Ethylene-vinyl acetate copolymer EVA resin for extrusion coating and preparation method thereof

Abstract

The invention discloses an ethylene-vinyl acetate copolymer (EVA) resin for extrusion coating and a preparation method thereof. A kettle type polymerization method is adopted, 75-85 wt% of ethylene monomers and 15-25 wt% of vinyl acetate monomers are subjected to a polymerization reaction in the presence of an initiator and a chain transfer agent, the polymerization temperature is 150-250 DEG C, the polymerization pressure is 150-230 MPa, and the polymerization time is 30-100 seconds. The EVA resin prepared by the invention has low thermal shrinkage rate and relatively high fluidity, and edge shrinkage and coating leakage are avoided in a coating process.

Description

Technical Field

[0001] The invention belongs to the technical field of EVA resins, and in particular relates to an ethylene-vinyl acetate copolymer EVA resin for extrusion coating and a preparation method thereof. Background Art

[0002] The pre-coated film is processed by an environmentally friendly method, that is, an extrusion composite process. Generally, hot melt adhesive is used as an adhesive, which is plasticized and melted by an extruder, and extruded through a T-die to form a molten film. When the adhesive layer is in a molten state, it is extruded and bonded with the plastic film (substrate) by a composite roller under certain conditions. After cooling and shaping, it becomes a composite material with new properties, which is then applied to the printing industry. The above method does not require the use of toxic solvents, and can be well composited with printed materials by simply heating the composite roller. The use process is completely environmentally friendly and no toxic gas is released.

[0003] Pre-coated film is mainly used for laminating various books, packaging materials, and advertising products to improve their surface strength, smoothness, clarity, and color vividness. According to the different substrates used, pre-coated film products are divided into two categories: BOPP pre-coated film and BOPET pre-coated film. The main problem in the promotion of pre-coated film is cost. In order to reduce the cost of pre-coated film, many companies control costs by adjusting the thickness of the film substrate, the amount of hot melt adhesive coating, and increasing the coating rate. When producing pre-coated film, the thickness of the substrate has been reduced from the original 20um to 12um. At present, some companies are still developing new products with 10um thickness substrate and 8um hot melt adhesive, and the thinnest thickness reaches 3um. At the same time, the speed of film coating is also accelerating, and the fastest can reach 200-300m / min.

[0004] Therefore, as the thickness of the pre-coating film gradually decreases and the laminating speed increases, the requirements for materials and processes are becoming more and more stringent. During the processing of ordinary EVA materials, the film is prone to shrinkage due to large thermal shrinkage, and it is also easy to leak coating during the laminating process, which affects the coating quality. Summary of the invention

[0005] Purpose of the invention: The purpose of the present invention is to provide an ethylene-vinyl acetate copolymer EVA resin for extrusion coating and a preparation method thereof in view of the deficiencies of the prior art. The coating EVA product involved in the present invention is produced by a kettle process. Compared with other EVA products, it has the characteristics of a wider molecular weight distribution and good fluidity, a lower thermal shrinkage rate, and can reduce the shrinkage and leakage phenomenon during high-speed coating.

[0006] Technical solution: The purpose of the present invention is achieved through the following technical solution:

[0007] The invention provides a method for preparing ethylene-vinyl acetate copolymer EVA resin for extrusion coating. The method adopts a kettle polymerization method, in the presence of an initiator and a chain transfer agent, 75-85wt% of ethylene monomer and 15-25wt% of vinyl acetate monomer are polymerized, the polymerization temperature is 150-250°C, the polymerization pressure is 150-230MPa, and the polymerization time is 30-100 seconds.

[0008] At present, EVA materials are mainly produced by two polymerization processes, a tubular process and a kettle process. The EVA materials produced by these two processes are different in structure. The product produced by the tubular process has a narrow molecular weight distribution and short branch chains; while the product produced by the kettle process has a wide molecular weight distribution and long branch chains. Therefore, the present invention performs polymerization on the kettle process.

[0009] Preferably, the initiator is selected from peroxide-based initiators and is added in an amount of 30-60 kg / h.

[0010] Further, the initiator is one or more of di-(2-ethoxyhexyl) peroxydicarbonate, butyl peroxypivalate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxyacetate or tert-butyl peroxyneodecanoate. Different initiators have different decomposition temperatures, and the present invention controls the polymerization temperature by the selected initiator.

[0011] Preferably, the chain transfer agent is added in an amount of 50 to 150 kg / h.

[0012] Furthermore, the chain transfer agent is selected from one or more of propylene, propionaldehyde, propane, 1-butene or isobutylene. Different chain transfer agents have different transfer efficiencies. The chain transfer agent selected in the present invention is beneficial to controlling the melt index and molecular weight of EVA.

[0013] The present invention further realizes the control of the polymerization temperature of EVA and the melt index and molecular weight of EVA by limiting the addition amount of the initiator and the chain transfer agent.

[0014] The present invention also provides an ethylene-vinyl acetate copolymer EVA resin prepared by the above preparation method. According to the method for preparing the copolymer resin of the present invention, an EVA resin with low heat shrinkage and good fluidity can be easily obtained, and the copolymer resin is suitable for preparing BOPP pre-coating film or BOPET pre-coating film.

[0015] The weight average molecular weight (Mw) of the EVA resin is 70,000 to 110,000 g / mol.

[0016] The molecular weight distribution Mw / Mn of the EVA resin is 3-5.

[0017] The EVA resin has a melt mass flow rate in the range of 7 to 30 g / 10 min at 190° C. and a load of 2.16 kg according to standard GB / T 3682-2000.

[0018] Beneficial effects:

[0019] (1) The present invention adopts a method of feeding monomers to a kettle reactor to prepare an ethylene-vinyl acetate copolymer (EVA) resin for extrusion coating. The present invention obtains an EVA product that meets performance requirements by controlling a special reactor type and a reaction process.

[0020] (2) The EVA resin prepared by the present invention has a lower thermal shrinkage than the resin based on the high pressure tube reactor with the same content of vinyl acetate. The EVA resin prepared by the present invention has a low thermal shrinkage and high fluidity, which can significantly improve the coating process and reduce the problems of missing coating and edge shrinkage. DETAILED DESCRIPTION

[0021] The technical solution of the present invention is described in detail below through specific embodiments, but the protection scope of the present invention is not limited to the embodiments.

[0022] The performance data of the polymer in the present invention are measured using the following method:

[0023] The melt mass flow rate of EVA resin is tested at 190°C and a load of 2.16 kg in accordance with standard GB / T 3682-2000.

[0024] The VA content of EVA resin is tested according to the standard GB / T 31984-2015, and the VA content is determined by thermogravimetric analysis (TGA) according to the following formula.

[0025] W VA =m L ×1.434

[0026] Where m L is the mass loss in the first stage of the thermogravimetric curve, and 1.434 is the ratio of the relative molecular mass of vinyl acetate (86.09) to the relative molecular mass of acetic acid (60.05).

[0027] The average molecular weight (Mn, Mw) of EVA resin is measured by gel permeation chromatography.

[0028] The heat shrinkage test of EVA film is to cut the film into 10*10cm size, immerse it in 80℃ water for 5min, and cool it at room temperature, and test the transverse length MD and longitudinal length TD. The smaller the length change, the smaller the heat shrinkage.

[0029] The monomer needs to be cooled in a cooler before being added to the reactor.

[0030] During the reaction, the entire polymerization equipment is in a closed and anaerobic state.

[0031] Example 1

[0032] 81 wt% of ethylene monomer and 19 wt% of vinyl acetate monomer are fed into a tank reactor for polymerization, 30 kg / h of a mixture of di-(2-ethoxyhexyl) peroxydicarbonate and tert-butyl peroxy-2-ethylhexanoate (mixing weight ratio is 20:80) are added, 80 kg / h of propylene is added as a chain transfer agent, and then polymerization is carried out under the conditions of a polymerization temperature of 176° C., a polymerization pressure of 190 MPa, and a polymerization time of 40 seconds to obtain an ethylene-vinyl acetate copolymer (EVA) resin.

[0033] Example 2

[0034] 75wt% of ethylene monomer and 25wt% of vinyl acetate monomer are fed into a tank reactor for polymerization, 40kg / h of a mixture of di-(2-ethoxyhexyl) peroxydicarbonate, tert-butyl peroxy-2-ethylhexanoate and butyl peroxypivalate (mixing weight ratio is 20:60:20) are added, and 100kg / h of a mixture of propylene and propionaldehyde (mixing weight ratio is 80:20) is added as a chain transfer agent, and then polymerization is carried out under the conditions of a polymerization temperature of 150°C, a polymerization pressure of 230MPa and a polymerization time of 100 seconds to obtain an ethylene-vinyl acetate copolymer (EVA) resin.

[0035] Example 3

[0036] 85wt% of ethylene monomer and 15wt% of vinyl acetate monomer are fed into a tank reactor for polymerization, 40kg / h of a mixture of di-(2-ethoxyhexyl) peroxydicarbonate, tert-butyl peroxy-2-ethylhexanoate and tert-butyl peroxyacetate (mixing weight ratio is 20:50:30) are added, 50kg / h of a mixture of propylene and 1-butene (mixing weight ratio is 80:20) is added as a chain transfer agent, and then polymerization is carried out under the conditions of a polymerization temperature of 250°C, a polymerization pressure of 150MPa and a polymerization time of 30 seconds to obtain an ethylene-vinyl acetate copolymer (EVA) resin.

[0037] Example 4

[0038] 81wt% of ethylene monomer and 19wt% of vinyl acetate monomer are fed into a tank reactor for polymerization, 60kg / h of a mixture of di-(2-ethoxyhexyl) peroxydicarbonate, tert-butyl peroxy-2-ethylhexanoate and tert-butyl peroxyneodecanoate (mixing weight ratio is 20:60:20) are added, 140kg / h of a mixture of propylene and isobutylene (mixing weight ratio is 80:20) is added as a chain transfer agent, then, polymerization is carried out under the conditions of a polymerization temperature of 182°C, a polymerization pressure of 200MPa and a polymerization time of 40 seconds to obtain an ethylene-vinyl acetate copolymer (EVA) resin.

[0039] Comparative Example 1

[0040] EVA copolymer resin is prepared according to a conventional method for preparing copolymer resin, which uses a high-pressure tubular reactor, adds 81wt% of ethylene monomer and 19wt% of vinyl acetate monomer, and adds 60kg / h of tert-butyl peroxy-2-ethylhexanoate thereto, adds 140kg / h of propylene as a chain transfer agent, and then polymerizes under polymerization conditions of 265MPa polymerization pressure or less and polymerization temperature of 225°C to obtain ethylene-vinyl acetate copolymer (EVA) resin.

[0041] Comparative Example 2

[0042] 81 wt % of ethylene monomer and 19 wt % of vinyl acetate monomer are fed into a tank reactor for polymerization, and 30 kg / h of a mixture of di-(2-ethoxyhexyl) peroxydicarbonate and tert-butyl peroxy-2-ethylhexanoate (mixing weight ratio is 20:80) is added, and then polymerization is carried out under the conditions of a polymerization temperature of 176° C., a polymerization pressure of 190 MPa, and a polymerization time of 40 seconds to obtain an ethylene-vinyl acetate copolymer (EVA) resin.

[0043] Film formation conditions:

[0044] The copolymer resins prepared according to the examples and comparative examples were extruded through an extruder having a T-die having a screw diameter of 35 mm and a width of 300 mm at an extrusion temperature of 95° C. and a screw speed of 180 r / min to prepare sheet-type films each having a thickness.

[0045] The performance parameters of the prepared sheet membrane are shown in Table 1:

[0046] Table 1 Performance parameters

[0047]

[0048] As shown in the table, the copolymer resin prepared in the examples of the present invention exhibits a broader molecular weight distribution. Compared with the film prepared from the copolymer resin prepared in the comparative example, the film prepared from the copolymer resin prepared in the examples of the present invention exhibits a lower heat shrinkage.

[0049] As described above, although the present invention has been shown and described with reference to specific preferred embodiments, it should not be construed as limiting the present invention itself. Various changes in form and details may be made without departing from the spirit and scope of the present invention as defined in the appended claims.

Claims

1. A method for preparing an ethylene-vinyl acetate copolymer (EVA) resin for extrusion coating, characterized in that: The kettle polymerization method is adopted to polymerize 75-85wt% of ethylene monomer and 15-25wt% of vinyl acetate monomer in the presence of an initiator and a chain transfer agent, the polymerization temperature is 150-250°C, the polymerization pressure is 150-230MPa, and the polymerization time is 30-100 seconds.

2. The preparation method according to claim 1, characterized in that: The initiator is selected from peroxide-based initiators and is added in an amount of 30-60 kg / h.

3. The preparation method according to claim 2, characterized in that: The initiator is one or more of di-(2-ethoxyhexyl) peroxydicarbonate, butyl peroxypivalate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxyacetate or tert-butyl peroxyneodecanoate.

4. The preparation method according to claim 1, characterized in that: The chain transfer agent is added in an amount of 50 to 150 kg / h.

5. The preparation method according to claim 4, characterized in that: The chain transfer agent is selected from one or more of propylene, propionaldehyde, propane, 1-butene or isobutylene.

6. An ethylene-vinyl acetate copolymer (EVA) resin prepared by the preparation method according to any one of claims 1 to 5.

7. The EVA resin according to claim 6, characterized in that The weight average molecular weight (Mw) of the EVA resin is 70,000 to 110,000 g / mol.

8. The EVA resin according to claim 6, characterized in that The molecular weight distribution Mw / Mn of the EVA resin is 3-5.

9. The EVA resin according to claim 6, characterized in that The EVA resin has a melt mass flow rate in the range of 7 to 30 g / 10 min at 190° C. and a load of 2.16 kg according to standard GB / T3682-2000.

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