Low-ash polypropylene resin and preparation method thereof

Abstract

The invention discloses low-ash polypropylene resin and a preparation method thereof. The preparation method comprises the following steps: polymerizing propylene serving as a raw material to obtain a polypropylene crude product; and purifying the obtained polypropylene crude product to obtain the low-ash polypropylene resin. The purification comprises the following steps: preparing a dried polypropylene crude product into slurry by using a composite solvent, and stirring for purification; then carrying out solid-liquid separation on the slurry to obtain wet polypropylene powder, and drying to obtain low-ash polypropylene resin powder; the composite solvent comprises a hydrocarbon solvent and a difunctional group additive, and the structure of the difunctional group additive is # imgabs0, in the formula, R1 is a C1-5 alkane group, R2 is a C1-3 alkane group, and R3 is a C1-4 alkane group. According to the invention, a composite solvent of a hydrocarbon solvent and a difunctional group additive is used to simply wash and purify a polypropylene crude product once, so that the polypropylene resin with low ash content can be obtained.

Description

Technical Field

[0001] The present invention relates to the field of polypropylene resin preparation, and particularly relates to a low-ash polypropylene resin and a preparation method thereof. Background Art

[0002] Polypropylene, abbreviated as PP, is a colorless, odorless, non-toxic, semi-transparent solid substance. Polypropylene is a thermoplastic synthetic resin with excellent properties and is a colorless semi-transparent thermoplastic lightweight general-purpose plastic. It has chemical resistance, heat resistance, electrical insulation, high-strength mechanical properties, and good high-abrasion processing properties, etc. Since its advent, polypropylene has been rapidly developed and applied in many fields such as machinery, automobiles, electronic appliances, construction, textiles, packaging, agriculture, forestry, fishery, and the food industry. In recent years, with the rapid development of industries such as packaging, electronics, and automobiles, it has greatly promoted the development of the industry.

[0003] Polypropylene can be used to make the insulating housing of household appliances and the inner liner of washing machines, and is commonly used as the insulating material for wires, cables, and other electrical appliances. When preparing barrier films, traditional film-making processes can be used for production, with relatively simple processes and low production costs.

[0004] CN103540020A discloses a production method for a special-purpose material for a homogeneous polypropylene barrier film of a lithium-ion battery. A propylene polymerization catalyst with high activity and moderate hydrogen regulation sensitivity is used, and the propylene polymerization activator is triethylaluminum, with its addition amount controlled below 7.0 L / h. The external electron donor is prepared by dicyclopentyl dimethoxysilane and tetraethoxysilane in a molar ratio of 1:1 to 1:8. The isotacticity of the product is greater than 98%, the melt flow rate is 2.6 - 3.0 g / 10 min, and the ash content is 50 - 100 ppm. The ash content index cannot meet the requirements of the high-end electrical application field.

[0005] CN102574939A discloses a catalyst component for olefin polymerization, using a glycol ester type and a diether type internal electron donor. It also discloses a preparation method of the catalyst component, a catalyst containing the catalyst component, and an olefin polymerization method using the catalyst for preparing a low-ash polypropylene product. The ash content of the product prepared by this technology is as low as 30 ppm, but it still cannot meet the requirements of high-end dry-type power electronic capacitors.

[0006] CN109666092A discloses a production process for a special material for microporous membranes of polypropylene lithium batteries, comprising the following steps: (1) After refining, polymer-grade propylene is mixed with polymer-grade ethylene, pressurized and fed into a polymerization kettle. At the same time, a configured main catalyst, co-catalyst, internal electron donor, external electron donor and hydrogen are added to carry out a polymerization reaction. The polymerization temperature of the reaction kettle is 70-80 °C; the polymerization pressures in the four reaction kettles are 3-4 MPa, 2.7-3.5 MPa, 1.7-2.0 MPa, and 1.5 MPa in sequence; during the production process, the hydrogen injection amount in the first reaction kettle is controlled between 5.5 and 6.5 Nm 3 / h, and the liquid level and the feeding and discharging amounts of the first reaction kettle are controlled to control the melt flow rate of the first reaction kettle at 10-18 g / 10 min; (2) Control the melt flow rate of the second reaction kettle at 7.5-12 g / 10 min; only hydrogen is added to the first reaction kettle for producing lithium battery film material, and the hydrogen in the first reaction kettle enters the second reaction kettle with the slurry; (3) Control the melt flow rate of the third reaction kettle at 4-7 g / 10 min. No hydrogen is added to the third reaction kettle for producing lithium battery film material. The hydrogen in the third reaction kettle is brought in with the slurry of the second reaction kettle, and liquid propylene is vaporized by the reaction heat in the third reaction kettle; (4) Control the melt flow rate of the fourth reaction kettle at 2.0-3.5 g / 10 min. The hydrogen in the fourth reaction kettle is brought in by the discharging system of the third reaction kettle; (5) The polymer slurry obtained in the fourth reaction kettle is washed with liquid propylene in a countercurrent manner to remove the catalyst and amorphous substances, and then heated and dried after flash separation and catalyst inactivation to obtain polymer powder; a halogen absorbent and an antioxidant are added to the polymer powder, and then pelletized and packaged into a special material product for polypropylene lithium battery membranes. However, the activity of the catalyst used is only 30,000 times, the ash content of the produced polypropylene powder is relatively high, and the electrical properties of the product are poor.

[0007] CN111019025A discloses a method for polymerizing high-purity polypropylene. The polymerization method comprises the following steps: After the catalyst and propylene monomer are pre-polymerized, subjected to two liquid-phase polymerization reactions and two gas-phase polymerization reactions in sequence, polypropylene slurry powder is formed; the polypropylene slurry powder is subjected to super flash evaporation secondary deashing; the deashed polypropylene slurry powder is subjected to secondary drying; the catalyst uses magnesium chloride as a carrier, polyamine and diester derivatives as internal electron donors, and silane as an external electron donor. However, the effect of removing metal ion impurities by the super flash evaporation method is poor.

[0008] CN110016092A discloses a method for continuously preparing polyolefins, especially a method for continuously preparing polyolefin elastomers or their mixtures. The method comprises the following steps: S1. Prepolymerization: The reaction materials are continuously added from the bottom of a kettle reactor. Under temperature control and stirring conditions, the material level continuously rises until it reaches the discharge level. The reaction materials include polymerization monomers, a catalyst system, a molecular weight regulator, a polymerization regulator, and a solvent; S2. Static mixing polymerization: After the reaction materials in step S1 are discharged, they are sent to a static mixer for continuous reaction. Under temperature control conditions, the material level continuously rises until it reaches the discharge level. Optionally, at least one of the polymerization monomers, the molecular weight regulator, the polymerization regulator, and the solvent is added as needed; S3. Screw extrusion polymerization: After the reaction materials in step S2 are discharged, they are sent to a reactive screw extruder for continuous reaction. Optionally, at least one of the polymerization monomers, the molecular weight regulator, the polymerization regulator, and the solvent is added as needed. The prepolymerization is in one stage or multiple stages. If it is in multiple stages, a transfer pump is used to transfer the materials between each stage. The disclosed method for continuously preparing polyolefins is more suitable for high-conversion and high-viscosity polymerization reactions, and has no significant effect on improving the catalyst activity and reducing the ash content of polyolefin resins.

[0009] CN109912734A discloses a method for washing a polypropylene resin. A mixed detergent composed of an alkane, an alcohol solvent, and a complexing agent is used for washing. The alcohol solvent washes elements such as Ti and Mg through the dissolution of hydroxyl groups, and the complexing agent removes Al elements by complexing with O having a lone pair of electrons in the molecule. The production method comprises the following steps: a) Primary washing: The raw polypropylene resin to be treated is put into a washing kettle, and a first mixture composed of a hydrocarbon solvent and a complexing agent is added. After heating, it is stirred in a closed state and centrifuged for discharging to obtain a first solid resin material; b) Secondary washing: A second washing agent composed of a hydrocarbon solvent, a complexing agent, and an alcohol solvent is added, and centrifuged for discharging to obtain a second solid resin material; c) Tertiary washing: A third washing agent composed of a complexing agent and an alcohol solvent is added. After heating, it is stirred in a closed state and centrifuged for discharging to obtain a third solid resin material; d) Quaternary washing: An alcohol solvent is added, and centrifuged for discharging to obtain a fourth solid resin material with an ash content of ≤50 ppm.

[0010] CN116410373A discloses a method for preparing a polypropylene resin, which includes a step of purifying with a composite solvent. The composite solvent is a mixed solvent of at least one alkyl alcohol having 1 to 10 carbon atoms and at least one alkane that is liquid at room temperature and has 5 to 20 carbon atoms.

[0011] CN116836314A discloses a method for preparing low-ash polypropylene, which comprises the following steps: a) Under closed conditions, heating and stirring a polypropylene resin with a first washing liquid, and filtering out the washing liquid; in step a), the components of the first washing liquid include an impregnating agent, a metal complexing agent and a pressure regulator; b) Under closed conditions, heat-stirring the polypropylene resin treated in step a) with a second washing liquid, and filtering out the washing liquid; the components of the second detergent in step b) include a low-boiling alcohol and / or a low-boiling alkane; c) Centrifuging and drying the polypropylene resin treated in step b) to obtain low-ash polypropylene.

[0012] The following problems exist in the preparation of the above-mentioned existing polypropylene resins: the washing method is complex, the number of washing times is large, the amount of detergent used is large, and the solvent separation and purification process will be increased during industrial promotion. Summary of the Invention

[0013] The purpose of the present invention is to provide a low-ash polypropylene resin and a preparation method thereof. By using a composite solvent of a hydrocarbon solvent and a bifunctional group additive to simply wash and purify the polypropylene crude product once, a low-ash polypropylene resin can be obtained.

[0014] In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0015] The present invention provides a method for preparing a low-ash polypropylene resin, wherein the preparation method includes:

[0016] The step of polymerizing propylene as a raw material to obtain a polypropylene crude product; and

[0017] The step of purifying the obtained polypropylene crude product to obtain the low-ash polypropylene resin;

[0018] The purification includes:

[0019] Preparing the dried polypropylene crude product into a slurry with a composite solvent, stirring for purification; then separating the solid and liquid of the slurry to obtain wet polypropylene powder, and drying to obtain low-ash polypropylene resin powder;

[0020] Among them, the composite solvent includes a hydrocarbon solvent and a bifunctional group additive, and the bifunctional group additive has the structure shown in the following formula:

[0021]

[0022] In the formula, R 1 is an alkane group of C 1-5 R 2 is an alkane group of C 1-3 R 3 is an alkane group of C 1-4an alkane group.

[0023] According to some specific embodiments of the present invention, preferably, in the purification process, the mass ratio of the hydrocarbon solvent in the configured slurry is 46% - 89%, the mass ratio of the bifunctional group additive is 6% - 40%, and the mass concentration of the crude polypropylene is 5% - 30%.

[0024] According to some specific embodiments of the present invention, preferably, the bifunctional group additive is selected from at least one of ethyl acetoacetate, methyl acetoacetate, butyl acetoacetate, and ethyl neopentanoylacetate.

[0025] According to some specific embodiments of the present invention, preferably, the bifunctional group additive is ethyl acetoacetate and / or ethyl neopentanoylacetate.

[0026] According to some specific embodiments of the present invention, preferably, the hydrocarbon solvent is selected from at least one of C 5～20 alkanes and C 6～12 aromatic hydrocarbons.

[0027] According to some specific embodiments of the present invention, preferably, the hydrocarbon solvent is selected from at least one of n-pentane, isopentane, n-hexane, n-heptane, n-octane, isooctane, n-nonane, n-decane, toluene, ethylbenzene, xylene, and trimethylbenzene.

[0028] According to some specific embodiments of the present invention, preferably, the hydrocarbon solvent is selected from at least one of n-hexane, n-heptane, n-decane, and toluene.

[0029] According to some specific embodiments of the present invention, preferably, the purification temperature is 60 - 140 °C and the time is 0.2 - 4 h.

[0030] According to some specific embodiments of the present invention, preferably, the purification temperature is 60 - 100 °C and the time is 1 - 3 h.

[0031] According to some specific embodiments of the present invention, preferably, the mass ratio of the solvent in the wet polypropylene powder is 15% - 35% (based on the total mass of the wet polypropylene powder being 100%); more preferably 15% - 30%.

[0032] According to some specific embodiments of the present invention, preferably, the ash content of the low-ash polypropylene resin is less than 20 ppm.

[0033] According to some specific embodiments of the present invention, preferably, the solid-liquid separation can be completed by centrifugation.

[0034] According to some specific embodiments of the present invention, preferably, after obtaining the low-ash polypropylene resin powder, there is also a step of extrusion granulation;

[0035] The extrusion granulation includes:

[0036] Mix the purified low-ash polypropylene resin powder with the antioxidant composite additive and then extrude and granulate to obtain the low-ash polypropylene resin.

[0037] The ash content of the low-ash polypropylene resin obtained by the extrusion granulation of the present invention is less than 20 ppm.

[0038] According to some specific embodiments of the present invention, preferably, the antioxidant composite additive includes an acid scavenger, a primary antioxidant, and a secondary antioxidant.

[0039] According to some specific embodiments of the present invention, preferably, the acid scavenger is selected from at least one of metal stearates and hydrotalcites; preferably zinc stearate and calcium stearate; the mass ratio of the low-ash polypropylene resin powder to the acid scavenger is (10,000 - 100,000):1.

[0040] According to some specific embodiments of the present invention, preferably, the primary antioxidant is selected from at least one of pentaerythritol tetra[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-S-triazine-2,4,6-(1H,3H,5H)trione, octadecyl β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, and 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane; preferably pentaerythritol tetra[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (i.e., antioxidant 1010) and 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-S-triazine-2,4,6-(1H,3H,5H)trione.

[0041] According to some specific embodiments of the present invention, preferably, the mass ratio of the low-ash polypropylene resin powder to the primary antioxidant is (1,000 - 10,000):1.

[0042] According to some specific embodiments of the present invention, preferably, the secondary antioxidant is selected from at least one of dilauryl thiodipropionate, distearyl thiodipropionate, bis(tetradecyl) thiodipropionate, tris(nonylphenyl) phosphite, dipentaerythritol diphosphite distearyl ester, tris(2,4-di-tert-butylphenyl) phosphite, and bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite; preferably dilauryl thiodipropionate and tris(2,4-di-tert-butylphenyl) phosphite (i.e., antioxidant 168).

[0043] According to some specific embodiments of the present invention, preferably, the mass ratio of the low-ash polypropylene resin powder to the auxiliary antioxidant is (100 - 10,000):1.

[0044] According to some specific embodiments of the present invention, preferably, the step of polymerizing propylene to obtain a polypropylene crude product includes:

[0045] Propylene prepolymerization:

[0046] In a prepolymerization reactor, a catalyst slurry is prepared, and the catalyst slurry includes a main catalyst, a cocatalyst, and a solvent; then propylene is introduced for prepolymerization reaction to obtain a prepolymer slurry.

[0047] Liquid-phase bulk polymerization:

[0048] Using propylene as a raw material, a polymerization reaction is carried out in the presence of the prepolymer slurry, a cocatalyst, an external electron donor, and hydrogen to obtain a polypropylene crude product slurry; the external electron donor is 9,9-bis(methoxymethyl)fluorene.

[0049] Gas-phase polymerization:

[0050] The polypropylene crude product slurry obtained by liquid-phase bulk polymerization enters a gas-phase fluidized bed reactor for polymerization reaction.

[0051] Polypropylene drying:

[0052] The material at the outlet of the gas-phase fluidized bed reactor is first subjected to gas-solid separation, and the solid component enters a dryer for drying to obtain the polypropylene crude product.

[0053] The following is a detailed description of each process for the preparation of the polypropylene crude product:

[0054] I. Propylene prepolymerization: In a prepolymerization reactor, a catalyst slurry is prepared, and the catalyst slurry includes a main catalyst, a cocatalyst, and a solvent; then propylene is introduced for prepolymerization reaction to obtain a prepolymer slurry.

[0055] During the prepolymerization process, a thin layer of polypropylene is formed on the surface of the catalyst particles.

[0056] Preferably, during the prepolymerization process, the mass ratio of propylene to the main catalyst is 2.5 - 10 kg / kg, and the mass ratio of the cocatalyst to the main catalyst is 0.027 - 0.075 kg / kg.

[0057] According to some specific embodiments of the present invention, preferably, the prepolymerization reaction includes two stages: in the first stage, the prepolymerization temperature is -5°C to 15°C, the propylene feeding rate is 10 to 45 kg / h, and the prepolymerization time is 0.5 to 3 h; in the second stage, the prepolymerization temperature is 5°C to 35°C, and the propylene feeding rate is 15 to 55 kg / h until all the propylene for prepolymerization is added to the prepolymerization reactor.

[0058] According to some specific embodiments of the present invention, preferably, the solvent in the catalyst slurry is selected from alkanes having 4 to 10 carbon atoms. More preferably, the alkanes having 4 to 10 carbon atoms are selected from at least one of butane, pentane, hexane, heptane, octane, nonane, and decane; most preferably hexane.

[0059] The main catalyst of the present invention is described in Chinese Patent 201080045745.X, which is incorporated herein by reference; it can also be obtained commercially. That is, as long as the main catalyst of the present invention meets the activity requirements, the activity of the main catalyst is at least 100,000 times; the specific calculation method of the catalyst activity is the mass ratio of the polymer obtained by the catalyst catalyzing propylene polymerization per unit time to the catalyst.

[0060] According to some specific embodiments of the present invention, preferably, the main catalyst is a Z-N series catalyst, including MgCl 2 support, TiCl 4 active center and internal electron donor; the internal electron donor is selected from at least one of alkyl-substituted 1,3-diether, aryl-substituted 1,3-diether, succinate, malonate, and glycol ester.

[0061] According to some specific embodiments of the present invention, preferably, the aryl of the aryl-substituted 1,3-diether is selected from monocyclic or polycyclic C 4-16 aryl. More specifically, the aryl of the aryl-substituted 1,3-diether is selected from one of cyclopentadienyl, indenyl, fluorenyl, and their derivatives.

[0062] According to some specific embodiments of the present invention, preferably, the structure of the alkyl-substituted 1,3-diether is shown in the following formula (I), and the structure of the aryl-substituted 1,3-diether is shown in the following formula (II) or formula (III):

[0063]

[0064] wherein, R 1 , R 2 , R 3 , R 4 , R 5 , R 6 are the same or different, and each independently is C 1-10Linear or branched alkyl groups.

[0065] According to some specific embodiments of the present invention, preferably, R 1 , R 2 , R 3 , R 4 , R 5 , R 6 are the same or different and are each independently methyl, ethyl, propyl, butyl, pentyl, heptyl, octyl, nonyl or decyl.

[0066] According to some specific embodiments of the present invention, preferably, the structures of the succinate and malonate are shown by the following formulas (IV) and (V) respectively:

[0067]

[0068] Wherein, R 7 , R 8 , R 9 and R 10 are the same or different and are each independently a linear or branched alkyl group of C 1-10 ; more preferably, R 7 , R 8 , R 9 and R 10 are each independently methyl, ethyl, propyl, butyl, pentyl, heptyl, octyl, nonyl or decyl.

[0069] According to some specific embodiments of the present invention, preferably, the cocatalyst is an alkylaluminum compound, and more preferably, the cocatalyst is selected from at least one of trialkylaluminum, alkylaluminoxane, a combination of trialkylaluminum and alkylaluminum halide, or a combination of trialkylaluminum and alkylaluminum hydride; most preferably triethylaluminum.

[0070] II. Liquid-phase bulk polymerization: Using propylene as a raw material, a polypropylene crude product slurry is obtained by carrying out a polymerization reaction in the presence of the prepolymer slurry, the cocatalyst, the external electron donor and hydrogen; the external electron donor is 9,9-bis(methoxymethyl)fluorene.

[0071] According to some specific embodiments of the present invention, preferably, the liquid-phase bulk polymerization includes the following steps:

[0072] Propylene, prepolymer slurry, cocatalyst, external electron donor, and hydrogen are added to the first reaction vessel, and a polymerization reaction is carried out at a temperature of 60°C to 75°C and a pressure of 2.5 to 3.5 MPa; the melt flow rate of the reaction slurry (the obtained polypropylene powder melt) at the outlet of the first reaction vessel is 4.0 to 4.5 g / 10 min; the hydrogen concentration in the first reaction vessel is 1000 to 3000 ppm; then the reaction slurry is added to the second reaction vessel, and hydrogen is introduced, and a polymerization reaction is carried out at a temperature of 60°C to 75°C and a pressure of 2.5 to 3.5 MPa. The melt flow rate of the reaction slurry (the obtained polypropylene powder melt) at the outlet of the second reaction vessel is 3.5 to 4.5 g / 10 min, and the hydrogen concentration in the second reaction vessel is 1000 to 2000 ppm.

[0073] According to some specific embodiments of the present invention, preferably, in the liquid-phase bulk polymerization process, the mass ratio of the main catalyst to propylene is 0.03 to 0.05 kg / t; the mass ratio of the cocatalyst to propylene is 0.015 to 0.0225 kg / t; the mass ratio of the external electron donor to the cocatalyst is 0 to 0.015 kg / kg, excluding 0.

[0074] According to some specific embodiments of the present invention, preferably, both the first reaction vessel and the second reaction vessel are kettle polymerization reactors, which are connected in series (forming a series polymerization process of continuously stirred tank reactors).

[0075] III. Gas-phase polymerization: The polypropylene crude product slurry obtained from the liquid-phase bulk polymerization enters a gas-phase fluidized bed reactor for a polymerization reaction.

[0076] According to some specific embodiments of the present invention, preferably, in the gas-phase polymerization, the polymerization temperature is 75 to 85°C, the polymerization pressure is 1.5 to 2.0 MPa, the melt flow rate of the polypropylene powder melt at the outlet of the gas-phase fluidized bed reactor is 2.8 to 3.2 g / 10 min, the ash content is less than 30 ppm, and the isotacticity is greater than 98.5%.

[0077] IV. Polypropylene drying: The material at the outlet of the gas-phase fluidized bed reactor is first subjected to gas-solid separation. The solid component (polypropylene powder) (the separated gas phase part enters the recovery system for recycling) enters a dryer for drying to obtain the polypropylene crude product.

[0078] According to some specific embodiments of the present invention, preferably, the solid component enters the dryer and is dried by convective inert gas. The inert gas is preferably nitrogen, and the drying temperature is preferably 100 - 110°C.

[0079] According to some specific embodiments of the present invention, preferably, the drying temperature is 95 to 115°C, more preferably 100 - 110°C.

[0080] On the other hand, the present invention provides a low-ash polypropylene resin prepared by the above preparation method.

[0081] The beneficial effects of the present invention include:

[0082] 1) The present invention uses an additive containing a bifunctional group and a hydrocarbon solvent as a polymer composite washing solvent, which significantly improves the complexing ability of the composite solvent with metal ions. Compared with the prior art, the polypropylene prepared by the present invention has lower ash content, and the product has good electrical properties and can meet the requirements of high-end capacitors.

[0083] 2) The washing solvent used in the present invention has simple composition, fewer washing times and less consumption of washing solvent. Compared with the prior art, it is more energy-saving and environmentally friendly and is more conducive to engineering applications. Detailed Embodiments

[0084] To illustrate the present invention more clearly, the present invention will be further described below in conjunction with preferred embodiments. Those skilled in the art should understand that the specific content described below is illustrative rather than restrictive, and should not be used to limit the protection scope of the present invention.

[0085] All numerical designations in the present invention (such as temperature, time, concentration, weight, etc., including the range of each of them) can generally be approximate values that can be appropriately changed (+) or (-) in increments of 0.1 or 1.0. All numerical designations can be understood as having the term "about" in front.

[0086] Source of raw materials or equipment:

[0087] Polypropylene, prepared by a polypropylene model device;

[0088] The main catalyst is provided by Beijing Aoda Catalyst Factory;

[0089] Triethylaluminum is provided by Yingkou Xiangyang Catalyst Factory;

[0090] Cyclohexylmethyldimethoxysilane is provided by Shandong Lujing Chemical Technology Co., Ltd.;

[0091] Hydrogen is provided by Lanzhou Petrochemical Company;

[0092] n-Hexane: provided by Tianjin Jiangtian Chemical Industry;

[0093] Ethyl acetoacetate: provided by Shanghai Macklin;

[0094] n-Heptane: provided by Tianjin Jiangtian Chemical Industry;

[0095] Methyl acetoacetate: provided by Shanghai Macklin;

[0096] Ethyl pivaloylacetate: provided by Shanghai Macklin;

[0097] Toluene: provided by Tianjin Jiangtian Chemical Industry;

[0098] n-Decane: provided by Tianjin Jiangtian Chemical Industry;

[0099] Acetylacetone: provided by Shanghai Macklin;

[0100] Ethyl acetate: provided by Shanghai Macklin;

[0101] Ethanol: provided by Tianjin Jiangtian Chemical Industry;

[0102] Butyl acetate: provided by Shanghai Macklin.

[0103] Evaluation and analysis method:

[0104] 1. Polymer ash content: Tested by inductively coupled plasma mass spectrometry.

[0105] Put the polypropylene resin into a microwave digestion instrument, add 6 mL of concentrated nitric acid and 2 mL of hydrogen peroxide to completely digest it into a transparent solution, then make up the volume to 50 mL, and conduct sample analysis through an inductively coupled plasma mass spectrometer. Determine the ash content of each element in the digestion solution by comparing with the standard curve, and calculate according to the following formula:

[0106]

[0107] In the formula: W is the metal ash content in polypropylene, μg / g (mass of a certain element / mass of polypropylene); ρ is the element concentration in the digestion solution, μg / ml; m is the mass of polypropylene weighed, g.

[0108] 2. Polymer isotacticity: Determined in accordance with GB / T 2412-2008.

[0109] 3. Electrical strength of the film: Tested in accordance with GB / T13542.2-2021.

[0110] Example 1

[0111] Propylene prepolymerization:

[0112] Add 500 kg of hexane and 7 kg of triethylaluminum solution (in hexane solvent, with a mass concentration of 15%) to the catalyst preparation tank. Then add 25 kg of titanium metal catalyst (Beijing Aoda HA-R catalyst) supported by magnesium chloride, and stir to form a homogeneous suspension. Use chilled brine to lower the temperature of the preparation tank below 5°C, and then add 1500 kg of fresh hexane to adjust the concentration of the catalyst slurry. Start stirring at 80 r / min. In the first stage of prepolymerization, open the automatic gas-phase propylene feeding valve, adjust the propylene feeding rate to 10 kg / h, start stirring at 40 r / min, and lower the temperature of the preparation tank below -5°C for 1 h of prepolymerization. In the second stage of prepolymerization, adjust the propylene feeding rate to 15 kg / h, start stirring at 40 r / min, and raise the temperature of the preparation tank to 5°C. Continue prepolymerization until all 100 kg of propylene is consumed to obtain the propylene prepolymer slurry. Transfer the above propylene prepolymer slurry to a storage tank for standby.

[0113] Liquid-phase bulk polymerization:

[0114] Add the propylene prepolymer slurry, propylene, cocatalyst, external electron donor, and an appropriate amount of hydrogen to the first stirred reactor for liquid-phase bulk polymerization. The gas-phase propylene is refluxed into the reactor through the top condenser. The polymerization temperature is 70°C, the polymerization pressure is 3.0 MPa, the liquid level is 49%, the feeding rate of the prepolymer slurry is 14.0 kg / h, the propylene feeding rate is 5.7 t / h, the feeding rate of the triethylaluminum solution is 0.8 kg / h, the feeding rate of 9,9-bis(methoxymethyl)fluorene is 0.07 kg / h, and the hydrogen feeding rate is 4.9 Nm 2 / h, and the hydrogen concentration is 2890 ppm. The flow rate of the reaction slurry at the outlet is 4.5 t / h.

[0115] The slurry from the outlet of the first stirred reactor enters the second stirred reactor for polymerization. The temperature of the second polymerization reactor is 64°C, the polymerization pressure is 2.6 MPa, the liquid level is 44%, and hydrogen is added to the second stirred polymerization reactor. The hydrogen feeding rate is 2.0 Nm 2 / h, and the hydrogen concentration is 1300 ppm. The flow rate of the reaction slurry at the outlet is 4.5 t / h.

[0116] Gas-phase polymerization:

[0117] The slurry from the outlet of the second stirred reactor enters the gas-phase fluidized-bed reactor for polymerization. The polymerization temperature is 80°C, the polymerization pressure is 1.8 MPa, and the bed level is 51%.

[0118] Polypropylene drying:

[0119] The material from the outlet of the gas-phase fluidized-bed reactor first enters the cyclone separator for gas-solid separation of the material. The separated solid-phase polypropylene powder enters the dryer and is dried with convective nitrogen gas. The drying temperature is 105°C.

[0120] Purification:

[0121] The polypropylene powder is dried by inert gas and then transported to the polymer deep purification tank, and a certain amount of composite solvent (a composite solvent of n-hexane and ethyl acetoacetate, with a mass ratio of 2:1) is added to form a polymer powder slurry. The slurry concentration is 10%. Polymer deep purification is carried out under certain process conditions (stirring speed 50 rpm, slurry feeding speed 20 t / h). The purification temperature is 60 °C and the purification time is 1.5 h. The polymer slurry after deep purification is centrifuged to obtain polymer wet powder. The mass ratio of the solvent in the wet powder is 25%. Then, through a multi-stage drying process (drying temperature 105 °C), volatile organic compounds are removed. Then it is transported to the extrusion granulation unit to obtain low-ash polypropylene resin for capacitor film.

[0122] Extrusion granulation:

[0123] A low-ash long-acting antioxidant composite additive (antioxidant 1010 is 2000 mg / kg, 168 is 1000 mg / kg, calcium stearate is 80 mg / kg) and the dried polypropylene powder are simultaneously added to a twin-screw extruder for extrusion granulation (granulation temperature is 225 °C, screw speed is 120 rpm) to obtain low-ash polypropylene resin for capacitor film.

[0124] Example 2

[0125] The preparation process is the same as that of Example 1.

[0126] The difference is that: the polypropylene powder is dried by inert gas and then transported to the polymer deep purification tank, and a certain amount of composite solvent (a composite solvent of n-hexane and ethyl acetoacetate, with a mass ratio of 2:1) is added to form a polymer powder slurry. The slurry concentration is 15%. Polymer deep purification is carried out under certain process conditions (stirring speed 50 rpm, slurry feeding speed 20 t / h). The purification temperature is 60 °C and the purification time is 1.5 h. The polymer slurry after deep purification is centrifuged to obtain polymer wet powder. The mass ratio of the solvent in the wet powder is 25%. Then, through a multi-stage drying process (drying temperature 105 °C), volatile organic compounds are removed. Then it is transported to the extrusion granulation unit to obtain low-ash polypropylene resin for capacitor film.

[0127] Example 3

[0128] The preparation process is the same as that of Example 1.

[0129] The differences are as follows: The polypropylene powder is dried by inert gas and then transported to the polymer deep purification tank, and a certain amount of composite solvent (a composite solvent of n-hexane and ethyl acetoacetate with a mass ratio of 2:1) is added to form a polymer powder slurry with a slurry concentration of 30%. Polymer deep purification is carried out under certain process conditions (stirring speed 50 rpm, slurry feeding speed 20 t / h), the purification temperature is 60 °C, and the purification time is 1.5 h. The polymer slurry after deep purification is centrifuged to obtain wet polymer powder, and the mass ratio of the solvent in the wet powder is 25%. Then, through a multi-stage drying process (drying temperature 105 °C), volatile organic compounds are removed. Then it is transported to the extrusion granulation unit to obtain low-ash polypropylene resin for capacitor film.

[0130] Example 4

[0131] The preparation process is the same as that of Example 1.

[0132] The differences are as follows: The polypropylene powder is dried by inert gas and then transported to the polymer deep purification tank, and a certain amount of composite solvent (a composite solvent of n-heptane and ethyl acetoacetate with a mass ratio of 5:3) is added to form a polymer powder slurry with a slurry concentration of 10%. Polymer deep purification is carried out under certain process conditions (stirring speed 50 rpm, slurry feeding speed 20 t / h), the purification temperature is 80 °C, and the purification time is 1 h. The polymer slurry after deep purification is centrifuged to obtain wet polymer powder, and the mass ratio of the solvent in the wet powder is 25%. Then, through a multi-stage drying process (drying temperature 105 °C), volatile organic compounds are removed. Then it is transported to the extrusion granulation unit to obtain low-ash polypropylene resin for capacitor film.

[0133] Example 5

[0134] The preparation process is the same as that of Example 1.

[0135] The differences are as follows: The polypropylene powder is dried by inert gas and then transported to the polymer deep purification tank, and a certain amount of composite solvent (a composite solvent of n-hexane and ethyl pivaloylacetate with a mass ratio of 2:1) is added to form a polymer powder slurry with a slurry concentration of 10%. Polymer deep purification is carried out under certain process conditions (stirring speed 50 rpm, slurry feeding speed 20 t / h), the purification temperature is 80 °C, and the purification time is 1 h. The polymer slurry after deep purification is centrifuged to obtain wet polymer powder, and the mass ratio of the solvent in the wet powder is 25%. Then, through a multi-stage drying process (drying temperature 105 °C), volatile organic compounds are removed. Then it is transported to the extrusion granulation unit to obtain low-ash polypropylene resin for capacitor film.

[0136] Example 6

[0137] The preparation process is the same as that of Example 1.

[0138] The difference is as follows: The polypropylene powder is dried by inert gas and then transported to the polymer deep purification tank, and a certain amount of composite solvent (a composite solvent of n-hexane and ethyl pivaloylacetate, with a mass ratio of 13:1) is added to form a polymer powder slurry. The slurry concentration is 10%. Polymer deep purification is carried out under certain process conditions (stirring speed 50 rpm, slurry feeding speed 20 t / h), the purification temperature is 80 °C, and the purification time is 1 h. The polymer slurry after deep purification is centrifuged to obtain polymer wet powder, and the mass ratio of the solvent in the wet powder is 25%. Then, through a multi-stage drying process (drying temperature 105 °C), volatile organic compounds are removed. Then it is transported to the extrusion granulation unit to obtain low-ash polypropylene resin for capacitor film.

[0139] Example 7

[0140] The preparation process is the same as that of Example 1.

[0141] The difference is as follows: The polypropylene powder is dried by inert gas and then transported to the polymer deep purification tank, and a certain amount of composite solvent (a composite solvent of n-hexane and ethyl pivaloylacetate, with a mass ratio of 3:2) is added to form a polymer powder slurry. The slurry concentration is 10%. Polymer deep purification is carried out under certain process conditions (stirring speed 50 rpm, slurry feeding speed 20 t / h), the purification temperature is 80 °C, and the purification time is 1 h. The polymer slurry after deep purification is centrifuged to obtain polymer wet powder, and the mass ratio of the solvent in the wet powder is 25%. Then, through a multi-stage drying process (drying temperature 105 °C), volatile organic compounds are removed. Then it is transported to the extrusion granulation unit to obtain low-ash polypropylene resin for capacitor film.

[0142] Comparative Example 1

[0143] The preparation process is the same as that of Example 1.

[0144] The difference is as follows: The polypropylene powder is dried by inert gas and then transported to the polymer deep purification tank, and a certain amount of hexane solvent is added to form a polymer powder slurry. The slurry concentration is 10%. Polymer deep purification is carried out under certain process conditions (stirring speed 50 rpm, slurry feeding speed 20 t / h), the purification temperature is 60 °C, and the purification time is 1.5 h. The polymer slurry after deep purification is centrifuged to obtain polymer wet powder, and the mass ratio of the solvent in the wet powder is 25%. Then, through a multi-stage drying process (drying temperature 105 °C), volatile organic compounds are removed. Then it is transported to the extrusion granulation unit to obtain polypropylene resin.

[0145] Comparative Example 2

[0146] The preparation process is the same as that of Example 1.

[0147] The differences are as follows: The polypropylene powder is dried by inert gas and then transported to the polymer deep purification tank, and a certain amount of ethyl acetoacetate solvent is added to form a polymer powder slurry with a slurry concentration of 10%. Polymer deep purification is carried out under certain process conditions (stirring speed 50 rpm, slurry feeding speed 20 t / h), the purification temperature is 60 °C, and the purification time is 1.5 h. The polymer slurry after deep purification is centrifuged to obtain polymer wet powder, and the mass ratio of the solvent in the wet powder is 25%. Then, through a multi-stage drying process (drying temperature 105 °C), volatile organic compounds are removed. Then it is transported to the extrusion granulation unit to obtain polypropylene resin.

[0148] Comparative Example 3

[0149] The preparation process is the same as that of Example 1.

[0150] The differences are as follows: The polypropylene powder is dried by inert gas and then transported to the polymer deep purification tank, and a certain amount of ethanol solvent is added to form a polymer powder slurry with a slurry concentration of 10%. Polymer deep purification is carried out under certain process conditions (stirring speed 50 rpm, slurry feeding speed 20 t / h), the purification temperature is 60 °C, and the purification time is 1.5 h. The polymer slurry after deep purification is centrifuged to obtain polymer wet powder, and the mass ratio of the solvent in the wet powder is 25%. Then, through a multi-stage drying process (drying temperature 105 °C), volatile organic compounds are removed. Then it is transported to the extrusion granulation unit to obtain polypropylene resin.

[0151] Comparative Example 4

[0152] The preparation process is the same as that of Example 1.

[0153] The differences are as follows: The polypropylene powder is dried by inert gas and then transported to the polymer deep purification tank, and a certain amount of composite solvent (hexane and ethanol solvent, mass ratio 4:1) is added to form a polymer powder slurry with a slurry concentration of 10%. Polymer deep purification is carried out under certain process conditions (stirring speed 50 rpm, slurry feeding speed 20 t / h), the purification temperature is 60 °C, and the purification time is 1.5 h. The polymer slurry after deep purification is centrifuged to obtain polymer wet powder, and the mass ratio of the solvent in the wet powder is 25%. Then, through a multi-stage drying process (drying temperature 105 °C), volatile organic compounds are removed. Then it is transported to the extrusion granulation unit to obtain polypropylene resin.

[0154] Comparative Example 5

[0155] The preparation process is the same as that of Example 1.

[0156] The differences are as follows: The polypropylene powder is dried by inert gas and then transported to the polymer deep purification tank, and a certain amount of composite solvent (hexane and acetylacetone solvent, with a mass ratio of 2:1) is added to form a polymer powder slurry. The slurry concentration is 10%. Polymer deep purification is carried out under certain process conditions (stirring speed 50 rpm, slurry feeding speed 20 t / h), the purification temperature is 60 °C, and the purification time is 1.5 h. The polymer slurry after deep purification is centrifuged to obtain polymer wet powder, and the mass ratio of the solvent in the wet powder is 25%. Then, through a multi-stage drying process (drying temperature 105 °C), volatile organic compounds are removed. Then it is transported to the extrusion granulation unit to obtain polypropylene resin.

[0157] Comparative Example 6

[0158] The preparation process is the same as that of Example 1.

[0159] The differences are as follows: The polypropylene powder is dried by inert gas and then transported to the polymer deep purification tank, and a certain amount of composite solvent (hexane and butyl acetate solvent, with a mass ratio of 2:1) is added to form a polymer powder slurry. The slurry concentration is 10%. Polymer deep purification is carried out under certain process conditions (stirring speed 50 rpm, slurry feeding speed 20 t / h), the purification temperature is 60 °C, and the purification time is 1.5 h. The polymer slurry after deep purification is centrifuged to obtain polymer wet powder, and the mass ratio of the solvent in the wet powder is 25%. Then, through a multi-stage drying process (drying temperature 105 °C), volatile organic compounds are removed. Then it is transported to the extrusion granulation unit to obtain polypropylene resin.

[0160] Comparative Example 7

[0161] The preparation process is the same as that of Example 1.

[0162] The differences are as follows: The polypropylene powder is dried by inert gas and then transported to the polymer deep purification tank, and a certain amount of composite solvent (hexane, acetylacetone and butyl acetate solvent, with a mass ratio of 4:1:1) is added to form a polymer powder slurry. The slurry concentration is 10%. Polymer deep purification is carried out under certain process conditions (stirring speed 50 rpm, slurry feeding speed 20 t / h), the purification temperature is 60 °C, and the purification time is 1.5 h. The polymer slurry after deep purification is centrifuged to obtain polymer wet powder, and the mass ratio of the solvent in the wet powder is 25%. Then, through a multi-stage drying process (drying temperature 105 °C), volatile organic compounds are removed. Then it is transported to the extrusion granulation unit to obtain polypropylene resin.

[0163] The products obtained from Examples 1-7 and Comparative Examples 1-7 were subjected to ash content test and isotacticity test, as shown in Tables 1 and 2 below.

[0164] The polypropylene resin product was made into a film with a thickness of 5 μm, and its electrical strength was tested in accordance with GB / T 13542.2-2021, as shown in Table 3.

[0165] Table 1 Total ash content and elemental content of the polypropylene resin product

[0166]

[0167] Table 2 Isotacticity of the polypropylene resin product

[0168]

[0169] Table 3 Breakdown voltage of the polypropylene film

[0170]

[0171]

[0172] In Examples 1 to 3, as the slurry concentration of the washing solvent continuously increased, the ash content data of the polypropylene resin product increased from 12 ppm to 18 ppm, the isotacticity was between 98.5% and 98.8%, and the breakdown strength of the prepared film increased from 659 v / μm to 673 v / μm. In Examples 4 to 7, when the washing temperature was increased to 80 °C, the isotacticity of the polypropylene resin was ≥90%, and the ash content data was basically the same as that in Example 3. Due to the relatively high isotacticity of the resin, the breakdown strength of the film increased to 684 v / μm.

[0173] In Comparative Examples 1 to 3, when the washing solvent was only one of hydrocarbons, bifunctional group additives or alcohols, the ash residue of the washed polypropylene resin was still relatively high. The composite solvent must simultaneously exert the synergistic effect of hydrocarbons and bifunctional group additives to have a good washing effect.

[0174] In Comparative Examples 4 to 6, when the additives in the washing solvent only contained groups such as hydroxyl, keto or ester groups, the effect of washing the catalyst residue ash in the polypropylene resin was inferior to that of the composite solvent containing bifunctional groups, but still better than that in Comparative Examples 1 to 3.

[0175] Although the washing solvent in Comparative Example 7 contained hydrocarbons, keto and ester groups at the same time, the keto and ester groups were not within one molecule and could not exert an electron conduction effect on each other, and its complexing ability was inferior to that of the bifunctional group additive of the present invention.

[0176] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, rather than limitations on the implementation manners of the present invention. For those of ordinary skill in the art, other different forms of changes or variations can be made based on the above description. It is impossible to list all the implementation manners here. Any obvious changes or variations derived from the technical solutions of the present invention still fall within the protection scope of the present invention.

Claims

1. A preparation method of a low-ash polypropylene resin, wherein, the preparation method comprises: a step of polymerizing propylene as a raw material to obtain a crude polypropylene product; and a step of purifying the obtained crude polypropylene product to obtain the low-ash polypropylene resin; the purification comprises: configuring the dried crude polypropylene into a slurry using a composite solvent, stirring for purification; then performing solid-liquid separation on the slurry to obtain wet polypropylene powder, and drying to obtain low-ash polypropylene resin powder; the composite solvent comprises a hydrocarbon solvent and a bifunctional group additive, and the bifunctional group additive has a structure shown by the following formula: In the formula, R 1 is an alkane group of C 1-5 , R 2 is an alkane group of C 1-3 , and R 3 is an alkane group of C 1-4 .

2. The preparation method according to claim 1, wherein, during the purification process, the mass ratio of the hydrocarbon solvent in the configured slurry is 46% - 89%, the mass ratio of the bifunctional group additive is 6% - 40%, and the mass concentration of the crude polypropylene product is 5% - 30%.

3. The preparation method according to claim 1, wherein, the bifunctional group additive is selected from at least one of ethyl acetoacetate, methyl acetoacetate, butyl acetoacetate, and ethyl neopentanoylacetate.

4. The preparation method according to claim 1, wherein, The hydrocarbon solvent is selected from at least one of alkanes having C 5～20 and aromatic hydrocarbons having C 6～12 .

5. The preparation method according to claim 1, wherein, the hydrocarbon solvent is selected from at least one of n-pentane, isopentane, n-hexane, n-heptane, n-octane, isooctane, n-nonane, n-decane, toluene, ethylbenzene, xylene, and mesitylene.

6. The preparation method according to claim 1, wherein, the temperature of the purification is 60 - 140 °C, and the time is 0.2 - 4 h.

7. The preparation method according to claim 1, wherein, the mass ratio of the solvent in the wet polypropylene powder is 15% - 35%.

8. The preparation method according to claim 1, wherein, the ash content of the low-ash polypropylene resin is less than 20 ppm.

9. The preparation method according to claim 1, wherein, after obtaining the low-ash polypropylene resin powder, it further comprises a step of extrusion granulation; the extrusion granulation comprises: mixing the purified low-ash polypropylene resin powder with an antioxidant composite additive and then performing extrusion granulation to obtain the low-ash polypropylene resin.

10. The preparation method according to claim 1, wherein, the antioxidant composite additive comprises an acid scavenger, a primary antioxidant, and a secondary antioxidant; the acid scavenger is selected from at least one of metal stearates and hydrotalcite; the primary antioxidant is selected from at least one of pentaerythritol tetra[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-S-triazine-2,4,6-(1H,3H,5H)trione, octadecyl β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, and 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane; The co-antioxidant is selected from at least one of dilauryl thiodipropionate, distearyl thiodipropionate, bis(tetradecyl) thiodipropionate, tris(nonylphenyl) phosphite, dipentaerythritol distearyl diphosphite, tris(2,4-di-tert-butylphenyl) phosphite, and dipentaerythritol bis(2,4-di-tert-butylphenyl) diphosphite.

11. The preparation method according to claim 1, wherein, the step of obtaining the polypropylene crude product by polymerizing propylene as a raw material includes: Propylene prepolymerization: In a prepolymerization reactor, a catalyst slurry is prepared, and the catalyst slurry includes a main catalyst, a cocatalyst, and a solvent; then propylene is introduced to carry out a prepolymerization reaction to obtain a prepolymer slurry; Liquid-phase bulk polymerization: Using propylene as a raw material, a polymerization reaction is carried out in the presence of the prepolymer slurry, a cocatalyst, an external electron donor, and hydrogen to obtain a polypropylene crude product slurry; the external electron donor is 9,9-bis(methoxymethyl)fluorene; Gas-phase polymerization: The polypropylene crude product slurry obtained by liquid-phase bulk polymerization enters a gas-phase fluidized bed reactor for a polymerization reaction; Polypropylene drying: The material at the outlet of the gas-phase fluidized bed reactor is first subjected to gas-solid separation, and the solid component enters a dryer for drying to obtain the polypropylene crude product.

12. The preparation method according to claim 11, wherein, During the prepolymerization process, the mass ratio of propylene to the main catalyst is 2.5 - 10 kg / kg, and the mass ratio of the cocatalyst to the main catalyst is 0.027 - 0.075 kg / kg.

13. The preparation method according to claim 11, wherein, The prepolymerization reaction includes two stages: in the first stage, the prepolymerization temperature is -5°C to 15°C, the propylene feeding rate is 10 - 45 kg / h, and the prepolymerization time is 0.5 - 3 h; in the second stage, the prepolymerization temperature is 5°C to 35°C, the propylene feeding rate is 15 - 55 kg / h until all the propylene for prepolymerization is added to the prepolymerization reactor.

14. The preparation method according to claim 11, wherein, The solvent in the catalyst slurry is selected from at least one of butane, pentane, hexane, heptane, octane, nonane, and decane.

15. The preparation method according to claim 11, wherein, The main catalyst is a Ziegler-Natta catalyst series, including MgCl 2 support, TiCl 4 active center and internal electron donor; the internal electron donor is selected from at least one of alkyl-substituted 1,3-diether, aryl-substituted 1,3-diether, succinate, malonate and glycol ester.

16. The preparation method according to claim 11, wherein, The cocatalyst is an alkylaluminum compound.

17. The preparation method according to claim 11, wherein, The liquid-phase bulk polymerization includes the following steps: Propylene, prepolymer slurry, cocatalyst, external electron donor and hydrogen are added to the first reaction vessel, and a polymerization reaction is carried out at a temperature of 60 °C to 75 °C and a pressure of 2.5 to 3.5 MPa; the flow rate of the reaction slurry at the outlet of the first reaction vessel is 4.0 to 4.5 g / 10 min; the hydrogen concentration in the first reaction vessel is 1000 to 3000 ppm; then the reaction slurry is added to the second reaction vessel, and hydrogen is introduced, and a polymerization reaction is carried out at a temperature of 60 °C to 75 °C and a pressure of 2.5 to 3.5 MPa, and the flow rate of the reaction slurry at the outlet of the second reaction vessel is 3.5 to 4.5 g / 10 min, and the hydrogen concentration in the second reaction vessel is 1000 to 2000 ppm.

18. According to the preparation method described in claim 11, wherein, in the liquid-phase bulk polymerization process, the mass ratio of the main catalyst to propylene is 0.03 to 0.05 kg / t; the mass ratio of the cocatalyst to propylene is 0.015 to 0.0225 kg / t; the mass ratio of the external electron donor to the cocatalyst is 0 to 0.015 kg / kg, excluding 0.

19. According to the preparation method described in claim 11, wherein, in the gas-phase polymerization, the polymerization temperature is 75 to 85 °C, the polymerization pressure is 1.5 to 2.0 MPa, the melt flow rate of the polypropylene powder at the outlet of the gas-phase fluidized bed reactor is 2.8 to 3.2 g / 10 min, the ash content is less than 30 ppm, and the isotacticity is greater than 98.5%.

20. A low-ash polypropylene resin prepared by the preparation method according to any one of claims 1-19.

Images