CN107304264B

CN107304264B - Polyethylene composition, polyethylene film and preparation method thereof

Info

Publication number: CN107304264B
Application number: CN201610250982.9A
Authority: CN
Inventors: 李汝贤; 于鲁强; 方园园; 王路生; 高岩; 高达利; 施红伟
Original assignee: Sinopec Beijing Research Institute of Chemical Industry; China Petroleum and Chemical Corp
Current assignee: Sinopec Beijing Research Institute of Chemical Industry; China Petroleum and Chemical Corp
Priority date: 2016-04-21
Filing date: 2016-04-21
Publication date: 2020-12-18
Anticipated expiration: 2036-04-21
Also published as: CN107304264A

Abstract

The invention relates to the field of polyethylene films, and particularly provides a polyethylene composition, a polyethylene film and preparation methods thereof. The polyethylene composition comprises the product prepared according to the following method: carrying out first polymerization reaction on ethylene and alpha-olefin in the presence of a catalyst, and then carrying out second polymerization reaction on the first polymerization reaction product, the ethylene and the alpha-olefin, wherein the conditions of the first polymerization reaction and the second polymerization reaction are that the weight average molecular weight Mw of the obtained second polymerization reaction product is 8-20 ten thousand g/mol, the Mw/Mn is more than or equal to 4.5, the Mz/Mw is more than or equal to 4.5, and the Mz +1/Mw is more than or equal to 7.5. When the polyethylene composition is used for preparing a polyethylene film by adopting a flat film drawing method, the drawing multiplying power is large, the film forming rate is high, the film forming property is very good, and the film prepared from the polyethylene composition also has excellent mechanical property and optical property.

Description

Polyethylene composition, polyethylene film and preparation method thereof

Technical Field

The invention relates to the field of polyethylene, in particular to a polyethylene composition, a preparation method of the polyethylene composition, the polyethylene composition prepared by the method, a polyethylene film, a preparation method of the polyethylene film and the polyethylene film prepared by the method.

Background

The Biaxially Oriented Polyethylene (BOPE) film is a film material formed by Biaxially stretching Polyethylene (PE) resin with a special molecular structure. In the forming and processing process of the BOPE film, after the film is stretched, the PE macromolecular chains and the crystal structure are highly oriented, so that the tensile strength of the film is obviously improved, the tensile breaking elongation is reduced, and the grain size in the film is reduced by the stretching crystallization in the biaxial stretching process, so that the film has lower haze, better transparency and higher glossiness. Therefore, the BOPE film can be widely used in the aspects of packaging bags, heavy packaging bags, vacuum heat-sealing films, low-temperature packaging films, composite films, medical and sanitary products, agricultural greenhouse films, mulching films and the like, and has the advantages of high mechanical strength, good puncture resistance and impact resistance, excellent optical performance, energy conservation, environmental protection and the like compared with polyethylene film products manufactured by the existing extrusion blow molding process and extrusion casting process.

The two-way stretching processing method of the plastic film comprises a flat film stretching method and a tube bubble stretching method at present. The flat film stretching method is already applied to the Processing of Polypropylene (PP), Polyamide (PA), polyethylene terephthalate (PET) and other film materials, and the process is mature. Compared with a tube bubble stretching method, the flat film stretching method has the advantages of large stretching ratio (the transverse stretching ratio can reach more than 10 times), high forming speed (the highest rolling speed can reach hundreds of meters per minute), high production efficiency, better mechanical strength, optical performance and thickness uniformity of the obtained film, obvious influence of process condition fluctuation on film forming, large film stretching processing difficulty and higher requirements on film raw materials. Most of the existing biaxially oriented polyethylene raw materials are limited to the preparation of BOPE films by a tube bubble stretching method, and when the existing biaxially oriented polyethylene raw materials are used for preparing BOPE films by a flat film stretching method, the defects of poor film forming property (low stretching speed and stretching ratio) and easy film cracking exist, namely, the biaxially oriented polyethylene raw materials are basically not suitable for preparing films by the flat film stretching method. Therefore, in order to fully utilize the above advantages of the flat film stretching method, it is required to develop a polyethylene raw material suitable for preparing a BOPE thin film by the flat film stretching method, which has good film forming property and is less likely to cause film cracking.

Disclosure of Invention

The invention aims to overcome the defects that when the existing polyethylene raw material is used for preparing a polyethylene film by a flat film stretching method, the film forming property is poor, the film is easy to break, and the film is not suitable for preparing the film by the flat film stretching method, and provides a novel polyethylene composition, a preparation method of the polyethylene composition, the polyethylene composition prepared by the method, a polyethylene film, a preparation method of the polyethylene film and the polyethylene film prepared by the method.

Specifically, the invention provides a polyethylene composition comprising the product produced according to the following process: carrying out first polymerization reaction on ethylene and alpha-olefin in the presence of a catalyst, and then carrying out second polymerization reaction on the first polymerization reaction product, the ethylene and the alpha-olefin, wherein the conditions of the first polymerization reaction and the second polymerization reaction are that the weight average molecular weight Mw of the obtained second polymerization reaction product is 8-20 ten thousand g/mol, the Mw/Mn is more than or equal to 4.5, the Mz/Mw is more than or equal to 4.5, and the Mz +1/Mw is more than or equal to 7.5.

The invention also provides a preparation method of the polyethylene composition, which comprises the steps of carrying out first polymerization reaction on ethylene and alpha-olefin in the presence of a catalyst, and then carrying out second polymerization reaction on the first polymerization reaction product, the ethylene and the alpha-olefin, wherein the conditions of the first polymerization reaction and the second polymerization reaction are that the weight-average molecular weight Mw of the obtained second polymerization reaction product is 8-20 ten thousand g/mol, the Mw/Mn is more than or equal to 4.5, the Mz/Mw is more than or equal to 4.5, and the Mz +1/Mw is more than or equal to 7.5.

The invention also provides the polyethylene composition prepared by the method.

The invention also provides a polyethylene film, which at least comprises one polyethylene layer prepared from the polyethylene composition.

The invention also provides a preparation method of the polyethylene film, which comprises the steps of carrying out extrusion casting on the polyethylene composition and stretching the obtained cast sheet.

In addition, the invention also provides the polyethylene film prepared by the method.

After intensive research, the inventor of the invention finds that the polyethylene composition with the weight-average molecular weight Mw of 8-20 ten thousand g/mol, Mw/Mn of more than or equal to 4.5, Mz/Mw of more than or equal to 4.5 and Mz +1/Mw of more than or equal to 7.5, which is obtained by adopting two-stage series polymerization reaction, has the advantages of large stretching multiplying power and high film forming rate when a film is prepared by adopting a biaxial stretching method, can meet the higher requirements of a flat film stretching method on polyethylene raw materials, and has more excellent mechanical properties and optical properties, such as high stretching strength, good puncture resistance and impact resistance, low haze, high glossiness and the like, thereby having great industrial application prospect.

According to a preferred embodiment of the present invention, when the polyethylene composition has a temperature rising elution curve measured by an analytical temperature rising elution fractionation method, the temperature rising elution curve includes a high temperature elution peak and a low temperature elution peak, the peak temperature of the high temperature elution peak is 90-110 ℃, the peak temperature of the low temperature elution peak is 50-90 ℃, the total area of the high temperature elution peak and the low temperature elution peak is taken as a reference, the area of the high temperature elution peak is less than or equal to 80%, and the area of the low temperature elution peak is greater than or equal to 20%, the polyethylene composition has better film forming property, and a film prepared from the polyethylene composition has more excellent mechanical property and optical property.

Additional features and advantages of the invention will be set forth in the detailed description which follows.

Detailed Description

The following describes in detail specific embodiments of the present invention. It should be understood that the detailed description and specific examples, while indicating the present invention, are given by way of illustration and explanation only, not limitation.

The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value, and such ranges or values should be understood to encompass values close to those ranges or values. For ranges of values, between the endpoints of each of the ranges and the individual points, and between the individual points may be combined with each other to give one or more new ranges of values, and these ranges of values should be considered as specifically disclosed herein.

The polyethylene composition provided by the invention contains a product prepared according to the following method: carrying out first polymerization reaction on ethylene and alpha-olefin in the presence of a catalyst, and then carrying out second polymerization reaction on the first polymerization reaction product, the ethylene and the alpha-olefin, wherein the conditions of the first polymerization reaction and the second polymerization reaction are that the weight average molecular weight Mw of the obtained second polymerization reaction product is 8-20 ten thousand g/mol, the Mw/Mn is more than or equal to 4.5, the Mz/Mw is more than or equal to 4.5, and the Mz +1/Mw is more than or equal to 7.5. Preferably, the second polymerization product has a weight average molecular weight Mw of from 10 to 15 ten thousand g/mol, and Mw/Mn of from 4.5 to 9.0, Mz/Mw of from 4.5 to 7.0, and Mz +1/Mw of from 7.5 to 13.5. In the present invention, Mw, Mn, Mz, and Mz +1 are measured by a high temperature Gel Permeation Chromatograph (GPC).

In order to obtain better film-forming properties in the resulting polyethylene composition and better mechanical and optical properties in films made from the polyethylene composition, according to the present invention, preferably the first polymerization product has a weight average molecular weight Mw of from 8 to 50 ten thousand g/mol, and an Mw/Mn of from 2.5 to 8.5, an Mz/Mw of from 2.0 to 6.5, and an Mz +1/Mw of from 4.5 to 12.0; more preferably, the weight average molecular weight Mw of the first polymerization reaction product ranges from 8 to 30 ten thousand g/mol, and Mw/Mn ranges from 3.0 to 7.0, Mz/Mw ranges from 2.5 to 5.5, and Mz +1/Mw ranges from 4.5 to 9.5.

The polyethylene composition provided according to the present invention has a temperature rising elution profile, as measured by analytical Temperature Rising Elution Fractionation (TREF), comprising a high temperature elution peak and a low temperature elution peak, in addition to a room temperature soluble peak, and the peak temperature of the high temperature elution peak is preferably 90 to 110 ℃, more preferably 95 to 100 ℃; the peak temperature of the low temperature elution peak is preferably 50 to 90 ℃, more preferably 60 to 90 ℃. It should be noted that the peak with a temperature of 90 ℃ in the temperature rising elution curve belongs to the category of the low temperature elution peak. Wherein the area of the room temperature soluble peak is usually not more than 5%. In addition, an excessively large area of the high-temperature elution peak may result in deterioration of optical properties of a biaxially oriented film produced from the polyethylene composition, and an excessively large area of the low-temperature elution peak may result in deterioration of mechanical properties of a biaxially oriented film produced from the polyethylene composition. Therefore, in order to enable the polyethylene composition to have good film forming performance and simultaneously balance the optical performance and mechanical performance of the obtained film better, the area of the high-temperature elution peak is preferably less than or equal to 80 percent and the area of the low-temperature elution peak is preferably more than or equal to 20 percent based on the total area of the high-temperature elution peak and the low-temperature elution peak; more preferably, the total area of the high-temperature elution peak and the low-temperature elution peak is taken as a reference, the area of the high-temperature elution peak is less than or equal to 70%, and the area of the low-temperature elution peak is more than or equal to 30%; most preferably, the area of the high-temperature elution peak is 10-60% and the area of the low-temperature elution peak is 40-90% based on the total area of the high-temperature elution peak and the low-temperature elution peak.

According to the present invention it is particularly preferred that the polyethylene composition has a melt index at a temperature of 190 ℃ under a load of 2.16kg of from 0.1 to 20g/10min, most preferably from 0.5 to 10g/10 min. On the basis of obtaining the above-mentioned polyethylene composition by two-stage tandem polymerization, controlling the melt index of the whole polyethylene composition within the above-mentioned preferred range enables the resulting polyethylene composition to have very excellent film forming properties, and the resulting polyethylene film has higher tensile strength and optical properties. In the present invention, the melt index is measured according to the method specified in GB/T3682-2000, wherein the test conditions include a temperature of 190 ℃ and a load of 2.16 kg.

Generally, if the melting temperature of the polyethylene composition is too high, the solidification speed of the cast sheet in the subsequent casting process is too high, which is not favorable for the flatness of the cast sheet; if the melting temperature of the polyethylene composition is too low, the solidification speed of the cast sheet in the subsequent casting process is too low, which is not favorable for improving the processing efficiency. Therefore, in order to provide the polyethylene composition with good film forming properties and excellent mechanical and optical properties, as well as good tape casting processability and high processing efficiency, the polyethylene composition preferably has a melting peak temperature of 100-.

The content of the structural unit derived from the alpha-olefin in the first polymerization reaction product and the second polymerization reaction product is not particularly limited in the present invention, and may be the same as or different from each other, and the content of the structural unit derived from the alpha-olefin in each of the first polymerization reaction product and the second polymerization reaction product may be each independently 0.2 to 20 mol%, preferably 2 to 10 mol%, based on the total content of the ethylene structural unit and the structural unit derived from the alpha-olefin in the first polymerization reaction product and the second polymerization reaction product, respectively. In the present invention, the ethylene structural unit means a structural unit formed by polymerization of ethylene; the structural unit derived from an alpha olefin means a structural unit formed by polymerization of an alpha olefin. In the present invention, the contents of the ethylene structural unit and the structural unit derived from the alpha olefin are employed¹³C nuclear magnetic resonance spectroscopy (NMR).

The alpha-olefins used in the first polymerization reaction and the second polymerization reaction may be the same or different and may be C independently₃-C₂₀At least one of olefins. The alpha-olefin used in the first polymerization reaction and the second polymerization reaction is preferably propylene, 1-butene, 2-butene, 3-methyl-1-butene, 4-methyl-1-butene, 1-pentene, 3-methyl-1-pentene, 4-methyl-1-pentene, 3-dimethyl-1-pentene, 3, 4-dimethyl-1-pentene, 4-dimethyl-1-pentene, 1-hexene, 4-methyl-1-hexene, 5-methyl-1-hexene, 1-heptene, 2-heptene, 1-octene, 1-decene, 1-dodecene, etc., from the viewpoint of availability of raw materials, At least one of 1-tetradecene, 1-hexadecene, 1-octadecene and 1-eicosene, and more preferably at least one of 1-butene, 1-hexene and 1-octene.

According to the present invention, there is provided a polyethylene composition, preferably, further comprising a lubricant, which is capable of improving extrusion processability of the polyethylene composition. The type and amount of the lubricant may be conventionally selected in the art, and for example, the lubricant may be selected from at least one of polyethylene glycol (PEG) type lubricant, fluoropolymer type lubricant, silicone type lubricant, fatty alcohol type lubricant, fatty acid ester type lubricant, stearic acid amide type lubricant, fatty acid metal soap type lubricant, alkane and alkane oxide type lubricant, and micro-nano particle type lubricant. Specifically, the PEG-based lubricant may be, for example, PEG molecule with molecular weight of 500-50000, which may be subjected to capping, grafting, crosslinking treatment, or other chemical or physical modification. The fluoropolymer lubricant may be at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyhexafluoropropylene, and the like, or may be another unimodal or multimodal fluoropolymer or a crystalline or semicrystalline fluoropolymer. The organic silicon lubricant can be various compounds which take carbon and silicon atoms as molecular main chains and take oligomers or oligomers of organic groups such as methyl, phenyl, alkoxy, vinyl and the like as side chains. The fatty alcohol-based lubricant may be, for example, at least one of a soft fatty alcohol, a hard fatty alcohol, a tallow fatty alcohol, and the like. The fatty acid based lubricant may be, for example, stearic acid and/or 12-hydroxystearic acid. The fatty acid ester lubricant may be at least one of butyl stearate, monoglyceride stearate, cetyl palmitate, stearyl stearate, and the like. The stearamide-based lubricant may be, for example, at least one of stearamide, oleamide, erucamide, n-Ethylenebisstearamide (EBS), and the like. The fatty acid metal soap lubricant may be at least one of lead stearate, calcium stearate, magnesium stearate, synthetic calcium acetate, and the like. The alkane and the oxidized alkane lubricant may be at least one of liquid paraffin, solid paraffin, polyethylene wax, polypropylene wax, ethylene oxide wax, and the like. The micro-nano particle lubricant can be powder rubber and/or silica gel particles. Further, the lubricant may be contained in an amount of 0.05 to 5 parts by weight, preferably 0.5 to 3 parts by weight, based on 100 parts by weight of the total weight of the second polymerization product.

In addition, the polyethylene composition may also contain various other conventional additives commonly used in polyethylene resins and polyethylene films, and the other additives do not adversely affect the stretch film forming property, the mechanical property and the optical property of the polyethylene composition provided by the invention. Such other adjuvants include, but are not limited to: at least one of an antioxidant, a halogen absorbent, a heat stabilizer, a slipping agent, an antistatic agent, an anti-sticking agent, an antifogging agent, an anti-aging agent, a weather-resistant stabilizer, a colorant, a filler and the like. In addition, the amount of the other additives can be selected conventionally in the art, and those skilled in the art can know the amount of the other additives, and the details are not described herein.

The polyethylene composition provided by the invention can only contain the second polymerization reaction product, also can consist of the second polymerization reaction product, the lubricant and other auxiliary agents, and also can consist of the second polymerization reaction product, the lubricant, other auxiliary agents and the existing polyethylene. Wherein the second polymerization product is present in an amount of not less than 80 weight percent, preferably not less than 90 weight percent, more preferably not less than 95 weight percent, based on the total weight of the polyethylene composition.

The preparation method of the polyethylene composition comprises the steps of carrying out first polymerization reaction on ethylene and alpha-olefin in the presence of a catalyst, and then carrying out second polymerization reaction on the first polymerization reaction product, the ethylene and the alpha-olefin, wherein the conditions of the first polymerization reaction and the second polymerization reaction are that the weight-average molecular weight Mw of the obtained second polymerization reaction product is 8-20 ten thousand g/mol, the Mw/Mn is more than or equal to 4.5, the Mz/Mw is more than or equal to 4.5, and the Mz +1/Mw is more than or equal to 7.5. Preferably, the second polymerization product has a weight average molecular weight Mw of from 10 to 15 ten thousand g/mol, and Mw/Mn of from 4.5 to 9.0, Mz/Mw of from 4.5 to 7.0, and Mz +1/Mw of from 7.5 to 13.5.

According to the invention, in the specific reaction process, the two reactors are connected in series, the polymerization processes of different reactors can be the same or different, the process operation can be operated independently or in linkage, and different polymers formed in different reactors are controlled by adjusting the reaction conditions (such as temperature, pressure, raw material composition and the like) in different reactors, so that the performance of the finally obtained polymer product is controlled.

The amount of each substance used in the first polymerization reaction and the second polymerization reaction is not particularly limited in the present invention, and for example, the amount of ethylene and the α -olefin used in the above two polymerization reaction processes may be such that the content of the structural unit derived from the α -olefin in the first polymerization reaction product and the second polymerization reaction product obtained is each independently 0.2 to 20 mol%, preferably each independently 2 to 10 mol%. In addition, generally speaking, hydrogen can be introduced during the first polymerization reaction and the second polymerization reaction to adjust the melt index of the polymerization reaction product, which is known to those skilled in the art and will not be described herein.

The alpha-olefins used in the first and second polymerization reactions may be the same or different and may each independently be C₃-C₂₀At least one of the olefins, the specific species of which has been described above, is not described in detail here.

The first polymerization reaction needs to be carried out in the presence of a catalyst, which may be, for example, a ziegler-natta catalyst. The ziegler-natta catalysts generally consist of a magnesium/titanium compound and an organoaluminum compound, and optionally an electron donor, and are well known to those skilled in the art and will not be described in detail herein.

In the present invention, the conditions of the first polymerization reaction and the second polymerization reaction are not particularly limited. For example, the temperature of the first polymerization reaction may be 50 to 130 ℃, preferably 50 to 95 ℃, and the pressure may be 0.5 to 5MPa, preferably 0.5 to 2.5 MPa. The temperature of the second polymerization reaction may be 50 to 130 ℃, preferably 50 to 95 ℃, and the pressure may be 0.5 to 5MPa, preferably 0.5 to 2.5 MPa. In the present invention, the pressures are gauge pressures.

In addition, after the second polymerization reaction product is obtained, at least one of a lubricant, other additives, and an existing polyethylene, etc. may be optionally added, and accordingly, the obtained polyethylene composition may contain only the second polymerization reaction product, may be composed of the second polymerization reaction product, the lubricant, and other additives, and may be composed of the second polymerization reaction product, the lubricant, other additives, and an existing polyethylene. Wherein the amounts of each are such that the resulting polyethylene composition comprises the second polymerization product in an amount of not less than 80 wt%, preferably not less than 90 wt%, more preferably not less than 95 wt%, based on the total weight of the polyethylene composition. In addition, the types of the lubricant and other additives are described above and are not described in detail herein.

The invention also provides a polyethylene film, which at least comprises one polyethylene layer formed by the polyethylene composition.

The polyethylene film may have a single-layer structure or a multi-layer structure. When the polyethylene film is a multilayer structure, at least the main layer (typically the layer of greatest thickness) is a polyethylene layer formed from the polyethylene composition. For example, the polyethylene film may have a composite structure of an upper skin layer, a core layer and a lower skin layer, and at least the core layer is a polyethylene layer formed of the polyethylene composition. Generally, the thickness of the polyethylene film may be 10 to 200. mu.m, preferably 10 to 100. mu.m. Further, when the polyethylene film has a composite structure of an upper skin layer, a core layer and a lower skin layer, the thicknesses of the upper skin layer and the lower skin layer are each independently 1 to 25%, preferably 0.5 to 5 μm, of the thickness of the polyethylene film.

The stretching may be unidirectional stretching or bidirectional stretching, preferably bidirectional stretching, and more preferably bidirectional stretching of a flat film. Specifically, in the process of preparing the polyethylene film by adopting a flat film biaxial stretching method, the polyethylene composition is added into an extrusion casting device for extrusion casting, and then the obtained casting sheet is stretched and formed in a film biaxial stretching device. In the extrusion casting, the cast sheet die may be selected according to the structure of a film to be obtained, for example, when a film having a single layer structure is to be obtained, a single layer die may be used; when it is desired to obtain a film having a multilayer structure (a film having a three-layer structure of an upper skin layer, a core layer and a lower skin layer), a multilayer-structure composite die may be used, and at least one layer (core layer) of the multilayer-structure composite die is communicated with an extruder hopper containing the above-mentioned polyethylene composition, so that at least one layer (core layer) of the obtained film is a polyethylene layer formed of the above-mentioned polyethylene composition. In the extrusion process, the extrusion temperature may be 160-260 ℃ and the temperature of the casting chill roll may be 15-85 ℃. In addition, the biaxial stretching may be performed by a simultaneous faradaic stretching process (i.e., stretching in the Machine Direction (MD) and the Transverse Direction (TD)) or a stepwise stretching process (i.e., stretching in the machine direction and then stretching in the transverse direction). The synchronous stretching process comprises the following specific steps: after the tape casting sheet is fully preheated, longitudinal direction stretching and transverse direction stretching are simultaneously carried out, wherein the preheating temperature can be 110-145 ℃, the stretching temperature can be 100-140 ℃, the longitudinal direction (MD) stretching ratio is more than or equal to 4 times and preferably 4-8 times, the Transverse Direction (TD) stretching ratio is more than or equal to 5 times and preferably 5-15 times, and the transverse direction stretching speed is more than or equal to 80%/s. The step-by-step stretching process comprises the following specific steps: the casting sheet is fully preheated, and then longitudinally stretched and transversely stretched, wherein the preheating temperature can be 70-138 ℃, the stretching temperature can be 75-135 ℃, the longitudinal (MD) stretching ratio is more than or equal to 4 times, preferably 4-8 times, the Transverse (TD) stretching ratio is more than or equal to 5 times, preferably 5-15 times, and the transverse stretching rate is more than or equal to 80%/s. After the stretch-molding of the film, the film may be subjected to annealing and setting treatment without being subjected to setting treatment. When the annealing setting process is performed, the film setting process temperature may be 110-145 ℃. And finally, performing surface corona treatment, edge cutting and winding treatment on the film to finally obtain the film.

The biaxially oriented film prepared by the flat film biaxially oriented method has high mechanical strength and good optical performance. The performance of the biaxial stretching film can meet the following requirements: the longitudinal (MD) tensile strength is more than or equal to 55MPa, preferably more than or equal to 60 MPa; the Transverse Direction (TD) tensile strength is more than or equal to 80MPa, preferably more than or equal to 90 MPa; the puncture strength is more than or equal to 2.5N, preferably more than or equal to 3N; the haze is less than or equal to 6 percent, and preferably less than or equal to 4.5 percent; the tensile elongation at break is less than or equal to 400 percent, preferably less than or equal to 350 percent. In the present invention, the machine direction tensile strength and the transverse direction tensile strength are measured according to the method defined in GB/T1040.3-2006. The puncture strength was measured according to the method defined in GB/T10004-2008, wherein the thickness of the thin film sample was 25. + -.5. mu.m. The haze was measured according to the method specified in GB/T2410-. The tensile elongation at break is measured according to the method specified in GB/T1040.3-2006.

The invention also provides the polyethylene film prepared by the method.

The polyethylene composition provided by the invention has the advantages of low production cost, simple processing technology and good universality, is suitable for preparing various BOPE film products, such as packaging films, composite films, agricultural films, barrier films, matte films, paper-like films and the like, widens the application field of polyethylene films, and improves the performance of the products.

The present invention will be described in detail below by way of examples.

In the following examples and comparative examples:

the film biaxial stretching apparatus was purchased from Bruckner, Germany, and was of the type Karo IV.

The properties of the polyethylene composition were tested according to the following methods:

(1) molecular weight and molecular weight distribution index (M)_wMw/Mn, Mz + 1): measuring by using a PL-GPC 220 type gel permeation chromatograph manufactured by Polymer Laboratories, UK, combined with an IR5 type infrared detector, wherein the chromatographic columns are 3 series-connected Plgel 10 mu m MIXED-B columns, the solvent and mobile phase are 1,2, 4-trichlorobenzene, the column temperature is 150 ℃, the flow rate is 1.0mL/min, and the general calibration is performed by adopting EasiCal PS-1 narrow-distribution polystyrene standard sample of the PL company;

(2) analytical temperature rising leaching test: the measurement is carried out by using a TREF 300 type temperature rising leaching analyzer manufactured by Spain Polymer Char S.A. company, and the specific steps are as follows: putting 80mg of polyethylene composition into a container, injecting 40mL of solvent (1,2, 4-trichlorobenzene, and the adding amount of antioxidant BHT is 0.03 wt%), heating to 160 ℃ under the protection of nitrogen, keeping the temperature at the stirring speed of 200rpm for 60 minutes, then transferring 2mL of solution into an analytical column, rapidly cooling (the cooling rate is 40 ℃/min) to 95 ℃, keeping the temperature for 45 minutes, slowly cooling to 35 ℃ at the speed of 0.1 ℃/min, keeping the temperature at 35 ℃ for 30 minutes, then heating at the speed of 1.0 ℃/min, rinsing the analytical column at the speed of 0.5mL/min, measuring the concentration of a leaching solution by using an infrared detector to obtain the concentration of a sample dissolved at different temperatures, and obtaining a temperature rising rinsing curve after normalization.

(3) Melting temperature test: the alloy is obtained by performing DSC test by adopting a Perkin-Elmer DSC-7 differential scanning calorimeter under the protection of nitrogen, wherein the temperature and the heat flow are corrected by adopting indium, and the dosage of a sample is 5 mg. Firstly, heating a sample to 180 ℃ at the speed of 10 ℃/min, then keeping the temperature for 5min to eliminate the thermal history, then cooling to 0 ℃ at the speed of 10 ℃/min, keeping the temperature at 0 ℃ for 1min, then heating to 180 ℃ again at the speed of 10 ℃/min, and determining the melting temperature from the heat flow curve recorded by the first heating, cooling and re-heating.

(4) Content of structural units derived from alpha-olefin: adopts BRUKER AVANCE III 400Hz nuclear magnetic resonance spectrometer¹³C nuclear magnetic resonance spectroscopy (NMR) method, wherein a 10mm probe is used, the test temperature is 125 ℃, 90 pulses, waltz16 decoupling, the sampling time AQ is 5s, and the delay time D1 is 10 s. Sample preparation: the sample was dissolved in deuterated o-dichlorobenzene at a concentration of 10% w/v, and 130-140Heating and dissolving in oil bath at the temperature of DEG C.

(5) Melt Index (MI): the measurement was carried out according to the method specified in GB/T3682-2000, wherein the test temperature was 190 ℃ and the load was 2.16 kg.

(6) Density: the measurement was carried out according to the method specified in GB/T1033.2-2010 and by the density gradient column method.

Example 1

This example illustrates the polyethylene composition and polyethylene film provided by the invention and their preparation.

(1) Preparation of polyethylene composition:

the polyethylene composition described in this example was prepared by direct polymerization in a series of two fluidized bed reactors (300 mm in internal diameter and 1000mm in height, both described below), wherein the product from polymerization reactor 1 and the product from polymerization reactor 2 were both ethylene/alpha olefin copolymerized Linear Low Density Polyethylene (LLDPE). Specifically, the recycle gas 1 is subjected to a first polymerization reaction in the polymerization reactor 1, and the first polymerization reaction product is introduced into the polymerization reactor 2, while the recycle gas 2 is introduced into the polymerization reactor 2 to perform a second polymerization reaction. Wherein all the alpha-olefins used in the reactor were 1-butene, the catalyst used in the polymerization reactor 1 was a Ziegler-Natta catalyst (the Ziegler-Natta catalyst was the Ziegler-Natta catalyst prepared in CN101838351A example 1, used in an amount of 0.2g/hr, the same applies hereinafter), and no catalyst was added to the polymerization reactor 2. The specific operating conditions were as follows:

the reaction temperature of a polymerization reactor 1 is 85 ℃, the reaction pressure is 1.9MPa, the composition of circulating gas 1 is 35 percent (mol) of ethylene, the ratio of butyl to ethyl is 0.45(mol/mol), the ratio of hydrogen to ethyl is 0.1(mol/mol), and the gas velocity of the circulating gas is 1.0 m/s; the reaction temperature of the polymerization reactor 2 is 85 ℃, the reaction pressure is 1.9MPa, the composition of the circulating gas 2 is 35 percent (mol) of ethylene, the ratio of butyl to ethyl is 0.5(mol/mol), the ratio of hydrogen to ethyl is 0.3(mol/mol), and the gas velocity of the circulating gas is 1.0 m/s; the ratio of the polymer yields in the two reactors was controlled to be 4.0, i.e., the first reactor polymer weight W1/(second reactor polymer weight-first reactor polymer weight) W2 was 4.0. Wherein specific properties of the first polymerization product and the second polymerization product are shown in Table 1.

Adding lubricant into the second polymerization reaction product to mix and homogenize. Wherein the lubricant is PEG lubricant (number average molecular weight is 10000) produced by Switzerland, and the lubricant is added in an amount of 0.1 part by weight per 100 parts by weight of the second polymerization product. And then, adding the homogenized mixture into a feeder of a double-screw extruder of the company W & P, feeding the materials into the double screws through the feeder, keeping the temperature of the screws between 180 ℃ and 240 ℃ in the processing process, melting and mixing the materials uniformly through the screws, extruding, granulating and drying to obtain polyethylene composition granules, wherein the properties of the polyethylene composition granules are shown in Table 2.

(2) Preparation of polyethylene film:

drying the polyethylene composition granules prepared in the step (1), adding the dried polyethylene composition granules into a multilayer extrusion casting machine of a Swedish Labtech company, wherein the model of the LCR400 is used for melt extrusion and casting sheets, wherein the polyethylene composition granules are respectively added into a core layer extruder, an upper surface layer extruder and a lower surface layer extruder, inorganic anti-sticking agents (silicon dioxide and the same below) are also required to be added into the upper surface layer extruder and the lower surface layer extruder, the weight ratio of the anti-sticking agents added into the upper surface layer extruder and the lower surface layer extruder to the polyethylene composition granules added into the upper surface layer extruder and the lower surface layer extruder is 0.02:1, and in the process of casting the sheets, the temperature of a casting chill roll is set to be 85 ℃, so that the polyethylene thick casting sheets are prepared, wherein the polyethylene thick casting sheets comprise an upper surface.

Placing the polyethylene thick cast sheet into a stretching clamp of film biaxial stretching equipment, and forming by adopting a biaxial step-by-step stretching process of stretching in a longitudinal direction (MD) and then stretching in a Transverse Direction (TD), wherein the process conditions of each step are as follows: the MD preheat temperature was 100 ℃, MD stretch temperature was 110 ℃, MD stretch ratio was 4 times, TD preheat temperature was 100 ℃, TD stretch temperature was 115 ℃, TD stretch ratio was 5 times, TD stretch rate of the film was 150%/s, film setting temperature was 120 ℃, resulting in a film with an average thickness of 25 μm, consisting of an upper surface layer, a core layer, and a lower surface layer, both of which had a thickness of 1.5 μm.

Example 2

(1) Preparation of polyethylene composition:

the polyethylene composition described in this example was prepared by direct polymerization using a two fluidized bed reactor series process, wherein the product of polymerization reactor 1 and the product of polymerization reactor 2 were both ethylene/alpha olefin copolymerized Linear Low Density Polyethylene (LLDPE). Specifically, the recycle gas 1 is subjected to a first polymerization reaction in the polymerization reactor 1, and the first polymerization reaction product is introduced into the polymerization reactor 2, while the recycle gas 2 is introduced into the polymerization reactor 2 to perform a second polymerization reaction. Wherein, the alpha olefin adopted in the reactor is 1-hexene, the catalyst adopted in the polymerization reactor 1 is Ziegler-Natta catalyst, and no catalyst is added in the polymerization reactor 2. The method comprises the following specific steps:

the reaction temperature of a polymerization reactor 1 is 80 ℃, the reaction pressure is 1.9MPa, the composition of circulating gas 1 is 35 percent (mol) of ethylene, the hexane-ethylene ratio is 0.08(mol/mol), the hydrogen-ethylene ratio is 0.01(mol/mol), and the gas velocity of the circulating gas is 1.0 m/s; the reaction temperature of the polymerization reactor 2 is 88 ℃, the reaction pressure is 1.9MPa, the composition of the circulating gas 2 is 35 percent (mol) of ethylene, the hexane-ethyl ratio is 0.13(mol/mol), the hydrogen-ethyl ratio is 0.5(mol/mol), and the gas velocity of the circulating gas is 1.0 m/s; the ratio of the polymer yields in the two reactors was controlled to 1.0, i.e., the first reactor polymer weight W1/(second reactor polymer weight-first reactor polymer weight) W2 was 1.0. Wherein specific properties of the first polymerization product and the second polymerization product are shown in Table 1.

Adding lubricant into the second polymerization reaction product to mix and homogenize. Wherein the lubricant is PEG lubricant (number average molecular weight is 10000) produced by Switzerland, and the lubricant is added in an amount of 3 parts by weight per 100 parts by weight of the second polymerization product. And then, adding the homogenized mixture into a feeder of a double-screw extruder of the company W & P, feeding the materials into the double screws through the feeder, keeping the temperature of the screws between 180 ℃ and 240 ℃ in the processing process, melting and mixing the materials uniformly through the screws, extruding, granulating and drying to obtain polyethylene composition granules, wherein the properties of the polyethylene composition granules are shown in Table 2.

(2) Preparation of polyethylene film:

drying the polyethylene composition granules prepared in the step (1), adding the dried polyethylene composition granules into a multilayer extrusion casting machine of a Swedish Labtech company, wherein the model of the LCR400 is used for melt extrusion and casting sheets, wherein the polyethylene composition granules are respectively added into a core layer extruder, an upper surface layer extruder and a lower surface layer extruder, inorganic anti-sticking agents are also required to be added into the upper surface layer extruder and the lower surface layer extruder, the weight ratio of the anti-sticking agents added into the upper surface layer extruder and the lower surface layer extruder to the polyethylene composition granules added into the upper surface layer extruder and the lower surface layer extruder is 0.02:1, and in the process of casting the sheets, the temperature of a casting chill roll is set to be 25 ℃ to prepare the polyethylene thick casting sheets which comprise an upper surface layer, a core layer and a lower surface layer.

Placing the polyethylene thick cast sheet into a stretching clamp of film biaxial stretching equipment, and forming by adopting a biaxial step-by-step stretching process of stretching in a longitudinal direction (MD) and then stretching in a Transverse Direction (TD), wherein the process conditions of each step are as follows: the MD preheat temperature was 100 ℃, MD stretch temperature was 110 ℃, MD stretch ratio was 4 times, TD preheat temperature was 100 ℃, TD stretch temperature was 115 ℃, TD stretch ratio was 6 times, TD stretch rate of the film was 100%/s, film setting temperature was 120 ℃, resulting in a film with an average thickness of 25 μm, consisting of an upper surface layer, a core layer, and a lower surface layer, both of which had a thickness of 1.5 μm.

Comparative example 1

This comparative example serves to illustrate a reference polyethylene composition and polyethylene film and the preparation thereof.

A commercially available polyethylene raw material is selected, a flat film method biaxial stretching process is adopted to prepare the polyethylene film, and the stretching film forming property and the film property of the raw material are compared with those of the polyethylene film.

(1) Polyethylene raw material:

selecting film-grade linear low-density polyethylene (7042) produced by Yanshan petrochemical company of China petrochemical group, wherein the film-grade linear low-density polyethylene is prepared by adopting Ziegler-Natta (Ziegler-Natta) catalyst, the melt index MI is 2.0g/10min, and the density rho is 0.920g/cm³The specific properties are shown in tables 1 and 2.

(2) Preparation of polyethylene film:

a polyethylene film was produced by following the procedure of example 1, except that the polyethylene composition pellets were replaced with the polyethylene raw material of the step (1) of this comparative example, and stretching film rupture occurred in all of the attempts, and the film could not be stretched to form. In addition, after the TD stretching rate in the preparation process of the polyethylene film is reduced to 50%/s, the film is still stretched and broken after a plurality of attempts, and the film cannot be stretched and formed.

Comparative example 2

A commercially available polyethylene raw material is selected, and a polyethylene film is prepared by adopting an extrusion blow molding process, and the film performance is compared with that of the invention.

(1) Polyethylene raw material:

the polyethylene feedstock used in this comparative example was the same as that of comparative example 1.

(2) Preparation of polyethylene film:

the polyethylene film of the comparative example was obtained by extrusion blow molding using an up-blowing film blowing apparatus of dr.collin, germany, in the following specific manner: adding the polyethylene raw material selected in the step (1) into a hopper of an extruder of an up-blowing film blowing device, fully melting and plasticizing the raw material by the extruder, extruding the raw material by an annular die of a machine head to prepare a melt film tube, blowing by compressed air (blowing ratio is 2.5 times), and cooling by an air ring to prepare the polyethylene film, wherein the film is of a single-layer structure with the average thickness of 25 mu m.

Comparative example 3

(1) Preparation of polyethylene composition:

first polymerization product, second polymerization product and polyethylene composition pellets were produced in the same manner as in example 1, except that the hydrogen/ethylene ratio in the recycle gas 1 during the production of the first polymerization product was 0.2 by mol. Wherein the specific properties of the first polymerization product and the second polymerization product are shown in Table 1, and the specific properties of the polyethylene composition pellets are shown in Table 2.

(2) Preparation of polyethylene film:

the preparation of a polyethylene film was carried out in the same manner as in example 1, except that the surface layer and core layer materials of the extruded flow-delay film were replaced with the polyethylene composition pellets of the above step (1), and as a result, it was revealed that the maximum stretching ratio of the biaxially oriented film was only 2 Times (TD) × 2 times (MD) and the maximum stretching ratio of the film TD was only 10%/s through many trials, that is, both the stretching ratio and the stretching rate were too low, and the stretch film forming property was poor and not practical.

Comparative example 4

The polyethylene composition is prepared by adopting a parallel process in the comparative example, and the specific steps are as follows:

(1) preparation of polyethylene composition:

first polymerization product, second polymerization product and polyethylene composition pellets were produced in the same manner as in example 2, except that the hydrogen to ethylene ratio in the recycle gas 1 during the production of the first polymerization product was 0.3 by mol. Wherein the specific properties of the first polymerization product and the second polymerization product are shown in Table 1, and the specific properties of the polyethylene composition pellets are shown in Table 2.

(2) Preparation of polyethylene film:

a polyethylene film was produced by following the procedure of example 1 except that the polyethylene composition pellets were replaced with the polyethylene composition pellets of step (1) of this comparative example, and the results showed that the maximum draw ratio of the biaxially oriented film was only 2 Times (TD) × 2 times (MD) and the maximum draw ratio of the TD of the film was only 10%/s through a number of trials, that is, both the draw ratio and the draw rate were too low, and the stretch film forming property was poor and not practical.

TABLE 1

TABLE 2

Test example

Test examples are presented to illustrate the testing of the properties of polyethylene films and reference polyethylene films.

(1) Haze: the measurement was carried out according to the method specified in GB/T2410-2008, wherein the thickness of the thin film sample was 25. + -. 5 μm, and the results are shown in Table 3;

(2) tensile strength, modulus and tensile elongation at break: the results of measurements carried out according to the method specified in GB/T1040.3-2006 are shown in Table 3;

(3) puncture strength: the results of measurements carried out according to the method defined in GB/T10004-2008, wherein the thickness of the film samples was 25. + -. 5 μm, are shown in Table 3.

TABLE 3

From the results of table 3, the following conclusions can be drawn:

(1) embodiments 1-2 adopt the polyethylene composition provided by the present invention to prepare a polyethylene film, and compared with a polyethylene film prepared by using the existing polyethylene raw material, the polyethylene film has the advantages of large film forming rate, fast stretching rate, high mechanical strength and good optical performance, and the properties of the prepared film can satisfy the requirements that the longitudinal (MD) tensile strength is greater than or equal to 55MPa, the Transverse (TD) tensile strength is greater than or equal to 80MPa, the puncture strength is greater than or equal to 2.5N, the haze is less than or equal to 4.5%, and the tensile elongation at break is less than or equal to 350%.

(2) As can be seen from the comparison of example 1 with comparative examples 1 and 2, the commercial polyethylene raw material prepared by the prior art cannot be stretched into a film, and thus is not suitable for processing by the biaxial stretching process by the flat film method. Compared with the biaxially oriented polyethylene film prepared by the flat film method, the blow-molded film prepared by the commercially available polyethylene raw material by adopting the extrusion film blowing process has lower mechanical strength and puncture strength, higher haze and larger difference in performance compared with the result of the invention.

(3) As can be seen from the comparison of example 1 with comparative examples 3 and 4, when the polyethylene composition does not have the composition and structure of the polyethylene composition provided by the present invention, the stretch film forming property is not good, and it has no utility for the preparation of the existing biaxial stretching process film by the flat film method.

The preferred embodiments of the present invention have been described in detail, however, the present invention is not limited to the specific details of the above embodiments, and various simple modifications may be made to the technical solution of the present invention within the technical idea of the present invention, and these simple modifications are within the protective scope of the present invention.

It should be noted that the various features described in the above embodiments may be combined in any suitable manner without departing from the scope of the invention. The invention is not described in detail in order to avoid unnecessary repetition.

In addition, any combination of the various embodiments of the present invention is also possible, and the same should be considered as the disclosure of the present invention as long as it does not depart from the spirit of the present invention.

Claims

1. A polyethylene composition, characterized in that the polyethylene composition comprises the product produced according to the process of: performing a first polymerization reaction on ethylene and alpha-olefin in the presence of a catalyst, and then performing a second polymerization reaction on the first polymerization reaction product, the ethylene and the alpha-olefin, wherein the conditions of the first polymerization reaction and the second polymerization reaction are that the weight average molecular weight Mw of the obtained second polymerization reaction product is 8-20 ten thousand g/mol, the Mw/Mn is more than or equal to 4.5, the Mz/Mw is more than or equal to 4.5, and the Mz +1/Mw is more than or equal to 7.5, the weight average molecular weight Mw of the first polymerization reaction product is 8-50 ten thousand g/mol, the Mw/Mn is 2.5-8.5, the Mz/Mw is 2.0-6.5, and the Mz +1/Mw is 4.5-12.0.

2. The polyethylene composition of claim 1, wherein the second polymerization product has a weight average molecular weight, Mw, of from 10 to 15 ten thousand g/mol, and Mw/Mn of from 4.5 to 9.0, Mz/Mw of from 4.5 to 7.0, and Mz +1/Mw of from 7.5 to 13.5.

3. The polyethylene composition of claim 1, wherein the first polymerization product has a weight average molecular weight, Mw, of 8-30 ten thousand g/mol, and Mw/Mn, of 3.0-7.0, Mz/Mw, of 2.5-5.5, and Mz +1/Mw, of 4.5-9.5.

4. The polyethylene composition according to any of claims 1-3, wherein said polyethylene composition has a temperature rising elution profile as measured by analytical temperature rising elution fractionation, comprising a high temperature elution peak and a low temperature elution peak, and wherein the peak temperature of said high temperature elution peak is from 90 ℃ to 110 ℃ and the peak temperature of said low temperature elution peak is from 50 ℃ to 90 ℃; and taking the total area of the high-temperature elution peak and the low-temperature elution peak as a reference, wherein the area of the high-temperature elution peak is less than or equal to 80%, and the area of the low-temperature elution peak is more than or equal to 20%.

5. The polyethylene composition according to claim 4, wherein the peak temperature of the high temperature elution peak is 95-100 ℃ and the peak temperature of the low temperature elution peak is 60-90 ℃; and taking the total area of the high-temperature elution peak and the low-temperature elution peak as a reference, wherein the area of the high-temperature elution peak is less than or equal to 70%, and the area of the low-temperature elution peak is more than or equal to 30%.

6. The polyethylene composition according to any of claims 1 to 3, wherein the polyethylene composition has a melt index at a temperature of 190 ℃ and a load of 2.16kg of from 0.1 to 20g/10 min.

7. The polyethylene composition according to claim 6, wherein the polyethylene composition has a melt index at a temperature of 190 ℃ under a load of 2.16kg of from 0.5 to 10g/10 min.

8. The polyethylene composition according to any of claims 1-3, wherein the polyethylene composition has a melting temperature as measured by differential scanning calorimetry of 100-130 ℃.

9. The polyethylene composition according to claim 8, wherein the polyethylene composition has a melting temperature as measured by differential scanning calorimetry of 110-.

10. The polyethylene composition according to any of claims 1-3, wherein the alpha olefin used in the first and second polymerization reactions is each independently C₃-C₂₀At least one of olefins.

11. The polyethylene composition according to claim 10, wherein the alpha-olefin used in the first and second polymerization reactions is, independently of each other, propylene, 1-butene, 2-butene, 3-methyl-1-butene, 1-pentene, 3-methyl-1-pentene, 4-methyl-1-pentene, 3-dimethyl-1-pentene, 3, 4-dimethyl-1-pentene, 4-dimethyl-1-pentene, 1-hexene, 4-methyl-1-hexene, 5-methyl-1-hexene, 1-heptene, 2-heptene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, or a mixture thereof, At least one of 1-hexadecene, 1-octadecene and 1-eicosene.

12. The polyethylene composition according to claim 11, wherein the alpha olefin used in the first and second polymerization reactions is each independently at least one of 1-butene, 1-hexene, and 1-octene.

13. A process for preparing a polyethylene composition, the process comprising subjecting ethylene and an alpha-olefin to a first polymerization reaction in the presence of a catalyst, and then subjecting the first polymerization reaction product to a second polymerization reaction with ethylene and an alpha-olefin, the conditions of the first polymerization reaction and the second polymerization reaction being such that the second polymerization reaction product obtained has a weight average molecular weight Mw of from 8 to 20 ten thousand g/mol, and Mw/Mn of 4.5 or more, Mz/Mw of 4.5 or more, and Mz +1/Mw of 7.5 or more; wherein the first polymerization product has a weight average molecular weight Mw of from 8 to 50 ten thousand g/mol, and Mw/Mn of from 2.5 to 8.5, Mz/Mw of from 2.0 to 6.5, and Mz +1/Mw of from 4.5 to 12.0.

14. The process of claim 13, wherein the weight average molecular weight Mw of the second polymerization product is from 10 to 15 ten thousand g/mol, and Mw/Mn is from 4.5 to 9.0, Mz/Mw is from 4.5 to 7.0, and Mz +1/Mw is from 7.5 to 13.5.

15. The process of claim 13, wherein the weight average molecular weight Mw of the first polymerization reaction product is from 8 to 30 ten thousand g/mol, and Mw/Mn is from 3.0 to 7.0, Mz/Mw is from 2.5 to 5.5, and Mz +1/Mw is from 4.5 to 9.5.

16. The process according to claim 13, wherein the first polymerization reaction is carried out at a temperature of 50 to 130 ℃ and a pressure of 0.5 to 5 MPa; the temperature of the second polymerization reaction is 50-130 ℃, and the pressure is 0.5-5 MPa.

17. The process of any of claims 13-15, wherein the alpha olefin used in the first and second polymerization reactions is each independently C₃-C₂₀At least one of olefins.

18. The process of claim 17, wherein the alpha-olefin used in the first and second polymerization reactions is, independently, propylene, 1-butene, 2-butene, 3-methyl-1-butene, 1-pentene, 3-methyl-1-pentene, 4-methyl-1-pentene, 3-dimethyl-1-pentene, 3, 4-dimethyl-1-pentene, 4-dimethyl-1-pentene, 1-hexene, 4-methyl-1-hexene, 5-methyl-1-hexene, 1-heptene, 2-heptene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, or a mixture thereof, At least one of 1-hexadecene, 1-octadecene and 1-eicosene.

19. The process of claim 18, wherein the alpha olefin used in the first and second polymerization reactions is each independently at least one of 1-butene, 1-hexene, and 1-octene.

20. A polyethylene composition produced by the process of any one of claims 13-19.

21. A polyethylene film comprising at least one polyethylene layer produced from the polyethylene composition of any one of claims 1-12 and 20.

22. A process for the production of a polyethylene film comprising extrusion casting a polyethylene composition according to any one of claims 1 to 12 and 20 and stretching the cast sheet obtained.

23. The method as claimed in claim 22, wherein the stretching method is biaxial stretching, and the longitudinal stretching ratio is more than or equal to 4 times, and the transverse stretching ratio is more than or equal to 5 times; the transverse stretching speed is more than or equal to 80%/s.

24. The method of claim 23, wherein the stretching is biaxial stretching and the longitudinal stretching magnification is 4-8 times and the transverse stretching magnification is 5-15 times.

25. A polyethylene film produced by the process of claim 22 or 23.