vinyl acetate, adding anionic emulsifier, and controlling it to stop adding 20 - 25 minutes after the vinyl acetate is added up.

9. Vinyl acetate-ethylene copolymer emulsion prepared by the method according to any one of claims 1 - 8.

10. Application of the vinyl acetate-ethylene copolymer emulsion according to claim 9 in paper and packaging.