Polyethylene base resin composition for a film and a method for producing the same

Abstract

The application discloses a polyethylene base resin composition for film and a preparation method thereof. The polyethylene base resin composition for film comprises a low-molecular-weight polyethylene resin and a high-molecular-weight polyethylene resin. The low-molecular-weight polyethylene resin has a weight average molecular weight of 8000-25000 and a molecular weight distribution of 3-5; the high-molecular-weight polyethylene resin has a weight average molecular weight of 250000-360000 and a molecular weight distribution of 10-15; and the mass of the low-molecular-weight polyethylene resin accounts for 20%-36% of the total mass of the low-molecular-weight polyethylene resin and the high-molecular-weight polyethylene resin. The high-molecular-weight polyethylene resin is generated in situ in the low-molecular-weight polyethylene resin, the mixing performance between the high-molecular-weight resin and the low-molecular-weight resin is improved, the resin composition has good uniformity, the toughness is improved, the processing performance is improved, and the appearance is excellent.

Description

Technical Field

[0001] This invention relates to the field of polyolefin resins, and more specifically to a polyethylene base resin composition for films and its preparation method. Background Technology

[0002] Polyethylene is classified into high-density polyethylene (HDPE, with a density of 0.941 g / cm³). 3 Medium-density polyethylene (MDPE, density 0.926-0.940 g / cm³) or higher), and medium-density polyethylene (MDPE, density 0.926-0.940 g / cm³). 3 Low-density polyethylene (LDPE, density 0.910-0.925 g / cm³) 3 And linear low-density polyethylene (LLDPE, with a density of 0.910-0.925 g / cm³), and linear low-density polyethylene (LLDPE, with a density of 0.910-0.925 g / cm³). 3 High-density polyethylene (HDPE), LLDPE, and LDPE are widely used in film production, particularly in applications requiring high strength and stiffness. Due to their good stiffness, tensile strength, and tear resistance, HDPE films are widely used in supermarket shopping bags, heat-shrink packaging films (PE), garbage bags, food and pharmaceutical packaging films, and industrial linings. Under the same performance conditions, HDPE films can be about 50% thinner than LDPE and 20%-30% thinner than LLDPE, while maintaining the same physical and mechanical properties (tensile strength, tear resistance, etc.), thus further reducing usage costs. When high transparency is not a primary concern, HDPE films can be blended with LDPE, LLDPE, or mLLDPE to improve stiffness and tensile strength.

[0003] With the rapid development of the national economy and the continuous improvement of people's living standards, the demand for HDPE film materials with high tensile strength, good stiffness, and easy opening is increasing year by year. Currently, there are relatively few HDPE film grades on the domestic market that truly achieve high mechanical strength, high stiffness, corrosion resistance, and meet the hygiene requirements for food and pharmaceutical packaging. These grades are characterized by maintaining the original stiffness and tensile strength without reduction while thinning the film. On the other hand, in recent years, with increasing attention to the hygiene and safety of plastics used in food and pharmaceutical packaging, the testing of hygiene standards for plastics used in food and pharmaceutical packaging has become increasingly important. Hexane extract testing has become an important testing item in the hygiene standards for plastics used in food and pharmaceutical packaging. Its principle is to determine the substances precipitated after hexane extraction from the plastic sample, representing substances that can be leached by oils in daily life. HDPE film, due to its high relative molecular mass, high density, and high crystallinity, has a more prominent advantage in this aspect compared to LDPE and LLDPE films.

[0004] Currently, foreign HDPE film grades mainly include PTT (Thailand), Honam Chemical (South Korea)'s 7000F, SCG (Thailand)'s H5604F, Iranian Petrochemical's 7000F, and ExxonMobil's HTA108 (homopolymer). Domestically, the main grades are Jilin Chemical Company's 9455F and Yangzi Petrochemical's 7000F.

[0005] High-density polyethylene (HDPE) typically has a linear homopolymer or copolymer molecular structure, and the comonomer can be 1-butene or 1-hexene. CN201410515973.9 discloses a high-stiffness polyethylene blown film material and its preparation method. The base resin of this blown film material is polyethylene obtained by copolymerizing ethylene and hexene using a chromium-based catalyst. This material can be used to blow-form HDPE films with high tensile strength and good film stiffness. Although the film material produced by this technology has excellent overall performance, the chromium-based catalyst requires sophisticated production processes, and the source of the comonomer 1-hexene is limited, making it more expensive than the currently commonly used 1-butene comonomer.

[0006] CN201410487910.7 discloses a low-ash high-density polyethylene resin and its preparation method, wherein the weight-average relative molecular mass is 3.0 × 10⁻⁶. 5 ~9.0×10 5 Its relative molecular mass distribution is 5–9, and its density is 0.940–0.965 g / cm³. 3 The preferred preparation method utilizes three slurry reactors connected in series, employing a supported titanium-based main catalyst, and continuously polymerizing in the presence of a co-catalyst. This high-density polyethylene resin has a high molecular weight and narrow molecular weight distribution, primarily used for forming wet-process lithium-ion battery separators; its processing speed is limited when used for blown film production.

[0007] Various processes can be used to produce polyethylene. In ethylene slurry polymerization, a diluent such as hexane is used to dissolve ethylene monomers, comonomers, and hydrogen, and the monomers are polymerized with a catalyst. After the polymerization reaction, the resulting polymer product exists as a slurry suspended in a liquid medium. In typical multi-reactor cascade processes (e.g., as shown in WO 2012 / 028591 A1, U.S. Patent No. 6,204,345 B1, and WO 2005 / 077992 A1), monomers, hydrogen, catalysts, and diluents are fed to the first of three reactors, where polymer particles contained in the diluent and unreacted monomers form a slurry. The reactors can be operated in parallel or in series, and the type / amount of monomers and conditions in each reactor can vary, resulting in a wide variety of polyethylene materials, including unimodal (molecular weight distribution) or multimodal polyethylene materials. Such multimodal compositions are used in a variety of applications; for example, WO 2012 / 069400 A1 discloses a trimodal polyethylene composition for blow molding.

[0008] CN1717448A discloses a polyethylene composition with a multi-peaked molecular weight distribution, having a molecular weight distribution of 0.955-0.960 g / cm³ at 23°C. 3 The MFI190 / 5 composition, with a density in the range of 0.8-1.6 dg / min, comprises 45-55% by weight of low molecular weight ethylene homopolymer A, 20-35% by weight of high molecular weight copolymer B made from ethylene and another 1-olefin having 4-8 carbon atoms, and 20-30% by weight of ultra-high molecular weight ethylene copolymer C. The polyethylene composition is prepared by a three-stage polymerization reaction, wherein the molecular weight of the polyethylene prepared in each stage is adjusted by means of hydrogen.

[0009] CN106317546A discloses a high-density polyethylene resin for large hollow containers, which adopts a three-reactor series slurry polymerization process. Ethylene is added to the first reactor and polymerized under the action of a catalyst to form a low molecular weight ethylene homopolymer. Ethylene and 1-butene are added to the second reactor and copolymerized to form a high molecular weight ethylene copolymer. Ethylene and 1-butene are added to the third reactor.

[0010] CN103387628A discloses a system and method for olefin polymerization, wherein the system includes three or more reactors connected in series; the first reactor is one or more loop reactors; the second reactor is one or more stirred tank reactors; and the third reactor is one or more gas-phase fluidized bed reactors. A catalyst and reactants enter the first reactor for polymerization, the reaction product containing the catalyst enters the second reactor, and feedstock is added to the second reactor for further reaction. The reaction product from the outlet of the second reactor enters the third reactor, and feedstock is added to the third reactor for further reaction. Finally, the final product is obtained in the third reactor.

[0011] Polyolefins with an excellent combination of properties are so-called bimodal or multimodal polyolefins. These polymers are typically prepared in a cascade of two or more polymerization reactors with different polymerization conditions. The composition of individual particles of the polyolefin powder obtained in such a polymerization method can vary considerably. Therefore, special effort is required in the granulation step to homogenize these polyethylenes. For example, WO 2004 / 096523 A1 discloses a specific extruder configuration for melting and homogenizing multimodal or bimodal polyolefins.

[0012] Bimodal or multimodal polyolefins exhibit highly variable molecular weight distributions among individual particles, resulting in significantly different melt viscosities and elastic properties. Large particles can be formed by the aggregation of smaller particles. Typically, this microscopic inhomogeneity presents problems when processing into homogeneous polyolefins as the final product. During the preparation of the aforementioned powder mixture, individual particles are melted, but high-molecular-weight particles, and thus high-viscosity and high-elasticity particles, remain in the low-viscosity molten matrix. Consequently, they cannot deform or separate sufficiently in a shear field, or may not deform or separate locally at all, leading to an inhomogeneous matrix in the microstructure. When this poorly prepared mixture for film manufacturing is sheared into particles as the final product, these particles exhibit inhomogeneity, resulting in numerous fisheyes when the film is formed.

[0013] CN200580033876.5 discloses a multimodal polyethylene composition with improved homogeneity. The base resin comprises three ethylene homopolymer or copolymer portions (A), (B), and (C) with different weight-average molecular weights (Mw), wherein portion (A) has an MFR... 21 The weight average molecular weight of part (B) is equal to or lower than 20 g / 10 min, and the weight average molecular weight of part (C) is lower than that of part (A).

[0014] CN200980145898.9 discloses a method for improving the uniformity of polyethylene using a series multi-step reaction method, comprising two slurry reactors and at least one gas-phase reactor connected in series, wherein a low molecular weight ethylene homopolymer or copolymer component with an MFR2 of 100-2000 g / 10 min is polymerized in reactor (A), a low molecular weight ethylene homopolymer or copolymer component with an MFR2 of 100-2000 g / 10 min is polymerized in reactor (B), and a high molecular weight ethylene copolymer component is polymerized in reactor (C).

[0015] CN201580069911.2 discloses a method for producing multi-peak polyethylene in-situ blends including ultra-high molecular weight components. The method involves first polymerizing an ultra-high molecular weight component with a weight-average molecular weight Mw equal to or greater than 500 kg / mol to equal to or less than 10000 kg / mol using a continuous multi-stage process; then polymerizing a lower molecular weight component in a second reaction step to obtain a first polyethylene resin (A) with a weight-average molecular weight Mw of 150 kg / mol to equal to or less than 1500 kg / mol; and then blending this first polyethylene resin (A) with a second polyethylene resin (B) having a weight-average molecular weight Mw equal to or greater than 50 kg / mol to less than 500 kg / mol.

[0016] CN201780069383.X discloses a polymer composition comprising a base resin and a method for preparing the composition. The base resin comprises an extremely high molecular weight component, a low molecular weight component, and a high molecular weight component, wherein the weight-average molecular weight of the high molecular weight component is higher than that of the low molecular weight component but lower than that of the extremely high molecular weight component. The amount of the extremely high molecular weight component in the base resin is 0.5 to 8% by weight. The extremely high molecular weight component has a viscosity-average molecular weight greater than 1100 kg / mol. All three components are copolymers of ethylene and comonomers.

[0017] The methods described above improve the uniformity of multi-peak products to some extent, but they are mostly applicable to products such as pipes. Apart from improving uniformity, no improvement has been observed in the toughness of the products. In addition, some polymerization methods require the combined use of circulating slurry reactors and gas-phase reactors. Therefore, there is still a need to study simple and easy-to-implement methods for preparing high-strength film resin compositions. Summary of the Invention

[0018] The main objective of this invention is to provide a polyethylene base resin composition for film and its preparation method, so as to overcome the defects of poor uniformity and poor toughness of polyethylene base resin compositions for film in the prior art.

[0019] To achieve the above objectives, the present invention provides a polyethylene base resin composition for films, comprising low molecular weight polyethylene resin and high molecular weight polyethylene resin, wherein the low molecular weight polyethylene resin has a weight-average molecular weight of 8,000 to 25,000 and a molecular weight distribution of 3 to 5; the high molecular weight polyethylene resin has a weight-average molecular weight of 250,000 to 360,000 and a molecular weight distribution of 10 to 15; and the mass of the low molecular weight polyethylene resin accounts for 20% to 36% of the total mass of the low molecular weight polyethylene resin and the high molecular weight polyethylene resin.

[0020] In one embodiment of the polyethylene base resin composition for films of the present invention, the low molecular weight polyethylene resin has a melting peak temperature of 130-133°C and a melting enthalpy of 230-260 J / g; the high molecular weight polyethylene resin has a melting peak temperature of 132-137°C and a melting enthalpy of 160-185 J / g.

[0021] In one embodiment, the polyethylene base resin composition for films of the present invention has a melt flow rate of 5-15 g / 10 min at a temperature of 190°C and a load of 21.6 kg, and a density of 0.945-0.955 g / cm³. 3 Nominal fracture strain ≥1000%, notched impact strength ≥60KJ / m 2 Yield stress ≥ 28 MPa.

[0022] In one embodiment, the polyethylene base resin composition for films of the present invention exhibits a non-uniformity of less than 0.01 mm, characterized by the average value of the largest non-uniform aggregate size. 2 The characteristic relaxation time of the polyethylene base resin composition for the film, obtained by fitting the relationship between viscosity and shear rate using a Cross model at a temperature of 190°C, is 2.0–5.0 s.

[0023] In one embodiment, the polyethylene base resin composition for films described in this invention is used at a temperature of 210°C, a die diameter of 2 mm, and a tensile acceleration of 6 mm / s. 2 Under the specified conditions, the melt tension of the polyethylene base resin composition for the film at fracture is 0.35N or higher.

[0024] In one embodiment, the polyethylene base resin composition for film of the present invention further includes a composite additive, wherein the total mass of the low molecular weight polyethylene resin and the high molecular weight polyethylene resin accounts for 90%-100% of the mass of the polyethylene base resin composition for film; the high molecular weight polyethylene resin is generated in situ in the low molecular weight polyethylene resin.

[0025] To achieve the above objectives, the present invention also provides a method for preparing the above-mentioned polyethylene base resin composition for films, which employs a three-reactor tandem slurry polymerization process, comprising the following steps:

[0026] Step 1: Add catalyst, ethylene, and hydrogen to the first reactor to carry out the polymerization reaction, and then input the product into the second reactor.

[0027] Step 2: α-olefins are added to the second reactor, along with ethylene and hydrogen, to carry out the polymerization reaction. The product after the reaction is then fed into the third reactor.

[0028] Step 3: Add catalyst, ethylene, hydrogen and α-olefin to the third reactor to carry out polymerization reaction and obtain the product;

[0029] Step 4: Mix the product obtained from polymerization in the third reactor with the composite additive, and then extrude and granulate to obtain the polyethylene base resin composition for film.

[0030] In one embodiment of the method for preparing the polyethylene base resin composition for films according to the present invention, the product generated in the first reactor has a melt flow rate of 170-200 g / 10 min at a temperature of 190°C and a load of 2.16 kg, and a density of not less than 0.960 g / cm³. 3 The product generated in the second reactor has a melt flow rate of 2.0–3.0 g / 10 min at a temperature of 190℃ and a load of 21.6 kg, and a density not exceeding 0.960 g / cm³. 3 The product generated in the third reactor has a melt flow rate of 5.0–15.0 g / 10 min at a temperature of 190℃ and a load of 21.6 kg, and a density not exceeding 0.955 g / cm³. 3 .

[0031] In one embodiment of the method for preparing the polyethylene base resin composition for film according to the present invention, the amount of ethylene added in step 1 accounts for 40% to 55% of the total amount of ethylene added in steps 1, 2, and 3; the amount of ethylene added in step 2 accounts for 10% to 30% of the total amount of ethylene added in steps 1, 2, and 3; and the amount of ethylene added in step 3 accounts for 20% to 45% of the total amount of ethylene added in steps 1, 2, and 3. Furthermore, the amount of α-olefin added in step 2 accounts for 1 to 3 wt% of the amount of ethylene added in step 2, and the amount of α-olefin added in step 3 accounts for 1 to 3 wt% of the amount of ethylene added in step 3.

[0032] In one embodiment of the method for preparing the polyethylene base resin composition for films according to the present invention, the hydrogen / ethylene volume ratio in the first reactor is 3.0–6.0, the temperature of the first reactor is 78–85°C, and the pressure is 0.40–0.60 MPa; the hydrogen / ethylene volume ratio in the second reactor is 0.01–0.1, the temperature of the second reactor is 70–80°C, and the pressure is 0.10–0.40 MPa; and the hydrogen / ethylene volume ratio in the third reactor is 0.21–0.80, the temperature of the third reactor is 75–83°C, and the pressure is 0.30–0.50 MPa.

[0033] In one embodiment of the method for preparing the polyethylene base resin composition for films according to the present invention, the α-olefin has 3 to 8 carbon atoms, the catalyst includes a main catalyst and a co-catalyst, the main catalyst is a Ziegler-Natta catalyst, and the co-catalyst is an alkylaluminum.

[0034] The beneficial effects of this invention are:

[0035] This invention improves the mixing performance between high-molecular-weight polyethylene resin and low-molecular-weight polyethylene resin by in-situ generating the high-molecular-weight resin, resulting in better uniformity of the resin composition. Therefore, under similar melt flow rates and densities, the notched impact strength of the resin composition of this invention is significantly superior to that of existing products. Simultaneously, the resin composition of this invention has high melt strength and a suitable characteristic relaxation time, providing a wider processing window, thus improving processing efficiency and enabling film processing speeds to reach 45 m / min. The final film product exhibits improved appearance, with fewer than 2 fisheyes (less than 0.4 mm in size) per 1520 cm². 2 It is superior to existing products.

[0036] The final product obtained using the three-reactor tandem polymerization process of this invention exhibits a broad peak distribution. The low molecular weight homopolymer improves processability during processing, possesses high crystallinity during molding, and provides product rigidity. The high molecular weight fraction, with its high comonomer content, facilitates the formation of more ligand molecules during crystallization, resulting in a good balance of rigidity and toughness in the resin composition. The larger high molecular weight component in the composition provides sufficient melt strength during melt extrusion, and the characteristic relaxation time of the composition can be controlled by designing the proportion of the high molecular weight component, enabling the composition to maintain bubble stability at high traction rates, facilitate smooth blown film production, and provide a wide processing and molding window.

[0037] This invention first generates a low molecular weight component, then partially generates a high molecular weight component in situ. Simultaneously, in a third reactor, ethylene and α-olefins re-initiate polymerization, resulting in a mixture of low and high molecular weight components. This provides internal lubrication and partially eliminates the entanglement of high molecular weight chains. Furthermore, this method avoids the problems of sequentially increasing molecular weight, small differences in molecular weight and viscosity between components, and difficulty in uniformly dispersing high and low molecular weight components in existing three-reactor series preparation methods. This invention ultimately improves product strength, toughness, and overall product uniformity. Detailed Implementation

[0038] 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.

[0039] This invention discloses a polyethylene base resin composition for films, comprising low molecular weight polyethylene resin and high molecular weight polyethylene resin. The low molecular weight polyethylene resin has a weight-average molecular weight of 8,000–25,000 and a molecular weight distribution of 3–5; the high molecular weight polyethylene resin has a weight-average molecular weight of 250,000–360,000 and a molecular weight distribution of 10–15; the mass of the low molecular weight polyethylene resin accounts for 20%–36% of the total mass of the low molecular weight polyethylene resin and the high molecular weight polyethylene resin; the mass of the high molecular weight polyethylene resin accounts for 64%–80% of the total mass of the low molecular weight polyethylene resin and the high molecular weight polyethylene resin.

[0040] This invention improves the mixing performance between high and low molecular weight resins by in-situ generating high molecular weight polyethylene resin in low molecular weight polyethylene resin, resulting in better uniformity, increased toughness, improved processing performance, and excellent appearance of the resin composition.

[0041] In one embodiment, the melting peak temperature of the low molecular weight polyethylene resin is 130–133°C, and the melting enthalpy is 230–260 J / g; the melting peak temperature of the high molecular weight polyethylene resin is 132–137°C, and the melting enthalpy is 160–185 J / g.

[0042] The polyethylene base resin composition for films obtained by this invention has a melt flow rate of 5-15 g / 10 min at a temperature of 190°C and a load of 21.6 kg, and a density of 0.945-0.955 g / cm³. 3 Nominal fracture strain ≥1000%, notched impact strength ≥60KJ / m 2 Yield stress ≥ 28 MPa.

[0043] The polyethylene base resin composition for films of the present invention, when characterized by the average value of the largest non-uniform aggregate size, exhibits a non-uniformity of less than 0.01 mm. 2 Specifically, the polyethylene base resin composition for films of the present invention was tested using the method of GB / T 18251. Slices (thickness < 60 micrometers, diameter 3–5 mm) from different locations of six particle samples were observed. The non-uniformity, characterized by the average size of the largest non-uniform aggregates, was less than 0.01 mm. 2 .

[0044] The polyethylene base resin composition for films of this invention exhibits a characteristic relaxation time of 2.0–5.0 s obtained by fitting the viscosity-shear rate relationship using a Cross model at 190°C. At 210°C, with a die diameter of 2 mm and a tensile acceleration of 6 mm / s², the same composition is used. 2 Under the specified conditions, the melt tension at fracture of the polyethylene base resin composition for films of the present invention is 0.35 N or more.

[0045] The polyethylene base resin composition for films of the present invention further includes a composite additive, wherein the total mass of low molecular weight polyethylene resin and high molecular weight polyethylene resin accounts for 90%-100% of the mass of the polyethylene base resin composition for films; and the composite additive accounts for less than 10% of the mass of the polyethylene base resin composition for films, for example 0-10%.

[0046] This invention does not particularly limit the type of composite additive; commonly used additives in this technical field are acceptable. In one embodiment, the composite additive of this invention includes an antioxidant and a stabilizer. The antioxidant may be pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (antioxidant 1010), tris(2,4-di-tert-butylphenyl) phosphite (antioxidant 168), or octadecyl β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (antioxidant 1076); the stabilizer may be zinc stearate, calcium stearate, hydrotalcite, zinc oxide, etc.

[0047] In another embodiment, based on 1000‰ of the weight of the polyethylene base resin composition for film, the composite additive of the present invention comprises: 0.3-1.0‰ of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid] (antioxidant 1010), 0.5-1.0‰ of tris(2,4-di-tert-butylphenyl) phosphite (antioxidant 168), and 0.5-1.2‰ of zinc oxide.

[0048] In another embodiment, the polyethylene base resin composition for films of the present invention may also contain small amounts of pigments, stabilizers, deacidifying agents, UV stabilizers, antistatic agents, processing aids, and other additives used in polyolefins.

[0049] In one embodiment, the method for preparing the above-mentioned polyethylene base resin composition for films of the present invention employs three-reactor tandem slurry polymerization, comprising the following steps:

[0050] Step 1: Add catalyst, ethylene, and hydrogen to the first reactor to carry out the polymerization reaction, and then input the product into the second reactor.

[0051] Step 2: α-olefins are added to the second reactor, along with ethylene and hydrogen, to carry out the polymerization reaction. The product after the reaction is then fed into the third reactor.

[0052] Step 3: Add catalyst, ethylene, hydrogen and α-olefin to the third reactor to carry out polymerization reaction and obtain the product;

[0053] Step 4: Mix the product obtained from polymerization in the third reactor with the composite additive, and then extrude and granulate to obtain the polyethylene base resin composition for film.

[0054] In existing technologies, HDPE film preparation techniques either employ chromium-based catalysts or utilize a dual- or triple-reactor series setup with a continuous slurry method. In each reactor, the molecular weight of the product gradually increases, the melt viscosity gradually increases, and the fluidity gradually decreases. By the time the product reaches the final reactor, its fluidity is already very poor. At this point, the small amount of ultra-high molecular weight component generated exhibits poor mixing uniformity with other components, potentially leading to microphase separation in the final product, thus affecting its appearance and macroscopic mechanical properties.

[0055] The improvement of the method for preparing high-strength film compositions by continuous polymerization of ethylene in this invention lies in the following: the low-molecular-weight ethylene homopolymer product generated in the first reactor enters the second reactor. Due to the supplementary addition of ethylene and α-olefins, some of the low-molecular-weight product continues to undergo copolymerization of ethylene and α-olefins to generate a high-molecular-weight ethylene copolymer. Since no fresh catalyst is added to the second reactor, almost no homopolymerization and copolymerization of ethylene to generate low-molecular-weight polymers occur. The unreacted low-molecular-weight ethylene homopolymer product forms a mixture with the high-molecular-weight ethylene copolymer generated in the second reactor, which facilitates the uniform dispersion of the high-molecular-weight product and improves its flowability. In the third reactor, a fresh catalyst is added to initiate ethylene polymerization to generate low-molecular-weight polymers. Simultaneously, the high-molecular-weight ethylene copolymer generated in the second reactor and some of the unreacted low-molecular-weight ethylene homopolymer product continue to react, introducing more comonomers. The resulting low-molecular-weight polymer and high-molecular-weight copolymer mix in situ to form a medium-molecular-weight product, improving product uniformity and thus enhancing the mechanical and processing properties of the final product.

[0056] In one embodiment, the present invention achieves the purpose of controlling the properties of the polyethylene base resin composition by controlling the melt flow rate (MFR) and density of each reactor. Furthermore, the amount of catalyst, α-olefin, and hydrogen introduced can be adjusted by measuring the melt index of the product powder in each reactor, thereby achieving the purpose of controlling the properties of the polyethylene base resin composition.

[0057] In one embodiment, the low molecular weight homopolymer generated in the first reactor has a melt flow rate (MFR) of 170–200 g / 10 min at a temperature of 190°C and a load of 2.16 kg, and a density of not less than 0.960 g / cm³. 3 Preferably, the concentration is not less than 0.965 g / cm³. 3 The high molecular weight copolymer generated in the second reactor has a melt flow rate (MFR) of 2.0–3.0 g / 10 min at a temperature of 190℃ and a load of 21.6 kg, and a density not exceeding 0.960 g / cm³. 3 Preferably, the concentration is no greater than 0.955 g / cm³. 3 The high molecular weight copolymer generated in the third reactor has a melt flow rate (MFR) of 5.0–15.0 g / 10 min at a temperature of 190℃ and a load of 21.6 kg, and a density not exceeding 0.955 g / cm³. 3 .

[0058] Since the basic resin preparation method in this invention is a multi-reactor continuous polymerization, the product from the first reactor continues to react in the second and third reactors, and the product from the second reactor continues to react in the third reactor. Therefore, the technical control parameters of each reactor are only used for quality control of the product, and the characteristic parameters of each reactor are different from the characteristic parameters of the components in the final product.

[0059] In one embodiment, the amount of ethylene added in step 1 accounts for 40% to 55% of the total amount of ethylene added in steps 1, 2, and 3; the amount of ethylene added in step 2 accounts for 10% to 30% of the total amount of ethylene added in steps 1, 2, and 3; and the amount of α-olefin added in step 2 accounts for 1 to 3 wt% of the amount of ethylene added in step 2, and the amount of α-olefin added in step 3 accounts for 1 to 3 wt% of the amount of ethylene added in step 3.

[0060] In one embodiment, the hydrogen / ethylene volume ratio in the first reactor is 3.0–6.0, the temperature of the first reactor is 78–85°C, and the pressure is 0.40–0.60 MPa; the hydrogen / ethylene volume ratio in the second reactor is 0.01–0.1, the temperature of the second reactor is 70–80°C, and the pressure is 0.10–0.40 MPa; and the hydrogen / ethylene volume ratio in the third reactor is 0.21–0.80, the temperature of the third reactor is 75–83°C, and the pressure is 0.30–0.50 MPa.

[0061] In one embodiment, the α-olefin of the present invention has 3 to 8 carbon atoms, preferably 4 to 8 carbon atoms; the catalyst used in the present invention is any catalyst suitable for catalyzing ethylene polymerization, preferably including a main catalyst and a co-catalyst, the main catalyst being, for example, the Ziegler-Natta catalyst commonly used in the art, and the co-catalyst being, for example, alkylaluminum, and more preferably triethylaluminum. The amount of main catalyst and co-catalyst added in the reactor is adjusted according to the product process parameters.

[0062] The technical solution of the present invention will be further described below. The measurement methods used in the following embodiments and comparative examples are as follows:

[0063] (1) The determination of each component in the polyethylene base resin composition for film is as follows: First, the base resin is classified into solutions, and then the obtained components are tested to obtain the characteristic parameters of each component.

[0064] The solution fractionation method is as follows: Weigh an appropriate amount of base resin, heat it to 130°C with xylene and stir to dissolve for 1 hour, then add the precipitant ethylene glycol ethyl ether, maintain the temperature for 1 hour, filter out the precipitate at high temperature, and dry it in a vacuum oven to obtain high molecular weight polyethylene resin (component B). Let the filtrate stand at room temperature for 5 hours, cool it to crystallize, filter out the precipitate, and dry it in a vacuum oven to obtain low molecular weight polyethylene resin (component A).

[0065] (2) Weight-average molecular weight and distribution were determined by gel permeation chromatography. Test conditions: 3 Mixed-B withguard tandem column, solvent 1,2,4-trichlorobenzene, test temperature 150℃, flow rate 1mL / min, differential detector, narrow distribution polystyrene standard.

[0066] (3) The melting peak temperature and enthalpy of each component were measured using a differential scanning calorimeter (DSC). The temperature was first raised to 190℃, held for 5 minutes to eliminate thermal history, then lowered to 40℃, and then raised to 190℃ again. The two heating curves were recorded and analyzed. The heating and cooling rate was 20℃ / min.

[0067] (4) Non-uniformity is tested according to the method of GB / T 18251. That is, the non-uniformity of the sample is characterized by measuring the area of ​​non-uniform aggregates in the granule composition after one extrusion granulation (e.g., when 2.5 wt% carbon black is added during extrusion granulation, the final extruded granules will exhibit visible color non-uniformity). The steps are as follows: At least 6 sections (thickness < 60 micrometers, diameter 3-5 mm) of different parts of the granule samples are evaluated at 100x magnification to identify the largest unstained inclusions ("white spots", agglomerates, granules) or aggregates of pigment, fish eyes, and carbon black, and the area is measured in mm. 2 The maximum size of unstained inclusions or aggregates in each section was evaluated. The maximum size of all section samples was statistically analyzed, and the average value was used to characterize the inhomogeneity. The resin composition provided by this invention has an inhomogeneity of less than 0.01 mm. 2 .

[0068] (5) Melt strength is usually expressed as melt tensile tension, which determines the processing characteristics of the material during blown film production. High melt tension indicates good melt stretchability. The melt tension test conditions are as follows: A Gottfert RT2000 advanced capillary rheometer equipped with a Rheotens module is used. During the test, the plunger moves downward, extruding the melt downward through the capillary die. The extruded melt wire passes through the force measuring wheel and is then pulled onto the winding wheel. The vertical distance between the force measuring wheel and the die is approximately 450 mm. The force measuring wheel is placed on a balance, and the winding wheel rotates at a uniform acceleration, tending to pull the force measuring wheel away from the balance. The change in the balance reading reflects the magnitude of the tension. The die length is 20 mm and the inner diameter is 2 mm; the plunger moving speed is 0.2 mm / s; the test temperature is 210℃; the melt wire is subjected to an initial velocity of 11.3 mm / s and an acceleration of 6 mm / s. 2 The melt tension at fracture, induced by the winding reel, is characterized as melt strength. The resin compositions provided by this invention all exhibit melt tensions of 0.35 N or higher at fracture.

[0069] (6) The characteristic relaxation time was obtained by measuring the dynamic rheological properties of the sample in the linear viscoelastic region using a rotational rheometer. The test conditions were: frequency scan range 0.01 to 100 Hz, strain set at 0.1%, and test temperature 190 ℃.

[0070] According to the first Cox-Merz relation (Equation 1), when the shear rate is comparable to the oscillation frequency, the absolute value of the complex viscosity of a polymer material in dynamic testing is equal to the value of its apparent shear viscosity in steady-state measurement.

[0071] |η * (ω)|=|η a (γ)| γ＝ω Formula 1

[0072] In the formula, η *For complex viscosity, Pa·s; η a ω is the apparent shear viscosity, Pa·s; ω is the oscillation frequency, rad / s; γ is the shear rate, s. -1 The Cox-Merz relation was used to convert the dynamic rheological test results into the melt rheological curves of the samples.

[0073] For most polymer melts, the dependence of viscosity on shear rate can be described by the Cross model (Equation 2):

[0074] η a =η0 / [1+(τ0·γ) 1-n Equation 2

[0075] Where η0 is the zero-shear viscosity of the resin, Pa·s; τ0 is the viscous characteristic relaxation time, s; and γ is the shear rate, s. -1 ; n is the power exponent.

[0076] The rheological curve of the sample obtained by converting dynamic rheological test results can be quantitatively described by fitting it with a Cross model, thereby obtaining parameters such as characteristic relaxation time. Characteristic relaxation time is closely related to blown film process control and film quality. If the relaxation time is too long, stress cannot be released; although the film bubble is stable, it hinders further stretching and easily leads to film bubble rupture. If the relaxation time is too short, stress relaxes too quickly, making it difficult to control the film bubble diameter and thickness, and easily causing wobbling. Therefore, there is an appropriate characteristic relaxation time for blown film processing. The resin composition provided by this invention has a characteristic relaxation time of 2.0–5.0 s, preferably 2.2–3.5 s.

[0077] Example 1

[0078] This invention provides a method for preparing high-density polyethylene resin for high-strength films, comprising the following steps:

[0079] (1) Polymer-grade ethylene, after purification, is pressurized and fed into the first reactor. Hexane is added as a solvent, along with a pre-prepared main catalyst (Beijing Aoda Company BCE catalyst) and co-catalyst (triethylaluminum), and hydrogen for polymerization. The polymerization temperature in the reactor is 75–85°C. During production, the hydrogen-to-ethylene ratio in the first reactor is controlled, as are the reactor level and feed / discharge rates, to maintain the melt flow rate in the first reactor at 170–200 g / 10 min (190°C, 2.16 kg load).

[0080] (2) The slurry from the first reactor overflows into the second reactor. Without adding a catalyst, butene-1 is added as a comonomer to continue the reaction. The melt flow rate is controlled at 2.0-3.0 g / 10 min (190℃, 21.6 Kg load) by controlling the hydrogen-to-ethylene ratio.

[0081] (3) The slurry from the second reactor overflows into the third reactor, and the catalyst and butene-1 are added again as comonomers. The melt flow rate of the third reactor is controlled at 5.0 to 15.0 g / 10 min (190℃, load 21.6 Kg) by controlling the hydrogen-to-ethylene ratio.

[0082] (4) The product from the third reactor is centrifuged and heated to dry to obtain polymer powder. Composite additives are added to the polymer powder (the additives and their weight ratios to polyethylene products are: antioxidant 1010:0.5‰, antioxidant 168:1‰, calcium stearate 0.8%), and then granulated and packaged into polyethylene special material products.

[0083] The main process conditions are shown in Table 1. The amount of main catalyst and co-catalyst is expressed in mmol / L solvent, with hexane as an example.

[0084] Table 1

[0085]

[0086]

[0087] Example 2

[0088] The preparation method is basically the same as in Example 1, and the main process parameters are shown in Table 2:

[0089] Table 2

[0090]

[0091] Example 3

[0092] The preparation method is basically the same as in Example 1, and the main process parameters are shown in Table 3:

[0093] Table 3

[0094]

[0095]

[0096] Comparative Example 1

[0097] (1) Polymer-grade ethylene, after purification, is pressurized and fed into the first reactor. Hexane is added as a solvent, along with a pre-prepared main catalyst (Beijing Aoda Company BCE catalyst) and co-catalyst (triethylaluminum), and hydrogen for polymerization. The polymerization temperature in the reactor is 75–85°C. During production, the hydrogen-to-ethylene ratio in the first reactor is controlled, as are the reactor level and feed / discharge rates, to maintain the melt flow rate in the first reactor at 170–200 g / 10 min (190°C, 2.16 kg load).

[0098] (2) The slurry from the first reactor overflows into the second reactor. Without adding a catalyst, butene-1 is added as a comonomer to continue the reaction.

[0099] (3) The slurry from the second reactor overflows into the third reactor. Without adding a catalyst, butene-1 is added as a comonomer. The melt flow rate of the third reactor is controlled at 5.0 to 15.0 g / 10 min (190℃, load 21.6 Kg) by controlling the hydrogen-to-ethylene ratio.

[0100] (4) The product from the third reactor is centrifuged and heated to dry to obtain polymer powder. Composite additives are added to the polymer powder (the weight ratio of each additive to the polyethylene product is: antioxidant 1010:0.5‰, antioxidant 168:1‰, calcium stearate 0.8%), and then granulated and packaged into polyethylene special material products.

[0101] The main process conditions are shown in Table 4.

[0102] Table 4

[0103]

[0104]

[0105] Comparative Example 2

[0106] The preparation method is basically the same as that of Comparative Example 1, and the main process parameters are shown in Table 5:

[0107] Table 5

[0108]

[0109] Comparative Example 3

[0110] The preparation method is basically the same as that of Comparative Example 1, and the main process parameters are shown in Table 6:

[0111] Table 6

[0112]

[0113]

[0114] Table 7. Basic Resin Characteristic Parameters of Examples

[0115]

[0116] Table 8. Basic Resin Properties of Examples and Comparative Examples

[0117]

[0118] Note: The bimodal feedstock used is Jilin Petrochemical 9455F, and the chromium-based broad-peak feedstock is Daqing Petrochemical 6097.

[0119] Table 9. Featured Performance of Examples and Comparative Examples

[0120]

[0121] Note: The bimodal feedstock used is Jilin Petrochemical 9455F, and the chromium-based broad-peak feedstock is Daqing Petrochemical 6097.

[0122] As shown in Tables 8 and 9, the film-specific resin composition prepared by this invention has a melt flow rate of 5-15 g / 10 min and a density of 0.945-0.955 g / cm³. 3 Tensile yield stress ≥28MPa, nominal strain at break ≥1000%, notched impact strength ≥60KJ / m 2 The non-uniformity is less than 0.01 mm. 2 The melt strength at 210℃ is greater than 0.35 N, the characteristic relaxation time is 2.5–3.5 s, and the processing speed can reach over 45 m / min, exhibiting excellent processing performance. The films prepared using the compositions in the embodiments of this invention have fewer than 2 fisheyes per 1520 cm² with a size smaller than 0.4 mm. 2 The film products have an excellent appearance.

[0123] As shown by the comparison of the data from the examples and comparative examples in Tables 8 and 9, the test materials prepared using existing technologies with comparable basic properties exhibited poorer uniformity than the resin composition of the present invention, with non-uniformity more than three times that of the resin composition of the present invention. Although the tensile yield stress of the comparative examples was close to that of the examples of the present invention, the nominal strain at break and impact strength were significantly lower than those of the examples of the present invention, and the melt strength was also lower than that of the present invention, with a processing traction speed below 30 m / min. Regarding the film appearance, the film prepared using the comparative example composition had more than 3 fisheyes (smaller than 0.4 mm) per 1520 cm². 2 .

[0124] Of course, the present invention may have other various embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the claims of the present invention.

Claims

1. A polyethylene-based resin composition for films, characterized in that, The composition comprises low molecular weight polyethylene resin, high molecular weight polyethylene resin, and composite additives. The high molecular weight polyethylene resin is generated in situ within the low molecular weight polyethylene resin. The total mass of the low molecular weight polyethylene resin and the high molecular weight polyethylene resin accounts for 90%-99.05% of the mass of the polyethylene base resin composition for film. The low molecular weight polyethylene resin has a weight-average molecular weight of 8,000-25,000 and a molecular weight distribution of 3-5. The high molecular weight polyethylene resin has a weight-average molecular weight of 250,000-360,000 and a molecular weight distribution of 10-15. The mass of the low molecular weight polyethylene resin accounts for 20%-36% of the total mass of the low molecular weight polyethylene resin and the high molecular weight polyethylene resin. The low molecular weight polyethylene resin has a melting peak temperature of 130-133°C and a melting enthalpy of 230-260 J / g. The high molecular weight polyethylene resin has a melting peak temperature of 132-137°C and a melting enthalpy of 160-185 J / g. The polyethylene base resin composition for the film has a melt flow rate of 5-15 g / 10 min at a temperature of 190°C and a load of 21.6 kg, and a density of 0.945-0.955 g / cm³. 3 Nominal fracture strain ≥1000%, notched impact strength ≥60KJ / m 2 Yield stress ≥ 28 MPa; The non-uniformity of the polyethylene base resin composition for films, characterized by the average size of the largest non-uniform aggregates, is less than 0.01 mm. 2 The characteristic relaxation time of the polyethylene base resin composition for the film, obtained by fitting the relationship between viscosity and shear rate using a Cross model at a temperature of 190°C, is 2.0–5.0 s.

2. The polyethylene-based resin composition for films according to claim 1, characterized in that, At a temperature of 210℃, a die diameter of 2mm, and a tensile acceleration of 6mm / s², 2 Under the specified conditions, the melt tension of the polyethylene base resin composition for the film at fracture is 0.35N or higher.

3. The method for preparing the polyethylene base resin composition for films according to claim 1 or 2, comprising three-reactor tandem slurry polymerization, characterized in that, Includes the following steps: Step 1: Add catalyst, ethylene, and hydrogen to the first reactor to carry out the polymerization reaction, and then input the product into the second reactor. Step 2: α-olefins are added to the second reactor, along with ethylene and hydrogen, to carry out the polymerization reaction. The product after the reaction is then fed into the third reactor. Step 3: Add catalyst, ethylene, hydrogen and α-olefin to the third reactor to carry out polymerization reaction and obtain the product; Step 4: Mix the product obtained from polymerization in the third reactor with the composite additive, and then extrude and granulate to obtain the polyethylene base resin composition for film.

4. The method for preparing the polyethylene base resin composition for films according to claim 3, characterized in that, The product generated in the second reactor has a melt flow rate of 2.0–3.0 g / 10 min at a temperature of 190 °C and a load of 21.6 kg, and a density not exceeding 0.960 g / cm³. 3 The product generated in the third reactor has a melt flow rate of 5.0–15.0 g / 10 min at a temperature of 190℃ and a load of 21.6 kg, and a density not exceeding 0.955 g / cm³. 3 .

5. The method for preparing the polyethylene base resin composition for films according to claim 4, characterized in that, The product generated in the first reactor has a melt flow rate of 170–200 g / 10 min at a temperature of 190 °C and a load of 2.16 kg, and a density of not less than 0.960 g / cm³. 3 The product generated in the second reactor has a melt flow rate of 2.0–3.0 g / 10 min at a temperature of 190℃ and a load of 21.6 kg, and a density not exceeding 0.960 g / cm³. 3 The product generated in the third reactor has a melt flow rate of 5.0–15.0 g / 10 min at a temperature of 190℃ and a load of 21.6 kg, and a density not exceeding 0.955 g / cm³. 3 .

6. The method for preparing the polyethylene base resin composition for films according to claim 3, characterized in that, In step 1, the amount of ethylene added accounts for 40% to 55% of the total amount of ethylene added in steps 1, 2, and 3; in step 2, the amount of ethylene added accounts for 10% to 30% of the total amount of ethylene added in steps 1, 2, and 3; and in step 3, the amount of ethylene added accounts for 20% to 45% of the total amount of ethylene added in steps 1, 2, and 3.

7. The method for preparing the polyethylene base resin composition for films according to any one of claims 3-6, characterized in that, In step 2, the amount of α-olefin added is 1-3 wt% of the amount of ethylene added in step 2, and in step 3, the amount of α-olefin added is 1-3 wt% of the amount of ethylene added in step 3.

8. The method for preparing the polyethylene base resin composition for films according to any one of claims 3-6, characterized in that, The hydrogen / ethylene volume ratio in the first reactor is 3.0–6.0, the temperature is 78–85℃, and the pressure is 0.40–0.60 MPa; the hydrogen / ethylene volume ratio in the second reactor is 0.01–0.1, the temperature is 70–80℃, and the pressure is 0.10–0.40 MPa; the hydrogen / ethylene volume ratio in the third reactor is 0.21–0.80, the temperature is 75–83℃, and the pressure is 0.30–0.50 MPa.

9. The method for preparing the polyethylene base resin composition for films according to any one of claims 3-6, characterized in that, The α-olefin has 3 to 8 carbon atoms, and the catalyst includes a main catalyst and a co-catalyst. The main catalyst is a Ziegler-Natta catalyst, and the co-catalyst is an alkylaluminum.

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