Tubular low density polyethylene composition for extrusion coating and preparation method thereof

Abstract

The invention belongs to the technical field of extrusion coatings, and particularly relates to a tubular low-density polyethylene composition for an extrusion coating and a preparation method of the tubular low-density polyethylene composition. The low-density polyethylene composition is prepared from the following raw materials in parts by weight: 90 to 95 parts of low-density polyethylene, 5 to 10 parts of ethylene-vinyl acetate copolymer, 0.05 to 0.10 part of grafting agent, 0.005 to 0.010 part of initiator and 0.05 to 0.10 part of antioxidant, and the sum of the parts by weight of the low-density polyethylene and the ethylene-vinyl acetate copolymer is 100. The low-density polyethylene composition disclosed by the invention has good tensile and adhesive properties, can be processed at a high speed, is relatively small in shrinkage, and overcomes the defects of poor adhesion and large shrinkage of LDPE (Low-Density Polyethylene) of an original tubular method.

Description

Technical Field

[0001] The invention belongs to the technical field of extrusion coating, and in particular relates to a tubular low-density polyethylene composition for extrusion coating and a preparation method thereof. Background Art

[0002] Extrusion coating is to coat molten polymer evenly on substrates such as paper, cardboard, woven bags, BOPP, aluminum foil, etc. by extrusion, so that the substrate can obtain functions such as waterproof, moisture-proof, and not easy to break. It can also be used as a connecting layer between two substrates to play a barrier and bonding role. Typical applications include coatings for liquid cartons such as milk and juice, heat-sealing layers of multi-layer film structures, and non-woven fabrics and woven bag coatings that provide moisture barrier effects.

[0003] Low-density polyethylene (LDPE) resins for extrusion coating are required to meet the following requirements: (1) good adhesion between substrate and polyethylene; (2) good thermal stability during high-temperature processing; (3) good fluidity; and (4) good heat sealing properties. Since kettle-processed LDPE has the characteristics of wide molecular weight distribution, many branches, and easy coating, most of the common coating LDPE on the market is produced by the kettle process. Although the kettle process has a smaller shrinkage when producing coatings, its coating speed is not as good as that of tubular-processed LDPE. Due to its low tensile strength, the strength of the film after formation is also low, which limits its application. Due to process limitations, tubular-processed LDPE has a relatively narrow molecular weight, low branching, poor adhesion during processing, and large shrinkage, which limits its application in the coating field.

[0004] Chinese invention patent CN114144462A Neck suppression of tubular LDPE-based polymers for extrusion coating at medium and high line speeds discloses a mixture consisting of 90wt% tubular LDPE and 1wt% to 10wt% ethylene acrylate copolymer, the mixture has good necking, at a line speed of 300 (500) m / min, a temperature of 320°C, a slit die of 1050 mm width, and a speed of 24 (12) g / m 2 When the coating weight is applied to kraft paper, the film has a shrinkage value of less than 100 mm. However, this method will affect the bonding effect when processing at high speeds in a simple blending manner. Summary of the invention

[0005] The purpose of the present invention is to provide a tubular low-density polyethylene composition for extrusion coating, which has good tensile and bonding properties, can be processed at high speed, and has a small shrinkage, thus overcoming the shortcomings of the original tubular LDPE with poor bonding and large shrinkage. The present invention also provides a preparation method thereof.

[0006] The tubular low-density polyethylene composition for extrusion coating of the present invention comprises the following raw materials in parts by weight: Low density polyethylene 90~95 parts Ethylene-vinyl acetate copolymer 5~10 parts Grafting agent 0.05~0.10 parts Initiator 0.005~0.010 parts Antioxidant 0.05~0.10 parts; The total weight of the low-density polyethylene and the ethylene-vinyl acetate copolymer is 100 parts.

[0007] The low-density polyethylene is a tubular low-density polyethylene, and its melt mass flow rate (MFR, 190°C, 2.16kg) is 8.0g / 10min~12.0g / 10min, and its density is 0.916~0.920g / cm 3 , molecular weight distribution (three-test high temperature GPC method) is 10.0~15.0.

[0008] The ethylene-vinyl acetate copolymer has a vinyl acetate content of 15% to 18% and an MFR of 10.0 g / 10 min to 20.0 g / 10 min.

[0009] The grafting agent includes maleic anhydride, unsaturated silane or acrylic acid, preferably maleic anhydride.

[0010] The initiator is an initiator containing a peroxy group, including dicumyl peroxide, benzoyl peroxide, tert-butyl benzoyl peroxide or methyl ethyl ketone peroxide, preferably dicumyl peroxide.

[0011] The antioxidant includes hindered phenols, hindered amines or phosphites, preferably 4,4'-thiobis(6-tert-butyl-3-methylphenol).

[0012] The method for preparing the tubular low-density polyethylene composition for extrusion coating of the present invention comprises the following steps: (1) Mix the grafting agent and the initiator, dissolve them in a solvent, stir them evenly in a closed mixer, and set aside; (2) After mixing low-density polyethylene, ethylene-vinyl acetate copolymer and antioxidant, add the mixture of step (1) thereto and mix well; (3) The raw materials mixed in step (2) are added into a twin-screw extruder for granulation, and the output MFR is controlled to be 4.0 g / 10 min-5.0 g / 10 min, and the exhaust port is maintained at a negative pressure.

[0013] In step (1), the grafting agent and the initiator are first dissolved in a solvent to improve the uniformity of material mixing. The solvent is preferably acetone. The stirring time is 3 to 5 minutes and the mixer speed is 150 to 300 rpm.

[0014] In step (3), the aspect ratio of the twin-screw extruder is ≥33, the number of shear blocks of the screw is ≥6 groups, the screw temperature is 160-210°C, the melt temperature is 180-185°C, and the screw speed is 150-500 r / min; the exhaust port maintains a negative pressure of 0.1-0.2 MPa, because the exhaust needs to be kept unobstructed during the extrusion process to ensure that the unreacted grafting agent and solvent are discharged.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. The low-density polyethylene composition of the present invention has uniform texture and good adhesion, which can effectively reduce the shrinkage of the coating and is mainly used for coatings of various substrates, such as cardboard, various film surfaces, woven bags, etc. Tubular low-density polyethylene has good processing performance and strength, but due to its low viscosity and relatively poor shrinkage, it not only shrinks greatly when used for high-speed coating, but also easily separates from the base layer, causing hollowing or delamination; ethylene-vinyl acetate copolymer itself has good processing performance and certain viscosity. After being mixed with tubular low-density polyethylene in small amounts, it can form an interlaced structure in the tubular low-density polyethylene. Under the action of initiators and grafting agents, the grafting can be made more uniform, and a lower grafting rate can achieve adhesion between the composition and the substrate, which can not only ensure bonding strength and reduce shrinkage, but also reduce the amount of initiators and grafting agents used, which is more environmentally friendly; 2. The present invention adopts a melt grafting method. The grafting agent, under the action of an initiator, grafts functional groups onto the molecular chain of low-density polyethylene to increase the melt viscosity. The antioxidant can delay or inhibit the oxidation process of the polymer, thereby preventing the aging of the polymer and extending its service life. Since the coating requires a high processing temperature, the present invention preferably uses 4,4'-thiobis(6-tert-butyl-3-methylphenol) as the antioxidant. 3. When the low-density polyethylene composition of the present invention is granulated, the output MFR needs to be controlled to be 4.0g / 10min~5.0g / 10min. If the MFR is controlled too high, although it will be beneficial to high-speed processing, it will cause increased shrinkage and decreased adhesion; if the MFR is controlled too low, although the adhesion will be improved, there will be a certain degree of hollowing phenomenon after the processing speed exceeds 150m / min. DETAILED DESCRIPTION

[0016] The present invention will be further described below in conjunction with the embodiments.

[0017] It should be noted that: in actual operation, the temperature control allows a temperature fluctuation of 2°C. Unless otherwise specified, all raw materials used in the embodiments and comparative examples are commercially available.

[0018] Embodiment 1~4 The tubular low-density polyethylene composition for extrusion coating of the present invention comprises the following raw materials: The low-density polyethylene is a tubular low-density polyethylene with an MFR of 7.8g / 10min and a density of 0.9185g / cm 3 , molecular weight distribution 12.0; The MFR of ethylene-vinyl acetate copolymer is 15 g / 10 min, and the vinyl acetate content is 15%; The grafting agent is maleic anhydride, technical grade; The initiator is dicumyl peroxide, and the content of dicumyl peroxide is 99%; The antioxidant is 4,4'-thiobis(6-tert-butyl-3-methylphenol); The amounts of the above raw materials are shown in Table 1 in parts by weight.

[0019] The preparation method thereof comprises the following steps: (1) Mix the grafting agent and the initiator and dissolve them in acetone, stir them evenly in a closed mixer and set aside; stir for 3 minutes at a mixer speed of 150 rpm; (2) After mixing low-density polyethylene, ethylene-vinyl acetate copolymer and antioxidant, add the mixture of step (1) thereto and mix well; (3) The raw materials mixed in step (2) are added to a twin-screw extruder for granulation. The discharge MFR is controlled to be 4.5 g / 10 min. During the extrusion process, the exhaust needs to be kept unobstructed to ensure the discharge of unreacted grafting agent and solvent. The negative pressure at the exhaust port is 0.1 MPa. The aspect ratio of the twin-screw extruder is 34, the screw has 10 shear blocks, the screw temperature is 160°C~192°C, the melt temperature is 182°C, and the screw speed is 300 r / min.

[0020] Comparative Example 1 Directly use low-density polyethylene for processing. The low-density polyethylene is tubular low-density polyethylene with an MFR of 7.8g / 10min and a density of 0.9185g / cm 3 , molecular weight distribution 12.0.

[0021] Comparative Example 2 Low-density polyethylene is directly used for processing. The low-density polyethylene is a kettle-processed low-density polyethylene with an MFR of 7.5g / 10min and a density of 0.9171g / cm 3 , molecular weight distribution 18.0.

[0022] Comparative Example 3 The preparation method is similar to that of Example 1, except that the amounts of the raw materials are shown in Table 1 in parts by weight, the screw temperature is 170°C to 200°C, the melt temperature is 188°C, and the MFR is controlled at 3.0 g / 10 min.

[0023] Comparative Example 4 The preparation method is similar to that of Example 1, except that the amounts of the raw materials are shown in Table 1 in parts by weight, the screw temperature is 160°C to 190°C, the melt temperature is 180°C, and the MFR is controlled at 6.5 g / 10 min.

[0024] The above-mentioned Examples 1 to 4 and Comparative Examples 1 to 4 were subjected to performance tests, and the results are shown in Table 2. The test methods are as follows: Melt flow rate (MFR): tested according to GB / T 3682.1-2018, feed amount 3.5g; Density: Test according to GB / T 1033.2-2010, take MFR strips without bubbles and burrs for testing; Peel strength: 180° peel strength is tested according to GB / T 2790-1995. The sample is 25 mm wide and 150 mm long. One end of the sample is torn open by about 20 mm and clamped in a universal electronic tensile machine with a peeling speed of 100 mm / min. Neck shrinkage: The die length of the coating equipment used is 0.8m, the gap is 0.8mm, the line speed is 450m / min, the extrusion temperature is 300℃, and the substrate is a film.

[0025] Table 1 Raw material ratios of examples and comparative examples Table 2 Performance test results of examples and comparative examples Among them, the maximum processing speed of comparative example 3 is 180m / min, and hollowing phenomenon occurs when the speed exceeds 180m / min.

Claims

1. A tubular low-density polyethylene composition for extrusion coating, characterized in that: The invention comprises the following raw materials in parts by weight: Low density polyethylene 90~95 parts Ethylene-vinyl acetate copolymer 5~10 parts Grafting agent 0.05~0.10 parts Initiator 0.005~0.010 parts Antioxidant 0.05~0.10 parts; The total weight of the low-density polyethylene and the ethylene-vinyl acetate copolymer is 100 parts.

2. The tubular low-density polyethylene composition for extrusion coating according to claim 1, characterized in that: The low-density polyethylene is a tubular low-density polyethylene, and its melt mass flow rate is 8.0 g / 10min-12.0 g / 10min, and its density is 0.916-0.920 g / cm 3 , molecular weight distribution is 10.0~15.

0.

3. The tubular low-density polyethylene composition for extrusion coating according to claim 1, characterized in that: The vinyl acetate content of the ethylene-vinyl acetate copolymer is 15% to 18%, and the melt mass flow rate thereof is 10.0 g / 10 min to 20.0 g / 10 min.

4. The tubular low-density polyethylene composition for extrusion coating according to claim 1, characterized in that: The grafting agent includes maleic anhydride, unsaturated silane or acrylic acid.

5. The tubular low-density polyethylene composition for extrusion coating according to claim 1, characterized in that: The initiator is an initiator containing a peroxy group.

6. The tubular low-density polyethylene composition for extrusion coating according to claim 5, characterized in that: These include dicumyl peroxide, benzoyl peroxide, tert-butyl benzoyl peroxide or methyl ethyl ketone peroxide.

7. The tubular low-density polyethylene composition for extrusion coating according to claim 1, characterized in that: The antioxidant includes hindered phenols, hindered amines or phosphites.

8. A method for preparing a tubular low-density polyethylene composition for extrusion coating according to any one of claims 1 to 7, characterized in that: The following steps are involved: (1) Mix the grafting agent and the initiator, dissolve them in a solvent, stir them evenly in a closed mixer, and set aside; (2) After mixing low-density polyethylene, ethylene-vinyl acetate copolymer and antioxidant, add the mixture of step (1) thereto and mix well; (3) The raw materials mixed in step (2) are added into a twin-screw extruder for granulation, and the output MFR is controlled to be 4.0 g / 10 min-5.0 g / 10 min, and the exhaust port is maintained at a negative pressure.

9. The method for preparing a tubular low-density polyethylene composition for extrusion coating according to claim 8, characterized in that: In step (3), the aspect ratio of the twin-screw extruder is ≥33, the number of shear blocks of the screw is ≥6 groups, the screw temperature is 160-210°C, the melt temperature is 180-185°C, and the screw speed is 150-500r / min.

10. The method for preparing a tubular low-density polyethylene composition for extrusion coating according to claim 8, characterized in that: In step (3), the exhaust port maintains a negative pressure of 0.1~0.2MPa.

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