Extremely low density polyethylene and preparation method thereof

Abstract

The invention provides ultra-low density polyethylene and a preparation method thereof, and the preparation method comprises the following steps: ethylene and alpha-olefin are subjected to a gas phase polymerization reaction under the action of a composite catalyst to obtain the ultra-low density polyethylene, the preparation method of the composite catalyst comprises the following steps: step 1, activating silica gel; step 2, enabling the activated silica gel to react with amino silane, and then mixing and reacting with an acyl halide compound to obtain modified silica gel; and step 3, mixing the modified silica gel with a zirconium compound for reaction, and then mixing with cyclopentadienyl metal for reaction to obtain the composite catalyst. The ultra-low density polyethylene powder has good particle morphology and high bulk density, and can be suitable for slurry method and gas phase method polymerization processes. The cast film product prepared from the extremely low density polyethylene has better toughness, heat sealability and puncture resistance.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical product preparation, and particularly relates to a very low density polyethylene and a preparation method thereof. Background Art

[0002] Polyethylene cast films are widely used in fields such as stretch wrap films, food wrap films, and food packaging films. Major foreign companies have successively developed production processes such as gas phase process, solution process, and high-pressure tubular process to prepare polyethylene, with the gas phase process being the most widely used.

[0003] CN112356543A discloses a high-gloss and high-transparency polyethylene cast film and a preparation method thereof. The polyethylene cast film comprises a corona layer of 25-30%, an intermediate layer of 40-50%, and a heat-sealing layer of 20-30%. The raw materials for preparing the corona layer and the intermediate layer are both linear low-density polyethylene resin a, and the raw material for preparing the heat-sealing layer is linear low-density polyethylene resin b with a slip agent and an anti-blocking agent. The linear low-density polyethylene resin of this invention is a product catalyzed by a Z-N catalyst, and its particle morphology is uneven.

[0004] There is still a need in the art to further study low-density polyethylene that can be used for cast films. Summary of the Invention

[0005] The main object of the present invention is to provide a very low density polyethylene and a preparation method thereof, so as to overcome the defects in the prior art such as uneven particle morphology of polyethylene, poor toughness, heat-sealing property, and puncture resistance of the prepared cast film.

[0006] To achieve the above object, the present invention provides a preparation method of a very low density polyethylene, comprising the following steps:

[0007] Carrying out gas phase polymerization reaction on ethylene and α-olefin under the action of a composite catalyst to obtain very low density polyethylene;

[0008] Wherein, the preparation method of the composite catalyst comprises the following steps:

[0009] Step 1, activating silica gel;

[0010] Step 2, reacting the activated silica gel with an amino silane, and then mixing and reacting with an acyl halide compound to obtain modified silica gel;

[0011] Step 3, mixing and reacting the modified silica gel with a metal compound, and then mixing and reacting with a cyclopentadienyl metal to obtain a composite catalyst.

[0012] In the preparation method of the very low density polyethylene of the present invention, the step of activating silica gel is: treating silica gel at 20-200 °C.

[0013] The preparation method of very low density polyethylene according to the present invention, wherein the structure of the aminosilane is: Si(OR 1 ) 3 R 2 NH 2 , where R 1 is methyl, ethyl, propyl or butyl, and R 2 is methyl, ethyl, propyl, butyl or pentyl; the reaction temperature of the activated silica gel and the aminosilane is 60-150°C.

[0014] The preparation method of very low density polyethylene according to the present invention, wherein the aminosilane is aminopropyltriethoxysilane or aminopropyltrimethoxysilane.

[0015] The preparation method of very low density polyethylene according to the present invention, wherein the acyl halide compound is at least one of acetyl chloride, formyl chloride and propionyl chloride; after the reaction of the activated silica gel and the aminosilane in step 2, a basic compound is also added during the mixing with the acyl halide compound; and / or, the basic compound is an organic amine.

[0016] The preparation method of very low density polyethylene according to the present invention, wherein the metal compound is a zirconium compound, a titanium compound or a hafnium compound; and / or, the zirconium compound is zirconium tetrachloride, the titanium compound is titanium tetrachloride, and the hafnium compound is hafnium tetrachloride; the cyclopentadienyl metal is at least one of sodium cyclopentadienyl and potassium cyclopentadienyl.

[0017] The preparation method of very low density polyethylene according to the present invention, wherein the reaction temperature of the modified silica gel and the metal compound is 30-80°C.

[0018] The preparation method of very low density polyethylene according to the present invention, wherein the preparation of the composite catalyst is carried out in an inert gas atmosphere and in an organic solvent.

[0019] The preparation method of very low density polyethylene according to the present invention, wherein in the gas-phase polymerization reaction, H 2 is used as a molecular weight regulator, the molar ratio of α-olefin to ethylene is 0.01-0.1, the molar ratio of H 2 to ethylene is 0.1-0.5, the reaction temperature is 75-95°C, and the reaction pressure is 1.0-5.0 MPa.

[0020] In order to achieve the above object, the present invention also provides very low density polyethylene obtained by the above preparation method.

[0021] Advantages of the present invention:

[0022] In the catalyst for preparing polyethylene of the present invention, an amide group is grafted onto the surface of silica gel, thereby realizing the in-situ synthesis of a metallocene structure on the surface of silica gel. The carrier modified with an amide group is loaded with a composite catalyst formed by, such as ZrCp 2 Cl 2 etc., which has high polymerization activity. In addition, the introduction of the amide group moves the active center of the metallocene catalyst away from the silica gel surface, avoiding the poisoning effect of the surface silanol groups on the active center. Moreover, grafting the metallocene structure onto the modified silica gel surface in an in-situ chemical bonding manner can effectively prevent the catalytic active center from falling off the carrier surface during the olefin polymerization reaction, not only improving the stability of the catalyst, facilitating the full exposure of the catalytic active center, but also effectively avoiding the phenomenon of polymer sticking to the reactor.

[0023] The very low density polyethylene powder of the present invention has good particle morphology and high bulk density, and can be applied to slurry polymerization and gas phase polymerization processes. The cast film products prepared from the very low density polyethylene of the present invention have good toughness, heat sealability and puncture resistance. Description of the Drawings

[0024] Figure 1 It is the SEM characterization map of the polymer prepared in Example 4 of the present invention. Detailed Embodiments

[0025] The technical solutions of the present invention are described in detail below. The following embodiments are implemented on the premise of the technical solutions of the present invention, and the detailed implementation processes are given. However, the protection scope of the present invention is not limited to the following embodiments. For the structures or experimental methods without specific conditions noted in the following embodiments, they are usually in accordance with conventional conditions.

[0026] The present invention provides a method for preparing very low density polyethylene, which includes the following steps:

[0027] Carry out gas phase polymerization reaction of ethylene and α-olefin under the action of a composite catalyst to obtain very low density polyethylene;

[0028] Among them, the preparation method of the composite catalyst includes the following steps:

[0029] Step 1, activate silica gel;

[0030] Step 2, react the activated silica gel with an amino silane, and then mix and react with an acyl halide compound to obtain modified silica gel;

[0031] Step 3, mix and react the modified silica gel with a metal compound, and then mix and react with a cyclopentadienyl metal to obtain a composite catalyst.

[0032] The present invention does not particularly limit the silica gel, and commercially available silica gel can be used. In one embodiment, the step of activating the silica gel is: treating the silica gel at 20-200°C, preferably at 60-160°C, to activate the silanol groups present on the surface of the silica gel. In another embodiment, the silica gel is activated in the presence of an organic solvent, such as toluene.

[0033] Step 2 of the present invention is: reacting the activated silica gel with an aminosilane, and then mixing and reacting with an acyl halide compound to obtain the modified silica gel.

[0034] In one embodiment, the structure of the aminosilane of the present invention is: Si(OR 1 ) 3 R 2 NH 2 , where R 1 is an alkyl group having 1-4 carbons, such as methyl, ethyl, propyl, butyl, where propyl can be n-propyl or isopropyl, and butyl can be n-butyl, isobutyl, or neobutyl; R 2 is an alkyl group having 1-5 carbons, such as methyl, ethyl, propyl, butyl, pentyl, where propyl can be n-propyl or isopropyl, and butyl can be n-butyl, isobutyl, or neobutyl, and pentyl can be n-pentyl, isopentyl, neopentyl, etc. In another embodiment, the aminosilane of the present invention is aminopropyltriethoxysilane or aminopropyltrimethoxysilane.

[0035] In one embodiment, the reaction temperature between the activated silica gel and the aminosilane is 60-150°C, preferably 80-120°C, and the reaction time is 1-20 h, preferably 8-20 h. In another embodiment, the reaction between the activated silica gel and the aminosilane is carried out in an organic solvent, such as toluene. After the reaction, the mixture is separated into solid and liquid, and then the solid is washed with an organic solvent to obtain the amino-modified silica gel. The present invention does not particularly limit the method of solid-liquid separation, such as vacuum filtration.

[0036] Then, the obtained amino-modified silica gel is mixed and reacted with an acyl halide compound to obtain the amide-modified silica gel.

[0037] In one embodiment, the acyl halide compound of the present invention is at least one of acetyl chloride, formyl chloride, and propionyl chloride. In another embodiment, a basic compound is added to the reaction system of the amino-modified silica gel and the acyl halide compound, and the basic compound is an organic amine, more specifically, triethylamine. The addition amount of the organic amine is, for example, the same as the molar amount of the aminosilane. In yet another embodiment, the dosage ratio of the acyl halide compound to the silica gel is 0.1-10 mmol / g.

[0038] In a specific embodiment, the amino-modified silica gel is mixed with an organic amine, and an organic solvent such as tetrahydrofuran is also added during the mixing process. Then, the mixture is cooled to 0 °C and slowly mixed with an acyl halide compound. Specifically, the acyl halide compound can be added dropwise to the mixture of the amino-modified silica gel, the organic amine, and the organic solvent. The reaction between the amino-modified silica gel and the acyl halide compound is carried out at room temperature, such as 20 - 30 °C, and the reaction time is, for example, 1 - 10 h, preferably 2 - 8 h. Then, solid-liquid separation is carried out, the solid is washed, and dried to obtain the amide-modified silica gel.

[0039] Step 3 of the present invention is: mixing and reacting the amide-modified silica gel with a metal compound, and then mixing and reacting with a cyclopentadienyl metal to obtain a composite catalyst.

[0040] In one embodiment, the metal compound of the present invention is a zirconium compound, a titanium compound or a hafnium compound; wherein, the zirconium compound is, for example, zirconium tetrachloride, the titanium compound is, for example, titanium tetrachloride, and the hafnium compound is, for example, hafnium tetrachloride. The amount of the metal compound used satisfies that the mass content of the metal compound in the obtained catalyst based on the metal is 1 - 30 wt%. In another embodiment, the reaction between the amide-modified silica gel and the metal compound is carried out in an organic solvent, such as toluene, the reaction temperature is 30 - 80 °C, preferably 40 - 70 °C, the reaction time is 1 - 15 h, preferably 4 - 12 h, solid-liquid separation is carried out, and the solid is washed to obtain the metal-loaded silica gel.

[0041] Then, the metal-loaded silica gel is mixed and reacted with the cyclopentadienyl metal. In one embodiment, the reaction is carried out in an organic solvent, such as toluene. Solid-liquid separation is carried out on the reaction mixture, and the solid is washed to obtain the composite catalyst. In another embodiment, the cyclopentadienyl metal is at least one of sodium cyclopentadienyl and potassium cyclopentadienyl. In yet another embodiment, the ratio of the amount of the cyclopentadienyl metal used to the amount of the silica gel is 10 - 500 μmol / g.

[0042] The preparation of the composite catalyst of the present invention is carried out in an inert gas atmosphere. The present invention does not particularly limit the type of the inert gas atmosphere, such as nitrogen, argon, etc. The activity of the composite catalyst prepared by the present invention is 200 - 20000 g / gcat.h, preferably 400 - 2000 g / gcat.h.

[0043] Ethylene and an α-olefin are subjected to a gas-phase polymerization reaction under the action of the above-prepared composite catalyst to obtain very low density polyethylene.

[0044] In one embodiment, the gas-phase polymerization reaction is carried out in a reactor, and H 2 As a molecular weight regulator, the molar ratio of the α-olefin to ethylene is 0.01 - 0.1, and H in the reactor 2The molar ratio of the α-olefin to ethylene is 0.1 to 0.5, the reaction temperature is 75 to 95 °C, and the reaction pressure is 1.0 to 5.0 MPa. Preferably, the molar ratio of the α-olefin to ethylene is 0.01 to 0.08, and H in the reactor 2 The molar ratio to ethylene is 0.1 to 0.4, the reaction temperature is 75 to 80 °C, and the reaction pressure is 1.0 to 3.0 MPa.

[0045] The granular morphology of the very low density polyethylene powder obtained by the method of the present invention is uniform and good, and the bulk density is high, which can be applied to the slurry polymerization process and the gas phase polymerization process.

[0046] The linear very low density polyethylene prepared by the present invention can be used to prepare a polyethylene cast film. In one embodiment, the method for preparing the polyethylene cast film is as follows: mixing the very low density polyethylene prepared above with a compound additive at normal temperature and pressure for 3 to 10 minutes, preferably 5 to 8 minutes, to obtain a mixture; then controlling the temperature at 180 to 210 °C, preferably 200 to 210 °C, and using a twin-screw extruder to granulate, thereby obtaining a special material for the very low density polyethylene cast film.

[0047] The present invention does not particularly limit the types and amounts of the compound additives, and common additives in the art can be compounded. The amount can be adjusted according to needs.

[0048] The melt index range of the special material for the very low density polyethylene cast film obtained by the method of the present invention is 3.0 - 4.5 g / 10 min, the density range is 0.90 - 0.92 g / cm 3 , and the bulk density range is 0.4 - 0.5 g / cm 3 , the relative molecular mass range is 10,000 to 100,000, and it has good processing performance and optical performance, and can be applied to packaging film products, such as packaging films and hoses with special requirements.

[0049] The very low density polyethylene cast film product of the present invention has irreplaceable advantages over other packaging materials in terms of toughness, heat sealability and puncture resistance. In addition, the very low density polyethylene cast film of the present invention has strong impact resistance and good transparency, is not hard or brittle in daily use, is not easy to break, has good heat sealability, and is suitable for long-term storage of items.

[0050] The technical solution of the present invention will be further described in detail below through specific examples.

[0051] Example 1

[0052] (1) Preparation of metallocene composite catalyst: Under a nitrogen atmosphere, using anhydrous toluene as a solvent, 1 g of pre-dried and pre-treated silica microspheres were activated at 80 °C to activate the silanol groups present on the silica surface. In an 80 °C environment, aminopropyltriethoxysilane was added to the reaction system, and the reaction was carried out for 10 h. After vacuum filtration and separation, the solid was washed several times with anhydrous toluene. The modified silica was added to a mixed solution of tetrahydrofuran and triethylamine. The molar amount of triethylamine was the same as that of aminopropyltriethoxysilane. The entire reaction system was placed in an ice-water bath at 0 °C, and then formyl chloride was added dropwise to the system. The molar ratio of aminopropyltriethoxysilane to formyl chloride was 1:1. After the dropwise addition, the reaction was carried out at room temperature for 3 h. The solid was separated by vacuum filtration, the solid was washed, and vacuum dried to obtain a stepwise modified silica support. The obtained silica support was placed in anhydrous toluene solvent and heated to 40 °C. Zirconium tetrachloride was added to the system. The molar ratio of aminopropyltriethoxysilane to zirconium tetrachloride was 0.1:20. After 12 h of reaction, solid-liquid separation and several washes were carried out. The obtained solid product was added to an anhydrous toluene solution of sodium cyclopentadienide for reaction. The molar ratio of zirconium tetrachloride to sodium cyclopentadienide was 1:1.2. After the reaction was completed, the solid was obtained by vacuum filtration, and after several washes and vacuum drying, the composite catalyst was obtained.

[0053] (2) Preparation of very low density polyethylene powder: Using the above composite catalyst, H 2 as a molecular weight regulator, ethylene and α-olefin were subjected to gas-phase polymerization reaction in a reactor to obtain linear low density polyethylene powder; during the reaction, the molar ratio of the α-olefin to ethylene was 0.020, and the molar ratio of H 2 to ethylene in the reactor was 0.16, the reaction temperature was 80 °C, and the reaction pressure was 2.0 MPa.

[0054] (3) Preparation of special material for very low density polyethylene cast film: The linear low density polyethylene powder obtained in the above step (2) was mixed with a compound additive at normal temperature and pressure for 8 minutes to obtain a mixture; among them, the compound additive was antioxidant 1010, antioxidant 168 and hydrotalcite, and the molar ratio of the three was 1:(1 - 3):(0.2 - 1). The addition amount of the compound additive accounted for 1200 ppm of the total amount of the special material. Then, the temperature was controlled at 200 °C, and granulation was carried out using a twin-screw extruder to obtain a special material for very low density polyethylene cast film.

[0055] Example 2

[0056] (1) Preparation of metallocene composite catalyst: Under a nitrogen atmosphere, using anhydrous toluene as a solvent, 1 g of pretreated silica microspheres was activated at 110 °C to activate the silanol groups present on the silica surface. In a 90 °C environment, aminopropyltriethoxysilane was added to the reaction system and reacted for 11 h. After vacuum filtration and separation, the solid was washed several times with anhydrous toluene. The modified silica was added to a mixed solution of tetrahydrofuran and triethylamine. The molar amount of triethylamine was the same as that of aminopropyltriethoxysilane. The entire reaction system was placed in an ice-water bath at 0 °C, and then acetyl chloride was added dropwise to the system. The molar ratio of aminopropyltriethoxysilane to acetyl chloride was 1:1. After the addition was completed, the reaction was carried out at room temperature for 6 h. The solid was separated by vacuum filtration, the solid was washed, and vacuum dried to obtain a stepwise modified silica support. The obtained silica support was placed in anhydrous toluene solvent and heated to 80 °C. Titanium tetrachloride was added to the system. The molar ratio of aminopropyltriethoxysilane to titanium tetrachloride was 0.1:20. After 12 h of reaction, solid-liquid separation and several washes were carried out. The obtained solid product was added to an anhydrous toluene solution containing an equal amount of sodium cyclopentadienide for reaction. The molar ratio of titanium tetrachloride to sodium cyclopentadienide was 1:1.2. After the reaction was completed, the solid was obtained by vacuum filtration, and after several washes and vacuum drying, the composite catalyst was obtained.

[0057] (2) Preparation of very low density polyethylene powder: Using the above composite catalyst, H 2 as a molecular weight regulator, ethylene and α-olefin were subjected to gas-phase polymerization reaction in a reactor to obtain linear low density polyethylene powder; during the reaction, the molar ratio of the α-olefin to ethylene was 0.025, and the molar ratio of H 2 to ethylene in the reactor was 0.18, the reaction temperature was 79 °C, and the reaction pressure was 2.0 MPa.

[0058] (3) Preparation of special material for very low density polyethylene cast film: The linear low density polyethylene powder obtained in the above step (2) was mixed with a compound additive (the type and dosage of the additive were the same as those in Example 1) at normal temperature and pressure for 7 minutes to obtain a mixture; then the temperature was controlled at 195 °C, and granulation was carried out using a twin-screw extruder to obtain a special material for very low density polyethylene cast film.

[0059] Example 3

[0060] (1) Preparation of metallocene composite catalyst: Under a nitrogen atmosphere, using anhydrous toluene as a solvent, 0.5 g of pretreated silica microspheres were activated at 120 °C to activate the silanol groups present on the silica surface. In an environment of 110 °C, aminopropyltriethoxysilane was added to the reaction system and reacted for 8 h. After vacuum filtration and separation, the solid was washed several times with anhydrous toluene. The modified silica was added to a mixed solution of tetrahydrofuran and triethylamine, and the molar ratio of triethylamine to aminopropyltriethoxysilane was 1.2:1. The entire reaction system was placed in an ice-water bath at 0 °C, and then acetyl chloride was added dropwise to the system. The molar ratio of aminopropyltriethoxysilane to acetyl chloride = 1:1. After the dropwise addition, the reaction was carried out at room temperature for 5 h. The solid was separated by vacuum filtration to achieve solid-liquid separation, the solid was washed, and vacuum dried to obtain a stepwise modified silica support. The obtained silica support was placed in anhydrous toluene solvent and heated to 100 °C. Zirconium tetrachloride was added to the system, and the molar ratio of aminopropyltriethoxysilane to zirconium tetrachloride = 0.1:20. After 6 h of reaction, solid-liquid separation and several washings were carried out. The obtained solid product was added to an anhydrous toluene solution containing an equal amount of potassium cyclopentadienide for reaction. The molar ratio of zirconium tetrachloride to potassium cyclopentadienide = 1:1.2. After the reaction was completed, the solid was obtained by vacuum filtration and vacuum dried after several washings to obtain the composite catalyst.

[0061] (2) Preparation of very low density polyethylene powder: Using the above composite catalyst, H 2 as a molecular weight regulator, ethylene and α-olefin were subjected to gas-phase polymerization reaction in a reactor to obtain linear low density polyethylene powder; during the reaction, the molar ratio of the α-olefin to ethylene was 0.030, and the molar ratio of H 2 to ethylene in the reactor was 0.20, the reaction temperature was 78 °C, and the reaction pressure was 1.9 MPa.

[0062] (3) Preparation of special material for very low density polyethylene cast film: The linear low density polyethylene powder obtained in the above step (2) was mixed with a compound additive (the type and dosage of the additive were the same as in Example 1) at normal temperature and pressure for 8 minutes to obtain a mixture; then the temperature was controlled at 190 °C, and granulation was carried out using a twin-screw extruder to obtain a special material for very low density polyethylene cast film.

[0063] Example 4

[0064] (1) Preparation of metallocene composite catalyst: Under a nitrogen atmosphere, using anhydrous toluene as a solvent, 0.5 g of pretreated silica microspheres were activated at 80 °C to activate the silanol groups present on the silica surface. In a 100 °C environment, aminopropyltriethoxysilane was added to the reaction system and reacted for 12 h. After vacuum filtration and separation, the solid was washed several times with anhydrous toluene. The modified silica was added to a mixed solution of tetrahydrofuran and triethylamine. The molar amount of triethylamine was the same as that of aminopropyltriethoxysilane. The entire reaction system was placed in an ice-water bath at 0 °C, and then acetyl chloride was added dropwise to the system. The molar ratio of aminopropyltriethoxysilane to acetyl chloride was 1.2:1. After the dropwise addition, the reaction was carried out at room temperature for 4 h. The solid was separated by vacuum filtration to achieve solid-liquid separation, the solid was washed, and vacuum dried to obtain a stepwise modified silica support. The obtained silica support was placed in anhydrous toluene solvent and heated to 50 °C. Zirconium tetrachloride was added to the system. The molar ratio of aminopropyltriethoxysilane to zirconium tetrachloride was 0.1:20. After 8 h of reaction, solid-liquid separation and several washings were carried out. The obtained solid product was added to an anhydrous toluene solution containing an equal amount of sodium cyclopentadienide for reaction. The molar ratio of zirconium tetrachloride to sodium cyclopentadienide was 1:1.2. After the reaction was completed, the solid was obtained by vacuum filtration and vacuum dried after several washings to obtain the composite catalyst.

[0065] (2) Preparation of very low density polyethylene powder: Using the above composite catalyst, H 2 as a molecular weight regulator, ethylene and α-olefin were subjected to gas-phase polymerization reaction in a reactor to obtain linear low density polyethylene powder; during the reaction, the molar ratio of the a-olefin to ethylene was 0.035, and the molar ratio of H 2 to ethylene in the reactor was 0.22, the reaction temperature was 77 °C, and the reaction pressure was 1.9 MPa.

[0066] (3) Preparation of special material for very low density polyethylene cast film: The linear low density polyethylene powder obtained in the above step (2) was mixed with a compound additive (the type and dosage of the additive were the same as those in Example 1) at normal temperature and pressure for 5 minutes to obtain a mixture; then the temperature was controlled at 205 °C, and granulation was carried out using a twin-screw extruder to obtain a special material for very low density polyethylene cast film.

[0067] Figure 1 SEM characterization pattern of the polymer prepared in Example 4 of the present invention. As Figure 1 shown, the particle morphology of the polyethylene powder obtained by the method of the present invention is uniform and good.

[0068] Example 5

[0069] (1) Preparation of metallocene composite catalyst: Under a nitrogen atmosphere, using anhydrous toluene as a solvent, 2 g of pretreated silica microspheres were activated at 150 °C to activate the silanol groups present on the silica surface. In a 120 °C environment, aminopropyltrimethoxysilane was added to the reaction system and reacted for 16 h. After vacuum filtration and separation, the solid was washed several times with anhydrous toluene. The modified silica was added to a mixed solution of tetrahydrofuran and triethylamine. The molar amount of triethylamine was the same as that of aminopropyltrimethoxysilane. The entire reaction system was placed in an ice-water bath at 0 °C, and then acetyl chloride was added dropwise to the system. The molar ratio of aminopropyltrimethoxysilane to acetyl chloride was 1:1. After the addition was completed, the reaction was carried out at room temperature for 5 h. The solid was separated by vacuum filtration, the solid was washed, and then vacuum dried to obtain a stepwise modified silica support. The obtained silica support was placed in anhydrous toluene solvent and heated to 40 °C. Zirconium tetrachloride was added to the system. The molar ratio of aminopropyltrimethoxysilane to zirconium tetrachloride was 0.1:18. After 5 h of reaction, solid-liquid separation and several washings were carried out. The obtained solid product was added to an anhydrous toluene solution containing an equal amount of sodium cyclopentadienide for reaction. The molar ratio of zirconium tetrachloride to sodium cyclopentadienide was 1:1.2. After the reaction was completed, the solid was obtained by vacuum filtration and was washed several times and vacuum dried to obtain the composite catalyst.

[0070] (2) Preparation of very low density polyethylene powder: Using the above composite catalyst, H 2 as a molecular weight regulator, ethylene and α-olefin were subjected to gas-phase polymerization reaction in a reactor to obtain linear low density polyethylene powder; during the reaction, the molar ratio of the α-olefin to ethylene was 0.040, and the molar ratio of H 2 to ethylene in the reactor was 0.24, the reaction temperature was 76 °C, and the reaction pressure was 2.0 MPa.

[0071] (3) Preparation of special material for very low density polyethylene cast film: The linear low density polyethylene powder obtained in the above step (2) was mixed with a compound additive (the type and dosage of the additive were the same as those in Example 1) at normal temperature and pressure for 6 minutes to obtain a mixture; then the temperature was controlled at 200 °C, and granulation was carried out using a twin-screw extruder to obtain a special material for very low density polyethylene cast film.

[0072] Example 6

[0073] (1) Preparation of metallocene composite catalyst: Under a nitrogen atmosphere, using anhydrous toluene as a solvent, 2 g of pretreated silica microspheres were activated at 130 °C to activate the silanol groups present on the silica surface. In an environment of 110 °C, aminopropyltriethoxysilane was added to the reaction system, and the reaction was carried out for 9 h. After vacuum filtration and separation, the solid was washed several times with anhydrous toluene. The modified silica was added to a mixed solution of tetrahydrofuran and triethylamine. The molar amount of triethylamine was the same as that of aminopropyltriethoxysilane. The entire reaction system was placed in an ice-water bath at 0 °C, and then acetyl chloride was added dropwise to the system. The molar ratio of aminopropyltriethoxysilane to acetyl chloride was 1:1. After the addition was completed, the reaction was carried out at room temperature for 7 h. The solid was separated by vacuum filtration to achieve solid-liquid separation, the solid was washed, and vacuum dried to obtain a stepwise modified silica support. The obtained silica support was placed in anhydrous toluene solvent and heated to 70 °C. Zirconium tetrachloride was added to the system. The molar ratio of aminopropyltriethoxysilane to zirconium tetrachloride was 0.1:20. After 11 h of reaction, solid-liquid separation and several washes were carried out. The obtained solid product was added to an anhydrous toluene solution containing an equal amount of sodium cyclopentadienide for reaction. The molar ratio of zirconium tetrachloride to sodium cyclopentadienide was 1:1. After the reaction was completed, the solid was obtained by vacuum filtration, and after several washes and vacuum drying, the composite catalyst was obtained.

[0074] (2) Preparation of very low density polyethylene powder: Using the above composite catalyst, H 2 as a molecular weight regulator, ethylene and α-olefin were subjected to gas-phase polymerization reaction in a reactor to obtain linear low density polyethylene powder; during the reaction, the molar ratio of the α-olefin to ethylene was 0.035, and the molar ratio of H 2 to ethylene in the reactor was 0.25, the reaction temperature was 75 °C, and the reaction pressure was 2.0 MPa.

[0075] (3) Preparation of special material for very low density polyethylene cast film: The linear low density polyethylene powder obtained in the above step (2) was mixed with a compound additive (the type and dosage of the additive were the same as those in Example 1) at normal temperature and pressure for 8 minutes to obtain a mixture; then the temperature was controlled at 200 °C, and granulation was carried out using a twin-screw extruder to obtain a special material for very low density polyethylene cast film.

[0076] Comparative Example 1

[0077] (1) Preparation of catalyst: Metal Na was used in a tetrahydrofuran solution or n-butyllithium (n-BuLi) was used in a hexane solution or an ether solution to carry out a substitution reaction with cyclopentadiene to form the corresponding sodium cyclopentadienide or lithium cyclopentadienide salt, and then continue to react with Me 3 SiCl to form cyclopentadienyltrimethylsilane, and then react with TiC1 4 to prepare cyclopentadienyltitanium trichloride.

[0078] (2) Preparation of very low density polyethylene powder: Using the above-mentioned catalyst cyclopentadienyl titanium trichloride, H 2 as a molecular weight regulator, subject ethylene and α-olefin to gas-phase polymerization reaction in a reactor to obtain linear low density polyethylene powder; during the reaction, the molar ratio of the α-olefin to ethylene is 0.030, and the molar ratio of H 2 to ethylene in the reactor is 0.26, the reaction temperature is 80 °C, and the reaction pressure is 2.0 MPa.

[0079] (3) Preparation of special material for very low density polyethylene cast film: Mix the linear low density polyethylene powder obtained in the above step (2) with a compound additive (the types and dosages of the additives are the same as those in Example 1) at normal temperature and pressure for 8 minutes to obtain a mixture; then control the temperature at 190 °C and use a twin-screw extruder for pelletizing to obtain a special material for very low density polyethylene cast film.

[0080] The test results of the performance of the cast films obtained in the examples and comparative examples are shown in Table 1. Among them, the melt flow index is measured under a 2.16 kg weight.

[0081] Table 1 Test results of the performance of special material for very low density polyethylene cast film

[0082]

[0083] As shown in Table 1, the catalyst used in the present invention effectively avoids the phenomenon of agglomeration of the polymerization product in the kettle, and the generated very low density polyethylene powder has good particle morphology. The very low density polyethylene cast film prepared by the method of the present invention has a lower density and better puncture resistance compared with the comparative example, and has a higher melt flow index, which is more suitable for various processing application scenarios.

[0084] Of course, the present invention may also 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 deformations according to the present invention, but these corresponding changes and deformations should all fall within the protection scope of the claims of the present invention.

Claims

1. A method for preparing very low density polyethylene, characterized in that, it comprises the following steps: subjecting ethylene and an α-olefin to a gas-phase polymerization reaction under the action of a composite catalyst to obtain very low density polyethylene; wherein, the preparation method of the composite catalyst comprises the following steps: Step 1, activating silica gel; Step 2, reacting the activated silica gel with an aminosilane, and then mixing and reacting with an acyl halide compound to obtain modified silica gel; Step 3, mixing and reacting the modified silica gel with a metal compound, and then mixing and reacting with a cyclopentadienyl metal to obtain a composite catalyst.

2. The method for preparing very low density polyethylene according to claim 1, characterized in that, the step of activating silica gel is: treating the silica gel at 20-200 °C.

3. The method for preparing very low density polyethylene according to claim 1, characterized in that, The structure of the amino silane is: Si(OR 1 ) 3 R 2 NH 2 , where R 1 is methyl, ethyl, propyl or butyl, and R 2 is methyl, ethyl, propyl, butyl or pentyl; the reaction temperature of the activated silica gel and the amino silane is 60-150°C.

4. The method for preparing very low density polyethylene according to claim 1, characterized in that, the aminosilane is aminopropyltriethoxysilane, aminopropyltrimethoxysilane.

5. The method for preparing very low density polyethylene according to claim 1, characterized in that, the acyl halide compound is at least one of acetyl chloride, formyl chloride, propionyl chloride; during the mixing process of the activated silica gel and the acyl halide compound after the reaction with the aminosilane in Step 2, a basic compound is also added; and / or, the basic compound is an organic amine.

6. The method for preparing very low density polyethylene according to claim 1, characterized in that, the metal compound is a zirconium compound, a titanium compound or a hafnium compound; and / or, the zirconium compound is zirconium tetrachloride, the titanium compound is titanium tetrachloride, the hafnium compound is hafnium tetrachloride; the cyclopentadienyl metal is at least one of sodium cyclopentadienyl, potassium cyclopentadienyl.

7. The method for preparing very low density polyethylene according to claim 1, characterized in that, the reaction temperature of the modified silica gel and the metal compound is 30-80 °C.

8. The method for preparing very low density polyethylene according to claim 1, characterized in that, the preparation of the composite catalyst is carried out in an inert gas atmosphere and in an organic solvent.

9. The method for preparing very low density polyethylene according to claim 1, characterized in that, In the gas-phase polymerization reaction, H 2 as a molecular weight regulator, the molar ratio of α-olefin to ethylene is 0.01 to 0.1, and H 2 has a molar ratio to ethylene of 0.1 to 0.5, the reaction temperature is 75 to 95 °C, and the reaction pressure is 1.0 to 5.0 MPa.

10. Very low density polyethylene obtained by the preparation method according to any one of claims 1-9.

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