Polyethylene composite film and preparation method thereof

Abstract

The invention provides a polyethylene composite film and a preparation method thereof. The polyethylene composite film comprises an outermost layer, a middle layer and an innermost layer which are arranged in sequence. The outermost layer and the innermost layer comprise 85.0%-94.8% of medium density polyethylene, 5.0%-14.8% of ultra-high molecular weight polyethylene and 0.2%-0.5% of calcium stearate. The middle layer is prepared from 70.0 to 90.0 percent of low density polyethylene, 10.0 to 30.0 percent of metallocene polyethylene and 0 to 5.0 percent of polyethylene oligomer. The outermost layer and the innermost layer contain a certain amount of calcium stearate, so that the crystallinity of a blend is remarkably improved, the softening / melting temperatures of the inner layer, the outer layer and the middle layer can be remarkably distinguished in cooperation with ultra-high molecular weight polyethylene, and a relatively large blow-up ratio is adopted during secondary blow-up, so that the oriented crystallization of polyethylene is improved, and the overall performance of the film is improved; therefore, the prepared film can replace a polypropylene / polyethylene / polypropylene composite film to be applied to a packaging film.

Description

Technical Field

[0001] The present invention relates to the technical field of multilayer structures, in particular to a film, and more particularly to a polyethylene composite film and a preparation method thereof. Background Art

[0002] In recent years, with the continuous expansion of the application fields of polymer materials, the performance requirements for polymer materials have become higher and higher. In many application fields, single-layer products with a single material can no longer meet the user's requirements for performance, functionality and appearance. It is usually necessary to build a multi-layer structure film material to solve this problem. Since the multi-layer film is composed of a variety of polymer materials, it allows the multi-layer film to organically combine the multiple properties of each polymer material with different properties, so that the product has the excellent characteristics of several different materials, thereby meeting the needs of different performance and functions of terminal applications.

[0003] Polypropylene / polyethylene / polypropylene composite film products have a wide range of applications and can replace PE, PVC, etc. to make various plastic bags. Especially in the field of food packaging films that require high barrier properties, the market prospects are broad and have great economic and social significance for promoting the development of the new packaging materials industry.

[0004] However, the main raw materials of the composite film of polypropylene / polyethylene / polypropylene include polypropylene and polyethylene. The physical and chemical properties of polypropylene and polyethylene are different, incompatible, and the degradation products are different. Therefore, if the composite film of polypropylene / polyethylene / polypropylene is recycled, it is not only difficult, but also the processing performance of the mixture obtained after recycling is greatly affected. This composite film is not easy to recycle like a single material film. Summary of the invention

[0005] Based on the above problems, the purpose of the present invention is to provide a polyethylene composite film and a preparation method thereof. The polyethylene composite film is made of polyethylene materials, and is a composite film of a single material, which can be easily recycled. Moreover, the polyethylene composite film is not only recyclable, but also has better mechanical properties and friction resistance, and can replace the application of polypropylene / polyethylene / polypropylene composite film in packaging film.

[0006] To achieve the above object, the first aspect of the present invention provides a polyethylene composite film, comprising an outermost layer, an intermediate layer and an innermost layer arranged in sequence. The outermost layer and the innermost layer are symmetrically distributed with the intermediate layer as the axis, and the outermost layer and the innermost layer comprise 85.0-94.8% of medium-density polyethylene, 5.0-14.8% of ultra-high molecular weight polyethylene and 0.2-0.5% of calcium stearate by weight percentage, and the intermediate layer comprises 70.0-90.0% of low-density polyethylene, 10.0-30.0% of metallocene polyethylene and 0-5.0% of polyethylene oligomer.

[0007] Compared with the prior art, the present invention has the outermost layer and the innermost layer symmetrically distributed with the middle layer as the axis, that is, the entire polyethylene composite film has a symmetrical structure. When affected by external force, each layer in the polyethylene composite film can maintain the uniformity of force to prevent local deformation and even tearing due to uneven force. At the same time, the material of the middle layer is low-density polyethylene, metallocene polyethylene and polyethylene oligomer, and the material of the outermost layer and the innermost layer is medium-density polyethylene, ultra-high molecular weight polyethylene and calcium stearate. Each layer is a polyethylene material. It is a composite film of a single material and can be easily recycled. In addition, calcium stearate, as a nucleating agent, can significantly improve the crystallization state of the blend in the outermost layer and the innermost layer, increase the crystallinity, form a large number of small and uniform spherulites, reduce the stress in the system, and is not easy to cause cracks. At the same time, crystallization restrains the movement of macromolecular chains, enhances the material's ability to resist deformation, and can cooperate with the ultra-high molecular weight polyethylene added to the inner and outer layers to further increase the softening temperature of the inner and outer polyethylene layers. The addition of low-density metallocene polyethylene and polyethylene oligomers in the middle layer can reduce the melting temperature of the middle layer polyethylene, thereby ensuring that during the heating and softening secondary inflation process, the middle layer polyethylene is in a molten state, while the inner and outer layers of polyethylene are only in a softened state, thereby increasing the stability of the film bubble and enabling large-scale horizontal and vertical secondary inflation. Large-scale secondary inflation can greatly improve the mechanical properties of polyethylene, so that the prepared film can replace polypropylene / polyethylene / polypropylene composite film for packaging film applications.

[0008] As a technical solution of the present invention, the thickness ratio of the outermost layer, the middle layer and the innermost layer is 1.5-2.5:3-6:1.5-2.5.

[0009] As a technical solution of the present invention, the density of the medium density polyethylene is 0.935-0.950 g / cm 3 , the melt index is <0.5g / 10min.

[0010] As a technical solution of the present invention, the weight average molecular weight of the ultra-high molecular weight polyethylene is 150,000 to 500,000.

[0011] As a technical solution of the present invention, the density of the low-density polyethylene is 0.910-0.930 g / cm 3 , the melt index is 1.0~3.0g / 10min.

[0012] As a technical solution of the present invention, the density of the metallocene polyethylene is 0.903-0.910 g / cm 3 , the melt index is 2.0~6.0g / 10min.

[0013] As a technical solution of the present invention, the weight average molecular weight of the polyethylene oligomer is 1000-3000.

[0014] As a technical solution of the present invention, the melting point of the middle layer is lower than that of the outermost layer, the melting point of the middle layer is lower than that of the innermost layer, and the melting point of the middle layer is 120-130°C.

[0015] The second aspect of the present invention provides a method for preparing a polyethylene composite film, wherein the raw materials of the outermost layer, the middle layer and the innermost layer are mixed evenly according to a formula ratio, and are respectively sent to the extruder of a three-layer co-extrusion film blowing unit, and then sequentially subjected to extrusion, water cooling, traction, heating and softening at 120-130°C, secondary blowing, and winding and shaping.

[0016] The three-layer film prepared by the method of the present invention has different materials, the middle layer is made of low-density polyethylene, metallocene polyethylene and polyethylene oligomer, and the outermost layer and the innermost layer are made of polyethylene, ultra-high molecular weight polyethylene and calcium stearate. Different films have different melting temperatures, and the middle layer is easier to melt, so when the middle layer begins to melt, the innermost layer and the outermost layer are in a softened state. Therefore, when the middle layer begins to melt and the innermost layer and the outermost layer are in a softened state, inflation is performed to obtain a three-layer composite film with a large inflation ratio. This three-layer composite film is easy to process and has better performance.

[0017] As a technical solution of the present invention, the secondary inflation ratio of the method for preparing the polyethylene composite film is 5-9. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Schematic diagram of the three-layer structure of the polyethylene composite film of the present invention. DETAILED DESCRIPTION

[0019] The technical solution of the present invention is further illustrated below through specific implementation modes, but does not constitute any limitation to the present invention.

[0020] For ease of description, some substances in the present invention are abbreviated, such as polyethylene is PE. Medium-density polyethylene MDPE, ultra-high molecular weight polyethylene UHMPE, which indicates that the molecular weight is extremely high. Low-density polyethylene is LDPE, which indicates the density angle. Metallocene polyethylene is mPE, which indicates polyethylene polymerized under a metallocene catalyst. Polyethylene oligomer is OPE, which indicates that the degree of polymerization of ethylene is not high and is a product of oligomerization.

[0021] like Figure 1As shown, the polyethylene composite film of the present invention comprises an outermost layer 1, an intermediate layer 2 and an innermost layer 3 arranged in sequence, the outermost layer 1 and the innermost layer 3 are symmetrically distributed with the intermediate layer 2 as the axis, and the thickness ratio of the outermost layer 1, the intermediate layer 2 and the innermost layer 3 is 1.5-2.5:3-6:1.5-2.5. Preferably, it is 1.5-2.5:4-6:1.5-2.5, specifically, but not limited to, 1.5:3:1.5, 2.0:3:2.0, 2.5:3:2.5, 1.5:4:1.5, 2.0:4:2.0, 2.5:4:2.5, 1.5:5:1.5, 2.0:5:2.0, 2.5:5:2.5, 1.5:6:1.5, 2.0:6:2.0, 2.5:6:2.5.

[0022] In terms of weight percentage, the outermost layer 1 and the innermost layer 3 include 85.0-94.8% of medium density polyethylene (MDPE), 5.0-14.8% of ultra-high molecular weight polyethylene (UHMPE) and 0.2-0.5% of calcium stearate. The content of MDPE can be, but is not limited to, 85.0%, 85.5%, 86.0%, 86.5%, 87.0%, 87.5%, 88.0%, 88.5%, 89.0%, 89.5%, 90.0%, 90.5%, 91.0%, 91.5%, 92.0%, 92.5%, 93.0%, 93.5%, 94.0%, 94.5%, 94.8%. The density of MDPE is 0.935-0.950 g / cm 3 , the melt index is <0.5g / 10min. Preferably, the density of PE is 0.935-0.945g / cm 3 , the melt index is 0.1-0.3 g / 10 min. UHMPE can be, but not limited to, 5.0%, 6.0%, 7.0%, 8.0%, 9.0%, 10.0%, 11.0%, 12.0%, 13.0%, 14.0%, 14.5%, 14.8%. The weight average molecular weight of UHMPE is 150,000-500,000. Preferably, the weight average molecular weight of UHMPE is 150,000-250,000. The content of calcium stearate can be, but not limited to, 0.2%, 0.3%, 0.4%, 0.5%.

[0023] The middle layer 2 includes 70.0-90.0% of low density polyethylene (LDPE), 10.0-30.0% of metallocene polyethylene (mPE) and 0-5.0% of polyethylene oligomer (OPE). The content of LDPE can be, but is not limited to, 70.0%, 71.0%, 72.0%, 73.0%, 74.0%, 75.0%, 76.0%, 77.0%, 78.0%, 79.0%, 80.0%, 81.0%, 82.0%, 83.0%, 84.0%, 85.0%, 86.0%, 87.0%, 88.0%, 89.0%, 90.0%. The density of LDPE is 0.910-0.930 g / cm 3 , the melt index is 1.0-3.0 g / 10 min. Preferably, the density of LDPE is 0.920-0.925 g / cm 3 , melt index is 1.5-2.5g / 10min. mPE can be but not limited to 10.0%, 12.0%, 14.0%, 16.0%, 18.0%, 20.0%, 22.0%, 24.0%, 26.0%, 28.0%, 30.0%. The density of mPE is 0.903-0.910g / cm 3 , the melt index is 2.0-6.0 g / 10 min. Preferably, the density of mPE is 0.905-0.910 g / cm 3 , the melt index is 2.5-4.5g / 10min. The content of OPE can be but is not limited to 0, 1.0%, 2.0%, 3.0%, 4.0%, 5.0%. The weight average molecular weight of the polyethylene polymer is 1000-3000. Preferably, the weight average molecular weight of the polyethylene polymer is 1500-2500. The melting point of the middle layer is lower than that of the outermost layer and the innermost layer, and the melting point of the middle layer is 120-130°C. That is to say, with the surface contacting the article to be packaged as the inner side, the polyethylene composite film includes the outermost layer 1, the middle layer 2 and the innermost layer 3 in sequence, and the thickness of the polyethylene composite film is 8-40um.

[0024] The polyethylene composite film of the present invention can be prepared by the following method, which comprises: mixing the raw materials of the outermost layer, the middle layer and the innermost layer uniformly according to the formula ratio, respectively sending them to the extruder of the three-layer co-extrusion film blowing unit, and then sequentially extruding, cooling by water, pulling, heating and softening at 120-130° C., secondary blowing, and winding and shaping. The secondary blowing ratio can be 5-9, preferably, the secondary blowing ratio can reach 7.

[0025] The polyethylene composite film of the present invention will be described in detail below through specific examples, but the implementation scope of the present invention is not limited thereto.

[0026] Example 1

[0027] The polyethylene composite film includes an outermost layer, a middle layer and an innermost layer in sequence. The total thickness of the polyethylene composite film is 40 μm. The performance parameters of each layer are as follows.

[0028] Outermost layer: 10 μm thick, comprising 93.6 wt.% of medium-density polyethylene, 6.0 wt.% of ultra-high molecular weight polyethylene and 0.4 wt.% of calcium stearate.

[0029] Middle layer: with a thickness of 20 μm, the middle layer comprises 80.0 wt.% of low-density polyethylene, 18.0 wt.% of metallocene polyethylene and 2.0 wt.% of polyethylene oligomer.

[0030] Innermost layer: 10 μm thick, comprising 93.6 wt.% of medium-density polyethylene, 6.0 wt.% of ultra-high molecular weight polyethylene and 0.4 wt.% of calcium stearate.

[0031] The parameters of various materials are:

[0032] The density of medium density polyethylene is 0.940g / cm 3 , melt index is 0.2g / 10min;

[0033] The weight average molecular weight of ultra-high molecular weight polyethylene is 200,000 to 250,000;

[0034] The density of low-density polyethylene is 0.921 g / cm 3 , melt index is 2.0g / 10min;

[0035] The density of metallocene polyethylene is 0.907 g / cm 3 , melt index is 3.5g / 10min;

[0036] The weight average molecular weight of the polyethylene oligomer is 1500-2000.

[0037] The corresponding raw materials of the three layers are mixed evenly according to the formula ratio, and sent to the extruder of the three-layer co-extrusion film blowing unit respectively, and then sequentially extruded, water cooled, pulled, heated and softened at 125±3℃, secondarily blown, and wound and shaped.

[0038] Example 2

[0039] The polyethylene composite film includes an outermost layer, a middle layer and an innermost layer in sequence. The total thickness of the polyethylene composite film is 38 μm. The performance parameters of each layer are as follows.

[0040] Outermost layer: 7 μm thick, comprising 93.6 wt.% of medium-density polyethylene, 6.0 wt.% of ultra-high molecular weight polyethylene and 0.4 wt.% of calcium stearate.

[0041] Middle layer: with a thickness of 14 μm, the middle layer comprises 80.0 wt.% of low-density polyethylene, 18.0 wt.% of metallocene polyethylene and 2.0 wt.% of polyethylene oligomer.

[0042] Innermost layer: 7 μm thick, comprising 93.6 wt.% of medium-density polyethylene, 6.0 wt.% of ultra-high molecular weight polyethylene and 0.4 wt.% of calcium stearate.

[0043] The parameters of various materials are:

[0044] The density of medium density polyethylene is 0.940g / cm 3 , melt index is 0.2g / 10min;

[0045] The weight average molecular weight of ultra-high molecular weight polyethylene is 200,000 to 250,000;

[0046] The density of low-density polyethylene is 0.921 g / cm 3 , melt index is 2.0g / 10min;

[0047] The density of metallocene polyethylene is 0.907 g / cm 3 , melt index is 3.5g / 10min;

[0048] The weight average molecular weight of the polyethylene oligomer is 1500-2000.

[0049] The corresponding raw materials of the three layers are mixed evenly according to the formula ratio, and respectively sent to the extruder of the three-layer co-extrusion film blowing unit, and then sequentially extruded, water cooled, pulled, heated and softened at 120-130°C, secondarily blown, and rolled for shaping.

[0050] Example 3

[0051] The polyethylene composite film includes an outermost layer, a middle layer and an innermost layer in sequence. The total thickness of the polyethylene composite film is 36 μm. The performance parameters of each layer are as follows.

[0052] Outermost layer: 9 μm thick, comprising 86.0 wt.% of medium-density polyethylene, 13.5 wt.% of ultra-high molecular weight polyethylene and 0.5 wt.% of calcium stearate.

[0053] Middle layer: with a thickness of 18 μm, the middle layer comprises 90.0 wt.% of low-density polyethylene and 10.0 wt.% of metallocene polyethylene.

[0054] Innermost layer: 9 μm thick, comprising 86.0 wt.% of medium-density polyethylene, 13.5 wt.% of ultra-high molecular weight polyethylene and 0.5 wt.% of calcium stearate.

[0055] The parameters of various materials are:

[0056] The density of medium density polyethylene is 0.940g / cm 3 , melt index is 0.2g / 10min;

[0057] The weight average molecular weight of ultra-high molecular weight polyethylene is 200,000 to 250,000;

[0058] The density of low-density polyethylene is 0.921 g / cm 3 , melt index is 2.0g / 10min;

[0059] The density of metallocene polyethylene is 0.907 g / cm 3 , melt index is 3.5g / 10min;

[0060] The weight average molecular weight of the polyethylene oligomer is 1500-2000.

[0061] The corresponding raw materials of the three layers are mixed evenly according to the formula ratio, and respectively sent to the extruder of the three-layer co-extrusion film blowing unit, and then sequentially extruded, water cooled, pulled, heated and softened at 120-130°C, secondarily blown, and rolled for shaping.

[0062] Example 4

[0063] The polyethylene composite film includes an outermost layer, a middle layer and an innermost layer in sequence. The total thickness of the polyethylene composite film is 40 μm. The performance parameters of each layer are as follows.

[0064] Outermost layer: 10 μm thick, comprising 93.6 wt.% of medium-density polyethylene, 6.0 wt.% of ultra-high molecular weight polyethylene and 0.4 wt.% of calcium stearate.

[0065] Middle layer: with a thickness of 20 μm, the middle layer comprises 75.0 wt.% of low-density polyethylene, 20.0 wt.% of metallocene polyethylene and 5.0 wt.% of polyethylene oligomer.

[0066] Innermost layer: 10 μm thick, comprising 93.6 wt.% of medium-density polyethylene, 6.0 wt.% of ultra-high molecular weight polyethylene and 0.4 wt.% of calcium stearate.

[0067] The parameters of various materials are:

[0068] The density of medium density polyethylene is 0.935g / cm 3 , melt index is 0.3g / 10min;

[0069] The weight average molecular weight of ultra-high molecular weight polyethylene is 300,000 to 350,000;

[0070] The density of low-density polyethylene is 0.925 g / cm 3 , melt index is 1.5g / 10min;

[0071] The density of metallocene polyethylene is 0.905g / cm 3 , melt index is 4.5g / 10min;

[0072] The weight average molecular weight of the polyethylene oligomer is 2,000 to 2,500.

[0073] The raw materials corresponding to the three layers are mixed evenly according to the formula ratio, and sent to the extruder of the three-layer co-extrusion film blowing unit respectively, and then sequentially extruded, water cooled, pulled, heated and softened at 120-130°C, secondarily inflated, and wound and shaped.

[0074] Comparative Example 1

[0075] The polyethylene composite film includes an outermost layer, a middle layer and an innermost layer in sequence. The total thickness of the polyethylene composite film is 40 μm. The performance parameters of each layer are as follows.

[0076] Outermost layer: 10 μm thick, comprising 94.0 wt.% of medium-density polyethylene and 6.0 wt.% of ultra-high molecular weight polyethylene.

[0077] Middle layer: with a thickness of 20 μm, the middle layer comprises 80.0 wt.% of low-density polyethylene, 18.0 wt.% of metallocene polyethylene and 2.0 wt.% of polyethylene oligomer.

[0078] Innermost layer: 10 μm thick, comprising 94 wt.% of medium-density polyethylene and 6.0 wt.% of ultra-high molecular weight polyethylene.

[0079] The parameters of various materials are:

[0080] The density of medium density polyethylene is 0.940g / cm 3 , melt index is 0.2g / 10min;

[0081] The weight average molecular weight of ultra-high molecular weight polyethylene is 200,000 to 250,000;

[0082] The density of low-density polyethylene is 0.921 g / cm 3 , melt index is 2.0g / 10min;

[0083] The density of metallocene polyethylene is 0.907 g / cm 3 , melt index is 3.5g / 10min;

[0084] The weight average molecular weight of the polyethylene oligomer is 1500-2000.

[0085] The raw materials corresponding to the three layers are mixed evenly according to the formula ratio, and sent to the extruder of the three-layer co-extrusion film blowing unit respectively, and then sequentially extruded, water cooled, pulled, heated and softened at 120-130°C, secondarily inflated, and wound and shaped.

[0086] Comparative Example 2

[0087] The polyethylene composite film includes an outermost layer, a middle layer and an innermost layer in sequence. The total thickness of the polyethylene composite film is 40 μm. The performance parameters of each layer are as follows.

[0088] Outermost layer: 10 μm thick, comprising 93.6 wt.% of medium-density polyethylene, 6.0 wt.% of ultra-high molecular weight polyethylene and 0.4 wt.% of talc.

[0089] Middle layer: with a thickness of 20 μm, the middle layer comprises 80.0 wt.% of low-density polyethylene, 18.0 wt.% of metallocene polyethylene and 2.0 wt.% of polyethylene oligomer.

[0090] Innermost layer: 10 μm thick, comprising 93.6 wt.% of medium-density polyethylene, 6.0 wt.% of ultra-high molecular weight polyethylene and 0.4 wt.% of talc.

[0091] The parameters of various materials are:

[0092] The density of medium density polyethylene is 0.940g / cm 3 , melt index is 0.2g / 10min;

[0093] The weight average molecular weight of ultra-high molecular weight polyethylene is 200,000 to 250,000;

[0094] The density of low-density polyethylene is 0.921 g / cm 3 , melt index is 2.0g / 10min;

[0095] The density of metallocene polyethylene is 0.907 g / cm 3 , melt index is 3.5g / 10min;

[0096] The weight average molecular weight of the polyethylene oligomer is 1500-2000.

[0097] The raw materials corresponding to the three layers are mixed evenly according to the formula ratio, and sent to the extruder of the three-layer co-extrusion film blowing unit respectively, and then sequentially extruded, water cooled, pulled, heated and softened at 120-130°C, secondarily inflated, and wound and shaped.

[0098] Comparative Example 3

[0099] The polyethylene composite film includes an outermost layer, a middle layer and an innermost layer in sequence. The total thickness of the polyethylene composite film is 40 μm. The performance parameters of each layer are as follows.

[0100] Outermost layer: 10 μm thick, comprising 93.95 wt.% of medium-density polyethylene, 6.0 wt.% of ultra-high molecular weight polyethylene and 0.05 wt.% of calcium stearate.

[0101] Middle layer: with a thickness of 20 μm, the middle layer comprises 80.0 wt.% of low-density polyethylene, 18.0 wt.% of metallocene polyethylene and 2.0 wt.% of polyethylene oligomer.

[0102] Innermost layer: 10 μm thick, comprising 93.95 wt.% of medium density polyethylene, 6.0 wt.% of ultra-high molecular weight polyethylene and 0.05 wt.% of calcium stearate.

[0103] The parameters of various materials are:

[0104] The density of medium density polyethylene is 0.940g / cm 3 , melt index is 0.2g / 10min;

[0105] The weight average molecular weight of ultra-high molecular weight polyethylene is 200,000 to 250,000;

[0106] The density of low-density polyethylene is 0.921 g / cm 3 , melt index is 2.0g / 10min;

[0107] The density of metallocene polyethylene is 0.907 g / cm 3 , melt index is 3.5g / 10min;

[0108] The weight average molecular weight of the polyethylene oligomer is 1500-2000.

[0109] The raw materials corresponding to the three layers are mixed evenly according to the formula ratio, and sent to the extruder of the three-layer co-extrusion film blowing unit respectively, and then sequentially extruded, water cooled, pulled, heated and softened at 120-130°C, secondarily inflated, and wound and shaped.

[0110] Comparative Example 4

[0111] The polyethylene composite film includes an outermost layer, a middle layer and an innermost layer in sequence. The total thickness of the polyethylene composite film is 40 μm. The performance parameters of each layer are as follows.

[0112] Outermost layer: 10 μm thick, comprising 93.0 wt.% of medium-density polyethylene, 6.0 wt.% of ultra-high molecular weight polyethylene and 1.0 wt.% of calcium stearate.

[0113] Middle layer: with a thickness of 20 μm, the middle layer comprises 80.0 wt.% of low-density polyethylene, 18.0 wt.% of metallocene polyethylene and 2.0 wt.% of polyethylene oligomer.

[0114] Innermost layer: 10 μm thick, comprising 93.6 wt.% of medium density polyethylene, 6.0 wt.% of ultra-high molecular weight polyethylene and 1.0 wt.% of calcium stearate.

[0115] The parameters of various materials are:

[0116] The density of medium density polyethylene is 0.940g / cm 3 , melt index is 0.2g / 10min;

[0117] The weight average molecular weight of ultra-high molecular weight polyethylene is 200,000 to 250,000;

[0118] The density of low-density polyethylene is 0.921 g / cm 3 , melt index is 2.0g / 10min;

[0119] The density of metallocene polyethylene is 0.907 g / cm 3 , melt index is 3.5g / 10min;

[0120] The weight average molecular weight of the polyethylene oligomer is 1500-2000.

[0121] The raw materials corresponding to the three layers are mixed evenly according to the formula ratio, and sent to the extruder of the three-layer co-extrusion film blowing unit respectively, and then sequentially extruded, water cooled, pulled, heated and softened at 120-130°C, secondarily inflated, and wound and shaped.

[0122] Comparative Example 5

[0123] The polyethylene composite film includes an outermost layer, a middle layer and an innermost layer in sequence. The total thickness of the polyethylene composite film is 40 μm. The performance parameters of each layer are as follows.

[0124] Outermost layer: 10 μm thick, comprising 99.6 wt.% of medium-density polyethylene and 0.4 wt.% of calcium stearate.

[0125] Middle layer: with a thickness of 20 μm, the middle layer comprises 80.0 wt.% of low-density polyethylene, 18.0 wt.% of metallocene polyethylene and 2.0 wt.% of polyethylene oligomer.

[0126] Innermost layer: 10 μm thick, comprising 99.6 wt.% of medium-density polyethylene and 0.4 wt.% of calcium stearate.

[0127] The parameters of various materials are:

[0128] The density of medium density polyethylene is 0.940g / cm 3 , melt index is 0.2g / 10min;

[0129] The density of low-density polyethylene is 0.921 g / cm 3 , melt index is 2.0g / 10min;

[0130] The density of metallocene polyethylene is 0.907 g / cm 3 , melt index is 3.5g / 10min;

[0131] The weight average molecular weight of the polyethylene oligomer is 1500-2000.

[0132] The corresponding raw materials of the three layers are mixed evenly according to the formula ratio, and sent to the extruder of the three-layer co-extrusion film blowing unit respectively, and then sequentially extruded, water cooled, pulled, heated and softened at 125±3℃, secondarily blown, and wound and shaped.

[0133] The polyethylene composite films prepared in Examples 1 to 4 and Comparative Examples 1 to 5 were tested for dart drop impact performance, tensile properties, hardness and abrasion resistance, respectively. The testing methods are as follows.

[0134] Drop impact performance: The drop impact performance test is carried out using the GB9639-1988 plastic film free-fall dart impact testing machine testing method to obtain the corresponding drop impact strength value.

[0135] Tensile performance test: GB13022-1991 film tensile performance test method was used to test the tensile performance and obtain the corresponding tensile strength value.

[0136] Tear performance: GB / T16578.1-2008 Determination method for tear resistance of plastic films and sheets was used to test the tear performance and obtain the corresponding tear strength value.

[0137] Friction resistance test: GB / T 13519-2016 "Polyethylene heat shrinkable film for packaging" is used to test the friction coefficient.

[0138] Table 1 Properties of polyethylene composite films of Examples 1 to 4 and Comparative Examples 1 to 5

[0139]

[0140]

[0141] From the results in Table 1, it can be seen that the drop impact strength, tensile strength and tear strength of Examples 1 to 4 are better than those of Comparative Examples 1 to 5, that is to say, the mechanical properties of the polyethylene composite films in Examples 1 to 4 are better than those of Comparative Examples 1 to 5, mainly because the outermost layer and the innermost layer contain a certain amount of calcium stearate, which significantly improves the crystallinity of the blend, significantly distinguishes the softening / melting temperature of the inner and outer layers and the middle layer, and achieves a larger blowing ratio during secondary blowing, thereby improving the oriented crystallization of polyethylene and improving the overall performance of the film.

[0142] In Comparative Examples 1 and 2, no calcium stearate or other fillers are added to the inner and outer layers, which cannot effectively promote the crystallization of polyethylene and the compatibility of polyethylene with ultra-high molecular weight polyethylene, resulting in a very low secondary blowing ratio and poor film performance.

[0143] In Comparative Example 3, the addition ratio of calcium stearate is low, and the crystallinity of the inner and outer layers is not improved much, so that the difference in softening / melting point between the inner and outer layers and the middle layer is not obvious, and a secondary inflation with a large inflation ratio cannot be formed, resulting in little improvement in performance after the secondary inflation.

[0144] In Comparative Example 4, the calcium stearate content is too high. When the amount of calcium stearate added is further increased, the calcium stearate agglomerates to a certain size, which hinders the movement of the PE molecular chain and forms stress concentration points inside the PE, causing the mechanical properties of the material to decrease and also reducing the inflation ratio of the secondary inflation.

[0145] In Comparative Example 5, no ultra-high molecular weight polyethylene was added to the inner and outer layers. The effect of increasing the softening point of the inner and outer layers by simply increasing the crystallinity of polyethylene with calcium stearate was not obvious, resulting in the softening point of the inner and outer layers being close to the melting point of the middle layer during the secondary inflation, and the inflation ratio being small, which reduced the mechanical properties.

[0146] The above description of the disclosed embodiments is intended to enable those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles disclosed herein.

Claims

1. A polyethylene composite film, comprising an outermost layer, an intermediate layer and an innermost layer arranged in sequence, characterized in that: The outermost layer and the innermost layer are symmetrically distributed with the middle layer as the axis. By weight percentage, the outermost layer and the innermost layer include 85.0-94.8% of medium-density polyethylene, 5.0-14.8% of ultra-high molecular weight polyethylene and 0.2-0.5% of calcium stearate, and the middle layer includes 70.0-90.0% of low-density polyethylene, 10.0-30.0% of metallocene polyethylene and 0-5.0% of polyethylene polymer.

2. The polyethylene composite film according to claim 1, characterized in that: The thickness ratio of the outermost layer, the middle layer and the innermost layer is 1.5-2.5:3-6:1.5-2.

5.

3. The polyethylene composite film according to claim 1, characterized in that: The density of the medium density polyethylene is 0.935-0.950 g / cm 3 , the melt index is <0.5g / 10min.

4. The polyethylene composite film according to claim 1, characterized in that: The weight average molecular weight of the ultra-high molecular weight polyethylene is 150,000 to 500,000.

5. The polyethylene composite film according to claim 1, characterized in that: The density of the low-density polyethylene is 0.910-0.930 g / cm 3 , the melt index is 1.0~3.0g / 10min.

6. The polyethylene composite film according to claim 1, characterized in that: The density of the metallocene polyethylene is 0.903-0.910 g / cm 3 , the melt index is 2.0~6.0g / 10min.

7. The polyethylene composite film according to claim 1, characterized in that: The weight average molecular weight of the polyethylene oligomer is 1000-3000.

8. The polyethylene composite film according to claim 1, characterized in that: The melting point of the middle layer is lower than that of the outermost layer, the melting point of the middle layer is lower than that of the innermost layer, and the melting point of the middle layer is 120-130°C.

9. The method for preparing a polyethylene composite film according to any one of claims 1 to 8, characterized in that: include: The raw materials of the outermost layer, the middle layer and the innermost layer are mixed evenly according to the formula ratio, and respectively sent to the extruder of the three-layer co-extrusion film blowing unit, and then sequentially extruded, water cooled, pulled, heated and softened at 120-130°C, secondarily inflated, and rolled for shaping.

10. The method for preparing a polyethylene composite film according to claim 9, characterized in that: The secondary blowing ratio is 5-9.

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