Composition for preparing linear low density polyethylene film and method for preparing the same

Abstract

The application discloses a composition for preparing linear low-density polyethylene film, which comprises the following components in parts by weight based on the total weight of the composition: 85-94 parts of linear low-density polyethylene, 5-14 parts of low-density polyethylene, 0.5-1 part of antioxidant and 0.05-0.5 part of processing aid. The composition can make the prepared film have excellent tensile yield stress and breaking nominal strain, and improve the processing performance of the film.

Description

Technical Field

[0001] This invention belongs to the field of polyethylene resin, and specifically relates to a composition for preparing linear low-density polyethylene film and a method for preparing the same. Background Technology

[0002] Polyethylene film has excellent properties such as easy processing, easy molding, good mechanical properties, and wide operating temperature range. It is widely used in the packaging field and agricultural production, such as food packaging bags, cling film, stretch film, heat shrink film, heavy packaging film, and greenhouse film. It has now become the most widely used film product in terms of volume and application range.

[0003] The raw materials for polyethylene cast film are mainly low-density polyethylene and linear low-density polyethylene. The processing involves first plasticizing and melting the raw materials using an extruder. The film is then extruded through a T-shaped die, forming a sheet that is cast onto the surface of a smoothly rotating cooling roller. The film is cooled and shaped on the cooling roller, then drawn, trimmed, and wound up. Unlike blown film, which forms a tubular preform, this method produces a sheet-like preform. Films formed using the casting method have a more uniform thickness, better gloss, transparency, and better heat-sealing properties than blown film.

[0004] Metallocene polyethylene (MPE) is produced by the polymerization of ethylene using a catalyst system composed of metallocene organic compounds and co-catalysts. Benefiting from the excellent copolymerization and molecular design capabilities of metallocene catalysts, its relative molecular mass distribution and comonomer incorporation methods can be flexibly adjusted. The narrow relative molecular mass and compositional distribution of MPE results in films with low haze and high transparency, making them suitable for applications such as greenhouse films, mulch films, and packaging films. It also possesses good toughness, allowing for a 20% reduction in thickness while maintaining the same toughness. Furthermore, MPE films exhibit high heat-sealing strength and a low heat-sealing initiation temperature, making them suitable for heavy-duty packaging films. However, the narrow relative molecular mass distribution can lead to a narrow processing window for metallocene films, poor shear rate sensitivity, and poor melt flowability. Moreover, its melt strength is lower than that of LDPE produced by high-pressure processes, making film processing more difficult to control. Therefore, providing a method to improve the processing performance of MPE is of significant practical importance.

[0005] Chinese Patent CN107556569 B discloses a linear low-density polyethylene composition with high gloss and puncture resistance, comprising the following raw materials in weight percentages: 99.50-99.66% ethylene-butene linear polyethylene copolymer, 0.25-0.35% nano-EVA composite nucleating agent, 0.03-0.05% primary antioxidant, and 0.06-0.10% secondary antioxidant. The nano-EVA composite nucleating agent comprises EVA, sodium stearate, sodium dodecylbenzenesulfonate, nucleating agent, and hydrated magnesium silicate powder, wherein the dry weight ratio of EVA:sodium stearate:sodium dodecylbenzenesulfonate is 90-98:1-5:1-5, and the ratio of nucleating agent:EVA:hydrated magnesium silicate is 12-15:10-15:3-5. The preparation method of this composition is also disclosed. The composition provided by this invention has excellent gloss and high puncture resistance. However, although this technology utilizes a combination of nano-EVA composite nucleating agents and antioxidants to improve the performance of the film, it has failed to solve the problem of poor processing performance of metallocene linear low-density polyethylene.

[0006] Chinese patent CN 108342012B discloses a mLLDPE resin composition for plastic greenhouse films and its preparation method. The composition consists of the following raw materials, with the following weight percentages: mLLDPE resin 100; peroxide 0.03-0.15; maleic anhydride grafted monomer masterbatch 6-14; electron donor 0.05-1.5; antioxidant 0.1-0.6; rheology modifier 0.04-0.08; lubricant 0.04-0.12. This mLLDPE resin composition exhibits high melt viscosity and melt elongation, enabling the production of greenhouse films and other films with excellent mechanical properties using a conventional single-layer blown film extrusion unit, thus achieving the preparation of single-layer films. The invention also provides a method for preparing the maleic anhydride grafted monomer masterbatch. However, while this technology utilizes a combination of peroxide, maleic anhydride, electron donor, and other additives to improve the mechanical properties of polyethylene films for plastic greenhouses, it fails to address the problem of poor processing performance of metallocene linear low-density polyethylene.

[0007] Chinese patent CN114031813A discloses an antioxidant composition for linear low-density polyethylene (LLDPE) and a LLDPE resin composition. This antioxidant composition includes a hindered phenolic antioxidant, a phosphite antioxidant, and an acid remover, wherein the hindered phenolic antioxidant is a semi-hindered phenolic antioxidant and / or a hindered phenolic antioxidant containing three phenolic hydroxyl groups. The antioxidant combination provided by this invention, when added to LLDPE, significantly improves its resistance to nitrogen oxide fumigation, thereby effectively improving the yellowing problem of LLDPE. However, while this technology significantly improves its resistance to nitrogen oxide fumigation and improves the yellowing problem of LLDPE, it fails to solve the problem of poor processing performance of LLDPE. Summary of the Invention

[0008] The purpose of this invention is to solve the problem of the difficulty in processing metallocene polyethylene films, and to provide a composition for preparing linear low-density polyethylene films and a method thereof. The composition of this invention enables the prepared film to have excellent tensile yield stress and nominal strain at break, and improves the processing performance of the film.

[0009] To achieve the above objectives, the present invention provides a composition for preparing linear low-density polyethylene film, the composition comprising: 85-94 parts of linear low-density polyethylene, 5-14 parts of low-density polyethylene, 0.5-1 parts of antioxidant, and 0.05-0.5 parts of processing aid, based on a total weight of 100 parts of the composition.

[0010] In this invention, based on a total weight of 100 parts of the composition, there are 88-92 parts of linear low-density polyethylene, 7-11 parts of low-density polyethylene, 0.8-0.95 parts of antioxidant, and 0.05-0.2 parts of processing aid.

[0011] In this invention, the relationship between the melt mass flow rate MFR1 of the linear low-density polyethylene and the melt mass flow rate MFR2 of the low-density polyethylene is: 1≤MFR2-MFR1≤6, and the relationship between the density ρ1 of the linear low-density polyethylene and the density ρ2 of the low-density polyethylene is: 0.001≤ρ1-ρ2≤0.02, 3≤(MFR2-MFR1) / 100(ρ1-ρ2)≤8.

[0012] In this invention, the relationship between the melt mass flow rate MFR1 of the linear low-density polyethylene and the melt mass flow rate MFR2 of the low-density polyethylene is: 2≤MFR2-MFR1≤4, and the relationship between the density ρ1 of the linear low-density polyethylene and the density ρ2 of the low-density polyethylene is: 0.003≤ρ1-ρ2≤0.01.

[0013] In this invention, the linear low-density polyethylene has a density of 0.920–0.934 g / cm³. 3 The melt flow rate is 3-4 g / 10 min; the molecular weight distribution (PD) of the linear low-density polyethylene is 2-3.2.

[0014] In this invention, the preparation method of the linear low-density polyethylene is not particularly limited. In one embodiment, the linear low-density polyethylene can be prepared, for example, by the following method: using a gas-phase fluidized bed process, a metallocene catalytic system to catalyze the polymerization reaction, 1-hexene as a comonomer to polymerize with ethylene, and hydrogen as a molecular weight regulator in the polymerization reaction; wherein the polymerization reaction temperature is 82-87°C, the reaction pressure is 2.1-2.4 MPa, the molar ratio of hydrogen to ethylene is 0.0002-0.001, and the molar ratio of 1-hexene to ethylene is 0.005-0.01. The metallocene catalytic system is not particularly limited, and it can be a conventionally used metallocene catalytic system, such as a supported metallocene catalyst with an activity of 8000-10000 gPE / gCat, but this invention is not limited to this.

[0015] In this invention, the preparation method and source of the low-density polyethylene are not particularly limited. The low-density polyethylene can be prepared by conventional techniques in the art or can be obtained by purchase.

[0016] In this invention, the antioxidant includes one or more of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], octadecyl β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, and tris[2,4-di-tert-butylphenyl]phosphite.

[0017] In this invention, the processing aid is fluoroelastomer PPA.

[0018] The present invention also provides a method for preparing the above-mentioned composition for preparing linear low-density polyethylene film, the method comprising: mixing linear low-density polyethylene, low-density polyethylene, antioxidant and processing aid and then granulating to obtain the composition.

[0019] In this invention, the granulation temperature is 190–200°C; this invention does not particularly limit the preparation process of mixing and granulation, which can be carried out using conventional methods in the art.

[0020] The present invention also provides a method for preparing linear low-density polyethylene film, the method comprising: adding particles of the above composition to an extruder for extrusion casting, wherein the die temperature of the extruder is 190-210°C, the roller cooling temperature is 24-28°C, and the thickness of the cast film is 28-32 μm.

[0021] The narrow molecular weight distribution of metallocenes leads to a narrow processing window, poor shear rate sensitivity, and poor melt flowability. Furthermore, their melt strength is lower than that of LDPE produced by high-pressure processes, making film bubble control difficult during blown film production. The low-density polyethylene used in this invention has a long-chain branched structure. This long-chain structure significantly alters the shear-dependent behavior of the material, making its melt elastic characteristics more pronounced. The introduction of the long-chain structure also affects the crystallization behavior of linear low-density polyethylene, reducing the material's crystallinity and thus improving the film's tensile properties. This invention solves the problem of difficult processing of metallocene polyethylene films by using a composition of linear low-density polyethylene, low-density polyethylene, antioxidants, and processing aids. This results in films with excellent tensile yield stress and nominal strain at break, as well as improved processing performance. Detailed Implementation

[0022] The following provides a detailed description of the embodiments of the present invention: These embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and processes. However, the scope of protection of the present invention is not limited to the following embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions.

[0023] All ranges disclosed herein include endpoints and are independently combinable. The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, which should be understood to include values ​​close to them.

[0024] The compositions of this invention can be used to prepare thin films, particularly cast films. The compositions of this invention enable the prepared films to possess excellent tensile yield stress and nominal strain at break, as well as improved film processing properties.

[0025] The preparation process of the compositions in the following examples is as follows:

[0026] In the polymerization reaction, ethylene is used as the raw material, 1-hexene as the comonomer, and hydrogen as the molecular weight regulator. Under polymerization conditions of 82–87°C and 2.1–2.4 MPa, ethylene, 1-hexene, and hydrogen are introduced into the gas-phase reactor, initiated by a metallocene catalyst. The gas enters the reactor from the bottom of the fluidized bed through a circulating gas pipeline. The high-speed circulating gas blows the powdered resin particles in the reactor through a distribution plate. Upon passing through the expansion section, the resin returns to the bottom of the fluidized bed, while the gas exits from the top and returns to the bottom. When the material level reaches a set height, the discharge system is triggered to discharge the material, yielding linear low-density polyethylene resin. Based on a total weight of 100 parts, 85-94 parts of linear low-density polyethylene, 5-14 parts of low-density polyethylene, 0.5-1 parts of antioxidant, and 0.05-0.5 parts of processing aid are mixed, extruded and granulated. During the granulation process, the extruder die temperature is 190-200°C to obtain the composition of the present invention for preparing linear low-density polyethylene film.

[0027] The metallocene catalyst used in the above polymerization reaction can be a supported metallocene catalyst with an activity of 8000-10000 gPE / gCat.

[0028] The specific preparation process of the thin film obtained in the following examples is as follows:

[0029] The above-obtained composition particles were added to an extruder for extrusion casting. The die temperature of the extruder was 200°C, the roll surface cooling temperature was 26°C, and the thickness of the cast film was 30 μm, thus obtaining a thin film.

[0030] Analysis and testing methods for linear low-density polyethylene:

[0031] The melt mass flow rate was tested according to the method specified in GB / T 3682, with a test temperature of 190℃ and a test load of 2.16 kg.

[0032] The density was tested according to the method specified in GB / T 1033.

[0033] Analytical and testing methods for compositions used in the preparation of linear low-density polyethylene films:

[0034] Rheological properties: Measurements were performed using a rotational rheometer with a plate diameter of 25 mm, a plate gap of 1 mm, a temperature of 190℃, and a dynamic frequency scan of 100–0.01 Hz, employing a high-to-low frequency scanning method. The strain was set to 1%. After the above rotational rheological test, the relationship between the complex viscosity (η*) and angular frequency (ω) of the product can be obtained. This relationship can be fitted using a Cross model.

[0035]

[0036] Where, η 0 λ is the zero-shear viscosity, λ is the relaxation time, and n is the shear-variance index.

[0037] Based on the Cross model, the zero-shear viscosity and relaxation time of the sample can be obtained, which can then characterize the sample's processing performance.

[0038] Analysis and testing methods for thin films:

[0039] The tensile properties of the film were tested according to the method specified in GB 13022.

[0040] Example 1

[0041] This embodiment describes the preparation of linear low-density polyethylene (LDPE) as mentioned in this invention using a metallocene catalyst and a gas-phase fluidized bed process. The reaction temperature was 84.9℃, the reaction pressure was 2.21 MPa, the molar ratio of hydrogen to ethylene was 0.00057, and the molar ratio of 1-hexene to ethylene was 0.0072. The basic properties of the obtained linear LPE were tested, and the results are shown in Table 1.

[0042] Take 90 parts of the above-mentioned linear low-density polyethylene, and select a melt flow rate of 7.0 g / 10 min and a density of 0.920 g / cm³. 3 Nine parts of low-density polyethylene, 0.9 parts of antioxidant pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 0.1 parts of processing aid fluoroelastomer PPA were used. The above mixture was extruded and granulated, and the rotational rheological test results of the resulting composition are shown in Table 2. The relationship between the melt flow rate MFR1 and the melt flow rate MFR2 of linear low-density polyethylene is: MFR2-MFR1=3.62, and the relationship between the density ρ1 and the density ρ2 of linear low-density polyethylene is: ρ1-ρ2=0.006, (MFR2-MFR1) / 100(ρ1-ρ2)=6.03.

[0043] The above-mentioned composition particles were cast to obtain a polyethylene cast film. The tensile yield stress and nominal strain at break of the cast film were tested, and the results are shown in Table 3.

[0044] Example 2

[0045] This embodiment describes the preparation of linear low-density polyethylene (LDPE) as mentioned in this invention using a metallocene catalyst and a gas-phase fluidized bed process. The reaction temperature was 85.1℃, the reaction pressure was 2.23 MPa, the molar ratio of hydrogen to ethylene was 0.00064, and the molar ratio of 1-hexene to ethylene was 0.0068. The basic properties of the obtained LLDPE were tested, and the results are shown in Table 1.

[0046] Take 89 parts of the above-mentioned linear low-density polyethylene, and select those with a melt flow rate of 6.5 g / 10 min and a density of 0.921 g / cm³. 3 The mixture consisted of 11 parts of low-density polyethylene, 0.9 parts of antioxidant pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 0.1 parts of processing aid fluoroelastomer PPA. The mixture was extruded and granulated. The rotational rheological test results of the resulting composition are shown in Table 2. The relationship between the melt flow rate MFR1 and the melt flow rate MFR2 of linear low-density polyethylene was: MFR2-MFR1=2.88. The relationship between the density ρ1 and the density ρ2 of linear low-density polyethylene was: ρ1-ρ2=0.008, (MFR2-MFR1) / 100(ρ1-ρ2)=3.6.

[0047] The above-mentioned composition particles were cast to obtain a polyethylene cast film. The tensile yield stress and nominal strain at break of the cast film were tested, and the results are shown in Table 3.

[0048] Example 3

[0049] This embodiment describes the preparation of linear low-density polyethylene (LDPE) as mentioned in this invention using a metallocene catalyst and a gas-phase fluidized bed process. The reaction temperature was 84.7℃, the reaction pressure was 2.18 MPa, the molar ratio of hydrogen to ethylene was 0.00071, and the molar ratio of 1-hexene to ethylene was 0.0079. The basic properties of the obtained linear LPE were tested, and the results are shown in Table 1.

[0050] Take 91 parts of the above-mentioned linear low-density polyethylene, and select those with a melt flow rate of 7.0 g / 10 min and a density of 0.918 g / cm³. 3 The mixture consisted of 8 parts of low-density polyethylene, 0.95 parts of antioxidant pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 0.05 parts of processing aid fluoroelastomer PPA. The mixture was extruded and granulated, and the rotational rheological test results are shown in Table 2. The relationship between the melt flow rate MFR1 and the melt flow rate MFR2 of linear low-density polyethylene was: MFR2-MFR1=3.29. The relationship between the density ρ1 and the density ρ2 of linear low-density polyethylene was: ρ1-ρ2=0.009, (MFR2-MFR1) / 100(ρ1-ρ2)=3.7.

[0051] The above-mentioned composition particles were cast to obtain a polyethylene cast film. The tensile yield stress and nominal strain at break of the cast film were tested, and the results are shown in Table 3.

[0052] Example 4

[0053] This embodiment describes the preparation of linear low-density polyethylene (LDPE) as mentioned in this invention using a metallocene catalyst and a gas-phase fluidized bed process. The reaction temperature was 84.7℃, the reaction pressure was 2.15 MPa, the molar ratio of hydrogen to ethylene was 0.00068, and the molar ratio of 1-hexene to ethylene was 0.0082. The basic properties of the obtained linear LPE were tested, and the results are shown in Table 1.

[0054] Take 92 parts of the above-mentioned linear low-density polyethylene, and select a melt flow rate of 6.0 g / 10 min and a density of 0.922 g / cm³. 3 The mixture consisted of 7 parts of low-density polyethylene, 0.92 parts of antioxidant β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate n-octadecyl alcohol ester, and 0.08 parts of processing aid fluoroelastomer PPA. The mixture was extruded and granulated, and the rotational rheological test results are shown in Table 2. The relationship between the melt flow rate MFR1 and the melt flow rate MFR2 of linear low-density polyethylene was: MFR2-MFR1=2.18. The relationship between the density ρ1 and the density ρ2 of linear low-density polyethylene was: ρ1-ρ2=0.003, (MFR2-MFR1) / 100(ρ1-ρ2)=7.3.

[0055] The above-mentioned composition particles were cast to obtain a polyethylene cast film. The tensile yield stress and nominal strain at break of the cast film were tested, and the results are shown in Table 3.

[0056] Example 5

[0057] This embodiment describes the preparation of linear low-density polyethylene (LDPE) as mentioned in this invention using a metallocene catalyst and a gas-phase fluidized bed process. The reaction temperature was 85.1℃, the reaction pressure was 2.31 MPa, the molar ratio of hydrogen to ethylene was 0.00049, and the molar ratio of 1-hexene to ethylene was 0.0075. The basic properties of the obtained linear LPE were tested, and the results are shown in Table 1.

[0058] Take 88 parts of the above-mentioned linear low-density polyethylene, and select a melt flow rate of 5.5 g / 10 min and a density of 0.921 g / cm³. 3The mixture consisted of 11 parts of low-density polyethylene, 0.8 parts of antioxidant β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate n-octadecyl alcohol ester, and 0.2 parts of processing aid fluoroelastomer PPA. The mixture was extruded and granulated. The rotational rheological test results of the resulting composition are shown in Table 2. The relationship between the melt flow rate MFR1 and the melt flow rate MFR2 of linear low-density polyethylene was: MFR2-MFR1=2.13. The relationship between the density ρ1 and the density ρ2 of linear low-density polyethylene was: ρ1-ρ2=0.007, (MFR2-MFR1) / 100(ρ1-ρ2)=3.0.

[0059] The above-mentioned composition particles were cast to obtain a polyethylene cast film. The tensile yield stress and nominal strain at break of the cast film were tested, and the results are shown in Table 3.

[0060] Comparative Example 1

[0061] Compared to Example 1, this comparative example did not include low-density polyethylene.

[0062] The linear low-density polyethylene mentioned in this invention was prepared using a metallocene catalyst and a gas-phase fluidized bed process. The reaction temperature was 84.9℃, the reaction pressure was 2.21 MPa, the molar ratio of hydrogen to ethylene was 0.00057, and the molar ratio of 1-hexene to ethylene was 0.0072. The basic properties of the obtained linear low-density polyethylene were tested, and the results are shown in Table 1.

[0063] Take 99 parts of the above-mentioned linear low-density polyethylene, 0.9 parts of antioxidant pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 0.1 parts of processing aid fluoroelastomer PPA. Extrude and granulate the above mixture. The rotational rheological test results of the obtained composition are shown in Table 2.

[0064] The above-mentioned composition particles were cast to obtain a polyethylene cast film. The tensile yield stress and nominal strain at break of the cast film were tested, and the results are shown in Table 3.

[0065] Comparative Example 2

[0066] Compared to Example 2, the melt mass flow rate of low-density polyethylene in this comparative example is lower than that of linear low-density polyethylene.

[0067] The linear low-density polyethylene mentioned in this invention was prepared using a metallocene catalyst and a gas-phase fluidized bed process. The reaction temperature was 85.1℃, the reaction pressure was 2.23 MPa, the molar ratio of hydrogen to ethylene was 0.00064, and the molar ratio of 1-hexene to ethylene was 0.0068. The basic properties of the obtained linear low-density polyethylene were tested, and the results are shown in Table 1.

[0068] Take 89 parts of the above-mentioned linear low-density polyethylene, and select a melt flow rate of 1.5 g / 10 min and a density of 0.918 g / cm³. 3 The mixture consisted of 11 parts of low-density polyethylene, 0.9 parts of antioxidant pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 0.1 parts of processing aid fluoroelastomer PPA. The mixture was extruded and granulated. The rotational rheological test results of the resulting composition are shown in Table 2. The relationship between the melt flow rate MFR1 and the melt flow rate MFR2 of linear low-density polyethylene was: MFR2-MFR1=-2.12. The relationship between the density ρ1 and the density ρ2 of linear low-density polyethylene was: ρ1-ρ2=0.011, (MFR2-MFR1) / 100(ρ1-ρ2)=-1.9.

[0069] The above-mentioned composition particles were cast to obtain a polyethylene cast film. The tensile yield stress and nominal strain at break of the cast film were tested, and the results are shown in Table 3.

[0070] Comparative Example 3

[0071] Compared with Example 3, the relationship between melt mass flow rate and density of linear low-density polyethylene and low-density polyethylene in this comparative example does not satisfy 1≤MFR2-MFR1≤6, 0.001≤ρ1-ρ2≤0.02, 3≤(MFR2-MFR1) / 100(ρ1-ρ2)≤8.

[0072] To utilize a metallocene catalyst, a gas-phase fluidized bed process was used to prepare the linear low-density polyethylene mentioned in this invention. The reaction temperature was 84.7℃, the reaction pressure was 2.18 MPa, the molar ratio of hydrogen to ethylene was 0.00071, and the molar ratio of 1-hexene to ethylene was 0.0079. The basic properties of the obtained linear low-density polyethylene were tested, and the results are shown in Table 1.

[0073] Take 91 parts of the above-mentioned linear low-density polyethylene, and select a melt flow rate of 4.5 g / 10 min and a density of 0.921 g / cm³. 3The mixture consisted of 8 parts of low-density polyethylene, 0.95 parts of antioxidant pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 0.05 parts of processing aid fluoroelastomer PPA. The mixture was extruded and granulated. The rotational rheological test results of the resulting composition are shown in Table 2. The relationship between the melt flow rate MFR1 and the melt flow rate MFR2 of linear low-density polyethylene was: MFR2-MFR1=0.79. The relationship between the density ρ1 and the density ρ2 of linear low-density polyethylene was: ρ1-ρ2=0.006, (MFR2-MFR1) / 100(ρ1-ρ2)=1.3.

[0074] The above-mentioned composition particles were cast to obtain a polyethylene cast film. The tensile yield stress and nominal strain at break of the cast film were tested, and the results are shown in Table 3.

[0075] Comparative Example 4

[0076] Compared to Example 4, the amount of low-density polyethylene added in this comparative example is 20 parts.

[0077] This embodiment describes the preparation of linear low-density polyethylene (LDPE) as mentioned in this invention using a metallocene catalyst and a gas-phase fluidized bed process. The reaction temperature was 84.7℃, the reaction pressure was 2.15 MPa, the molar ratio of hydrogen to ethylene was 0.00068, and the molar ratio of 1-hexene to ethylene was 0.0082. The basic properties of the obtained linear LPE were tested, and the results are shown in Table 1.

[0078] Take 79 parts of the above-mentioned linear low-density polyethylene, and select a melt flow rate of 6.0 g / 10 min and a density of 0.922 g / cm³. 3 The mixture consisted of 20 parts of low-density polyethylene, 0.92 parts of antioxidant β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate n-octadecyl alcohol ester, and 0.08 parts of processing aid fluoroelastomer PPA. The mixture was extruded and granulated, and the rotational rheological test results are shown in Table 2. The relationship between the melt flow rate MFR1 and the melt flow rate MFR2 of linear low-density polyethylene was: MFR2-MFR1=2.18. The relationship between the density ρ1 and the density ρ2 of linear low-density polyethylene was: ρ1-ρ2=0.003, (MFR2-MFR1) / 100(ρ1-ρ2)=7.3.

[0079] The above-mentioned composition particles were cast to obtain a polyethylene cast film. The tensile yield stress and nominal strain at break of the cast film were tested, and the results are shown in Table 3.

[0080] Table 1. Basic physical property test results of linear low-density polyethylene in the examples and comparative examples.

[0081]

[0082] Table 2. Rotational rheological test calculation results of the compositions in the examples and comparative examples.

[0083]

[0084]

[0085] Table 3. Test results of linear low-density polyethylene films in the examples and comparative examples.

[0086]

[0087] The rotational rheological test results and film test results of the compositions of Example 1 and Comparative Example 1 show that, in Example 1, the addition of low-density polyethylene to the composition resulted in a higher zero-shear viscosity than that of Comparative Example 1, indicating a more pronounced shear-dependent olefinic behavior. This suggests that Example 1 has better processing performance and is easier to process. The relaxation time of Example 1 is longer than that of Comparative Example 1, indicating that the transition from the Newtonian to the non-Newtonian region occurs at a lower frequency during shearing, further suggesting better processing performance. Comparing the film properties of Example 1 and Comparative Example 1, the film of Example 1 exhibits higher tensile yield stress and nominal strain at break.

[0088] A comparison between Example 2 and Comparative Example 2, and between Example 3 and Comparative Example 3, reveals that when the relationship between the melt mass flow rate and density of linear low-density polyethylene and low-density polyethylene in the examples satisfies 1≤MFR2-MFR1≤6, 0.001≤ρ1-ρ2≤0.02, and 3≤(MFR2-MFR1) / 100(ρ1-ρ2)≤8, the composition exhibits more prominent zero-shear viscosity, relaxation time, tensile yield stress of the film, and nominal strain at break.

[0089] A comparison of Example 4 and Comparative Example 4 reveals that when the amount of low-density polyethylene added in the examples is within the range of 5-14 parts of the present invention, the composition has higher zero-shear viscosity and relaxation time, and the resulting film has higher tensile yield stress and nominal strain at break.

[0090] This invention can be summarized in other forms. Those skilled in the art can make various corresponding changes and modifications based on this invention without departing from its spirit and essence, but such corresponding changes and modifications should all fall within the scope of the claims of this invention.

Claims

1. A composition for preparing linear low-density polyethylene film, characterized in that, include: Based on a total weight of 100 parts, the composition comprises 85-94 parts linear low-density polyethylene, 5-14 parts low-density polyethylene, 0.5-1 part antioxidant, and 0.05-0.5 parts processing aid. The relationship between the melt mass flow rate MFR1 and the melt mass flow rate MFR2 of the linear low-density polyethylene is: 1≤MFR2-MFR1≤6, and the relationship between the density ρ1 and the density ρ2 of the linear low-density polyethylene is: 0.001≤ρ1-ρ2≤0.02, 3≤(MFR2-MFR1) / 100(ρ1-ρ2)≤8.

2. The composition according to claim 1, characterized in that, Based on a total weight of 100 parts of the composition, the composition comprises 88-92 parts of linear low-density polyethylene, 7-11 parts of low-density polyethylene, 0.8-0.95 parts of antioxidant, and 0.05-0.2 parts of processing aid.

3. The composition according to claim 1, characterized in that, The relationship between the melt mass flow rate MFR1 of the linear low-density polyethylene and the melt mass flow rate MFR2 of the low-density polyethylene is: 2≤MFR2-MFR1≤4, and the relationship between the density ρ1 of the linear low-density polyethylene and the density ρ2 of the low-density polyethylene is: 0.003≤ρ1-ρ2≤0.

01.

4. The composition according to claim 1, characterized in that, The linear low-density polyethylene has a density of 0.920~0.934 g / cm³. 3 The melt flow rate is 3-4 g / 10 min; the molecular weight distribution (PD) of the linear low-density polyethylene is 2-3.

2.

5. The composition according to claim 1, characterized in that, The method for preparing linear low-density polyethylene includes the following steps: using a gas-phase fluidized bed process, a metallocene catalytic system to catalyze the polymerization reaction, 1-hexene as a comonomer to polymerize with ethylene, and hydrogen as a molecular weight regulator in the polymerization reaction; wherein, the polymerization reaction temperature is 82~87℃, the reaction pressure is 2.1~2.4MPa, the molar ratio of hydrogen to ethylene is 0.0002~0.001, and the molar ratio of 1-hexene to ethylene is 0.005~0.

01.

6. The composition according to claim 1, characterized in that, The antioxidants include one or more of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], octadecyl β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, and tris[2,4-di-tert-butylphenyl]phosphite.

7. The composition according to claim 1, characterized in that, The processing aid is the fluoroelastomer PPA.

8. A method for preparing the composition for preparing linear low-density polyethylene film according to any one of claims 1-7, characterized in that, include: The composition is obtained by mixing linear low-density polyethylene, low-density polyethylene, antioxidants and processing aids and then granulating them.

9. A method for preparing a linear low-density polyethylene film, characterized in that, include: The particles of the composition according to any one of claims 1-7 are added to an extruder for extrusion casting, wherein the die temperature of the extruder is 190~210℃, the roller cooling temperature is 24~28℃, and the thickness of the cast film is 28~32μm.

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